Outermost Surface Research Articles

Achieving stable lithium metal anode via constructing lithiophilicity gradient and regulating Li3N-rich SEI

Three-dimensional (3D) current collectors are studied for the application of Li metal anodes in high-energy battery systems. However, they still suffer from the preferential accumulation of Li on the outermost surface, resulting from an inadequate regulation of the Li+ transport. Herein, we propose a deposition regulation strategy involving the creation of a 3D lithiophilicity gradient structure of MoN on Cu3N nanowire-grown Cu foam (MCNCF) to induce a “bottom-up” Li deposition. During the initial Li deposition, the reaction between Li and Cu3N leads to the formation of Li3N while the lithiophilic MoN located at the bottom promotes the downward Li+ migration, resulting in the generation of a Li3N gradient. Such a “bottom-up” Li3N distribution results in the formation of a stable and Li3N-rich solid electrolyte interphase layer, facilitating the Li⁺ transport and promoting a uniform Li nucleation. Computational simulations and experimental results corroborate the preferential deposition of Li on the bottom of the substrate, leading to a uniform Li nucleation and growth throughout the electrode. The MCNCF electrode offers a significantly improved reversibility of the Li deposition, achieving a lifespan of more than 1200 h at a current density of 1 mA cm−2 in symmetric Li||Li cells. Furthermore, full-cells incorporating MCNCF@Li as the negative electrode and LiFePO4 cathodes exhibit outstanding electrochemical performance with a capacity retention of over 99.5 % after 250 cycles at 1 C, which significantly surpasses the performance achieved with CF@Li or CCF@Li electrodes. This innovative design strategy for 3D metallic current collectors, featuring a lithiophilicity gradient, provides new perspectives for the development of stable Li metal anodes and, as a result, for the advancement of Li-metal batteries.

Elucidating the effect of annealing temperature on the atomic-level surface structure evolution and electrochemical performance of Mo doped LiMn2O4 cathode materials

Critical barriers to the extensive applications of the spinel LiMn2O4 cathodes include grievous capacity degradation and structural collapse. Bulk elemental doping combining surface modification, which is a common approach to address these challenges. Here, synchronous bulk Mo-doped and in-situ surface reconstructed layer coated LiMo0.02Mn1.98O4 cathodes are prepared by tuning the annealing temperatures. Using the spherical aberration-corrected scanning transmission microscopy (Cs-STEM) technique, we demonstrate that part of Mo6+ ions dopes into the octahedral Mn 16d sites to form LiMo0.02Mn1.98O4 that strengthens bulk structural stability. The other part of Mo6+ ions or the Mn atom occupying the Mn 16c sites of the spinel to form a surface reconstructed layer on the outermost surface that suppresses the side reaction and slows down the decomposition of the electrolyte. Specifically, the LiMo0.02Mn1.98O4 cathode calcined at 750 °C with an appropriate surface reconstructed layer exhibits an outstanding capacity retention of 76.28 % after 1000 cycles at 10C at 25 °C. Our work provides a simple and novel way toward high electrochemical performance tuning for spinel LiMn2O4 cathodes.

Microstructure and high temperature tribological behavior of nickel-based superalloy with addition of revert

The recycling of revert plays a crucial role in cost-efficiency and green manufacturing of superalloy. In this study, optical microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy, tensile test, and tribological analysis were employed to investigate the microstructure, mechanical properties, and tribological behaviors of vacuum induction melted superalloys of 100% original material (Sample 1) and 40% original material + 60% revert (Sample 2). Dry sliding tests were conducted at 300 and 730 °C. The addition of revert promotes microstructure refinement, hardness increase, and carbide formation, at the cost of 18.5% ductility loss. During the high-temperature dry sliding process, abrasive, adhesive, and oxidative wear mechanisms were observed on the contact surfaces. Sample 2 exhibited a lower wear rate and coefficient of friction at both 300 and 730 °C. Additionally, a more continuous and wear-resistant tribolayer was formed on the wear track of Sample 2, effectively mitigating matrix oxidation compared to Sample 1. At 730 °C, a thick tribolayer developed on the wear surface of Sample 2, characterized by a hard and wear-resistant glaze layer on the outermost surface. The continuous sliding induced matrix heating, which facilitates recrystallization in the subsurface of Sample 2, induced by increased inclusions and carbides from the revert. The analysis of wear debris further illustrated the variations of wear mechanism at different temperatures. These findings are meaningful for understanding the high-temperature behavior of superalloys and optimizing alloy recycling process.

“Bamboo-like” strong and tough sodium alginate/polyacrylate hydrogel fiber with directional controlled release for wound healing promotion

Skin, as the biggest organ and outermost surface of the human body, is prone to injury due to various challenges, especially the expanding potential of accidents, which bring a huge social and economic burden. Hydrogels are emerging as the most promising candidate for wound dressings, which not only fulfill the varied requirements of dressings but also serve as drug carriers. But limited breathability, rapid drug release, and inadequate mechanical properties remains a significant challenge. Herein, we report a strong and tough sodium alginate/polyacrylate hydrogel fibers-based dressing with directional controlled drug release for wound healing promotion. Mimicking the bamboo structure, the drug solution is encapsulated within the fiber, and the rate of drug release can be modulated by controlling the wall thickness of the fiber. A cross-network structure in the hydrogel fiber through hydrogen bond and calcium ion crosslinking resulted in a 38 % increase in tensile strength. By precisely controlling the feeding process during weaving, drug-loaded fibers can be prepared at specific locations to facilitate targeted delivery to skin wound sites. Drug-loaded fabric has the breathability and biocompatibility required for dressings to promote wound healing. The findings highlight the potential of alginate/polyacrylate hydrogel fabrics for effective wound treatment.

Li2ZnCu3 modified Cu current collector to regulate Li deposition

Rationally designing a current collector that can maintain low lithium (Li) porosity and smooth morphology while enduring high‐loading Li deposition is crucial for realizing the high energy density of Li metal batteries, but it is still challengeable. Herein, a Li2ZnCu3 alloy‐modified Cu foil is reported as a stable current collector to fulfill the stable high‐loading Li deposition. Benefiting from the in‐situ alloying, the generated numerous Li2ZnCu3@Cu heterojunctions induce a homogeneous Li nucleation and dense growth even at an ultrahigh capacity of 12 mAh cm‐2. Such a spatial structure endows the overall Li2ZnCu3@Cu electrode with the manipulated steric hindrance and outmost surface electric potential to suppress the side reactions during Li stripping and plating. The resultant Li||Li2ZnCu3@Cu asymmetric cell preserves an ultrahigh average Coulombic efficiency of 99.2% at 3 mA cm‐2/6 mAh cm‐2 over 200 cycles. Moreover, the Li‐Li2ZnCu3@Cu||LiFePO4 cell maintains a cycling stability of 87.5% after 300 cycles. After coupling with the LiCoO2 cathode (4 mAh cm‐2), the cell exhibits a high energy density of 407.4 Wh kg‐1 with remarkable cycling reversibility at an N/P ratio of 3. All these findings present a doable way to realize the high‐capacity, dendrite‐free, and dense Li deposition for high‐performance Li metal batteries.

Li2ZnCu3 Modified Cu Current Collector to Regulate Li Deposition.

Rationally designing a current collector that can maintain low lithium (Li) porosity and smooth morphology while enduring high-loading Li deposition is crucial for realizing the high energy density of Li metal batteries, but it is still challengeable. Herein, a Li2ZnCu3 alloy-modified Cu foil is reported as a stable current collector to fulfill the stable high-loading Li deposition. Benefiting from the in situ alloying, the generated numerous Li2ZnCu3@Cu heterojunctions induce a homogeneous Li nucleation and dense growth even at an ultrahigh capacity of 12 mAh cm-2. Such a spatial structure endows the overall Li2ZnCu3@Cu electrode with the manipulated steric hindrance and outmost surface electric potential to suppress the side reactions during Li stripping and plating. The resultant Li||Li2ZnCu3@Cu asymmetric cell preserves an ultrahigh average Coulombic efficiency of 99.2 % at 3 mA cm-2/6 mAh cm-2 over 200 cycles. Moreover, the Li-Li2ZnCu3@Cu||LiFePO4 cell maintains a cycling stability of 87.5 % after 300 cycles. After coupling with the LiCoO2 cathode (4 mAh cm-2), the cell exhibits a high energy density of 407.4 Wh kg-1 with remarkable cycling reversibility at an N/P ratio of 3. All these findings present a doable way to realize the high-capacity, dendrite-free, and dense Li deposition for high-performance Li metal batteries.

A New Mechanism for the Inhibition of SA106 Gr.B Carbon Steel Corrosion by Nitrite in Alkaline Water.

The purpose of this study was to investigate the composition of oxide films formed on SA106 Gr.B carbon steel in nitrite solutions at 35 °C for 1000 h. The product of the reduction of nitrite during the corrosion inhibition process was also examined. The X-ray photoelectron spectroscopy results revealed that a thin Fe3O4 film was formed and ammonium ions were adsorbed on the outermost surface of the oxide film. The presence of ammonium ions was also demonstrated by ion chromatography. These results indicate that nitrites are reduced to ammonium ions, which in turn promotes the formation of the protective Fe3O4 film.

Multi-layer formation by oxidation and solid-state crystallization of cold sprayed amorphous coatings during dry sliding wear

Amorphous alloy coatings, known for the exceptional wear resistance, have emerged as a key solution for enhancing the wear performance of magnesium alloys under harsh environments. In this study, Fe-based amorphous alloy coatings were deposited on magnesium alloy by cold spraying technology, and the influence of microstructural evolution on the wear performance of coatings under dry sliding wear conditions was discussed. The results showed that a dense adherent oxide layer with a thickness of ∼700 nm comprising nanograins of less than 8 nm was formed at the outmost surface, which played a role of self-lubricating. Underneath, a 1 μm thick nanocrystalline-amorphous layer with nanograins of ∼20 nm dispersed in the amorphous alloy matrix was formed through in-situ crystallization induced by flash temperature. This composite structure prevented the formation of shear bands in amorphous alloys and enhanced the durability. Therefore, the transition from abrasive wear to adhesive wear was a consequence of the microstructural evolution from a dual-phase composite layer to a self-lubricating oxide layer.

In-situ formation of surface “self-protective” graphitic layer on phenolic resin-based thermal protection composites

Phenolic resin (PR) composites are widely utilized as thermal protection materials for high-speed aircraft, where their protection mechanisms have been investigated in earlier studies. However, it remained indistinct about the evolution behavior of the outermost surface and its subsequent effects on the ablation performance. Herein, this work re-investigated the morphological transformation and chemical conversion on the outermost surface of a silica fiber-reinforced PR board after ablation by means of combined experimental characterizations at micro- and nanoscale (SEM, TEM, Raman spectroscopy, XRD, XPS) and computational study (ReaxFF-based molecular dynamic simulation). For the first time, our study revealed that its outermost ablated surface demonstrated a distinct evolution behavior in terms of both the in-situ formation of “self-protective” graphitic layer and the radial redistribution of surface carbonaceous substances with different degrees of graphitization, leading to varied ablation resistance along the radial direction of the PR board. In addition, the computational study investigates the ablation-induced graphitization and its influence on the ablative resistance of PR surface. It indicated that, at equivalent energy flux density, PR with graphitized structures exhibited improved thermal protection performance, which can be attributed to decreased thermal conductivity and increased density, leading to a reduced ablation recession rate. Such revelation provides an alternative route in the design of PR-based ablative materials with enhanced ablation resistance.

Enhanced Surface Determination beyond Photoemission via Auger Photoelectron Coincidence Spectroscopy.

The study of surface properties at the nanoscale plays a crucial role in material science applications. This paper demonstrates the capabilities of Auger PhotoElectron Coincidence Spectroscopy (APECS) to obtain data with varying surface sensitivities from a single measurement. This makes it possible to extract the spectrum from the outermost surface layer even when faced with strongly overlapping surface and bulk spectral features, which we demonstrate by accurately extracting the surface component in Au 4f photoemission. Leveraging high energy resolution, transmission efficiency, tunable photon energy, and remarkable surface sensitivity of the APECS setup, we propose that optimal experimental conditions can be tailored to determine surface spectra accurately for a diverse range of materials. This opens new avenues for advancing our understanding of nanoscale surface phenomena across various material systems.

Structural evolution of cobalt for the production of long-chain paraffins by CO2 hydrogenation

A new role of alkali metals in the direct hydrogenation of CO2 to C5+ species over Co catalysts was proposed. During the CO2 hydrogenation, Na-promotion encouraged a new structural evolution (i.e., a thin oxygen vacancy (Ov)-rich Co3O4/Co2C shell and a Co0 core). This facilitated the migration of CHxO and CO species produced at the shell to the adjacent Co2C shell and Co0 core, where they undergo further chain growth. In contrast, Li- and K-promotion resulted in the generation of a thick and Ov-poor Co3O4 shell without Co2C, while in the absence of alkali metal promotion, Co0 was exposed as the outmost surface; in both the cases, methanation dominated. The ability of Na-promotion to remove –OH* from the Co surface helped maintain the thickness and valance state of the Co oxide shell. Thus, the Na-promotion developed the chain growth and CO producing core–shell structure, rather than any electronic promotional effects.

Surface reconstructed layer with bulk high-valence Mo doping to achieve long-life LiMn2O4 cathode material

Mo doping plays a striking role in the stability enhancement of LiMn2O4 cathode materials. However, the underlying microscopic mechanism is still unclear. Here, we elucidate that the relationship between the atomic position of Mo doping in LiMn2O4 and the stability enhancement of Mo-doped LiMn2O4 by utilizing the spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM). It exhibits that the part of Mo6+ ions occupy empty Mn 16c site to form a surface reconstructed layer of rock-salt like phase on the outermost surface and other Mo6+ ions dope into the Mn octahedral 16d sites to form LiMoxMn2-xO4 in bulk, which is beneficial for inhibiting the parasitic side reactions. Concomitantly, this dual modification of bulk structure and surface can significantly enhance electrode reversibility and Li+ diffusion. As a result, excellent long cycling stability of the as-designed optimal LiMo0.01Mn1.99O4 after 1500 cycles with 61.61 % capacity retention at 10 C (1 C = 148 mAh g−1) is presented, with an initial discharge capacity of 88.30 mAh g−1. Our research provides a clever approach for regulating the surface structure/bulk architecture in LiMn2O4 cathode materials.

Analysis of Subsurface Damage Structures of Gallium Nitride Substrates Induced by Mechanical Polishing with Diamond Abrasives

Subsurface damage (SSD) structures induced by mechanical polishing of gallium nitride (GaN) substrates are comprehensively investigated using atomic force microscopy, cathodoluminescence (CL) imaging, and cross‐sectional transmittance electron microscopy. The low removal rate of the CMP process is a barrier to high productivity of a GaN wafering process; therefore, reducing the chemical mechanical polishing (CMP) process time by reducing the depth of SSD induced by mechanical processing is an active research area. To better understand the SSD structures, the surface roughness, SSD depth, and SSD distributions induced by mechanical polishing are quantitatively evaluated in this study. The SSD structures induced by mechanical polishing can be quantitatively exhibited as the SSD distribution with the damage strength at the outermost surface and the damage propagation, which are obtained by CMP process time‐resolved CL imaging method. On the basis of the analysis results, a schematic model of the SSD structures for mechanically polished GaN substrates is proposed.

Depth to the minimal surface and radius of the entrance to the black hole

An asymptotically flat manifold is the union of an almost flat region and a compact non flat region. If there exists a black hole, we will call the boundary of non flat region the entrance to the black hole. Also, we will call the distance from the entrance to the outermost minimal surface the depth. We use an integral norm of Ricci curvature to obtain positive lower bounds on the depth and the radius of the entrance. Also we obtain an upper bound of the number of ends from the integral norm of Ricci curvature.

Surface Insights of Mesoporous Fragile Organic Materials Under Ultra‐Low‐Voltage Directed by Gemini Column

AbstractMesoporous materials find widespread applications due to their high specific surface area, contributing to enhanced performance. Scanning electron microscopy (SEM) observation of mesoporous materials provides valuable insights into their mesostructures. However, direct observation of the outermost surfaces, which often dictate material properties, remains challenging, especially for highly insulating and fragile compounds. In this study, utilizing SEM observation with ultra‐low‐voltage acceleration directed by Gemini column is proposed to achieve direct surface imaging of mesoporous organic materials with highly insulating and fragile characteristics. By observing mesostructured polymers obtained through a soft‐templating method without washing, stuffed pores are identified allowing the differentiation of micelle templates and polymer walls. Moreover, leveraging SEM measurements, a polymerization mechanism is proposed for dopamine on the polymer micelles adhered to the graphene oxide nanosheets. These findings demonstrate the potential of SEM measurements with ultra‐low accelerating voltage in facilitating surface observations of polymer‐derived nanomaterials characterized by high insulating properties and a fragile framework.

Femtosecond laser ultrafast photothermal exsolution

Exsolution, as an effective approach to constructing particle-decorated interfaces, is still challenging to yield interfacial films rather than isolated particles. Inspired by in vivo near-infrared laser photothermal therapy, using 3 mol% Y2O3 stabilized tetragonal zirconia polycrystals (3Y-TZP) as host oxide matrix and iron-oxide (Fe3O4/γ-Fe2O3/α-Fe2O3) materials as photothermal modulator and exsolution resource, femtosecond laser ultrafast exsolution approach is presented enabling to conquer this challenge. The key is to trigger photothermal annealing behavior via femtosecond laser ablation to initialize phase transition from monoclinic zirconia (m-ZrO2) to tetragonal zirconia (t-ZrO2) and induce t-ZrO2 columnar crystal growth. Fe-ions rapidly segregate along grain boundaries and diffuse towards the outmost surface, and become ‘frozen’, highlighting the potential to use photothermal materials and ultrafast heating/quenching behaviors of femtosecond laser ablation for interfacial exsolution. Triggering interfacial iron-oxide coloring exsolution is composition and concentration dependent. Photothermal materials themselves and corresponding photothermal transition capacity play a crucial role, initializing at 2 wt%, 3 wt%, and 5 wt% for Fe3O4/γ-Fe2O3/α-Fe2O3 doped 3Y-TZP samples. Due to different photothermal effects, exsolution states of ablated 5 wt% Fe3O4/γ-Fe2O3/α-Fe2O3-doped 3Y-TZP samples are totally different, with whole coverage, exhaustion (ablated away) and partial exsolution (rich in the grain boundaries in subsurface), respectively. Femtosecond laser ultrafast photothermal exsolution is uniquely featured by up to now the deepest microscale (10 μm from 5 wt%-Fe3O4-3Y-TZP sample) Fe-elemental deficient layer for exsolution and the whole coverage of exsolved materials rather than the formation of isolated exsolved particles by other methods. It is believed that this novel exsolution method may pave a good way to modulate interfacial properties for extensive applications in the fields of biology, optics/photonics, energy, catalysis, environment, etc.

Geometric surprises in the Python's lunch conjecture

A bulge surface, on a time reflection-symmetric Cauchy slice of a holographic spacetime, is a non-minimal extremal surface that occurs between two locally minimal surfaces homologous to a given boundary region. According to the python’s lunch conjecture of Brown et al., the bulge’s area controls the complexity of bulk reconstruction, in the sense of the amount of post-selection that needs to be overcome for the reconstruction of the entanglement wedge beyond the outermost extremal surface. We study the geometry of bulges in a variety of classical spacetimes, and discover a number of surprising features that distinguish them from more familiar extremal surfaces such as Ryu-Takayanagi surfaces: they spontaneously break spatial isometries, both continuous and discrete; they are sensitive to the choice of boundary infrared regulator; they can self-intersect; and they probe entanglement shadows, orbifold singularities, and compact spaces such as the sphere in AdS_p× S^qp×Sq. These features imply, according to the python’s lunch conjecture, novel qualitative differences between complexity and entanglement in the holographic context. We also find, surprisingly, that extended black brane interiors have a non-extensive complexity; similarly, for multi-boundary wormhole states, the complexity pleateaus after a certain number of boundaries have been included.

Exploring surface properties and premelting in crystals.

Crystal surfaces play a pivotal role in governing various significant processes, such as adsorption, nucleation, wetting, friction, and wear. A fundamental property that influences these processes is the surface free energy, γ. We have directly calculated γ(T) for low-index faces of Lennard-Jones (LJ), germanium, and silicon crystals along their sublimation lines using the computational cleavage technique. Our calculations agree well with experimental values for Si(111) and Ge(111), highlighting the accuracy of the method and models used. For LJ crystals, we identified a premelting onset at Tpm = 0.75Tm, marked by a sharp increase in atom mobility within the second outermost surface layer. Notably, Tpm closely aligned with the endpoint of the LJ melting line at negative pressures, Tend = 0.76Tm. We hypothesize that the emergence and coexistence of a liquid film atop the LJ crystal at Tpm < T < Tm correspond to the metastable melting line under negative pressures experienced by stretched crystal surfaces. Furthermore, our study of thin LJ crystal slabs reveals that premelting-induced failure leads to recrystallization below the homogeneous freezing limit, offering a promising avenue to explore crystal nucleation and growth at extremely deep supercoolings. Finally, no evidence of premelting was detected in the model crystals of Ge and Si, which is consistent with the experimental observations. Overall, our findings offer valuable insights into crystal surface phenomena at the atomic scale.

Study on the corrosion fatigue properties of ultrasonic surface strengthened axle steel

Owing to fatigue damage and environmental erosion, locomotive axles are prone to premature failure during service. It is therefore very important to improve the corrosion fatigue properties of axles. In this paper, the fatigue behavior of ultrasonic surface strengthened axle steel under corrosive environment was studied. The surface integrity of ultrasonic strengthened axle steel was characterized by microscopic analysis and hardness test. The fatigue data and fatigue fracture of axle steel under corrosion environment were compared. Also, the mechanism of the corrosion environment on fatigue crack initiation and propagation was analyzed. Results show that serious plastic deformation is generated in the surface of axle steel after ultrasonic strengthening. The grains in surface layer are obviously refined and get coarse gradually as deep into the matrix. The hardness of surface layer is greatly improved as well. The maximum value is located on the outermost surface, about 2 times of the matrix. Under the same fatigue load, the fatigue cycle number for ultrasonic strengthened axle steel is more than that of original sample. This is due to grain refinement, work hardening, and residual compressive stress induced by ultrasonic strengthening, which inhibits fatigue crack initiation in corrosion environment.

Triacylglycerol-droplet-induced bilayer spontaneous curvature in giant unilamellar vesicles

This study investigated the incorporation of triacylglycerol droplets in the bilayers of giant unilamellar vesicles (GUVs) using four triacylglycerols and four phosphatidylcholines by confocal laser scanning microscopy. The triacylglycerol droplets were incorporated between the monolayer leaflets of the GUVs. Among the spherical droplets protruding on only one side of the bilayers, the droplets bound to the outer leaflets outnumbered those bound to the inner leaflets. The more frequent droplet binding to the outer leaflet caused transbilayer asymmetry in the droplet surface density. A vesicle consisting of a single-bilayer spherical segment and a double-bilayer spherical segment was also observed. The yield of these vesicles was comparable with or higher than that of the droplet-incorporating GUVs for many of the phosphatidylcholine-triacylglycerol combinations. In a vesicle consisting of single-bilayer and double-bilayer segments, most of the triacylglycerol droplets were localized on the outermost membrane surface along the segment boundary and in the double-bilayer segment. To rationalize the formation of these vesicle structures, we propose that the transbilayer asymmetry in the droplet surface density induces spontaneous curvature of the bilayer, with the bilayer spontaneously bending away from the droplets. Energy calculations performed assuming the existence of spontaneous curvature of the bilayer corroborated the experimentally determined membrane shapes for the vesicles consisting of unilamellar and bilamellar regions.