Double-anode anodization of metal Ti in two beakers

Thermal decomposition synthesis of CuO on TiO2 NTs as promising photocatalysts for effective photoelectrocatalytic hydrogen evolution and pollutant removal

The preparation strategy is the important factor to obtain the effective photocatalyst, and the thermal decomposition could be used to prepare photocatalysts with high crystallinity and photoactivity. In this paper, thermal decomposition method was used to deposit CuO nanoparticles on TiO2 nanotube arrays (TiO2 NTs), and the TiO2 NTs/CuO exhibited remarkably high visible light absorption and photoelectrocatalytic performances toward dye degradation and Cr(VI) reduction. The potential degradation pathway and toxicities of rhodamine B (RhB) dyes and intermediates were investigated. Moreover, the high photoelectric conversion with visible light photocurrent of 25.94 μA/cm2 and interface photovoltage of -0.12 V was produced, and the visible light-induced photoelectrocatalytic water splitting performance was outstanding. The mechanisms of hybrid photocatalyst preparation and photocatalytic progress were analyzed, and the p-n heterojunction was the decisive factor of the excellent photocatalytic property. The preparation strategy of TiO2 NTs/CuO provides the reference and guidance for the effective photocatalysts used in industrial waste water remediation and new energy generation.

The Effect of Anodization and Thermal Treatment on Mixed-Oxide Layer Formation on Ti–Zr Alloy

The anodization or thermal treatments applied to alloys of titanium and zirconium have a substantiated effect on the mixed-oxide layer formation compared to the naturally occurring one. A Ti–Zr 50%/50% alloy was chosen for a comparative study. Controlled, thermally treated, and anodized samples obtained with controlled procedures were analyzed in terms of morphological and compositional analysis (using SEM and EDX analysis) as well as for the determination of hardness variations. Substantial differences were observed depending on the applied functionalization method (compact of structured mixed-oxide nanotubes when the samples are subjected to the anodization procedure); there was an increase of more than six folds in the mixed-oxide layer hardness and D Shore scale, when subjected to thermal treatment, and hence, this lead to the conclusion that one may control the morphology, composition and/or the hardness of the mixed-oxide layer by applying one or another or a combination of functionalization methods.

Characterization of modified titanium surfaces by anodisation and immersion tests

This paper explores the features of both nanoporous and compact TiO2 films formed by titanium anodisation in two experimental conditions, given that they have completely different morphologies and properties than those found by our previous studies. After anodisation, samples have been subjected for 20 days to immersion tests in different media (H2O, H3PO4, and KOH). Surface morphology, phase composition and wettability of anodised films were investigated using FESEM, FTIR, Raman spectroscopy, contact angle measurements and XPS, and the hydrophilicity of modified surfaces was investigated by immersion tests. Nanoporous films exhibited hydrophobic surfaces, but contact angle values gradually decreased after immersing films in H2O, H3PO4 and KOH media, respectively. However, compact films produced superhydrophilic surfaces, both before and after immersion tests, with the exception of immersing the film in a H3PO4 medium due to film removal by acid attack. As for compact films, an unusual morphology revealed by the presence of cone-shaped particles might be responsible for the adsorption of -OH groups arranged so as to favour anatase phase formation.

A phosphoric anion layer inhibits electronic current generation and nanotube growth during anodization of titanium

Nowadays the formation mechanism of anodic TiO2 nanotubes has attracted extensive attention. Field-assisted dissolution (TiO2 + 6F- + 4H+ → [TiF6]2- + 2H2O) has been considered as the causal link to the formation and growth of nanotubes. But it is hard for this theory to explain three stages of the current-time curve. Here, the anodization of titanium was studied by adding different concentrations of H3PO4 (0%, 4 wt%, 6 wt%, 8 wt%, and 10 wt%) in ethylene glycol containing the same concentration of NH4F (0.5 wt%). The results prove that under the action of the same concentration of NH4F, the growth rate of nanotubes decreases obviously with the increase of H3PO4 concentration, and the second stage of the current-time curve is also prolonged simultaneously. These experimental facts cannot be interpreted by field-assisted dissolution theory and the viscous flow model. Here, an anion layer formed by H3PO4 and the electronic current theory are ably used to explain these facts reasonably for the first time.

Unraveling the six stages of the current-time curve and the bilayer nanotubes obtained by one-step anodization of Zr

The application and growth mechanism of anodic TiO2 nanotubes have been a hot topic in the last ten years, but the formation mechanism of anodic ZrO2 nanotubes has rarely been studied. In one-step constant voltage anodization of Al and Ti, the typical current-time curve has three stages. Moreover, the current-time curves of the three stages can last for 10 min or even 10 hours, resulting in a single layer of nanotubes with the same diameter due to the constant voltage in one-step anodization. However, in this paper, it was found for the first time that the three stages of the current-time curve appeared twice in succession during one-step constant voltage anodization of Zr for only 900 seconds, and bilayer nanotubes with increased diameter were obtained. This six-stage current-time curve cannot be explained by classical field-assisted dissolution and field-assisted flow or stress-driven mechanisms. Here, the formation mechanism and growth kinetics of bilayer ZrO2 nanotubes have been clarified rationally by the theories of ionic current, electronic current and oxygen bubble mold. The interesting results presented in this paper are of great significance for revealing the anodizing process of various metals and the formation mechanism of porous structures.

Debunking the essential effect of temperature and voltage on the current curve and the nanotube morphology

The formation mechanism of anodic TiO2 nanotubes remains to be unclear till now. Many researchers study the influence of temperatures above 0 °C instead of below 0 °C. Few papers before have explained the relationship between the current-time curve and the morphology of the nanotubes. In this study, the innovative 'oxygen bubble model' and the ionic current and electronic current theories were introduced to explain the growth of nanotubes below 0 °C. The length of anodic TiO2 nanotubes at 15 °C, 0 °C, -10 °C were 1.28 μm, 0.93 μm and 0.21 μm, respectively, but the diameter of anodic TiO2 nanotubes was almost the same, at about 164 nm. When the temperature was low, the magnitude of electronic current and the ionic current was small, the mold effect was weak and nanotubes could not be formed. At the same time, this study shows that the dissolution reaction of the field-assisted solution theory has no electron gain or loss, and it has nothing to do with the current, which negates the field-assisted dissolution theory. A novel two-step anodization was used to verify the conclusion. It was found that nanotubes could be obtained when the anodizing current was decreasing or increasing. Also, ginseng-shaped nanotubes are formed at a particular voltage sequence. Based on the 'oxygen bubble model' and the ionic current and electronic current theories, the formation process of nanotubes of two-step anodization is explained clearly.

Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides

Anodic TiO2 nanotubes have been studied widely for two decades because of their regular tubular structures and extensive applications. However, the formation mechanism of anodic TiO2 nanotubes remains unclear, because it is difficult to find convincing evidence for popular field-assisted dissolution or field-assisted injection theories and the oxygen bubble model. Here, in a bid to find direct evidence that oxygen bubbles form nanotube embryos, a new method is applied to handle this challenge. Before nanotube formation, a dense cover layer was formed to make nanotubes grow more slowly. Many completely enclosed nanotube embryos formed by oxygen bubbles were found beneath the dense cover layer for the first time. The formation of these enclosed and hollow gourd-shaped embryos is convincing enough to prove that the nanotubes are formed by the oxygen bubble mold, similar to inflating a football, rather than by field-assisted dissolution. Based on the 'oxygen bubble model' and ionic current and electronic current theories, the formation and growth process of nanotube embryos is explained clearly for the first time. These interesting findings indicate that the 'oxygen bubble model' and ionic current and electronic current theories also apply to anodization of other metals.