Craig Klauber

Sulphur speciation of leached chalcopyrite surfaces as determined by X-ray photoelectron spectroscopy

Abstract A key factor in improving the bioleaching route for chalcopyrite processing is a better understanding of the surface speciation that exists under chemical leaching conditions that mimic the acid bioleach. The surface sulphur speciation of chalcopyrite under such conditions has been revisited using X-ray photoelectron spectroscopy (XPS). Objectives of the study were to resolve the issue of possible passivation candidates and to understand the relative roles of ferric and ferrous ions in the oxidative leaching process. Neither severely metal-deficient sulphides nor polysulphides were found to be major surface layer components during initial leaching. The primary surface species produced with an acid ferric leach is elemental sulphur. This is largely volatilisable and coats the underlying unleached sulphide mineral but not adjacent minor sulphate domains. It is important to distinguish between the loss of multilayer elemental sulphur and the loss of monolayer or submonolayer quantities of the same species, especially as it impacts on polysulphide identification. The second major leach product on the chalcopyrite surface is disulphide S 2 2− . Although the cation association of the S 2 2− is not known, evidence from the Cu 2p spectra discounts the formation of any CuS 2 type species. Both acidic ferric and ferrous leaches produced the elemental sulphur and disulphide surface, though the more aggressive ferric produced a greater quantity of elemental sulphur. Evidence for polysulphides with a chain length greater than 2 remains an open question. There is some evidence that such polysulphides might form with acidic ferrous leaching, but the prime candidate for any initial leaching inhibition (prior to jarosite formation) is elemental sulphur.

A combined ab initio and photoelectron study of galena (PbS)

Abstract The results of a study of the physical, elastic and electronic properties of PbS using both first principles theoretical and experimental X-ray and ultraviolet photoelectron techniques are presented. We have investigated the influence of the computational approximations such as the basis set, pseudopotential and the treatment of exchange and correlation on the accuracy of the theoretical results. The current study work confirms that the binding energy of the S 3s state lies at 13 eV, in agreement with previous experimental work and contrary to earlier ab initio Hartree–Fock (HF) calculations. We establish that the reason for the discrepancy between previous experimental and theoretical determinations of the electronic structure of PbS is due to the treatment of the exchange interaction. We demonstrate that calculations using the HF approximation tend to overestimate the binding energies of the valence and core states yielding densities of states in poor agreement with experiment whereas calculations performed with density functional theory within either the local density or the generalised gradient approximation give a significantly better description of the electronic structure. Finally, we confirm that hybrid functional techniques such as B3LYP provide an excellent description of the physical and electronic structure of PbS.

Combined application of QEM-SEM and hard X-ray microscopy to determine mineralogical associations and chemical speciation of trace metals.

We describe the application of quantitative evaluation of mineralogy by scanning electron microscopy in combination with techniques commonly available at hard X-ray microprobes to define the mineralogical environment of a bauxite residue core segment with the more specific aim of determining the speciation of trace metals (e.g., Ti, V, Cr, and Mn) within the mineral matrix. Successful trace metal speciation in heterogeneous matrices, such as those encountered in soils or mineral residues, relies on a combination of techniques including spectroscopy, microscopy, diffraction, and wet chemical and physical experiments. Of substantial interest is the ability to define the mineralogy of a sample to infer redox behavior, pH buffering, and mineral-water interfaces that are likely to interact with trace metals through adsorption, coprecipitation, dissolution, or electron transfer reactions. Quantitative evaluation of mineralogy by scanning electron microscopy coupled with micro-focused X-ray diffraction, micro-X-ray fluorescence, and micro-X-ray absorption near edge structure (mXANES) spectroscopy provided detailed insights into the composition of mineral assemblages and their effect on trace metal speciation during this investigation. In the sample investigated, titanium occurs as poorly ordered ilmenite, as rutile, and is substituted in iron oxides. Manganeses spatial correlation to Ti is closely linked to ilmenite, where it appears to substitute for Fe and Ti in the ilmenite structure based on its mXANES signature. Vanadium is associated with ilmenite and goethite but always assumes the +4 oxidation state, whereas chromium is predominantly in the +3 oxidation state and solely associated with iron oxides (goethite and hematite) and appears to substitute for Fe in the goethite structure.

Semi-quantitative methods for studying disorder and hydrogenation in hydrogenated amorphous silicon using Auger lineshape analysis

Simple methods have been developed to enable the X-ray excited silicon L2,3VV, and the experimentally more difficult L1L2,3V, Auger spectra to be treated routinely using numerical debroadening and deconvolution to obtain an indication of the valence band transition densities of states for the surface. A method based on the simplex algorithm has then been applied to enable both the Si L2,3VV and Si L1L2,3V spectra to be decomposed (decoupled) into their component (pp-, sp- and ss-like) peaks. Changes in these components have been compared before and after disordering and hydrogenation of the surface to quantitatively probe the effect of these treatments on the surface valence band densities of states. It is shown that both the L2,3VV and L1L2,3V lines give a simple semi-quantitative method for monitoring hydrogen incorporation in hydrogenated amorphous silicon. Examples of these methods applied to artificially and naturally hydrogenated amorphous silicon surfaces are presented to illustrate their usefulness for studying materials fabricated for use in solar cells.

Auger lineshape analysis of disorder and rehydrogenation induced changes in the electronic structure of silicon surfaces

X-ray excited Si LZ3 VV spectra have been measured for both disordered crystalline silicon and chemical vapour deposited amorphous silicon. A method, based on the Simplex algorithm, has been applied to enable the Si LZ3VV peak to be decomposed into its L23MJM23, L23Mz3M23 and localised two-hole components. Changes in the relative amounts of the LZ3 VV components have been compared with changes upon hydrogenation by direct hydrogen ion bombardment for both the disordered c-Si and a,Si samples. Recent published results related to monitoring the effects of disorder and then hydrogenation on silicon surfaces using the Si LZ3 VV Auger line shape are reviewed and show good agreement with the results obtained. In agreement with others it is concluded that the hydrogen decreases the localised defect states in the bandgap and creates new states in the valence band. It is shown that the effect is more pronounced in disordered a-Si than in disordered coSio Hydrogenation of this sort is effective in improving the electrical properties of the films, especially for chemical vapour deposition films.

Choice of substrate for amorphous silicon solar cells

ABSTRACT There is considerable difference of opinion in the literature about the best method of preparing stainless steel for use in amorphous silicon solar cells. We have used a chemical vapour deposition (CVD) method to deposit thin films of a-Si:H on various stainless steel surfaces. Our experiments include three commonly-available grades of stainless steel which were prepared in several different ways. The a-Si:H films deposited on these stainless steel substrates were fabricated into Schottky barrier photovoltaic cells and their performance was compared for different substrates and preparation techniques. Considerable differences were found between the performance of the a-Si:H devices prepared on different stainless steel surfaces. The reasons for these differences are explained by performing XPS depth profiling and scanning electron microscopy on the amorphous silicon/substrate interfaces. As a result of this work we are able to recommend both the most suitable grade of stainless steel to use and an optimized substrate preparation procedure for amorphous silicon solar cells prepared by CVD.

A semi-quantitative study of disorder in argon ion-bombarded crystalline silicon using Auger lineshape analysis

It is important to have a means of determining the effect that various degrees of disorder (amorphousness) have on the valence band densities of states in thin film amorphous semiconducting materials. Crystalline silicon has been bombarded with Ar+ ions of different energies to produce a surface analogous to amorphous material with varying degrees of disorder. X-ray excited Si L2,3-VV and L1-L2,3V spectra have been obtained and numerically treated to obtain an indication of the valence band densities of states (DOS) for different degrees of surface disorder. A method based on the simplex algorithm has been applied to these spectra to decompose them into their component (pp-, sp- and ss-like) contributions. Changes in these components, before and after inducing differing degrees of disorder, have been compared in order to semi-quantitatively probe the effect of disorder on the surface DOS. It is shown that both the Si L2,3-VV and Si L1-L2,3V Auger lines may be used to differentiate between an ordered and a disordered surface and the L1-L2,3V line can also be used to semi-quantitatively monitor different degrees of disorder. It is also shown that, despite it being initially counter-intuitive, increasing the bombarding ion energy results in less disorder on the surface.