ZnO/CdS core/shell nanorod arrays were fabricated by a two-step method. Single-crystalline ZnO nanorod arrays were first electrochemically grown on SnO(2):F (FTO) glass substrates. Then, CdS nanocrystals were deposited onto the ZnO nanorods, using the successive ion layer adsorption and reaction (SILAR) technique, to form core/shell nanocable architectures. Structural, morphological and optical properties of the nanorod heterojunctions were investigated. The results indicate that CdS single-crystalline domains with a mean diameter of about 7 nm are uniformly and conformally covered on the surface of the single-crystalline ZnO nanorods. ZnO absorption with a bandgap energy value of 3.30 ± 0.02 eV is present in all optical transmittance spectra. Another absorption edge close to 500 nm corresponding to CdS with bandgap energy values between 2.43 and 2.59 eV is observed. The dispersion in this value may originate in quantum confinement inside the nanocrystalline material. The appearance of both edges corresponds with the separation of ZnO and CdS phases and reveals the absorption increase due to CdS sensitizer. The photovoltaic performance of the resulting ZnO/CdS core/shell nanorod arrays has been investigated as solar cell photoanodes in a photoelectrochemical cell under white illumination. In comparison with bare ZnO nanorod arrays, a 13-fold enhancement in photoactivity was observed using the ZnO/CdS coaxial heterostructures.
Vertically aligned ZnO/Cu2O heterostructure nanopillar arrays consisting of a ZnO core and a Cu2O shell were fabricated by a two-step electrochemical deposition method. Morphological, structural and optical properties of the nanopillar heterojunctions were investigated. The surface of the single-crystalline ZnO nanopillars was coated uniformly, conformally and densely over the entire nanopillar length by numerous Cu2O nanocrystals (25–35 nm mean diameter), constituting a conformal shell layer 90 nm thick, integrating these two materials into an electronically intimate composite. The optical properties can be interpreted, by appropriate fittings of each feature, as being due to the properties of the bare ZnO nanopillar array plus the increased absorption of Cu2O. This study demonstrates that electrodeposition is a suitable and accessible technique for large-scale fabrication of nanopillar heterostructures and to achieve conformal coverage of nanostructured samples.
The optical properties of bare ZnO nanorods and sensitized nanostructures, with Cu 2 O and CdS, are comparatively studied. These nanostructures may show improved photovoltaic performance compared to planar ones. ZnO nanorod arrays were grown by electrochemical deposition. In a second step, Cu 2 O was also deposited electrochemically, while for CdS successive ion layer adsorption and reaction techniques were used. The experimental results are interpreted using numerical simulation based on an effective medium theory. Bare nanorod samples reveal mainly the direct ultraviolet absorption edge of ZnO (between 3.25 and 3.30 eV) and a monotonically increasing transmittance from the ultraviolet into the red. This increase is originated in light scattering, probably by the nanometric structure of the samples. For the sensitized samples reduced transmittance in the solar spectrum region is observed and several well-defined absorption edges appear. Spectral absorption edge shifts are interpreted comparing with numerical simulations. For CdS the measured shifts are larger than the ones obtained from numerical simulations. The difference may be due to the combined influence of sub-bandgap absorption, light scattering in the nanorod array and quantum confinement in the nanocrystalline structure of sensitizer layers. For Cu 2 O its more complex electronic structure gives larger dispersion in the results although major absorption edges are clearly observed.
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