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 ...

An innovative route is presented to obtain arrays of single-crystal ZnO nanotubes with tailored dimensions. The three-step process combines electrochemical and chemical approaches. The first step consists in the electrodeposition of ZnO nanowire arrays from the O 2 reduction in an aqueous solution of zinc chloride (ZnCl 2 ) and potassium chloride (KCl). In the second step the core of ZnO nanowires is selectively etched in a KCl solution, resulting in the formation of tubular structures. The influence of KCl concentration, temperature, and immersion time in the ZnO nanotube formation process is investigated, with the finding that the dissolution of the nanowire core occurs for [KCl] g 1 M and the etching rate is enhanced with the temperature. Arrays of ZnO nanotubes with tailored dimensions (200-500 nm external diameter and 1-5 µm length) are obtained by varying the conditions of nanowire array deposition and taking into account the dimensions of the nanowires to adjust the dissolution time. A precise control of the nanotube wall thickness is achieved by performing a further electrodeposition step. The whole process occurs at low temperature (80°C) in aqueous chloride solution at neutral pH, in a couple of hours. The structural properties of obtained ZnO nanotubes are analyzed by transmission electron microscopy, showing their single-crystal character.

Arrays of ordered hollow urchin‐like ZnO single‐crystal nanowires with controlled core dimensions and wire morphology can be obtained by a new method that combines electrochemical deposition with colloidal templating rendered electrically conductive, as reported by Jamil Elias and co‐workers . This method opens new opportunities for processing novel metal oxide or hydroxide materials based on a similar growth mechanism. Such ordered structures exhibit superior optical reflectance compared to nanowire arrays.