The Electronic Structure and Reactivity of Sulfide Surfaces: Combining Atomic-Scale Observations with Theoretical Calculations

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Virginia Tech

The electronic structure of clean pyrite {100} and covellite {001} surfaces have been investigated in ultra-high vacuum (UHV) for the purpose of understanding the nature of sulfide surface reactivity. Using primarily scanning tunneling microscopy and spectroscopy (STM/STS), the electronic structure at atomic sites on these surfaces was directly probed, and chemical insight into the results was provided by ab-initio calculations. Pyrite is the most abundant sulfide at the earth's near surface. Its oxidation influences a wide variety of natural and industrial chemical process, but very little is known about the stepwise oxidation reactions involved. For this reason, the first two chapters are directed at understanding the surface electronic structure and fundamental reactivity of pyrite surfaces at the atomic scale. UPS spectra show a characteristic peak at ~ 1 eV forming the top of the valence band for the near surface. Ab-initio calculated densities of states for the bulk crystal suggest that this band is comprised primarily of non-bonding Fe 3d t2g and lesser S 3p and Fe 3d eg states. Ab-initio slab calculations predict that the broken bonding symmetry at the surface displaces a Fe 3dZ2 dangling bond state into the bulk band gap. Evidence confirming the presence of this surface state is found in low bias STM imaging and normalized single-point tunneling spectra, which are in remarkable agreement with calculations of the LDOS at surface Fe and S sites. The results predict that due to the dangling bond surface states, Fe sites are energetically favored for redox interaction with electron donors or acceptor species. STM/STS observations of O₂/H₂O exposed surfaces are consistent with this assertion, as are ab-initio cluster calculations of adsorption reactions between O₂/H₂O derived species and the {100} surface. Furthermore, an enhancement in the "rate" of oxidation was discovered using UPS on pyrite surfaces exposed to a mixture of O₂/H₂O. Cluster calculations of adsorption energies reveal a similar result for the case where both O₂ and H₂O are dissociated on the surface and sorbed to Fe sites.

Covellite, similar to pyrite, is a natural semiconducting metal sulfide. In contrast, however, precious metal bearing solutions have a curiously lower affinity for covellite surfaces than for pyrite. At the same time, its unique combination of low resistivity and perfect basal cleavage represented a unique opportunity to improve our ability to interrogate metal sulfide surfaces using STM/STS at the atomic scale. Ab-initio calculations predict that cleaving covellite exposes two slightly different surfaces, one is expected to have dangling bonds, the other is not. Atomic-scale STM images and LEED patterns indicate that the surface structure is laterally unreconstructed. The STM images are predicted to show Cu sites as high tunneling current sites on the dangling bond covered surface, and S sites on the other. Based on tunneling spectra and tip-induced effects therein, reasonable arguments are presented which allow one to uniquely differentiate between the two possible surfaces.

For both pyrite and covellite, the combination of experiment and theoretical calculations afforded much more insightful conclusions than either would have alone. The calculations provided the necessary chemical framework with which to make interpretations of the experimental data and, in this sense, contribute information obtainable by no other means. This point is further developed in an investigation of Si-O interactions and the electron density distribution in the model silicate coesite, which is presented in the appendix. In addition, it breaks new ground by delving into differences and similarities between periodic vs. cluster calculations of minerals.

Pyrite, covellite, STM, tunneling spectroscopy, Oxidation