The mechanism of silver(I) oxide formation on polycrystalline silver in alkaline solution. Determination of nucleation and growth rates

The nucleation and growth of Ag(I) oxide layers on polycrystalline Ag electrodes is studied by potentiodynamic and potentiostatic current transients in 0.1 M NaOH. A model involving the diffusion controlled growth of a basal layer followed by the nucleation and growth of a secondary layer is proposed. Various nucleation parameters are estimated by comparing experimental data with simulated transients, and the results are discussed in terms of the atomistic theory of electrochemical phase formation. The influence of the nucleation parameters on the shape of the current transients is analysed and some controversial results in the literature are explained.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

The surface diffusion of gold atoms on gold electrodes in acid solution and its dependence on the presence of foreign adsorbates

The surface diffusion coefficient (Ds) of Au atoms in 0.5 M H2SO4 at 298 K has been determined from the time dependence of the surface roughness factor of electrodispersed Au electrodes in 0.5 M H2SO4. The value of Ds for 0.5 M H2SO4 is 5 × 10−14 cm2 s−1, a figure which depends considerably on the presence of adsorbable species in solution. Thus, the adsorption of pyridine produces a remarkable decrease of Ds, whereas the adsorption of Cl− ion results in the opposite effect. The corresponding values of Ds are 1.2 × 10−14 cm2 s−1 for 0.5 M H2SO4 + 0.1 M pyridine, and 2.5 × 10−13 cm2 s−1 for 0.5 M H2SO4 + 5 × 10−4 M KCl. These results are interpreted through the influence of the adsorbate on the Au surface atom diffusion mechanism.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Cover Crop Impacts on Soil Water Status

Water is a primary concern for producers in the Great Plains; as such, research is warranted to quantify how much cover crops affect the amount of soil water available to subsequent cash crops. Cover crop mixes have been marketed as a means to conserve water in no-till cropping systems following winter wheat (Triticum aestivum L.) harvest. The objectives of this study are to quantify changes in soil profile water content in the presence of different cover crops and mixtures of increasing species complexity, to quantify their biomass productivity and quality, and to quantify the impact of cover crops on subsequent corn (Zea mays L.) yields. We hypothesized the change in soil water brought on by the cover crop treatments would be correlated to the quantity of biomass produced and the species composition, rather than mixture complexity. Soil moisture was measured using a neutron probe to a depth of 9 ft. Results from 2013–14 showed no difference in water use between cover crop mixtures and single species. Cover crops depleted the soil profile by a maximum of 3.5 in. during growth, but fallow was able to gain 0.75 in. of water during the same period. At the time of corn planting, soil moisture under all cover crops had replenished to levels at cover crop emergence, except for the brassicas, which had extracted water from deeper in the profile. Corn yields were reduced following the grass cover crops and the six-species mix. Corn yields were more closely related to the carbon:nitrogen (C:N) ratio of the cover crop residue than to profile soil moisture at corn emergence. The fact that yields were similar for corn after fallow and for corn after brassica cover crops implied that water was not the cause of yield reductions after the other cover crops

The mechanism of silver(I) oxide formation on polycrystalline silver in alkaline solution. Determination of nucleation and growth rates

The nucleation and growth of Ag(I) oxide layers on polycrystalline Ag electrodes is studied by potentiodynamic and potentiostatic current transients in 0.1 M NaOH. A model involving the diffusion controlled growth of a basal layer followed by the nucleation and growth of a secondary layer is proposed. Various nucleation parameters are estimated by comparing experimental data with simulated transients, and the results are discussed in terms of the atomistic theory of electrochemical phase formation. The influence of the nucleation parameters on the shape of the current transients is analysed and some controversial results in the literature are explained.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

The surface diffusion of gold atoms on gold electrodes in acid solution and its dependence on the presence of foreign adsorbates

The surface diffusion coefficient (Ds) of Au atoms in 0.5 M H2SO4 at 298 K has been determined from the time dependence of the surface roughness factor of electrodispersed Au electrodes in 0.5 M H2SO4. The value of Ds for 0.5 M H2SO4 is 5 × 10−14 cm2 s−1, a figure which depends considerably on the presence of adsorbable species in solution. Thus, the adsorption of pyridine produces a remarkable decrease of Ds, whereas the adsorption of Cl− ion results in the opposite effect. The corresponding values of Ds are 1.2 × 10−14 cm2 s−1 for 0.5 M H2SO4 + 0.1 M pyridine, and 2.5 × 10−13 cm2 s−1 for 0.5 M H2SO4 + 5 × 10−4 M KCl. These results are interpreted through the influence of the adsorbate on the Au surface atom diffusion mechanism.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Fractal surfaces of gold and platinum electrodeposits. Dimensionality determination by scanning tunneling microscopy

The structure of gold and platinum deposits grown on gold and platinum wire cathodes, respectively, by electroreduction of the corresponding oxide layers has been analyzed in terms of surface fractals by measuring the perimeter L and area A of intergranular voids. Perimeter and area were determined from scanning tunneling microscopy (STM) topographic images of the deposit surfaces. Fractal behavior, L ∞ AD'/2 with D'= 1.5 ± 0.1, is found for both platinum and gold deposits. From these measurements, the fractal dimension of the surfaces was 2.5. The D' value of the gold deposits decreases by aging in the electrolyte at 325 K as a consequence of surface diffusion processes.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Fractal surfaces of gold and platinum electrodeposits. Dimensionality determination by scanning tunneling microscopy

The structure of gold and platinum deposits grown on gold and platinum wire cathodes, respectively, by electroreduction of the corresponding oxide layers has been analyzed in terms of surface fractals by measuring the perimeter L and area A of intergranular voids. Perimeter and area were determined from scanning tunneling microscopy (STM) topographic images of the deposit surfaces. Fractal behavior, L ∞ AD'/2 with D'= 1.5 ± 0.1, is found for both platinum and gold deposits. From these measurements, the fractal dimension of the surfaces was 2.5. The D' value of the gold deposits decreases by aging in the electrolyte at 325 K as a consequence of surface diffusion processes.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Component integration strategies in metamorphic 4-junction III-V concentrator solar cells

Progressing beyond 3-junction inverted-metamorphic multijunction solar cells grown on GaAs substrates, to 4-junction devices, requires the development of high quality metamorphic 0.7 eV GaInAs solar cells. Once accomplished, the integration of this subcell into a full, Monolithic, series connected, 4J-IMM structure demands the development of a metamorphic tunnel junction lattice matched to the 1eV GaInAs subcell. Moreover, the 0.7 eV junction adds about 2 hours of growth time to the structure, implying a heavier annealing of the subcells and tunnel junctions grown first. The final 4J structure is above 20 Pm thick, with about half of this thickness used by the metamorphic buffers required to change the lattice constant throughout the structure. Thinning of these buffers would help reduce the total thickness of the 4J structure to decrease its growth cost and the annealing time. These three topics: development of a metamorphic tunnel junction for the 4th junction, analysis of the annealing, and thinning of the structure, are tackled in this work. The results presented show the successful implementation of an antimonide-based tunnel junction for the 4th junction and of pathways to mitigate the impact of annealing and reduce the thickness of the metamorphic buffers

Purification, kinetics and spectral characterisation of a new versatile peroxidase from a Bjerkandera Sp. isolate

From the extracellular fluid of a novel strain of Bjerkandera sp., it was isolated, purified and identified the main enzyme responsible for Remazol Brilliant Blue R dye decolourisation. Such an enzyme is able to oxidise manganese, as well as veratryl alcohol and 2,6-dimethoxyphenol in manganese-independent reactions; hence, it can be included in the new group of versatile peroxidases. The molecular mass of said enzyme is ca. 45 kDa, and the N-terminal amino acid sequence obtained by Edman degradation is VAXPDGVNTA. The enzyme substrate range for oxidation of several phenolic and non-phenolic aromatic compounds was determined and the corresponding Michaelis-Menten kinetic constants calculated. Furthermore, spectrophotometric assays showing the Soret band and allowing observation of band shifts of the enzyme led to the conclusion that Bjerkandera strains may also synthesise at least two different versatile peroxidases, as happens with Pleurotus eryngii