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

The structural, electronic, and photocatalytic properties of two-dimensional Ti2 CO2 , Zr2 CO2 , and Hf2 CO2 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 Ti2 CO2 , 8% for Zr2 CO2 , and 13% for Hf2 CO2 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 Ti2 CO2 , Zr2 CO2 , and Hf2 CO2 systems can be used to oxidize H2 O into O2 .