TEMPO Monolayers on Si(100) Electrodes: Electrostatic Effects by the Electrolyte and Semiconductor Space-Charge on the Electroactivity of a Persistent Radical

Abstract

This work demonstrates the effect of electrostatic interactions on the electroactivity of a persistent organic free radical. This was achieved by chemisorption of molecules of 4-azido-2,2,6,6-tetramethyl-1-piperdinyloxy (4-azido-TEMPO) onto monolayer-modified Si(100) electrodes using a two-step chemical procedure to preserve the open-shell state and hence the electroactivity of the nitroxide radical. Kinetic and thermodynamic parameters for the surface electrochemical reaction are investigated experimentally and analyzed with the aid of electrochemical digital simulations and quantum-chemical calculations of a theoretical model of the tethered TEMPO system. Interactions between the electrolyte anions and the TEMPO grafted on highly doped, i.e., metallic, electrodes can be tuned to predictably manipulate the oxidizing power of surface nitroxide/oxoammonium redox couple, hence showing the practical importance of the electrostatics on the electrolyte side of the radical monolayer. Conversely, for monolayers prepared on the poorly doped electrodes, the electrostatic interactions between the tethered TEMPO units and the semiconductor-side, i.e., space-charge, become dominant and result in drastic kinetic changes to the electroactivity of the radical monolayer as well as electrochemical nonidealities that can be explained as an increase in the self-interaction "a" parameter that leads to the Frumkin isotherm.

Switchable Interfaces: Redox Monolayers on Si(100) by Electrochemical Trapping of Alcohol Nucleophiles

“…As shown in Figure 1d, refinement of the N 1s emission gives two fitted peaks with binding energies of 400.5 and 401.7 eV in an approximate 3:1 ratio. The position of the low binding energy line in the S-2 samples is in agreement with the 400.6 eV previously assigned to nitroxide nitrogen atoms in thin films [22]. Electrons from the triazole heterocycle also contributed to the 400.5 and 401.7 emissions with an expected 2:1 ratio.…”

“…A common laboratory approach to attach covalently organic molecules on silicon substrates is a three-step wet chemical process. Removal of the native silica layer with fluoride-containing solutions is followed by chemical passivation [19] of the hydrogen-terminated silicon surface by means of hydrosilylation of terminal alkenes or alkynes [20][21][22] and finally chemical derivatization of the aliphatic monolayer [16]. In this work we seek to expand the chemical repertoires of this last step and explore the synthetic scope of reactive carbocations in the context of surface chemistry.…”

Square Wave Voltcoulommetry Analysis of the Influence of the Electrostatic Environment on the Electrochemical Functionality of Redox Monolayers

“…A i;n and z i are the activity and charge number, respectively, of species i, and m 0;j i and � j n are the standard chemical potential and the Galvani potential in phase j. Subscript "n" in Eqs. (5) refers to the potential pulse E n being applied. Note that surface excess of the electro-inactive co-adsorbate P is not affected by the application of the potential pulse sequence and therefore it does not depend on E n .…”

“…Self-assembling procedures of conducting or semi-conducting interfaces with electroactive species have become a broadly used methodology for endowing them of new properties that could improve their chemical/electrochemical functionality. [1][2][3][4][5] The resulting modified interfaces have a great number of applications as are the development of new sensors/ biosensors, [6,7] molecular and organic electronics, [8][9][10] and energy storage systems, [11,12] among many others, together with the possibility of exploring in detail fundamental aspects of charge transfer processes as well as the chemical reactivity at electrified interfaces. [13][14][15] In order to adjust optimally the emerging properties of these modified interfaces to a specific purpose, it is necessary to carry out their physicochemical characterization, and specifically, a detailed analysis of their electrochemical functionality.…”

Electrochemical Microscopy Based on Spatial Light Modulators: A Projection System to Spatially Address Electrochemical Reactions at Semiconductors

“…Silicon functionalization. To ensure protection of the silicon electrode against anodic decomposition 18,19 we grafted an organic layer of 1,8-nonadiyne by means of UV-assisted hydrosilylation reactions to ensures the silicon stability against anodic decomposition. 20 The a-Si wafers were cut into 1 × 1 cm pieces, cleaned with dichloromethane and water, immersed in piranha solution (100…”