3D Lithiophilic CuZrAg Metallic Glass Based-Current Collector for High-Performance Lithium Metal Anode.

Lithium metal anodes face several challenges in practical applications, such as dendrite growth, poor cycle efficiency, and volume variation. 3D hosts with lithiophilic surfaces have emerged as a promising design strategy for anodes. In this study, inspiration from the intrinsic isotropy, chemical heterogeneity, and wide tunability of metallic glass (MG) is drew to develop a 3D mesoporous host with a lithiophilic surface. The CuZrAg MG is prepared using the scalable melt-spinning technique and subsequently treated with a simple one-step chemical dealloying method. This resultes in the creation of a host with a homogeneously distributed abundance of lithium affinity sites on the surface. The excellent lithiophilic property and capability for uniform lithium deposition of the 3D CuZrAg electrode have been confirmed through theoretical calculations. Therefore, the 3D CuZrAg electrode displays excellent cyclic stability for over 400 cycles with 96% coulomb efficiency, and ultra-low overpotentials of 5 mV for over 2000 h at 1.0 mA cm-2 and 1.0 mAh cm-2 . Additionally, the full cells partied with either LiFePO4 or LiNi0.8 Co0.1 Mn0.1 O2 cathode deliver exceptional long-term cyclability and rate capability. This work demonstrates the great potential of metallic glass in lithium metal anode application.

Lévy walk of quasiballistic phonons in nanowires.

Phonon transport in square-cross-section nanowires is studied using spectral Monte Carlo simulations. Our results show the evolution of the different transport regimes described by Lévy statistics as a function of the surface roughness-to-thermal wavelength ratio σ/λ. More precisely, the relationship between the Lévy index γ describing the mean free path distribution Ψ(Λ) and σ/λ is established for the classical diffusive regime, the superdiffusive regime, and the ballistic regime in the nanowire. Besides the conventional superdiffusive regime that is marked by Ψ(Λ) with a single heavy-tailed peak, we reveal an unconventional superdiffusive subregime featuring Ψ(Λ) with sawtooth oscillations when σ/λ∼0.01. Investigation of the direction of propagation of phonons shows a significant narrowing of the angular distribution around the long axis of the nanowire due to the diffuse scattering at rough boundaries when σ/λ>0.1. These results shed light on the transport mechanisms of quasiballistic phonons and will help in nanowire design for specific applications.

Cobalt-based MOF nanoribbons with abundant O/N species for cycloaddition of carbon dioxide to epoxides.

Chemical fixation of CO2 with epoxides is an effective option to achieve sustainable synthesis of cyclic organic carbonates. Although metal-organic frameworks (MOFs) are promising catalysts for this reaction, their low stability in aqueous solutions makes this application infeasible. In an effort to overcome this limitation, cobalt-based metal-organic framework (Co(II)MOF) nanoribbons have been prepared by coordinating the Co(II) ions with a new ligand (C16H12N4O4) full of oxygen and nitrogen moieties. Strong chemical interactions occur between the adsorbed CO2 and oxygen/nitrogen atoms in this porous MOF structure. Co(II)-MOF nanoribbons with tetra-n-butylammonium bromide acted as cocatalysts with ∼97% yield of cyclic carbonate (reaction kinetic rate of 14.7 × 106 µmol g-1 h-1) upon the cycloaddition of epichlorohydrin (ECH) to CO2 (>99% reaction selectivity under solvent-free reaction condition at 80 °C, 3 h and 1 MPa CO2 pressure). This work may open a new avenue for chemical fixation of CO2 by rational design of the components and morphology of MOF-based catalysts.

Controlled Synthesis of Perforated Oxide Nanosheets with High Density Nanopores Showing Superior Water Purification Performance.

A method for creating genuine nanopores in high area density on monolayer two-dimensional (2D) metallic oxides has been developed. By use of the strong reduction capability of hydroiodic acid, active metal ions, such as FeIII and CoIII, in 2D oxide nanosheets can be reduced to a divalent charge state (2+). The selective removal of FeO2 and CoO2 metal oxide units from the framework can be tuned to produce pores in a range of 1-4 nm. By monitoring of the redox reaction kinetics, the pore area density can be also tuned from ∼0.9 × 104 to ∼3.3 × 105 µm-2. The universality of this method to produce much smaller pores and higher area density than the previously reported ones has been proven in different oxide nanosheets. To demonstrate their potential applications, ultrasmall metal organic framework particles were grown inside the pores of perforated titania oxide nanosheets. The optimized hybrid film showed ∼100% rejection of methylene blue (MB) from the water. Its water permeance reached 4260 L m-2 h-1 bar-1, which is 1-3 orders of that for reported 2D membranes with good MB rejections.

Understanding the Surface Reconstruction on Ternary Wx CoBx for Water Oxidation and Zinc-Air Battery Applications.

Here, the synthesis of a series of pure phase metal borides is reported, including WB, CoB, WCoB, and W2 CoB2 , and their surface reconstruction is studied under the electrochemical activation in alkaline solution. A cyclic voltammetric activation is found to enhance the activity of the CoB and W2 CoB2 precatalysts due to the transformation of their surfaces into the amorphous CoOOH layer with a thickness of 3-4 nm. However, such surface transformation does not happen on the WB and WCoB due to their superior structure stability under the applied voltage, highlighting the importance of metal components for the surface reconstruction process. It is found that, compared with CoB, the W2 CoB2 surface shows a quicker reconstruction with a larger active surface area due to the selective leaching of the W from its surface. In the meantime, the metallic W2 CoB2 core underneath the CoOOH layer shows a better promotion of its oxygen evolution reaction (OER) performance than CoB. Therefore, the ternary W2 CoB2 shows better OER performance than the CoB, as well as the WB and WCoB. It is also found that the mixture of W2 CoB2 with Pt/C as the catalysts in air electrode for rechargeable Zn-air battery (ZAB), shows better performance than the IrO2 -Pt/C couple-based ZAB.

Ternary Mo2 NiB2 as a Superior Bifunctional Electrocatalyst for Overall Water Splitting.

Transition metal borides are considered as promising electrocatalysts for water splitting due to their metallic conductivity and good durability. However, the currently reported monometallic and noncrystalline multimetallic borides only show generic and monofunctional catalytic activity. In this work, the authors design and successfully synthesize highly crystalline ternary borides, Mo2 NiB2 , via a facile solid-state reaction from pure elemental powders. The as-synthesized Mo2 NiB2 exhibits very low overpotentials for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), that is, 280 and 160 mV to reach a current density of 10 mA cm-2 , in alkaline media. These values are much lower from the ones observed over monometallic borides, that is, Ni2 B and MoB, and the lowest among all nonprecious metal borides. By loading Mo2 NiB2 onto Ni foams as both cathode and anode electrode for overall water splitting applications, a low cell voltage of 1.57 V is required to achieve a current density of 10 mA cm-2 , comparable with the value required from the Pt/C||IrO2 /C couple (1.56 V). The proposed synthesis strategy can be used for the preparation of cost-effective, multi-metallic crystalline borides, as multifunctional electrocatalysts.

Photo-promoted in situ reduction and stabilization of Pd nanoparticles by H2 at photo-insensitive Sm2O3 nanorods.

Controlled synthesis of noble metal nanoparticles with well-defined size and good dispersion on supports has been a long-standing challenge in heterogeneous catalysis. Here we report a facile photo-assisted H2in situ reduction process to synthesize monodispersed Pd nanoparticles with 2-4 nm size on photo-insensitive Sm2O3 rare-earth metal oxide with nanorod morphology. Thanks to the contribution of UV irradiation, the photoelectrons generation in the Sm2O3 support accelerates the H2 reduction of Pd2+ ions into Pd0 and stabilize the growth of very small Pd nanoparticles homogeneously dispersed on the support. The homogeneous distribution of the Pd NPs on the surface of Sm2O3 is most likely attributed to the profuse nucleation sites created by the UV irradiation and the abundance of hydroxyl groups on the support. The hydrogenation of styrene to ethylbenzene was studied as a model reaction. As a result, the UV radiated sample shows an excellent TOF value of 7419 h-1, which is quadruple of the sample without UV irradiation, under the condition of 0.1 MPa H2 at a content of 1.0 wt% Pd. Besides, UV radiated sample shows a negligible performance degradation during the repeated cycling process. This photo-promoted H2 reduction process provides a convenient and straightforward route for assembling materials with novel structures and functions for nanotechnology applications.

Heat conduction tuning by wave nature of phonons.

The world communicates to our senses of vision, hearing, and touch in the language of waves, because light, sound, and even heat essentially consist of microscopic vibrations of different media. The wave nature of light and sound has been extensively investigated over the past century and is now widely used in modern technology. However, the wave nature of heat has been the subject of mostly theoretical studies because its experimental demonstration, let alone practical use, remains challenging due to its extremely short wavelengths. We show a possibility to use the wave nature of heat for thermal conductivity tuning via spatial short-range order in phononic crystal nanostructures. Our experimental and theoretical results suggest that interference of thermal phonons occurs in strictly periodic nanostructures and slows the propagation of heat. This finding expands the methodology of heat transfer engineering to the wave nature of heat.

Heat guiding and focusing using ballistic phonon transport in phononic nanostructures.

Unlike classical heat diffusion at macroscale, nanoscale heat conduction can occur without energy dissipation because phonons can ballistically travel in straight lines for hundreds of nanometres. Nevertheless, despite recent experimental evidence of such ballistic phonon transport, control over its directionality, and thus its practical use, remains a challenge, as the directions of individual phonons are chaotic. Here, we show a method to control the directionality of ballistic phonon transport using silicon membranes with arrays of holes. First, we demonstrate that the arrays of holes form fluxes of phonons oriented in the same direction. Next, we use these nanostructures as directional sources of ballistic phonons and couple the emitted phonons into nanowires. Finally, we introduce thermal lens nanostructures, in which the emitted phonons converge at the focal point, thus focusing heat into a spot of a few hundred nanometres. These results motivate the concept of ray-like heat manipulations at the nanoscale.

Thermal resistance at a solid/superfluid helium interface.

Kapitza in 1941 discovered that heat flowing across a solid in contact with superfluid helium (<2 K) encounters a strong thermal resistance at the interface. Khalatnikov demonstrated theoretically that this constitutes a general phenomenon related to all interfaces at all temperatures, given the dependence of heat transmission on the acoustic impedance (sound velocity × density) of each medium. For the solid/superfluid interface, the measured transmission of heat is almost one hundred times stronger than the Khalatnikov prediction. This discrepancy could be intuitively attributed to diffuse scattering of phonons at the interface but, despite several attempts, a detailed quantitative comparison between theoretical and experimental findings to explain the occurrence of scattering and its contribution to heat transmission had been lacking. Here we show that when the thermal wavelength λ of phonons of the less dense medium (liquid (4)He) becomes comparable to the r.m.s. surface roughness σ, the heat flux crossing the interface is amplified; in particular when σ ≈ 0.33λ, a spatial resonant mechanism occurs, as proposed by Adamenko and Fuks. We used a silicon single crystal whose surface roughness was controlled and characterized. The thermal boundary resistance measurements were performed from 0.4 to 2 K at different superfluid pressures ranging from saturated vapour pressure (SVP) to above (4)He solidification, to eliminate all hypothetical artefact mechanisms. Our results demonstrate the physical conditions necessary for resonant phonon scattering to occur at all interfaces, and therefore constitute a benchmark in the design of nanoscale devices for heat monitoring.