Vibrational Property Tuning of MXenes Revealed by Sublattice N Reactivity in Polar and Nonpolar Solvents

MXenes, a family of two-dimensional (2D) materials based on transition metal carbides and nitrides, have desirable properties, such as high conductivity, high surface area, and tunable surface groups, for electrocatalysis. Nitride MXenes, in particular, have shown excellent electrocatalytic performance for the nitrogen and oxygen reduction reactions, but a fundamental understanding of how their structures evolve during electrocatalysis remains unknown. Equally important and yet unknown is the effect of the reactivity of the lattice nitrogen on the vibrational behavior of nitride MXenes and the resulting implications in electrocatalysis. Here, we investigate the reactivity of lattice nitrogen and the vibrational properties of titanium nitride MXenes in relevant electrocatalytic solvents using confocal Raman spectroscopy. We found that the vibrational modes of titanium nitride MXenes are attenuated in polar solvents, which is revealed through the alteration of the Raman scattering in solvents. Contrary to polar solvents, the vibrational modes remain unchanged in nonpolar solvents like hydrocarbons due to the inactivity of the lattice nitrogen. We found that this behavior is unique to nitrides because the Raman characteristics of carbides and sulfides are unaffected by the solvent types. However, the inclusion of nitrogen into the carbide structure does exhibit Raman-solvent behavior similar to that of nitrides, suggesting that replacing carbon with nitrogen affects MXene-light interactions. We demonstrated a proof-of-concept utilizing lattice nitrogen reactivity to enhance the electrocatalytic nitrogen reduction reaction for ammonia production. In summary, we elucidate the vibrational properties of nitride MXenes in solvents and demonstrate the tunability of MXene vibrational properties via lattice atom substitution, which in turn can be exploited to advance the applications of MXenes in electrocatalysis.