Article

Electrocatalytic activity of Au nanoparticles onto TiO2 nanotubular layers in oxygen electroreduction reaction: Size and support effects

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Abstract

Electrocatalytic activity of both bare high-ordered TiO2 nanotubes (TNTs) and gold nanoparticles (Au NPs) loaded TNTs toward oxygen reduction reaction (ORR) has been examined by cyclic voltammetry (CV). Cyclic voltammograms for Au NPs-TNT electrodes are characterized by an additional wave observed at less negative potentials which is responsible for oxygen electroreduction on the surface of gold NPs. The overpotential for O2 reduction on the Au surface grows with increasing the temperature of TNT annealing and the Au NPs size. The nature of the effects observed was explained by peculiarities of the electron transport through Schottky barrier formed at the Au NPs – TiO2 interface. The width of the Schottky barrier, determined by semiconductor doping level and Au NPs size, plays a key role in the mechanism of electron transport through the space charge region.

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... Recently, the nanocomposite system of AuNPstitania nanotubular layers (TNT) has raised interest as an electrocatalysts for oxygen electroreduction [35,36]. Macak et al. demonstrated that TiO 2 nanotubular layers significantly enhance the electrocatalytic activity of the Au-TiO 2 system in acidic solutions when compared to flat titania films [35]. ...
... Macak et al. demonstrated that TiO 2 nanotubular layers significantly enhance the electrocatalytic activity of the Au-TiO 2 system in acidic solutions when compared to flat titania films [35]. Oxygen electroreduction reaction was also studied at TiO 2 nanotubular layers loaded with AuNPs in alkaline solutions [36]. It was shown that the electrocatalytic activity of AuNPs on TNT depends on the size of gold nanoparticles and the doping level of TiO 2 support. ...
... A series of AuNPs-TNT composites was prepared by deposition of AuNPs onto TiO 2 nanotubes from aqueous colloidal solutions (hereafter referred to as TNT-Au sol ) to compare their electrocalatytic activity with AuNPs-TNT electrodes produced by UV-assisted anchoring of gold nanoparticles (Fig. 1c). Colloidal solutions of AuNPs with an average diameter of 5, 15 and 25 nm were synthesized according to the methods described in [36,41]. Then, the synthesized colloids of AuNPs were applied to the TiO 2 nanotubular layers in an amount of 3 µg/cm 2 . ...
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Gold nanoparticles (AuNPs) have been deposited on titania nanotubular layers (TNT) via photocatalytic deposition, and the activity of the obtained AuNPs-TNT systems toward oxygen electroreduction reaction (ORR) in an alkaline medium has been studied and compared with the activity of AuNPs-TNT composites prepared from Au colloidal solutions. Two photodeposition methods were utilized for anchoring of AuNPs: direct UV-irradiation of a TNT electrode immersed into a HAuCl4 containing solution (TNT-Au1 composites) and initial adsorption of AuCl4⁻ ions on TNT followed by UV-irradiation (TNT-Au2 composites). The size, spatial distribution, structure and surface of AuNPs deposited on TNT layers were studied via scanning and transmission electron microscopy, optical spectroscopy and underpotential deposition of lead ad-atoms on gold. It was found that these parameters depend on the photodeposition method. The TNT-Au2 composites have smaller size, higher surface concentration and more uniform distribution of AuNPs in the TNT layers as compared with the TNT-Au1 systems. The electrocatalytic efficiency of Au-TNT electrodes in ORR was found to depend on various factors such as doping level of TNT support (governed by annealing temperature), AuNPs size and their loading amount. The electroreduction of oxygen was observed at less negative potentials when Au nanoparticles were grown on the TNT surface by photoreduction in comparison with the TNT electrodes modified with AuNPs from sols. The enhanced activity of the photocatalytically prepared AuNPs-TNT composites can be explained by the consolidation of the interface between gold nanoparticles and TiO2 support and the absence of ligands on the AuNPs surface.
... Besides the bare titania nanotubular films, samples covered with gold were obtained using a colloidal solution of gold nanoparticles with an average size of 5 nm. According to Ref. [31], after adding colloidal gold, the samples were successively dried in a vacuum and heated at 200 C for 1 h. Thus obtained films coated with Au were found to differ from the bare TiNT films by the electrocatalytic properties. ...
... Titania in the form of TiNTs obtained by electrochemical oxidation of titanium is usually nonstoichiometric and amorphous. EPR signals are not typically registered in such samples [18,30,31]. During heat treatment, a crystallization of titania occurs, and, consequently, different types of defects appear in the structure being formed. ...
Article
Electron paramagnetic resonance (EPR) spectroscopy was used to study the nature of defects in black titanium dioxide nanotubes (TiNTs) prepared by electrochemical oxidation of titanium in ethylene glycol. The TiNTs that have intensive black color were formed under thermal treatment of amorphous TiNTs in hydrogen at 500 °C. The work aimed to reveal the role of various defects in the black color of TiNTs. Amorphous carbon has been found to contribute mainly to the deep black color of anatase-type TiNTs. Considerable growth in •С-radicals concentration was detected by the EPR method in amorphous TiNTs under their heat-treatment in hydrogen. The radicals are resulted from the reduction of carbon-containing products of electrochemical and thermal decomposition of ethylene glycol. The dependence of the •С-radical signal position in EPR spectra on the axis of the applied magnetic field testifies for a predominant orientation of graphite-like networks along the direction of TiNTs growth. The Gaussian shape and broadness of the EPR lines are caused by the superposition of several signals from different carbon states. The conditions of synthesis that might result in the occurrence of Ti³⁺ state and oxygen vacancies in titanium oxide were analyzed. The absence of Ti³⁺ signals in the EPR spectra of TiNTs reduced in hydrogen can be explained by the high concentration of Ti³⁺ centers as well as by the formation of [Ti³⁺–VO–Ti³⁺] or [VO–Ti–VO]n– complexes, which are inactive in EPR. The indicated defects might also contribute to titanium dioxide color.
... Functionalization of TNT with AuNPs are commonly used in the literature and similarly to AgNPs, its deposition is carried out via sputtering (Li et al., 2014), photoreduction (Noothongkaew et al., 2017;Maltanava et al., 2016), electrodeposition (Hosseini and Momeni, 2010;Feil et al., 2010;Bai et al., 2016;Hu et al., 2016;Tan et al., 2020) methods. AuNPs/TNT layers developed so far have not been satisfactory due to the non-homogenous and non-spherical shape of AuNPs (Li et al., 2014;Noothongkaew et al., 2017), AuNPs not uniformly deposited (Bai et al., 2016), agglomerated (Feil et al., 2010;Hu et al., 2016;Feil et al., 2010Feil et al., , 2010Bai et al., 2016;Hu et al., 2016;Tan et al., 2020) or completely covering the TNT structure (Hosseini and Momeni, 2010). ...
Article
Nano-modification of titanium nanotubes (TNT), especially the incorporation of metal nanoparticles is very common and allows to achieve unique properties. However, current research is focused on developing a new method of covering with metal nanoparticles and specifying its properties, completely omitting the mechanism of nanoparticle growth on nanotubes and its stability. The aim of this work was to elaborate the titanium nanotube-silver/gold nanoparticle hybrid structure, examine its stability in aqueous solution, i.e. phosphate buffered saline (PBS), and Milli-Q water, as well as carry out the electrochemical characteristics of a hybrid structure. The first significant observation is no differences in nanotubes-nanoparticles structure is observed in PBS solution, which is commonly used by other authors. The obtained results measured in Milli-Q water indicated that gold nanoparticles were definitely less stable on titanium nanotubes surface than the silver nanoparticles. Stability ranges for AgNPs and AuNPs on the surface of TNT were established. The optimal volume used in washing procedure was 1−10 ml per 1 cm2 of TNT for AgNPs/TNT and 10−33 μl per 1 cm2 of TNT for AuNPs/TNT. Additionally, the electrochemical analysis confirmed that AgNPs/TNT are characterized by higher resistance, and lower reactance, which suggests that this kind of functionalization is more appropriable for biosensing. However, AuNPs/TNT are characterized by more positive open circuit potential, and redox stability, favorable for protein/drug immobilization.
... Photoreduction is a multistage and long-lasting technique [12]. The soaking at high temperature and direct adsorption includes long-lasting processes depending on the size and number of the AuNPs [13][14][15]. These methods depend on many factors that influence the reproducibility of the nanoparticle deposition process. ...
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The goal of this research was to find the best conditions to prepare titanium dioxide nanotubes (TNTs) modified with gold nanoparticles (AuNPs). This paper, for the first time, reports on the influence of the parameters of cyclic voltammetry process (CV) -based AuNP deposition, i.e., the number of cycles and the concentration of gold salt solution,on corrosion resistance and the capacitance of TNTs. Another innovation was to fabricate AuNPs with well-formed spherical geometry and uniform distribution on TNTs. The AuNPs/TNTs were characterized using scanning electron microscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and open-circuit potential measurement. From the obtained results,the correlation between the deposition process parameters, the AuNP diameters,and the electrical conductivity of the TNTs was found in a range from 14.3 ± 1.8 to 182.3 ± 51.7 nm. The size and amount of the AuNPs could be controlled by the number of deposition cycles and the concentration of the gold salt solution.The modification of TNTs using AuNPs facilitated electron transfer, increased the corrosion resistance, and caused better adsorption properties for bovine serum albumin.
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The title subject is studied by slow galvanostatic and potentiostatic transient and dc capacitance determination on potentiostatically well stabilized (20 h) passive titanium electrodes in acidic sulphate (pH 1.4) and neutral acetate (pH 7.2) solutions at 25°C. Combined with film thickness data and model studies, the results reveal information on the roughness factor of the electrodes used, on the kink-site dependent metal-ion transfer at the metal—oxide interface and on the ionic transport, the dielectric constant, the space charge and the electric field in the passive film.The interfacial metal-ion transfer appears to occur by four-valent titanium ions and the ionic transport in the film mainly by fully ionized oxygen vacancies. Under steady-growth conditions, the surface density of kinks on the metal (s) and the volume density of oxygen vacancies in the passive film at the metal—oxide interface (No) respectively obey d log s/d log j ≈ and d log N0/d log j ≈ where j is the cu density of metal oxidation.
Article
The present work is concerned with analysis of the nonlinear Mott-Schottky plots obtained from the measured impedances of anodically passivating amorphous and polycrystalline TiO2 films including multiple donor levels. The passivating amorphous TiO2 films were prepared on titanium in 0.5 M H2SO4 solution galvanostatically at 2 mA cm-2 at formation potentials of 2, 3 and 5 V/SCE, and the polycrystalline films at 5 mA cm-2 at potentials of 20 and 30 V/SCE. The analysis was made by obtaining a numerical solution to a modified Mott-Schottky equation by using the Euler method. Based upon the analysis, the applied potential dependence of the donor distribution across the amorphous and polycrystalline TiO2 films was determined for various film thicknesses and measuring frequencies. Ionized donor concentration increases with increasing applied potential, regardless of film thickness and measuring frequency. The thinner film and the lower measuring frequency enhance the donor concentration. It is suggested from the analysis of the nonlinear Mott-Schottky plots that the applied potential dependence of the donor distribution is attributable to the presence of multiple donor levels and that the donor concentration as a function of applied potential is definitely determined by the film thickness and measuring frequency.
Article
Zusammenfassung In der neuen Theorie der unipolaren Vorgänge an der Grenze Metall-Halbleiter spielt der Halbleiter nicht mehr die Rolle der schlecht emittierenden Elektrode einer Diodenanordnung, in der beide Glühelektroden durch einen Potentialberg getrennt sind (Abschnitt 1), sondern erscheint, wie das Vakuum in einer Glühelektronenröhre, als Träger eines zwischen den beiden Metallelektroden in ungleicher Dichte vorhandenen Elektronen- (oder Defektelektronen-) Gases, das allerdings anderen Strömungsgesetzen gehorcht als im Vakuum (Abschnitt 2). Bei größeren Schichtdicken ist auch diese Vakuumanalogie nicht ausreichend; wegen der in ihm vorhandenen thermisch dissoziierbaren Störstellen verhält sich vielmehr der Halbleiter ähnlich wie ein mit Natriumdampf von Glühtemperatur erfüllter Raum zwischen zwei thermisch emittierenden Elektroden. Besondere Übergangswiderstände an der Grenze Metall-Halbleiter entstehen, wenn die thermische Randdichte der Elektronen kleiner ist als die durch den Störstellenghalt bedingte Elektronendichte des neutralen Halbleiterinnern. Es bildet sich dann eine Raumladungsrandschicht aus, die sich bei einer in den Halbleiter hinein gerichteten Elektronenbewegung ausdehnt und dadurch ihren Widerstand vergrößert, während sie sich in umgekehrter Stromrichtung bis auf Null zusammenzieht (Abschnitt 3). Diese Deutung der Gleichrichterwirkung, die schon bei völlig gleichmäßiger Störstellenverteilung zu Übergangswiderständen und unipolarer Leitfähigkeit führt, wird in Abschnitt 4 und 5 durch die Betrachtung des zusätzlichen Einflusses einer ungleichmäßigen Störstellenverteilung erweitert. Besonders wird eine Störstellenverarmung des Halbleiters an der Metallgrenze („chemische Sperrschicht“) betrachtet; allgemein wird darauf hingewiesen, daß jede durch besondere Störstellenverteilung oder durch Randwirkungen hervorgerufene Ausbildung einer Raumladungszone innerhalb des Halbleiters als Ursache nichtlinearer und unipolarer Leitungsvorgänge in schwachen Feldern wirksam ist. Nachdem in Abschnitt 6 die Voraussetzungen dieser Raumladungstheorie noch einmal zusammengestellt sind, wird in den folgenden Paragraphen versucht, eine erste Übersicht über ihre Bedeutung für das ganze vorliegende Beobachtungsmaterial zu geben. In allen Fällen scheinen die sich aus der Raumladungstheorie ergebenden Schichtdicken (etwa 10-6 bis 10-3 cm, je nach der Leitfähigkeit des Störstellenhalbleiters) den tatsächlichen Abmessungen der anomal leitenden Gebiete an der Grenze Metall-Halbleiter zu entsprechen. Für Flächengleichrichter kann das aus Kapazitätsmessungen, für Spitzendetektoren durch eine Analyse der noch als „Spitzen“ wirksamen Berührungsflächen nachgewiesen werden (Abschnitt 7), wobei von der Annahme einer besonderen Sperrschicht an der Berührungsstelle Spitze-Halbleiter im allgemeinen kein Gebrauch gemacht wird. In Abschnitt 8 werden unter ähnlichen Geschichtspunkten „künstliche Sperrschichten“ behandelt, ferner die Vorgänge beim Gegenpressen von Folien, bei der losen Berührung verschiedener Halbleiter untereinander und beim Auftreten von Störzonen im Innern massiver Halbleiter. Abschnitt 9 gibt einen historischen Überblick über die Vorgeschichte der Theorie und ihre Stellung im Rahmen der sonstigen heute vorhandenen Erklärungsversuche.
Article
Photocurrent spectroscopy has been used to examine the early stages of anodic film growth on titanium. Analysis of the photocurrent conversion efficiency as a function of wavelength for films formed at different potentials has shown that the film is essentially pure TiO2 above ca.1.5 V vs. SCE (at pH 0). Photocurrent spectroscopy has also been used to follow changes in the structure and thickness of the anodic film during breakdown at higher voltages.
Article
Schottky nanocontacts are formed when nm-sized metal particles are located on the planar surface of a doped semiconductor. The charge distribution on and near such nanocontacts is analyzed in the case of disc-shaped particles. The results of calculations are presented as a function of particle size, semiconductor permittivity, dopant concentration, and Fermi level difference. In contrast to macroscopic junctions, the charging of the metal particle is demonstrated to be proportional to the Fermi level difference and accordingly to the potential difference between the metal and semiconductor, so that the junction exhibits a constant capacitance. The charging of the metal-vacuum metal surfaces may be appreciable, especially for relatively low values of the semiconductor permittivity. The tunneling barrier width at half height is shown to be close to, or less than, 3/8 of the disc diameter.
Article
Isolated, nanoscale (5.0–20.0 nm diameter) Cu clusters on a reduced TiO2 (110) surface exhibit the initiation of the Schottky effect. Apparent height changes of isolated clusters occur in scanning tunneling microscopy imaging as bias conditions are changed. This apparent height change is directly related to current flow through the cluster-oxide interface barrier. Further, depletion zones along the substrate surface adjacent to the clusters exhibit the same bias dependence indicating that changes are associated with local band-bending, analogous to that of macroscopic Schottky barriers. Barrier-height variations with cluster size and with applied voltage are quantified. When compared to models of edge effects in finite-sized systems a direct correlation between geometry and barrier formation is made.
Article
We have studied the interaction of small Aun (n = 1–3) particles with the defect-free (stoichiometric) and defected (partially reduced) TiO2 anatase (101) surface using density functional calculations within a slab geometry. On the stoichiometric surface, gold particles prefer anion sites and “standing” geometries, in agreement with simple MO theory arguments. On the defected surface, Au strongly binds to the two cations close to the bridging oxygen vacancy. For both Au2 and Au3, “lying” adsorption geometries are now more likely, and starting from n = 3 there is a tendency towards mixed binding, where cation and anion sites are simultaneously involved. Clustering of Au atoms is favored on both the stoichiometric and the reduced surfaces. CO strongly interacts with gold particles adsorbed on the stoichiometric surface. As found in previous gas-phase investigations, the CO-cluster bond is stronger when the cluster carries a positive charge. Accordingly, a weak interaction is computed for a gold atom supported on the reduced surface. © 2002 American Institute of Physics.
Article
The impedance behavior of nanotubular self-organized porous (PTO) films with thickness of , pore diameter, and average spacing formed on titanium by anodization in solution was investigated and compared to the behavior of compact layers (CTO). At potentials close to the flatband potential , PTO had an apparent interfacial capacitance at that was greater than the capacitance of CTO. This effect, however, was not only related to the larger effective area of PTO. Differentiation between both surface conditions vanished both at potentials significantly higher than and with increasing test frequency to . These findings together with observations of appreciable frequency dispersion suggest that the pore walls are rich in deep-lying localized states, which become evident only at lower test frequencies and at potentials negative enough that depletion zones do not merge at pore walls. PTO did not show a transmission line effect at frequencies as high as . Model calculations based on pore dimensions and electrolyte conductivity predicted that those effects could only take place at much higher test frequencies, but the slow response of deeper states might impair observation anyway.
Article
X-ray photoelectron spectroscopy (XPS) was employed to study the surface composition and electronic structure of Au/TiO2 catalysts in comparison with TiO2 (anatase) and to reveal time-dependent X-ray irradiation damage of the samples. The occurrence of Au nano-sized particles on a TiO2 support was found to result in a slight shift of Ti 2p core-level spectrum and in changes of the valence band and X-ray induced Auger spectra, compared to TiO2-only. It was shown that for different means of energy referencing the charge-corrected Au 4f7/2 binding energy in Au/TiO2 catalysts was 0.15–0.45 eV lower than that in pure bulk Au. Exposure to X-rays of Au/TiO2 catalysts and pure TiO2 caused a reduction of Ti 4+ oxidation state and desorption of oxygen from the surface. As a result, the surface chemical composition and electronic structure of the samples changed with time. The X-ray irradiation affected charge transfer processes in Au/TiO2 so that the pattern of X-ray induced damage in the Au-based catalyst turned out to be quite different from that in TiO2, with some characteristics displaying the very opposite features. Decreasing of the Au 4f7/2 binding energy and concurrent increasing of the fraction of Ti3+ species observed in the beginning of X-ray irradiation of Au/TiO2 may be taken as direct evidence for charge transfer from oxygen vacancies created by irradiation to Au particles.
Article
Gold can be deposited on TiO2 as hemispherical fine particles with diameters smallerthan 4nm by deposition precipitation. Gold hydroxide precipitates with high and homogeneous dispersion on TiO2, most probably on specific surface sites, in the pH range 6 to 10. The calcination of TiO2 with Au(OH)3 in reducing gas atmospheres, such as H2 and CO led to smaller gold particles than in air. In the case of Au/TiO2 samples prepared by deposition precipitation, the catalytic activity for low-temperature CO oxidation was very high and could be observed even at temperatures below 0°C. Physically mixed Au/TiO2 samples though of much lower catalytic activity, showed gradual improvement with increasing calcination temperature. An increase in calcination temperature not only caused particle coagulation but also brought about a stronger interaction with the TiO2 support. The above results therefore indicate that the catalytic activity of Au/TiO2 is sensitive to the structure of the perimeter interface between Au and TiO2.
Article
The capacity of thin passive films on metal electrodes is investigated. Two contributions are reported: one from the space charge in the film and one from the surface charge on the underlying metal. In the usual Schottky-Mott plot such films also exhibit a linear region, but this cannot ordinarily be used to obtain the carrier concentration and the flat-band potential. These calculations agree well with experimental data.
Article
A reflectance method has been used to assess conduction band edge energies (Ecb) for nanocrystalline TiO2(anatase) electrodes in contact with aqueous electrolytes. The measurements, which were made over a range of nearly 40 pH units, reveal a Nernstian dependence of Ecb upon pH over most of this range, i.e., a −64 mV shift per unit decrease in log(proton activity) between H0 = −8 and H- = +23. Electrochemical quartz crystal microbalance (EQCM) measurements have established that charge compensating proton uptake occurs at potentials negative of Ecb. Uptake occurs over the entire EQCM-accessible pH range (H0 = −5 to pH = +11). The combined findings are inconsistent with Ecb control solely via surface protonation and deprotonation reactions, whose pKa's occur in the vicinity of pH 4 and 10. They are consistent, however, with a mechanism whereby:  (a) electrochemical generation of Ti(III) trap sites, in the log(proton activity) range from H0 = −8 to H- = +23, is accompanied quantitatively by proton intercalation, (b) conversion of the trap sites back to oxidation state IV is accompanied quantitatively by proton expulsion, and (c) the conduction band edge energy is controlled by the pH-dependent trap-based Ti(III/IV) couple. The pH independence found for Ecb above H- = +23 and below H0 = −8 is ascribed to an eventual decoupling of proton intercalation from electron addition.
Article
The photocurrents measured by a scanning tunneling microscope have been used to analyze the electronic properties of nanoscale-modified WSe2 semiconductor surfaces. On uncovered crystals in ambient air, space charges along steps can be analyzed. On copper-modified semiconductor surfaces, the space charge zones around pulse-deposited metal particles and their dependence on the size of the metal particle can be determined and provide a first direct proof of size-dependent barrier heights of nanosized Schottky contacts. Also, the time evolution of the electronic properties of a metal-modified WSe2 surface under corrosion conditions was followed by photocurrent measurements.By combination of photocurrent measurements and local current/voltage curves, the influence of recombination and of charges on the photocurrent can be separated.
Article
The dependence of the kinetics of electrocatalytic oxygen reduction in basic electrolyte on the size of Au nanoparticles was determined for 3 and 7 nm clusters supported on carbon. The size-selected nanoparticles were prepared by reverse micelle encapsulation using PS−P2VP diblock copolymer, and the kinetic current for oxygen reduction was measured with a rotating disk electrode (RDE). The kinetic current was found to be 2.5 times higher for the 3 nm gold nanoparticles compared to the 7 nm gold nanoparticles at 23 °C. The 3 nm particles were found to facilitate four-electron electroreduction, whereas a two-electron electroreduction was inferred from the RDE data on the 7 nm particles. From experiments of the temperature-dependent current the apparent activation energy for the 3 nm clusters was found to be half that of the 7 nm clusters (0.1 and 0.2 eV, respectively).
Article
We describe a comparative study of the oxygen reduction reaction on two carbon-supported Pt-based alloy catalysts in aqueous acidic electrolyte at low temperature. Both alloys have the bulk compositions of 50 and 75 at. % Pt, with the alloying elements being Ni and Co. Comparison is made to a pure Pt catalyst on the same carbon support, Vulcan XC-72, having the same metal loading (20 wt %) and nominally the same particle size (4 ± 2 nm). High-resolution electron microscopy was used to determine the size and shape of the particles as well as the particle size distribution on all catalysts. Electrochemical measurements were performed using the thin-film rotating ring−disk electrode method in 0.1 M HClO4 at 20−60 °C. Hydrogen adsorption pseudocapacitance was used to determine the number of Pt surface atoms and to estimate the surface composition of the alloy catalysts. Kinetic analysis in comparison to pure Pt revealed a small activity enhancement (per Pt surface atom) of ca. 1.5 for the 25 at. % Ni and Co catalysts, and a more significant enhancement of a factor of 2−3 for the 50 at. % Co. The 50 at. % Ni catalyst was less active than the Pt standard and unstable at oxygen electrode potentials. Ring-current collection measurements for peroxide indicated no significant differences between the Pt−Co catalysts or the 25 at. % Ni catalyst and pure Pt, while the 50 at. % Ni catalyst had a higher peroxide yield. Together with the observed Tafel slopes and activation energies, it was concluded that the kinetic enhancement is contained in the preexponential factor of the conventional transition state theory rate expression. It is, however, not clear why the alloying with Ni or Co produces this change in the preexponential factor.
Article
Polycrystalline Au nanoparticles (NPs) of 3, 6, and 8 nm were synthesized via solution phase reaction of HAuCl4·3H2O with tert-butylamine borane and oleylamine in tetrahydronaphthalene. The sizes of the Au NPs were tuned by varying the reaction temperatures (40−3 °C). For polycrystalline Au NPs made at room temperature or above, smaller NPs showed more positive onset potential in catalyzing oxygen reduction reaction (ORR) in 0.5 M KOH media. However the most active Au NPs were the 8 nm ones that were synthesized at 3 °C. We rationalized the ORR activity of these Au NPs by the ease of oleylamine surfactant removal and the degree of disorder in the polycrystalline structure. This was further confirmed by the low activity observed from the same Au NPs passivated with hexadecanethiol, or from the Au NPs with higher degree of crystallinity made from the etching of the composite Au−Fe3O4 NPs.
Article
Während des Wachstums von Oxidschichten auf Titan in verschiedenen wäßrigen Elektrolytlösungen ist die aus Schichtdicke und Formierungsspannung ermittelte Feldstärke eine lineare Funktion des Logarithmus des Gesamtstromdichte. Die Stromausbeute für das Schichtwachstum hängt von der Stromdichte, dem pH-Wert und der Schichtdicke ab. Die Dielektrizitätskonstante der Schicht sinkt mit der Feldstärke zunächst von ε = 110 auf ε = 7 ab und wächst bei hohen Feldstärken und dicken Schichten wieder erheblich an. Die Auflösungsgeschwindigkeit anodisch erzeugter Oxidschichten auf Titan wurde in Abhängigkeit vom pH-Wert, der Chlorid-konzentration und der Temperatur untersucht. Während der Alterung der Schichten in destilliertem Wasser nimmt die Auflösungsgeschwindigkeit um bis zu 3 Zehnerpotenzen ab. Growth and dissolution of anodically obtained oxide layers on titanium Oxide films were grown by anodic polarization of titanium in various aqueous electrolytes. The field strength derived from film thickness and voltage is linearly related to the logarithm of the total current density. The current efficiency for film formation depends upon current density, pH-value, and film thickness. The dielectric constant of the film decreases with field strength from ε = 110 to ε = 7, but rises again at high field strengths and thick films. The dissolution rate of anodic oxide films on titanium was investigated as a function of pH-value, concentration of chloride, and temperature, respectively. Aging of the films in distilled water causes the dissolution rate to decrease by up to 3 order of magnitude.
Article
The present work reports on key factors that influence the degree of order in anodic TiO2 nanotube layers. We show that the anodization voltage and the Ti purity are of crucial importance for the ideality of self-organization within the nanotube layers and that repeated anodization can significantly improve hexagonal ordering. Optimizing each factor significantly reduces the variation in the average pore diameter and strongly reduces the areal density of polygon ordering/packing errors. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Article
The capacity and photocurrent behaviour of passive titanium electrodes has been investigated in dependence on the film thickness and the electrode potential, the photocurrents additionally on the wavelength. From capacity measurements a high donor concentration of 1020 cm−3 and a dielectric constant of D = 9 can be concluded. Compared to bulk TiO2 electrodes, the onset of the photocurrent is shifted to higher photon energies. The band gap is estimated to be Eg = 3.4 eV. At constant electrode potential ε the photocurrent decreases with increasing film thickness though the absorption of the film should be higher. This indicates the strong influence of the field strength in the film on the photocurrent which increases with thickness at constant field.
Article
The electronic interaction between TiO2(1 1 0) surfaces and Au clusters has been investigated by photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM) to elucidate the high catalytic activity of Au/TiO2. With increasing Au amount, the shift of O 2p nonbonding peak towards the Fermi level and the decrease in the Ti 3d peak area were precisely observed in the valence band PES spectra of Au/TiO2(1 1 0). These changes in the valence band structure are attributed to the electron transfer from the surface defects on TiO2(1 1 0) into the Au clusters. At the low coverage region, the STM images consistently show that the Au clusters nucleate at the surface defects of TiO2(1 1 0). Here, we discuss the electronic interaction between the Au clusters and the TiO2 surface relating to the catalytic activity of Au clusters on surfaces.
Article
The present work focuses on the investigation of the effect of the different crystallographic orientation of titanium grains on the formation of anodic oxide films and consequently their dielectric and semiconductive properties. By using a microcapillary cell the formation process and the electrochemical impedance spectroscopy (EIS) can be performed at high lateral resolution on variously orientated single grains of polycrystalline titanium. The oxide films were potentiodynamically formed by cyclovoltammetry. EIS measurements immediately followed by the oxide formation were used for a detailed investigation of the film properties, in particular, the relative permittivity ɛr and the donor concentration ND. In contrast to the most publications it was found that under the chosen conditions the crystallographic orientation of titanium substrate has no significant influence on the oxide thickness d, the relative permittivity ɛr or on the donor concentration ND of the oxide films. The relative permittivity ɛr is approximately 50. The donor concentration depends on the film thickness and amounts to approximately 3 × 1018 cm−3 in minimum.
Article
The semiconducting properties of anodic grown TiO2-nanotubes were analyzed by means of the potential dependence of the interfacial capacity under different electrolyte and illumination conditions. The experimental results offer evidence for a duplex oxide film conformed by the bottom and wall of pores with different density of donors and concentration of surface states. The surface extension offered by the walls of oxide tubes is manifested in surface related effects such as the shift of the flat band potential and trapping of photoelectrons.
Article
Nanostructured TiO 2 films can be reversibly doped by applying a potential of À0.6 V Ag/AgCl in aqueous 0.1 M HClO 4 for several min-utes. This charge transfer reductive doping causes significant changes of the cyclic voltammograms in the dark. Furthermore, an up to 6-fold increase of the efficiency for water photooxidation is observed upon electrochemical reductive doping. Both observations are explained by the build-up of a space charge layer through agglomerates of appropriately sized and shaped nanoparticles.
Article
On the basis of kinetic and photoelectrochemical studies we show that the formation of amorphous or strongly disordered TiO2 films on electropolished titanium rods can occur upon anodization in 0.5 M H2SO4 solution in a range of thickness which depends on the anodization rate. This finding is confirmed both by the changes in the shape of the photocurrent vs. potential curves with the energy of the incident photons, and by the impedance behaviour of the junction.Our data indicate that TiO2 films having different degree of disorder are formed depending on the anodization rate and oxide thickness. Crystalline films are formed at very low growth rates since very low thicknesses. Amorphous or strongly disordered films can be grown at high oxidation rates (≥ 100 mV/s) before the onset of a crystallization process. At higher thicknesses a gradual crystallization process takes place, as revealed both by the decrease of the anodizing current during the tensiodynamic growth and by the changes in the shape of the photocurrent vs. potential curves. The onset of the crystallization process of the film occurs at higher thicknesses with increasing the growth rate.
Article
Gold can be highly dispersed on a variety of metal oxides by coprecipitation and deposition-precipitation followed by calcination in air. The small gold particles are hemispherical in shape and stabilized by epitaxial contact, dislocations, or contact with an amorphous oxide layer. Such supported gold differs in catalytic nature from unsupported gold particles and exhibits high catalytic activities for low-temperature oxidation of CO. Especially, gold supported on TiO2, α-Fe2O3, Co3O4, NiO, Be(OH)2, and Mg(OH)2 is very active even at temperatures below 0°C. Among the gold catalysts supported on TiO2, α-Fe2O3, and Co3O4 the turnover frequencies for CO oxidation per surface gold atom are almost independent of the kind of support oxides used and increase sharply with a decrease in diameter of gold particles below 4 nm. Small gold particles not only provide the sites for the reversible adsorption of CO but also appreciably increase the amount of oxygen adsorbed on the support oxides. In the temperature range −10 to 65°C, the activation energies for CO oxidation were 8.2 kcal/mol (Au/TiO2), 8.4 kcal/mol (Au/α-Fe2O3), and 3.9 kcal/mol (Au/Co3O4). The rate of CO oxidation is zero order with respect to CO for the three catalysts, and 0.2-0.3 for Au/TiO2 and Au/Co3O4 and zero order for Au/α-Fe2O3 with respect to O2. By taking into consideration TPD and FT-IR data, a mechanism is proposed in which CO adsorbed on gold particles migrates toward the perimeter on support oxides and there it reacts with adsorbed oxygen to form bidentate carbonate species. The decomposition of the carbonate intermediate is considered to be rate-determining.
Article
Stress variations of semiconducting n-TiO2 films prepared by anodic oxidation in concentrated alkaline solution are studied. The relationship between stress and photoelectrochemical response is investigated. The presence of sodium ions in the reticular lattice is found to be responsible for the particular behaviour of the oxide. Results are compared with TiO2 prepared by thermal oxidation in air.
Article
Oxygen reduction in acidic electrolyte solution has been studied at various Au/C catalysts with different support materials and different gold particle sizes. The peak potential in cyclic voltammogramms (CVs) during oxygen reduction depends on the potential applied to the catalyst during electrochemical pre-treatment. This effect is attributed to a shift of oxide reduction on the nanoparticles to more negative potentials compared to bulk gold, as can be seen from CV. Quantitative assessment of the oxygen reduction activity has been carried out with rotating disc electrode (RDE) measurements. A catalyst with a gold loading of 0.5 wt% gold turned out to be the most active in terms of mass specific activity. However, the surface specific activity was neither dependent on the particle size in the range 2.7–42.3 nm nor on the support material. It has been found the average number of electrons transferred per oxygen molecule is two to three, independent on gold particle size.
Article
We propose a high-voltage (>1000 V) pulsed discharge method of preparation of thin anodic films on valve metals under conditions of extremely high rates of the film growth. The peculiarities of this method have been demonstrated on the anodic oxidation of titanium in sulfuric acid solutions.A light flash at the electrode was generated upon the pulsed anodization of titanium. The emission spectra consist of narrow lines assigned to electronically excited O, H and Ti atoms, indicating that a hot plasma is created at the electrode surface. The current efficiency of TiO2 anodic film formation exceeds markedly 100% for the pulsed Ti anodization. This non-faradaic yield was associated with the contribution of the plasma-generated high-energy radicals to the oxide growth processes. We applied various techniques to compare the composition, morphology, structure and semiconducting properties of the anodic films prepared by the high-voltage pulsed discharge and the conventional galvanostatic anodization. The anodic TiO2 films prepared by the high-voltage pulsed discharge have nearly ideal stoichiometric composition and are more amorphous as compared to galvanostatically grown ones. Thin TiO2 films (50–150 nm) formed in the pulsed regime exhibit significantly higher photocurrent quantum yields at λ < 310 nm and a significantly lower donor concentration.
Article
Au nanoparticles dispersed over a self-organized nanotubular TiO2 matrix can be used as a highly efficient catalyst system for the electrochemical oxygen reduction reaction in aqueous solutions. For the same loading of Au nanoparticles, the nanotubular support provides a manifold increase in the reaction rate in comparison with a flat TiO2 support, or a pure Au sheet electrode.
Article
ac Impedance measurements and photocurrent spectroscopy have been used to examine the dependence on growth rate of the solid state properties of thin (<20 nm) anodic oxide films on titanium. At room temperature, the relative permittivity and defect concentration profiles in the oxide were found to depend markedly on growth rate. The effects on the photocurrent of recombination and trapping were most pronounced when the films are grown rapidly. It was found that the high defect densities associated with rapidly grown oxide films could be decresed by growing or annealing the oxide films at higher temperatures.
Article
The cathodic reduction of oxygen in 1 mol dm−3 sodium hydroxide solutions has been investigated at several types of titanium oxide cathodes. Layers of TiO2 were prepared on titanium by spraying and thermal decomposition of solutions of titanium n-butoxide in 2-propanol and titanium tetrachloride in methanol+water and also by anodization; the reduction was also studied at Ebonex®, a conducting ceramic consisting mainly of Ti5O9. In all cases, the reduction of oxygen occurs largely by a 4e− reaction to water and the reaction occurs at potentials close to −1.0 V vs. SCE. Cyclic voltammetry in the absence of oxygen shows that, at these potentials, the surfaces undergo reduction and the electrochemistry of the Fe(CN)64− /Fe(CN)63− couple has been used to probe further the properties of the TiO2 surfaces.
Article
It is suggested that there may be several effects contributing to the special catalytic properties of supported nanosized gold particles, and that it is useful to order them in a hierarchy. The most important effect is related to the availability of many low-coordinated gold atoms on the small particles. Effects related to the interaction with the support may also contribute, but to a considerably smaller extent. We base the analysis on a new set of experimental results comparing the CO oxidation rates over gold supported on different reducible and nonreducible oxides, on an analysis of a large number of published activity data, and on an analysis of density-functional calculations of the effect of metal coordination numbers in comparison to the role of charge transfer, layer thickness, and interactions with the support.