Semiconductor heterojunctions have higher photocatalytic performance than a single photocatalytic material. However, the energy band offset and the photocatalytic reaction mechanism of these heterojunctions remain controversial. Here, tungsten disulfide (WS2)/graphitic carbon nitride (GCN) heterojunction photocatalytic water splitting is investigated with the hybrid density functional method. The band structures and the density of states (DOS) indicate that the WS2/GCN heterojunction is a type-II heterojunction, and its valence band offset and conduction band offset are 0.27 and 0.04 eV, respectively. The differential charge density distribution and the work function calculation indicate that a built-in electric field is formed in the WS2/GCN heterojunction. The potential of the built-in electric field is 0.16 V, and its direction is from the GCN surface to the WS2 surface. The built-in electric field separates the photogenerated electrons and the holes in space, effectively improving the photocatalytic efficiency of the WS2/GCN heterojunction. Our work provides insights into the electronic properties and the photocatalytic hydrogen evolution mechanism of the WS2/GCN heterojunction.
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