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Observation of Semiconductor Electrode-Dye Solution Interface by Means of Fluorescence and Laser-Induced Photoacoustic Spectroscopy Tamotsu Iwasaki,+ Tsuguo Sawada, * Hltoshl Kamada, Aklra Fujishima, and Kenlchl Honda Department of Synthetic Chemistry, Facuny of Engineering, The Unlversity of Tokyo, Hongo, Bunkyo-ku, Tokyo (Received February 20, 1979) Publication costs assisted by The University of Tokyo
In situ observation of a spectral sensitizing dye at a semiconductor electrode-solution interface was attempted by means of fluorescence and laser-induced photoacoustic spectroscopy. Dependences of fluorescence lifetime, fluorescence intensity, photoacoustic signal of the dye, and the sensitized photocurrent on the voltage applied to the SnOz electrode were measured. Direct evidence that the applied voltage affected a recombination process of electrons in the SnOz conduction band with the oxidized dye molecules at the electrode surface was obtained.
Introduction Spectral sensitization of semiconductors has been extensively studied by means of electrochemical techniques in recent years. Use of these techniques has provided much information on the spectral sensitization such as the correlation between energy level of dyes and that of the conduction band of the s e m i c o n d ~ c t o r , ' ~the J ~direction ~~~ of sensitized photocurrent3lZJ6and the effects of coexisting s u b ~ t a n c e s . ' - ~ However, J ~ ~ ~ few studies have been carried 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 paper discusses in some detail problems and results of our attempts to observe a region immediately at the semiconductor electrode-dye solution interface by means of fluorescence and laser-induced photoacoustic spectroscopy. Combination of these two spectroscopic methods is considered to be one of the most effective ways to obtain useful information, because a fluorescence signal and a photoacoustic signal are complementary since absorbed light energy by the dye must appear either radiatively in the fluorescence signal or nonradiatively in the photoacoustic signal. An optically transparent electrode made of SnOz was chosen as the semiconductor not only because it is suitable for internal reflection spectroscopic observation of electrode-solution interface,22but also because the characteristics of spectral sensitization of SnOz are well kno~n.l~-~l Experimental Section Fluorescence and photoacoustic signal measurements of a sensitizing dye were made with the experimental setup depicted in Figure 1. Fluorescence lifetime and intensity of the dye were measured by an apparatus made in this l a b o r a t ~ r y .This ~ ~ consists of a Nzflash lamp with duration half-time of 1 ns and a time-correlated single-photoncounting detection system. Light flashed at a frequency of 30 kHz was incident on a tin-oxide-coated prism at 75O, an angle of incidence which attained total reflection at the electrode-dye solution interface.22The wavelength of the irradiating light was longer than 470 nm to eliminate the intrinsic absorption of SnOz. This was achieved by using a Toshiba cutoff filter V-47. The wavelength of fluorescence detection was around 530 nm with the use of an interference filter (Koshin Kogaku Co. Ltd.). The N
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fluorescence spectra were measured by varying the monocromator wavelength of this apparatus after the N2 flash lamp was changed to a Dz lamp. The tin-oxide-coated glass electrode was obtained from Mitorika Glass Co. Ltd. The SnOz layer, about 2200 A thick, was coated on an optically flat trapezoidal quartz prism. The prism was cemented to an electrolyte glass cell and a piezoelectric ceramic with an epoxy resin so that the tin-oxide coated surface was in contact with an electrolyte solution, An Ag/AgCl reference electrode was placed in the cell, and a platinum wire served as the counter electrode. Bias potential to the SnOz electrode was controlled by a potentiostat. The immersed electrode area was about 14.5 cm2 and the cell contained approximately 20.5 mL of solution. Sodium fluorescein, purchased from Wako Junyaku Co. Ltd., was used as a sensitizing dye because of its high fluorescence efficiency. The electrolyte solution was made with distilled water and contained lo-* M dye with 0.1 N KNOB. Measurements of the photoacoustic signal were carried out after the N2 flash lamp was changed to an argon ion laser (Spectra Physics Model 164-03),operating in a single line mode of 488 nm. The laser beam was modulated at a frequency of 185 Hz by a light chopper and had an angle of incidence on the tin-oxide-coated prism at 75". The pressure fluctuation induced in the dye solution by absorbed radiation was detected by a semicylindrical piezoelectric ceramic (NPM, N-21 supplied by Tohoku Kinzoku Co. Ltd.).24A lock-in amplifier/preamplifier (NF Co. Ltd., Model LI-574) was used to amplify the modulated output signal. The piezoelectric ceramic acts simultaneously as a part of the electrolyte cell and a pressure sensor. The cell was placed inside an airtight chamber which was secured to a vibration-free stand to prevent pressure fluctuations caused by vibrations from external sources. Measurements of the photocurrent of the SnOzelectrode sensitized by sodium fluorescein were carried out with an instrument described elsewhere.20This consists of a 500-W xenon lamp, a Shimazu monochromator, and a Keithley picoammeter Model 417. The electrode-solution interface was irradiated through a collecting lens under conditions similar to those of fluorescence and photoacoustic signal measurements. Changes of the sensitized photocurrent were also measured simultaneously with photoacoustic signal measurements under argon ion laser irradiation. All the electrolyte solutions for fluorescence and photocurrent measurements were deoxygenated by bubbling 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 16, 1979
Observation of Semiconductor Electrode-Dye Interface Photoacoustic Signal
i
Fluorescence Measurement
1.5-
Measurement
/\Argon
I o n Laser
Flgure 1. Block diagram of the apparatus for the fluorescence and photoacoustic signal measurements: PM, photomultipier (RCA 8850); CFTD, constant fraction timing discriminator (Ortec 453); TAC, time to amplitude converter (Ortec 437A); FD, fast discriminator (Osaka Denpa MPS-1237); MCPHA, multichannel pulse height analyzer (Naig 0-161, D-171); SCPHA, single channel pulse height analyzer (Osaka Denpa MPS-1233). N
E +
q
? -1.0
0 5
C
F
a c K 0
Q
Flgure 2. Current-potential curves for the SnO, electrode in 0.1 N KNOB aqueous solution with M sodium fluorescein: (a) in the dark (solid curve); (b) under 500-W xenon lamp irradiation at 490 nm (broken curve).
with purified nitrogen during the measurements. The same procedure was carried out for at least 30 min prior to the photoacoustic signal measurement, and then a stopcock was closed in order to keep the cell oxygen-free for this measurement.
Results and Discussion Electrochemical properties of the S n 0 2semiconductor electrode are well known,25and it has shown distinctive advantages such as optical transparency, near metallic conductivity, high oxygen overvoltage, chemical durability, and excellent mechanical stability in photoelectrochemical ~tudies.'~-~OFigure 2 shows typical current-potential curves for the SnOz electrode in the dark and the potential dependence of a photocurrent sensitized by sodium fluorescein. Action spectra of the sensitized photocurrents with different applied voltages are shown in Figure 3. Owing to a large forbidden gap (3.7 eV)25and a small thickness (2200 A),the background anodic photocurrent due to the intrinsic absorption of SnOz was negligibly small in this spectral region and had a measurable value at wavelengths below 400 nm. The sensitized photocurrent was observed under potential-controlled conditions in the
0 4 V vs. Ag/AgCI
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The Journal of Physical Chemistry, Vol. 83,No. 16, 1979
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Figure 4. Potential dependence of the fluorescence intensity at the SnO, electrode-dye solution interface: (a) M sodium fluorescein in 0.1 N KN03 aqueous solution (solid curve); (b) lob4 M sodium fluorescein with IO-* M hydroquinone (broken line). Numbers from 1 to 9 on the solid curve are the order in which the measurements are taken.
creased, and it recovered when the voltage decreased. However, this potential dependence disappeared when reducing agents such as hydroquinone were added, as shown in Figure 4. As the fluorescence lifetime rf for the dye at the S n 0 2 electrode-solution interface is written as 1 7f = kf + ki, + kist + lz, where kf is the natural radiative rate constant, kic and kisc are the rate constants for the intramolecular radiationless processes of internal conversion and intersystem crossing, and k , is the rate constant for sensitization of the S n 0 2 electrode, the fluorescence lifetime's independence from the applied voltage may be due to the fact that the rate constant for sensitization k, is not affected by the applied voltage or to the fact that the quantumn efficiency for generation of the sensitized photocurrent is not very great. The latter may be the main cause in this experiment, since the quantumn efficiency estimated by measurements of the sensitized photocurrent is less than 0.03. On the other hand, the decrease in the fluorescence intensity just at the electrode-solution interface when the applied voltage increased must be due to the decrease in the dye concentration at the interface. This is supported by the fact that the dependence of the fluorescence intensity on the applied voltage disappeared on addition of hydroquinone, since hydroquinone could immediately regenerate the dye from oxidized dye molecules.1° The potential dependence of the sensitized photocurrent in Figure 3 correlates very closely with that of the fluorescence intensity in Figure 4,and it can be interpreted that the applied voltage affects a recombination process of electrons in the §noz conduction band with the oxidized dye molecules at the electrode surface. Electron injection from excited dye molecules in a spectral sensitization process leads at once to the formation of oxidized dye molecules and electrons at the crystal surface.lJ0S2' As anodic polarization gives bending of the S n 0 2conduction band in the space charge layer to promote immediate migration of injected electrons into the bulk of the crystal,27the sensitized photocurrent increases when the applied voltage increases. On the contrary, recombination of the oxidized dye molecules with electrons at the crystal surface decreases when the applied voltage increases, and this may lead to a decrease of the fluorescence intensity
Flgure 5. Potential dependence of the photoacoustic signal of sodium fluorescein M) at the SnOp electrode-solution interface In 0.1 N KN03 aqueous solution at 488 nm.
of the dye. The photocurrent due to recombination of oxidized dye molecules with electrons in the conduction band was observed by Pettinger et ala2with a highly doped ZnO electrode, rhodamine B, and the light pulse of an argon ion laser. They measured the transient behavior of the sensitized anodic photocurrent and simultaneously observed the cathodic transient current due to the reduction of the oxidized dye molecules by tunneling conduction electrons, which disappeared on addition of hydroquinone. Our results seem to very closely parallel this result, because similar tunneling effects may occur in the S n 0 2 electrode.2s Decrease in the dye concentration just a t the electrodesolution interface when the applied voltage increased was also measured by the laser-induced photoacoustic absorption technique, as shown in Figure 5. The photoacoustic signal of the dye decreased when the applied voltage increased, and it recovered when the voltage decreased. On the contrary, simultaneous measurement of the sensitized photocurrent under laser irradiation showed that the photocurrent increased when the applied voltage increased. The potential dependence of the photoacoustic signal disappeared on addition of hydroquinone. These results may also support the interpretation mentioned above, though a quantitative estimate has not yet been completed. Therefore, it can be expected that the laser-induced photoacoustic absorption technique will produce useful information in photoelectrochemicalstudies because of its high sensitivity and of its other distinctive features.24 Direct detection of the oxidized dye molecules' spectrum a t the electrode-solution interface by this technique during measurements of the sensitized photocurrent will give more certain evidence, which could not be obtained at this stage of the investigation. As the measurements of fluorescence and photoacoustic signals by the time-correlated single-photon-counting detection system and the laser-induced photoacoustic spectrometer can be carried out under a condition of one total reflection at the electrode-solution interface because of their high sensitivity, they can be applied to other semiconductor electrodes even of small size. Though the fluorescence lifetime of the dye did not change with the applied voltage variations made in this study, it still does not rule out the possibility that the rate constant for sensitization of other semiconductor electrodes may be affected by the polarization condition of the electrodes. In the case that the quantum efficiency for spectral sensitization is high enough and energy level of the excited state of a sensitizing dye is just above that of the conduction band of a semiconductor, the fluorescence lifetime of the dye at the electrode-solution interface may change with the applied voltage variations. Measurement of fluorescence lifetime changes will be the subject of further
Relaxation Time of Hydrated Protein
investigations with other combinations of semiconductors and sensitizing dyes.
Acknowledgment. The authors are especially grateful t o Mr. Shohei Oda for some of the photoacoustic signal measurements and Dr. Tadashi Watanabe for his constructive suggestions. References and Notes (1) H. Tributsch and H. Gerischer, Ber. Bunsenges. Phys. Chem., 73, 251, 850 (1969); 72, 437 (1968). (2) B. Pettinger, H. R. Schoppel, and H. Gerischer, Ber. Bunsenges. Phys. Chem., 77, 960 (1973). (3) E. Daltrozzo and H. Tributsch, Photogr. Sci. fng., 19, 308 (1975). (4) H. Gerischer, Faraday Discuss., Chem. Soc., 58, 219 (1974); H. Gerischer and M. Lubke, Z. Phys. Chem. (Frankfurtam Main), 98, 317 (1975). (5) K. Hauffe, H. Pusch, and J. Range, Z. Phys. Chem. (Frankfurt am Main), 64, 122 (1969); U. Bode, K. Hauffe, Y. Ishikawa, and H. Pusch, ibid., 85, 144 (1973); K. Hauffe and 0. Haeggqwist, bid., 85, 191 (1973). (6) H. J. Danzmann and K. Hauffe, Ber. Bunsenges. Phys. Chem., 79, 438 (1975). (7) K. Hauffe and U. Bode, Faraday Discuss., Chem. Soc., 58, 281 (1974); K. Hauffe, Photogr. Sci. fng., 20, 124 (1976). (8) W. P. Gomes and F. Cardon, Ber. Bunsenges. Phys. Chem., 75, 914 (1971). (9) M. Matsumura, Y. Nomura, and H. Tsubomura, Bull. Chem. Soc. Jpn., 49, 1409 (1976). (10) A. Fujishima, T. Iwase, T. Watanabe, and K. Honda, J . Am. Chem. Soc., 97, 4134 (1975); T. Watanabe, A. Fujishima, and K. Honda,
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Ber. Bunsenges. Phys. Chem., 79, 1213 (1975). (11) T. Watanabe, A. Fujishima, 0. Tatsuoki, and K. Honda, Bull. Chem. Soc. Jpn., 49, 8 (1976). (12) M. T. Spitler and M. Calvin, J . Chem. Phys., 66, 4294 (1977). (13) W. D. K. Clark and N. Sutin, J . Am. Chem. Soc., 99, 4676 (1977). (14) T, Iwasaki, S. Sumi, A. Fujishima, and K. Honda, Photogr. Sci. Eng., 23, 1 (1979); T. Abe, S. Ohkouchi, M. Sukigara, and K. Honda, Nippon Kagaku Kaishi, 569 (1976). (15) H. Gerischer and H. Selzle, flectrochim. Acta, 18, 799 (1973). (16) R. Memming and H. Tributsch, J . Phys. Chem., 75, 562 (1971); R. Memming, Photochem. Phofobiol., 16, 325 (1972). (17) R. Memming, Faraday Discuss., Chem. SOC.,58, 261 (1974); M. Gleria and R. Memming, Z. Phys. Chem. (Frankfurt am Main), 98, 303 (1975). (18) H. Kim and H. A. Laitinen, J. Nectrochem. SOC.,122, 53 (1975). (19) T. Osa and M. Fujihira, Nature (London),264, 349 (1976). (20) T. Myasaka, T. Watanabe, A. Fujishima, and K. Honda, J. Am. Chem. Soc., 100, 6657 (1978). (21) H. Gerischer and F. Willig, Top. Curr. Chem., 61, 31 (1976). (22) W. N. Hansen, T. Kuwana, and R. A. Osteryoung, Anal. Chem., 38, 1810 (1966); N. Winograd, H. N. Blount, and T. Kuwana, J . Phys. Chem., 73, 3456 (1969). (23) T. Sawada and H. Kamada, Jpn. Anal., 22, 882 (1973). (24) S. Oda, T. Sawada, and H. Kamada, Anal. Chem., 50, 865 (1978); Proc. Jpn. Acad., 54, B 189 (1978). (25) H. Lerner, Photogr. Sci. fng., 13, 103 (1969); Z. M, Jarzebski and J. P. Marton, J . Electrochem. SOC.,123, 333C (1976). (26) W. West and P. B. Gilman, "The Theory of the Photographic Process", 4th ed, T. H. James, Ed., Macmillan, New York, 1977, pp 251-290. (27) V. A. Myamlin and Y. V. Pleskov, "Electrochemistry of Semiconductors", Plenum Press, New York, 1967. (28) F. Mollers and R. Memming, Ber. Bunsenges. Phys. Chem., 76, 469, 475 (1972).
Nuclear Magnetic Resonance Spin-Spin Relaxation Time in Hydrated Protein Powders. A Two Site Dynamic Exchange Model J. Raul Grlgerat Departamento de Biofisica, IMBICE, C.C. 403, 1900 La Plata, Argentina, and Unlversidad Nacional de La Plafa, La Plata, Argentina (Received October 6, 1978; Revised Manuscript Received February 12, 1979)
The NMR spinspin relaxation time of water in hydrated protein powder is analyzed by using a two-site dynamic exchange model. An expression is derived for the population fraction of stronger bound water molecules by using Guggenheim adsorption isotherm. The model is applied to the published experimental results from the NMR of lysozyme. A linear relationship is found between the spin-spin relaxation time of water fixed to primary binding sites and water content.
Introduction NMR spectra of water in hydrated protein powder show a single spin-spin relaxation time (T& Zimmerman and Brittin' have shown that two or more exchangeable spin populations with different dynamic states can give a single observable transverse relaxation time if a fast exchange condition is satisfied. The formulation of a detailed model for the dynamical behavior of water in hydrated protein presents serious difficulties. Even the simplest exchange model (two sites) that one can propose needs independent determination of, or, what is more serious, severe assumptions, on different parameters. The purpose of this work is to show how a two-state dynamical model can adequately describe the NMR transverse relaxation behavior of water in hydrated protein powder. The main point is the use of the population fraction of primary binding sites derived from the adsorption isotherm. The model is worked out by using Address correspondence t o this author at t h e Departamento de Biofisica, IMBICE, C.C. 403, 1900 L a Plata, Argentina
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published experimental data.
The Exchange Model The inverse of the spin-spin relaxation time can be expressed, assuming chemical fast exchange conditions, as l/TZobsd= Cp;/Tz,
(1)
1
where T2,is the spin-spin relaxation time corresponding to the ith state and p Lthe probability of finding a molecule in such a state (Le., the population fraction). For the case of a two state model, eq 1 is reduced to 1 / T Z O b 8 d = f/T2b + (1 - f ) / T 2 m (2) where Tzbis the relaxation time of the strongly bound molecules, with population fraction f , and Tzmis the time corresponding to the more freely adsorbed water ("multilayer" water). If Tzbremains constant with changes in water content we have a static model, otherwise we obtain a dynamic model. From the point of view of NMR experiments eq 2 has three unknowns: TZb,T,", and f . We will consider first
0 1979 American
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