Fluoride-containing Electrolyte Research Articles

Photoeletrocatalytic Activity of a Cu2O-Loaded Self-Organized Highly Oriented TiO2 Nanotube Array Electrode for 4-Chlorophenol Degradation

Differently sized Cu2O nanoparticles have been assembled photocatalytically on the surface of self-organized highly oriented TiO2 nanotubes obtained by anodization of a Ti sheet in fluoride-containing electrolytes. X-ray diffraction analysis identifies an anatase structure and fine preferential orientation of (101) planes. The UV-vis absorption edge of the TiO2 nanotube arrays shift to lower energy after Cu2O loading. The composite array electrode exhibits a higher photovoltage response than the TiO2 powders directly deposited on a Ti sheet. The highest photoconversion efficiencies observed for the Cu2O-loaded electrode are 17.2% and 0.82% under UV light and visible light irradiation, respectively. Especially, the composite array electrode shows a higher efficiency than the nonloaded one for the photoelectrocatalytic decomposition of 4-chlorophenol. The improved photoeletrocatalytic activity of the TiO2/Cu2O composite array electrode is attributed to the synergistic effect of Cu2O nanoparticles and TiO2 nanotube arrays. The Cu2O nanoparticles could enhance the efficiency of photon harvesting and reduce the chances of electron-hole recombination by sending the electrons to the conduction band of TiO2 nanotubes. The accumulated electrons in the conduction band of TiO2 nanotubes would reduce oxygen to form peroxides for enhanced advanced oxidation. The byproducts were identified by high-performance liquid chromatography.

Influence of Surface Condition on Nanoporous and Nanotubular Film Formation on Titanium

The present study examines the influence of surface pretreatment on the evolution of nanotubular (T) and nanoporous (P) oxide film morphologies on titanium foil anodized potentiostatically, at , in fluoride-containing electrolyte. The foil was used in the untreated, mechanically/chembreically polished or preanodized conditions. The preanodization resulted in fluoride-free barrier films. Mainly T regions were produced on polished foil during subsequent anodizing in fluoride-containing electrolyte. In contrast, the P regions dominated on polished foil preanodized to . Both P and T regions resulted with rolled foil preanodized to or polished foil preanodized to . Raman spectroscopy disclosed anatase and rutile at P regions, whereas T regions are mainly amorphous. The P film thickened at a constant rate of , while the T film reached a limiting thickness of due to the relatively high solubility of the amorphous oxide. The presence of P film resulted in increased currents during anodizing. Furthermore, the results suggest that the formation of T film depends upon the access of ions to the titanium/film interface, resulting in the stabilization of a mainly amorphous film structure.

Nitrogen doped anodic TiO 2 nanotubes grown from nitrogen-containing Ti alloys

N-doped TiO 2 nanotubes were produced by anodization of a TiN alloy. The alloy was prepared to contain approximately 5 at.% of N from high-purity Ti and TiN powders using an arc-melting and consisted of a two-phase structure with different N-contents. Anodization of the alloy in fluoride-containing electrolyte results, under optimized conditions, in the growth of an ordered TiO 2 nanotube layer on both phases. On the N-rich phase significantly smaller nanotubes are grown while on the low N-concentration phase nanotubes with larger diameter were formed. However, XPS and photoelectrochemical measurements demonstrate successful nitrogen doping of the resulting nanotube layers, which leads to a significant visible photoresponse from this material.

Time-dependent growth of biomimetic apatite on anodic TiO 2 nanotubes

We report on the initial and later stages of apatite formation from simulated body fluid on titania with different surface morphologies (compact or nanotubular) and different crystal structures (anatase or amorphous). The nanotubular layers were fabricated by electrochemical anodization in fluoride-containing electrolytes. The study investigates the enhanced apatite deposition on titania nanotubes. In the initial stages of apatite growth, more nuclei are formed on the nanotubular surface than on flat compact TiO 2. While the crystallographic structure of the substrate plays a less important role than the morphology in the initial nucleation stages, it is of great importance in the later stages of apatite crystal growth. The nanotubular morphology combined with an anatase structure leads to the formation of apatite layers with a thickness of >6 nm in less than 2 days. No stable apatite layers can be observed on amorphous TiO 2 films, neither on compact nor on nanotubular substrates. XPS, FT-IR and XRD measurements reveal that carbonated hydroxyapatite (CHA) of low crystallinity is formed on annealed nanotubular and compact titania. Electrochemically grown and annealed TiO 2 nanotube arrays having anatase structure are expected to be a good precursor system for the formation of CHA and thus for the preparation of osseointegrative implants.

Anodic TiO[sub 2] Layer Conversion: Fluoride-Induced Rutile Formation at Room Temperature

The present work demonstrates that an amorphous anodic oxide on titanium can be converted to a micrometer thick nanoporous rutile layer upon extended polarization in a fluoride-containing electrolyte, whereas extended polarization in a fluoride-free electrolyte leads to a conversion to anatase. Titanium was preanodized in fluoride-free 1 M H 3 PO 4 for 10 min, then anodization was either continued for 5 h in the presence or absence of fluoride ions and the resulting layers characterized. Scanning electron microscopy, X-ray diffraction, and Raman and photocurrent spectroscopy confirm ion-selective structure conversion and extended layer growth at room temperature.

Formation of Vertically Oriented TiO2 Nanotube Arrays using a Fluoride Free HCl Aqueous Electrolyte

Described is the synthesis of TiO2 nanotube array thin films by anodization of Ti foil in an aqueous HCl electrolyte. This process represents an alternative electrolyte that can be utilized instead of fluoride-containing electrolytes. Nanotube arrays up to 300 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained by using a 3 M HCl aqueous electrolyte for anodization potentials between 10 and 13 V. An anodization voltage of up to 20 V could be used if it was increased stepwise with a resulting nanotube length of approximately 600 nm; however, the resulting nanotubes are not as well-ordered as those fabricated by a constant anodization voltage. The addition of a low concentration of H3PO4 as a buffering medium in the concentration range 0.01−0.1 M expands the anodization voltage up to 14 V but the architectures formed more closely resemble rods than tubes.

Photoactive Silicon-Based Nanostructure by Self-Organized Electrochemical Processing

A solar cell design is presented that allows energy conversion in solid-state photovoltaics as well as in photoelectrochemical and photoelectrocatalytic cells. The energy-converting structure uses a Schottky-type, (i.e., metallic) nanoemitter, prepared by an oscillatory (photo)electrochemical process. Silicon shows (photo)current oscillations in fluoride-containing electrolytes that form an oxide with interspersed nanopores. Spatially selective electrodeposition of Schottky-barrier metals into these pores produces the nanoemitter contacts. The electrolyte structure shows pronounced photoactivity in a photoelectrochemical cell. Light-induced hydrogen evolution is obtained for structures made with . The energy-band alignments of the structures are discussed and routes for preparation of efficient solid-state devices are outlined.

Fast migration of fluoride ions in growing anodic titanium oxide

The rapid inward migration of fluoride ions in growing anodic titanium oxide under a high electric field has been elucidated by anodizing a Ti–12 at% silicon alloy, where film growth proceeds at nearly 100% efficiency in selected electrolytes. Further, incorporated silicon species in the anodic film are immobile, acting as marker species. The migration rate of fluoride ions is determined precisely by three-stage anodizing, consisting of initial anodic film formation at a constant current density to 50 V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by reanodizing to 55 V in ammonium fluoride electrolyte and, finally, anodizing again in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by glow discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of the silicon marker species. The fluoride ions migrate inward at twice the rate of O 2− ions. Consequently, anodizing of titanium in fluoride-containing electrolytes develops a fluoride-rich layer that separates the alloy substrate from the anodic oxide, with eventual detachment of the film from the substrate.

Photoelectrochemical properties of N-doped self-organized titania nanotube layers with different thicknesses

The present work reports nitrogen doping of self-organized TiO2 nanotubular layers. Different thicknesses of the nanotubular layer architecture were formed by electrochemical anodization of Ti in different fluoride-containing electrolytes; tube lengths were 500 nm, 2.5 μm, and 6.1 μm. As-formed nanotube layers were annealed to an anatase structure and treated in ammonia environment at 550 °C to achieve nitrogen doping. The crystal structure, morphology, composition and photoresponse of the N-doped were characterized by scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and photoelectrochemical measurements. Results clearly show that successful N-doping of the TiO2 nanotubular layers can be achieved upon ammonia treatment. The magnitude of the photoresponse in ultraviolet and visible light is strongly dependent on the thicknesses of the layers. This effect is ascribed to recombination effects along the tube length.

TiO2–Nb2O5 Nanotubes with Electrochemically Tunable Morphologies

Made to measure: TiO2–Nb2O5 nanotubes are grown on a Ti–Nb alloy (Ti45Nb) by anodization in a fluoride-containing electrolyte (see micrograph), and their lengths and diameters can be controlled by adjusting the potential. The low rate of chemical dissolution of Nb2O5 in the electrolyte allows the formation of long nanotubes with regular morphology.

Hydroxyapatite growth on anodic TiO2 nanotubes

In the present work, we study the growth of hydroxyapatite formation on different TiO(2) nanotube layers. The nanotube layers were fabricated by electrochemical anodization of titanium in fluoride-containing electrolytes. To study various nanotube lengths, layers with an individual tube diameter of 100 nm were grown to a thickness of approximately 2 mum or 500 nm. The ability to form apatite on the nanotube layers was examined by immersion tests combined with SEM, XRD and FT-IR investigations. For reference, experiments were also carried out on compact anodic TiO(2) layers. The results clearly show that the presence of the nanotubes on a titanium surface enhances the apatite formation and that the 2-mum thick nanotube layer triggers deposition faster than the thinner layers. Tubes annealed to anatase, or a mixture of anatase and rutile are clearly more efficient in promoting apatite formation than the tubes in their "as-formed" amorphous state.

Self‐Organized High‐Aspect‐Ratio Nanoporous Zirconium Oxides Prepared by Electrochemical Anodization

The electrochemical formation of self-organized porous structures has recently attracted much attention due to the broad range of nanoscale applications that become accessible through nanopore arrays made of materials such as Al2O3, [1–12] Si, and InP. Particularly, the anodic oxidation of Al has been studied in detail and a variety of highly ordered porous oxide structures with a high-aspectratio pore geometry have been successfully prepared. In addition to direct applications, the nanostructures have also been employed as a template for the deposition and fabrication of secondary nanostructures. Applications include magnetic recording media, optical devices, functional electrodes, and waveguides. 7, 8] Recently we reported that under optimized electrochemical conditions, self-organized pore formation on Ti can be achieved to grow TiO2 nanotubes with aspect ratios of more than 20. Herein, we report the electrochemical fabrication of highly ordered porous zirconium oxide layers of nanotubular shape with very high aspect ratios (>100) by anodization of Zr. This is a clear improvement in comparison with other ongoing attempts. Key to achieving the high aspect ratio is an adjustment of the electrochemical parameters to an optimized competition between dissolution and formation of the oxide. Zirconium oxide is an important functional material that plays a key role as an industrial catalyst and catalyst support with particular acid catalysis properties. 22] Furthermore, zirconium oxide is known to have excellent ion-exchange properties that are exploited in various applications. Highsurface-area structures are often required to provide an enhanced performance. These structures are typically synthesized by sol–gel methods or colloid electrodeposition. A well-established fact in the literature is that when Zr or other so-called “valve metals” are anodized in most electrolytes, a compact oxide layer of up to several hundred nanometers thickness is formed. A unique feature in comparison to other valve metals is that the growth of these compact ZrO2 layers at room temperature in most electrolytes leads to a crystalline film. However, in electrolytes containing fluoride ions, the oxide formation process is fundamentally changed. Figure 1 shows SEM images of porous zirconium oxide layers observed after anodization in fluoride-containing electrolytes. Figure 1a shows the surface morphology, and

Determination of Oxygen in Gas Media Using a Gas-Diffusion Electrode Based on Manganese Dioxide

It was shown that manganese dioxide isolated from a fluoride-containing electrolyte can be used as an electrode material for determining oxygen in gas media.

Electrochemical Synthesis of Black Cobalt

The possibility of depositing black cobalt coatings from fluoride-containing electrolytes based on cobalt sulfate was studied.

Composition, Properties, and Electrochemical Behavior of Nonstoichiometric Manganese Dioxide Obtained in Fluoride-Containing Electrolytes

The composition, properties, and structure of manganese dioxide samples are studied. Deviation of the compound composition from stoichiometry affects the ionic component of the conductivity as well as catalytic and electrochemical activities of the compound. The higher the MnO2 deviation from stoichiometry, the better the electric parameters of prototype batteries with cathodes based on MnO2.

The voltage–time behaviour for porous anodizing of aluminium in a fluoride-containing oxalic acid electrolyte

A systematic investigation has been undertaken of voltage–time behaviour for galvanostatic porous anodizing of aluminium in 0.4 mol dm −3 H 2C 2O 4 modified by the addition of variable amounts of NH 4F, in circumstances of varying current density and stirring conditions. Both the concentration of NH 4F in the electrolyte and stirring of the fluoride-containing (FC) electrolyte appeared to affect the voltage–time response for anodizing aluminium, causing a decrease in the forming voltage. The voltage-reducing effect is further substantially enhanced while the solution is stirred. Despite a tenfold decrease in the anodizing voltage in the FC electrolyte, no apparent changes in pore morphology are determined from SEM observation. The phenomena described are explained by considering formation of FC electrolyte-derived species and their incorporation into growing oxide film.

The morphology, structure and mechanism of growth of chemical conversion coatings on aluminium

The morphology and structure of the chemical conversion coating developed on aluminium in an acid chromate/fluoride solution have been examined by transmission electron microscopy of stripped films and ultramicrotomed sections. The use of annealed, high purity aluminium specimens which had been electropolished and subsequently etched prior to the conversion coating treatment allowed a clear identification of the anodic and cathodic sites for the coating growth process. Thus, for the first time, it has been found that the morphology of the coating is strongly dependent on the crystallographic orientation of the substrate which, in turn, appears to control the preferential deposition of the hydrated chromium oxide at grain boundaries or cellular boundaries. Such boundaries predominantly contain the presumed flaw sites due to impurity segregation in the substrate. Based on these observations, the mechanism of conversion coating growth has been developed. Deposition of hydrated chromium oxide is observed at cathodic sites, associated with metal ridges developed on the aluminium surface as a result of the initial pretreatment. The anodic sites lie between the metal ridges, where the initial air-formed film is chemically dissolved in the fluoride-containing electrolyte. Dissolution of aluminium proceeds through the chemically thinned film, revealing scalloped regions between the regions of cathodically developed chromium oxide.