High performance photocatalytic and thermoelectric two-dimensional asymmetrically ordered Janus-like MXene alloys

Abstract

MXenes are two-dimensional transition metal carbides, nitrides, and/or carbonitrides which display extensive versatility in their elemental composition for tuning and optimization of their functional properties. In this work, by using a multiscale approach of density functional theory calculations, cluster expansion method, and Monte Carlo simulations, we systematically investigate the Ti_2(1−x)Mo_2xCO₂ alloy system of two-dimensional Ti₂CO₂ and Mo₂CO₂ MXenes with 0 < x < 1. From the alloy formation energies, we identify the stable ground-state and report the possible existence of an asymmetrically ordered Janus-like MXene alloy TiMoCO₂. This alloy is theoretically validated to be stable thermodynamically, mechanically, and thermally, with a high order–disorder phase transition temperature of ∼640 K. Importantly, TiMoCO₂ is a semiconductor with a band gap of 0.60 eV and its valence band maximum and conduction band minimum are located on opposite sides of the MXene monolayer. As such, the asymmetric configuration generates an intrinsic dipole moment, which realigns the conduction and valence band edges, permitting photocatalytic redox water-splitting to favorably occur with a higher solar absorption efficiency in the visible-infrared region due to the smaller band gap. From the transport calculations, we propose the TiMoCO₂ MXene alloy to exhibit high room temperature n- and p-type power factors of 49.8 μW cm⁻¹ K⁻² and 15 μW cm⁻¹ K⁻², respectively, outperforming other related experimentally synthesized Ti–Mo based MXene alloys and even similar to that of p-doped SnSe which is one of the best performing thermoelectric materials. All these indicate the stable asymmetrically ordered Janus-like TiMoCO₂ MXene alloy to be a promising high performance water-splitting photocatalyst and thermoelectric material, paving the way for more stable Janus-like MXenes with interesting and outstanding functional properties to be discovered.