**2. Methods of preparation of nanomaterials**

Numerous methods of nanomaterials synthesis have been developed. Generally, the two approaches top-down and bottom-up are used.


The method of synthesis of nanoparticle is given in **Figure 1. Synthesis of nanoparticles:**

There are four methods used for the synthesis of nanoparticles:


**Figure 1.** *Methods of synthesis of nanomaterials.*

*Quantum Dots Sensitized Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.107266*

3.Biological

4.Hybrid

### **2.1 Physical methods**

It is broadly divided into two types, namely mechanical and vapor-based methods [1–3].

### *2.1.1 Mechanical methods*

In this process, containers are used to make nanoparticles, hard balls of tungsten carbide, zirconia, or steel are plunged in the containers along with the materials whose nanoparticles are to be made. So this high-energy ball milling is a type of mechanical method.

## *2.1.2 Vapor-based methods*

The nanostructures are formed by evaporating different material on various types of substrates. It is divided into **a**. Physical vapor deposition (PVD) and **b.** Chemical vapor deposition (CVD).

### *2.1.2.1 Physical vapor deposition (PVD) method*

It is general term used to describe any of the variety of methods to deposit thin films by the condensation of vaporized form of the desired films material on to various working surfaces (e.g., onto semiconductor wafers). PVD is used in the manufacturing of semiconductor devices.

### *2.1.2.1.1 LASER ablation*

LASER ablation is the process of removing material from a solid (or occasionally liquid) surface by using pulse of LASER beam. At low LASER flux, material is heated by the absorbed energy and evaporates or sublimates, and at high flux, the material is converted to plasma. Single-wall carbon nanotubes are synthesized by LASER ablation method.

#### *2.1.2.1.2 Biological method or green synthesis*

Synthesis of nanomaterial using biological ingredient, which is divided by using microorganism such as fungi, yeasts, and bacteria, plants extracts or enzymes and use of templates such as DNA, membrane, viruses, diatoms.

#### *2.1.2.1.3 Hybrid method*

Hybrid method utilized for the preparation of nanoparticles involves the process such as electromechanical, chemical vapor deposition, particle arresting in glass or zeolites or polymers micro-emulsion.

#### *2.1.2.2 Chemical vapor deposition*

There are techniques depending on the source of excitation and the conditions of deposition, which include Atmospheric-Pressure CVD (APCVD), Low-Pressure CVD (LPCVD), Ultrahigh-Vacuum CVD (UVCVD), Metal Organic CVD (MOCVD), and Hot Filament CVD (HFCVD). In this method, when a substrate is exposed to one or two volatile precursors, they chemically react in vapor phase or decompose on its surface under thermal, plasma, or laser excitation and form nanostructures of high quality.

#### **2.2 Chemical methods**

These are less expensive, simple, and easily scalable techniques and used to synthesize nanostructures of different shape, morphologies, and particle sizes.

Various chemical methods are discussed as follows:

#### *2.2.1 Colloidal synthesis*

Colloids are the class (few nm sizes) of material where two or more phases exist with a size less than a micron. Particles are generally suspended in some host matrix. The colloidal particles are stabilized by columbic repulsion or stearic hindrance against aggregation.

Colloidal synthesis is carried out generally in a three-neck flask and to avoid processes such as oxidation of products; the reaction is carried out in inert atmosphere.

#### *2.2.2 Chemical bath deposition*

The high-quality metal chalcogenide nanocrystalline thin films (metallic or nonmetallic substrate) are fabricated by using this technique. This requires the metal and chalcogen ions. In other way, deposition of the film is possible if the ionic product should exceed the solubility product.

Thin film deposition proceeds as nucleation on the surface of the substrate, i.e., formation of stable second phase by the combination of a minimum number of ions or molecules in contact with a solution. The adsorbed cations/anions deposited on the substrate act as nucleation centers, and subsequent deposition takes place through adsorption of more ionic species present in the solution that results in the homogeneous and uniform film. The film deposition takes place ion by ion or cluster by cluster. The former results into uniform, adherent thin film, whereas latter thick, powdery, and diffusely reflecting films. The pH is also one of the deciding factors, at higher pH values, solubility product exceeds ionic product, i.e., unfavorable condition for deposition. Also as the temperature increases, metal ion gets dissociated from the complexing agent and film deposits through nucleation. Moreover, concentration, type of complexing agent, stirring rate of the solution also decide the quality of films [4, 5].

#### **Doctor blade method:**

Doctor blade method is very simple technique for coating of substrate, in this method precursor, solution is dropped/placed on the substrate. The film is produced by moving the blade over the substrate. Drying and annealing steps are used to form a solid film on the substrate. Various steps involved in doctor blade method are illustrated in **Figure 2**.

In doctor blade technique, well-mixed slurry of nanoparticles along with other constituents such as binder, dispersants, etc., are used. The slurry is spread over on a

**Figure 2.** *Doctor blade technique for the fabrication of photoanode.*

substrate and thin layer is produced, thickness of films is depending on the distance between blade and substrate [6, 7]. The advantages of Doctor Blade method are

