Strain engineering of electronic structures and photocatalytic responses of MXenes functionalized by oxygen

The structural, electronic, and photocatalytic properties of two-dimensional Ti₂CO₂, Zr₂CO₂, and Hf₂CO₂ MXenes were investigated by first-principles (PBE and hybrid) calculations. Transition from an indirect to a direct band gap was achieved for the biaxial tensile strain of 3% for Ti₂CO₂, 8% for Zr₂CO₂, and 13% for Hf₂CO₂ while the nature of the band gap remained indirect in the case of the compressive strain. The size of the band gap passed through a maximum under tensile strain and decreased monotonically under compressive strain. Analysis of Bader charge distribution showed that the tensile strain decreased the transfer of charge from the Ti, Zr, and Hf atoms to the C atom. Phonon spectra suggested that these systems are stable under a wide range of strains from compression to tension. The photocatalytic properties showed that unstrained and biaxial tensile strained Ti₂CO₂, Zr₂CO₂, and Hf₂CO₂ systems can be used to oxidize H₂O into O₂.