We have prepared and employed TiO2/CdZnS/CdZnSe electrodes for photochemical water splitting. The TiO2/CdZnS/CdZnSe electrodes consisting of sheet-like CdZnS/CdZnSe nanostructures (8–10？ m in length and 5–8？nm in width) were prepared through chemical bath deposition on TiO2 substrates. The TiO2/CdZnS/CdZnSe electrodes have light absorption over the wavelength 400–700？nm and a band gap of 1.87？eV. Upon one sun illumination of 100？mW？cm？2, the TiO2/CdZnS/CdZnSe electrodes provide a significant photocurrent density of 9.7？mA？cm？2 at ？0.9？V versus a saturated calomel electrode (SCE). Incident photon-to-current conversion efficiency (IPCE) spectrum of the electrodes displays a maximum IPCE value of 80% at 500？nm. Moreover, the TiO2/CdZnS/CdZnSe electrodes prepared from three different batches provide a remarkable photon-to-hydrogen efficiency of 7.3？±？0.1% (the rate of the photocatalytically produced H2 by water splitting is about 172.8？mmol·h？1·g？1), which is the most efficient quantum-dots-based photocatalysts used in solar water splitting. 1. Introduction Developing environmentally clean energy resources from abundant solar energy has attracted considerable attention these years [1]. Hydrogen production by photochemical water splitting is a promising route because hydrogen has the highest energy density values per mass (140？MJ？kg？1) and its oxidation product (H2O) is more eco-friendly [2–4]. Hitherto, many semiconductors with band-gap energy exceeding the oxidation potential of water (1.23？V versus normal hydrogen electrode (NHE)) at pH 1.0 have been employed for water splitting [5]. Albeit metal oxides including TiO2, ZnO, and their derivatives are the most common photocatalysts used in water splitting, yet they provide low overall photon-to-hydrogen efficiency ( ) attributed to their wide band gaps [6, 7]. To overcome these limitations, doping other metal or inorganic ions to TiO2 and ZnO materials has been demonstrated [8]. However, this strategy is not quite successful, mainly because their band gaps are greater than 2.0？eV (620？nm) [9], whereas photocatalysts having band gaps less than 2.0？eV can absorb solar light in the visible to near-infrared region more efficiently. Quantum dots (QDs) such as CdTe [10], CdS [11–13], and CdSe [14–18] have been anchored to TiO2 and ZnO electrodes to harvest visible light for more efficient water splitting. Photoelectrochemical cells (PECs) incorporating QDs-sensitized TiO2 electrodes provide several advantages: (i) ease of fabrication, (ii) generation of multiple electron/hole [19], (iii) high visible light
References
[1]
M. Gratzel, “Photoelectrochemical cells,” Nature, vol. 414, pp. 338–344, 2001.
[2]
L. Schlapbach and A. Zuttel, “Hydrogen-storage materials for mobile applications,” Nature, vol. 414, pp. 353–358, 2001.
[3]
K. Maeda and K. Domen, “Photocatalytic water splitting: recent progress and future challenges,” Journal of Physical Chemistry Letters, vol. 1, no. 18, pp. 2655–2661, 2010.
[4]
P. V. Kamat, “Manipulation of charge transfer across semiconductor interface. A criterion that cannot be ignored in photocatalyst design,” The Journal of Physical Chemistry Letters, vol. 3, no. 5, pp. 663–672, 2012.
[5]
Y. Xu and M. A. A. Schoonen, “The absolute energy positions of conduction and valence bands of selected semiconducting minerals,” American Mineralogist, vol. 85, no. 3-4, pp. 543–556, 2000.
[6]
J. H. Park, S. Kim, and A. J. Bard, “Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting,” Nano Letters, vol. 6, no. 1, pp. 24–28, 2006.
[7]
A. Wolcott, W. A. Smith, T. R. Kuykendall, Y. Zhao, and J. Z. Zhang, “Photoelectrochemical water splitting using dense and aligned TiO2 nanorod arrays,” Small, vol. 5, no. 1, pp. 104–111, 2009.
[8]
M. Kitano, M. Matsuoka, M. Ueshima, and M. Anpo, “Recent developments in titanium oxide-based photocatalysts,” Applied Catalysis A: General, vol. 325, no. 1, pp. 1–14, 2007.
[9]
O. Khaselev and J. A. Turner, “A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting,” Science, vol. 280, no. 5362, pp. 425–427, 1998.
[10]
H. M. Chen, C. K. Chen, Y. C. Chang et al., “Quantum dot monolayer sensitized ZnO nanowire-array photoelectrodes: true efficiency for water splitting,” Angewandte Chemie—International Edition, vol. 49, no. 34, pp. 5966–5969, 2010.
[11]
C. F. Chi, Y. L. Lee, and H. S. Weng, “A CdS-modified TiO2 nanocrystalline photoanode for efficient hydrogen generation by visible light,” Nanotechnology, vol. 19, no. 12, Article ID 125704, 2008.
[12]
L. M. Peter, D. J. Riley, E. J. Tull, and K. G. U. Wijayantha, “Photosensitization of nanocrystalline TiO2 by self-assembled layers of CdS quantum dots,” Chemical Communications, no. 10, pp. 1030–1031, 2002.
[13]
I. Robel, V. Subramanian, M. Kuno, and P. V. Kamat, “Quantum dot solar cells. Harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films,” Journal of the American Chemical Society, vol. 128, no. 7, pp. 2385–2393, 2006.
[14]
G. Wang, X. Yang, F. Qian, J. Z. Zhang, and Y. Li, “Double-sided CdS and CdSe quantum dot co-sensitized ZnO nanowire arrays for photoelectrochemical hydrogen generation,” Nano Letters, vol. 10, no. 3, pp. 1088–1092, 2010.
[15]
Y. L. Lee, C. F. Chi, and S. Y. Liau, “CdS/CdSe co-sensitized TiO2 photoelectrode for efficient hydrogen generation in a photoelectrochemical cell,” Chemistry of Materials, vol. 22, no. 3, pp. 922–927, 2010.
[16]
J. Hensel, G. Wang, Y. Li, and J. Z. Zhang, “Synergistic effect of CdSe quantum dot sensitization and nitrogen doping of TiO2 nanostructures for photoelectrochemical solar hydrogen generation,” Nano Letters, vol. 10, no. 2, pp. 478–483, 2010.
[17]
N. Chouhan, C. L. Yeh, S. F. Hu et al., “Array of CdSe QD-sensitized ZnO nanorods serves as photoanode for water splitting,” Journal of the Electrochemical Society, vol. 157, no. 10, pp. B1430–B1433, 2010.
[18]
L. Amirav and A. P. Alivisatos, “Photocatalytic hydrogen production with tunable nanorod heterostructures,” Journal of Physical Chemistry Letters, vol. 1, no. 7, pp. 1051–1054, 2010.
[19]
A. J. Nozik, “Quantum dot solar cells,” Physica E, vol. 14, pp. 115–120, 2002.
[20]
B. A. Gregga and M. C. Hanna, “Comparing organic to inorganic photovoltaic cells: theory, experiment, and simulation,” Journal of Applied Physics, vol. 93, pp. 3605–3614, 2003.
[21]
Z. Yang, C. Y. Chen, C. W. Liu, and H. T. Chang, “Electrocatalytic sulfur electrodes for CdS/CdSe quantum dot-sensitized solar cells,” Chemical Communications, vol. 46, no. 30, pp. 5485–5487, 2010.
[22]
G. Y. Lan, Z. Yang, Y. W. Lin, Z. H. G. Lin, H. Y. Liao, and H. T. Chang, “A simple strategy for improving the energy conversion of multilayered CdTe quantum dot-sensitized solar cells,” Journal of Materials Chemistry, vol. 19, no. 16, pp. 2349–2355, 2009.
[23]
Y. L. Lee and Y. S. Lo, “Highly efficient quantum-dot-sensitized solar cell based on co-sensitization of CdS/CdSe,” Advanced Functional Materials, vol. 19, no. 4, pp. 604–609, 2009.
[24]
N. N. Rao and S. Dube, “Photoelectrochemical generation of hydrogen using organic pollutants in water as sacrificial electron donors,” International Journal of Hydrogen Energy, vol. 21, no. 2, pp. 95–98, 1996.
[25]
A. Thibert, F. Andrew Frame, E. Busby, M. A. Holmes, F. E. Osterloh, and D. S. Larsen, “Sequestering high-energy electrons to facilitate photocatalytic hydrogen generation in CdSe/CdS nanocrystals,” The Journal of Physical Chemistry Letters, vol. 2, no. 21, pp. 2688–2694, 2011.
[26]
X. B. Chen, S. H. Shen, L. J. Guo, and S. S. Mao, “Semiconductor-based photocatalytic hydrogen generation,” Chemical Reviews, vol. 110, no. 11, pp. 6503–6570, 2010.
[27]
G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, and C. A. Grimes, “Enhanced photocleavage of water using titania nanotube arrays,” Nano Letters, vol. 5, no. 1, pp. 191–195, 2005.
[28]
Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide, and L. Han, “Dye-sensitized solar cells with conversion efficiency of 11.1%,” Japanese Journal of Applied Physics, vol. 45, no. 24–28, pp. L638–L640, 2006.
