Lewis-Acid Sites of TiO2 Surface for Adsorption of Organic Dye Having

Jul 17, 2013 - Having Pyridyl Group as Anchoring Unit. Yutaka Harima,* Takuya Fujita, Yuta Kano, Ichiro Imae, Kenji Komaguchi, Yousuke Ooyama, and Joj...
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Lewis-Acid Sites of TiO2 Surface for Adsorption of Organic Dye Having Pyridyl Group as Anchoring Unit Yutaka Harima,* Takuya Fujita, Yuta Kano, Ichiro Imae, Kenji Komaguchi, Yousuke Ooyama, and Joji Ohshita Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan S Supporting Information *

ABSTRACT: Adsorption of an organic dye having a pyridyl group as an anchoring unit (NI4) is studied on nanocrystalline TiO2 surface by being compared with a similar dye having a carboxyl anchor (NI2). Adsorption of both dyes followed a Langmuir isotherm, and analysis of the isotherms showed that an adsorption equilibrium constant of NI4 is (1.1 ± 0.1) × 103 M−1, much smaller than (0.6 ± 0.05) × 105 M−1 for NI2, and amounts of adsorbed dyes at saturation are similar to each other: [(1.5 ± 0.2) and (1.7 ± 0.3)] × 1014 cm−2 for NI2 and NI4, respectively. Coadsorption experiments with 4-carboxy TEMPO (4CT) and either NI2 and NI4 revealed that the adsorption sites of NI2 and NI4 are different from each other. This was supported by measurements of average nearest-neighbor interspin distances of 4CT radicals coadsorbed on TiO2 by a spin-probe ESR technique. The above findings and elaborate FT-IR studies demonstrated that NI2 and NI4 adsorb, respectively, on Brønsted- and Lewis-acid sites of TiO2 surface. The number of Lewis-acid sites evaluated from the temperature-programmed desorption experiments of TiO2 was in good agreement with the amount of NI4 adsorbed on TiO2 at saturation, providing a further confirmation for Lewis-acid sites acting as a predominant adsorption site of NI4. literature.14−17 A possible use of the pyridyl group replacing a traditional carboxyl anchor is of great importance because it can defuse a restriction imposed on the design and synthesis of photosensitizing organic dyes. However, details of adsorption behaviors of organic dyes having the pyridyl anchor on the TiO2 surface have not yet been clarified, although adsorption of pyridyl dyes on Lewis-acid sites of TiO2 was suggested in our previous studies on the basis of FT-IR measurements of dyes adsorbed on TiO2.11−13 In the present study, adsorption natures of organic dyes having a pyridyl group as an anchoring unit were investigated in detail by using NI4 as a typical pyridyl-anchor dye, in comparison with those of NI2, which is similar to NI4 except an anchoring unit. Adsorption properties of NI2 and NI4 on TiO2 were clarified on the basis of adsorption isotherms and their analyses. Coadsorption experiments of 4CT radicals having a carboxyl anchor and either NI2 and NI4, coupled to a spin-probe ESR technique, were carried out to distinguish adsorption sites of TiO2 surface available for NI2 and NI4. From FT-IR measurements and temperature-programmed desorption (TPD) experiments with ammonia, along with the adsorption experiments, we were led to the conclusion that NI4

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have received increasing interest as a promising alternative to conventional Si-based solar cells because they consist of low-cost materials and can be assembled by a simple and facile technique. Their performances have been steadily improved by intensive studies made from different viewpoints including molecular design and synthesis of efficient photosensitizing dyes. Initial DSSC studies were commenced by Grätzel with Ru−metal complexes as photosensitizers. However, Ru is one of the rare metals, and its use in the mass-marketed devices such as solar cells is not preferable. From this point of view, a number of studies have been focused on the development of efficient organic dyes.1−9 Organic photosensitizers have the following advantages over Ru sensitizers: (1) high molar absorption coefficients capable of harvesting light energy over a wide spectral region of sunlight, (2) facile designs and modifications of molecular structures leading to optimized photophysical and energetic properties, and (3) easy and low-cost synthesis and purifications. In the DSSC studies made so far, a carboxyl group has been exclusively employed as a unit for anchoring a sensitizing dye to the surface of nanocrystalline particles of TiO2. Very recently, we have proposed a pyridyl group as a novel anchoring unit and revealed that it acts efficiently as an anchoring and electron-withdrawing unit in photosensitizers.10−13 After our reports, some DSSC studies with dyes having pyridyl anchor have already appeared in the © 2013 American Chemical Society

Received: June 13, 2013 Revised: July 17, 2013 Published: July 17, 2013 16364

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outer lines of the ESR signal due to 4CT (d1/d) were employed.19 (See Figure S1 in the Supporting Information.) The xesr values of 4CT were evaluated by comparing the d1/d values with the calibration curve of d1/d versus xesr prepared by using TEMPO molecules (99% spin active, Aldrich) in a mixed solvent (dichloromethane/toluene 1:4 v/v).19 The amount of NI2 or NI4 coadsorbed with 4CT was evaluated by measuring an absorption spectrum of the solution obtained by transferring the coadsorbed TiO2 powder into the mixed solution to desorb the dye. 4CT was transparent in the visible region so that it did not disturb spectroscopic observations of NI2 and NI4. TPD measurements with ammonia as a probe gas were performed with a TPD-1-AT (BEL Japan), where helium was used as a carrier gas. TiO2 samples were heated to 500 °C for 1 h and cooled to lower temperatures for adsorption of ammonia. Then, the temperature was increased to 500 °C at a heating rate of 10 °C min−1 under a mild flow of helium gas (50 mL min−1) to detect ammonia by means of a quadrupole mass spectrometer. FT-IR spectra of NI2, NI4, or 4CT powders and dyes adsorbed on TiO2 were recorded on a FT-IR spectrometer (Perkin−Elmer Spectrum One) by the attenuated total reflectance (ATR) method.

adsorbs preferentially on Lewis-acid sites of TiO2 surface while NI2 does so on Brønsted-acid sites.

2. EXPERIMENTAL SECTION Two kinds of dyes NI2 and NI4 were synthesized according to the method previously described.11 4-Carboxy TEMPO (4CT, Tokyo Chemical Industry) was used without further purification. Structures of these chemicals are shown in Figure 1. Tetrahydrofuran (THF, Kanto Chemical) was purified by

3. RESULTS AND DISCUSSION Adsorption Isotherms. Figure 2 depicts adsorption isotherms for NI2 and NI4 adsorbed at the nanocrystalline

Figure 1. Structures of chemicals used in this study.

distillation and used as solvent for dye adsorption. The TiO2 paste (PST-18NR, JGC Catalysts and Chemicals) was deposited on a fluorine-doped tin oxide (FTO, Nippon Sheet Glass) substrate by doctor-blading. The TiO2 paste on FTO was sintered according to a temperature program (from room temperature to 450 °C for 50 min and at 450 °C for 35 min). A specific surface area of the sintered TiO2 powder was determined by nitrogen adsorption experiments at 77 K. When the temperature of the TiO2 electrode was cooled to 40 °C, the sintered electrode was immersed into THF solutions containing various concentrations of NI2 or NI4 for adsorption. Coadsorption of 4CT with NI2 or NI4 was carried out by immersing the TiO2 electrode into THF solutions containing 0.1 mM 4CT and either NI2 or NI4 at different concentrations. In all cases, the dye adsorption was performed for 24 h in an incubator kept at 25 °C. The dye-adsorbed TiO2 electrode was cautiously soaked in pure THF for several seconds to remove a trace of dying solution from the void of nanoporous TiO2 electrode. Subsequently, dye molecules adsorbed on TiO2 were desorbed in a mixed solution (THF/ DMSO/H2O (1 M NaOH) 5:4:1 v/v/v) for 3 h, and the amount of adsorption for NI2 and NI4 dyes was determined by taking absorption spectra of the solutions on a spectrophotometer (Shimadzu UV-3150). In the coadsorption experiments, the dye-adsorbed TiO2 powder was scraped from the FTO substrate and its weight was measured. The TiO2 powder was then transferred to a capillary with the help of a small quantity of THF and subjected to electron spin resonance (ESR) measurements at 77 K. The adsorption amount of 4CT was determined by a double integration of the ESR signal obtained by an ESR spectrometer (JEOL JES-RE1X). In addition, average nearest-neighbor interspin distances (xesr) of 4CT molecules adsorbed on TiO2 were determined from the ESR spectral broadening of 4CT radicals due to dipole−dipole interaction between electron spins.18 As a measure of the spectral broadening, the relative intensities of the center and

Figure 2. Adsorption isotherms of NI2, NI4, and 4CT on nanoporous TiO2 electrodes. Fitting curves are drawn on the basis of equations obtained by the linear-least-squares analysis of the plots in Figure 3.

TiO2 surface in a double-logarithmic representation, along with the one for 4CT, which has the same anchoring unit as NI2. The amount of adsorbed dyes (Cad) on the ordinate is expressed by the number of dye molecules per unit area of the TiO2 surface calculated using a specific surface area of 83 m2 g−1 for PST-18NR. The abscissa denotes concentrations of NI2, NI4, and 4CT in equilibrium with dye-adsorbed TiO2 electrodes in THF. They were obtained by subtracting the adsorption amounts of dyes from concentrations of dye solutions before adsorption starts, although they did not differ much from the initial concentrations except for at a lowconcentration region for the NI2 and 4CT adsorption. It is seen clearly from Figure 2 that the adsorption trends of the two dyes NI2 and NI4 differ greatly from each other, reflecting the difference of the anchoring unit between NI2 and NI4. The adsorption amount of NI4 is very small compared with that of NI2, consistent with our previous result.12,13 The adsorption amount of 4CT is close to that of NI2 at concentrations as low as 0.01 mM, while it becomes smaller than that of NI2 at higher 16365

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close to those for Ru and Zn metal complexes having carboxyl anchors: 2.8 × 104 M−1 for an N3 dye,20 1.0 × 105 M−1 for a black dye,21 and 2.0 × 104 and 1.1 × 105 M−1 for ZnPCA and ZnPDCA, respectively.24 A large difference in K between molecules having carboxyl and pyridyl groups as anchor is noted: [(0.6 ± 0.05) and (1.0 ± 0.05)] × 105 M−1 for NI2 and 4CT, and (1.1 ± 0.1) × 103 M−1 for NI4. The fact that the K value for NI4 is much smaller than that for NI2 and 4CT indicates that the pyridyl group is a very weak anchor to the TiO2 surface compared with the carboxyl group. It is worth noting here that short-circuit photocurrents of DSSCs based on NI4 were greater than those for NI2 when photocurrents were compared at the same adsorption amounts of the dyes.11,12 This may imply that an injection probability of an electron from a photoexited dye to the conduction band of TiO2 is unrelated to the strength of anchoring to TiO2 surface. We also note that the C0 values for NI2 and NI4 are close to each other and they are almost twice the value of 4CT. The disagreement in C0 between NI2 and 4CT, which have the same carboxyl anchor, will be discussed later. Temperature-Programmed Desorption Measurements. There are two kinds of acid sites on surfaces of metal oxides such as TiO2: Brønsted-acid sites (surface-bound hydroxyl group, Ti−OH) and Lewis-acid sites (exposed Tin+ cation).31−33 It is likely that the former may act as an adsorption site for carboxyl anchor, while the latter acts primarily as an adsorption site for pyridyl group. To characterize these acid sites on TiO2 surface, we made TPD experiments with ammonia as a probe molecule. The TiO2 (PST-18NR) powder was obtained by scraping the TiO2/FTO electrode prepared in a similar way as described in the adsorption experiments. Figure 4a illustrates a TPD spectrum of TiO2 obtained when ammonia was adsorbed at 100 °C. A single broad peak is seen in the TPD spectrum. The broad peak was decomposed into L and H components with peak temperatures of 182 and 279 °C, respectively, and surface densities were evaluated to be 0.63 × 1014 cm−2 for L component and 1.4 × 1014 cm−2 for H component. In view of the difference in adsorption strength of ammonia on both acid sites, it is reasonable to ascribe L component to Brønsted-acid sites and H component to Lewis-acid sites. Namely, the TPD spectrum of Figure 4a predicts that the TiO2 surface has 0.63 × 1014 cm−2 of Brønsted-acid sites and 1.4 × 1014 cm−2 of Lewisacid sites. The surface density of 1.4 × 1014 cm−2 for Lewis-acid sites corresponds well to the C0 value of (1.7 ± 0.3) × 1014 cm−2 for NI4, which adsorbs on Lewis-acid sites through a coordinate bonding. However, the surface density of 0.63 × 1014 cm−2 for Brønsted-acid sites evaluated from the TPD experiments is too small compared with the C0 value of (1.5 ± 0.2) × 1014 cm−2 for NI2. The disagreement may arise from the fact that desorption of ammonia from Brønsted-acid sites occurs, in part, at the adsorption temperature of 100 °C. To make sure of this possibility, a similar TPD measurement was made at an adsorption temperature of 50 °C. As is shown in Figure 4b, the TPD spectrum obtained at an adsorption temperature of 50 °C was broader than that of Figure 4a and decomposed reasonably into three Gaussian components, H, L, and L′. H component had a peak at 280 °C, and its surface density was 1.5 × 1014 cm−2, corresponding well to the one in Figure 4a. The peak temperature of L component in Figure 4b was shifted from 182 to 164 °C, and the surface density was increased greatly from (0.63 to 1.0) × 1014 cm−2, although the value of 1.0 × 1014 cm−2 was still smaller than the surface

concentrations. The three adsorption isotherms were analyzed by the use of the following equation: Cad−1 = (KC0)−1[Dye]−1 + C0−1

derived from a Langmuir isotherm:

(1)

20−26

Cad /C0 = K[Dye]/(1 + K[Dye])

(2)

where Cad, C0, K, and [Dye] denote an amount of adsorbed dye, amount of dye adsorbed at saturation, adsorption equilibrium constant, and equilibrium concentration of dye, respectively. Figure 3 represents Cad−1 versus [Dye]−1 plots

Figure 3. Double-reciprocal plots of the data shown in Figure 2. Best fitting lines were obtained from the linear-least-squares method.

obtained from the data of Figure 2 for NI2, NI4, and 4CT. The plots fit straight lines well, demonstrating that adsorptions of NI2, NI4, and 4CT on TiO2 surfaces follow a Langmuir isotherm. The Langmuir isotherm holds when the following conditions are valid: monolayer adsorption, equivalent adsorption site, and negligible interaction between adsorbed molecules. Dye molecules adsorbed on TiO2 nanoparticles are known to tend to aggregate, and the dye aggregation could be responsible for lowering of photovoltaic performances of DSSCs.27−30 Indeed, we have recently found that 4CT molecules aggregate on TiO2 surface on the basis of ESR measurements of nearest-neighbor intermolecular distances (xesr) of 4CT molecules adsorbed on TiO2.19 Nevertheless, the fact that adsorption of 4CT on TiO2 surface follows the Langmuir isotherm over a wide concentration range demonstrates that the attractive interaction between 4CT molecules leading to aggregation is weak enough not to affect the Langmuir isotherm. The K and C0 values were obtained by the linear leastsquares method as (0.6 ± 0.05) × 105 M−1 and (1.5 ± 0.2) × 1014 cm−2 for NI2, (1.1 ± 0.1) × 103 M−1 and (1.7 ± 0.3) × 1014 cm−2 for NI4, and (1.0 ± 0.05) × 105 M−1 and (0.8 ± 0.1) × 1014 cm−2 for 4CT, respectively. The results are summarized in Table 1, together with correlation coefficients (R) in the linear regression analysis. The K values for NI2 and 4CT, both of which have carboxyl anchors, are on the order of 105 M−1, Table 1. Adsorption Parameters of NI2, NI4, and 4CT Obtained from Langmuir Isotherms K/M−1 NI2 NI4 4CT a

C0/cm−2

(0.6 ± 0.05) × 10 (1.1 ± 0.1) × 103 (1.0 ± 0.05) × 105 5

Ra

(1.5 ± 0.2) × 10 (1.7 ± 0.3) × 1014 (0.8 ± 0.1) × 1014 14

0.9901 0.9968 0.9915

R denotes a correlation coefficient. 16366

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Figure 4. Desorption spectra of nanoporous TiO2 powder following adsorption of ammonia at (a) 100 and (b) 50 °C. The spectra are decomposed into Gaussian components shown by broken curves.

Figure 5. Coadsorptions of 4CT with (a) NI2 and (b) NI4. Concentration of 4CT was fixed at 0.1 mM, while concentrations of NI2 and NI4 were varied. SUM means the sum of adsorbed amounts of 4CT and NI2 or 4CT and NI4. Curves are to guide the eyes.

density of Brønsted-acid sites of TiO2 evaluated from the analysis of the adsorption isotherm of NI2. Most likely, the adsorption temperature of 50 °C is not sufficiently low so that desorption of ammonia may partially occur even at this temperature. It was difficult to lower the adsorption temperature further because the third component (L′) ascribable to ammonia molecules adsorbed on the first ammonia layer increased its intensity and tended to hamper a precise evaluation of the number of L component. It is to be noted here that even though the evaluations of the numbers of acid sites could be made precisely on the basis of TPD measurements, we will be unable to specify adsorption sites for NI2 and NI4 because of no appreciable difference in C0 between NI2 and NI4. As to the difference in C0 between NI2 and 4CT, which have the same carboxyl anchor, we tentatively consider that both NI2 and 4CT adsorb on Brønsted-acid sites in different adsorption modes: bidentate chelating for NI2 and bidentate bridging for 4CT,34 although detailed examinations will be required to conclude this. Coadsorption of NI2 and NI4 with 4CT. To specify possible sites on TiO2 surface for adsorption of NI2 and NI4, we used 4CT as coadsorbate, where the concentration of NI2 or NI4 was changed, with that of 4CT being kept constant at 0.1 mM. Figure 5 shows the results of coadsorption experiments of (a) NI2 and (b) NI4. As is seen in Figure 5a, with the increase in [NI2], the adsorption amount of NI2 increases and that of 4CT decreases. In contrast, Figure 5b illustrates that the adsorption amount of 4CT is almost independent of [NI4], while the adsorption amount of NI4 increases with the increase in [NI4]. Both NI2 and 4CT have the same anchoring unit (carboxyl group) and thus occupy Brønsted-acid sites. Therefore, adsorption of NI2 may compete with that of 4CT, and by increasing of [NI2] the adsorption

amount of NI2 may increase, being accompanied with the decrease in that of 4CT, consistent with the results shown in Figure 5a. An increase in the total adsorption amount at higher concentrations of NI2 may be accounted for in terms of the C0 value of NI2 being greater than that of 4CT. NI4 and 4CT have different anchors and may adsorb on Lewis- and Brønstedacid sites of TiO2 surface, respectively. If this is the case, NI4 and 4CT may not interfere with each other, and their adsorptions occur independently. Actually, the adsorption amounts of 4CT are almost constant at (0.6 to 0.8) × 1014 cm−2 independent of increased concentrations of NI4, the value being very close to (0.8 ± 0.1) × 1014 cm−2 obtained from the isotherm of 4CT in Figure 2. Furthermore, the adsorption amounts of NI4 in Figure 5b are very close to those of NI4 in Figure 2, irrespective of coadsorption of 4CT (0.1 mM). To provide a further confirmation for the adsorption sites of NI2 and NI4, we evaluated nearest-neighbor interspin distances (xesr) of 4CT molecules on TiO2 that coadsorb with NI2 and NI4. The results are shown in Figure 6. In the case of NI4, the interspin distance of 4CT does not change appreciably with a change in concentration of NI4 ranging from 0.1 to 2 mM, suggesting that NI4 and 4CT adsorb independently on TiO2 surface. In contrast, the interspin distance of 4CT increases with the increase in the concentration of NI2. This may be explained by the fact that NI2 and 4CT adsorb on the same acid sites and the increase in [NI2] leads to the decrease in the amount of adsorption of 4CT by competing adsorption of NI2. FT-IR Measurements. In our previous studies on pyridylanchor dyes,11,12 we speculated on the basis of a FT-IR spectroscopy that NI4 and other pyridyl dyes adsorb on Lewisacid sites. Figure 7 shows FT-IR spectra of NI2 and NI4 powders, together with those of NI2 and NI4 adsorbed on TiO2. As shown in Figure 7b, the NI4 powder has a peak at 16367

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Figure 6. Changes of xesr of 4CT with concentrations of NI2 and NI4 in the coadsorption experiments of Figure 5. Curves are to guide the eyes.

Figure 8. FT-IR spectra of NI4, adsorbed from THF solutions containing 0.25, 0.50, and 1.0 mM NI4 on TiO2, where spectra are shifted up and down for clarity without changing a scale of ordinate. Inset shows a change of ratio of transmission peak intensity at 1596 cm−1 (peak B) to that at 1614 cm−1 (peak A) with the adsorption amount of NI4.

1590 cm−1 which is close to 1580 cm−1, which has been assigned to one of the breathing modes of a pyridine ring.35,36 When NI4 is adsorbed on the TiO2 surface, the 1590 cm−1 band is shifted to a high-energy side. It is well known that adsorption states of pyridine on metal oxides can be assessed from the magnitude of the wavenumber shift.31 According to this criteria, the intense peak A observed at 1614 cm−1 is a strong indication that NI4 is adsorbed on Lewis-acid sites through a coordinate bonding. It is worth noting here that along with peak A an additional peak (peak B) is seen at 1596 cm−1, which was not discussed in our previous reports.11,12 According to the same criteria, a possible assignment of this peak is a breathing of a pyridyl ring adsorbed on Brønsted-acid sites of TiO2 through a hydrogen bonding. If this is the case, as the adsorption amount of NI4 decreases peak B may lose its intensity more rapidly than peak A because of the adsorption strength of NI4 on Brønsted-acid sites being weaker than that on Lewis-acid sites. To confirm this possibility, NI4 was adsorbed on TiO2 surface at different adsorption amounts, and their FT-IR spectra were measured. The results are depicted in Figure 8, where the adsorption amounts of NI4 were (0.35, 0.60, and 0.85) × 1014 cm−2, corresponding to 0.25, 0.50, and 1.0 mM, respectively, for the equilibrium concentrations of NI4. The intensity ratio of peak A to peak B was 0.69 to 0.76 almost independent of the adsorption amount of NI4 ranging from (0.35 to 0.85) × 1014 cm−2 (inset of Figure 8), hinting that the additional peak is not due to NI4 adsorbed on Brønsted-acid sites. (See Figure S2 in the Supporting Information for evaluations of peak intensities.) We now speculate that the broad 1590 cm−1 band for the NI4 powder consists of two peaks and peak B is assignable to a CC stretching vibration in benzene ring or carbazole ring. In fact,

the FT-IR spectrum of 9-butylcarbazole powder depicted in Figure 9 exhibits a peak at 1592 cm−1 close to 1590 cm−1 observed for the NI4 powder.

Figure 9. FT-IR spectra of NI4 and 9-butylcarbazole powder.

NI2, which has a carboxyl anchor, shows a somewhat complicated peak at ∼1600 cm−1 in a powder state, and when NI2 is adsorbed on TiO2 we clearly see a peak at 1597 cm−1 (Figure 7a). This peak is very close in position to peak B for NI4 adsorbed on TiO2 (1596 cm−1), suggesting strongly that peak B originates from a CC stretching vibration in benzene ring or carbazole ring of NI4 adsorbed on TiO2. To provide a further confirmation for the adsorption site of NI4, we measured the FT-IR spectrum of NI4 and 4CT coadsorbed on the TiO2 powder, where the concentrations of NI4 and 4CT used for adsorption were 1.0 and 0.1 mM, respectively

Figure 7. FT-IR spectra of (a) NI2 powder and NI2 adsorbed on TiO2 and (b) NI4 powder and NI4 adsorbed on TiO2. 16368

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Figure 10. FT-IR spectra of (a) NI4 adsorbed on TiO2 from 1.0 mM NI4 solution, and NI4 and 4CT coadsorbed from solution of 1.0 mM NI4 and 0.1 mM 4CT and (b) 4CT powder and 4CT adsorbed on TiO2 from 0.1 mM 4CT solution.

Tsunoji and Masahiro Sadakane of Hiroshima University for TPD measurements. This work was partially supported by a Grant-in-Aid for Scientific Research (B) (no. 25288085) from the Ministry of Education, Science, Sports and Culture of Japan.

(Figure 10a). The FT-IR spectrum of 4CT powder and 4CT adsorbed on TiO2 is also shown in Figure 10b. No clear peaks are seen at ∼1600 cm−1. It has been already found that NI4 and 4CT adsorb independently on Lewis- and Brønsted-acid sites of TiO2, respectively (Figures 5 and 6). This implies that under the coadsorption condition Brønsted-acid sites are occupied by 4CT and thus NI4 is unable to adsorb on Brønsted-acid sites. As shown in Figure 10a, however, we still see peaks at 1614 and 1596 cm−1, in good agreement with peaks A and B in the FTIR spectrum for the NI4- and 4CT-coadsorbed TiO2. This strongly demonstrates that peak B is indifferent to NI4 adsorbed on Brønsted-acid sites.

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4. CONCLUSIONS Adsorption of NI4, one of the organic dyes having a pyridyl anchor developed recently in our research group, on nanocrystalline TiO2 surface is investigated by measuring adsorption isotherms and FT-IR spectra and by carrying out coadsorption experiments with 4-carboxy TEMPO (4CT). It is concluded that NI4 adsorbs preferentially on Lewis-acid sites of TiO2 surface through a coordinate bonding, in contrast to Brønstedacid sites available for adsorption of conventional photosensitizing dyes having a carboxyl anchor. NI2 may adsorb on Lewis-acid sites as well. However, we presume that it adsorbs on Brønsted-acid sites alone in the concentration range of NI2 studied. The present study demonstrates that the two sorts of acid sites should be recognized independently in a surface engineering of TiO2 and be utilized synergistically for a further development of DSSCs.

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ASSOCIATED CONTENT

S Supporting Information *

Analysis of ESR signal (d1/d) for obtaining interspin distances and evaluations of intensities of peaks A and B in FT-IR spectra of Figure 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +81 82 424 6534. Fax: +81 82 424 5494. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Helpful discussions with Hiroshi Abe and Shigeki Nomura of Sekisui Chemical are appreciated. We also acknowledge Nao 16369

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp405835y | J. Phys. Chem. C 2013, 117, 16364−16370