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Novel construction of CdS-encapsulated TiO2 nano test tubes corked with ZnO nanorods
Materials Letters 64 (2010) 2194–2196
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Novel construction of CdS-encapsulated TiO2 nano test tubes corked with ZnO nanorods Ya-nan Zhang, Guohua Zhao ⁎, Yanzhu Lei, Zhiyuan Wu, Yuning Jin, Mingfang Li Department of Chemistry, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 14 May 2010 Accepted 2 July 2010 Available online 16 July 2010 Keywords: Composite materials Semiconductors Photoelectrocatalytic activities UV–vis region
a b s t r a c t A novel TiO2 nanotube array/CdS nanoparticle/ZnO nanorod (TiO2 NT/CdS/ZnO NR) photocatalyst was constructed by chemical assembling CdS into the TiO2 NTs, and then laying ZnO NRs on the surface. The SEM results showed that the TiO2 NTs looked like many “nano test tubes” and the ZnO NRs served as the corks to seal the nozzle. This photocatalyst exhibited a wide absorption range (200–535 nm) in both ultraviolet and visible regions (UV–vis region), and maintained very high photoelectrocatalytic (PEC) activities. The maximum photoelectric conversion efficiencies (η) of TiO2 NT/CdS/ZnO NRs are 31.8 and 5.98% under UV light (365 nm) and visible light (420–800 nm), respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the recent years, there has been a growing interest in finding new effective photocatalysts for the elimination of environmental pollutants, which should possess a high photoelectrocatalytic (PEC) activity both in UV and visible regions, utilizing the whole solar energy spectrum as much as possible. TiO2 is a widely used photocatalyst due to its high oxidative power, good resistance to photocorrosion and non-toxicity [1–3]. However, the optical quantum efficiency of TiO2 is relatively low and it can only absorb the UV light (b387 nm) that is accounted for only 4–5% of the solar spectrum. Coupling TiO2 with other semiconductors to get better PEC activity has attracted more and more attention recently. ZnO is also an excellent n-type semiconductor and the TiO2/ZnO composite nanostructure has been demonstrated to exhibit higher PEC oxidation efficiency than TiO2 [4–6]. CdS is a typical narrow-band semiconductor which is often used to sensitize TiO2 and broaden the photoabsorption edge to a visible region [7–10]. In this paper, our interest is to obtain an excellent photocatalyst by chemical assembling the above three semiconductors with a novel structure. Hereon, a new attempt has been made to construct a TiO2 NT/CdS/ZnO NR photocatalyst by encapsulating the CdS NPs in the TiO2 NTs and subsequently building a ZnO NR layer on the TiO2 NTs/ CdS surface. It assumes that this composite structure owns several advantages. First, TiO2 NT arrays are chosen as the substrate materials, which possess high adsorption capacity and abundant active sites. Second, CdS assembled into the nanotubes in the form of NPs with a
⁎ Corresponding author. Tel.: +86 21 65981180; fax: +86 21 65982287. E-mail address: [email protected] (G. Zhao). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.013
high loading amount. Third, when appropriate ZnO NRs cover on the surface of TiO2 NTs, a “Corking Nano Test Tubes” structure is obtained which serves as the stable shell to protect the encapsulated CdS nanoparticles from contacting the solution directly, enhancing the photostability of the photocatalyst. This work provides a new idea for constructing an excellent photoelectrocatalyst with high efficiency and stability in the UV–vis region.
2. Material and methods The preparation of TiO2 NTs was described previously [11]. The obtained TiO2 NTs were placed vertically into a buffer bottle under vacuum conditions. Then, CdS nanoparticles were deposited into the vacuum treated TiO2 NTs by sequential chemical bath deposition method with 0.05 M Cd(NO3)2 and 0.05 M Na2S aqueous solutions [12]. The resultant film was annealed under pure nitrogen (N2) at 500 °C for 2 h. The TiO2 NT/CdS substrates were suspended in a solution containing equal molar (0.01 mol L−1) Zn(NO3)2 and (CH2)6 N4 and installed in a Teflon-lined stainless steel autoclave at 90 °C for 4 h. The TiO2 NT/CdS/ZnO NRs were obtained after the sample was washed with deionized water and dried at room temperature. The detailed schematic diagram is shown in Scheme 1. Field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800) and X-ray diffraction (XRD, Bruker Co., Ltd., Germany) were used to characterize the morphology and crystal structure of the asprepared electrodes. The optical absorption characteristics of the photocatalysts are determined by UV–vis diffuse reflectance spectroscopy (UV–vis DRS, Model BWS002, BWtek). The PEC performance and the photoelectric conversion efficiency (η) of the
Y. Zhang et al. / Materials Letters 64 (2010) 2194–2196
2195
Scheme 1. Schematic illustration for the construction of the TiO2 NT/CdS/ZnO NRs photocatalyst.
resulting materials are further investigated by linear-sweep photovoltammetry, which is calculated in accordance with the following formula: [13] ηð%Þ = ½ðtotal power output − electrical power inputÞ = light power input × 100 = jp ðErev −jEapp jÞ × 100 = I0 :
3. Results and discussion 3.1. Characterization of TiO2 NT/CdS/ZnO NRs FE-SEM is used to characterize the morphologies of the photocatalysts, as shown in Fig. 1. The in situ grown TiO2 NTs are uniform in size and vertically aligned (Fig. 1A). Every nanotube looks like a “nano test tube” with a diameter of 60 nm. The cross-sectional view of the TiO2 NTs/CdS (Fig. 1B) shows that many CdS nanoparticles are introduced onto the side wall of TiO2 NTs, and there're also some CdS particles on the outer walls of TiO2 NTs. From Fig. 1C, the ZnO NR layer is observed to be relatively denser and the NRs exhibit irregular sixprism with the diameter of about 150–200 nm which corked the open
end of nanotubes as the lid. The “Corking Nano Test Tubes” structure has been obtained and the CdS NPs are encapsulated in the tubes with a good protection, resulting in a high photostability. The XRD results show that a composite crystalline form of the rutile and anatase exits when TiO2 NTs are annealed at 500 °C (Fig. 1D). It can be seen that the characteristic diffraction peaks of CdS crystal (111), (220) and (311) appear (curve b), suggesting that a cubic phase CdS is obtained (JCPDS 10-454). Some new diffraction peaks can be observed in curve c at 31.9°, 34.6° and 36.4°, corresponding to the (100), (002), and (101) facets of ZnO, which is in good agreement with the values in the standard card (JCPDS 36-1451). 3.2. PEC activity of photocatalysts The UV–vis diffuse reflection spectra (DRS) for these photocatalysts indicate that the TiO2 NTs have an absorption edge at 385 nm with a band gap of 3.22 eV (Fig. 2), according to the equation Eg = 1240/λg [14]. An obvious red-shift of the absorption edge from 385 nm to 535 nm is observed (curve b), further indicating that the sensibilization of CdS particles for TiO2 NTs can expand the photoabsorption range to visible light. The absorption edge of TiO2
Fig. 1. FE-SEM images of as-prepared photocatalysts. (A) cross-sectional view of TiO2 NTs, (B) cross-sectional view of TiO2 NTs/CdS, and (C) TiO2 NT/CdS/ZnO NRs. (D) XRD patterns of samples.
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Y. Zhang et al. / Materials Letters 64 (2010) 2194–2196
transfer may occur towards the opposite direction. The better charge separation can improve the photoelectrocatalytic activity greatly. 4. Conclusions In this work, we report a “Corking Nano Test Tubes” structure of TiO2 NT/CdS/ZnO NRs by filling one-dimensional TiO2 nanotube arrays (NTS) with CdS nanoparticles (NPs), and then constructing ZnO nanorods (NRs) on the TiO2 NTs/CdS surface. This photocatalyst exhibits a wide absorption (200–535 nm) response in the UV–vis region, and maintains very high PEC activities in both ultraviolet (UV) and visible regions. This resolution is novel and feasible, providing a composite photocatalyst of the new structure with higher efficiency and stability. Acknowledgements Fig. 2. UV–vis diffuse reflection spectra of photocatalysts.
Table 1 Photoelectric conversion efficiency for photocatalysts under UV light (3 mW cm−2) irradiation and visible light (100 mW cm−2). Photocatalysts
η (UV)
η (visible)
TiO2 NTs ITO/CdS TiO2 NTs/CdS TiO2 NT/CdS/ZnO NRs
8.6% 9.6% 28.0% 31.8%
– 0.50% 5.09% 5.98%
NT/CdS/ZnO NRs (curve c) is the same as that of TiO2 NTs/CdS since ZnO is transparent in the visible light. The final photocatalyst has a strong and wide absorption from 200 to 535 nm in UV–vis (curve c). The PEC performance of the resulting materials is listed in Table 1. No obvious light response is observed for TiO2 NTs in the visible light region. The maximum η of TiO2 NTs/CdS is much higher than that of ITO/CdS in both UV and visible regions. It indicates that TiO2 NTs are in favor of the dispersion of CdS NPs compared to ITO, due to the large specific surface and adsorption capacity. After covering with ZnO NRs, the remarkable maximum photoconversion efficiencies are achieved for TiO2 NT/CdS/ZnO NRs in both UV region (5.98%) and visible region (5.09%) at an applied potential of −0.8 V versus SCE. The overall photoelectric conversion efficiency of the composite semiconductor mainly depends on the position of its conduction and valence bands. The edge of the conduction band of CdS is higher than that of TiO2 and ZnO. So the energetic differences between the conduction bands of the semiconductors bring about a driving force, providing a better charge separation. Under visible irradiation, only CdS is excited and the electron transfer may occur from the conduction band of CdS towards that of ZnO, then to that of TiO2. Under UV irradiation, all semiconductors of the coupled system (TiO2/ ZnO/CdS) are activated, and the photogenerated holes consequently cascade quickly from TiO2 to ZnO, then to CdS, when the electron
This work was supported jointly by the National Natural Science Foundation of China (NO.20877058), 863 Program (NO.2008AA06Z329) from the Ministry of Science, and the Nanometer Science Foundation of Shanghai (NO. 0852nm01200). References [1] Grimes CA. Synthesis and application of highly ordered arrays of TiO2 nanotubes. J Mater Chem 2007;15:1451–7. [2] Tachikawa T, Fujitsuka M, Majima T. Mechanistic insight into the TiO 2 photocatalytic reactions: design of new photocatalysts. J Phys Chem C 2007;14: 5259–75. [3] Yurdakal S, Palmisano G, Loddo V, Augugliaro V, Palmisano L. Nanostructured rutile TiO2 for selective photocatalytic oxidation of aromatic alcohols to aldehydes in water. J Am Chem Soc 2008;5:1568–9. [4] Zhang ZH, Yuan Y, Fang YJ, Liang LH, Ding HC, Jin LT. Preparation of photocatalytic nano-ZnO/TiO2 film and application for determination of chemical oxygen demand. Talanta 2007;3:523–8. [5] Kostedt WL, Ismail AA, Mazyck DW. Impact of heat treatment and composition of ZnO-TiO2 nanoparticles for photocatalytic oxidation of an azo dye. Ind Eng Chem Res 2008;5:1483–7. [6] Lei YZ, Zhao GH, Liu MC, Zhang ZN, Tong XL, Cao TC. Fabrication, characterization, and photoelectrocatalytic application of ZnO nanorods grafted on vertically aligned TiO2 nanotubes. J Phys Chem C 2009;44:19067–76. [7] Kohtani S, Kudo A, Sakata T. Spectral sensitization of a TiO2 semiconductor electrode by CdS microcrystals and its photoelectrochemical properties. Chem Phys Lett 1993;206:166–70. [8] Anil K, Arvind KJ. Photophysics and photochemistry of colloidal CdS–TiO2 coupled semiconductors—photocatalytic oxidation of indole. J Mol Catal A Chem 2001;165: 265–73. [9] Hsu MC, Leu IC, Sun YM, Hon MH. Fabrication of CdS@TiO2 coaxial composite nanocables arrays by liquid-phase deposition. J Cryst Growth 2005;4:642–8. [10] Jang JS, Kim G, Joshi UA, Jang JW, Lee JS. Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation. Int J Hydrogen Energy 2008;21:5975–80. [11] Li PQ, Zhao GH, Cui X, Zhang YG, Tang YT. Constructing stake structured TiO2-NTs/ Sb-doped SnO2 electrode simultaneously with high electrocatalytic and photocatalytic performance for complete mineralization of refractory aromatic acid. J Phys Chem C 2009;6:2375–83. [12] Sun WT, Yu Y, Pan HY, Gao XF, Chen Q, Peng LM. CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes. J Am Chem Soc 2008;4:1124–5. [13] Khan SUM, Al-Shahry M, Ingler WB. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002;297:2243–5. [14] Sang LX, Zhong SH, Ma CF. The study on DRS and Raman spectroscopy of surface modified TiO2/SiO2. Spectrosc Spectr Anal 2007;27:720–4.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Novel construction of CdS-encapsulated TiO2 nano test tubes corked with ZnO nanorods Ya-nan Zhang, Guohua Zhao ⁎, Yanzhu Lei, Zhiyuan Wu, Yuning Jin, Mingfang Li Department of Chemistry, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 14 May 2010 Accepted 2 July 2010 Available online 16 July 2010 Keywords: Composite materials Semiconductors Photoelectrocatalytic activities UV–vis region
a b s t r a c t A novel TiO2 nanotube array/CdS nanoparticle/ZnO nanorod (TiO2 NT/CdS/ZnO NR) photocatalyst was constructed by chemical assembling CdS into the TiO2 NTs, and then laying ZnO NRs on the surface. The SEM results showed that the TiO2 NTs looked like many “nano test tubes” and the ZnO NRs served as the corks to seal the nozzle. This photocatalyst exhibited a wide absorption range (200–535 nm) in both ultraviolet and visible regions (UV–vis region), and maintained very high photoelectrocatalytic (PEC) activities. The maximum photoelectric conversion efficiencies (η) of TiO2 NT/CdS/ZnO NRs are 31.8 and 5.98% under UV light (365 nm) and visible light (420–800 nm), respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the recent years, there has been a growing interest in finding new effective photocatalysts for the elimination of environmental pollutants, which should possess a high photoelectrocatalytic (PEC) activity both in UV and visible regions, utilizing the whole solar energy spectrum as much as possible. TiO2 is a widely used photocatalyst due to its high oxidative power, good resistance to photocorrosion and non-toxicity [1–3]. However, the optical quantum efficiency of TiO2 is relatively low and it can only absorb the UV light (b387 nm) that is accounted for only 4–5% of the solar spectrum. Coupling TiO2 with other semiconductors to get better PEC activity has attracted more and more attention recently. ZnO is also an excellent n-type semiconductor and the TiO2/ZnO composite nanostructure has been demonstrated to exhibit higher PEC oxidation efficiency than TiO2 [4–6]. CdS is a typical narrow-band semiconductor which is often used to sensitize TiO2 and broaden the photoabsorption edge to a visible region [7–10]. In this paper, our interest is to obtain an excellent photocatalyst by chemical assembling the above three semiconductors with a novel structure. Hereon, a new attempt has been made to construct a TiO2 NT/CdS/ZnO NR photocatalyst by encapsulating the CdS NPs in the TiO2 NTs and subsequently building a ZnO NR layer on the TiO2 NTs/ CdS surface. It assumes that this composite structure owns several advantages. First, TiO2 NT arrays are chosen as the substrate materials, which possess high adsorption capacity and abundant active sites. Second, CdS assembled into the nanotubes in the form of NPs with a
⁎ Corresponding author. Tel.: +86 21 65981180; fax: +86 21 65982287. E-mail address: [email protected] (G. Zhao). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.013
high loading amount. Third, when appropriate ZnO NRs cover on the surface of TiO2 NTs, a “Corking Nano Test Tubes” structure is obtained which serves as the stable shell to protect the encapsulated CdS nanoparticles from contacting the solution directly, enhancing the photostability of the photocatalyst. This work provides a new idea for constructing an excellent photoelectrocatalyst with high efficiency and stability in the UV–vis region.
2. Material and methods The preparation of TiO2 NTs was described previously [11]. The obtained TiO2 NTs were placed vertically into a buffer bottle under vacuum conditions. Then, CdS nanoparticles were deposited into the vacuum treated TiO2 NTs by sequential chemical bath deposition method with 0.05 M Cd(NO3)2 and 0.05 M Na2S aqueous solutions [12]. The resultant film was annealed under pure nitrogen (N2) at 500 °C for 2 h. The TiO2 NT/CdS substrates were suspended in a solution containing equal molar (0.01 mol L−1) Zn(NO3)2 and (CH2)6 N4 and installed in a Teflon-lined stainless steel autoclave at 90 °C for 4 h. The TiO2 NT/CdS/ZnO NRs were obtained after the sample was washed with deionized water and dried at room temperature. The detailed schematic diagram is shown in Scheme 1. Field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800) and X-ray diffraction (XRD, Bruker Co., Ltd., Germany) were used to characterize the morphology and crystal structure of the asprepared electrodes. The optical absorption characteristics of the photocatalysts are determined by UV–vis diffuse reflectance spectroscopy (UV–vis DRS, Model BWS002, BWtek). The PEC performance and the photoelectric conversion efficiency (η) of the
Y. Zhang et al. / Materials Letters 64 (2010) 2194–2196
2195
Scheme 1. Schematic illustration for the construction of the TiO2 NT/CdS/ZnO NRs photocatalyst.
resulting materials are further investigated by linear-sweep photovoltammetry, which is calculated in accordance with the following formula: [13] ηð%Þ = ½ðtotal power output − electrical power inputÞ = light power input × 100 = jp ðErev −jEapp jÞ × 100 = I0 :
3. Results and discussion 3.1. Characterization of TiO2 NT/CdS/ZnO NRs FE-SEM is used to characterize the morphologies of the photocatalysts, as shown in Fig. 1. The in situ grown TiO2 NTs are uniform in size and vertically aligned (Fig. 1A). Every nanotube looks like a “nano test tube” with a diameter of 60 nm. The cross-sectional view of the TiO2 NTs/CdS (Fig. 1B) shows that many CdS nanoparticles are introduced onto the side wall of TiO2 NTs, and there're also some CdS particles on the outer walls of TiO2 NTs. From Fig. 1C, the ZnO NR layer is observed to be relatively denser and the NRs exhibit irregular sixprism with the diameter of about 150–200 nm which corked the open
end of nanotubes as the lid. The “Corking Nano Test Tubes” structure has been obtained and the CdS NPs are encapsulated in the tubes with a good protection, resulting in a high photostability. The XRD results show that a composite crystalline form of the rutile and anatase exits when TiO2 NTs are annealed at 500 °C (Fig. 1D). It can be seen that the characteristic diffraction peaks of CdS crystal (111), (220) and (311) appear (curve b), suggesting that a cubic phase CdS is obtained (JCPDS 10-454). Some new diffraction peaks can be observed in curve c at 31.9°, 34.6° and 36.4°, corresponding to the (100), (002), and (101) facets of ZnO, which is in good agreement with the values in the standard card (JCPDS 36-1451). 3.2. PEC activity of photocatalysts The UV–vis diffuse reflection spectra (DRS) for these photocatalysts indicate that the TiO2 NTs have an absorption edge at 385 nm with a band gap of 3.22 eV (Fig. 2), according to the equation Eg = 1240/λg [14]. An obvious red-shift of the absorption edge from 385 nm to 535 nm is observed (curve b), further indicating that the sensibilization of CdS particles for TiO2 NTs can expand the photoabsorption range to visible light. The absorption edge of TiO2
Fig. 1. FE-SEM images of as-prepared photocatalysts. (A) cross-sectional view of TiO2 NTs, (B) cross-sectional view of TiO2 NTs/CdS, and (C) TiO2 NT/CdS/ZnO NRs. (D) XRD patterns of samples.
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transfer may occur towards the opposite direction. The better charge separation can improve the photoelectrocatalytic activity greatly. 4. Conclusions In this work, we report a “Corking Nano Test Tubes” structure of TiO2 NT/CdS/ZnO NRs by filling one-dimensional TiO2 nanotube arrays (NTS) with CdS nanoparticles (NPs), and then constructing ZnO nanorods (NRs) on the TiO2 NTs/CdS surface. This photocatalyst exhibits a wide absorption (200–535 nm) response in the UV–vis region, and maintains very high PEC activities in both ultraviolet (UV) and visible regions. This resolution is novel and feasible, providing a composite photocatalyst of the new structure with higher efficiency and stability. Acknowledgements Fig. 2. UV–vis diffuse reflection spectra of photocatalysts.
Table 1 Photoelectric conversion efficiency for photocatalysts under UV light (3 mW cm−2) irradiation and visible light (100 mW cm−2). Photocatalysts
η (UV)
η (visible)
TiO2 NTs ITO/CdS TiO2 NTs/CdS TiO2 NT/CdS/ZnO NRs
8.6% 9.6% 28.0% 31.8%
– 0.50% 5.09% 5.98%
NT/CdS/ZnO NRs (curve c) is the same as that of TiO2 NTs/CdS since ZnO is transparent in the visible light. The final photocatalyst has a strong and wide absorption from 200 to 535 nm in UV–vis (curve c). The PEC performance of the resulting materials is listed in Table 1. No obvious light response is observed for TiO2 NTs in the visible light region. The maximum η of TiO2 NTs/CdS is much higher than that of ITO/CdS in both UV and visible regions. It indicates that TiO2 NTs are in favor of the dispersion of CdS NPs compared to ITO, due to the large specific surface and adsorption capacity. After covering with ZnO NRs, the remarkable maximum photoconversion efficiencies are achieved for TiO2 NT/CdS/ZnO NRs in both UV region (5.98%) and visible region (5.09%) at an applied potential of −0.8 V versus SCE. The overall photoelectric conversion efficiency of the composite semiconductor mainly depends on the position of its conduction and valence bands. The edge of the conduction band of CdS is higher than that of TiO2 and ZnO. So the energetic differences between the conduction bands of the semiconductors bring about a driving force, providing a better charge separation. Under visible irradiation, only CdS is excited and the electron transfer may occur from the conduction band of CdS towards that of ZnO, then to that of TiO2. Under UV irradiation, all semiconductors of the coupled system (TiO2/ ZnO/CdS) are activated, and the photogenerated holes consequently cascade quickly from TiO2 to ZnO, then to CdS, when the electron
This work was supported jointly by the National Natural Science Foundation of China (NO.20877058), 863 Program (NO.2008AA06Z329) from the Ministry of Science, and the Nanometer Science Foundation of Shanghai (NO. 0852nm01200). References [1] Grimes CA. Synthesis and application of highly ordered arrays of TiO2 nanotubes. J Mater Chem 2007;15:1451–7. [2] Tachikawa T, Fujitsuka M, Majima T. Mechanistic insight into the TiO 2 photocatalytic reactions: design of new photocatalysts. J Phys Chem C 2007;14: 5259–75. [3] Yurdakal S, Palmisano G, Loddo V, Augugliaro V, Palmisano L. Nanostructured rutile TiO2 for selective photocatalytic oxidation of aromatic alcohols to aldehydes in water. J Am Chem Soc 2008;5:1568–9. [4] Zhang ZH, Yuan Y, Fang YJ, Liang LH, Ding HC, Jin LT. Preparation of photocatalytic nano-ZnO/TiO2 film and application for determination of chemical oxygen demand. Talanta 2007;3:523–8. [5] Kostedt WL, Ismail AA, Mazyck DW. Impact of heat treatment and composition of ZnO-TiO2 nanoparticles for photocatalytic oxidation of an azo dye. Ind Eng Chem Res 2008;5:1483–7. [6] Lei YZ, Zhao GH, Liu MC, Zhang ZN, Tong XL, Cao TC. Fabrication, characterization, and photoelectrocatalytic application of ZnO nanorods grafted on vertically aligned TiO2 nanotubes. J Phys Chem C 2009;44:19067–76. [7] Kohtani S, Kudo A, Sakata T. Spectral sensitization of a TiO2 semiconductor electrode by CdS microcrystals and its photoelectrochemical properties. Chem Phys Lett 1993;206:166–70. [8] Anil K, Arvind KJ. Photophysics and photochemistry of colloidal CdS–TiO2 coupled semiconductors—photocatalytic oxidation of indole. J Mol Catal A Chem 2001;165: 265–73. [9] Hsu MC, Leu IC, Sun YM, Hon MH. Fabrication of CdS@TiO2 coaxial composite nanocables arrays by liquid-phase deposition. J Cryst Growth 2005;4:642–8. [10] Jang JS, Kim G, Joshi UA, Jang JW, Lee JS. Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation. Int J Hydrogen Energy 2008;21:5975–80. [11] Li PQ, Zhao GH, Cui X, Zhang YG, Tang YT. Constructing stake structured TiO2-NTs/ Sb-doped SnO2 electrode simultaneously with high electrocatalytic and photocatalytic performance for complete mineralization of refractory aromatic acid. J Phys Chem C 2009;6:2375–83. [12] Sun WT, Yu Y, Pan HY, Gao XF, Chen Q, Peng LM. CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes. J Am Chem Soc 2008;4:1124–5. [13] Khan SUM, Al-Shahry M, Ingler WB. Efficient photochemical water splitting by a chemically modified n-TiO2. Science 2002;297:2243–5. [14] Sang LX, Zhong SH, Ma CF. The study on DRS and Raman spectroscopy of surface modified TiO2/SiO2. Spectrosc Spectr Anal 2007;27:720–4.