Colloids and Surfafnces A: Physicochemicnl (lnd Engineering Aspects, 87 (1994) 25-31 0927-7757/94/$07.00 0 1994 ~ Elsevier Science B.V. All rights reserved.

25

In situ steric stabilization of titanium dioxide particles synthesized by a sol-gel process

V.J. Nagpal, R.M. Davisa**, J.S. Riffleb

aDepartment of Chemical Engineering, Virginiu Polytechnic Institute & State University, Blacksburg, VA 24061, USA bDepartment of Chemistry, Virginia Polytechnic Institute & State University, Blacksburg, VA 24061, USA

(Received 8 February 1993; accepted 3 December 1993)

Abstract

Spherical titanium dioxide particles were synthesized via the hydrolysis of tetraethylorthotitanate (TEOT) in ethanol in the presence of hydroxypropyl cellulose (HPC) which served as an in situ steric stabilizer. This work focused on the effect that water concentration had on the HPC-TiO, interactions leading to steric stabilization. The water concentration varied in the range [H,O]/[TEOT]=5.3-60. The amount of adsorbed HPC increased threefold over this water concentration range. This correlates with measurements of the preferential adsorption of water on TiO, in ethanol. The particle size decreased fivefold in the presence of HPC over the range of water concentrations studied owing to the combined effects of increased HPC adsorption and increased nucleation rates. Mean particle diameters as small as

70 nm were obtained.

Key words: Sol-gel process; Steric stabilization; Synthesis; Titanium dioxide particles

Introduction

The synthesis and processing of fine ceramic

particles less than 100 nm in diameter have received

considerable attention in recent years owing to

their novel optical, electronic and densification

properties [l-5]. Many studies have focused on

particle formation from the hydrolysis and conden-

sation of metal alkoxides [4-61. This paper con-

cerns the effect of water concentration on the

formation of TiO, particles from the hydrolysis

and condensation of tetraethylorthotitanate

(TEOT) in the presence of the steric stabilizer

hydroxypropyl cellulose (HPC). The reactions are

written in abbreviated form [43 as

Ti(OC,H5)4+4H20-+Ti(OH)4+4C2H50H

Hydrolysis

*Corresponding author.

SSDI 0927-7757(93)02735-W

Ti(OH),+TiO,*xHzO + (2 - ?c)H,O

Condensation

The ratio of water molarity to TEOT molarity,

defined as R = [H,OJ/[TEOT], is a crucial parameter controlling the hydrolysis reaction

kinetics and the resulting metal oxide morphology.

Particle formation requires R > 2.5 [ 7,8]. Previous studies of TiO, formation from alkoxides have

focused on how particle growth and the final size

distribution are affected by electrostatic [ 9- 111

and hydrodynamic effects [ 121, as well as by alcohol structure [ 131 and steric stabilization

[14-181. In this last case, the reactions occurred

in a solution of the polymer HPC. The polymer

adsorbed onto the particles as they formed, thus

generating repulsive steric forces that limited fur-

ther particle aggregation and resulted in spherical

particles with relatively narrow size distributions.

26 T/J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31

A significant reduction in particle size occurred

especially above a critical HPC concentration

where the particle size reached a plateau value

corresponding to nearly complete surface coverage

by HPC [17].

With the use of a steric stabilizer, the water

concentration in the reactions of TEOT takes on

added significance since previous theoretical and

experimental studies have shown that solvent com-

position strongly affects the extent of polymer

adsorption and the resulting adsorbed layer struc-

ture which directly affects stabilization [ 19,201.

While the adsorption of HPC on TiO, particles in

anhydrous ethanol was measured by Jean and

Ring to prove that in situ steric stabilization was

the stabilization mechanism [14,17], the effect of

water concentration on HPC adsorption was not

studied. In addition, no particle growth experi-

ments using HPC as a stabilizer have been reported

for water concentrations higher than R = 6.7.

In this paper, we examine the effects of water concentration on HPC adsorption. We demon-

strate for the first time that the amount of HPC

adsorbed onto TiO, increases markedly with water

concentration. This is shown to correlate with the

preferential adsorption of water on TiO, in

ethanol. We also report the effect of water

concentration on the mean diameter of TiO,

particles made over a much broader range of water

concentrations, 5.3

V.J. Nagpal et al.lColloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31 21

were filtered before mixing through a 0.2 urn filter.

The TEOT solutions were handled in a nitrogen

glove bag to prevent premature hydrolysis of the

ethoxide. The water concentrations in these experi-

ments varied over the range 5.3

28 1/J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31

o.5 I

I . M = B&M Kc/mole v MT = 1150 Kg/male

0.4 T 1

z .i 0.3 - E

E,z-\

s

;; \ 1 .z

0.1 - k- $

1 0.0 / I I I I I

0 10 20 30 40 50 60

R=[Water]/[TEOT]

Fig. 2. TEM mean diameter (urn) as a function of water concen- tration R ( = [H*O]/[ TEOT]) for [ TEOT] = 0.075 M, Cn,c = 1.7 g 1-r and different HPC molecular weights. The bars represent 10.

decreased from 60 to 43% as the HPC concen-

tration C,rc increased from 0.42 to 1.7 g 1-r. The

extent of conversion after 24 h increased with water

concentration, rising from 35% at R= 5.3 to over 99% for R>30 and was independent of HPC.

The reduction in particle size with water concen-

tration in the reaction mixture at a fixed polymer

and TEOT concentration can be due to the effect

of water concentration on (i) the kinetics of the

hydrolysis and condensation reactions, (ii) the

adsorption of HPC on TiO,, and (iii) the solubility

of HPC in the reaction medium. The effect of water

concentration on electrostatic stabilization of the

particles was not considered important here since

particles grown without HPC formed large, irregu-

lar aggregates at all water concentrations. The

effect of water concentration on the kinetics of the reactions of TEOT is well-documented [4].

Factors (ii) and (iii) will be discussed later.

E#ect of water concentration on HPC adsorption

The mass of adsorbed HPC per gram TiO,, r nPC, increased threefold as the water mole fraction

X, increased from 0.01 (R=5.3) to 0.12 (R=60)

at a fixed HPC molecular weight and concentration

as seen in Fig. 3(A). The error bars represent 50%

(2a/3) confidence limits. The adsorbed amount

l- nPc also increased with HPC molecular weight

since the longer chains have more segments to

adsorb onto the TiO,. This agrees with previous

theoretical and experimental studies of homo-

polymer adsorption [ 19,201. For C,= 1.7 g 1-l and

X,=0.01 (R=5.3), FHpC=5 x lop4 (g adsorbed

HPC E) (g TiO,)-, about ten times lower than

values for HPC E at 1.7 gl- in pure ethanol

reported by Jean and Ring [ 171. The discrepancy

Mw=l150 Kg/mole

c 006

/

Mw=68.5 Kg/mole,

coo1 1 T/ J ;--? Yw=68.5 Kg/mole. Cp=0.24 g/l

i l

n rlnn l I u VU

0.000 0 025 0.050 0.075 0100 Cl25 0 150

(A) R=5 3 Mole fraction water, X_ R=60

Fig. 3. (A) HPC adsorption isotherm, F (grams adsorbed HPC per gram TiO,) vs. mole fraction water X,, as a function of HPC concentration and molecular weight. The bars represent 2a/3. (B) Proposed water bridging between the TiO, surface and HPC via hydrogen bonding. Adsorption of an ethanol molecule at a Ti-OH site blocks adsorption by HPC.

V.J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31 29

could be due to the difference in the powder

preparation technique. That previous study used

TiO, powders which were dried prior to the experi-

ments whereas the powder in the present study

was never dried in order to replicate as closely as

possible the conditions of adsorption of HPC on

the growing TiO, particles. The surface chemistry

of TiO, powder can change significantly depending

upon the washing and the drying conditions [7]

which, in turn, might have a significant affect on

the polymer adsorption isotherm. Figure 4 shows

that water preferentially adsorbed on TiO, in

ethanol. This effect has not been reported before.

The plateau in the water adsorption curve occurs

at approximately the same water concentration

where the plateaux in the HPC adsorption iso-

therms occur in Fig. 3(A).

Solubility of HPC

The effect of water concentration on the solubil-

ity of HPC in the reaction medium is important

as this can affect the adsorbed amount FHpC and

the thickness of the adsorbed layer on TiO,.

Table 1 shows that the hydrodynamic diameter D,,

for HPC H in a mixture of ethanol and water

I / I I / , I

600 c 4

-0 _ 500 - i x

-& 2

400 -

\ B

300 - ;;/-t I

0.000 0.025 0.050 0.075 0.100 0.125 0.150

R=5.3 Mole fraction water, X_ R=60

Fig. 4. Adsorption isotherm of water on TiOz in ethanol as a function of mole fraction water X, (no added HPC). The bars represent 2a/3.

Table 1 Dependence of hydrodynamic diameter of HPC H on water concentration

Mole fraction water, X,

0.0 0.01 (R = 5.3) 0.12 (R=60) 1.00

M,z 1150 kg mol-.

& (nm?

70 72 78

210

bFrom dynamic light scattering measurements at 25C and a scattering angle of 90 using the second cumulants method

~241.

increased by approximately 7% over the range of

water concentration 5.3

30 V.J. Nagpal et ul.jColloids Surfaces A: Physicochem Eng. Aspects 87 (1994) 25 ~31

particle size owing to the combined effects of

increased HPC adsorption and nucleation rates.

This latter point was evidenced by the time

required for the onset of turbidity. For R = 5.3 at

all HPC concentrations, the typical time required

for the reaction mixtures to become turbid after

the addition of water was greater than 10 min

whereas, for R 230, the mixtures became turbid within 5 s.

Previous experimental and theoretical studies of

polymer adsorption have shown that the amount

of adsorbed polymer, Fp, can be described in terms

of the mean field lattice theory of Scheutjens and

Fleer as a function of two parameters: (i) the

polymer segmental adsorption free energy xskT

which reflects attractive interactions between

polymer segments and surface; (ii) the Flory

polymer-solvent interaction parameter 31 which

characterizes the polymer solubility [ 19,201. For a

fixed value of 1, Ip increases with the segmental

adsorption free energy XskT. For a fixed value of

ilskT> Fp increases with decreasing solubility

(increasing 71). Given the observation that the

solubility of HPC increases with water concen-

tration, the increase in FHpC with water concen-

tration can be explained qualitatively as being due

to an increase in xskT. The preferential adsorption

of water on TiO, in ethanol suggests a possible

mechanism for the enhanced adsorption of HPC.

It is possible that water may preferentially

adsorb since water forms a more polar and hence

stronger hydrogen bond with Ti-OH sites than

does ethanol. A definitive explanation for this will

require further work. The adsorbed water may

enhance HPC adsorption through the formation

of hydrogen bond bridges between Ti-OH surface

sites and the hydroxyl groups on the HPC as

depicted in Fig. 3(B). By contrast, ethanol cannot

participate in hydrogen bond bridge formation

between the Ti-OH surface and an HPC chain.

Adsorption of an ethanol molecule at a Ti-OH

site would block an HPC chain from forming a

hydrogen bond with that site. Whatever the precise

mechanism of enhanced adsorption, it is clear that

the effect of solvent composition on polymer

adsorption must be considered when choosing

reaction conditions. It is interesting to note that

the preferential adsorption of water on TiO, may

be related to the short range repulsive hydration

force proposed by Look and Zukoski to account

for TiO, particle formation [ 11,121.

A model for particle growth that predicts particle

size distribution requires an accurate calculation

of the particle pair interaction energy [ 11,121. An

accurate estimation of the steric interaction energy

in the present case is precluded by the polydisper-

sity of the polymer and by the fact that homo-

polymer adsorption typically occurs in trains, tails

and loops [ 191. Homopolymer adsorption theories

at present provide only qualitatively accurate pre-

dictions of the adsorbed layer thickness and seg-

ment density profile. An attractive alternative to

HPC for in situ steric stabilization experiments is

a block copolymer with narrow molecular weight

distribution where the anchor block and tail block

structures and molecular weights are chosen so

that the adsorbed copolymer forms a well-defined

brush layer [ 271. This would permit an accurate

calculation of the steric interaction energy.

Conclusions

TiOz particles were formed from the hydrolysis

and condensation of TEOT in the presence of an

in situ steric stabilizer, HPC. The mean particle

diameter when grown in the presence of HPC

decreased about fivefold as the water concentration

increased from R= 5.3 to R= 60 owing to the

combined effects of increased HPC adsorption and

increased nucleation rates. Mean diameters as

small as 70 nm were obtained. The amount of

adsorbed HPC increased threefold as the water

concentration R( = [ H,O]/[TEOT]) increased

from 5.3 to 60. This correlated with the preferential

adsorption of water on TiO, in ethanol which has

not been reported previously. A mechanism for

enhanced hydrogen bonding of the HPC to Ti-OH

surface sites was proposed in which the adsorbed

water might form hydrogen bond bridges between

Ti-OH sites and hydroxyl groups on HPC. Future

V.J. Nagpal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 87 (1994) 25-31 31

work will probe further the effect of solvent com-

position on the adsorption of HPC and related

polymers on TiO,. The difficulties connected with

estimating the steric interaction energy for the

HPC-TiO, system may be overcome by using

well-defined block copolymers. Future work will

involve in situ steric stabilization experiments

using block copolymers where the anchor block

and tail block structures and molecular weights

are chosen so that the adsorbed copolymer forms

a well-defined brush layer.

Acknowledgments

This work was partially supported by the

National Science Foundation under grant number

DMR-9005148-02. Support for V.J.N. was pro-

vided by Hercules Incorporated and the Virginia

Center for Innovative Technology. The authors

wish to thank Drs. C.F. Zukoski, J.L. Look and

M.T. Harris for helpful discussions and for provid-

ing preprints prior to publication. The authors

wish to thank Dr. M. Konas for performing the

gel permeation chromatography analyses of HPC.

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