6870 J. Phys. Chem., Vol. 100, No. 17, 1996
Letters
(13) (a) Braddock, J. N.; Meyer, T. J. J. Am. Chem. Soc. 1973, 95, 3158.
(b) Fussa-Rydel, O.; Zhang, H. T.; Hupp, J. T.; Leidner, C. R. Inorg. Chem.
1989, 28, 1533.
(14) Rochester, C. H. Acidity Functions; Academic Press: New York,
1970.
(15) Tickle, P.; Briggs, A. G.; Wilson, J. M. J. Chem. Soc. B 1970, 65.
(16) Because of the increased optical density available, the transient
spectrum was obtained with a colloidal dispersion of the dye-coated
semiconductor, rather than a thin film. The decay kinetics for colloid versus
film samples were similar.
the putative surface state energetics remains unelucidated. A
second explanation focuses attention back upon the origin of
the phenomenon described by eq 1. Recent electrochemical
quartz crystal microbalance studies from our laboratory have
shown unequivocally that electron accumulation layer formation
is accompanied quantitatively by intercalation of charge com-
pensating cations.9 If the cation is H+, then ECB (effectively
the onset potential for accumulation layer formation) unavoid-
ably acquires a Nernstian pH dependence26 and interfacial ET
becomes a proton-coupled process. The detailed kinetics,
however, could entail stepwise electron and proton transfer. If
rate-determining ET preceded the proposed H+ transfer, then
the kinetically relevant driving force would differ from the
overall reaction driving force and could conceivably lack a pH
dependence.
(17) See, for example: Ohno, T.; Nozaki, K.; Ikeda, N.; Haga, M. AdV.
Chem. Ser. 1991, 228, 215.
(18) The injected electron also is expected to absorb in the red portion
of the spectrum. The comparatively small extinction coefficient for this
species (ca. 600 M-1 cm-1 at 780 nm (trapped electron))21 versus the
coordinated radical anion (∼2400 M-1 cm-1), however, makes the former
difficult to detect under the conditions of our experiment.
(19) As expected, at lower pH the slow component is lost at less positive
potentials (e.g., -0.7 V at pH ) 2.4).
(20) The admittedly complex transient signals were idealized via fits to
an exponential decay (short component) plus a constant (long component;
essentially invariant over first 400 ns). The open-circuit lifetime of the short
component was unchanged (20 ( 2 ns) over the available pH range but
decreased by ca. 50% when the dark assembly was potentiostatted near
ECB. Fits to a more complex function (two exponentials plus a constant)
also yielded a pH invariant fast component.
(21) (a) Redmond, G.; Fitzmaurice, D.; Gra¨tzel, M. J. Phys. Chem. 1993,
97, 6951. (b) O’Regan, B.; Gra¨tzel, M.; Fitzmaurice, D. Chem. Phys. Lett.
1991, 183, 89
Regardless of the ultimate explanation, the pH independence
shown in Figure 4 raises important questions concerning
fundamental interfacial reaction kinetics as well as design criteria
for photoelectrochemical cell optimization.27,28 We hope to
report shortly on additional experiments directed toward both
questions.
Acknowledgment. We thank the Office of Naval Research,
the DOD AASERT program, and the Northwestern Materials
Research Center (DMR-9120521) for support of our work.
J.T.H. also gratefully acknowledges the Dreyfus Foundation
(Teacher-Scholar Award, 1991-96) for unrestricted support.
(22) See also: Redmond, G.; Fitzmaurice, D. J. Phys. Chem. 1993, 176,
493.
(23) The absence of a transient bleaching signal implies the existence
of a very rapid excited-state decay process, in addition to (or as an alternative
to) elimination of injection. Although we can only speculate at present, a
candidate reaction would be interfacial energy transfer. Interestingly, the
analogous carboxylated dye reportedly displays a long-lived luminescence
(∼100 ns) when bound to TiO2 under accumulation conditions (i.e., rapid
quenching is absent; see: O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel.
M. J. Phys. Chem. 1990, 94, 8720).
(24) Preliminary studies with the the prototypical dye, Ru(4,4′-carboxylate-
2,2′-bipyridine)34-, were suggestive of behavior similar to that for the
phosphonated dye but were complicated by desorption at extreme pH’s.
(25) A reviewer has pointed out that Degussa P25, while predominantly
comprised of anatase, also contains rutile. Because the two differ slightly
with respect to band-edge energetics, we also examined reactivity on a purely
anatase electrode prepared from a colloidal dispersion that had been
generated by hydrolyzing titanium isopropoxide. For the phosphonated dye
on the pure anatase surface, we obtained open-circuit results similar to those
shown in Figure 2, except that τshort was ca. 45 ns. Consistent with results
shown in Figure 4, the decay time proved to be pH independent over the
range from 2 (the lowest pH examined) to 10 (the highest pH examined).
(26) For alternative explanations of pH/ECB correlations for TiO2 see:
Finklea, H., Semiconductor Electrodes; Elsevier, New York, 1988, Chapter
2.
References and Notes
(1) For a review see: Parkinson, B. A.; Spitler, M. T. Electrochim.
Acta 1992, 37, 943.
(2) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Nazeer-
uddin, 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.
(3) (a) Argazzi, R.; Bignozi, C. A.; Heimer, T. A.; Castellano, F. N.;
Meyer, G. J. Inorg. Chem. 1994, 33, 5741. (b) Heimer, T. A.; Bignozzi, C.
A.; Meyer, G. J. J. Phys. Chem. 1993, 97, 11987.
(4) See, for example: Gra¨tzel, M. Coord. Chem. ReV. 1991, 111, 167.
(5) Moser, J.; Gra¨tzel, M. Chem. Phys. 1993, 176, 493.
(6) See, for example: (a) Bolts, J. M.; Wrighton, M. S. J. Phys. Chem.
1976, 80, 2641. (b) Watanabe, T.; Fujishima, A.; Tatsuoki, O.; Honda, K.
Bull. Chem. Soc. Jpn. 1976, 49, 8. (c) Gerisher, H. Electrochim. Acta 1989,
34, 1005. (d) Hardee, K. L.; Bard, A. J. J. Electrochem. Soc. 1975, 122,
739.
(7) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem.
190, 104, 2977.
(8) Wang, Y.; Schanze, K. S. Chem. Phys. 1993, 176, 305.
(9) Lyon, L. A.; Hupp, J. T. J. Phys. Chem. 1995, 99, 15718.
(10) Gould, S.; Strouse, G. E.; Meyer, T. J.; Sullivan, B. P. Inorg. Chem.
1991, 30, 2942.
(27) At the suggestion of a reviewer, we also examined ET kinetics in
the absence of solvent. (No attempt was made, however, to remove
chemisorbed water.) Results similar to those shown in Figure 2 were
obtained.
(28) A reviewer has drawn our attention to a report by Sonntag and
Spitler (J. Phys. Chem. 1985, 89, 1453) which shows that dye-sensitized
photovoltaic reactivity for single-crystal SrTiO3 can be turned on and off
via pH changes. In view of the present findings, transient studies of this
system could be very informative.
(11) Hirao, T.; Maunga, T.; Ohshiro, Y.; Agawa, T. Synthesis 1981,
56.
(12) See also: Pechy, P.; Rotzinger, F. P.; Nazreeruddin, M. K.; Kohle,
O.; Zakeeruddin, S. M.; Humphry-Baker, R.; Gra¨tzel, M. J. Chem. Soc.,
Chem. Commun. 1995, 1, 65.
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