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In this work, three dimensional self-supported Co3O4@Ni3S2 nanowire arrays on nickel foam substrates are prepared by a multi-step method. The as-synthesized product harvests the advantages of the high electrical conductivity of the Co3O4 nanowire arrays, large surface area provided by the Ni3S2 nanosheets, and the efficient ion diffusion pathway offered by the self-supported structure. The sample is subsequently used as the electrode material for supercapacitors, at a current density of 2 mA cm−2 a charge storage capability of 2.41C cm−2 (5.5 F cm−2) was achieved. When the sample is charged and discharged at 50 mA cm−2 for 10000 cycles, a stable capacity of 0.86C cm−2 (1.5 F cm−2) can still be maintained at the end of the test. Such a performance is better than those values reported in the literature obtained with similar systems even at a lower current density. We also assemble an asymmetric supercapacitor by pairing the Co3O4@Ni3S2 nanoarrays with activated carbon, and it shows good cyclability over 10000 cycles. The device yields energy densities of 65 and 32.3 Wh kg−1 at power densities of 361 and 9090 W kg−1, respectively. Furthermore, it is able to light up 12 red light-emitting diodes, showing promising potential for practical applications.

The micro-galvanic interactions between Cu-Fe-Mn-Li-containing aluminides and the alloy matrix of aluminium alloy AA2099 during chloride-induced corrosion were investigated in-situ with real-time monitoring of the local contact potential difference (VCPD) using the scanning Kelvin probe force microscopy (SKPFM) at controlled relative humidities. The aluminides showed noble potentials and were able to ennoble their neighbouring matrix sites when a cluster of aluminides surrounded the matrix. The matrix, hence, adopted a more positive VCPD, towards that of the aluminides. The anode-to-cathode ratio changed throughout the corrosion exposure and was seen to show a dynamic character. Much higher local VCPD activities were recorded during the earliest stages of corrosion, when the Al-Li AA2099 surface was first exposed to high humidities, than in later RH cycles; a phenomenon not seen in other aluminium alloys.

The structural stability of the electrode material is very important to achieve a long cycle life of a supercapacitor. In this work, we demonstrate a multi-step preparation of metal organic framework (MOF)-reinforced Co9S8 self-supported nanowire arrays for high-performance supercapacitor with very high durability. Co-based nanowires with a composition of Co2(OH)2(CO3)2 are first synthesized on the nickel foam substrate by a facile hydrothermal method. After converting them into Co3O4 nanowires by calcination, leaf-like Co-based MOF (Co-MOF) has been deposited on the surface of the oxide nanoarrays (NAs), forming Co3O4 NAs@Co-MOF core-shell structure. Co9S8 nanoarrays are finally obtained by an in-situ hydrothermal sulfurization, and this process not only realizes the chemical transformation, but also changes the morphology of the Co-MOF from leaf-like microplatelets into nanosheets wrapping around the one-dimensional (1D) nanowires. When this sample has been applied for supercapacitor, it exhibits a 4.48 F cm−2 at a current density of 2 mA cm−2. The long-term cycling test is conducted at 25 mA cm−2, and the sample can maintain a high and stable capacitance of 1.6 F cm−2 after 100k cycles. This gives rise to a capacitance loss of only 5.1 × 10−4% per cycle, demonstrating an exceptional cycling stability. Furthermore, an asymmetric supercapacitor is assembled by pairing the Co9S8 nanoarrays with activated carbon, and the device delivers excellent cycle stability with a reversible capacitance of 416 mF cm−2 after 100k cycles, leading to a capacitance loss of 2.8 × 10−4% per cycle. Furthermore, a quasi-solid-state asymmetric supercapacitor is able to light up 16 red LEDs, demonstrating great potential for practical application with excellent cycle stability and mechanical flexibility.

This work describes a versatile hydrothermal approach to synthesize different types of hierarchical structures composed of single crystalline β-Ni(OH) 2 nanosheets. We use the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride (BmimCl) as a structure directing agent, which allows the formation of films consisting of β-Ni(OH) 2 nanosheets on the nickel foam substrate. It is found that the IL not only controls the formation of the sheet-like structure, but also promotes the growth of the nanosheets on the substrate, building an intimate contact between the active hydroxide film and the conducting metal foam. This IL agent is so effective that such a hierarchical film can also be obtained with different thickness and organization if different nickel salt precursor is used. When these film samples were applied as the electrode materials for pseudocapacitor, they exhibited promising capacitive properties with a high capacitance of about 700 F g ⿿1 even at a high charge-discharge current rate of 5 A g ⿿1 for prolonged cycling.

The electropolishing of p-type silicon has been investigated over a wide range of wafer conductivities (p− to p++) and HF concentrations (0.01–15 wt%) by potentiodynamic polarization. The rate of oxide dissolution was determined from the plateau current density observed in the electropolishing region of the IV curve; i.e. where the growth and dissolution rates of the anodic oxide film are equal. The IV curves of silicon are reminiscent of a corrosion process control by the dissolution of a salt film in which the rate of reaction is controlled by the removal of dissolved products away from the surface rather than reactants to the surface as previously proposed. For the silicon anodic oxide films this can be by either mass transport or further chemical reaction with HF species in solution. It is shown that this means the dissolution rate should be described by: Dissolution rate = D K sp [ HF ] 2 δ + k A K sp [ HF ] 6 This relationship is shown to hold for the whole concentration range investigated. Because hydrofluoric acid is a weak acid this rate equation converts to a cubic equation when written in terms of the total HF concentration as opposed to only the HF species, but in this form the connection with the dissolution mechanism is lost.

The electrodeposition of ZnO nanowire arrays from nitrate-based solutions was systematically studied as a function of electrodeposition parameters such as the concentration of zinc and nitrate ion precursors, applied current and charge. A model based on the local concentrations of Zn2+ and OH−, generated from the electrochemical reduction of NO3−, is proposed to explain the growth mechanism of ZnO nanowires. The ratio between OH− production and Zn2+ diffusion rates is identified as a crucial parameter, determining the nanowire growth regime and therefore the nanowire dimensions. As a result of the model and using NaNO3 as an additional source of NO3−, the versatility of ZnO deposition was enhanced in comparison with the deposition from sole Zn(NO3)2 solutions. Arrays of large aspect ratio (∼450) ZnO nanowires were successfully obtained at growth rates compatibles with large-scale production. The obtained ZnO nanowire arrays were characterized and analyzed by field emission scanning and transmission electron microscopies, X-ray diffraction, X-ray photoemission and optical spectroscopies.

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.

The role that applied potential has in controlling the properties of porous silicon formed on highly conductive p-type silicon in diluted HF has been investigated by studying the photoluminescence characteristics along the current–voltage curve and using high resolution transmission electron microscopy (HRTEM) evidence to support the conclusions drawn. A dramatic decrease in the average nanocrystal size was found to take place after the etching current density switched from an exponential dependence on the applied potential to a linear relationship. Importantly this event occurred prior to reaching the Ups potential (usually consider the onset of electropolishing). This rapid decrease in particle sizes has been explained in terms of the partial formation of an oxide film. The presence of oxygen terminated porous silicon allows a trapped exciton states model to be invoked, which removes the quantum confinement restrictions on the minimum particle size. Support for the presence of a partial oxide prior to Ups comes from both FTIR measurements and previous literature related to the location of the flat-band potential. © 2008 Elsevier Ltd. All rights reserved.

The influence of dissolved oxygen on titanium oxide film growth at various sweep rates was explored for both pure titanium and a titanium alloy. The results showed that the dissolved oxygen becomes only significant when the sweep rate is sufficiently low, but which can be neglected in the case of enough high sweep rates. It reasonably assumes that there is no adequate time for oxygen to move to the anodic surface when the sweep rate is sufficiently high so that the role of oxygen becomes insignificant. However, the situation was found to be different when the anodic oxide film was growing at a sufficiently low sweep rate dynamically. This is because the reduction of dissolved oxygen in the solution occurs and produces an additional current, which opposes the oxide growth current produced by adsorption and subsequent charge reaction.

The nature of the anodic oxide film that forms on titanium on titanium in 0.9% NaCl has been investigated using a wide range of techniques. A linear relationship was found between the critical current density required for passivation of titanium in 0.9% NaCl and the sweep rate. Anodic oxide films formed on titanium in 0.9% NaCl appear to consist of two layers, an inner compact layer, the growth of which continues to follow a high field growth law, and a porous less protective outer porous layer. XPS and XRD indicated that passive films on titanium consist mainly of TiO2. However, hydroxides and lower oxides are also present, especially in rapidly grown films. XRD data indicated that in 0.9% NaCl the anodic oxide film is formed through the preferential removal atoms in the plane of (0 0 2) in the course of electrochemical reaction. A model based analysis XPS spectra was proposed to explain the growth rate dependence of the degree of protection offered by anodic oxide films on titanium. XPS clearly demonstrated the present of Ti(III) and Ti(II) cations in the passive film. This is strong evidence that cation migration more likely dominates over anion migration in the growth mechanism of anodic oxide film. XPS data also revealed that the concentrations of Ti(III) and Ti(II) species within the oxide films increased as the oxide/metal interface was approached.

The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced.

Both potentiostatically and galvanostatically controlled electrochemical impedance spectroscopy were used to show that the impedance obtained from passivated titanium electrodes is dominated by the capacitance of the space-charge layer. As a result neither the thickness of the oxide layer nor its rate of thinning can be readily determined from impedance measurements. Furthermore, it appears that the corrosion rates of passive metals determined by EIS may significantly overestimate the true rate of dissolution. No significant differences were observed between the impedance data obtained from the galvanostatic and the potentiostatic measurements.

The mechanisms involved in the (photo)dissolution of silicon in fluoride solutions are discussed. Several methods, including intensity modulated photocurrent spectroscopy, in situ ir spectroscopy, rotating ring disc voltammetry and ellipsometry, have been used to examine the anodic dissolution process. The valence state of the dissolving silicon has been determined under different conditions, and the optical properties of the surface have also been characterized. The generation of hydrogen has been linked in the case of n-Si to intensity dependent photocurrent multiplication, and in the case of p-Si it has been related to different etching regimes. The current oscillations observed during the anodic dissolution of p-Si are also discussed.

Potential modulated reflectance (PMR) spectroscopy has been used to investigate thin anodic oxide films formed on titanium in sulphuric acid. The origin of the reflectance modulation is discussed and the experimental results are compared with theoretical calculations based on the three layer model. The overall shape and the dependence of the PMR spectra on film thickness have been modelled satisfactorily by assuming that the band gap of the oxide in the presence of an electric field is increased due to dielectric polarisation. The potential dependence of the modulated reflectance response for films of constant thickness was related to the electric field distribution in the oxide and the analysis showed that the modulated reflectance response is dominated by electroabsorption. The results demonstrate that PMR spectroscopy offers a more reliable method than Mott—Schottky plots for determining the flat-band potential of the oxide.

ac Impedance measurements and photocurrent spectroscopy have been used to examine the dependence on growth rate of the solid state properties of thin (

Titanium electrodes have been passivated by forming an anodic oxide film in acid solution are known to undergo a spontaneous passive—active transition at open circuit after a delay period that depends on the experimental conditions. In the present work, the reactivation of the electrode has been investigated, and it has been shown that it arises as the result of uniform chemical dissolution of the oxide film. The rate of dissolution, and hence the stability of the oxide, was found to depend on the rate at which the oxide film had been formed as well as on solution composition and temperature. The rate of film dissolution in sulphuric acid showed a first order dependence on proton concentration, but high dissolution rates were also measured in alkaline solution (pH 14). The activation energy for film dissolution in 3.0 mol dmt-3 H2SO4 was found to be 63 kJ molt-1. The reduction of the film under galvanostatic conditions was also investigated, and it was established that simultaneous chemical and electrochemical dissolution occurred uniformly across the electrode surface.

FTIR surface infrared spectroscopy of the mercury solution interface shows the adsorption of complex ions of mercury thiocyanate at potentials more positive than −200mV vs SCE, and indicates that thiocyanate ion is adsorbed by electrostatic physiadsorption at more negative potentials.

The corrosion of titanium in acidic solution leads to the accumulation of substantial amounts of hydrogen in the metal, modifying the subsequent voltammetric behaviour of the electrode. SIMS has been used to show that hydrogen penetrates into the metal to a depth of several microns during prolonged exposure to acid solutions and the SIMS images obtained suggest that crystalline platelets of titanium hydride are formed in a titanium matrix under these conditions. The hydride is only partially oxidised during subsequent anodisation of the titanium, so that successive passivation/activation cycles lead to the progressive build up of hydride.