Defect Engineering in Photocatalysts and Photoelectrodes: From Small to Big

ConspectusGreen 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.