Fluoride-based Electrolyte Research Articles

Long-term degradation performance and biocompatibility of plasma electrolytic oxidation coated Mg-0.45Ca-xZn alloys prepared using a fluoride-based electrolyte

In this work, plasma electrolytic oxidation (PEO) with a fluoride-based electrolyte was performed on Mg-0.45Ca-xZn (x = 0, 0.45, 1.0, 2.0 wt%) alloys to enhance their degradation properties. These PEO coatings were composed of a porous layer and a barrier layer for alloys with Zn ≥ 0.45 wt%, while the coating on Mg-0.45Ca alloy was consisted of a dense inner layer and a porous outer layer. The content of Zn in the alloy has no significant effect on the hardness, adhesion strength and composition of the PEO coating. All coated-Mg-0.45Ca-xZn samples in this work showed decent anti-degradation properties in the simulated body fluid (SBF). Among them, PEO coatings on Mg-0.45Ca-1.0Zn and Mg-0.45Ca-2.0Zn alloys could provide excellent protection to the substrate over 3 months and exhibited good biocompatibility with Grade 0–1 of cytotoxicity, which were expected to be applied in the biomedical field. Besides, PEO process in this work enabled a high-quality coating to be formed on Mg–Ca–Zn bone screws.

Silver nanoparticles doped poly(3, 4-ethylene dioxythiophene) on titania nanotubes for orthopaedic application

This research is focused on analysing the behaviour of electropolymerized PEDOT doped with AgNPs on TNTA surface (AgNPs-PEDOT/TNTA) to improve corrosion resistance, antibacterial activity and biocompatibility that suits the orthopaedic field of research. Electrochemical anodization was performed to develop a uniformly ordered TNTA in a fluoride-based electrolyte. AgNPs-PEDOT was evenly incorporated on anodized TNTA surface using electropolymerization process. The morphological images of anodized TNTA and AgNPs-PEDOT/TNTA were captured using HR-SEM analysis. XRD, XPS and ATR-FTIR spectroscopic analyses were performed to determine the crystallinity, chemical entities and functional groups of AgNPs-PEDOT/TNTA. The corrosion prevention of AgNPs-PEDOT/TNTA was examined by electrochemical impedance spectroscopy (EIS), scanning electrochemical microscopy (SCEM) and potentiodynamic polarization studies. In vitro immersion studies of AgNPs-PEDOT/TNTA were systematically carried out for 7, 14 and 21 days in Hanks’ solution. The viability and proliferation of MG63 cells with and without silver nanoparticles doped PEDOT were assessed using the MTT assay. The involvement of silver nanoparticles in the antibacterial performance of PEDOT electropolymerized TNTA was examined using gram-positive and gram-negative bacteria. The presence of AgNPs in PEDOT plays a favourable role in long term implantation with excellent antibacterial and biochemical activity.

Fluoride-free synthesis of anodic TiO2 nanotube layers: a promising environmentally friendly method for efficient photocatalysts.

TiO2 nanotube (TNT) layers are generally prepared in fluoride-based electrolytes via electrochemical anodization that relies on the field-assisted dissolution of Ti metal forming nanoporous/nanotubular structures. However, the usage of fluoride ions is considered hazardous to the environment. Therefore, we present an environmentally friendly synthesis and application of TNT layers prepared in fluoride-free nitrate-based electrolytes. A well-defined nanotubular structure with thickness up to 1.5 μm and an inner tube diameter of ∼55 nm was obtained within 5 min using aqueous X(NO3)Y electrolytes (X = Na+, K+, Sr2+, Ag+). For the first time, we show the photocatalytic performance (using a model organic pollutant), HO˙ radical production, and thorough characterization of TNT layers prepared in such electrolytes. The highest degradation efficiency (k = 0.0113 min-1) and HO˙ radical production rate were obtained using TNT layers prepared in AgNO3 (Ag-NT). The intrinsic properties of Ag-NT such as the valence band maximum of ∼2.9 eV, surface roughness of ∼6 nm, and suitable morphological features and crystal structure were obtained. These results have the potential to pave the way for a more environmentally friendly synthesis of anodic TNT layers in the future using the next generation of fluoride-free nitrate-based electrolytes.

Facile one-pot preparation of Ti3+, N co-doping TiO2 nanotube arrays and enhanced photodegradation activities by tuning tube lengths and diameters

Abstract Well aligned Ti3+, N co-doping TiO2 nanotube array (NTA) film on Ti foil substrate has been successfully prepared by anodic oxidation method using fluoride-based electrolyte in ethylene glycol solution with 3 vol.% H2O, followed by calcination at 400 °C for 2 h. The TiO2 NTA exhibits high UV and visible light absorption capability with a narrow band gap of 2.74 eV, which is attributed to Ti3+ self-doping, probably induced by N doping. The Ti3+ site was found to be 0.46 eV below the conduction band and the N 2p surface state is 1.04 eV above the valence band. Effects of the preparation parameters on tube lengths and diameters of the TiO2 NTAs were investigated based on series of control experiments. The photocatalytic activities of two series of Ti3+, N co-doping TiO2 NTAs with different tube lengths (similar diameter) and different diameters (similar tube length) were assessed by means of photodegradations of rhodamine B (RhB) solutions under simulated sunlight irradiation. The results show that the photocatalytic activity of the Ti3+, N co-doping TiO2 NTA can be enhanced by simply tuning the tube length and diameter in an appropriate range. The photocatalytic reaction mechanism was proposed, based on series of radical-trapping experiments. The results show that ⋅OH is the major active species for photodegradation of RhB.

A new simple approach to prepare rare-earth metals-modified TiO2 nanotube arrays photoactive under visible light: Surface properties and mechanism investigation

Abstract Application of Ti90RE10 alloys (RE = Ho, Er, Nd, Y, Ce, Tm) as a working electrode, instead of Ti pure foil in anodic oxidation in a fluoride-based electrolyte, resulted in formation of well-ordered nanotubes made of TiO2 and RE2O3 mixture, which could be efficiently used for pollutant removal from water and air phase upon UV and visible irradiation and easily separable from the reaction mixture to recycle. The as-prepared NTs were characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), diffuse reflectance spectroscopy (DRS), luminescence spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the synthesized samples was investigated used phenol and toluene as a model compounds to follow degradation kinetics. The influence of the RE metals on the photoelectrochemical performance of modified TiO2 NTs was investigated. Surface morphology analysis showed formation of uniform and vertically oriented NTs structures with an open tube top and smooth walls. The results of EDX, XRD and XPS analysis proved that RE ions exist as surface compounds (RE3+ oxides). Visible light induced photoactivity (both photocatalytic and photoelectrochemical) is mostly pronounced in the presence of nanotubes obtained by anodic oxidation of Ti90Ho10 alloy. These nanotubes are able to degrade toluene in the gas phase even by irradiation of low-powered light source, such as light emitting diodes (λmax = 465 nm). Photocatalytic test of phenol degradation in the presence of scavenger indicates that photogenerated electrons and superoxide radicals play a meaningful role in the photocatalytic degradation of pollutants under visible irradiation. In addition, the photoelectrochemical tests performed under the influence of visible light irradiation confirmed that the RE-modification of TiO2 NTs caused a significant increase of photocurrent (up to 10 times higher). The new and original results on the preparation of RE2O3/TiO2 nanotubes obtained in one step anodic oxidation and photocatalytic activity in aqueous and gas phases represent an important contribution then will benefit photocatalytic surfaces preparation technologies.

The microstructure and mechanical properties of titanium dioxide nanotubes synthesized in the fluoride-based electrolyte

Titanium is one of the biomaterials commonly used for prosthetic devices due to its bio-inert properties. The discovery of titanium dioxide nanotubes (TDNTs) has created a great interest in medical applications such as dental and orthopedic implants. The synthesizing of TDNTs can produce different morphology, sizes and mechanical properties of the nanotubes – depending on the applied method. In this study, an electrochemical anodization method was used for synthesizing the TDNTs. A 100 ml mixture of 99% of ethylene glycol (EG), 1% of deionized water and 1 wt.% of ammonium fluoride (NH4F) was used as the electrolyte of the electrochemical cell. Parameters such as anodization time and the voltage applied were used to alter the morphology of the TDNTs formed. The produced nanotubes were analyzed and characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and microhardness tester. The SEM results showed that the formed diameter of nanotubes was mainly affected by the anodizing voltage. The wall thickness was found to be irrelevant to the parameters conducted in this study. The diameter of nanotubes formed with an anodizing voltage of 30, 45 and 60 V have the diameters ranging from 46 nm to 71 nm. All of the TDNTs samples formed have a wall thickness between 11 nm and 13 nm. With the use of EG and NH4F as an electrolyte, the array of TDNTs with honeycomb structure was formed. In general, hardness test showed that the hardness of the nanotubes was inversely proportional with the anodizing time. The anodizing voltage only has little effect on the hardness of the nanotubes. The nanotubes formed by 60 V have about 3 to 5% lower hardness compared to those formed by 30 V for different anodizing times.

Optical interferometry and photoacoustics as in-situ techniques to characterize the porous silicon formation: a review

Abstract Porous Silicon (PSi) is a groundbreaking material because its physicochemical properties can be customized through its porosity. This means that monitoring and control of the growing parameters allows the fabrication of PSi-based systems with controlled properties. Interferometry and photoacoustics are non -invasive, non - contact, real-time (in-situ) techniques used to characterize the phenomena that takes place during the formation of PSi. This work presents the mathematical and experimental aspects related to the implementation of the techniques mentioned above, which are meant to characterize the PSi growth in fluoride-based electrolyte media. These methods can determine macroscopic parameters of PSi such as thickness, porosity profile trough effective medium approximation (EMA), refractive index, etching rate, and RMS roughness under 100 nm. The monitoring ability of these techniques is strongly dependent on the wavelength of radiation used. However, it is possible to monitor thickness from λ0/4 to ∼ 1/ α0, where α0 is the optical absorption coefficient at λ0. Also, these techniques can be implemented as feedback control on the etching processes for fabrication of PSi.

Manufacturing of Dysprosium-Iron Alloys by Electrolysis in Fluoride-Based Electrolytes. Electrolysis in a Laboratory-Scale Cell

Electrolytic production of light rare earth elements and rare earth alloys with transition elements takes place in a fluoride-based electrolyte using rare earth oxides as raw material. The optimization of this method, mainly in terms of the energy efficiency and environmental impact control, is rather challenging. Anode effects, evolution of fluorine-containing compounds and side cathode reactions could largely be minimized by good control of the amount of rare earth oxide species dissolved in the fluoride-based electrolyte and their dissolution rate. The Dy2O3 feed rate needed for stable cell operation was studied by following up the anode voltage and gas analysis. On-line analysis of the cell off-gases by FTIR showed that the electrochemical reaction for the formation of Dy-Fe alloy gives mainly CO gas and that CF4 is starting to evolve gradually at anode voltages of ca. 3.25 V. The limiting current density for the discharge of the oxide ions at the graphite anode was in the range of 0.1 to 0.18 A cm−2 at dissolved Dy2O3 contents of ca. 1 wt pct. Modeling of the laboratory cell reactor was also carried out by implementing two models, i.e., an electrical model simulating the current density distribution at the electrodes and a laminal bubbly flow model that explains the electrolyte velocity induced by gas bubble production at the anode.

Manufacturing of Dysprosium-Iron Alloys by Electrolysis in Fluoride-Based Electrolytes: Oxide Solubility Determinations

Electrolytic production of light rare earth elements and alloys takes place in a fluoride-based electrolyte using rare earth oxides as raw material. The optimization of this method, mainly in terms of the energy efficiency and environmental impact control, is rather challenging. Anode effects, evolution of fluorine-containing compounds, and side cathode reactions could largely be minimized by a good control of the amount of rare earth oxide species dissolved in the fluoride-based electrolyte and their dissolution rate. The oxide content of the fluoride melts REF3-LiF (RE = Nd, Dy) at different compositions and temperatures were experimentally determined by carbothermal analysis of melt samples. The highest solubility values of oxide species, added as Dy2O3 and Dy2(CO3)3, were obtained to be of ca. 3 wt pct (expressed as Dy2O3) in the case of the equimolar DyF3-LiF melt at 1323 K (1050 °C). The oxide saturation values increased with the amount of REF3 present in the molten bath and the working temperature.

High Surface Area NanoCOT Catalysts on Ni Foam Framework for Efficient Water Electrolysis

Carbon doped nanostructured TiO2 electrodes (NanoCOT) are prepared by anodizing titanium in a fluoride-based electrolyte followed by thermal annealing in an atmosphere of methane and hydrogen in the presence of Fe precursors. NanoCOT are highly conductive and contain more than 1 × 10^9 /cm2 of nanowires or nanotubes to enhance their double layer charge capacitance and electrochemical stability. This electrode material also exhibits enhanced oxygen evolution reaction (OER) catalytic activity in alkaline solution, providing a current density of 1.33 mA/cm2 at an overpotential of 0.42 V. This OER current density of a NanoCOT electrode is about 4 times higher than an oxidized Ir electrode and 15 times higher than a Pt electrode because of its nanostructured high surface area and favorable OER kinetics. The enhanced catalytic activity of NanoCOT is attributed to the presence of a continuous energy band of the titanium oxide electrode with predominantly reduced defect states of Ti (e.g., Ti^1+, Ti^2+, and Ti^3+) formed by chemical reduction with hydrogen and carbon. The OER performance of NanoCOT can also be further enhanced by decreasing its overpotential by 150 mV at a current density of 1.0 mA/cm2after coating its surface electrophoretically with 2.0 nm IrOx nanoparticles. Further improve can be obtained by using porous substrate such as Ni and Ti foam.

Nanostructured WO3 photoanodes for efficient water splitting via anodisation in citric acid

We report the production of nanostructured WO3 photoanodes for solar water splitting produced via anodisation using for the first time citric acid, a safer and more environmentally friendly alternative to fluoride-based electrolytes.

Fabrication and photo-catalytic properties of pure and cation doped TiO2 nanotube arrays

The highly ordered and vertically oriented pure TiO2 nanotube arrays (TONTAs) are synthesized by anodic oxidation of titanium foils in fluoride-based electrolytes under different temperatures and voltages. Then TONTAs are treated at different temperatures. X-ray diffraction (XRD) patterns show that its structure is anatase at 500 °C with the temperature increasing to 700 °C, the most of anatase will transform to rutile, the nanotube arrays form granular titanium oxide, while at 900 °C, anatase will transform to rutile completely. The experimental results of photocatalytic degradation of methylene blue solution show that the cobalt-manganese co-doped TONTAs have good photocatalytic degradation efficiency.

Templating Using Self‐Aligned TiO2 Nanotube Stumps: Highly Ordered Metal and Polymer Bumped Arrays

Over the past decades, self-ordered nanostructured oxides, prepared by simple but optimized anodization of a metal electrode, have attracted great interest. In 1995, Masuda et al. reported first on the growth of highly ordered alumina structures by anodizing Al under a set of optimized electrochemical conditions. Meanwhile, a wide range of metals and their alloys have been found to form highly ordered porous or tubular oxide structures. Key for this was the introduction of fluoridebased electrolytes that allow anodic oxide formation, and at the same time mild permanent oxide etching by fluoride complex formation. As a result, not only typical valve metals such as Ti, Nb, and Ta but also Fe, V and Co were over the years anodized to self-organized porous or tubular structures. Particularly the formation of highly ordered TiO2 nanotube layers found wide resonance, as TiO2’s functional features can find broad applications, for example, in dye-sensitized solar cells (DSSCs), 9] photocatalysis, 11] memristors, biomedicine, and sensors. Depending on the different application requirements, the oxide nanotube layers can be prepared with different thicknesses, individual tube diameters, with various morphology variations, and crystallinity. The nanotube layers usually show a high degree of lateral ordering for longer anodization times, that is, that lead to several mm to several 10 mm tube lengths. For many applications, for example, solar cells or photocatalysis, these tubes suit their purpose well, as several mm in length typically correspond to an optimum performance. Even though high-aspect-ratio tubes are occasionally reported to be of use for templating (e.g. polymers or metals), for these short-circuiting problems may occur for cathodic electrodeposition, and the deposit filling is still far from being ideally conformal. However, recently we reported on the formation of extremely well-ordered arrays of short TiO2 nanotubes. These TiO2 nanotube stumps (TiNTS) form when Ti is anodized in a HF containing concentrated phosphoric acid electrolyte and have a short aspect ratio (<5) with layer thicknesses of ~200 nm and individual tube diameters of ~80 nm. In the present work, we show that these TiO2 nanotube stumps with their open and short aspect morphology are ideal for conformal filling of the tubes by sputter deposition, and thus ideally serve as a template. We demonstrate this with a range of noble metals and even more using cast filling with a molten polymer. As a result, in other words, these structures can be exploited for the fabrication of highly ordered replica structures—which can be used, for example, to create highly defined corrugated surfaces with a virtually ideal hexagonal ordering of metal (or polymer) bumps. Figure 1 shows SEM images of the titania nanotube layers (TiTNS) used in this work, that is, layers that were formed by anodizing Ti in a HF/H3PO4 electrolyte for 2 h. From the SEM top views (in Figures 1a and c) it is clear that not only do

Highly Conductive Nanostructured C-TiO2 Electrodes with Enhanced Electrochemical Stability and Double Layer Charge Storage Capacitance

The present work reports the structural and electrochemical properties of carbon-modified nanostructured TiO(2) electrodes (C-TiO(2)) prepared by anodizing titanium in a fluoride-based electrolyte followed by thermal annealing in an atmosphere of methane and hydrogen in the presence of Fe precursors. The C-TiO(2) nanostructured electrodes are highly conductive and contain more than 1 × 10(10) /cm(2) of nanowires or nanotubes to enhance their double layer charge capacitance and electrochemical stability. Electrogenerated chemiluminescence (ECL) study shows that a C-TiO(2) electrode can replace noble metal electrodes for ultrasensitive ECL detection. Dynamic potential control experiments of redox reactions show that the C-TiO(2) electrode has a broad potential window for a redox reaction. Double layer charging capacitance of the C-TiO(2) electrode is found to be 3 orders of magnitude higher than an ideal planar electrode because of its high surface area and efficient charge collection capability from the nanowire structured surface. The effect of anodization voltage, surface treatment with Fe precursors for carbon modification, the barrier layer between the Ti substrate, and anodized layer on the double layer charging capacitance is studied. Ferrocene carboxylic acid binds covalently to the anodized Ti surface forming a self-assembled monolayer, serving as an ideal precursor layer to yield C-TiO(2) electrodes with better double layer charging performance than the other precursors.

N-Doped TiO&lt;sub&gt;2 &lt;/sub&gt;Nanotubes and its Photocatalytic Activity under UV Light

We review the fabrication and properties of highly ordered TiO2 nanotube arrays made by anodic oxidation of titanium in fluoride-based electrolytes. The N-doped TiO2 nanotubes were prepared by changing the composition of electrolyte. TiO2 nanotubes and N-doped TiO2 nanotubes characterized by various analytical methods, such as scanning electron microscope (SEM) and X-ray photoelectron emission spectroscopy (XPS). The photocatalytic efficiency was evaluated by the photodegradation of Methyl orange solution under UV irradiation. It has been confirmed that the recombination rate of electron-hole pairs declined significantly. The higher visible light activity is due to the codoping of nitrogen.

Electrochemically Assisted Photocatalytic Oxidation of Methanol on TiO 2 Nanotube Arrays

The influence of an externally applied bias on photocatalytic performance of crystallized TiO2/Ti nanotubular electrode formed by anodization in fluoride-based electrolyte were investigated and compared to the behavior of multiporous TiO2 electrode. The photoelectrocatalytic oxidation behavior of methanol over the nanotubular electrode has been studied by measuring photocurrent response, potentiodynamic polarization spectroscopy and using electrochemical impedance spectroscopy (EIS). It was found that the photoelectrocatalytic oxidation and the charge transfer rate constant of reaction on TiO2/Ti nanotubular electrode can significantly be increased by applying electrochemical bias. Moreover, based on the results, it was found that the nanotubular TiO2 electrodes have considerably better performance in comparison with porous samples in photoelectrocatalytic performance.

Fabrication of self-organised highly ordered titanium oxide nanotube arrays by anodic oxidation and characterisation

Self-organised and vertically oriented titanium oxide nanotube array (TNTA) has been synthesised by potentiostat anodisation of Ti foil in fluoride-based electrolyte. By varying the anodisation voltage from 8 V to 24 V it was possible to gradually change the topologies of nanotubes. The size of TNTAs was measured using SEM images and also determined based on a non-destructive optical method. In addition, photoelectrochemical properties of nanotubular TiO2/Ti electrodes were examined by anodic photocurrent response, potentiodynamic polarisation measurements and electrochemical impedance spectroscopy. A general equivalent circuit model was proposed for photoelectrochemical system consists of anodic nanotubular TiO2 thin film electrode and test solution.

A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications

Abstract We review the fabrication, properties, and solar energy applications of highly ordered TiO 2 nanotube arrays made by anodic oxidation of titanium in fluoride-based electrolytes. The material architecture has proven to be of great interest for use in water photoelectrolysis, photocatalysis, heterojunction solar cells, and gas sensing. We examine the ability to fabricate nanotube arrays of different shape (cylindrical, tapered), pore size, length, and wall thickness by varying anodization parameters including electrolyte concentration, pH, voltage, and bath temperature, with fabrication and crystallization variables discussed in reference to a nanotube array growth model. We review efforts to lower the band gap of the titania nanotubes by anionic doping. Measured optical properties are compared with computational electromagnetic simulations obtained using finite difference time domain (FDTD). The article concludes by examining various practical applications of the remarkable material architecture, including its use for water photoelectrolysis, and in heterojucntion dye-sensitized solar cells.

Intermediate temperature fuel cells using alkaline and alkaline earth fluoride-based electrolytes

Investigation of fuel cell applications using fluoride electrolytes, especially alkaline and alkaline earth fluorides, is a very new subject. Actually, most of the fluorides can function as fuel cell electrolytes, but the alkaline and alkaline earth fluorides are specially interesting since in composite materials they exhibit excellent performance for fuel cell electrolytes. Among the alkaline earth fluoride fuel cells, the best performance was achieved thus far for the fuel cell using the NaF–BaF 2–Al 2O 3 electrolyte and LiNiO 2 anode supported technique; it reached a power density of about 110 mW cm −2 at a current density of 250 mA cm −2 at 750°C. Both proton and oxygen ionic conduction may exist in the alkaline earth fluorides since, during the fuel cell operation, water was observed at both the anode and the cathode, but, in most cases, water was formed at the cathode side mainly. Thus, proton conduction predominates, resulting in excellent fuel cell performances. A possible ionic transport mechanism is also discussed.

Fluoride-based electrolytes

Fluoride-based electrolytes