Enhanced visible-light photocatalytic activity of g-C3N4–ZnWO4 by fabricating a heterojunction: investigation based on experimental and theoretical studies

We present a systematic investigation of the microscopic mechanism of interface interaction, charge transfer and separation, as well as their influence on the photocatalytic activity of heterojunctions by a combination of theoretical calculations and experimental techniques for the g-C3N4–ZnWO4 composite. HRTEM results and DFT calculations mutually validate each other to indicate the reasonable existence of g-C3N4 (001)–ZnWO4 (010) and g-C3N4 (001)–ZnWO4 (011) interfaces. The g-C3N4–ZnWO4 heterojunctions show higher photocatalytic activity for degradation of MB than pure g-C3N4 and ZnWO4 under visible-light irradiation. Moreover, the heterojunctions significantly enhance the oxidation of phenol in contrast to pure g-C3N4, the phenol oxidation capacity of which is weak, clearly demonstrating a synergistic effect between g-C3N4 and ZnWO4. Interestingly, based on the theoretical calculations, we find that electrons in the upper valence band can be directly excited from g-C3N4 to the conduction band, that is, the W 5d orbital of ZnWO4, under visible-light irradiation, which should yield well-separated electron–hole pairs, with high photocatalytic performance in g-C3N4–ZnWO4 heterojunctions as shown by our experiment. The microcosmic mechanisms of interface interaction and charge transfer in this system can be helpful for fabricating other effective hetero-structured photocatalysts.