ZnO nanotube arrays for high-efficiency photo-energy conversion applications

Electrochemistry Communications 13 (2011) 788–791

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Electrochemistry Communications 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 / e l e c o m

Preparation of coaxial TiO2/ZnO nanotube arrays for high-efficiency photo-energy conversion applications Yu-Long Xie, Zi-Xia Li, Zhu-Guo Xu, Hao-Li Zhang ⁎ State Key Laboratory of Applied Organic Chemistry (SKLAOC), Lanzhou University, Lanzhou, 730000, China Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China

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Article history: Received 16 March 2011 Received in revised form 23 April 2011 Accepted 4 May 2011 Available online 12 May 2011 Keywords: Dye-sensitized solar cell Coaxial TiO2/ZnO Nanotube arrays Electrochemical deposition

a b s t r a c t A novel type of coaxial TiO2/ZnO nanotube arrays has been prepared by electrochemical method. The TiO2/ZnO nanotube arrays can be applied as highly efficient photoanodes in dye-sensitized solar cells (DSSC). Such photoanode benefits from the capability of high sensitizer loading offered by the high specific surface, and the direct conduction path through the aligned nanotubes. Moreover, the heterojunction at the TiO2/ZnO interface favors charge separation and reduces the probability of charge recombination. With the coaxial TiO2/ZnO nanotubes as the photoanode, DSSC with an overall 2.8% energy-conversion efficiency was obtained, which was 40% higher than that using pure TiO2 nanotubes. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSSCs) have attracted great interest in both academic research and industrial applications owing to its low cost, easy handling and relatively high conversion efficiency [1]. The active electrode in DSSCs is usually composed of TiO2 mesoporous films covered by an organic dye [2,3]. Recently, highly ordered TiO2 nanotubes (NTs) have been introduced to DSSCs [4–10]. Using TiO2 NTs as photoelectrode may offer several advantages, such as simple synthesis process, inexpensive cost and high stability. The most important advantage of ordered TiO2 NTs is they provide direct conduction path for electrons that facilitate separation of the photoexcited charges, and hence result in higher charge collection efficiencies [11,12]. Recently, extensive investigations have been conducted on the design and synthesis of nanocomposite metal oxides for the applications in photocatalysis [13–16] and DSSCs [17,18]. Among the various materials under investigation, TiO2/ZnO nanocomposites are of particular interests, partly due to the high reactivity of TiO2 and the high electron mobility of ZnO, which improve the process of electrons and holes transfer between the corresponding conduction and valence bands. As a result, a better separation of photogenerated charge carriers can be achieved when compared with a single metal oxide [14,15]. To date, most of the reported TiO2/ZnO nanocomposites were disordered mesoporous structures, and highly ordered TiO2/ZnO NT array has not been reported.

⁎ Corresponding author. Tel./fax: + 86 931 8912365. E-mail address: [email protected] (H.-L. Zhang). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.05.003

In this paper, a novel coaxial TiO2/ZnO NT array was prepared by electrochemical method, which gave highly ordered vertically aligned NTs with uniform pore sizes. The potential application of the TiO2/ZnO NT arrays in high-efficiency DSSCs was explored. Compared with pure TiO2 NT arrays, the coaxial TiO2/ZnO NT array showed enhanced charge separation efficiency. 2. Experimental Highly ordered TiO2 NT arrays were prepared by anodization of Ti foils (99.9% purity) of 250 μm thickness in a two-electrode cell. The anodization was performed at room temperature in a solution of 0.3 wt % NH4F and 2.0 vol.% H2O in ethylene glycol[19,20]. The TiO2 NTs were annealed at 450 °C for 2 hours. For the ZnO deposition, the TiO2 NTs served as the working electrodes in a conventional three-electrode system, and a saturated calomel electrode (SCE) and a platinum electrode served as the reference and counter electrode, respectively. The electrolyte was aqueous solution of 10 mM Zn(NO3)2 and 10 mM EDTA. The coaxial TiO2/ZnO NT arrays were prepared by applying a cathodic potential at −1.0 V, with a typical deposition time of 1 h. The methylene blue (5 mg/L) photodegradation assay was conducted with under the 12 W UV light source (365 nm). To fabricate DSSCs, the TiO2/ZnO NT samples were sensitized with Ru-based N719 dye and then sandwiched together with a Pt coated fluorine doped glass counter electrode using a polymer spacer (Surlyn). The DSSCs were tested under an AM 1.5 illumination provided by a xenon lamp (OSRAM E-O 150 W) with an optical filter (AM 1.5 G) under a power of 100 mW/cm2 (calibrated by a standard silicon solar cell). The generated photocurrent was measured with a Model 4200-SCS Semiconductor Characterization System. The incident photon-to-

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electron conversion efficiency (IPCE) spectra were obtained with a Solar Cell Scan 100 (Zolix Instruments). The morphologies of samples were obtained by using a field-emission scanning electron microscopy (FE-SEM, Hitachi S4800). The crystalline orientations of TiO2 were confirmed by X-ray diffraction (XRD-6000, Bruker). 3. Results and discussion Fig. 1 shows the FESEM images of pure TiO2 NT and the coaxial TiO2/ ZnO NT electrodes. The as-prepared TiO2 NT show highly uniform arrays with all the tubes aligned perpendicularly to the Ti substrates. The magnified image (Fig. 1b) indicates that the NTs were well open at the top with a uniform wall thickness of approximately 15 nm and a mean inter-pore diameter around 130 nm. Fig. 1c and d shows that, after the electrochemical deposition of ZnO, the TiO2 NT arrays were turned into a clear coaxial structure. The ZnO layer was deposited as an inner layer inside the original TiO2 pores, with a highly uniformed layer thickness of ca. 30 nm. Both top view (Fig. 1c) and side view (Fig. 1d) FESEM images suggest that the pores remain open and the deposited ZnO layer was uniform, which suggests that the growth of the ZnO layer is along the surface of inner sidewall of the TiO2 NTs. A careful study on the ZnO layer suggests that it composes of very small ZnO nanoparticles, which give rough and nanoporous inner layers (Fig. 1d). Such rough inner surface of coaxial TiO2/ZnO NT is favorable to the dye loading and electrolyte transport. The crystal structures of the pure TiO2 NT and the coaxial TiO2/ZnO NT arrays were characterized by XRD (Fig. 2a). All the peaks in the XRD pattern of the pure TiO2 NT arrays can be well indexed to the TiO2 anatase phase, while the XRD pattern of the coaxial TiO2/ZnO NT sample reflects the characteristic of both anatase TiO2 and hexagonal wurtzite ZnO structure. Fig. 2b shows that the UV–vis diffuse reflectance spectra of the pure TiO2 NTs and the hybrid coaxial TiO2/ZnO NTs are very similar. Both samples show adsorption edges at around 400 nm, which is consistent with their band gap of around 3.2 eV. The photocatalytic activity of the

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hybrid coaxial TiO2/ZnO NTs was compared with the pure TiO2 NTs using the methylene blue (MB) assay. As shown in Fig. 2c, after UV irradiation for 150 min, 80% of MB was degraded over the coaxial TiO2/ ZnO NTs, whereas only 57% of MB was degraded over the pure TiO2 NTs. The superior photocatalytic performance of the hybrid coaxial TiO2/ZnO NTs is partly attributed to a higher specific surface area, which ensures more MB contact with the active surfaces inside the NTs. Another contributing factor is the more efficient light harvesting by the coaxial TiO2/ZnO NTs. Fig. 2d shows the proposed band diagram of the TiO2/ZnO heterojunction, based on their bulk band gap data. As can be seen, the ZnO and TiO2 have similar band gap, but the energy levels of the ZnO are slightly higher than that of the TiO2, which is expected to induce band bending at their interface. Such energy bias between the two sides favors photoelectrons flow towards the conduction band (CB) of TiO2 and holes diffusion towards the valance band (VB) of ZnO [18,21]. Therefore, the TiO2/ZnO nano-heterojunction shall enhance charge separation, which effect is desired in both photocatalysis and DSSC. Fig. 3a shows the I–V curves for the DSSCs derived from the 15 μm thick pure TiO2 NT and the coaxial TiO2/ZnO NT arrays. The DSSC made of the pure TiO2 NT exhibits a power-conversion efficiency (η) of 2.0%. The DSSC derived from the coaxial TiO2/ZnO NT demonstrates an improved η of 2.8%. As shown in Fig. 3a, short circuit current (Jsc) and fill factor (FF) of the coaxial TiO2/ZnO NTs are all substantially increased compared to that of the pure TiO2 NT, leading to a great enhancement of η. It is believed that the improved performance can be attributed to three factors. First, the nanoporous ZnO layer and high specific surface area of the hybrid coaxial TiO2/ZnO NTs increases the amount of the adsorbed dye. Second, the desired biased energy band structure of TiO2/ZnO nano-heterojunction enhances electron separation efficiency. Third, the ZnO layer inhibits electron back transfer from TiO2 to the redox electrolyte, which may contribute to the improvement in the FF. To further clarify the underlying factor that leads to a higher photocurrent density in the DSSCs containing coaxial TiO2/ZnO NT anodes, we measured the photocurrent action spectra of the DSSCs.

Fig. 1. (a) and (b) FESEM images of TiO2 NT with smooth walls, grown under constant-voltage conditions, (c) and (d) FESEM images of coaxial TiO2/ZnO NT after ZnO deposition.

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Fig. 2. (a) XRD patterns of TiO2 NT and coaxial TiO2/ZnO NT, (b) UV–vis diffuse reflectance spectra of the TiO2 NT and coaxial TiO2/ZnO NT, (c) photocatalytic degradation of MB (5 mg L− 1) over the TiO2 NT and coaxial TiO2/ZnO NT, and (d) schematic illustration of the band structure and charge separation in TiO2/ZnO hybrid.

The IPCE spectra illustrate that the cell using the hybrid coaxial TiO2/ZnO NT as photoelectrode has a higher photoresponse than that of the pure TiO2 NT electrode at all wavelengths (Fig. 3b).

The Nyquist plots (Fig. 3) exhibit a large semicircle at low frequencies and a small one at high frequencies, which were fitted with an equivalent circuit as shown in Fig. 3c similar to those reported in

Fig. 3. (a) Photocurrent density–voltage curves, (b) action spectra of the DSSCs with anodes made of TiO2 NT and coaxial TiO2/ZnO NT. (c) Nyquist plots, and (d) Bode phase plots.

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the literature [22–25]. The equivalent circuit consisted of Rs (ohmic resistance), Rct1 (charge transfer resistance at the counter electrode/ electrolyte), Rct2 (charge transfer resistance at the photoelectrode/ electrolyte) and Zw (resistance at the Warburg diffusion of the redox I −/I3− couple in electrolyte). In some cases, Warburg diffusion process of I −/I3− in the electrolyte is associated with a third set of semicircles. However, in our work the conventional diffusion resistance of the redox couple is apparently greatly overlapped by Rct2 due to a short length for the diffusion of the I −/I3− ions available with the thin spacer used (25 μm thick). CPE1 and CPE2 are constant phase elements of the capacitance corresponding to Rct1 and Rct2, respectively. The Bode spectra of DSSCs in Fig. 3d show that the characteristic frequency peak in the middle frequency range moves toward a lower frequency upon using the coaxial TiO2/ZnO NTs. The electrons in the coaxial TiO2/ZnO NT photoelectrodes have a longer lifetime (111 ms) than in the pure TiO2 NT photoelectrode (53 ms). The longer lifetime implies a lower recombination rate and an increased electron-collection efficiency, which explains the increased efficiency of the cell. The electron transport is extended to recombine with I −/I3− and thus increases the electron transfer resistance and increases the electron lifetime in the coaxial TiO2/ ZnO NT photoelectrodes. These consequently lead to a significantly enhanced η of the cell. 4. Conclusions In summary, we have successfully prepared coaxial TiO2/ZnO nanotube arrays by electrochemical method. Using the coaxial TiO2/ ZnO nanotube arrays as the photoanode materials, DSSC with an overall 2.8% energy-conversion efficiency was obtained, which showed a 40% improvement over the DSSC that using pure TiO2 NTs as photoanode. This improvement is mainly ascribed to the enhanced charge separation efficiency made possible by the heterojunction structure at the TiO2/ZnO interface. This study provides useful insights for the development of new nanoscale heterojunction structures of TiO2/ZnO for photocatalysis and photo-energy conversion applications.

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Acknowledgements The authors are grateful to the financial support from the National Natural Science Foundation of China (NSFC. 21073079, J0730425), the Fundamental Research Funds for the Central Universities, Chunhui Project and “111” Project.

References [1] B. O'Regan, M. Grätzel, Nature 353 (1991) 737. [2] M. Gräzel, Photochem. Photobiol. C 4 (2003) 145. [3] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Gräzel, J. Am. Chem. Soc. 127 (2005) 16835. [4] K.H. Yu, J.H. Chen, Nanoscale Res. Lett. 4 (2009) 1. [5] X.F. Gao, H.B. Li, W.T. Sun, Q. Chen, F.Q. Tang, L.M. Peng, J. Phys. Chem. C 113 (2009) 7531. [6] A. Ghicov, S.P. Albu, R. Hahn, D. Kim, T. Stergiopouls, J. Kunze, C.-A. Schiller, P. Falaras, P. Schmuki, Chem. Asian J. 4 (2009) 520. [7] P. Roy, S.P. Albu, P. Schmuki, Electrochem. Commun. 12 (2010) 949. [8] D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, J. Am. Chem. Soc. 130 (2008) 16454. [9] P. Roy, D. Kim, I. Paramasivam, P. Schmuki, Electrochem. Commun. 11 (2009) 1001. [10] D. Kim, A. Ghicov, P. Schmuki, Electrochem. Commun. 10 (2008) 1835. [11] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [12] C. Chen, Y. Xie, G. Ali, S.H. Yoo, S.O. Cho, Nanotechnology 22 (2011) 15202. [13] Q.H. Zhang, G.W. Fan, L. Gao, Appl. Catal. B 76 (2007) 168. [14] D. Barreca, E. Comini, A.P. Ferrucci, A. Gasparotto, C. Maccato, C. Maragno, G. Sberveglieri, E. Tondello, Chem. Mater. 19 (2007) 5642. [15] H.Y. Yang, S.F. Yu, S.P. Lau, X. Zhang, D.D. Sun, G. Jun, Small 5 (2009) 2260. [16] Z. Zhang, Y. Yuan, L. Liang, Y. Cheng, G. Shi, L. Jin, J. Hazard. Mater. 158 (2008) 517. [17] A. Vomiero, I. Concina, M.M. Natile, E. Comini, G. Faglia, M. Ferroni, I. Kholmanov, G. Sberveglieri, Appl. Phys. Lett. 95 (2009) 193104. [18] X. Lü, F. Huang, X. Mou, Y. Wang, F. Xu, Adv. Mater. 22 (2010) 3719. [19] M. Paulose, K. Shankar, S. Yoriya, H.E. Prakasam, O.K. Varghese, G.K. Mor, T.A. Latempa, A. Fitzgerald, C.A. Grimes, J. Phys. Chem. B 110 (2006) 16180. [20] H.-Y. Si, Z.-H. Sun, H.-L. Zhang, Colloids Surf. A 313–314 (2008) 604. [21] L.S. Zhang, K.H. Wong, Z.G. Chen, J.C. Yu, J.C. Zhao, C. Hu, C.Y. Chan, P.K. Wong, Appl. Catal. A 363 (2009) 221. [22] Z. Chen, Y. Tang, H. Yang, Y. Xia, F. Li, T. Yi, C. Huang, J. Power Sources 171 (2007) 990. [23] J. van de Lagemaat, N.-G. Park, A.J. Frank, J. Phys. Chem. B 104 (2000) 2044. [24] C. Longo, A.F. Nogueira, Marco-A De Paoli, J. Phys. Chem. B 106 (2002) 5925. [25] P. Luo, H. Niu, G. Zheng, X. Bai, M. Zhang, W. Wang, Electrochim. Acta 55 (2010) 2697.