Charge Dynamics in TiO2/MXene Composites

Abstract

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.