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 (Ti 3 C 2 T x , where T x stands for surface terminations, such as O or OH) and a TiO 2 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 Ti 3 C 2 T x comes in close contact with the TiO 2 semiconductor, band bending leads to the formation of a Schottky barrier at the contact junction. In this plasmonic TiO 2 /Ti 3 C 2 T x heterostructure, hot electrons are excited only from MXene upon photon absorption at wavelengths far below the TiO 2 band gap. Under these conditions, the Ti 3 C 2 T x -generated plasmonic electrons are transferred into the conduction band of the TiO 2 semiconductor over the Schottky barrier with a fast time constant of 180 fs. The strong electronic coupling between oxygen-terminated Ti 3 C 2 T x and TiO 2 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.

Unconventional Photoconversion from In‐Plane 2D Heterostructures of 2D Transition Metal Carbides/Semiconductors

“…Very recently, an out‐of‐plane Ti 3 C 2 /TiO 2 heterostructure was studied under the same conditions as us, which showed contradictory negative photoconductivity. [ 18 ] These results indicate that the super photothermal performance of A500 should be ascribed to the seamlessly constructed in‐plane 2D Ti 3 C 2 /TiO 2 rather than the out‐of‐plane type. Upon the ultrafast photoexcitation at 800 nm (≈1.55 eV), hot electrons were generated only from Ti 3 C 2 because the light wavelength is close to its resonance band (Figure 2B) but below the bandgap of TiO 2 .…”

“…By fitting the traces after the photoexcitation and before the relaxation in Figure 3A, [ 16,17 ] a time constant of ≈121 fs was obtained which suggests a faster electron transfer across the Ti 3 C 2 /TiO 2 heterointerface than that in the out‐of‐plane interface (180 fs). [ 18 ] Because of the coexistence of in‐plane and out‐of‐plane Ti 3 C 2 /TiO 2 interfaces in A500, we believe that the unique positive photoconductivity and accelerated electron injection process are the comprehensive results of the two interface types and the in‐plane type played the dominant roles during photothermal conversion.…”

“…[14,15] Moreover, the current limited understanding of MXene-based photothermal materials cannot guide their photoconversion modulations beyond LSPR and electromagnetic interference (EMI) theories. [3] Compared to the reported out-of-plane 2D metal/ semiconductor paradigms, [16][17][18][19] the in-plane 2D heterostructures might open up unprecedented opportunities owing to entire new charge transfer diagrams. The weak oxidation resistance of MXenes enables the possible construction of in-plane 2D MXene/semiconductor heterostructures by slight oxidation.…”

“…Banking on the aforementioned considerations, vacuum annealing was considered a suitable way to build in‐plane 2D MXene/semiconductor heterostructures for fundamental investigations due to the following rationales: i) the in‐plane 2D heterostructures can form in situ during annealing, [ 20 ] which keep ideal contact and minimal barrier for charge transfer compared to the assembled ones. [ 18 ] ii) The annealing way can control the construction of MXene/semiconductor by adjusting the related technical parameters. iii) Annealed MXenes keep much better stability than the original.…”

Selective Photocatalytic Activities of Amino Acids/Peptide-Ti3C2Tx-TiO2 Composites Induced by Calcination: Adsorption Enhancement vs Charge Transfer Enhancement

“…12,13 The Ti 3 C 2 T x -generated plasmonic electrons are transferred into the conduction band of TiO 2 over the Schottky barrier, and the strong electronic coupling between oxygen-terminated Ti 3 C 2 T x and TiO 2 is ascribed to their proximity. 14 Although protein materials have low chemical resistance and are liable to photodegradation, good compatibility of TiO 2 and wool keratin composed of amino acids/peptide has been recognized. 15 The S and N elements of wool keratin can be doped with TiO 2 to improve the visible-light-induced photocatalytic activity of TiO 2 -coated wool fibers.…”