Facile synthesis and enhanced visible light photocatalytic activity of N and Zr co-doped TiO2 nanostructures from nanotubular titanic acid precursors
Min Zhang
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Xinluan Yu
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Dandan Lu
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Jianjun Yang
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Key Laboratory for Special Functional Materials of Ministry of Education, Henan University
, Kaifeng 475004,
People's Republic of China
Zr/N co-doped TiO2 nanostructures were successfully synthesized using nanotubular titanic acid (NTA) as precursors by a facile wet chemical route and subsequent calcination. These Zr/N-doped TiO2 nanostructures made by NTA precursors show significantly enhanced visible light absorption and much higher photocatalytic performance than the Zr/N-doped P25 TiO2 nanoparticles. Impacts of Zr/N co-doping on the morphologies, optical properties, and photocatalytic activities of the NTA precursor-based TiO2 were thoroughly investigated. The origin of the enhanced visible light photocatalytic activity is discussed in detail.
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Background
Recently, nanoscale TiO2 materials have attracted
extensive interest as promising materials for its applications
in environmental pollution control and energy storage
[1]. However, TiO2 is only responsive to UV light ( <
380 nm, 3% to 5% solar energy) due to its large bandgap
energy (typically 3.2 eV for anatase). It hinders the
practical application of TiO2 for efficient utilization of solar
energy [2]. Many studies have been performed to extend
the spectral response of TiO2 to visible light and
improve visible light photocatalytic activity by doping
and co-doping with metals of V, Fe, Cu, and Mo or
nonmetals of N, B, S, and C [3,4]. Among the efforts of
mono-doping, nitrogen-doped TiO2 was considered to
be a promising visible light active photocatalyst. Asahi
et al. reported that the effect of N doping into TiO2
achieved enhanced photocatalytic activity in visible
region than 400 nm [5]. Theoretical works revealed that
the result of the narrowed bandgap is due to N
dopinginduced localized 2p states above the valence band [6].
However, these states also act as traps for
photogenerated carriers and, thus, reduce the photogenerated
current and limit the photocatalytic efficiency.
In order to reduce the recombination rate of
photogenerated carriers in the nitrogen-doped TiO2, co-doping
transition metal and N have been explored [7]. Recently,
theoretical calculations have reported that visible light
activity of TiO2 can be even further enhanced by a suitable
combination of Zr and N co-doping [8]. The Zr/N
co-doping of anatase TiO2 could narrow bandgap by about
0.28 eV and enhance the lifetimes of photoexcited carriers.
Previously, we had fabricated N-doped TiO2 with visible
light absorption and photocatalytic activity using
precursor of nanotubular titanic acid (NTA, H2Ti2O4 (OH)2) [9].
The visible light sensitization of N-doped NTA sample
was due to the formation of single-electron-trapped
oxygen vacancies (SETOV) and N doping-induced bandgap
narrowing. It was also found that the N-doped TiO2
prepared by NTA showed the highest visible light
photocatalytic activity compared with the TiO2 prepared by
different other precursors such as P25 [10]. To obtain
further enhanced photocatalytic performance, in this work,
we prepared Zr and N co-doped TiO2 nanostructures
using nanotubular titanic acid (NTA) and P25 as
precursors by a facile wet chemical route and subsequent
calcination. A systemic investigation was employed to reveal the
effects of Zr and N doping/codoping in the enhancement
of visible light absorption and photoactivity of the
codoped TiO2 made by NTA and P25. The results showed
that Zr/N-doped TiO2 nanostructures made by nanotubular
NTA precursors show significantly enhanced visible light
absorption and much higher photocatalytic performance
than the Zr/N-doped P25 TiO2 nanoparticles. This work
provided a strategy for the further enhancement of visible
light photoactivity for the TiO2 photocatalysts in practical
applications.
Methods
Synthesis of NTA precursors
The precursor of nanotubular titanic acid was prepared
and used as a co-doped precursor according to the
procedures described in our previous reports [11-13]. Briefly,
the Degussa P25 TiO2, a commercial standard TiO2
photocatalyst, reacted with concentrated NaOH solution to
obtain Na2Ti2O5 H2O nanotubes, and then, NTA was
synthesized by an ion exchange reaction of Na2Ti2O5 H2O
nanotubes with an aqueous solution of HCl.
Preparation of N and Zr co-doped TiO2
The as-prepared NTA was mixed with urea (mass ratio of
1:2) and dissolved in a 2% aqueous solution of hydrogen
peroxide, followed by the addition of pre-calculated
amount of Zr(NO3)4 5H2O (Zr/Ti atomic ratio, 0%, 0.1%,
0.3%, 0.6%, 1.0%, 5.0%, and 10%). The resultant mixed
solution was refluxed for 4 h at 40C and followed by a
vacuum distillation at 50C to obtain the product of x%
Zr/N-NTA. Final Zr/N co-doped TiO2 were prepared by
the calcination of x% Zr/N-NTA at a temperature range
of 300C to 600C for 4 h. The target nanosized TiO2
powder was obtained, denoted as x% Zr/N-TiO2
(temperature), for example 0.6% Zr/N-TiO2(500). For
reference, Degussa P25 TiO2 powders were used a (...truncated)