Surface-sensitive Analytical Techniques Research Articles

Deciphering the True FEC Reduction Mechanism and Its Implications to the Understanding of the SEI in Li-Ion Batteries

The main challenges in rechargeable batteries, especially when aiming for high energy density, arise from the limited electrochemical stability window of current state-of-the-art electrolytes. Negative electrode active materials, such as silicon, tin oxide and others, whose capacity is many times higher than that of graphite, often exhibit large volume expansion upon lithiation and therefore the SEI formed on their surface is unstable. The most popular strategy for improving their cycling stability is the use of electrolyte additives that are reduced at potentials more positive than the electrolyte solvents. Their decomposition products are incorporated in the SEI, leading to a more stable electrode/electrolyte interface.One of the most-investigated electrolyte additives is fluoroethylene carbonate (FEC), because it has been shown to lead to a significant performance enhancement of Si- and Sn-based anodes. Many reaction pathways have been proposed, however there is currently no agreement on the exact type of chemical compounds constituting the decomposition products, nor on the exact mechanism for FEC decomposition. To address these questions, we conducted a systematic study that tracked the morphological and chemical changes during electrochemical decomposition of FEC. We have found that despite FEC often being referred as a “film-forming” additive, the first stage of its decomposition leads instead to the formation of spherical particles, consisting mainly of lithium fluoride (see figure below);1 only later a carbonate-rich film covers the entire electrode, covering as well the LiF-rich spheres. This phenomenon has been overlooked before likely due to very simple reason, disclosed in our contribution.A detailed investigation using XPEEM—a surface-sensitive analytical technique with high lateral resolution—as a function of electrolyte composition shows that fluorine in the later-formed carbonate film comes from the electrolyte salt, while all fluorine from FEC decomposition resides in the spherical particles, formed upon FEC reduction. In addition, we have found that the size and amount of particles strongly depend on the cell medium, where electrolytes with higher dielectric constant lead to larger particle size, as does the presence of the high-capacity electroactive materials in the electrode, both contributing to the final properties of FEC-derived SEI. The results of this study provide a deeper understanding of how fluorine-containing additives work and enables tuning of the SEI properties to the desired morphological and chemical outcome by using the laws of simple crystal-growth theory by adjusting the inter-cell environment.This work demonstrates that, with the right analytical tools, the true nature of the even seemingly very-well studied compounds can be revealed and a full understanding of their decomposition mechanisms can be clarified. Acknowledgement This research was supported by InnoSuisse (Project number 18254.2) References Y. Surace, D. Leanza, M. Mirolo, Ł. Kondracki, C.A.F. Vaz, M. El Kazzi, P. Novák, S. Trabesinger, Energy Storage Materials 2022, 44, 156-167. Figure 1

Cathode Surface Segregating Modification for Boosting Oxygen Reduction Reactions: Coupling Theory and Experiment in Proton Conducting Solid Oxide Fuel Cells (H-SOFCs)

The surface electrochemical catalysis reaction is complex due to the unpredictable surface morphology and severe reaction environment, especially for cathodes in proton conducting solid oxide fuel cells (H-SOFCs) where water vapor is generated and evaporated at a high operating temperature. The first-generation cathode materials (LaxSr1-xMO3-δ, M=Fe, Co, Mn) tend to have A-site cation segregation, and this segregation layer reduces the performance. Surface segregation phenomena tends to be ignored by computer simulations because surface termination is described as a highly crystallized layer, misleading the development of material computing science. Therefore, a combination of theory and surface investigation is essential for future work in this community. In this work, we use density functional theory (DFT) for designing a higher-performing cathode material, La0.35Bi0.15Sr0.5FeO3-δ (LBSF), for improvement of both bulk and surface electrochemical character in an intrinsic manner, predicting a candidate material with good performance that can be used in solid oxide proton conducting fuel cells (H-SOFCs). By combining a sensitive surface analysis technique, low energy ion scattering (LEIS), surface segregation is finely considered and determined that the particles are fully covered with a Sr segregation layer on the outer most surface. Aberration-Corrected transmission electron microscopy (ACTEM) is also applied and shows an amorphous structure at the particle edge. With the determination of this segregation layer, a reality-closed model is given, and related surface reactions are modelled based on this layer instead of a conventional cleaved, well-crystallized surface. Also, as bismuth diffuses towards the surface due to the lone pair effect, the segregation becomes an oxygen reduction reaction preferred layer instead of a blocking layer. After computational prediction and surface analysis, LBSF is successfully applied into H-SOFCs and shows excellent performance with a peak power density of 1630 mW cm−2 at 700 oC. In addition, long term stability tests were conducted to ensure stability in future commercial use, and the experimental results show that LBSF works stable under extremely high temperature operating environments. In addition, a full cell was stabilized for about 300 hours, indicating a potential commercial application in the future.

Development of highly active and coke-resilient Ni-based catalysts for low-temperature steam reformation of methane

In this work, we report on the development of highly active and stable catalysts for low temperature steam reformation of methane. The Ni-based catalysts supported on alumina were synthesized by the single step solution combustion synthesis (SCS) method. A combination of various surface and bulk sensitive analytical techniques such as XRD, cyclic TPDRO, XPS and HRTEM-SAED was utilized for detailed characterization of the catalysts. The catalyst 5NC synthesized by the SCS method exhibited superior activity and excellent stability for steam reformation of methane during the investigated period on stream for around 200 h. The light-off for methane conversion over the 5NC catalyst started at a reaction temperature of 350 °C whereas for the 5NP catalyst no activity below 600 °C was observed. Moreover, full methane conversion over the 5NC catalyst was achieved at 700 °C. Under similar conditions at reaction temperature of 700 °C, the methane decomposition rates over the 5NC catalyst was around 20 times higher than that of the 5NP catalyst. The exceptional high performance of the 5NC catalyst was attributed to the presence of surface defects, generation of nickel aluminates (NiAl2O4) nano-crystallites, uniform distribution and smaller metal oxide particle size.

Design of hexagonal gold nano disks for surface-enhanced Raman scattering based sensing

Surface-enhanced Raman scattering (SERS), a powerful surface-sensitive analytical technique capable of amplifying weak Raman signals finds application for a wide variety of problems requiring sensitive and targeted analyte detection. To further expand its practical applications, immense research works are going on in the field of designing highly sensitive SERS substrates with the intention of enhancing the signal intensity and improving the detection limit. In this work, we have theoretically designed a three-dimensional SERS substrate based on Au hexagonal nanodisks capable of potential trace detection of chemical, and biological, analytes to the single molecule level. The structure design and the performance evaluations were carried out using the finite element method (FEM) in the visible to near-IR region. Hexagonal nanostructures seem extremely promising in improving Raman detection sensitivity due to the presence of a large number of spatially localized hotspots. Although the SERS effect is primarily focused on surface plasmon resonance from metal nanostructures, the gap between disks significantly contributes to the field enhancement as observed in the monomer, dimer, and trimer hexagonal systems. Various spatially oriented two-dimensional arrays of trimer nanostructures were compared in this study on account of EF to propose a highly efficient SERS sensing platform, enabling the trace detection of chemical, and biological analytes and to the single molecule level applications.

A nanoscale analysis method to reveal oxygen exchange between environment, oxide, and electrodes in ReRAM devices

The limited sensitivity of existing analysis techniques at the nanometer scale makes it challenging to systematically examine the complex interactions in redox-based resistive random access memory (ReRAM) devices. To test models of oxygen movement in ReRAM devices beyond what has previously been possible, we present a new nanoscale analysis method. Harnessing the power of secondary ion mass spectrometry, the most sensitive surface analysis technique, for the first time, we observe the movement of 16O across electrically biased SiOx ReRAM stacks. We can therefore measure bulk concentration changes in a continuous profile with unprecedented sensitivity. This reveals the nanoscale details of the reversible field-driven exchange of oxygen across the ReRAM stack. Both the reservoir-like behavior of a Mo electrode and the injection of oxygen into the surface of SiOx from the ambient are observed within one profile. The injection of oxygen is controllable through changing the porosity of the SiOx layer. Modeling of the electric fields in the ReRAM stacks is carried out which, for the first time, uses real measurements of both the interface roughness and electrode porosity. This supports our findings helping to explain how and where oxygen from ambient moisture enters devices during operation.

Surface Stoichiometry and Roughness of a Degraded A380 Alloy after Casting, Technical Cleaning and Packaging.

The surface stoichiometry of the degraded surface of a commercial Al–Si casting was investigated. The die-cast component was previously stored in a sealed plastic envelope. After that, surface stains in the form of white layers were observed. X-ray photoelectron spectroscopy (XPS) was used to study these layers. For comparison, a seemingly unaffected area as well as a freshly cut surface of the casting were also analysed. In order to additionally assess the surface condition, surface roughnesses were measured. Based on the binding energies (BEs) of the Al and O in the XPS spectra, and the stoichiometric results, it was concluded that the surface layers of the degraded and undegraded samples consist mostly of aluminium oxide and aluminium hydroxide. Furthermore, sodium phosphate from the leftover detergent and silicon oxide were detected in both analysed areas. Analyses of the Al KLL Auger transition were used to corroborate this. The relative shares of Al oxide vs. hydroxide based on the elemental concentrations were determined. The chemical compositions and chemical states of the elements in the top layers were thus obtained. The combination of surface-sensitive analytical techniques was found to be a suitable tool for the ex-post identification of the source of defects.

Why Do Tethered-Bilayer Lipid Membranes Suit for Functional Membrane Protein Reincorporation?

Membrane proteins (MPs) are essential for cellular functions. Understanding the functions of MPs is crucial as they constitute an important class of drug targets. However, MPs are a challenging class of biomolecules to analyze because they cannot be studied outside their native environment. Their structure, function and activity are highly dependent on the local lipid environment, and these properties are compromised when the protein does not reside in the cell membrane. Mammalian cell membranes are complex and composed of different lipid species. Model membranes have been developed to provide an adequate environment to envisage MP reconstitution. Among them, tethered-Bilayer Lipid Membranes (tBLMs) appear as the best model because they allow the lipid bilayer to be decoupled from the support. Thus, they provide a sufficient aqueous space to envisage the proper accommodation of large extra-membranous domains of MPs, extending outside. Additionally, as the bilayer remains attached to tethers covalently fixed to the solid support, they can be investigated by a wide variety of surface-sensitive analytical techniques. This review provides an overview of the different approaches developed over the last two decades to achieve sophisticated tBLMs, with a more and more complex lipid composition and adapted for functional MP reconstitution.

Surface chemistry of the ladybird beetle adhesive foot fluid across various substrates.

Nature has coevolved highly adaptive and reliable bioadhesives across a multitude of animal species. Much attention has been paid in recent years to selectively mimic these adhesives for the improvement of a variety of technologies. However, very few of the chemical mechanisms that drive these natural adhesives are well understood. Many insects combine hairy feet with a secreted adhesive fluid, allowing for adhesion to considerably rough and slippery surfaces. Insect adhesive fluids have evolved highly specific compositions which are consistent across most surfaces and optimize both foot adhesion and release in natural environments. For example, beetles are thought to have adhesive fluids made up of a complex molecular mixture containing both hydrophobic and hydrophilic parts. We hypothesize that this causes the adhesive interface to be dynamic, with molecules in the fluid selectively organizing and ordering at surfaces with complimentary hydrophobicity to maximize adhesion. In this study, we examine the adhesive fluid of a seven-spotted ladybird beetle with a surface-sensitive analytical technique, sum frequency generation spectroscopy, as the fluid interacts with three substrates of varied wettabilities. The resulting spectra present no evidence of unique molecular environments between hydrophilic and hydrophobic surfaces but exhibit significant differences in the ordering of hydrocarbons. This change in surface interactions across different substrates correlates well with traction forces measured from beetles interacting with substrates of increasing hydrophobicities. We conclude that insect adhesion is dependent upon a dynamic molecular-interfacial response to an environmental surface.

Measurement of Peptide Coating Thickness and Chemical Composition Using XPS.

X-ray photoelectron spectroscopy is a highly surface-sensitive analytical technique, capable of providing quantitative information on the chemical composition of materials within the top ∼10nm of their surface. For samples consisting of distinct underlayer and overlayer materials, the thickness of the coating can also be determined if it falls within this ∼10nm information depth, which is often the case for peptide layers. Such measurements are simple to perform for flat samples and can also be performed on nanoparticulate samples provided that either the core radius or total particle radius are known. Here, we describe a straightforward protocol for obtaining such measurements from peptide coatings on both flat surfaces and nanoparticles, including preparation of nanoparticle samples from suspension, data acquisition, and analysis.

Improved accuracy in multicomponent surface complexation models using surface-sensitive analytical techniques: Adsorption of arsenic onto a TiO2/Fe2O3 multifunctional sorbent

Novel composite materials are increasingly developed for water treatment applications with the aim of achieving multifunctional behaviour, e.g. combining adsorption with light-driven remediation. The application of surface complexation models (SCM) is important to understand how adsorption changes as a function of pH, ionic strength and the presence of competitor ions. Component additive (CA) models describe composite sorbents using a combination of single-phase reference materials. However, predictive adsorption modelling using the CA-SCM approach remains unreliable, due to challenges in the quantitative determination of surface composition. In this study, we test the hypothesis that characterisation of the outermost surface using low energy ion scattering (LEIS) improves CA-SCM accuracy. We consider the TiO2/Fe2O3 photocatalyst-sorbents that are increasingly investigated for arsenic remediation. Due to an iron oxide surface coating that was not captured by bulk analysis, LEIS significantly improves the accuracy of our component additive predictions for monolayer surface processes: adsorption of arsenic(V) and surface acidity. We also demonstrate non-component additivity in multilayer arsenic(III) adsorption, due to changes in surface morphology/porosity. Our results demonstrate how surface-sensitive analytical techniques will improve adsorption models for the next generation of composite sorbents.

Precise capture and dynamic relocation of nanoparticulate biomolecules through dielectrophoretic enhancement by vertical nanogap architectures

Toward the development of surface-sensitive analytical techniques for biosensors and diagnostic biochip assays, a local integration of low-concentration target materials into the sensing region of interest is essential to improve the sensitivity and reliability of the devices. As a result, the dynamic process of sorting and accurate positioning the nanoparticulate biomolecules within pre-defined micro/nanostructures is critical, however, it remains a huge hurdle for the realization of practical surface-sensitive biosensors and biochips. A scalable, massive, and non-destructive trapping methodology based on dielectrophoretic forces is highly demanded for assembling nanoparticles and biosensing tools. Herein, we propose a vertical nanogap architecture with an electrode-insulator-electrode stack structure, facilitating the generation of strong dielectrophoretic forces at low voltages, to precisely capture and spatiotemporally manipulate nanoparticles and molecular assemblies, including lipid vesicles and amyloid-beta protofibrils/oligomers. Our vertical nanogap platform, allowing low-voltage nanoparticle captures on optical metasurface designs, provides new opportunities for constructing advanced surface-sensitive optoelectronic sensors.

Application of Random Forests in ToF-SIMS Data

Abstract Surface analysis techniques are particularly important in the field of materials science, which help researchers to understand the mechanism behind complex chemical reactions and study the properties of different materials. Time-of-flight secondary ion mass spectrometry (ToF-SIMS), a highly sensitive surface analysis technique, allows the reliable determination of various materials. ToF-SIMS spectra of materials are usually enormously complex since typical raw data may include many peaks over large mass-to-charge ratio (m/z) ranges. Hence, the use of data-mining methods in processing ToF-SIMS data is becoming more popular and important. In this study we show that random forests model can be used to automatically classify several different lithium-containing materials and to extract representative peaks from ToF-SIMS spectra of these materials. Our study shows good performance in analyzing spectra of materials with similar and dissimilar compositions, which can provide researchers with the possibility of quick and automatic analysis of ToF-SIMS data.

In situ and sensitive monitoring of configuration-switching involved dynamic adsorption by surface plasmon-coupled directional enhanced Raman scattering.

Surface adsorption studies play a crucial role in numerous fields from surface catalysis to molecular separation. However, investigation on adsorption mechanisms has been restricted to limited analytes and approaches, which calls for an in situ and sensitive surface analysis technique capable of revealing the mechanisms as well as discriminating different adsorbates and their geometry at different adsorption stages. In this study, we employed surface plasmon-coupled directional enhanced Raman scattering (SPCR), a novel technique developed by coupling surface plasmon-coupled emission with SERS, to study conformation-switching involved dynamic adsorption with background suppression and improved sensitivity (nearly 30-fold). We obtained the isotherms for a conformation-changing Raman model analyte, malachite green. An S-type Langmuir model was fitted from the time-resolved SPCR signals sensitively and without any interference from the bulk solution. The reorientation of the analyte from a predominantly parallel configuration to a perpendicular one was captured by the dramatic increase in the intensity ratios of the adsorption-related peaks to the adsorption-unrelated peak. We believe that this new sensitive and selective SPCR technique will be a promising tool for surface adsorption kinetics analysis.

Low Temperature deNOx Catalytic Activity with C2H4 as a Reductant Using Mixed Metal Fe-Mn Oxides Supported on Activated Carbon

The selective catalytic reduction of NOx (deNOx) at temperatures less than or at 200 °C was investigated while using C2H4 as the reductant and mixed oxides of Fe and Mn supported on activated carbon; their activity was compared to that of MnOx and FeOx separately supported on activated carbon. The bimetallic oxide compositions maintained high NO conversion of greater than 80–98% for periods that were three times greater than those of the supported monometallic oxides. To examine potential reasons for the significant increases in activity maintenance, and subsequent deactivation, the catalysts were examined by using bulk and surface sensitive analytical techniques before and after catalyst testing. No significant changes in Brunauer-Emmett-Teller (BET) surface areas or porosities were observed between freshly-prepared and tested catalysts whereas segregation of FeOx and MnOx species was readily observed in the mono-oxide catalysts after reaction testing that was not detected in the mixed oxide catalysts. Furthermore, x-ray diffraction and Raman spectroscopy data detected cubic Fe3Mn3O8 in both the freshly-prepared and reaction-tested mixed oxide catalysts that were more crystalline after testing. The presence of this compound, which is known to stabilize multivalent Fe species and to enhance oxygen transfer reactions, may be the reason for the high and relatively stable NO conversion activity, and its increased crystallinity during longer-term testing may also decrease surface availability of the active sites responsible for NO conversion. These results point to a potential of further enhancing catalyst stability and activity for low temperature deNOx that is applicable to advanced SCR processing with lower costs and less deleterious side effects to processing equipment.

Auger Electron Spectroscopy-A Review

Auger electron spectroscopy (AES) is surface sensitive analytical technique mainly,it provides quantitative elemental and chemical state information from surfaces of solid materials.It utilizes a high current, finely focused electron beam as an excitation source.In AES,A sample is probed with electron beam with energy between 3 to 3o Kev which results in ejection of electron in core level, filled by an outer level electron with excess energy being used then, this excess energy used to emit an electron this, emitted electron is said as Auger electron. That result as function of auger electron energy. The resulting spectra is obtained used to determine the composition of samples. The Auger electron kinetic energies are characteristic of emitting atoms and the measurement of their energies is used to identify the elements variation of composition with depth can be determined by ion sputtering technique. Lastly,AES is applicable for detection of thin films, characterization of materials, metallurgy and corrosion science.

Nanotoxicity of different sizes of graphene (G) and graphene oxide (GO) in vitro and in vivo

Graphene family nanomaterials (GFNs) have attracted significant attention due to their unique characteristics and applications in the fields of biomedicine and nanotechnology. However, previous studies highlighted the in vitro and in vivo toxicity of GFNs with size and oxidation state differences are still elusive. Therefore, we prepared graphene (G) and graphene oxide (GO) of three different sizes (S-small, M-medium, and L-large), and characterized them using multiple surface-sensitive analytical techniques. In vitro assays using HEK 293T cells revealed that the small and large sizes of G and GO significantly reduced the cell viability and increased DNA damage, accompanying with activated reactive oxygen species (ROS) generation and induced various expressions of associated critical genetic markers. Moreover, the bacterial assays highlighted that G and GO caused strong acute toxicity on Tox2 bacteria. Effects of G were higher than GO and showed size dependent effect: L > M > S, while the medium size of GO induced mild genetic toxicity on RecA bacteria. In vivo assays revealed that exposure to G and GO caused the developmental toxicity, induced ROS generation, and activated related pathways (specifically GO) in zebrafish. Taken together, G showed stronger ability to decrease the survival rate and induce the acute toxicity, while GO showed obvious toxicity in terms of DNA damages, ROS generation, and abnormal gene expressions. Our findings highlighted that G and GO differentially induced toxicity based on their varying physical characteristics, especially sizes and oxidation state, and exposure concentrations and sensitivity of the employed in vitro and in vivo models. In short, this study provided deep insights on the negative effects of GFNs exposure.

A Multi-Technique Approach for a Complete Thin Film Characterisation

Here we provide an overview of a multi-technique investigation of layered thin film and ultra-thin film coatings. Surface sensitive analytical techniques are chosen for their different sampling depths and include a combination of conventional Al Kα and higher photon energy Ag Lα excited XPS, angle resolved X-ray photoelectron spectroscopy (ARXPS) with maximum entropy method (MEM) reconstruction of concentration depth profiles and argon cluster depth profiling.

A comparative study of diaryl carbene insertion reactions at polymer surfaces

A detailed investigation of the reactions of diaryldiazo compounds for the surface modification of several polymers has been conducted. This has revealed that the rate of reaction of diaryldiazo compounds is influenced by their substituents, that the reaction is exothermic, and that the polymer itself exerts an influence on the surface modification. The results are consistent with the formation and insertion of a carbene intermediate at the polymer surface. It was further shown that such surface modified polymers, characterized using surface sensitive techniques, display macroscopic behavior consistent with the presence of the newly introduced surface chemical functionality, characterized using a combination of surface sensitive and bulk analytical techniques. This approach is complementary to directed energy deposition approaches, offers an alternative route to functional polymers, and provide a direct link between surface chemistry and observable macroscopic properties.

Insights into the Oxidant/Polymer Interfacial Growth of Vapor Phase Polymerized PEDOT Thin Films

AbstractThe vapor phase polymerization (VPP) technique is used to produce thin films of poly(3,4‐ethylenedioxythiopene) (PEDOT) in which the Fe(III)Tosylate oxidant is altered. The oxidant is changed with the addition of an amphiphilic co‐polymer having different molecular weights, namely 2800 Da. and 5800 Da. Resulting PEDOT films produce conductivities of ≈1500 and ≈3000 S cm−1 respectively. Small angle X‐ray diffraction (SA‐XRD) indicates that the oxidant incorporating the larger molecular weight co‐polymer possesses ordered structure and that this in turn helps “template” the PEDOT during film formation. The structure and composition of the bottom (i.e. initial film formation) and top (i.e. final film formation) PEDOT surfaces are studied using surface sensitive analytical techniques; small angle X‐ray diffraction (SA‐XRD), ultraviolet photoelectron spectroscopy (UPS), 2D grazing incidence X‐ray diffraction (2D‐GIXD), metastable induced electron spectroscopy (MIES) and neutral impact collision ion scattering spectroscopy (NICISS). The results indicate that the increase in conductivity using the larger molecular weight co‐polymer additive is due to the film having larger lamella‐ and π‐stacking regions in addition to doping levels which remain unchanged throughout film formation. These conclusions are further supported by results obtained on a model PEDOT:Tosylate system using density functional theory (DFT) calculations.

Metal/Matrix Enhanced Time-of-flight Secondary Ion Mass Spectrometry for Single Cell Lipids Analysis

Abstract The chemical components analysis of single cell is important for the understanding of physiological processes such as cell growth, signal transduction and apoptosis. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a sensitive surface analysis technique with high spatial resolution, which has been used for single cell and micro-area analysis. However, relatively low ionization yield of biomolecules limited its wide applications in single cell analysis. Herein, we used metal substrate and matrix material to enhance the ionization yield of lipids. The signal intensity of phosphatidylcholine (PC 40:0) casted on the matrix/gold-coated silicon substrate was 65 times higher than that on the silicon wafer. The signal enhancement of phosphatidylcholine (PC 34:1) on single cell surface cultured on matrix/gold-coated silicon substrate was observed as well. Owing to the influence of irregular topography and complex chemical environment of cell, the increase of lipids signal was smaller. Delayed extraction mode of ToF-SIMS overcame the effects of cell topography, leading to further enhancement of the signal intensity of lipids. Meanwhile, simultaneous high spatial resolution of chemical imaging and high mass resolution of the mass spectra of single cells were obtained. Our strategies provided new insights into the study of cell metabolism and cell-environment interactions.