J. Phys. Chem.
1982,86, 2291-2293
to the few lowest rotational states of BC. Thus, we assume that the v,, = % = 0 state roughly represents the scattering from the lower initial rotational states. As it turns out in the present case, this assumption is not necessary because for the energy we consider here, other terms in the summations over U b and Vb’ would make a negligible contribution to the total summation. In parts A and B of Figure 1we have plotted da/dQ for F + HD(v=O) HF(v’=2) + D and FF(v’=3) H, respectively, at the total energy of 0.33 eV. This energy corresponds to an initial relative kinetic energy of 0.097 eV (2.24 kcal/mol) for HD(v=Oj=O). As seen the former reaction exhibits substantial forward/backward scattering and a highly oscillatory structure whereas the scattering for the reaction to form DF(v’=3) is smooth and totally backward. These results are in qualitative accord with experiment, at approximately the same energy.5 The explanation for these very different differential cross sections is straightforward with the help of parts C and D of Figure 1. There the partial wave reaction probabilities I S z ( E - Evb-Of - Evbt,..:- E J * )are ~ ~ plotted vs. J. The sharp peak in the former reaction probabilities at J = 14 is of course just a consequence of the sharp resonance in the collinear reaction probability? Thus,the J = 14 partial wave dominates the summation over J in eq 1and da/dQ takes on much of the character of IpJ=14(~08 e)I2, i.e., peaked near 8 = 0 and 180” and highly oscillatory. By contrast the partial wave reaction probabilities in Figure 1D are
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quite smooth and typical for a backward scattered, rebound reaction. It is, of course, simply a reflection of the smooth energy dependence of the collinear reaction probability. The sharp resonance in the F HD(v=O) HF(v’=2) reaction has been analyzed and explained with a simple one-dimensional vibrationally adiabatic model.g The absence of sharp resonances in the F + DH(v=O) DF($=3) + H reaction at the energies considered is perphaps due, in part, to the much larger skew angle for this reaction (57”) compared to that for F + HD(v=O) HF + D(37”). Hopefully, refinements in the potential energy surface, the dynamics calculations, and the experiments will all come in the near future and together further elucidate the fascinating role of quantum resonances in chemical reactions.
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Acknowledgment. We are very grateful to Dr. Jack Kaye and Professor Aron Kuppermann for sending us their collinear exact quantum scattering matrices for the F + HD HF D and F + DH DF + H reactions. We also thank Professor Y. T. Lee for useful discussions. Finally, we thank the Department of Energy (Contract No. DE-AC02 81ER10900) for its support.
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(9) Kuppermann, A. “Reactive Scattering Resonances and Their Physical Interpretation: The Vibrational Structure of the Transition State” In ‘Potential Energy Surfaces and Dynamical Calculations”; D. G. Truhlar, D. G., Ed.;Plenum Preas: New York, 1981; Chapter 16, p 375.
Photochemistry of Colloidal Cadmium Sulfide. 2. Effects of Adsorbed Methyl Viologen and of Colloidal Platinum A. Hengleln Hahn-h4eher-InstItut fbr Kemforschung Berlin GmbH, Bereich Strahienchemb, P 1000 Berlin 39, Federal Republic of Germany (Received: Februaty 22, 1982; In Final Form: May 3, 1982)
Methyl viologen (MV) strongly promotes the photodegradation of colloidal CdS in aerated solution. Colloidal platinum exerts a less-pronounced effect. When MV and Pt are present, simultaneously, Pt retards the degradation of CdS. Both MV and Pt efficiently quench the fluorescenceof CdS. These effects are understood in terms of electron scavenging by the additives at the surface of the colloidal particles. However, Pt can also partially remove positive holes from the colloid (short-circuit action). In deaerated solutions, the following M) were present, electrons and positive holes M) and Na2S (5 X effects were observed: When MV were picked up with a quantum yield of 0.6. When only Pt was present, CdS was decomposed and H2generated with a very low yield of 0.0002. When both N a 8 and Pt were present, hydrogen was formed with a larger quantum yield of 0.05. When only MV was present, small photostationary concentrations of some 10” M of semireduced MV were built-up. Introduction Both the photodegradation and the fluorescence of colloidal cadmium sulfide have recently been shown to be influenced by dissolved substances at low concentrations.’ Fast degradation occurs only in the presence of oxygen, and adsorbed cations such as T1+ and Pb2+ drastically promote this degradation. They also quench the fluorescence of colloidal cadmium sulfide. A mechanism has been proposed in which electrons and positive holes are formed by light absorption in colloidal CdS. The essential chemical steps of the overall degradation CdS + O2 + 2H+ Cd2++ S(col1oid) + H202 (1)
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(1) Henglein, A Ber. Buwenges. Phys. Chem. 1982,86,301-5.
are the localization of positive holes to form S- radical anions and the oxidation of S- by oxygen.2 In competition with this degradation, recombination of electrons with positive holes or with semioxidized sulfide anions take place, part of which is accompanied by light emission. The effects of the adsorbed metal ions can be explained by (2) The thermal degradation of CdS by H202requires reaction times of several days under our concentration conditions. It can be neglected in the present experiments where the illumination of aerated solutions did not exceed 10 min. The decomposition of CdS by 02-anions formed in reactions 3, 5, and 9 can also be neglected. This was shown by a radiation chemical experiment in which 02-was produced with a yield of 6/100 eV and decomposition of CdS occurred with a yield of only 0.1 molecules/100 eV. (In this experiment, a solution containing 2 x lo4 M CdS plus 5 X M sodium formate was irradiated under O2 atmospheres.)
0022-3654/82/2086-229 l$Ol.25/0 0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86,No. 13, 1982
pending on the sample.' Deaerated samples had been evacuated under a pressure of bar. Hydrogen yields were determined with a Toepler pump and a McLeod manometer.
Figure 1. Rate of degradation (left ordinate scale) and intensity of fluorescence (right scale) as functions of the concentration of added methyl viologen: 2 X l o 4 M CdS; pH 7.2; aerated solutions.
electron scavenging at the surface of the colloidal particles. Cadmium sulfide is a semiconductor which has been studied in various laboratories during the past years with respect to ita use as the light-absorbing component in photoelectrochemicalcells.H CdS suspensions have been shown to photosensitize the reduction of O2 and MV2+ (methyl viologen cation) by EDTA7as well as the evolution of hydrogen in the presence of colloidal platinum as a co-cataly~t.~>~ The degradation and fluorescence studies on colloidal CdS allow one to recognize the interactions between dissolved compounds and the colloid and to understand the photocatalytic action of CdS in a more detailed manner. In the present paper the effects of the platinum co-catalyst and of methyl viologen on the degradation and the fluorescence of CdS are described.
Experimental Section The preparation of colloidal CdS from Cd(C10J2 and Na2S by using a Ludox HS30 silicon sol (13 nm; 6 X M) from Dupont as stabilizer and the doping of the colloid with less than 1mol % Ag+ ions (to obtain a well fluorescing sample) have previously been described.' The mean diameter of the CdS particle was 37 nm as determined by fractional filtration through micropore filters. The platinum sol was prepared by reduction of H2PtC1, (3 X M) with sodium citrate (2 X M) at 100 "C (1h boiling). The sol was subsequently treated with amberlite until the specific conductivity was 2.5 p!Z' cm-'. The average size of Pt particles was 30 A as determined by electron microscopy. The Pt sol had a weak yellow color. The illuminations were carried out with a 500-W xenon lamp with a 10-cm water filter and color filters (A > 400 nm). In the quantum yield measurements, a monochromator was used to select a fl0-nm wavelength range around 410 nm. Actinometry was carried out with ferric oxalate. The degradation of colloidal CdS was followed by measuring the decrease in its 400-nm absorption. Fluorescence measurements were made with a commercial fluorimeter. The exciting light had a wavelength of 390 nm. The fluorescence maximum lay at 620-650 nm de(3) Gerischer, H. J. Electroanul. Chem. Znterfaciol Electrochem. 1975, 58, 263. (4) Hodes, G.; Manassen, J.; Cahen, D. Nature (London) 1976, 261, 403-4. ( 5 ) Ellis, A. B.; Kaiser, S. W.; Wrighton, M. S. J.Am. Chem. SOC.1976, 98,635-7. (6) Miller, B.; Heller, A. Nature (London) 1976,262, 680-1. (7) Harbour, J. R.; Hair, M. L. J. Phys. Chem. 1977, 81, 1791-3. Harbour, J. R.; Wolkow, R.; Hair, M. L. J.Phys. Chem. 1981,85,4026-9. (8) Darwent, J. R.; Porter, G. J. Chem. SOC.,Chem. Commun. 1981, 145-6. Darwent, J. R. Zbid. 1981, 77, 1703-9. (9) Kalyanasundaram, K.; Borgarello, E.; Griltzel, M. Helu. Chim. Acta 1981,64, 362-6.
Results and Discussion Methyl Viologen Containing Sols. Figure 1shows the relative rate of degradation and the relative intensity of fluorescence of colloidal CdS as functions of the concentration of MV2+in aerated solution. The rate of degradation in the absence of MV2+was put equal to one. As previously mentioned, the quantum yield of degradation is 0.04 CdS molecules per photon, when no additives are present in the aerated sol.' It can be seen from Figure 1 that MV2+accelerated the degradation, the relative rate being 6 at a concentration of M. At higher concentrations of MV2+,the rate did not significantly increase. The fluorescence intensity in the absence of Mv2+was put equal to one (right ordinate scale in Figure 1). With increasing MV2+ concentration, the fluorescence became weaker. At a MV concentration of 5 X lo* M, both the rate of degradation and the intensity of fluorescence had changed to half the effect that was finally attained at M. I t is thus concluded that concentrations above degradation and fluorescence have a common intermediate that is scavenged by the MV2+ molecules at the surface of the colloidal particles. These effects are understood in terms of the previously proposed mechanism.' MV2+ scanvenges electrons that are formed by light absorption in CdS and the product MV+ further transfers them to oxygen:
(The subscript ads indicates an adsorbed species.) As previously shown, oxygen itself does not pick up electrons in an efficient manner.' As electrons are removed via reactions 2 and 3, more positive holes escape neutralization and therefore more efficiently oxidize sulfide anions than in the absence of MV2+: h+
s- +
+ S20 2
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-s +
02-
(4) (5)
The 02-radical anions mainly form H202.2 The above mechanism is corroborated by the results obtained in the illumination of deaerated solutions. Under these conditions, MV+ appeared and reached a stationary concentration after about 40 min of illumination. The stationary concentration became greater with increasing MV2+concentration. At the highest concentration used, M, the stationary MV+ concentration was i.e., 1.5 X M. The rates of formation of MV+ according 1.3 X to reaction 2 and of the consumption via
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h+ + MVads+
MVads2+
(6)
were equal in the stationary state and no net conversions of MV2+ or CdS occurred here. The MV+ formation was strongly enhanced in sols that contained Na2S as a second additive, as has also been reported by Gratzel.lo Under these conditions, the positive holes were scavenged by adsorbed sulfide anions before they reoxidized MV+ ions: 2h+ + Sad:-
+
Sads
(10) Griitzel, M. Acc. Chem. Res. 1981, 14, 276.
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sorbed H atom (reaction 9). However, the accelerating effect of Pt is by far not as pronounced as that of MV2+ (Figure l),although the strong fluorescence quenching by Pt indicated an efficient electron pick-up from the CdS particles. These findings are understood in terms of the Pt particles being able to also pick up positive holes, the efficiency of this process being somewhat lower than that of electron scavenging: Pt-H + h+ Pt + H+ (10)
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