Unraveling and leveraging in situ surface amorphization for enhanced hydrogen evolution reaction in alkaline media.

Surface amorphization provides electrocatalysts with more active sites and flexibility. However, there is still a lack of experimental observations and mechanistic explanations for the in situ amorphization process and its crucial role. Herein, we propose the concept that by in situ reconstructed amorphous surface, metal phosphorus trichalcogenides could intrinsically offer better catalytic performance for the alkaline hydrogen production. Trace Ru (0.81 wt.%) is doped into NiPS3 nanosheets for alkaline hydrogen production. Using in situ electrochemical transmission electron microscopy technique, we confirmed the amorphization process occurred on the edges of NiPS3 is critical for achieving superior activity. Comprehensive characterizations and theoretical calculations reveal Ru primarily stabilized at edges of NiPS3 through in situ formed amorphous layer containing bridging S22- species, which can effectively reduce the reaction energy barrier. This work emphasizes the critical role of in situ formed active layer and suggests its potential for optimizing catalytic activities of electrocatalysts.

Salt-Induced High-Density Vacancy-Rich 2D MoS2 for Efficient Hydrogen Evolution.

Emerging non-noble metal 2D catalysts, such as molybdenum disulfide (MoS2 ), hold great promise in hydrogen evolution reactions. The sulfur vacancy is recognized as a key defect type that can activate the inert basal plane to improve the catalytic performance. Unfortunately, the method of introducing sulfur vacancies is limited and requires costly post-treatment processes. Here, a novel salt-assisted chemical vapor deposition (CVD) method is demonstrated for synthesizing ultrahigh-density vacancy-rich 2H-MoS2 , with a controllable sulfur vacancy density of up to 3.35 × 1014  cm-2 . This approach involves a pre-sprayed potassium chloridepromoter on the growth substrate. The generation of such defects is closely related to ion adsorption in the growth process, the unstable MoS2 -K-H2 O triggers the formation of sulfur vacancies during the subsequent transfer process, and it is more controllable and nondestructive when compared to traditional post-treatment methods. The vacancy-rich monolayer MoS2 exhibits exceptional catalytic activity based on the microcell measurements, with an overpotential of ≈158.8 mV (100 mA cm-2 ) and a Tafel slope of 54.3 mV dec-1 in 0.5 m H2 SO4 electrolyte. These results indicate a promising opportunity for modulating sulfur vacancy defects in MoS2 using salt-assisted CVD growth. This approach represents a significant leap toward achieving better control over the catalytic performances of 2D materials.

Ultrahigh Lubricity between Two-Dimensional Ice and Two-Dimensional Atomic Layers.

Low temperature and high humidity conditions significantly degrade the performance of solid-state lubricants consisting of van der Waals (vdW) atomic layers, owing to the liquid water layer attached/intercalated to the vdW layers, which greatly enhances the interlayer friction. However, using low temperature in situ atomic force microscopy (AFM) and friction force microscopy (FFM), we unveil the unexpected ultralow friction between two-dimensional (2D) ice, a solid phase of water confined to the 2D space, and the 2D molybdenum disulfides (MoS2). The friction of MoS2 and 2D ice is reduced by more than 30% as compared to bare MoS2 and the rigid surface. The phase transition of liquid water into 2D ice under mechanical compression has also been observed. These new findings can be applied as novel frictionless water/ice transport technology in nanofluidic systems and promising high performance lubricants for operating in low temperature and high humidity environments.

Controlled Adhesion of Ice-Toward Ultraclean 2D Materials.

The scalable 2D device fabrication and integration demand either the large-area synthesis or the post-synthesis transfer of 2D layers. While the direct synthesis of 2D materials on most targeted surfaces remains challenging, the transfer approach from the growth substrate onto the targeted surfaces offers an alternative pathway for applications and integrations. However, the current transfer techniques for 2D materials predominantly involve polymers and organic solvents, which are liable to contaminate or deform the ultrasensitive atomic layers. Here, novel ice-aided transfer and ice-stamp transfer methods are developed, in which water (ice) is the only medium in the entire process. In practice, the adhesion between various 2D materials and ice can be well controlled by temperature. Through such controlled adhesion of ice, it is shown that the new transfer methods can yield ultrahigh quality and exceptional cleanliness in transferred 2D flakes and continuous 2D films, and are applicable for a wide range of substrates. Furthermore, beyond transfer, ice can also be used for cleaning the surfaces of 2D materials at higher temperatures. These novel techniques can enable unprecedented ultraclean 2D materials surfaces and performances, and will contribute to the upcoming technological revolutions associated with 2D materials.

Direct Visualization of Atomic Structure in Multivariate Metal-Organic Frameworks (MOFs) for Guiding Electrocatalysts Design.

The direct utilization of metal-organic frameworks (MOFs) for electrocatalytic oxygen evolution reaction (OER) has attracted increasing interests. Herein, we employ the low-dose integrated differential phase contrast-scanning transmission electron microscopy (iDPC-STEM) technique to visualize the atomic structure of multivariate MOFs (MTV-MOFs) for guiding the structural design of bulk MOFs for efficient OER. The iDPC-STEM images revealed that incorporating Fe3+ or 2-aminoterephthalate (ATA) into Ni-BDC (BDC: benzenedicarboxylate) can introduce inhomogeneous lattice strain that weaken the coordination bonds, which can be selectively cleaved via a mild heat treatment to simultaneously generate coordinatively unsaturated metal sites, conductive Ni@C and hierarchical porous structure. Thus, excellent OER activity with current densities of 10 and 100 mA cm-2 are achieved over the defective MOFs at small overpotentials of 286 mV and 365 mV, respectively, which is superior to the commercial RuO2 catalyst and most of the bulk MOFs.

Metal-Organic Frameworks Offering Tunable Binary Active Sites toward Highly Efficient Urea Oxidation Electrolysis.

Electrocatalytic urea oxidation reaction (UOR) is regarded as an effective yet challenging approach for the degradation of urea in wastewater into harmless N2 and CO2. To overcome the sluggish kinetics, catalytically active sites should be rationally designed to maneuver the multiple key steps of intermediate adsorption and desorption. Herein, we demonstrate that metal-organic frameworks (MOFs) can provide an ideal platform for tailoring binary active sites to facilitate the rate-determining steps, achieving remarkable electrocatalytic activity toward UOR. Specifically, the MOF (namely, NiMn0.14-BDC) based on Ni/Mn sites and terephthalic acid (BDC) ligands exhibits a low voltage of 1.317 V to deliver a current density of 10 mA cm-2. As a result, a high turnover frequency (TOF) of 0.15 s-1 is achieved at a voltage of 1.4 V, which enables a urea degradation rate of 81.87% in 0.33 M urea solution. The combination of experimental characterization with theoretical calculation reveals that the Ni and Mn sites play synergistic roles in maneuvering the evolution of urea molecules and key reaction intermediates during the UOR, while the binary Ni/Mn sites in MOF offer the tunability for electronic structure and d-band center impacting on the intermediate evolution. This work provides important insights into active site design by leveraging MOF platform and represents a solid step toward highly efficient UOR with MOF-based electrocatalysts.

Anomalous fracture in two-dimensional rhenium disulfide.

Low-dimensional materials usually exhibit mechanical properties from those of their bulk counterparts. Here, we show in two-dimensional (2D) rhenium disulfide (ReS2) that the fracture processes are dominated by a variety of previously unidentified phenomena, which are not present in bulk materials. Through direct transmission electron microscopy observations at the atomic scale, the structures close to the brittle crack tip zones are clearly revealed. Notably, the lattice reconstructions initiated at the postcrack edges can impose additional strain on the crack tips, modifying the fracture toughness of this material. Moreover, the monatomic thickness allows the restacking of postcrack edges in the shear strain-dominated cracks, which is potentially useful for the rational design of 2D stacking contacts in atomic width. Our studies provide critical insights into the atomistic processes of fracture and unveil the origin of the brittleness in the 2D materials.

The Mobile and Pinned Grain Boundaries in 2D Monoclinic Rhenium Disulfide.

In bulk crystals, the kinetics of dislocations is usually hindered by the twining boundaries (TB) or grain boundaries (GB), rendering the well-known grain boundary strengthening effects. Nevertheless, here it is found that in 2D rhenium disulfide (ReS2), twinning is much easier than dislocation slip. Consequently, the highly mobile TBs or GBs are inversely pinned by the relatively immobile dislocations. Due to the strong in-plane covalent bonding, the GBs in high-symmetry 2D materials such as graphene which consists of defects are immobile at room temperature. In contrast, in monoclinic 2D ReS2 several types of GBs (including TBs) can be readily generated and driven by mechanical loading. A complete library of the GBs in 2D ReS2 is established by the (in situ) atomic-scale transmission electron microscopy (TEM) characterizations and density functional theory (DFT) calculations. The twinning (shear) stresses for 2D ReS2 are estimated as low as 4-30 MPa, one or two orders of magnitude lower than the traditional bulk materials. Full elucidation on the GB structures and especially the intriguing GB kinetics in such anisotropic 2D materials are of fundamental importance to understand the structure-property relationships and develop strain-tunable applications for 2D materials in future.

Site-specific electrical contacts with the two-dimensional materials.

Electrical contact is an essential issue for all devices. Although the contacts of the emergent two-dimensional materials have been extensively investigated, it is still challenging to produce excellent contacts. The face and edge type contacts have been applied previously, however a comparative study on the site-specific contact performances is lacking. Here we report an in situ transmission electron microscopy study on the contact properties with a series of 2D materials. By manipulating the contact configurations in real time, it is confirmed that, for 2D semiconductors the vdW type face contacts exhibit superior conductivity compared with the non-vdW type contacts. The direct quantum tunneling across the vdW bonded interfaces are virtually more favorable than the Fowler-Nordheim tunneling across chemically bonded interfaces for contacts. Meanwhile, remarkable area, thickness, geometry, and defect site dependences are revealed. Our work sheds light on the significance of contact engineering for 2D materials in future applications.

Critical Stable Length in Wrinkles of Two-Dimensional Materials.

The emergent two-dimensional (2D) materials are atomically thin and ultraflexible, promising for a variety of miniaturized, high-performance, and flexible devices in applications. On one hand, the ultrahigh flexibility causes problems: the prevalent wrinkles in 2D materials may undermine the ideal properties and create barriers in fabrication, processing, and quality control of materials. On the other hand, in some cases the wrinkles are used for the architecturing of surface texture and the modulation of physical/chemical properties. Therefore, a thorough understanding of the mechanism and stability of wrinkles is highly needed. Herein, we report a critical length for stabilizing the wrinkles in 2D materials, observed in the wrinkling and wrinkle elimination processes upon thermal annealing as well as by our in situ TEM manipulations on individual wrinkles, which directly capture the evolving wrinkles with variable lengths. The experiments, mechanical modeling, and self-consistent charge density functional tight binding (SCC-DFTB) simulations reveal that a minimum critical length is required for stabilizing the wrinkles in 2D materials. Wrinkles with lengths below a critical value are unstable and removable by thermal annealing, while wrinkles with lengths above a critical value are self-stabilized by van der Waals interactions. It additionally confirms the pronounced frictional effects in wrinkles with lengths above critical value during dynamical movement or sliding.

In Situ Scanning Transmission Electron Microscopy Observations of Fracture at the Atomic Scale.

The formation, propagation, and structure of nanoscale cracks determine the failure mechanics of engineered materials. Herein, we have captured, with atomic resolution and in real time, unit cell-by-unit cell lattice-trapped cracking in two-dimensional (2D) rhenium disulfide (ReS_{2}) using in situ aberration corrected scanning transmission electron microscopy (STEM). Our real time observations of atomic configurations and corresponding strain fields in propagating cracks directly reveal the atomistic fracture mechanisms. The entirely brittle fracture with non-blunted crack tips as well as perfect healing of cracks have been observed. The mode I fracture toughness of 2D ReS_{2} is measured. Our experiments have bridged the linear elastic deformation zone and the ultimate nm-sized nonlinear deformation zone inside the crack tip. The dynamics of fracture has been explained by the atomic lattice trapping model. The direct visualization on the strain field in the ongoing crack tips and the gained insights of discrete bond breaking or healing in cracks will facilitate deeper insights into how atoms are able to withstand exceptionally large strains at the crack tips.

Facile Doping in Two-Dimensional Transition-Metal Dichalcogenides by UV Light.

Two-dimensional (2D) materials have been emerging as potential candidates for the next-generation materials in various technology fields. The performance of the devices based on these 2D materials depends on their intrinsic band structures as well as the extrinsic (doping) effects such as surrounding chemicals and environmental oxygen/moisture, which strongly determines their Fermi energy level. Herein, we report the UV treatments on the 2D transition-metal dichalcogenides, to controllably dope the samples without damaging the crystal structures or quenching the luminescence properties. More surprisingly, both n-type and p-type doping can be achieved depending on the initial status of the sample and the UV treatment conditions. The doping mechanisms were elaborated on the atomic scale with transmission electron microscopy and ab initio calculations. The facile doping by UV light has potential to be integrated with photolithography processes, aiming for the large-scale integrated device/circuits design and fabrications.

Combinatorial selection of a two-dimensional 3d-TM-tetracyanoquinodimethane (TM-TCNQ) monolayer as a high-activity nanocatalyst for CO oxidation.

The CO oxidation reaction on single 3d-transition metal catalytic sites in experimentally realized tetracyanoquinodimethane (TM-TCNQ) monolayers (TM = Sc-Zn) is systematically investigated by means of first-principles calculations. Considering the stabilities, adsorption characteristics and thermodynamics of all the ten candidates (Sc-Zn), Sc-TCNQ is found to display the lowest activation energies and yield the highest catalytic activity for room temperature CO oxidation. Exploring the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms, we find that the rate-limiting step of CO oxidation catalyzed by Sc-TCNQ (CO + O2* → OOCO*) can follow the LH mechanism with free energy barriers as low as 0.73 eV at 300 K. The second step of CO + O* → CO2 can occur with rather small energy barriers via either LH or ER mechanisms. The high activity of Sc-TCNQ can be attributed to its unique structural and electronic features by possessing high stability, optimum adsorption energies with adsorbates, and fast reaction kinetics. These results have significant implications for the synthesis of two-dimensional single atom catalysis for CO oxidation with low-cost and high activity at low temperature.

Interplay of spin-dependent delocalization and magnetic anisotropy in the ground and excited states of [Gd2@C78]- and [Gd2@C80].

The magnetic properties and electronic structure of the ground and excited states of two recently characterized endohedral metallo-fullerenes, [Gd2@C78]- (1) and [Gd2@C80]- (2), have been studied by theoretical methods. The systems can be considered as [Gd2]5+ dimers encapsulated in a fullerene cage with the fifteen unpaired electrons ferromagnetically coupled into an S = 15/2 high-spin configuration in the ground state. The microscopic mechanisms governing the Gd-Gd interactions leading to the ferromagnetic ground state are examined by a combination of density functional and ab initio calculations and the full energy spectrum of the ground and lowest excited states is constructed by means of ab initio model Hamiltonians. The ground state is characterized by strong electron delocalization bordering on a σ type one-electron covalent bond and minor zero-field splitting (ZFS) that is successfully described as a second order spin-orbit coupling effect. We have shown that the observed ferromagnetic interaction originates from Hund's rule coupling and not from the conventional double exchange mechanism. The calculated ZFS parameters of 1 and 2 in their optimized geometries are in qualitative agreement with experimental EPR results. The higher excited states display less electron delocalization, but at the same time they possess unquenched first-order angular momentum. This leads to strong spin-orbit coupling and highly anisotropic energy spectrum. The analysis of the excited states presented here constitutes the first detailed study of the effects of spin-dependent delocalization in the presence of first order orbital angular momentum and the obtained results can be applied to other mixed valence lanthanide systems.

Mononuclear Clusterfullerene Single-Molecule Magnet Containing Strained Fused-Pentagons Stabilized by a Nearly Linear Metal Cyanide Cluster.

Fused-pentagons results in an increase of local steric strain according to the isolated pentagon rule (IPR), and for all reported non-IPR clusterfullerenes multiple (two or three) metals are required to stabilize the strained fused-pentagons, making it difficult to access the single-atom properties. Herein, we report the syntheses and isolations of novel non-IPR mononuclear clusterfullerenes MNC@C76 (M=Tb, Y), in which one pair of strained fused-pentagon is stabilized by a mononuclear cluster. The molecular structures of MNC@C76 (M=Tb, Y) were determined unambiguously by single-crystal X-ray diffraction, featuring a non-IPR C2v (19138)-C76 cage entrapping a nearly linear MNC cluster, which is remarkably different from the triangular MNC cluster within the reported analogous clusterfullerenes based on IPR-obeying C82 cages. The TbNC@C76 molecule is found to be a field-induced single-molecule magnet (SMM).

Hyperdislocations in van der Waals Layered Materials.

Dislocations are one-dimensional line defects in three-dimensional crystals or periodic structures. It is common that the dislocation networks made of interactive dislocations be generated during plastic deformation. In van der Waals layered materials, the highly anisotropic nature facilitates the formation of such dislocation networks, which is critical for the friction or exfoliation behavior for these materials. By transmission electron microscopy analysis, we found the topological defects in such dislocation networks can be perfectly rationalized in the framework of traditional dislocation theory, which we applied the name "hyperdislocations". Due to the strong pinning effect of hyperdislocations, the state of exfoliation can be easily triggered by 1° twisting between two layers, which also explains the origin of disregistry and frictionlessness for all of the superlubricants that are widely used for friction reduction and wear protection.

Triangular Monometallic Cyanide Cluster Entrapped in Carbon Cage with Geometry-Dependent Molecular Magnetism.

Clusterfullerenes are capable of entrapping a variety of metal clusters within carbon cage, for which the entrapped metal cluster generally keeps its geometric structure (e.g., bond distance and angle) upon changing the isomeric structure of fullerene cage, and whether the properties of the entrapped metal cluster is geometry-dependent remains unclear. Herein we report an unusual triangular monometallic cluster entrapped in fullerene cage by isolating several novel terbium cyanide clusterfullerenes (TbNC@C82) with different cage isomeric structures. Upon varying the isomeric structure of C82 cage from C2(5) to Cs(6) and to C2v(9), the entrapped triangular TbNC cluster exhibits significant distortions as evidenced by the changes of Tb-C(N) and C-N bond distances and variation of the Tb-C(N)-N(C) angle by up to 20°, revealing that the geometric structure of the entrapped triangular TbNC cluster is variable. All three TbNC@C82 molecules are found to be single-ion magnets, and the change of the geometric structure of TbNC cluster directly leads to the alternation of the magnetic relaxation time of the corresponding TbNC@C82 clusterfullerene.

Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 and Sc2 TiC2 @Ih -C80 : Metal Size Tuning of the Ti(IV) /Ti(III) Redox Potentials.

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal Sc-Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH4 and Ti/CH4 systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH4 system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH4 system are Sc4 C2 @C80 (the most abundant EMF from this synthesis), Sc3 C2 @C80 , isomers of Sc2 C2 @C82 , and the family Sc2 C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3 CH@C80 . The Sc-Ti/CH4 system produces the mixed-metal Sc2 TiC@C2 n (2 n=68, 78, 80) and Sc2 TiC2 @C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 , and Sc2 TiC2 @Ih -C80 , were characterized by NMR spectroscopy. The structure of Sc2 TiC@Ih -C80 was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc2 TiC- and Sc2 TiC2 -containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redox-active Ti ion inside the fullerene cage enables analysis of the cluster-cage strain in the endohedral fullerenes through electrochemical measurements.

Toward high permeability, selectivity and controllability of water desalination with FePc nanopores.

Nanoporous materials exhibit promising potential in water transportation applications, especially in ocean water desalination. It is highly desired to have great permeability, selectivity and controllability in the desalination performance of these nanopores. However, it is still a challenge to achieve all three features in one material or device. Here, we demonstrate efficient and controllable water desalination with a nanoporous 2D Fe phthalocyanine (FePc) membrane using molecular dynamics simulations. We find the FePc membrane not only conducts fast water flow, but it also suppresses ion permeation. The selectivity is attributed to a mechanism distinct from the traditional steric exclusion: cations are excluded due to electrostatic repulsion, whereas anions can be trapped in the nanopore and induce the reorganization of ions in the vicinity of the nanopore, which in turn creates a tendency for the trapped anions to move back into the saline reservoir. More interestingly, we find such mechanism is largely due to the sufficiently strong electrostatic interaction of the charged nanopore region with ions and is not restricted to the FePc nanopore. In addition, the number of protonated nitrogen atoms in FePc pores can be modulated by adjusting the pH value of the solution. The extent of the anion occupancy can thus be regulated, giving rise to control of the water flow. Taken together, great permeability, selectivity and controllability can be achieved with this nanosheet system. Moreover, our study suggests there is an alternative mechanism of water desalination which may be realized by intrinsically nanoporous materials such as FePc membranes.

Misorientation-angle-dependent electrical transport across molybdenum disulfide grain boundaries.

Grain boundaries in monolayer transition metal dichalcogenides have unique atomic defect structures and band dispersion relations that depend on the inter-domain misorientation angle. Here, we explore misorientation angle-dependent electrical transport at grain boundaries in monolayer MoS2 by correlating the atomic defect structures of measured devices analysed with transmission electron microscopy and first-principles calculations. Transmission electron microscopy indicates that grain boundaries are primarily composed of 5-7 dislocation cores with periodicity and additional complex defects formed at high angles, obeying the classical low-angle theory for angles <22°. The inter-domain mobility is minimized for angles <9° and increases nonlinearly by two orders of magnitude before saturating at ∼ 16 cm(2) V(-1) s(-1) around misorientation angle ≈ 20°. This trend is explained via grain-boundary electrostatic barriers estimated from density functional calculations and experimental tunnelling barrier heights, which are ≈ 0.5 eV at low angles and ≈ 0.15 eV at high angles (≥ 20°).