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Now showing items 1 - 16 of 1278

  • Hydrotalcite-wrapped Co-B alloy with enhanced oxygen evolution activity

    Leng, Xue   Wu, Kuang-Hsu   Su, Bing-Jian   Jang, Ling-Yun   Gentle, Ian R.   Wang, Da-Wei  

    Water oxidation is one of the most attractive techniques for intermittent renewable energy conversion and storage. The oxygen evolution electrocatalytic performance of an amorphous Co-B alloy and its derivatives were studied. These materials were chemically synthesized by reducing a Co salt with NaBH4. The amorphous Co-B alloy showed good electrocatalytic activity in oxygen evolution but its stability was poor. A hydrotalcite-wrapped Co-B alloy was synthesized by mild oxidation. The electrocatalytic activity of this material in the oxygen evolution reaction was better than that of a commercially available Ir/C catalyst. (C) 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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  • Engineering dielectric constants in organic semiconductors

    Armin, Ardalan   Stoltzfus, Dani M.   Donaghey, Jenny E.   Clulow, Andrew J.   Nagiri, Ravi Chandra Raju   Burn, Paul L.   Gentle, Ian R.   Meredith, Paul  

    The dielectric properties of three pairs of organic semiconductors that contain increasing numbers of cyclopentadithiophene-co-benzothiadiazole moieties (monomer, dimer and polymer) were studied and compared. The materials in each pair differed in the nature of the 'solubilizing groups', which are either alkyl- or glycol-based. At low frequencies (
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  • The structural impact of water sorption on device-quality melanin thin films

    Clulow, Andrew J.   Mostert, A. Bernardus   Sheliakina, Margarita   Nelson, Andrew   Booth, Norman   Burn, Paul L.   Gentle, Ian R.   Meredith, Paul  

    The melanins are a class of pigmentary bio-macromolecules ubiquitous in the biosphere. They possess an intriguing set of physico-chemical properties and have been shown to exhibit hybrid protonic-electronic electrical conductivity, a feature derived from a process termed chemical self-doping driven by the sorption of water. Although the mechanism underlying the electrical conduction has been established, how the sorbed water interacts with the melanin structure at the physical level has not. Herein we use neutron reflectometry to study changes in the structure of synthetic melanin thin films as a function of H2O and D2O vapour pressure. Water is found to be taken up evenly throughout the films, and by employing the contrast effect, the existence of labile protons through reversible deuterium exchange is demonstrated. Finally, we determine a sorption isotherm to enable quantification of the melanin-water interactions.
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  • Electroactive cellulose-supported graphene oxide interlayers for Li-S batteries

    Zeng, Qingcong (Ray)   Leng, Xue   Wu, Kuang-Hsu   Gentle, Ian R.   Wang, Da-Wei  

    Electroactive graphene oxide (GO) interlayers between cathode and separator are examined in a lithium-sulfur cell. The GO interlayer is bifunctional and capable for lithium storage and polysulfide trapping depending on the specific surface properties. The corresponding electrochemical behaviors were evaluated by using galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectroscopy. A series of GO with different oxidation degrees were synthesized by tuning modified Hummer's method. The results revealed that hydroxyl was formed at insufficient oxidation while epoxide appeared at strong oxidation. The residual graphitic structures in weakly oxidized GO can donate electrons to improve polysulfide utilization while the enriched oxygen functional groups can react with lithium ion giving rise to higher cathode capacity. (C) 2015 Elsevier Ltd. All rights reserved.
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  • Influence of Dopant Concentration and Steric Bulk on Interlayer Diffusion in OLEDs

    McEwan, Jake A.   Clulow, Andrew J.   Nelson, Andrew   Wang, Renjie   Burn, Paul L.   Gentle, Ian R.  

    The performance of organic light-emitting diodes (OLEDs) can change when they are subjected to thermal stress after manufacture. The effect of heat on OLED film stacks is studied, in which the emissive layer (EML) comprises either a phosphorescent iridium(III) dopant blended in a host at different concentrations or materials with alkyl substituents to increase the steric bulk of the host and/or dopant. Neutron reflectometry with in situ photoluminescence measurements shows that interdiffusion between the emissive and hole transport layers within the films occurs on thermal annealing. Interdiffusion occurs independent of dopant concentration or steric bulk of the EML components. Importantly, when held at relatively low temperatures, the EML materials are found to only partially diffuse into an adjacent charge transport layer. The movement of materials is found to correlate with the change in luminescence from the hole transport material and an initial enhancement of the emission from the iridium(III) dopant. The results provide an explanation for the burn-in often observed for OLEDs as well as the need to change the driving characteristics over time to ensure that pixels can be held at the requisite brightness.
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  • A water-dielectric capacitor using hydrated graphene oxide film

    Wang, Da-Wei   Du, Aijun   Taran, Elena   Lu, Gao Qing (Max)   Gentle, Ian R.  

    Despite the widespread use of paper, plastic or ceramics in dielectric capacitors, water has not been commonly used as a dielectric due to its tendency to become conductive as it easily leaches ions from the environment. We show here that when water is confined between graphene oxide sheets, it can retain its insulating nature and behave as a dielectric. A hydrated graphene oxide film was used as a dielectric spacer to construct a prototype water-dielectric capacitor. The capacitance depends on the water content of the hydrated GO film as well as the voltage applied to the device. Our results show that the capacitance per unit area of the GO film is in the range of 100-800 mu F cm(-2), which is 5-40 times that of the double layer capacitance per surface area of activated carbon.
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  • Dependence of Organic Interlayer Diffusion on Glass-Transition Temperature in OLEDs

    McEwan, Jake A.   Clulow, Andrew J.   Nelson, Andrew   Yepuri, Nageshwar Rao   Burn, Paul L.   Gentle, Ian R.  

    Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic-films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 degrees C above the glass-transition temperature (T-g) of the material:with the lowest Tg. Diffusion occurred when the material with the lowest Tg formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative Tg's. If the supercooled liquid formed at a temperature significantly lower than the Tg of the higher-T-g material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two Tg's were similar, then the two: materials can interdiffuse at similar rates. The and extent of diffusion observed can provide insight into and a partial explanation-for the "burn in" often Observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of:the materials from the different layers had a strong effect on The emission of the-film, with quenching generally observed. These results emphaSite the importance of using thermally stable materials in morphology changes.
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  • On the origin of electrical conductivity in the bio-electronic material melanin

    Mostert, A. Bernardus   Powell, Ben J.   Gentle, Ian R.   Meredith, Paul  

    The skin pigment melanin is one of a few bio-macromolecules that display electrical and photo-conductivity in the solid-state. A model for melanin charge transport based on amorphous semiconductivity has been widely accepted for 40 years. In this letter, we show that a central pillar in support of this hypothesis, namely experimental agreement with a hydrated dielectric model, is an artefact related to measurement geometry and non-equilibrium behaviour. Our results cast significant doubt on the validity of the amorphous semiconductor model and are a reminder of the difficulties of electrical measurements on low conductivity, disordered organic materials. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3688491]
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  • Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers

    McEwan, Jake A.   Clulow, Andrew J.   Nelson, Andrew   Jansen-van Vuuren, Ross D.   Burn, Paul L.   Gentle, Ian R.  

    Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the T-g of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.
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  • Reduction-induced surface amorphization enhances the oxygen evolution activity in Co3O4

    Leng, Xue   Zeng, Qingcong   Wu, Kuang-Hsu   Gentle, Ian R.   Wang, Da-Wei  

    Modifying crystalline Co3O4 by thermal H-2 annealing and air exposure produced an amorphous surface layer consisting of mixed hydrated cobalt hydroxide/carbonate species and remarkably enhanced the oxygen evolution activity.
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  • The production and verification of pristine semi-fluorinated thiol monolayers on gold

    Ford, Kym   Battersby, Bronwyn J.   Wood, Barry J.   Gentle, Ian R.  

    The presence of adventitious contamination of self-assembled monolayers (SAMs) is a well-known phenomenon that is often overlooked or underestimated in the literature. Herein, we demonstrate that it is possible to produce pristine self-assembled monolayers (SAMs) on gold surfaces that are devoid of adventitious species. The chemical purity or the pristine quality of the SAM was verified by the experimental relative atomic ratios measured by X-ray photoelectron spectroscopy (XPS) of all elements including carbon and corresponded to within 5% of the stoichiometric ratios. Perfluoro-octyl-thiolate (F8) was used as a model compound in this study, where monolayers were assembled from solutions of an acetylated F8 precursor. Quantitative elemental characterization of the acetylated F8 precursor by cold-stage XPS provided valuable reference data for the analysis of the subsequent SAMs. Comprehensive analysis of high-resolution XPS C 1s spectra proved to be essential for establishing the purity of the SAMs, since the peaks of the adventitious species were easily distinguished from those of the F8. Analyses of deliberately contaminated F8 SAMs showed that the adventitious species persisted during the process of self-assembly and therefore co-existed with the SAM in the interfacial region. The work also established that even a lengthy deposition time of 18 h was incapable of displacing the adventitious species present at the interface. (C) 2011 Elsevier Inc. All rights reserved.
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  • Enhanced Electroactivity of Facet-Controlled Co3O4 Nanocrystals for Enzymeless Biosensing

    Wu, Kuang-Hsu   Leng, Xue   Gentle, Ian R.   Wang, Da-Wei  

    In this work, we report enhanced electroactivity of Co3O4 nanocrystals (nanocubes, NCs and truncated nano-octahedra, TNO) on the exposed {111} facets as compared to the {001} facets in relation to the surface density and the activity of the octahedral Co-III species. Transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were employed to characterize the crystal facets and materials properties. The enhanced electroactivity of {111} crystal facets was evaluated by cyclic voltammetry and amperometric titration. Our results indicate that the {111} facets in TNO has a better electroactivity for enzymeless glucose sensing with a decent glucose sensitivity of 32.54 mu A (mmol/L)(-1) cm(-2). Copyright (C) 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
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  • Anodic chlorine/nitrogen co-doping of reduced graphene oxide films at room temperature

    Wang, Da-Wei   Wu, Kuang-Hsu   Gentle, Ian R.   Lu, Gao Qing (Max)  

    Room temperature simultaneous doping of reduced graphene oxide films with oxygen, nitrogen and chlorine was performed through anodic polarization in a neutral nitrogen-deaerated KCl electrolyte. The thermodynamic electrochemical windows of water, dissolved nitrogen and chlorine anions were analyzed on the basis of the Pourbaix diagram. Anode polarization demonstrated that the nitrogen, water and chlorine anions can be oxidized at an applied potential of 1.7 V vs. NHE. The oxidative products, i.e. oxygen, nitrate anion and hypochlorous acid, can react with the reduced graphene oxide surface. X-ray photoelectron spectroscopy proved the chlorine-nitrogen co-doping of the treated film, along with an increase of oxygen groups. Surface structure evolution was also confirmed by Raman and Fourier-transform infrared spectroscopies. The anodic doping can be hindered by covering the reduced graphene oxide surface with sulfate anions or forming stable carbon-nitrogen bonds. Incorporation of oxygen, nitrogen and chlorine also helps to enhance the supercapacitance of the doped film. (C) 2012 Elsevier Ltd. All rights reserved.
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  • Controlling Hierarchy in Solution-processed Polymer Solar Cells Based on Crosslinked P3HT

    Tao, Chen   Aljada, Muhsen   Shaw, Paul E.   Lee, Kwan H.   Cavaye, Hamish   Balfour, Michael N.   Borthwick, Robert J.   James, Michael   Burn, Paul L.   Gentle, Ian R.   Meredith, Paul  

    Understanding and controlling the morphology of donor/acceptor blends is critical for the development of solution processable organic solar cells. By crosslinking a poly(3-n-hexylthiophene-2,5-diyl) (P3HT) film we have been able to spin-coat [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) onto the film to form a structure that is close to a bilayer, thus creating an ideal platform for investigating interdiffusion in this model system. Neutron reflectometry (NR) demonstrates that without any thermal treatment a smaller amount of PCBM percolates throughout the crosslinked P3HT when compared to a non-crosslinked P3HT film. Using time-resolved NR we also show thermal annealing increases the rate of diffusion, resulting in a near-uniform distribution of PCBM throughout the polymer film. XPS measurements confirm the presence of both P3HT and PCBM at the annealed film's surface indicating that the two components are intermixed. Photovoltaic devices fabricated using this bilayer approach and suitable annealing conditions yielded comparable power conversion efficiencies to bulk heterojunction devices made from the same materials. The crosslinking procedure has also enabled the formation of patterned P3HT films by photolithography. Pillars with feature sizes down to 2 mu m were produced and after subsequent deposition of PCBM and thermal annealing devices with efficiencies of up to 1.4% were produced.
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  • Challenges in Fluorescence Detection of Chemical Warfare Agent Vapors Using Solid‐State Films

    Fan, Shengqiang   Zhang, Guanran   Dennison, Genevieve H.   FitzGerald, Nicholas   Burn, Paul L.   Gentle, Ian R.   Shaw, Paul E.  

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  • Elucidating the Spatial Arrangement of Emitter Molecules in Organic Light-Emitting Diode Films

    Tonnele, Claire   Stroet, Martin   Caron, Bertrand   Clulow, Andrew J.   Nagiri, Ravi C. R.   Malde, Alpeshkumar K.   Burn, Paul L.   Gentle, Ian R.   Mark, Alan E.   Powell, Benjamin J.  

    The effect of varying the emitter concentration on the structural properties of an archetypal phosphorescent blend consisting of 4,4'-bis(N-carbazolyl) biphenyl and tris(2-phenylpyridyl) iridium(III) has been investigated using nonequilibrium molecular dynamics (MD) simulations that mimic the process of vacuum deposition. By comparison with reflectometry measurements, we show that the simulations provide an accurate model of the average density of such films. The emitter molecules were found not to be evenly distributed throughout film, but rather they can form networks that provide charge and/or energy migration pathways, even at emitter concentrations as low as approximate to 5 weight percent. At slightly higher concentrations, percolated networks form that span the entire system. While such networks would give improved charge transport, they could also lead to more non-radiative pathways for the emissive state and a resultant loss of efficiency.
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