Electrocatalytic and Optoelectronic Characteristics of the Two-Dimensional Titanium Nitride Ti₄N₃T_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 Ti₄N₃T_x MXene (T_x = basal plane surface terminating groups) and show that this material exhbits both metallic and semiconducting behaviors. We provide complete structural characterization of exfoliated Ti₄N₃T_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 Ti₄N₃T_x MXene forms a hybrid with a thin surface oxide layer resulting in visible light absorbtion 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 Ti₄N₃T_x shows an overpotential of ∼300 mV at -10 mA cm^-2 and a Tafel slope of ∼190 mV dec^-1. Finally, we observe semiconducting behavior at temperatures below ∼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 Ti₄N₃T_x MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.