1. Introduction

The photocatalysis has become an important research area in the recent years. The interest on such area has accelerated the number of papers, since photocatalysts have diverse applications, for example, the photodegradation of several contaminants that are quite difficult to transform to innocuous substance as water vapor. Other important applications are the development of self-cleaning products as well as the production of highly effective fuels like hydrogen. Nowadays, the technological development based on photocatalysis has an important growth in regions such as Europe and Asia (12% annually), with an estimated market of about € 1500 million in 2015 [1].

© 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 eproduction 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.

Photocatalysts are based on semiconductor materials, which are activated by radiation with higher energy than the bandgap of the used semiconductor, in order to create hole-electron pairs, once the electron passes from valence to conduction band. The promoted electron, toward conduction band, and the hole remained in valence band react in the photocatalyst interface with adsorbed substances in order to create reactive entities (free radicals and/or radical anions), which interact with contaminants to degrade them. All these processes imply the sorption on the photocatalyst's interface, its activation by radiation, creation of reactive species, and the recombination of electron to hole. The last could occur very fast that the creation of reactive species does not take place at great extent, giving ineffective photodegradation process.

We consider at least three windows of opportunity to boost ZnO as photocatalyst in academy and industry. The first is related to the quantum yield in visible light rather than UV for the

disruptors and emergent contaminants in water. A second opportunity is the immobilization of the photocatalyst that could make possible its utilization on continuous mode and toward implementation at industrial scale. The third challenge deals with nonlinear mathematical modeling of the factors that rule the photocatalytic kinetics of the metallic-modified ZnO for optimization purposes. This is not a trivial problem in photocatalysis and artificial neural network tools (ANNs) do not demand of academic expertise for a fast implementation; for example, in the industry sector. The reader can consult [3] for the approach that Meimaroglou and collaborators follow to assess ZnO photocatalyst structure-photocatalytic performance associations toward the maximization of the photocatalytic efficiency. The first two challenges

Nowadays, solar energy, radiant light, and heat from the sun are the most abundant available sources of clean energy. Thus, research studies and development of materials that can efficiently harvest solar irradiation and used for green environmental pollution management are essential. Photocatalysis, which could use renewable solar energy to activate the chemical reactions via oxidation and reduction, such as that occurs in advance oxidation processes (AOPs), is a sustainable technology to provide solution for environmental issue. This photocatalysis system has attracted great interest from science community as the most promising way to solve the environmental problems, especially getting rid of residual pollutants

In the field of photocatalysis, ZnO has emerged as the leading candidate for green environmental management systems because of its unique characteristics, such as wide bandgap (3.37 eV) in the near-UV spectral region, a large electron exciton binding energy of 60 eV at room temperature, strong oxidation ability, and good photocatalytic property [4]. It is a wellknown fact that ZnO occurs as white hexagonal crystal or white powder known as white zinc. ZnO crystallizes in the wurtzite structure and is available as large bulk single crystals [5]. As an important semiconductor material, ZnO has been applied in catalysis, rubber and paint

Recently, the development of ZnO with precisely controllable features has gained significant scientific interest. The electrical, optical, and magnetic properties of ZnO can be altered or improved by the use of ZnO in nanoscale and efforts have been developed to improve the properties of ZnO photocatalyst [7]. Moreover, ZnO is an environmental friendly material as it is compatible with living organisms [8]. Since ZnO has almost the same bandgap energy as TiO2 (3.2 eV), its photocatalytic capability is anticipated to be similar to that of TiO2. However, ZnO is relatively cheaper compared to TiO2, whereby the usage of titanium dioxide is uneconomic for large-scale water treatment operations [9]. The greatest advantage of ZnO is the ability to absorb a wide range of solar spectrum and more light quanta than some semiconducting metal oxides including the capacity to absorb visible light energy, which is due to its

industries, ceramic bodies, varistors, fertilizers, and cosmetics [6].

) pairs during decomposition of endocrine

Modified Metallic Oxides for Efficient Photocatalysis http://dx.doi.org/10.5772/intechopen.80834 59

efficient utilization of the electron-hole (e/h<sup>+</sup>

are addressed in the present chapter.

from wastewater stream.

2.1. Use of ZnO/graphene photocatalysts

In this context, the research regarding photocatalysis, in recent years, is focused to decrease the bandgap of photocatalyst in order to use solar light rather than UV light, which implies additional cost during the treatment process. In addition, minimization of the recombination process, in order to improve the photocatalytic performance, is quite important too. Some approaches to face these issues consider the doping with metals; meanwhile other research groups also consider the modifications with electron-rich species like graphene.

TiO2 has been the photocatalyst by excellence and has been widely studied in its pristine form and/or doped with metals like gold and other elements. Nevertheless, other metal oxides, such as ZnO, have shown better photocatalytic performance, especially when visible light is used. ZnO has also been doped with metals like gold or silver. More recently, other metal oxides like bismuth oxychloride have gained attention for photocatalytic process.

In this chapter, discussion about the modification of ZnO with graphene is presented and discussed in terms of the implied mechanism (hybridization), in addition to the obtained results when such hybrid photocatalyst was used for the photodegradation of triclosan under visible light. Additionally, the modification method of ZnO with silver nanoparticles and its effect on photocatalytic performance for bisphenol A, Rhodamine B (RhB), and Triclosan is presented.

Finally, the use of an attractive photocatalyst, bismuth oxychloride (BiOCl), and its respective modification with silver and graphene oxide for RhB photodegradation along with the result and mechanism for the photocatalyst based on TiO2-BiOCl used for photodegradation of phenol is presented and discussed.

The three different photocatalysts show how effective photocatalyst can be obtained and modified. The presented and discussed results contribute to understand some of the key parameters implied in the photocatalysis.
