The development of new and stable 2D semiconductors is imperative for driving progress in optoelectronics and realizing more efficient photocatalytic water splitting. Herein, we identify two new low-energy ZnCdO2 monolayer phases with \(P\;\overline{3\;}\;m1\) and P21/m space groups, using the CALYPSO program combined with first-principles calculations. The structural stability, electronic, carrier transport, optical, and photocatalytic properties were investigated. Both phases are direct-bandgap semiconductors with band gaps of 1.89 eV ( \(P\;\overline{3\;}\;m1\) ) and 2.17 eV (P21/m). The \(P\;\overline{3\;}\;m1\) and P21/m structures possess ultrahigh electron mobilities of 104 and 103 cm2V−1s− 1, respectively, with strong light absorption covering visible to ultraviolet regions. Besides, the P21/m phase exhibits band-edge potentials spanning the redox potential of water under both acidic and neutral conditions, whereas the \(P\;\overline{3\;}\;m1\) phase can satisfy this band alignment requirement under neutral conditions. Without applied strain, the P21/m phase exhibits over 10% solar-to-hydrogen (STH) efficiency across broad ranges of pH 3.26–12.67, peaking at 13.68%, while the \(P\;\overline{3\;}\;m1\) phase exceeds 10% over a more alkaline range of pH 8.5–14.0, with a maximum of 15.65%. At pH = 7, biaxial strain enables both phases to achieve over 10% STH efficiency. However, at pH = 0, the P21/m phase maintains STH efficiency above 10% only under + 4% tensile strain. Our results suggest that the predicted materials with appropriate strain and pH engineering hold great promise for photocatalytic hydrogen production.
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