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Nemsak, Slavomir

Publications and source records attributed to Nemsak, Slavomir.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Shapeshifting Nanocatalyst for CO2 Conversion

The conversion of CO2 into high-value chemicals through a photoreduction reaction in water is a promising route to reduce the dependence on fossil fuels. Enhancing selectivity toward hydrocarbons or alcohols can be achieved by Ag-Cu alloys. However, the stabilized surface state created by Ag-Cu interactions is still poorly understood. In this work, multi-modal in situ X-ray experiments reveals underlying mechanisms and the evolution of Ag-Cu nanoparticles under CO2 reduction reaction (CO2RR) conditions. Both morphological and chemical changes of Ag and Cu species induced by diffusion mechanics are tracked during nanocatalyst operation. The initial spheroid Ag-Cu nanoparticles are composed of a Cu-rich shell and Ag-rich core. The reduction treatment promotes Ag migration toward the surface. During photocatalytic CO2 reduction reaction, Cu atoms migrate back to the surface, forming Ag-Cu-O species. The study observes the surface oxidation of Cu(0) to Cu+ and the presence of Ag at the sub-surface region. Furthermore, nanoparticles change their shape, decreasing their specific surface area, driven by Cu diffusion during the CO2 photoreduction reaction. The results provide invaluable insights into the dynamic restructuring of the catalyst under reaction conditions and into the active species responsible for CO2 conversion.

CO2 reduction reaction↗
DOE OSTI2023

Correlating Tomographic Chemical Inhomogeneity and Low Energy Electronic Structure in Layered Quantum Materials

Photoemission spectroscopy (PES) is a suite of experimental tools to learn about the electronic and chemical structure of materials and surfaces. Normally implemented in a surface-sensitive manner, the research performed under this grant focused on pushing PES into less explored regimes, to reveal bulk electronic structure, to reveal tomographic (layer-resolved) chemistry and electronic structure of layered materials and heterostructures, and to reveal interface phenomena at the junction of two different materials. Standing wave (SW) spectroscopies have also been applied to PdCoO2, a material of interest due to its high conductivity and electron-hydrodynamic tendencies. This material can be modeled as an alternating layered structure consisting of metallic Pd layers and insulating CoO2 layers. Using SW XPS, the total electronic structure has been decomposed into contributions from the two layers, and computations highlighted the different many-body interactions in the two layers (Comm. Phys. 4, 143 (2021)). We have also used hard-x-ray angle-resolved photoemission spectroscopy (ARPES), to investigate LaB6, a technologically important material with widespread application as a cathode material for electron microscopes. We measured the bulk electronic structure of this material and found that the one-step model of photoemission better captured the electronic structure and correlations. This model treats the three steps of the photoemission process—excitation, transport of the photoelectron to the crystal surface, and escape into the vacuum—as a single quantum mechanically coherent process (Phys. Rev. Mater. 5, 055002 (2021)). We also applied x-ray photoelectron spectroscopy, implemented in a near total reflection grazing incidence geometry to elucidate technologically relevant interfaces, such as those between a substrate and photoresist (J. Phys. D: Appl. Phys. 54 464002 (2021)).

36 MATERIALS SCIENCE↗
DOE OSTI2022

Probing the initial stages of iron surface corrosion: Effect of O 2 and H 2 O on surface carbonation

Iron plays a vital role in natural processes such as water, mineral, iron, and nutrient cycles. Iron undergoes reduction-oxidation and catalytic reactions to produce various corrosion films depending on its chemical environment. Near ambient pressure X-ray photoelectron spectroscopy, polarized modulated infrared reflection absorption spectroscopy, and Auger electron spectroscopy were used to study the key reactants, from O 2 (g), H 2 O vapor, Na + and Cl – on the initial stages of iron surface corrosion. Here, with increasing the ratio of O 2 and H 2 O, surface hydrocarbons were shown to oxidize into carbonates, while the Cl – was found to migrate into the interface. The effect of each individual reactant was measured separately and water was shown to have a first order rate dependence on the carbonate growth at low pressures, with little dependence for O 2 . Near ambient pressures, both H 2 O and O 2 were found to increase the carbonate growth, which was estimated using the Langmuir isotherm model, yielding Gibbs energies between –9.8 to –8.5 kJ/mol. A mechanism is suggested to explain the oxidation is catalyzed by NaCl on iron surfaces and the adventitious hydrocarbons served as the source for surface carbonation. These findings have implications for understanding other surface catalytic and redox interface chemistry in complex environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Two-dimensional electron systems in perovskite oxide heterostructures: Role of the polarity-induced substitutional defects

The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as LaAlO 3 and SrTiO 3 has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, the fundamental origins of the 2DES are still not understood, e.g., the microscopic mechanisms of coexisting interface conductivity and magnetism. In this paper, we report a comprehensive spectroscopic investigation on the depth profile of 2DES-relevant Ti 3d interface carriers using depth- and element-specific techniques like standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti 3d interface carriers, which give rise to the 2DES are located within three unit cells from the n-type interface in the SrTiO 3 layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first three unit cells of the opposing LaAlO3 layer (~10 Å). Our findings provide a microscopic picture of how the localized and mobile Ti 3d interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the LaAlO 3 /SrTiO 3 interface have disparate explanations as originating from different types of interface carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Evidence of a magnetic transition in atomically thin Cr 2 TiC 2 T x MXene

Two-dimensional (2D) transition metal carbides and nitrides known as MXenes have shown attractive functionalities such as high electronic conductivity, a wide range of optical properties, versatile transition metal and surface chemistry, and solution processability. Although extensively studied computationally, the magnetic properties of this large family of 2D materials await experimental exploration. 2D magnetic materials have recently attracted significant interest as model systems to understand low-dimensional magnetism and for potential spintronic applications. Here, we report on synthesis of Cr 2 TiC 2 T x MXene and a detailed study of its magnetic as well as electronic properties. Using a combination of magnetometry, synchrotron X-ray linear dichroism, and field- and angular-dependent magnetoresistance measurements, we find clear evidence of a magnetic transition in Cr 2 TiC 2 T x at approximately 30 K, which is not present in its bulk layered carbide counterpart (Cr 2 TiAlC 2 MAX phase). Overall, this work presents the first experimental evidence of a magnetic transition in a MXene material and provides an exciting opportunity to explore magnetism in this large family of 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hydroxylation and Cation Segregation in (La 0.5 Sr 0.5 )FeO 3-$δ$ Electrodes

Simultaneously achieving high activity and stability is the primary challenge when engineering (electro)catalysts. Transition metal perovskite oxides are employed as air electrodes for solid-oxide fuel cells and electrolyzers. However, degradation of oxygen exchange kinetics at the solid–gas interface, often linked to alkaline-earth cation segregation and precipitation, limits widespread commercialization. Here, we systematically investigated the surface degradation mechanism induced by gas-phase impurities in La 0.5 Sr 0.5 )FeO 3-$δ$ (LSF55) thin-film electrodes by varying the concentration of H 2 O, SO 2 , and CO 2 . Degradation of the area-specific resistance in ambient and humidified synthetic air is significantly greater than in dry ambient and dry synthetic air, pointing to the importance of water vapor. Time-resolved, in situ ambient pressure X-ray photoelectron spectroscopy performed in O 2 showed that nonbulk Sr is present on the surface before the exposure to water vapor. Upon introduction of water vapor, neither additional Sr segregation nor precipitation driven by water vapor is a necessary condition for degradation. Rather, hydroxylation of the surface induces irreversible and significant degradation. At the same time, we show that Sr migration driven by water vapor is partially reversible. These fundamental insights can be used for the rational design of electrodes with improved catalytic stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗