Abstract
In the present work, W–Bi–S-tridoped TiO2 nanoparticles were synthesised by a simple sol–gel method. The structure and morphology of as-prepared W–Bi–S-tridoped TiO2 were characterised by using X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy and UV–Vis diffuse reflectance spectrum. It was found that the W–Bi–S-tridoped TiO2 nanoparticles were well crystallised with a small size distribution. The absorption edge of TiO2 was extended into visible-light region obviously after being doped with W, Bi and S. The photocatalytic degradation of methylene blue was used as a probe reaction to evaluate the efficiency of W, Bi and S doping. The W–Bi–S-tridoped TiO2 exhibited the best photocatalytic activity, compared with TiO2, S-TiO2, W–S–TiO2 and Bi–S–TiO2. The mechanism that enhanced photocatalytic activity might be attributed to the synergistic effect of W, Bi and S.
Introduction
With the development of science and technology, a large amount of organic pollutants were emitted into environment, thus causing a series of environmental problems.1–3 The photocatalytic oxidation of organic pollutants into CO2 and H2O was considered as an efficient process for solving the environmental problems. In this field, TiO2 was found to be an outstanding photocatalyst due to its non-toxicity, good chemical stability, low cost and high oxidising capacity.4,5 However, TiO2 has a wide band gap (3.0 eV for rutile and 3.2 eV for anatase) and could only absorb the ultraviolet light (wavelength < 400 nm), which is only approximately 5% of the solar spectrum.6,7 This caused less utilisation of solar energy. Thus, an important strategy to enhance the photocatalytic activity of TiO2 is the expansion of light response range to visible light. In the past few decades, various attempts have been made, for example, surface photosensitisation, noble metal deposition, composites with other semiconductor, ion doping and so on.8–15 As one of the most widely used methods, doping ions had three effects: (i) inhibiting the recombination of electrons and holes; (ii) forming the doping level in the forbidden band, thus making the electrons of TiO2 semiconductor to be excited with visible light to produce electron–hole pairs and (iii) introducing defect position on the surface of the semiconductor or changing the crystallinity, thus forming a trap centre for electron or hole and improving the separation efficiency of the photo-generated electron–hole pairs.16–20 It is noted that previous studies mainly focused on single-element doping modification of TiO2.12,15,18–23 The doping elements were reported with different roles on the enhanced photocatalytic activities. Recently, two elements codoping TiO2 were mostly studied based on the synergistic effect of both the elements, in which the activity of photocatalyst always was significantly enhanced.24–27 Both the doping elements are not only beneficial to the separation of photogenerated electron and hole pairs, but also feasible to the absorption of visible light.
Although W, Bi and S single-element or double-element doped TiO2 have been reported in the previous literature with higher photocatalytic activity under visible light,28–31 the research for W–Bi–S-tridoped TiO2 had not been reported so far. In our previous work, keeping the amount of the sulphur constant throughout, we successfully prepared Mo–Bi–S-, Mo–Sb–S- and W–Sb–S-tridoped TiO2 by a simple sol–gel method, and found that the three elements also show synergistic effect for enhanced photocatalytic activity of TiO2. 32 We also found that W-doped TiO2 can significantly improve the photocatalytic activity when compared with Mo-doped TiO2. So, in the present work, we prepare W–Bi–S-tridoped TiO2 photocatalyst to evaluate the efficiencies and the synergetic effects of such doping elements on the photocatalytic activity of TiO2. The as-prepared W–Bi–S-tridoped TiO2 nanoparticles were characterised by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV–Vis diffuse reflectance spectrum (UV–Vis DRS). The photocatalytic activity of W–Bi–S-tridoped TiO2 was investigated by using the degeneration of methylene blue (MB) solution.
Experimental methods
Chemicals
Tetrabutyl titanate was purchased from Alfa with the purity of 99.8%. Sodium tungstate dihydrate (Na2WO4·2H2O, 99.5%), bismuth (III) nitrate pentahydrate (Bi(NO3)3·5H2O, 99.5%) and Thiourea (CS(NH2)2, 99%) were furnished by Aladdin. The templating agent Triblock copolymer polyoxyethylene–polyoxypropylene–polyoxyethylene (P123, PEO20–PPO70–PEO20, MW = 5800) was supplied by Sigma-Aldrich. All the other reagents were used with analytical grade (99%) and high-purity water was used throughout the experiments.
Fabrication of W–Bi–S-tridoped TiO2
The procedure for the fabrication of W–Bi–S-tridoped TiO2 consists of the preparation of several solutions, namely A, B, C and D. Typically, solution A was prepared by dissolving 2.0 g of P123 and 0.039 g of thiourea in 20 mL of anhydrous ethanol, until it became a colourless transparent solution; solution B was obtained by adding 10 mL of 10 mol L−1 HCl drop by drop into 6.8 mL of tetrabutyl titanate and then vigorously stirring for 20 min. Solution C was prepared by mixing solutions A and B under vigorous stirring. After being stirred for 2 h, an appropriate amount of Na2WO4·2H2O and Bi(NO3)3·5H2O with a molar ratio of W:Ti = 0.008 and Bi:Ti = 0.010 were added into solution C to obtain solution D. After that, solution D was vigorously stirred for 6 h and then aged for 24 h at room temperature. The obtained sol was further dried at 80°C for 12 h until a dry gel was obtained. The obtained dry gel was finally calcined at 500°C for 3 h with a heating rate of 5°C min−1. After being cooled, the product was ground and the W–Bi–S-tridoped TiO2 was obtained. For comparison, W–S-TiO2, Bi–S–TiO2, S–TiO2 and pure TiO2 were also prepared using the same experimental methods. The process flowchart is illustrated in Scheme 1.
Process flowchart for the preparation of the photocatalysts
Characterisations
The powder XRD patterns were recorded by the Philips X'Pert pro MPD X-ray diffractometer using Ni-filtered Cu Kα1 irradiation (36 kV, 20 mA). The surface morphology and composition analysis of the products were performed on a JEOL JSM-7610F SEM equipped with an energy-dispersive spectrometer (EDS). Transmission electron microscopy micrographs were recorded on a JEOL JEM-2100 microscope operating at 200 kV. UV–Vis diffuse reflectance spectrum for the evaluation of optical properties was measured on a Varian Cary-5000 spectrophotometer (America) equipped with an integration sphere, using BaSO4 as a reference.
Photocatalytic activity
The photocatalytic activities of various samples were evaluated by degrading MB (C16H18N3S) under irradiation of visible light. In a typical procedure, 0.10 g of catalysts were suspended in 150 mL MB (35 mg L−1) aqueous solution (before the experiment, the pH of MB solution was adjusted to 9.0 with ammonia). Prior to irradiation, the suspension was stirred in dark for 30 min magnetically to reach adsorption/desorption equilibrium. The solution was then irradiated by a 350 W xenon lamp with a 420 nm cut-off filter. The solution's temperature was maintained at 20°C by using circulating cooling water, from which 5 mL was taken at intervals and centrifuged immediately. The remaining concentration of MB was analysed by a visible-light spectrophotometer (N2-7230G, Shanghai Precision Instrument Co., Ltd.) at 650 nm.
Results and discussions
Characterisations of W–Bi–S-tridoped TiO2
The crystallographic structures of W–Bi–S–TiO2, as well as TiO2, S–TiO2, W–S–TiO2 and Bi–S–TiO2, were characterised by XRD. As shown in Fig. 1, all diffraction peaks of W–Bi–S–TiO2 exhibit the presence of anatase phase of TiO2 (PDF 21-1272) without any other TiO2 phases or any impurity phases,
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indicating the negligible influence from the doping elements. This might be attributed to the low concentration of the doping elements and the uniform dispersion in TiO2 crystal architecture. However, as for TiO2, S–TiO2, W–S–TiO2 and Bi–S–TiO2, there were diffraction peaks of rutile TiO2 phase (JCPDS No. 21-1276) besides the anatase phase,
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suggesting that the four samples were of mixed phases of TiO2. The position of the doped foreign ions in TiO2 depends on the ionic size.
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W6 + ions with a radius of 0.60 Å, which is slightly smaller than Ti4+ (0.69 Å),
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should play the role of substitution dopants of Ti4 + in the lattice of TiO2. Bi3 + ions with a radius of 1.03 Å might be incorporated between the lattice sites of TiO2. To some extent, the crystal phase and grain size of TiO2 were determined by the dopants of W, Bi and S simultaneously. Thus, we could conclude that tri-doping with W, Bi and S could effectively retard the phase transformation of TiO2 from anatase to rutile. The average grain size of W–Bi–S-tridoped TiO2 was estimated to be 12 nm, according to the Debye–Scherrer equation.
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Powder XRD patterns of pure TiO2 and doped TiO2 nanoparticles calcined at 500°C
Figure 2 shows the TEM images of W–Bi–S-tridoped TiO2, which was prepared at the initial molar ratio of W:Bi:S:Ti = 0.008:0.010:0.026:1. Obviously, all the particles with slight agglomeration exhibit sphere-like shape with a mean size of approximately 10–15 nm, which is well consistent with the XRD result.
Transmission electron microscopy image of the prepared W–Bi–S-tridoped TiO2 calcined at 500°C
Figure 3a demonstrated the SEM image of W–Bi–S-tridoped TiO2. As seen, the tridoped TiO2 nanoparticles consisted of numerous relatively uniform big particles with a diameter of 1–3 μm, which could be attributed to the agglomerates of W–Bi–S-tridoped TiO2 nanocrystals. It could also be seen that the surface for the samples is quite rough, which would increase the specific surface area and active sites and thus enhance the photocatalytic activity of W–Bi–S-tridoped TiO2. The elemental composition was further investigated by EDS as shown in Fig. 3b. The peaks were well indexed to be S, Bi, W, Ti and O, indicating the successful doping of S, Bi and W. The average molar ratio of W:Bi:S:Ti was calculated to be 0.0080:0.0113:0.0240:1, which was in good agreement with the preparation of tridoped TiO2 (0.008:0.010:0.026:1). So, it could be deduced that the present sol–gel method was efficient to prepare W-, Bi- and S-tridoped TiO2.
a Scanning electron microscopy image and b EDS spectrum of W–Bi–S-tridoped TiO2 nanoparticles calcined at 500°C
UV–Vis diffuse reflectance spectrum is an efficient means to detect the presence of framework- and non-framework-incorporated transition metal species in crystal structures, thus distinguishing the coordination states of the elements and measuring the optical properties of solid powders.32,35 To provide insight into the effect of W, Bi and S on optical property of TiO2, the UV–Vis DRS spectra of W–Bi–S-tridoped TiO2 were investigated and are shown in Fig. 4. Obviously, the introduction of W, Bi and S has modified the absorption characteristics of TiO2. The absorption intensity of W–Bi–S-tridoped TiO2 was stronger in UV–visible region and the absorption range shifted to a longer wavelength, which fell into the visible range as compared with the other samples. This means that the W–Bi–S doping can expand the photoresponse range and thus enhance the photovoltaic activity. As known, this is crucial for TiO2 material as the absorption region for TiO2 falls in the UV zone, which only consists of 5% in the solar light.
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This behaviour may be explained by the synergistic effect of W, Bi and S. And the cooperative effect of anion–cation had a strong mutual structural influence on the band structure, and hence changed the band gap value eventually.
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In terms of W–Bi–S-tridoped TiO2, the improvement in the absorption properties was much more pronounced, thus the number of photogenerated electrons and holes to participate in the photocatalytic reaction would be even more and further enhanced the photocatalytic activity of TiO2 ultimately.
UV–Vis diffuse reflectance spectra of the samples calcined at 500°C
Photocatalytic activity
To evaluate and compare the photocatalytic activity, the photocatalytic degradation of MB was performed under visible-light irradiation. Figure 5 shows temporal evolution of the concentration changes during the photodegradation of MB over the catalysts. All the doped samples showed a higher photocatalytic activity than undoped TiO2, which removed only 17.3% of the initial MB after 180 min. The trend of photodegradation activities were in the order of W–Bi–S–TiO2 > Bi–S–TiO2 > W–S–TiO2 > S-TiO2 > TiO2. As expected, W–Bi–S-tridoped TiO2 exhibited the best photocatalytic activity with 98.7% degradation rate of MB within 180 min. However, the degradation rate of MB for Bi–S–TiO2, W–S–TiO2 and S–TiO2 were 89.9, 74.9 and 66.4%, respectively. The reason for this result might be related to the synergistic effect of W, Bi and S, which caused the absorption edge extension to the visible-light range.
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The photocatalytic activities of TiO2 might also be attributed to the catalyst-specific surface area, crystalline phases and crystallinity.
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As discussed above, W–Bi–S-tridoped TiO2 nanoparticles had a relatively rough surface area with small grain size; thus, it could supply much more active sites for photocatalytic degeneration of MB.
Methylene blue degradation on the different samples under the visible-light irradiation
Metal ions with higher redox potential indoped TiO2 could efficiently capture the photogenerated electrons and holes, and thus enhance the photocatalytic activity. 39 In the case of W–Bi–S-tridoped TiO2, W6 + ion with positive reduction potential could serve as a powerful electron scavenger to trap the photogenerated electrons in the conduction band, while Bi3 + ion has strong oxidation potential and might act as the hole trapper during the visible-light irradiation. Both of them would be in favour of inhibiting the recombination of electrons and holes. In addition, S doping could engender an isolated intermediate valence band and narrow the band gap, 40 and thereby induce visible-light adsorption at sub-band gap energy. As a consequence, it was indeed and reasonable that the W–Bi–S-tridoped TiO2 displayed the highest photocatalytic activity resulting from the synergistic effects of S, Bi and W doping.
Conclusion
In summary, W–Bi–S-tridoped TiO2 photocatalysts have been successfully synthesised through a facile sol–gel method in the presence of P123. The photocatalytic performance of W–Bi–S-tridoped TiO2 in photocatalytic degradation of MB was extraordinarily higher than that of W–S–TiO2, Bi–S–TiO2, S–TiO2 and TiO2. Tri-doping with W, Bi and S elements could effectively retard the phase transformation of TiO2 from anatase to rutile. And the remarkable photocatalytic activity of the as-synthesised W–Bi–S–TiO2 could be ascribed to the synergistic effects of rough surface area, good atanase crystallinity, small crystalline size, the intense absorption in the visible-light range, the narrow band gap caused by the existence of S-doping level and the effective inhibition of the recombination of photoinduced charge carriers. There might be a potential opportunity to use the W–Bi–S-tridoped TiO2 photocatalysts for waste water treatment in the future.
Footnotes
Acknowledgements
This work was supported by the Basic and Advanced Technology Research of Henan Province (No. 132300410143) and Science & Technology Research Key Project of Henan Province Education Department (No. 13a430068), China.