The electrodes in photoelectrochemical cells responsible for the generation of hydrogen and oxygen by water splitting have been intensively studied because of their high photon-to-electron conversion efficiency. The morphology of nanostructures with these high-efficiency electrodes was systematically compared with the morphology of ZnO structures with vertically aligned nanorod arrays (NA), hollow hemisphere arrays (HA), urchin-like (UL) nanorod arrays, and thin films (TF). The UV-vis light absorption, photoresponse (current-voltage characteristics in the dark and under light), and photoelectrochemistry of the electrodes were measured. The highest photon-to-electron conversion efficiency of 65% at a specific UV wavelength for an electrode with a ZnO UL structure was derived from the UL morphology of high light-trapping efficiency and carrier collection efficiency. The UL morphology also produced a photon-to-electron conversion efficiency of 4.5% under a solar simulator by CdS-sensitization of the ZnO UL electrode. The value was the highest observed thus far among the ZnO-based electrodes. We demonstrated that photoresponse measurement is a practical and simple technique for the estimation of the photon-to-electron conversion efficiency of an electrode.
Zinc oxide (ZnO) hollow hemisphere (HS) and urchin-like (UL) structures were fabricated and examined for application to a gas sensor. Films of hollow ZnO-HS arrays floating over substrates were synthesized via Zn sputtering onto the template of a polystyrene sphere array followed by oxidation. Growing ZnO nanorods upon HS surfaces via a hydrothermal method formed hollow ZnO-UL structures. The thicknesses of the HS films and the lengths of nanorods in the UL structures were varied to obtain the maximum response to NO gas. Both sensor structures showed a sensing of tens of parts per billion of levels of NO concentrations with good response and gas selectivity. The highest response was realized through the thinness and the open porosity of the structures. The surface depletion determined the sensor response signal for the sensor geometry with the highest response.
Optimization of highly porous ZnO thin film electrode structures is carried out for use in photoelectrochemical (PEC) cell. Spherical voids are formed in ZnO films using a sacrificial template of polystyrene spheres of ∼400 nm sizes. The voids serve as light scattering centers for dramatically enhanced light absorption in the visible region of the solar spectrum, and as channels for electrolyte infiltration into the film. The UV-vis spectra reveal that light absorption at wavelength of 800 nm is enhanced to about 99%. The voids are further used as a route for the deposition of CdS nano-crystallites throughout the ZnO film. The CdS-decorated ZnO nanoparticulate structure shows a photon-to-electron conversion efficiency of 5.7% under a solar simulator. This efficiency value is the highest reported so far among ZnO-based electrodes, and is achieved by optimization of the nanostructure in the CdS-decorated ZnO films.
We report a heterojunction formed between single-wall carbon nanotubes (SWCNTs) and a WO 3 thin film. WO 3 thin films were fabricated by sputter deposition of W followed by oxidation and SWCNTs were fabricated by the arc-discharge method. The morphology of the structures was examined by scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The current-voltage characteristics of WO 3 thin films, SWCNTs films, and SWCNT/WO 3 heterojunctions were measured in darkness and under ultraviolet light. A rectifying heterojunction formation of p-SWCNT/n-WO 3 was confirmed. The observed unconventional optoelectronic properties were analyzed, and the results were used to explain the photoconduction phenomena occurring at the heterojunction. The heterojunctions and resistors were also examined for their photodetection performance.
Experimentalp-SWCNT/n-WO 3 heterojunction preparation: SiO 2 (300 nm)/Si substrates were sequentially ultrasonicated in acetone, methanol, and deionized water for 15 min each and then blow dried
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