Terms Of Surface Interactions Research Articles

Palladium/γ-Fe2O3 nanoparticle mixtures for acetone and NO2 gas sensors

The functionalization of Fe3O4/γ-Fe2O3 nanoparticles (NPs) gas sensors with Pd NPs was studied. NPs films were deposited from the mixtures of Fe3O4/γ-Fe2O3 and Pd NPs colloidal solutions by modified Langmuir-Schaefer technique onto Al2O3 substrates, the NP size being 6–7nm. Samples were composed from 4 Fe3O4/γ-Fe2O3 NP monolayers (MLs), either pure or doped by Pd at the different NPs Pd: Fe3O4/γ-Fe2O3 proportions. NPs and films were studied by DLS, GI XRD, SEM and EDS. Sensors were tested toward acetone vapors and NO2 at concentration range of 5–200ppm and 0.5–12ppm respectively in dry air and under working temperatures range 200–500°C. Before testing 1nm, surfactant capping was removed by UV irradiation. Pd-doped Fe3O4/γ-Fe2O3 sensors showed p-type gas response, higher conductance and higher conduction activation energies compared to pure Fe3O4/γ-Fe2O3 sensor that gives n-type gas response. Results are explained in terms of surface interactions with oxidizing and reducing species, and electrical conduction mechanisms.

The Conductive Property of ZnO QDs-SiO2 and ZnO QDs-SiOxNy Nanocomposite Films

The dc and ac conductive properties of ZnO QDs-SiO2 and ZnO QDs-SiO(x)N(y) nancomposite films prepared by the target-attached sputtering method are investigated. Both two nanocomposite samples reveal a typical characteristic of a metal-oxide varistor (MOV) on the J-E plots with distinct threshold electric fields (E(th)) which are affected by the molecular bonding configurations in the matrix and at the dot/matrix interfaces. The two systems exhibit dissimilar dependences of dc conductivity (sigma(dc)) on the ZnO content, revealing the limitation of conventional percolation models in which the surface interaction term is usually ignored. The ac conduction behaviors of the two nanocomposite systems were also analyzed and their percolation concentrations V(c) (22.75% for ZnO QDs-SiO2 and 20.78% for ZnO QDs-SiO(x)N(y)) are determined by the construction of master curves. Analytical results illustrated that the dielectric matrix type indeed affects the defect configuration and transport behaviors inside ZnO dots. Hence, manipulation of dielectric matrix type can possibly achieve various optical and electrical properties in ZnO QDs-dielectric nanocomposite systems.

Crystal tilting in GaN grown by pendoepitaxy method on sapphire substrate

Pendeoepitaxy of GaN on sapphire substrate with SiO2 mask is demonstrated and characterized by transmission electron microscopy and double crystal x-ray diffraction. A continuous layer of GaN with low dislocation density was achieved by this method. Parts of the GaN layer are tilted symmetrically toward [11-20] direction and have two kinds of coalesce and tilt boundaries. Each boundary was formed by a vertical array of piled up dislocations with the Burger’s vector of [11-20]. The tilting mechanism in pendeo-epitaxy is discussed in terms of surface interaction between the SiO2 mask and ELO-GaN.

Polymer Thin Films on Patterned Si Surfaces

The propagation of roughness from a patterned silicon surface by polymer thin films was measured as a function of film thickness, time, and surface interaction using atomic force microscopy and synchrotron X-ray reflection. In the presence of an interacting surface, the decay length of the surface modulation was much longer than that observed in simple liquids. By measuring the time dependence of the surface corrugation amplitude, we were able to extract a surface diffusion coefficient by applying a modified version of the Mullins theory for surface diffusion in crystalline solids. The measured diffusion coefficients were an order of magnitude smaller than in the bulk, and scaled as 1/M3/2, in agreement with previous SIMS results. The results are interpreted in terms of surface interactions confining polymer chains over distances larger than the radii of gyration.

Free energy of self-interacting uniform stars

We consider uniform star polymers on a variety of lattices, with an additional contact interaction between pairs of vertices which are unit distance apart and are not joined by an edge of the star. We present some rigorous results and other evidence which indicate that these systems have the same limiting free energy as self-interacting self-avoiding walks. We discuss the extension of these results to other homeomorphism types and to systems with an additional surface interaction term.

Surface-directed spinodal decomposition: Hydrodynamic effects

We present results from a numerical study of surface-directed spinodal decomposition in a binary fluid mixture, for critical and various off-critical concentrations. We explicitly add a long-range surface interaction term to the bulk free-energy functional leading to preferential attraction of one of the components of the fluid mixture to a solid surface. We also consider hydrodynamic interactions in these model calculations. Results of the wetting layer thickness and the domain size as a function of time and the scaling behavior of the density profile perpendicular to the surface are presented in the fluid inertia controlled regime.

A kinetic treatment of heterogeneous enantioselective catalysis

Kinetic data are reported for the liquid phase racemic hydrogenation and enantioselective hydrogenation of methyl acetoacetate (MAA) over Ni/SiO 2 and tartaric acid (TA)-treated Ni/SiO 2 , respectively, conducted in a slurry-type reactor. The contribution of mass transport to the overall reaction rate was considered by studying the effects of varying a number of process variables, such as catalyst particle size, catalyst weight, stirring speed and reaction temperature. Reaction conditions are identified wherein contributions due to mass transfer are slight and the hydrogenation proceeds under surface kinetic control. Reaction orders with respect to MAA, hydrogen and the product, methyl 3-hydroxybutyrate (MHB), were determined and activation energies over the temperature range 318 ⩽ T ⩽ 383 for the bare and TA-modified supported nickel are presented. The reaction rates were observed to pass through maxima as the MAA concentration was increased. The rate data are fitted to a Langmuir–Hinshelwood model where both reactants adsorb competitively on the surface; the experimentally determined rates agree to ±15% with the values predicted by the kinetic model. Modification of the catalyst with TA not only induced enantioselectivity but also promoted the rate of hydrogenation and this effect is discussed in terms of surface interactions and the proposed kinetic model. The ratio of MAA concentration in bulk solution to that at the catalyst surface under reaction conditions is examined and the dependence of enantioselectivity, in the case of the TA-treated sample, on MAA concentration is presented.

Surface-directed spinodal decomposition in a two-dimensional model.

We report results from a numerical study of a two-dimensional model for the surface-directed spinodal decomposition in a critical mixture. We explicitly add a long-range surface interaction term to the bulk free-energy functional leading to preferential attraction of one of the components of the mixtures to a free surface. We find that, for sufficiently long-range interactions, the thickness of the wetting layer l(t) varies as l(t)\ensuremath{\sim}${\mathit{t}}^{1/3}$. We also find that the magnitudes of the average domain sizes in the parallel and perpendicular directions are different. However, the exponent (n) characterizing the asymptotic growth of domains in these two directions is found to be the same (n=1/3). As well, the density profile and the pair correlation functions satisfy dynamical scaling behavior.

Positronium Decay in Silica Sol-Gels

ABSTRACTPositronium lifetimes inside porous silica are measured and discussed in terms of surface interactions, oxygen and nitrogen induced quenching and pore sizes. Surface interactions are studied for silica gels having surfaces covered solely with hydroxyl or methoxy groups. A rapid decay of positronium inside the pores filled with gaseous or liquid oxygen is reported and explained in terms of quenching mechanisms. An increase in the measured lifetimes for pores filled with nitrogen is attributed to the change in the pore distribution function.

An electrokinetic study of a natural coal associated mixture of kaolinite and montmorillonite in electrolytes

Clay fines washed from a minus 1-inch mine-run raw coal were characterized for size, ash, mineralogical composition and electrokinetic charge. The effect of pH and the clay surface interactions with electrolytes of varying anionic to cationic ratio composition and concentration were studied via electrophoresis. The responses are compared to pure minerals such as kaolinite, montmorillonite, silica and alumina, in terms of surface interactions, suspension stability, double layer theory and Schulze-Hardy rule. The clay was found to have optimum stability at pH 8.5 in the absence of electrolytes. Anions of increasing valence increase stability whereas cations of increasing valence decrease stability. Although the mineralogical analysis showed the clay to be a composite of kaolinite and montmorillonite, the clay responded more like kaolinite than montmorillonite, suggesting that the latter acted as a stabiliser to prevent edge-to-face interactions in the suspensions.

Towards a complete configurational theory of non-equilibrium polymer adsorption

A theory of non-equilibrium adsorption is presented in which a polymer profile is built up by the adsorption of incremental concentrations of polymer. Once each increment has been adsorbed, its surface concentration remains constant but the bulk profile can equilibrate, subject to this constraint, as more polymer is adsorbed. This is a more realistic theory of non-equilibrium adsorption than the theory presented by Barford et al. in J. Chem. Soc., Faraday Trans. 1, 1986, 82, 3233, as in that theory no equilibration of the adsorbate was allowed as more polymer was adsorbed. In this paper the free energy of the adsorbate as a function of adsorbate concentration, and hence the saturation concentration, and the polymer profiles at saturation are calculated as a function of a surface interaction term. It is shown that the results are essentially identical to those of Barford et al., leading to the conclusion that the simpler assumptions of Barford et al. were justified.

Influence of the surface interaction on threshold values in the cholesteric-nematic phase transition

The threshold values for the field-induced cholesteric to nematic phase transition are described as crossover points of the free energy levels of the textures concerned. Known expressions for these energy levels as a function of an electric field are extended with a surface interaction term. By interpretation of experimental results in terms of the equations derived, the surface interaction energy is calculated both for planar and homeotropically aligned cholesteric liquid-crystal mixtures. Besides differences between planar and homeotropic wall alignment a difference between two types of planar wall alignment can now be understood, too. Additionally, the method appears very useful for calculating the twist elastic constant K22.

Angular distribution of neutrons from precompound decay in inelastic scattering

Precompound angular distribution in inelastic scattering of 14 MeV neutrons are analysed in terms of surface interaction with the higher exciton states taken into account. This model is applied to some (p, α) reactions also.