J. Phys. Chem. 1986, 90, 1844-1849
1844
Electrochemical Photovoltaic Cells with Layered Molybdenum Diselenide Photoanodes Mohamed Etman Laboratoire d’Electrochimie Interfaciale du CNRS, 921 95 Meudon Principal Cgdex, France (Received: July 15. 1985)
Current-voltage characteristics of the illuminated n-MoSez/HzS04interface revealed that the dark flat-band potential and the steady-state photocurrent onset potential were virtually identical at very low light intensities. Increasing the photon flux gave rise to more positive onset and flat-band potentials. These shifts leveled off at a maximum value of +0.45 V relative to the dark flat-band potential. Higher light intensities did not enhance the shift any further. In the presence of potassium ferrocyanide, the dark flat-band potential remained invariant, and the anodic shifts engendered by illumination were considerably reduced. Furthermore, the dark flat-band potentials (estimated from differential capacity measurements and Mott-Schottky type plots) were shifted anodically by the presence of ceric sulfate. When [Ce4+]= 0.01 M, the magnitude of the shift was the same as the maximum shift observed due to illumination. Fast photocurrent transients at the n-MoSe2/sulfuric acid interface revealed a downward shift of the energy bands, due to an accumulation of holes in the surface states. Charge-transfer processes and the trapping of holes were studied in the presence of various redox systems.
perimental conditions. “Ideal responses” of the illuminated inIntroduction terface have been substantiated, where the “onset potential” of In transition-metal dichalcogenides, the valence and conduction the steady-state photocurrent was the same as the flat-band poband edges are, in principle, defined by filled and empty electron tential in the dark. This occurred in the absence of redox couples states which are not involved in the covalent bonding. Accordingly, whose standard potentials matched the semiconductor bandgap. optical excitation of charge carriers must necessarily proceed On the other hand, high light intensities or oxidizing agents such without breaking the chemical bonds which are associated with as Ce4+ in the dark engendered parallel anodic shifts of (a) sp-hybridized orbitals of the metal and chalcogenide species.l flat-band potentials and (b) onset potentials of steady-state Furthermore, the metal atoms in the layered lattice are confined photocurrents. Thermodynamic and kinetic implications are to planes, screened from the crystal surface by a layer of the assessed. chalcogenide material. On the basis of these energetic and structural considerations, it is reasonable to assume that the basal Experimental Section plane (which is perpendicular to the C axis of the crystal) should All electrochemical measurements were made in a single comnot be prone to anodic corrosion. Such inertness would obviously partment containing both working and counterelectrodes. A be important in solar cells, where n-type semiconductors are in saturated calomel reference electrode was connected via a salt contact with aqueous electrolytes.2 In practice, photocorrosion bridge. The cathode consisted of platinum and had a surface area of layered structure semiconductors has been n ~ t i c e d . ~Pho,~ of about 10 cm2. The anode was made of an n-MoSe2 single tocorrosion was attribute! to exposure of parallel planes to the crystal approximately 0.05-0.10 cm2 in area, embedded in a Kel-F C axis of the electrode (Cll) to the e l e c t r ~ l y t e . ~On the other holder. Leakage of solutions was obviated with the aid of Scotch hand, stability against corrosion has been observed in the presence cast (3M Co., Minneapolis, MN). An ohmic contact (see Figure of certain redox couple^.^^^.^-^ More detailed experimentation 1) was realized through a drop of silver ”lacquer” (200-DEMEseemed, in general, necessary to elucidate the relevant photoeT R O N supplied by DEMETRON, Hanau, West Germany) to lectrochemical processes. Correlations to crystal structure and the back of the molybdenum diselenide single crystal and heating other properties of layered materials (anisotropy in carrier mobility and difference in chemical reactivity) have been i n v e ~ t i g a t e d . ~ . ~ ’ ~to 70-80 O C for 30 min. Anode surfaces were periodically renewed by chemical etching with perchloric acid or by application of The inconsistency between theoretical expectations and actual adhesive tape, peeling off successive layers and exposing clean new findings warranted further investigation. Summaries of recent surfaces. findings have been published in two preliminary note^.'^,'^ Results For,,impedance measurements, a high-frequency (10-1 00 kHz) are presented and discussed in detail in the present communication. sinusoidal current was applied and corresponding resistances and A plausible mechanism is proposed to account for the behavior capacities were recorded. Semiconductor electrodes were illuof the n-MoSe,/electrolyte liquid junctions under various exminated with white light from a xenon-mercury lamp (whose output was adjustable between 400 and 1100 W). Light intensity was controlled further by placing attenuating gray filters in the ( I ) Mattheis, C. F. Phys. Reu. B 1973, 8, 3719. (2) Tributsch, H. Ber. Bunsenges. Phys. Chem. 1977, 81, 361. light beam. A rotating chopper (variable speed) provided in(3) Trfbutsch, H.; Bennett, J. C. J . Electroanal. Chem. 1977, 81, 97. termittent illumination for transient measurements. Illumination (4) Tributsch, H. Structure Bonding (Berlin) 1982, 49, 127. times were typically 50 ms. All chemicals were reagent grade (5) Lewerenz, H. J.; Gerischer H.; Lubke, M. J. Electrochem. SOC.1984, (supplied by Merck). Water was purified in a super Q Millipore 131, 100. (6) Gobrecht, J.; Gerischer, H.; Tributsch, H . Ber. Bunsenges. Phys. Chem. system. 1978, 82, 1331.
(7) Lewerenz, H. J.; Heller, A.; Di Salvo, F. J. J. Am. Chem. Soc. 1980, 102, 1877. (8) Parkinson, B. A.; Furtak, T. E.; Canfield, D.; Kain, K.; Kline, G. Faraday Discuss. Chem. SOC.1980, 70, 233. (9) Kautek, W.; Gerischer, H.; Tributsch, H. Ber. Bunsenges. Phps. Chem. 1979, 83, 1000. (10) Kautek, W . ;Gerischer, H. Surf. Sci. 1982, 119, 46. (1 1) Etman, M.; Mc Evoy, A. J.; Memming, R. Poster presented at the
workshop on Photoelectrochemistry and Catalysis organized by the Royal Netherlands Academy of Sciences, Amsterdam, Dec 12-14, 1984. (12) Mc Evoy, A. J.; Etman, M.; Memming, R. J. Electroanal. Chem. 1985, 190, 225. (13) Etman, M.; Levy, F. J. Electroanal. Chem. 1985, 183, 395. (14) Etman, M. J. Electroanal. Chem. 1985, 183, 401.
0022-36j4/86/2090-1844$01.50/0
Results Current-potential characteristics in darkness and under illumination are illustrated in Figure 2. The normalized3’ ordinate assignments correspond to currents which were steady, i.e. similar results were obtained regardless of the potential sweep rate (in a range between 0.003 and 0.5 V s-]). The onset potentials of the steady photocurrents were dependent on relative light intensity up to a limit shown in Figure 2, curve D. When the photon flux was further increased, the shape of the curve remained the same and the onset potential remained invariant. Results of differential capacity measurements of the dark interface are presented by (I) 0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1845
MoSe2 Photoanodes
-500
I
I
-200
-100
0 voltage
I
1
1100
+zoo
+4 4 N
I
1 rnv
-J
Y
Figure 1. Current-voltage relationship between two back contacts (using a drop of Ag “lacquer’’) on both MoSe2 single crystal surfaces: R u 250 Q and p N I O fl cm.
E
a
\ U
-300;
Yu
Y
2
-
0
3 Y
200
r
0
-
1
I
to5
Ev/V,
100
0
+I
vs
I-
S.C.E.
Figure 3. (I) Mott-Schottky plot in the dark for n-MoSe,/l M H2S04 interface. (11)T h e same a s curve D shown in Figure 2.
a 5
Onset potentials
20 Upper limit of onset potentia
1
15-
’I
E,/V.
VI
S.C.E.
Figure 2. Current-voltage curves of n-MoSe2 anode immersed in aqueous 1 M H2S04. T h e saturation photocurrent Io for each illumination is
10
taken as a parameter of the light intensity. T h e saturation (injection) current for B, C , and D are respectively 10, 100, and 500 MA cm-2. Photocurrent on the ordinate is normalized to the saturation current Io of 500 uA c d .
in Figure 3. It is important to note in Figure 3 the difference between the flat-band potential (V,) obtained by extrapolation of curve I and the onset potential of the steady photocurrent on ’ ~ - ~ ~ various illuminated curve 11. In previous s t ~ d i e s ’ ~ ,involving photoanodes the following findings were substantiated by “frequency analysis’! of impedance measurements: (a) in the saturation domain (plateau region on current-voltage curve), the impedance corresponded to a simple equivalent circuit; (b) that circuit consisted of the depletion layer capacity and a resistance in series. In the present investigation, Mott-Schottky plots were also obtained (in the dark and under illumination) in the presence of various redox couples. Capacity values were evaluated by assuming that conditions a and b held. In the absence of redox couples (Figure 4), the illuminated Mott-Schottky plots (curves B, C, D, ..., I) shifted anodically relative to their dark counterpart (curve A). The Mott-Schottky region slopes remained invariant, (15) Kelly, J. J.; Notten, P. H. L. J . Electrochem. SOC.1983, 130, 2452. (16) Meeraker, J. E. A. M. v. d.; Kelly, J. J.; Notten, P. H. L. J . Electrochem. SOC.1985, 132, 638. (17) Schrijder, K.; Memrning, R. Be?. Bunsenges. Phys. Chern. 1985,89, 385.
-
5-
0
+OS E,/V
t l
v s ’5.C.E
Figure 4. Dependence of the differential capacity of the n-MoSe2/ 1 M H2S04interface on the applied potential in darkness and under different light intensities. Saturation or injection currents (in PA) taken as parameters of light intensities are as follows: A, 0; B, = 0.37; C, = 0.4; D, = 0.5; E, = 1.0; F, = 1.5; G, = 2; H, = 2.2; and I, = 2.4. Electrode area
N
5 mm2.
i.e. the plots were parallel. In the presence ofpotassium ferrocyanide (Figure 5 ) , the illuminated Mott-Schottky plots yielded a more clustered family of curves than in the absence of potassium ferrocyanide (Figure 4). Ferrocyanide had no effect on the depletion layer capacity in the dark. The shift of the Mott-Schottky intercepts on the potential axis (C2 0) at various light intensities in the absence and in the presence of ferrocyanide is summarized
-
Etman
1846 The Journal of Physical Chemistry, Vol. 90, No. 9, 1986
-Peak
E:
O.lOV/SCE 7
[.I
S t e a d y Photocurrent
E:0.25
E:0.35 0
+ 1 Ev/V
r0.5
b
v s S.C.E.
Figure 5. Dependence of the differential capacity of the n-MoSe2/(l M H2S0, + IO-* M K,[Fe(CN),]) interface on the applied potential in darkness and under illumination. Saturation (injection) currents (in @A) are as follows: A, = 0; B, = 1.8; C, = 15; D, = 30; E, = 45; and F, = 60. Electrode area N 5 mm2. 0
20
10
30
Figure 7. Photocurrent transients of n-MoSe2/1 M H,S04 at various M electrode potentials in the absence [A] and presence [B] of K,[Fe(CN),I.
+ 0.
40
.
t
-A*-
[A
N
404
5 0.
/*
-
1-
a E \
//
;0. ? ¶
V
0.
-.-*-
100. .
y
e
*/
aoo
800 I n ~ e cIion
no
,
i
itoa
c Y r IL n t , p di Cm"
Figure 6. Displacement of the Mott-Schottky intercepts in the absence [A] and in the presence [B] of M K,[Fe(CN),] obtained with different light intensities and relative to the dark flat-band potential. in Figure 6. In this context, it should be noted that the injection current scale used for B in Figure 6 is thirtyfold greater than the scale of curve A. It is apparent that the displacements of the flat-band potential for the illuminated interface are minimized by the presence of ferrocyanide (the absorption of light by 0.01 M ferrocyanide was negligible under the experimental conditions). If we restrict the comparison (Figure 6 ) of flat-band potential shifts, obtained in the absence and in the presence of K,Fe(CN),, specifically to an injection current density of 30 p A cm-2, we can conclude that the presence of 0.01 M K,Fe(CN), diminished considerably the anodic shift, restoring the flat-band potential to its dark value. The potential range in which no steady-state photocurrent was observed (in acid electrolytes in the absence of redox couples) was associated with appreciable transient (relaxation) effects.1'-13J8*19 Under appropriately selected conditions ( I , > 10 1A cm-', the absence of oxidizable moiety, such as ferrocyanide), photocurrent transients were observed during the first 50 ms following the (18) Gerischer, H. J . Electroanal. Chem. 1983, 150, 5 5 3 . (19) Philips, M. L.; Spitler, M. T. J . Electrochem. Soc. 1981, 128, 2137.
to
/
I
dark
L 0 . 5
+1
EV/V
vs
S.C.E.
Figure 8. Plots of photocurrents vs. applied potential. Io = 0.5 mA cm-2. (A) Steady-statecurrent in the absence of K,[Fe(CN),]; (B) same as (A) but in the presence of 0.01 M K,[Fe(CN),]; (C) transient photocurrent peaks in the absence of ferrocyanide. For identifying transient and steady-state photocurrent domains, refer to Figure 7 . application of a given potential. A typical set of such transient photocurrent measurements is shown in Figure 7A. After rising very steeply to a peak value on the incidence of the illumination, the photocurrent decayed to a stationary value. The relationship between transient and steady photocurrents is shown in Figure 8. The main difference between the results presented in curves A and B of Figure 8 is that the onset potential of the steady photocurrent occurs at a much more negative potentials in the presence of 0.01 M K,Fe(CN),. Values of transient current peaks obtained for the illuminated interface in the absence of the electron
MoSe, Photoanodes
1847
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986
thO25V
th1.6SV
DARK
1 LLUMl NA T E D
Figure 10. Band edge displacements. Bandgap of 1 4 eV according 10 Nozik 22 E,S and E,S are respectively the conduction and valence band energies at the semiconductor/electrolyte interface
counted for by a previously postulated surface state model I ' The potential drop across the depletion layer is given b y
v,, = v, - Vb*Y = v, - vo, Figure 9. Effect of 0.01 M Ce(1V) on the behavior of n-MoSe2/l M H2S04interface in the dark A, current-voltage curve; B, Mott-Schottky plot in the presence of Ce(1V);C, Mott-Schottky plot in the absence of
Ce(1V). donor [Fe(CN),I4- (and at the same light intensity) gave rise to curve C in Figure 8. The transient current observed in the absence of ferrocyanide disappeared entirely in the presence of 0.01 M K,[Fe(CN),] (see Figure 7B). The photoelectrochemical characteristics of the n-MoSe2/electrolyte interface in the presence of weaker electron donors have also been investigated. It was found that the presence of cerous sulfate had no effect at all, and the presence of ferrous sulfate had only a small effect. On the other hand, the effect of 0.01 M Ce(1V) ions as hole donors is documented in Figure 9. To compare results given on curves B and C in Figure 9, we had to consider the position of standard potential of Ce(IV) ions relative to the energy bands of MoSe2. In fact, the very positive standard potential of Ce(1V) ions is approximately the same as the valence band energy of MoSe, (see Figure IO). So it is apparent that Ce(1V) ions are capable of injecting holes into the valence band of MoSe, in the dark, engendering an anodic shift which parallels the shift observed due to hole injection by light excitation (cf. Figure 4). The relevant impedance measurements were made at potentials more positioe than the onset potential of the cathodic dark current which corresponds to the electroreduction of Ce(IV) to Ce(II1). Furthermore, the process Ce(1V)
+e
-
Ce(II1)
(1)
was observed at the onset potential of the steady photocurrent obtained under strong illumination in the absence of any redox couple (cf. Figures 9A and 311). Evidently, the onset potential of the current due to the reaction 1 was comparable to the maximum flat-band potential under illumination. Undoubtedly reaction 1 could occur at more anodic potentials; however, holes are concomitantly consumed by semiconductor corrosion yielding a negligible net current.
Discussion Mechanisms of Electron Transfer. The results can be ac-
- AV$'
l 2 -C
(L)
where Vdenotbs the potentials, the superscripts 0 dnd h*dirlutL respectively dark and illumination conditions, and the suLscripts have the following meaning: Sc, semiconductor; E, elect1ode; fb, flat band; H, inner Helmholtz layer. The anodic shift of the flat-band potential and the associated change of the potential drop across the Helmholtz layer resulted in downward d i s p l a ~ e m n t s of band edges as illustrated in Figure 10. Undei &ad> stdir. conditions, any anodic potential drop at the interface l m b t I W essarily entail a corresponding accumulation of holes, whest density is given by
N,e = A Q = CHAVHhu
(3) where C and AQ denote respectively capacity and tne Lvrrc sponding charge. For instance, if C, N 10 p F cm-2, on6 finds Nt = 3 X l o i 3cm-2 using the experimental value A/', = 0 45 V. Alternatively, the density of accumulated holes can albo bc evaluated from the photocurrent transients shown in Figure 7 by
N , = lsmiph dt e o Equations 3 and 4 yielded N , assignments which NUX iri g d ~ d agreement. To explain the mechanism of hole accunluldtiun ;IC assumed that the relevant energies under illuminatiori are itliiii the forbidden bandgap. The surface states may be extrimit i n nature, Le. created by illumination, or intrinsic, i.e. a characteristiL feature (without illumination) of the semiconductor as suL.h. Similar hole accumulation mechanisms have previously been reported for classical semiconductors such as GaAs, CdSe, Gap, and CdS.23-25 However, the Mott-Schottky plot shifts observed with those materials (under Comparable experimental conditions) were smaller relative to anodic shifts detected in the present investigation. Anodic shifts of Mott--Schottky plots and/or transient (relaxation) responses have been already noticed with nonlayered n-type semiconductor/liquid j ~ n c t i o n s . ~ "Moreover, ~~ (20) Memming, R.; Kelly, J. J. In Proceedings of the 3rd Internarioncrl Conference on Photochemical Energy Conversion and Storage, Connolly, J . S . , Ed.; Academic Press: New York, 1981; p 243. (21) Kelly, J. J.; Memming, R. J . Electrochem. SOC.1982, 129, 730. (22) Nozik, A. J. In Proceeding of a NATO Advanced Study Instirute oti Photovoltaic and Photoelectrochemical Solar Energy Conversion, Gent, Belgium, 1980, Cardon, F., Gomes, W. P., Dekeyser, W., Eds.; Plenum Press: New York, 1981; p 302. (23) Memming, R. Electrochim. Acta 1980, 25, 77. (24) Freze, Jr., Karl W. J . Electrochem. SOC.1983, 130, 28. (25) Memming, R. Nato ASZ Ser. C.1985, 146, 107.
1848
Etman
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 E
electrolyte
i
I Figure 12. Surface steps on a 1a)ercd MoSe, aeinlconduztor dnd motion of photogenerated _holes. Ilole concentration profiles parallel and perpendicular to the C axis Lire dlso indicated Adapted from ref 5
1
Figure 11. Mechanisms of electron transfer. Electron donors I and I1 represent respectively [ Fe(CN),]’ and Ce3+;Es,scorrespond to surface states energy levels. E, and E, are conduction and valence band energies in the bulk of the semiconductor and E e is the Fermi-level energy.
negative shifts of the onset potentials of illuminated p-GaAs, p-Gap, and p-CdTe have been reported by Kelly and Memming, Peter and co-workers, and Fotouhi, r e s p e c t i ~ e l y . ~The ~ , ~shift ~~~~ described in this paper (MoSe2) was comparable to the one observed for another layered semiconductor, viz., n-WSe2.’‘ , I 2 Remarkably, the number of holes accumulated amounted to as much as -3% of the surface atoms. We have substantiated the proposed surface state model through experiments involving addition of various electron donors (reducing agents) to the electrolyte. The change in the voltammetric characteristics of the illuminated n-MoSe, interface due to the presence of 0.01 M K,[Fe(CN),] is documented in Figures 5, 6B, 7B, and 8C. It is apparent that addition of ferrocyanide shifted the onset potentials of the steady-state photocurrents cathodically, Le. nearer to the dark flat-band potential (the dark flat-band potential being the same regardless of the presence or absence of ferrocyanide). A smaller cathodic shift was engendered by ferrous sulfate. However, cerous sulfate had no effect whatsoever. The redox potential of ferro-ferricyanide lies within the bandgap of MoSe,. Thus, the photocurrent corresponds to an electron transfer from [Fe(CN),]& to surface states, followed by recombination with photoinduced holes. This mechanism is shown in Figure 11. The redox potential of the cerous-ceric couple is considerably mure positive favoring (26) Ezzaouia, H. ThCse 3e Cycle, UniversitC de Paris XI, 1982. (27) Allongue, P.; Cachet, H.; Horowitz, G.; J . Elecrrorhem. Sor. 1983, 130, 2352. (28) Allongue, P.; Cachet, H.; Fronieiit. M . 33th ISE Meeting, Erlangen, Sep 1983, Abstract No. 0719. (29) Ezzaouia, €1.; Heindl, K.;Parsons, K.;Tributsch, H. J . Eleciruanal. Chem. 1983, 145, 219, 1984, 165, 155. (30) Anderrnan, M.; Kennedy, J. €1. J . Electrochem. Sur. 1984. 131. 21 (31) Kennedy, J . H. Lecture presented at the workshop on Photoelectrochemistry and Catalysis organized by the Royal Netherlands Academy of Sciences, Amsterdam, Dec 12-14, 1984. (32) Allongue, P.; Cachet, 13. J . Elerrrocheni. Soc. 1985, 132, 45. (33) Etman. M . Guyot, hl.; Merceron, T.; Parsons, R. Poster presented at Journees d’Electrochimie 85, Florence, May 28-31, 1985. (34) Li, J.; Peat, R.; Peter, 1..M. J . E k c t r o m n l . Chrni. 1984. 165, 31 (35) Fotouhi. B., private communication
Figure 13. Microscopic mechanism of charge transfer via surface states. Electron donors Di and DIIare the reduced forms of redox couples whose standard potentials favor electron transfer respectively to surface states in the forbidden bandgap (e.g., ferrocyanide in Figure 1 1 ) and into the valence band (e.g., Ce3+ in Figure 11). So+ is a surface radical cation, v denotes reaction rates.
(in principle) electron transfer from Ce3+ions directly into the valence band of MoSe,. Nature of Surface States. Surface states may be engendered by defects of crystal structure, by thin films, by intermediates of corrosive dissolution, etc. The smooth basal planar surfaces of layered MoSez and WSe, are chemically rather inert. Accordingly, it appears reasonable to correlate the surface states with “steps”, illustrated in Figure 12, as proposed by Lewerenz et aL5 A “step” does not necessarily mean a discontinuity on a single crystalline surface but may involve several layers. This may explain the relatively large density of prevailing surface states. Microscopic Modeling oJ’Charge Transfers. Charge transfer via surface states is summarized schematically in Figure 13. Experimental measurements (Figures 4 and 5) have revealed that onset potential shifts attained a limiting value (beyond which further irradiation enhancement was inconsequential) at a critical lightflux (CLF). This finding can be rationalized by assuming that whenever I I CLF
(5)
the rates of reactions 6 and 7 are equal: generation of So+:
So +
LI
-*So+ I dl,
(6)
disappearance of SO’: So+ + Q --*S2+ (7) To understand better the meaning of eq 6 and 7 as well as Figure 13 it is important to note that @ and Sorepresent respectively holes created in the valence band due to illumination and surface states somewhere in the forbidden bandgap of the MoSe, semiconductor. The result of eq 6 is the accumulation of positive charges (surface radical cations Sot) in the forbidden bandgap responsible for the anodic shift of the onset photocurrent and flat-band potentials. This is continuing as far as steps on the semiconductor a r e still not fully occupied by So’. A s far as Z < CLF, q >> Cd,r and the C1.F can then be defined by the light intensity engendering holes giving rise through its reaction with So to a quantity of Sot enough to occupy all steps. C L F corresponds to the maximum accumulation of the positive charge on t h e serniconductor/electrolyte
J. Phys. Chem. 1986, 90, 1849-1853 junction. When light intensities exceed the CLF, there are no more free steps, no more positive charge accumulation, and VI
= vdis
(8)
regardless of the absolute values of the two rates (which may be still increasing). Reaction 7 represents a preferable choice to an alternative disappearance process,36viz So+ + OH- = S-OH
(9)
Conclusions Study of the illuminated n-MoSe2/ 1 M H2S04liquid junction has shown that, even in the presence of surface states, the photocurrent-potential response appears ideal when the light intensity ~~~~~~
~~
~
~~
(36) Gerischer, H.; Lubke, M. Ber. Bunsenges. Phys. Chem. 1983,87, 123. ( 3 7 ) In the range where the relationship between the saturation photocurrent I, and the light intensity is linear, we considered I , as a parameter of light intensity. The photocurrent is normalized with respect to I, after which the onset potential become light intensity independent.
1849
is very low. The onset potential of the steady photocurrent is the same as the flat-band potential in the dark when (and only when!) the involvement of surface states is negligible. This indicates that the band edges are pinned at the same positions in the dark. Under high light intensity or in the presence of Ce(1V) band edges are displaced anodically giving rise to transient photocurrents. The transient photocurrent is associated with the trapping of photogenerated holes in surface states. The onset potential of the steady photocurrent depends on the light intensity whenever irradiation yields an increase in the density of surface states. In contradistinction, the onset potential of the transient photocurrent does not depend on light intensity.
Acknowledgment. The author is very indebted to Professor Joseph Jordan at The Pennsylvania State University not only for editorial assistance but also for the very fruitful discussion and criticism Thanks are due to Professor F. Levy for semiconductor material and to Mr. Charles Mathieu for figure drawing. Registry No. n-MoSe2, 12058-18-3; 11,S04, 7664-93-9; potassium ferrocyanide. 13943-58-3; ceric sulfate, 13590-82-4
Determination of the Structural Parameters of Reverse Micelles after Uptake of Proteins G. G. Zampieri, H. Jackle, and P. L. Luisi* Institut fur Polymere, Eidgenossische Technische Hochschule, CH-8092 Zurich, Switzerland (Received: July 22, 1985; In Final Form: November 25, 1985)
A method for the determination of the structural parameters of protein-containing reverse micelles is presented. The method is based on analytical ultracentrifugation and utilizes two dyes which are monitored simultaneously with the UV-scanner device. The first one is completely water soluble; the second one is a cosurfactant In each case, the population of both filled and unfilled micelles is measured from sedimentation runs. By monitoring the first dye, which probes only the water pool of the reverse micelles, one determines the partition of water in filled (Le. containing the protein) and unfilled micelles; by monitoring the chromogenic cosurfactant, one can evaluate the relative distribution of surfactant in unfilled and filled micelles. On the basis of this, also the inner core radius of the aggregate can be evaluated. The procedure is applied to reverse micelles formed by sodium di-2-ethylhexylsulfosuccinate (AOT) in isooctane and water, with a-chymotrypsin, lysozyme, and myelin basic protein (MBP) as guest molecules and different w o values (wo = [H,O]/[AOT]) in the range 5.6-13.7. The data permit us to draw the following picture: protein uptake in the reverse micelles is generally attended by an increase of the micelle water content, which causes an increase of the dimensions of the micelle. The protein needs at least 150&3000 water molecules in order to be hosted in the AOT reverse micelles. As a consequence of the redistribution of material, the unfilled micella will become smaller than initially, so that the micellar solution will generally consist of larger protein-containing micelles and smaller unfilled ones. The influences of the protein structure and protein concentration on these structural parameters are discussed.
Introduction Reverse micelles are spheroidal aggregates which are formed when certain surfactants are dissolved in apolar solvents. The polar head groups are directed toward the interior of the aggregate, thus forming a polar core which can solubilize water (water pool). The physicochemical properties of reverse micelles, such as the thermodynamics and kinetics of aggregate formation, the form and dimensions, the dynamics of the molecular components, the uptake of guest molecules, have been investigated extensively over the past We and others have been concerned with the properties of biopolymers solubilized in hydrocarbon solvents via reverse micelle~.~-”Usually, protein-containing reverse micelles are formed ( I ) Membrane Mimetic Chemistry, Fendler, J . H., Ed.; Wiley: New York, 1982. (2) Eicke, 13.-F. In Topics in Current Chemisfr?; Vol. 87: Springer Verlag: Berlin. 1980. (3)’Reaerse Micelles, Luisi, P. I.., Straub, B. E., Ed.; Plenum Press: New York, 1984. (4) Surfactanfs inSolution. Mittol. K. I . , Lindman, B., Ed.; Plenum Press: New York, 1984.
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by adding the protein (from a stock aqueous solution or from the solid state) to a hydrocarbon solution containing the preformed reverse micelles. The mechanism of uptake has not been clarified yet, and also the structure of the “filled” micelles (i.e. containing the protein) is not well-known. In principle, several structural models can be envisaged on the basis of geometrical considerations,” and it is not as yet easy to determine on the basis of thermodynamic elaborations which is the most probable. The problem has been approached experimentally, for example with ultra~entrifugation,~~’~ dynamic light scattering,* small angle (5) Khmelnitsky, Y. L.; Levashov, A. V.; Klyachko, N . L.; Martinek, K. Usp. Khim. 1984, 53, 545-565 [Russ. Chem. Rel;., 1984 (4)]. (6) Fletcher, P. D. I.; Freedman, R. B.; Mead, J.; Oldfield, C.; Robinson, B. H . Colloids Surf. 1984, 10, 193-203. (7) Hilhorst, R.; Spruijt, R.; Laane, C.; Veeger, C. Eur. J . Biochem. 1984, 144, 459-466. (8) Chatenay, D.; Urbach, W.; Cazabat, A . M.: Vacher, M.; Waks, M., submitted for publication in Biophys. J . (9) Bonner, F. J.; Wolf, R.; Luisi, P.L. J . Solid-Phase Bioehem. 1980, 5, 255-268. (10) Barbaric, S.: Luisi, P. L. J Am. Chem Soc. 3981, 103, 4239-4244. ( 1 I ) Luisi, P L. Angew Chem. 1985. 97, 446 -460.
0 1986 American Chemical Society
