Surface Modifications of Ti 2 CO 2 for Obtaining High Hydrogen Evolution Reaction Activity and Conductivity: A Computational Approach.

The identification of hydrogen evolution reaction (HER) catalysts with high conductivity and high activity is the top priority in the development of renewable energy. Oxygen-terminated Ti2 C (Ti2 CO2 ), as one of the typical MXenes, shows good HER Gibbs free energy (ΔGH ) at low hydrogen coverage, whereas the large band gap of approximately 1.0 eV limits its conductivity capability. By doping phosphor (P) or sulfur (S) to partially substitute the surficial O, the average free energy (ΔGH a ) at various hydrogen coverages has been draggd to approach zero, and the conductivity is also significantly improved by narrowing the band gap to lower than 0.3 eV. Partial charge analysis indicates that doping P or S on the surface induces the diffusion of electrons on oxygen from O 2px to O 2pz , resulting in O 2pz reaction with H 1s. The facial overlapping of H 1s with O 2pz will strengthen the binding strength, hence lowering ΔGH . The energy shift toward Fermi level of Ti 3d after P or S doping contributes to the reduced band gap and high conductivity. Surficial O or P vacancies are created to evaluate the vacancy effect on HER performance, which not only improves ΔGH a and conductivity but also leads to a low reaction barrier of H2 O splitting (<0.2 eV). The armchair nanoribbon (ANR) displays improved HER activity by P-doping at 50% ratio. Our research shows that modification of end-group in MXenes can effectively improve HER catalytic activity and conductivity, which is expected to promote their potential applications in electrocatalysis and energy conversion.