Recent Advancements in Photocatalytic Nanocomposites

Introduction

Environmental pollution is one of the major challenges that the earth is facing nowadays. Increasing population and overuse of pollutants to ‘facilitate an easy life’, adversely affect the natural resources such as water, air and soil. Among the many pollutants, organic pollutants play a major role in causing water pollution. Majority of these pollutants come from industrial effluents, plastics, detergents, organic dyes, synthetic fibres and rubber, paints, food additives, pharmaceuticals and other organic wastes (Bhanvase, 2017). These pollutants get introduced not only into surface water but also to groundwater. Therefore, researchers are in a quest for an effective method to treat such pollutants. Among the different available methods, photocatalysis is gaining a lot of interest due to the capability of photocatalytic materials to harvest solar energy and undergo reaction at mild reaction conditions. Photocatalysis is the process that involves the utilization of a light-sensitive material to catalyze a reaction. It can be classified as an up-hill process or down-hill process, where up-hill reactions convert light to chemical energy whereas down-hill reactions convert reactants to products using light energy. Semiconductor metal oxides are widely used in photocatalysis due to their innate photosensitive properties (Ashkarran, 2015; Bhanvase, 2017; Divya, 2017; Hatamie, 2015; Li, 2007; Liu, 2018; Lui, 2013; Nasrollahzadeh, 2018; Pruna, 2018; Saravanan, 2013; Shejale, 2018; Štengl, 2011; Tang, 2018; Yue, 2010). With the emergence of nanotechnology, a new gate has opened resulting in the drastic increase of interest in photocatalysis and photocatalytically active materials. The ability to tune the size, shape and morphology of nanomaterials help to develop new and advanced physicochemical properties such as increased stability, surface area, optical, magnetic, and catalytic properties have revolutionized the use of photocatalytic active materials (Ashkarran, 2015; Ayati, 2014; Bhanvase, 2017; Giovannetti, 2017; Hatamie, 2015; H. Huang, 2014; Lin, 2012; Lui, 2013; Morales-Torres, 2014; Panwar, 2016; Pruna, 2018; Sanzone, 2018; Saravanan, 2013; Sha, 2014; Shejale, 2018; Shen, 2011; Sugano, 2012; Tang, 2018; Wen, 2011; Y. Yang, 2016; Yue, 2010). However, as a downside, these semiconductor photocatalysts possess a wide bandgap that impedes the electron transportation resulting in the recombination of photo-induced electrons (electron-hole recombination) (Bhanvase, 2017; Ghartavol, 2019; Giovannetti, 2017). This electron-hole recombination occurs so fast that it limits the photocurrent which in turn limits the efficiency of the photocatalyst. Thus, a rising need to improve the existing properties, decelerating the electron-hole recombination and improving the overall efficiency of the material is a topic of interest for the experts worldwide. The commonly used techniques to improve the photocatalytic activity of the generally used semiconductor metal oxides are doping the metal oxide with suitable dopant (Deng, 2011; Wang, 2014), using a composite (Divya, 2017; Kowsari, 2017; Nanakkal, 2017; Štengl, 2011), and using a composite nanostructure (Ayati, 2014; Chan, 2005; Chen, 2017; Hatamie, 2015; He, 2018; Li, 2018; Morales-Torres, 2014; Panwar, 2016; Qian, 2009; Yun, 2015). A suitable dopant upon introduction into the crystal lattice of the semiconductor will introduce additional energy bands which lowers the bandgap that slows down the fast charge recombination (Bhanvase, 2017; Farhangi, 2011; Štengl, 2011). This will further improve the efficiency of the photocatalyst. Another method to improve the performance of the photocatalyst is to use a nanocomposite. This strategy uses a combination of the photocatalytically active material along with other active components such as carbon nanomaterials, metal oxides etc. to overcome the existing barriers of the photocatalyst (Divya, 2017; Li, 2007; Tang, 2018; Yue, 2010). Another widely studied method to overcome as well as to improve the efficiency of the photocatalyst is to use nanostructures (Liu, 2018; Lui, 2013; Nasrollahzadeh, 2018; Williams, 2008). These nanostructures show enhanced properties that are contributed from the photocatalyst as well as its constituents. Moreover, the nanostructures will be designed in such a way that enables the reactants to utilize the photocatalyst to its full efficiency. Apart from the aforementioned, recent trends to improve the efficiency of the photocatalysis also includes the alteration in synthetic routes such as atomic layer deposition of photocatalytic nanocomposites (Hamza, 2019; Rangarajan, 2018) and to make numerical model analysis(Gao, 2018; Zhao, 2017) of different photocatalytic systems to evaluate the efficiency of various photocatalysts. However, the nanostructured photocatalytic nanocomposites being the hot topic in the current research world, this chapter gives an idea about the recent advancements in nanostructured photocatalytic systems along with the basic principles of photocatalysis, its advantage and disadvantages and some of the commonly discussed applications.