**1. Introduction**

Metal oxide nanoparticles (MONPs) are well-known for their outstanding role especially for environmental, sensor, biomedical and energy applications [1–10]. Among MONPs, TiO<sup>2</sup> nanoparticles are least toxic [11] and therefore synthesis of nanostructured TiO<sup>2</sup> with tailored properties has been most extensively investigated in recent years.TiO<sup>2</sup> occurs in three different phases [12], anatase, rutile and brookite with rutile as most stable phase and anatase as most desirable phase. TiO<sup>2</sup> is associated with outstanding properties including high stability,

© 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.

exceptional biocompatibility, corrosion resistance, high photosensitivity and reactivity, as well as cost-effective and easy synthesis [13–16]. Semiconducting nature of anatase TiO<sup>2</sup> (Band gap of 3.2 eV) [17] allows it to degrade toxic organic compounds into simple hydrocarbons such as CO<sup>2</sup> and H2 O under UV irradiation. Under UV irradiation of energy greater than or equal to energy gap of TiO<sup>2</sup> , electrons and holes are produced in valence band and conduction band, respectively. These electrons and holes result in formation of oxygen active species (OH\*, H2 O2 , O<sup>2</sup> ¯, 1 O2 ) at surface of TiO<sup>2</sup> , which reacts with toxic organic compounds and decompose them. Thus, TiO<sup>2</sup> is well known photocatalyst largely utilized for water reclamation, air purification, soil remediation, surface wettability adjustment, bacteria killing, solar cells, sensors, self-cleaning and anti-reflective surfaces [18–25].

general results in amorphous powder after drying process. The process of mixing is known as hydrolysis, while formation of 3-dimensional network is called as condensation. These two processes are further controlled by many parameters including nature of metal precursor, ratio of precursor to solvent, nature of solvent, capping agents (surfactants), pH and tempera-

Novel TiO2 Photocatalyst Using Nonaqueous Solvent-Controlled Sol-Gel Route

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Aqueous sol-gel route [41], as its name suggests, uses water as solvent to dissolve metal precursor and to complete hydrolysis process. The hydrolysis process is extremely fast due to high reactivity of water with precursors and therefore, generally there is little control over morphology and reproducibility. Nonaqueous sol-gel routes offer a good alternative to get rid of these difficulties [43, 44]. An organic solvent (alcohols, ketones, aldehydes or ethers) is used to complete the hydrolysis process instead of water. Besides, the oxygen required for metal-oxide formation is supplied by organic solvent in nonaqueous sol-gel route, whereas water plays the role of oxygen donor in aqueous sol-gel synthesis. However, inclusion or exclusion of some surfactant (consisting of hydrophilic and hydrophobic groups) in reaction solution further classifies the nonaqueous sol-gel route into surfactant assisted and solvent controlled (surfactant free) routes respectively. The main advantage of surfactant assisted nonaqueous sol-gel route is that the surfactant acts as capping agent and results in highly mono-dispersed nanoparticles. In addition a good control over particle size, morphology with outstanding reproducibility is direct consequence of surfactant-assisted nonaqueous sol-gel route. Moreover, the surface properties of nanoparticles can be easily tailored by exchanging surfactants with other functional groups. However this method is also prone to certain limitations like impurities in nanoparticles and toxic effects due to surfactants. These limitations impose restrictions on the surface sensitive applications (photocatalytic, biomedical and sensing) of nanoparticles.

A good alternative to surfactant assisted nonaqueous sol-gel route [45, 46] is solvent controlled nonaqueous sol-gel route. The solvent in itself plays role of oxygen donor necessary for oxide formation and stabilizing agent to control shape, size and morphology of nanoparticles. This modified sol-gel route also facilitates highly pure nanoparticles completely free

ture. **Figure 1** summarizes the various types of sol-gel routes.

**2.2. Solvent controlled nonaqueous sol-gel route**

**Figure 1.** Various types of sol–gel synthesis routes.

A large number of synthesis methods have been employed for designing of TiO<sup>2</sup> nanoparticles with controlled shape, size, good yield and high dispersibility (less agglomeration). The shape and size of nanoparticles greatly affect the photocatalytic performance of the photocatalyst [26, 27]. Highly pure metal oxides can be prepared by conventional solid state route [28], but high processing temperature requirement limits its frequent use for synthesis. The biological synthesis method [11, 29] leads to formation of cost-effective, mono-dispersed nanoparticles but reproducibility needs improvement. To overcome all these difficulties during nanoparticles synthesis, alternative well-known liquid phase synthesis methods such as sol-gel [30, 31], hydrothermal [32, 33], microemulsion [34, 35], reverse microemulsion [36], sonochemical [37, 38] and microwave [39, 40] are employed. Among these synthesis methods, the sol-gel synthesis route gets special attention because of following reasons:


Sol-gel route can also yield multifold nanostructures such as nanoparticle, nanorods, nanotubes, aerogels and zeolites at a single platform. In addition, good yield and reproducibility are the key features of sol-gel route.

The present chapter will highlight the features of nonaqueous solvent controlled sol-gel route for the synthesis of pure and metal doped TiO<sup>2</sup> nanoparticles. Effects of metal doping and synthesis strategy on structural and surface properties are correlated with photocatalytic activity of pure and metal doped TiO<sup>2</sup> photocatalyst.
