▎ 摘　　要

Industrial demands for sustainable and renewable energy resources have inspired studies on photonic and electronic properties of graphitic-carbon nitride (g-C3N4) as a promising photocatalyst without precious metal. The absorption and the yield by metal-free pristine g-C3N4 are, however, still limited with hydrogen/oxygen evolution reaction (HER/OER) mostly around ultraviolet-light. Here, we propose the graphene-decorated g-C3N4 as a metal-free photocatalyst under visible-light, based on our atomic-scale measurements and calculations. The g-C3N4 nanosheets on highly-oriented pyrolytic graphite (HOPG) exhibit band-gaps appropriate for visible-light absorption and work-functions tuned for band alignments to supply electrons and holes for HER/OER. Scanning probe microscopy (SPM) measurements for local density of states (LDOS) in atomic scale and work-functions in nanometer scale with ab initio calculations confirmed the various electronic transitions for each nitrogen and carbon atom in different atomic registries. The graphene-decorated g-C3N4, therefore, could provide a break through enabling the efficient water-splitting reactions under visible-light without precious metal.