Synergetic Effects of Strain Engineering and Carbon Vacancy Modification on Tin+1CnO2 MXene for Promising Catalytic Activity of Hydrogen Evolution Reaction

Developing efficient and low-cost catalysts is crucial to hydrogen generation through water electrolysis. Oxygen-functionalized titanium carbide MXene holds tremendous potential in catalyzing the hydrogen evolution reaction (HER). However, appropriate modulation approaches aiming at optimizing the HER performance of these MXenes have not been systematically investigated. In this work, density functional theory was employed to screen the optimal HER operating conditions of Tin+1CnO2 (n = 1–3) by introducing carbon (C) vacancy modification coupled with strain engineering. Terminal oxygen atoms of pristine Ti2CO2 are identified as excellent reactive sites at relatively low hydrogen coverage, while those of Ti3C2O2 and Ti4C3O2 can reach an ideal catalytic state at high hydrogen coverage. An effective fine-tuning of reactive sites on these pristine MXenes can be achieved by exerting a biaxial strain. Besides, C vacancy can breed its nearest neighbor O atoms as active sites for Ti3C2O2 and Ti4C3O2, and their intrinsically excessive hydrogen adsorption would be weakened with the Gibbs free energies regulated extremely close to 0 eV. Detailed analyses indicate that more electrons are transferred to O atoms adjacent to the introduced C vacancy with the O-2p band center downshifting toward lower energy, thus weakening MXenes' capability in capturing hydrogen. Taken together, by collaborating biaxial strain with a C vacancy, Ti3C2O2 and Ti4C3O2 can perform with more outstanding reactive sites at a wider hydrogen coverage, while Ti2CO2 can perform only at low coverage. Our theoretical results point out reasonable regulation strategies for the desirable HER catalytic performance of Tin+1CnO2.