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CHAPTER 2

CHARACTERIZATION TECHNIQUES

2.1 INTRODUCTION

The background, motivation and the current status of the present research

work have been discussed in the chapter 1. This chapter presents an overview of the

various principle and instrumentation techniques such as, X - ray diffraction

technique, Fourier transform infrared spectroscopy (FTIR), X - ray photoelectron

spectra (XPS), Field emission scanning electron microscopy (FESEM), Transmission

electron microscopy (TEM) and UV - visible absorption spectroscopy. Also, the

experimental set up employed for the photocatalytic experiments is presented at the

end of this chapter, which are used to understand the features of metal oxides

nanostructures.

2.2 POWDER X - RAY DIFFRACTION

X - ray diffraction (XRD) is a versatile, non - destructive technique that

reveals detailed information about the chemical composition and crystallographic

structure of natural and manufactural materials. XRD is an apt method to examine

whether a resultant material has amorphous or crystalline nature. In crystalline solids,

the constituent particles (atoms, ions or molecules) are arranged in a regular order.

An interaction of a particular crystalline solid with X - rays helps in investigating its

actual structure. In the present work, XRD patterns were recorded using X’per PRO

PANalytical and Rigaku X - ray diffractometer (RINT - 2200) with CuK radiation at

0.02 / sec step interval.

2.2.1 Principle

The principle behind the design of powder diffraction experiments is the

random orientation of crystals in a mineral powder. If the powdered crystals are

randomly oriented, then for all sets of planes (h k l) some of the crystals in the

powder will be in the correct orientation (horizontal) with respect to the X - ray

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source to satisfy Bragg’s law for the proper angle . In other words, at least a few of

the mineral grains in the powder will diffract for each of the planes (h k l) during a

scan through the angles .

Figure 2.1 Principle of X - Ray Diffraction

X - ray diffraction is based on constructive interference of monochromatic

X - rays from a crystalline sample. When a focused X - ray beam interacts with these

planes of atoms, the beam undergoes various modifications like transmission,

absorption, refraction, scattering and diffraction. The diffracted beam can provide

information about the d - spacing by applying Bragg’s law given by,

n = 2d sin (2.1)

where n is an integer, is the wavelength of incident wave, d is the spacing between

the planes in the atomic lattice and is the angle between the incident ray and the

scattering planes. The principle of X - ray diffraction is shown in Figure 2.1.

2.2.2 Instrumentation

A typical powder X - ray diffractometer consists of a source of radiation, a

monochromator to choose the wavelength, slits to adjust the shape of the beam, a

sample and a detector. A goniometer is used for fine adjustment of the sample and the

detector positions. The goniometer mechanism supports the sample and detector,

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allowing precise movement. The source X - rays contains several components; the

most common being K and K . The specific wavelengths are characteristic of the

target material (Cu, Fe, Mo, Cr). Monochromators and filters are used to absorb the

unwanted emission with wavelength K , while allowing the desired wavelength,

K to pass through. The X - ray radiation most commonly used is that emitted by

copper, whose characteristic wavelength for the K radiation is equal to 1.5418 Å.

The filtered X - rays are collimated and directed onto the sample as shown in the

Figure 2.2. When the incident beam strikes a powder sample, diffraction occurs in

every possible orientation of 2 . The diffracted beam may be detected by using a

moveable detector such as a Geiger counter, which is connected to a chart recorder.

The counter is set to scan over a range of 2 values at a constant angular velocity.

Routinely, a 2 range of 0 to 100 degrees is sufficient to cover the most useful part of

the powder pattern. The scanning speed of the counter is usually 2 of 2 min-1.

Figure 2.2 Photograph of the powder X - ray Diffraction assembly [87].

A detector records and processes this X - ray signal and converts the signal to

a count rate which is then fed to a device such as a printer or computer monitor. The

sample must be ground to fine powder before loading it in the glass sample holder.

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Sample should completely occupy the square glass well. Crystalline phases can be

identified by just comparing the interplanar distance ‘d’ values obtained from XRD

data with the fundamental data in Joint Committee on Powder Diffraction Standards

(JCPDS).

2.3 FIELD EMISSION SCANNING ELECTRON MICROSCOPE (FESEM)

For understanding the morphology of the synthesized products, FESEM

(JEOL JSM 7001F, Japan) with an accelerating voltage of 20 kV has been used in the

present work. The FESEM is a microscope that uses electrons instead of light. Since

their developments in early 1950’s, scanning electron microscopes have developed

new areas of study in the medical and physical science communities.

2.3.1 Principle

When a specimen is irradiated with a high energy electron beam,

interactions between the incident electrons and the constituent atoms in the specimen

produce various signals: The primary electron beam interacts with the sample in a

number of key ways,

(i) Primary electrons generate low energy secondary electrons, which are

related to the topographic nature of the specimen.

(ii) Primary electrons can be backscattered which produces images with a high

degree if atomic number (Z) contrast.

(iii) Ionized atoms can relax by electron shell - to - shell transitions, which lead

to either X - ray emission or Auger electron ejection. The X - rays emitted

are characteristic of the elements in the top few micrometer of the sample.

2.3.2 Instrumentation and working

Essential components of a FESEM instrument include electron source

(“Gun”), electron lenses, sample stage, detectors for all signals of interest, display

data output devices. The schematic representation of FESEM is shown in Figure 2.3.

High energy electrons generated by a field emission source in high vaccum

conditions are accelerated by a field gradient and allowed to pass through a set of

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electromagnetic lenses, focusing onto the specimen. The samples synthesized for the

present work are in the powder form. Small quantity of the powder is sprinkled on a

carbon tape. The excess amount of sample is blown away with compressed air. This

carbon tape containing the sample is mounted on the sample holder and used for

imaging. As a result of interaction of electron beam with the sample, different signals

are produced. In case of FESEM, detector detects the secondary electrons and an

image of the sample surface is constructed by comparing the intensity of these

secondary electrons to the scanning primary electron beam. The electron beam is

generally scanned in a raster scan pattern, and the beam's position is combined with

the detected signal to produce an image on a display device such as a monitor.

FESEM can achieve resolution better than 1 micrometer.

Figure 2.3 Field Mission Scanning Electron Microscope [88].

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2.4 TRANSMISSION ELECTRON MICROSCOPE (TEM)

Transmission Electron Microscopy is an effective direct observation method

to view the atomic and molecular arrangement. TEM is an effective probe to study

the size and shape of nanoparticles. The crystallanity and size of the particles can

also be determined from TEM analysis.

2.4.1 Principle

In this form of microscopy, a beam of electrons transmits through an

extremely thin specimen and then interacts with the specimen when passing through

it [96]. The sample must be thin enough to transmit sufficient electrons such that

enough intensity falls on the screen to give an image.

2.4.2 Instrumentation

TEM contains four parts: electron source, electromagnetic lens system,

sample holder and imaging system as shown in Figure 2.4. The electron beam

coming from the source is tightly focused by the electromagnetic lenses and the

metal apertures. The system only allows electrons within a small energy range to

pass through, so the electrons in the electron beam will have a well - defined energy.

This beam falls on the sample placed in the holder. The electron beam passes through

the sample. The transmitted beam replicates the patterns on the sample. This

transmitted beam is projected onto a phosphor screen. In the present work, TEM

images were recorded by JEOL JEM 2100F transmission electron microscope at an

accelerating voltage of 200 kV. To obtain the images, the powder samples were

dispersed in ethanol and it was ultrasonicated for 20 minutes. A drop of dispersion

was coated onto the copper grid and TEM images were obtained.

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Figure 2.4 Transmission Electron Microscope [89].

2.5 X - RAY PHOTOELECTRON SPECTROSCOPY (XPS)

X - ray photoelectron spectroscopy is a surface characterization technique

that can analyze a sample to a depth of 2 to 5 nanometers (nm). XPS reveals which

chemical elements are present at the surface and the nature of the chemical bond that

exists between these elements. It can detect all of the elements except hydrogen and

helium. XPS for all the synthesized samples, in the present work, were recorded

using Shimadzu ESCA 3400.

2.5.1 Principle

XPS is a surface - sensitive quantitative spectroscopic technique that

measures the elemental composition at the parts per thousand range, empirical

formula, chemical state and electronic state of the elements that exist within a

material. Bombarding a sample in high vacuum (P ~ 10 8 millibar) or ultra - high

vacuum (UHV; P < 10 9millibar) conditions with X - rays gives rise to the emission

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of electrons. These photons eject electrons (the photoelectron) from the solid, and the

kinetic energy (Ek) of the outgoing photoelectrons is directly related to the energy of

the irradiating photon source ( ) and the binding energy (EB) of the electron in the

solid is given by,

Ek = Eb – hv (2.2)

XPS system uses a switchable Mg or Al K line at 1253.6 eV or 1486.6 eV,

respectively. At these energies, it is primarily core electron energy levels that are

probed. Valence level spectra can be collected, but at relatively low resolution.

2.5.2 Instrumentation

XPS is a surface chemical analysis technique that can be used to analyze

the surface chemistry of a material in its as - received state, or after some treatment,

for example: fracturing, cutting or scraping in air or UHV to expose the bulk

chemistry, ion beam etching to clean off some or all of the surface contamination

(with mild ion etching) or to intentionally expose deeper layers of the sample (with

ion etching) in depth - profiling XPS. Irradiating a sample with X - rays of sufficient

energy results in electrons in specific bound states to be excited. In a typical XPS

experiment, sufficient energy is input to break the photoelectron away from the

nuclear attraction force of an element. Figure 2.5 shows the schematic representation

of XPS.

Figure 2.5 X – ray Photoelectron Spectroscopy [90].

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In a typical XPS spectrum, some of the photo - ejected electrons

inelastically scatter through the sample enroute to the surface, while others undergo

prompt emission and suffer no energy loss in escaping the surface and into the

surrounding vacuum. Once these photo - ejected electrons are in the vacuum, they

are collected by an electron analyzer that measures their kinetic energy. An electron

energy analyzer produces an energy spectrum of intensity (number of photo - ejected

electrons versus time) versus binding energy (the energy the electrons had before

they left the atom). Each prominent energy peak on the spectrum corresponds to a

specific element. Besides identifying elements in the specimen, the intensity of the

peaks can also tell how much of each element is in the sample. Each peak area is

proportional to the number of atoms present in each element. The specimen chemical

composition is obtained by calculating the respective contribution of each peak area.

2.6 FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR)

Fourier transform infrared spectroscopy is an analytical technique used to

identify functional groups present in the sample by standard KBr pellet method. The

term Fourier Transform Infrared Spectroscopy (FTIR) refers to a fairly recent

development in the manner in which the data is collected and converted from an

interference pattern to a spectrum. By analyzing the features of a recorded infrared

spectrum, the composition or the structure of chemical components can be

determined. Infrared spectra originate in transition between two vibrational levels of

the molecule in the ground state and are usually observed as absorption and

transmission spectra in the infrared region. The technique measures (%T)

transmission of infrared radiation by the sample material versus wavelength. The IR

region of the electromagnetic spectrum is considered to cover the range from 50 to

12,500 cm 1 approximately.

2.6.1 Principle

When infrared light is passed through a sample of organic compound, some

frequencies are absorbed, while other frequencies are transmitted without being

absorbed. The transitions involved in the infrared absorption are associated with the

vibrational changes in the molecule. Different bonds / functional groups have

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different vibrational frequencies and hence, the presence of these bonds in a

molecule can be detected by identifying this characteristic frequency as an absorption

band in the infrared spectrum. The plot between transmittance against frequency is

called infrared spectrum.

Figure 2.6 Functional block diagram of FTIR Spectrophotometer [91].

2.6.2 Instrumentation

Fourier transform spectrometers have recently replaced dispersive

instruments for most applications due to their superior speed and sensitivity. They

have greatly extended the capabilities of infrared spectroscopy and have been applied

to many areas that are very difficult or nearly impossible to analyze by dispersive

instruments. Instead of viewing each component frequency sequentially, as in a

dispersive IR spectrometer, all frequencies are examined simultaneously in Fourier

transform infrared (FTIR) spectroscopy. There are three basic spectrometer

components in an FT system: radiation source, interferometer and detector. The

functional block diagram of the FTIR spectrometer is shown in the Figure 2.6. The

IR radiation from a broadband source is first directed into an interferometer, where it

is divided and then recombined after the split beams travel different optical paths to

generate constructive and destructive interference. Next, the resulting beam passes

through the sample compartment and reaches to the detector. Sample preparation is

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very easy. Almost any solid, liquid or gas sample can be analyzed. The sample to be

analyzed (minimum of 10 µg) should be ground into KBr matrix or dissolved in a

suitable solvent (CCl4 and CS2 are preferred). Water should be removed from sample

if possible. In case of solid samples, it is mixed with solid KBr (transparent in the

mid - IR region), then ground and pressed. In the present work, Fourier Transform

Infrared measurements (FTIR) were performed using Labindia FTIR

spectrophotometer by standard KBr pellet technique.

2.7 UV - VISIBLE SPECTROSCOPY (UV - Vis)

The absorption spectroscopy employs electromagnetic radiations between

190 to 800 nm. Since the absorption of ultraviolet or visible radiation by a molecule

leads transition among electronic energy levels of the molecule, it is also often called

as electronic spectroscopy. When sample molecules are exposed to light having an

energy (E = where ‘E’ is energy in joules, ‘h’ is Planck’s constant 6.62 × 10 – 34 J s

and ‘ ’ is frequency in Hertz), that matches a possible electronic transition within the

molecule, some of the light energy will be absorbed as the electron is promoted to a

higher energy orbital. An optical spectrometer records the wavelengths at which

absorption occurs, together with the degree of absorption at each wavelength. The

resulting spectrum is presented as a graph of absorbance (A) versus wavelength ( ).

The optical properties of materials can be studied with the help of UV - Vis spectra.

2.7.1 Principle

The absorbance of light by molecules in the solution is based on the Beer

Lambert law,

0log IA b cI

(2.3)

where, I0 is the intensity of the reference beam and I is the intensity of the sample

beam, is the molar absorbtivity with units of L mol - 1 cm - 1, b = path length of the

sample in centimeters and c = concentration given solution expressed in mol L - 1

[92].

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2.7.2 Instrumentation

UV - Vis spectrophotometer consists of five components namely source,

monochromator, sample holder (cuvette), detector and signal processor. Figure 2.7

shows the functional block diagram of UV - Vis Spectrometer. The light source is

usually a deuterium discharge lamp for UV measurements and a tungsten - halogen

lamp for visible and NIR measurements. The instrument automatically lamp swaps

when scanning between the UV and visible regions. UV and visible light from the

source enters the monochromator through entrance slits. The beam is collimated to

strike the dispersing element at an angle. The beam is split into its component

wavelengths by the grating or prism. By moving the dispersing element or the exit

slit, radiation of only a particular wavelength is allowed to leave through the exit slit.

This monochromatic light passes through a set of mirrors resulting in splitting of

monochromatic beam into two halves. One half of the beam passes through the

sample and other half of the beam passes through the reference. Sample and

reference are kept in a transparent quartz cuvette. The faces of these cuvettes through

which the radiation passes are highly polished to keep reflection and scatter losses to

a minimum. In this present thesis, as synthesized samples were dispersed in ethanol

using a homogeniser. However, in the present work, the samples are prepared in such

a way that the absorbance is between 0.5 and 1 %. Known amount of sample and

ethanol are transferred into 25 ml of standard measuring flask and then dispersed

with the help of homogenizer. The two beams after passing through the sample and

reference falls on the detector. Photomultiplier tube is the most commonly used

detector, which amplifies the resulting spectrum. The detector is connected to a

computer to obtain the desired output. In the present work, UV – Visible

spectroscopy (UV) were performed using Labindia UV spectrophotometer by

solution technique.

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Figure 2.7 Functional block diagram of UV - vis Spectrophotometer [93].

2.8 PHOTODEGRADATION EXPERIMENTS

Photodegradation experiments were performed in a home made

photocatalytic reactor system. The schematic representation of this bench scale

system, shown in Figure 2.8, consists of an 500 W, 420 nm halogen lamp and a

magnetic stirrer. The halogen lamp was kept at a distance of 21 cm above the

reaction mixture. The reaction container is equipped with an outside jacket for

effective heat dissipation.

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Figure 2.8 Experimental set up used for photocatalytic reactions.

Accurately weighed amount of photocatalyst was mixed with MB solution

under magnetic stirring. The solution pH was controlled at the desired level by

addition of NaOH and HCl using a Kyoto electronic AT - 200 automatic titrator. The

suspension was magnetically stirred for 1 hour to stabilize and equilibrate the

absorption of MB on the catalyst surface. After attaining equilibrium, the

suspension was then exposed to halogen light irradiation under continual stirring.

Aliquots of 3 mL were withdrawn at regular time intervals to carry out the

constituent analysis on a UV-vis spectrophotometer. Methylene blue exhibits broad

absorption band centered at 664 nm. According to Beer’s law, concentration of the

solution is directly proportional to absorption intensity. The degradation of

methylene blue was monitored by decrease in absorption intensity at 664 nm. At any

point of time, the photodegradation efficiency [94] is calculated as,

0

0

% 100tC CDC

(2.4)

where C0 and Ct are the concentrations of MB at time 0 and t, respectively; and t is

the irradiation time in seconds. This experiment was repeated for different dye

concentrations and from the obtained values, a plot of degradation percentage versus

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time for various concentrations of dye is plotted to observe the optimum dye

concentration. For this particular dye concentration, the optimum photocatalyst

dosage was determined by varying the concentration of photocatalysts. In order to

determine the optimum photocatalyst dosage, the photocatalytic experiment was

repeated with optimum dye concentration and various concentrations of

photocatalysts. A graph is plotted between catalyst dosage and degradation

percentage. A plot of ln (C0/Ct) versus time was studied to examine the reaction

kinetics of MB degradation.

2.9 CONCLUSION

The above characterization techniques such as XRD, FTIR, XPS, FESEM,

TEM and UV - vis spectroscopy are employed for the study of various characteristic

properties of the synthesized nanostructures. Each of these instruments provides

unique information of the synthesized sample which can be correlated, in order to

determine the properties of the synthesized nanostructures. Schematic representation

of experimental setup for carrying out the photodegradation experiments and the

inferences has also been discussed.