Electrocatalytic and Optoelectronic Characteristics of Exfoliated Two-Dimensional Titanium Nitride Ti 4 N 3 T x mxene

A relatively new class of two-dimensional (2D) materials called MXenes have garnered tremendous interest in the field of energy storage and conversion. Thus far nearly all MXenes reported experimentally have been described as metals, with a lone report of a mixed-metal carbide phase exhibiting semiconducting character. Here, we report the optical, electrocatalytic and electrical properties of the 2D Ti 4 N 3 T x MXene (T x = basal plane surface terminating groups) and show this material exhibits both metallic and semiconducting behavior. We provide complete structural characterization of exfoliated Ti 4 N 3 T x MXene and assign T x = O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated Ti 4 N 3 T x MXene forms a hybrid with a thin surface oxide layer resulting in visible light absorption at energies greater than ~2.0 eV and an excitation wavelength-dependent defect-state emission over a broad range centered at ~2.9 eV. As an electrocatalyst for the hydrogen evolution reaction, the exfoliated Ti 4 N 3 T x shows an overpotential of ~300 mV at –10 mA cm –2 and a Tafel slope of ~190 mV/decade. Finally, we observe a clear semiconductor-to-metal transition at ~90 K from temperature-dependent transport measurements under 5 T magnetic field likely resulting from the thin oxide layer. These results unveil the intriguing optical, electrocatalytic, and electrical properties of this 2D Ti 4 N 3 T x MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.