Defect Chemistry of Er3+-Doped TiO2 and Its Photocatalytic Activity for the Degradation of Flowing Gas-Phase VOCs

A series of Er-doped TiO2 photocatalysts, using Er(NO3)3·6H2O precursor ranging from 0.5 to 2 mol %, were synthesized via sol–gel method with titanium(IV) isopropoxide solution as reactant. The structure and properties of as-prepared samples were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, thermogravimetric analysis, photoluminescence, electrochemical impedance spectroscopy, and photocurrent. The as-synthesized Er-doped TiO2 nanoparticles demonstrated improved photocatalytic activities for the photodegradation of selected volatile organic compounds, including acetaldehyde, o-xylene, and ethylene. Additionally, high photodegradation of acetaldehyde was recorded in the visible region (λ > 420 nm). XPS analysis revealed that a handful of Er in anatase TiO2 was presented in the form of Er2O3, which benefited by increasing the specific surface area ranging from 59.28 to 110.34 m2/g and consequently enhanced the adsorption capacity for acetaldehyde, o-xylene, and ethylene 3.3, 3.7, and 3.8 times, respectively, in contrast to the undoped TiO2. Electron spin resonance results showed that Er doping caused production of oxygen vacancies (VO) and Ti3+, which promoted the photocatalytic activity of the catalyst. The TiO2 samples containing 0.5–1.5% Er content exhibited higher photoactivity in comparison with other samples. The highest removal efficiency of acetaldehyde and o-xylene within 100 min was 99.2 and 84.6%, respectively, and ethylene degradation efficiency reached 22.4% within 180 min. Furthermore, the visible light elimination efficiency of acetaldehyde was 25.5%. Finally, the results suggested that Er doping predominantly inhibited the catalyst poisoning in the degradation of o-xylene.