Synthesis of WO 3 in nanoscale with the usage of sucrose ester microemulsion and CTAB micelle solution N. Asim a, ⁎ , S. Radiman a , M.A.bin Yarmo b a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia b School of Chemical Science and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor Darul Ehsan, Malaysia Received 21 June 2006; accepted 9 October 2006 Available online 30 October 2006 Abstract WO 3 nanoparticles were successfully prepared using first the low temperature hydrolysis method and second the chemical reaction method in water-in-oil sucrose ester microemulsion consisting of S1570, 1-butanol, tetradecane and aqueous phase. In this study WO 3 nanoparticles also were prepared using the CTAB micelle solution. The resultant WO 3 nanoparticles have been investigated with X-ray diffraction (XRD), transmission electron microscopy (TEM), variable pressure scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The shape and particles size of the resultant WO 3 nanoparticles from both methods in sucrose ester microemulsion show similar spherical shape and size range between 10 and 50 nm. The WO 3 nanoparticles prepared with the CTAB micelle solution show spherical shape with the size range average of 25–50 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: WO 3 ; Nanoparticles; Sucrose ester microemulsion; X-ray techniques 1. Introduction Tungsten trioxide is a simple compound in terms of stoichiometry, but it is complex in terms of structure and phase transitions. Tungsten oxide is known as a photochromic and electrochromic material since it changes color upon the absorp- tion of light and in response to an electrically induced change in oxidation state. WO 3 exhibits the electrochemical effect [1] and can be used for the fabrication of smart windows and displays [2].In addition WO 3 appears to be one of the best candidates for gas sensing [3]. It can be operated reversibly and usually has stable chemical and thermal properties over extended periods of use. Gas-sensitive resistors based on tungsten oxide are useful for the measurement of low level s of ozone in air [4,5]. Recently, WO 3 has drawn more attention because of its high efficiency in photocatalytic degradation of organic compounds, including a large fraction of environmental toxins [6].WO 3 is a novel purificatory ecomaterial suitable for application in energy renewal, energy stor age and environmental cleanup. In all of these applications, the morphological characteristics of the materials like grain size or shape are very important and depend strongly on the preparation method. In the last decade, there has been an increasing interest in the study of nanocrystalline materials owing to the different physical and chemical properties compa red with convent ional coarse-grained structures [7–10]. The surface-to-bulk ratio for a nanocrystalline material is much greater than for a material with large grains, which yields a large interface between the solid and a gaseous or liquid medium. Many different methods have been used for the production of nanometer particles. Microemulsions that contain surfactant, oil, water and sometimes co-surfactant have been used widely for the synthesis of nanomaterials in the last two decades. Sucrose esters are biodegradable surfactants that can be manufactured in various hydrophilic–lipophilic properties using different fatty acids varying in their lipophilic chain length. Sucrose esters exist in a large variety of HLB values. The physical properties of sucrose Materials Letters 61 (2007) 2652 – 2657 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +60 3 89214131; fax: +60 3 89269470. E-mail address: n_asim2001@yahoo.com (N. Asim). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.10.014 esters are somewhat unique. Unlike the alkyl ethoxylates, the sucrose esters do not significantly change their HLB with in- creasing temperature. Consequently, increasing the temperature does not induce a phase inversion in microemulsion systems based on sucrose esters, as was observed in microemulsions based on alkyl ethoxylates. Temperature-insensitive sucrose ester-based microemulsions are described in the literature [11–13].Itwas found in general that sucrose esters are not able to form micro- emulsions without co-surfactants [14]. Since the most common sucrose esters are hydrophilic it was not expected that these surfactants will form reverse micelles or w/o microemulsions. However, Garti et al. [15] showed that the addition of a co-solvent/ co-emulsifier, such as short- or medium-chain alcohols, induces the formation of reverse micelles and the solubilization of Fig. 1. The VPSEM images of WO 3 nanoparticles prepared in this study. Fig. 2. The TEM images of WO3 nanoparticles prepared in this study. 2653N. Asim et al. / Materials Letters 61 (2007) 2652–2657 significant amounts of water into the micellar core to form water- in-oil microemulsions. In this study WO 3 were prepared in nanosize with different size ranges using sucrose ester and CTAB surfactants and its chemical and physical properties were investigated. 2. Materials and methods 2.1. Materials Nonionic surfactant of food grade sucrose fatty acid ester (hereafter denoted as S1570 with HLB value=15) and tetradecane (99%) were supplied from Mitsubishi-Kagaku Foods Corporation and TCI respectively. Hexadecyl trimethyl ammonium bromide (CTAB) (purity approx. 99%) was purchased from Sigma. 1-butanol (N 98% GC) and 1-hexanol were purchased from Fluka. Tungsten (VI) chloride (99%) and ammonia solution (25%) were purchased from Aldrich and BDH respectively. Deionized and double distilled water was used for microemulsion and solution preparation. All the chemicals and solvents were used as received without further purifications. 2.2. Determination of phase diagr am for sucrose ester As describe elsewhere [16] for determination of phase diagram, alcohol, oil, sucrose ester mixtures were titrated with water. The behavior of the four-component system is described in pseudo-ternary phase diagrams in which the weight ratio of two components was fixed. Usually, the oil:alcohol weight ratio was held constant at 1:1. The construction of the phase diagram was conducted in a thermostatic bath (37 ± 1 °C). The weight ratio of tetradecane (oil phase) and 1-butanol (co- surfactant) was fixed at 1:1, whereas the surfactant phase consisted of sucrose ester (S1570). The sugar ester used is a commercial sucrose monoester of stearic acid (S1570, denoted as SES, HLB = 15, at least 70% monoester) in a mixture with di- and polyesters of stearic and palmitic acids. The oil phase consists of a 1:1 weight ratio of tetradecane and l-butanol. The addition of the co-solvent (l-butanol) to the oil phase turned the oil phase into a better solvent and allowed significant solubilization of the surfactant into the oil with the formation of inverse micelles. Fig. 3. The XRD diffractogram for WO 3 (a) bulk, (b) sample 1, (c) sample 2 and (d) sample 3, respectively. Fig. 4. The energy dispersive X-ray (EDX) results for WO 3 nanoparticles. 2654 N. Asim et al. / Materials Letters 61 (2007) 2652–2657 The co-solvent is necessary because of the hydrophilicity of the sucrose monostearate and hydrophobicity of the oil. Upon addition of 1-butanol to tetradecane , due to its amphiphilic character it will redistribute into the interface and must therefore be considered also as a co-surfactant and not just as a co-oil; i.e., it has the ability to participate in the self-assembly with the surfactant. The microemulsion system used is solid at room temperature, but liquefies and structures into a homogeneous microemulsion when heated above 37±1 °C [17,18]. 2.3. Preparation of WO 3 in nanoscale The typical microemulsion used in the present study has the following com position: 30 wt.% of S1570, 50 wt.% of Fig. 5. Peak-fitted W 4f ,O 1s and C 1s signals of WO 3 nanoparticles (a) sample 1, (b) sample 2 and (c) sample 3 respectively. 2655N. Asim et al. / Materials Letters 61 (2007) 2652–2657 tetradecane/1-butanol and 20 wt.% of aqueous solution. Aqueous solutions containing of 4 wt.% and 14 wt.% of tungsten (VI) chloride and ammonia solution respectively. Two methods have been used for the synthesis of WO 3 in nanoscale with the use of sucrose ester microemulsion. In the first method, a w/o microemulsion with the composition: 30 wt.% of S1570, 50 wt.% of tetradecane/1- butanol and 20 wt.% of aqueous solution containing of 4 wt.% tungsten (VI) chloride, was stirred vigorously for 2 h at about 45 °C. Then this solution was kept in 60 °C for 4 days and were washed several times with deionized water and absolute ethanol in order to remove the surfactant, residual reactants and byproducts. All the precipitates were place in the furnace at 500 °C for 2 h (hereafter denoted as sample 1). In the second method, two types of w/o microemulsion with the composition: 30 wt.% of S1570, 50 wt.% of tetradecane/1- butanol and 20 wt.% of aqueous solution (containing of 4 wt.% and 14 wt.% of tungsten (VI) chloride and ammonia solution respectively) have been prepared. After stirring and getting homogeny solutions, the micro- emulsion containing ammonia solution was added to the other microemulsion containing tungsten (VI) chloride. The mixed microemulsion was stirred for 3 h at about 45 °C. Then the mixed microemulsion was kept at room temperature for 3 days in order to precipitate. After washing several times with deionized water and absolute ethanol in order to remove the surfactant, residual reactants and byproducts, the precipitate was kept in the furnace at 500 °C for 2 h (hereafter denoted as sample 2). The CTAB micelle solution used in the present study has a composition of 30 wt.% CTAB, 54 wt.% 1-hexanol and 16 wt.% of aqueous solution. This chosen composition was found to belong to the reverse micelles region [19]. For WO 3 nanoparticles pre paratio n, two micelle solutions with the composition mentioned above with aqueous solutions contain- ing of 3.1 wt.% and 12.5 wt.% of tungsten (VI ) chloride and ammonia solution respective ly were prepared. After stirring and getting clear solutions, the micelle solution containing ammonia solution was added to the other micelle solution containing tungsten (VI) chloride. The mixed micelle solution was stirred for 4 h at about 50 °C. Then the mixed micelle solution was kept at room temperature for 3 days in order to precipitate. After washing several times with deioni zed water and absolute ethanol in order to remove the surfactant, residual reactants and byproducts, the precipitate was kept in the furnace at 500 °C for 2 h (hereafter denoted as sample 3). 2.4. Characterization of WO 3 nanoparticles The study of the morphology and composition of the cal- cinated WO 3 nanoparticles were done by variable pressure scanning electron microscope (VPSEM), (model Leo 1450, accelerating vo ltage at 30 kV) equipped with energy dispersive X-ray analysis (EDX) and transmission electron microscopy (TEM) (mod el Phillips, CM12) operated at 100 kV. The X-ray diffraction (XRD) measurements were performed by a Bruker D8 advance X-ray diffractometer with running step =0.02° in the range of 20–65° 2-Theta, using a monochromatized Cu K α radiation ( λ = 0.154 nm). The XPS analyses were performed using a XSAM-HS KRATOS X-ray photoelectron spectroscop y. X-ray source type MgK was used with 10 mA current and 12 kV voltage to run XPS analys is for samples at 10 − 9 Torr pressure. The pass energy was set at 160 eV for the survey spectra and at 40 eV for the high resolution spectra of all elements of interest. Data processing was performed using the Kratos software after Shirley baseline subtraction and using Schofield sensitivity factors corrected for instrumentation transmission function. 3. Results and discussion The mor phologie s a nd s ize of the prepa red n anopar ticles wer e studied by variable pressure scanning electron microscope (VPSEM) and transmission electron micro scopy (TEM). They give comparable informa- tion for morphology and s ize investigations. The VPSEM a nd TEM images are depicte d in Figs. 1 and 2 respectively and show that the WO 3 nanoparticles prepared via sucrose ester microemulsion for both methods have spherical shape and approximately the same size range between 10 and50nm.WO 3 nanoparticle s prepared via CTAB microemulsion also have a spherical shape but with bigger size range between 25 and 50 nm. The XRD patterns in Fig. 3 shows that the WO 3 was in the form of orthorhombic lattice for all of the nanoparticles. More evaluation of the composition and purity of prepared WO 3 nanoparticles has been done by energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The calculated stoichiometry for the prepared nanoparticles taken from the atomic ratio data of EDX measurements (Fig. 4)isWO 3 within the limits of the experimental error. The wide scan of XPS spectrum within the B.E. range of 0–1100 eV and the narrow scan have been done for the WO 3 samples. Fig. 5 shows peak analysis of W 4f 7/2 ,W 4f 5/2 ,O 1s and C 1s signals for WO 3 samples. Both of the XPS and EDX patterns reveal the existence of W, O and C in the nanoparticles. The existence of C impurity in nanoparticles is believed to originate from environmental contamination and also the residual surfactants absorbed on the nanoparticles. The C 1s peak in the XPS results (Fig. 5) is from carbon contamination that is very usual and in fact, it is often used to calibrate peak position and in this case we assumed it comes from the residual surfactant and the environment. The photoelectron peak of the W 4f region in all of WO 3 samples shows a well-resolved double peak due to the 4f 7/2 and 4f 5/2 components (spin orbit splitting) and reveals the W +6 state and oxide form of tungsten in compound according to XPS handbook [20].TheO 1s band was deconvoluted in 3 components, the first one from right assumed is terminal oxygen (_O) and the second one is linkage oxygen (–O–) and these peaks were associated to the O 2− state, and the third one is Table 1 Binding energy (eV) for relative peak of WO 3 samples (corrected using C 1s = 285 eV as a reference) Sample Particle size (nm), shape O 1s (1) O 1s (2) O 1s (3) W 4f (1) W 4f (2) W 4f (3) W 4f (4) WO 3 (bulk) 40–120, different shapes 530.3 532.1 533.5 35.7 37.8 WO 3 (sample 1) 10–50, spherical 530.6 532.2 533.6 36.0 38.0 WO 3 (sample 2) 10–50, spherical 530.5 532.3 533.8 35.9 38.1 37.3 39.7 WO 3 (sample 3) 25–50, spherical 530.7 532.4 533.6 36.1 38.2 2656 N. Asim et al. / Materials Letters 61 (2007) 2652–2657 assumed to come from different sources, probably coming from rooted OH groups or from humidity in ambience (Table 1) [20]. The binding energies show the blue shift for nanomaterials prepared in this study compared with the bulk one and the more study on XPS blue shift induced in WO 3 nanoparticles are still in progress. 4. Conclusion Sucrose ester microemulsion and CTAB micelle solution systems in the reverse mic elle region have been used as template for the synthesis of WO 3 nanoparticles. The resultant nanoparticles have been investigated with XRD, VPSEM, EDX TEM and XPS methods and the results have shown and revealed the successful synthesis of WO 3 nanoparticles. It seems that no obvious difference exists between WO 3 nanoparticles obtained from using the one sucrose ester microemulsion with heat aging process and using of the mixing two sucrose ester microemulsions process. In both methods using sucrose ester microemulsion as a template the shapes are spherical with the orthorhombic lattice and particles sizes are approximately between 10 and 50 nm. WO 3 nanoparticles prepared via the CTAB micelle solution have spherical shape with bigger size range between 25–50 nm and orthorhombic lattice. More work to optimize the reaction conditions like precursors concentration and temperature for all mentioned methods for preparing smaller size range with very narrow size distribution is still in progress. Final ly this study shows that t he sucrose ester (biodegradable surfactants) microemulsion are suitable for synthesizing WO 3 nanoparticles. Acknowledgements The author would like to thank the following UKM staff namely: Mr. Zaki, Mr. Zailan, Ms. Normala and Mr. Syed for helpingwiththeuseofVPSEM,XRD,TEMandXPS respectively. References [1] S.K. Deb, Philos. Mag. 27 (801) (1973). [2] J.S.E.M. Svensson, C.G. Granqvist, Sol. energy mater. 12 (1985) 391–402. [3] H.T. 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