Electrochemically Induced Morphology and Volume Changes in

Jan 6, 2004 - The Netherlands, Department of Chemistry, University of Bath, Bath BA2 7AY, ... Kingdom, and Max-Planck-Institute for Polymer Research, ...
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Langmuir 2004, 20, 891-897

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Electrochemically Induced Morphology and Volume Changes in Surface-Grafted Poly(ferrocenyldimethylsilane) Monolayers Ma´ria Pe´ter,† Mark A. Hempenius,† E. Stefan Kooij,‡ Toby A. Jenkins,§ Steve J. Roser,§ Wolfgang Knoll,| and G. Julius Vancso*,† MESA+ Research Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands, Department of Applied Physics, University of Twente, 7500 AE Enschede, The Netherlands, Department of Chemistry, University of Bath, Bath BA2 7AY, United Kingdom, and Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received September 5, 2003. In Final Form: November 9, 2003 Poly(ferrocenyldimethylsilanes), composed of alternating ferrocene and dimethylsilane units in their main chain and featuring a thiol end group, were self-assembled to redox-active monolayers on gold. Electrochemical atomic force microscopy was employed to study the morphology of the monolayers as a function of the applied potential in situ. Surface plasmon resonance spectroscopy and spectroscopic ellipsometry measurements, performed under electrochemical control, indicated thickness changes of up to 15% upon oxidizing and reducing the surface-grafted polymers. X-ray reflectivity measurements unambiguously showed a thickness increase upon electrochemical oxidation of the monolayers. The reversible thickness change was attributed to stretching of the polymer chains upon oxidation due to an increase in charge density and to the attraction of counterions and associated solvent molecules, which are released when the polymer film is reduced to its neutral state.

Introduction The modification of electrode surfaces with electroactive species is of growing interest due to potential applications in the areas of ion recognition, biosensors, enzyme electrodes, and molecular electronics.1-3 Various approaches exist for the preparation of uniform redox-active layers on metallic electrodes. Thiol- or disulfide functional redox-active molecules can be adsorbed onto gold surfaces to form self-assembled monolayers (SAMs).4 Electrostatic self-assembly on conductive surfaces, using polyions which incorporate redox-active groups, is another way to modify electrodes, e.g., for use in enzymatic catalysis.5 When redox-active polymers rather than small molecules are deposited on conductive surfaces, one has the opportunity to electrochemically control the charge density in the polymer chain. For conjugated polymers such as polypyrrole and polyaniline, significant volume changes have been observed upon electrochemical oxidation and reduction, thus rendering such polymers of interest as microactuators and artificial muscles.6-8 * To whom correspondence may be addressed. E-mail: [email protected]. Tel: ++31 53 489 2974. Fax: ++31 53 489 3823. † University of Twente, MESA+ Research Institute for Nanotechnology. ‡ University of Twente, Department of Applied Physics. § University of Bath, Department of Chemistry. | Max-Planck-Institute for Polymer Research. (1) Trippe´, G.; Oc¸ afrain, M.; Besbes, M.; Monroche, V.; Lyskawa, J.; Le Derf, F.; Salle´, M.; Becher, J.; Colonna, B.; Echegoyen, L. New J. Chem. 2002, 26, 1320. (2) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2000, 39, 1180. (3) Smalley, J. F.; Finklea, H. O.; Chidsey, C. E. D.; Linford, M. R.; Creager, S. E.; Ferraris, J. P.; Chalfant, K.; Zawodzinsk, T.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2003, 125, 2004. (4) Hortholary, C.; Minc, F.; Coudret, C.; Bonvoisin, J.; Launay, J.-P. Chem. Commun. 2002, 1932. (5) Calvo, E. J.; Etchenique, R.; Pietrasanta, L.; Wolosiuk, A.; Danilowicz, C. Anal. Chem. 2001, 73, 1161. (6) Smela, E.; Gadegaard, N. Adv. Mater. 1999, 11, 953. (7) Smela, E. J. Micromech. Microeng. 1999, 9, 1.

Macromolecules containing inorganic elements or organometallic units in the main chain combine potentially useful chemical, electrochemical, optical, and other interesting characteristics with the properties and processability of polymers.9 Poly(ferrocenylsilanes), composed of alternating ferrocene and dialkylsilane units in the main chain, belong to this class of organometallic polymers.10 Due to the presence of ferrocene units in the main chain, poly(ferrocenylsilanes) can be reversibly oxidized and reduced. Although the redox behavior of poly(ferrocenylsilanes) has been documented,11-14 no reports on morphology and volume changes of these polymers upon electrochemical oxidation/reduction have appeared in the literature.15 The attachment of a thiol end-function allows one to immobilize poly(ferrocenylsilanes) on gold surfaces.16,17 Due to their stability and well-defined distance to the electrode, the resulting monolayers are highly suitable for studying interfacial electron-transfer processes.3 In this paper, redox-induced morphology and volume changes of self-assembled poly(ferrocenylsilane) monolayers on gold are studied in situ by means of (8) Bay, L.; Jacobsen, T.; Skaarup, S.; West, K. J. Phys. Chem. B 2001, 105, 8492. (9) Abd-El-Aziz, A. S. Macromol. Rapid. Commun. 2002, 23, 995. (10) Kulbaba, K.; Manners, I. Macromol. Rapid. Commun. 2001, 22, 711. (11) Foucher, D. A.; Honeyman, C. H.; Nelson, J. M.; Tang, B. Z.; Manners, I. Angew. Chem., Int. Ed. Engl. 1993, 32, 1709. (12) Nguyen, M. T.; Diaz, A. F.; Dement’ev, V. V.; Pannell, K. H. Chem. Mater. 1993, 5, 1389. (13) Foucher, D.; Ziembinski, R.; Petersen, R.; Pudelski, J.; Edwards, M.; Ni, Y.; Massey, J.; Jaeger, C. R.; Vancso, G. J.; Manners, I. Macromolecules 1994, 27, 3992. (14) Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683. (15) For a preliminary study see Pe´ter, M.; Hempenius, M. A.; Lammertink, R. G. H.; Vancso, G. J. Macromol. Symp. 2001, 167, 285. (16) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (17) Peter, M.; Lammertink, R. G. H.; Hempenius, M. A.; van Os, M.; Beulen, M. W. J.; Reinhoudt, D. N.; Knoll, W.; Vancso, G. J. Chem. Commun. 1999, 359.

10.1021/la035656v CCC: $27.50 © 2004 American Chemical Society Published on Web 01/06/2004

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electrochemical atomic force microscopy (ECAFM),18 surface plasmon resonance spectroscopy (SPR), spectroscopic ellipsometry, and X-ray reflectivity, under electrochemical control. Experimental Section Materials. Thiol end-functionalized poly(ferrocenyldimethylsilanes), ES-PFS, were prepared by treating living poly(ferrocenyldimethylsilanes) with ethylene sulfide.17 Polymers with varying degrees of polymerization (DPn) were obtained. Their notation corresponds to the monomer/initiator ratios used. The following molar mass data were determined by gel permeation chromatography in THF, relative to polystyrene standards: ESPFS 25 (Mn ) 7750 g/mol, DPn ) 32, Mw/Mn ) 1.36), ES-PFS 50 (Mn ) 12500 g/mol, DPn ) 50, Mw/Mn ) 1.14), ES-PFS 100 (Mn ) 22600 g/mol, DPn ) 92, Mw/Mn ) 1.13). Substrates. Gold substrates for ECAFM (borosilicate glass, 2 nm Cr, 250 nm Au) were purchased from Metallhandel Schro¨er (Lienen, Germany). Au(111) samples were obtained by annealing these substrates in a high-purity H2 flame for 8 min. Substrates for SPR were prepared by the evaporation of a 45-50 nm gold film onto high refractive index LaSFN9 (Berliner Glass). Ellipsometry substrates were obtained by the evaporation of Ti (5 nm) followed by Au (50 nm) onto BK7 glass substrates. X-ray reflectivity substrates were obtained by evaporating Ti (2 nm) onto boron-doped Si (5-10 Ω cm-2), followed by Au (5-6 Å). Prior to use, the substrates were cleaned in piranha (30% H2O2/ 70% H2SO4), rinsed with Milli-Q water and ethanol, and dried in a nitrogen stream. Caution! Piranha solution is a very strong oxidant, reacts violently with organic materials, and should be handled with utmost care! Poly(ferrocenyldimethylsilane) monolayers were prepared by immersing gold substrates in polymer solutions in toluene (1.0 mg mL-1) for 24 h, to ensure maximum coverage. The samples were subsequently rinsed with toluene and dried in a stream of nitrogen. Techniques. Electrochemical Atomic Force Microscopy (AFM). ECAFM is contact mode AFM operated in liquid, with the option of imaging under electrochemical control. In the ECAFM experiments, the sample surface was scanned with a sharp tip situated on the probe cantilever, while a potential was applied to the sample. Electrochemical experiments can be performed at controlled currents or controlled potentials. During measurements, a voltammogram (current versus potential) and a current versus time graph are displayed together with the topographical image. Images can be recorded at constant potential but also during potential cycles. Thus, electrochemically induced morphological changes can be monitored in situ at the electrode/ electrolyte interface. ECAFM experiments were performed with a NanoScope III AFM (Digital Instruments (DI), Santa Barbara, CA) using an electrochemical liquid cell (DI) (volume 50 µL). The fluid cell incorporates a three-electrode system with working (polymer monolayer on gold), Pt wire reference, and Pt counter electrodes, under the control of a potentiostat (DI). NaClO4 in water (0.1 M) was used as electrolyte. Prior to use, the electrolyte was deaerated by passing nitrogen through the solution. The potential was cycled between -0.2 and 0.4 V at a scan rate of v ) 50 mV s-1. AFM imaging was performed in contact mode, using triangular Si3N4 cantilevers with a spring constant of 0.32 N/m. SPR Combined with Electrochemistry. SPR is an optical technique where changes in the refractive index and thickness near an interface can readily be detected.19 Experimentally, the “plasmon resonance angle” is determined, which is the angle under which light, reflected at a prism/metal interface in the so-called Kretschmann configuration, exhibits a minimum in the reflectance. The presence of a thin film on the metal surface leads to a shift of the minimum in the angular reflectivity scan. From this shift, the film thickness can be determined if the refractive index is known, or vice versa.19 SPR/electrochemistry experiments were performed on a custom-built setup, equipped with a Teflon cuvette, which allowed the simultaneous recording of electrochemical data and the application of a potential to the (18) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276. (19) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569.

Pe´ ter et al. sample.20 The polymer films on gold used for the excitation of surface plasmons served as the working electrode. The electrolyte was NaClO4 in water (0.1 M). Pt counter and Ag/AgCl (EAg/AgCl ) +0.22 V vs NHE, normal hydrogen) reference electrodes were used. The polymer/Au/LaSFN9 substrates were mounted against the Teflon cuvette with a Kalrez O-ring. The exposed electrode area was 0.80 cm2. The electrochemical experiments were controlled using an EG&G PAR 273A potentiostat. Cyclic voltammograms (CVs) were recorded by cycling the potential between 0.3 and 0.6 V (versus Ag/AgCl reference electrode) at a scan rate of 10 mV s-1. The cuvette was then mounted for investigation by SPR in the Kretschmann configuration. The glass slide was optically coupled to the base of a 90° LaSFN9 prism (n ) 1.85 at λ ) 632.8 nm) using an index-matching fluid (n ) 1.70). A He/Ne laser with a wavelength of λ ) 632.8 nm and 5 mW power was used to excite plasmon surface polaritons at the metal/dielectric interface. The measured angular reflectivity curves were fitted using the Winspall 2.01 program. Layer thicknesses in the reduced and oxidized states were determined by nonlinear least-squares fitting to a four-layer model (glass/ gold/ES-PFS/electrolyte), using the Fresnel theory.21,22 A refractive index of n ) 1.687 for the polymer layer was taken.23 Spectroscopic Ellipsometry Combined with Electrochemistry. Spectroscopic ellipsometry measures the change of polarization state of light upon reflection from a planar sample surface or a set of parallel interfaces as a function of wavelength or energy. The complex reflection coefficient F ) tan ψ ei∆ is expressed using the quantities Ψ and ∆, which reflect the intensity and phase angle, respectively. The method is routinely used to determine the optical constants of bulk materials or thin films. For thin films, the thickness can in principle also be determined. However, for film thicknesses much smaller than the wavelength of the light, only the product of the geometrical thickness and the complex refractive index dN ˜ of the film is obtained. This allows one to determine d if N ˜ is known, and vice versa, but unambiguous characterization of d and N ˜ is generally not possible. For the optical characterization of the polymer monolayers at various potentials, a custom-built spectroscopic ellipsometer (rotating polarizer-analyzer geometry) equipped with a xenon lamp and a scanning monochromator was used.24 Measurements were performed in the visible region between 310 and 820 nm (4.0-1.5 eV, photon energy Eph ) hc/λ ) 1240/λ (eV)) at a fixed incident angle of 70°. The ellipsometric parameters, Ψ and ∆, were recorded as a function of wavelength at various potentials. The ellipsometric data were analyzed using the Abeles matrix algorithm.24 Details of the original method are described by Azzam and Bashara.25 In situ measurements under potential control were performed in a PTFE (Teflon) cell. Absorption of light in the cell windows and the solution imposes a lower wavelength limit of approximately 350 nm. Electrochemical experiments (cyclic voltammetry) were undertaken using an Autolab PGSTAT10 potentiostat (ECOCHEMIE, Utrecht, The Netherlands) in a three-electrode configuration. Polymer-covered gold disk working, MSE reference (EHg/HgSO4 ) +0.61 vs VNHE, normal hydrogen electrode), and Pt counter electrodes were used. The electrolyte was NaClO4 in water (0.1 M). The cyclic voltammograms were recorded between -0.3 and 0.4 V (versus MSE) at v ) 10 mV s-1 scan rate. X-ray Reflectivity Combined with Electrochemistry. X-ray reflectivity can reliably measure the thickness of a large range of film thicknesses, ranging from a few to several thousand angstroms, with subangstrom resolution and without explicit assumptions about optical properties of the material.26 The X-ray reflection experiments were carried out with a home-built X-ray (20) Badia, A.; Arnold, S.; Scheumann, V.; Zizlsperger, M.; Mack, J.; Jung, G.; Knoll, W. Sens. Actuators, B 1999, 54, 145. (21) Advincula, R.; Aust, E.; Meyer, W.; Knoll, W. Langmuir 1996, 12, 3536. (22) Georgiadis, R.; Peterlinz, K. P.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166. (23) The refractive index was determined by V. Z.-H. Chan and R. G. H. Lammertink at the Massachusetts Institute of Technology, by profilometry combined with ellipsometry. (24) Kooij, E. S.; Wormeester, H.; Brouwer, E. A. M.; van Vroonhoven, E.; van Silfhout, A.; Poelsema, B. Langmuir 2002, 18, 4401. (25) Azzam, R. M. A.; Bashara, N. H. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1987.

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Figure 1. ECAFM experiment on ES-PFS 50 layers, recorded at v ) 50 mV/s: (a) cyclic voltammogram; (b) current versus time graph; (c) contact mode ECAFM image recorded while cycling the potential. Scan size is 0.6 µm × 0.6 µm, imaged at a scan rate of 10.17 Hz. Z range is 5 nm. reflectometer with energy-sensitive detection.27 The machine is mechanically straightforward, with two arms pivoting about a central drive shaft, and the X-ray optical path defined by Huber 50 µm variable slits mounted on the arms. The source was a 2 kW tungsten tube, driven by a Phillips X-ray generator. X-ray detection was accomplished by an EG&G high-purity germanium solid-state detector connected to a PC via ADCAM hardware and MAESTRO software, which perform the necessary analogue to digital conversions and amplification. The energy resolution of the detector is approximately 1.5%, with a maximum count rate of approximately 10 kilocounts/s. Normalization of the measured raw reflection profile involved dividing the data by the measured white beam profile. Measured data were fitted by means of the Parratt algorithm,28 using a five-layer model (0.1 M NaClO4/ES-PFS/Au/Ti/Si). For the electrochemical experiments, samples were fixed on the bottom of a Teflon cell and mounted as working electrodes. Pt counter and Ag wire reference electrodes were used. CVs were recorded in 0.1 M NaClO4 at v ) 20 mV s-1 scan rate. The potential was cycled between 0.05 and 0.5 V. A PGSTAT10 potentiostat controlled by Autolab software was utilized.

Results and Discussion Electrochemical atomic force microscopy (ECAFM) was used to study morphology and volume changes of the poly(ferrocenylsilane) monolayers in situ as a function of the applied potential.15 The topography of the redox-active monolayer was imaged while applying a triangular signal to the polymer-covered working electrode. The corresponding cyclic voltammogram, obtained by cycling the potential between -0.2 and 0.4 V, is shown in Figure 1a. Figure 1b shows a current versus time graph, generated as the triangular signal is applied. As the potential is cycled at a scan rate of v ) 50 mV s-1, the monolayer displays a periodic height contrast (Figure 1c). Lower regions in the image correspond to potential values close to the reduction potential. Bearing analysis29 of the ECAFM image in Figure 1c indicates a height difference of 0.6 nm ((2%) between the low and the high regions, (26) Richter, A.; Guico, R.; Wang, J. Rev. Sci. Instrum. 2001, 72, 3004. (27) Roser, S. J.; Felici, R.; Eaglesham, A. Langmuir 1994, 10, 3853. (28) Parratt, L. G. Phys. Rev. 1954, 95, 359. (29) Bearing provides a method of plotting and analyzing the distribution of surface height over a sample. The sample thickness in the neutral state, determined by SPR against 0.1 M NaClO4, was 6.4 nm (ref 15).

which corresponds to a height variation of about 10%. The observed morphological response is reversible. Figure 2a shows an AFM height image recorded at a constant potential of +110 mV. At this potential, the features are higher compared to those at -160 mV where the surface appears to be almost flat (Figure 2b). If the potential is cycled again and held near the oxidation potential at +100 mV, the features become higher again (Figure 2c). Surface Plasmon Resonance Spectroscopy. SPR combined with cyclic voltammetry was employed to further study the thickness changes induced upon electrochemical oxidation/reduction.20 A cyclic voltammogram of an ESPFS 50 monolayer on gold (Figure 3), obtained with the SPR setup, shows the two reversible redox peaks characteristic of poly(ferrocenylsilanes), indicating intermetallic coupling between neighboring iron centers in the polymer chain. A ferrocenium group increases the oxidation potential of neighboring unoxidized ferrocene centers and was even found to weakly affect the oxidation potential of next nearest ferrocene centers.14 Thus, the first oxidation wave is attributed to oxidation of ferrocene centers at alternating positions along the chain. In the second wave, at higher potentials, oxidation of the remaining ferrocene centers, in positions next to oxidized units, is completed. Due to intermetallic interactions with a variable number of oxidized neighboring units, broadening of the second wave occurs. Corresponding intermolecular interactions may also contribute to this broadening. Angular reflectivity scans were recorded at various electrode potentials. When the thickness or the refractive index of the layers changes as a function of the potential, the propagation characteristics of surface plasmons are altered.21,22 This causes a shift in the resonance angle. The critical incident angle for total internal reflection, θc, which is dependent upon the refractive indices of materials at an interface, is not perturbed by the presence of the polymer monolayer on the gold substrate or by changes occurring in the layer as a result of the electrochemical process.19 Figure 4 shows angular reflectivity scans recorded for a clean gold surface (1) and an ES-PFS 50 monolayer on gold in the reduced (2) and oxidized state (3). In the neutral state, a monolayer thickness of 6.4 nm was found against

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Figure 2. ECAFM images obtained for ES-PFS 50 layers on gold at various potentials: (a) E ) +110 mV; (b) E ) -160 mV; (c) E ) +100 mV. Scan size is 0.6 µm × 0.6 µm, imaged at a scan rate of 10.17 Hz. Z range is 5 nm.

Figure 3. Cyclic voltammogram recorded for an ES-PFS 50 monolayer on gold at a scan rate of v ) 10 mV s-1. The potential was referenced to an Ag/AgCl (E ) 0.22 V) electrode. A platinum counter electrode and 0.1 M NaClO4 were used.

Figure 4. Angular reflectivity scans recorded for (1) bare gold, (2) ES-PFS 50 monolayer on gold in the reduced state, and (3) ES-PFS 50 layer on gold in the oxidized state (at the second oxidation peak potential). The resonance angle difference between the oxidized and reduced states gives a thickness increase of 1.17 nm in the oxidized state if a refractive index n ) 1.687 is taken.

0.1 M NaClO4. As the potential was increased to approximately E1/2 of the first redox wave (E ) 430 mV), the thickness rose to 6.8 nm, which corresponds to a 6.2% increase. As the potential was further increased to the first oxidation peak maximum (E ) 445 mV), a layer thickness of 7.0 nm was found. This corresponds to a thickness change of about 9.4% with respect to the original value. Angular reflectivity scans were further recorded for the reduced state of the layer, then immediately after the first oxidation peak (E ) 475 mV), then at the second oxidation peak potential (E ) 530 mV), and again in the reduced state. Thickness values of 6.8, 7.4, 7.6, and 6.8

nm were found. The maximum thickness change was found to be ∆d ) 1.2 nm at the second oxidation peak potential. This corresponds to an increase of 18.8% compared to the original value. Angular reflectivity scans recorded for ES-PFS 25 and ES-PFS 100 layers show similar behavior compared to the ES-PFS 50 layer as they are brought from the neutral to oxidized and subsequently to reduced states. The thickness of the ES-PFS 25 layer was found to be 4.9 nm in the neutral state against 0.1 M NaClO4. At the first oxidation peak, a thickness of 5.4 nm was found, corresponding to a 10.2% increase of the original value. In the reduced state the thickness was 5.0 nm, slightly higher than found for the neutral state of the film. However, the difference is in the range of the experimental error ((0.2 nm).30 The film thickness of a neutral ES-PFS 100 monolayer was 7.7 nm. Angular reflectivity scans at the first oxidation peak potential, after the first peak, at the second oxidation peak potential, and in the reduced state gave corresponding thicknesses of 8.3, 8.4, 8.6, and 7.8 nm, respectively. These values represent changes of 7.8%, 9.1%, 11.7%, and 1.3%, respectively, with respect to the original thickness. To establish whether the shift in the resonance angle shown in Figure 4 was caused by the applied electric field, angular reflectivity scans at several values within the employed potential range (0.3-0.6 V versus EAg/AgCl) were recorded for clean gold substrates (Figure 5a) and 1-octadecanethiol self-assembled monolayers on gold (Figure 5b). Scans were made at open circuit potential (EOCP ) 410 mV), at EOCP + 15 mV, and EOCP + 125 mV. The experimental data were fitted with the corresponding three-layer21,22 (glass/gold/electrolyte) and four-layer models (glass/gold/ODT/electrolyte), respectively. A refractive index n ) 1.5 was assumed for the ODT layer.31 The results show that the position of the resonance angle is not influenced by the applied potential. Spectroscopic ellipsometry combined with electrochemistry was carried out to monitor redox-induced variations in thickness and in optical properties of the monolayers. Ellipsometric spectra were recorded as a function of energy for bare gold substrates and neutral poly(ferrocenyldimethylsilane) monolayers on gold, in air (30) The error in thickness determined by SPR is (0.2 nm for the first fit. In subsequent fits the error is less than 0.2 nm and is mainly determined by the refractive index. (31) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

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Langmuir, Vol. 20, No. 3, 2004 895 Table 1. Thickness Data and Fit Parameters for Neutral ES-PFS Monolayers on Gold in Aqueous NaClO4, Obtained by Spectroscopic Ellipsometry ES-PFS 25 ES-PFS 50 ES-PFS 100

d (nm)

C1 ) kλ (nm)

6.0 ( 0.2 5.5 ( 0.2 5.6 ( 0.2

13.6 ( 7.4 0.5 ( 7.2 0.1 ( 6.2

Table 2. Thickness Data and Fit Parameters for ES-PFS Monolayers in Oxidized and Reduced States in Aqueous NaClO4, Obtained by Spectroscopic Ellipsometry ES-PFS 25 (red) ES-PFS 25 (ox) ES-PFS 50 (red) ES-PFS 50 (ox) ES-PFS 100 (red) ES-PFS 100 (ox)

Figure 5. (a) Angular reflectivity scans recorded for a clean gold substrate at several fixed potentials. The first scan was taken at open circuit potential (EOCP ) 410 mV), the second at OCP + 15 mV, and the third at OCP + 125 mV. Clearly, the position of the resonance angle is not influenced by the applied potential. (b) Angular reflectivity scans recorded for a 1-octadecanethiol self-assembled monolayer on gold at various potentials. The first scan was taken at open circuit potential (EOCP ) 410 mV), the second at OCP + 20 mV, and the third at OCP + 120 mV. The position of the resonance angle is not influenced by the applied potential.

Figure 6. Ellipsometry spectra recorded for bare gold and ES-PFS monolayers on gold in air (a and b) and against 0.1 M NaClO4 (c and d).

(Figure 6a,b). Similar spectra obtained in 0.1 M NaClO4 are shown in parts c and d of Figure 6. Analysis of the spectra for the bare substrate using a four-layer model24 (BK7/Ti/Au/H2O) yield Au and Ti layer thicknesses of 48.4 ( 2.3 nm and 1.6 ( 2.4 nm, respectively. These values differ slightly from values measured in air, possibly due to the polarization of the sample surface in contact with the electrolyte.32 The ellipsometric spectra of the polymer-covered substrates differ markedly from those of the bare substrates. A five-layer model24 (BK7glass/Ti/Au/ES-PFS/H2O), using the thicknesses and incident angle determined for the (32) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, 1992.

d (nm)

C1 ) kλ (nm)

6.0 ( 0.2 6.8 ( 0.2 5.5 ( 0.2 6.2 ( 0.2 5.6 ( 0.2 6.5 ( 0.2

14.9 ( 7.8 18.6 ( 7.3 3.7 ( 8.2 4.2 ( 7.6 4.0 ( 6.2 7.3 ( 5.8

bare gold substrate, was employed to analyze the data. The refractive index N ˜ ) n + ik was modeled using a constant value of n ) 1.687 for the real part and k ) C1/λ (k is the extinction coefficient, C1 is a fit parameter) for the imaginary part.33 The fit results for the three neutral films in aqueous NaClO4 are summarized in Table 1. By cycling the potential between -0.3 and 0.4 V versus MSE, the films can be switched back and forth between reduced and oxidized states. The fully reduced state corresponds to an applied potential of about -0.3 V, while at 0.25 V the films are fully oxidized. The absolute spectra were analyzed employing the fitting procedure described above. The results are given in Table 2. The thickness change of the films upon oxidation amounts to 13.5%, 13.7%, and 14.7% for ES-PFS 25, ES-PFS 50, and ESPFS 100, respectively. It is important to note that these values correspond to a change of the product dN ˜ , but this translates into a thickness variation when the refractive index is considered to be constant for all redox states. Absolute ellipsometric spectra in the fully oxidized and reduced states are not distinguishably different, therefore the results were plotted as the difference of spectra measured in the oxidized and reduced states. Figure 7 shows the changes of ellipsometric parameters, δΨ and δ∆, for an ES-PFS 50 layer as a function of energy at various potentials. Clearly, the qualitative behavior of the bare gold substrate is considerably different from that of the monolayer-covered samples. As the spectra with and without monolayer are not scalable, there is a real, but small, optical effect induced by the presence of the ES-PFS layer.34 Figure 8a shows the electrochemical response of an ESPFS monolayer as the potential is cycled between -0.3 and 0.4 V, versus EMSE. Simultaneously, Ψ and ∆ were recorded at 3 eV (λ ) 413 nm, Figure 8b). As the potential is cycled, the optical properties of the films are switched (33) Tompkins, H. G.; McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry: A User’s Guide; John Wiley & Sons: New York, 1999. (34) An electric field applied to a gold substrate can, depending on the intensity, perturb the electronic structure of the metal, thus inducing changes in the optical properties: electroreflectance (ref 35).To verify whether the differences observed in Ψ and ∆ are not merely due to electroreflectance of the gold substrate, measurements were performed on bare gold substrates at potentials corresponding to the oxidized and reduced states of the monolayer films. The resulting differences in Ψ and ∆ are shown in Figure 7 by curve E. At an energy of 3 eV (413 nm), the electroreflectance of gold is the smallest (δΨ ) 0° and δ∆ ) -0.1°), thus the contribution of the films to the change in optical properties is maximal. (35) a) Reipa, V.; Monbouquette, H. G.; Vilker, V. L. Langmuir 1998, 14, 6563. (b) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90.

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Figure 9. Dependence of Ψ on the applied potential, plotted together with the charge (Q) as a function of time. Ψ was recorded at an energy of 3 eV (413 nm).

Figure 7. Variation of the ellipsometric parameters δΨ and δ∆ as a function of energy at several potentials recorded for an ES-PFS 50 layer on gold: (A) and (B) differences between spectra recorded in the reduced state at -0.25 V with respect to spectra recorded at -0.29 V; (C) difference between spectra recorded at 0.07 V with respect to spectra recorded in the reduced state; (D) difference between spectra recorded at 0.22 V with respect to spectra recorded in the reduced state. Curve E represents the difference between spectra recorded at 0.3 V and at -0.25 V for a bare gold substrate.

Figure 10. Reflectivity recorded as a function of scattering vector. Open circles represent measured data in the reduced state; closed circles correspond to the oxidized state. Solid lines represent the fitted curves.

Figure 10 for an ES-PFS 50 layer on gold. Data presented by open circles were recorded in the neutral state, closed circles correspond to the oxidized state. For the ES-PFS 50 monolayer film, a thickness increase upon oxidation of ∆d ) 0.72 nm was observed (dneutral ) 5.0 nm and doxidized ) 5.7 nm), which corresponds to 14.4% of the original thickness. This value correlates well with the SPR and spectroscopic ellipsometry results, confirming that the electrochemical oxidation of the films is accompanied by a thickness increase. Conclusions

Figure 8. (a) Excitation signal and cyclic voltammograms as a function of time for an ES-PFS 50 layer on gold. (b) Variation in time of Ψ and ∆ with the potential. Measurements were performed at an energy of 3 eV (413 nm).

between two limiting values, corresponding to the reduced and oxidized states of the film. The correlation is even more clear if one of the ellipsometric parameters, e.g., Ψ, recorded in time, is compared with the variation of the charge in the electrochemical process (Figure 9). Upon oxidation, the increase in charge is accompanied by an increase in Ψ and a decrease in ∆. X-ray Reflectivity, combined with electrochemistry, was utilized to monitor the redox-induced thickness changes in the monolayers. This method allows the determination of layer thickness without the need of knowing the refractive index. X-ray reflectivity spectra were taken in the reduced and fully oxidized states. The reflectance as a function of scattering vector is shown in

Electrochemical atomic force microscopy (ECAFM), surface plasmon resonance spectroscopy (SPR), spectroscopic ellipsometry, and X-ray reflectivity, combined with electrochemistry, were employed to monitor redox-induced morphology and thickness changes in surface-grafted poly(ferrocenyldimethylsilane) monolayers on gold. ECAFM revealed a reversible change in the morphology of the monolayer upon oxidation and reduction. The morphology and volume changes of these redox-active monolayers are associated with conformational changes in the polymer backbone upon oxidation, due to an increase in charge density, and with the concomitant attraction of counterions and associated solvent molecules by the surface-grafted polymer chains. These processes are reversed upon reducing the monolayer films. SPR combined with electrochemistry showed that the position of the resonance angle depends on the applied potential. Assuming that the refractive index does not change upon oxidation, a thickness increase of over 10% was found for all layers in the oxidized state, up to 18%. The electric field did not influence the position of the resonance angle in the angular reflectivity scans. X-ray reflectivity measurements, which showed thickness increases of 14%, confirmed the assumption that the observed optical responses in ellipsometry and SPR upon oxidation of the monolayer films can largely be attributed

Surface-Grafted Poly(ferrocenylsilanes)

Langmuir, Vol. 20, No. 3, 2004 897

to a geometric thickness increase rather than an increase of the refractive index.

support of the Nuffield foundation and the University of Bath.

Acknowledgment. The authors thank the University of Twente for financial support. ATAJ acknowleges the

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