Highly efficient photoelectrochemical hydrogen generation using a ZnO nanowire array and a CdSe/CdS co-sensitizer
Introduction
Hydrogen production from PEC water splitting has attracted considerable attention because it uses only earth-abundant water and solar energy without generating pollutants [1], [2]. A variety of nanostructured metal oxides have been investigated for photoanodes in PEC cells [3], [4], [5], [6]. In the present study, we employed ZnO NW arrays due to their well-developed methods of synthesis [7] and high electron mobility (205–300 cm2 Vs−1 for bulk ZnO, and 1000 cm2 Vs−1 for single NW) [8]. 1D NW structures offer new opportunities for enhancing the efficiency of PEC cells because they provide direct electrical pathways that ensure the rapid radial collection of photogenerated carriers [9] and facilitate light absorption by decreasing reflectance through light scattering and trapping [10]. However, the large bandgap in ZnO NWs restricts utilization of light in the visible region, which is the main part of the solar spectrum. Therefore, considerable effort has been devoted to improving the visible light response, including dye [11] or low bandgap quantum dot (QD) sensitization [12], [13] and doping [14].
Recently, we prepared CdS QD sensitized ZnO NW arrays by a chemical bath deposition method for application as photoanodes in PEC cells [12]. However, limited visible light absorption due to the bandgap of CdS, with an upper wavelength cutoff of 550 nm, produced a low PEC performance. Here, we report the synthesis of CdSe deposited ZnO/CdS core/shell NW (CdSe/CdS/ZnO NW) arrays, and we describe their potential application as photoanodes in PEC cells. Due to the improvement in photoresponse and charge carrier transfer caused by the synergetic effect of CdS and CdSe, our proposed NWs showed substantially enhanced photocurrent generation.
Section snippets
Experimental details
CdSe/CdS/ZnO NW arrays were prepared by a three-step solution-based method, which offered the advantages of low cost, process simplicity, and ease of scaling-up. Arrays of ZnO NWs were grown on substrates (Ti foil and ITO glass) by an ammonia solution method [7]. CdS QDs were coated onto the ZnO NWs via successive ion layer absorption and reaction (SILAR) [12]. Finally, CdSe QDs were deposited onto the ZnO/CdS core/shell NWs via chemical bath deposition (CBD) which was carried out by immersing
Results and discussion
Fig. 1(a) shows a SEM image of the as-prepared ZnO NW array. The dense well-aligned NW array grown on a Ti foil substrate is clearly visible, with an average diameter of 100 nm and an average length of 10 μm. The deposition of CdS (Fig. 1(b)) showed only slight morphological changes, with a slight increase in the diameters of the NWs. SEM and TEM images obtained after the deposition of CdSe (Fig. 1(c) and inset) clearly showed that the NW surfaces became rough. We suggest that the difference in
Conclusion
In summary, we developed CdSe-deposited ZnO/CdS core/shell NW arrays for use as photoanodes in PEC hydrogen generation. ZnO NW structures offered efficient charge carrier collection, and the stepwise band alignment of CdSe/CdS/ZnO allowed for co-sensitization of CdS and CdSe to absorb and utilize the visible light in the solar spectrum. Additionally, the CdS interlayer acted as a passivation layer to prevent recombination and as a seed layer to assist CdSe deposition. Upon illumination at a
Acknowledgements
This work was supported by grant from National Research Foundation (NRF2010-0009545), and by Korean Research Foundation Grants Funded by the Korean Government (MOEHRD)(KRF 2008 005 J00501).
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