Formation, structure and wettability of fluorescent nanolayers of oligoperoxide europium complexes adsorbed to glass surface

Formation kinetics and the nanolayer structure are determined for fluorescent complexes, which are composed of oligoperoxide and europium (OP-Eu), and are adsorbed to glass from water-ammonia solutions. The effective surface fraction x modified by OP-Eu is evaluated (with the Cassie equation) from receding contact angles. Its temporal behavior is repeated by the average adlayer thickness d, determined from ellipsometry, indicating rapid adsorption followed by gradual desorption. The fraction x increases with d to saturate at ∼ 90% for 57 nm. Atomic Force Microscopy micrographs and refractive indices do not reflect simple composite surfaces. Therefore a thickness effect on OP-Eu surface energy is also advocated. Fluorescent microscopy confirms luminescent properties of OP-Eu, applicable for protein detection.     

Current oscillations during chronoamperometric and cyclic voltammetric measurements in alkaline Cu(II)-citrate solutions

It is demonstrated that current oscillations can be observed during chronoamperometric and cyclic voltammetric experiments in solutions containing 0.4 M CuSO₄ and 1.2 M citrate at pH 11 and 50 °C. The oscillations, which are shown to originate from local variations in the pH, result in the deposition of nanostructured Cu and Cu₂O materials. It is concluded that the current oscillations are analogous to the previously described potential oscillations obtained under controlled current conditions in alkaline Cu(II)-lactate, -tartrate and -citrate solutions. Rotating disk electrode results clearly show that the reduction of the Cu(II)-complexes is kinetically controlled and that the rate of the reduction increases with increasing pH and temperature. It is also shown that the presence of a cathodic peak on the anodic scan in the cyclic voltammograms can be used to identify the experimental conditions leading to the spontaneous current (or potential) oscillations. Electrochemical quartz crystal microbalance results indicate that the cathodic peak stems from an increased rate of the reduction of the Cu(II)-citrate complexes due to a rapid increase in the local pH. This causes Cu₂O rather than Cu to be deposited which, however, results in a decrease in the local pH and a decreasing current. In situ ellipsometry data confirm that Cu₂O deposition replaces that of Cu in the potential region of the cathodic peak. The present findings should facilitate syntheses of nanolayered materials based on spontaneous potential or current oscillations.     

A new method for achieving nanoscale reinforcement of biaxially oriented polypropylene film

Nanolayers of poly(ethylene oxide) (PEO) produced by layer-multiplying coextrusion crystallize as single, high aspect ratio lamellae that resemble large single crystals. The confined crystallization habit imparts two orders of magnitude reduction in the gas permeability. We now demonstrate how the highly oriented lamellar nanolayers can be obtained with biaxial stretching. For this purpose, we chose biaxially oriented polypropylene (BOPP) film for modification and incorporated PEO nanolayers under conditions that mimicked the typical fabrication process. Sheet that contained a center core with 33 alternating layers of polypropylene (PP) and PEO was coextruded and subsequently biaxially oriented at 145 °C. Biaxial stretching reduced the PEO layer thickness from the spherulitic microscale to nanolayers of highly oriented PEO single lamellae. The nanolayers improved the oxygen barrier by an order of magnitude without sacrificing the high clarity and good tear resistance of BOPP film.     

Efficient solar-light-driven photoelectrochemical water oxidation of one-step in-situ synthesized Co-doped g-C3N4 nanolayers

Cobalt-doped g-C₃N₄ (Co-g-CN) nanolayers were prepared by a single-step thermal treatment with urea and cobalt nitrate. Different amounts of cobalt nitrate were tested to optimize the amount of cobalt dopant in the g-C₃N₄ (g-CN) matrix. Several characterization methods were used to explore the structural and optical properties along with the photoelectrochemical (PEC) performance. X-ray diffraction and Fourier transform infrared studies confirmed that g-CN nanolayers were successfully doped with cobalt without disturbing the basic 2-D structure and tris-triazine units of g-CN. Furthermore, microscopy images demonstrated that the cobalt effectively transformed the short nanosheets into long nanolayers. The cobalt-doping enhanced the visible absorption of g-CN and tuned the bandgap from 2.71 to 2.62 eV. An X-ray photoelectron spectroscopy (XPS) investigation discovered that cobalt entered into the g-CN network as Co²⁺ ions. XPS valence band spectra gave information on the modification in the valence and conduction band edge potentials due to cobalt doping. The photoluminescence intensity from the Co-g-CN samples was lesser than that from g-CN nanosheets, and the PEC activity of the Co-g-CN nanolayers was greater than that of as-prepared g-CN nanosheets. Co-g-CN samples prepared with 15 mg of cobalt nitrate hexahydrate showed a PEC performance of 3.2522 mA/cm², which was greater than that of g-CN nanosheets (1.9246 mA/cm²). The better PEC performance was ascribed to the synergistic consequence of the higher visible absorption obtained by tuning the bandgap and the host–guest interactions between cobalt and g-CN.     

High-flux H2 separation membranes from (Pd/Au)n nanolayers

A novel strategy for the preparation of supported PdAu alloy layers allows the facile and fast fabrication of highly permeable and selective H₂ separation membranes from refractory metals via electroless plating and low-temperature alloying. Homogenous alloying of multiple, separately deposited Pd and Au layers with thickness in the nm range required less than one week at 773 K under atmospheric H₂ as evidenced by X-ray diffraction and H₂ permeation measurements. The H₂ permeation rate JH₂ became stable within a day even, reaching 0.62 mol m⁻² s⁻¹ at 773 K and ΔPH₂ = 100 kPa. The corresponding N₂ leak rate remained constant during a 350 h experiment, resulting in an ideal H₂/N₂ selectivity of 1400 and demonstrating that such membranes tolerate extended operation at that temperature well.     

Nanolayer enhancement of biaxially oriented polypropylene film for increased gas barrier

An order of magnitude improvement in the oxygen barrier of biaxially oriented polypropylene (BOPP) films was achieved using a layer-multiplying, forced assembly process. The improvement was achieved without sacrificing clarity and toughness of the films. Sheets with 33 alternating layers of polypropylene (PP) (17 layers) and poly(?-caprolactone) (PCL) (16 layers) were coextruded using layer-multiplication and two thick PP skins were added to the multilayered core as the last step in the continuous coextrusion process. The sheets were subsequently biaxially oriented to draw ratios from 4 × 4 to 6 × 6. Biaxial orientation at elevated temperature reduced the thickness of the melted PCL layers from the microscale to the nanoscale, which created 2-dimensional confinement for subsequent crystallization of the PCL layers. It was anticipated that the PCL layers would recrystallize as highly oriented, in-plane lamellae that would resemble single crystals. However, the PCL lamellae were oriented perpendicular to the film surface, which actually facilitated oxygen permeation through the PCL layers and increased the oxygen permeability of the oriented films. Crystallization as on-edge lamellae was attributed to nucleation by the polypropylene surface. However, the surface nucleation was prevented by inserting buffer polystyrene (PS) layers in between the PCL and PP layers. In this case, the PCL lamellae were oriented in-plane. With the very high aspect ratio lamellar crystals oriented perpendicular to the flux direction, the permeation pathway of oxygen became very tortuous and the oxygen barrier was significantly improved.     

Effect of confinement on the relaxation behavior of poly(ethylene oxide)

It is widely thought that confinement of an amorphous polymer alters the chain mobility, which affects the temperature and intensity of the glass transition. The present study sought to determine whether the same effects extend to semicrystalline polymers. Confinement was achieved by forced assembly of hundreds of alternating layers of poly(ethylene oxide) (PEO) with either poly(ethylene-co-acrylic acid) or polystyrene. The confinement gradually reduced the intensity of the PEO β-relaxation as the layer thickness decreased from the microscale to the nanoscale. By considering the changes in crystalline morphology that accompanied layer confinement, it was possible to completely account for the reduction in relaxation intensity using standard mechanical models. The viscoelastic behavior of the amorphous phase was satisfactorily represented by a modified standard linear solid (SLS). The amorphous and crystalline contributions were combined using a combination of parallel and series coupling in accordance with the Takayanagi model. No adjustment in the viscoelastic parameters of the modified SLS was required, indicating that there was no significant change in amorphous chain dynamics even in layers as thin as 45 nm.     

Development and characterization of a nanomultilayer coating of pectin and chitosan – Evaluation of its gas barrier properties and application on ‘Tommy Atkins’ mangoes

A nanomultilayer coating made of food-grade, bio-based materials (consisting of five nanolayers of pectin and chitosan) was produced. This coating was firstly characterized in terms of the water vapor, oxygen and carbon dioxide permeabilities; these parameters exhibited values of 0.019 ± 0.005 × 10⁻¹¹, 0.069 ± 0.066 × 10⁻¹⁴ and 44.8 ± 32 × 10⁻¹⁴ g m/(Pa s m²), respectively, and are of the same order of magnitude of those found in other nanomultilayer systems. The nanomultilayer system was applied on whole ‘Tommy Atkins’ mangoes and the layers’ adsorption was confirmed by changes in the contact angle of the coated fruits’ skin. After 45 d of storage, uncoated mangoes presented a higher mass loss, higher total soluble solids and lower titratable acidity in comparison with coated mangoes. Uncoated mangoes had also a damaged and wrinkled appearance, showing evidence of microbial spoilage, and the flesh exhibited a slightly brownish color, in comparison with the coated mangoes. These results suggest a positive effect of the coating on gas flow reduction and on the consequent extension of the shelf-life of mangoes.