Nano-Materials as Photocatalysts for Degradation of Environmental Pollutants

Challenges and Possibilities

Pardeep Singh, Anwesha Borthakur, P.K. Mishra, Dhanesh Tiwary

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780128185988
• eBook ISBN: 9780128185995
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Nano-Materials as Photocatalysts for Degradation of Environmental Pollutants: Challenges and Possibilities contains both practical and theoretical aspects of environmental management using the processes of photodegradation and various heterogeneous catalysts. The book's main focus is on the degradation of harmful pollutants, such as petrochemicals, crude oils, dyes, xenobiotic pharmaceutical waste, endocrine disrupting compounds, and other common pollutants. Chapters incorporate both theoretical and practical aspects. This book is useful for undergraduate or university students, teachers and researchers, especially those working in areas of photocatalysis through heterogeneous catalysts. The primary audience for this book includes Chemical Engineers, Environmental Engineers and scientists, scholars working on the management of hazardous waste, scientists working in fields of materials science, and Civil Engineers working on wastewater treatment. - Reviews recent trends in the photodegradation of organic pollutants - Offers a bibliometric analysis of photocatalysis for environmental abatement - Includes many degradation mechanisms of organic pollutants using various catalysts - Includes examples on the degradation of organic pollutants from various sources, e.g., pharmaceuticals, dyes, pesticides, etc. - Discusses the effect of nanocatalysts on soil, plants and the ecosystem

Chapter 1

An overview of synthesis techniques for preparing doped photocatalysts

Indrajit Sinha; Arup Kumar De Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi, India

Abstract

Fossil fuels are the mainstay of most technologies in fashion currently, and their excessive use is also the primary reason for many of the environmental problems that are being encountered (Marschall & Wang, 2014). One major renewable energy technology is photocatalysis, which requires only solar radiation for energy production by artificial photosynthetic reactions and can degrade environmental pollutants to less harmful fractions (Khaki et al., 2017; Ong et al., 2016). An extensively practiced way for efficient photocatalyst construction is the doping of the crystal lattice of the native semiconductor by a metal or nonmetal ions. Doping introduces defects into the ideal crystal lattice of the native semiconductor and also improves the activity of the photocatalysts by modifying their electronic structures (Shao et al., 2018). Such defects can entrap electrons or holes formed during the photoexcitation process. Another important function of the defect sites like vacancies is to increase the catalytic activation of strong bonds kinetically facilitating reactions. Doping could also lead to additional charge carriers in the photocatalyst and form extra bands that may narrow or widen the original bandgap. Therefore, doping can affect light absorption, reduce recombination through trap sites, and alter VB or CB positions to change the photocatalytic activity towards a substrate molecule or reaction. The present chapter gives a concise review of the synthesis techniques used to produce doped WBG semiconductors. The method of preparation of doped semiconductors determines the amount of doping possible and the nanostructure formed. It is important to mention that only those research works which report shift in XRD peaks of the original phase due to doping have been discussed in this chapter.

Keywords

Photocatalysis; Semiconductors; Doping; Synthesis techniques; Characterization

1 General introduction

Energy and environmental problems are the most pressing topical issues of our planet today (Umezawa & Ye, 2012). The two topics go in a cycle. Fossil fuels are the mainstay of most technologies in fashion currently, and their excessive use is also the primary reason for many of the environmental problems that are being encountered (Marschall & Wang, 2014). Jettisoning fossil fuels and shifting focus to renewable energy technologies are the apparent solution to this vicious circle (Jiang et al., 2017; Marschall & Wang, 2014). One major renewable energy technology is photocatalysis, which requires only solar radiation for energy production by artificial photosynthetic reactions like water splitting and CO₂ reduction (Ong, Tan, Ng, Yong, & Chai, 2016). Additionally, it is a technique that can degrade environmental pollutants to less harmful fractions using only sunlight (Khaki, Shafeeyan, Raman, & Daud, 2017; Ong et al., 2016). Discovery of semiconductors and their science was the first step to the realization of the phenomenon of photocatalysis (Hashimoto, Irie, & Fujishima, 2005). The oil crisis of the 1970s spurred the governments of developed countries to sponsor extensive research on renewable energy sources and technologies (Meng, Xing, Li, & Li, 2015; Ong et al., 2016). Another key stepping stone was the first paper on photocatalytic water splitting by Honda and Fujishima (Fujishima & Honda, 1972).

Today, we know that there are several requirements for the design of an active photocatalyst for a target reaction. The starting point is efficient light absorption for exciting electrons from the valence band (VB) to the conduction band (CB) of the semiconductor (Asahi, Morikawa, Ohwaki, Aoki, & Taga, 2001). Most of the traditional photocatalysts like TiO₂, ZnO, and ZnS are wide bandgap (WBG) semiconductors, and therefore, photoexcitation requires irradiation by short-wavelength UV radiation (Kedziora, Strek, Kepinski, Bugla-Ploskonska, & Doroszkiewicz, 2012). On the other hand the visible range of radiation constitutes a significant part of the solar spectrum. Consequently the use of UV radiation by WBG photocatalysts results in inefficient use of solar energy resources (Kumar, Verma, Pal, & Sinha, 2018). Naturally, a large amount of current photocatalysis research is on designing nanocomposites that can utilize visible light (Wang et al., 2014). Another vital issue related to the absorption of light is the photoefficiency of the photocatalyst. Generally the photoefficiency of most photocatalysts is very poor and is ~ 1% in most cases (Ge, Han, & Liu, 2011; Li et al., 2011).

The next issue is the fate of the photogenerated electron–hole pair (or exciton) excited species. The electrons and holes thus formed travel to the surface of the catalyst, or they may recombine (Pesci, Wang, Klug, Li, & Cowan, 2013). The energy that has been absorbed could be released by various phenomena included in photoluminescence and also by energy release in the form of heat (Wojcieszak, Kaczmarek, Domaradzki, & Mazur, 2013; Zhang & Mu, 2007). Overall the increase in photocatalytic efficiency can be achieved by ensuring that excitons do go to the photocatalyst surface and survive long enough to harness their redox ability (Takanabe, 2017). In other words, it is vital to design photocatalysts with increased lifetime or charge separation of the photogenerated electron–hole species (Jiang et al., 2018). A crucial point is the VB and CB positions of the photocatalyst. The redox potential of the targeted oxidation or reduction reaction needs to be lesser or more than the VB and CB positions of the photocatalyst, respectively. However, semiconductors with good driving force due to valence band (VB) and conduction band (CB) positions also may not show good efficiency (Jiang et al., 2018; Schoonen & Xu, 2000). The main reason for this is the weak adsorption of reactants on the catalyst surface. Thus the adsorption characteristics of different components making up the photocatalyst concerning the target reactants also need to be taken into account (Crittenden, Suri, Perram, & Hand, 1997; Nguyen-Phan et al., 2011).

From the previous discussion, it is clear that the photocatalytic efficiency of a material is dependent on two types of characteristics. The first set of requirements is not specific to any reactant. In contrast to this the second set of conditions depends on the reactant. Photocatalytic material characteristics (which are not reactant dependent) include bandgap (visible or UV range), charge separation, and photoefficiency. On the other hand, fabricating materials with tailored band positions and having compatible adsorption properties are aspects specific to the reaction being considered.

It is evident that there are two contradictory requirements in photocatalyst development. Thus the CB and VB positions should be sufficiently negative and positive, respectively to induce the required the reduction and oxidation reactions for the process to be sustained only by light irradiation (Jafari et al., 2016). The required band positions indicate that a wide band gap (WBG) semiconductor should be used. But a wide band semiconductor would either be an insulator or its absorption would in the UV range (Yan, Wang, Yao, & Yao, 2013). Since the UV range is only 4% of the solar radiation, therefore, this would result in poor utilization of this natural resource. Thus, proper exploitation of solar resources necessitates photocatalysts that can operate using visible light. Hence, proper energy utilization requires small band gap semiconductors (Fox & Dulay, 1993), while required band positions dictate a wide bandgap photocatalysts.

Wide bandgap photocatalysts also face the problem of relatively easy recombination of photo-induced holes and electron (Fox & Dulay, 1993; Johar, Afzal, Alazba, & Manzoor, 2015). The recombination problem can be reduced by taking recourse to one of the following methods. A major path followed involves designing of new heterostructure nanocomposites (Johar et al., 2015). Thus two semiconductors may be combined in such manner that the (VB and CB) band positions of two photocatalysts have staggered alignment with respect to each other. In such cases, semiconductor heterojunction is formed, and such composites may be Z-scheme or p-n junction-type photocatalysts (Maeda, 2013; Sun et al., 2018). Both types of heterojunctions have their advantages. In both cases, charge separation is achieved, but in Z-scheme, the photocatalytic driving forces due to (lower) VB and (higher) CB positions are better. Such semiconductor heterostructures are generally made up of two visible range semiconductors. Similarly, deposition of a noble metal cocatalyst can also lead to the utilization of visibl...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Chapter 1: An overview of synthesis techniques for preparing doped photocatalysts
• Chapter 2: Low-dimensional nanomaterials for the photocatalytic degradation of organic pollutants
• Chapter 3: Recent advances in functionalized polymer-based composite photocatalysts for wastewater treatment
• Chapter 4: Solar light-induced photocatalytic degradation of pharmaceuticals in wastewater treatment
• Chapter 5: Hydrotalcite-like compounds and related materials as catalysts for the photodegradation of pharmaceutical compounds: Synthesis and catalytic performances
• Chapter 6: Metal-organic frameworks for photocatalytic degradation of pollutants
• Chapter 7: Photocatalytic degradation of petrochemical pollutants
• Chapter 8: Photodegradation of pharmaceutical waste by nano-materials as photocatalysts
• Chapter 9: Photocatalytical degradation of pesticides
• Chapter 10: Imidazole framework based metal oxide nanoparticles photocatalysts: An approach towards amputation of organic pollutants from water
• Chapter 11: Exploitation of antibiotics: Mechanism of resistance, consequences, challenges of conventional remediation, and promise of nanomaterials in mitigation
• Chapter 12: Photochemical and photocatalytical degradation of antibiotics in water promoted by solar irradiation
• Chapter 13: Carbon-supported semiconductor nanoparticles as effective photocatalysts for water and wastewater treatment
• Chapter 14: Synthesis of graphitic carbon nitride—Nanostructured photocatalyst
• Chapter 15: Advancement and modification in photoreactor used for degradation processes
• Chapter 16: Nanocatalyst types and their potential impacts in agroecosystems: An overview
• Chapter 17: Nanotechnology for soil remediation: Revitalizing the tarnished resource
• Chapter 18: Overview of nanomaterials synthesis methods, characterization techniques and effect on seed germination
• Index