1269
J. Phys. Chem. 1980, 84, 1269-1270
Photocurreints from Semiconductor-Liquid Ammonia Junctions Jacqueline Belloni, * Physico-Chimie des Rayonnements, Universltg Paris-Sud, 9 1405 Orsay, France
Genevigve Van Amerongen, Mlchel Herlem, Chimie Analyiique GgnBrale, 7533 1 Paris Cedex 05,France
Jean-Lou Sculfort, Electrochimie Interfaciale, CNRS Bellevue, 92 190 Meudon, France
and Rudolf Heindl Physico-Chlmie des MatBriaux, CNRS Bellevue, 92 190 Meudon, France (Received August 3, 1979) Publlcation costs assisted by CNRS
Semiconductor-electrolyte junctions have been so far mostly studied in aqueous solutions. The aim of this work was to extend the investigation to nonaqueous solvents and for the first time to liquid ammonia. The photodecomposition of semiconductors in contact with aqueous electrolytes is actually a serious obstacle to the development of photogalvanic cells in view of light energy conversion. A preliminary study of the stability of some semiconductors immersed in liquid ammonia or in sodium-ammonia solutioiis allowed the following conclusions: (i) The surface of seimiconductors such as silicon and gallium phosphide is unaffected by contact with solvent, even a solvent containing strong reducing species such as solvated electrons. (ii)l These semiconductors do not catalyze the slow thermal reaction of solvated electrons giving amide and hydrogen. These two observations support the protective role of liquid ammonia toward solvated electrons at the interface with these semiconductors and can be considered as an indication favorable to the production of e: by photoinjection from a semiconductor into this solvent. The study of the semicoinductor-liquid ammonia junction is also expected, from comparison with the corresponding aqueous electrolyte junction,lJ to provide a better understanding of the phenomenon of electron transfer controlled by the interface. The experimental cell, under vacuum, is connected through grouind glass joints to a piece of brass supporting the disk of semiconductor to two platinum counterelectrodes and t o a reference electrode located in a side arm separated from the cell by a sintered glass. The reference electrode is a silver wire in contact with silver cations (5 X lo4 mol d ~ n -obtained ~) by anodic oxidation of the metal in situ. The solvent ammonia was condensed into the experimental cell at --65 "C after removing traces of water by contact with potassium amide. The electrolyte KBr was dried under vacuum; its concentration was 10-1mol dm-3. The semiconductor with an area of 0.12 cm2 was irradiated by a monochromatic light beam (A,, = 490 nm) through the lbottom of the cell and an unsilvered dewar containing an acetone-dry ice bath. The source was an argon lamp (150 W), and the absorbed power at 490 nm was about 101hW. In the dark, the current at the cathode is observed with the n type semiconductor GaP only and is attributed to 0022-3654/80/2084-1269$01 .OO/O
i/nA
--10
fb,'
-2
.1
,/-1
) /
6b
/
v
-
/
/
10
Figure 1. Photocurrents from Gap, n and p types, in liquid amrrionia at -65 O C as a function of the potential. The light wavelength was 490 nm.
the solvation of electrons. The current at the anode is observed with GaP p type and is to be related with the oxidation of the solvent (or of the electrode). The potential-current curves for gallium phosphide (n and p types) under illumination are given in Figure 1. They are similar to those obtained in aqueous However, in the potential range where the electrodes are ideally polarizable, the photocurrent measured (13 nb) is markedly higher than in water. Other examples of photoinjection of electrons from semiconductors into ammonia were recently describeda6-' It is of interest that, for n-type Gal?, the potential at the threshold, which represents the flat band potential, is V , = -2.0 f 0.2 V relative to the silver electrode, while for p-type GaP it is V , = +0.75 f 0.05 V. When we take into account for the difference between the silver electrode and the normal hydrogen electrode in water: the values of flat band potentials relative to NHE become respectively -1.8 f 0.2 and +0.95 f; 0.05 V. The difference between the two flat band potentials, 2.75 f 0.25 V, corresponds to the transition of the gap. This value is close to that obtained for the same system from impedance measurement~.~ In conclusion, the high photocurrents observed at the semiconductor-liquid ammonia interphase can be ex0 1980 American Chemical Society
1270
J. Phys. Chem. 1980, 84, 1270-1275 (2) J. L. Sculfort, A. M. Baticle, and J. Gautron, C. R . Acad. Sci., Ser. C, 287, 317 (1978). (3) J. Belbni, G;Van Amej-ongen, R, kindl, M, Herlem,and j,L, Sculfofi, C. R . Acad. Sci., Ser. C, 288, 295 (1979). (4) M. HerJem and A. Thiebautt, BuiL Soc. Chim., 383 (1970). M. Herlem, ThBse, Paris, 1966. (5) C. E. Krohn and J. C. Thompson, Chem. Phys. Lett., 65, 132 (1979). (6) R. E. Malpas, K. Itaya, and A. J. Bard, J. Am. Chem. Soc., 101, 2535 (1979). ( 7 ) F. Fox and Kabir-Ud-Din, J. Phys. Chem., 83, 1800 (1979).
plained by electron solvation. Furthermore it is worthwhile noting that the potential of electron pllot&iection is about -1.8 V, that is ~ 0 . V 3 lower than for the same system2 in the solvent water. References and Notes (1) M. Madou, F. Cardon, and W.P. Games, J . Electrochem. Soc.,124, 1623 (1977).
Theory of Electron Transfer Reactions of Solvated Electrons Neil R. Kestner Department of Chemistry, Louishna State University?Baton Rouge, Louisiana 70803 (Received July 17, 1979) Publication costs assisted by the U.S. Department of Energy
In this paper we review electron transfer reactions with specid reference to solvated electron reactions. Some new results are presented on free energy relations and the most serious problems with the application of the theory are presented.
I. General Concepts The theoretical study of electron transfer reactions involving solvated electrons is a special case of the study of electron transfer reactions in various media, in general. Electron transfer reactions are of crucial importance in commercial processes and biological systems in catalysis, enzymes, photosynthesis, etc. In this paper we will try to address the general problem of electron transfer processes and we will also try to discuss some of the unique problems relating to reactions of the solvated electron. Many of the latter have not been adequately studied. The electron transfer reaction of interest can be represen ted by T- A -* T + A(1) where T- may be a trapped electron or any donor and A is some acceptor. This is the outer-sphere electron transfer type of process. We will neglect in this work the socalled “inner-sphere” process in which chemical bonds are ruptured in the process. The interesting feature of the above reaction is that it takes place in a medium. The role of the medium and the details of the molecules themselves are important since this process will only occur if the energy of the initial and final states are equal. The medium and the vibrational modes of the molecules involved can provide the energy necessary for this match. This example of the Franck-Condon principle was first pointed out by Franck and by Libby1 and later elaborated upon by Marcus2 and Levicha3 They emphasized that a static model was not adequate for an explanation of the process. Since the medium is of such critical importance here the interaction of the electron (or donors and acceptors) and the medium plays an important role. All of this is in addition to the basic quantum mechanics of the problem which involves the shift in energy levels of the donor-acceptor complex, basically the interaction matrix elements between species. Therefore a proper study of electron transfer reactions requires the study of the donor plus acceptor plus the medium with all of their vibrational modes (or at least those coupled with the transferred electron). There is one major simplification. Since the electron is being transferred and nuclear motion is slow, we can
+
0022-3654/80/2084-1270$01 .OO/O
usually consider the theory of electron transfer rates in two steps which we will call the microscopic and macroscopic parts. The latter is what is observed experimentally. It is obtained by averaging over the microscopic rates which is our terminology for the probability per unit time for the electron to exchange centers when they are separated by a fixed distance R. This separation is really the BornQppenheimer approximation applied in this special problem. This separation assumes that the electron transfer is independent of the dynamics of the solvent. Efrima and Bixon4 have recently confirmed, by a more detailed stochastic theory, that this assumption is justified in general. In order to define our notation let the macroscopic rate constant be denoted by k and the microscopic rate by W(R,T)(if diffusion can be ignored): k(T) = 4ajm 0 P ( R , T )W(R,T)R2 dR
(2)
where P(R,T) is the probability of finding a donor and acceptor a distance R apart a t temperature T. The form of P(R,T) obviously depends on the medium involved. It represents the classical part of the problem. The quantum aspects of the problem are contained in W(R,T). The semiclassical version of the theory was first treated in the classic papers of Marcus5 who introduced the medium by the use of classical polar fluid theory. Levich6 expanded upon the role of the medium in semiclassical theory. Following this work Dogonadze and coworkers’ presented quantum mechanical versions. At about the same time the Kestner, Logan, and Jortner8 utilized the similarity to radiationless transitions to develop expressions for W(R,T)valid at all temperatures. That approach has been continued by Jortner? Jortner and Ulstrup,lo Van Duyne and Fischer,ll Schmickler,12 and others. We shall use it in this paper. We shall also refer to recent theoretical works by SchmidP who has followed up on a number of important aspects of this problem. Certain general statements can be made concerning the nature of W(R,T). At low temperatures W(R,7‘)becomes temperature independent. This temperature region is often referred to as “tunneling”. However, it is not tunneling in the classic sense of, say, LY particle decay since 0 1980 American Chemical Society
