N2/Ar plasma induced doping of ordered mesoporous TiO2 thin films for visible light active photocatalysis

Abstract

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...

Synthesis and Catalytic Applications of Non-Metal Doped Mesoporous Titania

“…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.…”

“…Since then, different mesoporous structures of TiO 2 have been synthesized including 2D hexagonal, 3D cubic, and lamellar using methods including sol-gel, hydrothermal, solvothermal, microwave, and sonochemical methods [65,[72][73][74][75][76][77][78][79][80][81][82][83][84][85]. A variety of morphologies have also been reported due to the interplay between mesostructure nucleation, orientation of mesophases at the surface of surfactant/precursor aggregates, and growth of organic/titania composites.…”

“…Of all forms, thin mp-TiO 2 films are highly relevant to photoelectrocatalysis both for practical reasons (they can readily be deposited onto electrodes) and because they provide fundamental insight into the interplay of composition, processing, and surface chemistry. For instance, in our group, we have prepared mp-TiO 2 thin films using Pluronic triblock copolymer surfactants with cubic mesostructure [65] and orthogonally oriented 2D hexagonally close packed (o-HCP) cylindrical nanopores [82][83][84] using the evaporation induced self-assembly (EISA) technique by manipulating process conditions. Figure 2 shows computer-generated illustrations of typical mesoporous structures and mesophases such as Im3m cubic mesoporous material, o-HCP mesoporous film, parallel HCP mesoporous film, and lamellar mesophase where higher intensity (yellow and red) corresponds to a greater density of hydrophobic tails of the surfactant template.…”

“…For instance, when mp-TiO 2 is synthesized in an acidic medium, a nitrogen source that makes the precursor solution basic cannot be used. This can be especially limiting because acidic solutions are sometime used to prepare surfactant-templated mp-TiO 2 thin films in order to stabilize low-molecular weight titanates [65]. In this case, hydrazine cannot be added in the precursor solution because the increase in pH will induce titania precipitation.…”

“…Various strategies have been reported to introduce non-metal dopants into mp-TiO 2 , but the properties of the doped mp-TiO 2 product are strongly dependent on the dopant source and strategy. For instance, we recently reported that plasma treatment of mp-TiO 2 introduces a high level of substitutional nitrogen into the lattice, whereas methods based on chemical precursors are more prone to generating interstitial nitrogen [65]. The photocatalytic activity of the materials is closely related to its preparation method, so understanding the relationship between synthetic strategy, structure, and performance is key to moving towards highly efficient visible-light photocatalysis using mp-TiO 2 based materials.…”