Charge Dynamics in TiO<sub>2</sub>/MXene Composites
Shaun Debow, Tong Zhang, Xu‐Sheng Liu, Fuzhan Song, Yuqin Qian, Jian Han, Kathleen Maleski, Zachary Zander, William R. Creasy, Danielle L. Kuhn, Yury Gogotsi, Brendan G. DeLacy, Yi Rao
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.