Remarkable Enhancement of Photocatalytic Water Oxidation in N2/Ar Plasma Treated, Mesoporous TiO2 Films

Mesoporous TiO2 films treated with N2/argon plasma were studied for ultraviolet and visible-light-induced photocatalytic water oxidation activity. Compared to pristine TiO2 films, plasma-treated TiO2 films showed remarkable enhancement of photocurrents (up to 80–240 times) in both ultraviolet light and visible light, greatly surpassing enhancements previously reported in the literature. The cubic-ordered mesoporous TiO2 thin films were prepared by a surfactant-templated sol–gel method and were treated with N2/argon plasma, an approach hypothesized to capitalize on the high degree of disorder in the material and the high energy of the plasma species to achieve efficient nitrogen doping. The effects of reaction gas pressure and N2 gas flow rate on photoelectrochemical (PEC) response were investigated. UV–vis absorbance spectra indicated that the incorporated N atoms significantly reduced the band gap of TiO2 with the enhancement of visible-light absorption, and XPS analysis showed primarily substitutional N atom incorporation rather than interstitial. The photocatalytic activity of nitrogen-doped TiO2 (N-TiO2) films was evaluated by chronoamperometry and linear sweep voltammetry. The effect of light sources on PEC performance was explored using UV (365 nm), blue (455 nm), and green (530 nm) LEDs. N-TiO2 films showed 242× and 240× enhancement of photocurrent, compared to undoped TiO2 films under UV and blue LED irradiation, respectively. The N-doped films also showed overall enhancement of up to 70× and 92× with a broad spectrum Xe arc lamp and halogen bulb, respectively, and photocatalytic activity even with green LED illumination, compared to no measurable activity without doping. The present study shows that plasma-induced doping of sol–gel materials enables the efficient incorporation of heteroatoms into disordered metal oxide nanostructures, thereby leading to remarkable enhancement in visible-light-driven photoelectrochemical water oxidation.