First-Principles Assessment of Alkali and Alkaline-Earth p-Block Ternary Oxides for Photocatalytic Water Splitting*

Molecular hydrogen is a sustainable energy carrier of primary interest to draw down carbon dioxide emissions and accelerate the transition to renewable modes of energy production for transportation, commercial, and residential applications. Photocatalytic materials use solar energy to split water and generate hydrogen, emitting oxygen as the only byproduct; however, many of the known water-splitting photoactive semiconductors have limited solar-to-hydrogen conversion efficiency. We present a detailed investigation of 109 alkali and alkaline-earth p-block ternary oxides for use as efficient water-splitting photocatalysts. We screen these materials on the basis of their band gaps and band edges calculated at both the semilocal density-functional theory (DFT) and nonempirical DFT+U levels [1, 2]. Nonempirical Hubbard U parameters are determined from first principles for the O-2p states of each material [3, 4]. Our calculations support that the addition of alkali and alkaline-earth elements to p-block binary oxides shifts the band edges towards more cathodic potentials and decreases the electron effective mass of the oxide, thereby increasing the photocatalytic efficiency. Pourbaix diagrams are used to assess electrochemical stability under aqueous conditions and narrow down the candidate materials by considering their decomposition energy and their decomposition products. Using this screening protocol, 8 alkali and alkaline-earth p-block oxides show promise as water-splitting photocatalysts.

[1] I. Timrov, N. Marzari, and M. Cococcioni, Phys. Rev. B 98, 085127 (2018).

[2] I. Timrov, N. Marzari, and M. Cococcioni, Phys. Rev. B 103, 045141 (2021).

[3] N. Kirchner-Hall, W. Zhao, Y. Xiong, I. Timrov, and I. Dabo, Appl. Sci. 11, 2395 (2021). 

[4] Y. Xiong and Q. Campbell et al., Energy Environ. Sci. 14, 2335-2348 (2021).