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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.