Chapter 3

FTIR, THERMAL AND NMR STUDIES

3.1. Introduction

Research in nanomaterials opens many new challenges both in fundamental

science and technology. There are three key steps in the development of

nanoscience and nanotechnology: materials preparation, property characterization.

and device fabrication. Preparation of nanomaterials has been advanced by

numerous physical and chemical techniques as described in section.2.1. The

purification and size selection techniques can produce nanocrystals with well-

defined structure and morphology. The current most challenging tasks are property

characterization and device fabrication. Characterization contains two main

categories: structure analysis and property measurements. Structure analysis is

carried out using a variety of microscopy and spectroscopy techniques, while the

property characterization is rather challenging.

Materials with nanometer size have unusual structural properties.

Nanopowders, controlled to nanocrystalline size (<I00 nm), can show atom like

behaviours which result from higher surface energy due to their large surface area

and wider band gap when they are divided to near atomic size. Success in devising

and assembling systems on the scale of nanometers requires a deeper understanding

of the basic processes and phenomena involved. Hence, one of the current key

objectives is to adapt and develop a range of techniques that can characterize the

structural and thermal properties of nanost~ctured systems. Techniques, that

provide local information on the nanometer scale, are .important in obtaining a

complete picture of material properties.

Due to extreme selectivity of size and structure of nanomaterials, the

physical properties of nanomaterials are much different'. To maintain and utilize

the basic and technological advantages offered by the size specificity and selectivity

of nanomaterials, it is imperative to understand the principles and methodologies

for characterizing the physical properties of individual nanoparticles. The properties

of nanostructures depend strongly on their size and shape2. Therefore, an essential

task in nanoscience is property characterization3 of individual nanoparticles with

well-defined atomic structures.

Characterizing the properties of individual nanoparticles is a challenge to

many existing testing and measuring techniques because of the following

constrain^.^ First, the size (diameter and length) is rather small; prohibiting the

applications of the well established testing techniques. Secondly, the small size of

the nanostructures makes their manipulation rather difficult, and hence special

techniques are needed for picking up and installing individual nanostructures.

Finally, new methods and technologies must be developed to quantify the properties

of individual nanostructures.

The very high surface-to-volume ratio of nanometer-sized particlesS can lead

to novel and unexpected atomic arrangements, and may also have dramatic effects

on other physical or chemical attributes. Because of this, the precise determination

of nanoparticles structure is a fundamental issue. Nanomatenals are a bridge that ,-

7 links single elements with single crystalline bulk structures. Quantum mechanics ' --- --

has successfully described the electronic structures of single elements and single

crystalline bulks. The well-established bondings, such as ionic, covalent and

metallic are the basis of solid-state structures. Thus, a thorough understanding of

the structure of nanocrystals can provide deep insight in the structural evolution

from single atoms to crystalline solids. The finite size of the particles confines the

spatial distribution of electrons, leading to quantised energy levels. Nanocrystals are

idle systems to understand quantum effects in nanostructured systems, which could

lead to major discoveries in solid-state physics.

The large surface-to-volume ratio of nanocrystals greatly changes the role

played by surface atoms in determining their thermodynamic properties. The

reduced co-ordination number of the surface atoms greatly increases the surface

energy6 so that atom diffusion occurs at relatively lower temperatures. For example.

the melting temperature of Au particles drops to as low as 300 OC for particles with

diameters of smaller than 5 nm, much lower than the bulk melting temperature

1063OC for Au. The significant changes of the density of surface atoms for different

crystallographic planes m nanomaterials, leads to different thermodynam~c

properties.

FTIR, Thermal and NMR Studies 65

In this chapter, the characterisation of the three nano metal phosphates,

namely; aluminium phosphate (AIPO~), copper phosphate ( C U ~ P ~ O ~ ) , magnesium

phosphate (Mg2P207) and the nanocomposite magnesium phosphate -iron

phosphate (MgZPZ07-Fe3(P04)2) are done through FTIR spectroscopy, TG, DTA

and also by NMR studies.

3.2. FTIR Analysis of Nanocrystalline Metal Phosphates

To get insight of the surface structure of the present materials, the infrared

(IR) spectra in vacuum was taken at room The IR absorption spectra - --- of all the nanocrystalline metal phosphates were obtained using a Perkin-Elmer

FTIR spectrophotometer (the range of middle IR), using. a standard technique of

KBr pellets. All measurements were done at normal pressure. Powder samples of

specific forms are mixed with KBr in the weight ratio 1: 150.

3.2(i) FTIR analysis of nanocrystalline Alp04

Nanocrystalline A1P04 has been extensively used as a catalyst, ion-

exchanger, adsorbent and so on. In these usages, understanding of the interaction

between A1P04 surface and other substances9 is of great interest. This interaction

relates to the surface structure and properties of AIP04, such as surface functional

groups, surface charge, hydrophilicity and porosity. Many w ~ r k e r s ' ~ - l ~ reported that

the Alp04 surface possesses P-OH and AI-.OH groups. Therefore, the regulation of

these surface sites may be expected to give rise to many properties, such as acidity

and basicity, affinity and reactivity to molecules, and catalytic activity.

The FTIR spectrum of the bulk sample shows that the basic structural

elements of A1P04 are tetrahedral units,l5.I6 although there are two types of

tetrahedra in the framework structure, that is, [ A I O ~ ] ~ . and PO^]' Thus, there are

two different bonds present in the structure of AlP04: a stronger P-0, and a weaker

A:-3. For a pure bulk A1P04 sample, the individual P -0 symmetric stretching

(1 100 cm-I), asymmetric ~tretching (1250 cm-I), symmetric bending (480 cm-I) and

asymmetric bending (695 cn-l) modes are expected in the IR spectrum. l 2 Thus the

band observed in the FTIR spectrum of nanocrystalline Alp04 (Fig.3.1) around

11 15 cm-I corresponds to the symmetric stretching mode of PO^]'.. This produces

a blue shift of about 15 cm-' for the symmetric stretching mode. Similarly, the band

Chapter 3 66

&*' 'e":bserved around 1260 cm-I corresponds to the asymmetric ~t re tch~ng mode A blue / . shift of about 10 cm-' is observed in this case. The bands observed around 485 crn 1

and 725 cm-I are characteristic bending modes of PO^)'., and the corresponding

blue shifts are about 5 cm-' and 30 cm-I respectively. For a nano size grain, the

atomic arrangements on the boundaries differ greatly from that of bulk crystals:

both in coordination number and bond lengths, showing some extent of disorder."

Crystal symmetry is thus degraded in nano size grains. The degradation in crystal

symmetry results in the shifting of IR active modes. In the present case, all the IR

active modes are shifted to the high frequency side, mainly because of the nano

grain size of the sample."

In vibrational spectroscopy, the spectral features are affected not only by

structural symmetry but also by the character of chemical bonds. The alternate

sequence of positioning the tetrahedron in AlP04 results in a single tetrahedron

[Po4l3. being surrounded by A I ~ ' cations - which subsequently can be described as

Al[Po4]. Thus, there are two different bonds present10 in the structure of AIP04: a

stronger P - 0 and a weaker AI-0 and therefore i t is reasonable to consider its

structure as a group of [po413- tetrahedra 'isolated' from each other by aluminium

cations.

2000 1500 1000

wavenumber (cm-')

Fig.3.1. FTIR spectrum of nanocrystalline Alp04

FTIR, Thennal and NMR Studies 67

Analysis of IR spectra can show many aspects of the structure only after

assigning bands to the corresponding normal vibration modes. It is calculated that

the low temperature form of Alp04 (phospho-berlinite) have 30 vibrationsi8 of

[PO$, of which 18 are internal modes, and 6 modes each correspond to both

rotations and translations. Within this mode!. six modes corresponding to A1 can be -

described as pseudolattice vibrations. Among the nine internal vibration^'^.^^

corresponding with the tetrahedron [~04]'.of high temperature form of AIP04

(phospho-tridymite), two modes will be active in the infrared range, with bands at

about 480 cm-' and at about 1140 cm-I. The band at about 720 c m ' can be described

as a pseudolattice resulting from Al-0 vibrations.

There are many reasons for the difference in frequency of nanocrystall~ne

materials from their bulk counterparts. Cirain boundary interactions, surface

amorphousness, interfacial effects etc. are prominent in nanomaterials, which

greatly influence their structure''. Many authors reported important results from

infrared studies. The general conclusion from all these observations is that optical

absorption and scattering in the fundamental lattice absorption region are size

dependent.

3.2(ii) FTIR analysis of nanocrystalline CuzPzO7

The FTIR spectrum of nanocrystalline CuzP207 is shown in Fig. 3.2. In

copper phosphate, the infrared spectrum of the nanocrystalline sample is dominated

by the spectral characteristics of PzOs. The bands can be approximately identified

as given in table 3.1, using the results of previous workers2'.*' in this field: There

are two main differences when nanocrystalline copper phosphate infrared

absorption is compared with the absorption spectra of its bulk counterparr.

Firstly, the ( ~ 0 4 ) ~ - band at 500 cm-' in the bulk sample is replaced by a much

sharper band at 526 cm-I. This seems to be due to both the strong P - 0 and the weak

Cu-0 bands

Chapter 3 68 -

2000 1500 1000

wavenumber (cm-')

Fig.3.2. FTIR spectrum of nanocrystalline CuzP~07

/ P-0 stretching band / from 940-960 c m ' I

(PO$ group

Cu-0 stretching band

P-0-P ring frequency

I P=O double stretching bond ( from 1050-1200 c m l 1

at 526.6 cm-'

at 615.4 cm-'

at 737 cm-'

I Hz0 water impurities, 0 - H / at around 1710 cm.' /

in the nanosample. Cu-0 is known to have a fundamental band at 620 cm-', which

is being replaced by the broad band around 615 cm-'

Some workers, Khan et al." in a study of phosphate glasses, suggested a

combined effect of all constituents, for explaining the broadening and shifting of

the bands with the formation of Cu-0-P-0 units, in the region of 410-610 cm-'. This

suggestion is consistent with the nanocrystalline model made of four phosphate

tetrahedra.

FTIR, Thennal and NMR Studies 69

Secondly, the broadening and shifting of the P-0 band and P=O band is

noted in the spectra. Here perhaps the presence of the Cu-0 band in the sample has

altered the stretching frequency.24 It is noted that a shift of band position is occurred

when nanocrystalline CuO is added to P205, with broadening of the band. The

suggestion is that a partial bond such as P-0- Cu' may be created i n the region 900-

1250 cm-I and the band shifts with increasing nanocrystalline CuO content. I n

general, it can be said that infrared analysis shows the dominance of the phosphate

tetrahedra in lattice vibrations. Effects due to copper in the sample swallowed up

any other small effects, but the phosphate bonds dominated all others.

3.2(iii) FTIR analysis of nanocrystafiine MgzPz07

Magnesium phosphate has many applications. They are widely used as

bonding in refractories and mortars and as rapid setting cements. They are an

important ingredient in fertilizers and play a role in medical research. It is usually

used for special applications25, such as repair of pavements and concrete structures

or for resistance to certain aggressive chemicals. Magnesium phosphate is an

excellent chemically bonded phosphate ceramic (CBPC) binder. Its properties are

greatly altered when the corresponding nanocrystalline form is considered. Since

optical absorption and scattering in the fundamental lattice absorption region are

size dependent, it is expected that all the IR active modes may be shifted, mainly

because of the nano grain size of the

The FTIR spectrum of nanocrystalline magnesium pyrophosphate is given

in Fig.3.3. The bands at 1717 and 1638 cm-I correspond to in-plane H- 0- H

bending vibrations of at least two different types of water molecules. The other

bands are assigned to asymmetrical (at 1560 and 1508 crn-I) and symmetrical

(1458,1420 cm-I) stretching vibrations of ( ~ 0 4 ) ~ ' . out-of-plane vibrations of Hz0

(733,720 cm-I), and bending vibrations (PO# (501,485,435 and 418 cm-I).

27-29 Many workers canied out the FTIR analysis of the bulk sample of

magnesium phosphate. The strongest and broad bands in the IR spectrum of bulk

magnesium phosphate are observed7 in the region of 0-H stretching vibrations of

Hz0 molecules, at 3490 (strongest band), 3390 and 3050 cm-I. A weak'broad band

at 2410 cm-' and a weak narrow band at 2102 cm-' correspond to the stretching

Chapter 3 70

2000 1500 1000 400

wavenumber (cm-')

Fig.3.3. FTIR spectrum of nanocrystalline Mg~P207

vibrations of the PO- H+ bond and free H' ions respectively. The bands at 1695 and

1622 cm-I correspond to in-plane H-0-H bending vibrations of water molecules.

The other bands are assigned to asymmetrical (at 1555 and 1505 cm ' ) and

symmetrical (1433 cm-I) stretching vibrations of out-of-plane vibrations of

H20 (739,727 cm-'), and bending vibrations (~04)'- (478cm~').

From these observations, it can be seen that in the case of nanocrystalline

magnesium phosphate, almost all the bands in the bulk sample are shifted and also

new bands are created. The H-0-H bending vibrations of water molecules at 1717

and 1638 cm-I correspond to the bands at 1695 and 1622 cm-I of the bulk sample.

The asymmetrical stretching vibrations of ( ~ 0 4 ) ~ ' a t 1555 and 1505 cm-' in the bulk

sample have been replaced in the nanosample by the bands at 1560 and 1508 cm-'

and symmetrical stretching vibrations of (~04) ' - at 1433 cm-I has been replaced by

two bands at about 1458 and 1420 cm-' respectively. The two out-of-plane

vibrations of Hz0 at 739 and 727 cm-I are shifted to about 733 and 720 cm I

respectively. Corresponding to the bending vibrat~ons of ( ~ 0 4 ) ' - at 478 cm ' in the

polycrystalline sample, there are many vibrations in the nanocrystalline sample. The

bands at 501,485, 435 and 478 cm-' are prominent among them. Due to the small

size of grains and the large surface-to-volume ratio of nanocrystals, the atom~c

FTIR, Thermal and NMR Studies 7 1

arrangements on the boundaries differ grea~ly from that of bulk crystals, showing

some extent of d i ~ o r d e r . ~ ~ , ~ ' This disorder in crystal symmetry results in the shifting

of IR active modes and in the evolution of new bands.

3.2(iv) FTIR analysis of nanocomposite M ~ ~ P z O ~ - F ~ ~ ( P O ~ ) Z

In the past few years nanocomposite materials have become one of the muh~

extensively studied materials all over the world as they have shown to pohxbs

several applications such as effective quantum electronic devices, magnetic

recording materials, sensors etc32. j3. Moreover, nanocomposite materials composed

of oxides and conducting polymers have brought out more fields of applications

such as smart windows, toners in photocopying, conductive paints, drug delivery,

rechargeable batteries One of the most important recent developments in

materials science is the ability to engineer material microstructures at the atomic

level. While in its early stages of development, this capability offers the potential

for significant increases in the performance of structural materials. Now, since these

materials are becoming economically viable, it is very important that appropriate

materials characterization techniques be developed to quantitatively evaluate the

microstructural features of nanocomposites.

In scientific literature the favourable properties of nanocomposites are often

assigned to the extreme small dimensions. It is argued that in nanocomposites most

of the molecules are on the surface. This behaviour of the molecules enhances the

properties of nanocomposites in such a way that the resultant nanocomposite

material acquires favourable properties. In nanocomposite crystals, as the crystal

size decreases, the intensity of bulk mode also decreases and after a particular size

of the crystal, this bulk mode vanishes.32

Surface bands are observed in the forbidden region, and these bands shift to

the higher frequencies as the crystal size is further decreased. The observed

frequency shift has been accounted on the basis of the additional restoring force

created by surface polarization charges and the observation of the broad bands was

explained on the basis of surface phonons, characteristic of small particles. Reduced

wave number (cm")

Fig.3.4. FTIR spectrum of nanocomposite Mg2P207 -Fe3(P04)2

density and modified coordination numbers in the boundary regions in

nanocrystalline composites affect the interatomic bonds in the boundary region and

hence will be different from those of bulk crystals. Thus the vibrational spectra of

nanocrystalline composites may be expected to be different from their bulk

counterparts.

The FTIR spectrum of nanocomposite magnesium pyrophosphate-iron

phosphate (MgzPzO, -Fe3(P0&) shown in Fig.3.4 consists of broad bands rather

than sharp ones as observed in their individual nano forms. Several broad IR

absorption peaks are identified with resonance frequencies at 465.8,582.63,736.54,

956.88, 1387 and 3414.38 cm-'. Since IR data of the bulk nanocomposite of the

present sample is not available, the changes in the IR spectrum due to

nanocrystallization are not possible. Hence the IR data of the sample is compared

with the individual data of their corresponding nanocrystalline components; namely

nanocrystalline magnesium phosphate and iron phosphate. Compared with the FTIR

spectra of the above nanocrystals, it is found that, the lattice vibrations of the

present nanocomposite are quite different from its individual nanocomponents due 9

to mixing. Most of the bands are merged to form the broad bands as seen in the - -- hpectrum. The modified surface of the resulting nanocomposite revealed that the

iron and magnesium ions are coordinated to the surface via the oxygen atoms since

FTIR, Thermal and NMR Studies 7 3

1

the peaks assigned to Mg -0 and Fe -0 stretching vibrations (Mg-0, 1387cm ': Fe- 7 -~ ~

0, 957 cm-') are shifted to lower frequency than those of the individual stretching

vibrations (Mg-0, 1420 cm"; Fe-0, ~ 986 cm-I). ~ The strongest band in the IR 7 spectrum is observed in the region of OH stretching vibrations of H20 molecules, at

3414 cm.'. The bands at 736 cm-' and 582 cm-I correspond to out-of-plane

vibrations of Hz0 and bending vibrations ( ~ 0 4 ) ~ -

As far as nanocomposites are concerned, the quantum confinement effect33

on its band structure is to be considered. For a nanocomposite grain, the atomic

arrangement on the boundaries differs greatly from that of bulk crystals. both in

coordination numbers and bond lengths.34 The disorder thus created in

nanocrystalline composites results in the splitting of IR active modes leading to

broadening of bands. The change of positions of active modes of the present sample 7

is attributed to quantum -confinement effect and microstructure formation of -__- _ - -

nanosized grains. The quantum confinement effect dominates at low frequencies 7 /-

- -~ -- ~~. ~ - ---- ~.

and the small size effect at high frequencies. It is thus evident that the quantum --

confinement effect and small size effect in nanocomposites play important roles in

7 the lattice vibrations; including the strengthening of active modes, the appearance . _ -----

of new IR active modes, the change in positions of active modes and in the

broadening of bands.35

3.3. Thermal Analysis of Nanocrystalline Metal Phosphates

Thennal analysis of nanocrystalline materials is used to study the phase

transitions of nanoparticles, which are expected to be much different from bulk

crystals. Many worker^^^.^^ reported the phase transition studies of nanostructured

materials. The free energy of nanoparticles has been reported to be higher4' than

that of their polycrystalline counterparts. The changes occurred on the~r

microstructure and atomic configuration can well study when they are subjected to

thermal analysis.

41-44 Many authors observed new phases and additional exothermic and

endothermic peaks in nanostructured materials, which were not present in their bulk

counterparts. The new peaks and phases are explained on the basis of grain growth

process and their microstructure and atonuc configuration changes when exposed to

high temperature. A large fraction of atoms in nanoparticles are surface atoms and

these atoms have significant influence on the thermal properties of nanostmctured

materials. Structural changes can be expected in small particles because of the large

amount of free energy4' associated with their grain boundaries. The energy

associated with the interface regions becomes substantial when the size of the

crystallites is in nanosized regime and i t may influence phase transitions in

nanophase materials. Other types of lattice imperfections may also exist due to

small size of the particles. Hence the phase transitions in nanoparticles are expected

to exhibit modified behaviour from that of the bulk materials.

3.3(i) TG and DTA analysis of nanocrystalline Alp04

The nanocrystalline Alp04 sample synthesized has been subjected to

thermal analysis. Fig.3.5 shows the TG and DTA curves in air of nanocrystalline

Alp04 sample. TG curve of the sample shows continuous weight losses from room

temperature to about 700°C with an endothermic peak as shown in the DTA curve.

This weight loss is ascribed to the removal of adsorbed and coordinated water. The

results are tabulated in table 3.2.

The TG curve exhibits four weight losses: the first one is less than 1 %

occurring from room temperature to 35OC and the second one is about 7.5%

appearing between 35 and 115°C. Both are assigned to desorption of water

molecu~es'~ from the sample. The third weight loss is about 15% between 115 and

475OC. The final weight loss is more than 60% and is between 475 and about

700°C. The third and forth weight losses together contribute more than 75% and 1s

assigned to the phase transition of the sample from the low temperature phospho-

berlinite to high temperature phospho-tridymite'5. Correspondingly, the DTA curve

shows two small endothermic peaks at around 85'C and around 475'C. X-ray

powder diffraction indicates that nanocrystalline AIP04 is stable up to 700°C as

phospho-berlinite and when treated at temperatures above 750°C, the material

transforms to phospho-tridymite,9 This phase transition is also reflected in the

exothermic effect at about 580°C on the DTA curve for the as synthesutd

nanocrystalline Alp04 material.

FTfR, Thennal and NMR Studies 7 5

Temperature O C

Fig.3.5. TGA and DTA curves of nanocrystalline Alp04

I TG DTA 1

Many have done thermal characterization of aluminrum

phosphate molecular sieves. Qiuming Gao et al." reported the TG curve exhibits

two weight losses of which the first one is occurring from 150 to 270°C and the

other is appearing between 520 and 700°C. 'The DTA curve shows two endothermic

peaks and an exothermic peak, which indicated the phase transition of the material

from berlinite to tridymite. On the other hand, in the present study of

nanocrystalline AIPO4, the positions of the peaks are shifted and in addition new

peaks are observed. Hidekazu Tanaka et al.17 reported the original Alp04 possesses

hydrated HzO, which are released as temperature increases and the phosphate

molecules dissociate to phosphate ions and H+ ions. The dissociation of phosphste

!

1

2

Temp.

36

115

Wt. percent

99

92

1

2

Temp.

85

474

molecules enables the phase transformation of the bulk sample studied at about

400°C. Keen et al.42 reported that the transition from y to P phase of nanophase Cul

takes place at a higher temperature than that of bulk CuI.

The elevated temperature of phase transition of the present sample is in

accordance with this observation. The phase transition of the bulk form is found to

take place at about 700°C. When it becomes nanocrystalline, the phase transition

(phospho-berlinite transforms to phospho-tridymite) takes place at temperatures

above 750°C. This increase in the phase transition temperature is explained on the

basis of the large amount of free energy associated with the grain boundaries of

nanoparticles.

3.3(ii) TG and DTA analysis of nanocrystalline Cud'zOr

The thermal behaviour of nanocrystalline copper pyrophosphate has been

investigated using conventional thermal analysis techniques as described above. As

in the case of AlP04, TG curve of the nanocrystalline copper phosphate sample

shows continuous weight losses from room temperature to about 900°C with two

endothermic peaks of which one is stronger and the other weak as shown in the

DTA curve. The results are tabulated in table 3.3.

1 -5 0 200 400 600 800 1000

Temperature 'C

Fig.3.6. TGA and DTA curves of nanocrystalline CuzPz07

FTIR, T h e m 1 and NMR Studies 77'

Wt. percent

737 47 760

7 TG

The TGA curve can be divided into four regions: the first region occuning

from room temperature to 97°C where the loss of weight is about 3% .The second

zone is up to about 480 OC where the loss in mass scale is 9%. Both are assigned to

desorption of water molecules2' from the sample. The third region is between 480

and 737°C where the corresponding loss in weight is about 41%. The last region is

up to about 900°C where the sample is transformed in to the stable form with a loss

in weight of 9.5%. The two endothermic peaks between the temperature 450 OC to

770°C denote the two phase transitionsZ2 of the sample of which the first and the

stronger one is at about 550 'C and the second one (weak) is at about 760 "C.

DTA

As in the case of nano AIP04, here also there is an elevation of the phase

transition temperature. The phase transition of polycrystalline CuzPz07 takes place

at about 650°C while for nano CuZP207 it is at 770°C. This change in the phase

transition temperature is due to nanocrystallization and is explained on the basis of

the large amount of free energy associated with the grain boundaries of

nanoparticles. The two well-separated endothermal effects, correlated to mass loss

steps, characterize the thermal behavior of nano Cu2P207 With respect to the above

described structural features, the first mass loss is due to dehydration (up to 550 "C)

and the second (up to 770 "C) is due to the dissociation of phosphate molecules.

But, looking at the curve shape (no plateau), it is rather incomplete or, at least, more

complicated than a simple evaporation process. Due to its large hydration, it is

found that nanocrystalline Cu2P207 is unstable under heating.

3.3(iii) TG and DTA analysis of nunocrystalline Mg9207

The thermal character of nanocrystalline magnesium phosphate has been

analysed in this part. The TG/ DTA curve (Fig. 3.7) shows a total weight loss of

Chapter 3 78

11% between RT and 544'C. This loss corresponds well with the release of all four

water molecules42 from the nanocrystalline sample. The stepwise like weight loss in

the TGA diagram indicates the existence of less hydrated polymorphs.27 The next

region, between temperatures 544OC and 836 OC is accompanied by a weight loss of

about 48%. The corresponding DTA curve shows three endothermic peaks and an

exothermic peak representing the phase transformations occurred in the

nanocrystalline sample at various temperatures.

25-28 Many researchers had done the thermal analysis of polycrystalline

MgzPz07, According to Kjell Ove Kongshaug et aLZ5, the TG curve of

polycrystalline MgzPz07 shows a weight loss of 21% between 150 OC and 600 OC.

This loss is attributed to the release of water molecules from the sample. The

existence of less hydrated polymorphs was confirmed by high temperature powder

X -ray diffraction, which indicated the existence of such phases.26 The first phase

transition was found to complete at 200°C, the second phase around 275°C. the

third and fourth at 355OC and 538OC respectively. Several variably hydrated

magnesium phosphates are shown to experience similar water loss.27 On further

heating, polycrystalline MgzPz* was found to transform into an amorphous phase.

Temperature %

Fig.3.7. TGA and DTA curves of nanocrystalline MgzPz*

FTIR, Thermal and NMR Studies 79

1 TG I DTA 1

At a later stage the amorphous phase recrystallizes into anhydrous faningtonite, a

thermodynamically stable form of MgzP207.

1

2

3

Thermal analysis of the present nanocrystalline MgzP207 sample fits well

with the analysis of its polycrystalline counterpart. The phase transition regions and

the weight loss% are found to be different from the bulk samplez5; but the

behaviours are exactly identical. The phase transitions of nanophase MgzP207 take

place at higher temperatures than that of bulk MgzP207, which is confmed to be a

general characteristic of nanophase materials, as explained in section 3.3 (I). The

phase transition temperatures 326 OC, 527 OC, 617 O C and 726 OC are found to be

higher than the reported phase transition temperatures of the corresponding 1

polycrystsalline counterpart. The positions of the new peaks and phases are due to

grain growth process and their microstructure and atomic configuration changes

when exposed to high temperature, as explained earlier. Through this analysis, once

again it is confirmed that the large fraction of surface atoms having large amount of

free energy associated with their grain boundaries significantly influence the

thermal properties of nanostructured materials. \

3.3(iv) TG and DTA analysis of nunocomposite Mgfl2O7-Fe3(P04)2

Temp.

60

544

836

The favourable properties of nanocomposites are due to their extreme small

dimensions of the particles and also due to the presence of most of the particles on - 1 the surface. T ie crystallisation and melting behaviour of nanocomposites were

analysed to determine their thermal properties.

The TGlDTA curves of the as-prepared nanocompositeMg2?207 -Fe3(P04)2

is shown in Fig. 3.8. The TG curve shows a weight loss between RT and 544"C,

which is related to elimination of crystallisation wateZ5. This region can be divided

Wt. percent

95

89

42

1

2

3

4

Temp.

326

527

617

726

Chapter 3 80

in two areas: RT-144OC and 144 - 448OC. The respective weight losses are 6.3 and

7% by mass. The total mass loss is 13.3%. An exothermic peak at 425OC is

observed in this region as shown in the DTA curve. The next region in the TG

curve is between 448°C and 65S°C where the weight loss is more than 36%. A large

endothermic peak is accompanied in this region as shown in the DTA curve. At / higher temperatures there are three overlapping stages: 655-712.712-778. and 778-

900 "C. The respective weight losses are 2.4 and 2% by mass. The total mass loss

0 200 400 600 - 800 1000

Temperature O C

Fig.3.8. TGA and DTA curves of magnesium pyrophosphate - iron phosphate nanocomposite , ,

Table. 3.5.

FTIR, Thennal and NMR Studies 81

in this region is about 8%. The endothermic effect over the temperature region is

shown in the DTA curve, namely at 730°C, which is not accompanied by

appreciable weight loss in the TG curve. The TG /DTA curves show that the

elimination of crystallisation water occurs between RT and 400 'C.

The qualitative details of the analysis may be deduced from the TG-DTA

data. The nature of the TG-DTA curves of the nanocomposite is found to be

different from the respective curves of its components. The positions of the new

peaks and phases may be due to the formation of the nanocomposite when two

thermodynamically different nanocrystalline materials are mixed. The grain growth

process and their microstructural changes when exposed to high temperatures are

found to follow a similar character of other nanocrystalline materials, as explained

earlier.

3.4. NMR Studies of Nanocrystalline Metal Phosphates

NMR spectroscopy can be used as a probe to get information about the

electronic structure and environment of atoms in solids in a non-destructive manner.

As a surface technique, however, NMR suffers the disadvantages of having a

relatively low sensitivity and NMR of surfaces remains restricted to materials with

high surface areas. Hence, nanoparticles having high surface to volume ratio are

ideal candidates for NMR spectroscopy. Each NMR-active nucleus has its own

magnetic field as a result of its magnetic moment. Depending on the spin state of

the nucleus, it will either add to or subtract from the external magnetic field. Magic

angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR)~'~' has been

successfully used for investigating the structures of various P-containing metals.

MAS-NMR analyses were conducted on "P nucleus. 3 1 ~ MAS NMR

spectroscopy4'47 is a useful probe of the structural environment of P and has been

used to quantitatively describe compositionally induced changes in the structures of

phosphate ceramics. In the present study, 3 ' ~ MAS NMR spectroscopy is used to

study the structure of the nanocrystalline forms of aluminium phosphate and

magnesium pyrophosphate. 3 L ~ MAS NMR spectroscopy technique is used to

Chapter 3 82

characterize how the incorporation of these different metal polyhedra affects the

chemical environment of the phosphate tetrahedra. The analysis of chemical shifts

gives a lot of information about the number of electrons and type of electronic

environments present in a molecule. However, an equally important tool for

discovering organic structures lies in the analysis of coupling between nuclei.

3 1 ~ MAS NMR spectroscopy measurements were carried out at room

temperature using a DSX 300 MAS NMR spectrometer at a frequency 121.5 MHz.

3 1 ~ MAS NMR spectra were obtained using a 90' pulse of 2 ps duration, a repeat

time of 60 s, spinning rate of 13.5 kHz. 3 ' ~ has a nuclear spin ( I ) of Y2, and thus the

chemical shifts49 do not depend on the magnetic field strength (H,). Resolution does

not appear to depend on H,, but separation of the true peaks and spinning side

bands is best at the highest spinning speeds. "P chemical shifts are expressed in

ppm relative to an 85% H3P04 solution.

Nuclear magnetic resonance (NMR) is one of the key methods for studying

and understanding the atomic structure, particularly of coordination numbers of

atoms or ions, group distributions, bridging and non-bridging oxygens etc. of

nanocrystalline materials. At present NMR becomes increasingly important because

two-dimensional (2D) NMR techniquesms2 enable more detailed knowledge of the

atomic structure.

3.4(i) NMR studies of nanocrystalline AlP04

Alp04 has two 100% abundant NMR-sensitive nuclides, 3 1 ~ and "AI, and

is ideally suited for NMR s t ~ d ~ ' ~ . ' ~ . The room temperature "P MAS NMR

spectrum of the nanocrystalline AlP04 sample is selected for the present study and

is shown in Fig.3.9. It consists of a well-defined peak centred at -27.047 ppm. The

resolution of the spectrum at high field is much better than at low field due to

decreasing A1-P dipolar coupling in ppm with increasing field strength. The NMR

results of nanocrystalline AIFQ provide useful insight into its structure. The "P

NMR allows the specification of the phosphate network to be determined in terms

of the number of bridging oxygen per P atom.

FTIR, Thermal and NMR Studies 83

ppm

Fig.3.9. NMR spectrum of nanocrystalline Alp04 sample.

The 3 1 ~ MAS NMR spectrum of nanocrystalline Alp04 shows a single

symmetrical peak at about -27 pprn and two other small peaks at -84.27 pprn and

40.32 ppm. Many authors4648 reported that in freshly calcinated bulk Alp04 the

single wide symmetric peak was observed at -33 ppm. The resonance of P nucleus

to lower fields may be due to some crystalline disorders in nano crystalline AIP04.

In AIPO-21, a series of three resonance peaks were appeared in the "P MAS NMR

spectrum due to the differences in AI-0-P bond angles. Akporiaye and

suggested that the P environment can be affected both by the presence of

neighbouring framework of A1 and changes in AI-0-P bond angles. Hence the

appearance of the small peaks at - 84.27 pprn may be due to the coordinated Al.

Since the bound structure on the surface of a nanoparticle is distorted, the NMR

signal from the surface nucleus is expected to be shifted by different chemical shift

interaction^.'^ The appearance of the small peak at 40.32 pprn may be due to a

signal from surface P nucleus.

3.4Cu) NMR studies of nanocrystalline MgzP~07

Solid-state 3 ' ~ MAS NMR investigations have been canied out on

nanocrystalline magnesium pyrophosphate at room temperature as described above

for aluminium phosphate. The distribution of the phosphate units, obtained from

the simulation of 3 1 ~ MAS spectra, is close to a binary distribution of bridging and

Chapter 3 84

non-bridging oxygen atoms, indicating that the M ~ ~ + cations act as network

modifiers.26

NMR spectrum of nanocrystalline Mg2P207 shows large number of signals

consistent with the crystallographically distinct P sites. The chemical shift values of

the two most prominent resonances are at -13.41 and at -19.66 ppm respectively.

These two most shielded resonances are probably due to the two phosphate

tetrahedra being involved in edge sharing. The next four resonances at 40.3,34.06,

-67.14 and -73.42 are supposed to be due to phosphate tetrahedra being

involved in vertice sharing to Mg 0ctahedra.2~ The associated spinning side bands at

ppm

Fig.3.10. NMR spectrum of nanocrystalline Mg2P207 sample.

170.58,164.3, 94, 87.82, and -0.13 are assigned to the anisotropy of the chemical

shift interaction.

Magnesium phosphates have important applications. Hence they have been

the subjects of a number of s tud ie~ .~ ' -~~ jell Ove Kongshaug et al.2' were recorded

and investigated "P MAS NMR spectra of hydrated magnesium phosphate. The

four resonances were grouped in two pairs and each one was attributed to edge

sharing and vertice sharing of the phosphate tetrahedra to Mg octahedra. "P MAS

NMR study of magnesium phosphate glasses was done by Franck Fayon et aLZ6

F T R , Thermal and NMR Studies 85

through high resolution MAS NMR spectroscopy and through 3 1 ~ double quantum

MAS NMR spectroscopy. The characterization of phosphorus connectivity scheme

in crystalline phosphates was obtained through these studies.

High resolution MAS NMR has proved to be a powerful tool to resolve the

different Q, units (Bridging oxygen atoms per PO4 units) through the 3 1 ~ chemical

shift, which is sensitive to the phosphorus local environment. Three distinct

isotropic contributions can be assigned to Q3, Q2 and QI units according to the 3 ' ~

chemical shift changes in crystalline magnesium phosphates. Solid state "P MAS

NMR investigations have been carried out on nanocrystalline magnesium

pyrophosphate. Suzuya et al studied the 3 1 ~ resonances of MgzP207 and observed

two peaks belonging to Ql units at -20.2 ppm and -13.8 ppm. The same peaks with

a slight change (-19.66 ppm and -13.41 ppm) are observed in nanocrystalline

Mg2P207. The slight change in ppm may be due to some crystalline disorders in

nanocrystalline Mg2P207. For each type of Q, units, the line width is due to a

continuous distribution of 3 1 ~ isotopic shifts, reflecting the structural disorder such

as bond angle and bond length variations and higher coordination sphere disorder,

depending on the nature of the neighbouring Q, units." or QI units in crystalline

magnesium phosphates, the PO4 tetrahedron is close to an axial symmetry with the

symmetry axis along the single P-OB (OB - bridging oxygen) bond. The same is

true in nanocrystalline MgzP207 also. These two most shielded resonances may be

due to the two phosphate tetrahedra being involved in edge sharing. The resonances

at 40.3, 34.06, -67.14 and -73.42 ppm are supposed to be due to phosphate

tetrahedra being involved in vertice sharing to Mg. 26

Since the bond structure on the surface of nanoparticles is distorted, the

NMR signal from the surface nuclei are expected to be sfrifted by different chemical

shift interaction^.'^ The resonances at 170.58, 164.3, 94, 87.82 and -0.13 ppm are

assigned to the resonances of surface nuclei. All these resonances are at low fields,

indicating that the shielding of surface nuclei are less and different nuclei have

different electronic environments.

Chapter 3 86

3.5. Conclusion

The structure of nanophase materials are dominated by their ultrafine grain

sizes and their large number of grain boundaries. Other structural features such as

pores and their associated free surfaces, grain boundary junctions, and crystal lattice

defects are also important when their structural properties are concerned. m I R

spectra of the nano metal phosphates, aluminium phosphate (AIP04). copper

phosphate ( C U ~ P ~ O ~ ) , magnesium phosphate (MgzP207) and the nanocomposite

magnesium phosphate - iron phosphate (MgzP207 - Fes(P04)~) showed that due to

the small size of the grains and the large surface -to -volume ratio of nanocrystals,

the atomic atrangements on the boundaries differ greatly from that of bulk crystals,

showing some extent of disorder. This disorder in crystal symmetry results in the

shifting of IR active modes and in the evolution of new bands in all nanocrystalline

samples of the present study. The quantum confinement effect and small size effect

are responsible for the appearance of new IR active modes and the change in

positions of bands in the nanocomposite.

Thermal studies of all the samples were done through TG and DTA

methods. New phases and additional exothermic and endothermic peaks are found

in all samples, which were explained on the basis of grain growth process and their

microstructure and atomic configuration changes when they were subjected to high

temperature. Here the changes observed in the phase transition temperatures were

explained on the basis of the large amount of free energy associated with the grain

boundaries of nanoparticles.

3 ' ~ MAS NMR spectroscopy was used to study the structure of the

nanocrystalline forms of the metal phosphates and to characterize how the

incorporation of these different metals affects the chemical environment of the

phosphate tetrahedra. As NMR probes the structural units and their orientations

with respect to the external magnetic field, the relative orientation of the 3 ' ~

chemical shift of adjacent units are measured. At present NMR becomes

increasingly important hecause two-dimensional (2D) NMR technique enable more

detailed knowledge of the atomic structure. 2-D double quantum solid-state 3 ' ~

FTIR, Thermal and NMR Studies 87

MAS NMR measurements exploiting the dipole coupling between the phosphorus

atoms and the two-dimensional exchange using scalar coupling are the advanced

techniques for measuring the connectivities of phosphate tetrahedra in

nanocrystalline materials.

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