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CHAPTER 2

CHARACTERIZATION TECHNIQUES

2.1 INTRODUCTION

The characterization of materials is important for understanding their

properties and applications. This chapter describes the instruments and experimental

methods utilized for various measurements towards the characterization of the

synthesized TiO2 nanostructures. The techniques adopted to characterize the

nanoparticles are: X - ray diffraction (XRD), UV - visible spectroscopy, Fourier

transforms infrared spectroscopy (FT-IR), scanning electron microscopy (SEM),

transmission electron microscopy (TEM) and surface area analysis (BET). The

fabrication of DSSC is also discussed.

Spectroscopy is a powerful tool to study the structure of

nanocrystalline, organic and inorganic materials. Spectroscopy is a technique that

uses the interaction of energy with a sample to perform an analysis. The data

obtained from spectroscopy is called a spectrum. A spectrum is a plot of the intensity

of energy detected versus the wavelength (or mass or momentum or frequency, etc.)

of the energy [59].

A spectrum can be used to obtain information about atomic and

molecular energy levels, molecular geometries, chemical bonds, interactions of

molecules and related processes. Often, spectra are used to identify the components

of a sample (qualitative analysis).

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Table 2.1 Characterization techniques and their applications

TECHNIQUES APPLICATIONS

1. X-Ray Diffraction (XRD)

2.Ultraviolet-Visible

Spectroscopy (UV/Vis)

3. Fourier Transform Infrared

Spectroscopy (FT IR)

4.Scanning Electron Microscopy

(SEM)

5. Transmission Electron

Microscopy (TEM)

6. Surface area analysis (BET)

To study the crystalline properties of solid

substances

To analyze molecular (organic) and ionic species

capable of absorbing at UV or Visible

wavelengths in dilute solutions

To analyze only molecular compounds (organic

compounds, natural products, polymers, etc.)

To study the topography, structure and

compositions of metals, ceramics, polymers,

composites and biological materials

To study the local structures, morphology, and

dispersion of multi component polymers, cross

sections and crystallizations of metallic alloys,

semiconductors, microstructure of composites,

etc.

To study the surface area, pore volume, pore

diameter, and pore size distribution of

nanoparticles.

Spectra may also be used to measure the amount of material in a sample

(quantitative analysis). Table 2.1 shows the various characterization techniques and

their applications. This chapter also encompasses a detailed view of the theoretical

aspects, instrumentation techniques of the three major spectroscopic methods

namely powder X – ray Diffraction, UV-visible, and Fourier Transform Infrared

spectroscopy (FT - IR).

The next level of characterization is to examine the surface, arrangement

of surface atoms and the electronic structure of the nanoparticles. This can be done

only by viewing the particles in the nanometre range. There are two main kinds of

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microscopy. The first class of microscopy involves a stationary sample in line with a

high - speed electron gun. Both the scanning electron microscope (SEM) and

transmission electron microscope (TEM) are based on this technique. Detailed

information of surface area measurement and pore size measurement of BET

analysis has described in this chapter.

2.2 POWDER X - RAY DIFFRACTION (XRD)

2.2.1 Principle

Figure 2.1 shows a schematic representation of powder X - ray

diffractometer. X-rays are electromagnetic radiation with wavelength in the range

0.01 - 10 nm. It is used in diffraction experiments in the typical wavelength range

0.5 - 1.5 A. For electromagnetic radiation to be diffracted, the spacing in the grating

should be of the same order as the wavelength. In crystals, the typical inter atomic

spacing 2 - 3Å the suitable radiation is X-rays. Hence X-rays can be used to the

study crystal structures [60].

Figure 2.1 Powder X - ray diffractometer

In this technique, the primary X - rays are made to fall on the sample

substance to be investigated. Because of its wave nature, like light waves, X - ray

gets diffracted to a certain angle. This angle of diffraction, which differs from that of

the incident beam, will give the information regarding the crystal nature of the

substance. The wavelength of the X - rays can be varied for the application by using

a grating plate.

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2.2.2 Bragg’s Law

Bragg diffraction occurs when electromagnetic radiation or subatomic

particle waves with wavelength comparable to atomic spacings are incident upon a

crystalline sample, scattered by the atoms in the system and undergo constructive

interference in accordance to Bragg's law.

Figure 2.2 Bragg’s Law

Considering the conditions necessary to make the phases of the beams

coincide when the incident angle equals and reflecting angle (Figure 2.2). The rays

of the incident beam are always in phase and parallel up to the point at which the top

beam strikes the top layer at atom Z. The second beam continues to the next layer

where it is scattered by atom B. The second beam must travel the extra distance

AB + BC if the two beams are to continue traveling adjacent and parallel. This extra

distance must be an integral (n) multiple of the wavelength (λ) for the phases of the

two beams to be the same:

nλ = AB+BC (1)

Recognizing ‘d’ as the hypotenuse of the right triangle ABZ, we can use

trigonometry to relate ‘d’ and θ to the distance (AB + BC). The distance AB is

opposite θ so,

AB = d sinθ (2)

Because AB = BC eq. (1) becomes,

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nλ = 2AB (3)

Substituting eq. (2) in eq. (3) we have,

nλ = 2d⋅sinθ (4)

and Bragg's Law has been derived.

where,

n - order of diffraction

λ - the wavelength of x-rays

d - the spacing between the planes in the atomic lattice.

θ - the angle between the incident ray and the scattering planes.

Note that if only two rows of atoms are involved, the transition from

constructive to destructive interference as θ changes is gradual. However, if

interference from many rows occurs then the constructive interference peaks become

very sharp with mostly destructive interference in between. This sharpening of the

peaks as the number of rows increases that is very similar to the sharpening of the

diffraction peaks from a diffraction grating as the number of slits increases.

2.2.3 Instrumentation

A powder X - ray diffractometer consists of a X - ray source (usually a

X - ray tube), a sample stage, a detector and provision to change angle θ

(Figure 2.3). The X - ray is focused on the sample at some angle θ, while the

detector opposite to the source reads the intensity of the X - ray that receives at

2θ away from the source path. The incident angle is the increased over time while

the detector angle always remains 2θ above the source path.

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Figure 2.3 Powder X - ray diffraction

2.2.3.1 X - ray Tubes

The most common source of X - rays is an X - ray tube. The tube is

evacuated and contains a copper block with a metal target anode and a tungsten

filament cathode with a high voltage is applied between them. The filament is heated

by a separate circuit, and the large potential difference between the cathode and

anode electrons at the metal target. The accelerated electrons knock core electrons

out of the metal, and electrons in the outer orbitals drop down to fill the vacancies

and thereby, emitting X-rays. The X-rays exit the tube through a beryllium window.

Due to massive amounts of heat being produced in this process, the copper block

must usually be water cooled.

2.2.3.2 X-ray Detectors

Most modern equipment do use transducers that produce an electrical

signal when exposed to radiation. These detectors are often used as photon counters,

so intensities are determined by the number of counts in a certain period of time.

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2.2.3.3 Gas-Filled Transducers

A gas-filled transducer consists of a metal chamber filled with an inert

gas, with the walls of the chamber as a cathode and a long anode in the center of the

chamber. When X - rays enters the chamber, its energy ionizes many molecules of

the gas. The free electrons then migrate towards the anode and the cations migrate

towards the cathode. The electrons that reach the anode cause current to flow, which

can be detected. The sensitivity and dead time (when the transducer will not respond

to radiation) both depend on the voltage of the transducer is operated at. At high

voltage, the transducer is very sensitive but it has a long dead time, whereas at low

voltage, the transducer has a short dead time but low sensitivity.

2.2.3.4 Scintillation Counters

In a scintillation counter, phosphor is placed in front of a photomultiplier

tube. When X - rays strike on the phosphor, it produces flashes of light, which are

detected by the photomultiplier tubes.

2.2.3.5 Semiconductor Transducers

A semiconductor transducer has a gold coated p-type semiconductor

layered on lithium containing semiconductor intrinsic zone, followed by an n-type

semiconductor on the other side of the intrinsic zone. The semiconductor is usually

composed of silicon and germanium but germanium is used if the radiation

wavelength is very short. The n - type semiconductor is coated by an aluminum

contact, which is connected to a pre-amplifier. The entire crystal has a voltage

applied across it. When X - ray strikes the crystal, it elevates many electrons in the

semiconductor into the conduction band, which causes a pulse of current [61, 62]

2.2.4 Applications

The X - ray diffraction is a good tool to study the nature of the crystalline

substances. In crystals, the ions or molecules are arranged in well-defined positions

in planes in three dimensions. The impinging X - rays are reflected by each crystal

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plane. So the planes can not be same or identical for any two chemical substances,

this technique provides vital information regarding the arrangement of atoms and the

spacing in between them and also to find out the chemical compositions of

crystalline substances. The sample under study can be of either a thin layer of crystal

or in a powder form. Since, the power of a diffracted beam depends on the quantity

of the corresponding crystalline substance. It is also possible to carry out

quantitative determinations [63].

2.3 ULTRAVIOLET - VISIBLE SPECTROSCOPY

2.3.1 Principle

Molecules absorb energy and this energy can bring out translational,

rotational or vibrational motion or ionization of the molecules depending upon the

frequency of the electromagnetic radiation. Excited molecules are unstable and

quickly drop down to ground state again giving off the received energy in the form

electromagnetic radiation. The wavelength and intensity of the electromagnetic

radiation absorbed or emitted can be recorded to get a spectrum (Figure 2.4).

Spectral analysis yields qualitative and quantitative information about the materials

under study [64].

Figure 2.4 Electromagnetic spectrum

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Ultraviolet-visible (UV-vis) spectroscopy is useful tool to characterize

the absorption, transmission, and reflection of a variety of compounds and materials,

such as pigments, coatings etc. The UV - vis spectra has broad features including

sample identification and very useful for quantitative measurements.

2.3.2 Instrumentation

The instruments has the following components:

A light source that generates a broad band of electromagnetic

radiation

A dispersion device that selects a particular wavelength (or, more

correctly a waveband) from the broadband radiation of the source

A sample area (component)

One or more detectors to measure the intensity of radiation

Other optical components, such as lenses or mirrors, relay light

through the instrument.

A schematic representation of a UV/vis spectrophotometer is shown

in Figure 2.5. Normal working range for a spectrometer is

190 – 900 nm, working beyond 180 nm requires special

arrangements [65].

Figure 2.5 Functional block diagram of UV - visible spectrophotometer

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2.3.2.1 The Light Source

A deuterium discharge lamp for UV region (160-375 nm)

A tungsten filament lamp or tungsten-halogen lamp for Visible and

NIR regions (350 - 2500 nm)

The instrument automatically swaps lamps when scanning between

the UV and VIS-NIR regions

2.3.2.2 The Monochromator

All monochromators contain the following component parts:

An entrance slit

A collimating lens

A dispersing device

A focusing lens

An exit slit

Ideally, the output from a monochromator is monochromatic light.

However, in practice, the output is always a band, optimally symmetrical in shape.

2.3.2.3 Dispersion devices

Dispersion devices cause different wavelengths of light to be dispersed at

different angles. When combined with an appropriate exit slit, these devices can be

used to select a particular wavelength (or, more precisely, a narrow waveband) of

light from a continuous source. Two types of dispersion devices, prisms and

holographic gratings are commonly used in UV-vis spectrophotometers.

Light falling on the grating is reflected at different angles, depending on

the wavelength. Holographic gratings yield a linear angular dispersion with

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wavelengths and are temperature insensitive. However, they reflect light in different

orders, which may overlap. As a result, filters must be used to ensure that only the

light from the desired reflection order reaches the detector.

2.3.2.4 Detectors

A detector converts a light signal into an electrical signal. Ideally, it

should give a linear response over a wide range with low noise and high sensitivity.

Spectrophotometers normally contain either a photomultiplier tube detector or a

photodiode detector.

The photomultiplier tube combines signal conversion with several stages

of amplification within the body of the tube. It consists of a photoemissive cathode,

a number of dynodes (which emit several electrons for each electron striking them)

and an anode.

Photodiodes are increasingly being used as detectors in modern

spectrophotometers. Photodiode detectors have a wider dynamic range and are more

robust than photomultiplier tube detectors. In a photodiode, light falling on the

semiconductor material allows electrons to flow through it, thereby depleting the

charge in a capacitor connected across the material. The amount of charge needed to

recharge the capacitor at regular intervals is proportional to the intensity of the light.

2.3.2.5 Cells

These are containers for the sample and reference solutions. They must

be transparent to the radiation passing through.

For UV region: Quartz or fused silica cuvettes are usually used.

VIS/NIR regions: Silicate glass or plastic cuvettes (350 - 2000 nm)

can also be used [66].

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2.3.3 Applications

It is the most widely used technique for quantitative molecular analysis

and obeys Beer - Lambert law. Sometimes, it is used in conjunction with other

techniques such as NMR, IR, etc., in the identification and structural analysis, of

organic compounds. For qualitative analysis it provides valuable information

through the absorption spectrum which is unique for a given compound [67 - 72].

2.4 SCANNING ELECTRON MICROSCOPY (SEM)

2.4.1 Principle

In this technique, an electron beam is focused onto sample surface kept in

a vacuum by electro-magnetic lenses (since electron possesses dual nature with

properties of both particle and wave, hence an electron beam can be focused or

condensed like an ordinary light). The beam is then rastered or scanned over the

surface of the sample. The scattered electron from the sample is then fed to the

detector and then to a cathode ray tube through an amplifier, where the images are

formed, which gives the information of the sample [73].

2.4.2 Instrumentation

It comprises of a heated filament as a source of electron beam, condenser

lenses, aperture, evacuated chamber for placing the sample, electron detector,

amplifier, CRT with image forming electronics, etc.

The SEM is an instrument that produces a largely magnified image by

using electrons instead of light to form an image. A schematic diagram of the

FE-SEM is shown in Figure 2.6. A beam of electrons is produced at the top of the

microscope by an electron gun. The electron beam follows a vertical path through

the microscope, which is held within a vacuum chamber.

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Figure 2.6 Functional Block diagram of field emission scanning electron

microscope (FE-SEM)

The beam travels through electromagnetic fields and lenses, which focus

the beam down towards the sample. Once the beam hits the sample, electrons and

X - rays are ejected from the sample. Detectors collect these X - rays, backscattered

electrons and secondary electrons and convert them into a signal that is sent to a

screen similar to a television screen. This produces the final image. In this research

work, the powder samples were placed on the carbon tape which is attached to the

sample holder. JEOL JSM 6320F (FESEM), F E I Quanta FEG 200 (HRSEM) are

used to study the surface morphology of the sample.

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2.4.3 Applications

Scanning electron microscopy has been applied to the surface studies of

metals, ceramics, polymers, composites and biological materials for both topography

as well as compositional analysis. An extension of this technique is Electron Probe

Micro Analysis (EPMA), where the emission of X-rays, from the sample surface, is

studied upon exposure to a beam of high energy electrons. Depending on the type of

detectors used this method is classified in to two as: Energy Dispersive

Spectrometry (EDS) and Wavelength Dispersive Spectrometry (WDS).

This technique is used extensively in the analysis of metallic and ceramic inclusions,

inclusions in polymeric materials and diffusion profiles in electronic components.

2.5 ENERGY DISPERSIVE X - RAY ANALYSIS (EDAX)

Energy dispersive X - ray spectroscopy (EDS or EDX) is an analytical

technique used predominantly for the elemental analysis or chemical

characterization of a specimen. Being a type of spectroscopy, it relies on the

investigation of a sample through interactions between electromagnetic radiation and

matter, analyzing X - rays emitted by the matter in this particular case.

Its characterization capabilities are due in large part to the fundamental principle that

each element of the periodic table has a unique atomic structure allowing X - rays

that are characteristic of an element's atomic structure to be uniquely distinguished

from each other.

To stimulate the emission of characteristic X - rays from a specimen, an

high energy beam of charged particles such as electrons or protons or a beam of

X - rays is focused into the sample to be characterized. At rest, an atom within the

sample contains ground state (or unexcited) electrons situated in discrete energy

levels or electron shells bound to the nucleus. The incident beam may excite an

electron in an inner shell, prompting its ejection and resulting in the formation of an

electron - hole within the atom’s electronic structure. An electron from an outer,

higher - energy shell then fills the hole, and the difference in energy between the

higher - energy shell and the lower energy shell is released in the form of a X - ray. The

X - ray released by the electron is then detected and analyzed by the energy dispersive

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spectrometer. These X - rays are characteristic of the difference in energy between the

two shells, and the atomic structure of the element form which they were emitted.

Figure 2.7 The principle of EDX

The excess energy of the electron that migrates to an inner shell (in order

to fill the newly - created hole) can do more than emitting a X - ray. Often, the

excess energy is transferred to a third electron from a further outer shell, prompting

its ejection. This ejected species is called an Auger electron and the method for its

analysis is known as Auger Electron Spectroscopy (AES).

Energy Dispersive X-Ray spectroscopy (EDS or EDX) is a technique

used in conjunction with chemical microanalysis by scanning electron microscopy

(SEM) (Figure 2.7). EDS technique detects X - rays emitted from the sample during

bombardment by an electron beam to characterize the elemental composition of the

volume analyzed.

When a sample is bombarded by the electron beam in SEM, the electrons

are ejected from atoms comprising the sample surface. EDS X - ray detector

measures the relative abundance of X - rays against their energy. The detector is

typically a lithium-drifted silicon solid-state device. When an incident X - ray hits

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the detector, which creates a pulse of charge that is proportional to the energy of

X - ray. The pulse charge is converted to a pulse voltage (which is proportional to

the energy X - ray) by a charge sensitive preamplifier. The signal is then sent to a

multichannel analyzer where the pulses are sorted by the tension. The energy, as

determined by measuring the voltage per incident X - ray is sent to a computer for

display and further data evaluation. The spectrum of X - ray energy versus counts is

evaluated to determine the elemental composition of the sample volume.

2.6 TRANSMISSION ELECTRON MICROSCOPY (TEM)

2.6.1 Principle

In this technique, a beam of high-energy electrons (typically 100 – 400 keV)

is collimated by magnetic lenses and allowed to pass through a specimen under high

vacuum. The transmitted beam and a number of diffracted beams can form a

resultant diffraction pattern, which is imaged on a fluorescent screen kept below the

specimen. The diffraction pattern gives the information regarding lattice spacing and

symmetry of the structure under consideration. Alternatively, either the transmitted

beam or the diffracted beams can be made to form a magnified image of the sample

on the viewing screen as bright-and dark field imaging modes respectively. This

gives information about the size and shape of the micro-structural constituents of the

material. High - resolution image contains information about the atomic structure of

the material. This can be obtained by recombining the transmitted beam and

diffracted beams together [74, 75].

2.6.2 Instrumentation

It comprises of a tungsten filament or LaB6 or a field emission gun as

source of electron beam, objective lens, imaging lens, CCD camera, monitor, etc.

The ray of electrons is produced by a pin-shaped cathode heated up by current. The

electrons are vacuumed up by a high voltage at the anode. The acceleration voltage

is between 50 and 150 kV. The higher it is, the shorter are the electron waves and the

higher is the power of resolution, but this factor is hardly ever limiting. The power

of resolution of electron microscopy is usually restrained by the quality of the

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lens-systems and especially by the technique with which the preparation has been

made. Modern gadgets have powers of resolution that range from 0.2 - 0.3 nm.

Schematic representation of TEM image is shown in Figure 2.8.

The accelerated ray of electrons passes a drill-hole at the bottom of the anode.

Its following way is analogous to that of a ray of light in a light microscope.

The lens-systems consist of electronic coils generating an electromagnetic field.

The ray is first focused by a condenser and then passes through the object, where it

is partially deflected. The degree of deflection depends on the electron density of the

object. The greater the mass of the atoms, the greater is the degree of deflection.

Biological objects have only weak contrasts since they consist mainly of atoms with

low atomic numbers (C, H, N, O). Consequently, it is necessary to treat the

preparations with special contrast enhancing chemicals (heavy metals) to get at least

some contrast. Additionally, they are not thicker than 100 nm, because the

temperature rises due to electron absorption. It is generally impossible to examine

living things.

Figure 2.8 Functional Block diagram of transmission electron microscope

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After passing through the object, the scattered electrons are collected by

an objective. Thereby an image is formed, that is subsequently enlarged by an

additional lens - system (called projective with electron microscopes). The formed

image is made visible on a fluorescent screen or it is documented on photographic

material. Photos taken with electron microscopes are always black and white.

The degree of darkness corresponds to the electron density (differences in atom

masses) of the candled preparation.

2.6.3 Applications

Transmission electron microscopy is used to study the local structures,

morphology, dispersion of multi - component polymers, cross sections and

crystallization of metallic alloys semiconductors, microstructure of composite

materials, etc. The instrument can be extended to include other detectors like Energy

Dispersive Spectrometer (EDS) or Energy Loss Spectrometer (ELS) to study about

the local chemistry of the material similar to SEM technique [76]

2.7 FOURIER TRANSFORM INFRARED SPECTROSCOPY (FT-IR)

2.7.1 Principle

It involves the absorption of electromagnetic radiation in the infrared

region of the spectrum which results in changes in the vibrational energy of

molecule. Since, usually all molecules will be having vibrations in the form of

stretching, bending, etc. The absorbed energy will be utilized in changing the energy

levels associated with them. It is a valuable and formidable tool in identifying

organic compounds which have polar chemical bonds (such as OH, NH, CH, etc.)

with good charge separation (strong dipoles) [77 - 79].

2.7.2 Instrumentation

It was originally designed as a double beam spectrophotometer

comprising IR source (red hot ceramic material), grating monochromator,

thermocouple detector, cells made of either sodium chloride or potassium bromide

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materials, etc. In this process, the light is dispersed by the monochromator but, this

type of basic design for IR measurements has been outdated. Instead, a newer

technique termed Fourier Transform-Infrared (FT-IR) has been in practice.

This technique utilises a single beam of un-dispersed light has the instrument

components similar to the previous one.

Figure 2.9 Functional Block diagram of Fourier transform infrared

spectrometer

In FT-IR, the un-dispersed light beam is passed through the sample and

the absorbance at all wavelengths is received at the detector simultaneously.

A computerized mathematical manipulation (known as “Fourier Transform”) is

performed on this data to obtain absorption data for each and every wavelength.

To perform this type of calculations interference of light pattern is required for

which the FT-IR instrumentation contains two mirrors; one fixed and one moveable

with a beam splitter in between them. Before scanning the sample, a reference or a

blank scanning is required. The following is the simplified design of the instrument

(Figure 2.9).

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2.7.2 Applications

It finds extensive use in the identification and structural analysis of

organic compounds, natural products, polymers, etc. The presence of particular

functional group in a given organic compound can be identified. Since every

functional group has unique vibrational energy, the IR spectra can be seen as their

fingerprints.

2.8 BET ANALYSIS

2.8.1 Surface Area Determination

The surface of a material is the dividing line between a solid and its

surroundings, liquid, gas or another solid. Therefore, the amount of surface or

surface area is an important factor in the behavior of a solid. Surface area has strong

influence on many factors such as dissolution rates of pharmaceuticals, the activity

of industrial catalyst, adsorption capacity of air and water purifiers, and the

processing of most powders and porous materials, etc. Whenever solid matter is

divided into smaller particles, new surfaces are created thereby increasing the

surface area. Similarly, when pores are created within the particle interior

(by dissolution, decomposition or some other physical or chemical means) the

surface area is also increased. There may be more than 2000 m2

of surface area in a

single gram of activated carbon as an example for gas absorption. The true surface

area, including surface irregularities and pore interiors, cannot be calculated from

particle size information but is rather determined at the atomic level by the

adsorption of an unreactive gas or inert gas. The amount adsorbed, let’s call it X, is a

function not only of the total amount of exposed surface but also (i) temperature, (ii)

gas pressure and (iii) the strength of interaction between gas and solid. Because most

gases and solids interact weakly, the surface must be cooled substantially in order to

cause measurable amounts of adsorption - enough to cover the entire surface. Where

the gas pressure increases, more gas is adsorbed on the surface (in a non - linear

way). However, adsorption of a cold gas does not stop when it covers the surface in

a complete layer of one molecule thick (let’s call the theoretical monolayer amount

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of gas Xm)! As the relative pressure is increased, excess gas is adsorbed to form

“multilayers”. [80, 81]

So, gas adsorption - as a function of pressure - does not follow a simple

relationship, and we must use an appropriate mathematical model to calculate the

surface area. We use the BET equation as follows:

0 0

1 1 1

[( ) 1]m m

C P

x P P X C X C P

(5)

where P/P0 is the gas’s relative pressure and constant ‘C’ is related to the strength of

interaction between gas and solid.

2.8.2 The Principle

The gas most commonly used is nitrogen for a number of reasons. In the

classical manometric technique, relative pressures less than unity are achieved by

creating conditions of partial vacuum (absolute pressures of pure nitrogen below

atmospheric pressure). This method is easily automated and the amount of gas

adsorbed is made at a number different relative pressures. Usually, the analyzer

obtains at least three data points in the relative pressure range between 0.025 and

0.30 Pa. Experimentally measured data are recorded as pairs of values: the amount

of gas adsorbed is expressed as STP volume (VSTP) and the corresponding relative

pressure (P/Po). A plot of these data is called an isotherm.

The Principle of calculation, the computer program takes over and a least

- squares linear regression is used to fit the best straight line through a transformed

data set consisting of the following pairs of values: 1/VSTP (Po/P)-1 and P/Po.

The monolayer capacity, Vm, is calculated from the slope,

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1

m

CS

V C

(6)

and the intercept, i, of the straight line

1

m

iV C

(7)

Solving for Vm

1

mV

s i

(8)

The number of molecules in the monolayer is obtained through the

number of moles. Vm value is calculated by dividing Vm by the molar volume

(MV) for the number of moles. The number of molecules covering the surface in a

layer one molecule thick can be determined by multiplying moles by Avogadro’s

number. If we know how much area one molecule occupies, then the total area can

be calculated. Thus the area called as “cross - sectional area”. Therefore, the total

surface area, St, is then calculated from the below equation

m A V m

t

v

V L AS

M (9)

where LAV is Avogadro’s number and Am is the cross - sectional area. All surface

area results are finally reported normalized by sample weight, or mass, as square

meters per gram, written m2 /g.

2.9 SOLAR CELL FABRICATION

The DSSC consists of four components namely: photoanode, dye

molecules, electrolytes and photocathode.

For the DSSC fabrication, ruthenium (II) N - 719 dyes, iodide and

tri - iodide electrolytes and Pt - coated photocathodes were purchased from

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commercially available sources and utilized. The surface of a photoanode was

modified with synthesized TiO2 nanostructures by using spray deposition method.

The fabrication of DSSC is described clearly here. 1.5 g of synthesized TiO2

nanostructures and 50 ml of ethanol were grounded in a mortar for few minutes to

form colloidal suspensions. Thereafter, five drops of triton - X were added to the

solution as an organic binder. Fluorine doped tin oxide (FTO) substrates were

cleaned ultrasonically using a mixture of acetone and ethanol.

Figure 2.10 I - V curve measurements system at Prof. Kenji Murakami

laboratory, Shizuoka University

The TiO2 nanostructures suspension in ethanolic solution was sprayed

over the FTO substrate at a substrate temperature of 150 ˚C by spray deposition.

TiO2 coated FTO substrates (photoanodes) were annealed at 450 ˚C for 2 h.

Photoanodes were immersed in ethanolic solution with 0.03 M di - tetrabutyl

ammonium is - bis (isothiocyanato) bis (2,2″- bipyridyl 4,4′dicarboxylato)

ruthenium (II) (N - 719). The dye sensitized photoanode and Pt - coated counter

electrode were clamped using clips. Finally, an iodide redox electrolyte was filled

between the electrodes via capillary action and DSSC’s device were subjected in

I - V instrument JASCO, CEP - 25BX, as shown in Figure 2.10.