Atomic-scale insights into the formation of 2D crystals from in situ transmission electron microscopy

Two-dimensional(2D)crystals are attractive due to their intriguing structures and properties which are strongly dependent on the synthesis conditions.To achieve their superior properties,it is of critical importance to fully understand the growth processes and mechanisms for tailored design and controlled growth of 2D crystals.Due to the high spatiotemporal resolution and the capability to mimic the realistic growth conditions,in situ transmission electron microscopy(TEM)becomes an effective way to monitor the growth process in real-time at the atomic scale,which is expected to provide atomic-scale insights into the nucleation and growth of 2D crystals.Here we review the recent in situ TEM works on the formation of 2D crystals under electron irradiation,thermal excitation as well as voltage bias.The underlying mechanisms are also elucidated in detail,providing key insights into the nucleation and formation of 2D crystals.

Unraveling the reaction mechanisms of electrode materials for sodiumion and potassium‐ion batteries by in situ transmission electron microscopy

Sodium ion batteries(SIBs)and potassium ion batteries(PIBs)have caught numerous attention due to the low cost and abundant availability of sodium and potassium.However,their power density,cycling stability and safety need further improvement for practical applications.Investigations on the reaction mechanisms and structural degradation when cycling are of great importance.In situ transmission electron microscopy(TEM)is one of the most significant techniques to understand and monitor electrochemical processes at an atomic scale with real-time imaging.In this review,the current progress in unraveling reaction mechanisms of electrode materials for SIBs and PIBs via in situ TEM is summarized.First,the importance of in situ TEM is highlighted.Then,based on the three types of electrochemical reaction,i.e.,intercalation reac-tion,conversion reaction and alloying reaction,the structural evolution and reaction kinetics at atomic resolution,and their relation to the electrochemical performance of electrode materials are reviewed and described in detail.Fi-nally,future directions of in situ TEM for SIBs and PIBs are proposed.Therefore,the in‐depth understanding revealed by in situ TEM will give an instructive guide in rational design of electrode materials for high performance electrode materials of SIBs and PIBs.

In situ observation of the electrochemical behavior of Li–CO_(2)/O_(2)batteries in an environmental transmission electron microscope

Li–CO_(2)/O_(2)batteries,a promising energy storage technology,not only provide ultrahigh discharge capacity but also capture CO_(2)and turn it into renewable energy.Their electrochemical reaction pathways'ambiguity,however,creates a hurdle for their practical application.This study used copper selenide(CuSe)nanosheets as the air cathode medium in an environmental transmission electron microscope to in situ study Li–CO_(2)/O_(2)(mix CO_(2)as well as O_(2)at a volume ratio of 1:1)and Li–O_(2)batteries as well as Li–CO_(2)batteries.Primary discharge reactions take place successively in the Li–CO_(2)/O_(2)–CuSe nanobattery:(I)4Li^(+)+O_(2)+4e^(−)→2Li_(2)O;(II)Li_(2)O+CO_(2)→Li_(2)CO_(3).The charge reaction proceeded via(III)2Li_(2)CO_(3)→4Li^(+)+2CO_(2)+O_(2)+4e^(−).However,Li–O_(2)and Li–CO_(2)nanobatteries showed poor cycling stability,suggesting the difficulty in the direct decomposition of the discharge product.The fluctuations of the Li–CO_(2)/O_(2)battery's electrochemistry were also shown to depend heavily on O_(2).The CuSe‐based Li–CO_(2)/O_(2)battery showed exceptional electrochemical performance.The Li^–CO_(2)/O_(2)battery offered a discharge capacity apex of 15,492 mAh g^(−1) and stable cycling 60 times at 100 mA g^(−1).Our research offers crucial insight into the electrochemical behavior of Li–CO_(2)/O_(2),Li–O_(2),and Li–CO_(2)nanobatteries,which may help the creation of high‐performance Li–CO_(2)/O_(2)batteries for energy storage applications.

Electrospun advanced nanomaterials for in situ transmission electron microscopy:Progress and perspectives

Electrospun nanofibers(NFs)have shown excellent properties including high porosity,abundant active sites,controllable diameter,uniform and designable structure,high mechanical strength,and superior resistance to external destruction,which are ideal nanoreactors for in situ characterizations.Among various techniques,in situ transmission electron microscopy(TEM)has enabled operando observation at the atomic level due to its high temporal and spatial resolution combined with excellent sensitivity,which is of great importance for rational materials design and performance improvement.In this review,the basic knowledge of in situ TEM techniques and the advantages of electrospun nanoreactors for in situ TEM characterization are first introduced.The recent development in electrospun nanoreactors for studying the physical properties,structural evolution,phase transition,and formation mechanisms of materials using in situ TEM is then summarized.The electrochemical behaviors of carbon nanofibers(CNFs),metal/metal oxide NFs,and solidelectrolyte interphase for different rechargeable batteries are highlighted.Finally,challenges faced by electrospun nanoreactors for in situ TEM characterization are discussed and potential solutions are proposed to advance this field.

In situ atomic-scale tracking of unusual interface reaction circulation and phase reversibility in(de)potassiated alloy-typed anode

Alloy-typed anode materials,endowed innately with high theoretical specific capacity,hold great promise as an alternative to intercalation-typed counterparts for alkali-ion batteries.Despite tremendous efforts devoted to addressing drastic volume change and severe pulverization issues of such anodes,the underlying mechanisms involving dynamic phase evolutions and reaction kinetics have not yet been fully comprehended.Herein,taking antimony(Sb)anode as a representative paradigm,its microscopic operating mechanisms down to the atomic scale during live(de)potassiation cycling are systematically unraveled using in situ transmission electron microscopy.Highly reversible phase transformations at single-particle level,that are Sb←→KSb_(2)←→KSb←→K_5Sb_(4)←→K_(3)Sb,were revealed during cycling.Meanwhile,multiple phase interfaces associated with different reaction kinetics coexisted and this phenomenon was properly elucidated in the context of density functional theory calculations.Impressively,previously unexplored unidirectional circulation of reaction interfaces within individual Sb particle is confirmed for both potassiation and depotassiation.Based on the empirical results,the surface diffusion-mediated potassiation-depotassiation pathways at single-particle level are suggested.This work affords new insights into energy storage mechanisms of Sb anode and valuable guidance for targeted optimization of alloy-typed anodes(not limited to Sb)toward advanced potassium-ion batteries.

In Situ Electrochemical Transmission Electron Microscopy for Sodium-Ion Batteries

Sodium-ion batteries(SIBs)possess promising application prospects for large-scale energy storage systems due to the abundance of sodium ions as a resource and their low cost.Development of advanced SIBs requires a clear understanding of the structures and kinetic/dynamic processes occurring in the cells during the charging/discharging process.In situ transmission electron microscopy(TEM)is a powerful tool for direct visualization of the phase transitions as well as morphological and structural evolutions of the electrodes during the electrochemical reaction process.Herein,we summarize the state-of-the-art in situ TEM studies on SIBs with a specific focus on real-time observations of the electrochemical behavior of battery materials.This review emphasizes the necessity of in situ TEM to elucidate fundamental issues regarding the reaction mechanism,phase transformation,structural evolution,and performance degradation of SIBs.Finally,critical challenges and emerging opportunities for in situ TEM research about SIBs are discussed.

Correlating the fluctuated growth of carbon nanotubes with catalyst evolution by atmospheric-pressure environmental transmission electron microscopy

Rate-controlled growth of carbon nanotubes(CNTs)and catalyst design are considered efficient ways for the preparation of CNTs with specific structures and properties.However,due to the difficulties in capturing the growth process of the CNTs with tiny size under a complex growth environment,the growth kinetics of CNTs and their correlation with the catalyst seed have been seldom revealed.Here,we investigated the growth process of CNTs from Ni nanoparticles(NPs)in real-time under atmospheric pressure using transmission electron microscopy equipped with a closed gas cell.It was found that the growth rates of CNTs fluctuated,and a phase transition from Ni_(3)C to Ni,and a reshaping of the catalyst NPs occurred during the growth process.We demonstrated that CNTs dynamically interacted with the connected catalyst NPs and the fluctuated growth rates of CNTs were correlated with the structure change of catalyst NPs.The origin of the growth rate fluctuation is attributed to the change of carbon concentration gradient in catalyst NPs.

Interface catalytic reduction of alumina by nickle for the aluminum nanowire growth: Dynamics observed by in situ TEM

Metal nanowires show promise in a broad range of applications and can be fabricated via a number of methods,such as vapor–liquid–solid process and template-based electrodeposition.However,the synthesis of Al nanowires(NWs)is still challenging from the stable alumina substrate.In this work,the Ni-catalyzed fabrication of Al NWs has been realized using various Al_(2)O_(3) substrates.The growth dynamics of Al NWs on Ni/Al_(2)O_(3) was studied using in situ transmission electron microscopy(TEM).The effect of alumina structures,compositions,and growth temperature were investigated.The growth of Al NWs correlates with the Na addition to the alumina support.Since no eutectic mixture of nickel aluminide was formed,a mechanism of Ni-catalyzed reduction of Al_(2)O_(3) for Al NWs growth has been proposed instead of the vapor–liquid–solid mechanism.The key insights reported here are not restricted to Ni-catalyzed Al NWs growth but can be extended to understanding the dynamic change and catalytic performance of Ni/Al_(2)O_(3) under working conditions.

Structural and chemical transformations of CuZn alloy nanoparticles under reactive redox atmospheres:An in situ TEM study

Alloying metals to form intermetallics has been proven effective in tuning the chemical properties of metal-based catalysts.However,intermetallic alloys can undergo structural and chemical transformations under reactive conditions,leading to changes in their catalytic function.Elucidating and understanding these transformations are crucial for establishing relevant structureperformance relationships and for the rational design of alloy-based catalysts.In this work,we used CuZn alloy nanoparticles(NPs)as a model material system and employed in situ transmission electron microscopy(TEM)to investigate the structural and chemical changes of CuZn NPs under H_(2),O_(2)and their mixture.Our results show how CuZn NPs undergo sequential transformations in the gas mixture at elevated temperatures,starting with gradual leaching and segregation of Zn,followed by oxidation at the NP surface.The remaining copper at the core of particles can then engage in dynamic behavior,eventually freeing itself from the zinc oxide shell.The structural dynamics arises from an oscillatory phase transition between Cu and Cu_(2)O and is correlated with the catalytic water formation,as confirmed by in situ mass spectrometry(MS).Under pure H_(2)or O_(2)atmosphere,we observe different structural evolution pathways and final chemical states of CuZn NPs compared to those in the gas mixture.These results clearly demonstrate that the chemical state of alloy NPs can vary considerably under reactive redox atmospheres,particularly for those containing elements with distinct redox properties,necessitating the use of in situ or detailed ex situ characterizations to gain relevant insights into the states of intermetallic alloy-based catalysts and structure-activity relationships.

In situ observation of temperature-dependent atomistic and mesoscale oxidation mechanisms of aluminum nanoparticles

Oxidation is a universal process causing metals’corrosion and degradation.While intensive researches have been conducted for decades,the detailed atomistic and mesoscale mechanisms of metal oxidation are still not well understood.Here using in situ environmental transmission electron microscopy(E-TEM)with atomic resolution,we revealed systematically the oxidation mechanisms of aluminum from ambient temperature to^600℃.It was found that an amorphous oxide layer formed readily once Al was exposed to air at room temperature.At^150℃,triangle-shaped Al2O3 lamellas grew selectively on gas/solid(oxygen/amorphous oxide layer)interface,however,the thickness of the oxide layer slowly increased mainly due to the inward diffusion of oxygen.As the temperature further increased,partial amorphous-to-crystallization transition was observed on the amorphous oxide film,resulting in the formation of highly dense nano-cracks in the oxide layer.At^600℃,fast oxidation process was observed.Lamellas grew into terraces on the oxide/gas interface,indicating that the high temperature oxidation is controlled by the outward diffusion of Al.Single or double/multi-layers of oxide nucleated at the corners of the terraces,forming denseγ’-Al2O3,which is a metastable oxide structure but may be stabilized at nanoscale.

Atomic-scale imaging of the defect dynamics in ceria nanowires under heating by in situ aberration-corrected TEM

The defects in the ceria usually work as the active reaction sites in their industrial applications.In this article,we studied the formation and atomic process of the defects of ceria nanowires under heating by using in situ aberration-corrected transmission electron microscopy(Cs-TEM)method.With the temperature elevating,ceria nanowires are reduced and defects begin to appear and grow up.When temperature reaches 1,023 K,the defect morphology exhibits the rhombus or hexagon patterns,which are surrounded by{111}and{200}planes with lower surface energy,and the heated ceria still maintain the same cubic fluorite structure as their parent.It is also indicated that the formation of defects originates from the release of lattice oxygen and the volatilization of surface Ce ions.This work provides an important insight into designing ceria-based catalysts and ionic conductors.

Tailoring Bi_(2)Te_(3) edge with semiconductor and metal properties under electron beam irradiation

In pursuit of miniaturization in the semiconductor industry,two-dimensional(2D)materials are used to fabricate new electronic devices.The topological insulator(TI)material bismuth telluride(Bi_(2)Te_(3)),as an emerging 2D material,has potential applications in electronic and spintronic devices due to its unique electrical properties.It is well known that the surface-to-volume ratio increases as the thickness of the material decreases,resulting in a more prominent edge effect.Therefore,for a single-layer Bi_(2)Te_(3),the atomic structure of the edge plays a crucial role in its electrical properties.Here,combining first-principles calculations and in situ transmission electron microscopy(TEM)experimental studies,we report that there are two types of edge structures in single-layer Bi2Te3:semiconducting flat edges and metallic zigzag edges.The dynamic evolution process of the edge structure with atomic resolution shows that the proportions of these two edges change with continuous electron beam irradiation.Our findings demonstrate the viability to use electron beam as an effective tool to precisely tailor the edge of Bi_(2)Te_(3) with desired properties,which paves the way for implementation of single-layer Bi2Te3 in electronics and spintronics.

In-situ revealing the degradation mechanisms of Pt film over 1000℃

Degradation of a metallic film under harsh thermal-mechanical-electrical coupling field conditions determines its service temperature and lifetime.In this work,the self-heating degradation behaviors of Pt thin films above 1000℃were studied in situ by TEM at the nanoscale.The Pt films degraded mainly through void nucleation and growth on the Pt-SiN_(x)interface.Voids preferentially formed at the grain boundary and triple junction intersections with the interface.At temperatures above 1040℃,the voids nucleated at both the grain boundaries and inside the Pt grains.A stress simulation of the suspended membrane suggests the existence of local tensile stress in the Pt film,which promotes the nucleation of voids at the Pt-Si Nxinterface.The grain-boundary-dominated mass transportation renders the voids grow preferentially at GBs and triple junctions in a Pt film.Additionally,under the influence of an applied current,the voids that nucleated inside Pt grains grew to a large size and accelerated the degradation of the Pt film.

Reversible transformation between terrace and step sites of Pt nanoparticles on titanium under CO and O_(2) environments

Understanding the dynamic evolution of active sites of supported metal catalysts during catalysis is fundamentally important for improving its performance,which attracts tremendous research interests in the past decades.There are two main surficial structures for metal catalysts:terrace sites and step sites,which exhibit catalytic activity discrepancy during catalysis.Herein,by using in situ transmission electron microscopy and in situ Fourier transform infrared spectroscopy(FTIR),the transformation between surface terrace and step sites of Pt-TiO_(2) catalysts was studied under CO and O_(2) environments.We found that the{111}step sites tend to form at{111}terrace under O_(2) environment,while these step sites prefer to transform into terrace under CO environment at elevated temperature.Meanwhile,quantitative ratios of terrace/step sites were obtained by in situ FTIR.It was found that this transformation between terrace sites and step sites was reversible during gas treatment cycling of CO and O_(2).The selective adsorption of O_(2) and CO species at different sites,which stabilized the step/terrace sites,was found to serve as the driving force for active sites transition by density functional theory calculations.Inspired by the in situ results,an enhanced catalytic activity of Pt-TiO_(2) catalysts was successfully achieved through tuning surface-active sites by gas treatments.

In situ dynamics response mechanism of the tunable length-diameter ratio nanochains for excellent microwave absorber

Faster response benefits the high-performance of magnetic material in various live applications.Hence,enhancing response speed toward the applied field via engineering advantages in structures is highly desired.In this paper,the precise synthesis of Co nanochain with the tunable length-diameter ratio is realized via a magnetic-field-guided assembly approach.The Co nanochain exhibits enhanced microwave absorption performance(near to-60 dB,layer thickness 2.2 mm)and broader effective absorption bandwidth(over 2/3 of total S,C,X,Ku bands).Furthermore,the simulated dynamic magnetic response reveals that the domain motion in 1D chain is faster than that in 0D nanoparticle,which is the determining factor of magnetic loss upgrade.Meanwhile,based on the controllable magnetic field experiment via in situ transmission electron microscopy,the association between magnetic response and microstructure is first present at the nanometer-level.The real and imaginary parts of relative complex permeability are determined by the domain migration confined inside Co nanochain and the magnetic flux field surrounded outside Co nanochain,respectively.Importantly,these findings can be extended to the novel design of microwave absorbers and promising candidates of magnetic carriers based on 1D structure.

Hierarchical crystalline-amorphous nanocomposites with high strength and large deformability enabled by elemental diffusion

Amorphous/nanocrystalline dual-phase structures have recently emerged as an effective way for over-coming the strength-ductility trade-offand breaking the limitation of the reverse Hall-Petch effect.Here,we proposed a new strategy to develop a hierarchical and interconnected amorphous-crystalline nanocomposite arising from the nanoscale elemental interdiffusion and oxygen adsorption behavior dur-ing thermal treatment processes.The nanocomposite consisted of a three-dimensional(3D)hierarchical network structure where the crystalline phase(Cr-Co-Ni-Al)was embedded into the Al-O-based amor-phous phase network with critical feature sizes encompassing three orders of magnitude(from microm-eter to nanometer scale).It can achieve ultrahigh compression yield strength of-3.6 GPa with large homogeneous deformation of over 50%strain.The massive interstitial atoms induced lattice distortion and hierarchical amorphous phase boundary contributed to the strength improvement.in situ Uniaxial compression inside a transmission electron microscope(TEM)revealed that the exceptional deformability of the nanocomposites resulted from the homogenous plastic flow of nano-sized amorphous phase and the plastic co-deformation behavior restricted by the nano-architected dual-phase interface.The proposed dual-phase synthesis approach can outperform conventional nanolaminates design strategies in terms of the mechanical properties achievable while providing a pathway to easily tune the microstructure of these nanolaminates.

In situ TEM visualization of Ag catalysis in Li-O_(2)nanobatteries

Lithium-oxygen(Li-O_(2))batteries have been considered as an ideal solution to solving the global energy crisis.Silver(Ag)and Agbased catalyst have been extensively studied due to their high catalytic activities in Li-O_(2)batteries.However,it remains a challenge to track the catalytic mechanism during the charge/discharge process.Here,a nanoscale processing method was used to assemble a Li-O_(2)nanobattery in an aberration-corrected environmental transmission electron microscope(ETEM),where a single Ag nanowire(NW)was used as catalyst for O_(2)electrode.A visualization of the lithium ion insertion process during the electrochemical reactions was achieved in this nanobattery.Numerous Ag nanoparticles(NPs)were observed on the surface of the Ag NW,which were covered by the discharge product Li2O_(2).By simultaneously studying the evolution of the interface and the phase transformation,it can be concluded that these Ag NPs wrapped around Ag NW acted as catalyst during the subsequent charge/discharge reaction.Based on these studies,Ag NPs decorated on porous carbon were synthesized,it can simultaneously improve the cycling stability(100 cycles)and the maximum specific capacity(17,371 mAh·g^(−1)at a current density of 100 mA·g^(−1))in a coin cell Li-O_(2)battery.This study suggests that nanoscale Ag may be a promising catalyst for Li-O_(2)battery.

Visualizing the crystallization of sodium chloride under supersaturated condition

Crystallization in supersaturated solution plays a fundamental role in a variety of natural and industrial processes.However,a thorough understanding of crystallization phenomena in supersaturated solution is still difficult because the real-time visualization of crystallization processes under supersaturated condition is a great challenge.Herein,an electron beam-induced crystallization method was carried out in in situ liquid cell transmission electron microscopy(TEM)to visualize the crystallization of NaCl under supersaturated condition in real time.Crucial steps and behaviors in the crystallization of NaCl were captured and clarified,including the growth of NaCl nanocrystals with different morphologies,the formation of initial crystalline seeds from amorphous ion clusters,and the non-equilibrium growth behaviors caused by uneven distribution of precursor ions.This study provides the real-time visualization of detailed nucleation and growth behaviors in NaCl crystallization and brings an ideal strategy for investigating crystallization phenomena under supersaturated condition.

Atomistic dynamics of disconnection-mediated grain boundary plasticity:A case study of gold nanocrystals

Grain boundary(GB)mediated deformation is a vital contributor to the plasticity of polycrystalline materials,where the disconnection model has become a widely recognized approach to depict the GB dynamics.However,experimental understanding of the atomistic disconnection dynamics remains scarce.In this case study of gold nanocrystals,atomistic disconnection dynamics governing the shear-coupled migration of flat GBs have been systematically investigated via in situ transmission electron microscopy nanomechanical testing supported by molecular dynamics simulations.Specifically,the site-dependent nucleation,shear-driven propagation,and diverse interactions associated with distinct GB disconnections are systematically elucidated and quantitatively compared.Moreover,the disconnection-mediated GB plasticity proves to prevail among different tilt and mixed GBs in gold.Eventually,a conceptual map of disconnection-mediated GB dynamics is established,which would furnish a unified understanding of GB plasticity in metallic materials.

Mass transport induced structural evolution and healing of sulfur vacancy lines and Mo chain in monolayer MoS_(2)

Defects play vital roles in tailoring structures and properties of materials including the atomically thin two-dimensional(2D)materials,and increasing demands are requested to find effective ways to realize the defect engineering,i.e.,tuning the defects and thus the materials’structure–property in a well-controlled way.Herein,we propose a novel method to tune the structures and configurations of one-dimensional(1D)line defects in monolayer MoS2 via mass transport induced structural transformation.By using atomic-resolved annular dark-field scanning transmission electron microscopy(ADF-STEM),we demonstrate in situ that sulfur vacancy line defect can be healed locally into defect-free MoS_(2)lattice via the desorption of Mo atoms from vacancy lines and adsorption into a moving Mo cluster.Furthermore,directional transport of Mo atoms(or Mo cluster)along the sulfur vacancy lines can induce the formation of Mo chains.Such a mass transport induced defect tuning provides more operational routes for the rational defect designing and property tuning in MoS_(2)as well as other related 2D materials.