Chapter 1
Research Frontiers in Solar Light Harvesting

Srabanti Ghosh

CSIR - Central Glass and Ceramic Research Institute, Fuel Cell and Battery Division, 196, Raja S.C. Mullick Road, Kolkata, 700032, India

1.1 Introduction

In continuously growing technology-driven society, an urgent need for efficient solar light harvesting to achieve sustainable solutions in science and industry exists [1, 2]. The rapid growth of industries and some unavoidable human activities cause environment pollution to be a threat to the society. Solar-energy-mediated advanced oxidation process in water purification is a highly desirable approach [3]. To use the solar light, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. Over the last few decades, a significant effort has been made to develop active materials including inorganic, organic, ceramic, polymeric, and carbonaceous, their composites with tunable size and structures [4-6]. A broad range of materials including metal oxides, chalcogenides, carbides, nitrides, and phosphides of various compositions such as heterogeneous, plasmonic, conjugated polymers, porous carbon-based materials, and graphene-based materials has been explored to address/solve energy and environment-related research challenges [7-10]. In this context, oxide-based semiconductors, in particular, TiO2, have been recognized as efficient and widely explored photocatalysts. Semiconductor-oxide-based catalysts is essentially limited by low quantum yield which results from the fast charge carrier (e-/h+) recombination, and the necessity to use UV irradiation (5% of total sun energy) having wide bandgap [11, 12]. To overcome these limitations, surface-tuning strategies and modification of oxides on the nanometer scale have been developed via doping or surface modifications to produce visible-light-responsive photocatalysts. Indeed, TiO2 doped with N, C, or S or its modification with metal nanoparticles (Ag, Au, Pt, Cu, Bi) has extended its activity toward the visible region [13-16]. However, the photocatalytic activity of the modified materials in the visible light is still not sufficient for commercial applications. Research efforts are therefore increasingly being carried out to design and develop more efficient novel visible-light active catalysts for photocatalysis and solar energy conversion. A considerable number of novel synthetic strategies including fabrication of plasmonic-based novel catalysts, heterojunctions, and cocatalyst have been proposed to offer new visible-light-active photocatalytic materials as potential substitutes of TiO2 for the most relevant photocatalytic applications such as detoxification and disinfection, removal of inorganic pollutants, water splitting, and organic synthesis [17-19]. In this regard, the loading of cocatalysts or secondary semiconductors, which can act as either electron or hole acceptors for improved charge separation, is a promising strategy for enhanced catalytic activity. A more innovative implementation of this idea would be based on the use of polymer-based composites, which could allow enhanced charge separation with respect to the photocatalytic activity of the inorganic component alone. In this chapter, the state of the art on development of novel nanostructures and the concept of heterojunction for efficient visible-light-driven water splitting, organic or inorganic pollutant degradation, and organic transformation have been discussed. The structural features of various nanostructured catalysts and their correlation are explained in detail. An overview of recent research efforts in the applications of visible-light-active photocatalysts, which include semiconductor metal oxides (TiO2, Fe2O3, Cu2O, etc.), polymeric graphitic carbon nitride (C3N4), plasmonic nanostructures (Au, Ag, etc.), conducting polymers nanostructure (PEDOT, PANI, PDPB, etc.), heterostructures, and other novel materials in degradation of photocatalytic pollutants , hydrogen generation, CO2 reduction, and selective redox organic synthesis are summarized.

1.2 Visible-Light-Driven Photocatalysis for Environmental Protection

Environmental pollution issues prompted the finding of potential solutions to clean up water and environmental detoxification via exploring clean energy routes through solar-light-induced photocatalysis. Extensive research has been done in the area of photocatalytic removal of organic, inorganic, and microbial pollutants using semiconductor photocatalysts (e.g., TiO2, ZnO, and CdS) for wastewater purification [20-23]. The key to the success of solar energy conversion is the development of high-performance materials of well-matched photo absorption with solar spectrum (visible-light-harvesting capability), efficient photoexcited charge separation to prevent electron-hole recombination, and adequate energy of charges that carry out the photodegradation of dye and other toxic molecules. Continuous efforts have been made to generate active photocatalysts under visible light, but their efficiency is low due to fast charge recombination [24]. Many excellent reviews have also come up regarding the development of oxide-based semiconductors, in particular, TiO2, via fine-tune of several electronic characteristics (e.g., atomic configuration, bandgap energy, band position, and lifetime of electrons and holes) [25-27]. In addition to dye sensitization, doping with metals and nonmetals, formation of heterojunctions have been extensively used to enhance the visible-light response of TiO2 materials and discussed in detail in Chapters 2, 5, 6, 11, and 16. For example, TiO2 doped with N, C, F, or S or its modification with metal nanoparticles has extended its activity toward the visible region [15, 28-30]. Visible-light activities arise from the changes of bandgap structure of semiconductor via adsorbed modifiers (surface modification) or bandgap narrowing (doping). Synthesis of different materials, such as M/TiO2 (M=Cu, Ag, Au, Pt, Pd, Bi, AgAu, AgCu, AuCu, AgPt), and the effect of metal modification on the photocatalytic activity have been discussed in Chapter 6. Moreover, Chen et al. reported disordered TiO2 nanophase derived from hydroxylation through hydrogenation treatment, which marked as black TiO2 and a considerable enhancement in visible-light-induced photocatalytic activity [31]. It has been reported that hydrogenation treatment induced the oxygen vacancies and Ti3+ sites in black TiO2, resulting in the bandgap narrowing and the separation of photogenerated electrons and holes, which enhanced solar absorption and significantly improved the photocatalytic activity of TiO2 [32, 33]. A variety of synthetic strategies of black TiO2 are outlined, and the structural and chemical features, electronic properties, and catalytic activity of the black TiO2 nanomaterials are described in Chapter 5. Furthermore, oxygen-rich layered titanium oxide is also useful for enhanced visible-light photoactivity [34, 35]. Kong et al. reported TiOO coordination bond in layered titanium oxide (composed of TiO6 layers, and interstitial hydrated H+ ions) initiated visible-light-driven photocatalytic activity [36]. Presence of TiOO coordination bonds lowers the bandgap and promotes the charge separation of the photoinduced electron-hole pairs.

Another important example is combination of nanostructured plasmonic metals with a oxide-based semiconductor, which significantly enhanced the photocatalytic activity due to the local surface plasmon resonance (LSPR) effect with very large absorption and scattering cross sections [28, 29]. In fact, LSPR causes an optical antenna effect, which efficiently harvests light and localizes electromagnetic waves at the nanoscale, and the charge carrier formation with efficient separation is obtained at the semiconductor/liquid interface, which benefits the photocatalytic reactions [37-40]. A series of reactions have been tested on Ag, Au, and Cu surfaces, illustrating that low-intensity visible-photon illumination can significantly enhance the rates of chemical transformations as well as control reaction selectivity with different mechanisms as discussed in Chapter 6. Direct plasmonic photocatalysis is believed to occur through the transient transfer of energetic electrons to adsorbate orbitals and the nature of the adsorbate may have a significant impact to control selectivity in plasmon-driven reactions [17]. These heterogeneous oxide-based semiconductor photocatalysts have been also explored for the removal of inorganic wastewater pollutants including cyanide-containing waste and heavy metal pollutants, such as arsenic species and hexavalent chromium [41-43]. Notably, due to high toxicity and carcinogenicity of hexavalent chromium (Cr(VI)), the concentration of Cr(VI) in wastewater should be controlled in acceptable levels before its release in order to protect potable water supplies [44, 45]. Although, molecular CO2 has a very low electron affinity and is chemically inert as well as very stable, photogenerated energetic electrons from photocatalysts can reduce CO2 to methane (CH4) and carbon monoxide (CO). The photocatalytic reduction of CO2 using solar energy has drawn considerable attention, which mimics the biological photosynthesis in plants [46-48]. It combines the reductive half reaction of CO2 fixation with a well- matched oxidative half reaction of water oxidation, in order to achieve a carbon neutral cycle, which accomplished with the environment protection. Over the last few decades, various semiconductor photocatalysts, including metal oxide, sulfide, and...