**1. Introduction**

The rapid growth of global population as well as industrialization has led to a concomitant increase in environmental pollution. This has very negative effects on natural elements that are vital for life on earth such as air and water. It becomes very crucial therefore to find sustainable ways to mitigate pollution in order to provide a clean and safe environment for humans. Photocatalysis has attracted worldwide interest due to its potential to use solar energy not only to solve environmental problems but also provide a renewable and sustainable energy

© 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.

source. An efficient photocatalyst converts solar energy into chemical energy which can be used for environmental and energy applications such as water treatment, air purification, self-cleaning surfaces, hydrogen production by water cleavage and CO<sup>2</sup> conversion to hydrocarbon fuels.

been researched to overcome the limitations of TiO2

photocatalysis.

of the TiO2

TiO2

TiO2

metal-TiO2

**2.1. Metal doping**

band gap energy of TiO2

Doping of TiO2

visible light to photoactivate the TiO2

the metal nitrates was found to widen the TiO2

prevent the recombination of electrons and (e<sup>−</sup>

loading) resulted in a high surface area TiO2

crystal phase transformations that generated electrons (e<sup>−</sup>

mum catalytic performance and a reduction in the TiO2

porting photogenerated electrons from the surface of TiO2

and Au [38] have also been used to modify TiO2

**2. Modification of TiO<sup>2</sup>**

The properties of modified TiO<sup>2</sup>

and the level of success that these strategies have been able to achieve. Based on the current level of research in this field, we also present some perspectives on the future of modified TiO<sup>2</sup>

photocatalytic properties. These modifications have been done in many different ways which include metal and non-metal doping, dye sensitization, surface modification, fabrication of composites with other materials and immobilization and stabilization on support structures.

regards to light absorption, charge separation, adsorption of organic pollutants, stabilization

Metal doping has been extensively used to advance efforts at developing modified TiO<sup>2</sup>

.

tocatalytic water splitting for the Pt-doped samples. The 2.5 wt%Pt-TiO<sup>2</sup>

activity increases as the charge carriers recombination rate is decreased.

tocatalysts to operate efficiently under visible light. The photoactivity of metal-doped TiO<sup>2</sup> photocatalysts depends to a large extent on the nature of the dopant ion and its nature, its

the catalyst is used and the reaction conditions [32]. The mechanism of the lowering of the

sample, Ti existed as both Ti4+ and Ti3+ and the conversion between Ti4+ and Ti3+ was found to

with an enhanced electron storage capacity, leading to a minimization of the electron-hole recombination rate [34]. Noble metal nanoparticles such as Ag [35], Pt [34], Pd [36], Rh [37]

to efficiently hinder electron-hole recombination due to the resulting Schottky barrier at the

interface. The noble metal nanoparticles act as a mediator in storing and trans-

prepared by sol gel technique and doped with different levels of Pt (1–5 wt% nominal

 with metals results in an overlap of the Ti 3d orbitals with the d levels of the metals causing a shift in the absorption spectrum to longer wavelengths which in turn favours the use of

are always intrinsically different from the pure TiO<sup>2</sup>

with metal doping is shown in **Figure 1**. It is believed that doping

nanoparticles with Li, Na, Mg, Fe and Co by high energy ball milling with

) and holes (h+

particles.

 **photocatalysts**

A large number of research works have been published on TiO<sup>2</sup>

particles and ease of separation of TiO2

level, the method used in the doping, the type of TiO<sup>2</sup>

for general photocatalytic applications

http://dx.doi.org/10.5772/intechopen.79374

Modified Titanium Dioxide for Photocatalytic Applications

modification to enhance its

used as well as the reaction for which

visible light response range. In the Na-doped

) and holes (h+

with an enhanced catalytic performance in pho-

). The metal ion doping promoted

band gap energy from 3.00 to 2.34 eV

to an acceptor. The photocatalytic

for photocatalysis and have been reported

) [33]. Mesoporous

had showed the opti-

with

39

pho-

Research in the development of efficient photocatalytic materials has seen significant progress in the last 2 decades with a large number of research papers published every year. Improvements in the performance of photocatalytic materials have been largely correlated with advances in nanotechnology. Of many materials that have been studied for photocatalysis, titanium dioxide (TiO<sup>2</sup> ; titania) has been extensively researched because it possesses may merits such as high photocatalytic activity, excellent physical and chemical stability, low cost, non-corrosive, nontoxicity and high availability [1–4]. The photocatalytic activity of titania depends on its phase. It exists in three crystalline phases; the anatase, rutile and brookite. The anatase phase is metastable and has a higher photocatalytic activity, while the rutile phase is more chemically stable but less active. Some titania with a mixture of both anatase and rutile phases exhibit higher activities compared to pure anatase and rutile phases [5–7]. When titania is irradiated with light of sufficient energy, electrons from the valence band are promoted to the conduction band, leaving an electron deficiency or hole, h<sup>+</sup> , in the valence band and an excess of negative charge in the conduction band. The free electrons in the conduction band are good reducing agents while the resultant holes in the valence band are strong oxidizing agents and can both participate in redox reactions.

Titania however suffers from a number of drawbacks that limit its practical applications in photocatalysis. Firstly, the photogenerated electrons and holes coexist in the titania particle and the probability of their recombination is high. This leads to low rates of the desired chemical transformations with respect to the absorbed light energy [8, 9]. The relatively large band gap energy (~ 3.2 eV) requires ultraviolet light for photoactivation, resulting in a very low efficiency in utilizing solar light. UV light accounts for only about 5% of the solar spectrum compared to visible light (45%) [1, 10]. In addition to these, because titania is non-porous and has a polar surface, it exhibits low absorption ability for non-polar organic pollutants [10–13]. There is also the challenge to recover nano-sized titania particles from treated water in regards to both economic and safety concern [14]. The TiO2 nanoparticles also suffer from aggregation and agglomeration which affect the photoactivity as well as light absorption [15–18]. Several strategies have been employed in the open literature to overcome these drawbacks. These strategies aim at extending the wavelength of photoactivation of TiO<sup>2</sup> into the visible region of the spectrum thereby increasing the utilization of solar energy; preventing the electron/hole pair recombination and thus allowing more charge carriers to successfully diffuse to the surface; increasing the absorption affinity of TiO<sup>2</sup> towards organic pollutants as well as preventing the aggregation and agglomeration of the nano-titania particles while easing their recovery from treated water. Several reviews have been published in recent years on the development of strategies to eliminate the limitations of titania photocatalysis [1, 19–25]. Most of these however focus on pollutant removal from wastewater, water splitting for hydrogen production, CO<sup>2</sup> conversion and reaction mechanisms [1, 21, 25–31]. In this chapter, we review some of the latest publications mainly covering the last 5 years, on strategies that have been researched to overcome the limitations of TiO2 for general photocatalytic applications and the level of success that these strategies have been able to achieve. Based on the current level of research in this field, we also present some perspectives on the future of modified TiO<sup>2</sup> photocatalysis.

#### **2. Modification of TiO<sup>2</sup> photocatalysts**

A large number of research works have been published on TiO<sup>2</sup> modification to enhance its photocatalytic properties. These modifications have been done in many different ways which include metal and non-metal doping, dye sensitization, surface modification, fabrication of composites with other materials and immobilization and stabilization on support structures. The properties of modified TiO<sup>2</sup> are always intrinsically different from the pure TiO<sup>2</sup> with regards to light absorption, charge separation, adsorption of organic pollutants, stabilization of the TiO2 particles and ease of separation of TiO2 particles.
