Electrocatalytic and Optoelectronic Characteristics of the Two-Dimensional Titanium Nitride TiNT 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 TiNT MXene (T = basal plane surface terminating groups) and show that this material exhbits both metallic and semiconducting behaviors. We provide complete structural characterization of exfoliated TiNT MXene and assign T = O and/or OH and find that this material is susceptible to surface oxidation. Optical experiments indicate that the exfoliated TiNT 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 TiNT shows an overpotential of ∼300 mV at -10 mA cm and a Tafel slope of ∼190 mV dec. 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 TiNT MXene that expands the potential of these new 2D materials into electrocatalysis and (opto)electronic applications.