Investigation of the influence of calcination temperature on the photocatalytic performance of TiO₂ nanotubes doped with Ti₃C₂T_x MXene

This study investigated the critical role of calcination temperature in optimizing the photocatalytic performance of MXene Ti₃C₂T_x-modified TiO₂ nanotubes (TiNTs) for the degradation of formic acid under UV light. We synthesized a series of 1 wt% MXene/TiNT composites by preparing MXene via HF etching of the Ti₃AlC₂ MAX precursor and TiNT using an alkaline hydrothermal method with P25 at 130 °C for 20 h. These composites were then calcined at temperatures ranging from 100 to 400 °C. Extensive characterization using techniques such as BET surface area analysis, X-ray diffraction (XRD), Raman spectroscopy, UV–Vis diffuse reflectance spectroscopy, and electrochemical impedance spectroscopy (EIS) revealed profound shifts in the structural, morphological, and electronic properties of the composites with varying calcination temperatures. Remarkably, calcination at 200 °C significantly enhanced the photocatalytic efficiency, with a degradation rate constant 80 % higher than that under other calcination conditions promoting in this way high charge separation. Complementary ammonia TPD measurements revealed a pseudo-second-order desorption kinetic model for the photocatalyst. The desorption heat was 156 kJ/mol at 200 °C calcination, which decreased slightly at higher temperatures, reflecting the loss of hydroxyl groups. Increasing the calcination temperature subtly reduces the acidity owing to the gradual loss of surface hydroxyl groups. Intriguingly, SEM analysis revealed that partial oxidation at 400 °C generated TiO₂ domains with unsaturated Ti⁴⁺ centers, introducing Lewis acid sites that effectively compensated for the diminished Brønsted acidity, maintaining stable NH₃ adsorption despite surface evolution. However, high-temperature calcination (300–400 °C) also promoted the formation of new TiO₂ nanoparticles on the MXene surface, as confirmed by SEM and UV–visible spectroscopy. These nanoparticles act as charge recombination centers, increasing electrical resistance as identified by EIS, which significantly hinders the photocatalytic performance. This study, therefore, deeply emphasized the critical role of post-treatment conditions of MXene/TiO₂ systems on their photocatalytic performances and the necessity of avoiding overoxidation of the Ti₃C₂T_x component during calcination.