Optimization of Ta-, V-, or Nb-doped TiO2 for photocatalytic and electrophotocatalytic degradation of p-nitrophenol under UV–visible light

The objective of this study is to investigate the enhancement of TiO₂ photocatalytic activity under UV/visible and visible light by doping with tantalum (Ta), vanadium (V) or niobium (Nb) precursors. Thus, TiO₂-based photocatalysts are prepared at room temperature via aqueous sol–gel synthesis. These photocatalysts are doped with different molar ratios of dopants. The physicochemical properties of the obtained photocatalysts are characterized using various complementary techniques. X-ray diffraction (XRD) is used to determine the distribution of different crystalline phases of TiO₂ and the proportion of amorphous TiO₂ in the samples. BET measurements give textural properties with specific surface area reaching up to 292 m²/g and TEM images illustrating their spherical morphology. Inductively coupled plasma atomic emission spectroscopy (ICP-AES), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) analyses confirmed the presence of Ti, O and Ta, V or Nb elements in the samples. Additionally, XPS spectra highlights the incorporation of nitrogen in the undoped synthesized TiO₂. Additionally, band gap widths measured by UV–vis diffuse reflectance spectroscopy (DRUS) are done to see the impact of the dopants on the bandgap, with V-doping reducing its value from 2.95 to 1.88 eV. A screening of the photocatalytic activity of undoped and doped photocatalysts is carried out by evaluating the degradation of p-nitrophenol under UV–visible light (300 < λ < 800 nm) and visible light only (395 < λ < 800 nm). This study suggests that photocatalytic activity is significantly influenced by the nature and dopant content. Photocatalytic tests show an improvement in the activity of the photocatalyst when doped with tantalum and niobium (from 22 % with undoped TiO₂ under visible light to 37 % and 55 % with the best Ta and Nb-doped samples respectively), while a notable decrease in activity is observed with vanadium doping (dropping to 3–12 % with the V-doped series). Finally, a preliminary electrophotocatalysis experimental setup is implemented and appears to show an improvement in the mineralization of the PNP solution when the anode is coated with layers of the best doped-TiO₂ material (Nd doping), compared to an uncoated anode (the mineralization rate increases from 72 % to 94 %).