On three hypotheses in photo-polarography

EIectrochhica Acta. 1968, Voi. 13, pp. 1249 to 1252. Persamon Press. Printed in Northem Ireland

ON THREE HYPOTHESES

IN PHOTO-POLAROGRAPHY*

H. BERG Deutsche Akademie d. Wissenschaften, Institut ftir Mikrobiologie und Pxperimentelle Therapie, Jena, Abt. Biophysikochemie Abstract-Three hypotheses for the origin of photocurrents at electrodes are compared, and types of critical distinguishing experiments are suggested. R&m&-Comparaison de trois hypotheses rendant differamment compte de l’origine des photocourants aux electrodes et suggestion de divers types d’exptrienees eruciales qui permettraient une discrimination. Zusarmneufassung-Die drei hestehenden Hypothesen tiher die Ursache des Photostromes an belichteten Quecksilherelektroden werden verglichen und die experimentellen Miiglichkeiten zur Unterscheidung diskutiert. 1. INTRODUCTION AT PRESENTtime three hypotheses on the origin of the current in non-absorbing and depolarizer-free solutions under illumination exist. Barker’9 and Heyrovskj% assumptions represent two extreme stand-points whereas the hypothesis of Berg3 is of an intermediate kind. 2. TERMS Before introduces

discussing the hypotheses it is necessary to explain some terms. Berg the conception of photo-residual current, i,. This term means a current

arising in consequence of the illumination of the polarized electrode in depolarizer-free non-absorbing indifferent solution. Barker uses the term “photo-current” (Qu for the same current observed when a negatively charged electrode in contact with a neutral solution is irradiated by visible or uv light. Heyrovsk$‘s term “photocurrent” (i,), means the increment of current produced by illumination. This variety of conceptions complicates the comparison. But we may adopt the following connection, iv = (i& = (i& + i,, neutral solution,

where i, = residual current without illumination. A confrontation of the three hypotheses is given here for the case of aqueous solutions of simple neutral electrolytes only. 3. THE

THREE

HYPOTHESES

According to Barker’s experiments his “photo-current” is primarily caused by the photo-emission of electrons which after becoming thermalized and hydrated tend to be captured by hydrogen ions or other reducible ions or molecules, so-called scavengers,

in solution in a specific distance from the electrode surface. There the hydrated electrons form an electron cloud. Some of the hydrated electrons return to the electrode. * Prepared for the 18th meeting of CITCE, Elmau, April 1967; manuscript received 23 August 1967. 1249

H. BERG

1250

Heyrovslj assumes a charge-transfer complex of adsorbed water molecules with the electrode surface. This complex absorbs at longer wavelengths than non-adsorbed water does. The absorption of light leads to electron transfer from the electrode (the donor) to the water (the acceptor). The difference of currents with and without illumination of the electrode is his “photo-current”. Berg’s “photo-residual current” is due to direct emission and tunnelling in a region of short wavelengths only. These ejected electrons are captured directly by protons (almost without hydration of electrons). At longer wavelengths emission does not occur: only the excitation of the electrode takes place (“hot electrode”). In such a way all the electrochemical parameters (without illumination) of the system are modified, including those of the double layer. In the course of the restoration of the former double layer a capacitive current occurs in every case. Sign inversion of i, can be explained by photo-desorption especially in the anodic potential range. 4. THE

MAIN

DIFFERENCES

The three hypotheses may be characterized by the expressions: (a) “electron cloud” (Barker) (b) “charge-transfer layer” (Heyrovsky) (c) “hot electrode” (Berg) and the main differences are shown in the following scheme3 TABLE1 Substanzen

Hypothesen

Elektrodensubstanz

Barker (“electron cloud’)

&O 1 eaqScavenger

Reaktionen Emission von eSolvation in der Losung e,,-

+ H,O+ -+ H + H,O

Diffusion von eaq- in Losung, Rtickkehr von eaq- nach der Elektrode Berg (“hot electrode”)

Heyrovskjl (“charge-transfer

layer”)

Elektrodensubstanz

Anregung der Elektrodensubstanz

angeregte Elektrodensubstanz eH,O+ 1 eH,O 1 eaqKationen Anionen

Emission,

Elektrodensubstanz) Hz0 I CT-Komplex CT*

Durchtunnelung

e-+H,O+-+H+H,O Solvation Weiterreaktion Doppelschichtbildung Adsorption

bzw.

CT-Komplexbildung CT*-Komplex-Anregung CT* -+ (e&q-hd

b&q-)sd

-+e,

Scavenger (Kationen) eaqI

eaq-

+

HsO+ -+ H + H,O

von e-

On three hypothesesin photo-polarography

1251

5. POSSIBILITIES OF EXPERIMENTAL DECISION In a scavenger-free simple solution the following experiments may serve as crucial experiments : (a) Barker’s hypothesis A square-wave-formed excitation-flash could be used in order to separate the emission by light and the return of electrons in the dark. Under such conditions the current of the returning electrons should predominate and a sign inversion of current should be observed. (b) Heyrovskj?s hypothesis With aid of attenuated total reflectance spectroscopic techniques it should be possible to observe the absorption of the CT-layer coupling these techniques into a working photo-electrochemical system (Osteryoung, Hansen and Kuwana, presented at the 17th CITCE meeting, Tokyo 1966). (c) Berg’s hypothesis The capacitive character of the extremely small photocurrent in the long wavelength region, especially on the positive branch of the electrocapillary curve, must be proved. 6. THEORETICAL TREATMENT A general theoretical treatment on the basis of an unified continuum theory consisting of electromagnetics, optics, continuum mechanics, and irreversible thermodynamics is able in principle to describe all three hypotheses. The photocurrent of each of the hypotheses is described by a phenomenological relation, ie a relation between thermodynamic fluxes and thermodynamic forces. The fluxes and forces playing a role in our multicomponent two-phase system under illumination are taken from the entropy-production term of this special model. For each process, eg diffusion, photoemission, restoration of the double layer etc, a pair of thermodynamic fluxes and forces exists. Forming the phenomenological relations we find the most general dependences. In the different hypotheses the forces composing the photocurrent are of different importance. These further experiments to decide between the three hypotheses would be helpful in understanding the electrode processes under illumination, the behaviour of hydrated electrons, and the spectroscopic properties of adsorbed substances. Added in proof:

A critical discussion of results of Barker and Heyrovsky’ was published* recently and this wave length dependence: i, --(A - &Jslz was measured. DISCUSSION (on papers by Barker, Heyrovskj and Berg) J. Hale-The

observation of a photocurrent at an electrode requires complete charge separation at the interface. In the cathodic case for example, the electron emitted from the electrode must be transferred eventually to the bulk of the solution. The excited state of a charge-transfer complex is a bound state, however, so that it seems to me that absorption by this complex should occur at waveFurthermore, I would suggest that at the red lengths longer than the red limit for photo-emission. limit, transfer of the electron occurs directly to an acceptor on the outer Helmholtz plane. At shorter

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wavelengths excess kinetic energy should be dissipated by collisions in the liquid before capture can occur. IX Gerischer-A system must fulftl some special conditions for one to detect a photocurrent in a steady state situation. I think this can occur only when there is some irreversible process following the primary emission step. Otherwise, since the emitted electrons reach only a distance of 100 A, the steady state should be reached after a time of lo-’ s, when all the electrons emitted from the surface would migrate back to the surface and be recaptured. There is no problem if the activated complex at the surface reacts rapidly enough: in that case the mechanism that Dr. Hey-rovsk$ proposes seems to be likely. I think that the key to the problem will be found in the correlation between the energy threshold for the electron emission and the electrode potential. This relationship is found to be linear in most cases, but the slope is normally much less than 1, which seems to indicate the importance of the structure of the energy barrier at the interface for the rate of electron emission. In general, one would expect that interaction with a polarizable solvent (using the optical dielectric constant) should give a reduction of the emission energy, but this effect seems to be much smaller than might be expected. F. Lohmann-It may be interesting to study photo-emission of electrons at electrodes in contact with solvents in which electrons are stable, for instance ethylene&amine. This system should be a reversible one. I agree with Professor Gerischer that one will see photocurrents mainly in cases where, after photo-emission of electrons, irreversible reactions take place. In their absence, it seems to be sensible to measure open-circuit potentials, because there must be a change of charge distribution between electrode and electrolyte caused by emitted electrons that are in equilibrium with the excited electrode. R. A. Marcus-With regard to Dr. Barker’s point involving the solvation of the emitted electron, several contributions are (1) an exchange repulsion, due to the operation of the Pauli exclusion principle, between this electron and the electrons in nearly solvent molecules,t (2) a polarization of the latter’s electrons, for those that are not too close to it,$ and (3) an interaction of the smeared-out charge cloud of the electron with the re-oriented solvent dipoles and higher poles. The third contribution does not influence the threshold energy for emission of the electron from the metal, but any existing layer of solvent molecules on the electrode does. REFERENCES 1. 2. 3. 4.

G. BARKER,Electrochim. Acta 13, 1221 (1968). M. HEYROVSK~,unpublished. H. BERG, J. Electroanal. Chem. 14,351 (1967). T. PLESKOW,A. BRODSKIand W. LEW~SCH, Elektrochimija (run.) 3, 1302 (1967). t This is perhaps factor responsible for the cavity formed by an electron in liquid ammonia. $ A rough estimate is given in R. A. MARCUS,J. them. Phys. 43,3477 (1965).