Photocatalytic properties of titanium oxide coatings obtained from plasma electrolytic oxidation process: effect of phosphorus and tungsten doping

Integrating visible-light photocatalysis into wastewater treatment enhances sustainable water purification technologies. This study investigated the photocatalytic properties of phosphorus-doped and phosphorustungsten- doped TiO2 coatings, produced via plasma electrolytic oxidation (PEO) of titanium substrates, for the treatment of textile wastewater. The photocurrent spectra of the coatings revealed energy band gaps ranging from 2.30 to 2.90 eV, with the lowest values observed in co-doped coatings. This decreased band gap promotes electron-hole pair recombination, negatively impacting photocatalytic performance. Indeed, differential capacitance measurements indicated a higher donor concentration in co-doped coatings, which hinders charge separation, as evidenced by the minimized photocurrent intensities. As a result, phosphorus-doped TiO2 demonstrated a maximum methylene blue degradation rate of 59.7% after 180 min under visible light. This high level of degradation was attributed to its enhanced photocatalytic activity, which includes efficient charge separation due to its mixed anatase-rutile phase composition, an increased number of adsorption sites from its smaller anatase crystallite size and higher dislocation density, and a reduced recombination rate of electron-hole pairs, as indicated by the photoluminescence emission intensity. Interestingly, the degradation efficiency of the co-doped coatings improved with repeated cycles, remaining more effective than their initial performance even after multiple iterations. This enhancement was attributed to the reduced pore plugging over time.