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

Abstract

Mesoporous TiO 2 films treated with N 2 /argon plasma were studied for ultraviolet and visible light induced photocatalytic water oxidation activity. Compared to pristine TiO 2 films, plasma treated TiO 2 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 TiO 2 thin films were prepared by a surfactanttemplated sol-gel method and were treated with N 2 /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 N 2 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 TiO 2 with the enhancement of visible light absorption, and XPS analysis showed primarily substitutional N atoms incorporation rather than interstitial. The photocatalytic activity of nitrogen-doped TiO 2 (N-TiO 2 ) 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-TiO 2 films showed 242x and 240x enhancement of photocurrent, compared to undoped TiO 2 films under UV and blue LED irradiation, respectively. The N-doped films also showed overall enhancement of up to 70x and 92x 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 plasmainduced doping of sol-gel materials enables the efficient incorporation of heteroatoms into disordered metal oxide nanostructures, thereby leading to remarkable enhancement in visiblelight driven photoelectrochemical water oxidation.

Synthesis and Catalytic Applications of Non-Metal Doped Mesoporous Titania

“…Fewer studies have been performed on the application of non-metal-doped mp-TiO 2 in H 2 O splitting, and CO 2 reduction, but these are potentially important areas for the application of titania due to the alignment of its valence and conduction bands with the relevant electrochemical potentials required for these reactions. Significant enhancements in these reactions have been made through non-metal doping of titania, including a recent report of 240× enhancement in the rate of photoelectrochemical water oxidation in N 2 /Ar plasma-doped mp-TiO 2 films [173]. Although this review summarizes significant enhancements in visible-light photoactivity, further improvements in visible light absorption, separation of photogenerated charge carriers, and charge transfer to reactants will be necessary for widespread use of titania in practical photocatalytic applications.…”

“…Among available doping approaches, plasma-assisted doping has several critical advantages, such as (i) effectively incorporating heteroatoms into the TiO 2 host by providing reactive dopant species and (ii) being conducted at relatively low temperature so that the nanostructure of TiO 2 is maintained. Recently, we have reported the incorporation of nitrogen into surfactant templated cubic ordered mp-TiO 2 thin films by N 2 /Ar plasma treatments [65,173]. About 2-5 atom % substitutional nitrogen was found in the doped films, and the nitrogen was found to be uniformly distributed through the depth of the films.…”

“…In addition, the small particle size found in mp-TiO 2 helped reduce charge recombination, which contributed to the improved water splitting performance. Recently, our group has reported surfactant templated mp-TiO2 films treated with N2/argon plasma studied for ultraviolet and visible light induced photocatalytic water splitting activity [173]. The effect of light sources on PEC performance was explored using UV (365 nm), blue (455 nm), and green (530 nm) LEDs.…”

Solar Water Oxidation by a Visible‐Light‐Responsive Tantalum/Nitrogen‐Codoped Rutile Titania Anode for Photoelectrochemical Water Splitting and Carbon Dioxide Fixation

“…The superior property of TiO 2 :Ta/N for photocatalytic water oxidation suggests the possibility of TiO 2 :Ta/N as a photoanode for water oxidation. Some N‐doped TiO 2 films have been reported to exhibit anodic photocurrent in aqueous solution under visible‐light irradiation . Mullins et al .…”

Unexpected Chemical and Thermal Stability of Surface Oxynitride of Anatase TiO2 Nanocrystals Prepared in the Afterglow of N2 Plasma

“…Especially, N 2 plasmas may induce the formation of chemically modified structures such as the reduced phases (e.g., TiO 2-x ) [7] and the nitride phases [8,9] for enhanced absorption of visible light. However, the facile diffusion of nitrogen species into the bulk of TiO 2 under typical treatment conditions at elevated temperatures may pose a technological limit in 'surface-selective' modification.…”