X-ray Photoelectron Spectrometers Research Articles

(Invited) Advances in XPS Analysis of Battery Materials

Energy storage (batteries, capacitors) and conversion (solar cells) systems are increasing demand for vehicles, mobile electronics, and other areas as alternatives to fossil fuels.These systems are complex and multi-component with many materials and interfaces that are difficult to handle and characterize. To optimize materials properties and improve battery lifetimes, there is a pressing need for physicochemical characterization approaches with air-free handling capabilities, high spatial resolution, the ability to analyze buried interfaces without damaging the chemistry, and the need to bridge the gap by in situ and operando surface analysis.Developments in X-ray Photoelectron Spectrometers (XPS) open up new capabilities to address these challenges. In this talk, I will discuss how we can overcome obstacles in analyzing battery materials using: Inert environment transfer vessel: This allows for handling battery materials in an air-free environment, which is essential for preventing oxidation and other degradation reactions.Microprobe X-ray source with <5 µm spatial resolution: This provides high spatial resolution for analyzing small areas of interest, such as SEI, dendrites and other features.Hard X-ray source and cluster ion gun source: These allow for the analysis of buried interfaces not accessible by conventional soft X-ray XPS.In situ energy gap measurements: These allow for the study of the changes in the electronic structure of battery materials as they are cycled, which can provide insights into the mechanisms of charge storage and discharge.Operando XPS experiments: These allow for the study of the changes in the surface structure of battery materials as they are operating, which can provide insights into the factors that affect their performance. Figure 1

Plasma Enabled Lithophilic Host for Lithium Anodes

The surge in electric vehicles and portable electronics has sparked increased demand for high-energy-density, rechargeable batteries. Lithium (Li) metal is sought after as the highest energy density anodes. However, Li metal suffers from many challenges including dendritic growth during stripping and plating and safety issues. This talk aims to discuss strategies to mitigate dendritic growth by using a 3D carbon-based host, as well as scalable fabrication techniques to integrate Li metal into a desirable 3D matrix. Electron microscopy and X-ray photoelectron spectrometers are combined to investigate the surface chemistry and morphology, in order to better understand the interaction between the Li metal and 3D carbon-based host material. Identified shifts in the C1s and O1s peaks demonstrate added surface functional groups in the carbon-based host material. With surface functionalization, the proposed Li-metal anode demonstrates significantly improved stability of stripping and plating when compared with lithium metal foil. As a result, the cells assembled with our proposed Li metal anode and the LiNixMnyCozO2 (x+y+z=1) cathode displays enhanced cycling reversibility as compared to the cells assembled with the Li metal foil.

Summary of ISO/TC 201 International Standard ISO 18516:2019 Surface chemical analysis—Determination of lateral resolution and sharpness in beam‐based methods with a range from nanometres to micrometres and its implementation for imaging laboratory X‐ray photoelectron spectrometers (XPS)

ISO 18516:2019 Surface chemical analysis—Determination of lateral resolution and sharpness in beam‐based methods with a range from nanometres to micrometres revises ISO 18516:2006 Surface chemical analysis—Auger electron spectroscopy and X‐ray photoelectron spectroscopy—Determination of lateral resolution. It implements three different methods delivering parameters useful to express the lateral resolution: (1) the straight edge method, (2) the narrow line method and (3) the grating method. The theoretical background of these methods is introduced in ISO/TR 19319:2013 Surface chemical analysis—Fundamental approaches to determination of lateral resolution and sharpness in beam‐based methods. The revised International Standard ISO 18516 delivers standardized procedures for the determination of the (1) effective lateral resolution by imaging of square‐wave gratings, the (2) lateral resolution expressed as the parameter D12–88 characterizing the steepness of the sigmoidal edge spread function (ESF) determined by imaging a straight edge and (3) the lateral resolution expressed as the full width of half maximum of the line spread function (LSF), wLSF, determined by imaging a narrow line. The last method also delivers information on the shape of the LSF, which characterizes an individual imaging instrument. Finally, the implementation of all three standardized methods in the field of imaging laboratory X‐ray photoelectron spectroscopy (XPS) is shortly presented. This part of the letter is based on the use of a new test sample developed at ETH Zurich, Switzerland. This test sample displays a micrometre scaled pattern motivated by the resolving power of recent imaging XPS instruments.

In Situ Characterization of the Solid Electrolyte Interphase in Lithium Ion Batteries

The increasing global energy demand has fueled considerable effort into improving upon current energy storage technologies to keep in step with this demand. Improving current energy storage technologies, such as lithium ion batteries (LIBs), requires insight and understanding of the fundamentals of their operation. A key component of LIBs is the solid-electrolyte interphase (SEI), which develops at the interface between the electrode and the electrolyte as a result of reaction with, and degradation of, the battery electrolyte [1]. This passivating layer is vital to the operation and performance of batteries, but little is known of its exact structure since these electrode/electrolyte interfaces are buried deep within the battery, and so are difficult to “look” at using standard characterization techniques [2]. So-called “post-mortem” studies, which involve taking batteries apart inside a glove box, are methodologically simple, but may alter the SEI surface chemistry due to exposing highly reactive materials to contaminants inside a glove box.Here we introduce a platform for in situ characterization of electrode/electrolyte interfaces, capable of operation and disassembly of LIB components (anode, cathode and electrolyte) completely in situ without the need to transfer materials to a glove box prior to characterization. We demonstrate the capabilities of the platform using interface sensitive techniques, including data from one of the first lab-based hard X-ray photoelectron spectrometers, which is based at the Henry Royce Institute at the University of Manchester. Electrochemical data is also presented, demonstrating the electrochemical characterization abilities of the platform, and shows potential for extending beyond studies of LIB technology.

Increasing Bromine in Intracellular Organic Matter of Freshwater Algae Growing in Bromide-Elevated Environments and Its Impacts on Characteristics of DBP Precursors

Bromide elevation in surface freshwater is unavoidable due to climate change and anthropogenic activities. Freshwater impaired by algal activity and bromide elevation has become problematic worldwide. However, studies of their impact on water quality and treatment practices are limited. This study is the first to demonstrate that genera of freshwater algae (e.g., cyanobacterium Microcystis aeruginosa and green alga Chlorella sp.) can accumulate bromine within their intracellular organic matter (IOM) when bromide occurs in water (i.e., 0, 0.5, 1, and 4 mg/L), which substantially enhances brominated disinfection byproduct (Br-DBP) formation upon chlorination. Using energy-dispersive and X-ray photoelectron spectrometers, we discovered clear bromine signals on algal surfaces in both cultures and natural water. Bromide elevation in source water particularly increased bromine accumulation inside algae cells and intracellularly, with greater than 33%–85% found in IOM. This phenomenon dramatically increased Br-DBP formation from Br-rich IOM precursors to ∼80% of total DBPs at an elevated Br– level of 4 mg/L, resulting in the exponential rise of toxic potency of chlorinated water. These findings are significant to understand the origins of natural organic bromine in freshwater environments and provide further insights into how the co-impairment of algae and bromide elevation may influence finished water quality upon chlorination.

Versailles Project on Advanced Materials and Standards interlaboratory study on intensity calibration for x-ray photoelectron spectroscopy instruments using low-density polyethylene.

We report the results of a Versailles Project on Advanced Materials and Standards interlaboratory study on the intensity scale calibration of x-ray photoelectron spectrometers using low-density polyethylene (LDPE) as an alternative material to gold, silver, and copper. An improved set of LDPE reference spectra, corrected for different instrument geometries using a quartz-monochromated Al Kα x-ray source, was developed using data provided by participants in this study. Using these new reference spectra, a transmission function was calculated for each dataset that participants provided. When compared to a similar calibration procedure using the NPL reference spectra for gold, the LDPE intensity calibration method achieves an absolute offset of ∼3.0% and a systematic deviation of ±6.5% on average across all participants. For spectra recorded at high pass energies (≥90 eV), values of absolute offset and systematic deviation are ∼5.8% and ±5.7%, respectively, whereas for spectra collected at lower pass energies (<90 eV), values of absolute offset and systematic deviation are ∼4.9% and ±8.8%, respectively; low pass energy spectra perform worse than the global average, in terms of systematic deviations, due to diminished count rates and signal-to-noise ratio. Differences in absolute offset are attributed to the surface roughness of the LDPE induced by sample preparation. We further assess the usability of LDPE as a secondary reference material and comment on its performance in the presence of issues such as variable dark noise, x-ray warm up times, inaccuracy at low count rates, and underlying spectrometer problems. In response to participant feedback and the results of the study, we provide an updated LDPE intensity calibration protocol to address the issues highlighted in the interlaboratory study. We also comment on the lack of implementation of a consistent and traceable intensity calibration method across the community of x-ray photoelectron spectroscopy (XPS) users and, therefore, propose a route to achieving this with the assistance of instrument manufacturers, metrology laboratories, and experts leading to an international standard for XPS intensity scale calibration.

Computational study of the structures and photoelectron spectra of 12 azabenzenes

The molecular structures of 12 azabenzenes have been optimized with the Gaussian09 package at the level of coupled cluster singles and doubles with the basis set cc-pVTZ. The optimized geometry of each is used in the ADF13 program for the calculation of the vertical ionization energies of all the electrons. For both outer-shell and inner-shell valence electrons, the 2009 method of ΔPBE0(SAOP) is used, whereas the 1999 method of ΔPW86PW91 + Crel is employed for the core electrons. For degenerate orbitals, the alternative method chosen is to use localized orbitals, keeping the integer number of electrons, while giving up proper symmetry, rather than to keep symmetry with fractional electrons. The success of the computed results of valence ionization potentials of pyridine and the diazabenzenes gives confidence for the predicted values for the higher azabenzenes. The calculated results for core-electron binding energies provide incentive to experimentalists to measure them with X-ray photoelectron spectrometers and (or) synchrotron facilities.

Understanding Catalyst Surfaces during Catalysis through Near Ambient Pressure X-ray Photoelectron Spectroscopy.

Heterogeneous catalysis occurs on the surface of a catalyst particle in a gas or liquid environment of reactants. The surface of the catalyst particle acts as an active chemical agent directly participating in a chemical reaction performed at a solid-gas or solid-liquid interface. Thus, authentic surface chemistry and the structure of a catalyst particle during catalysis are key descriptors for understanding catalytic performance of this catalyst. However, identification of the authentic surface of a catalyst particle during catalysis is not a simple task. We are far from knowing the fact. Photoelectron spectroscopy is one of the main techniques for characterizing surface of a catalyst since it's a surface sensitive technique. When used to track the surface of a catalyst particle at relatively high temperature in gas phase in the torr pressure range, it is called near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) or AP-XPS for simplicity. In the last several years, AP-XPS has been used to observe surface chemistry of catalysts of single crystals and nanoparticles of metal, metal oxide, and carbide. In this review, instrumentation of the near ambient pressure X-ray photoelectron spectrometers and observation of catalyst surfaces in gases phase under reaction conditions and during catalysis with AP-XPS are discussed with the following objectives: (1) to present how the surface of a catalyst particle can be characterized in gas phase, (2) to interpret how surface chemistries observed during catalysis are correlated with measured catalytic performances, (3) to demonstrate how the uncovered correlations between surface structures and catalytic performances help to understand catalytic mechanisms at a molecular level, and (4) to discuss challenges and prospects of using AP-XPS to explore the authentic surface of a catalyst under a condition near to an industrial catalytic condition. This review focuses on the application of AP-XPS to studies of catalysis and how the insights gained from AP-XPS studies can be used to achieve fundamental understanding of the catalytic mechanism at a molecular level.

Calibrations and checks of the binding-energy scales of X-ray photoelectron spectrometers

A recent paper in this journal [B. V. Crist, J. Electron Spectrosc. Relat. Phenom. (in press)] describes current and historical calibration problems with core-electron binding energies (BEs) measured by X-ray photoelectron spectrometers and how these problems affect BE data in databases. Although Crist provides useful information about the variability of reported BE values, his article is incomplete or incorrect in some historical and other details. The present article corrects and extends information in the Crist paper and provides additional information on how calibrations and checks of the BE scale should be made.

XPS study of the effects of long-term Ar+ ion and Ar cluster sputtering on the chemical degradation of hydrozincite and iron oxide

Monoatomic ion guns mounted on X-ray photoelectron spectrometers are frequently used for depth profiling to determine the depth distribution of various chemical compounds, or for surface cleaning. Sputtering with single ions may cause severe damage to some materials. Hence, in this study the influence of different sputter parameters on the degradation kinetics was examined. For comparison, the potential of Ar cluster sputtering was tested with the same materials, namely hydrozincite and FeO – two representatives of corrosion products that are susceptible to degradation. Chemical damage could only be minimized by cooling or cluster sputtering within a narrow cluster energy window.

In-situ synthesis of nanomagnetites on poly(amidoamine)-modified graphite oxides and their novel catalytic performances towards the degradation of p-nitroaniline

An approach of loading nanomagnetites (NMs), obtained by in-situ synthesis, on graphite oxides (GO) or poly(amidoamine)-modified GO (pGO) was described in detail in this work. Compared with the GO-NM, the pGO-NM hybrids performed excellent catalysis on degradation of aqueous p-nitroaniline in surrounding air and aqueous medium with a mechanical agitation without light irradiations and additional hydrogen peroxides. These properties differed tremendously from those of most reported metallic oxide-GO hybrids. The catalytic decomposing of the p-nitroaniline was recorded by the successive measurements of an ultraviolet–visible spectrometer. The chemical modifications on GO were investigated with infrared and X-ray photoelectron spectrometers. The hybrids of GO-NM and pGO-NM are estimated with ultraviolet–visible spectrometer, X-ray diffraction, transmission electron microscopy, cyclic voltammetry measurements and thermogravimetric analyses. The experimental results showed that the dissolved oxygen and water were involved in the degradation, which provided solid evidences for the mechanism discussion.

Design and Performance Aspects of a Custom-Built Ambient Pressure Photoelectron Spectrometer toward Bridging the Pressure Gap: Oxidation of Cu, Ag, and Au Surfaces at 1 mbar O2 Pressure

The critical features of a custom-built laboratory version ambient pressure photoelectron spectrometer (Lab-APPES) are presented. A double front cone differential pumping arrangement and an aperture free design employed in the electrostatic lens regime improve the data collection and data quality. In contrast to the conventional X-ray photoelectron spectrometers (XPS) operating at ultrahigh vacuum (UHV), it is possible to explore the electronic structure of solid surfaces under working conditions or closer to working conditions with Lab-APPES. Especially surface-dependent phenomena can be explored up to 1 mbar pressure and up to 873 K by conventional heating methods and at least up to 1273 K by a laser heating method. Simultaneous XPS and reaction kinetic measurements on solid surfaces make the Lab-APPES an important tool to measure the dynamic electronic structure changes on material surfaces under reaction conditions. The interaction of O2 with polycrystalline foils of Cu, Ag, and Au from UHV to 1 mbar ...

Characteristic Features of Stone‐Wales Defects in Single‐Walled Carbon Nanotube; Adsorption, Dispersion, and Field Emission

Adsorption behaviors of dodecanethiol (C12H25SH) molecules are investigated on the surface of single‐walled carbon nanotubes (SWCNTs) with vibrational and X‐ray photoelectron spectrometers. The active adsorption sites are proved as Stone‐Wales (SW) defects (5–7 ring defects). The SW defect‐removed SWCNTs formed by reacting nanotubes with allyl acrylate molecules are compared with pristine SWCNTs in dispersion and field emission. The former shows higher dispersion and field emission than the latter.

Comparison of the accuracies of two methods for the determination of the surface normal for x-ray photoelectron spectroscopy

Determining the true angle of detection of emitted electrons is a major factor for the accurate determination of film thicknesses in the range below 10 nm by x-ray photoelectron spectroscopy. To do this, the orientation of the surface normal in the instrument first requires determination. A comparison is made of two recent methods for the definition of the surface normal in x-ray photoelectron spectrometers. The first method, which we call the crystal axis method, involves the measurement of the Si 2p oxide and substrate intensities in the range ±5° around the surface normal for a (1 0 0) Si wafer with approximately 1 nm of thermal oxide. The second method, which we call the overlayer thickness consistency method, involves measuring similar intensities at 10°, 20°, 30° and 40° from the surface normal, for an amorphous oxide on amorphous Si sample, with a view to obtaining a consistent thickness. It is shown that both can readily define the surface normal alignment better than 1° but that the crystal axis method, for similar data quality and numbers of spectra, generates uncertainties a factor of approximately three times better than that of the overlayer consistency method and has fewer bias contributions. Possible improvements in the method are shown to be best by recording more data in the crystal axis method rather than, for example, by changing the materials.

Repeatable intensity calibration of an X-ray photoelectron spectrometer

Ten years ago, NPL developed an infrastructure for calibrating the intensity response functions of electron spectrometers for Auger electron and for X-ray photoelectron spectroscopies. Two software systems were developed: one for Auger electron spectrometers or for Auger electron and X-ray photoelectron spectrometers combined, and one for X-ray photoelectron spectrometers on their own; the latter being applied if no suitable electron gun is available. The system for Auger electron and X-ray photoelectron spectrometers combined has been used regularly to calibrate the Metrology Spectrometer II at NPL and experience shows that this gives an instrumental intensity consistency of 0.4% over 10 years. Evaluations have not previously been reported at this level. The system for Auger electron and X-ray photoelectron spectrometers combined is used here in preference to the system solely for X-ray photoelectron spectrometers since it is more robust to the sample condition and can be used over a wider energy range. These issues, and how observed variations in the instrument intensity response may arise, are explained.

Summary of ISO/TC 201 standard: XXI. ISO 21270:2004—Surface chemical analysis—X‐ray photoelectron and Auger electron spectrometers—Linearity of intensity scale

AbstractThis International Standard specifies two methods for determining the maximum count rate for an acceptable limit of divergence from linearity of the intensity scale of Auger and x‐ray photoelectron spectrometers. It also includes methods to correct for intensity non‐linearities so that a higher maximum count rate can be employed for those spectrometers for which the relevant correction equations have been shown to be valid. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Summary of ISO/TC 201 Standard: VII ISO 15472 : 2001—surface chemical analysis—x‐ray photoelectron spectrometers—calibration of energy scales

AbstractThis international standard specifies a method for calibrating the binding energy scales of x‐ray photoelectron spectrometers, for general analytical purposes, using unmonochromated Al or Mg x‐rays or monochromated Al x‐rays. It is only applicable to instruments that have an ion gun for sputter cleaning. This international standard further specifies a method to establish a calibration schedule, to test for the binding energy scale linearity at one intermediate energy, to confirm the uncertainty of the scale calibration at one low and one high binding energy value, to correct for small drifts of that scale and to define the expanded uncertainty of the calibration of the binding energy scale for a confidence level of 95%. This uncertainty includes contributions for behaviour observed in inter‐laboratory studies but does not cover all of the defects that could occur. This international standard is not applicable to instruments with binding energy scale errors that are significantly non‐linear with energy, to instruments operated in the constant retardation ratio mode at retardation ratios of &lt;10, to instruments with a spectrometer resolution worse than 1.5 eV or to instruments requiring tolerance limits of ±0.03 eV or less. This international standard does not provide a full calibration check, which would confirm the energy measured at each addressable point on the energy scale and would have to be performed in accordance with the manufacturer's recommended procedures. © Crown Copyright 2001. Published by John Wiley &amp; Sons, Ltd.

Summary of ISO/TC 201 Standard: VII ISO 15472 : 2001—surface chemical analysis—x‐ray photoelectron spectrometers—calibration of energy scales

This international standard specifies a method for calibrating the binding energy scales of x-ray photoelectron spectrometers, for general analytical purposes, using unmonochromated Al or Mg x-rays or monochromated Al x-rays. It is only applicable to instruments that have an ion gun for sputter cleaning. This international standard further specifies a method to establish a calibration schedule, to test for the binding energy scale linearity at one intermediate energy, to confirm the uncertainty of the scale calibration at one low and one high binding energy value, to correct for small drifts of that scale and to define the expanded uncertainty of the calibration of the binding energy scale for a confidence level of 95%. This uncertainty includes contributions for behaviour observed in inter-laboratory studies but does not cover all of the defects that could occur. This international standard is not applicable to instruments with binding energy scale errors that are significantly non-linear with energy, to instruments operated in the constant retardation ratio mode at retardation ratios of <10, to instruments with a spectrometer resolution worse than 1.5 eV or to instruments requiring tolerance limits of ±0.03 eV or less. This international standard does not provide a full calibration check, which would confirm the energy measured at each addressable point on the energy scale and would have to be performed in accordance with the manufacturer's recommended procedures. © Crown Copyright 2001. Published by John Wiley & Sons, Ltd.

Method for determining the signal linearity in single and multidetector counting systems in X-ray photoelectron spectroscopy

Both predictable and unpredictable non-linearities occur in the electron counting systems which provide the intensity scales of different X-ray photoelectron spectrometers. The predictable effects are inherent in good system design and may be accurately corrected as they are both stable and repeatable. Unpredictable effects occur due to inadequate design, deterioration in service or poor setting-up procedures. We have therefore, devised a method to measure this non-linearity in X-ray photoelectron spectroscopy (XPS). A widescan (survey) spectrum is recorded from a copper or stainless steel sample at both high and low X-ray source emission currents. The ratio of these spectra, at each emission energy, allows non-linearities to be easily measured. We show results for one example of a predictable system exhibiting only dead time effects which can be corrected to give 1% accuracy up to 5 Mc/s. We also show results for an unpredictable multichannel detector system where the non-linearity exceeds 5% at counting rates below 20 kc/s and reaches 50% at 500 kc/s.

Measurement of data for and the development of an ISO standard for the energy calibration of X-ray photoelectron spectrometers

New values have been developed for the reference peak energies for the levels Au 4f 7/2, Ag 3d 5/2 and Cu 2p 3/2 used for calibrating the binding energy scales of X-ray photoelectron spectrometers. These values are traceable to previous values but are homogenised between X-ray sources by calculation and are now referenced to the Ag Fermi edge instead of the Ni Fermi edge used earlier. These values, which are now extended to include monochromatic Al X-rays, are typically 0.04 eV lower than previous values. These reference energies are included in a calibration procedure to allow the evaluation of the binding energy repeatability, the energy scale linearity, the energy offset and its drift with time. Evaluation of these parameters then permits the energy scale to be calibrated within a user-defined tolerance limit over the binding energy range 0 to 1040 eV by a simple regular procedure.