ABO3 and A1-xCxB1-yDy(O1-zEz)3: review of experimental optimisation of thin film perovskites by high-throughput evaporative physical vapour deposition.

The 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.

Reversible perovskite electrocatalysts for oxygen reduction/oxygen evolution.

The identification of electrocatalysts mediating both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are prerequisite for the development of reversible fuel cells and rechargeable metal-air batteries. The question remains as to whether a bifunctional catalyst, or a single catalyst site, will exhibit potentials converging to +1.23 VRHE. Transition metal-based perovskites provide tunable catalysts where site substitution can influence both ORR and OER, however substitution in the pseudo-binary phases results in an anti-correlation in ORR and OER activities. We reveal that La x Mn y Ni1-y O3-δ , compositions with lanthanum A-site sub-stoichiometry exhibit reversible activity correlating with the appearance of the Mn3+/Mn4+ redox couple. The Mn3+/Mn4+ couple is associated with Mn4+ co-existing with Mn3+ in the bulk, as La3+ is substituted by Ni2+ at the A-site to create a mixed valent system. We also show that a direct A-site substitution by the Ca2+ cation in La x Ca1-x Mn y O3-δ perovskites also results in the creation of Mn4+, the appearance of the Mn3+/Mn4+ redox couple, and a concomitant reversible activity. These results highlight a general strategy of optimizing oxide electrocatalysts with reversible activity.

Stoichiometric Engineering of Chalcogenide Semiconductor Alloys for Nanophotonic Applications.

A variety of alternative plasmonic and dielectric material platforms-among them nitrides, semiconductors, and conductive oxides-have come to prominence in recent years as means to address the shortcomings of noble metals (including Joule losses, cost, and passive character) in certain nanophotonic and optical-frequency metamaterial applications. Here, it is shown that chalcogenide semiconductor alloys offer a uniquely broad pallet of optical properties, complementary to those of existing material platforms, which can be controlled by stoichiometric design. Using combinatorial high-throughput techniques, the extraordinary epsilon-near-zero, plasmonic, and low/high-index characteristics of Bi:Sb:Te alloys are explored. Depending upon composition they can, for example, have plasmonic figures of merit higher than conductive oxides and nitrides across the entire UV-NIR range, and higher than gold below 550 nm; present dielectric figures of merit better than conductive oxides at near-infrared telecommunications wavelengths; and exhibit record-breaking refractive indices as low as 0.7 and as high as 11.5.

Compositionally controlled plasmonics in amorphous semiconductor metasurfaces.

Amorphous bismuth telluride (Bi:Te) provides a composition-dependent, CMOS-compatible alternative material platform for plasmonics in the ultraviolet-visible spectral range. Thin films of the chalcogenide semiconductor are found, using high-throughput physical vapor deposition and characterization techniques, to exhibit a plasmonic response (a negative value of the real part of relative permittivity) over a band of wavelengths extending from ~250 nm to between 530 and 978 nm, depending on alloy composition (Bi:Te at% ratio). The plasmonic response is illustrated via the fabrication of subwavelength period nano-grating metasurfaces, which present strong, period-dependent plasmonic absorption resonances in the visible range, manifested in the perceived color of the nanostructured domains in reflection.

High-Throughput Synthesis and Characterization of Eu Doped Ba xSr2- xSiO4 Thin Film Phosphors.

High-throughput techniques have been employed for the synthesis and characterization of thin film phosphors of Eu-doped Ba xSr2- xSiO4. Direct synthesis from evaporation of the constituent elements under a flux of atomic oxygen on a sapphire substrate at 850 °C was used to directly produce thin film libraries (415 nm thickness) of the crystalline orthosilicate phase with the desired compositional variation (0.24 > x > 1.86). The orthosilicate phase could be synthesized as a pure, or predominantly pure, phase. Annealing the as synthesized library in a reducing atmosphere resulted in the reduction of the Eu while retaining the orthosilicate phase, and resulted in a materials thin film library where fluorescence excited by blue light (450 nm) was observable by the naked eye. Parallel screening of the fluorescence from the combinatorial libraries of Eu doped Ba xSr2- xSiO4 has been implemented by imaging the fluorescent radiation over the library using a monochrome digital camera using a series of color filters. Informatics tools have been developed to allow the 1931 CIE color coordinates and the relative quantum efficiencies of the materials library to be rapidly assessed and mapped against composition, crystal structure and phase purity. The range of compositions gave values of CIE x between 0.17 and 0.52 and CIE y between 0.48 and 0.69 with relative efficiencies in the range 2.0 × 10-4-7.6 × 10-4. Good agreement was obtained between the thin film phosphors and the fluorescence characteristics of a number of corresponding bulk phosphor powders. The thermal quenching of fluorescence in the thin film libraries was also measured in the temperature range 25-130 °C: The phase purity of the thin film was found to significantly influence both the relative quantum efficiency and the thermal quenching of the fluorescence.

Particle size and support effects in electrocatalysis.

Researchers increasingly recognize that, as with standard supported heterogeneous 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 can determine 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 future.

High throughput methodology for synthesis, screening, and optimization of solid state lithium ion electrolytes.

A study of the lithium ion conductor Li(3x)La(2/3-x)TiO(3) solid solution and the surrounding composition space was carried out using a high throughput physical vapor deposition system. An optimum total ionic conductivity value of 5.45 × 10(-4) S cm(-1) was obtained for the composition Li(0.17)La(0.29)Ti(0.54) (Li(3x)La(2/3-x)TiO(3)x = 0.11). This optimum value was calculated using an artificial neural network model based on the empirical data. Due to the large scale of the data set produced and the complexity of synthesis, informatics tools were required to analyze the data. Partition analysis was carried out to determine the synthetic parameters of importance and their threshold values. Multivariate curve resolution and principal component analysis were applied to the diffraction data set. This analysis enabled the construction of phase distribution diagrams, illustrating both the phases obtained and the compositional zones in which they occur. The synthetic technique presented has significant advantages over other thin film and bulk methodologies, in terms of both the compositional range covered and the nature of the materials produced.

The Hydrogen Evolution Reaction and Hydrogen Oxidation Reaction on thin film PdAu alloy surfaces.

A high throughput synthetic and screening methodology has been applied to the study of the Hydrogen Evolution Reaction (HER) and Hydrogen Oxidation Reaction (HOR) activities of PdAu thin film alloy surfaces. A compositional gradient method has been used to produce non-equilibrium surfaces of PdAu solid solution alloys. XPS shows surface compositions for these un-annealed alloys to be the same as the bulk, in contrast to the observed and expected behaviour of annealed alloys. The surface redox behaviour of the alloys provides direct evidence for the existence of Pd ensembles with distinctive surface chemistry dominating the oxygen interaction at intermediate alloy compositions. CO stripping voltammetry indicates that alloying of Pd with Au results in a strong poisoning of CO adsorption with the exception of CO adsorbed on Pd monomers. The compositional dependence of the HER and HOR activities is strongly correlated on the alloy surfaces, with a maximum in geometric activity observed at 60 at% Au, corresponding to a maximum in specific activity at 70 at% Au, for both reactions. This activity is associated with Pd poor ensembles. There is some evidence that while the Pd monomer catalyses the HER, HOR is not sustained at compositions where the monomer is present.

The influence of support and particle size on the platinum catalysed oxygen reduction reaction.

A range of platinum deposits, equivalent thicknesses (delta) 0.2-2.5 nm, have been synthesised on carbon and reduced titania (TiO(x)) supports using physical vapour deposition on (10 x 10) arrays of electrodes. For delta < 1.0 nm, discrete platinum centres are formed and the TiO(x) supported platinum show two distinct characteristics: (a) a strong positive shift in the potential for the oxidation of monolayers of CO with decreasing loading of Pt leading to an inability to oxidise the CO on the lowest loadings and (b) a strong negative shift in the potential for the reduction of oxygen. Both observations can be understood in terms of an increase in the irreversibility of the Pt/PtO couple at such surfaces. The same trends, although significantly weaker, are seen with the carbon supported platinum, delta < 1.0 nm, and it is suggested that the Pt/PtO couple on carbon shows intermediate kinetics between Pt on TiO(x) and bulk Pt. These results have significant implications for understanding the mechanism of oxygen reduction on supported Pt catalysts and hence for the search for alternative supports to platinum for ORR electrocatalysts.

The influence of Pt particle size on the surface oxidation of titania supported platinum.

A range of reduced titania (TiO(x)) supported platinum electrocatalysts have been synthesised using physical vapour deposition on arrays of electrodes. Surfaces with equivalent thicknesses of platinum in the range 0.2-2.5 nm on a uniform layer of TiO(x) have been synthesised on 10 x 10 arrays. The arrays have been used to study the surface redox chemistry of the supported platinum as well as the oxidation of a monolayer of carbon monoxide on the platinum. It is shown that below an equivalent thickness of 0.8 nm, there is a positive shift in the potential for the oxidation of the platinum surface and a negative shift for the reduction of the oxide with decrease in the platinum loading. These shifts show that it is the kinetics of the platinum/platinum oxide couple that change with platinum loading; the couple becomes increasingly irreversible with decreasing loading. The peak potential for the oxidation of the monolayer of carbon monoxide also shifts positive and broadens with decreasing platinum loading; these trends are again particularly marked below an equivalent thickness of 0.8 nm while below 0.4 nm no CO oxidation peak is observed although it could be confirmed that CO is adsorbed on such surfaces. Again, these changes with platinum loading are associated with the irreversibility of the platinum/platinum oxide couple. At low equivalent thicknesses, it is impossible to form the oxidised platinum species within the carbon monoxide monolayer essential to the commencement of oxidation of the CO monolayer.

High-throughput synthesis and screening of hydrogen-storage alloys.

Libraries of mixed-metal hydride materials are synthesized on a silicon microfabricated array of "hot-plate" MEMS devices, which allow high-throughput screening using temperature programmed desorption and infrared thermography. The heating plate of the MEMS device is a membrane with low heat capacity, allowing fast and localized temperature control and the extraction of calorimetric data from thermography. The combination of the synthetic method and screening chip allows a fast determination of the desorption temperature and hydrogen content of the materials. Mixed metal hydrides are synthesized directly. The potential of the method is exemplified by presenting results for the sorption properties of Mg xNi 1- x hydride thin-film materials. The results are consistent with the literature, showing the highest hydrogen capacity and desorption temperature for the MgH 2 phase in Mg-rich compositions and the promotion of a lower temperature desorption from the Mg 2NiH 4 phase, with a concomitant reduction in hydrogen capacity.

A combinatorial approach to the study of particle size effects on supported electrocatalysts: oxygen reduction on gold.

A novel high-throughput technique has been developed for the investigation of the influence of supported metal particle size and the support on electrocatalytic activity. Arrays with a gradation of catalyst particle sizes are fabricated in a physical vapor deposition system that also allows selection of the support material. Simultaneous electrochemical measurements at all electrodes in the array, together with determination of the actual particle size distribution on each of the electrodes by transmission electron microscopy (TEM), then allows rapid determination of the activity as a function of catalyst center size. The procedure is illustrated using data for the reduction of oxygen on gold nanoparticles supported on both substoichiometric titanium dioxide (TiO(x)()) and carbon and the conclusions are verified using voltammetry at rotating disk electrodes. Gold centers with diameters in the range 1.4-6.3 nm were investigated and it is demonstrated that, with both supports, the catalytic activity for oxygen reduction decays rapidly for particle sizes below 3.0 nm. This may be observed as a decrease in current at constant potential or an increase in the overpotential for oxygen reduction.

Combinatorial approach to the study of particle size effects in electrocatalysis: synthesis of supported gold nanoparticles.

A high-throughput method for physical vapor deposition has been applied to the synthesis of libraries of supported gold particles on amorphous substoichiometric TiO(x)() and carbon supports. The TiO(x)() substrate stoichiometry can be varied or kept constant across a supporting sample, and subsequent deposition of particle sizes on supports are controlled through the nucleation and growth process. TEM measurements indicate nucleation and growth of Au particles takes place, with the smallest particles initially observed at 1.4 nm with a maximum density of 5.5 x 10(12) cm(-2) on titania, and 2.6 nm with concomitantly lower density on carbon. The 1.4-nm particles on titania exhibit a binding energy shift in the Au(4f) core level of 0.3 eV from bulk gold, and the shift is approximately 0.1 eV by the time particles grow to a mean size of 2.5 nm. These shifts are associated with final state effects, and the supported gold particles are metallic and appear to be relatively stable in air. When combined with appropriate substrates and screening techniques, this method provides a highly controllable method for the high-throughput synthesis of model supported catalyst.

High-throughput synthesis and screening of ternary metal alloys for electrocatalysis.

We report the application of a new method for the high-throughput synthesis and screening of thin film materials and its application to the discovery of electrocatalysts. Results are presented for the PtPdAu ternary alloy system with respect to activity for oxygen reduction. The results reveal an enhancement in activity for a range of PtPd alloy compositions over either of the pure elements. An optimum composition range of ternary alloys with significant activity was also identified. A correlation was also investigated between the surface reduction potential and the activity for oxygen reduction in both binary and ternary alloys. The results demonstrate the potential of the methodology for the discovery and optimization of electrocatalysts for a wide range of applications.

Physical vapor deposition method for the high-throughput synthesis of solid-state material libraries.

A method that combines co-evaporation 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.

Combinatorial electrochemical screening of fuel cell electrocatalysts.

Combinatorial methods have been applied to the preparation and screening of fuel cell electrocatalysts. Hardware and software have been developed for fast sequential measurements of cyclic voltammetric and steady-state currents in 64-element half-cell arrays. The arrays were designed for the screening of high-surface-area supported electrocatalysts. Analysis software developed allowed the semiautomated processing of the large quantities of data, applying filters that defined figures of merit relevant to fuel cell catalyst activity and tolerance. Results are presented on the screening of carbon-supported platinum catalysts of varying platinum metal loading on carbon (and thus, particle size) in order to demonstrate the speed and sensitivity of the screening methodology. CO electro-oxidation, oxygen reduction, and methanol oxidation on a series of such catalysts reveal clear trends in characteristics and activities. Catalysts with smaller particle sizes reveal structure in the CO stripping voltammetry that can be associated with edge sites in addition to the closely packed planes, and this is concomitantly reduced as particle size is increased. Specific activity for steady-state methanol oxidation and oxygen reduction at room temperature in H(2)SO(4) electrolyte is found to be a maximum for the largest particle sizes, in agreement with the literature. These trends in activity are significantly smaller than the differences in activities of promoted platinum-based alloy catalysts for the same reaction.

Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys.

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.