Electrochemical tuning of titania nanotube morphology in inhibitor electrolytes

The electrochemical behavior of fluorine containing electrolytes and its influence in controlling the lateral dimensions of TiO₂ nanotubes is thoroughly investigated. Potentiostatic anodization is carried out in three different electrolytes, viz., aqueous hydrofluoric acid (HF), HF containing dimethyl sulphoxide (DMSO) and HF containing ethylene glycol (EG). The experiments were carried out over a broad voltage range from 2 to 200 V in 0.1-48 wt% HF concentrations and different electrolytic compositions for anodization times ranging from 5 s to 70 h. The chemistry that dictates how the nature of electrolytes influences the morphology of nanotubes is discussed. Electrochemical impedance spectra were recorded for varying compositions of all the electrolytes. It was observed that composition of the electrolyte and its fluorine inhibiting nature has significant impact on nanotube formation as well as in controlling the aspect ratio. The inhibiting nature of EG is helpful in holding fluorine at the titanium anode, thereby allowing controlled etching at appropriate voltages. Thus our study demonstrates that HF containing EG is a promising electrolytic system providing wide tunability in lateral dimensions and aspect ratio of TiO₂ nanotubes by systematically varying the anodization voltage and electrolyte composition.     

Fabrication of high aspect ratio and open‐ended TiO2 nanotube photocatalytic arrays through electrochemical anodization

Well‐aligned, high aspect‐ratio and open‐ended TiO₂ nanotube arrays secured within a Ti foil (TiO₂ nanotubes cartridge) were successfully prepared through the double‐sided anodization method. With ～210 µm of nanotube length, the anodic growth of TiO₂ was accelerated and stabilized by the lactic acid‐containing ethylene glycol electrolyte. In the absence of lactic acid, the anodization led to detachment of nanotubes from the Ti foil after 5–6 h of high voltage (80 V) anodization. Transmission electron microscope image and Raman spectrum revealed that the as‐anodized TiO₂ nanotube arrays without annealing treatment were partially crystalline anatase and demonstrated photocatalytic activity in the mineralization of formic acid. © 2015 American Institute of Chemical Engineers AIChE J, 62: 415–420, 2016     

Control of morphology and composition of self-organized zirconium titanate nanotubes formed in (NH4)2SO4/NH4F electrolytes

We investigated the formation of self-organized zirconium titanate nanotubes by anodizing a Ti-35Zr alloy in 1 M (NH₄)₂SO₄ + 0.1-2.0 wt.% NH₄F electrolytes. The morphology and composition of the zirconium titanate nanotube are controlled by the applied electrochemical conditions. The outer diameter of nanotubes is controlled by the anodization potential in the range between 1 and 100 V (versus Ag/AgCl). Tubes with diameters from 14 to 470 nm can be grown. The nanotube length correlates with the anodic charge up to a length where significant dissolution of the nanotube layer is observed. The wall thickness, composition of the nanotubes and porosity of the nanotube layer are significantly affected by the fluoride ion concentration. The length limiting factor of the nanotube growth is found to be the diffusion of ionic species in the electrolyte.     

Polishing effect on anodic titania nanotube formation

Titania nanotubes represent exciting opportunities in solar cell, sensing, and catalytic applications. In this work, four different surface polishing conditions: as-received, chemical polishing, mechanical polishing, and electropolishing, are studied in order to understand the effect of different surface conditions on the anodization process and nanotube morphology. At the same anodization condition of 100 V in 0.1 M NH₄F ethylene glycol at 0 °C for 3 min, the as-received and mechanically polished samples show nano-tubular surfaces while the chemically polished and electropolished samples have oxide layers on the top of the nanotubes. The nanotube morphologies, anodization current vs. time curves, and the bottom barrier layers are all related to the Ti surface conditions. The electropolished surface leads to the most homogeneous TiO₂ nanotube formation.     

Fabrication of Ti–Al–Zr alloy oxide nanotube arrays in organic electrolytes by anodization

We report the formation of self-organized Ti–Al–Zr alloy oxide nanotube arrays at different fluoride concentrations in formamide and glycerol electrolytes. The nanotube morphology is strongly influenced by the fluoride concentration, applied potential and reaction time. Titanium alloy (Ti–Al–Zr) oxide nanotube arrays with a length of about 6.13 μm, a pore diameter of 116 nm and wall thickness of 55 nm were prepared at 50 V for 24 h in formamide and glycerol mixtures (volume ratio 1:1) with addition of 1.0 wt% NH₄F. The as-prepared nanotubes were amorphous and alloy oxide crystals appeared after annealing in air at 400 and 600 °C for 3 h.     

Growth of TiO2-based nanotubes on Ti–6Al–4V alloy

The possibility of tailoring titania nanotube films on Ti–6Al–4V alloy is investigated using electrolytes based on NH₄H₂PO₄ with the addition of different concentrations of NH₄F. Several morphologies from high aspect ratio nanotubes to barrier layers are achieved by the control of the electrolyte composition, regarding its pH and fluoride concentrations. The morphology and composition of the anodic layers were evaluated by scanning and transmission electron microscopy, Rutherford backscattering spectroscopy (RBS) and Wavelength dispersive X-ray fluorescence spectroscopy.The formation efficiency and the fluoride ions content in the nanotubes depend on the F concentration in the electrolyte. The higher concentration of fluoride ions in the electrolyte promotes an increase of the F incorporated in the nanotubes, about 12 at.% but, reduces the nanotube formation efficiency. However, no significant presence of phosphorus was detected into the films by means of the above-mentioned analytical techniques.     

Synthesis and growth mechanism of multilayer TiO2 nanotube arrays

High-aspect-ratio TiO(2) nanotube arrays formed by anodic oxidation have drawn extensive attention due to their easy fabrication and various excellent optical, electrical and biomedical properties. In contrast to conventional single-layer TiO(2) nanotubes prepared via constant-voltage anodization, we synthesize multilayer TiO(2) nanotube arrays with high surface area by using alternating-voltage anodization steps. This work presents synthesis and growth mechanisms of single-layer smooth TiO(2) nanotubes, bamboo-type nanotubes and double-layer nanotubes, by tuning various parameters such as voltage, time, and water content in the electrolyte. It is found that ion diffusion inside the nanotubes dominates growth of these three structures. A stable pH and ion-diffusion profile allows the steady growth of smooth TiO(2) tubes in NH(4)F-containing ethylene glycol (EG). The addition of a low-voltage anodization step reduces the pH and ion-diffusion gradient in the nanotubes and induces formation of bamboo-type nanotubes and double-layer nanotubes when a second high-voltage anodization is conducted. Ion diffusion through a nanotube takes time; thus formation of lower-layer TO(2) nanotubes costs more time if longer nanotubes are grown in the upper layer, since ions diffuse through these longer nanotubes. This ion-diffusion controlled growth mechanism is further confirmed by tailoring the water content (0-20 vol%) in the electrolyte and the voltage gaps to control the time needed for initiation of lower-layer TiO(2) nanotube arrays. The fundamental understanding of the growth characteristics of double-layer TiO(2) nanotubes presented in this paper offers us more flexibility in engineering morphology, tuning dimensions and phase compositions of multilayer TiO(2) nanotubes. In addition, we synthesize double-layer TiO(2) nanotube arrays composed of one layer of anatase phase and another layer of amorphous phase.     

Effects of anodization parameters on the formation of titania nanotubes in ethylene glycol

The effects of fluoride concentration, anodization temperature, and anodization applied potential difference on the formation and dimensions of the titania nanotubes in ethylene glycol/water systems were investigated. It was found that fluoride concentration and anodization temperature were the two critical parameters for controlling the nanotube formation whilst anodization applied potential difference mainly contributes to tuning the dimension of the nanotubes. Electrolytes containing a low fluoride concentration are beneficial for initiating nanotube formation, whilst a higher anodization temperature is helpful to the rapid growth of the nanotubes. It is shown that the current-time curve obtained during the anodization can be used as an effective tool to predict the morphology of titania nanotubes. A model based on the competition between electrochemical oxidation of the titanium and chemical dissolution by fluoride ions is developed to explain the experimental observations.     

PEG-based polymer coated proppants in supercritical CO2: A new approach in current fracturing protocols

Liquified carbon dioxide (CO₂) can be used in hydraulic fracturing fluids to provide nonaqueous alternatives to conventional water-based fluids as it is more environmentally benign and minimizing depletion of natural-source freshwater. However, conventional CO₂-based fluids are not sufficiently viscous to suspend proppants that are added to fracturing fluids to hold open subterranean fractures. Because of this reduced ability to suspend proppants, CO₂-based fluids have not found its niche yet as future fracturing fluids. Accordingly, a need exists for nonaqueous hydraulic fracturing fluids that adequately support and suspend proppant particles. The present study presents a green alternative of developing a poly(ethylene glycol) based polymers coated proppants which can swell in supercritical CO₂ so as to decrease the entire density of the proppant during fracturing process.     

Plasticized nanocomposite polymer electrolytes based on poly(oxyethylene) and cellulose whiskers

Nanocomposite polymer electrolytes based on a plasticized high molecular weight poly(oxyethylene) (POE) reinforced with high aspect ratio cellulose nanoparticles were reported. The influence of tetra(ethylene) glycol dimethyl ether (TEGDME) as plasticizer is investigated. The study mainly focuses on the dynamic mechanical behavior and ionic conductivity performances. The miscibility of the blend POE/TEGDME was investigated using both thermal and mechanical investigations. Viable polymer electrolytes can be obtained from this combination, conciliating acceptable ionic conductivities and outstanding mechanical performances on a large temperature range.     

Combined effects of nanotube aspect ratio and shear rate on the carbon nanotube/polymer composites

We use fiber-level simulations to investigate the combined effects of carbon nanotube (CNT) aspect ratio and shear rate on the microstructure and electrical properties of CNT/polymer composites. In our previous studies, we studied the effects of aspect ratio at a constant shear rate as well as the effects of shear rate for a constant aspect ratio. In this study electrical properties and microstructure changes (e.g. agglomeration/deagglomeration, network strength, nanotube orientation) of CNT/polymer composites are investigated for varying aspect ratios at different shear rates. When shear rate is increased, we observe a decrease in the electrical conductivity and an increase in the anisotropy factor due to the deagglomeration and flow induced orientation. Increasing aspect ratio shifts the conductivity vs. shear rate curve to larger values and anisotropy vs. shear rate curve to lower values showing that there is a tendency for tube agglomeration when high aspect ratio nanotubes are used. On the other hand when low aspect ratio nanotubes are used, conductive networks can be more easily destroyed by the shear forces because networks formed by low aspect ratio nanotubes have lower strength than those formed by high aspect ratio nanotubes. Our results show that electrical conductivity is anisotropic with a larger component in the flow direction. The critical shear rate defined as the shear rate where the conductive network is destroyed and all components of the composite conductivity decrease to the matrix conductivity, shifts to higher values when the aspect ratio is increased. Reduced alignment and increased entanglement are the reasons of this decrease.     

Preparation of separated and open end TiO2 nanotubes

Separated and open end TiO₂ nanotubes with large surface area and through-hole structure exhibit exciting functionalities in energy and environmental applications. In this study, TiO₂ nanotube membranes are created using high purity titanium sheets as raw materials by anodic oxidation at 100 V for 12 h in ethylene glycol+0.25 wt% NH₄F+5 vol% H₂O. The pore size of the nanotubes is 215 nm with uniform diameters. Close-packed TiO₂ nanotube arrays are separated by immersion in a 0.15 wt% HF water solution. With further dissolution by 40% HF water solution for 10 min, the barrier layer at the back of the TiO₂ nanotubes completely disappears and the nanotubes become open at both ends. This study offers a facile approach of making separated and open-end nanotubes for electrocatalytic purposes.     

Preparation of glycol esters of soybean oil fatty acids and their potential as coalescent aids in paint formulations

Soy oil glycol monoesters have been prepared through the transesterification of soybean oil with ethylene, diethylene, propylene, and dipropylene glycols. The molar ratio of soybean oil to glycol used in these reactions was 1∶6. The catalyst used in these reactions was lithium carbonate, 0.5%, based on soybean oil. The transesterifications were carried out at 180–190°C. The composition of soy oil glycol esters and their physical properties were determined. The soy oil glycol esters were incorporated in a water-based paint formulation as the coalescent aid, and the minimum film formation temperature was determined. The minimum film formation temperature results indicated that these esters can be used as coalescent aids in latex paint formulations to help in continuous film formation at or below room temperature.     

Time-dependent growth of biomimetic apatite on anodic TiO2 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 TiO₂. 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 TiO₂ 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 TiO₂ 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.     

In situ photopolymerization of PEGDA‐protein hydrogels on nanotube surfaces

Peptide nanotubes were used as templates for the growth of poly(ethylene glycol) diacrylate‐based nanoscale hydrogels via photopolymerization. A Rose Bengal di‐amine derivative comprised of a photoactivator and coinitiator within the same molecule was used as the photoinitiator to increase photopolymerization efficiency. The nanotubes were covalently bound to the protein BSA before formation of the hydrogels. We also examined the photopolymerization efficiency in reactions involving nanotubes in the absence of BSA. Although photopolymerization occurred efficiently under both conditions, higher yields of highly crosslinked nanostructures were obtained for the protein bound nanotube‐PEGDA hydrogels. It was observed that the swelling ratios were also dependent upon whether or not BSA was bound to the nanotubes before photopolymerization. The thermal properties of the nanocomposite hydrogels were investigated using differential scanning calorimetry analyses and the morphologies were examined using TEM, SEM, and AFM analyses. Such nanocomposites prepared by low cost, mild methods could be extremely efficient for the in situ preparation of three‐dimensional arrays of peptide nanotube grafted hydrogels. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Copolyester nanocomposites based on carbon nanotubes: reinforcement effect of carbon nanotubes on viscoelastic and dielectric properties of nanocomposites

Nanocomposites based on an amorphous copolyester, poly(ethylene glycol‐co‐cyclohexane‐1,4‐dimethanol terephthalate) and carbon nanotubes were fabricated using a simple melt processing technique. The reinforcement effect of carbon nanotubes in the copolyester was investigated experimentally using different approaches based on dynamic mechanical analysis, rheology and dielectric analysis. The nanocomposites show a mechanical reinforcement effect with significant increase in the stiffness especially in the rubbery regime with increasing nanotube content. An increase in T_g and a decrease in damping are seen, which are derived from the presence of a percolating superstructure of the filler. Rheological experiments show an increase in storage modulus up to four orders of magnitude. Viscolelastic characterization shows that the percolation threshold is at 3 wt% of nanotubes. Dielectric relaxation spectroscopy confirms the presence of this percolating structure. We conclude that the responses of both rheological and electrical properties are different, although both are related to the formation of a percolating network superstructure of the filler. Copyright © 2007 Society of Chemical Industry     

Toluenesulfonate-acyl esters of ethylene glycol and other 1,2-diols as industrial antioxidants with cupric ion

Ethylene glycol esters of soybean oil FA increased in viscosity much more slowly than methyl or glycerol esters when oxidized at 105°C in the presence of flowing air and colloidal copper. This increased stability was caused by a minor constituent of the ethylene glycol esters, which was shown by MS to be a mixed ethanediol fatty acylate p-toluenesulfonate (EFAT). The p-toluenesulfonate group came from the catalyst used in the formation of the ethylene glycol esters. EFAT was quantified by UV spectrometry, HPLC, or GC of the acyl group that it contains. EFAT could be synthesized in good yield by reacting ethylene glycol, a FA, and p-toluenesulfonic acid (TSA) in a 1∶1∶1 molar ratio using a benzene azeotrope to remove water of esterification. EFAT increased the time required for the polymerization of soybean oil by about 27 times but required concentrations of 2–5% by weight. EFAT made with a variety of FA were active in delaying viscosity increase. Ethyl and decyl p-toluenesulfonate were inactive. Replacing ethylene glycol by glycerol and 1,2-propylene glycol but not by 1,3-propylene glycol resulted in active EFAT. TSA itself delayed the polymerization of soybean oil, especially in the presence of free ethylene glycol and FA, but this probably was caused by formation of EFAT during the oxidation test. Colloidal copper could be replaced by cupric ion. EFAT-copper appeared to act as an antioxidant by destroying hydroperoxides without initiation of free radical chains.     

Electrochemical synthesis of silica-doped high aspect-ratio titania nanotubes as nanobioceramics for implant applications

The preparation of silica-doped high aspect-ratio TiO₂ nanotubes and their apatite-forming ability were demonstrated in this study. The high aspect-ratio TiO₂ nanotube layers were produced by electrochemical anodic oxidation of Ti in chloride-containing electrolytes. Nanotubes were doped with different concentrations of silica particles through anodization in NaCl electrolyte containing different concentrations of water glass (24 g/L or 48 g/L Na₂SiO₃). The biomimetic apatite deposition behavior was evaluated under simulated body fluid (SBF) with an ion concentration nearly equal to human blood plasma. The experimental results collectively demonstrate the successful silica doping of the resultant nanotube layers with significant abundant OH groups on their surfaces. The results of hydroxyapatite (HA) growth on nanotubes clearly show that the silica doping greatly enhances the fast nucleation and growth of HA, especially for the tubes in their “as-formed” amorphous state, which usually require a long time for apatite induction. The nanotubes doped with high silica content combined with an anatase or a mixture of anatase and rutile led to the formation of very thick and continuous apatite layers with a thickness of ∼7 μm in 21 days. In contrast, to the tubes doped with a low concentration of silica (grown in an electrolyte containing 24 g/L Na₂SiO₃), the HA deposited in the form of closely packed spheroid particles and never developed into continuous films. This effect could be attributed to the critical active-site density (silanol groups, >Si-OH), which provides the sterochemical match for apatite growth. Finally, the results of this study provide, for the first time, evidence for the dependence of HA morphology/microstructure on the crystallographic structure and the density of active sites (>Si-OH groups).     

Modulation of wall thickness of Al-doped ZnO nanotubes by nanolamination of atomic layer deposition for oxygen sensing

The high surface-to-volume ratio and feature dimensions of the gas sensors are the key factors for improving the gas response. In this study, a novel method to prepare an Al-doped ZnO (AZO) nanotube oxygen sensor with tunable wall thickness is reported via the ZnO–Al₂O₃ nanolamination of atomic layer deposition (ALD) using tris(8-hydroxyquinoline) gallium nanowire (GaQ₃NW) as a template. The ALD of Al₂O₃ significantly enhances wall uniformity and decreases the wall thickness of the AZO nanotubes. In addition, the incorporation of Al₂O₃ allows full coverage of AZO on GaQ₃NWs. With an increase in the Al₂O₃ fraction, the carrier concentration increases, but the depth of the depletion layer and gas response of the nanotube sensor are reduced. The gas response of the nanotubes is inversely proportional to wall thickness, suggesting that it is a function of the surface-to-volume ratio. When the wall thickness is decreased to 12 nm, the gas response of AZO nanotubes with 2% Al increases significantly to 7. This can be explained by the grain control model, because thin wall leads to the formation of fully charge-depleted nanotubes.     

Synergistic effect of CeO2 modified Pt/C catalysts on the alcohols oxidation

The effect of the addition of CeO₂ to Pt/C catalysts on electrochemical oxidation of alcohols (methanol, ethanol, glycerol, ethylene glycol) was studied in alkaline solution. The ratios of Pt to CeO₂ in the catalysts were optimised to give the better performance. The electrochemical measurements revealed that the addition of CeO₂ into Pt-CeO₂/C catalysts could significantly improve the electrode performance for alcohols oxidation, in terms of the reaction activity and the poisoning resistance, due to the synergistic effect. The electrode with the weight ratio of Pt to CeO₂ equals 1.3:1 with platinum loading of 0.30 mg/cm² showed the highest catalytic activity for oxidation of ethanol, glycerol and ethylene glycol.