Fluoride-containing Electrolyte Research Articles

Totally Tubular TiO2 Nanotubes as Semiconducting Material for Dye-Sensitized Photoelectrodes

The use of chemical bonds to store solar energy is a promising approach to address the intermittency of the sun by harnessing sunlight to drive chemical transformations. One strategy to drive these processes is to use a bespoke semiconductor surface implemented in a photoelectrode to absorb light and generate charge carriers that can drive redox reactions such as the direct oxidation of adsorbed species on the photoanode. To minimize recombination of photogenerated electron-hole pairs, molecular donor-chromophore-acceptor (D-C-A) systems can be used to sensitize wide bandgap semiconductors for visible light absorption and to generate (high energy) long-living charge separated states. Such assemblies represent a photoelectrode for dye-sensitized photoelectrochemical cells (DS-PECs). Presented here are recent findings using molecularly sensitized TiO2 nanotubes as a nanohybrid architecture for DS-PECs. The highly ordered and oriented 1D-nanostructures are purpose-grown using a self-organizing electrochemical anodization approach. The nanotube mouth is functionalized with a Cu(I)-based donor-chromophore-acceptor system by direct assembly at the surface. The resulting photoanodes are investigated against different water oxidation catalysts (e.g., [Cp*Ir(pyalk)OH], pyalk = 2-(pyridine-2-yl)propan-2-ol; CoOx) for their ability to generate oxidizing equivalents that can subsequently drive follow-on oxidation reactions. A more common TiO2 nanostructure morphology (e.g., nanospheres) and simpler Cu(I)-based chromophores are also examined for comparative purposes to highlight the advantages of the proposed assembly.It is well understood that the role of the semiconductor is to connect the molecular components and transfer/store charge carriers from the excited dye to drive redox processes. The TiO2 nanotubes used here are synthesized by self-organized electrochemical anodization (SOA); a process Involving an anode/cathode arrangement where Ti is used as the anode. This process is a balance between the electrochemical formation of TiO2 and simultaneous chemical dissolution in fluoride-containing electrolytes. The resulting semiconducting film is found to be a vertically oriented, size-controlled, and back-contacted nanostructured array of TiO2 on a foil of titanium. In addition to this composition inheriting high chemical stability, the morphology can grant added benefits such as two-dimensional confinement, directional charge transport and orthogonal charge separation.[1]A crucial limiting factor for the performance of this system is the recombination of the electron-hole pairs. By using a transition metal complex or chromophore (C) that is covalently linked with electron donating (D) and electron accepting (A) moieties, the energy levels can be aligned for charge-transfer processes between the electroactive groups for directional electron transport and formation of a charge-separated state.[2] Copper(I) D-C-A assemblies are also of interest when taking into consideration the low-cost and photophysical properties when compared to typical ruthenium(II) polypyridine chromophores used in these types of cells. Transient photocurrent measurements of the photoelectrode suggest that the TiO2 nanotubes’ mouths generate a base photocurrent, which could be associated with graphitic-like carbon impurities extending light absorption for photoactivity. Furthermore, once the surface is decorated with the copper(I) D-C-A assembly, the photocurrent is significantly increased, enabling light-driven oxidation of CoOx photoelectrochemically deposited on the surface of the photoelectrode.REFERENCES[1] Nguyen, N. T.; Altomare, M.; Yoo, J. E.; Taccardi, N.; Schmuki, P. Noble Metals on Anodic TiO2 Nanotube Mouths: Thermal Dewetting of Minimal Pt Co-Catalyst Loading Leads to Significantly Enhanced Photocatalytic H2 Generation. Adv. Energy Mater. 2016, 6, 1501926. DOI: 10.1002/aenm.201501926[2] Singh, Z.; Kamal, S.; Majewski, M. B. Copper(I) Donor−Chromophore−Acceptor Assembly for Light-Driven Oxidation on a Zinc Oxide Nanowire Electrode. J. Phys. Chem. C. 2022, 126, 39, 16732-16743. DOI: 10.1021/acs.jpcc.2c04652 Figure 1

Application of the Point Defect Model to the Oscillatory Anodic Oxidation of Illuminated n-Type Silicon in the Presence of Fluoride Ions Using Electrochemical Impedance Spectroscopy

This work aims to provide insight into the oscillations occurring during the anodic electrooxidation of Si in fluoride-containing electrolytes using electrochemical impedance spectroscopy (EIS). The EIS measurements were conducted within less than a tenth of the oscillation periods allowing changes in the electrical properties of the silicon/oxide/electrolyte interfaces to be monitored during an oscillatory cycle. Application of the power law model to the experimental data revealed a significant change in resistivity at the oxide/semiconductor interface while the properties at the oxide/electrolyte interface remained constant and the oxide layer varied only by about 1 nm around an average value of about 4.9 nm. The application of the point defect model to the semiconductor/oxide/F−-containing electrolyte interface suggests that the oscillations are linked to the time delay between the production of oxygen vacancies at the Si/oxide interface and their consumption at the oxide/electrolyte interface.

Electrochemical dissolution and retrieval of broken NiTi endodontic files from root canal using chloride based isotonic fluids as electrolytes - An in vitro study.

The electrochemical dissolution method of instrument retrieval emphasizes on the dissolution of the instrument rather than sacrificing dentine. Most of the studies conducted for electrochemical dissolution used fluoride-containing electrolytes and were performed inside a beaker. In this study, we used chloride-based fluids as electrolytes. Fifty extracted mandibular first premolars were divided into five groups based on the electrolytes used. Canals were enlarged to ProTaper Universal F2, and files were intentionally broken inside the canal. These specimens were subjected to electrochemical characterization by applying the potential of 9V for 20 min. Optical images were taken to assess the change in surface topography. The results were analyzed statistically by one-way analysis of variance (analysis of variance [ANOVA]). The rate of dissolution based on the electrolyte used decreased in the following order, viz. Tyrode's solution>artificial saliva>normal saline>Ringer's lactate/physiological serum. Apart from fluoride, chloride-based electrolytes could be an efficient alternative.

Formation of Pores on Pure Ti in Highly Concentrated Sulfuric Acids

TiO2 nanotubes formed by anodization in fluoride-containing electrolytes have been extensively examined due to their wide rage of applications. It is reported that fluoride ions move faster to the interface between anodic oxide layer and underlying Ti substrate, forming a fluoride-rich layer at the interface as the ionic radius of fluoride ion is smaller compared to that of oxygen ion. The fluoride-rich layer is also reported to decrease the adhesion of the anodic oxide layer to the substrate. Therefore, the formation of nanotubular oxide layer in fluoride-free electrolytes is highly desired. In the present work, we examined the formation of nanotubular oxide layers on pure Ti in highly concentrated sulfuric acids. Specimens with the dimension of 5 mm x 80 mm x 0.6 mm were prepared from pure Ti sheet (purity ; 99.5%). Prior to anodization, the specimens were cleaned in acetone, methanol and deionized water, successively. Anodization was carried out in highly concentrated sulfuric acids at elevated temperatures using two-electrode configuration cell with platinum counter electrode. The applied voltage was increased with the sweep rate of 1 V/s to desired voltages and then kept at the voltages for various durations. After the anodization, the surface of the specimens was cleaned with deionized water and dried. The structure of the surface was evaluated using FE-SEM. In a highly concentrated sulfuric acid, the anodic current increased with increasing the applied voltage and after switching constant voltage, decreased with time. The formation of oxide layer was confirmed at the surface, but no porous structure was visible. However, a porous structure was observed on the specimen above the electrolyte surface, indicating that the anodization occurred under the meniscus consisting of highly concentrated sulfuric acids. The optimization of water concentration in sulfuric acid and temperature led to the formation of porous structure on pure Ti. This indicates that porous oxide layers can be formed in fluoride-free electrolytes although further optimization is required to improve the ordering of pores.

Investigating Solid Electrolyte Interphase Layer Dynamics in the Electrochemical Li-Mediated Ammonia Synthesis

Due to its important role in the production of fertilizers ammonia is one of the most produced chemicals worldwide. In recent years there has been increasing interest in the electrochemical lithium mediated nitrogen reduction reaction (LMNRR), as a possible alternative to the Haber-Bosch process. The LMNRR is advantageous as it can be carried out under ambient temperatures and pressures [1].The LMNRR process involves electrochemical plating of metallic Li from an organic electrolyte. This plated Li reacts with elemental nitrogen forming an intermediate nitride species, which is subsequently protonated to form ammonia. To ensure high selectivity towards ammonia production the competing hydrogen evolution reaction has to be limited [2]. The solid electrolyte interphase (SEI), formed from electrolyte decomposition at the working electrode plays a crucial role in this as it influences mass transport to the electrode [3]. Studies using neutron reflectometry with one-minute time resolution investigated the initial stages of SEI formation in LiClO4 containing electrolytes [4]. Recent investigations show improved performance using LiBF4 containing electrolytes [5].In this study formation and dynamics of the SEI layer derived from LiClO4 and LiBF4 containing electrolytes were examined in operando using grazing incidence wide-angle X-ray scattering (GI WAXS) in a synchrotron setup. The use of GI WAXS provides a time resolution in the seconds range, enabling the identification of transient species and fluctuations of deposits on the electrode surface.Different SEI species and their dynamics have been observed and correlated with electrolyte composition and faradaic efficiency towards ammonia. Figure 1a) shows the dynamic behavior of diffraction features attributed to Li and LiF in an experiment using LiBF4 as electrolyte salt. In Figure 1b) the development of the corresponding peak intensities is shown and correlated with the start and end of chronopotentiometry (CP), used to study the cell. LiF was formed from the decomposition of LiBF4 and accumulated on the working electrode, showing that LiF is a significant component of the SEI in fluoride-containing electrolytes. Furthermore, metallic Li was found to be more stable when LiBF4 was utilized as electrolyte salt, in contrast to LiClO4. This finding suggests that the SEI derived from LiF has different mass transport characteristics and is more robust than that derived from LiClO4. This may explain the previously reported superior performance of electrolytes containing LiBF4 [5].

Effect of TiO2 nanoparticles on the corrosion resistance, wear, and antibacterial properties of microarc oxidation coatings applied on AZ31 magnesium alloy

Microarc oxidation (MAO) is a surface modification process that is often adopted to produce a hard oxide layer on magnesium alloys to improve their corrosion resistance and wear resistance. However, under continual corrosion, the characteristic porous microstructure of MAO coatings provides diffusion paths that allow corrosive species to penetrate the coatings and react with the substrate underneath. Therefore, the present study investigated the modification of the microstructure of MAO coatings when adding TiO2 nanoparticles to a fluoride-containing silicate electrolyte system under the bipolar current mode. The results indicate that as the concentration of TiO2 nanoparticles increased, the quantity of rutile (TiO2) incorporated in the MAO coating increased, which led to the coating exhibiting an elevated hardness and a color change from white to gray-black. When the TiO2 concentration in the electrolyte was increased from 2.5 to 7.5 g/L, the total impedance of the MAO coating in electrochemical impedance spectroscopy analysis increased from 107 to >600 kΩcm2, which indicates that the corrosion resistance of the coating increased with the number of defects on it decreased. The addition of TiO2 enhances the photocatalytic properties of MAO coatings. However, excessive addition of TiO2 can lead to an adverse effect on the wear resistance of MAO coatings.

Structure, morphology and phase composition of anodized titanium dioxide nanotubes loaded with Pt and Pd

This study focuses on titanium dioxide (TiO2) nanotubes on solid substrates, which may find wide applications as photocatalysts and catalysts. To address the need for more stable and active electrocatalysts with reduced noble metal content, the study explores TinO2n–1 suboxides as promising substrates for the electrocatalysts. Notably, the addition of water in the fluoride-containing electrolyte plays a critical role in shaping the morphology of TiO2 nanotubes, leading to the formation of ordered structures under specific water concentration conditions. The study also examines the effects of platinum and palladium deposition on TiO2 nanotubes, enhancing their surface crystallinity and structural arrangement. The presence of an unidentified phase, possibly titanium hydride, is observed in certain samples. The findings highlight the potential of TiO2 nanotubes as efficient electrocatalysts and the influence of water content and substrate choice on their properties, opening up new avenues for advanced applications in various fields.

Multi-method characterization of anodic oxidation of a titanium alloy in fluoride-containing electrolytes

Multi-method characterization of anodic oxidation of a titanium alloy in fluoride-containing electrolytes

Highly enhanced photocatalytic activity of nanotubular Fe2O3/Fe2WO6 nanocomposite film formed by anodizing FeW alloy

This study demonstrates the highly enhanced photocatalytic activity of Fe2O3 nanotubes by making a heterojunction with Fe2WO6 and doping of W6+ ions into the α-Fe2O3 phase for water splitting and decomposition of an organic pollutant. Both nanotubular and compact Fe2O3/Fe2WO6 films are formed by anodizing the magnetron-sputtered Fe-9 at% W alloy in fluoride-containing ethylene glycol electrolyte and subsequent annealing at 450 °C. Their photocatalytic activities are compared with W-free nanoporous Fe2O3 film formed on high-purity Fe. The nanotubular Fe2O3/Fe2WO6 film exhibits a markedly enhanced photoelectrochemical oxygen evolution reaction and photocatalytic degradation of methylene blue compared with the nanoporous W-free Fe2O3 film. Because of the larger surface area, the nanotubular Fe2O3/Fe2WO6 film reveals further increased activity than the compact Fe2O3/Fe2WO6 film. The STEM study discloses the dispersion of Fe2WO6 nanoparticles in the α-Fe2O3 phase after annealing at 450 °C, and XRD analysis shows the possible doping of W species into α-Fe2O3. The formation of the W-doped α-Fe2O3/Fe2WO6 heterojunction likely promotes the separation of photogenerated electron-hole pairs, enhancing photocatalytic activity. Hence, the nanotubular Fe2O3/Fe2WO6 film is a possible candidate for visible-light-driven photocatalysts.

The effect of anodizing temperature on the oxygen evolution reaction activity of anodized FeNiCo alloy in alkaline electrolyte

This study investigates the oxygen evolution reaction (OER) activity of the FeNiCo alloy anodized at several temperatures in a fluoride-containing ethylene glycol electrolyte. When the alloy is anodized at 10 V, the OER activity in KOH electrolyte is highly enhanced on the alloy anodized at 15 and 20 °C, at which non-uniform film growth proceeds. The fraction of the locally thick film regions increases with anodizing time, enhancing the OER activity. Only thin porous films are formed at ≥30 °C even though the anodizing current is high because of the promotion of film dissolution at high electrolyte temperatures. Because of the thickness limitation, the OER activity is relatively low when the anodic films are formed at ≥30 °C. A good correlation is found between the OER activity and the electric double-layer capacitance; thicker porous anodic films enhance the OER activity. The anodic film formed at each temperature consists of a rutile-type (FeNiCo)F2 phase, but in KOH electrolyte, it is converted readily to an oxyhydroxide phase, which is OER active. A ∼2 μm-thick film obtained under the optimum anodizing condition reduces the overpotential of OER to 245 mV at 10 mA cm–2 in 1.0 mol dm–3 KOH electrolyte.

Antibacterial properties of laser-encapsulated titanium oxide nanotubes decorated with nanosilver and covered with chitosan/Eudragit polymers.

To provide antibacterial properties, the titanium samples were subjected to electrochemical oxidation in the fluoride-containing diethylene glycol-based electrolyte to create a titanium oxide nanotubular surface. Afterward, the surface was covered by sputtering with silver 5nm film, and the tops of the nanotubes were capped using laser treatment, resulting in an appearance of silver nanoparticles (AgNPs) of around 30nm in diameter on such a modified surface. To ensure a controlled release of the bactericidal substance, the samples were additionally coated with a pH-sensitive chitosan/Eudragit 100 coating, also exhibiting bactericidal properties. The modified titanium samples were characterized using SEM, EDS, AFM, Raman, and XPS techniques. The wettability, corrosion properties, adhesion of the coating to the substrate, the release of AgNPs into solutions simulating body fluids at different pH, and antibacterial properties were further investigated. The obtained composite coatings were hydrophilic, adjacent to the surface, and corrosion-resistant. An increase in the amount of silver released as ions or metallic particles into a simulated body fluid solution at acidic pH was observed for modified samples with the biopolymer coating after three days of exposure avoiding burst effect. The proposed modification was effective against both Gram-positive and Gram-negative bacteria.

Anodic growth of Ta-Ti-O nanotube arrays on Ta/Ti6Al4V alloy layer

Anodizing behavior of Ta/Ti6Al4V (Ta/TC4) alloy layer with scalelike micro/nano-textured surface and (α' + β) microstructure in fluoride-containing ethylene glycol electrolyte has been investigated. Ta/TC4 was prepared by plasma surface Ta-alloying of TC4 alloy. In anodization, by turning applied voltage, time, H2O concentration, and NH4F concentration, Ta-Ti-O nanotube arrays/nanopore arrays (NTAs/NPAs) with different thicknesses and diameters can be grown on Ta/TC4 surface. Higher voltage, higher NH4F concentration, and longer anodization time are conducive to forming NTAs rather than NPAs. The excessive H2O accelerates the chemical dissolution of NTAs and leads to uneven distribution of NTAs diameter and decline of thickness. The NPAs formed at the initial stage of anodization will be replaced by NTAs retaining the (α' + β) characteristic of Ta/TC4 with anodization continues, so does the scalelike micro/nano-textured structure on Ta/TC4 surface. The electrochemical tests in the DMEM solution reveal the superior corrosion resistance of Ta/TC4 than TC4. However, Ta-Ti-O NTAs show deteriorated corrosion resistance which may be ascribed to the large specific surface area and massive lamellar gaps between Ta-Ti-O NTAs.

Surface characterization of TiO2 nanotube arrays produced on Ti6Al4V alloy by anodic oxidation

A highly well-ordered TiO2 (titanium dioxide) nanotube arrays were prepared at different voltages (20 V, 30 V, 40 V, 50 V, 60 V, 80 V and 100 V) for 60 min on Ti6Al4V (titanium alloy) substrate in the ammonium fluoride-containing ethylene glycol electrolyte by a DC (direct current) power supply through AO (anodic oxidation) technique. The anatase-based TiO2 phases were observed at post-heat treatment although the titanium phase was obtained on TiO2 nanotube surfaces at post-production AO technique. Up to 40 V, the nanotube structures were uniformly distributed and the surface roughness increased. The roughness and thickness of the nanotube surfaces were measured as 7.57 nm – 47.08 nm and 0.18 μm – 0.83 μm, respectively. The nanotube array surfaces indicated hydrophobic properties compared to uncoated titanium surface. The hardness and elastic modulus of plain titanium alloy and TiO2 nanotube surfaces were investigated under a Berkovich indenter by a nanoindentation test device. The elastic modulus values of TiO2 nanotube surfaces were close to bone structure with respect to plain Ti6Al4V substrate. Moreover, hardness values of nanotube surfaces were measured between ~203.7 MPa and ~ 932.2 MPa. The adhesion properties such as critical load of the nanotube surfaces were investigated by micro scratch tester. Adhesion strengths of the nanotube surface were obtained between ~2.2 N and ~4.6 N.

Corrosion mechanism of titanium dioxide nanotubes in Ringer's solution

Titanium dioxide nanotubes (TNTs) layer fabricated via anodizing process in fluoride-containing polar organic electrolyte. The corrosion mechanism of pure Ti and the TNTs immersed in Ringer's solution was investigated and compared using the electrochemical impedance spectroscopy (EIS) method. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis were also used to investigate the TNTs. Results showed better corrosion resistance of TNTs compare to the bare Ti. An electrical equivalent circuit (EEC) model including three time-constant was proposed to fit the experimental data obtained for the TNTs. This behavior is attributed to the tube’s wall composition which is consisted of two different regions including an outer region (OR) of the tube and a C- and F-rich inner region (IR). EDX analysis and AT-FTIR spectrum confirmed the presence of C and F

Birhythmicity, intrinsic entrainment, and minimal chimeras in an electrochemical experiment.

The coexistence of limit cycles in a phase space, so called birhythmicity, is a phenomenon known to exist in many systems in various disciplines. Yet, detailed experimental investigations are rare, as are studies on the interaction between birhythmic components. In this article, we present experimental evidence for the existence of birhythmicity during the anodic electrodissolution of Si in a fluoride-containing electrolyte using weakly illuminated n-type Si electrodes. Moreover, we demonstrate several types of interaction between the coexisting limit cycles, in part resulting in peculiar dynamics. The two limit cycles exhibit vastly different sensitivities with respect to a small perturbation of the electrode potential, rendering the coupling essentially unidirectional. A manifestation of this is an asymmetric 1:2 intrinsic entrainment of the coexisting limit cycles on an individual uniformly oscillating electrode. In this state, the phase-space structure mediates the locking of one of the oscillators to the other one across the separatrix. Furthermore, the transition scenarios from one limit cycle to the other one at the borders of the birhythmicity go along with different types of spatial symmetry breaking. Finally, the master-slave type coupling promotes two (within the experimental limits) identical electrodes initialized on the different limit cycles to adopt states of different complexity: one of the electrodes exhibits irregular, most likely chaotic, motion, while the other one exhibits period-1 oscillations. The coexistence of coherence and incoherence is the characteristic property of a chimera state, the two coupled electrodes constituting an experimental example of a smallest chimera state in a minimal network configuration.

(Invited) Highly Active OER Electrodes Derived from Fe-Doped Ni Oxyfluoride Formed by Anodizing of NiFe Electrodeposits

Because of the limited availability of fossil fuels and pertinent global environmental issues, hydrogen production via electrochemical water splitting using renewable resources is an important alternative for future energy economies. For efficient alkaline water electrolysis to produce hydrogen, we need to develop highly active and durable electrocatalysts, particularly for oxygen evolution reaction (OER) because of high overpotential. Anodizing of metals and alloys is a promising means of directly fabricating an OER electrocatalyst on a metallic current collector. Binder-free electrocatalysts are readily formed by anodizing. Since Fe-doped Ni oxyhydroxides and hydroxides are recently reported as highly active OER electrocatalysts in alkaline media. Here, we report anodizing of NiFe electrodeposits in fluoride-containing ethylene glycol electrolyte and the conversion of resultant nanoporous Fe-doped Ni oxyfluoride to highly OER active hydroxide during OER (1).NiFe alloy films containing 7.2 – 21.1 at% Fe were obtained by electrodeposition on Ni substrate. Then, the alloys were anodized at 10 V in ethylene glycol electrolyte containing ammonium fluoride and water. The anodizing developed a porous layer consisting of crystalline NiF2 phase. XPS analysis revealed the presence of Fe and O as well as Ni and Fe. Thus, porous Fe-doped Ni oxyfluoride was developed by anodizing. The as-anodized NiFe alloys were initially less active for OER in 1.0 mol dm-3 KOH, but during potential cycling to OER potential, the OER activity was gradually enhanced. The enhancement was dependent upon Fe content, and the highest activity was obtained for the Ni-11.8 at% Fe. After activation, the porous layer was converted to amorphous hydroxide, and the fluoride content was highly reduced. Thus, anodically formed Fe-doped oxyfluoride on the NiFe electrodeposits is a suitable precursor of highly active OER electrocatalyst. The galvanostatic durability test exhibited the stable overpotential, disclosing the high durability of this electrocatalyst.Reference N. Yamada, S. Kitano, Y. Yato, D. Kowalski, Y. Aoki and H. Habazaki, ACS Applied Energy Materials, in press, doi.org/10.1021/acsaem.0c02362.

Self-Organized Multifrequency Clusters in an Oscillating Electrochemical System with Strong Nonlinear Coupling.

We study the spatiotemporal dynamics of the oscillatory photoelectrodissolution of n-type Si in a fluoride-containing electrolyte under anodic potentials using insitu ellipsometric imaging. When lowering the illumination intensity stepwise, we successively observe uniform oscillations, modulated amplitude clusters, and the coexistence of multifrequency clusters, i.e., regions with different frequencies, with a stationary domain. We argue that the multifrequency clusters emerge due to an adaptive, nonlinear, and nonlocal coupling, similar to those found in the context of neural dynamics.

Insights into the Intrinsic Creation of Heterojunction-Based Ordered TiO2 Nanotubes Obtained from the One-Step Anodic Oxidation of Titanium Alloys

To gain insights into how the presence of the second metal in Ti alloys affects the morphology, crystallinity, chemical structure, and photocatalytic/photoelectrochemical activity of mixed-oxide nanotubes (TiO2–MxOy), five systems were electrochemically synthesized in a fluoride-containing ethylene glycol-based electrolyte by using Ti alloys, with 10 wt % of the second metal, as precursors. The level of self-organization and -orientation in the obtained samples followed the order: Ag < Cu < Mn < V < Co, being the Ti90Ag10_40V sample, the only presenting single Ag nanoparticles embedded in the oxide layer. The highest photocatalytic and photoelectrochemical activities were reported for the Ti90Ag10_40V sample, which can be attained to the heterojunction between the oxides, the formation of single Ag nanoparticles, and the high level of self-organization. Apparently, the intrinsic heterojunction between the oxides depends on the still-needed-to-be-estimated stability of the oxides produced during the anodization of each system.

(Invited) Multiscale Modelling of Titanium Oxide Growth and Dissolution in Fluoride-Containing Electrolytes

Nanostructured valve metal oxides have been extensively investigated due to the variety of applications related to their valuable chemical and physical properties, including mechanical and thermal resistance, catalytic and photocatalytic activity, and biocompatibility. The technology to form well defined porous anodic oxides on titanium will strongly benefit from an optimization of the process parameters based on deterministic modelling. The relatively large number of adjustable parameters often precludes the unambiguous interpretation of steady-state and transient electrochemical data in terms of a unique kinetic model.An approach to overcome this ambiguity by parameterization of the models of barrier film growth [1, 2] on Ti and its alloys (Figure 1) using a combination of in-situ EIS in a large frequency range, dynamic high-frequency impedance and photocurrent measurements, combined with ex-situ characterization of the oxides by X-ray photoelectron spectroscopy (XPS) is presented. Additional estimates of kinetic parameters are derived from density functional theory (DFT) modelling of adsorption of water and fluoride on Ti and TiO2 [3, 4]. Ways to extract information on the rate-limiting steps of the process of passive film formation, growth and restructuring are described and the significance of the obtained parameters for the kinetics of barrier film growth and dissolution is outlined. Stancheva, M. Bojinov, Electrochim. Acta 78, 65 (2012).Bojinov, I. Betova, V. Karastoyanov, Electrochim. Acta 344, 136137 (2020).Stancheva, B. Diawara, F. Lebreau, M. Bojinov, J. Electrochem. Soc. 161, E3188 (2014).Betova, M. Bojinov, V. Karastoyanov, M. Stancheva, Computational Mater. Sci.171, 109260 (2020). Acknowledgments: The financial support of the National Science Fund of Bulgaria under contract ДН19/3-2017 „Modeling, optimization and modification of nanoporous anodic oxide films as catalysts for water photoelectrolysis“is gratefully acknowledged. Figure 1

(Invited) Anodic Growth of Oxide Nanotube Layers on Titanium Alloys

Self-organized nanoporous and nanotubular oxide layers can be synthesized on a wide range of transition metals by anodization in fluoride-containing electrolytes. In particular, self-organized TiO2 nanotubular layers have attracted intense attention due to their diverse fields of applications. Anodization of titanium alloys also has been reported, in order to utilize self-organized oxide nanostructures to biomedical titanium alloys for improved biocompatibility and bone conductivity and also to introduce the other element than titanium into TiO2 nanotube matrix for enhanced performance in various applications. In this presentation, we show anodization of titanium alloys, focusing on composition and microstructure of the alloys.