Green Synthesis of Hexagonal-Shaped WO3·0.33H2O Nanodiscs

Sep 17, 2008 - Synopsis. A facile route for the preparation of WO3·0.33H2O hexagonal-shaped nanodiscs composed of nanosheets via hydrothermal treatme...
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CRYSTAL GROWTH & DESIGN

Green Synthesis of Hexagonal-Shaped WO3 · 0.33H2O Nanodiscs Composed of Nanosheets

2008 VOL. 8, NO. 11 3993–3998

Liang Zhou,† Jin Zou,‡ Minmin Yu,† Peng Lu,† Jing Wei,† Yuqiang Qian,† Yunhua Wang,† and Chengzhong Yu*,† Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai, 200433, P.R. China, and School of Engineering and Center for Microscopy and Microanalysis, The UniVersity of Queensland, Brisbane QLD 4072, Australia ReceiVed January 20, 2008; ReVised Manuscript ReceiVed July 26, 2008

ABSTRACT: A facile route for the preparation of WO3 · 0.33H2O hexagonal-shaped nanodiscs composed of nanosheets via hydrothermal treatment of an aqueous peroxo-polytungstic acid solution has been demonstrated for the first time. The hexagonalshaped nanodiscs obtained have an average diameter of 200 nm and a mean thickness of several tens of nanometers. The effects of the preparation conditions such as the hydrothermal temperature and precursor concentration on the crystalline phase and morphology of the products have been studied systematically. The WO3 · 0.33H2O nanodiscs with an orthorhombic structure and regular hexagonal morphology can be synthesized in a wide hydrothermal temperature range from 100 to 200 °C. However, in the absence of hydrogen peroxide, monoclinic tungsten oxide square nanoplates are obtained. The hexagonal-shaped nanodiscs morphology have been well explained based on the crystal structure of orthorhombic WO3 · 0.33H2O. It is expected that this material with a well-defined and accessible crystal surface may find applications in catalysis, gas sensors, and other research areas.

1. Introduction Tungsten oxide based materials, including tungsten oxides (WO3-x) and tungsten oxide hydrates (WO3 · nH2O), have been studied extensively because of their polytype structures,1 intriguing physical/chemical properties,2-7 and widespread applications.8-19 With outstanding electrochromic, photochromic, gaschromic, field emission, and photocatalytic properties, tungsten oxides have been used in many fields, such as smart windows,8 solar energy devices,9 gas sensors,11,12 field emission devices,13-16 and photocatalysts.17,18 A great number of crystalline tungsten oxide and tungsten oxide hydrate phases have been synthesized previously.12,20-25 Among the hydrates, the structures of WO3 · 2H2O, WO3 · H2O, and WO3 · 0.33H2O have been well documented. In particular, WO3 · 0.33H2O was first discovered by Gerand et al.20 WO3 · 0.33H2O micrometer sized platelets and nanoneedle aggregates were obtained by hydrothermal treatment of an aqueous suspension of either tungstic acid gel (prepared by acidification of sodium tungstate) or crystallized WO3 · 2H2O at 120 °C.20 On the other hand, Pfeifer et al.26,27 found that the residual sodium was a persistent contaminant, and the WO3 · 0.33H2O phase could not be obtained with a sodium content less than 160 ppm. Further studies revealed that the WO3 · 0.33H2O phase can also be obtained by thermal decomposition of titanium or vanadium containing peroxo-polytungstic acid28-30 or acid leaching of tungsten containing compounds such as LiVWO6, LiAlW2O8, and LiFeW2O8.31,32 However, the morphologies of the products prepared by such methods are hard to control, especially at the nanoscale. In this paper, we demonstrate, for the first time, a facile method for the preparation of WO3 · 0.33H2O nanodiscs via the hydrothermal treatment of an aqueous peroxo-polytungstic acid solution. This process is atom-economical, and the resultant WO3 · 0.33H2O nanodiscs have uniform and well-defined hex* To whom correspondence should be addressed. E-mail: [email protected]. Phone: +8621-55665103. Fax: +8621-65641740. † Fudan University. ‡ The University of Queensland.

agonal-shaped morphology, although the crystal structure of the resultant WO3 · 0.33H2O nanodiscs has a low orthorhombic symmetry.

2. Experimental Section 2.1. Preparation. In a typical synthesis, 2.5 g of tungstic acid (0.01 mol) was dissolved in a mixture of 10 mL of 30 wt % hydrogen peroxide (H2O2) and 30 mL of distilled water under stirring to form a clear and colorless peroxo-polytungstic acid solution.33,34 The solution was then hydrothermally treated at 200 °C for 24 h, and a white colloidal suspension was obtained. The products were collected by centrifugation. For the preparation of thin films, a piece of glass slide was dipped into the white colloidal suspension directly and withdrew at a rate of 30 cm/min. To study the influence of hydrothermal temperature, we varied the hydrothermal temperature from 100 to 200 °C while keeping other synthesis parameters unchanged. To investigate the influence of peroxo-polytungstic acid precursor concentration, we varied the precursor concentration from 0.25 to 1.00 mol/L while the hydrothermal temperature was kept at 150 °C. To study the effect of H2O2 amount, we varied the H2O2 amount (0, 5, 10 mL; corresponding to a weight percentage of 0%, 3.75%, 7.50%, respectively) while keeping the hydrothermal temperature at 200 °C. 2.2. Characterization. X-ray diffraction (XRD) patterns were recorded on a German Bruker D4 X-ray diffractometer with Ni-filtered Cu KR radiation. Transmission electron microscopy (TEM) images were recorded on a JEOL 2011 microscope operated at 200 kV. Scanning electron microscopy (SEM) images were obtained on a Philips XL30 microscope operated at 20 kV. Thermal gravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/SDTA851 apparatus under an air flow of 100 mL/min with a heating rate of 5 °C/min.

3. Results 3.1. Characterization of WO3 · 0.33H2O Nanodiscs. XRD patterns of the products (both powders and films) are shown in Figure 1. For the powder sample (Figure 1a), all diffraction peaks can be exclusively indexed by the orthorhombic WO3 · 0.33H2O with the lattice parameters of a ) 0.7359 nm, b ) 1.251 nm, c ) 0.7704 nm and a space group of Fmm2 (Joint Committee on Powder Diffraction Standards, JCPDS Card No. 35-0270), suggesting a high purity of the products. In great

10.1021/cg800609n CCC: $40.75  2008 American Chemical Society Published on Web 09/17/2008

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Figure 1. XRD patterns of WO3 · 0.33H2O powders (a) and films (b).

Figure 3. TEM images of WO3 · 0.33H2O hexagonal nanodiscs at (A) a low and (B) a high magnification. The inset of (B) shows the SAED pattern. HRTEM images of WO3 · 0.33H2O hexagonal nanodiscs viewed from (C) top and (D) side direction.

Figure 2. SEM images of WO3 · 0.33H2O powders.

contrast to the XRD pattern of the powder sample, only a strong (002) diffraction and a weaker (004) diffraction can be detected in the XRD pattern of the film sample (Figure 1b), indicating that the thin film has a strongly preferential orientation with the normal of the thin film parallel to the c-axis of WO3 · 0.33H2O with a orthorhombic structure. Figure 2 presents the SEM images of the WO3 · 0.33H2O powders. Submicrometer sized particles with 100% purity can be found in Figure 2A. As can be seen from Figure 2B, the product has hexagonal disk-like morphology with an average size of ∼ 200 nm and a mean thickness of ∼ 50 nm. The arrows in Figure 2B indicate a side view of some disk-like particles.

The SEM images with different magnifications of the WO3 · 0.33H2O films are displayed in Supporting Information, Figure S1. At a low magnification, a homogeneous and crackfree film can be seen; at high magnifications, it is shown that most of the hexagonal disk-like particles lie flat on the substrate surface. Combining the XRD and SEM results together, we conclude that the c-axis of the WO3 · 0.33H2O orthorhombic phase is parallel to the normal of the discs. To understand the detailed structural and morphological characteristics of the resultant products, the TEM technique was employed. Figure 3A is a typical TEM image of products and shows similar morphology as observed in SEM images. Figure 3B is a TEM image of a typical well-developed hexagonalshaped disk-like particle with the corresponding selected area electron diffraction (SAED) pattern inserted in Figure 3B. A set of electron diffraction spots indicates that the WO3 · 0.33H2O particle is single crystal, and the SAED pattern can be indexed to the [001] zone axis of the orthorhombic WO3 · 0.33H2O. It should be noted that, besides the strong diffraction spots such as (200) and (020), weak diffraction spots can also be seen at those forbidden sites (indicated by white arrows in the inset of Figure 3B) in the SAED pattern. These weak diffraction spots can be attributed to the diffractions from the high-order Laue zone caused by the combination of elongation of diffraction spots along the normal of thin discs and the large {001} atomic spacing (narrowed Laue zones along the [001] direction). The comparison of the TEM image and the corresponding SAED pattern confirmed that the long edges of the hexagonal disk are of the {010} planes, the short edges are of the {110} planes, and the top and bottom surfaces of the hexagonal nanodisc are of the {001} planes. Figures 3C and 3D are high resolution TEM (HRTEM) images viewed from top and side of the WO3 · 0.33H2O hexagonal disks, respectively. The atomic spacings shown in Figure 3C are 0.37 and 0.64 nm, which respectively correspond to the {200} and {020} atomic spacings. From Figure 3D, the {020} and {002} atomic spacings are also confirmed by

Hexagonal-Shaped WO3 · 0.33H2O Nanodiscs

Crystal Growth & Design, Vol. 8, No. 11, 2008 3995

Figure 4. TGA curve of WO3 · 0.33H2O.

Figure 6. SEM of WO3 · 0.33H2O powders prepared at (a) 100 and (b) 150 °C with a precursor concentration of 0.25M.

Figure 5. XRD patterns of WO3 · 0.33H2O powder prepared at (a) 100, (b) 120, (c) 150, (d) 180, and (e) 200 °C.

HRTEM. In Figure 3D (viewed from side), the thickness of the disk is not uniform along its edge, suggesting that the disk could be composited of several sheets (with thicknesses of several nanometers). In fact, this conclusion can also be drawn from detailed analysis of Figures 3A (stacking of several nanosheets - arrowed) and 3B (the brighter contrast at edges arrowed). It is of interest to note that, although a disk is composed of several nanosheets, their crystallographical orientations remain the same. The TGA curve (Figure 4) of the products displays two major weight loss steps: one weight loss of 3.89 wt % occurred between 100 and 330 °C and mainly corresponds to the loss of constituent water in WO3 · 0.33H2O; another weight loss step (2.36 wt %) between 330 and 900 °C may be associated with the reduction of WO3 because the color of the sample changed from white to yellowish green after the TGA experiment. It should be mentioned that the weight loss of 3.89 wt % between 100 and 330 °C was much greater than the theoretical weight

loss (2.52 wt. %) that would be expected on the basis of the molecular formula WO3 · 0.33H2O; the most possible reason is that the reduction also happens in this temperature range. 3.2. Influences of the Hydrothermal Temperature. According to the literature,20,26,27,35 the WO3 · 0.33H2O phase is very sensitive to the preparation conditions, such as the hydrothermal temperature, pH value, and residual sodium ions. For example, the pure WO3 · 0.33H2O phase can only be obtained at a hydrothermal temperature of 120 °C.20 Above this temperature, yellowish products composed of WO3 · 0.33H2O, WO3 · H2O, and monoclinic WO3 were obtained.20 In our study, the hydrothermal temperature is varied from 100 to 200 °C to investigate its influence on the structure. To our surprise, as can be seen from the XRD patterns of samples prepared at different temperatures (Figure 5), the WO3 · 0.33H2O phase can be obtained in the whole hydrothermal temperature range under investigation (100-200 °C). Furthermore, the hydrothermal temperature has little influence on the overall morphology of the products as shown in Figure 2 (prepared at 200 °C) and Figure 6. 3.3. Influences of the Precursor Concentration. The influence of the peroxo-polytungstic acid precursor concentration was also studied. The precursor concentration did not affect the crystalline phase of the products as demonstrated by the XRD patterns shown in Figure 7; however, the relative intensity of the (020) peak to the (111) peak increases slightly with the precursor concentration. By comparing the SEM images of the samples prepared at 150 °C with different concentrations (Figures 6b, 8a, and 8b corresponding to a concentration of 0.25, 0.50, and 1.00 mol/L, respectively), it can be seen that increasing the precursor concentration leads to decreased and more irregular particles.

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Figure 7. XRD patterns (a, b, c) of WO3 · 0.33H2O powder prepared at 150 °C with a precursor concentration of 0.25, 0.50, 1.00 mol/L, respectively.

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Figure 9. XRD patterns of monoclinic WO3 obtained (a) without the addition of H2O2 and (b) with the addition of 5 mL of H2O2.

hexagonal-shaped nanodiscs. Without the addition of H2O2, only monoclinic WO3 (JCPDS Card No. 83-0951) can be obtained (Figure 9a). The products had a square plate-like morphology with edge lengths of 200-400 nm as shown in Figure 10a. Even with the addition of 5 mL of hydrogen peroxide solution, no WO3 · 0.33H2O phase was observed in the products (Figure 9b), and monoclinic WO3 square plates with much larger sizes (edge lengths of 500-1000 nm) were obtained (Figure 10b). The important roles of hydrogen peroxide in the synthesis are highly related to the chelating property of peroxo ligands [O2]2-: tungstic acid is not dissolvable in distilled water but dissolved completely in hydrogen peroxide solution to form a transparent and colorless peroxo-polytungstic acid solution because of the chelating effect of [O2]2-.36 The formation of stable coordination compounds in solution as hydrothermal treatment precursors is responsible for both the crystalline phase and morphology control of the resultant WO3 · 0.33H2O nanosheets.

4. Discussion

Figure 8. SEM images (A and B) of WO3 · 0.33H2O powder prepared at 150 °C with a precursor concentration of 0.50 and 1.00 mol/L, respectively.

3.4. Influences of the H2O2 Amount. The H2O2 amount played the most important role in the synthesis of WO3 · 0.33H2O

To fully explain the formation of hexagonal-shaped nanodiscs obtained in our study, even that the crystal structure of WO3 · 0.33H2O is orthorhombic, the structure of WO3 · 0.33H2O should be addressed at first. As can be seen from Figure 11, the orthorhombic WO3 · 0.33H2O structure contains two types of [WO6] octahedra. For type I [WO6] indicated by solid arrows, six oxygen atoms surrounding the central tungsten atom connect six different octahedra by corner-sharing. While for type II octahedra [WO5(H2O)] indicated by dotted arrows, two of the oxygen atoms off the ab plane are substituted by a shorter terminate WdO bond and a longer WsOH2 bond, respectively. As a consequence, an infinite layer is formed by corner-sharing type I and type II octahedra in the ab plane. The WO3 · 0.33H2O structure is built up by stacking of these layers along the [001] direction with a shift of a/2 between adjacent layers (a second layer is shown with red colors in Figure 11).

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is formed. As shown earlier, the resultant WO3 · 0.33H2O nanodiscs are enclosed by {001}, {110}, and {010} facets. According to the Bravais law,37 the faces with high reticular density (in other words, the facets with small surface free energy) are expected to be the exposed surface(s) during crystal growth. For the orthorhombic WO3 · 0.33H2O structure, the reticular density of different faces has an order of {001} > {110} > {010} > {100}. On the basis of the above discussion, the {001}, {110}, and {010} facets are expected as the exposed surfaces in WO3 · 0.33H2O because of their minimum surface energy, and this expectation agrees well with our experiment results. The fact that the shape of the nanodiscs has the hexagonal form can be attributed to the ratio of a to b being ∼ 1.7 (≈ tan60°), which leads to a dihedral angle of ∼ 120° between {010} and {110} planes.

5. Conclusion In conclusion, a facile route for the preparation of WO3 · 0.33H2O hexagonal nanodiscs without the assistance of any surfactants or templates was demonstrated. The effects of the preparation conditions, such as hydrothermal temperature, precursor concentration, and hydrogen peroxide amount on the crystalline phase and morphology of the products have been studied systematically. The complete absence of protecting agents on the surface of the products is a crucial point for some potential applications such as ultrasensitive gas sensors, where a well-defined and easily accessible crystal surface is required.

Figure 10. SEM images of monoclinic WO3 obtained (a) without the addition of H2O2 and (b) with the addition of 5 mL of H2O2.

Acknowledgment. Supported by the State Key Research Program (2006CB0N0302), the Ministry of Education of China, CNSF (20421303), Shanghai Science Committee (06JC14011) and Shanghai LADP (B113). Supporting Information Available: SEM images of WO3 · 0.33H2O films. This material is available free of charge via the Internet at http:// pubs.acs.org.

References

Figure 11. Schematic illustration of the WO3 · 0.33H2O structure.

The hexagonal-shaped disk-like morphology must be directly related to the structure of WO3 · 0.33H2O. Within the ab plane, all tungsten atoms in different octahedral blocks are connected by covalent bonding. In contrast, viewed along the [001] direction, each tungsten atom in type II octahedra has two terminal WdO and WsOH2 bonds, which are impossible to bond to other W atoms covalently. Compared to the interaction within one layer, the relatively weaker interaction between adjacent layers may restrict their stacking along the c axis to form the bulk form. As a consequence, the disk-like morphology

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