Charge Dynamics in TiO₂/MXene Composites

Metal–semiconductor heterostructures are believed to improve hot-electron injection efficiency and influence the photocatalytic performance. Understanding the carrier dynamics at the heterostructure is essential for designing more efficient photocatalysts. Herein, we fabricated a Schottky heterostructure using two-dimensional (2D) titanium carbide MXene (Ti₃C₂Tₓ, where Tₓ stands for surface terminations, such as O or OH) and a TiO₂ semiconductor and examined the carrier dynamics at the heterostructure using time-resolved infrared techniques. MXene exhibits robust metallic properties in terms of photoconductivity comparable to those of high-quality 2D graphene materials. The photoexcitation of MXene greatly increases the scattering rate and leads to a decreased photoconductivity. When Ti₃C₂Tₓ comes in close contact with the TiO₂ semiconductor, band bending leads to the formation of a Schottky barrier at the contact junction. In this plasmonic TiO₂/Ti₃C₂Tₓ heterostructure, hot electrons are excited only from MXene upon photon absorption at wavelengths far below the TiO₂ band gap. Under these conditions, the Ti₃C₂Tₓ-generated plasmonic electrons are transferred into the conduction band of the TiO₂ semiconductor over the Schottky barrier with a fast time constant of 180 fs. The strong electronic coupling between oxygen-terminated Ti₃C₂Tₓ and TiO₂ is due to their proximity, and the resulting interactions are likely responsible for the fast electron transfer in the composites. Our results demonstrate a potential of 2D MXene materials in plasmonic applications and provide new insights into the design of MXene-based photocatalysts.