Hollow Urchin‐like ZnO thin Films by Electrochemical Deposition

Since the first report on ultraviolet lasing from ZnO nanowires (NWs), 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, nanostructured solar cells, electroluminescent, and field-emission devices. What is now a relevant area of focus in nanoscience involves the preparation of higher-order assemblies, arrays, and superlattices of these 1D nanostructures. 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, especially for application in dye and semiconductor-sensitized solar cells. To date, there are only two strategies to synthesize hollow urchin-like ZnO NWs. The first one 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 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 ZnONWs, 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. 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 case, the deposition of ZnO takes place, from the nucleation step, on the PS spheres and the conductive substrate, simultaneously. As a result, the spheres are homogenously covered by a thin film composed of single-crystal ZnO NWs connected together at their base. This approach provides a simple and versatile way to synthesize well-ordered mono/multilayers of ZnO hollow microspheres with the ability to control the sphere size in addition to the ZnO NW dimensions and morphology. A monolayer of commercially available carboxylate-modified PS spheres ( 4.3mm) is deposited directly on a transparent conductive oxide (TCO) substrate by using a self-assembly technique on a water surface. We have used the method of Zhou et al. with some modifications. A detailed description of our process is given in the experimental section. Figure 1a shows a tilted, low-magnification scanning electron microscopy (SEM) image of the self-assembled monolayer on TCO substrate. A well-organized monolayer of PS microspheres can be observed in addition to occasional point defects in some regions due to the presence of larger spheres in the commercial solution (circled region in Fig. 1a). This organization is observed throughout the entire TCO surface ( 1.5 cm). As a proof of that, the lower inset in Figure 1a shows an optical image of TCO/PS where the sample colour is perfectly homogeneous, reflecting the presence of only one PS domain (monolayer) on the substrate. The detailed organization of the spheres was investigated by a closer examination using high-magnification SEM (Fig. 1b and its inset), which shows a relatively large area of the self-assembled monolayer and a perfectly ordered array. The TCO/PS sample has then been immersed for 30min in 2 M ZnCl2 aqueous solution at room temperature and used as a working electrode in an electrochemical cell for the deposition of ZnO NWs. The electrolyte was an aqueous solution saturated by molecular O2, containing 5 10 4 M ZnCl2 (zinc precursor) and 0.1 M KCl