Metal ion and N co-doped TiO 2 as a visible-light photocatalyst
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Jun Nunoshige, Tsuyoshi Takata, Junko N. Kondo, Michikazu Hara, and Kazunari Domen Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan (Received 11 November 2003; accepted 17 March 2004)
Powders of TiO2 doped with a metal ion and N species were prepared by a polymerized complex method and the visible-light photocatalytic activities of the products are investigated. Of the metal ions studied (K+, Ca2+, Sr2+, Ba2+, Nb5+, Fe3+, Zn2+, and Al3+), the photocatalyst prepared with Sr2+ exhibits the highest activity for acetaldehyde decomposition under visible-light irradiation. Results obtained from x-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analyses suggest that the doped N species reside at interstitial lattice positions in the catalyst. It was also found by XPS and ESR measurements that the doped N species combine with lattice oxygen to give rise to a paramagnetic property. The visible-light response of the catalyst is driven by the formation of paramagnetic N species at interstitial positions in the TiO2 lattice.
I. INTRODUCTION
Photocatalysts have been studied extensively for application in water and air purification, and TiO2 is currently the most widely used photocatalyst, offering both high photocatalytic activity and excellent chemical stability. However, TiO2 photocatalysts are active under near-ultraviolet irradiation, with a band gap energy of 3.0–3.2 eV. Many researchers have devoted study to the preparation of visible light driven TiO2 by doping the catalyst with a transition metal to change the band gap energy.1–5 As an example, Cr3+-doped TiO2 prepared using a flame reactor has a response in the visible light region, but the photocatalytic activity is much lower.2 The doped Cr3+ acts as an electron-hole recombination site, significantly suppressing the photocatalytic activity even under ultraviolet light irradiation. TiO2 catalysts doped with Cr3+ by ion-implantation absorb visible light longer than 550 nm, and catalysts thus prepared have been demonstrated to decompose NO into N2, O2, and N2O under visible light irradiation.3–5 It has also been found that NOx-doped TiO2 photocatalysts exhibit activity under visible-light irradiation.6 The doped NOx generates impurity levels in TiO2 similar to metal ion dopants. Recently, N-doped TiO2 was found
DOI: 10.1557/JMR.2004.0269 2100
J. Mater. Res., Vol. 19, No. 7, Jul 2004
to exhibit visible-light photocatalytic activity.7 The doped N becomes fixed at O sites in the TiO2 lattice, observed as a 396 eV N1s peak in x-ray photoelectron spectra, narrowing the bandgap and allowing the absorption of visible light. It has been reported that doping TiO2 with N under NH3 calcination at high temperature and at higher dosages results in the formation of O vacancies and Ti3+, which act as electron-hole recombination sites to suppress photocatalytic activity.8 Considering these previous results, the present authors postulated that the amount of N that could be doped into the TiO2 l
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