Reaction of Gold Substrates with Diazonium Salts in Acidic Solution at

Aug 21, 2009 - *To whom correspondence should be addressed: Tel 64-3-3642501, Fax 64-3-3642110, e-mail [email protected]., §...
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Reaction of Gold Substrates with Diazonium Salts in Acidic Solution at Open-Circuit Potential Joshua Lehr,†,‡ Bryce E. Williamson,† Benjamin S. Flavel,†,§ and Alison J. Downard*,†,‡ †

Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand, and ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, Private Bag 4800, Christchurch 8140, New Zealand. §Permanent address: School of Chemistry, Physics and Earth Sciences, Flinders University, Bedford Park, SA 5042, Australia. Received June 4, 2009. Revised Manuscript Received July 25, 2009

The reaction of gold substrates with p-nitrobenzene diazonium tetrafluoroborate (NBD) in 0.1 M H2SO4 at open-circuit potential (OCP) is demonstrated to proceed by electron transfer from gold to the NBD cation. Electrochemical, atomic force microscopy, and X-ray photoelectron spectroscopy analyses reveal the formation of multilayer films with the same composition as electrografted films. The film growth characteristics (surface concentration and film thickness vs time) also follow those observed during electrografting, consistent with electron transfer from the substrate to the diazonium cation. The OCP of the gold substrate increases during the period of film growth (∼60 min) and then decreases to close to its initial value. The increase corresponds to accumulation of positive charge as electrons are transferred to NBD; the discharge process is tentatively attributed to slow oxidation of adventitious impurities in the reaction solution. Films formed at OCP or by electrografting from aqueous acid solution are markedly less stable to sonication in acetonitrile than are those electrografted from acetonitrile. Increased amounts of physisorbed material in films prepared in aqueous media or bonding of aryl groups to different gold sites in the two media are tentatively proposed to account for the different stabilities.

Introduction Modification of surfaces with carefully designed nanoscale organic films is a promising approach for tailoring surface properties. Grafting molecular layers from solutions of aryldiazonium salts1,2 has been widely investigated for surface modification with applications such as corrosion protection,3 preparation of composite materials,4 sensor fabrication,5 and molecular electronics.6 The popularity of diazonium salts for surface modification is largely due to the assumed (or, in some cases, firmly established) covalent linkage between the film and the surface, which is expected to give stability advantages over physisorption methods. Electrochemically assisted grafting from diazonium salt solutions has been explored in many studies. Reduction of the diazonium cation at the electrode surface eliminates N2, forming the aryl radical which appears to attack the substrate to form a covalent bond.7 As further radicals are produced, they either graft directly to the substrate or attack already grafted aryl groups, building up a disordered, multilayer film structure.8 There is strong spectroscopic evidence for the presence of azo groups in such films;9-11 these may result from reaction of diazonium *To whom correspondence should be addressed: Tel 64-3-3642501, Fax 643-3642110, e-mail [email protected]. (1) Downard, A. J. Electroanalysis 2000, 12, 1085–1096. (2) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439. (3) Combellas, C.; Delamar, M.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Chem. Mater. 2005, 17, 3968–3975. (4) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. Carbon 1997, 35, 801–807. (5) Gooding, J. J. Electroanalysis 2008, 20, 573–582. (6) Ranganathan, S.; Steidel, I.; Anariba, F.; McCreery, R. L. Nano Lett. 2001, 1, 491–494. (7) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201–207. (8) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947–5951. (9) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Chem. Mater. 2007, 19, 4570–4575. (10) Toupin, M.; Belanger, D. Langmuir 2008, 24, 1910–1917. (11) Yu, S. S. C.; Tan, E. S. Q.; Jane, R. T.; Downard, A. J. Langmuir 2007, 23, 11074–11082.

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cations with radical centers in the growing film. Time-of-flight secondary ion mass spectroscopy of films formed on glassy carbon (GC) has established the existence of a covalent linkage between the film and the surface,12 and X-ray photoelectron spectroscopy (XPS) analysis of films electrografted to iron surfaces shows a signal attributed to a Fe-C bond.3,13 Several studies have demonstrated that, at a given grafting potential, growth of diazonium-derived films is self-limiting with respect to surface concentration and thickness.14,15 It is assumed that, as the film grows, electron transfer to the aryldiazonium cations occurs over an increasing distance, and hence its rate decreases as the film grows. Eventually the film becomes so thick that reduction essentially stops and the film reaches its maximum thickness. As an alternative to electrochemical grafting, simply immersing a substrate in a solution of an aryldiazonium salt can lead to formation of surface layers.16 This so-called spontaneous, or open-circuit potential (OCP) reaction, has been reported for a range of metals, semiconductors, and carbon materials. Detailed studies have been made for copper, iron, nickel, zinc, and carbon substrates,10,17-23 yielding interesting mechanistic insights that are pertinent to the present study. (12) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Langmuir 2005, 21, 280–286. (13) Boukerma, K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. Langmuir 2003, 19, 6333–6335. (14) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038–5045. (15) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791–8798. (16) Barriere, F.; Downard, A. J. J. Solid State Electrochem. 2008, 12, 1231–1244. (17) Adenier, A.; Barre, N.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Surf. Sci. 2006, 600, 4801–4812. (18) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Vautrin-Ul, C. Chem. Mater. 2005, 17, 491–501. (19) Chamoulaud, G.; Belanger, D. J. Phys. Chem. C 2007, 111, 7501–7507. (20) Hurley, B. L.; McCreery, R. L. J. Electrochem. Soc. 2004, 151, B252–B259. (21) 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, 370–378. (22) Le Floch, F.; Simonato, J.-P.; Bidan, G. Electrochim. Acta 2009, 54, 3078–3085. (23) Seinberg, J.-M.; Kullapere, M.; M€aeorg, U.; Maschion, F. C.; Maia, G.; Schiffrin, D. J.; Tammeveski, K. J. Electroanal. Chem. 2008, 624, 151–160.

Published on Web 08/21/2009

DOI: 10.1021/la902002n

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The groups of McCreery20 and Belanger19 examined modification of copper using aryldiazonium salts in acetonitrile (ACN) and aqueous acid solutions at OCP. Copper, as both Cu(0) and Cu(I), is well-known to reduce diazonium cations, yielding the radical. In McCreery’s work, copper surfaces were used with the native oxide layer present and also largely removed.20 Surface modification was examined in both ACN and 0.1 M H2SO4 solutions of diazonium salts, and in both cases, multilayer films were formed. Films formed on native oxide samples from H2SO4 solutions were boiled in H2O or sonicated in acetone. After both treatments, spectroscopic measurements showed a significant loss of material but that a fraction of the film resisted the harsh treatments, consistent with covalent bonding to the surface. XPS analysis directly confirmed the presence of Cu-O-C bonds, and there was strong indirect evidence for Cu-C bonds. Belanger’s study of spontaneous grafting at (mainly) oxide-free copper included reactions in ACN and acetic acid solutions of the p-nitrobenzene diazonium (NBD) cation, formed in situ in the grafting medium.19 These authors reported that reaction in the aqueous medium gave monolayer surface coverages, whereas modification in ACN resulted in multilayer films. The differences in surface coverages were attributed to more negative OCP values in ACN than in the aqueous medium and also to corrosion of the copper surface in acidic conditions. The presence of NaNO2 in the aqueous grafting solution may also have influenced film formation. Spontaneous modification of iron, zinc, copper, nickel, and GC substrates in ACN solutions of NBD has been investigated by Vautrin-Ul and co-workers.18 Surface films formed on all of these substrates, and spectroscopic analysis confirmed that the diazonium moiety was not present in the films. The films had similar properties to those generated by electrografting with respect to the dependence of film growth on reaction time and NBD concentration and the ability to block the electrochemistry of a solution redox probe. Although direct evidence for covalent bonding between substrate and film was not obtained, the stability of films to ultrasonic cleaning was taken to be strong evidence for covalent attachment. Later, the same authors examined the reactivity of a range of para-substituted benzenediazonium salts toward nickel, zinc, and iron substrates in ACN.17 They found that the extent and morphology of the surface films depended on both the substrate and the diazonium cation. Films formed most rapidly on the most easily oxidized substrate using the most easily reduced diazonium cation. It was concluded that film formation occurs via a redox reaction between the diazonium cation and the substrate, although it was noted that substrate-derived oxidation products were not detected. In a very recent of description of spontaneous grafting of diazonium salts at GC surfaces, Bidan and co-workers found that films were formed in ACN solutions of NBD, but not in solutions of amine derivatives, which are more difficult to reduce.22 These observations were shown to be consistent with the measured OCP of GC (in the absence of diazonium salt), which was sufficiently negative to reduce NBD but not the other diazonium salts. Gold has received much recent attention as a substrate for electrografted diazonium-derived films. Interest has focused on the characterization of the modified surfaces24-28 as well as the (24) Liu, G. Z.; Bocking, T.; Gooding, J. J. J. Electroanal. Chem. 2007, 600, 335– 344. (25) Liu, G. Z.; Liu, J. Q.; Bocking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136–146. (26) Shewchuk, D. M.; McDermott, M. T. Langmuir 2009, 25, 4556–4563. (27) Laforgue, A.; Addou, T.; Belanger, D. Langmuir 2005, 21, 6855–6865. (28) Paulik, M. G.; Brooksby, P. A.; Abell, A. D.; Downard, A. J. J. Phys. Chem. C 2007, 111, 7808–7815.

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use of the surfaces for sensor construction.5 There are several reasons for the use of gold in these studies: it is a familiar and wellunderstood material for surface modification via self-assembly of alkanethiols and has some advantages over carbon substrates for film characterization studies. It is compatible with standard microfabrication procedures and can be readily integrated into complex electrical and electronic devices. Furthermore, it does not have a stable surface oxide and can be easily prepared or obtained commercially with low surface roughness and (in principle) highly reproducible properties. Electrografting to gold appears to generate films with essentially the same properties as those formed at other substrates.27,28 Direct evidence for a covalent bond between the film and the substrate has not been obtained, but several studies have demonstrated that, when subjected to harsh treatment, only some of film is lost from the surface, suggesting the presence of both physisorbed and covalently attached material.26-28 Spontaneous modification of gold surfaces at OCP in ACN solutions of diazonium salts has been reported, but details of the reaction have not been explored.29,30 Recently, Combellas and coworkers compared OCP grafting of diazoates (spontaneously formed from diazonium cations at pH 10) with that of diazonium salts in 10 mM H2SO4. They established that the amount of film growth (during 60 min) is similar in both media and that the films show similar stability to sonication in acetone.31 The authors proposed that, at pH 10, grafting occurs via spontaneous homolytic dediazonation of diazoates, which produces aryl radicals. On the other hand, they pointed out that diazonium salts are stable in acidic solution, and the mechanism of spontaneous grafting under those conditions is unclear. In the present work we have examined the spontaneous reaction of aryldiazonium salts with gold in aqueous acid and ACN solutions. Grafting studies and film characterization were undertaken using the NBD salt since the resulting nitrophenyl (NP) group is conveniently detected electrochemically.14 We have focused on acid solution because, under our conditions, we found no evidence for reaction in ACN. Our study sought to address the following questions: when aqueous acidic solutions of diazonium salts are in contact with gold surfaces at OCP, is the diazonium moiety eliminated from the molecule, what is the role of the gold substrate during formation of surface layers, and is there evidence for a covalent bond between the film and the gold surface?

Experimental Section Chemicals and Substrates. Potassium ferricyanide and hypophosphorous acid (Riedel De Haen AG), sulfuric acid (Aldrich, 98%), hydrogen tetrachloroaurate (Alfa Aesar, 99.99%), ethanol (EtOH, Merck, >99.9%), and sodium perchlorate (Merck) were used as received. Aryldiazonium salts were prepared using literature methods,32 dried under vacuum, and stored in the dark. Acetonitrile (ACN HPLC grade) was dried over CaH2 (2 days) and refluxed under N2 (2 h) prior to distilling in a N2 atmosphere. Milli-Q water, resistivity >18 MΩ cm, was used for all aqueous solutions. All glassware and electrochemical cells were dried and stored at 60 °C. Au/NiCr/Si substrates were used in all experiments except for depth profiling by atomic force microscopy (AFM) and (29) Fan, F.-R. F.; Yang, J.; Cai, L.; Price, David W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 5550–5560. (30) Fan, F.-R. F.; Yang, J.; Dirk, S. M.; Price, D. W.; Kosynkin, D.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 2454–2455. (31) Podvorica, F. I.; Kanoufi, F.; Pinson, J.; Combellas, C. Electrochim. Acta 2009, 54, 2164–2170. (32) Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Compounds, 3rd ed.; Edward Arnold: London, 1985.

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Lehr et al. accompanying surface concentration measurements. They were prepared by depositing 30 nm of thermally evaporated nichrome onto precut (10 mm  10 mm) Si(100) wafers (Silicon Quest and Micro Materials), followed by 250 nm of thermally evaporated gold. Au/mica substrates were prepared by thermally evaporating 250 nm of gold onto mica. Metal disks were then cemented to the exposed gold face using Epoxi-Patch (Dexter), and the assemblies were cured at 60 °C for 24 h. Subsequently, the mica was cleaved from the gold by immersion of the disks in liquid N2, leaving smooth gold surfaces. Surface Modification at OCP. Surface modifications in aqueous media were carried out in 20 mL of freshly prepared 10 mM diazonium salt in 0.1 M H2SO4. Modification was also investigated in 20 mL of 10 mM diazonium salt in dry ACN (in both the presence and absence of dry 0.1 M [Bu4N]BF4). All reactions were preformed in a controlled temperature room at 25 ( 2 °C in the absence of light. Solutions were degassed with N2 prior to immersion of the substrate and maintained under a flow of N2 for the duration of the experiment. After modification, samples were sonicated in Milli-Q water for 5 min. Electrochemistry. Cyclic voltammograms were recorded using an Ecochemie Autolab PGSTAT 302 potentiostat and a three-electrode cell with the gold substrate as working electrode, a saturated calomel electrode (SCE) reference electrode, and a Pt wire auxiliary. Solutions were degassed with N2 prior to electrochemical measurements and maintained under a flow of N2 during electroanalysis. Electroreduction of NP films on Au/NiCr/Si substrates was carried out while exposing the entire gold electrode area (0.79 cm2) to electrolyte solution. All other electrochemical measurements were undertaken by mounting the gold electrode horizontally on an insulated metal base plate under a glass solution cell held in place by four springs. A hole in the cell bottom was centered over two concentric Viton O-rings, which formed seals against the Au surface. The smaller O-ring defined the geometric area of the gold working electrode (0.18 cm2). The larger O-ring secured a thin Cu strip to the gold for electrical contact. NP films were electroreduced using two cyclic scans between þ0.6 and -1.2 V (scan rate = 100 mV s-1) in 0.1 M NaClO4 in EtOH-H2O (1:9 by volume). The surface concentration of electroactive NP groups was determined based on the peaks for reduction of NP groups and oxidation of hydroxyaminophenyl groups obtained during the first cyclic scan (see Results and Discussion). The charges associated with each redox reaction were determined from peak areas calculated using Linkfit curve fitting software as described previously.14 Taking account of the relationship between the NP reduction peak and the hydroxyaminophenyl oxidation peak and the number of electrons involved in each redox process, the charge was converted to surface concentration of electroactive NP groups. The blocking properties of modified surfaces were examined by cyclic voltammetry of 1 mM K3[Fe(CN)6] in 0.1 M phosphate buffer pH 7.4 with scan rate = 100 mV s-1. OCP measurements were made using a Digitech QM1320 multimeter. Other Apparatus and Analyses. AFM (Digital Instruments Dimension 3100) depth profiling measurements and the data treatment procedure to determine NP film thickness were carried out by a method described in detail elsewhere.14 Briefly, four scans of a CSC 12 (Ultrasharp) chip were used to remove two 10  1.25 μm2 sections of film from each sample. For each section, three transverse cross sections were chosen from a 10  10 μm2 image to yield three average line profiles. In turn, each profile gave two film thicknesses: one from the step on the right side of the section and the other from the left. Thus, the datum reported for each sample is the mean of 12 values, and the uncertainties are two standard deviations of the mean. X-ray photoelectron spectra (XPS) were obtained from gold surfaces using a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al KR source (1486.6 eV), operated at 150 W. Narrow scans were recorded with a step size of 0.1 eV and Langmuir 2009, 25(23), 13503–13509

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Figure 1. Repeat cyclic voltammetric scans (100 mV s-1), obtained in 0.1 M NaClO4/EtOH-H2O, of a Au/NiCr/Si electrode modified by immersion in aqueous NBD solution for 60 min: (;) first scan; (---) second scan. pass energy of 20 eV. Peak positions were referenced to aromatic carbon at 284.7 eV. Atomic absorption spectroscopy (AAS) analysis for gold was carried out using a Varian Spectra AA 220FS at a wavelength of 242.8 nm. The spectral slit width was 1 nm, and the lamp current was 4 mA. Calibration standards (100, 200, 400, and 600 ppb) were prepared using HAuCl4 in 0.1 M H2SO4.

Results and Discussion Grafting from Diazonium Salt Solutions. Gold (Au/NiCr/ Si) substrates were immersed in NBD solution as described above and, after removal from solution and sonication, were mounted in an electrochemical cell. Figure 1 shows repeat cyclic voltammograms, in a 0.1 M solution of NaClO4 in EtOH-H2O (1:9 by volume), of an electrode after immersion in aqueous NBD solution for 60 min. The first scan shows the characteristic, irreversible reduction (Ep=-0.92 V) of the NP group to aminophenyl (eq 1) and hydroxyaminophenyl groups (eq 2) and, on the return scan, oxidation (Ep =-0.3 V) of hydroxyaminophenyl to nitrosophenyl groups (eq 3). surface---Ph-NO2 þ 6Hþ þ 6e - f surface---Ph-NH2 þ 2H2 O

ð1Þ surface---Ph-NO2 þ 4Hþ þ 4e - f surface---Ph-NHOHþ H2 O ð2Þ surface---Ph-NHOH T surface---Ph-NO þ 2Hþ þ 2e - ð3Þ On the second cycle, the only significant feature is the nitrosophenyl/hydroxyaminophenyl redox couple. These voltammograms demonstrate that the gold surface has been modified with NP groups at OCP. The absence of evidence for reduction of the diazonium moiety (NBD is reduced at Ep ≈ 0.17 V in this medium) indicates that the intact NBD cation is not simply physisorbed to the surface, a finding that is confirmed by XPS (see below). Furthermore, electrochemical analysis of gold surfaces immersed in 10 mM p-nitroaniline in 0.1 M H2SO4 for 60 min showed no indications of surface modification, confirming that the diazonium moiety is essential for the formation of NP surface layers. Electrochemical analysis of gold substrates after immersion for 120 min in 10 mM solution of NBD in dry ACN (in the presence or absence of dry 0.1 M [Bu4N]BF4) revealed no redox processes within the solvent limits, indicating that film formation does not proceed under these conditions. In comparison, Tour and coworkers reported modification of gold surfaces after 24 h immersion in ACN solutions of diazonium salts.29,30 The use of different reaction times may account for the contrasting results, but we DOI: 10.1021/la902002n

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Figure 2. Plots of (a) electrochemically determined surface concentration and (b) AFM-determined film thickness as functions of immersion time for gold surfaces in 10 mM NBD-0.1 M H2SO4 at 25 °C in an N2 atmosphere and in the absence of light. (c) Plot of surface concentration vs film thickness for NP films spontaneously formed on gold. Data were obtained at Au/NiCr/Si (b, 2) and Au/mica (O) substrates. Regression line forced through zero with slope = 7.6  10-10 mol cm-2 nm-1, R2 = 0.94, and standard deviation = 0.43  10-10 mol cm-2 nm-1.

have focused our attention of modification in aqueous acid where the reaction appears to be substantially more facile. Film Growth and XPS Characterization. Film growth in aqueous solutions was monitored electrochemically and by AFM for samples removed from solution at selected times. Surface concentrations of electroactive NP groups were estimated from the integrated charges associated with the NP reduction and hydroxyaminophenyl oxidation waves in the cyclic voltammograms (eqs 1-3 and Figure 1). AFM depth profiles were measured for samples after sonication and drying. Figure 2a shows the NP surface concentration vs immersion time data obtained from 15 experiments;ten using Au/NiCr/Si substrates and five using Au/mica substrates. The surface concentration is seen to increase over 60 min to a constant, or near constant, value of ∼14  10-10 mol cm-2. Film thickness, determined by AFM depth profiling of the Au/mica samples, exhibits a very similar dependence (Figure 2b), reaching a value of ∼1.6 nm after 60 min. In comparison with an optimized monolayer thickness of 0.79 nm (calculated using Spartan) for NP groups oriented perpendicular to the surface, the last result is indicative of a multilayer structure. Figure 2c shows that there is a near-linear relationship between surface concentration and 13506 DOI: 10.1021/la902002n

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film thickness with a film density that remains constant throughout film growth at ∼(7.6 ( 1.5)  10-10 mol cm-2 nm-1. The estimated (20% uncertainty in the density arises not only from the error of the regression line in Figure 2c but also from the uncertainty of establishing an appropriate baseline when determining peak areas from cyclic voltammograms and the errors associated with AFM depth profiling (Figure 2b). Using the calculated monolayer thickness of 0.79 nm, the surface concentration is (6.0 ( 1.2)  10-10 mol cm-2 for a film thickness corresponding to a single layer of NP groups. The plots in Figure 2 are qualitatively the same as those reported for NP groups electrografted in aqueous acid to pyrolyzed photoresist film (PPF) at an applied potential 150 mV negative of the diazonium cation reduction peak. In that case, film growth was found to be self-limiting at a surface concentration of ∼6  10-10 mol cm-2 and a thickness of ∼2 nm.14 From the full set of surface concentration vs film thickness data, the surface concentration was estimated to be (2.5 ( 0.5)  10-10 mol cm-2 for a film thickness equivalent to a monolayer. In another study, the surface concentrations of carboxyphenyl and methylphenyl groups electrografted to gold surfaces from ACN solutions were estimated to be in the range of (3-4)  10-10 mol cm-2.28 As a third comparison, after 120 min immersion of PPF in acidic 10 mM NBD solution at OCP, NP films reached a thickness of 3.8 nm with a surface concentration of (3.2 ( 0.5)  10-10 mol cm-2 for film thickness equivalent to a monolayer.33 The films formed at OCP in the present study have a higher packing density than both their electrografted counterparts and the film formed spontaneously on PPF, but this may be due, at least in part, to small differences in substrate surface roughness and postgrafting treatment (cleaning) of the films (see below). We note that the packing densities for the films in the present study and the others described above are significantly less than that calculated for a close-packed layer of NP groups on a flat surface (∼12  10-10 mol cm-2).7 Low packing density appears to be a feature of diazonium cation-derived films and presumably arises from both the irreversible and (at least partly) unselective reactions of aryl radicals and the branched multilayer film structure.34 XPS was used to characterize the films formed at OCP, before and after electroreduction. Figure 3a shows a N 1s core level spectrum obtained at a gold surface after immersion in aqueous NBD solution for 240 min; Figure 3b shows the spectrum of a similarly modified surface after reduction in 0.1 M NaClO4/ EtOH-H2O. The spectrum prior to reduction shows strong signals at 405.6 eV (assigned to the NP group) and 400.1 eV, along with a very weak signal at 402.6 eV. The latter was also observed in electrografted NP films on GC and is attributed to trace amounts of hydroxyaminophenyl groups generated during grafting.11 Importantly, the XPS spectrum is not consistent with the presence of significant amounts of unreacted diazonium cations, which would give signals at 403.8 and 405.1 eV.35 We and others have previously shown that the 400.1 eV signal can be assigned to azo groups introduced to surface during film growth.9,11,36 From the integrated signal intensities, ∼35% of the total film nitrogen is present in this form. In earlier work concerning electrografted NP films on GC, similar analysis revealed that 34-42% of the total nitrogen was present as azo groups,11 while recently published XPS spectra of NP films electrografted to gold indicates a similar proportion.26 Hence, (33) 2007. (34) (35) (36)

Garrett, D. J.; Lehr, J.; Miskelly, G. M.; Downard, A. J. J. Am. Chem. Soc. Downard, A. J. Int. J. Nanotechnol. 2009, 6, 233–244. Finn, P.; Jolly, W. L. Inorg. Chem. 1972, 11, 1434–1435. Lyskawa, J.; Belanger, D. Chem. Mater. 2006, 18, 4755–4763.

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Figure 4. OCP of gold substrates vs immersion time in 0.1 M H2SO4. After 60 min (arrow), NBD was added to solution (a) (b) to give a final concentration of 10 mM; p-nitroaniline was added to solution (b) (1) to give a final concentration of 10 mM, and “blank” 0.1 M H2SO4 was added to solution (c) (O).

Figure 3. N 1s core level spectra obtained at a gold surface (a) after immersion in NBD solution for 240 min followed by the standard rinsing and sonication procedure and (b) a surface modified as in (a) and subsequently reduced (two scans between þ0.6 and -1.2 V) in 0.1 M NaClO4/EtOH-H2O.

in this respect also, films formed by spontaneous reaction at gold appear similar to those electrografted to gold and GC substrates. In the XPS spectrum after reduction (Figure 3b), the signals between 399.4 and 401.5 eV correspond to the reduced N moieties: azo, amino, ammonium, and hydroxylamino. The weaker signal at 405.2 eV reveals the presence of NP groups, which account for ∼17% of those initially present. Hence, we deduce that ∼17% of NP groups are electroinactive and that our electrochemically determined surface concentrations (Figure 2a) are underestimated by a corresponding amount. Very similar data were obtained for NP films electrografted to glassy carbon (GC) from ACN solutions and subsequently electrochemically reduced in 0.1 M KCl/EtOH-H2O,11 confirming that films prepared by the two routes on the two different substrates have a similar degree of electroactivity. The Role of the Substrate in Film Formation. As noted in the Introduction, spontaneous grafting at OCP onto copper, nickel, zinc, iron, and GC substrates appears to proceed via reduction of the diazonium cation by the substrate.17-20,22 The measured OCP values are assumed to give an indication of the reducing power of the substrate, and these values are consistent with the observed film formation. This immediately suggests the possible involvement of gold in the spontaneous surface derivatization observed in this work. Figure 2a,b shows that film formation at OCP on Au has qualitatively the same characteristics as for electrografted films; that is, the rate of growth decreases with time, and the film thickness is self-limiting. These characteristics are consistent with a model whereby electron transfer from the substrate underlies Langmuir 2009, 25(23), 13503–13509

both spontaneous and electrochemically assisted film growth, with the film constituting an increasingly resistive blocking layer as it grows. However, an alternative explanation for limiting behavior during the spontaneous reaction is depletion of NBD over the relatively long time scale of the reaction. To test the latter possibility, NP films were formed for 60 min on Au at OCP using standard conditions. The samples were then transferred to a freshly prepared NP solution of the same composition, in which they were immersed for a further 180 min. Electrochemical analysis after the second immersion gave surface concentrations that were the same, within experimental uncertainty, as for films formed by a single 240 min immersion (Figure 2b). To test the possibility that further film growth had occurred during the second immersion but could not be detected electrochemically, another sample was spontaneously modified in 5 mL of grafting solution for 60 min, at which time a 100 μL aliquot of 1 M hypophosphorous acid was added. Hypophosphorous acid is wellknown to reduce diazonium salts and to lead to film grafting.37 Electrochemical analysis of these samples gave a surface concentration of 40  10-10 mol cm-2, demonstrating that any additional film growth can be detected electrochemically. Hence, it can be concluded that the measured surface concentration of films grown at OCP is not limited by factors associated with the age of the diazonium salt solution, nor the detectability of the NP groups. Direct evidence for the involvement of the substrate in film growth was obtained by monitoring the OCP of gold substrates in NBD and control solutions (Figure 4). Three samples were “equilibrated” in 0.1 M H2SO4 (10 mL) for 60 min, during which their OCPs reached near-steady baseline values ranging over ∼100 mV (all samples tested fell within a 150 mV range in OCP values). Between-sample variation in the OCP is not surprising because the gold electrode cannot reach equilibrium in this medium (there is no redox couple controlling the potential of the gold electrode), and the OCP will be sensitive to surface structure, cleanliness, and residual solution O2 concentration.38 After equilibration,10 mL aliquots of 0.1 M H2SO4 containing 20 mM NBD or 20 mM p-nitroaniline or a “blank” aliquot of 0.1 M H2SO4 were added to each solution to afford samples (a), (b), and (c), respectively. As shown in Figure 4, additions of p-nitroaniline or the blank had no effect on the slow decay of the OCP. But, in stark contrast, addition of NBD caused a sudden jump of ∼80 mV, followed by a slower increase of about 200 mV over 60 min and then a decay over ∼180 min to a value close (37) Pandurangappa, M.; Lawrence, N. S.; Compton, R. G. Analyst 2002, 127, 1568–1571. (38) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley: New York, 2001; pp 5-9.

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Figure 5. Cyclic voltammograms of gold substrates in 0.1 M H2SO4 with scan rate=100 mV s-1: (a) bare gold (initial potential=0 V); (b) after immersion in NBD solution for 120 min (initial potential=0.9 V).

to those of the control samples. This behavior was reproducible (using freshly prepared gold substrates), although the absolute potentials were variable, probably for the reasons described above. An increase in the OCP of a conductor is indicative of the accumulation of positive charge on its surface. The behavior of the OCP for sample (a) in Figure 4 indicates that at least two processes follow addition of NBD: the first is responsible for accumulation of positive charge and the second for the discharge of that accumulation. The first process is assigned to electron transfer from the gold to the diazonium cation. The discharge of accumulated positive charge is an oxidation process whose details remain unclear. The OCP values seem too low to be connected to formation of gold oxide or soluble salts of Au(I) or Au(III), but nevertheless we investigated these possibilities experimentally. Scan (a) of Figure 5 shows the voltammetric response (initial potential=0 V, scan rate=100 mV s-1) of an unmodified gold substrate in 0.1 M H2SO4 with gold oxide formation and reduction peaks clearly evident at ∼1.2 and ∼0.7 V, respectively. Scan (b), with an initial potential of 0.9 V, was obtained from a gold substrate after immersion in NBD solution for 120 min under standard conditions. There is no evidence for gold oxide reduction, indicating that insignificant amounts of gold oxide result from growth of the NP film. An identical voltammogram was obtained after 30 min immersion. For an electrode area of 0.18 cm2, the formation of a film with a surface concentration of 14  10-10 mol cm-2 would require ∼25 μC of transferred charge. If this were associated, stoichiometrically, with the formation of Au2O3, a reduction wave with the same integrated charge would be expected at ∼0.7 V. The charge associated with the reduction wave in scan (a) is 71 μC, and so as little as 10 μC would be easily detected in scan (b). From its absence, we conclude that the discharge process is not associated with the formation of gold oxide. Another possibility is that the discharge is due to the loss of gold ions into solution. To investigate this, a high surface area (∼28 cm2) crumpled gold wire electrode was placed in 2.5 mL of NBD solution. After immersion for 240 min, the electrode was removed and AAS was used to analyze the reaction solution for gold. The AAS detection limit of ∼100 ppb is an order of magnitude lower than the Au(III) concentration of ∼1000 ppb expected for a stoichiometric reaction associated with formation of an NP film with surface concentration 14  10-10 mol cm-2. In fact, no gold was detected, and hence it can be concluded that soluble gold salts are not generated as a major oxidation product. At this time, we tentatively assign the discharge process to oxidation of adventitious impurities in the reaction solution. The role of the gold substrate is thus essentially to mediate transfer of electrons from solution impurities to the diazonium cation. Of course, the situation is more complex than this because film formation accompanies electron transfer. 13508 DOI: 10.1021/la902002n

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The time scale of 60 min in Figure 4 for the increase in the OCP of the gold surface after introduction of NBD coincides with the period over which film growth reaches a maximum in the electrochemical and AFM experiments (Figure 2a,b). This is consistent with the proposed role of the substrate in film growth. While the OCP is low enough, electrons can be transferred to the diazonium cation and the film grows. As the OCP becomes too positive for NBD reduction, film growth slows to a stop with the oxidation of solution impurities, which is presumably concomitant with, but slower than, reductive film growth, becoming dominant. It might be expected that film growth would resume as the potential drops; however, the thicker film in the discharge time regime would have an insulating effect, slowing transfer of electrons from the metal to the film surface where any further growth would be expected. In fact, the scatter of data in Figure 2a makes it difficult to equivocally rule out film growth after 60 min, and a slow increase in surface concentration is possible. The OCP-time behavior during the charge-accumulation period of Figure 4 is qualitatively similar to that reported for OCP grafting of NBD to GC in 0.1 M [Bu4N]BF4-ACN22 and of the diazonium salt of 1-aminoanthraquinone to Vulcan XC72 carbon black.39 However, in neither of those earlier studies was the longer term behavior monitored, and hence the existence of a “spontaneous” discharge processes is untested. The data in Figure 4 are also strikingly similar to OCP vs time data obtained during the self-assembly of alkanethiols at gold surfaces in 0.1 M KOH/ETOH-H2O solution, except that the potential changes for that system were in the opposite sense and much faster: ∼40 s of negative charge accumulation and 60 s of discharge.40 Assembly of alkanethiols occurs with the transfer of negative charge from the sulfhydryl group to the gold, and the discharge process has been attributed to reduction of protons, which are highly mobile in aqueous solution.40 The much slower discharge in our experiments is consistent with the oxidation of adventitious and less mobile impurities at the film-coated electrode. It is interesting to note that, for the case of alkanethiols on gold surfaces, several authors have attempted to account for the magnitude of the potential shift during self-assembly.40,41 They showed that use of a simple parallel plate capacitor model gives a predicted shift in potential that is an order of magnitude greater than that observed and concluded that a better model is required. On the basis of this earlier work, it is evident that the parallel plate capacitor model would also greatly overestimate the potential shift expected in our system, and hence the relationship between the amount of film formed and the magnitude of the potential shift remains unclear. Further support for the proposed film formation mechanism from diazonium salt solution was obtained from the spontaneous reaction of p-methoxybenzene diazonium (MeOBD) salt. With a reduction peak potential of approximately -0.11 V vs SCE (1 mM in 0.1 M H2SO4, scan rate = 100 mV s-1), MeOBD is considerably more difficult to reduce than NBD (þ0.15 V under the same conditions). Cyclic voltammograms of Fe(CN)63- were obtained (not shown) for gold substrates before and after immersion in acidic solutions of MeOBD for 240 min under the same conditions as used for the NBD reactions. Voltammograms recorded at the “modified” surfaces were highly irreproducible: some were essentially the same as for bare gold, while others showed large peak separations or even no peaks attributable to Fe(CN)63-. (In contrast, cyclic voltammograms obtained at gold (39) Smith, R. D. L.; Pickup, P. G. Electrochim. Acta 2009, 54, 2305–2311. (40) Zhong, C.-J.; Woods, N. T.; Dawson, G. B.; Porter, M. D. Electrochem. Commun. 1999, 1, 17–21. (41) Cohen-Atiya, M.; Mandler, D. J. Electroanal. Chem. 2003, 550, 267–276.

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surfaces after 240 min reaction with NBD at OCP consistently showed very low background currents and no evidence for reduction of Fe(CN)63- over a wide potential range.) Evidently, film formation from MeOBD solutions varied from insignificant to films sufficiently thick to substantially slow electron transfer to the redox probe. We attribute this finding to sample-to-sample variations in the base OCP of the gold substrates (Figure 4) in combination with the low reduction potential of MeOBD. For some samples, the base OCP of the substrate was low enough for electron transfer to MeOBD to occur at a significant rate, while for other substrates with higher OCP values, the rate was negligible. Finally, we note that addition of NBD to ACN solutions (with or without added [Bu4N]BF4) caused no change in the OCP of the immersed gold substrate over 300 min, consistent with our findings that the spontaneous reaction does not give an electrochemically detectable film under those conditions. Stability of Films. Direct evidence for covalent bonding between aryl groups and gold substrates has not yet been obtained, but film stability can be used as a general indicator of the strength of film attachment. Shewchuk and McDermott have undertaken a careful study of NP films electrografted to gold from ACN solutions.26 They propose a multilayer film structure comprising physisorbed species (possibly dimers formed by radical-radical coupling in solution) and a smaller proportion of chemisorbed (covalently bound) aryl groups. The former can be removed by extended sonication or refluxing in ACN. The latter, which are resistant to the refluxing and sonication, were classified into two categories: those that could not be displaced by long-chain alkanethiols were proposed to be strongly bound to high-energy step sites while others more weakly bonded to terraces could be displaced by alkanethiols. This model is broadly consistent with both Belanger’s27 and our28 earlier findings that films electrografted to gold comprise some material that is relatively easily removed from the surface and some that is strongly attached. To test the stability of NP films formed on gold at OCP, surface concentrations of electroactive NP groups were measured after 30 min sonication in ACN and compared with those of films subjected to the standard sonication procedure (5 min in H2O). The experiments were repeated with films electrografted from 0.1 M H2SO4 solution and from 0.1 M [Bu4N]BF4-ACN. Films with approximately the same surface concentration were prepared in each case. The results in Table 1 show that for all films sonication for 30 min in ACN removes a significant amount of material not removed by sonication for 5 min in H2O. Furthermore, 30 min sonication in ACN removes significantly more of the material from films prepared in aqueous acid (by both electrografting (84%) and at OCP (83%)) than by electrografting in ACN (61%). Thus, it appears that the medium for film preparation, rather than the method (spontaneous vs electrochemical), is the important factor determining film stability. While there is no firm evidence that stability to 30 min sonication in ACN is an accurate indicator of covalent attachment (especially since Au-C covalent bonding is expected to be weak42), it is clear that there is a major difference between films (42) Jiang, D. E.; Sumpter, B. G.; Dai, S. J. Am. Chem. Soc. 2006, 128, 6030– 6031.

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Article Table 1. Effect of Sonication Time and Medium on the Surface Concentration (Γ) of NP Films Γ (10-10 mol cm-2)a film preparation

5 min, H2O

30 min, ACN

electrografted in ACN 10.9 4.3 electrografted in aqueous acid 11.5 1.8 grafted at OCP in aqueous acid 11.0 1.9 a Surface concentration of film remaining after sonication procedure.

prepared in the two media, and we tentatively suggest that films prepared in aqueous acid are largely physisorbed. A film structure that incorporates a large proportion of physisorbed material may explain why our spontaneously formed NP films have a greater density than films electrografted from ACN solutions. Physisorption may be more important in aqueous acid than in ACN solution because dimers or oligomers formed in solution (but near the surface) will have a lower solubility in the aqueous medium. Physisorption will block the surface and hence prevent direct reaction of radicals with the surface. An alternative explanation for the different stabilities of films prepared in aqueous acid and ACN is that gold surface rearrangement during reaction may result in aryl groups bonding to different surface sites in the two media. In line with Shewchuk and McDermott’s model that the strength of bonding depends on the nature of the surface sites (step vs terrace),26 differences in the relative proportions of such sites would be reflected as apparently different film stabilities in the two media.

Conclusions Nitrophenyl films form on gold surfaces by spontaneous reaction with NBD at OCP in aqueous acid. Gold transfers electrons to the NBD cation and positive charge accumulates on the substrate as the film grows. The accumulated charge is discharged through a process tentatively assigned to oxidation of adventitious impurities in solution. Films prepared at OCP show growth behavior and composition that are very similar to those electrografted from ACN solution, but their stability to extended sonication in ACN is significantly less. This effect is correlated with the reaction medium, and it is proposed that films prepared in aqueous H2SO4 may incorporate substantially greater proportions of physisorbed species. Alternatively, the origin of the difference may be that different surface rearrangement occurs during reaction in the two media, resulting in aryl groups bonding predominantly to different surface sites. In ongoing work we are investigating the stabilities of films prepared in other media and on alternative substrates. Acknowledgment. This work was supported by the MacDiarmid Institute for Advanced Materials and Nanotechnology. J.L. thanks the Tertiary Education Commission for a doctoral scholarship, and B.S.F. thanks the Australian Government’s Endeavour Research Fellowship program. We thank Dr. John Loring for use of Linkfit curve fitting software and Dr. Bryony James, University of Auckland, for XPS measurements.

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