The effect of atmospheric pressure on the growth rate of TiO2 nanotubes: Evidence against the field-assisted dissolution theory

Surface characterization, electrochemical properties and in vitro biological properties of Zn-deposited TiO2 nanotube surfaces

In this work, to improve antibacterial, biocompatible and bioactive properties of commercial pure titanium (cp-Ti) for implant applications, the Zn-deposited nanotube surfaces were fabricated on cp-Ti by using combined anodic oxidation (AO) and physical vapor deposition (PVD-TE) methods. Homogenous elemental distributions were observed through all surfaces. Moreover, Zn-deposited surfaces exhibited hydrophobic character while bare Ti surfaces were hydrophilic. Due to the biodegradable behavior of Zn on the nanotube surface, Zn-deposited nanotube surfaces showed higher corrosion current density than bare cp-Ti surface in SBF conditions as expected. In vitro biological properties such as cell viability, ALP activity, protein adsorption, hemolytic activity and antibacterial activity for Gram-positive and Gram-negative bacteria of all surfaces were investigated in detail. Cell viability, ALP activity and antibacterial properties of Zn-deposited nanotube surfaces were significantly improved with respect to bare cp-Ti. Moreover, hemolytic activity and protein adsorption of Zn-deposited nanotube surfaces were decreased. According to these results; a bioactive, biocompatible and antibacterial Zn-deposited nanotube surfaces produced on cp-Ti by using combined AO and PVD techniques can have potential for orthopedic and dental implant applications.

Synthesis and Characterization of Nanostructured Oxide Layers on Ti-Nb-Zr-Ta and Ti-Nb-Zr-Fe Biomedical Alloys

Nanoporous/nanotubular complex oxide layers were developed on high-fraction β phase quaternary Ti-Nb-Zr-Ta and Ti-Nb-Zr-Fe promising biomedical alloys with a low elasticity modulus. Surface modification was achieved by electrochemical anodization aimed at the synthesis of the morphology of the nanostructures, which exhibited inner diameters of 15–100 nm. SEM, EDS, XRD, and current evolution analyses were performed for the characterization of the oxide layers. By optimizing the process parameters of electrochemical anodization, complex oxide layers with pore/tube openings of 18–92 nm on Ti-10Nb-10Zr-5Ta, 19–89 nm on Ti-20Nb-20Zr-4Ta, and 17–72 nm on Ti-29.3Nb-13.6Zr-1.9Fe alloys were synthesized using 1 M H3PO4 + 0.5 wt% HF aqueous electrolytes and 0.5 wt% NH4F + 2 wt% H20 + ethylene glycol organic electrolytes.

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.

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.