Journal of Molecular Structure 1044 (2013) 99–103
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Synthesis and characterization of WO3 nanowires and metal nanoparticle-WO3 nanowire composites Mária Szabó a,⇑, Péter Pusztai a, Anne-Riikka Leino b, Krisztián Kordás b, Zoltán Kónya a,c, Ákos Kukovecz a,d a
Department of Applied and Environmental Chemistry, University of Szeged, H-6720 Szeged, Rerrich ter 1, Hungary Microelectronics and Materials Physics Laboratories, Department of Electrical and Informatical Engineering, University of Oulu, P.O. Box 4000, Oulu FI-90014, Finland c MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, H-6720 Szeged, Rerrich ter 1, Hungary d MTA-SZTE ‘‘Lendület’’ Porous Nanocomposites Research Group, H-6720 Szeged, Rerrich ter 1, Hungary b
h i g h l i g h t s " Hydrothermal synthesis of hexagonal WO3 nanowires. " Decoration of the nanowires with metal nanoparticles via wet impregnation. " Characterization of the samples using XRD, TEM, SEM, IR, UV–Vis.
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Article history: Available online 24 November 2012 Keywords: Tungsten-trioxide Hydrothermal Metal decoration Electron microscopy Spectrometry
a b s t r a c t Tungsten-trioxide nanowire bundles were prepared using a simple hydrothermal method. Sodium-tungstate was used as precursor and sodium-sulfate as structure directing agent. All the reflections of the Xray diffractogram of the synthesized wires belong to the hexagonal phase of the tungsten trioxide. The nanowires were successfully decorated with metal nanoparticles by wet impregnation. The TEM investigation showed that using different metal precursors resulted in different particle sizes and coverage on the surface. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanotechnology and nanostructured materials have been in the center of the attention in the past few decades [1]. Nanotechnology is, ‘‘the design, characterization, production, and application of structures, devices, and systems by controlled manipulation of size and shape at the nanometer scale (atomic, molecular, and macromolecular scale) that produces structures, devices, and systems with at least one novel/superior characteristic or property’’ [2]. The unique properties of these new materials are different from the properties of the bulk matter and that is the reason why they can be used widely. In the fabrication of nanoscale devices, optoelectronic, electrochemical devices or in mesoscopic physics one dimensional (1D) nanostructures can play an important role [3,4]. 1D structures, such as wires, rods, belts and tubes, can be fabricated using techniques, like nano-lithographic techniques [5–7], vapor-phase or ⇑ Corresponding author. Tel./fax: +36 62 544 619. E-mail address:
[email protected] (M. Szabó). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.11.041
vapour–liquid–solid methods [8], and solution-phase methods [9,10]. Hydrothermal or solvothermal methods are widely used in the preparation of 1D nanostructures [11–13]. In this technique the solubility of the solids increases because of the generated higher temperature and pressure above the critical point of the used solvent. The great advantage of this method is that – because of the extreme conditions in the autoclave – almost every material can be made soluble [14,15]. Due to their wide applicability the different tungsten-oxides are very interesting materials. The complexity of the tungsten–oxygen system is demonstrated by the existence of various stoichiometric and non-stoichiometric structures. The structure of these oxides is defined by the WO6 octahedra. When these octahedra are connected through their tips forming a chessboard-like structure we get the most common tungsten-oxide, the tungsten trioxide which has three different allotropes generated by temperature changes (triclinic 330 °C). The other common oxide is the hexagonal tungsten-trioxide where the octahedra form tunnels [16].
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Because of the good chemical and physical properties these materials can be used widely, for example in photoelectrochemical or electrochemical cells [17], catalysis [18], sensors [19–21], electrochrom/photochrom/thermochrom applications [22,23] or as photocatalysts [24,25]. In this paper we present the hydrothermal synthesis of hexagonal tungsten trioxide nanowires, the decoration of these nanowires with metal nanoparticles via wet impregnation and the characterization of these materials. 2. Experimental 2.1. Chemicals All the chemicals were analytical grade and they were used without any purification. Sodium-tungstate and sodium sulphate was purchased from Reanal, hydrochloric acid solution (concentrated), acetone, and ethanol from Merck, silver-nitrate, gold-chloride, diamminedichloropalladium, tetraamineplatinum nitrate and nickel acetylacetonate from Sigma–Aldrich. And also deionized water was used. 2.2. Synthesis of tungsten-trioxide nanowires by hydrothermal process Sodium-tungstate (2.5 g) and sodium-sulfate (3.0 g) were dissolved in distilled water (80 ml). Hydrochloric acid (3 M) was added dropwise to the clear solution under continuous stirring, the pH of the solution was set to 1.5. After 10 min of stirring the mixture was transferred into a Teflon-lined stainless steel autoclave and was kept at 180 °C for 48 h. The product was collected by centrifugation and washed with distilled water and ethanol and dried at 60 °C in air. 2.3. Synthesis of the tungsten-trioxide nanowire/metal nanoparticle composites
Fig. 1. SEM image of the hydrothermally synthesized nanowire bundles without (a) and with (b) revolving the autoclave.
The earlier synthesized nanowires were suspended in the mixture of ethanol and acetone (1:1) using ultrasonication. To this suspension the appropriate amount of metal salt (silver-nitrate, goldchloride, diamminedichloropalladium, tetraamineplatinum nitrate and nickel acetylacetonate) was added to get 1% (compared to the mass of the nanowires) metal content on the surface. After 12 h stirring the solvent was evaporated and the product was dried at 80 °C followed by a heat treatment at 300 °C in air and a reduction step at 350 °C in N2/H2 atmosphere.
tral resolution of 4 cm 1. For Infrared measurements the KBr pellet technique was applied. 3. Results and discussion In the synthesis of WO3 nanowires sodium-tungstate was used as precursor and sodium-sulfate as structure directing agent. Sulfate ions adsorb on specific crystal faces and this way the surface energy decreases at these faces thus a kinetic control over the crystal growth occurs [26].
2.4. Characterization methods The morphology of the nanowires and the nanoparticle/nanowire composites were characterized by scanning electron microscopy (SEM; Hitachi S-4700) and by transmission electron microscopy (TEM, Fei Technai G2 20 X Twin). For SEM the samples were spread on the surface of a piece of carbon tape attached to the aluminum sample holder. For TEM the samples were sonicated in ethanol before being dropped on a copper mounted holey carbon film and dried. Powder X-ray diffraction (XRD) patterns were obtained from powder samples mounted on silicon slides in a Rigaku Miniflex II XRD instrument operating with Cu Ka radiation (k = 1.5406 Å). The reflectance spectra of the powder samples were obtained with an Ocean Optics USB4000 spectrometer using a DH-2000BAL UV–Vis-NIR light source, equipped with a diffuse reflectance probe. The infrared absorption spectra were recorded on a Bruker Vertex 70 FTIR instrument in the range of 400–1400 cm 1 with a spec-
Fig. 2. XRD pattern of the WO3 nanowires (top) without and (bottom) with revolving autoclave.
M. Szabó et al. / Journal of Molecular Structure 1044 (2013) 99–103
From earlier works [27,28] of our group it is known that revolving of the autoclave during the synthesis can affect the structure of the product. To investigate this effect in this particular case, two different samples were prepared. There are no visible differences between the nanowire bundles of the two methods as it can be seen on the SEM images shown on Fig. 1. The images display that the samples consist of nanowire bundles. These bundles are a few hundreds of nanometers wide and micron-long. The diameter of the individual wires is a few tens of nanometers. But the X-ray diffractograms (Fig. 2) indicate significant changes when revolving was applied. First of all the intensity decrease and the disappearing of the reflections indicate less crystal-
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linity in case of the sample prepared with revolving autoclave. The appearing reflections (marked with ) are thought to belong to some non-stoichiometric tungsten-oxide phase. Decoration of the nanowires with metal nanoparticles was successful in all cases. The color of the initially white nanowires turned into grey after the calcination step. The XRD study of the dried and the heat treated samples did not show any structural changes and the color of the only WO3 containing sample did not change due to calcination. Thus the appearing grey color of the decorated nanowires indicates the presence of the nanoparticles. And although the XRD pattern of the nanowire/nanoparticle composites does not show reflections relating to any metal or metal
Fig. 3. The TEM images of the metal nanoparticle decorated WO3 nanowires.
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Fig. 4. UV–Vis spectrum of a typical nanowire sample (small graph) and its Kubelka–Munk transformation.
but the dispersity is better in case of the nickel decorated sample. The platinum particles completely cover the surface of the wires and although the size distribution is not homogenous all the particles are smaller than 10 nm. Silver particles have heterogeneous size distribution and most of the particles seem to be between the wires and not necessarily attached to the surface. From UV–Vis spectra band gaps were calculated using the Kubelka–Munk equation. A typical spectrum and its Kubelka–Munk transformation are shown on Fig. 4. The band gaps calculated from the spectra were 3.5 eV before the impregnation step and after calcination and reduction changed to 3.8–4.0. This change can be explained by the oxygen loss due to reduction [29]. Because of this oxygen loss the hexagonal tungsten-trioxide phase transforms into a non-stoichiometric tungsten oxide phase (tungsten blue oxide) and with this transformation the white color of the initial wires turns into royal blue. On Fig. 5a the infrared absorption spectrum of the hydrothermally prepared nanowires is presented. Because of the fundamental vibrations of the W@O, WAO, WAOAW chromophores [30,31] a characteristic region appears on the spectra (1100–500 cm 1). The bands found at 1600 cm 1, 3500 cm 1 and 3600 cm 1 originate from moisture (adsorbed water). Fig. 5b shows a typical spectrum of the impregnated, calcined and reduced samples. According to these spectra there are no significant structural changes due to heat treatment and the reduction step.
4. Conclusion The synthesis of hexagonal tungsten trioxide nanowires and the nanowire/metal nanoparticle composites was successful. The XRD study confirmed the crystal structure of the wires. The morphology of the wires and the composites was examined by electron microscopy. It can be said according to these results that the prepared tungsten trioxide samples consist of bundles of individual nanowires and that the nanoparticles cover the surface. The dispersity and homogeneity of the nanowire/nanoparticle composites is the best in case of the nickel and platinum decorated wires. Band gaps were calculated from UV–Vis spectra. Their increase due to calcination and reduction is explained by the oxygen loss of the stoichiometric tungsten trioxide. According to the IR spectra there is no significant structural change after the heat treatment and the reduction step. Further research will be focused on the application of these materials in sensors and as catalysts.
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Fig. 5. IR spectra of the wires (a) and the metal decorated sample (b).
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