First-principles study of biaxial strain effect on NH3 adsorbed Ti2CO2 monolayer

Functionalized MXenes possess versatile potential applications in the field of electrochemical energy storage, hydrogen storage, and sensor materials. Ti2CO2 monolayer absorbed by NH3 (NT-PT system) under biaxial strain is investigated via first-principles calculations. The positive adsorption energy under compressive strain indicates the repulsive interaction between NH3 and Ti2CO2 and is unfavorable to adsorption. The negative adsorption energies under tensile strain indicate the strong interaction between NH3 gas and Ti2CO2. Strain has little effect on the adsorption site. NT-PT system is the most stable adsorption system among all the possible configurations. NH3 is chemisorbed on Ti2CO2 monolayer with strain larger than 2%. NT-PT system with 2% strain is suitable as a gas sensor for NH3 detection because of the short recovery time and the sensitivity. The band gap of NT-PT system undergoes the indirect-direct transition under tensile strain. Ti, O, and C atoms have the main contribution to the energy level at the valence band maximum (VBM) and conduction band minimum (CBM), while N and H atoms have little contribution. The analysis of bond character of NT-PT system and the competitive adsorptions of NH3 with some molecules on pristine Ti2CO2 monolayer are further explored.