Enhancing the Photocatalytic Performance of MXenes via Stoichiometry Engineering of Their Electronic and Optical Properties.

Combining both density functional theory and the cluster expansion method, we investigate 3 binary MXene alloy systems of semiconducting Ti2CO2, Zr2CO2, and Hf2CO2, where the transition metals substitute one another (i.e., Ti2(1- x)Zr2 xCO2, Ti2(1- x)Hf2 xCO2, and Zr2(1- x)Hf2 xCO2). We show that this group of MXene alloys forms the solid-solution phase across all compositions. Special quasirandom structures are generated to model the solid-solution phase of these alloys, using which we demonstrate how their structural, mechanical, electronic, and optical properties are tuned via stoichiometry engineering. These alloys exhibit outstanding mechanical strength and stability. They possess indirect band gaps of 1.25-1.80 eV. For Ti2(1- x)Zr2 xCO2 and Ti2(1- x)Hf2 xCO2, they display higher absorbance in the solar spectrum than their constituent Zr2CO2 and Hf2CO2, respectively. Most of the MXene alloys also show appropriately aligned band edges for water splitting. We predict the Ti2(1- x)Zr2 xCO2 alloy with x = 0.2778 to be the most promising water-splitting photocatalyst among the MXenes studied here, outperforming its constituents, Ti2CO2 and Zr2CO2, when solar absorbance performance and band-edge alignments are simultaneously considered. This work demonstrates that alloying can be used to effectively tune photocatalytic performance.