J . Phys. Chem. 1986, 90, 6521-6522 at w < 30 is a Poisson distribution. The variation of the quenching rate constant with w gives information on the location of probes in the micellar aggregate: for solute solubilized inside the water pool, k, depends on 1/w3 whereas for solute localized at the interface k , depends on 1/w2. Such a change in the variation of k, against w can be extended to the proteins. When the pH of the water pool is changed, a protein previously located in the water pool can migrate at the interface.
6521
With cytochrome c at w > 30, some drastic changes occur in the system which could be explained by the fact that cytochrome acts as a cosurfactant. Acknowledgment. We thank Professor P. Luisi for proposing the use of ribonuclease to confirm the model, Dr. P. Millie for fruitful discussions, Dr. Perly for critical reading, and J. Potier for technical assistance.
Effect of Light Intensity on the Quantum Yield of Photoinduced Electron Transfer from Colloidal Cadmium Sulfide to Methylviologen Yoshio Nosaka' and Marye Anne Fox* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712 (Received: May 27, 1986)
The quantum yield for the reduction of methylviologen on pulse-irradiated colloidal CdS particles was measured as a function of incident light intensity. The experimental data were successfully simulated with a numerical kinetic analysis. The relationship between @ and light intensity and the rate constant for electron-hole recombination were obtained.
Interfacial electron transfer occurring at the solid semiconductor-liquid electrolyte junction is important in understanding solar energy conversion.2 Colloidal semiconductor sols are convenient for investigating the detailed kinetics of primary photophysical processes occurring on these optically transparent species. Using transient absorption spectroscopy, several research groups have characterized photochemical reactions on semiconductor colloid^.^-^ Recently, Graetzel and co-workers have determined the rate constant for electron-hole recombination, k,,on colloidal TiOz by laser-induced transient spectroscopy.6 Although they also reported the rate constant for electron transfer from the conduction band of colloidal CdS to adsorbed methylviologen (MV2+),7they did not report the quantum yield of the electron transfer. In the present study we have carefully measured the quantum yield for the reduction of MV2+ as a function of the excitation light intensity. The experimental data was then subjected to kinetic simulation, allowing us to independently obtain the electron-hole recombination rate constant. Colloidal CdS was prepared by injecting a deaerated 2-mL aqueous solution of 10 mM Cd(N03)2( M = mol dm-3) into a deaerated solution of 2 mL of 5 m M Na2S (from anhydrous crystals) and 2 mL of 0.2% sodium polyacrylate (buffered to pH 12 with NaOH) as a stabilizing agent. The absorption threshold of the resulting colloidal sol is about 490 nm. From the estimations by Henglein and co-workers,* this value of absorption edge corresponds to an average particle diameter of about 5 nm. Since the colloidal particle is very small compared to the excitation wavelength, light scattering is negligible in the absorption measurement. Furthermore, the total optical absorption can be regarded as proportional to the total colloidal mass.9 Solutions (1) Visiting scientist on the Japan-US. Cooperative Photoconversion and Photosynthesis Research Program. Permanent address; Department of Chemistry, Technological University of Nagaoka, 949-54 Japan. (2) Fox, M. A.; Lindig, B.; Chen, C. C. J . A m . Chem. Soc. 1982, 104, 5828-5829. (3) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 241-246. (4) Duanghong, D.; Ramsden, J.; Graetzel, M. J . A m . Chem. SOC.1982, 104, 2977-2985. ( 5 ) Kuczynski, J.; Thomas, J. K. Chem. Phys. Lett. 1982,88,445-447: J . Phys. Chem. 1983, 87, 5498-5503. (6) Rothenbergen, G.; Moser, J.; Graetzel, M.; Serpone, N.; Sharma, D. K. J . A m . Chem. SOC.1985, 107, 8054-8059. (7) Serpone, N.; Sharma, D. K.; Graetzel, M.; Ramsden, J. Chem. Phys. Lett. 1985, 115, 473-476. (8) Weller, H.; Schmidt, H. M.; Koch, U.; Fojtik, A,; Baral, S.: Henglein, A.; Kunath, W.; Weiss, K . ; Dieman, E. Chem. Phys. Lett. 1986, 124, 557-560. (9) Rossetti, R.; Ellison, L. J.; Gibson, J. M.; Brus, L. E. J . Chem. Phys. 1984, 80, 446444469,
0022-3654/86/2090-6521$01.50/0
containing 0.3-1.5 mM of colloidal CdS and 10 mM of MVZ+, deaerated by bubbling N,, were excited with the third harmonic of Q-switched Nd:YAG laser (Quantel YG-581 and YG-481) blazed at 355 nm. For excitation at 450 nm, a dye laser head (TDL 111) was used. The light intensity of the laser pulse, varied with glass filters and/or a concave lens, was measured by observing the intensity of the transient triple-state absorption for naphthalene triplet produced by sensitization of a deaerated hexane solution of 5 mM benzophenone and 0.1 M naphthalene3J0 at 355-nm excitation or for the luminescent excited state of Ru(bpy),zf in an aqueous solution of tris(bipyridine)ruthenium chloride" at 450 nm. The observed concentration of the excited states can then give, via a calibration curve, the concentration of absorbed photons for the solution having an absorbance of 0.1 in a 0.5-cm sample cell. According to this definition, a photon concentration of 10" M at 355 nm is produced by an energy flux of 0.7 mJ/cm2. Upon excitation by the laser pulse, a transient absorption at 397 and 606 nm, attributable to MV", was produced without time delay. By using a 30-ps pulse of a mode-locked laser, the growth time of the 606-nm absorption was found to be shorter than the time resolution of the detector system (ca. 2 ns). This observation With a highis consistent with the reported values for kc.7,12,13 intensity excitation flash, transient absorptions at 368 and 530 nm attributable to methylviologen radical dimer ( MV'+)22+were also observed. These transients decay, with a lifetime of about 100 ns, indicating the bleaching of (MV'f)2 by the photoinduced valence-band h01es.I~ The quantum yield for the photoinduced reduction of viologen, calculated from the reported molar extinction coefficient of MV" (13700 M-' cm-' at 606 nmI5), is plotted in Figure 1 for each excitation wavelength. The data points shown comprise experimental results for colloidal CdS solutions from several different batches. Reproducibility from different batch-to-batch preparations was excellent with variance between these solutions always being less than 10% of observed values. The quantum yield increased upon decreasing the photon concentration, indicating (10) Bensasson, R.; Land, E. J. Trans. Faraday SOC.1971,67, 1904-1915. ( 1 1 ) Creuz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N . J . A m . Chem. SOC.1980,102, 1309-1319. Kalyanasundaram, K. Coord. Chem. Reo. 1982, 46, 159-244. (12) Albery, W. J.; Brown, G. T.; Darwent, J. R.; Saievar-Iranizad, E. J . Chem. SOC.,Faraday Trans. 1 1985, 81, 1999-2007. (13) Rossetti, R.; Bech, S. M.; Brus, L. E. J . A m . Chem. SOC.1984, 106, 980-984. Rossetti, R.; Brus, L. E. J . Phys. Chem. 1986, 90, 558-560. (14) Nakahira, T.; Graetzel, M . J . Phys. Chem. 1984, 88, 4006-4010. (15) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86,2617-2619.
0 1986 American Chemical Society
Nosaka and Fox
6522 The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 extinction coefficient, [photon]:
' A
A ,
t,
and from the photon concentration
No = g(t) dt = 1.004 .n
Q
L
IC'
_
_
-
4
L
.
U
,
,
1
,
I
.
L
'0
1 0 6 At is realized. Then the only unknown parameter is k,/k,. The solid curve superimposed on the data points shown in Figure 1 is calculated with k,/k, of 8.8 X cm3. By adopting the reported value for k , (( 1.0 f 0.2) X lo9 s-l),' k, is obtained as (9 f 4) X 10-l' cm3 s-l. This reasonable value for electron-hole recombination on CdS thus and Si (5 X lies between those reported for T i 0 2 (3.2 X 10-9).17
These reaction rate constants can then be used to simulate the dashed curve shown in Figure 1 if the extinction coefficient at 450 nm is employed. Therefore, the rate data from the two experiments is self-consistent and the observed dependence of the quantum yield on excitation wavelength can be solely attributed to the difference in the extinction coefficients at the two wavelengths. Graetzel and co-workers6 claimed, at very low light intensities, that electron-hole recombination on colloidal TiO, becomes first order. However, in the present study, we calculate that photoexcitation produces about one electron-hole pair per particle (1.2 pairs/particle at a photon concentration of lo-' M) and still find good agreement for second-order electron-hole recombination on colloidal CdS, even at a low incident light intensity. Since in the present study recombination competes with electron transfer in the same colloidal particle, second-order kinetics seem applicable. Acknowledgment. Support of this work from the U S . Department of Energy, Fundamental Interactions Branch, is gratefully acknowledged. We are also thankful to the Japan-US. Program of Cooperation in Photoconversion and Photosynthesis for fellowship support for Y.N. The use of the equipment at the Center for Fast Kinetics Research, a facility supported by the National Institutes of Health and by the University of Texas at Austin, is greatly appreciated. (17) Landsberg, P. T.; Kouski, G. S. J . Appl. Phys. 1984, 56, 1696-1700.
