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2009, 113, 18444–18447 Published on Web 10/01/2009
Reduction of I2/I3- by Titanium Dioxide John Rowley and Gerald J. Meyer* Departments of Chemistry and Materials Science & Engineering, Johns Hopkins UniVersity, 3400 North Charles Street, Baltimore, Maryland, 21218 ReceiVed: July 29, 2009; ReVised Manuscript ReceiVed: September 17, 2009
A photoelectrochemical cell was designed that allowed the reactivity of oxidized iodide species with mesoporous nanocrystalline (anatase) TiO2 thin films to be quantified spectroscopically on nanosecond and longer time scales in half molar iodide acetonitrile solutions. Under forward bias conditions, TiO2 did not react with photogenerated iodine radical anions, I2-•, that were found instead to disproportionate with a rate constant that was within experimental error the same as that measured in fluid acetonitrile solution, k ) 3 × 109 M-1 s-1. The absence of reactivity with I2-• was unexpected. It appears that the reduction of I2-• by TiO2(e-) does not complete kinetically with rapid I2-• disproportionation. In contrast, TiO2(e-) was found to decrease the concentration of tri-iodide, I3-, and presumably molecular iodine, I2, that was expected to be present in low equilibrium concentrations. The findings have relevance to unwanted charge recombination processes in dye sensitized solar cells. The important loss mechanism in dye sensitized solar cells is considered to be recombination of TiO2 electrons with acceptors present in an organic electrolyte that contains mixtures of iodine and iodide.1,2 Under one sun and lower irradiance, recombination rarely occurs such that incident-photon-to-current efficiencies (IPCEs) are near unity at the short circuit condition and decrease only slightly at the power point. Because recombination is inefficient, the mechanism remains poorly understood.1-8 Voltammetry responses under forward bias (i.e., negative applied potentials) as well as photovoltage measurements of dye sensitized solar cells (DSSCs) have provided insights into how the sensitizer binding9 and molecular structure10-13 influence the thermodynamics and kinetics of charge recombination. However, such electrochemical data is limited in the information it provides and key mechanistic details, such as the identity of the acceptor(s), remain elusive. In principle, transient optical measurements of sensitized electrodes under forward bias in inert electrolytes can provide such molecular level information.4,14-18 Application of this approach to redox active electrolytes in DSSCs is nontrivial, as the applied bias results in irreversible halide reduction. Here, we describe a photochemical approach that circumvents this problem and report the first direct observation of tri-iodide/iodine reduction by TiO2 electrons. The new idea was to use photochemistry to initiate known iodide redox reactions within a potentiostatically controlled mesoporous TiO2 thin film. Shown in Figure 1 is a simplified scheme that displays the key features of the photoelectrochemical cell. Mesoporous nanocrystalline (anatase) TiO2 doctorbladed and sintered onto a fluorine doped tin oxide (FTO) conductive electrode in two parallel strips separated by about 1 cm served as the working electrode with Vycor-capped nonaqueous silver/silver nitrate reference and Pt mesh counter * To whom correspondence should be addressed. E-mail: meyer@ jhu.edu.
10.1021/jp907265x CCC: $40.75
electrodes. A quartz microscope slide secured over the FTO/ TiO2 thin film fixed the path length to be ∼9 µm. The uppermost TiO2 strip serves as a spacer to ensure that the quartz microscope slide remained parallel to the FTO electrode. This sandwich cell arrangement thus contained two distinct experimental zones: one comprised only of 0.5 M tetrabutyl ammonium iodide (TBAI) electrolyte and the second comprised of the mesoporous TiO2 film and the same electrolyte. Nanosecond 266 nm laser pulses and an orthogonal white light probe beam were used in a font-face illumination configuration. The transmitted white light was dispersed and single wavelength intensities recorded with a photomultiplier tube (PMT). Vertical translation of the photoelectrochemical cell allowed for the illumination and spectroscopic monitoring of either zone. Forward bias of a mesoporous TiO2 thin film in the photoelectrochemical cell solely produced the well-known absorption spectrum of TiO2 electrons (TiO2(e-)), Figure 2a.19 This arrangement allowed the TiO2(e-) concentration to be varied in the presence of iodide solutions with no observed electrolyte reduction. The oxidized forms of iodide were generated photochemically. Ultraviolet excitation of iodide solutions is known to yield iodine atoms, iodine radical anions (I2-•), solvated electrons, and tri-iodide (I3-), by reactions 1-4 (see Table 1).20-24 Reduction of I2 and I3- by solvated electrons has also been characterized; see reactions 5 and 6. The rate constants for the second-order reactions have previously been reported.20-28 On the basis of the known extinction coefficient of iodide (ε266 nm ) 935 M-1 cm-1), approximately 60% of the photons were absorbed by 0.5 M tetrabutylammonium iodide (TBAI)/ acetonitrile solution in the 9 µm path length. Band gap excitation of TiO2 is also known to result in iodine oxidation and I2-• formation; see reactions 7 and 8 (see Table 2).5,6 Light absorption forms an electron-hole pair (e-/h+), and the valence band hole is a potent oxidant capable of iodine oxidation.29 The 266 nm photons utilized exceed the 3.2 eV 2009 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18445
Figure 1. (a) Side and front view of the photoelectrochemical cell utilized that consisted of a Ag/AgNO3 reference electrode (RE, orange), a Pt counter electrode (CE, pink), and mesoporous TiO2 thin films (blue) deposited on a FTO substrate. A side view is also shown that exaggerates the 9 µm separation between the quartz coverslip (yellow) and the FTO substrate. (b) Top view of the cell and the optical alignment of the 266 nm excitation, white light probe beams, dispersive element, and detector. The Surlyn films that contain the TBAI/acetonitrile solution are shown.
Figure 2. (a) The UV-vis absorption spectra of a TiO2 thin film at the indicated applied potentials (vs Ag/AgNO3), in 0.5 M TBAI/acetonitrile. (b) Transient absorption spectra measured 0.1 µs (red squares) and 50 µs (blue circles) after pulsed 266 nm light excitation of mesoporous TiO2 in 0.5 M TBAI/acetonitrile. Solid lines show simulations of I2-• disproportionation to I3- at the indicated delay times.
band gap, and reactions 7 and 8 could be operative. The reaction with valence band holes might promote I2-• adsorption to the semiconductor surface. However, the spectroscopic properties of I2-• and the disproportionation kinetics (see below) were indistinguishable from those generated by 266 nm excitation of iodide in fluid acetonitrile solution. Pulsed 266 nm light excitation, with laser irradiances 100 suns of AM 1.5 irradiation.27 Nanosecond transient absorption studies of the mesoporous TiO2 thin film under forward bias in 0.5 M TBAI/acetonitrile revealed the following: (1) Dispropor-
Figure 3. (a) Representative transient absorbance changes monitored at 380 nm, where both I2-• and I3- absorb strongly, at the indicated applied forward biases (vs Ag/AgNO3) after pulsed 266 nm light excitation. The data was normalized at 1 µs. (b) Transient absorbance changes monitored at 705 nm, where I2-• absorbs light strongly. A fit to a second-order kinetic model is shown in yellow from which a rate constant of 3 × 109 M-1 s-1 was abstracted.
Letters tionation of I2-• occurs within the mesoporous TiO2 thin film with a rate constant that was within experimental error of that measured in fluid solution. (2) There was no evidence for a reaction between TiO2(e-) and I2-•, even with I2-• concentrations twice as large as those for I3-. (3) The I3- concentration decreased rapidly in the presence of TiO2(e-), indicating that I3-, or I2 which is in equilibrium with I3-, is the relevant electron acceptor. It is significant that I3- reduction by TiO2(e-) could be quantified spectroscopically at high TiO2(e-) concentrations. The absence of reactivity with I2-• was unexpected;25 while the origin of this is unclear, it appears that the reduction of I2-• by TiO2(e-) does not complete kinetically with rapid disproportionation. It should be emphasized that these measurements were performed with ultraviolet light and under conditions of high I2-• and TiO2(e-) concentrations in the absence of molecular sensitizers; therefore, extrapolation of these results to dye sensitized solar cells should be done with caution. Acknowledgment. This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FC02-96ER14662. References and Notes (1) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (2) Ardo, S.; Meyer, G. J. Chem. Soc. ReV. 2009, 38, 115. (3) Green, A. N. M.; Chandler, R. E.; Haque, S. A.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2005, 109, 142. (4) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 12203. (5) Fitzmaurice, D. J.; Eschle, M.; Frei, H.; Moser, J. J. Phys. Chem. 1993, 97, 3806. (6) Behar, D.; Rabani, J. J. Phys. Chem. B 2001, 105, 6324.
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18447 (7) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 8916. (8) Huang, S. Y.; Schlichthorl, G.; Nozik, A. J.; Gratzel, M.; Frank, A. J. J. Phys. Chem. B 1997, 101, 2576. (9) Kilsa, K.; Mayo, E. I.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S.; Winkler, J. R. J. Phys. Chem. B 2004, 108, 15640. (10) O’Regan, B.; Walley, K.; Juozapavicius, M.; Anderson, A.; Matar, F.; Ghaddar, T.; Zakeeruddin, S. M.; Klein, C. d.; Durrant, J. R. J. Am. Chem. Soc. 2009, 131, 3541. (11) O’Regan, B. C.; Lopez-Duarte, I.; Martinez-Diaz, M. V.; Forneli, A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres, T.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 2906. (12) Yanagida, M.; Yamaguchi, T.; Kurashige, M.; Hara, K.; Katoh, R.; Sugihara, H.; Arakawa, H. Inorg. Chem. 2003, 42, 7921. (13) Hoertz, P. G.; Thompson, D. W.; Friedman, L. A.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 9690. (14) O’Regan, B.; Moser, J.; Anderson, M.; Graetzel, M. J. Phys. Chem. 1990, 94, 8720. (15) Kamat, P. V.; Bedja, I.; Hotchandani, S.; Patterson, L. K. J. Phys. Chem. 1996, 100, 4900. (16) Haque, S. A.; Tachibana, Y.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. B 1998, 102, 1745. (17) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Moser, J. E.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. B 2001, 105, 7424. (18) O’Regan, B.; Moser, J.; Anderson, M.; Graetzel, M. J. Phys. Chem. 1990, 94, 8720. (19) Rothenberger, G.; Fitzmaurice, D.; Graetzel, M. J. Phys. Chem. 1992, 96, 5983. (20) Treinin, A.; Hayon, E. Int. J. Radiat. Phys. Chem. 1975, 7, 387. (21) Xia, C.; Peon, J.; Kohler, B. J. Chem. Phys. 2002, 117, 8855. (22) Nagarajan, V.; Fessenden, R. W. J. Phys. Chem. 1985, 89, 2330. (23) Ichino, T.; Fessenden, R. W. J. Phys. Chem. A 2007, 111, 2527. (24) Devonshire, R.; Weiss, J. J. J. Phys. Chem. 1968, 72, 3815. (25) Gardner, J. M.; Giaimuccio, J. M.; Meyer, G. J. J. Am. Chem. Soc. 2008, 130, 17252. (26) Grossweiner, L. I.; Matheson, M. S. J. Phys. Chem. 1957, 61, 1089. (27) Peter, L. M. J. Phys. Chem. C 2007, 111, 6601. (28) Schwarz, H. A.; Bielski, B. H. J. J. Phys. Chem. 1986, 90, 1445. (29) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 95, 735.
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