1,721,141 research outputs found
The discovery and optimisation of solid state materials for the energy and the electronics sectors using MBE based methodologies
No full textParticle size and support effects in electrocatalysis
No full textResearchers increasingly recognize that, as with standard supportedheterogeneous catalysts, the activity and selectivity of supported metal electrocatalysts are influenced by particle size, particle structure, and catalyst support. Studies using model supported heterogeneous catalysts have provided information about these effects. Similarly, model electrochemical studies on supported metal electrocatalysts can provide insight into the factors determining catalytic activity.High-throughput methods for catalyst synthesis and screening candetermine systematic trends in activity as a function of support and particle size with excellent statistical certainty. In this Account, we describe several such studies investigating methods for dispersing precious metals on both carbon and oxide supports, with particular emphasis on the prospects for the development of low-temperature fuel-cell electrocatalysts.One key finding is a decrease in catalytic activity with decreasing particle size independent of the support for both oxygen reduction and CO oxidation on supported gold and platinum. For these reactions, there appears to be an intrinsic particle size effect that results in a loss of activity at particle sizes below 2–3 nm. A titania support, however, also increases activity of gold particles in the electrooxidation of CO and in the reduction of oxygen, with an optimum at 3 nm particle size. This optimum may represent the superposition of competing effects: a titania-induced enhanced activity versus deactivation at small particle sizes. The titania support shows catalytic activity at potentials where carbon-supported and bulk-gold surfaces are normally oxidized and CO electrooxidation is poisoned. On the other hand, platinum on amorphous titania shows a different effect: the oxidation reduction reaction is strongly poisoned in the same particle size range. We correlated the influence of the titania support with titania-induced changes in the surface redox behavior of the platinum particles. For both supported gold and platinum particles in electrocatalysis, we observe parallels to the effects of particle size and support in the equivalent heterogeneous catalysts.Studies of model supported-metal electrocatalysts, performs efficiently using high throughput synthetic and screening methodologies, will lead to a better understanding of the mechanisms responsible for support and particle size effects in electrocatalysis, and will drive the development of more effective and robust catalysts in the futur
Physical vapor deposition method for the high-throughput synthesis of solid-state material libraries
No full textA method that combines coevaporation of pure elements from multiple finite-size sources on temperature-controlled substrates with independently controlled source shutters has been used for the synthesis of solid-state material combinatorial libraries. The source shutters are positioned to achieve a controlled gradient of the deposited elements across the substrate and are fixed during the course of deposition. Choice of the shutter position and the rate of deposition for each source allow the direct synthesis of continuous and controlled materials of varying composition. There are significant advantages of the method over alternatives which rely on sequential deposition and subsequent heat treatment to produce thin film materials. The parameters governing the creation of gradients have been identified and defined. Simulations and experimental data have been compared in the case of a single source. Results are presented for the synthesis of a ternary alloy library to demonstrate the methodology
Structural, dielectric and ferroelectric properties of (Bi,Na)TiO3–BaTiO3 system studied by high throughput screening
Full text linkThin-film materials libraries of the Bi2O3–Na2O–TiO2–BaO system in a broad composition range have been deposited in ultra-high vacuum from elemental evaporation sources and an oxygen plasma source. A high throughput approach was used for systematic compositional and structural characterization and the screening of the dielectric and ferroelectric properties. The perovskite (Bi,Na)TiO3–BaTiO3 phase with a Ba concentration near the morphotropic phase boundary (ca. 6 at.%) exhibited a relative dielectric permittivity of 180, a loss tangent of 0.04 and remnant polarization of 19 ?C/cm2. Compared to published data, observed remnant polarization is close to that known for epitaxially grown films but higher than the values reported for polycrystalline films. The high throughput methodology and systematic nature of the study allowed us to establish the composition boundaries of the phase with optimal dielectric and ferroelectric characteristics
ABO<sub>3</sub> and A<sub>1</sub>−<sub><i>x</i></sub>C<sub><i>x</i></sub>B<sub>1</sub>−<sub><i>y</i></sub>D<sub><i>y</i></sub>(O<sub>1</sub>−<sub><i>z</i></sub>E<sub><i>z</i></sub>)<sub>3</sub>: review of experimental optimisation of thin film perovskites by high-throughput evaporative physical vapour deposition
No full textThe development of functional perovskites for future technologies can be achieved though the combinatorial synthetic method of evaporative physical vapour deposition (HT-ePVD) which provides a direct low temperature route to anion stoichiometric materials. When combined with the ability to control and vary precisely the composition of thin film libraries of materials, high-throughput methods of screening and characterisation provides a rapid experimental determination of the structure/function relationship. This review of the use of HT-ePVD shows that controlled cationic substitutions in A and/or B sites can easily be explored, as can the effect of anionic substitution. This is exemplified in using this approach for a wide range of perovskite systems, where the tuning of the functional properties through cation substitution has application in a broad range of technologies
A simultaneous screening of the corrosion resistance of Ni–W thin film alloys
No full textA range of Ni–W thin film alloys have been synthesised by evaporation from atomic sources to produce a compositional gradient across the substrate, and characterised by X-ray diffraction (phase identification and crystallite size) and EDX (composition). The corrosion resistance of 100 discrete alloy compositions has been measured simultaneously in identical environments using a high-throughput screening protocol based on cyclic voltammetry. The corrosion resistances and potentials are extracted from potentiodynamic data by both the linear polarisation resistance (LPR) and Tafel extrapolation methods; results from these are shown to be self-consistent. The high-throughput approach allows, for the first time, a direct correlation to be made between the bulk crystalline phases and morphology, and the corrosion behaviour of Ni–W alloys. The incorporation of W in the Ni–W solid solution at low W compositions results in a continuous reduction in crystallite size, a small cathodic shift in corrosion potential, and a small reduction in corrosion resistance. A large cathodic shift in corrosion potential together with a significant increase in the corrosion resistance is observed for alloy compositions greater than 24%at. W. This composition range coincides with the formation of the amorphous phase
Oxygen reduction and oxygen evolution on SrTi1 − xFexO3 − y (STFO) perovskite electrocatalysts
No full textCompositionally graduated thin films of a SrTi1 − xFexO3 − y (STFO) perovskite electrocatalysts were successfully prepared by High Throughput Physical Vapour Deposition (HT-PVD) using evaporative sources. X-ray diffraction confirmed a continuous solid solution of the cubic perovskite structure with an increase in the lattice parameter with increasing x from 0.392 ± 0.001 nm for SrTiO3 to 0.386 ± 0.001 nm for SrFeO3. A Raman mode corresponding to an O-stretching vibration was observed which is disallowed by symmetry in the cubic structure suggests a localised lattice distortion. The perovskites exhibited poor conductivity for low values of x (ρ < 7 × 10− 8 S cm− 1) but conductivity increased with increasing Fe content before reaching a plateau at ρ = 0.041 S cm− 1 for x > 0.75. Increasing electrocatalytic activity towards the oxygen evolution reaction (OER) with increasing Fe content was observed, characterised by a 100 μA onset potential varying monotonically from 1.52VRHE (x = 0.2) to 1.40VRHE (x = 0.85). The high OER activity was however found to correlate with low electrode stability, consistent with the participation of lattice oxygen in the OER mechanism. The latter was evidenced by the redox electrochemistry associated with reversible oxygen intercalation. SrTi0.5Fe0.5O3 − y exhibited the optimal composition with good OER activity and electrode stability. Low electrocatalytic activity towards the oxygen reduction reaction (ORR) was observed for all oxygen stoichiometric compositions. The ORR did not occur until after reduction of the films suggesting that surface reduction is required for the creation of the active surface sites. ORR activity on the oxygen sub-stoichiometric perovskites showed the opposite trend with compositional variation to OER activity on the oxygen stoichiometric perovskites
High throughput optimisation of PdCu alloy electrocatalysts for the reduction of nitrate ions
No full textA high-throughput methodology was used to synthesise compositional gradient thin films of PdCu alloys. The expected equilibrium phases of the PdCu alloys as a function of composition were identified using XRD. The electrochemical behaviour of the alloys in NaOH base electrolyte was measured simultaneously on a micro-fabricated array of 100 electrodes. Small concentration of Pd addition to the Cu resulted in a reversible Cu derived surface redox, and higher concentrations decreased its equilibrium potential (a destabilisation of the surface oxide). The former is due to the mediation of the redox reaction by Pd and the latter due to electronic interaction between Pd and Cu. The most active reduction catalyst (rate at constant overpotential or minimum overpotential for a fixed rate) is in a narrow compositional region around 84%at. Cu. The trend in reactivity can be understood by the promotion of nitrate reduction through hydrogen spillover from Pd at low concentrations in the alloy. The reduction in activity as the palladium concentration increases further is concomitant with the destabilisation of oxygen on copper, and therefore likely to be due to the inability to stabilise the nitrate ion at the surface, or extract the first oxygen atom to produce nitrite
TiO2(110)-(1×1) supported Cu particles: An FT-RAIRS investigation
No full textThe morphology of TiO2(110)-(1 × 1) supported Cu particles has been investigated by Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS), employing adsorbed CO as a probe molecule sensitive to local surface structure. For Cu coverage (deposited at 300 K) less than 2.85 MLE nucleated Cu particles in the range 2 nm–4 nm are formed, as indicated by a final state shift in the core level Cu(2p3/2) binding energy and by the existence of only transmission bands in the FT-RAIRS spectra for adsorbed CO. ?S(CO) indicates that these small particles expose sites similar to those of the stepped Cu surfaces Cu(211), Cu(311), and Cu(755). At Cu coverages in the range of 6 MLE and above, corresponding to particle sizes above 4.6 nm, ?S(CO) indicates the predominance of (110), (100) and (111) adsorption sites. Annealing the Cu layers to 650 K results in the slight growth of the particle sizes, and transformation of the CO adsorption sites corresponding to the close packed facets. The transformation of the local dielectric from that of titania to that dominated by the Cu particle is shown to take place between 3.7 and 4.2 nm, and this change is also to a smaller extent sensitive to the dispersion of the particles.<br/
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