Reaction Mechanisms at the n ‐ FeS2 /  I  Interface: An Electrolyte Electroreflectance Study

Abstract

Monocrystalline normaln‐FeS2 (pyrite) in contact with the I−/In− redox couple was studied by electrolyte electroreflectance (EER) and conventional electro‐ and photoelectrochemical techniques. The EER signal originating from the 3.13 normaleVS:normalp→normalFe:enormalg , direct transition in pyrite can be used to determine precisely its flatband potential false(VFBfalse) , 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 VFB measurements are possible under situations where capacitance studies are extremely complicated. Experimental evidence is given on strong interaction of I− and In−‐normalions with the FeS2 surface, via complex formation with Fe3+ lattice ions generated by hole capture during anodic polarization or illumination. Small VFB shifts of about 50 mV toward negative potentials indicate that specific adsorption is stronger for In− than for I− species. Moreover, VFB 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 VFB (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.

Studies of incipient oxidation of coal-pyrite for improved pyrite rejection. First quarterly technical progress report, October 1, 1992--December 31, 1992

“…The overall behavior suggests that sulfur deposition neutralizes the charge in intrinsic surface states on pyrite, relaxing the upward bent bands and thereby decreasing the photocurrent.With sufficient neutralization of the charge in surface states, an apparent positive shift of about i 1 V occurs in the flat band potential. This is the same behavior observed by others for several i different redox couples(11)(12)(13)(14)(15). It should be noted that the photocurrent does not go entirely to zero with sulfur deposition; thus, the actual flat band potential is not determined.…”

“…7) represents the oxidation of HS to S, and the cathodic peak at -0.61 the reverse process (16). At potentials between -0.76 and -0.16 V on the positive potential sweep, the photocurrent is similar to the behavior observed in the absence of hydrosulfide, i.e., zero photocurrent induced by cathodic dissolution of pyrite at -0.76 V and a gradual increase in photocurrent with increasing potential; however, sulfur deposition causes a sudden decrease in the photocurrent at potentials greater than -0.06 V. The decrease may be associated with either a decrease in band bending at the surface (i.e., an apparent shift in the flat band potential to more positive potentials, similar to the model proposed in references (11,12)), or to the formation of a passivating sulfur layer at the surface, which inhibits charge transfer. The decrease in current on the voltammetry curve with sulfur deposition indicates a passivated surface.…”

“…intrinsic surface state is also consistent with studies of the photoelectrochemical behavior of pyrite with the redox couples V2+/V 3., Fe2+/Fe 3+, Br/Bh, and CI/C12. Jaegermann and Tributsch (11) and Salvador et al(12) studied the anodic photocurrents for these redox couples and found that the results were consistent with the band edges shifting with the reversible redox couples, implying that the flat band potential also shifts with the redox•couple. They attributed the shifts to a high density of band gap states associated with FeqS 3…”

Ionization of High-Density Deep Donor Defect States Explains the Low Photovoltage of Iron Pyrite Single Crystals

“…Furthermore, it has been previous suggested that the I − /I 2 redox couple could facilitate electron transfer through a direct ligand field interaction with the surface, which could reduce recombination at the semiconductor/electrolyte interface. 23,65,66 The favorable interaction of I − /I 2 redox couple with the surface may also be supported by the outstanding stability of iron pyrite PEC cells and high electrocatalytic activity of pyrite compounds for the iodine/iodide reaction. 28,65−68 In such a case, inner-sphere electron transfer explains the observed larger photocurrents.…”

Controlling incipient oxidation of pyrite for improved rejection. Final report

“…In addition, several An intrinsic surface state is also consistent with studies of the photoelectrochemical behavior of pyrite with the redox couples V 2+ N 3+ , Fe2+lFe3+, Br'/Br2, and C17CI2. Jaegermann and Tributsch [1983] and Salvador et al [1991 J studied the anodic photocurrents for these redox couples and found the results were consistent with the band edges shifting with the reversible potential of the redox couples. These shifts in the band edges with the potential of the redox couple imply that the flat band potential also shifts with the redox couple.…”