POLAROGRAPHY AND VOLTAMMETRY OF ULTRASMALL COLLOIDS - INTRODUCTION TOA NEW FIELD

Small colloidal particles of a semiconductor nature yield polarographi c and voltammetric diffusion-controlled currents which differ, by the form as well as by the information contents, from the currents obtaine d with true solutions. The electrode reaction proceeds here either as a volume or as a surface process according to whether Faradaic charge is transferred to the conduction band and consumed in the volume of th e particle or is transferred to and consumed in the surface of the par ticle. In polydisperse colloidal solutions each particle contributes t o the net current according to its size; different size implies differ ent diffusion coefficient, different Faradaic charge, and different re duction/oxidation potential. Hence, in polarography the slope of the w ave and the half-wave potential and in voltammetry the peak potential depend on the particle size distribution. Basic experimental material was gathered with ultrasmall SnO2, TiO2, and mixed TiO2Fe2O3 colloids. In the electroreduction of protonated SnO2 and TiO2 colloids in acidi c solutions, where the electrode process starts by reduction of the su rface protons, the smaller particles are reduced at less negative pote ntials than the bigger ones.