Bifunctional Square-Planar NiO4 Coordination of Topotactic LaNiO2.0 Films for Efficient Oxygen Evolution Reaction.

The high-efficient and low-cost oxygen evolution reaction (OER) is decisive for applications of oxide catalysts in metal-air batteries, electrolytic cells, and energy-storage technologies. Delicate regulations of active surface and catalytic reaction pathway of oxide materials principally determine thermodynamic energy barrier and kinetic rate during catalytic reactions, and thus have crucial impacts on OER performance. Herein, a synergistic modulation of catalytically active surface and reaction pathway through facile topotactic transformations switching from perovskite (PV) LaNiO3.0 film to infinite-layer (IL) LaNiO2.0 film is demonstrated, which absolutely contributes to improving OER performance. The square-planar NiO4 coordination of IL-LaNiO2.0 brings about more electrochemically active metal (Ni+ ) sites on the film surface. Meanwhile, the oxygen-deficient driven PV- IL topotactic transformations lead to a reaction pathway converted from absorbate evolution mechanism to lattice-oxygen-mediated mechanism (LOM). The non-concerted proton-electron transfer of LOM pathway, evidenced by the pH-dependent OER kinetics, further boosts the OER activity of IL-LaNiO2.0 films. These findings will advance the in-depth understanding of catalytic mechanisms and open new possibilities for developing highly active perovskite-derived oxide catalysts.

A Highly Strained Phase in PbZr0.2Ti0.8O3 Films with Enhanced Ferroelectric Properties.

Although epitaxial strain imparted by lattice mismatch between a film and the underlying substrate has led to distinct structures and emergent functionalities, the discrete lattice parameters of limited substrates, combined with strain relaxations driven by film thickness, result in severe obstructions to subtly regulate electro-elastic coupling properties in perovskite ferroelectric films. Here a practical and universal method to achieve highly strained phases with large tetragonal distortions in Pb-based ferroelectric films through synergetic effects of moderately (≈1.0%) misfit strains and laser fluences during pulsed laser deposition process is demonstrated. The phase possesses unexpectedly large Poisson's ratio and negative thermal expansion, and concomitant enhancements of spontaneous polarization (≈100 µC cm-2) and Curie temperature (≈800 °C), 40% and 75% larger than that of bulk counterparts, respectively. This strategy efficiently circumvents the long-standing issue of limited numbers of discrete substrates and enables continuous regulations of exploitable lattice states in functional oxide films with tightly elastic coupled performances beyond their present levels.

Modulation of Abnormal Poisson's Ratios and Electronic Properties in Mixed-Valence Perovskite Manganite Films.

Epitaxy and misfit strain imposed by underlying substrates have been intensively used to tailor the microstructure and electronic properties of oxide films, but this approach is largely restricted by commercially limited substrates. In contrast to the conventional epitaxial misfit strains with a positive Poisson's constant, we show here a tunable Poisson's ratio with anomalous values from negative, zero, to positive. This permits effective control over the out-of-plane lattice parameters that strongly correlate the magnetic and transport properties in perovskite mixed-valence La1- xSr xMnO3 thin films. Our results provide an unconventional approach to better modulation and understanding of elastic-mediated microstructures and physical properties of oxide films by engineering the Poisson's ratios.

Flexible Quasi-Two-Dimensional CoFe2O4 Epitaxial Thin Films for Continuous Strain Tuning of Magnetic Properties.

Epitaxial thin films of CoFe2O4 (CFO) have successfully been transferred from a SrTiO3 substrate onto a flexible polyimide substrate. By bending the flexible polyimide, different levels of uniaxial strain are continuously introduced into the CFO epitaxial thin films. Unlike traditional epitaxial strain induced by substrates, the strain from bending will not suffer from critical thickness limitation, crystalline quality variation, and substrate clamping, and more importantly, it provides a more intrinsic and reliable way to study strain-controlled behaviors in functional oxide systems. It is found that both the saturation magnetization and coercivity of the transferred films can be changed over the bending status and show a high accord with the movement of the curvature bending radius of the polyimide substrate. This reveals that the mechanical strain plays a critical role in tuning the magnetic properties of CFO thin films parallel and perpendicular to the film plane direction.

Negative Poisson's Ratio in Modern Functional Materials.

Materials with negative Poisson's ratio attract considerable attention due to their underlying intriguing physical properties and numerous promising applications, particularly in stringent environments such as aerospace and defense areas, because of their unconventional mechanical enhancements. Recent progress in materials with a negative Poisson's ratio are reviewed here, with the current state of research regarding both theory and experiment. The inter-relationship between the underlying structure and a negative Poisson's ratio is discussed in functional materials, including macroscopic bulk, low-dimensional nanoscale particles, films, sheets, or tubes. The coexistence and correlations with other negative indexes (such as negative compressibility and negative thermal expansion) are also addressed. Finally, open questions and future research opportunities are proposed for functional materials with negative Poisson's ratios.

Spectrophotometric determination of turbid optical parameters without using an integrating sphere.

Spectrophotometric quantification of turbidity by multiple optical parameters has wide-ranging applications in material analysis and life sciences. A robust system design needs to combine hardware for precise measurement of light signals with software to accurately model measurement configuration and rapidly solve a sequence of challenging inverse problems. We have developed and validated a design approach and performed system validation based on radiative transfer theory for determination of absorption coefficient, scattering coefficient, and anisotropy factor without using an integrating sphere. Accurate and rapid determination of parameters and spectra is achieved for microsphere suspension samples by combining photodiode-based measurement of four signals with the Monte Carlo simulation and perturbation-based inverse calculations. The three parameters of microsphere suspension samples have been determined from the measured signals as functions of wavelength from 400 to 800 nm and agree with calculated results based on the Mie theory. It has been shown that the inverse problems in the cases of microsphere suspension samples are well posed with convex cost functions to yield unique solutions, and it takes about 1 min to obtain the three parameters per wavelength.

Effects of Interfaces on the Structure and Novel Physical Properties in Epitaxial Multiferroic BiFeO3 Ultrathin Films.

In functional oxide films, different electrical/mechanical boundaries near film surfaces induce rich phase diagrams and exotic phenomena. In this paper, we review some key points which underpin structure, phase transition and related properties in BiFeO3 ultrathin films. Compared with the bulk counterparts, we survey the recent results of epitaxial BiFeO3 ultrathin films to illustrate how the atomic structure and phase are markedly influenced by the interface between the film and the substrate, and to emphasize the roles of misfit strain and depolarization field on determining the domain patterns, phase transformation and associated physical properties of BiFeO3 ultrathin films, such as polarization, piezoelectricity, and magnetism. One of the obvious consequences of the misfit strain on BiFeO3 ultrathin films is the emergence of a sequence of phase transition from tetragonal to mixed tetragonal & rhombohedral, the rhombohedral, mixed rhombohedral & orthorhombic, and finally orthorhombic phases. Other striking features of this system are the stable domain patterns and the crossover of 71° and 109° domains with different electrical boundary conditions on the film surface, which can be controlled and manipulated through the depolarization field. The external field-sensitive enhancements of properties for BiFeO3 ultrathin films, including the polarization, magnetism and morphotropic phase boundary-relevant piezoelectric response, offer us deeper insights into the investigations of the emergent properties and phenomena of epitaxial ultrathin films under various mechanical/electrical constraints. Finally, we briefly summarize the recent progress and list open questions for future study on BiFeO3 ultrathin films.