Abstract: Trivalent cerium ions in CeO2 are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce3+ species on the surface and in the bulk of Sm-doped CeO2(100) in hundreds of millitorr of H2–H2O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce3+ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface’s 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce3+ over wide ranges of...

Abstract: A single-layer graphene is synthesized on Cu foil in the absence of H2 flow by plasma enhanced chemical vapor deposition (PECVD). In lieu of an explicit H2 flow, hydrogen species are produced during methane decomposition process into their active species (CHx<4), assisted by the plasma. Notably, the early stage of growth depends strongly on the plasma power. The resulting grain size (the nucleation density) has a maximum (minimum) at 50 W and saturates when the plasma power is higher than 120 W because hydrogen partial pressures are effectively tuned by a simple control of the plasma power. Raman spectroscopy and transport measurements show that decomposed methane alone can provide sufficient amount of hydrogen species for high-quality graphene synthesis by PECVD.

Abstract: Praseodymium-Cerium Oxide (PrxCe1-xO2−δ; PCO), a potential three way catalyst oxygen storage material and solid oxide fuel cell (SOFC) cathode, exhibits surprisingly high levels of oxygen nonstoichiometry, even under oxidizing (e.g. air) conditions, resulting in mixed ionic electronic conductivity (MIEC). In this study we examine the redox kinetics of dense PCO thin films using impedance spectroscopy, for x = 0.01, 0.10 and 0.20, over the temperature range of 550 to 670°C, and the oxygen partial pressure range of 10−4 to 1 atm O2. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to electrode area, pointing to surface exchange rather than bulk diffusion limited kinetics. The large electrode capacitance (10−2F) was found to be consistent with an expected large electrochemically induced change in stoichiometry for x = 0.1 and x = 0.2 PCO. The PCO films showed surprisingly rapid oxygen exchange kinetics, comparable to other high performance SOFC cathode materials, from which values for the surface exchange coefficient, k q , were calculated. This study confirms the suitability of PCO as a model MIEC cathode material compatible with both zirconia and ceria based solid oxide electrolytes.

Abstract: Graphene has attracted considerable interest as a potential material for future electronics. Although mechanical peel is known to produce high quality graphene flakes, practical applications require continuous graphene layers over a large area. The catalyst-assisted chemical vapor deposition (CVD) is a promising synthetic method to deliver wafer-sized graphene. Here we present a systematic study on the nucleation and growth of crystallized graphene domains in an atmospheric pressure chemical vapor deposition (APCVD) process. Parametric studies show that the mean size of the graphene domains increases with increasing growth temperature and CH4 partial pressure, while the density of domains decreases with increasing growth temperature and is independent of the CH4 partial pressure. Our studies show that nucleation of graphene domains on copper substrate is highly dependent on the initial annealing temperature. A two-step synthetic process with higher initial annealing temperature but lower growth temperature is developed to reduce domain density and achieve high quality full-surface coverage of monolayer graphene films. Electrical transport measurements demonstrate that the resulting graphene exhibits a high carrier mobility of up to 3000 cm2 V−1 s−1 at room temperature.

Abstract: ZnO thin films have been produced by reactive sputtering with different oxygen contents in the sputtering gas. As a result of transmission electronic microscopy observation, each film consist of two layers: an interfacial layer close to the substrate, with a thickness of about 200 nm, composed of very fine crystal grains and an upper layer above the interfacial layer, composed of column-shaped grains aligned along the out-plane direction. The grain diameter ranges from 35 to 100 nm depending on the oxygen partial pressure. The in-plane and out-plane thermal conductivity have been measured at room temperature. The out-plane thermal conductivity of the interfacial layer is 2.3 W m−1 K−1, independent on the oxygen partial pressure. The out-plane thermal conductivity of the upper layer is 5.4, 7.1, and 4.0 W m−1 K−1, and the in-plane thermal conductivity 4.86, 6.01, and 2.66 W m−1 K−1, for the O2 30%, 60%, and 90% ZnO film, respectively. Both out-plane and in-plane thermal conductivity decrease with the decre...

Abstract: The response characteristics of amorphous-InGaZnO 4 (a-IGZO 4 ) thin films toward reducing/oxidizing gases (H 2 /NO 2 ), at sensor operating temperatures, are reported for the first time. The lack of grain boundaries eliminates a major source of electrical, microstructural and chemical inhomogeneities associated with polycrystalline semiconducting metal oxides (SMOs), rendering a-IGZO 4 a highly promising model sensor system. Gas sensor tests were carried out in the temperature range of 200–400 °C by monitoring changes in DC resistance during cyclic exposure to trace concentrations (between 1.25 and 50 ppm) of H 2 or NO 2 in dry air. The response ( S ) to H 2 was found to go through a temperature maximum (e.g. S ∼ 0.7 at 350 °C for pH 2 = 12.5 ppm) that value being a function of pH 2 . The response to NO 2 , on the other hand, decreased with increasing temperatures with the highest recorded values at 200 °C (e.g. S ∼ 33 at 200 °C for pNO 2 = 5 ppm). The response followed an approximate power law dependence on gas partial pressure ( p ), S = Ap β , with β taking on values of ∼0.5–1.0 as temperature increased from 200 to 400 °C. Response times were found to range from 10 s to greater than 1000 s as temperature decreased. The hysteretic behavior exhibited by a-IGZO films between 150 and 400 °C, under temperature sweep conditions, is attributed to kinetically limited adsorption/desorption and reaction rates.

Abstract: Tin oxide SnO2 films were prepared by RF magnetron sputtering. The effects of oxygen partial pressure percentage on the SnO2 property have been investigated to obtain relatively high-resistivity SnO2 films which could be used as buffer layers to optimize the performance of CdTe/CdS solar cells. The oxygen partial pressure percentage varied in the range of 1%~10%. The results show that the introduction of oxygen would suppress the deposition and growth of SnO2 films. Electrical measurement suggests that the film resistivity decreases with the increase of oxygen pressure. The SnO2 films with resistivity of 232 Ω cm were obtained in pure Ar atmosphere. All SnO2 films fabricated with different oxygen partial pressure percentage have almost the same optical band gap.

Abstract: A simple oxygen permeation model was developed based on the theoretical analysis of the role of interfaces of mixed conducting membranes. the developed model equations contain three resistance constants, which can be determined by correlating oxygen permeation flux to oxygen partial pressure on each side. a series of experimental measurements of oxygen fluxes of ba0.5sr0.5co0.8fe0.2o3-d membranes over a wide range of temperature and oxygen partial pressures were tested for the regression of three resistance constants with good correlation (r > 0.997). with this model, the interfacial exchange resistances of each side can be well distinguished from the bulk-diffusion resistance under a wide-temperature range. the kinetics parameters, including interfacial exchange coefficients on each side and ionic diffusion coefficient, can be obtained through the three resistance constants. parametric studies can predict the influences of membrane thickness, oxygen partial pressures on oxygen flux, distribution of permeation resistances, and characteristic thickness. (c) 2011 american institute of chemical engineers aiche j, 58: 17441754, 2012

Abstract: A systematic study for the optimization of the deposition process of ZnO thin films grown by dc magnetron sputtering at room temperature was carried out using different oxygen partial pressures and deposition times. We have established a correlation between the oxygen partial pressure, the chemical composition and the crystalline structure of the films.Stoichiometric and highly oriented ZnO thin films along the (0 0 2) crystal plane with very good optical performance were obtained for a relative oxygen gas flow of 20% in the gas mixture. Higher O2 concentrations resulted in non-stoichiometric ZnO with an excess of oxygen, which exhibited a lower degree of crystallinity and slightly higher band-gap energy.X-ray absorption near edge structure (XANES) analysis indicated that this excess of oxygen was incorporated in molecular form inducing a reduction in the crystallinity of the material. Post-deposition annealing treatments up to 500 °C significantly improved their crystallinity as confirmed by x-ray diffraction and XANES. Therefore, it has been found that it is possible to grow ZnO at room temperature with high crystal quality and good optical response by controlling the growth conditions.

Abstract: The AC conductivity of fluorite-structured La2Ce2O7 ceramic was measured under air and argon with different humidity between 250 and 550 °C. It was observed that the total conductivity in wet air and argon was higher than that under dry atmospheres. The effect of water vapor partial pressure () on the conductivity of La2Ce2O7 in air was investigated in detail. The total conductivity increased remarkably with the water vapor partial pressure, and this phenomenon became more notable at lower temperatures. The enhancement of the conductivity was attributed to the proton conduction behavior of La2Ce2O7 in wet atmospheres, and the proton conductivity reached 6.68 × 10–5 S cm–1 in wet air (3% H2O) at 550 °C. The relationship between the proton conductivity (σH) and in wet air could be fitted to . The estimated proton transport number increased with increasing water vapor partial pressure and decreasing temperature, and varied between 0.05 and 0.89 in this study.

Abstract: The oxygen surface exchange kinetics of mixed conducting perovskite oxides SrTi1−xFexO3−δ (x = 0, 0.01, 0.05, 0.35, 0.5) has been investigated as a function of temperature and oxygen partial pressure using the pulse-response 18O–16O isotope exchange (PIE) technique. Arrhenius activation energies range from 140 kJ mol−1 for x = 0 to 86 kJ mol−1 for x = 0.5. Extrapolating the temperature dependence to the intermediate temperature range, 500–600 °C, indicates that the rate of oxygen exchange, in air, increases with increasing iron mole fraction, but saturates at the highest iron mole fraction for the given series. The observed behavior is concomitant with corresponding increases in both electronic and ionic conductivity with increasing x in SrTi1−xFexO3−δ. Including literature data of related perovskite-type oxides Ba0.5Sr0.5Co0.8Fe0.2O3−δ, La0.6Sr0.4Co0.2Fe0.8O3−δ, La0.6Sr0.4CoO3−δ, and Sm0.5Sr0.5CoO3−δ, a linear relationship is observed in the log–log plot between oxygen exchange rate and oxide ionic conductivity with a slope fairly close to unity, suggesting that it is the magnitude of the oxide ionic conductivity that governs the rate of oxygen exchange in these solids. The distribution of oxygen isotopomers (16O2, 16O18O, 18O2) in the effluent pulse can be interpreted on the basis of a two-step exchange mechanism for the isotopic exchange reaction. Accordingly, the observed power law dependence of the overall surface exchange rate on oxygen partial pressure turns out to be an apparent one, depending on the relative rates of both steps involved in the adopted two-step scheme. Supplementary research is, however, required to elucidate which of the two possible reaction schemes better reflects the actual kinetics of oxygen surface exchange on SrTi1−xFexO3−δ.

Abstract: TiO2 and Co-doped TiO2 (CTO) thin films deposited at various oxygen partial pressures by pulsed laser deposition exhibit room temperature ferromagnetism (RTFM) independent of their phase. Films deposited at 0.1?mTorr oxygen partial pressure show a complete rutile phase confirmed from glancing angle x-ray diffraction and Raman spectroscopy. At the highest oxygen partial pressure, i.e. 300?mTorr, although the TiO2 film shows a complete anatase phase, a small peak corresponding to the rutile phase along with the anatase phase is identified in the case of CTO film. An increase in O to Ti/(Ti+Co) ratio with increase in oxygen partial pressure is observed from Rutherford backscattering spectroscopy. It is revealed from x-ray photoelectron spectroscopy (XPS) that oxygen vacancies are found to be higher in the CTO film than TiO2, while the valency of cobalt remains in the +2 state. Therefore, the CTO film deposited at 300?mTorr does not show a complete anatase phase unlike the TiO2 film deposited at the same partial pressure. We conclude that RTFM in both films is not due to impurities/contaminants, as confirmed from XPS depth profiling and cross-sectional transmission electron microscopy (TEM), but due to oxygen vacancies. The magnitude of moment, however, depends not only on the phase of TiO2 but also on the crystallinity of the films.

Abstract: Precise measurements of the surface tension of water in air vs humidity at 5, 10, 15, and 20 °C are shown For constant temperature, surface tension decreases linearly for increasing humidity in air These experimental data are in good agreement with a simple model based on Newton's laws here proposed It is assumed that evaporating molecules of water are ejected from liquid to gas with a mean normal component of the speed of "ejection" greater than zero A high humidity in the air reduces the net flow of evaporating water molecules lowering the effective surface tension on the drop Therefore, just steam in air acts as an effective surfactant for the water-air interface It can partially substitute chemical surfactants helping to reduce their environmental impact

Abstract: We report here measured adsorption capacities for a natural chabazite zeolite at pressures ranging from (5 to 3000) kPa, at temperatures of (244 and 305) K, for pure N2, CH4, and CO2, and for gas mixtures of CH4 + CO2. The pure gas data sets from this work and from the literature were in good agreement (10 %) and were regressed to Toth models over a wide range of pressure and temperature. We show that extrapolation of models that were fit only to low pressure data (below 120 kPa) can lead to a 30 % deviation in adsorption capacities predicted at high pressures. Similarly, models fit only to high pressure pure fluid data resulted in unreliable predictions for mixture adsorption capacities particularly when the component's partial pressure was low. The experimental results indicate that, while the chabazite is unlikely to be useful for N2/CH4 separation, it may have potential for removing bulk CO2 from natural gas, particularly at low temperatures. A feed gas mixture of 0.95CH4 + 0.05CO2 placed in contact w...

Abstract: Ni/NiO powdered nanoparticles with average sizes 10–30 nm were prepared by a levitation-jet method involving the condensation of Ni metal vapour in a mixture of helium with various amounts of air or oxygen. The process was undertaken with the application of a DC electric field up to 6.5 kV cm−1. The particles were characterized by X-Ray diffraction, transmission electron microscopy, BET adsorption and vibrating sample magnetometry. It was found that the intensity of the applied electric field and partial oxygen pressure correlated with the main structural and magnetic parameters of the nanoparticles, such as average particle size, residual ratio of nickel, coercivity and maximum magnetisation. The specific surface area of the particles correlated with the magnitude of the external electric field. Room-temperature hysteresis loops of weakly oxidized nanoparticles show ferromagnetic-like behaviour, whereas the strongly oxidized ones exhibit a low-field ferromagnetic feature superimposed to a paramagnetic signal, regardless of the particle size. Magnetic measurements allowed for the estimation of the residual metal Ni content in the powdered nanoparticles, which can be as low as 0.04 at.% depending on oxygen partial pressure and external electric field strength.