980
J. Phys. Chem. 1984,88, 980-984
The mechanism of C H 2 0current doubling is, we believe, related to its pH dependence. Formaldehyde hydrolyzes readily in aqueous solution to form methylene glyc01,~~-'~ which at high pH with an excess of OH- ions would exist in the ionic form of -OCH20H or even -OCH,0-.e6 We propose therefore that formaldehyde shows current-doubling behavior only in these ionic forms. The results using the gold electrode further suggest that formaldehyde is only oxidized in basic solution in the voltage range of interest. The current multiplication of the corrosion process at pH 7 may also compete with and mask any slight reaction of the formaldehyde. The flat-band shift and slope change of the Mott-Schottky plot with pH may be due to the initial growth of a thin selenium film. Since glyoxal undergoes an intramolecular disproportionation in basic medium that is similar to the Cannizzaro reaction of formaldehyde, its current-doubling behavior supports our conclusions concerning formaldehyde. As mentioned, none of the other chemicals tested showed current-doubling effects. Also,none undergo Cannimro reactions. The gold electrode results also support this relationship between species that current double (are oxidized easily) and undergo the Cannizzaro reaction. Gomes' group' reported current doubling with the formate ion, a result not observed here. They also reported methanol to be electrochemically inert, whereas we detected evidence of a small reaction. The sample they used was polished as a c-face (OOOT) crystal whereas ours was an a-face
(1 120). However, it would seem unlikely that this would cause these discrepancies. We observed that even large layers of selenium (- 1 pm) on CdSe had no effect on the C H 2 0 current doubling even though the Mott-Schottky plot shows a flat-band shift. The change in slope would seem to indicate an increase in the surface area of the electrode. The flat-band shift can be reconciled to a double layer across the selenium layer. But the lack of change in current doubling is hard to reconcile with Frese's model of the holes moving through the valence band of the ~ e l e n i u m . ~Frese , ~ found no photocurrent due to the photons absorbed in the selenium and concluded that electrons in the selenium layer were immobilized. Similarly, electrons injected during current doubling should be immobilized. Our observation of unchanging current doubling is most easily explained if the hole capture is always at the CdSe surface, not the Se surface-viz. a porous Se layer. As a possible catalyst for disproportionation of higher aldehydes and sugars, CdSe does not appear promising. Further experiments that could be tried include analysis of the products of the current doubling of CHzO to determine the relative proportions of reactants due to the CdSe electrode and the alkaline-induced Cannizzaro reaction. The response of a vacuum-evaporated selenium layer on CdSe could also be tested. Acknowledgment. We wish to acknowledge valuable discussions with W. R. Richards of the Chemistry Department at Simon Fraser University. Registry No. Glyoxal, 107-22-2;acetaldehyde, 75-07-0; methanol, 67-56-1; ethanol, 64-17-5; cadmium selenide, 1306-24-7; selenium, 7782-49-2; formaldehyde, 50-00-0.
Photophysical Properties of Cadmium Sulfide in Nafion Film J. P.Kuczynski, B. H. MilosavIjevic,+and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 28, 1983; In Final Form: October IO, 1983)
CdS was synthesized in a Nafion polymer film type N-125. The small crystalline particles obtained exhibit properties (bandgap = 2.42 eV; dE,,,/dT = -4.5 X lo4 eV/K) which are similar to those of CdS single crystals. X-ray analysis as well as the emission maximum dependence on the excitation light intensity indicates a crystalline structure perturbed by a large number of defects. Due to an interaction with the host polymer the luminescence lifetime approaches 1 p at room temperature. Upon irradiation with 488-nm light electron transfer to MV2+was observed with a quantum yield of 4.2 X lo4. In spite of the long-lived luminescence it was found from both transient emission and absorption data that the luminescence quenching is predominantly static. This quenching behavior is discussed in light of the reduced quencher mobility in the clusters formed in the hydrated Nafion film.
Introduction The promotion of interfacial electron transfer reactions by colloidal semiconductors upon irradiation with visible light has lately received much A good deal of the work has been focused on T i 0 2 colloids since this semiconductor is stable with respect to anodic di~solution.~However, since the bandgap of T i 0 2 is relatively large the major thrust of the research has shifted toward CdS, a semiconductor with a much smaller bandgap and enhanced spectral response in the visible region.sJO Several problems are associated with colloidal CdS employed in interfacial electron transfer reactions. Irradiation of a CdS particle generates an electron/hole pair whose lifetime (radiative or nonradiative) at room temperature is very short (7 < 1 ns). Therefore, only those molecules directly adsorbed onto the CdS surface undergo
therein. (3) Kraeutler, B.; Bard, A. J . Am. Chem. SOC.1978, ZOO, 4317. Izumi, I.; Fan, F. F.; Bard, A. J . Phys. Chem. 1981, 85, 218. Bard, A. Zbid. 1982, 86, 172, and references therein. (4) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133. Thomas, J. K. Chem. Rev. 1980,80, 283. Kuczynski, J.; Thomas, J. K. Chem. Phys. Lett. 1982, 88, 445. Kuczynski, J.; Thomas, J. K. J . Phys. Chem., in press. (5) Henglein, A. J . Phys. Chem. 1982, 86, 2291. Henglein, A. Ber. Bunsenges. Phys. Chem. 1982, 86, 201. (6) Nakato, Y.; Tsumura, A,; Tsubomura, M. Chem. Phys. Lett. 1982,85,
'On leave from Boris Kidrich Institute for Nuclear Science, Radiation Chemistry Department, 11001 Belgrade, P.O.Box 522, Yugoslavia.
(7) Sreva, E. F.; Olin, G. R.; Hair, J. R. J . Chem. SOC.,Chem. Commun. 1980, 401. (8) Darwent, J. R.; Porter, G. Chem. Commun.1981, 4, 145.
electron transfer reactions. This necessitates very high bulk concentrations of the electron transfer reagent in order to ensure (1) Fendlcr, J. H. J . Phys. Chem. 1980, 84, 1485. (2) Kalyanasundarum, K.; Borgarello, E.; Gratzel, M. Helv. Chem. Acta 1981, 64, 362. Gratzel, M. Acc. Chem. Res. 1981, 14, 376, and references
387.
0022-3654/84/2088-0980$01.50/00 1984 American Chemical Society
Photophysical Properties of CdS in Nafion Film
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 981
a sufficient surface concentration. However, such concentrations tend to destabilize the colloid resulting in flocculation. Furthermore, it is well established that CdS is unstable with respect to photoanodic dissolution.” Limited success in preventing this photocorrosion has been achieved by intercepting the hole by coating the CdS with a conducting polypyrole film.I2 In an effort to circumvent those problems we have synthesized colloidal CdS within an inert polymer matrix that provides indefinite stability against precipitation. Recently Memming and co-workers” imbedded CdS grains within a polyurethane membrane in order to investigate the role of catalysts in the direct photoelectrolysis of water. However, no mention was made concerning the photophysical or photochemical properties of the CdS particles incorporated within the polyurethane membrane. In our studies we chose to synthesize CdS within a perfluorsulfonic acid Nafion membrane since these membranes exhibit exceptional chemical, mechanical, and thermal stability. According to DuPont’s data the chemical structure of the Nafion membrane may be formulated as follow^:'^ UCFZ
-C F2 ),CF
T
L
I
-CF2
- 3 7
I I
c F2
I
SO3H
5-13.5
n = -1000 ,Y=
1, 2, 3 , .
0.6
z
U m 0.4
cn
m Q
0.2
WAVE LENGTH ( n m 1 Figure I. (a) Absorption spectrum of Nafion vs. air a t room temperature, (b) Absorption spectrum of CdS in Nafion vs. Nafion at room temperature.
CF2
RZ =
W 0
1,
1
i F 0
0.8
..
Furthermore, the proton from the sulfonic acid moiety is exchangeable for a sodium ion which alters the physiochemical properties of the membrane. Extensive ~ t u d i e s l ~ -of~ ’Nafion membranes indicate that the sulfonic acid groups and water molecules form a clusterlike structure depending upon the degree of hydration of the membrane.’* In this paper we report the photophysical and photochemical behavior of colloidal CdS prepared within a Nafion membrane as well as the nature of the microenvironment within the membrane.
Experimental Section Materials. Cadmium sulfide (99.99+% pure, Aldrich Chemical Co.) cadmium chloride (J. T. Baker Chemical Co.), sodium sulfide (Mallinckrodt) methanesulfonic acid (Aldrich Chemical Co.), pyrene-3-carboxaldehyde(Aldrich Chemical Co.), and tetrasodium ethylenediaminetetraacetate (Fisher Chemical Co.) were used as supplied. Methyl viologen, N,N’-dimethyl-4,4’-bipyridinium dichloride (Aldrich Chemical Co.), was recrystallized three times from methanol; pyrene (Aldrich Chemical Co.) was recrystallized twice from ethanol; perfluoroheptanoic acid (Minnesota Mining ~~~
~
(9) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72, 83. Kawai, T.; Sakata, T. Chem. Phys. Lett. 1982, 7 2 , 87. (10) Duonghong, D.; Ramsden, J.; Grltzel, M. J . Am. Chem. SOC.1982, in4 2977 . -, .
(1 1) Ellis, A. B.; Kaiser, S. W.; Bolts, J. M.; Wrighton, M. S . J . Am. Chem. Soc. 1977, 99, 2839. ( 1 2 ) Frank, A. J.; Honda, K. J. Phys. Chem. 1982, 86, 1933. (13) Meissner, D.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983,
-
96 . -, 34..
(14) Vaughan, D. J. DuPont Innovation 1973, 4, 10. (15) Fujimura, M.; Hashimoto, T.; Kawei, H. Macromolecules 1981, 14, 1309. (16) Roche, E. J.; Pineri, M.; Duplessix, R.; Levelut, A. M. J . Polym. Sci., Polym. Phys. Ed. 1981, 19, 1. (17) Rodmaca, B.; Pineri, M.; Coey, J. M. D. Rev. Phys. Appl. 1980, 25, 1179
(18) Ostrowska, J.; Narebska, A. Colloid Polym. Sci. 1983, 261, 43.
and Manufacturing Co.) and perfluoro oil FC 77 (Minnesota Mining and Manufacturing Co.) were used as received. The Nafion perfluoro membrane was kindly supplied by E. I. DuPont Du Nemours and Co. and was pretreated prior to use by boiling in water for approximately 45 min. The H+ or Nat exchanged forms of the Nafion membrane were prepared by soaking precut sections of the treated membrane for at least 24 h in either 1 M HC1 or 1 M NaCl, respectively. Incorporation of CdS into the Nafion membrane was accomplished by soaking the nafion in 5 X M CdC12 for 24 h followed by vacuum drying the membrane and, finally, rapidly submerging the dried film in a 0.1 M Na2S bath. Excess Na2Swas removed by washing the membrane in a water bath-several water changes were conducted in order to ensure removal of the unreacted sulfide. A similar procedure was used for incorporation of either pyrene or l-pyrenecarboxaldehyde (PCHO) into the nafion film. Instrumentation. Pulsed irradiation studies were conducted either with the 337411-1beam (8-mJ energy, 6-11s pulse width) from a Lambda Physik X 100 laser or with the 337-nm line (30-fiJ energy, 120-ps half-width) from a P.R.A. Nitromite N, laser. The short-lived transients produced were monitored by fast spectrophotometry (response I 1 ns) and the data were captured by a Tektronix 79 12A digitizer with subsequent processing by a 4052 A minicomputer. Steady-state absorption and emission spectra were recorded on a Perkin-Elmer 552 spectrophotometer and a Perkin-Elmer M P F 44B spectrofluorimeter, respectively. Quantum yields were determined through use of the 488-nm line from a Spectra Physiks argon ion laser and a Scientech light meter. X-ray diffraction data were obtained with a Diano X-ray diffractometer using Cu K a radiation. Steady-state and pulsed luminescence quenching studies of CdS in Nafion by methyl viologen (MV*+) were performed by positioning the film at a 45’ angle to the incident excitation source and the detector. This geometry minimizes scatter off of the film by directing scattered light away from the detector. Since the emission intensity is strongly position dependent the Nafion membrane was cut to fit securely into a standard quartz cell in order to prevent the film from moving upon addition of quencher.
Results and Discussion Photophysical Properties. Special attention has been centered on the absorption and emission spectra of the perfluorosulfonic acid membrane due to its possible interference with the CdS luminescence. Figure 1 shows the absorption spectrum of the Nafion N-125 membrane in its hydrogen form following standard pretreatment by boiling in distilled water for 30 min.’* Lee and Meisel noted that, due to some nonexchangeable impurity (the
982
t
Kuczynski et al.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
l
6.0r
0
z
8 400 v, W
z
f
2 200 W
2
t
a
-1
w
a
200
W A V E LENGTH ( n m )
Figure 2. (a) Luminescence spectrum of CdS in Nafion at room temperature, excitation wavelength = 400 nm. (b) Excitation spectrum of CdS in Nafion at room temperature, emission wavelength = 700 nm.
amount of which varies from batch to batch) present in the film as supplied by the manufacturer, excitation of the membrane in the UV region results in an emission band at 390 nm.19 We have not observed such luminescence even at the highest possible sensitivity of our instrumentation indicating that our Nafion samples were free from such impurities. The membrane became slightly yellow after being stored in air at room temperature for 6 months. However, no luminescence was detected from this sample. The yellowish-colored compound, which is probably a degradation and/or oxidation product of the polymer, can be removed by several successive cycles of boiling in distilled water followed by sonication. Nevertheless, since all excitation and emission studies of CdS in Nafion were conducted at greatly reduced (at least four orders of magnitude) instrumental sensitivity the possibility that the observed luminescence is connected with the membrane is ruled out. Also shown in Figure 1 is the absorption spectrum of CdS synthesized within the Nafion membrane. The film is bright orange and is totally transparent. The observed spectrum appears identical with that of colloidal CdS stabilized with sodium dodecyl sulfate (SDS)-the absorption onset begins at 520 nm, which coincides with the 2.42-eV bandgap of CdS, and the absorption rapidly rises toward the UV. If the Nafion film is allowed to soak in the 0.1M Na2S bath for a prolonged period the membrane becomes translucent and the absorbance becomes greatly enhanced at all wavelengths. Decreasing the temperature from 295 to 77 K resulted in a blue shift of 18 nm in the absorption spectrum. This corresponds to an increase in the bandgap of CdS to 2.50 eV, an increase of 0.08 eV. Though this value is slightly lower then that obtained for CdS single crystals (dEgp/dT = -5 X 10-4eV/K)m it does indicate that the CdS particles within the Nafion film exhibit optical properties similar to CdS single crystals. Figure 2 shows both the emission and excitation spectra of CdS in Nafion at low-intensity (150-W Xe arc lamp) excitation. Note that the excitation and absorption spectra are identical. This implies that the emission truly arises from direct bandgap excitation of the CdS particles and that the possibility of the luminescence originating from impurity excitation can be excluded. The CdS emission occurs as a broad, structureless band; the half-width and emission maximum at 620 nm are virtually identical with the luminescence properties of colloidal CdS stabilized with SDS. An X-ray diffraction analysis of CdS in Nafion was conducted by comparison to the diffraction pattern obtained for polycrystallinic CdS powder. For the powder, the interplanar spacing d values as well as the peak intensities correlate well with the (19)
Lee, P. C.; Meisel, D. J . Am. Chem. SOC.1980, 102, 5477.
(20) Balkaivski, M.; Waldron, R. D. Phys. Rev. 1958, Z12, 123.
600 IO00 (ns) Figure 3. CdS in Nafion luminescence decay at room temperature monitored at 600 nm (upper curve); log intensity vs. time plot (lower curve). TIME
published data.21 Furthermore, the three most intense peaks are quite distinct and separate. The peaks for the CdS in Nafion sample, however, have merged into essentially one broad peak exhibiting slight shoulders on either side of the maximum. Even though the peaks have coalesced the X-ray data indicates that the CdS in Nafion does possess a crystalline structure. Peak broadening suggests that the sample consists of small microcrystallites and/or that the crystal lattice is perturbed by a nonstoichiometric geometry or incorporation of a large number of defect sites. A further similarity between CdS in Nafion and colloidal CdS stabilized with SDS should be noted. For both samples a blue shift in the emission peak maximum is observed upon increasing the excitation light intensity. This effect is attributed to the large number of defects incorporated either into the CdS lattice or upon the particle surface.22 These defects serve as either electron- or hole-trapping centers located within the bandgap such that radiative recombination at these sites occurs at longer wavelengths than direct conduction band-valence band transitions. At high excitation intensities these trapping centers become saturated and the majority of the luminescence subsequently arises from direct conduction to valence band transitions. As a result, the emission maximum is blue shifted. Figure 3 shows the luminescence decay curve of CdS synthesized in Nafion at room temperature. The luminescence lives for approximately 1 ps, at least three orders of magnitude longer than colloidal CdS.'O Also shown in the same figure is a log intensity vs. time plot from which it can be seen that the emission decay is not simply exponential. Additionally, a log Z/Zo vs. log time plot is also nonlinear implying that the decay kinetics do not follow a power dependence on time as previously suggested for molecular s0lids.2~ The complex nature of the luminescencedecay precludes any quantitative treatment since the correlation between the kinetics and transient decay is presently unknown. Several attempts were made in an effort to understand the nature of such long-lived luminescence of CdS incorporated into Nafion. In this regard, fluorescent probe molecules were absorbed within the perfluorosulfonic acid membrane in order to provide information about the microenvironment present within the Nafion film. Pyrenecarboxaldehyde (PCHO) was chosen since its emission maximum is known to be strongly dependent on the dielectric constant of the media.24 PCHO, Cd2+,and S2- are transported by solvent into the pores of the film. CdS is formed at this location and hence it is reasonable to assume that the luminescence properties of PCHO, which define its environment, (21) Powder Diffraction File, ASTM, 1967. (22) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J . Phys. Chem.,
in press. (23) Debye, P.; Edwards, J. 0. J . Chem. Phys. 1952, 20, 263. (24) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1977.81,2176.
Photophysical Properties of CdS in Nafion Film also comment on the CdS environment. The fluorescence maximum of PCHO in Nac-exchanged Nafion, under vacuum, occurred at 464 nm. This corresponds to a dielectric constant of approximately 65, indicative of a very polar medium. In the H+-exchange form of the Nafion film the PCHO emission maximum, under vacuum, shifted to 526 nm. This result indicates an extremely polar medium. However, there is no significant difference in the photophysical behavior of CdS in either the Na+-exchanged or H+-exchanged Nafion membrane. A possible explanation of this result is that the sulfonic acid group, SO3-, does not function as either an electron or hole trap. The fine structure of pyrene fluorescence is quite solvent dependent25making it an excellent probe for the micropolarity of the Nafion membrane. Peaks I (A = 371 nm) and I11 (A = 393 nm) exhibit the greatest solvent depdencency and may be correlated with medium polarity.26 Pyrene incorporated into H+-exchanged Nafion from a M pyrene/methanol bath yielded a 1II:I ratio of 0.8, indicative of a polar environment. Since pyrene forms a 1:l hydrogen-bonded complex with methanol27it seems reasonable that this complex favors the more polar environment present in the Nafion film. Upon evaporation of the methanol under vacuum the pyrene remains within the vicinity of the sulfonic acid moeity. Absorption of pyrene into Na+-exchanged Nafion from a 10" M pyrene/methanol bath resulted in a 1II:I ratio of 1.6, indicative of a very nonpolar environment. This suggests that pyrene is noi solubilized within close proximity to the sulfonic acid residue but rather somewhere along the perfluoroethylene backbone of the polymer. Furthermore, the 1II:I ratio of pyrene in C7FI4(perfluoro oil FC 77) was very similar to that of pyrene in Na+ Nafion supporting the assignment of pyrene to the nonpolar skeleton of the film. A dispersion of crystallinic CdS powder in the perfluoro oil did not produce any long-lived luminescence which seems to indicate that the fluorine atom does not serve as an electrontrapping center. Water has a dramatic effect upon the luminescence quantum yield of CdS in Nafion. The quantum yield for CdS emission from dry films was approximately five times greater than luminescence from wet films. Degassed water (vigorous N 2 bubbling for 1 h) produced the same effect. These results imply that dissolved oxygen is not responsible for the drastic decrease in the emission intensity of CdS in wet films. Similar results were observed with methanol: the luminescence intensity is severely decreased in the wet film. However, acetonitrile does not affect the quantum yield for emission. It was found that both water and methanol swell the Nafion membrane by approximately 10% by weight (depending on the quantity of CdS incorporated in the film) whereas acetonitrile produced no such effect. Therefore, it is suggested that a possible explanation for the observed decrease in luminescence intensity resides in the CdS surface-polymer interaction. Soaking the membrane in either methanol or water causes the membrane to swell thereby reducing the number of contacts between the CdS particle and the polymer film. This effectively decreases the number of electron- and/or hole-trapping sites provided by the membrane-the emission intensity is decreased. Acetonitrile, since it does not swell the membrane, has no effect on the number of trapping centers and, therefore, no effect on the luminescence quantum yield. In order to gain further insight into the nature of electron/hole recombination of excited CdS in Nafion a transient absorption experiment was performed. The sample was positioned at a 45O angle to both the laser beam and analyzing light so that any scattered light was directed away from the detector. The absorbance at 475 nm was monitored. A sharp decrease in the absorbance was observed immediately following the laser pulse. Additionally, if the analyzing light is blocked there is no detectable luminescence signal. To our knowledge this is the first time that (25) Kalyanasundaram, K.; Thomas, J. K. J. A m . Chem. Soc. 1977, 99, 2039. (26) Nakajima, A. Bull. Chem. Sac. Jpn. 1971, 44, 3272. (27) Lianos, P.; Georghiou, G . Photochem. Photobiol. 1979, 29, 843.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 983
1
, L A S E R PULSE
200
400 TIME
600
800
1000
(ns)
Figure 4, CdS in Nafion luminescence decay a t 77 K monitored at 575 nm (solid curve); transient absorption decay a t 77 K monitored at 475 nm (dotted curve).
I
[MV"]
2
x IO3
Figure 5. Stern-Volrner plot of CdS in Nafion luminescence quenching by MV2+.
a ground-state bleach has been observed in a colloidal semiconductor. Shown in Figure 4 is a shape comparison plot of luminescence from CdS in Nafion and the ground-state bleach. The decays are identical. This implies that the lifetimes for both radiative and nonradiative electron/hole recombination are the same, leading to the important conclusion that once the luminescence event is over there are no further electrons available for redox reactions. Photochemical Properties. The CdS in Nafion luminescence can be quenched by addition of methyl viologen, MV2+. Shown in Figure 5 is a Stern-Volmer plot of MV2+quenching of the CdS in Nafion emission. The plot is linear with a slope of 2.2 X lo3 M-I. However, since the luminescence decay in the absence of quencher is not simply exponential, a reliable value for k,, the quenching rate constant, cannot be obtained. Futhermore, due to an interaction of MV2+with the Nafion film the precise nature of the quenching mechanism is unclear. A dry sample of CdS in Nafion, which had been soaked in 0.1 M MV2+,did not exhibit any luminescence quenching indicating that the MV2+ is not adsorbed on the CdS surface. Due to the uncertainty concerning the distribution of MV2+ between the CdS surface and the active sites in Nafion it is impossible to draw a reliable conclusion about the mechanism of quenching from the steady-state data. In order to determine whether the luminescence quenching is static (decrease in luminescence yield without a change in the lifetime), dynamic (decrease in both luminescence yield and lifetime), or both, a pulsed laser irradiation study was performed. Upon addition of MV2+ to the CdS in Nafion sample it was observed that the initial emission intensity (directly after the laser pulse) decreased with increasing quencher concentration. This result strongly suggests static quenching. Because of the complex nature of the luminescence decay it is difficult to give a proper quantitative treatment. However, if the last portion of the emission is approximated by an exponential decay it can be seen that a dynamic component of the quenching is present (Figure 6). The decay between 400 ns and 1 bs, in the absence of MV2+,has a
984
J. Phys. Chem. 1984,88, 984-989
200
400 TIME
600 (ns)
800
plus MV2+sample in order to increase the quantum yield for MV+. production by intercepting the hole. However, it was observed that EDTA4- had no effect on either the quantum yield or the shape of the transient decay curve due to MV'.. This is probably due to electrostatic expulsion of EDTA4- from the negatively charged Nafion film. Note that Memming and co-workers13also found that EDTA4- did not react with the photogenerated hole in CdS grains incorporated into a polyurethane membrane. Additionally, the quantum yield for MV+. production was approximately 3.5 times greater in Na+-exchanged Nafion as compared to H+-exchanged Nafion. This could be due to a side reaction of the disproportionation products of MV+. with H + 2 8 in the H+ exchanged film: 2MV+*
++
Figure 6. Shape comparison plot of CdS in Nafion luminescence decay: (a) no MV2+;(b) 6 X lo4 M MV2+.
half-life of 230 ns; in the presence of 6 X lo4 M MV2+the half-life decreased to 150 ns suggestive of dynamic quenching. In an effort to confirm the above-mentioned observation a transient absorption experiment was performed. It has been shown that CdS emission is quenched by MV2+via electron transfer4v5 to produce MV+, a stable radical cation (in the absence of oxygen) with a strong absorption at 605 nm. The growth in the absorbance at 605 nm follows the laser pulse implying that MV2+is adsorbed on the CdS surface. That is, coupled with the emission results, the quenching of the luminescence appears to be mostly static. Although the CdS luminescence in Nafion is very long lived compared to CdS colloidal emission it is still relatively short lived when compared to the diffusion of MV2+ in water clusters in Nafion.lg Therefore, the majority of quenching occurs by MV2+ adsorbed on the CdS surface. The rate constant for the backreaction, MV+. h+ MV2+ heat, is 8 X lo6 M-' s-l based on the bulk concentration of MV2+-the actual rate constant is much lower since only those methyl viologen molecules that are adsorbed on the CdS surface undergo electron transfer reactions. Nevertheless, on the millisecond time scale it is possible to observe some MV+. that persists indefinitely. In fact, the quantum yield for MV+. production was determined to be 4 . 3 X lo4 which is comparable to that reported for CdS colloid^.^ Ethylenediamminetetraacetate, EDTA", was added to the CdS in Nafion
+
+
+
MV2+
+ MVo H
CH3-N
+*~j
-
(1)
1
polymeric prduc+s
(3)
Although the equilibrium in (1) lies far to the left the presence of protic acids shifts the equilibrium drastically toward the right via protonation of MV+. (2). The protonated species absorbed in the UV but we were unable to detect it due to the huge background absorption of CdS. Acknowledgment. J.P.K. thanks the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work; B.H.M. acknowledges partial support from the National Science Foundation (CHE 82-01226) and the Army Research Office (DAA 6 29-80-K-0007). Registry No. Cadmium sulfide, 1306-23-6;Nafion, 39464-59-0. (28) Bard, A. J.; Ledwith, A.; Shine, H. J. in "Advances in Physical Organic Chemistry";Gold, V., Ed.;Academic Press: New York, 1970; Vol. 13.
Laser PhotolysWResonance Fluorescence Study of the Rate Constants for the Reactions of OH Radicals with CpH, and C,H, R. Zellner* and K. Lorenz Znstitut f u r Physikalische Chemie, Universitat Gottingen, 3400 Gottingen, F.R.G. (Received: March 30, 1983; In Final Form: August 2, 1983)
Absolute rate constants for the reactions of OH radicals with (1) CZH4 and (2) C3H6have been measured by using excimer laser photolysis to generate OH and time-resolved resonance fluorescence as its monitor. Reaction 1 was studied at total pressures between 4 and 130 mbar and over the temperature region 296-524 K. k , was found to show a "falloff" behavior with the extrapolated high-pressure limit given by k,"(T) = (2.0 f 0.8) X 10l2exp[(320 & 150)K/T] cm3/(mol.s). The observed pressure dependence is consistent with the accepted mechanism OH + C2H4 e C2H40H*2% C2H40H. Possible modifications of k @ ) due to bimolecular product channels are discussed. Reaction 2 was studied at 298 K and total pressures of Ar between 1.3 and 130 mbar. kz is also found to show a falloff behavior. In the limit of high pressure kto = (1.8 f 0.3) X 10" cm3/(mol.s).
-
Introduction OH-olefin reactions have attracted kineticists' interest for two reasons: (i) They are of substantial practical relevance to both hydrocarbon flame chemistry and photochemical air pollution and, therefore, modeling these systems requires an accurate kinetic data base over a wide range of temperature and pressure. (ii) They constitute a class of reactions where, due to the electrophilic 0022-3654/84/2088-0984$01.50/0
addition of OH to the *-bond system, a chemically activated species is formed whose subsequent reactions create a complex dependence of the overall rate coefficient on Pressure and ternPerature. A large number of rate measurements for both O H + C2H4l-I4 (1) N. R.. Greiner, J . Chem. Phys., 53, 1284 (1970).
0 1984 American Chemical Society
