J. Phys. Chem. 1995, 99, 4106-4112
4106
Effect of Structure on the Electrochemistry of Langmuir-Blodgett Monolayers of 16-Ferrocenylhexadecanoic Acid on a Self-Assembled Alkanethiol Monolayer Liang-Hong Guo,? John S. Facti,*,$ and George McLendon*9+ NSF Center for Photoinduced Charge Transfer, Department of Chemistry, University of Rochester, New York 14627, and Xerox Corporation, Webster Center for Research and Technology, 114-390, 800 Phillips Road, Webster, New York 14580 Received: May 18, 1994; In Final Form: August 18, 1994@
The voltammetry of bilayers of a Langmuir-Blodgett monolayer of 16-ferrocenylhexadecanoic acid (FCAC) transferred onto a decanethiol self-assembled monolayer on Au is presented. Monolayers of FCAC and mixed monolayers containing hexadecanoic acid (HDAC) are transferred onto decanethiol-modified Au electrodes by vertical dipping and horizontal touching techniques, resulting in the ferrocene groups residing at the hydrophobic monolayer-monolayer interface. In-trough voltammetic measurements are carried out immediately after monolayer transfer without recrossing the aidwater interface. Surface pressure-area isotherms of monolayers of FCAC and its mixtures with HDAC are collected at the aidwater and aidaqueous electrolyte interfaces in order to investigate the monolayer and mixed monolayer structures at these airkquid interfaces. The structures of the pure and mixed FCAC monolayers at the aidliquid interface depend greatly on the monolayer composition and the nature and concentration of the subphase electrolyte and pH. The monolayer structure in turn has a dramatic impact on the electrochemical response of the ferrocene groups in the bilayer. While voltammetic waves are observed in loosely packed LB monolayers of pure FCAC in a 1 M NaC104 subphase (PH 6.8) and in immiscible LB films of 1:4 FCAC/HDAC in 1 M HC104, no faradaic response was obtained with 1:4 FCAC/HDAC monolayers in 1 M NaC104. Well-defined voltammetry was best observed in expanded monolayers obtained by horizontal touch transfer. Isotherm results suggest a more rigid and better molecular packing of 1:4 FCAC/HDAC on the NaC104 subphase than on the HC104 subphase. The electrochemical behavior is interpreted on the basis of control of the counterion motion by the structure of the hydrophobic tail region of the outer LB monolayer. Complementary experiments using Au electrodes modified with 12-ferrocenylalkanethiol as LB transfer substrates support this interpretation.
Introduction Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs) have emerged during the last decade or so as two of the most convenient and useful methods of preparing molecularly tailored interfaces with controlled spatial and geometric constraints and have been examined widely from both fundamental and applied perspectives.' Kuhn and co-workers' pioneering investigations in this area involved the use of LB films to organize and orient amphiphiles in order to separate functional groups by a known distance. This was followed by several publications from their group3 as well as other^^-^ that addressed the effects of distance and medium on electron transfer (ET) and energy transfer. Recently, SAMs of both unfunctionalized and redox-terminated long-chain alkanethiols adsorbed on Au electrodes were used to control ET distances at electrochemical interfaces. Several groups have reported heterogeneous ET rates of redox species that either were chemically bonded to the hydrophobic terminus of an alkanethiol monolayer'. or were freely diffusing in solution7by electrochemical techniques. The rates were shown to decay exponentially with increasing alkanethiol chain length in accordance with theory,s and the decay constant was about 1.0 per methylene unit (ca. 0.8 kl) in these report^.^-^ Of particular interest to us is the utility of SAMs and LB films in the study of electron transfer reaction mechanisms, i.e. the dependence of ET rates on the donor-acceptor distance and
*
t Universitv of Rochester. @
Xerox &rp. Abstract published in Advance ACS Abstracts, March 1, 1995.
intervening medium. We propose that bilayer films assembled on Au substrates may be used for electrochemical investigation of the distance and medium effect on the interfacial ET reaction. As a test of this proposition, bilayers have been assembled which consist of an impermeable inner spacer monolayer of an alkanethiol on Au and an outer electroactive ferrocene-containing LB monolayer. The envisioned structure is shown in Scheme 1. When the ferrocene groups are located at the hydrophobic terminus of the amphiphile, they must necessarily reside at the SAMLB monolayer interface. One of the attributes of the SAM/LB bilayer assembly is that it takes advantages of the desirable features of both SAMs (pinhole-free films, variable thickness, and ease of preparation) and LB monolayers (compositional flexibility and external control of monolayer structure). Since in the bilayer arrangement the ET distance between the donor (ferrocene) and the acceptor (Au) is determined by the inner layer thickness, it should be possible to vary the ET distance simply by varying the alkanethiol chain length. To avoid uncertainties about the ET distance, it is important that the inner layer be structurally well-ordered and relatively defect-free. We previously reported9 on the spectroscopic and electrochemical characterization of these layers as well as the atomic force microscopic characterization of the surface topology of various Au substrates upon which the bilayer was intended to be assembled. Electrochemical examination of the alkanethiol monolayers self-assembled on freshly deposited near atomically smooth Admica substrates revealed the existence of a large number of pinhole defects in the monolayer. These substrates were therefore deemed not useful. We employ, on the other hand, a simple chemicaVelectrochemica1 procedure
0022-365419512099-4106$09.0010 0 1995 American Chemical Society
Structure of 16-FerrocenylhexadecanoicAcid Monolayers
J. Phys. Chem., Vol. 99, No. 12, 1995 4107
SCHEME 1: Illustration of the Idealized Structure on Gold of a Bilayer Composed of an Inner Self-Assembled Monolayer (SAM) of Alkanethiol and a n Outer Ferrocene Containing a Langmuir-Blodgett Monolayer CH, CH,
[
- sHC
COOH
COOH
-s4 -s-[/
COOH
CH3 AU
COOH
- 3 H c
SAM
for cleaning Au films on silicon wafers (see the Experimental Section), which ultimately resulted in densely packed alkanethiol monolayers that inhibited the electrochemistry of dissolved ferricyanide species, Le. SAMs which are virtually pinhole-free. FTIR spectra of these SAMs9showed the spectral characteristics of highly ordered and organized films and were in excellent agreement with earlier published work.l0 Ellipsometric thickness determinations showed that the film thickness increased monotonically with the carbon number, in good agreement with earlier work.l0 In previous electrochemical studies of LB f i i s of ferrocenederivatized amphiphiles, the ferrocene moiety was located at the hydrophilic end of the amphiphile." Assembly of a twomonolayer bilayer in which the ferrocene is located at the monolayer-monolayer interface, on the other hand, requires that the ferrocene moiety be located at the hydrophobic terminus of the amphiphile. LB monolayers of 16-ferrocenylhexadecanoic acid (FCAC) are therefore employed as the outer monolayer component of the bilayer. Surface pressure-area (IT-A) isotherms of pure FCAC and its mixtures with octadecanoic acid have also been reported earlier12 on a subphase containing little or no electrolyte. However, the monolayer structural characterization on such subphases may not be directly translatable to voltammetric electrochemical studies which require a much higher ionic strength electrolyte/subphase. In order to understand the electrochemical behavior of these bilayer systems, it is desirable to first examine the influence of the structure of the outer monolayer on bilayer electrochemical behavior. We report, therefore, the isotherm behavior of pure FCAC monolayers and its mixtures with hexadecanoic acid (HDAC) as a function of the nature and concentration of the subphase electrolyte and pH. The voltammetric response of the pure and mixed monolayers transferred onto decanethiol SAMs by both vertical dipping and horizontal touching as a function of surface pressure, pH, and electrolyte composition is also reported, and the results are interpreted in terms of the isotherm behavior. Experimental Section Materials and Apparatus. Monolayer grade water for the makeup of the Langmuir trough subphase was purified in succession by reverse osmosis, organic impurity removal (activated carbon), deionization, ultrdiltration, and double distillation from permanganate under high-purity Ar. Highpurity HDAC (ClsH31COOH) was purchased from Aldrich and recrystallized from ethanol. FCAC (FcC~~H~OCOOH, Fc = ferrocene) was a gift from Professor David Whitten (University of Rochester) and was synthesized by a published procedure.12 Cadmium chloride, NaHCO3, and NaC104 (Aldrich) were recrystallized from monolayer water. Perchloric acid was doubly distilled from Vycor and obtained from GFS. Spectro-
i
CH3
COOH
L-6 grade chloroform (Fisher) was used to prepare solutions of amphiphiles for spreading of monolayers. The LB trough was occasionally cleaned with 100%(undenatured) ethanol between runs to insure the purity of the subphase. 12-Ferrocenyldodecanethiol was a kind gift from Dr. S. Creager (Indiana University, Bloomington). Gold-coated silicon wafers, obtained from Polishing Corp. of America, were fabricated on (100) Si wafers which were highly polished on both sides and sputtered with 100 nm Au on both sides. A 10 nm Ti layer was deposited prior to Au deposition to improve Au adhesion to the Si wafer. AdSi wafers were diced with a scribe into pieces of 1 x 2 cm2 area prior to each experiment. They were cleaned by extensive ethanol rinsing and brief immersion (10 s) in hot 1:3 H202/H2SO4, followed by repeated electrochemical potential cycling (0 to -0.SV) in 1 M KCl.9 The cleaned electrodes were transferred immediately to an ethanol solution of the alkanethiol and kept immersed overnight. Surface pressure-area (IT-A) isotherms were obtained on a Lauda MGW trough interfaced to a Hewlett-Packard computer as previously described.lla The temperature of the subphase was maintained at 20 zk 1 "C. Cyclic voltammetric results were collected either on a Cypress computer-controlled potentiostat or with a Princeton Applied Research model 173 potentiostat, model 175 signal programmer, and Hewlett-Packard X-Y chart recorder. All potentials are referenced to a sodium chloride saturated calomel electrode (SCE). Procedures. Experimental details concerning amphiphile spreading and isotherm collection can be found in a previous publication.' la In monolayer transfer experiments that involved electrochemical characterization, the amphiphiles were first spread on the aqueous electrolyte subphase. After solvent evaporation, the moving barrier was compressed to the desired area or surface pressure and the monolayer was annealed, usually for 10-15 min. To transfer the monolayer by vertical dipping, an alkanethiol-modified planar Au electrode was vertically affixed to a motorized lift and dipped slowly (-2 d m i n ) into the subphase until a substrate area of 2 cm2 was immersed. Horizontal transfer was done by attaching the electrode to a horizontal stage connected to the lift. A Pt wire was attached to the Au coating on the back side of the wafer. Electrical connection to the front side was insured by the presence of a contiguous Au coating that goes around the edge of the wafer, connecting the front and back sides. The electrode was then moved down slowly toward the subphase parallel to the water surface and its motion stopped at the point of contact. Electrical contact with the subphase electrolyte was determined by observing the charging current.' IC Electrochemical measurements were carried out in the trough immediately after monolayer transfer to minimize any potential changes or disruption of the monolayer structure.
Guo et al.
4108 J. Phys. Chem., Vol. 99, No. 12, 1995 6o
g
E v
e k
so 40
so 20
8
f
a
m
m
10
0 0
0
50
100
150
200
Area (k'/FCAC)
Figure 1. Surface pressure-area
isotherms of mixtures of 16ferrocenylhexadecanoic acid (FCAC) and hexadecanoic acid (HDAC) at molar ratios of (a) 0:1, (b) 1:0, (c) 1:1, (d) 1:2, and (e) 1:4. Subphase: 0.2 mM CdC12/0.5 mM NaHC03 (pH 6.8). Note that in all the isotherms the horizontal axis corresponds to the FCAC molecular area, except in a, where it is the molecular area of HDAC.
Results and Discussion Surface Pressure-Area Isotherms. Surface pressure-area isotherms of HDAC, FCAC, and the mixtures of the two amphiphiles at various molar ratios on a subphase of low ionic strength (0.2 mM CdCld0.5 mM NaHC03) are illustrated in Figure 1. The molecular area axis of the mixture in the isotherm is obtained by dividing the water surface area by the number of FCAC molecules in the mixture, rather than the total number of molecules. The isotherm of HDAC is identical to those reported before13 and was used as a check on the cleanliness of the trough and water surface. Pure FCAC (Figure 1, curve b) shows two steep rises in pressure at molecular areas of -48 and -27 Az,respectively, with a transition between these values. Extrapolation of the first pressure rise to zero pressure gives a molecular area of 47 A2, close to the cross section area of ferrocene head group (50 The second pressure rise climbs to 45-50 mN/m without collapse and asymptotes toward 20 A2/molecule, which is significantly smaller than the ferrocene molecular area. Since this molecular area corresponds to that of an alkyl chain, the isotherm behavior suggests that the ferrocene heads groups are becoming staggered by a slight lifting out of the plane of the head groups. The latter result is somewhat different from the FCAC isotherm behavior reported earlier12where a molecular area of ca. 7-10 A2 and a relatively low collapse pressure were observed. A slightly higher cadmium chloride concentration and pH were used in our experiments, as well as a different spreading solvent. In an equimolar mixture of FCAC and HDAC (Figure 1, curve c), the phase transition between the two pressure increases is less pronounced and the extrapolated molecular area at the second rise (higher pressures) is ca. 50 A2, which corresponds to that of the ferrocene molecular area. Similar observations have been reported for the 1:1 mixture of a charged ferrocene amphiphile and stearic acid.lla The fact that the molecular area occupied by each FCAC in the 1:1 monolayer is equal to the ferrocene size, not the sum of FCAC and HDAC, implies that HDAC is accommodated in the space between the alkyl chains of the FCAC molecules. Incorporation of HDAC into FCAC films apparently leads to improved molecular packing in the monolayer and thus eliminates to a large extent lifting or squeezing out of FCAC molecules. This is clearly shown in
100
Area
200
so0
(A2IFCAC)
Figure 2. Surface pressure-area isotherms of mixtures of 16-
ferrocenylhexadecanoic acid and hexadecanoic acid at molar ratios of (a) 1:0, (b) 1:1, (c) 1:2, and (d) 1:4. Subphase: 1 M NaC104/0.5 mM CdCldO.5 mM NdC03 (pH 6.8). The horizontal axis corresponds to the FCAC molecular area. the high-pressure behavior of the isotherm in curves b and c. The FCAC molecular area in the 1:2 and 1:4 mixtures with HDAC (Figure 1, curves d and e, respectively) can be easily understood by adding one and three more hydrocarbon chains of HDAC into the 1:l film. There are two pieces of evidence that demonstrate the miscibility of the mixed films. Firstly, as more HDAC is added to the mixture, the transition, characteristic of a pure FCAC monolayer, becomes a less prominent feature. Components of an immiscible film would behave independently and exhibit features independent of comp~sition.'~Secondly, only one collapse pressure (at 60-65 mN/m) was observed for the mixed film. An immiscible film would show two distinctive collapse pressures, each representing the collapse of one of the components. In electrochemical measurements, electrolytes of higher concentration are needed to make the solution sufficiently conductive. For the cyclic voltammetric experiments described below, II-A isotherms of both pure and mixed FCAC films were collected on subphases containing 1 M NaC104 and 0.1 M NaC104, respectively, in the presence of 0.2 mM CdC12 (pH 6.8). On a 1 M NaC104 subphase, the ll-A isotherm of pure FCAC (Figure 2, curve a) again consists of two steeper rises in pressure and a gradual transition phase between these two regions, similar to the low ionic strength subphase (Figure 1). In contrast with the isotherm on the low ionic strength subphase, the second rise in pressure occurs at an even smaller molecular area (-15 A2), suggesting film collapse. Because of this we decided to investigate mixed monolayer films. In the mixed films, the FCAC features were prominent at 1:1, 1:2, and 1:4 molar ratios as long as the pressure was below 30 mN/m. Above 30 mN/m, the isotherm steepens, characteristic of a solid condensed phase, and the molecular areas determined by extrapolation to zero pressure are the same as those obtained on the low ionic strength subphase, both typical of fully packed hydrocarbon chains. A single collapse pressure of the mixed films indicates that the two components are mutually miscible. On the basis of these observations, we postulate that, in the mixed LB films, incorporation of HDAC molecules into the space between FCAC alkyl chains is favorable on the low ionic strength subphase. However, on a high ionic strength subphase, this process is much less favorable as indicated by the longer transition region. The initial pressure rise was observed at much larger areas on the high ionic strength subphase, indicating a
J. Phys. Chem., Vol. 99,No. 12, 1995 4109
Structure of 16-FerrocenylhexadecanoicAcid Monolayers 70
50 60
40
15
1
I
-
20 10 30
0
V
-10
-250 I
I
I
I
1
I
I
I
I
0
250
500
750
Potential (mV,vs. SCE)
Area (A'IFCAC)
Figure 3. Surface pressure-area isotherm of a mixture of 16ferrocenylhexadecanoic acid and hexadecanoic acid at a molar ratio of 1:4.Subphase: 1 M HC104. The horizontal axis corresponds to the FCAC molecular area.
more expanded film. The presence of Cd2+ cations added in the subphase electrostatically cross-links the carboxylate end of the fatty acid, promoting the formation of a condensed film. The results shown in Figure 2 suggest that the electrostatic crosslinking effect of CdZ+is diminished at high ionic strength, as indicated by the higher lift-off molecular areas. Further evidence to support this assertion is found in the isotherms of the same mixed films on a subphase containing 0.1 M NaC104 and 0.2 mM CdCl2 (not shown). In the isotherms, the initial rise in pressure takes place at an area of 180 AZ/FCAC,whereas the lift-off area in 1 M NaC104 is larger than 300 A2/FCAC (Figure 2, curve d). In addition, the slope of the initial rise in 0.1 M NaC104 was steeper than that in 1 M NaC104, also indicative of a less expanded film. Because perchlorate counterions can have an insolubilizing effect on monolayer films, isotherms were collected on 1 M NaCl subphases (not shown) for comparison. The latter are in effect the same as those collected on 1 M NaC104. Thus it does not appear to us that the nature of the counterion drives the differences between Figures 1 and 2 but rather the ionic strength. The pH of the subphase also has a significant influence on the monolayer structure. On a 1 M HC104 subphase, the isotherm of 1:4 FCAC/HDAC (Figure 3) is quite different from the isotherms collected on NaC104 subphases. In contrast with the NaC104 subphase, film collapse is observed at -37 mN/m. This suggests that the two components in the mixed film are immiscible. Addition of Cd2+ to the subphase did not cause any change to the isotherm, as expected. As demonstrated below, such profound changes in film structure have a dramatic impact on the electrochemical reactivity of the ferrocene groups in the monolayer. Bilayer Voltammetry. There have been several reports describing the cyclic voltammetric response of SAMs of pure w-ferrocenylalkanethiols and their mixtures with unsubstituted alkanethiols of various chain lengths adsorbed on gold elect r o d e ~ . ~In%addition, ~ * ~ ~ several electrochemical investigations used electrode surfaces modified with LB films of fatty acid ferrocene derivatives.llaJ1fJ1g One such effortlla involved the study of dimethylferrocenyloctadecylammonium perchlorate, ClgFc+, on Au electrodes. In these studies, LB transfer was accomplished upon withdrawal of a hydrophilic Au electrode from the subphase. The electrochemical analysis was carried out by passing the LB film a second time through the aidwater interface. The influence of the second transfer on the structural reorganization of the LB film is unknown but may be important.
E -0.5
"_. -1
0
200
400
600
Potential (mV,vs. SCE)
Figure 4. In-trough cyclic voltammograms of a mixed LB monolayer of (a) 1:0 and (b) 1:4 16-ferrocenylhexadecanoicacid and hexadecanoic acid in 1 M NaC104/0.5 mM CdCld0.5 mM NaHC03 (pH 6.8). Scan rate: 100 mV/s. The LB monolayer was transferred by downward vertical dipping of a decanethiol-modified planar Au electrode through the &/water interface at a surface pressure of 25 mN/m.
In contrast, the FCAC amphiphile used in the present work incorporates the neutral ferrocene group at the hydrophobic terminus of the hydrocarbon chain. Thus, in the bilayer modified electrodes shown in Scheme 1, the ferrocene is positioned at the hydrophobic-hydrophobic interface between the SAM and LB film. In this case, a single LB monolayer is transferred when the electrode is immersed into the subphase through the aidwater interface. Consequently, electrochemical analysis of the transferred film is done in the trough subphase with a minimum of structural reorganization. When a pure FCAC monolayer was transferred onto CloSW Au by the vertical dipping technique into a 0.1 M NaC104 subphase containing 0.2 mM Cd2+ at a pressure of 25 mN/m after an annealing time of 15 min, two redox couples are observed in the cyclic voltammogram as shown in Figure 4a. The midpoint potentials (E112) for the two couples are estimated as $0.22 and +0.40 V, respectively. The peak current of the anodic wave of the higher E112 couple increases linearly with the potential scan rate, provided that the scan rate was below 100 mV/s. The linear dependence indicates an electrode process involving a surface-attached redox species. Integration of the anodic charge under the two peaks at 50 mV/s yields a value of 25 pC/cm2 (r = 2.6 x 10-lo moYcm2), which is slightly less than that for a fully packed ferrocene fatty acid monolayer (32 pC/cmZ). However, the integrated charge decreases as the potential scan rate increases and at 500 mV/s is 14 pC/cm2, about one-half the value at low sweep rates. In contrast, CloSH monolayer films on Au immersed in 1 mM Fe(CN)63- in the subphase electrolyte exhibited blocking behavior as previously r e p ~ r t e d .This ~ suggests that charge transfer may not be 'due to direct reaction of the ferrocene sites at defects in the
Guo et al.
4110 J. Phys. Chem., Vol. 99, No. 12, 1995 9000
-
1
N$
2
.-F
-1=
n
g
I
O
'
I
I
400
600
70 W
5000
30M
low (1000)
(3000)
-6
-100
100
300
500
700
Potential (mV vs. SCE) Figure 5. In-trough cyclic voltammograms of (a) 16-ferrocenyldodecanethiol and dodecanethiol (5:95 molar ratio) coadsorbed on a Au electrode in a subphase electrolyte of 1 M NaC104/0.5 mM CdC12/0.5 mM NaHC03 (pH 6.8) at potential scan rates of 20, 50, 100, and 200 mV/s and (b) the electrode in a coated with a mixed LB monolayer of 16-ferrocenylhexadecanoicacid and hexadecanoic acid (1:4) transferred by downward vertical dipping. Potential scan rate: 100 mV/s.
inner monolayer but rather to tunneling through the inner layer. Convincing proof of this, however, would require the use of a ferrocene probe. Nonetheless, the scan rate dependence of the coverage is interpreted to be a consequence of slow ion permeation through the hydrophobic tails of the LB monolayer. This is supported by the results described below. Multiple-wave voltammograms with similar E112 values have already been reported for SAMs of ferrocene-terminated alkanethiols at high surface ~0verages.l~The reason for the multiple waves has been postulated to be the result of possible aggregation at high coverages, leading to adsorbate domains of different redox potentials. This argument, however, does not seem to be applicable here, as many ferrocene-containing LB and SAM monolayers in aqueous electrolyte exhibit single-wave voltammograms, regardless of surface coverage. An alternative interpretation of the multiple waves is discussed below. Shown in Figure 4b is the voltammogram of a single LB monolayer of 1:4 FCACMDAC on ClOSWAu. The LB monolayer was transferred by vertical dipping in 1 M NaC104 (pH 6.8, 0.2 mM Cd2+) at a surface pressure of 15 mN/m. No faradaic response is seen. The same result was obtained when the LB transfer was carried out at lower transfer pressures (e.g., 10 and 5 mN/m). In addition to a unity transfer ratio, ellipsometric evidence suggests the formation of a complete bilayer on the electrode. Ellipsometric thickness measurements were obtained on two LB layers of 1:4 FCAC/HDAC transferred at 15 mN/m onto a CloSWAu substrate by vertical immersion and emersion. Using a refractive index of 1.45 for the multilayer on Au (two LB monolayers and one decanethiol monolayer), an average film thickness of 56.6 8, for several replicate spots was obtained. This agrees rather well with a calculated value of 60 8, based on the measured values of 10 8, for the CloSH layer and 25 8, for each FCAC layer. This implies that the absence of faradaic waves is not due to the absence of the FCAC monolayer but rather is related to the blocking effect of the LB monolayer. To further demonstrate the effect of an LB adlayer on the electrochemical reactivity of redox groups buried at the monolayer-monolayer interface, a clean bare Au electrode was modified by adsorption from ethanol of a mixture of 5 mol % 12-ferrocenyldodecanethioland 95 mol % tetradecanethiol. Figure 5a presents the cyclic voltammetric behavior of the modified electrode in the same 1 M NaC104 (pH 6.8, 0.2 mh4
(5000)
(70 00)
0
200
800
Potential (mV, vs. SCE) Figure 6. In-trough cyclic voltammograms of LB monolayers of 1:4 mole ratio of 16-ferrocenylhexadecanoic acid and hexadecanoic acid transferred onto decanethiol-modified Au electrodes by vertical dipping in 1 M HClOI. Molecular areas of 16-ferrocenylhexadecanoicacid: (a) 300, (b) 190, and (c) 95 A2. Scan rate: 100 mV/s. Vertical scale: 20 PA.
Cd2+) subphase electrolyte. The figure illustrates well-defined waves characteristic of the surface-immobilized ferrocene, as l ~ ~surface coverage of the reported by Creager et ~ 1 . The ferrocene groups is estimated from the integrated charge of the redox peaks as 2.1 x lo-" mol/cm*, which corresponds to 6.4 mol % coverage. The electrode was then removed from the electrolyte, and an LB monolayer of 1:4 FCACMDAC was transferred vertically onto the modified Au under the same conditions as used for the transfer of the LB monolayer in Figure 4b. The results are shown in Figure 5b. The voltammetric waves of the ferrocene groups buried by the 1:4 FCACMDAC layer now become completely inhibited. This observation demonstrates explicitly the effect of a vertically transferred 1:4 FCAC:HDAC monolayer on the electrochemical reactivity of the redox groups buried at the monolayer-monolayer interface. Analogously, the absence of electrochemical waves in the 1:4 FCAC/HDAC monolayer in 1 M NaC104 results from highly hindered counterion permeation to the redox sites as a consequence of the very dense packing of the mixed monolayer. The isotherm behavior of the 1:4 FCACMDAC and pure FCAC is consistent with the cyclic voltammetric behavior of these monolayers transferred to CloSH substrates. As discussed above, improved molecular packing in 1:4 FCACMDAC monolayers results from a filling in by HDAC amphiphiles of the space in the tail groups of the FCAC amphiphiles which results from the difference between the molecular areas of the ferrocene head and alkyl tail groups of FCAC. In comparison, pure FCAC monolayers cannot be expected to exhibit the same high degree of molecular packing because of the disparity between the molecular areas of the head and tail groups. Thus the fraction of the ferrocene groups which are electrochemically reactive is much greater in the pure FCAC LB films on CIOSWAu. We have postulated that mixed FCACMDAC films on HClO4 are immiscible and domains of FCAC molecules may exist. Since redox waves were observed with pure FCAC monolayers in NaC104 (see Figure 4a), voltammetric peaks arising from faradaic reaction of the FCAC domains in the mixed FCAC/ HDAC films in HC104 is expected. Figure 6a presents a cyclic voltammogram of a film of 1:4 FCAC/HDAC spread on 1 M
J. Phys. Chem., Vol. 99, No. 12, 1995 4111
Structure of 16-FerrocenylhexadecanoicAcid Monolayers 1W.W
(50 W)
0
200
400
600
Potential (mV,vs. SCE)
Figure 7. In-trough cyclic voltammograms of LB monolayers of 1:4 mole ratio 16-ferrocenylhexadecanoic acid and hexadecanoic acid transferred onto decanethiol-modified Au electrodes by horizontal touching in 1 M HCIOI. Molecular areas of 16-ferrocenylhexadecanoic acid of (a) 450 and (b) 325 A*. Scan rate: 100 mVls. Vertical scale: 30 PA.
HC104 and transferred by vertical dipping onto a CloSWAu substrate. The transfer was done at a ferrocene area of 300 A2/molecule. The curve consists of only one redox couple ( E O ' = 0.44 V), indicative of a more homogeneous film than the pure FCAC monolayer on NaC104. The scan rate dependence of the peak current is linear, and the integrated area under the peak provides a total charge of -7 pC/cm2 (7 x lo-" mol/ cm2, 230 A2/molecule). This result implies that all of the redox sites are electrochemically reactive. Figure 6, parts b and c, illustrate the cyclic voltammograms of the mixed LB monolayers transferred at areas of 190 and 95 A2/ferrocene,respectively. Although the concentration of FCAC in the mixed film on the subphase (and presumably its surface coverage on the electrode) was increased by a factor of 2 when going from part b to part c, the integrated charge actually decreases from 10 pC/cm2 (160 A2/ferrocene) to 8.5 pC/cm2 (190 A*/ferrocene). Increasing the FCAC and HDAC coverage at transfer also increases the monolayer density, causing the LB monolayer to become more impermeable to counterion motion and hence the decrease in apparent surface charge (coverage). Attempts to transfer LB films by vertical dipping at molecular areas greater than 300 A2/ferrocene proved not to be feasible because of a poor transfer ratio. In order to study the voltammetry of bilayers containing highly expanded outer LB monolayers, we resorted to a horizontal touching transfer which also has the advantage that the voltammetry can be done in situ in the trough. Depicted in Figure 7 are the cyclic voltammograms of LB films of 1:4 FCACMDAC in 1 M HC104 transferred by horizontal touching onto CloSWAu substrates at FCAC molecular areas of 450 and 325 A2, respectively. The good correlation between the charge calculated from the cyclic voltammogram, 2.7 pC/cm2 (590 A2/ ferrocene) and 5.3 pC/cm2 (300 A2/ferrocene) in Figure 7, parts a and b, respectively, and ferrocene area at transfer indicates a high degree of electrochemical reactivity and is consistent with unrestricted counterion diffusion in expanded LB monolayers. The remarkably different electrochemical responses of the ferrocene moiety in the three types of LB monolayers, namely the pure FCAC in 1 M NaC104, 1:4 FCAC/HDAC in 1 M NaC104, and 1:4 FCACMDAC in 1 M HC104, can be related to the difference of structural integrity of the three films and can be analyzed by the treatment of Smith and White.16 The treatment predicts a strong dependence of the voltammetric wave shape and apparent redox potential on the distribution of counterions. In the LB film of pure FCAC, dense packing of
the hydrocarbon tails is not readily achieved even at relatively high surface pressures, due to the mismatch in molecular area between the ferrocene moiety and the alkyl chain. Voltammetric waves in this system are observed at "normal" (0.22 V) and upshifted potentials (ca. 0.40 V). Presumably the redox sites at the normal oxidation potential are associated with defects in the LB film (imperfections) which may be associated with free counterion permeation while those at higher oxidation potential are associated with hindered counterion permeation, Le. ferrocenes in a more hydrophobic environment. In the mixed monolayer in 1 M NaC104, each FCAC molecule is on average surrounded by four HDAC molecules that (at least partially) fill the otherwise empty space between FCAC hydrocarbon chains. Ion permeation through the packed film is thus expected to be negligible. Consequently, the voltammetric waves are broadened and upshifted to such an extent that they are not distinguishable from the background current. This situation is described by Figure 7 of ref 16. It is worth noting that such distorted faradaic waves are evident in both curves a and b of Figure 4, which shows an ill-defined departure of current from the background at potentials greater than the ferrocene formal potential. This current behavior is not manifested with Au electrodes that are modified with CloSH alone. The mixed monolayer in HC104 is a case intermediate between the two just analyzed. The formal potential of the ferrocene moiety in the LB film merits further discussion. The alkylferrocene formal potential when the ferrocene moiety is fully exposed to the 1 M HC104 solvent, is about +0.19 V, as previously reported.lSb Creager and Rowe investigated the ferrocene formal potential in 1 M HC104 in mixed self-assembled monolayers containing 6-ferrocenylhexanethiol and c4-c12 alkanethiols. The ferrocene redox potential increased systematically from +O. 19 V to about f 0 . 3 8 V as the alkanethiol chain length increased progressively over this range. Alkyl chains longer than c6 create an increasingly hydrophobic environment around the ferrocene groups, resulting in a positive shift in the formal potential because the ferricinium cation is destabilized relative to ferrocene. The formal potential of the ferrocene measured in our experiments with the mixed film in 1 M HC104 (E"' = 0.44 V) is consistent with the ferrocene present in a hydrophobic environment. We recognize that white's treatmenP can also lead to the same potential upshift. Following the latter analysis, we may attribute the two redox potentials observed with the pure FCAC monolayers in NaC104 (Figure 4a) to the existence of two populations of FCAC molecules. One population is exposed to the polar solvent potentially through film defects and exhibits a "normal" redox potential (0.22 V), and the other is in a relatively hydrophobic environment and thus shows an upshifted potential of 0.40 V.
Summary Bilayers consisting of a LB monolayer of 16-ferrocenylhexadecanoic acid transferred to a self-assembled monolayer of decanethiol on Au have been prepared as a function of the structure of the outer LB layer. The structure of this monolayer is controlled by the transfer pressure, the monolayer composition (pure or mixed monolayer), and the nature and concentration of the subphase electrolyte. Because the ferrocene groups are located at the monolayer-monolayer interface (Scheme l), counterion permeation through the tails of the LB monolayer is an important variable in the bilayer voltammetric response. Highly expanded monolayers formed by horizontal touching transfer resulted in a well-defined surface voltammetric response characterized by electrochemical reactivity of all the ferrocene
4112 J. Phys. Chem., Vol. 99, No. 12, 1995 groups. The expanded monolayer is thought to have an open structure which does not substantially hinder coupled counterion motion. Vertical monolayer transfer results in a higher chain packing density, which results in low or no faradaic response depending on the tightness of the packing. Complementary experiments involving self-assembled monolayers of 12-ferrocenyldodecanethiol show a dramatic decrease in the electrochemical reactivity of the redox species when the monolayer is covered with a densely packed 1:4 FCACMDAC monolayer transferred by the vertical immersion technique. The latter result supports our assertion of a remarkable level of control of electrochemical reactivity by monolayer structure.
Acknowledgment. We thank Prof. Stephen Creager (Indiana University) for useful discussions and the kind gift of 12ferrocenyldodecanethiol. The authors gratefully acknowledge the National Science Foundation for a Science and Technology Center Grant (CHE-9 120001). References and Notes (1) (a) Robert, G. G. Langmuir-Blodgeft Films; Plenum: New York, 1990. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, MA, 1991. (2) (a) Kuhn, H.; Mobius, D.; Bucher, G. Physical Methods of Chemistry; Weissberger, A., Rossiter, B., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B, p 597.
Guo et al. (3) Mobius, D. Acc. Chem. Res. 1981, 14, 63 and references therein. (4) Hsu, Y.; Penner, T. L.; Whitten, D. G. J. Phys. Chem. 1992, 96, 2790. (5) Chidsey, C. E. D. Science 1991, 251, 919. (6) Finklea, H. 0.;Hanshew, D. D. J. Am. Chem. SOC.1992,114,3173. (7) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (8) (a) Simmons, J. G. J. Appl. Phys. 1963, 34, 1793. (b) Frese, K. W., Jr. J. Phys. Chem. 1981, 85, 3911. (c) Beratan, D. N.; Hopfield, J. J. J. Am. Chem. SOC. 1984, 106, 1584. (9) Guo, L.-H.; Facci, J. S.;McLendon, G. Langmuir, 1994,10,4588. (10) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987, 109, 3559. (11) (a) Facci, J. S. Techniques of Chemistry; Murray, R. W., Ed.; Wiley-Interscience: New York, 1993; Vol. XXII, Chapter 3. (b) Fujihira, M.; Nishiyama, K.; Hamayuchi, Y. J. Chem. SOC.,Chem. Commun. 1986, 823. (c) Zhang, X.; Bard, A. J. J. Am. Chem. SOC.1989, 111, 8089. (d) Miller, C. J.; Bard, A. J. Anal. Chem. 1991, 63, 1707. (e) Obeng, Y. S.; Founta, A,; Bard, A. J. New J. Chem. 1992,16, 121. (0Miller, C. J.; Madja, M. J. Am. Chem. SOC. 1986, 108, 3118. (g) Widrig, C. A,; Madja, M. Langmuir 1989, 5, 689. (12) Seiders, R. P.; Brookhart, M.; Whitten, D. G. Isr. J. Chem. 1979, 18, 272. (13) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Intersciences: New York, 1966. (14) Facci, J. S.; Falcigno, P. A,; Gold, J. M. Langmuir 1986, 2, 732. (15) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC. 1990, 112, 4301. (b) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (c) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (16) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398.
JF9412 160
