Convective Boiling Between 2D Plates: Microgravity Influence on Bubble Growth and Detachment

The experiment detailed in this paper presents results obtained on the nucleation, growth and detachment of HFE-7100 confined vapour bubbles. Bubbles are created on an artificial nucleation site between two-dimensional plates under terrestrial and microgravity conditions. The experiments are performed by varying the shear flow by changing the convective mass flow rate, and varying the bubble nucleation rate by changing the heat flux supplied. The experiments are performed under normal (1 g) and reduced gravity (μg). The distance between the plates is equal to 1 mm. The results of these experiments are related to the detachment diameters of bubbles on the single artificial nucleation site and to the associated effects on the heat transfer by the confinement influence. The experimental device allows the observation of the flow using both visible video camera and infrared video camera. Here, we present the results obtained concerning the influence of gravity on the bubble detachment diameter and the images of 2D bubbles obtained in microgravity by means of an infrared camera. The following parameters: nucleation site surface temperature, bubble detachment diameter and bubble nucleation frequency evidence modifications due to microgravity.     

RETRACTED ARTICLE: Cooperative Time-Reversal Symmetry Breaking Induced by a Magnetic Field in a Quasi-2D d-Wave Superconductor

A model of quasi-two-dimensional d-wave superconductor, with strong nesting properties of the Fermi surface is considered. The orbital effect of a moderate magnetic field applied perpendicularly to the conducting planes is studied in the mean field approximation. It is shown that the field can induce a time-reversal symmetry-breaking spin density wave order coexisting with the superconducting order and can open a gap over the whole Fermi surface. The anomalies recently observed in the heat conductivity, penetration depth, and zero-bias conduction in cuprates might be ascribed to this effect.     

Catalysis of a Single Transition Metal Site for Water Oxidation: From Mononuclear Molecules to Single Atoms

Low-cost, nonprecious transition metal (TM) catalysts toward efficient water oxidation are of critical importance to future sustainable energy technologies. The advances in structure engineering of water oxidation catalysts (WOCs) with single TM centers as active sites, for example, single metallic molecular complexes (SMMCs), supported SMMCs, and single-atom catalysts (SACs) in recent reports are examined. The efforts made on these configurations in terms of design principle, advanced characterization, performances and theoretical studies, are critically reviewed. A clear roadmap with the correlations between the single-TM-site-based structures (coordination and geometric structure, TM species, support), and the catalytic performances in water oxidation is provided. The insights bridging SMMCs with SACs are also given. Finally, the challenges and opportunities in the single-TM-site catalysis are proposed.     

Nanobubbles at Water-Solid Interfaces: Calculation of the Contact Angle Based on a Simple Model

Nanobubbles have been found to form at the interface of water and solid surfaces. We examine the conditions for such bubbles to form and estimate the pressure inside the bubble based on thermodynamic considerations. Using a simple model we calculate the contact angle for a wide range of temperatures and hypothetical substrates possessing a continuous range of strengths. We show that as the temperature increases the shape of a bubble changes continuously from a spherical cap with low curvature to a complete sphere. An equivalent effect results from either increasing the strength of the solid or decreasing the surface tension. A model of a substrate formed by layers of materials is proposed to obtain a nanobubble with a particular contact angle.     

Frictionless 2D Contact formulations for finite deformations based on the mortar method

In this paper two different finite element formulations for frictionless large deformation contact problems with non-matching meshes are presented. Both are based on the mortar method. The first formulation introduces the contact constraints via Lagrange multipliers, the other employs the penalty method. Both formulations differ in size and the way of fulfilling the contact constraints, thus different strategies to determine the permanently changing contact area are required. Starting from the contact potential energy, the variational formulation, the linearization and finally the matrix formulation of both methods are derived. In combination with different contact detection methods the global solution algorithm is applied to different two-dimensional examples.     

Defect Effects on TiO2 Nanosheets: Stabilizing Single Atomic Site Au and Promoting Catalytic Properties

Isolated single atomic site catalysts have attracted great interest due to their remarkable catalytic properties. Because of their high surface energy, single atoms are highly mobile and tend to form aggregate during synthetic and catalytic processes. Therefore, it is a significant challenge to fabricate isolated single atomic site catalysts with good stability. Herein, a gentle method to stabilize single atomic site metal by constructing defects on the surface of supports is presented. As a proof of concept, single atomic site Au supported on defective TiO₂ nanosheets is prepared and it is discovered that (1) the surface defects on TiO₂ nanosheets can effectively stabilize Au single atomic sites through forming the Ti–Au–Ti structure; and (2) the Ti–Au–Ti structure can also promote the catalytic properties through reducing the energy barrier and relieving the competitive adsorption on isolated Au atomic sites. It is believed that this work paves a way to design stable and active single atomic site catalysts on oxide supports.     

Contact Angle of Liquid 4He on Cs: Evidence for Ripplons at the He-Cs Interface

Two independent methods to measure the contact angle, , of isotopically pure ⁴ He on Cs are described, and applied to the same Cs sample. The two methods agree well. The contact angle as a function of temperature is measured and clearly shows a first order surface phase transition with a wetting temperature T _w = 2 K. From (T) the difference in surface free energy of the Cs-vapour interface and the Cs-liquid interface is calculated. Its large temperature dependence suggests the existence of ripplons at the liquid He-Cs interface. This surprising possibility is discussed.     

Nitrogen Vacancies on 2D Layered W2N3: A Stable and Efficient Active Site for Nitrogen Reduction Reaction

Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon‐free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new 2D layered W₂N₃ nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h^?1 mg_cata^?1 (3.80 ± 0.32 × 10^?11 mol cm^?2 s^?1) and Faradaic efficiency of 11.67 ± 0.93% at ?0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron‐based characterizations prove that the nitrogen vacancies on 2D W₂N₃ are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W₂N₃ can provide an electron‐deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR.     

Environmentally‐Friendly Exfoliate and Active Site Self‐Assembly: Thin 2D/2D Heterostructure Amorphous Nickel–Iron Alloy on 2D Materials for Efficient Oxygen Evolution Reaction

A class of 2D layered materials exhibits substantial potential for high‐performance electrocatalysts due to high specific surface area, tunable electronic properties, and open 2D channels for fast ion transport. However, liquid‐phase exfoliation always utilizes organic solvents that are harmful to the environment, and the active sites are limited to edge sites. Here, an environmentally friendly exfoliator in aqueous solution is presented without utilizing any toxic or hazardous substance and active site self‐assembly on the inert base of 2D materials. Benefiting from thin 2D/2D heterostructure and strong interfacial coupling, the resultant highly disordered amorphous NiFe/2D materials (Ti3C2 MXene, graphene and MoS₂) thin nanosheets exhibit extraordinary electrocatalytic performance toward oxygen evolution reaction (OER) in alkaline media. DFT results further verify the experimental results. The study emphasizes a viable idea to probe efficient electrocatalysts by means of the synergistic effect of environmentally friendly exfoliator in aqueous solution and active site self‐assembly on the inert base of 2D materials which forms the unique thin 2D/2D heterostructure in‐suit. This new type of heterostructure opens up a novel avenue for the rational design of highly efficient 2D materials for electrocatalysis.     

单丸粒撞击金属靶材的有限元分析

本文应用商用有限元软件ABAQUS对单个丸粒撞击金属靶材进行了三维有限元分析,建立模拟弹丸撞击靶材的有限元模型。首先将单个丸粒撞击靶材产生残余应力的模拟结果与Meguid等人用ANSYS软件所模拟的结果进行对比,从而来验证本文模拟结果的可靠性。然后本文系统地研究了不同弹丸参数对于残余应力场分布规律的影响。模拟结果表明,弹丸参数对于喷丸残余应力场分布规律的影响各不相同。但是,无论这些参数如何变化,靶材次表层的最大残余压应力值所处的位置基本保持不变,始终位于靶材表面以下25%dshot处。根据有限元模拟的结果,可对喷丸强化工艺参数进行优化控制,提高喷丸强化的加工工艺水平,以实现更好的喷丸效果。     

Single Cobalt Atoms Immobilized on Palladium-Based Nanosheets as 2D Single-Atom Alloy for Efficient Hydrogen Evolution Reaction

Single-atom alloys (SAAs) display excellent electrocatalytic performance by overcoming the scaling relationships in alloys. However, due to the lack of a unique structure engineering design, it is difficult to obtain SAAs with a high specific surface area to expose more active sites. Herein, single Co atoms are immobilized on Pd metallene (Pdm) support to obtain Co/Pdm through the design of the engineered morphology of Pd, realizing the preparation of ultra-thin 2D SAA. The unsaturated coordination environments combined with the unique geometric and electronic structures realize the modulation of the d-band center and the redistribution of charges, generating highly active electronic states on the surface of Co/Pdm. Benefiting from the synergistic interaction and spillover effect, the Co/Pdm electrocatalyst exhibits outstanding hydrogen evolution reaction (HER) performance in both acid and alkaline solutions, especially with a Tafel slope of 8.2 mV dec⁻¹ and a low overpotential of 24.7 mV at 10 mA cm⁻² in the acidic medium, which outperforms commercial Pt/C and Pd/C. This work highlights the successful preparation of 2D ultra-thin SAA, which provides a new strategy for the preparation of HER electrocatalyst with high efficiency, activity, and stability.     

Fluorescent labeling of disulfide proteins on 2D gel for screening allergens: a preliminary study

An experimental protocol was established to detect disulfide proteins as fluorescent spots on 2D gel. In summary, free sulfhydryl groups in a protein mixture were capped with nonfluorescent iodoacetamide, followed by chemical reduction of disulfide bonds, and labeling of newly exposed sulfhydryl groups with the fluorescent probe, monobromobimane. Disulfide proteins were detectable as fluorescent spots under the UV lamp. As accumulating evidence suggests that disulfide bonds are responsible for the allergenicity of many proteins, we sought to use this protocol to find new allergens. In a model experiment using soybean trypsin inhibitor, a well-known allergen with disulfide bonds, and myoglobin, which has a free sulfhydryl group, only the former was labeled with fluorescence. Application of the protocol to mite and pollen extracts facilitated detection of known allergens or putative allergens exhibiting sequence similarities to known allergens. In this note, we report the protocol as a complementary tool for screening allergens, which is now solely dependent on immunological recognition.     

Heterostructures Based on 2D Materials: A Versatile Platform for Efficient Catalysis

The unique structural and electronic properties of 2D materials, including the metal and metal‐free ones, have prompted intense exploration in the search for new catalysts. The construction of different heterostructures based on 2D materials offers great opportunities for boosting the catalytic activity in electo(photo)chemical reactions. Particularly, the merits resulting from the synergism of the constituent components and the fascinating properties at the interface are tremendously interesting. This scenario has now become the state‐of‐the‐art point in the development of active catalysts for assisting energy conversion reactions including water splitting and CO₂ reduction. Here, starting from the theoretical background of the fundamental concepts, the progressive developments in the design and applications of heterostructures based on 2D materials are traced. Furthermore, a personal perspective on the exploration of 2D heterostructures for further potential application in catalysis is offered.     

On the Controlled Loading of Single Platinum Atoms as a Co-Catalyst on TiO2 Anatase for Optimized Photocatalytic H2 Generation

Single-atom (SA) catalysis is a novel frontline in the catalysis field due to the often drastically enhanced specific activity and selectivity of many catalytic reactions. Here, an atomic-scale defect engineering approach to form and control traps for platinum SA sites as co-catalyst for photocatalytic H₂ generation is described. Thin sputtered TiO₂ layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO₂ layers are reduced in Ar/H₂ under different conditions (leading to different but defined Ti³⁺-O_v surface defects), followed by immersion in a dilute hexachloroplatinic acid solution. HAADF-STEM results show that only on the thin-film substrate can the density of SA sites be successfully controlled by the degree of reduction by annealing. An optimized SA-Pt decoration can enhance the normalized photocatalytic activity of a TiO₂ sputtered sample by 150 times in comparison to a conventional platinum-nanoparticle-decorated TiO₂ surface. HAADF-STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO₂. Importantly, the density of the relevant surface exposed defect centers—thus the density of Pt-SA sites, which play the key role in photocatalytic activity—can be precisely optimized.     

Temperature and Pinning Effects on Driving a 2D Electron System on a Helium Film: A Numerical Study

Using numerical simulations we investigated the dynamic response to an externally driven force of a classical two-dimensional (2D) electron system on a helium film at finite temperatures. A potential barrier located at the center of the system behaves as a pinning center that results in an insulator state below a threshold driving force. We have found that the current-voltage characteristic obeys the scaling relation I=f ^ξ , with ξ ranging from ～(1.0–1.7) for different pinning strengths and temperatures. Our results may be used to understand the spread range of ξ in experiments with typical characteristic of plastic depinning.     

Hybrid lattice particle modeling: Theoretical considerations for a 2D elastic spring network for dynamic fracture simulations

A new hybrid lattice particle modeling (HLPM) scheme is proposed. The particle–particle interaction is derived from lattice modeling (LM) theory, whereas the computational scheme follows particle modeling (PM) technique. The newly proposed HLPM considers different particle interaction schemes, involving not only particles in the nearest neighborhood, but also the second nearest neighborhood. Different mesh structures with triangular or rectangular unit cells can be used. The current paper is concerned with the mathematical derivations of elastic interaction between contiguous particles in 2D lattice networks, accounting for different types of linkage mechanism and different shapes of lattice. Axial (α) and combined axial-angular (α − β) models are considered. Derivations are based on the equivalence of strain energy stored in a unit cell with its associated continuum structure in the case of in-plane elasticity. Conventional PM technique was restricted to a fixed Poisson’s ratio and had a strong bias in crack propagation direction, as a result of the geometry of the adopted lattice network. The current HLPM is free from the above-mentioned deficiencies and can be applied to a wide range of impact and dynamic fracture failure problems. Although the current analysis is based on the linear elastic spring model, inelastic considerations can be easily implemented, as HLPM has the same force interaction scheme as PM, based on the Lennard–Jones potential.