BaCu, a Two-Dimensional Electride with Cu Anions.

The electron-rich characteristic and low work function endow electrides with excellent performance in (opto)electronics and catalytic applications; these two features are closely related to the structural topology, constituents, and valence electron concentration of electrides. However, the synthesized electrides, especially two-dimensional (2D) electrides, are limited to specific structural prototypes and anionic p-block elements. Here we synthesize and identify a distinct 2D electride of BaCu with delocalized anionic electrons confined to the interlayer spaces of the BaCu framework. The bonding between Cu and Ba atoms exhibits ionic characteristics, and the adjacent Cu anions form a planar honeycomb structure with metallic Cu-Cu bonding. The negatively charged Cu ions are revealed by the theoretical calculations and experimental X-ray absorption near-edge structure. Physical property measurements reveal that BaCu electride has a high electronic conductivity (ρ = 3.20 µΩ cm) and a low work function (2.5 eV), attributed to the metallic Cu-Cu bonding and delocalized anionic electrons. In contrast to typical ionic 2D electrides with p-block anions, density functional theory calculations find that the orbital hybridization between the delocalized anionic electrons and BaCu framework leads to unique isotropic physical properties, such as mechanical properties, and work function. The freestanding BaCu monolayer with half-metal conductivity exhibits low exfoliation energy (0.84 J/m2) and high mechanical/thermal stability, suggesting the potential to achieve low-dimensional BaCu from the bulk. Our results expand the space for the structure and attributes of 2D electrides, facilitating the discovery and potential application of novel 2D electrides with transition metal anions.

Centimeter-sized diamond composites with high electrical conductivity and hardness.

Achieving high-performance materials with superior mechanical properties and electrical conductivity, especially in large-sized bulk forms, has always been the goal. However, it remains a grand challenge due to the inherent trade-off between these properties. Herein, by employing nanodiamonds as precursors, centimeter-sized diamond/graphene composites were synthesized under moderate pressure and temperature conditions (12 GPa and 1,300 to 1,500 °C), and the composites consisted of ultrafine diamond grains and few-layer graphene domains interconnected through covalently bonded interfaces. The composites exhibit a remarkable electrical conductivity of 2.0 × 104 S m-1 at room temperature, a Vickers hardness of up to ~55.8 GPa, and a toughness of 10.8 to 19.8 MPa m1/2. Theoretical calculations indicate that the transformation energy barrier for the graphitization of diamond surface is lower than that for diamond growth directly from conventional sp2 carbon materials, allowing the synthesis of such diamond composites under mild conditions. The above results pave the way for realizing large-sized diamond-based materials with ultrahigh electrical conductivity and superior mechanical properties simultaneously under moderate synthesis conditions, which will facilitate their large-scale applications in a variety of fields.

Longevous Sodium Metal Anodes with High Areal Capacity Enabled by 3D-Printed Sodiophilic Monoliths.

Sodium metal anode, featured by favorable redox voltage and material availability, offers a feasible avenue toward high-energy-density devices. However, uneven metal deposition and notorious dendrite proliferation synchronously hamper its broad application prospects. Here, a three-dimensional (3D) porous hierarchical silver/reduced graphene oxide (Ag/rGO) microlattice aerogel is devised as a sodiophilic monolith, which is realized by a direct ink writing 3D printing technology. The thus-printed Na@Ag/rGO electrode retains a durable cycling lifespan over 3100 h at 3.0 mA cm-2/1.0 mAh cm-2, concurrently harvesting a high average Coulombic efficiency of 99.80%. Impressively, it can be cycled for 340 h at a stringent condition of 6.0 mA cm-2 with a large areal capacity of 60.0 mAh cm-2 (∼1036.31 mAh g-1). Meanwhile, the well-regulated Na ion flux and uniform deposition kinetics are systematically probed by comprehensive electroanalytical analysis and theoretical simulations. As a result, assembled Na metal full battery delivers a long cycling sustainability over 500 cycles at 100 mA g-1 with a low per-cycle decay of 0.85%. The proposed strategy might inspire the construction of high-capacity Na metal anodes with appealing stability.

A pressure-induced high-pressure metallic GeTe phase.

As an important phase-change material, GeTe has many high-pressure phases as well, but its phase transitions under pressure are still lack of clarity. It is challenging to identify high-pressure GeTe crystal structures owing to the phase coexistence in a wide pressure range and the reversibility of phase transitions. Hence, first-principles calculations are required to provide further information in addition to limited experimental characterizations. In this work, a new orthorhombic Cmca GeTe high-pressure phase has been predicted via the CALYPSO method as the most energetically favorable phase in the pressure range between ∼30 and ∼38.5 GPa, which would update the GeTe high-pressure phase transition sequence. The crystal structure of the Cmca phase is composed of alternate stacking puckered layers of Ge six-membered rings and Te four-membered rings along the b direction. The high density of states near the Fermi level and delocalization of electrons from the two-dimensional electron localization function indicate a strong metallic property of the Cmca phase. Electron-phonon coupling calculations indicate that the Cmca phase is superconductive below ∼4.2 K at 35 GPa. The simulated x-ray diffraction pattern of the Cmca phase implies that this phase might coexist with the Pnma-boat phase under high pressure. These results offer further understanding on the high-pressure structural evolution and physical properties in GeTe and other IV-VI semiconductors.

Carbon Nanodots with Nearly Unity Fluorescent Efficiency Realized via Localized Excitons.

Carbon nanodots (CDs) have emerged as an alternative option for traditional nanocrystals due to their excellent optical properties and low toxicity. Nevertheless, high emission efficiency is a long-lasting pursuit for CDs. Herein, CDs with near-unity emission efficiency are prepared via atomic condensation of doped pyrrolic nitrogen, which can highly localize the excited states thus lead to the formation of bound excitons and the symmetry break of the π-electron conjugation. The short radiative lifetimes (<8 ns) and diffusion lengths (<50 nm) of the CDs imply that excitons can be efficiently localized by radiative recombination centers for a defect-insensitive emission of CDs. By incorporating the CDs into polystyrene, flexible light-converting films with a high solid-state quantum efficiency of 84% and good resistance to water, heating, and UV light are obtained. With the CD-polymer films as light conversion layers, CD-based white light-emitting diodes (WLEDs) with a luminous efficiency of 140 lm W-1 and a flat-panel illumination system with lighting sizes of more than 100 cm2 are achieved, matching state-of-the-art nanocrystal-based LEDs. These results pave the way toward carbon-based luminescent materials for solid-state lighting technology.

Regulating Na deposition by constructing a Au sodiophilic interphase on CNT modified carbon cloth for flexible sodium metal anode.

Na metal anode has attracted increasing attentions as the anode of sodium ion batteries (SIBs) due to its high theoretical capacity, low redox potential and high abundance. However, the formation of uncontrollable Na dendrite during repeated plating/stripping cycles hinders its further development and application. Herein, a sodiophilic Na metal anode host is developed by sputtering gold nanoparticles (Au NPs) into interconnected carbon nanotube modified carbon cloth (CNT/CC) to form a Au-CNT/CC architecture. Sodiophilic Au NPs effectively guide the Na metal uniform deposition and three-dimensional (3D) microporous structure offers a large surface area for nucleation and reducing the current densities. The regulated uniform Na metal deposition mechanism is investigated by the in-situ optical microscopy and simulation analysis. As a result, Au-CNT/CC electrode exhibits a low nucleation overpotential (2.2 mV) and stable cycle performance for 1600 h at 1 mA cm-2 with 2 mAh cm-2. Moreover, it even exhibits a long cycle stability for more than 800 h at 5 mA cm-2 with 2 mAh cm-2. To explore its application, a full cell coupled with a sodium vanadium phosphate coated with carbon layer (NVP@C) cathode is assembled and delivers an average discharge capacity of 80.6 mAh g-1 and coulombic efficiency of 99.6% for 400 cycles at 100 mAh g-1. Furthermore, a flexible pouch cell with Na@Au-CNT/CC as the anode is fabricated and demonstrated good flexibility and future application of wearable electronics.

Robust VS4@rGO nanocomposite as a high-capacity and long-life cathode material for aqueous zinc-ion batteries.

Although vanadium (V)-based sulfides have been investigated as cathodes for aqueous zinc-ion batteries (ZIBs), the performance improvement and the intrinsic zinc-ion (Zn2+) storage mechanism revelation is still challenging. Here, VS4@rGO composite with optimized morphology is designed and exhibits ultrahigh specific capacity (450 mA h g-1 at 0.5 A g-1) and high-rate capability (313.8 mA h g-1 at 10 A g-1) when applied as cathode material for aqueous ZIBs. Furthermore, the VS4@rGO cathode presents long-life cycling stability with capacity retention of ∼82% after 3500 cycles at 10 A g-1. The structural evolution, redox, and degradation mechanisms of VS4 during (dis)charge processes are further probed by in situ XRD/Raman techniques and TEM analysis. Our results indicate that the main energy storage mechanism is derived from the intercalation/deintercalation reactions in the open channels of VS4. Notably, an irreversible phase transition of VS4 into Zn3(OH)2V2O7·2H2O (ZVO) during the charging process and the further transition from ZVO to ZnV3O8 during long-term cycles are also observed, which might be the main reason leading to the capacity degradation of VS4@rGO. Our study further improves the electrochemical performance of VS4 in aqueous ZIBs through morphology design and provides new insights into the energy storage and performance degradation mechanisms of Zn2+ storage in VS4, and thus may endow the large-scale application of V-based sulfides for energy storage systems.

Bi and Sn Co-doping Enhanced Thermoelectric Properties of Cu3SbS4 Materials with Excellent Thermal Stability.

Cu3SbS4-based materials composed of nontoxic, low-cost, and earth-abundant elements potentially exhibit favorable thermoelectric performance. However, some key transport parameters and thermal stability have not been reported. In this work, the effects of Bi and Sn co-doping on thermoelectric properties and the thermal stability of Cu3SbS4 were studied by experiment and theoretical validation. Bi and Sn doping can effectively tune the electrical properties and the electronic band structure. The Bi and Sn doping leads to an increased carrier concentration from 6.4 × 1017 to 7.4 × 1020 cm-3 and a decreased optical band gap from 0.85 to 0.73 eV. The effective mass was increased from ∼3.0 me for Bi-doped samples to ∼4.0 me for Bi and Sn co-doped samples. An enhanced power factor of 1398 µW m-1 K-2 at 623 K was obtained for Cu3Sb1-x-yBixSnyS4 (x = 0.06, y = 0.09). The measurements of elastic properties exhibited a large Grüneisen parameter (γ ∼2) for Cu3SbS4-based materials. Finally, a maximum zT of 0.76 ± 0.02 at 623 K was achieved for Cu3Sb1-x-yBixSnyS4 (x = 0.06, y = 0.05) sample. In addition, Cu3SbS4 materials possess excellent thermal stability after thermal treatment in vacuum at 573 K for totally 500 h and dozens of heating-cooling thermal cycles (300-623-300 K). It indicates that Cu3SbS4 is a robust alternative for Te-free thermoelectric materials at an intermediate temperature range. This work provides feasible guidance to survey the thermal stability of chalcogenides.

An effective method to improve the growth rate of large single crystal diamonds under HPHT processes: optimized design of the catalyst geometric construction.

In this work, we presented the influence of catalyst geometric construction on temperature distribution, flow structure, the transport processes of the carbon atoms, and the resulting diamond growth in the process of HPHT diamond synthesis. Several catalyst geometry models were tested, and the experimental results of growth rates were compared with numerical simulations. We revealed that increasing the protrusion diameter of the convex-shaped catalysts could significantly improve the growth rate of diamonds. The diamond growth rate was improved from 1.6 mg h-1 to 4 mg h-1 when the protrusion diameter was enlarged by 2 mm. These results will be discussed through the characteristic distributions of the temperature and convection fields in the process of diamond growth.

A Porous and Conductive Graphite Nanonetwork Forming on the Surface of KCu7S4 for Energy Storage.

A flexible all-solid-state supercapacitor is fabricated by building a layer of porous and conductive nanonetwork on the surface of KCu7S4 nanowires supported on the carbon fiber fabric, where the porous and conductive nanonetwork is assembled by graphite nanoparticles. This porous graphite layer plays a key role in providing ion diffusion channels to access the KCu7S4 through the pores for electrochemical reactions and forming electron transport pathways from the graphite network to the electronic collector of the carbon fiber fabric. This flexible supercapacitor exhibits excellent electrochemical performance with high specific capacitance of 408 F g-1 at a current density of 0.5 A g-1 and high energy density of 36 Wh kg-1 at a power density of 201 W kg-1. Moreover, it is cost-effective, easy to scale up and environmentally friendly with high flexibility. Our investigation demonstrates that such a porous and conductive nanonetwork could be used to improve the charge storage efficiency for a wide range of electrode materials.