Anna T. Valota

Electrochemical Behavior of Monolayer and Bilayer Graphene

Results of a study on the electrochemical properties of exfoliated single and multilayer graphene flakes are presented. Graphene flakes were deposited on silicon/silicon oxide wafers to enable fast and accurate characterization by optical microscopy and Raman spectroscopy. Conductive silver paint and silver wires were used to fabricate contacts; epoxy resin was employed as a masking coating in order to expose a stable, well-defined area of graphene. Both multilayer and monolayer graphene microelectrodes showed quasi-reversible behavior during voltammetric measurements in potassium ferricyanide. However, the standard heterogeneous charge transfer rate constant, k°, was estimated to be higher for monolayer graphene flakes.

Influence of Water Content on the Growth of Anodic TiO2 Nanotubes in Fluoride-Containing Ethylene Glycol Electrolytes

The effect of water additions, from 0 to 50 vol %, to an ammonium fluoride/ethylene glycol electrolyte on the composition and morphology of titania-based nanotubes was investigated by scanning and transmission electron microscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. Further to the presence of units of TiO 2 , the films contained fluorine, carbon, nitrogen, and probably hydrogen species derived from the electrolyte. The compositions of the films appeared to be relatively independent of the water content of the electrolyte. Following a small increase in efficiency of film growth for additions between 0 and 1 vol % water, the efficiency decreased from about 25 to about 10% between water contents of 1 and 25 vol %, respectively, possibly due to a combination of loss of titanium species to the electrolyte and evolution of oxygen. The densities of the nanotube layers were estimated to be in the range of 1.25―1.75 g cm ―3 , with an average value of 1.47 g cm ―3 .

Electron Transfer Kinetics on Mono- and Multilayer Graphene

Understanding of the electrochemical properties of graphene, especially the electron transfer kinetics of a redox reaction between the graphene surface and a molecule, in comparison to graphite or other carbon-based materials, is essential for its potential in energy conversion and storage to be realized. Here we use voltammetric determination of the electron transfer rate for three redox mediators, ferricyanide, hexaammineruthenium, and hexachloroiridate (Fe(CN)(6)(3-), Ru(NH3)(6)(3+), and IrCl(6)(2-), respectively), to measure the reactivity of graphene samples prepared by mechanical exfoliation of natural graphite. Electron transfer rates are measured for varied number of graphene layers (1 to ca. 1000 layers) using microscopic droplets. The basal planes of mono- and multilayer graphene, supported on an insulating Si/SiO(2) substrate, exhibit significant electron transfer activity and changes in kinetics are observed for all three mediators. No significant trend in kinetics with flake thickness is discernible for each mediator; however, a large variation in kinetics is observed across the basal plane of the same flakes, indicating that local surface conditions affect the electrochemical performance. This is confirmed by in situ graphite exfoliation, which reveals significant deterioration of initially, near-reversible kinetics for Ru(NH3)(6)(3+) when comparing the atmosphere-aged and freshly exfoliated graphite surfaces.

Dye-sensitization of self-assembled titania nanotubes prepared by galvanostatic anodization of Ti sputtered on conductive glass

Self-organized porous TiO(2) nanotubes (NTs) were prepared on conductive glass by galvanostatic anodizing of sputtered titanium in an NH(4)F /glycerol electrolyte. DC magnetron sputtering at an elevated substrate temperature (500 degrees C) was used to deposit 650 nm thick titanium films. After anodizing, NTs, 830 nm long, with an average external diameter of 92 nm, were grown; this gave a high conversion rate of oxide from titanium (1.9), with a 220 nm thick layer of titanium, which was not oxidized, located at the base of the tubes. The NTs revealed a mainly amorphous structure, which transformed mostly to anatase upon thermal treatment in air at 450 degrees C. The tubes were sensitized by the N719 complex and the resultant photoelectrodes were incorporated into liquid dye solar cells (DSCs) and further tested under back-side illumination. High values of V(oc) (714 mV) were obtained under 1 sun (AM 1.5), assigned to low dark current magnitude and large recombination resistance and electron lifetime. In addition, typical values of fill factors (of the order of 0.62) were attained, in agreement with the estimated ohmic resistance of the cells in combination with low electron transfer resistance at the platinum/electrolyte interface. The overall moderate power conversion efficiency (of the order of 0.3%) was mainly due to the low short-circuit photocurrents (J(sc) = 0.68 mA cm(-2)), which was confirmed further by the corresponding IPCE values (5.2% at 510 nm). The magnitude of J(sc) was attributed to absorbed light losses due to back-side illumination of the cells, the low dye loading (due to the limited thickness of anodic titania) and the high charge transfer resistance at the TiO(2)/conductive substrate due to the presence of barrier layer(s) underneath the tubes. These preliminary results encourage the DSC community to explore further the galvanostatic anodizing of titanium in order to produce highly efficient porous TiO(2) NTs directly on conductive glass. Current work is focusing on achieving complete anodizing of the metal substrate and full transparency for the photoelectrode in order to increase and optimize the resultant cell efficiencies.

Electrochemistry in a drop: a study of the electrochemical behaviour of mechanically exfoliated graphene on photoresist coated silicon substrate

A micro apparatus for electrochemical studies on individual high quality graphene flakes is presented. A microinjection-micromanipulator system has been employed to deposit droplets of aqueous solutions containing redox-active species directly on selected micro-scale areas of mechanically exfoliated graphene layers on polymer coated silicon wafers. This approach allows the clear distinction between the electrochemical activity of pristine basal planes and the edges (defects) or steps to be measured. Voltammetric measurements were performed in a two-electrode configuration, and the standard heterogeneous electron transfer rate (k°) for reduction of hexachloroiridate (IrCl62−) was estimated. The kinetics of electron transfer were evaluated for several types of graphene: mono, bi, and few layer basal planes, and the k° was estimated for an edge/step between two few layer graphene flakes. As a comparison, the kinetic behaviour of graphite basal planes was measured for the deposited aqueous droplets. The appearance of ruptures on the graphene monolayer was observed after deposition of the aqueous solution for the case of graphene on a bare silicon/silicon oxide substrate.

Influence of Applied Potential on Titanium Oxide Nanotube Growth

The effect of applied potential on the morphology, growth efficiency, and composition of self-organized titanium oxide nanotubes, generated in 0.2 M N H4 F /glycerol electrolyte, has been examined by electron microscopy, Rutherford backscattering spectroscopy, and nuclear reaction analysis. A linear increase in nanotube diameter, length, and wall thickness, with increasing applied potential is evident up to 40 V, whereas the growth efficiency decreases with increase of the applied potential. Due to the semiconductive properties of the titanium oxide, at potentials above 40 V some of the oxygen anions migrating across the barrier layer lose electrons and generate oxygen gas within the oxide. This results in a morphological transition above 40 V, characterized by the degeneration of nanotube geometry and uniformity, and the appearance of oxygen-filled voids within the barrier layer and nanotubes walls.