Fabrication of Combined One-Dimensional and Three-Dimensional

Jun 14, 2013 - Hajime Wagata, Maki Fujisawa, Yusuke Mizuno, Nobuyuki Zettsu, Shuji Oishi, and Katsuya Teshima*. Department of Environmental Science ...
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Fabrication of Combined One-Dimensional and Three-Dimensional Structure of Potassium Tungstate Crystal Layers by Spray Deposition with Polystyrene Colloidal Crystal Templates Hajime Wagata, Maki Fujisawa, Yusuke Mizuno, Nobuyuki Zettsu, Shuji Oishi, and Katsuya Teshima* Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan ABSTRACT: Well-aligned honeycomb-designed layers of potassium tungstate (K0.33WO3.165) were successfully fabricated by spray deposition and microsphere lithography using polystyrene (PS) colloidal crystals as templates. A densely packed monolayer of monodisperse PS microspheres was formed on a silica glass surface, using the Langmuir−Blodgett thin film technique. Subsequently, a (NH4)10W12O41·5H2OKCl aqueous solution was sprayed as a micromist on the PS templates. While the K0.33WO 3.165 crystal layers were synthesized by chemical reaction between W and K sources during the heating, the PS templates were thermally decomposed, generating an inverse opal surface structure where the interstitial space of the densely hexagonal packed PS microspheres had been. Numerous hexagonal rodlike crystals were grown on the honeycomb-designed layer of K0.33WO3.165. X-ray diffraction analysis, energy-dispersive X-ray spectroscopy analyses, and X-ray photoelectron spectroscopy were performed to analyze the combined one-dimensional and three-dimensional honeycomb-designed structure.

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surface structure consisting of a high-quality crystal with a specific exposed crystal plane and a porous structure with a high specific surface area will provide a significant benefit for application of optical, electrical, and catalytic materials. In fact, a single-crystalline porous photocatalyst possessing both characteristics exhibited a much better water oxidation property than conventional particulate ones.18 Tungsten oxide compounds have attracted research interest as a means to obtain these properties.19−24 It is well-known that there are many stoichiometric compositions of K2O and WO3 in potassium tungstate compounds, including 1:1, 2:3, 1:2, 1:3, and 1:4. Although many attempts have been made to prepare nano/microstructured potassium tungstate crystals at relatively high temperatures25,26 or using the hydrothermal method,27 there is no research on the 1D and 3D combined structure. Since potassium tungstate, potassium-promoted WO3, and WO3 were reported to be effective for partial oxidation of methane,28 the 1D and 3D combined structured potassium tungstate will contribute improvement of the property. Herein, we report the facile fabrication of honeycomb-designed potassium tungstate layers coated with 1D potassium tungstate nanowires by spray deposition, using polystyrene (PS) colloidal crystals as templates. Furthermore, effects of the heating time on the nano/microstructures are investigated. A schematic illustration of the experimental procedure is shown in Figure 1. First, the Langmuir−Blodgett thin film technique was used to prepare densely packed monolayers of

ormation of highly functional crystal layers on various material surfaces is greatly desired for the production of leading-edge nano/microdevices. In particular, well-ordered three-dimensional (3D) porous structures have recently been of great interest because of their large surface area and unique structurally derived optical, electrical, and catalytic properties.1−4 There have been many studies on the fabrication of well-ordered porous 3D structures using techniques such as sol−gel template, electrochemical anodization, inkjet, and laser lithography methods.1−8 Among these methods, microsphere lithography has been of great interest because of its technological advantages, such as environmental friendliness, simplicity, and low cost. 9−13 In this process, densely hexagonally packed microsphere templates are prepared using the Langmuir−Blodgett film technique, and then the target materials are fabricated in the gaps between the microspheres. The microsphere template is then removed by thermal treatment or chemical etching, leaving a 3D inverse opal structure made up of the target material. However, the 3D porous structures usually have low crystal quality, which limits their potential for utilization of a specific crystal plane because they are generally constructed from aggregates of nanosized inorganic materials. To fabricate a surface with specific crystal planes with good crystal quality, various direct fabrication processes for producing array structures consisting of one-dimensional (1D) materials such as nanorods, nanowires, nanotubes, fibers, whiskers, and belts have been researched intensively.14−17 In this case, however, it is difficult to increase the specific surface area while maintaining the specific crystal planes because precise control of the crystal growth is required. A combined © 2013 American Chemical Society

Received: May 8, 2013 Revised: June 12, 2013 Published: June 14, 2013 3294

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Figure 1. Schematic illustration of the experimental procedure: (a) Langmuir−Blodgett thin film technique, (b) densely hexagonally packed PS colloidal crystals on glass surface, (c) spray deposition, and (d) heating.

monodisperse PS microspheres on a silica glass (10 × 10 × 0.5 cm3) surface. The PS microspheres had diameters of 1.62 ± 0.13 μm and were dispersed in isopropyl alcohol (IPA). The IPA suspension was carefully dropped on a clean water surface by a micropipet, and two-dimensional colloidal crystal films of PS microspheres were formed. Silica glass substrates with smooth hydrophilic surfaces were immersed with a slope angle of 45° into the water surface to transfer these colloidal crystal films onto the substrates. The PS microspheres were densely arrayed on the substrate. These colloidal crystal films were then used as templates for microsphere lithography. In particular, honeycomb-designed potassium tungstate layers were fabricated by spray deposition on the PS microsphere template and subsequent heating. During the deposition process, the sprayed precursor droplets undergo evaporation, solute condensation, and thermal decomposition, which ultimately result in the layer formation. In this study, the precursor solutions for the crystal layers were 5 × 10−3 M of (NH4)10W12O41·5H2O (Wako Pure Chemical Industries, Ltd.) and 3.0 × 10−2 M of KCl (Wako Pure Chemical Industries, Ltd.). The molar ratio of K/W was fixed at 0.5. The deposition temperature was maintained at 120 °C to dry the precursor droplets and to avoid thermal decomposition of the PS colloidal crystal film. When the precursors were sprayed on the PS colloidal crystal templates, they could permeate the template spaces. Finally, the substrates on which the PS array template was imbued with the precursor solutions were heated in air at 500 °C in an electric furnace so that the (NH4)10W12O41·5H2O and KCl would react into potassium tungstate and so that the PS microspheres would thermally decompose. The heating times were 1 and 5 h. The substrates were then cooled at a rate of 100 °C/h to room temperature. The obtained honeycomb-designed crystal layers were observed with a field emission scanning microscope (FESEM, JEOL, JSM-7000F) operated at an accelerating voltage of 15 kV. All samples were sputter-coated with Pt for the SEM measurements. The crystal phases of the prepared honeycombdesigned layers were studied by X-ray diffraction (XRD, RIGAKU, MiniflexII) with Cu Kα radiation (λ = 0.15418 nm). Chemical bonds were investigated by X-ray photoelectron

spectroscopy (XPS, JPS-9010MC, JEOL) using nonmonochromated Mg Kα radiation (1253.6 eV) with a 10 mA emission current and a 10 kV accelerating voltage. The profiles were fit using a Gaussian−Lorentzian function, and the fraction of the Gaussian was set to over 90% for all the component curves. The peak positions were normalized by positioning the C 1s peak at 284.5 eV. Well-ordered honeycomb-designed structures with numerous nanowires were successfully fabricated on the silica glasses by spray deposition using PS colloidal crystals as templates. The template was successfully prepared using the Langmuir− Blodgett thin film technique because the PS microspheres self-assembled on the water surface. The PS microspheres with 1.62 μm diameter were highly ordered, densely arranged, and closely hexagonally packed on the substrate. After the precursor solution was sprayed and the subsequent decomposition of the PS template by heating, nanowires on the surfaces of the honeycomb-designed walls on the substrates were obtained. Figure 2 shows SEM images of the PS colloidal crystal layer heated for 1 h. After this treatment, the PS microspheres were completely decomposed, and the chemical reaction between the (NH4)10W12O41·5H2O and KCl precursors had proceeded steadily. As a result, the long-range surface structure was successfully obtained on the silica glass (Figure 2, panels a and c), and relatively long nanowires were directly grown on the wall surfaces of the honeycomb structures (Figure 2, panels b and d). The PS microspheres were apparently decomposed after the formation of the honeycomb-designed layer because the porous surface structures were maintained. The honeycomb-designed layer had a thickness of approximately 1 μm, which corresponds to the radius of the PS microspheres. The thickness of the layer might be less than the diameter of the PS microspheres because of hydrophilic interactions between the substrate and solution and hydrophobic interactions between the PS microspheres and the solution. The precursor solutions could penetrate into the spaces between PS microspheres and spread on the silica glass surface because of these hydrophilic interactions, but the hydrophobic surfaces of the PS microspheres readily repelled the precursor solution. Therefore, the 3295

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morphologies of the crystal layers could be controlled by changing the heating time. Figure 4 shows XRD patterns of the honeycomb-designed layers fabricated with different heating times of 1 and 5 h. The

Figure 2. SEM images of the honeycomb-designed layer heated for 1 h: (a and b) tilt angle of 0° and (c and d) tilt angle of 30°.

Figure 4. XRD patterns of the honeycomb-designed layer with the 1D nanowires grown with heating times of (a) 1 h and (b) 5 h.

thickness of the layer was less than the diameter of the PS microspheres. The expansion of the spaces between the PS microspheres and the preservation of the densely hexagonally packed arrangement after spraying were due to the successful formation of a potassium tungstate crystal layer before the decomposition of the PS microspheres. The structure of the honeycomb-designed wall and the morphologies of the individual 1D crystals were greatly dependent on the heating time, as shown in Figure 3. Figure

diffraction lines did not change with increasing heating time. The crystal phase of the honeycomb-designed layers was identified as K0.33WO3.165 and/or WO3. No diffraction lines attributed to byproducts were observed. The spray-deposited precursor containing ammonium tungstate and KCl reacted to form 1D potassium tungstate, releasing gaseous ammonium and chlorine molecules. Since the growth of 1D structures occurred by extending the heating time after spray deposition, there should be mass transport to form the 1D structure. Although several models can be considered to explain the phenomenon, such as vapor−liquid−solid (VLS) growth, vapor-solid (VS) growth, or growth with potassium salt catalysts,29 it is difficult to conclude which process occurred to form the 1D potassium tungstate at this stage. The chemical composition and chemical bonding states of the honeycomb-designed layers fabricated at the heating time of 1 h were also investigated. Figure 5 shows a highmagnification FE-SEM image and the corresponding EDX mappings of the honeycomb-designed layer fabricated with a heating time of 1 h. The EDX mapping indicates that the W, K, and O atoms were present in both the honeycomb-designed layer and the nanowires, in which they were homogeneously distributed. Si atoms from the silica substrate were detected in the gaps within the honeycomb-designed layer. Chlorine atoms from KCl were not detected, which is consistent with the XPS result. The atomic ratio of K/W calculated from the EDX results was 0.32, which is comparable to that of K0.33WO3.165. Figure 6 shows XPS spectra of the honeycomb-designed layer fabricated with a heating time of 1 h. In the wide-scan XPS spectrum in Figure 6a, there are W, O, and Pt peaks. The W and O originate from the honeycomb-structured K0.33WO3.165, and the Pt is attributed to the Pt film sputtered before the SEM observation. Figure 6b shows the narrow scan for W 4f. There were two intense peaks at binding energies 34.91 and 37.00 eV, which corresponded to the literature values for the W 4f5/2 and W 4f7/2 states of potassium tungstate.28,30 These results indicate that the KCl and (NH4)10W12O41·5H2O reacted to form K0.33WO3.165 and that the other impurities were evaporated during the spray deposition and subsequent heating. The difference in the K/W ratio between the raw materials and the final product is considered to arise during the spray deposition process. Since KCl has a higher solubility in water than

Figure 3. SEM images of the honeycomb-designed layer heated for 5 h: (a and b) tilt angle of 0° and (c and d) tilt angle of 30°.

3 (panels a and b) show SEM images of the crystal layer heated for 5 h, and Figure 3 (panels c and d) show tilted SEM images of the same layer. Complete honeycomb structures remained, and numerous nanowires with relatively high aspect ratios were obviously grown on the wall surfaces of the honeycomb structure, indicating the promotion of crystal growth during the heating. The basic form of the rodlike crystals was a hexagonal cylinder, and their surfaces were relatively flat. These SEM images also show that the sizes of the individual 1D crystals increased with an increasing heating time. Thus, the 3296

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Figure 5. FE-SEM image and corresponding EDX mappings of the honeycomb-designed layer heated for 1 h.

Figure 6. (a) Wide-scan and (b) W 4f narrow-scan XPS spectra of the honeycomb-designed layer heated for 1 h.

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(NH4)10W12O41·5H2O, the KCl deposited on the substrate might more easily redissolve and flow away in the subsequently sprayed solution than would (NH4)10W12O41·5H2O. In summary, highly crystalline honeycomb-designed layers of K0.33WO3.165, consisting of numerous well-developed 1D nanowire crystals were successfully fabricated on silica glasses by spray deposition and subsequent heating. The colloidal crystal layer for use as a template for microsphere lithography was prepared by self-assembly of PS microspheres using the Langmuir−Blodgett thin film technique. After the precursor solutions were sprayed on the templates, the honeycombdesigned layers were formed through simple heating. The morphologies of the honeycomb-designed layers were greatly dependent on the heating conditions. Nanowires were directly grown on the wall surfaces of the honeycomb structure within a heating time of 1 h, and the width of the individual nanowires on the honeycomb walls increased with increasing heating time. EDX analysis of the honeycomb-designed layers showed that the layer was K0.33WO3.165, and Cl atoms were not detected on the layer. This technique combining spray deposition and microsphere lithography will be a promising method for fabricating combined 1D and 3D structures made of functional inorganic materials.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-26-269-5556 Fax: +81-26-269-5550. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by a Grant-in-Aid for Scientific Research (A) Number 25249089 from the Japan Society for the Promotion of Science (JSPS).

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