Short communication Hydrothermal synthesis of uniform WO 3 submicrospheres using thiourea as an assistant agent X.T. Su a , F. Xiao a , J.L. Lin a , J.K. Jian b , Y.N. Li a , Q.J. Sun a , J.D. Wang a, ⁎ a Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, No. 14 Shengli Road, Urumqi 830046, China b College of Physics Science and Technology, Xinjiang University, No. 14 Shengli Road, Urumqi 830046, China ARTICLE DATA ABSTRACT Article history: Received 23 February 2010 Received in revised form 27 April 2010 Accepted 28 April 2010 Nearly monodisperse tungsten trioxide submicrospheres have been synthesized with tungsten acid and HCl as the starting materials and thiourea as a structure-directing agent through a facile hydrothermal method. The obtained products were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and energy dispersive X-ray, respectively. The results show that the WO 3 submicrospheres are monodisperse with a diameter of about 800–1000 nm. The morphology of the products gradually evolutes from rods to spheres with increase of the reaction time. The formation mechanism of the WO 3 submicrospheres is primarily discussed. © 2010 Elsevier Inc. All rights reserved. Keywords: WO 3 Hydrothermal Submicrospheres Thiourea 1. Introduction Tungstentrioxide (WO 3 ) is an important semiconductor that has drawnsignificant attention in the applications of electrochromic devices, gas sensors and photocatalysts due to its relative narrower band gap (between 2.6 and 2.8 eV) [1–3]. Many research efforts have been devoted to the fabrication of WO 3 nanostruc- tures by different methods, including thermal evaporation method, hydrothermal process, reverse micelles route and physical vapor deposition technique [4–7]. Among them, the hydrothermal process is one effective method to prepare different WO 3 nanostructures because of its simple manner and low cost. It has been verified that the morphology and composition of the WO 3 nanostructures can be tuned by some assistingagentssuchassurfactant[8], inorganicsalt[9], complex agent [10] and some dissoluble organic acid [11].However,most of the products are one dimensional nanostructure. It is still difficult to obtain spherical WO 3 nanospheres under hydrother- mal conditions. It is known that the crystal faces and the shape of nanoparticles play an essential role in determining the catalytic properties [12]. Wang F et al. reported that spherical bismuth particles are the best catalysts for the solution–liquid– solid (SLS) growth of diameter-controlled semiconductor nano- wires and nanorods [13]. In addition, it has been reported that the sphericallymonodisperse morphologyisanimportantfactor for the low-light scattering at the surfaces, as well as the high- packing densities [14]. Recently, our group has synthesized monodisperse WO 3 ·2H 2 O nanospheres and WO 3 square nanoplates with L(+) tartaric acid or citric acid as a structure-directing agent [14,15].In MATERIALS CHARACTERIZATION 61 (2010) 831– 834 ⁎ Corresponding author. Tel.: +86 991 8581018; fax: +86 991 8582807. E-mail address: laofuzi193@163.com (J.D. Wang). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.04.014 available at www.sciencedirect.com www.elsevier.com/locate/matchar this paper, we report a hydrothermal synthesis of uniform WO 3 submicrospheres using thiourea as an assistant agent. Usually, thiourea is used as the sulfur source to prepare tungsten sulfide or other sulfides [16–18]. However, in this work, we found that tungsten oxide instead of tungsten sulfide was obtained when thiourea was used in the hydrothermal treatment of tungsten acid. The morphology of specific products gradually evolves into spheres from rods with the increase of reaction time. To our best knowledge, this is the first time to prepare tungsten oxide submicrospheres using thiourea as structure-directing agent by a hydrothermal process. Moreover, the formation mechanism for the WO 3 submicrospheres has also been primarily discussed. 2. Experimental All of the chemical reagents used in the experiment were of analytical grade. The detailed synthesizing process was as follows: 10 g of Na 2 WO 4 ·2H 2 O was resolved in 50 ml of distilled water, and several milliliters of 6 M HCl aqueous solution was introduced into the above solution, resulting in a yellow tungsten acid precipitation to adjust the pH value to 0.5. After stirring for 10 min, the resulting tungsten acid precipitation was filtered, washed with distilled water and dried at room temperature in air for 2 h. Then 0.5 g of the above tungsten acid powder and 1 g of thiourea were mixed into 30 ml distilled water. After 10 min of stirring, the mixture was transferred into a Teflon container with capacity of 100 mL, which was filled with distilled water up to 80% of the total volume, sealed and heated at a constant temperature of 180 °C. The reactions were performed for 3 h, 12 h and 24 h. After the reaction was completed, the resulting solid products were centrifuged, washed with distilled water and ethanol repeatedly to remove the ions possibly remaining in the final products, and finally dried at 60 °C in air for 60 min. X-ray analysis was made with a Rigaku D/max-ga diffrac- tometer, equipped with a graphite monochromator, copper Kα radiation with wavelength λ =1.54178 Å. A scanning electron microscope (LEO 1450VP) was used to perform morphological analysis. The elemental composition was determined using energy dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) micrograph was obtained in a Hitachi H-600 transmission electron microscope operated at 75 kV. 3. Results and Discussion The XRD pattern of the sample is shown in Fig. 1a. It can be seen that all the reflection peaks could be indexed to a hexagonal WO 3 phase (JCPDS No. 33-1387). No diffractions raised from impurities appear in the XRD pattern. EDX analysis is employed to confirm the compositions of the products. As shown in Fig. 1b, only oxygen and tungsten exist in the products with the molar ratio of about 3:1 (O/W), which agrees well with the XRD result. No elemental sulfur could be detected, which impl ies that pure WO 3 products were obtained under the current synthetic route. Typical scanning electron microscopy (SEM) images of the products prepared at 180 °C with different reaction time are shown in Fig. 2. It can be seen from Fig. 2a, rod-shaped products with diameters of several hundred nanometers and lengths up to several micrometers are obtained when the reaction system is conducted for 3 h. However, when the reaction time is prolonged to 12 h, spherical particles with submicrometer sizes are produced besides rod-shaped pro- ducts (Fig. 2b). When the reaction system is proceeding to 24 h, all of the rod-shaped particles disappear and only highly uniform submicro-sized spherical particles are obtained (Fig. 2c). Fig. 2d shows a high magnification SEM image of the WO 3 products, indicating that those submicrospheres are monodisperse with the sizes of 800–1000 nm. It can be seen that the surface of those spheres is rough, which implies that those spheres may be self-assembled from smaller nanopar- ticles. TEM image of the submicrospheres is shown in Fig. 3, revealing the similar morphological feature of the products as observed by SEM. Thiourea is widely used as a source of sulfur in the hydrothermal synthesis of transition metal sulfides [13–15]. However, in our case, tungsten sulfide hasn't been obtained under the hydrothermal conditions with the addition of thiourea. According the previous work [13],thereaction temperature is the key factor to obtain transition metal sulfides under hydrothermal conditions. It is reported that when the hydrothermal process preceded under 240 °C, there is no WS 2 produced through the reaction between thiourea Fig. 1 – (a) The XRD pattern and (b) EDX pattern of WO 3 prepared by thiourea-assisted hydrothermal method at 180 °C for 24 h. 832 MATERIALS CHARACTERIZATION 61 (2010) 831– 834 and WO 3 [13]. Therefore, in this experiment, only WO 3 products are obtained. Based on the above reaction pathway and the experimental results, we proposed a possible mech- anism to address the formation of WO 3 submicrospheres, which is depicted in Scheme 1. At the starting stage of the reaction, tungsten acid solid decomposes to generate rod- shaped WO 3 particles, which is similar with the reports [8,13]. These newly formed rod-shaped WO 3 particles could absorb thiourea molecules and be stabilized by them. The study has shown that the reaction of thiourea and water can produce CO 2 ,H 2 S, and NH 3 [13]. H 2 N À CS À NH 2 þ 2H 2 O→2NH 3 þ H 2 S↑ þ CO 2 ↑ It is known that the solu bility of WO 3 is influenced by the pH value of solution. In our experiments, the rod- shaped WO 3 particles could decompose into primary nano- particlesduetothechangeofpHvalueofthesolution caused by the decomposition of thioure a. It is well known that aggregation is energetic ally favored, so the primary nanoparticles tend to assemble into larger sp herical a ggre- gates du e to high surface energ y. Finally, all the primary particles gradually self- assemble to submicroscale spheres with the proceeding of the reaction. Such self-assembly of nanoparticles is favorable to decrease the surface energy of the system. Fig. 2 – SEM images of the samples prepared by thiourea-assisted hydrothermal treatment at 180 °C with different reaction times: (a) 3 h, (b) 12 h, (c) 24 h, low magnification, (d) 24 h, high magnification. Fig. 3 – TEM image of the sample prepared by thiourea- assisted hydrothermal treatment at 180 °C for 24 h. Scheme 1 – Schematic of the process for the formation of WO 3 submicrospheres. 833MATERIALS CHARACTERIZATION 61 (2010) 831– 834 4. Summary In summary, a novel and simple hydrothermal method was used to prepare uniform monodisperse WO 3 submicrospheres employing thiourea as assisting agent. A morphology evolu- tion of WO 3 from rod-like to spherical shapes was observed as the reaction proceeded. A thiourea-assisted self-assembly process was proposed to address the formation of the WO 3 submicrospheres. The method reported here may be applied to synthesize other spherical oxides such as MoO 3 or MoO 2 in the solution-based approach. Acknowledgements We appreciate the financial supports of Key Scientific Project of Xinjiang Province (No. 200732139) and Doctoral Foundation of Xinjiang University (No. BS080115). REFERENCES [1] Wang JM, Khoo E, Lee PS, Ma J. Synthesis, assembly, and electrochromicproperties of uniform crystalline WO 3 nanorods. J Phy Chem C 2008;112(37):14306–12. [2] Bendahan M, Guérina J, Boulmania R, Aguira K. WO 3 sensor response according to operating temperature: experiment and modeling. Sensors Actuat B: Chem 2007;124(1):24–9. [3] Hong SJ, Jun HC, Borse PH, Lee JS. 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Short communication Hydrothermal synthesis of uniform WO 3 submicrospheres using thiourea as an assistant agent X.T www.sciencedirect.com www.elsevier.com/locate/matchar this paper, we report a hydrothermal synthesis of uniform WO 3 submicrospheres using thiourea as an assistant agent. Usually, thiourea