J. Phys. Chem. 1991,95,3771-3779 B A
@
Figure 7. Two illustrated cases with the same surface area and the same quantity of adsorbed xenon; but A has a smaller pore space.
Consequently the inherent chemical shift, bo, reflects the surface functionality. Figure 6 also indicates that all the activated carbon samples give similar slopes in the 6 vs p plot, independent of the nitric acid treatment, the specific surface area, or the pore size distribution. Referring to this result, two situations can be considered as is shown in Figure 7. The two cases A and B in Figure 7 have the same total surface area and the same surface concentration of the adsorbed xenon. Compared with B, situation A may appear to have a greater xenon-xenon interaction term for 6 in eq 2 since the xenon atoms are likely to have more frequent collisions within the narrower space between the two surfaces. If so, the proporitonality constant 6, should depend on the average pore size, and therefore information on the pore size could be obtained from the slope of the 6 vs p plot. Our result seems to contradict such an assumption, and this can be rationalized by understanding that the main origin of the xenon-xenon interaction term in eq 2 is indeed the interaction between the adsorbed xenon atoms at the surface rather than the gas-phase xenon-xenon collision taking place within the pore volume; the gas-phase collision can occur
3771
much less frequently than the two-dimensional surface collision between the adsorbed xenon atoms, since the density of xenon in the adsorbed phase is much greater compared with the bulk gas phase and thus xenon exists most time as adsorbed on the surface wall. The pore size and the pore shape may also affect more or less the xenon-xenon interaction on the surface. More precise measurements of the chemical shift by using macroscopically more uniform samples will be necessary to study this seemingly small p dependence of 6,.
Conclusions The chemical shift of xenon adsorbed on activated carbons used in this work can be given as a sum of terms arising from the xenonsurface and the xenon-xenon interactions. At sufficientlv low xenon concentrations undertaken in this study, the chemicil shift or adsorbed xenon on similar amorphous materials may be plotted linearly with respect to the surface concentration of xenon which is the mole number of the adsorbed xenon per unit surface area. The intercept in such a plot of the chemical shift is sensitive to the nature of the surface since it reflects the xenonsurface interaction term. It is this term that makes the '29Xe N M R spectroscopy of the adsorbed xenon a powerful probe for the characterization of the amorphous carbon. It is, however, difficult to find any relation between the xenon-xenon interaction term and the specific surface area or the pore-size distribution under the same concentration of the adsorbed xenon in this study. This is due to that fact that the xenon-xenon interaction by the gasphase collision within the pore volume is probably small compared with the xenon-xenon interaction on the surface. The NMR spectrum taken at a given pressure may also be used as a fingerprint for the identification of the amorphous carbon, due to the difference in the chemical shift and the heterogenous line broadening of the 129XeN M R signal. Registry No. '29Xe,13965-99-6; Xe, 7440-63-3; carbon, 7440-44-0.
Photosensitized Electron InJectionfrom Xanthene Dyes Incorporated in Langmuir-Biodgett Films into SnO, Electrodes C.Biesmans, M. Van der Auweraer,* C. Cathy, D. Meerschaut, F. C. De Schryver, Laboratory for Molecular Dynamics and Spectroscopy, Chemistry Department, K . U.Leuven, Celestijnenlaan CZOOF, 3030 Heverlee, Belgium
W. Storck, and F. Willig Fritz Haber Institute, Faradayweg 4-6, 1000 Berlin 33, BRD (Received: September 25, 1990) Anodic photocurrents are obtained upon excitation of xanthene dyes incorporated into Langmuir-Blodgett films deposited on Sn02 The observed quantum yield of the photocurrent stronglydepends upon the applied electrode potential, the regenerator (hydroquinone)concentration, and the pH of the electrolyte. The photocurrent is nearly independent of the chemical structure of the dye or the amount of dye incorporated in the monolayer. For 3,6-bis(N-ethyl-N-octadecyl)aminoxanthyliumperchlorate (PYR18) and N,N'-dioctadecylrhodamine 119 (RH18), the photocurrent action spectra resemble the absorption spectra of the mixed monolayer or a dilute solution of the dye in chloroform. Interactions between the chromophore and the cadmium arachidate matrix shift the spectral sensitization maximum of the surface-active bis(N-ethyl-N-octadey1)rhodamine (RBI 8) to shorter wavelengths than the absorption maximum of the mixed monolayers or the action spectra of adsorbed rhodamine B. As observed for adsorbed rhodamine B the action spectra give no indication for the formation of sandwich dimers for RB18 and RH18. Analysis of Mott-Schottky plots obtained at low frequency indicates that decreasing the electrolyt pH leads to an anodic shift of the flat-band potential (E,) and that the electrode is only partially covered by the deposited Langmuir-Blodgett film.
Introduction The study of the photosensitized charge injection in wide bandgap semiconductors by adsorbed dyes has been initiated by the possible application to photoelectrochemical conversion of solar energy.14 These investigations were focused on systems with (1) Roberts, G . Sensors Acruarors 1984, 4, 131.
rhodamine B as the sensitizing dye and SnO, or ZnO as the semicondu~tor.~'~ Fromherz and Ardenma described the charge ( 2 ) Roberts, G . Adu. Chem. Ser. 1988, 218, 225. R. Electroanal. Chem. 1979, 11, 1. (4) Memming, R. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 353. (5) Memming, R.Far. Discuss. Chem. Soc. 1974,85. 261. (3) Memming,
0022-3654/91/2095-3771$02.50/00 1991 American Chemical Society
3772 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 injection into an In203electrode from a carbocyanine dye depasited on the electrode surface by the Langmuir-Blodgett method. This allowed a better control of the overall two-dimensional concentration and to some extent of the molecular orientation of the sensitizer. While Fromherz and Arden used cyanine dyes as sensitizer and allylthiourea as regenerator, most experiments with the adsorbed dyes were performed using rhodamine B and hydroquinone as, respectively, sensitizer and regenerator. To allow a better comparison of the behavior of adsorbed dyes and that of dyes incorporated in Langmuir-Blodgett films, the photosensitized injection of electrons into S n 0 2 by derivatives of xanthene dyes, which could be incorporated in Langmuir-Blodgett films, was investigated. To obtain more information on the relative efficiency of dye monomers and aggregated dye molecules,24the two-dimensional concentration of the dye in the LangmuirBlodgett film was varied from 5.0 X l o f 2to 1.6 X lo1, molecules/cm2. The determination of the electrode capacity as a function of the electrode potential2528 allows, under certain conditions, determination of the flat-band potential (Eb)and the density of the donors. Comparing the properties of an uncovered electrode surface and an electrode surface covered by a Langmuir-Blodgett film yields information on the contact between the LangmuirBlodgett film and the electrode surface.21 By changing the frequency of the ac signal used to determine the electrode impedance, we can obtain further information about the homogeneity of the electrode surfaceB or the presence of deeplying impurity near the electrode surface.
Experimental Methods Materials. Arachidic acid (Janssen) was crystallized from ethanol prior to use; bis(N-ethyl-N-octadecy1)-rhodamine (RB18) and 3,6-bis(N-ethyl-N-octadecyl)aminoxanthyliumperchlorate (PYRl8) were prepared as described After pu(6) Mdlers, F.; Memming, R. Ber. Bunsen-Ges. Phys. Chem. 1972, 76, 469. (7) Frippiat, A,; Kirsch-De Mesmaeker, A,; Nasielski, J. J . Electrochem. SOC.1983, 130, 237. (8) Frippiat, A.; Kirsch-De Mesmaeker, A. J. Phys. Chem. 1985,89,1285. (9) Frippiat, A.; Kirsch-De Mesmaeker, A. J . Electrochem. SOC.1987, 134. 66. ~. (lo) Kirsch-De Mesmaekcr, A,; Kanicki, J.; Leempoel, P.; Nasielski, J. Bull. Chem. Soc. Belg. 1978, 87, 849. ( I 1 ) Kirsch-De Mesmaeker, A.; Ltempoel, P.; Nasielski, J. Now. J. Chim. 1978. 2. 497. (12) 'Nasielski, J.; Kirsch-De Mesmaeker, A.; Leempoel, P. Electrochim. Acto 1978, 23, 605. (1 3) Nasielski, J.; Kirsch-De Mesmaeker, A.; Leempoel, P. Noun J. Chim. i9is,;2,497. (14) Hashimoto.. K.:. Hiramoto.. M.:. Sakata. T. Chem. Phvs. Lett. 1988. ir&2is. (15) Hashimoto, K.; Hiramoto, M.; Sakata, T. J . Phys. Chem. 1988,92, 4272. (16) Hickman, J.; Wessel, S.; Mackintosh, A.; Colbow, K. Semicond. Sci. Technol. 1987, 2, 207. (17) Shimura, M.; Shakushiro, K.; Shimura, Y. J . Appl. Electrochem. 1986., 16.683. ~ . ~~. . (18) Nakao, M.; Itoh, K.; Watanabe, T.; Honda, K. Ber. Bunsen-Ges. Phys. Chem. 1985,89, 134. (19) Itoh, K.; Nakao, M.; Honda. K. Chem. Phys. Lett. 1984,111,492. (20) Arden, W.; Fromherz, P. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 868. (21) Arden, W.; Fromherz, P. J . Electrochem. Soc. 1980, 127, 370. (22) Fromherz, P.; Arden, W. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 1045. (23) Fromherz, P.; Arden, W. J. Am. Chem. Soc. 1980. 102, 6211. (24) Kuhn, H.; Mbbius, D.; BBcher, N. In Physicol Methods in Chemistry; Wcissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. I, Part 3B, p 577. (25) Gomes, W.; Cardon, F. Prog. Sur/. Sci. 1982, 12, 155. (26) Dean, M. H.; Stimming, U. J . Electroonul. Chem. 1987, 228, 135. (27) Braun. C.; Fujishima, A,; Honda, K.; Nadjo, L. Surj Sci. 1986,176, 367. (28) Roberts, G. I.; Crowell, C. R. J . Appl. Phys. 1970, 41, 1767. (29) Nogami, G. J . Electrochem. Soc. 1985, 132, 76. (30) Nogami, G. J . Electrochem. Soc. 1986, 133, 525. (31) Van der Auweraer, M.; Verschuere, B.; De Schmer. F. C. funmuir I*, 4, 583. (32) Ioffe. 1. S.; Shapiro, A. L. J . Org. Chem. USSR 1970, 6,356. (33) Verschuere. B.: Van der Auweraer.. M.:. De Schrwer, F. C. Chem. Phis. 1991, 149, 385. ~
~~
Biesmans et al. rification by column chromatography on silica only one spot was observed by TLC in using different solvents as eluent. N,N'Dioctadecylrhodamine 1 19 (RH 18) was synthesized according to ref 32. Dilute solutions ( 5 X lo-' M) of arachidic acid and the different surface active dyes were prepared in spectroscopic grade chloroform (Janssen). Glass slides covered on one side with a layer of 1 pm S n 0 2 doped with fluoride were purchased from Glaverbel. Their resistance was 14 ohm/square. Monolayers. Mixed monolayers of the different dyes and arachidic acid were prepared on a Milli Q subphase containing 5.0 X lo4 mol L-' Cd(C104)2 in a circular trough" (Mayer Feintechnik). The pH was adjusted to 5.5 or 7 using NaOH (pa) (Janssen) and HClO, (pa) (Aldrich). The substrata were cleaned as described The monolayers were compressed to 30 mN m-I and deposited onto Sn02 with a deposition speed of 6.4 mm SI. Photoelectrochemical Experiments. The electrode potential was applied with a Heka Electronic 28c potentiostat. The dark and photocurrents were determined with the current amplifier of the potentiostat. For the determination of the photocurrent a 1500-W xenon lamp was used as light source. After passing through a monochromator (Applied Photophysics High Radiance) with an entrance slit of 5 mm and exit slit between 0.5 and 5 mm and a chopper (Scitec Instruments 300) operating at 10 Hz, the light was focused (IO-cm lens) to a spot with a diameter of 0.2 or 2 cm2 on the electrochemical cell. As the dark current (up to A) was of the same magnitude or larger than the photocurrent (up to 10" A), the output of the current amplifier of the polarograph was fed to a lock-in amplifier (Stanford Research Systems SR530) which allowed us to obtain the more accurate values of the photocurrent. The sample was excited at 560 or 530 nm with an intensity of approximately o+'l einstein s-l cm-2. The dark and photocurrents were digitized by using a Tecmar Lab Master Board and the data were transferred to an Olivetti M24 personal computer where subsequent analysis was performed. To correct the action spectra of the sensitized electron current (SEC) for the wavelength dependence of the intensity of the excitation light, the output of the lock-in amplifier was divided (BURRBROWN analog divider) by the output of a photodiode (RCA C30839) to which a small part of the excitation light was directed by a beam splitter and a 4-cm focusing lens. The light intensity was determined with a IL700 Research Radiometer from International Light. Electrical contact to the semiconductor was established by using Electrodag silver paint and an alligator clip. The electrode was mounted onto an electrochemical cell exposing a defined area (2 or 0.2 cm2) to the excititing light. As the excitation light entered the cell through the electrode the dye was illuminated through the SnO,. A platinum electrode was used as counter electrode and a saturated calomel electrode served as reference. The electrolyte contained M NaC10, (pa) (Janssen) and was buffered by using phosphate buffers. Hydroquinone (HQ) (pa) (Janssen) was added to the electrolyte in various concentrations. The pH of the electrolyte was adjusted with a 1.0 X lo-) mol L-' phosphate buffer, and 1.0 X mol L-' NaClO, was added as carrier electrolyte. Mott-Schottky curves were measured by using the internal function generator of the lock-in amplifier. An ac signal with an amplitude of 10 mV and a frequency of between 100 Hz and 10 kHz was superimposed on a constant applied electrode potential. The electrode potential was varied in steps of 50 mV between -600 and 800 mV versus SCE. The frequency dispersion of the capacitance and resistance of the electrochemical cell with a Sn02 electrode was recorded from 1 to lo4 Hz using N.F. frequency response analyzer Model 5720 combined with a potentiostat in a three-electrode configuration. The frequency dispersion of the impedance and phase shift were (34) Fromherz, P. Reo. Sci. Instrum. 1975. 4, 1380. (35) Biesmans, G.; Van der Auweraer, M.; De Schryver, F. C. Longmuir 1990,6, 277.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3113
Electron Injection from Xanthene Dyes into SnOz A
TABLE I: Influence of the Electrolyte pH on E& and nD for an Sn02
Electrode0
A A
En, mV pH(e1ec) vs SCE 3 5 7
A A
aa
A
A
a O
A
-10
-08
-06
-OL
a
300
9
0 9 o +
02
0
V v s
SCE
02
0 L
06
0 8
a
-08 -06
-OB
0
-02 Vvs
E b , mV pH(elec) vs SCE
nD,
B
-420
1.95 X IOzo 1.59 X IOzo 1.45 X 10"
-750
1.28 X
IO"
3 5 7
-140 -385 -420
9
-730
7.43 X IOi9 2.48 X IOi9 2.61 X loi9 3.07 X IOl9
"The area of the electrode amounts to 2 cm2;the electrolyte contains 1.0 X mol L-I NaCIO4 and 1.0 X IO-' mol L-' phosphate buffer; the frequency of the ac voltage amounts to 100 Hz. A, uncovered
f
1
-10 b
n
A -50 -350
nD, cm-3
02
OL
06
0
08
SCE
Figure 1. Mott-Schottky plot for SnOz electrodes as a function of the pH of the electrolyte; the electrolyte contains 1.O X 10-2 mol L-' NaC104 and 1.0 X IO" mol L-' phosphate buffer, the area of the electrode amounts to 2 cm2,the frequency of the ac signal amounts to 100 Hz; (0) pH 3, (+) pH 5, ( 0 )pH 7, (A) pH 9; (a) uncovered electrodes; (b) electrodes covered by a monolayer of arachidic acid, deposited from a subphase of pH 7 with 5.0 X IO-' mol L-' Cd(C104)2.
determined for an electrolyte containing 1.0 X mol L-' phosphate buffer of p H 6 and 1.0 X or 2.0 X lo-' mol L-' NaCIO,. The contact of the external circuit to the electrode was obtained by using silver paint or indium. Due to experimental error in the determination of the intensity of the incident light and small differences in the surface roughness of the electrodes, quantum yields obtained with different electrodes are only reproducible within 50%. For a series of experiments (e.g., as a function of hydroquinone concentration) performed with the same electrode the relative experimental uncertainty will be decreased to 25%. As it was not possible, due to interference effects, to determine directly the absorption spectra of monolayers deposited on SnO,, the absorbance of monolayers of the same composition deposited on glass was used to determine the quantum yield of the SEC. As the spectral properties of dyes in Langmuir-Blodgett films depend slightly upon the substratum on which the LangmuirBlodgett film is deposited,35the quantum yields of the SEC will be subjected to a determinate error that will depend upon the mixing ratio r between RBI8 and arachidic acid. This error will not influence the relative precision of experiments where the influence of the electrolyte pH or the regenerator concentration on the quantum yield of the SEC is considered.
Results Mort-Schottky Plots. For uncovered SnOz electrodes or S n 0 2 electrodes covered by a monolayer of arachidic acid a linear Mott-Schottky plot is always obtained at a frequency of 100 Hz up to respectively 0.5 V versus SCE at pH 5 or 0.2 V versus SCE at pH 9 (Figure 1). At more anodic electrode potentials a slight decrease of the slope of the Mott-Schottky plot is observed for
semiconductor; B, apparent values of En and nD for an Sn02electrode covered with an arachidic acid monolayer deposited from a subphase with pH 5 and 5.0 X IO4 mol L-' Cd(C104)2. uncovered electrodes. Increasing the electrolyte pH leads from a flat-band potential of -0.14 f 0.10 V versus the SCE at pH 3 to respectively -0.27 f 0.08 V versus the SCE at pH 5, -0.36 f 0.06 V versus the SCE at pH 7, and -0.66 f 0.09 V versus the SCE at pH 9. Increasing the ac frequency leads to a shift of the flat-band potential ( E f i )to more anodic potentials and to an increase of the slope. For frequencies higher than 500 Hz the slope of the Mott-Schottky plots increases by more than 2 orders of magnitude and dramatic deviations from a linear relationship, yielding a negative slope at electrode potentials more anodic than -0.2 V versus the SCE a t pH 5, are observed. The dispersion of the impedance and phase shift (Figure 2, a and b) indicate that at a carrier electrolyte concentration of 1.0 X lo-* mol L-'the impedance is nearly purely resistive at frequencies higher than 100 Hz. With a carrier electrolyte concentration of 2.0 X lo-' mol L-'an accurate determination of the impedance and phase shift is possible up to 1000 Hz (Figure 2, c and d). The results obtained for electrodes contacted by using indium and silver paint agree within experimental error. Contrary to the results obtained at more anodic potentials, where the phase 4 approaches asymptotically to 90° at low frequencies, the phase shift 4 levels at considerably smaller values at potentials more cathodic than 0 V versus the SCE. At 100 Hz a cathodic shift of the flat-band potential is observed upon increasing the electrolyte pH. Increasing the electrolyte pH also led to an increase of the slope of Mott-Schottky plot for electrodes covered by a monolayer of arachidic acid, indicating an (apparent) decrease of the doping of the semiconductor (nD) (Table I). Dark Currents. On S n 0 2 electrodes covered by a monolayer of N,"-bis(ethyloctadecy1)rhodamine at an electrolyte pH of 5.5 anodic dark currents were obtained in the presence of hydroquinone when the electrode potentials became more anodic than 120 mV versus the SCE. The measured dark currents were strongly dependent on the hydroquinone concentration (Figure 3a) and for hydroquinone concentrations larger than lO-' mol L-' they always exceeded A at electrode potentials more anodic than 600 mV (pH 5.5). For a hydroquinone concentration of IO-' mol L-' they already amount to A at 300 mV. Replacing the monolayer of RB18 by a mixed monolayer of RB18 and arachidic acid (mixing ratio's between 1/0 and 1/100)did not influence the dark current significantly. Also they did not differ in a significant way from the dark currents obtained for SnOz electrodes not covered by a monolayer. Increasing the electrolyte pH induced a large increase of the dark currents, which for an electrolyte pH of 9 amounted to already A at -50 mV versus the SCE (Figure 4a). Deposition of an extra double layer of arachidic acid led to a significant decrease of the dark current. The observed dark currents are attributed to the oxidation of the hydroquin~ne.'~.'~ Photocurrents. Upon excitation of a mixed monolayer of arachidic acid and RB18 deposited on an S n 0 2 electrode an anodic photocurrent is observed. The maximum of the action spectrum of the photocurrent (Figure 5 ) shifts to from 530 nm for a mixing ratio, r, between RB18 and arachidic acid of 1/50 to values between 548 and 558 nm for a mixing ratio of 1/0. For an
3774 The Journal of Physical Chemistry, V O ~ .95, NO.9, 1991
Biesmans et al. 90. I
80
I
8 0.
70.
GO
60.
4
50. 4
LO
LO
I
30. 2 0.
20. 10.
la!
0
10
20
30
LO
10
IC1 0
logf i l i z !
20
30
logf ( H z l
L 2
h
36 -30
c: N
-Z 2
L
t (b I
0.0
10
20
30
40
Id 1
0
I O
20
30
LO
logf I H z l
log f i H z !
Figure 2. Phase shift 6 (a, c) and logarithm of the impedance Z (b, d) observed for uncovered SnOz electrodes as a function of the logarithm of the frequencyf. The pH of the electrolyte equals 6; the electrolyte contains 1.0 X mol L-I NaClO, (a, b) or 2.0 X 10-I mol L-I NaCI04 (c, d) and 1.0 X lo-) mol L-I phosphate buffer: the area of the electrode amounts to 2 cm2; (-) 0.4 V versus SCE, (---) 0.0 V versus SCE, (---)-0.2 V versus SCE.
electrolyte pH between 3 and 9 the density of the photocurrent is proportional to the incident light intensity for intensities ranging from 3.0 X 10l2to 3.0 X 10" photons cmz). Upon excitation a t 560 nm the photocurrent obtained for an undiluted monolayer of RB18 decreased with 33% in 20 min, and excitation at 530 nm revealed no decrease in the photocurrent. The magnitude of the photocurrent indicates that in this period about 2.3 X l0ls electrons/cm2 traverse the electrode. As the surface concentration of chromophores in an undiluted monolayer equals 2.2 X lOI4 molecules/cm2 the turnover per chromophore equals 10.5 in this period. No bleaching could be observed by applying a potential to the electrochemical cell in the dark. For a mixing ratio of 1/2 an onset of the photocurrent is observed a t an electrode potential of -1 30 mV versus the SCE at pH 5 (Figure 4b at a hydroquinone concentration of 1.O X 10-I mol L-I. Increasing the electrode potential initially leads to an increase of the photocurrent which then reaches a maximum between 300 and 700 mV versus the SCE, depending upon the hydroquinone concentration (Figure 3, b-d). The features of the current-voltage plots do not depend upon the mixing ratio between the dye and arachidic acid (compare Figure 3, b and c, or Figure 4, b and c). Also, the quantum yield of the SEC, which remains always below 5 8 , does not depend on the mixing ratio between RB18 and arachidic acid (Table 11). As long as the hydroquinone concentration remains smaller than 2.0 X lW3 mol L-I, increasing the hydroquinone concentration leads to an increase of the quantum yield of the SEC. A further increase of the hydroquinone concentration does not lead to an additional increase of the SEC (Figure 3, b-d). A decrease of the electrolyte pH raises the quantum yield for the SEC (Table 11) and leads to a cathodic shift of the onset of the photocurrent (Figure 4, b and c). The combination of this shift and the larger maximum values of quantum yields at a lower
TABLE II: Influence of the Mixing Ratio between RB18 and Arachidic Acid and the Electrolyte pH on the Maximum Value of the Ourntum Yield for the SECO QSEC
electrolyte pH 3 5 I
9
r = 110 0.050 f 0.01 0.030 f 0.006 0.020 f 0.004 0.011 f 0.001
r = 112 0.065
r = 1/20
0.0216 0.0076 0.0076 O.00Sb
0.026 f 0.005 0.025 f 0.005 0.012 f 0.003 0.011 f 0.003 0.005 h 0.001 0.009 f 0.002 0.003 f 0.001 0.003 f 0.001 'The hydroquinone concentrationamounts to 1.0 X lWz mol L-' and the excitation occurred at 560 nm. The monolayer was deposited from a subphase with pH 5.5 and 5.0 X lo-' mol L-' Cd(C104)2. bExcitationat 530 nm. 3 5 I 9
pH leads to a crossing of the different curves in Figure 4b. Increasing the electrolyte pH also leads to a hypsochromic shift of the maximum of the action spectrum of the photocurrent of the undiluted monolayer (Figure 6). The pH of the subphase from which the Langmuir-Blodgett films are deposited does not influence the features of the current-voltage plots, the maximal value of the quantum yield of the SEC, or the action spectrum of the SEC. Photosensitized Injection by RH18. Also for mixed monolayers of R H l 8 and arachidic acid deposited on SnOz a SEC is observed for electrode potentials more anodic than -150 mV versus the SCE (Figure 7). The quantum yield of the SEC (0.023 i 0.010) does not depend upon r, the mixing ratio between RH18 and arachidic acid. The maximum of the action spectrum shifts from 523 nm for r equal to 1/50 to 530 nm for r equal to 1/0 (Figure 8). The
The Journal of Physical Chemistry, Vol. 95, NO. 9, 1991 3775
Electron Injection from Xanthene Dyes into S n 0 2 8.0
r
1.6
I
r
-5 10 A i
LO-
_.
.-.120 mV v s .
I
mV v s S C E
/
I
SCE
A
I
.........
4
0
0
4
4
m
4 0
A
0
0 0
4 0
,000
600 mV v s . S C E
1200
b L
I
!?
d
01
I 01
I
I 03
I
I
I
05
I 07
I
I 09
1 / [ H Q ] ( l o 3 I rnol-ll
Figure 3. (a) Dark current as a function of applied potential and the hydroquinone concentration for an SnOzelectrode covered by an undiluted monolayer of RBI 8; the electrolyte pH amounts to 5.5 and the electrode area to 2 cm2. (-) 1.O X IO-’ mol L-’ hydroquinone; 1.O X 1W2 mol L-l hydroquinone; (---) 5.0 X mol L-I hydroquinone; (---) 1.0 X mol L-l hydroquinone; (-.-) no hydroquinone. (b) Photocurrent as a function of applied (e..)
potential and the hydroquinone concentration for an SnOz electrode covered by an undiluted npnolayer of RB18 deposited at pH 5.5, from a subphase containing 5.0 X IO4 Cd(CIO,),; the excitation wavelength amounts to 560 nm, the electrolyte pH to 5 , and the electrode area to 0.2 cm2. (-) no hydroquinone; (- --) 5.0 X 1W’ mol L-I hydroquinone; (-..) 1.0 X 1W2 mol L-I hydroquinone; (--) 1.0 X 10-I mol L-l hydroquinone. (c) Photocurrent as a function of applied potential and the hydroquinone concentration for an Sn02electrode covered by a mixed monolayer of RBI8 and arachidic acid with a mixing ratio, r, equal to 1/20, deposited at pH 5.5, from a subphase containing 5.0 X lo4 Cd(CIO,),; the excitation wavelength amounts to 560 nm, the electrolyte pH to 5 and the electrode area to 2 cm2. (---) no hydroquinone; (---) 1.0 X mol L-I hydroquinone; 1.0 X 1W2 mol L-I hydroquinone; (-) 1.O X 1W’ mol L-’hydroquinone. (d) Influence of the inverse hydroquinone concentration and the electrode potential (versus the SCE) on the photocurrent for a mixed monolayer of RBI8 and arachidic acid ( r = 1/5) deposited at pH 5.5 from a subphase containing 5.0 X lo-‘ Cd(CIOA)2;the excitation wavelength amounts to 560 nm, the electrolyte pH to 5.5, and the electrode area to 2 cm2. (0)E = 0 mV; (e)E = 100 mv; (0j E = 200 mv; (A)E = 300 mv; (m) E = 400 mv. (e..)
action spectra obtained for RH18 correspond, except for a bathochromic shift of IO nm for the more concentrated monolayers, to the absorption spectra of a dilute solution of RH18 in chloroform. Photosensitized Injection by PYR18. Figure 9 shows that for mixed monolayers of PYRl8 and arachidic acid deposited on Sn02 a SEC can be observed for electrode potentials more anodic than -150 mV versus the SCE. The quantum yield of the SEC (0.010 f 0.005) is considerably smaller than observed for RB18 and RH18. It apparently decreases slightly upon decreasing the mixing ratio r between PYR18 and arachidic acid. The action spectra (Figure 10) observed for different mixing ratios resemble the absorption spectra of the mixed monolayers.”
Discussion Mott-Schottky Plots. The occurrence of linear Mott-Schottky plots is in agreement with the results obtained by Frippiat et a1.8 for the same n-type SnOl from the same source. At each pH the flat-band potentials obtained experimentally were 0.24.3 V more negative than the values reported in the l i t e r a t ~ r e . ~ . ~ *The ~~.” small deviations from linearity a t more anodic potentials could be due to the presence of deep donor sites or surface states.26.28-30 Those states should form a band f0.7 V below the conduction (36) Morrison, S . In Electrochemistry at Semiconductors and Oxidized Meral Electrodes: Plenum Press: New York. 1980. ( 3 7 ) Brcssel, B. Dissertation, Berlin, 1982.
band. The donor density obtained from the initial part of the Mott-Schottky plotx for the uncovered electrode amounts to (1.8 f 0.5) X 10” ~ m - ~ This . value is 3 times larger than the value obtained by Frippiat.8 This should be connected to the fact that the surface resistance of our samples amounts to 14 Q cm-2 while Frippiat et al. obtained 40 Q cm-2. The frequency dispersion of the impedance and phase shift reveal that the dramatic increase of the slope and the deviations from linearity of the Mott-Schottky plots, observed for a 1.0 X mol L-’NaC10, solution upon increasing the frequency, are artifacts due to the very small capacitive component of the cell impedance at frequencies larger than 100 Hz. For a NaClO, concentration of 2.0 X lo-’ mol L-’ linear Mott-Schottky plots could be obtained up to 1000 Hz. For this concentration of the carrier electrolyte the slope of the Mott-Schottky plots decreased by less than 30% between 10 and 1000 Hz. The decrease of nD observed upon increasing the frequency could also be due to the presence of donors that exchange electrons only slowly with the conduction band.28-30 The leveling of the phase shift a t values considerably smaller than 90°, observed for potentials more cathodic than 0.0 V, could be due to the presence a resistive component parallel with the space charge layer. This could indicateZSthe onset of faradaic proc e ~ s e sor ~ ’ the ~ ~ charging of surface states close to the edge of the conduction band a t cathodic potentials. The cathodic shift of the flat-band potential, observed upon increasing the electrolyte pH, can be due to an equilibrium between
Biesmans et al.
3776 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
c
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Figure 5. Influence of the mixing ratio, r, between RB18 and arachidic acid on the action spectra of the photocurrent obtained at 300 mV versus the SCE for an Sn02electrode covered by a mixed monolayer of RB18 and arachidic acid; the deposition occurred at pH 5.5 from a subphase containing 5.0 X lo4 Cd(CI04)2; the hydroquinone concentration amounted to 5 X mol L-' and the electrolyte pH to 5.5. (-) r = I/@ (...) r = 1/10; ( - - - ) r = 1/20; (---) r = 1/100.
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Figure 6. Influence of the electrolyte pH on the photocurrent action spectra obtained for an SnO, electrode covered by an undiluted monolayer of RB18; the electrode potential amounted to 400 mV versus the SCE, the hydroquinone concentrationto 1.O X 10-2 mol L-l, and the area of the electrode to 0.2 cm2;the monolayer was deposited at pH 5.5 from a subphase containing 5.0 X IO-' Cd(CI0J2 (-) pH 3, (---) pH 5 , (---) pH 7, (-*-) pH 9. ,000
1200
600 mV v s .
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Figure 4. (a) Dark current as a function of applied potential and the pH of the electrolyte for an Sn02electrode covered by an undiluted monolayer of RBI8 deposited at pH 5.5 from a subphasecontaining 5.0 X 10-4 Cd(CI04),; the electrode area amounts to 2 cm2. (-) pH 3; pH 5 ; (- - -) pH 7; (---) pH 9; the hydroquinone concentration amounts to 1.0 X 10-1 mol L-I. (b) Photocurrent as a function of applied potential and the pH of the electrolyte for an Sn02electrode covered by an undiluted monolayer of RBI8 deposited at pH 5.5 from a subphase containing 5.0 X l p Cd(CIO,),; the excitation wavelength amounts to 560 nm and the electrode area to 2 cm2. (-) pH 3; pH 5 ; (-- -) pH 7; (---) pH 9; the hydroquinone concentration amounts to 1.0 X 10-2 mol L-I. (c) photocurrent as a function of applied potential and the pH of the electrolyte for a mixed monolayer of RBI8 and arachidic acid (r = 1/20) deposited at pH 5.5 from a subphase containing 5.0 X IO-' Cd(CIO,),; the excitation wavelength amounts to 560 nm and the electrode area to 2 cm'; excitation at 560 nm (-) pH 3; (...) pH 5 ; (-- -) pH 7; the hydroquinone concentration amounts to 1.0 X IO-' mol L-I. (-e.)
(..e)
-OH and -0- groups a t the electrode s u r f a ~ e . The ~ ~ *shift ~ ~ of the flat-band potential as a function of the electrolyte pH exceeded the theoretical expected value of -59 ~ V / P H Similar . ~ ~ results were reported by Armstrong3" for Sn02. Fromherz and Arden*B corrected the experimentally obtained values of the capacitance of an In203 electrode covered by a Langmuir-Blodgett film for the fact that the electrode was only partly covered by the Langmuir-Blodgett film. Assuming that the capacity of the space charge layer showed a linear MottSchottky plot and that the donor density (nD)was not influenced by the deposition of the monolayer allowed one to determine the fraction of the electrode that was not covered by the Langmuir-Blodgett film. The bad coverage of the electrode surface by the monolayer could be due to the fact that the monolayer bridges some pores in a rough electrode surface rather than to pinholes in the monolayer. When the results of Table I are analyzed by using the same formalism as employed by Arden and (38) Armstrong, N.; Lin, A.; Fujihara, M.;Kuwana, T. AMI. Chem. 1976,
48, 74 1.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3777
Electron Injection from Xanthene Dyes into SnOz
1.50
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Figure 8. Influence of the mixing ratio, r, between RH18 and arachidic acid on the action spectra of the photocurrent obtained at 300 mV versus the SEC for an Sn02electrode covered by a mixed monolayer of RH 18 and arachidic acid; the deposition occurred at pH 5.5 from a subphase containing 5.0 X 10-4 Cd(CI04),; the hydroquinone concentration amounted to 5 X 1W2mol L-' and the electrolyte pH to 5 (-) r = 1/0; (...) r 1/5; (---) r = 1/20; (---) r = 1/100. Fromherz, the apparent change of nD as a function of the electrolyte pH can also be explained by a deterioration of the contact between the monolayer and the electrode when (Table 111) the electrolyte pH is decreased. On the average 40% of the electrode was not shielded by the monolayer from a direct contact with the electrolyte. This fraction is considerably larger than the 5% obtained by Arden and FromherzS2) for In203covered by a Langmuir-Blodgett film. The uneven surface of the Sn02elect r o d e ~ ~will * not prevent the d e E i t i o n of a Langmuir-Blodgett film. It has baen demonstrated3 that Langmuir-Blodgett films can be deposited on rough surfaces. In this case the films will remain planar and bridge holes or grooves in the surface. This will, however, lead to a deterioration of the in the
'
(39) Bikerman, J. Proc. R. Soc. London 1939, A170, 130. (40)Gainea, J. L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (41) Aveyard, R.; Haydon, D.Introduction to the Principles of Surface Chemistry; Cambridge University Press: Cambridge, UK. 1972. (42) Dierker,S.; Murray, C.; Legrange, S.;Schlattcr,N. Chem. Phys. Lctr. 1987, 137, 453. (43) Skita, V.; Richarson, W.; Filipokowski, M.; Garito, A.; Blaise, M. J . Phys. (Lcs Ulis. Fr.) 1986. 47, 1849.
555
585
WAVELENGTH l n m l
Figure 10. Influence of the mixing ratio, r, between PYR18 and arachidic acid on the action spectra of the photocurrent obtained at 300 mV versus the SCE for an SnOz electrode covered by a mixed monolayer of PYR18 and arachidic acid; the deposition occurred at pH 5.5 from a subphase containing 5.0 X lo4 Cd(CI04),; the hydroquinone concentration amounted to 5.0 X 1W2 mol L-'and the electrolyte pH to 5 (-) r = 1/0; r = 1/5; (---) r = 1/20; (---) r = 1/100. (as.)
TABLE III: Fraction ( b ) of tbe Area of the Semiconductor Not in Contact with the hngmuir-Blodgett Filmo pH(e1ec) C ,, pF Can,pF cm-, h, 96 3 5 I 9
6.6
5 4.5 1 3.43
3.6 1.82 1.60 1.51
52 31 29 31
"The area of the electrode surface amounts to 2 cm2. The electrolyte contains 1.0 X lo-, mol L-l NaCIO, and 1.0 X mol L-I phosphate buffer. The arachidic acid monolayer was deposited from a subphase with pH 5 and 5.0 X lo-' mol L-' Cd(C104)2. C, = capacitance of the space charge layer; C, = experimentally determined cell capacitance. Langmuir-Blodgett film and to decreased contact between the film and the surface. The influence of the pH on the contact between the Langmuir-Blodgett film and the electrode can be due to the different molecular packing of monolayers of arachidic (44) (45)
Swalen, J. D. Thin Solid Films 1987, 152. 151. Butler, M. J. Appl. Phys. 1977, 48, 1914.
3778 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
Biesmans et al.
acid and cadmium arachidate.&% The acid would be less firmly attached to the surface compared to the cadmium salt.5’ The Dark Currents. The conditions under which the dark currents are observed and their density resemble those of the dark currents observed for S n 0 2covered by adsorbed rhodamine B in the presence of hydroquin~ne.~~J’,’~ They are probably due to According to Bre~sel’~ the oxidation of hydroq~inone’~’~*’~.~~J3.” this would occur at 0.370 V versus the SCE. As the determined values of the flat-band potential indicate that the edge of the conduction band is situated a t potentials that are considerably more negative, tunneling of the electrons through the space charge layer is necessary. As an increase of the pH will shift this edge to more negative potentials, the latter effect will balance the cathodic shift of the standard redox potential c a d by an increase of the pH. This explains why (as long as the pH is lower than 9) the features of the current-voltage plots do not depend upon the pH. When the pH exceeds 9 dissociation of hydroquinone leads to an increase of the dark current and to a shift of the current-voltage curves to more cathodic electrode potentials. Photocurrents. The features of the current-voltage plots for the anodic photocurrents resemble those obtained for adsorbed rhodamine B13J3J938 and for monolayers of chlorophyll deposited on Sn02,5S For the different dyes discussed in this contribution and for different concentrations of the dye or hydroquinone the onset of the current-voltage plots occurred, at pH 5.5, always at -200 f 50 mV versus the SCE. If one tries to estimateS6the position of the LUMOSbS* of the dye versus the edge of the conduction band one observes that the injection of the electron is always exergonic by 0.6-0.75 eV. This electron transfer can therefore occur very rapidly.5g If this electron transfer is indeed extremely fast, one can explain why the quenching of the sensitized hole current@)*61 or the f l u o r e ~ c e n c e ~observed l * ~ ~ , ~ ~upon increasing the concentration of the xanthene dyes in the monolayer does affect the SEC only slightly. One can expect that electron injection is always much faster than energy transfer to the dimers or that even for the excited state of the dimers the electron injection in S n 0 2 is sufficiently exergonic to compete successfully with other decay processes. The latter argumentM is also used to explain why the quantum yield of the SEC obtained for adsorbed rhodamine B does not dependu6 upon the concentration of rhodamine B in solution.
In spite of this very efficient injection the overall quantum yield for the SEC remains low. This can be due to (1) incomplete contact between the monolayer and the electrode and (2) efficient recombinati0n~-”~’~~~J~3~ between electrons in the conduction band and oxidized dye molecules. The first argument is supported by the information obtained from the Mott-Schottky plots that suggest that a large fraction of the electrode surface is not shielded by the deposited Langmuir-Blodgett film from direct contact with the electrolyte. This would have as a consequence that a large fraction of the dye molecules in the Langmuir-Blodgett film is not in direct contact with the electrode and can therefore not contribute to the photocurrent. However, as the fraction of the electrode that is not in contact with the LB film does not exceed 50% this argument can explain the small quantum yield for the SEC only partially. The second argument will be much more important. This is supported by the influence of the electrolyte pH and the hydroquinone concentration on the quantum yield of SEC. Increasing the hydroquinone concentration will increase the relative rate with which the oxidized rhodamine B is regenerated. This will decrease the rate of the recombination process. An increase of the pH leads to a cathodic shift of the onset of the SEC. This shift parallels the shift of the flat-band potential determined from the MottSchottky plots and exceeds 59 mV per pH unit. Increasing the pH will increase the potential over the space charge layer at a given electrode potential or will allow one to obtain a given potential difference over the space charge layer at a more cathodic electrode potential. As the recombination of an electron in the conduction band with an oxidized dye molecule needs tunneling through the space charge layer or the thermal escape of the electrons over the barrier of the space charge layer, this process will for a given electrode potential become slower at a higher pH. For the recombination several mechanisms have been proposed.’2J3J8937~67 The observed linear dependence of the photocurrent on the light intensity can exclude nongeminate recombination between an injected electron and an oxidized dye molecule. A more detailed discussion of the recombination processes will be given elsewhereaS2When the recombination is mainly geminate,56 as proposed by B r e ~ s e l ?the ~ photocurrent would be expected to depend on the hydroquinone concentration only at the onset of the photocurrent-voltage plots. This is not observed. Therefore, one can expect that “alien”’o*11J3recombination with electrons of the conduction band is the most important recombination process. The Photocurrent Action Spectra. For PYRl8 the action spectra of the photocurrent agree with the absorption spectra of the Langmuir-Blodgett films in which this dye is incorporated.” This indicates that monomers and dimers of this dye have the same quantum yield for i n j e c t i ~ n ~ @ of .an ~ ~electron or that as soon as absorption of the dimer becomes important also energy transfer from the excited monomer to the dimer becomes a predominant decay process@ of the monomer. For RH18 the action spectra of the photocurrent agree, except for a bathochromic shift of 5-10 nm with the absorption spectrum of this dye in solution. Increasing the mixing ratio between the dye and arachidic acid leads to a bathochromic shift of the maximum from 523 nm for r = 1/50 to 530 nm for r = 1/0. This is due to an increase of the polarizability of the monolayer. The features of the action spectra do, however, give no indication for the presence of a second maximum at smaller wavelengths when the concentration of the dye in the monolayer is increased. In this aspect the photocurrent action spectra of RH18 show the same behavior as those of adsorbed molecules of rhodamine B.56 Also the absorption spectra of molecules of rhodamine B70 adsorbed on quartz or of molecules
~
~~~
~~~
(46) Peng, J.; Ketterson, J.; Dutta, P. Lungmuir 1988, 4, 1198. (47) Outka, D.; StBhr, J.; Swalen, J. J. Chem. Phys. 1988, 88, 4076. (48) Outka. D.; StBhr, J.; Swalen, J.; Rotermund, H. Chem. Rev. krr. 1987, 59, 1321. (49) Rate, J.; Swalm, J.; Outka. D.; StBhr, J. Thin Solid F i l m 1988,159, 215. (50) Kobayashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1988,160, 267. ( 5 1 ) Garoff, S. Thin Solid Films 1987, 152, 49. (52) Biesmans, G.; Van der Auweraer, M.; Cathry, C.; De Schryver, F.
C.; Yonezawa, Y.; Sato. T. To be submitted for publication. (53) Elliott, D.; Zellmer, D. L.; Laitinen, H. A. J. Elecrrochem. Soc. 1970, 117, 1343. (54) Laitinen, H. A.; Vincent, C. A.; Bcdnarski, T. M. J . Elecrrochem. Soc. 1968, 115, 1024. (55) Watanabe, T.; Miyasaka, T.; Fujishima, A,; Honda, K. Chem. Len. 1978,443. (56) Gerischer, H.; Willig, F. Top. Curr. Chcm. 1976, 61, 31. (57) Picchowski, A. P. J . Elec?roanal. Chem. 1983, 145, 67. (58) Loutfy. R. D.; Sharp, J. H. Phorogr. Sci. Eng. 1976, 20, 165. (59) Eichberger, R.; Willig, F. Chem. Phys. 1990, 141, 159. (60) Van der Auweraer, M.; Willig, F. Isr. J. Chcm. 1985, 25, 274. (61) Van der Auweraer, M.; Verschuere, B.; Biesmans. G.; De Schryver, F. C . Lmgmuir 1987, 3, 992. (62) Tamai, N.; Yamazaki, T.; Yamazaki, I.; Mizuma, A.; Mataga, N. Chcm. Phys. krr. 1988.147, 25, (63) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990.94,511. (64) Itoh, K.; Nakao. M.; Honda, K. J . Am. Chem. Soc. 1984,106, 1620. (65) Liang, Y.; Moy, P. F.; Poole, J. A.; Ponte-Goncalves, A. M.J . Phys. Chem. 1984,88, 24s 1. (66) Tributsch, H.; Gerischer, H. Bcr. Bunsen-Ges. Phys. Chsm. 1%9,73, 351.
(67) Yamase, T.; Gerischer, H.; LLlbke, M.; Pettinger, B. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 1041. (68) Sato, H.; Kawasaki, M.; Kasatani, H.; Nakashima, N.; Yoshihara. K. J . Phys. Chem. 1988, 92,754. (69) Killesreiter, H.; BHssler, N. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 503. (70) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K.J . Chem. Phys. 1986, 90,5094.
Electron Injection from Xanthene Dyes into S n 0 2 of RBI 831v62incorporated in Langmuir-Blodgett films are not dependent upon the concentration of the dye. This is probably due to the fact that the bulky phenyl group does not allow the formation of sandwich dimers. On the other hand the current action spectra obtained for RB18 are characterized by a significant hypsochromic shift compared to the absorption spectra of RB18 incorporated in LangmuirBlodgett films31.62or current action spectra of rhodamine B.56 Their features, however, do not depend upon the concentration of RB18 in the Langmuir-Blodgett film. It is not likely that this shift is due to a (partial) dealkylation of the tetraalkylrhodamine as was observed by Watanabes5 for adsorbed rhodamine B or by Kirsch-De Mesmaeker7' and Frippiat* for rhodamine B linked covalently to Sn02 where by photooxidation the rhodamine B loses successively all four alkyl groups. For those samples the shift of the action spectra was considerably larger than the shift observed here. This would indicate that for RB 18 in Langmuir-Blodgett films only a partial dealkylation would occur, e.g., the loss of two alkyl groups. In this case the action spectra obtained for RB18 would resemble those obtained for R H 18. The maximum of the action spectra observed for RB 18 is, however, situated at longer wavelengths than that of RH18. Furthermore, the action spectra observed for the SEC generated by RB18 resemble those of the SHC33generated by RB18, where the dye is reduced, which makes oxidative dealkylation very improbable. Another explanation could be the interaction between the rhodamine chromophore and anion^^"^ or cations76present in the electrolyte or the Langmuir-Blodgett film. Those interactions could lead to a partial localization of the positive charge in the chromophore and lead in this way to a hypsochromic shift. As these interactions could be larger at defects in the monolayer or the electrode surface, one could explain why this shift is larger for a batch of Sn02that is characterized by more defects. The action spectrum is also characterized by a hypsochromic shift when the electrolyte pH is increased. This shift is also larger than that observed for rhodamine B in solution.77 Interactions (71) Kirsch-De Mesmacker, A.; Dewit, R. Electrochim. Acta 1981.126, 297. (72) Butgereit, G.; Scheibe, G. Ber. Bunsen-Ges. Phys. Chem. 1%5,69, 301. (73) Kemula. W. Bull. Acra Polon. Sci. 1967, 15, 43. (74) McKay, R. B.;Hillson, P. J. J . Chem. Soc., Faraday Trans. 1961, 61, 1800. (75) Sheppard, S.;Geddes, A. J . Am. Chem. Soc. 1944,66, 1995. (76) Kortum. G.; Vogel, J . Chem. Ber. 1960, 93, 706. (77) Ferguson, J.; Mau, W. H. Chem. Phys. Lett. 1972, 17, 543.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3779 of the negatively charged carboxylate groups with neighboring chromophore moieties could influence the electronic distribution of the *-electrons. This would give rise to a hypsochromic shift of the absorption maximum and explains the observed band broadening upon raising the electrolyte pH.
Conclusions Upon excitation of xanthene dyes incorporated in LangmuirBlodgett films deposited on Sn02electrodes anodic photocurrents are observed at potentials more anodic than -150 mV versus the SCE. Due to the large free energy gain associated with the injection of the electrons in the conduction band the efficiency of this process, for the dyes investigated in this contribution, does not depend upon the oxidation potential of the excited species. For the same reason the possibility of energy transfer to dimers of larger aggregates has only a minor influence on the photocurrent. The small overall quantum yield for the sensitized electron current is due to incomplete contact between the monolayer and the electrode surface and to efficient competition between recombination processes involving the oxidized dye and electrons in the conduction band and the reduction of the oxidized dye by a regenerator. In this way the influence of the regenerator concentration and the electrolyte pH on the photocurrent can be explained. For a derivative of pyronine the action spectra resemble the absorption spectra of the monolayer. Due to the absence of a bulky phenyl group this dye can form sandwich type dimers in the Langmuir-Blodgett film which is testified by the absorption spectra of the Langmuir-Blodgett film and the current action spectra. For derivatives of rhodamine B (RB18) the action spectrum of the photocurrent is shifted to shorter wavelengths compared to the action spectra of adsorbed molecules of rhodamine B or the absorption spectra of RB18 incorporated in dry monolayers. This is due to interactions between the chromophore and anions or cations in the Langmuir-Blodgett film in the presence of electrolyte. As observed from the action spectra of adsorbed rhodamine B or the absorption spectra of RB18, no formation of sandwich dimers occurs due to the bulky phenyl group.
Acknowledgment. M.V.d.A. is a research associate of the F.K.F.O. G.B. thanks the I.W.O.N.L. and the K.U. Leuven for financial support. We thank the Belgian Ministry of Scientific Programmation and the F.K.F.O. for financial support to the laboratory. We thank Prof. Doblhofer of the Fritz Haber Institut for the possibility to determine the frequency dispersion of the cell impedance.
