sol-gel chemistry of transition metal...

Prog. Solid St. Chem. Vol, 18, pp. 250341, 1988 007%6786/88 $0.00 + .50 Printed in Great Britain. All rights reserved Copyright © 1989 Pergamon Press plc

SOL-GEL CHEMISTRY OF TRANSITION METAL OXIDES

J. Livage, M. Henry and C. Sanchez

Laboratoire de Chimie de la Mati6re Condensde, CNRS (UA 302), Universit6 Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France

i. INTRODUCTION

The sol-gel process provides a new approach to the preparation of glasses and

ceramics. Starting from molecular precursors, an oxide network is obtained via inorganic

polynterization reactions. These reactions occur in solutions and the term "sol-gel

processing" is often broadly used to describe the synthesis of inorganic oxides by "wet

chemistry" methods. These processes offer many advantages as compared to the conventional

"powder" route, such as :

Homogeneous multi-component systems can be easily obtained by mixing the molecular

precursor solutions 1,2

Temperatures required for material processing can be noticeably lowered leading to unusual

glasses or ceramics 3

The rheological properties of sols or gels allow the formation of fibers, films or

composites by such techniques as spinning 4, dip-coating 5 or impregnation 6

This explains why the sol-gel process has received so much scientific and

technological attention during the last decade. Several international meetings are now

devoted mainly to this topic, namely the "International Workshop on Glasses and Glass-

Ceramics from Gels" 7-10, "Ultrastructure Processing of Ceramics, Glasses and Composites"

11-13 and "Better Ceramics through Chemistry" 14,15

One unique property of the sol-gel process is the ability to go all the way from

the molecular precursor to the product, allowing a better control of the whole process and

the synthesis of "tailor-made" materials. Therefore, a real mastery of the sol-gel process

would require an emphasis which relates chemical reactivity to gel formation and powder

morphology. The present paper reviews successively the chemistry of the molecular

precursors, the aggregation phenomena involved in the sol-gel to material transformation,

the physical properties and applications of transition metal oxide gels.

The chemistry of the sol-gel process is based on hydroxylation and condensation of

molecular precursors. These reactions have been extensively studied in the case of silica

16. Unfortunately, much less data is available for transition metal oxide precursors. Two

different routes are usually described in the literature depending on whether the precursor

is an aqueous solution of an inorganic salt or a metal organic compound. The aqueous

chemistry of transition metal ions is described in the first section of this paper. This

topic can be quite complicated because of the numerous molecular species which can exist

depending on the oxidation state, the pH or the concentration. Moreover, in the case of non

tetravalent cations, oxides, hydroxides and even oxo-hydroxides can be obtained 17,18

The most versatile precursors for the sol-gel synthesis of oxides are undoubtely

JPSSC 18:4-A 259

260 J. Livage et al.

19 metal alkoxides which are very reactive toward nucleophilic reagents such as water

Hydrolysis and condensation of transition metal alkoxides are described in the second

section. These alkoxides appear to be much more reactive than silicon alkoxides. They must

be handled with great care, in a dry environment and are often stabilized via chemical

modification 20 This high chemical reactivity is due to the lower electronegativity of the

metal as compared with silicon, and the metal atom's ability to exhibit several

coordination states. As a result of the latter property, coordination expansion

spontaneously occurs when the metal alkoxide reacts with water.

Structural evolution during the sol to gel and gel to solid transitions need to be

fully understood before a real mastery of the sol-gel process can be reached. The

properties of a gel and its response to heat treatment are very sensitive to the structure

already created during the sol stage. Therefore the formation of colloidal aggregates often

determines the main properties of the resulting powder and its ability for the extent to

which the powder can be sintered. By varying the chemical conditions under which silica is

polymerized, structures can be formed which range from randomly branched polymers to

colloidal particles 21 The aggregation of colloidal SiO 2 particles and the growth of

silica polymers have been extensively studied during the last few years. They usually give

rise to very tenuous objects which have very low densities even for large radii of gyration

and can be described as fraetal aggregates 22

Monodispersed transition metal oxide colloids are currently synthesized which can

exhibit anisotropic shapes 23. Particle-particle interactions then lead to the formation of

anisotropic aggregates in which all individual particles are mutually oriented. These

ordered aggregates, called "tactoids" 24 will be described in the third section. They can

lead to anisotropic coatings that behave as host structures for intercalation 25

Sols and gels are usually considered as intermediates in the processing of glasses

and ceramics. Therefore, drying and densification are very important processes that need to

be fully understood 26-28. The present paper does not intend to describe the properties and

applications of transition metal oxide-based materials obtained via the gel route. These

will be reviewed briefly at the end of the paper. The fourth section shows that transition

metal oxide gels are actually diphasic materials made of solvent molecules trapped in a

solid network. Specific electronic and ionic properties arising from the two phases

together with their interface have been observed. They lead to new applications such as

antistatic coatings or electrochromic devices 29

A survey of the literature shows that most studies are concerned with the sol-gel

processing of silicates 30. Fewer papers have been published about A1203, TiO 2 or ZrO 2 and

very few papers deal with other transition metal oxides 29. Therefore, the present article

is mainly concerned with transition elements of the "d" group. However, most of the ideas

described here can be extended to other elements belonging to the "p" (B, AI, P .... ) or

"f" (rare-earths) groups.

2. AQUEOUS CHEMISTRY OF INORGANIC PRECURSORS

The aqueous chemistry of inorganic salts is quite complicated owing to the occur-

rence of hydrolysis reactions which convert the ions to new ionic species or to

precipitates. The hydrolysis of salts can involve the cation, the anion or even both. We

will start by considering the hydrolysis of metal cations, which was first studied by N.

Bjerrum at the beginning of the 20th century 31 At the same time, A. Werner 32 and P.

Pfeiffer 33 proposed the concept of "aquo-acidity" which describes cation hydrolysis as

Sol-Gel Chemistry of Transition Metal Oxides 261

the stepwise removal of protons from hydrating water molecules. However, until the work of

L.G. Sillen 34 , the formation of polynuclear hydrolysis products was almost ignored. This

author proposed a mechanism of hydrolysis in which hydroxyl groups are added to the cation

which leads to the formation of condensed species. Iso o and heteropoly oxometalates are now

well known 3S , and detailed experimental data on the hydrolysis of cations can be found in

the literature 18,36 Unfortunately, it is difficult to account for these data

quantitatively on a theoretical basis. However, a model was recently proposed which allows

the calculation of the partial charge distribution of any complex in order to predict their

chemical reactivity. When two atoms combine, a partial electron transfer occurs so that

each atom acquires a positive or negative partial charge 6 i . It is usually assumed that the

electronegativity Xi of an atom changes linearly with its charge 38 :

xi = x~ + ~i~i (1)

where X~ ° is the electronegativity of the neutral atom and N~ is the "hardness" which may

be defined as 37.

.~ - k/f T (2)

that depends on the electronegativity scale (k = 1.36 for Pauling's

to the principle of electronegativity equalization stated by R.T.

charge transfer should stop when the electronegativities of all

k is a constant

scale). According

Sanderson 39 , the

constituent atoms become equal to the mean electronegativity X given by 37 :

= Zi Pi/~ + kz

Zi (pi/~) (3)

where Pi corresponds to the stoichiometry of the i th atom in the compound and z is the

total charge of the ionic species. Electronegativity actually corresponds to the electronic

chemical potential and electronegativity equalization is nothing else than the well-known

thermodynamic principle of chemical potential equalization in the equilibrium state. The

partial charge 6 i can be deduced from eq. (1),(2) and (3) leading to:

6~ = (~ - x[)Ik~ (4)

6 i can be easily calculated knowing the electronegativity X~ of all neutral atoms, the

stoichiometric composition for the ionic species and its charge z. The Partial Charge Model

can be applied to both inorganic and metal-organic precursors. It is based on simple ideas

and is easy to handle. It corresponds to a thermodynamic approach and leads to a relatively

good quantification of inductive effects. However, several limitations do arise, namely :

In its present form, the Partial Charge Model does not take into account the real

structure of the chemical species.

- Resonance effects and ~ overlapping are not included.

It is difficult to account for coordination variations which occur during the chemical

process.

Nevertheless, this model can be applied successfully to describe the chemical

reactions involved in the sol-gel process and provides a useful guide for inorganic

polymerization reactions.

2.1. Hydrolysis of metal cations

2. i.I. Formation of inorganic precursors. When dissolved in pure water, a cation M z+

becomes solvated by the surrounding water molecules according to :

M z ÷ + :0 ~ M +-- 0


In the case of transition metal ions, this solvation leads to the formation of a

partially covalent bond. A partial charge transfer occurs from the filled 3a I bonding

orbital of the water molecule to the empty d orbitals of the transition metal ion. The

positive partial charge on the hydrogen atoms then increases and the water molecule, as a

whole, becomes more acidic. Depending on the magnitude of the electron transfer, the

following reactions occur :

[M - OH2 ]z+ = [M-OH] (z'l)+ + H + = [M=O] (z'2)+ + 2H +

Three kinds of ligands must then be considered in a non complexing aqueous medium : aquo

ligands (OH2) , hydroxo ligands (-OH), and oxo ligands (=0).

Let N be the number of water molecules covalently bound to the cation M z+

(coordination number). The rough formula for any inorganic precursor can then be written as

[MONH2,.h ](z'h)+, where h is defined as the molar ratio of hydrolysis. When h=0, the pre-

cursor is an "aquo-ion" [M(OH2)N] z+ while for h=2N , it is an "oxo-ion" [MON](2N'Z)"

If o<h<2N, the precursor can be either an oxo-hydroxo complex [MOx(OH)N.x ](N+x'z)"

(h>N), an hydroxo-aquo complex [M(OH)h(OH2)N.h ](z'h)+ (h<N) or an hydroxo complex :

[M(OH),] ("'z)" (h=N).

2.1.2. The "charge-pH" diagram. Let us consider a typical transition metal such as

chromium, which exhibits two stable oxidation states, namely Cr(Vl) and Cr(lll). Only three

Cr(Vl) precursors have been experimentally characterized in aqueous solutions 18 .

[CrO (OH) z]° h=6

[CrO] (OH) ] h=7

[CrO~'] 2" h=8

Cr(VI) gives rise to only oxo-hydroxo or oxo complexes but never to aquo

complexes. For Cr(III) however, five precursors have been reported 18 o

As a

aqueous solutions but never oxo-complexes,

[Or(OH2)6 ]3* [Or(OH) (OH 2)s ]2+ [ Cr (OH)2 (OH2)4 ]+

[Cr(OH)3 (0H2)3 ]0

[Cr(OH)4 ]"

consequence, Cr(III)

+8

0 2-

OH-

+3

+1

0 7 14 pH

Fig. i : The "charge-pH" diagram.

h=0

h=l

h=2

h=3

h=4

forms only aquo, aquo-hydroxo or hydroxo complexes in

These observations can be summed up in a

qualitative way using a "charge-pH" diagram

17,40 as shown in figure i. This diagram gives

the nature of the precursors as a function of

the formal charge z of the cation M z+ and the

pH of the aqueous solution. Three domains can

be defined namely : "aquo" [M(OH2)N] z+ ,

[MONH2N.h ](z'h)+, and "oxo" "hydroxo"

[MON](2N'z)'. Such a diagram shows that low-

valent cations (z<+4) give rise to aquo-

hydroxo and/or hydroxo complexes over the

whole range of pH, while high-valent cations

(z>+5) form oxo-hydroxo and/or oxo complexes

over the same range of pH. Tetravalent cations

(z=+4) are on the border line, and therefore

lead to a large number of possible precursors.


2.1.3. Quantitative analysis. The Partial Charge Model can be used in order to calculate

the magnitude of charge transfer between ligands (oxo, hydroxo, aquo) and cations M z÷ . A

"charge-pH" diagram can thus be established in close agreement with experimental data.

Using the model, acidic or basic forms of a given cation in an aqueous solution can also be

predicted 37 Under acidic conditions the reaction to be considered is the cleavage of the

O-H bond arising from the metal atom's large polarization :

~+ S" 6 ÷ M - O - H + H20 = M - 0" + H30 +

This occurs as long as 8(OH)>O in the [MONH2N.p] (z'p)+ precursor, leading to the

reaction :

[MONH2N ]z+ + PH20 = [MONH2N.p] (z'p)+ + pH30 +

The limiting condition 6(OH)=0 leads to the following relations :

- mean electronegativity X = ~(OH) = 2.71

z - n&(H)-6(M) charge conservation p =

1-8(H)

Partial charges ~(H) and ~(M) can thus be calculated leading to :

p = 1.45z - 0.45 N - 1.07(2.71-X~)/~ (5)

Relation (5) shows that the number of protons p removed through spontaneous

hydrolysis directly depends on the formal charge z, the coordination number N and the

electronegativity X~ of the metal. These last two parameters are a direct function of the

size of the cation M z+ which can thus be taken into account. When applying relation (5)

three possible cases have to be considered (cf Table i) :

i) p<0, (2N-p>2N) : the [M(OH2)H] z+ precursor does not exhibit any acidic behavior.

A base such as OH" must be added in order to initiate hydrolysis. This situation occurs for

Ag + and Mn 2+ cations for example.

M Z N X~ 2N-p Couple

Ru +8 4 1,78 -i,I [Ru04]°

Mn +7 4 1,63 0,5 [MnO4]'/[MnO3(OH)]°

Cr +6 4 1,59 2,1 [CrO2(OH)2]°/[CrO(OH)3 ]+

V +5 6 1,56 8,4 [VO2(OH2)4]+/[VO(OH)(OH2)4] 2+

Ti +4 6 1,32 10,2 [TiO(OH2)5]2+/[Ti(OH)(OH2)5] 3÷

Zr +4 8 1,29 15,1 [Zr(OH)(OH2)7]3+/[Zr(OH2)8] 4÷

Fe +3 6 1,72 11,2 [Fe(OH)(OH2)5]2+/[Fe(OH2)6] 3+

Mn +2 6 1,63 12,7 [Mn(OH2)6] 2+

Ag +i 2 1,68 4,3 [Ag(OH2)e] ÷

Table 1 - Some inorganic precursors in their most acid forms.

ii) p>2N, (2N-p<0) : the [MON](2N'z)" precursor does not exhibit any basic behavior

and cannot be protonated by H3 O+ in aqueous solutions. A typical example is RuO 4 .

iii) 0<p<2N, (0<2N-p<2N) : under acidic conditions, two species corresponding to

h=E(p) and h=E(p+l) are in equilibrium where E(p) indicates the whole part of p. Typical

examples are Mn(VII), Cr(VI), V(V), Ti(IV) and Fe(III).

Under basic conditions, the limiting reaction is the cleavage of the M-O bond

arising from the low polarization of the metal atom :

___+ M + (M-OH)aq aq + OHaq

This reaction occurs as soon as a hydroxyl ion can be formed through solvation. The

(z-q)+ precursor, leading to 37 : limiting case corresponds to S(OHaq)=-i in the [MONH2N.q]a q

q = i + 1.25z - 0.92(2.49-X~)/~ (6)


(2N-q) corresponds to the number of protons that cannot be removed from the precursor even

at very high pH. Two cases can be encountered when applying relation (6) (cf.Table 2).

M Z N X~ 2N-q Couple

Ru +8 5 1.78 -0.5 [RuO~] 2"

Mn +7 4 1.63 -i.I [Mn04]"

Cr +6 4 1.59 0.2 [CrO~]/[CrO3(OH)]"

V +5 4 1.56 1.4 [VO3(OH)]2"/[VO2(OH2)]

Ti +4 5 1.32 5.0 [MO(OH)4]2"/[M(OH)5]" Zr 1.29

Fe +3 4 1.72 3.8 [FeO(OH)3]2"/[Fe(OH)4]"

Mn +2 3 1.63 3.1 [Mn(OH)B]'/[Mn(OH)2(OHz)] °

Ag +I 2 1.68 2.3 [Ag(OH)2]'/[Ag(OH)(OH2)] °

Table 2 - Some inorganic precursors in their most basic forms.

i) q>2N (2N-q<0) : The most basic form of M is an oxo-ion [MON](2N'z)'. Typical

examples are Ru(VlII), Mn(VII).

ii) O<q<2N (0<2N-q<2N) : twe species corresponding to h=E(q) and h-E(q+l) are in

equilibrium at very high pH. These may be exo-hydroxe complexes (V(V), Ti(IV), Zr(IV) and

Fe(lll)) or hydroxe-aquo complexes (Mn(ll), Ag(1)).

2.1.4. Initiation of condensation reactions. Condensation in aqueous solutions can occur

through two simple mechanisms that can be related to the coordination unsaturation :

i) If the preferred coordination is already fulfilled in the molecular precursor,

condensation occurs via a substitution reaction. In this case an entering group OX and a

leaving group OY must be present around M :

X M - OX + M - OY ~ M - 0 - M + OY

in order to keep the coordination number of the metal unchanged.

ii) If the preferred coordination is not fulfilled in the molecular precursor,

addition reactions become possible :

X M - OX + M - OY ~ M - 0 - M - OY

An increase of the coordination number occurs so that no OY group need to be eliminated.

In aqueous solutions, three kinds of precursors have to be considered according to

the "charge-pH" diagram (cf. Fig.l).

- Oxo-ions [MON](2N'z)" • the partial charge on M is usually slightly positive while the

partial charge on 0 is strongly negative (6(0)<<0). As a consequence oxo ligands are very

good nucleophiles but very poor leaving groups. Condensation therefore occurs only via

addition when the precursor is unsaturated.

- aquo-ions [M(OH2)N] z+ : the partial charge en M is usually strongly positive (6(M)>>0),

while the charge on the H20 molecule is slightly positive (6(H20)>0). Aquo ligands thus

show no nucleophilic property and act only as leaving groups. Condensation cannot occur

with such precursors because no entering group is available.

precursors [MONH2N.h] Cz'h)+ : both nucleophilic ligands (oxo or hydroxo) Other and

leaving ligands (hydroxo or aquo) are present around the metal. Condensation through

substitution reactions can thus begin as soon as one hydroxo ligand appears in the

coordination sphere. Following the "charge-pH" diagram this means that we must move into

the hydroxo domain in order to get condensed species (oligomers, sols, gels or

precipitates). This can be done by :


- adding a base or an oxidizer to an aquo precursor :

[Fe(OH2)6] 3+ + 3 OH" ~ [Fe(OH)3(OHz)3] ° + 3 HzO

[Mn(OH2)6] 2+ + H202 , [Mn(OH)4(OH2)2 ]° + 2 H + + 2 H20

- adding an acid or a reducing agent to an oxo precursor :

[WO412" + 2 H3 O+ , [WO2(OH)2(OH2)2] °

2[Mn04]" + 3 H202 + 6 H20 , 2[Mn(OH)4(OH2)2 ]° + 30 Z + 2 OH"

- or even via thermohydrolysis of an aquo precursor :

[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2+ + H30 +

In this case, the temperature has to be increased because the enthalpy change All

hydrolysis reaction is positive 18

of the

2 .2 . Condensation via olation

2.2.1. Mechanism. According to the literature 41, "olation" leads to the formation of a

hydroxo or "oi" bridge M-OH-M. Such a condensation process occurs with hydroxo-aquo

precursors [M(OH)x(OH2)N.x ](z'x)+ where x < N. Basically it corresponds to a nucleophilic

substitution (SN) in which M-OH is the nueleophile and H20 the leaving group. Several kinds

of bridges can occur as shown in figure 2 . Following Baran 42 bridges will be symbolized

~- ~+ ~+ H M - - O H ~ +..~.~M~jOH 2 ~ M--O--M + H20 2(OH)1

M _ M \

M / ~ M ~ . 9H2 ~ M / O H - M + H20 3(0H)I

~- H ,~+ (~ + / 0 H-- - - - - -~ ~+ ~+ cO~

H20-- M - - * ~ M - - OH 2 . M M ~o~_ o

+ 2 H20 2(0H)2

H H /o

H20 OH ,~+ ,~_ H

+ H20 2 ( OH )3

AI 3+

I G a 3*

I I

o

z+ [M(OH2)N ] z * - - - [M(OH2)N_I ] * H20 S N 1

Be 2. Mg2 ÷

I I ,r ,r in 3* TI 3* Zn 2. Cd 2*

J I I I V2" Fe 3. C J * Ti3*Ni 2+ d + F,Z*Mn=*A, g* C~*C,u =*

I I I I I I I I I, I i , ~ i J , r , 1 2 3 4 5 6 7 8

Li* N a*K*RIoCs*

Hg 2+

I Mn 3÷

I log ~(s") 9 10

Fig.2. Olation mechanisms and lability of some aquo-ions.


as × (OH)y where x is the number of metal atoms linked by one "oi" bridge and y the number

of bridges between these x metal atoms. As oxygen cannot form more than four covalent

bonds, the limiting value for x is 3.

In all cases an aquo ligand must be removed from the coordination sphere. The

kinetics of olation therefore strongly depends on the lability of the M-OH 2 bond. This

lability depends mainly on the charge, size, electronegativity and electronic configuration

of the M atom as shown in figure 2 43,44 " the smaller the charge and the larger the ionic

radius, the faster the M-OH 2 bond will be broken. In addition, it is well known that

transition elements whose electronic configuration is d 3 (Cr B+ ,V 2+) ,d 6 low spin (Co 3+) or

d 8 (Ni 2+) are kinetically inert owing to their high crystal field stabilization energy in

octahedral coordinations 45. For these elements the rate constant for solvent exchange

ranges typically between 10 .4 and 10 .6 s "I 46

In other cases, olation can be extremely fast especially for low valent precursors

(O~_z-h<2) and is limited only by diffusion (k>10 ? M" I s" I). Rates are much slower for highly

charged precursors (z-h~2), particularly when the size of the cation is small. The

dimerization rate constant k of the Fe 3+ precursors is rather low 47 :

H 2(0H)I: [Fe(OH)(OH2)5] 2+ + [Fe(OH2)6] 3+ = [(H20)sFe-O-Fe(OH2)5] 5+ + H20 (k=2.5.10 "2 M'Is "I) o}

]4+ 2(0H)2: 2[Fe(OH)(OH2)5] 2÷ = [(H20)4Fe e(OH2) 4 + 2 H20 (k = 10"1-103 M'Is "I)

0

while it is much faster for VO 2÷ or Cu 2+ 4?,48 .

H 2(0H)I: [VO(OH)(OH2)4 ]+ + [VO(OH2)5] 2÷ = [(H20)40V-O-VO(OH2)4] 3+ + H20 (k = i M'Is "I )

2(0H)2:2[VO(OH)(OH2)41+ = [(H20)30 0(0H2)3 ]2+ + 2 H20 (k = 104 MIs "I)

-OH

2(0H)2 : 2[Cu(OH)(OH2)5 ]+ = [(H20)4C /0~ - o,CU(OH2)412÷~ ~ + 2 H20 (k = 108 M'Is "I)

2.2.2. Polycations. Charged precursors (z-h ~I) cannot condense indefinitely to form a

solid phase. This is mainly due to the fact that the nucleophilic strength of the hydroxo

group 6(OH) varies during the condensation process. In the typical dimerization reaction of

Cr(lll) :

° I 2[Cr(OH)(OH2)5] 2+ = [(H20)4Cr \ ~ Cr(OH2)4] 4+ + 2 H20

O

OH groups are negatively charged in the monomer (6(OH)=-0.02) while they become positively

charged in the dimer (6(OH)~+0.OI). The partial charge of hydroxo groups can change in sign

during the condensation process, owing to the departure of donor water molecules. From a

chemical stand point, this means that OH loses its nucleophilic power in this polycationic

compound. Condensation is then limited to dimers mainly for entropic reasons. More

condensed polycations can however be formed if the nucleophilic strength of the starting

monomer is higher. As an example, let us consider the dimerization of Ni(ll) species :

2[Ni(OH)(OH2)31÷ = [(H20)2Ni\ { i(OH2)212+ + 2 H20

O

6(OH)=-0.07 in the monomer and °0.03 in the dimer. The hydroxo group remains negatively

charged and keeps some nucleophilic character. Condensation can proceed further towards a


tetramer whose presumed structure is shown in figure 3 (structure E) :

2[Ni2(OH)2(OH2)4 ]2+ = [Ni4(OH)4(OH2)4] 4+ + 4 H20

The partial charge becomes 6(0H)=+0.06 in this tetramer and condensation stops

stage in agreement with experiments 49

at this

A (A) [M2 (OH) (OH2)× ] 3+

M = Mn 2+, Co 2+ Ni2+ 18

C

D

(B) [M 2(OH) 2(OH2) x](2z'2)+ M = VO 2+, Cr3+, Fe3+, Ti3+, Cu2+ 18

(C) [Cr2(OH)(OH2)I0 ]5+ 50

(D) [M 4 (OH)6 (OH 2)12 ]6+

M = Cr 3+ 51

(E) [M 4 (OH)4 (OH 2 )4 ]4+ M = Co 2+ , Ni 2+ 49

(F) [M 4 (OH)8 (OH 2 )I 6 ]8+

M = Zr 4+ , Hf 4+ 52,53

oM ®oH OHio Fig.3. Transition metal polycations,

Figure 3 gives other examples of transition metal polycations 18'49"53.It is easy to

show that in each case the partial charge on the hydroxo ligand is close to zero or weakly

positive. The Partial Charge Model is thus able to explain why condensation stops before an

infinite network is formed. These polycations must then be considered as end points in

hydrolysis and condensation reactions of monomeric precursors in a given range of pH.

Precursor 6(OH) 6(M)

[Ti(OH)2(OH2)4] 2+ - 0.01 + 0.88

[V(OH)2(OH2)4 ]2+ + 0.01 + 0.68

[Zr(OH)2(OH2)6 ]2+ - 0.07 + 0.87

[Hf(OH)2(OH2)6 ]2+ - 0.06 + 0.81

Table 3 - Nucleophilic strength of h = 2 precursors of tetravalent elements.

The formation of oxo-aquo precursors for tetravalent metals can also be easily

explained by the Partial Charge Model. Table 3 compares the nucleophilic strength of OH

groups for several h=2 precursors. The nucleophilic strength is quite low for Ti(IV) and

V(IV) which means that condensation is difficult. As condensation is inhibited, a

prototropic transfer between the two geminal hydroxo groups can occur :

6 + OH O

OH OH 2

The oxo l i g a n d t h u s fo rmed c a n make a s t r o n g d o u b l e bond w i t h t h e h i g h l y e l e c t r o p h i l i c

m e t a l and c a n n o t be e a s i l y p r e t e n a t e d a g a i n . As a c o n s e q u e n c e t h e s t a b l e fo rm o f t h e h~2


precursor is an oxo-aquo precursor [MO(OH2)5] 2+ rather than a geminal dihydroxo-aquo

precursor [M(OH)2(OH2)4] 2÷ in good agreement with experiments 54,55,56. Such a mechanism

does not occur with zirconium and hafnium. Hydroxo groups in the h-2 precursors are

nucleophilic enough to initiate further condensation. Therefore cyclic tetramers with

2(OH) 2 bridges are formed rather than monomeric oxo-aquo ions 52,53

2.2.3. Precipitation and gelation. Zero charged precursors (h-z) are able to nucleate a

solid phase through infinite condensation of "oi" groups. The final term of this process

must then be a hydroxide M(OH)z provided oxolation does not occur. In order to know whether

oxolation has to be taken into account when considering aquo-hydroxo precursors

[M(OH)h(OH2)N.h] (z'h)+ or hydroxides M(OH)z, let us consider the following equilibrium :

6 ÷ 6" 6 ÷ 6

--M-- -- = --M--~. .-

This reaction is basically a 1,3 electrophilic rearrangement where a proton jumps between

two adjacent hydroxo ligands, with at least one of them being in a bridging position. The

partial charge of the water molecule created by this prototropic transfer can be either

positive or negative :

i) 6(H20 ) < 0 : There is a net attractive force between the cation M(6 +) and the

aquo llgand (6"). Water elimination is thus prevented and the reverse prototropic transfer

occurs reforming the "oi" bridge which was originally broken. In such a situation the "oi"

bridge remains stable and oxolatlon does not occur.

ii) 6(H20 ) > 0 : There is a net repulsive force between the cation M(6 +) and the

aquo ligand (6+). Water can be removed and the reverse transfer becomes impossible leading

to the irreversible formation of an oxo bridge. In such a situation the "oi" bridge is

unstable and oxolation can compete with olation.

Table 4 gives the calculated values of 6(H20) for some transition metal aquo-

hydroxo precursors and hydroxides. It is seen that as soon as 6(H20)<0 an hydroxide M(OH) z

can be isolated 57 This is no more the case when 6(H20)>0 for oxolation can now occur. In

such conditions an oxy-hydroxide can be obtained with trivalent

Solid hydroxide Soluble precursor 6(H20 ) formed by pure 6(H20)

olation

[Mn(OH)2(OH2)4] ° - 0,02 Mn(OH) 2 - 0,06

[Fe(OH)2(OH2)4] ° - 0,01 Fe(OH) 2 - 0,02

[M(OH)2(OH2)4] °(*) - 0,003 M(OH) 2 - 0,01

[ Sc (OH)3 (OH 2 )3 ] ° Sc(OH)3 - 0,05 - 0,i0

[Y(OH)3 (OH2)3 ] ° Y(OH)3

[V(OH)3(OH2)3 ]° + 0,01 V(OH) 3 + 0,02

[Cr(OH)3(OH2)3 ]° + 0,01 Cr(OH) 3 + 0,03

[Mn(OH)3(OH2)3 ]° + 0,02 Mn(OH) 3 + 0,04

[Fe(OH)3(OH2)3 ]° + 0,03 Fe(OH) 3 + 0,07

[Co(OH)3(OH2)3 ]° + 0,03 Co(OH) 3 + 0,08

[TiO(OH)2(OH2)3 ]° + 0,01 TiO(OH)2 + 0,02

[VO(OH)2(O~)3 ]° + 0,05 VO(OH) 2 + 0,12

[Zr(OH)4(OH2)4] ° + 0,002 Zr(OH) 4 + 0,005

[Hf(OH)4(OH2)4] ° + 0,01 Hf(OH) 4 + 0,03

(*)M = Co, Ni, Cu.

Table 4 - Stability of hydroxides M(OH) z

elements while hydrous

Crystalline phases known

Mn(OH)2,MnO

Fe(OH)2,FeO

M(OH)2, MO

Y(OH)~ YOOH Sc(OH)~,ScO.OH

Y203 , Sc203

VO.OH, V203

CrO.OH,Cr203

MnO.OH, Mn203

FeOOH, Fe203

CoOOH

T i O 2

VO 2

ZrO 2

HfO 2

deduced from the Partial Charge Model.


oxides are obtained only with tetravalent elements. However, it must be pointed out that

these oxy hydroxides are formed under very specific conditions. They should not be

considered as the final term of nucleation and growth processes which would lead to the

oxide MOz/2 if 6(HzO)>0.

The formation of a gel rather than a precipitate from aquo-hydroxo inorganic pre- 58.

cursors is a rather complicated process which depends critically upon many parameters

- A pH-gradient is induced by the gelifying agent which may be NaOH, NH 3 , NaHCO 3 , Na2CO 3 ,

(NH2)2CO, or any hydroxyl exchanger.

The concentration of both reagents may be quite different.

- The addition mode and the speed of agitation of the solution must be controlled.

- The order of mixing of the reactants and the geometry of the vessel play a role.

- The temperature can either favor or inhibit gel formation.

The chemical composition of the aqueous solution can induce modification of the

precursors at a molecular level.

All these parameters must be taken into account because nucleation and growth

involve mainly olation reactions which are diffusion-controlled processes. As a

consequence, colloidal gels are obtained which are not very stable when prepared in a pure

form. Metals that lead to stable "oi" bridges give rise to well defined hydroxides M(OH) z

59 Other metals that do not form stable hydroxo bridges lead to hydrated amorphous

gelatinous precipitates MOx/2(OH)z.x.YH20 when a base is added to the aquo precursors.

These precipitates are not well defined. They lose water continuously through oxolation

finally leading to the oxide MOz/2 60,61,62 Other complications can arise with

multivalents elements such as Mn, Fe and Co because electron transfers may occur in the

solution, the solid phase, or even at the oxide-water interface. The following examples

will briefly show how these different reactions may be analyzed.

2.2.4. Sols and gels of divalent metal oxides. We will consider mainly Co 2+, Ni 2+ and Cu 2+

cations because other divalent metals (V 2÷ , Cr 2+ , Mn 2+ and Fe 2+) are easily oxidized in

aqueous solution.

Green transparent nickel hydroxide gels can be obtained by dissolving the freshly

precipitated hydroxide in tartric acid and adding sodium or potassium hydroxide in molar

proportions (>0.5 M) 63. Similar results are obtained when nickel acetate is dissolved in

glycerol and treated by an alcohol solution of potassium hydroxide 64 After dialysis and

dessication, the solid phase is Ni(OH)2 and not NiO showing the stability of the ol bridges

in this system. No structural characterization has been undertaken for these gels.

Owing to the easy oxidization of Co 2+ in strongly alkaline solutions, different

results are obtained with cobalt. In this case gelation is slower and the color changes

from pink to purple to green and after many days to brown 63 Oxidation of Co 2+ towards

Co 5÷ obviously occurs under such conditions :

3 Co 2+ + 3 H20 + 1/2 02 , Co304 + 6H +

This reaction was used by Sugimoto and Matijevie to produce monodispersed Co304 sols 65 In

this case it is interesting to point out that sols can be obtained only in the presence of

acetate ions. No precipitation is observed under the same conditions when other Co (II)

salts (nitrate, chloride and sulfate) are used.

Copper hydroxide gels are more difficult to produce and the following conditions

must be fulfilled in order to make them 66,67 .

i) The starting precursor must be copper (II) acetate. Nitrates, chlorides or sulfates

always give rise to gelatinous precipitates.

ii) The added base must be diluted ammonia without any excess.


iii) A small amount of sulfate ions must be added in order to get a stable gel.

These gels are highly anisotropic and show interesting aggregation phenomena which

have been studied in our laboratory.

Copper (II) hydrous-oxide sols can also be made by heating a solution of nitrate or

sulfate or a mixture of copper (II) nitrate and potassium phosphate 68. By heating copper

tartrate complexes (Fehling's solution) with glucose, uniform copper (I) hydrous oxide sols

can be obtained with various particle shapes and sizes 69

2.2.5. Sols and gels of trivalent metal oxides. Hydrous chromic oxide gels can be made

by treating Cr(III) sulfate, nitrate, chloride or acetate precursors with ammonia or

potassium hydroxide 70,71 Highly vibrant monolithic gels can be produced only when acetate

ions are present in excess 70-72. The color of these gels is blue-grey when NH 3 is used and

bottle-green with KOH . This difference may well be due to complexation between Cr 3+ and

NH 3 . These gels are amorphous to X-rays, but small fractions of crystalline CrOOH and

microcrystalline Cr(OH) 3 can sometimes be detected 73,74,?5. EXAFS measurements have shown

that the gels have the stoichiometry [Cr(OH)3(OH2)3].nH20 and that hydroxyl groups condense

to form Cr-O-Cr bonds without decreasing the coordination number of Cr 3+ 76 The final term

of this oxolation is ~-Cr203 with no intermediate phase such as CrOOH, which is in

agreement with the predicted instability of ol bridges in the h ~ 3 precursor (table 4). By

ageing chromium salts such as KCr(SO4)2.16H20, Cr2(SO4) 3 and Cr(NO3) 3 at high

temperature hydrous chromic oxide sols can be prepared 77,78 Some sulfate and phosphate

ions are necessary in order to obtain monodispersed sols 78

The behaviour of Fe 3+ is quite different despite similar eleetronegativity and

coordination number. Gelatinous precipitates are obtained instead of gels when a base

such as NH 3 or NaOH is added to precursors such as chlorides, sulphates, nitrates,

perchlorates, acetates or oxalates. This may be correlated with the rate of olation of the

aquo-hydroxo precursors :Fe 3+ (3d 5) exhibits no crystal field stabilization in an

octahedral symmetry. Consequently, olation is fast as shown by the rate of dimerization of

the [Fe(OH)(OH2)5] 2+ ion : k- 450 M'Is "I at 25oc 79. In contrast, Cr 3+ (3d 3) shows a high

crystal field stabilization in the same symmetry. This implies a low reactivity of Cr 3+

ions towards nucleophilie substitution and thus olation rates must slow down in a rather

drastic way. In agreement, the rate of dimerization of the [Cr(OH)(OH2)(C204)2]2"ion is k =

10 .5 MIs "I at 25°C 80. As monolithic gels are preferentially formed when the rate of

condensation is slow, gels are easily formed with Cr 3+ while only gelatinous precipitates

are obtained with Fe 3+ .

These gelatinous precipitates are amorphous and seem to have a composition

intermediate between ~-FeOOH (goethite) and o-Fe203 (haematite) 81'82. A crystal structure

has been proposed for a compound whose composition is close to 2Fe203.FeOOH.4H2 O83 . The gel

is supposed to be an amorphous form of this material 84,85. Upon aging, ~-FeOOH is formed

at pH>I0 while ~-Fe203 is obtained at pH<4 86,87. In agreement with the high partial charge

6(H20 ) in the h=3 [Fe(OH)3(OH2)3 ]° precursor, no microerystalline Fe(OH) 3 similar to

Cr(OH) 3 can be detected. Another difference between

hydrolysis kinetics of the aquo-ion 88,89 .

[Or(OH2)6] 3+ + H20 = [Cr(OH)(OH2)5] 2+ + H3 O+

[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2 + H3 O÷

[Fe(OH)(OH2)5 ]2+ + H20 = [Fe(OH)2(OH2)4] + + }{30 ÷

As a result, acidic ferric

Fe 3+ and Cr 3+ lies in the

kl : 1.4 105s'I

kl - 3. i0 zs" I

% ~ 6.1 104s "I

solutions are highly unstable and precipitate through

spontaneous hydrolysis. The mechanism of this precipitation was extensively studied 90-97

and appears to proceed as follows 98,99,100 :


- The h=l precursor [Fe(OH)(OH2)5 ]2+ can undergo a dimerization reaction and nucleate the

~-FeOOH phase through mixed olation/oxolation reactions.

- At room temperature the h=2 precursor [Fe(OH)2(OH2)5] + can form a polycation whose mean

composition is [Fe403(OH)4 ]2n+n with a molecular weight around 104 g/mole (n=25)~ This

polycation gives rise to spheres about 2-4 nm in diameter which are responsible for the

brown-red color of the colloidal solutions. Mixed oxo-hydroxo bridges 2(O)i, 2(0)2, 2(0H)I

and 2(0H)2 seem to be present in this polycation. A structure was proposed 101 , in which

the iron atoms are in a tetrahedral coordination in the core and in an octahedral

coordination near the surface. However, other results 102 suggest that all iron atoms are

octahedrally coordinated. Upon ageing, or adding a base, aggregation occurs leading to ~-

FeOOH needles with the same diameter as the original polycation. These needles then undergo

an oriented aggregation process giving rod-like particles which can form fibrous tactoids

responsible for the gelatinous aspect of the precipitate. In the presence of chloride ions

fl-FeOOH precipitates are formed rather than ~-FeOOH 103,104 while in the presence of

sulfate ions a basic salt precipitates 105,106 The synthesis of this Fe-polycation has

been reviewed 107

- At high temperature, the h=2 precursor does not form a polycation. It nucleates directly

into ~-Fe203 particles that may exhibit various morphologies 108,109

Iron oxide sols or gels can also be made through the oxidation of Fe(II) precursors

or the reduction of Fe(III) salts. Depending on the experimental conditions, the solid

phases thus formed can be Fe304 , ?-Fe203 or 6-FeOOH.

i) Magnetite Fe304 can be made through reduction of ~-Fe203 with hydrazine and is

formed following a dissolution recrystallization mechanism 110. The situation appears much

more complex when it is made by slow-oxidation of Fe(OH) 2 .

- Under basic conditions, nucleation takes place near the surface of Fe(OH) 2 particles and

the growth involves a contact-recrystallization mechanism inside the gel phase 111,112

Under neutral or weakly acidic conditions, some Fe(II) precursors are oxidized snd

copreeipitation of the ferric hydroxo complexes thus formed, with ferrous precursors leads

to a green product called green-rust 111 Surface oxidation of the green-rust particles

leads to colloidal magnetite Fe304 . Such a mechanism is probably involved when mixed

Fe(II)/Fe(III) precursors are used in order to obtain ferrofluids 113. Typically, an

aqueous mixture of ferric chloride and ferrous chloride is added, under strong agitation,

to an ammonia solution. A black gelatinous precipitate is instantaneously formed and can be

isolated from the solution by centrifugation or magnetic decantation without washing with

water. An alkaline ferrofluid is then made by peptization with tetramethylammoni~1

hydroxide. An acidic sol is obtained when the precipitate is stirred with aqueous

perchloric acid, centrifuged and peptized by adding distilled water. In all cases

peptization is possible only when the Fe(II)/Fe(III) ratio is lower than 0.15114'115

ii) The final term for the oxidation of Fe304 is ~-Fe203116 This transformation

is induced by air, H3 O÷, Fe 3+, Fe(OH)3 , Ag ÷ or other oxidizing agents 116-118 All these

reactions are characterized by an electron transfer at the water-solid interface, coupled

with an other electron transfer between Fe(ll) and Fe(lll) ions inside the particle.

Chemisorption at the interface induces a reduction of surface Fe 3+ cations in oetahedral

positions by trapping electrons from the solid phase which are normally deloealized.

Desorption or in-situ oxidation of this reactive Fe(ll) occurs, while charge compensation

leads to Fe 3+ migration from the core towards the surface with the creation of oxygen

vacancies (cf 5.4.2). The final product of these processes is aggregated 7-Fe203 particles.

iii) Finally, fast oxidation of Fe(II) by H202 leads to either crystalline 6-

FeOOH 111,119 or amorphous phases 120


2.2.6. Sols and gels of tetravalent metal oxides. Hydroxo-aquo precursors of tetravalent

Ti, Zr or Hf can be easily hydrolyzed 121'122'123. These cations can therefore form stable

sols. However, their growth mechanism mainly involves olation so that clear gels are

rather difficult to obtain. TiO 2 gels have been made by adding sodium carbonate to an

aqueous solution 124,125,126 or by acid peptization 127 . Similarly, ZrO 2 gels can be made by

neutralization of chloride or nitrate precursors with urea or by peptization. The structure

of these colloidal gels remains unknown, but mixed oxo/hydroxo bridges seem to be

present 61 , 62,128 The only structural study concerns amorphous ZrO 2 for which a sheet-like

structure with zirconium atoms linked through 3(O)1 and/or 3(OH)1 bridges was proposed 129

With other dioxide sols and gels such as VO 2 ,CrO 2 or MnO2, redox reactions cannot

be neglected. Sols of MnO 2 are readily obtained by reduction of KMnO 4 with As(OH)3130 ,

Na2S204131 , Mn2+132 , NH 4 + 133 , glucose, fructose or galactose 134.Gels have also been

formed 59.

2.3. Condensation via oxolation

2.3.1. Mechanism. Oxolation leads to the formation of oxo bridges M-O-M between two metal

cations M. Such a condensation process is observed when no aquo ligand is available in the

coordination sphere of the metal. Typically, this occurs for oxo-hydroxo precursors

[MOx(OH)N.x ](N+x'z)" where x<N. Two basic mechanisms have to be considered for oxolation

reactions.

i) When the metal coordination is not fully saturated, nucleophilic

addition (AN) with M-OH and/or M-O as nucleophiles can occur, as shown in figure 4. Ligands

need not be removed and chains or cycles are formed very rapidly 135,136,13Z. Typical

examples are given by [MO3(OH)]" species (M - W,Mo) which form cyclic tetramers

[M4OIz(OH)4] 4" . The kinetic constants of such reactions are larger than lOSM-~s "I in

agreement with a pure addition mechanism 138

2(0)2 or face bridges 2(0)3 are easily formed.

/

< A

According to this mechanism, edge bridges

s

O / \

+ O -- M - ' - ~O-~ - 2(O)3

-%

Fig. A. Formation of small polymers according to a

nucleophilic addition mechanism.

(A)(B)(C) chains ; (D) cycles.

ii) When the metal coordination is

already fully saturated, nucleo-

philic substitution must occur

with M-OH as a nucleophile and OH"

or HzO as leaving groups. This

reaction can be decomposed into

two basic steps :

- a nucleophilic addition leading

to an unstable 2(0H)I bridge :

6" 6 + H M-OH + M-OH , M-O-M-OH

followed by a fl-elimination

leading to the departure of one

water molecule :


6 + ~" H OH

- M - O - M - , M - O - M + H20

This basic mechanism will be called ANflE i in order to indicate the two step process and the

prototropie transfer within the transition state.

The first step can be catalyzed by bases which strongly favour the nucleophilic

attack :

M - OH + OH" , M - O" + H20

M - O" + M - OH , M - O - M + OH"

This mechanism will be called AN~E 2 in order to indicate a concerted elimination.

The second step can be catalyzed by acids which strongly favor the elimination of

the leaving group :

H 6" H M - 0 - M - OH + H 30 + , [ M - O - M - OH2] + + H20

1 H20 H ]+

M - O - M + H 30 + c [ M - O - M + H 20

The positive charge of the "oi" bridge greatly increases its acidity, favoring proton

removal. As a water molecule is eliminated from the transition state this mechanism will be

called AN~E I .

These mechanisms explain why, in contrast to olation, oxolation occurs ever a

wide range of pH. Moreover, as a proton has to be transferred before elimination occurs,

the rate limiting step can be either the proton transfer (AN~Ei) or the elimination of the

leaving group (ANflE I and AN~E2). Oxolation kinetics thus strongly depends on both the metal

M and the pH. The reaction rate usually goes through a minimum around the isoleetric point

of the solution (precursor [MOz. N(OH)2N.Z] ° predominent in solution). Considering the h = 7

precursor, the dimerization reaction of Cr(VI) can be written as follows 138,139,140 .

[HCrO4] + [HerO4]" = [Cr207 ]2" + H20 k = i M'Is "I and k ~ 5.10 .4 M'Is "I

while the polymerization of vanadates leads to 141.

[VO3 (OH) ]2" + [VOa(OH)2 ]" = [V206 (OH) ]3" + H20 ~ = 3.1 104 M'Is "I

[VO3(OH)] 2" + [V204(OH)3]" = [V309] 3" + 2 H20 k = 5 102 M'Is "I

Oxolation following an ANflE mechanism is a slow process and cannot proceed as fast

as olation as it is never under diffusion control. The different types of bridges that can

be formed via oxolation are shown below :

M---OH + HO---M M--0--M +

/~OH + HO--M /~O--M +

Isolated 3(0)I and 4(0)I bridges are not

H~O 2 (°)1

H20 3 (0)1

+ H20 4 (O)I

[M30(OAc)6 (0H2)3 ]÷ (M = Cr,Fe,Ru) and in [Cu40Cl 6 (Ph3PO) 4 ] complexes

+ H2° 4 ( ° ) I

very common. They can be found in

142

2.3.2. Polyanions. One of the main differences between aquo-hydroxo and oxo-hydroxo

precursors, is the fact that even when the charge is zero (x=z-N), condensation through

oxolation may not go beyond a limited degree of polymerization. This is again due to the

loss of nucleophilie strength of hydroxo groups after condensation has occurred :


2[CrO2(OH)2] ° ) [(HO)O2Cr-O-CrO2(OH)] ° + H20

6(OH) = -0.01 8(OH) = +0.04

This dimer behaves as an acid and can lose protons to form polyanions :

{ [Cr205(0H) 2]° = [Cr206(OH)]" + H +

[Cr206 (OH) ]" = [Cr207] 2" + H +

However, as condensation must occur before ionization takes place, such species are often

called "polyacids". Depending on M, more or less condensed species can be obtained. Let us

consider as an example the decavanadic acid that can be formed by the polycondensation of

h=5 precursors [VO(OH)31% :

I0 [VO(OH)3]° ) [H6V10028 ]° + 12 H20

6(OH)= -0,09 6(OH) = +0.003

Ten vanadium atoms are required to make the hydroxo group positive. Spontaneous

deprotonation leads to a polyoxy-ion [HzVI0028 ]4" whose structure is shown in figure 5G. At

higher pH further deprotonation occurs leading to :

[H2V10028 ]4" = [V10028 ]6" + 2 H* Figure 5 shows also the structures of some well known transition metal polyanions

35,143-161. These polyanions are probably formed through a mixed AN/AN~E mechanism as shown

in figure 6 137when the rate of the AN~E reaction is fast. Geometric constraints lead to

more open structures particularly when the reaction rate is slow (figure 7) 35,162-171

Fig.5. Structures of compact isopolyanions

(A) [W4012(OH)4 ]4" , (B) [W4016] 8" 35,143

(C) [M6019] 8" M = Nb, Ta 144,145

[M6019 ]2" M = W 146, Mo14?,148

(D) [MzO24] 6" M = W 35,143, Mo 149,150

(E) ~-[Mo8026] 4" 151-154

(F) [MOsO26(OH)2 ]6" 155,156

(G) [MI0028] 6" M = V157"159; Nb 16°

(H) [Au206] 6" 161

A

B

o o

~\ Oo AN

Fig.6. Mechanism of formation of isopoly-

anions according to Tytko and Glemser 137

Formation of the [M4011(OH)5] 3" (A) and

[M4012 (OH)4 ]4- (B) tetramers through

successive addition of [MO3(OH)]'tetrahedra

and protonation. (C) Structure of the

[M4OI2(OH)4] 4" tetramer and growth (D) of

the isopolyanion through an AN~E mechanism.


¢

V

Fig.7. Structures of non compact

isopolyanions.

(A) [M207] 2 M = Cr 162 Mo 35

[M207] 4 M = V 16S

(B) [Cr3010 ]2" 164

(C) [VsO9 Is 16s

(D) [Cr4013 ]2" 165

(E) [V4012] 4 ss,16s

(F) ~-[Mo8026 ]4" 153,154,166

(G) [H2W12042 ]I0" 167

(H) [H2W12040 ]6" 168

(I) [W10032 ]4" 169,170

(j) [Mo360112(OH2)1618- 171

It should be noted that the formation of most isopolyanions involves a change in the

coordination of the metal from 4 to 6. This change occurs because protonation increases the

electrophilic strength of the metal M as shown in table 5. As 6(M) becomes larger than

+0.50 octahedral coordination is preferred because it allows a larger charge transfer

towards the metal. This explains why pyrovanadates (precursor h=7 [VO3(OH)]2" ) and

metavanadates (precursor h=6 [VO2(OH)2]" ) have a tetrahedral structure 35'143 while vanadium 172

oxide gels and decavanadates (precursor h=5 [VO(OH)3] °) have an octahedral structure

With niobium in the h=6 precursor, the higher positive charge explains why niobium must

keep octahedral coordination even at very high pH (hexaniobate ion) :

6[NbO2(OH)2 ] " , [H2Nb6019 ]6" + 5 H20

With Mo(VI) and W(VI), the h=7 precursors [M03(OH)]" are on the border line between

both coordinations while h=6 precursors [MO2(OH)2] ° appear unstable in tetrahedral

coordination. Thus for Mo(VI) a great variety of polyanions can be formed in which this

element can have two different coordinations as in ~-[Mo8026 ]4"

Precursor X 6(0) 6(OH) 6(M) pK

[V04] 3 1.583 -0.74 +0.01

[HV0412" 2.056 -0.57 -0.59 +0.29 14.4

[H2V04] 2.378 -0.44 -0.30 +0.48 8.95

[H3V04] ° 2.611 -0.35 -0.09 +0.62 3.74

[Mo0412 2.046 -0°57 +0.29

[I{Mo04 ]" 2.431 -0.42 -0.25 +0.51 3.89

[H2MoO4 ] ° 2.693 -0.32 -0.02 +0.67 3.61

[WO4 ]2 2.055 -0.57 +0.27

[HWO 4 ] 2.439 -0.42 -0.25 +0.50 3.50

[H2WO 4]° 2.701 -0.31 -0.01 +0.64 4.60

[Nb04 ]3 1.550 -0.77 +0.07

[H2Nbo4]- 2.027 -0.58 -0.61 +0.38

Table 5 - Variation of partial charges with protonation for some tetrahedral inorganic

precursors.

JPSSC 18:4-B


At very low pH, positively charged oxo-aquo precursors are formed owing to the

low nucleophilic strength of hydroxo groups (cf.2.2.2). Condensation through oxo bridges

can occur leading to acidic polycations such as [Mo20(OH2)x] 2+ or [Mo20(OH)(OH2)×] ÷ 173

Tetrahedral species such as [CrO2(OH)2] ° cannot condense beyond a certain point,

leading to the formation of polyacids. This will no longer be the case if hydration occurs.

Hydrated phases can nucleate which have very different structures and can be transformed

into the anhydrous oxide MOz/2 upon heating. Moreover as coordination becomes saturated in

the h=z precursor, only slow AN~E mechanisms are involved, leading to the formation of

clear gels when an acid is added. Some of the probable growth mechanisms for these gels

will now be described.

2.3.3. Sols and gels of pentavalent metal oxides. Vanadium pentoxide gels can be made by

adding nitric acid to a vanadate salt or by hydration of the amorphous oxide V205 174 The

best method however, is to use a proton exchange resin which yields a relatively pure

product quite rapidly, without dialysis or washing 175. Polyvanadic acid solutions can

thus be prepared by ion exchange in a resin from sodium or ammonium metavanadate solutions

176,177. The freshly prepared decavanadic acid is yellow and turns dark-red within a few

hours. Decavanadic acid (M.W.-lOO0g/mole) predominates below 10"3M and transforms into

polymeric species (M.W.=2.106g/mole) above 10"3M 176. Aggregation occurs above 2.10"2M and

finally gelation is observed if the vanadium concentration is larger than 0.i M. Some

vanadium reduction occurs during the polycondensation process and about 1% of the vanadium

ions are in the V(IV) oxidation state as shown by ESR 178

The fibrous nature of the

............. gel is well established

(figure 8). Electron and X-

ray diffraction studies 172

have shown that these

fibers actually look like

flat ribbons about i00 A

wide and IOA thick. Accor-

ding to the 2D structure

observed along the ribbons,

V205 layers are formed by

fibrils 27A wide linked to-

gether side by side. Water

molecules can be inter-

calated leading to a gel or

Fig.8. Fibrous texture of V205 gels. a colloidal solution. The

xerogel obtained by drying

these gels at room temperature has a water content about 1.6 H20 per V205 which correspond

to one interfoliar water layer 179. Swelling of this xerogel can be followed by SAXS and

SANS 180

Acidification of vanadate solutions around pH-2 leads to the formation of the h=5

precursor which can be formulated [VO(OH)3]°. In this monomeric precursor with tetrahedral

structure, vanadium is highly electrophilic (6(V) - + 0.62). Addition of any nucleophilic

ligand is thus expected and transition towards an octahedral coordination must occur.

This can be achieved in two ways :

i) tetrahedral'h=5 precursors are acidic species : [VO(OH)3 ]° = [VO2(OH)2 ]" + H +

6(0)=-0.35 6(0)=-0.44


Addition and condensation of several such tetrahedral nucleophilic precursors lead to the

formation of decavanadic acid as shown in 1.3.2. (x~4) :

(10-x)[VO(OH)3]° + x[VO2(OH)2 ]" = [H6.×V10028] x" + 12 H20

ii) when x = 0 in the previous equilibrium, water molecules (6(0)=-0.40) appear to be

better nucleophiles than h=5 precursors (6(0)=-0.35) and figure 9 shows a possible pathway

towards fiber formation. The first step corresponds to an increase in the coordination of

the V(V) atom from four to six through the addition of two water molecules. An octahedral

complex is formed with a long V-OH 2 bond along the z axis, opposite to the short V=0 double

bond. The other water molecule has an hydroxo ligand in a trans position. Olation can occur

readily leading to a chain compound whose stoichiometry corresponds to [VO(OH)3(OH2)] ~. In

this case olation occurs before oxolation because the same complex contains both a good

leaving group (6(H20)=+0.I0) and a good nucleophile (6(OH)=-0.14). Once the chains are

formed, condensation through oxolation can occur between two chains in order to transform

unstable 2(0H)I bridges into stable 3(0)I bridges. Further condensation between these

double chains leads to a fibre-like structure as evidenced by electron diffraction 172

The coexistence of decavanadic acid o o

/}'k"~',OH OH 2

ODH -

5 . 7 5 ~ i b

3 . 6 0

Fig.9. Mechanism of formation of V205

fibers through olation (SN) and

oxolation (AN~E) from the monomeric

h=5 precursor.

of NbCI 5 or TaCI 5 with ammonia or

Peptization of this precipitate by

and fibrous polymeric species in V205 gels

can thus be understood in a very simple

manner. Both species are in an equilibrium

which can be shifted in either direction

by varying the V(V) concentration.

Niobium and tantalum behave quite

differently from vanadium : VCI 5 is un-

known while NbCI 5 is stable as dimeric

molecules Nb2CIIo with an octahedral

structure 57 Also, Nb=O and Ta=O double

bonds are not stable, which explains why

VOCI 3 is a monomerie tetrahedral complex

while NboCI 3 is an infinite octahedrally

coordinated polymer in which condensation

has occurred through 2(0)I and 2(CI)2

bridges. Therefore, formation of mixed

aquo-hydroxo-oxo complexes is not possible

with Nb(V)and Ta(V) inorganic precursors

mainly because they remain octahedrally

coordinated even at very high pH.

Consequently, amorphous gelatinous

precipitates are formed through hydrolysis

acidification of an alkali-niobate or tantalate.

washing or dialysis leads to sols and gels 181

2.3.4. Sols and gels of hexavalent metal oxides. Colloidal tungstic acid is usually

obtained by adding hydrochloric acid to a sodium tungstate solution 182 As with vanadium,

acidification can be performed with a proton exchange resin in order to obtain colloidal

solutions free of foreign ions 183,184 After exchange, a clear yellow-colored solution is

obtained which becomes progressively turbid and turns to a gel and then to a precipitate

within a few hours. Light-yellow precipitates are obtained when the tungsten concentration

is low (< 0.5M) while the precipitates are dark-yellow at higher concentrations (>0.7M). X-

ray diffraction has shown that the light-yellow xerogel corresponds to W03.2 H20 hydrate


Fig.10. Lamellar structure of WO3.H20 xerogels obtained

through polycondensation of [WO(OH)4(OH2)]°.

while the dark-yellow one is

WO3.H20 hydrate. The colloidal

particles thus obtained have a

plate-like shape and are able to

form long range ordered tactoids

184 (figure i0).

As in the case of vanadium,

the h=6 precursor formed by

acidification around pH=2

[MO2(OH)2 ]° is able to change

its coordination number from 4

(tetrahedral) to 6 (oetahedral)

owing to the high partial charge

on tungsten atom (6(W)=+0.64).

Addition of nucleophilic ligands

can occur again in two ways :

i)Tetrahedral h=6 species are acidic: [MO 2(OH) 2] ° = [MO 3(OH)]" + H +

6(O)=-0.31 6(0)=-0.42

Addition and condensation of these tetrahedral precursors lead to isopolyanions •

(10-x)[WO2(OH)2] ° + x [WO3(OH)]" ' [H4.xW10032 ]x" + 8 H20

- , ] x - + 5 H20 M = Mo,W (6-x)[MO 2(0H)2] ° + x [MO 3(OH)] [Hz-xM6019

ii) If x ~ O, water molecules ean enter into the coordination sphere. As the h=6

precursor has two oxo ligands, two water molecules can be added in a trans position

relative to the short M=O double bonds. In this case condensation can occur only through

oxolation leading to linear or cyclic species because the functionality of the precursor is

f= 2 :

I o

H ~ * W ",,o % //? %"oH

? JVo~ H206-

n[MO2(OH)2(OH2)z] ° , [MO3(OH2)2]" + n H20

Ho/i\o. 0"2

/

Fig.ll. Mechanism of formation of

WO3.H20 layers

No precipitation occurs because low

molecular weight cycles are easily formed.

This explains why Mo(Vl) does not give

rise to precipitates nor gels when ion-

exchange techniques are used 185,186,167

Mo(VI) thus behaves in a similar way as

Cr(VI), but seems to have an octahedral

coordination 188 with two water molecules

preventing precipitation. Such behavior

is not observed with W(VI) because

another possibility (figure ii) is the

disso-ciation of one water molecule by the

reverse mechanism proposed in 2.2.2. The

other oxo ligand remains stable and the

water molecule in the trans position plays

the same role as before. An oetahedral

[WO(OH)4(OH2)] ° h=6 precursor is formed

which can grow in a bidimensional way

through oxolation because olation is

prevented. The sheets thus formed can make

hydrogen bridges leading to the layered

structure of tungstic hydrates WO3.2H~O


and WO3.H20 189 shown in figure I0. The water dissociation process is very slow with Mo(VI)

but occurs upon ageing or heating leading to isostructural hydrates MoO3.2H20190'191 and

MoO3.H20 192,193,194 or to ~-MoO3.H20 a white-colored hydrate 195,196

2.4. Role of the anions

In our previous discussion on the hydrolysis of cations, the role of the counter

anion was completely neglected. The metal atom was assumed to be surrounded by aquo,

hydroxo or oxo species only. This situation occurs when pH modifications are obtained with

an ion exchange resin. However, in most cases a counter anion is present when an inorganic

2 F m , A C e . . . . . . B

Q l e " t' ' t " i ' , . , % ,

,IV r~ ~ A _ . . . ~ • , . ~ . ~

• . . ~ ' . . . ~ / ~ ' ~ f M ~

i I I ' i t l I 11 l l l l ~ i • g

, I , . i . , ' - • • ~ V ~ ' ~ A : k ~ ; : : ~td._. / ; ?

t ' : ~I i t Y / " ~ " i k " ! D

. f

t - i , . -

e- Y

~ ~ - ~ F

Fig. 12. Various morphologies of particles as a function of the type of counter-ions present in solution according to E. Matijevic.

(A) Cl" 23 (~.Fe203) (E) H2PO ~ 23 (~_Fe203)

(B) CIO4 108 (~_Fe203) (F) Cl" 23 (~-FeOOH)

(C) NO3 108 (~_Fe203) (G) HSO 4 105 (Fe3(OH)s(SO4)2.2H20)

(D) CI'/EtOH 109(~_Fe203)


salt is dissolved into water. In some cases organic or inorganic anionic species are added

to the solution in order to control the precipitation process. It is well known that other

anions besides hydroxide ions play a decisive role in homogeneous precipitation of metal

oxides 23,197-202. Some anions are strongly coordinated to metal cations and thus end up in

the precipitate while others can be removed by leaching. In most cases anions strongly

affect the particle morphology and colloid stability (figure 12) 23'105'108'109'201

Many techniques are now available to produce a large number of well-defined

monodispersed colloidal particles. However it is still difficult, if not impossible, to

predict the morphology of these particles. Anions seem to play both a chemical and a

physical role. At the beginning of the process, they are able to coordinate the metal ion

giving rise to a new molecular precursor whose chemical reactivity toward hydrolysis and

condensation is expected to be different. Once colloidal species are formed, the anions

change the double layer composition and the ionic strength of the solution therefore

modifying aggregation processes.

This section will attempt to describe the chemical role of anions in the aqueous

chemistry of inorganic precursors in order to show how they can orient the chemical

composition and the structure of colloidal particles. The following discussion shows that

the morphology of the particles cannot be a unique function of the chemistry involved

during nucleation and therefore physico-chemical factors must also play a decisive role. At

the present time, it is difficult to make a clear difference between growth by monomeric

units and growth through aggregation. As a result, no attempt will be made to correlate the

morphology observed by TEM and the chemical role of anions.

2.4.1 Complexation of metal cations. Associated species [M(OH)h(X)(OH2)~.h.I ](z'h'1)+ can

be formed when both positively charged hydrolyzed cations [M(OH)h(OH2)N.h] (z'h)+ and

negatively charged anions X" are simultaneously present in an aqueous solution. Such M-X

associations have been clearly shown by optical spectroscopy 203,204, N.M.R 205,206 or X-

ray scattering 207,208. The full coordination N of a metal cation in an aquo or hydroxo-

aquo precursor is already satisfied. Therefore the coordination of the anionic species X"

with such a precursor occurs via a nucleophilic substitution. However, the question arises

whether one can predict if such species remain stable in an aqueous medium or whether they

readily dissociate. Water actually plays a double role. It behaves as a solvent with a high

static dielectric constant (e=80) which favors the dissociation of ionic species. It is

also ~ a-donor molecule which reacts as a nucleophilic ligand. Therefore we have to check

whethe~ the M-X bond is stable against both ionic dissociation and hydrolysis.

i) Let us consider an associated species in which a monovalent anion X'is

coordinated to the hydrolyzed cation. Ionic dissociation corresponds to the following

reaction : 1(z.h.1) + H20 [M(OH)h(X)(OH2)N . . . . h lJaq. + H20 - [M(OH)h(OH2)N h 1(z'h)+aq. + X'aq. (7)

A partial charge transfer occurs between M and X within the M-X chemical bond leading to a

modification of the negative charge of the anion. Two possible cases arise :

- X" is more electronegative than H20 ligands (x(H20) - 2.49). Electrons are attracted

by X and the overall transfer goes from the precursor to X, increasing the negative charge

of the anion (6(X)<-!) The M-X bond become~ more ionic and the high dielectric constant of

the aqueous solvent favors ion-pair formation. Equilibrium (7) is displaced toward the

right and the associated species are not stable against ionic dissociation. X'does not

exhibit any ability to complex with the metal cation.

- X is less eleotronegative than H20 ligands. Electrons can be transferred from X to

the precursor. The negative charge of the anion decreases (6(X)>-I) giving rise to a more


covalent M-X bond which is not dissociated by the solvent. Equilibrium (7) is displaced

toward the left and the anion X" remains coordinated to the metal atom.

The ability of an anionic species X x" to form complexes with a cation M z+ will

therefore depend mainly on the magnitude of electron transfer from X to M within the M-X

bond. This electron transfer leads to a charge variation Ax of the anion, before (x) and

after (6(X)) eomplexation, which can be easily calculated with the Partial Charge Model:

Ax=x+6(X). A rough estimate of how much equilibrium (7) is displaced toward the left can be

made by looking at the relative charge variation of the anion :

ax 6(X) - - = 1 + - - (8) x x i00

In the case of monovalent anions this leads to : Ax = 1 + 6(X). Anion X" does not complex

when Ax<O. Its ability to form complexes increases with Ax when Ax>0, i.e. when the

electronegativity of the anion decreases. The above considerations are based on

electrostatic interactions only. Entropic and resonance effects observed with chelating

anions (E.D.T.A., ~-hydroxy acids) can increase their ability to form complexes and

209-213 therefore such species are often used to control precipitation processes

ii)The associated species with Ax>O [M(OH)h(X)(OH2)N.h.I ](z'h'1)+ are in the

presence of a large excess of water molecules. Therefore, they must also be stable against

hydrolysis :

] ( z - h - I ) + + H2 0 ](z-h-l)+ + (9 ) [ M ( O H ) h ( X ) ( O H 2 ) N . h . 1 ,aq " = [ M ( O H ) h + l ( O H 2 ) N . h . l , a q " HXaq "

This equilibrium goes through a transition state in which a proton can be

transferred from a water molecule towards the X group : [M(OH)h+ I(HX)(OH2)N.h.I ](z'h'1)+

Again, charge considerations lead to two possibilities :

6(HX)<O : from purely electrostatic considerations, the negatively charged HX species

remains attracted by the positively charged M z÷ cation. Equilibrium (9) is displaced

towards the left and the anion X" remains coordinated to the metal.

6(HX)>0 : nucleophilic substitution by water molecules becomes possible and the

associated species is not stable towards hydrolysis. It could be stable however in the

presence of aprotic solvents such as DMSO.

2.4.2. Complexation of Fe 3+ aqueous precursors. The hydrolysis of [Fe(OH2)613+ species has

been described previously. As an illustration, let us consider now whether this aqueous

precursor can be complexed by a monovalent anion X" such as CIO4, NO3, HSO4, H2PO ~ or

CH3COO'. According to the literature, such anions behave as bidendate ligands and should be

able to replace two water molecules giving rise to [Fe(X)(OH2)4] 2+ species. According to

the previous discussion, this complexed species has to be stable against : 3÷

Ionic dissociation : [Fe(X)(OH2)4] . = [Fe(OH2)6]aq + Xaq.

,y rolysis : = +

Depending on the strength of the acid HX in aqueous solution, the hydrolyzed species can be

reprotonated leading to the non hydrolyzed [Fe(OH2)6] 3+ precursor. Table 6 reports charge

calculations performed on both coordinated species using the Partial Charge Model.

x- ClO~ .% .so i He% CH 3 COl

2.86 2.76 2.64 2.49 2.24

6(X) -0.92 -0.84 -0.50 -0.34 +0.40

Ax +0.08 +0.18 +0.50 +0.66 +1.40 i

6(HX) I -0.52 -0.42 -0.15 +0.02 +0.70

Table 6 . Partial Charges $(X) and 6(HX) in [Fe(X)(OH2)4]2+ and [Fe(OH)(HX)(OH2)3 ]2+

species respectively, as a function of the mean electronegativity X of the anion Xaq.


According to table 6, the M-X bond becomes less and less ionic when the

electronegativity of X" decreases. The complexed species then become more stable towards

ion pair formation. On the other hand, 6(HX) increases so that hydrolytic dissociation

becomes possible as soon as 6(HX)>0.

'~,x

-+1.0 \ -1 .0 -

-+0 .5 - 0 . 5 -

A©O CI-

- - 0 . 5 + 0.5

- - 1 , 0 + 1 . 0 -

Fig.13. Variation of Ax - I+6(X) and 6(HX) versus X for some

[Fe(X)(OH2)4] 2÷ precursors.

~(HX)~

X" monovalent anions in

An electronegativity range can be estimated graphically if we plot Ax=l+6(x) and

6(HX) versus X as shown in figure 13. Anion X" remains coordinated to the metal (Ax>0,

~(HX)<0) for intermediate electronegativities only, roughly speaking, between 2.55<~<2.90

for the example shown in figure 13. Ionic dissociation prevails for higher electro-

negativities (Ax<0) while hydrolytic dissociation occurs for lower electronegativities

(6(HX)>0). Therefore, HCO3, CI" and CH3CO0" cannot give stable complexes in these

experimental conditions. The ability to form complexes for a given anion X" also depends on

the hydrolysis ratio h of the precursor, i.e. on the pH of the solution. Table 7 reports

partial charge calculations performed on hydrolyzed neutral species corresponding to h=2.

One can see that highly electronegative anions such as perchlorates are not able

to coordinate Fe 3+ ions because of ionic dissociation. They behave as counter ions.


However, the hydrolysis ratio h can be decreased by lowering the pH and complexation

occurs under highly acidic conditions as shown in table 6 and in agreement with

experimental observations 207,208 A similar behavior has been reported for Ti(IV) in

highly concentrated HCIO 4 214 On the other hand, less eleetronegative anions such as HC03

are able to coordinate the metal cation when the hydrolysis ratio is high, i.e. at high

pH. As a consequence, they behave mostly as counter ions except under basic conditions.

X ~ 6(x) Ax 6(HX)

CIO4 2.86 -1.26 -0.26 -0.94

HSO4 2.64 -0.92 +0.08 -0.65

HC03 2.49 -0.72 +0.28 -0.45

Table 7. Partial charges 6(X) and 6(HX) in the neutral species [Fe(OH)2X(OH2)2 ]°,

I,=3

I':211 I I h:, .=0 2.0 2.1 2.2 I 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0

Aoo- c,- HCO; HSO; NO~ C,O;

Fig.14. Electronegativity range as a function of the hydrolysis ratio h for which X"

monovalent anions form stable complexes [Fe(OH)h(X)(OH2)4.h ](z'h)+

As a rough guide, the electronegativity range for which anions form stable

complexes with metal cations shifts towards low electronegativities when the pH of the

aqueous solution increases as shown in figure 14. As a consequence, highly electronegative

anions usually behave as counter ions except at very low pH. Anions having a low

electronegativity also behave as counter ions or molecules (because of hydrolytic

dissociation), except at high pH. Some anions however, having a mean electronegativity

close to that of H20 (X=2.49) are able to form stable complexes over the whole range of pH.

Such anions (sulfates) will therefore have a strong effect on both hydrolysis and

condensation processes. They will induce deep modifications of the structure and morphology

of colloids and precipitates. They can even remain coordinated to the metal cation up to

the end of the precipitation giving rise to basic salts such as those observed when Fe 3+

ions are precipitated in the presence of SO~'. On the other hand, pure ~-Fe203 is obtained

with other anions which, depending on the pH, can be complexing or not (cf. figure 12).

This discussion concerning Fe 3÷ inorganic precursors can be easily extended to any other

aquo-hydroxo complex. For each element, electronegativity ranges may be computed as

previously described allowing a quantitative description of complexation phenomena in

aqueous solutions.

2.4.3. Hydrolysis and condensation of Fe 3+ . Anion complexation of metal cations leads to

new precursors whose chemical reactivity can be noticeably different. The modification of

Fe 3+ aqueous species by a strong chelating ligand such as EDTA,[(OOCCH2)2N-CH2-CH2-

N(CH2COO)2 ]4" , has been studied carefully. Both hydrolysis and condensation constants were

measured before 89,99,203,215 and after 216 complexation leading to the following results :

Hydrolysis :

[Fe(OH2)6] 3+ + H20 = [Fe(OH)(OH2)5] 2" + H30 + ~ = i0 "5

[Fe(OH2) 2 EDTA]" + H20 = [Fe(OH)(OH2)EDTA] 2" + H3 O+ ~ = 10 .25


Condensation :

2[Fe(OH)(OH2)5] 2+ = [Fe2(OH)2(OH2)8] 4+ + 2 H20

2[Fe(OH)(OH2)2EDTA]2" = [Fe2(OH)2(EDTA)2] 4" + 2 H20

It can be seen that hydrolysis is prevented by

condensation is favored. This is mainly due to charge

complexation. A partial charge calculation shows that (table 8):

Precursor 6(Fe) 6(H) 6(OH)

[Fe(OH2)6] 3+ + 0.59 + 0.34

[Fe(OH2)2(EDTA)]" + 0.43 + 0.20

[Fe(OH)(OH2)5] 2÷ + 0.55 + 0.30 - 0.01

[Fe(OH)(OH2)(EDTA)] 2" + 0.40 + 0.17 - 0.25

Table 8 : Complexation of h-0 and h-i aquo precursors of Fe 3+ by EDTA 4" (EDTA=C10HI208N2).

K d = 6.10 .4

K~ - 102.95

EDTA complexation while

modifications induced by

The positive partial charge on the protons of the water molecules in the non hydrolyzed

precursors decreases upon complexation. The EDTA modified precursor is therefore a weaker

acid and the deprotonation of coordinated water molecules is more difficult.

The condensation process begins with a nucleophilic attack by the negatively charged OH

group onto the positively charged metal atom. This process therefore is easier as 6(OH)

becomes more negative and 6(M) more positive. Table 8 shows that EDTA complexation leads to

a decrease of 6(Fe) and an increase of 6(OH). Therefore, it is not obvious to determine

which factor will prevail. However, a rough estimate of the condensation ability could be

given by the product 6(M).6(OH) that varies from -5.10 .3 up to -10 "I upon complexation. The

larger variation comes from 6(OH) and dimerization of the modified precursor should be

easier as confirmed by the equilibrium constants.

2.4.4. Formation of basic salts. Complexing anions coordinated to the dissolved metal ion

do not only change the charge distribution within the aqueous precursor. They can also play

a role as network formers in the structure of condensed phases. Some of them end up in the

solid giving rise to the precipitation of basic salts.

Figure 14 shows the structures which can nucleate from the h-2 aqueous zirconium

precursor [Zr(OH)2(OH2)6] 2+. All these structures have been experimentally determined by X-

ray diffraction 217-221 Non-eomplexing anions (~-0%) such as CI" or CIO4 are not able to

displace water molecules. They are not involved in the formation of condensed species and

hydrous zirconia ZrOz.nH20 can precipitate at high pH. A cyclic tetramer [Zr(OH)2(OH2)4]~ +

is formed via 2 (OH)2 bridges in which zirconium is surrounded by four terminal water

molecules and four bridging OH groups (square antiprism). Complexation occurs with nitrate

which exhibits a weak complexing ability (~-4%). Two terminal water molecules are replaced

by one NO3 group and a chain polymer [Zr(OH)2(NO3)(OH2)z] ~ is formed in which zirconium

remains in eight-fold coordination (dodecahedron). It should be mentioned that nitrates

remain as terminal groups : they do not link chains together and should not be considered

as network formers. Sulfates have a higher complexing ability (~-32%). Thus, they are able

to replace all coordinated water molecules leading to [Zr(OH)2SO4] n species in which the

zirconium is eightfold coordinated (square antiprism). Moreover, SO~'anions behave as

network formers, bridging three different [Zr(OH)2 ]2n+,n chains together. Stronger

complexation is expected with HPO~" (a-50%) or CrO~" (~-53%) ions. Chromate compounds

exhibit a layered structure in which [Zr3(OH)6CrO4]~ n+ sheets are linked together by CrO~"

tetrahedra. Zirconium exhibit both eightfold (dodecahedron) and sevenfold (pentagonal

bipyramid) coordinations. Another structure was suggested for the phosphate derivative 219


b

C O H 20

d

o>

Fig. 15. X-ray structures of some basic salts of zirconium.

(A) {[Zr4(OH)s(OH2)1618+,8CI04 " ) 52,53 : 6(CI04)=.I.I 7

(B) ([Zr(OH)2(NO3)(OH2)2]n+,nNO" ) 217

(C) [Zr(OH)2(SO4)(OH2) ] 218 :

(D) [Zr(OH)2(S04) ] 217,219 .

(E) [Zr(OH)2(Cr04) ] 220 .

[Zr(OH)2(H2P04)2 ] 221 :

: 6 (NO 3 )=-0.96

~ (HS04)=-0.37

6 (HS04)=-0.68

6 (HerO 4)=-0.47

6 (H2 P04 ) =- 0.50

Stronger complexation of chro-

mate ions (~=64%) can be obser-

ved if h is reduced to 1.5.

[Zr4(OH)6(CrO4~lSn+-.n chains with

zirconium in sevenfold coordi-

nation (pentagonal bipyramid)

linked together by Cr04"

tetrahedra are found in the

resulting compound.

2.4.5. Monodispersed chromium

hydrous oxide sols. The produc-

tion of monodispersed powders is

of the utmost importance for the

ceramic industry. Therefore

great efforts have been made in

order to control nucleation and

growth processes that lead to

the formation of a precipitate.

It appears that the fundamental

requirement for the preparation

of monodispersed particles in

aqueous solutions is to control

of the rate of generation of the

solutes species that are precur-

sors to precipitates 222,223

The goal is to reach a critical

supersaturation of the particle

forming species so that only one

burst nucleation occurs. Care

must be taken to avoid secondary

nucleation 201 This effect has

been well illustrated by E.

Matijevic et al. who showed that

spherical amorphous particles of

chromium hydrous oxide can be

generated by hydrothermal ageing of solutions containing sulfate 224,225 or phosphate

ions 78 but not in the presence of CI', NO3 or CH3CO0" ions. It appears that complexing

anions have a specific role in the nucleation process. Under identical experimental

conditions, but in the absence of these anions, only solute hydrolysis products are

obtained, and no solid particles precipitate. Electron microscopy shows that strands of

polymeric materials are obtained prior to the formation of spherical particles 226

Moreover, chemical analysis indicates that sulfates are bound in both solute chromium

complexes and polymeric species but not in the spherical chromium hydroxide particles

225,226,227 Therefore, the role of sulfates seems to be restricted to the nucleation step,

or to the cross-linking of polymeric chromium hydroxide chains. As nucleation involves

mainly neutral species, the following monomeric precursors have to be taken into account

22Z : [Cr(OH)3(OH2)3]o, [Cr(OH)2(HSO4)(OH2)2 ]° . Polymeric species giving rise to embryos

are formed from the condensation of these monomers. At high sulfate concentrations,


condensation involves mostly modified precursors. This gives rise to chain polymers formed

through olation and linked together by sulfate bridges. Partial charge calculations show

that in such polymers, HSO~ remains coordinated to the chromium atom. At lower sulfate

concentrations, condensation between modified and non modified precursors becomes more

important :

[Cr(OH)2(HSO4)(OH2)2] ° + [Cr(OH)3(OH2)3] ° ~ [Cr2(OH)5(HSO4 )(0H2)4] ° + 2H20

However, in this case the Cr-HSO 4 bond becomes more ionic (6(HSO4)<-I) and sulfate ions

lose their complex-forming ability giving rise to ion pair formation as follows :

[Cr2(OH)5(HSO4)(OH2)2 ]° = [Cr2(OH)6(OH2)4] ° + HSO4 + H3 O+

Hydrous chromium oxide should then precipitate, free from sulfate ions, in agreement with

Matijevic's results 225 At intermediate sulfate concentrations, both condensation

processes occur simultaneously. However, chromium oxide precipitation displaces the

previous equilibrium. As a consequence, the polymeric basic salt should be progressively

transformed into hydrous Cr203 .

3. METAL ORGANIC MOLECULAR PRECURSORS

Metal alkoxides M(OR) n are versatile molecular precursors for the sol-gel

synthesis of metal oxides. They are known for almost all transition metal elements,

including the lanthanides 19. The number and stability of transition metal alkoxides

decreases from left to right across periodic table. The alkoxy group OR (R = saturated or

unsaturated organic group) is a hard ~-donor and stabilizes the highest oxidation state of

the metal. Therefore, alkoxides of main group elements and d O transition metals (Ti, Zr)

are rather well-known, while those corresponding to the soft d n late transition metals have

been much less studied 228. Moreover, the chemistry of electron-rich metal alkoxides has

long been restricted by oligomerization reactions which lead to the formation of insoluble

polymeric species (Fe, Co, Ni, Cu...) 229. Some alkoxides which are already being widely

used in industry are commercially available at relatively low cost (Si, Ti, AI, Zr). Many

others can be found for small-scale applications but at much higher prices (V, Mn, Fe, Co,

Ni, Cu, Y, Nb, Ta) 230 Otherwise, transition metal alkoxides have to be prepared in the

laboratory following the usual methods for the synthesis of metal alkoxides 19 The sol-gel

processing of silicates from silicon alkoxides has been extensively studied 28,30,231

Unfortunatly, there is a lack of data concerning the hydrolysis and condensation of

transition metal alkoxides. Therefore, the chemical reactivity of these alkoxides, mostly

Ti(OR)4, will be compared to the chemical reactivity of the corresponding silicon alkoxides

Si(OR) 4 . The main differences arise from the following two points :

- The lower electronegativity of transition elements leads to a much higher electrophilic

character of the metal.

- The possibility exists for most transition metals to exhibit several coordinations so

that full coordination is usually not satisfied in the molecular precursor, which allows

coordination expansion.

As a result, transition metal alkoxides are much more reactive. They must be handled with

care, in the absence of moisture. They readily form precipitates rather than gels when

water is added.

3.1. Hydrolysis and condensation of metal alkoxides

Electronegative alkoxo groups (OR) make the metal atom highly prone to


nucleophilic attack. Metal alkoxides are therefore extremely reactive with water leading te

the formation of hydroxides or hydrous oxides. The overall reaction can be written as

follows :

M(OR)n + nH20 ----+ M(OH)n + nROH

This reaction is actually much more complex than it might seen. Two chemical processes,

namely hydrolysis and condensation, are involved in the formation of an oxide network from

metal alkoxides. Hydrolysis of the alkoxide occurs upon adding water or a water/alcohol

solution, and a reactive M-OH hydroxo

usually proposed in the literature 20,37

H-I + M-OR(a) , H~ : H/O --+ M-OR

(b)

group is generated. A three steps mechanism is

/ H0-M +-- O~ M-0H + R0H

(c) kH (d)

The first step (a) is a nucleophilic addition of a water molecule to the positively charged

metal atom M. This leads to a transition state (b) where the coordination number of M has

increased by one. The second step involves a proton transfer within (b) leading to the

intermediate (c). A proton from the entering water molecule is transfered to the nega-

tively charged oxygen of an adjacent OR group. The third step is the departure of the

better leaving group which should be the most positively charged species within the

transition state (c).

The whole process, (a) to (d), follows a nucleophilic substitution mechanism. Charge

distribution governs the thermodynamics of this reaction which will be highly favored

when:

The nucleophilic character of the entering molecule and the electrophilic character of

the metal atom are strong : 6(0)<<0 and 6(M)>>O.

The nucleofugal character of the leaving molecule is high : 6(ROH)>>O.

On the other hand, the rate of the nucleophilic substitution depends on :

The coordination unsaturation of the metal atom in the alkoxide given by the difference

between the maximum coordination number N of the metal atom in the oxide and its oxidation

state z. The larger (N-z), the lower the activation energy associated to the nucleophilic

addition of step (a) should be.

The ability of the proton to be transferred within the intermediate (b). The more acidic

the proton, the lower the activation energy associated with this transfer will be.

Condensation is also a complex process and can occur as soon as hydroxo groups are

generated. Depending on experimental conditions, three competitive mechanisms have to be

considered namely : alcoxolation, oxolation and elation.

i) Alcoxolation is a reaction by which a bridging oxo group is formed through the

elimination of an alcohol molecule. The mechanism is basically the same as for hydrolysis

with M replacing H in the entering group :

M-~ + M-OR --+ M-Ok:--+ M-OR ~ M-O-M +-- O~/*" ~ M-O-M+ ROH

(a) ~ (b) (c) ~ (d)

Consequently, the thermodynamics and kinetics of this reaction are governed by the

same parameters as for hydrolysis.

ii) Oxolation follows the same mechanism as alcoxolation, but the R group of the

leaving species is a proton

M-O + M-OH M-0:--+ M-OH

(a) ~ (b) The leaving group is thus a water molecule.

I M-O-M '~ - - :0~ :' M-O-M + H20

(c) ~ (d)


iii) Olation can occur when the full coordination of the metal atom is not

satisfied in the alkoxide (N-z~O). In this case bridging hydroxo groups can be formed

through the elimination of a solvent molecule. This latter can be either H20 or ROH

depending on the water concentration in the medium :

M - OH + M +-- O , M - O - M + ROH

M - OH + M +-- O , M - O - M + H20

The thermodynamics of this nucleophilic substitution are governed by the charge

distribution. The reaction is strongly favored when the nucleophilic character of the

entering group and the electrophilic strength of the metal are high : 6(0)<<0 and 6(M)>>0.

Moreover, since no proton transfer is involved within the transition state and since the

metal coordination is not saturated, the reaction rate is usually quite fast.

These four reactions (hydrolysis, alcoxolation, oxolation and olation) may be

involved in the transformation of a molecular precursor into an oxide network. The

structure and morphology of the resulting oxide strongly depend on the relative

contribution of each reaction. These contributions can be optimized by carefully adjusting

the experimental conditions which are related to both internal (nature of the metal atom

and alkyl groups, structure of the molecular precursor) and external (water/alkoxide ratio,

catalyst, concentration, solvent, temperature) parameters.

3.1.1. Nature of the metal atom. Since transition elements are more electropositive than

silicon, hydrolysis of transition metal alkoxides is much easier. Actually it is well known

that silicon alkoxides are not very reactive with water. On the other hand, transition

metal alkoxides react vigorously and a strongly exothermic reaction is observed as soon as

the alkoxide is brought into contact with water. A rough estimate of the partial charge

distribution (table 9) in metal alkoxides shows that the partial positive charge is much

higher for transition metals than for silicon. This explains why transition metal

alkoxides are very unstable towards hydrolysis 19,20,181,232. They must be handled very

carefully, in a dry environment and stabilizing agents are often added in the sol-gel

processing of transition metal oxides 20

Alkoxide Zr(OEt)4 Ti(OEt)4 Nb(OEt)s Ta(OEt)5 VO(OEt)3 W(OEt)6 Si(OEt)4 I ~(M) + 0.65 + 0.63 + 0.53 + 0.49 + 0.46 + 0.43 + 0.32 1

Table 9 : Positive partial charge on M for some metal ethoxides.

Another peculiarity of transition metal alkoxides is that coordination expansion

of the metal readily occurs upon hydrolysis. Hydrolysis rates are thus expected to be much

higher than for Si(OR)4 where the fourfold coordination of silicon is already satisfied. A

survey of the literature concerning hydrolysis rates of Tetraethoxysilane (~) gives values

ranging between 10 .4 and 10 .6 M'Is "I at pH-3 233-238 Extrapolation of this constant at

pH=7 gives ~=5 10 .9 M'Is "1 234. Although very little data is available for most transition

metal alkoxides, a minimum value of ~=lO'3M'Is "I at pH-7 can be roughly estimated for

Ti(OR) 4 239-243 which is at least five orders of magnitude larger than for Si(OR)4.

Hydrolyzed species such as M(OR)3(OH ) (M - Si,Ti) can undergo two condensation processes :

a l c o x o l a t i o n M(OR)3OH + RO-M(OR)2OH , (RO)3M-O-M(OR)2OH + ROH

oxolation M(OR)30H + HOoM(OR)3 , (RO)3M-O-M(OR)3 + H20


The charge distribution calculated within the transition states M2(OR)6(OH) 2 for M = Si, Ti

and R = Et are given in table i0 :

M 6(M) 6(OH) 6(H20) 6(EtOH)

Ti +0.64 -0.36 -0.25 +0.02

Si +0.33 -0.34 -0.21 +0.13

Table i0 : Charge distribution for two transition states during condensation.

In both cases the hydroxo groups are highly negatively charged allowing a nucleophilic

attack of the positively charged metal atom. After proton transfer, a positively chsrged

species must be removed. Table i0 shows that in both cases, water retains a negative

partial charge while ethanol carries a positive one. Therefore condensation of hydrolyzed

alkoxides should proceed via alcoxolation rather than oxolation. This conclusion has been

checked for Tetramethoxysilane by IR absorption 244 and for Tetraethoxysilane by 29Si NMR

233,245,2:46 In the case of transition metal alkoxides, alcoxolation leads to well defined

oxoalkoxides which can be isolated as single crystals. Their structures has been resolved

by X-ray diffraction for M = Ti 247 Nb 248 and Zr 249 and are shown in figure 16:

3 Ti(OEt)4 + 4 Ti(OEt)3(OH ) ~ TiFO4(OEt)20 + 4 EtOH

2 Nb(OEt)3(OH)2 + 6 Nb(OEt)4(OH ) , NbsO6(OEt)10 + i0 EtOH

5Zr(OMe) 4 + 8 Zr(OMe)3(OH ) , Zr1308(OMe)36

B

C

Fig.16. X-ray structures of some transition

metal oxo-alkoxides : (A) TizO 4 (OEt)20

(]3) NbsO10(OEt)20 ; (C) Zr1308(OMe)36

+ 8 MeOH

The main feature of these molecular

compounds is that the usual metal atom

coordination number is always satisfied.

The low coordination of the metal atom

in non-hydrolyzed transition metal alke-

xides must be correlated with their high

rates of condensation. Condensation is a

rather slow process for silicon alkoxides,

a global rate kc=10 "4 M-ls-1 where has

been measured for TEOS 250,251 . Such rate

constants are difficult to measure for

Ti(OR)4, owing to the rapid precipitation

of the oxide. However, a global rate cons-

tant of about 30 s "I was found for TiO 2

precipitated from Ti(OEt)4 243 Conden-

sation of Ti(OPri)4 is also extremely fast

as evidenced by the following linear

growth-rate deduced from turbidity

measurements 242 : r(nm.min'1)=0.9 [Ti] 4"I

This means that for [Ti]=0.1 M the growth-

rate is about 1.8 nms "I which is rather p

high. Similar behavior has been found for

the hydrolysis rates of Zr(oPrn)4 in

ethanol where the time t elapsed between

mixing and precipitation is given by 252 .

t-1(s'1)=0.9[H20] 3 [Zr(OPrn)4] I (mol.l "I)

3.1.2. Nature of the organic ligand. The hydrolysis rate constants for a series of silicon

alkoxides were measured by several authors 234-237 who pointed out that the rate of


hydrolysis decreases with increasing size of the alkyl groups (table Ii). It seems that

these results can be extended to transition metal alkoxides : hydrolysis of titanium n-

alkoxides also becomes slower when the size of the alkyl group increases 239,253. Table ii

shows that the partial charge distribution in the alkoxide depends on the alkyl group,

giving rise to more or less polar M-OR bonds 37

R 6(Ti) -6 (OR) 6(H) 6(Si) 6(OR) 6(H) ~I02M" Is I [H+] I

CH 3 +0.66 -0.16 +0.12 +0.36 -0.09 +0.14 571

CzH 5 +0.63 -0.16 +0.i0 +0.32 -0.08 +0.ii 1.9

n-C4~ +0.61 -0.15 +0.09 +0.30 -0.08 +0.09 0.83

n-C6HI 3 +0.60 -0.15 +0.08 +0.29 -0.07 +0.08 0.3

n-CgHI 9 +0.59 -0.15 +0.07 +0.28 -0.07 +0.08

Ti(OR) 4 Si(OR) 4

Table Ii : Charge distribution in Ti(OR) 4 and Si(OR) 4 n-alkoxides.

The positive partial charge of the metal atom (M ~ Si,Ti) decreases with the

length of the alkyl chain. The sensitivity of the alkoxide towards hydrolysis should then

decrease, in agreement with experiments 235,239,253. Moreover, the positive partial charge

of the hydrogen atom decreases in the same way. Proton transfer should then become more

difficult, which is an effect that could be related to the decrease of the kinetic constant

235

Experimental results are often explained in terms of steric hindrance as well. It

has been shown that, for isomeric titanium butoxides, the hydrolysis rate are in the order

tertiary > secondary > normal 239. A reverse behavior was observed for silicon butoxides

where the measured gelation time is 32 h for Si(OBun)4, 236 h for Si(OBun)(oBut)3 and 500 h

for Si(OBuS)4 236,23?. The hydrolysis of titanium tetra-tert-amyloxide behaves the same way

as required on the basis of effective

slow but becomes much faster for

distribution alone cannot explain the

effects of the alkyl chain should also

Mesomeric effects may also

Bistan and Gomory 253 studied the

shielding, i.e. hydrolysis of the first OR group is

the following ones 254. Steric hindrance and charge

hydrolysis rate of metal alkoxides. Inductive + I

probably be taken into account.

affect the hydrolysis of metal-organic precursors.

hydrolysis reaction of a number of alkoxides and

aryloxides Ti(OR)4 (R = C6H5, C6HsCH2, m-CH3C6H4). They concluded that aryloxides are more

resistant to hydrolysis than aliphatie alkoxides. The same behavior is observed with

W(OC6H5) 6 and W(OEt)6 255 On the other hand, silicon phenoxide Si(OC6H5) 4 appears to be

more reactive with water than Si(OEt)4 255,256 This difference in reactivity between

transition metals and silicon may he explained by two competing effects :

- The -I inductive effect of the aryloxo group increases the positive charge on the

metal atom.

- The mesomeric +E effect of the aromatic ring increases the ~-donor ability of aryloxo

groups and reduces the positive charge of the metal atom. The magnitude of this +E effect

strongly depends on the availability of the metal d-orbitals.

For silicon phenoxide the -I inductive effect is probably the strongest rendering

the silicon atom highly prone to nucleophilic attack. Meanwhile, for transition metal

aryloxides mesomerie effects should be predominent. This is also supported by the fact

that the hydrolytic stability also depends upon the nature of the aryloxo groups, i.e.

ortho-and para-substituted groups are more stable than classical phenoxides.

Condensation is also strongly affected by the nature of the alkyl chain. K.C. Chen

and J.D. Mackenzie showed that the gelation time of silicon alkoxides increases with the

length of the alkyl chain 257 For transition metal alkoxides, under neutral or basic


conditions, and without any chemical modification, gelation is never possible. Depending on

the chain length precipitates or polymer colloids are formed. Precipitation of TiO 2 from

Ti(OR) 4 is observed when R=Et 240 ,241 pr i 240,242 while linear polymers seem to be formed

when R=Bu n 239,258,259 or R=Am n 260 Experiments performed in our laboratory confirm that

precipitation cannot be avoided, even under mild hydrolysis conditions, when R = Et, Pr n or

Pr i . However, stable sols can be obtained when Ti(OBun)4 or Ti(OAmt) 4 are hydrolyzed under

the same conditions. Analytical ultracentrifuging of these sols leads to the following

mean molecular weights : M.W.=5600 g/mole for R=Bu n and M.W.=3800 g/mole for R=~n t .

This corresponds to molecular species containing at most several tens of titanium atoms.

This supports the formation of small polymeric species whose degree of condensation depends

on the R group. The larger the R group is, the smaller the resulting polymer.

The main characteristics of oxide powders (particle size, surface area,

morphology and crystalline phases), obtained via hydrolysis and condensation of metal

alkoxides strongly depend on the identity of the alkyl group. For example, both anatase and

rutile phases can be present in a TiO 2 powder obtained after calcination of a gel. The

ratio rutile/anatase can be varied by changing the molecular weight of the metal-organic

precursor 261~262 The same phenomenon has been found recently in the hydrolytic

condensation of zirconium alkoxides 263. The alkyl group affects the morphology and the

particle size of the resulting materials. These in turn affect the sintering and

monoelinic-tetragonal transformation of ZrO 2.

3.1.3. Molecular structure of the alkoxide. - Oligomerization : The above discussion does

not take into account one of the main features of transition metal alkoxides. In such

compounds the oxidation state z of the metal is generally smaller than its normal

coordination number N. The full coordination of the metal is therefore not satisfied in

monomerie alkoxides M(OR)z. Consequently, the metal atom tends to increase its coordination

number by using its vacant d orbitals to accept oxygen or nitrogen lone pairs from

nucleophilic ligands. In non polar solvents one finds that coordination expansion of the

metal occurs via alkoxy-bridging which leads to the formation of more or less condensed

oligomers in which the metal attains a higher coordination number. This oligomerization is

basically a nucleophilic addition of a negatively charged OR group to a positively charged

metal atom M. It corresponds to an alcolation reaction which could proceeds as follows:

The degree of association depends on the nature of the metal atom. Within a given

group, the molecular complexity increases with the atomic size of the metal (table 12).

According to Bradley 264 alkoxides should adopt the smallest possible structural unit

consistent with all atoms attaining their higher coordination number. The insolubility of

divalent transition metal alkoxides (Cu, Fe, Ni,Co,Mn) may thus be attributed to their

highly polymeric nature 265

Compound Ti(OEt)4 Zr(OEt)4 Hf(OEt) 4 Th(OEt) 4

Covalent radii (•) 1.32 1.45 1.44 1.55

Molecular complexity 2.9 3.6 3.6 6.0

Table 12 : Degree of oligomerization for some transition metal ethoxides as a function of

metal size.

The molecular complexity also depends on the nature of the alkoxy group. It

decreases with increasing branching and bulkiness of the OR group because of steric

JPSSC 18:4-C


hindrance effects 19. The molecular complexity of metal alkoxides is usually estimated from

molecular weight measurements in solution or by mass spectrometry 19,228 Direct evidence

for the oligomerization of titanium alkoxides was recently provided by X-ray absorption

experiments 266,26z. The shape and intensity of the prepeak observed before the absorption

edge show that titanium is tetracoordinated in Ti(OPri)4 and Ti(OAmt)4 while it is

pentacoordinated in Ti(OEt)4 , Ti(OBu")4 and Ti(OPrn)4. Moreover Ti...Ti distances of about

3.09A were clearly found in the EXAFS spectra of these last alkoxides showing that

oligomers are formed, while bulky (OPr i ) and (OAm t ) groups lead to monomers showing no

Ti...Ti correlations. In the case of silicon alkoxides the oxidation state z = 4 of Si and

its usual coordination number N - 4 are identical. Therefore Si(OR)4 precursors are always

monomeric and exhibit a tetrahedral structure 19

Solvate formation : Metal alkoxides are often dissolved in organic solvents before

hydrolysis is performed. These solvents usually correspond to the parent alcohol and are

far from being chemically inert with respect to the alkoxide. As a general rule, dilution

should lead to lower association. However, the nature of the solvent has to be taken into

account. Bradley observed that while Ti(OEt)4 remains trimeric in an inert solvent such as

benzene, the same was no longer true in a polar solvent such as EtOH 19. This was due to

the nucleophilic properties of the alcohol which causes dissociation and solvation of the

oligomer as follows:

2[Ti3(OEt)12 ] + 6 EtOH = 3[Ti2(OEt)6, 2 EtOH]

These experiments point out a very important property of metal alkoxides. Alkoxy

bridging is not the only method for coordination expansion. The alternative process of the

addition of a solvent donor molecule is also found. Metal alkoxides then behave as Lewis

acids and react with Lewis bases leading to solvate formation. Because of solvation, the

molecular structure of alkoxide precursors generally depends on the nature of the solvent

19 Zirconium alkoxides, for instance, exhibit a reduced molecular complexity when

dissolved in the parent alcohol rather than in an inert solvent. This is due to the

tendency of zirconium to expand its coordination with alcohol molecules instead of alkoxo

groups. The stability of such solvates increases with the positive charge of the metal atom

and its tendency to acquire a higher coordination number. Therefore, stable Zr(OPri)4.iPrOH

(6(Zr)=+0.64 ; N-z=3) 268,269 and Ce(OPi)4.iPrOH (6(Ce)-+0.75 ; N-z=4) 270 solvates can be

isolated as single crystals, while Ti(OEt)4.EtOH (6(Ti)-+0.63 ; N-z=2) can only be observed

in the solution at low temperature 2ZI No solvate has ever been characterized for Si(OEt)4

in ethanol (6(Si)=+0.32; N-z=0) 19

Hydrolysis/Condensation reactions : Alkoxy bridges appear to be more stable towards

hydrolysis than solvate bonds. Starting from a given alkoxide, different molecular

precursors can be obtained depending on the solvent used. Therefore, different hydrolysis

rates are expected which leads to completely different oxide materials. Precipitation

occurs when Zr(OPrn)4 is dissolved in n-propanol. This can however be avoided when

Zr(OPrn)4 is dissolved in a non-polar aprotic solvent such as cyclohexane, and leads to the

formation of polymeric gels and ZrO 2 monoliths 2Z2 As a result, hydrolysis and

condensation rates of Zr(OPrn)4 are much faster when the alkoxide is dissolved into

propanol than cyclohexane owing to the presence of solvate bonds in the former and alkoxy

bridges in the latter.

The same phenomenon was observed for titanium alkoxides. Monodispersed TiO 2 powders

have been synthesized by controlled hydrolysis of Ti(OEt) 4 in EtOH 240,241. Precipitation

also occurs when Ti(OPri)4 is dissolved in iPrOH but the monodispersity is lost 240-242

Ti(OEt)4 exhibits an oligomeric structure through ethoxy bridges while Ti(OPrl)4 remains

monomeric. Therefore, hydrolysis is much faster for this latter precursor than for the


former. Condensation, being a fast process in both cases, means that monodispersed TiO 2

powders can be obtained with Ti(OEt) 4 where hydrolysis rates are lower than condensation

rates. This is not possible with Ti(OPri)4 where hydrolysis and condensation rates are of

the same order of magnitude.

All these experiments show that the molecular structure of the precursor has to be

taken into account in order to describe its chemical reactivity. This was first

demonstrated by Bradley et al. 254 who carefully studied the hydrolysis of a large number

of alkoxides. They proposed different structural models to account for the hydrolysis

behavior of transition metal alkoxides (Ti, Zr, Nb, Ta, Ce). For each model the molecular

complexi1~y deduced from ebulliometric experiments can be related to the hydrolysis ratio h

assuming a sixfold coordinated metal atom. The structure of the molecular precursor is not

supposed to be modified upon hydrolysis, and condensation occurs between oligomerized

species. Four structural models names paqb have been proposed to account for experimental

results :

- The p3q4 model based on non solvated trimeric units Ti3(OR)12 (R=Et, Pr n, Bun).

- The p2q3 model based on solvated dimeric units M2(OR)8(ROH)2 (M=Zr, Ce, Ti).

- The plq3 model based on solvated monomers M(OR)4(ROH) 2 (M=Ti, Zr, Ce).

- And the plq2 model based on solvated monomers M(OR)5(ROH) of pentaalkoxides (M =

Ta, Nb).

The pioneer work of Bradley emphasizes the fact that both alcolation and solvate

formation can play a decisive role in hydrolysis/condensation reactions.

3.1.4. Hydrolysis ratio. The main external parameter is the hydrolysis ratio h which can be

defined as :

h [H20] (i0)

[M(OR) z ]

Bradley showed that for a given model paqb a mathematical relation can be established

between the average condensation degree n and the hydrolysis ratio 271 .

I/n = i/a - I/h (ii)

This relation shows that condensation could be adjusted by a careful control of the

hydrolysis ratio. However, quantitative predictions show some discrepencies with

experiments. For instance, the trimeric structure proposed for Ti(OEt) 4 in which titanium

atoms have a sixfold coordination 254 does not agree with XANES-EXAFS experiments 266,267

which suggest a fivefold coordination in Ti(OR) 4 (R = Et, Bun). This may explain why the

first hydrolysis product predicted from Bradley's model, Ti604(OEt)16, does not correspond

to the X..ray data on single crystals giving TiTO4(OEt)20 (figure 15) 247,274 Three main

domains could be considered in a rough qualitative analysis :

- h<l : In this domain condensation is mainly governed by alcolation and alcoxolation

reactions. The functionality of precursor towards alcoxolation is always smaller than one,

while for alcolation it could go up to z-i (i.e. three for a tetravalent metal). Under such

conditions, an infinite network is seldom obtained. Gelation or precipitation cannot occur

as long as hydrolysis remains carefully controlled (no local excess of water). Both

processes, alcolation or alcoxolation lead to molecular transition metal oxo-alkoxides

which can be isolated as single crystals from the solution (figure 16). Alcolation cannot

occur with silicon alkoxides owing to the fact that N-z=0. However, molecular compounds can

be for~ed through alcoxolation (dimers, trimers, tetramers .... ) which have been

characterized by 29Si NMR in solution 246

Oxo-alkoxides are the organic counterparts of polyanions and polycations which can

be obtained in aqueous solutions under careful control of the pH. Moreover, the structure

294

of these molecular clusters is

isostructural with Mo706 ~__ and

paratungstate Z [ H 2 W 1 2 0 4 2 1 1 0 "

J. Livageetal.

close to their inorganic analogs. Ti704(OEt)20 is

Nb8010(OEt)20 has the same structural units as

- ishs z : Charge calculations are reported in table 13, in order to describe the role of

the hydrolysis ratio. Ti(OPri)4 was chosen as an example because of its monomeric

structure. Table 13 gives the results obtained for h~0,1,2,3 or 4, in the transition states

Ti(OH)h(OR)4.h(OH2) and hydrolyzed species Ti(OH)h(OR)4. h :

Precursor h 6(OPr i ) 6(OH) 6(priOH) 6(H20) 6(Ti)

Ti (OPri)4 0 -0.15

Ti(OPri )4 (OH2) I -0.08 -0.38 +0.01 -0.28 +0.62

Ti (OPr i )30H 1 -0.08 -0.38 +0.02 -0.28 +0.62

Ti(OPri)3(OH)(OH 2 ) 2 -0.00 -0.36 +0.i0 -0.26 +0.64

Ti(OPri)2(OH)2 2 +0.04 -0.36 +0.15 -0.25 +0.64

Ti(OPri)2(OH)2(OH 2) 3 +0.13 -0.34 +0.25 -0.22 +0.65

Ti(OPri)(OH) 3 3 +0.28 -0.32 +0.41 -0.18 +0.67

Ti(OPri)(OH)3(OH2) 4 +0.38 -0.30 +0.52 -0.16 +0.68

Ti(OH) 4 4 -0.19 +0.01 +0.76

Table 13 : Influence of the hydrolysis ratio h upon the charge distribution in monomeric

precursors.

This table shows that the first steps of hydrolysis (h<2) can readily occur when

6(OR)<0 and 6(Ti)>0. As previously discussed, competition between oxolation and

alcoxolation may occur in this domain. Owing to the positive partial charge on ~PrOH,

alcoxolation should be favored thermodynamically . Under such conditions, chain polymers

can be obtained according to the following simplified scheme :

I I I IZ Ii n M ( O H ) ( O R ) 3 ~ . . . - 0 - R - O - R - 0 - R - 0 - O - 0 - . . . + n R O H

Such polymers were first obtained by Boyd and Winter with Ti(OBun)4 239,258. Under

similar conditions, spinnable sols were synthetized by Kamiya et al. 275,276, from which

SiO 2 or Tie 2 fibers could be drawn.

Upon further hydrolysis, the partial charge of the OR group becomes more and more

positive. This means that the prototropic transfer could become the rate limiting step. As

a consequence, hydrolysis may not go to completion even when h=4. This prediction is in

agreement with experimental data showing that the fourth alkoxy group is very difficult to

remove via hydrolysis or alcoxolation 239,241,258-260 Therefore, condensation via

oxolation becomes highly competitive when the full coordination is already satisfied (TMOS,

TEES). However, in the case of transition metal alkoxides, the alternative pathway,

elation, can occur preferentially because the required charge conditions (6(OH)<<0,

6(M)>>0 and N-z>>0) are fulfilled. The formation of elated polymers in this domain is

strongly supported by the fact that upon ageing, solvent is released via syneresis.

- h>z : Cross-linked polymers, particulate gels or precipitates can be obtained when an

excess of water is added to the alkoxide. It has been observed 263,277,278 that the

hydrolytic ratio strongly affects the mean size and weight of macromolecules which can be

formed. This observation seems to be general for Si, Ti, Zr alkoxides. Using an excess of

water, monodispersed powders based on Ti02, ZrO 2 and Ta205 have been obtained via

controlled precipitation of Ti(OEt)4 240, Zr(OPrn)4 279 and Ta(OEt) 5 280. As precipitation

is an extremely fast process in these experiments, it is highly probable that elation and

not oxolation is the predominent pathway for condensation.


3.1.5. Role of the catalyst. Another way to control hydrolysis and condensation processes

is to adjust the pH of the water used to perform hydrolysis. This can be done with an acid

such HCI or HNO3, or a base such as NH 3 or NaOH.

- Acid catalysis : Negatively charged OR groups can be easily protonated by H3 O+ ions :

M - OR + H30 + , M +--:O~ + H20

Under such conditions, the prototropic transfer and the departure of the leaving

group can no longer be the rate limiting steps. As a consequence, all OR groups can be

hydrolyzed as long as enough water is added. Hydrolysis rates can thus be greatly improved

by using an acid catalyst. This seems to be a general conclusion for all alkoxides

234,235,281,282. In the presence of H30 + , condensation occurs between these rapidly formed

hydrolyzed species M(OH)x(OR)z.x. Let us consider a typical polymer such as :

H O - - 0 - . . . - 0 - - 0 - . . . - 0 - - 0 - . . . - 0 - - O R A C

(charge calculations performed on different moieties of this polymer (A,B,C,D) are gathered

in table 14 :

SITE 6(OR) 6(Ti)

A -0.01 +0.70

B +0.22 +0.76

C +0.04 +0.71

D -0.08 +0.68

Table 14 : Charge distribution along a titanium oxo polymer.

It is easily seen that reactivity towards protonation decreases in the order :

D>>A>C>>B. OH groups are thus preferentially generated at the end of chains which leads to

rather linear polymers 283,284. The control of gelation rates is thus possible by using

acid catalysts together with substoichiometric hydrolysis ratio. Under such experimental

conditions spinnable sols 275,276 or monolithic gels 244,277 can be reproducibly obtained.

It must be pointed out that more acidic conditions (close to [H÷]=[Ti]), strongly inhibit

the condensation process. Protonation of the hydroxo group becomes possible, leading to

mixed aquo-hydroxo species such as those encountered with inorganic precursors.

The use of hyperacid catalysts such as trifluoromethanesulfonic acid (CF3SO3H) or

trifluoroacetic acid (CF3COOH) is also possible. However in this case,the reaction pathway

may be completely different and involves extremely reactive intermediates such as

sililenium ion (>Si ~) 285

- Base catalysis : Under acidic conditions hydrolysis and condensation can be uncorrelated

233. This is no longer the case with basic catalysts. Using NH 3 as a catalyst, it was shown

that hydrolysis of silicon alkoxides was activated 234,235 This could be due to a

nucleophilic activation of silicon through the coordination of the nitrogen lone pair.

Conversely, using NaOH as a catalyst, Bradley 281 showed that hydrolysis of Ti(OBuS)4 was

more difficult than under neutral or acidic conditions. In this case, nucleophilic addition

of OH" can occur which decreases the positive charge of the titanium atom.

Using NH 3 or NaOH, condensation is always activated through the formation of

highly nucleophilic species such as M - O" :

M - OH + :B - M - O" + BH + (B = OH', NH3)

This reactive condensation precursor will attack the more positively charged metal


atom. According to table 14, the order of reactivity will be B>>C A>D. Strongly cross-

linked polymers are expected to be formed in agreement with literature 283,284. Under such

conditions, depending upon the hydrolitic ratio, non-spinnable sols or particulate gels are

obtained. This is also the case if olation is a competitive pathway for condensation.

3.1.6. Other physical parameters. The hydrolysis ratio and nature of the catalysts are the

most important external parameters in sol-gel processing. However, other parameters such as

concentration, nature of the solvent, and temperature can also play a decisive role in

reactions pathways.

Dilution, for instance, could help to separate hydrolysis and condensation

processes when acid catalysts and high hydrolytic ratios are used. This has been shown for

TEOS by several authors using 29Si NMR 233,238,286,287

Another effect of dilution is to prevent growth through aggregation. According to

Yoldas, 263'277'2T8 the mean polymer size decreases as the precursor concentration increases

for Ti(OR)4 and Zr(OR)4 systems. This is intimately linked to the occurence of sol-gel

transition which is strongly affected by aggregation processes.

Solvent effects are much more subtle. Solvents having a high dielectric constant

(formamide, propylene carbonate and water in large excess) can induce different pathways

for hydrolysis and condensation reactions through the cleavage of the polar M - O - C

bonds. It is usually assumed that cleavage occurs at the M - 0 bond 288,289. However, this

may not be the case when tertiary alkoxides are hydrolyzed. In such conditions-highly

reactive intermediates such as carbocations may not be neglected.

Increasing the temperature generally activates both hydrolysis and condensation

processes. As a consequence, with poorly reactive precursors such as Si(OR)4 , the

temperature may be increased to activate the sol-gel transition. On the other hand, for

strongly reactive precursors such as transition metal alkoxides, the temperature must be

lowered in order to slow down hydrolysis and condensation processes as shown by Rehspringer

et al. 290 in BaTiO 3 processing.

3.2. Chemical modification of metal alkoxides

One of the main drawback or advantages of transition metal alkoxides is their high

reactivity with water. They must be handled with great care, in a dry box and precipitation

is usually observed rather than gelation. A survey of literature shows that chemical

additives are almost always used in order to improve the sol-gel process and obtain better

materials. Such additives can be solvents 257, acidic or basic catalysts 282 stabilizing

agents, 291,292 or drying control chemical additives 293,294. In most cases they are

nucleophilic XOH molecules that react with the alkoxide giving rise to a new molecular

20 precursor

M(OR) n + x XOH = M(OR)n.x(OX)x + xROH

The chemical reactivity of the alkoxide with nucleophilic species mainly depends

on the following :

- The electrophilic power of the metal atom increases when its electronegativity decreases.

- The ability of the metal atom to increase its coordination that can be estimated as the

difference (N-z) between its usual coordination number N in the oxide and its oxidation

state z. For a given group, (N-z) increases when going down the periodic table.

- The nucleophilic strength of the chemical modifiers.

Addition or substitution reactions lead to new molecular precursors which react

differently with respect to hydrolysis and condensation. The charge distribution among the


metal atom and its ligands is modified leading to enthalpy changes while entropy changes

occur when the coordination number increases. Both effects lead to a modification of the

nucleophilic reactions together with a differentiation of the ligand reactivity for

hydrolysis and condensation. It should be noted that the chemical reactivity and the

funetiennality of a M(OR)z.x(OX)x mixed alkoxide is not often simply deduced from the

behavior of the parent compounds M(OR)z and M(OX)z. Substitution reactions decrease the

functionnality while addition reactions leave it unchanged. Thus, substitution promotes a

decoupling between hydrolysis and condensation. Less electronegative ligands are first and

rather quickly removed upon hydrolysis while more electronegative ones (the modifiers)

should be mainly removed during condensation reactions. As a consequence, the growth of the

particles becomes more anisotropic which promotes the formation of polymeric gels.

Molecular modifications have a strong effect on parameters such as gelation time,

particle morphology, porosity, etc... The sol-gel transition in polymer chemistry is

usually given by equation (12) 295,296

t = [C0k(f2-2f)]'1 (12)

Three parameters can then be varied in order to optimize the sol-gel process, namely C O

(monomer concentration), k (bimoleeular condensation rate) and f (functionality which

depends on the hydrolysis rate). A good rule of thumb for the sol-gel chemist is reported

in table 15. It suggests that, depending on the relative hydrolysis and condensation rates,

different products can be obtained.

Hydrolysis Condensation Result rate rate

SLOW SLOW COLLOIDS/SOLS

FAST SLOW POLYMERIC GELS

FAST FAST COLLOIDAL GEL OR GELATINOUS PRECIPITATE

SLOW FAST CONTROLLED PRECIPITATION

Table 15 : Products obtained according to the relative rates of hydrolysis

condensation.

and

3.2.1. Alcohol interchange. Metal alkoxides react with a variety of alcohols to set up the

following equilibrium :

M(OR) z + x R'OH = M(OR)z.x(OR')x + x ROH

In general, the facility for interchange increases when the steric hindrance of the alkoxy

R group decreases ; OMe>OEt>OPri>OBu t " The facility for the interchange reaction also

depends strongly on the nature of the metal atom 19 More particularly, transition metal

alkoxides exhibit faster exchange rates than silicon alkoxides. This point can be

illustrated by NMR experiments performed on the metal probe. Recent 29Si NMR measurements

have clearly shown that exchange between silicon alkoxides and solvent molecules can take

place at room temperature 233,245 . The following reaction has been studied 233 :

Si(OEt) 4 + x HOPr i ) Si(OEt)4.x(HOPri)x + x EtOH

It has been shown, using 29Si NMR (figure 17a), that exchange between ethoxy group and

isopropanol molecule takes place under acidic catalysis on a time scale of about twenty

hours. Conversely, 51V NMR experiments performed in our laboratory have shown that the

following exchange reaction :

VO(OPri)3 + x (HOAm t) ~ VO(OPr~)3.× (OAmt) x + x HOPr ~

takes place instantaneously at room temperature (figure 17b) without a catalyst. Such

alcohelysis reactions are widely used for the synthesis of metal alkoxides. It is well


¢o

A " B ~

iJl . . . . . . ' l l i l _ _ . ,

PPml i i J J i J I , I , I , i 8 ( )

- 8 0 - 1 0 0 - 1 2 0 - 4 0 0 - 5 0 0 - 6 0 0 -700 - O'

Fig.17. (a) 29Si NMR spectrum of a solution of Si(OEt) 4

in propanol-2 with an acid catalyst after

20 hours.

(b) 51V NMR spectrum of a solution of VO(OAmt)3

in iPrOH with no acid catalyst after I0

minutes.

known that hydrolysis and conden-

sation rates depend on the nature

of the alkyl group. Therefore, it

should be possible to adjust the

rate of gelation of a given

alkoxide by using different

solvents 257

Similar experiments have been

performed in our laboratory with

titanium alkoxides. TiO 2 preci-

pitates are readily formed when a

stoichiometrie amount of water

(H20/Ti~2) is added to Ti(OPri)4

while stable colloidal solutions

are obtained with Ti(OAmt)4. On

the other hand, gelation occurs

within a few minutes when

Ti(OPri)4 is dissolved into AmtOH

prior to hydrolysis. Formation of

a mixed Ti(OPri)2(OAmt) 2 alkoxide

occur in which (OAm t ) groups

should be first hydrolyzed

according to partial charge

calculations 20

3.2.2. Metal chloride alkoxldes. Metal alkoxides are known to react with halogen or

hydrogen halides giving rise to halide alkoxides. Chloride alkoxides can also be very

easily obtained through the reaction of metal chlorides with alcohols 19

TiCI 4 + 3 EtOH , TiCI2(OEt)z.EtOH + 2 HCI

The reactivity of metal chlorides decreases with increasing electropositive character of

the metal, i.e. when going down the periodic table. The reaction of SiCI 4 with EtOH can be

pushed to completion with the formation of Si(OEt) 4 . Under the same conditions TiCI 4 and

ZrCI 4 undergo only partial substitution : TiCI2(OEt)2.EtOH 297 and ZrCI3(OEt).EtOH 298

while ThCI 4 forms only addition compounds i.e. alcoholates : ThCI4.4EtOH 19 Such chloride

alkoxides can be considered to be chemical modifications of the alkoxides. They are very

easy to synthesize and can be used as molecular precursors for the sol-gel processing of

transition metal oxides.

Niobium pentoxide gels are quite difficult to obtain from inorganic, NbCI 5 , or

metal organic, Nb(OEt)5, precursors. Both are highly reactive with water and precipitation

occurs rather than gelation. Niobium chloride alkoxides are readily formed when NbCI 5 is

dissolved into an alcohol 181 :

NbcI 5 + 3 ROH , NbCI2(OR)3 + 3 HCI

Solutions of these chloride alkoxides are quite stable. They can be stored in a

dry environment without any special care. Gels can be easily obtained through hydrolysis of

these solutions with an excess of water. The rate of gelation depends on the alcohol used.

It is much faster the longer the alkyl chain. Gelation occurs within a few seconds with

PriOH, a few hours with EtOH and several days with MeOH 181

Electrochromie WO 3 layers have been made from tungsten chloride alkoxides 20

Tungsten hexaehloride is dissolved in ethanol where upon a violent reaction the solution


turns blue. The chemical reaction can be written as follows 19 .

WCI 6 + 2 EtOH , WCI3(OEt)2 + 1/2 CI 2 + 2 HCI

Reduction of W(VI) to W(V) can be avoided by using WOCI 4 , instead of WCI 6 . Stable

solutions of oxychloride alkoxides are thus obtained. They can be kept for months and used

for making electrochromic layers by dip-coating 20

3.2.3. Acetic acid. Stable metal alkoxo-acylates can be formed when acetic acid is added to

an alkoxide 299 Acetic acid is often used as an acid catalyst in the sol-gel processing of

metal alkoxides M(OR)n : M = Si 275, AI 300, Ti 301,302 or Zr 20,263. Acid catalysis is

known to increase hydrolysis rates and acetic acid is currently used to decrease the

gelation time of Si(OR)4282. A reverse effect was actually observed with transition metal

alkoxides such as Ti(0R) 4 or Zr(OR) 4 . Precipitation readily occurs when pure water is added

to the alkoxide while homogeneous and transparent TiO 2 or ZrO 2 gels are obtained in the

presence of acetic acid 20,301 Gelation times then increase up to a few minutes or even

days. This can be attributed to the complexing ability of the acetate ligand.

An exothermic reaction takes place when acetic acid is added to Ti(OBun)4 , which

leads to a clear solution. X-ray absorption experiments on the Ti(OBun)4 precursor shows

that the coordination number of Ti increases from 5 to 6 upon acetic acid addition 266 13 C

and IH NMR of the modified precursor show that acetate groups are bonded to titanium while

infra-red spectra indicate that CH3CO0" behaves as a bidentate ligand (chelating and

bridging). A stoichiometric chemical reaction takes place for a one to one ratio which can

be written as follows :

Ti(OBun)4 + AcOH , Ti(OBun)3OAc + BuOH

Infra-red and NMR experiments show that (BunOH) groups are first removed upon hydrolysis

while chelating acetates remain bonded much longer to titanium and thereby slow down the

gelation process 301 Calculations based on the Partial Charge Model are in agreement with

these experiments. Titanium has a high positive charge (6=+0.61) in Ti(OBun)4 and its full

coordination is not satisfied. Therefore, nucleophilic addition of AcOH is possible giving

rise to the intermediate : Ti(OBun)4(AcOH). The charge distribution in this intermediate

shows that AcOH is negatively charged (6--0.7) while BuOH is positively charged (6=+0.1).

An alcohol molecule is then removed which leads to the substituted alkoxide

Ti(OBun)3(OAc). Hydrolysis of this new precursor begins via a nucleophilie addition of H20:

Ti(OBun)3(OAc) + H20 ........ ~ Ti(OR)3(OAc)(OH 2)

A charge distribution calculation shows that AcO remains negatively charged (6=-0.6) while

(Buno) is positively charged (6=+0.2). (OBu n) groups are then removed first upon hydrolysis

in agreement with NMR experiments 266 As acetate groups are not immediately removed

through hydrolysis or condensation, the functionality of Ti(OBun)3(OAc) is smaller than

that of Ti(OBun)4. The more (OAc) groups located around titanium, the smaller the

functionality will be and therefore the slower gelation occurs. In agreement with this

analysis, the gelation time strongly increases as the molar ratio HOAc/Ti approaches 2 301

3.2.4. Chelating ligands. - Acetylacetone is known to be a rather strong chelating ligand

and many metal ~-diketonates have already been reported in the literature 303 The enolic

form of ~-diketones contains a reactive hydroxyl group which reacts readily with metal

alkoxides 19,303 Therefore, acetylacetone has often been reported in the sol-gel

literature as a stabilizing agent for metal alkoxide precursors : W(OEt) 6 304, Zr(OPri)4

291,305, Ti(OPri)4 266, Ti(OBun)4 306 or Al(OBuS)3 292. Patents have even been obtained in

which acetylacetone is used to improve the process 307,308. Recently, TiO 2 colloids have

been stabilized up to high pH with acetylacetone 309 X-ray absorption experiments show


that Ti(OPri)4 is a four fold coordinated monomer. An exothermic reaction takes place when

acetylacetone is mixed to Ti(OPri)4 in a one to one ratio. IH and 13C NMR spectra, together

with infra-red experiments show that acac ligands are bonded to titanium. XANES suggests

that the coordination number increases up to 5 20 A stoichiometric reaction takes place

which can be written as follows :

Ti(OPri)4 + acaeH , Ti(OPri)3(acac) + priOH

Titanium coordination turns to 6 as soon as water is added to the new precursor. Ti...Ti

correlations become visible in the EXAFS spectrum showing that hydrolysis leads to

condensed species. NMR and I.R. spectra show that (OPr i ) groups are hydrolyzed first. All

acac ligands cannot be completely removed, even when a large excess of water is added.

Precipitation or gelation was not observed. Small colloidal particles about 5 nm in

diameter are obtained. These colloids are much smaller than those obtained without acac

modification (15 nm) which shows that this new ligand prevents condensation 20

- Hydrogen peroxide : Some papers 181,292,305 report the formation of gels in the presence

of H202. According to R. Roy et al. 310 the reaction of H202 with alkoxides results in

aerohydrogels that appear to have a fibrillar microstructure. Monolithic Nb202 gels can

also be easily obtained when H202 is added to NbCI 5 rather than H20 181. In both cases, the

resulting gels exhibit a yellow-orange color arising from the formation of peroxy species

311,312 Complex polymerization processes involving peroxy compounds are thus involved

during gelation. Peroxy ions 022" are known to be strong chelating ligands that are able to

react with the metal atom and increase its coordination 311 . Coordination numbers up to 7

have been found for peroxotitanium (IV) compounds 312. Let us suppose that peroxy species

can be formed as follows :

Ti(OEt)4 + H202 , Ti(OEt)202 + 2 EtOH

A charge distribution calculation shows that the peroxy group is negatively charged

(6(O2)~-0.89). It should therefore be strongly bound to titanium. A transition state

Ti(OEt)202.H20 could be formed when one water molecule is added to the new precursor. The

negatively charged peroxy group remains bound to Ti while the positively charged EtOH

molecule can be removed, giving rise to hydrolyzed species such as :

Ti(OEt)202 + H20 , Ti(OEt)202.H20 , Ti(OEt).O2(OH) + EtOH

O~" ligands increase the positive partial charge of the metal atom and the alkoxy

groups, making both the nueleophilic attack of water molecules and the departure of alkoxy

groups more facile. Moreover, these peroxo groups are not removed upon hydrolysis which

decreases the functionality of the alkoxide to a value close to 2. This could explain the

fibrillar microstructure observed by TEM 310

3.2.5. Organically modified gels. Organically modified silicates (ORMOSILS) have been

recently developed 314-316 In these compounds, non-hydrolyzable Si-C bonds are formed

which behave as network modifiers or network formers depending on the chemical reactivity

of the organic group 317 Such a modification cannot be extended to transition metal oxides

because the more ionic M-C bond would be destroyed upon hydrolysis. Organic modification

could however be performed using polyhydroxylated compounds such as polyols (glycerol,

polyethyleneglycol or polyethanolamine) or o-hydroxyacids (glycolic, salicylic or mandelic

acids). These species can react with metal alkoxides giving rise to mixed alkoxide

derivatives 306,318-320. These derivatives appear to be very stable because of chelate and

steric hindrance effects. Therefore, they are not removed during hydrolysis and

condensation leading to new, mixed organic-lnorganic materials. These compounds can be

calcinated in order to obtain a ceramic powder 290. They can also be used as such, like

ORMOSILS, and offer a wide range of new possibilities.


Electrolyte gels have been made via the reaction of a polyol (glycerol) and a

carboxylic acid (acetic acid) with a titanium alkoxide Ti(OBun)4 321. More or less viscous

gels are obtained upon hydrolysis in which both organic (Ti-OH2C-CHOH-CH20-Ti) and

inorganic (Ti-O-Ti) bridges are formed. They remain stable even upon heating at 80°C.

Layers deposited from these gels exhibit high proton conductivities at room temperature

(o=5.10 .4 Scm'1). Such gels have been used as electrolytes for making electrochromic

display devices 321

Reactions with maeromolecules such as cellulosic 322 or polysaccharides 323 lead

to other organically modified TiO 2 gels. In such compounds a good control of the cross

linking of hydroxy group is achieved by merely mixing the non-hydrolyzed alkoxide with the

polymeric material in different amounts. Application of these materials are manyfold :

production of high viscosity fluids for hydraulic fracturing 323

gelation of cellulosics or textile materials to make water repelant or flame retardant

fabrics 324,325

Very few papers concerning organic-inorganic copolymers involving transition

metals have been described 326,327. A good example is provided with TiO 2 gels organically

modified with vinyl acetylacetone. A Ti(OBun)4 alkoxide first reacts with the organic

modifier in a one to one ratio. A double polymerization process is then initiated via

partial hydrolysis of the alkoxy groups and radical polymerization of the vinyl functions

using azobisisobutyronitrile as a catalyst. A viscous product is obtained that can easily

be deposited onto a substrate giving photochromic coatings which turn blue upon U.V.

irradiation 20

4. ORDERED AGGREGATION AND INTERCALATION

All colloidal systems have an in-built tendency to become unstable and aggregate

spontaneously. This is a random process governed by Brownian motion, and as a result,

colloidal aggregates usually exhibit disordered open structures. The concept of fractal

geometry is often used to describe such aggregates 328. The fractal dimension can be

measured by Small Angle X-ray (SAXS) or Neutron Scattering (SANS). It is deduced from the

slope of the scattering curve in the Porod region 329,330. Models such as "cluster

aggregation" or "diffusion limited aggregation" are then computed in order to account for

the observed fractal dimensions. Most of the studies published in the literature deal with

silica gels. Transition metal oxide colloids however may exhibit a large variety of shapes

201 and two possibilities have to be considered for aggregation 24

i) if for any mutual orientation of the colloidal particles, the potential energy maximum

is less than kT, all collisions will be non-elastic and the multi-particle aggregate

ultimately formed will be completely disordered and isotropic.

ii) if for a particular orientation of the two colliding particles, the potential energy

maximum is less than kT, while it is in excess of kT for other orientations, an ordered

and, therefore anisotropic aggregate is bound to result. Such aggregates usually occur when

colloidal particles are strongly anisotropic (platelets, rods). They lead to sols or gels

that exhibit specific properties such as streaming birefringence, rheopexy or chemical

intercalation.

4.1. Anlsotropic Aggregates

4.1.1. Tactoid formation. Electrostatic repulsion between charged colloidal particles


usually prevents aggregation and flocculation. In some cases, these interactions can lead

to a long-range ordering in which charged colloids are placed along a periodic array as in

a crystal. Such systems, known as "colloidal crystals" 331, are observed with monodispersed

spherical colloidal particles such as latex or SiO 2 . The distance between particles is of

the same order as the optical wavelength giving rise to visible light scattering. These

colloidal crystals then appear iridescent. The natural opals are a well-known example of

such a long-range ordering.

More interesting is the case of colloidal particles that exhibit a strongly

anisotropic shape such as rods or platelets. Colloidal solutions of these non-spherical

particles may separate under suitable conditions into a concentrated, anisotropic phase and

a dilute, isotropic phase. Interactions between solid particles are quite strong in the

concentrated phase where colloidal particles are mutually oriented giving rise to the so-

called "tactoids". Colloidal particles are randomly dispersed into the isotropic dilute

phase called "atactosol" 24,332 Two main types of orientations have been observed :

Platelike colloids lead to sediments which have a periodicity along the axis

perpendicular to the layers. The tactoids which are formed by such oriented aggregates may

be called "smectic" tactoids. They are characterized by a brilliant luster giving rise to

the so-called "schiller layers". Tungstic acid or ~-FeOOH are typical examples of such

systems 333,334

Rodlike particles are arranged with their main axis parallel to each other. They may be

called "nematic" tactoids. The best known examples are V205 sols that give rise to

typically ellipsoidal tactoids 335 The special shape of these tactoids results from the

competition between two energy terms ; the interaction between rodlike particles that lead

to a cylinder and the interfacial energy between a viscous taetoid and the surrounding

dilute sol that lead to a spherical droplet.

The size of tactoids increases with the concentration of the sol. In the case of

V205, they can reach a length of about 250 ~m. Such tactoids are made of approximately 1013

colloidal particles. Below a critical concentration, tactoids are not formed and the

colloidal solution remains isotropic unless a shear stress is applied. Vanadium pentoxide

sols exhibit unusual properties such as thixotropy or streaming birefringence 336 arising

from the anisotropy of colloidal V205 particles. This can also explain the amazing

phenomenon of "rheopexy" ; on rolling a test tube, containing a liquified thixotropic V205

sol, between the palms of the hands, it is observed that gelation of the sol is accelerated

significantly 337. The mean distance between colloidal particles can be progressively

reduced by suitable changes of the dispersion medium. On careful addition of electrolytes,

V205 tactoids gradually shrink to a fraction of their original size, maintaining and even

enhancing their internal anisotropy. Ordered, reversible aggregates can then be changed

into ordered, irreversible aggregates giving rise to crystalloids in which all particles

are mutually oriented 332 Smectic tactoids of ~-FeOOH can,by drying slowly, be readily

changed into solid sheets in which individual layers are maintained, reminiscent of

structures like mica 338

4.1.2. Anisotropie layers deposited from ordered colloids. The spontaneous orientation of

anisotropic colloidal particles can be preserved and even enhanced, upon slow removal of

the solvent. Anisotropic coatings can therefore be obtained that exhibit specific

properties. One of the best known examples is undoubtely the V205 layers deposited from

gels that have been extensively studied during the last decade 339

V205 gels are made of entangled fibers (figure 8). Electron microscopy shows that

these fibers actually look like flat ribbons approximately I x i0 x 102 nm in size. X-ray


and electron diffraction experiments 172 show that these ribbons exhibit a two-dimensional

structure defined by a unit cell : a = 27.0 A and b = 3.6 A . This 2D structure is not

modified upon swelling and seems to be closely related to the layered structure of

orthorhombic V205 . Fibers are built of basic blocks containing I0 vanadium atoms along the

a direction. Some strongly bound water molecules, or OH groups, link these blocks together

giving rise to the corrugated structure of the ribbons 340 X-ray absorption experiments

show that vanadium is surrounded by five oxygen ate,ms with a short V=0 distance (1.58 A) as

in crystalline V205 . There is, however, no evidence for a long V-O bond between adjacent

layers 341

Under ambient conditions, the water content of V2Os.nH20 xerogels corresponds to

n=l.8. Thermal analysis 179,342 shows that water can be removed reversibly , upon heating

or under vacuum, down to a composition V205,0.5H20. Below this value further condensation

occurs and the thermal dehydration process becomes no longer reversible, leading to

crystalline V205 . According to infra-red and Raman studies the nature of water molecules

depends on the water stoichiometry 343,344. For high water content (n>l.8) water molecules

exchange hydrogen bonds with the oxide network while for low water contents (n=0.5) they

are directly bonded to vanadium atoms 345, in agreement with ESR and ENDOR experiments 178

001 j 003

, ,; S 10 15 20 25 30 ~ o

U

5

002 003

II II t,0ob I ]

10 115 20 25 30 ~ O

Fig.18. X-ray diffraction pattern of a V205.nH20

layer :

a) n = 1.8, basal spacing d = 11.5

b) n = 0.5, basal spacing d = 8.7 A

V205 layers deposited from gels

exhibit an anisotropic structure that

can be easily detected by X-ray

diffraction. Reflection geometry X-

ray diffraction patterns are typical

of a one-dimensional order corres-

ponding to the turbostratic stacking

of the ribbons one upon another along

a direction perpendicular to the

substrate 179 All diffraction peaks

can be indexed as 001 (figure 18).

The anisotropy of these coatings was

also clearly demonstrated by E.S.R.

178,infra_red 345 and polarized EXAFS

346 spectroscopies. The basal spacing

d, deduced from the position of the

001 peaks, increases with the amount

of water in the sample : d=8.7 A for

a xerogel dried under vacuum

(V205,0.5H20) and d=ll.5 A for a

xerogel dried under ambient condi-

tions (V205,1.8 H20 ). By comparison

with similar layered clay systems,

the 2.8 A increase of the d-spacing

was attributed to the reversible intercalation of one water molecule layer between the

V205 ribbons.

The swelling process of V2Os.nH20 xerogels at low water contents (n<20) was

followed by X-ray diffraction and Wide Angle Neutron Scattering 180. A stepwise swelling

process was first observed up to about n=6. The basal distance d increases by steps of 2.8

corresponding to the thickness of a single water layer. In this domain, interactions

between the oxide ribbons remain quite strong and the swelling process can be described as

the intercalation of water molecules into a layered host lattice. Beyond n=6, the basal


spacing d increases progressively and a continuous swelling seems more appropriate to

describe the water uptake process (figure 19). Interactions between particles become weaker

and their mean distance increases continuously with the amount of water added, as in usual

colloidal solutions. The composition V205.6H20 corresponds to a turning point between solid

state chemistry (n<6) and colloidal chemistry (n>6).

4 0

3 0

2 0

10

~d(A)

__1]_

_.~_1]._. WATER LAYERS

5 10 15 n i l20

Ln(I)

% k I'"' ++ % 7 x ' / i .HI.# n=192

+~. .i ~#tll, p, " ~. f [~,.,.~+~+ +%+, ,j~/~ I1~4 'd'~'~+ltllll#l n. 15 5 IE ~,. '+++ ~+l+,~++.+~°,. _

.% %1- n= Sl -1~- ~,~. ~.~

0.0 2.25 4.5 6.75 9.0 10"2Q[~-1)

Fig. 19. Variation of the basal distance d as

a function of the water content of a

V20 s.nH20 layer.

n f 1 2 0 / V 2 0 5 800 300 100 50 20 10 5 1

Lna(A) _ ' . . . . . . I ~(,~)

: ~ r S e n C ~ . T ". ~ 2510::

First "'- ~ 50 F ~ j _ T:ainr:~t IOn ~ ' " ~ " 3 0

2 I Regl/e I I I I lo -6 -5 -4 -3 -2 -1 0

Ln~

Fig,21. Swelling of V205.nH20 gels

as a function of the oxide

volume fraction.

Fig.20. Scattering curve of V205.nD20

gels as a function of the

amount of water.

Small Angle Neutron Scattering experiments

were also performed to follow the swelling 347 process to higher water content

Scattering curves for V 205.nD20 samples in

the concentration range 80<n<200 clearly

exhibit maxima in the angular dependence

(figure 20). The d-values corresponding to

these maxima are plotted in figure 21 as a

function of the volume fraction ¢ of V205 .

They can be described by : in(d) = kln(¢).

Assuming additivity of the volume

fractions of V20 s and water, the slope of

about -I observed in the concentrated

regime should correspond to a ID swelling

procedure of plate-like particles. In

fact, this concentration range can be

divided into two parts. The first part (regime I) has a slope of -0.9 for n<6 (or d<25 A).

The gel looks like an hydrated powder and the swelling procedure is characterized by an

increase of the basal spacing by steps of about 2.8 A. The second part has a slope of about

-I.i for 10<n<80 (or 50<d<250 ~). The gel is in a thixopic elastic state. A first

transition occurs in the range 25-50 ~, where the gel becomes an inelastic, pasty material.

The slope -0.60 observed in the more diluted regime II, where the gel turns from a

thixotropic liquid to a slightly viscous one, suggests a 2D swelling. The range between I00


and 200A corresponds to a second transition range, where the swelling process progressively

turns from ID to 2D.

Such behavior can be described using the following model. The thickness of the

V205-ribbons was estimated around 8.8 A by X-ray and neutron powder diffraction. The value

obtained from the extrapolation of the one dimensional swelling regime to the dry state is

approximately the same (7.4 A). In regime I, swelling is governed by the increase of the

interparticular distance perpendicular to the largest surface of the particles. When the

mean distance between ribbons reaches values comparable to the width of the particles, the

swelling becomes two dimensional• As a result, d-spacing that reflects the interparticular

distance increases more slowly with water content.

4.1.3. Magnetic ordering in 7-Fe203 colloids. Ferrimagnetic spinel iron oxide particles

about i0 nm in diameter can be prepared by increasing the pH of an aqueous mixture of Fe 2÷

and Fe 3+ salts. Stable sols are then obtained by peptizing the flocculate in an acidic or a

basic medium 113 Aggregation of the colloidal particles depends mainly on the sign and

the magnitude of surface charges 114 in relation to the acidity of the medium. Magnetic

dipolar interactions (=kT) are much smaller than electrostatic interactions (=lOkT) and are

not likely to contribute to the primary aggregation process. However, magnetic ordering has

been observed in colloidal aggregates that seem to behave as superferromagnets rather than

348 superparamagnets

Colloidal aggregates can be frozen in place by adding a water soluble polymer 349

or by surfaction and dispersion in a toluene-polymer mixture, in order to perform

correlative electron microscopy and M6ssbauer spectroscopy experiments. Figure 22 clearly

shows the effect of surface charges on the aggregation state : small clusters (n=5) are

formed in an acidic medium (figure 22A), while small chains (n=15) are observed at slightly

higher pH (figure 22B). Much longer strings (n=50) or large compact aggregates are found

around the point of zero charge (figure 22C).

o

A /'. .~; : '

• , - . % .

% • .. ,.%.

l-

• r . . . , < z.: ?~ .~, :

ij :j ;i ::: !: .: - ~. -

VELOCITY (ram/s)

Fig.22. Electron micrographs and M6ssbauer spectra (300K) of 7-Fe203 colloids in frozen-in

sols.


Small clusters give M6ssbauer spectra A or B depending on the aggregate concentration.

These spectra are typical of magnetically uniaxial particles which undergo

superparamagnetic relaxation. The change A~B corresponds to a weak evolution of the

particle magnetic coupling. Strings exhibit the same features (A~B) depending on their

size. Because of neighboring effects in the small clusters case and of branching and

coiling in the strings case, the change A~B is related to the average number of first

neighbors per particle. It occurs when one particle gets around 3 neighbors. Spectrum C is

typical of large, compact aggregates. It is nearly identical to the M6ssbauer spectrum of

bulk 7-Fe203 and was interpreted as enhanced interparticle interactions. Exchange coupling

between facing spins at the surface of adjacent particles likely becomes operative, leading

to superferromagnetic ordering 349. The magnetic moments of all elementary particles then

tend to be parallel to each other.

Large aggregates can also be obtained by adding an electrolyte to the aqueous sol.

Anionic charges can compensate positive surface charges in cationic sols. Anions and

hydration water molecules separate the iron oxide colloids. Magnetic interactions between

particles are then expected to decrease, especially those due to exchange coupling : super-

ferromagnetic ordering must vanish. In agreement, superferromagnetic ordering decreases

when the complexing ability of the anions increases : NO3"<CIO4"<SO42<HPO42 350

4.2. Intercalation properties of V205 ~els

Intercalation of guest species into host lattices with layered structures has

received ever increasing attention during the last decade. Intercalation is a reversible

process. The host matrix retains its basic structural integrity during the course of

forward and backward reactions while expansion of the lattice perpendicular to the layered

planes is observed. Host lattices must exhibit a strong 2D anisotropy, therefore very few

oxides are able to give rise to reversible intercalation. In the case of orthorhombic V205

for instance, intercalation seems to be restricted to small cations such as Li + 351 V205

actually behaves as a three-dimensional framework rather than a van der Waals host. Li+ions

are inserted into the channels of the orthorhombic network and the weak V-O bonds between

layers persist in LixV205 352

Vanadium pentoxide gels exhibit a layered structure in which the internal 2D

structure of the ribbons is closely related to that of the (a,c) planes of orthorhombic

V205 . Vanadium-oxygen bonds between the layers are however much weaker in the gel than in

the crystalline oxide. Therefore, reversible intercalation of guest species becomes

possible. This was already observed in the case of water intercalation leading to a

stepwise swelling process during the first hydration stages. V205 gels actually offer a

very versatile host structure for intercalation. Intercalation reactions involving

crystalline compounds are quite slow. They usually require heating under reflux for several

days. Gels are much more reactive species, full intercalation can be performed, at room

temperature, within a few hours or even minutes 25

4.2.1. Intercalation of metal cations. Ion exchange between protons of the hydrous

V205.nH20 xerogel and metal cations occurs as soon as the gel is dipped into an aqueous

solution of metal chlorides 353 The rate of ionic H+/M + exchange is controlled by

diffusion in the gel phase. It can be monitored by measuring the pH of the solution which

decreases when protons are released 354. Intercalation does not affect the monodimensional

order arising from the stacking of V205 ribbons. The basal spacing d increases, showing

that guest species are intercalated. The d increase however depends on the nature of the


intercalated cation. It varies with the charge/ionic radius ratio, related to the hydration

enthalpy U h of the cation. All data gathered are centered around two values : d=ll A and d

=13.6 A (figure 23) suggesting that the M + cation is intercalated with one or two water

layers 353 Intercalated species containing one water layer are obtained with cations

having a low U h value (mainly monovalent cations) whereas intercalation with two water

layers occurs with cations having a high U h value (mainly divalent cations). These

observations can be explained as resulting from the competition between two energy terms :

- The energy required to separate V205 layers increases with the basal spacing variation.

The energy required to remove water molecules from the solvation sphere of the cation

increases with the charge/radius ratio.

o d ( A )

Li + Ca2+C~2+Fe2+ Mg2+

~-27X

N ÷ Cs y R I ~ . H 4 = = I e / r 1 2 3 1=

Fig.23. Variation of the basal distance

d as a function of the charge/radius

ratio of cations intercalated into the

layered structure of V205 gels.

It is interesting to point out that V205

xerogels can be dissolved when dipped into pure

water. A non-limited swelling process is

observed, leading to a gel or a colloidal

solution. Such a phenomenon does not occur in

the aqueous solutions of metal cations. A

limited swelling process is observed which

stops after the intercalation of one or two

water layers. This is probably due to the

positive charge of the intercalated cations

that attracts the negatively charged V205

ribbons thus preventing further swelling by

additional water molecules. These results have

been recently extended to the intercalation of

alkali ions in the presence of non aqueous organic solvents 355 An increase of the basal

spacing d is observed which depends not only on the charge/ionic radius ratio, but also on

the nature of the solvent. Two solvation stages have been deduced from interlayer

distances. For water, they correspond to the intercalation of either one or two solvent

layers together with the metal cation.

Sodium intercalation has been recently used to synthesize vanadium bronzes at low

temperature 356 A V205,1.8 H20 xerogel is deposited onto a glass substrate, then dipped

into an aqueous solution of NaCI(IM). Intercalation occurs, leading to a Na0.33V205,1.8 H20

compound characterized by a basal spacing d=10.9 A. Water is then removed upon heating, and

crystallization of the monoclinic Na0.33V205 bronze occurs at 320°C instead of 700°C by

usual solid state reactions. The anisotropy of the layer is conserved even after

crystallization of the bronze so that the tunnels present in the structure remain parallel.

Ionic diffusion along these tunnels therefore becomes easier and such bronzes exhibit

remarkable properties as reversible cathodes in lithium batteries 356

4.2.2. Intercalation of molecular ions. Alkylammonium ions, CnH2n+1 N+ (CH3)3, with n

ranging from 1 to 18, have been intercalated into the layered structure of V205 gels. X-ray

diffraction patterns exhibit a serie of 001 peaks typical of the turbostratic stacking of

the V205 ribbons 357

Figure 24 reports the variation of the basal spacing d as a function of the number n of

carbon atoms in the alkyl chain. The orientation of the alkyl chain within the interlayer

volume can be easily deduced from this variation. The angle ~ between the chain and the

layer plane is given by 1.27sin ~=Ad/An. The 1.27 factor corresponds to the projection of a

C-C bond onto the main axis of the alkyl chain. Three domains are clearly seen:

Domain I (e=0) corresponds to short alkyl chains (n<6). Alkylammonium ions are

JPSSC 18:4-D


4(]

3C

20

10

dool (.~)

I ~90 °

.~- IT o I - " : / ~ ~ 5 3 •

7 - - j

, - /~- i:1 o . . . . .

I

i

I I I I I I i i I I "v 2 4 6 8 10 12 14 16 n a b

ooi

C

Fig.24. Variation of the basal distance d

of V205 .xH 20 gels as a function of n number

of carbon atoms of the alkyl chain of the

intercalated alkylammonium ions.

Fig.25. Position of alkylammonium ions

between the V205 layers as deduced from

figure 24 :(a) n<6, a=O °

(b) 6<n<12, 0°<=<90 ° ; (c) n>12, a=90 °

intercalated parallel to the layer planes in order to minimize the energy required to

separate the V205 layers (figure 25a).

- Domain III (==90 ° ) corresponds to long alkyl chains (n>12). Van der Waals interactions

between alkyl chains become predominant. Therefore, alkylammonium ions are aligned parallel

to one another in a direction perpendicular to the layer planes (figure 25c).

- Domain II (a=42 °) corresponds to an intermediate situation where both energy terms, layer

separation and Van der Waals interactions, are of the same order of magnitude. Alkyl chains

are still aligned parallel to one another, but they cannot stand perpendicular to the

layers (figure 25b).

Cobalticinium and ferricinium molecular ions have also been intercalated into V205

gels. In both cases, the basal distance increases up to about 13.2 ~ 358. The basic V205

ribbon structure is not modified upon intercalation. A noticeable improvement of the ID

stacking is even observed, especially with Co(C5H5); . The ~d=4.4 A increase of the basal

spacing suggests that cyclopentadienyl rings are perpendicular to the layers and somehow

inserted into the corrugated sheet structure of the ribbons.

4.2.3. Swelling in organic solvents. The first stages of the swelling process of V205 gels

in an aqueous solution can be described as the intercalation of one to several water

layers. The same process occurs when water is replaced by an organic solvent. The basal

spacing d between V205 ribbons increases by steps when the gel is dipped into a polar

organic solvent, while the internal 2D structure of the ribbons remains unchanged 179. Some

solvents (propylene carbonate) form a double-layer intercalate (d=21.5 A) while others

(DMSO) lead to a single layer compound (d=16.5 A). Some solvents however (DMF) do not

intercalate at all. The problem of reliable intercalation criteria is therefore opened.

This appears to be a rather difficult problem as many different interactions have to be

taken into account, namely solid-solid, solid-solvent and solvent-solvent interactions.

Neither dipole moments nor relative permittivities or even Gutman's donnor numbers allow

one to predict possible intercalation. The best parameter appears to be the Hildebrand

parameter 6 (square root of the cohesive energy density) which is related to the


vaporization energy and the molar volume of the solvent 359. No intercalation is usually

observed in V205 gels when 6 is smaller than 13 call/2cm "3/2.

In some cases however, a chemical reaction (proton or electron exchange) occurs

between the gel and the organic compound. A molecular ion is formed and intercalation

proceeds via an ion exchange process. Pyridine, benzidine and alkylamines for instance have

been intercalated into V205 .n H20 gels 360,361 Although some vanadium reduction occurs,

infra-red studies show that intercalation is mainly governed by a proton transfer, reaction

involving interlayer water molecules. This denotes the Bronsted acid character of these

gels. Protonation of organic bases leads to the formation of pyridinium, benzidinium or

alkylammonium ions. Intercalation of tetrathiofulvalene (TTF) has also been reported. A

black flocculate is obtained and the resulting material (TTFxV205 with x<l.8) is an ill-

organized, insoluble solid, that contains a large amount of water 362. The process appears

to be non reversible and can hardly be described as intercalation. A severe reduction of

vanadium ions by the organic molecules or solvent (ethanol) presumably occurs. The layered

structure of the gel is destroyed and the amount of water increases with the amount of

363 V(IV) ions as it was previously shown with other reducing reagents

5. PHYSICAL PROPERTIES AND APPLICATIONS OF TRANSITION METAL OXIDE GELS.

The sol-gel process is mainly used for making glasses, ceramics, films or fibers.

The gel state is then nothing more than an intermediate stage in the processing of these

materials. Drying and densification quickly follow the chemical synthesis of gels. Many

examples can be found in the literature in which transition metal oxide gels are used to

make ceramic powders 2,364 optical coatings 5 or fibers 4. Such applications will not be

described here. The discussion will rather be focused on the physical properties of gels or

xerogels before calcination in order to show that they can lead to new applications in the

field of materials science. Gels or xerogels are diphasic systems in which solvent

molecules (usually water) are trapped inside an oxide network. Such materials can be

considered to be water-oxide composites. Therefore they exhibit specific properties arising

from the intimate mixing of both phases. Transition metal ions often exhibit several

valence states giving rise to mixed valence compounds. Electronic properties due to a

hopping process within the solid phase can be observed. Water molecules are adsorbed at the

surface of the oxide particles. They can be more or less ionized, depending on the acidity

of the oxide, giving rise to H3 O÷ or OH" species. Ionic properties arising from ion

diffusion within the liquid phase can thus be expected. Both phases are involved in the

electrochemical properties of transition metal oxide gels. Electron diffusion occurs

through the solid phase and ion diffusion through the liquid phase. Because of the very

large interface between both phases, electron transfer at the oxide-water interface can be

greatly enhanced leading to specific photochemical properties.

5.1. Electronic properties

5.1.l. Small polaron hopping. A general condition for semieondueting behavior of transition

metal oxides is that the metal ions should be capable of existing in several valence

states, so that conduction can take place by electron transfer from low to high valence

states. A strong electron-phonon coupling is usually observed in transition metal oxides

leading to the formation of a so-called "small polaron" 365. The strong interaction between

the unpaired electron and the polar oxide network leads to a polarization of the lattice


and a displacement of the oxygen ions around the low valence transition metal ion. When

these distortions are limited to the nearest neighbors, the unpaired electron becomes

trapped in its own potential well 366. A "small-polaron" is formed, characterized by its

binding energy Wp which is usually about 0.5 eV for most transition metal oxides 367. Small

polaron hopping between two neighboring sites occurs when both sites have the same

potential energy. This is achieved by lattice distortions and phonons must be involved in

the hopping process. Conduction then has the character of a thermally activated process in

368 The hopping rate however which the activation energy W h should be given by Wh=I/2 Wp .

depends on two factors :

- A phonon term corresponding to the probability for both sites to have the same potential

energy.

- An electronic term corresponding to the probability for the electron to tunnel from one

site to the other during this coincidence.

A detailed analysis of small polaron diffusion is rather difficult and can be found in many

review papers 368. A general formula for electrical conductivity in transition metal oxides

was proposed by Austin and Mott 366.

e 2 W a = w -- c(l-c) exp(-2oR)exp(-~) (13)

RkT

where :

- u is a phonon frequency related to the Debye temperature 8 by hv-k0.

- R is the distance between transition metal ions.

c is the ratio of ion concentration in the low valence state reported to the total

concentration of transition metal ions.

a is the rate of the electronic wave function decay, exp(-2~R) corresponds to the

tunnelling transfer.

W is the thermal activation energy of the hopping process.

One of the most striking features of the small polaron conductivity is that the

thermal activation energy W decreases with the temperature.

At high temperature (T>8/2), the small polaron hopping is activated by an optical

multiphonon process. The activation energy is given by W =Wh+i/2Wd, where W d corresponds

to a disorder term in the case of non-crystalline oxides 369

- As the temperature is lowered the phonon spectrum freezes out and the polaron term W h

drops continuously to zero, leading to a decrease in the observed activation energy W

below 8/2. A detailed analysis of the electrical conductivity variation in this temperature

range was proposed by Schnakenberg 370

- At very low temperature (T<0/4) an acoustical phonon assisted hopping takes place and the

activation energy becomes W - 1/2 W d .

5.1.2. Semiconducting V205 xerogels. The semiconducting properties of V205 layers deposited

from gels have been extensively studied because of their potential application as

antistatic coatings in the photographic industry 371,372,373. Such xerogels, when dried

near room temperature, still contain some water and care must be taken in order to separate

electronic and ionic contributions to the electrical conductivity 3?4 Purely electronic

conductivity can be observed when the xerogel is under vacuum or in the presence of a dry

atmosphere. The water content then corresponds to V205,0.5 H20. The electrical behavior

appears to be purely ohmic, both a.c. and d.c. conductivities are identical and no

transient regime is observed when a d.c. voltage is applied across the sample 375. The

room-temperature conductivity depends on the amount of reduced vanadium ions. It increases

quite fast with the V 4+ concentration 376 Some discrepancies are observed in the


literature concerning the conductivity of V205 layers deposited from gels. Conductivities

as high as a=0.1 Scm "I at 300K have been reported 371. It seems that this value somehow

depends on the way electrodes are deposited onto the sample. All results however do agree

with the very low mobility of the charge carriers, values ranging between 10 .5 and 10 .6

em2V'Is "I are currently reported 373 The temperature dependence of the d.c. conductivity,

plotted as log(aT) versus (T "I) is shown in figure 26. The non-linear variation, together

with the low mobility of the charge carriers, is typical of a small polaron hopping

process. Theoretical models suggested by Mott

the experimental results 371,373

hn ~T.101

- . 4

'7 E . .~

- . 7

O . , a U

366 or Schnakenberg 370 fit quite well with

i i i i. , , L i. 0~ T(K "1) 3 . 2 3.6 4.0 4 4 4.8 5.2 5,6 6 0 10 /

,HI .... ..... , . . . . .

0

ii N

[ \\

\\

I \ \ \

V (volts) 10 V H 20 VTH

Fig.26. Temperature dependence of the

electrical conductivity of V205

layers deposited from gels.

Fig.27. Intensity-Potential characteristic

showing the switching effect in a

V205,1.8 H20 xerogel.

The small-polaron hopping process can be either thermally or optically activated.

According to the theory, the optical activation energy Wop t should roughly correspond to

Wopt=4 Wt h 369 Intervalence transfer then usually corresponds to a broad absorption around

leV, i.e. in the red part of the optical spectrum. Therefore, most mixed valence compounds

exhibit a typical blue color. An optical absorption study of V205 gels was performed by J.

Bullot et al. 377 They found an optical gap of 2.2eV close to the gap of crystalline V205.

An Urbach tail was observed on the low energy side, whose slope increases with the amount

of V 4+ . Moreover, the absorption due to the optically induced polaron hopping was detected

in the near infra-red region. The absorption band maximum (0.geV) is close to the energy

predicted from conductivity data. It depends on the V 4+ concentration and suffers a

redshift when the amount of V 4+ increases.

A threshold switching process was observed in V205 layers deposited from gels

378 Two gold electrodes 0.i mm apart were evaporated in a coplanar geometry at the surface

of the layer. The device is formed by applying a high voltage of about IOOV between the

electrodes. After a few cycles, the device starts switching. A typical I-V characteristic

is shown in figure 27. The threshold voltage is around 25 V, the minimum holding current

close to 500 #A and the on/off ratio 400. These values depend on the way the device is

made. On/off ratio as high as 800 were obtained but these results are hardly reproducible.

Optical microscopy shows that some filaments grow between the electrodes when the forming

voltage is applied. These filaments correspond presumably to the formation of VO 2 and the

switching effect should be due to the metal-insulator transition of VO 2 around 60°C. This

could explain why switching is no longer observed above this temperature .


5.2. Ionic properties

5.2.1. Particle hydrates. From a chemical stand point, transition metal oxides gels or

xerogels are hydrous oxides. They correspond to the general formula MOx.nH20 and can be

defined as "particle hydrates" according to the classification suggested by W.A. England et

al. 379. Following these authors, particle hydrates consist of charged particles separated

by an aqueous solution "S". The structure within a particle is that of the anhydrous oxide

and the particles are linked together to form agglomerates. The full coordination at the

surface of the particles is preserved by water molecules. Additional water molecules occupy

the inter-particle region to produce a connected, viscous liquid region through the

composite solid. A protonation equilibrium exists at the oxide-water interface. This makes

the liquid-like region acidic or basic depending on the nature of the oxide network. Acidic

dissociation is promoted by small metal atoms of high positive charge and basic

dissociation by large metal atoms of low charge. Particle hydrates exhibit some common

features such as good ion exchange properties or fast proton conduction 380. The liquid

content favours densifieation and particle hydrates can often be compressed into

transparent pellets by cold pressing.

5.2.2. Ion exchange. Inorganic ion-exchangers have been widely studied during the last

few decades. The rapid development in nuclear energy, hydrometallurgy, high-purity

materials, water purification, etc. has reinforced attempts to find new, highly selective

ion-exchanging materials, resistant to chemicals, temperature and radiation. Hydrous oxides

appear to be good candidates as ion-exchangers. They can compete with commercial organic

or natural inorganic (clays) products. Hydrous oxides of polyvalent metals behave as cation

or anion exchangers. Their dissociation can be represented schematically as follows 379 .

M-OH = M + + OH" (14)

M-OH = M-O" + H + (15)

Scheme (14) takes place in acid solutions where the hydrous oxide acts as an anionic ion

exchanger. Scheme (15) corresponds to a cationic ion exchanger in a basic medium.

Dissociation, near the isoelectric point of amphoteric oxides, such as ZrO2, TiO 2 or ThO 2

occurs in both ways which enables simultaneous development of both ion exchange processes.

The resultant charge on the particles may be switched reversibly from positive to negative

by changing pH. There is a characteristic pH value for any particular oxide at which the

overall charge on the surface is zero. This pH value is called the zero point of charge

(ZPC) and is readily determined by potentiometric titration. The shape of the pH titration

curve, however, depends on the preparation of the hydrous oxide.

The general formula for a tetravalent M(IV) hydrous oxide having a mean

oxygen/metal ratio R per particle can be written as follows 379 .

MOz.nH20 - [MOa(OH)b(H20)R. (a+b) ] 4 . ( 2 a + b ) + S

where the brackets correspond to the molar formula of the solid particle and S is the

aqueous region of the material. This region is basic, S = Sb, if (2a+b)<4 and acidic; S =

SA, if (2a+b)>4, where :

S A - (2a+b-4)H3 O+ + [(2+n-R)-(2a+b-4)]H20

S B - (4-2a-b)OH" + [(2+n-R)-(4-2a-b)]H20

The two equilibria may be represented by acidic and basic dissociation constants, K A and

~. Cation exchange corresponds to the replacement of the H3 O÷ ions in S by M ÷ ions coming

from an external solution S'. This equilibrium is characterized by a relative formation

constant ~. Cation exchange is then favored by a high pH, a large concentration of M + ions

or a large value of KAK N . For hydrous oxides, K M apparently increases with decreasing the


size of M ÷. Therefore, cation exchange of alkali metals decreases in the order

Cs+>Rb+>K+>Na+>Li+ "

The exchange properties of silica gels are already well known. Transition metal

oxide gels find applications in isolation, removal and treatment of radioactive materials

and purification of water. Among the main characteristics of these compounds are their

stability in strong radiation fields and retention of ion exchange properties above 100°C.

5.2.3 Fast proton conduction. The development of solid state proton conductors has received

stimulus from the practical side due to possible applications in low-temperature fuel-

cells, storage batteries and electrochromic devices. Hydrous oxides have been shown to be

rather good proton conductors and therefore proton diffusion has been extensively studied

during the last decade, mainly by a.c. conductivity measurements and 1H NMR.

The temperature dependence of the conductivity of hydrous oxides usually exhibits

a kink below 0°C, corresponding to the freezing of included water. Conductivity then

decreases faster and higher activation energies are observed. Above 60°C, some water

molecules leave the network and the conductivity drops. An analysis of the literature shows

that proton conductivity usually ranges between i0 "6 Scm "I and 10 .4 Scm "I with activation

energies around 0.30 eV. These values do not depend strongly on the nature of the oxide and

can be accounted for by proton diffusion through water molecules adsorbed at the surface of

small colloidal particles. Therefore, conductivity increases quite fast with water content

of the hydrous oxide MOx.nH20 , i.e. as a function of the water pressure above the sample.

Quite different values are found for hydroxides such as AI(OH)3.H20 (~=6.10 "8 Scm "~) or

framework hydrates such as antimonic acid Sb205,nH20 (a=7.5.10 "3 Scm "I) 381

A high proton conductivity necessitates a large concentration of mobile protons as

well as a high proton mobility. The first factor is optimised in highly acidic oxides which

contain high-valent cations. Proton motion in aqueous solutions usually occurs via the

classical drift of H3 O+ or via the tunnelling of a proton through an hydrogen bond. This

last process has a much higher probability in the case of solid hydrous oxides. However,

two conduction pathways are available. One is entirely within the interconnected liquid

phase between the particles, while the other is via the surface of the isolated particles.

It is not clear at this point whether proton diffusion occurs through the liquid or at the

surface. Nevertheless, good proton mobility should be expected in those hydrous oxides

which have a large water content and a high oxidation state.

Proton NMR relaxation times have been measured for several hydrous oxides 382,383

According to the authors, protons are found in three different environments :

i) at the surface as hydroxyl groups.

ii) in acid solution in micropores (diameter < i00 A).

iii) in acid solution in macropores (diameter > 103 A).

Pores result from the agglomeration of oxide particles. The solution in the

macropores should be almost liquid-like while that in the micropores will be more

constrained and viscous. Proton conduction involves chemical exchange between environments

of various viscosity. It must be pointed out however, that there is no simple link between

conductivity and NM_R data.

5.2.4 Mixed conduction in V205 gels. Hydrous oxides appear to be good proton conductors

379. When prepared from gels, they are easy to compress into pellets or to deposit as thin

layers. Therefore they would be very good candidates for solid-state ionic devices.

However, it is not clear whether the measured conductivity arises from the bulk or from

water adsorbed at the surface of the sample. Proton conductivity must then be studied as a


function of the water stoichiometry and related to the water adsorption isotherms 384

Figure 28 shows the dependence of the water content of a V205 layer deposited from gels as

a function of the water pressure above the sample. As shown by X-ray diffraction 180, the

water content increases by steps corresponding to the intercalation of one to several water

layers between the ribbon-like colloidal particles. For a relative humidity larger than 80%

a continuous swelling is observed that can lead, if enough water is added, to a colloidal

solution.

Electronic conductivity predominates at low water pressures. The water content of the

xerogel corresponds to VZ05, 0.5 H20 and the basal distance to d-8.7 A. This means that no

water remains intercalated between the ribbon-like vanadium oxide particles. The thermal

activation energy for conductivity decreases with the temperature. Such a behavior is

typical of small polaron hopping between V 4÷ and V 5+ ions in the oxide network 373

c / /

E

½ %

z

I I I J. 0 . 2 5 0.5 0 .75

relat ive I~umidity P/P$ (H20)

Fig.28. Water adsorption isotherm

of a V205.nil20 xerogel.

10

i_

0 . 2 5 0.5 0 . 7 5 I r

re lat ive h u m i d i t y P/Ps (H20)

Fig.29. Variation of the a.c. conductivity of a

V205,nH20 gel as a function of the relative

humidity of the surrounding atmosphere.

- Conductivity increases quite quickly with the water pressure above the sample (figure 29)

177,385. A high a.c. conductivity is observed in ambient conditions (o=10 .2 Scm "I at 300K).

The water content of the xerogel is V205,1.6 H20 and the basal distance d=ll.5 A

corresponds to the intercalation of one water layer. The log(aT) vs f(T "I ) curve shows two

Arrhenius behaviors with a kink around -10°C, typical of proton conductivity in particle

hydrates 177. A dielectric study of this xerogel in a broad frequency range (105-1010Hz)

suggests three different behaviors for the intercalated water 386 .

- A low frequency effect due to proton diffusion.

- Two dielectric relaxations due to water molecules which are strongly or weakly bound to

the ribbons.

- A dielectric relaxation which should be due to a fast rotation of H30 + ions.

It has to be pointed out that both curves in figure 28 and figure 29 are quite similar. A

plateau is observed around ambient conditions that corresponds to the intercalation of the

first water layer. This gives rise to the sigmoidal shape of the conductivity isotherm that

looks like a type II Brunauer isotherm. Such an isotherm is typical of a multilayer

adsorption process in which the first layer is much more strongly bound than the following

ones.

This study was extended to framework hydrates (HUO 2PO 4 ,nH20) and particle hydrates

(Ce(HPO4)2.nH20) . All these compounds have a bidimensional character as a common feature.

Conductivity occurs either in a layer or on a surface. The main difference then has to be

drawn between the bonding of water molecules responsible for proton conduction. The most

strongly bound water molecules give a well ordered lattice through which proton diffusion


will be solid-like. As soon as the water molecules are less tightly bound to the solid

network, they become disordered and give rise to a liquid like behavior 384

The example of V205 layers shows that the electrical conductivity of a gel, or

xerogel, cannot be fully described unless both the solid and the liquid phases are taken

into account. Mixed conduction occurs in V205 gels. Electron hopping is observed at low

water content while proton diffusion predominates as soon as the swelling process begins.

This could account for some discrepancies in the literature and explains one of the main

advantages of V205 antistatic coatings that keep their electrical properties under both dry

or humid atmospheres 387

5.3 Electrochemical properties

5.3.1 Electrochromic display devices. Electrochromie layers based on amorphous WO 3 thin

films have been extensively studied during the last decade 388 . Such films can exhibit two

stable states, one is transparent while the other one is blue. Reversible coloration and

bleaching can be easily obtained in an electrochemical cell. A double injection process is

observed that can be described as follows :

WO 3 + xe" + xM + = MxWO 3 (M + = H + , Li + )

Electrochromic WO 3 layers have been used to make display devices 389, rear-view mirrors 390

or smart windows 391

Amorphous WO 3 thin films are usually deposited by vacuum evaporation or

sputtering, however sol-gel derived eleetrochromic layers have also been made recently 392

Several techniques for the preparation of WO 3 films from solutions have been published

during the last few years. Amorphous WO 3 .nH20 can be formed upon hydrolysis of metal-

organic precursors such as tungsten hexaphenoxide 393 or tungsten ethoxide 302,394

Tungstic acid colloidal solutions have been obtained by ion exchange from an aqueous

solution of sodium tungstate 183 Peroxotungstic acid coated films were also investigated

for electrochromic applications 395 They are obtained by dissolution of a freshly

precipitated tungstic acid in an hydrogen peroxide solution. More recently, tungsten alkoxo

chlorides were obtained upon dissolution of WOCI 4 into an alcohol 20 Stable solutions are

obtained which can be easily deposited and hydrolyzed by dip-coating 20. Crystalline

WO3.nH20 (n=l,2) layers have recently been deposited from gels and colloidal solutions.

They appear to be strongly anisotropic as shown by X-ray diffraction and infrared dichroism

184 This lamellar structure of the film favors the intercalation of guest species.

W03.1~20 films chemically intercalate long-chain alkylammonium and electrochemically

intercalate Li + ions. They can therefore be used for making display devices.

Other transition metal oxides also exhibit electrochromic properties and can be

deposited via the sol-gel process. TiO 2 films have been made from Ti(OBun)4. They turn

from white to blue reversibly. V205 films deposited from a polyvanadic acid sol turn from

yellow to green upon an applied voltage of ± 1.5V. They have a memory effect of more than

20 hours 396

The sol-gel technique offers many advantages for making electrochromic devices :

Thin layers can be easily deposited under ambient conditions by dip-coating, spin-coating

or spraying. Large surfaces can be coated at low cost 5

According to the literature, eleetrochromic characteristics of WO 3 films are very

sensitive to the method of preparation It has been reported that sputtered films are

easier to color and bleach than evaporated ones. This is probably due to the smaller water

content of the latter. Water has to be incorporated into WO 3 films in order to obtain

faster coloration 397 Sol-gel deposited films always contain some water making


electrochemical ion diffusion easier. Moreover, evaporated amorphous WO 3 films always

adsorb water when placed in an ambient atmosphere. It has been shown that such hydrated

layers could be described as xerogels of hydrated tungsten hydroxy-oxides. The high

electrochromic reversibility and the short response time of these layers was attributed to

398 their porous, spongy structure

Multi-layer all-gel devices have recently been made in which all active layers

(electrochromic and electrolyte) are deposited from gels. Such cells exhibit a rather long

response time, a good cycling behavior, and a very long memory 321. They open the way for

new micro-ionic devices.

5.3.2 Reversible cathodes for lithium batteries. Lithium batteries, based on the

reversible insertion of Li + ions into a host lattice have been extensively studied 399

Most work was focused on transition metal chalcogenides that exhibit layered structures.

Vanadium oxides (V205, V6012... ) also appear to be good candidates for such applications.

They offer high stoichiometric energy densities : values up to 600 Wh/kg have been reported

for V205 352 However, as already mentioned, this oxide behaves as a 3D framework rather

than a Van der Waals host 352. It is hoped that better reversibility can be achieved with

amorphous oxides for which structural changes should be limited. Therefore vanadate glasses

have been suggested as reversible cathodes for lithium batteries 400,401. Amorphous V205

made by splat-cooling also exhibits interesting properties as a reversible cathode 402

Electrochemical experiments performed with LiAsF 6 in a cyclic ether as an electrolyte show

that up to 1.8 Li + ions per V205 can be inserted reversibly between 3.5 V and 2 V (vs.

Li+/Li). Contrary to crystalline V205, the open circuit voltage continuously decreases with

the amount of inserted Li + , suggesting that no phase transition occurs.

Reversible electrochemical intercalation of Li + ions into V2Os.I.6H20 xerogels was

reported a few years ago 403 Electrochemical experiments were made using a triple

electrode device and a LiCiO4-propylene carbonate solution as an electrolyte. Li + ions are

intercalated when a negative voltage is applied to the V205 electrode. Two LixV2Oscompounds

are formed upon reduction corresponding to x-l.l and x-l.6. The process appears to be

reversible and Li + ions are removed upon electrochemical oxidation. An X-ray study of the

layer shows that the well-ordered stacking of the V205 ribbons is destroyed upon insertion,

giving rise to a disordered material. However, the I-D order is restored during the

oxidation cycle. Another electrochemical study was published recently, using a V205 xerogel

as the cathode and metallic lithium as the negative electrode 404. This xerogel was

partially dehydrated at 230°C. The remaining water appears to be strongly bound, as no

adverse effect on the lithium counter electrode was detected. A polymeric electrolyte was

used in order to avoid swelling by the solvent. The discharge curve appears to be nearly

linear between 3.5-2.2V (vs Li), with none of the inflections and plateaux characteristic

of crystalline V205 . The average cycling efficiency during the first 46 cycles is 99.7%.

However this good performance is accompanied by a modest energy density : 420 Wh/kg for

insertion of i.i Li/V205 . Gels are quite suitable for making layers and should be quite

useful in the processing of thin film micro-batteries.

5.4. Interfaclal properties

5.4.l.Photochemistry of colloidal semiconductors. When a semiconductor is brought into

contact with an aqueous solution, an electron transfer occurs at the oxide-water interface

until the electrochemical potentials of both phases (Fermi level and mean redox potential)

become equal. As a result of this transfer, the oxide surface becomes charged with respect


to the solution. This charge is actually distributed over a region the thickness of which

depends on the doping level of the semiconductor. For a weakly doped oxide, the space

charge layer is typically more than i000 A. The electric field created by the electron

transfer can be described by the typical band bending model of the semiconductor-

electrolyte junction. Electron-hole pairs can be created when photons of energy larger than

the band gap (h~>Eg) are adsorbed at the surface of the oxide. The electrical field of the

junction provides electron-hole separation within the space charge region. For n-type

semiconductors, holes move toward the surface while electrons move toward the bulk. The

opposite is observed for p-type semiconductors. Redox reactions are thus expected at the

oxide-water interface 405,406

The photochemical properties of oxide suspensions have already been widely

studied. However, such studies are best carried out using colloidal particles with a

diameter smaller than i0 rim. Such solutions are optically transparent and offer a larger

oxide-water interface 406 Moreover, the size of colloidal particles may be smaller than

the space charge thickness so that the semiconductor-electrolyte junction model cannot be

applied. Charge separation at the junction is no longer effective and charge carrier

mobility should be described by usual diffusion theory 407. When the particles are small,

both charge carriers can reach the surface. Light-induced charge separation and redox

reactions can be coupled without intervention of bulk diffusion. Thus a single colloidal

semiconductor particle can be treated with appropriate catalysts so that different regions

of the same particle function either as anodes or cathodes 405

The photochemical properties of transition metal oxide colloids Fe203 408 WO 3

409,410 or MnO 2 411 have been widely studied, although most of the work has been performed

on TiO 2 in an aqueous or organic medium. This oxide can lead to a large variety of

photochemical reactions such as water splitting 412, photocatalysis 413 or photodegradation

of pollutants 414. Transparent TiO 2 sols are usually produced via hydrolysis of TiCI 4 in

water or hydrolysis of Ti(OPri)4 in acid aqueous solutions 415 Particles a few i00 A in

diameter are obtained which exhibit good photochemical characteristics. However these sols

only absorb U.V. light and are not stable above pH 3. Chemical modification of the alkoxide

with a strong chelating ligand such as acetylacetone was reported to give transparent sols

that remain stable up to pH i0. Moreover, charge transfer from the (acac) ligand to the Ti

atom gives rise to a strong absorption of visible light that improves the photochemical

efficiency of TiO 2 sols. These modified colloids are stronger reducing agents than other

TiO 2 colloids 416

A photoelectrochemical cell consists of two electrodes immersed into an aqueous

electrolyte and connected electrically by a wire. The main electrode is a semiconductor

with one face in contact with the electrolyte and the other face connected to the shorting

wire by an ohmic contact. The majority carriers move toward the bulk of the electrode where

they are collected by the wire and transferred to the counter electrode, usually a metal

that does not react chemically with the electrolyte 41T. In such a cell, the potential

corresponding to zero excess charge in the semiconductor (i.e. the point of zero charge) is

called the flat band potential Vfb. This is a very important parameter that gives an

estimate of the reducing power of the electrons generated upon illumination of the n-type

semiconductor 407 that behaves as a photoanode. Very few papers actually report the sol-gel

processing of photoelectrodes despite the fact that this process can offer many advantages:

- Oxide layers of large area can be easily deposited onto a metallic substrate and sintered

at relatively low temperature.

- Metastable crystalline phases can be obtained such as anatase in the case of TiO 2 instead

of rutile 418


- Oxides such as TiO 2 actually have a wide band gap (Eg=3.2eV). Therefore they are of

little interest for photoelectrolysis unless means are found to enhance both their

electrical conductivity and light response into the visible region. This can be obtained by

doping with impurities such as Cr 3+ or AI 3+ . Doping can be made at a molecular level by

mixing the appropriate solutions of molecular precursors which yields highly homogeneously

doped photoanodes 419,420

- Photoelectrochemical and photochemical 421,422 experiments permit such measurements as

flat band potential values that can give useful information to characterize the water oxide

(colloid or gel) interface. Moreover, the d.C. photocurrent variation as a function of the

incident wave length gives an access to the band profile of gels or xerogels.

5.4.2. Electron transfer at the Fe304 colloids interface. Interfacial properties of metal

oxides do not usually take into account the bulk of the solid network. Acid peptization of

metal oxides is usually described as arising from the protonation of surface M-OH groups

leading to positively charged particles. Such a description may become no longer valid when

metal ions exhibit several valence states. Electron hopping occurs in mixed valence oxides

and the whole oxide network may be involved in surface charge modifications. Such a process

can become especially important for colloidal particles having a large surface/volume

ratio. As an example, redox reactions of Fe304 colloids have been shown to involve the

whole spinel lattice and not only the surface of the particle. Such behavior results from

electron delocalization in mixed valence Fe304 together with close structural similarity

between the reduced (Fe304) and oxidized (7-Fe203) forms.

At room temperature Fe304 exhibits a cubic inverse spinel structure. Octahedral

sites are equally occupied by Fe 2+ and Fe 3+ ions while tetrahedral sites are occupied by

Fe 3+ ions only. It is a mixed valence compound where electron hopping leads to

delocalization over the octahedral sites. As a consequence, octahedrally coordinated iron

ions exhibit an average charge of +2.5. The molecular formula may thus be written as

Fe3+[Fe~'5+]O4 where brackets label octahedral sites. 7-Fe203 is also a spinel oxide with

almost the same lattice parameter, but it contains no Fe 2+ ions and octahedral sites are

iron deficient. It can be described as Fe3+[Fe~3DI/3]04 where D corresponds to iron

vacancies.

It has been shown that oxidation of Fe304 into 7-Fe203 can occur under anaerobic

conditions. One of the possible pathways for the process is outlined in figure 30 for Fe304

colloids in a weakly acidic medium (pH=2).In this case all Fe 2÷ ions are released from the

spinel framework while protons are consumed 118. Fe2+ desorption proceeds without

significant structural changes and the overall stoichiometry is found to be 2 H + consumed

per Fe 2+ ion released in solution. Surface protonation of Fe-OH hydroxyl groups appears to

be the driving force of the process. The potential energy at protonated octahedral sites is

lowered leading to electron localization and formation of -O-Fe-OH~ species (figure 30b).

As O-Fe 2+ bonds are weakened, hydrolysis occurs, leading to soluble [Fe(OH2)6] ++ with

formation of iron vacancies and bare oxygen at the surface (figure 30c). Simultaneously,

electron localization at the surface leaves unpaired Fe 3÷ ions in the core of the particle

which are acceptor states for mobile electrons. As Fe 2+ ions are released in solution, an

electron flux occurs towards the surface leaving a positively charged core. Charge

neutrality is locally maintained by outward migration of iron ions towards the surface

leaving vacancies within the lattice (figure 30d). The surface is thus progressively

renewed: new superficial iron ions are coordinated by water molecules while bared oxygen

adsorb protons from the solution (figure 30c). Peptization occurs as soon as

Fe(II)/Fe(III)s0.15, leading to a stable cationic sol while at the end of the process the


colloid is converted into ?-Fe203 . It must be pointed out that this transformation is

partially reversible. Once Fe 2÷ have been removed from Fe304 colloids, they can be

readsorbed just by raising the pH up to 5 or 6, but the particle does not transform into

Fe304 anymore. After adsorption of Fe 2÷ at the surface electron transfers towards

octahedral Fe 3+ and delocalization over the v-Fez03 lattice occurs but no migration of iron

ions inside the particle is observed. This process leads to an epitaxial growth of a Fe304

layer and adsorption stops as soon as all octahedral sites within the core have an average

charge of +2.5. As no significant iron diffusion occurs, it is supposed that simultaneous

proton diffusion towards the core is involved in order to maintain charge balance within

the lattice 116

( ~ ( ~ 3 ~ F e 2 5 *

H H H H H H IH H H Y H oFe 3÷ ...... ; 6 d d 6 6 ....... E ...... 6 H ~ H 6 6H"E)'H 6 ' ' ' ~ "Fe2*

0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 O ~ O u O ~ O ¢ O P O ~ O uFe vacancy 0 ¢ 0 ~ 0 ¢ 0 ~ 0 e 0 0 / ( 0 ¢ 0 ~ 0 a~"O @ 0 (~[Fe (OH2)612÷

OeOeO~OeO®OeO OoOeOeOo/OeOeO

= H ;H H I H H IH H: H H H H H H H ' - ' , i , , D ..... 6 6 6 6 ' = ........ 6 6,6 6 6,6--

o® o /O .6 . , 6 .o .6 .6 .oTo} oAo o o I ooo, ,o, ,ooo, ,o •

Od 0 ~ 0 cO-'6 O ~0 • 0 ODO®O ° O n O ~ O ~ O

hopping I electrons

Fig.30. Schematic process of the transformation Fe304 ~ 7-Fe2O 3 in weakly acidic medium.

(A) Configuration of the octahedral sublattice in Fe304.

(B) Protonation and electron localization at surface sites.

(C) Desorption of Fe 2÷ and migration of iron towards the surface.

(D) Fe 2÷ content has decreased and vacancies have appeared.

This transformation Fe304 ----+ 7-Fe203 is also observed under various conditions

(aerobic oxidation, Fe 3+ adsorption, etc...) 116"118. Analysis of this reaction reveals the

same electronic process: electron transfer through the interface relayed by electron

transfer within the particle. The intrinsic structural transformation is likely to be the

same but adsorption phenomena that induce electron transfer at the interface and outward

conditions that rule the behavior of the superficial Fe 2÷ may be quite different. The

fundamental role of electron delocalization in surface phenomena is nicely corroborated by

the fact that replacing Fe 2+ by another divalent cation such as Co 2÷ prevents electron

delocalization. In agreement with this result, surface hydrolysis of Fe2CoO 4 is strongly

inhibited in weak acid medium, and Co 2÷ adsorption onto 7-Fe203 is also very limited. This

explains the outstanding behavior of spinel iron oxide colloids which may be used as

colloidal electron exchanger in aqueous solutions.

6. MONOGRAPH

The present monograph provides a brief review of the published literature on

transition metal oxide sols and gels. The main elements are classified according to their

atomic number. Multicomponent systems are discussed separately at the end of the monograph.


This review mainly points out the nature of the precursors, the experimental procedure and

the main applications of the resulting materials.

6.1. Transition metal oxide gels

6.1.1. Titanium oxide. TiO 2 gels have been known for a long time. They can be made by

dissolving sodium titanate in concentrated hydrochloric acid, then adding a weak base such

as ~C03, (NH4)2CO 3 or Na2CO 3 in order to avoid high pH gradients 124,125,126 TiO 2 sols

can be easily obtained through thermohydrolysis of TiCI 4 or TiO(N03) 2 under acidic

conditions 60,423 The colloidal particles are crystalline and have anatase or rutile

structure depending on the pH and the nature of the counter-ions 60,423 Some authors have

studied the parameters which influence gel formation while others have focused their

attention on processing in order to obtain fibers, coatings or monodispersed powders.

Sol and gel formation : Most recent studies are devoted to metal-organic routes using

Ti(OR) 4 alkoxides precursors. Monolithic TiO 2 gels can be synthesized from Ti(OR) 4 (R= Et,

Bu n , Pr I , Pr n , Bu s ) using substoichiometric hydrolysis ratios (l<h<4) and inorganic acid

catalysts (HCI, HNO3) 244,277. TiO2.based gels or colloids can also be obtained after a

chemical modification of titanium alkoxides 20 . This modification is performed mainly with

acetic acid 266,309,420, acetylacetone 266,309 or hydrogen peroxide 310. A good review of

gel synthesis using inorganic precursors was published by Woodhead 424

- TiO 2 fibers : TiO 2 sols or gels allow fibers to be drawn when viscosity is carefuly

controlled. Spinnable sols can be made through acid hydrolysis (HCI) of Ti(OPri)4 in

ethanol using substoichiometric hydrolysis ratio 276 Chemical modification with

acetylacetone offers an alternative route. The modified precursor Ti(OPri)2(acac)2 is

hydrolyzed in ethanol in the presence of acidic (HCI) or basic (NH4OH) catalysts leading to

sols or transparent monoliths 425. Fibers can also be obtained by unidirectional freezing

of a gel made through partial neutralization of TiCI 4 with KOH followed by dialysis 426

- TiO 2 coatings : Coatings on various metals have been made by dip coating in various

inorganic sols. Dispersable sols can be obtained using phase-transfer or extraction

techniques 427. Membranes with controlled porosity for ultrafiltration have been obtained

through hydrolysis of Ti(OPri)4 or Ti(OBun)4 and peptization by HCI or HNO 3 in the presence

of cellulose 428,429. Porosity appears to depend mainly on the firing temperature.

- Powders for ceramics : Monodispersed submicronic TiO 2 powders can be obtained by using

either inorganic or organic precursors. The inorganic route involves thermohydrolysis of

TiOS04 430,431 or TiCI 4 in the presence of Na2SO 4 121 In both cases monodispersed spheres

about 0.4 ~m in diameter are obtained. With alkoxides precursors, two main routes are

available :

i) Controlled precipitation of Ti(OEt) 4 in EtOH with an excess of water 240,241,432,433

These powders can easily be doped with Nb(OEt) 5 and Ta(OEt) 5 434 Hydroxypropyl cellulose

can be used in order to improve the monodispersion 435

ii) Hydrolysis of Ti(OEt) 4 or Ti(OPri)4 aerosols leads to monodispersed spheres whose

diameter can be varied from 0.06 to 0.6 ~m 122,243,436

Other TiO2-based powders for ceramics can be obtained either by precipitation of

Ti(OPri)4 in iprOH 242,437,438 and Ti(OBui)4 in iPrOH 436 or by spray techniques using

Ti(OR)4/ROH mixtures (R - Et, Pr i , Bu n ) 261. This leads to dense TiO 2 ceramics when heated

around 900°C. TiO2-based anionic exchangers have been synthesized through acid hydrolysis

of Ti(OR) 4 439 Spherical TiO 2 powders with diameters in the range of 1-2000 ~m can also be

obtained from gels made from inorganic 440,441 or organic precursors 441


6.1.2. Vanadium pentoxide. V205 gels have been known for a long time and can be

synthesised by different routes.

- Acidification of sodium or ammonium metavanadate solutions by hydrochloric or nitric acid

followed by washing or dialysis 174

- Acidification of sodium or ammonium metavanadate solutions with a proton exchange resin 174-176

Dissolution of amorphous V205 prepared by splat cooling into water 442-444

Pouring the molten V205 oxide directly into water 445

- Hydrolysis of vanadium oxoalkoxides VO(OR)3 (R = Et, Pr i , Pr n , Bu n , Am t) in the presence

of excess water 446,447

The structure and properties of V205 gels have been reviewed recently 339,448

6.1.3. Chromium oxide. Monodispersed sols of hydrous chromic oxide have been synthetised

by thermohydrolysis of various Cr(III) salts (CrCI3, Cr(NO3)3, KCr(S04)2) in the presence

of sulphate or phosphate ions 78

Monolithic green or blue-grey hydrous chromic oxide gels are easily formed when

Cr(IIl) salts (CrCI 3, Cr(N03) 3, Cr2(SO4)3, Cr(OOCCH3)3) are treated by an aqueous basic

solution of NH40H or KOH with an excess of acetate ions 70,71,74,76 The structure of these

gels was studied by EXAFS, Infra-Red spectroscopy, TEM and magnetic measurements 74,76

Catalysis is one of the main applications of these gels 449

Hydrosols can also be obtained via hydrolysis and condensation of CrCI3.4EtOH ,

CrCI3.3EtOH, Cr(OEt)3.EtOH and Cr(OEt)3 in ethanol 450

6.1.4. Manganese oxides. Pure hydrosols of Mn(OH)2 can be obtained via hydrolysis and

condensation of Mn(OEt)2 in ethanol 450

Hydrous MnO 2 sols and colloids are readily obtained through the reduction of KMnO 4

with reducing agents such as As(OH)3130 , Na2S204131, Mn2+ 132,411,451, NH~ 133 or glucose

134. Transparent sols of manganese (IV) oxides and manganese III oxides can also be

prepared by 7-irradiation of KMnO 4 solutions 411 No structural studies have been performed

on these sols.

6.1.5. Hydrous ferric oxide. Monodispersed hydrous ferric oxide sols can be obtained

through controlled thermolysis of Fe(III) salts (chloride, nitrate, sulphate and

perchlorate) 108 Chlorides first lead to monodispersed acicular ~-FeOOH which, upon

further aging, give rise to monodispersed ~-Fe203 spheres. Monodispersed ellipsoidal ~-

Fe203 particles are directly formed with nitrates or perchlorates 108 If FeCI 3 is aged in

a water/ethanol mixture, fl-FeOOH is more rapidly formed and leads to monodispersed cubic ~-

Fe203 particles, 109 while with triethanolamine monodispersed ~-Fe203 discs are obtained

110. In the presence of a reducing agent such as hydrogen peroxide or hydrazine, the

Fe2+/Fe 3+ ratio can be adjusted until the formation of monodispersed Fe304 sols 110 occurs.

Finally, if Fe2(S04) 3 is used as a Fe 3+ precursor, monodispersed basic salts are obtained :

Fe3(SO4)2(OH)5.2H20 and Fe4(SO4)(OH)I 0 105. The formation of these hydrous iron oxide sols

and the aggregation process that occurs upon aging have been extensively studied 90-97 7-

Fe20 B sols can be obtained through peptization and oxidation of precipitates of hydrous

Fe304 using weakly polarizing bases such as tetramethyl ammonium hydroxide or a strong acid

such as perchloric acid 113,114 Pure hydrosols of hydrous ferric oxide can also be

obtained through hydrolysis and condensation of Fe(OEt)3 in ethanol 450

Gelatinous precipitates of hydrous ferric oxide can be obtained using a large

variety of techniques 107: dialysis, hydrolysis of inorganic precursors through dilution,


ionic exchange, phase transfer extraction with long chain organic amines, neutralization

with a weak base such as NaHCO 3 100, peptization of a precipitate 452 or decomposition of

ferrous oxalate by hydrogen peroxide 120

6.i.6. Cobalt nickel and copper oxides. Monodispersed cubic Co304 particles have been

synthesized by aging CoOOH precipitated from cobalt acetate 65. Pure hydrous cobalt oxide

hydrosols are obtained through hydrolysis and condensation of Co(OEt) 3 in ethanol 450

Ni(OH)2 and Co(OH) 2 gels can be synthetized upon dialysis of nickel or cobalt

tartrate precipates 63 Green Ni(OH)2 gels can also be obtained through neutralization of

nickel(II) acetate dissolved in glycerol with alcoholic KOH 64

Monodispersed Cu20 sols are formed upon ageing copper (If) tartrate in the presence

of glucose 69 Ellipsoidal CuO or Cu(OH)2 particles can be obtained upon ageing copper(II)

nitrates or sulphates 68

Sky-blue copper(II) hydroxide gels can be obtained through neutralization of

copper acetate with ammonia in the presence of a small amount of sulfate ions 66,6z or

through neutralization of CuCI 2 with NaOH 58

6.1.7. Hydrous yttrium oxide. Sols and gels can easily be obtained from yttrium nitrate by

ion exchange techniques 453 Structural characterization of such sols has been done by

EXAFS, SAXS, light scattering and TEM 454. Peptization of yttrium hydroxide precipitates

also leads to colloidal solutions 455

6.1.8. Zirconium oxide. Monolithic ZrO 2 gels can be synthesized from Zr(OR) 4 (R = Et, Pr I ,

Pr n , Bu n) using substoichiometric hydrolysis ratios (l<h<4) and inorganic acid catalysts

(HCl, HN03) 263. Stabilization of Zr(oPrn)4 via chemical modification 20 can be performed

with acetic acid 263, acetylacetone 291,305 or hydrogen peroxide 305. Using different

solvents also leads to monolithic gels upon mild hydrolysis of Zr(oPrn)4 272 or Zr(OBun)4

456. ZrO2 gels obtained from inorganic precursors were reviewed by Woodhead 424. Structural

studies have been performed on amorphous ZrO 2 gels 129,441

ZrO 2 fibers : Two main methods have been used to get ZrO 2 fibers :

- Extrusion and calcination of zirconium acetate 457

- Unidirectional freezing of aqueous solutions made from ZrOCI 2 426,458

ZrO 2 coatings: They have been made mainly by dip-coating from colloidal solutions using

inorganic precursors 42? or alkoxides 429,459,460,461. Chemical modification of Zr(oPrn)4

by acetic acid 429,459,460, acac or etac 461 and ethylene glycol 461 allows a better

control of the viscosity. The dip-coating process can thus be easily optimized.

Powders for ceramics : Thermohydrolysis is the cheapest way to obtain monodispersed ZrO 2

powders from inorganic precursors such as ZrOCI2, ZrO(NO3)2, ZrCI 4 or ZrO(S04)

455,462,463,464 The formation of monodispersed ZrO 2 sols was followed by TEM 465

Controlled precipitation of zirconium alkoxides, Zr(OPri)4 or Zr(oPrn)4 in EtOH

279,364,433,466, allows the synthesis of submieronie monodispersed ZrO 2 based powders.

6.1.9. Niobium and tantalum pentoxldes. Monodispersed Ta205 powders can be synthesized

through controlled precipitation of Ta(OEt)5 in an ethanol/butanol-i (1:4) mixture with an

excess of water (h=3-10) 280

Ta205 sols can be used to make storage capacitor dielectrics for microelectronies

by hydrolysis-condensation of Ta(OEt)5 in ethanol or toluene with an acid catalyst such as

HCI or CH3COOH 232,467. Thin films 1750 A thick were obtained from such sols by a spin

coating technique 467


Various methods can be used in order to synthesize Nb205 gels 181 :

- Hydrolysis of NbcI 5 followed by a careful washing and the addition of hydrogen

in order to remove chloride ions.

- Hydrolysis of chloride-alkoxides such as Nb(0R)3CI 2.

- Hydrolysis of niobium ethoxide in various alcohols.

peroxide

6.1.10. Tungsten oxide WO 3. Colloidal tungstic acid can be obtained when acidification of a

sodium tungstate solution with HCI is followed by washing or dialysis 468,469. A pure sol

can be easily obtained by acidification of a sodium tungstate solution through a proton-

exchange column 183,470 Electrochromic thin films have been deposited from WO 3 sols in

order to make display devices 183 These sols can also be made from tungsten chloride-

alkoxides 321 or alkoxides. In this case, a chemical stabilization of W(OEt) 6 by

acetylacetone in butanol must be made 304

6.1.11. Noble metal oxides. Gelification can also be achieved with noble transition

elements such as Au(lll). Hydrous Au203 gels have been synthesized through acidification of

Na Au(OH)4 with an inorganic acid 471 Colloids IrO2.xH20 can be prepared by hydrolysis of

hexachloroiridate (III) or (IV) at pH=7 472. Finally, colloidal RuO2.2H20 can be

synthesized by dissolving KRuO4and poly(styrene/maleic anhydride) (i:i) in water and adding

aqueous H202 at pH=7 473. The main application of these noble transition metal colloids is

in the field of photocatalytic materials 474

6.2. Materials

The sol-gel process is especially suitable for making multicomponent ceramics or

glasses. Only materials derived from sol-gels are rewiewed here. Other wet techniques such

as coprecipitation, freeze drying, spray drying and liquid drying will not be considered.

6.2.1. Ferroelectric ceramics. Barium titanate BaTiO 3 is the most often studied material

for high dielectric constant ceramic capacitors. The alkoxide route using Ti(OEt) 4 290,475

or Ti(OPri)4 476-478 and barium alkoxides such as Ba(OEt) 2 290, Ba(oPrn)2 476 or Ba(OH)2 in

CH30 H 478 have been mainly used to obtain thin films 478 or monolithic gels 290 Barium may

be substituted by strontium in these ceramics. Strontium titanate powders can be obtained

through controlled precipitation of a double alkoxide SrTi(OPri)6 479,480 W doped

strontium titanate can be synthesized giving c values as high as 40,000 through chemical

modification of Ti(OBun)4 with ethylene glycol and citric acid 481 Strontium is introduced

as Sr(N03)2, tungsten as tungstic acid H2WO 4 and hydrolysis is performed under acidic

(HN03) conditions. Finally complex formulations can be obtained by mixing Zr(OBun)4,

Ti(OBun)4 , Nb(OEt) 5, Sb(OEt) 3, La(N03)3, Ba(OH) 2 and Sr(OH) 2 in butanol-2. They lead to

fine perovskite powders which can be used for piezoelectric and electrooptic applications 482

Lead titanate PbTiO 3 can also be used in high dielectric constant ceramic

capacitors. It can be made as a monolithic gel from Pb(OAc)2.nH20 and Ti(OPri) 4 precursors

in methoxyethanol using acid catalyzed (HN03) hydrolysis 483,484,485. Thin films have been

made by spin-coating 486 The substitution of some titanium by zirconium lead to the so

called PZT compositions. Films of various compositions are obtained by dip-coating 487 or

spin coating 488,489 Titanium and zirconium are usually introduced as alkoxides :

Ti(OPri)4 487, Ti(OBun)4 488,489, Zr(oPrn)4 488,489, Zr(OEt)4 487 ; while lead is

introduced as lead acetate or lead ethyl-2 hexanoate 487,488

JPSSC 18:4-E


Alkali niobates and tantalates are also very important ferroelectric materials

which can be obtained by the sol-gel process. LiNbO 3 490,491 ; Nal. x Li x NbO3 492 ; KNbO 3 ;

KTaO 3 and K(Ta,Nb)O 3 476 have been made mainly from alkoxides.

6.2.2. Magnetic ceramics. Spherical mixed cobalt and nickel ferrite particles have been

synthesized by ageing Fe(II), Co(II) and Ni(II) hydroxides in the presence of nitrate or

sulfate ions 493. Ferromagnetic NiFe204 films are deposited by dip-coating from solutions

containing Nickel(II) ethyl-2-hexanoate and Fe(III) ethyl-2-hexanoate 487

Barium ferrite powders BaFe12 O19 can be obtained from a goethite gel and Ba(OR) 2

in ethanol 494. Through hydrolysis-condensation of Ba(OEt) 2 and Fe(OEt) 3 magnetic Ba2Fe204

can also be synthesized 495

6.2.3. Other ceramics. Many binary systems have been made by the sol-gel process :

. TiO2_AI203 (Ti(OPri)4/iproH) 432 and AI2TiO 5 from organometallic precursors 496

_ ZrO2.AI203 (Zr(OPrn)4/EtOH) 432, and AI203-ZrO 2 composites made by dispersing ZrO 2 fibers

in AI203 gel (Al(OBuS)3/qINO3 or HCI) 497

Y3AI5012 made from yttrium and aluminium alkoxides 480 or Y(NO3) 3 and Al(OPri)3 in

ethanol with base catalyst which leads to a translucent gel 498

- Y203-AI205 transparent gels from Al(OBuS)3 and yttrium acetate hydrolyzed at pH 5.5 499

- LaYO 3 thermomechanic ceramics made by basic (NH 3) hydrolysis of La(OEt) 3 and Y(OEt) 3 500

- TiO2-CeO 2 films made by dip-coating from a solution containing Ti(OPri)4, CeCI 4 and

acetic acid 501

- ZrO2-Ce203 thermomechanic ceramics from Ce(acac) 3 and Zr(OBun)4 in ethanol 502

ZrO2-Cr203 thin films by solvent extraction 503

TiO2-V205 fibrous gels from VO(OEt)3 or decavanadic acid sols 447

6.2.4. Glasses and vitroceramics. Low thermal expansion coefficient glasses are mainly

based on TiO2-SiO 2 systems. TEOS and titanium alkoxides Ti(OEt) 4 315,504, Ti(OPri)4 275

Ti(OBun)4 504,505,506 and Ti(OPri)2(acac)2 425 are mixed and hydrolyzed under acidic

conditions (HCI, CH3COOH , PTSA).

Alkali resistant glasses are obtained in the SiO2-ZrO 2 system. Gels can be made by

mixing TEOS and zirconium n-propoxide 275,315 Zr(acac) 4 507 or ZrO(NO3) 2 508. Coatings 50

nm thick can be obtained from Si(OEt)4/Zr(oPrn)4 mixtures hydrolyzed under an atmosphere of

95% relative humidity 509. Monolithic films or fibers can be obtained when hydrolysis is

performed under acidic conditions 275

Other vitreous compositions studied included :

SiO2-ZrO 2 photoresponsive polymers made by polymerization of Zr(OBun)4 in

tetrahydrofuran and other solvents such as : benzene, ethanol, CS 2 and acetone with

freshly crushed silica gel 456

Si02-Fe203 made from TEOS and Fe(OEt)3 2?5

Si02-Y203 high temperature glasses made from Y(NO3) 3 and TEOS/EtOH (1:3) 510

SiO2-TiO2-ZrO 2 films made by dip-coating from ethanolic solution containing TEOS,

Ti(OBun)4 and Zr(OPrn)4 with HCI and/or formamide 511

Si02-TiO2-ZrO 2 glasses obtained through hydrolysis of a mixture of TEOS, Ti(OBun)4 and

Zr(oPrn)4 in ethanol 512

SiO2-ZrO2-AI203-Na20 alkali resistant glasses obtained through hydrolysis of TEOS,

Zr(oPrn)4, Al(OBuS) 3 and NaOEt mixtures at h<l.8 with chemical modifiers (acetic acid,

ethylene glycol and pentanol) followed by a peptization process with nitric or perch~oric

acid 513


SiO2-TiO2-AI203-Li20 low thermal expansion coefficient glasses made through the

hydrolysis of a Si(OMe)4, Ti(OPri)4 , Al(OBuS)3 and LiOMe alkoxide mixture in methanol in

the presence of a DCCA such as formamide 514

Na20-B203-V205-SiO 2 gels made by mixing Si(OEt) 4, VO(OEt) 3, B(OBun)3 and NaOMe in

methanol using NH 3 as a catalyst and a wet atmosphere for hydrolysis 515 Similar colored

gels are obtained when Co(OAc) 2 or ~i(OAc) 2 is used instead of VO(OEt) 3 .

6.2.5. Catalysts. The sol-gel process offers many advantages for making catalysts. Powders

with high surface area and optimized pore size distributions can be obtained. Since

homogeneous mixing can be made at the molecular scale, the chemical reactivity of the oxide

surface can be greatly enhanced.

- Hydrogen adsorption is achieved at the surface of chromium oxide gels which provide good

catalysts for the dehydro-cyclization of paraffins 516. These gels have been prepared by

slow precipitation with dilute ammonia from dilute chromium nitrate solutions, and by

gelation from chromic acetate and reduction of chromic acid by alcohol or other reducing

agents such as dugar or oxalic acid 449 . The highest recorded rate for the dehydro-

cyclization of n-heptane was obtained from chromium oxide gels obtained through reduction

with oxalic acid 449 Chromium oxide microspheres for catalyzing fluorination processes can

also be obtained using sol-gel techniques 517 Hydrolysis is achieved by mixing chromium

chloride with ammonia and hexamethylenetetramine. Gelation occurs by injecting the solution

into a glass column using ethyl-2 hexanol as the extraction solvent. Highly dispersed

particles with narrow size distributions have been obtained from Ti(OPr~)4 and cobalt

nitrate dissolved in ethyleneglycol 518. The average size of the particles can be varied in

the range 30-120 A by diluting the alkoxide precursor during the synthesis. This leads to

modified catalytic activity for the hydrogenation of propionaldehyde.

If drying is performed in hypercritical conditions, a highly porous material called

"aerogel" is obtained. Aerogels exhibit better catalytic properties (activity, selectivity,

resistance to desactivation) than usual xerogel catalysts 519,520. Anatase TiO 2 aerogels

made from Ti(OPri)4 or Ti(OBun)4 allow partial oxidation, at room temperature under U.V.

irradiation, of paraffins, olefins and alcohols into ketones and aldehydes 521 NiO/AI203

or NiO/SiO2/AI203 aerogels made from nickel acetate in methanol, Al(OBuS)3 and Si(OMe) 4 are

almost 100% selective towards partial oxidation of paraffins or olefins. Isobutene can be

converted into methacroleine and acetone 519,522 Similar aerogels and Cr203/AI203 aerogels

also allow the conversion of olefins into nitriles 520, while Fe203/SiO 2 and Fe203/AI203

aerogels exhibit Fisher-Tropsch reaction rates two or three orders of magnitude higher than

those of the conventional reduced iron catalysts 523. Reduced oxides such as MoO 2 in

aerogel form can be made from Mo(acac)3 in methanol/ammonia solution. They have been used

for electrochemical generator catalysts 520 Finally, mixed oxide aerogels (TiO2-ZrO 2 ,

TiO2-SiO2, ZrO2-Si02, MgO-TiO2, MgO-ZrO2) made from Ti(OBun)4 , Zr(OPri)4 , Si(OMe) 4 and

Mg(OMe) 2 precursors, can replace SiO 2 or AI203 as substrates for catalysts 519,520

- The transition metal oxide catalytic phase can also be used as a coating on SiO 2 or

various glassy substrates. Amorphous TiO 2 coatings made from an ethanolic solution of

Ti(OBun)4 can be deposited onto glass spheres of the Si02/AI203/CaO/K20/MgO/Na20 system and

treated by a solution of Pd(CBHs) 2 in pentane 524. The catalytic activity of such catalysts

towards olefin hydrogenation is comparable with that of the best conventional systems. A

mono-atomic layer of amorphous niobium oxide can be deposited onto the surface of SiO 2 by

reacting surface silanol groups with Nb(OEt5) in dry hexane followed by chemical treatment

with H20 and 02 525 Such a catalyst is active and selective for ethene formation from

ethanol.


6.2.6. High Te Superconducting Ceramics. A tremendous effort has been applied to synthesize

high temperature superconducting ceramics by the sol-gel process. The versatility of this

process will allow one to obtain dense ceramics, fibers or films from sols or gels

intermediates. Bulk ceramics and thick films have already been made by solution techniques.

In the case of the 90K superconducting phase YBa2Cu307 such materials have been achieved

through coprecipitation 526-528 , by controlled precipitation with colloidal mixtures of

hydroxides and acetates 453,529,530, solutions of neodecanoates 531, ethylhexanoates 532 Some other solution processes have included the control of the rheology by use of

ethyleneglycol and citrates 535,534 or metacrylates 535. Because of the difficulty to

dissolve copper alkoxides very few processes have been described using alkoxides 536

However, some soluble alkoxides like Cu(OCH2CH2NEt2) 2 o'r Cu(OCH2CH2OBu)2 have been recently

successfully used 537

Unfortunately, these precursors decompose through oxides and barium carbonates around

500°C. The reaction leading to the pure material occurs then only around 850°C upon a long

time, and the sintering effect is weak. Although better homogeneity is achieved, the trans-

port properties of the superconducting ceramics obtained up to date by sol-gel processes

are still dominated by the grain boundaries and they do not show better critical currents

than conventional ones. The best results have actually been obtained with carbon-free

precursors like nitrates 538-541 or hyponitrites 537 that decompose easily to oxides upon

heating. The superconducting phase can then be obtained around 650=C, yielding submicronic

grains with, unfortunately, a poor diamagnetic signal intrinsic to the small grain size 537 .

7 . CONCLUSION

Interest in the sol-gel process began about 20 years ago 542. Many significant

results have been obtained since then and products such as optical coatings or fibers have

already appeared on the market. However, the future of the sol-gel technology still depends

on whether it will be able to make better and cheaper materials or even completely new

materials 543. Therefore a real mastery of the process is required from both scientific

knowledge and technological expertise point of view. One of the main advantage of the sol-

gel process is the ability to go all the way from the molecular precursor to the product,

making possible to synthesize tailor-made materials. However, many parameters are involved

along the process : chemistry during hydrolysis and condensation of the precursors,

physical chemistry of aggregation, gelation, drying and finally physics to account for the

properties of the material. Each step has to be optimized depending on the required

application.

The sol-gel process is based on inorganic polymerization reactions. Thus,

chemistry is one of the main points for further development of the process. The chemical

reactivity of silica precursors is beginning to be rather well understood but this is not

yet the case for transition metal oxides. Six chemical reactions are mainly involved in the

sol-gel process, namely ; hydrolysis, modification, olation, alcolation, oxolation and

alcoxolation. More reliable experimental data and accurate characterization of all the

chemical species involved in these reactions have to be obtained before a real science of

inorganic polymerization can be established. Many efforts are being made in order to adapt

the existing techniques to this problem. The ideal method should be able to give "in-situ"

and dynamical information all the way, from small molecular species to large colloidal

particles and gels. Spectroscopies such as X-ray absorption, IR-Raman, X-ray or Neutron

Scattering, high resolution liquid and solid state N.M.R. have been used during the last


few years. They appear to give significant results. From a theoretical point of view, the

chemical reactivity of a molecular MX n precursor mainly depends on the polarity of the M-X

bond and the nature of the solvent ROH. As shown in this paper, the thermodynamics of these

reactions can be described on the basis of electronic chemical potential # or

electronegativity X considerations (X=-#) 37. A molecular precursor ME n chemically reacts

with ROH when its mean electronegativity is smaller (x(MXn)<x(ROH)), while solvation occurs

in the reverse case (x(MXn)>X(ROH)). Water has a high electronegativity (X(H20) = 2.49) so

that alkoxides cannot be easily handled and are readily hydrolyzed in the presence of

moisture. Inorganic precursors must then be used in aqueous solutions. They are quite

electronegative and therefore will not react as readily with most ROH reagents. Their

chemical reactivity is quite low and they are not easily modified by chemical additives. On

the other hand, metal alkoxides have a low electronegativity so that they react with many

chemical reagents besides water. They offer a large variety of chemical reactions 20 and

are therefore versatile precursors for the sol-gel process.

Once the colloidal particles are formed, chemistry does not play such an important

role in the sol-gel process. Aggregation occurs which dictate the ultrastructure of the

oxide. However, it mainly depends on physico-chemical parameters such as particle-particle

or particle-solvent interactions. Thus, either colloidal or polymeric gels are obtained

278. According to the literature, polymeric gels are made almost exclusively from metal

alkoxides while colloidal gels are typically formed from metallic salt aqueous solutions 2

However, such a simplified classification should be considered with care. Polymeric V205

gels have been obtained from inorganic precursors 176 and colloidal SiO 2 from silicon

alkoxides 544 Small-angle scattering experiments are currently done in order to study

aggregation and gelification 330 Computer models have been proposed to account for the

observed scattering curves, and the fractal geometry of aggregates is still a matter of

debate 22

The functionality f of the molecular precursors is sometimes taken into account

329. As for organic polymers this will be a very important parameter. A three-dimensionnal

network is usually obtained when f is larger than 2, while chain polymers are expected when

f is close to 2. Gelation processes should therefore be strongly dependent on the

functionality of the precursors. As mentioned in this paper, the shape of the primary

colloidal particles is often governed by the chemical conditions. Strongly anisotropic

colloids can sometimes be obtained which lead to ordered aggregation and anisotropic

coatings. However, it must be pointed out that, up to now, it remains impossible to relate

chemistry and morphology.

Despite the present lack of knowledge, it may be assumed that the sol-gel

processing of transition metal oxides will continue to grow in the near future. It offers

unique advantages for making monodispersed powders 23, multicomponent ceramics 2, coatings

5 , fibers 4 or even completely new mixed organic-inorganic materials 317. However, one of

the main drawbacks of the sol-gel process remains the long time required for drying and

densification. Rapid drying causes cracking and monolithic materials are difficult to make.

Two general approaches have been proposed to circumvent this problem, namely hypercritical

drying 545 and the so-called DCCA (Drying Control Chemical Additives) 546. They have been

almost exclusively used for silica rather than for transition metal oxides. Theoretical

analysis of the drying process has been also recently proposed 28 This should provide the

necessary basis for the development of the sol-gel process. Anyway, as shown in this paper,

gels or xerogels, can actually be considered as liquid-solid composites. They exhibit some

interesting physical properties and can be used for making antistatic coatings, micro-

batteries or electrochromic display devices.


Acknowledgments : We are greatly indebted to Prof. E. MATIJEVIC for authorizing and

providing the reproduction of electron micrographs of figure 12 and to D r . K. CHEMSEDDINE

for providing electron micrograph of figure i0. Special thanks are also due to D r J.P.

JOLIVET and P. BARBOUX for helpful discussions and preparation of the final manuscript.

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sol-gel chemistry of transition metal...

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