Effect of Iron Doping in Ordered Nickel Oxide Thin Film Catalyst for the Oxygen Evolution Reaction.

Water splitting has emerged as a promising route for generating hydrogen as an alternative to conventional production methods. Finding affordable and scalable catalysts for the anodic half-reaction, the oxygen evolution reaction (OER), could help with its industrial widespread implementation. Iron-containing Ni-based catalysts have a competitive performance for the use in commercial alkaline electrolyzers. Due to the complexity of studying the catalysts at working conditions, the active phase and the role that iron exerts in conjunction with Ni are still a matter of investigation. Here, we study this topic with NiO(001) and Ni0.75Fe0.25O x (001) thin film model electrocatalysts employing surface-sensitive techniques. We show that iron constrains the growth of the oxyhydroxide phase formed on top of the Ni or NiFe oxide, which is considered the active phase for the OER. Besides, operando Raman and grazing incidence X-ray absorption spectroscopy experiments reveal that the presence of iron affects both, the disorder level of the active phase and the oxidative charge around Ni during OER. The observed compositional, structural, and electronic properties of each system have been correlated with their electrochemical performance.

Facet Dependence of the Oxygen Evolution Reaction on Co3O4, CoFe2O4, and Fe3O4 Epitaxial Film Electrocatalysts.

The main obstacle for the electrocatalytic production of "green hydrogen" is finding suitable electrocatalysts which operate highly efficiently over extended periods of time. The topic of this study is the oxygen evolution reaction (OER), one of the half-reactions of water splitting. It is complex and has intricate kinetics, which impairs the reaction efficiency. Transition metal oxides have shown potential as electrocatalysts for this reaction, but much remains unknown about the atomic scale processes. We have investigated structure-composition-reactivity correlations for Co3O4, CoFe2O4, and Fe3O4 epitaxial thin-film electrocatalysts exposing either the (001) or (111) surface facets. We found that for Co3O4, the (001) facet is more reactive, while for the other oxides, the (111) facet is more active. A Tafel-like evaluation reveals systematically smaller "Tafel" slopes for the (001) facets. Furthermore, the slopes are smaller for the iron-containing films. Additionally, we found that the oxyhydroxide skin layer which forms under OER reaction conditions is thicker on the cobalt oxides than on the other oxides, which we attribute to either a different density of surface defects or to iron hindering the growth of the skin layers. All studied skin layers were thinner than 1 nm.

Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction.

Water electrolysis to produce 'green H2' with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co3O4 with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe2O4(111) is found to be up to about four times more active than Co3O4(111) and about nine times more than Fe3O4(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe2O4(111), the oxyhydroxide's Fe/Co concentration ratio is stabilized by partial iron dissolution.

Adatom Bonding Sites in a Nickel-Fe3 O4 (001) Single-Atom Model Catalyst and O2 Reactivity Unveiled by Surface Action Spectroscopy with Infrared Free-Electron Laser Light.

Single-atom (SA) catalysis presently receives much attention with its promise to decrease the cost of the active material while increasing the catalyst's performance. However, key details such as the exact location of SA species and their stability are often unclear due to a lack of atomic level information. Here, we show how vibrational spectra measured with surface action spectroscopy (SAS) and density functional theory (DFT) simulations can differentiate between different adatom binding sites and determine the location of Ni and Au single atoms on Fe3 O4 (001). We reveal that Ni and Au adatoms selectively bind to surface oxygen ions which are octahedrally coordinated to Fe ions. In addition, we find that the Ni adatoms can activate O2 to superoxide in contrast to the bare surface and Ni in subsurface positions. Overall, we unveil the advantages of combining SAS and DFT for improving the understanding of single-atom catalysts.

Surface oxygen Vacancies on Reduced Co3 O4 (100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation.

The activation of molecular oxygen is a fundamental step in almost all catalytic oxidation reactions. We have studied this topic and the role of surface vacancies for Co3 O4 (100) films with a synergistic combination of experimental and theoretical methods. We show that the as-prepared surface is B-layer terminated and that mild reduction produces oxygen single and double vacancies in this layer. Oxygen adsorption experiments clearly reveal different superoxide species below room temperature. The superoxide desorbs below ca. 120 K from a vacancy-free surface and is not active for CO oxidation while superoxide on a surface with oxygen vacancies is stable up to ca. 270 K and can oxidize CO already at the low temperature of 120 K. The vacancies are not refilled by oxygen from the superoxide, which makes them suitable for long-term operation. Our joint experimental/theoretical effort highlights the relevance of surface vacancies in catalytic oxidation reactions.

Elucidating Surface Structure with Action Spectroscopy.

Surface Action Spectroscopy, a vibrational spectroscopy method developed in recent years at the Fritz Haber Institute is employed for structure determination of clean and H2O-dosed (111) magnetite surfaces. Surface structural information is revealed by using the microscopic surface vibrations as a fingerprint of the surface structure. Such vibrations involve just the topmost atomic layers, and therefore the structural information is truly surface related. Our results strongly support the view that regular Fe3O4(111)/Pt(111) is terminated by the so-called Fetet1 termination, that the biphase termination of Fe3O4(111)/Pt(111) consists of FeO and Fe3O4(111) terminated areas, and we show that the method can differentiate between different water structures in H2O-derived adsorbate layers on Fe3O4(111)/Pt(111). With this, we conclude that the method is a capable new member in the set of techniques providing crucial information to elucidate surface structures. The method does not rely on translational symmetry and can therefore also be applied to systems which are not well ordered. Even an application to rough surfaces is possible.

Surface core level BE shifts for CaO(100): insights into physical origins.

The relationship between the electronic structure of CaO and the binding energy, BE, shifts between surface and bulk atoms is examined and the physical origins of these shifts are established. Furthermore, the contribution of covalent mixing to the interaction, including the energetic importance, is investigated and found to be small. In particular, the small shift between surface and bulk O(1s) BEs is shown to originate from changes in the polarizable charge distribution of surface O anions. This relationship, which is relevant for the catalytic properties of CaO, follows because the BE shifts are dominated by initial state contributions and the relaxation in response to the core-ionization is similar for bulk and surface. In order to explain the dominance of initial state effects for the BE shifts, the relaxation is decomposed into atomic and extra-atomic contributions. The bonding and the core-level BE shifts have been studied using cluster models of CaO with Hartree-Fock wavefunctions. The theoretical shifts are compared with X-ray photoelectron spectroscopy measurements where both angular resolution and incident photon energy have been used to distinguish surface and bulk ionization.

Growth of well-ordered iron sulfide thin films.

In this paper a growth recipe for well-ordered iron sulfide films and the results of their characterisation are presented. The film was studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). XRD data reveal that the film has a NiAs-like structure with Fe vacancies, similar to iron sulfides such as pyrrhotite and smythite, although no indication of any ordering of these vacancies was observed. LEED and STM results show that the film exhibits a 2 × 2 surface reconstruction. XPS data provide additional evidence for a large number of Fe vacancies, and the oxidation states of the Fe and S in the film are analysed.

Oxidation of Reduced Ceria by Incorporation of Hydrogen.

The interaction of hydrogen with reduced ceria (CeO2-x ) powders and CeO2-x (111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce3+ species to hydrogen, which results in the formation of Ce4+ , and thus in oxidation of ceria.

Surface action spectroscopy with rare gas messenger atoms.

Action spectroscopy with inert gas messengers is commonly used for the characterization of aggregates in the gas phase. The messengers, often rare gas atoms or D2 molecules, are attached to the gas phase aggregates at low temperature. Vibrational spectra of the aggregates are measured via detection of inert gas desorption following a vibrational excitation by variable-energy infrared light. We have constructed an apparatus for the application of action spectroscopy to surfaces of solids with the aim of establishing a new method for the vibrational spectroscopy of surfaces and deposited clusters. Experiments performed for neon covered V2O3(0001) show that this method can provide information about surface vibrations. Besides the surface sensitive channel, there is also a bulk sensitive one as demonstrated with the example of CeO2(111) thin film data. Unlike infrared reflection absorption spectroscopy, normalization to a reference spectrum is not required for action spectroscopy data, and unlike high resolution electron energy loss spectroscopy, the action spectroscopy method does not suffer from moderate resolution nor from multiple excitations. Selective decoration of specific surface features with messenger atoms may be utilized to focus the spectroscopic information onto these features.