Non-destructive Depth Profiling Research Articles

Tungsten Oxide-Based Materials as Catalyst Support for Oxygen Evolution Reaction

The high cost of iridium (Ir) is a major concern for the large scale deployment of polymer electrolyte membrane water electrolysis (PEMWE). To mitigate its impact, the usage of Ir on the anode must be significantly reduced. Down-sizing the catalyst particle is a straight forward strategy but requires the use of appropriate support materials to disperse and anchor fine Ir nanoparticles. We have investigated the use of tungsten oxide-based materials as support for the oxygen evolution reaction (OER) Ir and ruthenium (Ru) catalyst. The synthesis of nanostructured substoichiometric tungsten oxide and the subsequent loading of ultrafine Ir/Ru-based nanoparticles is thoroughly studied. Due to the successful loading, the mass activity of the tungsten oxide-supported Ir/Ru-based catalyst shows significant improvements under rotating disk electrode (RDE) conditions compared with commercially available Ir/Ru-blacks. Furthermore, non-destructive depth profiling by synchrotron-based X-ray photoelectron spectroscopy (XPS) has revealed the strong catalyst-support interaction which suppressed the oxidation of the supported Ir/Ru-based catalysts, leading to much enhanced durability under accelerated durability testing conditions. To verify its performance under practical PEM electrolysis conditions, the tungsten oxide-supported OER catalyst is directly coated onto a polymer electrolyte membrane, i.e., forming a catalyst coated membrane (CCM), and tested under single cell testing conditions without pressure differential. The voltage dependent chemical state of the supported catalyst is also probed in-situ by synchrotron-based XPS where a potential dependent electron transfer between the catalyst and tungsten-oxide support is discovered. Our discovery is anticipated to aid the development of cost effective PEM electrolysis anodes.

Strong Catalyst-Support Interaction in WOx Nanowires Supported Iridium Nanocatalysts: A Pathway to Efficient Water Electrolysis

This paper investigates the use of sub-stoichiometric tungsten oxide nanowires (WOxNW) as a support material for Ir nanocatalysts in the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis (PEMWE). The results demonstrate that Ir@WOxNW has exceptional activity and durability, which paves the way for large-scale PEMWE deployment. The catalyst-support interaction plays a crucial role in explaining this performance boost.The preparation and characterization of Ir@WOxNW involved loading ultra-fine Ir nanoparticles onto WOxNW through hydrothermal treatment. High-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDS) mapping analysis revealed that the iridium nanoparticles were uniformly loaded on the WOxNW support material, indicating efficient loading and dispersion. X-ray photoelectron spectroscopy (XPS) analysis showed that the supported Ir catalysts had a mixed valence state of Ir0 and Ir4+. Ir@WOxNW was compared with three other Ir-based OER catalysts, outperforming them in terms of activity and durability.The Ir@WOxNW catalyst demonstrated superior OER activity compared to the other three catalysts, with a mass activity of 812 A/gIr at 1.55 V vs reversible hydrogen electrode (RHE). The Tafel slopes of the OER catalysts were grouped into two categories: Ir Black and Ir@WOxNW showed smaller values (about 40 mV/dec), and larger values for Ir@ATO and Ir Oxide (about 120 mV/dec), respectively. Electrochemical impedance spectroscopy (EIS) measurements indicated that the high mass activity of Ir@WOxNW was due to a high population of catalytic sites with fast intrinsic kinetics from the dispersed ultra-fine Ir catalyst on WOxNW. The durability of all catalysts was investigated using an accelerated durability test (ADT), and the Ir@WOxNW catalyst demonstrated unexpectedly high durability compared to the other catalysts.The postmortem XPS analysis showed that the Ir@WOxNW catalyst exhibited a subtle change in Ir0/Ir4+ ratio after the ADT, indicating suppressed Ir oxidation and greater stability compared to the other catalysts. Additionally, non-destructive depth profile analysis by synchrotron based XPS was conducted on the supported ultrafine Ir catalysts before and after the ADT test. The difference in oxidation state increased as the supported Ir catalysts were probed deeper into the core. The results showed that after oxidation potential cycles ADT, the WOxNW-supported Iridium exhibited strong resistance to oxidation. The core location remained a high percentage of metal peak, in contrast to the other three control catalysts.Density functional theory calculations suggested that the oxygen-deficient WOxNW support helps to maintain the OER-active metal@metal oxide core@shell-like form of the Ir catalyst particles by enhancing electron transfer from the WOxNW to the Ir surface, thereby weakening the binding of oxygen to the Ir surface and enhancing the oxidation resistance of the Ir metal to oxides.In conclusion, ultrafine Ir nanoparticles supported on WOxNW have significant potential for practical applications in electrochemical water splitting. The high activity and durability of Ir@WOxNW make it a promising candidate for large-scale PEMWE deployment.

Nondestructive initial-profile-free 3D elemental mapping inmultilayer thin film structures based onEDX anda quadratic programming problem.

We have demonstrated a new data analysis method that enables nondestructive depth profiling of a multilayer thin-film sample from energy-dispersive X-ray spectroscopy (EDX) data without the assumption of initial profiles. This method is based on a quadratic programming problem and allows for three-dimensional elemental mapping in the sample without destroying it, by performing depth profiling for all the pixels in the EDX two-dimensional mapping data. In this paper, first nondestructive depth profiling of two samples with different multilayer structures was performed using the proposed method. The results were compared with those obtained by cross-sectional observation to validate the accuracy and usefulness of the proposed method. Next, an example of the three-dimensional elemental mapping based on the proposed method was demonstrated. This method allows us to nondestructively obtain three-dimensional elemental distribution within a sample over a wide area on the order of mm, which is impossible to obtain using other analytical methods. The way to determine the hyperparameters, which significantly affects the calculation results, is fully described in this paper.

WOx nanowire supported ultra-fine Ir-IrOx nanocatalyst with compelling OER activity and durability

Ultra-fine iridium (Ir) nanocatalyst is successfully dispersed and stabilized on sub-stoichiometric tungsten oxide nanowires (WOxNW) forming an active and stable OER catalyst. With its abundant amount of exposed active surfaces, the WOxNW supported Ir catalyst shows a high mass activity of 812 A/gIr at 1.55 V vs RHE. This value is three times higher than Ir-black and 30 times higher than Ir supported on antimony-doped tin oxide. The Ir@WOxNW possesses much satisfactory stability, showing a loss of only 40 % compared with the nearly 100 % loss of Ir-black after 1,000 accelerated durability testing cycles. Non-destructive depth profile by synchrotron-based XPS and density functional calculations unambiguously reveal the strong catalyst-support interaction between the WOxNW and the supported Ir catalyst. Due to the inhibited oxidation, the OER active core–shell-like structure of the Ir-IrO2 is well-maintained on the WOxNW supports under OER testing conditions. The developed catalyst shows high potential to significantly reduce Ir usage for practical water electrolysis.

Operando NAP-XPS Studies of a Ceria-Supported Pd Catalyst for CO Oxidation

Supported Pd/CeO2 catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, operando near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO2 catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO2-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H2-TPR and Raman measurements. Adsorbed peroxide (O22−) species on the catalyst surface could also be detected by Raman spectra. Operando NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdOx and PdII ions in a PdxCe1−xO2−δ solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdOx was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the PdxCe1−xO2−δ phase, which was created at the Pd–CeO2 interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceria to Pd species for CO oxidation.

P-type conductivity in Sn-doped Sb2Se3

Antimony selenide (Sb2Se3) is a promising absorber material for thin-film photovoltaics. However, certain areas of fundamental understanding of this material remain incomplete and this presents a barrier to further efficiency gains. In particular, recent studies have highlighted the role of majority carrier type and extrinsic doping in drastically changing the performance of high efficiency devices (Hobson et al 2020 Chem. Mater. 32 2621–30). Herein, Sn-doped Sb2Se3 bulk crystals are shown to exhibit p-type conductivity using Hall effect and hot-probe measurements. The measured conductivities are higher than those achieved through native defects alone, but with a carrier density (up to 7.4 × 1014 cm−3) several orders of magnitude smaller than the quantity of Sn included in the source material. Additionally, a combination of ultraviolet, x-ray and hard x-ray photoemission spectroscopies are employed to obtain a non-destructive depth profile of the valence band maximum, confirming p-type conductivity and indicating a majority carrier type inversion layer at the surface. Finally, these results are supported by density functional theory calculations of the defect formation energies in Sn-doped Sb2Se3, showing a possible limit on the carrier concentration achievable with Sn as a dopant. This study sheds light on the effectiveness of Sn as a p-type dopant in Sb2Se3 and highlights avenues for further optimisation of doped Sb2Se3 for solar energy devices.

Non-destructive depth profile evaluation of multi-layer thin film stack using simultaneous analysis of data from multiple X-ray photoelectron spectroscopy instruments

Non-destructive depth profile evaluation of multi-layer thin film stacks using simultaneous analysis of angle-resolved X-ray photoelectron spectroscopy data from multiple instruments is demonstrated. The data analysis algorithm, called the maximum smoothness method, was originally designed to handle data from a single XPS instrument with a single X-ray energy; in this work, the algorithm is extended to provide a simultaneous analysis tool which can handle data from multiple instruments with multiple X-ray energies. The analysis provides depth profiles for multilayer stacks that cannot be obtained by conventional data analysis methods. In this paper, metal multi-layer stack samples with total thickness greater than 10 nm are analyzed with the maximum smoothness method to non-destructively obtain depth profiles, with precise information on the chemical states of atoms in the surface layer (<2 nm) and the overall layer stack structure, which can only be obtained by analyzing the data from multiple instruments.

Experimental evidence of ion migration in aged inorganic perovskite solar cells using non-destructive RBS depth profiling

Rutherford back scattering technique can be used to investigate ion migration in halide perovskites.

Elemental Depth Profiling of Intact Metal-Organic Framework Single Crystals by Scanning Nuclear Microprobe.

The growing field of MOF–catalyst composites often relies on postsynthetic modifications for the installation of active sites. In the resulting MOFs, the spatial distribution of the inserted catalysts has far-reaching ramifications for the performance of the system and thus needs to be precisely determined. Herein, we report the application of a scanning nuclear microprobe for accurate and nondestructive depth profiling of individual UiO-66 and UiO-67 (UiO = Universitetet i Oslo) single crystals. Initial optimization work using native UiO-66 crystals yielded a microbeam method which avoided beam damage, while subsequent analysis of Zr/Hf mixed-metal UiO-66 crystals demonstrated the potential of the method to obtain high-resolution depth profiles. The microbeam method was further used to analyze the depth distribution of postsynthetically introduced organic moieties, revealing either core–shell or uniform incorporation can be obtained depending on the size of the introduced molecule, as well as the number of carboxylate binding groups. Finally, the spatial distribution of platinum centers that were postsynthetically installed in the bpy binding pockets of UiO-67-bpy (bpy = 5,5′-dicarboxyy-2,2′-bipyridine) was analyzed by microbeam and contextualized. We expect that the method presented herein will be applicable for characterizing a wide variety of MOFs subjected to postsynthetic modifications and provide information crucial for their optimization as functional materials.

Nondestructive residual stress depth profile analysis at the inner surface of small boreholes using energy-dispersive diffraction under laboratory conditions.

Energy-dispersive diffraction under both laboratory and synchrotron conditions was applied to study the hoop stress in the near-surface region of the inner wall of boreholes with a small diameter of 2 mm. By use of different X-ray beam cross sections for the sin2ψ measurements, it is demonstrated that the borehole-to-beam-diameter ratio must be considered in the evaluation. A beam cross section which is comparable to the borehole diameter reduces the slope of the d hkl φψ-sin2ψ distributions and thus invalidates the result of stress analysis. A quantitative relationship is applied, which allows the results obtained under the above conditions to be scaled so that they reflect the actual residual stress state at the measurement position. Owing to the small diffraction angles, energy-dispersive diffraction proves to be the only suitable experimental technique that allows a nondestructive and depth-resolved analysis of the hoop stress component at the inner surface of boreholes with a large length-to-diameter ratio.

An overview on the non-destructive in-depth surface analysis of corrosion-resistant films: A case study of W-xCr deposits in 12 M HCl solution

Non-destructive in-depth analysis of the surface films formed on the sputter-deposited binary W-xCr (x = 25, 57, 91 at %) alloys in 12 M HCl solution open to air at 30 °C was investigated using an angle-resolved X-ray photoelectron spectroscopic (AR-XPS) technique to understand the synergistic corrosion resistance effects of showing very low corrosion rates, even lower than both alloying metals of the deposits. The average corrosion rates of these three tungsten-based sputter deposits found to be more than five orders of magnitude (between 3.1 × 10−3 and 7.2 × 10−3 mm/y) to that of chromium and also nearly one order of magnitude lower than that of tungsten metals. Such high corrosion resistance of the sputter-deposited W-xCr alloys is due to the formation of homogeneous passive double oxyhydroxide film consisting of Wox and Cr4+ cations without any concentration gradient in-depth after immersion in 12 M HCl solution open to air at 30 °C from the study of the non-destructive depth profiling technique of AR-XPS. Consequently, both alloying elements of tungsten and niobium are acted synergistically in enhancing high corrosion resistance properties of the alloys in such aggressive electrolyte.&#x0D; BIBECHANA 18 (2021) 201-213

Non-destructive elemental depth profiling of Ni/Ti multilayers by GIXRF technique

In thin film multilayer along with total interface widths, study of individual interface widths of the constituent elements of the multilayer is important for the knowledge of diffusion mechanism and diffusion kinetics. Element specific Grazing Incidence X-ray Fluorescence (GIXRF) technique has been used to study the individual interface diffusion of Ni and Ti in the Ni/Ti multilayer. The above measurements yield similar results on the interface qualities of the multilayer as obtained from more widely used Grazing Incidence X-ray reflectivity (GIXRR) technique and give additional information regarding Ni and Ti diffusion separately which cannot be obtained from GIXRR. Ni-fluorescence and Ti-fluorescence GIXRF spectra of 10-bilayer Ni/Ti multilayer films are fitted simultaneously to find the interface widths of Ni and Ti at the Ni-on-Ti and Ti-on-Ni interfaces. The Error function distribution has been used to describe the concentration of elements at the interfaces. Further, the variation of these individual diffusion and individual concentration profile of elements has been studied with annealing of the multilayer. It has been shown that non-destructive and relatively simpler GIXRF technique can be used for elemental depth profiling of a multilayer very precisely, which otherwise can be obtained by using complicated destructive technique like atom probe tomography.

Universal analytical formula for the emission depth distribution function for photoelectrons with kinetic energies up to 5000 eV

Numerous parameters needed for quantification of X-ray photoelectron spectroscopy can be derived from the emission depth distribution function (EMDDF) known for a given photoelectron line, an experimental configuration, and a solid. This function describes the probability that a photoelectron emitted at a certain depth enters an analyzer without energy loss. The EMDDF turns out to be distinctly affected by photoelectron elastic scattering effects. A useful measure of influence of elastic scattering is a closely related function named the correction factor (CF). This function is a useful tool for correcting the in-depth concentration profiles obtained from the procedure of non-destructive depth profiling. Due of complexity of the theoretical models describing transport of photoelectrons emitted by unpolarized X-rays in the surface region of solids, a typical computational approach involves Monte Carlo algorithms with different simulation strategies. However, convenient sources of EMDDFs (and CF functions) are analytical formulas that can be implemented in the software dedicated for practical surface analysis. The present report evaluates accuracy of different analytical formulae. A new expression is proposed which can be considered as a predictive formula, i.e., a simple analytical expression of reasonable accuracy for typical measurement conditions. This formula is applicable to photoelectrons emitted by unpolarized or circularly polarized X-rays. Furthermore, stress is put on the influence of non-dipolar parameters that define a high-energy photoemission cross section on the EMDDF for photoelectrons with kinetic energies exceeding 1500 eV. Eventually, a photoelectron kinetic energy of 5 keV was established as an upper limit of applicability of the presented formalism. Finally, applicability of the analytical formalism, derived for unpolarized X-rays, to photoelectrons emitted by linearly polarized radiation is analyzed. Due to the considerable influence of the position of the polarization vector on the photoelectron signal, the analytical formalism developed for unpolarized radiation cannot be recommended for use if the of X-ray beam is linearly polarized or has partial polarization.

In-depth distribution of elements and chemical bonds in the surface region of calcium-doped diamond-like carbon films

In the present work we determined surface composition and chemical bonds at the top surface and in a shallow sub-surface region of the calcium-doped diamond-like carbon (DLC) layers using angular-resolved photoelectron spectroscopy (ARXPS) aided by the Maximum Entropy Method (MEM). The resulting non-destructive depth profiles showed that the composition and the chemical bonding vary significantly on the way from the top surface to the shallow sub-surface region. We demonstrate, for the first time, that Ca doping induced (i) the accumulation of the calcium carbonate, CaCO3, at the surface, while the calcium oxide, CaO, is locatedin a deeper sub-surface region, (ii) structural conversion from the C sp2-rich top surface observed in the undoped DLC layer to the C sp3-rich top surface of the Ca-doped DLC samples, and (iii) conversion in the opposite direction which occurred in a sub-surface region. Our conclusions are consistent with Raman and Ion Surface Scattering spectra. Obtained results can be useful for deeper understanding the interaction between doped DLC layer surfaces with surroundings and particularly with living tissue.

Laboratory grazing-incidence X-ray fluorescence spectroscopy as an analytical tool for the investigation of sub-nanometer CrSc multilayer water window optics

Efficient multilayer optics for radiation in the water window range are difficult to manufacture due to extremely small layer thicknesses and severe intermixing of elements between the layers. Therefore, adequate analytics and short feedback loops are of utmost importance for manufacturers to improve performance and efficiency. We show the possibility for non-destructive elemental depth profiling with commercial laboratory equipment using four real-life CrSc multilayer samples. Comparative measurements at the laboratory of PTB at the synchrotron radiation facility BESSY II confirm the results and prove the potential of laboratory equipment for the reliable analysis of stratified materials with sub-nanometer layer thicknesses.

Complementary physicochemical analysis by ellipsometry and Auger spectroscopy of nano-sized protective coating layers

Protective SiC-rich nano layer was created by ion beam mixing of Si/C multilayers. The transformation of the Si and C layers into a homogeneous SiC layer was analyzed using complementary depth profiling by spectroscopic ellipsometry (SE) and Auger electron spectroscopy (AES). The distribution of elements and their chemical states was revealed by AES, whereas SE measured the accurate thicknesses, density, crystallinity, and the distribution of different phases. The optical properties of the created SiC layer determined by SE were almost identical to that of ion implantation-amorphized SiC. Amorphous Si and void formations were revealed by SE. The void profile determined by SE correlated well with the Xe distribution measured by AES. The complementary capabilities of SE and AES for a detailed chemical investigation were pointed out, such as the atomic (AES), structural (AES and SE) and non-destructive depth profiling (SE) features. It was shown that besides the synergism that the two methods reveal, SE is capable of a quick, sensitive and non-destructive testing of such layer structures and materials.

XPS study on the passivity of coarse-grained polycrystalline and electrodeposited nanocrystalline nickel-iron (NiFe) alloys

Electrodeposited nanocrystalline NiFe alloys are compared to their polycrystalline, coarse-grained counterparts to understand the effects of grain size reduction on the passivation kinetics and the chemical structure of the passive film. The passive film formed in acidic sulfate environment was characterized using parallel angle-resolved XPS (PARXPS) to obtain non-destructive compositional depth profiles. The composition (Ni:Fe) of the passive film was insensitive to grain size; however, grain size reduction resulted in increased anodic activity producing thicker films that contained increased concentrations of higher oxide species.

Non-destructive Depth Profiling of a Nano Surface by High-energy Synchrotron Radiation XPS

Non-destructive Depth Profiling of a Nano Surface by High-energy Synchrotron Radiation XPS

Comparison of historical violins by non-destructive MRI depth profiling

The back plates of master violins at the Ashmolean Museum in Oxford and the Museo del Violino in Cremona were analyzed by portable magnetic resonance imaging with the NMR-MOUSE in terms of depth profiles. Distributions of transverse NMR relaxation times were recorded versus depth at points of small curvature across the violin back plates providing insight into the physical states of the materials. Their analysis reveals the effects of surface treatments, wood treatment, and possible wood degradation. While conclusions about the type of treatment cannot be drawn, the profiles report surprising differences between the violins from the museums in Oxford and Cremona.

Non-Destructive Depth Profile Analysis of UV Offset Ink Films with Confocal Raman Microscopy

Non-Destructive Depth Profile Analysis of UV Offset Ink Films with Confocal Raman Microscopy