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Langmuir 1990, 6, 512-513
Picosecond Charge-Transfer Events in the Photosensitization of Colloidal TiO,+ Prashant V. Kamat Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556 Received August 21, 1989. I n Final Form: November 14, 1989
The charge transfer from the singlet excited anthracene-9-carboxylic acid (9AC) into the conduction band of a large band gap semiconductor, TiO,, and the recombination of injected charge with the cation radical 9AC'+ have been time resolved in the picosecond-nanosecond time domain with the aid of picosecond laser flash photolysis. Photosensitization of a stable large band gap semiconductor is of considerable interest in the photoelectrochemical conversion of visible light and in the applications of color photography.'*2 Investigation of primary photochemical processes that occur in the picosecondnanosecond time domain is important since these events control the efficiency of photosensitization. Laser flash p h o t o l y ~ i s ~and - ~ resonance Raman spectroscopy6 have been used to characterize the photochemical transients in colloidal semiconductor systems. However, very limited attempts have been made to probe the photosensitization process in the subnanosecond time domain.'^^ Recently, we have shown that 9AC*(Sl) can inject charge into the TiO, colloids, and such an injected charge can be utilized to reduce another substrate, N,N,N',N'tetraethyloxonine.' We report here subnanosecond photochemical events on colloidal TiO, surface which directly influence the sensitization of the semiconductor. Suspensions of colloidal TiO, ( D p 300 A) were prepared by the hydrolysis of titanium(1V) 2-propoxide in acetonitrile.' Picosecond laser flash photolysis experiments were performed with a 355-nm laser pulse (pulse width 18 ps) from a Quantel YG-501 DP Nd:YAG laser system. The excitation beam, which was spread over an area 10 mm X 1 mm, and the probe beam, which was generated by passing the residual fundamental output through a D,O/H,O solution, were at right angles to each other. The time zero in these experiments corresponds to the time at the end of the excitation pulse. Details of the experimental procedure are described elsewhere.lO"' All the samples were deaerated by bubbling Ar for 1520 min, and the experiments were performed at room temperature (23 OC). The interaction between 9AC and the surface hydroxyl groups leads to the broadening of the absorption bands of 9AC and the extension of its absorption into the
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P a r t 14 of t h e series, Photoelectrochemistry in Semiconductor Particulate Systems. (1) (a) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31. (b) Meier, H. Photochem. Photobiol. 1972, 16, 219. (2) Spitler, M.; Parkinson, B. A. Langmuir 1986,2, 549. (3) Moser, J.; Grltzel, M. J. Am. Chem. SOC.1984, 106, 6557. (4) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986,90,1389. (5) Kalyansundaram, K.; Vlachopoulos, N.; Krishnan, V.; Monnier, A.; Gratzel, M. J.Phys. Chem. 1987, 91, 2342. (6) Rossetti, R.; Brus, L. E. J. Am. Chem. SOC.1984, 106, 4336. (7) Kirk, A. D.; Langford, C. H.; Joly, C. S.; Lesage, R.; Sharma, D. K. J. Chem. SOC.,Chem. Commun. 1984,961.
(8)Moser, J.; Gratzel, M.; Sharma, D. K.; Serpone, N. Helv. Chim. Acta 1985, 68, 1686. (9) Kamat, P. V. J. Phys. Chem. 1989,93,859. (10) Ebbesen, T. W. Reu. Sci. Instrum. 1988,59, 1307. (11) Kamat, P. V.; Ebbesen, T. W.; Dimitrijevic, N. M.: Nozik, A. J. Chem. Phys. Lett. 1989, 157, 384.
0743-7463/90/2406-0512$02.50/0
The apparent association constant for the association between 9AC and TiO, was 6000 M-l. It was confirmed in a separate experiment that colloidal TiO, prepared in CH3CN cannot be excited directly with 355-nm light; hence, its interference during the selective excitation of 9AC (cgAC(355 nm) = 6000 M-' cm-') was negligible. The transient absorption spectra recorded at various time intervals after 355-nm laser pulse excitation of 9AC in colloidal T i 0 suspension exhibit absorption maximum at 720 nm." This absorption spectrum is very similar to electrochemically generated cation radical of anthracene derivatives.14 In the present study, cation radical, 9AC'+, is formed as a result of the charge injection from the excited 9AC*(Sl) into the conduction band of TiO,: 9AC*(S1)+ TiO,
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9AC"
+ TiO,(e)
(1)
Fluorescence quenching of 9AC*(SI) by TiO, colloids has shown that nearly 96% of the excited singlet participates in reaction 1. A triplet excited state which has an absorption maximum at 425 nm does not participate in the charge injection process. The preliminary estimate of the rate constant (5 X 10' s-l) for the charge injection process (reaction 1) was obtained from the quenching of the fluorescence lifetime of 9AC by TiO,.' However, the observed decrease in fluorescence quantum yield of 9AC was 5 times greater than the decrease in fluorescence lifetime as a result of its adsorption on TiO, colloid. This indicated that a faster quenching mechanism should be operative for the deacivation of 9AC*(S1). Because of the limitations of instrument response time, we could not resolve the emission lifetimes shorter than 500 ps. However, with the picosecond transient absorption spectroscopy, it was possible to probe this ultrafast charge injection process. The spectrum recorded at t = 0 ps (Figure 1) shows that 9AC'+ formation is completed within the laser pulse duration. Since the time constant for the growth of 9AC'+ is expected to be less than 20 ps, the observed rate constant for the charge injection process (reaction 1) should be greater than 5 X 1O1's-'. The observed rate constant for the heterogeneous electron transfer is higher than the values reported earlier for the sensitization of colloidal TiO, by eosin8 and phthalocyanine' but is comparable to the rate constant for the photoinduced (12) Kamat, P. V.; Ford, W. E. Chem. Phys. Lett. 1987, 135, 421. (13) Negligibly small transient absorption was observed when colloi-
dal TiO, was excluded from the system. (14) Masnovi, J. M.; Seddon, E. A.; Kochi, J. K. Can. J. Chem. 1984, 62, 2552.
0 1990 American Chemical Society
Letters
Langmuir, Vol. 6, No. 2, 1990 513
U Q 0.04
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n nn
-.I-
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WAVELENGTH, nm
Figure 1. Time-resolved transient absorption spectra recorded after 355-nm laser pulse excitation of 9AC (0.2 mM) adsorbed on colloidal TiO, (5 mM in acetonitrile).
heterogeneous electron transfer between colloidal CdS and zwitterionic viologen.'l The spectra recorded in Figure 1 show the decay of 9AC" in the subnanosecond time domain as a result of its reaction with the injected charge:
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9AC" + Ti02(e) 9AC + Ti02 (2) Only -10% of 9AC'+ survived this back reaction, possibly by diffusing away from the TiOz surface. The initial decay (
