Self Assembly and Properties of C:WO3 Nano-Platelets and C:VO2/V2O5 Triangular Capsules Produced by Laser Solution Photolysis
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- Mwakikunga, B.W., Forbes, A., Sideras-Haddad, E. et al. Nanoscale Res Lett (2010) 5: 389. doi:10.1007/s11671-009-9494-4
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Laser photolysis of WCl6 in ethanol and a specific mixture of V2O5 and VCl3 in ethanol lead to carbon modified vanadium and tungsten oxides with interesting properties. The presence of graphene’s aromatic rings (from the vibrational frequency of 1,600 cm−1) together with C–C bonding of carbon (from the Raman shift of 1,124 cm−1) present unique optical, vibrational, electronic and structural properties of the intended tungsten trioxide and vanadium dioxide materials. The morphology of these samples shows nano-platelets in WOx samples and, in VOx samples, encapsulated spherical quantum dots in conjunction with fullerenes of VOx. Conductivity studies revealed that the VO2/V2O5 nanostructures are more sensitive to Cl than to the presence of ethanol, whereas the C:WO3 nano-platelets are more sensitive to ethanol than atomic C.
The study of vanadium and tungsten oxides has been undertaken extensively in recent years due to their respective thermo-chromic and electro-chromic and hence gas-chromatic properties. Since the discovery of the metal-to-insulator transition (MIT) at 340 K of VO2 in 1959 by Morin  and electro-chromism of WO3 in 1975 by Faughnan [2, 3], and also due to the fact that the tungsten metal is, so far, the best known dopant in VO2 to reduce the MIT temperature to room temperature, the study of the two materials together is expected to yield a good understanding of their MIT behaviours especially at the nano-scale as discussed by this group and others previously [4–6]. To date, self assembly of these materials has been achieved by a number of techniques, including: hydrothermal techniques , employing templates either with polymers or pre-assembled carbon nanotubes , CVD epitaxial growth , sol–gel , ion implantation , hot-wire CVD , sputtering  and ultrasonic spray pyrolysis [14–18]. Also V2O5 capsules , WO3 nano-rods and nano-wires and nano-arrays [20–22] have previously been obtained using several techniques. Laser synthesis methods have been of particular interest and have been followed by this group previously [23, 24]. The coherent, intense and almost monochromatic laser light allows it to be tuned to selectively dissociate specific bonds in a precursor molecule either by resonance between the laser frequency and the bond’s natural frequency or via multi-photon absorption. This leads to products that can be unique and different from those obtained by traditional thermal deposition techniques. In this work, we followed a process called laser solution photolysis (LSP) that has been used previously to obtain FePt ultra-fine powders . Organo-metallic precursors containing Fe and Pt, respectively were employed in the presence of a polymer. The polymer was employed to reduce agglomeration of the nano-particles produced. Further examples of the technique include, gold nano-particles produced by UV light irradiation of gold chloride [26–28], iron-based nanoparticles produced by utilising UV light absorbing ferrocene and iron(II) acetylacetonate [29, 30] and laser ablation in a solid–liquid interface [31, 32]. In this study, we used, as precursors, metal ethoxides which were produced from metal chlorides.
Most of the HCl is lost as gas bubbles, which visibly effervesce from the liquid. Similar reaction routes are expected for the vanadium dioxide precursor solution.
For the synthesis of WO3, an aliquot of 5.3 mg of a dark-blue WCl6 powder was dissolved in 500 ml of ethanol in an argon environment, resulting in a light blue to light-yellow liquid. When this liquid was irradiated with 5,000 saw-tooth-shaped pulses from a 248-nm KrF excimer laser, with a fixed energy of 10 mJ at 8 Hz, the light yellow liquid turned to blue-black. For the production of VO2, one part of V2O5(in which molecule the V takes the valence of 5+) and two parts of VCl3 (where V has a valence of 3+) were dissolved in ethanol. The ratio was chosen to produce a stoichiometry of VO2 in which molecule the V atom has a valence of 4+.
Scanning electron microscopy was carried on a Gemini Neon 40 FEG SEM equipped with a focussed ion beam (FIB) gun. A drop of the as-irradiated liquid was dropped onto a glass slide and Si(111) surface. Raman spectroscopy was carried out using a Jobin–Yvon T64000 Raman spectrograph with a 514.5-nm line from an argon ion laser. The power of the laser at the post-annealed samples (0.384 mW) was small enough in order to minimise localised heating of the sample. The T64000 was operated in single spectrograph mode, with the 1,800 lines/mm grating and a 50× objective on the microscope. A drop of such liquid was also placed on carbon holey film supported by copper grids for high resolution transmission electron microscopy on a JEOL 2100 equipped with a LaB6 filament and a Gartan U1000 camera with 2,028 × 2,048 pixels.
Results and Discussion
When the same precursors are irradiated with the 248-nm beam from the KrF excimer laser, the W(OR)6 liquid turns blue-black before stabilising to a yellow colour after a few days, whereas no colour change is seen in the vanadium precursor. It is suggested that at this wavelength, a different bond is dissociated namely the C–H bond which has a frequency of between 3,000 and 3,300 cm−1 compared to the laser frequency of 40,322 cm−1(at λ = 248 nm). The C–H bond then has a higher probability of being dissociated than the O–C bond, since the C–H bond is only 10 times lower than the laser frequency when compared to the O–C which is 40 times lower. Also, the C–H bond is capable of oscillating closer to the laser frequency via other higher order vibrational modes.
Possible Mechanism of Formation of the Triangular Envelopes and VOxInorganic Fullerenes
Influence of Chlorine on the Conductance of VOxStructures
Effect of Ethanol on the Conductance of WOxPlatelets
Production of nano-platelets of carbon modified WO3 and 6-nm encapsulated VOx quantum dots by laser solution photolysis have been achieved. Conductivity studies revealed that the VO2/V2O5 nanostructures are more sensitive to atomic C than to the presence of ethanol, whereas the C:WO3 nano-platelets are more sensitive to ethanol than atomic C.
We acknowledge Nosipho Moloto for her assistance with FIB FEGSEM, Brian Yalisi for the KrF laser and Lerato Shikwambana and Malcolm Govender for the starting materials. Financial and infrastructural support from the CSIR National Laser Centre and characterisation facilitation of the CSIR National Centre for Nano-Structured Materials are acknowledged.
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