Elucidation of the Mechanism of Redox Grafting of Diazotated

Article pubs.acs.org/Langmuir

Elucidation of the Mechanism of Redox Grafting of Diazotated Anthraquinone Sergey Chernyy,† Antoine Bousquet,§ Kristian Torbensen,† Joseph Iruthayaraj,†,‡ Marcel Ceccato,† Steen Uttrup Pedersen,*,†,‡ and Kim Daasbjerg*,†,‡ †

Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus, Denmark Interdisciplinary Nanoscience Center (iNANO), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark § IPREM UMR CNRS 5254, Université de Pau et des Pays de l’Adour, Hélioparc 2 avenue du Président, Angot, 64053 Pau cedex 9, France ‡

S Supporting Information *

ABSTRACT: Redox grafting of aryldiazonium salts containing redox units may be used to form exceptionally thick covalently attached conducting films, even in the micrometers range, in a controlled manner on glassy carbon and gold substrates. With the objective to investigate the mechanism of this process in detail, 1-anthraquinone (AQ) redox units were immobilized on these substrates by electroreduction of 9,10-dioxo-9,10-dihydroanthracene-1-diazonium tetrafluoroborate. Electrochemical quartz crystal microbalance was employed to follow the grafting process during a cyclic voltammetric sweep by recording the frequency change. The redox grafting is shown to have two mass gain regions/phases: an irreversible one due to the addition of AQ units to the substrate/film and a reversible one due to the association of cations from the supporting electrolyte with the AQ radical anions formed during the sweeping process. Scanning electrochemical microscopy was used to study the relationship between the conductivity of the film and the charging level of the AQ redox units in the grafted film. For that purpose, approach curves were recorded at a platinum ultramicroelectrode for AQ-containing films on gold and glassy carbon surfaces using the ferro/ferricyanide redox system as redox probe. It is concluded that the film growth has its origin in electron transfer processes occurring through the layer mediated by the redox moieties embedded in the organic film.

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INTRODUCTION Electrodes coated with electroactive polymers have been intensively studied due to the promising electrocatalytic, optical, sensing, and charge storage properties of such films.1 One of the most versatile approaches for modification of carbon and metals involves the electroreduction of aryldiazonium salts.2−4 This technique has earned large success as evidenced by the increasing number of reviews on its mechanism and applications.2,4−7 It is now well established that the electrografting mechanism involves an electron transfer from the electrode surface to the aryldiazonium ion which promotes the formation of aryl radicals capable of attacking the surface and forming a covalent bond with it.8−11 Because highly reactive aryl radicals are involved in the grafting process, the film is usually a disordered and insulating polyarylene multilayer,8,12 providing the method with one of its most characteristic feature, i.e., a self-limiting film growth. In most cases the film growth comes to a halt at E0O/R with ET denoting the applied potential at the tip), the measured tip current will depend on the extent by which O may be regenerated at the AQ film. In the case that no charge transfer can take place at the film site, which is expected if the film is in its neutral and insulating state (i.e., ES > E0AQ with ES denoting the applied potential of the substrate), the only role of the substrate/film will be to physically block the free diffusion of R to the tip electrode, thus leading to a negative feedback situation. In contrast, if a reduction of O to R is feasible by the substrate through a mediating and charged AQ film (i.e., ES < E0AQ and E0AQ < E0O/R), then more R will effectively become available at the tip and an increase in the tip current is monitored as positive feedback. In these studies we employed three different AQ embedded films, Au-AQx, where x = 5, 20, or 52 denotes the film thickness in units of nanometers. Furthermore, Fe(CN)63−/4− served as = the solution-based redox system O/R in water (E0Fe(CN)3−/4− 6 9578

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Figure 5A shows a plot of log kapp against ES for a bare Au and the three Au-AQ films. For the bare Au electrode, a diffusion-controlled plateau is reached at very small overdue to the fact that a direct potentials with respect to E0Fe(CN)3−/4− 6 and unhindered charge transfer may take place at the surface. In contrast, for all three Au-AQ films the measured kapp values are substantially smaller. At low applied overpotentials until ES ≈ 0.4 and −0.1 V vs SCE for Fe(CN)63− and Fe(CN)64−, respectively, the approach curves of the two thickest film show negative feedback with kapp = 1.0 × 10−4 cm s−1. In comparison, kapp is higher (= (5−6) × 10−4 cm s−1) for Au-AQ5, indicating that either some permeation of the redox probe into the films and/or diffusion through pinholes is possible. This is in agreement with the conclusion derived from the cyclic voltammetric investigations of the films (vide supra). As the overpotential is increased, a raise in kapp is noticed until plateaus are reached in both potential directions at ES ≈ 0.9 V and ES ≈ −0.5 V vs SCE, respectively, with kapp = 2.6 × 10−2 cm s−1 for bare Au, 2.7 × 10−3 cm s−1 for Au-AQ5, 8.0 × 10−4 cm s−1 for Au-AQ20, and 3.5 × 10−4 cm s−1 for Au-AQ52, indicating that the charge transfers taking place at pinholes are governed by diffusion-controlled processes. As expected, the exact value of kapp and the overpotential required for reaching the plateaus are dependent on the film thickness, in that kapp is smallest and the overpotential largest for the thickest film. Pushing ES even further in a negative direction, a third region on the left-hand side of Figure 5A is observed, in which kapp increases once again to reach a new plateau value. In particular, for the Au-AQ5 and Au-AQ20 films this region is very distinct, corresponding exactly to the potential range for charging the AQ film described by Einit red,AQ = −0.5 and Ep,AQ = −1.035 V vs SCE. This suggests that the increase in kapp can be ascribed to an increase in the film conductivity and that at sufficiently high overpotentials the electron transfer occurring at the film/ solution interface is at its diffusion-controlled limit, with values of kapp = 1.7 × 10−2 cm s−1 for Au-AQ5, 1.0 × 10−2 cm s−1 for Au-AQ20, and 4.1 × 10−3 cm s−1 for Au-AQ52. To exclude that this raise in the conductivity of the films could be due to anything else but their charging, it would be pertinent to carry out a corresponding series of experiments, in which the overpotential was pushed further in the positive direction, exploiting that an oxidation of the Au-AQ films is precluded. Unfortunately, such a potential extension is made impossible by the appearance of other oxidation processes on the Au substrate. Hence, it was decided to include glassy carbon (GC) as substrate material, taking advantage of its larger applicable potential window. Three AQ-modified GC electrodes, GC-AQ7, GC-AQ22, and GC-AQ39, were therefore redox grafted as described in the Experimental Section and subjected to the same sort of investigation as the three Au-AQ electrodes. First of all, the cyclic voltammetric experiments carried out at these films using Fe(CN)64− as redox probe in aqueous solution (Figures S3 and S4, Supporting Information) showed an almost complete blockage of the charge-transfer processes. The fact that the blocking effect is larger on GC than Au (compare with Figures S1 and S2) points to the formation of a more compact film on the former material as already noticed and discussed by Gooding and co-workers59,60 However, the quality of the fitting of the approach curves to obtain kapp in SECM is equally good for the two materials as illustrated in the Supporting Information (Figures S6−S9).

0.135 V vs SCE), fulfilling the very important requirement of being, by and large, unable to permeate or diffuse through pinholes57 into the relatively hydrophobic organic film. At least this is true for the thickest films as evidenced directly from the high blocking effect in these cases toward Fe(CN)64− in cyclic voltammetry (Figures S1−S4; see the Supporting Information for a discussion of the characteristics of films on both Au and glassy carbon substrates). For Au-AQ5 a distorted signal can be discerned, indicating that some permeation or diffusion through pinholes may take place in this case. In the negatively charged state of the films, permeation would be expected to be even lower because of the strong electrostatic repulsion between the film and the negatively charged redox probe. A series of SECM approach curves were recorded using ET = 0.455 V vs SCE to secure a diffusion-controlled oxidation of the redox probe, Fe(CN)64−, while varying ES in consecutive steps of −0.1 V from −0.045 to −1.145 V vs SCE. In this manner the characteristic reduction region of the AQ film was passed, noting that the potential corresponding to the initial rise of the AQ wave in cyclic voltammetry, Einit red,AQ, = −0.5 V vs SCE and the pertinent peak potential, Ep,AQ, = −1.035 V vs SCE (Figure S5, Supporting Information). Figure 4 shows the approach curves recorded for the AuAQ20 film under these conditions together with the theoretical

Figure 4. Plots of the normalized tip current, iT/iT,inf, vs the dimensionless distance, L (= d/a), for consecutive SECM approach curves recorded for a Au-AQ20 film using 2 mM K4Fe(CN)6 as redox probe in 0.1 M KCl (●) along with theoretical fits (solid lines); iT,inf denotes the current recorded far away from the substrate, and a is the radius of the tip electrode. Obtained with a Pt-tip electrode [a = 12.5 μm and RG (glass to Pt-wire ratio of radii) = 4], ET = 0.455 V vs SCE, approach rate = 1 μm s−1, and ES varied in steps of −0.1 V going from −0.045 to −1.145 V vs SCE.

curves; results for other potential ranges, films, and electrode materials are collected in the Supporting Information (Figures S6−S9). Evidently, the response goes from negative to positive feedback as ES is becoming more negative. This development indicates that there is a potential-dependent increase in the rate of reduction of Fe(CN)63− at the AQ film/solution interface. By fitting the experimental data to theoretical calculations, the apparent heterogeneous electron transfer rate constant, kapp, at the substrate/solution interface may further be extracted as discussed in the Supporting Information.58 In precisely the same manner (and for comparative reasons) we have obtained kapp for a positively biased substrate with ES ranging from 0.255 to 0.955 V vs SCE and ET = −0.245 V vs SCE, using now Fe(CN)63− as redox probe for examining the charge transfer properties of the noncharged film. 9579

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Figure 5. Plots of log kapp vs ES for (A) bare Au (■), Au-AQ5 (●), Au-AQ20 (▲), and Au-AQ52 (⧫) and (B) bare GC (■), GC-AQ7 (●), GC-AQ22 (▲), and GC-AQ39 (⧫). The data on the left-hand sides of each plot were obtained with Fe(CN)64− and those on the right-hand side with Fe(CN)63− as solution-based redox probe.

Figure 5B shows a plot of log kapp as a function of ES for bare GC and the three GC-AQ electrodes with Fe(CN)63−/4− again serving as the solution based redox system. The main observation worth noting here is that a large increase in the charge transfer rate is once again observed upon increasing the applied negative substrate polarity, while the effect of a corresponding increase in the positive substrate polarity is modest. This shows that a large driving force is, in general, required for enforcing charge transport through the compact organic film, unless one exploits the fact that the AQ moieties in the film are reducible as on the left-hand side of Figure 5B. The effect on kapp of charging the film is even more pronounced for the films grafted on GC compared to those on Au. In spite of a great compactness of the films on GC as evidenced by the low value of kapp (≈ 5.3 × 10−5 cm s−1) at low overpotentials, the shape of the curves resembles those obtained for the bare substrate with a width that increases as the thickness of the film becomes larger but, nevertheless, ending up in quite similar plateau values for kapp, i.e., kapp = 1.9 × 10−2 cm s−1 for bare GC, 1.7 × 10−2 cm s−1 for GC-AQ7, 1.5 × 10−2 cm s−1 for GC-AQ22, and 1.1 × 10−2 cm s−1 for GCAQ39. Hence, reduction of the AQ moieties induces radical changes in the properties of the film, which allows it to behave like a true electrode with the charge transfer to and from the Fe(CN)63−/4− redox system occurring efficiently at the film/ solution interface.

electron transport through the layer and charge transfers at the film/solution interface to occur efficiently.

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ASSOCIATED CONTENT

S Supporting Information *

Blocking experiments, electrochemical characteristics of GCAQ films, and SECM approach curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +45 8942 3908, Fax +45 8619 6199, e-mail [email protected]. dk (S.U.P.); Tel +45 8942 3922, Fax +45 8619 6199, e-mail [email protected] (K.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Danish Agency for Science, Technology and Innovation, Grundfos A/S, and SP Group A/S are gratefully acknowledged for financial support.

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REFERENCES

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CONCLUSION Electroreduction of aryldiazonium salts may lead to the formation of thick organic films, depending on the conditions used for the grafting. For the so-called redox grafting method to be successful in this respect, diazonium salts containing a redox moiety must be reduced in a repetitive potential sweeping procedure. According to the EQCM study of the mechanism the redox grafting of the anthraquinone diazonium salt occurs via the mediated reduction of the already grafted AQ units in the film. It was observed that the mass increase of the film on the reductive sweep is due to the irreversible attachment of AQ units as well as the reversible association of Bu4N+ from the supporting electrolyte. At the same time expulsion of physisorbed species takes place as a result of the sweeping, thereby opening the layer structure for further grafting. Finally, SECM studies show that the film, upon charging of the AQ redox units, indeed becomes conductive, hence allowing 9580

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