We investigated the structural evolution of electrochemically fabricated Pd nanowires $\textit{in situ}$ by means of grazing-incidence transmission small- and wide-angle x-ray scattering (GTSAXS and GTWAXS), x-ray fluorescence (XRF) and 2-dimensional surface optical reflectance (2D-SOR). This shows how electrodeposition and the hydrogen evolution reaction (HER) compete and interact during Pd electrodepositon. During the bottom-up growth of the nanowires, we show that $\beta$-phase Pd hydride is formed. Suspending the electrodeposition then leads to a phase transition from $\beta$- to $\alpha$-phase Pd hydride. Additionally, we find that grain coalescence later hinders the incorporation of hydrogen in the Pd unit cell. GTSAXS and 2D-SOR provide complementary information on the volume fraction of the pores occupied by Pd, while XRF was used to monitor the amount of Pd electrodeposited.
X-ray diffractometers primarily designed for surface X-ray diffraction are often used to measure the diffraction from powders, textured materials and fiber-texture samples in 2θ scans. Unlike in high-energy powder diffraction, only a fraction of the powder rings is typically measured, and the data consist of many detector images across the 2θ range. Such diffractometers typically scan in directions not possible on a conventional laboratory diffractometer, which gives enhanced control of the scattering vector relative to the sample orientation. There are, however, very few examples where the measured intensity is directly used, such as for profile/Rietveld refinement, as is common with other powder diffraction data. Although the underlying physics is known, converting the data is time consuming and the appropriate corrections are dispersed across several publications, often not with powder diffraction in mind. This paper presents the angle calculations and correction factors required to calculate meaningful intensities for 2θ scans with a (2 + 3)-type diffractometer and an area detector. Some of the limitations with respect to texture, refraction and instrumental resolution are also discussed, as is the kind of information that one can hope to obtain.
During this thesis nanoporous alumina has been used as a template for the synthesis of nano-confined palladium structures. The nanoporous templates have been synthesized by anodization of aluminum and characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM) and small angle X-ray scattering (SAXS), resulting in good statistics of the pore diameter and interpore distance of these nanoporous structures. Palladium nanowires of different height and diameter has successfully been fabricated within the nanoporous alumina using electrodeposition from a neutral electrolyte. Palladium nanoparticles have been deposited onto nanoporous membranes using an electroless chemical deposition technique. The fabricated palladium nanostructures have been characterized using SEM. The catalytic activity of these palladium nanostructures has been characterized by mass spectrometry and planar laser induced flourescence. A difference in the catalytic conversion and in the onset temperature of the catalytic reactions has been observed as a function of pore size. The experiments should be repeated in a more controlled manner before any conclusion can be drawn about the effect of the nano-confinement on the catalytic activity. But this work has proved that palladium nanowires can be grown inside nanoporous alumina templates of different pore size and different depth and these palladium nanostructures show catalytic activity for CO oxidation. (Less)
Abstract In recent years, studies of surfaces at more realistic conditions has
advanced significantly, leading to an increased understanding of surface dynamics
under reaction conditions. The development has mainly been due to the development
of new experimental techniques or new experimental approaches. Techniques such as
High Pressure Scanning Tunneling/Force Microscopy (HPSTM/HPAFM), Ambient
Pressure X-ray Photo emission Spectroscopy (APXPS), Surface X-Ray Diffraction
(SXRD), Polarization-Modulation InfraRed Reflection Absorption Spectroscopy (PMIRRAS)
and Planar Laser Induced Fluorescence (PLIF) at semi-realistic conditions
has been used to study planar model catalysts or industrial materials under operating
conditions. 2D-Surface Optical Reflectance (2D-SOR) has recently received attention
as a useful experimental tool used in gaseous and liquid harsh conditions by providing
complementary experimental information on planar model samples as well as being a
powerful powerful experimental tool on its own. The simplicity of the approach and
the cost of the equipment makes it an attractive alternative and useful tool for surface
science studies under reaction conditions. In this topical review, we review some recent
studies that have been promoted by the technical development in optical components,
image acquisition and computational image analysis.
Abstract Corrosion results in large costs and environmental impact but can be controlled by thin oxide films that passivate the metal surfaces and hinder further oxidation or dissolution in an aqueous environment. The structure, chemistry, and thickness of these oxide films play a significant role in determining their anti-corrosion properties and the early-stage oxidation dynamics affect the properties of the developed oxide. Here, we use in situ X-ray Photoelectron Spectroscopy (XPS) to study the early-stage oxidation of a Ni-Cr-Mo alloy at room temperature and up to 400 °C. Cr and Mo begin to oxidize immediately after exposure to O 2 , and Cr 3+ , Mo 4+ , and Mo 6+ oxides are formed. In contrast, Ni does not contribute significantly to the oxide film. A self-limiting oxide thickness, which did not depend on temperature below 400 °C, is observed. This is attributed to the consumption of available Cr and Mo near the surface, which results in an enrichment of metallic Ni under the oxide. The self-limited oxide thickness is 6–8 Å, which corresponds to 3–4 atomic layers of cations in the oxide. At 400 °C, sublimation of Mo 6+ oxide is observed, resulting in the formation of an almost pure layer of Cr 2 O 3 on the alloy surface. Lastly, a mechanism is presented that explains the formation of the bi-layer oxide structure observed for Ni-Cr-Mo alloys, which involves the enhanced migration of hexavalent Mo ions in the electric field, which drives mass transport during oxidation according to both the Cabrera Mott model and the Point Defect Model.
Effect of tempering temperature on the composition of the passive film of a martensitic tool alloy was studied by synchrotron-based hard/soft X-ray photoelectron spectroscopy and electrochemical analyses. The contents of Cr and Mo in the passive film are affected by precipitation of tempering carbides. Increase of tempering temperature from 200 to 525°C leads to enhanced formation of Cr/Mo-rich tempering carbides and Cr depletion. Tempering at 525°C results in a Cr content < 11 at.% in the underlying metallic layer and formation of a Cr-deficient defective passive film, and thus loss of passivity for the tool alloy in corrosive conditions.
In this study, we present a systematic investigation of the controlled fabrication of Au–Pd barcode nanowires within nanoporous anodic aluminum oxide (NP-AAO) templates. By using a combination of in situ X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM), we elucidate the influence of template preparation methods on the resulting nanowire properties. The template treatment, involving either pore widening or barrier layer thinning, significantly impacts nanowire growth. Through the analysis of the XRD data, we observe sequential deposition of Au and Pd segments with lattice parameter variations and strain effects. Particularly, the lattice parameters of Au and Pd segments display intricate temporal dependencies, influenced by interfacial effects and strain caused by growth under confinement. FIB-SEM imaging reveals uniform and reproducible nanowire lengths in the template treated with pore widening. Furthermore, TEM analysis confirms the presence of distinct Au and Pd segments, while scanning TEM–energy-dispersive X-ray spectroscopy revealed minor evidence of interdiffusion between the first and the second electrodeposited segments. Our findings emphasize the potential of the electrodeposition process within nanoporous templates for producing barcode nanowires with precise segmental properties. The combination of in situ XRD and electron microscopy offers valuable insights into the growth dynamics and structural characteristics of the fabricated Au–Pd barcode nanowires. This controlled fabrication strategy opens doors to tailoring nanowire properties for diverse applications, particularly in catalysis.
The solid–liquid interface, where corrosion reactions occur, is notoriously difficult to study in operando conditions. Few experimental techniques can provide surface sensitivity and spatial and temporal resolution and are compatible with liquid environments. 2-Dimensional surface optical reflectance (2D-SOR) is an emerging optical technique providing microscopic information with high temporal resolution. It complements methods such as ambient pressure X-ray photoelectron spectroscopy or inductively coupled plasma-mass spectrometry, which provides valuable chemical information from the surface and the dissolving species but lacks spatial resolution. Here, we show how 2D-SOR can be used to measure the formation and growth of corrosion products in operando through oscillations in the reflected signal caused by constructive and destructive interference of the light in the film of growing corrosion products. The total thickness of the corrosion product film obtained with 2D-SOR agrees well with that measured using scanning electron microscopy ex situ. 2D-SOR has proved to be an excellent and valuable tool for monitoring the corrosion process with spatial resolution in operating conditions.
We have developed an electrochemical cell for in situ 2-Dimensional Surface Optical Reflectance (2D-SOR) studies during anodization and cyclic voltammetry. The 2D-SOR signal was recorded from electrodes made of polycrystalline Al, Au(111), and Pt(100) single crystals. The changes can be followed at a video rate acquisition frequency of 200 Hz and demonstrate a strong contrast between oxidizing and reducing conditions. Good correlation between the 2D-SOR signal and the anodization conditions or the cyclic voltammetry current is also observed. The power of this approach is discussed, with a focus on applications in various fields of electrochemistry. The combination of 2D-SOR with other techniques, as well as its spatial resolution and sensitivity, has also been discussed.
• In-situ ambient pressure XPS methodology for electrochemical oxide growth presented. • Quantitative analysis for ambient pressure XPS data further developed. • Enrichment of MoO 3 observed at higher applied potentials. Ambient Pressure X-ray Photoelectron Spectroscopy combined with an electrochemical setup is used to study, in situ, the electrochemical oxide growth on an industrial Ni-Cr-Mo alloy. The native oxide film was characterized in vacuum and in water vapor at 17 mbar, and was found to be 11.4 Å thick and rich in Cr 3+ . In 0.1M NaCl electrolyte, anodic growth of the oxide film at potentials up to 700 mV vs. Ag/AgCl nearly doubled the thickness of the oxide film. Moreover, a transformation of the oxide composition occurred, as the oxide became enriched in Mo 6+ with a chemical fingerprint more like that of pure MoO 3 . Both thermodynamics and kinetics of the oxidation of the alloying elements dictate the oxide film growth and composition. Furthermore, we develop the quantitative analysis of oxide composition and thickness to take into account the attenuation through the liquid water and the water vapor atmosphere. Finally, we discuss the differences between ex situ , UHV, in situ , and operando measurements. Our approach is robust, fast, simple, and suitable for systematically probing metal surfaces after aqueous exposure and electrochemical polarization, which promises wide applications for studies of solid-liquid interfaces in corrosion, batteries, fuel cells, and electrocatalysis.
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