Browsing by Author "Skyllas-Kazacos, M"

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    Anodically synthesized titania films for lithium batteries: effect of titanium substrate and surface treatment
    (Elsevier, 2009-05-01) Lindsay, MJ; Skyllas-Kazacos, M; Luca, V
    A number of titania films have been produced through anodising high purity titanium from different suppliers in either the as-received state or following polishing and etching. Anodising was carried out galvanostatically for a period of 10 min in 0.2 M H2SO4. The performance of the films was then evaluated as potential anode materials for lithium batteries. Raman spectroscopy showed these films had spectra characteristic of anatase with the presence of some rutile whilst the spectra of the lithiated state was characteristic of the orthorhombic phase of LixTiO2. The surface condition in particular was found to have an effect on the electrochemical performance and properties of the films most notably on capacity fade. Whilst the electrodes produced from as-received titanium demonstrated stable cycle capacities after the initial few cycles, those on polished and etched substrates faded over 50 cycles. The best performing films offered a capacity of at least 48 μAh cm−2 over the 50 cycles. All the electrodes examined however did show signs of the film having being damaged as a result of electrochemical cycling. With the wide range of anodising parameters that can be altered there is considerable scope for optimising the electrochemical performance of films produced through such a technique. © 2009, Elsevier Ltd.
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    Crystallite size dependence of lithium intercalation in nanocrystalline rutile
    (American Chemical Society, 2009-07-30) Milne, NA; Skyllas-Kazacos, M; Luca, V
    An electrochemical investigation has been undertaken into the dependence of particle size on the mechanism of lithium intercalation in rutile. A thoroughly characterized suite of rutile nanoparticles with diameters in the broad size range 4−50 nm was used in the study. Variation in the cyclic voltammetry of rutile over cycling is presented for the first time and numerous new electrode reactions identified. While a crystallite size effect was witnessed during fast voltammetry experiments, this effect disappeared as the rate was reduced and no variation in capacity was witnessed in galvanostatic cycling experiments. The intercalation potential was seen to increase with decreasing crystallite size, the opposite of what was predicted from quantum confinement. Enhanced lithium intercalation into the smaller nanoparticles has been ascribed to the slightly expanded structure of smaller nanocrystals relative to larger ones taking the dimensions of the tunnels into which Li inserts beyond a threshold value. This lattice expansion or volume dilation with decreasing crystallite size is due to the expected increase in surface tension with decreasing crystallite size. This would correspondingly increase the intercalation potential as the crystallite size decreased. © 2009, American Chemical Society
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    Sol-gel tungsten oxide/titanium oxide multilayer nanoheterostructured thin films: structural and photoelectrochemical properties
    (American Chemical Society, 2007-12-20) Luca, V; Blackford, MG; Finnie, KS; Evans, PJ; James, M; Lindsay, MJ; Skyllas-Kazacos, M; Barnes, PRF
    Multilayer structures of alternating thin titanium and tungsten oxide layers having dimensions of similar to 20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique. Single, double, and triple layer titanate and tungstate thin films were deposited on silicon substrates, and these films were initially annealed at 400°C. Structural and microstructural aspects of the films were investigated using a variety of techniques, including X-ray reflectometry, grazing incidence X-ray absorption spectroscopy (GIXAS), cross-sectional transmission electron microscopy (TEM), and secondary ion mass spectrometry. The dimensions of the films and the character of the interfaces were principally gauged by cross-sectional TEM and X-ray reflectometery. All films were continuous on a local scale and had relatively low surface roughness. At the treatment temperature of 400°C, only the tungsten oxide component showed appreciable crystallinity. The multilayer films had relatively diffuse interfaces, even after annealing in air at this temperature. At these temperatures, easily measurable diffusion of tungsten into the titanium oxide component was observed, whereas the diffusion of titanium into the tungsten oxide component occurred to a lesser degree. At higher temperatures, interdiffusion of components was found to be significant. TEM, X-ray diffraction, and Ti K-edge GIXAS measurements indicated that annealing at 400°C generated films in which the titanate component remained amorphous while the tungstate component crystallized in the tetragonal modification Of WO3, which is normally stable only at high temperatures. Grazing incidence X-ray absorption spectroscopy allowed the degree of distortion of the tungsten oxygen polyhedra to be monitored as a function of depth into the film. The photoelectrochernical activity of the multilayer film electrodes was investigated, and the activity for water photo-oxidation was assessed. The photoelectrochernical response was greatest when crystalline WO3 was bounded on both sides by amorphous TiO2 layers. In this bounded state, WO3 had unique structural characteristics. © 2007, American Chemical Society