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Langmuir 1998, 14, 2744-2749
Vibrational Spectroscopic Study of the Coordination of (2,2′-Bipyridyl-4,4′-dicarboxylic acid)ruthenium(II) Complexes to the Surface of Nanocrystalline Titania Kim S. Finnie,* John R. Bartlett, and James L. Woolfrey Materials Division, Australian Nuclear Science and Technology Organization, Private Mail Bag 1, Menai NSW 2234, Australia Received September 22, 1997. In Final Form: February 25, 1998 The coordination of photosensitizing Ru(II) dyes to a nanocrystalline titania film, as employed in the Gra¨tzel solar cell, has been examined by vibrational spectroscopy. The major infrared bands of the adsorbed dyes have been assigned by comparison with spectra (IR and Raman) of the parent dye molecules, and suggest a bidentate chelate or bridging coordination to the TiO2 surface via two carboxylate groups per dye molecule.
Introduction There is growing interest in photoelectrochemical cells based on dye-sensitized semiconductor films.1-3 In such cells, dyes are adsorbed onto the surface of wide band-gap (>3 eV) semiconductors, which themselves absorb only in the lower wavelength UV region. Suitable dyes have broad, strong absorption over the visible and near-infrared wavelength range and inject electrons into the conduction band of the semiconductor with high quantum yield, thus enabling efficient harvesting of visible light. Cells developed at the Ecole Polytechnique Fe´de´rale de Lausanne (EPFL) by Gra¨tzel, with photoelectrodes based on nanocrystalline TiO2 films sensitized with a ruthenium complex dye, have displayed solar conversion efficiencies of 10%.4 These cells have considerable potential to compete commercially with more conventional silicon-based cells, particularly for applications in which cell transparency and low, diffuse light levels are important factors. A study of the surface of the dyed TiO2 photoelectrode using vibrational spectroscopy has been undertaken to determine both how the dye is attached to the film, which is not well-understood, and the effect of various film surface treatments on dye performance. The ruthenium complex contains two bipyridine (bipy) ligands with 4,4′-substitution by carboxylic acid groups, through which the dye interacts with the TiO2 surface (Figure 1). In a previous publication, it was concluded from IR measurements (showing carboxyl bands at 1728 and 1610 cm-1) that both ester-like and chelating linkages are involved in binding the dye to the film.5 However, the authors do not appear to have considered the possibility of uncoordinated carboxylic acid groups contributing to the spectrum. Indeed, the CdO stretch of the acid group was measured at 1720 cm-1, which given the bandwidths involved, is not significantly shifted from the band attributed to a unidentate (ester-like) carboxylate. Recently, the IR spectra have been reported of dye adsorbed from aqueous solution onto thin films of both sol-gel (mostly amorphous) and P25 TiO2 cast onto the (1) Kamat, P. V. CHEMTECH 1995, 25 (6), 22. (2) Gra¨tzel, M. Renewable Energy 1994, 5 (1), 118. (3) Gra¨tzel, M. J. Sol-Gel Sci. Technol. 1994, 2, 673. (4) Gra¨tzel, M.; Kalyanasundaram, K. Curr. Sci. 1994, 66 (10), 716. (5) Murakoshi, K.; Kano, G.; Wada, Y.; Yanagida, S.; Miyazaki, H.; Matsumoto, M.; Murasawa, S. J. Electroanal. Chem. 1995, 396, 27.
Figure 1. Structure of the fully protonated dye, cis-di(thiocyanato)-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium (II).
surface of a ZnSe internal reflection element.6 Bands at 1380 and 1600 cm-1 were attributed to symmetric and antisymmetric stretching modes of carboxylate respectively, although the bipy bands which occur in this region (and particularly that at 1609 cm-1) were not discussed. The mode of coordination was determined to be bridging because of the minor wavenumber shifts in the carboxylate stretching modes with adsorption to the film. Here we report analysis by IR and Raman spectroscopy of a series of parent dyes and an in situ IR study of the corresponding dyed TiO2 photoelectrodes, to investigate the manner in which the dye molecules interact with the TiO2 surface. Experimental Section Samples. Three dyes were tested, all based on the dye favored by EPFL, cis-di(thiocyanato)-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II).7 In systematically investigating the vibrational spectra of the dye powders, a series of dyes, in which the acid group was progressively neutralized by addition of tetran-butylammonium hydroxide ([tba]OH), were compared: (1) contained no added [tba]+ (referred to as tba0), (2) two acid groups (6) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451. (7) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382.
S0743-7463(97)01060-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/14/1998
Vibrational Spectroscopy of Ruthenium(II) Complexes per molecule have been deprotonated (tba2), and all four acid groups have been deprotonated (tba4). The dyes are all intensely colored powders which are dissolved in organic solvents for application to the films. Vibrational analysis was carried out using the solids, in preference to the solutions, because of their limited solubility. In addition to the dye spectra, IR and Raman spectra of a number of extra species were measured to provide further spectral information: tetran-butylammonium iodide, to characterize the counterion vibrations, and benzoic acid, to compare its coordination to the TiO2 surface with that of the dye. In the latter case, spectra of benzoic acid and the corresponding benzoate salt (produced by neutralizing with tetramethylammonium hydroxide, [tma]OH) were measured in solution, to allow calculation of depolarization ratios. Titania films were produced by screen printing with an ANSTO formulated serigraphic paste onto a conducting glass substrate coated with fluorine-doped SnO2 (Libby Owens Ford, LOF Tech 15), which forms the photoanode of the Gra¨tzel cell. The fired films, with a thickness of 4 µm, consist of anatase nanocrystallites with crystallite sizes ranging from 15 to 25 nm. IR film spectra were measured of ∼(1 × 1)-cm2 films dyed with each of the three dyes, and also with adsorbed benzoic acid. The dyeing procedure was as follows: (a) predry the film at 450 °C for 15 min in air; (b) soak the film in dye solution held at 70 °C for 2 h; (c) rinse the film with dry ethanol and dry under flowing, dry nitrogen. In the case of benzoic acid, the predried film was held for 45 min in solution and then rinsed thoroughly with dry ethanol, before measuring. IR Spectra. IR spectra (nominal 4-cm-1 resolution) were measured using a Digilab FTS-40 spectrometer, equipped with a liquid N2 cooled MCT detector. A linearized detector was used for all quantitative spectra. The dye powders were held in KBr pellets in the case of tba2 and tba4, but the spectrum of tba0 was obtained using a photoacoustic accessory to avoid baseline defects. Solution spectra of benzoic acid, and the benzoate salt (both dissolved in 2-propanol), were measured using an attenuated total reflectance (ATR) accessory equipped with a single reflection ZnSe element. The films were measured as reflectanceabsorption spectra, with incidence and reflectance angles of 80°. However, it should be noted that the film spectra show no polarization dependence and therefore are treated as thick film, bulk samples. Spectral data were processed using Win-IR (Digilab), incorporating a curve-fitting procedure. Raman Spectra. Raman spectra (nominal 4-cm-1 resolution) were measured using a Digilab Raman II (FT) spectrometer, with a laser excitation wavelength of 1.064 µm (SpectraPhysics Nd:YAG laser). Raman scattering at 180° was detected with a liquid N2 cooled germanium detector and ratioed against a white light source. Since the Raman spectra of the dyes are dominated by bipy bands, the carboxylate bands appearing only weakly, the Raman spectra of the equivalent films have not been reported here.
Results and Discussion Dye Spectra. In analyzing the vibrational spectra of the dye powders, the comparison of the series of dyes where the acid group was progressively neutralized by the addition of tetra-n-butylammonium hydroxide ([tba]OH) has enabled assignment of the major spectral bands. The region of particular interest is between 1800 and 1000 cm-1, as the various C-O stretching bands which are found here indicate the types of C-O bonding which are present in the molecule.8 This region is complicated with vibrations of the bipy framework, [tba]+ ion, carboxylic acid, and carboxylate groups all contributing to the spectra; the isothiocyanato ligand has an intense ν(C-N) at 2100 cm-1 in IR and Raman and the [tba]+ ion has strong IR ν(C-H) bands at 2963, 2936, and 2875 cm-1. Although not discussed here, the latter were used for estimating (8) Lin-Vien, D. K.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, 1991; Chapter 9.
Langmuir, Vol. 14, No. 10, 1998 2745
Figure 2. Infrared absorption spectra (1800-1000 cm-1) of the dye powders tba0, tba2, and tba4.
Figure 3. Raman spectra (1800-1000 cm-1) of the dye powders tba0, tba2, and tba4.
the relative concentration of [tba]+ ions in the films, as there are no aliphatic C-H groups in the fully protonated dye molecule. In the concentrated state, the dyes are strongly hydrogen-bonded through the carboxylic acid groups; consequently, the corresponding O-H stretch is shifted to lower wavenumbers and is broadened compared to the band of isolated O-H species. The CdO stretch, which has significant IR intensity, is also shifted slightly to lower wavenumbers relative to the monomeric acid, as a result of partial electron withdrawal from the CdO bond. The IR and Raman spectra of the three dye species in the region 1800-1000 cm-1 are shown in Figures 2 and 3, respectively. tba0. The IR spectrum of tba0 exhibits CdO stretching modes at 1740 and 1708 cm-1. There are five bands in the region 1700-1400 cm-1, at 1609, 1549, 1470, 1440, and 1412 cm-1. The first four agree well with literature values for the ring stretching modes of [Ru(bpy)3]2+, reported at 1608, 1563, 1491, and 1450 cm-1,9 given that some shifts are expected due to ring substitution. There are a number of bands to lower energy (to ∼1000 cm-1) in [Ru(bpy)3]2+ which contain both C-C and C-N stretching and CCH deformation character.9 The corresponding region for tba0 in the Raman spectrum (Figure 3) is dominated by bipy modes, with bands at 1609, 1545, 1470, 1304, and 1266 cm-1. However, the CdO modes also appear weakly in the spectrum, at 1738 and 1704 cm-1. The presence of (9) Strommen, D. P.; Mallick, P. K.; Danzer, G. D.; Lumpkin, R. S.; Kincaid, J. R. J. Phys. Chem. 1990, 94, 1357.
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Figure 5. Three possible carboxylate coordination modes: (a) unidentate, (b) bidentate chelating, and (c) bridging. Figure 4. Infrared absorption spectra (1800-1300 cm-1) of (a) tba0 dye powder, and (b) tba0 adsorbed onto TiO2 film.
more than one CdO stretching mode in both IR and Raman spectra indicates that the four acid groups are not all equivalent, probably due to differences in the extent of hydrogen bonding. tba4. The absence of IR bands above 1700 cm-1 in the tba4 spectrum (Figure 2) indicates that there are no acid CdO bonds present. Instead, there is an intense, broad band at 1615 cm-1 (overlapping the highest bipy band), which is assigned to the antisymmetric stretch of -CO2-, where the negative charge is delocalized to give two equivalent (or nearly so) C-O bonds. There is a highenergy tail to the band indicating some variation in this bonding. The other intense band in this spectrum at 1364 cm-1 (shoulder at 1383 cm-1) is assigned as the symmetric stretch of -CO2-, since it is not assignable to the [tba]+ ion or bipy. There are two Raman bands at 1413 and 1339 cm-1 which do not appear in the tba0 spectrum (Figure 3) and are also not assignable to [tba]+ or bipy. The latter lies under the band envelope of the IR 1364cm-1 vibration, so the most likely assignment is as a symmetric stretch of -CO2-. The IR spectrum of tba4 is relatively featureless between 1300 and 1000 cm-1, suggesting that the strong bands in this region for tba0 and tba2 are due to singly bonded C-O stretching modes (with some admixture of C-OH bending).8 The [tba]+ ion is clearly present in the sample, as evidenced by the corresponding ν(C-H) bands in IR; however, in this region (1800-1000 cm-1) only the strongest [tba]+ band at 1480 cm-1 is apparent in the dye’s IR spectrum. tba2. The spectra of tba2 appear similar to those of tba0. There is now one CdO band at 1715 cm-1 in IR, and at 1723 cm-1 in Raman. There is no distinct IR band at 1615 cm-1, although there appears to be some broad absorption in this region, and there is an increase in intensity of the IR band at 1371 cm-1, indicating that some -CO2- is present. There are fewer strong IR bands in the C-O stretch region, with increased intensity of the 1230-cm-1 band and virtual loss of the band cluster around 1130 cm-1, consistent with the remaining hydrogenbonded carboxylic acid groups being more homogeneous in this sample. Spectra of Dye Adsorbed on TiO2 Films. The IR absorption spectra over the range 1800-1300 cm-1 of the tba0 dye in the neat state (a) (photoacoustic) and adsorbed onto the TiO2 films (b) are shown in Figure 4. The spectral region at lower wavenumbers is complicated by an interference band. Coordination of the dye molecule to the TiO2 surface is unlikely to significantly alter the position of the ring stretching modes, and indeed, bands
in the region 1700-1400 cm-1 were found at 1610, 1545, 1471, 1438, and 1408 cm-1, in good agreement with the spectrum of the neat dye. However, the IR band at 1610 cm-1 is broadened and has increased intensity relative to the lower lying bands, as has the intense band at 1380 cm-1, similar to the IR spectrum of tba4. Therefore, these are assigned as antisymmetric and symmetric ν(-CO2-) modes, respectively, of carboxylate. The carboxylic acid groups in neat tba0 are all protonated, so the carboxylate bands observed in the spectrum of sorbed tba0 must be due to carboxylates formed on coordination to the TiO2 surface. The splitting of carboxylate stretching bands (∆ ) νas(-CO2-) - νs(-CO2-)) has frequently been used in vibrational spectroscopic studies of metal complexes of carboxylic acids, to attempt to distinguish between possible modes of coordination (illustrated in Figure 5) of the carboxylate to the metal.10-13 In general, unidentate complexes are expected to exhibit a larger ∆ than the corresponding ionic species, due to the decrease in equivalence of the C-O bonds. Conversely, bidentate chelate coordination should result in significantly lower ∆ than that in the ionic species, whereas ∆ for bridging complexes should be close to the ionic value.10 A recent ab initio molecular orbital study has investigated the physical basis for the structure-frequency correlation and found that ∆ is related to changes in the CO bond length and the OCO angle.14 The splitting observed for the neat ionic tba4 carboxylates is 250 cm-1, which is somewhat higher than the largest ∆ observed in a study of alkali-metal benzoate salts (173 cm-1).15 The splitting in the tba0 film spectrum is 230 cm-1, slightly less than the corresponding ionic value. Indeed, in the spectra of adsorbed tba2 and tba4, ionic and coordinated carboxylates were not distinguishable. On the basis of the arguments outlined above, this would suggest that the carboxylate groups are coordinated via bridging or bidentate coordination to the titanium ions. This finding is at variance with that of Meyer et al., who concluded that a similar species, [Ru(bpy)2(4,4′-(CO2H)2bpy)]2+, attaches to TiO2 via unidentate coordination.13 (10) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. (11) Nakamoto, K. Infrared and Raman Spectra, 4th ed.; John Wiley & Sons: New York, 1986; p 232. (12) Doeuff, S.; Henry, M.; Sanchez, C.; Livage, J. J. Non-Cryst. Solids 1987, 89, 206. (13) Meyer, T.; Meyer, G. J.; Pfennig, B. W.; Schoonoover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Xiaohong, C.; Peek, B. M.; Wall, C. G.; Ou, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem. 1994, 33, 3952. (14) Nara, M.; Torii, H.; Tasumi, M. J. Phys. Chem. 1996, 100 (51), 19812. (15) Green, J. H. S.; Kynaston, W.; Lindsey, A. S. Spectrochim. Acta 1961, 17, 486.
Vibrational Spectroscopy of Ruthenium(II) Complexes
Figure 6. Infrared absorption spectra (1800-1300 cm-1) of (a) benzoic acid, (b) benzoate [tma] salt, and (c) benzoic acid adsorbed onto TiO2 film.
This assignment was based on the observation of a lowintensity CdO band at 1691 cm-1 and absence of νasym or νsym(-CO2-) bands in the Raman spectrum. However, we have found Raman spectroscopy to be less useful than IR for the detection of carboxylate bands of the coordinated dye molecules, because of the intensity of the bipy ring modes at 1609, 1540, and 1470 cm-1 which dominate the rather weak film spectra. Indeed, we were unable to detect the uncoordinated acid ν(CdO) bands in Raman, which were easily observed in IR at 1770 and 1740 cm-1. The νasym and νsym(-CO2-) modes were also not detected in the Raman spectra of the dyed films, but were observed in IR. Therefore, the discrepancy between the two results appears to be related to the differing sensitivity of the two measurement techniques to the -CO2- stretching modes in the presence of possibly resonantly (or preresonantly) enhanced bipy modes.13 To further investigate the coordination of the dye species to the TiO2 surface, benzoic acid was chosen as a model system for the dye, with phenyl replacing the pyridine ring. The IR absorption spectra of benzoic acid (a), the benzoate [tma] salt (b), and benzoic acid adsorbed onto a TiO2 film (c) are shown in Figure 6. When benzoic acid is neutralized by the addition of [tma]OH, the CdO bands at 1720 (sh) and 1699 cm-1 disappear, and intense bands appear at 1559 and 1378 cm-1 (shoulder at 1368 cm-1), which are assigned to νasym(-CO2-) and νsym(-CO2-), respectively. The measured depolarization ratios for the corresponding Raman bands at 1563 and 1381 cm-1 were 0.86 and 0.07, respectively. Although there is a significant error in the first ratio (since the 1563 cm-1 Raman band is extremely weak), the bands appear fully depolarized and polarized, respectively, supporting the assignments above. The spectrum of benzoic acid coordinated to TiO2 is similar to that of the benzoate ion (Figure 6), in that the highest wavenumber bands in the 1800-1300-cm-1 region are the ring modes at ∼1600 cm-1. Two unresolved bands at 1535 and 1505 cm-1 are assigned to νasym(-CO2-), and those at 1418 and 1405 to νsym(-CO2-), the splitting probably indicating two different coordination sites. Using the above “rule of thumb”, the largest possible separation ∆ ) νas(-CO2-) - νs(-CO2-) of 130 cm-1 for the coordinated benzoate, compared with ∆ ) 181 cm-1 for the benzoate [tma] salt, suggests either a bidentate chelate or bridging coordination for the benzoate to the TiO2 surface, rather than a unidentate, ester-like linkage. It should be noted that a number of exceptions have been found to the above rules in assessing the coordination
Langmuir, Vol. 14, No. 10, 1998 2747
Figure 7. Infrared absorption spectra (3800-3550 cm-1) of (a) TiO2 film (dried at 450 °C), (b) tba0 dyed film, and (c) film after washing with a weak organic base. All spectra have been baseline-adjusted to correct for an interference band.
of acetate groups to metals.16 We are unable to look for further evidence in the region below 1000 cm-1 (COO deformation bands)11 due to strong absorption by the titania film. Hence, we conclude that the separation ∆ ) νas(-CO2-) - νs(-CO2-) in the coordinated dye and benzoic acid spectra, compared with that in the salt spectra, indicate either a bidentate chelate or bridging coordination for the carboxylate ion, with unidentate coordination being considered unlikely. Dye Attachment to Surface. Clearly, on steric grounds, it is unlikely that the dye is anchored to the titania film through all four carboxylic acid groups. In contrast to the hydrogen-bonded nature of the bulk dye, at least a fraction of the acid groups are relatively isolated when the dye is adsorbed to the TiO2 surface, and a sharp ν(OH) is observed at 3582 cm-1 (Figure 7). While it is feasible that the dye molecule can also attach to the surface by H-bonding through the acid OH group, this would result in a broadened ν(OH) at lower wavenumbers (∼3000 cm-1). Hence, the sharp 3582-cm-1 band is indicative of the presence of non-H-bonded, uncoordinated acid groups in the adsorbed dye molecule. Two CdO bands are observed in the adsorbed tba0 dye’s IR spectrum, at 1770 and 1740 cm-1 (Figure 4), with the latter being the stronger and broader of the two. There are several types of CdO groups which could conceivably be present in the adsorbed dye. Clearly there must be CdO of non-H-bonded, uncoordinated acid, as evidenced by the ν(OH) at 3582 cm-1 (Figure 7). This would be expected at higher wavenumbers than observed in the H-bonded dye; a likely candidate is the 1770-cm-1 band. Evidence for this assignment is found on comparing the band intensities for the various adsorbed dyes; the 3582and 1770-cm-1 bands are present in similar ratios in all three dyes, whereas the intensity of the 1740-cm-1 band drops significantly in the adsorbed tba4 dye spectrum. In addition, the 3582- and 1770-cm-1 bands both disappear when the dyed film is treated with a weak organic base, thus deprotonating the dye (see Figure 7). Hydrogen-bonded acid groups, as observed in the dye powder sample, have ν(CdO) modes at ∼1740-1710 cm-1. A possible alternative assignment for a band in this region is to ν(CdO) arising from unidentate coordination via an ester-like linkage to the TiO2, as proposed by Murakoshi et al. (see Introduction).5 However, in our experiments, the intensity of the 1740-cm-1 band was also observed to (16) Edwards, D. A.; Hayward, R. N. Can. J. Chem. 1968, 46, 3443.
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Table 1. Normalized Band Intensities for Dyed Film Spectra and Intensity Ratios
assignment
3580 cm-1 a ν(OH) of non H-bonded acid
1770 cm-1 a ν(CdO) of non H-bonded acid
1740 cm-1 a ν(CdO) of H-bonded acid
total CdO
1610 cm-1 a νas(CO2-)
1545 cm-1 a bipy ring mode
1380 cm-1 a νs(CO2-)
tba0 tba2 tba4 tba2/tba0 tba4/tba0
0.11 0.037 0.037 0.33 0.33
0.21 0.051 0.061 0.25 0.29
1.9 1.0 0.26 0.54 0.14
2.1 1.1 0.32 0.51 0.16
2.4 3.4 5.3 1.4 2.2
1 1 1 1 1
6.1 9.0 11.6 1.5 1.9
a
Band position.
decrease, by approximately 50%, on treatment of the film with a weak base. This observation favors assignment of the band to H-bonded carboxylic acid groups, which would also be susceptible to deprotonation, rather than to esterlike linkages, which would not be expected to be affected by the surface treatment. The normalized intensities of the bands assigned above are recorded in Table 1, for the various dyes. The spectra have been normalized using the 1545-cm-1 bipy band, which is not expected to change with deprotonation of the dye. Similar results were obtained using the bipy band at 1440 cm-1. As the absolute intensities are very small, corresponding to an average ∆A ∼ 0.01, and were calculated using a curve fitting procedure, the numbers have an estimated error of at least 10%. Band ratios are also tabulated, to highlight spectral changes with the various dyes. Interestingly, the tba4 dye has the same ν(OH) intensity as does tba2, relative to tba0, despite the fact that the tba4 dye molecule has no protonated acid groups. The only source of protons during the dyeing step is the film surface itself. Surface hydroxyl species on the undyed TiO2 film exhibit stretching bands at 3692, 3676, and 3630 cm-1 (Figure 7). These band positions are similar to those observed in previous studies where IR spectroscopy was used to characterize TiO2 surfaces.17-19 The variations in wavenumber arise from different modes of coordination; for example, terminal (isolated) OH groups have stretching bands at higher wavenumbers (reflecting a higher force constant) than bridging OH groups. An assignment of these bands has not been attempted here, being the subject of a future study. However, as is shown in Figure 7, none of these bands are observed after the film has been dyed, suggesting that the surface OH groups are either H-bonded to the dye or displaced by dye attachment to the TiO2 surface. A study of the relative acidities of these groups on TiO2 has found values of pKa ranging from ∼3 for protonated Ti-OH2+, to ∼6 for bridging OH and ∼8 for terminal TiOH.20 With a pKa of 4-5 for tba0,21 it is clear that a -CO2- group can abstract a proton quite readily from these species (with the exception of terminally bound TiOH), although the majority of the -CO2- groups on tba2 and tba4 remain deprotonated, given the intensity of ν(OH) relative to that of tba0 dye is 0.33. In the extreme case for tba0, where at most three of the acid groups could remain uncoordinated to TiO2, and non-H-bonded, tba2 and tba4 would then have a maximum of one non-H(17) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75 (9), 1216. (18) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (19) Crocker, M.; Herold, R. H. M.; Wilson, A. E.; Mackay, M.; Emeis, C. A.; Hoogendoorn, A. M. J. Chem. Soc., Faraday Trans. 1996, 92 (15), 2791. (20) Venz, P.; Bartlett, J. R.; Frost, R. L.; Woolfrey, J. L. Sol-Gel Processing of Advanced Materials, Ceramic Transactions; Klein, L. C., Pope, E. J., Sakka, S., Woolfrey, J. L., Eds.; American Ceramic Society: Westerville, OH, 1998; Vol. 81, in press. (21) Calculated from pH data supplied by EPFL.
bonded acid group per dye molecule. However, comparison of the intensities of the CdO bands at 1770 and 1740 cm-1 (Figure 4) suggests that only a small fraction (∼10% for tba0) of the total CdO intensity arises from non-H-bonded acid groups. Even less is found in tba2 (5%), but the proportion is greater (∼20%) for tba4, related to the dramatic decrease in the 1740 cm-1 band intensity. Comparison of the change in total CdO intensity (sum of 1770- and 1740-cm-1 bands), compared with that of -CO2- intensity (1610- and 1380-cm-1 bands) from the different adsorbed dyes reveal trends which indicate how the dyes are attached to the film: (1) The tba2 dye has 0.51 times the CdO intensity of tba0, while the ratio drops to 0.16 for tba4 compared to that of tba0. (2) The relative intensities of the 1610- and 1380-cm-1 bands increase by 1.4 and 1.5, respectively, for tba2 compared to that of tba0, and 2.2 and 1.9 for tba4 compared to that of tba0. This general pattern, of -CO2- concentration increasing by ∼1.5, and then by ∼2 times the intensity through the series tba0, tba2, and tba4, coupled with the decrease in CdO concentration by ∼1/2, and ultimately to a small value, suggests that the adsorbed dyes contain the following species: (1) tba0 with 2 -CO2- and 2 CdO per molecule; (2) tba2 with 3 -CO2- and 1 CdO per molecule; (3) tba4 with 4 -CO2- and a only a small percentage substituted as CdO. As tba0 does not have any -CO2- groups in the parent dye, this suggests that the majority of the dye molecules are coordinated to the surface of the TiO2 via two -CO2- linkages. There is evidence that the same bonding arrangement holds for the tba2 and tba4 dyes, in the reduction of the [tba]+ ion ν(C-H) intensities observed in the dyed films, compared with the parent dyes. Of the three C-H bands, the band at 2875 cm-1 is the most reliable measure of [tba]+ ion concentration in the films, since the two bands at 2963 and 2936 cm-1 are not resolved. The intensity of the 2875-cm-1 band (normalized by the 1545-cm-1 bipy band, as above) of the adsorbed tba4 dye was reduced to 0.51 times that observed in the bulk tba4 dye, suggesting that, of the four [tba]+ ions per dye molecule, two remain attached to the dye when adsorbed to the TiO2 film, leaving two -CO2- groups to bind to the TiO2. In the case of tba2, the ν(CH) bands were too weak to be observed on top of an intense interference band in this region (present in all the film IR spectra). Analysis of a thinner film in which the interference band in question is shifted to higher wavenumbers, indicates that the normalized 2875-cm-1 band intensity is 40% of that measured for tba4. However, the bands are extremely weak in the very thin films, and there would be considerable error in the band area determination. Nevertheless, as two [tba]+ ions per molecule can be observed for the tba4 dye, the concentration for tba2 must be significantly less than this, and our interpretation of the bonding arrangement suggests that there is one [tba]+ ion present per tba2 dye molecule.
Vibrational Spectroscopy of Ruthenium(II) Complexes
Conclusions By comparison with bulk dye Raman and IR spectra, the major bipy, carboxylic acid, and carboxylate infrared bands in the region 1800-1300 cm-1 have been assigned for cis-di(thiocyanato)-N,N-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II) (and associated salts) adsorbed onto the surface of TiO2 films. The adsorbed dye carboxylic acid bands (ν(CdO)) appear at 1770 and 1740 cm-1. The latter is due to hydrogenbonded groups, and the former to the fraction (∼10%) of acid groups which remain isolated. The isolated acid groups give rise to a ν(OH) at 3580 cm-1. The presence of the ν(CdO) and ν(OH) bands in the spectrum of adsorbed dye in which all carboxylic acid groups had been neutralized indicates that a fraction of carboxylate groups (∼15%) have abstracted a proton from the TiO2 film surface. Carboxylate bands of adsorbed dye, due to both coordinated and ionic carboxylate groups, appear at 1610 cm-1 (νas(-CO2-)) and 1380 cm-1 (νs(-CO2-)). The separation νas(-CO2-) - νs(-CO2-) of the coordinated acid compared with that observed for the ionic
Langmuir, Vol. 14, No. 10, 1998 2749
carboxylate, measured for the adsorbed dye and also benzoic acid (which should show similar coordination), indicates that the carboxylate attaches via bidentate chelate or bridging coordination to the TiO2 surface. Observed trends in the intensities of the dye carboxylic acid and carboxylate bands, and those of the tetrabutylammonium counterion, are consistent with attachment of the dyes to the TiO2 surface via two coordinating carboxylate groups per dye molecule. Acknowledgment. These studies were part of an Energy Research & Development Corporation (Australian Federal Government) funded collaborative project with Sustainable Technologies Australia Ltd. and Monash University to commercialize the Gra¨tzel solar cell. We would particularly like to thank Dr. L. Y. Goh, Dr. L. Spiccia, and Professor D. MacFarlane, Department of Chemistry, Monash University, for supplying the ruthenium dye complexes. LA971060U
