Quantitative Characterization of Hexagonal Packings in Nanoporous Alumina Arrays: A Case Study

Herein is presented a methodology to quantify the degree of hexagonal order in nanoporous alumina arrays (NAA). The approach is inspired by the theory of two-dimensional melting, developed to describe phase transitions in two-dimensional systems that present liquid-crystal-like structures. A local order parameter (LOP) is defined to quantify the degree of hexagonal order of each pore without any arbitrary parameters. Using this LOP, three main qualitative and quantitative analytical tools were developed: (i) a color code to create a map of the LOP, which is a visual tool to identify the degree of order; (ii) quantitative measurements of the average hexagonal order of the sample by measuring the distribution of the LOP and the distribution of the number of neighbors of each pore, and (iii) a quantification of the spatial correlation of the LOP, which indicates how far the hexagonal order is spread in a sample. Because this approach has a strong support on tools developed in statistical mechanics, one can go beyond a simple characterization and interpret the results in terms of phases, as in other physical systems. This may help to unveil the mechanisms behind the self-organization process and long-range order observed in NAA. Moreover, this approach can be trivially extended to characterize other physical systems that form hexagonal packings

Micro and Nano-Texturization of Intermetallic Oxide Alloys by a Single Anodization Step: Preparation of Artificial Self-Cleaning Surfaces

Micro- and nanostructures of Ti-γCu (γ = 0, 30, 50, 70, and 100 wt %) intermetallic alloys were produced through a single anodization step. It was found that the original alloy composition influences the final oxide morphology obtained after anodization which presented formation of a microstructure with nanotubes, nanoparticles or nanopillars on the surface. Pure Ti and Cu oxide metals and their alloys presented hydrophilic or superhydrophilic properties immediately after anodization. When the anodized pure metal and/or Ti-γCu surfaces were functionalized with trimethoxypropylsilane (TPMSi), by dipping and coating with a thin perfluorinated layer, the treated substrates became in all cases superhydrophobic (water contact angles in the range of 152–166°), showing excellent self-cleaning properties with hysteresis below 3°. These results can be explained by a combination of nanomicro morphologies with low surface energy compounds in the topmost monolayers. The decrease in hysteresis was associated with a higher M–OH bond concentration on the anodized surfaces, which allowed for more complete TMPSi coating coverage. This study also indicates that easy and effective fabrication of superhydrophobic surfaces in pure metals and alloys is possible without involving traditional multistep processes

Photochemical Hydrogen Production of Ta₂O₅ Nanotubes Decorated with NiO Nanoparticles by Modified Sputtering Deposition

The use of metal/oxide nanoparticles (NPs) as cocatalysts in heterogeneous photocatalysis is an important strategy to improve the photocatalytic activity of semiconductors for hydrogen generation. This article reports the use of a modified sputtering deposition method to prepare ultrafine NiO NPs cocatalysts dispersed on anodic Ta₂O₅ nanotubes (NTs). <i>In situ</i> X-ray absorption near-edge spectroscopy (XANES) measurements revealed that after exposing the as-prepared Ni NPs to air atmosphere a mixture of 68% of Ni and 32% of NiO was formed. Pure phase NiO NPs was successfully obtained after a controlled thermal oxidation at 500 °C which was confirmed by <i>in situ</i> XANES and <i>ex situ</i> XPS analyses. The photocatalytic hydrogen production activity was evaluated using ethanol as a sacrificial agent. Ta₂O₅ NTs with 0.16 wt % of NiO showed superior photocatalytic activity (up to 7.7 ± 0.3 mmol h^–1 g^–1) as compared to pure Ta₂O₅ NTs (4.9 ± 0.3 mmol h^–1 g^–1) The observed higher photocatalytic activity suggests that NiO/Ta₂O₅ NTs is a promising material for photocatalytic hydrogen evolution

Ta₂O₅ Nanotubes Obtained by Anodization: Effect of Thermal Treatment on the Photocatalytic Activity for Hydrogen Production

Freestanding tantalum oxide nanotubes (Ta₂O₅ NTs) were easily fabricated by controlling only the electrolyte temperature during anodization in a sulfuric acid solution. When the electrolyte temperature decreased, the adherence of NTs to the Ta substrate increased. High electrolyte temperatures facilitated formation of freestanding NTs. Thermal treatment of the freestanding Ta₂O₅ NTs below 750 °C resulted in an amorphous structure. The orthorhombic crystalline phase appeared only at temperatures higher than 750 °C. The effect of thermal treatment on the crystalline structure and morphology of Ta₂O₅ NTs showed that the NTs retained their tubular shape up to 800 °C. In addition, it was shown that the crystallinity of the NTs was enhanced from 11% to 34% by increasing the treatment time for the NTs at 800 °C from 0.5 to 1 h. High crystallinity and low surface contamination increased the photocatalytic activity of the freestanding NTs for hydrogen production by water splitting using a water/ethanol solution under UV radiation. The sample annealed at 800 °C for 1 h showed the highest photocatalytic activity for hydrogen generation. Additionally, changes to the physicochemical properties of the surface and bulk of the photocatalyst showed decreased selectivity for minor products (C₂H₄ and C₂H₆)