3332
Langmuir 1994,10, 3332-3337
Chemically Modified Electrodes by Nucleophilic Substitution of Chlorosilylated Platinum Oxide Surfaces Chun-hsien Chen, James E. Hutchison, Timothy A. Postlethwaite, John N. Richardson, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received February 25, 1994@
Chlorosilylated platinum oxide electrode surfaces can be generated by reaction of Sic14 vapor with an electrochemicallyprepared monolayer of platinum oxide. A variety of nucleophilic agents (suchas alcohols, amines, thiols, and Grignard reagents) can be used to displace chloride and thereby functionalize the metal surface. Electroactivesurfacesprepared with ferrocenemethanol as the nucleophileshow that derivatization by small molecules can achieve coverages on the order of a full monolayer. Surfaces modified with longchain alkyl groups efficientlyblock electrode reactions of redox probes dissolvedin the contactingsolution, but other electrochemical(doublelayer capacitance and surface coverage)and contact angle measurements suggest that these molecule films are not highly ordered, self-assembled monolayers. It has now been established that electrode surfaces possessing unique and desired properties can be fabricated by chemical or chemi~orption~-~ of appropriately functionalized molecules on a suitable electrode substrate. These chemically modified electrodes have provided controllable molecular surfaces for a wide variety of electrochemical and other surface chemical studies. The recent advance^^,^ in alkanethiol self-assembled monolayers (SAMs)on Au surfaces have provided an entry into preparation of surface structures that are not only chemically controllable but also exhibit a high degree of structural order. This paper describes a straightforward, but important, extension of the chemistry available for modification of platinum electrodes, by means of a synthetic strategy similar to that briefly described for tin oxide electrodes by Fox et ale7 The procedure involves two conceptually simple steps, Scheme 1: that lead to chemically modified Pt electrodes2 starting from their monolayer-oxidized, conductive surfaces. (Rcan be an n-alkyl chain ranging from n = 4 to 18 or an electroactive moiety such as a ferrocene derivative.) The electrochemicallyformed platinum oxide surface is chlorosilylated by reaction with tetrachlorosilane vapor (reaction 1). The resulting C1terminated surface reacts with suitable nucleophilic molecules, e.g., alcohols, thiols, amines, and carbanions (reaction 21, to form the derivatized surface. Although a particular functionalized surface can in principle be built by synthesizing a particular reagent and then attaching it to the surface, cross reactions Abstract published inAdvance ACSAbstructs, August 15,1994. (1)Murray, R.InElectroanalytical Chemistry; Bard, A. J.,Ed.; Marcel Dekker: New York, 1984;Vol. 13,pp 191-368,and references therein. (2) Molecular Design of Electrode Surfaces; Murray, R., Ed.; Wiley: New York, 1992. (3)Hubbard, A. T.Chem. Rev. 1988,88, 633. (4)Chidsey, C.E. D. Science 1991,251,919-922. ( 5 ) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991,252,688-691. (6) Finklea, H. 0.; Hanshew, D. D. J. Am. Chem. SOC.1992,114, 3173-3181. (7)Fox, M.A.; Nobs, F. J.;Voynick, T. A. J.Am. Chem. SOC.1980, 102,4029-4036. (8) The other two bonds of the Si atom are not shown in the reaction schemes because several possibilities exist between which we cannot distinguish. The Si centers may be (i) bound in a siloxane network, (ii) attached to surface oxide, or (iii) retained as Si-Cl bonds in reaction 1that can lead to multiple functional group attachment per Si atom in reaction 2. @
Scheme 1 I
+ SiC14(g)-Pt-O-Si-CI
Pt-0-H
Pt-0-k-C1 I
+ HOR--+Pt-0-Si-OR HSR HNRR’ MR (M = MgCI)
I
(1)
1
I
(2)
-”
-SR -R
between the reagent’s head (anchoring) group and its functionalized tail group can interfere with this approach. Chloro- and alkoxysilanes are, for example, common anchoring groups for oxide surfaces (including Pt oxide) because a robust film can be assembled through crosslinked siloxane networks. 1,9 These silyl functionalities are, however, often incompatible with tail groups of interest. Even those reagents which can be synthesized with noninteracting silyl and tail group functionalities can be difficult to isolate as pure materials due to the sensitivity of the silyl group to hydrolysis and/or polymerization by trace amounts of water.1° To avoid such complications, electrodes modified with siloxane networks have been assembled in a stepwise manner, using first chlorosilanes or alkoxylsilanes bearing compatible but reactive tail groups, which are then further functionalized in a n additional reaction step to form the target molecular The first and most common chemically modified electrodes, amine-functionalized and other nucleophilic surfacesl1J2 assembled by solution phase1 or vapor phase13J4 reactions of alkylaminosilanes with surface oxides and then reacted with suitable, functionalized electrophiles,15 were made in this manner. Surfaces terminated with acid chlorides, reactive to nucleophilic substitution, can be prepared by reactions of thionyl chloride (SOC12) or pentachlorophosphate (PC15) with surface carboxylic acids. This method can be used (9)Ulman, A. An Introduction to Ultrathin Organic Films; Academic: San Diego, CA, 1991. (10)Fischer, A. B.;Kinney, J. B.; Staley, R. H.; Wrighton, M. S. J. Am. Chem. SOC.1979,101,6501-6506. (11)Xu,C.; Sun,L.;Kepley, L. J.; Crooks, R. M.; Ricco, A. J.Anal. Chem. 1993,65,2102-2107. (12)Kurth, D. G.;Bein, T. Langmuir 1993,9,2965-2973. (13)Haller, I. 3.Am. Chem. SOC.1978,100, 8050-8055. (14)Kurth, D. G.;Bein, T. Angew. Chem., Int. Ed. Engl. 1992,31, 336-338. (15)See, for example: Balachander, N.; Sukenik, C. N. Langmuir 1990,6,1621-1627,and references therein.
Q743-7463/94/241Q-3332$04.50/00 1994 American Chemical Society
Chemically Modified Electrodes on oxidized carbon surfaces (such as graphite electrodes16 or polyethylene films") and those obtained by selfassembly of acid-terminated alkanethiols on gold.le I n a study of SnOzelectrode modification by arenes, Fox et al.' prepared chlorosilylated surfaces by exposing the SnOz surface to the vapor of refluxing tetrachlorosilane in a procedure analogous to reaction 1. The reactivity of the SnOz surface was explored only for reactions with lithiated arenes. More recently, Cl3SiCo(CO)4was synthesized and utilized as a n electrophilic linking agent on indium-tinoxide (ITO)surfaces.lg In the current work, chlorosilylated platinum oxide surfaces prepared by the straightforward reaction 1are shown to exhibit a range of reactivity similar to that of the cobalt carbonyl modified IT0 surface^.'^ Among the reactions investigated in reaction 2 were those in which R is a long chain alkyl group, in order to explore whether such reactions might product selfassembled ordered layers analogous to those k n o w n for alkanethiols chemisorbed on Au surface^.^^^ We describe contact angle and electrochemical measurements (double layer capacitances (Cad and blocking effect experiments) on these alkyl-functionalized surfaces. These measurements are standard approaches to characterize alkanethiols on Au substrate^^^^^ and other self-assembled monolayer systems. C , values for alkanethiol films chemisorbed on Au have been calculated based on Helmholtz theory and confirm, in those cases, the expected film thicknesses and dielectric constants.20 The blocking effect of the alkyl molecular film is described as the ratio of the redox current of a solution probe (blocked by the organic layer) to that current obtained a t a n unmodified naked Pt electrode. The blocking effect probes the degree to which defects exist in the electrode-supported molecular film and to which it exhibits permeability to contacting solutes. €'ti PtO electrodes offer a relatively wide potential window, and because charge integration of hydrogen adsorption waves afford values for the microscopic Pt electrode area and thereby monolayer oxide coverages,"' yields of Scheme 1 can be inferred from experiments in which R is electroactive, e.g., ferrocene.
Experimental Section Materials. Toluene, THF,acetonitrile, and octadecyltrichlorosilane (OTS) were purified and dried by distillation. Bicyclohexyl and chloroform were passed through alumina columns for waa purification.22 Tris(2,2'-bipyridine)cobalt(II), [Co(bpy)3I2+, prepared by mixing an aqueous CoClz solution with a 4-fold molar excess of the ligand.23 Tetrafluorotetracyanoquinodimethanide salt, TCNQF4-, was prepared by metathesis of tetrabutylammonium iodide ( B d I ) with neutral TCNQF4, which was obtained as crystals according to Wheland et al.24 All other reagents and electrolytes were ACS reagent grade or better and used as received. In-house distilled water was further purified by passage through a Barnstead Nanopure system ( > 18MSZ-cm). Electrode Preparation. Platinum films (ca. 150 nm thick) were sputtered (rfmagnetron,20 mTorr Ar, Supersystem I, Kurt J. Lesker Co.) onto piranha-etched oxidized silicon wafers. (Piranha solution is a 20% (v/v) mixture of 30% aqueous HzO2 and concentrated H2SO4. Warning: This solution can react (16)Watkins, B. F.; Behling, J. R.; Kariv, E.; Miller, L. L. J . Am. Chem. SOC.1976,97,3549-3550, and references therein. (17)Wilson, M. D.;Ferguson, G. S.; Whitesides, G. M. J.Am. Chem. SOC.1990,112,1244-1245. (18)Duevel, R. V.; Corn, R. M. Anal. Chem. 1982,64,337-342. (19)Chen, K.;Herr, B. R.; Singewald, E. T.; Mirkin, C. A. Langmuir 1992,8,2585-2587. (20)Porter, M. D.;Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987,109,3559-3568. (21)Hubbard, A. T.ACC.Chem. Res. 1980,13,177-184. (22)Moaz, R.;Sagiv, J. J . ColloidZnterfaceSci. 1984,100,465-496. (23)Chen, Y.-W.; Santhanam, K. S. V.; Bard, A. J. J . Electrochem. SOC.1982,129,61-66. (24)Wheland, R.C.; Martin, E. L. J . Org. Chem. 1975,40,3101.
Langmuir, Vol. 10,No. 9,1994 3333 violently with organic materials.) Adhesion of the platinum film was enhanced with a 250-A precoating of sputtered chromium. Platinum-coated wafers were cut into 2 x 1cm pieces to which contacts were cemented with silver epoxy. The contacts were covered and the working electrode's exposed surface area was defined with a layer of insulating epoxy. The Pt electrodes were cleaned by repeated potential cycling between -0.23 and +1.20 V us SCE in 0.5 M HzS04 until voltammograms characteristic of a clean Pt surface26 were obtained. The microscopic electrode surface areas were calculated by integration of the hydrogen adsorption waves.21 The roughness factor of the Pt electrode was 1.9; all area-dependent measurements are reported based on the microscopic area. The platinum oxide layer was generated by applying a potential of 1.20 V us SCE in 0.5 M H2S04 until the oxidation current decayed to zero, whereupon the electrode was removed from the solution while still under potential control (1.20 V), rinsed copiously with water, and dried at ca. 50 "C for at least 30 min. Electrode Modification and Characterization. The silanization and further modification (Scheme 1)reactions were conducted in a glovebox. Chlorosilylated surfaces were prepared by exposingthe electrochemicallygenerated Pt oxide surfaces to Sic14 vapor in a vial at ambient temperature for at least 30 min. After excess and physisorbed SiCL was removed in vacuo, the silanized surfaces were exposedto vapors of, or soaked in solutions (5% by weight in toluene, or THF)ofvarious nucleophilicreactants for from 2 h to 2 days. The modified electrodes were washed thoroughly with dry toluene or acetonitrile in the glovebox and removed from the glovebox to a fume hood where they were subsequently rinsed with methylene chloride and, for films prepared using Grignard reagents, rinsed with water. Pt oxide surfaces were reacted with octadecyltrichlorosilane solutions (WOTS films) in a fume hood under N2. The Pt oxide surfaces were soaked for 5 min in 5%(w/w)OTS solutions in 80% bicyclohexyV20%chloroform.22 The relative humidity in the hood was not under special control. Auger electron spectroscopy (AES)was performed on a PerkinElmer 595 spectrometer. The e-beam (5 kV) incident angle was 75" from the surface normal to enhance sampling of surface layers relative to bulk. Advancing and static contact angles of water on the modified surfaces were measured using a RamB-Hart Model 100 goniometer. Electrochemistry. Electrochemical measurements were made using an in-house designed potentiostat and waveform generator in a standard three-compartment cell. Potentials for aqueous electrochemistry are referenced to an SCE and measurements made in acetonitrile are referenced to a silver quasireference electrode (AgQRE). Platinum counter electrodes were used in all cases. All solutions were thoroughly sparged with nitrogen gas.
Results Pt Oxide Chlorosilylation. The coverages of active chlorosilyl surface groups attained in reaction 1,and thus of subsequently functional groups attached in reaction 2, depend on the amount of surface oxide initially present on the Pt surface prior to reaction 1. The quantity of Pt oxide was monitored through the different surface preparation steps using the integrated charge under the voltammetric peak for reducing the Pt oxide layer in 0.5 M H2S04solution. This reduction peak was examined by connecting the electrode to the potentiostat a t 0.9 V, placing it in the solution, and immediately sweeping the applied potential to more negative values. We observed that the charge measured for the Pt oxide reduction wave (435 pC/cm2) did not change from the value observed immediately following the electrode oxidation, after procedures in which the Pt oxide surface was rinsed, dried, or transferred into a glovebox. These results show that it is possible to consistently prepare and maintain a Pt oxide layer through a series of preparation steps. The only effect observed was that the F% oxide reduction peak (25)Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A. J. Electroanat. Chem. 1973,43,9-36.
3334 Langmuir, Vol. 10,No. 9, 1994
12
-6O ] , -12
Chen et al. et al.,27exhibit similar broadness when the redox sites have not been diluted (e.g., by other alkanethiols). Broadened surface voltammograms are believed to arise from electroactive neighbor-neighbor interactions and surface-structural inh0mogeneity.l The inset of Figure 1A shows that the voltammetric peak current is proportional to the potential scan rate,z6 consistent with the expectation that the ferrocene molecules reacting in Figure 1A are surface confined. The charges under the oxidative ferrocene voltammetric peaks in Figure 1A are scan rate independent (from 20 to 500 mV/s) and amount to 74 f 6 ,uC/cm2(7.6 x mol/ cm?. This charge density is equivalent to 92% derivatization of a full surface siloxane monolayerz8~z9 and to 1.7 monolayers of hexagonally close-packed ferrocenee30The coverage measurements suggest that a full monolayer of chlorosilane is bound onto the oxide surface and every Si unit is substituted by a hydroxymethylferrocene. A ferrocene terminated thiol with a longer alkyl spacer (FcC~HIGSH) was also reacted with chlorosilylated Pt electrodes under gas-phase and solution-phase conditions. The voltammograms (Figure 1B) are characteristic of surface-confined ferrocene groups. The coverages are variable from film to film however, ranging from 28% to 48% ((7.4 to 12.8) x 1013molecules/cm2)of a full monolayer of close-packed ferrocene. Surface Modifications with Aliphatic Reagents. Another aim of this work, beyond demonstrating new surface attachment chemistry on conductive oxide electrodes, was to explore whether the chlorosilylated surface formed by reaction 1was a useful platform for preparing highly ordered monolayers analogous to the self-assembled films resulting from chemisorption of alkanethiols onto Au(ll1) surfaces. To that end we inspected the reactivity of chlorosilylated Pt oxide surfaces with a variety of aliphatic alcohols, thiols, carbanions, and amines. Table 1gives a partial listing ofthe nucleophilic reagents tested, the expected modified electrode structure, and physical properties measured for each electrode. Also listed for comparison in Table 1are the properties of unmodified Pt surfaces and of OTS-coated Pt (PUOTS). The Pt/OTS films were prepared similarly to literature procedureszz in that special care was not taken to keep the reaction environment rigorously dry. A good PUOTS film is not in fact obtained when the substrate and deposition solution are rigorously dry, an observation reported by other groups.12,31-33Under ambient conditions, some WOTS films appeared cloudy and were discarded. The cloudiness signals OTS hydrolysis and polymerization in the deposition solution. The solutions generally became opaque after 2 h. Scheme 1offers the
;4, 1 0.8
0.4
0.0
-0.4
E (V vs. AgQRE) Figure 1. (A) Cyclic voltammograms of FcCHzOH-modified surface in dry CH&N containing 0.1 M B a P F 6 , at potential scan rates of 50,100, and 200 mV/s. The inset shows that the peak current density varies linearly as a function of scan rate. (B) Cyclicvoltammogram ofPt-I-o-si-s-c&&'c prepared by thermal evaporation of HSCaH16Fc and gas phase reaction with the chlorosilylated Pt electrode, at 100 mV/s. The calculated coverage is 40%.
shifted negatively ea. 50 mV following drying of the oxide layer (without a change in reduction charge). The reason for the potential shift is unclear. Oxidized electrodes were also examined by ex situ AES for all procedural steps prior to chlorosilylation. The only elements observed a t any point prior to chlorosilylation were Pt, 0, and adventitious carbon, which are expected. Peaks for Si or Si02 (expected a t 92 and 1619 or 76 and 1606 eV) were not observed. Following reaction of the Pt oxide surface with Sic14 vapor, AES showed the presence of Pt, Si, 0, and C. No halides (including C1, expected a t around 182 eV) or sulfur were seen, from which we infer that the room exposure of the chlorosilylated surface during its transfer from the glovebox to the spectrometer resulted in its complete hydrolysis (to SiOH) by atmospheric moisture. (This hydrolysis was minimized in our preparative work by handling substrates treated with SiC14exclusively in the glovebox.) Calculations based on peak-to-peak ratios estimate that atom percentages of Pt, Si, 0, and C near the surface were 13, 15, 63, and 9, respectively. The Pt/Si ratio suggests a full monolayer of Si is attained, and the SUO ratio is consistent with a Si04 surface stoichiometry. Surface Modifications with Ferrocene-Containingaeagents. Various nucleophiles used t o functionalize the chlorosilylated surface under gas-phase and/or liquidphase reaction conditions are listed in reaction 2. Gas phase reactions of hydroxymethylferrocene with chlorosilylated surfaces are expected to result in formation of a Pt-0-Si-0-CHz-Fc surface linkage (-Fc = -C5H4F~CF,HS). Figure 1A shows that thusly attached ferrocene can be oxidized in well-defined cyclicvoltammetric waves. The voltammograms, taken at several potential scan rates and without iR,,, compensation, are fairly reversible (AEpeak 40 mV>but are very broad (Ekhm= ea. 250 mV at 100 mV/s us ideally expectedz6(90 mV). Voltammoreported for surface-confined species, such as monolayers of ferrocene ester-tagged alkanethiol (i.e., FcC02C11H2zSH)chemisorbed on Au electrodes by Chidsey
(26) For a reversible,ideally behaved surface confined species, i, = (nzF/4RT)vATo*andEkhm= 90.6/n mV (25 "C). Bard, A. J.;Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 522. (27) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . A m . Chem. SOC.1990,112,4301-4306. (28)The surface X-ray scattering measurementsz9suggest that the average molecular area of an octahedralsiloxane (C18H&i03-) monoSi atoms layers is 20.2 f 0.2 Az.The surface thus contains 5.0 x per cm2. Ifonly one ferrocene binds to one Si, the charge density becomes 80 pC/cm2. (29) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosowsky, S. D.; Folkers, J. P.; Whitesides, G. M. J . Phys. Chem. 1991, 95, 28542861; J . Phys. Chem. 1993,98, 1754. (30) The diameter of the ferrocene group estimated by Chidsey et aLZ7is 6.6A. Therefore,a hexagonal close-packedmonolayerofferrocene has the a unit cell of 37.72 &/molecule and thus contains 2.65 x l O I 4 moleculeshmz or has the charge density of 42.5 pC/cm2. (31) LeGrange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993,9, 1749-1753. (32) Finklea, H. 0.; Robinson,L. R.; Blackbum,A,;Richter,B.; Allara, D.; Bright, T. Langmuir 1986,2, 239-244. (33) Wasserman, S. R.; Tao, Y.-T.;Whitesides, G. M. Langmuir 1989, 5, 1074-1087.
Langmuir, Vol. 10, No. 9, 1994 3335
Chemically Modified Electrodes
Table 1. Advancing Contact Angles, Double Layer Capacitances, and Blocking Effects of Representative Films nucleophile C4HsOWvapor C&3sSH/tO~Uene CisH37MgCnHF (ClsH37)2NWtoluene
modified electrode Pt-I-O-Si-O-C&I9 F't-I-o-si-s-c16H33 F't-I-O-Si-C18H37 Pt-I-O-Si-N-(C18H37)2
C ~ ~ & ~ S i C l ~ i c y c l o h e xWOTS' yl unmodified F't fdm
O(H~O) (deg) 102 f 2 90 f 5
Cdla
CuFlcm2)
92f2 95 f 5
27 f 5 22f2 25 f 5
112f2 wet
24f5 87 k 10
% blocking effectb,c (1- (&osted/iuncoated))
79.1-83.4 85.0-92.2 92.0-98.5 88.0-97.0 92.7-99.2 95.0-99.2 -100
solution 1 mM [Ru(NH&]3+ in 1M KC1 1 mM [Ru(NH3)6I3+in 1 M KCl 1 mM [Ru(NH3)6I3+in 1 M KCl 1 mM [Ru(NH3)6I3+in 1 M KCl 1 mM Fc in CH3CNd 1 mM TCNQF4- in CH3CNd 1 mM [C0(bpy)3]~+ in CH3CNd
75.5-84.2
1 mM [Ru(NH&I3+ in 1 M KC1
a Obtained at 10 and 100 mV/s scan rates in solutions of 1 M KCl and of 0.1 M Bu4"Fs in acetonitrile. Peak currents measured at 100 mV/s. The silane films, except those prepared by Grignard reagents, were hydrolyzed easily. Therefore, only the first few scans were taken for the electrochemicalmeasurements. In solutions of dry acetonitrile, the voltammograms were stable at least 1 h. The supporting electrolyte was 0.1 M BwNPFs. e WOTS films were prepared in a solution of bicyclohexyl-chloroform mixture solution.
300
200
200 100
-E
ksi
100
0
0 -100
-100
-200
-200
-300 1.o
0.5
0.0 1.0
0.5
0.0 -0.5
1.0
0.0
-1.0
E (V vs. AgQRE) Figure 2. Cyclic voltammetryat 100mV/s of bare and Pt-I-O-Si-N(C18H37)2 (dashed,vertical expansion by the number indicated) electrodes in nonaqueous solutions containing 0.1 M Bu4NPFe and 1 mM (A)Fc, (B) TCNQFI-, and (C) [C0(bpy)3]~+in CH3CN.
advantage of avoiding polymerization concerns in the preparation process. The characteristics of the alkyl-modified Pt surfaces were compared with those of previously prepared organic thin films on metal surfaces. Contact angles of water on 1-butanol-substituted chlorosilylated surfaces (prepared with reaction 2) are 102 f 2", a value in good agreement with measurement^^^ of n-butylsiloxane films on Si/SiO2. In that the alkyl length was varied and monolayers formed on Si/SiO2 substrates by long-chain alkyltrichlorosilanes exhibited optimum advancing contact angles of 112". We observed that films prepared with Scheme 1 where R was an alkyl group (such as C ~ G H ~ ~C18H37SH, MgCl, and (C18H3,)2NH) exhibit significantly higher wettability. The measured contact angles ofca. 90" show that the films obtained are not highly hydrophobic and by inference that the alkyl chains are not densely packed and the terminal CH3- groups not well-ordered a t the film-water interface.34 The double layer capacitance of a derivatized surface is another measure of the degree of hydrophobic film order.20 We observe that electrodes modified by various long chain alkyl groups all have Cas of about 25 pF/cm2 (Table l), as measured from the voltammetric charging envelope between -0.4 and +0.6 V in 1M KC1 solution. This Cdl value is only 25-35% ofthat observed on a naked Pt electrode surface (87 pF/cm2) and is substantially greater than the 1 pF/cm2 measured for well-ordered n-octadecanethiol SAMs on Au electrodes.20 Surprisingly Pt/OTS films exhibited a contact angle (112", Table 1) identified to the optimum value observed on monolayers formed on Si/SiOz substrates by long-chain alkyltrichlorosilanes (vide infra), yet also displayed intermediate Ca values ca. 24pF/cm2(Table 1). These large &values, for Pt/OTS electrodes and those prepared by Scheme 1using long chain alkyl groups, suggest that these monolayers
have a significant permeability for the electrolyte ions (K+and/or C1-) through the organic layer to its interface with the metal surface. Although the Cd values measured in aqueous KC1 solutions are large, the films prepared by Scheme 1are typically fairly effective a t blocking permeation from aqueous media by electroactive probes such as those in Table 1. In a 1mM solution of [Ru(NH&I3+in 1M KCl(,,,, the redox currents of Scheme 1 modified and WOTS electrodes are roughly 15%of the current controlled solely by diffusion a t a naked Pt electrode (i.e.,the blocking effect is higher than 85%). The observed scatter in the 85% current attenuation is ca. lo%, while that seen for octadecyltrichlorosilane films on Pt electrodes in this study and previously on Au electrodes32ranged from 12 to 97%. Table 1shows that the blocking of currents for several electroactive probes studied in dry CH3CN solutions at Pt-O-Si-N(Cl8H3,)2 eIectrodes is even more effective than that seen in aqueous media. The results indicate (Table 1)that the blocking is more effective for the more bulky5redox species although this is complicated by their differing charges. The dramatic attenuation of voltammetric currents a t naked us modified electrodes for the probes ferrocene, TCNQF4-, and [Co(bpy)3I2+in dry CH3CN solutions is shown in Figure 2 (parts A, B, and C, respectively). No measurable faradaic currents could be observed a t the potentials for the [C0(bpy)31~+'~+ and [Co(bpy)3I2+/+ reactions. The current-potential profiles for ferrocene and TCNQF4- a t the modified electrodes are notably plateau-like instead of the peak-shaped form (34) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J.Phys. Chem. 1992,96,5097-5105. (35)(a)Accordingtocrystalstructures andgeneralbondlen the calculated sizes of redox probes are 34,150,and 510 for Fc, LiTCNQ, and Os(bipy)&ln respectively. (b) Pressprich, K. A. Ph.D. Thesis, University of North Carolina, 1989;p 9.
P
3336 Langmuir, Vol. 10, No. 9, 1994 characteristic of linear diffusion; the plateau waveshape is normally associated36with radial or mixed linear-radial diffusion. The possible significance of the waveshape is discussed later. Except for those prepared by addition of carbanions, the organic monolayers produced by reaction 2 are expected to exhibit some hydrolytic in~tability.~'In dry acetonitrile solutions purged with dry nitrogen gas, voltammograms such those in Figures 1and 2 are stable for a t least 1h so the Pt-0-Si-X-C (X = 0,S, N) linkage is electrochemically stable. When the nucleophile is a Grignard reagent, the film appears resistant to hydrolysis in aqueous KC1 because observed blocking effects are stable for at least 1 h. The Pt/OTS films show similar hydrolytic stability in aqueous medium. In aqueous solutions, the other alkyl monolayers on the other hand display rapid decreases in blocking effects even over the first few voltammetric potential scans. The blocking effect observed on the second cyclical potential scan is, for example, typically 3-5% less than that of the first scan. In general, the blocking effect decreases from the initial 85% to ea. 60-65% over the first 20 min and thereafter to values of 45 to 60% where it becomes more or less constant for periods of 2 h and more. These changes in blocking effect are induced to similar extents and over similar periods of time simply by contact of the alkyl monolayer with the aqueous solution, without potential control. These results indicate that the electrode coatings are indeed hydrolytically unstable toward water contact, resulting in a loss of material from the coating. To minimize the effect of this instability, the Cd and blocking effect values listed in Table 1for aqueous solutions were obtained from the first voltammetric scan, which was conducted immediately after placing the electrode in the cell solution. Even with this precaution, some initial hydrolysis may have taken place, with the result of decreasing the observed blocking effects and increasing the observed double layer capacitance values. This may explain why the blocking effects measured for films prepared using a Grignard reagent are substantially higher than films prepared with other nucleophiles in reaction 2, and why the blocking effects measured for Pt-O-Si-N(Cl~H3,)2 modified electrodes (uidesupra) are larger in dry acetonitrile than in aqueous solutions.
Discussion The chlorosilylation of Pt oxide successfully provides a new type of reactive surface for chemically modified electrodes based on Pt. The synthetic steps to prepare chlorosilylated Pt oxide surfaces are straightforward, and given attention to moisture sensitivity, the subsequent reaction 2 works for a wide range of readily available nucleophiles. The gas phase attachment of small electroactive molecules such as FcCHzOH produced coverages on the order of a full monolayer, for the molecule with a larger alkyl chain, F c C ~ H ~ ~ this S H ,method gave a lower coverage. The limitations of this method are that the adlayers (except those with Si-C bonds) are hydrolytically unstable and that preparation of well-ordered long-chain alkyl monolayers has not yet been achieved. This section will discuss features ofthe Scheme 1method and compare it to the previous method (i.e., soaking the P m t O electrodes in a silane solution). Reactions of the Grignard reagent (C18H37MgCl)with the chlorosilylated Pt electrode produce in principle the (36)Longmire, M. L.; Watanabe, M.; Zhang, H.; Wooster, T. T.; Murray, R. W. Anal. Chem. 1990,62, 747-752. (37)Bazant, V.;Chvalovsky, V.; Rathousky, J. Organosilicon Compounds; Academic: New York, 1965;Vol. 1.
Chen et al. same Pt-O-Si-Cl8H3, surface linkage as that resulting in the Pt/OTS monolayers. Although films prepared by the Grignard reagent have smaller contact angles, they have similar c d l and better blocking effects than Pt/OTS films. Films modified by C18H37MgCl and by other CISnucleophiles show smaller contact angles than those modified by molecules with short-chain alkyl groups, such as butanol, an indication of worse in-plane ordering for long-chain molecules. This result is inconsistent with the general understanding of self-assembly of alkane monolayers, where better ordering is typically driven by the larger lateral van der Waals forces for molecules with longer alkyl chains. The contact angle measurements suggest that Scheme 1 method does not readily allow assembly of well-ordered films of long-chain alkylsiloxanes on oxide surfaces. The plateau-like diffusion currents for ferrocene and TCNQFI- a t the modified electrodes in CH3CN solutions are normally associated with radial diffusion conditions. The radial diffusion of the probes would presumably be to defect sites in the film that are suffkiently well separated that the radial profiles do not overlap (otherwise linear diffusion would prevail). Given that the blocking effects a t Pt-O-si-N(Cl&I3,)2 electrodes depend on the electroactive probe size (or charge) (Table l ) ,these defects must have dimensions of near molecular size (as opposed to large bare patches of Pt exposed to the solution, which would not discriminate as to the nature of the probe). Thus, the organic films do not consist of well-ordered alkylsiloxane islands surrounded by large patches bare Pt, as has been reported with OTS on Al oxide surfaces,38 but is instead a relatively uniform organic adlayer similar to the incompletelyformed monolayers of OTS on Si/Si02.29 The organic adlayer resulting from formation of covalent bonds between chlorosilylated surface and the incoming nucleophile does not equilibrate with the unreacted nucleophile, a t least under the conditions chosen here. Once the Si-X-C bonds are formed, the alkyl chains are irreversibly bound to the surface (unlike the case of alkanethiols on Au9) and interchain van der Waals interactions correspondingly cannot drive assembly of a closed-packed film. Also, the long alkyl chains may fold in such a way as to sterically block complete reaction of the active Si-C1 sites with nucleophiles and thus prevent complete formation of the organic adlayer. Although the Ca values are similar for films prepared by Scheme 1and by reaction of Pt oxides with chlorosilanes (e.g., WOTS),the differencein their contact angles suggest that the c d l measurement is not a sensitive indicator of the hydrophobic quality of siloxane films. Also, the Cdl values are substantially larger than those for Au/SC18H3, electrodes.20 This difference in ion permeability of the films is undoubtedly related to the alkyl chain packing structures. Octadecanethiol monolayers are chemisorbed on Au with tilt angles20~39-42 of about 30",with a vertical cross section of 18.3 Azwhich is essentially the same as that of crystalline p ~ l y e t h y l e n e . The ~ ~ crystalline-like chain packing is supported by infrared reflection experi(38)Cohen, S.R.; Naaman, R.; Sagiv, J. J.Phys. Chem. 1986,90, 3054-3056. (39)This tilt angle was obtained by different techniques such as ellipsometry,2OFTIR,40X-ray and theoretical calculation^.^^ (40)Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao,Y.-T.; Parikh, A. N.; Nuzzo, R. G. J . Am. Chem. SOC.1991,113,7152-7167. (41)Fenter, P.;Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447-2450. Ulman, A.; Shnidman,Y.; Eilers, J. E. J . A m . Chem. (42)Sellers, H.; SOC.1993,115,9389-9401. Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6,682-691. (43)
Chemically Modified Electrodes m e n t ~ . ~ ~No , " detailed information about alkyl-chain cross section is available for OTS films, but X-ray diffraction and reflectivity data show that the in-plane structure of OTS films on Si/SiOz is l i q ~ i d - l i k e .It ~ ~is probable that the actual difference in mol/cm2 coverage between crystalline-like and liquid-like structures is rather small, but the result of less regular packing in the latter leads to a significantlyhigher permeability to small electrolyte ions as reflected in Cd values. Films prepared with Cl~,H37MgClexhibit better and more reproducible blocking effects than the WOTS films. As noted above, the OTS reaction requires the presence of some moisture. Under dry reaction conditions, WOTS films exhibited much lower and irreproducible blocking (44) Nuzzo, R. G.;Dubois, L. H.;Allara, D.L. J . Am. Chem. SOC. 1990,112, 558-569.
Langmuir, Vol. 10, No. 9, 1994 3337 effects. Transmission IR studies of moist OTS deposition solutions34suggest that hydrolysis and polymerization reactions can precede deposition. We suspect that pre.hydrolyzed OTS probably forms some relatively wellordered patches on the Pt oxide surface and that the defects dowing eledroactive probe permeation are at the boundaries of such patches. The prepolymerization problem is avoided in Scheme 1.
Acknowledgment. We thank Jorge Duarte and Mark Ray of the Microelectronics Center of North Carolina for help with the AES measurements. J.E.H. is the recipient of a National Science Foundation Postdoctoral Fellowship (Grant CHE-9203585). The research was supported in part by grants from the National Science Foundation and the Office of Naval Research.
