Sensors and Actuators B 140 (2009) 623–628 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Nanoplates of ␣-SnWO 4 and SnW 3 O 9 prepared via a facile hydrothermal method and their gas-sensing property Hui Dong, Zhaohui Li ∗ , Zhengxin Ding, Haibo Pan, Xuxu Wang, Xianzhi Fu ∗ Research Institute of Photocatalysis, Fuzhou University, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, PR China article info Article history: Received 19 March 20 09 Received in revised form 2 May 2009 Accepted 11 May 2009 Available online 19 May 2009 Keywords: SnWO 4 SnW 3 O 9 Nanoplate Hydrothermal Gas-sensing abstract Nanoplates of ␣-SnWO 4 and SnW 3 O 9 were selectively synthesized in large scale via a facile hydrothermal reaction method. The final products obtained were dependent on the reaction pH and the molar ratio of W 6+ to Sn 2+ in the precursors. Theas-prepared nanoplates of ␣-SnWO 4 and SnW 3 O 9 were characterized by X-ray powder diffraction (XRD), N 2 -sorption BET surface area, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The XPS results showed that Sn exists in divalent form (Sn 2+ )inSnW 3 O 9 as well as in ␣-SnWO 4 . The gas- sensing performances of the as-prepared ␣-SnWO 4 and SnW 3 O 9 toward H 2 S and H 2 were investigated. The hydrothermal prepared ␣-SnWO 4 showed higher response toward H 2 than that prepared via a solid- state reaction due to the high specific surface area. The gas-sensing property toward H 2 S as well as H 2 over SnW 3 O 9 was for the first time reported. As compared to ␣-SnWO 4 , SnW 3 O 9 exhibits higher response toward H 2 S and its higher response can be well explained by the existence of the multivalent W (W 6+ /W 4+ ) in SnW 3 O 9 . © 2009 Elsevier B.V. All rights reserved. 1. Introduction Since both SnO 2 and WO 3 are well-known materials in the semiconductor gas-sensor field and have found applications in commercial sensor devices, the studies on the gas-sensing prop- erty of ternary Sn–W–O systems have also attracted a lot of interest [1–3]. Most already reported Sn-based gas-sensing semiconduc- tors contain Sn 4+ as in the case of SnO 2 [4–6]. The inclusions of both Sn 4+ and Sn 2+ have also been reported in some Sn–W–O gas- sensing semiconductors. For example, Solis and Lantto [3] reported the gas-sensing property of Sn x WO 3+x with the atomic ratio x between 1.25 and 2.5. The only reported Sn 2+ -based gas-sensing semiconductor is ␣-SnWO 4 . Since tin in the divalent form may enable an electron transfer between the Sn 2+ lattice ions at the sur- face and surface adsorbates, the developments of other Sn–W–O gas-sensing materials with Sn in divalent state would be interest- ing. ␣-SnWO 4 is an n-type semiconductor with an orthorhom- bic crystal structure and both Sn and W atoms have distorted octahedral oxygen coordinations [1,2]. Traditional methods in the preparations of the ternary Sn–W–O mixed oxides, including ␣- SnWO 4 , are solid-state reactions [3,7,8] or the direct redox reaction between metal Sn and the tungstenic acid [9]. Since Sn 2+ can be ∗ Corresponding authors. Tel.: +86 591 83779105; fax: +86 591 83779105. E-mail addresses: zhaohuili1969@yahoo.com (Z. Li), xzfu@fzu.edu.cn (X. Fu). easily oxidized to Sn 4+ , these reactions have to be carried out under N 2 atmosphere. The other disadvantage of these reaction meth- ods is the difficulty in the preparations of nanocrystalline products with small particle size and high surface area. The as-obtained products are therefore not favorable for the applications in the gas-sensor field since a high “surface accessibility” is crucial in obtaining a high sensitivity of the semiconductor material [10–12]. According to the generally accepted theory, the gas sensitivity of a semiconductor material is generated by the gas–solid interactions, i.e., the adsorption/desorption and reactions on the semiconduc- tor surface [13]. Therefore nanocrystalline semiconductor materials with small particle sizes and high active surface area are expected to exhibit superior gas-response property to their bulk counter- part since they can provide more active surface for the adsorbates [14–16]. Although sometimes the instability of the sensitivity is observed due to the evolution of the fine microstructure during the working of the sensor at high temperature, the application of the nanomaterials for gas-sensing still attracted much recent inter- est. For example, a recent report showed that the sensor made of hierarchical Cu 2 O microspheres with hollow and multilayered con- figuration exhibited much higher gas-sensing property than bulk Cu 2 O [15]. The applications of low temperature hydrothermal method in the preparations of pure crystalline nanomaterials with small par- ticle size, narrow grain size-distribution and large specific surface area without high temperature treatment have been well docu- mented. To make the hydrothermal method more attractive is that 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.05.010 624 H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 it can give products with tunable morphology and particle size by simply adjusting the reaction temperature, time and reaction pH as well as using the additives [17]. Herein, we report the selective syntheses of nanoplates of ␣- SnWO 4 and SnW 3 O 9 in large scale via a facile low temperature hydrothermal route. The reaction pH and the molar ratio of W 6+ to Sn 2+ in the precursors played important roles in the final prod- ucts obtained. Although the hydrothermal method hasbeen applied in the preparations of many tungstates [18–20], to the best of our knowledge, it has never been reported in the preparations of ternary Sn–W–O mixed oxides. The gas-sensing performances of the as-prepared ␣-SnWO 4 and SnW 3 O 9 toward H 2 S and H 2 were also investigated. SnW 3 O 9 represents another ternary Sn(II)–W–O semiconductor which shows gas-sensing property. Our result also revealed that the hydrothermal prepared ␣-SnWO 4 showed higher sensitivity toward H 2 than that prepared via a solid-state reaction. 2. Experimental 2.1. Syntheses Nanocrystalline ␣-SnWO 4 and SnW 3 O 9 were prepared by the hydrothermal method. All of the reactants and solvents were ana- lytical grade and were used without further purifications. In a typical procedure for the preparation of ␣-SnWO 4 , SnCl 2 ·2H 2 O (1.128 g, 5 mmol), (NH 4 ) 5 H 5 [H 2 (WO 4 ) 6 ]·H 2 O (1.335 g, 0.833 mmol) (molar ratio of Sn 2+ to W 6+ at 1:1)were added to 65 mL aqueous solution. The pH of the resulting mixture was adjusted to 1, 3, 5, 7, 9 and 11 with sodium hydrate solution (2 mol L −1 ) under vigorous stirring. The resulting suspension was transferred to a 100 mL Teflon-lined stainless steel autoclave and sealed tightly. The autoclaves were kept at 200 ◦ C for 48 h. After cooling to room tem- perature, the precipitate was collected, washed with distilled water and then dried in air at 80 ◦ C. The procedure for the preparation of pure SnW 3 O 9 is similar to that of ␣-SnWO 4 except that the molar ratio of Sn 2+ to W 6+ is 1:2 and the pH value is lower than 1. For comparison, bulk ␣-SnWO 4 sample was prepared from SnO and WO 3 using a conventional solid-state synthesis route [3].To prevent the oxidation of Sn 2+ to Sn 4+ , SnO and WO 3 was heated in an argon atmosphere at 600 ◦ C for 15 h to obtain the sample. 2.2. Characterizations X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with CuK ˛ radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respec- tively. Data were recorded at a scanning rate of 0.02 ◦ s −1 in the 2 range of 10–70 ◦ . It was used to identify the phase present and their crystallite size. The crystallite size was calculated from X-ray line broadening analysis by Scherer equation: D = 0.89/ˇcos Â, where D is the crystal size in nm, is the CuK ˛ wavelength (0.15406 nm), ˇ is the half-width of the peak in rad, and  is the corresponding diffrac- tion angle. The Brunauer–Emmett–Teller (BET) surface area was measured with an ASAP2020M (Micromeritics Instrument Corp.). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were measured by JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. A suspension in ethanol was sonicated and a drop was dripped on the sup- port film. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a VG Scientific ESCA Lab Mark II spectrom- eter equipped with two ultra-high vacuum 6 (UHV) chambers. All binding energies were referenced to the C 1s peak of the surface adventitious carbon at 284.8 eV. 2.3. Gas-sensing property measurements The sensor structure and the testing principle were similar to that reported previously [10,11,21]. The electrode for measurement was composed of a pair of four-fingered gold electrodes of 120 m width and 40 m spacing between fingers on an alumina substrate. The overlap distance of the fingers was 400 m, and a Ni–Cr heater (37 ) was made on the opposing face of the substrate. The thick film was coated with a layer of sensor materials of about 10 m thick. After drying at 150 ◦ C for 2 h in air to improve the stabil- ity, the electrical contact was made through connecting the four platinum wires with the instrument base by silver paste. During the measurement, the sensors were hosted in a closed plastic tube equipped with appropriate inlets and outlets for gas flow. A given amount of gas such as H 2 SorH 2 was injected into the chamber by a micro-injector. The resistance of a sensitive material is measured in air (R air ) and in air–gas mixtures (R gas ) under the same operating current. The gas response magnitude (S) was defined as the ratio of R air to R gas (S =R air /R gas ). 3. Results and discussion 3.1. Syntheses The pH value plays an important role in controlling the compo- sition of the final products. Fig. 1 shows the XRD patterns for the products obtained from the hydrothermal treatment of the precur- sors with a 1:1 molar ratio of W 6+ to Sn 2+ at 200 ◦ C for 48 h under different pH values. It is observed that pure phase of ␣-SnWO 4 (JCPDS no. 29-1354) can only be obtained at aneutralpH value (from 6 to 9) although the formation of the phase of ␣-SnWO 4 starts at pH of 2. The products obtained in the acidic condition (pH from 2 to 5) are a mixture of ␣-SnWO 4 , SnW 3 O 9 (JCPDS no. 86-628) and SnO 2 . With pH decreasing to 1, the phase of ␣-SnWO 4 cannot be obtained and the product obtained is a mixture of SnW 3 O 9 and SnO 2 . With pH increasing to basic condition, a mixture of SnO 2 and SnO can be obtained. The molar ratio of W 6+ to Sn 2+ also plays an important role in the final product. Fig. 2 shows the representative XRD patterns of the products obtained when treated hydrothermally at 200 ◦ Cata pH value lower than 1 under different molar ratio of W 6+ to Sn 2+ . It is found that pure SnW 3 O 9 can only be obtained when the molar ratio of W 6+ to Sn 2+ is 2:1. A lower molar ratio of W 6+ to Sn 2+ (<2:1) only gives a mixture of SnO 2 and SnW 3 O 9 . It is not strange that pure Fig. 1. XRD patterns of the samples prepared at 200 ◦ C for 48 h with different pH values, (a) pH 1; (b) pH 3; (c) pH 5; (d) pH 7; (e) pH 9; (f) pH 11. (᭹) SnW 3 O 9 ; (*) SnO 2 ;() ␣-SnWO 4 ;() SnO. H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 625 Fig. 2. XRD patterns of the samples prepared with the different molar ratio of Sn 2+ to W 6+ under the strong acidity condition (pH < 1) at 200 ◦ C for 48 h, (a) 1:1; (b) 1:1.5; (c) 1:2; (d) 1:2.5; (e) 1:3. (*) SnO 2 ; (#) H 2 W 1.5 O 5.5 ·H 2 O. phase of SnW 3 O 9 cannot be obtained at the stoichiometric molar ratio of W 6+ to Sn 2+ at 3:1 since part of Sn and W are engaged in the redox reaction as evidenced from the following XPS result. Instead, a mixture of SnW 3 O 9 and H 2 W 1.5 O 5.5 ·H 2 O is obtained. The reason why pure SnW 3 O 9 can only be obtained when the molar ratio of W 6+ to Sn 2+ is 2:1 is not very clear. We proposed that Sn 2+ and NH 4 + in (NH 4 ) 5 H 5 [H 2 (WO 4 ) 6 ]·H 2 O are responsible for the reduction of partofW 6+ to give W 4+ and lead to the formation of SnW 3 O 9 . The exact reactions occurring may be very complicated. However, the involvement of NH 4 + in the formation of SnW 3 O 9 can be confirmed by the fact that no SnW 3 O 9 can be obtained when Na 2 WO 4 instead of (NH 4 ) 5 H 5 [H 2 (WO 4 ) 6 ]·H 2 O is used as the starting material. 3.2. Characterizations XPS analyses were carried out on the as-prepared SnW 3 O 9 and ␣-SnWO 4 . XPS spectra of both SnW 3 O 9 and ␣-SnWO 4 in the Sn 3d region show binding energies of Sn 3d 5/2 and Sn 3d 3/2 at around 486.6 and 495.1 eV respectively and suggest that in both samples Sn exist in the chemical states of Sn 2+ [22] (Fig. 3a). The high- resolution XPS spectra of the W 4f region for ␣-SnWO 4 show peaks around 35.5 eV for W 4f 7/2 and 37.8 for W 4f 5/2 , which indicates that WexistasW +6 in ␣-SnWO 4 [23] (Fig. 3b). For SnW 3 O 9 , the high- resolution XPS spectra of the W 4f 7/2 region can be deconvoluted into two peaks around 34.2 and 35.6 eV respectively and suggests that W exist in multi-valency in SnW 3 O 9 . The binding energy at 34.2 eV can be ascribed to W 4+ while the other peak at 35.6 eV orig- inates from W 6+ [24,25]. The atomic ratio of W 4+ /W 6+ as evidenced from the XPS result is around 1/2. This indicates that part of the W is reduced from W 6+ to W 4+ . It is possible that Sn 2+ and NH 4 + in (NH 4 ) 5 H 5 [H 2 (WO 4 ) 6 ]·H 2 O are responsible for the reduction of W 6+ to W 4+ and the formation of SnW 3 O 9 . This can also explain that the stoichiometric 3:1 molar ratio of W 6+ to Sn 2+ cannot give the pure phase of SnW 3 O 9 . The high-resolution XPS spectra of the O 1s peaks can be deconvoluted into oxygen in lattice (O 2− ) at binding energy of 530.5 eV and surface adsorbed oxygen (O − ) at 532.0 eV [26] (Fig. 3c). The TEM image shows that the as-prepared ␣-SnWO 4 sample consists of thin irregular nanoplates with the dimension range from several tens of nanometers to several hundred nanometers. (Fig. 4a) The HRTEM image (Fig. 4b) shows clear lattice fringes. The fringes of d = 0.577 and 0.375 nm correspond to (0 2 0) and (1 0 1) crystallographic plane of ␣-SnWO 4 , respectively. The typical TEM image of SnW 3 O 9 shows that it consists of hexagonal nanoplates Fig. 3. XPS spectra of ␣-SnWO 4 and SnW 3 O 9 (a) Sn 3d; (b) W 4f; (c) O 1s. with dimension in the range of 60–150 nm. (Fig. 4c) The clear lat- tice fringes of d = 0.321 nm observed in the HRTEM image (Fig. 4d) correspond to the (2 0 0) crystallographic plane of SnW 3 O 9 . N 2 -sorption isotherm for both ␣-SnWO 4 and SnW 3 O 9 exhibits stepwise adsorption and desorption (type IV isotherm), indicative of porous solids (Fig. 5). Due to the smaller average crystallite size (17.9 nm) as determined from the XRD result, ␣-SnWO 4 has a higher BET specific surface area of 40.0 m 2 g −1 as compared to that of SnW 3 O 9 (27.2 m 2 g −1 ), which has a larger average crys- tallite size of 35.7 nm. The BET surface area of the hydrothermal prepared ␣-SnWO 4 is also much higher than that of ␣-SnWO 4 pre- pared via a solid-state reaction (3.4 m 2 g −1 ). Therefore, compared 626 H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 Fig. 4. (a) TEM and (b) HRTEM of ␣-SnWO 4 ; (c) TEM and (d) HRTEM of SnW 3 O 9 . to the conventional solid-state method, the hydrothermal method is a practical method to prepare nanocrystalline SnWO 4 samples with small particle size and large BET specific surface. 3.3. Gas-sensing property The gas-sensing performance toward H 2 of the hydrothermal prepared ␣-SnWO 4 was investigated. The operating temperature 100 ◦ C was chosen in this study according to our previous study that the response toward 1000 ppm H 2 is best when the operat- ing temperature is 10 0 ◦ C. Fig. 6 shows the response of both the Fig. 5. N 2 -sorption isotherm and the pore size distribution plot for ␣-SnWO 4 and SnW 3 O 9 . The pore size distribution was estimated from the desorption branch of the isotherm. hydrothermal prepared and solid-state prepared ␣-SnWO 4 toward H 2 at a working temperature of 373 K. Since ␣-SnWO 4 is an n-type semiconductor, the free carriers are originated from the oxygen vacancies. Therefore ␣-SnWO 4 is expected to adsorb both moisture in the form of hydroxyl groups and oxygen in the ambient envi- ronment. The adsorbed O 2− and HO − groups can trap the electrons from the conduction band of ␣-SnWO 4 and induce the formation of a depletion layer on the surface. When exposed to a test gas, gas molecules are chemi-adsorbed at the active sites on the surface and will be oxidized by the adsorbed oxygen and lattice O 2− . During the oxidation process, electrons will transfer to the surface of ␣-SnWO 4 Fig. 6. Response of the sensors made of the as-prepared ␣-SnWO 4(Hy) and ␣- SnWO 4(SSR) samples toward H 2 S at an operating temperature of 100 ◦ C. H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 627 Fig. 7. Response of the sensors made of the as-prepared ␣-SnWO 4 and SnW 3 O 9 samples toward H 2 S at an operating temperature of 100 ◦ C. to lower the number of trapped electrons, inducing a decrease in the resistance. Therefore the gas response for ␣-SnWO 4 can b e defined as the ratio of the stationary electrical resistance of the sensor in air (R air ) and in the test gas (R gas ), i.e., S = R air /R gas . As shown in Fig. 6, the response of the hydrothermal prepared ␣-SnWO 4 nanocrys- tals toward 100 ppm H 2 is estimated to be 2.8, which is nearly two times as that of the ␣-SnWO 4 prepared via a solid-state reaction (1.6). The higher specific surface area of the hydrothermal prepared ␣-SnWO 4 is responsible for the higher response since it can provide more active site for the gas chemisorption. Although the gas-sensing property of ternary Sn–W–O system has well been documented, to the best of our knowledge, the gas- sensing property of SnW 3 O 9 has never been reported previously. Herein the gas-sensing performance of nanoplates of SnW 3 O 9 and ␣-SnWO 4 toward H 2 S was investigated. It is observed that both semiconductors exhibit excellent gas-sensing performance toward H 2 S(Fig. 7). The response of the nanoplates of SnW 3 O 9 toward 100 ppm H 2 S is estimated to be 20, while that of the hydrother- mal prepared ␣-SnWO 4 is about 9. We note that even at a very low H 2 S concentration of 20 ppm, both semiconductors still exhibit a very impressive sensing response (8.0 for SnW 3 O 9 and 5.2 for ␣- SnWO 4 ). It is a little weird to observe that SnW 3 O 9 nanoplates, with a lower specific surface area (27.2 m 2 g −1 ), show a higher response toward H 2 S than ␣-SnWO 4 nanoplates with a higher specific sur- face area (40.0 m 2 g −1 ). This relative higher response of SnW 3 O 9 can be explained by the existence of multivalent W (W 6+ /W 4+ ), which can promote the chemi-adsorption of H 2 S and is beneficial to the change of the resistance for the n-type semiconductor like SnW 3 O 9 . The as-prepared SnW 3 O 9 also shows response to other gas, like H 2 . The response of the as-prepared SnW 3 O 9 is estimated to be 2.30 toward 500 ppm H 2 . SnW 3 O 9 is another ternary Sn(II)–W–O semiconductor which shows promising application as the gas sen- sor. 4. Conclusions In summary, nanoplates of ␣-SnWO 4 and SnW 3 O 9 can be selec- tively synthesized in large scale via a facile hydrothermal reaction method. The final products obtained are strongly dependent on the pH and the molar ratio of W 6+ to Sn 2+ in the precursors. Due to the high specific surface area, ␣-SnWO 4 nanoplates show higher response toward H 2 than that prepared via a solid-state reaction. The as-prepared SnW 3 O 9 haxagonal nanoplates show gas-sensing performance for both H 2 S and H 2 . As compared to ␣-SnWO 4 , SnW 3 O 9 exhibits higher response toward H 2 S and its higher response can be well explained by the existence of the mul- tivalent W (W 6+ /W 4+ )inSnW 3 O 9 . Acknowledgments The work was supported by National Natural Science Foundation of China (20537010, 20677009), National Basic Research Program of China (973 Program: 2007CB613306, 2007CB616907), grant from Fujian Province (E0710009). This work was also supported by Pro- gram for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818). Z. Li thanks program for New Century Excellent Talents in University (NCET-05-0572), State Education Ministry of P.R. China. References [1] J.L. Solis, V. Lantto, A study of gas-sensing properties of sputtered ␣-SnWO 4 thin films, Sens. Actuators B 24–25 (1995) 591–595. [2] J.L. Solis, V. 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Galatsis, Y.X. Li, Investigation of thin films of mixed oxides for gas-sensing applications, Surf. Interface Anal. 34 (2002) 672–676. [26] A. Gurlo, N. Barsan, U. Weimar, M. Ivanovskaya, A. Taurino, P. Siciliano, Polycrystalline well-shaped blocks of indium oxide obtained by the sol- gel method and their gas-sensing properties, Chem. Mater. 15 (2003) 4377–4383. 628 H. Dong et al. / Sensors and Actuators B 140 (2009) 623–628 Biographies Hui Dong received BSc degree in chemistry from Taishan University in 2006 and MSc degree in inorganic Chemistry from Fuzhou University in 2009. His interest is in the synthesis and application of nanomaterials. Zhaohui Li received BSc degree in chemistry from Fudan University in 1990, MSc degree from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science in 1996, and PhD degree in chemistry from National University of Singa- pore in 2000. She is currently a professor in Department of Chemistry and Chemical Engineering of Fuzhou University. Her research interest includes nanostructured materials and photocatalysis. Zhengxin Ding received PhD degree in chemistry from Fuzhou University in 2003. He is currently an associate professor in Department of Chemistry and Chemical Engineering of Fuzhou University. His major research interests focus on photocatal- ysis. Haibo Pan received BSc degree in physics from Huaqiao University in 1984, MSc degree from Shanghai University in 1990. He is currently an associate profes- sor at Chemistry and Chemical Engineering Collage, Fuzhou University. His major research interests include gas sensor by nanomaterials, computer simulation for structure and properties of nanomaterials, and organic/inorganic compound by nanomaterials. Xuxu Wang received PhD degree in Department of chemistry from Ecole Supérieure de Chimie Physique Electronique de Lyon, France, in 2000. He is currently a profes- sor in Department of Chemistry and Chemical Engineering of Fuzhou University. His major research interests focus mainly on photocatalytic mechanism and nano- photocatalytic materials. Xianzhi Fu received his BSc and PhD degree in Department of Chemistry from Beijing University, China, in 1982 and 1991. He is currently a professor in Department of Chemistry and Chemical Engineering of Fuzhou University. His research focused mainly on photocatalytic mechanism and nano-photocatalytic materials. . surface area, ␣-SnWO 4 nanoplates show higher response toward H 2 than that prepared via a solid-state reaction. The as -prepared SnW 3 O 9 haxagonal nanoplates. 2009 Keywords: SnWO 4 SnW 3 O 9 Nanoplate Hydrothermal Gas-sensing abstract Nanoplates of ␣-SnWO 4 and SnW 3 O 9 were selectively synthesized in large scale via a facile hydrothermal reaction