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

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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

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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

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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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