Jamil Elias

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

Electrodeposition of gold thin films with controlled morphologies and their applications in electrocatalysis and SERS

Here, an easy and effective electrochemical route towards the synthesis of gold thin films with well-controlled roughness, morphology and crystallographic orientation is reported. To control these different factors, the applied potential during deposition played a major role. A tentative nucleation and growth mechanism is demonstrated by means of electrochemical characterizations and a formation mechanism is proposed. Interestingly, the differences in geometry and orientation of the different gold deposits have shown a clear correlation with the electrocatalytical activity in the case of oxygen sensing. In addition, not only the electrocatalytical activity but also the surface-enhanced Raman scattering of the gold deposits have been found to depend both on the roughness and on the size of the surface nanostructures, allowing a fine tuning by controlling these two parameters during deposition.

Structural and optical characterization of electrodeposited CdSe in mesoporous anatase TiO2 for regenerative quantum-dot-sensitized solar cells

We investigated CdSe-sensitized TiO(2) solar cells by means of electrodeposition under galvanostatic control. The electrodeposition of CdSe within the mesoporous film of TiO(2) gives rise to a uniform, thickness controlled, conformal layer of nanostructured CdSe particles intimately wrapping the anatase TiO(2) nanoparticles. This technique has the advantage of providing not only a fast method for sensitization ( < 5 min) but also being easily scalable to the sensitization of large-area panels. XRD together with SAED analysis highlight that the deposit of CdSe is exclusively constituted of the hexagonal polymorph. In addition, hierarchical growth has also been shown, starting from the formation of a TiO(2)-CdSe core-shell structure followed by the growth of an assembly of CdSe nanoparticles resembling cauliflowers. This assembly exhibits at its core a mosaic texture with crystallites of about 3 nm in size, in contrast to a shell composed of well-crystallized single crystals between 5 and 10 nm in size. Preliminary results on the photovoltaic performance of such a nanostructured composite of TiO(2) and CdSe show 0.8% power conversion efficiency under A.M.1.5 G conditions-100 mW cm(-2) in association with a new regenerative redox couple based on cobalt(+III/+II) polypyridil complex (V(oc ) = 485 mV, J(sc ) = 4.26 mA cm (-2), ff=0.37).

Well Ordered Hollow Urchin-Like ZnO by Electrodeposition

Ordered arrays of 1D Single-crystal ZnO nanowires have emerged as promising building blocks for a new generation of devices in different technological domains such as optoelectronics, solar cells, gas sensing, field emission and piezoelectrics. Among the various deposition techniques, electrochemical deposition appears as a versatile method well suited to synthesize ZnO [1] with a large variety of morphologies and dimensions [24]. 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 In this contribution, we will present an original and new approach to fabricate, in a time period compatible with large-scale production, thin films composed of wellordered urchin-like ZnO nanowires (NWs) hollow from inside (Figure 1a and b) and with controlled NW and core dimensions. The synthesis method combines together the formation of a polystyrene (PS) microspheres 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 for the first time that a compound can be electrodeposited directly on the surface of polystyrene spheres [5]. We will also show that the same process allows to deposit multilayers of ZnO urchins on transparent conductive oxide (TCO) substrates. The dimensions of the NWs can be also controlled by varying the constitution of the electrolyte. Our new approach has enabled us to produce a wide range of hollow ordered urchin ZnO NWs on relatively large surfaces of TCO. We expect that these structures will be useful in many applications and nanodevices. As preliminary experiments, we investigated the geometry influence on solar light scattering. A spectacular increase (more than 50%) of the reflectance was observed throughout the entire visible wavelength range by comparing normal NW arrays with the ordered hollow urchin-like structures (Figure 2). In conclusion, this mechanism opens up new opportunities for processing novel metal oxide or hydroxide materials based on a similar growth mechanisms to that of ZnO (e.g. CdO, TiO2, CeO2, Co(OH)2, Ni(OH)2, etc.). Importantly, preliminary experiments have confirmed a strong improvement of light scattering by the urchin-like ZnO NW thin films, offering promising applications as building-blocks in different types of nanostructured solar cells (organic, hybrid and dye sensitized solar cells) and potentially useful for a wide variety of applications in optical devices, photonic crystals, and nanodevices. Fig. 1 SEM micrographs of: a) hollow ordered urchinlike ZnO NWs deposited on TCO substrate, the inset represent a higher magnification image, b) Mechanically broken single urchin structure.

Influence of Experimental Parameters on the Synthesis of Gold Nanoparticles by Electroless Deposition

We report a simple synthetic route based on electroless deposition (galvanic displacement) and natural lithography to synthesize organized Au nanoparticles (NPs). We will study the influence of different experimental parameters (time, temperature, pH and additives) on the synthetic process. We show for the first time the formation of organized extended domains of Au nanorings using gelatin as an additive. A wide range of applications can be envisaged for these nanostructures, as for instance chemical- and bio-sensors using the SERS effect.