Interfacial charge transfer and interaction in the MXene/TiO2 heterostructures

Hybrid materials of MXenes [two-dimensional (2D) carbides and nitrides] and transition-metal oxides have shown great promise in electrical energy storage (EES) and 2D heterostructures have been proposed as the next-generation electrode materials to expand the limits of current technology. Here we use first principles density functional theory to investigate the interfacial structure, energetics, and electronic properties of the heterostructures of MXenes (${\mathrm{Ti}}_{\mathrm{n}+1}{\mathrm{C}}_{\mathrm{n}}{T}_{2}$; T = terminal groups) and anatase $\mathrm{Ti}{\mathrm{O}}_{2}$. We find that the greatest work-function differences are between OH-terminated MXene (1.6 eV) and anatase $\mathrm{Ti}{\mathrm{O}}_{2}(101)$ (6.4 eV), resulting in the largest interfacial electron transfer ($\ensuremath{\sim}0.9\phantom{\rule{0.16em}{0ex}}e/\mathrm{n}{\mathrm{m}}^{2}$ across the interface) from MXene to the $\mathrm{Ti}{\mathrm{O}}_{2}$ layer. This interface also has the strongest adhesion and is further strengthened by hydrogen bond formation. For O--, F--, or mixed O--/F-- terminated ${\mathrm{Ti}}_{\mathrm{n}+1}{\mathrm{C}}_{\mathrm{n}}$ MXenes, electron transfer is minimal and interfacial adhesion is weak for their heterostructures with $\mathrm{Ti}{\mathrm{O}}_{2}$. The strong dependence of the interfacial properties of the $\mathrm{MXene}/\mathrm{Ti}{\mathrm{O}}_{2}$ heterostructures on the surface chemistry of the MXenes will be useful to tune the heterostructures for EES applications.