Electrochimica Acta 130 (2014) 222–231 Contents lists available at ScienceDirect Electrochimica Acta j our na l ho me pa g e: www.elsevier.com/locate/electacta ZnS shielded ZnO nanowire photoanodes for efficient water splitting Ajay Kushwaha, Mohammed Aslam ∗ Department of Physics, National Center for Photovoltaic Research and Education (NCPRE), Indian Institute of Technology Bombay, Powai, Mumbai-400076, India a r t i c l e i n f o Article history: Received 7 February 2014 Accepted 7 March 2014 Available online 18 March 2014 Keywords: Zinc oxide Nanowire Core-shell Photoluminescence Water splitting Open circuit photopotential a b s t r a c t 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 2p 3/2 and S 2p 1/2 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 geom- etry also results in an order of increment in charge carrier density (5.3 × 10 18 cm -3 to 2.8 × 10 19 cm -3 ), i.e. results in an improvement of electrical conductivity and photoelectrochemical performance. Elec- trochemical 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 compar- ison 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. © 2014 Published by Elsevier Ltd. 1. Introduction Electrochemical solar driven water splitting is an attractive, environment friendly and non-conventional energy resource [1–3]. Light active semiconductors or metal oxides have shown great prospects for this method of hydrogen generation [4–6]. Titanium dioxide has been explored extensively among various other metal oxides such as ZnO, Fe 2 O 3 , and WO 3 due to stable and non-corrosive nature, however, poor electrical conductivity in TiO 2 is one of the bottlenecks for the technological applications [7–10]. Zinc oxide has similar characteristics as TiO 2 in exception to 10–100 times higher electron mobility [11], which renders large electrical con- ductivity and makes ZnO more promising photoanode [11,12]. However, wide band gap in ZnO limits the photoelectrochemi- cal (PEC) water splitting performance due to poor absorption of visible light [13]. Therefore, ZnO is doped to tailor the band gap towards enhancement of light absorption [13–17]. The general objective of doping is upward shifting of the valence band to reduce the band gap leading towards higher absorption [13]. Doped ZnO films have also shown increased carrier density which resulted ∗ Corresponding author. Tel.: +91 22 25767585. E-mail address: m.aslam@iitb.ac.in (M. Aslam). in improvement of hydrogen generation efficiency than undoped ZnO. However, large concentration of impurity yields higher den- sity of recombination centers, which hinders the PEC performance, therefore, further investigations toward functionalization of ZnO photoanode is required [18,19]. Remarkable advancement in fabrication of variety of ZnO nano- structures and doping therein has put resurgence in the activity of PEC water splitting. Confined dimensionality (1D - 2D) offers an improvement in light absorption due to large active surface area and ultrafast charge transport behaviour [20,21]. In specific, large surface area and vertical orientation of one dimensional nano- structure renders enhanced light scattering and promotes multiple absorption of light [22]. In addition, nitrogen, carbon and hydrogen doped ZnO nanowires have shown improvement in device effi- ciency for PEC water splitting [10,11,19,23,24]. Though, pure and doped ZnO nanowire electrodes have many favorable properties, but thermodynamical instability in electrolyte solution, photo cor- rosion and quick recombination of photo induced electron–hole pairs are few of the persisting issues in ZnO [25,26,27,28,29]. Unsta- bility against photo-corrosion and degradation can be effectively addressed by applying a protective shell over the nanowire surface [26,29]. A suitable choice of shell material not only stabilize the core against corrosion but also reduces the recombination losses, as well enhance the electron collection efficiency. Different metal or http://dx.doi.org/10.1016/j.electacta.2014.03.008 0013-4686/© 2014 Published by Elsevier Ltd.