Atomic force microscopy of the electrochemical reductive dissolution of sub-micrometer sized crystals of goethite immobilized on gold electrodes

Goethite (alpha-FeOOH) crystals with edge lengths between 100 and 1000 nm were deposited on polycrystalline gold electrodes and dissolved electrochemically by reduction. The reductive dissolution was followed by in situ atomic force microscopy. The study proves that multidomain and twinned crystal aggregates are reductively dissolved at more positive potentials than single domain crystals. The {1 1 0} face of the crystals was identified as the most reactive crystal face. The dissolution kinetics of single crystals exhibited a surprising variation with respect to rate laws, whereas the average of seven single crystals could be modelled by the shrinking disc kinetics with a rate constant of 2 x 10(-3) s(-1), when an oxalate buffer was used. This value is in agreement with previous studies of goethite powders under the same conditions. Further, the formation of a relatively thick hydration layer can be observed when dry goethite crystals are exposed to wet nitrogen. This hydration layer facilitates the electrochemical reduction slightly by decreasing the peak potential by about 17 mV. (C) 2003 Elsevier B.V. All rights reserved.