Titanium oxide nanotubes prepared in phosphate electrolytes

In the present communication, we report the electrochemical formation of self-organized titanium oxide nanotubes (π-TiO₂) prepared in fluoride ion containing phosphate electrolytes. The morphology of the π-TiO₂ layers (particularly the pore diameter and length) is affected by the electrochemical conditions used (applied potential, electrolyte composition, pH, and anodizing time). Under specific sets of conditions highly self-organized titanium oxide nanotubes are formed with diameters varying from approx. 40 nm to 100 nm and length from approx. 100 nm to 4 μm. XPS investigations show that the nanotubes formed in phosphate solutions contain a significant amount of phosphorous species.     

Growth and field crystallization of anodic films on Ta–Nb alloys

The growth behavior of amorphous anodic films on Ta–Nb solid solution alloys has been investigated over a wide composition range at a constant current density of 50 A m⁻² in 0.1 mol dm⁻³ ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising a thin outer Nb₂O₅ layer and an inner layer consisting of units of Ta₂O₅ and Nb₂O₅. The outer Nb₂O₅ layer is formed as a consequence of the faster outward migration of Nb⁵⁺ ions, compared with Ta⁵⁺ ions, during film growth under the high electric field. Their relative migration rates are independent of the alloy composition. The formation ratio, density, and capacitance of the films show a linear relation to the alloy composition. The susceptibility of the anodic films to field crystallization during anodizing at constant voltage increases with increasing niobium content of the alloy.     

Self-organized porous and tubular oxide layers on TiAl alloys

The present paper describes the electrochemical fabrication of nanostructured oxide films on a TiAl intermetallic compound. The alloy is investigated under conditions where the individual alloying elements show the growth of ordered oxide structures, i.e. anodization is carried out in fluoride containing and fluoride free H₂SO₄ electrolytes. In 1 M H₂SO₄ the alloy shows randomly ordered nanoporous oxide structures while in HF-containing electrolytes highly ordered films can be formed. The key factor that affects the morphology is the anodizing potential. At low potentials (∼10 V) self-organized nanopores are formed whereas at higher potentials (∼40 V) separation of the pore walls and therefore formation of nanotubes can be observed. The results clearly indicate that on TiAl a wide range of nanoscale morphologies can be achieved ranging from random porous to organized pores to organized tubes.     

Effect of Fluoride or Chloride Ions on the Morphology of ZrO2 Thin Film Grown in Ethylene Glycol Electrolyte by Anodization

0.3 wt % ammonium fluoride (NH₄F) or ammonium chloride (NH₄Cl) was added to ethylene glycol (EG) as an active ingredient for the formation of anodic oxide comprising of ZrO₂ nanotubes (ZNTs) by anodic oxidation of zirconium (Zr) at 20 V for 10 min. It was observed that nanotubes were successfully grown in EG/NH₄F/H₂O with aspect ratio of 144.3. Shorter tubes were formed in EG/NH₄F/H₂O₂. This could be due to higher excessive chemical etching at the tip of the tubes. When fluoride was replaced by chloride in both electrolytes, multilayered oxide resembling pyramids was observed. The pyramids have width at the bottom of 3-4 μm and the top is 1-2 μm with 10.7 μm height. Oxidation of Zr in EG/NH₄Cl/H₂O₂ was rater rapid. The multilayered structure is thought to have formed due to the re-deposition of ZrO₂ or hydrated ZrO₂ on the foil inside pores formed within the oxide layer. XRD result revealed an amorphous structure for as-anodized samples regardless of the electrolytes used for this work.     

Permanent electrochemical promotion of C3H8 oxidation over thin sputtered Pt films

The effect of “permanent electrochemical promotion of catalysis” (P-EPOC) was studied for the first time in the catalytic deep oxidation of C₃H₈ over a thin (～ 150 nm) sputtered Pt film on YSZ, under excess of oxygen at 350 °C. Short positive potential application (+ 1 V) resulted in a 5.6-fold increase of the catalytic rate, where C₃H₈ conversion reached 33%, while the apparent Faradaic efficiency was ～ 330. After positive current interruption the catalytic rate remained in a highly active steady-state, determined by the total charge of the anodic polarization step. Restoration of the catalytic activity to the initial value occurred only by a similar negative potential imposition. This new stable steady-state after current interruption can be interpreted by storage of a non-reactive oxygen species upon anodic polarization at the proximity of the Pt/YSZ interface and subsequent enhanced migration of spillover O^δ? species from the electrolyte support to the Pt/gas interface under open-circuit conditions.     

Highly increased capacitance and thermal stability of anodic oxide films on oxygen-incorporated Zr-Ti alloy

Heat treatment of Zr-24 at% Ti alloy with barrier-type dielectric anodic oxide films was conducted at 473 K in air to examine the thermal stability of the dielectric oxide films for possible electrolytic capacitor application. The anodic oxide film was formed by anodizing of the alloy at 50 V for 30 min in 0.1 mol dm^?3 ammonium pentaborate electrolyte. The anodic oxide film of 125 nm thickness was crystalline, containing both monoclinic and tetragonal ZrO₂ phase. It was found that marked thickening of the oxide film with generation of cracks occurred during heat treatment at 473 K. Thus, the dielectric loss was largely increased along with the capacitance increase. In contrast, the anodic oxide film formed on the oxygen-incorporated alloy remained uniform, and no significant increase in dielectric loss was observed even after the heat treatment. The capacitance of the anodic film became as high as 4.8 mF m^?2, which was nearly twice that on Ta. The high capacitance was associated with the preferential formation of tetragonal ZrO₂ phase in the anodic oxide film on the oxygen-incorporated alloy. Findings indicated that the oxygen-incorporated Zr-Ti alloy is a promising novel material for capacitor application.     

Relation between Morphology and Photoelectrochemical Performance of TiO2 Nanotubes Arrays Grown in Ethylene Glycol/Water

The TiO₂ nanotube films were prepared by anodizing Ti plates in 0.2 M NH₄F ethylene glycol/10% H₂O under different formation voltages keeping fixed the time length, or by keeping fixed the formation voltage and varying the time length. The morphology of the TiO₂ film obtained was observed by SEM images and different morphological parameters were derived from them. Furthermore, the optical and semiconducting properties of TiO₂ films were also measured. The photoelectrochemical performance toward water oxidation of the TiO₂ only showed to be dependent with the inner diameter of the nanotubes, that could be related to the interaction of the film with the light and the transport of species in the electrolyte inward or outward the film.     

In situ AFM studies of SEI formation at a Sn electrode

Early stages of the solid electrolyte interphase (SEI) formation at a tin foil electrode in an ethylene carbonate (EC) based electrolyte were investigated by in situ AFM and cyclic voltammetry (CV) at potentials >0.7 V, i.e., above the potential of Sn–Li alloying. We detected and observed initial steps of the surface film formation at ～2.8 V vs. Li/Li⁺ followed by gradual film morphology changes at potentials 0.7 < U < 2.5 V. The SEI layer undergoes continuous reformation during the following CV cycles between 0.7 and 2.5 V. The surface film on Sn does not effectively prevent the electrolyte reduction and a large fraction of the reaction products dissolve in the electrolyte. The unstable SEI layer on Sn in EC-based electrolytes may compromise the use of tin-based anodes in Li-ion battery systems unless the interfacial chemistry of the electrode and/or electrolyte is modified.     

Effect of anodizing conditions on the cell morphology of anodic films on AA2024-T3 alloy

The effects of applied current density, anodizing time, and electrolyte temperature on the cell and pore morphology of anodic films and the voltage-time response obtained during galvanostatic anodizing of AA2024-T3 alloy in sulphuric acid electrolytes have been studied. Scanning electron microscopy was employed to observe the film morphology. Sponge-like porous structure was promoted by anodizing at relatively low current density and high electrolyte temperature. In contrast, linear porous structure was favoured under the converse conditions. Intermediate conditions resulted in films containing either sequential layers of the 2 morphologies or a morphology incorporating features of the 2 types; such conditions were associated with anodizing voltages in the range 25 to 35 V. The reasons for the morphological differences are proposed to be due to interactions between film growth stresses and stresses arising from oxygen evolution on the development of the alumina cells.     

Formation of hexagonally ordered nanoporous anodic zirconia

The present work demonstrates that highly ordered porous anodic zirconia (PAZ) arrays with cell diameters ranging from 70 to 120 nm can be grown in fluoride containing glycerol electrolytes. We show that this morphology (in contrast to the typically observed nanotubular layers) can simply be obtained by controlling the water content in the electrolyte during the anodization process. It is proposed that the morphology transition from pores to tubes is based on the rate of preferential etching at the hexagonal cell triple points in the oxide. This finding allows producing void free highly defined nanoporous layers for various applications.     

Formation of mesoporous germanium double layers by electrochemical etching for layer transfer processes

We produce uniform mesoporous single- and multilayers on 4 in. p-type Ge wafers by means of electrochemical etching in highly concentrated HF-based electrolytes. Pore formation by anodic etching in germanium leads to a constant dissolution of the already formed porous layer plus substrate. Alternating the etching bias from anodic to cathodic bias enhances the passivation of the pore walls and substrate. The formation of porous multilayers is possible, since the starting layer is not dissolved during the formation of the separation layer. We report on the production of mesoporous double layers in Ge with different porosities. The change in the porosity of the porous layers is achieved by varying the anodic etching current and the HF concentration of the electrolyte. Porosities in the range of 25–65% are obtained for etching current densities of 1–15 mA cm^?2 with the specific resistivity of the Ge substrates lying in the (0.020–0.032) Ω cm range and electrolyte HF concentrations in the range of 35–50 wt.%.     

Titania nanotube formation in chloride and bromide containing electrolytes

Titania nanotubes and nanofibers were grown by anodization of titanium in fluoride-free electrolytes composed of NaCl and KBr dissolved in either water, ethylene glycol (EG), glycerol, or 50/50 mixtures of water and EG, and water and glycerol. The tubes and fibers grew out of pits in the titanium foil. The 15 nm diameter of the nanotubes was significantly smaller, and the growth rates were much faster than those of tubes developed in fluoride solutions. Nanotubes were nucleated in all electrolytes investigated, but the tubes’ lengths were limited to a few nms in EG and glycerol. Nanofibers produced in the aqueous solutions and nanotubes formed in the 50/50 aqueous mixtures grew to many tens of microns in less than 60 s.     

Fabrication of irregular-layer-free and diameter-tunable Ni–Ti–O nanopores by anodization of NiTi alloy

Ni–Ti–O nanopores (NPs) free of irregular surface layers and with tunable diameters are prepared by anodizing NiTi alloy in glycerol containing H₂O and NaCl. In an electrolyte composed of glycerol, 10 vol% H₂O and 0.6 M NaCl, NPs with diameters between 23 and 39 nm can be produced at anodization voltages between 20 and 80 V. In this electrolyte system, the irregular oxide layer on the surface can be completely removed chemically and/or mechanically during anodization. The resulting Ni–Ti–O NPs with tunable diameters should prove useful, for example, in energy, environmental and biomedical applications.     

Electrochemical formation of self-organized zirconium titanate nanotube multilayers

The present work reports the formation of multilayers of self-organized zirconium titanate nanotubes by anodizing a Ti–35Zr alloy in 1 M (NH₄)₂SO₄ + 0.5 wt% NH₄F electrolytes. It was found that multilayers consisting of different diameter nanotubes can be produced by repeated anodization steps under different conditions. Formation of new nanotubes starts in the gaps between the existing tubes. The process allows the formation of multilayer stacks consisting of layers of several 100 nm in length and adjustable nanotube diameters in a range from 50 to 180 nm.     

Investigation of immiscible alloy system of Al–Sn thin films as anodes for lithium ion batteries

The Al–Sn, which is immiscible alloy, film was prepared by e-beam deposition to explore the possibility as anode material for lithium ion batteries for the first time. The film has a complex structure with tiny Sn particles dispersed homogeneously in the Al active matrix. The diffusion coefficients of Li⁺ in these Al–Sn alloy films were determined to be 2.1–3.2 × 10⁻⁸ cm²/s by linear sweep voltammetry. The film electrode with high Al content (Al–33wt%Sn) delivered a high initial discharge capacity of 972.8 mA h g⁻¹, while the film electrode with high Sn content (Al–64wt%Sn) with an initial discharge capacity of 552 mA h g⁻¹ showed good cycle performance indicated by retaining a capacity of about 381 mA h g⁻¹ after 60 cycles. Our preliminary results demonstrate that Al–Sn immiscible alloy is a potential candidate for anodic material of lithium ion batteries.     

Li2SO4-polyacrylamide polymer electrolytes for 2.0 V solid symmetric supercapacitors

A neutral polymer electrolyte comprised of lithium sulfate (Li₂SO₄) and polyacrylamide (PAM) was developed. The Li₂SO₄-PAM electrolyte film shows an ionic conductivity up to 10 mS cm^− 1 in 45%RH conditions. Solid double layer capacitors were demonstrated using CNT-graphite electrodes and Li₂SO₄-PAM solid electrolytes. The voltage window of the solid cell was about 2.0 V, identical to that of a Li₂SO₄ liquid cell used as baseline. The demonstrated voltage window is significantly larger than that reported for proton- or hydroxyl-conducting electrolytes, suggesting that the Li₂SO₄-PAM electrolyte is a promising system for high energy density supercapacitors. The solid device also demonstrated excellent rate capability (up to 5 V s^− 1) and good cycle life (beyond 10,000 charge/discharge cycles).     

TiO2 nanotube arrays fabricated by anodization in different electrolytes for biosensing

Titania nanotube arrays were fabricated by anodic oxidation of titanium foil in different electrolytes. The morphology, crystallinity and composition of the as-prepared nanotube arrays were studied by XRD, SEM and EDX. Electrochemical impedance spectroscopy (EIS) was employed to investigate their electrical conductivity and capacitance. Titania nanotube arrays co-adsorbed with horseradish peroxidase (HRP) and thionine chloride (Th) were studied for their sensitivity to hydrogen peroxide by means of cyclic voltammetric and galvanostatic measurements. The experiments showed that TiO₂ nanotube arrays possessed appreciably different sensitivities to H₂O₂ due to their different conductivity. Further experiments revealed that TiO₂ nanotubes have noticeably different ability of adsorbing HRP and Th, and the best sensitivity was achieved when the density of HRP is the highest. The TiO₂ nanotube arrays fabricated in potassium fluoride solution demonstrated the best sensitivity on hydrogen peroxide in the range of 10⁻⁵–3 × 10⁻³ M at pH 6.7 and at a potential of −600 mV (vs. Ag/AgCl).     

High aspect ratio ordered nanoporous Ta2O5 films by anodization of Ta

In this work we report the electrochemical formation of self-organized high aspect ratio Ta₂O₅ porous layers, grown by anodization of Ta in non-aqueous electrolytes consisting of glycerol and small additions of NH₄F. Under optimized electrochemical conditions in these glycerol based electrolytes, the porous layers can be grown up to thickness of 15 μm, with a pore diameter in the range of 10–40 nm. The dimensions and the morphology of the layers can be strongly influenced by the fluoride concentration, the applied potential and the water content in the electrolyte.     

Effect of current density and behaviour of second phases in anodizing of a Mg-Zn-RE alloy in a fluoride/glycerol/water electrolyte

Anodizing of a Mg-Zn-RE alloy was carried out at constant current densities from 0.1 to 10 mA cm⁻² in a fluoride/glycerol/water electrolyte. Rutherford backscattering spectroscopy, nuclear reaction analysis and analytical transmission electron microscopy revealed barrier-type films composed of oxide and fluoride species. The films were formed by outward migration of cations and inward migration of anions. The transport number of cations in the film above the matrix was in the range ∼0.5 to 0.6, and ∼0.1 in the film above the grain boundary Mg-Zn-RE phase. From the oxidation behaviour of the Zn-Zr phases, it is suggested that anions and cations migrate through short-circuit paths in the film.

     

Amorphous silicon thin films as a high capacity anodes for Li-ion batteries in ionic liquid electrolytes

High lithiation capacity at low red-ox potentials in combination with good safety characteristics makes amorphous Si as a very promising anode material for rechargeable Li batteries.Thin film silicon electrodes were prepared by DC magnetron sputtering of silicon on stainless steel substrates. Their behavior as Li insertion/extraction electrodes was studied by voltammetry and chronopotentiometry at room temperature in the ionic liquid (IL) 1-methyl-1-propylpiperidinium bis(trifluoromethylsuphonil)imide containing 1 M Li bis(trifluoromethylsuphonil)imide. Li/Si cells containing this electrolyte showed good performance with a stable Si electrodes capacity of about 3000 mA h g⁻¹ and a relatively low irreversible capacity. Preliminary results on cycling Si–LiCoO₂ cells using this IL electrolyte are also presented.