Synthesis of self-organized mixed oxide nanotubes by sonoelectrochemical anodization of Ti-8Mn alloy

Self ordered arrays of titanium manganese mixed oxide nanotubes were prepared by anodization of Ti8Mn alloy (UNS R56080) under ultrasonication in diluted ethylene glycol containing fluoride. The dimensions of the nanotubes (diameter: 20-100 nm and length: 0.5-2.0 μm) could be tuned by changing the synthesis parameters. The as-anodized nanotubes showed a stoichiometry of (Ti,Mn)O₂. Upon annealing at 500 °C in oxygen atmosphere, the nanotubes contained a mixture of anatase + rutile phases of TiO₂ and Mn₂O₃. The composition of the oxide nanotubes was influenced by the chemistry of the phases present in the alloy. More manganese content was observed in the oxide formed on the β-phase than in the oxide layer of α-phase. Anodization in the ultrasonic field increased the kinetics of nanotubular oxide formation and resulted in homogeneous ordering of the nanotubular arrays as compared to the anodization by conventional stirring in the fluoride containing ethylene glycol solution. Whereas, anodization in aqueous acidified fluoride solutions resulted in severe attack of the β-phase and did not show presence of nanotubular oxide structure.     

Electrochemical synthesis of self-organized TiO2 nanotubular structures using an ionic liquid (BMIM-BF4)

We show that an ionic liquid consisting of imidazolium salt with a BF₄ counter ion (BMIM-BF₄) can directly be used to grow well-defined layers of self-organized TiO₂ nanotubes. For this a Ti metal substrate is anodized in this electrolyte for potential range between 3 V_Ag/AgCl and 10 V_Ag/AgCl without addition of free fluoride species (fluorides are used in all previous tube growth procedures). Key factors that influence the morphology and geometry of the resulting nanotubular layer are the anodic potential, the anodization time and particularly the water content in the ionic liquid. The resulting nanotubes layers have thickness in the range of approximately 300-650 nm; with individual tubes that have diameters between 27 nm and 43 nm.     

Formation of self-ordered nano-tubular structure of anodic oxide layer on titanium

Room temperature anodization of titanium foil specimens was carried out in 0.5 M phosphoric acid solution with addition of various halide ions. Addition of 0.138 M HF or NaF resulted in self-ordered nano-tubular oxide structure. Addition of bromide and chloride ions initiated only pitting and nano-pores were not observed during anodization. Acidified fluoride solution is found to be necessary to obtain ordered nano-structure as neutral fluoride solution did not form nano-pores. Instability of the oxide layer during anodization and formation of the self-ordered structure can be explained by the perturbation theory. Separation of individual nano-tubes of titanium oxide layer from the inter-connected nano-pores could be attributed to the possible repulsion forces of the cation vacancies.     

Formation and characterization of iron oxide nanoparticles loaded on self-organized TiO2 nanotubes

The iron oxide nanoparticles were loaded onto self-organized TiO₂ nanotube layers grown by anodization of Ti in fluoride containing electrolytes. The nanoparticles were obtained by electrodepositing method in glycerol/water/FeCl₃·6H₂O electrolytes at room temperature. The X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) measurements showed that the nanoparticles consisted of iron nanocrystalline (Fe) and magnetite (Fe₃O₄). The hematite (α-Fe₂O₃) structure was obtained by annealing in air at 450 °C. The growth mechanism of the nanoparticles and their morphology were also described. Furthermore, the nanoparticles exhibited good ferromagnetic properties at room temperature.     

Electrochemically multi-anodized TiO2 nanotube arrays for enhancing hydrogen generation by photoelectrocatalytic water splitting

An enhanced hydrogen production by photoelectrocatalytic water splitting was obtained using extremely highly ordered nanotubular TiO₂ arrays in this work. Highly ordered TiO₂ nanotube arrays with a regular top porous morphology were grown by a facile and green three-step electrochemical anodization. The well ordered hexagonal concaves were uniformly distributed on titanium substrate by the first anodization, served as a template for further growth of TiO₂ nanotubes. As a result, the TiO₂ nanotube arrays constructed through the third anodization showed appreciably more regular architecture than that of the sample by conventional single anodization under the same conditions. The enhanced photoelectrochemical activity was demonstrated through the hydrogen generation by photoelectrocatalytic water splitting, with an exact H₂ evolution rate up to 420 μmol h⁻¹ cm⁻² (10 mL h⁻¹ cm⁻²) in 2 M Na₂CO₃ + 0.5 M ethylene glycol. The photocurrent density of the third-step anodic TiO₂ nanotubes is about 24 mA cm⁻² in 0.5 M KOH, which is 2.2 times higher than that of the normal TiO₂ nanotubes (∼11 mA cm⁻²) by a single electrochemical anodization.     

Influence of water content on nanotubular anodic titania formed in fluoride/glycerol electrolytes

A study has been carried out of nanotubular anodic films formed on titanium at 20 V in fluoride/glycerol electrolyte, containing up to 50 vol.% water. Anodizing was terminated at a charge of 1 C cm⁻². Addition of water resulted in an increased current and significantly reduced tube length associated with increased oxygen gas evolution. Films formed in the absence of added water were amorphous by electron diffraction, whereas water addition also led to the formation of anatase and rutile. The barrier layer was relatively thin for electrolyte of low water content, due to either a voltage drop in the electrolyte close to the anode or to a change in the film composition affecting the electric field across the layer. Ribbing of the external walls of the nanotubes was more evident in the presence of water. It is suggested that dissolution of a fluoride-rich layer, which separates the nanotubes, accompanies the nanotube growth, with the dissolution allowing transient film formation at the external walls of the nanotubes when the residual layer is sufficiently thin.     

Facile synthesis of robust free-standing TiO2 nanotubular membranes for biofiltration applications

Robust monodisperse nanoporous membranes have a wide range of biotechnological applications, but are often difficult or costly to fabricate. Here, a simple technique is reported to produce free-standing TiO₂ nanotubular membranes with through-hole morphology. It consists of a three-step anodization procedure carried out at room temperature on a Ti foil. The first anodization (1 h at 80 V) is used to pattern the surface of the metallic foil. Then, the second anodization (24 h at 80 V) produces the array of TiO₂ nanotubes that will constitute the final membrane. A higher voltage anodization (3–5 min at 180 V) is finally applied to detach the TiO₂ nanotubular layer from the underlying Ti foil. In order to completely remove the barrier layer that obstructs some pores of the membrane, the latter is etched 2 min in a buffered oxide etch solution. The overall process produces 60-μm-thick TiO₂ nanotubular membranes with tube openings of 110 nm on one side and 73 nm on the other side. The through-hole morphology of these membranes has been verified by performing diffusion experiments with glucose, insulin, and immunoglobulin G where in differences in diffusion rate are observed based on molecular weight. Such biocompatible TiO₂ nanotubular membranes, with controlled pore size and morphology, have broad biotechnological and biomedical applications.     

Electrochemical tuning of titania nanotube morphology in inhibitor electrolytes

The electrochemical behavior of fluorine containing electrolytes and its influence in controlling the lateral dimensions of TiO₂ nanotubes is thoroughly investigated. Potentiostatic anodization is carried out in three different electrolytes, viz., aqueous hydrofluoric acid (HF), HF containing dimethyl sulphoxide (DMSO) and HF containing ethylene glycol (EG). The experiments were carried out over a broad voltage range from 2 to 200 V in 0.1-48 wt% HF concentrations and different electrolytic compositions for anodization times ranging from 5 s to 70 h. The chemistry that dictates how the nature of electrolytes influences the morphology of nanotubes is discussed. Electrochemical impedance spectra were recorded for varying compositions of all the electrolytes. It was observed that composition of the electrolyte and its fluorine inhibiting nature has significant impact on nanotube formation as well as in controlling the aspect ratio. The inhibiting nature of EG is helpful in holding fluorine at the titanium anode, thereby allowing controlled etching at appropriate voltages. Thus our study demonstrates that HF containing EG is a promising electrolytic system providing wide tunability in lateral dimensions and aspect ratio of TiO₂ nanotubes by systematically varying the anodization voltage and electrolyte composition.     

Control of morphology and composition of self-organized zirconium titanate nanotubes formed in (NH4)2SO4/NH4F electrolytes

We investigated the formation of self-organized zirconium titanate nanotubes by anodizing a Ti-35Zr alloy in 1 M (NH₄)₂SO₄ + 0.1-2.0 wt.% NH₄F electrolytes. The morphology and composition of the zirconium titanate nanotube are controlled by the applied electrochemical conditions. The outer diameter of nanotubes is controlled by the anodization potential in the range between 1 and 100 V (versus Ag/AgCl). Tubes with diameters from 14 to 470 nm can be grown. The nanotube length correlates with the anodic charge up to a length where significant dissolution of the nanotube layer is observed. The wall thickness, composition of the nanotubes and porosity of the nanotube layer are significantly affected by the fluoride ion concentration. The length limiting factor of the nanotube growth is found to be the diffusion of ionic species in the electrolyte.     

Stable TiO2 nanotube arrays with high UV photoconversion efficiency

This work is intended to define an optimal methodology for preparing highly ordered TiO₂ nanotube arrays by a 60-V anodization in a glycol ethylene solution. In order to obtain a mechanically stable structure with a high UV photoconversion efficiency, it is necessary to carefully control the growth mechanism through the anodization process. For this reason, the nanotube arrays have to be formed upon a compact titanium dioxide layer with well-defined thickness. Besides, both the fluoride concentration and anodization time are strictly correlated, because elevated concentrations and/or a long anodization time produce unstable structure with low photoconversion efficiency. The best result in the terms of reproducibility has been obtained previously for a three-minute galvanostatic oxide growth on the pickled titanium sheet, and anodic growth in ethylene glycol solution containing 1 wt.% H₂O and 0.20 wt.% NH₄F for a period lower than 4.5 h. The UV photoconversion efficiency was measured and a maximum value of 28.3% has been obtained, which is the highest result in the literature.     

Formation of self-organized Zircaloy-4 oxide nanotubes in organic viscous electrolyte via anodization

This work reports the formation of self-organized Zircaloy-4 (Zr-4) oxide nanotubes in viscous organic ethylene glycol (EG) electrolyte containing a small amount of fluoride salt and deionized (DI) water via an electrochemical anodization. The structure, morphology, and composition of the Zr-4 oxide nanotubes were studied using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), EDX, and XPS. SEM results showed that the length of the nanotubes is approximately 13 μm, and TEM results showed that the inner diameter of the Zr-4 oxide nanotubes is approximately 20 nm with average wall thickness of approximately 7 nm. XRD and selected area electron diffraction pattern (SAED) results confirmed that the as-anodized Zr-4 oxide nanotubes have cubic crystalline structure. Both cubic and monoclinic phases were found after annealing of Zr-4 oxide nanotubes. The tubular structure morphology of Zr-4 oxide nanotubes did not remain intact after annealing which is attributed to the elimination of F species from the annealed nanotubes.     

Designing micro-nano structure of anodized iron oxide films by metallographic adjustment on T8 steel

In this study, the anodized iron oxide films with micro-nano structure were prepared by a combination of heat treatment and anodization. The adjustment of these oxide films at the micron-scale was achieved by heat treatment on substrate to modify the position of the cementite and ferrite. The distribution and existing forms of carbon on cementite and ferrite determine the morphology of their respective anodization. It was found annealing at 300 °C can improve the crystallinity of iron oxide and maintain the original micro-nano structure. Although the crystal quality of iron oxide can be further improved with higher annealing temperature, it would result in the collapse of nanoporous structures and the reconstruction of the surface morphology. Furthermore, anodized samples made from those with lamellar ferrite and cementite show better specific capacitance. The iron oxide film on the annealed substrate obtained the highest specific capacitance of 35.3 mF/cm² at a scan rate of 20 mV/s in our samples.     

CVD synthesis of Al2O3 nanotubular structures using a powder source

Alumina (Al₂O₃) 1D nanotubular structures were synthesized by chemical vapor deposition (CVD) using aluminum (Al) and Al₂O₃ powder sources at a temperature of 1300 °C and a pressure of 100 Pa. At present, no research has been published regarding the synthesis of α-phase Al₂O₃ nanotubes using only a powder source. In this work, we attempted to grow α-phase Al₂O₃ nanotubes without catalysts. As-deposited nanotubular structures had an irregular and ragged surface morphology that was related to the boundary layer thickness. Moreover, complementary nanotubular structures can be obtained via an annealing process. A model to describe the irregular nanotubular structure formation was suggested, and the effect of the boundary layer thickness was demonstrated in our experimental conditions.     

Enhanced inactivation of E. coli bacteria using immobilized porous TiO2 photoelectrocatalysis

Immobilized TiO₂ nanotube electrodes with high surface areas were grown via electrochemical anodization in aqueous solution containing fluoride ions for photocatalysis applications. The photoelectrochemical properties of the grown immobilized TiO₂ film were studied by potentiodynamic measurements (linear sweep voltammetry), in addition to the calculation of the photocurrent response. The nanotube electrode properties were compared to mesoporous TiO₂ electrodes grown by anodization in sulfuric acid at high potentials (above the microsparking potential) and to 1 g/l P-25 TiO₂ powder. Photocatalyst films were evaluated by high resolution SEM and XRD for surface and crystallographic characterization. Finally, photoelectrocatalytic application of TiO₂ was studied via inactivation of E. coli. The use of the high surface area TiO₂ nanotubes resulted in a high photocurrent and an extremely rapid E. coli inactivation rate of ∼10⁶ CFU/ml bacteria within 10 min. The immobilized nanotube system is proven to be the most potent electrode for water purification.     

Characteristics of the removal of mercury vapor in coal derived fuel gas over iron oxide sorbents

The characteristics of a novel method for Hg removal using H₂S and sorbents containing iron oxide were studied. Previously, we have suggested that this method is based on the reaction of Hg and H₂S over the sorbents to form HgS. However, the reaction mechanism is not well understood. In this work, the characteristics of the Hg removal were studied to clarify the reaction mechanism. In laboratory made sorbents containing iron oxide were used as the sorbent to remove mercury vapor from simulated coal derived fuel gases having a composition of Hg (4.8 ppb), H₂S (400 ppm), CO (30%), H₂ (20%), H₂O (8%), and N₂ (balance gas). The following results were obtained: (1) The presence of H₂S was indispensable for the removal of Hg from coal derived fuel gas; (2) Hg was removed effectively by the sorbents containing iron oxide in the temperature range of 60-100 °C; (3) The presence of H₂O suppressed the Hg removal activity; (4) The presence of oxygen may play very important role in the Hg removal and; (5) Formation of elemental sulfur was observed upon heating of the used sample.     

Visible light photoelectrochemical responsiveness of self-organized nanoporous WO3 films

Visible light-responsive WO₃ nanoporous films with preferential orientation of the (0 0 2) planes were prepared by anodization in neutral F⁻-containing strong electrolytes. The pore diameter of the self-organized structure was estimated to be in the region of 70-90 nm. Voltages were applied by stepping, which positively influenced passivity breakdown and played a significant role in the formation of self-organized nanoporous films. Under visible light irradiation, the photocurrent density (at 1.6 V vs. Ag/AgCl) and maximum photoconversion efficiency generated by the annealed nanoporous film were 3.45 mA/cm² and 0.91%, respectively. The annealed nanoporous WO₃ films show maximum incident photon-to-current conversion efficiency of 92% at 340 nm at 1.2 V vs. Ag/AgCl. These values are higher than that of annealed compact WO₃ film due to the large interfacial heterojunction area. The photoelectrochemical activities and electronic conductivities were also enhanced by annealing crystallization, which removed the recombination centers.     

Porous niobium oxide films prepared by anodization in HF/H3PO4

Ordered porous niobium oxide with the diameter of less than 10 nm and the aspect ratio of more than 20 is prepared by anodization of niobium foils at 2.5 V in the mixture of 1 wt% HF and 1 M H₃PO₄ for 1 h. In this study, the effects of the mixed electrolytes, anodic potential and anodization time on the preparation of porous niobium oxide are described based on the current-time transients during anodization and morphological observations. It is founded that a single HF electrolyte leads to the formation of pores as well as the fast dissolution of formed pores at the surface. The dissolution of the formed oxide is significantly retarded by the addition of appropriate amount of H₃PO₄.     

An electrochemical and surface analytical study of the formation of nanoporous oxides on niobium

In the present paper, the anodization of Nb in mixed sulphate + fluoride electrolytes resulting in the formation of a nanoporous oxide film has been studied. Chronoamperometry and electrochemical impedance spectroscopy have been employed to characterise in situ the kinetics of the oxidation process. In addition, the evolution of the layer structure and morphology has been followed by ex situ scanning electron microscopy. Particularly, local electrochemical impedance spectroscopy has been used to discern between the mesoscopic 2D and 3D distributions of time constants at the electrode surface. The similarity between local and global impedance spectra during anodic oxidation of Nb demonstrates the presence of an inherent 3D distribution of the high-frequency time constant, which is interpreted as in-depth variation of the steady state conductivity of the passive film. The experimental and calculational results are discussed in relation to the micro- and nanoscopic structure of the formed oxide.     

The anodic oxide film on iron in neutral solution

The composition of the anodic passive oxide film on iron in neutral solution has been investigated by cathodic reduction, chemical analysis and ellipsometry. The cathodic reduction using a borate solution of pH 6·35 containing arsenic trioxide as inhibitor estimates iron in the film to be all iron (III), indicating that no magnetite layer is present. Oxygen in the film is estimated from the ellipsometric thickness to be in excess of the stoichiometric ferric oxide, suggesting the presence of bound water. The average composition is represented as Fe₂O₃.0·4H₂O, in which hydrogen may be replaced partly with iron-ion vacancy. The anodic oxide film is composed of an inner anhydrous ferric oxide layer, which thickens with the potential and an outer layer of hydrous ferric oxide whose thickness depends on the condition of passivation and environment.     

Ozone-oxygen gas enhanced passivation of pure iron

Pure iron samples were passivated in borate buffer solution and were exposed to ozone-oxygen gas to study the possibility of improving the surface nanostructure such as atomically flat terrace structure. The treated samples were analyzed with spectroscopic ellipsometry and scanning tunneling microscopy, and the test of corrosion resistance was also carried out. The thickness of the oxide film increased by passivation at 800 mV (Ag/AgCl) and increased by ozone-oxygen gas exposure, but the oxidation film thickness decreased in air due to the reconstruction after rapid growth of oxides by passivation. The reconstruction of the oxide film was estimated by the change of the film thickness and compositions which were analyzed by spectroscopic ellipsometry using several layer models of Fe(OH)₃, γ-Fe₂O₃ and Fe₃O₄ and so on. The widest terrace width of the oxidation film was obtained on large particles based on the reconstruction after the combination treatment of passivation at 800 mV as well as subsequent ozone-oxygen gas treatment. The terrace width of the oxide film after passivation and ozone-oxygen gas treatment was three times larger than that of air-formed oxide film. The terrace width with atomic scale flatness was correlated with the corrosion resistance except for the increase in oxide film thickness.