Nanocomposites for Visible Light-induced Photocatalysis

This introductory chapter discusses the rapid development of nanotechnology for the application of visible light-induced photocatalysis, which is driven by the unique material properties arising from the nanoscale dimensions. It includes the description of the carbon-based nanomaterials developed first in the early development such as fullerene, carbon nanotube, and graphene. Conductive polymers were then described as photocatalysts with different dimensional nanostructures. Moreover, semiconductors were presented as potential materials for photocatalysis. For the practical visible light applications, photocatalysts need to be modified either by narrowing the band gap or by inhibiting the recombination of charge carriers via the formation of heterojunction nanocomposites. As the focus of this book, nanocomposites have been reported as a promising strategy for high-activity visible light-driven photocatalysis. This chapter is also complemented with some examples of industrial applications of photocatalysis for practical use.

Photocatalyst is a gifted method which can be used for various purposes like degradation of various organic pollutants in wastewater, production of hydrogen, purification of air, and antibacterial activity. When compared with other methods, photocatalysis is rapidly growing and gaining more attention from the researchers due to its several advantages such as low cost and attractive efficiency. Photocatalysis is a unique process for rectifying energy and environmental issues. In this connection, this chapter deals with basic principles, classification, mechanism, limitations, and operating parameters of photocatalytic processes. Furthermore, the most efficient photocatalytic materials, its mechanism, its challenges, and their solution of rectification were discussed in detail.

Photocatalysis is a promising technique for solving the worldwide energy and environmental crisis. The key challenge in this technique is to develop efficient photocatalysts that have to satisfy several criteria such as high chemical and photochemical stability as well as effective charge separation and strong light absorption. Synthesis of semiconducting nanocomposites is considered to be a promising way to achieve efficient photocatalysts. This improved photocatalytic activity of the nanocomposite photocatalysts is attributed to the enhancement of the charge separation, irradiation absorption, and photo and chemical stability. This chapter summarizes many research studies on semiconducting nanocomposites for different photocatalytic applications. Different consistencies for photocatalytic organic transformations have been discussed herein.

Heterogeneous photocatalysis has become an encouraging reaction technique to combat energy crisis and global environmental issues. Visible light (~400 nm–750 nm)-driven photocatalysis is the most imperative heterogeneous photocatalysis because of its selective product delivery, easy operation, and utilization of abundant available clean energy resource. In this context, utilization of clean, and available sunlight (having 44% visible light) could be a pleasant platform for solving energy and environmental problems. Thus visible light-driven photocatalysis is highly demanding, and so designing of such photocatalysts and their exploitation in catalysis under visible light has become a central research theme in catalysis. Surface plasmon resonance (SPR) active nanomaterials or composites are very effective to carry out catalytic redox reactions in presence of visible light due to the electron–hole formation, and termed as visible light plasmonic photocatalyst. Processes can be demonstrated through oxidation by “hole” and reduction by “hot electron”. Herein, we discussed on fabrication or synthesis of visible light plasmonic photocatalysts, and their application on catalytic reaction under visible light illumination. Visible light-induced SPR with detailed understanding of the fate of generated electron and hole on the redox reactions has been discussed. We have depicted various types of catalytic reactions such as photodegradation of large organic dyes (organic transformation), oxidation reaction, reduction reaction, hydroxylation, imine synthesis, water splitting reaction, biaryl synthesis, and CO₂ reduction.

Mixed metal oxide nanocomposite assisted photocatalysis has gained enormous interest among the scientists as a potential candidate for degrading environmentally harmful pollutants. This chapter reviews the recent advancement in the field of photocatalysis, focusing on the scientific challenges and opportunities offered by semiconducting mixed metal oxide materials. This review begins with a literature review to explore the suitable material and to optimize their energy band configurations for visible light active photocatalytic applications. This continues with examining the design and fabrication of hybrid nanocomposite materials for efficient photocatalytic performance. Finally, the discussion is meant on the synthesis methods for understanding the key aspects to engineer the nanocomposites for its use as an efficient and sustainable photocatalytic materials. This chapter also emphasizes vital problem that should be noted in upcoming research activities.

Aside from chemical modification, increasing the surface area of a photocatalyst material is a very important strategy to improve its photocatalytic activity. Although nanoparticles possess large surface area, problems arise from the aggregation of nanoparticles in solutions and difficulties related to the recycling of nanoparticles limit their practical applications. However, introduction of porosity into a photocatalyst structure not only provides a large surface area for adsorption of organic molecules and their subsequent photodegradation, but also improves the light harvesting by increasing the optical path length of the incident light inside the porous structure. The porous structure of a photocatalyst also provides a medium for better diffusion of reactants and products and can add selectivity to the properties of the photocatalyst material. Moreover, nanoporous photocatalyst materials can be easily recycled and reused which is very important in practical applications. In this chapter, the most common methods for the preparation of nanoporous nanocomposites for photocatalytic applications are presented. The parameters controlling the morphological characteristics of nanoporous structures together with the photocatalytic activity of these structures are discussed.

TiO₂ photocatalysts have been applied in treating wide range of organic contaminants, ranging from dye effluents to persistent organic pollutants. Discharging of those contaminants has polluted our natural water resources and reduced the quality and quantity of the clean water for our daily usage. Although TiO₂ photocatalysts show high removal efficiency towards most of the pollutants, the fast recombination rate and large bandgap impede its practical use under visible light irradiation. Considerable efforts have been employed to immobilize TiO₂ onto different substrates, particularly on polymer owing to their highly abundance and low cost. This chapter highlighted the various types of polymer-supported TiO₂ photocatalyst in degrading organic pollutants.

This paper gives a brief overview of the progress in the development of carbon-based nanocomposites for visible light-induced photocatalysis, based on graphene, graphene oxide, g-C₃N₄, [60]-fullerenes, and carbon nanotubes nanocomposites. In particular, recent progresses on the emerging strategies for tailoring carbon-based nanocomposites photocatalysts to enhance their photoactivity including elemental doping, heterostructure design and functional architecture assembly are discussed. The reported examples are collected and analyzed; and the reaction mechanism, the influence of various factors on the photocatalytic performance, the challenges involved, and the outlooks of carbon-based nanocomposites as photocatalyst are discussed in detail. Finally, some important applications such as photocatalytic degradation of pollutants, photocatalytic H₂ production, and photocatalytic CO₂ reduction are reviewed.

The carbonaceous π-conjugated/polymeric materials have been emerging as suitable materials to synthesize nanocomposites because of their attractive nanoporous structure, controllable surface chemistry, mechanical strength and favourable interactions with the semiconducting materials. The photocatalytic performances of the traditional polymeric materials are generally poor. Their performances can be greatly improved by coupling with a host semiconducting material. This is mainly due to their unique crystal structure, stability, high conductivities, nature of formation, efficient catalytic activity, promising electrochemical and optical properties. These polymeric nanocomposites act as photo sensitizers and good visible light absorbers due to π–π* electronic transitions. In this chapter the preparation methods, microstructure analysis and photocatalytic mechanism of graphitic carbon nitride (g-C₃N₄) and various carbonaceous π-conjugated/polymeric material composite catalysts are focused. In particular, modification of g-C₃N₄ by various carbonaceous π-conjugated/polymeric materials result in hybridization owing to strong π–π stacking interaction, which stabilizes the hybrid nanostructure and efficiently utilize the solar spectra by extending the photocatalytic applications in NO removal, CO₂ reduction and oxygen reduction reactions, water splitting to liberate H₂ fuel and degradation of pollutants. The challenges of various π-conjugated/polymeric material modified nanocomposites of g-C₃N₄ in the field of photocatalysis are also highlighted in this chapter to extend their applications in sustainable energy development.

The concept of photocatalysis is not new, but the photocatalyst used for the process of photocatalysis is improving day by day. To take the concept of photocatalysis in advanced manner, titanium-based mixed metal oxide nanocomposites photocatalyst has been introduced in the field of photocatalysis. A brief study on the photocatalytic activity of the titanium-based mixed metal oxide nanocomposites (by categorizing blockwise into s, p, d, f groups) has been given in this chapter. The mechanism behind the improved photoactivity of the nanocomposite, due to the efficient charge separation at the heterojunction interface, is summarized. Various structures adopted by titanium-based mixed metal oxides like perovskite, pyrochlore, ilmenite, etc. by considering their ionic radii are reviewed here. Morphology, surface area, lattice and energy level matching etc. are some of the key factors responsible for the improved photoactivity with examples are discussed briefly. The photocatalytic activity of mixed metal oxide nanocomposites beyond titanium is also reviewed here in the last section. This book chapter may give a new insight for the development of research on nanocomposite in the field of photocatalysis as well as other fields such as supercapacitor and sensors.

As the present chapter of the book is located in the concluding section, it was important to highlight the main applications of composite materials focusing especially on applications, which exploit other peculiarities of the materials besides photocatalysis. This will be done, by introducing those materials and their composites that are most studied, or were found to exhibit interesting behavior. In many of the presented cases, the main structural, morphological, or optical property of the given composite will be discussed to understand its functioning mechanism, and its role in the current scientific approaches. Additionally, this chapter aims to give a perspective regarding the composite-based nanoscience, and points out important research directions for the further developments of composite materials.