A DFT investigation of photocatalytic water splitting properties of the InS/GaTe heterostructure: direct Z-scheme vs. traditional type-II

Abstract

Density Functional Theory is used to predict the structural, electronic, and optical properties, as well as the reaction energetics, of the InS/GaTe heterostructure. The system is stable and found to have an ideal band gap of 1.34 eV, significantly lower than its monolayer counterparts. This makes it more effective in absorbing light in the visible region, as confirmed by our analysis of its optical properties. The oxygen evolution reaction (OER) was investigated for both the direct Z-scheme and the type-II mechanisms. The photogenerated hole potential for the Z-scheme ranges from 2.37 eV for pH = 0 to 4.02 eV for pH = 14, while that for the type-II mechanism is from 1.44 eV (pH = 0) to 3.09 eV (pH = 14). Based on the analysis of the electronic properties of the InS/GaTe heterostructure, and its Gibbs free energy reaction pathway for OER when the light is turned on, the transfer mechanism of the photogenerated electrons and holes in InS/GaTe is predicted to follow the direct Z-scheme mechanism. Notably, the OER reaction is predicted to be spontaneous for a wide pH range: 2 ≤ pH ≤ 14 (Z-scheme) and 3 ≤ pH ≤ 14 (type-II). This makes the InS/GaTe heterostructure more promising for OER compared to many other catalysts. While the type-II mechanism cannot facilitate HER, the Z-scheme mode of InS/GaTe is predicted to have good performance for HER, with an ideal Gibbs free energy of −0.02 eV at pH = 7. The solar-to-hydrogen efficiency is predicted to be 44.8%, which is higher than that of many other photocatalysts, and is far higher than the 10% threshold for commercial applications. These results strongly indicate that the InS/GaTe heterostructure, in its Z-scheme mode, holds high potential as a photocatalyst to facilitate both OER and HER for water splitting applications.