synthesis and microstructural characterization of...

Chapter 2

SYNTHESIS AND MICROSTRUCTURAL CHARACTERIZATION OF NANOSTRUCTURED MATERIALS

2.1. Introduction

Artificially synthesized nanostructured materials have structures in zero to

three dimensions. Nanostructured materials thus include zero dimensionality atom

clusters and cluster assemblies, one dimensionally modulated multilayers, and their

three-dimensional analogues, nanophase materials.' There are three key steps in the

development of nanoscience and nanotechnology: materials preparation, property

characterization, and device fabrication. Preparation of nanomaterials is being

advanced by numerous physical and chemical techniques. The purification and size

selection techniques developed can produce nanocrystals with well-defined

structure and morphology.2s3 Due to size and structure selectivity of nanomaterials,

the physical property of nanomaterials could be quiet diverse. It is known that the

properties of nanostructures depend strongly on their size and

The synthesis of nanostructured materials (NSMs) from atomic or molecular

sources depends on the control of a variety of "nanoscale" attributes desired in the

final product. The first technique involves the production of isolated, ultrafine

crystallites having uncontaminated free surfaces followed by a consolidation

process either at rooni temperature or at elevated temperatures.6 Nanocomposites

can be produced by depositing chemically different molecules simultaneously or

consecutively.' By introducing defects in a formerly perfect crystal such as

dislocations or grain boundaries, new classes of NSMs can be synthesized.

Different physical methods are currently in use for the synthesis and

commercial production of NSMs. The first and most widely used technique

involves the synthesis of single-phase metals and ceramic oxides by the inert-gas

evaporation technique. The evaporated atoms or molecules undergo a homogeneous

condensation to form atom clusters via collisions with gas atoms or molecules in

the vicinity of a cold - powder collection surface. The clusters once formed must be

removed from the region of deposition to prevent further aggregation and

Chapter 2 28

coalescence of the cluster^.^" Sputtering is another technique to produce NSMs as

well as a variety of thin films. This method involves the ejection of atoms or

clusters of designated materials by subjecting them to an accelerated and highly

focused beam of inert gas such as argon or helium. The third physical method

involves the generation of NSMs via severe mechanical deformation. In this

method, NSMs are produced by structural degradation of coarser-grained structure5

induced by the application of high mechanical energy. The nanometer-sized grains

nucleate within the shear bands of the deformed materials converting a coarser-

grained structure to an ultrafine powder. The heavy deformation of the coarser

material is effected by means of a high-energy ball mill or by a high-energy shear

stress process. 6-8

Chemistry has played a major role in developing new materials with novel

and technologically important properties. The advantage of chemical synthesis is its

versatility in designing and synthesizing new materials that can be refined in to the

final product. The primary advantage that the chemical process offer over other

methods is good chemical homogeneity, as chemical method offers mixing at

molecular leveLg A basic understanding of the principles of crystal chemistry,

thermodynamics, phase equilibrium, and reaction kinetics is important to take

advantage of the many benefits that chemical processing has to offer.

Solution chemistry is used sometimes to prepare the precursor, which i s

subsequently converted to the nanophase particles by nonliquid phase chemical

reactions. Precipitation of a solid from a solution is a common technique for the

synthesis of fine particles. The growth of the nuclei after formation usually

proceeds by diffusion, in which case concentration gradient and reaction

temperatures are very important in determining the growth rate of the particles. For

instance, to prepare unagglomorated particles with a very narrow size distribution,

all the nuclei must form at flearly the same time and subsequent growth must occur

without further nucleation Zr agglomeration of the particles.1° The particle size and

stze distribution, the physical properties such as crystallinity and crystal structure,

the degree af dispersion etc. ciul be affected by reaction In addition, the

concentratiorr of reaLtants, the reaction temperature, the pH and the order of

Synthesis and Microstrucrur~zl Characterization of Nanusrrucrured Mareriols 29

addition of reactants to the solution are also important.9-" Thus, chemical

homogeneity and stoichiomem/ requires a very careful control of reaction

conditions.

In this chapter, the synthesis of the nanocrystalline forms of aluminium

phosphate (AIP04), copper pyrophosphate (CuzP207). magnesium pyrophosphate

(MgzP207) and the nanocomposite magnesium pyrophosphate-iron phosphate

(Mg2P~07 - Fe3(P04)2) ancl their characterization details are presented. Each sample

is prepared in three different grain sizes by varying the molar concentration of the

reactants. The sintered samples are subjected to X-ray diffraction analysis, through

which the crystal structures, grain sizes and the changes in lattice parameters are

analyzed. The phase changes with temperature of nano Alp04 sample are studied

through transmission electron micrographs and selected area diffraction patterns.

2.2. Synthesis of Nanocrystalline Metal Phosphates

For the present study, nanocrystalline powders of three different metal

phosphates; namely a1u:niniunl phosphate (AlP04). copper pyrophosphate

(Cu2P207) & magnesium pyrophosphate (MgzPz07) were prepared in three different

grain sizes, by changing the molar concentration of the reactants. The

nanocomposite Mg2P207 - Fe3(P04)2 for different iron phosphate concentrations

were also prepared. All the samples were synthesized from a polymer matrix based

precursor solution through a versatile, efficient and technologically simple method.

A detailed description of the synthesis route is given in the literature.'' Analytical

grade (AR) chemicals of high purity were used for the synthesis of all samples in

the present study.

To produce nanociystalline aluminium phosphate (n-AIP04), aluminium

nitrate [A1(N03)2.9Hz0] (I moVlL HzO) is mixed with ammonium dihydrogen

phosphate [NH4(HzP04)] ( I moV 500 ml H20) and then with dilute HNO,, such that

pH of the solution = 1. To this mixture 3 moles of sucrose and 10% wlv aqueous

solution of PVA (0.05 mol:~ are added, so that the total volume of the solution after

mixing is 3.0 L. The solution is well stirred and then evaporated to a viscous liquid

with the evolution of brown fumes of the decomposed nitrates. The PVA is acting

as the matrix and sucrose as the fuel. The fluffy, voluminous, carbonaceous,

Chapter 2 30

pyrolysed mass produced during the complete evaporation of the precursor solution

of metal nitrate-PVA-sucrose is then thermolysed from room temperature to 900 'C

to get the nanosized powder. The resulting powder is then calcinated at 900 "C for 4

hours to obtain the crystalline phase of the required sample.lz Samples with

different grain sizes are prepared by changing the molar concentration of the

reactants." Molar concentrations of 1.0 mol~ . ' , 0.5 m o l ~ . ' and 0.1 m o l ~ . ' are

selected for the present study. The overall reaction can be represented as:

Aluminium phosphate (AIP04) thus formed becomes more crystalline and

nanosized under further heat treatments in presence of sucrose and PVA. The

ammonium nitrate (NH4 N03) decomposes to NH3 and NO2

Nanocrystalline copper pyrophosphate (n-CuzP207) and magnesium

pyrophosphate (n-Mg2P207:l are also prepared in a similar manner. Copper nitrate

[Cu(N03)~.3HzO] and magnesium nitrate [Mg(N03)~.6H201 are respectively used

instead of aluminium nitrate. Copper phosphate samples were calcinated at 600 OC

for 4 hours and magnesium phosphate samples were calcinated at 900 OC for 4

hours to get the required crystalline phase. The corresponding reactions are:

Copper phosphate I[Cu3(P04)2] and magnesium phosphate [Mg3(P04)2]

formed here transform to copper pyrophosphate (CUZPZO~) and magnesium1

pyrophosphate (Mg2P207) above 160°C. Ammonium nitrate (NHdNO3) decomposes

to NH3 and NOz.

The nanocomposite MgzP207 -Fe3(P04)2 with different weight percent of

iron phosphate (= 5%, lo%, & 15%) are prepared, for the present study, by mixing

freshly prepared n-Mg2P207 and n-Fe3(P04)z in an agate mortar and grounded well.

The method of synthesis of n-Fe3(P04)z is exactly similar t~ that descnbed above

for the other samples. The prepared samples were tnen calcinated at 900°C for 4

hours to obtain the desired clystalline phase.

Synthesis and Microstructural Characterization of Nanosrructured Materials 3 1

2.3. Consolidation of Nanoparticle Samples

In the fabrication of nanostmctured materials, full densification of the

powder is to be achieved while simultaneously retaining the nanoscale

microstructure. To reduce grain growth during sintering, a high-density

homogeneous sample with minimum pore size is required.I4 Thus, prior to sintering

a sample composed of nanosize powders, it is essential to compact the powder at

high pressures. For the present case, the nanopowder samples were consolidated in

the form of cylindrical pellets of 13 mm diameter and thickness about 1-2 rnm. For

micro hardness ~tudies, '~.~' a uniaxial force ranging 3-6 tons is applied for 2

minutes using a hand operated hydraulic press. For dielectric studies the uniaxial

force applied was 4 ton:$ for all the samples. Conducting silver paste was coated on

both faces of the pellets to serve as electrodes. The pellets were air heated at 80°C

for 30 minutes in a hot air oven for electrode c ~ r i n ~ . ' ~ . ~ ~

2.4. X-ray Diffraction (XRD) Analysis of Nanocrystalline Materials

Scattering and imaging techniques are usually employed to determine the

nature of nanoparticle crystal s t ~ c t u r e . ~ ' The versatile and most widely used

technique is X - ray diffraction and scattering. X -ray diffraction method is used as

a tool for the determination of the crystal structure, particle size and changes in the 22-24 lattice parameters of all the samples in the present investigation. When

clystallites are of nanometer size appreciable broadening in the X-ray diffraction

lines will occur. The observecl line broadening can be used to estimate the average

size of the particles. The diffraction patterns were recorded using a Philips

Analytical PW1710 BASED diffractometer. The X -ray generator was operated at

35 KV and at 20 mA. Copper K, (1 = 1.5418 A) radiation was used as source. The

'd' values obtained from XRD were compared with standard 'd' values obtained

from the Internarional Center for Drffraction Data - Powder Dljfraction Files

(ICDD - P D e . The expansion or contraction of the lattice due to reduction in the

physical dimension of all the crystalline samples is systematically analysed. The

particle sizes were determined through the line broadening of the diffraction lines

using Scherrer formula.2s

Chapter 2 - 32

X-ray diffraction techniques are widely accepted for the characterization of

small particles. Broadness and shape of the measured diffraction lines are

characteristic of the crystallite size of the sample. Very small crystallites can be

considered to be domains, which are diffracting incoherently with respect to one

another, resulting in line broadening.

Reduction in the physical dimension of a crystalline solid leads to changes

in the lattice parameters. The expansion or contraction of the lattice in nanocrystals

with reducing particle size depends primarily on the nature of the interatomic bonds'

(ionic, covalent or metallic).26 It is reported2' that the unit cell should contract with

decreasing particle size in ionic crystals. In covalent and metallic crystals, on the

other hand, a decrease in the particle size is accompanied by an increase in the

lattice parameters, leading to an enhancement in the unit cell volume. For cubic

lattices, a size dependent intra-crystalline pressure is observed due to the

electrostatic interaction between the atoms of the lattice. This interaction energy is

positive in certain crystals that contain ions with opposite charges in the atomic

planes parallel to the crystal axes. The interaction energy is negative in certain other

crystals due to the presence of Ions of like charges in atomic planes.

In covalent cubic crystals, there is a small expansion in the lattice parameter

with decreasing size. This is explained using a point charge modelz8, which predicts

that the magnitude of the size effect should be directly proportional to the square of

the excess negative charge localized on bond lines between neighboring atoms. The

ions in the outermost layer are incompletely coordinated and posses unpaired

electronic orbitals. Each of these dangling bonds forms an electric dipole, resulting

in a parallel array of dipoles originating in the boundary layer of each nanoparticle.

The repulsive inter-dipolar forces thus formed would tend to increase the

equilibrium values of lattice constants. The lattice expansion is also due to some

other reasons such as capping layers formation at the surface29 due to chemical

impurities, such as C, N, and 0 that penetrate into the surface and expand the

spacing between the atomic layers

Synthesis and Microsrructural' Characterization of Nanosrructured Materials 33

In nano crystalline materials, the surface atoms amount to 40.50% of the

total atoms depending on the geometry. Every atom on the particle surface

contributes half of its surface area to the whole surface area of nanoparticle. As a

result, the surface energy of nanoparticles decreases which will tend to contract

their sizes by distorting their crystal lattice elastically. The best description of

nanoparticle structure includes bond length contraction and random disorder3'

characterized by correlated. atomic displacements. Bond length contraction makes

the nanoparticles much stiffer than expected. It is well known that the lattice

constant of small particles is different from the bulk, owing to the existence of

surface or interface stress.31 Obviously; the variation of lattice constants will result

in a change of the electron density in the particles. Since, changes and disorder in ---- the lattice p w s will separately modify electronic properties, inclusion of

3 -

these effects is essential for accurate nanoparticle calculations. - ---

2.5. Results and Discussion

2.5 (i) XRD Analysis of Nanostructured Alp04

Nano-sized AIP04 (n-AIP04) samples of different grain sizes were prepared

and the effects of thermal treatment on these powders were studied. Fig.2.1 presents

the XRD spectra of 'as prepared sample' and samples with different heat treatment

conditions. An examination of these spectra leads to the conclusion that the 'as

prepared sample' and samples at low heat treatment conditions are amorphous in

nature as shown in Fig.2.1 (a, b, c). The crystalline nature gradually increases as the

, annealing temperature and annealing time are increased. The broad bands 7 - corresponding to the annealing temperature at 9 0 0 ' ~ for 5 hours showiin Fig.2.l(d)

clearly indicate the crystalline nature of the sample at higher annealing conditions.

The mean crystallite sizes of the samples were determined using Schemer formula.

It is estimated that the average crystallite size of the sample is 16.34 nm. For the

present study nano-sized samples of Alp04 with average grain sizes of 16.34 nm, P.

13.59 nm and 12.04 nm, were prepared, by changing the concentration of the

reactants3' to 1.0 mol~- ' , 0.5 mol1;' and 0.1 mol~. ' (hereafter referred to as samples 1 Al , A2 & A3) respectively. Fig.2.2 shows the XRD pattern of all the three samples ---- I

1 of AIP04 prepared with different grain sizes. All the samples are of cubic structure.

r . Diffraction angle

* R D WWY AIP04 sample A1 under different heat treatment sample' (b) sample sintered at 700 'C for 4 hrs (c)

sample sintered at 900 OC' for 2 hrs (d) sample sintered at 900 'C for 4 hrs.

The '8 values obtained from XRD are compared with the standard ICDD-PDF)~

data of cubic Alp04 The results are tabulated in table 2.1, table 2.2. and table 2.3.

The PDF analysis of nano Alp04 samples indicates more and more bond

length compression as particle size decreases. The unit cell contraction with

reduction iri particle size is characteristic of ionic crystals. For an ideal crystal

lattick, the lattice parameter contraction is proportional to the diameter a of

nanoparticle, Lien

Synthesis and Microstructural Characterization of Nanosfrucfured Materials 35

Diffraction angle 2 ,? 5:

Fig.2.2 XRp spectraf nanocrystalline Alp04 samples (a) sample A 1 (b) sample A2 and (c) sample A3; sintered at 900 'C for 4 hrs.

Table 2.1 XRD pattern of nanocrystalline Alp04 sample Al

Table 2.2 XRD pattern of nanocrystalline Alp04 sample A2

of m a t t l ~ / d values A d values A contraction

Table 2.3 XRD pattern of nanocrystalline Alp04 sample A3

where Aa and a are the lattice parameter of nanoparticle and the lattice parameter

of bulk material respectively. K =xG/ (2y0) , where yo is the surface energy per unit

area at room temperature.34 This equation is the basic relation to account for size

effect on nanoparticles due to the variation of lattice constants. 1 f ,

It can be seen from tahle 2.1 that the mean compression for 'd' values of --- sample A1 (size16 nm) is 1.1%. For sample A2 (size13 nm), the shift in the 'd'

values shows a contraction of approximately 3% and for sample A3 (size12 nm), i t

is 4.5 %, as given in table 2.2 and table 2.3. There are several features of the

nan~~art ic les '~ that may be responsible for the lattice contraction as seen above

Since a reasonable fractim af wirticles are on the surface, nano ALP04 sample tnes 7 -- -

to minimize its surface energy and tends to contract. As particle size decreases the

Synthesis and Microstructural Characterization of Nanostructured Materials 37

percentage of surface atoms increases, which explains the increase in lattice

compression as particle size decreases. Opposite charges located in atomic planes

parallel to the crystal axes of nano Alp04 samples give a positive electrostatic

interaction between the elements of the lattice. The intra-crystalline pressure36 thus

produced is also responsible for the contraction of lattice parameters observed in

nanocrystalline Alp04 samples.

2.5 (ii) XRD Analysis of'Nanocrystalline CuzPz07

The XRD pattern of nanocrystalline copper pyrophosphate (Cu2P207)

prepared with reactant concentration 1.0 molL-' (sample C1) is shown in Fig.2.3.

Examining the observed 'd' values with the standard ICDD-PDF~~ values show that

the crystal structures of the particles are monoclinic. The average grain size of the

particles is estimated using Schemer's equation and is found to be 34.46 nm. The

results are tabulated in table 2.4. The XRD patterns of the other two samples of

nanocrystalline copper pyrophosphate (Cu2P207) prepared by changing the reactant

concentrations to 0.5 molL-' and 0.1 m o l ~ . ' (samples C2 &C3) along with sample

C1 are shown in Fig.2.58.The observed results are tabulated in table 2.5. and table

2.6. The crystal structures of all the samples are similar; i.e. monoclinic. The mean

grain sizes of the Cu~Pz07 samples prepared with reactant concentrations 0.5

mo l~ . ' and 0.1 mol~ . ' are 32.26 nm and 27 nm respectively.

X - ray analysis of nano CuzPz07 samples indicates slight expansion of bond

length as particle size decreases. It is found from table 2.4 that the mean bond - length expansion for sample C1 of size 34 nm is 0.4 %. As grain size of the sample

decreases, the degree of lattice expansion increases. For sample C2 (size32 nm), the ?

--- percentage of lattice expansion is about 0.45 and for sample C3 of size27 nm, the -- -- corresponding variation is 0.5 %. The results are tabulated in table 2.5 and table2.6.

Quantitative X-ray diffraction measurements on the Cu~P207 samples

revealed that the peak. positions of the X-ray diffraction lines of Cu~P207 I nanocrystallites are higher than those of the corresponding bulk CuzP207 crystal and

7--

that the deviation increases with a decrease in grain size. Further investigations

showed that the lattice parameter of the Cu2P207 nanocrystallite is significantly

increased by refining the grain size. The above results were qualitatively interpreted 1

_C_-

in terms ot the thermodynanuc theory, which suggests that a supersaturation of

Diffraction angle

Fig.2.3. XRD spectra of sample C1 sintered at 6 0 0 ' ~ for 4 hrs -- Observed ICDD-PDF % of lattice ( h k I ) 1 2e 1 d values b / d values b / contraction I plane 1

Table 2.4. XRD pattern of nanocrystalline Cu2P207 sample ('1


X * .- 2 Y c -

I 60

I 50

I 40

I 30

I 20

Diffraction angle - d e ?'

Fig.2.4. XRD spectraof nanocrystalline C U ~ P Z O ~ samples (a) sample C1 sintered at 600 'C fo;2 hrs (b) sample C1 (c) sample C2 and (d) sample C3; sintered at 600 'C

for 4 hrs

Chapter 2 40

) , B p G G d J I C D D - P D r V l d values A, d values A contraction

Table 2.5. XRD pattern of nanocrystalline Cu2Pz07 sample C2

Obsewed I ICDD-PDF % of lattice ( h k 1 ) I 2B I d values A c~ vaiues A I contraction I plane I

Table 2.6. XRD pattern of nanocrystalline Cu2P207 sample C3

Synthesis and Microstructural Characterization qf Nanostructured Materials 41

vacancies in the lattice structure of a nm-grained system is obtained as the grain

size is decreased down to a few nanometres. 38

Lattice variations and bonding characteristics in Ti02 nanocrystals were

examined39 by x-ray diffraction spectroscopy. With a reduction in the physical

dimensions, Ti02 nanocrystals show a linear lattice expansion and an anomalous

covalency enhancement. A surface defect dipole model is proposed to explain these

physical phenomena in terms of the strong interactions among the surface dipoles

that produce an increased negative pressure. The covalency enhancement is

interpreted according to the critical properties of the increased Ti-0 bond lengths in

the expanded lattice.

The observation of lattice expansion of Cu2P207 nanocrystallite is in perfect

agreement with this theory. The broadening of the strong P-0 and the weak Cu-0

bands of nanocrystalline CuzPz07 samples are observed by FTIR analysis as

described in section 3.2 (ii). In covalent crystals, as explained in section 2.5, unit

cell volume increases as particle size decreases, which is accompanied by an

increase in the lattice parameters.28 Nanocrystalline Cu2P207 showing a similar

character may thus be considered as a covalent crystal. The ions in the outermost

layer of covalent crystals are incompletely coordinated and these dangling bonds

form an electric dipole of the same character in the boundary layer of each

nanoparticle. The repulsive inter-dipolar forces thus formed increases the

equilibrium values of lattice constants. Capping layers formation at the surface due

to chemical impuritiesz9 is also responsible for lattice expansion in nano CuzP207,

2.5 (iii) XRD analysis of nanocrystalline MgzPzO,

The XRD pattern of nanocrystalline magnesium pyrophosphate (MgzPz07)

prepared with reactant concentration 1.0 mol~. ' (sample MI) is shown in Fig.2.5.

The observed 'd' values are compared with the standard ICDD-PDF values4' and

the crystal structures of the particles are found to be monoclinic. Using Scherrer

formula the average grain size of the particles is estimated and is found to be 25.37

nm. The results are tabulated in table 2.7. The XRD patterns of the other two

samples of nanocrystalline magnesium pyrophosphate (MgzP207) prepared by

changing the reactant concentrations to 0.5 mol~ . ' and 0.1 mol~. ' (samples M2 &

Chapter 2 42

M3) along with sample MI are shown in Fig.2.6. The observed results are tabulated

In table 2.8. and in table 2.9. The crystal structures of all the samples are similar;

i.e. monoclinic. The average grain sizes of the MgzPz07 samples prepared with

reactant concentrations 0.5 mol~. ' and 0.1 rnol~.' are 22.22 nm and 20.9 nm

respectively.

Detailed X-ray diffraction studies of nano MgzPz07 samples (Fig. 2.6) show

both expansion and contraction of bond lengths for all samples. From table 2.7, for

the sample M1 with grain size 25 nm, it is found that lattice contraction and

expansion are simultaneously occurred, but with an average lattice expansion of 0.5

%. For sample M2 of size 22 nm, the mean lattice expansion increases to 1.5 %. For

sample M3 (size 21 nm), the lattice expansion is about 5 %. The observed and

calculated values are tabulated in table 2.8 and table 2.9.

60 50 40 30

Diffraction angle

Fig.2.5. XKD spectrun: of nanoc~stallineMg2Pfi sample M1 sintered at 900 C for 4 hrs


Table 2.7. XRD pattern of nanocrystalline Mg2P207 sample M1

50 40 30

Diffraction angle

Fig.2.6. XRD spectra 0f.n- MgzPz07 samples (a) sample MI sintered at 900 'C for 2 hrs (b) sample M1 (c) sample M2 and (d) sample M3; sintered at 9 0 0 ~ ~ for 4 hrs

As in the case of nano Cu2P207 samples, the positions of the X-ray peaks of

Mg2P207 nanocrystallites are shifted with respect to their bulk crystal and the mean

deviation increases with a decrease in grain size. The increased negative pressure,

created by the strong interactions among the surface defect dipoles enhances the

covalency,28 which change the bond lengths in the expanded lattice. Lattice


!6:8i 1.541 1 :z 1 -1.248 1 1 3 3 1 1.522 -2.148 -5 2 3

62.3 1.489 1.476 -0.881 2 4 2

Table.2.8. XRI) pattern of nanocrystalline MgzPz07 sample M2

, I

20

2 1

22.6

23

25

27.5

29.6

31

33.6 - 35

36.6

Observed d values A

4.229

3.933

3.86:s

3.56

3.242

3.01'7

2.883

2.666

2.562

2.454

ICDD-PDF d values A

4.149

3.86

3.492

3.181

3.126

3.008

2.929

2.795

2.62

2.416

% of lattice contraction

-1.928

-1.891

-10.682

-11.914

-3.71 1

-0.299

1.571

4.615

2.214

-1.573

( h k 1 ) plane

0 2 0

1 1 1

0 2 1

-2 0 2

0 1 2

-30 1

2 2 0

-3 1 1

3 1 0

-2 2 2

Table.2.9. XRD pattern of nanocrystalline Mg2P207 sample M3

28

2 1

22.5

22.7

25

25.3

27.5

28.7

29.9

32.5

33.9

35.5

% of lattice contraction

- 1.928

-2.332

-12.142

-11.914

-12.572

-7.779

-6.145

-6.869 -- -5.115

-9.396

-8.223

( h k I ) plane

0 2 0

1 1 1

0 2 1

-2 0 2

0 1 2

-301

2 2 0

-3 1 1

3 1 0

-2 2 2

-3 2 1

Observed d values A

4.229

3.95

3.916

3.56

3.519

3.242

3.109

2.987

2.754

2.643

2.527

ICDD-PDF d values A

4.149

3.86

3.492

3.181

3.126

3.008

2.929

2.795

2.62

2.416

2.335


expansions have been measured in monodisperse Ce02 nanoparticles4' and in

BaTiO, single nanoparticles4' by electron diffraction. X-ray photoelectron

spectroscopy studies on Ce02 nanoparticles and on BaTi03 clusters show that the

origin of lattice expansion is the decrease of electrostatic force caused by valence

reduction of Ce ions and the increase in ionicity of Ti ions, respectively.

Thus the lattice parameter change of ionic nanoparticles with the increase in

ionicity would depend on the structure of the particles. In fact, a shift of the lattice

parameters with particle size variation may result from several different effects,30

such as changes of optical interband transitions, changes of electronic band

structure, changes of the effective mass of the conduction electrons etc.

Accordingly both contraction and expansion of the lattice parameters are possible in

the same crystal. It is evident that the relative importance of these effects for the

change of lattice parameters is different, depending on the experimental conditions

and materials.

The observation of the simultaneous expansion and contraction of the lattice

parameters in Mg2P207 nanocrystallites is in accordance with this argument. Due to

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

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

showing some extent of disorder. This disorder in crystal symmetry leads to the

broadening and shifting of many bands in the crystal The details of this observation

are given in the FTIR analysis of nano MgzP207 in section 3.2 (iii) of this thesis.

Since the resulting effect is bond length expansion, it may be assumed that

MgzP207 nanocrystallites contain ions with like charges4' in the atomic planes

parallel to the crystal axes. The negative intra-crystalline pressure thus produced

due to the electrostatic interaction between the elements of the lattice is responsible

for the expansion of lattice parameters. Bond length expansion may also be due to

the formation of capping layersz9 at the surface due to chemical impurities.

2.5 (iv) XRD Analysis of Nanocomposite Mg2P207- Fe3 (P04)2

The XRD pattern of nanocrystalline composite MgzP207 - Fe3 (PO& prepared

with Fe3(P04)2 content 5%. 10% and 15%, (samples S1, S2, & S3); all with

Chapter 2 48

Diffraction angle

Fig.2.7. X p spectra of nanocrystalline Mg~P207-Fe3 (PO& composite samples (a) sample S1 (b) sample S2 and (c) sample S3; sintered at 900 'C for 4 hrs

reactant concentrations 1.0 m o l ~ " are shown in Fig.2.7. The results are tabulated in

table 2.10, table 2.1 1 and table 2.12.

The observed 'd' values are compared with the standard ICDD-PDF values

of magnesium pyrophosphate Mg2P207 40 and also with iron phosphate Fe3(PO& 43

Using Scherrer formula the average grain sizes of the particles in the prepared

composites samples S1, S2 & S3 are estimated and are found to be 36.01 nm, 36.87

Synthesis and Microstrucrurul Characterization o f Nanostructured Materials 49

Table.2.10. XRD pattern of nanocrystalline MgzP2O-r-Fe3 (PO& sample S1

Table.2.11. XRD pattern of nanocrystalline Mg:P207-Fe3 (Pod2 sample S2


Observed d Values A d values A

Table.2.12. XRD pattern of nanocrystalline Mg~P207-Fe3 (PO& sample S3

Chapter 2 P

52

nm and 39.92 nm respectively. It is thus evident that as the weight percentage of

Fe3(P04)2 in the prepared nanocomposite MgzP~O~-Fe3(P04)2 increases, the grain ' / . . slze also increases.

A random displacement can be used to describe the substitution of

nano Fe3(P04)2 into the host nano Mg~P207. This allows the use of an empirical law

that relates the statistical substitution of a guest lattice into the host lattice with the

experimentally observed degree of lattice change with increasing defect lattice

concentration. Statistical substitution into a lattice site is predicted to lead to a

lattice contraction for smaller ions and a lattice expansion for larger ions." The

shift in the lattice parameter in table 2.10 shows a lattice contraction of

approximately 0.1% as the concentration of the doped material increases to 5 %

(sample Sl). The observation of a linear contraction with increasing Fe,(P04)2

concentration suggests a displacement occurs at the lattice sites through a purely

statistical process."The mean lattice contraction in the XRD data increases to 0.15

% for sample S2 with increasing Fe3(P04)z content to 10 % , as given in table 2.1 1.

The lattice contraction for sample S3, with gust lattice concentration 15%, is about

0.5 % as observed from table 2.12.

For isolated particles, the lattice contraction plays an important role in the

size evolution due to their weak free path effect and strong surface stress.44 The

statistical substitution of a guest lattice into the host lattice will enhance the degree

of lattice change with the increase of defect ion concentration. It is to be noted that

the mean lattice contraction of each sample in the present investigation is extremely

small comparing with the values of their bulk counterparts. This may be due to the

fact that the lattice contraction produced in smaller ions is opposed by the lattice

expansion of larger ions as suggested by the random clisp!acement model." The

PDF analysis of nanocrystalline Mg2P20, with reactant concentrations 1.0 mo l~ . '

gives a mean lattice expansion as observed in section 2.5 (iii). Nanocrystalline

Fe3(P04)2 has a mean lattice c~ntract ion.~~ The combined effect of these two

natures is appearing as a resultant contraction when botk the samples are mixed to

form the present nanocomposite. Though both type of icr:s. hhvlng like charges and

unlike charges, are present in atomic planes parallel 1.2 the crysta! axes, there is a


net positive interaction energy due to the resultant intra-crystalline pressure, which

gives the net contraction of the lattice parameters of the present nanocomposite.

2.6. TEM Analysis of Nanocrystalline Materials

Transmission electron microscopy (TEM) is a unique method to get direct

imaging of nanoparticles, although some stmctural features can be raveled by X-ray

and neutron diffraction. TEM can provide a real space image on the atom

distribution in the nanocrystal and on its surface. It can also be used to get atomic

resolution lattice images and chemical information at a spatial resolution of 1 nm or

better, allowing direct identification of the chemistry of a single nanocrystal. With a

finely focused electron probe, the structural characteristics of a single nanoparticle

can be fully characterized.

Transmission electron microscopy allows imaging of individual crystallites

and the development of a statistical description of the size and shape of the particles

in a nanocrystalline sample.30 High magnification imaging with lattice contrast

allows the determination of individual crystallite morphology. It is found that the

morphology of the particles do not change with the change in the molar

concentration of the reactants during synthesis. The overall degree of aggregation is

better revealed by TEM image3', while the XRD image provides a better resolution

on determining the grain sizes. ' E M observations show the presence of

nanoparticles dispersed homogeneously within the matrix. In agreement with the

XRD spectra, dark field observations indicate the amorphous character of the

majority of the nanoparticles. It is possible to find particles that give rise to 3 ---- - - - . -

diffraction rings through which they could be identified. The selected area - . ~ - - -

.-

diffraction (SAD) show a series of rings ascribed to the main reflections

of the corresponding nanocrystalline phase.

Three types of contrast46 usually dominate images in TEM. First diffraction

contrast, which is produced due to distortion in the o~ientation of the crystal so that

the diffracted intensity of the incident electron beam is perturbed, leading to

contrast observed in bright field image. For nanocrystals, most of the grains are

Uefect-free in volume, while high densities of defects are localized at surfaces and 7

grain boundaries.Diffraction contrast is useful for capturing strain distribution in

Chapter 2 54

nanocrystals. Secondly, phase contrast, which is produced by the phase modulation

of the incident electron wave when transmits through the crystal potential. This type

of contrast is sensitive to the atom distribution in the specimen and it is the basis of

high resolution TEM. Finally, mass-thickness or atomic number produced contrast.

Atoms with different atomic numbers exhibit different powers of scattering and the

image contrast is sensitive to the average atomic number along the beam direction.

Nanocrystals exhibiting distinctly different properties from the bulk are mainly

due to their large portions of surface atoms and the size effect. The high-resolution

capability of TEM and the complimentary applications of the techniques make it a

powerful tool in identifying and quantifying the chemical and electronic structure of

nanomaterials. High-spatial resolution analysis is vitally important for solving many

of the practical problems of nanomaterials. Spectroscopy analysis of the solid-state

effects46 is a new direction of quantitative microscopy. The exploration of the fine

structure in TEM is likely to reveal rich information about bonding in crystals and

at interfaces. Also TEM is important for characterizing and measuring the

properties of individual nanostructures, from which the structure-property

relationship of nanostructures can be clearly registered.

At high temperatures many nanophase ceramic materials undergo phase

transitions4' as they convert from metastable crystal structures to the stable

thermodynamic phase. These transitions are of importance as their occurrence can

lead to large volume changes, resulting in cracks or defects in dense sintered

ceramics. In smaller particles the greater surface area can have drastic effects on the

thermodynamic stability as well as the kinetics of phase transformations. 48-50 If

such effects are known, it is possible to fabricate nanocrystalline samples, which

adopt the stable structure before sintering. The temperature behaviour of

nanocrystalline Alp04 is studied in this section using transmission electron

microscopy.

2.60) TEM analysis of nanocrystalline Alp04 \

The TEM studies of nanocrystalline Alp04 sintered from mom temperature

up to W C is described in this part. The transmission electron micrographs (TEM)

Synthesis and Microstructural Characterization of Nanostructured Materials 5 5

Fig.2.8.(b): TEM micrograph of nano AlP04 sample at 450 "C

Scale: 15mm= 1 pn

Fig.2.8.(a): SAD pattern of ' nano A P 0 4 sample at 450 OC

Fig.2.9.(a): SAD pattern of nano AlP04 sample at 750 "C

Fig.2.9.(b):TEM micrograph of nano AlP04 sample at 750 "C

Scale: 15mm=500 nm

Chapter 2 56

and their corresponding selected area diffraction (:SAD) patterns of nanocrystalline

Alp04 sintered at two different typical temperatures (450°C and 750°C) are shown

in Fig.2.8. and Fig.2.9. The TEM images were taken using the JOEL 200CX

electron microscope operated at an acceleration voltage of 160 kV. For TEM

studies, the sample powder was dispersed in methanol using an ultrasonic bath and

a drop of suspension was placed on a copper grid coated with holey carbon fi ~ r n . ' ~

The TEM bright-field images in Fig.2.8 and Fig.2.9 provide good reviews of the

sample surfaces. When this sample is subjected to heat treatments in a furnace it

clearly transform from the low temperature phospho-berlinite to high temperature

phospho-tridymite. In this case the berlinite phase is clearly seen at temperatures as

low as 450°C while at 750"C, it transforms to the stable phospho-tridymite phase.

Such data is in good agreement with numerous thermal studies of Alp04 nanophase

ceramics. These phase transitions are studied in detail using TG and DTA methods

in section 3.3(i) of this thesis. In addition, the peak widths are observed to decrease

sharply when the transition to tridymite occurs. 'This indicates that significant grain

growth occurs during the phase transformation. Above 450°C a number of changes

are observed. The most striking of these is the increased fluctuation in the shape of

the particles, significantly during sintering. Strong selected area electron diffraction

is observed from these nanocrystals, which appear bright in dark field illumination

indicating that a large fraction of the material is crystalline. The micrographs show

an aggregation appeared in part of the particles. This observation is also consistent --- with the fact that average nanoparticle sizes of the present sample increase between A

temperatures 400°C and 900°C with final sizes as large as 50 nm. In addition, the

sample shows new rings and spots in selected area electron diffraction patterns at

750°C, matching with other observations of this crystalline product. The

diffractogram of high-resolution image of the heated nanocrystal shows spots that I

correspond to the d-spacings of the crystal. Extensive examinations show that the

common crystallite defects, stacking faults, twin planes etc. are minimum in the

present nanocrystalline sample.


2.7. Conclusion

Three different nanocrystalline metal phosphates; namely, aluminium

phosphate (AlP04). copper pyrophosphate (CuzPz07) and magnesium

pyrophosphate (Mg2PZ07), in three different reactant concentrations (1.0 molL ',0.5

m o l ~ . ' & 0.1 mol~. ' ) were synthesized, by following a versatile, efficient and

simple chemical route. The nanocrystalline composite Mg2P207 - Fe3 (P04)2 was

also prepared in a similar manner by changing the content of iron phosphate in three

different weight percent. The grain size and crystal structure of all the prepared

samples were analyzed, as a function of sintering temperature and time, using X-ray

diffraction and transmission electron microscopy techniques. The XRD and TEM

observations are shown in figures. The results are tabulated. It is observed that as

the reactant concentration decreases, the size of the grains also decreases. The mean

crystallite sizes of the Alp04 samples were estimated to be16 nm, 13.5 nm and 12

nm, for reactant concentrations 1.0 m o l ~ ~ ' , 0.5 m o l ~ . ' & 0.1 m o l ~ . ' respectively.

All the samples are of cubic structure. The average grain sizes of the Cu2P207

samples prepared with reactant concentrations 1.0 mol~. ' , 0.5 m o l ~ ~ ' and 0.1 molL-

' are 34.5 nm, 32 nm and 27 nm respectively. The crystal structure of all the

samples in this case was found to be monoclinic. For the Mg2P207 samples, the

average grain sizes were estimated to be 25nm, 22 nm and 21 nm respectively. In

the case of the nanocomposite, the grain size was found to increase as the iron

phosphate content in the composite increases. The average grain sizes of the

different composite samples Mg~P207 - Fe3(P04)2 with different Fe3 (PO& weight

percent (= 5%, 10% and 15%) prepared with reactant concentrationl.0 mol~ . ' were

found to be 36 nm , 37 nm and 40 nm respectively. The well-defined peaks in the

XRD spectrum of each sample clearly indicate the crystalline nature at higher

sintering conditions.

The quantitative determination of nanoparticle crystallinity and disorder,

using a PDF-based method is adopted in this work. It is found that the best % description of nanoparticle structure includes bond length contraction and

expansion, random disorder, and a type of disorder characterized by correlated

atomic displacements. The PDF analysis of nano AIP04 samples indicates more and

Chapter 2 58

more bond length compression as particle size decreases, which is characteristic of

ionic crystals. In covalent crystals, unit cell volume increases as particle size

decreases, which is accompanied by an increase in the lattice parameters.

Nanoclystalline Cu2P207 is showing an exactly similar character. The observation

of the simultaneous expansion and contraction of the lattice parameters in Mg2P207

nanocrystallites is due to several different effects, such as changes of optical

interband transitions, changes of electronic band structure, changes of the effective

mass of the conduction electrons etc. which are the features of ionic crystals. The

changes in the lattice parameters of MgZP207-Fe3(P04)2 nanocomposite is explained

using a random displacement model. From the PDF analysis, it is observed that the

sample has a mean lattice contraction.

The TEM and SAD patterns are used to study the phase changes with

temperature of n- AlP04 samples. The TEM bright-field images show that the

berlinite phase at 450°C, transforms to the stable phospho-tridymite phase at 750°C,

consistent with the TG-DTA analysis. The rings and spots in selected area electron

diffraction patterns indicate that a large fraction of the material is crystalline.

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