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J. Phys. Chem. 1980, 84, 1060-1061
trolyte-plus-salt solution.2 This leads indeed to a negligible value of 6, less than 1%. If, instead of (2), we propose following values, estimated from experimental data on the investigated polyelectrolyte4J
k, = 0.14
(6) M. Adam, M. Delsanti, and G. Pouyet, J. Phys. Leff., 40, L-435 (1979). (7) J. Roots, B. Nystrom, L. 0. Sundelof, and B. Porsh, Polymer, 20, 337 (1979). (8) M. Stephen, J . Chem. Phvs.. 55, 3878 11971). (9) M. Stephen, J . Chem. Piys., 61, 1598 (1974j. (10) M. Nagasawa, I. Noda, T. Takahashi, and N. Shimamoto, J . Phys. Chem.. 76. 16 (1972). (11) Y. Suzuki, I.’N&, and M. Nagasawa, J. phys. Chem.,73, 797 (1969).
kl = 0.40
(3) then one obtains 6 = -0.23. Hence, we agree with Dr. Manning to claim that, for the present analysis, a departure from the Debye-Huckel approximation cannot alone explain the massive discrepancy between apparent and effective polyion valences. Another important aspect of our Discussion1concerns the polymer concentrations used in our experiments. Let us clearly state that, in our work, both polyion transport parameters (u3and D3) were not obtained in the same concentration regime. Self-diffusion data belong to the dilute regime and electrophoresis data to the semidilute regime (C, = 0.5%), because we were not able to attain, in electrophoretic light scattering, concentrations below Cp*, the concentration at which polymer chains start to overlap. For high-molecular-weight polyelectrolytes (M,,, k lo6), Cp* is very low, typically less or much less than 0.1% (weight percent) depending on polyion ionization and salt concentration. This has two consequences. (i) In the field of high-molecular-weightpolyelectrolytes almost all published results on electrophoresis were obtained in the semidilute regime, where transport coefficients are molecular-weight independent, but become concentration dependent,‘j a result which reasonably explains the different power laws we observed for the polyion friction coefficients. (ii) The experimental u3values might be underestimated with respect to their dilute regime values, because of increased friction with the environment, if we refer to experiments performed on neutral polymer^.^ For example, with the polystyrene/toluerie system at 25 “ C ( M , = 110000) the polymer friction coefficient is increased by a factor of 1.5, when the concentration rises from 0 to 0.5% This effect probably contributes to lower u3 and thus z ~ ~ P Finally, let us point out that the observation of low apparent valences could also be due to overestimated polyion self-diffusion coefficients. The spectrum of light scattered by charged macromolecules is indeed complex, and the first theoretical analysis of this difficult subject has been given by Stephen,@ for the model of a rigid rod. However, Stephen’s theory is only valid if the Debye shielding length is much larger than the interparticle spacing, a hypothesis which does not apply under excess of salt conditions, for a dilute polymer solution. Thus, a complete theory of the charge effects on light scattering is not yet available. In any case, if quasi-elastic light scattering only measures an apparent self-diffusion coefficient of the polyion, this cannot provide the key explanation for our results, for we also observed very low 2sap values with data published by Nagasawa et ala1O9l1who used quite different experimental techniques. At the outset of this comment we could agree with Dr. Manning in stating that there is yet no clear-cut explanation for our experimental observations, which need to be confirmed.
Centre de Recherches sur les Macromol6cules 6 7083 Strasbourg-Cedex, France Received September 18, 1979
Study of the Photochemical Reaction of Sensitizing Dyes Adsorbed on Semiconductor Powder by Means of Photoacoustlc Spectroscopy
.’
References and Notes J. P. Meullenet, A. Schmitt, and M. Drifford, J . Phys. Chem., 83, 1924 (1979). A. Schmitt, J. P. Meullenet, and R. Varoqui, Biopo/ymers, 17, 1249 (1978). A. Schmitt, R. Varoqui, and J. P. Meullenet, J . Phys. Chem., 81, 1514 11977). J. P. Meullenet, A. Schmitt, and R. Varoqui, J . Phys. Chem., 83, 2603 (1979). J. P. Meullenet, These de Doctorat d’Etat Strasbourg, 1979.
Adrlen Schmltt
.
Sir: Spectral sensitization of semiconductorsby a dyestuff is a very interesting phenomenon in photochemistry and photographic science. Though a large number of investigations have been made on spectral sensitization of the conventional photographic system112and of the electrochemical system of semiconductor electrodes: few studies have been carrier out by using in situ observation of a sensitizing dye after excitation at a semiconductor surface, which is expected to afford useful information on sensitization mechanisms. The present research applies the technique of photoacoustic spectroscopy to investigations of the photochemical reaction of sensitizing dyes adsorbed on semiconductor powder as an extension of our previous work by using this technique to observe spectral sensitization processes at the semiconductor electrode-dye solution interfaces4 ZnO powder was mainly used as the semiconductor. chloride were Rhodamine B and l,11-diethyl-2,2’-cyanine chosen as the spectral sensitizing dye because they are well-known dyes in the photographic and photoelectrochemical systems. The sensitizing dyes were allowed to adsorb on ZnO powder in their aqueous solution. After the solution was centrifuged, ZnO powder with the dyes was dried by vacuum drying and kept in a desiccator. Measurements of photoacoustic spectra of the dyes adsorbed on the surface of the semiconductor powder were carried out with a single-beam photoacoustic apparatus made in this laboratory. This consists of a 500-W xenon lamp, monochromator (Japan Spectroscopic Co. Ltd., Model CT-30F), a light chopper, a specially designed closed cell with a condenser microphone (Bruel and Kjaer Ltd., Model 4144),5and a lock-in amplifier/preamplifier (NF Co. Ltd., Model LI-574). The light beam was modulated at a frequency of 80 Hz. In order to cause the photochemical reaction before measurement of the photoacoustic spectra, we irradiated the sensitizing dyes adsorbed on the ZnO powder with light from a 500-W xenon lamp. The wavelength of the excitation light was longer than 500 nm to eliminate the intrinsic absorption of the ZnO powder. All measurements and preparation of samples were carried out in a dark room. The photoacoustic spectra of some commercial ZnO powder and l,l’-diethyl-2,2’-cyanine chloride adsorbed on ZnO are shown in Figure 1. These spectra have been normalized by dividing the experimental spectra by the power spectra of the lamp in order to remove the structure of the light source. The photoacoustic spectrum of 1,l’diethyl-2,2’-cyanine fits its well-known absorption spec-
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J. Phys. Chem. 1980,84, 1061-1063
ation. There was no change in the spectra when nonsemiconducting solid materials such as A1203were employed. These results seem to support the theory that the excited dyes can inject an electron into the conduction band of the semicond~ctor~-~ and that the oxidative N-dealkylations of dyes are preceded on the semiconductor powders by radical cation f ~ r m a t i o n .Further ~,~ investigations are now being undertaken in an attempt at direct detection of the radical cation’s spectrum by this technique, as this could easily solve the problem of in situ observation of the sensitizing dyes adsorbed on semiconductor powders because of its high sensitivity and of its other distinctive features.6J0
Acknowledgment. The authors gratefully thank Drs. Tsuguo Sawada and Akira Fujishima for their valuable suggestions.
Wavelength ( nm )
Figure 1. Change in the photoacoustic spectra on irradiation (A >500 nm) of l,l’diethyl-2,2’-cyanine chloride adsorbed on ZnO powder: (1) ZnO powder; (2)before irradiation; (3)30-s irradiation; (4)90 s; (5)
270 s; (6)870 s.
References and Notes (1) S. Dahne, fhotogr. Sci. Eng., 23, 219 (1979). (2) W. West and P.8. Gilman, “The Theory of the PhotographicProcess”, 4th ed,T. H. James, Ed., Macmillan, New York, 1977,pp 251-290. (3) H. Gerlscher and F. Wlllig, Top. Curr. Chem., 61, 31 (1976). (4) T. Iwasaki, T. Sawada, H.Kamada, A. Fujishima, and K. Honda, J .
fhys. Chem., 83, 2142 (1979). (5) T. Sawada and M. Yamamoto, Bunseki, 172 (1979). (6) 0. W. Byers, S. Gross, and P. M. Henrichs, Phofochem. Photobiol., 23,37 (1976). (7) T. Watanabe, T. Takizawa, and K. Honda, J. fhys. Chem., 81,1845 (1977). (8) T. Takizawa, T. Watanabe, and K. Honda, J. Phys. Chem., 82, 139t (1978). (9)T. H. James, fhotogr. Sci. Eng., 18, 100 (1974). (10) A. Rosencwaig, Anal. Chem., 47, 592A (1975). Department of Industrial Chemistry.
450
500 Wavelength (
550
600
nm )
Flgure 2. Change In the photoacoustic spectra on irradiation (A > 500 nm) of rhodamine B adsorbed on ZnO powder: (1) before irradiation; (2) 5-s irradiation; (3)15 s; (4)30 s; (5)45 s; (6)60 s; (7)90 s;
Departments of Synthetic and Industrial Chemistry Faculty of Engineering The University of Tokyo Hongo, Bunkyo-ku, Tokyo, Japan
Tamotsu Iwasaki Shohel Oda”’ Hltoshi Kamadat Kenlchl Honda
Received December 31, 1979
(8)250 s; (9)430 s; (IO) 1030 s.
trum closely. Signal maxima at around 525 and 575 nm are due to the molecular state and the J aggregate of this dye, respectively.2 The photoacoustic signal decreased with exposure to the excitation light without any appreciable shift of the spectrum, which may be related to a rather drastic decomposition of the dye chromophores.6 Figure 2 shows changes in the spectra of rhodamine B on ZnO with exposure to light. The maximum of the spectra exhibited a gradual hypsochromic shift in the course of exposure, which is considered to be due to the occurrence of an efficient N-deethylation of the dyea7pg N-Deethylated compounds such as N,N,N’-triethylrhodamine, N,N’-diethylrhodamine, N-ethylrhodamine, and rhodamine have been chromatographically detected during the photochemical reaction of CdS-suspended aqueous solution of rhodamine B (N,N,N’,N’-tetraethylr h ~ d a m i n e ) .In ~ general, the rate of disappearance of the 510-nm maximum species was negligibly small compared with that of the spectral shift from 565 to 510 nm. The injected electron from the excited dye into the conduction band of ZnO must be subsequently removed by interaction with oxygen (or oxygen and water) to form 02-, which subsequently reacts with other electron acceptor^.^-^ Similar changes of the spectra of the sensitizing dyes were also observed by using other semiconductor powders such as Ti02, CdS, AgC1, and AgBr instead of ZnO, or by exciting the semiconductor powders with ultraviolet irradi-
A Theoretical Study of the Site Selectivity of the Zeolite Cation. 3. Change in Site Selectlvlty of the Zinc(I1) Ion with the Degree of Ion Exchange in Dehydrated Zn( 1I)K-A Publlcation costs assisted by Hokkaido University
Sir: Zeolite A contains 12 monovalent cations per unit cell. These cations can be equivalently exchanged by various others. The kind and distribution of cations introduced into the crystal are the primary factors controlling the adsorptive and catalytic properties of zeolite A. It has been reported by Takaishi et al.’ that ZnK-A shows some peculiar properties when fully Kt ion-exchanged zeolite A, K12-A, was exchanged with Zn2+ions to a certain extent. The exchange isotherm of Zn2+ion for K+ ion in zeolite A has a distinct step at about 60% exchange. The isobaric adsorption of SiH4rises at 67% exchange. In practical use, Zn,Kl2-&-A, 2 5 x 5 4, activated at 400 “C effectively sorbs trace amounts of the principal impurity, PH3,from SiH4 The silane thus refined can be used in the preparation of silicon for semiconductor with ultrahigh purity. In order to explain these properties, Takaishi assumed that the fifth and sixth Zn2+ions probably occupy the eight-membered oxygen ring sites (sites 11), despite the presence and ready availabilit,yof vacant six-membered oxygen ring sites (sites I) preferred in the initial stage of ion exchange. This
0022-3654/80/2084-1061$01.00/00 1980 American Chemical Society
