Photocatalysis

Principles and Applications

Rakshit Ameta, Suresh C. Ameta

• Publisher: CRC Press
• Year: 2016
• ISBN:
  9781315355139
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Photocatalysis, reactions carried out in the presence of a semiconductor and light, is rapidly becoming one of the most active areas of chemical research, with applications in areas such as electrochemistry, medicine, and environmental chemistry, Photocatalysis: Principles and Applications stresses the development of various types of photocatalytic semiconductors, including binary, ternary, quaternary, and composite, and their modifications by metallization, sensitization, and doping to enhance their photocatalytic activities. In addition to describing the principles and mechanisms of photocatalysis, it also discusses other possible applications of photocatalysis such as use as antifouling agents, controlling air pollution by degrading contaminants present in the environment, self-cleaning of glasses and tiles in the presence of light/artificial light, green composites, wastewater treatment, hydrogen generation, and inactivation of microorganisms. The book also describes medical applications and summarizes efforts in the field of photosplitting of water as a newer energy source and photoreduction of carbon dioxide for providing synthetic fuels and also a step towards mimicking photosynthesis.

• 
  
  

• Introduces the basic principle of photocatalysis.

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• Provides an overview of the types of semiconductors, their immobilization, and modifications to make them more active.

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• Gives possible applications of photocatalysis in wastewater treatment and strategy to combat against different kinds of pollutions like water, air, and soil.

• 
  
  

• Summarizes efforts in the field of photosplitting of water as a newer energy source and photoreduction of carbon dioxide for providing synthetic fuels and as a step towards mimicking photosynthesis.

• 
  
  

• Discusses inactivation of different kinds of microorganisms.

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• Covers medical applications.

Features

• 
  
  

• Introduces the basic principle of photocatalysis.

• 
  
  

• Provides an overview of the types of semiconductors, their immobilization, and modifications to make them more active.

• 
  
  

• Gives possible applications of photocatalysis in wastewater treatment and strategy to combat against different kinds of pollutions like water, air, and soil.

• 
  
  

• Summarizes efforts in the field of photosplitting of water as a newer energy source and photoreduction of carbon dioxide for providing synthetic fuels and as a step towards mimicking photosynthesis.

• 
  
  

• Discusses inactivation of different kinds of microorganisms.

• 
  
  

• Covers medical applications.

1 Introduction

1.1 HISTORY

The Greek word photocatalysis is a combination of two words: photo (“phos” means light) and catalysis (“katalyo” means to break apart, decompose). Generally, the term photocatalysis is used to describe a process where light is used to excite a photocatalyst and the rate of chemical reaction is accelerated without involving the photocatalyst. Although the term photocatalysis is quite confusing and there has been a long debate on the definition of this term, the International Union of Pure and Applied Chemistry (IUPAC) has finally decided that the term photocatalysis is reserved for the reactions carried out in the presence of a semiconductor and light. Plotnikow (1936) mentioned photocatalysis in his book entitled Allgemeine Photochemie. Almost four decades later, some researchers actively started conducting surface studies on photocatalysts such as TiO₂ and ZnO. In the meantime, some workers also thought on the possibility of using sunlight as the energy source. Fujishima and Honda (1972) conducted the photolysis of water using a semiconductor electrode (TiO₂) in a photoelectrochemical cell, which gave momentum to the field of photocatalysis. This was considered as the real beginning of this field. In the 1980s and 1990s, many efforts were made to understand the fundamental process and improve the photocatalytic efficiency of titania. In the last few years, semiconductor materials have been used as a photocatalyst for air and water remediation, mineralizing hazardous organic pollutants, and industrial and health applications (Rengel et al. 2012).

1.2 PHOTOCATALYSIS

A photoinduced reaction, which is accelerated in the presence of a catalyst (semiconductor), is termed photocatalysis. There are two types of photocatalysis: homogeneous and heterogeneous.

• Homogeneous photocatalysis
  When the reactant and photocatalyst exist in the same phase, the reaction is called homogeneous photocatalysis. Coordination compounds, dyes, natural pigments, and so on are the most common examples of homogenous photocatalysts.
• Heterogeneous photocatalysis
  In this type, the reactant and photocatalyst exist in different phases. Transition metal chalcogenides are the most common examples of heterogeneous photocatalysts, which have some unique characteristics.

Photocatalytic reactions are initiated by the absorption of a photon with appropriate energy that is equal to or higher than the band gap energy of the photocatalyst. The absorbed photon creates a charge separation as the electron is elevated from the valence band (VB) of a semiconductor to the conduction band (CB), creating a hole (h⁺) in the VB. This excited electron can reduce any substrate or react with electron acceptors such as O₂ present on the semiconductor surface or dissolved in water, reducing it to superoxide radical anion O₂^−•. On the other hand, the hole can oxidize the organic molecule to form R⁺, or react with ⁻OH or H₂O, oxidizing them to ^•OH radicals.

Other highly oxidant species such as peroxide radicals are also responsible for the heterogeneous photodecomposition of organic substrates. ⁻OH is quite a strong oxidizing agent which can oxidize most of the azo dyes and other pollutants to the minerals as end products (Konstantinou and Albanis 2014). The field of photocatalysis has been reviewed by different researchers from time to time (Fox and Dulay 1993; Fujishima et al. 2000; Ameta et al. 2003; Reloez et al. 2012).

1.3 SEMICONDUCTING MATERIALS

Currently, semiconductor photocatalysis is becoming one of the most active areas of research and has been studied in different streams such as catalysis; photochemistry; electrochemistry; inorganic and organic chemistries; physical, and polymer, environmental chemistry; and so on. Mainly binary semiconductors such as TiO₂, ZnO, Fe₂O₃, CdS, and ZnS have been used as photocatalysts because of a favorable combination of their electronic structure, light absorption properties, charge transport characteristics, and excited-state lifetime.

Apart from the binary chalcogenides, some ternary chalcogenides have also been a subject of investigation; these include SrZrO₃, PbCrO₄, CuInS₂, Cu₂SnS₃, and so on (Tell et al. 1971; Guo et al. 2014; Chen et al. 2015; Miseki et al. 2015).

Very little work has been carried out on the use of quaternary oxides and sulfides in comparison to binary and ternary chalcogenides. Some such photocatalysts are Bi₂AlVO₇, Cu₂ZnSnS₄, FeZn₂Cu₃O_6.5, and so on (Luan et al. 2009; Reshak et al. 2014; Kumawat et al. 2015).

1.4 MODIFICATIONS

Usually semiconductors providing promising solutions for environmental pollution problems and solar energy crisis are selected as photocatalysts. Considering the benefits and limitations of these photocatalytic materials, some researchers have attempted to enhance photocatalytic activity of these materials using various techniques. Different strategies have been used from time to time, such as surface and interface modification by controlling morphology and particle size, composite or coupling materials, transition metal doping, nonmetal doping, codoping (metal–metal, metal–nonmetal, nonmetal–nonmetal), noble metal deposition, and surface sensitization by organic dye and metal complexes, to enhance the photocatalytic properties.

1.4.1 DOPING

The addition of impurities to a very pure substance is known as doping, which is divided into the following two categories: (1) cationic doping and (2) anionic doping. In cationic doping, the semiconductor is doped with cations, for example Al, Cu, V, Cr, Fe, Ni, Co, Mn, and so on, while in anionic doping, anions are used, for example N, S, F, C, and so on. Each type of dopant has its own unique impact on crystal lattice of the photocatalyst. Metal and nonmetal ion doping on the surface of a photocatalyst increases its photoresponsiveness to the visible region by creating new energy levels (or impurity state) between the VB and CB to reduce its band gap. The electrons excited by light are shifted from the impurity state to the CB.

The photocatalytic activity of different nanoparticles such as Fe-doped TiO₂, WO₃/ZnO, and Fe-doped CeO₂ was examined by Siriwong et al. (2012). Metal-doped SrTiO₃ photocatalyst was prepared by Chen et al. (2012) for water splitting while Zhang et al. (2013) explained the effect of nonmetal dopants such as B, C, N, F, P, and S as anions on electronic structures of SrTiO₃. Anandan et al. (2012) studied the photocatalytic activity of TiO₂ and ZnO after doping by rare-earth metal La. Maeda and Yamada (2007) also studied doping of Cu, Al, and Fe with TiO₂ semiconductor.

Codoping of metal and nonmetal also increases the photocatalytic activity of the photocatalysts. Codoping of Cr + N in ZnO, Cu + Al codoped ZnO, Ga + N codoped TiO₂, and W + C codoped TiO₂ nanowires have been reported by various researchers (Wu et al. 2011; Li et al. 2012; Cho et al. 2013; Nibret et al. 2015).

1.4.2 COMPOSITES/COUPLING

Coupling of semiconductors or composite is another method to make photocatalysts effective in the visible light for different applications. Here, a large band gap semiconductor is coupled with a small band gap semiconductor having a more negative CB level. As a result, the electrons of CB can be injected from the small band gap semiconductor to the large band gap semiconductor. The dye sensitization process is similar to this method, but the only difference is that the electrons move from one semiconductor to another. Hydrogen production by coupled SnO₂, CdS, CdS/Pt–TiO₂, and NiS/Zn_xCd_1–xS/reduced graphene oxide has been studied (Gurunathan et al. 1997; Park et al. 2011; Zhang et al. 2014).

1.4.3 METALLIZATION

Noble metals such as Ag, Au, Pt, Ni, Cu, Rh, Pd, and so on, have been used to improve the photocatalytic activity of a semiconductor. This process decreases the possibility of electron–hole recombination, and causes efficient charge separation and higher rates of photocatalytic reaction. Noble metals having these properties can assist in electron transfer, leading to higher photocatalytic activity.

1.4.4 DYE SENSITIZATION

Dye sensitization is a promising method for surface modification of photocatalysts to utilize the visible light for energy conversion. Dyes have redox properties and visible light sensitivity, which can be used in solar cells as well as in photocatalytic systems. When dyes are exposed to the visible light, they can inject electrons to the CB of semiconductors to start a catalytic reaction.

To convert absorbed light directly into electrical energy with higher efficiency in solar cells or through generation of hydrogen, a fast electron injection and slow backward reaction are the major requirements.

1.5 APPLICATIONS

Different semiconductors are used in the water and air purification, self-cleaning, self-sterilization, antifogging, antimicrobial activity, and so on. In these areas, the TiO₂ photocatalyst has attracted much attention because of its high catalytic efficiency, chemical stability, economy, low toxicity, and good compatibility with traditional construction materials. It is also useful for damaging microorganisms, for example bacteria and viruses, and even in inactivating some cancer cells, as well as for the photosplitting of water to produce hydrogen gas, the fuel of the future.

1.5.1 WATER TREATMENT

Many binary and ternary semiconductors have been used as photocatalysts in wastewater treatment. TiO₂ and ZnO photocatalysts have been quite commonly used in wastewater purification. ZnO is an excellent photocatalytic oxidation material widely used in wastewater treatment in industries such as pharmacy, printing and dyeing, paper and pulp, and so on. TiO₂ nanotubes (TNTs) are the most promising photocatalysts for photocatalytic decontamination of water. Benjwal et al. (2015) reported that the graphene oxide–TiO₂/Fe₃O_4- based ternary nanocomposites have potential applications in wastewater treatment.

1.5.2 REMOVING TRACE METALS

Some trace elements such as Hg, Cr, Pb, and other metals are extremely hazardous to human health. These toxic metals can be successfully removed, even at lower level concentrations such as parts per million, by heterogeneous photocatalysis to maintain water quality and human health.

1.5.3 WATER SPLITTING

A number of oxides, sulfides, and selenides have been prepared as photocatalysts for water-splitting reactions. Nanosized TiO₂, many coupled semiconductor CaFe₂O₄/TiO₂, heterojunction WO₃/BiVO₄, core/shell nanofibers CdS/ZnO, and so on, offer a promising way to produce hydrogen from water (Su et al. 2011; Yang et al. 2013; Reddy et al. 2014).

1.5.4 SELF-CLEANING FUNCTIONS

The TiO₂ photocatalyst has attracted much attention as a photofunctional material, because cleaning glass and tile surfaces involves high-energy depletion, chemical detergents, and high costs. The organic and inorganic molecules remain adsorbed and easily degraded on the TiO₂-based self-cleaning surface. Then, it can be washed with water due to the high hydrophilicity of titania film. This function of titania is effective only when the number of incident solar photons per unit time is greater than the rate of adsorption of the or...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• Authors
• Chapter 1 Introduction
• Chapter 2 Photocatalysis
• Chapter 3 Binary Semiconductors
• Chapter 4 Ternary Semiconductors
• Chapter 5 Quaternary Semiconductors
• Chapter 6 Metallization
• Chapter 7 Doping
• Chapter 8 Sensitization
• Chapter 9 Composites
• Chapter 10 Immobilization
• Chapter 11 Hydrogen Generation
• Chapter 12 Solar Cells
• Chapter 13 Wastewater Treatment
• Chapter 14 Reduction of Carbon Dioxide
• Chapter 15 Artificial Photosynthesis
• Chapter 16 Medical Applications
• Chapter 17 Other Applications
• Chapter 18 Photoreactors
• Chapter 19 Future Trends
• Index