Effect of Ordered TiO2 Nanotube Array Substrate on Photocatalytic

Oct 11, 2013 - The composite photocatalyst with layered nanoarray structure, CdS-sensitized ZnO nanorod arrays (ZnONRA/CdS) grown on ordered TiO2 nano...
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Effect of Ordered TiO2 Nanotube Array Substrate on Photocatalytic Performance of CdS-Sensitized ZnO Nanorod Arrays Liangpeng Wu,† Juan Li,† Shaohong Zhang,† Lizhen Long,† Xinjun Li,*,† and Chaoping Cen*,‡ †

Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China ‡ The Key Laboratory of Water and Air Pollution Control of Guangdong Province, South China Institute of Environmental Science, Ministry of Environmental Protection, Guangzhou 510640, China ABSTRACT: The composite photocatalyst with layered nanoarray structure, CdSsensitized ZnO nanorod arrays (ZnONRA/CdS) grown on ordered TiO2 nanotube arrays (TiO2NTA), was constructed. The performance of composite photocatalyst toward to hydrogen production from water splitting was investigated. The ZnONRA/CdS composite photocatalyst with the substrate layer of ordered TiO2NTA has the enhanced rate of hydrogen production and the improved photostability. It may be attributed to the onedimensional structure of TiO2NTA at the bottom of ZnONRA/CdS composite photocatalyst, which provides a direct transfer pathway of photoinjected electrons along the photoanode to enhance charge-collection efficiency and consequently reduce electron−hole recombination.

1. INTRODUCTION Since Fujishima and Honda reported photoelectrochemical water splitting using a TiO2 electrode in 1972,1 extensive efforts have been made to develop numerous semiconductor photocatalysts.2 In 2001, Zou et al. reported that the photocatalytic efficiency of splitting water has been improved over the oxide semiconductor photocatalyst under visible light irradiation.3 In recent years, spectral sensitization of wide band gap semiconductors has aroused considerable attention of scientists.4 Cadmium sulfide (CdS) with a band gap of around 2.4 eV is often used to sensitize wide band gap semiconductors (e.g., TiO2 and ZnO) severed as photoelectrodes,5 which has been used in quantum dots-sensitized solar cells as well as in visible light photocatalytic production of hydrogen6,7 and degradation of organics.8,9 The efficient charge separation and transport of the photogenerated electron−hole pairs would improve the photoelectrochemical performance of the photoelectrodes. One-dimensional (1D) nanostructured photoelectrode with many desirable properties such as nanorod/nanotube/nanwire arrays can provide direct electron pathways for the photogenerated electrons to rapidly collect and transport and consequently enhance the photoelectrochemical performance of the photoelectrode.10,11 ZnO nanorod arrays have been studied for use in many fields, and CdS-sensitized ZnO nanorod arrays have high conversion efficiency.12 Yong et al. fabricated the photoanode of CdSe-deposited ZnO/CdS core/ shell nanowire arrays, on which highly efficient photoelectrochemical (PEC) hydrogen generation was achieved.13 The structure and property of the substrate could affect the transport and separation of photogenerated electron−hole © 2013 American Chemical Society

pairs, which have a great influence on the photoactivity and photostability of photoelectrode.14 A report by Haque et al. demonstrated that the introduction of a compact layer as the substrate in DSSCs between the interface of the fluorine-doped tin oxide (FTO) glass as current collectors and porous TiO2 as the photocatalyst has been proven theoretically and practically effective to block the electron recombination via the indirect route.15 Thus, it is important to explore a fast electron transport material serving current collectors, such as metal foils, indium doped tin oxide (ITO), carbon paper,16 silicon wafer17 and FTO.18 Yu et al. synthesized a compact TiO2 layer on FTO for the fabrication of DSSCs, compared with the traditional DSSCs without this compact layer, the solar energy-to-electricity conversion efficiency, short-circuit current and open-circuit potential of the DSSCs with the compact layer were improved by 33.3%, 20.3%, and 10.2%, respectively.19 Lee et al. report that the power conversion efficiency of CdSe quantum dotsensitized TiO2 nanotubes solar cells increased by 25.9% in the presence of the ZnO thin layer.20 Yang et al. fabricated mesoporous Ni−Co oxides nanowire arrays on ordered TiO2 nanotubes; the pseudocapacitor material has the much improved capacity and cycling stability.21 Ordered TiO2 nanotube arrays with high specific surface areas, well-aligned nanostructures, and good chemical stability can provide a fast transfer pathway for photogenerated electrons, which facilitates electron transfer and subsequently improves the electron transfer efficiency.22 In this paper, a new Received: August 26, 2013 Revised: October 11, 2013 Published: October 11, 2013 22591

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2.2. Materials Characterizations. A model JEOL JSM6700F field-emission scanning electron microscopy (FESEM) was used to characterize the morphologies of the samples. The crystalline phase of the samples were investigated with X-ray diffraction (XRD, X’Pert-PRO, PANalytical, Holland) equipped with Cu Kα radiation (λ = 0.154 056 nm) at an accelerating voltage of 40 kV and a current of 40 mA. The patterns were recorded with 2θ range from 5° to 80° at a scan rate of 1.5°/ min. The absorption spectrum was recorded by a UV−vis spectrophotometer (LAMBDA 750) equipped with an integrating sphere and with BaSO4 as a reference in the range of 250−700 nm. The width of slit was 2.0 nm, and the step was 0.5 nm. 2.3. Photoeletrochemical Measurement. Photoelectrochemical measurements were carried out in a three-electrode photoelectrochemicalcell with a quartz window under the illumination of a 300 W Xe lamp (Changtuo Co. Ltd., Beijing) at room temperature. Transient photocurrent (iph) and electrochemical impedance spectroscopy (EIS) were measured using a CHI660A workstation (CHI Co.) in a standard threeelectrode configuration with the as-prepared TiO2NTA/ ZnONA/CdS or ZnONA/CdS as the photoanode, a Pt electrode as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode in an aqueous solution containing 0.15 M Na2SO3 and 0.15 M Na2S. Water splitting reactions were carried out in a homemade photochemical cell described as shown in Figure 1. Before

attempt has been made to construct CdS-sensitized ZnO nanorod arrays (ZnONRA) on ordered TiO2 nanotube arrays (TiO2NTA) (Scheme 1). The photoelectrochemical behavior and the photocatalytic performance were investigated. Scheme 1. Schematic Illustration for the Construction of TiO2NTA/ZnONRA/CdS and ZnONRA/CdS

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. The ordered TiO2NTA was prepared in HF solution using anodic oxidation, following the typical procedure.23 Prior to anodization, the Ti foils were mechanically polished with different abrasive paper, soaked in a mixture of HF and HNO3 acids (the mixing ratio of HF:HNO3:H2O = 1:4:5 in volume), and rinsed with deionized water. The TiO2NTA was first prepared by anodization of the clear Ti foil with area of 1.5 × 5 cm2 in an electrolyte containing 5 wt % hydrofluoric acid aqueous solution at 20 V and room temperature for 1.5 h. The as-anodized amorphous TiO2NTA was annealed in dry air environment at 500 °C for 2 h with heating and cooling rate of 2 °C min−1. ZnONRA was fabricated on TiO2NTAand Ti foil substrates through hydrothermal process. A typical method for synthesizing the ZnONRA was carried out as in the literature.24 First, ZnO seed layer films were deposited on TiO2NTA and Ti foil substrates via the dip-coating method.25 Second, the precursor aqueous solution for the growth of ZnONRA were prepared in advance by mixing Zn(NO3)2·4H2O and (CH2)6N4 keeping their molar ratio at 1:1 in deionized water with stirring at icebathing condition. Then, the TiO2NTA and Ti foil substrates with ZnO seed layer were immersed into the precursor solutions at 85 °C for 6 h without any stirring. Finally, the resultant TiO2NTA/ZnONRA and ZnONRA were washed with deionized water and annealed at 500 °C for 2 h. TiO2NTA/ZnONA/CdS and ZnONRA/CdS were prepared by depositing CdS on TiO2NTA/ZnONRA and ZnONRA via chemical bathing deposition. TiO2NTA/ZnONRA and ZnONRA were immersed into the clear reactant solution containing 0.01 M Cd(NO3)2 and 0.01 M C2H5NS at a certain temperature for 100 min. CdS growth was observed by the emergence of a yellow color on the sample surface. The modified film was thoroughly washed by deionized water in order to remove the surface residue and finally dried in the air. After the treatment, the samples were annealed again at 350 °C in N2 atmosphere for 60 min.

Figure 1. Schematic diagram of the photochemical cell for hydrogen generation.

testing the solution with the catalyst was purged several times with Ar to ensure complete air removal. A 300 W Xe lamp was used as the light source, the distance between the photoanode and the light source was fixed at 20 cm, and the power under this condition was calibrated to be 100 mW/cm2. The photoanode was immersed in a 25 mL aqueous solution containing 0.15 M Na2S and 0.15 M Na2SO3. The gas was analyzed by gas chromatography (Agilent 6890) with thermal conductivity detector equipped with a 5A molecular sieve using Ar as carrier gas.

3. RESULTS AND DISCUSSION Figures 2a and 2b show SEM images of Ti foil and TiO2NTA. Uniform TiO2NTA with high density is formed, and the average length of nanotube is ∼400 nm. ZnONRA has been fabricated on TiO2NTA and Ti foil substrates by the 22592

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Figure 2. SEM images of (a) Ti foil, (b) TiO2NTA, (c) ZnONRA, (d) TiO2NTA/ZnONRA, (e) ZnONRA/CdS, (f) TiO2NTA/ZnONRA/CdS, (g) SEM image and EDS spectrum of the edge region of TiO2NTA/ZnONRA/CdS, and the corresponding EDS elemental mapping of (h) Cd, (i) S, (j) Zn, (k) Ti, and (l) O.

Energy-dispersive X-ray spectroscopy (EDX) analysis is carried out to identify the elemental composition of TiO2NTA/ZnONRA/CdS, and the EDX spectrum is shown in Figure 2g. The Mα peak of Cd and S can be seen clearly at 2.8 and 2.35 keV, respectively. Besides the strong Kα and Kβ peaks of the Zn element appearing at 1.0 and 8.65 keV, a moderate Kα peak of the element O can also be observed at 0.52 keV. A moderate Kα peak of the element Ti can be observed at 0.4, 4.65, and 4.95 keV.13 The quantitative analysis reveals the atomic composition of Ti, Zn, Cd, S, and O is 18.56%, 4.08%, 16.88%, 18.48%, and 42.01%, respectively. The calculated molar ratio of Cd to S is close to 1:1, confirming the formation of CdS. The atomic ratio of Zn and Ti to O confirms the formation of ZnO and TiO2. In order to further verify the layer nanoarrays structure of TiO2NTA/ZnONRA/CdS, electron mapping image analysis of the sample are recorded and shown (Figure 2h−l). The above edge region clearly shows cylindrical shell contrast reflecting the overlapped double layers,

hydrothermal process. The SEM images of ZnONRA and TiO2NTA/ZnONRA are shown in Figures 2c and 2d, respectively. It can be clearly observed that highly dense ZnONRA with hexagonal structure and rather smooth surface has grown on Ti foil and TiO2NTA substrates, and their morphologies are almost the same. Ti foil and TiO2NTA substrates can be also observed from the cross-section images (insets of Figures 2c and 2d), which verified the structure of TiO 2 nanotube still preserved after the hydrothermal deposition of ZnONRA. Figures 2e and 2f show the images of ZnONRA/CdS grown on Ti foil and TiO2NTA substrates, respectively. After chemical bathing deposition, ZnONRA/CdS retain their hexagonal geometry, but they exhibit the increment in diameter and the roughened surface. At the same time, the cross-section images further illuminate that the nanoscale CdS coating is densely and uniformly packed on the surface of ZnONRA (insets of Figures 2e and 2f). 22593

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and the EDS elemental mapping confirmed localized Zn and O in the filled core area. And the elemental mapping of Ti and O (Figures 2k and 2l) indicates the presence of TiO2 in the bottom of TiO2NTA/ZnONRA/CdS. Figure 3 shows XRD patterns of TiO2NTA, ZnONRA, TiO2NTA/ZnONRA, ZnONRA/CdS, and TiO2NTA/ZnON-

Figure 4. UV−vis absorption spectrum of (a) ZnONRA/CdS and (b) TiO2NTA/ZnONRA/CdS.

plots of ZnONRA/CdS and TiO2NTA/ZnONRA/CdS as photoelectrodes are shown in Figure 5a. The obtained radius of curvature of TiO2NTA/ZnONRA/CdS is obviously smaller than that of ZnONRA/CdS. Yu et al. reported that the compact TiO2 film substrate between the porous TiO2 film and FTO in DSSCs could provide more effective electron pathways, and electrons can be collected quickly at the conduction band of the photoanodes and transferred to external circuit, resulting in the improved performance of the DSSC.19 For as-prepared TiO2NTA/ZnONRA/CdS photoelectrode, the one-dimensional structured TiO2 nanotubes array as the substrate plays the same role, which can provide 1D tranfer pathway, enhance the electron transport, and improve the separation efficiency of the photogenerated electron−hole pairs. The characteristic frequency peaks in Bode phase plots, which can be related to as the inverse of the recombination lifetime (τr) or electron lifetime (τe),29 are shown in Figure 5b. The characteristic frequency peak shifted to lower frequency in the presence of the TiO2NTA substrate. This further indicates that the collection and transport of electrons in TiO2NTA/ZnONRA/ CdS photoelectrode with the TiO2NTA substrate was faster than that in ZnONRA/CdS photoelectrode, thereby reducing the electron recombination and extending the electron lifetime. Iph is generated from the excited photoelectrons from the valence band to the conduction band and mainly reflects the conductance value and the number of free photogenerated carriers in the semiconductor. The photocurrent responses of TiO2NTA/ZnONRA/CdS and ZnONRA/CdS are shown in Figure 6. The current responses in the dark were negligible. In the case of visible light illumination, an apparently boosted photocurrent response appeared, indicating the happening of photoelectrocatalysis. The iph of TiO2NTA/ZnONRA/CdS photoelectrode is much higher and more stable than that of ZnONRA/CdS photoelectrode. This indicates that more free carriers could be generated and rapidly transferred in TiO2NTA/ZnONRA/CdS than that in ZnONRA/CdS. The accumulative amount of hydrogen evolution and the average hourly hydrogen evolution as a function of irradiation time on ZnONRA/CdS and TiO2NTA/ZnONRA/CdS photoelectrodes are shown in Figures 7a and 7b. In the first hour, the hydrogen evolution rates for ZnONRA/CdS and TiO2NTA/ ZnONRA/CdS photoelectrodes are 34.2 and 41.1 μmol cm−1, respectively. The hydrogen generation rate of TiO2NTA/ ZnONRA/CdS is much higher than that of ZnONRA/CdS, and total amounts of hydrogen generated increase very steadily

Figure 3. XRD patterns of TiO2NTA (a), ZnONRA (b), TiO2NTA/ ZnONRA (c), ZnONRA/CdS (d), and TiO2NTA/ZnONRA/CdS (e).

RA/CdS. The peak values of 25.3° and 48.05° can be attributed to the crystal planes (101) and (200) of the anatase phase (Figure 3a); the other peaks come from the Ti foils substrate. The peaks with values of 31.8°, 34.5°, and 36.3° corresponding to (100), (002), and (102) can be exactly ascribed to ZnO with the hexagonal-shaped crystal structure (Figure 3b). The characteristic diffraction peaks of anatase TiO2 and hexagonal ZnO can be observed for TiO2NTA/ZnONRA sample (Figure 3c). And ZnONRA/CdS and TiO2NTA/ZnONRA/CdS exhibit new diffraction peaks corresponding to the CdS phase appearing at 2θ = 26.6°, 28.3°, and 43.9° (Figure 3d,e). Besides, TiO2NTA/ZnONRA/CdS shows the characteristic diffraction peaks of anatase TiO2. These results illuminate that the TiO2NTA/ZnONRA/CdS sample possesses anatase crystal TiO2, hexagonal crystal ZnO, and CdS, which is in agreement with the results of SEM. Figure 4 shows UV−vis diffuse reflection spectra of ZnONRA/CdS and TiO2NTA/ZnONRA/CdS. ZnONRA/ CdS shows an absorption edge at ∼560 nm, corresponding to the energy band gap about 2.24 eV. TiO2NTA/ZnONRA/ CdS exhibits a small red-shift in the band gap. The conduction band edge of TiO2 is below that of ZnO, suggesting that the photogenerated electrons can be transferred effectively from ZnONRA to the collection electrode of TiO2NTA, which promotes the separate efficiency of photogenerated electron− hole pairs and extends the range of excited spectrum.26 Electrochemical impedance spectroscopy (EIS) and transient photocurrent (iph) are effective methods to evaluate the photoelectrochemical properties of photocatalysts. It is well established that EIS Nyquist plots are associated with the charge transfer resistance and the separation efficiency of the photohole−electron pairs.27 A larger circular radius usually represents a larger charge transfer resistance and, therefore, a lower separation efficiency of the photohole−electron pairs.28 In our experiments, EIS measurements were carried out over the frequency range of 1000 kHz−0.01 Hz. The EIS Nyquist 22594

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Figure 5. Electrochemical impedance spectra results of ZnONRA/CdS and TiO2NTA/ZnONRA/CdS under irradiation: (a) Nyquist plots; (b) Bode phase plots.

charge separation. The conduction band (CB) and valence band (VB) edge potentials of CdS, ZnO, and TiO2 were at −0.52 and 1.88 eV, −0.31 and 2.89 eV, and −0.29 and 2.91 eV, respectively.30 Under visible-light irradiation, the electrons in the VB of CdS are excited to its CB, then inject to CB of ZnO, subsequently transfer to CB of TiO2 (Figure 8).31 The structure and property of the substrate could affect the transport and separation of photogenerated electron−hole pairs. Yu et al. reported that a compact TiO2 film substrate between FTO and TiO2 nanoparticles could reduce effectively recombination rate of the photoelectrons due to the efficient electron transport network.32 The 1D structure substrate of TiO2NTA allows for an oriented and much shorter random walk path for the photogenerated carriers to increase the charge-collection efficiency and improve electron lifetimes.33 For TiO2NTA/ZnONRA/CdS composite photocatalyst, the ordered TiO2NTA substrate can collect and quickly transfer the electrons to external circuit via more effective electron pathways, resulting in the improvement of charge-carriers separation efficiency at the band of CdS and the enhanced photocatalytic performance for hydrogen evolution. Meanwhile, the efficient separation and the quick transfer of the photogenerated carriers could decrease the photocorrosion and enhance the photostability.34 TiO2NTA with 1D arrays structure as the substrate reduced photogenerated carriers’ recombination as well as resulted in an enhancement in the photostability and the photocatalytic efficiency of TiO2NTA/ ZnONRA/CdS composite photocatalyst. The present work would provide a new possibility for the design and preparation of photocatalysts with higher efficiency and stability for utilizing visible light as an energy source.

Figure 6. Photocurrents of (a) ZnONRA/CdS and (b) TiO2NTA/ ZnONRA/CdS.

during a testing course of 5 h, suggesting the good photostability of TiO2NTA/ZnONRA/CdS composite photocatalyst. However, the relatively low photocatalytic H 2 production rate and bad stability were also observed for ZnONRA/CdS composite photocatalyst (Figure 7b). The reason may be that the ordered TiO2NTA layer structure can ensure good mechanical adhesion of ZnONRA/CdS, effective separation of electrons and holes, and fast electron transport in TiO2NTA/ZnONRA/CdS composite photocatalyst. ZnONRA/CdS composite photocatalyst with the substrate layer of ordered TiO2 nanotubes has a higher hydrogen production rate and better photostability than that without the substrate layer. The reasons is that 1D layered nanoarray structure of the composite material can provide a direct transfer pathway for charge transport and enhance the efficiency of

Figure 7. Time-profiled photocatalytic hydrogen productionon ZnONRA/CdS and TiO2NTA/ZnONRA/CdS under 300 W Xe lamp in a photoelectrochemical cell. 22595

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Figure 8. Schematic illustration of the band gap energy and charge separation of the combined TiO2NTA/ZnONRA/CdS photocatalyst. (7) Li, C. L.; Yuan, J.; Han, B. Y.; Jiang, L.; Shangguan, W. F. TiO2 Nanotubes Incorporated with CdS for Photocatalytic Hydrogen Production from Splitting Water under Visible Light Irradiation. Int. J. Hydrogen Energy 2010, 35, 7073−7079. (8) Bavykin, D. V.; Lapkin, A. A.; Plucinski, P. K.; TorrenteMurciano, L.; Friedrich, J. M.; Walsh, F. C. Deposition of Pt, Pd, Ru, and Au into the Surface of Titanate Nantubes. Top. Catal. 2006, 39 (3−4), 151−60. (9) Kundu, P.; Deshpande, P. A.; Madras, G.; Ravishankar, N. Nanoscale ZnO/CdS Heterostructures with Engineered Interfaces for High Photocatalytic Activity under Solar Radiation. J. Mater. Chem. 2011, 21, 4209−4216. (10) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Photosensitization of ZnO Nanowires with CdSe Quantum Dots for Photovoltaic Devices. Nano Lett. 2007, 7, 1793−1798. (11) Tak, Y.; Hoog, S. J.; Lee, J. S.; Yong, K. Fabrication of ZnO/CdS Core/Shell Nanowire Arrays for Efficient Solar Energy Conversion. J. Mater. Chem. 2009, 19, 5945−5951. (12) Sun, B.; Hao, Y. Z.; Guo, F.; Cao, Y. H.; Zhang, Y. H.; Li, Y. P.; Xu, D. S. Fabrication of Poly(3-hexylthiophene)/CdS/ZnO Core− Shell Nanotube Array for Semiconductor-Sensitized Solar Cell. J. Phys. Chem. C 2012, 116 (1), 1395−1400. (13) Shankar, K.; Bandara, J.; Paulose, M.; Wietasch, H.; Varghese, O. K.; Mor, G. K.; Latempa, T. J.; Thlakkat, M.; Grimes, C. A. Highly Efficient Solar Cells Using TiO2 Nanotube Arrays Sensitized with a Donor-Antenna Dye. Nano Lett. 2008, 8, 1654−1659. (14) Ouyang, J. L.; Chang, M. L.; Li, X. J. CdS-Sensitized ZnO Nanorod Arrays Coated with TiO2 Layer for Visible Light Photoelectrocatalysis. J. Mater. Sci. 2012, 47, 4187−4193. (15) Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.; Klug, D. R.; Durrant, J. R. Charge Separation Versus Recombination in Dye-Sensitized Nanocrystalline Solar Cells: The Minimization of Kinetic Redundancy. J. Am. Chem. Soc. 2005, 127, 3456−3462. (16) Na, J. S.; Gong, B.; Scarel, G.; Parsons, G. N. Surface Polarity Shielding and Hierarchical ZnO Nano-Architectures Produced Using Sequential Hydrothermal Crystal Synthesis and Thin Film Atomic Layer Deposition. ACS Nano 2009, 3, 3191−3199. (17) Vayssieres, L. Growth of Arrayed Nanorods and Nanowires of ZnO from Aqueous Solutions. Adv. Mater. 2003, 15, 464−466. (18) Konenkamp, R.; Boedecker, K.; Lux-Steiner, M. C.; Poschenrieder, M.; Zenia, F.; Levy-Clement, C. Thin Film Semiconductor Deposition on Free-Standing ZnO Columns. Appl. Phys. Lett. 2000, 77, 2575−2577. (19) Yu, H.; Zhang, S. Q.; Zhao, H. J.; Will, G.; Liu, P. An Efficient and Low-Cost TiO2 Compact Layer for Performance Improvement of Dye-Sensitized Solar Cells. Electrochim. Acta 2009, 54, 1319−1324. (20) Lee, W.; Kang, S. H.; Kim, J. Y.; Kolekar, G. B.; Sung, Y. E.; Han, S. H. TiO2 Nanotubes with a ZnO Thin Energy Barrier for Improved Current Efficiency of CdSe Quantum-Dot-Sensitized Solar Cells. Nanotechnology 2009, 20, 335706. (21) Yang, F.; Yao, J. Y.; Liu, F. L.; He, H. C.; Zhou, M.; Xiao, P.; Zhang, Y. H. Ni-Co Oxides Nanowire Arrays Grown on Ordered TiO2

4. CONCLUSIONS In summary, the TiO2NTA/ZnONRA/CdS composite photocatalyst was fabricated by building CdS-sensitized ZnONRA via the hydrothermal method on electrochemical anodization formation of ordered TiO2 nanotubes arrays surface. The composite photocatalyst of ZnONRA/CdS with the substrate layer of ordered TiO2 nanotubes has the enhanced rate of hydrogen production and the better photostability. The existence of the one-dimensional structure of TiO2NTA, serving as an electron collector and transporter, could provide a direct and quick electron pathway of photoinjected electrons along the photoanode and reduce electron−hole recombination.

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AUTHOR INFORMATION

Corresponding Authors

*Tel 86 20 87057781; Fax 86 20 87057677; e-mail lixj@ms. giec.ac.cn (X.L.). *Tel 86 20 85525914; Fax 86 20 85524451; e-mail [email protected] (C.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (No. 51172233), the Foundation of the key laboratory of water and air pollution control of Guangdong province, China (GD2012A05), and National 973 project of China (No. 2009CB220002).

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. SemiconductorBased Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503−6570. (3) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Direct Splitting of Water under Visible Light Irradiation with an Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625−627. (4) Tak, Y.; Kim, H.; Lee, D.; Yong, K. Type-II CdS NanoparticleZnO Nanowire Heterostructure Arrays Fabricated by a Solution Process: Enhanced Photocatalytic Activity. Chem. Commun. 2008, 38, 4585−4587. (5) Hu, Y.; Gao, X. H.; Yu, L.; Wang, Y. R.; Ning, J. Q.; Xu, S. J.; Lou, X. W. Carbon-Coated CdS Petalous Nanostructures with Enhanced Photostability and Photocatalytic Activity. Angew. Chem., Int. Ed. 2013, 52, 5636−5639. (6) Wang, X. W.; Liu, G.; Lu, G. Q.; Cheng, H. M. Stable Photocatalytic Hydrogen Evolution from Water over ZnO-CdS CoreShell Nanorods. Int. J. Hydrogen Energy 2010, 35, 8199−8205. 22596

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The Journal of Physical Chemistry C

Article

Nanotubes with High Performance in Supercapacitors. J. Mater. Chem. A 2013, 1, 594−601. (22) Zhang, Y. H.; Xiao, P.; Zhou, X. Y.; Liu, D.; Garcia, B. B.; Cao, G. Z. Carbon Monoxide Annealed TiO2 Nanotube Array Electrodes for Efficient Biosensor Applications. J. Mater. Chem. 2009, 19, 948− 953. (23) Zhang, Y. Y.; Li, X. J.; Feng, M. Z.; Zhou, F. L.; Chen, J. Z. Photoelectrochemical Performance of TiO2-Nanotube-Array Film Modified by Decoration of TiO2 via Liquid Phase Deposition. Surf. Coat. Technol. 2010, 205, 2572−2577. (24) Wang, Y. X.; Li, X. Y.; Lu, G.; Quan, X.; Chen, G. H. Highly Oriented 1-D ZnO Nanorod Arrays on Zinc Foil: Direct Growth from Substrate, Optical Properties and Photocatalytic Activities. J. Phys. Chem. C 2008, 112 (19), 7332−7336. (25) Zhou, F. L.; Li, X. J.; Shu, J.; Wang, J. Synthesis and Visible Light Photo-Electrochemical Behaviors of In2O3-Sensitized ZnO Nanowire Array Film. J. Photochem. Photobiol. A 2011, 219, 132−138. (26) Mane, R. S.; Lee, W. J.; Pathan, H. M.; Han, S. H. Nanocrystalline TiO2/ZnO Thin Films: Fabrication and Application to Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 24254− 24259. (27) Ningshen, S.; Kamachi Mudali, U.; Amarendra, G.; Gopalan, P.; Dayal, R. K.; Khatak, H. S. Hydrogen Effects on The Passive Film Formation and Pitting Susceptibility of Nitrogen Containing type 316L Stainless Steels. Corros. Sci. 2006, 48, 1106−1121. (28) Zheng, J.; Yu, H.; Li, X.; Zhang, S. Enhanced Photocatalytic Activity of TiO2 Nano-Structured Thin Film with a Silver Hierarchical Configuration. Appl. Surf. Sci. 2008, 254, 1630−1635. (29) Schlichthorl, G.; Park, N. G.; Frank, A. J. Evaluation of the Charge-Collection Efficiency of Dye-Sensitized Nanocrystalline TiO2 Solar Cells. J. Phys. Chem. B 1999, 103, 782−791. (30) Roh, S. J.; Mane, R. S.; Min, S. K.; Lee, W. J.; Lokhande, C. D.; Han, S. H. Achievement of 4.51% Conversion Efficiency Using ZnO Recombination Barrier Layer in TiO2 Dye-Sensitized Solar Cell. Appl. Phys. Lett. 2006, 89, 253512−253514. (31) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (32) Yu, H.; Zhang, S.; Zhao, H.; Xue, B.; Liu, P.; Will, G. HighPerformance TiO2 Photoanode with an Efficient Electron Transport Network for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2009, 113, 16277−16282. (33) Kim, D.; Ghicov, A.; Albu, S. P.; Schmuki, P. Bamboo-Type TiO2 Nanotubes: Improved Conversion Efficiency in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 16454−16455. (34) Lei, Y. Z.; Zhao, G. H.; Liu, M. C.; Zhang, Z. N.; Tong, X. L.; Cao, T. C. Fabrication, Characterization, and Photoelectrocatalytic Application of ZnO Nanorods Grafted on Vertically Aligned TiO2 Nanotubes. J. Phys. Chem. C 2009, 113, 19067−19076.

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dx.doi.org/10.1021/jp408527r | J. Phys. Chem. C 2013, 117, 22591−22597