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CdS quantum dots co-sensitized TiO2 nanotube array photoelectrode for highly efficient solar cells
Electrochimica Acta 79 (2012) 175–181
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
CdSe/CdS quantum dots co-sensitized TiO2 nanotube array photoelectrode for highly efficient solar cells Yuekun Lai a,b , Zequan Lin a , Dajiang Zheng a , Lifeng Chi b , Ronggui Du a , Changjian Lin a,∗,1 a b
State Key Laboratory of Physical Chemistry of Solid Surfaces, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Physikalisches Institute and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Münster D-48149, Germany
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Article history: Received 9 April 2012 Received in revised form 18 June 2012 Accepted 28 June 2012 Available online 5 July 2012 Keywords: TiO2 nanotube arrays (TNAs) Anodic oxidation CdS/CdSe Chemical bath deposition (CBD) Quantum dots sensitized solar cells (QDSSCs)
a b s t r a c t In this work, we designed and fabricated a novel one-dimensional CdSe/CdS@TiO2 core–shell nanotube array for quantum dots co-sensitized solar cells (QDSSCs) application. The three-component core–shell nanotube array structure was formed cascade by coating CdS nanoparticles with a successive ionic layer adsorption and reaction process and a thin CdSe layer by chemical bath deposition onto the vertical TiO2 nanotube arrays (TNAs), which enhanced the optical absorption in the visible region and presented an stepwise band-edge level structure to improve the charge separation. Under optimum conditions, the CdS/CdSe co-sensitized QDSSC demonstrated a power conversion efficiency (PCE) of 2.40% under 100 mW/cm2 illumination of simulate sunlight. Furthermore, an improved QDSSC with a PCE up to 2.74% was obtained by sealed annealing of TNAs, due to the transformation of thin and smooth nanotube to thick and rough particle nanotube. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Quantum dot sensitized solar cells (QDSSCs) have attracted great attention in the past decade because of the recent popularity of the synthesis of well-defined QDs to construct cost-effective photovoltaic solar cell [1,2]. Of particular interest are CdX (X = S, Se, and Te) QDs, which have small and size-dependent band gaps and thus provide new opportunities for harvesting light energy in the visible and infrared regions of solar light [3–5]. In addition, through the impact ionization effect, it is possible to generate multiple excitons from single-photon absorption in QDs [6,7]. In the case of the QDSSCs, excited electrons of CdX nanocrystals are injected into a large band gap semiconductor (e.g. TiO2 and ZnO), and holes are reacted with a redox couple. However, QDSSCs have not been demonstrated as an efficient inorganic dye than expected. The major challenge of improving the performance of QDSSCs is to inhibit charge recombination at the semiconductor surface. Since the new architecture of vertically aligned TiO2 nanotube arrays (TNAs) by electrochemical anodization, it has been verified to be an ideal photoanode in photoelectrocatalytic devices due to its efficient path for the transportation of photogenerated excitons [8–16]. Nevertheless, there are still some issues to be resolved
∗ Corresponding author. Tel.: +86 592 2189354; fax: +86 592 2186675. E-mail address: [email protected] (C. Lin). 1 ISE member. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.06.105
(e.g. visible light harvest and larger surface area), which limit their potential applications. Previously, Kang and co-workers prepared the CdS/CdSe coupled TiO2 nanofibrous electrode with a maximum PCE of 2.69% [17], while Shen et al. fabricated a CdSe QDs-sensitized TNAs photoelectrode with an optimal photovoltaic conversion efficiency of 1.80% [18]. To our knowledge, there are few works dedicated to the multiple semiconductor QDs-sensitized solar cells based on the TNAs [19–23]. In this work, we report a novel QDSSCs with coupled CdS/CdSe QDs-sensitized one-dimension vertically aligned TiO2 nanocrystal arrays and further improve the PCE by a sealed annealing process. For such QDSSC using CdS/CdSe QDs co-sensitized TNAs electrode, a PCE of 2.74% is achieved under optimum parameters. 2. Experimental 2.1. Preparation of TNAs and TiO2 nanocrystal decoration Titanium foils (15 mm × 10 mm of 0.1 mm thickness; 99.7% purity) were sonicated successively in acetone, ethanol and deionized water for 5 min respectively, and dried in air. The electrochemical anodization was used to prepare TNAs on Ti sheets in electrolyte of ethylene glycol containing NH4 F (0.5 wt%) and water (2 vol%) at 50 V for 3 h with a platinum foil counter electrode, similar with our previous work [24]. The obtained sample was ultrasonicated with isopropanol for 10 min and dried in air. In order to form crystalline anatase TiO2 phase, the as-prepared samples were
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Fig. 1. Top view and cross-sectional SEM images of TiO2 nanotube arrays with various deposition cycles by conventional AA treatment (a and b); CdS(5c)@TNAs (c and d); CdS(10c)@TNAs (e); CdS(15c)@TNAs (f).
treated to 450 ◦ C for 2 h by ambient annealing (AA) in air or sealed annealing (SA) in a sealed container with a heating rate of 5 ◦ C/min and natural cooling afterwards. For comparison, a common TiCl4 treatment was also applied for TiO2 nanocrystal decoration. The TiCl4 treatment was performed by immersing pre-annealed TNAs in 100 ml of 0.2 M TiCl4 aqueous solution in a 60 ◦ C oil bath for 30 min, followed by rinsing and annealing at 450 ◦ C for 30 min [25].
was added to 0.25 M sodium sulfite (Na2 SO3 ) aqueous solution, then stirred and heated at 70◦ for 10 h to obtain 0.1 M sodium selenosulfate (Na2 SeSO3 ) solution. Secondly, the CdS@TNAs sample was immersed in same volume ratio of 0.1 M Na2 SeSO3 solution and 0.08 M Cd(NO3 )2 + 0.16 M trisodium nitrilotriacetate (Na3 NTA) solution at 5 ◦ C for a certain time to form cascade co-sensitized CdSe/CdS@TNAs electrodes.
2.2. Synthesis of CdS@TNA and CdSe/CdS@TNA composites
2.3. Characterizations and measurements
CdS QDs were assembled into TNAs by a successive ionic layer adsorption and reaction (SILAR) process similar to previously reported approach [26]. Typically, the TNA samples were immersed successively in 0.4 M Cd(NO3 )2 ethanol solution and 0.1 M Na2 S methanol solution for 2 min, respectively. Between each immersion step, the samples were rinsed with ethanol adequately to remove excess ions that were in the TNAs. Such operation process was repeated for several cycles, and after finishing the coating, the samples were rinsed with deionized water. With the aim of improving the stability and PEC efficiency of the constructed photoanodes, a thin shell of CdSe QDs was coated onto the surface of CdS@TNAs core by a chemical bath deposition (CBD) process with the following processes. Firstly, 0.1 M selenium (Se) powder
Platinum (Pt) counter electrode was prepared by dropping 2 mg/ml H2 PtCl6 isopropanol solution on FTO glass, followed by heating at 450 ◦ C for 15 min. The redox electrolyte used in the study was a mixture of methanol and water (v/v = 30/70) containing 0.6 M Na2 S, 0.2 M sulfur powder (S) and 0.2 M KCl. The electrolyte was injected between two electrodes and driven by capillary force through the hole on the hot-melt sealed film. As the Ti foil was not transparent, light had to enter the cell through the Pt-coated FTO glass, yielding QDs-sensitized hierarchically structured TiO2 nanotube solar cells in a backside illumination mode. The current–voltage characteristics were measured with Oriel I–V test station by Newport. A solar simulator (SoLux Solar Simulator) was used to simulate sunlight for an illumination intensity
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Fig. 2. Top view and cross-sectional SEM images of CdSe(30 h)/CdS(5c)@TNAs sample by conventional AA treatment (a and b); EDX spectrum of CdSe(30 h)/CdS(5c)@TNAs sample (c); XRD spectra of corresponding electrodes (d). A, T, and C represent anatase TiO2 , Ti and CdSe, respectively.
of 100 mW/cm2 as calibrated with a Daystar Meter. The morphology of the prepared samples was observed using field-emission scanning electron microscopy (FE-SEM, S4800). The crystal structure was analyzed with an X-ray diffractometer (Philips, Panalytical X’pert, CuK␣ radiation). The absorption properties of the nanotube array samples were investigated using a diffuse reflectance UV–vis spectrometer (Varian, Cary 5000).
3. Results and discussion Fig. 1a and b shows a typical SEM image of the TNAs fabricated by electrochemical anodization. Compared to the electrolyte of HF (0.5 wt%) aqueous solution, ethylene glycol organic solution containing NH4 F (0.5 wt%) and water (2 vol%) was found to dramatically increase the thickness and growth rate of TiO2 nanotube [27]. The average inner diameter and the tube length of these TNAs are about 120 nm and 22 m, respectively. Fig. 1c and d presents the SEM image of CdS(5c) QDs on TNAs obtained after 5 cycles of SILAR process. The ethanol solutions with lower surface tension allow CdS QDs to uniformly form on the surface of TNAs, therefore a core–shell structured CdS@TNAs composite can be successfully constructed. The average diameter of CdS QDs on both outside and inside nanotube wall is about 4 nm. With the increase of SILAR cycles (Fig. 1e), the increase in CdS QDs deposited on the nanotube wall leads to the obvious decrease in inner diameter for the composited CdS(10c)@TNAs nanotube. When the SILAR
process increases up to 15 cycles (Fig. 1f), a large ratio of TiO2 nanotubes is covered by the aggregation of the CdS particles. Thus, the CdS(5c)@TNAs sample was chosen to conduct the CdSe adsorption time study as discussed below. After CdSe coating by a CBD process for 30 h (Fig. 2a), the nanoparticles size and tube diameter increase to about 40 nm and 170 nm, respectively. The cross-sectional image clearly indicates that the TiO2 nanotube wall is fully coated with uniform CdSe/CdS QDs (Fig. 2b). The corresponding EDX spectrum in Fig. 2c reveals that the obtained thin films are composed of Ti, O, Cd, Se and S. The XRD spectra indicate the successfully coating of CdSe nanocrystals on the CdS(5c)@TNAs sample (Fig. 2d). Except of peaks from anatase TiO2 and Ti substrate with labels of A and T, it only shows two broad peaks, C(2 2 0) and C(3 1 1), belonging to a single cubic phase of CdSe in accordance with the data reported in ASTM cards (JCPDF Card No 19-191). It is difficult to collect the signal of the crystalline phase of CdS particles due to the small and uniform CdS quantum dots distributed on the TNAs. The UV–vis diffuse reflection spectra of TNAs and corresponding QDs-sensitized TNAs are displayed in Fig. 3. It is apparent that QDs-sensitized TNAs samples exhibited higher light absorption in both UV and visible light regions. This is ascribed to the light trapping of multi-layer composite structures and the complementary effect in visible light harvest of low band gap QDs. The broad absorption peaks of the TNAs (curve 1) and CdS(5c)@TNAs (curve 2) may be attributed to the sub-band gap state of the special tube structures [28–30]. Table 1 summarizes the device performance of the resulting CdS/CdSe core–shell QDs-sensitized TNA solar cells by AA
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Fig. 3. Diffuse reflection spectra. (1) TNAs; (2) CdS(5c)@TNAs; (3) CdSe (30 h)/CdS(5c)@TNAs.
treatment. The current–voltage (J–V) characteristics of representative samples are shown in Fig. 4. We found that all coupled CdS/CdSe QDs-sensitized hybrid electrodes (Fig. 4b) exhibited a greatly enhanced short-circuit current density (Jsc ) as compared to pure TNAs and single CdS or CdSe QDs-sensitized TNAs electrodes (Fig. 4a). This is due to the combination of CdS and CdSe QDs has a complementary effect in light harvest, surface area increment and a stepwise band-edge level structure of CdSe/CdS@TNAs beneficiated for electron injection into vertical TiO2 nanotube [31]. When only CdS(5c) or CdSe(30 h) was used as sensitizer, the short circuit current densities are 0.62 and 1.48 mA/cm2 , respectively. For the CdSe(30 h)/CdS(5c) co-sensitized TNAs, the current density markedly increases to 10.82 mA/cm2 . We found that double the electrolyte concentration just slightly increase the current density of CdSe(30 h)/CdS(5c)@TNAs electrode from 10.82 to 11.37 mA/cm2 (see supporting information of Fig. S1 and Table S1). Therefore, the increase in current density can be attributed to the considerably decreased charge transfer resistance at the interface of the various electrodes and polysulfide electrolyte. In case of the CdSe and CdS(5c) co-sensitized TNAs electrodes by conventional AA, the PCE firstly increased with the increase of the CdSe growth duration and the highest performance (PCE ≈ 2.20%) was achieved from the CdSe(30 h)/CdS(5c)@TNAs photoanode with a CdSe growth duration of 30 h. Further increase the CdSe coating duration, the PCE decrease. This is attributed primarily to the thicker CdSe layer on CdS nanoparticles not only affected the electron transport but also retarded the infiltration of electrolyte into nanotubes. To further improve the solar cell efficiency, a common and widely used TiCl4 pre-treatment process was applied to coat TiO2 nanoparticles on TNAs surface. Fig. 5a and b shows the morphology of the nanotube sample after TiCl4 treatment. It can be seen that Table 1 Photovoltaic parameters of single or coupled QDs-sensitized TNAs solar cells by AA treatment. Sensitizers
Voc (V)
Jsc (mA/cm2 )
Fill factor
Efficiency (%)
Non CdS(5c) CdSe(30 h) CdSe(10 h)/CdS(5c) CdSe(15 h)/CdS(5c) CdSe(20 h)/CdS(5c) CdSe(25 h)/CdS(5c) CdSe(30 h)/CdS(5c) CdSe(35 h)/CdS(5c)
0.27 0.78 0.50 0.51 0.54 0.52 0.54 0.53 0.51
0.28 0.62 1.48 6.23 7.51 9.18 9.82 10.82 13.19
0.19 0.28 0.27 0.30 0.34 0.35 0.36 0.38 0.32
0.014 0.14 0.093 0.94 1.37 1.70 1.93 2.20 2.09
Fig. 4. (a) J–V characteristics of the TNAs electrode and the single CdS(5c) or CdSe(30 h) QDs modified TNAs electrodes. (b) Effect of the CdSe growth duration on J–V characteristics of the coupled CdS(5c)/CdSe QDs-sensitized TNAs photoelectrodes by conventional AA treatment.
uniform TiO2 nanoparticles were successfully decorate on smooth nanotube wall. The wall thickness increased to about 35 nm while the pore size reduced to approximately 110 nm after TiCl4 treatment. It has been reported that TiO2 nanoparticles were produced due to the hydrolysis of Ti4+ ions and decorated the nanotube wall [32]. Similarly, the wall thickness further increase with the coat of CdS(5c) and CdSe(30 h) quantum dots on the TiCl4 pretreated TNA film (Fig. 5c and d). The PCE of the modified CdSe/CdS@TNAs electrode (Fig. 6), exhibiting an open circuit voltage, Voc of 0.59 V, a short circuit current, Jsc of 14.60 mA/cm2 , a fill factor of 0.35, and a PCE of 2.40%, about 9.1% increase in PCE as compared to that of the CdSe/CdS@TNAs electrode without the treatment of TiCl4 (2.20%). This is ascribed to the increase in surface area and decrease in recombination centers with the introduction of TiO2 nanocrystals by TiCl4 treatment. To improve the photoelectrochemical performance of solar cells, a developed SA process was applied to one-step construct TiO2 nanocrystals on TNAs (Fig. 7a and b). After SA treatment the smooth TiO2 nanotubes became comparatively rough and covered by a well-defined nanocrystal as revealed by the SEM measurement (Fig. 7a). The cross-sectional SEM image showed that the nanotubular wall was decorated with substantial nanoparticles (Fig. 7b), forming hierarchically structured nanotube arrays. The average size of the formed TiO2 nanoparticles on top nanotube surface and inner nanotube wall is about 60 nm and 30 nm, respectively. It is noteworthy that the wall thickness increased from 10 nm to 60 nm after
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Fig. 5. Top view and cross-sectional SEM images of TNAs with TiCl4 treatment (a and b), CdSe(30 h)/CdS(5c) QDs-sensitized TiCl4 treated TNAs (c and d).
SA treatment, and meanwhile the inner tube diameter approximately decreased from 120 nm to 30 nm. We proposed that in the SA process, the TiO2 nanoparticles were mainly evolved from the smooth nanotubes via a catalytic reaction and capping agent stabilization of fluorine ion residues in long nanotubes during the anodizing process in fluorine ions containing electrolyte [33–37]. This multilayer-structured film is similar to that treated by TiCl4 and have been verified for efficient internal light trapping, fast electron transport and fluent redox diffusion [38–40]. After coupled CdSe(40 h)/CdS(5c) QDs-sensitization (Fig. 7c and d), the particle size on the top of TNA surface and the nanotube wall thickness further increased. The combined SA pre-treated and CdSe/CdS QDs co-sensitized TNAs were then utilized as photoanodes to assemble QDSSCs,
and measured the cell performance (Fig. 8). Table 2 summarized the photovoltaic parameters of coupled CdSe/CdS(5c) QDs cosensitized TNAs solar cells as a function of CdSe QDs adsorption duration with the SA treatment of TNAs. We found that the parameter of short-circuit current density (Jsc ) sharply increased with the increase in CdSe QDs adsorption times up to 30 h and slowly approached a platform for 40 h. However, the increase in adsorption times resulted in deterioration of open-circuit voltage and fill factor after a certain adsorption time. High CBD duration of CdSe QDs coating might cause an increase in recombination centers, poor penetration of electrolyte and the decrease in surface area. Under optimum parameters, the QDSSC assembled with a CBD duration of CdSe QDs for 40 h exhibited a short-circuit current density of 14.30 mA/cm2 , an open-circuit voltage of 0.58 V, a fill factor of 0.33, and a PCE of 2.74%, this represented a 25% increase in PCE as compared to that of the optimal photoanode by AA treatment (2.20%) and even surpass that of the similar TiO2 nanoparticles coated nanotubes composite structures prepared by TiCl4 treatment (2.40%). This result was caused by the excellent scattering property of the hybrid QDs on hierarchical TiO2 structures with a higher surface area by SA which had better light capture ability inside the particles coated nanotube than smooth nanotube by AA. It is expected that further improvements in the cell efficiency should be possible with longer CdSe growth duration.
Table 2 Photovoltaic parameters of coupled QDs-sensitized TNAs solar cells by SA treatment.
Fig. 6. J–V characteristics of coupled CdSe(30 h)/CdS(5c) QDs-sensitized TNAs photoelectrodes with or without TiCl4 treatment by conventional AA treatment. The inset shows the detail parameters of the coupled CdSe(30 h)/CdS(5c)@TNAs photoelectrodes with or without TiCl4 treatment.
Sensitizers
Voc (V)
Jsc (mA/cm2 )
Fill factor
Efficiency (%)
CdSe(15 h)/CdS(5c) CdSe(20 h)/CdS(5c) CdSe(25 h)/CdS(5c) CdSe(30 h)/CdS(5c) CdSe(35 h)/CdS(5c) CdSe(40 h)/CdS(5c)
0.49 0.52 0.54 0.57 0.59 0.58
9.95 11.47 12.37 13.57 14.19 14.30
0.33 0.34 0.32 0.31 0.31 0.33
1.65 2.02 2.16 2.38 2.60 2.74
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Fig. 7. Top view and cross-sectional SEM images of pure TNAs by SA treatment (a and b), CdSe(40 h)/CdS(5c)@TNAs composited sample by SA pretreatment (c and d).
Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (51072170, 21021002), the National High Technology Research and Development Program of China (2009AA03Z327), the National Basic Research Program of China (2012CB932900), and the Alexander von Humboldt (AvH) Foundation of Germany. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2012.06.105. References Fig. 8. Effect of CdSe growth duration on J–V characteristics of coupled CdS/CdSe QDs-sensitized TNAs photoanodes by SA treatment.
4. Conclusion In summary, uniform CdS and CdSe QDs were successfully assembled on TNAs to construct CdS/CdSe core–shell co-sensitized QDSSCs. A developed SA treatment of TNAs resulted in a morphology change from thin TiO2 nanotube wall to well-defined nanocrystal tube wall due to the presence of fluorine ion residues in long nanotubes during the anodization in fluorine containing electrolyte. Moreover, an optimum PCE of up to 2.74% can be achieved by the CdSe(40 h)/CdS@TNAs sample with SA treatment under simulated AM 1.5 G irradiation of 100 mW/cm2 , a 25% increase in PCE as compared to the optimal CdSe(30 h)/CdS@TNAs sample by conventional AA treatment and even higher than that of the best CdSe(30 h)/CdS@TNAs photoanode with conventional TiCl4 pretreatment.
[1] Z.S. Yang, C.Y. Chen, P. Roy, H.T. Chang, Chemical Communications 47 (2011) 9561. [2] P.V. Kamat, Journal of Physical Chemistry C 112 (2008) 18737. [3] W.T. Sun, Y. Yu, H.Y. Pan, X.F. Gao, Q. Chen, L.M. Peng, Journal of the American Chemical Society 130 (2008) 1124. [4] M. Yang, N.K. Shrestha, P. Schmuki, Electrochimica Acta 55 (2010) 7766. [5] J.Z. Zhang, Journal of Physical Chemistry B 104 (2000) 7239. [6] A.J. Nozik, Physica E 14 (2002) 115. [7] V.I. Klimov, D.W. McBranch, Physical Review Letters 80 (1998) 4028. [8] J.J. Gong, C.J. Lin, M.D. Ye, Y.K. Lai, Chemical Communications 47 (2011) 2598. [9] W.X. Guo, C. Xu, X. Wang, S.H. Wang, C.F. Pan, C.J. Lin, Z.L. Wang, Journal of the American Chemical Society 134 (2012) 4437. [10] K.S. Raja, V.K. Mahajan, M. Misra, Journal of Power Sources 159 (2006) 1258. [11] P. Pillai, K.S. Raja, M. Misra, Journal of Power Sources 161 (2006) 524. [12] J.M. Macak, H. Tsuchiya, A. Ghicov, P. Schmuki, Electrochemistry Communications 7 (2005) 1133. [13] D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, Journal of the American Chemical Society 130 (2008) 16454. [14] J. Yoon, S. Bae, E. Shim, H. Joo, Journal of Power Sources 189 (2009) 1296. [15] Y.K. Lai, H.F. Zhuang, L. Sun, Z. Chen, C.J. Lin, Electrochimica Acta 54 (2009) 6536. [16] A. Ghicov, S.P. Albu, R. Hahn, D. Kim, T. Stergiopoulos, J. Kunze, C.A. Schiller, P. Falaras, P. Schmuki, Chemistry – An Asian Journal 4 (2009) 520. [17] P. Sudhagar, J.H. Jung, S. Park, Y.G. Lee, R. Sathyamoorthy, Y.S. Kang, H. Ahn, Electrochemistry Communications 11 (2009) 2220. [18] Q. Shen, A. Yamada, S. Tamura, T. Toyoda, Applied Physics Letters 97 (2010) 123107.
Y. Lai et al. / Electrochimica Acta 79 (2012) 175–181 [19] S.L. Cheng, W.Y. Fu, B.Y. Hai, L.N. Yhang, J.W. Ma, H. Zhao, M.L. Sun, L.H. Yang, Journal of Physical Chemistry C 116 (2012) 2615. [20] J.F. Yan, F. Zhou, Journal of Materials Chemistry 26 (2011) 9406. [21] X.F. Guan, S.Q. Huang, Q.X. Zhang, X. Shen, H.C. Sun, D.M. Li, Y.H. Luo, R.C. Yu, Q.B. Meng, Nanotechnology 22 (2011) 465402. [22] S.Q. Huang, Q.X. Zhang, X.M. Huang, X.Z. Guo, M.H. Deng, D.M. Li, Y.H. Luo, Q. Shen, T. Toyoda, Q.B. Meng, Nanotechnology 21 (2010) 375201. [23] X.F. Gao, W.T. Sun, G. Ai, L.M. Peng, Applied Physics Letters 96 (2010) 153104. [24] J.J. Gong, Y.K. Lai, C.J. Lin, Electrochimica Acta 55 (2010) 4776. [25] M.D. Ye, X.K. Xin, C.J. Lin, Z.Q. Lin, Nano Letters 11 (2011) 3214. [26] Z.Q. Lin, Y.K. Lai, R.G. Hu, J. Li, R.G. Du, C.J. Lin, Electrochimica Acta 55 (2010) 8717. [27] Y.K. Lai, X.F. Gao, H.F. Zhuang, J.Y. Huang, C.J. Lin, L. Jiang, Advanced Materials 21 (2009) 3799. [28] D.W. Bahnemann, R. Dillert, P.K.J. Robertson, Chemical Physics of Nanostructured Semicondutors, XSP, BV, Eindhoven, 2003. [29] Y.K. Lai, L. Sun, Y.C. Chen, H.F. Zhuang, C.J. Lin, J.W. Chin, Journal of the Electrochemical Society 153 (2006) D123.
181
[30] Y.K. Lai, J.Y. Huang, H.F. Zhang, V.P. Subramaniam, Y.X. Tang, D.G. Gong, L. Sundar, L. Sun, Z. Chen, C.J. Lin, Journal of Hazardous Materials 184 (2010) 855. [31] Y.L. Lee, C.F. Chi, S.Y. Liau, Chemistry of Materials 22 (2010) 922. [32] C.C. Chen, H.W. Chung, C.H. Chen, H.P. Lu, C.M. Lan, S.F. Chen, L. Luo, C.S. Hung, E.W.G. Diau, Journal of Physical Chemistry C 112 (2008) 19151. [33] Y. Alivov, Z.Y. Fan, Journal of Physical Chemistry C 113 (2009) 12954. [34] Y. Alivov, Z.Y. Fan, Nanotechnology 20 (2009) 405610. [35] S.W. Liu, J.G. Yu, M. Jaroniec, Chemistry of Materials 23 (2011) 4085. [36] L. Chen, L.F. Shen, P. Nie, X.G. Zhang, H.S. Li, Electrochimica Acta 62 (2012) 408. [37] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Nature 453 (2008) 638. [38] Y.K. Lai, J.J. Gong, C.J. Lin, International Journal of Hydrogen Energy 37 (2012) 6438. [39] W.K. Tu, C.J. Lin, A. Chatterjee, G.H. Shiau, S.H. Chien, Journal of Power Sources 203 (2012) 297. [40] C.J. Lin, W.K. Tu, C.K. Kuo, S.H. Chien, Journal of Power Sources 196 (2011) 4865.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
CdSe/CdS quantum dots co-sensitized TiO2 nanotube array photoelectrode for highly efficient solar cells Yuekun Lai a,b , Zequan Lin a , Dajiang Zheng a , Lifeng Chi b , Ronggui Du a , Changjian Lin a,∗,1 a b
State Key Laboratory of Physical Chemistry of Solid Surfaces, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Physikalisches Institute and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Münster D-48149, Germany
a r t i c l e
i n f o
Article history: Received 9 April 2012 Received in revised form 18 June 2012 Accepted 28 June 2012 Available online 5 July 2012 Keywords: TiO2 nanotube arrays (TNAs) Anodic oxidation CdS/CdSe Chemical bath deposition (CBD) Quantum dots sensitized solar cells (QDSSCs)
a b s t r a c t In this work, we designed and fabricated a novel one-dimensional CdSe/CdS@TiO2 core–shell nanotube array for quantum dots co-sensitized solar cells (QDSSCs) application. The three-component core–shell nanotube array structure was formed cascade by coating CdS nanoparticles with a successive ionic layer adsorption and reaction process and a thin CdSe layer by chemical bath deposition onto the vertical TiO2 nanotube arrays (TNAs), which enhanced the optical absorption in the visible region and presented an stepwise band-edge level structure to improve the charge separation. Under optimum conditions, the CdS/CdSe co-sensitized QDSSC demonstrated a power conversion efficiency (PCE) of 2.40% under 100 mW/cm2 illumination of simulate sunlight. Furthermore, an improved QDSSC with a PCE up to 2.74% was obtained by sealed annealing of TNAs, due to the transformation of thin and smooth nanotube to thick and rough particle nanotube. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Quantum dot sensitized solar cells (QDSSCs) have attracted great attention in the past decade because of the recent popularity of the synthesis of well-defined QDs to construct cost-effective photovoltaic solar cell [1,2]. Of particular interest are CdX (X = S, Se, and Te) QDs, which have small and size-dependent band gaps and thus provide new opportunities for harvesting light energy in the visible and infrared regions of solar light [3–5]. In addition, through the impact ionization effect, it is possible to generate multiple excitons from single-photon absorption in QDs [6,7]. In the case of the QDSSCs, excited electrons of CdX nanocrystals are injected into a large band gap semiconductor (e.g. TiO2 and ZnO), and holes are reacted with a redox couple. However, QDSSCs have not been demonstrated as an efficient inorganic dye than expected. The major challenge of improving the performance of QDSSCs is to inhibit charge recombination at the semiconductor surface. Since the new architecture of vertically aligned TiO2 nanotube arrays (TNAs) by electrochemical anodization, it has been verified to be an ideal photoanode in photoelectrocatalytic devices due to its efficient path for the transportation of photogenerated excitons [8–16]. Nevertheless, there are still some issues to be resolved
∗ Corresponding author. Tel.: +86 592 2189354; fax: +86 592 2186675. E-mail address: [email protected] (C. Lin). 1 ISE member. 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.06.105
(e.g. visible light harvest and larger surface area), which limit their potential applications. Previously, Kang and co-workers prepared the CdS/CdSe coupled TiO2 nanofibrous electrode with a maximum PCE of 2.69% [17], while Shen et al. fabricated a CdSe QDs-sensitized TNAs photoelectrode with an optimal photovoltaic conversion efficiency of 1.80% [18]. To our knowledge, there are few works dedicated to the multiple semiconductor QDs-sensitized solar cells based on the TNAs [19–23]. In this work, we report a novel QDSSCs with coupled CdS/CdSe QDs-sensitized one-dimension vertically aligned TiO2 nanocrystal arrays and further improve the PCE by a sealed annealing process. For such QDSSC using CdS/CdSe QDs co-sensitized TNAs electrode, a PCE of 2.74% is achieved under optimum parameters. 2. Experimental 2.1. Preparation of TNAs and TiO2 nanocrystal decoration Titanium foils (15 mm × 10 mm of 0.1 mm thickness; 99.7% purity) were sonicated successively in acetone, ethanol and deionized water for 5 min respectively, and dried in air. The electrochemical anodization was used to prepare TNAs on Ti sheets in electrolyte of ethylene glycol containing NH4 F (0.5 wt%) and water (2 vol%) at 50 V for 3 h with a platinum foil counter electrode, similar with our previous work [24]. The obtained sample was ultrasonicated with isopropanol for 10 min and dried in air. In order to form crystalline anatase TiO2 phase, the as-prepared samples were
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Fig. 1. Top view and cross-sectional SEM images of TiO2 nanotube arrays with various deposition cycles by conventional AA treatment (a and b); CdS(5c)@TNAs (c and d); CdS(10c)@TNAs (e); CdS(15c)@TNAs (f).
treated to 450 ◦ C for 2 h by ambient annealing (AA) in air or sealed annealing (SA) in a sealed container with a heating rate of 5 ◦ C/min and natural cooling afterwards. For comparison, a common TiCl4 treatment was also applied for TiO2 nanocrystal decoration. The TiCl4 treatment was performed by immersing pre-annealed TNAs in 100 ml of 0.2 M TiCl4 aqueous solution in a 60 ◦ C oil bath for 30 min, followed by rinsing and annealing at 450 ◦ C for 30 min [25].
was added to 0.25 M sodium sulfite (Na2 SO3 ) aqueous solution, then stirred and heated at 70◦ for 10 h to obtain 0.1 M sodium selenosulfate (Na2 SeSO3 ) solution. Secondly, the CdS@TNAs sample was immersed in same volume ratio of 0.1 M Na2 SeSO3 solution and 0.08 M Cd(NO3 )2 + 0.16 M trisodium nitrilotriacetate (Na3 NTA) solution at 5 ◦ C for a certain time to form cascade co-sensitized CdSe/CdS@TNAs electrodes.
2.2. Synthesis of CdS@TNA and CdSe/CdS@TNA composites
2.3. Characterizations and measurements
CdS QDs were assembled into TNAs by a successive ionic layer adsorption and reaction (SILAR) process similar to previously reported approach [26]. Typically, the TNA samples were immersed successively in 0.4 M Cd(NO3 )2 ethanol solution and 0.1 M Na2 S methanol solution for 2 min, respectively. Between each immersion step, the samples were rinsed with ethanol adequately to remove excess ions that were in the TNAs. Such operation process was repeated for several cycles, and after finishing the coating, the samples were rinsed with deionized water. With the aim of improving the stability and PEC efficiency of the constructed photoanodes, a thin shell of CdSe QDs was coated onto the surface of CdS@TNAs core by a chemical bath deposition (CBD) process with the following processes. Firstly, 0.1 M selenium (Se) powder
Platinum (Pt) counter electrode was prepared by dropping 2 mg/ml H2 PtCl6 isopropanol solution on FTO glass, followed by heating at 450 ◦ C for 15 min. The redox electrolyte used in the study was a mixture of methanol and water (v/v = 30/70) containing 0.6 M Na2 S, 0.2 M sulfur powder (S) and 0.2 M KCl. The electrolyte was injected between two electrodes and driven by capillary force through the hole on the hot-melt sealed film. As the Ti foil was not transparent, light had to enter the cell through the Pt-coated FTO glass, yielding QDs-sensitized hierarchically structured TiO2 nanotube solar cells in a backside illumination mode. The current–voltage characteristics were measured with Oriel I–V test station by Newport. A solar simulator (SoLux Solar Simulator) was used to simulate sunlight for an illumination intensity
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Fig. 2. Top view and cross-sectional SEM images of CdSe(30 h)/CdS(5c)@TNAs sample by conventional AA treatment (a and b); EDX spectrum of CdSe(30 h)/CdS(5c)@TNAs sample (c); XRD spectra of corresponding electrodes (d). A, T, and C represent anatase TiO2 , Ti and CdSe, respectively.
of 100 mW/cm2 as calibrated with a Daystar Meter. The morphology of the prepared samples was observed using field-emission scanning electron microscopy (FE-SEM, S4800). The crystal structure was analyzed with an X-ray diffractometer (Philips, Panalytical X’pert, CuK␣ radiation). The absorption properties of the nanotube array samples were investigated using a diffuse reflectance UV–vis spectrometer (Varian, Cary 5000).
3. Results and discussion Fig. 1a and b shows a typical SEM image of the TNAs fabricated by electrochemical anodization. Compared to the electrolyte of HF (0.5 wt%) aqueous solution, ethylene glycol organic solution containing NH4 F (0.5 wt%) and water (2 vol%) was found to dramatically increase the thickness and growth rate of TiO2 nanotube [27]. The average inner diameter and the tube length of these TNAs are about 120 nm and 22 m, respectively. Fig. 1c and d presents the SEM image of CdS(5c) QDs on TNAs obtained after 5 cycles of SILAR process. The ethanol solutions with lower surface tension allow CdS QDs to uniformly form on the surface of TNAs, therefore a core–shell structured CdS@TNAs composite can be successfully constructed. The average diameter of CdS QDs on both outside and inside nanotube wall is about 4 nm. With the increase of SILAR cycles (Fig. 1e), the increase in CdS QDs deposited on the nanotube wall leads to the obvious decrease in inner diameter for the composited CdS(10c)@TNAs nanotube. When the SILAR
process increases up to 15 cycles (Fig. 1f), a large ratio of TiO2 nanotubes is covered by the aggregation of the CdS particles. Thus, the CdS(5c)@TNAs sample was chosen to conduct the CdSe adsorption time study as discussed below. After CdSe coating by a CBD process for 30 h (Fig. 2a), the nanoparticles size and tube diameter increase to about 40 nm and 170 nm, respectively. The cross-sectional image clearly indicates that the TiO2 nanotube wall is fully coated with uniform CdSe/CdS QDs (Fig. 2b). The corresponding EDX spectrum in Fig. 2c reveals that the obtained thin films are composed of Ti, O, Cd, Se and S. The XRD spectra indicate the successfully coating of CdSe nanocrystals on the CdS(5c)@TNAs sample (Fig. 2d). Except of peaks from anatase TiO2 and Ti substrate with labels of A and T, it only shows two broad peaks, C(2 2 0) and C(3 1 1), belonging to a single cubic phase of CdSe in accordance with the data reported in ASTM cards (JCPDF Card No 19-191). It is difficult to collect the signal of the crystalline phase of CdS particles due to the small and uniform CdS quantum dots distributed on the TNAs. The UV–vis diffuse reflection spectra of TNAs and corresponding QDs-sensitized TNAs are displayed in Fig. 3. It is apparent that QDs-sensitized TNAs samples exhibited higher light absorption in both UV and visible light regions. This is ascribed to the light trapping of multi-layer composite structures and the complementary effect in visible light harvest of low band gap QDs. The broad absorption peaks of the TNAs (curve 1) and CdS(5c)@TNAs (curve 2) may be attributed to the sub-band gap state of the special tube structures [28–30]. Table 1 summarizes the device performance of the resulting CdS/CdSe core–shell QDs-sensitized TNA solar cells by AA
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Fig. 3. Diffuse reflection spectra. (1) TNAs; (2) CdS(5c)@TNAs; (3) CdSe (30 h)/CdS(5c)@TNAs.
treatment. The current–voltage (J–V) characteristics of representative samples are shown in Fig. 4. We found that all coupled CdS/CdSe QDs-sensitized hybrid electrodes (Fig. 4b) exhibited a greatly enhanced short-circuit current density (Jsc ) as compared to pure TNAs and single CdS or CdSe QDs-sensitized TNAs electrodes (Fig. 4a). This is due to the combination of CdS and CdSe QDs has a complementary effect in light harvest, surface area increment and a stepwise band-edge level structure of CdSe/CdS@TNAs beneficiated for electron injection into vertical TiO2 nanotube [31]. When only CdS(5c) or CdSe(30 h) was used as sensitizer, the short circuit current densities are 0.62 and 1.48 mA/cm2 , respectively. For the CdSe(30 h)/CdS(5c) co-sensitized TNAs, the current density markedly increases to 10.82 mA/cm2 . We found that double the electrolyte concentration just slightly increase the current density of CdSe(30 h)/CdS(5c)@TNAs electrode from 10.82 to 11.37 mA/cm2 (see supporting information of Fig. S1 and Table S1). Therefore, the increase in current density can be attributed to the considerably decreased charge transfer resistance at the interface of the various electrodes and polysulfide electrolyte. In case of the CdSe and CdS(5c) co-sensitized TNAs electrodes by conventional AA, the PCE firstly increased with the increase of the CdSe growth duration and the highest performance (PCE ≈ 2.20%) was achieved from the CdSe(30 h)/CdS(5c)@TNAs photoanode with a CdSe growth duration of 30 h. Further increase the CdSe coating duration, the PCE decrease. This is attributed primarily to the thicker CdSe layer on CdS nanoparticles not only affected the electron transport but also retarded the infiltration of electrolyte into nanotubes. To further improve the solar cell efficiency, a common and widely used TiCl4 pre-treatment process was applied to coat TiO2 nanoparticles on TNAs surface. Fig. 5a and b shows the morphology of the nanotube sample after TiCl4 treatment. It can be seen that Table 1 Photovoltaic parameters of single or coupled QDs-sensitized TNAs solar cells by AA treatment. Sensitizers
Voc (V)
Jsc (mA/cm2 )
Fill factor
Efficiency (%)
Non CdS(5c) CdSe(30 h) CdSe(10 h)/CdS(5c) CdSe(15 h)/CdS(5c) CdSe(20 h)/CdS(5c) CdSe(25 h)/CdS(5c) CdSe(30 h)/CdS(5c) CdSe(35 h)/CdS(5c)
0.27 0.78 0.50 0.51 0.54 0.52 0.54 0.53 0.51
0.28 0.62 1.48 6.23 7.51 9.18 9.82 10.82 13.19
0.19 0.28 0.27 0.30 0.34 0.35 0.36 0.38 0.32
0.014 0.14 0.093 0.94 1.37 1.70 1.93 2.20 2.09
Fig. 4. (a) J–V characteristics of the TNAs electrode and the single CdS(5c) or CdSe(30 h) QDs modified TNAs electrodes. (b) Effect of the CdSe growth duration on J–V characteristics of the coupled CdS(5c)/CdSe QDs-sensitized TNAs photoelectrodes by conventional AA treatment.
uniform TiO2 nanoparticles were successfully decorate on smooth nanotube wall. The wall thickness increased to about 35 nm while the pore size reduced to approximately 110 nm after TiCl4 treatment. It has been reported that TiO2 nanoparticles were produced due to the hydrolysis of Ti4+ ions and decorated the nanotube wall [32]. Similarly, the wall thickness further increase with the coat of CdS(5c) and CdSe(30 h) quantum dots on the TiCl4 pretreated TNA film (Fig. 5c and d). The PCE of the modified CdSe/CdS@TNAs electrode (Fig. 6), exhibiting an open circuit voltage, Voc of 0.59 V, a short circuit current, Jsc of 14.60 mA/cm2 , a fill factor of 0.35, and a PCE of 2.40%, about 9.1% increase in PCE as compared to that of the CdSe/CdS@TNAs electrode without the treatment of TiCl4 (2.20%). This is ascribed to the increase in surface area and decrease in recombination centers with the introduction of TiO2 nanocrystals by TiCl4 treatment. To improve the photoelectrochemical performance of solar cells, a developed SA process was applied to one-step construct TiO2 nanocrystals on TNAs (Fig. 7a and b). After SA treatment the smooth TiO2 nanotubes became comparatively rough and covered by a well-defined nanocrystal as revealed by the SEM measurement (Fig. 7a). The cross-sectional SEM image showed that the nanotubular wall was decorated with substantial nanoparticles (Fig. 7b), forming hierarchically structured nanotube arrays. The average size of the formed TiO2 nanoparticles on top nanotube surface and inner nanotube wall is about 60 nm and 30 nm, respectively. It is noteworthy that the wall thickness increased from 10 nm to 60 nm after
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Fig. 5. Top view and cross-sectional SEM images of TNAs with TiCl4 treatment (a and b), CdSe(30 h)/CdS(5c) QDs-sensitized TiCl4 treated TNAs (c and d).
SA treatment, and meanwhile the inner tube diameter approximately decreased from 120 nm to 30 nm. We proposed that in the SA process, the TiO2 nanoparticles were mainly evolved from the smooth nanotubes via a catalytic reaction and capping agent stabilization of fluorine ion residues in long nanotubes during the anodizing process in fluorine ions containing electrolyte [33–37]. This multilayer-structured film is similar to that treated by TiCl4 and have been verified for efficient internal light trapping, fast electron transport and fluent redox diffusion [38–40]. After coupled CdSe(40 h)/CdS(5c) QDs-sensitization (Fig. 7c and d), the particle size on the top of TNA surface and the nanotube wall thickness further increased. The combined SA pre-treated and CdSe/CdS QDs co-sensitized TNAs were then utilized as photoanodes to assemble QDSSCs,
and measured the cell performance (Fig. 8). Table 2 summarized the photovoltaic parameters of coupled CdSe/CdS(5c) QDs cosensitized TNAs solar cells as a function of CdSe QDs adsorption duration with the SA treatment of TNAs. We found that the parameter of short-circuit current density (Jsc ) sharply increased with the increase in CdSe QDs adsorption times up to 30 h and slowly approached a platform for 40 h. However, the increase in adsorption times resulted in deterioration of open-circuit voltage and fill factor after a certain adsorption time. High CBD duration of CdSe QDs coating might cause an increase in recombination centers, poor penetration of electrolyte and the decrease in surface area. Under optimum parameters, the QDSSC assembled with a CBD duration of CdSe QDs for 40 h exhibited a short-circuit current density of 14.30 mA/cm2 , an open-circuit voltage of 0.58 V, a fill factor of 0.33, and a PCE of 2.74%, this represented a 25% increase in PCE as compared to that of the optimal photoanode by AA treatment (2.20%) and even surpass that of the similar TiO2 nanoparticles coated nanotubes composite structures prepared by TiCl4 treatment (2.40%). This result was caused by the excellent scattering property of the hybrid QDs on hierarchical TiO2 structures with a higher surface area by SA which had better light capture ability inside the particles coated nanotube than smooth nanotube by AA. It is expected that further improvements in the cell efficiency should be possible with longer CdSe growth duration.
Table 2 Photovoltaic parameters of coupled QDs-sensitized TNAs solar cells by SA treatment.
Fig. 6. J–V characteristics of coupled CdSe(30 h)/CdS(5c) QDs-sensitized TNAs photoelectrodes with or without TiCl4 treatment by conventional AA treatment. The inset shows the detail parameters of the coupled CdSe(30 h)/CdS(5c)@TNAs photoelectrodes with or without TiCl4 treatment.
Sensitizers
Voc (V)
Jsc (mA/cm2 )
Fill factor
Efficiency (%)
CdSe(15 h)/CdS(5c) CdSe(20 h)/CdS(5c) CdSe(25 h)/CdS(5c) CdSe(30 h)/CdS(5c) CdSe(35 h)/CdS(5c) CdSe(40 h)/CdS(5c)
0.49 0.52 0.54 0.57 0.59 0.58
9.95 11.47 12.37 13.57 14.19 14.30
0.33 0.34 0.32 0.31 0.31 0.33
1.65 2.02 2.16 2.38 2.60 2.74
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Fig. 7. Top view and cross-sectional SEM images of pure TNAs by SA treatment (a and b), CdSe(40 h)/CdS(5c)@TNAs composited sample by SA pretreatment (c and d).
Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (51072170, 21021002), the National High Technology Research and Development Program of China (2009AA03Z327), the National Basic Research Program of China (2012CB932900), and the Alexander von Humboldt (AvH) Foundation of Germany. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2012.06.105. References Fig. 8. Effect of CdSe growth duration on J–V characteristics of coupled CdS/CdSe QDs-sensitized TNAs photoanodes by SA treatment.
4. Conclusion In summary, uniform CdS and CdSe QDs were successfully assembled on TNAs to construct CdS/CdSe core–shell co-sensitized QDSSCs. A developed SA treatment of TNAs resulted in a morphology change from thin TiO2 nanotube wall to well-defined nanocrystal tube wall due to the presence of fluorine ion residues in long nanotubes during the anodization in fluorine containing electrolyte. Moreover, an optimum PCE of up to 2.74% can be achieved by the CdSe(40 h)/CdS@TNAs sample with SA treatment under simulated AM 1.5 G irradiation of 100 mW/cm2 , a 25% increase in PCE as compared to the optimal CdSe(30 h)/CdS@TNAs sample by conventional AA treatment and even higher than that of the best CdSe(30 h)/CdS@TNAs photoanode with conventional TiCl4 pretreatment.
[1] Z.S. Yang, C.Y. Chen, P. Roy, H.T. Chang, Chemical Communications 47 (2011) 9561. [2] P.V. Kamat, Journal of Physical Chemistry C 112 (2008) 18737. [3] W.T. Sun, Y. Yu, H.Y. Pan, X.F. Gao, Q. Chen, L.M. Peng, Journal of the American Chemical Society 130 (2008) 1124. [4] M. Yang, N.K. Shrestha, P. Schmuki, Electrochimica Acta 55 (2010) 7766. [5] J.Z. Zhang, Journal of Physical Chemistry B 104 (2000) 7239. [6] A.J. Nozik, Physica E 14 (2002) 115. [7] V.I. Klimov, D.W. McBranch, Physical Review Letters 80 (1998) 4028. [8] J.J. Gong, C.J. Lin, M.D. Ye, Y.K. Lai, Chemical Communications 47 (2011) 2598. [9] W.X. Guo, C. Xu, X. Wang, S.H. Wang, C.F. Pan, C.J. Lin, Z.L. Wang, Journal of the American Chemical Society 134 (2012) 4437. [10] K.S. Raja, V.K. Mahajan, M. Misra, Journal of Power Sources 159 (2006) 1258. [11] P. Pillai, K.S. Raja, M. Misra, Journal of Power Sources 161 (2006) 524. [12] J.M. Macak, H. Tsuchiya, A. Ghicov, P. Schmuki, Electrochemistry Communications 7 (2005) 1133. [13] D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, Journal of the American Chemical Society 130 (2008) 16454. [14] J. Yoon, S. Bae, E. Shim, H. Joo, Journal of Power Sources 189 (2009) 1296. [15] Y.K. Lai, H.F. Zhuang, L. Sun, Z. Chen, C.J. Lin, Electrochimica Acta 54 (2009) 6536. [16] A. Ghicov, S.P. Albu, R. Hahn, D. Kim, T. Stergiopoulos, J. Kunze, C.A. Schiller, P. Falaras, P. Schmuki, Chemistry – An Asian Journal 4 (2009) 520. [17] P. Sudhagar, J.H. Jung, S. Park, Y.G. Lee, R. Sathyamoorthy, Y.S. Kang, H. Ahn, Electrochemistry Communications 11 (2009) 2220. [18] Q. Shen, A. Yamada, S. Tamura, T. Toyoda, Applied Physics Letters 97 (2010) 123107.
Y. Lai et al. / Electrochimica Acta 79 (2012) 175–181 [19] S.L. Cheng, W.Y. Fu, B.Y. Hai, L.N. Yhang, J.W. Ma, H. Zhao, M.L. Sun, L.H. Yang, Journal of Physical Chemistry C 116 (2012) 2615. [20] J.F. Yan, F. Zhou, Journal of Materials Chemistry 26 (2011) 9406. [21] X.F. Guan, S.Q. Huang, Q.X. Zhang, X. Shen, H.C. Sun, D.M. Li, Y.H. Luo, R.C. Yu, Q.B. Meng, Nanotechnology 22 (2011) 465402. [22] S.Q. Huang, Q.X. Zhang, X.M. Huang, X.Z. Guo, M.H. Deng, D.M. Li, Y.H. Luo, Q. Shen, T. Toyoda, Q.B. Meng, Nanotechnology 21 (2010) 375201. [23] X.F. Gao, W.T. Sun, G. Ai, L.M. Peng, Applied Physics Letters 96 (2010) 153104. [24] J.J. Gong, Y.K. Lai, C.J. Lin, Electrochimica Acta 55 (2010) 4776. [25] M.D. Ye, X.K. Xin, C.J. Lin, Z.Q. Lin, Nano Letters 11 (2011) 3214. [26] Z.Q. Lin, Y.K. Lai, R.G. Hu, J. Li, R.G. Du, C.J. Lin, Electrochimica Acta 55 (2010) 8717. [27] Y.K. Lai, X.F. Gao, H.F. Zhuang, J.Y. Huang, C.J. Lin, L. Jiang, Advanced Materials 21 (2009) 3799. [28] D.W. Bahnemann, R. Dillert, P.K.J. Robertson, Chemical Physics of Nanostructured Semicondutors, XSP, BV, Eindhoven, 2003. [29] Y.K. Lai, L. Sun, Y.C. Chen, H.F. Zhuang, C.J. Lin, J.W. Chin, Journal of the Electrochemical Society 153 (2006) D123.
181
[30] Y.K. Lai, J.Y. Huang, H.F. Zhang, V.P. Subramaniam, Y.X. Tang, D.G. Gong, L. Sundar, L. Sun, Z. Chen, C.J. Lin, Journal of Hazardous Materials 184 (2010) 855. [31] Y.L. Lee, C.F. Chi, S.Y. Liau, Chemistry of Materials 22 (2010) 922. [32] C.C. Chen, H.W. Chung, C.H. Chen, H.P. Lu, C.M. Lan, S.F. Chen, L. Luo, C.S. Hung, E.W.G. Diau, Journal of Physical Chemistry C 112 (2008) 19151. [33] Y. Alivov, Z.Y. Fan, Journal of Physical Chemistry C 113 (2009) 12954. [34] Y. Alivov, Z.Y. Fan, Nanotechnology 20 (2009) 405610. [35] S.W. Liu, J.G. Yu, M. Jaroniec, Chemistry of Materials 23 (2011) 4085. [36] L. Chen, L.F. Shen, P. Nie, X.G. Zhang, H.S. Li, Electrochimica Acta 62 (2012) 408. [37] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Nature 453 (2008) 638. [38] Y.K. Lai, J.J. Gong, C.J. Lin, International Journal of Hydrogen Energy 37 (2012) 6438. [39] W.K. Tu, C.J. Lin, A. Chatterjee, G.H. Shiau, S.H. Chien, Journal of Power Sources 203 (2012) 297. [40] C.J. Lin, W.K. Tu, C.K. Kuo, S.H. Chien, Journal of Power Sources 196 (2011) 4865.