Flatband λ-Ti3O5 towards extraordinary solar steam generation.

Solar steam interfacial evaporation represents a promising strategy for seawater desalination and wastewater purification owing to its environmentally friendly character1-3. To improve the solar-to-steam generation, most previous efforts have focused on effectively harvesting solar energy over the full solar spectrum4-7. However, the importance of tuning joint densities of states in enhancing solar absorption of photothermal materials is less emphasized. Here we propose a route to greatly elevate joint densities of states by introducing a flat-band electronic structure. Our study reveals that metallic λ-Ti3O5 powders show a high solar absorptivity of 96.4% due to Ti-Ti dimer-induced flat bands around the Fermi level. By incorporating them into three-dimensional porous hydrogel-based evaporators with a conical cavity, an unprecedentedly high evaporation rate of roughly 6.09 kilograms per square metre per hour is achieved for 3.5 weight percent saline water under 1 sun of irradiation without salt precipitation. Fundamentally, the Ti-Ti dimers and U-shaped groove structure exposed on the λ-Ti3O5 surface facilitate the dissociation of adsorbed water molecules and benefit the interfacial water evaporation in the form of small clusters. The present work highlights the crucial roles of Ti-Ti dimer-induced flat bands in enchaining solar absorption and peculiar U-shaped grooves in promoting water dissociation, offering insights into access to cost-effective solar-to-steam generation.

Coexisting Ferroelectric and Ferrovalley Polarizations in Bilayer Stacked Magnetic Semiconductors.

It has long been expected that the coexistence of ferroelectric and ferrovalley polarizations in one magnetic semiconductor could offer the possibility to revolutionize electronic devices. In this study, monolayer and bilayer YI2 are studied. Monolayer YI2 is a ferromagnetic semiconductor and exhibits a valley polarization up to 105 meV. All of the present bilayer YI2 regardless of stacking orders show antiferromagnetic states. Interestingly, the bilayer YI2 with 3R-type stackings shows not only valley polarization but also unexpected ferroelectric polarization, proving the concurrent ferrovalley and multiferroics behaviors. Moreover, the valley polarization of 3R-type bilayer YI2 can be reversed by controlling the direction of ferroelectric polarization through an electric field or manipulating the magnetization direction using an external magnetic field. The amazing phenomenon is also demonstrated in 2D van der Waals LaI2 and GdBr2 bilayers. A design idea of multifunctional devices is proposed based on the concurrent ferrovalley and multiferroics characteristics.

Reactive Molecular Dynamics Simulations of Polystyrene Pyrolysis.

Polymers' controlled pyrolysis is an economical and environmentally friendly solution to prepare activated carbon. However, due to the experimental difficulty in measuring the dependence between microstructure and pyrolysis parameters at high temperatures, the unknown pyrolysis mechanism hinders access to the target products with desirable morphologies and performances. In this study, we investigate the pyrolysis process of polystyrene (PS) under different heating rates and temperatures employing reactive molecular dynamics (ReaxFF-MD) simulations. A clear profile of the generation of pyrolysis products determined by the temperature and heating rate is constructed. It is found that the heating rate affects the type and amount of pyrolysis intermediates and their timing, and that low-rate heating helps yield more diverse pyrolysis intermediates. While the temperature affects the pyrolytic structure of the final equilibrium products, either too low or too high a target temperature is detrimental to generating large areas of the graphitized structure. The reduced time plots (RTPs) with simulation results predict a PS pyrolytic activation energy of 159.74 kJ/mol. The established theoretical evolution process matches experiments well, thus, contributing to preparing target activated carbons by referring to the regulatory mechanism of pyrolytic microstructure.

A Tale of Two Sites: Neighboring Atomically Dispersed Pt Sites Cooperatively Remove Trace H2 in CO-Rich Stream.

Single-atom catalysts (SACs) exhibit distinct catalytic behavior compared with nano-catalysts because of their unique atomic coordination environment without the direct bonding between identical metal centers. How these single atom sites interact with each other and influence the catalytic performance remains unveiled as designing densely populated but stable SACs is still an enormous challenge to date. Here, a fabrication strategy for embedding high areal density single-atom Pt sites via a defect engineering approach is demonstrated. Similar to the synergistic mechanism in binuclear homogeneous catalysts, from both experimental and theoretical results, it is proved that electrons would redistribute between the two oxo-bridged paired Pt sites after hydrogen adsorption on one site, which enables the other Pt site to have high CO oxidation activity at mild-temperature. The dynamic electronic interaction between neighboring Pt sites is found to be distance dependent. These new SACs with abundant Pt-O-Pt paired structures can improve the efficiency of CO chemical purification.

Quinary High-Entropy-Alloy@Graphite Nanocapsules with Tunable Interfacial Impedance Matching for Optimizing Microwave Absorption.

Designing heterogeneous interfaces and components at the nanoscale is proven effective for optimizing electromagnetic wave absorption and shielding properties, which can achieve desirable dielectric polarization and ferromagnetic resonances. However, it remains a challenge for the precise control of components and microstructures via an efficient synthesis approach. Here, the arc-discharged plasma method is proposed to synthesize core@shell structural high-entropy-alloy@graphite nanocapsules (HEA@C-NPs), in which the HEA nanoparticles are in situ encapsulated within a few layers of graphite through the decomposition of methane. In particular, the HEA cores can be designed via combinations of various transition elements, presenting the optimized interfacial impedance matching. As an example, the FeCoNiTiMn HEA@C-NPs obtain the minimum reflection loss (RLmin ) of -33.4 dB at 7.0 GHz (3.34 mm) and the efficient absorption bandwidth (≤-10 dB) of 5.45 GHz ranging from 12.55 to 18.00 GHz with an absorber thickness of 1.9 mm. The present approach can be extended to other carbon-coated complex components systems for various applications.

Feasibility study on Ti-15Mo-7Cu with low elastic modulus and high antibacterial property.

Titanium and titanium alloy with low density, high specific strength, good biological, excellent mechanical compatibility and easy to process have been widely used in the medical materials, but their application in orthopedics and dentistry often face bacterial infection, corrosion failure and stress shielding. In this paper, Ti-15Mo-7Cu (TM-7Cu) alloy was prepared by high vacuum non-consumable electric arc melting furnace and then treated by solution and aging treatment. The microstructure, mechanical properties, antibacterial properties and cytocompatibility were studied by X-ray diffraction, microhardness tester, electrochemical working station, antibacterial test and Live/Dead staining technology. The results have shown that the heat treatment significantly influenced the phase transformation, the precipitation of Ti2Cu phase, the elastic modulus and the antibacterial ability. With the extension of the aging time, the elastic modulus slightly increased and the antibacterial rate obviously increased. TM-7Cu alloy with a low elastic modulus of 83GPa and a high antibacterial rate of > 93% was obtained. TM-7Cu alloy showed no cytotoxicity to MC3T3. It was suggested that TM-7Cu might be a highly competitive medical material.

Two-Dimensional Frank-Kasper Z Phase with One Unit-Cell Thickness.

Z phase is one of the three basic units by which the Frank-Kasper (F-K) phases are generally assembled. Compared to the other two basic units, that is, A15 and C15 structures, the Z structure is rarely experimentally observed because of a relatively large volume ratio among the constituents to inhibit its formation. Moreover, the discovered Z structures are generally the three-dimensional ordered Gibbs bulk phases to conform to their thermodynamic stability. Here, we confirmed the existence of a metastable two-dimensional F-K Z phase that has only one unit-cell height in the crystallography in a model Mg-Sm-Zn system, using atomic-scale scanning transmission electron microscopy combined with the first-principles calculations. Self-adapted atomic shuffling can convert the simple hexagonal close-packed structure to the topologically close-packed F-K Z phase. This finding provides new insight into understanding the formation mechanism and clustering behavior of the F-K phases and even quasicrystals in general condensed matters.

Twin Boundary Superstructures Assembled by Periodic Segregation of Solute Atoms.

Twinning is a common deformation mechanism in metals, and twin boundary (TB) segregation of impurities/solutes plays an important role in the performances of alloys such as thermostability, mobility, and even strengthening. The occurrence of such segregation phenomena is generally believed as a one-layer coverage of solutes alternately distributed at extension/compression sites, in an orderly, continuous manner. However, in the Mn-free and Mn-containing Mg-Nd model systems, we reported unexpected three- and five-layered discontinuous segregation patterns of the coherent {101̅1} TBs, and not all the extension sites occupied by solutes larger in size than Mg, and even some larger sized solutes taking the compression sites. Nd/Mn solutes selectively segregate at substitutional sites and thus to generate two new types of ordered two-dimensional TB superstructures or complexions. These findings refresh the understanding of solute segregation in the perfect coherent TBs and provide a meaningful theoretical guidance for designing materials via targeted TB segregation.

Nonsymmetrical Segregation of Solutes in Periodic Misfit Dislocations Separated Tilt Grain Boundaries.

Interfacial segregation is ubiquitous in mulit-component polycrystalline materials and plays a decisive role in material properties. So far, the discovered solute segregation patterns at special high-symmetry interfaces are usually located at the boundary lines or are distributed symmetrically at the boundaries. Here, in a model Mg-Nd-Mn alloy, we confirm that elastic strain minimization facilitated nonsymmetrical segregation of solutes in four types of linear tilt grain boundaries (TGBs) to generate ordered interfacial superstructures. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy observations indicate that the solutes selectively segregate at substitutional sites at the linear TGBs separated by periodic misfit dislocations to form such two-dimensional planar structures. These findings are totally different from the classical McLean-type segregation which has assumed the monolayer or submonolayer coverage of a grain boundary and refresh understanding on strain-driven interface segregation behaviors.

Enhanced resistive switching performance in yttrium-doped CH3NH3PbI3 perovskite devices.

In this study, yttrium-doped CH3NH3PbI3 (Y-MAPbI3) and pure CH3NH3PbI3 (MAPbI3) perovskite films have been fabricated using a one-step solution spin coating method in a glove box. X-ray diffractometry and field-emission scanning electron microscopy were used to characterize the crystal structures and morphologies of perovskite films, respectively. It was found that the orientation of the crystal changed and the grains became more uniform in Y-MAPbI3 film, compared with the pure MAPbI3 perovskite film. The films were used to prepare the resistive switching memory devices with the device structure of Al/Y-MAPbI3 (MAPbI3)/ITO-glass. The memory performance of both devices was studied and showed a bipolar resistive switching behavior. The Al/MAPbI3/ITO device had an endurance of about 328 cycles. In contrast, the Al/Y-MAPbI3/ITO device exhibited an enhanced performance with a long endurance up to 3000 cycles. Moreover, the Al/Y-MAPbI3/ITO device also showed a higher ON/OFF ratio of over 103, long retention time (≥104 s), lower operation voltage (±0.5 V) and outstanding reproducibility. Additionally, the conduction mechanism of the high resistance state transformed from space-charge limited current for a Y free device to the Schottky emission after Y doping. The present results indicate that the Al/Y-MAPbI3/ITO device has a great potential to be used in high-performance memory devices.

High-Entropy-Alloy Nanoparticles with Enhanced Interband Transitions for Efficient Photothermal Conversion.

Photothermal materials with broadband optical absorption and high conversion efficiency are intensively pursued to date. Here, proposing by the d-d interband transitions, we report an unprecedented high-entropy alloy FeCoNiTiVCrCu nanoparticles that the energy regions below and above the Fermi level (±4 eV) have been fully filled by the 3d transition metals, which realizes an average absorbance greater than 96 % in the entire solar spectrum (wavelength of 250 to 2500 nm). Furthermore, we also calculated the photothermal conversion efficiency and the evaporation rate towards the steam generation. Due to its pronounced full light capture and ultrafast local heating, our high-entropy-alloy nanoparticle-based solar steam generator has over 98 % efficiency under one sun irradiation, meanwhile enabling a high evaporation rate of 2.26 kg m-2 h-1 .

Interface hybridization and spin filter effect in metal-free phthalocyanine spin valves.

Spin-orbit coupling (SOC) has long been regarded as the core interaction to determine the efficiency of spin conserved transport in semiconductor spintronics. In this report, a spin-valve device with a Co/metal-free phthalocyanine (H2Pc)/Co stacking structure is fabricated. The magnetoresistance effect was successfully obtained in the device. It is also found that the magnetoresistance response is relatively smaller than that of metallic phthalocyanines, clearly implying that SOC is not the key factor to affect the magnetoresistance in phthalocyanine spin-valves. The dominant mechanism that determines the spin transport efficiency in the present H2Pc devices was systemically explored by combining both experimental measurements and first-principles calculation analysis. It was noticed that both the crystalline structure and molecular orientation of the H2Pc layer could be modified by the contact under-layer materials, which changes the magnetization intensity of the ferromagnetic metallic electrode due to the strong interface hybridization of Co/H2Pc. Meanwhile, the theoretical calculations clearly demonstrated that the spin filter effect from the second H2Pc layer should be responsible for the decrease of the magnetoresistance response in the present spin-valves compared to those using metallic phthalocyanine layers. This investigation may trigger new insights into the role of SOC strength and interface hybridization in organic spintronics.

Control of Catalytic Activity of Nano-Au through Tailoring the Fermi Level of Support.

The catalytic properties of nanometals are strongly dependent on their electronic states which, are influenced by the interaction with the supports. However, a precise manipulation of the electronic interaction is lacking, and the nature of the interaction is still ambiguous. Herein, using Au/ZnFex Co2- x O4 (x = 0-2) as a model system with continuously tuned Fermi levels of supports, the electronic structure of the Au catalyst can be precisely controlled by changing the Fermi level of the support, which arises from the charge redistribution between the two phases. A higher Fermi level of ZnFe2 O4 support makes nano-Au negatively charged and thus facilitates the oxidation of CO, and in contrast, a lower Fermi level of ZnCo2 O4 support makes nano-Au positively charged and is preferential to the oxidation of benzyl alcohol. This work represents a solid step towards exploration of advanced catalysts with deliberate design of electronic structure and catalytic properties.

Unusual interfacial magnetic interactions for τ-MnAl with Fe(Co) atomic layers.

The interfacial magnetic interaction and coupling mechanism for τ-MnAl with Fe(Co) atomic layers have been studied using first principles calculations. The stable surface and interface were firstly determined by the surface energy of τ-MnAl and interface energy of τ-MnAl/Fe(Co) films, respectively. Their magnetic coupling interactions were investigated by varying the Fe(Co) atomic layer numbers. It is noted that both Fe and Co exhibited ferromagnetic coupling with τ-MnAl. Interestingly, an unusual oscillation phenomenon of magnetic coupling for τ-MnAl with Fe(Co) atomic layers was observed depending on the layer thickness of Fe(Co). Moreover, Fe and Co showed different oscillation modes. The energy difference between antiferromagnetic and ferromagnetic states is larger for τ-MnAl/Fe and τ-MnAl/Co when the Fe(Co) layer numbers are even and odd, respectively. Their mechanisms were analyzed based on the band structures and the confinement of electrons in quantum wells. It is found that the magnetic coupling oscillation in τ-MnAl/Fe originated from both the spin up Δ1 band and spin down Δ5 band at the [capital Gamma, Greek, macron] points. Comparatively, the oscillation of τ-MnAl/Co is due to the spin up band at the X[combining macron] point. The present results could provide insight to further understand interfacial exchange interactions among magnetic layers.

Effect of heat treatment on the bio-corrosion properties and wear resistance of antibacterial Co-29Cr-6Mo-xCu alloys.

Co-Cr-Mo alloys have been widely used in hip implants due to their good corrosion resistance and good wear resistance. However, complaint is still raising due to infection and inflammation. The addition of Cu has been proven to be an effective way to develop a new kind of Co-based alloy with good antibacterial properties. In this paper, the effect of heat treatment on the corrosion property, the tribology property and the antibacterial property of Cu containing Co-based alloys were investigated in detail. The microstructure observation showed that the as-cast alloys mainly consisted of a dendritic matrix with carbide dispersion at grain boundaries and a fine Cu-rich phase in the matrix and at the carbide/matrix interface. The carbide precipitates and the distribution of Cu phases affected significantly the friction coefficient and wear resistance of Co-xCu alloy. Annealing at 1060 °C/24 h promoted the precipitation of carbide and in turn increased the hardness and wear resistance markedly. Heat treatments, including annealing, solid solution and ageing treatment, enhanced the corrosion resistance of Co-xCu alloy without reduction in antibacterial properties. However, the addition of Cu increased the corrosion resistance and antibacterial properties but reduced the wear resistance especially at high Cu content.

New Structured Laves Phase in the Mg-In-Ca System with Nontranslational Symmetry and Two Unit Cells.

All of the AB_{2} Laves phases discovered so far satisfy the general crystalline structure characteristic of translational symmetry; however, we report here a new structured Laves phase directly precipitated in an aged Mg-In-Ca alloy by using aberration-corrected scanning transmission electron microscopy. The nanoprecipitate is determined to be a (Mg,In)_{2}Ca phase, which has a C14 Laves structure (hcp, space group: P6_{3}/mmc, a=6.25 Å, c=10.31 Å) but without any translational symmetry on the (0001)_{p} basal plane. The (Mg,In)_{2}Ca Laves phase contains two separate unit cells promoting the formation of five tiling patterns. The bonding of these patterns leads to the generation of the present Laves phase, followed by the Penrose geometrical rule. The orientation relationship between the Laves precipitate and Mg matrix is (0001)_{p}//(0001)_{α} and [11[over ¯]00]_{p}//[112[over ¯]0]_{α}. More specifically, in contrast to the traditional view that the third element would orderly replace other atoms in a manner of layer by layer on the close-packed (0001)_{L} plane, the In atoms here have orderly occupied certain position of Mg atomic columns along the [0001]_{L} zone axis. The finding would be interesting and important for understanding the formation mechanism of Laves phases, and even atom stacking behavior in condensed matter.

Ultralight Fe@C Nanocapsules/Sponge Composite with Reversibly Tunable Microwave Absorption Performances.

Microwave absorbers are usually designed to solve electromagnetic interferences at a specific frequency, while the requirements may be dynamic during service life. Therefore, a recoverable tuning for microwave absorption properties in response to an external stimulus would be highly desirable. We herein present a micro/nano-scale hybrid absorber, in which high-performance Fe@C nanocapsule absorbents are integrated with a porous melamine sponge skeleton, exhibiting multiple merits of light weight, strong absorption and high elasticity. By mechanically compressing and decompressing the absorber, microwave absorption performances can be effectively shifted between 18 GHz and 26.5 GHz. The present study thus provides a new strategy for the design of a 'dynamic' microwave absorber.

Gigahertz Dielectric Polarization of Substitutional Single Niobium Atoms in Defective Graphitic Layers.

We synthesize two Nb/C composites with an order of magnitude difference in the density of single niobium atoms substituted into defective graphitic layers. The concentration and sites of single Nb atoms are identified using aberration-corrected scanning transmission electron microscopy and density functional theory. Comparing the experimental complex permittivity spectra reveals that a representative dielectric resonance at ∼16 GHz originates from the intrinsic polarization of single Nb atom sites, which is confirmed by theoretical simulations. The single-atom dielectric resonance represents the physical limit of the electromagnetic response of condensed matter, and thus might open up a new avenue for designing electromagnetic wave absorption materials. Single-atom resonance also has important implications in understanding the correlation between the macroscopic dielectric behaviors and the atomic-scale structural origin.

(Ti/Zr,N) codoped hematite for enhancing the photoelectrochemical activity of water splitting.

In this theoretical study, first-principles calculations were carried out to explore the photocatalytic activity of cation (Ti or Zr) and anion (N) compensated codoped hematite based on density functional theory (DFT). For (Ti/Zr,N) codoped hematite, the band edges of the conduction band and the valence band move close to each other, leading to an obvious bandgap reduction. Compared with the pure hematite, the optical absorption coefficient of codoped hematite is significantly enhanced in the visible light region. The charge distribution at the conduction band minimum (CBM) and valence band maximum (VBM) is spatially separated after codoping, which is beneficial for extending the carrier lifetime. More interestingly, the CBM becomes electronically delocalized in (Ti,N) doped hematite, which indicates better carrier transport properties in the bulk system. Due to these special features of (Ti/Zr,N) codoped hematite, an improved photocatalytic performance can be expected.

Hydrogen generation by water splitting on hematite (0001) surfaces: first-principles calculations.

The surface chemical activity is a critical factor affecting the photocatalytic efficiency of hematite. In this study, we investigate systematically the reaction kinetics of water heterolytic dissociation (H2O-OH(-) + H(+)) and hydrogen generation by water splitting on four kinds of hematite (0001) surfaces, namely perfect and defective O- and Fe-terminated surfaces, at the electronic level based on first-principles calculations. The simulation results illustrate that the chemical reaction rate for the dissociation and hydrogen generation is sensitive to the morphology of the hematite (0001) surface. For water heterolytic dissociation, the hydrogen atom is apt to drop from water molecules on the perfect O-terminated (0001) surface without energy consumption. However, the Fe-terminated (0001) perfect surface is a preferable candidate for hydrogen generation, on which the whole photoelectrochemical process needs to overcome a rate determined barrier of 2.77 eV. Our investigation shows that O- or Fe-vacancy on hematite (0001) surfaces is not conductive to hydrogen generation by water splitting.