Electron transfer processes mediated by nanostructured materials assembled at electrode surfaces underpin fundamental processes in novel electrochemical sensors, light energy conversion systems and molecular electronics. Functionalisation of electrode surfaces with hierarchical architectures incorporating self-assembling molecular systems and materials, such as metal nanostructures, quantum dots, carbon nanotubes, graphene or biomolecules have been intensively studied over the last 20 years. Important steps have been made towards the rationalisation of the charge transfer dynamics from redox species in solution across molecular self-assembling systems to electrode surfaces. For instance, a unified picture has emerged describing the factors which determine the rate constant for electron transfer processes across rigid self-assembling molecular barriers. An increasing bulk of evidence has recently shown that the incorporation of nanomaterials into self-assembling monolayers leads to an entirely different electrochemical behaviour. This perspective rationalises some of the key observations associated with nanoparticle mediated charge transfer, such as the apparent distance independent charge transfer resistance observed for redox species in solution. This behaviour only manifests itself clearly in the case where the probability of direct charge transfer from the redox probe to the electrode is strongly attenuated by self-assembling molecular barriers. Here we will highlight specific issues concerning self-assembled monolayers as blocking barriers prior to discussing the effect of nanoparticles on the electrochemical response of the system. Selected examples will provide conclusive evidence that the extent of charge transfer mediation is determined by the overlap between the density of states of the nanostructures and the energy levels of redox species in solution. Only in the case where a strong overlap exists between the energy levels of the two components, the nanostructures behave as "electron launchers", allowing efficient charge transfer across insulating molecular layers.

ZnO nanowire structures are used today in a variety of applications, from gas/chemical sensing to photocatalysis, photovoltaics and piezoelectric actuation. Electrochemical anodization of zinc foil allows rapid formation of high aspect ratio ZnO nanowires under mild reaction conditions compared to more common fabrication methods. In this study we demonstrate, for the first time, how 3D hierarchical ZnO nanowire structures can be fabricated by controlling the type of electrolyte and anodization voltage, temperature and time. Optimization of the reaction conditions yields growth rates of up to 3.2 μm min -1 and the controlled formation of aligned arrays of nanowires, flower-like nanostructures, and hierarchical, fractal nanowire structures. Annealing of the nanowires produces high surface area (55 m 2 g -1 ) nanowires with slit-type pores perpendicular to the nanowire axis. In depth analysis of the anodization process allows us to propose the likely growth mechanisms at work during anodization. The findings presented here not only contribute to our knowledge of the interesting area of zinc anodization, but also enable researchers to design complex hierarchical structures for use in areas such as photovoltaics, photocatalysis and sensing.

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Nanostructured zinc oxide films were produced through anodization of zinc foil by using different electrolytes at different voltages, temperatures and over different time periods. The ZnO films were characterised by studying their surface morphology using FESEM, crystal structure using XRD, wetting behaviour through contact angle measurement, and also measuring the profile of the ZnO layer and band gap. Results show that the type of electrolyte and its concentration determine the morphology and size of the nanostructures. Voltage, time and temperature affect the distribution and density of the nanostructures along the surface. The band gaps of the films were in the range of 3.27 to 3.50 eV. Although ZnO is a hydrophilic material, some of the films display hydrophobic and super-hydrophobic behaviour. The data obtained in this study was combined with literature studies and used to devise design guidelines to obtain ZnO films with specific nanostructures and macroscopic properties by controlling the anodization parameters.

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