Surface reduction increases free electron concentration in MXene for better photothermal performance

The photothermal properties of MXenes, a class of two-dimensional (2D) transition metal carbides and nitrides, are primarily driven by their high free electron concentration due to localized surface plasmon resonance. However, this electron concentration is often limited by suboptimal d-orbital occupancy and electron-withdrawing effects from electronegative terminations. A new sodium-mediated surface reduction strategy in molten salts has been developed to optimize the surface coordination environment of MXenes. This method mitigates electron-withdrawing and scattering effects while injecting electrons into the Ti-3d states, significantly enhancing free electron concentration, carrier mobility, and electrical conductivity.

The optimized reduced Ti3C2 MXene achieves a photothermal conversion efficiency of 91.66% under 808-nanometer laser irradiation, marking a substantial improvement over pristine MXene. This advancement is demonstrated in a photothermal antibacterial woundplast with ultralow MXene content, which shows a high bacterial kill rate. The study highlights an effective method for tuning the photothermal properties of MXenes, with potential applications in areas requiring tailored surface chemistry and high electron concentration.

MXenes are typically synthesized by selectively etching the A layer from the MAX phase using etchants such as hydrofluoric acid or Lewis acidic molten salts. This process results in materials with the formula Mn+1XnTx, where M is an early transition metal, X is carbon or nitrogen, and Tx denotes surface terminations. The unique structure of MXenes provides exceptional metallic conductivity, mechanical flexibility, hydrophilicity, and tunable electronic properties, driving their exploration in energy storage, catalysis, and biomedicine.

To improve the photothermal performance of MXenes, strategies such as surface decoration with plasmonic noble metal nanoparticles or intrinsic electron modulation have been explored. However, these methods can introduce synthesis complexity and stability concerns. A more direct approach involves regulating surface chemistry to reduce electronegative terminations and restore intrinsic electronic properties. Conventional methods like high-temperature annealing or chemical reduction often result in incomplete termination removal or structural damage.

The sodium-mediated molten salt reduction strategy achieves dual electronic optimization by modifying surface terminations and directly injecting electrons into the Ti-3d states. This process saturates Ti-3d states near the Fermi level and optimizes surface coordination, resulting in significant improvements in electron concentration, mobility, and conductivity. The optimized reduced Ti3C2 MXene exhibits the highest reported photothermal conversion efficiency for MXenes under 808-nm laser irradiation.

The synthesis process involves reacting Ti3C2Clx with metallic sodium in a LiCl-KCl molten salt under an inert atmosphere. The choice of Ti3C2Clx and sodium as reactants is based on thermodynamic considerations, such as bond dissociation energies and Gibbs free energy changes. The reduction process effectively removes -Cl terminations and injects electrons, leading to enhanced photothermal properties.

X-ray diffraction and scanning electron microscopy analyses reveal notable structural changes in reduced Ti3C2 samples, including increased interlayer spacing and removal of -Cl terminations. These changes facilitate the exfoliation of reduced MXene into nanosheets without additional intercalation treatments. The surface chemistry of the MXene samples is further characterized by x-ray photoelectron spectroscopy and Raman spectroscopy, confirming the successful reduction and electron injection.

The study demonstrates that the sodium-mediated reduction strategy not only enhances the photothermal properties of MXenes but also simplifies their preparation process. This approach holds promise for the development of MXene-based applications in biomedicine and other fields requiring advanced carbon materials.