**1. Introduction**

Titanium dioxide photocatalyst is a well-known and well researched photocatalyst due to its interesting properties which include stability, non-toxicity, biocompatibility, optical and electrical properties. It exists mainly in three different forms namely anatase, rutile and brookite and their structures are shown in **Figure 1** [1]. Upon heating, both anatase and brookite convert to rutile which is more stable at all temperatures and pressures below 60 kbar, according to thermodynamic calculations [2]. **Table 1** shows some of the structural and physical properties of the anatase and rutile phase of titanium dioxide [3].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and photocatalysis [6] compared to nanoparticulate forms of titanium oxide. Recently, bundles and arrays of TNTs with different qualities have been synthesized by a variety of different techniques such as template-assisted, sol-gel, hydrothermal, electro-anodization, chemical vapor

these include reverse micelles, the sol-gel process, the metal organic chemical vapor deposition (MOCVD) [8], gas phase (aerosol) synthesis [9], wet-chemical synthesis by precipitation of hydroxides from salts, microemulsion-mediated methods [10] and electrochemical synthesis. These methods can be divided into five general groups namely sol-gel, deposition methods, sonochemical and microwave-assisted methods, hydro/solvothermal methods and

In these methods, materials in the vapor state are condensed to form a solid phase material. The process is normally carried out in a vacuum chamber and if a chemical reaction takes place, it is called chemical vapor deposition (CVD) and physical vapor deposition (PVD) if no reaction occurs. Examples of CVD include electrostatic spray hydrolysis, diffusion flame pyrolysis, thermal plasma pyrolysis, ultrasonic spray pyrolysis, laser-induced pyrolysis and

) nanoparticles and

a = 4.5936 c = 2.9587

Synthetic Methods for Titanium Dioxide Nanoparticles: A Review

http://dx.doi.org/10.5772/intechopen.75425

153

1.949 (4) 1.980 (2)

There are other various techniques for preparing titanium dioxide (TiO2

**Property Anatase Rutile** Molecular weight (g/mol) 79.88 79.88 Melting point (°C) 1825 1825 Boiling point (°C) 2500–3000 2500–3000

Specific gravity 3.9 4.0

Light absorption (nm) λ ≤ 385 nm λ ≤ 415 nm Mohr's hardness 5.5 6.5 to 7 Refractive index 2.55 2.75 Dielectric constant 31 114

Crystal structure Tetragonal Tetragonal

c = 9.515

1.965 (2)

) 3.79 4.13

deposition and physical vapor deposition [7].

Lattice constants (Å) a = 3.784

Ti─O bond length (Å) 1.937 (4)

**Table 1.** Properties of the anatase and rutile phase of titanium dioxide [3].

oxidation methods.

Density (g/cm3

**2. Synthetic methods**

**2.1. Deposition methods**

**Figure 1.** Different forms titanium dioxide [1].

The recent development of nanotechnology has proved that nanomaterials such as nano-sized titanium dioxide photocatalysts can have high activity in the photodegradation of a wide range of organic and inorganic contaminants in water. It is believed that photocatalysis will soon be recognized as one of the most effective means of dealing with various kinds of wastewater since organic pollutants can be completely degraded to harmless matter under normal conditions of temperature and pressure.

It is capable of degrading pollutants such as herbicides, carboxylic acids and alcohols completely to carbon dioxide, water and simple minerals [4]. For it to be very effective, it should have certain properties such as suitable particle size, shape, crystallinity and a good ratio of anatase to rutile. This is the reason why most researchers have been trying to use different methods to get particles with suitable characteristics for environmental remediation or for other applications of interest. Several studies have proved that TiO2 nanostructures, in particular, titanium dioxide nanotubes (TNTs) have their performance improved in photovoltaics [5]


**Table 1.** Properties of the anatase and rutile phase of titanium dioxide [3].

and photocatalysis [6] compared to nanoparticulate forms of titanium oxide. Recently, bundles and arrays of TNTs with different qualities have been synthesized by a variety of different techniques such as template-assisted, sol-gel, hydrothermal, electro-anodization, chemical vapor deposition and physical vapor deposition [7].

There are other various techniques for preparing titanium dioxide (TiO2 ) nanoparticles and these include reverse micelles, the sol-gel process, the metal organic chemical vapor deposition (MOCVD) [8], gas phase (aerosol) synthesis [9], wet-chemical synthesis by precipitation of hydroxides from salts, microemulsion-mediated methods [10] and electrochemical synthesis. These methods can be divided into five general groups namely sol-gel, deposition methods, sonochemical and microwave-assisted methods, hydro/solvothermal methods and oxidation methods.
