Atomic-Resolution Interfacial Reaction Mechanism between Bi2Te3-Based Alloys and Ni Electrodes.

Interdiffusion and solid-solid phase reaction at the interface between thermoelectric (TE) materials and the electrode critically influence interfacial transport properties and the overall energy conversion efficiency during service. Here, the microstructural evolution and diffusion mechanisms at the interfaces between the most widely used Bi2Te3-based TE materials, n-type Bi2Te2.7Se0.3 (BTS) and p-type Bi0.5Sb1.5Te3 (BST), and Ni electrodes were investigated at atomic resolution using spherical aberration-corrected scanning transmission electron microscopy (STEM). The BTS(0001)/Ni and BST(0001)/Ni interfaces were constructed by depositing Ni nanoparticles on mechanically exfoliated BTS and BST bulk materials and subsequent annealing. The interfacial reaction is initially dominated by Ni diffusion into the TE matrix to form NiAs-type NiM intermetallics, while Ni trans-quintuple-layer diffusion only occurs in Sb-rich BST. The Bi-rich BTS is more influenced by the Ni-Te preferential reaction, resulting in NiM abnormal grain growth and the formation of tilted and rotated interfaces. Bi diffusion into the BTS matrix forms a Bi double layer at the interface or Bi2[Bi2(Te,Se)3] as the annealing temperature increases, while Bi diffusion into the Ni thin film greatly accelerates the interfacial reaction rate, as elucidated by in situ heating STEM. The results provide essential structural details to understand and prevent the degradation of TE device performance.

Single-atom Iron Catalyst with Biomimetic Active Center to Accelerate Proton Spillover for Medical-level Electrosynthesis of H2 O2 Disinfectant.

Electrosynthesis of H2 O2 has great potential for directly converting O2 into disinfectant, yet it is still a big challenge to develop effective electrocatalysts for medical-level H2 O2 production. Herein, we report the design and fabrication of electrocatalysts with biomimetic active centers, consisting of single atomic iron asymmetrically coordinated with both nitrogen and sulfur, dispersed on hierarchically porous carbon (FeSA -NS/C). The newly-developed FeSA -NS/C catalyst exhibited a high catalytic activity and selectivity for oxygen reduction to produce H2 O2 at a high current of 100 mA cm-2 with a record high H2 O2 selectivity of 90 %. An accumulated H2 O2 concentration of 5.8 wt.% is obtained for the electrocatalysis process, which is sufficient for medical disinfection. Combined theoretical calculations and experimental characterizations verified the rationally-designed catalytic active center with the atomic Fe site stabilized by three-coordinated nitrogen atoms and one-sulfur atom (Fe-N3 S-C). It was further found that the replacement of one N atom with S atom in the classical Fe-N4 -C active center could induce an asymmetric charge distribution over N atoms surrounding the Fe reactive center to accelerate proton spillover for a rapid formation of the OOH* intermediate, thus speeding up the whole reaction kinetics of oxygen reduction for H2 O2 electrosynthesis.

Structure and Performance Optimization of Co Magnetic Thin Films Deposited by Vacuum Evaporation Coating.

Co magnetic films are widely used in high-frequency magnetic recording and vertical magnetic recording due to their high saturation magnetization and magnetocrystalline anisotropy. In this work, ferromagnetic Co magnetic films were prepared on copper substrate by vacuum evaporation combined with heat treatment (H2 atmosphere), to investigate the impact of film thickness and annealing temperature on microstructure and magnetic properties. The results show that with the increase in annealing temperature, the Co thin film physical phase does not change significantly, the crystallinity increases, and the grain size increases, which is consistent with the results obtained from the SEM morphology map of the sample surface, leading to an increase in coercivity. By annealing experiments (atmospheric atmosphere) on Co magnetic films with and without an Al protective layer, as shown by scanning electron microscopy microscopic characterization results, it was verified that the Al layer can protect the inner Co layer from oxidation. As the film thickness increases from 10 to 300 nm, the magnetic properties of Co films change significantly. The saturation magnetization gradually increases from 0.89 to 5.21 emu/g, and the coercivity increases from 124.3 to 363.8 Oe. The remanence ratio of the 10 nm magnetic film is 0.82, which is much higher than the film remanence ratio of 0.46 at 50 nm. This is because when the thickness of the film is between 10 and 50 nm, the magnetic moments partially deviate from the in-plane direction, and the out-of-plane component reduces the film remanence ratio. This study shows that optimizing annealing temperature and film thickness can effectively control the structure and magnetic properties of Co magnetic films, which is of great significance for the development of the magnetic recording field.

Direct observation of cation diffusion driven surface reconstruction at van der Waals gaps.

Weak interlayer van der Waals (vdW) bonding has significant impact on the surface/interface structure, electronic properties, and transport properties of vdW layered materials. Unraveling the complex atomistic dynamics and structural evolution at vdW surfaces is therefore critical for the design and synthesis of the next-generation vdW layered materials. Here, we show that Ge/Bi cation diffusion along the vdW gap in layered GeBi2Te4 (GBT) can be directly observed using in situ heating scanning transmission electron microscopy (STEM). The cation concentration variation during diffusion was correlated with the local Te6 octahedron distortion based on a quantitative analysis of the atomic column intensity and position in time-elapsed STEM images. The in-plane cation diffusion leads to out-of-plane surface etching through complex structural evolutions involving the formation and propagation of a non-centrosymmetric GeTe2 triple layer surface reconstruction on fresh vdW surfaces, and GBT subsurface reconstruction from a septuple layer to a quintuple layer. Our results provide atomistic insight into the cation diffusion and surface reconstruction in vdW layered materials.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis.

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO2 ER) to produce multicarbon (C2+ ) feedstocks (e.g., C2 H4 ). However, the high energy barriers for CO2 activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C2 H4 selectivity of 62.5 % and an industrial-level current density of 160 mA cm-2 at a low potential of -0.72 V were achieved. Our studies revealed that the isolated NiN3 active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO2 ER to C2 H4 .

Local Spin-State Tuning of Iron Single-Atom Electrocatalyst by S-Coordinated Doping for Kinetics-Boosted Ammonia Synthesis.

The electrochemical nitrogen reduction reaction (e-NRR) is envisaged as alternative technique to the Haber-Bosch process for NH3 synthesis. However, how to develop highly active e-NRR catalysts faces daunting challenges. Herein, a viable strategy to manipulate local spin state of isolated iron sites through S-coordinated doping (FeSA -NSC) is reported. Incorporation of S in the coordination of FeSA -NSC can induce the transition of spin-polarization configuration with the formation of a medium-spin-state of Fe (t2g 6 eg 1), which is beneficial for facilitating eg electrons to penetrate the antibonding π-orbital of nitrogen. As a consequence, a record-high current density up to 10 mA cm-2 can be achieved, together with a high NH3 selectivity of ≈10% in a flow cell reactor. Both experimental and theoretical analyses indicate that the monovalent Fe(I) atomic center in the FeSA -NSC after the S doping accelerates the N2 activation and protonation in the rate-determining step of *N2 to *NNH.

Fabrication and Excellent Performances of Bismuth Telluride-Based Thermoelectric Devices.

The barrier layer between thermoelectric (TE) legs and electrodes has crucial impact on the electrothermal conversion efficiency of the TE device; however, the interfacial reaction of the Ni metal barrier layer with TE legs in traditional Bi2Te3-based devices is harmful to the device performance. Herein, a high-quality barrier layer of a Ni-based alloy has been fabricated on both n-type and p-type Bi2Te3-based TE legs by the electroplating method. The in situ XRD results indicate that the as-prepared Bi2Te3-based TE legs with a Ni-based alloy barrier layer remain stable even at 300 °C. The high-resolution high-angle annular dark field scanning transmission electron microscopy images reveal that the Ni-based alloy barrier layer has more excellent stability than that of the Ni metal barrier layer. The Bi2Te3-based TE devices with excellent structural and performance stabilities were assembled with the as-grown high-performance n-type and p-type Bi2Te3-based leg with a Ni-based alloy barrier layer, which have lower internal resistance and higher cooling and power generation performances. A maximum cooling temperature difference over 65 K and a maximum cooling capacity of 55 W were obtained for the high-performance Bi2Te3-based TE devices. This work provides a new strategy for high-temperature applications of commercial Bi2Te3-based TE devices.

Electronic Localization Derived Excellent Stability of Li Metal Anode with Ultrathin Alloy.

Lithium metal is an ideal anode for next-generation high-energy-density batteries. However, lithium dendrite growth has impeded its commercial application. Herein, fabricating Li-based ultrathin alloys with electronic localization and high surface work function via depositing Bi, Al, or Au metals on the surface of copper foil for in situ alloying with lithium is proposed. It is discovered that the electronic localization can induce self-smoothing effect of Li ions, as a result, significantly suppressing the growth of dendritic lithium. Meanwhile, the high surface work function can effectively alleviate side reactions between the electrolyte and lithium. With the as-obtained ultrathin alloys as anodes, excellent cycling performance is achieved. The half cells run stably after more than 120 cycles under high capacity of 4 mAh cm-2 . The S||Bi/Cu-Li full cell delivers a specific capacity of 736 mAh g-1 after 200 cycles. This work provides a new strategy for fabricating long-life and high-capacity lithium batteries.

Promoting CO2 Electroreduction Kinetics on Atomically Dispersed Monovalent ZnI Sites by Rationally Engineering Proton-Feeding Centers.

Electrocatalytic reduction of CO2 (CO2 RR) to value-added chemicals is of great significance for CO2 utilization, however the CO2 RR process involving multi-electron and proton transfer is greatly limited by poor selectivity and low yield. Herein, We have developed an atomically dispersed monovalent zinc catalyst anchored on nitrogenated carbon nanosheets (Zn/NC NSs). Benefiting from the unique coordination environment and atomic dispersion, the Zn/NC NSs exhibit a superior CO2 RR performance, featuring a high current density up to 50 mA cm-2 with an outstanding CO Faradaic efficiency of ≈95 %. The center ZnI atom coordinated with three N atoms and one N atom that bridges over two adjacent graphitic edges (Zn-N3+1 ) is identified as the catalytically active site. Experimental results reveal that the twisted Zn-N3+1 structure accelerates CO2 activation and protonation in the rate-determining step of *CO2 to *COOH, while theoretical calculations elucidate that atomically dispersed Zn-N3+1 moieties decrease the potential barriers for intermediate COOH* formation, promoting the proton-coupled CO2 RR kinetics and boosting the overall catalytic performance.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates.

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form SW and S2W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS2 monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS2 . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS2 monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Atomically Dispersed Zinc(I) Active Sites to Accelerate Nitrogen Reduction Kinetics for Ammonia Electrosynthesis.

Developing highly active and stable nitrogen reduction reaction (NRR) catalysts for NH3 electrosynthesis remains challenging. Herein, an unusual NRR electrocatalyst is reported with a single Zn(I) site supported on hollow porous N-doped carbon nanofibers (Zn1 N-C). The Zn1 N-C nanofibers exhibit an outstanding NRR activity with a high NH3 yield rate of ≈16.1 µg NH3 h-1 mgcat -1 at -0.3 V and Faradaic efficiency (FE) of 11.8% in alkaline media, surpassing other previously reported carbon-based NRR electrocatalysts with transition metals atomically dispersed and nitrogen coordinated (TM-Nx ) sites. 15 N2 isotope labeling experiments confirm that the feeding nitrogen gas is the only nitrogen source in the production of NH3 . Structural characterization reveals that atomically dispersed Zn(I) sites with Zn-N4 moieties are likely the active sites, and the nearby graphitic N site synergistically facilitates the NRR process. In situ attenuated total reflectance-Fourier transform infrared measurement and theoretical calculation elucidate that the formation of initial *NNH intermediate is the rate-limiting step during the NH3 production. The graphitic N atoms adjacent to the tetracoordinate Zn-N4 moieties could significantly lower the energy barrier for this step to accelerate hydrogenation kinetics duing the NRR.

Nickel particle-enabled width-controlled growth of bilayer molybdenum disulfide nanoribbons.

Transition metal dichalcogenides exhibit a variety of electronic behaviors depending on the number of layers and width. Therefore, developing facile methods for their controllable synthesis is of central importance. We found that nickel nanoparticles promote both heterogeneous nucleation of the first layer of molybdenum disulfide and simultaneously catalyzes homoepitaxial tip growth of a second layer via a vapor-liquid-solid (VLS) mechanism, resulting in bilayer nanoribbons with width controlled by the nanoparticle diameter. Simulations further confirm the VLS growth mechanism toward nanoribbons and its orders of magnitude higher growth speed compared to the conventional noncatalytic growth of flakes. Width-dependent Coulomb blockade oscillation observed in the transfer characteristics of the nanoribbons at temperatures up to 60 K evidences the value of this proposed synthesis strategy for future nanoelectronics.

Excellent Thermoelectric Performance from In Situ Reaction between Co Nanoparticles and BiSbTe Flexible Films.

Low-cost flexible thermoelectric (TE) films with excellent cooling performance are critical for the in-plane heat dissipation application based on the TE film refrigeration technology. In this work, a flexible film epoxy/Bi0.5Sb1.5Te3 is developed by the incorporation of ferromagnetic Co nanoparticles to improve the electrical transport and cooling performance. The magnetic properties and microstructures clearly indicate that part of Co nanoparticles in situ reacts with Te from Bi0.5Sb1.5Te3 to form CoTe2, as well as BiTe' antisite defects. The electric conductivity is greatly enhanced because of the increased carrier density, while a large Seebeck coefficient is well maintained because of the extra magnetic scattering. The power factor of the flexible film with 0.2 wt % Co nanoparticles reached 2.28 mW·m-1·K-2 at 300 K, increased by 34% compared to the epoxy/Bi0.5Sb1.5Te3 film. The maximum cooling temperature difference is 1.5 times higher compared with the epoxy/Bi0.5Sb1.5Te3 film. This work provides a general method to improve the electrothermal conversion performance of BiSbTe-based flexible films through in situ reaction.

Magnetically enhanced thermoelectrics: a comprehensive review.

Thermoelectric (TE) materials have great potential for waste-energyrecycling and solid-state cooling. Their conversion efficiency has attracted huge attention to the development of TE devices, and largely depends on the thermal and electrical transport properties. Magnetically enhanced thermoelectrics open up the possibility of making thermoelectricity a future leader in sustainable energy development and offer an intriguing platform for both fundamental physics and prospective applications. In this review, state-of-the-art TE materials are summarized from the magnetism point of view, via diagrams of the charges, lattices, orbits and spin degrees of freedom. Our fundamental knowledge of magnetically induced TE effects is discussed. The underlying thermo-electro-magnetic merits are discussed in terms of superparamagnetism- and magnetic-transition-enhanced electron scattering, field-dependent magnetoelectric coupling, and the magnon- and phonon-drag Seebeck effects. After these topics, we finally review several thermal-electronic and spin current-induced TE materials, highlight future possible strategies for further improvingZT, and also give a brief outline of ongoing research challenges and open questions in this nascent field.

Dimensional Control of Octahedral Tilt in SrRuO3 via Infinite-Layered Oxides.

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO3 (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO2 (SCO) layers. The infinite-layered CuO2 exhibits a structural transformation from "planar-type" to "chain-type" with reduced film thickness. Two orientations dramatically modify the polyhedral connectivity at the interface, thus altering the octahedral distortion of SRO. The local structural variation changes the spin state of Ru and orbital hybridization strength, leading to a significant change in the magnetoresistance and anomalous Hall resistivity. These findings could launch investigations into adaptive control of functionalities in quantum oxide heterostructures using oxygen coordination.

Electrochemical activation induced multi-valence variation of (NH4)2V4O9 as a high-performance cathode material for zinc-ion batteries.

In this paper, we found that (NH4)2V4O9 undergoes an electrochemical activation process in the first charging process at ∼1.4 V (vs. Zn2+/Zn), leading to a significant improvement of capacity and cycling stability. The activated vanadium oxides delivered a high specific capacity of 477 mA h g-1 at 50 mA g-1 and outstanding cycling stability with 97.7% capacity retention after 5000 cycles at 15 A g-1.

Proton Capture Strategy for Enhancing Electrochemical CO2 Reduction on Atomically Dispersed Metal-Nitrogen Active Sites*.

Electrocatalysts play a key role in accelerating the sluggish electrochemical CO2 reduction (ECR) involving multi-electron and proton transfer. We now develop a proton capture strategy by accelerating the water dissociation reaction catalyzed by transition-metal nanoparticles (NPs) adjacent to atomically dispersed and nitrogen-coordinated single nickel (Ni-Nx ) active sites to accelerate proton transfer to the latter for boosting the intermediate protonation step, and thus the whole ECR process. Aberration-corrected scanning transmission electron microscopy, X-ray absorption spectroscopy, and calculations reveal that the Ni NPs accelerate the adsorbed H (Had ) generation and transfer to the adjacent Ni-Nx sites for boosting the intermediate protonation and the overall ECR processes. This proton capture strategy is universal to design and prepare for various high-performance catalysts for diverse electrochemical reactions even beyond ECR.

Intrinsic Defects in MoS2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers.

Pulsed laser deposition (PLD) can be considered a powerful method for the growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) into van der Waals heterostructures. However, despite significant progress, the defects in 2D TMDs grown by PLD remain largely unknown and yet to be explored. Here, we combine atomic resolution images and first-principles calculations to reveal the atomic structure of defects, grains, and grain boundaries in mono- and bilayer MoS2 grown by PLD. We find that sulfur vacancies and MoS antisites are the predominant point defects in 2D MoS2. We predict that the aforementioned point defects are thermodynamically favorable under a Mo-rich/S-poor environment. The MoS2 monolayers are polycrystalline and feature nanometer size grains connected by a high density of grain boundaries. In particular, the coalescence of nanometer grains results in the formation of 180° mirror twin boundaries consisting of distinct 4- and 8-membered rings. We show that PLD synthesis of bilayer MoS2 results in various structural symmetries, including AA' and AB, but also turbostratic with characteristic moiré patterns. Moreover, we report on the experimental demonstration of an electron beam-driven transition between the AB and AA' stacking orientations in bilayer MoS2. These results provide a detailed insight into the atomic structure of monolayer MoS2 and the role of the grain boundaries on the growth of bilayer MoS2, which has importance for future applications in optoelectronics.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites.

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO3 (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO2 (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO6 octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 µB /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.