Atomic-Scale Mechanism on Nucleation and Growth of Mo2C Nanoparticles Revealed by in Situ Transmission Electron Microscopy

Vacuum Evaporation Plating Enabling ≤ 10 µm Ultrathin Lithium Foils for Lithium Metal Batteries

Lithium (Li) metal is widely recognized as a viable candidate for anode material in future battery technologies due to its exceptional energy density. Nevertheless, the commercial Li foils in common use are too thick (≈100 µm), resulting in a waste of Li resources. Herein, by applying the vacuum evaporation plating technology, the ultra-thin Li foils (VELi) with high purity, strong adhesion, and thickness of less than 10 µm are successfully prepared. The manipulation of evaporation temperature allows for convenient regulation of the thickness of the fabricated Li film. This physical thinning method allows for fast, continuous, and highly accurate mass production. With a current density of 0.5 mA cm-2 for a plating amount of 0.5 mAh cm-2, VELi||VELi cells can stably cycle for 200 h. The maximum utilization of Li is already more than 25%. Furthermore, LiFePO4||VELi full cells present excellent cycling performance at 1 C (1 C = 155 mAh g-1) with a capacity retention rate of 90.56% after 240 cycles. VELi increases the utilization of active Li and significantly reduces the cost of Li usage while ensuring anode cycling and multiplication performance. Vacuum evaporation plating technology provides a feasible strategy for the practical application of ultra-thin Li anodes.

Atomically Resolved Defects on Thin Molybdenum Carbide (α-Mo2C) Crystals

Thin transition metal carbides (TMCs) garnered significant attention in recent years due to their attractive combination of mechanical and electrical properties with chemical and thermal stability. On the other hand, a complete picture of how defects affect the physical properties and application potential of this emerging class of materials is lacking. Here, we present an atomic-resolution study of defects on thin crystals of molybdenum carbide (α-Mo₂C) grown via chemical vapor deposition (CVD) by way of conductive atomic force microscopy (C-AFM) measurements under ambient conditions. Defects are characterized based on the type (enhancement/attenuation) and spatial extent (compact/extended) of the effect they have on the conductivity landscape of the crystal surfaces. Ab initio calculations performed by way of density functional theory (DFT) are employed to gather clues about the identity of the defects.

Thermal Stability and Sublimation of Two-Dimensional Co9Se8 Nanosheets for Ultrathin and Flexible Nanoelectronic Devices

An understanding of the structural and compositional stability of nanomaterials is significant from both fundamental and technological points of view. Here, we investigate the thermal stability of half-unit-cell thick two-dimensional (2D) Co₉Se₈ nanosheets that are exceptionally interesting because of their half-metallic ferromagnetic properties. By employing in situ heating in the transmission electron microscope (TEM), we find that the nanosheets show good structural and chemical stability without changes to the cubic crystal structure until sublimation of the nanosheets starts at temperatures between 460 and 520 °C. The real-time observations of the sublimation process show preferential removal at {110} type crystal facets. From an analysis of sublimation rates at various temperatures, we find that the sublimation occurs through noncontinuous and punctuated mass loss at lower temperatures while the sublimation is continuous and uniform at higher temperatures. Our findings provide an understanding of the nanoscale structural and compositional stability of 2D Co₉Se₈ nanosheets, which is of importance for their reliable application and sustained performance as ultrathin and flexible nanoelectronic devices.

In Situ Kinetic Observations on Crystal Nucleation and Growth

Nucleation and growth are critical steps in crystallization, which plays an important role in determining crystal structure, size, morphology, and purity. Therefore, understanding the mechanisms of nucleation and growth is crucial to realize the controllable fabrication of crystalline products with desired and reproducible properties. Based on classical models, the initial crystal nucleus is formed by the spontaneous aggregation of ions, atoms, or molecules, and crystal growth is dependent on the monomer's diffusion and the surface reaction. Recently, numerous in situ investigations on crystallization dynamics have uncovered the existence of nonclassical mechanisms. This review provides a summary and highlights the in situ studies of crystal nucleation and growth, with a particular emphasis on the state-of-the-art research progress since the year 2016, and includes technological advances, atomic-scale observations, substrate- and temperature-dependent nucleation and growth, and the progress achieved in the various materials: metals, alloys, metallic compounds, colloids, and proteins. Finally, the forthcoming opportunities and challenges in this fascinating field are discussed.

Multistep nucleation visualized during solid-state crystallization

Mechanisms of nucleation have been debated for more than a century, despite successes of classical nucleation theory. The nucleation process has been recently argued as involving a nonclassical mechanism (the "two-step" mechanism) in which an intermediate step occurs before the formation of a nascent ordered phase. However, a thorough understanding of this mechanism, in terms of both microscopic kinetics and thermodynamics, remains experimentally challenging. Here, in situ observations using transmission electron microscopy on a solid-state nucleation case indicate that early-stage crystallization can follow the non-classical pathway, yet proceed via a more complex manner in which multiple metastable states precede the emergence of a stable nucleus. The intermediate steps were sequentially isolated as spinodal decomposition of amorphous precursor, mass transport and structural oscillations between crystalline and amorphous states. Our experimental and theoretical analyses support the idea that the energetic favorability is the driving force for the observed sequence of events. Due to the broad applicability of solid-state crystallization, the findings of this study offer new insights into modern nucleation theory and a potential avenue for materials design.

Highly Efficient Porous Carbon Electrocatalyst with Controllable N-Species Content for Selective CO2 Reduction

We report a straightforward strategy to design efficient N doped porous carbon (NPC) electrocatalyst that has a high concentration of easily accessible active sites for the CO₂ reduction reaction (CO₂ RR). The NPC with large amounts of active N (pyridinic and graphitic N) and highly porous structure is prepared by using an oxygen-rich metal-organic framework (Zn-MOF-74) precursor. The amount of active N species can be tuned by optimizing the calcination temperature and time. Owing to the large pore sizes, the active sites are well exposed to electrolyte for CO₂ RR. The NPC exhibits superior CO₂ RR activity with a small onset potential of -0.35 V and a high faradaic efficiency (FE) of 98.4 % towards CO at -0.55 V vs. RHE, one of the highest values among NPC-based CO₂ RR electrocatalysts. This work advances an effective and facile way towards highly active and cost-effective alternatives to noble-metal CO₂ RR electrocatalysts for practical applications.

Scalable Two-Dimensional Lateral Metal/Semiconductor Junction Fabricated with Selective Synthetic Integration of Transition-Metal-Carbide (Mo2C)/-Dichalcogenide (MoS2)

The construction of manufacturable, stable, high-quality metal/semiconductor junction structures is of fundamental importance to implement higher-level devices and circuit systems. Owing to the unique features of two-dimensional (2D) materials, namely, that intralayer atoms are covalently bonded, whereas interlayer atoms are held together by weak attractive interactions, there are several studies on the fabrication and identification of the peculiar properties of various 2D heterostructures. However, large-scale 2D lateral metal/semiconductor junction structures with acceptable levels of manufacturability and quality have not yet been demonstrated, which is among the critical technological hurdles to overcome for the realization of 2D material-based electronic and photonic devices. This paper reports the fabrication of a manufacturable large-scale metal (Mo₂C)/semiconductor (MoS₂) junction via selective synthetic integration and a lithographically patterned SiO₂ masking layer. It is demonstrated that whereas chemical conversion to Mo₂C occurs in the exposed chemical vapor deposition-grown MoS₂ part, the MoS₂ layer under the SiO₂ masking layer is protected from chemical conversion, so that a scalable Mo₂C/MoS₂ heterostructure is integrated down to nanometer-scale dimensions. Excellent contact resistance of 2.1 kΩ·μm is achieved from this lateral junction structure, providing a manufacturable and highly stable metal/semiconductor building block for real implementation of 2D material-based nanoscale device integration.

Big to Small: Ultrafine Mo2 C Particles Derived from Giant Polyoxomolybdate Clusters for Hydrogen Evolution Reaction

Due to its electronic structure, similar to platinum, molybdenum carbides (Mo₂ C) hold great promise as a cost-effective catalyst platform. However, the realization of high-performance Mo₂ C catalysts is still limited because controlling their particle size and catalytic activity is challenging with current synthesis methods. Here, the synthesis of ultrafine β-Mo₂ C nanoparticles with narrow size distribution (2.5 ± 0.7 nm) and high mass loading (up to 27.5 wt%) on graphene substrate using a giant Mo-based polyoxomolybdate cluster, Mo₁₃₂ ((NH₄ )₄₂ [Mo₁₃₂ O₃₇₂ (CH₃ COO)₃₀ (H₂ O)₇₂ ]·10CH₃ COONH₄ ·300H₂ O) is demonstrated. Moreover, a nitrogen-containing polymeric binder (polyethyleneimine) is used to create MoN bonds between Mo₂ C nanoparticles and nitrogen-doped graphene layers, which significantly enhance the catalytic activity of Mo₂ C for the hydrogen evolution reaction, as is revealed by X-ray photoelectron spectroscopy and density functional theory calculations. The optimal Mo₂ C catalyst shows a large exchange current density of 1.19 mA cm^-2 , a high turnover frequency of 0.70 s^-1 as well as excellent durability. The demonstrated new strategy opens up the possibility of developing practical platinum substitutes based on Mo₂ C for various catalytic applications.

Observable Two-Step Nucleation Mechanism in Solid-State Formation of Tungsten Carbide

The nucleation of crystals from ubiquitous solid-state reactions impacts a wide range of natural and synthetic processes and is fundamental to physical and chemical synthesis. However, the microscopic organization mechanism of amorphous precursors to nanoscale clusters of ordered atoms (nucleus) in an all-solid environment is inaccessible by common experimental probes. Here, by using in situ transmission electron microscopy in combination with theoretical simulations, we show in the reactive formation of a metal carbide that nucleation actually occurs via a two-step mechanism, in which a spinodal-structured amorphous intermediate reorganizes from an amorphous precursor and precedes the emergence of a crystalline nucleus, rather than direct one-step nucleation from classical consideration. We further isolated a series of sophisticated dynamics during formation and development of the nucleus in real-space and interpreted them by thermodynamic favorability. We anticipate that such an indirect organization mechanism which contains a metastable intermedium among the free energy gap between precursors and nanocrystals has its chance in underlying most solid-state crystallizations, whereas the as-established experimental method represents a step forward in exploring fundamentals in chemical reaction, material engineering, etc.

Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS2 Films

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS₂) films. To this end, we deposit a model heterostructure of thin amorphous MoS₂ films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS₂ into crystalline MoS₂ domains, thereby emulating widely used elevated temperature MoS₂ synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS₂ films in situ and at the atomic scale. Our observations suggest that during MoS₂ processing, various MoS₂ polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS₂ layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS₂ crystallization can be influenced by a weak MoS₂/graphene support epitaxy. Our atomic-scale in situ approach thereby visualizes multiple fundamental processes that underlie the varied MoS₂ morphologies observed in previous ex situ growth and processing work. Our work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.