Low-temperature synthesis of mesoporous boron carbides as metal-free photocatalysts for enhanced CO2 reduction and generation of hydroxyl radicals

Boron-rich semiconductors make significant contributions to the family of explored metal-free photocatalysts, which have attracted much attention in recent years. Boron carbide (B 4 C) belongs to a typical metal-free boron-rich photocatalyst which is facing difficulties in further optimization mainly due to the extreme conditions required for the synthesis of this material. In the present work, five different transition-metal catalysts (Fe, Co, Ni, Cu, and Zn) were investigated for lowering the crystallization temperature of B 4 C. Ni is the best catalyst for the reaction, and the crystalline mesoporous B 4 C powders can be obtained at merely 850 °C with a surface area of 130.55 m 2  g −1 , which is 27 times larger than commercial B 4 C. The photocatalytic properties of B 4 C prepared with Ni catalyst at different calcination temperatures were further evaluated by photocatalytic CO 2 reduction and generation of hydroxyl radicals. Both the crystallinity and surface area of the B 4 C would influence the final photocatalytic properties. For B 4 C photocatalysts, we firstly found that the crystallinity would influence the photogenerated holes more significantly while the surface area would have more significant influence on the photogenerated electrons. The B 4 C obtained at 950 °C exhibits the best photocatalytic activities for both CO 2 reduction and generation of ·OH radicals, which are 3.1 and 2.1 times higher than the commercial B 4 C, respectively. This present study may provide crucial references for the low-temperature synthesis of crystalline B 4 C and new opportunities for the application of the metal-free B 4 C photocatalysts to solar energy conversion.