Naofumi Nagai

Fabrication of boehmite films with cage-like pores and their properties as enzyme immobilization supports

The texture of porous films prepared from boehmite nanofibres changed from the lengthwise orientation to the random orientation with the addition of ammonia, and with this change, the shape of the pores in the films turned from slit-like into mesh-like while increasing both the pore size and pore volume. The pore size and volume of the films also increased with an increase in the length of the fibres. When the porous films were used for the immobilization of four lipases with different sizes, the films with the mesh texture exhibited higher immobilization ability than the films with the slit texture. It was found that not only the pore size and volume but also the surface charge and hydroxyl groups are important for the immobilization of enzymes. Among the four lipases used, the lipase with the biggest size (ca. 9.5 nm) was most firmly immobilized on the film with the peak top of pore diameter at 10 nm in the distribution curve, and this immobilized lipase exhibited higher activity in the hydrolysis of 2,3-dimercaptopropanol trilactate than the corresponding native one and was recyclable.

Surface treatment- and calcination temperature-dependent adsorption of methyl orange molecules in wastewater on self-standing alumina nanofiber films

A novel form of alumina, free-standing and transparent alumina nanofiber thin films has been synthesized and utilized as highly efficient adsorbents for organic dye removal and decoloration, and their adsorption capacities are sensitively dependent on surface treatment and calcination temperature. Compared with commercial alumina powders, the films exhibited totally different adsorption behaviors and enhanced adsorption capacities, which may originate from their different pore sizes in the two forms. The as-prepared dye-adsorbed alumina nanofiber films may open up new possibilities for some potential applications, e.g., pH indicator paper for pH testing derived from the dye-adsorbed alumina films was enabled based on different dye-desorption characteristics in different solutions.

Properties of boehmite and Al2O3 thin films prepared from boehmite nanofibres

Transparent and nanoporous self-standing films consisting of oriented boehmite nanofibres were fabricated via the self-assembly of the fibres. The film-forming ability of boehmite nanofibres was closely related to their aspect ratio, and the fibres of 100 to 5000 in aspect ratio were favourable to fabricate the films. Crystallinity and stability of boehmite fibres and the films increased with an increase in the aspect ratio of the fibres. The boehmite films were easily transformed to γ-, θ- and α-Al2O3 films by calcination while keeping the original shapes of the films. Transformation temperatures to γ-, θ- and α-Al2O3 films tended to rise, respectively, with the aspect ratio of the fibres forming the films. When the boehmite films were calcined at the temperatures of 300 °C to 800 °C, the obtained γ-Al2O3 films exhibited bright bluish-white emission under the excitation of a UV lamp. The emission intensity tended to increase with a decrease in the aspect ratio of the fibres forming films.

Fabrication of boehmite and Al2O3 nonwovens from boehmite nanofibres and their potential as the sorbent

Boehmite nonwovens with hierarchical texture could be fabricated by a simple procedure consisting of mixing a boehmite nanofibre sol with an organic solvent and filtering the mixture. However, whether the nonwovens were obtained or not depended on the solvents. High polar solvents such as methanol and ethanol caused gelation of the mixture, and low polar solvents such as toluene and ether yielded condensates in the mixture. The medium polar solvents with solubility parameters of 9–12.5, concretely, mixtures of butanol with methanol, ethanol and isopropanol, were favourable for obtaining the nonwovens. The nonwovens consisted of bundles of boehmite nanofibres, and the texture, pore volume, bulk density and porosity of the nonwovens roughly depended on the thickness of the bundles. Thus, thinner bundles and thicker bundles gave dense and loose textures, respectively. The bundles became thicker by increasing the butanol content in the mixed alcohols, and thinner by increasing the methanol, ethanol and isopropanol content. The nonwovens could be transformed from boehmite to θ-Al2O3via γ-Al2O3 in the crystal phase while maintaining seemingly the original texture when the calcination temperature was raised to 1000 °C stepwise. The nonwovens were tested as sorbents for polycyclic organics such as chromaqurol B, stilbazo and alizarin yellow R in water, and it was found that the nonwovens absorb chromaqurol B and stilbazo effectively, but alizarin yellow R slightly.

Self-standing microporous films of arrayed alumina nano-fibers including Schiff base molecules: effect of the environment around the molecules on their photo-luminescence

A self-standing, flexible and transparent microporous film made of alumina nano-fibers is applied as a host to design new functional hybrid materials. Luminescent Schiff base molecules, N-salicylidene-p-toluidine, were found to fully, stably, and homogeneously occupy the slit-like micropore of the film from X-ray diffraction, optical spectra, and N2 gas adsorption isotherm measurements. Photoluminescence (PL) intensity of the hybrid film was sensitive to the type of transition metal ions dispersed in aqueous solution. For example, Zn2+ and Cu2+ ions in the solutions were involved with the apparent increase and decrease of the PL intensity, respectively. This unique alumina film has high potential to be a candidate of a host matrix for designing new hybrid materials with noble functions.

A Highly Photoreflective and Heat‐Insulating Alumina Film Composed of Stacked Mesoporous Layers in Hierarchical Structure

An alumina film with highly photoreflective and heat-insulating properties can be simply synthesized using a sol of fibrous boehmite with an additive. The entangled fibers bring about mesopores among them and form stacked 2D nonwoven-like nanosheets. The porosity and the layered structure of alumina accompanying the heat resistivity provide the upper properties that are usually difficult to realize simultaneously.

Properties of Boehmite AlO(OH) Nanoparticles as the Coatings and Fillers

Boehmite nanoparticles with different shapes and lengths were used in the preparation of the boehmite films and coatings, and as the fillers for an organic polymer. Then, it was investigated how the shapes and lengths of the particles reflected the characteristics of the films, coatings and fillers. The films were prepared by casting an aqueous sol of the nanoparticles on a Teflon-lined plate, drying and peeling. The coatings were fabricated on porous alumina tubes or graphite sheets by dipping the substrates into a sol of the nanoparticles. The boehmite-polyvinyl alcohol (PVA) composite films, in which the nanoparticles are fillers, were prepared by casting a mixture of a boehmite nanoparticle sol and PVA. The films, coatings and composites were characterized by TEM, TG-DTA, gas permeability, pore size distribution and thermal expansion measurements. In the films, fibrous particles with aspect ratios over 50 aggregated in parallel to each other to form slit-like micropores, on the other hand, plate- and rod-like particles with aspect ratios below 10 randomly aggregated to form meso pores. Corresponding to this, for the coatings on porous alumina tubes and graphite sheets, the fibrous particles gave much low gas permeability and high thermostability, respectively, compared with the plate-like and rod-like particles. For the boehmite-PVA composite films, the fibrous particles gave lower thermal expansion than the plate-like and rod-like particles.