Lena Mazeina

Size-Driven Structural and Thermodynamic Complexity in Iron Oxides

Iron oxides occur ubiquitously in environmental, geological, planetary, and technological settings. They exist in a rich variety of structures and hydration states. They are commonly fine-grained (nanophase) and poorly crystalline. This review summarizes recently measured thermodynamic data on their formation and surface energies. These data are essential for calculating the thermodynamic stability fields of the various iron oxide and oxyhydroxide phases and understanding their occurrence in natural and anthropogenic environments. The competition between surface enthalpy and the energetics of phase transformation leads to the general conclusion that polymorphs metastable as micrometer-sized or larger crystals can often be thermodynamically stabilized at the nanoscale. Such size-driven crossovers in stability help to explain patterns of occurrence of different iron oxides in nature.

Functionalized Ga2O3 nanowires as active material in room temperature capacitance-based gas sensors.

We report the first evidence for functionalization of Ga(2)O(3) nanowires (NWs), which have been incorporated as the active material in room temperature capacitance gas-sensing devices. An adsorbed layer of pyruvic acid (PA) was successfully formed on Ga(2)O(3) NWs by simple room temperature vapor transport, which was confirmed by Fourier transform infrared spectroscopy. The effect of the adsorbed PA on the surface properties was demonstrated by the change in the response of the NW gas-sensing devices. Results indicate that the adsorption of PA reduced the sensitivity of the Ga(2)O(3) NW device to common hydrocarbons such as nitromethane and acetone while improving the response to triethylamine by an order of magnitude. Taking into account the simplicity of this functionalization together with the ease of producing these capacitance-based gas-sensing devices, this approach represents a viable technique for sensor development.

SURFACE ENTHALPY OF GOETHITE

High-temperature oxide-melt solution calorimetry and acid-solution calorimetry were used to determine the heat of dissolution of synthetic goethite with particle sizes in the range 2–75 nm and measured surface areas of 30–273 m2/g (27–240 × 103 m2/mol). Sample characterization was performed using X-ray diffraction, Fourier transform infrared spectroscopy, the Brunauer, Emmett and Teller method and thermogravimetric analysis. Water content (structural plus excess water) was determined from weight loss after firing at 1100°C. Calorimetric data were corrected for excess water assuming this loosely adsorbed water has the same energetics as bulk liquid water. The enthalpy of formation was calculated from calorimetric data using enthalpies of formation of hematite and liquid water as reference phases for high-temperature oxide-melt calorimetry and using enthalpy of formation of lepidocrocite for acid-solution calorimetry. The enthalpy of formation of goethite can vary by 15–20 kJ/mol as a function of surface area. The plot of calorimetric data vs. surface area gives a surface enthalpy of 0.60±0.10 J/m2 and enthalpy of formation of goethite (with nominal composition FeOOH and surface area = 0) of −561.5±1.5 kJ/mol. This surface enthalpy of goethite, which is lower than values reported previously, clarifies previous inconsistencies between goethite-hematite equilibrium thermodynamics and observations in natural systems.

Thermodynamic properties of CaTh(PO4)2 synthetic cheralite

Abstract The mineral cheralite [CaTh(PO4)2] allows for the incorporation of tetravalent actinides in monazitebased crystalline phases. Experimental determination of its thermodynamic properties is crucial for defining its stability and subsequent long-term ability to immobilize radionuclides. Low-temperature heat capacity from 0.5 to 300 K, enthalpy increments from 485 to 1565 K, and the enthalpy of formation of cheralite from the oxides were measured and reported on for the first time. At 298.15 K, S° = (201.6 ± 2.6) J/(K·mol), which includes the configurational entropy of Ca and Th mixing, ΔHfox = -(506.4 ± 9.5) kJ/mol, ΔHfel = -(3872.8 ± 10.2) kJ/mol, ΔGfox = -(501.6 ± 9.6) kJ/mol, and ΔGfel = -(3635.5 ± 10.2) kJ/mol. In aqueous environments, cheralite is able to form from whitlockite or apatite and thorianite. Under anhydrous conditions, cheralite can form by solid-state reaction only if the resultant product includes very stable Ca salts instead of CaO.

Incorporation of novel ternary Sn-Ga-O and Zn-Ga-O nanostructures into gas sensing devices

Novel ternary nanostructures (ZnO-Ga2O3 nanobrushes, SnO2-Ga2O3 heterostructures and Sn-doped Ga2O3 nanowires) are excellent materials for gas sensing applications due to their large surface areas and structural defects. Also, these nanostructures consist of different materials with different degrees of crystallinity and defect densities thus broadening their gas sensing capabilities. Gas sensing devices, developed in our laboratory based on room temperature capacitance measurements, were first fabricated by standard photolithography and lift-off techniques to pattern platinum (Pt) pads and interdigitated fingers acting as conducting paths. The nanostructures, which were characterized by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and photoluminescence (PL), were then incorporated by the catalyst-assisted growth directly onto the devices. The most efficient devices were those with high yield of nanostructures and with low-resistivity of the Pt pads. To achieve that, different catalysts (nickel, Ni; copper, Cu, and gold, Au) were used for different nanostructures. For example, the best catalyst for the device fabrication of Sn-doped Ga2O3 nanowires was Ni whereas for nanostructures with high Sn content Cu was the best catalyst. Challenges and successes of device fabrication for capacitance-based gas sensing devices are discussed in this work together with some sensing results for such analytes as acetone, acetic acid, isopropanol, dichoropentane, nitrotolouene and nitromethane.

Structure and Orientation Determination of Metal-Oxide Nanostructures by Electron Backscatter Diffraction

Crystal growth directions, epitaxial relationships, and identities of crystallographic surfaces are all fundamental parameters that will influence novel electronic, chemical, mechanical or optical behavior in nanostructures. Electron backscatter diffraction (EBSD) stands as a robust approach for rapidly determining the crystal structure and orientation of nanostructures. However, aside from a few recent studies, including analysis of GaAs [1] and GaN [2] nanowires, EBSD has not been widely applied towards studying nanostructures.

Below bandgap excitation of SnO 2 nanowires: The relaxation of trap states

Carrier relaxation of SnO2 nanowires is investigated by excitation at 3.2 eV, ∼0.4 eV below the bandgap. The excited state transmission spectrum from 1.9–2.7 eV is intensity-dependent and recovers uniformly with a biexponential relaxation route.