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Fundamentals and Functional Mechanisms of Photocatalysis in Water Treatment

Rohit Sharma1, Parteek Mandyal1, Shabnam Sambyal1, Baizeng Fang2, Vineet Kumar3, Peyman Gholami4, Aashish Priye5 and Pooja Shandilya1,*

1 School of Advanced Chemical Sciences, Shoolini University, Solan (HP), India
2Department of Chemical and Biological engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, V6P 1Z3, Canada
3 Chemistry and Bioprospecting Division, Forest Research Institute, Dehradun, India
4Department of Chemistry, University of Helsinki, Helsinki, Finland
5 Department of Chemical Engineering, University of Cincinnati, Ohio, United States
*Corresponding author

1.1 Introduction

The continuous hike in world population, industrialization, and agricultural development has increased the trend of organic pollutants in water sources. Fortunately, many biodegradable pollutants can be removed by sedimentation, biodegradation, filtration, adsorption processes, etc. But, the pollutants discharged in the textile, agricultural and pharmaceutical industries usually contain nonbiodegradable, toxic, and carcinogenic pollutants. The extensive usage of industrial chemicals and their partial removal not only pollutes the larger water bodies but also damages the quality of ground water [1, 2]. Water having a low ratio of biological oxygen demand to chemical oxygen demand carries pollutants with a complex composition that is difficult to remove. The use of an oxidizing agent for the water remediation process is one of the facile strategies that can be frequently applied. From the past few years, the realm of photocatalysis has been flourishing as an excellent and green approach toward wastewater remediation.

The term photocatalysis is generally adopted to explicate the activation of chemical reactions under light irradiation. Photocatalysis is based on advanced oxidation processes (AOPs) capable of decomposing the complex and less biodegradable contaminants. Furthermore, photocatalysis is a sustainable, economical, energy-saving, and green technology with zero generation of a secondary pollutant. Previously, various photocatalysts like metal oxides, metal phosphides, carbonaceous materials, layered double hydroxides, metal sulfide, etc. have been broadly investigated for photodegradation [3-8]. The reactive oxidation species (ROS) generated on irradiation are generally accountable for photodegradation. These reactive oxygen species have strong redox potential, which can degrade almost all types of organic pollutants from water. Thus, ideal photocatalysts must possess high light absorption ability, suitable bandgap value, low recombination process, and large surface area.

The construction of an ideal photocatalyst cannot be accomplished easily, however, and there are several infirmities such as wide bandgap, inappropriate separation, migration of charge carriers, high recombination rates of excitons, and low light absorption ability that hindered wider applicability. Especially for bare photocatalysts, these drawbacks are more prominent. For efficient solar light harvesting, semiconductors with a small bandgap are required; on the contrary, such semiconducting materials will speed up the recombination process. These two conditions contradict each other, thereby eventually reducing the overall quantum efficiency. Several modifications were thus done to overcome these drawbacks.

Heterojunction formation is one of the most common and efficient techniques where the different materials with suitable band edge potential and bandgap were fabricated together. Such heterojunction-based photocatalysts provide good charge transfer ability with high charge separation and better oxidation and reduction capability. This chapter, therefore, summarizes the characteristic properties and heterojunctions of various metal oxides, metal phosphides, carbonaceous materials, and other layered materials. Also, the fundamental and functional mechanisms of varied photocatalysts utilized in the water decontamination process are briefly described [5, 9-13]. The mechanism of charge transfer in different heterojunctions, specifically, type II, Z-scheme, and S-scheme are described in detail. Different characterization techniques including X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) calculation, trapping experiments, and electron paramagnetic resonance (EPR) are also elucidated to confirm the successful fabrication of a heterojunction and charge transfer route. Finally, the applications of different metal oxides, metal phosphides, and carbonaceous material based heterojunctions are considered; a summary is then appended.

1.2 Different Photocatalytic Materials for Water Treatment

1.2.1 Metal Oxides Based Photocatalysts

Metal oxides having closely packed structures are generally formed by the coordination of metal ions and oxides. Such a configuration provides stoichiometric diversity and compositional simplicity to metal oxides; thus, they have been vastly investigated in several physical and chemical phenomena like magnetism, electron transport, gas sensing, and photocatalysis. Additionally, the physical, chemical, optical, and electronic characteristics of metal oxides can be enhanced by changing reaction conditions, doping with heteroatoms, and forming nanocomposites of metal oxides. Particularly, when the size of metal oxide particles is reduced to nanoscale dimensions, a large concentration of related atoms is present on their surface. This increases the surface area, which corresponds to its high efficiency, since a large number of active sites will be available for reaction. The absorption capacity of nanomaterials at nanoscale improves in comparison to their bulk counterpart as the reduction in particle size increases the surface-to-volume ratio and surface energy of adsorbent. Furthermore, good recyclability is an advantageous characteristic of metal oxides, which makes them persistent nanomaterials studied in applications from environmental remediation to energy conversion; this is true specifically for transition metal oxides owing to their good crystallinity, versatile surface characteristics, well-controlled structure, nontoxicity, high stability, cost-effectiveness, and nature as an earth-abundant material suitable for the formation of an ideal photocatalyst [14].

Typically, metal oxide semiconductors have a large bandgap value (E g) of > 3 eV and are extensively employed in photocatalysis. They also demonstrate great potential for the generation of both reducing and oxidizing species on irradiation. The valance band maximum (VBM) and conduction band minimum (CBM) majorly consist of oxygen 2p and metal ns or nd orbitals, respectively. When photons of energy equal to or more than the bandgap value strike on the semiconducting surface, the photoexcited electrons accumulate on the CB while the holes remain on the VB. Once these charge species are created, they are typically trapped on metal oxide surfaces and can oxidize or reduce the pollutant. The basic photocatalytic mechanism is attained by following four steps: (i) light absorption and charge separation, (ii) electron migration to acceptor, (iii) oxidation of donor, and (iv) charge recombination. The dye photosensitization of metal oxide semiconductors occurs in the following steps: (i) visible light absorption, (ii) electron transport from the dye (excited state) to CB of the metal oxide semiconductor, (iii) electrons shifting to acceptor, (iv) charge recombination, and (v) regeneration of sensitizer. Similarly, the ligand-to-metal charge transfer (LMCT) photosensitization is accomplished by (i) visible light stimulated LMCT shifting, (ii) electron transport to acceptor, (iii) charge recombination, and finally (iv) regeneration of adsorbate or degradation of adsorbate in the presence and absence of electron donor. Despite several favorable features of metal oxide semiconductors, various properties should be precisely optimized for practical applications [14, 15].

Generally, wide bandgap and high recombination of excitons are the main drawbacks of metal oxide semiconductors, which reduces their visible light absorption response and also decreases the density of available charge carriers to carry photocatalysis. Wide bandgap allows the absorption of light radiation under the UV region, which consists of only 3-4% of the solar spectrum. Additionally, UV radiation possesses highly energetic photons that can directly activate the chemical bonds of some organic substances and cause the formation of radical intermediates, initiating the nonselective reactions. Furthermore, the study of UV light active metal oxide semiconductors in the laboratory needs specialized glassware and high-cost UV light sources. Several modifications, therefore, such as doping, tuning of surface, defects creation, and heterostructure formation, are commonly used to enhance charge separation and visible light response of semiconductors. Crystallinity, geometry, surface area, and conductivity are some other influencing factors that affect the photocatalytic performance of metal oxide-based photocatalysts. These semiconductors exhibit both n and p-type conductivity depending upon the interaction between oxides and metal orbitals. Usually, O 2p orbitals are more localized whereas oxide orbitals are highly dispersed, resulting in a smaller effective mass of electrons in comparison to holes. Since carrier mobility is inversely...