Vertically Oriented TiO2 Nanotube Arrays Grown on Ti Meshes for Flexible Dye-SensitizedSolar 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 electrochemicalanodization and their application as flexible electrodes for dye-sensitized solar cells (DSC). The dependenceof physical features (i.e., diameter, wall thickness, and length) of TiO2 nanotube arrays on the anodizationduration is systematically studied. Our results indicate that this type of flexible electrodes features transparencyand high bend ability when subjected to external force. Varying the length of nanotube arrays has beendemonstrated to critically influence the solar-to-electric conversion efficiency of DSCs. In combination witha low-volatility organic electrolyte and a rigid platinized conductive glass counter electrode, TiO2 nanotubearrays 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 lightweightflexible DSC is also fabricated using the same nanotube arrays combined with a flexible Pt/ITO/PET counterelectrode, which generates a conversion efficiency of 1.23%.

1. Introduction

Dye-sensitized solar cells (DSCs) are attracting widespreadresearch interest for converting sunlight to electricity becauseof their low cost and high efficiency.1-5 A dye-sensitizedmesoporous nanocrystalline TiO2 film on conductive glass isthe heart of DSCs and has achieved a record efficiency of 11%in combination with a volatile I3

-/I- electrolyte and a platinizedconductive glass counter electrode.6-8 However, the rigid andfragile glass substrate restricts the flexibility, shape, weight, andoverall thickness of the devices, resulting in some complexitiesin transport and installation. Lightweight flexible substrates, onthe other hand, show an impressive potential to overcome thesedisadvantages. In the past several years, indium tin oxide coatedpoly(ethylene terephthalate) (ITO/PET) films have been con-sidered as an attractive flexible substrate for DSCs. A varietyof strategies have been explored to deposit mesoporous TiO2

nanoparticulate film on ITO/PET.9-16 However, this polymersubstrate can only withstand a low-temperature sintering process,which results in a poor crystallization and interparticle connec-tion, hampering the photovoltaic performance.17-20

Alternatively, metal foils are another kind of potential flexiblesubstrates for DSCs owing to their low electric resistance andhigh heat-resistant ability.17-19 A high conversion efficiency of7.2% 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 anod-ization are attracting significant attention in flexible DSCsapplication,21,22 because of the intuitive one-dimensional electricchannel and large internal surface area. Detailed studies haveshown that the charge recombination in nanotube-array-basedDSCs 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 arerestricting some possible applications of flexible DSCs. Metalmeshes can act as a promising substitute for metal foils as the

flexible substrates of DSCs,28 since they can withstand a high-temperature sintering process, simultaneously possessing highflexibility and transparency. Therefore, development of moreefficient TiO2 nanomaterials on metal mesh is very necessaryto potentially improve the conversion efficiency of mesh-basedflexible 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 nanotubearrays on Ti wires using the anodization method. An improvedphotocatalytic activity toward a textile dye degradation com-pared to nanotube arrays over foils is noted.29 In this article,we fabricated vertically oriented TiO2 nanotube arrays withcontrollable lengths on Ti meshes by electrochemical anodiza-tion in ethylene glycol-based electrolyte and investigated theirpotential application as flexible electrodes for DSCs. Our resultsindicate that this type of flexible electrodes features transparencyand high bend ability when subjected to external force. Varyingthe length of nanotube arrays systematically has been demon-strated to critically influence the conversion efficiency of DSCs.Combined with a rigid platinized conductive glass counterelectrode, TiO2 nanotube arrays with an optimized length of 40µm on a 50-mesh substrate achieve a conversion efficiency of1.47% (calculated from the geometric area of Ti mesh) at 97.4mW/cm2 AM 1.5 simulated full sunlight.

2. Experimental Section

Vertically oriented TiO2 nanotube arrays on Ti meshes wereprepared by electrochemical anodization in a two-electrodeelectrochemical cell with a platinum foil as counter electrode.30-34

To anodize all the interlaced Ti wires (horizontal and longitu-dinal wires) in a mesh, a square Ti mesh (Alfa Aesar, 50-meshor 30-mesh) was folded along its diagonal (Figure 1a) and thetwo overlapped edges were cohered together using conductivesilver paste (SPI Supplies). The resulting triangular double-layerTi mesh (Figure 1b) was used as a working electrode, whichwas connected to a DC power source (E3649A, Agilent Tech-

* To whom correspondence should be addressed. Telephone: (775) 784-1603. E-mail: misra@unr.edu.

J. Phys. Chem. C 2009, 113, 14028–1403314028

10.1021/jp903342s CCC: $40.75 2009 American Chemical SocietyPublished on Web 07/01/2009

nologies) via the conductive silver paste. The silver paste shouldbe kept away from the electrolyte to avoid oxidation. Theinterval between the end of working electrode and counterelectrode was about 3 cm. The electrolyte consisted of 0.25 wt% ammonium fluoride (Fisher Scientific) and 2 vol % Milli-Qwater in ethylene glycol (Sigma-Aldrich). The Ti mesh wasanodized at 60 V for 1-16 h to prepare TiO2 nanotube arrayswith varying length. The as-prepared TiO2 nanotube arrays onTi meshes were annealed at 500 °C under ambient air for 3 hto make TiO2 crystallize. When cooled to about 80 °C, thesamples were stained by immersing them into 0.5 mM cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthe-nium(II) bistetrabutyl ammonium (N719, Solaronix) in 1:1(v/v) acetonitrile/tert-butanol (Aldrich) for 24 h at roomtemperature. A similar approach for quantum dot solar cells hasalso shown that elevated temperature can expedite depositionover TiO2.35 After being rinsed with acetonitrile (Aldrich) toremove the physically adsorbed dye molecules, two edges ofthe mesh were coated with conductive silver paste and usedfor working electrodes of DSCs.

In our work, two kinds of counter electrodes were used toassemble the DSCs. In the first case, a rigid counter electrodewas fabricated by thermal decomposition of hexachloroplatinicacid (Sigma-Aldrich) on conductive glass (TEC15, 15 Ω/sq,Hartford Glass).36 In the second case, a flexible counter electrodewas prepared by sputtering Pt on ITO/PET (Sigma-Aldrich, 45Ω/sq) in vacuum. The morphology of the counter electrodes isshown in Figure S1 of the Supporting Information. Subse-quently, open rigid and flexible DSCs were assembled byclamping together the flexible working electrode and thecorresponding counter electrode. Schematic diagram of ourDSCs is shown in Figure S2 of the Supporting Information.

Once the electrodes were assembled, several drops of low-volatility organic electrolyte, which contains 1.0 M 1-methyl-3-propylimidazolium iodide (Fluka), 0.15 M iodine (Sigma-

Aldrich), 0.1 M guanidine thiocyanate (Sigma-Aldrich), and 0.5M 1-methyl benzimidazole (Sigma-Aldrich) in 3-methoxypro-pionitrile (Sigma-Aldrich),37 were deposited on the surface ofthe working electrode. The electrolyte could penetrate theinterval between the working electrode and counter electrodevia 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 poten-tiostat (SI 1286, Schlumberger). The illuminated geometric areaon the working electrode was 0.283 cm2. On the basis of thevalues of short-circuit photocurrent (Isc), open-circuit photo-voltage (Voc), and maximal power output (Pmax) derived fromthe I-V curves, the fill factor (FF) and solar-to-electricconversion efficiency (η) of the DSCs were calculated usingthe following equations.1,38 It should be noted that the η wascalculated on the basis of the geometric area of Ti mesh.

Desorption of N719 dye from the working electrodes wasachieved by immersing the N719-sensitized TiO2 nanotubearrays on Ti meshes into 5 mL of 0.01 M KOH (FisherScientific) in 1:1 (v/v) ethanol/water solution for 2 h.39 Theamounts of adsorbed dye were determined by measuring theUV-visible absorption spectra (UV-2401 PC, Shimadzu), basedon a precalibrated standard curve at 519-nm absorption.

The morphology of TiO2 nanotube arrays on Ti meshes wascharacterized using a Hitachi S-4700 field emission scanningelectron microscope (FESEM). The internal diameter and thewall thickness of nanotube arrays were ascertained from thetop-view FESEM images, and the length was determined fromthe cross-sectional images. X-ray diffraction (XRD) patternswere measured on a PANalytical X’Pert PRO X-ray diffractionmeter using Cu KR radiation in the range of 20-60°.

3. Results and Discussion

Before electrochemical anodization, the conductivity of the entireTi mesh should be considered. For a fresh Ti mesh (Figure 1a),before anodization, the intimate connections at the cross pointsbetween interlaced horizontal and longitudinal wires ensure a goodconductivity throughout the mesh. However, the anodization maydestroy these intimate connections because of the formation of aTiO2 nanotube layer on the surface of the wires. Our triangularconfiguration for Ti meshes (Figure 1b) can overcome thisdrawback, which ensures the connection of both horizontal andlongitudinal Ti wires to the power source directly at anytime duringanodization via the conductive silver paste. Figure 1c shows arepresentative photograph of a Ti mesh (50-mesh) after anodizationat 60 V for 3 h. The mesh shows a uniform dark color throughoutthe entire anodized region, indicating that anodization is consistent.This is further confirmed by FESEM measurements as will bediscussed later. Further as expected, the transparency of theanodized mesh is retained, as evidenced by the explicit “UNR”characters on the background in Figure 1c. It is noteworthy tomention that the transparency of the mesh may decrease if the meshnumber increases. Figure 1d shows that the anodized mesh exhibitshigh bend ability (i.e., flexibility) when subjected to external force,

Figure 1. (a) Photographs of a Ti mesh (size: 50-mesh) and its foldingdirection before anodization. (b) Triangular configuration of a Ti meshafter folding along the diagonal and its connection to a conductive clipvia conductive silver paste. (c, d) Pictures of TiO2 nanotube arrays onTi mesh (50-mesh) fabricated by anodization at 60 V for 3 h. (c) Theanodized mesh is transparent. (d) The anodized mesh features high bendability when subjected to external force.

FF )Pmax

Isc × Voc

η(%) )Pmax

I× 100 )

Isc × Voc × FF

I× 100

TiO2 Nanotube Arrays Grown on Ti Meshes J. Phys. Chem. C, Vol. 113, No. 31, 2009 14029

because of the slight relative displacement of horizontal andlongitudinal wires, which can release some stresses.28 Transparencyand high flexibility are additional and important merits for meshconfiguration which foils do not have.

Figure 2 shows the typical top view and cross-sectional FESEMimages of TiO2 nanotube arrays on Ti mesh (50-mesh, 3-hanodization). As shown in Figure 2a, the diameter of the wires inthe mesh is 0.1 mm; the opening width (distance between twowires) is 0.4 mm. The percentage of open area is then calculatedto be 64%. From the cross section of single wire in the mesh (Figure2b), we can determine that TiO2 nanotube arrays grew in a radiallyoutward direction around the Ti wire uniformly and compactly.Such a compact packing of photoactive materials can reduce thecontact between the conductive Ti substrate and the electrolyte,which is considered to be vital to suppress the dark current in theDSCs.40 Magnified top view and cross-sectional FESEM images(Figure 2c-f) indicate that TiO2 nanotube arrays with similarlengths (of the order of micrometers, ∼13 µm), internal diametersof ∼80 nm, and wall thickness of ∼27 nm form on both horizontaland longitudinal wires, which confirms that the anodization isconsistent throughout the mesh. One can also notice that the imagesshow TiO2 nanotube arrays are vertical to the Ti wire substrate,featuring a highly parallel one-dimensional nanostructure.

We studied the effects of anodization duration on the physicalfeatures of TiO2 nanotube arrays on Ti mesh (50-mesh). As shownin Figure 3, the length of nanotube arrays can be increased from5.6 to 48 µm by simply increasing the anodization duration from 1to 16 h, accompanying with an increased internal diameter from61 to 120 nm and a decreased wall thickness from 37 to 9 nm. It

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 imageof 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 ofTiO2 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 imagesof the cross sections.

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 nanotubearrays following anodization at 60 V for different durations.

14030 J. Phys. Chem. C, Vol. 113, No. 31, 2009 Liu et al.

has been reported that the lengths of TiO2 nanotube arrays aredetermined by the difference between electrochemical etching andchemical dissolution rates of TiO2, which will increase withanodization duration until these two etching rates are equal.41 Inthis work, the lengths of nanotube arrays experience a nonlinearincrement with anodization duration (1-16 h) because of thereduced electrochemical etching rate caused by the increased lengthof nanotube arrays with anodization duration. The reduction in thewall thickness contributes to the widening of internal diameter,which can be explained by the chemical dissolution of the nanotubewalls. XRD measurements indicate that nanotube arrays annealingat 500 °C under ambient air for 3 h are composed of pure anatase-phase 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 inthe Supporting Information). Figure S3 also shows that thediffraction intensity of anatase crystal faces becomes strongerfollowing the increased lengths of nanotube arrays, clearly indica-tive of the increased amount of TiO2 on the mesh substrate.

I-V curves of DSCs based on flexible TiO2 nanotube arrays onTi meshes (50-mesh) and rigid conductive glass counter electrodesare shown in Figure 4a. The variations of Isc, Voc, FF, and η withthe lengths of nanotube arrays are shown in Figure 4b,c. Althoughthe mesh has cavities, the η was calculated using the geometricalarea of Ti mesh. Figure 4b shows that increasing the lengths ofnanotube arrays from 5.6 to 40 µm improves the Isc of DSCs from1.94 to 4.95 mA/cm2, accompanied with a decreased FF from 0.60to 0.56. The decrease in FF is ascribed to the increased seriesresistance of DSCs caused by the increased length of nanotubearrays. Figure 4b also indicates that the Voc of DSCs shows noremarkable dependence on the lengths of nanotube arrays from5.6 to 40 µm. In general, the Voc of DSCs is determined by thedifference between the quasi-Fermi level of electrons in TiO2 andthe redox potential of the I3

-/I- couple. The similarity in Voc meansthat the quasi-Fermi levels of electrons in the nanotube arrays withlengths from 5.6 to 40 µm are similar. The illumination can inducea high concentration of electrons in the conduction band of TiO2,which makes the quasi-Fermi levels in all the nanotube arrays (5.6to 40 µm) approach the conduction band edge of TiO2.42 It is notedthat the DSCs using TiO2 nanotube arrays on Ti meshes show alower open-circuit photovoltage (∼0.52 V) compared with usingnanotube arrays on Ti foil.23-27 As discussed in the previous section(Figure 2b), the anodization of Ti meshes will produce radialgrowth of TiO2 nanotube arrays around the Ti wires. In theassembled DSCs, the complete mesh is actually soaked in theelectrolyte. However, only one-half of the lateral area of the wirescan be illuminated directly by the sunlight. The other half of thewires has no significant contribution to the performances of DSCs.On the contrary, it will increase the dark current of the DSCs andreduce the open-circuit photovoltage.

Since there was a great photocurrent improvement with thelength of nanotube arrays, we decided to examine the dye loadingin the DSCs quantitatively. As shown in Figure 4c, the amount ofadsorbed N719 dye by nanotube arrays behaves in an almost linearincrement from 0.034 to 0.142 µmol/cm2 with the lengths from5.6 to 40 µm. This suggests that the enhancement in Isc is due tothe increased loading of the N719 dye. The DSCs using 40-µm-long nanotube arrays on Ti mesh (50-mesh) generated an Isc of4.95 mA/cm2, a Voc of 0.52 V, and a FF of 0.56, resulting in a bestη of 1.47%. It is also noted that further extending the length ofnanotube arrays to 48 µm can still increase the amount of adsorbeddye to 0.19 µmol/cm2. However, the nanotube arrays are quitebrittle because of this large length. Care should be taken to assemblethe DSC device. The DSCs generate a lower η of 1.17%, resulting

from a slightly decreased Isc (4.77 mA/cm2), an abruptly decreasedFF (0.48), and a slightly decreased Voc (0.5 V). This reduced ηmay be related to the limited optical penetration depth22 or theelectron diffusion length when illuminated from the side of TiO2

nanotube arrays, which results in a high series resistance of DSCsand low quasi-Fermi level of electrons in TiO2.

Restricted by the commercial unavailability for Ti meshes withhigh mesh number, in the present work, we used Ti mesh with amesh number of 50 as a substrate for DSCs. However, it isreasonable to expect that increasing the mesh number shouldimprove the η, since a larger surface area can be available forproducing nanotube arrays and depositing dye molecules. Toconfirm if our hypothesis is true, we anodized a Ti mesh with asmaller mesh number of 30 (wire diameter is also 0.1 mm) for12 h to get TiO2 nanotube arrays that are about 40-µm long. Figure5 compares the I-V curves of DSCs on the basis of nanotube arrayson Ti meshes with mesh numbers of 30 and 50. The mesh number

Figure 4. (a) I-V curves of DSCs with TiO2 nanotube arrays of lengthsof 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 arigid thermally platinized conductive glass. Solid symbols representmeasurements taken under AM 1.5 simulated full sunlight (97.3 mW/cm2).Open symbols represent measurements taken in the dark. The aperturearea of a photo mask is 0.283 cm2. (b) Effects of the lengths of nanotubearrays 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).

TiO2 Nanotube Arrays Grown on Ti Meshes J. Phys. Chem. C, Vol. 113, No. 31, 2009 14031

is noted to significantly influence the Isc. However, interestinglyenough, 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-meshsubstrate. Our results indicate that the η of the DSCs increasesalmost proportionally as the mesh number increases for almostidentical lengths of nanotube arrays.

In a related work, Fan et al.28 prepared a mesh-based flexibleDSC using TiO2 nanoparticle-coated stainless steel mesh (120-mesh) by a proprietary technique. The wire diameter in their meshwas 0.067 mm, and the open width was 0.134 mm. They obtainedan η of 1.49% at 100 mW/cm2 AM 1.5 illumination combiningwith a N3 dye, a Pt foil counter electrode, and volatile acetonitrile-based electrolyte. Considering the much lower mesh number ofour substrate (50-mesh), high resistance of conductive-glass-basedcounter electrode, and low-volatility electrolyte, our present η of1.47% calculated from the geometric area of Ti mesh can beconsidered as an improvement in the preparation of metal mesh-based flexible DSCs following the work of Fan and co-workers.To further compare the η more reasonably, we calculated the ηusing the real surface area of Ti wires in Ti mesh by correctingthe 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%; thecorresponding value in our system is 2.60%. This indicates about52% improvement when TiO2 nanotube arrays on Ti meshes withour design configuration are utilized to support the dye.

We consider that several particular features of TiO2 nanotubearrays on Ti mesh contribute to such a high conversion efficiencywhen calculated from the real surface area of Ti wires in Ti mesh.First, TiO2 nanotube arrays prepared by electrochemical anodizationin the liquid electrolyte can effectively and completely cover thesurfaces of Ti wires in the meshes (as shown in Figure 2b).Moreover, a natural TiO2 barrier layer forms at the TiO2 nanotubearray/Ti metal interface during anodization.23,41 This compact TiO2

coating can separate the Ti conductive substrate and the redoxelectrolyte effectively, which can suppress the generation of darkcurrent greatly. Further, the underlying Ti backbone not only canform a stable anchoring structure for TiO2 nanotube arrays,32 butalso is an extremely effective transporter for the photogeneratedelectrons. The oriented one-dimensional structures of TiO2 nanotubearrays provide a unidirectional grain boundary free channel forelectron transport to the underlying Ti backbone, which alsocontributes to enhancing the charge collection efficiency of TiO2

layer by suppressing the charge recombination,23-25 resulting inimproved conversion efficiency.

Lightweight flexible DSCs using 40-µm-long TiO2 nanotubearrays on Ti mesh (50-mesh) combined with a flexible Pt/ITO/PET counter electrode were also fabricated. Herein, 40-µm-longnanotube arrays are mechanically stable under a small bendingangle (e90°). However, if the bending angle is larger than 90°,some nanotube arrays are broken because of the stress, especiallythe nanotube arrays at the cross points between interlaced horizontaland longitudinal wires (Figure S4 in the Supporting Information).Therefore, under a small bending angle, the mechanical strengthof nanotube arrays is enough for the application of flexible DSCs.I-V curves of the flexible DSCs are shown in Figure 6. The flexibleDSCs generate an Isc of 4.66 mA/cm2, a Voc of 0.52 V, and a FFof 0.50, resulting in an η of 1.23%. The Voc and Isc of the flexibleDSCs are similar to those of the DSCs using the rigid glass counterelectrode. However, the FF and η show a little reduction in theflexible DSCs performance. This difference can be explained bythe higher resistance of conductive ITO/PET (45 Ω/sq) comparedwith that of conductive glass (15 Ω/sq)10 (Figure S5 in theSupporting Information). We also investigated the η of the actualflexible DSCs under bending. Because of the Pt/ITO/PET counterelectrode 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 smallbending angles qualitatively. Figure S6 in the Supporting Informa-tion shows that the bending of the actual flexible DSCs does notshow significant effects on the η of the flexible DSCs.

4. Conclusions

To the best of our knowledge, this is the first report that describesthe application of vertically oriented TiO2 nanotube arrays grownon Ti meshes as a flexible working electrode for DSCs. The lengthsof TiO2 nanotube arrays can be systematically altered by theduration of anodization. Varying the lengths of nanotube arrayshas been demonstrated to critically influence the η of DSCs. Incombination with a low-volatility organic electrolyte and rigidconductive glass counter electrode, TiO2 nanotube arrays with anoptimized length of 40 µm on a 50-mesh substrate achieve an ηof 1.47% (calculated from the geometric area of Ti mesh) at 97.4mW/cm2 AM 1.5 simulated full sunlight. Such a high η can beascribed to the uniform and compact formation of TiO2 nanotubearrays over an underlying network of Ti backbone, which enhancesan already improved charge collection efficiency resulting fromthe oriented one-dimensional nanostructure. This work also dem-

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

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 sputteredPt on ITO/PET as flexible counter electrode. Solid symbols representmeasurements taken under AM 1.5 simulated full sunlight (97.3 mW/cm2).Open symbols represent measurements taken in the dark. The aperturearea of a photo mask is 0.283 cm2.

14032 J. Phys. Chem. C, Vol. 113, No. 31, 2009 Liu et al.

onstrates the fabrication of a lightweight flexible DSC using 40-µm-long TiO2 nanotube arrays on Ti mesh combined with a flexiblePt/ITO/PET counter electrode. The assembly generates an η of1.23% (calculated from the geometric area of Ti mesh), which isa little lower than the η of a similar DSC that has a rigid counterelectrode. The variations in the performance of the two DSCs canbe attributed to the higher resistance of ITO/PET. Verticallyoriented TiO2 nanotube arrays grown on Ti mesh thus can be animportant component of flexible DSCs. Additionally, the materialscan also make significant contributions to improving other practicalapplications, such as photocatalytic air/water filter44 and photo-electrocatalytic water purification.45

Acknowledgment. This work was supported by the U.S.Department of Energy through DOE Grant No. DE-FC36-06GO86066. We thank Dr. Wen-Ming Chien for XRDmeasurements.

Supporting Information Available: SEM images of counterelectrodes fabricated by thermal decomposition (rigid) and sput-tering (flexible). Schematic diagram of DSCs using TiO2 nanotubearrays grown on a Ti mesh combined with a platinized conductiveglass counter electrode and I3

-/I- redox electrolyte. XRD patternsof TiO2 nanotube arrays on the Ti mesh (50-mesh) with lengthsof 5.6, 21, and 40 µm. SEM images for the tops of 40-µm-longTiO2 nanotube arrays on Ti mesh (50-mesh) under different bendingangles (qualitative). Cyclic voltammetry curves of different counterelectrodes in 10 mM 1-methyl-3-propylimidazolium iodide and0.15 mM iodine in 3-methoxypropionitrile containing 0.1 M sodiumperchlorate supporting electrolyte. Normalized conversion efficien-cies of flexible DSCs using 40-µm-long TiO2 nanotube arrays onTi mesh under two qualitative small bending angles. This materialis available free of charge via the Internet at http://pubs.acs.org.

References and Notes

(1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737–740.(2) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.;

Sekiguchi, T.; Gratzel, 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. Cells2003, 80, 315–326.

(6) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.;Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gratzel, 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.; Gratzel, 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) Lindstrom, 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.

(17) Kang, M. G.; Park, N.-G.; Ryu, K. S.; Chang, S. H.; Kim, K.-J.Sol. Energy Mater. Sol. Cells 2006, 90, 574–581.

(18) Ito, S.; Ha, N. C.; Rothenberger, G.; Liska, P.; Comte, P.;Zakeeruddin, S. M.; Pechy, P.; Nazeeruddin, M. K.; Gratzel, M. Chem.Commun. 2006, 4004–4006.

(19) Yun, H.-G.; Jun, Y.; Kim, J.; Bae, B.-S.; Kang, M. G. Appl. Phys.Lett. 2008, 93, 133311.

(20) Nakade, S.; Matsuda, M.; Kambe, S.; Saito, Y.; Kitamura, T.;Sakata, T.; Wada, Y.; Mori, H.; Yanagida, S. J. Phys. Chem. B 2002, 106,10004–10010.

(21) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.;Sumioka, K.; Zakeeruddin, S. M.; Gratzel, M. ACS Nano 2008, 2, 1113–1116.

(22) Lin, C.; Yu, W.; Chien, S. Appl. Phys. Lett. 2008, 93, 133107.(23) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007,

7, 69–74.(24) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker,

A. B. J. Am. Chem. Soc. 2008, 130, 13364–13372.(25) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes,

C. A. Nano Lett. 2006, 6, 215–218.(26) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Hardin,

B.; Grimes, C. A. Nanotechnology 2006, 17, 1446–1448.(27) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Grimes,

C. A. J. Phys. D: Appl. Phys. 2006, 39, 2498–2503.(28) Fan, X.; Wang, F.; Chu, Z.; Chen, L.; Zhang, C.; Zou, D. Appl.

Phys. Lett. 2007, 90, 073501.(29) Kar, A.; Smith, Y. R.; Subramanian, V. EnViron. Sci. Technol. 2009,

43, 3260–3265.(30) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese,

O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys.Chem. B 2006, 110, 16179–16184.

(31) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.;Grimes, C. A. J. Phys. Chem. C 2007, 111, 7235–7241.

(32) Liu, Z.; Zhang, X.; Nishimoto, S.; Jin, M.; Tryk, D. A.; Murakami,T.; Fujishima, A. J. Phys. Chem. C 2008, 112, 253–259.

(33) Albu, S. P.; Kim, D.; Schmuki, P. Angew. Chem., Int. Ed. 2008,47, 1916–1919.

(34) Raja, K. S.; Gandhi, T.; Misra, M. Electrochem. Commun. 2007,9, 1069–1076.

(35) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem.Soc. 2006, 128, 2385–2393.

(36) Papageorgiou, N.; Maier, W. F.; Gratzel, M. J. Electrochem. Soc.1997, 144, 876–884.

(37) Shi, D.; Pootrakulchote, N.; Li, R.; Guo, J.; Wang, Y.; Zakeeruddin,S. M.; Gratzel, M.; Wang, P. J. Phys. Chem. C 2008, 112, 17046–17050.

(38) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269–277.(39) Yanagida, M.; Yamaguchi, T.; Kurashige, M.; Hara, K.; Katoh,

R.; Sugihara, H.; Arakawa, H. Inorg. Chem. 2003, 42, 7921–7931.(40) Ito, S.; Liska, P.; Comte, P.; Charvet, R.; Pechy, P.; Bach, U.;

Schmidt-Mende, L.; Zakeeruddin, S. M.; Kay, A.; Nazeeruddin, M. K.;Gratzel, M. Chem. Commun. 2005, 4351–4353.

(41) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes,C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011–2075.

(42) Alarcon, H.; Boschloo, G.; Mendoza, P.; Solis, J. L.; Hagfeldt, A.J. Phys. Chem. B 2005, 109, 18483–18490.

(43) We defined several parameters for a Ti mesh: D, diameter of the Tiwires in the mesh; d, opening width of the mesh; Remp, percentage of openingarea in the mesh; Sc, area of geometric 1 cm2 Ti mesh after correcting thecavities and lateral area of the wire; η, calculated conversion efficiency basedon geometric 1 cm2 Ti mesh. The ηc was calculated using the followingequations, considering reasonably only one half of the lateral area of the wirecan be illuminated directly by the incident light.

(44) Liu, Z.; Zhang, X.; Nishimoto, S.; Murakami, T.; Fujishima, A.EnViron. Sci. Technol. 2008, 42, 8547–8551.

(45) Li, X. Z.; Liu, H. L.; Yue, P. T.; Sun, Y. P. EnViron. Sci. Technol.2000, 34, 4401–4406.

JP903342S

ηc )ηSc

Sc )1 × (1 - Remp) × 3.14 × D × 0.5

D) 1.57 × (1 - Remp)

Remp ) d2

(D + d)2

TiO2 Nanotube Arrays Grown on Ti Meshes J. Phys. Chem. C, Vol. 113, No. 31, 2009 14033