Two-Dimensional Transition Metal Oxides and Chalcogenides for Advanced Photocatalysis: Progress, Challenges, and Opportunities

Sunlight-driven catalytic reactions are appealing for resolving energy and environmental problems. Transition metal oxides (TMOs) and chalcogenides (TMCs) comprise one of the most popular categories of photocatalysts, thanks to their high stability, low cost, Earth abundance, and outstanding catalytic activity. Downsizing TMOs and TMCs to 2D materials offers additional opportunities to finely tune their surface, electronic, and catalytic properties. However, 2D TMOs and TMCs fall into a less mature field than other well-established 2D materials. Less is known about their "form-to-function" relationship, and mechanisms for their synthesis await more research. Herein, the progress toward the rational design of layered and nonlayered 2D TMOs and TMCs is summarized, as well as principles to engineer their nanosheets (NSs) into 3D architectures for practical application. The formation mechanisms and crystal growth models of these 2D materials are included. The key factors that determine the electronic, surface structures, and catalytic properties of 2D TMOs and TMCs are examined in particular, which are key considerations in tuning their performance in light absorption, charge carrier transfer/separation, molecule capture and activation, etc. Finally, the present challenges and future research directions in this promising field are illustrated.