Highly Crystalline Carbon Nitride Nanosheets for Ultrahigh Photocatalytic Hydrogen Evolution

Photocatalytic activity for H-2 evolution of carbon nitrides (CNs) is very sensitive to synthetic procedures. CNs prepared under different conditions are featured with varied crystallinity and texture, which in turn rule over their intrinsic electronic and chemical states as well as band energy level, thus determining their photocatalytic activities towards H-2 evolution. In comparison with CN obtained by polymerization of melamine in air (CN-a), CN prepared in an outgassed and sealed tube (CN-s) exhibits much improved photocatalytic activity towards H-2 evolution, increasing from 90 to 594mol/g/h under visible light. Exfoliation of CN-s into ultrathin CN nanosheets (CN-n) further leads to an ultrahigh photocatalytic H-2 evolution rate, rising to 2570 mu mol/g/h, which is among the highest photocatalytic activities for CNs reported to date. Detailed characterization reveals that the superior photocatalytic performance originates from the high crystallinity, extended visible-light absorption, efficient separation of photoexcited electron-hole pairs, enhanced electron delocalization, as well as increased active sites in CNs. This study uncovers the positive effects over photocatalytic activity of CNs and, hence, could provide new inspiration towards CN syntheses for improved photocatalytic performance.