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CHAPTER 2

MATERIAL PREPARATION AND

CHARACTERIZATION METHODS

2.1 INTRODUCTION

Mesoporous materials have been paid much attention in both

scientific researches and practical applications. In this review, we focus on

recent developments on preparation of mesoporous materials, adopted in our

laboratory. Microwave irradiation technique and direct hydrothermal

synthesis are the preparative methods used to synthesize the mesoporous

molecular sieves. The characterization and the study of physical properties is

another important aspect of the prepared materials. In this chapter, various

types of synthesis techniques used for the preparation of mesoporous

molecular sieves are discussed. The principle, the experimental set-up and the

measurement procedure of various characterization methods used in our study

also presented.

2.2 PREPARATION METHODS

2.2.1 Microwave Irradiation Technique

Microwave irradiation as a novel, selective dielectric heating

method which offers great advantages such as faster, simpler and very energy

efficient for synthesizing inorganic solids. Because of the energy transfer

from microwaves to the material occurs either through resonance or

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relaxation, is widely used as the basis for the reaction mechanism. Further,

microwave-assisted synthesis methods are unique in providing scaled-up

processes which avoid thermal gradient effects, compared with conventional

heating methods. Therefore, they have been leading to a potentially and

industrially important advancement in the large-scale synthesis of

nanomaterials (Panda et al 2006; Vadivel Murugan et al 2001). The aim of

present study was to obtain the metal adulterated mesoporous materials SnO2

and TiO2 by microwave irradiation technique.

2.2.2 Direct Hydrothermal Synthesis

Directly synthesizing the functional mesoporous materials by

adding acetate, nitrate and chloride salts into the initial synthetic mixture,

followed by evaporation and calcination and to get the composites with

essential properties is a one-pot process. Several metals, such as Al, Ti, and

Zr, have been incorporated into SBA-15 framework during the process of

hydrothermal synthesis (Yue et al 1999; Newalkar et al 2001), but there is no

report on the direct synthesis of oxides modified KIT-6 and SBA-15 with

alkaline earth or transition metals. This prompted us to try and prepare such

new functional mesoporous materials directly using hydrothermal synthesis

technique.

2.3 CHARACTERIZATION METHODS

Different methods have been employed to characterise the as-

synthesised, calcined and modified mesoporous materials. In the present

work, the following physico-chemical characterisation techniques have been

utilized.

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2.4 POWDER X – RAY DIFFRACTION ANALYSIS

2.4.1 Bragg's law

A beam of X-rays of wavelength is directed to the crystal at an

angle to the atomic planes. The interaction between X-rays and the electrons

of the atoms is visualized as a process of reflection of X-rays by the atomic

planes. This is an equivalent description of the diffraction effects produced by

a three dimensional grating. The atomic planes are considered to be semi-

transparent, that is, they allow a part of the X-ray to pass through and reflect

the other part, the incident angle being equal to the reflected angle (called

the Bragg angle). Referring to Fig. 2.1, there is a path difference between rays

reflected from plane 1 and the adjacent plane 2 in the crystal. The two

reflected rays will reinforce each other, only when this path difference is

equal to an integral multiple of the wavelength. If d is the interplanar spacing,

the path difference is twice the distance d sin , as indicated in Fig. 2.1.

Figure 2.1 Illustration of Bragg's Law

The Bragg condition for reflection can therefore be written as

2dhkl sin = n (2.1)

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where,

n is an integer (order of diffraction)

is the wavelength of the X-Ray used.

is the Bragg angle.

d is inter planar spacing.

2.4.2 X-ray Powder Diffractometer

The powder method of diffraction was devised independently by

Debye and Scherrer. It is the most useful of all diffraction methods and when

properly employed, can yield a great deal of structural information about the

material under investigation. Powder diffraction method involves the

diffraction of monochromatic X-rays by a powder specimen. Monochromatic

usually means a strong characteristic Kα component of the filtered radiation

from a X- ray tube operated above the Kα excitation potential of the target

material.

Selection of Kα renders the incident beam to be a highly

monochromatised one. The focussing monochromatic geometry results in

narrower diffracted peaks and low background at low angles. The sample is

mounted vertically to the Seemann-Bohlin focussing circle with the

scintillation counter tube moving along the circumference of it. It is possible

to record the diffracted beam from 2 to 160 degrees. The diffractometer is

connected to a computer for data collection and analysis. The scintillation

counter tube can be moved in step of 0.01 degree by means of a stepper motor

and any diffracted beam can be closely scanned to study the peak profile.

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The conditions for obtaining X-ray diffraction from crystalline

materials is as mentioned. For the present work Rigaku X-ray Diffractometer

was used for the XRD studies as shown in Fig. 2.2.

The powder diffraction pattern of a substance is characteristic of

the substance and forms a sort of fingerprint of the substance to be identified.

The peaks of the X-ray diffraction pattern can be compared with the standard

available data for the confirmation of the structure. For the purpose of

comparison, many standards are available, some of which are, Willars Hand

book, Joint Committee on Powder Diffraction Standards (JCPDS) and

National Bureau of Standards.

Figure 2.2 Photograph of Rigaku X- ray diffractometer

The powder X-ray diffraction patterns of samples were collected on a

Rigaku diffractometer using CuKα (λ = 1.5406Ǻ) radiation. The

diffractograms were recorded in the 2θ range of 0.7-10o with 2θ step size of

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0.01o and a step time of 6 s for low angle diffraction and the 2θ range from 10

to 80° with 0.02°/min for high angle diffraction.

2.5 CHEMICAL AND TOPOLOGICAL STUDIES

Identification and quantitative estimation of chemical species are

the first characterizations to be carried out after preparing any material. Non-

destructive techniques like Scanning Electron Microscopy / Energy

Dispersive Analysis of X-rays /X-ray fluorescence and photoelectron

spectroscopy etc., make use of electron energy levels / velocities of electrons

emitted as fingerprints of chemical species present in the material. These are

widely used for quantitative analysis. The added advantage of these

techniques is the feasibility of mapping chemical species present in the

selected region.

2.5.1 Scanning Electron Microscopy (SEM)

The scanning electron microscope (SEM) is a type of electron

microscope capable of producing high resolution images of a sample surface.

Due to the manner in which the image is created, SEM images have a

characteristic three-dimensional quality and are useful for judging the surface

structure of the sample.

In a typical SEM configuration, electrons are thermionically

emitted from a tungsten or lanthanum hexaboride LaB6 cathode filament

towards an anode; alternatively electrons can be emitted via field emission

(FE). The electron beam, which typically has an energy ranging from a few

KeV to 50 KeV, is focused by two successive condenser lenses into a beam

with a very fine spot size (~ 5 nm). The beam then passes through the

objective lens, where pairs of scanning coils deflect the beam either linearly

or in a raster fashion over a rectangular area of the sample surface.

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As the primary electrons strike the surface they are inelastically

scattered by atoms in the sample. Through these scattering events, the primary

beam effectively spreads and fills a tear-drop shaped volume, known as the

interaction volume, extending about 1µm to 5µm into the surface. Interactions

in this region lead to the subsequent emission of electrons which are then

detected to produce an image. X-rays, which are also produced by the

interaction of electrons with the sample, may also be detected in an SEM

equipped for Energy dispersive X-ray spectroscopy. The block diagram of

SEM instrument is shown in Fig. 2.3. A photograph of SEM instrument is

shown in Fig. 2.4.

The most common imaging mode monitors low energy (<50 eV)

secondary electrons. Due to their low energy, these electrons must originate

within a few tenths of a nanometer from the surface. The electrons are

detected by a scintillator-photomultiplier device and the resulting signal used

to modulate the intensity of a CRT that is rastered in conjunction with the

raster-scanned primary beam. Because the secondary electrons come from the

near surface region, the brightness of the signal depends on the surface area

that is exposed to the primary beam. This surface area is relatively small for a

flat surface, but increases for steep surfaces. Thus steep surfaces and edges

(cliffs) tend to be brighter than flat surfaces resulting in images with good

three-dimensional contrast. Using this technique, resolutions of the order of 5

nm are possible.

In addition to the secondary electrons, backscattered electrons

(essentially elastically scattered primary electrons) can also be detected.

Backscattered electrons may be used to detect both topological and

compositional details, although due to their much higher energy

(approximately the same as the primary beam) these electrons may be

scattered from fairly deep within the sample. This results in less topological

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contrast than for the case of secondary electrons. However, the probability of

backscattering is a weak function of atomic number, thus some contrast

between areas with different chemical compositions can be observed

especially when the average atomic number of the different regions is quite

different.

Additionally, backscattered electrons cannot be "collected" with a

positive bias on a standard Everhart-Thornley detector as is the case with

secondary electrons. Use of a dedicated backscatter detector greatly improves

the collection of backscattered electrons through improvement of the

placement of the detector and by using a detector design that is only sensitive

to the high-energy backscattered electrons. There are usually 2-10 times more

backscattered electrons emitted from a sample than there are secondary

electrons. The Everhart-Thornley detector has low geometric efficiency since

it is located on one side of the sample and is highly directional in its

collection.

The spatial resolution of the SEM depends on the size of the

electron spot which in turn depends on the magnetic electron-optical system

which produces the scanning beam. The resolution is also limited by the size

of the interaction volume, or the extent of material which interacts with the

electron beam. The spot size and the interaction volume are both very large

compared to the distances between atoms, so the resolution of the SEM is not

high enough to image down to the atomic scale.

The SEM has compensating advantages, though, including the

ability to image a comparatively large area of the specimen; the ability to

image bulk materials (not just thin films or foils); and the variety of analytical

modes available for measuring the composition and nature of the specimen.

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Figure 2.3 Schematic diagram of SEM instrument

2.5.2 Energy Dispersive Analysis of X-rays (EDS)

The energy dispersive X-ray spectroscopy (EDS) is a method used

to determine the energy spectrum of X-ray radiation emitted by the sample. It

is mainly used in chemical analysis, in a X-ray fluorescence spectrometer

(especially portable devices), or in an Electron Microprobe (e.g. inside an

scanning electron microscope).

The detector is a semiconductor, usually silicon doped with lithium

(Si : Li detector). The semiconductor is polarised with a high voltage; when a

X-ray photon hits the detector, it creates electron-hole pairs that drift due to

the high voltage. The electric charge is collected, it is like charging a

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condenser; the increment of voltage of the condenser is proportional to the

energy of the photon, it is thus possible to determine the energy spectrum.

The condenser voltage is reset regularly to avoid saturation. To reduce the

electronic noise, the detector is cooled by Peltier effect or best by liquid

nitrogen.

Figure 2.4 Photograph of the scanning electron microscope Hitachi

S-4800

In recent years, a new type of EDS detector has become

commercially available based on a superconducting microcalorimeter. This

microcalorimeter spectrometer has the simultaneous detection capabilities of

the EDS combined with the high spectral resolution of the WDS. Unlike the

semiconductor EDS the microcalorimeter measures the temperature change

caused by the absorption of the x-ray photon in the detector, as such the

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detector must be maintained at ultra low temperatures (~100mK) by the use of

liquid helium and/or an adiabatic demagnetisation refrigerator (ADR). In

essence, the microcalorimeter is a super sensitive thermometer. The

microcalorimeter EDS has suffered from a number of drawbacks compared

with conventional detectors which scientists are now addressing, these

include; low count rates, poor collection efficiencies and small detector areas.

These drawbacks have been overcome somewhat by the use of arrays of

detectors and x-ray focusing optics.

2.6 ICP-OES

The inductively coupled plasma - optical emission spectrometer

(ICP-OES) Seiko Instruments Inc. SPS1700HVR is used to determine

concentrations of a wide range of elements in solution. The instrument is

typically directed to determinations of the lighter elements in the periodic

table, principally the alkali metals, Li - Rb; alkaline earth metals, Be - Ba;

transition metals Sc - Mo; Al and metalloids and non-metals, Si, P, S & As.

For the present elemental analysis, the samples have been dissolved

in HNO3+ HF at 105ºC. The analysis of heteroatom was performed by atomic

absorption spectroscopy (Seiko Instruments Inc. SPS1700HVR). The

calibration was done by using known concentration of metal salt solution.

2.7 NITROGEN ADSORPTION

Nitrogen adsorption and desorption isotherms were measured at -

196oC on a Quantachrome Autosorb 1 sorption analyzer as shown in

Fig. 2.5. Depending on the surface area of the sample, about 50 to 120 mg of

sample was used for the analysis. All the mesoporous samples were outgassed

for at least 3 h at 250 °C under vacuum (p < 10-5 hPa ) in the degas port of the

adsorption analyzer whereas the samples loaded with proteins or vitamins

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were outgassed for 24 h at 100 °C under vacuum (p < 10-5 hPa ) prior to the

analysis. The specific surface area was calculated using the BET equation

(2.2)

where n is the amount of gas adsorbed at a relative pressure P0 and nm is the

amount adsorbed constituting a monolayer surface coverage. The BET

constant C is related to the energy of adsorption in the first layer and

consequently its value mirrors the adsorbent-adsorbate interactions.

Figure 2.5 Photograph of the Quantachrome Autosorb - 1 absorption

analyzer

The total surface area (St) of the sample can be obtained using the

following equation (2.3)

St = nm Acs N (2.3)

1

n (P0/P)–1

1

nm C

(C – 1)

nm C(P/P0) = +

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mm

WnM

(2.4)

m cst

W NASM

(2.5)

where N is Avagadro’s number (6.023 x 1023 molecules/mol), M is the

molecular weight of the adsorbate Wm is the weight of the adsorbate

constituting a monolayer surface coverage, nm is the amount absorbed

constituting a monolayer surface coverage and Acs is the molecular cross-

sectional area of the adsorbate molecule. The specific surface area (S) of the

solid can be calculated from the total surface area (St) and the sample weight

(m) after degassing using the equation (2.6)

tSSm

(2.6)

The total pore volume can be calculated by converting the volume

of nitrogen adsorbed (Vads) into the volume of liquid nitrogen (Vliq) by using

the equation (2.3)

ad mliq

PV VVRT

(2.7)

in which P and T are ambient pressure and temperature respectively, Vm is the

molar volume of the nitrogen (34.7 cm3 mol-1). Specific pore volume (VP) can

be calculated from equation (2.8)

liqp

VV

m (2.8)

where m is the weight of the adsorbent after degassing. The pore size

distributions were obtained from the adsorption and desorption branch of the

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nitrogen isotherms by means of the BJH method using the corrected form of

the Kelvin equation as proposed by Kruk et al (1997).

(2.9)

In equation (2.9), VL is the molar volume of the liquid adsorbate, γ

is its surface tension (8.88 x 10-3 N/m), R is the gas constant (8.314 J mol-1 K-

1) and T is the absolute temperature (-196oC). The statistical film thickness of

nitrogen adsorbate (t(P/P0)) in the mesopores as a function of the relative

pressure (P/P0) can be calculated from equation (2.10).

t(P/P0)nm = 0.1 [60.65 / 0.03071 – log (P/P0)] 0.3968 (2.10)

The diameter of KIT-6 cavity was calculated using equation

a0 = 61/2dhkl (2.11)

In equation (2.11), a0 is the diameter of the cavity of a cubic unit

cell of length a, and ν is the number of cavities present in the unit cell.

The diameter of SBA-15 cavity was calculated using equation (2.12)

a0= 2dhkl/√3 (2.12)

In equation (2.11), a0 is the diameter of the cavity of a unit cell of

length a, and ν is the number of cavities present in the unit cell.

2.8 DIFFUSE REFLECTANCE SPECTROSCOPY (DRS)

The electronic band structure of semiconductors and metals is

determined by their optical properties. The optical absorption is a result of

interaction between the material and light. When a frequency of light is in

2VL

RT ln(P0/P) r(P/P0) = + t(P/P0) + 0.3 nm

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resonance with the energy difference between states the transition allowed or

partly allowed by selection rules, a photon is absorbed by the material.

This results in a decrease of transmission or an increase in

absorbance of the light passing through the sample. By measuring the

transmission or absorbance of sample as a function of the frequency of the

light, one can obtain a characteristic absorption spectrum of the material.

Diffuse reflectance spectroscopy (DRS) on powders and or pellets is roughly

analogous to transmission measurements on thin films.

In the present study, a Perkin Elmer Lambda 750 spectrophotometer is

used for recording the reflectance spectra in the range of 200-2000 nm at

room temperature. This contains double beam and double pass

monochromator system with good resolving power and photometric

efficiency in the UV, VIS and IR regions. It is possible to carryout accurate

spectral measurements due to its sensitive dual microprocessor based system.

The block diagram of optical Perkin Elmer Lambda 750 spectrophotometer is

shown in Fig. 2.6. A photograph of Perkin Elmer Lambda 750

spectrophotometer is shown in Fig. 2.7.

The light beam from either a Tungsten or Deuterium lamp (after

passing through the filter (F) and slit (S) is focused onto the grating by a

concave mirror. The beam is chopped by chopper (BC) three times per second

is converted into a pulse beam. The pulsating beam can easily be

differentiated from the background radiation for accurate optical

measurements. This beam is again reflected by the grating and is directed to

the partial reflecting mirror (R) which in turn splits the pulsating beam into

two paths, one through the sample under investigation and the other through a

reference sample. These two beams of light are directed onto a detector.

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Figure 2.6 Functional block diagram of Perkin Elmer lambda 750

spectrophotometer

Figure 2.7 Photograph of Perkin Elmer Lambda 750 UV-VIS-NIR

spectrometer

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Lock in amplifier measures the light intensity by eliminating

background and unwanted intensities of light. At the detector the relative

beam intensities of reference and experimental samples alternatively striking,

facilitate accurate measurements of the radiation. A photomultiplier tube is

used as a detector in UV and VIS regions where as lead sulphide

photosensitive element is used in IR region. In this spectrometer the detector

is selected by means of automatic test function.

2.9 PHOTOLUMINESCENCE STUDIES

In this experiment, the energy levels in a semiconductor quantum

well structure are investigated using the technique of photoluminescence

(PL). A laser is used to photoexcite electrons in a semiconductor and when

they spontaneously de-excite they emit luminescence. The luminescence is

analyzed with a spectrometer and the peaks in the spectra represent a direct

measure of the energy levels in the semiconductor. The importance of

electronic devices using semiconductor material is second only to devices

using the more ubiquitous semiconductor, Si.

The photoluminescence spectra of the powder samples of the

present system were recorded in the wavelength range 400-700 using a

F-3010 Hitachi fluorescence spectrophotometer and functional block diagram

shown in Fig. 2.8. The light emitted from the Xe-lamp enters the excitation

monochromator. The beam splitter splits the light emerging from the

excitation monochromator and a fraction of it is directed to the monitor

detector. A shutter is provided between the excitation monochromator and

the sample, which is placed in the optical path as commanded from the

operation panel. All the driving components, i.e., the wavelength drive

motors, slit control motors and rotary solenoid for shutter are operated by

signals sent from the computer.

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Figure 2.8 Functional block diagram of Hitachi F-3010 fluorescence

spectrophotometer

The configuration of the optical system of Model F-3010

fluorescence spectrophotometer is shown in Fig. 2.9. The radiation coming

from the Xe-lamp is converged at the entrance slit S1 of the excitation

monochromator through lenses L1 and L2. Only the light dispersed by the

excitation concave grating (excitation beam) enters the exit slit S2. The

excitation beam from the exit slit S2 is split by a beam splitter BS and a part

of the split beam is diverted to the monitor detector via lens L3 and diffusion

plate for measurement of its intensity.

On the other hand, most of the split beam after BS is reflected by

the mirror M1 and converges at the sample cell through lens L4. The

fluorescence coming out of the sample is restricted into the entrance slit S3 of

the emission monochromator through lenses L5 and L6. The fluorescence

dispersed by the emission concave grating passes through the exit slit S4 and

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is converged at the photomultiplier via concave mirror M2 for intensity

measurements. The emission shutter is provided in order to protect the

photomultiplier. It automatically closes upon opening the lid of the sample

compartment.

Figure 2.9 Optical system configuration of Hitachi F – 3010 Fluorescence

Spectrophotometer

2.10 TRANSMISSION ELECTRON MICROSCOPY (TEM)

Novel nano-structured materials, nano-crystals and nano-particles

require 2D and 3D characterization and qualification. The imaging and

analysis tools are capable of an imaging and analytical range from a few

nanometer resolutions down to sub-Ângström resolution. Wide-range

scanning and transmission electron microscopy (SEM and TEM) provides the

resolution required to qualify these materials for nanomaterials preparation

and processes. Structural information, such as morphology and

crystallography, as well as chemical, magnetic and electrical, strain and stress

33

information can be obtained with various degrees of resolution and a wide

variety of sample classes. Dual Beam microscopy adds the power of the

focused ion beam (FIB) for site-specific cross-sectioning in order to gain a

better understanding of the materials below their surfaces.

Figure 2.10 Schematic diagram of TEM

The electrons from the electron gun are accelerated to very high

voltages (100 – 200 keV) which are allowed to pass through a specimen and

focusing lenses. The lens-systems consist of electronic coils generating an

electromagnetic field. The ray is first focused by a condenser. It then passes

through the specimen, where it is partially deflected. The degree of deflection

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depends on the electron density of the specimen. The greater the mass of the

atoms, the greater is the degree of deflection. If the intermediate lens (shown

in Fig. 2.10) is adjusted so that its object plane is the image plane of the

projector lens, then an image is projected onto the viewing screen. If the back

focal plane of the objective lens acts as the object plane for the intermediate

lens, then the diffraction pattern is projected in the viewing screen (Williams

et al 1996). The TEM has the advantage that it is able to resolve to the order

of a few Å. In this work a HITACHI (HF-2000) (Fig. 2.11) is used to study

the morphology of the nanoparticles with a resolution of 2–3 Å. The samples

were examined under the TEM after dispersing them in acetone and placing a

few drops of the mixture in the Cu grid. The above experiment techniques are

utilized for the characterization of present prepared samples, which are

discussed in the following chapters.

Figure 2.11 Photograph of the Transmission Electron Microscope