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Letters Sensitization of Nanocrystalline TiO2 Initiated by Reductive Quenching of Molecular Excited States David W. Thompson, Craig A. Kelly, Fereshteh Farzad, and Gerald J. Meyer* Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218 Received July 6, 1998. In Final Form: November 17, 1998 An alternative mechanism for dye sensitization of wide bandgap semiconductors has been realized: reductive quenching of the dye excited state(s) followed by thermal interfacial electron transfer. The processes were identified using nanosecond-time-resolved absorption and photoluminescence spectroscopies after selective excitation of Ru(deeb)(bpy)2(PF6)2, where bpy is 2,2′-bipyridine and deeb is 4,4′-(COOC2H5)22,2′-bipyridine, sensitizers anchored to a nanocrystalline (anatase) TiO2 film immersed in a 0.1 M tetrabutylammonium perchlorate acetonitrile solution with phenothiazine electron donors. With the electrolyte changed to 0.1 M lithium perchlorate, this same assembly undergoes the generally accepted mechanism for dye sensitization: the sensitizer excited state(s) transfer an electron to the semiconductor and is subsequently reduced by the phenothiazine donor.
Introduction An attractive approach for solar energy conversion has been to utilize wide bandgap semiconductors sensitized with inorganic coordination compounds in regenerative solar cells.1 In the commonly accepted model for dye sensitization, the sensitizer excited state(s) transfers an electron to the semiconductor, Scheme 1a.2 Here we present spectroscopic data that demonstrate an alternative mechanism for dye sensitization of colloidal anatase TiO2, Scheme 1b. In this mechanism, the sensitizer excited state(s) are first quenched by an external donor, D, and subsequently transfers an electron across the semiconductor interface. While precedence for this alternative mechanism exists from photoelectrochemical studies at planar semiconductor electrodes,3 to our knowledge this Letter represents the first direct spectroscopic observation. Experimental Section The materials under study are nanocrystalline (anatase) TiO2 films sensitized with Ru(deeb)(bpy)2(PF6)2, where bpy is 2,2′bipyridine and deeb is 4,4′-(CO2C2H5)2-2,2′-bipyridine, in the presence of phenothiazine, PTZ, donors shown below. The
Scheme 1
diameter particles interconnected in a mesoporous ∼10 µm thick film. Surface attachment occurs after soaking the films in millimolar Ru(deeb)(bpy)2(PF6)2 acetonitrile solutions with mild heating overnight.4 The electrodes, abbreviated Ru(deeb)(bpy)22+/ TiO2, are rinsed and immersed in argon-saturated acetonitrile solutions. Electron transfer kinetics and mechanisms are measured by monitoring absorption or luminescence changes observed after pulsed 532 nm light excitation as previously described.4 Spectral grade acetonitrile was obtained from Burdick & Jackson and used as received. Tetrabutylammonium perchlorate (TBAP), LiClO4, ferrocene, and trimethylamine were used as received from Aldrich. Phenothiazine (Aldrich, 99.9%) was recrystallized twice.
Results and Discussion The excited states of Ru(deeb)(bpy)22+ bound to TiO2 in a 0.1 M TBAP acetonitrile or neat acetonitrile are longlived. Under these conditions relatively little interfacial
preparation of anatase films follows a sol-gel route previously described in the literature.4 The procedure produces ∼20 nm (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Hagfeldt, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49.
(3) (a) Ortmans, I.; Moucheron, C.; Kirsch-De Mesmaeker, A. Coord. Chem. Rev. 1998, 168, 233 and references therein. (b) We also note that this approach is similar in concept to the well-known photogalvanic effect. See, for example: Albery, W. J. Acc. Chem. Res. 1982, 15, 142 and references therein. Kamat, P. V.; Fox, M. A. J. Electrochem. Soc. 1984, 131, 1032. (4) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319.
10.1021/la980809d CCC: $18.00 © 1999 American Chemical Society Published on Web 12/29/1998
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550 nm are due to the reduced sensitizer.6,7 On a longer time scale than is shown, [RuII(deeb-)(bpy)2+, PTZ+] recombines by electron transfer to ground-state products with second-order kinetics. The transient absorption difference spectra shown in Figure 2b were obtained after pulsed excitation of a Ru(deeb)(bpy)22+/TiO2 film immersed in acetonitrile, [PTZ] ) 3.05 mM, [TBAP] ) 0.1 M. The initial spectrum observed immediately after excitation corresponds to the MLCT excited state which evolves into an absorption difference spectrum consistent with the formation of an interfacial charge separated state, [TiO2(e-), PTZ+]. A weak positive absorption for PTZ+ at ∼510 nm and no features that would correspond to the reduced sensitizer are observed. We therefore infer that reductive quenching of the MLCT excited states, eq II is followed by rapid electron injection into TiO2 to form [TiO2(e-), PTZ+], eq III. We note that
Ru(deeb)(bpy)22+*/TiO2 + PTZ f Figure 1. Photoluminescence spectral changes for Ru(deeb)(bpy)22+/TiO2 in argon-saturated acetonitrile solution as a function of added PTZ. The PTZ concentrations were from top to bottom, 0, 0.1, 0.2, 0.5, 1.0, and 5 mM. The inset shows this same data recast in a Stern-Volmer plot PLI(0)/PLI vs [PTZ], where PLI(0) is the integrated PL intensity at [PTZ] ) 0 and PLI is the integrated PL intensity for each [PTZ] concentration, from which a Stern-Volmer constant of 1300 ( 100 M-1 was abstracted from the slope. The samples were excited with 460 ( 4 nm light.
electron transfer occurs.5 The excited states are readily quenched by the addition of triethylamine, PTZ, 10methyl-PTZ, ferrocene, or dioxygen to the external acetonitrile solution. PTZ quenching was studied in most detail as an extension of previous studies and for its favorable spectroscopic and redox properties.6 The average lifetime and relative photoluminescence, PL, quantum yield of Ru(deeb)(bpy)22+*/TiO2 excited states are quenched by addition of PTZ to the acetonitrile solution. Figure 1 shows the PL spectra taken under increasing amounts of PTZ. We note that the initial PL spectra can be restored if the sensitized film is again exposed to 0.1 M TBAP acetonitrile solution. The steady-state PL quenching is well described by the Stern-Volmer model from which a Stern-Volmer constant of 1300 ( 100 M-1 is obtained, Figure 1 inset. Figure 2a was measured with Ru(deeb)(bpy)22+ in fluid 0.1 M TBAP acetonitrile solution that also contains 3.5 mM PTZ. The initial absorption difference spectra is assigned to the metal-to-ligand charge transfer (MLCT) excited states.6 This spectrum evolves into an absorption difference spectrum consistent with reductive excited state electron transfer to form the charge-separated state [RuII(deeb-)(bpy)2+, PTZ+], eq I. The positive absorption
Ru(deeb)(bpy)22+* + PTZ f II
-
+
Ru (deeb )(bpy)2 + PTZ
+
RuII(deeb-)(bpy)2+/TiO2 + PTZ+ (II) kinj
RuII(deeb-)(bpy)2+/TiO2 + PTZ+ 98 Ru(deeb)(bpy)22+/TiO2(e-) + PTZ+ (III) TiO2(e-) is expected to absorb light very weakly in the UV-vis region and is difficult to unambiguously identify in the presence of oxidized sensitizers and phenothiazines.8 The interfacial charge separated state recombines with second-order equal concentration kinetics, eq IV. Un-
Ru(deeb)(bpy)22+/TiO2(e-) + PTZ+ f Ru(deeb)(bpy)22+/TiO2 + PTZ (IV) certainties in the appropriate path length lead to significant errors in calculating the rate constant. However, recombination occurs on a microsecond time scale under these conditions. Figure 2c shows the absorption difference spectra after pulsed light excitation of Ru(deeb)(bpy)22+/TiO2 in 0.1 M LiClO4 with 3.5 mM PTZ. The initial spectrum observed corresponds to that of the oxidized sensitizer indicating that electron injection into TiO2 occurs within the 8 ns laser pulse, to form the interfacial charge separated state [TiO2(e-), Ru(III)], eq V. PTZ subsequently reduces the oxidized sensitizer to form the same interfacial charge separated state created by the alternative mechanism, [TiO2(e-), PTZ+], eq VI. We have recently reported similar kinj
Ru(deeb)(bpy)22+*/TiO2 98 RuIII(deeb)(bpy)23+/TiO2(e-) (V) RuIII(deeb)(bpy)23+/TiO2(e-) + PTZ f
(I)
Ru(deeb)(bpy)22+/TiO2(e-) + PTZ+ (VI)
at 510 nm is assigned to PTZ+ and the features at 390 and
behavior under conditions of efficient, fast electron injection for sensitized aqueous colloidal solutions and sensitized thin films in propylene carbonate electrolyte.6 The interfacial charge separated state also recombines with second-order equal concentration kinetics, eq IV. The
(5) Photoinduced electron injection is not observed for Ru(deeb)(bpy)22+*/TiO2 assemblies in neat acetonitrile because the acceptor states in TiO2 are not energetically accessible. Li+ is a potential determining ion for TiO2. See, for example: (a) Yates, D. E.; Healy, T. W. J. Chem. Soc., Faraday Trans. 1 1980, 76, 9. (b) Enright, B.; Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1994, 98, 6195. (c) Kelly, C.; Thompson, D.; Farzad, F.; Meyer, G. J. To be submitted for publication. (6) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. J. Phys. Chem. B 1997, 101, 2591.
(7) Mulazzani, Q. G.; Emmi, S.; Fouchi, P. G.; Hoffman, M. Z.; Venturi, M. J. Am. Chem. Soc. 1978, 100, 981. (8) Stipkala, J. M.; Castellano, F. N.; Heimer, T. A.; Kelly, C. A.; Livi, K. J. T.; Meyer, G. J. Chem. Mater. 1997, 9, 2341 and references therein.
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Figure 2. Absorption difference spectra obtained after pulsed 532 nm light excitation (9 mJ/pulse, 8-10 ns fwhm) under the following conditions. (a) Ru(deeb)(bpy)22+ in argon-purged 0.1 M TBAP acetonitrile solution that contains 3.5 mM PTZ. The spectra were recorded 0 (diamonds), 20 (filled squares), 40 (filled circles), 80 (open triangles), 150 (filled triangles), and 300 (open circles) ns after pulsed excitation. (b) A Ru(deeb)(bpy)22+/TiO2 film immersed in argon-purged 0.1 M TBAP, 3.05 mM PTZ acetonitrile solution recorded 0 (diamonds), 20 (filled squares), 40 (filled circles), 80 (open triangles), 120 (filled triangles), and 500 (open circles) ns after pulsed excitation. (c) A Ru(deeb)(bpy)22+/TiO2 film immersed in an argon-purged 0.1 M LiClO4, 3.5 mM PTZ acetonitrile recorded 0 (diamonds), 10 (filled squares), 25 (filled circles), 50 (open triangles), 200 (filled triangles), and 500 (open circles) ns after pulsed excitation.
uncertainties in appropriate path length mentioned above are also problematic here. For pH ) 2 aqueous TiO2 colloid solutions, where the path length is better defined, recombination occurs with a rate constant of k ) (1.34 ( 0.05) × 107 M-1 s-1 for a related water soluble PTZ derivative, promethazine.6
In summary, the spectroscopic data demonstrate sensitization of TiO2 to visible light by an alternative mechanism: reductive quenching of Ru(II) excited states followed by rapid interfacial electron injection. The PL quenching studies demonstrate dynamic quenching of the surface-bound excited states, and the transient absorption
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data show that the quenching is by reductive electron transfer. Thermal electron injection occurs on a time scale faster than we can time resolve, kinj > 108 s-1, to form the charge-separated state, [TiO2(e-), PTZ+]. For comparison, the same charge-separated state was formed by the commonly accepted mechanism under conditions that promote rapid interfacial electron transfer, 0.1 M LiClO4 acetonitrile electrolyte. The recent realization of efficient solar energy conversion from dye-sensitized anatase films in regenerative solar cells with iodide/triiodide acetonitrile electrolyte has renewed interest in these processes.1 For the operational solar cell, strong evidence exists that sensitization occurs by the commonly accepted mechanisms shown in Scheme 1a. Femtosecond electron injection rates recently reported are much faster than diffusional bimolecular excited-state quenching.9 Furthermore, as recently shown by Kamat, the sensitizer excited states are not potent enough oxidants to efficiently oxidize iodide.10 Nevertheless, the possibility for fabricating efficient solar cells that operate by the alternative mechanism reported here appears very real.
For this to be realized, sensitizers that are strong oxidants with long-lived excited states are required.3 Alternatively, sensitizers need to be designed that promote rapid reductive quenching through intramolecular electron tranfer6,11 or ion-pairing.12 A potential advantage of the alternative mechanism is that sensitizers that are not thermodynamically capable of excited-state electron transfer may inject electrons into the semiconductor from their reduced forms, as observed here.13 For Ru(deeb)(bpy)22+, the Ru(3+/2+*) potential is -0.6 V and the Ru(2+/+) potential is -1.0 V vs SCE, thereby providing an additional 400 mV of driving force for interfacial charge separation. It may therefore be more straightforward to sensitize TiO2 over broad spectral regions by the reductive quenching mechanism than by the generally accepted mechanism.
(9) (a) Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. J. Phys. Chem. 1996, 100, 20056. (b) Heimer, T. A.; Meyer, G. J. J. Lumin. 1996, 70, 468. (c) Hannappel, T.; Burfeindt, B.; Storck, W.; Willig, F. J. Phys. Chem. B 1997, 101, 6799. (d) Heimer, T. A.; Heilweil, E. J. J. Phys. Chem. B 1997, 101, 10990. (10) Nasr, C.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B 1998, 102, 4944.
(11) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. J. Am. Chem. Soc. 1995, 117, 11815. (12) (a) Mallouk, T. E.; Krueger, J. S.; Mayer, J. E.; Dymond, C. M. G. Inorg. Chem. 1989, 28, 3507. (b) Clark, C. D.; Hoffman, M. Z. J. Phys. Chem. 1996, 100, 7526. (13) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1994, 33, 5741.
Acknowledgment. We thank the National Science Foundation (CHE-9708222) for support of this research. LA980809D
