New interpretations of pyrrhotite and pentlandite surface electronic structures: A route to the identification of minority species

Author: Zoe Pettifer

Pettifer, Zoe, 2020 New interpretations of pyrrhotite and pentlandite surface electronic structures: A route to the identification of minority species, Flinders University, College of Science and Engineering

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Transition metal sulfides have complex electronic structures which make them potentially useful as catalysts or for use in electronic devices. While photoemission spectroscopy is the best method to study these materials, accurate interpretation can be difficult for materials with complex electronic structures. With increasing interest in the applications of sulfides as catalysts, there is a growing need for clear and accurate interpretation of photoemission spectra. Improvements in sample preparation, resolution of spectroscopy techniques and recent developments of new spectromicroscopy techniques mean that the tools required to analyse surface electronic structure of minerals are more readily available. Increasing availability of these techniques is driving the necessity for improved understanding of the electronic structure and more accurate interpretations of spectra.

The original contribution to knowledge presented in this thesis is the advanced interpretation of photoemission spectra for pyrrhotite and pentlandite by the examination of electronic structure, and the subsequent enhanced ability to detect minority species on sulfide surfaces using photoemission spectroscopy and microscopy techniques. Empirical line shapes were defined for core level sulfur spectra for pyrrhotite (Fe7S8) and pentlandite ([Fe,Ni]9S8), which allows additional surface species to be detected and quantified on reacted surfaces of these minerals. In addition to ultra-high vacuum spectroscopy techniques, scanning transmission X-ray microscopy is presented as a new method for sulfide analysis, which allows for the mapping of chemical species and detection of minority species on sulfide surfaces in oxidising conditions.

Photoemission spectroscopy was used with varied analysis depths to distinguish surface and bulk states on vacuum fracture surfaces of pyrrhotite and pentlandite. Pyrrhotite fracture surfaces were observed to form an undercoordinated surface monosulfide in addition to a disulfide and polysulfide species. Pentlandite fracture surfaces are interpreted to have two undercoordinated surface monosulfide species, which arise from the loss of at least one bonding partner as a result of fracture. Pentlandite’s surface monosulfide species were seen to overlap with the bulk monosulfide species which has previously made the interpretation of pentlandite sulfur spectra difficult; however as a result of this study the correct interpretation is now provided. The difference in surface species reflect the polar and non-polar surfaces which are exposed on fracture surfaces of pyrrhotite and pentlandite, respectively.

By distinguishing the bulk line shapes of the core level spectra from the surface species, asymmetric line shapes were observed. Broadening on the high binding energy side of the sulfur spectra for both pyrrhotite and pentlandite were determined to originate from ligand to metal charge transfer final states. These asymmetric line shapes were fitted with an empirical set of peaks, which subsequently enables the identification of minority surface species. These line shapes were vital for identifying and quantifying the surface species which result from oxidation of the samples.

The identification of minority species was further enhanced with the implementation of Scanning Transmission X-ray Microscopy (STXM). STXM is a novel spectromicroscopy technique, for investigating the changes to a mineral’s surface species in situ, thus negating the requirement for UHV which may result in altered chemical states due to a change in environmental conditions. It was shown that the spatial resolution and chemical mapping ability of STXM was able to identify minority species on the mineral’s surface which could not previously be distinguished from the bulk spectral contributions. This technique also provided the extended ability for in situ analysis of the electrochemical reduction of a mineral particle. 

Keywords: Synchrotron X-ray Photoelectron Spectroscopy, X-ray Absorption Spectroscopy, Scanning Transmission X-ray Microscopy, sulfide mineral surface characterisation, surface species, electronic structure, XPS curve fitting

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Professor Sarah Harmer