Résumé
Electrocatalytic behavior of TEMPO derivative SAMs on gold has been studied in the presence of benzyl alcohol. The results demonstrate that interfacial activity of the SAMs can be enhanced by diluting the TEMPO moiety with an alkyl passive matrix. The absolute catalytic activity exhibits a maximum for an intermediate value of the surface coverage of catalytic centers. The most significant feature is the monotonic increase of the turnover (relative activity) until a limit value reached for low TEMPO surface concentrations. The electrocatalytic performances seem to be governed by a combination of two factors: the physical accessibility (by alcohol molecules in solution) and the regeneration (via the comproportionation of oxoammonium and hydroxylamine before electrochemical reoxidation) of the catalytic centers. ■ INTRODUCTION Self-assembled monolayers (SAMs) of thiolate derivatives have been widely studied and are known to form well-organized structures on various substrates. 1 The self-organization of monolayers leads to packed structures and allows high surface coverage. 2 Employing redox SAM-modified electrodes for catalysis, recognition, and sensing has resulted in some elegant examples that incorporate sophisticated receptors on the electrode surfaces. 3,4 However, one of the limitations of such architecture is the low structural quality exhibited by the monolayers when the structure-building forces are disturbed by the use of bulky active elements. In such cases, the strong interactions between the head groups result in poorly organized SAMs and possible ungrafted zones. 5 A second disadvantage can come from the strong mutual interactions of the functional groups in confined spaces. Indeed, the transposition of the properties of an active moiety from the solution to the surface can only be successful if the available space around the immobilized active center is sufficient. 6 Few works have resorted to the elaboration of mixed SAMs in order to favor active molecule accessibility 7 or enhance its stability, 8 but, to the best of our knowledge, quantification of the interfacial activity of an immobilized center as a function of its surface concentration has not been examined in detail. The present study investigates the reactivity of TEMPO derivative SAMs and mixed SAMs toward the electrocatalytic oxidation of benzyl alcohol in solution. Our goal was to demonstrate that interfacial activity of the SAMs can be enhanced by diluting the TEMPO moiety with an alkyl passive matrix. ■ EXPERIMENTAL SECTION Au substrates were prepared by deposition of ca. 5 nm of chromium followed by ca. 100 nm of gold onto a glass substrate through a shadow mask (MECACHIMIQUE/France) using physical vapor deposition system (PVD ME300 PLASSYS/France) and were made immediately before use. This protocol provides reproducible Au(111) surfaces with high crystallographic quality, low roughness (Ra less than 2 nm). They do not undergo post-treatment after completion. Mixed SAMs were elaborated by successive adsorptions of aminoxylalkanethiol C15T (Scheme 1) and dodecanethiol C12. Synthesis and characterization of C15T were described in ref 9 and dodecanethiol was used as received from Sigma-Aldrich. Cyclic voltammetry (CV) was performed in a three-electrode cell controlled at a temperature of 293K. Working electrodes were functionalized Au substrates. Counter electrodes were platinum plates. Reference electrodes were Ag/AgNO 3 (0.01 M CH 3 CN). Cyclic voltammograms were recorded in dry HPLC-grade dichloromethane (CH 2 Cl 2) and the supporting electrolyte was tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6). X-ray photoelectron spectroscopy (XPS) data have been collected using a Kratos Axis Ultra spectrometer. The X-ray source is Al K working at 1486.6 eV and using a spot size of 0.7 × 0.3 mm 2. Semiquantitative XPS analysis has been performed using a pseudo-Voigt function constrained by full width at half-maximum (fwhm) Scheme 1. Derivative of TEMPO Radical: C15T Article pubs.acs.org/Langmuir