Development of in situ characterization techniques in molecular beam epitaxy

Ex situ characterization techniques in molecular beam epitaxy(MBE)have inherent limitations,such as being prone to sample contamination and unstable surfaces during sample transfer from the MBE chamber.In recent years,the need for improved accuracy and reliability in measurement has driven the increasing adoption of in situ characterization techniques.These techniques,such as reflection high-energy electron diffraction,scanning tunneling microscopy,and X-ray photoelectron spectroscopy,allow direct observation of film growth processes in real time without exposing the sample to air,hence offering insights into the growth mechanisms of epitaxial films with controlled properties.By combining multiple in situ characterization techniques with MBE,researchers can better understand film growth processes,realizing novel materials with customized properties and extensive applications.This review aims to overview the benefits and achievements of in situ characterization techniques in MBE and their applications for material science research.In addition,through further analysis of these techniques regarding their challenges and potential solutions,particularly highlighting the assistance of machine learning to correlate in situ characterization with other material information,we hope to provide a guideline for future efforts in the development of novel monitoring and control schemes for MBE growth processes with improved material properties.

In situ characterizations of advanced electrode materials for sodium-ion batteries toward high electrochemical performances

Energy storage is an ever-growing global concern due to increased energy needs and resource exhaustion.Sodium-ion batteries(SIBs)have called increasing attention and achieved substantial progress in recent years owing to the abundance and even distribution of Na resources in the crust,and the predicted low cost of the technique.Nevertheless,SIBs still face challenges like lower energy density and inferior cycling stability compared to mature lithium-ion batteries(LIBs).Enhancing the electrochemical performance of SIBs requires an in-deep and comprehensive understanding of the improvement strategies and the underlying reaction mechanism elucidated by in situ techniques.In this review,commonly applied in situ techniques,for instance,transmission electron microscopy(TEM),Raman spectroscopy,X-ray diffraction(XRD),and X-ray absorption near-edge structure(XANES),and their applications on the representative cathode and anode materials with selected samples are summarized.We discuss the merits and demerits of each type of material,strategies to enhance their electrochemical performance,and the applications of in situ characterizations of them during the de/sodiation process to reveal the underlying reaction mechanism for performance improvement.We aim to elucidate the composition/structure-per formance relationship to provide guidelines for rational design and preparation of electrode materials toward high electrochemical performance.

Design and operando/in situ characterization of precious-metal-free electrocatalysts for alkaline water splitting

Electrochemical water splitting has attracted considerable attention for the production of hydrogen fuel by using renewable energy resources.However,the sluggish reaction kinetics make it essential to explore precious-metal-free electrocatalysts with superior activity and long-term stability.Tremendous efforts have been made in exploring electrocatalysts to reduce the energy barriers and improve catalytic efficiency.This review summarizes different categories of precious-metal-free electrocatalysts developed in the past 5 years for alkaline water splitting.The design strategies for optimizing the electronic and geometric structures of electrocatalysts with enhanced catalytic performance are discussed,including composition modulation,defect engineering,and structural engineering.Particularly,the advancement of operando/in situ characterization techniques toward the understanding of structural evolution,reaction intermediates,and active sites during the water splitting process are summarized.Finally,current challenges and future perspectives toward achieving efficient catalyst systems for industrial applications are proposed.This review will provide insights and strategies to the design of precious-metalfree electrocatalysts and inspire future research in alkaline water splitting.

In situ characterizations of solid-solid interfaces in solid-state batteries using synchrotron X-ray techniques

The solid-solid electrode-electrolyte interface represents an important component in solid-state batteries(SSBs),as ionic diffusion,reaction,transformation,and restructuring could all take place.As these processes strongly influence the battery performance,studying the evolution of the solid-solid interfaces,particularly in situ during battery operation,can provide insights to establish the structure-property relationship for SSBs.Synchrotron X-ray techniques,owing to their unique penetration power and diverse approaches,are suitable to investigate the buried interfaces and examine structural,compositional,and morphological changes.In this review,we will discuss various surface-sensitive synchrotron-based scattering,spectroscopy,and imaging methods for the in situ characterization of solid-solid interfaces and how this information can be correlated to the electrochemical properties of SSBs.The goal is to overview the advantages and disadvantages of each technique by highlighting representative examples,so that similar strategies can be applied by battery researchers and beyond to study similar solid-solid interface systems.

Revealing the Catalytic Conversion via in Situ Characterization for Lithium–Sulfur Batteries

Probing effective strategies to accelerate the transformation of sulfur species and alleviate the accumulation of lithium polysulfides is of profound significance for breaking through the bottlenecks of the intrinsic redox kinetics and shuttle effect of lithium–sulfur batteries(LSBs).Introducing catalysts is regarded as a straightforward approach to reduce the conversion barrier of sulfur species for enhancing the performance of LSBs.However,the catalytic mechanism is elusive due to the time-varying,process-dependent,and enclosed reaction processes.Therefore,monitoring the evolution of catalysts and sulfur species by in situ characterization during the full process of the redox reaction is essential to reveal the kinetics and the mechanism of catalytic conversion,which may promote novel and efficient catalyst design.This review outlines the recent progress of in situ characterization techniques to investigate the catalytic mechanism.We focus on the evaluation of the catalytic effect and clarification of the catalytic mechanism by in situ characterization techniques.In addition,a perspective on improving the in situ characterization methods and linked data analysis are proposed to offer research suggestions in the field.

In Situ Characterization of Polymer Matrices for Bio-electrode Applications

1 Results Electropolymerized azines are considered an important group of mediators for NAD+/ NADH-based biocatalytic applications[1].Characterizing these electroactive polymers in situ on electrode surface is vital to understand their behavior and properties.We recently studied the polymer deposition on electrodes using imaging ellipsometry (IE) as an in situ technique[2].The observation of surface morphology development can be conducted in cyclic voltammetric cycles in a nanometer scale.We then combine...

Electrolyte-dependent formation of solid electrolyte interphase and ion intercalation revealed by in situ surface characterizations

The formation of solid electrolyte interphase(SEI) and ion intercalation are two key processes in rechargeable batteries, which need to be explored under dynamic operating conditions. In this work, both planar and sandwich model lithium batteries consisting of Li metal | ionic liquid electrolyte | graphite electrode have been constructed and investigated by a series of in situ surface analysis platforms including atomic force microscopy, Raman and X-ray photoelectron spectroscopy. It is found that the choice of electrolyte, including the concentration and contents, has a profound effect on the SEI formation and evolution, and the subsequent ion intercalation. A smooth and compact SEI is preferably produced in highconcentration electrolytes, with FSI^(-) salt superior to TFSI^(-) salt, facilitating the lithiation/delithiation to achieve high capacity and excellent cycle stability, while suppressing the co-intercalation of electrolyte solvent ions. The innovative research scenario of well-defined model batteries in combination with multiple genuinely in situ surface analysis methods presented herein leads to insightful results, which provide valuable strategies for the rational design and optimization of practical batteries, and energy storage devices in general.

IN SITU X-RAY DIFFRACTION CHARACTERIZATION OF SILVER/SOLUTION INTERFACES

In situ x-ray diffraction electrochemical method is used to study the activation of silver electrode in KCl solution and UPD lead on silver electrode surface. We found that the activation makes the silver crystal thicker in (111), and the arrangement of water molecules on the silver electrode surface with UPD lead is partially ordered.

A review of in situ transmission electron microscopy study on the switching mechanism and packaging reliability in non-volatile memory

Non-volatile memory(NVM)devices with non-volatility and low power consumption properties are important in the data storage field.The switching mechanism and packaging reliability issues in NVMs are of great research interest.The switching process in NVM devices accompanied by the evolution of microstructure and composition is fast and subtle.Transmission electron microscopy(TEM)with high spatial resolution and versatile external fields is widely used in analyzing the evolution of morphology,structures and chemical compositions at atomic scale.The various external stimuli,such as thermal,electrical,mechanical,optical and magnetic fields,provide a platform to probe and engineer NVM devices inside TEM in real-time.Such advanced technologies make it possible for an in situ and interactive manipulation of NVM devices without sacrificing the resolution.This technology facilitates the exploration of the intrinsic structure-switching mechanism of NVMs and the reliability issues in the memory package.In this review,the evolution of the functional layers in NVM devices characterized by the advanced in situ TEM technology is introduced,with intermetallic compounds forming and degradation process investigated.The principles and challenges of TEM technology on NVM device study are also discussed.

Photothermal Catalytic Selective Oxidation of Isobutane to Methacrylic Acid over Keggin-Type Heteropolyacid

Thermal and photothermal catalytic selec-tive oxidation of isobutane to methacrylic acid(MAA)are comparatively studied over a keggin-type Cs2.9Cu0.34V0.49PMo12O40 het-eropolyacid acid.An introduction of light was observed to enhance both the i-C4H10 conversion and the MAA selectivity,and consequently the MAA formate rate,particularly at low temperatures.Characterization re-sults show that oxidation of methacrolein(MAL)to MAA is the rate-limiting step while UV light illumination promotes the oxidation ofσ-bonded MAL with OH groups toσ-bonded MAA on the catalyst surface.These results demonstrate a synergistic effect of thermal cataly-sis and photocatalysis in selective oxidation of isobutane to MAA,which suggests photother-mal catalysis as a promising strategy to catalyze the selective oxidation of higher hydrocar-bons at relative mild reaction conditions.

In situ/operando characterization techniques for electrochemical CO_(2)reduction

Utilizing CO_(2)as a carbon feedstock for producing fuels and useful chemicals is attractive due to the advantages of being abundant,nontoxic,and economical.Electrochemical CO_(2)reduction(CO_(2)RR)provides an avenue to close the anthropogenic carbon cycle.However,the reaction process of multi-electronic products of CO_(2)RR is quite complex.It is hard to yield a target product with high selectivity,high current density,low overpotential,and good stability simultaneously.In recent years,in situ/operando characterization techniques have played important roles in the catalysis field via establishing the structure-reactivity/selectivity relationships of catalysts and thereby obtaining information about mechanisms.As a result,it is necessary to apply in situ/operando characterization technologies to clarify the reaction pathway of CO_(2)RR.In this mini-review,we discuss recent progress on the in situ/operando characterizations for electrochemical CO_(2)RR,including microscopies,infrared spectroscopy,Raman spectroscopy,X-ray photoelectron spectroscopy,and X-ray absorption fine spectroscopy.Moreover,the capabilities of these in situ/operando characterizations and the remaining challenges are also discussed.

Application of in situ/operando characterization techniques in heterostructure catalysts toward water electrolysis

The key to achieving the breakthrough of hydrogen energy from marginal energy sources in large scale applications lies in the development and design of efficient electrocatalysts for the electrochemical oxidation and reduction of water.The unique heterostructure endows the catalyst with a mass of functional interfaces that are decisive for the enhancement of catalyst activity,stability,and reaction kinetics.Although some cutting-edge reviews have focused on the synthesis strategies,constitution,and applications of heterostructure catalysts,the field still lacks a detailed discussion of the actual reaction processes occurring at the interface,which is detrimental to the understanding of the true catalytic mechanism.Relying on advanced in situ/operando characterization techniques to understand the working mechanism of heterostructure catalysts is essential for rational design of advanced catalysts.In this review,we first present the advantages of heterostructure catalysts applied to electrolyzing water.Subsequently,the application of in situ/operando techniques in probing three aspects of heterostructure catalyst surface reconstruction,reaction mechanism,and the role of each component is highlighted with classical case studies.Finally,the current challenges and prospects for the design of heterostructure electrocatalysts are discussed in detail.

Electrochemical Kinetic Modulators in Lithium–Sulfur Batteries:From Defect-Rich Catalysts to Single Atomic Catalysts

Lithium–sulfur batteries exhibit unparalleled merits in theoretical energy density(2600 W h kg^(-1))among next-generation storage systems.However,the sluggish electrochemical kinetics of sulfur reduction reactions,sulfide oxidation reactions in the sulfur cathode,and the lithium dendrite growth resulted from uncontrollable lithium behaviors in lithium anode have inhibited high-rate conversions and uniform deposition to achieve high performances.Thanks to the“adsorption-catalysis”synergetic effects,the reaction kinetics of sulfur reduction reactions/sulfide oxidation reactions composed of the delithiation of Li_(2)S and the interconversions of sulfur species are propelled by lowering the delithiation/diffusion energy barriers,inhibiting polysulfide shuttling.Meanwhile,the anodic plating kinetic behaviors modulated by the catalysts tend to uniformize without dendrite growth.In this review,the various active catalysts in modulating lithium behaviors are summarized,especially for the defect-rich catalysts and single atomic catalysts.The working mechanisms of these highly active catalysts revealed from theoretical simulation to in situ/operando characterizations are also highlighted.Furthermore,the opportunities of future higher performance enhancement to realize practical applications of lithium–sulfur batteries are prospected,shedding light on the future practical development.

Oxygen-Containing Functional Groups Regulating the Carbon/Electrolyte Interfacial Properties Toward Enhanced K^(+)Storage

Oxygen-containing functional groups were found to e ectively boost the K^(+)storage performance of carbonaceous materials,however,the mechanism behind the performance enhancement remains unclear.Herein,we report higher rate capability and better long-term cycle performance employing oxygen-doped graphite oxide(GO)as the anode material for potassium ion batteries(PIBs),compared to the raw graphite.The in situ Raman spectroscopy elucidates the adsorption-intercalation hybrid K^(+)storage mechanism,assigning the capacity enhancement to be mainly correlated with reversible K^(+)adsorption/desorption at the newly introduced oxygen sites.It is unraveled that the C=O and COOH rather than C-O-C and OH groups contribute to the capacity enhancement.Based on in situ Fourier transform infrared(FT-IR)spectra and in situ electrochemical impedance spectroscopy(EIS),it is found that the oxygen-containing functional groups regulate the components of solid electrolyte interphase(SEI),leading to the formation of highly conductive,intact and robust SEI.Through the systematic investigations,we hereby uncover the K^(+)storage mechanism of GO-based PIB,and establish a clear relationship between the types/contents of oxygen functional groups and the regulated composition of SEI.

Quantifying Electrochemical Processes in Batteries and Beyond

The correlation of electrochemical measurements with materials characterization has advanced our understanding of operation and degradation mechanisms in electrochemical energy storage and many other fields.Yet,often these correlations are qualitative,preventing the unambiguous identification of both operational principles and the root causes of performance losses.Here we suggest quantitative approaches to define competing mechanisms and determine their relative contributions.We illustrate the importance of quantitative methodologies over a range of electrochemical systems and highlight the need to consider the effect of the experimental design and measurement itself.These approaches will reveal the most detrimental degradation mechanisms and enable the development of strategies to suppress,stabilize or eliminate them,leading to materials and devices with longer lifetimes,reduced environmental impact,and improved performance.

Diagnosing Battery Degradation via Gas Analysis

Interfacial reactions in lithium-ion batteries often involve gaseous reaction products.Mechanistic investigation of material degradation processes requires a technique to identify and quantify these gases in battery cells.Online electrochemical mass spectrometry(OEMS)is an operando gas analysis method that continuously samples the headspace of a custom battery cell.Real-time gas analysis by quantitative OEMS was used to create mechanistic understanding of battery degradation reactions,some of which will be highlight in this article.

Dynamic investigation of oxygen defects on transition metal-based electrocatalysts:formation,characterization,and mechanism during alkaline oxygen evolution reaction

Oxygen defects play a critical role in the electrocatalytic oxygen evolution reaction(OER).Therefore,in-depth understanding the structure-activity-mechanism relationship of these defects is the key to design efficient OER electrocatalysts.This relationship needs to be understood dynamically due to the potential for irreversible phase transitions during OER.Consequently,significant efforts have been devoted to study the dynamic evolution of oxygen defects to shed light on the OER mechanism.This review critically examines and analyzes the dynamic processes occurring at oxygen defect sites during OER,including defect formation and defect evolution mechanisms,along with the advanced characterization techniques needed to understand these processes.This review aims to provide a comprehensive understanding of high-efficiency electrocatalysts,with a particular emphasis on the importance of in situ monitoring the dynamic evolution of oxygen defects,providing a new perspective towards efficient OER electrocatalyst design.

Reduction and carburization of iron oxides for Fischer–Tropsch synthesis

The activation of iron oxide Fischer–Tropsch Synthesis(FTS) catalysts was investigated during pretreatment: reduction in hydrogen followed by carburization in either CO or syngas mixture, or simultaneously reduction and carburization in syngas. A combination of different complementary in situ techniques was used to gain insight into the behavior of Fe-based FTS catalysts during activation. In situ XRD was used to identify the crystalline structures present during both reduction in hydrogen and carburization. An increase in reduction rate was established when increasing the temperature. A complete reduction was demonstrated in the ETEM and a grain size dependency was proven, i.e. bigger grains need higher temperature in order to reduce. XPS and XAS both indicate the formation of a small amount of carbonaceous species at the surface of the bulk metallic iron during carburization.

Interatomic electron transfer promotes electroreduction CO_(2)-to-CO efficiency over a CuZn diatomic site

Diatomic site catalysts(DACs)with two adjacent atomic metal species can provide synergistic interactions and more sophisticated functionalities to break the bottleneck of intrinsic drawbacks of single atom catalysts(SACs).Herein,we have designed a CuZn diatomic site(CuZn-DAS)electrocatalyst with unique coordination structure(CuN_(4)-ZnN_(4))by anchoring and ordering the spatial distance between the metal precursors on the carbon nitride(C_(3)N_(4))derived N-doped carbon(NC)substrate.The CuZn-DAS/NC shows high activity and selectivity for electroreduction CO_(2)into CO.The Faradaic efficiency for CO of CuZn-DAS/NC(98.4%)is higher than that of Cu single atomic site on NC(Cu-SAS/NC)(36.4%)and Zn single atomic site on NC(Zn-SAS/NC)(66.8%)at-0.6 V versus reversible hydrogen electrode(vs.RHE).In situ characterizations reveal that the CuZn-DAS is more favorable for the formation and adsorption of^(*)COOH than those of the electrocatalysts with single atomic site.Theorical calculations show that the charge redistribution of Zn site in CuZn-DAS/NC caused by the considerable electron transfers from Zn atoms to the adjacent Cu atoms can reduce the adsorption energy barriers for^(*)COOH and^(*)CO production,improving the activity and CO selectivity.

Surface-Enhanced Raman Spectroscopy:Principles,Methods,and Applications in Energy Systems

Surface-enhanced Raman spectroscopy(SERS)has advanced significantly since its inception.Numerous experimental and theoretical efforts have been made to understand the SERS effect and demonstrate its potential.Due to its extremely high sensitivity and selectivity and ability to provide molecular fingerprint information,SERS has a wide range of applications in surface and interfacial chemistry,energy,materials,biomedicine,environmental analysis,etc.This review aims to provide readers with an understanding of the principles,methodologies,and applications of SERS.We briefly introduce the fundamental theory of the SERS enhancement mechanism and summarize the details of the preparation of SERS-active substrates.Recent applications of SERS in energy systems are then highlighted,including probing surface reactions and interfacial charge transfer of batteries and electrocatalysts.Finally,the challenges and prospects of SERS research are discussed.