A study on the formation of titania nanopillars during porous anodic alumina through-mask anodization of Ti substrates

The formation of anodic titania during porous anodic alumina (PAA) through-mask anodization has been analysed for varying anodization conditions on mechanically polished bulk Ti surfaces. Titania nanopillars were formed through the porous masks in both oxalic and phosphoric acid electrolytes. For applied potentials above 40 V the titania formed along narrow channels through the alumina pore bottoms resulting in root-like attachments of the titania pillars to the Ti substrate. We further demonstrated that high-field anodization can be used for PAA through-mask anodization. The formation of titania changed with increased current density which resulted in more efficient oxide growth through the alumina pores. When the Al/Ti samples were immersed in the electrolyte without exclusively exposing the Al surface to the electrolyte the titania formed solely on top of the alumina pore bottoms which resulted in that the titania structures were detached from the Ti substrates during selective removal of the PAA templates.     

Effect of the solvent on growth of titania nanotubes prepared by anodization of Ti in HCl

The effect of the solvent on the anodic growth of titania nanotubes in HCl dissolved in water, ethylene glycol and 2-propanol was studied. These nanotubes grow with locally rapid breakdown of the passive TiO₂ film forming a forest of nanotubes-bearing microtowers with the background of passive TiO₂ Film. These bundles of assembled-groups of titania nanotubes look like Pillars corals. The low relative permittivity of the 2-propanol led to lowering of dissociation of HCl and hence lowering the activity of H⁺ and Cl⁻ ions which in turn led to suppress of dissolution of titania and increasing the growth rate of the titania nanotubes. The X-ray diffraction pattern showed that the titania nanotubes after annealing change to the crystalline anatase phase. The anodic films showed characteristic coloration with intensity and color that changes (qualitatively) with time of anodization.     

Formation of porous anodic titanium oxide films in hot phosphate/glycerol electrolyte

The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm⁻³ K₂HPO₄ + 0.2 mol dm⁻³ K₃PO₄/glycerol electrolyte containing only 0.04% water at 433 K. The growth efficiency is reduced by increasing the formation voltage to 20 V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20 V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m⁻². The pore diameter is not significantly dependent upon the formation voltage, being ∼10 nm.     

Formation of anodic films on sputtering-deposited Al-Hf alloys

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO₂ and Al₂O₃ distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al³⁺ and Hf⁴⁺ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al³⁺-O²⁻ and Hf⁴⁺-O²⁻ bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V⁻¹ respectively.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

The effect of electrolyte anions incorporated into anodic oxide films on the experimental transport numbers of ions

The growth of barrier anodic film is considered theoretically with regard to the migration of three ionic carriers: oxygen and metal ions and electrolyte anions. It is shown that the consideration of anion transport leads to the conclusion that the film grows at three interfaces: the metal/oxide and oxide/electrolyte interfaces and the interface between an oxide layer containing electrolyte anions (contaminated layer) and the oxide layer free of them (“pure” layer). The error in the measured transport numbers of metal and oxygen, which is caused by ignoring a contribution of electrolyte anions to the total charge transport, is maximum in the absence of anion motion.     

Self-organized porous titanium oxide prepared in Na2SO4/NaF electrolytes

The anodic formation of nanoporous TiO₂ on titanium was investigated in Na₂SO₄ electrolytes containing low concentrations of NaF (0.1-1 wt.%). It was found that under optimized electrolyte conditions and extended polarization, a self-organized nanostructure consisting of porous TiO₂ is obtained. The porous structure is arranged in sections of arrays with single pore diameters of typically 100 nm and an average spacing of 150 nm. The pores are open at the top and covered by oxide at the bottom. Compared with earlier work, we show that using a neutral NaF electrolyte significantly thicker porous layers can be obtained than in acidic solutions.     

Growth of porous anodic films on sputtering-deposited aluminium incorporating Al-Hf alloy nanolayers

Formation of porous anodic films on sputtering-deposited aluminium incorporating Al-Hf tracer layers has been examined at constant current in sulphuric and phosphoric acids. Hafnium was selected as the tracer species since the migration rates of Hf⁴⁺ and Al³⁺ ions are similar in barrier-type anodic alumina. The distribution of hafnium in the films was determined using ion beam analysis, scanning electron microscopy and transmission electron microscopy. Increases in the anodizing voltage and barrier layer thickness accompany the oxidation of hafnium and the migration of Hf⁴⁺ ions through the barrier layer region of the porous film. Hf⁴⁺ and Al³⁺ ions that migrate to the pore bases are lost to the electrolyte. Other Hf⁴⁺ ions are incorporated into the cell walls. For films formed in phosphoric acid, with relatively thick barrier layers, channelling of the ion current leads to accelerated outward transport of Hf⁴⁺ ions toward the pore base, while a U-shaped inner edge of the hafnium distribution beneath the pores is associated with more slowly transported hafnium species. The tracer behaviours for films formed in both acids are consistent with the transport of Hf⁴⁺ ions in the barrier layer regions by a combination of flow of film material and ion migration, the flow being a key factor in the development of the pores. The percentage losses of Hf⁴⁺ and Al³⁺ ions from the films to the electrolyte are relatively similar, correlating with their similar migration rates, and contrast with the retention in the film of slow migrating W⁶⁺ ions, found previously, due to a more dominant role of flow.     

Morphological and structural studies of nanoporous nickel oxide films fabricated by anodic electrochemical deposition techniques

Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate, nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved Ni²⁺ to Ni³⁺, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small pores is obtained by CV deposition in a potential range of 0.7-1.1 V. A galvanostatically deposited film is structurally denser near the surface of the substrate, and becomes less dense further away from the surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration, potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited films resemble closely to cubic NiO when the annealing temperature exceeds 200 °C.     

Formation of porous anodic films on Ti-Si alloys in hot phosphate-glycerol electrolyte

Porous anodic films, with pore size of ∼10 nm, have been developed by anodizing of magnetron sputtered Ti-25 at.% Si alloy at constant formation voltages in glycerol electrolyte containing dibasic potassium phosphate at 433 K. The films, of amorphous structure, contain titanium and silicon species, as units of TiO₂ and SiO₂, throughout the film thicknesses, with negligible amounts of phosphorus species. The silicon is enriched in the film relative to the composition of the alloy, the level of enrichment suggesting that anion migration is increased in comparison with amorphous film growth at ambient temperature. In contrast to the behaviour of the alloy, essentially barrier films were formed on commercially pure titanium in the glycerol electrolyte, when a main anodic reaction was generation of oxygen, which was probably promoted by the development of anatase.     

Influence of molybdenum species on growth of anodic titania

TiMo, bcc, solid-solution alloys, containing 11.5-37.0 at.% molybdenum, have been anodised galvanostatically in 0.1 mol dm⁻³ ammonium pentaborate and 1.0 mol dm⁻³ phosphoric acid electrolytes, with resultant anodic films characterised by scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy and glow discharge optical emission spectroscopy. Uniform amorphous films are formed at high current efficiency to >100 V, with formation ratios of 2.3 and 2.2 nm V⁻¹ in the respective electrolytes, contrasting with the amorphous-to-crystalline transition of anodic titania on titanium that occurs at ∼20-50 V. Apart from minor incorporation of electrolyte species, the films comprise an outer layer of TiO₂ and an inner oxide layer containing Ti⁴⁺ and Mo⁶⁺ ions. The films grow by migration of both cations and anions, with Ti⁴⁺ ions migrating faster than Mo⁶⁺ ions that is related to the energies of Ti⁴⁺O and Mo⁶⁺O bonds.     

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.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

A flow model of porous anodic film growth on aluminium

The development of pores in a classical porous anodic film formed on aluminium in phosphoric acid solution is investigated. The study employs a tungsten tracer layer that is incorporated into the anodic film from the aluminium substrate, followed by detection of the tracer by transmission electron microscopy and Rutherford backscattering spectroscopy. Distortions of the tungsten layer on entry into the film and retention of tungsten species in the film are compatible with porosity arising mainly from flow of anodic oxide beneath the pore bases towards the cell walls. The behaviour is contrary to expectations of a dissolution model of pore formation.     

Development and study of anodic Ti/TiO2 electrodes and their potential use as impedimetric immunosensors

Titanium dioxide films were anodically formed at various potentials up to 65 V in 1 M H₂SO₄. Oxide films were characterized by performing various techniques, including electrochemical impedance spectroscopy, scanning electron microscopy, Raman spectroscopy, ellipsometry and diffuse reflectance FT-IR spectroscopy. Low voltage anodization (up to 10 V) results to amorphous TiO₂, whereas at higher applied potentials (up to 65 V), anatase is the predominant form. Anatase films were further hydroxylated with an acidic agent and the effect of this treatment on the overall impedance of the electrodes was studied with impedance spectroscopy. The potential use of anodic (anatase) Ti/TiO₂ electrodes in the development of impedimetric immunosensors is also demonstrated by monitoring the immunoreaction of avidin/anti-avidin with different instrumental approaches based on a FRA analyzer, an LCR-meter and a home-built charge integrator (Multipulser).     

Influence of silicon on the growth of barrier-type anodic films on titanium

Amorphous anodic titania, stabilised by incorporation of silicon species, is shown to grow to high voltages on sputter-deposited, single-phase Ti-Si alloys during anodizing at a constant current density in ammonium pentaborate electrolyte. The films comprise two main layers, with silicon species confined to the inner layers. An amorphous-to-crystalline transition occurs at ∼60 V on the Ti-6 at.% Si alloy, while the transition is suppressed to voltages above 140 V on alloys with 12 and 26 at.% silicon. The crystalline oxide, nucleated at a depth of ∼40% of the film thickness, is associated with the presence of a precursor of crystalline oxide in the pre-existing air-formed oxide. The modified structure of the air-formed oxide due to increased incorporation of silicon species suppresses the amorphous-to-crystalline transition until the onset of dielectric breakdown. The transport numbers of cations and anions during growth of the anodic oxides are independent of the concentration of silicon species in the inner layer, despite the marked change in the field strength.     

Optimized observation of tungsten tracers for investigation of formation of porous anodic alumina

Improved methods are presented for investigating the flow of anodic alumina during the formation of porous anodic films on aluminium in phosphoric acid. In particular, the use of tungsten nanolayer tracers with increased flatness is shown to result in enhanced definition of the influences of flow on the tracer distribution, as observed by electron microscopy. Additionally, taper sectioning of films, by ultramicrotomy, and parallel sectioning of films, by sputtering using an Ar⁺ ion plasma, enable tungsten distributions across cells to be determined. In the case of an Al–3.5 at.%W alloy, the flow results in an inner alumina layer lining the pore walls that is relatively free of tungsten species and an outer cell layer of comparatively high tungsten content. Both sectioning procedures also show the presence of fine cell boundary bands of relatively high tungsten content, which are suggested to result from the transport of tungsten, enriched in the alloy immediately beneath the film, toward the cell boundary by the alloy/film interface.     

Stress-affected and stress-affecting instabilities during the growth of anodic oxide films

The objective of the current paper is to (re-)address the question whether internal stress is a fundamental parameter driving some generic cases of growth instabilities commonly encountered during the growth of anodic oxide films, namely breakdown and pore initiation. This has been done by unraveling possible correlations between a key electrochemical characteristic of the instability event and the internal stress evolution, the latter being measured in situ during the very same anodising experiment. As such, we have been able to make more conclusive statements as compared to the merely speculative arguments in the literature whether these instabilities have a mechanical origin or not. In the case of breakdown, the two well-documented types of breakdown events encountered during galvanostatic Zr anodising were both found to be stress-affected: instantaneous compressive internal stresses were identified as the driving force for both the densifying phase transformation responsible for type-I breakdown, as well as for the buckling-induced delamination events observed during type-II breakdown. Pore initiation in anodic Al₂O₃ on the other hand was found not be stress-affected. Instead, pore formation is rather believed to induce itself a modification in the mechanical behaviour, and was therefore classified as stress-affecting.     

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.     

Growth of anodic oxide films on oxygen-containing niobium

The present study is directed at understanding of the influence of oxygen in the metal on anodic film growth on niobium, using sputter-deposited niobium containing from about 0-52 at.% oxygen, with anodizing carried out at high efficiency in phosphoric acid electrolyte. The findings reveal amorphous anodic niobia films, with no significant effect of oxygen on the field strength, transport numbers, mobility of impurity species and capacitance. However, since niobium is partially oxidized due to presence of oxygen in the substrate, less charge is required to form the films, hence reducing the time to reach a particular film thickness and anodizing voltage. Further, the relative thickness of film material formed at the metal/film interface is increased by the incorporation of oxygen species into the films from the substrate, with an associated altered depth of incorporation of phosphorus species into the films.