- Email: [email protected]
Cu2O heterostructures by electrochemical method and their photoelectrochemical properties
Materials Letters 92 (2013) 239–242
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled synthesis of coaxial core–shell TiO2/Cu2O heterostructures by electrochemical method and their photoelectrochemical properties Jinbo Xue a,b, Qianqian Shen b, Wei Liang a,b,n, Xuguang Liu a,c, Bingshe Xu a,b a
Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China c College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
a r t i c l e i n f o
abstract
Article history: Received 20 August 2012 Accepted 31 October 2012 Available online 9 November 2012
Coaxial heterostructures based on n-type TiO2 nanotubes (TNTs) arrays enclosed with p-type Cu2O nanoparticles were prepared through a combination of anodic oxidation and subsequent electrodeposition. Cu2O nanoparticles were electrodeposited into the inter-tubular space of TNTs to form the coaxial core–shell TNTs@Cu2O heterostructures. The Cu2O nanoparticles were preferentially deposited on the ribs of TNTs arrays, and the amount of thiourea added during electrodeposition of Cu2O played an important role in formation of coaxial core–shell TNTs@Cu2O heterostructures. The heterostructures exhibited excellent photoelectrochemical performance under visible light. Such heterostructures would serve as a promising candidate for solar energy conversion. & 2012 Elsevier B.V. All rights reserved.
Keywords: Coaxial core–shell heterostructure Microstructure Semiconductors Solar energy materials
1. Introduction Vertically oriented self-organized TiO2 nanotubes (TNTs) arrays, synthesized by electrochemical anodization [1,2], are currently generating great interest owing to orthogonalization of charge separation and charge transport [3]. However, the wide band gap (3.2 eV) for anatase corresponds to the utilization of only 5% of sunlight. Therefore, much effort has been dedicated to expanding the photocatalytic function of the TNTs to the visible light region [4]. In addition, TiO2 suffers from dissatisfactory quantum efficiency. The rapid recombination of photoinduced electron–hole pairs greatly limits the energy conversion efficiency[5]. Therefore, to enhance the efficiency in the visible range and charge separation, considerable efforts have been taken to form heterostructure between TiO2 and narrow-band-gap semiconductor. The p-type cuprous oxide (Cu2O) has recently been reported to be a very good sensitizer to improve the photocatalytic activity of TiO2 in solar energy utilization [6–9]. Han and co-workers prepared TiO2/Cu2O composite films that exhibited better photocatalytic efficiencies than TiO2 and Cu2O do alone under visible light irradiation [10]. Hou et al. reported the deposition of Cu2O nanoparticles onto TNTs by a simple photoreduction deposition strategy to prepare Cu2O/TiO2 heterojunction arrays [11,12]. The as-prepared TiO2/Cu2O composites were found to be highly
n Corresponding author at: Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China. Tel./fax: þ86 351 6018398. E-mail address: [email protected] (W. Liang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.127
efficient for photoelectrocatalysis under visible light. However, Cu2O particles reported above were supported mainly on the top surface of the TNTs rather than into the inner and external walls of the nanotubes. Therefore, the large surface area of the threedimensional (3D) TNTs frameworks was not fully utilized, leading to insufficiently effective electron pathways between the Cu2O and TiO2. In this work, Cu2O nanoparticles were electrodeposited into the inter-tubular space of TNTs to form coaxial core–shell TNTs@Cu2O heterostructures. The formation mechanism for the coaxial core– shell TNTs@Cu2O composite was proposed. The photoelectrochemical performance of the as-prepared TNTs@Cu2O was evaluated under visible light.
2. Experiment Preparation of TNTs arrays and coaxial core–shell TNTs@Cu2O heterostructures: The TNTs arrays were prepared by anodization of Ti foil in 0.05 M phosphoric acid (H3PO4) solution with 0.1 M ammonium fluoride (NH4F). The following anodization process was performed in two-electrode configuration with a titanium foil as the working electrode and platinum foil as the counter electrode under 20 V for 4 h at 40 1C. After anodic oxidation, the samples were annealed at 773 K for 2 h. The TNTs@Cu2O heterostructures were fabricated by electrochemical deposition of Cu2O nanoparticles into TNTs. The electrodeposition was conducted on a CHI-660D electrochemical workstation with TNTs as the working electrode, platinum foil as the counter electrode, and saturated calomel electrode (SCE) as
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the reference electrode. The electrolyte was prepared by adding different concentrations thiourea (0, 0.1, 0.3, 0.5 and 1 mM) in 0.02 M Cu(Ac)2 solution. During electrochemical deposition, potentiostatic method was adopted with deposition potentials of 0.1, 0.2, and 0.3 V vs. SCE. Characterization of coaxial core–shell TNTs@Cu2O heterostructures: The crystalline phase of the heterostructures was identified by X-ray diffraction (XRD) on a Bruxel D8 diffractometer. The chemical nature of Cu was analyzed by X-ray photoelectron spectroscopy (XPS) using a Mg mono Ka X-ray source operated at 15 kV and 150 W. The morphologies of samples were characterized on a JEOL JSM-6700F field emitting scanning electron microscope (FESEM). The energy dispersive X-ray spectrum (EDS) was recorded with an energy dispersive detector of Oxford Corporate equipped on the SEM. The high resolution transmission electron microscopy (HRTEM) observation was carried out with a JEOL JEM-2010 electron microscope. A UV–vis spectrometer (Lambda750S PerkinElmer) was used to record the UV–vis absorption spectra. Photoelectrochemical analysis was conducted by a three-electrode cell in which the TNTs@Cu2O heterostructure, Pt foil, and SCE served as working, counter, and reference electrodes,
respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution, which was purged with N2 gas for 30 min prior to electrochemical measurement. The photocurrent was recorded on a CHI-660D electrochemical workstation, and a 500 W xenon lamp with 420 nm filter was used as the light source.
3. Results and discussion The crystal structure of the heterostructures was confirmed by XRD analysis (Fig. 1a). The heterostructures mainly consisted of anatase (TiO2), cuprous oxide (Cu2O) and Ti substrate under the potential of 0.05 V and 0.1 V. As the reduction potential decreased to 0.3 V, metal copper phase began to appear in the heterostructures. The EDS spectra for the TNTs@Cu2O heterostructures fabricated under the potential of 0.1 V (Fig. 1b) show that the heterostructures consisted of Ti, Cu and O elements. Further evidence for composition of the products was obtained by XPS analysis. The typical XPS peaks of the Cu(2p) at 952.6 and 932.5 eV for TNTs@Cu2O heterostructures demonstrated the existence of Cu þ , as shown in Fig. 1c.
Fig. 1. (a) XRD patterns of TNTs@Cu2O heterostructures prepared under the potential of 0.05V, 0.1V and 0.3 V (vs. SCE) with 0.5 mM thiourea; (b) EDS and (c) the core–level Cu(2p) XPS spectrum of TNTs@Cu2O heterostructures prepared under the potential of 0.1 V (vs. SCE) with 0.5 mM thiourea.
Fig. 2. Cross-section SEM images of the TNTs (a) and TNTs@Cu2O heterostructures prepared under the potential of 0.1 V (vs. SCE) with different thiourea concentrations: (b) 0.1 mM, (c) 0.3 mM, (d) 0.5 mM and (e) 1.0 mM. (f) Proposed formation mechanism of coaxial core–shell TNTs@Cu2O heterostructures.
J. Xue et al. / Materials Letters 92 (2013) 239–242
Fig. 2 shows the cross-section images of the coaxial core–shell TNTs@Cu2O heterostructures. For pure TNTs without deposited Cu2O nanoparticles (Fig. 2a), the majority of the nanotubes were bridged by ribs (noted by arrows). Under low concentration of thiourea (0.1 mM), some Cu2O nanoparticles were deposited on the ribs (Fig. 2b). As the concentration of thiourea increased, the Cu2O nanoparticles on the ribs increased in amount and merged, as indicated by the circles in Fig. 2c. When the concentration of thiourea was upto 0.5 mM, the connected Cu2O nanoparticles developed into Cu2O film on external walls of TNTs (indicated by the circles in Fig. 2d) and the ribs became unclear. However, when the thiourea concentration was further increased to 1.0 mM, the amount of Cu2O nanoparticles deposited on ribs decreased, as shown in Fig. 2e, as a result of the formation of complex between thiourea and Cu ion [13] and correspondingly the difficult
241
precipitation of Cu2O nucleus from the electrolyte. From these results, it can be concluded that Cu2O nanoparticles were favorably deposited on the ribs. This is because the protuberant ribs from smooth TNTs resulted in accumulation of charge in the micro-zone by applying a cathodic potential on the TNTs arrays, which induced the preferential adsorption of Cu2 þ ions on the ribs and promoted the reduction of Cu2 þ to Cu þ ions. Furthermore, the ribs worked as the sites of heterogeneous nucleation for facilitating deposition of Cu2O nanoparticles. Therefore, the Cu2O nucleus formed on the ribs and then merged with each other to form coaxial TNTs@Cu2O heterostructures with increasing thiourea concentration. The proposed formation mechanism is shown in Fig. 2f. In order to further understand the crystal structure of the coaxial core–shell TNTs@Cu2O heterostructures, TEM observations
Fig. 3. (a) Cross-section TEM images of the TNTs@Cu2O heterostructures; and (b) corresponding SAED pattern; (c) HRTEM image of the selected region of TEM in (a).
Fig. 4. (a) UV–vis DR spectra of coaxial TNTs@Cu2O heterostructures; (b) photocurrent density profiles (Iph) of the TNTs@Cu2O heterostructures at a bias potential of 0.0 V (vs. SCE) under visible light.
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were conducted (Fig. 3). As shown in Fig. 3a, the ribs bridged the nanotubes, and some Cu2O nanoparticles were deposited on the ribs. The corresponding SAED pattern (Fig. 3b) indicates that the heterostructures consisted of polycrystalline phase of TiO2 and Cu2O. Fig. 3c is the HRTEM image of selected area in Fig. 3a, in which the lattice fringes with interplanar spacing of d¼0.25 nm and d¼0.35 nm are consistent with those of the Cu2O (111) plane and TiO2 (101) plane, respectively. It indicates that Cu2O nanoparticles covered TiO2 nanotubes, and the coaxial core–shell TNTs@Cu2O heterostructures were fabricated. Fig. 4a shows the UV–vis/DR spectra of TNTs arrays and the coaxial core–shell TNTs@Cu2O heterostructures. The absorption edge of the annealed TNTs arrays was 375 nm. After being heterojunctured with Cu2O nanoparticles, the absorption edge shifted to 600 nm. These results indicate that the deposition of Cu2O nanoparticles extended the absorption of the TNTs arrays into the visible light region. The photoelectrochemical behavior of heterostructured electrodes is presented in Fig. 4b. No significant current was observed in the dark for all samples. Under visible light (l 4400 nm), the annealed TNTs arrays did not show observable photocurrent response. However, the average photocurrent increased as the amount of Cu2O nanoparticles increased when the thiourea concentration changed from 0.1 mM to 0.5 mM. This coincides with the UV–vis DR measurement perfectly, suggesting that the increase of the photocurrent was attributed to the enhancement of the visible light adsorption of the samples. In addition, the photocurrent of heterostructure decayed with prolonging time, because the reduction of Cu2O induced by photogenerated electrons and the photocorrosion of Cu2O induced by the photogenerated holes would result in degradation of its photochemical properties[14,15]. In order to improve the stability of photocurrent of the heterostructure, photogenerated holes should be consumed or transferred. This will be discussed in detail in our later works.
4. Conclusion Coaxial core–shell TNTs@Cu2O heterostructures were fabricated by using electrochemical method. The ribs formed during anodic oxidation and thiourea added during electrodepostition of Cu2O nanoparticles played important roles in formation of such heterostructures. The coaxial core–shell TNTs@Cu2O heterostructures exhibited excellent photoelectrochemical performance under visible light.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51175363), Program for Changjiang Scholar and Innovative Research Team in University (No. IRT0972). References [1] Macak JM, Zlamal M, Krysa J, Schmuki P. Small 2007;3:300–4. [2] Paramasivam I, Macak JM, Schmuki P. Electrochem Commun 2008;10:71–5. [3] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Nano Lett 2005;5:191–5. [4] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Science 2001;293:269–71. [5] Berger T, Sterrer M, Diwald O, Knozinger E, Panayotov D, Thompson TL, et al. J Phys Chem B 2005;109:6061–8. [6] Bessekhouad Y, Robert D, Weber JV. Catal Today 2005;101:315–21. [7] Li JL, Liu L, Yu Y, Tang YW, Li HL, Du FP. Electrochem Commun 2004;6:940–3. [8] Tang YW, Chen ZG, Jia ZJ, Zhang LS, Li JL. Mater Lett 2005;59:434–8. [9] Zhang YG, Ma LL, Li JL, Yu Y. Environ Sci Technol 2007;41:6264–9. [10] Han CH, Li ZY, Shen JY. J Hazard Mater 2009;168:215–9. [11] Hou Y, Li XY, Zhao QD, Quan X, Chen GH. Appl Phys Lett 2009;95:093108/ 1–093108/3. [12] Hou Y, Li XY, Zhou XJ, Quan X, Chen GH. Environ Sci Technol 2009;43:858–63. [13] Szymaszek A, Biernat J, Pajdowski L. Electrochim Acta 1977;22:359–64. [14] Huang L, Peng F, Yu H, Wang HJ. Solid State Sci 2009;11:129–38. [15] Bessekhouad Y, Robert D, Weber JV. Catal Today 2005;101:315–21.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled synthesis of coaxial core–shell TiO2/Cu2O heterostructures by electrochemical method and their photoelectrochemical properties Jinbo Xue a,b, Qianqian Shen b, Wei Liang a,b,n, Xuguang Liu a,c, Bingshe Xu a,b a
Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China c College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
a r t i c l e i n f o
abstract
Article history: Received 20 August 2012 Accepted 31 October 2012 Available online 9 November 2012
Coaxial heterostructures based on n-type TiO2 nanotubes (TNTs) arrays enclosed with p-type Cu2O nanoparticles were prepared through a combination of anodic oxidation and subsequent electrodeposition. Cu2O nanoparticles were electrodeposited into the inter-tubular space of TNTs to form the coaxial core–shell TNTs@Cu2O heterostructures. The Cu2O nanoparticles were preferentially deposited on the ribs of TNTs arrays, and the amount of thiourea added during electrodeposition of Cu2O played an important role in formation of coaxial core–shell TNTs@Cu2O heterostructures. The heterostructures exhibited excellent photoelectrochemical performance under visible light. Such heterostructures would serve as a promising candidate for solar energy conversion. & 2012 Elsevier B.V. All rights reserved.
Keywords: Coaxial core–shell heterostructure Microstructure Semiconductors Solar energy materials
1. Introduction Vertically oriented self-organized TiO2 nanotubes (TNTs) arrays, synthesized by electrochemical anodization [1,2], are currently generating great interest owing to orthogonalization of charge separation and charge transport [3]. However, the wide band gap (3.2 eV) for anatase corresponds to the utilization of only 5% of sunlight. Therefore, much effort has been dedicated to expanding the photocatalytic function of the TNTs to the visible light region [4]. In addition, TiO2 suffers from dissatisfactory quantum efficiency. The rapid recombination of photoinduced electron–hole pairs greatly limits the energy conversion efficiency[5]. Therefore, to enhance the efficiency in the visible range and charge separation, considerable efforts have been taken to form heterostructure between TiO2 and narrow-band-gap semiconductor. The p-type cuprous oxide (Cu2O) has recently been reported to be a very good sensitizer to improve the photocatalytic activity of TiO2 in solar energy utilization [6–9]. Han and co-workers prepared TiO2/Cu2O composite films that exhibited better photocatalytic efficiencies than TiO2 and Cu2O do alone under visible light irradiation [10]. Hou et al. reported the deposition of Cu2O nanoparticles onto TNTs by a simple photoreduction deposition strategy to prepare Cu2O/TiO2 heterojunction arrays [11,12]. The as-prepared TiO2/Cu2O composites were found to be highly
n Corresponding author at: Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China. Tel./fax: þ86 351 6018398. E-mail address: [email protected] (W. Liang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.10.127
efficient for photoelectrocatalysis under visible light. However, Cu2O particles reported above were supported mainly on the top surface of the TNTs rather than into the inner and external walls of the nanotubes. Therefore, the large surface area of the threedimensional (3D) TNTs frameworks was not fully utilized, leading to insufficiently effective electron pathways between the Cu2O and TiO2. In this work, Cu2O nanoparticles were electrodeposited into the inter-tubular space of TNTs to form coaxial core–shell TNTs@Cu2O heterostructures. The formation mechanism for the coaxial core– shell TNTs@Cu2O composite was proposed. The photoelectrochemical performance of the as-prepared TNTs@Cu2O was evaluated under visible light.
2. Experiment Preparation of TNTs arrays and coaxial core–shell TNTs@Cu2O heterostructures: The TNTs arrays were prepared by anodization of Ti foil in 0.05 M phosphoric acid (H3PO4) solution with 0.1 M ammonium fluoride (NH4F). The following anodization process was performed in two-electrode configuration with a titanium foil as the working electrode and platinum foil as the counter electrode under 20 V for 4 h at 40 1C. After anodic oxidation, the samples were annealed at 773 K for 2 h. The TNTs@Cu2O heterostructures were fabricated by electrochemical deposition of Cu2O nanoparticles into TNTs. The electrodeposition was conducted on a CHI-660D electrochemical workstation with TNTs as the working electrode, platinum foil as the counter electrode, and saturated calomel electrode (SCE) as
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the reference electrode. The electrolyte was prepared by adding different concentrations thiourea (0, 0.1, 0.3, 0.5 and 1 mM) in 0.02 M Cu(Ac)2 solution. During electrochemical deposition, potentiostatic method was adopted with deposition potentials of 0.1, 0.2, and 0.3 V vs. SCE. Characterization of coaxial core–shell TNTs@Cu2O heterostructures: The crystalline phase of the heterostructures was identified by X-ray diffraction (XRD) on a Bruxel D8 diffractometer. The chemical nature of Cu was analyzed by X-ray photoelectron spectroscopy (XPS) using a Mg mono Ka X-ray source operated at 15 kV and 150 W. The morphologies of samples were characterized on a JEOL JSM-6700F field emitting scanning electron microscope (FESEM). The energy dispersive X-ray spectrum (EDS) was recorded with an energy dispersive detector of Oxford Corporate equipped on the SEM. The high resolution transmission electron microscopy (HRTEM) observation was carried out with a JEOL JEM-2010 electron microscope. A UV–vis spectrometer (Lambda750S PerkinElmer) was used to record the UV–vis absorption spectra. Photoelectrochemical analysis was conducted by a three-electrode cell in which the TNTs@Cu2O heterostructure, Pt foil, and SCE served as working, counter, and reference electrodes,
respectively. The electrolyte was 0.5 M Na2SO4 aqueous solution, which was purged with N2 gas for 30 min prior to electrochemical measurement. The photocurrent was recorded on a CHI-660D electrochemical workstation, and a 500 W xenon lamp with 420 nm filter was used as the light source.
3. Results and discussion The crystal structure of the heterostructures was confirmed by XRD analysis (Fig. 1a). The heterostructures mainly consisted of anatase (TiO2), cuprous oxide (Cu2O) and Ti substrate under the potential of 0.05 V and 0.1 V. As the reduction potential decreased to 0.3 V, metal copper phase began to appear in the heterostructures. The EDS spectra for the TNTs@Cu2O heterostructures fabricated under the potential of 0.1 V (Fig. 1b) show that the heterostructures consisted of Ti, Cu and O elements. Further evidence for composition of the products was obtained by XPS analysis. The typical XPS peaks of the Cu(2p) at 952.6 and 932.5 eV for TNTs@Cu2O heterostructures demonstrated the existence of Cu þ , as shown in Fig. 1c.
Fig. 1. (a) XRD patterns of TNTs@Cu2O heterostructures prepared under the potential of 0.05V, 0.1V and 0.3 V (vs. SCE) with 0.5 mM thiourea; (b) EDS and (c) the core–level Cu(2p) XPS spectrum of TNTs@Cu2O heterostructures prepared under the potential of 0.1 V (vs. SCE) with 0.5 mM thiourea.
Fig. 2. Cross-section SEM images of the TNTs (a) and TNTs@Cu2O heterostructures prepared under the potential of 0.1 V (vs. SCE) with different thiourea concentrations: (b) 0.1 mM, (c) 0.3 mM, (d) 0.5 mM and (e) 1.0 mM. (f) Proposed formation mechanism of coaxial core–shell TNTs@Cu2O heterostructures.
J. Xue et al. / Materials Letters 92 (2013) 239–242
Fig. 2 shows the cross-section images of the coaxial core–shell TNTs@Cu2O heterostructures. For pure TNTs without deposited Cu2O nanoparticles (Fig. 2a), the majority of the nanotubes were bridged by ribs (noted by arrows). Under low concentration of thiourea (0.1 mM), some Cu2O nanoparticles were deposited on the ribs (Fig. 2b). As the concentration of thiourea increased, the Cu2O nanoparticles on the ribs increased in amount and merged, as indicated by the circles in Fig. 2c. When the concentration of thiourea was upto 0.5 mM, the connected Cu2O nanoparticles developed into Cu2O film on external walls of TNTs (indicated by the circles in Fig. 2d) and the ribs became unclear. However, when the thiourea concentration was further increased to 1.0 mM, the amount of Cu2O nanoparticles deposited on ribs decreased, as shown in Fig. 2e, as a result of the formation of complex between thiourea and Cu ion [13] and correspondingly the difficult
241
precipitation of Cu2O nucleus from the electrolyte. From these results, it can be concluded that Cu2O nanoparticles were favorably deposited on the ribs. This is because the protuberant ribs from smooth TNTs resulted in accumulation of charge in the micro-zone by applying a cathodic potential on the TNTs arrays, which induced the preferential adsorption of Cu2 þ ions on the ribs and promoted the reduction of Cu2 þ to Cu þ ions. Furthermore, the ribs worked as the sites of heterogeneous nucleation for facilitating deposition of Cu2O nanoparticles. Therefore, the Cu2O nucleus formed on the ribs and then merged with each other to form coaxial TNTs@Cu2O heterostructures with increasing thiourea concentration. The proposed formation mechanism is shown in Fig. 2f. In order to further understand the crystal structure of the coaxial core–shell TNTs@Cu2O heterostructures, TEM observations
Fig. 3. (a) Cross-section TEM images of the TNTs@Cu2O heterostructures; and (b) corresponding SAED pattern; (c) HRTEM image of the selected region of TEM in (a).
Fig. 4. (a) UV–vis DR spectra of coaxial TNTs@Cu2O heterostructures; (b) photocurrent density profiles (Iph) of the TNTs@Cu2O heterostructures at a bias potential of 0.0 V (vs. SCE) under visible light.
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were conducted (Fig. 3). As shown in Fig. 3a, the ribs bridged the nanotubes, and some Cu2O nanoparticles were deposited on the ribs. The corresponding SAED pattern (Fig. 3b) indicates that the heterostructures consisted of polycrystalline phase of TiO2 and Cu2O. Fig. 3c is the HRTEM image of selected area in Fig. 3a, in which the lattice fringes with interplanar spacing of d¼0.25 nm and d¼0.35 nm are consistent with those of the Cu2O (111) plane and TiO2 (101) plane, respectively. It indicates that Cu2O nanoparticles covered TiO2 nanotubes, and the coaxial core–shell TNTs@Cu2O heterostructures were fabricated. Fig. 4a shows the UV–vis/DR spectra of TNTs arrays and the coaxial core–shell TNTs@Cu2O heterostructures. The absorption edge of the annealed TNTs arrays was 375 nm. After being heterojunctured with Cu2O nanoparticles, the absorption edge shifted to 600 nm. These results indicate that the deposition of Cu2O nanoparticles extended the absorption of the TNTs arrays into the visible light region. The photoelectrochemical behavior of heterostructured electrodes is presented in Fig. 4b. No significant current was observed in the dark for all samples. Under visible light (l 4400 nm), the annealed TNTs arrays did not show observable photocurrent response. However, the average photocurrent increased as the amount of Cu2O nanoparticles increased when the thiourea concentration changed from 0.1 mM to 0.5 mM. This coincides with the UV–vis DR measurement perfectly, suggesting that the increase of the photocurrent was attributed to the enhancement of the visible light adsorption of the samples. In addition, the photocurrent of heterostructure decayed with prolonging time, because the reduction of Cu2O induced by photogenerated electrons and the photocorrosion of Cu2O induced by the photogenerated holes would result in degradation of its photochemical properties[14,15]. In order to improve the stability of photocurrent of the heterostructure, photogenerated holes should be consumed or transferred. This will be discussed in detail in our later works.
4. Conclusion Coaxial core–shell TNTs@Cu2O heterostructures were fabricated by using electrochemical method. The ribs formed during anodic oxidation and thiourea added during electrodepostition of Cu2O nanoparticles played important roles in formation of such heterostructures. The coaxial core–shell TNTs@Cu2O heterostructures exhibited excellent photoelectrochemical performance under visible light.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51175363), Program for Changjiang Scholar and Innovative Research Team in University (No. IRT0972). References [1] Macak JM, Zlamal M, Krysa J, Schmuki P. Small 2007;3:300–4. [2] Paramasivam I, Macak JM, Schmuki P. Electrochem Commun 2008;10:71–5. [3] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Nano Lett 2005;5:191–5. [4] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Science 2001;293:269–71. [5] Berger T, Sterrer M, Diwald O, Knozinger E, Panayotov D, Thompson TL, et al. J Phys Chem B 2005;109:6061–8. [6] Bessekhouad Y, Robert D, Weber JV. Catal Today 2005;101:315–21. [7] Li JL, Liu L, Yu Y, Tang YW, Li HL, Du FP. Electrochem Commun 2004;6:940–3. [8] Tang YW, Chen ZG, Jia ZJ, Zhang LS, Li JL. Mater Lett 2005;59:434–8. [9] Zhang YG, Ma LL, Li JL, Yu Y. Environ Sci Technol 2007;41:6264–9. [10] Han CH, Li ZY, Shen JY. J Hazard Mater 2009;168:215–9. [11] Hou Y, Li XY, Zhao QD, Quan X, Chen GH. Appl Phys Lett 2009;95:093108/ 1–093108/3. [12] Hou Y, Li XY, Zhou XJ, Quan X, Chen GH. Environ Sci Technol 2009;43:858–63. [13] Szymaszek A, Biernat J, Pajdowski L. Electrochim Acta 1977;22:359–64. [14] Huang L, Peng F, Yu H, Wang HJ. Solid State Sci 2009;11:129–38. [15] Bessekhouad Y, Robert D, Weber JV. Catal Today 2005;101:315–21.