J Sol-Gel Sci Technol (2009) 50:98–102
DOI 10.1007/s10971-009-1903-8
ORIGINAL PAPER
Phosphorous, nitrogen, and molybdenum ternary co-doped TiO2:
preparation and photocatalytic activities under visible light
Yanfang Shen Æ Tianying Xiong Æ Hao Du Æ
Huazi Jin Æ Jianku Shang Æ Ke Yang
Received: 23 October 2008 / Accepted: 16 January 2009 / Published online: 6 March 2009
Ó Springer Science+Business Media, LLC 2009
Abstract P, N, and Mo ternary co-doped nano TiO2
photocatalysts ((P, N, Mo)-TiO2) were prepared by a single
step sol–gel method, which show much enhanced photocatalytic activities over Mo-TiO2, (P, N)-TiO2, un-doped
TiO2 and Degussa P25 under visible light irradiation. The
degradation rate of 0.72Mo–P-TiO2 is as high as 65.3%,
which is about 6.7 times of that of Degussa P25. Possible
reasons for the improvement of photocatalytic activities
were analyzed.
Keywords TiO2 (P, N, Mo)-ternary Co-doping Visible-light Photocatalysis
1 Introduction
TiO2 is a kind of promising photocatalyst in great variety
areas, such as photo decomposition of water to reserve
hydrogen energy [1, 2], dye-sensitized solar cells [3, 4],
anti-bacteria and anti-virus [5, 6], photo degradation of
pollutants in water, and air [7, 8], and so on. However,
there exists a bottle neck for TiO2, its large band gap (about
3.2 eV for anatase); makes it almost impossible to absorb
Y. Shen T. Xiong (&) H. Du H. Jin K. Yang
Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
e-mail: [email protected]
Y. Shen
Graduate School of Chinese Academy of Sciences, Beijing
100086, China
J. Shang
Department of Materials Science and Engineering, University of
Illinois at Urbana-Champaign, Urbana 1L61801, USA
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visible light. It is known that visible light is the main part
of solar spectrum, containing about 48% of solar energy.
Therefore, it is important to make TiO2 sensitive to visible
light.
Inserting new energy levels of defects in the forbidden
gap by doping is an accessible way to develop visible light
sensitive TiO2 photocatalysts. So far, a lot of works have
been reported on the metal doped TiO2 and non-metal
doped TiO2 [9–13]. There have also been some reports on
the metal and non-metal ions, or two kinds of non-metal
ions co-doped TiO2 as visible light induced photocatalysts.
Sakatani et al. prepared the metal ion and nitrogen
co-doped TiO2 as a visible light photocatalyst by a polymerized complex method [14]. Gao et al. prepared the TiO2–
N–x%WO3 composite photocatalysts by introducing WO3
into nitrogen-doped TiO2 [15]. Liu et al. synthesized (S,
La2O3)-co-doped TiO2 by situ hydrothermal method [16].
In this work, P, N, and Mo co-doped TiO2 photocatalysts were prepared by a sol–gel method, which is easilyoperated and likely-industrialized. Their photocatalytic
activities under visible light were also investigated. Furthermore, the reason for the enhanced photocatalytic
activities is discussed.
2 Experimental
In the experiment, all chemicals used were of analytical
reagent grade. Ammonium hydrogen molybdenum phosphate, (NH4)3H4[P4(Mo2O7)6], was used as the precursor of
P, N, and Mo. The P, N, and Mo ternary co-doped TiO2
photocatalysts were named as x%Mo–P–N-TiO2 by the
atomic ratio of Mo. For a typical synthesis, 1 g Cetyl
Trimethyl Ammonium Bromid (CTAB) was dissolved in a
mixture of 10 mL Tetrabutyl Titanate (TTOB), 100 mL
J Sol-Gel Sci Technol (2009) 50:98–102
dry ethanol, and 6 mL nitric acid (70%) to make a
homogeneous solution. Then 15 mL 0.064 M water solution of (NH4)3H4[P4(Mo2O7)6] was added dropwise under
strong stirring. The obtained sol was left in a thermost at
40 °C for 24 h, and then dried at 60 °C in an oven to get
the gel. At last, the gel was calcined at 500 °C in static air
for 4 h to get the 0.36Mo–P-TiO2. Mo-doped TiO2 were
prepared by the same process in which ammonium
molybdate was used as the precursor of Mo. (P, N)-binary
co-doped TiO2 was prepared by mechanical alloying
method using (NH4)2HPO4 as the source of P and N. Undoped TiO2 was also prepared as a reference.
X-ray diffraction (XRD) patterns of the samples were
recorded using an XD-3A diffractometer (Shimadazu
Corp., Japan, Cu Ka). UV-Vis absorption spectra were
recorded using a UV-550 (Jasco, Japan.). The chemical
states of the P and Mo doped into the TiO2 were analyzed
using a PHI5300 photoelectron spectrometer system (PE
Corp., USA.) with an Al Ka source (1486.6 eV). N2
adsorption–desorption isotherms at 77 K were measured
using a Micrometrics ASAP2010 system to get the Brunauer-Emmett-Teller (BET) surface areas of the products.
To evaluate the photocatalytic activities of the catalysts,
sulfosalicylic acid (SSA) and methylene blue (MB) were
used as photodegradation targets. A 13 watt fluorescent
lamp was used as the light source, and a filter was adopted
to exclude the UV light (less than 5%). The initial
concentrations of SSA and MB solutions were 40 mg/l
and 20 mg/l, respectively. The doze of catalysts added was
1 g/l.
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Fig. 1 XRD patterns of (P, N, Mo)-TiO2 and Mo-TiO2 photocatalysts: a 0.36Mo–P–N-TiO2, b 0.72Mo–P–N-TiO2, c 0.36Mo-TiO2, d
0.72Mo-TiO2, e (P, N)-TiO2, f un-doped TiO2
3 Results and discussions
XRD patterns of (P, N, Mo)-ternary co-doped TiO2 photocatalysts are presented in Fig. 1. In case of (P, N, Mo)TiO2 photocatalysts, anatase (JCPDS, No. 21-1272) is the
only phase in the samples, except for 0.72Mo–P–TiO2, in
which MoO3 (JCPDS, No. 65-2421) as a second phase.
This indicates that MoO3 will form in the (P, N, Mo)-TiO2
photocatalysts when the amount of Mo is large enough.
The co-existence of anatase and MoO3 is beneficial for
improving the photocatalytic activities of (P, N, Mo)-TiO2
photocatalysts, which was proven by the results of photodegradation experiment as shown in Fig. 5a, b. In case of
Mo-TiO2 and un-doped TiO2 photocatalysts, anatase is the
only phase in the samples. For (P, N)-TiO2 photocatalysts,
both anatase and rutile phases were detected, but the
amount of anatase is more than rutile.
TEM image of 0.72Mo–P–N-TiO2 is presented in
Fig. 2. It can be seen that the morphology of the samples is
of spherical shape and the average particle size is about
50 nm.
Fig. 2 TEM image of 0.72Mo–P–N-TiO2
N2 adsorption–desorption isotherms of (P, N, Mo)-TiO2
are presented in Fig. 3. It can be seen that the adsorption–
desorption isotherms of the samples belong to type IV, the
characteristic of mesoporous materials. A hysteresis loop
(0.4 \ P/P0 \ 0.7) appears on the adsorption–desorption
isotherm. The type of the hysteresis loop is H1, suggesting
the existence of pores with homogeneous size and morphology. The BET surface areas of 0.36Mo–P–N-TiO2 and
0.72Mo–P–N-TiO2 are 55 and 158 m2/g, respectively.
The UV-Vis absorption spectra of the (P, Mo, N)-TiO2,
Mo-TiO2 and (P, N)-TiO2 photocatalysts are recorded in
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J Sol-Gel Sci Technol (2009) 50:98–102
Table 1 The absorption-thresholds and band gap of the prepared
samples
Fig. 3 N2 adsorption–desorption isotherms of (P, Mo)-TiO2 photocatalysts: a 0.36Mo–P–N-TiO2, b 0.72Mo–P–N-TiO2
Fig. 4. All these photocatalysts show much enhanced
absorption in the visible light region than the un-doped
TiO2 and Degussa P25. And the absorbance of (P, N, Mo)TiO2 in the visible light region is better than those of MoTiO2 and (P, N)-TiO2, which are also sensitive to visible
light. According to Asahi’s method [11], the absorptionthresholds of 0.36Mo–P–N-TiO2 and 0.72Mo–P–N-TiO2
are 565 and 570 nm, respectively, and the band gap of
which are 2.19 and 2.18 eV, respectively. For comparison,
the absorption-thresholds and band gap of the prepared
samples are listed in Table 1. It is worth pointing out that
the absorbance of 0.72Mo–P–N-TiO2 begins to increase
from about 475 nm. The results indicate that (P, N, Mo)ternary co-doping is better than Mo-doping and (P, N)binary co-doping in enhancing the absorption abilities of
TiO2 in the visible light region, which suggests that (P, N,
Mo)-ternary co-doping may efficiently reduce the band gap
Fig. 4 UV-Vis absorption spectra of (P, Mo, N)-TiO2, Mo-TiO2, and
(P, N)-TiO2 photocatalysts: a 0.36Mo–P–N-TiO2, b 0.72Mo–P–NTiO2, c 0.36Mo-TiO2, d 0.72Mo-TiO2, e P25, f (P, N)-TiO2, g undoped TiO2
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Samples
Abs.-threshold (nm)
Band gap (eV)
0.36Mo–P–N-TiO2
565
2.19
0.72Mo–P–N-TiO2
570
2.18
0.36Mo-TiO2
510
2.43
0.72Mo-TiO2
505
2.46
P25
400
3.1
(P, N)-TiO2
450
2.76
Un-doped TiO2
440
2.82
of TiO2 compared with Mo-doping and (P, N)-binary
doping. Like many other metal or unmetal doped TiO2
catalysts, (P, N, Mo)-ternary co-doped TiO2 catalysts are
colorful. The colors of (P, N, Mo)- TiO2, Mo-TiO2 catalysts are both green, and the color of (P, N)-TiO2 catalysts
are vivid yellow.
Surface chemical states of P, N, and Mo in the sample of
0.72Mo–P–N-TiO2 are shown in Fig. 5. The graphical
separation of complex spectra bands was fulfilled by means
of PEAKFIT program. The P2p peak locating at 133.4 eV
could be deconvoluted into contributions from the P5?
oxidation state, corresponding to the phosphate anion
([PO4]3-) [17]. The Mo3d state can be fitted into three
peaks, locating at 229.7, 232.65, and 235.6 eV, which can
be further denoted to the oxidation states of Mo4?, Mo5?,
and Mo6?, respectively [18]. It should be pointed out that
XPS is much more sensitive than XRD, and it can only give
the chemical states of surfaces. This is the reason why no
corresponding phases of Mo4? and Mo5? were detected by
XRD (seen in Fig. 1). Figure 5c presents the XPS spectra
of N1s and Mo3p3/2 peaks. PEAKFIT program was used to
fit the whole peak into two peaks. The one located at
396.4 eV belongs to N1s state, originating from the
stretching of Ti–N–O band [11]. While the other one
locating at 393.9 eV is the typical peak of Mo3p3/2.
In order to evaluate the photocatalytic activities of (P, N,
Mo)-TiO2, experiments on photodegradation of MB and
SSA were carried out under visible light irradiation. Blank
experiments without adding catalysts in the target solutions
were also done. Before irradiation, the target solutions with
and without catalysts were lay in the dark under strong
stirring for 30 min to reach adsorption–desorption balance
on the surfaces of the catalysts.
Kinetics of (P, N, Mo)-TiO2 and Mo-TiO2 in SSA
photodegradation under visible light are presented in
Fig. 6a. All the (P, N, Mo)-TiO2, Mo-TiO2 and (P, N)-TiO2
photocatalysts display enhanced photocatalytic activities
over un-doped TiO2 and P25 under visible light. The
photocatalytic activities of (P, N, Mo)-TiO2 are the best
among all the samples, indicating P, N, and Mo ternary
J Sol-Gel Sci Technol (2009) 50:98–102
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Fig. 6 a Kinetics of (P, N, Mo)-TiO2 and Mo-TiO2 in SSA
photodegradation under visible light; b Kinetics of (P, N, Mo)-TiO2
and Mo-TiO2 photocatalysts in MB photodegradation under visible
light: a 0.36Mo–P–N-TiO2, b 0.72Mo–P–N-TiO2, c 0.36Mo-TiO2, d
0.72Mo-TiO2, e (P, N)-TiO2, f pure TiO2, g P25, h Blank
Fig. 5 XPS spectra of 0.72Mo–P–N-TiO2 after sputtering with Ar?
for 60 s: a P2p peak; b Mo3d peaks, c N1s and Mo3p3/2 peaks
co-doping is more effective on improving the photocatalytic activities of TiO2 than just Mo-doping and (P, N)binary co-doping. On the other hand, the photodegradation
rate of 0.72Mo–P-TiO2 is as high as 65.3%, which is about
6.7 times of Degussa P25.
Kinetics of (P, N, Mo)-TiO2 and Mo-TiO2 in MB photodegradation under visible light are presented in Fig. 6b.
Similar with the results in SSA photodegradation, (P, N,
Mo)-TiO2 exhibits the best photodegradation abilities
among all the samples. And the photodegradation abilities
of (P, N)-TiO2 are better than those of Mo-TiO2, un-doped
TiO2, and P25. The results suggest that (P, N, Mo)-ternary
co-doping can improve the photocatalytic activities of TiO2
distinctly. The possible reason is (P, N, Mo)-ternary
co-doping can introduce P-, N- acceptor levels above the
valance band and the Mo- donor level below the conduction band of TiO2, which can reduce the band gap of TiO2
and make TiO2 absorb visible light. Further experiments
are under going to clarify the mechanism of (P, N, Mo)ternary co-doping on improving the photocatalytic activities of TiO2 under visible light.
4 Conclusions
(P, N, Mo)-ternary co-doped TiO2 photocatalysts were
synthesized by a sol–gel method, and show highly efficient
photocatalytic activities in SSA and MB photodegradation
under visible light. The photocatalytic activities of (P, N,
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Mo)-TiO2 are far better than those of Mo-TiO2, (P, N)TiO2, un-doped TiO2 and P25. The results suggest that P,
N, and Mo ternary co-doping may narrow the band gap of
TiO2, resulting in great enhancement of the photocatalytic
activities under visible light.
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