Optimization, Yield Studies and Morphology of WO3Nano-Wires Synthesized by Laser Pyrolysis in C2H2and O2Ambients—Validation of a New Growth Mechanism
© to the authors 2008
Received: 28 July 2008
Accepted: 3 September 2008
Published: 25 September 2008
Laser pyrolysis has been used to synthesize WO3nanostructures. Spherical nano-particles were obtained when acetylene was used to carry the precursor droplet, whereas thin films were obtained at high flow-rates of oxygen carrier gas. In both environments WO3nano-wires appear only after thermal annealing of the as-deposited powders and films. Samples produced under oxygen carrier gas in the laser pyrolysis system gave a higher yield of WO3nano-wires after annealing than the samples which were run under acetylene carrier gas. Alongside the targeted nano-wires, the acetylene-ran samples showed trace amounts of multi-walled carbon nano-tubes; such carbon nano-tubes are not seen in the oxygen-processed WO3nano-wires. The solid–vapour–solid (SVS) mechanism [B. Mwakikunga et al., J. Nanosci. Nanotechnol., 2008] was found to be the possible mechanism that explains the manner of growth of the nano-wires. This model, based on the theory from basic statistical mechanics has herein been validated by length-diameter data for the produced WO3nano-wires.
Amongst many transition metal oxides, WO3 has excellent electro-chromic, gaso-chromatic and photo-chromatic properties. At room temperature it adopts the distorted monoclinic structure of ReO3. For this reason, WO3 has been used to construct flat panel displays, photo–electro–chromic ‘smart’ windows [2–4], writing–reading–erasing optical devices [5, 6], optical modulation devices [7, 8], gas sensors and humidity and temperature sensors [9–11]. Self assembly of these materials has been achieved by hydrothermal techniques, additive-free hydrothermal means, templating either with a polymer or pre-assembled carbon nano-tubes, epitaxial growth, sol-gel, electro-chemical means and hot-wire CVD methods. Recently, WO3 nano-rods produced by a facile chemical route and CVD have been reported [12, 13] in this journal. In laser pyrolysis, authors have reported synthesis of, for instance, ceramics, silicon and silicon compounds, carbon compounds, olefins, chromium oxides, diamond, fullerenes and many other classes of materials. These experiments have largely been performed at high laser powers and hence at high temperatures. At such high levels, where anharmonicity cannot be ruled out, laser pyrolysis is equivalent to traditional pyrolysis with the photo-thermal process overwhelming the photo-chemical one. However, it has long been realized that even at low intensity, the CO2 laser has successfully been used in the synthesis of boron compounds from BCl3 [14, 15]. At these low power values, the laser is used to selectively excite the reactant to a relatively low vibrational level from which a chemical reaction with other reactants present is initiated. One expects to achieve product formation distinctly different from that achieved by traditional pyrolysis for the same chemical reaction provided that the laser energy absorbed is channelled mainly into the chemical process rather than into heating.
In this Letter, we report optimization of parameters that led to the synthesis of WO3nano-spheres and thin films at relatively low laser power (50 W in a 2.4-mm focal region). We demonstrate the role of thermal annealing in the conversion of the spheres and slabs into nano-wires. We also show the morphological differences and yields when carrier gases—C2H2or O2—are used during the synthesis.
The experiment parameter used to obtain the WO3samples by laser pyrolysis
Gas 1 (8 cm3/min)
Gas 2 (8 cm3/min)
Gas 3 variable
WCl6 + Ethanol
Slabs + Rods
WCl6 + Ethanol
Sphere + Rod
Lengths and corresponding diameters of the nano-wires were measured by means of a software package ImageTool. As is the required procedure, calibration is initially made against the marker of known length in both the image scale and the real space scale. Then the distance between two points is measured for each point with accuracy that heavily depends on (1) the pixel density of the projecting screen, (2) the random errors from operator’s hand and (3) the magnification of the image.
For us to understand the novel growth of these nano-wires, it is important to briefly review some related growth mechanisms available in literature. Sir Frederick Frank proposed the ‘screw dislocation theory’ in 1949. Central to this dislocation theory were Polanyi, Orowan, Taylor, Burger and Mott & Nabarro . Defects and dislocation in the initial crystals initiate one-dimensional growth; “…the crystal face always has exposed molecular terraces on which growth can continue, and the need for fresh 2D nucleation never arises…” . In 1964, detailed studies on the morphology and growth of silicon whiskers by Wagner & Ellis  led to a new concept of crystal growth from vapour, which was called the vapour–liquid–solid (VLS) mechanism. The new growth mechanism was built around three important facts: (a) silicon whiskers did not contain an axial screw dislocation (b) an impurity was essential for whisker growth and (c) a small globule was always present at the tip of the whisker during growth. From fact (a), it was clear that growth from vapour did not occur according to Frank’s screw dislocation theory and from, facts (b) and (c), it was important that a new growth mechanism be studied.
The main characteristics of VLS mechanism are (1) the presence of a catalyst and (2) direct proportionality of the diameter of the nanostructure to the growth rate. Thick whiskers grow longer than thinner ones because this growth can be afforded by the continual supply of building blocks in the CVD system. Plotting the growth rate, V,  or terminal length l ∞  of the whisker versus D gives curves with a positive ascent. A plot of V 1/n versus 1/D gives a straight line graph with a negative slope .
where l 0 is the final length and α is the growth or decay coefficient.
In this Letter, we introduce for the first time the statistical-mechanical aspects of this proposed SVS model and fit the ensuing mathematical expressions to the data.
E A is the activation energy of the atoms.
Parameter ζ is a function of temperature T and also depends on the geometry of the source of the atoms. The higher the annealing temperature, T, the higher the slope, ζ. This fact may mean that thinner nano-wires can be obtained at higher annealing temperatures. But there must be a lower limit to how thinner the nano-wires can get in the SVS mechanism since at much higher temperatures all solid-state starting material should evaporate away leaving nothing to form the nano-wires with. These limits are yet to be determined. The same question has been asked if there is a thermo-dynamical lower limit to the nano-wires growth by VLS . It can be seen that if the source is equally crystalline then the ratio of the densities in the source to the final structure is unity. By quick inspection, one can see that the geometry described by the summation in Eq. 11 is proportional to the total surface area of all atomic or molecular layers in the source. A plot of l versus 1/D 2 should be a positive straight line graph with a y-intercept of zero and a slope of ζ. Similarly a plot of aspect ratios l/D versus 1/D 3 is supposed to be a positive straight line going through the origin and having the slope, ζ.
In summary, liquid atomization and subsequent laser pyrolysis were carried out using a CO2 laser tuned at its 10P20 line of wavelength 10.6 μm. SEM characterization of the as-produced WO3 samples showed that selective photochemical reactions by the laser have a part to play in initiating self assembly growth centres even without the need for a catalyst. Self assembly is only continued by further annealing. We have shown that oxygen carrier gas gives a higher yield of WO3 nano-wires by laser pyrolysis than acetylene. The latter also shows trace amounts of multi-walled carbon nano-tubes. The transmission electron microscopy reveals that the nano-wires are core-shell structures of a mixture of Au, Pd and C in the shell and WO3 at the core. The shell is due to the prior-to-SEM coating to improve imaging. The absence of catalysts in addition to the analysis of the nano-wire length-and-diameter data has validated a new growth mechanism, which we have called SVS growth as proposed earlier .
Authors would like to thank Prof. Michael Witcomb, Mr. Mthokozisi Masuku, Mr. Henk van Wyk and Ms. Retha Rossouw. The South African Department of Science and Technology (DST) project for the African Laser Centre, the National Research Foundation (NRF), the DST/NRF Centre for Excellence in Strong Materials and the CSIR National Centre for Nano-Structured Materials are acknowledged.
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