Fluorinated TiO₂ Hollow Spheres for Detecting Formaldehyde under UV Irradiation

The fluorinated titanium dioxide (F-TiO₂) hollow spheres with varying F to Ti molar ratios were prepared by a simple one-step hydrothermal method followed by thermal processing. The diameter of the F-TiO₂-0.3 hollow spheres with a nominal ratio of F:Ti = 0.3:1 was about 200-400 nm. Compared with the sensor based on pristine TiO₂ sensing materials, the F-TiO₂-0.3 sensor displayed an enhanced sensing performance toward gaseous formaldehyde (HCHO) vapor at room temperature under ultraviolet (UV) light irradiation. The F-TiO₂-0.3 sensor demonstrated an approximately 18-fold enhanced response (1.56) compared to the pristine TiO₂ sensor (0.085). The response and recovery times of the F-TiO₂-0.3 sensor to 10 ppm HCHO were about 56 s and 64 s, respectively, and a limit-of-detection value of 0.5 ppm HCHO was estimated. The F-TiO₂-0.3 sensor also demonstrated good repeatability and selectivity to HCHO gas under UV light irradiation. The outstanding HCHO gas-sensing properties of the F-TiO₂-0.3 sensor were related to the following factors: the excitation effect caused by the UV light facilitated surface chemical reactions with analyte gas species; the hollow sphere structure provided sufficient active sites; and the surface fluoride (≡Ti-F) created additional chemisorption sites on the surface of the TiO₂ material.