Chapter 2
Synthesis and Characterization of Nanostructured
Materials
This chapter presents succinctly an overview of the recent trends in
materials synthesis methodologies to obtain multifunctional materials. The
state of the art includes a description of the methods and understanding of
the various aspects of the formation mechanism of the nanostructures for
applications in photocatalysis and DSSCs. This chapter also discusses the
importance and the information obtained from the various characterization
techniques employed during the course of the present work.
** Part of the published article: Chemical Sensors, Review Article
Chapter 2
45
2.1. INTRODUCTION:
Nanoscience and nanotechnology are interdisciplinary emerging areas that
are expected to have wide ranging implications in all fields of science and
technology including materials science, medicine, biology, electronics, aerospace,
environment and energy sector etc. Since historical times, the development of new
synthesis procedures for the design and fabrication of nanoscale materials with
controlled shape and size has been an exciting field. Significant advances have been
made in several directions followed by an understanding of basic principles
underlying the methods to obtain materials with desired properties and subsequently
fine tuning and tailoring the procedures to obtain the product in the desired
morphology. The selected approach would also be tailored appropriately to meet the
requirements of energy conservation and stipulated green technology principles with
the expediency of up-scaling. Adopting these practices centre around economic
viability and meeting the industrial demand which practically would lead to simpler
techniques and versatility to be routinely adopted for similar materials leading to
generic procedures. Synthesis of materials for a select application entails imparting
all the necessary criteria that leads to rendering the materials with the desired
properties. This chapter explicitly examines different techniques that can be used for
the synthesis of metal oxide nanostructures including hydro/solvothermal,
microwave assisted, template assisted, sol-gel, atomic layer deposition (ALD),
chemical vapor deposition (CVD) and electrochemical methods. Characterization of
nanomaterials is a essential component of Materials research to ascertain the
suitability of the synthesiszed material for the desired application. This chapter
outlines various characterization techniques and information made available by the
techniques employed during the course of the present work.
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46
2.2. SYNTHESIS METHODS:
2.2.1. Hydro/Solvothermal Method:
Hydro and solvothermal techniques are the most important and well
established methods for the synthesis of nanomaterials under controlled temperature
and pressure.1-5 These reactions are normally conducted in teflon lined stainless steel
vessels called autoclaves under high pressure. The fabrication of nanomaterials with
facile autoclave synthesis offers great advantages, such as more economic advantage,
environmentally friendly and large scale production of the fine particles. The key
advantage in hydro/solvothemal methods is that the reaction parameters like
operating temperature, reaction time, solute volume, choice of solvents and additives,
concentration and the choice of different geometries of autoclaves that are can be
altered readily.1 The process can be described as follows. When the solvent is heated
up to its boiling point in a closed vessel, the autogenous pressure far exceeds
ambient pressure. Performing a chemical reaction under such conditions is referred
as solvothermal, where water is used as solvent it is referred to as hydrothermal
processing.2,6 Solvent at elevated temperature plays an essential role in the precursor
to material transformation because the vapor pressure is much higher and the
structural/physical properties of solvent at elevated temperatures is different from
that at room temperature. At elevated temperature and under closed conditions the
solvents act as supercritical a fluid that readily dissolves all inorganic substances and
increases the reactivity thereby allowing the subsequent crystallization of dissolved
substances. The products of hydro/solvothermal reactions are usually crystalline and
do not require any post annealing treatments.6 The changes mentioned above offer
additional controlling parameters to produce a variety of high-quality nanoparticles
and nanotubes, which are not possible at low temperatures.7 In spite of the flexibility
Chapter 2
47
offered by the hydro/solvothermal methods, it is difficult to understand the exact
mechanism which would enable predicting a prior the desired phases and
morphologies that could be obtained.1
2.2.2. Microwave Assisted Synthesis:
Microwave assisted synthetic technique (MAS) is a viable method and opens
up a new promising energy effective approach for the synthesis of nanostructured
materials. Microwaves are electromagnetic waves which consist of electric and
magnetic field components in the range of 1 mm to 1 m and their corresponding
frequencies between 0.3 to 300 GHz. 2.45 GHz is most commonly used frequency
for chemical reactions and is selectively absorbed by polar molecules (solvents or
reagents).8 The energy of microwave photon in the above frequency range is too low
to break the chemical bonds. This implies that the microwave photon does not
induce the chemical reaction but provides only heat energy to initiate the chemical
reaction.9.10 Several hypothesis have been put forth to explain the rapid heating
caused by the microwave process.11-14 The microwave heating process is the transfer
of electromagnetic energy to thermal energy based on two basic mechanisms, the
dipolar mechanism and ion conducting mechanism. These phenomena depend on the
absorption capability of specific polar molecules, which means greater the polarity
of molecule greater the microwave effect in terms of transfer of heat. During the
process of irradiation of samples at microwave frequencies, ions and dipoles attempt
to align with the external electric field. In this process ions and dipoles undergo
collision with each another and release heat energy.15 Microwaves directly contact
with material and can penetrate through the material, heat can be generated
throughout the volume of the material resulting in volumetric heating. In contrast, in
conventional heating methods heat energy transfer to the reactants is through the
Chapter 2
48
walls of the vessel which is slow and inefficient to activate the reactants. Figure 1
shows the differences in the conventional heating and microwave heating methods.
Figure 1. Difference in heating rates of conventional and microwave heating techniques
(left), inverted temperature gradients microwave vs oil bath heating: Temperature profile
after 60 s as affected by microwave irradiation compared to oil bath heating. Microwave
rises the temperature of the whole volume simultaneously where as in oil bath heating the
reaction vessel gets heated first.1
The advantage of microwave assisted method over conventional heating
method is the faster reaction rates, shorter time, improved yield, small size particles,
high purity materials and enhanced physical properties. Additionally, this method
also offers the possibility of varying the experimental parameters such as the
precursor concentration ratio, surfactant choice, solvent, time of reaction and
temperature.
In summary microwave assisted synthesis technique is an attractive process
currently being explored for the synthesis of numerous materials in large scale
industrial production. This method is time, cost, energy saving and provides rapid
heating opening up new challenging environment for experimental design. Even
though the mechanism is not well understood, the potential use of microwave
assisted synthesis is well established for wide range of complex nanostructured
materials.
Chapter 2
49
2.2.3. Template Directed Synthesis:
Template directed synthesis strategy is an unchallenging straightforward
approach to fabricate the nanostructures with precise control of their shape and size.
In this method, prefabricated or pre-existing templates are employed as structural
framework to manipulate the formation and growth of nanostructures within the
spatially confined space. The obtained nanostructure gets cast into dimensions
similar to the template such as its size and morphology. Several reviews have been
documented to exploit the structural features, formation and mechanism of template
based synthesis for various dimensional nanostructures.16,22 Based on the nature of
the template, there are two kinds of templates used and named as soft template and
hard template. In general, soft templates can be composed of a variety of materials
including biological scaffolds such as peptides and lipids, polymers, micelles,
naturally occurring gels, liquid crystals and block copolymers, As against track-
etched polycarbonate membrane (PCM), anodic alumina membranes (AAO), mono-
dispersed silica spheres, polymer latex colloids, carbon spheres and carbon
nanotubes that can be considered as hard templates. Basic structure and some
representative examples of hard templates are shown in figure 2.23 Hard templates
provide a powerful scaffold for fabricating complex 1D nanostructures, which are
difficult to fabricate using other methods. These templates allow for varying the
composition of the nanostructures in both axial and radial directions.20 The
advantage of hard template is that they are readily available in large quantities with a
wide range of narrow size distribution and well known simple synthetic
formulations.
In general, hard template-directed process for the synthesis of nanostructures
involves three major steps, i.e., pre-fabrication of template materials, formation or
Chapter 2
50
deposition of target nanostructures within or around the templates, and removal of
templates by suitable techniques, such as chemical etching or calcinations.20,25
Deposition and filling of target precursor solution into or around a template that can
be easily obtained with other synthetic techniques, such as electrochemical
deposition26, chemical deposition27, sol–gel deposition28, CVD 29, and atomic layer
deposition (ALD)30. However, alongside the advantages, there are some major
disadvantages with hard templating methods. Firstly, the chemical etching of
template has to be carried out by either strong acid (HF) or strong base (NaOH) and
the target synthetic materials must be stable against these etching agents. Second,
another synthetic solution step is required to introduce target precursor materials and
this limits the range of materials whose stability in solution is a requirement.19
Figure 2. A compilation of images that show representative examples for certain categories
of soft templates. The following materials are presented: (a) a plant stem (b) freeze-dried
starch (c) a polymeric colloidal crystal (d) a three-dimensionally ordered macroporous
structure (e) a polyurethane foam (f) an AAO membrane (g) an in situ NaCl crystal template
(h) a polymer produced from an AAO membrane (i) individual colloidal spheres (j) and rod-
shaped nanoparticles.24
On the other hand, soft templating method is a simple and reliable pathway
for the synthesis of well-ordered mesoporous materials. Most of soft templates are
known surfactants and are classified as cationic, anionic and non-ionic based on the
carrying hydrophilic or hydrophobic head group at neutral pH. The cationic and
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51
anionic surfactants interact with precursor materials through electrostatic interaction
and nonionic surfactants are rarely involved in electrostatic interaction. These
surfactants easily dissolve in proper solvents and easily interact with precursor
molecules. These surfactants also can generate supramolecular self-aggregates
called micelles at their critical concentration and undergone phase transition to
crystalline state. The advantages of soft template methods are that the templates are
low cost, easily available, can be carried out in mild reaction condition and soluble
in proper solvents. It is also possible to get a large variety of mesoporous structures
by organizing the concentration and composition of templates. Although it provides
a simple and versatile root to synthesize mesoporous structures with various
templates there are still some issues that need to be addressed. The methods
involved are complicated, sol gel and hydrolysis based solution phase methods that
are difficult to control. Though it is easy to remove the template by selective
method, the process itself could cause reformulating of the mesoporous structure.
2.2.4. Sol-Gel Method:
The sol-gel synthesis procedure is a wet-chemical technique extensively used
for the fabrication of various nanostructured materials31. This procedure comprises
of four steps viz hydrolysis, condensation, drying, and thermal decomposition.32,33
The flexibility of the sol-gel process enables optimization of sensor parameters via
control of the conditions involved in each of the above four steps. In a typical sol-gel
process, a suspension of nanometer sized colloidal particles (sol) is obtained by the
hydrolysis of precursor molecule.34-36 The colloidal suspensions of particles (sol)
react with each other or interact by Vander-Waals forces or hydrogen bonds forming
a three dimensional oxide network called gel. The three dimensional network of gel
can easily shape materials into complex geometries in a gel state. Drying or thermal
Chapter 2
52
evaporation of gel to remove the liquid phase leads to the final products.37 The rate
of hydrolysis and condensation plays a very important role in controlling the size
and morphology of the final products. More significantly slower and controlled
hydrolysis results in the precise control in their size and more importantly
properties.38 Generally, metal salts and metal alkoxides are used as precursors in sol-
gel process. This sol-gel procedure offers several advantages to obtain bulk as well
as nanomaterials in high purity and with precise control of the composition and
texture. Perhaps, it is a straightforward method to synthesize and control the
composition and homogeneity of mixed metal oxide materials. The sole flexibility of
sol–gel method is that it involves a large number of adjustable parameters including
nature and concentration of precursors, temperature, solvent, aging and drying
conditions.39 This method facilitates synthesis of porous materials, excellent
composition control, homogeneity and industrially large scale yield. Although sol-
gel chemistry has several advantageous for the synthesis of bulk materials, it has
some limitations when it comes to preparation of nanoscale materials. The high
reactivity of metal-oxide precursors could accelerate the hydrolysis,that slightly
alters the experimental conditions resulting in distorted structures as well as
reproducibility issues.40 The synthesized nanostructures exhibit poor crystallinity,
and the post annealing treatment could destroy or alter the crystalline material
morphology.
2.2.5. Chemical Vapour Deposition (CVD):
Chemical vapour deposition is a powerful technique that can be used to produce
high quality thin films. CVD is one of the best processing method for the deposition of
amorphous, single-crystalline, polycrystalline thin films and coatings for a wide range of
applications.41 In a typical CVD process, the substrate is exposed to gas precursor, which
Chapter 2
53
reacts and/or decomposes in activated environment (heat, light, plasma) to produce the solid
films.42,43 The deposition process occurs either in gas phase, near activated substrate or on
the substrate surface. The inert gas transports the precursor molecule into the reaction
chamber and facilitates physisorption of precursor molecules onto the substrate surface. The
physisorbed precursor molecule decomposes and generates atoms and by-products on the
surface. The adsorption of atoms on the surface nucleates the growth process accompanied
by exhaustion of by-products and unreacted precursors out of the chamber. Deposition of
the atoms leads to the final solid thin film of desired materials. The crystal structure, film
morphology and the film thickness strongly depends on the precursor molecule,
temperature, chemical reaction and deposition rate. Several chemical reactions are involved
in CVD process include thermal decomposition, reduction, hydrolysis, oxidation, and
carbonization.44 There are a variety of modified CVDs to enhance deposition process, which
involves the use of plasmas, ions, photons, lasers and hot filaments.45 There are also variants
in CVD technologies such as metal-organic chemical vapor deposition (MOCVD),
(OMCVD), plasma enhanced CVD (PECVD), low pressure CVD (LPCVD), atmospheric
pressure CVD (APCVD), laser Assisted CVD (LACVD) and liquid injection CVD, which
are used depending on the type of precursor materials available.41,46-49 The CVD offers a
route to create pure thin films in good uniform and controlled composition with good
adhesion. Due to the high deposition rate thick coating films can readily be obtained. On
the other side, there are some disadvantages with CVD technique. The most important
disadvantage is the requirement for the precursor involved, has to be volatile at room
temperature. Some precursors are deposited at elevated temperatures, but this could restrict
the range and type of substrates. It is difficult or tedious to deposit multicomponent mixtures
with good stoichiometry using multi-source precursors because different precursors have
different vaporisation rates. Moreover the CVD process exhausts the toxic by-products.50
Nevertheless, Chemical vapor deposition (CVD) is a cost-effective and versatile method to
Chapter 2
54
produce films with a wide variety of morphologies and capable of controlling porosity,
grain size and film thickness.
2.2.6. Atomic Layer Deposition:
Atomic layer deposition is a kind of chemical vapor deposition (CVD)
technique for the deposition of material layers with precise control at angstrom or
atomic layer.51 Unlike the CVD, ALD offers many advantages including accurate
thickness control, excellent conformality of the deposited films, high uniformity
over a large area, good reproducibility, low defect density and low growth
temperatures.52 ALD is surface and successive controlled process wherein the
growth of film and thickness is dictated by the self-terminating gas surface
reaction.53-55 ALD is the reaction between precursors materials are separated into
successive surface reactions. In this manner, the precursor material is introduced
separately into the substrate surface to be adsorbed as a thin film in a self-limiting
process, and each surface reaction is separated by a purge step to remove the
unreacted precursor and the by-product (Figure 3).56,57
Figure 3. Schematic representation of ALD process. In the step (a) the precursor molecule
is exposed on substrate surface to adsorb precursor and then excess precursor removed by
purging with inert gas (step (b)). In step (c) the precursor molecule deposited as second
layers which react with first layer and purge step is repeated to remove extra precursor. The
process is repeated to get desire thickness of film.51
Chapter 2
55
The film growth takes place in a cyclic manner and the growth sequence are
repeated as many number of times as required to get the desired film thickness.
ALD process can provide accurate thickness that can be controlled in every step.
Temperature, precursor material and substrate are the three parameters that
determine the deposition features. Prior to introducing the precursor material into the
substrate surface, the reactive sites should be created on the substrate to chemisorb
the precursor molecules. The interaction of first precursor with surface active sites
provides a new layer with terminating new reactive sites. Similarly, the second
precursor creates another new layer with new functional group and this is also
responsible for terminating the further reaction.55 ALD offers facile doping and
provides smooth uniform layer over large area without pin holes. SnO2 nanofibers
were fabricated by combining the electrospinning and ALD techniques choosing
polyacrylonitrile (PAN) electro-spinning material and electrospun PAN used as
template for SnO2 coating by ALD process.
2.2.7. Electrochemical Methods:
Electrochemical method is also a viable method to fabricate various 1D, 2D
and quasi dimensional metal oxide nanostructures at room temperature.
Electrochemical method offers distinctive advantages over other methods for the
synthesis of ordered arrays of nanochannels with high surface area and aspect ratio.
The electrochemical method consists of two electrodes called anode and cathode in
contact with electrolyte solution. The redox reaction occurs separately at anode and
cathode by transferring electrons from one species to another. There are two types of
approaches by means of electrochemical process that occur at the cathode and anode
that are electrode deposition and electrochemical anodisation. Electrode deposition
and anodization can as well be referred to as bottom-up and top-down respectively.
Chapter 2
56
In both processes the chemical reaction takes place at the interface between solid
and solution as a result of current flow through this interface.58 Electrochemical
method does not require expensive instrumentation, high temperatures, and is also
not a time-consuming process. The morphology, shape, size, crystallinity and other
parameters of the resulting nanostructures can be tailored conveniently by changing
the electrolyte concentration, applied voltage, temperature and anodization or
deposition time. However, there are some disadvantages with electrochemical
methods. Since the reaction is performed at room temperature, poor crystalline
product could be obtained. This method is only applicable to electrically conductive
materials such as metals, alloys, semiconductors this could restrict the range of
materials.59
Electrochemical deposition is an emerging technique typically used for the
deposition of metals and alloys at industrial level, and providing new avenues for the
synthesis of metal oxide nanostructures from an economic and academic point of
view.60 Electrochemical deposition of metal oxides typically proceeds by either
reduction or oxidation of metal ions in a solution through the electron transfer
between the electrode and the electro-active species present in the solution.61 In both
cases deposition of metal oxide proceeds by the dissociation of the corresponding
strong oxidant metal complex and precipitates onto the electrode. The deposition
technique is less advanced for studying gas sensing materials. Electrodeposition
technique can be employed as a fast, easy, and reliable process to produce stable
metal oxide thin films for large-scale production.
Electrochemical anodization is another class in electrochemical method,
where highly ordered mesoporous metal oxides can be obtained by the anodic
oxidation of corresponding metal sheet. The best example to illustrate the anodic
Chapter 2
57
oxidation is the synthesis of Aluminium oxide membrane (AAO). The self-anodic
oxidation of aluminum foil in a strong acid electrolyte at high potentials ranging
from 2 to 500 volts results in the highly ordered porous membrane. The porous
structure such as pore diameter, pore wall thickness and pore density etc. can be
readily altered by the applied potential, the anodization time proportionally
controlling the length of the pores. The anodization method can be effectively
utilized to fabricate various metal oxide nanotubes and the detail mechanism has
been discussed.62-65
2.3. CHARACTERIZATION TECHNIQUES:
The fundamental characteristic of nonmaterials lies in the fact is that the
properties of materials change dramatically when their size is reduced to nanometer
range. Measurements of nano dimension, studying their properties and establishing
the structure property relationship of nano materials is not an easy task. This the
primary motivation to undertake such studies. This has led to an upsurge in research
activities coupled with the discovery of sophisticated characterization tools to
facilitate control of the size, dimension in the nano range and study their optical,
electronic properties as well. Therefore characterization of nanomaterials is also an
emerging field posing a lot of challenges to scientist. The part of this chapter
discusses the importance and applications of various characterization techniques
employed during the course of work.
2.3.1. Powder X-Ray Diffraction Technique (XRD):
X-ray powder diffraction (XRD) is a non-destructive analytical technique
primarily used for the determination of a crystallographic structure and unit cell size
of natural and synthesized materials. X-ray diffraction is also used to measure the
Chapter 2
58
crystallite size in a powder sample.66 For the present work powder X-ray diffraction
(XRD) patterns were recorded on a Siemens (Cheshire, UK) D5000 X-ray
Diffractometer over a 2 range of 2o to 60o using CuKα (=1.5406Å) radiation at
40 kV and 30 mA with a standard monochromator using a Ni filter.
2.3.2.Transmission Electron Microscopy (TEM):
The transmission electron microscope (TEM) is used to examine the
structure, composition, and properties of specimens in submicron detail. It also
enables the investigation of crystal structures, orientations and chemical
compositions of phases, precipitates and contaminants through diffraction pattern,
characteristic X-ray, and electron energy loss analysis. For the present work
transmission electron microscope (TEM) (Philips Tecnai G2 FE1 F12, operating at
80-100 kV) was extensively used to investigate the morphology and size of the
particles. The samples for TEM were prepared by dispersing the material in ethanol
by ultrasonication and drop drying onto a formvar coated copper grid. HRTEM was
carried out on a JEOL TEM 2010 microscope operating at 200 kV.
2.3.3. Scanning Electron Microscopy (SEM):
The scanning electron microscope (SEM) is one of the most versatile
instruments available for the examination and analysis of the microstructure
morphology, topography, grain orientation and chemical composition of materials.
For the present work scanning electron microscopic (SEM) analysis of the prepared
materials were performed by using Hitachi S–3000N Scanning Electron Microscope
operated at 10 kV.
2.3.4. UV-Vis SPECTROSCOPY:
The UV-Vis spectra have broad features that are of limited use for sample
Chapter 2
59
identification but are very useful in analytical chemistry for quantitative
determination of various analytes. UV spectroscopy is used for measuring the
absorption, emission and transmission of the ultraviolet and the visible wavelengths
by matter. The concentration of an analyte in solution can be determined by
measuring the absorbance at some wavelength and applying the Beer-Lambert Law.
UV absorption spectroscopy is one of the best methods for structure identification of
organic molecules determination of impurities in organic molecules. This is the
simple method for estimating the band gap energy values of materials. For the
present work the UV-Vis spectroscopy studies was carried on Varian Cary 5000
spectrophotometer in the wavelength range of 200 - 800 nm.
2.3.5. UV-Vis Diffuse Reflectance Spectroscopy:
UV-Vis Diffuse Reflectance Spectroscopy is an ideal tool for characterizing
optical and electronic properties of solid samples. This technique is very useful for
measuring the reflectance, transmittance and absorbance of solid samples as well as
thin films. For the present work the UV-DRS (ultraviolet diffuse reflectance
spectroscopy) analysis was done on a Varian Cary 5000 spectrophotometer using
KBr diluted pellets of solid samples and pure KBr was used as the reference.
2.3.6. Fourier Transform Infrared Spectroscopy (FTIR):
IR spectroscopy is primarily used to identify bond types, structures, and
functional groups in organic and inorganic compounds. For the present work FT-IR
spectra of the solid samples were recorded on Bruker Alpha spectrometer equipped
with a DTGS-KBr detector over a range of 4000 cm-1- 400 cm-1.
2.3.7. Raman Spectroscopy:
For the present work Micro Raman spectra were recorded using HORIBA
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60
Jobin Yvon Raman Spectrometer, equipped with an 17 mW internal excitation
source of He−Ne laser (632.8 nm) CCD camera and a scan resolution held at 2 cm-1.
2.3.8. Photoluminescence Spectroscopy:
For the present work the room-temperature photoluminescence (PL) spectra
were recorded by means of a Jobin–Yvon Fluorolog-3 spectrofluorimeter using the
xenon lamp (450 W) as light source. The samples for PL studies were prepared by
dispersing small amounts of synthesized samples in water by ultrasonication for 10
min.
2.3.9. Thermogravimetric Analysis (TGA):
Thermogravimetric analysis (TGA) is one of the thermal analysis technique
that is performed on samples to determine changes in weight in relation to change in
temperature. TGA is commonly employed for materials, to determine degradation
temperatures, absorbed moisture content of materials, the level of inorganic and
organic components in materials, decomposition points of explosives, and solvent
residues. For the present work thermogravimetric analysis was done with TA Q50
analyser in N2 atmosphere, with a heating rate of 10o/min from 25 OC to 800 OC.
2.3.10. BET Surface Area & BJH Pore Size Distribution Analysis:
Gas sorption (both adsorption and desorption) at the clean surface of dry
solid powders is the most popular method for determining the surface area of these
powders as well as the pore size distribution of porous materials. For the present
work N2 -sorption studies were performed at 77 K on a Micromeritics ASAP 2020
and Quantachrome Autosorb automated gas sorption system (Nova 4000e). The
calcined samples were pretreated at 200 oC for over 6 hours prior to the sorption
analysis. The specific surface area and the pore-size distribution (PSD) were
Chapter 2
61
calculated by the Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH)
methods, respectively.
2.4. CONCLUSIONS:
In conclusion, this chapter presents an overview of some of the environmentally
favourable methodologies for the fabrication of a wide range nanomaterials. The
methods include hydro/solvothermal and microwave assisted synthesis, several wet
deposition techniques, CVDs, template mediated methodologies etc. In particular
the last decade, has witnessed a major research focus on hydro/solvothermal
synthesis of nanostructured materials because of its noteworthy advantages, such as
relatively high yield, being cost-efficient, convenience in handling and ease in
composition control. Hydrothermal synthesis is a distinctive technique endowed
with the ability to deliver a great variety of pure, doped, single and mixed oxides,
facilitating growth of preferred crystal phase and planes by controlling the synthesis
parameters. On the other hand, microwave assisted synthetic processes have shown
significant advances to produce wide variety of materials, which allow the rapid and
scalable synthesis of metal oxide with tunable properties. Apart from conventional
synthetic methods, several deposition techniques including sol-gel, CVD, ALD and
electrochemical techniques have been discussed. The advances in the synthesis of
these techniques have paved the way to fabricate a wide range of new functional
materials, in particular 1D and 2D nanostructured materials with many potential
applications. Among them ALD emerged as the best choice to create complex
materials because of its slow process, high-throughput production and homogenous
coating that are difficult to obtain with other deposition techniques. Most of these
methods combine with hard or soft template approaches to generate a wealth of new
hierarchical materials with different morphologies. It is noteworthy that high quality
Chapter 2
62
hetero-architecture materials can be obtained directly with template directing
method. Creating such materials by depositing, coating or incorporating into existing
nanostructures is a real challenge.
Chapter 2
63
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