J Mater Sci (2015) 50:21–27 DOI 10.1007/s10853-014-8441-7
Synthesis and characterization of TiO2/WO3 composite nanotubes for photocatalytic applications Xiaofei Qu • Dandan Xie • Lei Gao Lixin Cao • Fanglin Du
•
Received: 17 March 2014 / Accepted: 1 July 2014 / Published online: 16 October 2014 Ó Springer Science+Business Media New York 2014
Abstract TiO2/WO3 composite nanotubes were synthesized in an anodic aluminum oxide (AAO) template by a sol–gel method. The prepared nanotubes were characterized by transmission electron microscopy, scanning electron microscopy, powder X-ray diffraction, and Brunauer– Emmett–Teller surface area. Using the nanotubes embedded in the AAO templates as catalysts, photocatalytic degradation of methyl orange aqueous solution was carried out under UV light irradiation. The results showed that the TiO2/WO3 composite nanotubes with the thickness about 50 nm could be successfully synthesized by this method. TiO2 showed anatase phase and WO3 displayed monoclinic phase. The composite nanotubes (TiO2/WO3) exhibited higher photocatalytic activity than the pure nanotubes (WO3 or TiO2). The possible reason for improving the photocatalytic activity was also discussed.
Introduction As we all know, azo dye is one of toxic substances to human and animals with one or more azo bonds chromophores to an aromatic ring [1]. It is difficult to be degraded and causes several environment problems [2, 3]. Numerous efforts have been devoted to deal with the problems, such as physical,
X. Qu D. Xie L. Gao F. Du (&) College of Materials Science and Engineering, Qingdao University of Science and Technology, Zhengzhou Road 53, Qingdao 266042, China e-mail:
[email protected] L. Cao (&) Institute of Materials Science and Engineering, Ocean University of China, Songling Road 238, Qingdao 266100, China e-mail:
[email protected]
chemical, and biologic means. However, they all suffer from the incomplete degradation of the pollutants, which could cause secondary pollution. Recently, the technology of photodegradation based on TiO2 has been proved to be an effective method to degrade azo dye. For this technology, there are several advantages such as wide application, no toxicity, and a strong destructive power to the pollutants [4]. Although TiO2 owns many advantages, most applications of TiO2 still suffer from the easy recombination of the electron–hole pairs and the absorption of light only at ultraviolet (UV) wavelengths. In order to improve the TiO2 photocatalytic activity, studies have been focused on two main directions: (1) reduce the recombination rate of photogenerated electron–hole and extend its useful operating range, and (2) increase its surface area and modify the adsorption properties [5]. Up to now, many researches have been reported that the photocatalytic properties can be effectively modified by dye sensitization [6, 7], deposition of noble metals [8–10], and coupling with other semiconductors, such as SnO2 [11], ZnO [12], CdS [13], ZrO2 [14], Fe2O3 [15], WO3 [16, 17], and SiO2 [18]. TiO2 coupled with another semiconductor can produce the so-called charge separation effect to extend the lifetime of electron–hole pairs [19, 20]. Among the coupled materials, TiO2/WO3 is one of such compositions that hold great promise [21–23]. Since WO3 has a band gap of *2.8 eV, which is a little narrower than TiO2 (*3.2 eV), it can absorb more visible light from the sun. In addition, WO3 is resistant to acidic media, which makes it a powerful catalyst in the presence of organic acids [24]. Up to now, TiO2/WO3 coupled structures have been prepared by various methods, such as sol–gel [4, 17], templating [25], co-precipitation [26, 27], and hydrothermal synthesis [28, 29]. Among these methods, the sol–gel technique has attracted a lot of attention because of low
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cost and processing simplicity. The morphologies of the TiO2/WO3 prepared by sol–gel method have been mainly disordered nanoparticles [30, 31] and films [32–34]. For disordered nanoparticles, the separation of these powdered photocatalysts from suspended solution could be very difficult. For the thin films, they have the coarse surface and infirm force shortcomings [35]. To the best of our knowledge, this is the first report related to the preparation of the coaxial TiO2/WO3 core– shell nanotubes using sol–gel method in an anodic aluminum oxide (AAO) template. TiO2/WO3 nanostructure in the form of nanotube array would own large surface area and adjustable structure which could improve its photocatalysis efficiency to some extent. And comparing to the other structures, it is more convenient to separate from suspended solution for the nanotube arrays. The process is feasible and environment friendly.
Experimental Materials AAO templates (*200 nm) were purchased from Whatman Co. Ammonium tungstate hydrate (H40N10O41W12xH2O), TiF4, alcohol, and H2SO4 (95–98 wt%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Methyl orange was purchased from Tianjin Guangcheng Chemical Reagent Co., Ltd. The distilled water was used for the preparation of all the catalysts, as well as to dilute methyl orange solution.
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with X-ray powder diffraction (XRD, D/MAX-rA). The specific surface area was determined by the Brunauer– Emmett–Teller (BET, SA-3100) method using a Surface Area Analyzer after being degassed at 200 °C for 2 h with nitrogen as the adsorbent. Characteristics of photocatalytic activity Photocatalytic activity of TiO2/WO3 composite nanotubes was determined by degrading methyl orange (MO) (5 mg/L, pH 3) at room temperature. The maximum absorbance for MO at 506 nm was used as the wavelength for monitoring MO degradation. The size of the sample used in the photocatalytic degradation depended on the volume of MO solution (1.5 mL/cm2). After stirred in the dark for 2 h, MO solution was irradiated under a Hg lamp (300 W) with the main wavelength of 254 nm. The proper amount of MO solution was taken out every 30 min to measure the absorption with the assistant of UV–Vis spectrometer (UV-2550). In order to see the structure of the prepared samples clearly, for TEM analyses, nanotube materials were extracted from the AAO templates by dissolving the AAO templates in 3 M NaOH solutions. For SEM test, some samples were also free from the AAO templates. For XRD analyses and photocatalytic activity, the samples were all embedded in the AAO templates.
Results and discussion Characterization of TiO2/WO3 composite nanotubes
Synthesis of TiO2/WO3 composite nanotubes Morphology TiO2/WO3 composite nanotubes were synthesized in an AAO template by a sol–gel method. In our case, a pressure infiltration process was applied. Briefly, AAO template was first immersed in the ammonium tungstate solution (0.001 M) and the process was kept at 80 °C for 12 h during infiltration. After that, the sample was taken out and baked at 80 °C for 1 h. Then, the AAO template with the WO3 precursor in it was immersed in TiF4 solution (0.04 M), and the process was kept at 60 °C for 9 min. Finally, an organized array of TiO2/WO3 nanotubes with TiO2 on the inner surface of the WO3 nanotubes was obtained after heating at 600 °C for 3 h. In order to make a comparison, pure TiO2 and pure WO3 arrays were also prepared using the similar process. Materials characterization The morphological studies were carried out with transmission electron microscopy (TEM, JEOL JEM-2100) and scanning electron microscopy (SEM, JSM-6700F). The crystallographic structure of the samples was determined
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The structures of TiO2, WO3, and TiO2/WO3 nanotubes were investigated by TEM. Figure 1a, b is the typical TEM images of TiO2 nanotube and WO3 nanotube, respectively. They all exhibited the structure of single-layer nanotube. The wall thickness of TiO2 nanotube (Fig. 1a) and WO3 nanotube (Fig. 1b) was *30 and *20 nm, respectively. For TiO2/WO3 composite nanotubes (Fig. 1c), the wall thickness was *50 nm, which was thicker than that of pure nanotube (TiO2 or WO3). In addition, an obvious double-layer structure could be seen from Fig. 1c, which confirmed that after coating, WO3 nanotubes could be uniformly covered with TiO2 layer. The morphologies of the samples were further characterized by SEM. In order to see the nanotubes clearly, samples were all free from the AAO templates except that shown in the inset of Fig. 2c. For TiO2 (Fig. 2a) and WO3 (Fig. 2b), the structure of nanotube could be clearly seen. Comparing to the thickness of TiO2 or WO3 nanotubes, it was obviously thicker for the thickness of TiO2/WO3
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Fig. 1 TEM analyses of a TiO2 nanotubes, b WO3 nanotubes, and c TiO2/WO3 composite nanotubes
composite nanotubes (Fig. 2c). The inset of Fig. 2c showed the TiO2/WO3 composite nanotube array embedded in the AAO template. It was obvious that the AAO template was the texture with honey comb inside. And comparing to the pore diameter of the empty AAO template (*200 nm), it was smaller for the TiO2/WO3 composite nanotube array (*90 nm). So it was safe to say that the TiO2/WO3 composite nanotubes could be successfully synthesized using the sol–gel method assisted with the AAO template.
Fig. 2 SEM analyses of a TiO2 nanotubes, b WO3 nanotubes, and (c) TiO2/WO3 composite nanotubes
Phases XRD patterns of the samples prepared at 600 °C are shown in Fig. 3. From Fig. 3d, five main peaks at 2h = 25.2°, 37.8°, 47.9°, 55.0°, and 62.7° corresponded to the (1 0 1), (0 0 4), (2 0 0), (2 1 1), and (2 0 4) reflections of anatase
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♣
(2 0 0)
(2 0 4)
♦
♦
♣
(c)
♦
160 140 120 100 80 60 40 20 0
(2 0 4)
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0.0
(b)
30
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50
(a)
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70
2θ/(°)
Fig. 3 XRD patterns of (a) AAO template, (b) TiO2 nanotubes imbedded in an AAO template, (c) WO3 nanotubes imbedded in an AAO template, and (d) the TiO2/WO3 composite nanotubes imbedded in an AAO template
Quantity Adsorbed (cm 3/g STP)
♦ TiO2 JCPDS#21-1272 ♣ WO3 JCPDS#43-1035
0.2
0.4
0.6
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1.0
Relative Pressure (P0/P) 60
20
(a)
Adsorption Desorption
180
-20 (2 1 1)
(0 0 4)
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(2 0 0)
(1 0 1)
Intensity/a.u.
(d)
Quantity Adsorbed (cm 3/g STP)
♣♣
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(2 1 1)
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(0 0 4)
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200
(0 0 2) (2 0 0) (1 2 0) (-1 1 2)
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(1 0 1) (-1 1 2)
(0 0 2)
24
(b)
Adsorption Desorption
50 40 30 20 10 0 0.0
0.2
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1.0
Relative Pressure (P0/P)
BET surface area analysis Figure 4 shows the N2 adsorption–desorption isotherms. Results from nitrogen sorption measurement showed typeIV isotherms. The BET surface areas of TiO2 nanotube, WO3 nanotube, and TiO2/WO3 nanotube were 119.04, 23.96, and 31.75 m2/g, respectively (Table 1). The increase of surface area would benefit the adsorption of dye and could increase the photocatalytic activity. Photocatalytic application of TiO2/WO3 composite nanotubes The photocatalytic activities of all prepared photocatalysts were evaluated by the degradation of MO in aqueous
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Quantity Adsorbed (cm 3/g STP)
TiO2 (JCPDS no. 21-1272), and the other peaks at 2h = 23.1°, 28.6°, and 45.4° could be indexed to the (0 0 2), (-1 1 2), and (-3 1 2) reflections of monoclinic phase of WO3 (JCPDS no. 43-1035). In addition, XRD patterns taken from pure TiO2 nanotubes (Fig. 3b) and pure WO3 nanotubes (Fig. 3c) were in good agreement with the reported literatures [36, 37].
(c)
Adsorption Desorption
80
60
40
20
0 0.0
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Relative Pressure (P0 /P)
Fig. 4 Nitrogen adsorption–desorption isotherm of a TiO2 nanotubes, b WO3 nanotubes, and c TiO2/WO3 composite nanotubes
solution (pH 3) and the results are shown in Fig. 5. For all samples, the decrease of the absorbance at 506 nm was used to monitor the degradation of the dye. For TiO2/WO3 nanocatalyst (Fig. 5c), the maximum absorption peaks of
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0.7
Sample
TiO2
WO3
TiO2/WO3
0.6
SBET (m2/g)
119.04
23.96
31.75
0.5
j/min
0.0060
0.0043
0.0180
R2
0.999
0.996
0.930
Absorbance
Table 1 Surface area and apparent rate constant j/min of TiO2 nanotubes, WO3 nanotubes, and TiO2/WO3 composite nanotubes
(a) 0 min 30 min 60 min 90 min 120 min 150 min 180 min
0.4 0123h
0.3 0.2 0.1 0.0 350
400
450
500
550
600
650
Wavelength (nm) 0.8 0 min 30 min 60 min 90 min 120 min 150 min 180 min
0.7
Absorbance
0.6 0.5 0.4
0123h
(b)
0.3 0.2 0.1 0.0 350
400
450
500
550
600
650
Wavelength (nm) 0.7
0 min 30 min 60 min 90 min 120 min 150 min 180 min
0.6 0.5
Absorbance
methyl orange at 506 nm diminished gradually and almost disappeared for 150 min, while it still existed after 180 min exposure time for the pure TiO2 (Fig. 5a) and pure WO3 (Fig. 5b) nanocatalysts. In order to see the result of photocatalytic activity directly, the photographs for the corresponding materials are shown in the inset of Fig. 5. Among all the samples, the better photocatalytic activity could be directly seen by our naked eyes for the TiO2/WO3 composite nanotubes embedded in AAO template. It was reported that the enhanced photocatalytic activity of TiO2/ WO3 could be attributed to the presence of the intimately bonded TiO2/WO3 surface hetero-structure. The structure was helpful to the separation of the photogenerated electrons and holes, resulting in the decrease of the electron– hole pair recombination rate [38]. The methyl orange degradation efficiency of all the samples over a period of time is exhibited in Fig. 6. The degradation efficiency of the as-synthesized samples was defined as C/C0, where C0 was the initial concentration of MO after equilibrium adsorption, and C was the concentration during the reaction. For TiO2/WO3 photocatalyst, the degradation efficiency after 180 min was about 97 %, while it was about 70 and 55 % for the TiO2 and WO3, respectively. In addition, the variations in ln(C0/C) as a function of irradiation time (t) are given in Fig. 7. In order to infer the reaction kinetics of photocatalytic degradation of MO, the data of ln(C0/C) are also calculated according to the equation ln(C0/C) = jt, where j was the apparent rate constant. The result indicated that the photocatalytic degradation reaction followed pseudo-first-order kinetics. The calculated j data for each photocatalyst are listed in Table 1. From the data, we could see that the photocatalyst TiO2/WO3 composite nanotubes owned the best photocatalytic properties. The corresponding correlation coefficients R2 for the samples are also shown in Table 1. The increased photocatalytic activity of the coupled TiO2/WO3 photocatalyst is mainly attributed to the synergistic effect on photocatalytic properties. Comparing to the energy level of TiO2, the potentials of the conduction band (CB) and the valence band (VB) of WO3 are charged a bit more positive. When WO3 (Eg = 2.8 eV) and TiO2 (Eg = 3.2 eV) form a coupled photocatalyst, upon UV irradiation, the photoexcited electrons of TiO2 conduction band will be transferred to conduction band of WO3. At the
0.4
(c)
0123h
0.3 0.2 0.1 0.0 350
400
450
500
550
600
650
Wavelength (nm)
Fig. 5 Adsorption spectra of methyl orange solutions in the presence of a TiO2 nanotubes imbedded in an AAO template, b WO3 nanotubes imbedded in an AAO template, and c TiO2/WO3 composite nanotubes imbedded in an AAO template irradiated by a UV lamp at different periods of time
same time, the photogenerated holes will migrate to valence band of TiO2, causing effective separation of photoexcited electron–hole pairs within the TiO2/WO3
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Conclusions
1.0 0.9 0.8
C/C0
0.7 0.6 0.5
(b)
0.4
(a)
0.3 0.2 0.1
(c)
0.0 0
20
40
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80
100
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140
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180
Time (min)
Fig. 6 Comparison of photocatalytic decomposition rates of methyl orange of (a) TiO2 nanotubes imbedded in an AAO template, (b) WO3 nanotubes imbedded in an AAO template, and (c) TiO2/WO3 composite nanotubes imbedded in an AAO template
3.5
(c)
3.0
TiO2/WO3 composite nanotubes were successfully synthesized via sol–gel process assisted with porous AAO template. The wall thickness was *50 nm, which was thicker than that of pure nanotube (TiO2 or WO3). The length of the coaxial TiO2/WO3 core–shell nanotubes was about 50 lm, corresponding to the thickness of the AAO template. The technique could be extended to produce other large size and well-ordered core–shell arrays. The photocatalytic activities of the prepared samples (TiO2, WO3, TiO2/WO3 nanotubes) were evaluated by the degradation of MO in aqueous solution (pH 3). The photocatalytic degradation followed pseudo-first-order kinetics. Compared with pure WO3 and pure TiO2 nanotubes, TiO2/WO3 composite nanotubes exhibited a higher photocatalytic activity, which could be attributed to the enhancement in charge separation efficiency. Acknowledgements This work was financially supported by Natural Science Foundation of China (Grant No. 51272115) and The Scientific Research Encouragement Foundation for Outstanding Young and Middle Aged Scientists of Shandong Province, China (Grant No. BS2013CL025).
2.5
ln(C0 /C)
2.0
References
1.5
(a) 1.0
(b) 0.5 0.0 -0.5 0
30
60
90
120
150
180
Irradiation time / min
Fig. 7 Linear transform ln(C0/C) = f (t) of the kinetic curves of MO degradation of (a) TiO2 nanotubes imbedded in an AAO template, (b) WO3 nanotubes imbedded in an AAO template, and (c) TiO2/WO3 composite nanotubes imbedded in an AAO template
photocatalyst. The enhanced charge separation related to the TiO2/WO3 composite photocatalyst was helpful to the higher activity of the TiO2/WO3 photocatalyst. It is worthy to note that the main purpose of the paper is to report a new method for preparing well-ordered arrays of TiO2/WO3 nanotube coaxial core–shell heterojunctions, which are deposited directly into the pore channels of porous AAO templates ordered in a honeycomb arrangement. Furthermore, comparing to the disordered powders, photocatalysts in the form of nanotube array in AAO templates are more convenient for separating from suspended solution.
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