Synthesis and Characterization of Zirconium Tungstate Ultra-Thin Fibers Lifeng Zhang,† Jane Y. Howe,‡ Yan Zhang,*,§ and Hao Fong*,† Department of Chemistry, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, South Dakota 57701-3995, Materials Science and Technology DiVision, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6064, and School of Physics and Materials Science, Anhui UniVersity, Hefei, Anhui 230039, P. R. China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 667–670
ReceiVed NoVember 18, 2008; ReVised Manuscript ReceiVed December 24, 2008
Negative thermal expansion (NTE), dimensions decreasing as temperature increases, is a rare but highly desired property in solidstate materials. There are two (i.e., anisotropic and isotropic) types of NTE materials. Anisotropic NTE materials (e.g., cordierite1) decrease dimensions in some crystallographic directions while increasing dimensions in other directions; in such a case, an overall volume expansion usually occurs with the increase of temperature. Zirconium tungstate (ZrW2O8) is one of a few isotropic NTE materials, and has recently attracted growing attentions because of its relatively large decrease in dimensions in all crystallographic directions over a wide temperature range from 0.3 to 1443 K.2,3 Although ZrW2O8 was first discovered in 1959, its structure and properties had not been fully understood until the 1990s.2-6 An important application of ZrW2O8 is for making composites with high dimension stability at elevated temperatures; and this can be achieved by incorporation of ZrW2O8 in a matrix material with positive coefficient of thermal expansion (CTE). Composites with precisely tailored CTEs (preferably zero through adjusting the volume fraction of ZrW2O8) are particularly useful for aerospace applications such as high-precision optical systems and/or electronic devices. ZrW2O8 is meta-stable at the room temperature yet is thermodynamically stable in the temperature range from 1380 to 1530 K.3,7 In the past, ZrW2O8 was synthesized by first heating a mixture of ZrO2 and WO3 above 1380 K, then holding the temperature in the stable range for a period of time followed by a rapid cooling. This method requires a high synthesis temperature, and it is often difficult to obtain phase-pure ZrW2O8 because of the volatility of WO3 in that high temperature range.8,9 Recently, several other methods including solid-combustion, coprecipitation, sol-gel, and hydrothermal synthesis have been developed to prepare ZrW2O8 at relatively low temperatures.7-10 Nonetheless, no matter which method is adopted, the synthesized ZrW2O8 is usually a powder with particle/rod sizes ranging from nanometers to micrometers and aspect ratios of less than 10.9,10 This study aimed to synthesize ZrW2O8 ultrathin fibers with large aspect ratios of 100 or higher. Ultrathin ZrW2O8 fibers could be utilized to develop innovative one-dimensional NTE composites, which might have improved properties as compared to the currently available one-dimensional NTE composites containing spherically, cylindrically, and/or irregularly shaped ZrW2O8 particles. Electrospinning has been repeatedly demonstrated as a straightforward and cost-effective method to prepare polymer, ceramic, and carbon/graphite fibers with diameters ranging from submicrometers to nanometers.11-15 Electrospun ceramic nanofibers are generally prepared by electrospinning spin dopes containing ceramic precursors and carrying polymers followed by high-temperature * Corresponding author. Phone: 86-551-386-1255(Y.Z.); (605) 394-1229(H.F.). Fax: 86-551-576-9883 (Y.Z.); (605) 394-1232 (H.F.). E-mail: zhangyaner2005@ 163.com (Y.Z.);
[email protected] (H.F.). † South Dakota School of Mines and Technology. ‡ Oak Ridge National Laboratory. § Anhui University.
pyrolysis. The ZrW2O8 precursors selected in this study were zirconium(IV) oxychloride octahydrate (ZOO, product number 31670, Sigma-Aldrich) and ammonium metatungstate hydrate (AMH, product number 16906, Sigma-Aldrich).7 Polyvinylpyrrolidone (PVP, product number 437190, Mw ≈ 1 300 000, SigmaAldrich) was used as the carrying polymer because it decomposes before melting starts, thus the fiber morphology can be well preserved during pyrolysis.16 Additionally, PVP molecules with lone pair electrons on N and O atoms can coordinate with metal ions and/or metal-containing particles; thus PVP can also serve as the colloidal stabilizer for the spin dope containing ZrW2O8 particles.17 Morphologies and structures of the synthesized ZrW2O8 ultrathin fibers and their precursors were examined by SEM (Zeiss Supra 40VP field-emission), XRD (Rigaku Ultima Plus), and TEM (Hitachi HF-3300). Two synthetic approaches were explored during this study to prepare the ZrW2O8 ultrathin fibers. The following were the general procedures of the two approaches: (1) mixing ZrW2O8 precursors (i.e., ZOO and AMH) with PVP in a solvent to prepare a spin dope, electrospinning the spin dope to obtain precursor fibers, pyrolyzing the precursor fibers to burn/remove PVP, and finally, converting the product into ZrW2O8 through refluxing in 6 M hydrochloric acid for 2 days and aging at the room temperature for 7 days;7 and (2) synthesizing a ZrW2O8 powder in advance according to a reported procedure,7 mixing the synthesized ZrW2O8 powder with PVP in a solvent to prepare a spin dope, electrospinning the spin dope to prepare precursor fibers, and finally, pyrolyzing the precursor fibers to make ZrW2O8 fibers. We experimented with both approaches and revealed that approach (1) was not successful: although ZrW2O8 crystalline structure formed after refluxing and aging treatments, the final product did not possess the fiber morphology. Therefore, we concluded that refluxing and aging treatments had to be undertaken prior to electrospinning, and approach (2) was thus adopted as the synthetic method. The ZrW2O8 ultrathin fibers studied in this research were synthesized using the following procedure: (1) dissolving ZOO and AMH in distilled water separately followed by mixing the two solutions under vigorous mechanical stirring; (2) refluxing the precipitate in 6 M hydrochloric acid (diluted from product number 435570, SigmaAldrich) for 2 days followed by aging it at room temperature for 7 days; (3) filtering and drying the mixture, then heating it at 600 °C for 10 h to make a powder consisting of ZrW2O8 particles; (4) mixing the ZrW2O8 particles in N,N-dimethylformamide (DMF, product number 227056, Sigma-Aldrich) followed by ultrasonication to facilitate the separation and uniform dispersion of ZrW2O8 particles; (5) preparing a spin dope by adding PVP into the mixture of ZrW2O8 and DMF; (6) electrospinning the spin dope to prepare precursor fibers; and (7) pyrolyzing the precursor fibers first at 325 °C for 8 h then at 600 °C for 10 h to make the final ZrW2O8 in ultrathin fiber form. Figure 1A is a SEM image showing the representative morphology of the synthesized ZrW2O8 powder, which consisted of tiny rods/particles with diameters in tens of nanometers and aspect ratios
10.1021/cg801272m CCC: $40.75 2009 American Chemical Society Published on Web 01/12/2009
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Figure 1. (A)SEM image showing the morphology of the synthesized ZrW2O8 powder, (B) SEM image showing the morphology the ZrW2O8 powder after ultrasonication, and (C) high-resolution TEM image showing that a representative particle in (B) was polycrystalline with the thickness of surface amorphous layer being several nanometers.
of 10 or smaller. Because the ZrW2O8 rods/particles were agglomerated, a 230 W Branson ultrasonic cleaner was used to separate and disperse the rods/particles in DMF. Figure 1B is a representative SEM image showing that the ZrW2O8 powder after ultrasonication consisted of spherically and/or irregularly shaped particles with sizes ranging from tens to hundreds of nanometers and aspect ratios of 1-2. High-resolution TEM revealed that these ZrW2O8 particles were polycrystalline having an amorphous surface layer with the thickness of ∼5 nm (Figure 1C). XRD (Figure 2A) indicated that the ZrW2O8 powder had a small amount of impurity (likely WO3), which was consistent with the TEM observation. According to the Bragg equation and the Scherrer equation as shown below, the average interplanar spacing d and crystallite size parameter Lc could be determined using the XRD results.
nλ 2sin θ 0.9λ Lc ) βcos θ d)
Where n is an integer number determined by the order of diffraction, λ is the wavelength of X-ray (λ ) 0.154 nm in this study), θ is the scattering angle, and β is the full-width at halfmaximum of the diffraction peak (in radians); the coefficient of 0.9 is included in the Scherrer’s eequation because the assumption of spherically shaped crystallites is adopted to analyze the peak. The strongest diffraction peak centered at the 2θ angle of 21.68° is attributed to the (210) crystallographic plane in ZrW2O8;2 accordingly, d(210) equals to 0.41 nm and Lc(210) equals 23 nm. The calculated d(210) value is consistent with the high-resolution TEM observation of the interplanar spacing (Figure 2A). A uniform and stable spin dope that contained 5 wt % ZrW2O8 and 20 wt % PVP was prepared by adding PVP into the colloidal
mixture of ZrW2O8 particles in DMF. Electrospinning was conducted under the ambient condition using a specially designed setup as reported previously;16,18 and the applied voltage during electrospinning was set at 10 kV. It is noteworthy that the process was extremely stable, and the electrospinning jet could run steadily without breaking for many hours. The obtained fiber mat on an aluminum collector had a thickness of ∼40 µm and a mass per unit area of ∼30 g/m2, whereas the fibers had diameters of ∼1 µm (Figure 3A). The as-electrospun precursor fibers contained very few microscopically identifiable beads and/or beaded-nanofibers and had relatively small variations in diameters; TEM revealed that the ZrW2O8 particles were randomly distributed in the fibers (Figure 3D). These fibers were subsequently subjected to pyrolysis first at 325 °C for 8 h and then at 600 °C for 10 h in air. The overall fiber morphology was well-preserved in both stages (images B and C in Figure 3). It is noteworthy that pyrolysis at 325 °C was essential to retain the fiber morphology, probably because the partial decomposition of PVP at 325 °C resulted in the formation of a three-dimensional char network. If the precursor fibers were pyrolyzed directly at 600 °C, more broken fibers would be obtained (results not shown). Pyrolysis at 600 °C removed organic and/or carbonaceous components and resulted in the formation of the final ZrW2O8 ultrathin fibers. The final ZrW2O8 fibers (imag es C and F in Figure 3) had diameters in the submicrometer range, which was slightly smaller than those of the as-electrospun precursor fibers and the 325 °C heat-treated intermediate fibers (images A, C, D, and F in Figure 3). XRD (Figure 2B) indicated that the final ZrW2O8 ultrathin fibers possessed a higher degree of purity/crystallinity than the previously obtained ZrW2O8 powder. The strongest diffraction peak corresponding to the (210) crystallographic plane in the ZrW2O8 ultrathin fibers was centered at the same 2θ angle as that of the ZrW2O8 particles; and the calculated d(210) value was
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Crystal Growth & Design, Vol. 9, No. 2, 2009 669
Figure 2. XRD curves of (A) the synthesized ZrW2O8 powder and (B) the final ZrW2O8 ultrathin fibers. The (C) curve is the standard XRD spectrum of ZrW2O8. The insets are the high-resolution TEM images indicating the interplanar spacing in both A and B is ∼0.4 nm, whereas the surface amorphous layer in B is significantly thinner than that in A.
Figure 3. (A-C) SEM and (D-F) TEM images showing the representative morphologies and structures of (A, D) the as-electrospun precursor fibers, (B, E) the intermediate fibers after the heat treatment of the precursor fibers at 325 °C, and (C, F) the final ZrW2O8 ultrathin fibers after the heat treatment of the intermediate fibers at 600 °C.
consistent with the high resolution TEM observation of the interplanar spacing (Figure 2B). The Lc(210) value, however,
increased to 29 nm, which was higher than that of the ZrW2O8 particles (23 nm). Figure 4 is a high-resolution TEM image showing
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Acknowledgment. This research was supported by the U.S. Air Force Research Laboratory (AFRL) under the Cooperative Agreement Number (CAN) of FA9453-06-C-0366. TEM study was sponsored by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency & Renewable Energy, Office of FreedomCAR and Vehicle Technologies, though the High Temperature Materials Laboratory (HTML) at the Oak Ridge National Laboratory (ORNL).
References
Figure 4. High-resolution TEM image showing the crystalline morphology of a representative single-crystalline ZrW2O8 crystallite in the final ZrW2O8 ultrathin fibers. The inset is the fast Fourier transform electron diffraction pattern of the crystallite.
the representative crystalline morphology of the crystallites in the final ZrW2O8 ultrathin fibers. These single-crystalline ZrW2O8 crystallites had sizes of ∼30 nm and surface roughness of several nanometers, which were consistent with the XRD result; additionally, the average thickness of the surface amorphous layer in the final ZrW2O8 crystallites was reduced to 1 nm or less, which was significantly thinner than that in the ZrW2O8 particles (∼5 nm) (Figure 1C). These results indicated that electrospinning followed by pyrolysis not only led to the formation of ZrW2O8 ultrathin fibers, but also improved the purity/crystallinity of ZrW2O8 and resulted in larger ZrW2O8 crystallites. As a conclusion, this study demonstrated that electrospinning followed by pyrolysis was an innovative method to prepare ZrW2O8 ultrathin fibers with diameters in hundreds of nanometers; and the fibers were made of single-crystalline ZrW2O8 crystallites with sizes being tens of nanometers. The ZrW2O8 ultrathin fibers are expected
(1) Service, R. F. Science 1996, 272, 30. (2) Mary, T. A.; Evans, J. S. O.; Vogt, T.; Sleight, A. W. Science 1996, 272, 90–92. (3) Duan, N.; Kameswari, U.; Sleight, A. W. J. Am. Chem. Soc. 1999, 121, 10432–10433. (4) Kowach, G. R. J. Cryst. Growth 2000, 212, 167–172. (5) Evans, J. S. O.; Mary, T. A.; Vogt, T.; Subramanian, M. A.; Sleight, A. W. Chem. Mater. 1996, 8, 2809–2323. (6) Evans, J. S. O.; Hu, Z.; Jorgensen, J. D.; Argyriou, D. N.; Short, S.; Sleight, A. W. Science 1997, 275, 61–65. (7) Closmann, C.; Sleight, A. W.; Haygarth, J. C. J. Solid State Chem. 1998, 139, 424–426. (8) Kameswari, U.; Sleight, A. W.; Evans, J. S. O. Int. J. Inorg. Mater. 2000, 2, 333–337. (9) Xing, Q.; Xing, X.; Yu, R.; Du, L.; Meng, J.; Luo, J.; Wang, D.; Liu, G. J. Cryst. Growth 2005, 283, 208–214. (10) Sullivan, L. M.; Lukehart, C. M. Chem. Mater. 2005, 17, 2136–2141. (11) Dzenis, Y. Science 2004, 304, 1917–1919. (12) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670– 5703. (13) Fong, H. In Polymeric Nanostructures and Their Applications; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2007; pp 451-474. (14) Lu, X.; Liu, X.; Zhang, W.; Wang, C.; Wei, Y. J. Colloid Interface Sci. 2006, 298, 996–999. (15) Wang, G.; Ji, Y.; Huang, X.; Yang, X.; Gouma, P. I.; Dudley, M. J. Phys. Chem. B 2006, 110, 23777–23782. (16) Liu, Y.; Sagi, S.; Chandrasekar, R.; Zhang, L.; Hedin, N. E.; Fong, H. J. Nanosci. Nanotechnol. 2008, 8, 1528–1536. (17) Lu, X.; Zhao, Y.; Wang, C. AdV. Mater. 2005, 17, 2485–2488. (18) Liu, Y.; Cui, L.; Guan, F.; Gao, Y.; Hedin, N. E.; Zhu, L.; Fong, H. Macromolecules 2007, 40, 6283–6290.
CG801272M
