Controllable Synthesis and Photocatalytic Applications of Two-dimensional Covalent Organic Frameworks

In the past century, the extensive consumption of fossil fuels has caused significant environmental pollution and energy resource crisis. As a result, seeking to develop clean and renewable alternatives to traditional fossil fuels has recently aroused great concern. Clean and abundant solar energy is undoubtedly an ideal solution. Inspired by natural photosynthesis, tremendous efforts have been devoted over past decades to build artificial photocatalytic systems that aims to convert solar energy to valuable chemical energy, such as H-2, CO, CH4 and H2O2 etc. To do this, developing advanced semiconductor photocatalysts with superior optoelectronic properties is of fundamental importance and has become one of the main challenges. Two-dimensional covalent organic frameworks (2D COFs), as a new class of crystalline, organic porous materials, have recently attracted increasing attention for photocatalytic applications. Specially, they feature periodic molecular architectures and it-unit arrays that are assembled from organic subunits via covalent bonds and pi-pi interaction. Benefiting from these unique structural characters, 2D COFs usually possess broad visible light absorption range and much enhanced charge separation efficiency in comparison with their amorphous counterparts. Moreover, their molecular topology structures and optoelectronic properties can be readily tuned by selecting suitable monomers and synthetic procedures, making them to be a versatile platform for task-oriented synthesis of advanced photocatalysts with predesigned structures and functionalities. Therefore, the development of 2D COFs photocatalysts will undoubtedly provide new opportunities for efficient utilization of solar energy. In this paper, we first overviewed the synthetic methods of 2D COFs and the viable strategies for modulating their optoelectronic properties, and subsequently summarized current research advances of 2D COFs on photocatalytic water splitting, CO2 reduction and H2O2 production. Finally, the opportunities and challenges in this emerging field are prospected.