Title page for ETD etd-51498-1128


Type of Document Dissertation
Author Rosso, Kevin Michael
Author's Email Address kmr@vt.edu
URN etd-51498-1128
Title The Electronic Structure and Reactivity of Sulfide Surfaces: Combining Atomic-Scale Observations with Theoretical Calculations
Degree PhD
Department Geological Sciences
Advisory Committee
Advisor Name Title
Hochella, Michael F. Jr. Committee Chair
Cox, David F. Committee Member
Dillard, John G. Committee Member
Gibbs, Gerald V. Committee Member
Rimstidt, james Donald Committee Member
Keywords
  • Pyrite
  • covellite
  • STM
  • tunneling spectroscopy
  • oxidation
Date of Defense 1998-06-05
Availability unrestricted
Abstract
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.

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