2D Mg2M2X5 (M = B, Al, Ga, In, Tl; X = S, Se, Te) monolayers: Novel stable semiconductors for water splitting photocatalysts

Abstract

Two-dimensional (2D) semiconductors with appropriate band gaps can be promising candidates for being efficient water splitting photocatalysts and are also applicable in optical devices. Herein, a new series of ternary 2D monolayer materials Mg2M2X5 (M = B Al, Ga, In, Tl and X = S, Se, Te) is designed based on density functional theory. Structural, thermal, dynamical, and optoelectronic properties of 15 distinct Mg2M2X5 monolayer nanomaterials are discussed. We show that all designed monolayer structures are thermally, dynamically, and thermodynamically stable. All of the 2D Mg2M2X5 monolayers are semiconductors, except for monolayer Mg2B2Te5 which has metallic properties. The 2D Mg2Al2Se5, Mg2B2S5, Mg2B2Se5, Mg2Ga2S5, Mg2Ga2Se5, Mg2Ga2Te5, Mg2In2S5, and Mg2Tl2S5 monolayers have direct band gaps (0.80–2.50 eV), whereas the rest of the investigated 2D Mg2M2X5 monolayers are semiconductors with indirect gaps (0.40–3.05 eV). Based on our calculations, Mg2Ga2S5, Mg2Al2Se5, and Mg2B2S5 monolayers possess extremely high anisotropies with high electron mobilities of 1208.28, 629.94 and 6144.28 cm2V-1s−1 along the Y direction, respectively. Remarkably, these new 2D monolayers have pronounced strong optical absorption (>105 cm−1) over a broad range of the visible light spectrum. The Mg2Ga2S5, Mg2B2S5, and Mg2Al2Se5 monolayers exhibit excellent potential for application in solar water splitting due to the moderate band gaps, high electron mobilities and ideal band alignments for photon excitation for both hydrogen and oxygen evolution reactions.