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Proton-Controlled Electron Injection from Molecular Excited States to the Empty States in Nanocrystalline TiO2 Ping Qu and Gerald J. Meyer* Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218 Received June 21, 2001. In Final Form: August 7, 2001 The excited-state and redox properties of Ru(deeb)(bpy)2(PF6)2, Ru(dcb-H2)(bpy)2(PF6)2, Ru(bpy)2(ina)2(PF6)2, and Ru(dpbp)(bpy)2(PF6)2, where bpy is 2,2′-bipyridine, deeb is 4,4′-(CO2Et)2-bpy, dcb-H2 is 4,4′(CO2H)2-bpy, dpbp is 4,4′-(PO(OEt)2)2-bpy, and ina is isonicotinic acid, bound to nanocrystalline TiO2 and colloidal ZrO2 films have been studied in acetonitrile at room temperature as a function of the interfacial proton concentration. High surface proton concentrations favor a “carboxylic acid” type linkage(s) where low proton concentrations favor “carboxylate” type binding mode(s) for Ru(II) compounds with ethyl ester or carboxylic acid functional groups. The “carboxylic acid” linkages are unstable when Lewis acids such as Li+ are present in acetonitrile, while desorption is absent for the carboxylate binding under the same conditions. The kinetics for binding are faster when the interfacial proton concentration is high; however, the saturation surface coverage is about 1/3 lower than for base-pretreated samples. The spectroscopic properties are consistent with ester hydrolysis by the base-pretreated metal oxide surfaces. The efficiency for intermolecular RuIII/II electron “hopping” between surface bound compounds approaches zero when the proton concentration is low. Protons or lithium cations promote rapid and reversible oxidation-reduction of all the surface bound compounds. The origin of this cation effect is speculative but may reflect the translational mobility of the surface bound compounds. Small changes in the RuIII/II formal reduction potentials, 108 s-1. The quantum yields for charge injection into TiO2 were quantified as a function of surface pretreatment from pH 11 to 1 for Ru(deeb)(bpy)22+ in acetonitrile. A sharp onset of injection yields were observed as the pH decreased from pH 3.0 to 2.0, and the yield increased from 0.1 to 0.8. The pH 1 pretreated sample had a quantum yield of unity. The charge recombination kinetics, monitored at the excited stateground state isosbestic point 403 nm, were well modeled by a sum of two second-order rate constants as previously described.12 Over ∼3 pH unit change in surface pretreatment conditions, there was no significant change in the recombination kinetics. The properties of Ru(dpbp)(bpy)22+ and Ru(bpy)2(ina)22+ were qualitatively similar to MLCT excited states observed on ZrO2 and base treated TiO2 and interfacial charge separated states observed for pH 1 pretreated TiO2 with injection quantum yields of 1.0 and 0.7, respectively. Cyclic voltammograms of Ru(deeb)(bpy)22+ bound to acid- and base-pretreated ZrO2 recorded at 50 mV/s scan rates are shown in Figure 5. Quantitatively similar behavior was observed on TiO2. With 0.1 M TBAClO4 as the supporting electrolyte, the acid-pretreated samples display a quasi-reversible wave with E1/2(RuIII/II) ) 1.34 V vs Ag/AgCl. Significantly less Faradaic current and no clear anodic or cathodic peak currents were observed on base-pretreated surfaces under the same conditions. With 0.1 M LiClO4 as the electrolyte, both acid and basepretreated surfaces gave quasi-reversible waves, with E1/2(RuIII/II) ) 1.34 and 1.27 V vs Ag/AgCl, respectively. Considerable surface desorption was observed with the acid-pretreated surfaces in the 0.1 M LiClO4 acetonitrile electrolyte. Similar electrochemical behaviors were observed for Ru(dpbp)(bpy)22+ and Ru(bpy)2(ina)22+ (Table 1). For the base-pretreated samples with LiClO4 electrolyte and the acid-pretreated samples with TBAClO4, the surface bound sensitizers were stable in the Ru(II) and Ru(III) states. Stepping the potential positive of the RuIII/II reduction potential lead to the reversible oxidation of all the surface bound sensitizers as determined by absorption spectroscopy. The base treated samples with TBAClO4 as the supporting electrolyte, on the other hand, displayed negligible changes in absorption with applied potentials as positive as the effective electrochemical window would allow, i.e., +1.8 V vs Ag/AgCl. The excited-state reduction potentials were calculated by eq 3, where ∆Ges is the free energy stored in the thermally equilibrated excited states. ∆Ges was estimated from a tangent line to the higher energy side of the corrected PL spectrum of the surface bound complex in
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Figure 4. Time-resolved absorption difference spectra of Ru(deeb)(bpy)22+ on (A) acid-pretreated ZrO2 surface, (B) base-pretreated ZrO2 surface, (C) acid-pretreated TiO2 surface, and (D) base-pretreated TiO2 surface in argon-saturated acetonitrile. The difference spectra were recorded after pulsed 532.5 nm light excitation (∼14 mJ cm-2, 8 ns fwhm), and absorbance change at different delay times: 10 ns (squares), 200 ns (circles), 1000 ns (up triangles), and 2000 ns (down triangles).
neat acetonitrile. The excited- and ground-state reduction potential of three sensitizers bound to TiO2 or ZrO2 are given in Table 1.
E1/2(RuIII/II*) ) E1/2(RuIII/II) - ∆Ges
(3)
The photocurrent action spectra of TiO2 surfaces pretreated at pH ) 1, 2, 5, and 11 sensitized with Ru(deeb)(bpy)22+ in either 0.5 M TBAI/0.05 M I2 or 0.5 M LiI/0.05 M I2 acetonitrile electrolyte are shown in Figure 6. With TBAI/I2, the pH ) 1 and pH ) 2 acid-pretreated surfaces demonstrated high incident photon-to-current efficiency (IPCE) with photocurrent action and absorptance spectra that agree well, while the pH 5 and pH 11 pretreated samples gave very small IPCE. With LiI in the electrolyte, the IPCE of pH 5 and pH 11 pretreated TiO2 were dramatically improved. The IPCE for the pH 1 and pH 2 pretreated samples are also high; however, desorption of the sensitizer was observed, and almost all of the surface bound sensitzers were lost after one IPCE measurement, leading to significant uncertainty in the absolute conversion efficiency. The open-circuit photovoltages were within experimental error the same for pH 2 pretreated samples with 0.5 M TBAI/0.05 M I2 and for pH 11 pretreated sample with 0.5 M LiI/0.05 M I2 as the electrolyte. Typical values are 500 ( 20 mV when excited with 460 nm light under the same conditions as for the IPCE measurements. Discussion This study demonstrates the profound impact interfacial proton concentration can have on the behavior of molecular excited states bound to semiconductor and insulator surfaces. Our initial intention was to quantify the excitedstate behavior in both aqueous and nonaqueous solutions. However, we found that the stability of these surface bound complexes in aqueous solution was generally insufficient
Figure 5. Cyclic voltammograms of Ru(deeb)(bpy)22+/ZrO2 in (A) 0.1 M TBAClO4/ACN electrolyte or (B) 0.1 M LiClO4/ACN electrolyte recorded at 50 mV/s. The solid lines represent the acid-pretreated ZrO2 surfaces, and the dashed lines are the base-pretreated ZrO2 surfaces. The TiO2 surfaces shows similar behavior.
for mechanistic spectroscopic studies that required significant signal averaging.15 We therefore adopted the procedure of equilibrating the metal oxide films with
Nanocrystalline TiO2
Langmuir, Vol. 17, No. 21, 2001 6725 Scheme 1
Figure 6. Incident photon to current conversion efficiency (IPCE) of Ru(deeb)(bpy)22+/TiO2 in (A) 0.5 M TBAI/0.05 M I2 electrolyte and (B) 0.5 M LiClO4/0.05 M I2 electrolyte. The TiO2 surfaces were pretreated with pH 1 (squares), pH 2 (circles), pH 5 (up triangles), and pH 11 (down triangles) aqueous solution.
aqueous solutions of known pH prior to surface binding and mechanistic studies in acetonitrile solution. We find that the surface attachment, excited states, charge transport, and injection are all profoundly impacted by this aqueous surface pretreatment. Below we elaborate on these points in more detail. Spectroscopic Properties of Ru(II) Compounds in Aqueous Solution. Acid-base properties of ligands coordinated to Ru(II) have been the subject of many studies.16 These studies provide a direct measure of how σ donation and π back-bonding influence the metal-toligand charge transfer (MLCT) excited states.16 The electron-withdrawing carboxylic acid (or ester) groups stabilize the π* levels of bipyridine, and this results in a red-shifted MLCT absorption and emission relative to the unsubstituted and deprotonated carboxylate forms. These bathochromic shifts are significant and are diagnostic of the protonation state of the complex both in solution16 and on the metal oxide surfaces (vide infra). Wrighton and co-workers first investigated the spectroscopic properties of Ru(dcb-H2)(bpy)22+ as a function of pH. A single inflection point was observed spectroscopically at pH ) 2.4 that led the authors to conclude that both carboxylic acid groups were deprotonated simultaneously.16a Sasse and co-workers later reinvestigated this chemistry and (15) Bignozzi, C. A.; Costa, E.; Alebbi, M.; Gillaizeau-Gauthier, I.; Odobel, F.; Qu, P.; Meyer, G. J. Inorg. Chem., in press. (16) (a) Giordano, P. J.; Bock, C. R.; Wrighton, M. S.; Interrante, L.; Williams, R. F. X. J. Am. Chem. Soc. 1977, 99, 3187. (b) Ferguson, J.; Mau, A. W.-H.; Sasse, W. H. F. Chem. Phys. Lett. 1979, 68, 21. (c) Shimidzu, T.; Iyoda, T.; Izaki, K. J. Phys. Chem. 1985, 89, 642. (d) Mesmaeker, A. K.-D.; Jacquet, L.; Nasielski, J. Inorg. Chem. 1988, 27, 4451. (e) Nazeeruddin, M. K.; Kalyanasundaram, K. Inorg. Chem. 1989, 28, 4251.
reported more detailed titration studies.16b They found two inflection points and reported pKa1 ) 1.75 and pKa2 ) 2.80. Later studies were consistent with the work of Sasse.16d,e The carboxylic acid groups on isonicotinic acid, ina, ligands are in the para positions of pyridine. For Ru(bpy)2(ina)22+, the Ru(II) f ina charge-transfer bands are below 400 nm and are not accessible with visible light.11 Therefore, acid-base chemistry on the pyridine ligand modulates the RuIII/II potential through inductive effects but does not significantly alter the π* levels of the chromophoric bpy ligands. The carboxylic acid groups exert an electron-withdrawing effect relative to the carboxylate form, and spectral shifts opposite to that in Ru(dcb-H2)(bpy)22+ are observed. The spectra of the fully deprotonated compound is red-shifted relative to that of the fully protonated compound.11 Neither compound is emissive in fluid solution at room temperature due to the presence of low-lying ligand field states.11 The Ru(dpbp)(bpy)22+ complex was studied for comparison purposes as it binds to metal oxide surfaces through phosphonic acid groups.17 The hydrolyzed form of this complex has four protons that can be deprotonated. Recent spectroscopic data are consistent with a pKa1 ) 1.75 and a pKa2 ) 12.0.9 The fully protonated compound displays emission and absorption bands that are redshifted from the deprotonated forms. Surface Attachment. The nature of the linkage between a sensitizer and a semiconductor surface can influence excited-state and interfacial electron-transfer behavior.1 The surface linkage may also affect the sensitizer stability, electronic coupling, and the redox potentials of the sensitizer and the semiconductor. Carboxylic acid groups are by far the most popular anchoring groups in the dye-sensitized solar cell, and a deeper understanding of the binding kinetics and bond nature is therefore important.18 Several studies that have addressed how carboxylic acids interact with TiO2 and other metal oxide surfaces.18,19 In most vibrational studies, an asymmetric C-O stretch consistent with carboxylate binding has been reported (Scheme 1A).18,19c Evidence for ester linkages (Scheme 1B) and a hydrogen-bonded network have also been reported.19a,b The studies here are consistent with previous work and provide a reasonable explanation for the apparent discrepancies in behavior reported by different research groups.19 The base-pretreated metal oxides results in a “carboxylate” linkage (Scheme 1A), and acid pretreatment yields a stretch coincident with the uncomplexed carboxylic acid termed a “carboxylic acid” linkage in Scheme 1B. (17) Pe`chy, P.; Rotzinger, F. P.; Nazeeruddin, M. K.; Kohle, O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1995, 65. (18) (a) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (19) (a) Umpathy, S.; Cartner, A. M.; Parker, A. W.; Hester, R. E. J. Phys. Chem. 1990, 94, 1357. (b) Meyer, T. J.; Meyer, G. J.; Pfenning, B.; Schoonover, J. R.; Timpson, C.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek, B. M.; Wall, C. G.; Ou, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem. 1994, 33, 3952. (c) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1994, 33, 5741. (d) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1998, 14, 2744.
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Unfortunately, the IR data do not give clear information on the surface site(s) that form the chemical bond and essentially reports only on the C-O bond order, 2 for the acid-pretreated samples and 1.5 for the base-pretreated surface.20 The carboxylate linkage may be chelated to one surface site, bridging two (as shown), or electrostatic in nature. The stability of the linkage in high ionic strength solutions strongly suggests that the linkage is not completely electrostatic in nature. The carboxylic acid linkage may be to a metal center, Ti or Zr, or to a surface proton in a hydrogen-bonding-type mode. The asymmetric stretch for the compound bound to ZrO2 and TiO2 are within experimental error the same as that measured for the protonated and ethyl ester compounds in KBr, which argues for a surface proton site. This linkage is stable in high ionic strength solutions, but addition of Lewis acids, such as Li+, results in rapid and complete desorption of the surface bound complex. For TiO2 films that were not pretreated with acid or base, a mixture of carboxylate- and carboxylic acid-type linkages were observed. It is not clear whether the spectra correspond to sensitizers with one carboxylate and one carboxylic acid type bond or a mixture of fully protonated and the fully deprotonated surface bound complexes. If the former scenario were correct, it would suggest that the pKa of the untreated TiO2 surface is between 1.75 and 2.8.16b We have previously reported evidence that the protonation state of surface bound Ru(II) compounds critically depend on the metal oxide preparation conditions.10 It is interesting to note that previous workers have correlated a red shift in the photocurrent action spectra, relative to the absorption spectra in fluid solution, with the degree of electronic coupling between the sensitizer and the semiconductor.21 Goodenough first proposed that the conduction band of single-crystal rutile TiO2 can overlap strongly with the π* system of Ru(dcb-H2)(bpy)22+* through an ester linkage. Similar behavior has been reported for sensitized anatase nanocrystallites.21d In the current study, we find nearly identical spectroscopic behavior for the Ru(II) sensitizers anchored to TiO2 and ZrO2. The results strongly suggests that the spectral shifts observed here are due to Bronsted acid-base chemistry and not to stabilization of the excited states by the semiconductor conduction band. Adduct formation constants of 104-105 M-1 have been reported for equilibrium binding of Ru(II) complexes with dcb-H2 ligands to nanocrystalline TiO2.8,17,22 However, to our knowledge, the kinetics have not been previously reported. Here we find that on acid-pretreated surfaces sensitizer binding is fast with equilibrium established within 3 h. With base-pretreated metal oxide surface, the sensitizer attachment is kinetically sluggish, and timedependent spectral shifts and surface concentration increases are observed over periods of days. Initially, the absorption spectrum resemble that of the fully protonated (or ethyl ester) compound, which evolves over a period of days, to spectra consistent with the formation of carboxylate bonds. We note that acid, base, and metal oxide
surface catalyzed hydrolysis of ester linkages are wellknown.23 Excited States. The MLCT absorption and PL spectral data of the Ru(II) compounds anchored to metal oxide surfaces support the IR data and are excellent reporters of surface acid-base chemistry. In fact, the absorption spectra of the aqueous and surface bound complexes in acetonitrile agree well and allow facile determination of the surface linkage.16 The absorption and PL properties of the compounds anchored to acid-pretreated metal oxides are consistent with the carboxylic acid form while the base-pretreated surfaces yield spectra consistent with the carboxylate.16 The first-order time-resolved PL decays for sensitizers bound to ZrO2 surfaces reflect a homogeneous distribution of noninteracting surface bound excited states. The appearance of a second-order component in the excitedstate relaxation on TiO2 is indicative of excited stateexcited state annihilation processes that result from fast intermolecular energy transfer across the semiconductor interface.10,24 The observed second-order rate constant, k2, is a function of the excited-state concentration. Under the current experimental conditions, we do not have independent evaluation of the excited-state concentration and therefore report observed values. The excited-state behavior of Ru(bpy)2(ina)22+ is particularly notable. In fluid solution at room temperature, the presence of low-lying ligand field states renders this compound photochemically unstable and nonemissive with an excited-state lifetime less than 10 ns. Upon attachment to the metal oxide surfaces the complex becomes emissive with long-lived excited states and high photochemical stability in both the protonated and deprotonated forms. Detailed temperature-dependent behavior of this complex on untreated metal oxide surfaces has been the subject of a recent paper.11 Intermolecular Charge Transfer. We and others have found that redox-active molecules bond to the mesoporous nanocrystalline TiO2 films can be electrochemically oxidized and reduced in a reversible fashion.8,25 Since the reduction potentials of the molecules exist near mid-band gap, the process does not involve the VB or CB of the semiconductor. In fact, very similar behavior is observed on insulating ZrO2. Instead, the accepted mechanism involves initial oxidation of compounds bound to the tin oxide substrate followed by intermolecular electron hopping across the nanoparticle surfaces. For the entire film to be oxidized, this mechanism requires electronic communication between all the surface bound complexes. In fact, Gra¨tzel and co-workers have quantified the percolation threshold concentration necessary for complete oxidation of amines bound to related TiO2 films.25a Chronoamperometry experiments with optical detection have allowed the effective diffusion constants for intermolecular hopping with RuIII/II(dcb)(bpy)2/ TiO2 to be quantified, D ) (8 ( 2) × 10-9 cm2/s in 0.1 M LiClO4 acetonitrile.25b In this work an unexpected cation dependence for intermolecular electron hopping was discovered. The efficiency of intermolecular charge transport is near zero
(20) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. (21) (a) Anderson, S.; Constable, E. C.; Dare-Edwards, M. P.; Goodenough, J. B.; Hamnett, A.; Seddon, K. R.; Wright, R. D. Nature 1979, 280, 571. (b) Goodenough, J. B.; Hamnett, A.; Dare-Edwards, M. P.; Campet, G.; Wright, R. D. Surf. Sci. 1980, 101, 531. (c) Gulino, D. A.; Drickamer, H. G. J. Phys. Chem. 1984, 88, 1173. (d) 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. (22) Nazeeruddin, M. K.; Liska, P.; Moser, J.; Vlachopoulos, N.; Gra¨tzel, M. Helv. Chim. Acta 1990, 73, 1788.
(23) (a) Torrents, A.; Stone, A. T. Environ. Sci. Technol. 1991, 25, 143. (b) Torrents, A.; Stone, A. T. Soil Sci. Soc. Am. J. 1994, 58, 738. (24) Farzad, F.; Thompson, D. W.; Kelly, C. A.; Meyer, G. J. J. Am. Chem. Soc. 1999, 121, 5577. (25) (a) Bonhote, P.; Gogniat, E.; Tingry, S.; Barbe, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Gra¨tzel, M. J. Phys. Chem. B 1998, 102, 1498. (b) Farzad, F. Molecular Level Energy and Electron-Transfer Processes at Nanocrystalline Titanium Dioxide Interfaces. Thesis Johns Hopkins University, 1999. (c) Trammell, S. A.; Meyer, T. J. J. Phys. Chem. B 1999, 103, 104.
Nanocrystalline TiO2
Langmuir, Vol. 17, No. 21, 2001 6727 Scheme 2
for base-pretreated TiO2 and ZrO2 in tetrabutylammonium perchlorate electrolytes. Essentially no sensitizer oxidation is observed by cyclic voltammetry, chronoamperometry, or spectroelectrochemistry under these conditions. The presence of small cations, such as H+ or Li+, at the interface, on the other hand, results in rapid and efficient intermolecular charge transfer across the nanocrystalline surfaces. An explanation for the cation-induced changes in intermolecular charge-transfer efficiency is unknown but may reflect a greater translational mobility of the carboxylic acid bound compounds than the carboxylate compounds. Alternatively, small cations at the interface may lower the reorganization energy for RuIII/II hopping. We note that protons and/or alkali ions are also required for high electrical conduction in oxide glasses.26 Small shifts in the RuIII/II potentials between the acidand base-pretreated surfaces,
