and (iv) hydrogen. This is because of the multi‐electron, multi‐hole, and multi‐proton reactions. The mechanism and selectivity were examined. The strategies for facing the challenges in this exciting area were proposed at the end of the chapter.
Chapter 3: Natural and Artificial Photosynthesis. This chapter provides an overview of the most important process, i.e. natural photosynthesis. Such a process has been literally powered the whole planet with various species and the ecosystem. It introduces the detailed processes, including light harvesting, charge separation and accumulation, water oxidation, and nitrogen fixation. With the insights into them, inspirations are achieved for artificial photosynthesis, which leads to the fantastic explorations on solar‐to‐chemical conversions.
Chapter 4: Photocatalytic Hydrogen Evolution. This chapter mainly introduces heterogeneous photocatalytic reactions for hydrogen evolution. The basic principles involved in photocatalytic hydrogen evolutions are introduced. Following that, photocatalytic hydrogen evolution reactions under ultraviolet (UV) light, visible light, and near‐infrared light irradiations were investigated in detail. Because of the light absorption ability, which is determined by the band gap of a semiconductor, specific semiconductors only work better in a specific condition. As a result, titanium dioxide and its modified counterparts are mainly introduced in UV reactions, carbon nitride and various modified ones are presented in visible‐light photocatalysis, and upconversion materials are discussed for near‐infrared light reactions. Comprehensive survey on materials for the specific light regions was also available. For providing the insights into the reactions, the roles of sacrificial reagents and reaction pathways were also discussed in this chapter.
Chapter 5: Photoelectrochemical Hydrogen Evolution. The status, opportunities, and challenges in the hydrogen energy are discussed. Other than powdered photocatalysis, this chapter concerns photoelectrochemical (PEC) and photovoltaic‐driven water electrocatalysis (PV + EC). The key of these processes is the efficient photoelectrode fabrication, which determines the light harvest, charge separation and transfer, and surface reactions. The configurations of PEC and the strategies for promoting charge transfer through the semiconductor film and providing strong driving force for carriers' separation are reviewed. The catalyst design for achieving such strategies was paid with extra attention.
Chapter 6: Photocatalytic Oxygen Evolution. Water oxidation half‐reaction has been regarded to be the primary barrier in solar fuel production, because such a reaction requires multiple protons and electrons to be involved. This chapter identifies the critical challenge of water oxidation to be the design of efficient, low‐cost catalysts with excellent stability. The morphology, structure, and photocatalytic water oxidation performances of various earth‐abundant materials are reviewed. Both homogeneous and heterogeneous reactions are discussed. Moreover, quantum size effect, localized surface plasmon resonance, active facet exposure defect engineering, and heterojunction construction in low‐dimensional materials are discussed in terms of enhanced photocatalytic water oxidation.
Chapter 7: Photoelectrochemical Oxygen Evolution. This chapter reviews the fundamentals of PEC oxygen evolution toward higher efficiencies as compared with powdered or homogeneous photocatalytic oxygen evolution. It discusses the factors affecting the photoanodic current, the relationship between electrode potentials and pH in the electrolyte, the evaluation method of PEC performance of photoanode materials, the relationship between flat band potential and photocurrent onset potential, the selection strategy of photoanode materials, and PEC device for water splitting. The determining parameters, such as nanostructuring, morphology control, donor doping, cocatalyst loading, heterojunction formation, and electron‐conductive materials, are discussed for better design of photoanode materials. It concludes that the rational material design and the knowledge of thermodynamics and kinetics are required and the low‐cost photoanode materials composed of inexpensive earth‐abundant elements control the feasibility of this technology.
Chapter 8: Photocatalytic and Photoelectrochemical Overall Water Splitting. After the discussions of the individual hydrogen or oxygen evolution processes, this chapter focuses on the overall water splitting processes. The fundamental scientific requisites, mechanism aspects, and development horizons for overall water splitting are comprehensively covered. Then the development of photocatalytic technologies and hybrid systems integrating photovoltaic devices and photoelectrodes in photoelectrochemical platforms for overall water splitting are introduced. An extensive library of light‐responsive semiconductor‐based materials, attractive cocatalysts, and plasmonic nanostructures and assessed synthesis approaches, e.g. the construction of heterojunctions between state‐of‐the‐art semiconductors, is presented. It is expected that the development of practical and adequate materials for solar‐driven overall water splitting systems can further advance this exciting system.
Chapter 9: Photocatalytic CO2 Reduction. After hydrogen/oxygen evolutions, another main topic in solar fuels, i.e. CO2 reduction, starts in this chapter. Photocatalytic CO2 reduction has been regarded to be economical, recyclable, and safe, therefore holding a great promise for addressing worldwide energy and environmental problems. In this chapter, the whole process of photocatalytic CO2 reduction is analyzed in terms of energy and mass flow. Discussions are made to illustrate the effects of the flow of solar energy through concentrator, reactor, reaction solution, and photocatalyst and the mass flow from the adsorption and activation of reactants (such as CO2) on the surface of the photocatalysts to the formation of CO, CH4, and other products through photocatalytic reaction. Contributions are also made to present the energy loss and obstacles contained in the transformation processes and the possible strategies to improve the overall reaction efficiency.
Chapter 10: Photoelectrochemical CO2 Reduction. As inspired by PEC water splitting, PEC reduction of CO2 can also integrate and optimize both photocatalysis and electrocatalysis toward an improved CO2 reduction system with a higher efficiency and stability. This chapter first introduces the fundamentals and reaction parameters in the concerned systems; then research advances on the development catalyst materials are surveyed. This includes the semiconductors, cocatalysts, and hybrid semiconductors. Novel reactor systems for PEC CO2 reduction are introduced. At last, research opportunities and challenges are identified.
Chapter 11: Photocatalytic and Photoelectrochemical Nitrogen Fixation. Using solar energy to convert nitrogen in air to valuable chemicals is a direct mimicking system to natural photosynthesis. Artificial photocatalytic and PEC reduction of N2 to ammonia emerged as a fantastic venue to this end. This chapter introduces the recent advances in photocatalytic and photoelectrochemical N2 reduction to NH3. The fundamental principles on N2 reduction are presented. Then, the current design strategies, mainly including the defect engineering, structural regulation, interface control, heterojunction construction, cocatalyst engineering, and biomimetic engineering, for the preparation of heterogeneous catalysts are summarized. At last, the remaining challenges as well as future perspectives in this very exciting research field are outlined.
Chapter 12: Photocatalytic Production of Hydrogen Peroxide Using MOF Materials. In theory, oxygen and water can be employed to produce hydrogen peroxide using solar energy, which fortunately was demonstrated to be feasible through photocatalysis. It was reported that synthesis of H2O2 from O2 reduction reaction using metal organic frameworks (MOFs) is attractive and promising. In this chapter, the H2O2 production through visible‐light‐induced O2 reduction by an MOF, MIL‐125‐NH2, coupled with oxidative reaction