Facile Surface Engineering of TiO2 Nanosheets for Enhanced Isopropanol Sensing under UV Irradiation

Surface termination and defects of metal oxide semiconductors are crucial in the process of gas adsorption–desorption and signal transduction, thereby determining their sensing performance. Herein, a facile solvent-assisted surface engineering strategy was demonstrated to synthesize anatase TiO₂ nanosheets (TNS) for an ultraviolet (UV) light-activated isopropanol (IPA) gas sensor. Surface-fluorinated TiO₂ nanosheets (F-TNS) were first synthesized by the hydrofluoric acid-assisted hydrothermal method and followed by hydrothermally treating in Na₂S solutions with different concentrations. The effect of the progressive removal of fluorides was discussed in detail based on X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), electrochemical impedance spectroscopy (EIS), and in situ Fourier transform infrared (FTIR) spectroscopy analyses. Compared with F-TNS, the chemiresistive sensor based on the TNS with a trace amount of fluorine exhibited a 324% increase in the sensitivity to 50 ppm of isopropanol at 50 °C under UV irradiation (λ = 365 nm, 30 mW/cm²), while it exhibited a 45% decrease in the recovery time. The enhanced isopropanol sensing performance could be attributed to the high surface area, rational surface terminations, oxygen vacancies, and UV photoexcited charge carriers, which further modulate the surface reaction and charge transfer. These findings offer a facile strategy for the rational design of oxide-based sensing materials, which help in understanding the function of surface terminations and defects in gas sensing.