Cezarina Cela Mardare

Investigations on Bactericidal Properties of Molybdenum–Tungsten Oxides Combinatorial Thin Film Material Libraries

A combinatorial thin film material library from the molybdenum-tungsten refractory metals oxides system was prepared by thermal coevaporation, and its structural and morphological properties were investigated after a multiple step heat treatment. A mixture of crystalline and amorphous oxides and suboxides was obtained, as well as surface structuring caused by the enrichment of molybdenum oxides in large grains. It was found that the oxide phases and the surface morphology change as a function of the compositional gradient. Tests of the library antimicrobial activity against E. coli were performed and the antimicrobial activity was proven in some defined compositional ranges. A mechanism for explaining the observed activity is proposed, involving a collective contribution from (i) increased local acidity due to the enrichment in large grains of molybdenum oxides with different stoichiometry and (ii) the release of free radicals from the W18O49 phase under visible light.

Electrocatalysis on copper–palladium alloys for amperometric formaldehyde sensing

Co-evaporation of Cu and Pd was used for the deposition of a relatively small gradient concentration thin film combinatorial library with Pd amounts between 4 and 14 atomic percent (at%). Screening for electrocatalytic oxidation of formaldehyde was performed by scanning droplet cell microscopy along the Cu–Pd compositional spread in alkaline solution and a best material performance for this process was identified for 7.5 at% Pd in Cu confirming results from co-sputtering studies. However, the microstructure and crystallographic analysis of Cu–Pd thin film alloys showed a compositionally induced gradual change of properties without any significant discontinuity. This indicates that the Cu–Pd atomic ratio is the main factor defining the electrocatalytic activity of the investigated alloys. This finding is also confirmed for bulk Cu–Pd alloys where the reproducibility of significant formaldehyde oxidation electrocatalytic activity when using Cu-7.5 at% Pd was demonstrated. An amperometric formaldehyde sensor was fabricated and its reproducibility, repeatability and stability were assessed. During successive anodic formaldehyde current oxidation peak observations a standard deviation value of 8% was measured. Multiple efficient successive use of the same sensor (5 to 10 times) were demonstrated and a maximum of 5% decrease in the current density was observed after 21 days of normal environment storage during a shelf-lifetime evaluation of the sensor. Overall, the study reveals inexpensive approaches for fabrication of multiple use formaldehyde sensors via thermal evaporation or bulk alloy casting, as well as the transfer of the main feature (i.e. maximum current density for formaldehyde oxidation) from a thin film combinatorial library to bulk samples.

Silver-, calcium-, and copper molybdate compounds: Preparation, antibacterial activity, and mechanisms

Developing novel compounds with antimicrobial properties can be an effective approach to decreasing the number of healthcare-associated infections, particularly in the context of medical devices and touch surfaces. A variety of molybdate powders (Ag2MoO4, CaMoO4, CuMoO4 and Cu3Mo2O9) were synthesized and characterized, and Escherichia coli was used as a model gram-negative bacterium to demonstrate their antimicrobial properties. Optical density measurements, bacterial colony growth, and stained gel images for protein expression clearly showed that silver- and copper molybdates inhibit bacterial growth, whereas CaMoO4 exhibited no bactericidal effect. All tests were performed in both daylight and darkness to assess the possible contribution of a photocatalytic effect on the activity observed. The main mechanism responsible for the antibacterial effect observed for Ag2MoO4 is related to Ag+ release in combination with medium acidification, whereas for compounds containing copper, leaching of Cu2+ ions is proposed. All these effects are known to cause damage at the cellular level. A photocatalytic contribution to the antibacterial activity was not clearly observable. Based on the pH and solubility measurements performed for powders in contact with various media (ultrapure water and bacterial growth medium), silver molybdate (Ag2MoO4) was identified as the best antibacterial candidate. This compound has great potential for further use in hybrid powder-polymer/varnish systems for touch surfaces in healthcare settings.

Spectroscopic ellipsometry for compositionally induced bandgap tuning of combinatorial niobium–tantalum anodic oxides

Variable angle spectroscopic ellipsometry (VASE) was used for optical property mapping of anodic oxides grown on a wide spread Nb–Ta thin film combinatorial library (Nb content ranging between 8 and 84 at%) with 2 at% resolution. The microstructure of the parent metal alloys was tuned by modifying the library deposition conditions in order to avoid compositionally induced microstructure dissimilarities between the alloys. This allowed an interpretation of the mixed oxides properties based on a single Tauc–Lorentz oscillator for data fitting. Upon anodization up to 10 V vs. SHE, VASE was used to fit the oxide thickness in order to map the oxide formation factors with the results showing a good agreement with the electrochemical data. The measured bandgap values of the Nb–Ta oxides show a deviation from the linear model based on mixing individual Nb and Ta electronegativities proportional to their atomic fractions. This behavior was explained via an in-depth compositional gradient in the oxides. Minority species are depleted at the metal/oxide interface triggering a non-linear shift in the resulting alloy electronegativity. The Nb–Ta mixed oxides bandgap tuning via compositional tuning of the parent metal has been demonstrated and absorption close to the edge of the visible spectrum was identified above 30 at% Nb.

Basic properties mapping of anodic oxides in the hafnium–niobium–tantalum ternary system

ABSTRACT A thin film combinatorial library deposited by co-sputtering of Hf, Nb and Ta was employed to characterise fundamental properties of the Hf-Nb-Ta system. Compositional mappings of microstructure and crystallography revealed similarities in alloy evolution. Distinct lattice distortion was observed upon addition of hexagonal Hf, leading to amorphisation of alloys containing more than 32 at.% Hf and less than 27 and 41 at.% Nb and Ta, respectively. Volta potential and open circuit potential mappings indicated minimal values for the highest Hf concentration. Localised anodisation of the library by scanning droplet cell microscopy revealed valve metal behaviour. Oxide formation factors above 2 nm V−1 were identified in compositional zones with high amounts of Nb and Ta. Fitting of electrochemical impedance spectroscopy data allowed electrical permittivity and resistivity of mixed oxides to be mapped. Their compositional behaviours were attributed to characteristics of the parent metal alloys and particularities of the pure oxides. Mott–Schottky analysis suggested n-type semiconductor properties for all Hf–Nb–Ta oxides studied. Donor density and flat-band potential were mapped compositionally, and their variations were found to be related mainly to the Nb amount. Synergetic effects were identified in mappings of Hf-Nb-Ta parent metals and their anodic oxides. Graphical Abstract