Voltammetry of Metallic Powder Suspensions on Mercury Electrodes

Abstract

Ultrafine powder samples of Cu, Fe, Ni, Mo and W were examined voltammetrically in aqueous suspensions with hanging mercury drop electrode. According to chemical interaction of the metallic powder particles with the dispersing medium the voltammetric curves either consist of a set of individual current peaks given by particulate reduction of surface oxides on impingements of particles from their suspension upon the electrode, or correspond to electroactivity of dissolved products of chemical reaction between particles and the medium. In connection with increasing importance of metallic powders in contemporary industrial technology we attempted to find a way of their characterization by an electrochemical method. Although the present literature on electrochemistry of metallic nanoparticles is fairly rich (e.g. [1 -3]), we did not find so far any paper, which would compare voltammograms of powder suspensions of different metals.When the dispersed electroactive particles are of the size of several nanometers, their diffusion coefficients are near to those of molecules, and their motion towards the electrode is controlled by the rate of diffusion, as in the case of true solutions [4]. In our present case the dimensions of the particles are of the order of hundred nanometers and their movements in the solution follow more or less the laws of Brownian motion. The voltammetric curves of some of the powder samples dispersed in neutral and/or alkaline solutions displayed characteristic irregular peaks of irreversible cathodic current, due obviously to electroreduction of surface oxides, notoriously present on the surfaces of metallic powder particles kept open to air (cf. [5]). Suspensions of different insoluble compounds had been studied several decades ago with polarography [6,7] and with voltammetry [8]; in contradistinction to those results we obtained current signals without simultaneously stirring the suspensions, both when using dropping mercury electrode or HMDE. That was due to the smaller size of our particles, which allowed their relatively faster thermal motion in the suspension.In cases when the particles do not undergo a chemical reaction with the supporting solution, their electroactivity is limited to reduction of the surface oxides by electron transfer when they, in course of their thermal motion, approach the electrode surface sufficiently closely. This approach depends on thermal energy of the particle, and on electric charge and surface tension of the mercury electrode; it varies from a direct physical contact at positive potentials, to a distance, which allows electron tunneling, at negative potentials. Starting from the positive side, with increasing negative potential the surface tension of the mercury electrode increases to its maximum at the point of zero charge. There the particles have the highest affinity to the mercury surface and hence the highest tendency to adsorb on it. Further in the negative direction the surface tension gradually decreases, and the orientation of polarized water...

Electrochemistry of Nanoparticles

“…This approach can be traced to the studies of Heyrovsky et al, who showed that the reduction of polydisperse ceramic semiconductor NP colloids contributed to the cyclic voltammetry of an Hg drop electrode by a summation of cathodic steps that had an onset potential dependent on the particle size. [323] It has also been demonstrated that the faradaic current at a UME performing a redox reaction decreased in discrete steps upon the addition of insulator microparticles. Schematic representation of the single NP experiment by Stimming and co-workers.…”

Facilitating electron transfer in bioelectrocatalytic systems

“…145 Heyrovsky in 2006, demonstrated detection of faradaic current transients during collisions of metal colloids particles for the first time. 146 The methodology was further developed by Bard 147 and Compton 148 using two different approaches.…”

Dispersion of silver particles in aqueous solutions visualized by polarography/voltammetry

“…In our papers on the voltammetric behavior of ultrafine metallic powders [1][2][3] we have shown that different degrees of dispersion of electroactive species react at mercury electrodes in different characteristic ways. It is known that silver compounds can exist in aqueous media in various states of dispersion [4] and that they undergo complex mechanisms of particle formation [5][6][7].…”

“…Colloidal solutions of these species already show some differences from true solutions, due to different sizes and related properties of individual particles; however, their transport to and from the electrode still occurs mostly by diffusion [10]. In fine suspensions the electroactive particles, of average dimensions between about 100 nm and 1 m, move in the medium by irregular Brownian motion and generate electrolytic current by particulate mechanism due to fortuitous impingements of the particles upon the electrode [1][2][3]. The "impingement" can be either direct mechanical contact of the electrode surface by the particle in a proper orientation, or an approach of the particle to the electrode within a distance over which electron tunneling can take place.…”