Spectroscopic ellipsometry and XPS studies of anodic aluminum oxide formation in sulfuric acid

Influence of humic acid and dihydroxy benzoic acid on the agglomeration, adsorption, sedimentation and dissolution of copper, manganese, aluminum and silica nanoparticles - A tentative exposure scenario

This work focuses on kinetic aspects of stability, mobility, and dissolution of bare Cu, Al and Mn, and SiO₂ NPs in synthetic freshwater (FW) with and without the presence of natural organic matter (NOM). This includes elucidation of particle and surface interactions, metal dissolution kinetics, and speciation predictions of released metals in solution. Dihydroxy benzoic acid (DHBA) and humic acid adsorbed rapidly on all metal NPs (<1 min) via multiple surface coordinations, followed in general by rapid agglomeration and concomitant sedimentation for a large fraction of the particles. In contrast, NOM did not induce agglomeration of the SiO₂ NPs during the test duration (21 days). DHBA in concentrations of 0.1 and 1 mM was unable to stabilize the metal NPs for time periods longer than 6 h, whereas humic acid, at certain concentrations (20 mg/L) was more efficient (>24 h). The presence of NOM increased the amount of released metals into solution, in particular for Al and Cu, whereas the effect for Mn was minor. At least 10% of the particle mass was dissolved within 24 h and remained in solution for the metal NPs in the presence of NOM. Speciation modeling revealed that released Al and Cu predominantly formed complexes with NOM, whereas less complexation was seen for Mn. The results imply that potentially dispersed NPs of Cu, Al and Mn readily dissolve or sediment close to the source in freshwater of low salinity, whereas SiO₂ NPs are more stable and therefore more mobile in solution.

Subtle vertical structures of porous anodic alumina films for use as waveguides

Since porous anodic alumina (PAA) is a frequently-used optical waveguide material, accurate characterization of its structure parameters and optical properties is in urgent need. To characterize PAA, spectroscopic ellipsometry is preferred due to its undamaged detection, no sample pretreatment, and having a coverage area relatively larger than that of scanning electron microscopy. For spectroscopic ellipsometric data fitting, previous studies usually adopted a four-layer model, which displays a large bias from the raw data. Here, a modified six-layer model is built in consideration of the more elaborate porous layer that is the dominating contributor for the optical property of a PAA film. By using this six-layer model, PAA films with different thicknesses and under different oxidation voltages were analyzed, and the disperse curves of the porous layer were provided. This study will be helpful for learning the subtle structure of PAA and widen its applications for optical purposes.

Optical and Physical Properties of Cobalt Oxide Films Electrogenerated in Bicarbonate Aqueous Media

For the first time, cobalt oxide films that are highly protective against localized corrosion and depicting a wide variety of bright and uniform colors due to light interference, have been successfully electrogenerated on polycrystalline cobalt disk electrodes under potentiostatic polarization in a mild aqueous bicarbonate medium. Open circuit potential measurements have shown the formation of a film with a bilayered structure, organized as a thin Co3O4 outer layer and a thick CoO inner layer. The existence of Co3O4 as a thin outer layer, previously postulated from galvanostatic reduction experiments, has been confirmed from XPS analysis. Raman spectroscopy, performed using a very low laser intensity, has shown that the films are mainly composed of CoO. The broadness of the Raman bands observed is associated to the amorphous character of the film, a result that has been confirmed by spectroscopic ellipsometry and X-ray diffraction analysis. Overall film thicknesses, well controlled by the anodization duration, were determined and correlated using mechanical (atomic force microscopy and profilometry) and spectroscopic (specular UV-vis-NIR reflectance and ellipsometry) techniques. Spectroscopic ellipsometry, using a simple amorphous dispersion model, has proved efficient for measuring thicknesses of films ranging from 31 to 290 nm with very low standard deviations. The real part of the complex refractive indices of these films, ranging from 1.8 to 2.2 (at lambda = 632.8 nm) depending on the anodization duration, is in good agreement with values reported in the literature for CoO. The film with the highest refractive index, and consequently the more densely packed structure, was obtained following a 30-minute anodization period.