Grafting Aryl Diazonium Cations to Polycrystalline Gold: Insights into

The structure and properties of thin organic films electrografted to conducting surfaces by reduction of the corresponding diazonium salts are not wel...
0 downloads 0 Views 105KB Size
Download PDF
7808

J. Phys. Chem. C 2007, 111, 7808-7815

Grafting Aryl Diazonium Cations to Polycrystalline Gold: Insights into Film Structure Using Gold Oxide Reduction, Redox Probe Electrochemistry, and Contact Angle Behavior Matthew G. Paulik, Paula A. Brooksby,* Andrew D. Abell, and Alison J. Downard* MacDiarmid Institute for AdVanced Materials and Nanotechnology, Department of Chemistry, UniVersity of Canterbury, PriVate Bag 4800, Christchurch, New Zealand ReceiVed: January 25, 2007; In Final Form: March 12, 2007

The structure and properties of thin organic films electrografted to conducting surfaces by reduction of the corresponding diazonium salts are not well understood. In this work we used electrochemistry and contact angle measurements to characterize multilayer carboxyphenyl and methylphenyl films grafted to Au surfaces. Freshly grafted films contain material that can be readily removed during potential cycling and sonication. Observation of Au oxide surface electrochemistry confirms a porous bulk film structure as previously found for films electrografted to carbon surfaces. The charge associated with Au oxide reduction was used to estimate the upper limit for surface concentration of modifiers directly attached to the surface after careful preparation of the Au surface prior to and after grafting. Values for the surface concentration of the modifier in the range of 3-4 × 10-10 mol cm-2 indicate that the first layer of the films is loosely packed. The close correspondence of surface concentration with that previously found for a “monolayer equivalent” of nitrophenyl film covalently grafted to smooth carbon surfaces ((2.5 ( 0.5) × 10-10 mol cm-2) supports a similar film formation mechanism, that is, formation of a Au-C bond. We also show that sonication of the Au surface considerably alters the Au oxide reduction charge of an initially well-defined bare Au surface. Hence, Au oxide electrochemistry cannot be used to monitor film changes after sonication. Examination of the voltammetric response of Fe(CN)63-/4- and measurements of water contact angles at the grafted electrodes give insight into the filmsolution interface. Sonication of films in solvents of different polarities leads to different interfacial electrochemistry and hydrophilicity, consistent with a dynamic film structure that can reorganize in response to the environment. The reliable interpretation of changes in redox probe responses thus requires careful consideration of the dynamic mechanical properties of these loosely packed films.

Introduction The electrochemical reduction of a diazonium cation in aqueous and nonaqueous solutions at carbon surfaces is well documented.1,2 Electroreduction generates a radical species that attacks the surface to give a covalent C-C bond.3 Interest in this grafting method, at metal surfaces,3-14 arises from the desire to generate covalently bound organic films with good mechanical and thermal stability. The study of the reduction of aryl diazonium cation on metal surfaces is facilitated by the fact that the metal interface can be well defined. However, this is not true of amorphous graphitic carbon materials with the exception of highly oriented pyrolytic graphite. The unique oxide (hydroxide) electrochemistry of metal surfaces provides a means to examine film coverage directly by observing how oxide (hydroxide) electrochemistry is affected in the presence of a film. Unambiguous identification of a metal-C bond was first demonstrated with X-ray photoelectron spectroscopy (XPS) detection of a Fe-C linkage4 and subsequently of other M-C bonds.10,11,13 Au-C bonds have not been detected spectroscopically, but other evidence strongly supports formation of a Au-C covalent bond.5-17 The electrochemical response of a NO2-functionalized modifier is the most frequently used method for establishing the coverage of diazonium-derived films, but this technique has been * To whom correspondence should be addressed. Phone: 64-3-3667001. Fax: 64-3-3642110. E-mail: P.A.B., [email protected]; A.J.D., [email protected].

shown to be inaccurate in some circumstances.18 The NO2 moiety can be electroinactive at the expected potential due to electrochemically inaccessible nitro groups buried within the film, formation of cyclohexadienyl groups resulting from the film growth mechanism,19 and radical attacks at the nitro centers during grafting. The voltammetry of a solution redox probe is an alternative method of inferring film structure, and while it is routinely used, the results can be difficult to interpret.20 In recent years, electrochemical impedance spectroscopy (EIS), XPS, and vibrational spectroscopy (Raman and infrared) have been used for film characterization to reveal new molecular and structural detail.3,13,14,21 Grafted films have been shown to be a mixture of strongly bound organic layers and weakly bound and entrapped solvent and solute species that are at least partly removed during postgrafting washing procedures.10 The height (thickness) of electrografted films on carbon substrates is known to change in response to solvent and electrolyte treatments.18,22,23 Direct evidence for shrinking and swelling effects have been observed for nitroazobenzene films grafted to carbon when subjected to potential cycling treatments in acetonitrile and water solutions.18 Height changes from 6 to 2 nm were measured for the film grafted in acetonitrile and later treated in water. The implications are that these films are both porous and flexible, and hence, the outer boundary of the film, at the film-solution interface, may be able to reorganize in response to the solution. The purpose of the present work is to (a) use gold oxide electrochemistry to examine films grafted to Au surfaces and

10.1021/jp0706578 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/05/2007

Grafting Aryl Diazonium Cations (b) investigate the dynamic behavior of grafted films on Au surfaces as a direct response to their immediate environment. Film responses to potential cycling and solvent sonication procedures are examined using Au oxide electrochemistry, Fe(CN)63-/4- solution electrochemistry, and contact angle analysis. The Au oxide electrochemistry reflects the nature of the electrode-film boundary and may give information not easily obtained for films grafted to carbon surfaces. Electrochemical probe and contact angle analyses can be used to interrogate film-solution boundary properties. This complements results from the Au oxide studies. Carboxyphenyl (CP) and methylphenyl (MP) films were chosen for this study since the carboxylic acid and methyl functional groups possess very different polarities and hydrophilicity, thus influencing the interfacial behavior in measurably but different ways. Experimental Section Chemicals. H2O2 (40%, Wilsons Chemical Ltd.), H2SO4 (Aldrich), HClO4 (BDH, AnalaR), NaClO4‚H2O (Scharlau), K3Fe(CN)6 (Riedel De Hae¨n AG), and KCl (BDH, AnalaR) were used as received. Piranha solution was made with 1:3 H2O2:concentrated H2SO4. Caution: piranha solution is a strong oxidant and reacts Violently with organic matter. The surface modifiers 4-nitrobenzenediazonium tetrafluoroborate (BF4‚N2(C6H4)NO2, NBD), 4-carboxylbenzenediazonium tetrafluoroborate (BF4‚N2(C6H4)COOH, CBD), and 4-methylbenzenediazonium tetrafluoroborate (BF4‚N2(C6H4)CH3, MBD) salts were prepared from their amine precursors (Aldrich) using literature methods,24 dried under vacuum, and stored in the dark. Tetrabutylammonium tetraflouroborate ([Bu4N]BF4) was prepared by standard methods and dried under vacuum at 80 °C for 2 days. Acetonitrile (HPLC grade) was dried over CaH2 for 2 days and refluxed under N2 for 2 h prior to distilling in an N2 atmosphere. Milli-Q water, >18 MΩ cm, was used for all aqueous solutions. Petroleum ether (reagent grade) was used as received. Gold Electrode Preparation and Cleaning. A polycrystalline gold electrode, with a diameter of 6 mm and roughness factor of 1.5 (calculated using 400 µC cm-2 as the charge required to reduce a monolayer of Au oxide), was used for solution electrochemistry. The electrode was cleaned by mechanical polishing on a microcloth with successively finer grades of alumina down to 0.05 µm particle size. Mechanical polishing was performed periodically. The surface was sonicated in numerous water and acetone solutions to remove the polishing debris. Typically, the electrode was dipped into piranha solution for 1 min, flame annealed using H2 gas, cooled to room temperature, and then continuously cycled in aqueous 0.01 M HClO4 solution between -0.2 and 1.45 V (SCE) at 50 mV s-1 until the cyclic voltammograms did not show changes between consecutive cycles over a 15-min period. Cycling was halted at -0.2 V, giving an oxide-free surface. The cycling period was typically 2-4 h. The Au surfaces used for contact angle measurements were thermally deposited Au (200 nm) onto a Si (100) substrate. An adhesion layer of Ti (30 nm) or NiCr (30 nm) was deposited between the Si and Au. These surfaces were stored in a desiccator for no longer than 2 weeks. Electrochemistry. All electrochemical measurements were performed using a computer-controlled EG&G PAR model 273A potentiostat or a PAR model 362 potentiostat operated with ADI Powerlab Software. The solution electrochemistry with the polycrystalline gold electrode was performed in a standard three-electrode cell with a gold secondary electrode and either a SCE (for aqueous solutions) or a Ag wire pseudo-reference

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7809 electrode (for acetonitrile solutions). The potential of the Fc/Fc+ couple was 0.37 V. The reference electrode was cleaned prior to each modification, and its potential, with respect to the diazonium cation reduction peak, was stable between experiments. The working electrode made contact with the solution using the hanging meniscus method. The cell used for electrochemical modification of the Si/Ti(NiCr)/Au substrates has been described in detail elsewhere.25 The geometric area of the gold working electrode surface was 0.13 cm2. The secondary electrode was a gold wire, and the reference electrode was a Ag wire described above. Solutions for electrochemistry were degassed with N2, and all measurements were made at 22 ( 2 °C. Film Preparation. Unless stated otherwise, films were grafted to the polycrystalline Au surface prepared by piranha treatment, flamed, and potential cycled to a steady state. Freshly prepared Si/Ti/Au surfaces had no further surface treatments prior to grafting. The modifier solution contained 1 mM diazonium cation + 0.1 M [Bu4N]BF4 in acetonitrile. A single cyclic voltammogram was recorded to determine the location of the diazonium cation reduction peak (Epred) where the negative potential limit of the voltammogram did not exceed 300 mV past Epred. Controlled potential electrolysis was then used to step the working electrode potential from 0.4 V (Ag wire) to the modification potential (Eapp) for a known time, τ. The value of Eapp was determined from Epred, where Eapp ) Epred -150 mV. After modification, the solution with the diazonium was discarded; the cell and modified gold surface was washed with pure acetonitrile and then water and dried with nitrogen gas. The Si/Ti/Au wafer, with the grafted film, was removed from the cell and rinsed and dried as described in the text. Contact Angle Measurements. Static contact angle measurements were performed with a 2.0 µL droplet of water imaged 5-10 s after the drop was applied. Each computed contact angle was obtained by averaging 12 contact angle measurements at two positions (rotated 90° with respect to each other) and repeating the procedure for a second droplet at a different location on the surface. Hence, for the one surface the quoted contact angle is an average of 24 measurements with the uncertainty determined to be (2° for that surface. Curve Fitting. Voltammetric peak analysis was performed by curve fitting the data.26 Lorentzian, Gaussian, or mixed Lorentzian-Gaussian curves were fitted to the voltammetric peaks using the Levenberg-Marquardt algorithm. Polynomial baselines were either fitted during nonlinear least-squares iteration or solved for and subtracted from the voltammogram prior to the curvefit. Results and Discussion The sections to follow describe (i) the electrochemical behavior of Au and its oxides and the preconditioning of the surface for all further work, (ii) CP and MP film preparation and treatments and their effects on Au oxide electrochemistry, and (iii) an illustration of how contact angle and electrochemical probe analyses reveal the film structure at an interface and its response to the solution environment. Gold Surface Behavior to Pretreatments. Oxidation of Au involves one adsorbed and two nearby water molecules,27-30 as shown in reaction 1. The subsequent electrochemical reduction of the oxide is given in reaction 2.

Au‚H2Os + 2H2O h Au(OH)3,s + 3H+ + 3e-

(1)

Au(OH)3,s + 3e- f Aus + 3OH-

(2)

7810 J. Phys. Chem. C, Vol. 111, No. 21, 2007

Paulik et al.

Figure 1. Voltammetric scans of bare Au in 0.01 M HClO4 at 100 mV s-1 following different surface treatments: (a) flame annealed, (b) cycled at 50 mV s-1 for 3 h, (c) surface prepared as in (b) and subsequently immersed in 1 M HClO4 at OCP overnight, and (d) surface prepared as in (b) and subsequently immersed in water at OCP overnight.

Figure 2. Voltammetric scans in 0.1 M [Bu4N]BF4-acetonitrile solution for (A) 1 mM CBD and (B) 1 mM MBD. Scan rate ) 100 mV s-1. The cycle number of each voltammogram is shown.

The solution pH, concentration, and identity of electrolyte anions, anodic polarization time, and temperature all have an effect on the electrochemistry of oxide formation and dissolution.28 In the current study, acidic solutions were used to avoid significant pH changes at the electrode-(film) interface. The voltammograms shown in Figure 1 were recorded in 0.01 M HClO4 at 100 mV s-1 where (a) is the initial scan after flame annealing and (b) is the same surface after 3 h of potential cycling at 50 mV s-1. The gold oxide reduction peak becomes larger with slow potential cycling until the surface stabilizes and there is no further change to the peak. This surface response is reproducibly obtained and therefore an ideal initial interface for use in voltammetric experiments. Parts (c) and (d) are the initial scans from surface (b) following immersion at open circuit potential (OCP) overnight in aqueous 1 M HClO4 and water, respectively. Parts (c) and (d) show the degree to which the oxide reduction peak can change with simple immersion in solvent and electrolyte, the pure water treatment giving the greatest change to the oxide reduction peak. Short immersion times (90%) upon repeatedly alternating between water and PE. The voltammograms show that the apparent electrontransfer rate to Fe(CN)63-/4- at PE-treated CP films is slower than that at the water-treated films. We attribute these results to interaction between the film and the solvent. The carboxyl groups in the CP film are expected to be solvated by water but not by PE. Hence, a CP film sonicated in water is predicted to be more porous since water can permeate into the film and cause swelling. Conversely, in PE the carboxyl film would be expected to repel solvent, causing film shrinkage and decreasing film porosity. Hence, the apparent rate of electron transfer to Fe(CN)63-/4- at PE- and water-treated CP films should be different with the PE-treated film having a slower rate. The solid lines in Figure 5B show the Fe(CN)63-/4- response from acetonitrile- and water-treated MP films with a relatively faster electron-transfer rate being observed at acetonitrile-treated films compared to water. The Fe(CN)63-/4- voltammetric response at a MP film is also expected to respond to changes in polarity and hydrophilicity caused by solvent sonication treatments. A water-treated film is likely to shrink and become less porous due to the methyl group preferring not to be exposed to water, hence leading to a slower electron-transfer rate for the redox probe. By comparison, acetonitrile sonication treatment swells the film, exposing pinholes and defects, giving a relatively faster electron-transfer rate. Probe scans 3 and 4 in Figure 5A represent the film behavior when alternating between sonication in different solvents. However, the initial probe scan of a freshly grafted film obtained before sonication (that is, scan 2) cannot be recovered. We speculate that the origin of this difference is due to loss of entrained unbound (or weakly associated) diazonium ions and electrolyte remaining from grafting and an initial film structure that is irreversibly altered upon sonication. Similar conclusions were reached by Be´langer.10 In summary, a freshly prepared film undergoes an irreversible change when sonicated. After initial sonication, each subsequent sonication treatment gives reproducible Fe(CN)63-/4- behavior, where the solvent and modifier combination with the best matched hydrophilicity and polarity give faster apparent Fe(CN)63-/4- responses. The probe voltammetry is consistent with reversible changes to the film structure after sonication in the two solvents, giving more or less blocking films. On the basis of our results, we also note that interpreting changes in redox probe responses as evidence that film losses are or are not occurring is flawed without first considering solvation effects on the film. Water Contact Angle Measurements at CP and MP Films. Modification of the surface free energy of a solid has a fundamental impact on measured contact angles.41 Changes to the hydrophilic or hydrophobic properties of the interface, the underlying structure of the surface film (which affects the packing density of groups in the surface), surface roughness, and heterogeneity all affect the contact angle. Flat Au surfaces

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7813

Figure 6. Plot showing the measured static contact angle on bare and (A) CP- and (B) MP-modified Au surfaces. The labels on the abscissa are W ) water, PE ) petroleum ether, and ACN ) acetonitrile. Closed symbols ) 0 °C, open symbols ) 45 °C, circles are τ ) 5 min, and triangles are τ ) 1 min.

were electrochemically modified with either CP or MP (τ ) 1 and 5 min), rinsed with acetonitrile, and sonicated for 20 min in water and PE (CP film) or water and acetonitrile (MP film). The sonication temperature was kept constant at either 0 or 45 °C. Plots of the static contact angle of a 2.0 µL drop of water on the surface of bare Au, freshly prepared films on Au, and after sonication treatments are given in Figure 6 for (A) CP and (B) MP films, where W ) water, PE ) petroleum ether, and ACN ) acetonitrile. The initially hydrophobic bare Au surface has a contact angle between 76° and 83°. Sonication of bare Au in water and PE gave contact angles in the range 75-85°. However, sonication in acetonitrile reduced the contact angle to ∼55°, consistent with the specific interaction of acetonitrile with the surface. Freshly prepared CP films are more hydrophilic (20-30°) than MP films (53-67°), as expected from the carboxyl functional groups. Figure 6 shows that temperature has a small influence on the measured contact angle, but the choice of solvent is more significant. The changes in contact angle, relative to the sonication solvent used, are attributed to solvent polarity and subsequent reorganization of the film to maximize exposure of either the functional groups or the phenyl rings at the interface. For example, sonication of a CP film in water is expected to expose the carboxyl groups at the surface, maximizing the surface hydrophilicity. Sonication of the CP film in PE causes film reorientation with the aryl rings preferentially exposed, maximizing the hydrophobicity of the surface. Thus, the measured contact angle will be larger after sonication in PE than with sonication in water. Cycling of the surfaces between solvents with different polarity to give surfaces with different wettabilities was reproducible as were the redox probe results described previously. The contact angle of the newly prepared CP film was always lower than those measured after sonication treatments, indicating that the freshly prepared surface gives the most hydrophilic interface. This implies that the population of COO-/COOH groups at the interface is greatest for a freshly prepared film and that sonication leads to fewer carboxyl groups. As noted above, the sonication-induced loss of weakly bound (noncovalent) species from aryl diazonium grafting to Au surfaces has

7814 J. Phys. Chem. C, Vol. 111, No. 21, 2007

Paulik et al.

been established by other workers.10 The contact angle behavior observed here confirms that there are accompanying irreversible structural changes to the film surface, and the same conclusion can be drawn from the Fe(CN)63-/4- voltammetry results described earlier in which the initial probe response was always different from subsequent behavior following sonication. Following the initial sonication step continued sonication of CP films in solvents with differing polarity gave changes in contact angles that tracked solvent polarity. However, these changes are not fully reversible and there is a trend toward higher contact angle as the total sonication time increases. This suggests a continuing slow change in the film, consistent with the loss of film material observed by earlier workers.10 The contact angles measured on the MP film always show a dependence on the polarity of the solvent used for sonication, but there is no apparent underlying irreversible change with continued sonication. The initial contact angle, on a freshly electrogenerated film in acetonitrile, is not the same as that obtained on the subsequent acetonitrile treatments; rather an intermediate angle between acetonitrile and water is initially measured. The unique initial contact angle also correlates with the distinctive initial Fe(CN)63-/4- scan.

film losses are occurring. Redox probe and contact angle data demonstrate that the solution-film interface is very much dependent on the solvent treatment, and this additionally affects the apparent kinetics of electron transfer through the bulk film in a way that is not well understood at this time. Moreover, underlying reasons for the oscillatory behavior of the electrochemical probe and contact angle results may not be the same. This study demonstrates that, with due care, electrochemistry and contact angle measurements are suitable tools to interrogate diazonium cation reduction on metal surfaces and, importantly, critical evaluation of the results can provide much information on the surface-film interface and the dynamic nature of the solution-film interface.

Conclusions

(1) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883. (2) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119, 201. (3) Pinson, J.; Podvorica, F. Chem. Soc. ReV. 2005, 34, 429. (4) Boukerma, K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. Langmuir 2003, 19, 6333. (5) Bernard, M.-C.; Chausse´, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450. (6) Wang, J.; Firestone, M. A.; Auciello, O.; Carlisle, J. A. Langmuir 2004, 20, 11450. (7) Maldonado, S.; Smith, T. J.; Williams, R. D.; Morin, S.; Barton, E.; Stevenson, K. J. Langmuir 2006, 22, 2884. (8) Combellas, C.; Delamar, M.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Chem. Mater. 2005, 17, 3968. (9) Ghilane, J.; Delamar, M.; Guilloux-Viry, M.; Lagrost, C.; Mangeney, C.; Hapiot, P. Langmuir 2005, 21, 6422. (10) Laforgue, A.; Addou, T.; Be´langer, D. Langmuir 2005, 21, 6855. (11) Liu, G.; Liu, J.; Bo¨cking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136. (12) Liu, G.; Bo¨cking, T.; Gooding, J. J. J. Electroanal. Chem. 2006, 600, 335. (13) Lyskawa, J.; Be´langer, D. Chem. Mater. 2006, 18, 4755. (14) Ricci, A.; Bonazzola, C.; Calvo, E. J. Phys. Chem. Chem. Phys. 2006, 8, 4297. (15) Adenier, A.; Bernard, M.-C.; Chehimi, M. M.; Cabet-Deliry, E.; Desbat, B.; Fagebaume, O.; Pinson, J.; Podvorica, F. J. Am. Chem. Soc. 2001, 123, 4541. (16) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370. (17) Adenier, A.; Barre´, N.; Cabet-Deliry, E.; Chausse´, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Surf. Sci. 2006, 600, 4801. (18) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791. (19) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Langmuir 2005, 21, 280. (20) Cruickshank, A. C.; Tan, E. S. Q.; Brooksby, P. A.; Downard, A. J. Electrochem. Commun. 2007, doi:10.1016/j.elecom.2007.02.004. (21) Anariba, F.; Viswanathan, U.; Bocian, D. F.; McCreery, R. L. Anal. Chem. 2006, 78, 3104. (22) Brooksby, P. A.; Downard, A. J.; Yu, S. S. C. Langmuir 2005, 21, 11304. (23) Downard, A. J.; Yu, S. S. C. e-J. Surf. Sci. Nanotech. 2005, 3, 294. (24) Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Compounds, 3rd ed.; Edward Arnold: London, 1985. (25) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038. (26) Loring, J. S. Ph.D. Thesis, University of California at Davis, 2000. (27) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (28) Hamelin, A.; Martins, A. M. J. Electroanal. Chem. 1996, 407, 13. (29) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429.

The present study investigated the Au oxide reduction electrochemistry in the presence and absence of two (assumed) covalently grafted surface films. The films were grafted by the electroreduction of CBD and MBD cations under conditions that produce multilayer films. These films were subject to sonication and solvent treatments, and the resulting filmsolution interface was examined using electrochemical probe and contact angle analysis. Au oxide-grafted film electrochemistry under the best possible conditions, where film porosity and channels are present and the surface has been reconstructed before and after grafting, provides a good estimate of the upper limit of modifier surface concentration. Under ideal conditions for a reconstructed Aufilm surface the surface coverage for CP and MP was found to be 20-30% of a possible full layer, corresponding to Γ ) 3-4 × 10-10 mol cm-2. Therefore, the bulk film is expected to be largely vacant with the interstitial space occupied by solvent and ions. Sonication of a grafted film (on Au) changes the Au oxide peak uncontrollably, and it is impossible to interpret film coverage information. Au oxide reduction voltammetry, redox probe voltammetry, and contact angle measurements all confirm the findings of other workers that the grafted films include noncovalently bound material that can be removed during potential cycling and sonication. On the other hand, there is no evidence that phenyl groups directly bound to Au are lost during the same treatments, and we provide strong evidence to the contrary. The observed reversible increase and decrease of contact angle and redox probe responses for CP and MP films for solvents of different polarity and hydrophilicity is accounted for by the dynamic mechanical properties of the film. The flexibility of the porous films permits the solution-film interface to rearrange; thus, the wettability of the interface is a function of the solvent treatment. This may have important applications for microfluidic systems where thin films can be used to control fluid dynamics at the solution-film interface. Last, electrochemical redox probe scans can provide evidence for the nature of the film-solution interface. However, care has to be taken when using probe responses alone as it is easy to overinterpret the results to erroneously conclude that bulk

Acknowledgment. P.A.B. and M.G.P. thank Dr. John Loring for use of the Linkfit curve fitting software. Supporting Information Available: Voltammetry of NB electroreduction to Au and subsequent reduction of the nitro group to an amine group. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Grafting Aryl Diazonium Cations (30) Juodkazis, K.; Juodkazyte˘ , J.; Sˇ ebeka, B.; Lukinskas, A. Electrochem. Commun. 1999, 1, 315. (31) Gao, X.; Hamelin, A.; Weaver, M. J. Phys. ReV. Lett. 1991, 67, 618. (32) Ocko, B. M.; Helgesen, G.; Schardt, B.; Wang, J.; Hamelin, A. Phys. ReV. Lett. 1992, 69, 3350. (33) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929. (34) Silva, F.; Martins, A. Electrochim. Acta 1998, 44, 919. (35) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947.

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7815 (36) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837. (37) Xia, S. J.; Liu, G.; Birss, V. I. Langmuir 2000, 16, 1379. (38) D’Amours, M.; Be´langer, D. J. Phys. Chem. B 2003, 107, 4811. (39) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. (40) Saby, C.; Ortiz, B.; Champagne, G. Y.; Be´langer, D. Langmuir 1997, 13, 6805. (41) Surfactant Science Series; Berg, J. C., Ed.; Marcel Dekker, Inc.: New York, 1993; Vol. 49.