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J. Phys. Chem. C 2010, 114, 10–14
Ethanol-Promoted Fabrication of Tungsten Oxide Nanobelts with Defined Crystal Orientation Suphaporn Daothong,†,‡ Naratip Songmee,† Nuchjira Dejang,† Thomas Pichler,§ Hidetsugu Shiozawa,| Yan Jia,⊥ David Batchelor,# Esko Kauppinen,‡ Somchai Thongtem,† Paola Ayala,*,§ and Pisith Singjai*,† Nanomaterials Research Unit, Department of Physics, Faculty of Science, Chiang Mai UniVersity, Chiang Mai 50200, Thailand, Helsinki UniVersity of Technology, P.O. Box 5100, FI-02015 TKK, Finland, Faculty of Physics, UniVersity of Vienna, Strudlhofgasse 4, A-1090, Vienna, Austria, AdVanced Technology Institute, UniVersity of Surrey, Guildford, GU2 7XH, U.K., School of Physics, Northeast Normal UniVersity, Changchun, 130024, China, and BESSY II, D-12489, Berlin, Germany ReceiVed: June 26, 2009
A controlled method for the production of tungsten oxide nanobelts through metal oxidation in presence of ethanol is proposed. At the optimal synthesis conditions, up to 20 µm long vertically aligned ribbon-like structures with a narrow rectangular cross section can be obtained in a tuned manner with preferential formation of WO2 in the presence of graphitic like carbon. Bulk and local-scale studies suggest that carbon diffusion to the surface of the material leads to the formation of mainly WO3 nanobelts from simple annealing treatments at 450 °C. This represents one alternative method to the common tungsten oxidation in air, opening the possibility to use C-containing compounds with negligible formation of carbide traces. An in-depth characterization of these materials has been performed, and the possible growth mechanisms are here discussed. 1. Introduction Nanostructures made of one-dimensional (1D) transitionmetal oxides are ideal candidates for nanotechnological applications due to a wide variety of outstanding properties profitable for fundamental and applied research. In particular, tungsten oxide is one of these materials, which has encountered an extremely wide variety of applications. Some of the tungsten oxide properties are widely profited in many electrochromic and gasochromic devices such as gas sensors, humidity sensing, and water splitting.1-4 Suspended nanoparticles can be used as nanofluid dispersions and coatings. Surface functionalized nanoparticles and sputtered thin films have also encountered promising uses. Bioapplications regarding diagnosis and sensing are in this scope, as well as nanomaterials for use in composites, polymers, and textiles. Also, energy research has found great interest in this material for fuel cell layers and solar energy materials. However, the nanoscale structure of tungsten oxide requires a very deep understanding if the target is nanoelectronics and nanophotonics applications, such as micro- and nanoelectromechanical systems (MEMS and NEMS), and bionano materials. Quantum dots are also part of the applications that require structural control for conducting and semiconducting uses, and for their mechanochemical and optical properties. For this reason, further extensive research needs to be done taking into account potential electrical, dielectric, magnetic, optical, imaging, catalytic, and biomedical properties. However, one of the main obstacles in the tungsten oxide research is that it adopts * Corresponding authors. E-mail:
[email protected] (P.A.); singjai@ chiangmai.ac.th (P.S.). Tel: +6653941922, ext. 610. Fax: +66 53892271. † Chiang Mai University. ‡ Helsinki University of Technology. § University of Vienna. | University of Surrey. ⊥ Northeast Normal University. # BESSY II.
different stoichiometric configurations. WOx (2 e x e 3) has been observed in all the so far reported nanostructures. The methods of 1D tungsten oxide preparation found in the literature comprise a templating method for the synthesis of WO3 nanowires,5 mild solution-based colloidal approaches to produce WO3 nanofibers,6 physical vapor deposition (PVD) used to prepare the WO3 nanorods and nanobelts (NBs),7 nanorod synthesis with WO2 and WO2.9 configurations via thermal oxidation processes,8,9 and WO3 NB synthesis via hydrothermal reactions.10 These 1D structures represent an ideal system for fully understanding dimensionally confined phenomena in these types of functional oxides. Nowadays, the controlled formation of some elongated nanostructures such as nanofibers, nanorods, nanotubes, nanobuds, nanowires, and NBs is highly desired, and they are intensively investigated with special emphasis on the optimal synthesis processes. In this contribution, we present an alternative highly controllable method to produce tungsten oxide NBs employing a direct growth technique based on an ethanol-promoted chemical vapor deposition. This process employs thermal oxidation of tungsten-wire substrates in an ethanol vapor ambient.11 An advantage of our method lies in the fact that the substrate can be kept clean up to the reaction temperature, followed by a very controlled process. The optimal synthesis conditions yield up to 20 µm long vertically aligned ribbonlike structures with a narrow rectangular cross section and with a preferential WO2 configuration. This crystallization results in a very defined WO3 configuration with short annealing times. Bulk and local-scale studies on these novel structures suggest a carbon diffusion to the surface of the material from an annealing treatment. A very detailed characterization of this material as well as its possible growth mechanism are here discussed.
10.1021/jp9085975 2010 American Chemical Society Published on Web 12/16/2009
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2. Experimental Section The tungsten oxide NBs were grown using a high purity tungsten wire (99.8% from Advent Research Materials, Ltd.) as the substrate. The starting material had 0.25 mm of diameter. This tungsten substrate was sonicated while immersed in ethanol for 5 min prior to its insertion in the center of the reaction system (furnace with a quartz tube, i.d. ) 38 mm) coupled to a rotary pump. Once the pressure of the reactor reached 0.01 Torr, the temperature was set to 450 °C and a mixture of hydrogen (50 sccm) and argon (300 sccm) gases was fed through the system inducing a pressure of 300 Torr for 30 min. The purpose of this flow was to reduce the native oxide. The chamber was then evacuated to the initial base pressure, and then the temperature was raised again to values between 600 and 750 °C. The oxide growth was made via the introduction of an ethanol vapor into the furnace at the pressure of 10 Torr. The experiments were performed in a wide range of time intervals (10 to 180 min). A later annealing process in air at 450 °C was applied to react with the amorphous carbon for time intervals between 30 and 180 min. The overall morphology of the NBs was observed using scanning electron microscopy (SEM, JEOL JSM-6335F) and transmission electron microscopy (TEM, JEOL JEM-2010). Raman spectroscopy (Jobin Yvon, T64000 spectrometer with a 514.5 nm argon ion laser) and X-ray diffraction (XRD, PANalytical X’Pert Pro MPD with Cu KR radiation) were also used for bulk probing of the samples. Photoemission (PES) experiments were carried out at the beamline UE52PGM at BESSY II, where a hemispherical photoelectron energy analyzer Scienta R4000, with an energy resolution of 10 meV, was used. The base pressure in the setup was kept below 5 × 10-10 mbar, and all experiments were performed under ambient temperature. The samples were first inspected with an X-ray photoelectron spectroscopy (XPS) survey. 3. Results and Discussion The proposed method provides an alternative path for the formation of tungsten oxide nanostructured materials. At the optimal synthesis conditions, up to 20 µm long vertically aligned ribbon-like structures with a narrow rectangular cross section can be obtained in a controlled manner. The optimal NB growth conditions were systematically investigated resulting in a mainly “temperature-restricted” process. The initial morphological studies are focused on SEM, showing the samples to consist primarily of long, flat nanostructures varying in length according to the growth conditions. The micrographs in Figure 1a-d show the hefty formation of elongated material grown on the substrates for 30 min at different growth temperatures (600, 650, 700, and 750 °C). Insets at higher magnification are also depicted in order to better observe the critical difference of the samples according to synthesis temperature variations. It is clear that the morphology is extremely compromised with the changes in the synthesis temperature. For instance, at 650 °C in Figure 1b, the first roots of structures with a belt-like morphology are observed. However, this differs drastically from the samples obtained with a growth temperature of 700 °C, where a dense growth of NBs is clearly seen (Figure 1c). Again, the NB growth was obviously diminished with the use of temperatures higher than 700 °C (see Figure 1d). From these overall observations, the optimal growth temperature to analyze the formation evolution with the time was set at the value of 700 °C. The SEM investigations were extended to inspect the cross sections of the substrates grown at this same temperature but with different reaction times.
Figure 1. SEM images of as-grown NBs, synthesized at temperatures of (a) 600 °C, (b) 650 °C, (c) 700 °C, and (d) 750 °C for 30 min.
The second set of SEM images shown in the upper panel in Figure 2 are a cross section view of the substrates with the NBs, which were synthesized at a temperature of 700 °C at representative reaction times. Previously reported methods have suggested that the WO2.9 nanowires grow perpendicular to the substrate, obeying the typical crowding effect that rules the growth of vertically aligned carbon nanotubes.12,13 However, we cannot strictly apply this mechanism to the NBs grown with this method. The W-fed mechanism seems to promote a growth preferentially perpendicular to the substrate, which is more evidently seen in Figure 2c,d. With longer reaction times, the crowding of the grown material starts to destroy that alignment, mainly compromised by the increase of the material produced. The influence of the reaction time at a temperature of 700 °C is mainly evidenced in the length of the NBs, as depicted in the lower panel of Figure 2. Initially, the length of NBs increased rapidly; however, after ca. 20 min of synthesis, the process starts to be inhibited, restricting the growth to the following trend, L ) 5.78(t - 9.96)0.14 (where L is the length of NBs and t is the growth time). Spectroscopic techniques were used further examine the structural properties of these NBs. First, the Raman spectra in Figure 3 are shown for the NB samples before and after annealing with heat treatments at 450 °C during different times (30, 60, 120, and 180 min). The lower spectrum corresponding to the as-grown material exhibits five peaks at 286.0, 515.5, 599.4, 623.2, and 784.3 cm-1 that can be assigned to the monoclinic structure of WO2. The position of the first peak (at 286 cm-1) corresponds to the W-O-W bending mode, whereas the peak at 784.3 cm-1 is assigned to a stretching mode.14,15 The broad peaks at 1340 and 1577 cm-1 correspond to graphitized carbon,16,17 which is considered a byproduct of the ethanol decomposition. The intensity of amorphous carbon decreases progressively with the annealing treatment at 450 °C; meanwhile, the structure reorganizes to the WO3, as explained in detail later. Interestingly, as observed in the upper spectra of Figure 3, a thermal treatment of 180 min results in practical elimination of the two bands corresponding to the graphitized carbon. The NBs without annealing treatment where further characterized with TEM as observed in Figure 5. The selected area diffraction pattern (SADP) clearly shows the NB is single crystalline with the monoclinic structure of WO2. In the highresolution image, the lattice spacing distance was measured as
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Figure 2. Left panel: SEM cross-section images of the NBs, grown at 700 °C for (a) 10 min, (b) 20 min, (c) 30 min, (d) 60 min, (e) 120 min, and (d) 180 min. Right panel: Length of the as-grown NBs versus the growth time.
Figure 3. From bottom to top: Raman spectra of the NBs (a) as-grown and annealed at 450 °C for (b) 30 min, (c) 60 min, (d) 120 min, and (e) 180 min.
Figure 4. XRD patterns of (a) the as-grown and (b) annealed NBs at the temperature of 450 °C for 180 min.
0.346 nm, which corresponds to the (011) interplanar distance. According to these results, in the as-grown samples, the NB preferential growth along the [011] direction is also in good agreement with the highest intensity peak of the XRD spectrum. Now turning back to the annealed NBs, the Raman studies shown on the upper spectra in Figure 3 exhibit peaks at 274, 326, 718, and 804 cm-1, corresponding to the monoclinic structure of WO3. The bending mode of WO3 is seen at 275 cm-1, whereas bands at 718 and 804 cm-1 are assigned to the W-O-W stretching mode (it has been suggested that the
Figure 5. Transmission electron study of an as-grown sample at 700 °C with a highly selective WO2 configuration: (Left) Bright-field image of the sample. (Right) Electron diffraction pattern and high-resolution image.
W-O-W bonds at 719 cm-1 are longer than the binding at 804 cm-114). Deepening in the analysis of the XRD patterns of NBs with and without annealing treatment shown in Figure 4, we can see that in both cases the signals corresponding to the tungsten substrate (JCPDS card No. 04-0806) are a common feature (three peaks at 40.4°, 58.4°, and 73.3°). In pattern a, corresponding to the as-grown sample (JCPDS card No. 32-1393) prepared at the optimal synthesis temperature for 30 min (with previously suggested monoclinic WO2 crystalline structure), the lattice constants are a ) 0.55754 nm, b ) 0.48995 nm, c ) 0.55608 nm, and β ) 118.86°. On the other hand, for the upper pattern b, corresponding to the annealed material (JCPDS card No. 72-1465) at 450 °C for 180 min, the XRD signals can be indexed to the monoclinic structure of WO3 with lattice constants a ) 0.7300 nm, b ) 0.7530 nm, c ) 0.7680 nm, and β ) 90.90° given the displayed peaks at 23.2° 23.7°, 24.4°, 26.7°, 29.0°, 33.4°, 34.2°, 41.8°, and 50.4°. These results confirm the previous analysis that suggest that the annealing process favors the changes from a monoclinic WO2 structure to monoclinic WO3. It is worth noting the absence of XRD signals corresponding to carbon, suggesting either the negligible formation of carbonaceous species or its presence in amorphous form, which does not contradict the occurrence of the carbon corresponding to peaks in the Raman spectrum of the nonannealed sample.
Ethanol-Promoted Production of Tungsten Oxide Nanobelts
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Figure 6. TEM micrographs at different magnifications of the annealed WO3 nanowires.
On the other hand, the TEM micrographs in Figure 6 clearly show the intact remaining NB morphology (low magnification) and its ordered crystallization (high-resolution image). Now, regarding the formation mechanism of these structures, it is important to consider that the clean tungsten wire acts as the substrate and tungsten supply, whereas the main oxygen source is ethanol (although a minimal contribution of remaining air in the reactor cannot be disregarded because the pressure observed was between ∼3 and 10 Pa).18 With these considerations, the following synthesis reactions can be contemplated: 700°C
W(s) + 3C2H5OH(g) + 5O2(g) 98 WO2(s) + 4C(s) + 2CO(g) + 9H2O(g) 700°C
W(s) + C2H5OH(g) + 3O2(g) 98 WO2(s) + 2CO(g) + 3H2O(g) 700°C
W(s) + C2H5OH(g) + 2O2(g) 98 WO2(s) + 2C(s) + 3H2O(g) These are all vapor-solid (VS) mechanisms, considering that the pressure at which the reaction occurs is mainly ruled by the vapor pressure of the ethanol, and therefore depends on the pumping rate applied in the experiments. Previously reported experiments under different chemical vapor deposition conditions using flat substrates and a carboncontaining precursor (methane) have suggested the formation of tungsten carbide in an intermediate step, which would promote the nanowire synthesis. In that study, Klinke et al.19 proposed that the formation of crystalline arrays of tungsten oxide (WO3) nanowires occurs, exposing oxidized tungsten films to hydrogen and methane. Such mechanism considers the formation of tungsten carbide stimulating the growth of nanowires. Although that method represents the closest process to the one discussed in this contribution (considering that this synthesis route also employs a carbon-containing precursor), we believe
Figure 7. Variable energy PES spectra of atungsten oxide nanowire sample annealed at 450 °C.
that the formation of carbide has a minimal effect in the growth process of the nanowires here described. In any case, we cannot discard an initial favorable decomposition of the ethanol on the surface of the W substrate. In order to corroborate this, variable energy PES experiments were carried out. The spectra shown in Figure 7 were recorded using X-rays with photon energies of 1250, 650, 463.7, and 125 eV. At high and low energies, it is clear that the W corresponding to the substrate is visible. However, at 650 eV, the maximum surface sensitivity is reached, and therefore the peaks at 33.6 and 31.4 eV corresponding to metal tungsten (W4f7/2) do not appear in this spectrum. The inset in Figure 7 corresponds to the oxygen O1s PES signal recorded with the 650 eV beam where the spectral decomposition yields at least two components at 530.8 and around 532.3 eV. The first can be assigned to the oxygen-bonding environment in WO3, whereas the higher energy peak is related to H adsorbed to the sample, as a remainder of the synthesis process with ethanol.20 The annealing process in oxygen atmosphere most likely induces the change of WO2 into WO3, while the amorphous carbon is oxidized through the following reactions: 450°C
WO2(s) + C(s) + O2(g) 98 WO3(s) + CO(g)
(1)
450°C
2WO2(s) + C(s) + 2O2(g) 98 2WO3(s) + CO2(g)
(2) Hydrogen is also present in ethanol, and we believe its role is to enhance the oxide diffusion, as such a phenomenon is wellknown to occur in certain semiconducting materials. 4. Conclusions This contribution presents a method to controllably fabricate tungsten oxide NBs. The tungsten metal wire oxidation in the
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presence of ethanol exhibits an optimal synthesis condition at which up to 20 µm long vertically aligned ribbon-like structures with a narrow rectangular cross section and with improved formation of the preferential WO2 configuration are formed. It has been corroborated by a number of techniques that NBs of even WO2 material with traces of C are transformed into WO3 after annealing at 450 °C. Acknowledgment. The authors would like to thank the Electron Microscopy Research and Service Center, Mr. Ekkapong Kuntaruk for help with Raman spectroscopy measurements, and Dr. Hua Jiang from TKK for TEM assistance. S.D. is grateful for her Ph.D. scholarship from the Commission on Higher Education and the Graduate School of Chiang Mai University. H.S. thanks the EPSRC for support through a Portfolio Partnership grant. P.A., T.P., and H.S. are grateful to R. Schoenfelder, D. Batchelor and R. Huebel for technical assistance. References and Notes (1) Lee, K.; Fang, Y.; Lee, W.; Ho, J.; Chen, K.; Liao, K. Sens. Actuators B 2000, 69, 96–99. (2) Liao, C.; Chen, F.; Kai, J. Sol. Energy Mater. Sol. Cells 2006, 90, 1147–1155. (3) Georg, A.; Graf, W.; Neumann, R.; Wittwer, V. Thin Solid Films 2001, 384, 269–275. (4) Poirier, G.; Nalin, M.; Messaddeq, Y.; Ribeiro, S. J. L. Solid State Ionics 2007, 178, 871–875.
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