Facet Junction Engineering for Photocatalysis: A Comprehensive Review on Elementary Knowledge, Facet‐Synergistic Mechanisms, Functional Modifications, and Future Perspectives

Abstract A facet junction (also called facet heterojunction or surface heterojunction) is defined as a complex polyhedral single crystal exposed to two, three, or four types of crystallographic planes, and is a promising platform for achieving multifunctional photocatalysis. Compared with simple polyhedral counterparts enclosed by the same crystallographic planes or conventional heterogeneous junctions, facet junctions have notable facet‐crosslinking effects arising from different electronic structures of heterogeneous facets, which is beneficial for promoting the transfer and separation of photogenerated charge carriers in a single‐crystalline semiconductor without the introduction of external species or interfaces. However, there are few specialized review articles on facet junction engineering of photocatalysts. Herein, an overview of facet junction‐based photocatalysts is provided based on the following aspects: elementary knowledge, microstructural features, intrinsic facet‐synergistic mechanisms, functional modifications, enhanced photocatalytic mechanisms, challenges and issues, and directions for future investigations. This review article will act as a theoretical base for researchers who are focused on facet‐dependent effects to design and fabricate new photocatalysts.