Self-organized porous alumina nanostructures fabricated by the anodization of aluminum have attracted considerable attention in both scientific and commercial fields as an indispensable part of nanotechnology. This has been fuelled by their versatile applications in fields of electronics or optoelectronics, [1] magnetics, [2,3] energy storage, [4] photocatalysis, [5] photonics, [6] and biosensors. [7] To facilitate various practical applications and nanodevices, fabricating highly ordered porous anodic alumina films with low cost and by a simple process on a large scale is an essential and urgent task and has yet not been solved. Porous anodic alumina (PAA) films with parallel nanopores are known as having a honeycomb-like structure that has short-distance ordering (in several tens to hundreds of nanometers) but long-distance disordering for pore arrangement. [8] To achieve a highly ordered pore arrangement over a large area, many studies have so far elaborated a variety of pretreatments or pretexturing techniques.[9±15] For instance, Masuda and Fukuda [9] first proposed a two-step anodization process, in which the dents on aluminum formed in the first anodization step (several days) worked as the initial sites of pore growth in the second anodization step, thus improving the overall pore arrangement to some extent. To achieve highly ordered PAA films, Masuda and co-workers also invented a pretexturing process, [10,11] i.e., using a textured SiC molder to produce ordered patterns on aluminum by a mechanical indentation prior to anodization. The shallow concaves on aluminum induced the pore initiation during anodization and led to an ideally ordered pore arrangement within the stamped areas (e.g., 4 mm 4 mm). Recently, some modified pretexturing methods, such as pre-patterning on aluminum by optical diffraction grating, [12] atomic force microscope scanning probe, [13] focused-ion-beam, [14] and polystyrene beads, [15] have been also attempted, to perform direct or mold-less patterning on aluminum exclusive of the expensive SiC master fabrication. To facilitate pretexturing, aluminum samples were usually annealed in nitrogen or argon at 400± 500 C to remove mechanical stress and to recrystallize the aluminum. They were then electro-polished in mixed acid solutions (e.g., a mixture of HClO 4 and C 2 H 5 OH) to smooth the surface for precise and uniform imprints. However, all of the pretexturing processes mentioned above, i.e., controlling the initial sites of pore growth, are utilizing external methods or extrinsic factors to achieve ordered PAA films. The decisive method or the intrinsic factor for pore ordering, however, is actually the anodizing process itself, i.e., the optimally combined anodizing conditions such as solution, temperature, and potential or current density. Therefore, raising the self-organizing ability of porous alumina films through an anodizing process, exclusive of any external assistance, is the radical solution for lowering production cost and exploring new applicable fields, which are sig...
Various ordered nanoporous alumina films with arbitrary pore intervals from 130 to 980 nm were fabricated on aluminum by a critical-potential anodization approach with sulfuric, phosphoric, oxalic, glycolic, tartaric, malic, and citric acid electrolytes under 70-450 V. The pore intervals of the porous alumina films were linearly proportional to applied potentials, with corresponding dominated territories to the electrolytes. In addition to pore interval, the self-ordering extent of pore arrangement was also improved with increasing anodizing potentials, leading to highly ordered porous alumina films at critical-high potentials. A cell separation phenomenon occurred for the films formed in sulfuric and glycolic acid solutions at the critical potentials, thus leading to the formation of highly ordered alumina nanotubule arrays. The critical-potential anodization in the other electrolytes produced self-organized porous alumina films with two-layered pore walls and pore bases. The basic principle for achieving porous alumina films with desired pore intervals is controlling the balance of the growth of barrier layer and the pore generation by adjusting the acidity, the concentration, and temperature of electrolytes. The porous alumina films formed in various electrolytes were transparent, and the transmittances of the films were inversely proportional to the applied potentials or the pore intervals.
Two-dimensional monoclinic WO(3) nanoplates with high specific surface areas are synthesized through a novel conversion process using tungstate-based inorganic-organic hybrid micro/nanobelts as precursors. The process developed involves a topochemical transformation of tungstate-based inorganic-organic hybrid belts into WO(3) nanoplates via an intermediate product of H(2)WO(4) nanoplates, utilizing the similarity of the W-O octahedral layers in both H(2)WO(4) and WO(3). The as-obtained WO(3) nanoplates show a single-crystalline nanostructure with the smallest side along the [001] direction. The WO(3) nanoplates are 200-500 nm x 200-500 nm x 10-30 nm in size, and their specific surface areas are up to 180 m(2) g(-1). Photocatalytic measurements of visible-light-driven oxidation of water for O(2) generation in the presence of Ag(+) ions indicate that the activity of the as-obtained WO(3) nanoplates is one order of magnitude higher than that of commercially available WO(3) powders.
The effect of SiO 2 addition on the anatase-to-rutile phase transition was investigated by DTA, XRD, FTIR, and XPS. TiO 2 xerogels containing SiO 2 up to 20 mol% were prepared by mixing and hydrolyzing titanium tetraisopropoxide (TTIP) and tetraethylorthosilicate (TEOS) with HNO 3 as a catalyst. With increased amounts of SiO 2 in the xerogels, the following results were obtained: (1) the crystallization temperature of anatase increased from 415°C in pure TiO 2 to 609°C in 20-mol%-SiO 2 -containing xerogel in the DTA curves; (2) the formation temperature of rutile, according to quantitative XRD analysis, increased with increased SiO 2 content up to 5 mol% SiO 2 but became constant at higher SiO 2 contents; (3) the crystallinity of anatase became lower; and (4) the lattice parameter a of the anatase decreased slightly, but the parameter c decreased greatly up to 20 mol% SiO 2 . Although the added silicon atoms were considered from these results to be incorporated into the amorphous TiO 2 and anatase structures, the 29 Si MAS NMR spectra of the xerogels containing 10 mol% SiO 2 showed only tetrahedral silicon, with no indication of silicon in octahedral coordination. When calcined at higher temperatures, the xerogel showed polymerization of the SiO 4 tetrahedra in the NMR spectra and the Si-O-Si vibration in the FTIR spectra. The chemical composition of the xerogel surfaces, measured using XPS, showed increased SiO 2 content with increased calcining temperature, indicating the expulsion of silicon from inside the particles to form an amorphous SiO 2 surface layer. The formation of this amorphous SiO 2 surface layer was considered to be important in retarding the anataseto-rutile phase transition by suppressing diffusion between anatase particles in direct contact and limiting their ability to act as surface nucleation sites for rutile. These effects of silicon additions were similar to those observed in the ␥-Al 2 O 3 -to-␣-Al 2 O 3 transition.
The effect of the crystallinity of boehmite powders on the temperatures of c-Al 2 O 3 formation and the h-to a-Al 2 O 3 transformation was investigated using boehmite powders of varying crystallite size prepared under various hydrothermal conditions. With increasing crystallite size of the boehmite powders, the specific surface area decreased and the expanded (020) d-spacing approached the reported value. Thermogravimetric ( TG) profiles of more poorly crystalline boehmite indicated the presence of excess water molecules of different binding energy located on the surface or in the interlayer. The crystallite size of boehmite also showed a strong correlation with the formation temperature of c-Al 2 O 3 and the phase transition temperature of h-to a-Al 2 O 3 . Since both these temperatures are increased with increasing the crystallite size of boehmite, this is an important factor in determining the conditions for obtaining c-, h-and a-Al 2 O 3 from boehmite. By relating the TG weight loss to the water and OH contents in boehmite and c-Al 2 O 3 , respectively, the crystallite size of boehmite can be related to the water content. This indicates that the c-Al 2 O 3 transformed from boehmite of smaller crystallite size contains larger amounts of OH groups, implying more poorly crystalline c-Al 2 O 3 . The reported topotactic transformation of boehmite via c-to h-Al 2 O 3 indicates a relationship between the crystallite size of the boehmite and that of the resulting transition alumina, which could explain the change of the phase transition temperature from h-to a-Al 2 O 3 .been performed to clarify the relation between the boehmite
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