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The journal of physical chemistry / C 127(8), 4021 - 4032 (2023). doi:10.1021/acs.jpcc.2c07650, The state of the surface near-region during CO hydrogenation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C., Published by Soc., Washington, DC

ACS applied nano materials 6(9), 8004 - 8016 (2023). doi:10.1021/acsanm.3c01306, We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain anatomistic picture of the catalyst’s structure under in situ oxidizing and reducing conditions.The Cu/ZnO model catalysts were investigated at elevated gas pressures by highenergy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectronspectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu$_2$Ounder atmospheric conditions at room temperature. As the nanoparticles swell duringoxidation, they maintain their epitaxy on basal ZnO (000±1) surfaces, whereas on thevicinal ZnO (10$\bar{14}$) surface, the nanoparticles undergo a coherent tilt. We find thatthe oxidation process is fully reversible under H$_2$ flow at 500 K, resulting in predominantlywell-aligned nanoparticles on the basal surfaces, whereas the orientation of CuNPs on vicinal ZnO was only partially restored. The analysis of the substrate crystaltruncation rods evidences the stability of basal ZnO surfaces under all gas conditions.No Cu-Zn bulk alloy formation is observed. Under CO$_2$ flow, no diffraction signalfrom the nanoparticles is detected, pointing to their completely disordered state. TheAP-XPS results are in line with the formation of CuO. Scanning electron microscopyimages show that massive mass transport has set in, leading to the formation of largeragglomerates., Published by ACS Publications, Washington, DC

Performing fundamental operando catalysis studies under realistic conditions is a key to further develop and increase the efficiency of industrial catalysts. Operando X-ray photoelectron spectroscopy (XPS) experiments have been limited to pressures, and the relevance for industrial applications has been questioned. Herein, we report on the CO oxidation experiment on Pd(100) performed at a total pressure of 1 bar using XPS. We investigate the light-off regime and the surface chemical composition at the atomistic level in the highly active phase. Furthermore, the observed gas-phase photoemission peaks of CO$_2$, CO, and O$_2$ indicate that the kinetics of the reaction during the light-off regime can be followed operando, and by studying the reaction rate of the reaction, the activation energy is calculated. The reaction was preceded by an in situ oxidation study in 7% O$_2$ in He and a total pressure of 70 mbar to confirm the surface sensitivity and assignment of the oxygen-induced photoemission peaks. However, oxygen-induced photoemission peaks were not observed during the reaction studies, but instead, a metallic Pd phase is present in the highly active regime under the conditions applied. The novel XPS setup utilizes hard X-rays to enable high-pressure studies, combined with a grazing incident angle to increase the surface sensitivity of the measurement. Our findings demonstrate the possibilities of achieving chemical information of the catalyst, operando, on an atomistic level, under industrially relevant conditions.

We investigated Cu/ZnO model catalysts for methanol synthesis to obtain an atomistic picture of activation and deactivation processes under in situ oxidizing and reducing conditions. We have investigated Cu nanoparticles with different shapes and aspect ratios grown epitaxially on basal and vicinal ZnO surfaces at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu2O under atmospheric conditions at room temperature. During oxidation, they maintain their epitaxy on basal ZnO (000-1) surfaces, whereas on the vicinal ZnO (10-14) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H2 flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas a random orientation is preferred for the (10-14) surface. Under CO2 flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No proof for Cu-Zn alloy formation is found. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates, which is detrimental to the catalyst’s performance.

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO 2 /CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO 2 /CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO 2 gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO 2 methanol synthesis.

Physical review / B 106(5), 054424 (2022). doi:10.1103/PhysRevB.106.054424, The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly dispersive nearly fully polarized Eu $4f$ bands, as well as O $2p$ levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrate the exchange splitting in O $2p$ and its vanishing at the Curie temperature. Our calculations with a Hubbard U term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of indirect exchange in the O $2p$ levels by analyzing the orbital resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu $4f$ band manifold. The analysis of the shape of spin-resolved spectra in the Eu $4f$ spectral region reveals signatures of hybridization with O $2p$ states, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature, and demonstrate that they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material., Published by Inst., Woodbury, NY

ACS catalysis 12, 7609 - 7621 (2022). doi:10.1021/acscatal.2c00905, Carbide formation on iron-based catalysts is an integral and, arguably, the most important part of the Fischer–Tropsch synthesis process, converting CO and H2 into synthetic fuels and numerous valuable chemicals. Here, we report an in situ surface-sensitive study of the effect of pressure, temperature, time, and gas feed composition on the growth dynamics of two distinct iron–carbon phases with the octahedral and trigonal prismatic coordination of carbon sites on an Fe(110) single crystal acting as a model catalyst. Using a combination of state-of-the-art X-ray photoelectron spectroscopy at an unprecedentedly high pressure, high-energy surface X-ray diffraction, mass spectrometry, and theoretical calculations, we reveal the details of iron surface carburization and product formation under semirealistic conditions. We provide a detailed insight into the state of the catalyst’s surface in relation to the reaction., Published by ACS, Washington, DC

Journal of the American Chemical Society 144(16), 7038 - 7042 (2022). doi:10.1021/jacs.2c00300, The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra., Published by American Chemical Society, Washington, DC

Journal of vacuum science & technology / A 40(1), 013215 (2022). doi:10.1116/6.0001491, The tunability of the oxygen content in complex oxides and heterostructures has emerged as a key to designing their physical functionalities. Controlling the interface reactivity by redox reactions provides a powerful means to deliberately set distinct oxide phases and emerging properties. We present routes on how to control oxygen-driven redox mechanisms in ultrathin ferro(i)magnetic and ferroelectric oxide films and across oxide interfaces. We address the growth and control of metastable EuO oxide phases, the control of phase transitions of binary Fe oxides by oxygen migration, the in operando determination of NiFe$_2$O$_4$/SrTiO$_3$ interface band alignments, as well as the role of interfacial oxide exchange in ferroelectric HfO$_2$-based capacitors—uncovered by the unique capabilities of photoelectron spectroscopy and, in particular, using hard x-rays., Published by Inst., New York, NY

Angewandte Chemie 134(3), e202111021 (2022). doi:10.1002/ange.202111021, Subsurface oxygen has been proposed to be crucial in oxide-derived copper (OD-Cu) electrocatalysts for enhancing the binding of CO intermediates during CO$_2$ reduction reaction (CO$_2$RR). However, the presence of such oxygen species under reductive conditions still remains debated. In this work, the existence of subsurface oxygen is validated by grazing incident hard X-ray photoelectron spectroscopy, where OD-Cu was prepared by reduction of Cu oxide with H$_2$ without exposing to air. The results suggest two types of subsurface oxygen embedded between the fully reduced metallic surface and the Cu$_2$O buried beneath: (i) oxygen staying at lattice defects and/or vacancies in the surface-most region and (ii) interstitial oxygen intercalated in metal structure. This study adds convincing support to the presence of subsurface oxygen in OD-Cu, which previously has been suggested to play an important role to mitigate the σ-repulsion of Cu for CO intermediates in CO$_2$RR., Published by Wiley-VCH, Weinheim

Nature Communications 12(1), 6117 (2021). doi:10.1038/s41467-021-26372-y, Heterogeneous catalyst surfaces are dynamic entities that respond rapidly to changes in theirlocal gas environment, and the dynamics of the response is a decisive factor for the catalysts���action and activity. Few probes are able to map catalyst structure and local gas environmentsimultaneously under reaction conditions at the timescales of the dynamic changes. Here weuse the CO oxidation reaction and a Pd(100) model catalyst to demonstrate how suchstudies can be performed by time-resolved ambient pressure photoelectron spectroscopy.Central elements of the method are cyclic gas pulsing and software-based event-averaging byimage recognition of spectral features. A keyfinding is that at 3.2 mbar total pressure ametallic, predominantly CO-covered metallic surface turns highly active for a few secondsonce the O2:CO ratio becomes high enough to lift the CO poisoning effect before masstransport limitations triggers formation of a���5 oxide., Published by Nature Publishing Group UK, [London]

Journal of physics / Condensed matter 33(23), 233001 (1-44) (2021). doi:10.1088/1361-648X/abeacd, Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities., Published by IOP Publ., Bristol

Physical review / B 101(8), 085121 (1-10) (2020). doi:10.1103/PhysRevB.101.085121, We report the observation of a two-dimensional electron system (2DES) at the (110) surface of the transparent bulk insulator SnO$_2$ and the tunability of its carrier density by means of temperature or Eu deposition. The 2DES is insensitive to surface reconstructions and, surprisingly, it survives even after exposure to ambient conditions—an extraordinary fact recalling the well known catalytic properties SnO$_2$. Our data show that surface oxygen vacancies are at the origin of such 2DES, providing key information about the long-debated origin of n-type conductivity in SnO$_2$, at the basis of a wide range of applications. Furthermore, our study shows that the emergence of a 2DES in a given oxide depends on a delicate interplay between its crystal structure and the orbital character of its conduction band., Published by Inst., Woodbury, NY

We demonstrate the emergence and control of magnetic phases between magnetite (Fe

Physical review materials 3(6), 061401 (2019). doi:10.1103/PhysRevMaterials.3.061401, We demonstrate a route to prepare thin films of the ferromagnetic insulator europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO$_3$ as the supplier of oxygen. The process allows tuning the electronic, magnetic, and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis., Published by APS, College Park, MD

We demonstrate the emergence and control of magnetic phases between magnetite (Fe3O4), a ferrimagnetic halfmetal, and SrTiO3, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition (TV) was detected to persist from bulk-like down to ultrathin Fe3O4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe2+ ions with decreasing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe2O3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γ-Fe2O3 that finally leads to stoichiometric and ferrimagnetic Fe3O4/Sr...

13th International Conference on Synchrotron Radiation Instrumentation, SRI2018, Taipei, Taiwan; AIP conference proceedings 2054(1), 040010 (2019). doi:10.1063/1.5084611, A new undulator beamline (P22) for hard X-ray photoelectron spectroscopy (HAXPES) was built at PETRA III (DESY, Hamburg) to meet the increasing demand for HAXPES-based techniques. It provides four special instruments for high-resolution studies of the electronic and chemical structure of functional nano-materials and catalytic interfaces, with a focus on measurements under operando and/or ambient conditions: (i) a versatile solid-state spectroscopy setup with optional wide-angle lens and in-situ electrical characterization, (ii) a HAXPEEM instrument for sub-µm spectro-microscopy applications, (iii) an ambient pressure system (> 1 bar) for operando studies of catalytic reactions and (iv) a time-of-flight spectrometer as a full-field k-microscope for measurements of the 4D spectral function ρ(EB,k). The X-ray optics were designed to deliver high brightness photon flux within the HAXPES energy range 2.4 – 15 keV. An LN$_2$-cooled double-crystal monochromator with interchangeable pairs of Si(111) and (311) crystals is optionally combined with a double channel-cut post-monochromator to generate X-rays with variable energy bandpass adapted to the needs of the experiment. Additionally, the beam polarization can be varied using a diamond phase plate integrated into the beamline. Adaptive beam focusing is realized by Be compound refractive lenses and/or horizontally deflecting mirrors down to a spot size of ∼20x17 µm$^2$ with a flux of up to 1.1x10$^{13}$ ph/s (for Si(111) at 6 keV)., Published by AIP Publ., Melville, NY

Journal of materials chemistry / C 4(9), 1813 - 1820(2016). doi:10.1039/C6TC00170J, As a prototypical all-oxide heterostructure, the ferromagnetic insulator europium monoxide (EuO) issynthesized on transparent and conductive indium tin oxide (ITO) virtual substrates. Non-destructivehard X-ray photoelectron spectroscopy is employed to depth profile the chemical composition of themagnetic layer and the buried oxide–oxide interface. We find that thermally activated oxygen diffusionfrom ITO affects the EuO growth process. We present how to control the oxygen reactivity at the interfaceand discuss its origin in a thermodynamic analysis. Our complementary methodical strategy allowsfor a significant improvement of the EuO chemical quality with sizeable magnetic properties. Generally,our approach derives guidelines for the proper choice of oxide substrates and buffer layer materials forfunctional all-oxide heterostructures., Published by RSC, London [u.a.]

We create a two-dimensional electron system (2DES) at the interface between EuO, a ferromagnetic insulator, and ${\mathrm{SrTiO}}_{3}$, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. This is achieved by a controlled in situ redox reaction between pure metallic Eu deposited at room temperature on the surface of ${\mathrm{SrTiO}}_{3}$---an innovative bottom-up approach that can be easily generalized to other functional oxides and scaled to applications. Additionally, we find that the resulting EuO capping layer can be tuned from paramagnetic to ferromagnetic, depending on the layer thickness. These results demonstrate that the simple, novel technique of creating 2DESs in oxides by deposition of elementary reducing agents [T. C. R\"odel et al., Adv. Mater. 28, 1976 (2016)] can be extended to simultaneously produce an active, e.g., magnetic, capping layer enabling the realization and control of additional functionalities in such oxide-based 2DESs.