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Dec 22, 2015 - Electrografting of 4‑Nitrobenzenediazonium Ion at Carbon. Electrodes: Catalyzed and Uncatalyzed Reduction Processes. Lita Lee,. †. ...
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Electrografting of 4‑Nitrobenzenediazonium Ion at Carbon Electrodes: Catalyzed and Uncatalyzed Reduction Processes Lita Lee,† Paula A. Brooksby,† Philippe Hapiot,‡ and Alison J. Downard*,† †

MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 8140 ‡ Institut des Sciences Chimiques de Rennes (Equipe MaCSE), CNRS, UMR 6226, Université de Rennes 1, Campus de Beaulieu, Bat 10C, 35042 Rennes, Cedex, France S Supporting Information *

ABSTRACT: Cyclic voltammograms for the reduction of aryldiazonium ions at glassy carbon electrodes are often, but not always, reported to show two peaks. The origin of this intriguing behavior remains controversial. Using 4nitrobenzenediazonium ion (NBD), the most widely studied aryldiazonium salt, we make a detailed examination of the electroreduction processes in acetonitrile solution. We confirm that deposition of film can occur during both reduction processes. Film thickness measurements using atomic force microscopy reveal that multilayer films of very similar thickness are formed when reduction is carried out at either peak, even though the film formed at the more negative potential is significantly more blocking to solution redox probes. These and other aspects of the electrochemistry are consistent with the operation of a surface-catalyzed reduction step (proceeding at a clean surface only) followed by an uncatalyzed reduction at a more negative potential. The catalyzed reduction proceeds at both edge-plane and basal-plane graphite materials, suggesting that particular carbon surface sites are not required. The unusual aspect of aryldiazonium ion electrochemistry is that unlike other surface-catalyzed reactions, both processes are seen in a single voltammetric scan at an initially clean electrode because the conditions for observing the uncatalyzed reaction are produced by film deposition during the first catalyzed reduction step.

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INTRODUCTION It is well-established that the one-electron reduction of aryldiazonium ions leads to the formation of aryl radicals which may chemisorb to the electrode surface via a covalent bond.1−3 A multilayer film is usually formed via attack of radicals on already grafted groups and, to a lesser extent, through coupling of aryldiazonium ions to the film as well as to the surface.4 Interestingly, reported cyclic voltammograms (CVs) obtained at carbon electrodes in aryldiazonium ion solutions often, but not always, show two reduction peaks. For example, Figure 1 shows CVs of 4-nitrobenzenediazonium salt (NBD) in acetonitrile (ACN) solution, where the first scan in Figure 1a shows two reduction peaks, labeled peak 1 and peak 2, whereas the first scan in Figure 1b5 shows only one peak, close to the peak 2 potential. (These peak labels are used throughout the present report.) The variability of response on the first scan (that is, the appearance of one or two reduction peaks) is not limited to a particular aryldiazonium ion, and when two peaks are observed, different relative currents for the peaks are frequently reported for the same aryldiazonium ion.6,7 A survey of the literature confirms, as shown in Figure 1b, that when a film is deposited on the electrode during CV scans and only one reduction peak is obtained, the peak appears close to the peak 2 potential. An exception to this observation is when only very small amounts of film are deposited during CV scans (see below). © XXXX American Chemical Society

The second and subsequent CV scans of Figure 1a,b demonstrate the typical electrochemical behavior during electrografting from aryldiazonium ion solutions. The second scans show no reduction peaks and only low current consistent with electron transfer at an electrode surface blocked by an insulating film. On further scans (Figure 1b), the currents decrease further indicating formation of an increasingly blocking surface film. From scrutiny of the numerous CVs reported in the literature, the second and subsequent CV scans obtained during grafting aryldiazonium salts appear similar regardless of whether one or two peaks are observed on the first scan. Several interpretations have been proposed for the origin of the two reduction peaks for aryldiazonium ions on carbon electrodes, as exemplified by the CVs of Figure 1a. In an early study of three aryldiazonium ions (benzenediazonium, 4methylbenzenediazonium, and NBD), Andrieux and Pinson showed that at high scan rates (ν ≥ 1 V s−1) the aryl radical that is responsible for the grafting reaction can be reduced to an aryl anion, giving a small peak at a potential more negative than that for reduction of the aryldiazonium ion itself.2 The second peak was only observed when CVs were obtained in low Received: August 28, 2015 Revised: November 16, 2015

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Figure 1. Repeat CVs (ν = 0.2 V s−1) obtained in 0.1 M [Bu4N]BF4−ACN at GC of (a) 1 mM NBD; solid line: first scan; dashed line: second scan; (b) a−c: scans 1−3 in 2 mM NBD; d: scan in the absence of NBD. (a) This work (see below). (b) Reprinted with permission from ref 5.

that on freshly polished GC peak 1 corresponds to the reduction of the aryldiazonium moiety of NBD with concomitant grafting, and peak 2 is the reduction of the grafted nitro groups to hydroxylamine groups.11 Clearly this conclusion does not provide an explanation for the common observation of two reduction peaks in ACN solution: as the authors point out, in an aprotic medium, nitro groups cannot be reduced to hydroxylamine groups but instead are reduced to the corresponding radical anion at very negative potential. Hence, Richard and co-workers’ study sheds no light on the two reduction peaks observed by many others for NBD in aprotic media, or for aryldiazonium ions lacking an electroactive substituent, in aqueous media. Here we expand on Cline and co-workers’ study of the reduction of aryldiazonium ions in ACN solution to gain further understanding of the origin of the two reduction peaks at carbon electrodes. The study is focused on reduction of NBD at GC electrodes as GC the most commonly used carbon electrode material for electrografting and NBD has become the benchmark aryldiazonium ion. We use ACN as the solvent to avoid complications arising from the reduction of nitro groups in protic media. The mechanism of formation of films from aryldiazonium salts and the structure and properties of the layers have been thoroughly studied, for NBD in particular, by a range of spectroscopic and microscopy techniques. Reference 4 provides an excellent review of important work in this area.

concentration solutions of aryldiazonium salts (0.1−0.6 mM) where deposition of film was minimized. For NBD, the authors reported that the second peak was very small, consistent with the very high reactivity of the phenyl radical. In contrast, the commonly observed second reduction peak appears in CVs obtained at relatively slow scan rates and higher concentrations and clearly has a different origin. Hence, reduction of aryl radicals will not be considered in the following discussion. Cline and co-workers8 made a careful and detailed investigation of the origin of the two reduction peaks seen on GC for NBD in 0.1 M [Bu4N]BF4−ACN. They convincingly demonstrated that both peaks correspond to reduction of the aryldiazonium ion and tentatively postulated that the two reduction peaks arise from two different sites on the electrode for the adsorption of aryldiazonium ion and/or attachment of aryl radicals. They tentatively suggested that at peak 1 reduction leads to formation of a bond between an aryl group and the surface, whereas peak 2 may correspond to attachment of radicals at a different type of surface site or to already attached aryl groups.8 Cline and co-workers’ study also explained the conflicting literature reports of either one or two reduction peaks for a given aryldiazonium ion, namely, that this variation is likely due to differences in “cleanliness” or “past history” of the GC electrodes, and the amount of time the electrodes are in an aryldiazonium ion solution before the CVs are collected. With respect to the latter point, it is well-known that films can be grafted to GC spontaneously at open-circuit potential from aryldiazonium ion solutions,5,9 and the extent of grafting is expected to depend on the immersion time of GC in the aryldiazonium ion solution. CVs obtained at freshly polished and spontaneously grafted GC electrodes are therefore expected to be different. Benedetto and co-workers10 investigated the electroreduction of the aryldiazonium salt C6F13-S-Ar-N2+ in 0.05 M tetraethylammonium perchlorate−ACN on Au surfaces with several crystallographic orientations. They found a number of distinct peaks which they attributed to the one-electron reduction of aryldiazonium ion on different crystalline sites. The authors suggested that at GC, also, the two reduction peaks may originate from different crystalline sites; however, they did not carry out experimental investigations on carbon surfaces. More recently, Richard and co-workers11 investigated the reduction of NBD at GC surfaces in an aqueous acidic medium (0.1 M HCl). They showed that CVs obtained at a “fully polished” electrode had two reduction peaks, while at an “unpolished” electrode (electrode was polished with 9 μm diamond powder only) only one reduction peak was observed; this appeared at the peak 2 potential. The authors concluded

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EXPERIMENTAL SECTION

Materials and Reagents. Unless stated otherwise, all reagents were used as received. ACN from VWR BDH was HPLC grade; Millipore Milli-Q water (resistivity >18 MΩ cm) was used for all aqueous solutions and washing. Tetrabutylammonium tetrafluoroborate ([Bu4N]BF4), pyrolyzed photoresist film (PPF),7 and aryldiazonium salts12−14 were synthesized using literature procedures. Electrochemistry. All electrochemical measurements were performed using an Eco Chemie Autolab PGSTAT302 potentiostat/ galvanostat. The working electrode was a GC disk (area = 0.071 cm2) or a GC, PPF, or HOPG plate with an area defined by an O-ring (0.29 cm2 for CVs in NBD solution and 0.11 cm2 for subsequent reduction of nitrophenyl groups). A Pt mesh auxiliary electrode was used, and the reference electrode was a saturated calomel electrode (SCE) for measurements in aqueous solutions and a calomel electrode with 1 M LiCl(aq) (CE) for nonaqueous solutions. The ferrocene/ferrocenium couple appeared at E1/2 = 0.36 V vs CE (1 M LiCl) in 0.1 M [Bu4N]BF4−ACN solution. GC disk electrodes were used in a standard three-electrode cell. For GC, PPF, and HOPG plate electrodes, a glass cell with a hole in the base was clamped on top of a Kalrez O-ring sitting on the plate electrode. Electrical contact was made through a strip of copper. CVs of aryldiazonium salts and electrode modifications were performed in 0.1 M [Bu4N]BF4−ACN solution degassed for 5 min by B

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Langmuir purging with N2(g). Unless specified otherwise, electrochemical measurements were initiated immediately after the working electrode was immersed in the solution. Before and between each experiment, GC electrodes were freshly polished with a slurry of 1 μm alumina in water, followed by sonication in water for 5 min and drying with a stream of N2(g). A fresh HOPG surface was prepared between each experiment using adhesive tape to remove the top graphite layers. PPF was sonicated in ACN for 10 s before use. CVs of nitrophenylmodified electrodes were obtained after sonicating the modified electrodes in ACN for 1 min (for plate electrodes) or 5 min (for GC disks electrode) and drying with a stream of N2(g). The electrodes were subsequently scanned between 0.6 and −1.2 V (initial potential = 0.6 V) in 0.1 M H2SO4 at ν = 0.1 V s−1. The surface concentration of nitrophenyl groups was obtained from the charge associated with reduction of nitrophenyl groups and oxidation hydroxylamine groups,15 as described previously.16 A monolayer of carboxyphenyl groups (Ar-COOH) was prepared by grafting from a solution of 5 mM 4-[(9-H-fluoren-9-ylmethoxy)carbonyl]benzene-1-diazonium ion (N2+-Ar-COO-Fm)14 using five potential cycles between 0.80 and −0.75 V at ν = 0.05 V s−1. The modified electrode was rinsed with acetone, sonicated in ACN for 5 min, and dried with N2(g). The protecting Fm group was cleaved from the layer by immersing the modified electrode in a stirred solution of 20% piperidine in DMF for 40 min, followed by rinsing with acetone and water. Atomic Force Microscopy (AFM). Measurements were made with a Dimension 3100 and Nanoscope IIIa controller (Digital Instruments, Veeco, USA). Film thicknesses were measured on modified PPF working electrodes by profiling across a scratch in the film.7 As described previously,13 the reported film thicknesses are the means of at least 16 values obtained from three scratches, and the uncertainty is the standard deviation of the mean.

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RESULTS To limit the amount of film deposition at open-circuit potential, unless stated otherwise, all CVs were obtained immediately after electrodes were exposed to the aryldiazonium ion solution. Under these conditions, CVs of NBD obtained at freshly polished GC electrodes always showed peak 1 and usually peak 2 (see below). Although the results below concern NBD almost exclusively, the generality of the behavior was investigated by obtaining CVs of ten aryldiazonium salts at freshly polished GC (1 mM in 0.1 M [Bu4N]BF4−ACN at ν = 0.1 V s−1). These CVs, shown in Figure S1 (Supporting Information), confirm that under these conditions all derivatives exhibit two reduction processes on the first scan, although in some cases peak 1 appears as a shoulder on peak 2. Consideration of the data in terms of the electronic properties or steric bulk of the substituents did not reveal any systematic relationship between these properties and relative peak currents or potential separation between the peaks. General CV Behavior. Figures 2a,b show first scan CVs of 1 mM NBD in 0.1 M [Bu4N]BF4−ACN obtained at ν = 0.02− 0.5 V s−1. Except at ν = 0.5 V s−1 (CV 8), all CVs show two reduction peaks. For ν ≤ 0.1 V s−1 (CVs 1−4, Figure 2a) the reduction current is very low on the return scan, consistent with the presence of a surface film which blocks electron transfer to aryldiazonium ions in solution. As scan rate increases above 0.1 V s−1 (CVs 5−8), there is an increasing reduction current in the return scan, indicating, as expected, that less film is deposited on the forward scan during the shorter time scale experiments and that reduction of aryldiazonium ion and presumably, deposition of film, continues at a significant rate on the reverse scan. Scrutiny of CVs shown in other reports confirms that when conditions prevent or significantly limit film deposition, peak 2

Figure 2. CVs obtained at GC in a solution of 1 mM NBD in 0.1 M [Bu4N]BF4−ACN at (a, b) ν = 0.02 V s−1 (1), 0.05 V s−1 (2), 0.08 V s−1 (3), 0.1 V s−1 (4), 0.2 V s−1 (5), 0.3 V s−1 (6), 0.4 V s−1 (7), 0.5 V s−1 (8); (c) ν = 0.5 V s−1, first scan (9), second scan (10).

is absent. For example, this is apparent in CVs shown by Combellas and co-workers, who examined reduction of aryldiazonium ions for which steric hindrance at the ortho positions prevents grafting.17,18 Similarly, when an excess of radical scavenger (2,2-diphenyl-1-picrylhydrazyl) was added to a solution of NBD, thereby limiting film grafting, there was only one irreversible reduction peak, appearing at the peak 1 potential.19 In Cline and co-workers’s work, only peak 1 is seen in CVs obtained at ν = 0.2 V s−1 in low concentration (0.02 and 0.4 mM) NBD solutions.8 Hence, the single-peak CV obtained on the first scan at ν = 0.5 V s−1 (CVs 8 and 9 in Figures 2b and 2c, respectively) is consistent with the deposition of only a small amount of film during the forward scan. A second scan under the same conditions (Figure 2c, CV 10) shows a peak at Epc ≈ −0.15 V, which is assumed to be peak 2 moved to more negative potentials than in the first scans at slower scan rates. This assumption seems reasonable because a negative shift in peak 2 potential on the second scan at high scan rate implies that the grafted film is more blocking to electron transfer than the film grafted at the peak 1 potential during the first scan at slower scan rates but is less blocking than the film present on the second scan at slower scan rates where reduction of NBD is completely blocked. The appearance of peaks in repeat CV scans in discussed further below. Plots of peak 1 current against ν1/2 and ν are shown in the Supporting Information (Figure S2); neither plot is linear over the scan rate range 0.02−0.5 V s−1. In the absence of film C

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films formed by scanning through each NBD reduction peak, the voltammetry of dopamine and ferrocene was examined at the modified surfaces. A layer prepared by holding the potential at 0.45 V for 300 s in NBD solution was also included to ensure that the film was formed by the electron transfer process associated with peak 1 only. The voltammetry of dopamine was examined because it is an inner-sphere redox probe for which the rate of electron transfer is affected by its ability to interact directly with the surface.20,21 In contrast, ferrocene is an outersphere redox probe for which electron transfer occurs by tunneling.22 Figure 4 shows the responses of a GC electrode in

deposition, Andrieux and Pinson showed that reduction of aryldiazonium ions is diffusion controlled.2 However, a linear i vs ν1/2 relationship is not expected at scan rates where significant film deposition occurs because the film is blocking toward electron transfer (see below) and the amount of film deposited varies with scan rate (Figure 2). The plots of Figure S2 are discussed in the Supporting Information. Electrografting at Peak 1. To explore the origin of the two reduction peaks, we followed the strategy used by Benedetto and co-workers for Au electrodes10 and examined the films formed by reduction at peak 1 only and by reduction also involving peak 2. Five repeat scans were recorded in NBD solution over the potential range of peak 1 (0.75−0.25 V, Figure 3, black lines), immediately followed by another five

Figure 4. CVs (ν = 0.1 V s−1) obtained in (a) 1 mM dopamine in 0.1 M H2SO4; (b) 1 mM ferrocene in 0.1 M [Bu4N]BF4−ACN at a GC electrode after modification in a solution of 1 mM NBD (in 0.1 M [Bu4N]BF4−ACN) by five cycles from 0.75 to 0.25 V followed by five cycles from 0.75 to −0.45 V (black lines, procedure I); one cycle from 0.75 to −0.45 V (green line, procedure II); five cycles from 0.75 to 0.25 V (red line, procedure III); applying 0.45 V for 300 s (blue line, procedure IV). Yellow line is 1 mM dopamine in 0.1 M H2SO4 at polished GC.

Figure 3. CVs (ν = 0.1 V s−1) obtained in 1 mM NBD in 0.1 M [Bu4N]BF4−ACN. Black lines: five repeat scans, 0.75 to 0.25 V, at freshly polished GC; red lines: scans 6−10, 0.75 to −0.45 V, recorded immediately after scans 1−5; gray dashed line: one scan, 0.75 to −0.45 V, at freshly polished GC.

cycles through both reduction processes (0.75 to −0.45 V, red lines). As shown by the black lines, after scanning once through peak 1 subsequent scans have very low current, and only on the second scan is there evidence of a very small reduction peak. When the sixth scan is extended to −0.45 V (red line), peak 2 is obtained. On further repeat scans to −0.45 V only low currents are obtained. For the first scan extended to −0.45 V, the size of peak 2 is similar to that in a single CV scan from 0.74 to −0.45 V at a freshly polished electrode (gray dashed line). Importantly, in a separate experiment, after repeat scanning between 0.75 and 0.25 V the electrode was removed from solution, sonicated for 5 min in ACN, and then returned to the NBD solution. The first scan on this surface was the same as that shown in red in Figure 3, indicating that film deposited at peak 1 is stably attached to the surface and that blockage of pores or channels in the film by physisorbed material is not influencing the electrochemical response. The experiments described above are consistent with Cline and co-workers’ finding that reduction of NBD occurs at both peaks.8 The two reduction peaks behave similarly on repeat scans: after the first scan there is a dramatic decrease in reduction current, indicating the formation of a film that is passivating toward NBD reduction. Considering this behavior, it is not surprising that peak 2 is little affected by the number of prior scans through the peak 1 potential range; i.e., based on the low currents for scans 2−5, little further change to the electrode surface or solution would be expected after repeat scans through peak 1. Blocking Properties of the Grafted Layers. The CVs in Figure 3 demonstrate that although reduction of NBD is selflimiting at the peak 1 potential, the film deposited during this reduction step does not block electron transfer at the peak 2 potential. To further investigate the blocking properties of the

solutions of dopamine (1 mM in 0.1 M H2SO4) and ferrocene (1 mM in 0.1 M [Bu4N]BF4−ACN) after four different modification procedures in NBD solution (Table 1). Table 1. Modification Procedures at GC Electrodes in 1 mM NBD Solution and the Corresponding Surface Concentration of Electroactive Nitrophenyl (NP) Groups procedure I II III IV

ΓNPa (× 10−10 mol cm−2)

modification scan 5× from 0.75 to 0.25 V followed by 5× from 0.75 to −0.45 at 0.1 V s−1 scan 1× from 0.75 to −0.45 at 0.1 V s−1 scan 5× from 0.75 to 0.25 at 0.1 V s−1 apply potential of 0.45 V for 300 s

18.2 ± 0.6 18.8 ± 0.9 14.5 ± 0.2 17.5 ± 0.2

a

Calculated from CVs recorded in 0.1 M H2SO4. Average value of two repeat experiments and the uncertainties indicate the range of values obtained.

After modifying the GC electrode by scanning five cycles from 0.75 to 0.25 V followed by five cycles from 0.75 to −0.45 V (procedure I) or scanning one cycle from 0.75 to −0.45 V in NBD solution (procedure II), the dopamine response at the modified is totally blocked between 0.1 and 0.9 V (Figure 4a, black and green lines, respectively). When a GC electrode is scanned five cycles from 0.75 to 0.25 V (procedure III) or has a potential of 0.45 V applied for 300 s in NBD solution (procedure IV), a well-defined dopamine couple is observed at the resulting surfaces (Figure 4a, red and blue lines, respectively). However, this response, with ΔEp ≈ 600 mV, indicates significantly slower electron transfer than that of dopamine at polished GC (Figure 4a, yellow line). Following D

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Evidently, morphology differences are below the resolution limits achieved. Effect of Surface Pretreatment. The study of Cline and co-workers demonstrated that GC surface pretreatment and history had a strong effect on peak 1 for reduction of NBD.8 After spontaneous grafting by immersion in NBD solution at open circuit potential, peak 1 for reduction of NBD was diminished compared with that at a polished GC electrode, but peak 2 was unchanged. Simply immersing polished GC in Nanopure water also decreased the magnitude of peak 1; this was attributed to adsorption of adventitious impurities. Figure S4 shows the results of similar experiments, performed in this work, which confirm that peak 1, but not peak 2, is sensitive to the presence of a prior spontaneously grafted film or to electrode surface contamination. To establish the generality of this behavior, we examined grafting of a different species, 4((triisopropylsilyl)ethynyl)benzenediazonium ion (TIPS-EthAr-N2+).26 At a freshly polished GC electrode, the CV shows two peaks (Figure 5, black line). After repolishing, the electrode

Duvall and McCreery’s analysis of dopamine electrochemistry,20,21 at polished GC the electron transfer is catalyzed by dopamine adsorbed on bare GC, but at electrodes modified by procedures I to IV, the response is typical of uncatalyzed electron transfer, presumably because adsorption sites are blocked by the grafted nitrophenyl films. For ferrocene, after modification at peak 1 using procedures III and IV, the CVs (Figure 4b, superimposed red and blue lines) are indistinguishable from those obtained at polished GC (not shown), indicating electron tunneling across the layer. A clear decrease in electron transfer rate after grafting by scanning more negative than peak 2 (Figure 4b green and black lines) is consistent with the presence of more blocking layers. Although the use of different media do not allow further comparisons between the responses of ferrocene and dopamine, both redox probes demonstrate that films formed by scanning through peak 2 are significantly more blocking than those prepared by restricting the potential to the peak 1 region. Surface Concentrations and Film Thicknesses. Table 1 lists the surface concentrations of electroactive nitrophenyl groups, ΓNP, in films prepared by the four procedures described above. ΓNP values were obtained by measuring the charge associated with the reduction and oxidation processes of the films, as described in the Experimental Section. For GC electrodes modified by scanning past peak 2 (procedures I and II), ΓNP is ∼(18−19) × 10−10 mol cm−2. A monolayer equivalent of nitrophenyl groups on a flat surface has been estimated to be (2.5 ± 0.5) × 10−10 mol cm−2, and hence this coverage corresponds to a multilayer film.7 Modification procedure II is expected to deposit less film than procedure I because there are fewer potential scans past peak 2, and confirming this, the CVs of Figure 4 show a less blocking film after procedure II (green lines) than procedure I (black lines). Hence, the finding that both modifications give the same ΓNP indicates that only nitrophenyl groups located on the outside of the layer remain electroactive, a phenomenon previously reported for thick films.23,24 The measured surface concentration is therefore assumed to underestimate the number of nitrophenyl groups in the film deposited by procedure I and possibly procedure II. When GC electrodes were modified by reduction at peak 1 only (procedures III and IV), ΓNP values also indicate multilayer films although ΓNP for the film prepared by procedure III is lower than for films prepared by the other procedures. The thickness of films prepared by procedures I−III was measured by AFM depth profiling. For these experiments, films were grafted to PPF, which is a GC-like material but with a lower surface roughness that enables measurement of nanoscale films.25 The measured thicknesses were 2.9 ± 0.5, 1.9 ± 0.5, and 2.2 ± 0.8 nm for films prepared by procedures I, II, and III, respectively. Although the films prepared on PPF cannot be assumed to have exactly the same surface concentrations as those prepared on GC (Table 1), AFM measurements confirm that multilayer films with similar thicknesses are formed by all procedures. This is surprising considering that the CVs of Figure 4 indicate that films prepared by applying potentials more positive than peak 2 are significantly less blocking to dopamine and ferrocene electrochemistry than are films prepared at potentials more negative than peak 2. AFM imaging did not reveal any differences in morphology for films prepared by grafting at the peak 1 potential and more negative than the peak 2 potential (Figure S3, Supporting Information).

Figure 5. First scan CVs (ν = 0.05 V s−1) obtained in a solution of 5 mM TIPS-Eth-Ar-N2+ in 0.1 M [Bu4N]BF4−ACN at freshly polished GC (black) and GC grafted with a monolayer of Ar-COOH groups (red).

was modified with a sparse monolayer of Ar-COOH groups using a protection−deprotection strategy,14 and the modified electrode was used to obtain a CV of TIPS-Eth-Ar-N2+ (Figure 5, red line). In comparison with the CV of TIPS-Eth-Ar-N2+ obtained at polished GC, the CV obtained at the pregrafted electrode has a much diminished peak 1. This is analogous to the decrease in peak 1 current for NBD after spontaneous grafting in NBD solution (Figure S4). Hence, the data in Figure 5 show that the high sensitivity of peak 1 to the presence of small amount of grafted film and the relative insensitivity of peak 2 are not unique to NBD. It is important to note that under some conditions the potential and current for peak 2 are sensitive to the amount of film on the surface. This has been noted above for repeat scans of NBD at υ = 0.5 V s−1 (Figure 2c). Another example is seen in CVs of 4-trifluoromethylbenzenediazonium ion reported by Ranganathan and McCreery.25 In repeat CVs on PPF, the peak potential progressively moves to more negative potentials with a concomitant decrease in peak current. Repeat CV scans of the same aryldiazonium ion on GC, shown in Figure S1h, also show a significant shift in the peak 2 potential. For this aryldiazonium ion and others shown in Figure S1d−j, reduction at peak 1 is completely blocked after a single scan, but the second reduction process is observed at more negative potential on the second scan, albeit in some cases with low current. All these examples are consistent with slow film formation such that the film deposited on the first scan is not sufficiently thick to completely block reduction of aryldiazonium ion on the following scan(s). E

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reduction peaks on the first scan. However, the peak 1 current at PPF is smaller than at GC, suggesting that the self-inhibition effect is stronger at PPF than GC. The CVs recorded at HOPG were not entirely reproducible: sometimes “peak 1” appeared as two closely merged peaks as shown in Figure S5a. The variability in response at HOPG is attributed to differences in the fresh HOPG surfaces used for each experiment. Nevertheless, at HOPG peak 1 is always relatively large and at a more negative potential compared to that at GC and PPF. The shift in peak potential is attributed to a slower rate of electron transfer at HOPG than at GC and PPF. Peak 2 is very small at HOPG, but there is significant reduction current over a broad potential range on the first negative-going scan. Examination of NBD reduction at HOPG at ν = 0.01−0.5 V s−1 (Figure S5b) shows that as the scan rate increases significant reduction currents are detected on the return scan. A plot of peak 1 current (in the first scan) against ν1/2 (Figure S5c) is linear, indicating diffusion controlled reduction of NBD and little evidence of self-inhibition by film deposition. Consistent with this observation, in contrast to GC, scans obtained at HOPG at ν = 0.5 V s−1 show that peak 1 is present on repeat scans although with lower current (Figure 6b). The relative amounts of film deposited at the three carbon materials under the same conditions is revealed by CVs of the modified surfaces obtained in 0.1 M H2SO4 (Figure 6c). The surface concentration of electroactive nitrophenyl groups was similar on GC and PPF and much greater than on HOPG. This confirms that the differences between the CVs of NBD reduction at HOPG and GC (and PPF) are due to the much smaller amount of film deposited at HOPG. A low yield for film formation at HOPG is consistent with our recent study which revealed that the film deposited at HOPG basal plane via reduction of aryldiazonium ions is physisorbed, and there is stable attachment only at graphite edges.27

Negative shifts in the peak 2 potential arise from electron transfer across an increasingly thick film. Electrochemical Response on Other Carbon Surfaces. The response of NBD at HOPG and PPF was briefly examined and compared to that at GC (Figure 6). To enable easy

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DISCUSSION Andrieux and Pinson estimated a rate constant of ∼0.003 cm s−1 for reduction of benzenediazonium and 4-methylbenzenediazonium ions at GC and noted that the low rate constant is consistent with electron transfer concerted with loss of N2.2 While a strong influence of electrode surface condition is expected for redox couples with such low rate constants, the electrochemical response of NBD, and other aryldiazonium ions (Figure S1), under film-forming conditions, cannot be simply explained by a slow electron transfer rate. As the film grows during the CV scan, there is not a continuous negative shift in the reduction potential as electron transfer progressively slows at the thickening film; this would give a single broad reduction feature. Instead, as the potential becomes more negative, reduction at peak 1 stops and a different process operates (peak 2). Further, peak 1 is sometimes totally absent (Figure 1b5 and Figure 3, first red scan) whereas the potential and current of peak 2 is similar when the surface has been pregrafted at peak 1 or has physisorbed impurities.8,11 On the other hand, when peak 2 is present on repeat scans because of slow film formation, its behavior is typical of a kinetically slow redox reaction occurring at an increasingly blocked surface (see, for example, Figure 2c, Figure S1d−j, and Figure 4B in ref 25). The response of NBD on repeat scanning at GC over the peak 1 potential range (Figure 3) matches that described in a detailed report by Pinson, Savéant, and co-workers.1 They satisfactorily modeled their experimental data based on a selfinhibiting irreversible electrochemical reaction mechanism.28 In

Figure 6. CVs obtained in a solution of (a, b) 1 mM NBD in 0.1 M [Bu4N]BF4−ACN at a scan rate of (a) 0.05 V s−1 and (b) 0.5 V s−1. Solid line: first scan; dashed line: second scan; dotted line: third scan. (c) 0.1 M H2SO4 at ν = 0.1 V s−1 of the electrodes modified by the CV scans as shown in (a). Black line: GC plate; red line: HOPG; blue line: PPF.

comparison of data, all CVs were obtained at plate electrodes using the same O-ring to define the electrode area. For these experiments CVs were recorded 2 min after the electrodes were exposed to the aryldiazonium ion solution. Because of the cell setup used for the plate electrodes, after the introduction of solution to the cell some time was taken to ensure there was no leakage and no gas bubbles trapped between the O-ring and the glass cell. Therefore, 2 min was adopted as the standard “wait” time. Figure 6a shows CVs obtained in a solution of 1 mM NBD in 0.1 M [Bu4N]BF4−ACN solution at GC (black line), PPF (blue line), and HOPG (red line). Comparing the GC response to that obtained at the same scan rate but without a wait time (Figure 2a, CV 2) reveals that the wait time has led to a small decrease in the peak 1 current:peak 2 current ratio. This is expected, based on the effects of electrode treatment described above and more specifically on the effects of spontaneous grafting as shown in Figure S4. The response at PPF (Figure 6a) is similar to that obtained at GC, with two distinct F

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rapidly as at the other substrates. This is expected given that the HOPG surface is comprised mainly of basal plane, and film deposition is known to occur rapidly on defects and edges, but more slowly on the basal plane of HOPG.27,29,30 This response also indicates that the catalytic reduction of NBD proceeds on the HOPG basal plane, albeit at a slower rate than at GC and PPF, as indicated by the negative shift in peak 1 potential. GC and PPF are largely (at least) edge plane materials31 and clearly the catalytic reduction proceeds at these materials, too, and hence we have no evidence that specific carbon surface sites are required for catalysis. Our study provides no other information on the nature of the surface interaction; however, it is interesting to note that spontaneous grafting of aryldiazonium ions to carbon nanotubes,32−34 amorphous carbons,35,36 and graphene37 has been studied in detail, and a two-step mechanism has been proposed. In the first step, an electron transfer mediated adsorption yields a reduced aryldiazonium species. In the second step, dediazoniation gives an aryl radical which covalently bonds to the substrate. Although the relationship between mechanisms operative during spontaneous grafting and electrografting is unknown, it is possible that the process giving rise to peak 1 is similar to step 1 described above. Whether such a mechanism could account for the observed electrochemical behavior would depend on the kinetics of each step, and formation of a multilayer film at peak 1 would require desorption of the reduced aryldiazonium species before or after dediazoniation and radical attack at already grafted groups, rather than only at the substrate surface. Finally, the varying responses of different aryldiazonium ions at GC (for example, NBD and TIPS-Eth-Ar-N2+ (Figures 2 and 4, respectively) and Figure S1) and at different graphitic carbon materials can be qualitatively accounted for by the operation of catalyzed and uncatalyzed reduction steps. The rates of electron transfer, grafting to the surface and film growth, and the inherent blocking properties of the aryl derivative are expected to vary with the aryldiazonium ion and the substrate and will influence the observed voltammetric response.

this mechanism, a solution-based irreversible electron transfer reaction gives the aryl radical, followed by two competing reactions: chemisorption of the radical and reaction of the radical in solution. Importantly, they assumed that the modified electrode surface was completely inhibiting toward reduction of the aryldiazonium ion. Assuming the self-inhibiting irreversible electrochemical reaction mechanism correctly describes the process at peak 1, this raises the question of why grafting of aryl groups completely inhibits (rather than progressively slows) further reduction of aryldiazonium ions over that potential range. Drawing on parallels with the voltammetry of dopamine at GC,20,21 we propose that a catalytic reaction is involved in reduction at peak 1 and that catalysis requires a direct interaction between the aryldiazonium ion and the carbon surface. The requirement for direct interaction with bare surface explains why reduction at peak 1 is self-limiting: as the reduction proceeds, aryl groups are deposited until access to bare surface is blocked and the catalytic reduction then stops. Similarly, the presence of just a monolayer of physisorbed impurities will prevent catalysis, and peak 1 will be absent at a “dirty” electrode. The formation of a multilayer film at peak 1, as evidenced by film thickness measurements as well as surface concentration measurements, indicates that reduction at peak 1 does not require grafting of aryl radicals directly to the surface; rather, it is the reaction(s) leading to the production of radicals that require interaction with the surface. Under conditions where the catalytic reaction is inhibited, reduction of aryldiazonium ion can occur via an uncatalyzed reaction at the more negative peak 2 potential. Hence, the unusual two peak behavior for reduction of aryldiazonium ions can be accounted for by operation of catalyzed and uncatalyzed reactions. Unlike the situation for other species (e.g., dopamine) which undergo catalyzed and uncatalyzed electrochemical reactions, for aryldiazonium ions both responses can be observed in a single voltammogram because the film deposited during the catalyzed reduction produces the conditions for the uncatalyzed reduction at a more negative potential. This explains why when reduction of aryldiazonium ion at peak 1 leads to only very low, or no film deposition (as for example when bulky groups on the aromatic ring impede grafting17,18 or the “grafting” scan rate is high relative to film growth, Figure 2c), peak 2 is not observed. The similar film thicknesses of films grafted at peaks 1 and 2 (Table 1), but their significantly different blocking properties toward dopamine and ferrocene (Figure 5) point to different film structures. For the film formed at peak 1, a structure which allows dopamine to approach the surface but not to interact directly with the surface is indicated, suggesting a loosely packed, inhomogeneous film with both thin areas and thick multilayer structures. Based on the dopamine response, the film structure does not appear to be influenced by its preparation method: very similar blocking properties are seen at films generated by repetitive cycling through the peak 1 potential or by controlled potential electrolysis at a potential positive of the peak. During reduction at peak 2, electron transfer to aryldiazonium ion occurs across the film and will be fastest at the thin areas, effectively filling in the loose-packed structure giving a denser and more blocking, although not significantly thicker, film. Comparing NBD reduction at HOPG, PPF, and GC electrodes (Figure 6a), a much larger peak 1 current at HOPG indicates that the catalytic reduction is not inhibited as

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CONCLUSION An inherently slow electron transfer rate coupled with the deposition of an insulating film underlies the unusual two-peak voltammetry for reduction of NBD at graphitic carbon substrates. Our study suggests that under the common experimental conditions used for grafting (clean GC electrode, NBD concentration ≥1 mM, cyclic voltammetry with ν ≤ 0.2 V s−1, or controlled potential electrolysis), the reduction can be catalyzed by an interaction with the electrode surface, giving the peak at more positive potential (peak 1). In contradiction to suggestions in other reports, we show conclusively that peak 1 does not correspond to reduction of the aryldiazonium ion accompanied by direct grafting to the surface only: film thickness measurements demonstrate that a multilayer film is grafted at peak 1. This loosely packed, inhomogeneous multilayer film grows until aryldiazonium ion cannot interact with the surface, and the catalytic reaction is prevented. In the absence of a catalytic pathway, reduction of aryldiazonium ion occurs at a more negative potential (peak 2). This reduction step gives a more blocking film, largely (at least) through fillingin the thin areas in the film formed at peak 1. Our study provides no evidence that specific surface sites are required for catalysis: the catalyzed reduction of NBD at GC, PPF, and HOPG surfaces suggests that catalysis is not limited to either G

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(10) Benedetto, A.; Balog, M.; Viel, P.; Le Derf, F.; Salle, M.; Palacin, S. Electro-Reduction of Diazonium Salts on Gold: Why Do We Observe Multi-Peaks? Electrochim. Acta 2008, 53, 7117−7122. (11) Richard, W.; Evrard, D.; Gros, P. New Insight into 4Nitrobenzene Diazonium Reduction Process: Evidence for a Grafting Step Distinct from No2 Electrochemical Reactivity. J. Electroanal. Chem. 2012, 685, 109−115. (12) Dunker, M. F. W.; Starkey, E. B.; Jenkins, G. L. The Preparation of Some Organic Mercurials from Diazonium Borofluorides. J. Am. Chem. Soc. 1936, 58, 2308−2309. (13) Lee, L.; Brooksby, P. A.; Leroux, Y. R.; Hapiot, P.; Downard, A. J. Mixed Monolayer Organic Films Via Sequential Electrografting from Aryldiazonium Ion and Arylhydrazine Solutions. Langmuir 2013, 29, 3133−3139. (14) Lee, L.; Ma, H. F.; Brooksby, P. A.; Brown, S. A.; Leroux, Y. R.; Hapiot, P.; Downard, A. J. Covalently Anchored Carboxyphenyl Monolayer Via Aryldiazonium Ion Grafting: A Well-Defined Reactive Tether Layer for on-Surface Chemistry. Langmuir 2014, 30, 7104− 7111. (15) Ortiz, B.; Saby, C.; Champagne, G. Y.; Bélanger, D. Electrochemical Modification of a Carbon Electrode Using Aromatic Diazonium Salts. 2. Electrochemistry of 4-Nitrophenyl Modified Glassy Carbon Electrodes in Aqueous Media. J. Electroanal. Chem. 1998, 455, 75−81. (16) Yu, S. S. C.; Tan, E. S. Q.; Jane, R. T.; Downard, A. J. An Electrochemical and Xps Study of Reduction of Nitrophenyl Films Covalently Grafted to Planar Carbon Surfaces (Vol 23, Pg 11074, 2007). Langmuir 2008, 24, 7038−7038. (17) Combellas, C.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Sterically Hindered Diazonium Salts for the Grafting of a Monolayer on Metals. J. Am. Chem. Soc. 2008, 130, 8576−8577. (18) Combellas, C.; Jiang, D. E.; Kanoufi, F.; Pinson, J.; Podvorica, F. I. Steric Effects in the Reaction of Aryl Radicals on Surfaces. Langmuir 2009, 25, 286−293. (19) Menanteau, T.; Levillain, E.; Breton, T. Electrografting Via Diazonium Chemistry: From Multilayer to Monolayer Using Radical Scavenger. Chem. Mater. 2013, 25, 2905−2909. (20) DuVall, S. H.; McCreery, R. L. Control of Catechol and Hydroquinone Electron-Transfer Kinetics on Native and Modified Glassy Carbon Electrodes. Anal. Chem. 1999, 71, 4594−4602. (21) DuVall, S. H.; McCreery, R. L. Self-Catalysis by Catechols and Quinones During Heterogeneous Electron Transfer at Carbon Electrodes. J. Am. Chem. Soc. 2000, 122, 6759−6764. (22) Bard, A. J. Inner-Sphere Heterogeneous Electrode Reactions. Electrocatalysis and Photocatalysis: The Challenge. J. Am. Chem. Soc. 2010, 132, 7559−7567. (23) Brooksby, P. A.; Downard, A. J. Multilayer Nitroazobenzene Films Covalently Attached to Carbon. An Afm and Electrochemical Study. J. Phys. Chem. B 2005, 109, 8791−8798. (24) Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Nitrophenyl Groups in Diazonium-Generated Multilayered Films: Which Are Electrochemically Responsive? Langmuir 2010, 26, 10812−10821. (25) Ranganathan, S.; McCreery, R. L. Electroanalytical Performance of Carbon Films with near-Atomic Flatness. Anal. Chem. 2001, 73, 893−900. (26) Leroux, Y. R.; Fei, H.; Noel, J. M.; Roux, C.; Hapiot, P. Efficient Covalent Modification of a Carbon Surface: Use of a Silyl Protecting Group to Form an Active Monolayer. J. Am. Chem. Soc. 2010, 132, 14039−14041. (27) Ma, H. F.; Lee, L.; Brooksby, P. A.; Brown, S. A.; Fraser, S. J.; Gordon, K. C.; Leroux, Y. R.; Hapiot, P.; Downard, A. J. Scanning Tunneling and Atomic Force Microscopy Evidence for Covalent and Noncovalent Interactions between Aryl Films and Highly Ordered Pyrolytic Graphite. J. Phys. Chem. C 2014, 118, 5820−5826. (28) Bhugun, I.; Savéant, J.-M. Derivatization of Surfaces and SelfInhibition in Irreversible Electrochemical Reactions: Cyclic Voltammetry and Preparative-Scale Electrolysis. J. Electroanal. Chem. 1995, 395, 127−131.

edge plane or basal plane graphitic sites. The similar features of CVs of NBD with other film-forming aryldiazonium ions suggests a common mechanism for all derivatives.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03233. Figures and discussion of CVs of benzene diazonium ions: NBD, Eth-Ar-N2+, TIPS-Eth-Ar-N2+, HOOC-ArN2+, CH3OC-Ar-N2+, NC-Ar-N2+, Br-Ar-N2+, F3C-ArN2+, H-Ar-N2+ and H3C-Ar-N2+; plots and discussion of relationship between CV peak current and scan rate for reduction of NBD at peak 1 on GC; topographical AFM images of films; figure showing effect of electrode pretreatment on CVs of NBD; figures showing CVs of NBD obtained on HOPG and relationship between CV peak current and scan rate (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +64 3 364 2501; fax +64 3 364 2110; e-mail alison. [email protected] (A.J.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MacDiarmid Institute for Advanced Materials and Nanotechnology. Lita Lee thanks the MacDiarmid Institute for a doctoral scholarship.

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REFERENCES

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