ZnO Core–Shell Nanotube

ARTICLE pubs.acs.org/JPCC

Fabrication of Poly(3-hexylthiophene)/CdS/ZnO Core
Shell Nanotube Array for Semiconductor-Sensitized Solar Cell Bao Sun,†,‡ Yanzhong Hao,*,‡ Fen Guo,† Yinhu Cao,‡ Yanhui Zhang,‡ Yingpin Li,‡ and Dongsheng Xu*,§ †

College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China College of Sciences, Hebei University of Science and Technology, Shijiazhuang 050018, P. R. China § Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡

ABSTRACT: In this article, we designed and fabricated a novel semiconductor-sensitized solar cell using a poly(3-hexylthiophene) (P3HT)/CdS/ZnO core
shell nanotube array as the photoanode. A facile sequent electrodeposition process was employed to prepare the photoanode. The one-dimensional three-component core
shell ordered nanoarray structure was formed by electrodepositing CdS nanoparticles and a thin P3HT layer onto the walls of ZnO nanotubes, which enhanced the optical absorption in the visible region and presented an interface between n-type CdS and p-type P3HT to improve the charge separation. An energy conversion efficiency up to 1.28% was obtained with the designed nanostructured solar cell based on the P3HT/CdS/ ZnO nanotube array.

’ INTRODUCTION In recent years, nanostructured solar cells have been developed rapidly due to their low-cost alternatives to the traditional inorganic solar cells.1
10 Among these alternative technologies, dye-sensitized solar cells (DSSCs) have attracted much attention because they make use of some new concepts, such as nanostructured configurations and dye absorbers, and exhibit promising conversion efficiencies of more than 11%.11
15 However, DSSCs still face several problems, such as dye degradation under full sun irradiation. As substitutes to conventional dyes, inorganic semiconductors (SCs) with narrow band gaps, such as CdS,16
18 PbS,19 CdSe,20,21 and CdTe,22 are used as new light-harvesting materials for SC-sensitized solar cells. They can offer several advantages over the conventional dyes, including high stability under irradiation, excellent optical properties, and good controllability in the fabrication process. Despite of the specific advantages of SCs, SC-sensitized solar cells still suffer from several problems. Efficient charge separation and fast charge transport in semiconductor nanostructures are the major challenges.23 Various strategies have been made to improve charge separation efficiency and accelerate charge transport processes.24
29 Plass et al.24 introduced an organic hole transport material, MeOTAD, onto the surface of a PbS-sensitized TiO2 film to create a multilayer structure in which the high surface area p
n heterjunction between TiO2 and MeOTAD could accelerate the charge transport from PbS nanoparticles to the neighboring p- and n-type materials. In addition, a number of works used one-dimension (1D) core
shell nanoarray structures as optimal architectures for smooth charge transport.25
29 In such architectures, charge transport is direct and fast due to the 1D nature, and charge separation is efficient and controllable because it takes place in the radial versus the axial direction, and the charge collection distance is easy to be tailored by tuning the shell thickness. r 2011 American Chemical Society

Building from these ideas, we propose a novel SC-sensitized solar cells based on 1D three-component core
shell architecture (p-type semiconducting polymer/inorganic semiconductor nanocrystals/n-type 1D semiconductor array) as the photoanode in which a thin layer of inorganic semiconductor nanocrystals as the sensitizer is coated onto the 1D n-type semiconductor, and a thin layer of p-type semiconducting polymer (such as P3HT) as the hole transport material and light absorber is deposited into the rest intervals. The designed structure can offer several specific advantages including (i) introducing an interface between p-type and n-type materials to improve the charge separation efficiency and (ii) employing the 1D core
shell structure to accelerate the charge transport from the inorganic semiconductor nanocrystals to the neighboring p-type and n- type materials dividedly and reduce the charge recombination. In addition, a facile and low-cost electrodeposition process is designed to offer a better bonding between the semiconductors. Recently, we reported an initial solar cell based on a P3HT/ CdSe/ZnO core
shell nanorod array and an energy conversion efficiency of 0.88% has been achieved under 100 mW/cm2 AM 1.5 irradiance.30 In this study, CdS nanocrystals instead of CdSe nanocrystals were investigated, and ZnO nanotubes instead of ZnO nanorods were prepared to provide higher surface areas for coating more CdS nanoparticles and efficient channels for charge separation and transportation. A novel SC-sensitized solar cell based on a P3HT/CdS/ZnO core
shell nanotube array was fabricated and an enhanced energy conversion efficiency up to 1.28% was achieved. Received: June 28, 2011 Revised: October 19, 2011 Published: November 28, 2011 1395

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’ EXPERIMENTAL SECTION Preparation and Characterization of P3HT/CdS/ZnO Core
Shell Nanotube Array. ZnO nanotube arrays were prepared by

electrochemical etching of the electrodeposited ZnO nanorod arrays in alkaline solution. The electrochemical processes were performed by potentiostatic mode in a homemade glass cell at 70 °C in which a platinum electrode (5  10 mm) and a saturated calomel electrode (SCE) served as the counter electrode and reference electrode, respectively. First, ZnO nanorod arrays were electrodeposited on an ITO substrate in 15 mL mixed solution containing 0.025 M Zn (NO3)2, 30 μL polyethylene glycol (PEG-400), and 10 μL ethylenediamine (EDA). The deposited electrode potential was
1.10 V vs SCE, and the deposition time was 2 h. Second, ZnO nanotube arrays were prepared by etching the obtained ZnO nanorod arrays in a aqueous solution of 0.1 M EDA at a potential of
0.60 V for 1 h. Then, the ITO substrate covered with ZnO nanotubes was rinsed by deionized water for several times and dried in air at room temperature. Subsequently, the obtained ITO substrates covered with ZnO nanotubes were used as working electrode to prepare CdS/ZnO core
shell nanotube arrays. A platinum electrode and an Ag/ AgCl electrode were served as the counter electrode and reference electrode in the electrodeposition system. The electrolyte was a dimethyl sulfoxide (DMSO) solution containing 0.1 M CdCl2, 0.1 M triethanolamine (TEA), and saturated elemental S. The electrodeposition was performed at an electrode potential of
1.00 V for different times (6, 9, and 12 min). The as-deposited samples were immediately dipped in pure hot DMSO, then rinsed with ethanol and deionized water, and finally annealed at 400 °C for 1 h. According to the deposition time of CdS, the asprepared CdS/ZnO core
shell nanotube arrays were named as CdS (6 min)/ZnO, CdS (9 min)/ZnO, and CdS (12 min)/ZnO (6 min, 9 min, and 12 min represent the deposition time for CdS nanoparticles). Finally, a thin layer of P3HT was electrodeposited onto the asprepared CdS/ZnO nanotubes according to our earlier report.30 Typically, the electrodeposition was carried out at room temperature in an acetonitrile solution containing 0.01 M tetrabutylammonium tetrafluoroborate and 0.1 M 3-hexylthiophene. The ITO substrate coated with CdS/ZnO nanotubes, a platinum electrode, and a SCE were served as the working electrode, counter electrode, and reference electrode, respectively. The applied potential was 1.8 V and the deposition time was 5 min. The as-deposited samples were rinsed with absolute ethanol and then annealed at 150 °C for 1 h. The morphologies and structures of the as-prepared samples were characterized by FESEM (Hitachi S4800
I, 10.0 kV), TEM (JEOL JEMCX200), HRTEM (H-9000NAR), and powder X-ray diffraction (XRD, Bruker D8-advance diffractometer with Cu Kα radiation, λ = 0.15418 nm). The UV
vis diffuse reflection spectra were recorded on a Hitachi UV
vis NIR spectrophotometer (U-4100). Preparation and Photovoltaic Measurements of the SCSensitized Solar Cells. To prepare the SC-sensitized solar cells, the core
shell nanotube arrays of CdS/ZnO and P3HT/CdS/ ZnO were all sandwiched together with platinized ITO counter electrodes. The electrolyte (0.05MLiI, 0.05 M I2, 0.6 M 1-propyl3-methylimidazolium iodiode (PMII), and 0.5 M 4-terbutylpyridine in 3-mathoxyproprionitrile) was injected into the cells from the edges by capillarity. For comparison, the solar cell based on the N719-sensitized CdS/ZnO core
shell nanotube

Figure 1. SEM images of (a) ZnO nanorod array and (b) top view and (c) cross-sectional view of the ZnO nanotube array. (d) TEM image of a single ZnO nanotube.

electrode was also assembled by dipping the CdS/ZnO core
shell nanotube array in an ethanol solution of 3  10
4 M cisbis(isothiocyanato)bis(2,20 -bipyridy1
4,4 0 -dicarboxylate)ruthenium(II)bis-tetrabutylammonium (N719, Solaronix SA, Switzerland) at 80 °C for 1 h. Photocurrent
voltage measurements of the as-prepared SCsensitized solar cells were performed on a Keithley 2400 sourcemeter using simulated AM 1.5 sunlight with an output power of 100 mW/cm2 produced by a solar simulator (Newport 69911).

’ RESULTS AND DISCUSSION Figure 1a shows the typical SEM image of the electrodeposited ZnO nanorod array prepared on an ITO substrate. All the nanorods have smooth surfaces and regular hexagonal cylindrical shape with a relatively uniform size of 200
300 nm in diameter. The top view SEM image of the ZnO nanotube array, shown in Figure 1b, reveals a dense and uniform array in accordance with the template of the ZnO nanorod array. Figure 1c gives the crosssection view of the ZnO nanotube array. It can be seen that the nanotubes are well-aligned with an average length of about 1.5 μm. Figure 1d displays a typical TEM image of an individual ZnO nanotube. It can be found that, by electrochemical etching, the ZnO nanorods are successfully changed into ZnO nanotubes with a wall thickness of 30 nm and inner diameter of 200 nm. The inset TEM image in Figure 1d indicates that the ZnO nanotube is open at both ends. Compared to the ZnO nanorod arrays in our earlier report,30 the as-prepared ZnO nanotube arrays can offer higher surface areas, which could be very favorable to increase the amount of electrodeposited CdS nanoparticles and then enhance the optical absorption. Figure 2a
d shows the top-view and cross-sectional SEM images of the CdS/ZnO core
shell nanotube arrays electrodeposited at
1.0 V for different times. It can be seen that CdS nanoparticles have been successfully and synchronously electrodeposited inside and outside the ZnO nanotube walls to form a double-layer core
shell structure, which could offer two channels (inside and outside the tubes) for efficient electron transport from the CdS shell to ZnO core. With prolonging the deposition 1396

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Figure 4. SEM image of the P3HT/CdS (9 min)/ZnO core
shell nanotube array. (9 min represents the deposition time for CdS.)

Figure 2. SEM images of the CdS/ZnO core
shell nanotube arrays electrodeposited at
1.0 V for different times: (a) 6 min, (b) 9 min, and (c) 12 min. (d) Cross-section view of the CdS (9 min)/ZnO core
shell nanotube array. (e) TEM image for a single CdS (9 min)/ZnO nanotube and (f) the EDX spectrum of the CdS (9 min)/ZnO core
shell nanotube array.

Figure 3. XRD patterns of the (a) ZnO nanotube array and (b) CdS/ ZnO core
shell nanotube array; b and 2 stand for the diffraction peaks of ZnO and CdS.

time, the thickness of the CdS shells increases obviously, as shown in the inset of Figure 2a
c. When the deposition time reaches 12 min, the ZnO nanotube array (Figure 2c) is nearly filled with CdS nanoparticles, which could limit the subsequent deposition of P3HT. Figure 2e displays a typical TEM image of an individual CdS (9 min)/ZnO nanotube. It reveals that CdS particles are crystallized onto the surface of a ZnO nanotube with an average diameter of about 15 nm. The corresponding EDX spectrum is shown in Figure 2f, and the elemental analysis data are inserted. The results indicate that the as-prepared core
shell

Figure 5. Optical absorpsion spectra for (a) pure ZnO nanotube electrode, (b) CdS (6 min)/ZnO, (c) CdS (9 min)/ZnO, (d) CdS (12 min)/ZnO, and (e) P3HT/CdS(9 min)/ZnO core
shell nanotube electrodes.

nanotubes are composed of O, Zn, Cd, and S. The molar ratio of Cd and S is approximately 1:1, in accordance with their stoichiometric proportion. Figure 3a shows the XRD pattern of the ZnO nanotube array. All the diffraction peaks are indexed to a hexagonal wurtzite structure of ZnO (JCPDS no. 36-1451). A relatively higher intensity is observed at 2θ = 34.4°, indicating that the as-prepared ZnO nanotubes are well-aligned along the c-axis and perpendicular to the ITO substrate. After electrodeposition of CdS, as shown in Figure 2b, the new diffraction peaks at 24.9°, 26.6°, 28.3°, 43.8°, and 51.8° are indexed to the wurtzite structure of CdS (JCPDS no. 1060316). This result confirms that wellcrystallized CdS nanoparticles were successfully electrodeposited onto the ZnO nanotubes. After electrodeposition of P3HT, some distinct changes appear in SEM image as shown in Figure 4. The outer average diameter of the nanotubes increases from 400 to 450 nm, and the interiors of the nanotubes are completely filled in. These observations indicate that P3HT molecules are successfully deposited onto the walls of the CdS (9 min)/ZnO nanotubes to form the designed three-component core
shell structure. Figure 5 shows the UV
vis diffuse reflectance spectra for the pure ZnO nanotube array, three different CdS/ZnO arrays (based on CdS (6 min)/ZnO, CdS (9 min)/ZnO, and CdS (12 min)/ZnO core
shell nanotube arrays) and the P3HT/CdS (9 min)/ZnO core
shell nanotube array. It can be seen from Figure 5a that the ZnO nanotube array absorbs in the UV region 1397

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Figure 6. J
V curves for the SC-sensitized solar cells fabricated with the electrodes of CdS (6 min)/ZnO, CdS (9 min)/ZnO, and CdS (12 min)/ZnO core
shell nanotube arrays.

Figure 7. J
V curves for the SC-sensitized solar cells fabricated with the electrodes of CdS (9 min)/ZnO, P3HT/CdS (9 min)/ZnO, and N719/CdS (9 min)/ZnO core
shell nanotube arrays.

Table 1. Photovoltaic Properties of the SC-Sensitized Solar Cells Based on Different Electrodes

Table 2. Photovoltaic Properties of the SC-Sensitized Solar Cells Based on Different Electrodes

samplea

Jsc (mA/cm2)

Voc (V)

FF

η (%)

sampleb

Jsc (mA/cm2)

Voc (V)

FF

η (%)

device A

2.353

0.654

0.34

0.52

device B

3.527

0.644

0.36

0.83

0.83

device E

5.181

0.688

0.36

1.28

0.71

device G

3.972

0.730

0.37

1.07

device B device C

3.527 3.185

0.644 0.620

0.36 0.36

a

Devices A
C are coded as SC-sensitized solar cells fabricated with CdS(6 min)/ZnO, CdS(9 min)/ZnO, and CdS(12 min)/ZnO core
shell nanotube electrodes, respectively.

with a band edge of ∼390 nm. After depositing CdS nanoparticles onto ZnO nanotubes, the CdS/ZnO core
shell nanotube arrays (Figure 5b
d) extend their optical absorptions to the visible region. With the deposition time of CdS increasing, the optical absorption in the visible region is gradually enhanced due to the increased amounts of deposited CdS nanoparticles, while the optical absorption in the UV region is sharply declined because the thicker CdS nanoparticle-layer (as shown in Figure 2a
c) would seriously block the absorption of ZnO in the UV region. For the P3HT/CdS (9 min)/ZnO core
shell nanotube array (Figure 5e), the deposition of P3HT results in a slight red-shift and a further enhancement in optical absorption, especially at the wavelength range of 400
530 nm. It can be deduced that photovoltaic properties with our designed SCsensitized solar cell based on the P3HT/CdS (9 min)/ZnO core
shell nanotube electrode could be improved by enhancing the optical absorption. In order to investigate the influence of CdS shell thickness (in accordance with the deposition time for CdS) on photovoltaic performance of the CdS/ZnO core
shell nanotube electrodes, J
V characteristics for SC-sensitized solar cells based on CdS(6 min)/ZnO, CdS(9 min)/ZnO, and CdS(12 min)/ZnO core
shell nanotube electrodes were measured. The results are displayed in Figure 6 and Table 1. With the deposition time of CdS increasing, the energy conversion efficiency (η) of the SCsensitized solar cells increases and then decreases. Optimized photovoltaic performance is obtained from device B (based on the CdS (9 min)/ZnO core
shell nanotube electrode), with a Voc of 0.644 V, a Jsc of 3.527 mA/cm2, an FF of 0.36, and a η of 0.83%. For device A (based on the CdS(6 min)/ZnO core
shell nanotube electrode), the poor photovoltaic property could be attributed to its thinner CdS nanoparticle-layer and the consequent

b

Device B, E, and G are coded as SC-sensitized solar cells fabricated with the electrodes of CdS(9 min)/ZnO, P3HT/CdS(9 min)/ZnO, and N719-coated CdS(9 min)/ZnO, respectively.

Figure 8. Photocurrent
potential plot of the P3HT/CdS (9 min)/ ZnO core
shell nanotube electrode. The wavelength of the incident light is 470 nm.

less optical absorption of visible light (as known from Figure 5b,c). For device C (based on the CdS(12 min)/ZnO core
shell nanotube electrode), the thicker CdS nanoparticle layer can result in an enhanced optical absorption of visible light (as shown in Figure 5d), but it also can increase the recombination chance of photogenerated carriers and then lead to the poor photovoltaic property. To improve the photovoltaic performance of the optimal CdS (9 min)/ZnO electrode (device B), a thin layer of P3HT was electrodeposited onto the CdS (9 min)/ZnO nanotubes to prepare the P3HT/CdS (9 min)/ZnO electrode (device D). For comparison, a N719-coated CdS (9 min)/ZnO electrode (device E) is also fabricated. J
V characteristics for different cells are shown in Figure 7, and the related parameters are listed in Table 2. It is found that the photovoltaic performance of device B 1398

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Figure 9. (a) Schematic configuration for the SC-sensitized solar cell and (b) charge-transfer process in the P3HT/CdS/ZnO core
shell nanotube electrode.

can be improved by coating the CdS (9 min)/ZnO electrode with either P3HT or N719. Sensitized with N719, device E shows a 29% increase in energy conversion efficiency, displaying a Voc of 0.730 V, a Jsc of 3.972 mA/cm2, an FF of 0.37, and a η of 1.07%. Coated with P3HT, device D reveals an improvement of 54% in energy conversion efficiency, exhibiting a Voc of 0.688 V, a Jsc of 5.181 mA/cm2, an FF of 0.36, and a η of 1.28%. A superior performance is presented in device D compared to that in device E. This is probably attributed to the introduced interface between CdS and P3HT in device D, where a p
n heterojunction might be formed (discussed later), and the charge separation efficiency could be improved. In addition, as an excellent hole-transport material, P3HT can efficiently transfer the generated holes to the redox couples, which could result in a higher photocurrent. Compared to our earlier reported SC-sensitized solar cell based on the structure of the P3HT/CdSe/ZnO core
shell nanorod array,30 the designed device D presents a 45% increase in energy conversion efficiency. This could be explained as follows: first, ZnO nanotubes can provide higher surface areas for coating more CdS nanoparticles to enhance optical absorption. Second, ZnO nanotubes can offer two channels (inside and outside the nanotubes) for more efficient charge separation and transportation. To confirm the formation of the p
n heterojunction between CdS and P3HT in the P3HT/CdS (9 min)/ZnO core
shell nanotube electrode, a photocurrent
potential plot of the electrode was measured in an ethanol solution containing 0.1 mol/L KSCN, as shown in Figure 8. Under illumination of 470 nm, anodic photocurrents were obtained in fairly positive potential, while cathodic photocurrents were obtained in relatively negative potential. This indicated that a p
n heterojunction might be formed between CdS and P3HT in the measured electrode. To better understand the operation mechanism of the designed SC-sensitized solar cell based on the P3HT/CdS/ZnO core
shell nanotube electrode, in Figure 9, we depicted the main configuration and charge-transfer processes among ZnO, CdS, and P3HT after being excited by light. In such a configuration, the visible light is absorbed by CdS and P3HT, and the excited electron
hole pairs are generated. Then, benefitting from the p
n heterojunction probably formed at the interface between n-type CdS and p-type P3HT, the photogenerated charges in CdS and P3HT are effectively separated. Subsequently, driven by the favorable energetic arrangement shown in Figure 9b, the separated electrons are efficiently transferred from the doublelayer CdS shells to the ZnO core and then quickly transferred to the ITO substrate along the direct path of the ZnO nanotubes. Meanwhile, the separated holes are transferred from CdS layers to P3HT layers and scavenged by the I
/I3
couple in the

electrolyte. According to the above discussion, CdS can serve as not only the photosensitizer but also the electronic mediator, and P3HT act as both the hole conductor and light absorber, which significantly improves the photocurrent and the overall conversion efficiency for the designed SC-sensitized solar cells based on the P3HT/CdS/ZnO core
shell nanotube structures.

’ CONCLUSIONS A novel P3HT/CdS/ZnO core
shell nanotube array was fabricated by a simple electrodeposition process and used as photoanode in solar cell. The 1D three-component core
shell ordered nanoarray structure was formed by electrodepositing CdS nanoparticles and a thin P3HT layer onto ZnO nanotubes, which enhanced the optical absorption in the visible region and resulted in the increase of energy conversion efficiency. The interface between CdS and P3HT existed in the structure, where a p
n heterojunction might be formed, can improve the charge separation and transportation. An energy conversion efficiency up to 1.28% was obtained with the designed nanostructured solar cell based on the P3HT/CdS (9 min)/ZnO nanotube array. Compared to our earlier reported solar cell based on the structure of the P3HT/CdSe/ZnO nanorod array, the designed nanostructured solar cell presented an enhanced photovoltaic performance. ’ AUTHOR INFORMATION Corresponding Author

* Tel: 86-311-81668528. Fax: 86-311-81668512. E-mail: yzhao@ hebust.edu.cn (Y.H.); [email protected] (D.X.).

’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (NSFC, grant nos. 21173065 and 20573031), MSTC (grant no. 2011CB808702) and Natural Science Foundation of Hebei Province of China (grant no. B2010000856). ’ REFERENCES (1) Kang, T. S.; Smith, A. P.; Taylor, B. E.; Durstock, M. F. Nano Lett. 2009, 9, 601–606. (2) Tan, B.; Wu, Y. Y. J. Phys. Chem. B 2006, 110, 15932–15938. (3) Zhu, G.; Pan, L.; Xu, T.; Sun, Z. ACS Appl. Mater. Interfaces 2011, 3, 3146–3151. (4) Chen, J.; Wu, J.; Lei, W.; Song, J. L.; Deng, W. Q.; Sun, X. W. Appl. Surf. Sci. 2010, 256, 7438–7441. (5) Farrow, B.; Kamat, P. V. J. Am. Chem. Soc. 2009, 131, 11124–11131. 1399

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(6) Ganesh, T.; Mane, R. S.; Cai, G.; Chang, J. H.; Han, S. H. J. Phys. Chem. C 2009, 113, 7666–7669. (7) Leventis, H. C.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Nano Lett. 2010, 10, 1253–1258. (8) Gao, X. F.; Li, H. B.; Sun, W. T.; Chen, Q.; Tang, F. Q.; Peng, L. M. J. Phys. Chem. C 2009, 113, 7531–7535. (9) Yasuaki, J.; Naya, S.; Tada, H. J. Phys. Chem. C 2010, 114, 16837–16842. (10) Guldin, S.; Huttner, S.; Kolle, M.; Welland, M. E.; Buschbaum, P. M.; Friend, R. H.; Steiner, U.; Tetreault, N. Nano Lett. 2010, 10, 2303–2309. (11) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737–740. (12) Gratzel, M. Nature 2001, 414, 338–344. (13) Gratzel, M. J. Photochem. Photobiol., A 2004, 164, 3–14. (14) Green, M. A.; Emery, K.; King, D. L.; Hishikawa, Y.; Warta, W. Prog. Photovoltaics 2006, 14, 455–461. (15) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Langmuir 2000, 16, 4688–4694. (16) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. J. Am. Chem. Soc. 2008, 130, 1124–1125. (17) Banerjee, S.; Mohapatra, S. K.; Das, P. P.; Misra, M. Chem. Mater. 2008, 20, 6784–6791. (18) Lee, W.; Min, S. K.; Dhas, V.; Ogale, S. B.; Han, S. H. Electrochem. Commun. 2009, 11, 103–106. (19) Gao, J. B.; Luther, J. M.; Semonin, O. E.; Ellingson, R. J.; Nozik, A. J.; Beard, M. C. Nano Lett. 2011, 11, 1002–1008. (20) Chong, L. W.; Chien, H. T.; Lee, Y. L. J. Power Sources 2010, 195, 5109–5113. (21) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (22) Seabold, J. A.; Shankar, K.; Wilke, R. H. T.; Paulose, M.; Varghese, O. K.; Grimes, C. A.; Choi, K. S. Chem. Mater. 2008, 20, 5266–5273. (23) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753. (24) Plass, R.; Pelet, S.; Krueger, J.; Gratzel, M.; Bach, U. J. Phys. Chem. B 2002, 106, 7578–7580. (25) Greene, L. E.; Law, M.; Yuhas, B. D.; Yang, P. J. Phys. Chem. C 2007, 111, 18451–18456. (26) Zhang, Y.; Wang, L. W.; Mascarenhas, A. Nano Lett. 2007, 7, 1264–1269. (27) Myung, Y.; Jang, D. M.; Sung, T. K.; Sohn, Y. J.; Jung, G. B.; Cho, Y. J.; Kim, H. S.; Park, J. ACS Nano 2010, 4, 3789–3800. (28) Lin, Z. Q.; Lai, Y. K.; Hu, R. G.; Li, J.; Du, R. G.; Lin, C. J. Electrochim. Acta 2010, 55, 8717–8723. (29) Das, K.; De, S. K. J. Phys. Chem. C 2009, 113, 3494–3501. (30) Hao, Y. Z.; Pei, J.; Wei, Y.; Cao, Y. H.; Jiao, S. H.; Zhu, F.; Li, J. J.; Xu, D. S. J. Phys. Chem. C 2010, 114, 8622–8625.

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