Dynamics of interfacial charge transfer in iron(III) oxide colloids

Abstract

The kinetics of electron and proton transfer to colloidal Fe(III) oxides have been examined as a function of pH, colloid size, crystal structure, and reductant concentration. Electron injection into the colloidal particles was achieved using pulse-radiolytically generated methylviologen and 1,1'-bis(3-sulfonatopropyl)bipyridium radicals. The rate of electron transfer is governed both by the free energy difference between the redox potential of the redox couple in the electrolyte and the Fermi level in the solid and by the electrostatic interaction between the reducing radical and the electric double layer at the interface. The contribution of the latter to the rate constant dominates in cases of highly charged reductants and in particular for positively charged donors. For small particles, proton consumption occurs concomitantly with electron transfer, whereas the proton uptake lags behind the electron transfer to the larger particles in the suspensions. However, the polarization lasts no more than a few milliseconds. In accord with previous results, evidence is also presented which shows that the injected electrons can be trapped either at the surface or in the interior of the particle. The lower the pH and the smaller the particle radius, the greater is the friction of electrons trapped in the surface. A fastmore » pH buffering by surface hydroxy groups when the colloid is near the point of zero charge (pzc) was also observed.« less