Langmuir 1993,9,323-329
323
Electrochemically Induced Transformations of Monolayers Formed by Self-Assembly of Mercaptoethanol at Gold Duane E. Weissham,+Mary M. Walczak,t and Marc D. Porter' Department of Chemiotry and Ames Laboratory-US. Department of Energy, Iowa State University, Ames, Iowa 50011 Received July 23,1992 The structural stability of monolayers formed by the self-assembly of mercaptoethanol (HOCH2CHr
SH)at annealed, mica-supported gold (ME/Au) has been characterized using electrochemical, infrared
spectroscopic, and X-ray photoelectron spectroscopic techniques. The study was motivated by the observation that the one-electron reductive desorption of long-chain n-alkanethiolatee by linear sweep voltammetrygenerally exhibitsasingle well-definedwave,' while that of ME/Au produces two waves. The relative magnitudes of the waves for ME/Au are dependent on the extent of exposure to the laboratory ambient. Electrochemical oxidation of ME/Au shows that three oxidative processes occur sequentially. In the fmt process, MEVAu is catalytically oxidized by a four-electron process to a eulfidoacetic acid monolayer (adsorbed mercaptoacetic acid, MAA/Au). In the second process, the electrogenerated MAA/Au undergoes C-S bond cleavage, which gives rise to the second reductive desorption wave. In this case,as the extent of oxidation increases, a third desorption wave is also produced as a result of gradual formation of adsorbed polysulfur species. In the third oxidativeprocess, the remaining sulfur monolayer is oxidatively desorbed from the surface. Our results, coupled with earlier literature reporta,*G2s that electrochemicaloxidation of thiolatea on gold is a multipath process with the favored path d e p e y z i on the reaction conditione and the chemical composition of the thiolate. The spontaneous oxidation of MEVAu may proceed via the same path as the electrochemical oxidation (ME/Au to MAA/Au to S/Au), or adsorption of thiolate may activate the C-S bond, leading to direct conversion from ME/Au to S/Au.
Introduction Alkanethiols,X(CH2)nSHfself-assembled at gold have been studied extensively as model interfacial structures by a variety of techniques, includingX-ray photoelectron, infrared, and Raman spectroscopies,contact angles, electrochemistry, and scanning tunneling microscopy.1a~b~2bJ The combined weight of these findings has revealed that thiols chemisorb as the corresponding thiolates at gold,1aJ'*3eJand for end groups comparable in size to the cross-sectionof the polymethylene chains (e.g., X = CH3, COOH, OH), form densely packed, ordered monolayers with the polymethylenechain tilted -30' from the surface normal.hfa It has generallybeen consideredthat the longchain ( n > 10) alkanethiolate (X = CH3) monolayers are
structurally stable for long periods of time (i.e., several months)." Recent studies, using sensitive surfaceanalysis techniques, have found evidencefor oxidized sulfurspecies in these monolayers, suggesting that the assumption may not be warranted.' Data presented here show that monolayers of shorter chains with other end groups (notablyX = OH or COOH)can be considerablylesestable than longer chain alkanethiolates. As part of a series of electrochemical studies,' we have shown that alkanethiolate monolayers can be desorbed from gold by the one-electron reductive path in reaction 1. Driving reaction 1by a linear voltage sweep produces AuS(CH2),X + le-
To whom correspondence should be addressed.
-
Au + X(CH2),S-
(1)
a well-defineddesorptionwave (e.g., Figure la) for thiolatea formed at annealed, mica-supported gold.6 The charge under the desorption wave provides a measure of the surface coverage, r, of the thi~late.l&~ After correction for surface roughness, I' for the n-alkanethiolates(n = 3-17) was determined to be (7.9 i 0.6) X 1 0 - l O moucm2, a value consistent with that expected for a densely packed monolayer.1b Our explorations of the electrode reactions of thiolate monolayers on gold have also determined that the peak position (Ep)of the reductive desorption wave is linearly dependent on the number of methylene groups in the chain (n),shifting to more negative voltages as ra incream. Thie dependence was modeled as an interfacial structure composed of two capacitors connected in series: sulfur head group (C.) and alkyl tail (Cn)." As n increases, the
t Permanent address:
Department of Chemistry, Augustma College, Sioux Falls, SD 57197. t Present address: Department of Chemistry, St. Olaf College, Northfield, MN. (1) (a)Widrig, C. A,; Chung, C.; Porter, M. D. J . Electroanal. Chem. 1991,310,335-59.(b) W a l d , M. M.; Popenoe, D.D.;Deinhammer, R. S.; Lamp, B. D.;C h u g , C.; Porter, M. D. Longmuir 1991,7,2687-93.(c) Weiaahaar, D.E.; Lamp, B. D.;Porter, M. D. J. Am. Chem. SOC.1992, 114,5806-2. (2)(a) Wierae, D.G.; Lohrengel, M. M.; Schultze, J. W. J.Electrwnal. Chem. 1978,92,121-31.(b) Finklea, H.0.;Avery, S.; Lynch, M.;Furtsch, T. Longmuir 1987, 3, -13. (e) Vandeberg, P. J.; Kowagoe, J. L.; J o h n , D. C. Anal. Chim. Acta, in press. (3)(a) Porter, M. D.; Bright, T. B.; h a , D.L.; Chidmy, C. E. D.J. Am. Chem. Soc. 1987,109,3559-68. (b) Sabatani, E.; Rubinstein, I. J . Phye. Chem. 1987,91,6663-9.(e) Stefely, J.; Markowitz, M. A.; Rsgen, S. L. J. Am. Chem. SOC.1988,110,7463-69. (d) Bain, C. D.;Troughton, E. B.; Tao, Y.T.; Evall, J.; Whiteeidee, G. M.;Nuzzo, R. G. J. Am. Chem. SOC.1989,111,321-35.(e)Bain, C. D.;Biebuyck, H.A.; Whitaides, G. M. Langmuir 1989,6,723-7.(0Nuzzo, R. G.; Duboh, L. H.;Allara, D. L. J. Am. Chem. SOC.1990,112,558-569. (e) Chidsey, C. E. D.;Loiacono, D. N.Langmuir lSSo,6,682-91.(h) Dubois, L. H.;Zegaraki,B. R.; Nuzzo, R. G. J. Am. Chem.Soc. 1990,112,570-79. (i) Evans, S.D.; Urankar, E.; Ulman, A. J. Am. Chem. SOC.1991,113,4121-31. c j ) Thomas, R. C.; Sun,L. Crwka, R. M.; Ricco, A. J. Langmuir 1991,7,62+2. (k) Collard, D.M.;Fox,M.A.Langmuir1991,7,1192-7. (l)Bryant,M. A.;Pemberton, J. E. J. Am. Chem. Soc. 1991,113,8284-93.(m) Alvee, C. A.; Smith, E. L.; Porter, M. D.J. Am. Chem. SOC.1992,114,1222-7. (n) Widrig, C. A.; Alvea, C.A.; Porter, M. D.J. Am. Chem. SOC.1991,113,2806-10.( 0 ) Nordyke, L. L.; Buttzy, D.A. Langmuir 1991, 7, 380-8. (p) Wang, J.; Frontman, L. M.; Ward, M.D. J. Phye. Chem. 1992,M,5224-28.
(4)(a) Tarlov, M.J.; Newman, J. G.Longmuir 1992,8,1308-1406. (b) Li, Y.;Huang, J.; McIver, R. T. Jr.; Hemminger,J. C. J. Am. Chem. SOC. 1992,114,2428-32. (6)The well-defined voltammetry at annealed, mica-8upport.d gold reflects the reductive desorption of thiolatea from a rtrmgly t a r r a d surface. In contrast, reductive desorptionfrom g b or sllicon-rupportul gold (not annealed) producea multiple w a m which we believe ropmeent desorption from terrace and one or more type^ of rtap d h . "he thiolrt4 coverage at all s u b t r a t a , after accounting for rurtace roughnoan, L consistent with that expected for a monolayer.8 (D
1993 American Chemical Society
-
1
-
-0.5
Weisshaar et aL.
VoL.9, No. 1,1993
324 Langmuir,
-
I
-0.7
.
r
-0.9
.
.
I
VOLTAGE VI Ag/AgCl/rot’d
1
-1.3
-1.1
KCI
Figure 1. LSV for the reductive desorption of (a) ethanethiolate/ Au and (b) MEVAu in 0.5 M KOH. The scan was initiated at -0.40 V at 100 mV/s.
fraction of the applied voltage dropped across the sulfur head group decreases. Therefore,E, occursat increasingly negative voltages with increasing chain length.s In addition, we have recently discovered that the reverse of reaction 1 can be used to electrodeposit thiolate monolayers at gold, and that the electrodeposited monolayers have structures and interfacial properties similar to their self-assembled analogs.lC These studies established that electron transfer for the electrodeposition is rapid compared to the voltammetric time scale employed (100 mV/s); therefore, the peak positions reflect the thermodynamics of the deposition reaction. To facilitate an in-depth study of the thermodynamics of the electrodepositionreaction, we sought to minimize the capacitivecontribution toE, by using monolayerswith short chain lengths. Our initial choice for a probe was mermptoethanol (HOCH&HdH, abbreviated ME),mainly because of its solubility in water. However, during the early stages of this investigation, we discovered that voltammetric curves for the reductive desorption of sulfidoethanol (thiolate of mercaptoethanol, abbreviated (MEIAu)produced two desorption waves (e.g., Figure lb) rather than the single wave typically found for n-alkanethiolate monolayers (e.g., Figure la). This report preeents the results of a study aimed at delineating this unexpected observation. In the following, we describe the fiidings from a multitschnique study of the electrochemical oxidation of ME/Au at annealed,mica-supportedgold. In situ infrared reflection spectroscopy (IRS) and X-ray photoelectron spectroscopy (XPS) are used to probe the composition of the layers, which are correlated with data from charactmhtion using various electrochemical techniques. Together these studies reveal that ME/Au is first oxidized to sulfidoacetic acid (MAA/Au) and, at more positive potentials, C-S bond cleavageoccurs. Pwible correlations of the electrochemical reactions to the spontaneous reactions of MEIAu and MAAIAu are briefly discussed.
Experimental Section Inrtrumentation. Electrochemical experiments were performed using either a CV-27 potentioatat (BioanalyticalSystems) (6) We suspect that the observed uncertainty in E, (126 mV) for a given chain length, which is Inrger than anticipated for voltammetric mmurementa of a redox proms, resdta from cdl variations caused by smallchangesinco~erage.~~ However,other fnctorssuchaesolvent effecta m y a h play a role in determinii peak mition. Studies to determine the source of the E, variability are continuing.
or a Model CYSY-1 electroanalysis system (Cypress Systems, Lawrence, KS). The CV-27 was interfaced to an IBM PC/AT via an Analog Devices RTI-815 data acquisition board that was controlled using Labtech Notebook/XE (LABTECH, Wilmington, MA) software. A conventional three-electrode cell was used with the area of the working electrode defined by the opening in an inert elastomer gasket (0.65 cm2). A Pt coil a d a r y electrode and a Ag/AgCVeaturated KCl reference electrode were used; all voltages are given with respect to this reference. The supporting electrolyte w a 0.5 M KOH in distilled, deionized water. The supporting electrolyte was purged continuously with water-saturated argon or nitrogen (Air Products, Des Moines, IA) in a vessel external to the cell and for an additional 3 min after introduction into the cell. For the in situ IRS,1024scans were collected by a Nicolet 740 FTIR spectrometer employing p-polarized light incident at BOo at the liquid/solid interface and a liquid nitrogen cooled HgCdTe detector. All spectra were collected at 4-cm-I resolution. The in situ cell has been described elsewhere.? The solution spectrum of 100mM mercaptoacetic acid in 0.5 M NaOD/D20 as obtained using a ZnSe CIRCLE cell (Spectra Tech, Inc., Stamford, CT). The in situ IRS data for the monolayers are presented in the form of differential spectra. In this mode, a single beam is collected at a base voltage (-0.45 V reflectance spectrum (Ro) here). The voltage is next stepped to and held at a selected value for 5 min after which another single beam reflectance spectrum (R)is acquired. The two single beam spectra are then combined to give a reflectance-absorbance spectrum (-log RIRo)). The XPS data were acquired with a Physical Electronics Industries (Eden Prairie, MN) Model 5500 multitmhnique surface analysissystemequipped with a hemisphericalanalyzer, a toroidal monochromator, and a multichannel detector. Monochromatic Al K a radiation (1486.6 eV) tit 300 W was used for excitation. Photoelectrons were collected at 45O in a constant transmission mode. The Au(4f?,2) emission was monitored as an internal reference for binding energy and sample charging. The base pressure of the ion-pumped ultrahigh vacuum (UHV) chamber was less than 1X 10-7 Pa during analysis. Acquisition times were less than 25 min with duration depending on signal strength; no signal degradation was observed during any of the measurements. Spectral resolution, estimated from the full width at halfmaximum (fwhm) of the Au(4f7,2) peak, was 0.8 eV. Chemicals. Absolute ethanol (punctilious grade, Quantum Chemical Corp., Cincinnati, OH),potassium hydroxide (semiconductor grade, Aldrich), sodium deuteroxide (40% in D20, Aldrich), D2O (99 atom 7% ,Aldrich), mercaptoethanol(99+ % , Aldrich) and mercaptoacetic acid (99+% ,Aldrich), were used as received. The purities of the mercaptoethanol and mercaptoacetic acid (neat and ethanol solutions) were determined by the Instrumental Services Group of the Iowa State University Chemistry Department using a FINNEGAN Model 4000 GC/ MS (results presented below). House-distilled water was further purified with a Millipore Mill-Q water system. Sample Preparation. The gold f i i substrateswere prepared by vapor deposition of about 300 nm of gold (99.9 % purity) onto freshly cleaved 75-mm X 25-mm green mica sheeta (AehevilleSchoonmaker Mica Co., Newport News, VA) at a rate of 0.3-0.4 nm/s. The pressure in the cryopumped Edwards E360A evaporator system was less than 9 X 1W Pa during the deposition. The Au fiis were then annealed at 300 OC for 4 h. Just prior to use, the gold f i i s were cleaned in an oxygen plasma (Harrik Scientific) at 1-2 Pa for 1min. The roughness factor of these annealedgold films on mica ie 1.L8 All reported I% are corrected for thisroughness factor, and their uncertainties are given as the standard deviation of at least 10 measurements. Gold f i b for in situ IRS were vapor deposited on stainless steel plungers in a procedure analogous to that at mica, but were not annealed. Prior to Au deposition, the plungers were primed with a 15-nm layer of chromium to improve adhesion of the gold f i i . ’ Monolayers were self-assembled by immersing the gold f i e in ethanol solutions of 10 mM thiol for 1-5 min. The samples (7) Popenoe, D. D.; Stole,5.M.; Porter, M. D. Appl. Spectrosc. 1992, 46, 79-87. (8) Walczak, M. M.; Alves, C. A.; Lamp, B. L.; Deinhammer,, FL S.; Porter, M. D. (Iowa State University). Manuscript in preparation.
Langmuir, Vol. 9, No.1, 1993 326
Self-Assembly of Mercaptoethanol at Gold
REGION 2
REGION 3 A
I I
0.5
0.3
0.1
.
n
U
100 pA
.
-0.3 -0.5 -0.7 -0.0 VOLTAGE va Ag/AgCl/rot'd KCI -0.1
REGION 1
d
1
-1.1
7
,
-1.3
Figure 2. CV for ME/Au in 0.5 M KOH initiated in a positive direction at -0.40 V at 100 mV/s. were then briefly rinsed with ethanol or water on a spin coater (Headway Research Inc., Garland, TX). Control experiments ehowed that monolayers prepared from mercaptoethanol and mercaptoacetic acid that were purified by vacuum distillation exhibited the same voltammetric behavior as those assembled from the etock, as-received thiols.
Results and Discussion 1. Preliminary Characterization of the Electrochemical Oddationof Mercaptoethanol Monolayers. As stated, the reductive desorption (reaction 1) of alkanethiolate monolayers usually produces a single, welldefined wave on annealed, mica-supportad gold. A linear sweep voltammogram (LSV) representative of such behavior is shown in Figure l a for an ethanethiolate monolayer; the large cathodic wave near -0.79 V reflects reaction 1. In contrast, the reductive desorption of ME/ Au typically exhibitetwo waves: one at about -0.65 V and one at about -0.95 V (Figure lb).g The magnitude of the wave at -0.65 V decreases and the magnitude of the wave at -0.95 V increases with increasing exposure to the assembling solution or to the laboratory ambient. Occasionally, we find that when the total time for assembly and exposure to the laboratory ambient is short (a few minutes at most), desorption of ME monolayersproduces only the wave at -0.65 V. If the assembly/exposuretime is longer than a few minutes, two waves are routinely observed. These observations suggest that a portion of ME/Au undergoesa spontaneouschemicaltransformation to a species that is desorbed at more negative voltages than ME i t d f . To gain further insights into the nature of the transformation of the ME/Au, we investigated the possibility of electrochemically inducing the spontaneous reaction. Figure 2 shows the cyclic voltammetric (CV) behavior of ME/Au in 0.5 M KOH between +0.50 and -1.30 V for a 100mV/s sweep initiated in a positive direction from -0.40 V. The reduction wave at -0.95 V, coupled with the absence of a reduction wave at 4 . 6 5 V, indicates that the positive sweepleads to a transformation of ME/ Au similar to the spontaneous reaction. The presence of three large anodic waves at +0.05, +0.25, and +0.44 V suggests the possibility of more than one spontaneous reaction at the gold surface. The reduction wave at +0.12 V correlates with the oxide reduction wave observed at an uncoated gold electrode at this pH. Slower positive scans (e.g., 1mV/s in Figure 3a) on ME/ Au result in a shift of the three oxidative waves to more (9) E, for the major wave for ME (-0.66 V) in significantly p i t i v e of that for ethanethiolate (-0.79 V). T h e difference in cdl for these layers is small, 80 the difference in the E, ruggesta that ME i lees strongly bound to gold than ethanethiolate.
I
0.2 0.0 -0.2 VOLTAGE va Ag/AgCl/sat'd KCI
0.4
-0.4
Figure 3. LSV for oxidation of (a) ME/Au and (b) MAA/Au monolayers in 0.5 M KOH initiated at -0.40 V at 1 mV/s.
I
150 1
-0.5
-0.7
-0.9
-1.1
-1.3
VOLTAGE VI Ag/AgCl/aat'd KCI
Figure 4. LSV for the reductive desorption of MAA/Au in 0.5 M KOH initiated at -0.40 V at 100 mV/s.
negativevoltages (-0.20, +0.14, and +0.28 V,respectively). The shift of these waves with scan rate suggests that the rates of electron-transfer or any follow-up chemical reactionsfor these anodicprocesses are slow in comparison to the voltammetric time scale. The slow delamination of the gold films from mica at such alkaline p H s precluded an evaluation at slower scan rates. A LSV obtained under identicalconditionsfor self-assembledMAA/Au,a possible oxidation product for ME/Au, is shown in Figure 3b. The two curves (Figure 3) are similar except for the absence of the wave at -0.20 V for MAA/Au. The difference suggests that the wave at -0.20 V for ME/Au represents the oxidation of ME/Au to MAA/ Au according to reaction 2. For subsequent discussion, the "regions of reactivityaround these waves are defined as region 1(-0.4 to +0.05 V), region 2 (+0.05 to +0.2 V), and region 3 (+0.2 to +0.5 VI. AuSCH~CH~OH + 50HAuSCH2COO- + 4H2O -C
+ 46- (2)
A further examination of the electrode reactions of MAA/Au revealsthree reductivedeeorptionwaves (Figure 4): a wave at -0.70 V, which is negative of the major desorption wave for ME/Au, and two overlapping waves at -0.89 and -0.95 V. ME/Au exposed to the laboratory ambient for longer periods of time (Le., overnight) also exhibits the third desorption wave at -0.89 V. This suggests that MAA/Au and MEIAu are transformed to
Weieshar et al.
326 Langmuir, Vol. 9, No.1, 1993 I
II Ill
0
I -0.3 -
.,, ...
., .. i
-0.5
-0.2
-0.1
0.0
0.1
0.2
0.3
VOLTAGE VI Ag/AgCl/rat'd KCI
=
-0.7
-0.9
-1.1
-1.3
VOLTAGE VI Ag/AgCl/rat'd KCI
Figure 5. LSV for reductive desorption of ME/Au (series 2) in 0.6 M KOH initiated at 4.40 V at 100 mV/s, following 5-min voltage steps to (a) no step, (b)-0.10 V, (c) +0.05 V, (d) +0.15 V, and (e) +0.20 V.
the same surface species and that the spontaneous transformation of M W A u is faster. To correlate each of the oxidative processes evidenced in F v e 3 with the voltammetryfor reductive desorption of ME/Au, two series of experiments were conducted wherein the voltage wae stepped from -0.40 V to a preselected, more positive value. After 1min (series 1)or after 6 min (series 2), the voltage was stepped back to -0.40 V and a negative-going linear sweep at 100mV/s was immediately initiated. Desorption LSV's obtained at selected step voltages for series 2 are shown in Figure 5. Parte a and b of Figure 6 present the reductive desorption curves for voltage steps into region 1. The increase in the magnitude of the wave at -0.96 V (wave111)after oxidation in region 1 reflects the greater extent of spontaneous reaction that occurs due to the longer exposure time for the oxidized samples (purge plus oxidation time vs purge time only) and not the electrochemical oxidation. The reductive desorption behavior depicted in Figure 6a,b is consistent with the conversion of ME/Au to W / A u by reaction 2 as speculated. For voltages in region 2 (Figure6c,d),wave 111increases, a wave at -0.89 V (wave 11)grows in, and wave I decreases and finally diaappeara. Waves I1 and III correlate with the overlapping waves for MAA/Au in Figure 4. This indicates that the transformation producing waves I1and 111is driven by the voltages in region 2. For step voltages into region 3, wave I is absent, and waves 11 and I11 decrease in magnitude (Figure 6e), suggestingthat the surface species is oxidatively desorbed at these voltages. A series of experiments on MAA/Au (not shown) obtained under series 2 conditions produced LSV's similar to those depicted in Figure 5 for MEYAu. Figure 6 summarizes the trends for surface coverages, calculated assuming a one-electron reductive desorption, as a functionof the oxidative atep voltage for series 1data. Three sets of series 1data are presented in Figure 6: the surface coverage under wave I (o),the sum of the surface coverages under waves I1 and 111(O),and the sum of the surface coverages under all three waves ( 0 1. These data follow the trends noted in the discussion of Figure 6. Importantly, the sum of the coverages for all three waves is constant ((7.0 f 0.9) X 10-10moVcmZ)until step voltages enter region 3. A similar treatment of the series 2 data
Figure 6. Surface coverage vs oxidative step voltage for a 1-min step in 0.5 M KOH (series 1): coverage for wave I (O), sum of coverages for waves 11 and I11 (O),and total coverage with error bars indicating the range of 2-4 measurementa ( 0 ) . Solid lines are cubic spline fit to a five-point Savitsky-Golay smoothing of each curve.
shown in Figure 5 exhibits the same trends and total coverageas the series 1data shown in Figure 6.lO Together, these data argue that the products of the oxidation are desorbed by a one-electron reaction. The followingthree sections discuss the resultsof a more detailed study of the electrochemicallyinduced oxidation of ME/Au and identify the oxidation processes occurring in regions 1-3. 2. Characterization of the Process in Region 1. As discussed, comparison of the LSV's in Figures 3 and 6 suggests that ME/Au is oxidized to W A u in region 1 according to reaction 2. To assess electrochemically the validity of this deduction, the number of electrons per mole of adsorbed ME (ne)for the oxidative process in region 1was determined by integrating the charge passed (Qrea&) for 1min after a voltage step from -0.40 to -0.10 V. The double layer charge (Qd) waa estimated by application of the same step on the oxidized monolayer. The monolayer was then reductively desorbed by a negative scan initiated at -0.40V at 100mV/s. The surface coverage of the oxidized ME (QME)was calculated from the charge under wave I in the reductive desorption curve." The number of electrons (ne = Qm* - Q~)/QME)was found to equal 3.7 (range &0.3),which is consistent with reaction 2.
In situ IRSand X P S were used to provide direct evidence for the conversion proposed in reaction 2. Samples for these characterizationswere initially produced by stepping the voltage from -0.40 to +0.06 or +0.20 V, dependingon the targeted treatment, for 1min in 0.5 M KOH (analogous to series 1). Sampleswere ab0 prepared by stepping from -0.40 to -0.10 or +0.13 V for 6 min (analogous to series 2). The series 1treatments produced essentially the same results as the series 2 treatments. Data for both aeries treatmenta will be presented. A set of in situ differential IRS spectra for ME/Au in 0.6 M NaOD and DzO for a base voltage of -0.46 V are shown in Figure 7. An IR spectrum for a 100 mM MAA solution in 0.6 M NaOD and DzO, referenced to a spectrum (10) The Elp for the transformation procen.9in region 2, odetermined from Figure 6 (wries 1 conditione),in about +0.10 V which in negative of the corresponding LSV peak in Figure 2 at +0.14 V. The El,z for the same p r o " obtained from the data in Figure 6 (aeries 2 conditions,not shown) is H . 0 5 V, which b 8hifted wen further p i t i v e . Thio shift of Elj2 with step time in consistent with the scan rate dependence of the anodic CV waves (Figures2 and 3). (11) The w n w occurriq eftar oxidation at voltages in region 1 (Le., waves I1 and 111) are the result of the spontaneow reaction and ere not used in the calculation of ne.
Langmuir, Val. 8, No. 1, 1993 32’7
Self-Assembly of Mercaptoethunol at Gold
17-00
1sbo 1500 14b0 WAVENUMBER (CM-1)
1300
Figure 7. Differential in situ IR spectra of ME/Au (series 2) at (a) -0.40V, (b)-0.10 V, and (c) +0.13 V, base voltage -0.45 V. (d) shows the spectrum of 100 m M MAA in 0.5 M NaOD/D20. See text for further details. S ie 0.001 au in (a)-(c) and 0.03 au in (d). of the electrolyte, is included for comparison. As expected from the voltammetry in Figure 5, there are no discernible featuresin the spectrum for ME/Au held at -0.40V (Figure
BlNMNG ENERGY ( 0 9
Figure8. X P S C(1s)region for ME/Au (aeriea 1): (a)unhW, (b) +0.06 V, 1 miq and (c) +O.N V, 1 min. The dotted line b the data smoothed by a ninepoint Saviteky&lay function.
7a). The two bands observed at 1393 and 1690cm-’ after treatment in region 1 (Figure 7b) follow the assignments of the two strongestbands inthe solution spectrum (Figure 7d) at 1386 and 1549 cm-l. These bands, assigned to the symmetric (v,(COO-))and asymmetric (v,(COO-))carboxylate stretching modes,respectively, verify that reaction 2 is occurring in region 1. The band at 1444 cm-l in the solution spectrum is tentatively assigned to a methylene wagging mode.12 A more detailed analysis of the spectra was not attempted.’3 T h e electrochemically induced changes in ME/Au were 1 also probed using X P S . Figure 8 presents the results of 1 s5 165 1 Sl 1 59 the characterization in the C(1s) region for a freshly prepared sample (Figure8a), a samplepretreated in region BlNMNG ENERGY (a@ 1 (Figure8b),and a sample pretreated in region 2 (Figure Figure9. XPS S(2p)region for ME/Au (se~ries 1): (a) untreat& &). Figure 9 presents the corresponding spectra of the (b)+0.05 V, 1 miq and (c) +0.20 V, 1 min. The dotted line b the data smoothed by a nine-point Savitaky-Golay function. S(2p) region. The data for these two spectral regions are summarized in Table I. Both the C(ls)and S(2p) spectra Table I. Binding Energion (eV) and Commitiond are consistent with the expected composition of an ME A 8 a ~ n t for r XPS Speatn of Untreated .nd monolayer. In Figure 8a, the C(ls)band at 284.4 eV is Electroohemidly Treat& Monolayers 8olf-As”bled characteristic of a C-H moiety and the high-energy from Memaptoethanol at Gold shoulder near 286 eV is indicative of a C-OHm0iety.l‘ In binding enem Figure 9a, the S(2p) doublet at 161.9 and 163.1 eV ( 2 ~ ~ 1 2 band untreated +o.W v +o.m v and 2~112,respectively) is consistentwith other reports of 83.9 83.9 83.9 thiolates at g0ld.%J5 87.6 87.6 87.6 After oxidation in region 1 (Figure 7b), the C(ls) 161.0 161.0 spectrum still exhibits the 284.4-eV band for a C-H moiety.
P k
(12)Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Halk New York, 1980;Vol. 2, p 17. (13)A comparieon of the relative absorbances of the u,(COO-) and u,(COO-) modea in Figure 7b,c given a clue to the spatial orientation of the carbo.ylate end group. In the solution p h , the absorbance of the u,(COO-) mode ir greater than that of the u,(COO-) mode,whereas the opposite ie true for the surface species. The transition dipole momenta for the v,(COO-) and u.(COO-) modea lie in the 04-0 plane; the former bieects the OC-0 bond angle, and the latter ie perpendicular to the bisector. In light of the relative absorbances and the infrared surface wlection rule, it folloan that the hnaition dipole moment for r,(COO-) for MAA/Au must be oriented claea to the surface normal. (lbs, A.; Liedberg, B.J. Colloid Interface Sci. 1991,144,282-92.) (14) Wagner, C. D., R Q ~ EW. , M.,Davis, L. E., Moulder, J. F., Muilenberg,G. E., Eda;H a d b o o k of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN,1979. (16)Smith, E. L.; Alvea, C. A,; Andregg, J. W.; Porter, M.D.; Siperko, L. M.Submitted for publication.
161.9 161.9 163.1
161.9
161.9 163.1 284.4
284.4
284.4
286 (eh)
288 (br) 292.7 296.5
The shoulder near 286 eV for the C-OH moiety has disappeared, and a broad band centered around 288 eV, consistent with the carboxylate carbon,l4 has appeared. The K(2psp) band at 292.7 eV and the K(2p1p) band at 296.3 eV are attributed to potaeeium ion from the supporting electrolyte, which functions aa a countdon for the carboxylate group. T h e retention of potandm
Webshaar et al.
328 Langmuir, Vol. 9, No.1, 1993 ion has also been reported for deprotonated aromatic acid monolayers at platinum.’s Potassium was detected only on samples treated in region l,I7 where we expect the presence of the carboxylatemoiety, and not in untreated samples or samples treated in regions 2 and 3. These observationsare consistent with the oxidative conversion of an alcohol to a carboxylic acid end group, i.e., reaction 2, in region 1. The spectrum in the S(2p) region (Figure 9b) for the sample oxidized in region 1also continues to exhibit the thiolate sulfur doublet at 161.9 and 163.1 eV. The similarityto the doublet shown in Figure 9a indicates that oxidation occurs at the alcohol end group, as expected for the conversionof ME/Auto MAAJAu,and not at the sulfur head group. A low-energyshoulder at 161.0 eV, developed as a result of this treatment, is attributed to partial oxidation of the newly formed MAA/Au to form a gold sulfide species. This band assignment and product description are discussed in more detail in section 3. Together, the IRS,X P S , and electrochemical data clearly‘show that the anodic wave in region 1of Figure 3a resulti’from the oxidative conversion of ME/Au to MAA/ A u This conclusion is consistentwith reports that alcohols in alkalinesolution are catalyticallyoxidized to carboxylic acids at gold by voltages in region l.18 The suggested mechanismfor this oxidation involvesoxygentransfer from incipient AuOH to the alcohol, forming first the aldehyde and then the carboxylic acid.18 At f i s t glance, it seems problematic that electrochemically generated MAA/Au (Figure 5b) is desorbed at more positive Voltages than self-assembled MAA/Au (Figure 4). We tentatively attribute this to a lower coverage for the electrohcemically generated MAA/Au than for selfassembled MAA,l9 which arises primarily because the average coveragefor freshly-prepared ME/Au ((7.0 f 0.9) X 10-10moVcm2)is lower than that for self-assembledMAA ((7.8 & 0.7) X 10-10 mol/cmz). We are currently investigating the reasons for the lower coverages for ME. 3. Characterization of the Process Occurring in Region 2. We have established that application of voltages within region 1will oridize ME/Au to MAA/Au. Parta c and d of Figure 5 indicate that the oxidation driven in region 2 produces desorption waves I1 and 111. This additional oxidationmay occur at the tail group, resulting in a decrease in the length of the alkyl chain (with COZas a possible product), or at the sulfur head group. The JRS data provideevidence that C-S bond cleavage occurs in region 2. Figure 7c is the in situ differential spectrum of ME/Au after successivetreatments in regions 1and 2. Treatment in region 1converts ME/Au to MAAI Au, as discussed. After treatment in region 2, the absorbances of both carboxylate modes have increased, and the relative absorbances are similar to that observed (16) Stem,D. A.; Laguren-Davidmn,L.; Frank, D. G.;Gui, J. Y.; Lin, C.-H.;Lu, F.; Salaita, G. N.; Walton, N.; Zapien, D. C.; Hubbard, A. T. J . Am. Chem. SOC.1989,111,877-91. (17) A comparison of areae for the K(2p3p) peak at 292.7 eV and the C(ls) peak at 284.4 eV in Figure 8b, after correction for XPS crosssections (XPS software, Physical Electronics Industries) but not escape depths, indicatee that potassium coverage is 15%of that expected for a 1:l binding stoichiometry. The coverage for potassium ion ie lees than a monolayer due to replacement by hydrogen ion during rinsing. (18) (a) Ocon, P.; Alonso, C.; Celdran, R.; Gonzalez-Velaeco, J. J. Eiectrwnal. Chem. 1986,2&3,179-96.(b)Gonzales Hernan, E.;Alonso, C.;Gondez-Vel”,J. J.Appl.Ekctrochem. 1987,17,866-76. (c)Larew, L. A.; Johnson, D. C. J. Electroonut. Chem. 1989,262,167-82. (19) h e t h i b l a t e monolayemaredenselypakmdandrelativelydefect (pinhole)free. Introduction ofdefecta in the monolayer (lower coverage) i n c r e w the permeability of the electrolyte which in turn c a w s an increw in cdl. In our model, thin increase in cdl appeare ae an apparent decrease in Chain length (increased (7”). Thus, changes in coverage result in a shift of Ep’
-
in solution (Figure 7d). Both findings argue that the carboxylate species has been released from the surface into the thin solution layer of the in situ cell. Since the spectra clearlyshow that carbonate ion (the expected form of COZat this pH) is absent,20the data further suggest that the C-C bond remains intact. Studies aimed at identifying the desorption product (i.e. comparisons with 1 R spectra of possible oxidationproducta such as succinate ion) have to date proven inconclusive. The XPS spectra in Figures 8 and 9 provide additional evidence that oxidation in region 2 leads to scission of the C-S bond. As mentioned, the S(2p) spectrum in Figure 9b (region 1treatment) shows the thiolate sulfur doublet at 161.9and 163.1 eV and the developmentof alow-energy shoulder at 161.0 eV. We ascribed this shoulder to the low-energyhalf of a newly formed doublet. After oxidation in region 2 (Figure 9c), the original doublet has almost disappeared. Additionally, the band at 161.0 eV has increased in magnitude, and ita high-energycompanion is apparent at 161.9 eV. In other words, the sulfur doublet has shifted to lower energy. In an XPS study of sulfur adsorption on gold, Buckley et aLZ1ascribed an S(2ps 2) band at 161.2 eV to a gold sulfide species. The doubiet in Figure 9c then suggests the presence of atomic sulfur bound to gold, a surface species that can only be produced by C-S bond cleavage. The C(ls) spectrum also provides evidence for cleavage of the C-S bond. The bands at 297.2 and 295.3 eV for potassium and at 288 eV for the carboxylatemoiety, which appeared after treatment in region 1 (Figure 8b), disappeared after treatment in region 2 (Figure 8c). Disappearance of these bands can arise from either a decarboxylation reaction or C-S bond cleavage. However, methanethiolate, the expected decarboxylation product, is reductively desorbed at a voltage more positive than that shown in Figure 1for ethanethiolate (-0.79 V). Thus, the growth of waves I1and 111,which occur at more negative voltages, further argues that C-S bond cleavage and not decarboxylation occurs in region 2. We note that an extension of the above conclusions to the trend in the signal intensities in the C(ls) region is complicated by adventitious adsorption, attributed to sample handling and transfer protocols. The reductive desorption behavior for waves I1 and I11 in Figure 5 also supports C-S bond cleavage. Briceno and Chander have reported that sulfur adsorbed on gold is reduced by a one-electron, pH-dependent process where the extrapolated Epin 0.5 M KOH is around -0).90.22 They also found a singlereduction wave for low sulfur coverages, and a second wave that grows in at a slightlymore positive voltage as sulfur coverage increases. The more positive wave was attributed to the desorption of a polyeulfur species that gradually forms at higher sulfur coverages.22b The scanning tunneling microscopic observation29 of the electrochemical conversion of gold-adsorbed, monatomic sulfur to what appears to be a polysulfide species and our observationz4that Seself-assembledon gold is reductively desorbed at -0.89 V provide additional support for the (20) Henberg, G.Molecular Spectra and Molecular Structure; Van Nostrand: Princeton, NJ, Vol. 111. (21) Buckley,A. N.; Hamilton, I. C.; Woods, R. J. Electroanat. Chem. 1987,216,213-27. (22) (a) Briceno, A.; Chander, S. J. Appl. Electrochem. 1990,20,w811. (b) Briceno, A,; Chander, S. J. Appl. Electrochem. 1990,20,512-17. (23) Gao, X.; Zhang, Y.; Weaver, M. J. Submitted for publication in J. Phye. Chem. (24) Lamp, B. D.;Porter,M. D. (Iowa State University). Unpublished work. (26) (a) Hamilton, I. C.; Woods, R. J. Appl. Electrochem. 19811, 13, 783-94. (b) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992,8,668-72.
Self-Assembly of Mercaptoethanol at Gold supposition of Briceno and Chander.22b The step voltage dependence of waves I1 and III in Figure 5 fits this model quite well. Only C-S bond cleavage can produce the surface-bound sulfur required by the model. We have, as yet, not attempted to determine the electron stoichiometry or to identify further the desorbed products of the C-S bond cleavage step. The decomposition of MAA/Au through C-S bond cleavage, however, is not surprising. We have found by GUMS that a 10 mM solution of MAA in ethanol stored for 5 days at room temperature produced H2S, acetaldehyde, the ethyl ester, and an as yet unidentified species. In contrast,neat MAA stored under refrigerationcontained only the corresponding cyclic thiwster dimer and water. It is the cleavage of the C-S bond in solution-phaseMAA that results in the formation of H2S. 4. Characterization of the Process Occurring in Region 3. We have concluded that oxidation of MAAIAu (self-"bled or electrochemicallygenerated)by voltages in region 2 produces a monolayer of sulfur on the gold surface by C-S bond cleavage. Due to the extensive we literature on the oxidation of sulfur at g01d,2a*21-2392s have not pursued astudy of the oxidativeprocess occurring in region 3. These studies have shown that the catalytic oxidation mechanism is a complex and multistep process that ultimately produces sulfate. The studies of the electrochemical oxidation of sulfur2a and thiolates1a*2b*c at gold indicate that the mechanistic picture we have presented thus far may be oversimplified. Vandeberg et have reported evidence that oxidation of thiourea at gold ultimately produces sulfate, but intermediate products such as adsorbed formamidine disulfide and atomic sulfur are also observed. Wierse et al.& found that adsorbedsulfur (monoatomicor polyatomic species) is completely desorbed from gold by an oxidative scan to +0.80V at 10oO VIS. We find that oxidative scans to the same voltage at 100 mV/s also exhaustively desorb ME and MAA monolayersfromgold. Our earlier studies,la as well as that of Finklea et al.,2b found similar behavior for n-propanethiolate and n-butanethiolate, but note that oxidative desorption of longer chain thiolates does not occur until the voltage is sufficiently positive to induce oxygenevolution. We also postulated that the oxidatively deaorbed species for n-propanethiolate is the corresponding sulfinate (RSOz-).l* Since the oxidative processes discussed here are slow, it is likely that the oxidation and subsequent desorption of ME and MAA at the faster scan rates follow a different pathway than at slower scan rates. That is, the oxidation of adsorbed thiolates may be a multipath process, with the favored path depending on reaction conditions as well as the chemical composition of the thiolate.
Conclusion The above findings have provided insights into the electrochemicaloxidationof ME/Au and W A u at gold incontact withelectrolytesolution. Theseresultssuggest
Langmuir, Vol. 9, No.1, 1993 329
that,in the spontaneousoxidationof ME, it is fmt oxidized to MAA which then decomposes via C-S bond cleavage. This mechanism is supported by preliminary measurements of the open circuit voltage for MEIAu in 0.5 M KOH (4.2 to 4 . 3 V), which is within region 1 where oxidationof ME/ Au to MAA/Au occurs. A reexamination of the reductive desorption experiments with the shorter chain alkanethiolates (n = 1-61, prompted by the findings reported here, has also revealed a second desorption wave at -0.95 V. However, the rate of transformation is slower than for MEIAu and decreases with increasing chain length. Since it is unlikely that the CH3 moiety will spontaneously oxidize to a carboxylic acid, the transformations of the alkanethiolates suggest that adsorption on gold activates the C-S bond. Since ME is a stable species (e.g., GC/MS shows that neat ME and &dayold, 10 mM ME solutions in ethanol contain only traces of the corresponding disulfide and water), activation of the C-S bond upon adsorption may facilitate the spontaneous decomposition of ME/Au also. Studies are underway to delineate further the spontaneous oxidation process, including the effects of the end group (X)and chain length. In addition to delineating various aspecte of the electrochemicaloxidationfor ME/Au, we have also shown that the stability of thiolate monolayers on gold cannot necessarily be assumed. The spontaneous oxidation of MEIAu and MAAIAu and our preliminary observations of similar behavior for alkanethiolates at gold further suggest that short-chain thiolate monolayers may be considerably less stable than their longer chain counterparts, a consequence of the weaker barrier properties of the short-chain s t r ~ c t u r e s . ~ ~ Acknowledgment. We gratefully acknowledge the expert assistance of Jan Beane of the Chemistry Instrumentation Services Group of the Iowa State University Chemistry Department for the GUMS work and of Jim Anderegg of Ames Laboratory for the XPS analyses, Darwin Popenoe and Brian Lamp for the in situ IRS experiments and to each for guidance in interpreting the data. Helpful discussions with Dennis Johnson and Pete Vandeberg about sulfur and thiol oxidations at gold and with Earl Smith and Brian Lamp about IRS mode assignments are also acknowledged. D.E.W. gratefully acknowledges sabbatical leave support from Auguatana College, Sioux Falls, SD, M.D.P. expresses appreciation for a Dow Corning Assistant Professorship and an Alcoa Foundation Faculty Development Award, and M.M.W. is grateful to the Institute for Physical Research and Technology for a postdoctoral fellowship. Amea Laboratory is operated for the U.S.Department of Energy by Iowa State University under Contract No. W-7405-eng82. This work was supported by the Summer Faculty Research Participation Program of the U.S. Department of Energy and by the Advanced Life Support Division of the NASA-Ames Research Center (Contract No. NAG2722.
