Recent Publications Feed (ADMIN)

The impact of plasma inhomogeneity on the sp² content of thin film (∼micron) boron doped diamond (BDD) electrodes, grown using microwave chemical vapour deposition (MW-CVD) under different methane (CH₄) concentrations (1% and 5%), is investigated. The sp²surface content (critical for interpreting electrochemical data) is comparatively assessed using a variety of electrochemical measurements: capacitance; solvent window analysis and quinone surface coverage. For all growths, distinctive regions containing appreciably differing amounts of sp² carbon are identified, across the wafer. For example, on the 1% CH₄ wafer, some areas exhibit electrochemical signatures indicative of high quality, minimal sp² content BDD, whereas others show regions comprising significant sp² carbon. Note Raman microscopy was unable to identify these variations. On the 5% CH₄ wafer, no region was found to contain minimal levels of sp² carbon. Changes in sp² content across the BDD films indicates spatial variations in parameters such as temperature, methane and atomic hydrogen concentrations during growth, in this case linked directly to the use of a commonly employed multi-moded (overmoded) chamber for MW-CVD BDD synthesis, operated under low power density conditions. Varying sp² levels can have significant impact on the resulting electrochemical behaviour of the BDD.

The kinetics of crystal growth of gypsum is determined by measuring the 3D time-evolution of isolated microcrystals (∼10 μm characteristic dimension) by in situ AFM. By coupling such measurements to a well-posed diffusion model, the importance of mass transport to the overall rate can be elucidated readily. Indeed, because microscale interfaces that act as source or sink sites are characterized by intrinsically high diffusion rates, it is possible to study crystal growth free from mass transport effects in many instances. In the present study, a particular focus is to elucidate how the ratio of Ca²⁺ to SO₄^2– ions at constant supersaturation influences the rate of growth at the major crystal faces of gypsum. It is found that growth at the {100} and {001} faces, in particular, is highly sensitive to solution stoichiometry, resulting in needle-like crystals forming in Ca²⁺-rich solutions and plate-like crystals forming in SO₄^2–-rich solutions. The maximum growth rate occurs with a stoichiometric solution of Ca²⁺:SO₄^2– The much slower growing basal {010} face also shows a stoichiometry dependence and growth is found to occur at step sites in growth hillocks. Importantly, overall growth rates derived by measuring the volumetric expansion of microcrystals by 3D in situ AFM are in reasonable agreement with previous bulk studies on suspensions. This study is a further illustration that the study of individual microcrystals is a powerful approach for resolving face-specific kinetics and in providing a link between microscopic observations and macroscopic rates in bulk systems. In this study it has further been possible to link face-specific kinetics to the resulting crystal morphology.

Fourier-transformed large amplitude alternating current voltammetry (FTACV) provides a sensitive analytical tool for the discrimination of electrode reactions that are complicated by surface heterogeneity. In this paper, it is shown how the FTACV response at a dual-electrode system comprised of different electrode materials having different heterogeneous charge transfer (k⁰₁ and k⁰₂) can be resolved into its individual electrode kinetics components without prior knowledge of the electrode size ratio (θ₁:θ₂). This is possible when one process is reversible and the other is quasi-reversible; achievable by careful selection of the FTACV frequency. The applicability of the FTACV method over a wide range of electrode kinetic values and size ratios is considered for conditions under which numerical simulations based on a 1D diffusion model are adequate to describe the mass transport problem.