Highly Active Nickel Nanoparticles Supported on TiO2 Nanotube Electrodes for Methanol Electrooxidation

Nickel nanoparticles/TiO₂ nanotubes/Ti electrodes were prepared by galvanic deposition of nickel nanoparticles on the TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride‐containing electrolyte. The morphology and surface characteristics of titanium dioxide nanotubes and Ni/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy‐dispersive X‐ray spectroscopy, respectively. The results indicated that nickel nanoparticles were homogeneously deposited on the surface of TiO₂ nanotubes. The electrocatalytic behaviour of nickel nanoparticles/TiO₂/Ti electrodes for the methanol electrooxidation was studied by electrochemical impedance spectroscopy, cyclic voltammetry, differential pulse voltammetry and chronoamperometry methods. The results showed that Ni/TiO₂/Ti electrodes exhibit a considerably higher electrocatalytic activity toward the oxidation of methanol.     

Fabrication parameter-dependent morphologies of self-organized ZrO<Subscript>2</Subscript> nanotubes during anodization

Zirconia (ZrO₂) nanotubes have been synthesized using a facile anodizing process in organic electrolyte systems containing a low content of fluoride. The nanotube architecture evolution was recorded at different anodization periods (1–24 h) by scanning electron microscopy. A compact layer was found between the Zr substrate and its upper tubular layer after 1 h of anodization, whereas after further anodization for 3 h the compact layer disappeared. Meanwhile, ZrO₂ nanotubes turned to a uniform structure from top to bottom. However, after 18–24-h-long anodization, the uniform tubular layer was replaced by a random layer composed of various structural defects. Since the compact layer was not completely dissolved, the retained compact layer yielded O-rings with double walls on the outer surface of the nanotubes.     

Self-organized porous and tubular oxide layers on TiAl alloys

The present paper describes the electrochemical fabrication of nanostructured oxide films on a TiAl intermetallic compound. The alloy is investigated under conditions where the individual alloying elements show the growth of ordered oxide structures, i.e. anodization is carried out in fluoride containing and fluoride free H₂SO₄ electrolytes. In 1 M H₂SO₄ the alloy shows randomly ordered nanoporous oxide structures while in HF-containing electrolytes highly ordered films can be formed. The key factor that affects the morphology is the anodizing potential. At low potentials (∼10 V) self-organized nanopores are formed whereas at higher potentials (∼40 V) separation of the pore walls and therefore formation of nanotubes can be observed. The results clearly indicate that on TiAl a wide range of nanoscale morphologies can be achieved ranging from random porous to organized pores to organized tubes.     

Formation of Nickel Sulfide Nanoframes from Metal–Organic Frameworks with Enhanced Pseudocapacitive and Electrocatalytic Properties

Nanoframe‐like hollow structures with unique three‐dimensional (3D) open architecture hold great promise for various applications. Current research efforts mainly focus on frame‐like noble metals and metal oxides. However, metal sulfides with frame‐like nanostructures have been rarely reported. Starting from metal–organic frameworks (MOFs), we demonstrate a novel structure‐induced anisotropic chemical etching/anion exchange method to transform Ni‐Co Prussian blue analogue (PBA) nanocubes into NiS nanoframes with tunable size. The reaction between Ni‐Co PBA nanocube templates and Na₂S in solution leads to the formation of well‐defined NiS nanoframes. The different reactivity between the edges and the plane surface of the Ni‐Co PBA nanocubes is found to be the key factor for the formation of NiS nanoframes. Benefitting from their structural merits including 3D open structure, small size of primary nanoparticles, high specific surface area, and good structural robustness, the as‐derived NiS nanoframes manifest excellent electrochemical performance for electrochemical capacitors and hydrogen evolution reaction in alkaline electrolyte.     

Electrochemical behavior in simulated body fluid of TiO2 nanotubes on TiAlNb alloy elaborated in various anodizing electrolyte

The present study is focused on tailoring the morphology of TiO₂ nanotubes obtained on Ti6Al7Nb alloy and evaluating their electrochemical behavior in simulated body fluid. The presence of the α and β phases on the Ti6Al7Nb alloy leads to a two‐scale organization of the nanotubes on the samples – which in turn affects the electrochemical stability. Furthermore, five different types of TiO₂ nanotubes were obtained in various electrolytes (e.g. Generation I, a mixture of Generation II and Generation III, Generation III). Electrochemical behavior analysis of all obtained nanotubes morphologies was composed of Tafel plots, cyclic voltammetry and electrochemical impedance spectroscopy and was correlated with morphology data obtained from SEM (nanotubes diameters from top‐view and nanotube length from cross‐section view). The electrochemical results showed that morphological modifications of the Ti6Al7Nb alloy's surface by electrochemical anodizing have induced changes to the electrochemical behavior of the material, evident in the corrosion rates. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrogen bubbles and formation of nanoporous silicon during electrochemical etching

Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etched nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of heavily n‐doped (R = 0.05–0.5 Ω·cm) silicon 〈100〉 single crystals at a current density of 10 mA cm^?2 in buffer oxide etch (BOE) electrolyte solution, defect craters containing textured nanopores were observed to occur in ring‐shaped patterns. The defect craters apparently originate at the hydrogen/BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density and high BOE viscosity allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current‐carrying Si? OH species are most likely responsible for the widening of the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the few nanopores formed at the ring edges are isolated from each other. Overall features observed in the photoluminescence intensity by XPS strongly emphasize the role of surface oxide that influences these properties. Copyright © 2005 John Wiley & Sons, Ltd.     

Hierarchical Porous ZnMn2O4 Hollow Nanotubes with Enhanced Lithium Storage toward Lithium‐Ion Batteries

We have purposefully developed a smart template‐engaged methodology to efficiently fabricate well‐defined ternary spinel ZnMn₂O₄ hollow nanotubes (NTs). The procedure involves coating carbon nanotubes (CNTs) with ZnMn₂O₄ nanosheets (NSs), followed by heating at high temperature in air to oxidize the CNT template. Physicochemical characterization demonstrated that the formed ZnMn₂O₄ NTs with a diameter of approximately 100 nm were composed of assembled NSs and/or nanoparticles (NPs) as building blocks and possessed numerous nanopores of several nanometers in the sidewall of the NTs. In favor of the intrinsic structural advantages, the resulting ZnMn₂O₄ NTs exhibited superior electrochemical lithium‐storage performance with a large capacity, good rate behavior, and excellent cyclability when evaluated as promising anodes for lithium‐ion batteries (LIBs). The remarkable electrochemical performance was rationally ascribed to the appealing one‐dimensional (1D) porous hollow tubular architecture with nanoscale subunits and mesopores in the sidewalls, which decreased the diffusion length for the Li⁺ ions, improved the kinetic process, and enhanced the structural integrity with sufficient void space to tolerate the volume variation during Li⁺‐ion insertion/extraction. These results highlight the promising application of 1D ZnMn₂O₄ NTs as anodes for high‐performance LIBs.     

The effect of annealing temperatures on surface properties,hydroxyapatite growth and cell behaviors of TiO2 nanotubes

Well‐ordered TiO₂ nanotubes were prepared by the electrochemical anodization of titanium in an ethylene glycol electrolyte containing 1 wt% NH₄F and 10 wt% H₂O at 20 V for 20 min, followed by annealing. The surface morphology and crystal structure of the samples were examined as a function of the annealing temperature by field emission scanning electron microscopy (FE‐SEM) and X‐ray diffraction (XRD), respectively. Crystallization of the nanotubes to the anatase phase occurred at 450 °C, while rutile formation was observed at 600 °C. Disintegration of the nanotubes was observed at 600 °C and the structure vanished completely at 750 °C. Electrochemical corrosion studies showed that the annealed nanotubes exhibited higher corrosion resistance than the as‐formed nanotubes. The growth of hydroxyapatite on the different TiO₂ nanotubes was also investigated by soaking them in simulated body fluid (SBF). The results indicated that the tubes annealed to a mixture of anatase and rutile was clearly more efficient than that in their amorphous or plain anatase state. The in vitro cell response in terms of cell morphology and proliferation was evaluated using osteoblast cells. The highest cell activity was observed on the TiO₂ nanotubes annealed at 600 °C. Copyright © 2010 John Wiley & Sons, Ltd.     

Nanoporous oxide formation by anodic oxidation of Nb in sulphate–fluoride electrolytes

The influence of hydrofluoric acid (HF) concentration and applied potential on the processes of anodic oxidation of Nb in sulphuric acid solution was studied by chronoamperometry, electrochemical impedance spectroscopy and scanning electron microscopy. During the first stage of the process, a compact barrier film is formed. On top of this film, a porous overlayer starts to form, then the nanopores grow into an ordered nanostructure. Subsequently, secondary 3D flower-shaped structures begin to form. These structures gradually spread all over the surface as an irregular multilayer film. The rates of the process of porous overlayer formation and subsequent growth of nanopore arrays increase with applied potential as well as with the HF concentration. The films have been characterised ex situ by electrochemical impedance spectroscopy at open circuit potential and capacitance vs. potential measurements to follow the different stages of nanoporous film formation with electrochemical methods. The impedance spectra and capacitance vs. potential curves have been interpreted using previously proposed models for the amorphous semiconductor/electrolyte interface. An attempt to rationalise the mechanism of nanoporous layer growth is presented by using the conceptual views of the mixed-conduction model and recent ideas for porous film formation on valve metals.     

Design of Advanced MnO/N‐Gr 3D Walls through Polymer Cross‐Linking for High‐Performance Supercapacitor

Three‐dimensional, vertically aligned MnO/nitrogen‐doped graphene (3D MnO/N‐Gr) walls were prepared through facile solution‐phase synthesis followed by thermal treatment. Polyvinylpyrrolidone (PVP) was strategically added to generate cross‐links to simultaneously form 3D wall structures and to incorporate nitrogen atoms into the graphene network. The unique wall features of the as‐prepared 3D MnO/N‐Gr hybirdes provide a large surface area (91.516 m² g^?1) and allow for rapid diffusion of the ion electrolyte, resulting in a high specific capacitance of 378 F g^?1 at 0.25 A g^?1 and an excellent charge/discharge stability (93.7 % capacity retention after 8000 cycles) in aqueous 1 m Na₂SO₄ solution as electrolyte. Moreover, the symmetric supercapacitors that were rationally designed by using 3D MnO/N‐Gr hybrids exhibit outstanding electrochemical performance in an organic electrolyte with an energy density of 90.6 Wh kg^?1 and a power density of 437.5 W kg^?1.     

Nickel oxide coated on ultrasonically pretreated carbon nanotubes for supercapacitor

Nickel oxide/carbon nanotubes (NiO/CNTs) composite materials for supercapacitor are prepared by chemically depositing nickel hydroxide onto carbon nanotubes pretreated by ultrasonication and followed by thermal annealing at 300 °C. A series of NiO/CNTs composites with different weight ratios of nickel oxide versus carbon nanotubes are synthesized via the same route. The high-resolution TEM and SEM results show that a lot of nicks, which favored the nucleation of the nickel hydroxide formed on the outer walls of carbon nanotubes due to ultrasonic cavitations, and then nickel oxide coated uniformly on the outer surface of the individual carbon nanotubes. The NiO/CNTs electrode presents a maximum specific capacitance of 523 F/g as well as a good cycle life during 1,000 cycles in 6 M KOH electrolyte. The good electrochemical characteristics of NiO/CNTs composite can be attributed to the three-dimensionally interconnected nanotubular structure with a thin film of electroactive materials.     

阳极氧化法制备TiO_2纳米管及其光催化性能

纳米TiO_2对诸多环境污染物有显著的光催化降解作用,光催化已发展成为新型环境污染治理技术.本文采用阳极氧化法制备出TiO_2纳米管,对比了四种电解液组成(A氟化铵+硫酸铵+水;B氟化铵+硫酸铵+乙酸+水;C氟化铵+硫酸铵+甘油+水;D氢氟酸+二甲基亚砜(DMOS)+乙醇)对催化剂表面形貌及光催化性能的影响.结果表明,电解液A和C都制备出了形貌清晰的TiO_2纳米管,管径约为60～74 nm.样品经400℃煅烧,TiO_2晶型主要为锐钛矿相;经500℃煅烧,出现少量金红石相;经700℃煅烧,晶型全部为金红石相.具有良好形貌的TiO_2纳米管同时具有良好的紫外光吸收能力.当亚甲基蓝初始浓度为10mg·L~(-1),经500℃煅烧的TiO_2纳米管光催化活性最佳,光照30 min亚甲基蓝的降解率达89.98%.亚甲基蓝光催化降解反应符合一级反应动力学,反应速率常数为0.079 30.     

Rapid anodic growth of TiO2 and WO3 nanotubes in fluoride free electrolytes

In the present work we report on the formation of bundles of high aspect ratio TiO₂ nanotubes and WO₃ nanopores structures with very thin tube or pore walls using anodization under “high voltage” conditions in perchlorate or chloride containing electrolytes. The bundles of TiO₂ nanotubes consist of separated tubes with diameters in the range of approximately 20–40 nm and the WO₃ nanopores consist of pores with diameters in the range of 30–50 nm. Growth occurs locally at specific surface locations. Both the TiO₂ and the WO₃ structures can be grown up to several dozens of micrometers in length within few minutes. We suggest that the growth of these high aspect structures is initiated by localized anodic breakdown event, triggered by a sufficiently high applied anodic field.     

A Comparative Study of Functionalized High‐Purity Carbon Nanotubes towards the V(IV)/V(V) Redox Reaction Using Cyclic Voltammetry and Scanning Electrochemical Microscopy

Functionalized high purity carbon nanotubes (CNTs) with various amounts of oxygen containing surface groups were investigated towards the relevant redox reactions of the all‐vanadium redox flow battery. The quinone/hydroquinone redox peaks between 0.0 and 0.7 V vs. Ag|AgCl|KCl_sat. were used to quantifying the degree of functionalization and correlated to XPS results. Cyclic voltammetry in vanadyl sulfate‐containing 3 M H₂SO₄ as a common supporting electrolyte showed no influence of the amount of surface groups on the V(IV)/V(V) redox system. In contrast, the reactions occurring at the negative electrode (V(II)/V(III) and V(III)/V(IV)) are strongly affected by oxygen surface groups. However, under modified experimental conditions, SECM experiments detecting the consumption of VO²⁺ molecules by CNT thin films in pH=2 solution show improved onset potentials with increased surface oxygen content up to ∼ 3 at%. Further increase in surface oxygen up to 8 at% led to minor improvement. These dissimilar results under different experimental conditions are rationalized by suggesting that oxygen functional groups do not form the active site for the V(IV)/V(V) reaction but wetting of the catalyst layer is of high importance.     

Controllable Synthesis of Ultrathin NiCo2O4 Nanosheets Incorporated onto Composite Nanotubes for Efficient Oxygen Reduction

Exploring non‐precious‐metal‐based oxygen reduction reaction (ORR) electrocatalysts featuring high efficiency, low cost, and environmental friendliness is of great importance for the broad applications of fuel cells and metal–air batteries. In this work, ultrathin NiCo₂O₄ nanosheets deposited on 1D SnO₂ nanotubes (SNT) were successfully fabricated through a productive electrospinning technique followed by a sintering and low‐temperature coprecipitation strategy. This hierarchically engineered architecture has ultrathin NiCo₂O₄ nanosheets uniformly and fully erected on both walls of tubular SNTs, which results in improved electrochemical activity as an ORR catalyst, in terms of positive onset potential and high current density, as well as superior tolerance to crossover effects and long‐term durability with respect to the commercial Pt/C catalyst. The excellent performance of SNT@NiCo₂O₄ composites may originate from their rationally designed hierarchical tubular nanostructure with completely exposed active sites and interconnected 1D networks for efficient electron and electrolyte transfer; this makes these composite nanotubes promising candidates to replace platinum‐based catalysts for practical fuel cell and metal–air battery applications.     

Titanium oxide nanotubes prepared in phosphate electrolytes

In the present communication, we report the electrochemical formation of self-organized titanium oxide nanotubes (π-TiO₂) prepared in fluoride ion containing phosphate electrolytes. The morphology of the π-TiO₂ layers (particularly the pore diameter and length) is affected by the electrochemical conditions used (applied potential, electrolyte composition, pH, and anodizing time). Under specific sets of conditions highly self-organized titanium oxide nanotubes are formed with diameters varying from approx. 40 nm to 100 nm and length from approx. 100 nm to 4 μm. XPS investigations show that the nanotubes formed in phosphate solutions contain a significant amount of phosphorous species.     

含氯离子电解液中二氧化钛纳米管的阳极氧化形成机理及应用

采用电化学阳极氧化的方法,以氯化铵(NH₄Cl)水溶液为电解液,在纯钛表面制备了二氧化钛(TiO₂)纳米管。考察了制备电压、氧化时间、Cl^-浓度和钛基体的退火处理对阳极氧化过程的影响规律,探讨了在含氯离子电解液中纳米管的形成机理,并基于上述含氯离子电解液中纳米管形成机制,通过两步阳极氧化法得到无支撑纳米管薄膜。     

Electrospun V2O5 Nanostructures with Controllable Morphology as High‐Performance Cathode Materials for Lithium‐Ion Batteries

Porous V₂O₅ nanotubes, hierarchical V₂O₅ nanofibers, and single‐crystalline V₂O₅ nanobelts were controllably synthesized by using a simple electrospinning technique and subsequent annealing. The mechanism for the formation of these controllable structures was investigated. When tested as the cathode materials in lithium‐ion batteries (LIBs), the as‐formed V₂O₅ nanostructures exhibited a highly reversible capacity, excellent cycling performance, and good rate capacity. In particular, the porous V₂O₅ nanotubes provided short distances for Li⁺‐ion diffusion and large electrode–electrolyte contact areas for high Li⁺‐ion flux across the interface; Moreover, these nanotubes delivered a high power density of 40.2 kW kg^?1 whilst the energy density remained as high as 201 W h kg^?1, which, as one of the highest values measured on V₂O₅‐based cathode materials, could bridge the performance gap between batteries and supercapacitors. Moreover, to the best of our knowledge, this is the first preparation of single‐crystalline V₂O₅ nanobelts by using electrospinning techniques. Interestingly, the beneficial crystal orientation provided improved cycling stability for lithium intercalation. These results demonstrate that further improvement or optimization of electrochemical performance in transition‐metal‐oxide‐based electrode materials could be realized by the design of 1D nanostructures with unique morphologies.     

Electrolyte Effects on Charge Transport Behavior of [Os(bpy)2(PVP)10Cl]Cl and [Ru(bpy)2(PVP)10Cl]Cl Redox Polymers in Ultra‐Thin Films of Polyions

Metallopolymer films have important applications in electrochemical catalysis. The alternate electrostatic layer‐by‐layer method was used to assemble films of [Ru(bpy)₂(PVP)₁₀Cl]Cl (denoted as ClRu‐PVP) and [Os(bpy)₂(PVP)₁₀Cl]Cl (ClOs‐PVP) metallopolymers onto pyrolytic graphite electrodes. Film thickness estimated by quartz crystal microbalance was 6–8 nm. The effects of pH, electrolyte species and concentration on the electrochemical properties of these electroactive polymers were studied using cyclic voltammetry (CV). Behavior in various electrolytes was compared. Also the mass changes within the ultra‐thin film during redox of Os^2+/3+ were characterized by in situ electrochemical quartz crystal microbalance (EQCM). The results indicate rapid reversible electron transfer, and show that both ClRu‐PVP and ClOs‐PVP have compact surface structures while ClOs‐PVP is a little denser than ClRu‐PVP. Although hydrogen ions do not participate in the chemical reaction of either film, the movement of Na⁺ cation and water accompanies the redox process of ClOs‐PVP films.     

碳纳米管的活化处理及对其电化学容量影响的研究

采用KOH为活性剂,对碳纳米管进行活化处理,经透射电子显微镜和高分辨透射电子显微镜从不同角度观察,发现得到了两端开口,管长较短,管壁粗糙的活性碳纳米管．用氮气自动吸附仪测试了活化前后两种碳纳米管的比表面积,发现活性碳纳米管具有比活化前碳纳米管更高的有效比表面积,将这两种碳纳米管分别作为电极材料应用于电化学超级电容器,经测试,发现活化后的碳纳米管的电化学容量大大提高,在有机电解液中达到了50F／g．