Grazing Incidence X-ray Absorption Spectroscopy Research Articles

Substrate-Assisted Atomic Dispersion of Cobalt for Alkaline Water Electrolysis.

Atomically dispersed single-atom catalysts have recently attracted broad research interest due to their high atom efficiency and unique catalytic performance. In this study, atomic dispersion of cobalt is achieved using a chemical bath deposition method on a highly stable alkali titanate film (Ti/KTiO). These films were characterized using a variety of techniques, with atomic dispersion confirmed via grazing incidence X-ray absorption spectroscopy and ab initio modeling of single-atom systems. This modeling indicated that the alkali ion incorporated into the film facilitates atomic dispersion. Experimentally, the Ti/KTiO-supported Co(OH)2 catalysts exhibited remarkable electrochemical performance, with an overpotential of 163 mV to achieve a current density of 10 mA cm-2 with a catalyst loading of ∼0.1 mg cm-2 and high stability. These results show the potential of Ti/KTiO/Co(OH)2 catalysts for atomically efficient hydrogen production.

In-situ and operando Grazing Incidence XAS: a novel set-up and its application to model Pd electrodes for alcohols oxidation

Abstract In this article, we present an in-situ and operando time-resolved X-ray Absorption Spectroscopy (XAS) technique which exploits a combination of Grazing Incidence XAS (GIXAS) and Fixed Energy X-ray Absorption Voltammetry (FEXRAV), the Grazing Incidence FEXRAV (GI-FEXRAV). A case-study is also outlined. Palladium ultra-low loadings were deposited above Au polycrystalline iso-oriented substrates adopting three different deposition methods: surface-controlled electrochemical methods, direct electrodeposition, and physical vapour deposition (PVD). These catalytical surfaces were prepared for the investigation by GI-FEXRAV of the Pd oxidation/dissolution phenomenon that could occur when the metal is used in the anodic compartment of Direct Alcohol Fuel Cells (DAFCs) or in electrochemical reformers. Moreover, we report a robust, low cost and versatile procedure to obtain wide and flat iso-oriented gold substrates that can mimic monocrystalline gold (1 1 1) in the electrochemical response. The use of GI-FEXRAV for the operando characterization of the catalysts, in conjunction with the designed experimental cell and our flexible Au-based electrochemical substrates show an invaluable potential in the operando study of fundamental phenomena in heterogeneous electrocatalysis model systems and, due to its versatility, paves the way to further studies on a wide selection of electrochemical systems.

Probing the Electrode-Liquid Interface Using Operando total-Reflection X-Ray Absorption Spectroscopy

Traditional operando methods to study electrochemical (EC) processes, although successful, are based on EC measurements, lacking in direct sensitivity of chemical and structural changes. A fundamental understanding of the underlying mechanism occurring during an EC reaction could allow to improve performances and properties of applied material systems. EC processes are localized at the electrode-electrolyte interfacial regions, but several surface sensitive experimental techniques are often limited to ex situ conditions, owing to the need of a Ultra High Vacuum (UHV) environment, e.g., Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS), Low Energy Electron Diffraction (LEED), etc.1 On the contrary, synchrotron-based hard x-rays provide an intense and highly energetic source that can penetrate the electrolyte without undergoing significant attenuation. Furthermore, the x-ray beam can be focused on a flat metal surface adopting a grazing incidence geometry, increasing surface sensitivity up to a few nanometers in subcritical condition. The latter case is achieved when the incident angle is set below the critical angle for total external reflection (TER), characterized by the occurrence of an evanescent wave confined at the surface. As a result, x-ray photon-in photon-out techniques can be used to probe the electrode-liquid interface in working conditions.2,3 In this contribution, we performed operando Grazing Incidence X-ray Absorption Spectroscopy (GI-XAS) in total external reflection (TER) mode4 (Fig. 1) at P64 (DESY, Hamburg), using our EC setup designed for multimodal studies with hard x-ray scattering and spectroscopy techniques3. XAS can provide unique information about the local chemical and structural environment of absorbing atoms during electrochemical polarization. The electro-oxidation of polycrystalline gold, relevant for the understanding of electrolysis, and the surface passive film development and breakdown of Ni-Cr-Mo corrosion resistant alloys, have been investigated. Despite the presence of the electrolyte and the PEEK walls of the EC cell, 1-3 nm thick surface oxides film can be detected in TER mode using energies down to 8 keV. Advantages and challenges of this method will be discussed showing experimental results. Figure 1 Left: Schematic of our EC cell showing sample geometry for GI-XAS in TER mode, the incidence angle q is always lower than the critical angle. Right: GI-XAS in TER mode from a Ni 59 alloy sample at the Ni k-edge at increasing EC polarization in 1M NaCl and pH 12. Maurice, V. Progress in corrosion science at atomic and nanometric scales. Progress in Materials Science 40 (2018).Timoshenko, J. & Roldan Cuenya, B. In Situ / Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy. Chem. Rev. 121, 882–961 (2021).Långberg, M. Redefining passivity breakdown of super duplex stainless steel by electrochemical operando synchrotron near surface X-ray analyses. npj Materials Degradation, 3(1): p. 22. (2019).D. Gajdek. Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O 2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy, J. Phys. Chem. C, vol. 126, n. 7, pagg. 3411–3418 (2022) Figure 1

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Time-resolved in-situ investigation of Co-nitride thin film growth by grazing incidence X-ray absorption spectroscopy

The growth of cobalt nitride thin films prepared by reactive sputtering on float glass substrates was in-situ investigated using surface sensitive, grazing incidence X-ray absorption fine structure spectroscopy. Time-resolved measurements employing a quick-scanning monochromator allow to follow the growth on time-scales of a few seconds per spectrum, so that the film growth could be studied in detail. A quantitative modeling of the obtained data clearly show that a zinc blende type CoN is formed already during the initial stages of film growth, and the formation of small crystalline clusters finally lead to a 3D-growth of polycrystalline CoN-films.

Time-Resolved In-Situ Investigation of Co-Nitride Thin Film Growth by Grazing Incidence X-Ray Absorption Spectroscopy

Time-Resolved In-Situ Investigation of Co-Nitride Thin Film Growth by Grazing Incidence X-Ray Absorption Spectroscopy

Oxidation State and Surface Reconstruction of Cu under CO2 Reduction Conditions from In Situ X-ray Characterization.

The electrochemical CO2 reduction reaction (CO2RR) using Cu-based catalysts holds great potential for producing valuable multi-carbon products from renewable energy. However, the chemical and structural state of Cu catalyst surfaces during the CO2RR remains a matter of debate. Here, we show the structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS reveals that the surface oxide layer is fully reduced to metallic Cu before the onset potential for CO2RR, and the catalyst maintains the metallic state across the potentials relevant to the CO2RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO2. Quantitative analysis of the reconstruction profiles reveals that the degree of reconstruction increases with increasingly negative applied potentials, and it persists when the applied potential returns to more positive values. These findings show that the surface of Cu electrocatalysts is dynamic during the CO2RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis.

Use of in Situ Synchrotron Techniques to Probe the Oxidized Surface of Molybdenum Nitride Oxygen Reduction Electrocatalysis

The development of active and stable earth-abundant catalysts for the oxygen reduction reaction (ORR) is needed for widespread, economic development of fuel cell technologies. Designing and optimizing these non-platinum group metals is challenging, however, because they are susceptible to composition and structure changes, including dissolution, oxidation, and corrosion, both in air and under reaction conditions. To identify the active surface, and thus understand the properties that affect activity, the catalyst surface must be characterized in situ. Herein, we utilize a grazing incidence electrochemical cell to investigate in situ composition and morphology changes of a molybdenum nitride (Mo-N) thin film catalyst using grazing incidence x-ray absorption spectroscopy (GI-XAS) and x-ray reflectivity (XRR). In rotating ring disk electrode measurements, we find that the activity, selectivity, stability, and capacitance of the Mo-N catalyst is dependent on the maximum potential to which it has been exposed. Specifically, the overpotential required to reach -2 mA cm-2 geo decreases by over 90 mV when the maximum potential is increased from 0.3 to 0.8 V vs RHE (Figure 1). Because the Mo-N oxidizes rapidly in air, ex situ characterization methods including x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry can provide only limited insight into these in situ catalyst changes. Using in situ GI-XAS measurements at applied potentials between 0.3 and 0.9 V vs RHE, however, we are able to determine that the surface of the film oxidizes and becomes more amorphous when exposed to increasingly higher potentials (Figure 1). Furthermore, the surface remains oxidized on the order of several hours when returned to “ORR relevant potentials” (< 0.6 V vs RHE), indicating that this surface-oxidized nitride is the active surface for ORR. Using in situ XRR measurements, we find that there is no change in surface roughness at potentials below 0.7 V vs RHE, but the film roughens significantly at 0.8 V vs RHE, correlating with ex situ measurements of Mo dissolution at this potential (Figure 1). We therefore conclude that the intrinsic activity of the Mo-N catalyst increases when exposed to potentials up to 0.7 V vs RHE, while above that potential activity enhancements are due to the exposure of more active sites through dissolution. The in situ electrochemical surface-sensitive x-ray characterization as used here is a promising methodology for understanding and leveraging surface dynamics to improve the performance of non-traditional catalysts.Figure 1. RRDE measurements in O2-saturated 0.1M HClO4 at 1600 rpm and schematics illustrating how the catalyst composition and morphology change with applied potential. Figure 1

Examining the surface evolution of LaTiOxNy an oxynitride solar water splitting photocatalyst

LaTiOxNy oxynitride thin films are employed to study the surface modifications at the solid-liquid interface that occur during photoelectrocatalytic water splitting. Neutron reflectometry and grazing incidence x-ray absorption spectroscopy were utilised to distinguish between the surface and bulk signals, with a surface sensitivity of 3 nm. Here we show, contrary to what is typically assumed, that the A cations are active sites that undergo oxidation at the surface as a consequence of the water splitting process. Whereas, the B cations undergo local disordering with the valence state remaining unchanged. This surface modification reduces the overall water splitting efficiency, but is suppressed when the oxynitride thin films are decorated with a co-catalyst. With this example we present the possibilities of surface sensitive studies using techniques capable of operando measurements in water, opening up new opportunities for applications to other materials and for surface sensitive, operando studies of the water splitting process.

Synergetic effect on catalytic activity and charge transfer in Pt-Pd bimetallic model catalysts prepared by atomic layer deposition.

Pt-Pd bimetallic nanoparticles were synthesized on TiO2 support on the planar substrate as well as on high surface area SiO2 gel by atomic layer deposition to identify the catalytic performance improvement after the formation of Pt-Pd bimetallic nanoparticles by surface analysis techniques. From X-ray absorption near edge spectra of Pt-Pd bimetallic nanoparticles, d-orbital hybridization between Pt 5d and Pd 4d was observed, which is responsible for charge transfer from Pt to Pd. Moreover, it was found from the in situ grazing incidence X-ray absorption spectroscopy study that Pt-Pd nanoparticles have a Pd shell/Pt core structure with CO adsorption. Resonant photoemission spectroscopy on Pt-Pd bimetallic nanoparticles showed that Pd resonant intensity is enhanced compared to that of Pd monometallic nanoparticles because of d-orbital hybridization and electronic states broadening of Pt and Pd compared monometallic catalysts, which results in catalytic performance improvement.

Synchrotron radiation characterization of magnetron sputtered WC-Co thin films on mild steel substrate

Mild steel offers versatile properties at lower costs. This has given the alloy a large application base in the industry. However, the increasing complexity and severity of service environments have shifted the focus of many industries to structure modification techniques, like physical vapor deposition to improve material properties and performance. Synthesis of WC-Co thin films by physical vapor deposition technology has attracted great research interest owing to the outstanding mechanical properties of the material and its potential to be utilized extreme engineering applications such as in wear-resistance, heavy cutting and excavation industries. The growth in the use of WC-Co thin films in the general mechanical industry is however slow due to lack of data on the tribological characteristics of WC-Co coated materials. Control and manipulation of synthesis parameters are of significant concern in order to tailor such material properties and performance. The focus of this paper is to, therefore, investigate the effect of Rf magnetron power and deposition temperature on the structure and sliding wear behavior of WC-Co thin film. The surface morphology and nature were acquired using x-ray photoelectron spectroscopy (XPS) and Grazing Incidence x-ray absorption spectroscopy (GI-XAS). To validate the synchrotron results, additional analysis was acquired from Scanning electron microscopy (SEM), Raman spectroscopy and surface profilometry to predict and point out optimum synthesis parameters for best properties of the film. Finally, the wear performance of the film-substrate system was determined and reported.

Electrodeposition and Characterization of Ultra-Thin Films As Model Systems for Electrocatalysis

Platinum is nowadays considered the best catalyst for many anodic and cathodic reactions in fuel cells. However, its low abundance poses a limit for its usage in such devices. In recent years, much effort has been done to substitute this metal whit more abundant ones, in particular concerning the alcohol electrooxidation process in direct alkaline alcohol fuel cells (DAFC).In this respect, palladium (which shares with platinum a similar catalytic activity towards alcohol electrooxidation) may represent a viable substitute for the anodic compartment in DAFC, but its main limit resides in the deactivation process (oxidation of the metal) that occurs during the cell duty cycle. Aiming to study this phenomenon, different catalytically active bimetallic Au-Pd electrodes were preparedby combined Under-Potential Deposition (UPD) of copper and Surface Limited Redox Replacement (SLRR) with Pd, exploiting homemade Au (111) polycrystalline surfaces. This catalytic ultra-thin layers (with different Pd coverages above Au) proved to be robust and reproducible, both morphologically and electrochemically. Moreover, this method enabled the preparation of large, flat and homogeneous surfaces, enabling their use as model surfaces in synchrotron light experiments (performed at the BM-08 line of the ESRF in Grenoble), where a blend between the Grazing Incidence X-ray Absorption Spectroscopy (GIXAS) and the Fixed Energy X-Ray Absorption Spectroscopy (FEXRAV) was exploited. This novel experimental set-up, a grazing Incidence FEXRAV technique (GI-FEXRAV), allowed to follow in operando changes in the speciation of the Pd during voltammetric scans, and thus to disclose its deactivation/dissolution in systems with a very low catalytical load. Finally, with this novel set-up we just sampled the topmost catalytic layer, thus greatly enhancing the capability to study small modifications of the surface.

Characterization of Surface Changes on Molybdenum Nitride Thin Films during the Oxygen Reduction Reaction Using Operando Grazing Incidence X-Ray Absorption Spectroscopy

Coupled water electrolysis systems and proton-exchange membrane fuel cells (PEMFC) are a promising technology for clean energy storage and conversion, respectively.1 The efficiency of the hydrogen PEMFC is limited, in part, by the poor performance of catalysts for the oxygen reduction reaction (ORR). Due to problems with low activity and conductivity, the majority of non-platinum group catalysts for the ORR are conductive-carbon-supported nanoparticulate transition metal nitrides and/or carbides. It is difficult to determine the active surface of these catalysts during reaction conditions due to their high surface area, inhomogeneous size and distribution, complicated support-catalysts interactions, and surface oxidation in air. To overcome these complications, our work has focused on the operando structural and compositional characterization of the surface of active molybdenum nitride thin films. These films, prepared via reductive sputtering,2 are an ideal template for study due to their tunable thickness, lack of carbon support, moderate electrical conductivity, and well-defined surface. Using a specifically designed electrochemical cell for grazing incidence x-ray absorption and diffraction, we have been able to characterize changes in our Mo x N surface under ORR conditions. These same surface changes are not apparent at similar non-ORR reducing conditions, supporting our hypothesis that we are characterizing the active surface during ORR. ­ This work highlights the role of nitrogen, oxygen vacancies, and electrical conductivity on electrochemical activity and allows us to probe the structure and thickness of the active surface layer as well as the active site geometry. Understanding the role and importance of nitride bulk and the distinct surface structure will allow us to re-think our typical rational design of catalyst materials and help us design more active and stable catalysts in the future. References (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science (80-. ). 2017, 355, 1–12. (2) Hones, P.; Martin, N.; Regula, M.; Francis, L. Structural and Mechanical Properties of Chromium Structural and Mechanical Properties of Chromium Nitride , Molybdenum Nitride , and Tungsten Nitride Thin Films. J. Phys. D Appl. Phys. 2003, 36, 1023–1029.

Surface Chemistry and Long-Term Stability of Amorphous Zn–Sn–O Thin Films

Amorphous (a-) oxides form an important category of transparent conducting/semiconducting thin films used as electrodes and channel layers in thin film transistors. The compositional flexibility of amorphous states, through doping, makes it possible to fine-tune the electrical properties of films from conducting to semiconducting. However, surface chemistry and stability of these films are rarely addressed. Surface studies of amorphous materials, in general, are scarce due to disorder. Here, long-term surface stability of a–Zn–Sn–O films was investigated using grazing incidence X-ray absorption spectroscopy techniques. We present a detailed description of film surface structures and their evolution over time. It was found that the surface structure is, locally, a close analogue of the crystalline counterpart and that surface chemistry is governed by vacancies, strain, composition, and film density. It is shown that the long-term stability of a film is questionable when the film has a high Zn content and a...

Operando Grazing Incidence X-Ray Diffraction and X-Ray Absorption Spectroscopy for Electrochemical CO2 Reduction on Aupd, Pd and Au Electrodes

The electrochemical reduction of carbon dioxide to fuels and chemicals using electricity produced from renewable sources represents an appealing approach to creating a carbon neutral fuel cycle or carbon negative chemical production pathway. However, a lack of fundamental understanding of the surface of the catalyst during electrochemical experiments hinders the development of more active catalysts. Recent reports indicate that the surface of the catalyst can change drastically under reducing conditions, suggesting that the active form may differ from the catalyst as characterized ex situ. Consequently, accurately probing the surface of the catalyst under operando conditions is critical to observing structural and electronic changes in the catalyst, allowing key insights into the carbon dioxide reduction reaction. We report the design and development of such operando experiments at the Stanford Synchrotron Radiation Lightsource using grazing incidence x-ray diffraction and grazing incidence x-ray absorption spectroscopy. Using these techniques, we study changes in the surface physical and electronic structure of thin film Au, Pd, and AuPd catalysts during electrochemical carbon dioxide reduction.

Copper Cluster Size Effect in Methanol Synthesis from CO2

Size-selected Cun catalysts (n = 3, 4, 20) were synthesized on Al2O3 thin films using mass-selected cluster deposition. A systematic study of size and support effects was carried out for CO2 hydrogenation at atmospheric pressure using a combination of in situ grazing incidence X-ray absorption spectroscopy, catalytic activity measurement, and first-principles calculations. The catalytic activity for methanol synthesis is found to strongly vary as a function of the cluster size; the Cu4/Al2O3 catalyst shows the highest turnover rate for CH3OH production. With only one atom less than Cu4, Cu3 showed less than 50% activity. Density functional theory calculations predict that the activities of the gas-phase Cu clusters increase as the cluster size decreases; however, the stronger charge transfer interaction with Al2O3 support for Cu3 than for Cu4 leads to remarkably reduced binding strength between the adsorbed intermediates and supported Cu3, which subsequently results in a less favorable energetic pathway t...

Temperature-Dependent Evolution of the Oxidation States of Cobalt and Platinum in Co1–xPtx Clusters under H2 and CO + H2 Atmospheres

Co1–xPtx clusters of 2.9-nm size with a range of atomically precise Pt/Co atomic ratios (x = 0, 0.25, 0.5, 0.75, 1) were synthesized using the mass-selected low-energy cluster beam deposition (LECBD) technique and soft-landed onto an amorphous alumina thin film prepared by atomic layer deposition (ALD). Utilizing ex situ X-ray photoemission spectroscopy (XPS), the oxidation state of the as-made clusters supported on Al2O3 was determined after both a 1-h-long exposure to air and aging for several weeks while exposed to air. Next, the aged cluster samples were characterized by grazing-incidence X-ray absorption spectroscopy (GIXAS) and then pretreated with diluted hydrogen and further exposed to the mixture of diluted CO and H2 up to 225 °C at atmospheric pressure, and the temperature-dependent evolutions of the particle size/shape and the oxidation states of the individual metal components within the clusters were monitored using in situ grazing-incidence small-angle X-ray scattering and X-ray absorption s...

Grazing incidence X-ray absorption characterization of amorphous Zn-Sn-O thin film

We report a surface structure study of an amorphous Zn-Sn-O (a-ZTO) transparent conducting film using the grazing incidence X-ray absorption spectroscopy technique. By setting the measuring angles far below the critical angle at which the total external reflection occurs, the details of the surface structure of a film or bulk can be successfully accessed. The results show that unlike in the film where Zn is severely under coordinated (N < 4), it is fully coordinated (N = 4) near the surface while the coordination number around Sn is slightly smaller near the surface than in the film. Despite a 30% Zn doping, the local structure in the film is rutile-like.

Uranium Immobilization and Nanofilm Formation on Magnesium-Rich Minerals.

Polarization-dependent grazing incidence X-ray absorption spectroscopy (XAS) measurements were completed on oriented single crystals of magnesite [MgCO3] and brucite [Mg(OH)2] reacted with aqueous uranyl chloride above and below the solubility boundaries of schoepite (500, 50, and 5 ppm) at pH 8.3 and at ambient (PCO2 = 10(-3.5)) or reduced partial pressures of carbon dioxide (PCO2 = 10(-4.5)). X-ray absorption near edge structure (XANES) spectra show a striking polarization dependence (χ = 0° and 90° relative to the polarization plane of the incident beam) and consistently demonstrated that the uranyl molecule was preferentially oriented with its Oaxial═U(VI)═Oaxial linkage at high angles (60-80°) to both magnesite (101̅4) and brucite (0001). Extended X-ray absorption fine structure (EXAFS) analysis shows that the "effective" number of U(VI) axial oxygens is the most strongly affected fitting parameter as a function of polarization. Furthermore, axial tilt in the surface thin films (thickness ∼ 21 Å) is correlated with surface roughness [σ]. Our results show that hydrated uranyl(-carbonate) complexes polymerize on all of our experimental surfaces and that this process is controlled by surface hydroxylation. These results provide new insights into the bonding configuration expected for uranyl complexes on the environmentally significant carbonate and hydroxide mineral surfaces.

Structural change with the resistance drift phenomenon in amorphous GeTe phase change materials’ thin films

Ageing of the amorphous phase of chalcogenide phase change materials is characterized by a large increase of their resistivity with time. This phenomenon, known as resistance drift and commonly attributed to structural relaxation, the nature of which remains unknown, has until now hindered the development of ultra-high multilevel storage devices. The origin of the resistance drift of amorphous GeTe thin films is studied here by resistivity measurements and grazing incidence x-ray absorption spectroscopy (GIXAS). The local order around Ge atoms is investigated at the Ge K-edge on a-GeTe samples previously set at different resistance drift levels by thermal annealing. In all samples, Ge–Ge and Ge–Te bonds coexist. This study demonstrates that the drift phenomenon is concomitant with structural changes linked to Ge–Ge homopolar bonds.