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 ...

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.

No abstract

No abstract

An investigation of the electrodeposition of ZnO nanowire arrays from the reduction of dissolved molecular oxygen in solutions containing different anions (Cl-, SO4 2-, and CH3COO-) is reported. A significant variation of the diameter (65−110 nm) and length (1.0−3.4 μm) of the nanowires was obtained by changing only the nature of the anions in the solution. Nanowires exhibiting the lowest and highest aspect ratios were obtained in chloride and acetate solutions, respectively. The diffusion of Zn2+ to the cathode and the OH- generation rate were analyzed by cyclic voltammetry. The difference in adsorption behaviors of the different anions on the ZnO surface is proposed as one of the main reasons for the observed differences in the OH- production rate. The ratio between the OH- generation rate and the Zn2+ diffusion to the cathode is proposed as a major parameter in the electrodeposition of ZnO nanowires. Obtained samples were analyzed by X-ray diffraction, and no spurious phases were observed. Thus, changing the chemical nature of the anions in solution allows for the dimensions and the deposition rate of electrodeposited ZnO nanowire arrays to be controlled.

Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.