I. INTRODUCTION
In the past few years, one-dimensional (1D) nanostructured materials have attracted tremendous research interest owing to their unique physical, chemical, and optical properties (Shankar et al., Reference Shankar, Mor, Fitzzgerald and Grimes2007; Xia et al., Reference Xia, Yang, Sun, Wu, Mayers, Gates, Yin, Kim and Yan2003; Baeck et al., Reference Baeck, Choi, Jaramillo, Stucky and McFarland2003). Among all nanomaterials, tungsten oxides are of much importance because of their outstanding electrochromic, gaschromic, thermochromic, and optochromic properties, as well as their potential applications in electrochromic display, semiconductor gas sensors, and photocatalysts (Sanrato et al., Reference Sanrato, Odziemkowski, Ulmann and Augustynski2001; Pol et al., Reference Pol, Pol, Kessler, Seisenbaeva, Solovyov and Gedanken2005; Wang et al., Reference Wang, Lee and Ma2009). Particularly, 1D tungsten oxide can be used as a precursor for the preparation of tungsten disulfide nanotubes. As one important specie of the inorganic fullerenelike materials, tungsten disulfide nanotubes have many potential applications in electron device, catalyst, superior solid lubricant, and high performance composite (Tenne et al., Reference Tenne, Margulis, Genut and Hodes1992; Rothschild et al., Reference Rothschild, Sloan and Tenne2000). In a previous paper, we successfully prepared substoichiometric tungsten oxide nanowires by using a solution-based soft-chemistry method, and their features under thermal processing were systematically investigated (Sun et al., Reference Sun, Zhao, Xia, Zou, Min and Zhu2008). In this paper, we present an in-depth morphological and structural analysis on tungsten oxide nanowires using X-ray diffraction (XRD), selected area electron diffraction (SAED), and high-resolution transmission electron microscope (HRTEM) techniques.
II. EXPERIMENTAL
Bundled tungsten oxide nanowires were successfully synthesized by using a hydrothermal method with hexachloride and cyclohexanol as raw materials (Sun et al., Reference Sun, Zhao, Xia, Zou, Min and Zhu2008). First, 80 mg of WCl6 was dissolved in 2 ml of ethanol in a beaker to obtain a solution. 60 ml of cyclohexanol was then added to the solution, which was subsequently transferred to and sealed in a PTFE-line 120 ml autoclave. The reaction times in our experiments were from 6 to 24 h. After natural cooling to room temperature, the precipitate was removed from the autoclave and then centrifuged and washed with de-ionized water and acetone several times. The resulting products were then characterized by an X-ray diffractometer (Siemens D500, Cu K α radiation), a transmission electron microscope (TEM) (JEOL 2000FX, 200 KV), and a HRTEM (JEOL 4000FX, 400 KV).
III. RESULTS AND DISCUSSION
TEM results show that the resulting products, synthesized at a WCl6 concentration of 0.003M and with a reaction time of 6 h, exhibit bundled features with lengths up to a few microns and diameters of about 5 to 20 nm [see Figure 1a]. Detailed information on the nanowires characterization was previously reported by Sun et al. (Reference Sun, Zhao, Xia, Zou, Min and Zhu2008). Large different morphologies but only slightly different crystalline structures, depending on the concentration of WCl6 in cyclohexanol, have also been reported by Sun et al. (Reference Sun, Zou and Min2009).
Both the morphologies and the crystalline structures of the hydrothermal products obtained in this study vary with different reaction times. As shown in the bottom XRD pattern of Figure 2, for a short reaction time of 5 h, only a couple of weak diffraction peaks at 23.5° and 48.0°2θ can be assigned to the monoclinic W18O49 (010) and (020) reflections, respectively (Lamire et al., Reference Lamire, Labbe, Goreaud and Raveau1987). The broad and
Figure 1. TEM images of the product synthesized with a reaction time of 6 h: (a) bundled W18O49 nanowires and (b) single W18O49 nanowire. The inset in Figure 1b is the SAED pattern.
Figure 2. (Color online) XRD patterns of the as-synthesized products with different reaction times.
diffuse peak with a peak intensity appeared at around 27°2θ can be attributed to the presence of an amorphous phase. The XRD results indicate that the resulting product has very poor degree of crystallinity. Increasing the reaction time to 6 h, the W18O49 (010) peak becomes much sharper and stronger than that of the product obtained with a reaction time of 5 h, indicating that the W18O49 ⟨010⟩ direction is the dominant growth direction for the ultrathin nanowires. Meanwhile, the intensities of the amorphous phase become significantly weaker suggesting that a part of the amorphous phase had transformed into crystalline W18O49 (see the XRD pattern of the 6 h product in Figure 2). Increasing the reaction time further to 8 h, the XRD pattern is dominated by sharp and intensive diffraction peaks of monoclinic W18O49, while the broad amorphous peak at 27°2θ becomes almost invisible (see the XRD pattern for the 8 h product in Figure 2). Increasing the reaction time even further to 24 h, the W18O49 diffraction peaks are very strong, and no amorphous peak can be detected in the XRD pattern shown at the top of Figure 2. It should also be noted that the peak intensity of the W18O49 (104) reflection is also very strong similar to that of the strongest (010) peak, suggesting that the one-dimensional nanostructured feature in the hydrothermal product may likely disappear when the reaction time increases to 24 h. In fact, a mixture of irregular particles and thick rods was observed in the product with the reaction time of 24 h (not shown here).
After thorough dispersion by a sonication treatment, the bundled nanowires [shown in Figure 1a] can be dispersed to single flexible nanowires, and one of the nanowires is shown in Figure 1b. The SAED pattern [bottom right inset; Figure 1b] of the nanowire exhibits a row of well-defined spots running parallel to the nanowire growth axis with a basic lattice spacing of ∼0.38 nm and other rows of weak streak spots running nearly normal to the growth axis. The presence of the streak spots suggests the existence of crystal defects, such as dislocations, stacking faults, and/or oxygen vacancies. The HRTEM image shown in Figure 3a indicates that the lattice fringe separation is ∼0.38 nm for the (010) lattice plane of monoclinic W18O49, in agreement with the SAED and XRD results. A very thin amorphous layer can also be seen [see the black arrow in Figure 3a], confirming the XRD results. Moreover, HRTEM can give detailed information
Figure 3. HRTEM image of a W18O49 nanowire shown in Figure 1b. Figures 3b and 3c are the high magnification images of the corresponding sections in Figure 3a.
on possible defects presented in a nanowire. For example, by a careful examination of the squared areas in Figure 3a, edge dislocations or stacking faults can be clearly observed [see the white arrows shown in Figures 3b and 3c]. It is also worth pointing out that discontinuities of lattice fringe are presented in a larger nanowire. The presence of the observed discontinuities may be caused by a combination of the oxygen vacancies in substoichiometric W18O49 and an incomplete crystallization during the hydrothermal process. The crystal defects shown in the HRTEM images also correlated with the streaking spots in the SAED patterns [see Figure 1b].
The bundled tungsten oxide nanowires gradually undergo a series of morphological evolution with increased temperature, as reported in earlier works by Sun et al. (Reference Sun, Zhao, Xia, Zou, Min and Zhu2008, Reference Sun, Zou and Min2009). Based on HRTEM results obtained in this study, we propose a possible defect-induced mechanism. During thermal processing, crystal defects are highly activated after absorbing external energy and favor a morphological transformation of the nanowires. Hence, the defect-located areas will possibly be the preferential evolution source, especially the locations where individual nanowires are intersected within the bundles. With increasing temperature, the neighboring defective sites between single nanowires will then be combined, fractured, or broken, finally resulting in the apparent morphological evolution.
IV. CONCLUSION
In summary, the crystalline structures of the hydrothermal products with different reaction times were studied, and the structural analysis of the tungsten oxide nanowires using X-ray diffraction and transmission electron microscopy was also performed. The as-synthesized products exhibit increasing degree of crystallinity with increasing reaction time. It was found that the single dispersed nanowire has crystal defects, including stacking faults, edge dislocations, and/or oxygen vacancies. The results obtained in this study provide a better understanding of the temperature-dependent morphological evolution of the ultrathin nanowires under thermal processing, which can be beneficial to define the working environment for their potential applications.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China under Grant Nos. 50972063 and 50572041, the Natural Science Foundation of Shandong Province under Grant No. Y 2007F 64, the Science and Research Development Plan of Education Department in Shandong Province under Grant No. J 06A 02, the Tackling Key Program of Science and Technology in Shandong Province under Grant No. 2006GG2203014, and the Application Foundation Research Program of Qingdao under Grant No. 09-1-3-27-jch.
