Kathryn Prince

Nitrogen-rich indium nitride

K.S.A.B. would like to acknowledge the support of an Australian Research Council Fellowship. We would also like to acknowledge the support of the Australian Research Council through a Large grant and a Discovery grant; the support of a Macquarie University Research Development Grant, and the Australian Institute of Nuclear Science and Engineering for SIMS access.

The mechanism of copper activation of sphalerite

Abstract On the basis of recent SIMS and XAFS measurements in conjunction with already published XPS results, a mechanism for the adsorption/absorption of Cu onto sphalerite is proposed. Under conditions of high pH and high nominal surface coverage of the sphalerite by the Cu, Cu(OH)2 colloidal particles are observed on the sphalerite surfaces using SIMS. Under other conditions, SIMS measurements have indicated that adsorption of the Cu is essentially uniform over the sphalerite surface and is not related to low coordination sites on the surface of the sphalerite. Depth profiling of sphalerite surfaces with Cu adsorbed under conditions that do not result in Cu(OH)2 colloidal particles show that the Cu adsorbed/absorbed on the sphalerite surface is largely in the first few atomic layers. XAFS analysis of Cu activated sphalerite has indicated that the Cu occupies a distorted trigonal planar geometry, coordinated to three S atoms, in both surface and bulk sites. In addition Cu(1s), absorption edges in XAFS show that both bulk and surface adsorbed copper have an oxidation state less than +1 with the surface Cu being slightly more oxidised than the bulk absorbed Cu. On the basis of the combined XPS, SIMS, XAFS and solution studies, a model is proposed that, on surface adsorption of Cu, the surface Zn(II) atoms are replaced by Cu(II) atoms which are then reduced in situ to Cu(I). This reduction is accompanied by the oxidation of the three neighbouring S atoms to an oxidation state of approximately −1.5. On bulk absorption of Cu atoms into the sphalerite lattice a distorted trigonal planar configuration is achieved through the breakage of a formerly tetrahedral Zn–S bond. The breakage of this bond results in a 3-fold coordinated Cu plus one S 3-fold coordinated to Zn atoms. The breakage of this bond leads to a greater reduction of the Cu than on surface absorption and also oxidation of the 3-fold coordinated S atom to an approximately −0.5 oxidation state. This model does not invoke any polysulfite or S–S bonded species to explain the higher binding energy components of the S(2p) XPS spectra.

Defect chemistry and defect engineering of TiO2-based semiconductors for solar energy conversion

This tutorial review considers defect chemistry of TiO2 and its solid solutions as well as defect-related properties associated with solar-to-chemical energy conversion, such as Fermi level, bandgap, charge transport and surface active sites. Defect disorder is discussed in terms of defect reactions and the related charge compensation. Defect equilibria are used in derivation of defect diagrams showing the effect of oxygen activity and temperature on the concentration of both ionic and electronic defects. These defect diagrams may be used for imposition of desired semiconducting properties that are needed to maximize the performance of TiO2-based photoelectrodes for the generation of solar hydrogen fuel using photo electrochemical cells (PECs) and photocatalysts for water purification. The performance of the TiO2-based semiconductors is considered in terms of the key performance-related properties (KPPs) that are defect related. It is shown that defect engineering may be applied for optimization of the KPPs in order to achieve optimum performance.

Preparation of boron doped porous titania networks containing gold nanoparticles with enhanced visible-light photocatalytic activity

The ability to decrease the electron/hole recombination rate, and decrease the band gap of titania to allow photoactivity on irradiation with visible light is attracting more and more attention. Here, boron doping of the titania, the deposition of gold nanoparticles, along with a meso-macroporous structure were obtained using a facile agarose gel templating process combined with sol-gel chemistry. The Au/B/TiO(2) nanocomposites were characterized using SEM, TEM, XRD, N(2) gas sorption, diffuse UV-vis, photoluminescence, and SIMS. The photocatalytic activity was assessed by degradation of an organic probe molecule (methylene blue) under visible light (λ > 420 nm). The resulting materials achieved photocatalytic activities up to 50% greater than the commercial Degussa P25 under visible light. The enhancement in photocatalytic activity was primarily attributed to the decrease in band gap as a result of the boron doping and its influence on the anatase to rutile phase formation: The doped materials were highly crystalline and an optimum anatase to rutile ratio (3:1) was obtained with 0.25 wt % boron in the sample calcined at 650 °C. In addition, the presence of the gold nanoparticles decreased recombination between the photoexcited electrons and holes, which further improved the photocatalytic activity.

SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces

Abstract The surface oxidation of metal sulfides in air and aqueous solution is of central importance in mineral separation and environmental control of acid mine drainage. Mechanisms of oxidation, dissolution and surface restructuring have been extensively studied using XPS. High binding energy components in S 2p XPS spectra have been attributed to metal-deficiency, formation of polysulfide S n 2− , elemental sulfur and electronic defect structures (ie Cu(I)/ZnS). The assignment of these components in S 2p XPS spectra has, however, left significant uncertainties particularly in the formation of SS bonding in polysulfide species requiring confirmation from other surface analytical techniques. The use of static ToF-SIMS has provided a new avenue for identification of these species and their development in oxidation of the sulfide surfaces. For the iron sulfides, there is a systematic change in the FeS 2 /FeS fragment ratio from troilite (FeS) through pyrrhotite (Fe 1−x S) to pyrite (FeS 2 ) with ratios varying from 0.59, 1.2 to 32 respectively. Similarly, high ratios for FeS n /FeS are found for pyrite compared with pyrrhotite and troilite mirrored in the S n /S fragment ratios. Changes in surface oxidation, represented in atomic concentrations and S 2p XPS spectra, are seen in the ToF-SIMS signals for S n /SO n ratios in the same iron sulfide sequence. These mass markers, reflecting increased SS bonding, increase in surfaces after oxidation giving further confidence in XPS assignment to polysulfide species. Freshly cleaved galena PbS surfaces reacted in pH8 aqueous solution for increasing periods of time have also shown a systematic increase in S n /S ratios with increasing at.% of oxidised S n 2− species from XPS spectra. Statistical analysis of oxidised galena has shown that the ratios 206 PbO + / 206 Pb + and 208 PbOH + / 208 Pb + directly reflect the degree of oxidation of the surface lead species whilst the O − /S − , S − /total — ion yield and SO 3 − /S − are the best measures for following the oxidation of sulfur species. Results from these ratios suggest that initial air oxidation takes place predominantly on the S sites rather than Pb sites but, in solution at pH9, both sites are oxidised. The ToF-SIMS results appear to directly reflect the surface chemistry of the metal and sulfur species and are not consistent with recombination or fragmentation of secondary neutral or ionic species. The results strongly suggest increasing polymerisation of SS species with increasing oxidation in accord with the XPS assignment to polysulfide of increasing chain length.

Electrochemical behavior of olivine-type LiMnPO4 in aqueous solutions

The electrochemical behavior of olivine-type lithium manganese phosphate as a cathode material was investigated in a saturated aqueous lithium hydroxide electrolyte. The crystal structure and surface characterization of the olivine type and the products which are formed on its oxidation and subsequent reduction were studied. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and secondary ion mass spectrometry were used for these investigations. was found to be reversibly delithiated/lithiated on electro-oxidation/reduction

ORIGINS OF ND–SR–PB ISOTOPIC VARIATIONS IN SINGLE SCHEELITE GRAINS FROM ARCHAEAN GOLD DEPOSITS, WESTERN AUSTRALIA

Accessory gangue scheelite (CaWO4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique. Type I scheelite is strongly enriched in middle REE (MREE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial 87Sr/86Sr and eNd values ranging from 0.70141 to 0.70163 and +2.5 to +3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have 87Sr/86Sr<0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching of a Rb-rich mineral during hydrothermal alteration of the greenstone. The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial 87Sr/86Sr ratios and eNd values between 0.70130 and 0.70146, and +1.1 to +2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that 87Sr/86Sr ratios in Type II fluids were closer to those of the host dolerite (0.7008–0.7013), due to more extensive fluid interaction with the dolerite. A positive correlation between Na and REE suggests that REE3+ are accommodated by the coupled substitution REE3++Na+=2 Ca2+ into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small (<m) to medium (<km) scale processes involving mixing between “fresh” batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration. The very high-eNd values measured in some scheelites have been previously used to link gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny (<20 μm) grains of secondary hydroxyl-bastnasite were found within micro-fractures of one scheelite grain containing an extremely high-eNd signature. The hydroxyl-bastnasite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high-eNdi values observed in scheelite from Western Australia.

Nanostructured TiO2 membranes by atomic layer deposition

Conformal TiO2 films have been deposited on track-etched polycarbonate membranes by means of atomic layer deposition. Membranes with pore aspect ratios of 25 ∶ 1 and 50 ∶ 1 were selected for initial study. A suite of complementary characterisation techniques was used to probe the nanostructure, stoichiometry and uniformity of the TiO2 coatings. These established that both the surfaces and pores of the membranes were completely covered with an amorphous film. Further, the stoichiometry of the films was close to that of TiO2 though they were found to contain ∼6 atom% chlorine. While the internal walls of the membrane pores were covered with film over their full length, the coating thickness decreased in the direction of gas flow. This was attributed to a variation in reactant concentrations along the pores due to the process conditions. Gas conductance measurements were performed on several coated membranes. These showed that membrane conductance could be controlled by varying the coating thickness.

Atomic Layer Deposition of TiO2 / Al2O3 Films For Optical Applications

Atomic layer deposition (ALD) is an important technology for depositing functional coatings on accessible, reactive surfaces with precise control of thickness and nanostructure. Unlike conventional chemical vapour deposition, where growth rate is dependent on reactant flux, ALD employs sequential surface chemical reactions to saturate a surface with a (sub-) monolayer of reactive compounds such as metal alkoxides or covalent halides, followed by reaction with a second compound such as water to deposit coatings layer-by-layer. A judicious choice of reactants and processing conditions ensures that the reactions are self-limiting, resulting in controlled film growth with excellent conformality to the substrate. This paper investigates the deposition and characterisation of multi-layer TiO2 /Al2O3 films on a range of substrates, including silicon <100>, soda glass and polycarbonate, using titanium tetrachloride/water and trimethylaluminium/water as precursor couples. Structure-property correlations were established using a suite of analytical tools, including transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), X-ray reflectometry (XRR) and spectroscopic ellipsometry (SE). The evolution of nanostructure and composition of multi-layer high/low refractive index stacks are discussed as a function of deposition parameters.

Noble Metal-Modified Porous Titania Networks and their Application as Photocatalysts

In order to enhance the photocatalytic activity of titania materials, which suffer from high percentages of photon‐induced electron and hole pair recombination, noble metal (Pd, Au, Ag, and Pt) modified porous titania materials were prepared. A modified deposition precipitation technique was used to deposit the metal onto a porous TiO2 support preformed using an agarose gel templating technique coupled with sol‐gel chemistry. The final composites were characterized by use of SEM–EDX, TEM, XRD, diffuse reflectance UV/Vis spectroscopy, X‐ray photoelectron spectroscopy, FTIR, inductively coupled plasma mass spectroscopy, zeta potential, and secondary ion mass spectrometry. The photocatalytic activity of the samples was investigated by monitoring changes in methylene blue concentration under UV light irradiation. The photocatalytic activity was enhanced significantly, relative to the pure titania support, by Pd (63 %) and Au (31–37 %), and slightly by Ag (24 %) and Pt (12 %) at the respective optimum metal deposition amount. A uniform distribution of metal in the TiO2 network, the metallic form of the noble metal, and an increased organic pollutant adsorption contributed positively to the photocatalytic activity of the final materials.