Experimental and theoretical investigation of the control and balance of active sites on oxygen plasma-functionalized MoSe₂ nanosheets for efficient hydrogen evolution reaction

Plasma functionalization is an effective method to improve the electrocatalytic activity of catalysts for the hydrogen evolution reaction (HER), but the relationship between the plasma and catalytic activity is not clear. Herein, oxygen plasma processing is conducted on MoSe₂ nanosheets and the effects of the plasma parameters including ion energy and radical flux are investigated by plasma simulation and molecular dynamics (MD) to fathom the interactions between the plasma and catalyst. A moderate ion energy and flux produce doping effects leading to proper replacement of Se sites by oxygen atoms to balance vacancy generation. Consequently, the HER characteristics are improved as exemplified by a small overpotential of 165 mV at 10 mA cm⁻² and Tafel slope of 55.2 mV dec^-1. Based on first-principles density-functional theory calculation, increased polarization and state density distributions close to the Femi level introduced by oxygen and vacancies reduce the bandgap and ΔG_H at the Mo, Se, and O sites, consequently enhancing charge transfer between the catalyst and electrolyte. The results convey new fundamental knowledge about plasma surface functionalization of electrochemical catalysts and enable precise and optimal selection of plasma processing parameters to control and balance the active sites for efficient water splitting.