We theoretically propose a type of monolayer structure, H- or F-BX (X = As, Sb; Y = P, As), produced by surface hydrogenation or fluorination, with high stability, large band structures and high light absorption for photocatalytic water splitting. Based on first-principles calculations with the HSE06 functional, the electronic properties and optical properties were explored to reveal their potential performance in semiconductor devices. Additionally, owing to the Janus structure and high electronegativity of the monolayers, our calculations showed that surface fluorination can easily create an internal electric field compared with surface hydrogenation, which results in different trends of increasing bandgaps in monolayer H- and F-BX. We also found that the monolayers H- and F-BX have suitable band edges and high solar to hydrogen (STH) efficiency, enabling them to be photocatalysts for water splitting. Our work not only proposes eight monolayer semiconductors for expanding the number of two-dimensional semiconductors, but also provides a guide for how to regulate semiconductors for application in photocatalytic water splitting by using surface hydrogenation and fluorination.
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