Preparation and water-splitting photocatalytic behavior of S-doped WO3

Applied Surface Science 263 (2012) 157–162

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Preparation and water-splitting photocatalytic behavior of S-doped WO3 Wenzhang Li a , Jie Li a,∗ , Xuan Wang a,b , Qiyuan Chen a a Key Laboratory of Resources Chemistry of Nonferrous Metals (Ministry of Education), School of Chemistry and Chemical Engineering, Central South University, Changsha 410083 China b Shenyang Aluminum and Magnesium Engineering and Research Institute, Shenyang 110001 China

a r t i c l e

i n f o

Article history: Received 17 April 2010 Received in revised form 20 July 2012 Accepted 5 September 2012 Available online 13 September 2012 Keywords: Sulfur-doping Photocatalysis Tungsten oxide Water-splitting

a b s t r a c t In the present work, sulfur (S)-doped tungsten oxide (WO3 ) was studied by photoelectrochemical and photocatalytic methods in order to evaluate the photoactivity and the possibility of its application in water splitting. S-doped WO3 powders were prepared by solid-state annealing method using thiourea as S precursor. The crystal structure, composition and morphology of pure and S-doped WO3 were compared using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The influence of doping concentration and annealing temperature on the properties of S-doped WO3 powders and their photocatalytic activities under both ultraviolet (UV) and visible light (VIS) irradiation compared with WO3 were investigated. The results indicate that sulfur can be doped successfully into WO3 and the S-doped WO3 powder annealed at 500 ◦ C exhibited the highest photocatalytic activity under both UV and visible light irradiation. The highest average oxygen evolution rate of S-doped WO3 under UV (99.9 ␮mol L−1 g−1 h−1 ) and visible light (76.7 ␮mol L−1 g−1 h−1 ) irradiation is 1.25 and 1.57 times the value of that of undoped WO3 , respectively. © 2012 Elsevier B.V. All rights reserved.

1. Introduction After the Honda-Fujishima effect was discovered in 1972 [1], the possibility of water splitting using semiconductor photocatalysts was confirmed, and increasing attention was drawn to this research field. Tungsten oxide (WO3 ) was initially used as a kind of material for the photodecomposition of water by Hodes [2]. Compared with titanium oxide (TiO2 ), its smaller band gap (2.5–2.8 eV) guaranteed that WO3 would absorb more solar energy and thus generate a larger photocurrent. Moreover, WO3 is stabile against photocorrosion [3] and has satisfactory photoelectron transport properties [4]. It is also an effective photocatalyst for the evolution of oxygen. In fact, many reports discuss the use of WO3 as a stable photocatalyst for water-splitting [5–9]. Satisfactory photocatalysts for water splitting are few in number. The semiconductor material to be used must be chemically inert, conductive, and should have a band gap that can make full use of the solar spectrum (∼1.6 eV) effectively. The conductivity and band gap of such materials can be altered by chemical substitution or doping procedures. Doping of some metal ions may promote photocatalytic activity, but the metal-doping of semiconducting oxides is generally thermo-labile and introduces recombination between photogenerated electrons and holes [10]. Thus, the doping

∗ Corresponding author. Tel.: +86 731 8887 9616; fax: +86 731 8887 9616. E-mail addresses: [email protected] (J. Li), [email protected] (Q. Chen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.021

of nonmetal elements has attracted considerable attention [11–13]. Asahi was the first to demonstrate the possibility of modification by doping nonmetal elements into TiO2 through theoretical calculations [14]. It has been reported that the band gap of WO3 can be significantly narrowed to lower than 2.0 eV by N-doping [15–17], thus also supposedly greatly improving its photo absorption performance. Due to degradation of the electron transport properties resulting from its highly defective lattice, however, the additional carriers generated upon illumination cannot be extracted, and the actual photocurrent generated was found to be smaller than that of undoped WO3 . Thus, enhancing the conductivity of photocatalysts is an important point to consider. To the best of our knowledge, the sulfur-doping (S-doping) of WO3 for water-splitting has not been reported yet, though many researchers have discussed the possibility of modifying TiO2 by Sdoping [18–28]. S2− doping may be difficult to carry out due to the large formation energy required because its ionic radius is significantly larger than that of O2− [18,19,27]. In S-doped TiO2 samples prepared using thiourea as S precursor [21–23,25], Ti4+ ions were substituted by S4+ or S6+ ions, and S-doped TiO2 showed a strong visible light response. Considering the ionic radius of W6+ is close to that of Ti4+ , W6+ might be substituted by S cation. It is expected that the effective modification of WO3 could be realized by S-doping. In this study, a series of S-doped WO3 samples with different amounts of S were prepared via a solid-state reaction at lowheating temperature (∼500 ◦ C) using thiourea as sulfur precursor. The influence of doping concentration and annealing temperature

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on the properties of S-doped WO3 powders and their photocatalytic activities under both ultraviolet (UV) and visible light (VIS) irradiation compared with WO3 were investigated. 2. Experimental 2.1. Preparation process of WO3 and S-doped WO3 powders S-doped photocatalysts were prepared by low temperature solid-state annealing method. The WO3 powders used in this experiment were obtained by annealing ammonium tungstate ((NH4 )6 H5 [H2 (WO4 )6 ]·H2 O) for 5 h at 600 ◦ C. Certain amounts of thiourea (H2 NCSNH2 ) and WO3 were mixed in an organic aqueous solution according to the formula m(S)/m(WO3 ) × 100%(wt%) = ε%. After grinding in an agate mortar for 30 min, the precursors were dried at 80 ◦ C in a thermostatic air-blower-driven drying closet. The prepared precursors (ε = 2.0) were then annealed for 2 h at temperatures of 300, 400, 500, 600, and 700 ◦ C, obtaining SW-300, SW-400, SW-500, SW-600, and SW-700, respectively. The compounds SW1.0, SW-2.0, SW-3.0, and SW-4.0 were obtained when the samples were annealed at 500 ◦ C at ε = 1.0, 2.0, 3.0, and 4.0, respectively. 2.2. Characterization of WO3 and S-doped WO3 photocatalysts The morphologies and sizes of S-doped WO3 and WO3 particles obtained were observed by SEM (JSM-5600LV, JEOL Ltd., Japan). The crystalline structures of the samples were measured by X-ray diffraction (XRD, D/Max2250, Rigaku Corporation, Japan) using graphite monochromatic copper radiation (Cu K␣,  = 0.154056 nm) at 40 kV and 300 mA. All bonding energies were identified by XPS (X-ray photoelectron spectroscopy) with Mg K˛ radiation (energy = 1253.6 eV, 16 mA × 12 kV) (K-Alpha 1063, Thermo Fisher, Britain) and then calibrated to the C1s peak at 284.7 eV of the surface of adventitious carbon. Sulfur concentrations of S-doped WO3 samples were measured by an energy dispersive X-ray spectroscopy (EDAX, EDX-GENESIS 60S, USA). The resistivities were measured by a 4-point probe resistivity measurement system (RST-9, Probes Tech. China). UV–vis DRS measurements (versus BaSO4 ) were recorded on a Purkinje General U-1901 spectrophotometer (China). 2.3. Photoelectrochemical and photocatalytic properties of WO3 and S-doped WO3 photocatalysts A powder sample (WO3 and S-doped WO3 ) was sonicated in ethanol solution and drop-casted on the fluorine-doped tin oxide (FTO) electrodes with the aid of a micro-syringe. Drying under flowing air during the fabrication of photoelectrodes assisted fast evaporation of the ethanol, leaving the powder homogeneously deposited on FTO surface. Photoelectrochemical measurements were carried out in a standard three-compartment cell consisting of a working electrode, a Pt foil counter electrode, and an Ag/AgCl/satd. KCl electrode was employed as the reference electrode. The I–V characteristics were recorded by a potentiostat (Zennium, Zahner, Germany) in 0.5 M H2 SO4 electrolyte. A 500 W Xe lamp (CHF-XM35, Trusttech Co. Ltd, Beijing) served as the visible light source with an intensity of 100 mW/cm2 . A 400 nm cutoff filter was placed into the path of the Xe lamp to remove the UV irradiation. Photocatalytic reactions were carried out in a self-made hollow jacked quartz reactor with an interior illuminant (high-pressure mercury lamp, emission wavelength of 300–400 nm, 250 W; highpressure short arc xenon lamp, dominant frequency of 500 nm, 250 W). Two lamps served as simulative light sources of UV and VIS irradiation. Distilled water was boiled for 20 min to remove air in the system before the reactions and then used to fill the

Fig. 1. SEM images of the WO3 and SW-500 samples.

reactor together with the photocatalyst (3.57 g L−1 ) after cooling down to room temperature. The pH was kept to 2.0 at the beginning, with the concentration of the electron acceptor Fe3+ at 16.0 mmol L−1 . A magnetic stirrer was employed to maintain the suspension of the photocatalyst, and the reaction system was controlled at room temperature by external circulating water. The gas volumes were determined by measuring the volumes of the water that spilled out from the reactor. The obtained gases were analyzed by gas chromatography (Shimadzu GC-8A, Japan; Ar carrier). The rate of oxygen formation is expressed in millimoles of produced oxygen per gram of catalysts per hour in per liter of suspension (␮mol L−1 g−1 h−1 ). 3. Results and discussion 3.1. Morphologies of the WO3 and S-doped WO3 powders The SEM images of WO3 and SW-500 are shown in Fig. 1. The particle sizes are estimated to be in the region of 200 nm, with no significant variation when sulfur is incorporated into the compound. The unlisted SEM images of other samples are similar, indicating that annealing temperature exerts minimal effects on the shapes and sizes of S-doped WO3 particles. After a second annealing no agglomeration was observed. This may be caused by electrostatic repulsion of nonmetal ions, which can restrain the agglomeration [29]. 3.2. Crystal structures of the WO3 and S-doped WO3 powders The structures of the WO3 and S-doped WO3 powders were evaluated by X-ray diffraction (as shown in Fig. 2). Both WO3 and

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when annealed at 300 ◦ C, 42.4 nm at 400 ◦ C, 43.6 nm at 500 ◦ C, 43.9 nm at 600 ◦ C and 46.2 nm at 700 ◦ C. That is, the grain size increased with a corresponding increase in annealing temperature, which is in agreement with the research on the preparation of WO3 samples via an electrochemical deposition technique [30] and a gas evaporation method [31]. In contrast, Sun et al. [32] found that the grain size of their WO3 samples decreased with an increase in annealing temperature when prepared via a spray pyrolysis method. These differences are probably due to the different synthetic methods. In the preparation of N-doped WO3 and S-doped TiO2 , Cole et al. [17] and Yu et al. [25] found that the grain size decreased with an increase in doping concentration. Sun et al. [32] and Ho et al. [26] found that the grain size increased with an increase in doping concentration for C-doped WO3 and S-doped TiO2 , respectively. In the current research, when the concentration of sulfur is increased, the grain sizes of different S-doped WO3 samples grow larger (in Table 1). This suggests that the incorporation of sulfur leads to the crystal growth of monoclinic WO3 during annealing process, resulting in a larger crystallite size for the S-doped WO3 .

3.3. Compositions of the WO3 and S-doped WO3 powders Fig. 3(A) shows the high-resolution XPS spectra of the S 2p region of the S-doped WO3 powder. The sulfur atoms are all in the state of S6+ , with a peak in 168.5 eV [33]. It should be noted that no peaks were found around 160–161 eV, which corresponds to the W S bond when S atoms replaced O atoms on the WO3 surface. S2− ˚ doping may be difficult to carry out because its ionic radius (1.7 A) ˚ [18,19,27], which was significantly larger than that of O2− (1.22 A) leads to the large formation energy required for substitutionally forming W S bonding instead of W O bonding. Thus, the substitution of W6+ by S6+ is more favorable than replacing O2− with S2− , and subsequently W O S bonds form. This is consistent with the results reported by Ho et al. [26] and Dong et al. [33]. Fig. 3(B) shows the XPS spectra for the O 1s region of WO3 and SW-2.0 samples. On the O 1s region of the XPS spectrum of both WO3 and SW-2.0 samples, an unsymmetrical O 1s peak was observed, which means that two chemical forms of O-atoms exist in these powders. Therefore, the O 1s peak of the samples was deconvoluted into two separate peaks using Gaussian distributions. Fig. 3(B) gives the fitting results of the O 1s region of WO3 and SW2.0 samples. The O 1s region is contributed to by two aspects. The main contribution is the oxygen in the lattice (O2− ), whose binding energy is 530.30 eV [34]. The minor contribution is assigned to adsorbed oxygen in the form of OH on the surface, whose banding energy is 531.10 eV [35]. Based on the obtained XPS spectra, the contents of the lattice oxygen (O (a)) and adsorbed oxygen (O (b)) on the surface of WO3 and SW-2.0 samples were calculated and listed in Table 2. It can be seen from Table 2 that the content of surface hydroxyl groups is much higher in the S-doped sample than in the pure sample. The increase in OH content is beneficial for trapping more photogenerated holes and thus inhibiting electron–hole recombination. Fig. 3(C) shows the XPS spectra for W 4f before and after S-doping. Two doublet peaks and the Shirley background are combined to reproduce the W 4f spectra. The highest-intensity doublet

Fig. 2. XRD patterns of the samples of (A) SW-300–SW-700, and (B) WO3 and the samples of SW-1.0–SW-4.0.

Table 1 Material properties for WO3 and S-doped WO3 powders. Sample

D200 (nm)

S (at.%)

Resistivity (cm)

WO3 SW-1.0 SW-2.0 SW-3.0 SW-4.0

34.8 43.0 43.6 44.3 44.2

0.00 0.98 1.87 2.36 2.98

135.8 108.4 52.8 48.2 46.3

S-doped WO3 have significant diffraction peaks at 23.10◦ , 23.58◦ , and 24.34◦ , characteristically representing monoclinic tungsten oxide (PDF card # 72-0677), and the three main peaks at ∼23◦ are attributed to the (0 0 2), (0 2 0), and (2 0 0) lattice planes of monoclinic WO3 . The grain sizes of the samples based on the (2 0 0) peak (D200 ) calculated from the Scherrer formula [30] were found to be 41.6 nm Table 2 Chemical composition of the WO3 and SW-2.0 samples calculated by XPS. Sample

O (a) 2−

WO3 SW-2.0

W6+

O (b)

O

%

530.30 530.30

92.19 82.25

OH 531.10 531.10

W5+

%

4f7/2

4f5/2

%

4f7/2

4f5/2

%

7.81 17.75

35.43 35.55

37.42 37.55

94.26 90.20

34.50 34.80

36.50 36.80

5.74 9.80

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peak, labeled as a1 and a2, with binding energies of ∼35.5 eV (a1, W 4f7/2 ) and ∼37.5 eV (a2, W 4f5/2 ), respectively, is associated with photoelectrons emitted from W atoms in the +6 oxidation state (W6+ ). Other doublets in the spectra correspond to tungsten suboxides. The doublet found at the lower binding energy ∼0.8 eV (labeled as b1 and b2) is generated by photoelectrons emitted from W atoms in the +5 oxidation state (W5+ ). All these binding energies (as shown in Table 2) are in agreement with the literature values for WO3 [34–39]. According to the literature [40], the intensity ratio of the 4f5/2 to the 4f7/2 component is fixed at 0.75 (I[4f5/2 ]/I(4f7/2 ) = 0.75) when fitting the W 4f spectra. It can be seen from Table 2 that that S-doping caused an increase in W5+ and a decrease in W6+ on the surface of the sample. The sulfur concentrations of the S-doped WO3 samples were measured by EDS as listed in Table 1. It is clearly that the S-dopants concentrations of the S-doped WO3 samples increase with the precursor sulfur concentrations.

3.4. Electronic properties of the WO3 and S-doped WO3 powders Semiconductor resistivity is an expression of effective charge separation and carrier mobility, and samples’ resistivity directly affected its photocatalytic activity [15,17]. As shown in Table 1, the resistivity measured by 4-point probe measurements decreased from 135.8 cm (WO3 ) to 46.3 cm (SW-4.0) with increase of sulfur concentrations. Grain size in polycrystalline WO3 plays a key role in establishing effective charge separation. In our research, the grain sizes increase with the concentration of sulfur; the larger the grains, the smaller the number of grain boundaries that will impede free electron transfers, thus decreasing resistivity [17,41]. Besides the inherent properties and carrier concentration of the materials [15], oxygen vacancies are also factors that can affect resistivity. Large amounts of oxygen vacancies could generate crystallographic shear (CS) planes that can increase resistivity [15,17]. In our case, with reference to the XRD analysis, these CS planes were not observed. Moreover, oxygen vacancies also serve as electron traps and provide electron, which are considered free and move through the material via polaronic transportation [17]. The increase in adsorbed oxygen and substoichiometric WO3 is able to generate more oxygen vacancies. This can be explained by S-doping, which converts W6+ to W5+ (XPS measurements) by charge compensation, as supposed by Yu [42]. Meanwhile, carbonaceous residue from the organic aqueous solution could also act as a reducing agent to generate oxygen vacancies [43].

3.5. Absorption properties of the WO3 and S-doped WO3 powders

Fig. 3. High-resolution XPS spectra of the (A) S 2p region, (B) O 1s region and (C) W 4f region for WO3 and SW-2.0 samples.

In Fig. 4, the UV–vis reflectance of S-doped WO3 samples with different sulfur contents and annealed under different temperatures are compared with that of WO3 . Noticeable red-shifts of the fundamental absorption edge are observed. The edge of the samples shifted from 475 nm (2.61 eV) for WO3 to 490 nm (2.53 eV) for SW-2.0 sample. Moreover, the absorbance increases with the sulfur concentration of S-doped WO3 samples. These results indicate that the sulfur atoms are indeed introduction into the lattice of WO3 , thus altering its crystal and electronic structures. According to theoretical calculations for TiO2 [14,18,20,23], S-doping could form intermediate energy levels above the valence band (VB), which should play a significant role in the photoresponse of TiO2 . Thus, the S-doped material is expected to show high photocatalytic activity under visible light irradiation. Aside from the intrinsic absorption measured by DRS, surface state absorption and absorptions of different types of defects also contribute to the actual absorption spectrum [44]. Extrinsic

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Fig. 5. Photocurrentepotential curve of the samples of WO3 and SW-1.0–SW-4.0 in a 0.5 M H2 SO4 aqueous solution under VIS illumination.

Fig. 4. UV–vis DRS spectra of the samples of (A) SW-1.0–SW-4.0, and (B) SW300–SW-700.

absorption induced by the existence of oxygen vacancies is of great importance in the improvement of catalytic activity.

To compare the photocatalytic activity of the as-prepared samples, photocatalytic water splitting under UV and visible (VIS) light irradiation was performed. Fig. 6 shows the average oxygen evolution rates of WO3 and S-doped WO3 with different doping amounts under 12 h of UV and VIS irradiation. Under UV irradiation, the average amount of oxygen evolution of the sample of SW-2.0 reached its highest value at 99.9 ␮mol L−1 g−1 h−1 compared with 80.2 ␮mol L−1 g−1 h−1 for WO3 , an increase of 25%. With further increasing doping concentration, the photocatalytic activity of the S-doped WO3 decreases gradually. The activity orders of the as-prepared doped samples are in good agreement with their corresponding photocurrent density orders. Since the absorption of UV irradiation did not change significantly (as shown in Fig. 4), the results indicate that higher sulfur doping content could act as recombination centers for the electron/hole pairs [45]. Under VIS irradiation, the average amount of oxygen evolution of the sample of SW-2.0 reached its highest value at 76.7 ␮mol L−1 g−1 h−1 compared with 48.9 ␮mol L−1 g−1 h−1 for WO3 , an increase of 57%. In view of the photocatalytic activity under VIS was greatly improved by S-doping, it can be supposed that a intermediate energy level consisting S 2p orbits appears above

3.6. Photoelectrochemical and photocatalytic activity of the samples The photocurrent of a photoanode indirectly indicates the photocatalytic activity in a photoelectrochemical cell. Fig. 5 displays the photocurrent–potential curve of a photoelectrochemical test of the obtained WO3 and S-doped WO3 thin films in a 0.5 M H2 SO4 solution with illumination of 100 mW/cm2 . All samples exhibit pronounced photocurrent starting at ∼+0.4 V (vs. Ag/AgCl). In comparison to undoped WO3 , the doped sample of SW-2.0 shows a significant enhancement in photoresponse with the photocurrent density increasing from 0.62 to 0.87 mA/cm2 . Furthermore, it was demonstrated that the photocurrent density is strongly affected by doping concentration of sulfur from the linear sweep voltammograms collected from the S-doped WO3 photoelectrodes. The maximum photocurrent was obtained with the SW-2.0 sample and photocurrent decreased with further increase in doping concentration. This tendency may be attributed to the increasing sulfur concentration which will lower the quantum yields. In other words, the S-doping sites could also serve as recombination sites, which will decrease photocurrent density.

Fig. 6. Photocatalytic activity of the samples of WO3 and SW-1.0–SW-4.0 for water splitting to oxygen under UV and VIS irradiation.

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Central Universities (no. 2012QNZT12) and the Postdoctoral Science Foundation of Central South University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12] Fig. 7. Photocatalytic activity of the samples of WO3 and SW-300–SW-700 for water splitting to oxygen under UV and VIS irradiation.

the VB, then electron transition between VB and this level would be excited by VIS irradiation. However, further increasing the Sdoping source resulted in increased visible light absorption but decreased photocatalytic activity because of the fast recombination of electron/hole pairs caused by excessive dopants. The results suggest that the high visible light activity results from the balance between recombination rate of electron/hole pairs and visible light absorption. Annealing temperature is also a contributing factor when sulfur concentration is fixed. As shown in Fig. 7, different annealing temperatures influence average oxygen evolution rates. The photocatalytic activity was negatively influenced when the temperature was excessively low or high. This can be explained by the hypothesis low temperatures inhibited S-doping into the lattice of WO3 while high temperatures promote the evaporation of nonmetal sulfur.

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4. Conclusions The WO3 and S-doped WO3 powders prepared by low temperature annealing are both monoclinic tungsten oxides and the phases do not change significantly with S-doping. The grain size of S-doped WO3 is increased with the S concentrations and its resistivity is reduced. S-doping causes the amount of adsorbed oxygen and W5+ on the surface to increase, so that oxygen vacancies also increase. Moreover, oxygen vacancies can reduce resistivity and increase extrinsic absorption. There are noticeable red-shifts in the fundamental absorption edge of WO3 , and the absorption of visible light increases. Experiments show that, using S-doped WO3 as the photocatalyst, the highest average oxygen evolution rate obtained in an Fe3+ containing suspension is 99.9 and 76.7 ␮mol L−1 g−1 h−1 under UV and VIS irradiation, respectively, which is significantly higher than that of WO3 (80.2 and 48.9 ␮mol L−1 g−1 h−1 ). Acknowledgments This study was supported by the National High Technology Research and Development Program of China (863 Program, grant no. 2011AA050528), the National Nature Science Foundation of China (no. 51072232), the Fundamental Research Funds for the

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