Electrochemical impedance simulation of a metal oxide heterostructure/electrolyte interface: A review

The results of an analysis of the literature data obtained when investigating processes of charge transfer at interfaces of heterostructures formed by various methods, including the high-energy methods, are presented in this paper. The performed investigation of oxide layers at the titanium surface, with use made of impedance spectroscopy data made it possible to reveal the nature and influence of some processes and factors on the charge transfer mechanism realized at a metal oxide heterostructure/electrolyte interface. Simulating an oxide/electrolyte interface gives one a chance to identify, in a spectrum, the responses that characterize the behavior of porous and poreless layers, as well as the responses that are due to the space-charge region formed in the oxide material and to the corrosion and diffusion processes.     

Calculation of the Stoichiometric Composition of Nanostructures Synthesized by Molecular Layer Deposition on the Surface of Solid Matrices

Calculational formulas for determining from the results of chemical and thermogravimetric analyses the true relative contents of components in products formed in molecular layer deposition of nanostructures on the surface of solid matrices are suggested. The approaches considered are illustrated by the example of evaluation of the chemical composition of titanium oxide (nitrogen) and silicon-nitrogen nano-layers synthesized by molecular layer deposition on the silica surface.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 3, 2005, pp. 373–379.Original Russian Text Copyright © 2005 by Sosnov, Malkov, Malygin.     

Optimum synthesis of heterogeneous multisectional resistive—capacitive circuits simulating the generalized Warburg impedance

Procedures that are usually used for synthesizing multisectional RC circuits capable of real-time simulation of the generalized Warburg impedance are briefly reviewed. The review shows different procedures to synthesize circuits with substantially different numbers of required sections at the same requirements with regard to the frequency interval and the simulation accuracy. The results of the development of optimum numerical synthesis of such circuits, which leads to a substantial decrease in the number of sections at the same initial data, are presented.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 219–223.Original Russian Text Copyright © 2005 by Vyaselev, Glebov.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Synthesis and Electrosurface Properties of One- and Two-Component Nanostructures

One- and two-component (titanium and aluminum oxides) oxide nanostructures are synthesized by molecular layer-by-layer deposition from the gas phase onto silicon oxide and boehmite substrates. Two-component nanostructures are prepared by the consecutive deposition of nanolayers of aluminum and titanium oxides onto a dispersed silica substrate. Electrosurface properties of thus-prepared samples are studied and compared. It is shown that the positions of the isoelectric point and the point of zero charge of a 5TiO₂/5Al₂O₃/SiO₂ composite sample are governed by an outer titanium oxide nanolayer and are similar to those of bulk titanium oxide and a nanostructured film of titanium oxide deposited onto an aluminum hydroxide substrate.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 469–474.Original Russian Text Copyright © 2005 by Bogdanova, Ermakova, Chikhachev, Sidorova, Aleksandrov, Savina.     

Surface characteristics of titanium–silver alloys in artificial saliva

Titanium and its alloys are widely used in biomedical and dental fields because of their excellent corrosion resistance and biocompatibility. It is well known that titanium is protected from corrosion because of the stability of the passive film that controls and determines the corrosion resistance and biocompatibility of titanium and its alloys. The purpose of this study was to evaluate the electrochemical properties of titanium–silver alloys and the surface characteristics of passive film in artificial saliva. We designed titanium–silver alloys with silver contents ranging from 0 to 5 at.%, in 1% increments. These alloys were arc‐melted, homogenized, hot‐rolled to 2 mm thickness, and finally solution heat‐treated for 1 h and quenched. Potentiostatic testing was performed, and the open circuit potentials of the alloys were measured in artificial saliva, at 37 °C. The passive films of the titanium–silver alloys were analyzed via XPS. Titanium–silver alloys maintained low current density and showed stable passive region and also had high open circuit potential as compared with pure titanium. The open circuit potential of titanium–silver alloys increased as silver addition increased. With regard to the fraction of oxygen species, a component of over 80% was found to be comprised of oxide. Therefore, the titanium surface mainly consisted of titanium oxide and, on the titanium–silver alloys, this film was composed of TiO₂, Ti₂O₃, and TiO. As silver content increased, the TiO₂ fraction also increased, as did the thickness of the titanium oxide layer formed. Copyright © 2005 John Wiley & Sons, Ltd.     

Flexible Polymer–Organic Solar Cells Based on P3HT:PCBM Bulk Heterojunction Active Layer Constructed under Environmental Conditions

In this study, some crucial parameters were determined of flexible polymer–organic solar cells prepared from an active layer blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) mixed in 1:1 mass ratio and deposited from chlorobenzene solution by spin-coating on poly(ethylene terephthalate) (PET)/ITO substrates. Additionally, the positive effect of an electron transport layer (ETL) prepared from zinc oxide nanoparticles (ZnO np) on flexible photovoltaic elements’ performance and stability was investigated. Test devices with above normal architecture and silver back electrodes deposed by magnetron sputtering were constructed under environmental conditions. They were characterized by current-voltage (I–V) measurements, quantum efficiency, impedance spectroscopy, surface morphology, and time–degradation experiments. The control over morphology of active layer thin film was achieved by post-deposition thermal treatment at temperatures of 110–120 °C, which led to optimization of device morphology and electrical parameters. The impedance spectroscopy results of flexible photovoltaic elements were fitted using two R||CPE circuits in series. Polymer–organic solar cells prepared on plastic substrates showed comparable current–voltage characteristics and structural properties but need further device stability improvement according to traditionally constructed cells on glass substrates.     

Ellipsometric Study of Anodic Oxide Films on Alloys of the Ti–Al System

Anodic oxide films (AOF) on neat titanium and on alloys of the Ti–Al system (Ti–10 at. % Al, Ti–24 at. % Al, Ti–50 at. % Al, Ti–75 at. % Al) formed in borate buffer solution are studied by ellipsometry. A model is proposed, which adequately describes the shape of the alloy surface with and without AOF. The model takes into account the roughness factor, allowing one to more correctly determine optical parameters of AOF and dependence of its thickness on anodic potential. It is found that the alloy surface roughness is effectively smoothed with increasing applied potential.     

Redox polymer poly-N,N′-2,3-dimethylbutane-2,3-diyl-bis(salicylideniminato)nickel: An impedance spectroscopy study

Films of poly-N,N-2,3-dimethylbutane-2,3-diyl-bis(salicylideniminato)nickel are studied by cyclic voltammetry and faradaic impedance methods in acetonitrile solutions containing 0.1 M tetraethylammonium tetrafluoroborate. The resistance to charge transfer and the double-layer capacitance, calculated from the semicircle in the high-frequency spectrum portion, are referred to the film/electrolyte interface. From the low- and mid-frequency impedance spectra portions, the low-frequency capacitance and the effective diffusion coefficient of charge carriers are determined. The assumption about a noticeable film porosity is shored up by direct scanning electron microscopy examination of film surfaces.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 433–441.Original Russian Text Copyright © 2005 by Dmitrieva, Logvinov, Kurdakova, Kondratev, Malev, Timonov.     

The chemistry of adsorbed Cu(II) and Mn(II) in aqueous titanium dioxide suspensions

We studied the adsorption behavior of Cu(II) and Mn(II) on the surface of titanium dioxide over the pH range from 2.0 to 11.5. The titanium dioxide we used in these experiments was prepared by hydrolyzing TiCl₄ and had a surface area of 113.7 m² g⁻¹. All suspensions, which were 9.04 × 10⁻³ M in NaClO₄, contained 20 m² liter⁻¹ of oxide surface and divalent metal ion concentrations sufficient (at full adsorption from solution) to cover the available surface with one-half, one, and four layers of close-packed, hydrated ions. Both divalent ions began adsorption below titanium dioxide's isoelectric point (pH = 6.2). Cu²⁺ adsorption was accompanied by net OH⁻ uptake from solution and it was inferred that the titania surface also provided OH⁻ for Cu²⁺ adsorption. ESR spectra demonstrate the coexistence of two distinct forms adopted by these metal ions on the surface. A portion of the adsorbed metal ions occupies sites magnetically isolated one from another, as evidenced by the paramagnetic behavior of this form. The majority of the metal ions, however, exist in hydrous-metal-ion clusters in which spin-exchange coupling of the electron dipoles determines the magnetic behavior. Electrophoretic mobility measurements indicate that ions adsorbed at isolated sites exert a stronger influence on the electrophoretically measured charge of the suspension particles than ions in clusters. Even though these experiments were performed in the absence of oxygen, we observed the oxidation of a limited amount of the Mn(II) on the surface as low as pH = 5. Presumably this occurs as a result of electron transfer between photo-induced electron holes and Mn(II) on the surface.     

Forms of adsorption of methanol on anatase and directions of their transformation

The adsorption of methanol on titanium(IV) oxide in the anatase crystalline modification was investigated by IR spectroscopy and thermal desorption. It was established that coordinationally bonded alcohol and two types of methoxides are formed on the surface of the titanium dioxide (anatase). The adsorption centers of the methanol were found. The thermal stability and the degree of particípation in oxidation were determined for the surface structures of the adsorbed alcohol. The possibility of the conversion of the coordinationally bonded alcohol and the weakly bonded methoxyl into partial oxidation products and the strongly bonded methoxyl into total oxidation products was demonstrated. In the investigated system formate complexes at concentrations sufficient for detection by IR spectroscopy were only formed with the participation of the oxygen of the gas phase through further oxidation of the strongly bonded methoxyls.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 707–712, November–December, 1988.The authors thank V. A. Gerasimova and S. K. Anan'in for assistance in the production of the experimental results.     

Monochromatized x‐ray photoelectron spectroscopy of the AM60 magnesium alloy surface after treatments in fluoride‐based Ti and Zr solutions

The behaviour of the 6% aluminium–magnesium alloy (AM60) surface in zirconium or titanium fluoride aqueous acid solutions was studied. X‐ray photoelectron spectroscopy was used to investigate modifications in the surface chemistry with respect to the composition of the surface treatment solution. The surface film is composed of magnesium hydroxide and hydroxyfluoride, zirconium oxide, oxyhydroxide or oxyfluoride, titanium oxide and structural and adsorbed water. Optimal parameters leading to the formation of a zirconium‐ or titanium‐rich film were determined. A mechanism is proposed for the formation of zirconium‐ or titanium‐based films. Copyright © 2005 John Wiley & Sons, Ltd.     

Interfacial electrical properties of DNA-modified diamond thin films: intrinsic response and hybridization-induced field effects

We have investigated the frequency-dependent interfacial electrical properties of nanocrystalline diamond films that were covalently linked to DNA oligonucleotides and how these properties are changed upon exposure to complementary and noncomplementary DNA oligonucleotides. Frequency-dependent electrical measurements at the open-circuit potential show significant changes in impedance at frequencies of >10(4) Hz when DNA-modified diamond films are exposed to complementary DNA, with only minimal changes when exposed to noncomplementary DNA molecules. Measurements as a function of potential show that at 10(5) Hz, the impedance is dominated by the space-charge region of the diamond film. DNA molecules hybridizing at the interface induce a field effect in the diamond space-charge layer, altering the impedance of the diamond film. By identifying a range of impedances where the impedance is dominated by the diamond space-charge layer, we show that it possible to directly observe DNA hybridization, in real time and without additional labels, via simple measurement of the interfacial impedance.     

Properties of Adsorbed Oxygen Forms on a Defective Ag(111) Surface. DFT Analysis

A cluster model of an Ag12–3O (AS_V) adsorption center using layered silver oxide as a prototype is proposed. The model includes a cation vacancy V on the Ag(111) surface and oxide type subsurface oxygen atoms O_ox. Density functional theory (DFT) (B3LYP/LANL1MB approximation) is used to analyze the electronic structure of AS_V and oxygen adsorption on this center, AS_V+O AS–O. As shown by the calculations, the adsorbed oxygen is associated with the subsurface oxygen atoms O_ss to form structures similar to metal ozonides — Ag–O_ss–O_ep–O_ss–Ag–O_ox–Ag, containing electrophilic oxygen O_ep along with the oxide oxygen O_ox. The optical spectra of the AS_V and AS–O centers were calculated by the configuration interaction method with single excitations (CIS). For AS_V, the most intense absorption bands were obtained in the region 500-700 nm. Oxygen association is accompanied by a sharp decrease in spectrum intensity in the range 600-700 nm and an increase in the intensity of the peak at 500 nm. Vibration frequencies and (IR) intensities were determined for the AS_V and AS–O centers. The AS_V center exhibits a characteristic spectrum in the region 350-500 cm^–1, which corresponds to the frequency spectrum of the surface oxide Ag₂O. For associated oxygen forms (AS–O center), the calculations predict additional peaks around 980, 640 and 230 cm^–1. These peaks are due to the vibrations of the O_ss–O_ep–O_ss structural unit, stabilized at the cation vacancy.     

Structure Peculiarities of Reduced Vanadium–Titanium Oxide Catalysts

The properties of vanadium–titanium oxide catalysts, which contain coherent phase boundaries formed by V₂O₅ and TiO₂ crystallites during reduction by hydrogen at 150–500°C, are examined. The phase boundary is preserved over the entire examined temperature range regardless of the structure of vanadium oxide, which is formed. The state of vanadium ions at the phase boundary is determined. The presence of a phase boundary in the catalyst is responsible for the V₂O₅ V₂O₃ transition without the formation of intermediate structures.     

Photoelectrochemical properties of indium doped iron oxide

Oxygen photoanodes formed by reactive sputtering of iron oxide onto conducting indium tin oxide (ITO) substrates held at 350 ° C have been investigated by conventional photoelectrochemical, impedance, XPS and auger spectroscopic methods. This fabrication procedure leads to films containing 8 to 20 atomic % indium in the front portion of the film, increasing to much higher values near the ITO interface (back portion of the film). Two interesting effects are observed with the thin-film iron oxide formed in this way. The first is that the as formed films must be vacuum annealed before an appreciable dc photoanodic response is observed. Secondly, films 250 nm thick display the property of giving about double the quantum yield for back face, than for front face, illumination over the spin and parity forbidden transition centered at 535 nm. On correcting for transmission and reflection losses, the resulting true quantum efficiencies satisfy the same inequality, a result that can only be accounted for by a higher ( > 2 × ) primary quantum efficiency for the back, versus the front portion of the film. As these films show substantially higher quantum efficiencies than do indium free films of the same thickness, it is concluded that the indium in the films is responsible for the increase in primary quantum efficiency. This result is discussed in terms of a localized states model for α-Fe₂O₃.     

等离子体修饰TiO2及其响应光谱红移

TiO2在光催化方面应用前景十分广阔,而阻碍其应用的是它的宽禁带,因此研究开发响应光谱红移的TiO2就成为当前光催化剂研究的重要课题.等离子体处理是修饰TiO2的一种有效方法,目前等离子体修饰TiO2及其响应光谱红移的研究取得了一定进展,等离子体修饰的TiO2表面生成了氧空穴,并形成了原子掺杂,减小了禁带宽度,使其响应光谱红移.本文综述了目前该领域的研究现状,并对今后的研究做了展望.     

等离子体修饰TiO_2及其响应光谱红移

TiO2在光催化方面应用前景十分广阔,而阻碍其应用的是它的宽禁带,因此研究开发响应光谱红移的TiO2就成为当前光催化剂研究的重要课题。等离子体处理是修饰TiO2的一种有效方法,目前等离子体修饰TiO2及其响应光谱红移的研究取得了一定进展,等离子体修饰的TiO2表面生成了氧空穴,并形成了原子掺杂,减小了禁带宽度,使其响应光谱红移。本文综述了目前该领域的研究现状,并对今后的研究做了展望。     

Effects of thermal treatment on hydrophilicity and corrosion resistance of Ti surface

Surface treatment of titanium (Ti) surface has been extensively studied to improve its properties for biomedical applications, including hydrophilicity, corrosion resistance, and tissue integration. In this present work, we present the effects of thermal oxidation as surface modification method on metallic titanium (Ti). The Ti foils were oxidized at 300°C, 400°C, 500°C, and 600°C under air atmosphere for 3 hours, which formed oxide layer on Ti surface. The physicochemical properties including surface chemistry, roughness, and thickness of the oxide layer were evaluated in order to investigate how these factors affected surface hydrophilicity, microhardness, and corrosion resistance properties of the Ti surface. The results revealed that surfaces of all oxidized samples were modified by formation of titanium dioxide layer, of which morphology, phase, and thickness were changed according to the oxidized temperatures. Increasing oxidation temperature led to the formation of thicker oxide layer and phase transformation of anatase to rutile. The presence of the oxide layer helped the improvement of corrosion resistance and microhardness. The most improvement in surface roughness was found in the specimens treated at 400°C, which significantly improved surface hydrophilicity. But both surface roughness and hydrophilicity reduced when oxidized at 500°C and 600°C, suggesting that hydrophilicity was dominated by the surface roughness. In addition, this surface treatment did not reduce the biocompatibility of the metallic Ti substrates against murine osteoblasts (MC3T3).     

The influence of fluoride on the physicochemical properties of anodic oxide films formed on titanium surfaces

The influence of fluoride (and its concentration) on the electrochemical and semiconducting properties of anodic oxide films formed on titanium surfaces was investigated by performing electrochemical measurements (potentiodynamic/pontiostatic polarization, open circuit potential (OCP), and capacitance measurements) for a titanium/oxide film/solution interface system in fluoride-containing 1.0 M HClO(4) solution. On the basis of the Mott-Schottky analysis, and with taking into account both the surface reactions (or, say, the specifically chemical adsorption) of fluoride ions at the oxide film surface and the migration/intercalation of fluoride ions into the oxide film, the changes in the electrochemical behavior of titanium measured in this work (e.g., the blocked anodic oxygen evolution, the increased anodic steady-state current density, the positively shifted flat band potential, and the positively shifted film breakdown potential) were interpreted by the changes in the surface and the bulk physicochemical properties (e.g., the surface charges, surface state density, doping concentration, and the interfacial potential drops) of the anodic films grown on titanium. The fluoride concentrations tested in this work can be divided into three groups according to their effect on the electrochemical behavior of the oxide films: < or =0.001 M, 0.001-0.01 M, and >0.01 M. By tracing the changes of the OCP of the passivated titanium in fluoride-containing solutions, the deleterious/depassive effect of fluoride ions on the titanium oxide films was examined and evaluated with the parameter of the film breakdown time. It was also shown that the films anodically formed on titanium at higher potentials (>2.5 V) exhibited significantly higher stability against the fluoride attack than that either formed at lower potentials (<2.5 V) or formed natively in the air.     

XPS study on surface modification of NiTi alloy by acidic solution immersion and subsequent heating in air

Changes in the surface chemical state of a nearly equiatomic nickel–titanium (NiTi) alloy caused by immersion in aqueous solutions of HNO₃ and H₂SO₄ as well as subsequent heating in air at 723 K were analyzed using X-ray photoelectron spectroscopy (XPS). An XPS analysis using angle-resolved technique and a mathematical deconvolution technique revealed that a passive layer formed in an ambient atmosphere contained TiO₂ as a major state and Ni(OH)₂ and NiO as minor states. The Ni(OH)₂ on the alloy remained in the region even when heated in air at 723 K. Therefore, the resulting layer became a Ti-oxide layer with Ni segregated region at the surface, which was NiO formed via dehydration of Ni(OH)₂. However, immersion in an aqueous solution of HNO₃ or H₂SO₄ enables Ni(OH)₂ state to dissolve in the passive layer of a NiTi alloy; thereby, the Ni segregated region rarely appeared in the oxide layer by heating. The Ni segregated region at the surface becomes an obstacle for the inward diffusion of oxygen; thus, the annihilation of such a segregated region results in an increase in the thickness of the oxide layer.