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.2s3Due 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

28

Chapter 2

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 coarsergrained 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~zlCharacterization 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 m o l ~ . ' , 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)~.6H201are 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-Mg2P207and 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

31

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 a uniaxial force ranging 3-6 tons is applied for 2 micro hardness ~tudies,'~.~' 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 lattice parameters22-24 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 surface29due 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.31Obviously; 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

3

s will separately modify electronic properties, inclusion of

-

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

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 m o l ~ - ' 0.5 , mol1;' and 0.1 mol~.' (hereafter referred to as samples A l , A2 & A3) respectively. Fig.2.2 shows the XRD pattern of all the three samples

----

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

1 I 1

7

r.

Diffraction angle

* R D WWY sample sintered at 900

OC'

AIP04 sample A1 under different heat treatment sample' (b) sample sintered at 700 'C for 4 hrs (c) for 2 hrs (d) sample sintered at 900 'C for 4 hrs.

The '8values 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

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

35

Table 2.2 XRD pattern of nanocrystalline Alp04 sample A2

d values A

d values A

of m a contraction

t t l ~ /

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 = x G / ( 2 y 0 ) ,where yo is the surface energy per unit area at room temperature.34This 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 n a n ~ ~ a r t i c l e that s ' ~ 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 pressure36thus 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~Pz07samples prepared with reactant concentrations 0.5 m o l ~ . and ' 0.1 m o l ~ . '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~P207samples 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 _C_-

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

1

Diffraction angle

--

Fig.2.3. XRD spectra of sample C1 sintered at 6 0 0 ' ~for 4 hrs

1

2e

1

Observed d values b

/

ICDD-PDF d values b

/

% of lattice

contraction

I

(h

kI) plane

Table 2.4. XRD pattern of nanocrystalline Cu2P207sample ('1

1

39


X .-*

-2 Y c

I

60

I

50

Diffraction angle '?

I

I

40

-

30

I

20

de

Fig.2.4. XRD spectraof nanocrystalline C U ~ P Zsamples O~ (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

40

Chapter 2

),BpGGdJICDD-PDrVl d values A

d values A,

contraction

Table 2.5. XRD pattern of nanocrystalline Cu2Pz07 sample C2

I

2B

I

dObsewed values A

I

ICDD-PDF c~ vaiues A

I

% of lattice

contraction

I

(hk1)

plane

Table 2.6. XRD pattern of nanocrystalline Cu2P207 sample C3

I

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 impuritiesz9is 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 m o l ~ . '(sample M I ) 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 m o l ~ . 'and 0.1 m o l ~ . '(samples M2 &

42

Chapter 2

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

43

50

40

30

Diffraction angle

Fig.2.6. XRD spectra 0f.n- MgzPz07samples (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 Cu2P207samples, 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


-

20

Observed d values A

ICDD-PDF d values A

% of lattice contraction

( h k 1)

21

4.229

4.149

-1.928

020

22.6

3.933

3.86

-1.891

11 1

23

3.86:s

3.492

-10.682

02 1

25

3.56

3.181

-11.914

-2 0 2

27.5

3.242

3.126

-3.711

0 12

29.6

3.01'7

3.008

-0.299

-30 1

31

2.883

2.929

1.571

220

33.6

2.666

2.795

4.615

-3 1 1

35

2.562

2.62

2.214

310

36.6

2.454

2.416

-1.573

-2 2 2

6!8:i

1.541 1.522 62.3

1.489

plane

1 :z 1 1 1 1.476

-1.248

133

-2.148

-5 2 3

-0.881

242

Table.2.8. XRI) pattern of nanocrystalline MgzPz07sample M2

, I

45

28

Observed d values A

ICDD-PDF d values A

% of lattice

contraction

k I) plane

21

4.229

4.149

- 1.928

020

22.5

3.95

3.86

-2.332

111

22.7

3.916

3.492

-12.142

02 1

25

3.56

3.181

-11.914

-2 0 2

25.3

3.519

3.126

-12.572

0 12

27.5

3.242

3.008

-7.779

-301

28.7

3.109

2.929

-6.145

220

29.9

2.987

2.795

--

-6.869

-3 1 1

32.5

2.754

2.62

-5.115

310

33.9

2.643

2.416

-9.396

-2 2 2

35.5

2.527

2.335

-8.223

-3 2 1

(h

Table.2.9. XRD pattern of nanocrystalline Mg2P207 sample M3


47

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 Mg2P207nanocrystallites 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 layersz9at 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

48

Chapter 2

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

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

49

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

51

Chapter 2

52 P

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)2increases, 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)zcontent 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.44The 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 m o l ~ . ' gives a mean lattice expansion as observed in section 2.5 (iii). Nanocrystalline Fe3(P04)2 has a mean lattice c ~ n t r a c t i o n .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


53

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 . -

-

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 contrast46usually dominate images in TEM. First diffraction contrast, which is produced due to distortion in the o~ientationof 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

3

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 effects46is 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)

\


'

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

55

Fig.2.8.(b): TEM micrograph of nano AlP04 sample at 450 "C Scale: 15mm=1pn

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 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.

I


57

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 m o l ~ . ' ) 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 m o l ~ . '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 Cu2P207is 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)2nanocomposite 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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