Synthesis of high crystallinity g-C3N4 hollow spheres for efficient photocatalytic removal of U(VI) under visible light

Graphite carbon nitride is a promising photocatalyst, however, its performance needs to be improved due to its low specific surface area, high carrier recombination rate and poor visible light utilization. Here, we report that hollow g-C3N4 nanospheres (HCN-R, R = Air, N2, NH3) with different crystallinity were obtained by adjusting the calcination atmosphere. The hollow sphere structure is conducive to fully exposing the surface active sites, shortening the electron transmission distance, and the incident light can be reflected and refracted multiple times within the hollow sphere to improve light utilization efficiency. In addition, NH3 atmosphere can improve the crystallinity of hollow spherical g-C3N4. The increase in crystallinity facilitates the elimination of electron capture centers and promotes intra-layer charge transfer in the two-dimensional π-conjugate plane. HCN-NH3 has the fastest photocatalytic removal rate of U(VI) (0.0286 min−1) compared with HCN-Air (0.0190 min−1) and HCN-N2 (0.0161 min−1). After 5 cycles of photocatalytic experiments, the removal rate of HCN-NH3 can still be maintained above 96%, showing the characteristics of high stability and easy recovery. The photocatalytic mechanism indicates that e– and •O2– radicals promote the removal process of U(VI), and U(VI) is deposited in the form of (UO2)O2·2H2O under air.