REACTION-MECHANISMS AT THE N-FES2/I INTERFACE - AN ELECTROLYTE ELECTROREFLECTANCE STUDY

Monocrystalline n-FeS2 (pyrite) in contact with the I-/I(n)- redox couple was studied by electrolyte electroreflectance (EER) and conventional electro- and photoelectrochemical techniques. The EER signal originating from the 3.13 eV S: p --> Fe: e(g), direct transition in pyrite can be used to determine precisely its flatband potential (V(FB)), even in the presence of a high concentration of bandgap surface states, which is typical for this semiconducting transition metal sulfide. We have also been able to show that dynamic V(FB) measurements are possible under situations where capacitance studies are extremely complicated. Experimental evidence is given on strong interaction of I- and I(n)(-)-ions with the FeS2 surface, via complex formation with Fe3+ lattice ions generated by hole capture during anodic polarization or illumination. Small V(FB) shifts of about 50 mV toward negative potentials indicate that specific adsorption is stronger for I(n)- than for I- species. Moreover, V(FB) shifts of more than 1 V towards negative values can also be observed under cathodic polarization. This behavior is correlated to surface accumulation of electrons which accompanies FeS2 electroreduction. A comprehensive model for surface photoreactions is proposed. This takes into account the catalytic role of the semiconductor surface in kinetics of charge transfer to the electrolyte. Both the very positive V(FB) (high electronic affinity) and the high density of bandgap states, probably associated with Fe2S3 lattice impurities, seem to determine the photovoltage limitations which are inherent to the FeS2/I- system.