Vertically Oriented TiO2 Nanotube Arrays Grown on Ti Meshes for

Jul 1, 2009 - We describe here the growth of vertically oriented TiO2 nanotube arrays on Ti meshes by electrochemical anodization and their applicatio...
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Vertically Oriented TiO2 Nanotube Arrays Grown on Ti Meshes for Flexible Dye-Sensitized Solar Cells Zhaoyue Liu, Vaidyanathan (Ravi) Subramania, and Mano Misra* Department of Chemical and Metallurgical Engineering, MS 388, UniVersity of NeVada, Reno, NeVada 89557 ReceiVed: April 10, 2009; ReVised Manuscript ReceiVed: May 19, 2009

We describe here the growth of vertically oriented TiO2 nanotube arrays on Ti meshes by electrochemical anodization and their application as flexible electrodes for dye-sensitized solar cells (DSC). The dependence of physical features (i.e., diameter, wall thickness, and length) of TiO2 nanotube arrays on the anodization duration is systematically studied. Our results indicate that this type of flexible electrodes features transparency and high bend ability when subjected to external force. Varying the length of nanotube arrays has been demonstrated to critically influence the solar-to-electric conversion efficiency of DSCs. In combination with a low-volatility organic electrolyte and a rigid platinized conductive glass counter electrode, TiO2 nanotube arrays with an optimized length of 40 µm on a 50-mesh substrate achieve a conversion efficiency of 1.47% (calculated from the geometric area of Ti mesh) at 97.4 mW/cm2 AM 1.5 simulated full light. A lightweight flexible DSC is also fabricated using the same nanotube arrays combined with a flexible Pt/ITO/PET counter electrode, which generates a conversion efficiency of 1.23%. 1. Introduction Dye-sensitized solar cells (DSCs) are attracting widespread research interest for converting sunlight to electricity because of their low cost and high efficiency.1-5 A dye-sensitized mesoporous nanocrystalline TiO2 film on conductive glass is the heart of DSCs and has achieved a record efficiency of 11% in combination with a volatile I3-/I- electrolyte and a platinized conductive glass counter electrode.6-8 However, the rigid and fragile glass substrate restricts the flexibility, shape, weight, and overall thickness of the devices, resulting in some complexities in transport and installation. Lightweight flexible substrates, on the other hand, show an impressive potential to overcome these disadvantages. In the past several years, indium tin oxide coated poly(ethylene terephthalate) (ITO/PET) films have been considered as an attractive flexible substrate for DSCs. A variety of strategies have been explored to deposit mesoporous TiO2 nanoparticulate film on ITO/PET.9-16 However, this polymer substrate can only withstand a low-temperature sintering process, which results in a poor crystallization and interparticle connection, hampering the photovoltaic performance.17-20 Alternatively, metal foils are another kind of potential flexible substrates for DSCs owing to their low electric resistance and high heat-resistant ability.17-19 A high conversion efficiency of 7.2% has been achieved for flexible DSCs based on TiO2 nanoparticle-coated Ti foil.18 Recently, vertically oriented TiO2 nanotube arrays on Ti foil prepared by electrochemical anodization are attracting significant attention in flexible DSCs application,21,22 because of the intuitive one-dimensional electric channel and large internal surface area. Detailed studies have shown that the charge recombination in nanotube-array-based DSCs is much slower than that in nanoparticle-based DSCs, which results in improved charge collection efficiency.23-27 However, the opacity and low flexibility of metal foils are restricting some possible applications of flexible DSCs. Metal meshes can act as a promising substitute for metal foils as the * To whom correspondence should be addressed. Telephone: (775) 7841603. E-mail: [email protected].

flexible substrates of DSCs,28 since they can withstand a hightemperature sintering process, simultaneously possessing high flexibility and transparency. Therefore, development of more efficient TiO2 nanomaterials on metal mesh is very necessary to potentially improve the conversion efficiency of mesh-based flexible DSCs. Electrochemical anodization has the versatility to grow TiO2 nanotube arrays on any Ti substrate independent of its geometry. Recently, we demonstrated the preparation of TiO2 nanotube arrays on Ti wires using the anodization method. An improved photocatalytic activity toward a textile dye degradation compared to nanotube arrays over foils is noted.29 In this article, we fabricated vertically oriented TiO2 nanotube arrays with controllable lengths on Ti meshes by electrochemical anodization in ethylene glycol-based electrolyte and investigated their potential application as flexible electrodes for DSCs. Our results indicate that this type of flexible electrodes features transparency and high bend ability when subjected to external force. Varying the length of nanotube arrays systematically has been demonstrated to critically influence the conversion efficiency of DSCs. Combined with a rigid platinized conductive glass counter electrode, TiO2 nanotube arrays with an optimized length of 40 µm on a 50-mesh substrate achieve a conversion efficiency of 1.47% (calculated from the geometric area of Ti mesh) at 97.4 mW/cm2 AM 1.5 simulated full sunlight. 2. Experimental Section Vertically oriented TiO2 nanotube arrays on Ti meshes were prepared by electrochemical anodization in a two-electrode electrochemical cell with a platinum foil as counter electrode.30-34 To anodize all the interlaced Ti wires (horizontal and longitudinal wires) in a mesh, a square Ti mesh (Alfa Aesar, 50-mesh or 30-mesh) was folded along its diagonal (Figure 1a) and the two overlapped edges were cohered together using conductive silver paste (SPI Supplies). The resulting triangular double-layer Ti mesh (Figure 1b) was used as a working electrode, which was connected to a DC power source (E3649A, Agilent Tech-

10.1021/jp903342s CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

TiO2 Nanotube Arrays Grown on Ti Meshes

J. Phys. Chem. C, Vol. 113, No. 31, 2009 14029 Aldrich), 0.1 M guanidine thiocyanate (Sigma-Aldrich), and 0.5 M 1-methyl benzimidazole (Sigma-Aldrich) in 3-methoxypropionitrile (Sigma-Aldrich),37 were deposited on the surface of the working electrode. The electrolyte could penetrate the interval between the working electrode and counter electrode via capillary force. Photocurrent density-photovoltage (I-V) curves under AM 1.5 simulated full sunlight with an intensity (I) of 97.3 mW/cm2 provided by a solar simulator (model 69911, Newport-Oriel) were recorded by a computer-controlled potentiostat (SI 1286, Schlumberger). The illuminated geometric area on the working electrode was 0.283 cm2. On the basis of the values of short-circuit photocurrent (Isc), open-circuit photovoltage (Voc), and maximal power output (Pmax) derived from the I-V curves, the fill factor (FF) and solar-to-electric conversion efficiency (η) of the DSCs were calculated using the following equations.1,38 It should be noted that the η was calculated on the basis of the geometric area of Ti mesh.

FF ) Figure 1. (a) Photographs of a Ti mesh (size: 50-mesh) and its folding direction before anodization. (b) Triangular configuration of a Ti mesh after folding along the diagonal and its connection to a conductive clip via conductive silver paste. (c, d) Pictures of TiO2 nanotube arrays on Ti mesh (50-mesh) fabricated by anodization at 60 V for 3 h. (c) The anodized mesh is transparent. (d) The anodized mesh features high bend ability when subjected to external force.

nologies) via the conductive silver paste. The silver paste should be kept away from the electrolyte to avoid oxidation. The interval between the end of working electrode and counter electrode was about 3 cm. The electrolyte consisted of 0.25 wt % ammonium fluoride (Fisher Scientific) and 2 vol % Milli-Q water in ethylene glycol (Sigma-Aldrich). The Ti mesh was anodized at 60 V for 1-16 h to prepare TiO2 nanotube arrays with varying length. The as-prepared TiO2 nanotube arrays on Ti meshes were annealed at 500 °C under ambient air for 3 h to make TiO2 crystallize. When cooled to about 80 °C, the samples were stained by immersing them into 0.5 mM cisbis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bistetrabutyl ammonium (N719, Solaronix) in 1:1 (v/v) acetonitrile/tert-butanol (Aldrich) for 24 h at room temperature. A similar approach for quantum dot solar cells has also shown that elevated temperature can expedite deposition over TiO2.35 After being rinsed with acetonitrile (Aldrich) to remove the physically adsorbed dye molecules, two edges of the mesh were coated with conductive silver paste and used for working electrodes of DSCs. In our work, two kinds of counter electrodes were used to assemble the DSCs. In the first case, a rigid counter electrode was fabricated by thermal decomposition of hexachloroplatinic acid (Sigma-Aldrich) on conductive glass (TEC15, 15 Ω/sq, Hartford Glass).36 In the second case, a flexible counter electrode was prepared by sputtering Pt on ITO/PET (Sigma-Aldrich, 45 Ω/sq) in vacuum. The morphology of the counter electrodes is shown in Figure S1 of the Supporting Information. Subsequently, open rigid and flexible DSCs were assembled by clamping together the flexible working electrode and the corresponding counter electrode. Schematic diagram of our DSCs is shown in Figure S2 of the Supporting Information. Once the electrodes were assembled, several drops of lowvolatility organic electrolyte, which contains 1.0 M 1-methyl3-propylimidazolium iodide (Fluka), 0.15 M iodine (Sigma-

η(%) )

Pmax Isc × Voc

Pmax Isc × Voc × FF × 100 ) × 100 I I

Desorption of N719 dye from the working electrodes was achieved by immersing the N719-sensitized TiO2 nanotube arrays on Ti meshes into 5 mL of 0.01 M KOH (Fisher Scientific) in 1:1 (v/v) ethanol/water solution for 2 h.39 The amounts of adsorbed dye were determined by measuring the UV-visible absorption spectra (UV-2401 PC, Shimadzu), based on a precalibrated standard curve at 519-nm absorption. The morphology of TiO2 nanotube arrays on Ti meshes was characterized using a Hitachi S-4700 field emission scanning electron microscope (FESEM). The internal diameter and the wall thickness of nanotube arrays were ascertained from the top-view FESEM images, and the length was determined from the cross-sectional images. X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert PRO X-ray diffraction meter using Cu KR radiation in the range of 20-60°. 3. Results and Discussion Before electrochemical anodization, the conductivity of the entire Ti mesh should be considered. For a fresh Ti mesh (Figure 1a), before anodization, the intimate connections at the cross points between interlaced horizontal and longitudinal wires ensure a good conductivity throughout the mesh. However, the anodization may destroy these intimate connections because of the formation of a TiO2 nanotube layer on the surface of the wires. Our triangular configuration for Ti meshes (Figure 1b) can overcome this drawback, which ensures the connection of both horizontal and longitudinal Ti wires to the power source directly at anytime during anodization via the conductive silver paste. Figure 1c shows a representative photograph of a Ti mesh (50-mesh) after anodization at 60 V for 3 h. The mesh shows a uniform dark color throughout the entire anodized region, indicating that anodization is consistent. This is further confirmed by FESEM measurements as will be discussed later. Further as expected, the transparency of the anodized mesh is retained, as evidenced by the explicit “UNR” characters on the background in Figure 1c. It is noteworthy to mention that the transparency of the mesh may decrease if the mesh number increases. Figure 1d shows that the anodized mesh exhibits high bend ability (i.e., flexibility) when subjected to external force,

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Figure 2. FESEM images of TiO2 nanotube arrays on Ti mesh (50-mesh) fabricated by anodization at 60 V for 3 h. (a) Low-magnified overall image of the anodized mesh. (b) Low-magnified cross section of TiO2 nanotube arrays on a single Ti wire. (c-f) Magnified top view and cross-sectional images of TiO2 nanotube arrays on horizontal Ti wire (c, d) and on longitudinal Ti wire (e, f). Insets in (d) and (f) are the corresponding high-magnification images of the cross sections.

because of the slight relative displacement of horizontal and longitudinal wires, which can release some stresses.28 Transparency and high flexibility are additional and important merits for mesh configuration which foils do not have. Figure 2 shows the typical top view and cross-sectional FESEM images of TiO2 nanotube arrays on Ti mesh (50-mesh, 3-h anodization). As shown in Figure 2a, the diameter of the wires in the mesh is 0.1 mm; the opening width (distance between two wires) is 0.4 mm. The percentage of open area is then calculated to be 64%. From the cross section of single wire in the mesh (Figure 2b), we can determine that TiO2 nanotube arrays grew in a radially outward direction around the Ti wire uniformly and compactly. Such a compact packing of photoactive materials can reduce the contact between the conductive Ti substrate and the electrolyte, which is considered to be vital to suppress the dark current in the DSCs.40 Magnified top view and cross-sectional FESEM images (Figure 2c-f) indicate that TiO2 nanotube arrays with similar lengths (of the order of micrometers, ∼13 µm), internal diameters of ∼80 nm, and wall thickness of ∼27 nm form on both horizontal and longitudinal wires, which confirms that the anodization is consistent throughout the mesh. One can also notice that the images show TiO2 nanotube arrays are vertical to the Ti wire substrate, featuring a highly parallel one-dimensional nanostructure.

Figure 3. Effects of anodization duration on the physical features of TiO2 nanotubes arrays on Ti mesh (50-mesh). Variations in the lengths (O), internal diameters (ID, 0), and wall thicknesses (WT, ]) of TiO2 nanotube arrays following anodization at 60 V for different durations.

We studied the effects of anodization duration on the physical features of TiO2 nanotube arrays on Ti mesh (50-mesh). As shown in Figure 3, the length of nanotube arrays can be increased from 5.6 to 48 µm by simply increasing the anodization duration from 1 to 16 h, accompanying with an increased internal diameter from 61 to 120 nm and a decreased wall thickness from 37 to 9 nm. It

TiO2 Nanotube Arrays Grown on Ti Meshes has been reported that the lengths of TiO2 nanotube arrays are determined by the difference between electrochemical etching and chemical dissolution rates of TiO2, which will increase with anodization duration until these two etching rates are equal.41 In this work, the lengths of nanotube arrays experience a nonlinear increment with anodization duration (1-16 h) because of the reduced electrochemical etching rate caused by the increased length of nanotube arrays with anodization duration. The reduction in the wall thickness contributes to the widening of internal diameter, which can be explained by the chemical dissolution of the nanotube walls. XRD measurements indicate that nanotube arrays annealing at 500 °C under ambient air for 3 h are composed of pure anatasephase TiO2, as evidenced by the strong diffraction peaks at 2θ ) 25.5, 38.1, and 48.3°, which can be indexed, respectively, to the (101), (004), and (200) crystal faces of anatase TiO2 (Figure S3 in the Supporting Information). Figure S3 also shows that the diffraction intensity of anatase crystal faces becomes stronger following the increased lengths of nanotube arrays, clearly indicative of the increased amount of TiO2 on the mesh substrate. I-V curves of DSCs based on flexible TiO2 nanotube arrays on Ti meshes (50-mesh) and rigid conductive glass counter electrodes are shown in Figure 4a. The variations of Isc, Voc, FF, and η with the lengths of nanotube arrays are shown in Figure 4b,c. Although the mesh has cavities, the η was calculated using the geometrical area of Ti mesh. Figure 4b shows that increasing the lengths of nanotube arrays from 5.6 to 40 µm improves the Isc of DSCs from 1.94 to 4.95 mA/cm2, accompanied with a decreased FF from 0.60 to 0.56. The decrease in FF is ascribed to the increased series resistance of DSCs caused by the increased length of nanotube arrays. Figure 4b also indicates that the Voc of DSCs shows no remarkable dependence on the lengths of nanotube arrays from 5.6 to 40 µm. In general, the Voc of DSCs is determined by the difference between the quasi-Fermi level of electrons in TiO2 and the redox potential of the I3-/I- couple. The similarity in Voc means that the quasi-Fermi levels of electrons in the nanotube arrays with lengths from 5.6 to 40 µm are similar. The illumination can induce a high concentration of electrons in the conduction band of TiO2, which makes the quasi-Fermi levels in all the nanotube arrays (5.6 to 40 µm) approach the conduction band edge of TiO2.42 It is noted that the DSCs using TiO2 nanotube arrays on Ti meshes show a lower open-circuit photovoltage (∼0.52 V) compared with using nanotube arrays on Ti foil.23-27 As discussed in the previous section (Figure 2b), the anodization of Ti meshes will produce radial growth of TiO2 nanotube arrays around the Ti wires. In the assembled DSCs, the complete mesh is actually soaked in the electrolyte. However, only one-half of the lateral area of the wires can be illuminated directly by the sunlight. The other half of the wires has no significant contribution to the performances of DSCs. On the contrary, it will increase the dark current of the DSCs and reduce the open-circuit photovoltage. Since there was a great photocurrent improvement with the length of nanotube arrays, we decided to examine the dye loading in the DSCs quantitatively. As shown in Figure 4c, the amount of adsorbed N719 dye by nanotube arrays behaves in an almost linear increment from 0.034 to 0.142 µmol/cm2 with the lengths from 5.6 to 40 µm. This suggests that the enhancement in Isc is due to the increased loading of the N719 dye. The DSCs using 40-µmlong nanotube arrays on Ti mesh (50-mesh) generated an Isc of 4.95 mA/cm2, a Voc of 0.52 V, and a FF of 0.56, resulting in a best η of 1.47%. It is also noted that further extending the length of nanotube arrays to 48 µm can still increase the amount of adsorbed dye to 0.19 µmol/cm2. However, the nanotube arrays are quite brittle because of this large length. Care should be taken to assemble the DSC device. The DSCs generate a lower η of 1.17%, resulting

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Figure 4. (a) I-V curves of DSCs with TiO2 nanotube arrays of lengths of 5.6 µm (short line), 13 µm ([), 21 µm (triangle), 32 µm (9), 40 µm (b), and 48 µm (*) on a 50-mesh Ti substrate. The counter electrode is a rigid thermally platinized conductive glass. Solid symbols represent measurements taken under AM 1.5 simulated full sunlight (97.3 mW/cm2). Open symbols represent measurements taken in the dark. The aperture area of a photo mask is 0.283 cm2. (b) Effects of the lengths of nanotube arrays on Isc (0, left y-axis), Voc (], right y-axis), and FF (O, right y-axis). (c) Effects of the lengths of nanotube arrays on conversion efficiency (0, left y-axis) and amount of adsorbed N719 dye (O, right y-axis).

from a slightly decreased Isc (4.77 mA/cm2), an abruptly decreased FF (0.48), and a slightly decreased Voc (0.5 V). This reduced η may be related to the limited optical penetration depth22 or the electron diffusion length when illuminated from the side of TiO2 nanotube arrays, which results in a high series resistance of DSCs and low quasi-Fermi level of electrons in TiO2. Restricted by the commercial unavailability for Ti meshes with high mesh number, in the present work, we used Ti mesh with a mesh number of 50 as a substrate for DSCs. However, it is reasonable to expect that increasing the mesh number should improve the η, since a larger surface area can be available for producing nanotube arrays and depositing dye molecules. To confirm if our hypothesis is true, we anodized a Ti mesh with a smaller mesh number of 30 (wire diameter is also 0.1 mm) for 12 h to get TiO2 nanotube arrays that are about 40-µm long. Figure 5 compares the I-V curves of DSCs on the basis of nanotube arrays on Ti meshes with mesh numbers of 30 and 50. The mesh number

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Figure 5. I-V curves of DSCs with 40-µm-long TiO2 nanotube arrays on 50-mesh (b,O) and 30-mesh (9,0) Ti substrate. The counter electrode is rigid thermally platinized conductive glass. Solid symbols represent measurements taken under AM 1.5 simulated full sunlight (97.3 mW/cm2). Open symbols represent measurements taken in the dark. The aperture area of a photo mask is 0.283 cm2.

is noted to significantly influence the Isc. However, interestingly enough, little influence on Voc is noted with varying mesh size. The DSC-based 30-mesh substrate generates an Isc of 2.91 mA/ cm2, a Voc of 0.52 V, and a FF of 0.51, resulting in an η of 0.78%. This lower η can be attributed to the smaller amount of N719 dye (0.083 µmol/cm2) adsorbed by TiO2 nanotube arrays on 30-mesh substrate. Our results indicate that the η of the DSCs increases almost proportionally as the mesh number increases for almost identical lengths of nanotube arrays. In a related work, Fan et al.28 prepared a mesh-based flexible DSC using TiO2 nanoparticle-coated stainless steel mesh (120mesh) by a proprietary technique. The wire diameter in their mesh was 0.067 mm, and the open width was 0.134 mm. They obtained an η of 1.49% at 100 mW/cm2 AM 1.5 illumination combining with a N3 dye, a Pt foil counter electrode, and volatile acetonitrilebased electrolyte. Considering the much lower mesh number of our substrate (50-mesh), high resistance of conductive-glass-based counter electrode, and low-volatility electrolyte, our present η of 1.47% calculated from the geometric area of Ti mesh can be considered as an improvement in the preparation of metal meshbased flexible DSCs following the work of Fan and co-workers. To further compare the η more reasonably, we calculated the η using the real surface area of Ti wires in Ti mesh by correcting the cavities of the mesh and the lateral area of the Ti wire (determined by wire diameter), which is defined as ηc.43 The ηc derived from the data reported by Fan et al. is about 1.71%; the corresponding value in our system is 2.60%. This indicates about 52% improvement when TiO2 nanotube arrays on Ti meshes with our design configuration are utilized to support the dye. We consider that several particular features of TiO2 nanotube arrays on Ti mesh contribute to such a high conversion efficiency when calculated from the real surface area of Ti wires in Ti mesh. First, TiO2 nanotube arrays prepared by electrochemical anodization in the liquid electrolyte can effectively and completely cover the surfaces of Ti wires in the meshes (as shown in Figure 2b). Moreover, a natural TiO2 barrier layer forms at the TiO2 nanotube array/Ti metal interface during anodization.23,41 This compact TiO2 coating can separate the Ti conductive substrate and the redox electrolyte effectively, which can suppress the generation of dark current greatly. Further, the underlying Ti backbone not only can form a stable anchoring structure for TiO2 nanotube arrays,32 but also is an extremely effective transporter for the photogenerated electrons. The oriented one-dimensional structures of TiO2 nanotube arrays provide a unidirectional grain boundary free channel for electron transport to the underlying Ti backbone, which also contributes to enhancing the charge collection efficiency of TiO2

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Figure 6. I-V curves of lightweight flexible DSCs with 40-µm-long TiO2 nanotube arrays on Ti mesh (50-mesh) as working electrode and sputtered Pt on ITO/PET as flexible counter electrode. Solid symbols represent measurements taken under AM 1.5 simulated full sunlight (97.3 mW/cm2). Open symbols represent measurements taken in the dark. The aperture area of a photo mask is 0.283 cm2.

layer by suppressing the charge recombination,23-25 resulting in improved conversion efficiency. Lightweight flexible DSCs using 40-µm-long TiO2 nanotube arrays on Ti mesh (50-mesh) combined with a flexible Pt/ITO/ PET counter electrode were also fabricated. Herein, 40-µm-long nanotube arrays are mechanically stable under a small bending angle (e90°). However, if the bending angle is larger than 90°, some nanotube arrays are broken because of the stress, especially the nanotube arrays at the cross points between interlaced horizontal and longitudinal wires (Figure S4 in the Supporting Information). Therefore, under a small bending angle, the mechanical strength of nanotube arrays is enough for the application of flexible DSCs. I-V curves of the flexible DSCs are shown in Figure 6. The flexible DSCs generate an Isc of 4.66 mA/cm2, a Voc of 0.52 V, and a FF of 0.50, resulting in an η of 1.23%. The Voc and Isc of the flexible DSCs are similar to those of the DSCs using the rigid glass counter electrode. However, the FF and η show a little reduction in the flexible DSCs performance. This difference can be explained by the higher resistance of conductive ITO/PET (45 Ω/sq) compared with that of conductive glass (15 Ω/sq)10 (Figure S5 in the Supporting Information). We also investigated the η of the actual flexible DSCs under bending. Because of the Pt/ITO/PET counter electrode and the silver conductive paste on the working electrode, the actual flexible DSCs cannot be bent with a large angle. Herein, we only investigated the η of the flexible DSCs under two small bending angles qualitatively. Figure S6 in the Supporting Information shows that the bending of the actual flexible DSCs does not show significant effects on the η of the flexible DSCs. 4. Conclusions To the best of our knowledge, this is the first report that describes the application of vertically oriented TiO2 nanotube arrays grown on Ti meshes as a flexible working electrode for DSCs. The lengths of TiO2 nanotube arrays can be systematically altered by the duration of anodization. Varying the lengths of nanotube arrays has been demonstrated to critically influence the η of DSCs. In combination with a low-volatility organic electrolyte and rigid conductive glass counter electrode, TiO2 nanotube arrays with an optimized length of 40 µm on a 50-mesh substrate achieve an η of 1.47% (calculated from the geometric area of Ti mesh) at 97.4 mW/cm2 AM 1.5 simulated full sunlight. Such a high η can be ascribed to the uniform and compact formation of TiO2 nanotube arrays over an underlying network of Ti backbone, which enhances an already improved charge collection efficiency resulting from the oriented one-dimensional nanostructure. This work also dem-

TiO2 Nanotube Arrays Grown on Ti Meshes onstrates the fabrication of a lightweight flexible DSC using 40µm-long TiO2 nanotube arrays on Ti mesh combined with a flexible Pt/ITO/PET counter electrode. The assembly generates an η of 1.23% (calculated from the geometric area of Ti mesh), which is a little lower than the η of a similar DSC that has a rigid counter electrode. The variations in the performance of the two DSCs can be attributed to the higher resistance of ITO/PET. Vertically oriented TiO2 nanotube arrays grown on Ti mesh thus can be an important component of flexible DSCs. Additionally, the materials can also make significant contributions to improving other practical applications, such as photocatalytic air/water filter44 and photoelectrocatalytic water purification.45 Acknowledgment. This work was supported by the U.S. Department of Energy through DOE Grant No. DE-FC3606GO86066. We thank Dr. Wen-Ming Chien for XRD measurements. Supporting Information Available: SEM images of counter electrodes fabricated by thermal decomposition (rigid) and sputtering (flexible). Schematic diagram of DSCs using TiO2 nanotube arrays grown on a Ti mesh combined with a platinized conductive glass counter electrode and I3-/I- redox electrolyte. XRD patterns of TiO2 nanotube arrays on the Ti mesh (50-mesh) with lengths of 5.6, 21, and 40 µm. SEM images for the tops of 40-µm-long TiO2 nanotube arrays on Ti mesh (50-mesh) under different bending angles (qualitative). Cyclic voltammetry curves of different counter electrodes in 10 mM 1-methyl-3-propylimidazolium iodide and 0.15 mM iodine in 3-methoxypropionitrile containing 0.1 M sodium perchlorate supporting electrolyte. Normalized conversion efficiencies of flexible DSCs using 40-µm-long TiO2 nanotube arrays on Ti mesh under two qualitative small bending angles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737–740. (2) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402–407. (3) Chen, R.; Yang, X.; Tian, H.; Wang, X.; Hagfeldt, A.; Sun, L. Chem. Mater. 2007, 19, 4007–4015. (4) Hamann, T. W.; Martinson, A. B. F.; Elam, J. W.; Pellin, M. J.; Hupp, J. T. AdV. Mater. 2008, 20, 1560–1564. (5) Zhang, X.; Sutanto, I.; Taguchi, T.; Meng, Q.; Rao, T.; Fujishima, A.; Watanabe, H.; Nakamori, T.; Uragami, M. Sol. Energy Mater. Sol. Cells 2003, 80, 315–326. (6) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835–16847. (7) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys., Part 2 2006, 45, L638–L640. (8) Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. J. Am. Chem. Soc. 2008, 130, 10720–10728. (9) Longo, C.; Nogueira, A. F.; De Paoli, M.-A.; Cachet, H. J. Phys. Chem. B 2002, 106, 5925–5930. (10) Lindstro¨m, H.; Holmberg, A.; Magnusson, E.; Lindquist, S. E.; Malmqvist, L.; Hagfeldt, A. Nano Lett. 2001, 1, 97–100. (11) Kumar, R.; Sharma, A. K.; Parmar, V. S.; Watterson, A. C.; Chittibabu, K. G.; Kumar, J.; Samuelson, L. A. Chem. Mater. 2004, 16, 4841–4846. (12) Zhang, D.; Yoshida, T.; Oekermann, T.; Furuta, K.; Minoura, H. AdV. Funct. Mater. 2006, 16, 1228–1234. (13) Pichot, F.; Ferrere, S.; Pitts, R. J.; Gregg, B. A. J. Electrochem. Soc. 1999, 146, 4324–4326. (14) Zhang, D.; Yoshida, T.; Minoura, H. AdV. Mater. 2003, 15, 814– 817. (15) Ikegami, M.; Miyoshi, K.; Miyasaka, T.; Teshima, K.; Wei, T. C.; Wan, C. C.; Wang, Y. Y. Appl. Phys. Lett. 2007, 90, 153122. (16) Miyasaka, T.; Ikegami, M.; Kijitori, Y. J. Electrochem. Soc. 2007, 154, A455–A461.

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Sc )

η Sc

1 × (1 - Remp) × 3.14 × D × 0.5 ) 1.57 × (1 - Remp) D Remp )

d2 (D + d)2

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