Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites

Hindawi Publishing Corporation Advances in Materials Science and Engineering Volume 2015, Article ID 934587, 22 pages http://dx.doi.org/10.1155/2015/934587 Review Article Photocatalysis and Bandgap Engineering Using ZnO Nanocomposites Muhammad Ali Johar,1 Rana Arslan Afzal,1 Abdulrahman Ali Alazba,1 and Umair Manzoor1,2 1 Alamoudi Water Research Chair, King Saud University, P.O. Box 2460, 11451 Riyadh, Saudi Arabia Centre for Micro & Nano Devices, Department of Physics, COMSATS Institute of Information Technology, 44000 Islamabad, Pakistan 2 Correspondence should be addressed to Umair Manzoor; Received 20 August 2015; Revised 12 October 2015; Accepted 13 October 2015 Academic Editor: Filippo Giannazzo Copyright © 2015 Muhammad Ali Johar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Nanocomposites have a great potential to work as efficient, multifunctional materials for energy conversion and photoelectrochemical reactions. Nanocomposites may reveal more improved photocatalysis by implying the improvements of their electronic and structural properties than pure photocatalyst. This paper presents the recent work carried out on photoelectrochemical reactions using the composite materials of ZnO with CdS, ZnO with SnO2 , ZnO with TiO2 , ZnO with Ag2 S, and ZnO with graphene and graphene oxide. The photocatalytic efficiency mainly depends upon the light harvesting span of a material, lifetime of photogenerated electron-hole pair, and reactive sites available in the photocatalyst. We reviewed the UV-Vis absorption spectrum of nanocomposite and photodegradation reported by the same material and how photodegradation depends upon the factors described above. Finally the improvement in the absorption band edge of nanocomposite material is discussed. 1. Introduction A humongous amount of water pollutants is discharged into the environment by the industries on daily basis which causes many hefty problems for humans, amphibious environments, and microorganisms [1–11]. The main sources of the water pollutants are fertilizers [12–14], microorganisms [15–18], application of pesticides and chemicals to soils [19–25], sewage [26–29] and wastewater [30–32], septic tanks [33–36], underground storage and tube leakages [37], atmospheric deposition [38–41], industrial waste which usually contains sulphur [42], asbestos, lead, mercury, nitrates and phosphates, oils, textile dyes, and so forth. These water pollutants cause the death of aquatic animals [43–49], disruption of food chains, different human diseases [50–59], destruction of ecosystems, and so on. To decontaminate the contaminated water, researchers have taken many steps and have suggested many pollutants remediation techniques. One method is to treat the wastewater on site by the treatment plants, as it has a great potential [60–63]. There are a variety of water treatment processes like chemical, physical, and biological techniques, but each has its limitations for the application, cost, and effectiveness point of view. The pollutants are being transferred to solid phase from liquid phase by physical techniques like adsorption, precipitation, or air stripping; hence the pollutants are not destroyed. Chemical oxidation may be slow to moderate in the rate and selective or rapid but nonselective, hence generating oxidant cost. When the feed is inhibitory or toxic to bioculture, the limitation of biological oxidation takes place. Rest of the techniques are limited due to oxidative potential, economics, or tendency to farm harmful byproducts [64, 65]. Due to these limitations there may be offered an effective particular process which may be the combination of the available techniques in such a way to exploit their individual strength, thus an appropriate solution obtained within the economic constraints. Nowadays the most appropriate techniques for the water treatment are advanced oxidation processes (AOPs) which have very fewer limitations [66– 70]. Among AOPs, heterogeneous photocatalysis is a tertiary 2 Advances in Materials Science and Engineering Rocksalt Zinc blende Wurtzite (a) (b) (c) Figure 1: Stick and ball representation of different ZnO crystal structures: (a) cubic rocksalt, (b) cubic zinc blende, and (c) hexagonal wurtzite. The shaded gray and black spheres denote Zn and O atoms, respectively [96]. water treatment process and has attracted the interest of researchers due to its ability to completely decompose the target pollutants [71–73]. There is a great potential for the mitigation of the toxic chemicals from the polluted water by photocatalytic degradation using nanostructured semiconductors [74, 75]. Currently, the hot issue among the most important challenges faced by science researchers for clean energy, pollutant-free water and air is designing new materials for the maximal harvesting of solar radiation. An extensive work has been carried out on ZnO and TiO2 for the application of photocatalysis and photovoltaic cells due to their advantage of high stability against photocorrosion, suitable bandgap, and good photovoltaic and photocatalysis efficiencies [76–82]. The photocatalytic behavior of the nanocomposites varies with morphologies [83–93]. For ZnO, the difference in photocatalytic behavior occurs due to polar planes, surface areas, and oxygen vacancies. Xu et al. synthesized different morphologies of ZnO by solvothermal method and used them as photocatalyst for the degradation of phenol [83]. They suggested that NPs and nanoflowers exhibited enhanced photodegradation results compared to nanorods, nanotubes, nanoflowers, and hour-glass-like ZnO spheres. Liu et al. prepared TiO2 nanostructures with different morphologies like NPs, nanorods, and microspheres via hydrothermal route and applied them for the photodegradation of phenol [87]. They observed excellent photodegradation results when nanorods were used as photocatalyst. Although ZnO has been studied since 1935, new techniques and advance equipment make it possible to explore its remarkable properties [94]. ZnO is now considered to be the future material for various optoelectronics devices and sensors and as a catalyst. The characteristic of ZnO as photocatalyst becomes more prominent due to the enhanced photocatalytic efficiency of ZnO in the pure and doped forms and as a physical mixture. The figure of merits of doped and undoped ZnO nanomaterials is high carrier mobility, environmental sustainability, high photocatalytic efficiency, facile, simple tailoring of structures, nontoxicity, low cost for massive synthesis, and so forth. 2. ZnO Properties and Crystal Structure ZnO occurs as a white powder. ZnO is an amphoteric oxide. ZnO is II-VI compound semiconductor whose iconicity lies at the borderline between ionic and covalent semiconductors. ZnO has three crystal (...truncated)