ARTICLE pubs.acs.org/JPCC
Grafting of Thin Organic Films by Electrooxidation of Arylhydrazines Kristoffer Malmos,† Joseph Iruthayaraj,† Ryosuke Ogaki,‡ Peter Kingshott,§ Flemming Besenbacher,‡ Steen U. Pedersen,*,†,‡ and Kim Daasbjerg*,†,‡ †
Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark § Industrial Research Institute Swinburne (IRIS), Swinburne University of Technology, 543-545 Burwood Road, Hawthorn, Victoria 3122, Australia ‡
bS Supporting Information ABSTRACT: Anodic oxidation of arylhydrazines in aqueous solution leads to formation of a chemisorbed monolayer on the electrode surface of both glassy carbon and gold. The grafting process occurs in a three-electron process, in which the arylhydrazine is oxidized to the actual grafting agent, that is, the corresponding aryl radical, via the formation of the aryldiazene as intermediate. Analysis of the modified surface by X-ray photoelectron spectroscopy and polarization modulation infrared reflection absorption spectroscopy shows conclusively that the bonding to the surface does not occur through the hydrazine group but rather through the aryl group after expulsion of nitrogen. In general, the method is found to be tolerant toward variations in the experimental conditions, but at high pH (>7), the grafting rate increases considerably and multilayers may be formed. The more efficient grafting at high pH can be attributed to an increase of the rate of the deprotonation steps in the oxidation mechanism, the result of which will be that the aryl radical formation takes place close to the surface to be modified. In general, the monolayer exhibits a low susceptibility toward further grafting due to a lowering of the electron transfer rate and the occurrence of physisorption. In particular, at high concentrations of the arylhydrazine, physisorbed products formed during the grafting process are layered on top of the chemisorbed film, hence enhancing the blocking effect and precluding further growth.
’ INTRODUCTION In the search for modified surfaces with high electrochemical and chemical stability, much focus has been on covalently anchored layers of organic molecules. Self-assembled monolayers (SAMs) of thiols on gold have an advantage in terms of equilibrium formation of a well-ordered film but the lack of stability due to the relatively weak AuS bond is a severe drawback for this approach.1 In recent years several electrochemically based methods have been developed to form more strongly attached layers on substrates using grafting precursors such as aryldiazonium,2 diaryliodonium,3,4 or triarylsulfonium5 salts and alkyl6 or aryl amines.7 The first three methods are employing the aryl radical as grafting agent while the reactive intermediate in the case of amines is the aminyl radical. Both kinds of radicals are highly efficient grafting agents which, in turn, makes it difficult to control the process and hence the layer formation. The by far most common approach of the above-mentioned is the electroreduction of aryldiazonium salts which has been employed on a large variety of conducting materials such as carbon, metals, and semiconductors.2 The electroreduction can r 2011 American Chemical Society
be performed either by applying a fixed potential8 or by sweeping the potential9 at the surface to be modified. For successful modification of nonconducting materials, a reducing agent such as hypophosphorous acid in solution may be used in the presence of high concentrations of the diazonium salt.10 The reductions lead to the formation of the corresponding aryldiazenyl radicals which decompose rapidly by expulsion of nitrogen to generate aryl radicals.11,12 The radicals will either combine with each other, react with the solvent or the parent diazonium salt, or diffuse to and react with the substrate surface. This procedure usually leads to disordered multilayer formation, unless special precautions are taken.1318 Recently, we have introduced a new method for forming monolayers of aryl groups on glassy carbon (GC) and gold through a mild oxidation of arylhydrazines in water.19 This method is complementary to the diazonium salt approach which Received: March 29, 2011 Revised: June 1, 2011 Published: June 21, 2011 13343
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Scheme 1. Proposed Mechanism for the Anodic Oxidation of Arylhydrazines
is carried out under mild reductive conditions. Electrografting of amines is like for the arylhydrazines taking place under oxidative conditions but at much more positive potentials,6,7 where most metals will be corroded. The new method was found to be tolerant toward variations in the experimental conditions, that is, grafting time, grafting potential, the nature of the substituent on the aryl group, and to a large extent pH. Hence, it is a very promising methodology, if the aim is to form thin, well-defined, and covalently assembled aryl layers on surfaces. Concerning the mechanistic aspects a number of studies have been published on the chemical2025 and electrochemical2628 oxidation of hydrazine derivatives and, in general, there is agreement on the presence of aryl radicals as intermediates.21,22 Most studies also agree on having aryldiazenes (Ar—NdNH) and/or aryldiazenyl radicals (Ar—NdN 3 ) as precursors of the aryl radicals.24,29 Specifically, the aryl radicals are suggested to be formed by homogeneous3033 or anodic oxidation of aryldiazenes, followed by a first-order cleavage reaction.11 The unstable arylhydrazinyl radical (Ar-NH-NH 3 ) intermediate has also been trapped.22 In preparative experiments the type of products isolated is strongly dependent on the solvent. In organic solvents they consist mainly of arenes and for certain substituents also azobenzenes,30,32,33 while in aqueous solvents biphenyl and phenylderived products are found.21 Other studies suggest based on the electron stoichiometry that the anodic oxidation of arylhydrazines proceeds via the aryldiazene to the corresponding aryldiazonium salt followed by a nucleophilic attack of water (solvent) to form phenolic compounds.28,34 In general, electroanalytical investigations are greatly affected by products adsorbing to the electrode surface.26,35 The layer formed has been described as being impossible to remove by any means other than mechanical polishing.28 The adsorbed layer identified as phenolic derivatives was suggested to be formed through the diazonium salt intermediate.35 Kuhr and co-workers have proposed a method for modification of surfaces either by reduction or oxidation of biotinhydrazide.36,37 They suggested that the oxidative mechanism of the grafting of the latter resembled that of amine grafting with the intermediate formation of aminyl radicals, or in a more recent publication that of diazonium grafting.38 Hazard et al. have proposed the intermediacy of an acylium cation from the direct oxidation of hydrazides.39 Concerning the nature of the bonding between the surface and the organic moiety, covalent bond formation between aryl groups and carbonaceous material has been proposed for fullerenes and carbon nanotubes heated in a solution containing arylhydrazine derivates.4042 In addition, formation of a σ-type bond between aryl groups and gold salts was verified by infrared and Raman spectroscopy in a process involving oxidation of arylhydrazines.43 The grafting mechanism used as a working hypothesis in our previous study includes both the aryldiazene and aryl radical as
intermediates in the anodic oxidation of arylhydrazine (Scheme 1).19 In this work our aim is to look further into this mechanism and, in particular, clarify the reasons for the preference of monolayer formation under a variety of experimental conditions. The question to be addressed is, how this can be the case, if the grafting as for the aryldiazonium approach involves highly reactive aryl radicals? Cyclic voltammetry was used as the main technique to perform and analyze the grafting on GC and Au of six parasubstituted arylhydrazines (R = OCH3, H, Br, Cl, COOH, or NO2; 1af) and the corresponding aryldiazene intermediate 2f at different pH values in water. Chronoamperometry was applied to determine the number of electrons involved in the overall oxidation process. Ellipsometry was employed to measure the thickness of the layer formed and through this prove the existence of a physisorbed layer. The grafted film was further investigated using surface sensitive spectroscopy such as X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The anodic oxidation of arylhydrazine and subsequent covalent surface modification were also undertaken in an aprotic polar solvent such as dimethyl sulfoxide (DMSO) to show the diversity of the process.
’ EXPERIMENTAL SECTION Chemicals. 4-Methoxyphenylhydrazine (1a), phenylhydrazine (1b), 4-bromophenylhydrazine hydrochloride (1c), and 4-chlorophenylhydrazine hydrochloride (1d) were purchased from Sigma-Aldrich and 4-nitrophenylhydrazine (1f) from Struers. They were all used as received without further purification. 4-Hydrazinylbenzoic acid (1e) was synthesized following a published procedure.44 4-Nitrophenyldiazene (2f) was synthesized from 1f by homogeneous oxidation using ferricyanide as oxidizing agent (see Supporting Information).24 Acetonitrile (anhydrous, 99.9%) and dimethyl sulfoxide (DMSO) were purchased from Lab-Scan and Fluka, respectively, and used as received. Water was triple distilled. Electrodes. GC rods (Sigradur G, HTW, 1 mm diameter) were embedded in epoxy resin and carefully polished (with 180, 500, 1000, and 2000 grit sandpaper) followed by treatment with diamond suspensions (Struers, 9, 3, 1, and 0.25 μm grain size). Afterward the electrodes were washed thoroughly with triple distilled water and ethanol (96%). Finally, the electrodes were sonicated 5 min in ethanol (99.99%). GC plates (Sigradur G, HTW, 10 mm 10 mm 1 mm) were sonicated in hexane and acetone for 10 min, respectively, prior to ellipsometry measurements. Gold sputtered glass plates (10 mm 10 mm 0.5 mm) consisting of 200 nm gold on top of 20 nm titanium were purchased from Polyteknik (Denmark). Prior to use the plates were immersed in a “piranha” solution, that is, 1:3 (v:v) 30% 13344
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The Journal of Physical Chemistry C H2O2:H2SO4 for 5 min, rinsed with triple distilled water followed by ethanol (99.99%), and then dried under a stream of argon. Electrochemical Setup. A standard three-electrode electrochemical setup (CH Instruments 660B or 601C) consisting of GC or Au as working electrode, a platinum wire as auxiliary electrode and a standard calomel electrode (SCE) as reference electrode was used in all electrochemical experiments. Electrochemical Grafting. Modification of the electrode surface was performed in a 10 mL aqueous solution of compounds 1af. Five cyclic voltammetric sweeps between 0.2 and 0.6 V or 0.8 V versus SCE at a sweep rate of 0.2 V s1 were carried out followed by potentiostatic electrolysis for 300 s at 0.6 or 0.8 V versus SCE. The grafting solution was purged with argon prior to the modification step. Modification in 0.1 M NaClO4/ DMSO was done following the procedure described above, although a longer electrolysis time of 600 or 1800 s was applied. Modified surfaces were cleaned by rinsing with the solvent used for grafting and ethanol, followed by sonication in these two solvents and acetonitrile for 10 min in each case. Finally, the surface was dried under a stream of argon. The effect of pH on the grafting was tested on compound 1f in 0.01 M HClO4 (pH 2), 0.1 M potassium hydrogen phthalate (100 mL) þ 0.1 M HCl (0.2 mL) (pH 4), 0.1 M KH2PO4 (pH 5), 0.1 M KH2PO4 (100 mL) þ 0.1 M NaOH (11.2 mL) (pH 6), 0.1 M KH2PO4/Na2HPO4 buffer (pH 7), 0.1 M KH2PO4 (100 mL) þ 0.1 M NaOH (93.4 mL), 0.025 M Na2B4O7 (100 mL) þ 0.1 M HCl (9.2 mL) (pH 9), 0.05 M NaHCO3 (100 mL) þ 0.1 M NaOH (21.4 mL) (pH 10) and 0.01 M NaOH (pH 12). Chronoamperometry. Determination of the number of electrons was performed using 0.2 mM 1f in 10 mL 0.01 M HClO4. A GC electrode was employed as working electrode and polished prior to the measurements. The step time was 500 ms and potential steps were from 0.1 to 0.7 V versus SCE and from 0.1 to 0.9 V versus SCE. The solution was stirred between the steps. Ellipsometry. Thickness of the electrografted films in the dry state was measured using a rotating analyzer ellipsometer (Dre, Germany). The GC plates (1 cm 1 cm) were measured at 65° angle of incidence. The ellipsometric parameters of the bare (Δs, Ψs) and the film covered substrates (Δf, Ψf) were measured in air at ambient temperature, where Δ is the phase shift and tan Ψ is the amplitude ratio upon reflection. The complex refractive index of the bare substrate was calculated from the measured Δs and Ψs values. A three-layer optical model consisting of a substrate with a complex refractive index, the grafted layer with a refractive index nf and thickness df, and the surrounding medium (air) was used to calculate the overall reflection coefficients for in-plane (Rp) and out-of-plane (Rs) polarized lights using the method described elsewhere.45,46 A constant value of nf = 1.55 was assumed for all the electrografted films in order to calculate the thickness. Each data point is the average of 612 measurements performed on three different spots on each of two to four plates (from the same batch) before and after grafting. PM-IRRAS. PM-IRRAS spectra were recorded on a Bio-Rad FTS 65A (Randolph, MA) FTIR spectrometer equipped with an external PM-IRRAS module with a narrow bandwidth mercury cadmiumtelluride (MCT) detector cooled in liquid nitrogen. The infrared beam was modulated at 74 kHz between s and p polarization by combining a gold wire polarizer with Hinds zincselenide photoelastic modulator (PEM-90/II/ZS37). The PEM was adjusted so that the s polarization was linear for wavenumbers at 1500 cm1. The gold substrates were irradiated
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Scheme 2. Reduction and Oxidation Pathways for 1f in Strongly Acidic Medium
with an incident grazing angle of 80°. The two signals, Rp Rs, and, Rp þ Rs, were extracted with a high-pass filter (40 kHz; EG&G model 189) and a lock-in amplifier (SR 810 DSP) and digitized sequentially as 20 spectra of each signal in 20 cycles (in total 800 spectra). The differential surface reflectivity spectra were obtained with a 4 cm1 resolution. The experimental PMIRRAS spectra were normalized with respect to a bare substrate and finally baseline-corrected using cubic splines in the Digilab Resolution Pro 4.0 program. All spectra were recorded at room temperature in a dry atmosphere. Analysis of CV Data. The total charge (Q) used for the reduction of the surface grafted electroactive groups was obtained by numerical integration of the background-subtracted electrochemical response recorded in cyclic voltammetry. First the background was subtracted using a fourth degree polynomial (obtained from a fitting of the data on either side of the wave) in order to describe the steadily rising background signal usually occurring while surpassing the potential region of the peak. The charge Q was then determined by numerical integration of the electrochemical signal. It is important to note that for film-coated electrodes there is an uncertainty associated with establishing an appropriate baseline since the properties of the film change during sweeping. For this reason the uncertainty could be higher than 20%. XPS. XPS data acquisition was carried out on a Kratos Axis Ultra DLD instrument (Kratos Analytical Ltd., Telford, U.K.) equipped with a monochromated Al KR X-ray source (hν = 1486.6 eV) operated at 15 kV and 15 mA (225 W). Survey spectra (binding energy (BE) of 01400 eV with pass energy of 160 eV) were used for element identification and quantification. The acquired data were converted to VAMAS format and analyzed using CASAXPS software. Spectra were calibrated at Au 4f7/2 = 84.0 eV.
’ RESULTS Electron Stoichiometry. Chronoamperometry is a unique technique for the determination of the number of electrons, n, involved in an electrochemical process.47 In particular, if the experiment is carried out under diffusion-controlled conditions, one may employ the Cottrell equation which in its time-normalized form is given by eq 1.
pffiffi nFACD1=2 iðtÞ t ¼ in ðtÞ ¼ π1=2
ð1Þ
The parameter i(t) denotes the recorded current, in(t) is the time normalized diffusion-controlled current, F is Faraday's constant, A the area of the electrode surface, C the bulk concentration, D the diffusion coefficient, and t is the time of the chronoamperometric experiment. A notable feature of the molecular structure of 1f is that it possesses both a reducible nitrophenyl group and an oxidizable phenylhydrazine group (Scheme 2). It is well-known that parasubstituted nitrophenyl compounds can be reduced in strongly acidic solution to the corresponding amine in a 6e/6Hþ process (at E1/2 ∼ 0.55 V versus SCE),48 thus allowing this process to 13345
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The Journal of Physical Chemistry C be used as an internal reference for the oxidation process. This exploits the fact that the diffusion coefficient (same compound) and electrode area (same electrode) for the two experiments are the same. An important assumption would be that only a limited amount of fouling at the electrode surface is allowed during the measurement in both directions. In the case of electrooxidation of arylhydrazines this assumption can be fulfilled only if the bulk concentration is low. All experiments were therefore carried out using 0.2 mM aqueous solutions of 1f. Voltammograms recorded of 1f in the potential window from 0 to 1.0 V versus SCE and from 0 to 0.8 V versus SCE exhibit a reduction peak pertaining to the nitrophenyl group at 0.7 V versus SCE and an oxidation wave pertaining to the arylhydrazine moiety at 0.5 V versus SCE, respectively (Figure S1, Supporting Information). On this basis the step potentials selected for recording diffusion-controlled currents, in,red(t) and in,ox(t), in the chronoamperometric experiments were 0.9 and 0.7 V versus SCE, respectively. In √ Figure 1 the recorded time-normalized currents, in(t) (=i(t) t), are depicted for both the reduction and oxidation processes in the 100500 ms time interval. Data points below 100 ms were disregarded to avoid contributions from double layer charging effects as well as incomplete reduction of the
Figure 1. Plot of the time-normalized current, in, vs t obtained from chronoamperograms of 0.2 mM 1f in 0.01 M HClO4 at a GC electrode. The normalized chronoamperograms shown are obtained as an average of measurements at five different electrodes with the dotted lines representing the 95% confidence interval. For the reduction process the potential was stepped from 0.1 to 0.9 V vs SCE (red curve), while for the oxidation process a potential step from 0.1 to 0.7 V vs SCE was employed (black curve). The solution was stirred between steps.
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nitrophenyl group. Above 500 ms unwanted contributions from natural convection or spherical diffusion would be present, not to mention possible effects from the occurrence of a partial grafting. In agreement with eq 1 both in,red(t) and in,ox(t) are reasonably constant in the time interval selected but with different absolute values because of differences in n. Since nred = 6, nox can easily be calculated to be 2.7 ( 0.6 from eq 2. nox ¼ nred
in, ox ðtÞ in, red ðtÞ
ð2Þ
Chronoamperometric measurement of the corresponding oxidation of 1a under the same conditions resulted in a timenormalized current of the same magnitude as for the oxidation of 1f (Figure S2, Supporting Information). On the reasonable assumption that 1a and 1f have similar diffusion coefficients, this indicates that the oxidation processes of the various arylhydrazines follow the same three-electron mechanism in accordance with the overall mechanism outlined in Scheme 1. Electrografting of Arylhydrazines. Figure 2 shows the cyclic voltammograms of the anodic oxidation of 2.0 mM 1f at a GC working electrode in aqueous pH 5 (A) and 10 (B) buffer solutions. In the first sweep an irreversible wave is observed with a peak potential, Ep, at 0.33 V versus SCE at pH 5 and an ill-defined peak at 0.20 V versus SCE at pH 10. The peak potential shifts to more positive values, and the peak current decreases upon successive sweeping. After potentiostatic electrolysis at 0.8 V versus SCE at pH 5 (Figure S3, Supporting Information) and 0.6 V versus SCE at pH 10 for 300 s, the cyclic voltammogram becomes featureless, indicating that an insulating layer has been formed and that the surface is completely blocked toward further oxidation of 1f. Also it may be concluded from a comparison of the two figures that the grafting proceeds much more efficiently at high pH. The values of Ep and the dry film thickness (measured by ellipsometry) of the modified GC plates (GC1f) are shown in Figure 3 as a function of pH. Evidently, Ep decreases linearly from pH 2 to 7 (ΔEp/ΔpH = 53 mV) and then becomes constant in basic medium. Along with this development the opposite is seen for the film thickness, in that it is constant (∼0.7 nm) in acidic medium but increases sharply as pH is raised above 7. Grafting at pH 5 from 1b and 1c using the same conditions as for 1f shows the same trends (Figure S4, Supporting Information). The thicknesses of the organic layer obtained for pH 5 on GC plates are determined to be 0.8 ( 0.3 nm for 1b and 0.5 ( 0.1 nm for 1c. For all three cases the numbers obtained
Figure 2. Cyclic voltammograms of 2.0 mM 1f recorded at a GC electrode using a sweep rate of 0.2 V s1 in aqueous solution at pH 5 (A) and pH 10 (B). Numerals refer to sweep numbers. Voltammogram “AE” was obtained after potentiostatic electrolysis at 0.8 V (A) and 0.6 V (B) vs SCE for 300 s. Solution was stirred between cycles and during electrolysis. 13346
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Figure 3. Plot of peak potential, Ep, (blue solid circle) of the anodic oxidation of 2.0 mM 1f and dry film thickness (red solid square) obtained as a function of pH. Lines are drawn to guide the eye. The error bar for the dry film thicknesses corresponds to pooled standard deviation obtained from 6 to 12 measurements made on two to four GC substrates.
Figure 4. Plot of thickness of chemisorbed layer vs concentration of 1f obtained after the standard grafting and cleaning procedures (see Experimental Section). Ellipsometric determination of the thickness was carried out in the dry state. The error bar corresponds to pooled standard deviation obtained from 6 to 12 measurements made on two to four GC substrates.
correspond to the formation of monolayers. Theoretical MM2 calculations (ChemBio3D Ultra 12.0 software; CambridgeSoft) of the molecular lengths give 0.62 nm for 1f, 0.54 nm for 1b, and 0.56 nm for 1c. The effect of varying the concentration of arylhydrazine was examined in the 0.55 mM concentration range in aqueous pH 7 solution for 1f. A plot of thickness against concentration reveals a systematic change with the largest concentration resulting in a low film thickness of ∼0.1 nm and the lowest concentration giving the thickest film of ∼1 nm thickness (Figure 4). Electrografting of 4-Nitrophenyldiazene (2f). Aryldiazenes are quite unstable compounds which in the absence of oxygen disproportionate with a rate of (1 3) 102 M1 s1.32 In open air the stability is even lower. For this reason 2f had to be transferred immediately to the electrochemical cell upon being synthesized. Figure 5 shows the cyclic voltammogram of 2f with its characteristic broad oxidation wave (Ep = 0.45 V versus SCE). Although the exact concentration of 2f is unknown because of its instability, an approximate concentration of 3 mM can be determined by comparing the peak oxidation current (oneelectron process) with that of the arylhydrazine (three-electron
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Figure 5. Cyclic voltammogram of ∼3 mM 2f recorded at a GC electrode in 0.1 M KH2PO4/water/ethanol (vol % 4:1; pH 5) using a sweep rate of 0.2 V s1. Numerals refer to sweep numbers. Solution was stirred between cycles.
process; Figure 1). The peak current of the subsequent two cyclic voltammograms recorded diminishes while the peak is shifted toward higher potentials due to the electrode being blocked by a film that hinders further oxidation of 2f. After the grafting process a clearly visible layer was observed on the substrate, but since it was easily removed by the cleaning process as revealed by ellipsometry, it was assumed to consist of mainly physisorbed material. Diluting the grafting solution of 2f by a factor of 10 to ∼0.3 mM resulted in a slower blocking behavior, but now a chemisorbed surface film having a thickness of 1.3 ( 0.3 nm remained after cleaning. The blocking of the electrode using 2f as grafting agent occurs faster than that seen for 1f at the same pH (=5). Most likely, this can be attributed to the fact that appreciable amounts of aryl radicals are formed close to the electrode surface in the presumed one-electron oxidation of 2f. In comparison, the three-electron oxidation of 1f may involve several slow deprotonation steps before the aryl radicals are formed and then in a relatively larger distance from the surface. The electron stoichiometry could not be determined for the oxidation of 2f because of the uncertainty associated with the determination of the concentration as well as the occurrence of a fast fouling of the surface, even at low concentrations of 2f. However, the important thing to notice here is that 1f and 2f are oxidized at essentially the same potential, meaning that if 2f is formed as an intermediate in the grafting process of 1f, it will be further oxidized. XPS. Detailed XPS analysis was carried out on Au1c, Au1d, Au1f, and GC1d plates to extract information regarding the surface composition of the layer formed on the surface. In Table 1 the atomic concentrations are listed for these films (survey spectra are collected in Figure S5, Supporting Information). The low amount of nitrogen after modification from 1c and 1d shows that the hydrazine functional group must be lost during the grafting process. The carbon to halogen ratio of about 10:1 found for samples Au1c and Au1d is larger than the theoretically expected 6:1, but this might be due to the presence of organic impurities on Au surfaces as seen from the analysis of the bare Au sample. In sample Au1f the high chemical concentration of N1s (8 atom %) originates from peaks appearing at both 400 and 406 eV (ratio 1:1) with the latter corresponding to the N1s of the nitro group. The presence of the 400 eV peak in this case suggests 13347
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Table 1. XPS Surface Composition of Modified Gold and GC Plates atomic concentration (%) electrode
Au4f
C1s
O1s
N1s
Cl2p
Br3p
Au bare 51.2 ( 0.7 36.1 ( 1.3 11.7 ( 1.0 1.0 ( 1.5
a
Au1c
34.5 ( 4.7 55.9 ( 4.4
Au1d Au1f
31.5 ( 0.7 46.4 ( 0.1 16.9 ( 0.7 1.2 ( 1.0 3.9 ( 0.4 19.8 ( 3.1 58.4 ( 0.8 13.9 ( 1.7 8.0 ( 1.7a
3.9 ( 0.6 0.3 ( 0.4
GC bare
94.5 ( 0.4
5.2 ( 0.4 0.1 ( 0.2
GC1d
94.4 ( 0.1
4.5 ( 0.4 0.2 ( 0.2 0.9 ( 0.3
5.2 ( 1.2
Appearance of two peaks at 400 and 406 eV in a 1:1 ratio.
Figure 6. Cyclic voltammograms of 1f recorded in aqueous pH 5 solution using a sweep rate of 0.2 V s1 at a freshly polished GC electrode (black line) and a GC electrode cleaned by standard procedure after first grafting (red line) and first (blue line) and second (green line) regrafting.
the existence of nitrogen in lower oxidation states, for example, as azobenzene, hydrazine, or amine moieties, the latter presumably being formed during the XPS experiment from the reduction of the nitrophenyl group.49 Regrafting and Physisorption. First, it was noted that the thickness of the organic layer upon grafting of 1f at pH 9 changed from 5.4 ( 0.4 nm before ultrasonic cleaning to 1.1 ( 0.1 nm after as measured by ellipsometry. This indicates the presence of a several nanometer thick physisorbed layer which could be removed by extensive sonication in organic solvents, hence leaving only the chemisorbed part of the organic film on the surface. In an experiment carried out at pH 4 a cleaned modified surface was regrafted and then cleaned under the exact same conditions as employed the first time. Interestingly, the thickness of the film measured before and after regrafting (and cleaning) was the same within 0.1 nm, and even three repetitive regraftings could not change this result. However, if the regrafting was carried out in a solution with the same concentration of 1f but now at higher pH (going from pH 4 to 9), the film thickness increased by 0.4 nm. In Figure 6 cyclic voltammograms recorded for grafting and regrafting experiments are collected. Importantly, all these electrodes have been sonicated thoroughly before recordings and any physisorbed layers should therefore be removed. As expected the electrode obtained after the first grafting exhibits a certain blocking effect toward 1f compared to a bare electrode as evidenced by a decrease of the peak current by ∼40% and a shift of Ep by ∼100 mV. However, the voltammetric wave is still
Figure 7. PM-IRRAS spectra of Au1f before (solid line) and after (dotted line) sonication. Peaks pertaining to the nitro group are present at νasym = 1530 cm1 and νsym = 1350 cm1.
Figure 8. Cyclic voltammograms recorded on a GC1f electrode for the first (solid line) and second (dotted line) cycle using a sweep rate of 0.2 V s1 in 0.1 M H2SO4.
clearly visible which is consistent with the presence of a thin layer as measured by ellipsometry. The effect of repeated grafting is small showing that once the electrode is covered by the first layer no further growth occurs. Also this result is consistent with those obtained from ellipsometry. The contribution from physisorption also becomes very evident using PM-IRRAS to characterize the layer. Spectra were obtained of a gold plate electrografted from 2.0 mM 1f in aqueous pH 5 solution before and after sonication in water, ethanol, and acetonitrile (5 min in each solvent). As seen in Figure 7 the characteristic peaks of the nitro group at νasym = 1530 cm1 and νsym = 1350 cm1 remain after this procedure but with only about one-third left of the original intensity. Note also that there is no sign of NH signals from hydrazine.50 A PM-IRRAS spectrum of Au1e showing the characteristic signals of the benzoic acid moiety is presented in the Supporting Information (Figure S6). Chemical analysis of the constitution of the physisorbed layer as well as the products extracted from the electrolysis solution of 1f revealed that the three main products were 4,40 -dinitroazobenzene (3), 4,40 -dinitro-1,2-diphenylhydrazine (4), and 4,40 -dinitrobiphenyl (5) formed in the ratio 2:4:1 (see Supporting Information). In one experiment 2-naphthol was added with the aim of trapping diazonium salts, if formed, but no coupling products were detected. Direct Electrochemical Analysis. The immobilized nitrophenyl group on the electrografted GC1f electrode was analyzed by cyclic voltammetry in 0.1 M H2SO4. Under these conditions nitrophenyl is reduced to a mixture of the corresponding hydroxylamine and amine.51 As shown by the cyclic voltammograms in Figure 8, a cathodic reduction wave can be observed with 13348
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The Journal of Physical Chemistry C
Figure 9. Plot of Γ vs pH of the grafting medium for GC1f, where the coverage is determined from cyclic voltammograms recorded in 0.1 M H2SO4. The line is drawn to guide the eye.
Ep = 0.55 V versus SCE on the first sweep and an oxidation wave with Ep = 0.30 V versus SCE on the reverse sweep. The first wave corresponds to the irreversible reduction of nitrophenyl and the second to the oxidation of the generated hydroxylamine to the corresponding nitroso compound. On the subsequent sweep a new reduction wave around 0.2 V versus SCE appears assigned to the reversible reduction of nitroso to hydroxylamine. At the same time the reduction wave at 0.55 V versus SCE is almost absent showing that the transformation of the electroactive nitrophenyl groups on the first sweep has been essentially complete. The surface coverage, Γ, was determined from an integration of the electrochemical signals (see Experimental Section). Figure 9 presents a plot of Γ versus pH, showing that Γ is essentially constant until pH 7, whereafter the tendency is a weak increase. In average, Γ ∼ 5 1010 mol cm2, which is similar to what is found for the electrografting of 4-nitrobenzenediazonium and 3,30 -dinitrophenyliodonium salts.3 Spontaneous Grafting. The possibility of having spontaneous grafting was investigated by immersing the electrode into a 2 mM aqueous solution of 1f under argon at pH 4, 7, and 9. For all pH values the coverage of 4-nitrophenyl groups detected after 40 min of immersion was less than 5% of that obtained in the electrografting procedure. Surface Modification in DMSO. Grafting in an aprotic polar organic solvent, DMSO, was performed in an attempt to diminish physisorption, considering that the solubility of the organic products would be higher than that in water. In Figure 10 successive cyclic voltammograms are collected with the first one showing an anodic oxidation wave with Ep = 0.55 V versus SCE. On subsequent sweeps a weak blocking behavior can be observed as evidenced by the small but continuous decrease of ip for each sweep. Even after electrolysis for 1800 s at a potential 400 mV positive of Ep, the blocking of the electrode is relatively small. The thickness of the film measured after modification in DMSO and subsequent cleaning depends on the grafting time. Potentiostatic electrolysis at 1.0 V versus SCE for 600 and 1800 s results in thicknesses of 1.2 ( 0.4 and 1.9 ( 0.1 nm, respectively. As expected physisorption is a smaller problem in DMSO than in aqueous solution since the thickness of the film in the latter case before cleaning was measured to be 2.7 ( 0.3 nm. It may be noted that appreciable amounts of physisorbed material are also formed in grafting media such as methanol and ethanol.
ARTICLE
Figure 10. Cyclic voltammograms of 2.0 mM 1f recorded at a GC electrode using a sweep rate of 0.2 V s1 in 0.1 M NaClO4/DMSO. Numerals refer to sweep numbers. Voltammogram “AE” was obtained after potentiostatic electrolysis at 1.0 V vs SCE for 1800 s. Solution was stirred between cycles and during electrolysis.
’ DISCUSSION General Observations. In Figure 2 the cyclic voltammogram of 1f in aqueous solution at pH 5 shows a single irreversible peak corresponding to the oxidation of arylhydrazine. During continuous scanning the electrode surface becomes blocked as evidenced by the decrease of ip and increase of Ep. Several sweeps and/or electrolysis are needed to completely block the electrode surface toward further oxidation of arylhydrazine. The slow decrease of ip during continuous scanning indicates that the grafting efficiency is low. This behavior is similar to what has been observed during the reduction of iodonium3,4 and sulfonium5 salts and oxidation of amines,6 while the grafting of diazonium salts proceeds much faster.2 Immersion of the GC electrode in the grafting solution up to 40 min did not contribute significantly to any layer formation on the surface. This is by far exceeding the time scale used when employing electrooxidation; hence, spontaneous grafting can be excluded as a contributing factor during the electrografting process. The film formed on the surface of the electrode as a result of the anodic oxidation consists of both a physisorbed layer that can be removed by sonication and a layer chemically bonded to the substrate as deduced from its robustness against sonication. Previously, we have shown by PM-IRRAS that the hydrazine functionality of the arylhydrazine was completely absent in the chemisorbed layer,19 which is in accordance with the lack of NH signals in Figure 7. This is further confirmed in this paper by XPS analysis of Au1c, Au1d, and GC1d, where the amount of nitrogen was low (
