Characterization of oxide layers formed on electrochemically treated Ti by using soft X-ray absorption measurements

The oxide layers of electrolytic oxidized titanium (Ti) were characterized using Ti L_2,3 and O K edge X-ray absorption. The spectra show that the structure of the oxide layers that are formed during a 1 min treatment are dependent on the concentration of the electrolyte (H₂SO₄ or Na₂SO₄) with which the Ti surface was treated, and also on the magnitude of the potential that was applied during the anodic oxidation process (100 V or 150 V). It is found that a potential of 150 V and an electrolyte concentration of 0.5 M or 1.0 M produces a layer of TiO₂ having rutile crystal structure.     

Characterization of native and anodic oxide films formed on commercial pure titanium using electrochemical properties and morphology techniques

Potentiostatically anodized oxide films on the surface of commercial pure titanium (cp-Ti) formed in sulfuric (0.5 M H₂SO₄) and in phosphoric (1.4 M H₃PO₄) acid solutions under variables anodizing voltages were investigated and compared with the native oxide film. Potentiodynamic polarization and electrochemical impedance spectroscopy, EIS, were used to predicate the different in corrosion behavior of the oxide film samples. Scanning electron microscope, SEM, and electron diffraction X-ray analysis, EDX, were used to investigate the difference in the morphology between different types of oxide films. The electrochemical characteristics were examined in phosphate saline buffer solution, PSB (pH 7.4) at 25 °C. Results have been shown that the nature of the native oxide film is thin and amorphous, while the process of anodization of Ti in both acid solutions plays an important role in changing the properties of passive oxide films. Significant increase in the corrosion resistance of the anodized surface film was recorded after 3 h of electrode immersion in PSB. On the other side, the coverage (θ) of film formed on cp-Ti was differed by changing the anodized acid solution. Impedance results showed that both the native film and anodized film formed on cp-Ti consist of two layers. The resistance of the anodized film has reached to the highest value by anodization of cp-Ti in H₃PO₄ and the inner layer in the anodized film formed in both acid solutions is also porous.     

Growth process of atomically flat anodic films on titanium under potentiostatical electrochemical treatment in H2SO4 solution

The as-deposited titanium film on silicon wafer was electrochemically treated in potential sweep and potential step modes in 0.1 M H₂SO₄ solution at 30 °C. Under the anodization conditions of potential sweep and properly modulated cyclic voltammetry (CV), nanoscale grains, step-terrace structure and atomic images were clearly observed on the surface of anodic oxide film on titanium. Under potential step conditions, if the anodization time was short (1 s), no grains could be found on the anodic oxide film surface, even though the potential was high up to 9000 mV. Moreover, whatever potential sweep or potential step mode was performed, sufficient time (low sweep rate means a prolonged anodization time) was needed for the formation of nanoscale grains, atomically flat surface and step-terrace structure on the anodized titanium film.     

Reactivities of ultrathin alumina films exposed to intermediate pressures of H2O: Substrate-mediated mechanism for growth and loss of surface order

The response of ordered ultrathin Al₂O₃ films on NiAl(1 1 0) and Ni₃Al(1 1 0) substrates to sequential exposures at varying pressures of H₂O between 10⁻⁷ Torr and 10⁻³ Torr, ambient temperature, was characterized by LEED, AES and density functional theory (DFT) calculations. In all cases, an increase in average oxide thickness, as determined by AES, was observed, consistent with a field-induced oxide growth mechanism. Ordered oxide films of initial average thicknesses of 7 Å and 12 Å grown on NiAl(1 1 0) achieved a limiting thickness of 17(1) Å, while films of initial thickness of 7 Å and 11 Å grown on Ni₃Al(1 1 0) achieved a limiting thickness of 12(1) Å. The LEED patterns for the thinner (7 Å) films were not observed after exposure to 10⁻⁵ Torr (NiAl(1 1 0)), or 10⁻⁴ Torr (Ni₃Al(1 1 0)). In contrast, LEED patterns for the films of greater initial thickness persisted after exposures to 10⁻³ Torr UHV. DFT calculations indicate an Al vacancy formation energy that is significantly greater (by ∼0.5 eV) on the surface that has the thicker oxide film, directly opposite to what may be naively expected. A simple coordination argument supports these numerical results. Therefore, the greater limiting oxide thickness observed on NiAl(1 1 0) demonstrates that the rate determining step in the oxide growth process is not Al removal from the metal substrate and transport across the oxide/metal interface. Instead, the results indicate that the determining factor in the oxide growth mechanism is the kinetic barrier to Al diffusion from the substrate bulk to the oxide/metal interface. The persistence of the LEED patterns observed for the films of greater initial oxide thickness indicates that the surface disorder generally observed for alumina films grown on aluminide substrates and exposed to intermediate pressures of H₂O is due to the growth of a disordered alumina layer over an ordered substrate, rather than to direct H₂O interaction with terrace sites.     

Investigation of IrO2-SnO2 thin film evolution from aqueous media

The thermal evolution process of IrO₂-SnO₂/Ti mixed oxide thin films of varying noble metal content has been investigated under in situ conditions by thermogravimetry-mass spectrometry, Fourier transform infrared emission spectroscopy and cyclic voltammetry. The gel-like films prepared from aqueous solutions of the precursor salts Sn(OH)₂(CH₃COO)_2−xCl_x and H₂IrCl₆ on titanium metal support were heated in an atmosphere containing 20% O₂ and 80% Ar up to 600 °C.The thermal decomposition reactions practically take place in two separate temperature ranges from ambient to about 250 °C and between 300 and 600 °C. In the low temperature range the liberation of solution components and - to a limited extent - an oxidative cracking reaction of the acetate ligand takes place catalyzed by the noble metal. In the high temperature range the evolution of chlorine as well as the decomposition of surface species formed (carbonyls, carboxylates, carbonates) can be observed. The acetate ligand shows extreme high stability and is decomposed in the 400-550 °C range, only.Since the formation and decomposition of the organic surface species can significantly influence the morphology (and thus the electrochemical properties) of the films, the complete understanding of the film evolution process is indispensable to optimize the experimental conditions of electrode preparation.     

XPS study of the deposited Ti layer in a magnetron-type sputter ion pump

X-ray photoelectron spectroscopy (XPS) was used to study the surface chemical composition of the anode deposits in a miniature magnetron ion pump. The pump was mounted on an UHV system with the ultimate pressure of 1 × 10⁻⁹ mbar. A stable discharge was established in the nitrogen atmosphere with some traces of CO at about 10⁻⁷ mbar. The cathode was made of pure titanium. The sputtered titanium atoms deposited on the anode, where they reacted with gases to form a film of titanium compounds. The thickness of the deposited titanium layer on the anode was about 100 nm. The results from XPS investigations indicate that active gases such as O₂ and N₂ react with Ti forming TiO₂ and TiN. While carbon containing molecules just adsorb on the surface and do not form carbide. In the bulk of the deposited layer almost pure TiN was found with some traces of oxygen and carbon. The part of carbon was bonded to TiC, which can be caused by ion sputtering during the depth profiling.     

High-density plasma etching of indium-zinc oxide films in Ar/Cl2 and Ar/CH4/H2 chemistries

The dry etching characteristics of transparent and conductive indium-zinc oxide (IZO) films have been investigated using an inductively coupled high-density plasma. While the Cl₂-based plasma mixture showed little enhancement over physical sputtering in a pure argon atmosphere, the CH₄/H₂/Ar chemistry produced an increase of the IZO etch rate. On the other hand, the surface morphology of IZO films after etching in Ar and Ar/Cl₂ discharges is smooth, whereas that after etching in CH₄/H₂/Ar presents particle-like features resulting from the preferential desorption of In- and O-containing products. Etching in CH₄/H₂/Ar also produces formation of a Zn-rich surface layer, whose thickness (∼40 nm) is well-above the expected range of incident ions in the material (∼1 nm). Such alteration of the IZO layer after etching in CH₄/H₂/Ar plasmas is expected to have a significant impact on the transparent electrode properties in optoelectronic device fabrication.     

Electrodeposited ruthenium oxide thin films for supercapacitor: Effect of surface treatments

Amorphous and porous ruthenium oxide thin films have been deposited from aqueous Ru(III)Cl₃ solution on stainless steel substrates using electrodeposition method. Cyclic voltammetry study of a film showed a maximum specific capacitance of 650 F g⁻¹ in 0.5 M H₂SO₄ electrolyte. The surface treatments such as air annealing, anodization and ultrasonic weltering affected surface morphology. The supercapacitance of ruthenium oxide electrode is found to be dependent on the surface morphology.     

Pure and Nb-doped TiO1.5 films grown by pulsed-laser deposition for transparent p-n homojunctions

Pure and Nb-doped titanium oxide thin films were grown on sapphire substrates by pulsed-laser deposition in vacuum (10⁻⁷ mbar). The PLD growth leads to titanium oxide thin films displaying a high oxygen deficiency (TiO_1.5) compared with the stoichiometric TiO₂ compound. The structural and electrical properties (phase, crystalline orientation, nature and concentration of charge carriers, etc.) of these titanium oxide films were studied by XRD measurements and Hall effect experiments, respectively. The undoped TiO_1.5 phase displayed a p-type semiconductivity. Doping this titanium oxide phase with Nb⁵⁺ leads to an n-type behaviour as is generally observed for titanium oxide films with oxygen deficiency (TiO_x with 1.7 < x < 2). Multilayer homojunctions were obtained by the stacking of TiO_1.5 (p-type) and Nb-TiO_1.5 (n-type) thin films deposited onto sapphire substrates. Each layer is 75 nm thick and the resulting heterostructure shows a good transparency in the visible range. Finally, the I-V curves obtained for such systems exhibit a rectifying response and demonstrate that it is possible to fabricate p-n homojunctions based only on transparent conductive oxide thin films and on a single chemical compound (TiO_x).     

Oxidation of 4H-SiC covered with a SmSix surface alloy

Oxidation of Sm/4H-SiC is studied by X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED). In particular, we report kinetic information from the oxidation of a SmSi_x (1 × 1) surface alloy formed on (0 0 0 1) 4H-SiC. During the initial oxidation of the SmSi_x alloy, a (2 × 2)-LEED pattern is observed. Furthermore, the Sm 2+ valency observed from the clean SmSi_x surface alloy, which is related to surface samarium atoms, disappear at 15 L oxygen exposure. The oxygen atom is consequently deduced to be located at bridge or hollow sites involving one Sm atom. The initial oxidation result in an oxygen deficit SmSiO_x interface oxide, probably as a consequence of the high oxidation temperatures in this work (900-1050 °C). We report that in a prolonged oxidation (longer than 10 kL) a SiO₂ layer forms on top of the samarium silicon oxide interface layer.     

Coating formation by plasma electrolytic oxidation on ZC71/SiC/12p-T6 magnesium metal matrix composite

Plasma electrolytic oxidation (PEO) of a ZC71/SiC/12p-T6 magnesium metal matrix composite (MMC) is investigated in relation to coating growth and corrosion behaviour. PEO treatment was undertaken at 350 mA cm⁻² (rms) and 50 Hz with a square waveform in stirred 0.05 M Na₂SiO₃.5H₂O/0.1 M KOH electrolyte. The findings revealed thick, dense oxide coatings, with an average hardness of 3.4 GPa, formed at an average rate of ∼1 μm min⁻¹ for treatment times up to 100 min and ∼0.2 μm min⁻¹ for later times. The coatings are composed mainly of MgO and Mg₂SiO₄, with an increased silicon content in the outer regions, constituting <10% of the coating thickness. SiC particles are incorporated into the coating, with formation of a silicon-rich layer at the particle/coating interface due to exposure to high temperatures during coating formation. The distribution of the particles in the coating indicated growth of new oxide at the metal/coating interface. The corrosion rate of the MMC in 3.5% NaCl is reduced by approximately two orders of magnitude by the PEO treatment.     

Transparent titania nanotubes of micrometer length prepared by anodization of titanium thin film deposited on ITO

Transparent TiO₂ nanotube arrays of micrometer lengths were prepared by anodization of titanium thin film RF sputtered on indium tin oxide (ITO) which was coated on glass substrate. The sputtering process took place at elevated temperature of 500 °C. The structures of the films were studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD) while the optical properties of the films were investigated using UV-visible spectroscopy. Two types of electrolytes were used in this work: an aqueous mixture of acetic acid and HF solution and a mixture of NH₄F and water dissolved in ethylene glycol. The concentration of NH₄F, voltage and the thickness of the sputtered titanium film were varied to study their effect on the formation of TiO₂ nanotube arrays. It is demonstrated in this work that the nanoporous layer is formed on top of the ordered array of TiO₂ nanotubes. Furthermore, the optical transmittance of TiO₂ nanotubes annealed at 450 °C is much lower than the non annealed TiO₂ nanotubes in the visible wavelength region.     

Influence of titanium ions implantation on corrosion behavior of zirconium in 1 M H2SO4

In order to study the effect of titanium ion implantation on the aqueous corrosion behavior of zirconium, specimens were implanted with titanium ions with fluence ranging from 1 × 10¹⁶ to 1 × 10¹⁷ ions/cm², using a metal vapor vacuum arc (MEVVA) source at an extraction voltage of 40 kV. The valence states and depth distributions of elements in the surface layer of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), respectively. The potentiodynamic polarization technique was employed to evaluate the aqueous corrosion resistance of implanted zirconium in a 1 M H₂SO₄ solution. It was found that a significant improvement was achieved in the aqueous corrosion resistance of zirconium implanted with titanium ions. The larger the fluence, the better is the corrosion resistance of implanted sample. Finally, the mechanism of the corrosion behavior of titanium-implanted zirconium was discussed.     

NbTi0.5Ni0.5O4 as anode compound material for SOFCs

NbTi_0.5Ni_0.5O₄ (NTNO) has been prepared using solid state synthesis and investigated as a potential anode material. The oxide form of NTNO has single phase rutile-type structure with tetragonal (P42/mnm) space group. The reduced form is a composite of nano-scaled particles of metallic Ni and Nb_1.33Ti_0.67O₄ phase. Reduced NTNO showed high electronic conductivity up to 280 S.cm^− 1 at 900 °C in reducing atmosphere, but suffers from low CTE equal to 3.78 10^− 6 K^− 1. Studies of NTNO as anode material were carried out in a three electrode - electrochemical half cell configuration under pure humidified H₂ at 900 °C using a 2 mm thick zirconia electrolyte and without any additional current collector material. The results show a reasonable series resistance (R_s) equal to 2.7 Ωcm² (about 50% higher than for metallic gold layers) indicating a good current collection performance for a 10 μm layer of material. The polarization resistance (R_p) was equal to 33 Ωcm² and is attributed to a poor density of three phase boundaries (TPB) and shortage of oxide ion conduction in the anode layer. The results show the potential of NTNO as an anode material, especially after optimization of the microstructure towards the increase of TPB length.     

Hydrogen peroxide treatment on ZnO substrates to investigate the characteristics of Pt and Pt oxide Schottky contacts

We utilize hydrogen peroxide (H₂O₂) treatment on (0 0 0 1) ZnO substrates to investigate the characteristics of Pt and Pt oxide Schottky contacts (SCs). X-ray rocking curves show the mosaicity structure becomes larger after H₂O₂ treatment. Photoluminescence (PL) spectra show the yellow-orange emission peaking at ∼576-580 nm with respect to deep level of oxygen interstitials introduced by H₂O₂ treatment. The threshold formation of ZnO₂ resistive layer on H₂O₂-treated ZnO for 45 min is observed from grazing-incidence X-ray diffraction. The better electrical characteristic is performed by Pt oxide SC with the larger barrier height (1.09 eV) and the lower leakage current (9.52 × 10⁻¹¹ A/cm² at −2 V) than Pt SC on the H₂O₂-treated ZnO for 60 min. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometer (SIMS) examinations indicate the promoted interface oxide bonding and Zn outdiffusion for Pt oxide contact, different from Pt contact. Based on current-voltage, capacitance-voltage, X-ray diffraction, PL spectra, XPS, and SIMS results, the possible mechanism for effective rectifying characteristic and enhanced Schottky behavior is given.     

Controlled preparation of well-ordered transition metal oxide layers on a metallic surface

A method to create various well-ordered two dimensional transition metal oxide films on a metallic substrate has been exploited. The formation of an intermediate amorphous layer with controllable metal-oxygen stoichiometry serves as an important precursor condition for the final transformation into a mono-phase, crystalline oxide layer via mild annealing. As a key ingredient serves a Cu₃Au(1 0 0) substrate covered by oxygen. The flat Cu-O topmost layer stops completely intermixing of the substrate material with the subsequently evaporated transition metal film. Likewise the wetting of the surface is considerably enhanced and a homogeneous oxidation of the film is strongly promoted. The proposed technique appears to be widely efficient for preparation of various two dimensional oxide films covering the entire Cu₃Au(1 0 0) substrate. Its usefulness is demonstrated successfully for vanadium, niobium and molybdenum to produce a set of single-phase transition metal oxides of different stoichiometry and geometrical structure. All created oxides are found to be thermally stable at least up to a substrate temperature of 800 K.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.     

Oxidation of Ni(Pt)Si by molecular vs. atomic oxygen

X-ray photoelectron spectroscopy (XPS) has been used to characterize the oxidation of a clean Ni(Pt)Si surface under two distinct conditions: exposure to a mixed flux of atomic and molecular oxygen (O + O₂; PO+O₂ = 5 × 10⁻⁶ Torr) and pure molecular oxygen (O₂; PO₂ = 10⁻⁵ Torr) at ambient temperatures. Formation of the clean, stoichiometric (nickel monosilicide) phase under vacuum conditions results in the formation of a surface layer enriched in PtSi. Oxidation of this surface in the presence of atomic oxygen initially results in formation of a silicon oxide overlayer. At higher exposures, kinetically limited oxidation of Pt results in Pt silicate formation. No passivation of oxygen uptake of the sample is observed for total O + O₂ exposure <8 × 10⁴ L, at which point the average oxide/silicate overlayer thickness is 23 (3) Å (uncertainty in the last digit in parentheses). In contrast, exposure of the clean Ni(Pt)Si surface to molecular oxygen only (maximum exposure: 5 × 10⁵ L) results in slow growth of a silicon oxide overlayer, without silicate formation, and eventual passivation at a total average oxide thickness of 8(1) Å, compared to a oxide average thickness of 17(2) Å (no silicate formation) for the as-received sample (i.e., exposed to ambient.) The aggressive silicon oxidation by atomic oxygen, results in Ni-rich silicide formation in the substrate and the kinetically limited oxidation of the Pt.     

Effect of Ge surface termination on oxidation behavior

Sulfur-termination was formed on the Ge(1 0 0) surface using (NH₄)₂S solution. Formation of Ge-S and the oxidation of the S-terminated Ge surface were monitored with multiple internal reflection Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. In the 0.5, 5, or 20% (NH₄)₂S solution, H-termination on the Ge(1 0 0) surface was substituted with S-termination in 1 min. When the S-terminated Ge(1 0 0) surface was exposed in air ambient, the oxidation was retarded for about 3600 min. The preservation time of the oxide layer up to one monolayer of S-terminated Ge(1 0 0) surface was about 120 times longer than for the H-terminated Ge(1 0 0) surface. However, the oxidation of S-terminated Ge(1 0 0) surface drastically increased after the threshold time. There was no significant difference in threshold time between S-terminations formed in 0.5, 5, and 20% (NH₄)₂S solutions. With the surface oxidation, desorption of S on the Ge surface was observed. The desorption behavior of sulfur on the S-terminated Ge(1 0 0) surface was independent of the concentration of the (NH₄)₂S solution that forms S-termination. Non-ideal S-termination on Ge surfaces may be related to drastic oxidation of the Ge surface. Finally, with the desulfurization on the S-terminated Ge(1 0 0) surface, oxide growth is accelerated.     

Oxidation mechanism of hydrogen-terminated Ge(1 0 0) surface

Control of the surface chemistry to prepare a robust termination on the Ge surface is crucial for the development of high-end Ge devices. In this study, oxidation of a H-terminated Ge surface was studied in air ambient and H₂O using a multiple internal reflection Fourier transform infrared spectroscopy (MIR FT-IR) technique. Ge surface treated in less diluted HF exhibited a stronger Ge-H peak intensity, and the surface was easily oxidized in the air ambient. Therefore, it is believed that the treatment of the Ge surface in highly diluted HF solution has an advantage in suppressing the oxidation of Ge in the air ambient. For the oxidation of Ge(1 0 0) surface in air ambient, the Ge surface is attacked by oxidizing agents to break Ge-H and Ge-Ge bonds, and the transition GeO_x layer is first formed, followed by a layer-by-layer GeO₂ formation with the increase in exposure time. When the H-terminated Ge surface was treated in H₂O, GeO_x was mainly formed, the thickness of the oxide layer was not changed with an increase in treatment time, and the Ge surface was maintained in a suboxide state, which exhibits a different oxidation mechanism from that in air ambient.