Janus Group-III Chalcogenide Monolayers and Derivative Type-II Heterojunctions as Water-Splitting Photocatalysts with Strong Visible-Light Absorbance

Search for two-dimensional (2D) water-splitting photocatalysts is crucial to solve energy crises and environmental problems. In this research, we study the electronic and photocatalytic properties of single-layer Ga2X1X2 (Ga2SeTe, Ga2STe, and Ga2SSe) and newly proposed alpha-Ga2S3/Ga2SSe-A, alpha-Ga2S3/Ga2SSe-B, and alpha-Ga2S3/Ga2SSe-C van der Waals heterojunctions using first principles calculations. Theoretical results indicate that Ga2X1X2 monolayers present suitable band edges. 2D alpha-Ga2S3/Ga2SSe-B and alpha-Ga2S3/Ga2SSe-C belong to type-II heterojunctions and under biaxial strains embody suitable band edges. Comparisons of the valence band maximum charge and electric dipole of alpha-Ga2S3/Ga2SSe-A and alpha-Ga2S3/Ga2SSe-B demonstrate that it is possible to achieve suitable band edges for water splitting by switching electric dipoles. Especially, the three Ga2X1X2 monolayers and alpha-Ga2S3/Ga2SSe-B and alpha-Ga2S3/Ga2SSe-C heterojunctions absorb a large amount of visible light, thus proving that they are promising photocatalysts for water splitting. More importantly, we find that the optical absorption coefficients of 2D monolayers and heterojunctions in previous calculations are several times underestimated because the effective volume is not taken into consideration. To obtain reliable absorption coefficients, the real and imaginary parts of dielectric function must be renormalized.