The carrier density of ZnO nanowires has been determined by means of electrochemical impedance spectroscopy. A model taking into account the geometry of ZnO nanowires has been developed and the differences with the standard flat model, as curved Mott-Schottky plots, are discussed. The as-grown electrodeposited samples present a high donor density of 6.2×1019cm−3, dramatically reduced by two orders of magnitude after an annealing in air at 450°C during 1h. The results show that the surface of the ZnO nanowires is active; therefore this system appears as a useful structure to support a functionalized nanostructured devices.

A systematic study of the role of KCl on the electrodeposition of ZnO nanowire arrays from the reduction of oxygen in ZnCl 2 solutions was performed. Besides its role as a supporting electrolyte, other major effects were found. An increase of KCl concentration ([KCl]) considerably decreased the rate of O 2 reduction. The consequent decrease in OHproduction rate resulted in an augmentation of the ZnO deposition efficiency, from a value around 3% for [KCl] ) 5 × 10 -2 M to more than 40% for [KCl] ) 3.4 M. The increase of the deposition efficiency mainly resulted in an enhancement of the longitudinal growth rate. However, high [KCl] (>1 M) also favored the lateral growth of the ZnO nanowires, resulting in diameters as big as 300 nm (in comparison to the diameter of 80 nm obtained for [KCl] < 1 M). The observed effects were discussed in terms of Clion adsorption on the cathode surface. The possible preferential adsorption of the anion on the (0001) ZnO surface was emphasized. Transmission electron microscopy revealed that the ZnO nanowires were single crystals, irrespective of [KCl] in the electrolyte. Thus, playing with the chloride content in the solution is an interesting way to obtain ZnO single-crystal nanowire arrays with tailored dimensions under controlled deposition rates. The influence of the nanowire dimensions on the optical properties was also discussed, showing the interest of this study in the frame of nanostructured solar cells.

Since the first report on ultraviolet lasing from ZnO nanowires (NWs), [1] remarkable effort has been dedicated to the development of novel synthesis routes for 1D ZnO nanostructures. Ordered arrays of 1D ZnO NWs have a promising future as applications in electronic and optoelectronic devices, because they are expected to improve the performance of various nanodevices such as short-wavelength lasers, [1] nanostructured solar cells, [2,3] electroluminescent, [4] and field-emission devices.[5]What is now a relevant area of focus in nanoscience involves the preparation of higher-order assemblies, arrays, and superlattices of these 1D nanostructures. [6] Recently, many efforts have focused on the integration of 1D nanoscale building blocks into 3D architectures. Hollow urchin-like ZnO NWs that combine properties of 3D and 1D materials may emerge as a more interesting alternative than simple arrays of NWs due to the higher specific surface and porosity, [7] especially for application in dye and semiconductor-sensitized solar cells. [3,8] To date, there are only two strategies to synthesize hollow urchin-like ZnO NWs. The first one [9] is a wet-chemical route that uses a modified Kirkendall process, by which zinc powders that are spherical in shape are transformed into hollow urchin-like ZnO NWs dispersed in solution. The second strategy [10][11][12] is based on the calcination of metallic Zn microsphere powders at relatively high temperature (500-750 8C). With these two approaches, ZnO nanostructures are often randomly distributed (in size and organization), which may limit their practical applications as building blocks in nanodevices. Nevertheless, it is essential for the fabrication of nanodevices to assemble NW-structured hollow spheres with a uniform size in ordered arrays, since such an organisation combines the merits of patterned arrays and nanometer-sized materials. Until now, a suitable technique is still missing for the fabrication of ordered arrays of hollow urchin-like ZnO NWs with tunable sizes.In this paper, we report on a novel approach to fabricate well-ordered hollow urchin-like single-crystal ZnO NWs with controlled NW and core dimensions. The method combines the formation of a polystyrene (PS) microsphere colloidal mono/ multilayer and the electrodeposition of ZnO NWs, followed by the elimination of the PS microspheres, which play the role of a template. It is shown that the light scattering properties of such an ordered architecture exceed those of ZnO NW arrays. Applications as 3D building blocks in the field of nanostructured solar cells are discussed.Mono/multilayers of PS spheres covering conductive substrates have been used as templates to electrodeposit inverse opal structures. [13,14] In such cases the nucleation of ZnO took place at the interstitial sites (on a conductive substrate) between the PS spheres leading to different morphologies depending on the employed method. Our strategy of electrodeposition differs from those previously described by the mode of nucleation and growth. In our ...