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© 2020 Elsevier Ltd and Techna Group S.r.l. The triboelectric effect has recently demonstrated its great potential in environmental remediation and even new energy applications for triggering a number of catalytic reactions by utilizing trivial mechanical energy. In this study, Ba4Nd2Fe2Nb8O30 (BNFN) submicron powders were used to degrade organic dyes via the tribocatalytic effect. Under the frictional excitation of three PTFE stirring rods in a 5 mg/L RhB dye solution, BNFN demonstrates a high tribocatalytic degradation efficiency of 97% in 2 h. Hydroxyl radicals (•OH) and superoxide radicals (•O2－) were also detected during the catalysis process, which proves that triboelectric energy stimulates BNFN to generate electron-hole pairs. The tribocatalysis of tungsten bronze BNFN submicron powders provides a novel and efficient method for the degradation of wastewater dye by utilizing trivial mechanical energy.

© 2020 World Scientific Publishing Co. Pte Ltd. All rights reserved. Sodium niobate (NaNbO3)-based dielectrics have received much attention for energy storage applications due to their low-cost, lightweight, and nontoxic nature. The field-induced metastable ferroelectric phase in NaNbO3-based dielectrics, however, leads to a large hysteresis of the polarization-electric field (P-E) loops and hence deteriorate the energy storage performance. In this study, the hysteresis was successfully reduced by introducing Bi3+and Ti4+into A-site and B-site of NaNbO3, respectively. MnO2addition was added to further increase the ceramic density and enhance the cycling reliability. As a result, a high recoverable energy density of 4.3 J/cm3and a high energy efficiency of 90% were simultaneously achieved in the ceramic capacitor at an applied electric field of 360 kV/cm. Of particular importance is that the ceramic capacitor exhibits a stable energy storage properties over a wide temperature range of -70 to 170 °C, with much improved electric cycling reliability up to 105cycles.

Sodium niobate (NaNbO3)-based lead-free ceramics have been actively studied for energy storage applications because of their antiferroelectric and/or relaxor features achieved in modified systems. The P-E loops of NaNbO3-based ceramics are usually hysteretic because of the existence of a metastable ferroelectric phase at room temperature. In this study, by introducing aliovalent cations and A-site vacancies, the relaxor characteristics are greatly enhanced in (Na1-2xBix)(Nb1-xZrx)O3 ceramics, leading to a high energy storage efficiency of above 90%. In addition, sintering aid CuO and a special ramp-to-spike sintering profile were employed to decrease the sintering temperature and reduce the grain size. The modified ceramic exhibits improved insulating properties and hence a higher breakdown strength, leading to a high recoverable energy density of 4.9 J/cm3 and a high energy efficiency of 88% at 430 kV/cm. The ceramic also exhibits satisfactory temperature stability over a wide temperature range from 25 to 125 °C and charge-discharge performance, making it a promising candidate for high-power dielectric energy storage applications.

Both spin gapless semiconductors (SGSs) and nodal ring states (NRSs) have aroused great scientific interest in recent years due to their unique electronic properties and high application potential. However, since their advent, all SGSs and NRSs have been predicted in independent materials. In this work, we proposed a novel type of material, nodal ring spin gapless semiconductor (NRSGS), which combines both states of the SGSs and NRSs. The synthesized material Mg2VO4 is selected as a potential candidate. Detailed band structure analysis reveals that there are gapless crossings in the spin-up direction, which are from multiple topological nodal rings located exactly at the Fermi energy level. Mg2VO4 combines the advantages inherited from both NRSs and SGSs in terms of the innumerable gapless points along multiple nodal rings with all linear dispersions and direct contacts. In addition, Mg2VO4 also shows strong robustness against both the spin orbit coupling effect and strain conditions. Therefore, for the first time, we propose the concept of an NRSGS, and the first such material candidate Mg2VO4 can immediately advance corresponding experimental measurements and even facilitate real applications.

© 2020 American Physical Society. Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

© The Royal Society of Chemistry. Eco-friendly energy harvesters with high output for effectively harvesting mechanical energy over a broad temperature range are highly desirable. Here, a lead-free, flexible, and efficient hybrid energy harvester was demonstrated by the combination of a single electrode triboelectric energy harvester (TEH) and an interdigital electrode piezoelectric energy harvester (PEH), which are based on a 0-3 composite of high temperature BiFeO3-BaTiO3 (BF-BT) piezoelectric ceramic particles and polyimide (PI) matrix. With an appropriate external connection, the hybrid piezo-triboelectric energy harvester (P-TEH) generates an open-circuit voltage and a short-circuit current of 175 V and 600 nA, respectively, together with high power density of 4.1 mW cm-3. This work demonstrates that the combination of the interdigital electrode PEH with TEH can enable high electrical output due to minimum internal interference between the two energy harvesting parts. Of particular significance is that the flexible P-TEH shows great high temperature stability up to 200 °C with an open-circuit voltage of 150 V and a short-circuit current of 560 nA, exhibiting excellent potential for using at high temperature.

© 2020 IOP Publishing Ltd. To date, a handful of topological semimetals (TMs) with multiple types of topological nodal line (TNL) states have been theoretically predicted in novel materials. However, their TNLs are often affected by many factors, such as spin-orbit coupling (SOC) effect, strain, and the extraneous bands near the band crossing points, and therefore, the TNL states cannot be easily verified by experiments. Here, by using first-principles calculations, we report that the Pnma LiBH is a potential TM with twofold and quadruple degenerate topological nodal lines. These TNLs situate very close to the Fermi level, and do not coexist with other extraneous bands. More importantly, the TNLs of this material are very robust to the effect of SOC, uniform strain, and biaxial strain. The nontrivial band structure in LiBH produces drum-head-like surface states in the (001) surface projection. Our result reveals that LiBH material is an excellent candidate to study the multiple kinds of TNLs.

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Designing cost-effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active-center-transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO2) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO2 nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO2 acts as the co-catalyst for HER, whereas the active center transfers to LiCoO2 and Pt turns into the co-catalyst for OER. The unique architecture of Pt/LiCoO2 heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO2 heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm−2 for HER and OER in alkaline medium, respectively.

© 2020 The Royal Society of Chemistry. The concept of composite material has been increasingly applied for the significant improvement in the thermoelectric performance because of the predictable effective medium properties and the unique interfacial correlated thermal and electrical transport mechanism. Herein, we report that the graphene inclusion can lead to a significant reduction in thermal conductivity and improve the overall thermoelectric figure-of-merit in SnSe. We demonstrate a systematic investigation on the microstructures and electrical and thermoelectric properties of the SnSe/graphene composite. HRTEM reveals the uniform distribution of graphene nanosheets in the SnSe matrix, forming a sharp interface with refined SnSe grain sizes and defects nearby the interfaces. Thermal conductivity decreases with graphene addition and can significantly reduce to as low as ∼0.18 W m-1 K-1, resulting in an enhanced figure of merit (ZT) of the SnSe/graphene composite by at least 50% compared with pristine SnSe. The significant reduction in thermal conductivity is attributed to the phonon scattering by densely distributed phase interfaces as well as defects and grain boundaries. The carbon element is also believed to potentially reduce long-range tin diffusion by acting as a confinement barrier to restrict heat and ion diffusion. Our work proves that the graphene secondary phase could enhance the ZT of the SnSe matrix, which might pave the way for achieving high-performance thermoelectric properties in carbon-induced composite materials.

© 2020 Elsevier Ltd and Techna Group S.r.l. Micrometre-sized pure BiFeO3 (BF) and Ce-doped Bi0.9Ce0.1FeO3 (BCF) powders were synthesized by using conventional hydrothermal (H) and microwave assisted hydrothermal (MH) methods. The BF-H and BCF-H show obviously enhanced weak ferromagnetic characteristic, in which the BCF-H exhibits excellent magnetic performance, with significantly increased maximum magnetization of ∼3.6 emu/g (under 50 kOe magnetic field) and remanent magnetization of ∼0.268 emu/g at room temperature. The superior magnetic property of BCF-H could be attributed to the suppression of spiral structure and the modulation of canting angle of spins in antiferromagnetic sublattices.

© 2020 The perovskite oxide interface has attracted extensive attention as a platform for achieving strong coupling between ferroelectricity and magnetism. In this work, robust control of magnetoelectric (ME) coupling in the BiFeO3/BaTiO3 (BFO/BTO) heterostructure (HS) was revealed by using the first-principles calculation. Switching of the ferroelectric polarization of BTO induce large ME effect with significant changes on the magnetic ordering and easy magnetization axis, making up for the weak ME coupling effect of single-phase multiferroic BFO. In addition, the Dzyaloshinskii-Moriya interaction (DMI) and the exchange coupling constants J for the BFO part of the HSs are simultaneously manipulated by the ferroelectric polarization, especially the DMI at the interface is significantly enhanced, which is three or four times larger than that of the individual BFO bulk. This work paves the way for designing new nanomagnetic devices based on the substantial interfacial ME effect.

2020 Science China Press Oxygen vacancy is one of the pivotal factors for tuning/creating various oxide properties. Understanding the behavior of oxygen vacancies is of paramount importance. In this study, we identify a metastable oxygen vacancy ordering state other than the well-known Magnéli phases in TiO2 crystals from both experimental and theoretical studies. The oxygen vacancy ordering is found to be a zigzag chain along the [0 0 1] direction in the (1 1 0) plane occurring in a wide temperature range of 200-500 °C. This metastable ordering state leads to a first-order phase transition accompanied by significant enhancement of dielectric permittivity and a memristive effect featuring a low driving electric field. Our results can improve oxide properties by engineering oxygen vacancies.

We study, in the absence of a magnetic field, the stability of skyrmions in a disk-shaped ferromagnetic multilayer in the presence of locally pinned spins at the edge. This particular boundary condition suppresses the formation of helical states yet enhances the stabilization of states with rotational symmetry, such as Neél skyrmions. Numerical simulation shows that the size of the disk is a viable parameter to control the creation and annihilation of skyrmionic states. We also find that the demagnetization field renormalizes the Dzyaloshinskii-Moriya interaction. This study opens an alternative avenue to the generation, stabilization, and control of magnetic skyrmions in field-free heterostructures.

Advances in smart and wearable devices are driving innovations in multifunctional flexible materials at a tremendous pace. Here, drawing support from the unique flexible fluorophlogopite mica platform, we present a promising all-inorganic bendable Mn-modified 0.65(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.35SrTiO3 (NBBST) film with dual use in electrocaloric (EC) refrigeration and energy storage via a cost-effective transfer-free process. An appreciable room-temperature EC effect with adiabatic temperature change of 12 K and isothermal entropy of 18 J K-1 kg-1 was realized in the NBBST film, which benefits from the large change in dipolar ordering near depolarization temperature. Also, the film exhibits a broad operating temperature span over 25 °C because of its relaxor feature. Most importantly, the film can maintain a high EC performance either under bending deformation at 5 mm radius or after undergoing 104 bending-unbending cycles. Meanwhile, the flexible NBBST film possesses good energy storage property with a recoverable energy density of 56 J cm-3 and an efficiency of 66%. This is the first example of a lead-free all-inorganic multifunctional film capacitor toward the flexible EC refrigeration and energy storage devices. This work shows bright prospects in the emerging flexible e-market.

2020 Elsevier B.V. Very recently, a novel Dirac half-metal has been investigated in the perovskite-type lanthanum manganite (LaMnO3) by Ma et al. [1] and the half metallicity with 100% spin polarization is discovered. More remarkably, multiple linear Dirac crossings are observed around the Fermi energy level in the spin-up direction. Inspired by this work, another perovskite-type material YbBO3, which has been synthesized before, is investigated by first principles calculation. Results confirm the presence of half metallicity in this material with also multiple Dirac-like linear and parabolic band crossings around the Fermi level. The estimated Curie temperature is 469.4 K and the mechanic and dynamic stabilities have been verified. Besides, the elastic anisotropy is revealed with the directional dependent Young's modulus and shear modulus, together with the acoustic wave properties. The effect of the uniform strain, electron doping and spin-orbital coupling on the electronic structures have been also accessed. In particular, nearly spin-gapless semiconducting behavior with linear or parabolic energy dispersions is obtained by electron doping. Moreover, several thermodynamic properties have been examined under temperature from 0 to 500 K and pressure from 0 to 5 GPa. In combination, we expect YbBO3 can attract more research attention and even facilitate the spintronic application.

Due to its physical and chemical stability in addition to its transparency in the visible region with ultraviolet protective activity, ZnO can be used in sunscreen applications. In our article, Na-doped zinc oxide nanoparticles were prepared in different concentrations by sol–gel and solvothermal techniques to reduce their photocatalytic activity. The photocatalytic activity of the doped samples was suppressed effectively at a concentration of 0.03 at% Na doping up to 90% by conducting the sol–gel process rather than the solvothermal process which resulted in about 70% photocatalytic reduction in a period of time from (0–30 min) when exposed to ultraviolet and visible light. In addition, the nanoparticles resulted by sol–gel route show a reduction in photoactivity under solar simulation about 98% rather than those resulted via solvothermal process which shows a reduction around 92% for 30 min which is the same period of time used for the photocatalytic degradation test. The particle size was around 64.6–84.6 nm for samples prepared by both methods.

2020 Author(s). BiFeO3, a typical multiferroic compound, owns complicated interactions among its charge, lattice, and spin degrees, while largely remaining a mystery. In this work, the BiFeO3 single crystal has been investigated in pulsed high magnetic fields up to 50 T. The lattice, spin, and charge orderings are found to couple with each other strongly. High magnetic field could drive the spin cycloid phase to a canted antiferromagnetic phase while accompanied by a clear magnetostriction response and weakening of ferroelectric polarization. The critical magnetic field for the magnetic phase transition was shifted to the lower magnetic field region by an external electric field over a broad temperature range, which indicates the complex interaction among magnetization, electrical polarization, and crystal lattice. This work has provided a clear picture to show the synergetic response of multiple ordering parameters during the magnetic phase transition in BiFeO3, which could benefit the design of BiFeO3 based spintronic devices and novel multiferroic materials with strong magnetoelectric coupling.

2020 by the authors. Due to the enhanced demand for numerous electrical energy storage applications, including applications at elevated temperatures, dielectric capacitors with optimized energy storage properties have attracted extensive attention. In this study, a series of lead-free strontium bismuth titanate based relaxor ferroelectric ceramics have been successfully synthesized by high temperature solid-state reaction. The ultrahigh recoverable energy storage density of 4.2 J/cm3 under 380 kV/cm, with the high efficiency of 88%, was obtained in the sample with x = 0.06. Of particular importance is that this ceramic composition exhibits excellent energy storage performance over a wide work temperature up to 150 °C, with strong fatigue endurance and fast discharge speed. All these merits demonstrate the studied ceramic system is a potential candidate for high-temperature capacitors as energy storage devices.

© 2020 Lead-free (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) possesses comparable piezoelectric constant with lead zirconate titanate (PZT), but its poor temperature electric performances stability and low Curie temperature limit its application. Here we designed composition graded BZT-BCT films with improved temperature stability of piezoelectric, ferroelectric, and dielectric performances over a wide temperature range, and the d33 reaches 21 pm/V with hysteresis loop even at 180 °C, which is far above the Curie temperature of BZT-BCT ceramic and BZT-0.5BCT film. The excellent temperature stability is ascribed to the lattice distortion and strain gradient in the grains caused by ions diffusion, and could suppress phase transition. This work could bring forward a feasible design for dielectric/piezoelectric/ferroelectric devices operating in harsh temperature environment.

The layered oxychalcogenides BiCuOCh (Ch = S, Se, and Te) have gained extensive attention due to their low lattice thermal conductivity, moderate Seebeck coefficient, and tunable electrical conductivity. However, the thermoelectric properties of their Ag-based isomorphic compounds are still unclear. Here, by means of first‐principles calculations and Boltzmann transport theory, we identify that three materials exhibit intrinsically low lattice thermal conductivity. Because of large unit volumes and relatively low sound velocities, the lattice thermal conductivities for BiAgOS, BiAgOSe, and BiAgOTe are only 0.63, 0.58, and 0.49 W/mK, respectively. Combined with the analysis of electrical transport measurements results, the p-type BiAgOSe along a-direction shows preferable thermoelectric character with the maximum ZT value of 2.38 at 300 K. Besides, the ZT values for p-type BiAgOS and BiAgOTe at the same temperature along a-direction could reach to 1.75 and 1.90. Our results reveal that the family of oxychalcogenides BiAgOCh is potential p-type thermoelectric material. Especially, although n-type BiAgOCh cannot be a comparable thermoelectric material (ZT = 0.04-0.71), our results imply that optimum carrier concentration for n-type BiAgOS and BiAgOSe could be proved as an effective regulation strategy for relatively large ZT value. This is mainly due to the fact that ZT value has more than one extreme points from low carrier concentration (1 x 1018 cm−3) to high carrier concentration (1 x 1022 cm−3). Furthermore, this finding opens a new avenue for these oxychalcogenides compounds and significantly accelerates the process of the applications in oxides as potential high temperature thermoelectric materials.

The potential spin gapless semiconductor CoFeMnSi is one of most promising materials used in the potential spintronics devices. In this work, we realized the semiconductor-like behavior of CoFeMnSi thin film deposited on the fluorophlogopite substrate, and achieved a flexible CoFeMnSi film by exfoliating the substrate. The film grown on the fluorophlogopite substrate exhibits a partial L21 ordered structure after annealing at 300 °C. The bending tunability of the transport and magnetic properties were investigated. The semiconductor-metallic like transition can be varied through bending. The in-plane coercivity could be tuned up to 440% under a bending curvature of 1.9 mm. Interestingly, an exchange bias was observed in CoFeMnSi thin film. XPS analyses show that there exists the bonding of oxides at the interface between CoFeMnSi layer and substrate. Thus, it could be deduced that the antiferromagnetic coupling occurs along the interface, causing an exchange bias.

Graphitic carbon nitride (g-C3N4, CN) with nitrogen vacancies was synthesized by a controlled thermal etching method in a semi-closed air-conditioning system. The defect-modified g-C3N4shows an excellent photocatalytic performance demonstrated by water splitting under visible light irradiation. With proper heat-treatment durations such as 2 h (CN2) and 4 h (CN4) at 550 °C, the hydrogen production rates significantly increase to 100 µmol h-1and 72 µmol h-1, which are 11 times and 8 times the rate of the pristine CN (8.8 µmol h-1) respectively. The excellent hydrogen production performance of nitrogen defect modified CN2 is due to the synergy effect of the decreased band gap, enlarged specific surface area and increased separation/migration efficiency of photoinduced charge carriers. This simple defect engineering method provides a good paradigm to improve the photocatalytic performance by tailoring the electronic and physical structures of g-C3N4.

© 2020 World Scientific Publishing Co. Pte Ltd. All rights reserved. Sodium niobate (NaNbO3)-based dielectrics have received much attention for energy storage applications due to their low-cost, lightweight, and nontoxic nature. The field-induced metastable ferroelectric phase in NaNbO3-based dielectrics, however, leads to a large hysteresis of the polarization-electric field (P-E) loops and hence deteriorate the energy storage performance. In this study, the hysteresis was successfully reduced by introducing Bi3+and Ti4+into A-site and B-site of NaNbO3, respectively. MnO2addition was added to further increase the ceramic density and enhance the cycling reliability. As a result, a high recoverable energy density of 4.3 J/cm3and a high energy efficiency of 90% were simultaneously achieved in the ceramic capacitor at an applied electric field of 360 kV/cm. Of particular importance is that the ceramic capacitor exhibits a stable energy storage properties over a wide temperature range of -70 to 170 °C, with much improved electric cycling reliability up to 105cycles.

© 2020 Elsevier Ltd and Techna Group S.r.l. The triboelectric effect has recently demonstrated its great potential in environmental remediation and even new energy applications for triggering a number of catalytic reactions by utilizing trivial mechanical energy. In this study, Ba4Nd2Fe2Nb8O30 (BNFN) submicron powders were used to degrade organic dyes via the tribocatalytic effect. Under the frictional excitation of three PTFE stirring rods in a 5 mg/L RhB dye solution, BNFN demonstrates a high tribocatalytic degradation efficiency of 97% in 2 h. Hydroxyl radicals (•OH) and superoxide radicals (•O2－) were also detected during the catalysis process, which proves that triboelectric energy stimulates BNFN to generate electron-hole pairs. The tribocatalysis of tungsten bronze BNFN submicron powders provides a novel and efficient method for the degradation of wastewater dye by utilizing trivial mechanical energy.

© 2020 IOP Publishing Ltd. To date, a handful of topological semimetals (TMs) with multiple types of topological nodal line (TNL) states have been theoretically predicted in novel materials. However, their TNLs are often affected by many factors, such as spin-orbit coupling (SOC) effect, strain, and the extraneous bands near the band crossing points, and therefore, the TNL states cannot be easily verified by experiments. Here, by using first-principles calculations, we report that the Pnma LiBH is a potential TM with twofold and quadruple degenerate topological nodal lines. These TNLs situate very close to the Fermi level, and do not coexist with other extraneous bands. More importantly, the TNLs of this material are very robust to the effect of SOC, uniform strain, and biaxial strain. The nontrivial band structure in LiBH produces drum-head-like surface states in the (001) surface projection. Our result reveals that LiBH material is an excellent candidate to study the multiple kinds of TNLs.

Both spin gapless semiconductors (SGSs) and nodal ring states (NRSs) have aroused great scientific interest in recent years due to their unique electronic properties and high application potential. However, since their advent, all SGSs and NRSs have been predicted in independent materials. In this work, we proposed a novel type of material, nodal ring spin gapless semiconductor (NRSGS), which combines both states of the SGSs and NRSs. The synthesized material Mg2VO4 is selected as a potential candidate. Detailed band structure analysis reveals that there are gapless crossings in the spin-up direction, which are from multiple topological nodal rings located exactly at the Fermi energy level. Mg2VO4 combines the advantages inherited from both NRSs and SGSs in terms of the innumerable gapless points along multiple nodal rings with all linear dispersions and direct contacts. In addition, Mg2VO4 also shows strong robustness against both the spin orbit coupling effect and strain conditions. Therefore, for the first time, we propose the concept of an NRSGS, and the first such material candidate Mg2VO4 can immediately advance corresponding experimental measurements and even facilitate real applications.

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Designing cost-effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active-center-transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO2) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO2 nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO2 acts as the co-catalyst for HER, whereas the active center transfers to LiCoO2 and Pt turns into the co-catalyst for OER. The unique architecture of Pt/LiCoO2 heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO2 heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm−2 for HER and OER in alkaline medium, respectively.

© 2020 American Physical Society. Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

© 2020 Elsevier Ltd and Techna Group S.r.l. Micrometre-sized pure BiFeO3 (BF) and Ce-doped Bi0.9Ce0.1FeO3 (BCF) powders were synthesized by using conventional hydrothermal (H) and microwave assisted hydrothermal (MH) methods. The BF-H and BCF-H show obviously enhanced weak ferromagnetic characteristic, in which the BCF-H exhibits excellent magnetic performance, with significantly increased maximum magnetization of ∼3.6 emu/g (under 50 kOe magnetic field) and remanent magnetization of ∼0.268 emu/g at room temperature. The superior magnetic property of BCF-H could be attributed to the suppression of spiral structure and the modulation of canting angle of spins in antiferromagnetic sublattices.

© The Royal Society of Chemistry. Eco-friendly energy harvesters with high output for effectively harvesting mechanical energy over a broad temperature range are highly desirable. Here, a lead-free, flexible, and efficient hybrid energy harvester was demonstrated by the combination of a single electrode triboelectric energy harvester (TEH) and an interdigital electrode piezoelectric energy harvester (PEH), which are based on a 0-3 composite of high temperature BiFeO3-BaTiO3 (BF-BT) piezoelectric ceramic particles and polyimide (PI) matrix. With an appropriate external connection, the hybrid piezo-triboelectric energy harvester (P-TEH) generates an open-circuit voltage and a short-circuit current of 175 V and 600 nA, respectively, together with high power density of 4.1 mW cm-3. This work demonstrates that the combination of the interdigital electrode PEH with TEH can enable high electrical output due to minimum internal interference between the two energy harvesting parts. Of particular significance is that the flexible P-TEH shows great high temperature stability up to 200 °C with an open-circuit voltage of 150 V and a short-circuit current of 560 nA, exhibiting excellent potential for using at high temperature.

© 2020 The perovskite oxide interface has attracted extensive attention as a platform for achieving strong coupling between ferroelectricity and magnetism. In this work, robust control of magnetoelectric (ME) coupling in the BiFeO3/BaTiO3 (BFO/BTO) heterostructure (HS) was revealed by using the first-principles calculation. Switching of the ferroelectric polarization of BTO induce large ME effect with significant changes on the magnetic ordering and easy magnetization axis, making up for the weak ME coupling effect of single-phase multiferroic BFO. In addition, the Dzyaloshinskii-Moriya interaction (DMI) and the exchange coupling constants J for the BFO part of the HSs are simultaneously manipulated by the ferroelectric polarization, especially the DMI at the interface is significantly enhanced, which is three or four times larger than that of the individual BFO bulk. This work paves the way for designing new nanomagnetic devices based on the substantial interfacial ME effect.

© 2020 Lead-free (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) possesses comparable piezoelectric constant with lead zirconate titanate (PZT), but its poor temperature electric performances stability and low Curie temperature limit its application. Here we designed composition graded BZT-BCT films with improved temperature stability of piezoelectric, ferroelectric, and dielectric performances over a wide temperature range, and the d33 reaches 21 pm/V with hysteresis loop even at 180 °C, which is far above the Curie temperature of BZT-BCT ceramic and BZT-0.5BCT film. The excellent temperature stability is ascribed to the lattice distortion and strain gradient in the grains caused by ions diffusion, and could suppress phase transition. This work could bring forward a feasible design for dielectric/piezoelectric/ferroelectric devices operating in harsh temperature environment.

Graphitic carbon nitride (g-C3N4, CN) with nitrogen vacancies was synthesized by a controlled thermal etching method in a semi-closed air-conditioning system. The defect-modified g-C3N4shows an excellent photocatalytic performance demonstrated by water splitting under visible light irradiation. With proper heat-treatment durations such as 2 h (CN2) and 4 h (CN4) at 550 °C, the hydrogen production rates significantly increase to 100 µmol h-1and 72 µmol h-1, which are 11 times and 8 times the rate of the pristine CN (8.8 µmol h-1) respectively. The excellent hydrogen production performance of nitrogen defect modified CN2 is due to the synergy effect of the decreased band gap, enlarged specific surface area and increased separation/migration efficiency of photoinduced charge carriers. This simple defect engineering method provides a good paradigm to improve the photocatalytic performance by tailoring the electronic and physical structures of g-C3N4.

© 2020 The Royal Society of Chemistry. The concept of composite material has been increasingly applied for the significant improvement in the thermoelectric performance because of the predictable effective medium properties and the unique interfacial correlated thermal and electrical transport mechanism. Herein, we report that the graphene inclusion can lead to a significant reduction in thermal conductivity and improve the overall thermoelectric figure-of-merit in SnSe. We demonstrate a systematic investigation on the microstructures and electrical and thermoelectric properties of the SnSe/graphene composite. HRTEM reveals the uniform distribution of graphene nanosheets in the SnSe matrix, forming a sharp interface with refined SnSe grain sizes and defects nearby the interfaces. Thermal conductivity decreases with graphene addition and can significantly reduce to as low as ∼0.18 W m-1 K-1, resulting in an enhanced figure of merit (ZT) of the SnSe/graphene composite by at least 50% compared with pristine SnSe. The significant reduction in thermal conductivity is attributed to the phonon scattering by densely distributed phase interfaces as well as defects and grain boundaries. The carbon element is also believed to potentially reduce long-range tin diffusion by acting as a confinement barrier to restrict heat and ion diffusion. Our work proves that the graphene secondary phase could enhance the ZT of the SnSe matrix, which might pave the way for achieving high-performance thermoelectric properties in carbon-induced composite materials.

We study, in the absence of a magnetic field, the stability of skyrmions in a disk-shaped ferromagnetic multilayer in the presence of locally pinned spins at the edge. This particular boundary condition suppresses the formation of helical states yet enhances the stabilization of states with rotational symmetry, such as Neél skyrmions. Numerical simulation shows that the size of the disk is a viable parameter to control the creation and annihilation of skyrmionic states. We also find that the demagnetization field renormalizes the Dzyaloshinskii-Moriya interaction. This study opens an alternative avenue to the generation, stabilization, and control of magnetic skyrmions in field-free heterostructures.

Advances in smart and wearable devices are driving innovations in multifunctional flexible materials at a tremendous pace. Here, drawing support from the unique flexible fluorophlogopite mica platform, we present a promising all-inorganic bendable Mn-modified 0.65(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.35SrTiO3 (NBBST) film with dual use in electrocaloric (EC) refrigeration and energy storage via a cost-effective transfer-free process. An appreciable room-temperature EC effect with adiabatic temperature change of 12 K and isothermal entropy of 18 J K-1 kg-1 was realized in the NBBST film, which benefits from the large change in dipolar ordering near depolarization temperature. Also, the film exhibits a broad operating temperature span over 25 °C because of its relaxor feature. Most importantly, the film can maintain a high EC performance either under bending deformation at 5 mm radius or after undergoing 104 bending-unbending cycles. Meanwhile, the flexible NBBST film possesses good energy storage property with a recoverable energy density of 56 J cm-3 and an efficiency of 66%. This is the first example of a lead-free all-inorganic multifunctional film capacitor toward the flexible EC refrigeration and energy storage devices. This work shows bright prospects in the emerging flexible e-market.

2020 Science China Press Oxygen vacancy is one of the pivotal factors for tuning/creating various oxide properties. Understanding the behavior of oxygen vacancies is of paramount importance. In this study, we identify a metastable oxygen vacancy ordering state other than the well-known Magnéli phases in TiO2 crystals from both experimental and theoretical studies. The oxygen vacancy ordering is found to be a zigzag chain along the [0 0 1] direction in the (1 1 0) plane occurring in a wide temperature range of 200-500 °C. This metastable ordering state leads to a first-order phase transition accompanied by significant enhancement of dielectric permittivity and a memristive effect featuring a low driving electric field. Our results can improve oxide properties by engineering oxygen vacancies.

Sodium niobate (NaNbO3)-based lead-free ceramics have been actively studied for energy storage applications because of their antiferroelectric and/or relaxor features achieved in modified systems. The P-E loops of NaNbO3-based ceramics are usually hysteretic because of the existence of a metastable ferroelectric phase at room temperature. In this study, by introducing aliovalent cations and A-site vacancies, the relaxor characteristics are greatly enhanced in (Na1-2xBix)(Nb1-xZrx)O3 ceramics, leading to a high energy storage efficiency of above 90%. In addition, sintering aid CuO and a special ramp-to-spike sintering profile were employed to decrease the sintering temperature and reduce the grain size. The modified ceramic exhibits improved insulating properties and hence a higher breakdown strength, leading to a high recoverable energy density of 4.9 J/cm3 and a high energy efficiency of 88% at 430 kV/cm. The ceramic also exhibits satisfactory temperature stability over a wide temperature range from 25 to 125 °C and charge-discharge performance, making it a promising candidate for high-power dielectric energy storage applications.

2020 Author(s). BiFeO3, a typical multiferroic compound, owns complicated interactions among its charge, lattice, and spin degrees, while largely remaining a mystery. In this work, the BiFeO3 single crystal has been investigated in pulsed high magnetic fields up to 50 T. The lattice, spin, and charge orderings are found to couple with each other strongly. High magnetic field could drive the spin cycloid phase to a canted antiferromagnetic phase while accompanied by a clear magnetostriction response and weakening of ferroelectric polarization. The critical magnetic field for the magnetic phase transition was shifted to the lower magnetic field region by an external electric field over a broad temperature range, which indicates the complex interaction among magnetization, electrical polarization, and crystal lattice. This work has provided a clear picture to show the synergetic response of multiple ordering parameters during the magnetic phase transition in BiFeO3, which could benefit the design of BiFeO3 based spintronic devices and novel multiferroic materials with strong magnetoelectric coupling.

Due to its physical and chemical stability in addition to its transparency in the visible region with ultraviolet protective activity, ZnO can be used in sunscreen applications. In our article, Na-doped zinc oxide nanoparticles were prepared in different concentrations by sol–gel and solvothermal techniques to reduce their photocatalytic activity. The photocatalytic activity of the doped samples was suppressed effectively at a concentration of 0.03 at% Na doping up to 90% by conducting the sol–gel process rather than the solvothermal process which resulted in about 70% photocatalytic reduction in a period of time from (0–30 min) when exposed to ultraviolet and visible light. In addition, the nanoparticles resulted by sol–gel route show a reduction in photoactivity under solar simulation about 98% rather than those resulted via solvothermal process which shows a reduction around 92% for 30 min which is the same period of time used for the photocatalytic degradation test. The particle size was around 64.6–84.6 nm for samples prepared by both methods.

2020 by the authors. Due to the enhanced demand for numerous electrical energy storage applications, including applications at elevated temperatures, dielectric capacitors with optimized energy storage properties have attracted extensive attention. In this study, a series of lead-free strontium bismuth titanate based relaxor ferroelectric ceramics have been successfully synthesized by high temperature solid-state reaction. The ultrahigh recoverable energy storage density of 4.2 J/cm3 under 380 kV/cm, with the high efficiency of 88%, was obtained in the sample with x = 0.06. Of particular importance is that this ceramic composition exhibits excellent energy storage performance over a wide work temperature up to 150 °C, with strong fatigue endurance and fast discharge speed. All these merits demonstrate the studied ceramic system is a potential candidate for high-temperature capacitors as energy storage devices.

2020 Elsevier B.V. Very recently, a novel Dirac half-metal has been investigated in the perovskite-type lanthanum manganite (LaMnO3) by Ma et al. [1] and the half metallicity with 100% spin polarization is discovered. More remarkably, multiple linear Dirac crossings are observed around the Fermi energy level in the spin-up direction. Inspired by this work, another perovskite-type material YbBO3, which has been synthesized before, is investigated by first principles calculation. Results confirm the presence of half metallicity in this material with also multiple Dirac-like linear and parabolic band crossings around the Fermi level. The estimated Curie temperature is 469.4 K and the mechanic and dynamic stabilities have been verified. Besides, the elastic anisotropy is revealed with the directional dependent Young's modulus and shear modulus, together with the acoustic wave properties. The effect of the uniform strain, electron doping and spin-orbital coupling on the electronic structures have been also accessed. In particular, nearly spin-gapless semiconducting behavior with linear or parabolic energy dispersions is obtained by electron doping. Moreover, several thermodynamic properties have been examined under temperature from 0 to 500 K and pressure from 0 to 5 GPa. In combination, we expect YbBO3 can attract more research attention and even facilitate the spintronic application.

The layered oxychalcogenides BiCuOCh (Ch = S, Se, and Te) have gained extensive attention due to their low lattice thermal conductivity, moderate Seebeck coefficient, and tunable electrical conductivity. However, the thermoelectric properties of their Ag-based isomorphic compounds are still unclear. Here, by means of first‐principles calculations and Boltzmann transport theory, we identify that three materials exhibit intrinsically low lattice thermal conductivity. Because of large unit volumes and relatively low sound velocities, the lattice thermal conductivities for BiAgOS, BiAgOSe, and BiAgOTe are only 0.63, 0.58, and 0.49 W/mK, respectively. Combined with the analysis of electrical transport measurements results, the p-type BiAgOSe along a-direction shows preferable thermoelectric character with the maximum ZT value of 2.38 at 300 K. Besides, the ZT values for p-type BiAgOS and BiAgOTe at the same temperature along a-direction could reach to 1.75 and 1.90. Our results reveal that the family of oxychalcogenides BiAgOCh is potential p-type thermoelectric material. Especially, although n-type BiAgOCh cannot be a comparable thermoelectric material (ZT = 0.04-0.71), our results imply that optimum carrier concentration for n-type BiAgOS and BiAgOSe could be proved as an effective regulation strategy for relatively large ZT value. This is mainly due to the fact that ZT value has more than one extreme points from low carrier concentration (1 x 1018 cm−3) to high carrier concentration (1 x 1022 cm−3). Furthermore, this finding opens a new avenue for these oxychalcogenides compounds and significantly accelerates the process of the applications in oxides as potential high temperature thermoelectric materials.

The potential spin gapless semiconductor CoFeMnSi is one of most promising materials used in the potential spintronics devices. In this work, we realized the semiconductor-like behavior of CoFeMnSi thin film deposited on the fluorophlogopite substrate, and achieved a flexible CoFeMnSi film by exfoliating the substrate. The film grown on the fluorophlogopite substrate exhibits a partial L21 ordered structure after annealing at 300 °C. The bending tunability of the transport and magnetic properties were investigated. The semiconductor-metallic like transition can be varied through bending. The in-plane coercivity could be tuned up to 440% under a bending curvature of 1.9 mm. Interestingly, an exchange bias was observed in CoFeMnSi thin film. XPS analyses show that there exists the bonding of oxides at the interface between CoFeMnSi layer and substrate. Thus, it could be deduced that the antiferromagnetic coupling occurs along the interface, causing an exchange bias.

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Designing cost-effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active-center-transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO2) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO2 nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO2 acts as the co-catalyst for HER, whereas the active center transfers to LiCoO2 and Pt turns into the co-catalyst for OER. The unique architecture of Pt/LiCoO2 heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO2 heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm−2 for HER and OER in alkaline medium, respectively.

© 2020 Elsevier Ltd and Techna Group S.r.l. The triboelectric effect has recently demonstrated its great potential in environmental remediation and even new energy applications for triggering a number of catalytic reactions by utilizing trivial mechanical energy. In this study, Ba4Nd2Fe2Nb8O30 (BNFN) submicron powders were used to degrade organic dyes via the tribocatalytic effect. Under the frictional excitation of three PTFE stirring rods in a 5 mg/L RhB dye solution, BNFN demonstrates a high tribocatalytic degradation efficiency of 97% in 2 h. Hydroxyl radicals (•OH) and superoxide radicals (•O2－) were also detected during the catalysis process, which proves that triboelectric energy stimulates BNFN to generate electron-hole pairs. The tribocatalysis of tungsten bronze BNFN submicron powders provides a novel and efficient method for the degradation of wastewater dye by utilizing trivial mechanical energy.

© 2020 Elsevier Ltd and Techna Group S.r.l. Micrometre-sized pure BiFeO3 (BF) and Ce-doped Bi0.9Ce0.1FeO3 (BCF) powders were synthesized by using conventional hydrothermal (H) and microwave assisted hydrothermal (MH) methods. The BF-H and BCF-H show obviously enhanced weak ferromagnetic characteristic, in which the BCF-H exhibits excellent magnetic performance, with significantly increased maximum magnetization of ∼3.6 emu/g (under 50 kOe magnetic field) and remanent magnetization of ∼0.268 emu/g at room temperature. The superior magnetic property of BCF-H could be attributed to the suppression of spiral structure and the modulation of canting angle of spins in antiferromagnetic sublattices.

© 2020 American Physical Society. Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

2020 Author(s). BiFeO3, a typical multiferroic compound, owns complicated interactions among its charge, lattice, and spin degrees, while largely remaining a mystery. In this work, the BiFeO3 single crystal has been investigated in pulsed high magnetic fields up to 50 T. The lattice, spin, and charge orderings are found to couple with each other strongly. High magnetic field could drive the spin cycloid phase to a canted antiferromagnetic phase while accompanied by a clear magnetostriction response and weakening of ferroelectric polarization. The critical magnetic field for the magnetic phase transition was shifted to the lower magnetic field region by an external electric field over a broad temperature range, which indicates the complex interaction among magnetization, electrical polarization, and crystal lattice. This work has provided a clear picture to show the synergetic response of multiple ordering parameters during the magnetic phase transition in BiFeO3, which could benefit the design of BiFeO3 based spintronic devices and novel multiferroic materials with strong magnetoelectric coupling.

Advances in smart and wearable devices are driving innovations in multifunctional flexible materials at a tremendous pace. Here, drawing support from the unique flexible fluorophlogopite mica platform, we present a promising all-inorganic bendable Mn-modified 0.65(0.94Na0.5Bi0.5TiO3-0.06BaTiO3)-0.35SrTiO3 (NBBST) film with dual use in electrocaloric (EC) refrigeration and energy storage via a cost-effective transfer-free process. An appreciable room-temperature EC effect with adiabatic temperature change of 12 K and isothermal entropy of 18 J K-1 kg-1 was realized in the NBBST film, which benefits from the large change in dipolar ordering near depolarization temperature. Also, the film exhibits a broad operating temperature span over 25 °C because of its relaxor feature. Most importantly, the film can maintain a high EC performance either under bending deformation at 5 mm radius or after undergoing 104 bending-unbending cycles. Meanwhile, the flexible NBBST film possesses good energy storage property with a recoverable energy density of 56 J cm-3 and an efficiency of 66%. This is the first example of a lead-free all-inorganic multifunctional film capacitor toward the flexible EC refrigeration and energy storage devices. This work shows bright prospects in the emerging flexible e-market.