Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides.

The electrochemical conversion of nitrate to ammonia is a way to eliminate nitrate pollutant in water. Cu-Co synergistic effect was found to produce excellent performance in ammonia generation. However, few studies have focused on this effect in high-entropy oxides. Here, we report the spin-related Cu-Co synergistic effect on electrochemical nitrate-to-ammonia conversion using high-entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O. In contrast, the Li-incorporated MgCoNiCuZnO exhibits inferior performance. By correlating the electronic structure, we found that the Co spin states are crucial for the Cu-Co synergistic effect for ammonia generation. The Cu-Co pair with a high spin Co in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O can facilitate ammonia generation, while a low spin Co in Li-incorporated MgCoNiCuZnO decreases the Cu-Co synergistic effect on ammonia generation. These findings offer important insights in employing the synergistic effect and spin states inside for selective catalysis. It also indicates the generality of the magnetic effect in ammonia synthesis between electrocatalysis and thermal catalysis.

Large Electromechanical Response in a Polycrystalline Alkali-Deficient (K,Na)NbO3 Thin Film on Silicon.

The demand for large electromechanical performance in lead-free polycrystalline piezoelectric thin films is driven by the need for compact, high-performance microelectromechanical systems (MEMS) based devices operating at low voltages. Here we significantly enhance the electromechanical response in a polycrystalline lead-free oxide thin film by utilizing lattice-defect-induced structural inhomogeneities. Unlike prior observations in mismatched epitaxial films with limited low-frequency enhancements, we achieve large electromechanical strain in a polycrystalline (K,Na)NbO3 film integrated on silicon. This is achieved by inducing self-assembled Nb-rich planar faults with a nonstoichiometric composition. The film exhibits an effective piezoelectric coefficient of 565 pm V-1 at 1 kHz, surpassing those of lead-based counterparts. Notably, lattice defect growth is substrate-independent, and the large electromechanical response is extended to even higher frequencies in a polycrystalline film. Improved properties arise from unique lattice defect morphology and frequency-dependent relaxation behavior, offering a new route to remarkable electromechanical response in polycrystalline thin films.

Oxygen Plasma Induced Nanochannels for Creating Bimetallic Hollow Nanocrystals.

Platinum-based metal catalysts are considered excellent converters in various catalytic reactions, particularly in fuel cell applications. The atomic structure at the nanocrystal surface and the metal interface both influence the catalytic performance, controlling the efficiency of the electrochemical reactions. Here we report the synthesis of Ag/Pt and Ag/Pd core/shell nanocrystals and insight into the formation mechanism of these bimetallic core/shell nanocrystals when undergoing oxygen plasma treatment. We carefully designed the oxidation treatment that determines the structural and compositional evolution. The accelerated oxidation-triggered diffusion of Ag toward the outer metal shell leads to the Kirkendall effect. After prolonged oxygen plasma treatment, most core/shell nanocrystals evolve into hollow spheres. At the same time, a minor fraction of the metal remains unchanged with a well-protected Ag core and a monocrystalline Pt or Pd shell. We hypothesize that the O2 plasma disturbs the Pt or Pd shell surface and introduces active O species that react with the diffused Ag from the inside out. Based on EDX elemental mapping, combined with several electron microscopic techniques, we deduced the formation mechanism of the hollow structures to be as follows: (I) the oxidation of Ag within the Pt or Pd lattice causes a disrupted crystal lattice of Pt or Pd; (II) nanochannels arise at the defect locations on the Pt or Pd shell; (III) the remaining Ag atoms pass through these nanochannels and leave a hollow crystal behind. Our findings deepen the understanding of interface dynamics of bimetallic nanostructured catalysts under an oxidative environment and unveil an alternative approach for catalyst pretreatment.

Diverse Defects in Alkali Niobate Thin Films: Understanding at Atomic Scales and Their Implications on Properties.

Defects in ferroelectric materials have many implications on the material properties which, in most cases, are detrimental. However, engineering these defects can also create opportunities for property enhancement as well as for tailoring novel functionalities. To purposely manipulate these defects, a thorough knowledge of their spatial atomic arrangement, as well as elastic and electrostatic interactions with the surrounding lattice, is highly crucial. In this work, analytical scanning transmission electron microscopy (STEM) is used to reveal a diverse range of multidimensional crystalline defects (point, line, planar, and secondary phase) in (K,Na)NbO3 (KNN) ferroelectric thin films. The atomic-scale analyses of the defect-lattice interactions suggest strong elastic and electrostatic couplings which vary among the individual defects and correspondingly affect the electric polarization. In particular, the observed polarization orientations are correlated with lattice relaxations as well as strain gradients and can strongly impact the properties of the ferroelectric films. The knowledge and understanding obtained in this study open a new avenue for the improvement of properties as well as the discovery of defect-based functionalities in alkali niobate thin films.

Sound Absorption Improvement in Porous Ferroelectret Polyethylene with Effective Piezoelectric Mechanism.

Airborne sound absorption in porous materials involves complex mechanisms of converting mechanical acoustic energy into heat. In this work, the effective piezoelectric properties of polyethylene ferroelectret foams on sound absorption were investigated by comparable samples with and without the piezoelectric response. Corona poling and thermal annealing treatments were applied to the samples in order to enable and remove the piezoelectric property, respectively, while the microstructure and the mechanical properties remained substantially unchanged. The effective piezoelectric properties and airborne sound absorption coefficients of the polyethylene foam samples before and after material treatments were measured and analyzed. Our experimental results and theoretical analysis showed that the open-cell ferroelectret polymer foam with an effective piezoelectric property provides an additional electromechanical energy conversion mechanism to enhance the airborne acoustic absorption performance.

Hofmeister Effect Mediated Strong PHEMA-Gelatin Hydrogel Actuator.

Hydrogels have become popular in biomedical applications, but their applications in muscle and tendon-like bioactuators have been hindered by low toughness and elastic modulus. Recently, a significant toughness enhancement of a single hydrogel network has been successfully achieved by the Hofmeister effect. However, little has been conducted for the Hofmeister effect on the hybrid hydrogels, although they have a special network structure consisting of two types of polymer components. Herein we fabricated hybrid poly(2-hydroxyethyl methacrylate) (PHEMA)-gelatin hydrogels with high mechanical performance and stimuli response. An ideal bicontinuous phase separation structure of the PHEMA (rigid) and gelatin (ductile) was observed with embedded microdisc-like gelatin in the three-dimensional polymeric network of PHEMA. A significant enhancement of mechanical performance by the Hofmeister effect was attributed to the salting-out-induced stronger and closer interphase interaction between PHEMA and gelatin. A superior comprehensive mechanical performance with fracture elongation over 650%, tensile strength of 5.2 MPa, toughness of 13.5 MJ/m3, and modulus of 45.6 MPa was achieved with the salting-out effect. More specifically, the synergy of phase separation and Hofmeister effect enable the hydrogel to contract with an enhanced modulus in high-concentration salt solutions, while the same hydrogel swells and relaxes in dilute solutions, exhibiting an ionic stimulus response and excellent shape-memory properties like those of most artificial muscle. This is manifested in highly stretched, twisted, and knotted hydrogel strips that can rapidly recover their original shape in a dilute salt solution. The high strength and modulus, ionic stimuli response, and shape memory property make the hybrid hydrogel a promising material for bioactuators in various biomedical applications.

Reversible Tuning of Mie Resonances in the Visible Spectrum.

Dielectric optical nanoantennas are promising as fundamental building blocks in next generation color displays, metasurface holograms, and wavefront shaping optical devices. Due to the high refractive index of the nanoantenna material, they support geometry-dependent Mie resonances in the visible spectrum. Although phase change materials, such as the germanium-antimony-tellurium alloys, and post-transition metal oxides, such as ITO, have been used to tune antennas in the near-infrared spectrum, reversibly tuning the response of dielectric antennas in the visible spectrum remains challenging. In this paper, we designed and experimentally demonstrated dielectric nanodisc arrays exhibiting reversible tunability of Mie resonances in the visible spectrum. We achieved tunability by exploiting phase transitions in Sb2S3 nanodiscs. Mie resonances within the nanodisc give rise to structural colors in the reflection mode. Crystallization and laser-induced amorphization of these Sb2S3 resonators allow the colors to be switched back and forth. These tunable Sb2S3 nanoantenna arrays could enable the next generation of high-resolution color displays, holographic displays, and miniature LiDAR systems.

Exploring Reusability of Disposable Face Masks: Effects of Disinfection Methods on Filtration Efficiency, Breathability, and Fluid Resistance.

To curb the spread of the COVID-19 virus, the use of face masks such as disposable surgical masks and N95 respirators is being encouraged and even enforced in some countries. The widespread use of masks has resulted in global shortages and individuals are reusing them. This calls for proper disinfection of the masks while retaining their protective capability. In this study, the killing efficiency of ultraviolet-C (UV-C) irradiation, dry heat, and steam sterilization against bacteria (Staphylococcus aureus), fungi (Candida albicans), and nonpathogenic virus (Salmonella virus P22) is investigated. UV-C irradiation for 10 min in a commercial UV sterilizer effectively disinfects surgical masks. N95 respirators require dry heat at 100 °C for hours while steam treatment works within 5 min. To address the question on safe reuse of the disinfected masks, their bacteria filtration efficiency, particle filtration efficiency, breathability, and fluid resistance are assessed. These performance factors are unaffected after 5 cycles of steam (10 min per cycle) and 10 cycles of dry heat at 100 °C (40 min per cycle) for N95 respirators, and 10 cycles of UV-C irradiation for surgical masks (10 min per side per cycle). These findings provide insights into formulating the standard procedures for reusing masks without compromising their protective ability.

Synthesis and optical and electronic properties of one-dimensional sulfoxonium-based hybrid metal halide (CH3)3SOPbI3.

We report the synthesis and optical and electronic properties of a one-dimensional sulfoxonium-based hybrid metal halide in an orthorhombic crystal system with a Pnma space group. To provide direct insights, a method is developed to calculate tolerance factors with the ionic radii of non-spherical cations from X-ray crystallographic data.

Effect of molecular orientation on mechanical property of single electrospun fiber of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate].

Electrospun poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) fibers were collected by using a counter electrode collector or a rotating disk collector. The molecular orientation and mechanical property of single PHBV fiber were studied. 2-D wide-angle X-ray diffraction and polarized Fourier transform infrared spectra of the macroscopically aligned fibers confirmed the orientation of polymer chains, with PHBV chains preferentially oriented along the fiber axis. The degree of orientation increased with increasing fiber take-up velocity. X-ray diffraction pattern also indicates the development of beta-form crystal in electrospun PHBV fibers collected at an angular velocity of 1500 rpm. The thermal behavior of electrospun PHBV fibers was studied using modulated differential scanning calorimetry. The tensile properties of single electrospun PHBV fibers were studied using a nanotensile tester. Our results indicate that electrospun PHBV fiber with a higher degree of molecular orientation exhibits a higher tensile modulus and strength but lower strain at break.