Fluoride-containing Electrolyte Research Articles

Photocurrent Enhancement of TiO2 Nanotubes Decorated with PbS Quantum Dots

In this paper, ordered TiO2 nanotubes (TNTs) were fabricated by electrochemical anodization in a fluoride-containing electrolyte solution. In order to extent absorption spectrum to visible light band, annealed TNTs with different PbS QDs contents, acting as light sensitizer, was loaded by ultrasonic-assisted dip coating approach. The QDs sensitized TNTs were characterized by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and UV-vis spectroscopy analysis. The average diameter of nanotube and PbS QD were approximately ~150nm and ~30nm, respectively. The TNTs decorated with PbS QDs present strong visible-light absorption.

Interfacial structure of the firmly adhered TiO2 nanotube films to titanium fabricated by a modified anodization

Abstract TiO2 nanotube films firmly adhered to Ti substrate were fabricated by a modified anodization, which involved in a traditional nanotubular structure formation stage in a fluoride-containing electrolyte and an additional fluoride (F−) ion sedimentation stage by addition of (CH3COO)2 Mg during the later period of anodization. The obtained results showed that the adhesion strength of the modified anodization derived TiO2 nanotube films, as characterized by critical load (Lc) using a scratch test, could be significantly improved from 3.2 ± 0.8 N for the traditional anodization derived one to a maximum value of 17.5 ± 1 N. There was no obvious change in surface morphologies of the amorphous TiO2 nanotube films prepared by the traditional and modified anodization, respectively. For the traditional anodization derived nanotube film, there was an ultra-thin fluoride-rich layer (about 15 nm in thickness) existing between the nanotube bottoms and the Ti substrate, weakening the adhesion strength. However, the modified anodization derived TiO2 nanotube film with the highest Lc value consisted of two layers: an outer nanotube layer and an inner compact layer adjacent to the Ti substrate composed of amorphous titanium oxide free of fluoride ions. Such structure of TiO2/Ti interface led to a considerable improvement in the adhesion strength between the nanotube film and the Ti substrate.

The influence of the electrolyte on chemical and morphological modifications of an iron sulfide thin film negative electrode.

The chemical and morphological modifications of FeS thin film as anode material for LiBs have been studied in detail in two classical electrolytes usually used in Li-ion batteries: 1 M LiClO4-PC and 1 M LiPF6-EC/DMC. The X-ray photoelectron spectroscopic (XPS) analysis evidenced the formation of a solid electrolyte interphase (SEI) that contains a more significant amount of inorganic salt residues formed in LiPF6-EC/DMC than in LiClO4-PC, which is likely to increase the ionic resistivity of the SEI, thus impeding the lithiation-delithiation in the first cycles while improving its reversibility. Ion depth profiles performed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) show volume expansion-shrinkage of the thin film leading to cracking and pulverization of the electrode material, which is also confirmed by scanning electron microscopy (SEM) analysis. The prolonged cycling results in penetration and accumulation of the electrolyte in a bulk electrode with accumulation of the inorganic species in the inner part of the SEI enhanced in a fluoride-containing electrolyte. Cycling in these two different electrolytes leads also to formation of two different electrode morphologies: with a compact electrode structure formed in LiClO4-PC and a foam-like, porous structure in LiPF6-EC/DMC. A model of this conversion-type thin film electrode modification based on these thorough spectroscopic and microscopic analyses induced by cycling in two different electrolytes is proposed.

Corrosion protection of iron using porous anodic oxide/conducting polymer composite coatings.

Conducting polymers (CPs), including polypyrrole, have attracted attention for their potential in the protection of metals against corrosion; however, CP coatings have the limitation of poor adhesion to metal substrates. In this study, a composite coating, comprising a self-organized porous anodic oxide layer and a polypyrrole layer, has been developed on iron. Because of electropolymerization in the pores of the anodic oxide layer, the composite coating showed improved adhesion to the substrate along with prolonged corrosion protection in a NaCl aqueous corrosive environment. The anodic oxide layers are formed in a fluoride-containing organic electrolyte and contain a large amount of fluoride species. The removal of these fluoride species from the oxide layer and the metal/oxide interface region is crucial for improving the corrosion protection.

Self-organization of TiO2 Nanobamboos by Anodization with Deep Eutectic Solvent

We report an unconventional electrochemical spontaneous oscillation of current in Ti anodization by adding deep eutectic solvent (DES) of 2–10% to a fluoride-containing electrolyte; nanobamboos (NBs), i.e., nanotubes decorated with periodic exterior rings, were formed by the DES-assisted anodization. Spontaneous formation of NBs occurred upon application of a constant 20V voltage, together with a self-induced current oscillation that corresponds well to the NB structures. The initial voltage must be higher than a threshold of approximately 14V to induce the self-organization of the NBs. Once a dense layer of TiO2 is formed at 10V or lower, formation of the NBs is suppressed even when a higher voltage (above 15V) is applied. The NBs formed with the spontaneous current oscillation were densely packed (ca. 1.54×1010cm−2) with each NB possessing many rings spaced 20–35nm apart. The thickness of each ring was ca. 10nm. By increasing the DES concentration to 10 wt%, the diameter of the NBs increased from 45 to 110nm and their growth rate increased from 0.16 to 0.3μm/h. The spontaneous current oscillation we observed differs from the conventional electrochemical oscillation because (1) oscillation period is quite long (7min) and (2) the oscillation results in the periodic modulation of TiO2 NBs. The self-organized nanobaoboos were characterized for various anodization conditions, and a mechanism was proposed with the basis of the high conductivity and high viscosity of DES.

Formation and field-assisted dissolution of anodic films on iron in fluoride-containing organic electrolyte

Magnetron-sputtered iron films were potentiodynamically anodized at two different sweep rates to 50 V in an ethylene glycol electrolyte containing ammonium fluoride and water. At a high sweep rate of 1.0 V s−1, a barrier-type anodic film was formed even though the current efficiency was as low as ∼50%. In contrast, a nanoporous anodic film developed at a low sweep rate of 0.05 V s−1, and the film-formation efficiency reduced to 37%. The main part of the anodic films consists of iron (III) hydroxyfluoride with a thin inner layer composed of FeF3. The inner fluoride layer is formed owing to the faster inward migration of fluoride ions compared to that of the oxygen species. During immersion or re-anodizing of the iron specimen with an approximately 100-nm-thick, barrier-type anodic film at and below 15 V, thinning of the anodic film proceeded uniformly and film dissolution was enhanced by applying an electric field. The impact of the electric field on film formation and dissolution is discussed.

Coupling between dissolution and passivation revisited – Kinetic parameters of anodic oxidation of titanium alloys in a fluoride-containing electrolyte

In the present paper, the anodic film growth on Ti alloys in water- and fluoride-containing ethylene glycol electrolyte were investigated using electrochemical and surface analytical techniques. Steady-state current–potential curves and electrochemical impedance spectra as depending on potential and fluoride content pointed to two parallel reaction pathways – film growth/dissolution and titanium dissolution through the film. The chemical composition of the anodic films was estimated by X-ray photoelectron spectroscopy (XPS). XPS analyses revealed the presence of a non-stoichiometric oxide containing mainly Ti4+ and a certain amount of Ti3+, with a certain degree of hydroxylation. Mo and Al were found to be enriched in the oxides, and evidence of transpassive oxidation/dissolution of Mo was found. A kinetic model of the process is proposed and its parameters are estimated by quantitative comparison with the current–potential and EIS data. The main features of the XPS data, namely film thickness, concentration of Ti(III) and fluoride as depending on potential are also reproduced by the model.

Titania Nanotube Arrays Load with Ibuprofen by Solvothermal and its Release Behavior

In this study, the vertically aligned titania nanotube arrays (TNTs) was fabricated on the surface of titanium substrate in fluoride-containing electrolytes via self-ordering electrochemical anodization. The prepared TNTs loaded with ibuprofen (IBU) by solvothermal to achieve a local drug delivery system and its release properties were investigated. The samples were characterized by field emission scanning electronic microscope (FE-SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR). The results indicated that the IBU drug molecules were successfully loaded on the surface of TNTs and the drug release shows a sustained release process. It suggested that TNTs as drug carrier had the properties of sustained release which have a good application prospect in biomedical field.

Effect of TiO2 nanotubes lateral spacing on photovoltaic properties of dye-sensitized solar cells

Vertically aligned TiO2 nanotube arrays (~8 μm long, ~110 nm wide) have been fabricated through anodic oxidation of Ti-metal sheet in fluoride-containing electrolyte. By changing the volume ratio of ethylene glycol and diethylene glycol in the electrolyte, TiO2 nanotube arrays with different tube-to-tube lateral spacing, i.e., closely packed, just separated, and fully separated, have been synthesized and applied as photoanodes for dye-sensitized solar cells (DSSCs). Photovoltaic efficiency of 2.99 %, 3.34 %, and 3.44 % has been obtained for DSSCs based on the closely packed, just separated, and fully separated TiO2 nanotube arrays, respectively, illustrating the effect of tube-to-tube lateral spacing of TiO2 nanotube arrays on the performances of DSSCs. It is suggested that fully separated TiO2 nanotube arrays are beneficial to the conversion efficiency of DSSCs due to higher dye loading and faster electron transfer.

Efficient suppression of nanograss during porous anodic TiO 2 nanotubes growth

When Ti foil was anodized in fluoride-containing electrolyte for a long time, undesired etching-induced “nanograss” would inevitably generate on the top of porous anodic TiO2 nanotubes (PATNTs). The nanograss will hinder the ions transport and in turn yield depressed (photo) electrochemical performance. In order to obtain nanograss-free nanotubes, a modified three-step anodization and two-layer nanostructure of PATNTs were designed to avoid the nanograss. The first layer (L1) nanotubes were obtained by the conventional two-step anodization. After washing and drying processes, the third-step anodization was carried out with the presence of L1 nanotubes. The L1 nanotubes, serving as a sacrificed layer, was etched and transformed into nanograss, while the ultralong nanotubes (L2) were maintained underneath the L1. The bi-layer nanostructure of the nanograss/nanotubes (L1/L2) was then ultrasonically rinsed in deionized water to remove the nanograss (L1 layer). Then much longer nanotubes (L2 layer) with intact nanotube mouths could be obtained. Using this novel approach, the ultralong nanotubes without nanograss can be rationally controlled by adjusting the anodizing times of two layers.

Synthesis and characterization of hybrid micro/nano-structured NiTi surfaces by a combination of etching and anodizing

The purpose of this study was to generate hybrid micro/nano-structures on biomedical nickel–titanium alloy (NiTi). To achieve this, NiTi surfaces were firstly electrochemically etched and then anodized in fluoride-containing electrolyte. With the etching process, the NiTi surface was micro-roughened through the formation of micropits uniformly distributed over the entire surface. Following the subsequent anodizing process, self-organized nanotube structures enriched in TiO2 could be superimposed on the etched surface under specific conditions. Furthermore, the anodizing treatment significantly reduced water contact angles and increased the surface free energy compared to the surfaces prior to anodizing. The results of this study show for the first time that it is possible to create hybrid micro/nano-structures on biomedical NiTi alloys by combining electrochemical etching and anodizing under controlled conditions. These novel structures are expected to significantly enhance the surface biofunctionality of the material when compared to conventional implant devices with either micro- or nano-structured surfaces.

Forming Process of Anodic TiO2 Nanotubes under a Preformed Compact Surface Layer

Anodic TiO2 nanotubes (NTs) have been studied extensively for many years. However, the growth kinetics still remains unclear, because it is hardly derived by direct in situ methods. Here, an interesting approach is proposed to overcome this challenge. A combinatorial anodization was exploited to monitor the pore initiation and nanotube growth under a preformed compact surface layer (CSL). The preformed CSL and the NTs under the CSL (UCSL-NTs) were formed in fluoride-free and fluoride-containing electrolytes, respectively. The forming process of UCSL-NTs was discussed as compared with that of the general NTs, mainly focusing on the differences of current-time curves and electric charge quantity (Coulomb). The results show that pore embryos of UCSL-NTs have already been achieved under the CSL before the CSL is dissolved. There are five stages in the current-time curve of UCSL-NTs, which is significantly different from three stages of the general NTs. A new growth model, based on a comprehensive review of the existing theories, is proposed to explain the current decrease and increase. And the forming process of TiO2 NTs is considered to be dominated by the oxide plastic flow around the oxygen bubbles.

Improved performance of Ag-nanoparticle-decorated TiO2 nanotube arrays in Li-ion batteries

The present work investigates the electrochemical response of silver nanoparticle (Ag-NP)-decorated TiO2 nanotube (NT) layers as an anode material for a lithium-ion battery. Self-organized nanotube layers with a thickness of approximately 1 µm and a diameter of approximately 100 nm were grown by anodization of Ti in a fluoride-containing aqueous electrolyte. Ag NPs (average particle size of ∼10 nm) were deposited both inside and outside the nanotube geometry in a well-distributed manner through a simple and efficient photocatalytic reduction process. The morphology and the chemical composition of the resulting materials were characterized by using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). Our results show that the TiO2 NT layers decorated with Ag NPs had a superior electrochemical response in terms of charge/discharge capacity, rate capability, cyclic performance and columbic efficiency. The enhanced performance is attributed to the improved electronic and ionic conductivity, obtained by providing highly conductive paths to electrons flowing through a well-distributed Ag NPs deposition on the walls of the highly-oriented NTs.

Supported nanohydroxyapatite on anodized titanium wire for solid-phase microextraction

In this paper, a simple and versatile route was introduced to prepare solid-phase microextraction coatings on the chemically inert titanium wire. Titania nanotube array film can be created on metallic substrates by electrochemical anodization in fluoride-containing electrolytes and subsequently support various secondary reactions to prepare functional surfaces. In the present work, titania nanotube array-coated titanium wire was successfully modified by nanostructured hydroxyapatite by a simple solution-based in situ chemical deposition method. This coating has a high surface-to-volume ratio with a thickness of about 10 μm. Extraction performance of the fiber was assessed on several polycyclic aromatic hydrocarbons in water solutions. The nanohydroxyapatite-coated fiber showed good precision (<7.4 %), low detection limits (1.79-4.89 ng/L), and wide linearity (0.1-200 μg/L) under the selected conditions. The repeatability of fiber to fiber was 1.9-18.2 %. The new solid-phase microextraction fiber has a lifetime of over 150 extractions due to the hydroxyapatite nanoslices uniformly and strongly deposited on the wire surface. The environmental water sample was used to test the reliability of the solid-phase microextraction-gas chromatography method; some analytes were detected and quantified.

Self-Ordering Process of Hexagonal Cells in Porous Anodic TiO&lt;sub&gt;2&lt;/sub&gt; Nanotubes

Cylindrical TiO2nanotubes and hexagonal TiO2nanotubes were obtained in the anodizing process of titanium in fluoride-containing electrolyte. Based on the experimental findings and viscous flow model, a mechanism of self-ordering process of hexagonal cells arrangement for porous anodic TiO2nanotubes (PATNT) is proposed in this paper. The analysis results show that oxygen evolution in the pore bottoms plays an important role in the self-ordering process. Oxide grows around the oxygen bubbles in the pore bottoms, which results in the formation of cylindrical TiO2nanotubes. Volume expansion of TiO2takes place during the anodizing process. Plastic deformation and repulsive force caused by volume expansion are responsible for self-ordering process of PATNT hexagonal cells.

Miniature H2/O2 fuel cells using TiO2 nanotube substrates and sputtered Pt catalyst

Modified designs for a miniature H2/air fuel cell are described using Pt sputtered on titanium oxide nanotube (Pt/TONT) as a catalyst at low mass loadings. Titanium microgrids or Ti evaporated onto a silicon substrate with holes at the center were used as substrates that were then anodized in fluoride-containing electrolytes to form the TONT arrays. Self-organized tubes grew as anodizing time increased, resulting in the highly enhanced surface area for electrochemical reactions. High cathodic current on cyclic voltammogram of Pt/TONT catalyst indicated high catalytic activity for the oxygen reduction reaction (ORR). The miniature fuel cells were successful, and showed high power densities up to 9.64 mW cm−2 at potentials from 0.7 V to 0.2 V.

Thin film nonstoichiometric chromium oxide-based cathode material for rechargeable and primary lithium batteries

The application of nonstoichiometric chromium oxide-based thin film cathodes in lithium rechargeable and primary batteries operating at high rates has been demonstrated. Films of varying composition have been obtained by anionic Cr (VI) species electrodeposition on a 1X18N10Т grade stainless steel cathode from fluoride-containing electrolytes. The effect of film doping by Li+ ions during its electrosynthesis has been сonsidered. As-prepared films were studied by scanning and transmission electron microscopies, 3D optical profiler, thermogravimetric analysis, chemical analysis, and X-ray diffraction (XRD). The main phase components of the electrodeposited films regardless of Li+ in an electrolyte are Cr2O3, α-CrOOH, β-CrOOH, and metallic chromium as shown by XRD pattern refinement. The electrochemical reduction rate in a non-aqueous electrolyte (1 M LiClO4 in PC/DME) correlates with the chromium oxide-hydroxide component content of film. Primary CrO x -Li CR2325 mock-up cathode coating can be discharged in a pulsed mode at 10 Ω external resistance with 80–84 mA cm−2 current densities for 10–100 ms. Thin film cathodes electrodeposited in the presence of lithium ions become rechargeable when the lithium content of the film reaches 0.02 wt.%. Mock-ups of CR2325 coin battery with a thin film cathode doped with lithium ions can be discharged more than 40 times with 136 mAh g−1 specific capacity, 461 Wh kg−1 specific energy and 154 W kg−1 power density at 30 kΩ external resistance. The simplicity of thin film preparation makes this technology promising for thin film lithium batteries.

Fabrication and Formation Mechanism of Triple-Layered TiO2 Nanotubes

The formation mechanism of porous anodic TiO2 nanotubes (PATNT) still remains unclear. A special approach is proposed in this paper to investigate the forming process of nanopores in the preformed nanotubes. A novel and not easily brittle nanostructure, called triple-layered TiO2 nanotube array, has been fabricated by changing the electrolytes during the electrochemical anodizing processes. The first porous layer was fabricated in fluoride-containing electrolyte, the middle compact layer was formed in fluoride-free electrolyte and the second porous layer was formed in the same fluoride-containing electrolyte. The results show that middle compact layer becomes thicker with the increase of the third time anodizing voltage. At the same time, it needs more time for the fourth time anodization to reach the equilibrium current, where the nanotubes begin to develop steadily. Furthermore, a possible mechanism for the growth of the triple-layered nanotubes is discussed by comparison with the normal PATNT. The present results may be helpful to understand the mechanism of PATNT and facilitate assembling diverse nanostructures for extensive applications in photocatalysis, dye-sensitized solar cells, and biomedical devices.

Preparation of Cu2O/TiO2Nanotube Arrays Heterojunction and Their Photoelectrochemical Response in Visible Light

Self-organized highly oriented TiO2 nanotube arrays have been obtained by anodization of a Ti sheet in fluoride-containing electrolytes. Cu2O/TiO2 nanotube arrays heterojunction have been fabricated by electrochemical methods. The content of Cu2O loaded on the arrays was controlled by varying the electrodeposition time. The samples have been char- acterized by scanning electron microscopy (SEM) and UV-vis absorption spectrum. In addition, visible photoelectric conver- sion and photocatalytic water splitting performance of composite electrode materials were characterized under visible light with the measured density of 100 mVcm -2 . The results demonstrated that all Cu2O/TiO2 nanotube arrays heterojunction had a strong absorption in the visible region which suggested that the composite Cu2O/TiO2 nanotubes could make more efficient use of visible light compared with the unmodified TiO2 nanotubes. Simultaneously, p-n heterojunction was formed between Cu2O and TiO2 and the p-n junction was equivalent to a single directional conductive diode that allows electrons to flow from the n-type to the p-type, but the backward flow was prohibited. They could enhance the charges transport and reduce the re- combination rate of electrons and holes effectively. Separating the electron-hole pairs in different semiconductors could in- duce a potential difference at the heterojunction interface. Moreover when the electrodeposition time was 30 min, Cu2O par- ticles were uniform and almost covered the surface of TiO2 nanotubes. Comparing with the composite Cu2O/TiO2 nanotubes prepared by other electrodeposition time, Cu2O/TiO2 nanotube arrays heterojunction fabricated by electroplating for 30 min, noted by Cu2O/TiO2NTs-30, had better performance of photoelectrochemical response in visible light, the visible photoelec- tric conversion and photocatalytic water splitting efficiency were both the highest. Compared with TiO2 nanotube arrays, though the open-circuit voltage (Voc) of Cu2O/TiO2NTs-30 was -0.629 V and reduced 0.046 V, the short-circuit current density (Isc) enhanced 4.5 times, indicating that there were many more photoelectrons accumulated on the surface of the Cu2O/TiO2 nanotube arrays heterojunction. The improved Isc was also reflected in the increased incident monochromatic photo-to-current conversion efficiency (IPCE), the IPCE response at the maximum absorption wavelength was enhanced almost 6 times. Keywords TiO 2 nanotube arrays; Cu2O; visible light; photo-splitting water; photoelectric conversion

Flow-through TiO2 nanotube arrays: a modified support with homogeneous distribution of Ag nanoparticles and their photocatalytic activities

Silver decorated TiO2 nanotube arrays (TNTAs) show great potential applications for photocatalysis and gas sensors. In this work, we report an improved strategy to modify the morphology of flow-through TiO2 nanotube arrays (f-TNTAs) for homogeneous Ag nanoparticle loading, and their photocatalytic activities are also investigated. Firstly, TNTAs were fabricated by potentiostatic anodization in fluoride-containing electrolytes. Subsequently, a high voltage was immediately exerted, and then a low potential was applied at the end of anodization process. The as-prepared f-TNTAs with improved bottom morphologies were finally obtained. This new kind of support was immersed in AgNO3 solution, and then the absorbed silver ions were reduced to metallic Ag0 by UV light. Compared with conventional TiO2 nanotube arrays (c-TNTAs), the modified f-TNTAs show a better ability for the dispersion of Ag nanoparticles (Ag NPs) in different regions (upper, central and bottom region) of the nanotubes. A series of testing measures (XPS, EDX, SEM and XRD) were adopted to confirm this facile process. Ag decorated f-TNTAs were used as photocatalysts for the degradation of Methyl Orange (MO) under UV light. The degradation rate could reach 54% in 10 min, and the complete degradation of MO was observed after 30 min. These results were much better than that of Ag decorated c-TNTAs. The modified f-TNTAs via our method can also be used to couple with other noble metals or compound semiconductors. These composite structures are expected to find potential applications in photoelectric devices, gas sensors, and photocatalysis.