Enhanced electrocatalytic activity on TEMPO mixed film grafted by diazonium reduction
Introduction
The booming of 2D-nanomaterials in a wide range of applications involves the transposition of chemical events from solution to surface in which electrochemistry plays a central role. However, when immobilizing a chemical structure, the conservation of its properties is not obvious. Indeed, the properties expression is often related to the molecule accessibility, its interaction with the substrate [1] and the organization of molecular buildings [2,3]. Ideally, each function must be able to interact both with the substrate, to easily exchange electron, and the medium, in which the event takes place. It seems essential to take into account this point in the design of new 2D materials in order to optimize interfacial reactivity and reaction kinetics. The dilution of the active species by the formation of mixed films represents an interesting way to modulate the environment of the active species because it does not require any particular synthesis efforts. Many examples of mixed layers based on the self-assembled monolayer technique were reported [[4], [5], [6], [7], [8], [9], [10], [11]], [[4], [5], [6], [7], [8], [9], [10], [11]] [[4], [5], [6], [7], [8], [9], [10], [11]] but are rarely focused on a systematic reactivity comparison. However, it has been shown that the dilution of the patterns may have a positive impact on the interfacial activity [[12], [13], [14]]. In recent years, the diazonium salts electrografting technique has become a technique of choice for surface modification because it offers the advantage of forming more robust films, on a very wide range of substrates [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. The major drawbacks of this technique remain the lack of control of the proportion of the two entities on the surface during a mixed layers preparation [[24], [25], [26], [27], [28]], [[24], [25], [26], [27], [28]] [[24], [25], [26], [27], [28]] and the generation of multilayers [29], inherent to the radical nature of the mechanism. In this context, we have recently developed an approach to better control the proportion of surface patterns by using the post-functionalization of a pre-formed azide platform [30]. Used in combination with a technique allowing to limit the multilayer formation, it becomes possible to obtain mixed organic films with controlled structure. Here we propose to finely investigate the interfacial reactivity of a TEMPO-modified layer [31] exploiting a model electrocatalytic reaction to draw structure-properties relationships. Azido layers were prepared under the form of multi- and monolayer by the use of DPPH as redox inhibitor, and the TEMPO surface concentration was varied by dilution with phenyl moieties. Atomic force microscopy was used to characterize the reactive organic platforms and electrochemical/electrocatalytical characteristics were extracted from cyclic voltammograms.
Section snippets
Synthesis of ((4-ethynylphenyl)carbamoyl)TEMPO (φ -TEMPO)
A mixture of 4-carboxy-TEMPO (245 mg, 1.2 mmol), 4-ethynylaniline (281 mg, 2.4 mmol), 4-(dimethylamino)pyridine (293 mg, 2.4 mmol), and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (425 μL, 2.4 mmol) was stirred in 30 mL of dichloromethane overnight at room temperature. The organic layer was washed with aqueous 4% HCl (3 × 20 mL), dried over MgSO4, and evaporated under reduced pressure. The resulting residue was purified by column chromatography on silica gel (dichloromethane/ethyl acetate
Results and discussion
The post-functionalization technique described in this paper involves a Huisgen 1,3-dipolar cycloaddition to anchor acetylene derivatives on the pre-formed reactive platform (Scheme 1).
For this purpose, a glassy carbon electrode was modified by 10 voltammetric cycles between 0.4 and −0.6 V vs Ag/AgNO3 in the presence of a millimolar solution of 4-azidobenzene-1-diazonium (4-BD-N3) in acetonitrile/nBu4NPF6 (0.1 M) (see Figure S1). A characteristic reduction peak was observed at −0.42 V, and the
Conclusion
It has been demonstrated through this study that it is possible to control the structure of functional mixed layers covalently immobilized on electrode surface. Without such control, no structure/properties relationships would be possible and consequently, no improvement could be brought to the surface activity. Two main results stem from interfacial reactivity investigations. Firstly, the dilution of immobilized TEMPO moieties leads to the marked enhancement of the relative catalytic
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
CRediT authorship contribution statement
Marius Cesbron: Investigation, Methodology, Resources. Sylvie Dabos-Seignon: Investigation, Methodology, Data curation, Formal analysis. Christelle Gautier: Conceptualization, Validation, Methodology, Writing - original draft, Visualization, Supervision. Tony Breton: Conceptualization, Validation, Methodology, Writing - original draft, Visualization, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank MATRIX SFR of the University of Angers and more precisely the ASTRAL and CARMA platforms for the characterization of organic compounds and AFM measurements. We also thank Marylène Dias for her help in the 4-azidobenzenediazonium synthesis. This work was supported by the “Centre National de la Recherche Scientifique” (CNRS France) and the “Agence Nationale de la Recherche” (ANR France). The Ministère de la Recherche is acknowledged for the PhD grant of Marius Cesbron.
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