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Introduction:
In order to explore novel physical properties and phenomena
and realize potential applications of nanostructures and
nanomaterials, the ability to fabricate and process nanomaterials
and nanostructures is the first corner stone in nanotechnology.
There exist a number of methods to synthesize the nanomaterials
which are categorized in two techniques “top down and bottom up”.
Solid state route, ball milling comes in the category of top down
approach, while wet chemical routes like sol-gel, co-precipitation,
etc. come in the category of bottom up approach. Secondly,
characterization of nanomaterials is necessary to analyze their
various properties. Therefore, this chapter describes the various
methods of synthesis and characterization of nanomaterials.
Characterization techniques include XRD, SEM, TEM, EDAX, UVVisible
spectroscopy, FTIR spectroscopy, etc.
2.2. Synthesis of Nanomaterials:
Fabrication of nanomaterials with strict control over size,
shape, and crystalline structure has become very important for the
applications of nanotechnology in numerous fields including
catalysis, medicine, and electronics. Synthesis methods for
nanoparticles are typically grouped into two categories: “top-down” and “bottom-up” approach. The first involves the division of a
massive solid into smaller and smaller portions, successively
reaching to nanometer size. This approach may involve milling or
attrition. The second, “bottom-up”, method of nanoparticle
fabrication involves the condensation of atoms or molecular entities
in a gas phase or in solution to form the material in the nanometer
range. The latter approach is far more popular in the synthesis of
nanoparticles owing to several advantages associated with it. Fig. 2.1
shows the general overview of the two approaches. There are many
bottom up methods of synthesizing metal oxide nanomaterials, such
as hydrothermal, [1, 2] combustion synthesis [3], gas-phase methods
[4, 5], microwave synthesis and sol-gel processing [6]. Sol-gel
processing techniques will be discussed in detail in this chapter
because the materials reported in subsequent chapters were
fabricated using this method. However, an overview of other
techniques usually employed for the synthesis of nanomaterials is
also discussed hereunder.
2.2.1. Combustion route:
Combustion synthesis leads to highly crystalline particles with
large surface areas [7, 8]. The process involves a rapid heating of a
solution containing redox groups [9]. During combustion, the
temperature reaches approximately 650 °C for one or two minutes
making the material crystalline.
2.2.2. Hydrothermal method:
Hydrothermal synthesis is typically carried out in a
pressurized vessel called an autoclave with the reaction in aqueous
solution [10]. The temperature in the autoclave can be raised above
the boiling point of water, reaching the pressure of vapour
saturation. Hydrothermal synthesis is widely used for the
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preparation of metal oxide nanoparticles which can easily be
obtained through hydrothermal treatment of peptized precipitates of
a metal precursor with water [10, 11]. The hydrothermal method can
be useful to control grain size, particle morphology, crystalline phase
and surface chemistry through regulation of the solution
composition, reaction temperature, pressure, solvent properties,
additives and aging time [9].
2.2.3. Gas phase methods:
Gas phase methods are ideal for the production of thin films.
Gas phase synthesis can be carried out chemically or physically.
Chemical vapour deposition (CVD) is a widely used industrial
technique that can coat large areas in a short space of time [9].
During the procedure, metal oxide is formed from a chemical
reaction or decomposition of a precursor in the gas phase [12, 13].
Physical vapour deposition (PVD) is another thin film
deposition technique. The process is similar to chemical vapour
deposition (CVD) except that the raw materials/precursors, i.e. the
material that is going to be deposited starts out in solid form,
whereas in CVD, the precursors are introduced to the reaction
chamber in the gaseous state. The process proceeds atomistically
and mostly involves no chemical reactions. Various methods have
been developed for the removal of growth species from the source or
target. The thickness of the deposits can vary from angstroms to
millimeters. In general, these methods can be divided into two groups: evaporation and sputtering. In evaporation, the growth
species are removed from the source by thermal means. In
sputtering, atoms or molecules are dislodged from solid target
through impact of gaseous ions (plasma) [14].
2.2.4. Microwave synthesis:
Microwave synthesis is relatively new and an interesting
technique for the synthesis of oxide materials [15]. Various
nanomaterials have been synthesized in remarkably short time
under microwave irradiation [16, 17]. Microwave techniques
eliminate the use of high temperature calcination for extended
periods of time and allow for fast, reproducible synthesis of
crystalline metal oxide nanomaterials. Utilizing microwave energy for
the thermal treatment generally leads to a very fine particle in the
nanocrystalline regime because of the shorter synthesis time and a
highly focused local heating.
2.2.5. Sol-gel method:
The sol-gel process is a capable wet chemical process to make
ceramic and glass materials. This synthesis technique involves the
conversion of a system from a colloidal liquid, named sol, into a
semi-solid gel phase [18, 19, 20]. The sol-gel technology can be used
to prepare ceramic or glass materials in a wide variety of forms:
ultra-fine or spherical shaped powders, thin film coatings, ceramic
fibres, microporous inorganic membranes, monolithics, or extremely
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porous aerogels. An overview of the sol-gel process is illustrated in
Fig. 2.2.
This technique offers many advantages including the low
processing temperature, the ability to control the composition on
molecular scale and the porosity to obtain high surface area
materials, the homogeneity of the final product up to atomic scale.
Moreover, it is possible to synthesize complex composition materials,
to form higher purity products through the use of high purity
reagents. The sol-gel process allows obtaining high quality films up
to micron thickness, difficult to obtain using the physical deposition
techniques. Moreover, it is possible to synthesize complex
composition materials and to provide coatings over complex
geometries [18, 19, 20].
The starting materials used in the preparation of the sol are
usually inorganic metal salts or metal organic compounds, which by
hydrolysis and polycondensation reactions form the sol [18, 19, 20].
Further processing of the sol enables one to make ceramic materials
in different forms. Thin films can be produced by spin-coating or dipcoating.
When the sol is cast into a mould, a wet gel will form. By
drying and heat-treatment, the gel is converted into dense ceramic or
glass materials. If the liquid in a wet gel is removed under a
supercritical condition, a highly porous and extremely low density
aerogel material is obtained. As the viscosity of a sol is adjusted into
a suitable viscosity range, ceramic fibres can be drawn from the sol.
Ultra-fine and uniform ceramic powders are formed by precipitation,
spray pyrolysis, or emulsion techniques.
2.3. Characterization Techniques:
2.3.1. X-ray Diffraction:
The German Physicist, Von Laue in 1912 was the first who
took up the problem of X-ray diffraction (XRD) with the cause that,
“if crystals were composed of regularly spaced atoms which might act
as scattering centers for x-rays, and if X-rays were electromagnetic
waves of wavelength about equal to the inter atomic distances in
crystals, then it should be possible to diffract X-rays by means of
crystals” [21]. Now a days, X-ray diffraction is most extensively used
technique for the characterization of the materials. A lot of
information can be extracted from the XRD data. This is an
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appropriate technique for all forms of samples, i.e. powder and bulk
as well as thin film. Using this technique, one can get the
information regarding the crystalline nature of a material, nature of
the phase present, lattice parameter and grain size [22]. From the
position and shape of the lines, one can obtain information regarding
the unit cell parameters and microstructural parameters (grain size,
microstrain, etc), respectively. In case of thin films, the change in
lattice parameter with respect to the bulk gives the idea about the
nature of strain present in the system.
The interaction of X-ray radiation with crystalline sample is
governed by Bragg’s law, which depicts a relationship between the
diffraction angles (Bragg angle), X-ray wavelength, and interplanar
spacing of the crystal planes. According to Bragg’s law, the X-ray
diffraction can be visualized as X-rays reflecting from a series of
crystallographic planes as shown in Fig. 2.3. The path differences
introduced between a pair of waves travelled through the neighboring
crystallographic planes are determined by the interplanar spacing. If
the total path difference is equal to nλ (n being an integer), the
constructive interference will occur and a group of diffraction peaks
can be observed, which give rise to X-ray patterns. The quantitative
account of Bragg’s law can be expressed as:
2d
hkl
sin θ = nλ
where d is the interplanar spacing for a given set of hkl and θ the
Bragg angle.