Photocatalytic activity of defective black-titanium oxide photocatalysts towards pesticide degradation under UV/VIS irradiation

Wastewater treatment containing pesticides has become an intensively studied field in recent years. Titanium dioxide is the most efficient photocatalyst that uses UV radiation, but the photocatalytic properties can be improved under visible radiation conditions. The photocatalytic properties of titanium oxide are closely related to materials defect disorders, such as oxygen vacancy and Ti3+ defect. Therefore, defective TiO2 or black TiO2 nanoparticles are attractive materials for heterogeneous photocatalysis. Defective nanoparticles with superior properties to white TiO2 and efficient solar harvest in the photocatalytic application were synthesised in two convenient and low-cost steps via facile sol-gel and NaBH4 reduction techniques. In the reduction step of the white titanium dioxide powder, the defects were controlled by varying the TiO2: NaBH4wt ratio at 350 °C for 1 h in an argon atmosphere, obtaining powders with shades of grey to black. The characterisation measurements include structure (XRD, XPS, EDS), morphology (SEM), optical properties (UV–vis DRS spectra), and particles (surface area, porosity, DLS particle size distribution) were applied to investigate the properties of nanostructured defective TiO2. EDS analysis revealed that the O: Ti ratio for the white TiO2 powder sample was approximately 1.7, which decreased from 1.7 to 1.6 and 1.4 as the TiO2:NaBH4 ratio was 4:1, 2:1 and 1:1, respectively. Photocatalytic activity of black titanium oxide under visible light was over 90% of imidacloprid degradation and can be associated with the specific surface area, bandgap energy smaller than white TiO2, a large amount of surface defect Ti3+ and oxygen vacancies that promote charge carriers' separation.