J. Phys. Chem. 1984, 88, 4906-4907
4906
In experiments with y-ray irradiation, Decker and Mayo2 have found that hydroperoxides were the major oxidation product (66 mol %). their experiments the dose rate was low and the irradiation had to be carried out for long periods (e.g., 3 days). With electron beam irradiation, however, the fraction of R 0 2 H was found to be only 10 mol %. In this case the same total dose was achieved in 20 min.* It seems to us that hydroperoxide could be formed in the reaction ROy H. ROZH (8) This reaction could effectively compete with the hydrogen recombination reaction
+
+
He
+ He
+
H2
(9)
as well as with reaction 2 during the long irradiation periods of the V a y experiments* Acknowledgment. We thank Jacqueline M. Harmon, Robertine M. Gale, and David s. Rice for their dedicated assistance and John W. Mayfield for his encouragement and support. This work was sponsored by the Hercules Polypropylene Business Center, which is now part of HIMONT Incorporated. We thank HIMoNT for the paper for publication. Registry No. PP (homopolymer), 9003-07-0;oxygen-17, 13968-48-4.
Optimlzation of the Donor Density of a Semiconductor Electrode for Enhancement of the Quantum Yleld of the Dye-Sensitized Photocurrent Masashi Nakao,* Kiminori Itoh, Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan
and Kenichi Honda Division of Molecular Science, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo- ku, Kyoto 606, Japan (Received: July 3, 1984)
Photoelectrochemical and fluorescence measurements were performed to elucidate the mechanism of dye sensitization of a semiconductor electrode. The quantum yield of the dye-sensitized photocurrent (q) and the influence of a reducing agent on it varied remarkably with the donor density of the semiconductor electrode.
Electrochemical spectral sensitization by dyes adsorbed on semiconductor electrode surfaces is of interest as a model for solar energy conversion into electrical or chemical energy.'-5 In the photosynthetic and photocatalytic fields many workers have tried to attain a high-energy conversion efficiency by utilizing the elctrochemical spectral sensitization process. Systems having moderate photocurrent quantum yields, q, have been reported, e.g., 0.09 for the senitized photocurrent from chemically modified , ~ 0.22 for rose bengal (RB) on rhodamine B (RhB) on S ~ I O and Z ~ I O .We ~ have already pointed out that these q's are much smaller than the q expected from the data for fluorescence intensity and/or lifetime measurements on solid surfaces, including those of semi~onductors.6*~ Thus the q value for the RhB-Sn02 system is 0.4-0.7. Here we report the enhancement and the optimization of q by regulation of the donor density in n-type semiconductor substrates. We find that the donor density is an important factor for charge separation at the solid-solution interface. Transparent thin films of SnO, and ZnO were used as semiconductor electrodes. They were deposited on indium-tinoxide-coated glass plates either by vacuum deposition or by spray pyrolysis. Their donor density ( N d )was controlled by S b or F doping in SnO, and by In doping in ZnO. The carrier density was determined from Mott-Schottky plots. The at-band potentials were around -0.5 V vs. Ag/AgC104 f y he S n 0 2 electrodes and -0.8 V for the ZnO electrodes. T& photocurrent due
tO
(1) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31. (2) Tributsch, H.; Calvin, M. Photochern. Phorobiol. 1971, 14, 9. (3) Fujihira, M.; Osa, T.; Hursch, D.; Kuwana, T. J. Electroanal. Chem. 1978, 88, 285. (4) Matsumura, M.; Nomura, Y.; Tsubomura, H. Bull. Chem. SOC.Jpn. 1977, 50, 2533. (5) Spitler, M.; Calvin M. J. Phys. Chem. 1977, 67, 5193. (6) Itoh, K.; Chiyokawa, Y; Nakao, M.; Honda, K. J. Am. Chem. SOC. 1984, 106, 1630. . (7) Nakao, M.; Itoh, K.; Honda, K. Denki Kagaku 1984, 52, 378.
0022-3654/84/2088-4906$01.50/0
to the dye sensitization was measured by the conventional potentiostatic method.8 Pt and Ag wires were employed as counterelectrodes and reference electrodes, respectively. NaClO, (0.1 M) in acetonitrile was used as electrolyte. RhB and RB were used as photosensitizers and 0.01 M hydroquinone was added as a reducing agent (supersensitizer). Figure 1 shows the action spectra of the q's for the four systems. The value of q was calculated by using the following equation: q = iJeZ(1 -
where i , is the sensitized photocurrent (A), e is the elementary C), Z is the incident photon number, and charge (=1.6022 X Abs is absorbance of the adsorbed dye. At optimum Nd, q for the RB-Sn02 system appeared to be almost unity. The steadystate photocurrent reached ca. 20 p A under the following conditions: Z = 5.5 X 10l5photons s-l at 560 nm (500-W monochromatic light from a Xe lamp), lo4 M dye concentration (the surface dye concentration was 0.010 in absorbance, Abs in eq 1, consequently ca. 2.3% of incident light is absorbed). Other systems also showed about a threefold enhancement compared with conventional ones. The observed photocurrents did not change during the measurements because of the effective supersensiti~ation.~~~,~ As Figure 2 shows, the optimum Nd for the S n 0 2 system was about 4 X lozo~ m - ~A ,quite large photocurrent was observed for RhB as well as for RB at this Nd. Note that q 0.1 at Nd 1020cm-3 in the system of RhB-SnO, corresponds well to that reported previously for the same ~ y s t e m .On ~ the other hand, the optimum Nd for ZnO appeared to be around lo'* ~ m - ~ . The photocurrent caused by direct excitation of the semiconductor electrode varies with the Nd of the semiconductor,1° because
-
-
(8) Nakao, M.; Watanabe, T.; Itoh, K.; Fujishima, A,; Honda, K. Ber. Bunsenges. Phys. Chem. 1984, 88, 17. (9) Mesmaeker, A. K.; Dewitt, R. Electrochim. Acta 1981, 26, 29.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 4907
Letters
t/
=0.01
/
1019 ..
A / nm Figure 1. Action spectra of q for the four sensitization systems: RBSnOz (a), RB-ZnO (b), RhB-Sn02 (c), and RhB-ZnO (d). [RhB] = M, [RBI = lo4 M, and measured at +0.6 V vs. Ag wire, corresponding to about 1 V anodic bias from the flat-band potential.
of the well-known relationship between the light penetration depth and the thickness of the space charge layer, which depends on Nd. In addition, the charge separation efficiency also depends on the electric field of the space charge layer. In the case of spectral sensitization only the charge separation argument applies. One can quantitatively discuss the Nddependence of q, shown in Figure 2, by analyzing the effect of the electric field. We consider two competing processes, namely, charge separation and recombination. k, and k, are the respective rate constants. The electric field a t the electrode surface is proportional to the square root of Nd, thus k, 0: Nd1l2.Therefore, the relationship is q
a
k,/(k, i- ak,) = Nd1/2/(Nd’/2 i- a’k,)
(2)
where a and a’are constants. Choice of appropriate values for a’and k, results in the two dashed lines in Figure 2. With this simple simulation we can deduce the tendency of increasing q in the RhB-Sn02 system, but the steep increase of q remains to be clarified. As revealed by the present results, Nd is one of the factors affecting q. The Nd’semployed in the previous reports were not optimum for the spectral sensitization. For instance, S n 0 2 (10) Gautron, J.; Lemasson, P.; Marucco, J. F. Faraday Discuss. Chem. Soc. 1980, 70,81.
1O2O
Nd /
102’
Crn-3
Figure 2. Dependence of q on Nd of SnOz. Nd = 3.5 X 1020 cm-3 corresponds to curve c in Figure 1, and measured under same conditions as in Figure 1. Two simulation curves for eq 2 are shown by dashed lines.
-
transparent electrodes of Nd lo2’ cm-3 have been generally employed, mainly because they are commerically obtainable. The large dependence of q on Ndmay be interpreted by considering the decay process of electrons injected from the excited dye through possible surface states. Basically, the increase in Nd causes an increase in the electric field of the space charge region of the semiconductor substrates and hence suppresses these decays up to the optimum Nd. The electron mobility of ZnO is a few times larger than that of Sn02.” Therefore, the electric field strength necessary to separate electrons from the electrode surface may be much smaller than that of Sn02. This is qualitatively reasonable when considering that the optimum Ndfor ZnO was ca. 10l8 ~ m - ~ . The decrease in q for Nd is larger than the optimum Nd.This may be attributed to a leakage of the conduction electrons by tunneling through the thin space charge layer. The details of this mechanism remain, however, still to be clarified. Furthermore, these results suggest that a further increase in q can be expected by controlling the structure of the space charge region. For instance, a composite semiconductor structure consisting of a quite thin layer (
