CHAPTER 2 MATERIAL PREPARATION AND CHARACTERIZATION...
Transcript of CHAPTER 2 MATERIAL PREPARATION AND CHARACTERIZATION...
14
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
15
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.
16
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)
17
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.
18
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
19
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.
20
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
21
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.
22
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
23
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
24
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
25
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) = +
26
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
27
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
28
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.
29
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
30
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.
31
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
32
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
34
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