18

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

19

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,

20

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,

21

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.

22

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

23

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

24

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

25

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

26

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

27

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.

28

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

29

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

30

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

31

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

32

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

33

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.

34

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

35

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

36

2.5. REFERENCES

1. A. Kopp Alves et al., Novel Synthesis and Characterization of Nanostructured

Materials, Engineering Materials, DOI: 10.1007/978-3-642-41275-2-2.

2. P. Saravanan, R. Gopalan, and V. Chandrasekaran, Synthesis and

Characterisation of Nanomaterials, Defence Science Journal, (2008) 58, 4,

504-516.

3. Yury Gogotsi, Nanomaterials Handbook, Crc Press (2006).

4. Chetna Dhand, Neeraj Dwivedi, Xian Jun Loh, Alice Ng Jie Ying, Navin

Kumar Verma, Roger W. Beuerman, Rajamani Lakshminarayanan. Seeram

Ramakrishna, Methods and Strategies for the Synthesis of Diverse

Nanoparticles and their Applications: A Comprehensive Overview, RSC

Advances, DOI: 10.1039/C5RA19388E.

5. Hans-Georg Braun, Electron beam lithography, (2008).

6. F. Einar Kruis, Heinz Fissan, Aaron Peled, Synthesis Of Nanoparticles in the

Gas Phase for Electronic, Optical And Magnetic Applicationsða Review,

J. Aerosol Sci.( 1998) Vol. 29, No. 5/6, 511-535.

7. Mark T. Swihart, Vapor-phase synthesis of nanoparticles, Current Opinion in

Colloid and Interface Science (2003) 8, 127–133.

8. Phillip Wagener, Stephen Barcikowski, Niko Barch, Fabrication of

Nanoparticles and nanomaterials using laser ablation in liquids, Photonik

International (1- 2011).

37

9. Vincenzo Amendola, Moreno Meneghetti, Laser ablation synthesis in solution

and size manipulation of noble metal nanoparticles, Phys. Chem. Chem. Phys.,

(2009), 11, 3805–3821.

10. Borsella1, R. D’amato1, G. Terranova 1, M. Falconieri 2, F. Fabbri, Synthesis

Of Nanoparticles By Laser Pyrolysis:From Research To Applications, “Italy In

Japan 2011” InitiativeScience, Technology And Innovation.

11. Helmut Schmidt, Nanoparticles by chemical synthesis, processing to materials

and innovative applications, Appl. Organometal. Chem. (2001) 15: 331–343.

12. Larry L. Hench, Jon K. West, The Sol-Gel Process, Chem. Rev. 1990, 90.

33-7.2.

13. Amit Kumar, Nishtha Yadav, Monica Bhatt, Neeraj K Mishra, Pratibha

Chaudhary, Rajeev Singh, Sol-Gel Derived Nanomaterials and It’s

Applications: A Review, Res. J. Chem. Sci. (2015) Vol. 5(12), 98-105.

14. Pallab Ghosh, Preparation of Nanomaterials, NPTEL Material.

15. Controlled Synthesis of Nanoparticles in Microheterogeneous Systems,

Vincenzo Turco Liveri, Springer Science + Business Media (2006).

16. Rina Das, Dinesh Mehta, Harsh Bhardawaj, An Overview On Microwave

Mediated Synthesis, International Journal Of Research And Development In

Pharmacy And Life Science, June - July, 2012, Vol. 1, No.2, Pp. 32-39.

17. Saba Hasan, A Review on Nanoparticles: Their Synthesis and Types, Res.

J. Recent. Sci., (2015) 4, 9-11.