Optimization, Yield Studies and Morphology of WO3Nano-Wires Synthesized by Laser Pyrolysis in C2H2and O2Ambients—Validation of a New Growth Mechanism

Nanoscale Res Lett (2008) 3:372–380 DOI 10.1007/s11671-008-9169-6 NANO EXPRESS Optimization, Yield Studies and Morphology of WO3 Nano-Wires Synthesized by Laser Pyrolysis in C2H2 and O2 Ambients— Validation of a New Growth Mechanism B. W. Mwakikunga Æ A. Forbes Æ E. Sideras-Haddad Æ C. Arendse Received: 28 July 2008 / Accepted: 3 September 2008 / Published online: 25 September 2008
to the authors 2008 Abstract Laser pyrolysis has been used to synthesize WO3 nanostructures. 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 WO3 nanowires 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 WO3 nano-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 WO3 nano-wires. The solid–vapour– solid (SVS) mechanism [B. Mwakikunga et al., J. Nanosci. B. W. Mwakikunga (&)
C. Arendse CSIR, National Centre for Nano-Structured Materials, P.O. Box 395, Pretoria 0001, South Africa e-mail: B. W. Mwakikunga
E. Sideras-Haddad School of Physics, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050 Johannesburg, South Africa B. W. Mwakikunga Department of Physics and Biochemical Sciences, University of Malawi, The Polytechnic, Chichiri, Private Bag 303, Blantyre 0003, Malawi A. Forbes (&) CSIR National Laser Centre, P.O. Box 395, Pretoria 0001, South Africa e-mail: A. Forbes School of Physics, University of Kwazulu-Natal, Private Bag X54001, Durban 4000, South Africa 123 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 WO3 nano-wires. Keywords Laser pyrolysis
Tungsten trioxide
Nano-wires
Growth mechanism Introduction 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 [1]. 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 nanorods 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 Nanoscale Res Lett (2008) 3:372–380 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 WO3 nano-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—C2H2 or O2—are used during the synthesis. Experimental Our laser pyrolysis experimental set up was fully described in our previous publication [16]. Briefly, the method involves injecting a stream of very fine droplets of a precursor solution into an infrared laser beam and depositing the resulting aerosol onto a Corning glass substrate. A wavelength tuneable continuous wave (cw) CO2 laser was used in the experiments (Edinburgh Instruments, model PL6). By selecting a wavelength of 10.6 lm, the laser was 373 within, but not exactly on, the absorption region of the premade precursor (WCl6 in ethanol or tungsten ethoxide) for the production of WO3. From the fact that (1) the excitation wavelength of 10.6 lm is not exactly at the main resonance peak of the W-ethoxide precursor of 9.44 lm and (2) the laser power of 50 W (focussed into 2.4-mm beam diameter at the waist) is not low enough to rule out anharmonic effects in the excitation, the decomposition of this precursor could be due to both photochemical (resonance) and photo-thermal (anharmonic) processes. The as-produced materials showed decomposition of W-ethoxide into WO3 nano-particles suggesting that the photo-chemical process indeed occurred. Also worth describing here is the carrier gas system which is accomplished by a three-way nozzle having three concentric cylinders. The outer cylinder is connected to an argon supply. The argon guides the aerosol droplets which are carried by either C2H2 (supposedly nonreactive) or O2 (highly reactive) gases interchangeably in the middle and second cylinder. This is illustrated in Fig. 1. An aliquot of 5.4 mg of dark blue powder of WCl6 (Aldrich 99.99%) was dissolved in 500 mL of ethanol. Since WCl6 is highly reactive with air and moisture, its dissolution was conducted in an argon atmosphere. Particles from this process were collected on Corning glass substrates, placed on a rotating stage, at room temperature and at atmospheric pressure. The particle deposition showed a void at the centre (Fig. 1b) when the encapsulating carrier gas flow-rate was higher than the carrier gas driving the precursor droplets. When the flow-rates were reversed, the deposition showed the profile of a hump (Fig. 1a) showing there was more deposition at the centre Fig. 1 Laser pyrolysis illustration and the role of carrier gas and precursor relative flow-rates (a) when the precursor flow-rate is larger than the encapsulating carrier gas (Ar) and (b) when th (...truncated)