Defect Engineering in Photocatalysts and Photoelectrodes: From Small to Big

Abstract

Conspectus Green hydrogen as a clean energy carrier plays a critical role in tackling climate change. Solar-driven water splitting is regarded as one of the most promising strategies for green hydrogen production, but the solar-to-hydrogen (STH) conversion efficiency is still far from the industrial requirement. Defect engineering is an effective strategy to enhance the performance of photocatalysts and photoelectrodes. In a crystal, defects range from micro- to macro-levels, such as atomic-scale vacancies and dopants, distortion along the crystal, and defective surface, which all influence the photoresponse, electrical conductivity, and surface properties of semiconductor materials. The methodologies of creating defects have been vigorously explored, while accurate identification and characterization of defects have been focused on much less. Moreover, the widely reported benefits of defects in the photocatalytic system may not necessarily cancel out the negative spillover effect of charge recombination. As a Janus-role structure with different influences, the defect is an old, yet complex, notion, and in-depth understanding of defects is vital toward efficient solar-driven hydrogen generation. In this Account, we provide an overview of the engineering of defects with some typical examples, including atomic vacancies of anion and metal, long-range lattice distortion, and macro-range surface defects. We start with our effort to explore the controllable generation of vacancies, including oxygen vacancies, nitrogen vacancies, and metal vacancies via post-treatment and in situ synthesis. A comprehensive understanding of anion vacancies, especially the oxygen vacancies, has been discussed. The identification of metal vacancies and their role in the photoelectrochemical (PEC) process are further discussed. We then move on to the long-range defects of lattice distortion to understand their formation mechanism and contributions to PEC water splitting. Finally, the defective surface is discussed based on the case studies of two-dimensional (2D) mesoporous crystalline photocatalysts and photoelectrodes. Although significant advances have been achieved in solar-driven hydrogen production, there is still a long way ahead before reaching the industrial production target. To inspire innovative defect engineering for further improving the STH conversion efficiency, we provide the following perspectives. First, precise synthesis strategies should be developed to control the type and concentration of defects. Second, reliable techniques and measurements should be established to make accurate identification and characterization of defects. Last but not least, the interaction of different types of defects should be further explored to guide the rational design of material systems, which may lead to unique synergistic mechanisms in further promoting solar energy conversion efficiency.

Recent advances on g-C3N4-based Z-scheme photocatalysts for organic pollutant removal

“…To enhance the photocatalytic property of g-C 3 N 4 , various approaches have been proposed, e.g., doping, 52,53 obtaining porous architectures, 54,55 achieving highly crystalline structures, 56 metal loading, 57,58 heterojunction construction, [59][60][61] exfoliation, 62,63 defect engineering, 64,65 and molecular and electronic structure engineering. [66][67][68][69] Among these approaches, the integration of two semiconductors to fabricate type-II heterojunctions is a promising method for enhancing the photocatalytic performance since the band alignment between the two semiconductors will provide high separation efficiency of the photogenerated carrier.…”

Nanostructured Metal Chalcohalide Photocatalysts: Crystal Structures, Synthesis, and Applications

“…The photocatalytic activity of a nanomaterial can be influenced by shape and size architecting, ,,− surface passivation, − defect engineering, − doping, − and formation of heterojunctions. − The modulation of shape in the NCs and the generation of active facets or sites can increase the photocatalytic efficiency by accelerating the association followed by dissociation of reactants and products, respectively. Furthermore, tuning the active size of the materials can create an effective surface area.…”

Interfacial States in Au/Reduced TiO2 Plasmonic Photocatalysts Quench Hot-Carrier Photoactivity

“…To address these stringent requirements, one possible route consists in using hybrid metal–semiconductor nanostructures combining the former’s exceptional optical properties and the possibility of forming rectifying junctions. − Furthermore, the fabrication of properly designed hybrid nanosystems requires considerable effort in understanding both parts and, even more, their interfaces. For example, the optical properties of the nanostructures are linked directly to their size, shape, composition, and embedding media, while the generated charge carrier energy, lifetime, and availability for redox processes rely on their surface states (e.g., crystallinity, defects, etc..) and the metal–semiconductor interface as well as with the surrounding media status. − …”

“…Induced defects such as oxygen vacancies (V O s) and Ti 3+ states are well-known because of their simple formation process (e.g., thermal treatment in a reducing atmosphere) while significantly affecting the photocatalyst properties. − ,− For instance, Chen et al showed the narrowing of the TiO 2 optical band gap from 3.3 to 1.54 eV through lattice disorder, inducing defect states (DSs) stabilized by hydrogen atoms . The V O s, from which Ti 3+ states arise, introduced by thermal treatment under H 2 , correspond to defect levels 0.7–1 eV below the CB .…”