This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 3493–3495 3493 Cite this: Chem. Commun., 2011, 47, 3493–3495 Photocatalytic CdSe QDs-decorated ZnO nanotubes: an effective photoelectrode for splitting waterw Neelu Chouhan, a Chai Ling Yeh, b Shu-Fen Hu,* c Ru-Shi Liu,* a Wen-Sheng Chang d and Kuei-Hsien Chen e Received 13th December 2010, Accepted 26th January 2011 DOI: 10.1039/c0cc05548d Arrays of ZnO nanorods (NRs) were successfully converted into nanotubes (NTs), used as photoelectrodes in photoelectrochemical (PEC) cells after their sensitization with CdSe quantum dots (QDs) and a strong correlation between the PEC performance and geometrical structure of ZnO NTs@CdSe(QDs) and ZnO NRs@CdSe(QDs) was established under the same conditions. One-dimensional (1D) nanoarchitectures 1 of wide-band-gap semiconductors (WBGS) (ZnO, TiO 2 , SnO 2 and GaN) such as nanorods (NRs), nanotubes (NTs) and nanowires (NWs) exhibit excellent photon absorption and facilitate the efficient transport of photogenerated-charge carriers. 2,3 Modified WBGS in shape and size have been used to improve the performance of optoelectronic devices. But most of the WBGS are sensitive to the UV portion of sunlight (B5% of the whole sunlight spectrum). 4 Therefore, small-band-gap sensitizers (organic dyes or short-band-gap semiconductors) must be added or band-engineering adopted to functionalize WBGS in the visible part of the sunlight. Quantum dots (QDs) are favored as sensitizers for WBGS because of their uniquely tunable particle size, their convenience of multiple charge carrier generation from a single high-energy photon, and their ability to modulate vectorial charge transfer. However, to date numerous QD (CdS, CdSe, CdTe, PbS, CuInS 2 , InP, Bi 2 S 3 ) 5–11 -sensitized nanostructures (most of which are TiO 2 -based) have been investigated for use in photovoltaics. Recently, the arrays of CdSe nanofilms on ZnO nanowires have been studied as photoanodes in solar cells. 12,13 Additionally, the author’s group has stimulated research into the QDs decorated-1D ZnO nanodevices by investigating ZnO NRs@CdSe(QDs) and ZnO NWs@CdTe(QDs) arrays for the photoelectrochemical splitting of water, 14,15 but ZnO NTs@CdSe(QDs) has not been reported for the same by anyone till now. The uncooled ZnO samples (0 h) grew as rods and the array films in the mother solution were slowly cooled for different times to promote the hexagonal etching of NRs using NH 4 OH that was produced by the cleavage of HMT, facilitating their gradual conversion into NTs (Fig. 1). The ZnO NTs/NRs arrays films were washed with DIW, air-dried, annealed at 450 1C for 0.5 h. Hydrophobic oleic acid-encapsulated-CdSe QDs were produced using the method described by Qu and Peng et al. 17 with some modifications, which adopted hydro- philic character upon the exchange of ligands (oleic acid with 3-mercaptopropionic acid (MPA)). Finally, the aggregates of MPA-escorted CdSe QDs were deposited on the surface of the ZnO NTs/NRs arrays by sequential layer-by-layer deposition and the as-obtained ZnO NRs or NTs@CdSe(QDs) samples were annealed at 350 1C for 0.5 h. UV-visible absorption spectroscopy and X-ray diffraction (XRD) study confirmed the aforementioned deposition (Fig. S2aw, S2bw, S3cw, S3dw, S3ewand S4w), in close agreement with the high-resolution transmission electron microscopic (HRTEM, JEM-2100F microscope) results (Fig. 2). HRTEM images (Fig. 2a,b,d) and their corresponding selected area electron diffraction spectroscopic (SAED) patterns (Fig. 2c) reconfirmed the 5–8 nm thick loading of 3 nm sized-CdSe QDs 18 aggregate on the walls of the ZnO nano-surface. Fig. 2e,f present low- magnification field emission scanning electron microscopic (FESEM, JEOL JSM-6700F) images of arrays of pristine Fig. 1 Gradual transformation of ZnO NRs to NTs on cooling. a Department of Chemistry, National Taiwan University, Sec. 4, Roosevelt Road, Taipei 106, Taiwan. E-mail: rsliu@ntu.edu.tw b Institute of Electro-Optical Science and Technology, National Taiwan Normal University, 88, Sec. 4, Ting-Chou Road, Taipei 116, Taiwan. c Department of Physics, National Taiwan Normal University, 88, Sec. 4, Ting-Chou Road, Taipei 116, Taiwan. E-mail: sfhu.hu@gmail.com d Department of Nano-Tech Energy, Industrial Technology Research Institute, 421, Sec. 4, Zhong Xing Road, Jhun Dong, Hsinchu 310, Taiwan. e Institute of Atomic & Molecular Sciences Academia Sinica, Sec-4, Roosevelt Road, Taipei 106, Taiwan. w Electronic supplementary information (ESI) available: UV-Vis absorbance spectra, XRD patterns, FESEM images of ZnO NRs (0 h cooling), PL of CdSe QDs and photoresponses of ZnO NTs@CdSe(QDs). See DOI: 10.1039/c0cc05548d ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Downloaded by National Taiwan University on 30 March 2011 Published on 11 February 2011 on http://pubs.rsc.org | doi:10.1039/C0CC05548D View Online