Influence of Tripolyphosphate on Electronic Conductivity and Photothermal Relaxation Dynamics in Ti₃C₂T _x MXene

Ti₃C₂T _x MXene, the most extensively studied member of the MXene family, combines metallic conductivity, strong light absorption, and exceptional photothermal efficiency, enabling applications ranging from optoelectronics to thermal management and biomedical systems. However, its practical use has been challenged by limited environmental stability. While polyphosphate edge-capping has previously been shown to effectively suppress oxidation and degradation in aqueous suspensions, its influence on the intrinsic electronic properties and photothermal behavior of MXene films has remained unexplored. Here, we investigate the impact of sodium tripolyphosphate (TPP) introduced during aqueous processing on the electrical transport and photothermal dynamics of Ti₃C₂T _x films. Using terahertz time-domain spectroscopy (THz-TDS) and four-point probe measurements, we find that TPP addition does not significantly alter charge transport or carrier localization, indicating that the electronic structure of the films is preserved. Optical pump-THz probe spectroscopy reveals that, upon photoexcitation, all samples exhibit the characteristic transient suppression of conductivity associated with photothermal heating, followed by a slow recovery over hundreds of picoseconds. At higher TPP concentrations, the thermal relaxation is noticeably slower, suggesting that TPP residues at flake edges and interflake interfaces hinder phonon transport and heat dissipation. These findings demonstrate that addition of polyphosphate, while maintaining the excellent conductivity of Ti₃C₂T _x , can be used to control photothermal relaxation behavior and the thermal response of MXene-based functional materials.