Langmuir 1986, 2, 732-738
732
Characterization of Electroactive Langmuir-Blodgett Monolayers of (Ferrocenylmethy1)dimethyloctadecylammonium Sulfate at Gold and Air/Water Interfaces John S. Facci,” Pasquale A. Falcigno, and Jeffrey M. Gold Xerox Webster Research Center, Webster, N e w York 14580 Received March 21, 1986. I n Final Form: J u l y 9, 1986 Organized monolayer films of (ferrocenylmethy1)dimethyloctadecylammoniumhexafluorophosphate on aqueous 0.1 M Na2S04surfaces were transferred via the Langmuir-Blodgett (L-B) technique to hydrophilic Au electrodes. These transfers resulted in the “heads down” orientation of the polar electroactive head group on the electrode. The cyclic voltammetry of electrodes coated with single L-B monolayers was examined with increasing transfer pressure, rt. The fraction 0 of transferred electroactive sites which react faradaically with the electrode on the cyclic voltammetric time scale is consistently less than unity and passes through a maximum. It is postulated that this is due to dissolution from the Au/electrolyte interface which is maximized as a result of the competition of two factors: low film cohesiveness at low rt and intermolecular electrostatic repulsion at high rt. Detailed study of the surfactant behavior at the air/water interface is presented as it is thought that the structure of the monolayer on the electrode is influenced to a large degree by the film state at the air/water interface. Introduction Electrode surface modification has been the subject of recent intensive study, with areas such as electrocatalysis, display technology, charge rectification, energy conversion, and macromolecular electronics being touted as possible applications.’ Most electrode-coating techniques (e.g., covalent chemical attachment, electropolymerization, evaporative and spin coating) result in a random spatial and orientational arrangement of redox centers in the film. However, the ability to introduce large-scale order a t the molecular level may offer unique advantages in terms of novel device preparation. Also, control of surface molecular architecture could yield important insights into electron-transfer reactions a t interfaces. Recent renewed interest in the properties and technological applications of self-assembling monolayers a t the airlwater interface and monolayers transferred via the Langmuir-Blodgett (L-B) technique has led to advances in the understanding of the structure and properties of these system^.^-^ These systems are ideally suited to introducing large-scale molecular order at electrode surfaces. However, modification of electrode surfaces with transferred monolayers has received scant attention from elect roc he mist^.^,^
L-B monolayers of ambipolar surfactant systems are known to form monomolecular and multilayer films which are homogeneously thick and exhibit a high degree of organization5 and low pinhole density. Films of variable but controlled thicknesses are readily achieved by repeated transfer of monolayers from the airlwater interface to the substrate. In this way, rather complex “organizates” with (1) (a) Murray, R. W. In ElectroanalyticalChemistry;Bard, A. J.,Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191 and references therein. (b) Chidsey, C. E. D.; Murray, R. W. Science (Washington,D.C.) 1986, 231, 25. (2) Gaines, G. I., Jr. Insoluble Monolayers at Liquid-Gas Interfaces;
Wiley-Interscience: New York, 1966. (3) Gaines, G. L., Jr. Thin Solid Films 1980, 68, 1. (4) Kuhn, H.; Mobius, D. Angew. Chem., Znt. Ed. Engl. 1971,10,620. (5) Vincett, P. S.; Roberts, G. G. Thin Solid Films 1980, 68, 135. (6) (a) Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1985, 183, 1. (b) Daifuku, H.; Yoshimura, I.; Hirata, I.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1986, 199, 47. (c) Aoki, K.; Tokuda, K.; Matsuda, H. J . Electroajal. Chem. 1986, 199, 69. (7) (a) Fujihira, M.; Ohnishi, N.; Osa, T. Nature (London) 1977,268, 226. (b) Fujihira, M.; Poosittisak, S. J. Electroanal.Chem. 1986, 199, 481.
0743-7463/86/2402-0732$01.50/0
tailored electrical or electrochemical properties may be built up. Chromophore and electrophore orientation a t solid interfaces may also be achieved by judicious choice of the position of hydrocarbon substitution on the parent system.s Thus modification of electrode surfaces by L-B films is perceived to be advantageous in terms of implementing control over the spatial arrangement of redox molecules a t electrode interfaces. One may envision, for example, ultrathin rectifying contacts not based on semiconductor junctions. Toward this end, we have synthesized the surfactant redox system (ferrocenylmethy1)dimethyloctadecylammonium hexafluorophosphate (I),shown below. In this
paper, we shall examine the surface pressure vs. molecular area (*-A) behavior and monolayer stability of I a t the air/water interface a t various surface pressures. We shall also describe the modification of Au electrodes with single L-B monolayers of I oriented with the electroactive head group down on the electrode. Monolayer transfers to the electrode are performed a t a variety of surface transfer pressures rtin order to ascertain the effect of molecular packing density on film voltammetry. This is in contrast with a previous report6adescribing electrode modification by L-B films of surfactant derivatives of M(bpy),2+ (M = Ru, Os; bpy = 2,2’-bipyridine) in which film compression was regulated by a piston oil that provided only a single T~ 20 mN/m.
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Experimental Section Materials. (Ferrocenylmethy1)dimethyloctadecylammonium Hexafluorophosphate (I). 1-Iodo-n-octadecane (Aldrich was recrystallized from 2/ 1 acetone/ethanol. (Ferrocenylmethyl)dimethylamine, 2.0 g (8.5 mmol), was dissolved in 40 mL of THF. To this was added a slight excess (4.4g, 11.5 mmol) of the alkyl halide and the mixture refluxed under Ar. Reaction progress was qualitatively monitored via alumina thin-layer chromatography (eluting solvent 5050 toluene/acetonitrile) by checking for the disappearance of the spot for the (8) Nakahara, H.; Fukuda, K. Thin Solid Films 1983, 99, 45.
0 1986 American Chemical Society
Characterization of Electroactive L-B Monolayers parent amine. After 5 h, the reaction mixture was allowed to cool, the solvent was removed by a rotary evaporator, and the residue was washed first with water to remove THF and then with heptane to remove excess iodooctadecane. The desired product was isolated as the iodide salt, and was recrystallized 3 times from ethyl acetate and once from water/methanol (50/50), giving brilliant yellow platelets, mp 110-115 "C. Overall yield was 60%. (Ferrocenylmethy1)dimethyloctadecylammoniumhexafluorophosphate (I)was isolated by precipitation from a mixture of water/methanol solutions of the iodide salt and NH,PF, a t 50 "C. UV-visible spectra for I and the parent amine in CH,CN were essentially identical with that of ferrocene with c = 135 and 114, respectively, at A,, = 420 nm. NMR: 0.9, (3 H), 1.3 (32 H), 2.95, 3.05, 3.2 (8 H), 4.15 (7 H), 4.55, 4.9 ppm (4 H). IR spectra were obtained on solvent evaporated films cast onto KBr. IR (cm-'): 3050 (w), 2840, 2910 (s), 1510 (s), 1376 (m), 1230,1090 (sharp), 1000 (m, medium), 800-900 (four sharp bands), 710,735 (sharp d). Monolayer grade water used to prepare subphases was purified by reverse osmosis (Millipore RO-15), deionization (Millipore Super Q),and double distillation from alkaline permanganate and dilute sulfuric acid under a N2 atmosphere. Once purified, water was stored under a positive pressure of nitrogen. Na2S04(Aldrich, Gold Label) and Burdick and Jackson high-purity T H F were used as received. CHC1, and benzene used to make up spreading solutions were freshly distilled prior to use. Stock solutions of I (in 50/50 vol % CHCl,/benzene) used to spread monolayers at the air/water interface were kept tightly sealed in a desiccator and stored in the dark in order to minimize sample photooxidation. Instrumentation. UV-visible spectra were recorded on a Cary 17 spectrophotometer. IR spectra were obtained on a PerkinElmer Model 283 IR spectrometer. NMR spectra were obtained on a Bruker Model WP-80. All monolayer transfers and electrochemical measurements were performed in a Class 100 microelectronics clean room using a Lauda MGW film balance equipped with a floating barrier pressure transducer. Pressurearea isotherms were obtained under H P Model 9816 computer control and H P Model 3497A digital interface. Material handling and preparation were done in an external Class 1000 area. Potential uncertainties in the monolayer film structure a t the air/water interface which accrue from the use of the floating barrier pressure transducer are not addressed in this paper. A PAR Model 175 signal programmer provided the potential excitation to a PAR Model 173 potentiostat with Model 276 current to voltage converter. Current vs. potential curves were recorded on a Hewlett-Packard 7004B X-Y recorder. The same Au foil working electrode (2.5 X 2.5 X 0.05 cm) was used throughout and was mirror-polished on both faces with successively finer grades of diamond paste (9,3, 1, and 0.25 pm). The auxiliary electrode was in all cases a cylindrical Pt gauze which completely surrounded the working electrode. Electrode potentials are all referenced to a sodium chloride saturated calomel electrode (SSCE) of conventional design. The auxiliary, reference and working electrodes were each piaced in the subphase electrolyte (0.1 M Na,SO,) prior to film spreading. Following film spreading and compression to the desired surface pressure, the working electrode (Au) was slowly emersed resulting in monolayer transfer. The film was then expanded and the surface pressure was then brought to zero by opening the moving barrier. The working electrode was then slowly reimmersed in the subphase to obtain cyclic voltammetric data. This procedure resulted in the transfer of a single monolayer "heads down" on the electrode. Expanding the film to a negligible surface pressure avoids transfer of a second layer during reimmersion. In addition, as will be shown below, film deposition during immersion does not occur even at high surface pressures.
Results and Discussion Surfactant Behavior at the Air/ Water Interface. Monolayer films of I were spread at t h e air/water interface o n a film balance from 50/50 vol % ' chloroform/benzene solutions. Figure 1 shows a surface pressure vs. molecular area (a-A) isotherm for I at 19 " C o n 0.1 M Na2S04.The arrows indicate t h e compression and expansion cycles of the isotherm. The film is i n the liquid-expanded state at
Langmuir, Vol. 2, No. 6 , 1986 733
I 0k\-'+-
L 20
-
d
i
60 80 MOLECULAR AREA ( A MOLECULE] 40
1 '00
Figure 1. Compression-expansion TT-Aisotherms of I at the air/O.l M Na2S04interface. Significantly greater hysteresis in the *-A curve is observed when film collapse is allowed to occur (-) than when it is avoided Ahm,the molecular limiting area a t this interface, is given by the extrapolation (- - -) of the K-A curve to K = 0. (-e).
low pressure and liquid-condensed sstate at high pressure. Film collapse occurs under moving barrier conditions at a > 42 mN/m and molecular area A < 24-27 A2/rnolecule. T h e dashed line in Figure 1extrapolates the linear segment of t h e a-A c w v e in the high a region t o zero pressure. T h e molecular area indicated b y this extrapolation, t h e limiting molecular area (All,,,), is suggestive of the cross-sectional head group area.2 Limitations of molecular areas calculated from extrapolations to a = 0 m a y be f o u n d elsewhere.2 As indicated in the figure, All* = 51 A2/molecule on 0.1 M Na2S04. Molecular models indicate a molecular area of 49 A2. T h e strong inflection i n t h e expansion segment in Figure 1 suggests a spontaneous respreading of the collapsed film. When the film is compressed to a 42 m N / m , thereby avoiding t h e point of complete collapse, a vestigial hysteresis i n t h e a-A curve is observed as shown b y the d o t t e d curve i n Figure 1. T h i s implies that the kinetics of t h e molecular processes associated with monolayer compression and expansion a r e slow relative to t h e areal compression rates employed. This is supported b y noting t h e effect of periodically arresting t h e barrier movement for a period of 1 m i n during film compression and expansion cycles. T h e a-A isotherm of I o n 0.1 M Na2S04in Figure 2 shows that a decreases somewhat when film compression is halted but increases when expansion is halted. The decrease is due to the slow reorganization of molecules to a lower surface area which m i g h t be expected for expanded films or m a y result from a relatively slow collapse process. T h e increase is d u e to slow molecular expansion t o accommodate a larger surface area or from relatively slow film reconstruction f r o m a slightly collapsed state. Further indication of the tendency of these films to form self-assembling monolayers is gained from a determination of the equilibrium spreading pressure (ESP) o n water. When a small crystal of c o m p o u n d I is placed on a p u r e water subphase, a monolayer film spontanesouly forms on the water surface and an increase i n the surface pressure is observed. Figure 3 shows t h e a vs. time results. Within a short period of time, an ESP of ca. 18 m N / m develops, corresponding t o A = 40-50 A2/molecule [(4.2-3.3) X mol/cm2], slightly less than Allmmeasured for I on p u r e water. T h u s , I spontaneously forms an organized, closepacked monolayer at the a i r / w a t e r interface. T h e ionic strength of t h e subphase significantly affects the limiting molecular area of I at the air/water interface.
Facci e t al.
734 Langmuir, Vol. 2, No. 6, 1986 [
T
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T
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m
I
i I 4
I
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-3
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I
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1 ,
,
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I
I
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I
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Figure 2. Effect of periodic arrest on 7r during compression and expansion on the 7r-A isotherm of compound I. Surface pressure decreases when compression is halted and increases when expansion is halted.
--I 1
I
I
1
0
20
40
60
I
1
100 120
TIME (MINI
Figure 3. 7r vs. time behavior for a crystal of I placed on a pure water subphase, indicating an equilibrium spreading pressure (ESP) of ca. 18 mN/m.
A plot of Allmvs. Na2S04concentration, shown in Figure 4, passes through a minimum near 0.1 M Na2S04. The initial decrease in Ah as the ionic strength of the subphase increases reflects a diminution in charge repulsion between positively charged head groups, fixed at the interface, as a result of the charge screening by SO:- counterions. This allows molecular packing to increase. The rise in limiting area at high Na2S0, concentration is likely due to the inclusion of counterions among the head groups in the interfacial region. Na2S04 concentration of 0.1 M was selected as the subphase for the in situ electrochemical study of L-B film coated electrodes, since this electrolyte concentration both minimizes intersite charge repulsion and is sufficiently high to maintain electrolytic conductivity required for voltammetric studies. Monolayer Transfer to Au Electrodes. Au electrode surfaces described in the Experimental Section were prepared for transfer by hand polishing with 0.25-km diamond paste and copious rinsing with monolayer grade water.
Figure 4. Effect of the ionic strength of the subphase on the limiting molecular area of I at the air/water interface. The curve passes through a minimum of ca. 50 A2/molecule (I’+ = 3.3 X mol/cm*) at 0.03 M Na2S04. Electrochemicalexperiments were conducted near this minimum at 0.1 M Na2S04.
The electrode was then briefly immersed in 100 “C concentrated H2S04/HN03to remove residual organic surface impurities and finally in ethanolic KOH. After a final rinse with monolayer grade water, the electrode was immediately immersed in the trough subphase (reference and similarly cleaned auxiliary electrodes were previously immersed). A t this point the Au surface is quite hydrophilic as evidenced by a meniscus contact angle of ca. 0-10”. Following the spreading of a film on the subphase surface (initial area A = 80-120 A2/molecule), the barrier was closed at an areal compression rate of 8.0-5.3 (A2/molecule) min-’ depending on the number of molecules initially spread on the water surface. Because of the hysteresis and slow molecular reorganization processes shown in Figures 1 and 2, monolayers on the water surface were held at constant pressure for 3-5 min to determine the rate of film loss (e.g., collapse or dissolution) from the air/water interface. Transfer of the monolayer was accomplished by withdrawing the substrate through the L-B film coated subphase at 1.0 mm/min at constant pressure. Constant x was achieved via an electronic feedback mechanism which adjusted the rate of barrier movement to match the rate of film removal from the water surface. A t the air/ water interface the polar cationic head group of I [(ferrocenylmethyl)dimethylammonium]resides in the aqueous phase while the hydrocarbon “tail” resides out of the aqueous phase. Interaction of the hydrophilic head group with the hydrophilic Au surface results in the “heads down” orientation of the monolayer film on Au following transfer. As expected, all single monolayer-coated electrodes emerged dry from the subphase and were hydrophobic. The x-A curves for transfer of L-B films of I from 0.1 M Na2S04at five different surface pressures are shown in Figure 5 (curves A-E). Overlaid for comparison is curve F, the a-A curve from Figure 1. The pressure plateaus indicate that films were transferred at xt = 2.7, 7.2, 12.1, 17.4, and 26.9 f 0.3 mN/m (curves A-E. respectively). The right- and left-hand tick marks denote the beginning and end of film transfer, and the molecular areas associated with them are denoted by Ainit and Afin,respectively. Values of Ainitare summarized in Table I. The constant x region to the right of Ainitin curves A-E shows the decreasing film area a t constant pressure due to a slow film loss or reorganization mechanism. This region of the x-A curve was replotted as molecular area vs. time. The decrease in film area prior to transfer was found to be linear and the film loss rate [ (A2/molecule) s-l] due to slow reorganization or collapse is summarized in Table 1.1 AIoss is the area of the film on the air/water interface not transferred to Au over the time required to transfer due to the competing loss or reorganization mechanism. The
Langmuir, Vol. 2, No. 6, 1986 735
Characterization of Electroactive L-B Monolayers
II.’/
100-
I
I
IOp A /cm2
A 60 80 too 120 (8*/molecule) Figure 5. Curves A-E: *-A isotherms of the compression and transfer at constant A of films of I to hydrophilic Au electrodes. = 2.7, 7.2, 12.1, 17.4, and 26.9 i 0.3 Shown are transfers at rtTt mN/m, respectively. Overlaid is curve F, the *-A isotherm of Figure 1. Right and left tick marks denote the beginning and end of film transfer.
0
20
A
SWEEP RATE lmV/s)
40
AREA
Table I. Summary of Transfer Parameters for the Transfer of Langmuir-Blodgett Films of I from H 2 0 to Au Surfaces transfer logy, =tt film loss rate; &it, A2/moleculea mN/mb (A2/molecule) s-* ratiod mole 63.5 2.7 f 0.3 0.0015 1.04 2.72 54.2 7.2 k 0.3 0.0019 1.01 3.09 46.0 12.1 f 0.3 0.0038 1.01 3.65 38.6 17.4 0.0083 0.92 3.97 37.4 22.0 0.0054 0.73 3.27 32.4 26.9 0.0056 0.96 4.91 “Molecular area of I at the air/water interface at the beginning of film transfer. Surface pressure maintained throughout transfer. Linear least-squares slope of molecular area vs. time plot at constant P = rt made just prior to transfer. dRatio of the area swept by moving barrier (corrected for film loss) to the projected area of substrate. eNumber of moles transferred from the air/ water interface to Au (see eq 1).
number of moles of I transferred to the Au electrode from the airfwater interface is calculated from eq 1,where Y
Y = K(Ainit - Afin - AIoss)/WAinit)
(1)
is the number of moles transferred, K is the number of molecules spread, and N is Avogadro’s number. Values of Y a t all T
