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Eingeladener Vortrag
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With the increasing production of renewable energy and concomitant depletion of fossil resources, the demand for efficient water splitting electrocatalysts continues to grow. Iridium (Ir) and iridium oxides (IrOₓ) are currently the most promising candidates for an efficient oxygen evolution reaction (OER) in acidic medium, which remains the bottleneck in water electrolysis. Yet, the extremely high costs for Ir hamper a widespread production of hydrogen (H₂) on an industrial scale. Herein, the authors report a concept for the synthesis of electrode coatings with template-controlled mesoporosity surface-modified with highly active Ir species. The improved utilization of noble metal species relies on the synthesis of soft-templated metal oxide supports and a subsequent shape-conformal deposition of Ir species via atomic layer deposition (ALD) at two different reaction temperatures. The study reveals that a minimum Ir content in the mesoporous titania-based support is mandatory to provide a sufficient electrical bulk conductivity. After ALD, a significantly enhanced OER activity results in dependency of the ALD cycle number and temperature. The most active developed electrocatalyst film achieves an outstanding mass-specific activity of 2622 mA mg(Ir)⁻¹ at 1.60 V(RHE) in a rotating-disc electrode (RDE) setup at 25 °C using 0.5 m H₂SO₄ as a supporting electrolyte.
Ellipsometry-based approach for the characterization of mesoporous thin films for H2 technologies
(2021)
Porous thin layer materials are gaining importance in different fields of technology and pose a challenge to the accurate determination of materials properties important for their function. In this work, we demonstrate a hybrid measurement technique using ellipsometry together with other independent methods for validation. Ellipsometry provides information about the porosity of different mesoporous films (PtRuNP/OMC = 45%; IrOx = 46%) as well as about the pore size (pore radius of ca. 5 nm for PtRuNP/OMC). In addition, the electronic structure of a material, such as intraband transitions of a mesoporous IrOx film, can be identified, which can be used to better understand the mechanisms of chemical processes. In addition, we show that ellipsometry can be used as a scalable imaging and visualization method for quality assurance in production. These require accurate and traceable measurements, with reference materials playing an important role that include porosity and other related properties. We show that our novel analytical methods are useful for improving analytical work in this entire field.
Hydrogen production via water electrolysis will be an essential cornerstone in development of sustainable, fossil-free fuel and chemical production on a global scale. The activity and stability of each catalyst is highly dependent on the properties of the coating, i.e., phase composition, crystallinity, accessible surface area, and many other factors. The key to developing improved catalysts is a better understanding of the relationships between their performance, stability, and physicochemical properties. However, these relationships can be complex and are also strongly influenced by the reaction environment. Therefore, operando analysis of the catalyst material during catalysis at realistic potentials and current densities is highly desirable. However, many analytical techniques cannot be applied in liquid environments at realistic potentials and current densities.
We propose environmental ellipsometric analysis in a dedicated electrochemical flow cell as a method to evaluate gas evolution reactions operando under realistic working conditions. Figure 1 illustrates schematically the developed technique. Key factors to success are highly active model-type catalysts, a suitable sample environment, and a deep understanding of the appropriate model development, as well as concise cross validation with numerous other analytical techniques.
The method was developed and validated by analyzing a calcination series (300 – 600 °C) of mesoporous templated IrOx films ex-situ and operando under oxygen evolution reaction (OER) conditions. The employed environmental electrochemical spectroscopic ellipsometric (ECSE) analysis revealed during OER the change of optical and electronic properties, i.e. the dielectric functions, resistivity and band-to-band transitions (p-d band transitions). Film thickness and porosity were validated by means of scanning electron microscopy (SEM), X-ray reflectometry (XRR) or ellipsometric porosimetry (EP), electrical and electronic properties by means of conductivity measurements, X-ray photoelectron spectroscopy (XPS) or UV-Vis-NIR absorption spectroscopy. The electronic structures of the catalysts from valence electron energy loss spectra (VEELS) derived from ε1 and ε2 from SE measurements reveal a direct correlation with electrochemical activities in OER.
In the presentation reversible and irreversible potential-dependent changes of the catalyst properties during operation will be discussed along with the dynamics of gas formation, transport and dissolution at different potentials.
In recent years, the fabrication of laser-generated surface structures on metals such as titanium surfaces have gained remarkable interests, being technologically relevant for applications in optics, medicine, fluid transport, tribology, and wetting of surfaces.
The morphology of these structures, and so their chemistry, is influenced by the different laser processing parameters such as the laser fluence, wavelength, pulse repetition rate, laser light polarization type and direction, angle of incidence, and the effective number of laser pulses per beam spot area.
However, the characterization of the different surface structures can be difficult because of constraints regarding the analytical information from both depth and the topographic artifacts which may limit the lateral and depth resolution of elemental distributions as well as their proper quantification. A promising technique to investigate these structures even at the nano-scale is Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), a very surface sensitive technique that at the same time allows to perform depth-profiling, imaging and 3D-reconstruction of selected ion-sputter fragment distributions on the surface.
In this study we combine chemical analyses such as Energy Dispersive X-ray spectroscopy (EDX) and high-resolution scanning electron microscopy (SEM) analyses with ToF-SIMS to fully characterize the evolution of various types of laser-generated micro- and nanostructures formed on Ti and Ti alloys at different laser fluence levels, effective number of pulses and at different pulse repetition rates (1 – 400 kHz), following irradiation by near-infrared ultrashort laser pulses (925 fs, 1030 nm) in air environment or under argon gas flow.
We show how this combined surface analytical approach allows to evaluate alteration in the surface chemistry of the laser-generated surface structures depending on the laser processing parameters and the ambient environment.
Crystalline Silicon undergoes a complex phase-change dynamic of melting, amorphization, ablation and re-crystallization upon irradiation with high intensity ultra-short laser pulses [1]. The final state of such a modified surface spot depends on many factors, most notably the local fluence and the surface’s crystal orientation. In this study, we induced superficial structure and phase changes in Silicon <111> and <100> wafers using single femtosecond laser pulses (790 nm, 30 fs) for a range of different peak fluences. The resulting surface modifications were studied in great detail using a number of different techniques, including spectroscopic imaging ellipsometry (SIE), atomic force microscopy, high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray spectroscopy within scanning transmission electron microscopy (STEM-EDX).
Playing a pivotal role in this work, SIE provided non-destructive measurements for the calculation of the radial amorphous layer-thickness profiles of the irradiated spots using a two-layer thin-film model (Silicon dioxide and amorphous Silicon on a crystalline Silicon substrate). The measurements further allowed for the analysis of the oxide-layer modifications induced by the laser treatment. The results of the SIE-calculations were cross-checked by an in-depth material lamella via HRTEM and STEM-EDX.
Ellipsometry has been an extremely successful and fast expanding method in the past decades along with other related techniques using polarisation sensitive measurements. Opening new fields of application for a successful measurement technique brings some requirements and issues that have to be solved. From a metrological point of view, ellipsometry has the problem that uncertainties are difficult to determine for model-based analysis techniques in general. In this presentation, we will explore how the usefulness of polarimetric methods like ellipsometry can be increased.
Ellipsometry as a method could profit from several current developments which will be discussed in this presentation:
• Standardisation initiatives on national and international level developing standards for best practice when using ellipsometry. A series of at least six standards is currently developed on national German and international level covering different levels of sample complexity.
• Projects on traceability of ellipsometry and structured surface spectrometry as well as new dielectric function database initiatives.
• Metadata handling and data ontology providing a better framework for exchange and collaborative use of research data.
We will also explore the quantification of measurement uncertainty using examples from projects in which BAM is involved. Examples will be presented of multilayer and non-ideal materials as well as the determination of layer properties for technical applications such as thin layer catalysts and complex polymers. The definition of reference materials will be discussed.
Ellipsometry is a powerful tool, which allows the investigation of material properties over a broad spectral range. Over the course of several years, the ellipsometry lab at BAM has become an accredited testing lab according to ISO/IEC 17025 laying bare the need of better methods for accuracy and traceability. Despite its wide range of application in both research and development as well as industry, there have been no generally accepted standards dealing with model validation and measurement uncertainties.
Based on the first German standard DIN 50989 – 1: 2018 Ellipsometry - Part 1: Principles (currently international standard ISO 23131: 2021) and under consideration of GUM [1] a series of standards for ellipsometry was developed. The entire 6-part series covers several model-based application cases. This standards series avoids having narrow and material specific application cases but instead classifies applications of ellipsometry according to the sample complexity. The concept of ellipsometric transfer quantities (Ψ and Δ or alternatively the elements of transfer matrices) is implemented in the series. For each application case a model-based validation strategy was developed. Thus, the standards are applicable to all materials, instruments and measuring principles.
The uniform structure concept of the series facilitates its practical applicability for users. The standards include the model-based GUM-compliant determination/estimation of the measurement uncertainties. In addition, the appendices of the documents contain numerous measurement and simulation examples as well as recommendations for measuring practice.
In this contribution we present the application cases and basic structure of the standards developed in collaboration with Accurion GmbH and SENTECH Instruments GmbH in the project SNELLIUS.
Rising energy demand and the impending climate change require the development of a sustainable, fossil-free fuel and chemical production on a global scale. Hydrogen production via water electrolysis will be a fundamental cornerstone in this endeavor. The activity and stability of respective electrode coatings strongly depends on the coating's properties, i.e. phase composition, crystallinity, electrical conductivity, accessible surface, wettability and many other factors. The key to the development of improved catalysts is a better understanding of the relations between their performance, stability and physicochemical properties. However, those relations can be complex, and are strongly influenced also by the reaction environment. Hence, operando analysis of the catalyst material during catalysis at realistic potentials and current densities is highly desirable. Yet, many analytical techniques cannot be applied in liquid environments at realistic potentials and current densities.
We propose environmental ellipsometric analysis in a dedicated electrochemical flow cell as a new method to evaluate gas evolution reactions operando under realistic working conditions. Figure 1 illustrates schematically the developed technique. Key factors to success are highly active model-type catalysts with template-controlled porosity, a suitable sample environment, a deep understanding of the spectroscopic method and respective model development, as well as concise cross validation with numerous other analytical techniques.
The method was developed and validated by analyzing a calcination series (300 - 600°C) of mesoporous templated IrOx films ex-situ and operando under oxygen evolution reaction (OER) conditions. The employed environmental electrochemical spectroscopic ellipsometric analysis (ECSE) revealed during OER the change of optical and electronic properties, i.e. the dielectric functions (real ε1 and imaginary part ε2), electrical and electronic properties such as resistivity (ρ) and band-to-band transitions (p-d band transitions). Film thickness and porosity were validated by means of scanning electron microscopy (SEM), X-ray reflectometry (XRR) or ellipsometric porosimetry (EP), electrical and electronic properties by means of conductivity measurements, X-ray photoelectron spectroscopy (XPS) or UV-Vis-NIR absorption spectroscopy. The electronic structures of the catalysts from valence electron energy loss spectra (VEELS) derived from the real (ε1) and imaginary part (ε2) of the dielectric function from SE measurements reveal a direct correlation with electrochemical activities in OER.
In the presentation reversible and irreversible potential-dependent changes of the catalyst properties during operation will be discussed along with the dynamics of gas formation, transport and dissolution
Rising energy demand and the impending climate change require the development of a sustainable, fossil-free fuel and chemical production on a global scale. Hydrogen production via water electrolysis will be a fundamental cornerstone in this endeavor. The activity and stability of respective electrode coatings strongly depends on the coating's properties, i.e. phase composition, crystallinity, electrical conductivity, accessible surface, wettability and many other factors. The key to the development of improved catalysts is a better understanding of the relations between their performance, stability and physicochemical properties. However, those relations can be complex, and are strongly influenced also by the reaction environment. Hence, operando analysis of the catalyst material during catalysis at realistic potentials and current densities is highly desirable. Yet, many analytical techniques cannot be applied in liquid environments at realistic potentials and current densities.
We propose environmental ellipsometric analysis in a dedicated electrochemical flow cell as a new method to evaluate gas evolution reactions operando under realistic working conditions. Figure 1 illustrates schematically the developed technique. Key factors to success are highly active model-type catalysts with template-controlled porosity, a suitable sample environment, a deep understanding of the spectroscopic method and respective model development, as well as concise cross validation with numerous other analytical techniques.
The method was developed and validated by analyzing a calcination series (300 - 600°C) of mesoporous templated IrOx films ex-situ and operando under oxygen evolution reaction (OER) conditions. The employed environmental electrochemical spectroscopic ellipsometric analysis (ECSE) revealed during OER the change of optical and electronic properties, i.e. the dielectric functions (real ε1 and imaginary part ε2), electrical and electronic properties such as resistivity (ρ) and band-to-band transitions (p-d band transitions). Film thickness and porosity were validated by means of scanning electron microscopy (SEM), X-ray reflectometry (XRR) or ellipsometric porosimetry (EP), electrical and electronic properties by means of conductivity measurements, X-ray photoelectron spectroscopy (XPS) or UV-Vis-NIR absorption spectroscopy. The electronic structures of the catalysts from valence electron energy loss spectra (VEELS) derived from the real (ε1) and imaginary part (ε2) of the dielectric function from SE measurements reveal a direct correlation with electrochemical activities in OER.
In the presentation reversible and irreversible potential-dependent changes of the catalyst properties during operation will be discussed along with the dynamics of gas formation, transport and dissolution at different potentials.
Im Vorhaben SNELLIUS wurden in enger Zusammenarbeit der BAM mit den Firmen Accurion und SENTECH Instruments aufbauend auf der Norm DIN 50989-1 „Ellipsometrie - Teil 1: Grundlagen; Ellipsometry – Part 1: Principles“ insgesamt fünf Normenentwürfe erarbeitet, die fünf Modell-basierten Anwendungsfällen der Ellipsometrie entsprechen. Unter Berücksichtigung des neuartigen Modell-basierten Normungskonzepts wurde in den Normenentwürfen erstmals eine GUM- und DIN EN ISO/IEC 17025:2018 konforme Bestimmung bzw. Abschätzung der Messunsicherheit umgesetzt. Dies erforderte neben der textlichen Erstellung auch umfangreiche experimentelle Arbeiten bei den Firmen, Vergleichsversuche und komplementäre Modell- und Datenevaluierungen, da sowohl die verwendete ellipsometrische Hardware (unterschiedliche ellipsometrische Funktionsprinzipien) als auch die ellipsometrische Software (Modell-Randbedingungen, Fitalgorithmus) firmenspezifisch zum Teil sehr unterschiedlich sind. Hierbei mussten in einem Konsensprozess Lösungen gefunden werden, die die beiden deutschen Ellipsometer-Anbieter Accurion (Imaging-Ellipsometrie) und SENTECH Instruments (Mapping Ellipsometrie) gleichermaßen akzeptieren konnten und die zudem einen Transfer auf die Internationale Ebene (ISO Norm) ermöglichen, also vom Grundsatz her auch auf weitere Ellipsometerhersteller wie (J. A. Woollam Co., USA; SEMILAB, Ungarn; HORIBA, Japan) mit wieder anderen ellipsometrischen Funktionsprinzipien, Hard- und Softwarelösungen anwendbar sind. Die weitgehende Unabhängigkeit von ellipsometrischen Funktionsprinzipien, Hard- und Softwarelösungen wurde dadurch erreicht, dass normativ festgelegt wurde, dass die ellipsometrischen Größen PSI und DELTA im Sinne von DIN ENISO/IEC 17025 als Rohdaten fungieren.
The thermodynamic behavior of thin PVME films including the irreversible adsorbed layer on the substrate is investigated. In a first step, the growth kinetics of the adsorbed layer was studied combining a leaching technique and atomic force microscopy. Further, it was shown that there is a critical initial film thickness for the formation of a surface-filling adsorbed layer. Additionally, spectroscopic ellipsometry measurements were carried out to investigate the influence of the adsorbed layer on the glass transition temperature of the thin films. For 30 nm films and below, the influence of the adsorbed layer percolates strongly to the bulk-like layer of the film. Finally, the molecular dynamics of the adsorbed layer was studied by broadband dielectric spectroscopy, employing nanostructured-electrode systems. One process was revealed, which was assigned either to molecular fluctuations taking place in a loosely-bounded the part of the adsorbed layer, or to the desorption/adsorption of segments at the substrate.
In the face of rising energy demand and the impending climate change the development of a sustainable, fossil-free fuel and chemical production is of global importance. One possible goal is the development of electrochemical conversion processes using catalysts. Porous materials play an important role in such energy applications. The key to the development of improved catalysts is a better understanding of the relations between their performance, stability and physico-chemical properties. However, the complex morphology of such catalysts constitutes a challenge even for modern analytical techniques. Spectroscopic ellipsometry (SE) is a versatile method for studying material properties by using appropriate models (e.g. film thickness, optical and electronic properties). The fact that the material properties cannot be taken directly from the measured spectra, the developed models have to be validated. In a first step, the model for the ellipsometric fit studies of a calcination series of mesoporous iridium oxide films (300 – 600 °C) was investigated and validated with respect to their material properties. Moreover, the electronic structures of the catalysts reveal a direct correlation with electrochemical activities. The development of an environmental cell offers the possibility of investigations under real conditions. This will allow changes in the optical and electronic properties during the electrocatalytic oxygen evolution reaction.
Efficient water electrolysis requires highly active electrodes. The activity of corresponding catalytic coatings strongly depends on material properties such as film thickness, crystallinity, electrical conductivity, and chemical surface speciation. Measuring these properties with high accuracy in vacuum-free and nondestructive methods facilitates the elucidation of structure−activity relationships in realistic environments. Here, we report a novel approach to analyze the optical and electrical properties of highly active oxygen evolution reaction (OER) catalysts via spectroscopic ellipsometry (SE). Using a series of differently calcined, mesoporous, templated iridium oxide films as an example, we assess the film thickness, porosity, electrical resistivity, electron concentration, electron mobility, and interband and intraband transition energies by modeling of the optical spectra. Independently performed analyses using scanning electron microscopy, energy-dispersive X-ray spectroscopy, ellipsometric porosimetry, X-ray reflectometry, and absorption spectroscopy indicate a high accuracy of the deduced material properties. A comparison of the derived analytical data from SE, resonant photoemission spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy with activity measurements of the OER suggests that the intrinsic activity of iridium oxides scales with a shift of the Ir 5d t2g sub-level and an increase of p−d interband transition energies caused by a transition of μ1-OH to μ3-O species.
MISTRAL-Müller-Matrix-basierte Identifizierung von Inhomogenitäten und strukturellen Anomalien
(2020)
Mit abbildender Ellipsometrie (imaging ellipsometry: IE) können Nanometer-Beschichtungen/ Degradationen visualisiert und bezüglich Brechungsindex n, Extinktionskoeffizient k und Schichtdicke d charakterisiert werden. Materialseitig werden lokal Homogenität und Isotropie innerhalb eines/mehrerer Bildpunkte (field of analysis: FOA) und messtechnisch global die Ebenheit der Proben bezüglich aller Bildpunkte des Messfeldes vorausgesetzt. Für die Müller-Matrix-basierte abbildende Ellipsometrie (MM-IE), die den optischen Response anisotroper Materialien in jedem Bildpunkt über die bis zu 16 Müller-Matrix-Elemente erfasst, ist dies von besonderer Relevanz. Reale Substrate sind aber oft nicht eben, können lokal Noppen/Mulden (im FOA) aufweisen oder sind global konvex/ konkav im möglichen Messfeld (field of view: FOV) gekrümmt. Diese lokalen/globalen strukturellen Anomalien des Substrats werden schon für die IE messtechnisch/modellseitig oft nicht beherrscht und sind für die MM-IE in der Wirkung auf die MM-Elemente unbekannt. Diese Defizite sollen messtechnisch bzw. modellmäßig behoben und somit die Messung lokal/global gekrümmter Oberflächen ermöglicht werden (MM-CSIE, CS curved surface).
3D coherence scanning interferometry (CSI) is an optical, non-contact and rapide measurement technique using a defined bandwidth of white light at normal incidence. Based on this operational principle, white light interference microscopy (WLIM) provides three-dimensional surface topometry data up to a resolution of 0.4 μm lateral and 0.1 nm vertical. Three operating modi, i.e. surface, films and advanced films, enable measurements of step heights, roughness, wear volume, cone angle, surface pattern and layer thickness of transparent coatings.
The determination of layer thickness by WLIM requires the knowledge of optical constants, i.e.the refractive index n and the extinction coefficient k. For technical surfaces, data base values - if available at all - have to be determined or validated by spectroscopic ellipsometry (SE). From this oblique incidence technique both optical constants and layer thickness can be derived based on a model for at least semi-transparent coatings. For different layer thicknesses, a
comparison is made between WLIM and SE. Measurement uncertainty is
discussed for both topometric features and layer thickness for different use cases.
Traceability to SI system is ensured by certified standards (PTB/NIST) within a DAkkS DIN EN ISO/IEC 17025:2018 accredited lab.
Sufficient adhesion/tensile strength are basic requirements for any coating/composite material. For coatings, adhesive strength in N/mm2 is of Major interest for various applications such as decorative and water-repellent coatings on wood (paints and varnishes), optical coatings on glass and polymers (reflectors and filters), electrical coatings on semiconductors, glass and polymers (conducting and bondable layers), mechanical coatings on metals and polymers (wear-reduction, scratch-resistance) and adhesion-promoting layers. For composite materials, tensile strength in N/mm2 is also a key quantity for carbon fiber reinforced composites (CFC), laminates and adhesive-bonded joints. Centrifugal adhesion testing (CAT) transfers the single-sample tensile test from a tensile or universal testing machine into an analytical centrifuge as multiple-sample test of up to eight test pieces. The one-sided sample support instead of a two-sided sample clamping and the absence of mounting- and testing-correlated shear forces provides fast and reliable results both for adhesive strength and bonding strength by means of bonded test stamps. For bonding strength, the evaluation of failure pattern from microscopic inspection is required in order to determine the failure pattern according to ISO 10365 such as adhesive failure (AF), delamination failure (DF) and cohesive failure (CF). Hence, one test run by CAT-technology provides either statistics or ranking of up to eight samples at once. For adhesive strength of coatings, a variety of examples is discussed such as ALD-Al203 layers as adhesion promoters, evaporated Ag-layers on N-BK7 glass, sputtered Cr- and Al-layers on Borofloat 33 glass, evaporated Au-films on N-BK7 glass and sputtered SiO2 -layers on CR39 Polymer. Provided adhesive or bonding strength are high enough, the substrate or the joining part may also fail.