Lithiation Mechanism of Tunnel‐Structured MnO2 Electrode Investigated by In Situ Transmission Electron Microscopy

Manganese oxide (α‐MnO₂) has been considered a promising energy material, including as a lithium‐based battery electrode candidate, due to its environmental friendliness. Thanks to its unique 1D [2 × 2] tunnel structure, α‐MnO₂ can be applied to a cathode by insertion reaction and to an anode by conversion reaction in corresponding voltage ranges, in a lithium‐based battery. Numerous reports have attributed its remarkable performance to its unique tunnel structure; however, the precise electrochemical reaction mechanism remains unknown. In this study, finding of the lithiation mechanism of α‐MnO₂ nanowire by in situ transmission electron microscopy (TEM) is reported. By elaborately modifying the existing in situ TEM experimental technique, rapid lithium‐ion diffusion through the tunnels is verified. Furthermore, by tracing the full lithiation procedure, the evolution of the MnO intermediate phase and the development of the MnO and Li₂O phases with preferred orientations is demonstrated, which explains how the conversion reaction occurs in α‐MnO₂ material. This study provides a comprehensive understanding of the electrochemical lithiation process and mechanism of α‐MnO₂ material, in addition to the introduction of an improved in situ TEM biasing technique.     

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS₂) and bismuth telluride (Bi₂Te₃) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high‐reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS₂ and Bi₂Te₃ is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS₂ and Bi₂Te₃ crystals with unprecedented resolution. Real‐time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single‐crystalline free‐standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.     

扫描透射电子显微镜(STEM)在新一代高K栅介质材料的应用

扫描透射电子显微镜(STEM)原子序数衬度像(Z-衬度像)具有分辨率高(可直接“观察”到晶体中原子的真实位置)、对化学组成敏感以及图像直观易解释等优点, 成为原子尺度研究材料微结构的强有力工具。本文介绍了STEM Z-衬度像成像原理、方法及技术特点, 并结合具体的高K栅介质材料 (如铪基金属氧化物、稀土金属氧化物和钙钛矿结构外延氧化物薄膜)对STEM在新一代高K栅介质材料研究中的应用进行了评述。 目前球差校正STEM Z-衬度的像空间分辨率已达亚埃级, 该技术在高K柵介质与半导体之间的界面微结构表征方面具有十分重要的应用。对此, 本文亦进行了介绍。     

Gram‐Scale Aqueous Synthesis of Stable Few‐Layered 1T‐MoS2: Applications for Visible‐Light‐Driven Photocatalytic Hydrogen Evolution

Most recently, much attention has been devoted to 1T phase MoS₂ because of its distinctive phase‐engineering nature and promising applications in catalysts, electronics, and energy storage devices. While alkali metal intercalation and exfoliation methods have been well developed to realize unstable 1T‐MoS₂, but the aqueous synthesis for producing stable metallic phase remains big challenging. Herein, a new synthetic protocol is developed to mass‐produce colloidal metallic 1T‐MoS₂ layers highly stabilized by intercalated ammonium ions (abbreviated as N‐MoS₂). In combination with density functional calculations, the X‐ray diffraction pattern and Raman spectra elucidate the excellent stability of metallic phase. As clearly depicted by high‐angle annular dark‐field imaging in an aberration‐corrected scanning transmission electron microscope and extended X‐ray absorption fine structure, the N‐MoS₂ exhibits a distorted octahedral structure with a 2a ₀ × a ₀ basal plane superlattice and 2.72 Å Mo–Mo bond length. In a proof‐of‐concept demonstration for the obtained material's applications, highly efficient photocatalytic activity is achieved by simply hybridizing metallic N‐MoS₂ with semiconducting CdS nanorods due to the synergistic effect. As a direct outcome, this CdS:N‐MoS₂ hybrid shows giant enhancement of hydrogen evolution rate, which is almost 21‐fold higher than pure CdS and threefold higher than corresponding annealed CdS:2H‐MoS₂.     

Variable‐Wavelength Quick Scanning Nanofocused X‐Ray Microscopy for In Situ Strain and Tilt Mapping

Compression of micropillars is followed in situ by a quick nanofocused X‐ray scanning microscopy technique combined with 3D reciprocal space mapping. Compared to other attempts using X‐ray nanobeams, it avoids any motion or vibration that would lead to a destruction of the sample. The technique consists of scanning both the energy of the incident nanofocused X‐ray beam and the in‐plane translations of the focusing optics along the X‐ray beam. Here, the approach by imaging the strain and lattice orientation of Si micropillars and their pedestals during in situ compression is demonstrated. Varying the energy of the incident beam instead of rocking the sample and mapping the focusing optics instead of moving the sample supplies a vibration‐free measurement of the reciprocal space maps without removal of the mechanical load. The maps of strain and lattice orientation are in good agreement with the ones recorded by ordinary rocking‐curve scans. Variable‐wavelength quick scanning X‐ray microscopy opens the route for in situ strain and tilt mapping toward more diverse and complex materials environments, especially where sample manipulation is difficult.     

Direct Observations of the Formation and Redox‐Mediator‐Assisted Decomposition of Li2O2 in a Liquid‐Cell Li–O2 Microbattery by Scanning Transmission Electron Microscopy

Operando scanning transmission electron microscopy observations of cathodic reactions in a liquid‐cell Li–O₂ microbattery in the presence of the redox mediator tetrathiafulvalene (TTF) in 1.0 m LiClO₄ dissolved dimethyl sulfoxide electrolyte are reported. It is found that the TTF addition does not obviously affect the discharge reaction for the formation of a solid Li₂O₂ phase. The coarsening of Li₂O₂ nanoparticles occurs via both conventional Ostwald ripening and nonclassical crystallization by particle attachment. During charging, the oxidation reaction at significantly reduced charge potentials mainly takes place at Li₂O₂/electrolyte interfaces and has obvious correspondence with the oxidized TTF⁺ distributions in the electric fields of the charged electrode. This study provides direct evidence that TTF truly plays a role in promoting the decomposition of Li₂O₂ as a soluble charge‐transfer agent between the electrode and the Li₂O₂.     

Atomic‐Scale Structure Evolution in a Quasi‐Equilibrated Electrochemical Process of Electrode Materials for Rechargeable Batteries

Lithium‐ion batteries have proven to be extremely attractive candidates for applications in portable electronics, electric vehicles, and smart grid in terms of energy density, power density, and service life. Further performance optimization to satisfy ever‐increasing demands on energy storage of such applications is highly desired. In most of cases, the kinetics and stability of electrode materials are strongly correlated to the transport and storage behaviors of lithium ions in the lattice of the host. Therefore, information about structural evolution of electrode materials at an atomic scale is always helpful to explain the electrochemical performances of batteries at a macroscale. The annular‐bright‐field (ABF) imaging in aberration‐corrected scanning transmission electron microscopy (STEM) allows simultaneous imaging of light and heavy elements, providing an unprecedented opportunity to probe the nearly equilibrated local structure of electrode materials after electrochemical cycling at atomic resolution. Recent progress toward unraveling the atomic‐scale structure of selected electrode materials with different charge and/or discharge state to extend the current understanding of electrochemical reaction mechanism with the ABF and high angle annular dark field STEM imaging is presented here. Future research on the relationship between atomic‐level structure evolution and microscopic reaction mechanisms of electrode materials for rechargeable batteries is envisaged.     

高熵陶瓷固溶结构的透射电镜研究

高熵会带来热力学上的高熵效应、结构上的晶格畸变效应、动力学上的迟滞扩散效应以及性能上的"鸡尾酒"效应,通过高熵设计来提高陶瓷材料的性能是目前研究的热点,而通过透射电镜揭示高熵结构及其与性能相关性的研究还很缺乏。本研究以相应金属氧化物、碳化硼和石墨为原材料,在制备高熵硼化物和高熵碳化物粉体的基础上,利用放电等离子体烧结制备得到高熵(TiZrHfNbTa)B₂和(TiZrHfNbTa)C陶瓷。采用透射电子显微镜及其能谱分析手段对两种高熵陶瓷进行了纳米尺度和原子尺度的结构表征,发现过渡金属元素固溶后保持了晶体结构的完整性,五种元素分布均匀,但在原子尺度存在固溶元素的浓度振荡、原子离散和晶格应变。本工作获得的原子尺度的固溶结构信息将有助于对高熵陶瓷构效关系的理解,并为高熵陶瓷的组分和结构设计提供实验依据。     

Direct Bandgap Transition in Many‐Layer MoS2 by Plasma‐Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of many‐layer MoS₂ using low‐energy oxygen plasma treatment. Gas phase treatment of MoS₂ with oxygen radicals generated in an upstream N₂–O₂ plasma is shown to enhance the photoluminescence (PL) of many‐layer, mechanically exfoliated MoS₂ flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS₂ from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS₂. Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many‐layer MoS₂ flakes (2–15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS₂, while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS₂, without reducing sample thickness, and hence optical density.     

In Situ Synthesis of Few‐Layered g‐C3N4 with Vertically Aligned MoS2 Loading for Boosting Solar‐to‐Hydrogen Generation

In artificial photocatalytic hydrogen evolution, effective incident photon absorption and a high‐charge recombination rate are crucial factors influencing the overall efficiency. Herein, a traditional solid‐state synthesis is used to obtain, for the first time, novel samples of few‐layered g‐C₃N₄ with vertically aligned MoS₂ loading (MoS₂/C₃N₄). Thiourea and layered MoO₃ are chosen as precursors, as they react under nitrogen atmosphere to in situ produce the products. According to the quasi‐Fourier transform infrared reflectance and X‐ray diffraction measurements, the detailed reaction process is determined to proceed through the confirmed formation pathway. The two precursor units MoS₂ and C₃N₄ are linked by Mo? N bonds, which act as electronic receivers/conductors and hydrogen‐generation sites. Density functional theory is also carried out, which determines that the interface sites act as electron‐accumulation regions. According to the photoelectrochemical results, MoS₂/C₃N₄ can achieve a current of 0.05 mA cm^?2, which is almost ten times higher than that of bare g‐C₃N₄ or the MoS₂/C₃N₄‐R reference samples. The findings in the present work pave the way to not only synthesize a series of designated samples but also thoroughly understand the solid‐state reaction.     

<Emphasis Type="Italic">In situ</Emphasis> TEM observation of dissolution and regrowth dynamics of MoO<Subscript>2</Subscript> nanowires under oxygen

Direct observation of the dissolution behavior of nanomaterials could provide fundamental insight to understanding their anisotropic properties and stability.The dissolution mechanism in solution and vacuum has been well documented.However,the gas-involved dissolution and regrowth have seldom been explored and the mechanisms remain elusive.We report herein,an in situ TEM study of the dissolution and regrowth dynamics of MoO2 nanowires under oxygen using environmental transmission electron microscopy (ETEM).For the first time,oscillatory dissolution on the nanowire tip is revealed,and,intriguingly,simultaneous layer-by-layer regrowth on the sidewall facets is observed,leading to a shorter and wider nanowire.Combined with first-principles calculations,we found that electron beam irradiation caused oxygen loss in the tip facets,which resulted in changing the preferential growth facets and drove the morphology reshaping.     

Origin of Fracture‐Resistance to Large Volume Change in Cu‐Substituted Co3O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu‐substituted Co₃O₄ supplemented by first‐principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube‐on‐cube orientation relationship with Li₂O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube‐on‐cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li₂O in the discharge cycle and underpins the catalytic activity of Li₂O decomposition in the charge cycle.     

Single Ni Atoms Anchored on Porous Few‐Layer g‐C3N4 for Photocatalytic CO2 Reduction: The Role of Edge Confinement

It is greatly intriguing yet remains challenging to construct single‐atomic photocatalysts with stable surface free energy, favorable for well‐defined atomic coordination and photocatalytic carrier mobility during the photoredox process. Herein, an unsaturated edge confinement strategy is defined by coordinating single‐atomic‐site Ni on the bottom‐up synthesized porous few‐layer g‐C₃N₄ (namely, Ni₅‐CN) via a self‐limiting method. This Ni₅‐CN system with a few isolated Ni clusters distributed on the edge of g‐C₃N₄ is beneficial to immobilize the nonedged single‐atomic‐site Ni species, thus achieving a high single‐atomic active site density. Remarkably, the Ni₅‐CN system exhibits comparably high photocatalytic activity for CO₂ reduction, giving the CO generation rate of 8.6 µmol g^?1 h^?1 under visible‐light illumination, which is 7.8 times that of pure porous few‐layer g‐C₃N₄ (namely, CN, 1.1 µmol g^?1 h^?1). X‐ray absorption spectrometric analysis unveils that the cationic coordination environment of single‐atomic‐site Ni center, which is formed by Ni‐N doping‐intercalation the first coordination shell, motivates the superiority in synergistic N–Ni–N connection and interfacial carrier transfer. The photocatalytic mechanistic prediction confirms that the introduced unsaturated Ni‐N coordination favorably binds with CO₂, and enhances the rate‐determining step of intermediates for CO generation.