Exploiting direct current plasma electrolytic oxidation to boost photoelectrocatalysis

In this study, we report an investigation of the photoelectrochemical activity of TiO₂ films formed by DC plasma electrolytic oxidation (PEO) at a variable potential in a sulfuric acid electrolyte at 0 and 25^◦C. The surface morphology was mainly determined by the oxide-forming potential. X-Ray Diffraction and Raman analyses showed that the relative amount of the anatase and rutile phases varied from 100% anatase at low potential (110–130 V) to 100% rutile at high potential (180–200 V), while mixed-phase oxide films formed at intermediate potential. Correspondingly, the band gap of the TiO₂ films decreased from about 3.20 eV (pure anatase) to 2.94 eV (pure rutile) and was red-shifted about 0.1 eV by reducing the electrolyte temperature from 25^◦C to 0^◦C. Glow-Discharge Optical Emission Spectroscopy (GD-OES) and X-ray Photoelectron Spectroscopy (XPS) analyses evidenced S-containing species located preferentially close to the TiO₂/Ti interface. The photoelectrochemical activity was assessed by measuring the incident photon-to-current efficiency (IPCE) under Ultraviolet C (UV-C) irradiation, which showed a non-gaussian normal trend as a function of the PEO cell potential, with maximum values exceeding 80%. Photoelectrocatalytic activity was assessed by decolorization of model solutions containing methylene blue. Photoanodes having higher IPCE values showed faster decolorization kinetics.