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Developing inexpensive, noble metal-free, efficient, stable, and bifunctional electrocatalysts has attracted significant research interest in electrocatalysis and air battery-based energy storage d...

Volta potential measurements from Scanning Kelvin Probe Force Microscopy (SKPFM) have been used over the years to discuss the nobility of an alloy’s surface and its tendency to localised corrosion. This has led to classical interpretations that may (unfortunately) over-generalise the actual Volta potential behaviour of the surface during ongoing localised corrosion. Consequently, issues on correlating Volta potential measurements to corrosion potentials and mechanisms arise. With the advent of in-situ SKPFM, which allows for the Volta potential measurements during ongoing dynamic surface electrochemical and/or physicochemical reactions, novel observations regarding Volta potential evolution are available. This calls for an updated and re-evaluated interpretation of the Volta potential measurements from SKPFM relating to localised corrosion analysis. The novel interpretation includes an additional layer of information, as compared to classical interpretations; there is greater information on the role of interfacial parameters and time (kinetics). Optimistically, this will bridge the gap of Volta potential measurements and its correlation to corrosion potentials.

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 evolution of corrosion products that develop within the scratched region of a 96 wt% zinc-rich paint exposed to continuous salt spray for 1,000 h has been investigated by both electrochemical a...

Economical electro-catalysts for the oxygen reduction reaction (ORR) are highly desirable for a range of advance energy storage technologies. Chromium compounds have been suggested as one possible source of non-precious metal based catalysts for oxygen reduction reaction (ORR), especially chromia (Cr2 O3 ) which is the most stable form of Cr oxide at room temperature. Using density functional theory+U calculations, we investigate the 4-electron ORR on the hydroxylated Cr2 O3 surfaces alloyed with 17 different transition metals. On the one hand, we find that the ORR overpotential is lower when the Cr2 O3 surface alloyed with elements towards the end of both the first and second rows of transition metals. Among these elements, Cd alloyed Cr2 O3 surface is found to promote the ORR the most, but due to its high toxicity and price it loses out to Zn as the recommended alloyant. On the other hand, we find that the ORR overpotential is generally higher and less varied on the Cr2 O3 surface alloyed with the early-to-mid row transition metal elements (e. g. Zr, Ti). As Cr2 O3 is also a major component in the passive film on stainless steels, where a low ORR rate is desirable to reduce the impact of localized corrosion. This implies that alloying with early-to-mid row transition elements could be beneficial to stainless steels. The difference in oxygen reduction activity is attributed to the tendency of forming stable ORR intermediates during the oxygen reduction process.

Nanocrystalline and amorphous nickel-tungsten alloy with relatively high tungsten contents of up to 34 at.% were produced by electrodeposition. The Hall-Petch relationship and breakdown was studied in this alloy, with grain sizes ranging from 25 nm to fully amorphous. In this higher tungsten content alloy, evidence of solid-solution strengthening was found, which was previously absent in alloys with

Positively charged nanodiamond (ND) is used to decorate negatively charged graphene oxide (GO) to form a GO-ND hybrid nanomaterial by electrostatic force. Structural studies results showed that after the decoration, the aggregation of GO sheets is extensively hindered in both at the powder and dispersion states, with a clear reduction in the layer numbers in the latter. The mechanical properties of epoxy/GO, epoxy/ND and epoxy/GO-ND were investigated and compared. The results showed that the GO increased the ductility of epoxy, while the ND increased the rigidity. The best mechanical performance was found for the epoxy/GO-ND nanocomposites, at a GO:ND ratio of 1:5. The reinforcement mechanism of the nanophases was further illustrated by the fracture surface of SEM/optical images and TGA analysis. In addition, the anti-corrosion property of the thus developed epoxy nanocomposite coatings was revealed by electrochemical impedance spectroscopy (EIS), and the results demonstrated that the epoxy/GO-ND coatings exhibited better anti-corrosion property.

Microbiologically influenced corrosion (MIC) is a major concern in a wide range of industries, with claims that it contributes 20% of the total annual corrosion cost. The focus of this present work is to review critically the most recent proposals for MIC mechanisms, with particular emphasis on whether or not these make sense in terms of their electrochemistry. It is determined that, despite the long history of investigating MIC, we are still a long way from really understanding its fundamental mechanisms, especially in relation to non-sulphate reducing bacterial (SRB) anaerobes. Nevertheless, we do know that both the cathodic polarization theory and direct electron transfer from the metal into the cell are incorrect. Electrically conducting pili also do not appear to play a role in direct electron transfer, although these could still play a role in aiding the mass transport of redox mediators. However, it is not clear if the microorganisms are just altering the local chemistry or if they are participating directly in the electrochemical corrosion process, albeit via the generation of redox mediators. The review finishes with suggestions on what needs to be done to further our understanding of MIC.

Cathode materials are important in determining the performance of a capacitive deionization cell. In this work activated carbon cloth (ACC) grafted with tungsten oxide was employed as cathode, which was first grown on ACC with a flaky morphology by a self-anodization method. The oxide was uniformly distributed over the surface of the ACC. The desalination capacity of the obtained material is deduced from electrochemical characterization, based on the preliminary stage, in the static 1 M NaCl aqueous solution over a potential range from −1 V to 0.2 V. The modified ACC attained an enhanced ion removal ability, which gives promising potential in the further application on removing heavy ions from the wastewater of industries.

Toxic metal ions, such as Ni2+ and Mn2+, in industrial waste streams are nonbiodegradable and can cause damage to the human body. Electrochemical cleaning techniques are attractive as they offer more control and produce less sludge than do chemical/biological approaches without the high pressures needed for membranes. Here, nanoneedle-structured α-MnO2/carbon fiber paper (CFP) composites were synthesized by a hydrothermal approach and used as electrodes for combined electroadsorption and capacitive deionization removal of nickel and manganese ions from pseudoindustrial waste streams. The specific performance of α-MnO2/CFP (16.4 mg Ni2+ per g of active material) not only shows a great improvement in comparison with its original CFP substrate (0.034 Ni2+ mg per g), but also is over 6 times that of activated carbon (2.5 mg Ni2+ per g). The high performance of α-MnO2/CFP composites is attributed to their high surface area, desirable mesoporosity, pore-size distribution that permits the further access of ions, and their property as a pseudocapacitor, which contributes to a more efficient electron/charge transfer in the faradic process. Unfortunately, it was also found that some Mn2+ ions are released due to the partial reduction of MnO2 when operated as a negative electrode. For the removal of Mn2+ ions, an asymmetric arrangement, consisting of a MnO2/CFP positive electrode and an activated carbon negative electrode, was employed. This arrangement reduced the Mn2+ concentration from 100 ppm to less than 2 ppm, a vast improvement over the systematical two-activated carbon electrode system that could only reach 42 ppm under the same conditions. It was also observed that as long as the MnO2/CFP composite was maintained as a positive electrode, it was completely stable. The technique was able to reduce both Ni2+ and Mn2+ ions to well below the 10 ppm requirement for discharge into public sewers in Singapore.

The role of biogenic H 2 S in the microbiologically influenced corrosion (MIC) of carbon steel was investigated. Desulfovibrio vulgaris (ATCC 7757), a sulfate reducing bacterium, was tested against C1018 carbon steel in anaerobic vials with three different sizes, each filled with 40 mL of ATCC 1249 culture medium, providing headspace volumes of 10 mL, 85 mL and 160 mL, respectively for H 2 S to escape. Results showed that a larger headspace led to a lower H 2 S concentration in the culture medium, and this increased the sessile cell count and made the iron sulfide film thinner, resulting in increased MIC.

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.

Investigations in microbiological-influenced corrosion in recent years have suggested a positive correlation in steel corrosion with methanogens. However, many cases reported in the literature were performed in medium containing Na2S·9H2O or l -cysteine, from which the methanogens can produce H2S that in itself is corrosive. Here it is demonstrated that if all sources of H2S are removed, the methanogenic strain Methanosarcina barkeri only cause accelerated corrosion when deprived of traditional energy sources for methanogenesis, such as methanol and H2. Possible mechanisms for this phenomenon of starvation conditions triggering accelerated corrosion are discussed.

In this work, we have developed a system to prepare self-supported nickel-based nanobelts of similar morphology but varying chemical compositions. After a simple hydrothermal treatment, phase-pure self-supported nickel sulfate hydroxide (Ni(SO4)0.3(OH)1.4) nanobelts can be directly prepared on a nickel foam substrate, which can be converted into NiO nanobelts by calcination without any change in the morphology. Alternatively, the pristine Ni(SO4)0.3(OH)1.4 nanobelts can be transformed into Ni3S2 nanobelts via a facile hydrothermal sulfidization. When these nanobelts were applied in supercapacitors they demonstrated contrasting performances, with the Ni3S2 nanobelts showing a high reversible capacitance of about 3.5 F cm–2 at 10 mA cm–2 over 1000 cycles, while the Ni(SO4)0.3(OH)1.4 sample delivered insignificant capacitance. This huge difference in electrochemical activity relates to dramatically different amounts of charge being stored within their bulks, with charge and discharge rates limited by the bul...

Motivated by the primary benefit of reduced material cost, the thickness of crystalline silicon solar cells has been continuously reduced. Laboratory and industrial studies have explored ultrathin crystalline silicon solar cells below 50 μ m with ambitious endeavors toward thicknesses of only a few micrometers. Ultrathin crystalline silicon solar cells require compatible small-scale surface textures to enhance the optical absorption. For this purpose, a novel submicron periodic nanostructure—periodic upright nanopyramids (PuNPs)—is fabricated by an integrated process of laser interference lithography and anisotropic etching of silicon in an alkaline solution. By simulation and measurements, we demonstrate that PuNPs are able to reduce front surface reflectance more effectively than conventional micron-scale pyramid textures and previously investigated periodic inverted nanopyramids (PiNPs). With a silicon nitride antireflection coating, we predict that PuNPs reduce the front surface reflectance to below 1% at an angle of incidence of 8°, which is comparable to black silicon. The superior antireflective property of PuNPs contributes to an absorbed photocurrent density of 40.8 mA/cm2 for a 40 μ m silicon absorber layer, which is 0.7 mA/cm2 higher than PiNPs, 0.8 mA/cm2 higher than inverted pyramids and 1 mA/cm2 higher than upright pyramids.

Sensitized AA5083-H2 aluminum alloy was exposed to chloride-laden thin-film electrolyte at ambient temperature (20%–85% relative humidity) and the local Volta potential measured, in-situ and in real-time, using the Scanning Kelvin Probe Force Microscopy, with the intention to elucidate the earliest stage of localized corrosion. Positive Volta potentials vs alloy matrix were measured for magnesium silicides in ambient air, which, however, underwent a severe nobility loss during corrosion, causing their nobility to invert to active potentials (negative) relative to the alloy matrix. The reason for the nobility inversion was explained by the preferential dissolution of Mg2+, which resulted in an electropositive surface. Aluminides, both with and without silicon, were seen to form the main cathodes at all exposure conditions. The local alloy matrix next to closely-separated aluminides were seen to adopt the Volta potential of the neighbor aluminides, which, hence, resulted in local corrosion protection. The phenomenon of nobility adoption introduced in this work raises questions regarding the anode-to-cathode ratio, which was observed to change during corrosion, and the resulting impact to localized micro-galvanic corrosion. This work further demonstrates that it is necessary to measure the Volta potential during corrosion to reflect the true relationship between the Volta potential and corrosion potential or breakdown potential.

A perspective is provided on mechanisms for microbially influenced corrosion by sulfate-reducing bacteria and metal-oxidizing bacteria. The authors discuss the confusion introduced by cross-disciplinary discussions of MIC mechanisms and the divergent terminologies used by microbiologists and electrochemists, e.g. “anaerobic” and “direct electron transfer.” Examination of the cathodic depolarization theory for corrosion of carbon steel by sulfate-reducing bacteria suggests that the theory cannot explain the observations. Stainless steels containing

We report a rationally designed two-step method to fabricate self-supported Ni3S2 nanosheet arrays. We first used 2-methylimidazole (2-MI), an organic molecule commonly served as organic linkers in metal–organic frameworks (MOFs), to synthesize an α-Ni(OH)2 nanosheet array as a precursor, followed by its hydrothermal sulfidization into Ni3S2. The resulting Ni3S2 nanosheet array demonstrated superior supercapacitance properties, with a very high capacitance of about 1,000 F g–1 being delivered at a high current density of 50 A g–1 for 20,000 charge–discharge cycles. This performance is unparalleled by other reported nickel sulfide-based supercapacitors and is also advantageous compared to other nickel-based materials such as NiO and Ni(OH)2. An asymmetric supercapacitor was then established, exhibiting a very stable capacitance of about 200 F g–1 at a high current density of 10 A g–1 for 10,000 cycles and a surprisingly high energy density of 202 W h kg–1. This value is comparable to that of the lithium-io...

In this work, self-supported Ni 3 S 2 nanorod arrays on nickel foam substrate are prepared by a facile one-pot hydrothermal method. These phase-pure nanorods possess an interesting secondary structure: the surface is covered with ultrathin Ni 3 S 2 nanosheets. Such an integration of one-dimensional nanorod array with two-dimensional nanosheets combines the advantages of the two components, demonstrating enhanced pseudocapactive performance with a high and very stable areal capacitance over prolonged charge-discharge tests. When the as-prepared material is incorporated as the cathode in an asymmetric supercapacitor, where activated carbon is selected as the anode, the device exhibits highly reversible capacitance with insignificant decay over 3000 cycles. Excellent energy densities of 115 and 102 Wh kg −1 are achieved at power densities of 506 and 1094 W kg −1 , respectively. Such a performance can be attributed to the unique sheet-on-rod nanoarray on the nickel foam substrate.

Grades 2, 5 and Grade 7 were investigated in near anaerobic (

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.

Compared to graphene, graphene oxides (GO) have also been applied in many areas due to their unique structures and properties, such as tunable and functionalized surfaces, great processing and competition for scalable production. In this review, GO reinforced epoxy resins are systematically looked into, where they can be classified into two broad groups: epoxy/GO nanocomposites and epoxy/GO coatings, depending on the application types. In the epoxy/GO nanocomposites, the GO reinforcement on the different properties of epoxy matrices are described in details, like mechanical, toughness, thermal conductivity and so on. As for the epoxy/GO coatings, the GO effect on the anti-corrosion properties is one of the main concerns. For both of them, GO modification methods and dispersing routes in the respective epoxy matrices are among the most important considerations. There are some remaining unsolved issues, which will be highlighted together with the future perspectives of the GO reinforced epoxy resins.

Titanium is being considered as a candidate material for high-level nuclear waste container due to its superior corrosion resistance. However, problems may arise when titanium comes in contact with hydrogen-bearing environments, as it is vulnerable to hydrogen embrittlement. To assess the lifetime of titanium container, hydrogen content and hydrogen permeation efficiency were estimated in infiltrated water through bentonite prepared with simulated groundwater of Beishan, the preselected area in China, by electrochemical and metallurgical analysis. It is concluded that the absorbed hydrogen due to general corrosion will not be sufficient for hydrogen embrittlement to occur in titanium for at least 10,000 years after deep geological disposal. This represents more than 300 and 5,000 half-lives of Cs137 and Cs134 respectively; cesium being the most likely element to escape a repository due to its high solubility in groundwaters.

The electrolyte film thickness condensation and evaporation is an important parameter for complexity of atmospheric corrosion. Atmospheric corrosion rate of zinc at the west coast of Singapore was measured for one year using an electrical resistance monitoring system. The analysis of the data reveals that significant corrosion rates only occur at specific hours on dry days. The beginning of this period corresponds to falling temperature and rising relative humidity resulting in the formation of a film of moisture on the zinc surface and the end corresponds to a point that this film dries as the temperature increases. This finding allowed the drying rate of the moisture film to be estimated for input into a moving boundary simulation model of the galvanic corrosion in scratched and zinc coating samples. The simulation results showed that the maximum corrosion rate occurs at electrolyte thickness of about 8 μm. Moreover, the simulation suggested that cut-edge is a more harmful defect than scratch, which was confirmed by the appearance of iron corrosion products on atmospheric exposed cut-edge samples whereas scratched samples were not corroded after one week of exposure. Finally, moving boundary simulation allowed to predict the changes to the geometry of the corroding electrodes.

The mechanism of corrosion protection of steel substrate by a 96 wt% zinc primer that was damaged by scratch was simulated. The primer was approximated to pure zinc layer due to high percentage of zinc. Results show that while the middle of scratch is protected by sacrificial protection, at the corners of scratch a local cathodic inhibition results in barrier protection due to variations in the dissolved oxygen concentration. Moving boundary simulation allowed to predict the changes to the geometry of the corroding electrodes. Finally, taking into account zinc electrodes consumption, the period of sacrificial protection was predicted by simulations.

Financial support was provided by the Singapore Institute of Manufacturing Technology (SIMTech) under project No. U12-S-036SU. Y. T. Tan acknowledges the support of a NUS Graduate School (NGS) scholarship for his PhD studies.

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.

(Received July 22, 2015; Revised November 03, 2015; Accepted November 03, 2015)The pitting behaviours of 304L and 316L stainless steels were investigated at 3 °C to 90 °C in 1 M solutions of NaCl, NaBr and NaI by potentiodynamic polarization. The temperature dependences of the pitting potential varied according to the anion, being near linear in bromide but exponential in chloride. As a result, at low temperatures grades 304L and 316L steel are most susceptible to pitting by bromide ions, while at high temperatures both stainless steels were more susceptible to pitting by small chloride anions than the larger bromide and iodide. Thus, increasing temperature appears to favour attack by smaller anions. This paper will attempt to rationalise both of the above findings in terms of the point defect model. Initial findings are that qualitatively this approach can be reasonably successful, but not at the quantitative level, possibly due to insufficient data on the mechanical properties of thin passive films.Keywords: stainless steel, pitting corrosion, anion size

Materials of choice for offshore structures and the marine industry have been increasingly favoring materials that offer high strength-to-weight ratios. One of the most promising families of light-weight materials is titanium alloys, but these do have two potential Achilles` heels: (i) the passive film may not form or may be unstable in low oxygen environments, leading to rapid corrosion; and (ii) titanium is a strong hydride former, making it vulnerable to hydrogen embrittlement (cracking) at high temperatures in low oxygen environments. Unfortunately, such environments exist at deep sea well-heads; temperatures can exceed , and oxygen levels can drop below 1 ppm. The present study demonstrates the results of investigations into the corrosion behavior of a range of titanium alloys, including newly developed alloys containing rare earth additions for refined microstructure and added strength, in artificial seawater over the temperature range of to . Tests include potentiodynamic polarization, crevice corrosion, and U-bend stress corrosion cracking.

The critical chloride threshold concentration and the polarizability of the barrier layer/outer layer (as defined by the Point Defect Model) have been determined for AISI 1020 carbon steel in simulated concrete pore solutions of pH 9, 10, 11 and 12.5 with varying chloride and bicarbonate/carbonate concentrations. It was found that bicarbonate/carbonate ions have a beneficial effect on the charge transfer resistance, the critical chloride threshold concentration and the pitting potential. When the critical chloride threshold concentration for pitting is exceeded, the inhibitive effectiveness of bicarbonate/carbonate against pitting corrosion is around one order of magnitude lower compared to hydroxide.

The corrosion performance of commercial hard disk media which was subjected to bi-level surface modification has been reported. The surface treatment was carried out by bombarding the surface of the magnetic media with C+ ions at 350 eV followed by 90 eV using filtered cathodic vacuum arc (FCVA). The energy and embedment depth of the impinging C+ ions were adjusted by applying an optimized bias to the substrate and simulated by a Stopping and Range of Ions in Matter (SRIM) code which predicted the formation of a graded atomically mixed layer at the carbon-media interface. Cross-section transmission electron microscopy (TEM) revealed the formation of a 1.8 nm dense nano-layered carbon overcoat structure on the surface of the media. Despite an ~ 33% reduction in the thickness, the bi-level surface modified disk showed corrosion performance similar to that of a commercially manufactured disk with a thicker carbon overcoat of 2.7 nm. This improvement in the corrosion/oxidation resistance per unit thickness can be attributed to the formation of a dense and highly sp3 bonded carbon layer, as revealed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. This study demonstrates the effectiveness of the bi-level surface modification technique in forming an ultra-thin yet protective overcoat for future hard disks with high areal densities.

In this paper, the degradation of Acid Orange 7 (AO7), by persulfate activated from in situ electrochemically generated Cu+ (EC-Cu-PDS) was studied. The influence of key parameters such as pH, current density, copper concentration and electrode size were investigated, with the optimal operation condition being pH 2 with cathode to anode area of 4:1. The degradation efficiency of the EC-Cu-PDS after 30 min was compared to the efficiency of other electrocatalysis system using iron, cobalt or silver. The proposed EC-Cu-PDS process has a higher degradation efficiency of 70%, as compare to 56% of the EC-Fe-PDS process. Although the EC-Cu-PDS process has a lower degradation efficiency compared to 100% of Co-PMS and 80% of the EC-Ag-PDS processes, the EC-Cu-PDS process would be cheaper and safer to operate than the latter processes. The estimated electrical energy consumption of the EC-Cu-PDS processes was significantly lower than other electrocatalysis processes, ranging from 0.008 to 2.42 kW h m−3. The degradation of AO7 by the EC-Cu-PDS process was via oxidization of AO7 by both hydroxyl radicals generated at the anode and sulfate radicals generated by both electrochemically at the anode and Cu+ activation of PDS.

A sensor device is demonstrated based on changes in the impedance within a layer of porous silicon on exposure to the target analyte. Selectivity can be obtained by attaching functional organic groups onto the porous silicon. If the target molecules do not interact with the attached functional groups, then the sensor's response varies linearly with concentration. In the event however, when strong hydrogen-bonding occurs a much larger response is recorded on exposure to low concentrations than would be expected. The hydrogen-bonding either causes increased penetration of the solvent into the porous matrix or reduces the potential drop across the solvent/porous silicon interface, i.e. reduces magnitude of the space charge capacitance in the silicon nanorods.

Electrochemical impedance spectroscopy is used to study the etching of p-type silicon in ethanolic HF over the potential region where porous silicon forms. The impedance data fits a single equivalent circuit for all HF concentrations. The appearance of a finite length Warburg element at potentials negative of the flatband potential is interpreted as evidence that a submonolayer of oxide/hydroxide islands form in the Tafel region. Although this is contrary to previous studies it is consistent with published spectroscopy data. Low values for the double layer capacitance are attributed to the hydrophobic nature of the Si-H terminated surface, whereas an inductance loop or negative differential capacitance behavior seen at low frequencies is interpreted in terms of a potential-dependent surface roughness through the formation/dissolution of a submonolayer oxide. Interpretation of the resistive components of the circuit are in line with the known mechanism of etching, that is charge transfer changing to mixed control as the potential is made more positive and dissolution becomes limited by mass transport processes. (C) 2014 The Electrochemical Society. All rights reserved.

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.

A DC magnetron is used to sputter highly conductive ITO on the surface of porous silicon to form LEDs that not only emit the usual red-orange light in one bias direction, but stable white light when biased in the opposite direction. The direction of the bias required for white light emission depends on whether p- or n-type silicon is used. The white light is clearly visible to the naked eye and stable for >2 h of continuous operation. The likely source of the white light emission is from the intra- or inter-band transitions of hot carriers generated in an avalanche process.

The understanding of microbial influenced corrosion (MIC) in aerobic mixed biofilms benefits from advanced microscopy and microbial ecology characterization of biofilms. Here, the onset of MIC in stainless steel coupons was studied in both natural and artificial seawater. Rapid selection of biofilm-forming microorganisms from natural seawater was observed for field experiments. Potential ennoblement was observed only in natural seawater. A seawater derived mixed microbial consortium enriched in artificial seawater was used to characterize the effect of several parameters on MIC. The concentration of organic carbon was the major determinant of MIC, while shaking speed and polishing played minor roles. The biofilm was preferentially formed at the grain boundaries. These results outline the need for MIC onset characterization with mixed microbial consortia to predict long-term corrosion behavior of stainless steel in seawater. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

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.

We have used electrochemical impedance to investigate the improvement in photovoltaic performance in aging of ionic liquid dye-sensitized solar cells using a high-absorption coefficient organic dye (2E)-2-cyano-3-(5-(5-((E)-2-(10-(2-ethylhexyl)-10H-phenothiazin-7-yl)vinyl)thiophen-2-yl)thiophen-2-yl)acrylic acid, which is in contrast to N719-based devices. It was found that the enhancement is due to reduced recombination of the photoexcited electrons. The decreased recombination plausibly originates from molecular re-orientation along with cation adsorption, with Fourier transform infrared spectra lending support to the former mechanism. After aging, the photovoltaic device using the organic dye outperforms the counterpart by the ruthenium complex dye and achieves an impressive efficiency of 5.6% under AM 1.5 100 mW/cm2 illumination. Copyright © 2011 John Wiley & Sons, Ltd.

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.

Two novel organic dyes have been synthesized using electron rich phenothiazine as electron donors and oligothiophene vinylene as conjugation spacers. The two dyes (2E)-2-cyano-3-(5-(5-((E)-2-(10-(2 ...

Calcium phosphate coatings, in particular synthetic hydroxyapatite, are applied to the surfaces of titanium and its alloys so as to improve the biocompatibility and biological performance. Currently, plasma spraying is the clinically accepted technique for the deposition of calcium phosphate onto titanium. Electrochemical cathodic deposition is emerging as an alternative technique due to it being a nonline-of-sight technique. In this present study, it is demonstrated that increased thickness, crystallinity and adhesion of calcium phosphate coating on titanium is achieved by periodic pulsed low current densities compared to a constant current deposition method. It is believed that the “off” part of the AC deposition cycle gives the calcium and phosphate ions in the bulk solution sufficient time to diffuse to the titanium's surface maintaining more favourable conditions for HA growth. Unfortunately, although pulsed deposition at high current densities is able to produce thick coatings it cannot avoid problems associated with hydrogen bubbles and thus both AC and DC films deposited at high current densities have low crystallinity and poor adhesion.

To improve on the biological performance of titanium and its alloys hydroxyapatite coatings are applied to the surface. Although plasma spraying is currently the clinically accepted deposition technique, it has drawbacks including being a line-of-site method which greatly limits its uses for coating either irregular-shapes or the low elastic moduli porous materials and metallic sponges now being developed for implants. Electrochemical cathodic deposition is an alternative technique but this also has disadvantages, mainly the crystallinity of the deposited hydroxyapatite and its adhesion to the substrate. In the present study it is demonstrated that increased adhesion and crystallinity are obtained by either pre-treating the Ti substrate with NaOH followed by a heat treatment at 600 °C or the addition of H 2 O 2 to the electrolyte. The increased adhesion is believed to result in the former case from the development of a titanate binding layer. In the latter case the local alkaline conditions required for deposition are obtained from the reduction of the peroxide, thereby negating the need to evolve hydrogen that can damage the film. Rotating the substrate at low revolutions is also beneficial for adhesion however; at high rotation rates bulk precipitation is favoured over film formation.

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.