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Ordered Mesoporous SnO2-Based Photoanodes for High-Performance Dye-Sensitized Solar Cells Easwaramoorthi Ramasamy and Jinwoo Lee* Department of Chemical Engineering, School of EnVironmental Science and Engineering, Pohang UniVersity of Science and Technology (POSTECH), Pohang, Korea ReceiVed: August 9, 2010; ReVised Manuscript ReceiVed: NoVember 3, 2010
Ordered mesoporous SnO2 with three-dimensional bicontinuous cubic mesostructure, high surface area, and crystalline frameworks was synthesized by the solvent-free infiltration of tin precursor in KIT-6 silica template and employed as a photoanode in dye-sensitized solar cells (DSSCs). It is shown that coating an ultrathin TiO2 or Al2O3 layer on mesoporous SnO2 photoanode greatly improves the open-circuit voltage, short-circuit current, and fill factor, leading to more than a 3-fold improvement in the energy conversion efficiency. The superior photovoltaic performance of surface modified mesoporous SnO2 photoanode is mainly the result of inhibited electron recombination caused by passivation of reactive surface states and increased dye loading. Control cells fabricated with conventional nanoparticle SnO2/TiO2 photoanode exhibit a similar trend but with about 30% lower energy conversion efficiency, which is mainly because of limitations due to low dye loading and poor light scattering. 1. Introduction Dye-sensitized solar cells (DSSCs) based on metal oxide semiconductor photoanodes and redox electrolytes are of great interest for application to solar energy conversion.1 In such types of solar cells, photons with sufficient energy excite dye molecules, and this is followed by a rapid injection of electrons into the conduction band of the metal oxide semiconductor. Record efficiencies of about 11% have been realized for devices employing randomly oriented nanoparticle TiO2 photoanodes.2 Nevertheless further improvements in the photovoltaic performance have been limited because of slow electron transport and enhanced charge recombination in the TiO2 film.3 Several studies have addressed the use of alternative metal oxides such as ZnO,4a SnO2,4b Zn2SnO4,4c and Nb2O5.4d Among them, SnO2 possesses certain inherent advantages such as higher electron mobility (100-200 cm2V-1S1-)5 and more positive conduction band edge position than TiO2. The high electron mobility leads to faster transport of photoinjected electrons to a transparent conductive oxide current collector while the more positive conduction band edge position facilitates electron injection from photoexcited dye molecules, especially in low band gap organic sensitizers.6 Furthermore, in the case of SnO2, its larger band gap (3.6 eV), as compared with that of TiO2 (3.2 eV), creates fewer oxidative holes in the valence band under UV illumination than are created in the case of TiO2, thereby minimizing the dye degradation rate and improving the long-term stability of DSSCs.7 Surprisingly, the performance of DSSCs based on SnO2 photoanodes is much less than those consisting of TiO2. The inferior photovoltaic properties of a SnO2 photoanode DSSC are attributed to the faster electron recombination kinetics resulting from a 300 mV positive shift in the conduction band edge and poor dye uptake associated with the low isoelectric point.8 These issues have been recently overcome by engineering the SnO2 surface with conformal barrier layers using materials such as TiO2 or Al2O3.9 * To whom correspondence should be addressed. E-mail: jinwoo03@ postech.ac.kr.
The main shortcomings of any randomly oriented nanoparticle photoanode are the extent of the detrimental grain boundaries at the interparticle region and the disordered pore structure.10 Recent progress in the improvement of DSSC performance has resulted from the use of the one-dimensional (1D) metal-oxide architecture (i.e., nanowire or nanotube) photoanodes; these are expected to promote faster electron transport and slower recombination.11 However, the advantage offered by the network structure of a nanowire photoanode is limited because of the small surface area, and thus, the device shows low efficiency. In this regard, ordered mesoporous materials are of interest as candidates for electrode material as they offer a unique combination of high surface area and ordered mesoscale pores together with a continuous interwoven network.12,13 Furthermore, surface functionalization of ordered mesoporous materials may turn out to be a valuable method for exploring new areas of optoelectronics.14 In this work, we describe the synthesis of ordered mesoporous SnO2 by a hard template method and report its application as a high-performance photoanode in DSSC (Figure 1). The large surface area and three-dimensional (3D) bicontinuous cubic structure of the modified ordered mesoporous SnO2 photoanode offers greater dye loading capability and enhanced light scattering; thus resulting DSSC shows 40% higher efficiencies than that of conventional nanoparticle photoanodes. 2. Experimental Section Ordered mesoporous SnO2 (hereafter, meso-SnO2) was synthesized by a hard template method using SnCl2.2H2O as a precursor and KIT-6 silica as a template.15,16 In a typical synthesis, 1.5 g of SnCl2 · 2H2O (Sigma Aldrich) was finely ground with 1 g of KIT-6 silica and kept at 85 °C for 24 h under vacuum. The resultant sample was heated to 700 °C in air, to convert tin chloride to tin oxide within the pores of the KIT-6 silica template. Finally, the silica template was etched in a dilute aqueous solution of HF acid and meso-SnO2 was collected by filtration, washed with deionized water several times and dried at 50 °C.
10.1021/jp1074797 2010 American Chemical Society Published on Web 11/29/2010
Ordered Mesoporous SnO2-Based Photoanodes
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Figure 1. Schematic diagram of the synthesis of ordered mesoporous SnO2 particles with 3D bicontinuous cubic mesostructure and assembly of photoanodes. Conformal TiO2 or Al2O3 layer was deposited on ordered mesoporous SnO2 by solution or atomic layer deposition method, respectively (for clarity, only the SnO2 network replicated from single chiral channel is shown in the photoanode).
To fabricate photoanode for DSSCs, 0.2 g of meso-SnO2 powder was finely ground and mixed with 0.18 g of binder solution (5 wt % ethyl cellulose dissolved in R-terpineol). The obtained high-viscous paste was diluted by adding 0.25 g of R-terpineol and thoroughly mixing with a mortar and pestle for 1 h. The paste was then coated on F-doped tin oxide (FTO) glass substrate by a screen printing method, followed by heat treatment at 400 °C for 30 min in air. The thickness of the photoanode was estimated to be 3 µm (Figure S5 in the Supporting Information). The TiO2 coated mesoporous SnO2 photoanode (denoted as meso-SnO2/TiO2) was obtained by treating the meso-SnO2 electrodes in 0.2 mM aqueous TiCl4 solution at 80 °C for 30 min; rinsed with water and ethanol, and finally sintered at 500 °C for 30 min in air. For Brunauer-Emmett-Teller (BET) and XRD analyses, mesoSnO2/TiO2 powder was prepared by dipping the as-synthesized meso-SnO2 powder in 0.2 mM aqueous TiCl4 solution, similar to the method described for the photoanodes. The Al2O3 coated mesoporous SnO2 photoanode (denoted as meso-SnO2/Al2O3) was obtained by growing a conformal layer of Al2O3 on meso-SnO2 photoanode via atomic layer deposition (Qurous Co, Plus 200). The deposition was carried out at 150 °C using trimethylaluminium and water as precursors. Given that the growth rate of Al2O3 on a planar surface is 2 nm/10 cycle in optimum exposure conditions, 5 cycles were used to obtain ∼1 nm thick Al2O3 layer on meso-SnO2. The electrode preparation technique for the randomly oriented nanoparticle SnO2 photoanode (denoted as nano-SnO2) was similar to that previously described for the meso-SnO2 photoanode except that the paste was prepared from commercially available SnO2 powders (Alfa Aesar, particle size: 22-43 nm). Freshly sintered photoanodes were immersed in a 5 × 10-4 M solution of N719 dye (Solaronix) in a mixture of acetonitrile/ tert-butanol (volume ratio 1:1) for 24 h. The dye impregnated photoanode was rinsed in anhydrous ethanol and assembled with a Pt counter electrode in a sandwich configuration. The redox electrolyte of 0.6 M 1-butyl-3-methylimidazolium iodide, 0.1 M lithium iodide, 0.03 M iodine, 0.1 M guanidine thiocyanate, and 0.5 M 4-tert-butyl pyridine in acetonitrile was introduced into the cell via vacuum backfilling and the electrolyte injection hole was firmly sealed with Surlyn and a microscope cover glass. Photocurrent-voltage characteristics of the DSSCs were measured under simulated air mass 1.5 G solar spectrum. The intensity was adjusted to 100 mW · cm-2 using NREL certified silicon reference cell equipped with a KG-5 filter. Incident photon to current conversion efficiency (IPCE) spectrum was collected over the wavelength rage of 350 to 800 nm with a chopping frequency of 10 Hz (PV Measurements, Inc.). The lifetime of electrons in the conduction band was obtained by intensity modulated photovoltage spectroscopy (IMVS) measurements (K3400, Polaronix). Dye uptake was estimated by treating the dye-adsorbed electrodes in 0.01 M NaOH solution
in water and ethanol (volume ratio: 50:50) and measuring the absorption spectrum using a UV-vis spectrophotometer (DU 800, Beckman Coulter, Inc.). The diffused reflectance spectra of meso-SnO2 and nano-SnO2 photoanodes were obtained using an UV-vis spectrometer (UV-2401, Shimadzu Corporation) equipped with an integrating sphere. 3. Results and Discussion Figure 2a shows the scanning transmission electron microscopy (STEM) image of a meso-SnO2/TiO2 powder. The intact long-range periodic order obviously indicates that the mesoscale regularity is well preserved after the nanocasting and subsequent TiO2 layer deposition and annealing. The small-angle X-ray scattering (SAXS) pattern (Figure 2b) of meso-SnO2/TiO2 powder exhibits six well-resolved peaks that can be indexed as 211, 220, 321, 400, 420, and 332. This gives the spacing ratios of 3:4:7:8:10:11, corresponding to Bragg reflections of the 3D bicontinuous cubic mesostructure with Ia3d space group. As expected, no notable difference between the SAXS pattern of bare and TiO2 coated meso-SnO2 was observed, demonstrating the ordered mesostructure in the resulting photoanodes. Energy dispersive X-ray spectroscopy (EDS) elemental analysis was performed on a meso-SnO2/TiO2 powder in order to probe its mesostructural order. The EDS elemental mapping (Figure 2c and d) shows the presence of both Sn and Ti atoms distributed throughout the particle with a Ti/Sn atomic ratio of 0.118, demonstrating the coverage of TiO2 layer over the entire surface of meso-SnO2. It is worth mentioning that no residue silica was detected, indicating that the sample was virtually silica-free. The detailed structure of the meso-SnO2/TiO2 powder was elucidated using high-resolution transmission electron microscopy (HR-TEM). Figure 3 shows representative HR-TEM images of (a) bare meso-SnO2 particles and (b) particles that were treated with TiCl4 and subsequently sintered at 500 °C for 30 min in air. The bare meso-SnO2 shows well-defined lattice fringes with d-spacing of about 0.33 nm, corresponding to {110} planes of rutile phase. TiCl4 treatment resulted in an ultrathin (∼1 nm), TiO2 layer on the meso-SnO2 surface. Figure 4 compares the X-ray diffraction pattern (XRD) of meso-SnO2/ TiO2 powders along with bare meso-SnO2 and commercially available SnO2 nanopowders (nano-SnO2). The diffraction pattern of meso-SnO2 is well matched with nano-SnO2 and all peaks can be indexed to a rutile structure (JCPDS No. 41-1445). Interestingly, meso-SnO2/TiO2 powder did not exhibit any additional diffraction peaks associated with TiO2. Therefore, it is not possible to precisely determine the crystalline nature of the ultrathin TiO2 shell layer.15 Nevertheless, previous literature suggest that TiO2 layers prepared under similar experimental conditions adopted the rutile phase.6a The specific surface area and pore size distribution of bare meso-SnO2 and meso-SnO2/TiO2 powders were characterized
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Figure 2. (a) STEM image and (b) SAXS profile of meso-SnO2/TiO2 particles, along with the associated EDS elemental mapping data showing distribution of (c) Sn and d) Ti atoms throughout the particle.
Figure 3. High-resolution TEM images of (a) bare meso-SnO2, and (b) meso-SnO2/TiO2
Figure 5. (a) Nitrogen sorption isotherm and (b) BJH pore size distribution of bare meso-SnO2 and meso-SnO2/TiO2 powders.
Figure 4. X-ray diffraction patterns of nano-SnO2, meso-SnO2, and meso-SnO2/TiO2 powders.
using BET analysis and the results are shown in Figure 5. Bare meso-SnO2 exhibits type-IV N2 sorption isotherm with two distinct capillary condensation steps, corresponding to a bimodal pore system composed of 3 nm pores generated after the removal of the silica wall and ∼20 nm pores caused by the partial replication of KIT-6 silica in some regions (Figure S2 in the Supporting Information).18 The BET surface area and pore volume of bare meso-SnO2 are calculated to be 105 m2g-1 and 0.27 cm3g-1, respectively. After TiCl4 treatment and annealing,
the obtained meso-SnO2/TiO2 powder retains type-IV isotherm, indicating that formation of the ultrathin TiO2 shell layer does not significantly affect the morphology of underlying SnO2. However, the slight reduction in pore size resulted in a decrease in BET surface area (73 m2g-1) and pore volume (0.19 cm3g-1). Considering the bulk density of SnO2, these values are impressive and much higher than those of commercial SnO2 nanoparticles (Alfa Aesar, 25 m2g-1) and TiO2 coated SnO2 hollow microspheres (27 m2g-1).9c Figure 6a shows the IPCE spectra of bare and surface modified meso-SnO2 photoanode DSSCs. All devices exhibit maximum quantum efficiency at wavelengths in the 530-540 nm region. By modifying the meso-SnO2 surface with TiO2 coating, however, the IPCE was considerably increased over the entire spectral range. The photocurrent-voltage characteristics of the corresponding DSSCs measured under 1 sun condition (AM 1.5 G, 100 mW.cm-2) are shown in Figure 6b.
Ordered Mesoporous SnO2-Based Photoanodes
J. Phys. Chem. C, Vol. 114, No. 50, 2010 22035 TABLE 1: Photovoltaic Parameters of DSSCs Based on Various SnO2 Photoanodes,a under 1 Sun Illumination (100 mW · cm-2, AM 1.5G) dye uptake photoanode meso-SnO2 meso-SnO2/TiO2 meso-SnO2/Al3O3 nano-SnO2 nano-SnO2/TiO2
[10
-8
VOC -2
mol cm ] 2.64 3.96 4.32 1.68 2.63
JSC
η -2
[V]
[mA cm ]
FF
[%]
0.417 0.711 0.705 0.412 0.707
7.01 10.42 10.10 4.20 6.60
0.37 0.51 0.51 0.53 0.58
1.1 3.8 3.6 0.9 2.7
Thickness of the photoanode: ∼3 µm, Active area of the device: 0.25 cm2 a
Figure 6. (a) Incident photon to current conversion efficiency (IPCE), and (b) photocurrent-voltage characteristics of DSSCs with bare mesoSnO2 (black), TiO2 coated meso-SnO2 (red), and Al2O3 coated mesoSnO2 (green) photoanodes.
Devices comprising bare meso-SnO2 photoanodes exhibit an open-circuit voltage (VOC) of 0.417 V and short-circuit current (JSC) of 7.01 mA.cm-2, resulting in an energy conversion efficiency (η) of only 1.1%. Introduction of a TiO2 layer on meso-SnO2 caused η to increase to 3.8%. This >3-fold improvement in the device performance of meso-SnO2/TiO2 photoanode can be chiefly attributed to the modest increase in JSC and fill factor (FF), and much greater VOC. In order to confirm that the improved performance of mesoSnO2/TiO2 photoanode mainly arises from the meso-SnO2 core rather than TiO2 shell; we also fabricated meso-SnO2/Al2O3 photoanode DSSCs. Atomic layer deposition (ALD) was employed to deposit an Al2O3 layer on the meso-SnO2 photoanode. Given that the growth rate of Al2O3 on a planar surface is 2 nm/10 cycle in optimum exposure conditions, the thickness of Al2O3 layer grown by ALD for 5 cycles is estimated to be about 1 nm. The DSSC fabricated using meso-SnO2/Al2O3 photoanode shows much improved photovoltaic performance in comparison with bare meso-SnO2 (Figure 6). However, the IPCE and JSC are slightly lower than that of meso-SnO2/TiO2, yielding an energy conversion efficiency of 3.6%. As the photovoltaic parameters of DSSCs are greatly influenced by the Al2O3 shell layer thickness,19,20 further research is required to determine the optimum Al2O3 shell thickness on meso-SnO2 core for improved energy conversion efficiency. As summarized in Table 1, modifying the meso-SnO2 surface with higher isoelectric point TiO2 or Al2O3 layer increased the dye uptake capability via carboxyl linkage. Consequently, the IPCE and JSC of mesoSnO2/TiO2 or meso-SnO2/Al2O3 photoanodes are higher than those of bare meso-SnO2 photoanodes. The remarkable improvement in the VOC of meso-SnO2/TiO2 or Al2O3 photoanode may be attributed to slowed electron recombination predominantly caused by the passivation of subband-edge surface states.6a,19,21 Figure 7 shows the dark current-voltage characteristics of bare and surface modified meso-SnO2 photoanode DSSCs. In the dark and under forward bias, electrons enter into the cell through meso-SnO2 and react
Figure 7. Dark current density-voltage characteristics of DSSCs with bare meso-SnO2 (black), TiO2 coated meso-SnO2 (red), and Al2O3 coated meso-SnO2 (green) photoanodes.
Figure 8. Electron lifetime in bare and TiO2 coated meso-SnO2 photoanode DSSCs, measured by intensity modulated photovoltage spectroscopy (IMVS).
with I3- ions whereas the oxidation of I- ions takes place at Pt counter electrode.22 The shift in the dark-current onset to a higher potential confirms the reduced electron recombination in surface modified meso-SnO2 photoanodes. Minimizing the charge recombination in DSSC also gives rise to an increase in shunt resistance and thereby improved FF.23 Figure 8 shows the lifetime of photoinjected electrons (τn) in bare and surface modified meso-SnO2 photoanodes measured by intensity modulated photovoltage spectroscopy (IMVS). Over the range of measured light intensity, meso-SnO2/TiO2 photoanode possessed higher τn compared with similar thickness bare meso-SnO2 photoanodes. Introduction of an ultrathin TiO2 or Al2O3 layer on meso-SnO2 may have passivated the reactive low-energy SnO2 surface states; accordingly, the device shows an inhibited electron recombination rate and higher values of VOC. For comparison, DSSCs with commercially available nanoparticle SnO2 photoanodes (denoted as nano-SnO2) were also fabricated and tested under similar conditions. Figure 9a shows the IPCE of bare and TiO2-coated nano-SnO2 photoanode DSSCs. The surface modification of nano-SnO2 photoanodes
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Ramasamy and Lee is much higher than nano-SnO2 throughout the visible range, demonstrating an improved light scattering ability of meso-SnO2 particles. Interestingly, meso-SnO2/TiO2 photoanode exhibits further enhanced reflectance. Therefore, the higher JSC of the meso-SnO2/TiO2 photoanode compared with the nano-SnO2/ TiO2 photoanode may be attributed to the enhanced dye-uptake and light scattering abilities. 4. Conclusions
Figure 9. (a) Incident photon to current conversion efficiency (IPCE), and (b) current-voltage characteristics of DSSCs with bare nano-SnO2 and nano-SnO2 /TiO2 photoanodes.
We have shown that a surface modified ordered mesoporous SnO2 photoanode with high surface area and 3D bicontinuous cubic mesostructure is a superior candidate for realizing highperformance DSSCs. The developed device had a 3-µm-thick meso-SnO2/TiO2 photoanode exhibiting an η of 3.8% and 40% higher performance compared with conventional nanoparticle SnO2/TiO2 photoanodes. The superior photovoltaic performance of the meso-SnO2/TiO2 photoanode is attributed to its features of high dye-uptake capability and enhanced light scattering ability. Our results have greatly increased the scope of research into the role of ordered mesoporous materials in energy conversion device applications. Acknowledgment. This work was supported by a National Research Foundation of Korea Grant funded by the Korean Government (2009-0064640). Supporting Information Available: Synthesis of KIT-6 silica and scheme and SEM images of meso-SnO2 with bimodal pores and SEM cross-section images of various SnO2 photoanodes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 10. Diffused reflectance spectra of bare and TiO2 coated nanoSnO2 and meso-SnO2 films.
by TiCl4 treatment increased the IPCE, in a similar manner as the meso-SnO2 system. However, the notable difference in the IPCE spectra of nano-SnO2 and meso-SnO2 comes from substantially lower quantum efficiency in the short-wavelength region (400-600 nm) and poor sensitivity in the long wavelength region (600-700 nm). The relatively smaller IPCE of nano-SnO2/TiO2 in the short wavelength region is a result of low dye-loading associated with the small surface area of SnO2 nanoparticles. Furthermore, photoanodes made up of 20-30 nm size particles did not efficiently scatter the incident light, especially in the long wavelength region.24 Therefore, nanoSnO2/TiO2 photoanode devices display hardly any quantum efficiency in long wavelength regions. Figure 9b shows the current-voltage characteristics of bare and surface modified nano-SnO2 photoanode DSSCs. Devices incorporating bare nano-SnO2 photoanodes showed 1% energy conversion efficiency. Following TiCl4 treatment, the photovoltaic parameters (VOC, JSC, FF) were improved to 0.707 V, 6.6 mA.cm-2, and 0.58, respectively. The obtained η of 2.7% for a 3-µm-thick nano-SnO2/TiO2 photoanode is about 30% lower than that of a meso-SnO2/TiO2 photoanode (3.8%) of similar thickness. In addition to higher dye loading capability, the meso-SnO2/TiO2 photoanode appears to possess enhanced light scattering ability. Figure 10 compares the diffused reflectance spectra of bare and TiO2 coated meso-SnO2 photoanodes, along with those of nanoSnO2 photoanodes. The reflectance of meso-SnO2 photoanodes
(1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) (a) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. 2006, 45, L638. (b) Chen, C.-Y.; Wang, M.; Li, J.-Y.; Pootrakulchote, N.; Alibabaei, L.; Ngocle, C.; Decoppet, J.-D.; Tsai, J.-H.; Gra¨tzel, C.; Wu, C.-G.; Zakeeruddin, S. M.; Gra¨tzel, M. ACS Nano 2009, 3, 3103. (3) (a) Hendry, E.; Koeberg, M.; O’Regan, B.; Bonn, M. Nano Lett. 2006, 6, 755. (b) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (4) (a) Rensmo, H.; Keis, K.; Lindstro¨m, H.; So¨dergren, S.; Solbrand, A.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1997, 101, 2598. (b) Chappel, S.; Chen, S.; Zaban, A. Langmuir 2002, 18, 3336. (c) Tan, B.; Toman, E.; Li, Y.; Wu, Y. J. Am. Chem. Soc. 2007, 129, 4162. (d) Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Mater. 1998, 10, 3825. (5) Jarzebski, Z. M.; Marton, J. P. J. Electrochem. Soc. 1976, 123, 299C. (6) (a) Snaith, H. J.; Ducati, C. Nano Lett. 2010, 10, 1259. (b) Mishra, A.; Fischer, M. K. R.; Ba¨uerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474. (7) Senevirathna, M. K. I.; Pitigala, P.; Premalal, E. V. A.; Tennakone, K.; Kumara, G. R. A.; Konno, A. Sol. Energy Mater. Sol. Cells 2007, 9, 544. (8) Green, A. N. M.; Palomares, E.; Haque, S. A.; Kroon, J. M.; Durrant, J. R. J. Phys. Chem. B 2005, 109, 12525. (9) (a) Kay, A.; Gra¨tzel, M. Chem. Mater. 2002, 14, 2930. (b) Chapperl, S.; Chen, S.; Zaban, A. Langmuir 2002, 18, 3336. (c) Qian, J.; Liu, P.; Ziao, Y.; Jiang, Y.; Cao, Y.; Ai, X.; Yang, H. AdV. Mater. 2009, 21, 3663. (10) (a) Adachi, M.; Murata, Y.; Takao, J.; Sakamoto, M.; Wang, F. J. Am. Chem. Soc. 2004, 126, 14943. (b) Kang, S. H.; Choi, S.-H.; Kang, M.-S.; Kim, J.-Y.; Kim, H.-S.; Hyeon, T.; Sung, Y.-E. AdV. Mater. 2008, 20, 54. (11) (a) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mat. 2005, 4, 455. (b) Feng, X.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781. (12) (a) Zukalova, M.; Zukal, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Gra¨tzel, M. Nano Lett. 2005, 5, 1789. (b) Nedelcu, M.; Lee, J.; Crossland, E. J. W.; Warren, S. C. W.; Orilall, M. C.; Guldin, S.; Huttner, S.; Ducati, C.; Eder, D.; Wiesner, U.; Steiner, U.; Snaith, H. J. Soft Matter 2009, 5, 134. (c) Crossland, E. J. W.; Kamperman, M.; Nedelcu, M.; Ducati, C.; Wiesner, U.; Smilgies, D.-M.; Toombes, G. E. S.; Hillmyer, M. A.; Ludwigs, S.; Steiner, U.; Snaith, H. J. Nano Lett. 2009, 9, 2807. (d) Lee, J.; Oriall,
Ordered Mesoporous SnO2-Based Photoanodes M. C.; Warren, S. C.; Kamperman, M.; DiSalvo, F. J.; Wiesner, U. Nat. Mater. 2008, 7, 222. (13) (a) Ramasamy, E.; Lee, J. Chem. Commun. 2010, 46, 2136. (b) Ramasamy, E.; Lee, J. Carbon 2010, 48, 3715. (c) Ramasamy, E.; Chun, J.; Lee, J. Carbon 2010, 48, 4556. (14) Hou, K.; Puzzo, D.; Helander, M. G.; Lo, S. S.; Bonifacio, L. D.; Wang, W.; Lu, Z.-H.; Scholes, G. D.; Ozin, G. A. AdV. Mater. 2009, 21, 2492. (15) Park, K.; Zhang, Q.; Garcia, B. B.; Zhou, X.; Jeong, Y.-H.; Cao, G. AdV. Mater. 2010, 22, 2329. (16) Shon, J. K.; Kong, S. S.; Kim, Y. S.; Lee, J.-H.; Park, W. K.; Park, S. C.; Kim, J. M. Micropor. Mesopor. Mater. 2009, 120, 441. (17) Kim, T. W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 7601. (18) Shi, Y.; Guo, B.; Corr, S. A.; Shi, Q.; Hu, Y.-S.; Heier, K. R.; Chen, L.; Seshadri, R.; Stucky, G. D. Nano Lett. 2009, 9, 4215. (19) Prasittichai, C.; Hupp, J. T. J. Phys. Chem. Lett. 2010, 1, 1611.
J. Phys. Chem. C, Vol. 114, No. 50, 2010 22037 (20) (a) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. J. Phys. Chem. B. 2006, 110, 22562. (b) Lin, C.; Tsai, F.-Y.; Lee, M.-H.; Lee, C.-H.; Tien, T.-C.; Wang, L.-P.; Tsai, S.-Y. J. Mater. Chem. 2009, 19, 2999. (c) Tien, T.-C.; Pan, F.-M.; Wang, L.-P.; Tsai, F.-Y.; Lin, C. J. Phys. Chem. C. 2010, 114, 10048. (21) Gubbala, S.; Chakrapani, V.; Kumar, V.; Sunkara, M. K. AdV. Funct. Mater. 2008, 18, 2411. (22) Ito, S.; Liska, P.; Comte, P.; Charvet, R.; Pechy, P.; Bach, U.; Schmidt-Mende, L.; Zakeeruddin, S. M.; Kay, A.; Nazeeruddin, M. K.; Gra¨tzel, M. Chem. Commun. 2005, 4351. (23) Koide, N.; Islam, A.; Chiba, Y.; Han, L. J. Photochem. Photobiol. A: Chem. 2006, 182, 296. (24) Koo, H.-J.; Park, J.; Yoo, B.; Yoo, K.; Kim, K.; Park, N.-G. Inorg. Chim. Acta 2008, 361, 677.
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