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J . Phys. Chem. 1989, 93, 3258-3260
the surface layer can reduce the intensity of X P electrons from an underlying substance20 even when d/X is 0.1 or less. The invariance of the XPS intensity of Na( 1s) thus strongly suggests that the Na site is not covered by glucose; neither a dissolution of Na into the solvent nor an adsorption of an external N a contaminant explained the experimental result that the Na content measured by XPS was almost identical with the original Na content of the sample. (20) Hohmann, S.Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Wiley: New York, 1983; p 141.
The glucose adsorption onto the 0-A1 sites is probable considering the result obtained by the adsorption measurement described in section 3.1. It has been pointed out in section 3.1 that the adsorption capacity obtained by applying a Langmuir adsorption isotherm to the experimental result is equivalent to the amount of glucose necessary to cover the surface area of the adsorbent alumina. This clearly means that major components (0,Al) constituting the adsorbent surface provide glucose molecules with adsorption sites. Registry No. A1,03, 1344-28-1; N a , 7440-23-5; glucose, 50-99-7.
Adsorption-Controlled Redox Activity. Surface-Enhanced Raman Investigation of Cystine versus Cysteine on Silver Electrodes Tadashi Watanabe* and Hiroyuki M a e d a Institute of Industrial Science, University of Tokyo, Roppongi. Minato- ku, Tokyo 106, Japan (Received: July 12, 1988)
A quasi-reversible disulfide thiol redox process was clearly detected by surface-enhanced Raman spectroscopy on a silver electrode in a potential range between -0.3 and +0.3 V vs Ag/AgCl when cystine (disulfide) was adsorbed from the electrolyte solution. In contrast, no electrochemistry was observed when its reduced partner, cysteine (thiol), was adsorbed. Such a conspicuous difference in redox activity is rationalized in terms of both the strong Ag-S bonding and the operation of steric hindrance by adsorption of two cysteine molecules to neighboring silver sites.
Introduction Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for the in situ investigation of the nature and surface orientation of adsorbed species at an electrode/electrolyte interface, although the electrode material is limited to such metals as silver, gold, and copper.Iv2 Recently several workers studied aromatic (diphenyl and dipyridyl) disulfides on and gold6 electrodes by SERS and confirmed that all these disulfides are adsorbed as ionized thiols due to S-S bond cleavage. To date, no cases are known in which a disulfide retains the S-S bond on these electrode surfaces, and no S-S bond reformation by electrooxidation has been demonstrated in the framework of SERS investigation. In the present work we studied a pair of nonaromatic disulfide (cystine) and thiol (cysteine) on silver electrodes by SERS and found that adsorbed cystine undergoes a quasi-reversible disulfide + thiol interconversion electrochemically. Surprisingly, no such electrochemical process was observed when the reduced partner (cysteine) was first adsorbed on the silver electrode. A model is proposed to account for this unprecedented finding. Experimental Section Reagent grade L-cystine and L-cysteine (>98% purity) from Tokyo Kasei Kogyo Co., Ltd., were used without further purification. Water was used after deionization and ultrafiltration. One end of a 99.95% silver rod (6.0 mm in diameter), with the side face electrically isolated with thermally coated Teflon, was cut at an angle of 30°, and the resulting 0.57-cm2 oval area served as a working electrode. A platinum wire and an Ag/AgCl (1) Surface Enhanced Raman Scattering, Chang, R . K., Furtak, T. E., Eds.; Plenum: New York, 1982. (2) Papers collected in Surf. Sci. 1985, 158, no. 1-3. (3) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1981, 85, 248. (4) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J . Electroanal. Chem. 1984, 175, 341. (5) Takahashi, M.; Fujita, M.; Ito, M. Surf. Sci. 1985, 158, 307. (6) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J . Electroanal. Chem. 1985, 186, 299.
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electrode (KCl saturated) were employed as the counter and reference electrodes, respectively, and the potential of the working electrode was controlled with a Hokuto Denko Model NPGFZ2501 potentiostat. Raman measurements were conducted on a Jasco Model R-800 laser Raman spectrophotometer equipped with an NEC Model GLS-3200 argon ion laser. The excitation wavelength was 488.0 nm, the beam power about 50 mW, the wavenumber resolution 5 cm-I, and the wavenumber scan rate 0.5 cm-'/s throughout. For SERS measurements, the silver electrode surface was first submitted to seven oxidation-reduction cycles (reformation charge: -200 mC/cm2 in each) in a deoxygenated 0.5 M HCl aqueous solution to obtain a SERS-active roughened surface, and then an equal volume of a 3.5 M HCI aqueous solution and cystine or cysteine was added so that the final solution was 2 M in HC1 and 1-200 mM in the compound to be studied. The solution of a total volume of 2 mL in a Pyrex cell was kept under nitrogen atmosphere during measurements. The scattered light was collected at right angles to the excitation beam through a thin solution layer and a flat window.
Results and Discussion Figure 1 compares the normal Raman scattering (NRS) spectrum of powdery cysteine (A) with the SER spectrum from a silver electrode surface in contact with a 100 mM cysteine solution (B). At this concentration the N R S from the latter solution was negligible. The outstanding features in going from A to B are the disappearance of the S-H stretching peak at 2565 cm-I and significant broadening and wavenumber lowering of the 670 cm-'). This evidences, as in C-S stretching' peak (690 the case of thiourea on a silver electrode: the formation of a strong Ag-S bond (or more specifically a bond between the thiolate moiety -S- and surface Ag' ion9) on adsorption of cysteine. The
-
(7) Susi, H.; Byler, D. M.; Gerasimowicz, W. V. J . Mol. Struct. 1983, 102, 63. (8) Loo, B. H . Chem. Phys. Lett. 1982, 89, 346.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3259
Adsorption-Controlled Redox Activity
3000
2000
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I
I
I
1500
I
I
I
I
I
1000
I
I
I
I
500
I
L
Ramon s h i f t / cm-'
Figure 1. (A) Normal Raman spectrum for powdery cysteine and (B) SER spectrum of cysteine adsorbed on Ag electrode (bulk concentration, 100 mM; potential, -0.3 V vs Ag/AgCI). Raman s h i f t /cm-'
Figure 3. SER spectra for the C-S and S-S stretching frequency range at consecutive potentials in 100 mM cystine solution. The broken arrow from 0 to +0.3 V denotes incomplete recovery. I
,
;m 7
--
C-S str.
0 c ro
a .
Roman s h i f t
/ cm-'
:
~
~:
~
:
;
;
:
:
:
:
:
Figure 2. SER spectra for the C-S and S-S stretching frequency range at consecutive potentials in 100 mM cysteine solution.
strong Ag-S bonding is supported by an additional observation that the C-S stretching intensity remained practically unchanged on decreasing the bulk cysteine concentration from the initial value of 100 mM down to 0.01 mM by successive dilution with HC1 aqueous solutions. Further, a low solubility product of the cysteine silver salt (3.98 X M2)l0 is in line with the strong surface bonding. A series of SER spectra from the silver electrode surface in a 100 mM cysteine solution are displayed in Figure 2 as a function of electrode potential. Because of the redox activity of the substrate itself, the cysteine/cystine redox potential Eo has not been measured on silver electrodes. Kolthoff and co-workers' studied the cysteine/cystine redox reaction on a mercury electrode and obtained an Eo value of +0.08 V vs N H E in an aqueous solution of pH 4.9. This is converted to about +0.1 V vs Ag/AgCl in the pH 0.9 solution employed in the present work. However, even at a potential of + O S V vs Ag/AgCl, no peak is discernible in Figure 2 at around 500 cm-I where the S-S stretching mode of cystine is expected to occur.IZ The sole event is, as one generally encounters in SERS,13,14 the potential dependence of the C-S (9) Watanabe, T.; Kawanami, 0.;Honda, K.; Pettinger, B. Chem. Phys. Lett. 1983, 102, 565. (10) Krzewska, S.; Podsiadly, H. Polyhedron 1986, 5, 937. (1 1) Kolthoff, I. M.; Stricks, W.; Kapoor, R. C. J . Am. Chem. SOC.1955, 77, 4733. (1:) Simons, L.; Bergstrom, G.;Blomfelt, G.; Forss, S . ; Stenback, H.; Wansen, G.Comment. Phys.-Math. 1972, 42, 125. (13) Furtak, T. E.; Roy, D. Surf. Sci. 1985, 158, 126.
Figure 4. Relative SER intensity for C-S and S-S stretching modes in 100 mM cystine solution during a potential journey indicated by the arrow.
stretching intensity. Essentially the same behavior was observed in a cysteine concentration range from 1 m M to 1 M. This evidences the redox inactivity of cysteine adsorbed on a silver electrode. A completely different result was obtained from SERS measurements in cystine solutions in a potential range between +0.3 and -0.3 V vs Ag/AgCl. The S-S stretching peak (502 cm-') was clearly observed at potentials positive of about -0.05 V, as illustrated in Figure 3. The intensity of this peak decreases as the electrode potential is lowered, until it completely disappears at -0.3 V. On a backward potential journey from -0.3 V, the S-S stretching mode regains its intensity, and at the starting potential of +0.3 V t h e spectrum is approximately restored. Figure 4 shows the potential dependences of the C-S and S-S peaks in a 100 mM cystine solution. The intensity of each vibrational mode exhibits a hysteresis with potential; this may reflect a physical difference in the silver electrode surface between positive and negative potential scans. The elucidation of the details of these hystereses, including the large bump for the C-S stretching mode intensity (14) Kobayashi, M.; Imai, M. Surf.Sci. 1985, 158, 275
J . Phys. Chem. 1989, 93, 3260-3269
3260
Figure 5. Surface adsorption layer model to account for the experimental observations. The dotted, black, and white spheres represent carbon, oxygen, and hydrogen atoms, respectively. The atoms are scaled referring to the Ag-Ag distance (2.88 A) for the silver (1 1 1 ) plane together with known van der Waals radii and interatomic distances.
on the positive scan, is however beyond the scope of the present study. The quasi-reversibility of C-S and S-S stretching intensities at the two extremes of the electrode potential was noted also by further potential cycles and in cystine solutions of concentration ranging from 1 mM to 200 mM. This unambiguously indicates that cystine (RSSR) adsorbed on a silver electrode undergoes a quasi-reversible redox process with its reduced partner cysteine (RS-) as follows: (RSSR),d,
+ 2e-
F+
2(RS-),d,
To our knowledge, this is the first example where an adsorbed disulfide molecule as it is has been neatly detected by SERS. The failure to observe aromatic disulfides in their intact states by SERS in previous work^^-^ could be ascribed either to the S-S bond cleavage owing to the smaller bond energy (55 f 1.5 kcal/mol) than that of alkyl disulfides (74 f 2 kcal/mol)15 or to the flat alignment of S-S bonds on the electrode surface, for which surface Raman enhancement would be ~mal1.l~The flat alignment could
(16) Demuth, J. E.; Christmann, K.; Sanda, P. N. Chem. Phys. Letf. 1980, 76, 201. ( 1 7 ) Patterson, M. L.; Weaver, M. J. J . Phys. Chem. 1985,89, 1331, 5046.
(15) Benson, S. W. Chem. Rev. 1978, 78, 23.
Studies of the n-GaAs/KOH-Se;--Se*-
result, as Patterson and Weaver demonstrated for alkenes and alkynes on gold electrodes,” from an attractive interaction between the *-electron system of two aromatic rings and the silver surface. The redox inactivity of cysteine and the activity of cystine as revealed by the present SERS investigation may be rationalized in terms of a model for the adsorbed layer illustrated in ,Figure 5. The cystine molecule is adsorbed on the silver electrode probably with the S-S bond tilted to some extent against the mrface (A), in view of the fairly intense S-S stretching peak (Figure 3). On electrochemical reduction, it is converted to two ionized thiol (cysteine) fragments, and since the Ag-S bonding is apparently strong enough as mentioned above, the resulting two fragments may be trapped on the two neighboring silver atoms, in spite of some steric hindrance between the main bodies of the cysteine molecules. On electrooxidation, the bond could readily be formed between the closely sitting sulfur atoms to yield the starting molecule, cystine. On the other hand, in case where the silver electrode is in contact with a cysteine solution (Figure 5B), the steric hindrance operates first at the time of adsorption so that the “adjacent sitting” of two cysteine (of sulfide) molecules is energetically unfavorable. The strong Ag-S bonding, which acted in holding the two cysteine fragments close together in A, prevents their surface diffusion in case B, and the S-S bond formation by electrooxidation is strongly hampered. The importance of steric hindrance in suppressing the “adjacent sitting” of two cysteine molecules is supported indirectly by an additional SERS study on the ethanethiol/diethyl disulfide pair, having a much less bulky ethyl group attached to S than in the case of the cysteine/cystine pair. Thus, we were indeed able to observe a quasi-reversible ethanethiol/diethyl disulfide electrochemistry, reflected in a quasi-reversible SERS spectral change similar to that depicted in Figure 3, on Ag electrodes in a potential range between 4 . 4 and +0.3 V vs Ag/AgCI (unpublished results). The present results thus demonstrate the effectiveness of SERS in unraveling the surface electrochemistry on a molecular level. Extension of these measurements to other thiol/disulfide systems and other adsorbates is currently in progress. Registry No. Ag, 7440-22-4;L-Cystine, 56-89-3; L-Cysteine, 52-90-4; ethanethiol, 75-08-1;diethyl disulfide, 110-81-6.
Semiconductor/Liquid Junction
Bruce J. Tufts, Ian L. Abrahams, Louis G. Casagrande, and Nathan S. Lewis* Division of Chemistry and Chemical Engineering,t California Institute of Technology, Pasadena, California 91 125 (Received: July 20, 1988; In Final Form: October 7, 1988) The current-voltage characteristics of the n-GaAs/KOH-Se,*--Se*- semiconductor/liquid junction have been determined for a variety of conditions, including changes in the majority carrier density, the minority carrier diffusion length, and the incident light intensity. These data provide an experimental test of previous digital simulation calculationsand provide necessary data for use in further mechanistic studies of this system. Spectral response measurements have been performed to elucidate the anomalous increase in short circuit photocurrent density when the n-GaAs surface is textured to a low-reflectivity matte finish. Novel metal ion treatments have been discovered for n-GaAs photoanodes, and we have observed solar simulated efficienciesin excess of 16%for Os3+-treatedn-GaAs photoanodes. Chemisorption of metal ions that yielded beneficial effects on n-GaAs photoanode performance also yielded increased charge-transfer rates at p-GaAs, n+-GaAs, and Sn-doped In203 surfaces. Introduction Although the n-GaAs/KOH-Se-/Z-/C system was the first efficient and stable photoelectrochemical cell to be discovered,’ *Author to whom correspondence should be addressed. ‘Contribution No. 7815.
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surprisingly little is known about the behavior of this system when solution properties or bulk semiconductor parameters are varied. Such experiments are valuable in obtaining a microscopic un( 1 ) Chang, K. C.; Heller, A.; Schwartz, B.; Menezes, S.; Miller, B. Science (Washington, D.C.)1977, 196, 1097.
0 1989 American Chemical Society
