We report the fabrication of a three dimensional branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. Specifically, we demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature. The branching nanowire heterostructures offer direct functional integration of different materials for high efficiency water photoelectrolysis and scalable photoelectrodes for clean hydrogen fuel generation.
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"The specimen presented a high photodetection sensitivity with an on/off ratio larger than 250 and a peak photoresponsivity of 12.8 mA/W at 900 nm. They also used them in photoelectrochemical cells and found that the 3D nanowire heterostructures demonstrated large enhancement in photocathodic current density (an achieved value as high as 8 mA/cm2) and overall hydrogen evolution kinetics . Kim synthesized the ZnO/Si nanowire arrays by combining nanosphere lithography and solution process . "
[Show abstract][Hide abstract] ABSTRACT: A rational approach for creating branched ZnO/Si nanowire arrays with hierarchical structure was developed based on a combination of three simple and cost-effective synthesis pathways. The crucial procedure included growth of crystalline Si nanowire arrays as backbones by chemical etching of Si substrates, deposition of ZnO thin film as a seed layer by magnetron sputtering, and fabrication of ZnO nanowire arrays as branches by hydrothermal growth. The successful synthesis of ZnO/Si heterogeneous nanostructures was confirmed by morphologic, structural, and optical characterizations. The roles of key experimental parameters, such as the etchant solution, the substrate direction, and the seed layer on the hierarchical nanostructure formation, were systematically investigated. It was demonstrated that an etchant solution with an appropriate redox potential of the oxidant was crucial for a moderate etching speed to achieve a well-aligned Si nanowire array with solid and round surface. Meanwhile, the presence of gravity gradient was a key issue for the growth of branched ZnO nanowire arrays. The substrate should be placed vertically or facedown in contrast to the solution surface during the hydrothermal growth. Otherwise, only the condensation of the ZnO nanoparticles took place in a form of film on the substrate surface. The seed layer played another important role in the growth of ZnO nanowire arrays, as it provided nucleation sites and determined the growing direction and density of the nanowire arrays for reducing the thermodynamic barrier. The results of this study might provide insight on the synthesis of hierarchical three-dimensional nanostructure materials and offer an approach for the development of complex devices and advanced applications.
Full-text · Article · Jun 2014 · Nanoscale Research Letters
[Show abstract][Hide abstract] ABSTRACT: Hydrogenated ZnO nanorod arrays (NRAs) grown on F-doped SnO(2) (FTO) glass substrates yield a benchmark specific hydrogen production rate of 122,500 μmol h(-1) g(-1), and exhibit excellent stability and recyclability.
Full-text · Article · Apr 2012 · Chemical Communications
[Show abstract][Hide abstract] ABSTRACT: We report a nickel oxide (NiOx) thin film, from a cost-effective sol–gel process, coated n-type silicon (n-Si) as a photoanode for efficient photo-oxidation of water under neutral pH condition. The NiOx thin film has three functions: (i) serves as a protection layer to improve the chemical stability of the Si photoelectrode, (ii) acts as an oxygen evolution catalyst, and (iii) provides junction photovoltage to further reduce overpotential. The oxygen evolution onset potential is reduced to below the thermodynamic water oxidation level and oxygen evolution was observed at low overpotentials. Our results demonstrate the fabrication of robust photoelectrodes from low-cost NiOx and Si, which enable a practical solar water oxidation with high efficiency.
No preview · Article · Jun 2012 · Energy & Environmental Science