Tungsten Oxide Nanorods Array and Nanobundle Prepared by Using Chemical Vapor Deposition Technique
Tungsten oxide (WO3) nanorods array prepared using chemical vapor deposition techniques was studied. The influence of oxygen gas concentration on the nanoscale tungsten oxide structure was observed; it was responsible for the stoichiometric and morphology variation from nanoscale particle to nanorods array. Experimental results also indicated that the deposition temperature was highly related to the morphology; the chemical structure, however, was stable. The evolution of the crystalline structure and surface morphology was analyzed by scanning electron microscopy, Raman spectra and X-ray diffraction approaches. The stoichiometric variation was indicated by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.
KeywordsNanostructure Tungsten oxide Nanorod Nanobundle CVD
Nanostructured transition metal oxides are outstanding candidates for a wide range of applications including lithium-ion batteries, [1, 2] catalysts,  electrochromic materials, [4, 5] and sensors. [6, 7] Nanostructured tungsten oxide, as a typical transition metal oxide material, has been researched frequently these years.
The nanostructured tungsten oxide material exhibited many excellent properties because of their particular phase structure and huge surface areas, which depend greatly on the experimental parameters. In previous experiment of chemical vapor deposition (CVD), it was realized that several factors, such as filament temperature, electrical current, gas flow and the composition of gas, would affect the structure of the sample. The major factors could be the substrate temperature and the chamber pressure . Moreover, the effect of the reaction gas concentration on the sample properties was also preliminarily studied .
Based on the previous achievements, the focus of the present paper would be on two issues: to analyze the influence of oxygen gas concentration (OGC) on the stoichiometry phase, and to study an effect of substrate temperature on the crystalline structure of tungsten oxide nanorods array. All samples have been characterized by using Raman spectra, scanning electron microscopy (SEM); energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were also employed to characterize the samples.
Experimental Set Up
The nanostructured tungsten oxide materials were synthesized using a CVD technique. The Molybdenum (Mo) wafer was used as deposition substrate. Before placing the substrates in the CVD chamber, the mirror-like surface of the polished substrates were ultrasonically washed in a methanol solution for 5 min, rinsed with acetone, and dried with helium. After placing the substrate, the chamber was pumped down to 2.0 × 10−5 Torr before feeding the gases. Two kinds of gas mixture, 8.7% of CH4, 0.3% O2, and 91% H2and 8.3% of CH4, 0.7% of O2, and 91% H2gases were used. The flow rate of mixed gases was 5SCCM. The gas pressure inside the deposition chamber was maintained at 500 mTorr during the deposition. An AC power supply with electric current of 10 A and voltage of 8 V was used to heat the tungsten filament to temperature 2,400 °C to promote gas phase activation.
Results and Discussions
EDS elements component quantitative results
Based on the data above, the stoichiometry variation can be given. As seen under low OGC, the carbon content inside the sample was higher than that of oxygen. Therefore, two possible chemical states might coexist inside the sample. One was tungsten oxide together with tungsten carbide. Due to lack of oxide component, the stoichiometry phase of tungsten oxide should be WO3−x, wherex was related to the stoichiometry phase of tungsten carbide. The second possibility was that carbon atoms, tungsten atoms, and tungsten oxide mixed but independently existed, which mean there were no chemical bonds among them. This expectation has been confirmed by XPS or XRD measurements below.
The XPS profile of sample (b) prepared under high OGC was shown in Fig. 3b. The oxygen modified tungsten features remained unchanged, indicating the presence of stoichiometry tungsten oxide. The peaks of atomic tungsten vanished. Moreover, the shoulder peaks related to WO2 or WOxalso disappeared. This evidence strongly supported the stoichiometry phase evolution of the tungsten oxide.
Under high OGC, much more prominent Raman spectra peaks at 701 cm−1and 801 cm−1were indicated in Fig. 4b, which supported the existence of tungsten trioxide. A conclusion from Raman spectra was revealed: the crystalline tungsten oxide has been yielded under high OGC. Variation from the two weak humps in Fig. 4a to prominent peaks in Fig. 4b indicated that the crystalline structural evolution followed the change of OGC.
In summary, the stoichiometry phase evolution of tungsten oxide highly depended on the variation of OGC in mixture gases during deposition. Following an increase of OGC, the sub-stoichiometry tungsten oxide-WO2and WOx-would become stoichiometry tungsten oxide. It was also found that atomic tungsten and carbon without any chemical bond structure would be mixed inside the sample. The variation of OGC also determined the structural evolutions from amorphous to crystal. Low OGC caused yielding amorphous tungsten oxide, whereas high OGC resulted in producing polycrystalline WO3.
The morphologies of these three samples were prominently different. The sample of tungsten oxide prepared at 800 °C (Fig. 6a) was thin, sharp and short. Nanobundle was generated in this sample. The diameter of single nanorod was around 200 nm, and the length was 2 μm. The nanobundle was so compact that it tended to yield to larger nanorod. The sample yielded at 1,000°C was shown in Fig. 6b. The hump-like nanostructured tungsten oxide was obtained. The diameter of the hump at this temperature was 500 nm approximately. It could be assumed that the nanobundle in Fig. 6a gathered then became a hump. This dynamic phenomenon was similar to the little drip gathering to be a larger drip. Furthermore, the tungsten oxide nanorod was clearly shown in Fig. 6c. The diameter of the nanorod was more than 500 nm, and the length was longer than 5 μm.
Based on these three samples, it was concluded that the growth rate of tungsten oxide increased following the rising of temperature, resulting in that the sample diameter was large and the length was long. Similar result was reported by Chi et al.  and Pal et al. . It was especially mentioned that the root of each nanobundle was thinner than the main body . So it could be inferred that the thin nanobundle at lower temperature could be the base of the sample prepared at higher temperature, which was similar to the condition shown at former paragraph concerning the different OGC.
In conclusion, the variation of the properties of tungsten oxide highly depended on the OGC of the gas mixture. Slight rise of OGC from 0.3% to 0.7% in the mixture gas during deposition resulted in large change of the oxygen quantitative component in samples from 7% to 51%; the morphologies of the samples varied from particle-based film to nanorods array, and the chemical phase developed from sub-stoichiometry phase to stoichiometry. The crystalline structure also altered to crystalline tungsten oxide from amorphous structure. The sample prepared under high substrate temperature (800 °C–1,200 °C) was also investigated. The diameter and length of the samples’ nanostructure grew up by raising the substrate temperature. The evolution of morphology was prominent, whereas the structure was quite stable. Based on the evidences above, it could be concluded that the properties of tungsten oxide were highly sensitive to the experimental parameters during deposition, especially the OGC in the gas mixture.
This work has been supported by NSF-EPSCoR and DoD grants. We would like to thank Mr. William’s assistance of Raman measurements, Mr. Ortiz and Ms. Hernandez for SEM and EDS measurements, Mr. Esteban for XPS measurements and Mr. Wu for XRD measurements.
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