Green Photocatalysts for Energy and Environmental Process

The demand for sustainable materials in the area of energy and environment has been going forth among researchers over the past several years. This is imputable to the increasing demand globally in the field of energy and environmental sectors to conserve the materials for future needs, either in the form of reuse or reprocessing the materials by environmentally friendly style. The advancement in the green photoactive nanostructure materials for energy- and environmental-related applications has been increased gradually, and sophisticated progress has also been developed and carried out innovatively. Various synthesis approaches have been demonstrated to synthesize materials using environmentally benign chemicals. There are several green chemicals available in our ecosystem, including biomass, recycling chemical waste and carbon dioxide, etc. On the whole, the incorporation of green chemistry is an essential role to maintain vast technological development and the economic competitiveness of advanced social club as easily as for future generations.

The present concern in the energy sector is the very high rate of decrease of the reserves of fossil fuel of our planet. Therefore, the continuously increasing need for reliable energy supply has led to a boost in research and development of alternative energy sources. These alternative energy sources should essentially exhibit ready availability, renewability, sustainability, and environmental friendliness. Two prime examples of such green energy sources are solar energy and energy from readily available fuels (such as waste matter and biomass). This has led to the development of photofuel cells (PFCs), which combine the unique properties of solar cells and fuel cells. In addition, the use of photocatalysts and photoelectrocatalysts in normal fuel cells serves the purpose of trapping solar energy to cause oxidation of the fuel in fuel cells, which in turn results in generation of electrical energy.

The depleting fossil fuels and their serious environmental impact have heightened the need for an alternative renewable energy resource. The hydrogen obtained from sunlight-assisted water splitting is found to be a promising alternate to the fossil fuel. The proposal of generating hydrogen from solar water splitting has a great potential to solve the energy and environmental issues and introduce an energy revolution in sustainable and cleaner way. A variety of photocatalysts ranging from organic to inorganic materials have been studied, but their overall efficiency is limited due to varied reasons. The prevalent reason is the poor control over the recombination of photoexcited charge carriers. It has been investigated that integrating different materials in the form of heterojunction can improve the photocatalyst’s efficiency in multiple folds. These heterojunctions would enhance the charge carrier mobility and lifetime, absorption coefficient, stability, etc. which leads to improved charge separation at the heterointerface and hence their efficiency in H₂ generation by photo-assisted water splitting.

In view of this, the present chapter mainly focuses on reviewing TiO₂-based heterojunctions, a widely studied material for photocatalytic hydrogen generation. The basic working principle of different heterojunctions of TiO₂, their concerned drawbacks, and the recent impressive progress in developing other forms of TiO₂ heterojunctions are presented. In addition, the overviews of synthesis strategies of various TiO₂-based heterojunction materials are reviewed. The performance and stability of any heterojunction photocatalyst are dependent on the synergistic functioning of the interfaces between the individual materials forming heterojunction. The study of light matter interaction in these heterointerfaces is expected to provide crucial information helpful for exploiting the potential of these photocatalysts for commercial viability. Therefore, the present chapter also demonstrates the existing methodologies to probe these heterojunctions under light irradiation for charge carrier dynamic analysis. Finally, the chapter is concluded with the ups and downs of TiO₂-based heterojunction research and the future perspectives.

In recent years, environmental pollutions and energy issues have attracted much attention for the sustainable development of human life. It is of great challenge for the researcher to find a clean, biocompatible, nontoxic, and environment-friendly method for solving the problems associated with the environment. Photocatalysis is one of the effective strategies for mitigating the energy crisis and environmental pollutions. Several photocatalysts were developed for the efficient degradation of environmental pollutants into useful products. Mainly, quantum dot (QD)-based photocatalysts have attracted the researchers due to their attractive properties such as quantum confinement effect, large surface area, and high catalytic activity that makes its promising applications in the field of photocatalysis, sensing, light-emitting diodes, energy storage devices, bioimaging, and solar cell.

This chapter deals with the recent developments of QDs, (a) overview of quantum dots such as carbon, graphene, cadmium sulfide, zinc oxide, cadmium selenium, core-shell quantum dots and (b) photocatalytic applications of QDs such as carbon materials like carbon- and graphene-based QDs and metal-, metal sulfide-, and metal oxide-based QDs. The detailed discussions are made on the efficiency of photocatalytic behaviors of QDs, surface-modified QDs with different functionalities, and the doping of QDs with other elements like S, N, B, Si, etc. and also provided the reaction mechanisms of QDs in photocatalysis.

One exciting aspect of research is focused on the treatment of wastewater and complete mineralization of the pollutants by photocatalysis. The aromatic structure and complex nature of the pollutants cause difficulties in decomposition and complete mineralization. Progress in photocatalytic degradation would be very interesting if it were under moderate reaction conditions with a cost-effective catalyst with enhanced efficiency. Various nanocomposite materials have been used for this purpose. Here, we provide a detailed summary including the latest literature reports on the evolution of the zinc oxide (ZnO) photocatalyst used for the degradation of contaminants from wastewater.

The spatial arrangement of designed reaction centers with engineered porosity withdraws a special attention in exploring metal-organic frameworks (MOFs) for developing a wide range of photocatalyst in the last decade. This chapter targets to recapitulate the recent advancement of MOF-derived photocatalyst with their mechanism, types, structural engineering, and various practical uses.

Photobiocatalysis is a novel concept that aims at merging the features from photocatalysis and enzymatic catalysis. The process of photocatalysis involves the degradation of contaminants in water and air. It has also shown promising application in solar energy. However, many photocatalytic processes have low efficiency as compared to other enzymatic processes. Therefore, to improve the performance of such photocatalytic processes, a promising photobiocatalysis approach shows a great promise to enhance the effectiveness of the photocatalytic system. The combination of photocatalysis and biocatalysis technologies is an alternative to develop environmentally benign process for the synthesis of renewable chemicals. In photobiocatalysis, the semiconductor coupled with the enzyme which regularly needs a natural compound and a relay transferring charge carriers from the semiconductor. The enzyme diminution mediated by NAD⁺/NADH along with an electron relay utilized the conductivity band electrons of excited semiconductors for photobiocatalysis. The photosynthetic organisms are the natural source for photobiocatalysis.

The present write-up discusses the working mechanism and applications of the current photocatalytic system such as metal oxide photocatalyst and graphene-based photocatalyst. Advances in enzyme-mediated photocatalysis are particularly discussed. The critical factors to control the photobiocatalytic process and key enzymes involved in deciphering the reaction mechanism of photobiocatalysis are critically discussed. Cofactor vs mediator medicated photobiocatalysis is also discussed.

Among various renewable energy projects, harvestation of clean solar energy through semiconductor-based photocatalysis is now emerging as a feasible technology and has gained considerable interdisciplinary attention for its diversified potential in energy and environmental applications. Until now, although a good number of photocatalytic materials were reported, g-C₃N₄ is found to be a promising material in a variety of applications. To introduce desirable electronic properties and more number of surface active sites, designing of nanoporous g-C₃N₄ has been recognized as one of the most agreeable avenues to extend its potential applications. In nanoporous g-C₃N₄ network, the highly interconnected pores render the material with high surface area, offer numerous pathways for mass transport and multiple reflection of incident light, and favor strong adsorption at the active sites. Here, we have highlighted how further control over porosity and morphology can be achieved by using different templates during formation. Large available surface not only absorbs the organic/inorganic pollutants effectively by offering more number of active sites but also prevents aggregation of particles by accelerating diffusion kinetics. This chapter mainly focused on various types of templates used for the preparation of porous g-C₃N₄ and its applications in detail with special reference to dye degradation, reduction of hexavalent Cr, and reduction of CO₂ and for the evolution of H₂ photocatalytically.

Photocatalysis is one of the fastest growing technologies for the treatment of pollutants, utilizing the mechanism of reaction with the help of light (photo emissions). Photocatalysis has captured broad academic and research interest during the past three decades for its potential of controlling pollution in air and water. Its qualities, such as low cost and high efficiency, have caused researchers all over the world to focus on it and also promoted many industrial applications and much research. Photocatalysis has been used to remove major air pollutants, disinfect water, and oxidize various organic chemicals. In this connection, this chapter considers the properties of the ideal photocatalyst, available photocatalytic materials for air pollution control, common indoor air pollutants and their severe health effects, and purification techniques for indoor air pollution. Furthermore, photocatalytic oxidation techniques for the removal of volatile organic compounds are discussed in detail.

Energy and environmental crisis are two major challenges in present-day life. The advent usage of fossil fuels for electricity and transportation had resulted in global warming with deadly repercussions. Photocatalysis could be a good alternative to solve the present-day problems. Engineering the material for maximum solar irradiation harvesting, efficient charge generation, its transport to reaction centers, and high catalytic properties are the main priorities; in this sense, two-dimensional materials could be suitable for photocatalysis because of its physical, electrical, and chemical properties and the possibility to chemically tailor them. Photoinduced charge carriers could catalyze various chemical reactions for crucial applications. Hydrogen fuel could be used for hydrogen fuel cell to generate the electricity and power the electric vehicles. Major greenhouse gas, such as CO₂ emitted by various industries and conventional automobiles, could be reduced to high-energy fuel such as methanol, ethanol, or formate. Photodegradation could decompose toxic organic substances from water-polluted sources. Human effort and expenditures could be saved by self-cleaning photocatalysis in offices, hospitals, or photovoltaic panels. Photocatalytic materials may play a critical role in effective cancer therapy for its sensitizing property and ability to produce reactive oxygen species which kills cancer.

The multiferroic material bismuth ferrite (BiFeO₃, BFO) is a newly emerging photocatalytic material reported to be comparable with other oxide semiconductor materials. BFO photocatalysis encounters challenges based on its practical use. As it has a narrow energy bandgap (about 1.8–2.8 eV), the photocatalysis of visible and UV light such as hydrogen (H₂) generation by water splitting becomes possible. This chapter discusses the synthesis of single-phase BFO nanostructures approached by the hydrothermal method, and optimization of single-phase BFO nanostructures by tuning the particle size and morphology with assistance of sodium hydroxide (NaOH) and potassium hydroxide (KOH) as precipitating agents. Outlooks on the expansion of advanced BFO photocatalysts with possible improvement for the remediation of environmental pollution are discussed.