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The reduction of 4-nitrobenzene diazonium electrografted layer: An electrochemical study coupled to in situ sum-frequency generation spectroscopy

Abstract : This work describes an electrochemical study of 4-nitrobenzene diazonium (4-NBD) reduction onto glassy carbon (GC) electrode coupled to in situ sum-frequency generation (SFG) spectroscopy. After 4-NBD grafting at 0.3 V vs. saturated calomel electrode (SCE) onto GC, SFG allowed a clear signal assigned to the symmetrical vibration mode of the nitro (NO 2) groups to be observed at 1349 cm À1 or 1353 cm À1 depending on whether the spectrum was recorded in air or inside the solution. This result proved that 4-NBD grafting actually occurs at a potential as high as 0.3 V vs. SCE. The combination of SFG data and cyclic voltammetry (CV) also indicated that at such a potential, NO 2 groups did not experience reduction process into hydroxylamine (NHOH) or amine (NH 2) groups. The electrolysis of grafted NO 2 moieties at À0.1 V was followed by CV and in situ by SFG. The exponential decay of the NO 2 signal located at 1353 cm À1 vs. electrolysis time was in accordance with a charge transfer-limited reaction rate for a species immobilized at the electrode surface, and allowed a first order kinetic rate constant for NO 2 reduction to be estimated k ¼ 0.006 s À1. The integration of the peaks observed on the corresponding cyclic voltammograms (CVs) which were attributed to the NO/NHOH reversible system showed that the NO 2 reduction produced both hydroxylamine and amine groups and was not quantitative. The fact that SFG spectroscopy was silent for long electrolysis time values suggested the remaining nitro groups to be located far from the electrode surface, as a consequence of an electron tunneling efficiency which decreased throughout the film thickness. Further electrolysis at À0.8 V allowed the remaining nitro groups to be reduced into NH 2 with almost quantitative yields. All these results suggest the existence of a stratified layer during the electrolysis process, in which there is no limitation due to H þ diffusion in the organic film.
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Submitted on : Monday, May 13, 2019 - 10:18:26 AM
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William Richard, David Evrard, Bertrand Busson, Christophe Humbert, Laetitia Dalstein, et al.. The reduction of 4-nitrobenzene diazonium electrografted layer: An electrochemical study coupled to in situ sum-frequency generation spectroscopy. Electrochimica Acta, Elsevier, 2018, 283, pp.1640-1648. ⟨10.1016/j.electacta.2018.07.073⟩. ⟨hal-02111048⟩

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