Emmanuel Maisonhaute

Precise Adjustment of Nanometric-Scale Diffusion Layers within a Redox Dendrimer Molecule by Ultrafast Cyclic Voltammetry: An Electrochemical Nanometric Microtome

Performing cyclic voltammetry at scan rates into the megavolt per second range allows the exploration of the nanosecond time scale as well as the creation of nanometric diffusion layers adjacent to the electrode surface. This latter property is used here to adjust precisely the diffusion layer width within the outer shell of a fourth-generation dendrimer molecule decorated by 64 [Ru(II)(tpy)2] redox centers (tpy = terpyridine). Thus the shape of the dendrimer molecule adsorbed onto the ultramicroelectrode surface can be explored voltammetrically in a way reminiscent of an analysis with a nanometric microtome. The quantitative analysis developed here applied to the experimental voltammograms demonstrates that in agreement with previous scanning tunneling microscopy (STM) studies the adsorbed dendrimer molecules are no more spherical as they are in solution but resemble more closely hemispheres resting onto the electrode surface on their diametrical planes. The same quantitative analysis gives access to the apparent diffusion coefficient featuring electron hopping between the [Ru(II)/ Ru(III)(tpy)2] redox centers distributed on the dendrimer surface. Based on the electron hopping rate constant thus measured and on a Smoluchowski-type model developed here to take into account viscosity effects during the displacement of the [Ru(II)/Ru(III)(tpy)2] redox centers around their equilibrium positions, it is shown that the [Ru(II)/Ru(III)(tpy)2] redox centers are extremely labile in their potential wells so that they may cross-talk considerably more easily than they would do in solution at an equivalent concentration.

Surface acoustic cavitation understood via nanosecond electrochemistry. Part III: shear stress in ultrasonic cleaning

Acoustic cavitation is extensively used for cleaning purposes. However, little is known about the fundamental aspects of the cleaning process. Our previous electrochemical data suggested that acoustic bubbles were oscillating at a distance of only a few tens of nanometers above the surface [J. Phys. Chem. B 105 (2001) 12,087; E. Maisonhaute, B.A. Brookes, R.G. Compton, J. Phys. Chem. B 106 (2002) 3166-3172]. The flow velocities resulting from the bubble collapse lead to important drag and shear forces on the surface, responsible for cleaning and/or eroding the latter. We review here the forces acting on an adsorbed particle located on the surface, and develop arguments to explain why small adsorbates are harder to remove by sonication. Then, experimental results on particle desorption and surface effects brought about by ultrasound are presented and shown to agree with our theoretical predictions.

Ohmic drop compensation in cyclic voltammetry at scan rates in the megavolt per second range: access to nanometric diffusion layers via transient electrochemistry

Abstract A new concept of a three-electrode potentiostat involving positive feedback compensation of ohmic drop is discussed. This potentiostat allows the electrochemical investigation of nanosecond time scales by allowing the recording of ohmic drop-free voltammograms at scan rates in the megavolt per second range. This range of scan rate corresponds to the development of diffusion layers having only a few nanometers thickness. The principle and properties of the potentiostat are first demonstrated analytically based on a simplified equivalent circuit for the conditions used in this study (v

In situ liquid-cell electron microscopy of silver–palladium galvanic replacement reactions on silver nanoparticles

Galvanic replacement reactions provide an elegant way of transforming solid nanoparticles into complex hollow morphologies. Conventionally, galvanic replacement is studied by stopping the reaction at different stages and characterizing the products ex situ. In situ observations by liquid-cell electron microscopy can provide insight into mechanisms, rates and possible modifications of galvanic replacement reactions in the native solution environment. Here we use liquid-cell electron microscopy to investigate galvanic replacement reactions between silver nanoparticle templates and aqueous palladium salt solutions. Our in situ observations follow the transformation of the silver nanoparticles into hollow silver-palladium nanostructures. While the silver-palladium nanocages have morphologies similar to those obtained in ex situ control experiments the reaction rates are much higher, indicating that the electron beam strongly affects the galvanic-type process in the liquid-cell. By using scavengers added to the aqueous solution we identify the role of radicals generated via radiolysis by high-energy electrons in modifying galvanic reactions.

Do Molecular Conductances Correlate with Electrochemical Rate Constants? Experimental Insights

We measured single-molecule conductances for three different redox systems self-assembled onto gold by the STMBJ method and compared them with electrochemical heterogeneous rate constants determined by ultrafast voltammetry. It was observed that fast systems indeed give higher conductance. Monotonous dependency of conductance on potential reveals that large molecular fluctuations prevent the molecular redox levels to lie in between the Fermi levels of the electrodes in the nanogap configuration. Electronic coupling factors for both experimental approaches were therefore evaluated based on the superexchange mechanism theory. The results suggest that coupling is surprisingly on the same order of magnitude or even larger in conductance measurements whereas electron transfer occurs on larger distances than in transient electrochemistry.

Ultrafast cyclic voltammetry: performing in the few megavolts per second range without ohmic drop

A new concept of a three-electrode potentiostat involving positive feedback compensation of ohmic drop is used to investigate nanosecond time scales by allowing the recording of ohmic drop-free voltammograms at scan rates of a few megavolts per second. This range of scan rates corresponds to the development of diffusion layers whose widths are only a few nanometers thick. Independent tests on dummy cells (Bode plots) demonstrated that the potentiostat behaved excellently in the megavolt per second range. Examination of the well-established voltammetric reduction of anthracene in highly concentrated (0.9 M) supporting electrolyte confirmed that this potentiostat allowed the recording of undistorted ohmic drop-free voltammograms up to 2.25 MV s−1.

Voltammetric investigation of the anodic dimerization of p-halogenoanilines in DMF: Reactivity of their electrogenerated cation radicals☆

Abstract The electrodimerization mechanisms of p- chloro- and p- bromoaniline were studied in unbuffered DMF medium. By combined application of conventional and fast voltammetry (100 kV s −1 range), the primary radical cation intermediates, formed by the one electron oxidation of each p- halogenoaniline were characterized. The overall reaction path involves a dimerization via a N–C bond formation and de-halogenation at the para position. A detailed mechanistic investigation demonstrates that this proceeds through a fast reversible deprotonation of the primary radical cation followed by the subsequent N–C bond formation between the resulting radical and its parent radical cation which is the rate-determining step of the sequence (e-p-RRC-p kinetic sequence). The effect of a relatively strong, but weakly nucleophilic base, 2,6-lutidine, has been also investigated and confirmed the involvement of the fast deprotonation pre-equilibria. The fast voltammetric experiments were simulated and the apparent rate constants for the overall deprotonation/dimerization sequence obtained on the basis of peak potential shift analysis were thus confirmed.

The fabrication and characterization of adjustable nanogaps between gold electrodes on chip for electrical measurement of single molecules

This work reports on a new method to fabricate mechanically controllable break junctions (MCBJ) with finely adjustable nanogaps between two gold electrodes on solid state chips for characterizing electron transport properties of single molecules. The simple, low cost, robust and reproducible fabrication method combines conventional photolithography, chemical etching and electrodeposition to produce suspended electrodes separated with nanogaps. The MCBJ devices fabricated by the method can undergo many cycles in which the nanogap width can be precisely and repeatedly varied from zero to several nanometers. The method improves the success rate of the MCBJ experiments. Using these devices the electron transport properties of a typical molecular system, commercially available benzene-1,4-dithiol (BDT), have been studied. The I-V and G-V characteristic curves of BDT and the conductance value for a single BDT molecule established the excellent device suitability for molecular electronics research.

Electrochemically active phenylenediamine probes for transition metal cation detection

A novel family of tetraalkyl-p-phenylenediamine (TAPD)-based ligands has been efficiently prepared by reductive amination of heterocyclic aldehydes. The redox properties of these electrochemical active ligands change dramatically upon complexation of the transition metal cations Zn2+, Ni2+ and Cd2+ leading to large oxidation potential shifts of up to 950 mV depending on the nature of the ligand. Complexes with a metal to ligand ratio of 1 ∶ 2 were formed and 113Cd NMR revealed an octahedral coordination sphere of the metal. All pyridyl derivatives show a distinct chemoselectivity (Zn2+ > Cd2+ > Ni2+). The thiophenyl containing derivatives display a particularly high selectivity for zinc cations (Zn2+ ≫ Ni2+, Cd2+).

Fullerodendrimers with a tris-isothiocyanate core allowing their anchoring onto gold electrodes

Dendrimers with peripheral fullerene subunits and a tris-isothiocyanate core have been prepared and self-assembled onto a gold surface; detailed electrochemical studies revealed that electron transfer from the electrode to the fullerene subunits occurs through space at a short distance from the electrode.