Novel pentagonal MgX2 (X = O, S, Se, Te) monolayers: promising photocatalysts for overall water splitting and CO2 reduction

Abstract

The development of efficient photocatalytic systems capable of harvesting visible light is essential for sustainable water splitting and addressing the escalating environmental and energy crises. In this study, first-principles calculations are employed to design and evaluate two-dimensional (2D) pentagonal MgX₂ (X = O, S, Se, Te) monolayers for photocatalytic water splitting and CO₂ reduction. The structural integrity and stability of these materials are confirmed through formation energy analysis, phonon dispersion, ab initio molecular dynamics simulations, and elastic constant evaluations. Hybrid HSE06 functional calculations predict band gaps ranging from 1.488 to 4.471 eV, with penta-MgO₂ exhibiting a direct band gap, while the others possess indirect band gaps. All monolayers exhibit suitable band edge alignments for driving both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as for facilitating CO₂ reduction. Moreover, they exhibit high carrier mobilities and strong optical absorption in the UV–visible region, with absorption coefficients exceeding 10⁵ cm⁻¹. Gibbs free energy calculations confirm the thermodynamic viability of water splitting under the applied external potential. Notably, penta-MgTe₂ emerges as the most promising photocatalyst for CO₂ reduction due to its efficient stabilization of key intermediates (*CHO and *CH₃OH) and low-energy hydrogenation steps, favoring selective CH₄ production under solar-driven conditions. These findings highlight the potential of pentagonal Mg-based 2D materials as efficient and sustainable photocatalysts for clean energy conversion applications.