High‐Lithium‐Affinity Chemically Exfoliated 2D Covalent Organic Frameworks

Covalent organic frameworks (COFs) with reversible redox behaviors are potential electrode materials for lithium‐ion batteries (LIBs). However, the sluggish lithium diffusion kinetics, poor electronic conductivity, low reversible capacities, and poor rate performance for most reported COF materials limit their further application. Herein, a new 2D COF (TFPB‐COF) with six unsaturated benzene rings per repeating unit and ordered mesoporous pores (≈2.1 nm) is designed. A chemical stripping strategy is developed to obtain exfoliated few‐layered COF nanosheets (E‐TFPB‐COF), whose restacking is prevented by the in situ formed MnO₂ nanoparticles. Compared with the bulk TFPB‐COF, the exfoliated TFPB‐COF exhibits new active Li‐storage sites associated with conjugated aromatic π electrons by facilitating faster ion/electron kinetics. The E‐TFPB‐COF/MnO₂ and E‐TFPB‐COF electrodes exhibit large reversible capacities of 1359 and 968 mAh g^?1 after 300 cycles with good high‐rate capability.     

High‐Throughput Continuous Production of Shear‐Exfoliated 2D Layered Materials using Compressible Flows

2D nanomaterials are finding numerous applications in next‐generation electronics, consumer goods, energy generation and storage, and healthcare. The rapid rise of utility and applications for 2D nanomaterials necessitates developing means for their mass production. This study details a new compressible flow exfoliation method for producing 2D nanomaterials using a multiphase flow of 2D layered materials suspended in a high‐pressure gas undergoing expansion. The expanded gas–solid mixture is sprayed in a suitable solvent, where a significant portion (up to 10% yield) of the initial hexagonal boron nitride material is found to be exfoliated with a mean thickness of 4.2 nm. The exfoliation is attributed to the high shear rates ( > 10⁵ s^?1) generated by supersonic flow of compressible gases inside narrow orifices and converging‐diverging channels. This method has significant advantages over current 2D material exfoliation methods, such as chemical intercalation and exfoliation, as well as liquid phase shear exfoliation, with the most obvious benefit being the fast, continuous nature of the process. Other advantages include environmentally friendly processing, reduced occurrence of defects, and the versatility to be applied to any 2D layered material using any gaseous medium. Scaling this process to industrial production has a strong possibility of reducing the cost of creating 2D nanomaterials.     

Metallic Graphene‐Like VSe2 Ultrathin Nanosheets: Superior Potassium‐Ion Storage and Their Working Mechanism

Potassium‐ion batteries (KIBs) are receiving increasing interest in grid‐scale energy storage owing to the earth abundant and low cost of potassium resources. However, their development still stays at the infancy stage due to the lack of suitable electrode materials with reversible depotassiation/potassiation behavior, resulting in poor rate performance, low capacity, and cycling stability. Herein, the first example of synthesizing single‐crystalline metallic graphene‐like VSe₂ nanosheets for greatly boosting the performance of KIBs in term of capacity, rate capability, and cycling stability is reported. Benefiting from the unique 2D nanostructure, high electron/K⁺‐ion conductivity, and outstanding pseudocapacitance effects, ultrathin VSe₂ nanosheets show a very high reversible capacity of 366 mAh g^?1 at 100 mA g^?1, a high rate capability of 169 mAh g^?1 at 2000 mA g^?1, and a very low decay of 0.025% per cycle over 500 cycles, which are the best in all the reported anode materials in KIBs. The first‐principles calculations reveal that VSe₂ nanosheets have large adsorption energy and low diffusion barriers for the intercalation of K⁺‐ion. Ex situ X‐ray diffraction analysis indicates that VSe₂ nanosheets undertake a reversible phase evolution by initially proceeding with the K⁺‐ion insertion within VSe₂ layers, followed by the conversion reaction mechanism.     

Exfoliated Mesoporous 2D Covalent Organic Frameworks for High-Rate Electrochemical Double-Layer Capacitors

The electrochemical double-layer capacitors (EDLCs) are highly demanded electrical energy storage devices due to their high power density with thousands of cycle life compared with pseudocapacitors and batteries. Herein, a series of capacitor cells composed of exfoliated mesoporous 2D covalent organic frameworks (e-COFs) that are able to perform excellent double-layer charge storage is reported. The selected mesoporous 2D COFs possess eclipsed AA layer-stacking mode with 3.4 nm square-like open channels, favorable BET surface areas (up to 1170 m² g⁻¹), and high thermal and chemical stabilities. The COFs via the facile, scalable, and mild chemical exfoliation method are further exfoliated to produce thin-layer structure with average thickness of about 22 nm. The e-COF-based capacitor cells achieve high areal capacitance (5.46 mF cm⁻² at 1,000 mV s⁻¹), high gravimetric power (55 kW kg⁻¹), and relatively low τ₀ value (121 ms). More importantly, they perform nearly an ideal DL charge storage at high charge–discharge rate (up to 30 000 mV s⁻¹) and maintain almost 100% capacitance stability even after 10 000 cycles. This study thus provides insights into the potential utilization of COF materials for EDLCs.     

Ultrathin VSe2 Nanosheets with Fast Ion Diffusion and Robust Structural Stability for Rechargeable Zinc‐Ion Battery Cathode

The realizing of high‐performance rechargeable aqueous zinc‐ion batteries (ZIBs) with high energy density and long cycling life is promising but still challenging due to the lack of suitable layered cathode materials. The work reports the excellent zinc‐ion storage performance as‐observed in few‐layered ultrathin VSe₂ nanosheets with a two‐step Zn²⁺ intercalation/de‐intercalation mechanism verified by ex situ X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) characterizations. The VSe₂ nanosheets exhibit a discharge plateau at 1.0–0.7 V, a specific capacity of 131.8 mAh g^?1 (at 0.1 A g^?1), and a high energy density of 107.3 Wh kg^?1 (at a power density of 81.2 W kg^?1). More importantly, outstanding cycle stability (capacity retention of 80.8% after 500 cycles) without any activation process is achieved. Such a prominent cyclic stability should be attributed to its fast Zn²⁺ diffusion kinetics (D_Zn²⁺ ≈ 10^?8 cm^?2 s^?1) and robust structural/crystalline stability. Density functional theory (DFT) calculation further reveals a strong metallic characteristic and optimal zinc‐ion diffusion pathway with a hopping energy barrier of 0.91 eV. The present finding implies that 2D ultrathin VSe₂ is a very promising cathode material in ZIBs with remarkable battery performance superior to other layered transitional metal dichalcogenides.     

Chemically Tuned p‐ and n‐Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post‐silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom‐thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p‐ and n‐type semiconductors is essential for various device applications, such as complementary metal‐oxide‐semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4‐nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field‐effect transistors (FETs) to p‐ and n‐type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V^?1s^?1 for holes and electrons, respectively, which are much higher than those of the pristine WSe₂. The doping effects are studied by photoluminescence, Raman, X‐ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption ( ≈ 0.17 nW). Furthermore, a p‐n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC‐based advanced electronics.     

Parallel Aluminum-Cobalt Oxide Nanosheet Arrays with High-Temperature Ferromagnetism

Parallel nanomaterials possess unique properties and show potential applications in industry. Whereas, vertically aligned 2D nanomaterials have plane orientations that are generally chaotic. Simultaneous control of their growth direction and spatial orientation for parallel nanosheets remains a big challenge. Here, a facile preparation of vertically aligned parallel nanosheet arrays of aluminum-cobalt oxide is reported via a collaborative dealloying and hydrothermal method. The parallel growth of nanosheets is attributed to the lattice-matching among the nanosheets, the buffer layer, and the substrate, which is verified by a careful transmission electron microscopy study. Furthermore, the aluminum-cobalt oxide nanosheets exhibit high-temperature ferromagnetism with a 919 K Curie temperature and a 5.22 emu g⁻¹ saturation magnetization at 300 K, implying the potential applications in high-temperature ferromagnetic fields.     

Intrinsic Van Der Waals Magnetic Materials from Bulk to the 2D Limit: New Frontiers of Spintronics

2D van der Waals (vdW) magnets, which present intrinsic ferromagnetic/antiferromagnetic ground states at finite temperatures down to atomic‐layer thicknesses, open a new horizon in materials science and enable the potential development of new spin‐related applications. The layered structure of vdW magnets facilitates their atomic‐layer cleavability and magnetic anisotropy, which counteracts spin fluctuations, thereby providing an ideal platform for theoretically and experimentally exploring magnetic phase transitions in the 2D limit. With reduced dimensions, the susceptibility of 2D magnets to a large variety of external stimuli also makes them more promising than their bulk counterpart in various device applications. Here, the current status of characterization and tuning of the magnetic properties of 2D vdW magnets, particularly the atomic‐layer thickness, is presented. Various state‐of‐the‐art optical and electrical techniques have been applied to reveal the magnetic states of 2D vdW magnets. Other emerging 2D vdW magnets and future perspectives on the stacking strategy are also given; it is believed that they will excite more intensive research and provide unprecedented opportunities in the field of spintronics.     

Wafer‐Scale and Low‐Temperature Growth of 1T‐WS2 Film for Efficient and Stable Hydrogen Evolution Reaction

The metallic 1T phase of WS₂ (1T‐WS₂), which boosts the charge transfer between the electron source and active edge sites, can be used as an efficient electrocatalyst for the hydrogen evolution reaction (HER). As the semiconductor 2H phase of WS₂ (2H‐WS₂) is inherently stable, methods for synthesizing 1T‐WS₂ are limited and complicated. Herein, a uniform wafer‐scale 1T‐WS₂ film is prepared using a plasma‐enhanced chemical vapor deposition (PE‐CVD) system. The growth temperature is maintained at 150 °C enabling the direct synthesis of 1T‐WS₂ films on both rigid dielectric and flexible polymer substrates. Both the crystallinity and number of layers of the as‐grown 1T‐WS₂ are verified by various spectroscopic and microscopic analyses. A distorted 1T structure with a 2a₀ × a₀ superlattice is observed using scanning transmission electron microscopy. An electrochemical analysis of the 1T‐WS₂ film demonstrates its similar catalytic activity and high durability as compared to those of previously reported untreated and planar 1T‐WS₂ films synthesized with CVD and hydrothermal methods. The 1T‐WS₂ does not transform to stable 2H‐WS₂, even after a 700 h exposure to harsh catalytic conditions and 1000 cycles of HERs. This synthetic strategy can provide a facile method to synthesize uniform 1T‐phase 2D materials for electrocatalysis applications.     

Self‐Powered Chemical Sensing Driven by Graphene‐Based Photovoltaic Heterojunctions with Chemically Tunable Built‐In Potentials

Ultralow power chemical sensing is essential toward realizing the Internet of Things. However, electrically driven sensors must consume power to generate an electrical readout. Here, a different class of self‐powered chemical sensing platform based on unconventional photovoltaic heterojunctions consisting of a top graphene (Gr) layer in contact with underlying photoactive semiconductors including bulk silicon and layered transition metal dichalcogenides is proposed. Owing to the chemically tunable electrochemical potential of Gr, the built‐in potential at the junction is effectively modulated by absorbed gas molecules in a predictable manner depending on their redox characteristics. Such ability distinctive from bulk photovoltaic counterparts enables photovoltaic‐driven chemical sensing without electric power consumption. Furthermore, it is demonstrated that the hydrogen (H₂) sensing properties are independent of the light intensity, but sensitive to the gas concentration down to the 1 ppm level at room temperature. These results present an innovative strategy to realize extremely energy‐efficient sensors, providing an important advancement for future ubiquitous sensing.