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Spontaneous Grafting of Nitrophenyl Groups to Planar Glassy Carbon Substrates: Evidence for Two Mechanisms Joshua Lehr,§,†,‡ Bryce E. Williamson,† 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 ABSTRACT: The covalent grafting of nitrophenyl functionalities to planar carbon substrates by reaction with 4-nitrobenezene diazonium salt at open circuit potential has been studied in aqueous acid and acetonitrile solutions. Atomic force microscopy and electrochemical measurements reveal that the reaction proceeds through two distinct mechanisms. Rapid film growth occurs via reduction of the 4-nitrobenezene diazonium cation by the substrate, giving an aryl radical that couples to the surface. Film growth by this mechanism ceases once the film has reached a thickness at which electron transfer through the passivating film is no longer possible. Slow film growth via a secondary, potential-independent mechanism continues even after the substrate-dependent reaction has ceased. We tentatively propose that slow film growth involves grafting of an aryl cation originating from thermal heterolytic decomposition of the diazonium cation.
’ INTRODUCTION Modification with thin organic films is a useful method for tailoring surfaces with specific chemical and physical properties. In recent years, functionalization using aryldiazonium salts has been widely employed on carbon, metal, and semiconductor surfaces.1,2 The popularity of the method derives largely from the formation of covalent bonds between the surface and the modifiers, giving stable and robust molecular layers. Consequently, surfaces modified in this manner have been investigated for various applications including corrosion protection,3 sensor fabrication,4 catalytic electrodes,5 and molecular electronics.6 Modification using aryldiazonium salts is most commonly achieved by electrochemical reduction of the diazonium cation at the surface. Electron transfer from the substrate to the aryldiazonium cation results in the formation molecular dinitrogen and an aryl radical;7 the radical subsequently couples covalently to the surface. In most cases, the attack of aryl radicals on already attached aryl groups results in the formation of multilayer films.8,9 In addition to the electrochemically induced reaction, modification with aryldiazonium salts can be achieved spontaneously at open circuit potential (OCP) at metal,3,1012carbon,13,14 and semiconductor15 substrates by simply immersing a substrate in a solution containing diazonium salt in the absence of an applied potential. This method has proven particularly useful for modification of carbon powders16,17 and nanotubes1821 since it eliminates the need to immobilize the substrate particles on an electrode. In addition, we have recently demonstrated that the spontaneous grafting of aryldiazonium salts can be combined with microcontact printing to pattern surfaces at the microscale.22,23 r 2011 American Chemical Society
Despite the attention of several research groups, the mechanism for spontaneous grafting is less clear than that for its electrochemical counterpart. Vautrin-Ul and co-workers found that spontaneous reactions are more facile at more reducing metals (those with lower OCPs), suggesting they proceed via electron transfer from the substrate to the diazonium cation with formation of an aryl radical intermediate.24 Their mechanism is supported by observations that the OCPs of carbon16,25 and gold12 substrates increase during spontaneous modification, consistent with the build-up of positive surface charge concomitant with the loss of electrons. Spontaneously formed films at gold have been shown to be self-limiting (or near self-limiting) in terms of surface concentration and film thickness, which is also consistent with a mechanism involving reduction by the substrate: when the film becomes sufficiently thick, it passivates the surface by blocking electron transfer and further growth is inhibited.12 However, an alternative explanation for spontaneous grafting at gold surfaces has very recently been proposed.26 Deniau and co-workers found no experimental evidence for the involvement of radicals and suggested that either the diazonium cation or the aryl cation (formed by heterolytic dediazoniation of the diazonium cation) grafts spontaneously to the gold surface. The spontaneous reaction of diazonium salts with planar carbon substrates has not been studied extensively. Vautrin-Ul and co-workers demonstrated spontaneous grafting of nitrophenyl films to glassy carbon (GC) in acetonitrile (ACN) solutions with surface concentrations indicative of multilayers.10 In a Received: December 13, 2010 Revised: February 27, 2011 Published: March 18, 2011 6629
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The Journal of Physical Chemistry C related study, Tammeveski and co-workers spontaneously grafted anthraquinone to GC in ACN and aqueous conditions.14 A broader investigation by Bidan and co-workers demonstrated that immersion of GC in solutions of less-readily reduced diazonium salts does not lead to modification, suggesting that modification requires reduction of the diazonium cation by the substrate.25 Mechanistic studies of the modification of carbon black with nitrophenyl films have been undertaken by Belanger and co-workers; it was proposed that the carbon powder cannot be an effective electron donor for bringing about the reduction of the diazonium cation. They suggested that modification proceeds via a process analogous to the Kolbe reaction, with surface carboxylic acid groups acting as the electron source for homolytic cleavage of the diazonium moiety.17 In this mechanism, multilayer formation would not be expected, since the surface-bound carboxylic acid is consumed in the process of generating the radical. Interestingly, and in support of Belanger’s suggestion, Pickup and co-workers found low (0.1 monolayer) surface concentrations for diazonium-derived anthraquinone films at carbon black.16 In recent reports, Compton and co-workers suggest that spontaneous modification of graphite powder, glassy carbon powder27 and multiwalled carbon nanotubes20 from aqueous 4-nitrobenzenediazonium (NBD) salt solutions does not proceed via generation of aryl radicals. They propose instead the formation of the aryl cation intermediate by spontaneous, substrate-independent decomposition of the aryldiazonium cation. Here we describe a study of the spontaneous modification of GC in NBD salt solutions. NBD is a “benchmark” diazonium reagent since its reactions may be readily monitored electrochemically due to the presence of electroactive nitrophenyl (NP) groups in the resulting film. The work was undertaken in ACN and aqueous acid solutions using commercial GC and a GC-like pyrolyzed photoresist film (PPF). These substrates are of interest due to their wide use as electrode materials and because their planarity facilitates quantitative electrochemical and atomic force microscopy (AFM) investigations of the mechanism of film formation.
’ EXPERIMENTAL SECTION Aqueous solutions were prepared using Milli-Q water with resistivity >18 MΩ cm. Sulfuric acid (Aldrich, 98%) was used as received. ACN (HPLC grade) was dried over CaH2 for at least 2 days before refluxing for 2 h and subsequent distillation under a N2 atmosphere. NBD was prepared as described previously.28 GC disk electrodes (area = 0.07 cm2) were polished with a 1 μm alumina slurry and sonicated for >5 min in acetone prior to modification. PPF was prepared as reported previously.9 Surface modifications at OCP were undertaken by immersing the carbon substrate in a 10 mM solution of NBD in either 0.14 M H2SO4 or ACN with 0.1 M [Bu4N]BF4 electrolyte. Solutions were thoroughly degassed with N2 and modification reactions were carried out in the dark under a constant stream of N2. After modification, electrodes were sonicated in Milli-Q water for 5 min to remove any physisorbed material. Tests established that longer sonication times in aqueous media or sonication for up to 30 min in ACN did not cause further changes in the surface concentrations of films. Electrochemical experiments were carried out using an Ecochemie Autolab PGSTAT 302 potentiostat and a three-electrode cell. The auxiliary electrode was Pt wire and the reference
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Figure 1. Cyclic voltammograms of GC electrodes in 0.14 M H2SO4 after immersion for 60 min in (a) 10 mM NBD/0.1 M [Bu4N]BF4/ACN and (b)10 mM NBD/0.14 M H2SO4. Scan rate = 100 mV s1.
electrode was saturated calomel (SCE) or Ag/Agþ (102 M AgNO3 in 0.1 M [Bu4N]BF4/ACN) for aqueous and ACN solutions, respectively. Electroreduction of NP films was achieved by recording two consecutive cyclic voltammograms in 0.14 M H2SO4 between approximately 0.7 and 1.2 V vs SCE from an initial potential of 0.7 V. The charge associated with reduction, and hence the surface concentration of electroactive NP groups prior to reduction, was determined from the cyclic voltammograms using Linkfit curve-fitting software with thirdorder polynomial baselines.29 Electroreduction was performed in aqueous acid rather than ACN solution because the process is typically not fully chemically reversible in the latter. In ACN, an unknown fraction of the generated radical anions are protonated by adventitious water and are further reduced, so that the overall reduction involves more than one electron per NP group, and the stoichiometry is unknown.30 In aqueous conditions, the number of electroactive NP groups can be calculated from the relationship between the irreversible NP reduction and the hydroxyaminophenyl oxidation.31 OCP measurements were made using a Digitech QM1320 multimeter. The procedure for atomic force microscopy (AFM) film thickness measurements has been previously described.12
’ RESULTS Figure 1 shows cyclic voltammograms, obtained in 0.14 M H2SO4, of GC electrodes modified by immersion for 60 min in solutions of (a) NBD/0.1 M [Bu4N]BF4/ACN and (b) NBD/ 0.14 M H2SO4. Both exhibit the characteristic irreversible reduction (Epc ≈ 0.6 V vs SCE) of NP groups to aminophenyl (eq 1) and hydroxyaminophenyl groups (eq 2), as well as an anodic peak at Epa ≈ 0.35 V assigned to hydroxyaminophenyl oxidation (eq 3).32 These observations confirm surface modification by attachment of NP groups.22,24 There is no evidence for residual diazonium moieties, which, if present, would be reduced at Epc ≈ 0 V. GC-Ph-NO2 þ 6Hþ þ 6e- f GC-Ph-NH2 þ 2H2 O 6630
ð1Þ
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Figure 2. Effect on OCP of GC of addition, at time = 60 min, of (b) NBD, (O) p-nitroanaline, (1) or blank electrolyte to (a) 0.1 M [Bu4N]BF4/ACN and (b) 0.14 M H2SO4 solution.
GC-Ph-NO2 þ 4Hþ þ 4e f GC-Ph-NHOH þ H2 O ð2Þ GC-Ph-NHOH T GC-Ph-NO þ 2Hþ þ 2e
ð3Þ
As an initial step toward ascertaining the characteristics and mechanism of film formation, the responses of the OCPs of GC electrodes to addition of NBD were monitored. The electrodes were immersed in “blank” solutions of electrolyte in (0.1 M [Bu4N]BF4/ACN or 0.14 M H2SO4) for an “equilibration” period of 60 min. An equal volume (5 mL) of 20 mM NBD/electrolyte, 20 mM p-nitroaniline/electrolyte, or “blank” electrolyte was then added and the OCPs were monitored for a further 120 min (Figure 2). In both media, addition of NBD (b) causes an immediate and significant jump in the OCP. In contrast, additions of blank electrolyte (O) or p-nitroaniline (1), a structural analog of NBD without the diazonium functionality, cause no sudden change in OCP, indicating that the presence of the diazonium functionality is responsible for the response. The increase in OCP is followed by a slower decrease, which is particularly evident in the aqueous acid medium. The behavior before and after addition of NBD was reproducible when experiments were repeated using the same, carefully polished, GC electrode. When different electrodes were used, the behavior was qualitatively the same but the baseline potentials could differ by as much as ∼0.3 V. Two methods were used to more directly quantify film growth as a function of immersion time. In the first, cyclic voltammograms of GC surfaces were recorded in 0.14 M H2SO4, after grafting in a solution of NBD in 0.14 M H2SO4 or ACN for variable time periods up to 13 h. In the second, AFM was used to measure the thicknesses of NP films grown at OCP in acidic NBD solutions on PPF. PPF is a GC-like substrate with a very low surface roughness, making it suitable for AFM depth profiling.33 Figure 3a shows ΓNP, the surface concentrations of electroactive NP groups calculated from the charges associated with the
Figure 3. (a) ΓNP and (b) ENP pc vs immersion time for films prepared on GC in (b) NBD/ACN and (O) NBD/H2SO4. (c) Film thickness vs immersion time for films prepared on PPF in NBD/H2SO4.
redox processes of eqs 13, vs immersion time in NBD/ACN (b) and NBD/H2SO4 (O). In both media, ΓNP increases rapidly over the first ∼30 min, to a maximum of ∼40 50 1010 mol cm2, followed by a slower decrease, which is more marked for films grafted in aqueous acid. The surface concentrations throughout are consistent with multilayer films; monolayers of similar groups attached via diazonium salt reduction typically have ΓNP ≈ (4.5 ( 1.5) 1010 mol cm2.9,12,34 The peak potentials, ENP pc , for irreversible reduction of the NP groups (eqs 1 and 2) for the same samples are given in Figure 3b. In both media, ENP pc shows the behavior, reported previously for electrografted NP films,30,35 of becoming more negative at longer immersion times, indicating that reduction is becoming increasingly “difficult”. The values in ACN are slightly less negative, but the trends in the two media are roughly parallel; a more rapid initial drop followed by slower change that persists throughout the immersion times investigated. For films prepared in H2SO4 solution, the initial drop is steeper, occurring over ∼30 min compared with ∼200 min in ACN. Figure 3c shows that the thickness of the film prepared on PPF in acidic aqueous NBD solution is close to its maximum after ∼60 min immersion time, and that subsequent changes are small compared with the uncertainty of the measurements. Further experiments were performed at GC by applying potentials significantly more positive than the reduction peak for the diazonium moiety (Figure 4a,b); at these potentials the reduction of NBD by electron transfer from the substrate will be suppressed. A GC electrode was immersed in 5 mL of blank electrolyte (aqueous acid or 0.1 M [Bu4N]BF4/ACN), the 6631
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concentrations are lower still and become independent of the applied potential.
Figure 4. (a),(b) Cyclic voltammograms of 1 mM NBD in (a) 0.1 M [Bu4N]BF4/ACN and (b) 0.14 M H2SO4 solution; scan rate = 100 mV s1. (c) ΓNP and (d) ENP pc vs applied potential during grafting in NBD/ACN (b, vs Ag/Agþ for 4 h) and NBD/H2SO4 (O, vs SCE for 13 h).
selected potential was applied and then 5 mL of 20 mM NBD in the same electrolyte was added to give a NBD solution concentration of 10 mM. The potential was then maintained for 4 h (in ACN) or 13 h (in aqueous acid), after which the electrode was removed from solution, rinsed by sonication and analyzed by cyclic voltammetry in 0.14 M H2SO4. Figure 4c,d shows plots of ΓNP and ENP pc vs applied potential for GC electrodes after immersion in 0.1 M [Bu4N]BF4/ACN (b) and 0.14 M H2SO4 (O) solutions of NBD. NP films are formed in both media, with ΓNP and ENP pc values that indicate very slow growth. For example, the film formed from aqueous acid after application of 0.65 V (vs SCE) for 13 h has electrochemical parameters close to those of films formed at OCP after only a few minutes (Figure 3a,b). At higher applied potentials, the surface
’ DISCUSSION As noted in the Introduction, spontaneous grafting at planar GC surfaces from diazonium salt solutions has been generally assumed to follow the mechanism proposed for metal substrates involving reduction of the diazonium cation by electron transfer from the substrate. However, the data in Figure 4 reveal the growth of NP-containing films even at applied potentials where the electron-transfer mechanism will be strongly suppressed. The questions therefore arise: (1) what other possible mechanism(s) could be responsible for such growth and; (2) is more than one mechanism operative during spontaneous film formation at OCP? Plausible alternative grafting processes to electron transfer from the substrate involve formation of uncharged aryl radicals via reduction by agents other than the substrate36,37 (for example, the solvent), heterolytic cleavage of the diazonium cation to yield an aryl cation and dinitrogen,20,26,27,36,37 and reaction of NBD with surface carboxylate moieties resulting in coupling of NP groups and loss of CO2.17 Polished GC surfaces have submonolayer amounts of oxygen functionalities and hence the formation of multilayer films is not consistent with a mechanism relying on reaction of NBD with surface carboxylates. The initial, rapid increase in the OCP of GC electrodes on addition of NBD (Figure 2) closely matches the responses noted by other workers for reactions of diazonium salts with GC.25 It is indicative of the accumulation of positive charge on the substrate, which is incompatible with a process involving surface attack by (uncharged) radicals generated via a substrateindependent mechanism. It is however consistent with grafting by electron transfer from the substrate to NBD cations in solution or by attachment of cationic species. The subsequent decay in the OCP of GC in H2SO4 solution (Figure 2b) is very similar to that observed for the same process in the same medium at gold substrates,12 where it was attributed to the oxidation of adventitious solution impurities. We propose that similar mechanisms operate in this work, with the overall OCP curves arising from competition between a film-growth process (which charges the substrate) and oxidation of adventitious species (which discharges the substrate). The slower rate of discharge in ACN (Figure 2a) could be due to either an inherently slower discharge mechanism (for example different and/or less concentrated impurities) or sustained film growth that competes more effectively and over a longer period than in aqueous acid, possibilities that we discuss below. The AFM-determined film thicknesses at PPF in Figure 3c indicate that the electrochemically determined decay of ΓNP in Figure 3a for GC after ∼30 min is not associated with a concomitant decrease in the amount of grafted film. A more tenable explanation (demonstrated previously for films electrografted from diazonium salt solutions35,30) is that the decreased cyclic voltammetric response is due to diminished NP electroactivity. This is consistent with the change of ENP pc with immersion time and is probably due to inhibited transport of ions and/or solvent through multilayer blocking films as they become thicker and/or denser. The evolution of ENP pc with immersion time (Figure 3b) allows comparison of the film-growth characteristics in the two media. Over the first ∼30 min of immersion, the decrease of ENP pc is faster for films prepared in H2SO4 solution than in ACN. This may 6632
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The Journal of Physical Chemistry C indicate faster initial film growth in aqueous acid, but it could also be due to the films having inherently different blocking characteristics; electroreduction of NBD in aqueous acid has previously been found to give more passivating films than those formed in ACN.9 From ∼100 min onward, the changes in ENP pc are very similar in the two media. Although experimental uncertainties in the AFM profiling measurements preclude the determination of small changes in film thickness after ∼200 min (Figure 3c), the continuing drift of ENP pc to lower potential indicates that slow growth continues for at least 13 h. This growth may mainly “fill in” the film, increasing its density rather than its thickness. Given the similarities in the ENP pc vs immersion time trends, it seems reasonable to propose that the film growth and, by inference, the reaction mechanism(s) are the same in the two media. However, there is tentative evidence from the data in Figure 3 that the initial burst of film growth is slower in ACN, but sustained over a longer period than in aqueous acid. Hence, the apparently slower discharge of the substrate in ACN (Figure 2) may be due partially to sustained charging, as the film takes longer to become blocking to electron transfer in ACN. Nevertheless, this seems unlikely to be able to fully account for the very marked differences in discharge rates and we believe that an inherently slower discharge mechanism is almost certainly operating in ACN. Grafting of NP films at high applied potentials (Figure 4) provides important clues about the mechanism(s) of spontaneous grafting at OCP. The data for the H2SO4 system at 0.65 V point to electrografting at a rate that is very slow in comparison with spontaneous grafting at OCP (surface concentrations that take 13 h to accumulate at 0.65 V require only a few minutes at OCP). The cyclic voltammograms (Figure 4a,b) show that the reduction peak potential for NBD in ACN is ∼400 mV more negative than in aqueous acid. It is therefore understandable that slow electrografting can occur at this potential in aqueous acid but not in ACN. At applied potentials of 0.7 V and greater, the amount of film formed is independent of the applied potential, which is inconsistent with an electrochemical reaction. Thus we can conclude that electron transfer has been entirely suppressed at these potentials and that the continued slow growth of NPcontaining films must involve a potential- and substrate-independent mechanism. The significantly lower amount of NP film grafted at applied potentials g0.7 V than after the same reaction time at OCP indicates that the higher potentials have suppressed the reaction that is responsible for rapid growth of substantial films at OCP. In other words, the initial rapid grafting reaction at OCP must proceed through a pathway that is potential-dependent. Clearly, this is consistent with a primary OCP reaction involving reduction of NBD by electron transfer from the substrate. On the other hand, it does not seem to be consistent with a reaction pathway involving aryl cations. First, in the presence of excess supporting electrolyte (as used in these experiments), surface charge screening should make the rate of film growth via the cation mechanism independent of surface potential. And second, at the concentrations used in these experiments, the rate of formation of aryl cations by spontaneous (substrate-independent) heterolytic cleavage of the diazonium should be constant for the durations of the experiments. The initial film growth at OCP exhibits neither of these characteristics, reaffirming that reduction of NBD via electron transfer from the substrate is the dominant mechanism for film growth at short times.
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Although grafting via aryl cations cannot account for the rapid grafting reaction at short reaction times, it may account for the continued slow surface-independent film growth once a blocking layer has formed. Since ACN and aqueous acid solutions have relatively low nucleophilicity parameters, reduction of NBD by the solvent to generate aryl radicals is not expected to be a major dediazoniation pathway; rather thermal decomposition of NBD leading to the formation of aryl cations is a more likely candidate.37 We have not investigated the mechanism of this secondary grafting reaction but assume that it also operates in the early stages of spontaneous film grafting or electrografting as a minor background process concurrent with electron transfer. On the basis of observations from electrografted films, the rate of electron transfer across a 4 nm NP film is expected to be very slow.9 The plot of ENP pc vs immersion time in H2SO4 solution (Figure 3b) reveals a rapid decrease in ENP pc over the first ∼30 min, and then a change to a more gradual decrease, which presumably reflects a change in the film growth rate. AFM measurements show that films grafted spontaneously from H2SO4 solution (Figure 3c) have thicknesses close to 4 nm after ∼30 min. Hence, in aqueous acid solution, we propose that reduction of diazonium cations by electron transfer from the substrate dominates film growth for the first ∼30 min. However, during that initial period, the growing film significantly slows electron transfer and thus the secondary reaction becomes increasingly important at longer reaction times. The data of Figure 3b suggest that, in ACN, initial film growth is somewhat slower than in H2SO4 solution (at short reaction times, ENP pc is less negative for films grafted in ACN) and that electron transfer driven grafting dominates for the first ∼200 min, after which grafting through the secondary reaction becomes the important film growth mechanism.
’ CONCLUSIONS The spontaneous modification of GC with NBD at OCP, in both aqueous acid and ACN solutions, has been shown to proceed via two mechanisms. Fast film growth at short immersion times occurs primarily through reduction of NBD by electron transfer from the carbon substrate to produce aryl radicals that couple to the surface. As found for electrografted films, this film-formation process is self-limiting, ceasing when the film becomes too blocking for electron transfer from the surface to the NBD cation. A secondary pathway accounts for the continued slow growth of films for at least 13 h. It occurs via a solution-generated intermediate, tentatively proposed to be an aryl cation, and is probably operative as a minor background process even as the surface-facilitated process proceeds. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 64-3-3642501. Fax: 64-3-3642110. E-mail:
[email protected]. Present Addresses §
Discipline of Chemistry, University of Newcastle, Callaghan, NSW 2308, Australia.
’ ACKNOWLEDGMENT The authors thank the Department of Electrical and Computer Engineering, University of Canterbury, for use of AFM 6633
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The Journal of Physical Chemistry C facilities. We thank the MacDiarmid Institute for Advanced Materials and Nanotechnology and the University of Canterbury for funding.
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