29This study reports the rapid and effective nitrogen doping of ordered, mesoporous TiO 2 thin 30 films using nitrogen/argon (N 2 /Ar) plasma. The resulting nitrogen-doped TiO 2 (N-TiO 2 ) films 31 show a significant enhancement in both visible light absorption and photocatalytic activity. The 32 cubic ordered mesoporous TiO 2 thin films are prepared via a sol-gel method using titanium 33 tetrachloride (TiCl 4 ) as precursor and triblock copolymer Pluronic F127 as the template. 34 Following brief calcination, the TiO 2 films are treated with N 2 /Ar plasma under controlled 35 conditions of reactive gas pressure, microwave power, and plasma exposure duration. To vary 36 the degree of nitrogen doping, the plasma exposure time varied from zero up to 210 min. The 37 nitrogen content of the films increases with plasma exposure duration, up to over 3 at% N. X-ray 38 photoelectron spectroscopic (XPS) analyses and UV-vis absorbance spectra of N-TiO 2 films 39 indicate that the incorporated N atoms reduce the band gap of TiO 2 and thus enhance the 40 absorption of visible light. Finally, the visible-light photocatalytic activity of N-TiO 2 films is 41 determined from the photocatalytic degradation of methylene blue under visible-light 42 illumination (with a 455 nm LED). The N-TiO 2 films prepared by 150 min treatment show the 43 optimum photocatalytic activity with a pseudo-first order rate coefficient of 0.24 h -1 , which is six 44 times greater than that of undoped TiO 2 films. Treatment for excessive time (e.g. 210 min) leads 45 to a decline in photocatalytic activity due to coarsening of the porous structure. The present study 46 suggests that plasma-induced doping is a promising approach to enable the efficient 47 incorporation of heteroatoms into surfactant-templated TiO 2 thin films while maintaining their 48 nanostructures, thereby leading to the significant enhancement of visible-light photoactivity. 49 50 51 3 1. Introduction 52 Since Fujishima first reported H 2 generation by splitting water with a TiO 2 photocatalyst, 53 TiO 2 has attracted much attention due to many advantageous properties including its low cost, 54 high availability, chemical stability, and excellent optoelectronic properties [1-4]. These unique 55 properties have enabled titania to be utilized in a wide range of applications including solar 56 energy conversion, antimicrobial agents, whiteners in paint, ceramics, textiles, personal care 57 products, and catalysts for environmental remediation [4-12]. 58 Despite many attractive features of TiO 2 , one critical challenge is the innate inability of TiO 2 59 to absorb visible light [5]. The wide band gap of TiO 2 allows the absorption of solar light mainly 60 in the ultraviolet (UV) range, which corresponds to only 8% of the whole solar spectrum, while 61 visible light constitutes 47% [3]. To reduce the intrinsic band gap of TiO 2 , several strategies have 62 been tested including the incorporation of either metallic (e.g. Fe and Ni) or non-metallic (e.g. C, 63 F, N, S, P and B) atoms...

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.

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