Electronic Structure of Sulfide Minerals

Significant Achievements, Anticipated Outcomes and Future Work

Pristine fractured surfaces of minerals such as pyrite, chalcopyrite, loellingite, marcasite, arsenopyrite, bornite, molybdenite, etc have been obtained using conventional and synchrotron based XPS and XAS. Ab initio modelling of these minerals has been performed, using density functional quantum chemical models, to obtain the electronic and optical properties of these and other minerals. Understanding these properties enables qualitative prediction of the XPS and XAS spectra, leading to a more detailed knowledge of these structures useful for minerals extraction. The objective is to obtain electronic structures of both surfaces and bulk, as well as adsorption behaviour on such surfaces, which will further applications in materials design.

A study of Cu adsorption on pyrite fracture surfaces has now been completed, and submitted for publication. The reactivity of the pyrite surface can be understood from a Mulliken population analysis of a fracture surface after geometry optimization, which shows that the surface S monomers display the larges negative charges and hence the greatest reactivity, followed by the surface S dimers, which are less strongly charged and hence less reactive. We have performed geometry optimization calculations of Cu adsorption onto such a pyrite surface, and have found there to be several possible geometries for the adsorption of Cu onto pyrite. The geometry which is the most consistent with experimental XPS spectra is a diagonal bond, with the adsorbed Cu bound to a surface S monomer and a surface S dimer respectively. This study will be enhanced by further experiments obtaining XANES spectra of such surfaces, and by modelling these spectra using the ab initio package FEFF8.

A comparative study of surface reactivity has been performed which compares the surfaces of pyrite, chalcopyrite and molybdenite. The pyrite S 2p XPS displays two surface states of lower binding energy than the bulk S dimer. For chalcopyrite two surface states are observed, one each of higher and lower binding energies as compared to the bulk. Molybdenite displays a perfect bulk S 2p doublet with no discernible surface states observed. Ab inito geometry optimization and Mulliken population analysis calculations were performed in order to understand the differences between these types of spectra. The relaxed surface of pyrite displays two surface states of larger charge than the bulk S, consistent with the experimental observation and the assignment of these states to surface S monomers and dimers. Different chalcopyrite surfaces were investigated. Surfaces exposing S monomers relax to form surface S polymers, consistent with the experimental observation of an S 2p core line shift to higher binding energy and the assignment of such states to surface S polymers. Surfaces exposing S and metal atoms relax in such a way as to expose only S atoms, which are under-coordinated and therefore have larger charge than the bulk S atoms. This is consistent with the experimental observation of a core line shift towards lower binding energy and the assignment of such states to under-coordinated surface S atoms. Molybdenite surfaces expose fully co-ordinates S atoms which display the same charges than the bulk S atoms, consistent with the absence of any surface states in the experimental spectra.

Some calculations were performed on surface states of O deficient titania, a study which is now complete and accepted for publication.

Computational Techniques Used

The density of states, population analysis and band structure calculations were performed within the density functional theory framework, using the program CASTEP, a density functional theory plane-wave pseudopotential program. CASTEP offers the possibility of calculating irregular slabs, such as the different cuts through pyrite (100) surfaces and modifications thereof - polarity of slabs prevents convergence of the solution to such surfaces in many other quantum chemical codes.

We also use FEFF8 in order to model XANES spectra.

Publications, Awards and External Funding

External Funding and Awards

This work is supported as part of the ARC Special Research Centre on Particles and Materials Interfaces.

Publications

G. U. von Oertzen, W. M. Skinner, H. W. Nesbitt, A. R. Pratt and A. N. Buckley, Cu adsorption on pyrite fracture surfaces: Ab initio and spectroscopic studies, Geochimica, under review

G. U. von Oertzen, A. R. Gerson, O deficiency in the rutile TiO2 (110) surface: ab initio quantum chemical investigation of the electronic properties, International Journal of Quantum Chemistry, accepted February 2006.

G. U. von Oertzen, W. M. Skinner, H. W. Nesbitt, Surface States of Pyrite (100): Ab initio and x-ray photoemission spectroscopy study of the bulk and surface electronic structure of pyrite (100) with implications for reactivity, Physical Review B 72, 235427

G. U. von Oertzen, W. M. Skinner, H. W. Nesbitt, Ab initio and XPS studies of pyrite (100) surface states, Radiation Physics and Chemistry, accepted for publication (August 2005)