ZnS shielded ZnO nanowire photoanodes for efficient water splitting

• Facile and low temperature chemical conversion of ZnO nanowire to ZnO-ZnS core-shell nanowire is presented.
• High quality ZnS shell results in suppression of defect emission and improves the electrical conductivity of ZnO nanowire films.
• ZnS shielded ZnO nanowires lead to stable photoanode formation and specific band alignment enhances water splitting performance.
• An enhancement in open circuit photo-voltage and increase in life time of photo generated charge carriers is observed for ZnS modified ZnO nanowire anode.

A chemical conversion of zinc oxide to zinc sulfide is adapted to create ZnS shell over single crystalline ZnO nanowire. Appearance of an additional peak in x-ray diffraction corresponds to ZnS and strong S 2p3/2 and S 2p1/2 peaks in x-ray photoelectron spectroscopy due to Zn-S bonding indicates high quality ZnS shell growth. Reduction in visible light transmittance (approx 15%) is observed in ZnO-ZnS core-shell nanowires which renders higher light absorption. Suppression of defect emission in core-shell nanowires indicates a reduction in surface defects and chemically adsorbed oxygen species. The core-shell geometry also results in an order of increment in charge carrier density (5.3 × 1018 cm−3 to 2.8 × 1019 cm−3), i.e. results in an improvement of electrical conductivity and photoelectrochemical performance. Electrochemical solar to hydrogen conversion efficiency is increased to more than double (from 0.15% to 0.38%) due to an improvement in photo-charge carrier separation and collection properties. In comparison to pristine ZnO, photocurrent for ZnO-ZnS core-shell remains same without noticeable fluctuation for few hours, which indicates the ZnS functionalization adds stability to unstable ZnO photoanode. Higher conduction band position of ZnS enhances open circuit photovoltage and reduces photo carrier recombination rate leads to enhancement in photo carrier life time.

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