Letter pubs.acs.org/Langmuir
TiO2 Derived by Titanate Route from Electrospun Nanostructures for High-Performance Dye-Sensitized Solar Cells A. Sreekumaran Nair,*,†,⊥ Peining Zhu,‡,⊥ V. Jagadeesh Babu,† Shengyuan Yang,§ Thirumal Krishnamoorthy,∥ Rajendiran Murugan,† Shengjie Peng,∥ and Seeram Ramakrishna*,‡ †
Healthcare and Energy Materials Laboratory, Nanoscience and Nanotechnology Initiative, National University of Singapore, 117584, Singapore ‡ Department of Mechanical Engineering, National University of Singapore, 117574, Singapore § NUS Graduate School for Integrative Sciences and Engineering, Singapore, 117456, Singapore ∥ School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798, Singapore S Supporting Information *
ABSTRACT: We report the use of highly porous, dense, and anisotropic TiO2 derived from electrospun TiO2−SiO2 nanostructures through titanate route in dye-sensitized solar cells. The titanate-derived TiO2 of high surface areas exhibited superior photovoltaic parameters (efficiency > 7%) in comparison to the respective electrospun TiO2 nanomaterials and commercially available P-25.
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the action of concentrated alkali which was first reported by Kasuga et al.11,12) could offer a solution to this problem. Selective leaching of one of the components during the chemical transformation by NaOH from the composite fibers would create additional porosity and hence high surface area for the latter component. Thus, we have fabricated electrospun TiO2−SiO2 composite nanofibers and rice-shaped nanostructures that were treated with NaOH (5 M) at slightly elevated temperatures (80 °C, SiO2 is an acidic oxide which could be completely etched by NaOH,13 and we assumed that the etching will make the TiO2 highly porous) to get the titanates. The titanates were subsequently subjected to a hydrothermal treatment with HCl (0.1 M at 180 °C for 24 h in a stainless steel hydrothermal bomb) which converted them into high surface area TiO2. The titanate-derived TiO2 when employed in DSCs displayed an efficiency which was ∼50% higher than that of the respective electrospun TiO2 materials.
INTRODUCTION Dye-sensitized solar cells (DSCs)1,2 have received considerable attention in renewable energy research owing to favorable factors that include (a) simple nonvacuum processing conditions, (b) low cost due to the relatively high natural abundance of raw materials, and (c) properties that include being lightweight, high tolerance to impurities, comparable conversion efficiencies to that of amorphous Si solar cells, and suitability for building-integrated photovoltaics (in view of the optical transparency of the electrodes and the panoramic views offered by different dyes).3 High surface area TiO2 is the backbone of DSCs as the sensitizers (dyes) are loaded on it, and it also transports the photoexcited electrons from the sensitizer to the transparent conducting oxide substrate.1,2 Efforts to fabricate high surface area TiO2 are an ongoing process which ultimately will facilitate efficient DSCs’ fabrication by a single step process, thus eliminate the need for the conventional double layer configuration4 (consisting of active and scattering layers, which is a technology- and costintensive process) in DSCs. One of the issues with the electrospun TiO2 nanomaterials when employed in DSCs is the low conversion efficiency (typically ∼5%) due to poor surface areas (∼40−60 m2/g,5−9 however, there is also a report on DSC using electrospun TiO2 nanorods reporting an efficiency of >9%10). We anticipated that the titanate route (chemical transformation of TiO2 nanoparticles into rod/wire/sheet-like titanate/TiO2 structures by © 2012 American Chemical Society
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EXPERIMENTAL DETAILS
a. Fabrication of Rice-Shaped TiO2−SiO2 Composites. Riceshaped TiO2−SiO2 composites were prepared by electrospinning technique based on optimized procedures reported in our previous publications.14,15 Here is a brief account of the procedure. About 2.4 g of polyvinyl acetate (PVAc, Mw = 500 000, Sigma Aldrich) was added Received: January 13, 2012 Published: April 2, 2012 6202
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Figure 1. SEM images of the electrospun TiO2−SiO2−PVP (A) and TiO2−SiO2−PVAc (B) composite fibers. Panel (C) is a representative highresolution image of (B). Panels (D) and (E) show the SEM images of the sintered nanostructures from (A) and (B), respectively. Panel (F) shows the result of EDS mapping from (B) showing the elemental composition. to 20 mL of N,N-dimethyl acetamide (DMAc, 99.8%, Aldrich) under magnetic stirring. To this 4 mL of acetic acid (99.7%), 1.75 mL of titanium(IV) isopropoxide (TiP, 97%, Aldrich, Germany), and 0.25 mL of tetraethoxysilane (TES, 99%, Sigma-Aldrich, Germany) were added. The mixture was stirred for about 12 h to attain sufficient viscosity for electrospinning. The electrospinning was done at 30 kV at a flow rate of 1 mL/h using a commercial machine, NANON (MECC, Japan). The distance between the needle tip (27G 1/2) and the collector (which is a rotating drum wrapped with an aluminum foil) was ∼10 cm. The humidity level inside the electrospinning chamber was maintained between 50 and 60%. The as-spun composite fibers (TiO2−SiO2−PVAc) were removed from the collector in the form of a membrane and were sintered at 500 °C for 1 h to get the rice-shaped TiO2−SiO2 composite. The as-spun fibers and the sintered composites were characterized by spectroscopy and microscopy. b. Fabrication of TiO2−SiO2 Nanofibers. The nanofiber-shaped TiO2−SiO2 composites were prepared by a similar procedure as described above, except that the PVAc polymer was replaced with polyvinylpyrrolidone (PVP, Mw = 1.30 × 106 Aldrich). About 0.6 g of PVP was dissolved in 14 mL of absolute ethanol (Fischer Scientific). Four milliliters of acetic acid (99.7%) and a mixture of 1.75 mL of TiP and 0.25 mL of TES were subsequently added to the polymer solution. The mixture was stirred for 12 h and subjected to electrospinning under the conditions described in the experimental section a. The asspun composite fibers (TiO2−SiO2−PVP) were sintered at 500 °C for 1 h to get the TiO2−SiO2 nanofibers. The as-spun and the sintered nanofibers were characterized by spectroscopy and microscopy. c. Fabrication of TiO2 Nanostructures from the TiO2−SiO2 Composites. Five hundred milligrams of the TiO2−SiO2 composites (nanofibers and the rice grain-shaped separately) was treated with 100 mL of 5 M NaOH solution (in glass vials) at 80 °C in an oven for 24 h for structural rearrangement of TiO2 and in situ etching of SiO2. The treated materials (the sodium titanates) were washed several times with deionized water. The respective materials were subjected to hydrothermal treatment with HCl (0.1 M) in a hydrothermal bomb (lined with an inner Teflon coating) at 180 °C for 24 h. The material was collected and washed repeatedly with water and finally with
ethanol and was used for subsequent analysis by spectroscopy and microscopy. d. Fabrication of Dye-Sensitized Solar Cells. DSCs were fabricated according to a procedure reported previously.6,14 About 100 mg of the TiO2 was mixed with 100 μL of polyester (made by the polycondenzation of citric acid and ethylene glycol at elevated temperatures6) and sonicated for 12 h to get a smooth paste. The paste was screen printed twice and doctor-bladed once on TiCl4treated fluorine-doped tin oxide (FTO) plates (2.2 mm thick with a sheet resistance of 15 Ω/□ from Solaronix) to a thickness of 15 μm. This was sintered at 450 °C for 1 h (the thickness of TiO2 then reduces to 12 μm due to polymer evaporation), cooled down to 100 °C, and then soaked in N3 dye solution for 24 h for saturate dye loading. The active area of the (square-shaped) dye-loaded TiO2 was 0.25 cm2. This was sealed against the Pt counter electrode in presence of a spacer and iodide electrolyte for DSC measurements at Standard Solar Spectra AM1.5 G conditions. e. Characterization. Scanning electron microscopy (SEM) was done using a JEOL JSM-6701F field emission scanning electron microscope operated at 30 kV. High-resolution transmission electron microscopy (HRTEM) was performed using a JEOL 3010 machine operated at 300 kV. Brunauer−Emmett−Teller (BET) surface area was measured using a NOVA 4200E surface area and pore size analyzer (Quantachrome). The samples were dried under flowing N2 at 350 °C overnight prior to BET measurements under standard protocols at 77 K. Powder XRD was done using a Bruker-AXS D8 ADVANCE spectrometer. Photocurrent measurements were done under 1 sun illumination using an XES-151 S solar simulator (San-Ei, Japan) under AM1.5 G conditions and an Autolab PGSTAT30 (Eco Chemie B.V., The Netherlands), respectively, with automatic data acquisition. Incident photon-to-electron conversion efficiency (IPCE) was measured using an IPCE evaluation system for dye-sensitized solar cells (Bunkoh-Keiki Co. Ltd., CEP-2000). X-ray photoelectron spectroscopy (XPS) was done using a calibrated Thermo Scientific Theta Probe XPS instrument. Monochromatic Al Kα X-ray (hν = 1486.6 eV) was used for analysis with an incident angle 30° with respect to surface normal. Photoelectrons were collected at a takeoff angle of 50° with respect to the surface normal. The analysis area was 6203
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Figure 2. XRD spectra of the titanate-derived TiO2 obtained from TiO2−SiO2 fibers (A) and rice-shaped TiO2−SiO2 composite (B).
Figure 3. SEM images (low- and high-magnification, respectively) of the titanate-derived TiO2. (A) Fiber shaped, (D) rice-shaped. Inset of (A) shows a magnified image of the fibers. (B, E) High-resolution TEM images of the fibers and the rice-shaped TiO2. Inset of (B) shows a magnified image of a fiber. (C, F) Lattice resolved images. Insets of (C) and (F) show SAED patterns revealing the crystallinity of the TiO2. approximately 400 μm in diameter, while the maximum analysis depth was in the range of 4−10 nm. Survey- and high-resolution spectra were acquired for analysis of surface composition and for oxidation state identification, respectively. Carbon correction was made based on the C1s peak at 285.0 eV using the manufacturer’s standard software.
rice-shaped nanostructure is made of nearly spherical crystals of 15−20 and 10−15 nm diameters. The resolved images of the crystals indicate prominent lattice spacings corresponding to mainly of anatase TiO2 regions. No lattice spacing was seen for SiO2 in the HRTEM images implying that it could be amorphous. Powder XRD spectrum of the TiO2 −SiO 2 composite showed nearly well-defined peaks for TiO2 (again no peak was obvious for the SiO2 confirming its amorphous nature13 which further verified the observations from TEM). The TiO2 was mostly anatase in phase (major peaks are indexed in the spectrum) with a bit of rutile impurity (marked by R in the spectrum in Supporting Information 2). The BET surface areas of the metal oxide composites were 32 m2/g for the fibers and 36 m2/g for the rice-shaped ones. The elemental composition of the sample mapped using energy dispersive Xray spectroscopy (EDS) is given in Figure 1F which is consistent with the stated composition of the composite (TiO2−SiO2). Major elements detected were Ti, Si and O. The presence of Pt was from the thin conductive coating on the sample. The energy dispersive X-ray spectrum (EDS) further implied the absence of impurities such as C, N, etc. (probable from the polymer degradation) in the sintered samples. The material was additionally characterized by XPS (Supporting Information 3), which further validated the elemental
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RESULTS AND DISCUSSION Figures 1A and B show the SEM images of the TiO2−SiO2− PVP and TiO2−SiO2−PVAc composite nanofibers produced by electrospinning. A high-magnification image of the fibers is shown in Figure 1C. The fibers were smooth and continuous with diameters ranging from ∼50−350 nm. The nanofibers upon sintering resulted in fiber- and rice-shaped TiO2−SiO2 composites (Figure 1D and E). The mechanism behind the formation of nanofibers and rice-shaped TiO2 nano/mesostructures from TiO2−PVP and TiO2−PVAc composites has already been well-documented.5,7 The diameter of the sintered nanofibers was in the range of ∼110−200 nm and the reduction in fiber diameter was because of the evaporation of the polymer from the composite matrix. The dimensions of the rice-shaped composites were in the range of 280−510 nm in length and 50−110 nm in breadth. High-resolution TEM images of the sintered nanostructures are shown in Supporting Information 1. It is evident from the images that each fiber- and 6204
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Figure 4. (A) Current density vs voltage plots of titanate-derived fiber- and rice-shaped TiO2 (traces a and b). Trace c shows that of P-25 TiO2. (B) Respective IPCE traces.
composition of the composite. Only the Ti4+ oxidation state was prominent and no other intermediate ones. The O 1S peak was deconvoluted into three peaks at 530.7, 531.5, and 532.8 eV, respectively, which are indicative of the presence of Ti−O− Ti, Si−O−Si, and Ti−O−Si bonds which further imply that the SiO2 could be embedded into the TiO2 matrix through chemical (Ti−O−Si) bonds.13 Figure 2 shows the powder X-ray diffraction (XRD) data of the titanate-derived TiO2 showing high crystallinity of the both. Major peaks are assigned (purely of anatse phase) in the spectra itself (PCPDFWIN # 211272). XRD results further indicate the absence of impurity (e.g., SiO2 in case it is leaching from the TiO2 matrix is incomplete) in the samples. Figures 3A and D show the SEM images of the titanate-derived TiO2 obtained from the above samples as a result of the hydrothermal treatment. The nanofiber and rice morphologies were still retained for the materials under the hydrothermal conditions; however, they became porous owing to the NaOH-assisted dissolution of SiO2 from the TiO2 matrix (see Supporting Information 4 and 6 for high resolution images of the porous fibers and rice-shaped TiO2, respectively). A high magnification SEM image of the nanofibers is shown at the inset of Figure 3A. The average diameter of the nanofiber- and the rice-shaped TiO2 were slightly lower (80−150 nm for fibers and 250−400 nm in length and 50−110 nm in breadth for the rice-shaped ones) than that of the TiO2−SiO2 composites which could be attributed to the selective leaching of SiO2 from the TiO2−SiO2 matrix. The EDS spectrum of the samples (acquired from a large area) showed the complete leaching of SiO2 from the TiO2 matrix, implying that the hydrothermally treated materials were purely of TiO2 after washing and drying (Supporting Information 7). Figure 3B and E, respectively, show the TEM images of the titanate-derived fiber- and rice-shaped TiO2. A magnified image of a single nanofiber is shown in the inset of Figure 3B which reveals that each nanofiber is made-up of spherical crystals of 8−10 nm in diameters (see an enlarged image in Supporting Information 5). Figure 3E shows that the rice-shaped TiO2 is made-up of spherical crystals of ∼10 nm diameter. High-resolution TEM images of the fiber- and the rice-shaped TiO2 (Figure 3C and F, respectively) showed a lattice spacing of 0.35 nm corresponding to the anatase (101 lattice) phase of TiO2. The selected-area electron diffraction (SAED) patterns shown at the insets reveal the crystalline nature of the TiO2. XPS survey and high-resolution spectra further testified the absence of impurities and purity of the TiO2 (Supporting Information 8). Note the absence of Si in the sample which further proved the complete leaching of SiO2
from the TiO2 matrix. The BET surface areas were 123 m2/g for the fiber-based TiO2 and 110 m2/g for the rice-shaped TiO2 which are nearly 3.8 and 3.1 times compared to the starting materials (TiO2−SiO2 composites) and ∼1.8 and 2.8 times in comparison to the rice- and fiber-shaped TiO2 (BET surface areas of electrospun (directly sintered without hydrothermal treatment) rice- and fiber-shaped TiO2 were 60 and 44 m2/g, respectively). The pore volumes in the respective cases were ∼1.23 and ∼1.11 cm3/g, respectively, which are nearly an order of magnitude higher than that in the respective electrospun TiO2 nanomaterials (∼0.13 cm3/g).5−7 The titanate-derived TiO2 were exploited for DSC applications in view of their excellently high surface areas and large pore volumes. Traces a and b in Figure 4 show the photovoltaic responses of the DSCs. The best performance was shown by the fiber-shaped TiO2 (trace a) which showed the photovoltaic (PV) parameters of a current density (Jsc) of 14.38 mA/cm2 (14.12 mA/cm2 for the TiO2 obtained from riceshaped TiO2−SiO2 composite), an open-circuit voltage (Voc) of 0.75 V (0.74 V), a fill-factor (FF) of 64.6 (64.0), and an overall conversion efficiency (η) of 7.02% (6.72%). The slightly enhanced efficiency resulted from the large surface area of the fiber-shaped TiO2 in comparison to the rice-shaped one. This was additionally confirmed from the dye-loading measurements, the values of which were 1.67 × 10−7 and 1.62 × 10−7 mol cm−2, respectively, in the cases of the former and the latter. This is further reflected from the incident photon-to-electron conversion efficiency (IPCE) traces (Figure 4B). Note that the IPCE peaks (∼70% and 68%, respectively) for both the titanate-derived TiO2 were nearly the same which follows the same trend of the efficiency from the Jsc−V traces. The PV and IPCE parameters reported were the average values from four tested devices. It is also interesting to note that the PV parameters of the titanate-derived TiO2 were higher (∼ 50%) than that of the fiber/rice grain-shaped anatase TiO2 (4.63% and 4.49%)5−9 and commercial P-25 (trace c in Figure 4). The efficiency was even comparable to the standard TiO2 paste from Solaronix.16,17 The superior nature of the titanate-derived TiO2 over the electrospun TiO2 nanostructures and P-25 in photovoltaics justifies the objective of the present research. Further, the efficiency reported in the present paper by the methodology is one of the best among the category of titanate-derived TiO218−23 and electrospun TiO2 materials.5−9 In summary, we have found that the NaOH-assisted etching of SiO2 from electrospun TiO2−SiO2 composite nanofibers and rice-shaped nano/mesostructures and their subsequent hydro6205
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Sensitized Solar Cells Based on Electrospun TiO2 Nanorod Photoelectrodes. J. Phys. Chem. C. 2009, 113, 21453−21457. (11) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Formation of Titanium Oxide Nanotube. Langmuir 1998, 14, 3160− 3163. (12) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Titania Nanotubes Prepared by Chemical Processing. Adv. Mater. 1999, 11, 1307−1311. (13) Su, C.; Lin, K.-F.; Lin, Y.-H.; You, B.-H. Preparation and Characterization of High-Surface-Area Titanium Dioxide by Sol-Gel Process. J. Porous Mater. 2006, 13, 251−258. (14) Nair, A. S.; Peining, Z.; Babu, V. J.; Shengyuan, Y.; Shengjie, P.; Ramakrishna, S. Highly Anisotropic Titanates from Electrospun TiO2−SiO2 Composite Nanofibers and Rice Grain-Shaped Nanostructures. RSC Adv. 2012, 2, 992−998. (15) Nair, A. S.; Peining, Z.; Babu, V. J.; Shengyuan, Y.; Ramakrishna, S. Anisotropic TiO2 Nanomaterials in Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 21248−21261. (16) Zhu, P.; Nair, A. S.; Yang, S.; Peng, S.; Ramakrishna, S. Which is a Superior Material for Scattering Layer in Dye-Sensitized Solar CellsElectrospun Rice Grain- or Nanofiber-Shaped TiO2? J. Mater. Chem. 2011, 21, 12210−12212. (17) Peng, S.; Wu, Y.; Zhu, P.; Thavasi, V.; Mhaisalkar, S. G.; Ramakrishna, S. Facile Fabrication of Polypyrrole/Functionalized Multiwalled Carbon Nanotubes Composite as Counter Electrodes in Low-Cost Dye-Sensitized Solar Cells. J. Photochem. Photobiol., A 2011, 223, 97−102. (18) Ohsaki, Y.; Masaki, N.; Kitamura, T.; Wada, Y.; Okamoto, T.; Sekino, T.; Niihara, K.; Yanagida, S. Dye-Sensitized TiO2 Nanotube Solar Cells: Fabrication and Electronic Characterization. Phys. Chem. Chem. Phys. 2005, 7, 4157−4163. (19) Kim, G.-S.; Seo, H.-K.; Godble, V. P.; Kim, Y.-S.; Yang, O.-B.; Shin, H.-S. Electrophoretic Deposition of Titanate Nanotubes from Commercial Titania Nanoparticles: Application to Dye-Sensitized Solar Cells. Electrochem. Commun. 2006, 8, 961−966. (20) Shao, F.; Sun, J.; Gao, L.; Yang, S.; Luo, J. Growth of Various TiO2 Nanostructures for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2011, 115, 1819−1823. (21) Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, H.; Arakawa, H. Utilization of Titanate Nanotubes as an Electrode Material in DyeSensitized Solar Cells. J. Electrochem. Soc. 2006, 153, A1232−A1236. (22) Qiu, Y.; Chen, W.; Yang, S. Double-Layered Photoanodes from Variable-Size Anatase TiO2 Nanospindles: A Candidate for HighEfficiency Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2010, 49, 3675−3679. (23) Qu, J.; Li, G. R.; Gao, X. P. One-Dimensional Hierarchical Titania for Fast Reaction Kinetics of Photoanode Materials of DyeSensitized Solar Cells. Energy Environ. Sci. 2010, 3, 2003−2009.
thermal treatment with HCl resulted in high surface area TiO2 nanomaterials. The materials when employed in DSCs showed nearly 50% higher efficiency than the respective electrospun TiO2 nanomaterials (electrospun fiber and rice-shaped TiO2 nanostructures without hydrothermal conditions) and commercially available P-25. We believe that the facile two-stage fabrication of high surface area TiO2 will open up additional applications in photocatalysis, Li-ion batteries, water splitting, and so on.
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ASSOCIATED CONTENT
* Supporting Information S
TEM images, XRD spectra, and XPS data of the TiO2−SiO2 composite and large area SEM images, EDS, and XPS spectra of titanate-derived TiO2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (A.S.N.);
[email protected] (S.R.). Telephone: 65-6516 6593. Fax: +65-6773 0339. Author Contributions ⊥
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial assistance from National Research Foundation (NRF 2007 EWT-CERP 01-0531) and M3TC (Economic Development Board) (R-261-501-018-414) of Singapore.
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REFERENCES
(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Grätzel, M. Conversion of Sunlight to Electric Power by Nanocrystalline Dye-Sensitized Solar Cells. J. Photochem. Photobiol., A 2004, 164, 3−14. (3) GonÇ alves, L. M.; de Zea Bermudez, V.; Ribeiro, H. A.; Mendes, A. M. Dye-Sensitized Solar Cells: A Safe Bet for the Future. Energy Environ. Sci. 2008, 1, 655−667. (4) Sauvage, F.; Chen, D.; Comte, P.; Huang, F.; Heiniger, L. P.; Cheng, Y. B.; Caruso, R. A.; Grätzel, M. Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 Beads Achieve Power Conversion Efficiencies Over 10%. ACS Nano 2010, 4, 4420−4425. (5) Nair, A. S.; Shengyuan, Y.; Peining, Z.; Ramakrishna, S. Rice Grain-Shaped TiO2 Mesostructures by Electrospinning for DyeSensitized Solar Cells. Chem. Commun. 2010, 46, 7421−7423. (6) Nair, A. S.; Jose, R.; Shengyuan, Y.; Ramakrishna, S. A Simple Recipe for an Efficient TiO2 Nanofiber-Based Dye-Sensitized Solar Cell. J. Colloid Interface Sci. 2011, 353, 39−45. (7) Shengyuan, Y.; Peining, Z.; Nair, A. S.; Ramakrishna, S. Rice Grain-Shaped TiO2 Mesostructures-Synthesis, Characterization and Applications in Dye-Sensitized Solar Cells and Photocatalysis. J. Mater. Chem. 2011, 21, 6541−6458. (8) Song, M. Y.; Kim, D. K.; Ihn, K. J.; Jo, S. M.; Kim, D. Y. Electrospun TiO2 Electrodes for Dye-Sensitized Solar Cells. Nanotechnology 2004, 15, 1861−1865. (9) Kokubo, H.; Ding, B.; Naka, T.; Tsuchihira, H.; Shiratori, S. Multi-Core Cable-like TiO2 Nanofibrous Membranes for DyeSensitized Solar Cells. Nanotechnology 2007, 18, 165604. (10) Lee, B. H.; Song, M. Y.; Jang, S.-Y.; Jo, S. M.; Kwak, S.-Y.; Kim, D. Y. Charge Transport Characteristics of High Efficiency Dye6206
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