Due to their layered structure, two-dimensional Ruddlesden-Popper perovskites (RPPs), composed of multiple organic/inorganic quantum wells, can in principle be exfoliated down to few and single layers. These molecularly thin layers are expected to present unique properties with respect to the bulk counterpart, due to increased lattice deformations caused by interface strain. Here, we have synthesized centimetre-sized, pure-phase single-crystal RPP perovskites (CH(CH)NH)(CHNH)PbI (n = 1-4) from which single quantum well layers have been exfoliated. We observed a reversible shift in excitonic energies induced by laser annealing on exfoliated layers encapsulated by hexagonal boron nitride. Moreover, a highly efficient photodetector was fabricated using a molecularly thin n = 4 RPP crystal, showing a photogain of 10 and an internal quantum efficiency of ~34%. Our results suggest that, thanks to their dynamic structure, atomically thin perovskites enable an additional degree of control for the bandgap engineering of these materials.

Achieving homogeneous phase transition and uniform charge distribution is essential for good cycle stability and high capacity when phase conversion materials are used as electrodes. Herein, we show that chemical lithiation of bulk 2H-MoS distorts its crystalline domains in three primary directions to produce mosaic-like 1T' nanocrystalline domains, which improve phase and charge uniformity during subsequent electrochemical phase conversion. 1T'-LiMoS, a macroscopic dense material with interconnected nanoscale grains, shows excellent cycle stability and rate capability in a lithium rechargeable battery compared to bulk or exfoliated-restacked MoS. Transmission electron microscopy studies reveal that the interconnected MoS nanocrystals created during the phase change process are reformable even after multiple cycles of galvanostatic charging/discharging, which allows them to play important roles in the long term cycling performance of the chemically intercalated TMD materials. These studies shed light on how bulk TMDs can be processed into quasi-2D nanophase material for stable energy storage.

As a new member of the MXene group, 2D Mo C has attracted considerable interest due to its potential application as electrodes for energy storage and catalysis. The large-area synthesis of Mo C film is needed for such applications. Here, the one-step direct synthesis of 2D Mo C-on-graphene film by molten copper-catalyzed chemical vapor deposition (CVD) is reported. High-quality and uniform Mo C film in the centimeter range can be grown on graphene using a Mo-Cu alloy catalyst. Within the vertical heterostructure, graphene acts as a diffusion barrier to the phase-segregated Mo and allows nanometer-thin Mo C to be grown. Graphene-templated growth of Mo C produces well-faceted, large-sized single crystals with low defect density, as confirmed by scanning transmission electron microscopy (STEM) measurements. Due to its more efficient graphene-mediated charge-transfer kinetics, the as-grown Mo C-on-graphene heterostructure shows a much lower onset voltage for hydrogen evolution reactions as compared to Mo C-only electrodes.

Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe2) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe2 is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T‐VSe2 flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe2 surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room‐temperature ferromagnetism is confirmed on the exfoliated 2D VSe2 flakes using a superconducting quantum interference device (SQUID), X‐ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe2, although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

standards because of their high catalytic activity. But they suffer from high cost, scarcity, and moderate durability. [2] A variety of non-noble metal catalysts, such as transition metal oxides, phosphides, sulfides, and carbides, have been sought as the alternatives to noble metal catalysts. Their catalytic activities, however, are still far from satisfactory. [3] Currently, a major challenge of catalyst design is to achieve multifunctionality while maintaining high catalytic activity. [1c] Cathodic and anodic reactions usually require different catalysts and the optimal operating conditions for these catalysts differ (e.g., pH, electrolyte, and potential window). [4] Consequently, these issues unavoidably complicate the system construction, compromise the overall system performance, and increase the cost. For example, rechargeable metal-air batteries require oxygen reduction reaction (ORR) for discharging and oxygen evolution reaction (OER) for charging on cathode. [5] Pt/C is excellent for ORR, but not active for OER. On the other hand, Ru or Ir-based compounds are the benchmark electrocatalysts for OER, whereas their ORR activity is poor. Therefore, cathode has to load two types of catalysts, leading to the reduction of effective loading of each catalyst and complication of electrode fabrication. In addition, ORR (OER) catalysts may be reduced (oxidized) at OER (ORR) potentials. [6] Multifunctionality of a catalyst extends its application scope, making it more economically viable. Recently, single-atom catalysts (SACs) have attracted enormous attention due to their appealing properties, particularly including maximum atomic utilization and highly tunable electronic properties. [7] Usually, transition metal SACs are anchored onto carbon nanomaterials through coordination with nitrogen dopants. [8] Nevertheless, the strong electronegativity of N atom can undesirably alter the electronic properties of metal catalytic center and increase the free energy for adsorption of reaction intermediates. [9] To tackle this problem, secondary heteroatom dopant with relatively weak electronegativity (S and P) may be introduced to optimize the local coordination environment of active metal center. [10] For instances, S doped CuNC SACs exhibited superior ORR performance compared with undoped counterpart; [10c] P doping improves the catalytic performance of CoNC SACs for hydrogen evolution reaction (HER). [10d] Designing multifunctional catalysts with high activity, stability, and low-cost for energy storage and conversion is a significant challenge. Herein, a trifunctional electrocatalyst is synthesized by anchoring individually dispersed Co atoms on N and S codoped hollow carbon spheres (CoSA/N,S-HCS), which exhibits outstanding catalytic activity and stability for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction. When equipped in liquid or flexible solid-state rechargeable Zn-air batteries, CoSA/N,S-HCS endows them with high power and energy density as well as excellent long-term...

Mechanically stable and foldable air cathodes with exceptional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities are key components of wearable metal-air batteries. Herein, we report a directional freeze-casting and annealing approach for the construction of 3D honeycomb nanostructured, N, P-doped carbon aerogel incorporating in situ grown FeP/Fe 2 O 3 nanoparticles as the cathode for flexible Zn-air battery. The aqueous rechargeable Zn-air batteries assembled with this carbon aerogel exhibit a remarkable specific capacity of 648 mAh g -1 at current density of 20 mA cm -2 with good long-term durability, outperforming those assembled with commercial Pt/C+RuO 2 catalyst. Furthermore, such foldable carbon aerogel with directional channels can serve as freestanding air cathode for flexible solid state Zn-air battery without the use of carbon paper/cloth and additives, giving a specific capacity of 676 mA h g -1 and an energy density of 517 W

Interface confined reactions, which can modulate the bonding of reactants with catalytic centres and influence the rate of the mass transport from bulk solution, have emerged as a viable strategy for achieving highly stable and selective catalysis. Here we demonstrate that 1T′-enriched lithiated molybdenum disulfide is a highly powerful reducing agent, which can be exploited for the in-situ reduction of metal ions within the inner planes of lithiated molybdenum disulfide to form a zero valent metal-intercalated molybdenum disulfide. The confinement of platinum nanoparticles within the molybdenum disulfide layered structure leads to enhanced hydrogen evolution reaction activity and stability compared to catalysts dispersed on carbon support. In particular, the inner platinum surface is accessible to charged species like proton and metal ions, while blocking poisoning by larger sized pollutants or neutral molecules. This points a way forward for using bulk intercalated compounds for energy related applications.

oxygen evolution reaction or OER) processes. [1][2][3][4][5] To avoid the use of costly noble metal catalysts, nitrogen-doped porous carbon materials are proposed as the electrode materials in these batteries since they can be derived from naturally abundant biomass. The performance of these porous carbon materials as electrodes depends on the chemistry that results in the generation of OER-active pyridinic N and ORR-active quaternary N groups in a high density as well as the porosity of the materials. [6] This is because these factors determine the extent of exposure of the active sites to the relevant chemical species such as O 2 , OH − , and H 2 O and help prevent the rapid clogging of the planar electrode surface. [7][8][9][10] SiO 2 [8,11,12] and Al 2 O 3[13] microbeads have been employed widely as templates for generating nanopores in carbon-based catalysts. However, this requires multiple steps such as the bottom-up synthesis of the catalysts from carbon precursors as well as etching and purification processes to remove the templates, which increases the cost for mass production. [14][15][16][17][18] In addition, porous carbon materials synthesized by bottom-up methods are generally in the powder form and are thus not self-supporting. Thus, the fabrication of air electrodes requires an additional process, wherein the powder carbon materials are electrosprayed onto carbon cloth/paper, which then lead to the inevitable decreases Porous carbon electrodes have emerged as important cathode materials for metal-air battery systems. However, most approaches for fabricating porous carbon electrodes from biomass are highly energy inefficient as they require the breaking down of the biomass and its subsequent reconstitution into powder-like carbon. Here, enzymes are explored to effectively hydrolyze the partial cellulose in bulk raw wood to form a large number of nanopores, which helps to maximally expose the inner parts of the raw wood to sufficiently dope nitrogen onto the carbon skeletons during the subsequent pyrolysis process. The resulting carbons exhibit excellent catalytic activity with respect to the oxygen reduction and oxygen evolution reactions. As-fabricated cellulosedigested, carbonized wood plates are mechanically strong, have high conductivity, and contain a crosslinked network and natural ion-transport channels and can be employed directly as metal-free electrodes without carbon paper, polymer binders, or carbon black. When used as metal-free cathodes in zincair batteries, they result in a specific capacity of 801 mA h g −1 and an energy density of 955 W h kg −1 with the long-term stability of the batteries being as high as 110 h. This work paves the way for the ready conversion of abundant biomass into high-value engineering products for energy-related applications.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201900341.Rechargeable Zn-air batteries have emerged as a promising technology for coping with future energy demands owin...