Photoelectrochemical Behavior of Polychelate Porphyrin

Open Access ... University of Texas at Arlington. ... The influence of TiO2 nanotube array (NTA) morphology on dye-sensitized solar ... photoresponse ...
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J. Phys. Chem. C 2009, 113, 2996–3006

Photoelectrochemical Behavior of Polychelate Porphyrin Chromophores and Titanium Dioxide Nanotube Arrays for Dye-Sensitized Solar Cells Norma R. de Tacconi,*,† Wilaiwan Chanmanee,† Krishnan Rajeshwar,† Jonathan Rochford,‡ and Elena Galoppini*,‡ Center for Renewable Energy Science & Technology (CREST), Department of Chemistry & Biochemistry, UniVersity of Texas at Arlington, Arlington, Texas 76109-0065, and Chemistry Department, Rutgers UniVersity, Newark, New Jersey 07102 ReceiVed: September 12, 2008; ReVised Manuscript ReceiVed: December 9, 2008

The influence of TiO2 nanotube array (NTA) morphology on dye-sensitized solar cell performance was studied with five porphyrin chromophores containing up to four carboxylic acid anchor groups. The TiO2 NTAs were anodically grown on Ti foil in NH4F electrolyte containing additives such as poly(ethylene glycol) (PEG 400), ethylene glycol, and glycerol. The NTAs grown in the presence of PEG 400 had the largest nanotube diameter (∼120 nm) and also yielded the best photoresponse with all five porphyrins in iodide/ polyiodide electrolyte. Interestingly, nanoporous TiO2 “control” films, anodically grown on Ti foil in the absence of the organic additive, yielded an inferior photoresponse under the same conditions. Two of the porphyrins (P3 and P5), envisioned to have a spider-like configuration on the TiO2 NTA surface, yielded the best photoresponse among the five candidates in the Q(1,0) porphyrin spectral region. This result underlines that the molecular structure of the chromophore affects the directionality of electron transfer and its consequent ability to inject electrons (by the photoexcited porphyrin) into the oxide nanotube host. Finally, some perspectives on the use of TiO2 NTAs and porphyrins in DSSCs are given against the backdrop of the above data. 1. Introduction Dye-sensitized solar cells (DSSCs) based on mesoporous, nanocrystalline oxides1 are a promising low-cost alternative to conventional solid-state photovoltaic devices for the conversion of solar energy to electric power. Ruthenium- or osmium-based polypyridyl complexes were used as sensitizing dyes in the earlier studies;1 however, these dyes are not attractive in terms of practical and widespread deployment of DSSCs because of the high cost and low abundance of noble metals such as ruthenium and osmium. On the other hand, the use of porphyrins as light-harvesting agents and sensitizers in DSSCs is particularly attractive given their key role in the photosynthetic apparatus and the relative ease with which a variety of porphyrins can be synthesized.2 Thus it is hardly surprising that a large number of recent studies on DSSCs have been oriented toward this topic.3,4 In recent studies, the molecular structure and binding orientation and geometry were found to influence the photophysics and DSSC performance for a series of zinc polychelate porphyrins bound to ZnO nanorod4a and ZnO and TiO24b,c nanocrystalline films. We build on these results in this paper with studies on five polychelate zinc porphyrin/TiO2 nanotube array (NTA) interfaces in a DSSC device environment; Figure 1 schematically depicts the five porphyrins studied. Three of these compounds (P1, P3, and P5) have been studied before,4b albeit in the context of binding to TiO2 and ZnO nanocrystalline surfaces. The other two (P2 and P4) were included in a more recent study,4c again * To whom correspondence should be addressed. Telephone: (817) 2725421(N.R.T.); (973) 353-5317(E.G.). E-mail: [email protected] (N.R.T.); [email protected] (E.G.). † University of Texas at Arlington. ‡ Rutgers University.

with their binding studied on nanocrystalline TiO2 and ZnO films prepared by a sol-gel procedure5 and by a previously published solution technique, respectively.6 This study, by contrast, focuses on the binding and subsequent photoelectrochemical behavior of P1-P5 on TiO2 NTAs. The sensitization of this relatively new morphology of TiO2 by model dyes (designed to investigate the influence of structural factors on solar cells) is relatively little explored, and the molecules, P1-P5, could provide new insights into the properties of TiO2 NTAs in turn. Given that the transport of injected electrons through the oxide film is an important component of the overall operating mechanism of a DSSC,7 one would expect that the morphology of the oxide film should play a key role. Specifically, if the oxide film is oriented one-dimensionally, aligned perpendicular to the (rear) electron-collecting substrate, the charge collection efficiency in the DSSC could be improved by promoting faster transport and slower recombination. Indeed, this expectation is largely borne out by recent studies on TiO2 NTAs.8-10 The aim of the present study was to examine dye binding, aggregation, and orientation effects on TiO2 NTAs from a DSSC perspective using the new series of polychelate porphyrin chromophores (Figure 1). The influence of the NTA morphology (i.e., nanotube diameter) was also assessed using these chromophores. 2. Experimental Section 2.1. Chemicals and Materials. Zn(II)-5,10,15,20-tetra(4carboxyphenyl)porphyrin (P1), Zn(II)-5-(3,5-dicarboxyphenyl)10,15,20-trimesitylporphyrin (P2), Zn(II)-5,10,15,20-tetra (3-carboxyphenyl)porphyrin (P3), Zn(II)-5-(3,5-dicarboxyphenyl)phenyl-10,15,20-triphenylporphyrin (P4), and Zn(II)-5,10,15, 20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin (P5) were prepared and characterized as described elsewhere.4 Methanol (99.9%, Alfa Aesar), lithium iodide (Aldrich), iodine and 4-tert-

10.1021/jp808137s CCC: $40.75  2009 American Chemical Society Published on Web 01/27/2009

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Figure 1. Chemical structures of the five polychelate porphyrin chromophores (P1-P5) used in the sensitization of TiO2 NTAs. The center of the figure schematizes a TiO2 nanotube containing P5 anchored to its internal wall.

butylpyridine (Sigma, ACS reagent grade), and acetonitrile (Alfa Aesar) were used as received. 2.2. Preparation of TiO2 Nanotubes. The titanium foils (99.95%, Alfa Aesar) were 0.25 mm thick. They were polished to mirror-quality smooth finish using silicon carbide sandpaper of successively finer roughness (220, 240, 320, 400, 600, 800, 1000, and 1500 grit) and degreased by successively sonicating for 5 min in acetone, 2-propanol, and finally ultrapure water. They were then dried under a flowing N2 stream and used immediately. For the anodic NTA growth experiments, the titanium foil was contacted and then pressed against an O-ring in an electrochemical cell, leaving 0.7 cm2 exposed to the electrolyte. A two-electrode configuration was used for anodization. Titanium foil (14 × 14 × 0.25 mm) was used as anode, while a large platinum coil was used as cathode. Anodization experiments were carried out using a multioutput power supply (Switching System International, CA) connected to a digital multimeter. All NTA films were grown at constant voltage (20 V) at room temperature (25 ( 2 °C) and from 0.36 M NH4F electrolyte without and with medium modifiers: ethylene glycol (EG), glycerol:H2O (90:10), and poly(ethylene glycol) (PEG 400): H2O (80:20). All solutions were prepared from reagent grade chemicals and deionized water (18 MΩ cm). After formation

of the oxide, the anodized Ti foil was removed from the O-ring assembly and carefully washed by immersion in deionized water and then dried in a flowing N2 stream. The as-grown NTAs were annealed at 450 °C for 30 min in a furnace (Model 65014 Isotemp Programmable Muffle Furnace, Fisher Scientific) and were allowed to cool gradually back to the ambient condition. The thermal anneal consisted of a linear heat ramp (at 10 °C/min) from room temperature to a final temperature of 450 °C. The NTAs were packed in approximately hexagonal symmetry with an average inner diameter ranging from 55 to 120 nm and wall thickness of 13-16 nm, depending on the medium modifier used during their growth.11 The average nanotube length was in the 600-850 nm range, although much longer nanotubes could be grown (see below). Other details of TiO2 NTA preparation and characterization are given elsewhere.11 Table 1 provides a summary of the physical characteristics of the NTA films (i.e., tube length, wall thickness, and inner tube diameter) as obtained from scanning electron microscopy (SEM) images. A simple calculation was performed for obtaining the number of nanotubes per square centimeter (using their diameter and assuming a perfectly ordered array of circles) and thereby estimating the real area of the NTA films. The calculation considers that all the tubes are in contact with their neighbors and that the real area comes only from the internal

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TABLE 1: Physical Characteristics of the TiO2 NTA Filmsa and Calculated NTA Film Area for the Three Preparation Conditions preparationb

nanotube lengtha (nm)

wall thicknessa (nm)

inner diametera (nm)

number of nanotubes (cm-2)

nanotube individual area (cm2)

real area of NTA films (cm2)

PEG 400 glycerol ethylene glycol

725 ( 10 600 ( 10 835 ( 10

16.0 ( 0.4 13.0 ( 0.5 14.4 ( 0.6

120 ( 3 66 ( 3 55 ( 3

4.33 × 109 1.18 × 1010 1.42 × 1010

2.73 × 10-9 1.25 × 10-9 1.45 × 10-9

11.8 14.8 20.6

a

Data culled from ref 11d. b At 3 h, 20 V.

TABLE 2: Solution UV-Vis Absorption/Fluorescence Bands and Electrochemical Oxidation/Reduction Potentials of P1-P5a UV-vis absorption

porphyrinb P1 P2 P3 P4 P5

fluorescence

Soret λ max, Q(1,0) λ max, Q(0,0) λ max, nm (ε × 105, nm (ε × 104, nm (ε × 104, Q(0,0)*, Q(1,0)* M-1 cm-1) M-1 cm-1) M-1 cm-1) λmax, nm (Φ) 424 (2.78) 423 (5.49) 425 (4.44) 424 (5.66) 425 (5.51)

558 (1.39) 557 (1.72) 558 (2.09) 557 (1.64) 557 (2.77)

598 (0.46) 596 (0.51) 597 (0.66) 597 (0.61) 596 (0.96)

606, 658 (0.023) 602, 654 (0.02) 604, 657 (0.016) 602, 656 (0.034) 605, 659 (0.017)

oxidationc (V)

reductionc (V)

1st

2nd

1st

1.10 0.99 1.11 1.08 1.12

1.47 1.41 1.38 1.39 1.37

-1.13 -1.44 -1.08 -1.10 -1.08

2nd†

3rd†

-1.46 -1.65 -1.65 -1.45 -1.47 -1.31 -1.46

E0-0 (eV) E1/2(P+/P*) (eV) 2.06 2.07 2.07 2.07 2.07

-0.96 -1.08 -0.96 -0.99 -0.95

Data culled from ref 4 for the methyl ester derivatives. b P1 ) Zn(II)-5,10,15,20-tetra(4-carboxyphenyl)porphyrin; P2 ) Zn(II)-5(3,5-dicarboxyphenyl)-10,15,20-trimesitylporphyrin; P3 ) Zn(II)-5,10,15,20-tetra(3-carboxyphenyl)porphyrin; P4 ) Zn(II)-5-(3,5-dicarboxyphenyl)-10,15,20-triphenylporphyrin; P5 ) Zn(II)-5,10,15,20-tetra(3-(4-carboxyphenyl)phenyl)porphyrin. c Potentials are reported vs NHE. All cyclic voltammograms were measured at 0.1 V/s in dichloromethane with 0.1 M Bu4NBF4. a

TABLE 3: Surface Coverage for the Five Porphyrins on Three Types of TiO2 NTAs As Determined by Desorption surface coverage (molecules/cm2) porphyrin

TiO2 NTAs (EG)

TiO2 NTAs (Gly)

TiO2 NTAs (PEG 400)

P1 P2 P3 P4 P5

1.3 × 1014 4.2 × 1013 6.5 × 1014 4.4 × 1013 1.7 × 1014

1.6 × 1014 3.8 × 1013 9.8 × 1014 3.9 × 1013 4.2 × 1014

1.4 × 1014 4.2 × 1013 9.2 × 1014 4.9 × 1013 3.7 × 1014

wall of the tubes plus the ring top area of each individual tube; the results are contained in Table 1. (Note that the external area of nanotubes was deliberately not considered because intimate contact of the nanotubes with one another would lead to a significant uncertainty in the estimation of the available area. Further, the inaccessible regions of these films are not available for anchoring porphyrins or to perform efficiently with the triodide redox couple for the photon-to-electron transduction). 2.3. Dye Sensitization of TiO2 NTA Electrodes. Before dye adsorption, each TiO2 NTA film was calcined at 150 °C for 30 min in air to remove water adsorbed on the TiO2 surface, and then it was cooled to 80 °C and immediately soaked in a dye solution. Adsorption of the dye on the TiO2 surface was carried out by soaking the TiO2 electrode in a dry methanol solution of the dye (nominal concentration 5 × 10-4 M) at room temperature for 30 min, and then the electrode was washed with pure methanol to remove the physisorbed dye. In some instances, a TiCl4 solution pretreatment of the TiO2 NTA film was tested. All NTA films were stored in the dark and in a dry atmosphere. 2.4. Determination of Dye Coverage on TiO2 NTA Electrodes. The amount of adsorbed porphyrins was determined in 0.1 M NaOH aqueous solution for P1, P3, and P5 and in a 0.1 M NaOH solution in methanol:water (1:1) for P2 and P4. The solvent mixture was chosen because it was found that P1, P3, and P5 desorbed satisfactorily in 0.1 M NaOH whereas P2 and P4 did not. On the other hand, NTA-bound P2 and P4 were solubilized in the solvent mixture although the amounts stripped were found to be smaller when compared with P1, P3, and P5 (see below). The determination of the desorbed dye was performed by carefully measuring the volume containing the

desorbed dye as well as its optical absorption (absorbance) at 424 nm (Soret band). These values were then used to calculate the number of adsorbed molecules, using Beer’s law (with the corresponding ε value at 424 nm for 0.1 M NaOH in water and methanol:water (1:1) solutions, respectively) and then normalized with respect to the respective area of the NTAs from ethylene glycol (EG), glycerol (Gly), and poly(ethylene glycol) (PEG 400) (Table 1). 2.5. Photoelectrochemical Measurements and Other Characterization. The electronic spectra of the adsorbed porphyrins on TiO2 NTAs were measured by diffuse reflectance spectroscopy using a Perkin-Elmer Lambda 35 UV-vis spectrophotometer. For incident photon-to-current conversion efficiency (IPCE) measurements, the dye-sensitized semiconductor electrode was incorporated into a thin-layer sandwiched solar cell. The geometric area of the semiconductor electrode was ∼0.7 cm2. A transparent conducting oxide (TCO) glass with 15 Å of electron-beam evaporated transparent Pt served as the counter electrode. A sealing separator (60 µm thick) provided the electrolyte chamber and also served to avoid short-circuiting when the counter and working electrodes were clamped together. The electrolyte solution consisted of a mixture of LiI (0.3 M), iodine (15 mM), 4-tert-butylpyridine (0.2 M), ethanol (15% v/v), and acetonitrile. As the NTA electrodes were not optically transparent, the cell was illuminated from the top (i.e., through the counter electrode), and the light intensity was measured monochromatically to normalize the photocurrent response. The DSSC cell performance was quantified in terms of the incident photon-to-current conversion efficiency (IPCE) parameter:

IPCE(λ) ) 1240(Isc /λΦ)

(1)

where λ is the wavelength of incident light (in nm), Isc is the short-circuit current (in mA/cm2), and Φ is the incident radiation flux (in mW/cm2). As the real area of the NTA films depends on the medium modifier used in their preparation, the IPCE values were also normalized with respect to this active area. Raman spectra were recorded with a Horiba Jovin Yvon ARAMIS instrument using an excitation wavelength of 473

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Figure 2. Comparison of UV-vis spectrophotometric data for the five porphyrin compounds in solution (dashed-dotted lines) and adsorbed on TiO2 NTAs (solid lines). The spectra of the dissolved compounds in methanol were obtained in the transmittance mode, while those for the adsorbed species were acquired by diffuse reflectance with BaSO4 standard. The TiO2 NTAs were prepared by anodization at 20 V, 3 h in 0.36 M NH4F/ poly(ethylene glycol) (PEG 400):H2O (80:20).

nm and a grating with 600 lines/mm. The slit width was set at 10 µm, the time exposure was 10 s, and 10 scans were accumulated and averaged for each spectrum. Spectra were obtained for the methanolic solutions used for TiO2 sensitization as well as for TiO2 NTAs after porphyrin adsorption. The morphology and microstructure of the nanotubes were observed on a scanning electron microscope (Zeiss Supra 55) with a nominal electron beam voltage of 5 kV.

3. Results and Discussion 3.1. Porphyrin Chromophores Studied. The structures of the five polychelate Zn porphyrins (P1-P5) are shown in Figure 1 along with a schematic visualization of how one of these compounds (P5) would bind on the TiO2 NTA surface. Selected spectroscopic and electrochemical data on P1-P5, culled from previous reports,4 are compiled in Table 2. The key point to note is that the excited states of all five chromophores are capable of

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Figure 3. Resonance Raman spectra of P1 dissolved in methanol (A) and bound to TiO2 NTAs prepared from PEG 400:H2O (80:20) (B).

injecting photogenerated electrons from the porphyrin excited states into the TiO2 NTA conduction band (see redox levels, E1/2(P+/ P*), of the excited-state porphyrins in the last column of Table 2). 3.2. Binding of the Porphyrins to the TiO2 NTA Surface. Values of surface coverage are reported in Table 3, where the surface area was calculated using the nanotube dimensions measured by SEM (Table 1). The highest surface coverage was for P3 (reaching values of 9.8 × 1014 molecules/cm2 on TiO2 NTAs from glycerol) and the others were ordered thus: P5 > P1 > P2 > P4. The three types of NTAs provided relatively similar coverages for each type of porphyrin, although the coverages on NTAs prepared from glycerol were always higher, except for P2 and P4. The latter two showed low surface coverage as determined by the amount desorbed (Table 3), and this is most likely due to an irreversible adsorption and therefore incomplete desorption of these two chromophores from the TiO2 NTAs (see above). The fact that P2 and P4 are not completely removed (i.e., desorbed) from the nanotubes obviously indicates a stronger interaction with the semiconductor surface and also indicates that the desorption method is not quantitative for determination of the amounts of P2 and P4 in the nanotube surface. At variance, P1, P3 and P5 were found to be desorbed quantitatively. Assuming that P3 occupies a surface area of 3.3 × 10-14 cm2 (calculated from its x and y dimensions),4b a monolayer of planar molecular configuration (side-by-side) would contain 3.0 × 1013 molecules/cm2. Porphyrin P5 occupies a surface area of 4.7 × 10-14 cm2, and therefore, a planar molecular monolayer (side-by-side) corresponds to 2.1 × 1013 molecules/cm2. Note that these values for the theoretical surface coverage related to the occupancy of a monolayer are always smaller than the values reported in Table 3. The difference could arise from the fact that the nanotube areas were calculated just from the geometric parameters (length, diameter, and wall thickness) measured by SEM and neither the external area of nanotubes nor the roughness factor was considered. Assuming that porphyrin P5 could be used as a probe for the surface roughness of TiO2 NTAs in this study, the roughness

de Tacconi et al. factor would be ∼8 for EG, 20 for Gly, and 18 for PEG 400. Roughness factors reported by dye adsorption (methylene blue) in nanoporous TiO2 gave values of ∼17, while nitrogen adsorption afforded a much larger value of ∼305.12 The difference was attributed to the larger size of the methylene blue probe with respect to the nitrogen probe.12 Our roughness factor estimations are in line with these ideas. In a previous paper,4b the surface coverage of P3 was found higher than that of P5 for nanoporous TiO2, and now the same trend is observed here for the TiO2 NTAs regardless of the nanotube dimension considered. In terms of the possible binding geometries for P1-P5, three of the chromophores could bind via one or two (P1, P2, P4) carboxylic anchoring groups while P3 and P5 use up to four anchoring groups. Anchoring of sensitizing dyes to TiO2 surfaces has been achieved via a number of functional groups (as reviewed in ref 2), with the most widely used and successful to date being the carboxylic acid and phosphonic acid functionalities. 3.3. Spectroscopic Characterization. Figure 2 contains the UV-vis spectrophotometric data on the five compounds in methanol (dashed-dotted curves). All the spectra contain a series of bands between ∼400 and ∼650 nm arising from π-π* transitions involving the conjugated porphyrin macrocycle. Specifically, the Soret absorption band appears between ∼400 and ∼450 nm in each case and Q bands (Q(1,0) and Q(0,0)) appear between ∼500 and 650 nm (Figure 2). Figure 2 also displays diffuse reflectance UV-vis spectra of the TiO2 NTAs after adsorption of P1-P5 (solid lines). In these latter cases, the Soret band is appreciably broadened and slightly red-shifted relative to the spectra for the solution counterparts. The spectra of P1, P2, and P4 on TiO2 NTA also showed clear splitting of the Soret band, giving a shoulder on its short-wavelength (i.e., high energy) side (Figure 2). This pattern is attributed to faceto-face stacking of the aromatic macrocycle (H-aggregation). Similar trends have been reported, for example, for the binding of carboxyphenylethynyl zinc porphyrins on nanocrystalline TiO2 surfaces.3e The red shift of absorbed vs solution porphyrin spectra is noticeable in all five porphyrins and seems to reflect the perturbation of the electronic state of the porphyrin cores either by the anchoring to the TiO2 NTA surface or by a surface concentration effect. In fact, the visible spectrum of Zn porphyrins was reported to shift to the red when the concentration was changed from 10-5 to 10-3 M;13 molecular selfassembly in solid films was postulated to give this red shift and a more intense lowest Q band.13 In our case, the accumulation of porphyrins is on the TiO2 NTA surface and the simplest explanation would be that the red shift is due to side-by-side interaction (J-aggregates).3e Interestingly, the spectra for P3 and P5 do not show significant absorption at the blue side of the Soret bands, suggesting that there is a dominant side-by-side vs face-to-face stacking. At variance, P1, P2, and P4 show preferentially H-aggregates, i.e., face-to-face stacking. On the other hand, the additional possibility cannot be ruled out that the red shift arises also from interaction of the porphyrins with residual additives (EG, Gly, PEG 400, 4-tertbutylpyridine iodide/triiodide3h) from the NTA preparation. For instance, it was recently reported that interaction with foreign organic molecules, e.g., pyridine, leads to a red shift of both Soret and Q bands of Zn porphyrins and to more intense Q bands for Zn porphyrins,14 in agreement with our finding with the TiO2 NTAs. In ref 14, the red shift was assigned to a complex formation involving pentacoordination of the Zn atom.

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TABLE 4: Raman Bands for P1-P5 and Proposed Assignmentsa,c P1

P2

P3

P4

P5

A

B

A

B

A

B

A

B

A

B

assignmentb

665 707 840 890 998 1080 1176 1238 1290 1340 1350 1419 1451 1490 1546 1603 -

661 702 832 892 1008 1076 1184 1237 1288 1340 1351 1418 1451 1545 1590 1606 -

663 725 825 869 1001 1069 1155 1229 1300 1326 1353 1417 1448 1490 1546 1589 1638

666 690 725 826 867 1001 1068 1156 1229 1298 1326 1352 1489 1545 1657

665 691 730 850 866 1011 1073 1173 1238 1303 1352 1421 1456 1494 1547 1598 -

667 692 848 867 1011 1078 1235 1303 1352 1420 1455 1493 1547 1587 1597 -

643 730 837 888 1008 1077 1191 1236 1289 1354 1419 1455 1497 1551 1599 -

639 837 888 1011 1070 1185 1236 1293 1356 1423 1455 1497 1550 1599 -

634 899 1010 1075 1217 1303 1358 1502 1553 1593 -

650 713 842 921 1008 1075 1217 1305 1354 1419 1459 1498 1551 1595 1678

ω(CO2-), ν(ring) monosubstituted phenyl δa(CO2-), ν(pyrrole ring) δs(CO2-), symmetric bending ν(C-CO2-) phenyl ring breathing δ(Cβ-H) δ(Cβ-H) ν(Cm-phenyl) ν(CR-N) ν(CR-Cβ) + δ(Cβ-H) ν(CR-N) + δ(Cβ-N) νs(CO2-), symmetric stretching ν(CR-Cβ) + δ(Cβ-H) ν(CR-Cβ) pyrrole νa(CO2-) νa(CO2-) shoulder ν(CdC) phenyl ring stretching ν(CdO)

a See also Figures S1-S4 in the Supporting Information. NTA-bound forms of the chromophore.

b

For nomenclature, see Burke et al.15 c Columns A and B are for solution and

Figure 4. Resonance Raman spectra of P2, P4, and P5 bound to TiO2 NTAs prepared from PEG 400:H2O (80:20).

The intensity ratio (R) of the Q(1,0) and Soret bands is seen to increase upon adsorption. The R values range from 0.040 to 0.050 for P1-P5 in solution but are 2-5 times higher (i.e., 0.08 for P1, 0.14 for P2, 0.16 for P3, 0.12 for P4, and 0.24 for P5) in the adsorbed state (data taken from Figure 2). Clearly, R values are higher for P5 and P3 than for the other three, which showed H-aggregation. These findings will be correlated with the IPCE values at the Q(1.0) spectral region in a subsequent section below. Laser Raman spectroscopy was carried out to further characterize the chromophores (P1-P5) in solution and bound to the TiO2 NTA surface. Representative Raman spectra are contained in Figure 3 for P1 in solution and adsorbed on TiO2. Corresponding Raman spectra of solution and adsorbed P2-P5

are in Supporting Information (Figures S1-S4). Since the laser excitation wavelength for these spectra was 473 nm, resonance enhancement of the various Raman bands may be expected, especially in the spectra corresponding to the adsorbed state. In this regard, note that 473 nm falls on the long-wavelength tail of the broadened Soret band in each case; cf. Figure 2. Table 4 compiles the Raman band positions for P1-P5 and their assignments;15-18 data are tabulated both for the solution (columns A) and NTA-bound (columns B) forms of the chromophore. When P1 is bound to the TiO2 NTA, the band at 1546 cm-1 (νa(CO2-)) seen in the solution spectrum becomes much broader when the porphyrin is on the TiO2 surface and splits giving rise to a new broad band at 1590 cm-1. This splitting may be

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Figure 5. (a) Cross-sectional SEM images of TiO2 NTAs under the following anodic growth times: 3 h (left image), 8 h (middle), and 24 h (right). (b) Resultant photoaction spectra of these TiO2 NTAs after sensitization with P3. The four IPCE-wavelength plots correspond to TiO2 NTAs prepared with different anodization times (3, 8, 14, and 24 h).

TABLE 5: Photoelectrochemical Properties of Polychelate Porphyrins on TiO2 Nanotube Arrays and Nanoporous Films IPCE (%) Soret, TiO2/media modifier (tube diameter) porphyrin 430 nm

Q(1,0), 560 nma

Q(0,0), 600 nmb

EG (55 nm)

P1 P2 P3 P4 P5

38.4 37.4 51.8 39.1 53.6

4.2 (0.11) 2.0 (0.05) 19.2 (0.51) 9.5 (0.25) 24.6 (0.48) 13.1 (0.25) 15.5 (0.40) 12.0 (0.31) 26.6 (0.50) 23.1 (0.43)

glycerol (70 nm)

P1 P2 P3 P4 P5

24.9 35.2 47.8 48.9 48.8

3.0 (0.12) 1.9 (0.78) 14.2 (0.40) 7.5 (0.21) 22.2 (0.46) 12.0 (0.25) 23.1 (0.47) 16.0 (0.33) 29.5 (0.60) 24.2 (0.49)

PEG 400 (120 nm)

P1 P2 P3 P4 P5

34.1 39.1 47.9 54.2 57.1

4.9 (0.14) 3.2 (0.09) 14.9 (0.38) 7.2 (0.18) 26.2 (0.54) 13.5 (0.28) 19.0 (0.35) 8.7 (0.16) 32.3 (0.56) 20.8 (0.36)

none (nanoporous)

P1 P2 P3

19.1 34.8 43.2

2.8 (0.15) 2.1 (0.11) 9.3 (0.27) 4.9 (0.14) 20.4 (0.47) 11.3 (0.26)

a In parentheses are the Q(1,0) vs Soret peak intensity ratios. b In parentheses are the Q(0,0) vs Soret peak intensity ratios.

associated with attachment of the porphyrin to the oxide surface, although a ring vibration at the same frequency has also been reported.16 The symmetric δs(CO2-) bending at 840 cm-1 is shifted to 832 cm-1 in the bound condition, as expected for a

bidentate adsorption mode restraining bending motion. Finally, the band at 1288-1290 cm-1 is enhanced because of the contribution from stretching vibrations ν(C-O) of the carboxylic groups and ν(CR-N) of the porphyrin moiety. The Raman spectra of P2 and P4 (Figures S1 and S3 in the Supporting Information) share similarities both in solution and in the adsorbed state. The ν(CdO) mode at 1638 cm-1 in P2 seen in solution decreases significantly but does not disappear on the bound state. On the other hand, as expected upon adsorption, the νs(CO2-) at 1417 cm-1 in solution becomes broader and smaller upon binding to the TiO2 NTA surface. The νs(CO2-) feature in P3 (Figure S2 in the Supporting Information) at 1421 cm-1 is seen in both solution and adsorbed conditions because some free carboxylic acid groups are still present in the bound situation. The νa(CO2-) at ∼1547 cm-1 is clearly enhanced by binding, while the bending δa(CO2-) mode at 730 cm-1 almost disappears upon binding. The Raman spectra of P5 (Figure S4 in the Supporting Information) have similarities with those of P3. For P3 and P5, a broad band at 1417-1420 cm-1, related to the νs(CO2-) mode, increases in intensity upon adsorption, while this effect was not observed in P2 and P4. The splitting of the carboxylate stretching bands (νa - νs) is frequently used to distinguish between possible modes of coordination.19,20 In general, unidentate attachment is expected to exhibit a larger (νa - νs) value than the ionic species (i.e., species in solution in our case) due to a decrease in the equivalence of the C-O bonds. Bidentate chelate coordination results in significantly lower (νa - νs) splitting than ionic while the (νa - νs) for bridging should be close to the ionic value.19 Our data seem to point mainly to bridging coordination or

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J. Phys. Chem. C, Vol. 113, No. 7, 2009 3003

Figure 6. SEM images and photoaction spectra of TiO2 NTAs grown with ethylene glycol (a, b) and glycerol (c, d) as medium modifiers and sensitized with porphyrins P1-P5.

bidentate chelation in agreement with the data on mesoporous TiO2 films for P1 and P2 reported by us recently.4b Finally, comparing the Raman spectra of bound P2, P4, and P5 (Figure 4), interesting features can be seen related to the interaction of the porphyrin moieties with the semiconductor surface particularly between 1000 and 1600 cm-1 where C-C and C-N stretching and C-H deformation modes are observed: (i) The ν(CR-Cβ) + δ(Cβ-H) feature at ∼1332 cm-1 which is clearly seen in the solution spectra of P2 and P4 (Figures S1 and S3 in the Supporting Information) is however seen only as a small shoulder in the bound state of P2 and P4 and becomes more noticeable in P5, suggesting that this vibration is less affected by interaction with TiO2 in this latter case than in P2 and P4. (ii) For P4, no band contribution is seen in the 1290-1305 cm-1 region associated with ν(CR-N), while a band is seen for P2 at 1298 cm-1 and is even more noticeable for P5 (at 1305 cm-1). (iii) The 1599 cm-1 band associated with phenyl CdC stretching is quite noticeable in P4, but its intensity is very low in P2 and P5. A pronounced phenyl band was also seen for tetra(4-carboxyphenyl)porphyrin adsorbed on nanocrystalline TiO2.17 These tendencies may be indicative of an increased interaction of the porphyrin moiety with the NTA surface (in the order P4 > P2 > P5); however, we cannot neglect contributions from face-to-face aggregation seen in the adsorbed state (Figure 2). 3.4. Photoelectrochemical Behavior. As shown by us earlier,11a,c the anodization time in a constant potential (or

current, for that matter) film growth mode can be used to tune the nanotube length. Figure 5a contains cross-sectional SEM pictures of TiO2 NTAs varying in length from ∼0.6 to ∼5 µm as the anodization duration is varied from 3 to 24 h, respectively. The corresponding photoaction spectra of these samples after sensitization with P3 are shown in Figure 5b along with one other sample grown at 14 h. The quality of photoresponse (i.e., magnitude of photocurrent at a given wavelength, see eq 1 above) scales with the nanotube length. Presumably, this tendency will saturate as an increasing fraction of the dye molecules may be situated (within the NTAs, see Figure 1) at distances beyond the light penetration depth. However, this limit was not reached in the experiments considered in Figure 5b. Note also that the photoaction spectra mirror the corresponding diffuse reflectance profiles in Figure 2 in that the photocurrents are generated in wavelength regimes corresponding to the Soret and Q bands. The IPCE values in Figure 5b compare favorably with the best performing DSSCs reported in the literature on Znporphyrin-sensitized nanocrystalline TiO2 films (see, for example, refs 3c, d, and f). Our IPCE values for the Q bands, however, are lower than those reported for these best devices in the literature. It must be noted that DSSCs constructed from TiO2 NTAs and using Ru (e.g., N3 or N719) dyes (see, for example, refs 9a and b) have been reported to yield rather low IPCE values (in the ∼1-9% range), although this trend is definitely not intrinsic to NTAs as our data in Figure 5b and

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de Tacconi et al.

Figure 7. SEM images and photoaction spectra of TiO2 NTA (a, b) and TiO2 nanoporous film (c, d) grown with and without PEG 400 and sensitized with porphyrins P1-P5.

other reported performance figures-of-merit8,10 establish. Factors such as the incident photon flux and NTA details play a role here as will be shown from the data below. To assess the influence of the TiO2 NTA variables and the porphyrin binding geometry, shorter nanotubes (∼600 nm long, 3 h growth time, see above) were deployed in comparative experiments where all other variables were maintained constant. The results are summarized in Table 5 and in Figures 6 and 7. Significantly, of the five polychelate porphyrins considered here, P3 and P5 performed best in the Q(1,0) spectral region, indicating that the binding geometry of the chromophore is influencing the directionality of electron transfer and its consequent ability to inject photogenerated electrons into the oxide host. As elaborated elsewhere,11 the NTA anodic growth medium additive plays a key role in controlling the nanotube dimensions. Thus the photoelectrochemical performance of NTAs prepared from PEG 400 (average nanotube diameter ∼ 120 nm) (Figure 7b) are seen to be superior to counterparts generated from EG (average tube diameter ∼ 55 nm) (Figure 6b) or glycerol (average tube diameter ∼ 70 nm) (Figure 6d). Presumably more dye molecules can be packed in a given space when the nanotube diameter is larger, i.e., the nanotube dimensions are bigger. Moreover, larger nanotube diameters seem to provide better accessibility for the iodide/triiodide redox species to reach the oxidized adsorbed porphyrins, thus inhibiting recombination between the injected electrons and the oxidized dye molecules.

Importantly, the Q(1,0) vs Soret peak intensity ratios in the photoaction spectra (see Table 5) are higher than those obtained in the diffuse reflectance spectra (Figure 2) presumably because the TiO2 NTAs are efficiently draining the photogenerated electrons from the porphyrin to the back contact. This may be also due to a difference in injection efficiencies from the S1 and S2 excited states, i.e., Q and Soret states, respectively. The higher Q vs Soret band intensity ratio suggests that the spider orientation of P3 and P5 favors charge injection from the S1 state, possibly pointing to a role of the directionality of the dipole moment in the excited state. The influence of NTA quality and morphology on photoelectrochemical performance was assessed by including in the comparison “control” TiO2 films anodically prepared from Ti foil but omitting the organic additive in the NH4F electrolyte. Such films (as also discussed in refs 11a-d) have a nanoporous morphology as exemplified by the SEM picture in Figure 7c. Significantly, these films showed inferior photoresponse (see entries for P1, P2, and P3 in Table 5). Normalized IPCE data for the different types of nanotubes using the real area of each sample (based on the nanotube diameter, wall thickness and nanotube length; see the Experimental Section and Table 1) are compared in Figure 8, where the bar size is scaled to be proportional to the size of the nanotubes. When using the real area for normalizing the IPCE data, clearer trends appear: 1. Overall, the five porphyrins behave best in PEG 400derived nanotubes probably because these nanotubes, in addition

TiO2 NTAs for DSSCs

Figure 8. Bar plots of normalized IPCE data for the different types of nanotubes using the real area of each sample and at the Soret band (a) and the Q(1,0) band (b). The bar width was scaled to be proportional to the nanotube diameter.

to the vectorial injection of electrons to the back contact, facilitate the iodide/triiodide redox species to move easily for reducing the sensitizer back to its starting state. 2. The normalized IPCE values show a clear dependence on the nanotube diameter. 3. At 560 nm (S0 f S1 excitation), P5 is the best performer, the next is P3, then P4 and P2 behave similarly, and finally P1 is the worst performer. The better IPCE performance of P5 with respect to P3 indicates the importance of the longer linker bridge in the spider molecules for reduced charge recombination.4b,c Importantly, the order of performance at 560 nm seems also to follow an order similar to that of the respective values of extinction coefficients. Exceptions are P2 and P4, where the methyl groups in P2 maintain the porphyrin moiety slightly farther than in P4. 4. The IPCE data related to the Soret band (S0 f S2 excitation) of these five porphyrins again show that P5 is overall the best candidate followed by P4. This order of performance seems to indicate that P5 is on the surface on its four legs (like a spider). P4, although it has only two carboxylic groups, could interact with the semiconductor surface also through the porphyrin moiety (supported by Raman data) for electron injection. The lower performance of P1 could be linked to its lower extinction coefficient. 4. General Discussion and Perspectives While many studies already exist on the use of TiO2 NTAs in DSSC devices,6-8 the main contribution of the present study is to demonstrate the key roles played by the porphyrin aggregation (on the NTA surface) and the NTA morphology and nanotube dimensions on the resultant photoelectrochemical

J. Phys. Chem. C, Vol. 113, No. 7, 2009 3005 response. Thus, in a DSSC device environment and in a practical sense, the NTAs not only serve to sequester the dye chromophores within their confines but also drain the photogenerated electrons from them in an efficient manner. In this regard, our best normalized IPCE values for the Soret band (Figure 8) compare well with the highest values (∼80%) reported for porphyrin-mesoporous TiO2 combinations.3c,d,f The use of TiO2 NTAs grown on Ti foil in DSSCs requires “backside” illumination (i.e., illumination through the semitransparent counter electrode side) with attendant problems related to absorption of near-UV photons by the iodide/ polyiodide electrolyte and by light reflection losses at the Ptcoated TCO glass surface. Solutions to this problem have included growing the NTAs on FTO glass,10a using free-standing TiO2 NTAs,21 or developing detachment procedures for transferring the NTA from the Ti substrate to FTO glass (e.g., ref 8h). All of these approaches would be compatible with further optimization of porphyrin-based and TiO2 NTA based DSSC devices. Nanocrystalline TiO2 films for DSSC applications are several micrometers thick to ensure adequate dye loading and photon absorption. For corresponding DSSCs based on NTAs, this then entails the use of rather long nanotubes to secure a comparable performance. With the use of such long nanotubes, the touted advantage (see, for example, refs 8e and f) of vectorial electron transport intrinsic to one-dimensional materials (such as NTAs), relative to the tortuous electron transport pathway extant in mesoporous films, becomes less clear-cut. The issues of dye utilization for chromophores buried deep within the NTAs (see above) as well as light scattering within the NTA walls themselves also remain to be addressed. Nonetheless, the primary objective of this study was to assess the effects of chromophore binding geometry and NTA morphology on photoresponse quality from a DSSC perspective. We believe that this objective was successfully met with the use of rather short (∼600 nm) TiO2 nanotubes. Finally, while this paper was undergoing peer review, a related paper has appeared on the photoelectrochemical properties of meso- and β-functionalized porphyrin sensitizers for TiO2-based DSSCs.22 The electronic structures of the porphyrin macrocyclic core were reported to be strongly coupled with olefinic side chains resulting in broadened and red-shifted Soret and Q bands. This in turn enhanced the overall electron injection efficiency into the anatase nanoporous TiO2 layer.22 5. Conclusions This study substantially builds on the corpus of data on DSSCs based on TiO2 NTAs8-10 and also complements the recent impressive results on the use of porphyrin chromophores in mesoporous TiO2 film based DSSCs.2,3c,d,f,22 It has been shown that both the porphyrin binding geometry and the oxide NTA morphology (i.e., nanotube diameter) exert a crucial influence on the resultant performance of DSSCs built from such chromophore-oxide combinations. The porphyrin-sensitized TiO2 NTAs are shown to have enhanced charge-collection efficiency for all five porphyrins when compared with corresponding data for nanoporous TiO2 films grown by anodization. Finally, the IPCE data were found to be influenced by the molecular identity of the porphyrin and its molecular organization. Acknowledgment. Partial support of this research from the U.S. Department of Energy (Basic Energy Sciences) via separate grants to K.R. (DE-FG03-95ER14536) and E.G. (DE-FG0201ER15256) is gratefully acknowledged. The three anonymous

3006 J. Phys. Chem. C, Vol. 113, No. 7, 2009 reviewers are thanked for constructive criticism of an earlier manuscript version. Supporting Information Available: Laser Raman spectra of P2-P5 in methanolic solution and adsorbed on TiO2 NTAs are shown in Figures S1-S4, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 335, 737. (b) Gra¨tzel, M. Nature 2001, 414, 338. (c) Gra¨tzel, M. J. Photochem. Photobiol., C: Photochem. ReV. 2003, 4, 145. (2) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. ReV. 2004, 248, 1363. (3) (a) Odobel, F.; Blart, E.; Lagre´e, M.; Villieras, M.; Boujtita, H.; El Murr, N.; Caramori, S.; Bignozzi, C. A. J. Mater. Chem. 2003, 13, 502. (b) Watson, D. F.; Marton, A.; Stux, A. M.; Meyer, G. J. J. Phys. Chem. B 2004, 108, 11680. (c) Nazeeruddin, M. K.; Humphry-Baker, R.; Officer, D. F.; Campbell, W. M.; Burell, A. K.; Gra¨tzel, M. Langmuir 2004, 20, 6514. (d) Wang, Q.; Campbell, W. M.; Bonfanatni, E. F.; Jolley, K. W.; Officer, D. L.; Walsh, P. J.; Gordon, K.; Humphry-Baker, R.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Phys. Chem. B 2005, 109, 15397. (e) Lo, C. F.; Luo, L.; Diau, E. W.-G.; Chang, I.-Jy.; Lin, C.-Y. Chem. Commun. 2006, 1430. (f) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Gra¨tzel, M.; Officer, D. L. J. Phys. Chem. C 2007, 111, 11760. (g) Stromberg, J. R.; Marton, A.; Kee, H. L.; Kirmaier, C.; Diers, J. R.; Muthiah, C.; Taniguchi, M.; Lindsey, J. S.; Bocian, D. F.; Meyer, G. J.; Holten, D. J. Phys. Chem. C 2007, 111, 15464. (h) Morris, A. J.; Marton, A.; Meyer, G. J. Inorg. Chem. 2008, 47, 7681. (4) (a) Galoppini, E.; Rochford, J.; Chen, H.; Saraf, G.; Lu, Y.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. B 2006, 110, 16159. (b) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655. (c) Rochford, J.; Galoppini, E. Langmuir 2008, 24, 5366. (5) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. (6) Taratula, O.; Galoppini, E.; Wang, D.; Chu, D.; Zhang, Z.; Chen, H.; Saraf, G.; Lu, Y. J. Phys. Chem. B 2006, 110, 6506. (7) (a) Nelson, J.; Haque, S. A.; Klug, D. R.; Durrant, J. R. Phys. ReV. B 2001, 63, 205321. (b) Dlozik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. J. Phys. Chem. B 1997, 101, 10281. (8) (a) Ohsaki, Y.; Masaki, N.; Kitamura, T.; Wada, Y.; Okamoto, T.; Sekino, T.; Niihara, K.; Yanagida, S. Phys. Chem. Chem. Phys. 2005, 7, 4157. (b) Yoon, J. H.; Jang, S.-R.; Vittal, R.; Lee, J.; Kim, K.-J. J. Photochem. Photobiol., A: Chem. 2006, 180, 184. (c) Wei, M.; Konishi, Y.; Zhou, H.; Sugihara, H.; Arakawa, H. J. Electrochem. Soc. 2006, 153, A1232. (d) Akhtar, M. S.; Chun, J.-M.; Yang, O.-B. Electrochem. Commun. 2007, 9, 2833. (e) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (f) Zhu, K.; Vinzant, T. B.; Neale, N. R.; Frank, A. J. Nano Lett. 2007, 7, 3739. (g) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gra¨tzel, M. ACS Nano 2008, 2, 1113. (h) Park, J. H.; Lee, T.-W.; Kang, M. G. Chem.

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