Synthesis and characterization of WO 3 nanostructures prepared by an aged-hydrothermal method R. Huirache-Acuña a,b, ⁎ , F. Paraguay-Delgado c,1 ,M.A.Albiter d ,J.Lara-Romero d , R. Martínez-Sánchez c a CFATA-UNAM, Boulevard Juriquilla 3001, Juriquilla Querétaro, 76230, Mexico b Universidad La Salle Morelia, Av. Universidad 500, Mpio. Tarímbaro Mich., 58880, Mexico c Centro de Investigación en Materiales Avanzados, S.C. CIMAV, Laboratorio Nacional de Nanotecnología-Chihuahua, Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chih., 31109, Mexico d Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Morelia Mich., 58000, Mexico ARTICLE DATA ABSTRACT Article history: Received 7 November 2008 Receivedin revised form 5 March 2009 Accepted 9 March 2009 Nanostructures of tungsten trioxide (WO 3 ) have been successfully synthesized by using an aged route at low temperature (60 °C) followed by a hydrothermal method at 200 °C for 48 h under well controlled conditions. The material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Specific Surface Area (S BET ) were measured by using the BET method. The lengths of the WO 3 nanostructures obtained are between 30 and 200 nm and their diameters are from 20 to 70 nm. The growth direction of the tungste n oxide nanostructures was determined along [010] axis with an inter-planar distance of 0.38 nm. © 2009 Elsevier Inc. All rights reserved. Keywords: Nanostructures Tungsten trioxide (WO 3 ) Hydrothermal method 1. Introduction In recent years, studies of transition metal oxides nanos- tructures have become impor tant due to the different potential applications, such as nanoelectronics [1], gas sensors [2,3], optical devices [4], electrochromic windows [5,6], and catalysts [7 , 8] . Nanostructures of tungsten oxide having nanometer scale had been formed by using different condi- tions and preparation methods: electrospinning [9], oxidation of a substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen [10], by heating a tungsten filament in a partial oxygen atmosphere [11], by reacting WO(OMe) 4 under auto- genic pressure at elevated temperature followed by annealing [12], by hot filament chemical vapor deposition [13], physical vapor deposition process [14] and by ultrasonic spray and laser pyrolysis techniques [15–17]. Recently, Therese et al. [18] and Ha et al. [19] reported the synthesis of WO 3 nanostructures by following a hydrothermal route. They used as raw materials ammonium salts and some additives to control the formation of nanomaterials. In comparison with the methods mentioned before, the hydrothermal route is an economical preparation method of nanostructures since it does not require an expensive experimental setup. In this work, we report the synthesis and characterization of WO 3 nanostructures pre- pared by following an easy two step aged-hydrothermal MATERIALS CHARACTERIZATION 60 (2009) 932– 937 ⁎ Corresponding author. CFATA-UNAM, Boulevard Juriquilla 3001, Juriquilla Querétaro, 76230, Mexico. Tel.: +52 442 238 11 43; fax: +52 442 238 1 1 65. E-mail address: rafael_huirache@yahoo.it (R. Huirache-Acuña). 1 Present address: National Institute for Nanotechnology, 11421 Saskatchewan Drive Edmonton (AB) Canada T6G 2M9. 1044-5803/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2009.03.006 method at low temperature by using ammonium metatung- state as tungsten source and without the presence of additives. 2. Experimental 2.1. Synthesis of WO 3 Nanostructures WO 3 nanostructures were synthesized by using an aged- hydrothermal route. A saturated aqueous solution of ammo- nium metatungstate [(NH 4 ) 10 W 12 O 41 xH 2 O] (0.15 mol of W) was prepared and acidified with HNO 3 2.2 N (Normal) to produce a pH around 5 and then kept in a flask hermetically sealed with stirring by one week at 60 °C. Then, 5 ml of the aged solution was deposited into a Teflon-lined stainless steel autoclave and heated at 200 °C for 48 h. The material obtained was filtered and washed with deionized water and dried in the presence of air at room temperature. 2.2. Characterization A scanning electron microscope (JEOL JSM 5800 LV) was used to perform morphological analysis. Several fields were analyzed at different magnifications in order to get information of the prevalent features. The elemental composition was determined using energy dispersive spectroscopy (EDS) (Oxford Inca X- Sight). Specific surface area (S BET ) determination was made with a Quantachrome AUTOSORB-1 model by nitrogen adsorption at − 196 °C using the BET isotherm. Samples were degassed under flowing argon at 200 °C for 2 h before nitrogen adsorption. TEM and HRTEM micrographs were obtained in a Philips TECNAI F20 FEG transmission electron microscope operated at 200 kV. X-ray analysis was made with a Philips X Pert MPD diffractometer, equipped with a graphite monochromator, copper Kα radiation with wavelength λ=1.54056 Å, operated at 43 kV and 30 mA. Raman spectroscopy was performed using a Labram system model Dilor micro-Raman equipped with a 20 mW He–Ne laser emitting at 632.8 nm and a holographic notch filter made by Kaiser Optical Systems, Inc. (model supertNotch-Plus) with a 256×1024-pixel char ge-coupled device (CCD) used as the detector; and a computer-controlled XY stage with a spatial resolution of 0.1 µm with two interchangeable gratings (600 and 1800 g mm − 1 ) and a confocal microscope with 10, 50, and 100× objectives. All measurements were collected at room tempera- ture with no special sample preparation. Oxidation state and surface composition were analyzed by an X-ray photoelectron spectroscope, Energy Spectrometer EA 11 MCD, using Mg monoenergetic soft X-ray (Kα =1253.6 eV). 3. Results and Discussion The synthesis method presented in this work for procuring WO 3 nanostructures is a modification of the method reported by Albiter et al. for MoO 3 nanorods [20] and Therese et al. for WO 3 nanostructures [18]. The main difference between Therese et al. method and ours is that we did not use additives to form the nanostructures. On the other hand, nitric acid (HNO 3 ) was not used before the hydrothermal treatment as reported by Albiter et al. To convert W 12 O 41 10− anions to neutral W 12 O 36 , excess divalent oxygen anions must be removed. Stoichiometrically, five divalent oxygen anions per W 12 O 41 10− must be combined with protons from the acidic medium: W 12 O 10− 41 þ 10H þ →12WO 3 þ 5H 2 O ð1Þ According to reaction (1), high concentrations of both W 12 O 41 10− and H + would shift the reaction to the right ensuring the formation of WO 3 , although many intermediate steps and Fig. 1 – XRD pattern for WO 3 nanostructures. Where all reflections are indexed based on a hexagonal WO 3 cell. Table 1 – Crystallite size (ϕ) determi ned by Scherer equation (ϕ=Kλ/βCosθ), where K=0.9, λ=1.54 Å, β is FWHM in radians and θ is the glancing angle. (hkl) 100 001 200 t (Å) 209.221 443.202 210.485 Fig. 2 – N 2 adsorption–desorption isotherm at − 196 °C of tungsten oxide nanostructures. 933MATERIALS CHARACTERIZATION 60 (2009) 932– 937 thus compounds and phases may exist. It is thus anticipated that the formation of WO 3 should have a strong dependence on the acid medium. Furthermore, the aging time in solution and the hydrothermal treatment time inside the autoclave have a strong influence in the formation of nanostructures. Paraguay-Delgado et al. [21], concluded that two conditions are important for procuring nanostructures, the first being that for an extended-time-aged solution a short hydrothermal treatment is required (about 24 h) and the second being that for a short-time-aged solution (1 week) at least 36 h of hydrothermal treatment is required. The XRD pattern from WO 3 nanostructures is reported in Fig. 1 where a well crystallized phase was observed. The half- widths reflection indicates the presence of nanoscale tung- sten oxide which was corroborated measuring the crystallite size (ϕ) for more intense and representative reflections (100), (001) and (200) (Table 1) by using the Scherer equation: / = Kk bCosh ð2Þ The observed peaks could be indexed based in a hex- agonal cell with inter-planar spacings for tungsten t rioxide (ICSD 32,001, J CPDS 33-1387; a =7.298 Å, c=3.899 Å, space group P6/mm) [18,19]. It was also observed that there are not other impurity phase peaks. As observed, Fig. 2 shows the N 2 adsorption–desorption curve corresponding to a type IV isotherm (IUPAC Classifi- cation) with desorption step characteristic of mesoporous materials above t he re lative p ressure (P/Po) of 0.4 a nd specific surface area (S BET ) values between 34 and 35 m 2 (Table 2). The formation of a mesoporous material is due to the water vapor pressure inside the autoclave at 200 °C, at this time the exactly mechanism of formation is not right known [22]. Fig. 3a–c shows SEM micrographs at different magnifica- tions from separable WO 3 nanostructures with different size protruding out. The oxide nanostructures had smooth sur- faces and a not well-defined rectangular cross section. These particles were about 0.1 to 3 µm long, and 50–200 nm wide as determined from SEM images. As illustrated in Fig. 3a–c this method led to the formation of nanostructures in a wide range of thicknesses, most of which had shown an irregular shape. The oxygen (O) and tungsten (W) atomic contents were determined by Energy Dispersive Spectroscopy (EDS) analysis (1% error) and the results are reported in Table 2. The EDS spectrum presented in Fig. 3d reveals a 3:1 atomic ratio for oxygen and tungsten elements, which solely constitute the composition of WO 3 . Transmission electron microscopy (TEM) micrographs of WO 3 nanostructures are reported (Fig. 4a–b). A representative TEM image of the tungsten oxide nanostructures is given in Table 2 – Elemental analysis determined by EDS (% atomic) and specific surface area (S BET ). Sample % at. O % at. W S BET (m 2 /g) WO 3 (I) 75 25 35 WO 3 (II) 75 25 34.7 Fig. 3 – Scanning electron microscopy images of WO 3 nanostructures at different magnifications: (a) 5000×, b) 14,000× and c) 15,000× (d) EDS spectrum. 934 MATERIALS CHARACTERIZATION 60 (2009) 932– 937 Fig. 4a. This sample consists of very well separated particles with irregular shape and lengths between 30 and 200 nm and wide from 20 to 70 nm, aggregated together due to the high surface energy owing to their nanosize. Fig. 4b shows a TEM micrograph at a higher magnification. By using HRTEM we observed that the growth direction of the tungsten oxide nanostructures is along [010] axis with an inter-planar distance of 0.38 nm (Fig. 4b). It seems that the WO 3 growth proceeds layer by layer increasing the thickness and the width of the nanostructures. Raman spectroscopy was used to characterize this material since this technique is suitable to obtain details of the WO 3 chemical structure (Fig. 5). Three broad bands were clearly detected: high in the 900–1000 cm − 1 region, medium in the 600–800 cm − 1 region and low in the 200– 400 cm − 1 region. The most intense peak is centered at 780 cm − 1 with a shoulder at 730 cm − 1 and they are attributed to the symmetric and asymmetric vibrations of W 6+ –O bonds (O–W–O st re tch ing mod es ). T wo pea ks cen te red at 320 and 270 c m − 1 can be found in the 200–400 range and correspond to W–O–W bending modes of the bridging oxygen [23–25]. A peak at 910 cm − 1 with a s houlder positioned at 960 cm − 1 in the 900–1 000 cm − 1 can be observed. These peaks correspond to the WfOstretching mode of terminal oxyg en atoms th at are present on th e surface of the cluster (dangling bonds) or at the boundaries of nanometre grains [15,26].Thesmallfeatureobservedat 435 cm − 1 is attributed to the characteristic band of crystal- line WO 3 [27]. Note that these results confirm the formation of hexagonal WO 3 since the main features corresponding to monoclinic WO 3 typically reported at 807 and 715 cm − 1 are absent in the R aman spectrum [28–30]. The XPS collected spectra of the material for the peaks O 1 s and W 4f are shown in Fig. 6a and b respectively. Peaks position for O 1 s is 530.3 eV and binding energy peak located at 35.4 and 40.6 eV is attributed to W 4 f . According to the literature [31] it is WO 3 , this means that the surface of the material contains W 6+ and not other oxidation state for this metal was detected. This was also verified by calculating the O/W ratio using their relative peak areas (I) and atomic sensitivity factors (S), which is shown below (Eq. (3)): Atomic Ratio O W = I 0 S 0 I W S W = 47483:97 0:66 63692:91 2:75 =3:1: ð3Þ 4. Conclusions WO 3 nanostructures with hexagonal phase and mesopor- osity were obtained by using a two step aged-hydrothermal method. By using HRTEM we observed that the growth direction of the tungsten oxide nanostructures is along [010] Fig. 4 – Transmission electron microscopy (TEM) micrographs of WO 3 nanostructures. Fig. 5 – Typical Raman spectrum of hexagonal WO 3 nanostructures obtained by the aged-hydrothermal method. 935MATERIALS CHARACTERIZATION 60 (2009) 932– 937 axis with an inter-planar distance of 0.38 nm. Raman, EDS and XPS analysis confirmed that the chemical structure and oxidation states belong to tungsten oxide (WO 3 ). Acknowledgements The authors appreciate the valuable technical assistance of M.C. W. Antúnez, M. I.Q. 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