AVS 68 Session AS-ThP: Applied Surface Science Poster Session

Single-site metal centers in heterogeneous catalysts are highly desirable to improve reaction selectivity, due to the uniform active site geometry. It is often advantageous to have a predictable 2-D structure of single-site metal centers, making it necessary to integrate the single-site centers into an ordered array of receptor sites. One way to approach this challenge is through the formation of covalent organic frameworks (COFs). In the work reported here, we utilize a surface-assisted dehalogenation reaction of 1, 3, 5–tris(4-bromophenyl)benzene (TBB) to form a 2-D covalent organic framework on single-crystal metal surfaces. Following successful formation of the 2-D framework on the surface, favorable binding sites for metal catalysts (platinum) are formed by depositing 1, 10-phenathraline-5, 6-dione (PDO) on the surface. PDO becomes confined within the porous COF and is then able to stabilize vapor deposited metal and single atom sites. X-ray photoelectron spectroscopy confirms the oxidation of the platinum metal when deposited with PDO molecules, while scanning tunneling microscopy confirms the confinement of PDO within the pores, and the absence of any external 1-D Pt:PDO chains. The highly predictable nature of the covalent organic framework chemistry makes this an exciting result for single-site metal center formation.

Scanning tunneling microscopy, through the observation of molecular clusters and monolayers on surfaces, provides direct experimental evidence as to how intermolecular interactions result in the self-assembly of extended structures. A detailed understanding of the interplay of interactions will result in improved understanding and prediction of the behavior of self-assembling systems, with implications in a wide range of disciplines ranging from crystal engineering to supramolecular chemistry to protein secondary and tertiary structure. We present experimental results on the self-assembly of proline, as well as the results from computational modeling. Pulse deposition of d-proline on Au(111) reveals that it forms dimer or catamer chains and pentamer clusters. The dimer structures are chiral, as would be expected from an enantiopure adsorbate; however, the pentamers on the surface appear racemically mixed. Comparison of self-assembled structures to related molecules reveals similar behavior for proline, indole-2-carboxylic acid, and indoline-2-carboxylic acid, but substantial differences are observed for pyrrole-2-carboxylic acid.

X-ray photoelectron spectroscopy (XPS) is a highly quantitative and surface sensitive technique for analysis of the chemical composition of a given sample. However, it is important that the data obtained during a photoemission experiment is interpreted correctly – both instrumental and sample-related factors must be accounted for to obtain a quantitative understanding of the sample composition. It is necessary to know the energy-dependent spectrometer response function ('transmission function') of the XPS instrument, as well as the sample-dependent relative sensitivity factors (RSFs) which account for the differences in emitted electron intensities for each peak.

In a recent VAMAS interlaboratory study, the huge variability of transmission functions between different laboratories and instruments has been demonstrated, underlining the need for an ISO standard on XPS intensity calibration which is now being developed under the auspices of ISO TC201 “Surface Chemical Analysis”. Here we present instrument geometry-corrected reference spectra of low-density polyethylene (LDPE) for Al Kα instruments which are traceable to gold, silver, and copper reference spectra from the National Physical Laboratory (NPL). Sensitivity factors are required to account for differences in the intensity of emitted electrons from different materials within a sample. Such differences arise due to several factors, including the photoionisation cross-section; electron kinetic-energy-dependent variations in transport through a material; and the anisotropic emission of excited electrons. Theoretically determined sensitivity factors, such as the NPL AMRSFs for Al and Mg anode instruments,are commonly used.

For hard x-ray photoelectron spectroscopy (HAXPES), photoionisation cross-sections decrease significantly as photon energy increases, and anisotropy effects become more complex due to the increased contribution of non-dipolar effects. It is therefore increasingly important that a careful consideration be given to the estimation of RSFs. This is further complicated by the multiple photon energies and instrument geometries available in commercially available HAXPES instrumentation. Here we present a set of formulae, derived from fitting of theoretical database values, which allow the estimation of AMRSFs for instruments using any photon energy in the range 1.5 keV - 10 keV, with any instrument geometry. A few instrument geometries in which X-rays are polarised are insensitive to angular emission effects, and these are highlighted in this work.

XPS has become a mainstay of the suite of analytical tools available for characterisation of new materials and processes.It would be fair to say that XPS instrumentation is now viewed as a “tool” by many users and the expectation is therefore that results are automatically produced.Here we discuss the latest improvements and advancements which provide not just data but results and answers.We look at improving peak identification from unknown samples and automatic region spectral acquisitions.In addition, we show how known samples can be quickly and efficiently acquired and processed to provide comparison results for QA or trend analysis.

We have developed artificial intelligence based methodology that can be utilized to reliably analyze experimental results from Extended X-ray Absorption Fine Structure (EXAFS) measurements. This development will help to address the reproducibility problems that slow research progress and inhibit effective tech transfer and manufacturing innovation in these scientific disciplines. A machine learning approach was applied to the analysis of extended X-ray absorption fine structure (EXAFS) spectroscopy measurements collected using a synchrotron radiation facility. Specifically, a genetic algorithm was developed for fitting of the measured spectra to extract the relevant structural parameters. The current approach relies on a human analyst to suggest a potential set of chemical compounds in the form of feff.inp input files that may be present. The algorithm then attempts to determine the best structural paths from these compounds that are present in the experimental measurement. The automated analysis looks for the primary EXAFS path contributors from the potential compounds. It calculates a goodness of fit value that can be used to identify the chemical moieties present. The analysis package is called EXAFS Neo and is open source written in Python. It requires the use of Larch and Feff for calculating the initial EXAFS paths. We have recently extended the code to make use of Feff8.5lite so it can calculate the paths needed for populating the analysis from within the EXAFS Neo package. The code base has been expanded for the analysis of nanoindentation data and simple x-ray photoelectron spectroscopy measurements. The publication describing the analysis package and where to obtain the software can be downloaded at: https://doi.org/10.1016/j.apsusc.2021.149059 or by contacting the speaker.

Development of sustainable technologies that produce energy are extremely important to reduce the environmental impacts of current technologies. Proton exchange membrane fuel cells (PEMFCs) generate clean energy using hydrogen and oxygen with water as the main byproduct. The state-of-the-art catalyst for the oxygen reduction reaction occurring at the cathode is based on Pt nanoparticles dispersed on carbon support (Pt/Carbon). The cathode catalyst layer has a major influence on the overall cost of a PEMFC due to the loading of platinum that is needed to achieve a reasonable power density. Optimization of the catalyst layers to achieve the most efficient distribution of the ionomer coating which directly effects the mass transport and device efficiency, is one of the current needs especially for catalysts layers produced using scalable routes. Furthermore, there is a general lack of understanding about how the catalyst support, catalyst, and ionomer interact with one another on a molecular scale. This work utilizes X-ray Photoelectron Spectroscopy (XPS), a highly surface sensitive technique to characterize elemental and chemical speciation of the catalyst layers. XPS is a very common technique to study catalyst but has not been widely applied to investigate catalyst layers.This poster will provide a general background on sample preparation, and data acquisition and analysis of catalysts and catalysts layers made with platinum on High Surface Area Carbon (HSC) catalyst, including findings related to Nafion ionomer degradation. Furthermore, this poster will present comparison of chemistry of the catalyst before and after incorporation into catalyst layer, and will discuss composition of catalyst layers as a function of ionomer loading.

Additive manufacturing is an ever-growing area of research interest that depends upon the high quality of precursor materials in order to achieve a robust final printed product. In this work, an alloy powder material of CuCrZr used in the laser-powder bed diffusion process of 3D printing will be analyzed for grain boundary diffusion and for chemical variation among different particle sizes. Recent advances in Auger Electron Spectroscopy, including in situ FIB capabilities, put AES at the forefront of surface analysis techniques with respect to characterization of such defects on the nanoscale. The quantitative elemental information AES provides from solid surfaces combined with FIB tomography allows for in situ cross-sectioning and subsequent elemental characterization of the CuCrZr powder particles. The variation of chemistry across different sized particles will also be assessed via quantitative chemical analysis using XPS.

In this work, CuCrZr alloy powder particles were cross-sectioned and analyzed using a PHI 710 scanning Auger nanoprobe equipped with a 25kV Schottky field emission electron gun and a coaxial Cylindrical Mirror Analyzer (CMA). We demonstrate the use of AES in conjunction with a focused ion beam (FIB) to produce site-specific imaging of grain boundary diffusion within the alloy.

X-ray photoelectron spectroscopy (XPS) is widely used to probe the top 10 nm of materials. It is based on the photoelectric effect. Spectra acquired in XPS typically require peak fitting. To properly peak fit, various mathematical functions, including the background and peak shapes, are used. Additionally, different experimental conditions typically change the types of peaks that are required for a fit. We have surveyed the XPS reporting from 2021 papers in three major journals. About 900 papers were surveyed. Only 70% listed the XPS instrument that was used. Among publications with fitted XPS data, over 90% did not indicate the line shape(s) used and over 35% did not indicate the background used. In this presentation, we provide information on other aspects of reporting as well. To improve the reporting in the literature, which should improve the reproducibility of studies, we recommend that (at a minimum) the following be reported:

1. The manufacturer and model number of the photoelectron spectrometer
2. The X-ray anode and energy (for example, in conventional XPS, Mg K or Al K with energies of 1253.6 or 1486.6 eV, respectively)
3. The type of source: non-monochromatic or monochromatized
4. The X-ray power and spot size
5. The photon energy used in the measurement, along with energy resolution
6. The vacuum level during the analysis
   Additionally, information about the line shape(s), fitting program, peak positions, and background should also be included if fitting data.

Electrochemistry plays a pivotal role in scientific and technological advancements. In order to achieve the circular carbon neutral economy goals, we need to advance our infrastructure to study electrochemical processes such as hydrogen production, batteries, fuel cells, recycling, and corrosion. The common feature in all these processes are interface reactions and the key to further development is improving the understanding of these phenomena.

In fundamental research, we have seen increasing use of classical Ultra-High Vacuum (UHV) analysis techniques at higher pressures, although the pressure range is still well below atmospheric conditions and even more so below operating conditions of any real-world electrochemical device.¹ Therefore, we decided to mimic the real operating conditions for electrochemical devices and combined with UHV surface and interface analysis techniques. We integrate the electrochemistry into a UHV system for quasi-operando analysis to explore the energy storage and conversion research areas at nanoscale.

The major challenge with operando and in-situ clusters is to have an ideal electrochemical flow cell suitable for studying the solid/liquid interface dynamics. Most of the cells with easy transfer of working electrode have poor electrode geometries, which results in high ohmic resistance. Additionally, it is pertinent to prevent the presence of organic impurities in the cell as they may get adsorb at the surface and obstruct the surface-interface analysis. We have initiated the development of our experimental setup based on a cell designed by Olaf Brummel² and tried to overcome the above-mentioned limitations. To avoid the glass corrosion, the lower part of cell is designed with Teflon and further the geometry of the cell is modified in a unique way. I will present my initial results in this direction as well as an overview of electrochemical surface and interface analysis cluster being developed at TU Wien.

References:

1. A. Foelske-Schmitz, X-Ray Photoelectron Spectroscopy in Electrochemistry Research, In: Wandelt, K., (Ed.) Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier Inc. (2018), vol. 1, 591–606.
2. M. Bertram et. al, Cobalt Oxide-Supported Pt Electrocatalysts: Intimate Correlation between Particle Size, Electronic Metal–Support Interaction and Stability, J. Phys. Chem. Lett.2020, 11, 19, 8365–8371.

The precipitation of an anodic Mg-rich phase in 5xxx series aluminum alloys causes them to become sensitized and highly susceptible to corrosion, especially when used in marine-based applications. Heat treatments (>240°C) can dissolve the secondary phases back into the matrix but are unrealistic for service components. In this study, laser surface melting (LSM) is used to reverse the sensitization at the alloy’s surface. An excimer ns-pulsed laser was employed at a fluence of 1.5 J/cm 2 to rapidly melt and solidify the surface of highly sensitized AA5083 samples. Characterization of the composition, roughness, and corrosion resistance of these samples were performed to achieve a holistic understanding of how the alloy surface changes after laser melting. Results show that laser processing decreased the open circuit potential from -780 mV to -980 mV (vs. SCE) due to surface homogenization and dissolution of secondary phases, which contribute to micro-galvanic corrosion. A reduction in the corrosion rate was attributed to the dissolution of anodic precipitates into the solid aluminum solution within the melted region, approximately 7 um deep beneath the surface. This work illustrates the efficacy of using LSM to reverse the sensitization of Al-Mg alloys, leading the way toward a method of restoring the original corrosion resistance of sensitized in-service material

MXenes comprise a family of two-dimensional carbides and nitrides that has grown to encompass numerous structures and compositions – >30 single transition metal MXenes with a near infinite number of solid solutions ranging in thickness depending on the number of atomic layers, from 5-11. MXenes are promising for a variety of applications ranging from electrodes for energy storage to wireless communication, optoelectronics, and medicine because of their high electrical conductivity, redox-active surfaces, plasmonic behavior, and other attractive properties. MXenes are typically derived via topochemical etching of atomically thick layers from precursor layered MAX phases using corrosive halogenated aqueous or vapor etchants. Knowledge of the reaction mechanism and process kinetics are of fundamental importance for the synthesis and property control of MXenes. Prediction of the optimal synthesis approaches will facilitate new MXene composition discovery and prediction of optimal processing time as a function of various parameters will also facilitate scaling up the wet chemical synthesis of MXenes for industrial use. Despite their importance, such studies have been challenging because of the atomic thickness of the A-element layers being etched and the aggressive etchants that hinder in situ studies. Herein, we investigate the defects in the metal and carbon site in the parent MAX phase using ex situ photoemission spectroscopy and mass spectrometry to understand how MXenes develop from MAX phases. Moreover, we investigate the effect of such defects and MAX structure and chemistry influence the etching reaction mechanism through in situ optical profilometry and Raman spectroscopy of single MAX particles to monitor the structural and chemical transformation. Through these methods, we gain fundamental understandings of atomic etching mechanism and kinetics for layered materials and how the reaction starts, proceeds, and finishes.