CHAPTER 4

ANALYSIS OF FTIR SPECTRA OF NANOPARTICLES

OF MgA1204, SrA1204 AND NiA1204

4.1 Introduction

Spectroscopic studies can provide information on a large variety of physical

and chemical processes in solids. In many cases, aspects addressed and studied by

infrared spectroscopy are not accessible to the classical, conventional techniques of

solid-state chemistry. Infrared spectroscopy is associated with vibrational energy of

atoms or group of atoms in a material. The infrared region has prime importance for

the study of compounds. Since the IR spectra contain large number of bands, the

possibility that two compounds have the same infrared spectrum is exceedingly

small. Due to this reason IR spectrum is called the "finger print" of a molecule.

If two compounds give super imposed spectra, it means they are one and the same.

The IR spectrum of nanoparticles differs considerably from that of the bulk counter

part.' An overall estimation of the particle size and shape can also be made with the

help of IR spectroscopy.2 In the case of nanoparticles, the surface to volume ratio is

very high, when compared to bulk form. This is the reason for the distinguished

features exhibited by nanoparticles. The number of atoms that constitute the surface

can influence the vibrational spectra of nano particles.3 Infrared absorption in small

crystals has been studied in detail in many materials. FTIR spectroscopy is mainly

used to identity the elements and the phase of the elements. For example, when

water is in liquid phase, the fundamental vibtrational modes are at 3219 (v,), 1645

( b) and 3405 ( e ) cm-'. where as in vapour phase the infrared modes appear at 3652

(v,), 1595 (Y) and 7765(e) ~ m - ' . ~ Gold nanoparticles possess a wine - red

colouration due to the transverse oscillations of the surface electrons of the particle

on interaction with light of suitable wavelength, called surface plasmon resonance.

Surface plasmon band is greatly dependent on the nanoparticle size. The shape of

the particles plays an important role in deciding the position of this band. When the

shape of gold nanoparticle changes to the form of nanorods, the surface plasmon

resonance mode changes from 517 nm to 850 nm.' Thus FTIR is an effective tool

in detecting the shape of nanometer sized materials.

The insite diagnosis of small particles is of great interest, both for basic

physics investigation and for large technical plasma application. In astronomy,

FTIR spectroscopy together with the knowledge of size, composition and

morphology are very much needed for the identification and interpretation of

absorption pattern of extragalactic dust cloud and finally for the understanding of

the mechanisms leading to star formation. Also, in technical application involving

nanoparticles, the control of the processes requires the insite measurement of

concentration, size distribution, dielectric properties etc. FTIR spectroscopy can

effectively be used to measure the particle formation.' It is found that the width and

intensity of peaks in an IR spectrum have explicit dependence on the particle size.

As particle size increases, the width of the peak decreases and intensity increases.

FTIR spectroscopy is widely used to study the nature of surface adsorbents

in nanoparticles. Since the nanoparticles possess large surface area, the

modification of the surface by a suitable adsorbate can generate different properties.

The FTIR spectra of the nanoparticles, which contain some adsorbates, posses

additional peaks in comparison with the FTIR pattern of a bare nanoparticle. So the

property change with different adsorbates can easily be detected with FTIR

spectroscopy.7

Due to the high surface to volume ratio, the activity at the surface of the

nanoparticles would be significantly different from that of the bulk. From FTIR

data it is possible to study the oxidation levels of nanoparticles prepared at different

partial oxygen pressures. In the case of Fe nanoparticles, at oxygen partial pressure

below 200 Torr, the sample contains only Fe304. But at higher oxygen partial

pressure FTIR spectrum contains a peak at 450 cm-', which is characteristic peak of

Y - Fez03 .'

Recent research in nanoparticle field has focussed on a new hybrid nano

particle system that consists of a dielectric core surrounded by a thin noble metal

shell. This hybrid form is termed as "nano shell^".^ Nano shells possess unique

optical properties like strong optical absorbance and a large third order non-linear

optical susceptibility. The absorbance can be selectively tuned to any wavelength

simply by adjusting the ratio of the thickness of dielectric core to the metal over

layer.'' After growth of the metal over layer, the metal surface is coated with a self

assembled monolayer (SAM) for nano shell purification, reducing corrosions and

promoting the stability of the metal surface. FTIR spectroscopy offers a wealth of

information regarding the structure of SAMs on flat surfaces and on the surface of

nanoparticles. FTIR spectroscopy offers insight into the conformational order and

orientation of the alkyl chains of SAMs.

The transformation of a material from liquid phase to solid phase can be

studied with the help of FTIR. The decrease in the width of the peaks of the spectra,

when the material changes from liquid phase to solid phase, is a measure of the

transformation. The decrease in the width of the peaks also accounts for the

increase in the crystallinity.

The results from FTIR studies of small crystals are interesting. In the case of

sodium chloride nanocrystals the absorption bands reported are entirely different

from that of a bulk crystal. The restoring force created by surface polarisation

charge was responsible for the frequency difference.I2 From the comparative study

of nano materials and bulk materials, the difference in the frequency of vibrational

modes is attributed to dipolar interactions, interfacial effects, surface 13, 14 amorphousness, surface free energy etc. From the above mentioned studies it is

clear that the fundamental absorption and vibration frequencies of atoms or group

of atoms in nano materials are size dependent. In this chapter, the shift in absorption

frequency observed in the FTIR spectra of nanoparticles of MgA1204, SrA1204 and

NiA1204 due to difference in particle size is presented. Also, the deviation of the

nanoparticle spectrum from their bulk-material counterparts is discussed.

4.2 Theory

FTIR is one of the most widely used tools for the detection of functional

groups in pure compounds and mixtures and for compound comparison. Infrared

study is related to the vibrational motion of atoms or molecules. In terms of

frequency, the IR regions extends from 3 x 1012 Hz to 3 x l0I4 Hz and in terms of

wave number it extends from 100 cm-' to lo4 cm-'. This study is mainly used for

structure elucidation in organic and inorganic compounds. These compounds absorb

electromagnetic energy in the infrared region of the spectrum. IR radiation does not

have sufficient energy to cause the excitation of electrons. However, it causes atoms

or group of atoms to vibrate faster about the bonds, which connect them. The

compounds absorb energy from a particular region since the vibrations are

quantized. The position of a particular absorption band is specified by a particular

wave number.

In practice, infrared radiation of successively increasing wavelength is

passed through the sample and the percentage of transmittance is measured using an

infrared spectrophotometer. The graph which shows the variation of the percentage

of transmittance with wave number (cm-I) is called an infrared spectrum. Each dip

in a spectrum is called a peak and it represents absorption of infrared radiation at

that frequency. In the spectrum, 100% transmittance means 0% absorption and

vice-versa. During vibration, the covalent bonds behave as if the atoms are

connected by small springs. Atoms in a molecule are vibrating about an average

value of inter atomic distance. At room temperature, most of the molecules in a

given sample are in the lowest vibrational state. However, on absorption of light of

appropriate energy, the molecule becomes excited to the second vibrational levels.

Such absorption can occur only if the dipole moment of the molecule is different in

the two vibrational levels. The variation of the dipole moment with the change in

interatomic distance during the vibration corresponds to an oscillating electric field

that can interact with the oscillating electric field associated with electromagnetic

radiation. In general, molecular vibration, which gives rise to a change in dipole

moment will give rise to absorption bands in infrared region. Otherwise they are

called infrared inactive molecules.

Atoms in a molecule can make stretching vibration as well as bending

vibration. These vibrational frequencies are specific so that we can assign the mode

of vibration and atoms involved, from the frequency of vibration.

I \ symmetric asymmetric

Stretching Vibrations

rC kndlng (or deform,tk,n)

I

linplanel lour-of-plane)

Bending Vibrations

Fig.4.1 Molecular Vibration

By Hooke's law the frequency of vibration is directly proportional to square

root of force constant and reduced mass of the system.

4.3 Instrumentation

IR spectrophotometer consists of complex mechanical and electrical systems

that convert very small variation in energy absorbed into an accurately recorded

spectrum.

Fig. 4.2 Schematic diagram of a recording spectrophotometer.

Infrared radiation is produced by electrically heating a Globar (silicon

carbide) or an Ernest filament (sintered oxides of zirconium, cerium and thorium) at

1000 - 1800 O C . Sodium chloride prism is used for frequency dispersion. Gratings

can give better resolution compared to prisms.

Infrared spectra of both gases and liquids may be obtained by direct study of

the undiluted specimen. Solids can be examined in crystalline form by mixing it

with an alkali halide like KBr. Since the KBr does not absorb infrared radiation in

the region 4000-650 cm'l a complete spectrum of the solid is obtained. The pattern

of the IR spectrum in the finger print region (4000-500 cm-') is very sensitive and

changes even with minor variation in the structure of the material. Hence, this

method is an effective tool for the characterisation of the material. This can

effectively be used for the studies like minor chemical changes, stereo chemical

changes, variation in the particle sizes, morphology, inclusion of impurity etc.

4.4 Fourier Transform Infrared Spectroscopy (FTIR)

One of the major disadvantages of the conventional method of producing an

IR spectrum is its inherent slowness. In conventional recording each point of the

spectrum has to be recorded separately, while the frequency is swept smoothly

across the whole span of the spectrum. In recording the IR spectrum of a

compound, which has few peaks, most of the time is spent in recording the

background noise. With the technique "Fourier Transform Spectroscopy"

simultaneous and almost instantaneous recording of the whole spectrum in the

magnetic resonance, microwave and infrared region is possible.

In ordinary IR spectrophotometer the frequency v emitted by a sample is

detected by the detector as a function of time rather than as the function of

frequency. This method creates lot of problems when the system has to handle a

number of frequencies at a given instant. In order to avoid these difficulties, the

technique of Fourier Transform was developed. The computer, which receives the

detector output, would apply the Fourier Transform process to the stored data, and

the data is detected in frequency domain instead of time domain.

Fig. 4.3(a) shows the complex wave pattern in the time domain and Fig.

4.3(b) shows the corresponding frequency domain spectrum obtained by the

application of Fourier Transform technique.

F.T. 4 !I

Fig. 4.3 The use of the Fourier transform to convert the summed sine waves (a) into their frequency spectrum (b).

Thus in FTIR spectroscopy, the detector collects all the information

simultaneously and the computer with the aid of Fourier transform programme

converts it into frequency domain spectrum. So direct structural analysis of samples

within seconds is possible with the help of FTIR spectroscopy.

4.5 Results and Discussions.

4.5.1 Nanoparticles of MgAlzO4

Nanoparticles of MgA1204 were prepared by chemical route as described in

section 3.2.1. The XRD patterns of the particles were recorded and the sizes of the

particles estimated from the line broadening of the diffraction lines using Schemer

equation were found to be 9 nm, 19 nm and 33 nm. FTIR spectra of all particle

sizes were recorded and studied in the wave number range 400 - 4000 cm-I.

Fig. 4.4 FTIR Spectra of nanosized Magnesium aluminate having particle sizes (Mg-1) 9 nm, (Mg-2) 19 nm, (Mg-3) 33 nm.

FTIR spectra of MgA1204 samples having different particle sizes are shown

in Fig. 4.4. The precursor powder of MgA1204 annealed at 900 OC for 15 hrs yielded

particles of size 9 nm. Prominent absorption peaks were obtained at 514, 689, 1629

and 3463 cm-'. In the case of MgA1204 with particle size 19 nm, absorption peaks

were obtained at 452, 587, 682, 1640 and 3425 cm-'. At a higher particle size of 33

nrn only three peaks develop at 445, 588 and 3425 cm-'.

In the case of nanoparticles, quantum confinement effect, size effect, dipolar

interactions, interfacial effects, surface amorphousness, high internal stress etc play

some important role in the observed changes in vibrational frequencies. According

to earlier study by Walker et alls MgA1204 prepared by sol gel method, crystallizes

to the MgA1204 spinel form at 600 O C and becomes fully crystalline at 1030 OC with

a particle size of 7 - 100 nm. In their study the broad absorption peaks at 3456 cm-I

and 3400 cm-' were attributed to the presence of hydrogen - bonded water.15

According to Zawrah et all7 in MgA1204 prepared by co-precipitation of respective

nitrates absorption bands were formed at 1425, 1525, 1630 and 3450 cm-I, when

annealed at 500 O C . The band at 3450 cm-' corresponds to valence vibration of the

constitutional water.I6 The bands at 1425 cm-' and 1525 cm" correspond toy- NO,

vibrations, which may present because of inadequate decomposition of nitrates. The

. band at 1630 cm-' was assigned to the deformation vibration of water molecules (6

H~o)." In the case of MgA1204, the vibrational peaks at 3463, 3425 and 3452 cm"

corresponding to particles sizes 9 nm, 19 nm and 33 nm are due to hydrogen

bonded water. The area and intensity of the bands which accounts the presence of

water diminishes with rise in annealing temperature. The peak at 3463 cm-'

corresponding to annealing temperature 900 OC has maximum area and intensity.

The peak at 3452 cm-I developed during annealing at 1100 OC has the least area and

intensity. The bands at 1629 cm-I and 1641 cm-I are due to the deformation

vibrations of water molecules (6 H2O). This vibrational mode is absent for the

sample annealed at 1100 O C . The appearance of this mode up to 1000 OC is probably

due to the adsorption of water during the compaction of the powder specimens with

KBr. Above 900 O C , some prominent peaks appear in the range 690 - 445 cm-I.

These bands may be due to the existence of A106 groups building up the

magnesium spinel, which is the only crystallite phase at this temperature. In the

case of MgA1204 having particle size 9 nm, the vibrational frequencies at 514 cm-'

and 689 cm-' are due to A1 - 0 stretching in different A1 coordination states of A106

and ,4104. Below the calcination temperature 900 O C , the size of the particles was

small and they were not in spherical shape. Above 900 O C , the solid state reaction to

form magnesium aluminate occurs leading to a change of shape. So the

crystallisation of magnesium aluminate spinel occurs at a calcination temperature of

more than 900 "C. Several authors have reported the IR absorption bands of powder

MgA1204. However, due to particle size effects, the assignment of vibrational peaks

is not yet definite. Powdered natural crystals of MgA1204 show bands at 690, 538

and 300 ~ m - ' . ' ~ Synthetic spinel powder shows peaks at 692, 526, and 307 cm-I. l 9

Another authentic work shows broad bands at 688,522 and 3092 cm-'. In the case

of MgA1204 having particle of size 9 nm show prominent peaks at 689 cm-I and 5 14

cm". This confirms the phase formation at 900 OC. The small shift in the vibrational

frequencies may be due to the surface effect. According to Grimes and ~ o l l e t t ' ~ ,

absorption peaks in synthetic spinels were observed at 307,455, 526, 570, 692 and

720 cm-I. Halfner and ~aves" reported absorption peaks of synthetic MgA1204 at

538, 690 and 750 cm-' and that of natural spinel at 521, 578, 685 and 752 cm-I.

Preudhomme and ~ a r t e ~ ' reported the absorption peaks at 309, 522, 580, 688 and

760 cm". In the case of MgA1204 at 19 nm, prominent peaks are found at 452,587

and 682 cm-'. At 33 nm, the peak at 682 cm-' is absent. Only the peaks at 445 and

588 cm-' persist. When compared to the different vibrational frequencies of the bulk

material to the nano MgA1204, the vibrational frequencies show a shift. This shift

in the vibrational frequencies may rise due to size effect, dipolar interactions,

interfacial effects or surface amorphousness. When the particle size increases from

9 nm to 33 nm, new bands as well as shift in frequencies are observed. As the

particle size increases, the decrease in frequencies of certain bands may arise due to

the repulsive dipolar interactions and the increase in frequency of certain other

bands arise due to size effect. The evolution of new bands may be due to the change

in surface amorphousness.

4.5.2 Nanoparticles of SrAIz04

Fig. 4.5 shows the FTIR spectra of nanoparticles of SrA1204 annealed at

900 OC, 1000 OC and 1 100 OC for 15 hours.

Fig. 4.5 FTIR Spectra of nanosized Strontium aluminate having particle sizes (Sr-1) 9 nrn, (Sr-2) 22 nm and (Sr-3) 38 nm.

The spectrum of SrA1204 of size 9 nm prepared by annealing the precursor

at 900 "C consists of prominent peaks at 451, 593, 645, 1630 and 3463 cm-'.

SrAI2O4 belongs to distorted stuffed tridymite type of structure. Tridymite belongs

to nepheline family of structures consisting of the corner sharing tetrahedral

framework that distorts to form large cation occupying cavities. In SrA1204, the

framework is formed by AlO, tertrahedra and structural channels are occupied by

srZ+ ions.22 In general X04 framework with tridymite (Td) structure consists of four

degenerate modes of vibration, symmetric stretching (ys), antisymmetric stretching

(y,,), symmetric bending (A) and antisymmetric bending (c&,).~' As per report,

absorption peaks in the range 780 - 900 cm-' are due to antisymmetric stretching of

0 - A1 - 0 and peaks in the range 550 - 650 cm-' are due to antisymmetric

bending. The band due to symmetric bending of 0 - A1 - 0 appearsz4 in the range

420 - 447 cm-I. Thus in the case of SrAI2O4 the absorption peaks at 451 cm-I

corresponds to symmetric bending of 0 - Al - 0 . The bands at 593 and 645 cm?

are due to antisymmetric bending taking place in 0 - Al - 0 . The absorption peak

at 1630 cm-' shows the presence of adsorbed water accumulated on the sample

when it is mixed with KBr. The disappearance of this band at higher annealing

temperatures shows the presence of adsorbed water. The band at 3463 cm-'

corresponds to structural water. The width and intensity of this band decrease with

increase in annealing temperature.

When the annealing temperature is increased to 1000 OC, SrA1204

nanoparticles of size 22 nm are formed. The peak of 45 1 cm-' and 645 cm-' remains

intact while a new peak of low intensity appears at 498 cm-'. The peak at 593 cm-'

is shifted to 583 cm-I with a decrease in intensity. The band at 3485 cm-I represents

the presence of water in small amounts. The band at 1473 cm-' is due to adsorbed

water. As the annealing temperature is increased to 1100 OG the size of the particle

becomes 38 nm and splitting of absorption lines occurs and new peaks appear in the

absorption spectnun. The prominent peaks observed in the spectrum of SrA1204

having a particle size of 38 nm are at 447, 505, 525, 580, 636, 697, 730, 762, 842

and 3452 cm'l. The band at 447 cm-' is due to symmetric bending of 0 - A1 - 0.

The bands at 580 cmh' and 636 cm-' originate due to the antisymmetric bending.

The band at 842 cm-' represents the antisymmetric stretching of the bond. The other

bands are due to the splitting of bands due to nearest neighbour interaction. The

weak band at 3452 cm-' shows the traces water at annealing temperature 1 I00 OC.

The adsorbed water peak due to mixing with KBr, which was observed at lower

annealing temperature, was absent when the annealing was done at 1100 O C . With

change in particle size, appreciable changes are observed in the IR bands. The IR

absorption spectrum of nanoparticles of SrA1204 of average particle size 9 nm,

22 nm and 38 nm are compared with the IR absorption spectrum of bulk spectrum.

The IR spectrum of nano SrA1204 shows shift in frequencies as well as evolution of

new bands. Due to the small size of grains and the large surface-tc+volume ratio of

nanocrystals, the atomic arrangements on the boundaries differ greatly from that of

bulk crystals, showing some extent of disorder. This surface amorphousness results

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

The variation in the band position may be due to variation in the cation-

oxygen bond length resulting from the change in particle size. Also, the broadening

of the band with decrease in particle size may be due to the occupancy of cations of

the different characters on the same site. This observation shows that, as particle

size decreases the inversion nature of SrA1204 modifies.

The FTIR study has been canied out to characterize the nanosized

nickel aluminate particles. The FTIR spectra were recorded on an infrared

spectrophotometer with KBr pellets in the range 400 - 4000 cm-'. The infrared

spectra of the NiAI204 nano particle with different particle sizes are shown in

Fig. 4.6.

Pig.4.6 FTIR Spectra of nanosized Nickel aluminate having particle sizes (Ni-1) 10 nm, (Ni-2) 14 and (Ni-3) 30 nm.

FTIR studies show that vibrational peaks in the range 500-900 cm",

represent characteristic metal-xygen stretching frequencies associated with the 25-28 vibrations of Ni - 0 - A1 - 0 and Ni - 0 - Al bonds. Also, in the case of

substances with spinel like structure, the two Al - 0 stretching bands observed in

the range 500-900 cm-I can be assigned to different co-ordination states of Al atoms

(A106 and A104). There is also a probability for the overlap of metal-oxygen

vibrations of samples with Al - 0 bands. NiA1204 with sizes 10 nm, 14 nm and

30 nm were prepared by annealing the precursor powder at temperatures 900 OC,

1000 OC and 1100 O C . All the samples contain a common band 3450 cm-'. This

band accounts for the hydroxyl stretching frequency [u (OH)], which can be

assigned to the overlapping of bands due to surface adsorbed water. With increase

in temperature the width and intensity of this peak decreases showing the tendency

for complete extinction of adsorbed water at high annealing temperature. The Al-

0 stretching frequencies are found in the range 900 - 470 cm-l. The Ni - 0

stretching frequencies are found in the range 550 - 340 cm-I. The absorption peaks

at 498, 455 and 453 cm-I for particle sizes 10 nm, 14 nm and 30 nm represent the

characteristic Ni - 0 stretching frequency in NiA1204. Also, the absorption peaks at

724 cm-' for 10 nm, 595 cm'l and 700 cm-' for 14 nm and 503,595 and 720 cm-' in

30 nm particle size are due to the Al - 0 stretching frequencies. The peak at 722

cm-I is attributed to tetrahedrally co-ordinated Al - 0. Samples of all these particle

sizes show this specific vibration. The band at about 595 ern-' can be assigned to the

stretching vibration mode of Al - 0 for the octahedrally co-ordinated aluminium

ions.29 This specific band is absent in 10 nm particle sizes and appears only in 14

nm and 38 nm particle sizes. With increase in particles size the number of

absorption peaks also increases indicating more Ni - 0 or Al - 0 stretching

vibration. In spinel type materials, the bands between 1080 and 1160 cm-'

correspond to Al - OH bending modes?0 In the case of NiA1204 all the samples do

not contain any such bands, indicating the absence of Al -OH bending modes. The

band around 1630 cme' in all the cases is assigned to the deformation vibration of

water molecules (6H20). The appearance of this band is probably due to absorption

of water during pelletization with KBr. The FTIR spectra of nano NiA1204 shows

shift in the frequencies with that are observed in nano NiA1204 prepared in a

different approach. This shift in frequency is due to the change in surface

amorphousness. When the particle size varies from 30 nm to 10 nm the vibrational

frequencies show a blue shift in most cases, while in some cases red shift is

observed. The blue shift is due to the attractive electrostatic interactions while the

red shift can be accounted by the repulsive dipolar interactions.

4.6 Conclusion

Infrared spectroscopy can effectively be used for the characterisation of

nanoparticles. The advantages of using FTIR over conventional method have been

thoroughly reviewed. Absorption peaks with the FTIR spectrum of MgA1204,

SrA1204 and NiA1204 of different particle sizes were studied and explained. The

wave numbers corresponding to symmetric stretching, antisymmetric stretching,

symmetric bending and antisymmetric bending were assigned. From the FTIR

spectrum of NiAI204, the wave numbers associated with vibrations of four

fundamental modes were determined. Basically, Ni - 0, A1 - 0 and Ni - 0 - A1

bonds were involved in the vibration. The FTIR spectra of MA1204 powders (M =

Mg, Sr, or Ni) in all three cases, show metal-oxygen stretching frequencies in the

range 500 - 900 cm-' associated with the vibrations of M - 0, A1 -0, and M - 0 -

A1 bonds. Due to small size of grains and the large surface-to-volume ratio of

nano crystals the FTIR frequencies differ greatly from that of bulk crystals. The

evolution of new bands and shift in IR acting mode were due to surface

amorphousness.

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