Enhanced photoelectrochemical water splitting of micro-arc oxidized TiO₂ via anatase/rutile phase control and nitrogen doping

Titania and nitrogen-doped titania (TiO2) films were successfully prepared via micro-arc oxidation (MAO) of pure Ti plates. The films showed micro-porosity and high crystallinity with ∼10–20 nm nanocrystals of anatase or anatase/rutile. Through the variation of applied voltage and time for MAO processing the phase composition ratio of rutile-to-anatase could be controlled from 5:95 to 19:81. Nitrogen doping into titania was conducted by adding acetamide in an NaOH electrolyte solution during MAO and the nitrogen concentration was determined to be ∼1 ± 0.23% by X-ray photoelectron spectroscopy. UV/visible spectroscopy revealed the apparent redshift in nitrogen-doped samples, which evidences the nitrogen doping in TiO₂ lattices. Also, both pure and nitrogen-doped samples showed the gradual redshift according to rutile content. Nitrogen-doped 81% anatase-19% rutile samples showed the highest photocurrent density and photoconversion efficiency of 0.594 mA/cm² and 0.6685%, respectively at 0 voltage, while pure and 100% anatase TiO₂ samples showed the lowest ones of 0.036 mA/cm² and 0.0327%, respectively at 0 voltage. Photocurrent density could be further increased up to 0.8 mA/cm² using a mixed electrolyte of NaOH and KI.