chapter - ii synthesis methods of …...physical methods are like inert gas condensation, physical...
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CHAPTER - II
SYNTHESIS METHODS OF NANOPARTICLES
2.1. INTRODUCTION
There is a tremendous research interest in the area of nanotechnology to develop
reliable processes for the synthesis of nanomaterials over a range of sizes and chemical
compositions. Although the conventional methods of synthesis of metal sols, known
since the times of Michael Faraday, continue to be used for generating metal
nanoparticles, there have been several improvements and modifications in the methods
which provide a better control over the size, shape, and other characteristics of the
nanoparticles. These developments have enabled studies of quantum confinement as
well as other properties dependent on size, shape, and composition [1]. Ligating
nanoparticles with organic molecules and assembling these in one-, two-, or three-
dimensional meso structures have added another dimension to this field wherein
collective properties of nanoparticles have been of special interest.
The exciting potential of nanomaterials can utilized to nano device applications,
only with a combination of nano building units and strategies for assembling them.
Self-assembly of nanoparticles synthesized by the colloidal route on suitable supports is
one of the interesting techniques currently being investigated for realizing such
structures. Though the synthesis and organization of nanoparticles provide
complementary tools for nanotechnology, processing of nanoparticles or nano powders
into bulk shapes, retaining nanosized is another challenging aspect, as far as structural
and engineering applications are concerned. Synthesis and assembly strategies of
nanoparticles mostly accommodate precursors from liquid, solid or gas phase; employ
chemical or physical deposition approaches; and similarly rely on either chemical
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reactivity or physical compaction to integrate the nanostructure building blocks within
the final material structure [2,3]. The variety of techniques that can be classified in
top-down or bottom-up approaches are schematically illustrated in Fig. 1.
Fig. 1 Schematic diagram of top-down or bottom-up approaches of synthesis
These techniques are further classified into three categories namely physical
methods, chemical methods and Bio-assisted methods. Physical methods are like Inert
gas condensation, physical vapour deposition, laser pyrolysis, Flame spray pyrolysis,
electro spraying techniques, melt mixing. Chemical methods are like sol-gel synthesis,
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micro emulsion technique, hydrothermal synthesis, polyol synthesis, plasma enriched
vapour deposition.
2.2. PHYSICAL METHODS OF NANOPARTICLE SYNTHESIS
Top-down approach, where synthesis is initialized with the bulk counterpart that
leaches out systematically bit-after-bit leading to the generation of fine nanoparticles.
Physical methods apply mechanical pressure, high energy radiations, thermal energy or
electrical energy to cause material abrasion, melting, evaporation or condensation to
generate nanoparticles. These methods mainly operate on top-down strategy and are
advantageous as they are free of solvent contamination and produce uniform mono
disperse nanoparticles. At the same time, the abundant waste produced during the
synthesis makes physical processes less economical.
High energy ball milling, laser ablation, electro spraying, inert gas
condensation, physical vapour deposition, laser pyrolysis, flash spray pyrolysis, melt
mixing are some of the most regularly used physical methods to generate nanoparticles.
2.2.1. High Energy Ball Milling
The early Nano materials were made by a simple method called ball milling.
High energy ball milling (HEBM), first developed by John Benjamin in 1970 to
synthesize oxide dispersion strengthened alloys capable of withstanding high
temperature and pressure, is a robust and energy efficient synthesis method to generate
nanoparticles with varying shapes and dimensionalities. In high energy ball milling
process, the moving balls transfer their kinetic energy to the milled material. This
results in the breaking of their chemical bonds and rupturing of the milled materials
into smaller particles with newly created surfaces. Milling media, milling speed,
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ball-to-powder weight ratio, type of milling (dry or wet), type of high energy ball mill
(vibrator mill, planetary mill, attritor mill, tumbler ball mill, etc.), milling atmosphere
and duration of milling regulate the amount of energy transfer between the balls and the
material during the process, and thus affect the physical and morphological properties
of the resultant nanomaterials. The high energy ball milling process sometimes involve
very high local temperature (>1000 °C) and pressure (several GPa) conditions and thus
also considered as a mechano chemical synthesis process [4].
Fig. 2 (a) High energy ball milling (HEBM) system (b) Schematic representation
of the HEBM synthesis with and without surfactant
Currently, surfactant assisted high energy ball milling is used as an efficient
strategy for the synthesis of NPs with precise size and specific surface characteristics
(Fig. 2b). Surfactants are the surface active agents containing both hydrophobic and
hydrophilic properties and can be classified as anionic, cationic, zwitterionic and
nonionic depending upon the surface charge characteristics of their hydrophilic group.
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Following adsorption on the material surface, the surfactant molecules generate
electrostatic/steric forces which stabilize the milling particles, and thus minimize the
uncontrolled fracturing of particles. Surfactant can also lower the surface energy of the
freshly generated fine particles by forming thin organic layer and introducing long
range capillary forces that lower the energy for crack propagation. This prevents the
particles from agglomeration and cold welding that may lead to enhancement of
particle size. Nature and amount of the surfactant used during the HEBM tremendously
affect the physical characteristics of the nanoparticles.
2.2.2. Electron Beam Lithography
The origins of the use of lithography date from the 17th
century in applications
of ink Imprinting. Nowadays, the techniques and applications of lithography have been
diversified, but the concept keeps valid. Lithography is the process to transfer a pattern
from one media to another. Electron beam lithography appeared in the late 60s and
consists of the electron irradiation of a surface that is covered with a resist sensitive to
electrons by means of a focused electron beam. The energetic absorption in specific
places causes the intra molecular phenomena that define the features in the polymeric
layer. This lithographic process, capable of creating submicronic structures, comprises
three steps: exposure of the sensitive material, development of the resist and pattern
transfer. It is important to consider that these should not be realized independently and
the final resolution is conditioned for the accumulative effect of each individual step of
the process. A great number of parameters, conditions and factors within the different
subsystems are involved in the process and contribute to the EBL operation and result.
In a direct write EBL system, the designs are directly defined by scanning the energetic
electron beam, and then the sensitive material is physically or chemically modified due
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to the energy deposited from the electron beam. This material is called the resist, since,
later, it resists the process of transference to the substrate. The energy deposited during
the exposure creates a latent image that is materialized during chemical development.
For positive resists, the development eliminates the patterned area, whereas for
negative resists, the inverse occurs [5]. In consequence, the shape and characteristics of
the electron beam, the energy and intensity of electrons, the molecular structure and
thickness of the resist, the electron–solid interactions, the chemistry of the developer in
the resist, the conditions for development and the irradiation process, from the structure
design to the beam deflection and control, are determinant for the results, in terms of
dimensions, resist profile, edge roughness, feature definition, etc.
2.2.3. Inert Gas Condensation Synthesis Method
One of the earliest methods used to synthesize nanoparticles, is the evaporation
of a material in a cool inert gas, usually He or Ar, at low pressures conditions, of the
order of 1 mbar. It is usually called ‘inert gas evaporation’. Common vaporization
methods are resistive evaporation, laser evaporation and sputtering. A convective flow
of inert gas passes over the evaporation source and transports the nanoparticles formed
above the evaporative source via thermophoresis towards a substrate with a liquid N2
cooled surface. A modification which consists of a scraper and a collection funnel
allows the production of relatively large quantities of nanoparticles, which are
agglomerated but do not form hard agglomerates and which can be compacted in the
apparatus itself without exposing them to air. This method was pioneered by the group
of Birringer and Gleiter. Increased pressure or increased molecular weight of the inert
gas leads to an increase in the mean particle size. This so-called Inert Gas Condensation
method is already used on a commercial scale for a wide range of materials. Also
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reactive condensation is possible, usually by adding O2 to the inert gas in order to
produce nanosized ceramic particles. Another method replaces the evaporation boat by
a hot-wall tubular reactor into which an organo metallic precursor in a carrier gas is
introduced. This process is known as Chemical Vapor Condensation referring to the
chemical reactions taking place as opposed to the inert gas condensation method.
Fig. 3 explains the inert gas condensation method of nanoparticle synthesis [6].
Fig. 3(a-c) Schematic diagram of IGC system used for synthesis of HNPs
(a) aggregation zone, (b) aperture through which formed nano clusters moved
(c) deposition section
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2.2.4. Physical Vapour Deposition Method
Physical vapour deposition is a collective set of processes commonly used to
produce nanoparticles and to deposit thin layers of material, typically in the range of
few nanometers to several micrometers [7]. Physical vapour deposition (PVD) is an
environment friendly vacuum deposition technique consisting of three fundamental
steps:
1) Vaporization of the material from a solid source
2) Transportation of the vaporized material
3) Nucleation and growth to generate thin films and nanoparticles.
Most commonly used PVD methods for nanoparticles synthesis are Sputtering,
Electron beam evaporation, Pulsed laser deposition, Vacuum arc.
Fig. 4 Schematic representation of (a) plasma sputtering, (b) electron beam
evaporation, (c) pulsed laser deposition and (d) vacuum arc technique
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2.2.5. Laser Pyrolysis Method
The CO2 laser pyrolysis technique, first developed by Haggerty et al. at the
beginning of 1980s, is a vapour phase synthesis process. This method can be used to
synthesize nanoparticles of large variety of oxide (TiO2, SiO2, Al2O3, Fe2O3),
non-oxide (Si, SiC, Si3N3, MoS2) and ternary composites like Si/C/N and Si/Ti/C. The
CO2 laser pyrolysis technique is classified as a vapour - phase synthesis process for the
production of nanoparticles. In this class of synthesis routes, nanoparticle formation
starts abruptly when a sufficient degree of super saturation of condensable products is
reached in the vapour phase. Once nucleation occurs, fast particle growth takes place
by coalescence/coagulation rather than further nucleation. At sufficiently high
temperatures, particle coalescence (sintering) is faster than coagulation and spherical
particles are formed.
Fig. 5 Schematic representation of laser pyrolysis method
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At lower temperatures, coalescence slows down and partially sintered, non
spherical particles and/or loose agglomerates of particles are formed. In the process of
CO2 laser pyrolysis, the condensable products result from laser induced chemical
reactions at the crossing point of the laser beam with the molecular flow of gas or
vapour–phase precursors. The pre-requisite for energy coupling into the system, leading
to molecular decomposition, is that at least one of the precursors absorbs through a
resonant vibrational mode the infrared (IR) CO2 laser radiation tuned at about 10mm.
Alternatively, an inert photo-sensitizer is added to the vapour phase mixture. The high
power of the CO2 laser induces the sequential absorption of several IR photons in the
same molecule, followed by collision assisted energy pooling leading to a rapid
increase of the average temperature in the gas accompanied by the appearance of a
flame in the interaction volume. If the molecules are excited above the dissociation
threshold, molecular decomposition, eventually followed by chemical reactions, occurs
with the formation of condensable and/or volatile products. Nucleation and growth of
nanoparticles occurs in a very short time by coagulation and coalescence of the reaction
products and the growth is abruptly terminated as soon as the particles leave the
irradiation region. As a result, nanoparticles with average size ranging from 5 to 30 nm
and narrow size distribution are formed in the hot region. Fig. 5 shows the schematic
diagram of laser pyrolysis synthesis method of nanoparticles [8-10].
2.3. CHEMICAL SYNTHESIS METHODS
Sol-gel method, micro emulsion technique, hydrothermal synthesis, polyol
synthesis, chemical vapour synthesis and plasma enhanced chemical vapour deposition
technique are some of the most commonly used chemical methods for the nanoparticle
synthesis. These techniques are under the bottom up category of nanoparticle synthesis.
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2.3.1. Sol-Gel Method
Sol-gel is a methodology of producing small particles in material chemistry.
It is mostly used for the synthesis of metal oxides. The initial step of this process is
converting monomers or the starting material into a sol, i.e., a colloidal solution which
is the precursor for the further formation of a gel. This gel is made up of discrete
particles or polymers. Most commonly used precursors are chlorides or metal
alkoxides. These precursors are hydrolysed and poly condensed for the formation of
colloids. Sol-gel process is preferred due to its economical feasibility and the
low-temperature process which gives us control over the composition of the product
achieved. Small amounts of dopants like rare earth elements and organic dyes can be
used in the sol which homogeneously disseminates in the product formed finally. The
synthesized product is used as an investment casting material in the processing and
manufacture of ceramics. Thin metal oxide films can also be produced using this for
further uses.
Fig. 6 Schematic representation of sol-gel synthesis method
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Fig. 6 shows the steps involved in the sol-gel synthesis of nanoparticles.
Sol or colloidal solution is a solution where distribution of particles of the size ~ 0.1-1
μm takes place in a liquid in which the only suspending force is the Brownian motion.
A gel is formed when solid and liquid phases are dispersed in each other. In this
process, initially, colloidal particles are dispersed in a liquid forming a sol. Deposition
of this sol can produce thin coating on any substrate by the means of spraying, spinning
or coating. The particles in the sol are left to polymerize by removing the stabilizing
components and further produce a complex network gel. The remaining organic and
inorganic components pyrolyze in the end by heat treatments to form amorphous or
crystalline coatings. Sol-gel comprises of two major reactions: alcoholic group
hydrolysis and its condensation. Precursor sol which is obtained can be given a desired
shape using appropriate casting container. It can also be deposited on a substrate to
form a film by dip coating or spin coating or used to synthesize microsphere or
nanosphere powders. The steps involved in the sol-gel synthesis are Mixing, Casting,
Gelation, Aging, Drying and Densification [11-13].
2.3.2. Hydrothermal Synthesis
This method is used to fabricate NPs of metal oxide, iron oxide and lithium iron
phosphate keeping control over the characteristics of particles by varying the properties
of near or supercritical water by using different pressure and temperature conditions.
It can be performed in two types of systems, the batch hydrothermal or continuous
hydrothermal process. The former is able to carry out a system with the desired ratio
phases while the latter allows a higher rate of reaction to be achieved at a shorter period
of time. In a chemical solution, nanoparticles are produced from a colloidal system that
consists of two or more phases (solid, liquid or gas states) of matter (e.g. gels and
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foams) mixed together under controlled pressure and temperature [14]. The advantage
of using this method includes the capability to synthesize a huge amount of NPs with
an optimized size, morphology, composition and surface chemistry that is rationally
inexpensive. Hydrothermal is a facile and fast process for the synthesis of NPs of
various other materials such as CoFe2O4, Ag, FeWO4, La1-xSrxCrO3, CdS, Zr, ZnO, etc.
2.3.3. Polyol Synthesis
The polyol synthesis designates the liquid-phase synthesis in high-boiling,
multivalent alcohols and is mainly directed to nanoparticles. Chemically, the polyol
family starts with ethylene glycol (EG) as its simplest representative. From this huge
group of polyols, EG, DEG, GLY, and BD are generally most often applied to prepare
nanoparticles. The most important feature of the polyols is what can be considered as
water-equivalent but high-boiling solvents. Hence, polyols show solubilities of
compounds similar to water, which allows using simple, low-cost metal salts
(e.g., halides, nitrates and sulfates) as starting materials. Moreover, insolubility and
precipitation of products in the polyol as they are prerequisite for obtaining
nanoparticles can be assessed from their insolubility in water, too. Whereas, the
solubility of polar compounds and salts in water is driven by the enormous polarity.
However, the lower polarity is compensated by the chelating properties of the polyols,
which in sum results in a water-comparable solubility. The chelating effect of the
polyols, moreover, is highly beneficial for controlling particle nucleation, particle
growth and agglomeration of nanoparticles as the polyols adhere on the particle surface
(especially on oxides) and serve as colloidal stabilisers. To this concern, the
comparably high viscosity of the polyols also is a benefit. Polyol process is the
synthesis of metal-containing compounds using polyols as the reaction medium that
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plays a role of solvent, reducing agent and complexing agent at the same time, with
dissolved stabilizing/protecting agents [4]. This chemical process was used to
synthesize a wide range of
1) metal nanoparticles
2) metal oxide nanoparticles
3) nano scaled metal chalcogenides and non-metal main-group elements.
Fig. 7 The morphology evolution of iron oxide NPs in polyol processes using two
different polyols
2.3.4. Micro Emulsion Technique
The term micro emulsion was first assigned by Schulman et al. in 1959. Micro
emulsions can be defined as the thermally stable, macroscopically homogenous,
optically transparent and isotropic dispersions constituting minimum of three
components i.e., polar phase (generally water), non-polar phase (generally hydrocarbon
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liquid or oil) and surfactant. Surfactant molecules creates the interfacial layer
separating the aqueous and the organic phases, reduces the interfacial tension between
the micro emulsion and the excess phase and act as a steric barrier preventing the
coalescence of the droplets. Micro emulsion system consists of mono dispersed
spherical droplets (diameter ranging from 600 nm to 8000 nm) of water-in-oil (w/o) or
oil-in-water (o/w) depending on the surfactant used. The w/o reverse micellar system
acts as an excellent reaction site for the nanoparticles synthesis [15]. Reverse micelle is
water-in-oil micro emulsion where the polar head groups of the surfactant creating the
aqueous core and resides towards inside whereas the organic tails of the surfactant
molecules directed towards outside as shown in Fig. 8a. In general there are two micro
emulsion routes to synthesize the nanoparticles namely
1) one micro emulsion method
2) two micro emulsion method
One micro emulsion method can be further divided into two types i.e., energy
triggering method that needs a triggering agent to initiate the nucleation reaction within
the single micro emulsion containing the precursor and other is one micro emulsion
plus reactant method which is initiated by adding one of the reactant directly into micro
emulsion already carrying the second reactant One micro emulsion processes are
diffusion controlled since the second trigger/reactant has to diffuse through the
interfacial wall of the micro emulsion encapsulating the first reactant to accomplish the
nanoparticles synthesis.
In two micro emulsion method, the two micro emulsions carrying the separate
reactants are mixed together in appropriate ratios (Fig. 8c). Brownian motion of the
micelles helps them to approach each other resulting in inter-micellar collisions and
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sufficiently energetic collisions leads to the mixing of the micellar components. Once
both the reactant comes in a same micellar compartment, the chemical reaction takes
place in this nano reactor.
Fig. 8(a-c) shows (a) typical Reverse Micelle System, (b) various steps involved in
one Micro emulsion process and (c) reaction sequence involved in the two micro
emulsion nano particles synthesis
As the critical number of molecules attained inside the micelle, it initiates the
nucleation process and results in nanoparticles formation. Numerous inter-micellar
collisions are needed for the sufficient reactant exchange, their mixing and finally their
reaction to terminate at the end product. Micro emulsion technique was used most
commonly for the synthesis of the inorganic nanomaterials including metal
nanoparticles, semiconducting metal sulphite nanoparticles, metal salt nanoparticles,
metal oxide nanoparticles, magnetic nanoparticles and composite nanoparticles.
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2.3.5. Microwave Assisted Synthesis
Microwave-enhanced chemistry is based on the efficient heating of materials by
“microwave dielectric heating” effects. This phenomenon is dependent on the ability of
a specific material (solvent or reagent) to absorb microwave energy and convert it into
heat. Microwaves are defined as electromagnetic waves with vacuum wavelength
ranging between 0.1to 100cm or, equivalently, with frequencies between 0.3 to
300GHz. With microwave heating, the energy can be applied directly to the sample
rather than conductively, via the vessel. Heating can be started or stopped instantly, or
the power level can be adjusted to match the required. Microwave dielectric heating is a
non-quantum mechanical effect and its leads to volumetric heating of the samples [16].
Therefore, it is necessary to question whether it has any significant advantages
compared to thermal heating of chemical reactants.
The interest in the microwave assisted organic synthesis has been growing
during the recent years. With microwave heating energy can be directly applied to the
reaction not to the vessel where it takes time for the reaction to be completed and also
the time taken is less and there is the consumption of time. Microwave heating is based
on dielectric heating, i.e., molecule exhibiting a permanent dipole moment will try to
align to the applied electromagnetic field resulting in rotation, friction and collision of
molecules and, thus in heat generation. Microwave irradiation in chemical reaction
enhancement has been well recognized for increasing reaction rates and formation of
clear. Some of the major advantages include spectacular decrease in reaction time,
improved conversions, clean product formation and wide scope for the development of
new reaction conditions. Recent reports have shown that microwave heating can be
very convenient for use in a large number of organic synthetic methods. Microwave
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heating is instantaneous and very specific and there is no contact required between the
energy source and the reaction vessel. Microwave dielectric heating is a non quantum
mechanical effect and it leads to volumetric heating of the samples.
2.4. BIO-ASSISTED METHODS FOR THE SYNTHESIS OF NANOPARTICLES
Bio-assisted methods, biosynthesis or green synthesis provides an
environmentally benign, low-toxic, cost-effective and efficient protocol to synthesize
and fabricate nanoparticles. These methods employ biological systems like bacteria,
fungi, viruses, yeast, actinomycetes, plant extracts, etc.[17] for the synthesis of metal
and metal oxide nanoparticles. Bio-assisted methods can be broadly divided into three
categories:
i) Biogenic synthesis using microorganisms
ii) Biogenic synthesis using bio-molecules as the templates
iii) Biogenic synthesis using plant extracts
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