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Multiple-principal element alloys hold great promise for multifunctional material discovery (e.g., for novel electrocatalysts based on complex solid solutions) in a virtually unlimited compositional space. Here, the phase constitution of the noble metal system Ag-Ir-Pd-Pt-Ru was investigated over a large compositional range in the quinary composition space and for different annealing temperatures from 600 to 900 °C using thin-film materials libraries. Composition-dependent X-ray diffraction mapping of the as-deposited thin-film materials library indicates different phases being present across the composition space (face-centered cubic (fcc), hexagonal close packed (hcp) and mixed fcc + hcp), which are strongly dependent on the Ru content. In general, low Ru contents promote the fcc phase, whereas high Ru contents favor the formation of an hcp solid-solution phase. Furthermore, a temperature-induced phase transformation study was carried out for a selected measurement area of fcc-Ag5Ir8Pd56Pt8Ru23. With increasing temperature, the initial fcc phase transforms to an intermediate C14-type Laves phase at 360 °C, and then to hcp when the temperature reaches 510 °C. The formation and disappearance of the hexagonal Laves phase, which covers a wide temperature range, plays a crucial role of bridging the fcc to hcp phase transition. The obtained composition, phase and temperature data are transformed into phase maps which could be used to guide theoretical studies and lay a basis for tuning the functional properties of these materials.

Room temperature ionic liquids (RT-ILs) are promising electrolytes for electrocatalysis. Understanding the effects of the electrode–electrolyte interface structure on electrocatalysis in RT-ILs is important. Ultrafast mass transport of redox species in N-methyl- N-ethyl-pyrrolidinium polybromide (MEPBr2n+1) enabled evaluation of the reorganization energy ( λ), which reflects the solvation structure in the inner Helmholtz plane (IHP). λ was achieved by fitting the electron transfer rate-limited voltammogram at a Pt ultramicroelectrode (UME) to the Marcus–Hush–Chidsey model for heterogeneous electron transfer kinetics. However, it is time-consuming or even impossible to prepare electrode materials, including alloys of numerous compositions in the form of UME, for each experiment. Herein, we report a method to evaluate the λ of MEPBr2n+1 by scanning electrochemical cell microscopy (SECCM), which allows high throughput electrochemical measurements using a single electrode with high spatial resolution. Fast mass transport in the nanosized SECCM tip is critical for achieving heterogeneous electron transfer-limited voltammograms. Furthermore, investigating λ on a high-entropy alloy materials library composed of Pt, Pd, Ru, Ir, and Ag suggests a negative correlation between λ and the work function. Given that the potential of zero charge correlates with the work function of electrodes, this can be attributed to the surface-charge sensitive ionic structure in the IHP of MEPBr2n+1, modulating the solvation energy of the redox-active species in the IHP.

Complex solid solutions (“high entropy alloys”), comprising five or more principal elements, promise a paradigm change in electrocatalysis due to the availability of millions of different active sites with unique arrangements of multiple elements directly neighbouring a binding site. Thus, strong electronic and geometric effects are induced, which are known as effective tools to tune activity. With the example of the oxygen reduction reaction, we show that by utilising a data‐driven discovery cycle, the multidimensionality challenge raised by this catalyst class can be mastered. Iteratively refined computational models predict activity trends around which continuous composition‐spread thin‐film libraries are synthesised. High‐throughput characterisation datasets are then used as input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag‐Ir‐Pd‐Pt‐Ru. The method can identify optimal complex‐solid‐solution materials for electrocatalytic reactions in an unprecedented manner., Complex solid solutions (“high‐entropy alloys”) promise a paradigm change in electrocatalysis but expose the challenge of almost unlimited options in adjusting their compositions. We propose the utilisation of computational models, combined with high‐throughput experimentation for the verification of the model assumptions, which allows for model refinement in iterative loops, understanding of binding mechanisms, and discovery of the most active composition.

Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001)alpha-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K. The fcc-phase persists while the original texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 micron wide grains. Only the grains having specific orientation relationships to the sapphire grow; as in the case of pure fcc metal (M) films 4 orientation relationships (OR) are found: OR1 (M(111)[1-10]//alpha-Al2O3 (0001)[1-100]) and OR2 (M(111)[1-10]//alpha-Al2O3 (0001)[11-20]) and their twin-related variants (OR1t and OR2t). Below 1000 K, the film microstructure stabilizes into 10 micron wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures., 6 figures and 3 tables

Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it.

Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys.

Complex solid‐solution electrocatalysts (also referred to as high‐entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single‐phase solid solutions from five or more constituents, they offer unique capabilities of fine‐tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media., Elementary: The effect of changing elements within high‐entropy electrocatalysts with respect to changes in adsorption energies and the correlated characteristic activity curves is analyzed. This concept is verified for the oxygen reduction reaction (ORR) in alkaline media. A targeted design of complex solid‐solution electrocatalysts is significantly enhanced by only screening equiatomic alloys without the need to scan the whole composition range.

Refractory high-entropy alloys are of interest due to the potential of compositionally complex alloys to achieve combinations of mechanical properties such as room-temperature ductility and high-temperature strength rarely found in simpler alloys. To study a large compositional range of the system Nb–Mo–Ta–W, thin-film materials libraries were fabricated by combinatorial sputtering. High-throughput characterization methods were used to systematically determine composition-dependent properties: (I) the extent and stability of the complex solid solution range and (II) the mechanical properties (Young’s modulus, hardness). The whole investigated composition range of Nb20–59Mo9–31Ta10–42W12–32 crystallized in a bcc phase, independent of annealing temperatures ranging from 300 to 900 °C. Mechanical strength values of the Nb–Mo–Ta–W compositions were calculated using the Maresca–Curtin analytical model parameterized with experimental data. A strong positive correlation with measured hardness was observed that allows using this analytical model for optimization of the mechanical strength. We predict that compositions with high Mo contents provide the highest hardness values.

Discovery of new electrocatalysts requires the combination of high-throughput synthesis and high-throughput screening techniques to explore the vast compositional range of compositionally complex solid solutions (CCSS).

Despite outstanding accomplishments in catalyst discovery, finding new, more efficient, environmentally neutral, and noble metal-free catalysts remains challenging and unsolved. Recently, complex solid solutions consisting of at least five different elements and often named as high-entropy alloys have emerged as a new class of electrocatalysts for a variety of reactions. The multicomponent combinations of elements facilitate tuning of active sites and catalytic properties. Predicting optimal catalyst composition remains difficult, making testing of a very high number of them indispensable. We present the high-throughput screening of the electrochemical activity of thin film material libraries prepared by combinatorial co-sputtering of metals which are commonly used in catalysis (Pd, Cu, Ni) combined with metals which are not commonly used in catalysis (Ti, Hf, Zr). Introducing unusual elements in the search space allows discovery of catalytic activity for hitherto unknown compositions. Material libraries with very similar composition spreads can show different activities vs. composition trends for different reactions. In order to address the inherent challenge of the huge combinatorial material space and the inability to predict active electrocatalyst compositions, we developed a high-throughput process based on co-sputtered material libraries, and performed high-throughput characterization using energy dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (SEM), X-ray diffraction (XRD) and conductivity measurements followed by electrochemical screening by means of a scanning droplet cell. The results show surprising material compositions with increased activity for the oxygen reduction reaction and the hydrogen evolution reaction. Such data are important input data for future data-driven materials prediction.

High entropy alloys (HEA) comprise a huge search space for new electrocatalysts. Next to element combinations, the optimization of the chemical composition is essential for tuning HEA to specific catalytic processes. Simulations of electrocatalytic activity can guide experimental efforts. Yet, the currently available underlying model assumptions do not necessarily align with experimental evidence. To study deviations of theoretical models and experimental data requires statistically relevant datasets. Here, a combinatorial strategy for acquiring large experimental datasets of multi-dimensional composition spaces is presented. Ru–Rh–Pd–Ir–Pt is studied as an exemplary, highly relevant HEA system. Systematic comparison with computed electrochemical activity enables the study of deviations from theoretical model assumptions for compositionally complex solid solutions in the experiment. The results suggest that the experimentally obtained distribution of surface atoms deviates from the ideal distribution of atoms in the model. Leveraging both advanced simulation and large experimental data enables the estimation of electrocatalytic activity and solid-solution stability trends in the 5D composition space of the HEA system. A perspective on future directions for the development of active and stable HEA catalysts is outlined.

Complex solid solutions (“high entropy alloys” with a single solid-solution phase) hold great promise in electrocatalysis because of their nearly unlim- ited number of different active sites exposed at the surface. It has been shown by theoretical studies that multiple arrangements of different elements directly neighboring a binding site create millions of differently active catalytic sites. We report a zooming-in approach using scanning electrochemical cell microscopy (SECCM) to distinguish between the averaged electrochemical response of mul- tiple active sites and active site-specific electrochemical response. Using a thin film complex solid solution electrocatalyst and a range of SECCM single bar- rel capillaries with diameters from 1.2 µm to 50 nm, we observed an averaged electrochemical response for the oxygen reduction reaction with minor statisti- cal variations for the larger capillary diameters. In contrast, significant statistical heterogeneity among the measured spots is observed for small capillary diame- ters. This statistical heterogeneity is attributed to the ability of the smaller probe size to address a comparatively smaller number of active sites with high or low activity dominating the measured electrocatalytic currents. &nbsp

Complex solid solution (CSS) nanoparticles were recently discovered as efficient electrocatalysts for a variety of reactions. As one of many advantages, they exhibit the potential to replace noble-metal catalysts with multinary combinations of transition metals because they offer formation of new unique and tailorable active sites of multiple elements located next to each other. This Perspective reports on the current state and on challenges of the (combinatorial) synthesis of multinary nanoparticles and advanced electron microscopy characterization techniques for revealing structure–activity correlations on an atomic scale. We discuss what distinguishes this material class from common catalysts to highlight their potential to act as electrocatalysts and rationalize their nontypical electrochemical behavior. We provide an overview about challenges in synthesis, characterization, and electrochemical evaluation and propose guidelines for future design of CSS catalysts to achieve further progress in this res...

The anodic oxygen evolution reaction (OER) has significant importance in many electrochemical technologies. In proton exchange membrane water electrolyzers it plays a pivotal role for electrochemical energy conversion, yet sluggish kinetics and the corrosive environment during operation still compel significant advances in electrode materials to enable a widespread application. Up-to-date Iridium is known as the best catalyst material for the OER in acidic media due to its relatively high activity and long-term stability. However, scarcity of iridium drives the development of strategies for its efficient utilization. One promising way would be the formation of mixtures in which the noble catalyst element is dispersed in the non-noble matrix of more stable metals or metal oxides. A promising valve metal oxide is TiOx, yet the degree to which performance can be optimized by composition is still unresolved. Thus, using a scanning flow cell connected to an inductively coupled plasma mass spectrometer, we examined the activity and stability for the OER of an oxidized Ir–Ti thin film material library covering the composition range from 20–70 at.% of Ir. We find that regardless of the composition the rate of Ir dissolution is observed to be lower than that of thermally prepared IrO2. Moreover, mixtures containing at least 50 at.% of Ir exhibit reactivity comparable to IrO2. Their superior performance is discussed with complementary information obtained from atomic scale and electronic structure analysis using atom probe tomography and x-ray photoelectron spectroscopy. Overall, our data shows that Ir–Ti mixtures can be promising OER catalysts with both high activity and high stability.

International audience; The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by magnetron cosputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.

Due to the high demand for renewable and infrastructure compatible energy conversion and storage technologies, research on organic fuel cells receives increasing interest again recently. Organic fuels such as alcohols provide an attractive avenue to overcome the drawbacks of hydrogen as an energy carrier. Particularly interesting are secondary alcohols that almost exclusively form ketones as the final oxidation product, as they can be utilized in “zero emission” concepts without CO2 as a by-product. The state-of-the-art electrocatalyst in secondary alcohol oxidation is Pt-Ru, which demonstrates low onset potentials for the oxidation of the most facile secondary alcohol isopropanol. Yet, the achievable current densities are still relatively low and decrease rapidly due to the formed product acetone, which can poison the catalyst surface over time. Therefore, there is an inevitable need for the development of novel electrocatalyst materials circumventing these challenges. In this study, we employ a high-throughput electrochemical approach coupled to on-line inductively-coupled plasma mass spectrometry to map the composition-dependent activity and stability of PtxIr100-x alloy electrocatalysts toward the electro-oxidation of isopropanol. The activity and stability of magnetron sputtered PtxIr100-x material libraries are studied in 0.1 M HClO4 both in the absence and presence of isopropanol. The highest current densities are achieved for the sample containing the least amount of Ir (3.4 at.%), with a continuous decrease with the increasing amount of Ir. The alloys are inactive towards the oxidation of isopropanol when the amount of Ir exceeded 80 at%. The presence of isopropanol also has a notable effect on stability: while dissolution rates do not change in the case of pure Pt and Ir, a significant increase in stability is observed for the PtxIr100-x thin-film samples at all applied upper potential limits. This is explained by the strong adsorption of acetone on the surface of the catalyst that inhibits the formation of surface oxides.

Atom probe tomography was combined with transmission electron microscopy to characterize in atomic detail the structure of nanocrystalline high entropy alloy CrMnFeCoNi thin films before and after exposure to air at 500 °C for 5 min. Mn and Cr oxide scales were observed on the surface of the sample. These results on the nanoscale for short experimental time agree with literature reports on bulk counterparts after oxidation at 900 °C for 100 h, which means that the oxidation performance of complex materials can be tested in an accelerated way. Moreover, oxidation-related sub-surface depletion of Mn and Cr together with partial decomposition of the initial Cantor phase yield a FeCo-B2 sub-surface phase. In comparison to coarse-grained bulk material, the faster oxidation of the nanocrystalline material was attributed to an enhanced outwards diffusion of Cr and Mn along grain boundaries and within the newly formed B2 phase.

High-entropy alloys (HEAs) with multiple principal elements open up a practically infinite space for designing novel materials. Probing this huge material universe requires the use of combinatorial and high-throughput synthesis and processing methods. Here, we present and discuss four different combinatorial experimental methods that have been used to accelerate the development of novel HEAs, namely, rapid alloy prototyping, diffusion-multiples, laser additive manufacturing, and combinatorial co-deposition of thin-film materials libraries. While the first three approaches are bulk methods which allow for downstream processing and microstructure adaptation, the latter technique is a thin-film method capable of efficiently synthesizing wider ranges of composition and using high-throughput measurement techniques to characterize their structure and properties. Additional coupling of these high-throughput experimental methodologies with theoretical guidance regarding specific target features such as phase (meta)stability allows for effective screening of novel HEAs with beneficial property profiles.

The efficiency of the direct electrochemical CO 2 reduction can be improved by the development of new alloy catalysts, but to do so a highly resolved composition screening remains to be connected to complex sample preparation and time consuming analysis. We have developed a technique that allows a fast and easy initial catalyst composition screening by analyzing thin film composition spread samples, utilizing a scanning flow cell coupled to an online electrochemical mass spectrometer (SFC-OLEMS). As a first case example, the investigation of a Cu–Co thin film material library demonstrates the benefits and high potential of this approach. In particular, a shift in selectivity toward C 2 species for low Co content (5–15 at.%) has been found and is discussed as being related to changed adsorption energies of intermediate products and the consequent modification of reaction pathways.

Structural and magnetic properties of magnetron-sputtered Fe–P(–Mn) thin films with compositions around the Fe2P single phase region are reported, revealing the compositional range of the Fe2P-type structure and the change of the magnetic properties within this composition spread. The structural analysis shows that in order to obtain crystalline Fe–P phases the P content must be higher than (Fe0.97Mn0.03)2.33P. A maximum phase fraction of the Fe2P-type structure is obtained in the examined (Fe0.97Mn0.03)1.78P sample. The hysteresis loops for the Fe2P(–Mn) thin films show a two-step magnetic reversal with one part belonging to an amorphous phase fraction and the other to the Fe2P(–Mn) phase. A maximum coercivity of 0.36 T was measured for the Fe2P(–Mn) phase fraction also at the composition of (Fe0.97Mn0.03)1.78P. Furthermore, electrochemical properties of FeP2(–Mn) thin films as hydrogen evolution catalysts (HER) are studied. FeP2(–Mn) shows a HER onset potential about 200 mV lower than that of Pt. Chronoamperometric testing at − 11.5 mA/cm2 for over 3500 s revealed no obvious decay in current density, suggesting good stability under typical working conditions in a photoelectrochemical device.

Metallic iridium and ruthenium as well as their oxides are among the most active oxygen evolution (OER) electrocatalysts in acidic media, and are also of interest for the catalysis of the hydrogen evolution (HER). The stability of these materials under different operating conditions is, however, still not fully understood. In the current work, activity and stability of well-defined Ru, RuO2, Ir, and IrO2 thin film electrodes are evaluated in acidic and alkaline electrolytes using an electrochemical scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS). Identical experimental protocols are intentionally employed for all electrodes and electrolytes, to obtain unambiguous and comparable information on intrinsic activity and stability of the electrodes. It is found that independent of the electrolyte, OER activity decreases as Ru > Ir ≈ RuO2 > IrO2, while dissolution increases as IrO2 ≪ RuO2

High oxygen evolution reaction activity of ruthenium and long term stability of iridium in acidic electrolytes make their mixed oxides attractive candidates for utilization as anodes in water electrolyzers. Indeed, such materials were addressed in numerous previous studies. The application of a scanning flow cell connected to an inductively coupled plasma mass spectrometer allowed us now to examine the stability and activity toward oxygen evolution reaction of such mixed oxides in parallel. The whole composition range of Ir–Ru mixtures has been covered in a thin film material library. In the whole composition range the rate of Ru dissolution is observed to be much higher than that of Ir. Eventually, due to the loss of Ru, the activity of the mixed oxides approaches the value corresponding to pure IrO$_{2}$. Interestingly, the loss of only a few percent of a monolayer in Ru surface concentration results in a significant drop in activity. Several explanations of this phenomenon are discussed. It is concluded that the herein observed stability of mixed Ir–Ru oxide systems is most likely a result of high corrosion resistance of the iridium component, but not due to an alteration of the material's electronic structure.

After two decades of slumber, research on organic fuel cells is getting increasing interest. Organic fuels, such as alcohols provide an attractive avenue to overcome the drawbacks of hydrogen as an energy carrier however, catalysis is still a bottleneck for their efficient oxidation. In most studies almost exclusively primary alcohols (e.g., methanol and ethanol) are investigated as fuels. The major downside of this approach is that these are not “zero emission” systems since CO2 forms as a final product. Moreover, CO evolves as an intermediate, which poisons the catalyst surface leading to deteriorated device performance. A possible solution is to use secondary alcohols, such as isopropanol in which case the product is acetone. A prominent member of the bifunctional electrocatalyst family is PtRu, which proven to be the state-of-the-art electrocatalyst in alcohol oxidation. However, in the case of isopropanol the forming acetone poisons its surface, thus there is an inevitable need for the development of novel catalyst materials circumventing these issues. In general, most research efforts so far have addressed the activity and mechanism of oxidation reactions on bifunctional electrocatalysts, but less is known about the influence of alcohols on their stability during operation. On-line techniques allow an elegant option to study such processes besides activity and to track the formed intermediates in real time. In this study we employ on-line ICP-MS to map the activity and stability of Pt-rich PtRu and PtIr alloy catalysts towards the electrocatalytic oxidation methanol, ethanol and isopropanol. Material libraries were synthesized by combinatorial reactive magnetron sputtering that allowed precise control over the metal/metal ratio in each sample. To determine both the surface and bulk composition of PtRu and PtIr material libraries XRD, XPS and EDX measurements were performed. Activity and stability of the electrocatalyst samples were studied in both acidic (HClO4) and alkaline (KOH) media and in the presence of the alcohols. The observed trends in the activity and dissolution characteristics of both systems are going to be discussed in detail along with the identification of the possible underlying processes.

Owing to their superior electrocatalytic performance, non-stoichiometric mixed oxides are often considered as promising electrocatalysts for the acidic oxygen evolution reaction (OER). Their activity and stability can be superior to those of the state-of-the-art IrO2 catalysts, although the exact nature of this phenomenon is not yet understood. In the current work, a Ir0.7Sn0.3O2-x thin-film electrode is taken as a representative example for a thorough evaluation of OER activity of the non-stoichiometric oxides. Complementary activity and stability analysis of Ir0.7Sn0.3O2-x electrodes is achieved using a setup based on an electrochemical scanning flow cell and ICP-MS. The obtained ICP-MS data presents an unambiguous proof of the preferential dissolution of the less noble Sn from the mixed oxide during OER. While less than a monolayer of Ir is dissolved after a prolonged electrolysis of 1400 min during which its dissolution rate drops to near zero, the amount of Sn lost is ten monolayers. The latter finding is confirmed by XPS analysis, which besides showing Ir surface enrichment also indicates a gradual transformation of Ir0 to IrIII species. This transition is beneficial for electrode activity, as the overpotential for OER at j = 5 mA cm−2 was decreasing up to 300 mV. The increase in electrode activity is attributed to several mechanisms including generation of IrIII active sites and overall surface area increase. A generalized description of OER catalysis by Ir-based materials is given, including data from the current work as well as from other Ir-based mixed oxides, such as Ir-Ru-O and Ir-Ni-O.

Co–Mn–Ge is a system of interest for magnetocaloric applications as a solid state magnetic refrigerant. A thin film materials library covering a large fraction of the Co–Mn–Ge ternary composition space was fabricated by sputter deposition. After deposition, it was annealed at 600 °C for 3 h and quenched subsequently. An energy-dispersive X-ray spectroscopy and X-ray diffraction-based cluster analysis revealed the regions of existence for the CoMnGe and the Co2MnGe single phase areas. Furthermore, high intensity diffraction peaks revealed the presence of the hexagonal (Co, Mn)7Ge6 phase in a region that also featured the CoMnGe phase. In this region, a non-linear, symmetric, and hysteretic shift of the (200) diffraction peak of the (Co, Mn)7Ge6 phase was observed by temperature-dependent X-ray diffraction for Co23Mn33Ge44, indicating a structural phase transition taking place between 350 and 375 K upon heating and 325 and 300 K upon cooling. This coincides with a magnetic transition near 325 K from the ferromagnetic to the paramagnetic state. However, no magnetostructural coupling was identified in the temperature range from 330 to 300 K upon cooling. Magnetostriction and an undetected structural transition of the CoMnGe phase were ruled out as probable causes for the non-linear shifts.

Within the search for new catalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells, alloys of Pt with other transition metals are of great interest due to their increased specific and especially mass activity. However, the drawback of these catalysts is their reduced stability due to the dissolution of the less-noble metal from the alloy. To resolve the potential dependence of these dissolution processes, we investigate a material library of Pt-Cu thin-film alloys with compositions ranging from 0 at% Cu up to 60 at% Cu. Utilizing our combinatorial scanning flow cell technique coupled to a mass spectrometer several aspects of dissolution are revealed. The onset of relevant Cu dissolution was found to be around 0.9 VRHE, independent of the composition. Although this is well below the onset potential of the Pt dissolution (1.15 VRHE), the two dissolution processes are clearly correlated, especially when the surface is already depleted of Cu. In contrast to Pt, however, Cu preferentially dissolves during anodic polarization rather than during the cathodic sweep. Additionally, at Cu compositions above the parting limit between 50 and 57 at% Cu a breakdown of passivity and massive Cu dissolution leads to porosity formation. The critical potential for the alloy with 57 at% Cu was detected around 1.3 VRHE, which is above the stability potential of Pt. While the absolute activity during the porosity formation increases due to the formation of more active sites in the pores, the specific activity decays to a value close to pure Pt.

A Nb-Ta thin film compositional spread obtained from a co-sputtering process was analysed. The microstructure and crystallographic investigations revealed the presence of a compositional threshold at Nb-60 at.%Ta where the change from tetragonal to cubic symmetry was evidenced by a mixed tetragonal-cubic phase. The electrochemical properties of the anodic oxides were studied via cyclic voltammetry and the oxide formation factors were mapped along the entire compositional spread. Values ranging from 1.8 nm·V −1 at the Ta-rich side to 2.6 nm·V −1 at the Nb-rich side of the library were measured. All Nb-Ta mixed anodic oxides were found to exhibit a type-n semiconducting behaviour as evidenced by Mott-Schottky analysis. The chemical composition of the surface anodic oxides differed from the composition of the parent metal alloys and no clear trend could be identified regarding their mismatch.

Layered WO 3 /TiO 2 nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO 2 and WO 3 layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO 3 microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO 2 nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO 3 and TiO 2 films show thickness and microstructure dependence. Among individual WO 3 layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO 2 nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO 3 -wedge type layer with a layer of TiO 2 nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO 3 layers. The highest photocurrent that is measured in the combinatorial library of porous WO 3 /TiO 2 films is as high as 0.11 mA/cm 2 . Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.

A wide-range thin film Hf–Nb combinatorial library deposited by co-sputtering is studied. The microstructure and crystallographic properties of the thin film alloys locally investigated by SEM and GIXRD are mapped along the entire compositional spread from 14 to 94 at.% Nb. Scanning droplet cell microscopy (SDCM) is used for mapping the electrochemical properties of the naturally oxidised metallic surfaces. Anodisation of the Hf–Nb thin films alloys is achieved with a high throughput due to computer-controlled scanning, made with a composition resolution of 1 at.%. The electrical properties of the anodic oxides are mapped by EIS and a maximum electrical permittivity close to 75 was found for Hf–33 at.% Nb. Semiconducting properties of the mixed anodic oxides are studied using Mott–Schottky analysis and their composition and mixing is investigated by XPS depth profiling.

The transition from energy supply by fossil fuels and nuclear power to alternative solutions relies on efficient energy conversion and long-term storage possibilities. Hydrogen is a chemical-energy carrier with great potential, which can be generated through electrolysis during times of high supply of renewable electrical energy. Fuel cells can then regain electrical energy for mobile applications, thereby closing the sustainable hydrogen cycle. The systems of choice, especially in the mobile sector, are polymer electrolyte membrane fuel cells because of their low operation temperatures and short start-up times. However, for widespread commercialization of fuel-cell-driven cars, some issues still need to be resolved. A major aim is to improve the performance of the catalysts, which would reduce the required amount of the typically rare and expensive material that is used. One option is to utilize alloys of Pt with non-noble metals, which are known to increase the specific and mass activity towards the oxygen reduction reaction (ORR). To investigate the performance of these catalysts, that is, their activity and stability, we analyzed the dissolution behavior of Pt–Cu thin films of different compositions during electrochemical treatment. The combinatorial thin-film material library was deposited by using magnetron sputtering with a moving blade-type shutter to create a defined thickness gradient by progressively shadowing the deposition source. By repeatedly alternating between Pt and Cu sources, a multilayer stack of wedges was formed. These bilayers were sequentially deposited 39 times, as shown schematically in Figure 1a. The complete as-deposited materials library was annealed at 825 8C for 1 h under vacuum to mix the Pt and Cu layers in the thickness direction. The composition across the wafer was mapped by using energy-dispersive X-ray spectroscopy (EDX), as shown in Figure 1b. The resulting compositions range from almost pure Pt to approximately 65 at% Cu. The sample is homogeneous on the microscopic level as no segregation of Pt or Cu is observed in the grains or at the grain boundaries, for any composition, by using highresolution EDX. Prior to the electrochemical experiments, the surface compositions close to the specific measurement locations were determined by using X-ray photoelectron spectroscopy (XPS). The EDX and XPS results correspond well with each other, indicating that the same composition is present at the surface as in the bulk. The electrochemical experiments were performed with a scanning flow cell (SFC) coupled to an inductively coupled plasma mass spectrometer (ICP–MS). This technique allows for on-line monitoring of the dissolution of both Pt and Cu simultaneously, and it enables the measured amounts of Pt and Cu to be linked to the applied electrochemical potential sequences. 4] Material libraries are well suited for use in the SFC setup because the complete library is prepared in one single process, and the pretreatment of the different locations is always the same. This saves preparation time and reduces errors caused by sample handling. Moreover, screening of a continuous library can be easily performed with the SFC because of the computer-controlled positioning of the cell and automated measurement procedures. A particular potential protocol was chosen for this first comparative study, which is shown in Figure 2b. Each measurement location was contacted under potential control at 0.1 V versus a reversible hydrogen electrode (RHE), followed by a potential step to 0.8 V (vs RHE) after some equilibration time. For Figure 1. a) Preparation of the Pt–Cu material library, which was achieved by depositing alternating wedges of Cu and Pt (with a reference band of pure Pt). b) EDX map of the resulting compositions after annealing.

A Lia(NixMnyCoz)Or cathode materials library was fabricated by combinatorial magnetron sputtering. The compositional analysis of the library was performed by a new high-throughput approach for Li-content measurement in thin films, which combines automated energy-dispersive X-ray spectroscopy, Deuteron-induced gamma emission, and Rutherford backscattering measurements. Furthermore, combining this approach with thickness measurements allows the mapping of density values of samples from the materials library. By correlating the obtained compositional data with structural data from high-throughput X-ray diffraction measurements, those compositions which show a layered (R3̅m) structure and are therefore most interesting for Li-battery applications (for cathode (positive) electrodes) can be rapidly identified. This structure was identified as being most pronounced in the compositions Li0.6(Ni0.16Mn0.35Co0.48)O2, Li0.7(Ni0.10Mn0.37Co0.51)O2, Li0.6(Ni0.23Mn0.33Co0.43)O2, Li0.3(Ni0.65Mn0.08Co0.26)O2, Li0.3(Ni0.63Mn0.08Co0.29)O2, Li0.4(Ni0.56Mn0.09Co0.34)O2, Li0.5(Ni0.45Mn0.13Co0.42)O2, and Li0.6(Ni0.34Mn0.14Co0.52)O2.

The shape memory properties of the complete Ti–Ni–Cu thin film system were investigated using combinatorial methods, i.e. fabrication and high-throughput characterization of thin film materials libraries. Thin film composition spreads were deposited using a wedge-type multilayer technique and annealed at 500 °C, 600 °C and 700 °C for 1 h for alloy formation. The complete composition regions showing reversible phase transformations were identified for each annealing temperature. These regions are well extended in comparison to prior knowledge. Furthermore, the composition-structure-property relations governing the phase transformation characteristics in the thin film samples were determined. For films annealed at 500 °C and 600 °C the transformation temperatures are highest for compositions close to Ti50Ni50−xCux and decrease as the thin film compositions deviate. Similarly, the thermal transformation hysteresis is found to be smallest for “stoichiometric” (Ti50Ni50−xCux) compositions. Precipitation of Ti-rich and (Ni,Cu)-rich phases is found to be responsible. With increasing annealing temperature the transformation temperatures increase and the thermal hysteresis values decrease for compositions showing B2→B19 phase transformation paths, due to coarsening of the precipitate phases. The alloying process of the multilayer thin films leads to the formation of the equilibrium phases. The formation of Guinier–Preston zones is suppressed. For thin films annealed at 700 °C the transforming composition region is comparatively smaller and the phase transformation properties are influenced by Ti2Ni precipitates.

A frame store pn-junction CCD (pnCCD) detector was applied to study thermally induced interdiffusion in Fe/Pt thin film multilayers (MLs) in a temperature range between 300 and 585 K. Based on the energy resolution of the detector the reflectivity was measured simultaneously in a spectral range between 8 keV

The shape memory thin film system Ti–Ni–Hf was investigated with regard to its structural, phase transformation and functional fatigue properties by means of combinatorial and high-throughput methods. Temperature-dependent resistance measurements revealed a broad compositional region showing a reversible phase transformation. A ternary Laves phase was identified using X-ray diffraction as a precipitate phase within the transforming composition region. With increasing Ti content, the amount of the Laves phase increases, which results in an increase in the thermal hysteresis and a simultaneous decrease in the transformation temperatures. Shape memory properties were characterized by temperature-dependent stress change measurements using micromachined Si cantilever array wafers coated with Ti–Ni–Hf. The recovery stress was found to increase for small amounts of Laves phase precipitates. Strengthening of the matrix due to the Laves phase precipitates is concluded to be responsible for the observed increase in recovery stress and improved functional fatigue properties for (Ti,Hf)-rich alloy compositions (Ti40.0Ni47.5Hf12.5).

Polycrystalline WO3 thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O2 gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO3, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm−2 at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO3 films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm.

Thermal annealing of Fe/Pt multilayers (ML) is reported to reduce significantly the formation temperature of FePt hard magnetic thin films. The transformation mechanisms of [Fe 1.38 nm/Pt 2.24 nm]50 ML, prepared by magnetron sputtering, is investigated in the present communication by high temperature X-ray reflectivity using synchrotron radiation. Complete degradation of the ML periodic structure is observed at about 610 K. The variation with annealing temperature of the intensity of the first Bragg peak, the correlated vertical roughness, and the lateral correlation length of the ML show that the ML transform in two stages with a cross-over temperature of about 515 ± 15 K. This behavior cannot be simply explained by the change in the measured interdiffusion coefficient below and above the cross-over temperature, suggesting the formation of FePt nanograins along the interfaces.

This paper reports on a novel miniaturized deposition technique based on micro-hotplates which are used as microevaporation sources (MES) for a localized deposition of thin films and nanoparticles. The feasibility of this small-scale deposition technique and its general properties are shown for depositions of Ag on unpatterned and microstructured substrates. The deposited films are rotationally symmetric and show a distinct lateral thickness change. We take advantage of this latter effect, as, e.g., all stages of film condensation can be observed within one experiment on one sample, in a size suitable for transmission electron microscopy investigations. For realizing the most laterally confined depositions, a micro-Knudsen cell was used. It is shown that the use of MES is also very suitable for the fabrication and deposition of nanoparticles.

The Ti–Ni–W two-phase shape memory alloy (SMA) thin film system is presented as a prototype for new SMAs with tailorable thermal transformation hysteresis (ΔT). The concept is to combine the SMA TiNi with almost insoluble W to create the two-phase system (TiNi)–(β-W). This system behaves like a pseudobinary TiNi system. Phase transformation behavior for compositions above the solubility limit of W in TiNi exhibit a B2–R phase transformation with characteristically small ΔT. Moreover, ΔT is dependent on the amount of W and it can be tailored to zero and even negative. This phenomenon is rationalized as being due to the mechanical interaction between the phases B2-TiNi and β-W. The presented results are very promising for the development of high-speed Ti–Ni-based SMA actuators.

A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf–21 at.% Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal through an intermediate amorphous region. The local oxidation of Hf–Ta parent metal alloys at different compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric constants were mapped for the entire composition range and XPS depth profiles allowed investigation of the oxide compositions.

A new ferromagnetic shape memory thin film system, Fe–Pd–Cu, was developed using ab initio calculations, combinatorial fabrication and high-throughput experimentation methods. Reversible martensitic transformations are found in extended compositional regions, which have increased fcc–fct transformation temperatures in comparison to previously published results. High resolution transmission electron microscopy verified the existence of a homogeneous ternary phase without precipitates. Curie temperature, saturation polarization and orbital magnetism are only moderately decreased by alloying with nonmagnetic Cu. Compared to the binary system; enhanced Invar-type thermal expansion anomalies in terms of an increased volume magnetostriction are predicted. Complementary experiments on splat-fabricated bulk Fe–Pd–Cu samples showed an enhanced stability of the disordered transforming Fe70Pd30 phase against decomposition. From the comparison of bulk and thin film results, it can be inferred that, for ternary systems, the Fe content, rather than the valence electron concentration, should be regarded as the decisive factor determining the fcc–fct transformation temperature.

Thermal annealing of [Fe 1.65 nm/Pt 1.84 nm] 50 multilayers at 673 K for various annealing times between 60 and 12000 s leads to the direct formation of the fully ordered L1 0 FePt phase with (111) texture. The average grain sizes, determined from X-ray diffraction size–strain analysis, are smaller than the critical size for multi-domain FePt particles, suggesting the presence of single-domain (SD) grains. The coercivity increases with annealing time and increasing grain size and reaches values of about 955 kA/m. The remanence values are typical for randomly oriented weakly-interacting particles. A decrease of the remanence with annealing time suggests a decrease of the intergrain exchange interactions with annealing time. Analysis of minor loops and the initial magnetization curves shows the presence of a broad distribution of critical fields, which the individual SD particles have to overcome for the magnetization reversal.

Improving the functional stability of shape memory alloys (SMAs), which undergo a reversible martensitic transformation, is critical for their applications and remains a central research theme driving advances in shape memory technology. By using a thin-film composition-spread technique and high-throughput characterization methods, the lattice parameters of quaternary Ti-Ni-Cu-Pd SMAs and the thermal hysteresis are tailored. Novel alloys with near-zero thermal hysteresis, as predicted by the geometric nonlinear theory of martensite, are identified. The thin-film results are successfully transferred to bulk materials and near-zero thermal hysteresis is observed for the phase transformation in bulk alloys using the temperature-dependent alternating current potential drop method. A universal behavior of hysteresis versus the middle eigenvalue of the transformation stretch matrix is observed for different alloy systems. Furthermore, significantly improved functional stability, investigated by thermal cycling using differential scanning calorimetry, is found for the quaternary bulk alloy Ti 50.2 Ni 34.4 Cu 12.3 Pd 3.1 .

Composition spread thin film samples of Nb and Ti were prepared by co-sputtering. The composition range from Nb–5 at.% Ti to Nb–78 at.% Ti was achieved and characterized by high resolution field emission scanning electron microscopy and grazing angle X-ray diffraction. Nb stabilized the β-Ti phase over the entire range studied. The structure was cubic with a continuous change in the lattice constants with composition. Several distinct compositional zones were identified in the as-deposited film morphology. Then, anodic oxides were grown potentiodynamically and characterized by electrochemical impedance spectroscopy using a scanning droplet cell to yield a comprehensive description of oxide film properties including dielectric permittivity, resistivity, thickness and film formation factor. Mott–Schottky analysis of potentiostatically grown oxides showed how the alloy composition influences the flat band potential and the donor density of the mixed n-type semiconducting oxides. Complementary X-ray photo electron spectroscopy as a chemical analysis revealed differences in the oxide compositions as compared to the as-deposited metal resulting from the different ion transport numbers.

A comprehensive study of the anodic oxide formation on Hf–Ti alloys over the entire range of composition was conducted. Combinatorial thin film libraries were prepared by co-sputtering. HRSEM and XRD were used to characterize the thin films and to confirm that a continuous change of the composition was obtained for this fully miscible system. Electrochemical investigations were performed by means of an automatic scanning droplet cell with a droplet radius of 100 μm. Subsequent potentiodynamic scans with intermittent impedance measurements allowed a quantitative determination of film formation factor and dielectric constant for each composition with a resolution of 0.5 at.%. Mott–Schottky analysis of the oxides was used to evaluate the relationship between parent metal composition and oxide properties, namely flat band potential and donor density. The structure–property and composition–property relationships are discussed in detail.

A combinatorial study of the kinetics of anodic oxide formation was performed on a co-deposited Ta–Ti composition spread sample (from 2 to 76 at.% Ti). The samples were characterized by High Resolution Scanning Electron Microscopy (HRSEM) and Grazing Angle X-Ray Diffraction (GIXRD). High-throughput-screening was performed with an automated scanning-droplet-cell to anodise incrementally by cyclic voltammetry and perform electrochemical impedance spectroscopy. A comprehensive set of oxide parameters, including film formation factor, dielectric constant, specific oxide resistance, flat band potential and donor density was obtained. The observed changes could be directly correlated to the microstructure of the parent metal.

Thin metallic Mn–Co films (0.3 μm thickness) were produced by RF-magnetron co-sputtering on SiO 2 /Si and on ZMG232L steel substrates. The deposition was followed by a heat treatment in order to convert the metallic coatings to (Mn,Co) 3 O 4 spinel oxides. All steel samples were analyzed after 1 h heat treatment in order to confirm the presence of the spinel structure on top of the steel, as well as to investigate and characterise the evolution and growth of oxides, namely (Mn,Cr) 3 O 4 and Cr 2 O 3 , at the internal steel/coating interface. From the structural and morphological characterization, the presence of a well-adherent approximately 0.5 μm thick Mn 1.5 Co 1.5 O 4 coating was confirmed. After this preparatory annealing, selected samples were heat-treated in simulated cathodic or anodic atmospheres at 800 °C for 500 h in order to investigate the properties of the coatings after long time annealing. For the samples annealed in cathodic-like atmospheres, Cr was present on the surface of the coating, whereas for the samples annealed in anodic-like atmospheres, MnO was the main phase observed on the surface. Measurements of the area specific resistance performed on the steel coated with Mn 50 Co 50 showed a resistivity of around 59 mΩ cm after 601 h annealing in ambient air.

This paper presents the use of micro-hotplates (MHPs) as thermal processing and in situ characterization platforms for phase transformations in thin films. MHPs are fabricated by microsystem technology processes and consist of a SiO2/Si3N4 membrane (app. 1 μm) supported by a bulk Si frame. Several embedded Pt thin films serve as heater and electrical measurement electrodes. It is shown that the MHPs have unique properties for the controlled annealing of thin film materials (up to 1270 K), as the annealing temperature and heating/cooling rates can be precisely controlled by in situ measurements. These rates can be extremely high (up to 104 K/s), due to the low thermal mass of MHPs. The high cooling rates are especially useful for the fabrication of metastable phases (e.g. Fe70Pd30) by quenching. By measuring the resistivity of a thin film under test in situ as a function of the MHP temperature, microstructural changes (e.g. phase transformations) can be detected during heating and cooling cycles. In this paper, examples are presented for the determination of phase transitions in thin films using MHPs: the solid–liquid–gas phase transition (Al), the ferromagnetic–paramagnetic phase transition (Fe–Pt) and martensitic transformations (Ni–Ti–Cu, Fe–Pd). Furthermore, it is demonstrated that crystallization processes from amorphous to crystalline (Ni–Ti–Cu) can be detected with this method. Finally the application of MHPs in thin film combinatorial materials science and high-throughput experimentation is described.

This paper describes the use of thin film high-throughput experimentation methods for the efficient development of multifunctional materials, using Ni – Ti – X and ferromagnetic shape memory alloys as examples. The thin films were fabricated in the form of binary, ternary, and quaternary materials libraries by special magnetron sputter deposition processes. These materials libraries were subsequently processed and characterized by high-throughput experimentation methods in order to relate compositional information with structural and functional properties. For this, appropriate visualization of the data is necessary. Results show that the martensitically transforming regions in ternary thin films are generally larger than was known from literature. Within these regions, the variation of the functional properties can be mapped with respect to the composition and microstructure, and thus the most suitable materials for applications can be effectively selected.