Abstract
ZnO–ZnS 1D hetero-nanostructures were prepared by an easy and scalable processing route. It consists of ZnO nanorod electrodeposition on ITO substrate and surface sulfidation by ion exchange in an aqueous Na2S solution. Increasing the treatment contact time (tc) from 8 to 48 h involves different ZnS growth mechanisms leading to different structural and microstructural rod characteristics, even if the overall size does not change significantly. Grazing X-ray diffraction, high-resolution microscopy, energy-dispersive spectrometry and X-ray photoelectron spectroscopy describe the outer surface layer as a poly- and nanocrystalline ZnS blende shell whose thickness and roughness increase with tc. The ZnO wurtzite–ZnS blende interface goes from continuous and dense, at short tc, to discontinuous and porous at long tc, indicating that ZnS formation proceeds in a more complex way than a simple S2−/O2− ion exchange over the treatment time. This feature has significant consequences for the photoelectrochemical performance of these materials when they are used as photoanodes in a typical light-assisted water splitting experiment. A photocurrent (Jp) fluctuation of 45% for less than 5 min of operation is observed for the sample prepared with a long sulfidation time while it does not exceed 15% for that obtained with a short one, underlining the importance of the material processing conditions on the preparation of valuable photoanodes.
Highlights
Semiconducting core–shell hetero-nanostructures have been the focus of functional materials science and engineering
The ZnS shell formed has a polycrystalline cubic blende structure with a thickness which increases with the sul dation treatment time
Partial disorientation of native ZnO crystallites during ZnS formation is observed with the formation of interfacial voids when the sul dation is prolonged
Summary
Semiconducting core–shell hetero-nanostructures have been the focus of functional materials science and engineering. ZnO (band-gap 1⁄4 3.39 eV) and ZnS (band-gap 1⁄4 3.60 eV) are two direct wide-band gap semiconductors In one hand, their conduction and valence band-edges straddle the water-redox levels, and in the other hand, their relative band-energy positions offer a favorable charge separation. The thicker the formed ZnS shell is, the better PEC performances are.[14,15,33] Focusing on this material processing route, we wanted to explore its limits for the preparation of efficient ZnO–ZnS based PEC photoanodes. We denoted the bare ZnO NRs and their related ZnO–ZnS composites as ZO, ZOS8 and ZOS48, respectively We measured their PEC properties, working within a home-made threeelectrode cell and using a passive Na2SO4 electrolyte solution (0.5 M, pH 7). We discussed all the obtained results in the framework of our main goal
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