Chemical and Microstructural Nanoscale Homogeneity in Superconducting YBa2Cu3O7-x Films Derived from Metal-Propionate Fluorine-free Solutions.

Research involved in developing alternative energy sources has become a necessity to face global warming. In this context, superconductivity is an appealing solution to enhance clean electrical energy provided that lower production costs can be attained. By implementation of chemical solution deposition techniques and high-throughput growth methods, low-cost nanostructured epitaxial cuprate superconductors are timely candidates. Here, we present a versatile and tunable solution method suitable for the preparation of high-performance epitaxial cuprate superconducting films. Disregarding the renowned trifluoroacetate route, we center our focus on the transient liquid-assisted growth (TLAG) that meets the requirement of being a greener chemical process together with ultrafast growth rates beyond 100 nm/s. We developed a facile, fast, and cost-effective method, starting from the synthesis of metal-propionate powders of Y, Ba, and Cu of high purity and high yields, being the precursors of the fluorine-free solutions, which enable the chemical and microstructural nanoscale homogeneity of YBa2Cu3O7-x (YBCO) precursor films. These solutions present endured stability and enable precise tunability of the composition, concentration, porosity, and film thickness. Homogeneous precursor films up to thicknesses of 2.7 µm through eight layer multidepositions are demonstrated, thus establishing the correct basis for epitaxial growth using the fast kinetics of the TLAG process. YBCO films of 500 nm thickness with a critical current density of 2.6 MA/cm2 at 77 K were obtained, showing the correlation of precursor film homogeneity to the final YBCO physical properties.

Kinetic Control of Ultrafast Transient Liquid Assisted Growth of Solution-Derived YBa2 Cu3 O7 -x Superconducting Films.

Transient liquid assisted growth (TLAG) is an ultrafast non-equilibrium growth process mainly governed by kinetic parameters, which are only accessible through fast in situ characterizations. In situ synchrotron X-ray diffraction (XRD) analysis and in situ electrical resistivity measurements are used to derive kinetic diagrams of YBa2 Cu3 O7- x (YBCO) superconducting films prepared via TLAG and to reveal the unique peculiarities of the process. In particular, diagrams for the phase evolution and the YBCO growth rates have been built for the two TLAG routes. It is shown that TLAG transient liquids can be obtained upon the melting of two barium cuprate phases (and not just one), differentiated by their copper oxidation state. This knowledge serves as a guide to determine the processing conditions to reach high performance films at high growth rates. With proper control of these kinetic parameters, films with critical current densities of 2-2.6 MA cm-2 at 77 K and growth rates between 100-2000 nm s-1 are reached. These growth rates are 1.5-3 orders of magnitude higher than those of conventional methods.

Defining inkjet printing conditions of superconducting cuprate films through machine learning.

The design and optimization of new processing approaches for the development of rare earth cuprate (REBCO) high temperature superconductors is required to increase their cost-effective fabrication and promote market implementation. The exploration of a broad range of parameters enabled by these methods is the ideal scenario for a new set of high-throughput experimentation (HTE) and data-driven tools based on machine learning (ML) algorithms that are envisaged to speed up this optimization in a low-cost and efficient manner compatible with industrialization. In this work, we developed a data-driven methodology that allows us to analyze and optimize the inkjet printing (IJP) deposition process of REBCO precursor solutions. A dataset containing 231 samples was used to build ML models. Linear and tree-based (Random Forest, AdaBoost and Gradient Boosting) regression algorithms were compared, reaching performances above 87%. Model interpretation using Shapley Additive Explanations (SHAP) revealed the most important variables for each study. We could determine that to ensure homogeneous CSD films of 1 micron thickness without cracks after the pyrolysis, we need average drop volumes of 190-210 pl, and no. of drops between 5000 and 6000, delivering a total volume deposited close to 1 µl.

Real-Space Image of Charged Patches in Tunable-Size Nanocrystals.

The remarkable dual nature of faceted-charge patchy metal fluoride nanocrystals arises from the spontaneous selective coordination of anionic and cationic ligands on the different facets of the nanocrystals. In previous studies, the identification and origin of the charge at the patches were obtained by combining computer simulations with indirect experimental evidence. Taking a step further, we report herein the first direct real-space identification by Kelvin probe force microscopy of the predicted faceted-charge patchy behavior, allowing the image of the dual faceted-charge surfaces. High-resolution transmission electron microscopy reveals the detailed nanocrystal faceting and allows unambiguously inferring the hydrophilic or hydrophobic role of each facet from the identification of the surface atoms exposed at the respective crystallographic planes. The success of the study lies in a foresighted synthesis methodology designed to tune the nanocrystal size to be suitable for microscopy studies and demanding applications.

Combinatorial Screening of Cuprate Superconductors by Drop-On-Demand Inkjet Printing.

Combinatorial and high-throughput experimentation (HTE) is achieving more relevance in material design, representing a turning point in the process of accelerated discovery, development, and optimization of materials based on data-driven approaches. The versatility of drop-on-demand inkjet printing (IJP) allows performing combinatorial studies through fabrication of compositionally graded materials with high spatial precision, here by mixing superconducting REBCO precursor solutions with different rare earth (RE) elements. The homogeneity of combinatorial Y1-xGdxBa2Cu3O7 samples was designed with computational methods and confirmed by energy-dispersive X-ray spectroscopy (EDX) and high-resolution X-ray diffraction (XRD). We reveal the advantages of this strategy in the optimization of the epitaxial growth of high-temperature REBCO superconducting films using the novel transient liquid-assisted growth method (TLAG). Advanced characterization methods, such as in situ synchrotron growth experiments, are tailored to suit the combinatorial approach and demonstrated to be essential for HTE schemes. The experimental strategy presented is key for the attainment of large datasets for the implementation of machine learning backed material design frameworks.

Hybrid approach to obtain high-quality BaMO3 perovskite nanocrystals.

A novel hybrid solvothermal approach for perovskite nanocrystal formation via accurate control of the hydrolytic process is reported. This new synthetic methodology sets a whole general route to successfully tune the sizes of high-quality BaMO3 (M = Ti4+, Zr4+, and Hf4+) perovskite nanocrystals. Purely cubic-phase nanocrystals (stable in alcohol media) were obtained using controlled water amounts, combining the well-known aqueous sol-gel process with the classic solvothermal method. Exhaustive optimizations revealed feasibility of a fast (1 hour) and reproducible synthesis with small variations in the crystal size or agglomeration parameters. The study also reveals water content as the pivotal factor to achieve this wide range of sizes through a controlled hydrolytic step. Finally, the study of the hydrolytic process made it possible to shed some light on mechanistic insights of this synthetic route.

Control of nanostructure and pinning properties in solution deposited YBa2Cu3O7-x nanocomposites with preformed perovskite nanoparticles.

Solution deposited YBa2Cu3O7-x (YBCO) nanocomposites with preformed nanoparticles represent a promising cost-effective approach for superior critical current properties under applied magnetic fields. Nonetheless, the majority of YBCO nanocomposites with high nanoparticle loads (>20%) suffer from nanoparticle coalescence and degraded superconducting properties. Here, we study the influence of nanoparticle concentration (0-25% mol), size (5 nm-10 nm) and composition (BaHfO3, BaZrO3) on the generation of structural defects in the epitaxial YBCO matrix, key parameter for vortex pinning. We demonstrate that flash-heated superconducting nanocomposites with 20 mol% preformed BaHfO3 or BaZrO3 perovskite secondary phases feature discrete and small (7 nm) nanoparticles and high density of YBa2Cu4O8 (Y248) intergrowths. We identify a synergy between Y248 intergrowth density and small nanoparticles to increase artificial vortex pinning centers. Also, we validate the multideposition process to successfully increase film thickness of epitaxial nanocomposites with competitive critical currents Ic at 77 K.

Using evolved gas analysis - mass spectrometry to characterize adsorption on a nanoparticle surface.

The surface chemistry of nanoparticles is the key factor to control and predict their interactions with molecules, ions, other particles, other materials, or substrates, determining key properties such as nanoparticle stability or biocompatibility. In consequence, the development of new techniques or modification of classical techniques to characterize nanoparticle surfaces is of utmost importance. Here, a classical analysis technique, thermally evolved gas analysis - mass spectrometry (EGA-MS), is employed to obtain an image of the nanoparticle-solvent interface, unraveling the molecules present on the surface. As the use of complementary techniques is urged, the validity of EGA-MS characterization is corroborated by comparison with a previously reported surface characterization method. Previous studies were based on several experimental techniques and MD simulations using YF3 nano/supraparticles and LaF3 nanoparticles as model systems. We demonstrate the applicability of this technique in two differently sized systems and two systems composed of the same ions on their surface but with a different inorganic core (e.g. LaF3 and YF3 nanoparticles). The results described in this paper agree well with our previous results combining experimental techniques and MD simulations. EGA-MS not only revealed the ions attached to the nanoparticle surface but also shed light on their coordination (e.g. citrate attached to one or two carboxylate moieties). Thus, we show that EGA-MS is a useful and efficient technique to characterize the surface chemistry of nanoparticles and to control and predict their final properties.

Faceted-Charge Patchy LnF3 Nanocrystals with a Selective Solvent Interaction.

A fast and single-step preparation of patchy LnF3 faceted-charge nanocrystals are described. These hexagonal faceted nanocrystals allow the spontaneous selective adsorption of cations or anions in the different faces, producing stable and well-defined patches of different charge. The mechanism for the formation of the patches and the properties of the obtained nanocrystals were characterized by a combination of experimental techniques and all-atomic molecular dynamics simulations. The spontaneous dual-charged surface as well as the luminescence effects that can be achieved by doping host-LaF3 systems make these new nanocrystals interesting both from a fundamental point of view and for a wide range of applications.

Simple Synthesis of Biocompatible Stable CeO2 Nanoparticles as Antioxidant Agents.

Cerium oxide (IV) nanoparticles offer a high redox ability, while maintaining nontoxicity and high stability. Thus, dispersed nanoceria is a promising candidate as antioxidant material for human cells. In this work, we report on a fast and simple one-pot process that yield a final nanoparticle size of 2-4 nm in polar solvents such as water and alcohols. High boiling point solvents, namely, benzyl alcohol and triethylene glycol, are used to obtain high crystalline nanoparticles by thermal and microwave activation. Transmission electron microscopy investigations prove the narrow size distribution of the CeO2 nanoparticles and show that the shape can be tuned from spherical to cubic using an appropriate precursor. The main objective of this work is to produce nanoparticles, which are well-defined, biocompatible, and stable in highly concentrated colloidal solutions (up to 90 mM) for a long period of time to study their behavior as antioxidant agents in human cells under oxidative stress.

Tailoring the Synthesis of LnF3 (Ln = La-Lu and Y) Nanocrystals via Mechanistic Study of the Coprecipitation Method.

Here, 15 LnF3 nanocrystals are synthesized using coprecipitation method with citrate stabilization to allow the fast, easy, and reproducible synthesis of several nanoscaled structures in water. General trends related to the behavior of LnF3 nanocrystals are highlighted due to their broad range of application in several fields (e.g., medical applications). The same nature for all Ln3+ cations is expected due to the internal role of f orbitals. However, we found that the use of different lanthanide elements is crucial in the final size, shape, assembly, and crystalline structure. In addition, the decrease of the cation size of the lanthanide series changes the behavior of these compounds, resulting in hexagonal, orthorhombic, and cubic crystalline structures. In addition, we are able to tune the cubic crystalline phase to pure orthorhombic by modifying the pH of the system using HBF4 instead of tetramethylammonium citrate. Via 11B NMR, we demonstrated the mechanism of HBF4 as fluorinating agent if an additional source of F- is not added during the synthesis. 1H NMR and IR techniques were performed to unravel the picture of the surface chemistry of the two representative metal cations (Y and La). Finally, HRTEM and SAED were performed to uncover the shape of the obtained nanocrystals and the preferential orientation of the assembled particles, giving crucial information on the involved mechanisms. This study reveals not only the dependence of the crystalline structure on the used metal and pH but also ability to achieve LnF3 assembled particles depending on the final shape and temperature.

Tunable Self-Assembly of YF3 Nanoparticles by Citrate-Mediated Ionic Bridges.

Ligand-to-surface interactions are critical factors in surface and interface chemistry to control the mechanisms governing nanostructured colloidal suspensions. In particular, molecules containing carboxylate moieties (such as citrate anions) have been extensively investigated to stabilize metal, metal oxide, and metal fluoride nanoparticles. Using YF3 nanoparticles as a model system, we show here the self-assembly of citrate-stabilized nanostructures (supraparticles) with a size tunable by temperature. Results from several experimental techniques and molecular dynamics simulations show that the self-assembly of nanoparticles into supraparticles is due to ionic bridges between different nanoparticles. These interactions were caused by cations (e.g., ammonium) strongly adsorbed onto the nanoparticle surface that also interact strongly with nonbonded citrate anions, creating ionic bridges in solution between nanoparticles. Experimentally, we observe self-assembly of nanoparticles into supraparticles at 25 and 100 °C. Interestingly, at high temperatures (100 °C), this citrate-bridge self-assembly mechanism is more efficient, giving rise to larger supraparticles. At low temperatures (5 °C), this mechanism is not observed, and nanoparticles remain stable. Molecular dynamics simulations show that the free energy of a single citrate bridge between nanoparticles in solution is much larger than the thermal energy and in fact is much larger than typical adsorption free energies of ions on colloids. Summarizing our experiments and simulations, we identify as key aspects of the self-assembly mechanism the requirement of NPs with a surface able to adsorb anions and cations and the presence of multidentate ions in solution. This indicates that this new ion-mediated self-assembly mechanism is not specific of YF3 and citrate anions, as supported by preliminary experimental results in other systems.

Erratum: Neutron and X-ray diffraction study of ferrite nanocrystals obtained by microwave-assisted growth. A structural comparison with the thermal synthetic route. Corrigendum.

In the paper by Solano et al. ▶ [J. Appl. Cryst. (2014), 47, 414-420], Inés Puente Orench is missing from the list of authors. The complete list of authors should be Eduardo Solano, Carlos Frontera, Inés Puente Orench, Teresa Puig, Xavier Obradors, Susagna Ricart and Josep Ros.[This corrects the article DOI: 10.1107/S1600576713032895.].

Quatsomes: vesicles formed by self-assembly of sterols and quaternary ammonium surfactants.

Thermodynamically stable nanovesicle structures are of high interest for academia and industry in a wide variety of application fields, ranging from preparation of nanomaterials to nanomedicine. Here, we show the ability of quaternary ammonium surfactants and sterols to self-assemble, forming stable amphiphilic bimolecular building-blocks with the appropriate structural characteristics to form in aqueous phases, closed bilayers, named quatsomes, with outstanding stability, with time and temperature. The molecular self-assembling of cholesterol and surfactant cetyltrimethylammonium bromide (CTAB) was studied by quasi-elastic light scattering, cryogenic transmission electron microscopy, turbidity (optical density) measurements, and molecular dynamic simulations with atomistic detail, upon varying the cholesterol-to-surfactant molar ratio. As pure species, CTAB forms micelles and insoluble cholesterol forms crystals in water. However, our molecular dynamic simulations reveal that the synergy between CTAB and cholesterol molecules makes them self-assemble into bimolecular amphiphiles and then into bilayers in the presence of water. These bilayers have the same structure of those formed by double-tailed unimolecular amphiphiles.

Deposition of photocatalytically active TiO2 films by inkjet printing of TiO2 nanoparticle suspensions obtained from microwave-assisted hydrothermal synthesis.

In this paper, we present an inkjet printing approach suited for the deposition of photocatalytically active, transparent titanium oxide coatings from an aqueous, colloidal suspension. We used a bottom-up approach in which a microwave-assisted hydrothermal treatment of titanium propoxide aqueous solutions in the presence of ethylenediaminetetraacetic acid and triethanolamine was used to create suspensions containing titania nanoparticles. Different inkjet printing set-ups, electromagnetic and piezoelectric driven, were tested to deposit the inks on glass substrates. The presence of preformed titania nanoparticles was expected to make it possible to reduce the heating temperature necessary to obtain the functionality of photocatalysis which can widen the application range of the approach to heat-sensitive substrates. We investigated the crystallinity and size of the obtained nanoparticles by electron microscopy and dynamic light scattering. The rheological properties of the suspensions were evaluated against the relevant criteria for inkjet printing and the jettability was analyzed. The photocatalytic activity of the obtained layers was analyzed by following the decomposition of a methylene blue solution under UV illumination. The influence of the heat treatment temperature on the film roughness, thickness and photocatalytic activity was studied. Good photocatalytic performance was achieved for heat treatments at temperatures as low as 150 °C, introducing the possibility of using this approach for heat-sensitive substrates.

Highly hydrophobic polyfluorinated azo dyes grafted on surfaces.

Artificial hydrophobic surfaces have a great potential in a wide range of industrial applications owing to their self-cleaning, anti-fogging and anti-biofouling properties. A family of polyfluorinated reactive azo dyes has been prepared and some of them easily modified and grafted on cotton fabric and glass surfaces obtaining new coloured hydrophobic materials.

The nickel-catalyzed carbonylative cycloaddition of allyl halides and acetylenes: an efficient tool for cyclopentane annelation.

From a practical synthetic point of view, the nickel-mediated carbonylative cycloaddition of alkynes and allyl halides is a straightforward method for obtaining the cyclopentane skeleton in high yields and with controlled stereochemistry, especially when considering the efficiency of the intermolecular version of the reaction. The efforts to make the previously stoichiometric process catalytic in nickel, after experimental mechanistic observations, are reported herein. The unexpected intervention of iron as a reductant and the isolation of a final dimeric species that exhibits interesting tautomeric behavior are also presented. An extension of the reaction to new substrates has led to the conclusion that, although the steric and electronic effects of the alkyne substituents are generally irrelevant in relation to the adducts and their yields, those of the allylic counterpart may have a significant influence on the outcome of the reaction. However, the presence of the amine moiety in the alkyne completely inhibited the reaction. The feasibility of a multicentered reaction was verified with a triacetylene in which up to 12 bonds were created at once and in good yield.

Versatile chemoselectivity in Ni-catalyzed multiple bond carbonylations and cyclocarbonylations in CO(2)-expanded liquids.

Selective mono or double carbonylations could be achieved by using CO(2)-expanded liquids in [2 + 2 + 1] carbonylative reactions of alkenes or acetylenes with allyl bromides catalyzed by Ni(i).

The Ni-mediated cyclocarbonylation of allyl halides and alkynes made catalytic. Evidence supporting the involvement of pseudoradical Ni(i) species in the mechanism.

We report here on a highly efficient catalytic method to synthesize intermolecularly the cyclopentane skeleton from starting products as simple as allyl halides, alkynes, and carbon monoxide under very mild reaction conditions by means of a substoichiometric amount of iron, acetone, and a catalytic amount of Ni(II) iodide.

Reduction of cyclic group 6 Fischer carbene complexes: a case of exquisite regioselectivity.

The reduction of alpha,beta-unsaturated cyclic group 6 metal-carbene complexes strongly depends on the electronic profile of the groups attached both to the carbene and the beta-carbon and occurs with exquisite regioselectivity. Thus, for complexes 8 the reduction does not take place at the carbene carbon but exclusively at the gamma-carbonyl group. The resulting alkoxide 20 evolves to a tricyclic epoxide structure 21, which precludes additional hydride transfers. Complex 9 experiences the exclusive 1,4-reduction because of the imino character of the beta-carbon (due to the participation of the aromatic resonance form 22). In contrast, monocyclic carbene complexes 10 behave as their acyclic congeners and experience 1,2-hydride addition followed by the 1,3-migration of the metal center. In this case, the participation of eta(3)-Cr(CO)(5) species 31 allows us to understand the labeling pattern found in the final products.