Langmuir 2000, 16, 8525-8528
8525
Adsorption Studies of Counterions Carried by the Sensitizer cis-Dithiocyanato(2,2′-bipyridyl-4,4′-dicarboxylate) Ruthenium(II) on Nanocrystalline TiO2 Films Md. K. Nazeeruddin,* M. Amirnasr,† P. Comte, J. R. Mackay,‡ A. J. McQuillan,‡ R. Houriet,§ and M. Gra¨tzel* Laboratory for Photonics and Interfaces, Institute of Physical Chemistry, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland Received May 17, 2000. In Final Form: July 31, 2000 Adsorption studies of counterions carried by the ruthenium(II) complexes (Bu4N)2[Ru(Hdcbpy)2(NCS)2] (1), (Bu4N)4[Ru(dcbpy)2(NCS)2] (2), and [Ru(H2dcbpy)2(NCS)2] (3) (dcbpy ) 2,2′-bipyridyl-4,4′-dicarboxylate, Bu4N ) tetrabutylammonium) on TiO2 have been carried out by using thermoanalytical techniques, NMR, and ATR-FTIR spectroscopic methods. The thermogravimetric analysis (TGA) data of the adsorbed complexes 1 and 2 on TiO2 show the presence of 1 and e1.3 cations coadsorbed per ruthenium center, respectively. These complexes in the adsorbed state on TiO2 show remarkable stability in air atmospheres at high temperatures up to 180 °C. The only process that is observed at lower temperatures is the dehydration, which occurs between 40 and 110 °C. At high temperature the processes are deamination of the tetrabutylammonium counterion as well as decarboxylation and decomposition of the complex between 200 and 400 °C. The NMR data of the desorbed complexes 1 and 2 from the TiO2 surface show the presence of 1 and e1.5 cations, which is in close agreement with the TGA data. Comparative analysis of the ATRFTIR spectra of complexes 1 and 2 between the free and the adsorbed forms also indicates the presence of 1 and e1.5 cations per ruthenium center, respectively.
1. Introduction In recent years, dye-sensitized solar cells based on nanocrystalline TiO2 films have received significant attention because of their high efficiency and low cost compared to those of silicon solar cells. Many groups have been focusing their attention on fundamental aspects of dye-sensitized solar cell components.1-7 A dye-derivatized † On sabbatical leave from Isfahan University of Technology, 84154 Isfahan, Iran. ‡ Department of Chemistry, University of Otago, Dunedin, New Zealand. § Department of Materials Science, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland.
(1) Schlichtho¨rl, G.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 1999, 103, 782. (b) Huang, S. Y.; Schlichtho¨rl, G.; Nozik, A. J.; Gra¨tzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576. (c) Schlichthorl, G.; Huang, S. Y.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (2) Zaban, A.; Ferrere, S.; Sprague, J.; Gregg, B. A. J. Phys. Chem. B. 1997, 101, 55. (b) Ferrere, S.; Gregg, B. A. J. Am. Chem. Soc. 1998, 120, 843. (c) Lemon, B. I.; Hupp, J. T. J. Phys. Chem. B 1999, 103, 3797.(d) Langdon, B. T.; MacKenzie, V. J.; Asunskis, D. J.; Kelly, D. F., J. Phys. Chem. B 1999, 103, 11176. (3) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G. J. Langmuir, 1999, 15, 7047.(b) Thompson, D. W.; Kelly, C. A.; Farzad, F.; Meyer, G. J. Langmuir, 1999, 15, 650.(c) Trammell, S. A.; Moss, J. A.; Yang, J. C.; Nakhle, B. M. Slate, C. A.; Odobel, F.; Sykora, M.; Erickson, B. W.; Meyer, T. J. Inorg. Chem. 1999, 38, 3665. (4) Franco, G.; Gehring, J.; Peter, L. M.; Ponomarev, E. A.; Uhlendorf, I. J. Phys. Chem. B 1999, 103, 692. (b) Salafsky, J. S.; Lubberhuizen, W. H.; van Faassen, E.; Schropp, R. E. I. J. Phys. Chem. B 1998, 102, 766. (5) Solbrand, A.; Henningsson, A.; So¨dergren, S.; Lindstro¨m, H.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1999, 103, 1078. (b) Bando, K. K.; Mitsuzuka, Y.; Sugino, M.; Sughihara, H.; Sayama, K.; Arakawa, H. Chemistry Lett. 1999, 853. (c) Sughihara, H.; Sing, L. P.; Sayama, K.; Arakawa, H.; Nazeeruddin, Md. K.; Gra¨tzel, M. Chemistry Lett. 1998, 1005. (d) Murakoshi, K.; Kano, G.; Wada, Y.; Yanagida, S.; Miyazaki, H.; Matsumoto. M.; Murasawa, S. J. Electroanal. Chem. 1995, 396, 27. (6) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Meyer, G. J. Inorg. Chem. 1997, 36, 2. (b) Argazzi, R.; Bignozzi, C. A.; Hasselmann, G. M.; Meyer, G. J. Inorg. Chem. 1998, 37, 4533.
mesoporous titania film is one of the key components in such cells. The long-term stability of the sensitizer under the operating conditions of the cell,8 the interaction of the sensitizer with nanocrystalline TiO2 films,9-11 the dynamics of electron injection processes on the semiconductor,12,13 and the photophysical and electrochemical properties of the sensitizer have been investigated.14,15 Despite all of these studies, the chemical nature of the adsorbed sensitizer along with the counterions on the semiconductor surface has not been fully revealed. However, from evidence based on the crystal structure of complex 3, Shklover et al. have postulated upon the possible anchoring modes of the sensitizer on the TiO2.16 We are interested to find the number of counterions adsorbed along with the sensitizer on the TiO2 surface. Herein, we report on the basis of NMR, ATR-FTIR, and thermogravimetric methods the number of counterions adsorbed onto the TiO2 surface during the dyeing process. (7) Jing, B.; Zhang, H.; Zhang, M.; Lu, Z.; Shen, T. J. Mater. Chem. 1998, 8, 2055. (b) Tennakone, K.; Kumara, G. R. R. A.; Kottegoda, I. R. M.; Perera, V. P. S. Chem. Commun., 1999, 15. (c) Nasr, C.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B, 1998, 102, 4944. (d) Ihara, M.; Tanaka, K.; Sakaki, K.; Honma, I.; Yamada, K. J. Phys. Chem. B 1997, 101, 5153. (8) Kohle, O.; Gra¨tzel, M.; Meyer, A. F.; Meyer, T. B. Advanced Materials, 1997, 11, 904. (9) Fillinger, A.; Parkinson, B. A. J. Electrochem. Soc.1999, 146, 4559. (10) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir.1998, 14, 2744. (11) Duffy, N. W., Dobson, K. D., Gordon, K. C., Robinson, B. H., McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451. (12) Schwarzburg, K.; Willig, F.; J. Phys. Chem. B 1999, 103, 5743. (13) Haque, S. A.; Tachibana, Y.; Willis, R. L.; Moser, J.-E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. J. Phys. Chem. B 2000, 104, 538. (14) Nazeeruddin, Md. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover, V.; Fischer, C. H.; Gra¨tzel, M. Inorg. Chem. 1999, 38, 6298. (15) Bond, A. M.; Deacon, G. B.; Howitt, J.; MacFarlane, D. R.; Spiccia, L.; Wolfbauer, G. J. Electrochem. Soc.1999, 146, 648. (16) Shklover, V.; Ovchinnikov, Yu. E.; Braginsky, L. S.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Mater. 1998, 10, 2533.
10.1021/la000685g CCC: $19.00 © 2000 American Chemical Society Published on Web 10/03/2000
8526
Langmuir, Vol. 16, No. 22, 2000
Nazeeruddin et al.
2. Experimental Section 2.1. Materials. The ruthenium complexes were available from our previous studies.14 The solvents (puriss grade) and the chemicals (reagent grade) were obtained from Fluka and used as received. The preparation method of the nanostructured TiO2 films was described in our previous publication.17 2.2. Analytical Measurements. UV-vis and fluorescence spectra were recorded in a 1-cm path length quartz cell on a Cary 5 spectrophotometer and a Spex Fluorolog 112 spectrofluorometer, respectively. The emitted light was detected with a Hamamatsu R2658 photomultiplier operated in single-photon counting mode. The emission spectra were photometrically corrected using a calibrated 200 W tungsten lamp as reference source. The emission lifetimes were measured by exciting the sample with a pulse from an active mode-locked Nd:YAG laser, using the frequency doubled line at 532 nm. 2.3. Thermal Methods. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed using Mettler TA 4000 and Rheometric Scientific STA 1000 thermal analyzers. TGA experiments were carried out in a dynamic atmosphere (20 mL/min) of air or nitrogen with a heating rate of 5 and 50 °C/min in TBA measurement. Thermogravimetric and differential scanning calorimetry curves were recorded by heating the samples, that is, ∼1-5 mg of the solid dye and 1520 mg of dye-derivatized TiO2 powder, up to about 450 °C. Indium was used as reference to calibrate the temperature and heat flow. The dye-coated TiO2 powder was prepared using the following procedure. The dye solutions of complexes 1 and 2 (concentration 5 × 10-4) were prepared by dissolving the solid in a 1:1 acetonitrile and tert-butyl alcohol solution. The TiO2 electrodes, which were heated to 450 °C and subsequently cooled (60 °C), were plunged into the dye solution for 15 h and then rinsed thoroughly with acetonitrile. The dye-coated electrodes were dried under reduced pressure at 60 °C for 5 h. The scraped dye-coated TiO2 powder was used for TGA and ex situ ATRFTIR analysis. 2.4. NMR Spectra. 1H NMR spectra were obtained on a Bruker AC-P 200 MHz spectrometer, and the reported proton NMR chemical shifts were against TMS. The solutions for NMR spectra were prepared by desorbing the dye from the TiO2 electrode (2 × 10 cm, 13 µm thick) by immersing it in a pH 7 sodium phosphate buffer solution (≈0.6 mL) for 5 min. Then, the solvent was evaporated completely under reduced light using a rotaryevaporator and the remaining solid was dissolved in 0.5 mL of D2O. 2.5. IR Spectra. IR spectra of solutions and for the in situ adsorption work were obtained with a Bio-Rad Digilab FTS60 spectrometer (64 scans at 4 cm-1 resolution) and a single internal reflection (ATR) ZnSe prism with a Prism Liquid Cell accessory (Harrick). The reported spectra have not been corrected for dependence of penetration depth on wavelength. TiO2 films ∼2 µm thick were formed on the prisms by vacuum evaporation of solvent from acetonitrile suspensions of TiO2 particles (Degussa P25). These films adhered well to the ZnSe under the conditions of the adsorption experiments. Initially, solvent was flowed (∼0.5 cm3 min-1) over the TiO2-coated prism to obtain reference spectra followed by the adsorbate solution. Adsorption equilibrium was generally reached within 1 h of initial contact. Solution was flowed via PharMed tubing (Cole-Parmer) through a Kel-F block sealed to the ZnSe prism surface with a Teflon O-ring in a similar experimental setup to that previously reported.18 The ex situ IR spectra were measured using a Nicolet 510 FTIR spectrometer equipped with a "Golden Gate (TM)" (GrasebySpecac) single-bounce diamond-ATR accessory. The IR bench was flushed with nitrogen gas, and the spectra are the average of 50 accumulated scans.
3. Results and Discussion 3.1. Thermogravimetric Analysis. The combined thermogravimetric (TG), derivative thermogravimetric (17) Nazeeruddin, Md. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (18) Ekstro¨m, G. N.; McQuillan, A. J. J. Phys. Chem. B 1999, 103, 10562.
Figure 1. Combined thermogravimetric (TG), derivative thermogravimetric (DTG), and differential scanning calorimetry (DSC) curves of complex 1 adsorbed on TiO2 in dynamic air atmosphere.
Figure 2. Combined thermogravimetric (TG), derivative thermogravimetric (DTG), and differential scanning calorimetry (DSC) curves of complex 3 adsorbed on TiO2 in dynamic air atmosphere.
(DTG), and differential scanning calorimetry (DSC) curves of complex 1 in a dynamic atmosphere of air are presented in Figure 1. Three well-defined steps are observed in the thermograms. These steps are associated with loss of water, exothermic decomposition of amine (between 200 and 300°C), and decarboxylation (between 340 and 380 °C) followed by decomposition. The dramatic drop in mass around 170 °C is most likely due to the endothermic desorption of the species chemisorbed on the active centers of TiO2.19 The organic cation Bu4N+, coadsorbed with the dye onto TiO2, undergoes a very slow deamination reaction at low heating rates. Under these conditions, it, therefore, acts as a heat sink and increases the thermal stability of the dye toward decarboxylation relative to that of complex 3 by 30 °C (Figure 2). Nevertheless, at higher heating rates the deamination occurs more rapidly.19, 20 Figure 1 shows that the deamination occurs first: an endothermic release of Bu4N+ in a short period of time followed by an exothermic oxidation of the amine. During the TG analysis, the heating rate of 50 °C/min between (19) Amirnasr, M.; Nazeeruddin, Md. K.;.Gra¨tzel, M. Thermochim. Acta 2000, 348, 105. (20) Parasad, M. R. R.; Krishnamurthy, V. N. Thermochim. Acta 1991, 185, 1.
Adsorption of Counterions Carried by a Sensitizer
Langmuir, Vol. 16, No. 22, 2000 8527
Figure 4. ATR-IR spectrum of 9 × 10-2 M solution of 2 in acetonitrile with reference spectrum from acetonitrile on bare ZnSe prism. Figure 3. Combined thermogravimetric (TG), derivative thermogravimetric (DTG), and differential scanning calorimetry (DSC) curves of complex 2 adsorbed on TiO2 in dynamic air atmosphere.
30 and 310 °C followed by 2 min of isothermal heating at 310 °C and a heating rate of 5 °C/min were used. These conditions were essential to ensure a quick decomposition of Bu4N+ and proper separation of the decarboxylation and decomposition steps. From Figure 1, the number of Bu4N+ cations adsorbed on the surface was estimated quantitatively by measuring the relative mass loss in the deamination and decarboxylation steps. Since four carboxylate groups are oxidized in the exothermic decarboxylation process, a theoretical mass loss ratio of 1.37 is expected for the release of one Bu4N+ (242 g) in the first step relative to four CO2 (176 g) in the second.19 The experimental ratio of 1.4 obtained for the mass loss in the first step relative to the second indicates the presence of about one Bu4N+ per dye molecule on the TiO2 surface. The thermal behavior of complex 2 was comparable to that of complex 1 under identical experimental conditions. The TG, DTG, and DSC curves of complex 2 are shown in Figure 3. Similar to the case for complex 1, there are three major steps observed for complex 2. The experimental ratio of the mass loss in the first step relative to the second is in accord with the presence of e1.3 Bu4N+ per dye molecule. It is interesting to note that the decarboxylation and decomposition of the dye occur with no detachment of the dye from TiO2. 3.2. NMR Data. Complexes 1 and 2 in a D2O + NaOD solution shows six peaks in the aromatic region centered at δ 9.51 (d), 8.93 (s), 8.77 (s), 8.23 (dd), 7.81 (d), and 7.54 (dd), corresponding to two different dcbpy ring protons, in which two pyridine rings are trans to the NCS ligands and the remaining two are trans to each other.21 The resonance peaks in the aliphatic region centered at δ 3.18 (t), 1.64 (m), 1.35 (q), and 0.95 (t) ppm are due to tetrabutylammonium cations. The integrated ratio of the aliphatic to aromatic protons shows two and four tetrabutylammonium cations in complexes 1 and 2, respectively.14 The NMR spectra of the desorbed dyes in the aromatic and aliphatic regions show a number of peaks and peak positions similar to those of the free dye. However, the integrated ratio between the aromatic to aliphatic protons in the desorbed complexes 1 and 2 shows the presence of 1 and e1.5 tetrabutylammonium cations, respectively. (21) Shklover, V.; Nazeeruddin, Md. K.; Zakeeruddin, S. M.; Barbe, C.; Kay, A.; Haibach, T.; Steurer, W.; Hermann, R.; Nissen, H.-U.; Gra¨tzel, M. Chem. Mater. 1997, 9, 430.
The UV/vis absorption spectrum of complex 1 in pH 7 sodium phosphate buffer solution shows two metal-toligand charge-transfer (MLCT) absorption bands in the visible region at 502 and 372 nm. The band in the UV at 308 nm with a shoulder at 301 nm is assigned to an intraligand (π-π*) charge-transfer transition.14 The UV/ vis absorption spectra of the desorbed complex 1 from the TiO2 surface in pH 7 buffer solution show MLCT and π-π* charge-transfer transition bands in the same position as that observed for the complex 1. When excited in the lowest energy MLCT absorption band, complex 1, in pH 7 buffer solution at 298 K, exhibits a luminescence consisting of a single band with a maximum at 740 nm. Excitation of complex 1 at different wavelengths within the manifold of the MLCT bands gave the same emission maxima. The excited-state lifetime of complex 1 measured at 298 K under aerobic conditions is 60 (( 5) ns. It is striking to note that the desorbed complex 1 emission maxima and the lifetimes are comparable to those observed for the solid complex 1 dissolved in pH 7 buffer solution. The spectral data show that there is no change in the composition of the complex during the adsorption and desorption processes. The electronic and emission data of the desorbed complex 2 in pH ) 7 solution are comparable to those for the solid dissolved in the same buffer and to that of complex 1. 3.3. In Situ Infrared Spectra of 1, 2, and 3 Adsorbed on TiO2 from Acetonitrile Solutions. Of the dyes studied, only 2 was sufficiently soluble (∼0.1 M) in acetonitrile to obtain its spectrum in solution. Figure 4 shows the solution spectrum of complex 2 in the 22001200 cm-1 range with the most prominent bands occurring at 2105 cm-1 (NCS), 1623 cm-1 (COO-as), and 1345 cm-1 (COO-s). The absorption bands at 1542 and 1468 cm-1 are largely due to Bu4N+ with a small contribution from an underlying weak bipyridyl band.10 The adsorption of 2 onto TiO2 from 10-5 M solution gave a weaker spectrum than those of the other adsorbates 1 and 3, and as a consequence, its spectrum, shown in Figure 5a, is of lower quality. However, adsorption of 2 on TiO2 has significantly altered its spectrum in the 1300-1700 cm-1 region while the SCN group absorption remains at 2105 cm-1, indicating an unchanged environment. The most prominent band is the COO-s band at 1379 cm-1 while the corresponding COO-as band at 1608 cm-1 is weaker. The band associated with the bipyridyl ligand is at 1548 cm-1, and there is clear evidence of some coadsorbed Bu4N+ in the band at 1469 cm-1. Noting that the 1468 cm-1 band in Figure 4 corresponds to four Bu4N+ ions and comparing the ratio of absorbances of the Bu4N+ band to that of the bipyridyl
8528
Langmuir, Vol. 16, No. 22, 2000
Figure 5. ATR-IR spectra of species adsorbed on TiO2 (P25) particle films from 10-5 M solutions in acetonitrile of (a) 2, (b) 1, and (c) 3. Absorbances of the 2105 cm-1 peak in the spectra were (a) 0.0013, (b) 0.0025, and (c) 0.0022. The reference spectrum in each case was from acetonitrile on the TiO2 film.
band in Figures 4 and 5a shows that about 1.5 Bu4N+ ions are coadsorbed with each 2 molecule on TiO2. The spectrum of 1 adsorbed on TiO2 is shown in Figure 5b. The major difference in the spectrum compared with that of 2 adsorbed on TiO2 is the clear evidence of COOH groups remaining in the adsorbate with bands at 1734 cm-1 (CdO) and at 1219 cm-1 (CsO). There is also evidence of coadsorbed Bu4N+ in the presence of the band at 1470 cm-1, but it is a weaker band relative to that in Figure 5a. Comparison in Figures 4 and 5b of the ratio of absorbances of the Bu4N+ band suggests that about one tetrabutylammonium ion is coadsorbed with each 1 adsorbate on TiO2. The spectrum of 3 adsorbed on TiO2 is given in Figure 5c. This spectrum clearly shows from the band at 1734 cm-1 that 3 is adsorbed with retention of some COOH groups, but there is also clear evidence of carboxylate absorptions at 1604 and 1381 cm-1. The weakness of the bipyridyl absorption around 1470 cm-1, which is present in the spectrum of each compound, is evident in Figure 5c for the compound not containing Bu4N+. 3.4. IR Data of Complexes 1 and 2 Adsorbed on Nanocrystalline TiO2 Films. The ex situ infrared spectra of the dyes in the free and adsorbed form onto
Nazeeruddin et al.
TiO2 films were recorded in the 4000-400 cm-1 region, as a solid using ATR-FTIR (not shown). The infrared spectra of complex 1 as a solid sample show bands at 1709 and 1610 cm-1 due to carboxylic acid and carboxylate groups, respectively. The symmetric stretch of the carboxylate group was observed at 1362 cm-1. The intense peak at 1225 is assigned to the C-O stretch. The strong band at 2098 cm-1 is assigned to the υ(CN) of the thiocyanate ligand. The bands at 2873, 2931, and 2960 cm-1 are assigned to the υ(C-H) of tetrabutylammonium groups.22 The IR spectrum of complex 1 adsorbed onto the TiO2 surface shows the presence of characteristic bands due to NCS and tetrabutylammonium groups, at the same position as that observed for the free dye. However, the integrated intensity of the υ(C-H) band of tetrabutylammonium at 2960 cm-1 was reduced to half (normalized with respect to the υ(CN) band at 2098 cm-1) compared to that for complex 1. This clearly indicates that, out of two tetrabutylammonium cations, only one tetrabutylammonium cation was adsorbed on the TiO2 surface per ruthenium center. Nevertheless, the IR data of the adsorbed complex 2 show the presence of e1.5 tetrabutylammonium cations per ruthenium center. Our studies clearly show that when complex 1 or 2 is adsorbed on a nanocrystalline TiO2, all the cations that were associated with the complex are not coadsorbed. 4. Conclusions Thermoanalytical data display that complexes 1 and 2 are stable on the TiO2 surface and do not undergo any observable chemical change below 180 °C. The tetrabutylammonium cations introduced to replace the protons have a noticeable effect on the thermal stability of the complexes. The decarboxylation temperature increases by about 30 °C by replacing protons in complex 3 by tetrabutylammonium cations. The TGA, NMR, and IR data unambiguously show the coadsorption of one counterion on the TiO2 surface along with complex 1 and e1.5 counterions for complex 2. Acknowledgments are made to the Swiss Energy Office (OFEN) and the Institut fu¨r Angewandte Photovoltaik (INAP), Germany, for support of this work. We are grateful to Dr. R. Humphry-Baker for his assistance in measuring the lifetimes and Christian Spu¨hler, LGRCInstitut de Ge´nie Chimique, De´partement de Chimie, for his help in thermoanalytical measurements. LA000685G (22) Nakamoto, K. Infrared and Raman spectra of inorganic coordination compounds, 5th ed.; Wiley: New York, 1997.
