Applications of XPS to the study of inorganic compounds

The surface behaviour of materials is crucial to our everyday lives. Studies of the corrosive, reactive, optical and electronic properties of surfaces are thus of great importance to a wide range of industries including the chemical and electronics sectors. The surface properties of polymers can also be tuned for use in packaging, non stick coatings or for use in medical applications. Methods to characterise surface composition and reactivity are thus critical to the development of next generation materials. This report will outline the basic principles of X-ray photoelectron spectroscopy and how it can be applied to analyse the surfaces of inorganic materials. The role of XPS in understanding the nature of the active site in heterogeneous catalysts will also be discussed.

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)](ClO) has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S = 0) and high-spin (S = 2) states. Temperaturedependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T(?) = 223 and T(?) = 213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T, and an explanation is provided.

Sulphide ore minerals:surface chemical properties

The surfaces of iron-containing sulphide minerals were oxidised by a range of inorganic oxidants, and the resultant surface alteration products studied using various spectroscopic techniques. The characterisation of surface oxidation is relevant to the alteration of ores in nature and their behaviour during flotation and leaching, of importance to the metallurgical industry. The sulphides investigated included pyrite (FeS2), hexagonal pyrrhotine (Fe9S10), monoclinic pyrrhotine (Fe7Se), violarite (FeNi2S4), pentlandite ((FeiNi)9Se), chalcopyrite (CuFeS2) and arsenopyrite (FeAsS). The surfaces were oxidised by various methods including acid (sulphuric), alkali (ammonium hydroxide), hydrogen peroxide, steam, electrochemical and air/oxygen (in a low-temperature (150ºC) furnace), The surfaces were examined using surface sensitive chemical spectroscopic methods including x-ray photoelectron spectroscopy (ms), Auger electron spectroscopy (LES) and conversion electron Mössbauer spectroscopy (CEKS). Physical characterisation of the surfaces was undertaken using scanning electron microscopy (SM), spectral reflectance measurements and optical microscopy. Bulk characterisation of the sulphide minerals was undertaken using x-ray diffraction and electron microprobe techniques. Observed phases suggested to form in most of the sulphide surfaces include Fe204, Fe1-x0, Fe202, Fe00H, Fe(OH)3, with iron II & III oxy-sulphates. The iron sulphides show variable extents of oxidation, indicating pyrite to be the most stable. Violarite shows stability to oxidation, suggested to result from both its stable spinel crystal structure, and from the rapid formation of sulphur at the surface protecting the sub-surface from further oxidation. The phenomenon of sub-surface enrichment (in metals), forming secondary sulphides, is exhibited by pentlandite and chalcopyrite, forming violarite and copper sulphides respectively. The consequences of this enrichment with regard to processing and leaching are discussed. Arsenopyrite, often a hindrance in ore processing, exhibits the formation of arsenic compounds at the surface, the dissolution of which is discussed in view of the possible environmental hazard caused by the local pollution of water systems. The results obtained allow a characterisation of the sulphides in terms of their relative stability to oxidation, and an order of stability of the sulphide surfaces is proposed. Models were constructed to explain the chemical compositions of the surfaces, and the inter-relationships between the phases determined at the surface and in the sub-surface. These were compared to the thermo-chemically predicted phases shown in Eh/pH and partial pressure diagrams! The results are discussed, both in terms of the mineralogy and geochemistry of natural ores, and the implications for extraction and processing of these ore minerals.

Spectroscopic studies of field-induced electron emission from isolated microstructures

A detailed investigation has been undertaken into the field induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated and dielectric coated metallic electrodes. These processes have been investigated using two dedicated experimental systems that were developed for this study. The first is a novel combined photo/field emission microscope, which employs a UV source to stimulate photo-electrons from the sample surface in order to generate a topographical image. This system utilises an electrostatic lens column to provide identical optical properties under the different operating conditions required for purely topographical and combined photo/field imaging. The system has been demonstrated to have a resolution approaching 1m. Emission images have been obtained from carbon emission sites using this system to reveal that emission may occur from the edge triple junction or from the bulk of the carbon particle. An existing UHV electron spectrometer has been extensively rebuilt to incorporate a computer control and data acquisition system, improved sample handling and manipulation and a specimen heating stage. Details are given of a comprehensive study into the effects of sample heating on the emission process under conditions of both bulk and transient heating. Similar studies were also performed under conditions of both zero and high applied field. These show that the properties of emission sites are strongly temperature and field dependent thus indicating that the emission process is `non-metallic' in nature. The results have been shown to be consistent with an existing hot electron emission model.

The influence of defects on the electron-transfer and magnetic properties of RbxMn[Fe(CN)6]y·zH2O

The synthesis and detailed characterization of a few samples of the compound RbMn[Fe(CN)]·zHO are described. The composition of the materials significantly depends on the applied preparative conditions. Analysis of spectroscopic results (FTIR, Raman, Fe Mössbauer, XPS) and X-ray powder-diffraction data yielded a further assessment of the difference in structural features in terms of the amount of Fe(CN)6 vacancies and the associated number of water molecules. The characteristic individual magnetic behavior, as well as the metal-to-metal charge-transfer capabilities of the various samples, could be related to significant changes within the structures that appear to be associated with the synthetic method used.