Solar Energy Materials & Solar Cells 92 (2008) 1–10
The structure and photocatalytic studies of N-doped TiO
2
films
prepared by radio frequency reactive magnetron sputtering
Baoshun Liu
a,b,
Ã
, Liping Wen
a
, Xiujian Zhao
a
a
Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Ministry of Education, Wuhan, Hubei 430070, PR China
b
The Institution of Material Science and Technology, Wuhan University of Technology, Wuhan, Hubei 430070, PR China
Received 19 July 2007; accepted 20 July 2007
Abstract
N-doped TiO
2
films were prepared by a radio frequency reactive magnetron sputtering (RF-MS) deposition method from an undoped
TiO
2
target in a mixture of Ar/N
2
atmosphere on heated quartz glass substrates. The structures and properties of the N-doped were
studied by XRD, Raman, XPS, TEM, ultraviolet (UV)–vis and PL spectroscopy. By analyzing the structures and photocatalytic
activities of undoped and N-doped TiO
2
films under ultraviolet and visible light irradiation, the probable photocatalytic mechanism of
N-doped TiO
2
films was investigated. Because many oxygen defects are caused in films by nitrogen doping, it is presumed that nitrogen
doping and oxygen defect induced the formation of new states closed to the valence band and conduction band, respectively. The
cooperation of nitrogen and oxygen defects leads to a significant narrowing of the band gap and greatly improves the absorption in the
visible light region. It is found that the degradation efficiencies of N-doped TiO
2
films greatly decreased under ultraviolet irradiation, but
slowly improved under visible light irradiation, compared with the undoped TiO
2
film. It is suggested that the N-doped TiO
2
films are
formed for the nitrogen to occupy oxygen defect sites directly. The doped nitrogen ions and oxygen defects act as recombination centers
that reduce the lifetime of photo-induced electrons and holes, thereby resulting in the decrease of photocatalytic activity under ultraviolet
light illumination.
r 2007 Elsevier B.V. All rights reserved.
Keywords: N-doped TiO
2
films; Photocatalysis; Oxygen defects; Methyl orange
1. Introduction
TiO
2
is the most widely used photocatalyst for the
decomposition of various organic pollutants because of its
inexpensiveness and nontoxicity. Moreover, the photoca-
talysis based on TiO
2
is found to have several advantages
over conventional oxidation processes, such as complete
mineralization of the pollutants, use of the near- ultraviolet
(UV) or solar light, no addition of other chemicals,
operation at room temperature, and so on [1,2]. Although
it has been widely investigated in the past decade, some
problems still remain in its practical applications [3–7]. The
band gap energy of TiO
2
(3.0–3.2 eV) requires UV light
irradiation, and thus only a small portion of the solar
spectrum is absorbed in the UV region [8]. Hence, much
effort has been devoted to preparing a TiO
2
-based
photocatalyst that is capable of efficient utilization of the
visible light constituting the main part of the solar
spectrum. Up to now, many strategies, including the
doping of TiO
2
with transition metals, the synthesis of
reduced forms with TiO
2Àx
structure by plasma treatment,
and the doping of TiO
2
with anionic nonmetals have been
investigated.
Even though the modified TiO
2
by doping with
transition metals can be prepared successfully, only a few
can indeed improve the photocatalytic activity [9–11].
However, most of them reduce the photoactivity because of
their role as recombination centers [12,13]. Furthermore,
many transition metals are toxic and can diffuse from TiO
2
to the environment during the application, which may lead
to a second pollution and limit its further studies. In
ARTICLE IN PRESS
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doi:10.1016/j.solmat.2007.07.009
Ã
Corresponding author. The Institution of Material Science and
Technology, Wuhan University of Technology, Wuhan, Hubei 430070,
PR China. Tel./fax: +86 27 87669729.
E-mail address: liubaoshun@126.com (B. Liu).