ReviewThin film photocatalysis for environmental remediation: A status review
Graphical abstract
Introduction
Nowadays, a rapid increase in the world population and widespread industrialization is mainly responsible for huge environmental pollution caused by several hazardous waste and organic contaminants. The continuous discharge of organic pollutants such as dyes in water resources tends to water pollution and affects not only human health but also the aquatic life in the water bodies [1]. The point sources (refineries, mines, factories, power plants, etc.) are mainly responsible for surface and groundwater pollution than non-point sources (cars, buses, and trains, etc.) [2]. According to the Lancet Commission on Pollution and Health, around 1.8 million deaths worldwide are related to water-born diseases [3]. There are various types of pollutants which includes inorganic (heavy metal ions, metal oxides, metal complexes, salts, etc.), organic (dyes, pesticides, pharmaceutical ingredients, fertilizers, phenols, surfactants, etc.), nutrients and agricultural runoff, pathogens and so on. Among these, organic pollutants carry a major part due to its longer persistence, strong resistance and significant effect on human health. Most of the dyes from industrial activity are released directly into the water bodies produce unwanted color which causes non-aesthetic pollution, eutrophication, perturbations and restriction on biological activities of aquatic life [4,5]. Various organic compounds that are responsible for pollution along with their chemical structures are listed in Table 1. Therefore, society needs to focus on the degradation of such pollutants. Several techniques have been used to eliminate organic compounds such as biodegradation, chemical oxidation and electrochemical conversion/combustion, ozonation, Fenton or photo-Fenton systems, ultrasound, adsorption onto activated carbons, reverse osmosis, photolysis (UV), photolysis (UV/H2O2), and advanced oxidation processes (AOPs). Out of these, the advanced oxidation processes are found to be effective methods to degrade organic pollutants. It involves the formation of highly reactive oxidizing species for the oxidation of pollutants. Among the various AOPs, Photocatalysis has gained growing attention for degradation of organic pollutants due to its high efficiency, low-cost and green approach method [6,7].
Section snippets
Introduction to photocatalysis
Photocatalysis is a technique that utilizes light and semiconductor. It exhibits high stability, non-toxicity, corrosion resistance and no cause of secondary pollution [8]. In 1972, Fujishima and Honda for the first time reported the hydrogen production by water splitting with the help of n-type TiO2 semiconductor photocatalyst [9]. Fig. 1 indicates various applications of semiconductor photocatalysts. The photocatalytic activity of photocatalyst dominates the performance of the photocatalytic
Theoretical background of thin films
Thin film is a layer that extends infinitely along any two directions but restricted along the third direction and its thickness ranges from several nanometers to few micrometers. The main advantage of thin film technology is the reduction in size and tuning of material properties. Further thin films provide cost reduction and miniaturization of the device [11,18]. The properties of the films are mainly dominated by the structure of the film but the mechanical and thermal properties of films
Metal oxide thin film photocatalysts
Metal oxides are emerging as promising candidates for photocatalytic applications due to its biocompatibility, exceptional stability, ability to generate charge carriers, lifetime, favorable electronic structure and charge transport characteristics. The most popular photocatalyst are TiO2, ZnO, SnO2, WO3 and transformation from UV to visible region absorption were achieved by metal-non metal doping. The layer by layer deposition of metal oxide film enhances the photocatalytic performance [94,95
Metal sulfide thin film photocatalysts
Metal sulfide photocatalyst has gained considerable attention due to its wide light absorption range and higher conduction band position as compared to metal oxides [231]. Among the various metal sulfides, CdS and ZnS are widely studied for photocatalytic application and more attention is focused on the generation of solar fuel and degradation of environmental pollutants [232].
Conclusion and future prospects
Above mentioned literature elaborates on the photocatalytic technique emerging as a promising, environmentally friendly, efficient, advanced and cost-effective route for the photodegradation of various environmental pollutants. Thin film photocatalysts have gained wide attention to solve the post-separation problem associated with powder photocatalysts. The metal oxide and metal sulfide thin film photocatalysts can decompose a variety of organic pollutants into less harmful reaction products
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Author Rupesh pedanekar is thankful to the Chhatrapati Shahu Maharaj National Research Fellowship (CSMNRF-2019) from Government of Maharatshtra India for financial Support.
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