Non-Noble-Metal-Doped Carbon Nitride Photocatalysts for Water Splitting Screened Out by Empty Defect States and the d-Band Center

A rational design of water-splitting photocatalysts from the perspective of the electronic structure is highly desirable for optimizing catalytic activities. However, the structure-activity relationship is still unclear, which impedes the development of efficient catalysts. Herein, by comparing systematically the overall water-splitting capability of 20 kinds of metallic elements anchored at three sites (including cavity, carbon vacancy, and nitrogen vacancy) of graphitic carbon nitride (g-C3N4) through density functional theory calculations, we uncover that availability of in-gap empty defect states and the d-band center position are paramount parameters to determine activities of g-C3N4 on photocatalytic water splitting. In-gap empty states play a role in accommodating electrons from H2O to facilitate its splitting. A lower d-band center weakens the interaction between reaction intermediates and g-C3N4, thereby promoting O-2 desorption. Metals embedded at carbon vacancies are found to be superior to those at cavities and nitrogen vacancies because the former not only provides ample in-gap empty states but also has a lower d-band center. We also discover a rule that, for a reaction in which the bond order between the metal and intermediate enlarges (reduces), its reaction difficulty increases (decreases) with the increasing atomic number for elements in the same period. After screening, we find that non-noble metals Co, Ni, and Ga anchored at carbon vacancies possess catalytic performances comparable to Pd- and Pt-doped systems, with the rate-determining barriers less than 0.55 eV. Our findings may provide useful information for designing effective photocatalysts.