Publications - Publications

Electrocatalytic and structural investigation of trimetallic NiFeMo bifunctional electrocatalyst for industrial alkaline water electrolysis

Frederiksen, M. L., Oglou, R. C., Lauritsen, J. V., Bentien, A., Nielsen, L. P. — Mon, 01 Apr 2024 21:33:08 +0200

In pursuit of sustainable hydrogen production, alkaline water electrolysis offers fossil-free technology for generating hydrogen. Exploring new non-precious metal electrocatalysts plays a crucial role in this endeavor. Herein, we investigate a trimetallic NiFeMo material on a nickel foam support, serving as a bifunctional electrocatalyst for catalyzing both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Scanning electron microscopy reveals a nanosheet array structure with a uniform distribution of Ni, Fe, and Mo compounds on the electrode surface. Furthermore, the chemical surface composition of the pristine and spent electrodes is elucidated via x-ray photoelectron spectroscopy, displaying primarily oxidized species on the electrocatalyst surface. Bifunctional performance is assessed in a three-electrode setup, unveiling overpotentials of 70 mV for the HER and 140 mV for the OER, in a 30 wt% KOH electrolyte at 90 °C. Additionally, in an industrial electrolysis cell, the activated electrode is evaluated as cathode and anode for 28 days, which decreased the overpotential of 330–350 mV at 200 mA cm_geo⁻² compared with pristine nickel foam. The performance increase of the electroplated coating is attributed to the increased surface area and enhanced intrinsic activity. The electrolysis cell experiences a ∼6 % voltage loss during the experiment, indicating its robustness and suitability for industrial alkaline electrolysis applications.

A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites

Gammelgaard, J. J., Sun, Z., Vestergaard, A. K., et al. — Fri, 01 Sep 2023 21:33:08 +0200

Dewetting Transition of CoO/Pt(111) in CO Oxidation Conditions Observed In Situ by Ambient Pressure STM and XPS

Rattigan, E., Jensen, S., Sun, Z., et al. — Mon, 01 May 2023 21:33:08 +0200

Ultrathin cobalt oxide films supported on noble-metal surfaces have received much attention as interesting examples of low-temperature CO oxidation catalysts. It is expected that the activity of the cobalt oxides is closely linked with the structure and morphology of the film, but a direct operando correlation between CO oxidation activity, nanoscale structure, oxidation state, and surface composition has been missing. Here, we use a combination of operando ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy to investigate varying submonolayer coverages of CoO supported on Pt(111) under CO oxidation conditions. The goal is to compare the structural and spectroscopic features as the samples are exposed to O-rich CO/O₂ gas mixtures at millibar pressure and brought to temperatures where CO oxidation occurs. Upon first exposure to millibar gas mixtures, the initial bilayer CoO film is oxidized to trilayer CoO₂, characterized by a preserved film morphology and Co in a predominant 3+ oxidation state. However, upon temperature increase during the CO oxidation reaction, the cobalt oxide ultrathin film undergoes dewetting into nanoparticles. On the basis of the XPS signature, we conclude that these nanoparticles have a Co₃O₄-like structure. The results underline the importance of operando observations of surface structures. This new insight into the Co oxide/metal interface may aid in our understanding of reactivity of metal oxide coated noble-metal particles in general.

Origin of hydroxyl pair formation on reduced anatase TiO₂(101)

Adamsen, K. C., Petrik, N. G., Dononelli, W., et al. — Mon, 01 May 2023 21:33:08 +0200

The interaction of water with metal oxide surfaces is of key importance to several research fields and applications. Because of its ability to photo-catalyze water splitting, reducible anatase TiO₂ (a-TiO₂) is of particular interest. Here, we combine experiments and theory to study the dissociation of water on bulk-reduced a-TiO₂(101). Following large water exposures at room temperature, point-like protrusions appear on the a-TiO₂(101) surface, as shown by scanning tunneling microscopy (STM). These protrusions originate from hydroxyl pairs, consisting of terminal and bridging OH groups, OH_t/OH_b, as revealed by infrared reflection absorption spectroscopy (IRRAS) and valence band experiments. Utilizing density functional theory (DFT) calculations, we offer a comprehensive model of the water/a-TiO₂(101) interaction. This model also explains why the hydroxyl pairs are thermally stable up to ∼480 K.

The interface of in-situ grown single-layer epitaxial MoS₂ on SrTiO₃(001) and (111)

Haastrup, M. J., Bianchi, M., Lammich, L., Lauritsen, J. V. — Mon, 01 May 2023 21:33:08 +0200

SrTiO₃ (STO) is a versatile substrate with a high dielectric constant, which may be used in heterostructures with 2D materials, such as MoS₂, to induce interesting changes to the electronic structure. STO single crystal substrates have previously been shown to support the growth of well-defined epitaxial single-layer (SL) MoS₂ crystals. The STO substrate is already known to renormalize the electronic bandgap of SL MoS₂, but the electronic nature of the interface and its dependence on epitaxy are still unclear. Herein, we have investigated an in-situ physical vapor deposition (PVD) method, which could eliminate the need for ambient transfer between substrate preparation, subsequent MoS₂ growth and surface characterization. Based on this, we then investigate the structure and epitaxial alignment of pristine SL MoS₂ in various surface coverages grown on two STO substrates with a different initial surface lattice, the STO(001)(4 × 2) and STO(111)-(9/5 × 9/5) reconstructed surfaces, respectively. Scanning tunneling microscopy shows that epitaxial alignment of the SL MoS₂ is present for both systems, reflected by orientation of MoS₂ edges and a distinct moiré pattern visible on the MoS₂(0001) basal place. Upon increasing the SL MoS₂ coverage, the presence of four distinct rotational domains on the STO(001) substrate, whilst only two on STO(111), is seen to control the possibilities for the formation of coherent MoS₂ domains with the same orientation. The presented methodology relies on standard PVD in ultra-high vacuum and it may be extended to other systems to help explore pristine two-dimensional transition metal dichalcogenide/STO systems in general.

Steering carbon dioxide reduction toward C-C coupling using copper electrodes modified with porous molecular films

Zhao, S., Christensen, O., Sun, Z., et al. — Wed, 01 Feb 2023 21:33:08 +0100

Copper offers unique capability as catalyst for multicarbon compounds production in the electrochemical carbon dioxide reduction reaction. In lieu of conventional catalysis alloying with other elements, copper can be modified with organic molecules to regulate product distribution. Here, we systematically study to which extent the carbon dioxide reduction is affected by film thickness and porosity. On a polycrystalline copper electrode, immobilization of porous bipyridine-based films of varying thicknesses is shown to result in almost an order of magnitude enhancement of the intrinsic current density pertaining to ethylene formation while multicarbon products selectivity increases from 9.7 to 61.9%. In contrast, the total current density remains mostly unaffected by the modification once it is normalized with respect to the electrochemical active surface area. Supported by a microkinetic model, we propose that porous and thick films increase both local carbon monoxide partial pressure and the carbon monoxide surface coverage by retaining in situ generated carbon monoxide. This reroutes the reaction pathway toward multicarbon products by enhancing carbon-carbon coupling. Our study highlights the significance of customizing the molecular film structure to improve the selectivity of copper catalysts for carbon dioxide reduction reaction.

Atomic-Scale Site Characterization of Cu-Zn Exchange on Cu(111)

Mammen, M., Jensen, S., Andersen, M., Lauritsen, J. V. — Wed, 01 Feb 2023 21:33:08 +0100

An accurate understanding of the physicochemical properties of bimetallic heterogeneous catalysts relies on atomic-scale knowledge of the surface morphology and the atomic distribution. Alloys of Cu and Zn created during catalyst operation are frequently studied and debated in relation to a description of the active phase of Cu/ZnO/Al2O3 methanol synthesis catalysts. This makes it relevant to build a better understanding of Zn dissolution pathways in Cu surfaces and the resulting surface morphology. Herein, we use scanning tunneling microscopy to investigate surface morphology and the distinct atom site configurations of Zn and Cu on Cu(111) resulting from room-Temperature Zn exchange from a Zn monolayer into the topmost layer of Cu(111). A gradual dissolution of Zn islands induces an extensive element intermixing at room temperature, resulting in Zn alloying at Cu terrace lattice sites. In addition, we observe and address an interlayer element exchange between the Zn submonolayers in direct contact with Cu. The exchange process is driven by lattice strain and is strongly facilitated at the perimeter of Zn edges. The STM contrast associated with the resulting intermixed sites is reported together with the simulation of these sites based on density functional theory, showing that imaging of isolated Zn sites in Cu(111) is sensitive to the STM tip state. The findings provide new insight into the atomic-scale exchange for Zn/Cu bimetallic surfaces, which may be used onward for understanding the debated surface morphology that develops during reductive activation and alloy formation in the Cu/ZnO/Al2O3 methanol synthesis catalyst.

Growth Mechanism of Single-Domain Monolayer MoS₂Nanosheets on Au(111) Revealed by In Situ Microscopy

Ewert, M., Buß, L., Lauritsen, J. V., Falta, J., Flege, J. I. — Thu, 01 Dec 2022 21:33:08 +0100

The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness.

Can the CO₂Reduction Reaction Be Improved on Cu

Christensen, O., Zhao, S., Sun, Z., et al. — Thu, 01 Dec 2022 21:33:08 +0100

Cu is currently the most effective monometallic catalyst for producing valuable multicarbon-based (C₂₊) products, such as ethylene and ethanol, from the CO₂reduction reaction (CO₂RR). One approach to optimize the activity and selectivity of the metal Cu catalyst is to functionalize the Cu electrode with a molecular modifier. We investigate from a data standpoint whether any reported functionalized Cu catalyst improves the intrinsic activity and/or multicarbon product selectivity compared to the performance of bare Cu foil and the best single crystal Cu facets. Our analysis shows that the reported increases in activity are due to increased surface roughness and disappear once normalized with respect to electrochemical surface area. The intrinsic activity generally falls below that of the bare Cu foil reference, both for total and product-specific current, which we attribute to nonselective blocking of active sites by the modifier on the surface. Instead, an analysis of various polymer diffusion coefficients indicates that the modifier allows for easier diffusion of CO₂compared to H₂O to the surface, leading to greater selectivity for CO₂RR and C₂₊products. As such, our analysis finds no catalyst for CO₂RR that intrinsically outperforms bare Cu.

Electrically Tunable Reactivity of Substrate-Supported Cobalt Oxide Nanocrystals

Sánchez-Grande, A., Nguyën, H. C., Lauwaet, K., et al. — Tue, 01 Mar 2022 21:33:08 +0100

First-row transition metal oxides are promising materials for catalyzing the oxygen evolution reaction. Surface sensitive techniques provide a unique perspective allowing the study of the structure, adsorption sites, and reactivity of catalysts at the atomic scale, which furnishes rationalization and improves the design of highly efficient catalytic materials. Here, a scanning probe microscopy study complemented by density functional theory on the structural and electronic properties of CoO nanoislands grown on Au(111) is reported. Two distinct phases are observed: The most extended displays a Moiré pattern (α-region), while the less abundant is 1Co:1Au coincidental (β-region). As a result of the surface registry, in the β-region the oxide adlayer is compressed by 9%, increasing the unoccupied local density of states and enhancing the selective water adsorption at low temperature through a cobalt inversion mechanism. Tip-induced voltage pulses irreversibly transform α- into β-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.

The cobalt oxidation state in preferential CO oxidation on CoO_x/Pt(111) investigated by operando X-ray photoemission spectroscopy

Rattigan, E., Sun, Z., Gallo, T., et al. — Fri, 01 Apr 2022 21:33:08 +0200

The combination of a reducible transition metal oxide and a noble metal such as Pt often leads to active low-temperature catalysts for the preferential oxidation of CO in excess H₂ gas (PROX reaction). While CO oxidation has been investigated for such systems in model studies, the added influence of hydrogen gas, representative of PROX, remains less explored. Herein, we use ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy on a CoO_x/Pt(111) planar model catalyst to analyze the active phase and the adsorbed species at the CoO_x/Pt(111) interface under atmospheres of CO and O₂ with a varying partial pressure of H₂ gas. By following the evolution of the Co oxidation state as the catalyst is brought to a reaction temperature of above 150 °C, we determine that the active state is characterized by the transformation from planar CoO with Co in the 2+ state to a mixed Co²⁺/Co³⁺ phase at the temperature where CO₂ production is first observed. Furthermore, our spectroscopy observations of the surface species suggest a reaction pathway for CO oxidation, proceeding from CO exclusively adsorbed on Co²⁺ sites reacting with the lattice O from the oxide. Under steady state CO oxidation conditions (CO/O₂), the mixed oxide phase is replenished from oxygen incorporating into cobalt oxide nanoislands. In CO/O₂/H₂, however, the onset of the active Co²⁺/Co³⁺ phase formation is surprisingly sensitive to the H₂ pressure, which we explain by the formation of several possible hydroxylated intermediate phases that expose both Co²⁺ and Co³⁺. This variation, however, has no influence on the temperature where CO oxidation is observed. Our study points to the general importance of a dynamic reducibility window of cobalt oxide, which is influenced by hydroxylation, and the bonding strength of CO to the reduced oxide phase as important parameters for the activity of the system.

Adsorption and Reaction of NH₃on Rutile TiO₂(110)

Bühlmeyer, H., Adamsen, K. C., Xu, T., et al. — Fri, 01 Apr 2022 21:33:08 +0200

By means of high-resolution scanning tunneling microscopy (STM), we studied the adsorption and reaction of submonolayer NH3 on rutile TiO2(110) surfaces in different oxidation states. On a clean, reduced TiO2(110) surface with O vacancies, NH3 adsorbs at 120 K exclusively as single molecular species on surface Ti sites. On a TiO2(110) surface with H adatoms, we observed small amounts of NH3 monomers in close proximity to each other and NH3 dimers, in addition to the majority of isolated NH3 monomers. On such a surface, we found that NH3 and H adatoms can diffuse together along the [001] direction and NH3 can diffuse along the [11¯ 0] direction via "hydroxyl bridges". On an oxidized TiO2(110) surface with O adatoms (Oot) and O2 molecules, we found dimeric NH3 species and isolated OHt groups. Interestingly, there were also grouped adsorbates of NH2OH stoichiometry (singles and pairs). Following annealing at 330 K, the coverage of paired NH2OH species was much increased. We find a strong affinity of NH3 species to interact with Oot adatoms and Oot adatom pairs and propose dissociation of NH3 species at Oot adatoms. Finally, we discuss the faith of the observed adsorbates at elevated temperature.

WO₃ Monomers Supported on Anatase TiO₂(101), −(001), and Rutile TiO₂(110)

Xu, T., Adamsen, K. C., Li, Z., Lammich, L., Lauritsen, J. V., Wendt, S. — Tue, 01 Feb 2022 21:33:08 +0100

We combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to study the molecular and electronic structure of submonolayer tungsten oxide supported on anatase TiO₂(101), −(001), and rutile TiO₂(110) surfaces. We found that monomeric tungsten oxide species form on all three TiO₂ surfaces upon mild annealing at 400 K, with a geometry depending on the supporting facet. At ∼600 K, surface diffusion of the monomers sets in, but the monomers remain on the surface without diffusing into the bulk even at higher annealing temperatures. As-deposited tungsten oxide at monolayer coverage is stronger oxidized than thick layers. At elevated temperatures (400-900 K), significant reduction is observed, strongly dependent on the TiO₂ facet employed and bulk defects within the substrate. Among the TiO₂ surfaces studied, the weakest reduction by vacuum annealing was found for tungsten oxide supported on anatase TiO₂(001).

Water dissociation on mixed Co-Fe oxide bilayer nanoislands on Au(111)

Sun, Z., Rodríguez-Fernández, J., Lauritsen, J. V. — Fri, 01 Apr 2022 21:33:08 +0200

We investigate the hydroxylation behaviour of mixed Co-Fe oxide nanoislands synthesized on a Au(111) surface under exposure to water vapour at vacuum conditions. The pure Co and Fe bilayer oxides both become hydroxylated by water exposure in vacuum conditions, albeit to a very different extent. It is however an open question how mixed oxides, exposing sites with a mixed coordination to Fe and Co, behave. By forming surface O species with a mixed Fe/Co coordination, we can investigate the nature of such sites. By means of scanning tunnelling microscopy and x-ray photoelectron spectroscopy, we characterize a series of Co-Fe oxides samples with different Fe contents at the atomic scale and observe a scaling of the hydroxylation degree with the amount of Fe inside the Co-Fe oxides. Our results indicate that the Fe dopants within the Co-Fe oxides have opposing effects on edge and basal plane sites modifying the maximum hydroxylation degree of pure cobalt oxide, perturbing the original binding sites of H, releasing the absorbed H or blocking the diffusion pathway of H.

Iron carbide formation on thin iron films grown on Cu(1 0 0)

Rodríguez, D. G., Gleeson, M. A., Lauritsen, J. V., et al. — Mon, 30 May 2022 21:33:08 +0200

Thin iron films evaporated onto Cu(1 0 0) were carburized using ethylene to produce iron carbide surfaces for use as model systems in experimental research. XPS and AES confirm that ethylene dissociation produces a pure iron carbide. A maximum of 0.5 ML carbon can be deposited for film thicknesses below 12 ML where Fe grows as γ-iron (FCC). For thick, BCC-Fe(1 1 0) films, post-treatment with ethylene leads to carbon coverages beyond 0.5 ML where some carbon diffuses into the bulk. The film remains α-iron (BCC) and a different surface carbide with a (4 × 3) unit cell is found. On the thin FCC-Fe(1 0 0) films, carbon reconstructs the surface into a p4g(2 × 2)-Fe₂C layer which has a special stability and acts as a carbon trap that prevents carbon diffusion into the bulk. Fe₂C is thermally stable up to 700 K above which Fe diffuses into the copper substrate while leaving graphitic carbon behind. Carbon segregates to the surface during evaporation of iron on top of an Fe₂C-covered FCC-Fe film and causes the film to retain the FCC structure up to a thickness of at least 30 ML, far beyond 12 ML where BCC-Fe forms on Cu(1 0 0) in absence of surface carbon.

Applications of high-resolution scanning probe microscopy in hydroprocessing catalysis studies

Besenbacher, F., Lauritsen, J. V. — Mon, 01 Nov 2021 21:33:08 +0100

Scanning probe microscopy offers real space atomic and nanoscale imaging of surfaces and supported nanoparticles, and these techniques are used in a planar catalyst model system context to analyze important atomic-scale aspects of nanoparticle structure, particle edges, defects, vacancies and to image the resulting interaction of individual reactants with the active surfaces. Here we review applications of high-resolution scanning probe microscopy to hydroprocessing catalysis, to demonstrate how these atom-resolved imaging techniques have recently advanced to become tools that elucidate the catalytically active edge structures of metal sulfide hydrotreating catalyst, resolve the intramolecular structure of complex hydrocarbons, and provide atomic-scale information on the mechanisms in which heteroatom-bearing hydrocarbons interact with a catalyst particle. This mini-review addresses the advantages of scanning tunneling microscopy for studies of planar model systems relevant for hydroprocessing catalysis, including the emerging use of near-ambient pressure (operando) scanning tunneling microscopy together with examples of the use of low-temperature molecular imaging with non-contact atomic force microscopy.

Lateral Interfaces between Monolayer MoS₂Edges and Armchair Graphene Nanoribbons on Au(111)

Haastrup, M. J., Mammen, M. H.R., Rodríguez-Fernández, J., Lauritsen, J. V. — Thu, 01 Apr 2021 21:33:08 +0200

The realization of electronic devices based on heterostructures of metallic, semiconducting, or insulating two-dimensional materials relies on the ability to form structurally coherent and clean interfaces between them, vertically or laterally. Lateral two-dimensional heterostructures that fuse together two different materials in a well-controlled manner have attracted recent attention, but the methods to form seamless interfaces between structurally dissimilar materials, such as graphene and transition-metal dichalcogenides (TMDCs), are still limited. Here, we investigate the structure of the lateral interfaces that arise between monolayer MoS2 flakes on Au(111) and two families of armchair graphene nanoribbons (GNRs) created through on-surface assisted Ullmann coupling using regular organobromine precursors for GNR synthesis. We find that parallel alignment between the GNR armchair edge and MoS2 leads to van der Waals bonded nanoribbons, whereas a perpendicular orientation is characterized by a single phenyl-group of the GNR covalently bonded to S on the edge. The edge-on bonding is facilitated by a hydrogen treatment of the MoS2, and temperature control during growth is shown to influence the nanoribbon width and the yield of covalently attached nanoribbons. Interestingly, the temperatures needed to drive the intramolecular dehydrogenation during GNR formation are lowered significantly by the presence of MoS2, which we attribute to enhanced hydrogen recombination at the MoS2 edges. These results are a demonstration of a viable method to make laterally bonded graphene nanostructures to TMDCs to be used in further investigations of two-dimensional heterostructure junctions.

The Effect of Fe Dopant Location in Co(Fe)OOH_xNanoparticles for the Oxygen Evolution Reaction

Sun, Z., Curto, A., Rodríguez-Fernández, J., et al. — Mon, 01 Nov 2021 21:33:08 +0100

The addition of iron (Fe) can in certain cases have a strong positive effect on the activity of cobalt and nickel oxide nanoparticles in the electrocatalytic oxygen evolution reaction (OER). The reported optimal Fe dopant concentrations are, however, inconsistent, and the origin of the increased activity due to Fe dopants in mixed oxides has not been identified so far. Here, we combine density functional theory calculations, scanning tunneling microscopy, and OER activity measurements on atomically defined Fe-doped Co oxyhydroxide nanoparticles supported on a gold surface to establish the link between the activity and the Fe distribution and concentration within the oxyhydroxide phase. We find that addition of Fe results in distinct effects depending on its location on edge or basal plane sites of the oxyhydroxide nanoparticles, resulting in a nonlinear OER activity as a function of Fe content. Fe atom substitution itself does not lead to intrinsically more active OER sites than the best Co sites. Instead, the sensitivity to Fe promoter content is explained by the strong preference for Fe to locate on the most active edge sites of oxyhydroxide nanoparticles, which for low Fe concentrations stabilizes the particles but in higher concentrations leads to a shell structure with less active Fe on all edge positions. The optimal Fe content thereby becomes dependent on nanoparticle size. Our findings demonstrate that synthesis strategies that adjust not only the Fe concentration in mixed oxides but also its distribution within a catalyst nanoparticle can lead to enhanced OER performance.

Nanoscale Chevrel-Phase Mo₆S₈Prepared by a Molecular Precursor Approach for Highly Efficient Electrocatalysis of the Hydrogen Evolution Reaction in Acidic Media

Elgendy, A., Papaderakis, A. A., Byrne, C., et al. — Mon, 01 Nov 2021 21:33:08 +0100

Developing a simple, safe, and efficient route for the preparation of nanoparticulate ternary Chevrel phases MxMo6S8 (CPs; where M = metal) is of great interest because of their applications in energy conversion and storage technologies. Currently, the wide use of these materials is restricted by the prolonged reaction time, the high energy demands required for their synthesis, the complexity of the preparation process, and the ambiguity in the size of the resultant particles. Herein, we report a simple, efficient, and controllable molecular precursor approach for the synthesis of nanoscale CPs without the use of hydrogen gas as a reducing agent. A mixture of precursors based on molybdenum and copper dithiocarbamate complexes was subjected to thermolysis in the presence of finely divided molybdenum to furnish the copper CP, Cu2Mo6S8. The successful formation of the Cu2Mo6S8 CP is confirmed by X-ray diffraction analysis and Raman spectroscopy, while the surface chemistry of the material was examined by X-ray photoelectron spectroscopy photon depth profiling via tunable synchrotron radiation. Microscopic characterization results demonstrate that the synthesized material has a homogeneous structure at the nanoscale, in contrast to the microparticles obtained from conventional approaches previously reported. The prepared CP was assessed as an electrocatalyst for the hydrogen evolution reaction in acidic media. Because of its unique nanoscale texturing, the Cu-leached CP, Mo6S8, exhibits a highly promising electrocatalytic activity toward hydrogen evolution with an overpotential required to reach a current density of 10 mA cm-2 equal to 265 mV versus reversible hydrogen electrode. The overpotential reduces to 232 mV upon mixing of the catalyst with 20% w/w of high-conductivity carbon. It is expected that the proposed synthetic strategy, which represents a facile route to tailored CPs, can be extended to the preparation of versatile, easily tunable CP Mo6S8-based electrode materials for applications in electrocatalysis.

Size-dependent phase stability in transition metal dichalcogenide nanoparticles controlled by metal substrates

Bruix, A., Lauritsen, J. V., Hammer, B. — Tue, 01 Jun 2021 21:33:08 +0200

Nanomaterials based on MoS2 and related transition metal dichalcogenides (TMDCs) are remarkably versatile; MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and the hydrogen evolution reaction, and transition metal dichalcogenides in general have recently emerged as novel 2D components for nanoscale electronics and optoelectronics. The properties of such materials are intimately related to their structure and dimensionality. For example, only the edges exposed by MoS2 nanoparticles (NPs) are catalytically active, and extended MoS2 systems show different character (direct or indirect gap semiconducting, or metallic) depending on their thickness and crystallographic phase. In this work, we show how particle size and interaction with a metal surface affect the stability and properties of different MoS2 NPs and the resulting phase diagrams. By means of calculations based on the Density Functional Theory (DFT), we address how support interactions affect MoS2 nanoparticles of varying size, composition, and structure. We demonstrate that interaction with Au modifies the relative stability of the different nanoparticle types so that edge terminations and crystallographic phases that are metastable for free-standing nanoparticles and monolayers are expressed in the supported system. These support-effects are strongly size-dependent due to the mismatch between Au and MoS2 lattices, which explains experimentally observed transitions in the structural phases for supported MoS2 NPs. Accounting for vdW interactions and the contraction of the Au(111) surface underneath the MoS2 is further found to be necessary for quantitatively reproducing experimental results. We finally find that support-induced effects on the stability of nanoparticle structures are general to TMDC nanoparticles on metal surfaces, which we demonstrate also for MoS2 on Au(111), WS2 on Au(111), and WS2 on Ag(111). This work demonstrates how the properties of nanostructured transition metal dichalcogenides and similar layered systems can be modified by the choice of supporting metal.

A versatile electrochemical cell for hanging meniscus or flow cell measurement of planar model electrodes characterized with scanning tunneling microscopy and x-ray photoelectron spectroscopy

Sun, Z., Lauritsen, J. V. — Wed, 01 Sep 2021 21:33:08 +0200

We demonstrate the development of a portable electrochemistry (EC) cell setup that can be applied to measure relevant electrochemical signals on planar samples in conjunction with pre- and post-characterization by surface science methods, such as scanning tunneling microscopy and x-ray photoelectron spectroscopy. The EC cell setup, including the transfer and EC cell compartments, possesses the advantage of a small size and can be integrated with standard ultra-high vacuum (UHV) systems or synchrotron end-stations by replacing the flange adaptor, sample housing, and transfer arm. It allows a direct transfer of the pre-characterized planar sample from the UHV environment to the EC cell to conduct in situ electrochemical measurements without exposing to ambient air. The EC cell setup can operate in both the hanging meniscus and flow cell mode. As a proof of concept, using a Au(111) single crystal electrode, we demonstrate the application of the EC cell setup in both modes and report on the post-EC structure and chemical surface composition as provided by scanning tunneling microscopy and x-ray photoelectron spectroscopy. To exemplify the advantage of an in situ EC cell, the EC cell performance is further compared to a corresponding experiment on a Au(111) sample measured by transfer at ambient conditions. The EC cell demonstrated here enables a wealth of future electrocatalysis measurements that combine surface science model catalyst approaches to facilitate the understanding of nano- and atomic-scale structures of electrocatalytic interfaces, the crucial role of catalyst stability, and the nature of low-concentration and atomically dispersed metal (single atom) dopants.

Spectroscopic view of ultrafast charge carrier dynamics in single- and bilayer transition metal dichalcogenide semiconductors

Majchrzak, P., Volckaert, K., Čabo, A. G., et al. — Thu, 01 Jul 2021 21:33:08 +0200

The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS₂ and WS₂ on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9–2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS₂. These timescales are explained by Auger recombination involving MoS₂ and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.

Structural Dynamics of Ultrathin Cobalt Oxide Nanoislands under Potential Control

Stumm, C., Bertram, M., Kastenmeier, M., et al. — Wed, 24 Mar 2021 21:33:08 +0100

Cobalt oxide is a promising earth abundant electrocatalyst and one of the most intensively studied oxides in electrocatalysis. In this study, the structural dynamics of well-defined cobalt oxide nanoislands (NIs) on Au(111) are investigated in situ under potential control. The samples are prepared in ultra-high vacuum and the system is characterized using scanning tunneling microscopy (STM). After transfer into the electrochemical environment, the structure, mobility, and dissolution is studied via in situ electrochemical (EC) STM, cyclic voltammetry, and EC on-line inductively coupled plasma mass spectrometry. Cobalt oxide on Au(111) forms bilayer (BL) and double-bilayer NIs (DL), which are stable at the open circuit potential (0.8 V_RHE). In the cathodic scan, the cobalt oxide BL islands become mobile at potentials of 0.5 V_RHE and start dissolving at potentials below. In sharp contrast to the BL islands, the DL islands retain their morphology up to much lower potential. The re-deposition of Co aggregates is observed close to the reduction potential of Co²⁺ to Co³⁺. In the anodic scan, both the BL and DL islands retain their morphology up to 1.5 V_RHE. Even under these conditions, the islands do not show dissolution during the oxygen evolution reaction (OER) while maintaining their high OER activity.

Electronic properties of single-layer CoO2/Au(111)

Holt, A. J. U., Pakdel, S., Rodriguez-Fernandez, J., et al. — Thu, 01 Jul 2021 21:33:08 +0200

We report direct measurements via angle-resolved photoemission spectroscopy (ARPES) of the electronic dispersion of single-layer (SL) CoO2. The Fermi contour consists of a large hole pocket centered at the point. To interpret the ARPES results, we use density functional theory (DFT) in combination with the multi-orbital Gutzwiller Approximation (DFT+GA), basing our calculations on crystalline structure parameters derived from x-ray photoelectron diffraction and low-energy electron diffraction. Our calculations are in good agreement with the measured dispersion. We conclude that the material is a moderately correlated metal. We also discuss substrate effects, and the influence of hydroxylation on the CoO2 SL electronic structure.

NH₃on anatase TiO₂(101)

Adamsen, K. C., Kolsbjerg, E. L., Koust, S., et al. — Tue, 01 Dec 2020 21:33:08 +0100

We utilized scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations to study the diffusion of ammonia (NH3) on anatase TiO2(101). From time-lapsed STM imaging, we observed monomeric and dimeric diffusion channels, and a general tendency to higher diffusion rates with increasing NH3 coverage. In surface regions where several NH3 molecules are adsorbed within a few sites, we further observed the diffusion of NH3 molecules occurring in cascades, where the diffusion of one adsorbate triggers that of others. This eventually leads to apparent diffusion barriers that are lower than expected within a single-jump model. From the DFT calculations, we obtained mechanistic insights into the two observed NH3 diffusion channels. Within the dimeric NH3 diffusion channel, one NH3 swings around another adsorbed NH3 and experiences a reduced diffusion barrier, owing to the intermolecular bonding during the event.

Preparation and Characterization of V₂O₅/a-TiO₂(101) Model Catalysts

Koust, S., Adamsen, K. C., Xu, T., et al. — Tue, 01 Dec 2020 21:33:08 +0100

We prepared vanadia-titania model catalysts with V(+5) oxidation state by sublimating V2O5 powder onto clean anatase TiO2(101) [a-TiO2(101)]. The V2O5/a-TiO2(101) model catalysts with V2O5 in the sub-monolayer coverage range were studied with scanning tunneling microscopy, lab-source X-ray photoelectron spectroscopy, and synchrotron-radiation X-ray photoelectron spectroscopy. On freshly prepared V2O5/a-TiO2(101) samples, we find well-dispersed V2O5 clusters as the smallest species, together with larger particles of V2O5 stoichiometry. Upon vacuum-annealing at ∼500 K, small V2O5 clusters agglomerate into larger particles. Upon vacuum-annealing at ∼700 K, vanadia reduces and, eventually, disappears from the surface via V diffusion into the bulk. The V2O5/a-TiO2(101) model catalyst can be hydroxylated by an ice-assisted preparation. We find evidence for OH groups located at the V2O5/a-TiO2(101) interface as well as at the a-TiO2(101) support.

Monomeric two-dimensionally ordered WO₃clusters on anatase Ti O₂(101)

Xu, T., Adamsen, K. C., Falsig, H., et al. — Tue, 01 Dec 2020 21:33:08 +0100

We combined scanning tunneling microscopy and x-ray photoelectron spectroscopy experiments with density-functional theory calculations to study dispersed tungsta clusters on anatase TiO2(101). Following two different preparation methods, we found that monomeric WO3 species are the most stable configuration rather than WO3 trimers, (WO3)3. The WO3 monomers form tetrahedral WO4 structures on anatase TiO2(101), with one W-O bond and two W-O-Ti linkages per WO3 monomer. Locally, the WO3 monomers form well-ordered (2 × 1) structures. The discovered geometric structure of WO3 on anatase TiO2(101) opens up numerous opportunities for fundamental studies addressing tungsta and accurate structure-activity studies of WO3/TiO2 model catalysts.

Site-dependent reactivity of MoS₂ nanoparticles in hydrodesulfurization of thiophene

Sat, 01 Aug 2020 21:33:08 +0200

The catalytically active site for the removal of S from organosulfur compounds in catalytic hydrodesulfurization has been attributed to a generic site at an S-vacancy on the edge of MoS₂ particles. However, steric constraints in adsorption and variations in S-coordination means that not all S-vacancy sites should be considered equally active. Here, we use a combination of atom-resolved scanning probe microscopy and density functional theory to reveal how the generation of S-vacancies within MoS₂ nanoparticles and the subsequent adsorption of thiophene (C₄H₄S) depends strongly on the location on the edge of MoS₂. Thiophene adsorbs directly at open corner vacancy sites, however, we find that its adsorption at S-vacancy sites away from the MoS₂ particle corners leads to an activated and concerted displacement of neighboring edge S. This mechanism allows the reactant to self-generate a double CUS site that reduces steric effects in more constrained sites along the edge.

Cubes on a string

Deville, C., Folkjær, M., Reinholdt, P., et al. — Mon, 01 Jun 2020 21:33:08 +0200

A series of semicrystalline and amorphous one-dimensional (1D) polymeric chains consisting of cubane-like CoII4L4 units (L = S-1,2-bis(benzimidazol-2-yl)ethanol) and dicarboxylates were synthesized and characterized by single crystal diffraction and X-ray total scattering. The polycationic chains are composed of [Co4L4(dicarboxylate)]2+ monomeric units, while one molecular dicarboxylate counterion is balancing the charge of each monomer. The linear compound series has five members, and the crystal structures were solved for [Co4L4(tph)](tph) and [Co4L4(ndc)](ndc), where tph = terephthalate and ndc = 2,6-naphthalenedicarboxylate. Partly crystalline compounds were produced by slow assembly at elevated temperature (over days), while the amorphous compounds were formed by fast precipitation (within minutes). Pair distribution function (PDF) analysis based on X-ray total scattering data reveals the presence of the cubane-like entity in both the amorphous and semicrystalline samples. While the powders are non-porous, precipitation is a fast and versatile method to produce compounds with cubane-like centres with moderate surface areas of 17-49 m2 g-1 allowing for surface chemical reactions. The powders have a high concentration of Lewis base sites as verified by their selective adsorption of CO2 over N2. The use of an amorphous cubane-like polymer for the electrocatalytic oxygen evolution reaction was demonstrated.

Direct Integration of Few-Layer MoS₂ at Plasmonic Au Nanostructure by Substrate-Diffusion Delivered Mo

Ma, R., Haastrup, M. J., Wang, Z., et al. — Wed, 01 Jan 2020 21:33:08 +0100

The seamless combination of plasmonic structures with metal oxides and in particular with 2D materials has attracted significant recent interests. Here, a novel approach to grow molybdenum disulfide (MoS₂) is presented: an optically active 2D material, directly onto plasmonic nanoscale gold disks via a surface diffusion delivery of Mo from within the nanostructure and a sulfidation process. X-ray photoelectron spectroscopy and Raman spectroscopy measurements demonstrate the existence and the reaction process of few layered MoS₂. Delivery of Mo from within the nanostructure and direct growth of MoS₂ has the potential to form better defined metal/MoS₂ interfaces compared to exfoliation routes. This facile approach has the potential for application in photodetector or other devices.

Molecular Nanowire Bonding to Epitaxial Single-Layer MoS₂ by an On-Surface Ullmann Coupling Reaction

Wed, 01 Jan 2020 21:33:08 +0100

Lateral heterostructures consisting of 2D transition metal dichalcogenides (TMDCs) directly interfaced with molecular networks or nanowires can be used to construct new hybrid materials with interesting electronic and spintronic properties. However, chemical methods for selective and controllable bond formation between 2D materials and organic molecular networks need to be developed. As a demonstration of a self-assembled organic nanowire-TMDC system, a method to link and interconnect epitaxial single-layer MoS₂ flakes with organic molecules is demonstrated. Whereas pristine epitaxial single-layer MoS₂ has no affinity for molecular attachment, it is found that single-layer MoS₂ will selectively bind the organic molecule 2,8-dibromodibenzothiophene (DBDBT) in a surface-assisted Ullmann coupling reaction when the MoS₂ has been activated by pre-exposing it to hydrogen. Atom-resolved scanning tunneling microscopy (STM) imaging is used to analyze the bonding of the nanowires, and thereby it is revealed that selective bonding takes place on a specific S atom at the corner site between the two types of zig-zag edges available in a hexagonal single layer MoS₂ sheet. The method reported here successfully combining synthesis of epitaxial TMDCs and Ullmann coupling reactions on surfaces may open up new synthesis routes for 2D organic-TMDC hybrid materials.

Adsorption and reaction of methanol on Fe₃O₄(001)

Marcinkowski, M. D., Adamsen, K. C., Doudin, N., et al. — Sat, 01 Feb 2020 21:33:08 +0100

The interaction of methanol with iron oxide surfaces is of interest due to its potential in hydrogen storage and from a fundamental perspective as a chemical probe of reactivity. We present here a study examining the adsorption and reaction of methanol on magnetite Fe₃O₄(001) at cryogenic temperatures using a combination of temperature programmed desorption, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The methanol desorption profile from Fe₃O₄(001) is complex, exhibiting peaks at 140 K, 173 K, 230 K, and 268 K, corresponding to the desorption of intact methanol, as well as peaks at 341 K and 495 K due to the reaction of methoxy intermediates. The saturation of a monolayer of methanol corresponds to ∼5 molecules/unit cell (u.c.), which is slightly higher than the number of surface octahedral iron atoms of 4/u.c. We probe the kinetics and thermodynamics of the desorption of molecular methanol using inversion analysis. The deconvolution of the complex desorption profile into individual peaks allows for calculations of both the desorption energy and the prefactor of each feature. The initial 0.7 methanol/u.c. reacts to form methoxy and hydroxy intermediates at 180 K, which remain on the surface above room temperature after intact methanol has desorbed. The methoxy species react via one of two channels, a recombination reaction with surface hydroxyls to form additional methanol at ∼350 K and a disproportionation reaction to form methanol and formaldehyde at ∼500 K. Only 20% of the methoxy species undergo the disproportionation reaction, with most of them reacting via the 350 K pathway.

Atomic-Scale View of the Oxidation and Reduction of Supported Ultrathin FeO Islands

Li, Y., Adamsen, K. C., Lammich, L., Lauritsen, J. V., Wendt, S. — Tue, 01 Oct 2019 21:33:08 +0200

By means of scanning tunneling microscopy (STM) measurements, we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O₂) and hydrogen (H₂), respectively. The FeO islands respond very dynamically toward O₂, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O₂ dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O₂ molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites. In contrast, we found that H₂ molecules only dissociate when vibrationally excited through the ion gauge and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H₂ dissociation. Our STM results reveal how excess O atoms are incorporated and released in O₂ and H₂ and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.

Anisotropic iron-doping patterns in two-dimensional cobalt oxide nanoislands on Au(111)

Curto, A., Sun, Z., Rodríguez-Fernández, J., et al. — Sun, 01 Sep 2019 21:33:08 +0200

An integrated approach combining density functional theory (DFT) calculations and atomic resolution scanning tunneling microscopy (STM) is used to study well-defined iron-doped cobalt oxide nanoislands supported on Au(111). The focus is on the structure and distribution of Fe dopants within these nanoislands of CoO as a function of Fe to Co ratio. The DFT and STM results agree strongly and complement each other to allow for a more complete understanding of the dopant structure trends on the nanoscale. Using Fe as a marker, we first find that the stacking sequence of the moiré structure of the host cobalt oxide nanoislands can be identified unambiguously through a combination of DFT and STM. Using the distinct contrast of the embedded Fe dopant atoms as observed with atom-resolved STM, we find correlations between Fe dopant position and the CoO/Au(111) moiré pattern at varying Fe dopant densities. Formation of Fe-dopant clusters within the nanoislands is investigated in detail through DFT and found to agree with the dopant patterns observed in STM. We find that the structural effects of Fe dopants throughout the nanoislands with the basal planes and the two types of edges—the oxygen and metal edges—have different nature. Both DFT calculations and STM images show a strong preference for Fe dopants to be located directly on or near the oxygen edge of the nanoislands as opposed to being directly on or near the metal edge. Taken together, our results illustrate that Fe dopant incorporation and distribution within CoO nanoislands are highly anisotropic and governed by both the moiré structure of the basal planes as well as nano-size effects present at the under-coordinated edges of different local geometry and chemistries. [Figure not available: see fulltext.]

Basal plane oxygen exchange of epitaxial MoS₂ without edge oxidation

Gronborg, S. S., Thorarinsdottir, K., Kyhl, L., et al. — Tue, 01 Oct 2019 21:33:08 +0200

The intentional formation of defects in transition-metal dichalcogenides, such as MoS₂, is an attractive way to modify the electronic and chemical properties of this class of 2D materials. However, the mechanisms and methods available for selective doping or modification of the basal plane must be improved. Here we investigate the process of O defect formation in epitaxial single-layer MoS₂ on Au(1 1 1) using scanning tunneling microscopy (STM) and ambient pressure x-ray photoelectron spectroscopy (AP-XPS) during oxidation with O₂ and H₂O gas from low vacuum to the mbar range. Both oxidants result in exchange of S in the upper part of the basal plane with O, in line with air exposure experiments. Temperature-dependent measurements show that this is an activated process with an experimentally estimated reaction barrier of ∼0.79 ± 0.20 eV. We surprisingly find that the morphology of the MoS₂ flakes and their edges remain intact in O₂, even for relatively high degrees of basal plane O exchange, in contrast to the oxidation behavior of exfoliated single-layer MoS₂. From analysis of atom-resolved STM images of the MoS₂ edges, we can attribute this unusual stability to a passivating effect of excess edge sulfur species adsorbed under the sulfiding conditions of the MoS₂ synthesis in H₂S gas. We thus demonstrate that control over pre-sulfidation of the edges, temperature and pressure during oxidation can be used in a fast process to form strongly O doped single-layer MoS₂ with no degradation of the initial shape and edge structure of the epitaxial MoS₂ sheet.

Structure of CoOx Thin Films on Pt(111) in Oxidation of CO

Fester, J., Sung, Z., Rodriguez-Fernandez, J., Lauritsen, J. V. — Mon, 01 Jul 2019 21:33:08 +0200

The combination of noble Pt group metals and reducible metal oxides has been extensively studied and demonstrated to display superior catalytic properties in low-temperature CO oxidation or preferential CO oxidation reactions. Here, we investigate CoOx films and nanoislands on Pt(111) as inverse model catalysts for CO oxidation, in order to shed light on the nature of the 0 species in CoOx that participate in CO oxidation. Through temperature-programmed desorption, scanning tunneling microscopy, and X-ray photoelectron spectroscopy of carefully prepared and atomically defined CoOx film structures on Pt(111), we study the changes in the oxide structure upon interaction with CO and reoxidation in O-2. The combined approach reveals structural motives representing both reversibly and irreversibly reduced forms of the oxide after CO oxidation. Overall, a remarkably facile release of lattice oxygen compared to the isostructural thin-film islands supported on Au(111) underlines the attractive properties as a CO oxidation catalyst system, ascribed to the synergy between the oxide film and Pt(111). We also conclude that bilayer Co-O structures, reflecting Co in a low oxidation state, facilitate the CO oxidation at lower temperatures compared with fully oxidized CoO2 trilayers. This surprisingly points to a more facile O release from the bilayer structures.

Structure and stability of Au-supported layered cobalt oxide nanoislands in ambient conditions

Fester, J., Sun, Z., Rodríguez-Fernández, J., Walton, A. S., Lauritsen, J. V. — Mon, 01 Apr 2019 21:33:08 +0200

Cobalt oxide is a promising earth-abundant electrocatalyst for water splitting; however, the structural complexity of oxides coupled with the difficulty of characterizing it in its operating environment means that fundamental understanding of its catalytic properties remains poor. In this study, we go beyond vacuum studies and investigate the morphological evolution of a CoO _x /Au(111) model system from intermediate to high pressures of H ₂ O vapor by means of scanning tunneling microscopy and near-ambient pressure and vacuum X-ray photoelectron spectroscopy. At elevated H ₂ O pressure, we describe the formation of a well-defined Co(OH) ₂ nanoisland morphology with cobalt in the +2 oxidation state. In contrast, the presence of O ₂ , in air and liquid water, results in only partially hydroxylated Co ³⁺ phases comprising sheets of the CoO(OH _x ) trilayer, corresponding to a single sheet of cobalt(III)oxyhydroxide. We conclude that the oxyhydroxide structure, known to be the catalytically active phase for the oxygen evolution reaction, is stabilized by aerobic conditions, which inhibits further transformation into the catalytically inactive cobalt(II)hydroxide.

Structural and electronic properties of Fe dopants in cobalt oxide nanoislands on Au(111)

Rodríguez-Fernández, J., Sun, Z., Zhang, L., et al. — Mon, 28 Jan 2019 21:33:08 +0100

Mixed metal oxides of earth-abundant 3d transition metals are an interesting class of materials that show interesting magnetic properties and a significant synergistic effect as catalysts for electrochemical oxygen evolution compared to simple unary oxides. However, the exact atomic-scale nature of such mixed oxide phases and the link to their interesting physico-chemical properties are poorly understood. Here, a combination of scanning tunneling microscopy and x-ray photoemission spectroscopy reveals that Fe species embed in a facile way into CoO bilayers on Au(111) resulting in an Fe doped oxide. Density functional theory and the spectroscopic fingerprint from x-ray photoemission spectroscopy reveal that the Fe dopants in the cobalt oxide matrix assume a higher oxidation state than in the structurally corresponding unary bilayer oxide. Furthermore, the substituted Fe is structurally displaced further away from the Au than the metal in either of the corresponding pure unary oxides. Both O and to a smaller extent Co in the nearest coordination shell are also structurally and electronically perturbed. The interesting effects observed in the bilayer binary oxides may enable a better fundamental understanding of the nature of doping of metal oxides, in general, and promotion effects in catalytic applications.

Dissociation of water on atomically-defined cobalt oxide nanoislands on Pt(111) and its effect on the adsorption of CO

Wähler, T., Hohner, C., Sun, Z., et al. — Tue, 01 Jan 2019 21:33:08 +0100

We have investigated the adsorption and dissociation of water and its co-adsorption with CO on atomically defined cobalt oxide nanoislands on Pt(111). The CoO islands were prepared under ultrahigh vacuum (UHV) conditions by reactive deposition of Co metal in oxygen atmosphere. The island structure was characterized by scanning tunneling microscopy (STM), showing that the nanoislands consist of a CoO bilayer and are regularly shaped with island edges that are mainly terminated by Co²⁺ ions. D₂O was dosed in UHV onto the CoO islands on Pt(111) after pre-saturation with CO. D₂O dissociation was monitored in situ by isothermal and temperature programmed infrared reflection absorption spectroscopy (IRAS). Isotopic exchange experiments were performed with H₂O, D₂O, and D₂ ¹⁸O to elucidate the nature of the hydroxyl groups. Three principal types of OD species are identified: (i) isolated OD at the edges of the CoO islands (Co-O_eD), (ii) OD groups within larger hydroxylated areas on the CoO islands (Co-O_cD), and (iii) isolated OD groups on the CoO terraces (Co-O_tD). At 400 K, water adsorbs dissociatively on the CoO islands and forms isolated hydroxyl species (Co-O_eD) at the island edges only. At room temperature (300 K), the coverage of hydroxyl groups increases rapidly, in line with the water-assisted hydroxylation reaction suggested previously. Adsorption experiments with D₂ ¹⁸O suggest that two equivalent groups are formed from one water molecule after dissociation at island edges, leading to the formation of larger hydroxylated areas on the CoO islands (Co-O_cD) and, in addition, isolated OD species on the CoO terraces (Co-O_tD). While the initial step of D₂O dissociation is facile, the formation of larger hydroxylated areas is a slow and irreversible process. At 200 K, the formation of hydroxylated areas is accompanied by the co-adsorption of molecular water. The hydroxyl groups on the CoO islands are shown to interact with the CO preadsorbed on the CoO/Pt(111) model system. In particular, we observe a new CO species, stabilized by OD groups on the CoO islands, which adsorbs much stronger than CO on the OD-free CoO surface.

Atomically Defined Iron Carbide Surface for Fischer-Tropsch Synthesis Catalysis

Li, Y., Li, Z., Ahsen, A., et al. — Fri, 01 Feb 2019 21:33:08 +0100

With the purpose of investigating the reactivity of Fe carbide as an active phase in Fischer-Tropsch catalysis, we studied the formation of a well-defined Fe carbide surface structure resulting from carbon exposure of an Fe film on Au(111). Using two different sources of carbon (C), namely atomic carbon and ethylene gas, we used synchrotron X-ray photoelectron spectroscopy (XPS) to show that a 6 ML Fe film readily converts into a well-defined and thermodynamically stable carbide phase. Scanning tunneling microscopy (STM) showed that the surface of the Fe carbide film is crystalline and is dominated by Fe(110)-like facets perturbed into a (2 X 2) periodic structure due to insertion of C in the interstitial sites. The reactivity of the carbide film toward CO, H-2, and O-2 was furthermore probed by XPS under vacuum conditions. While the pristine Fe carbide surface was unreactive toward hydrogen gas at 500 K, we interestingly found that CO dissociation from a preadsorbed monolayer of CO takes place already at low temperature. This observation points to an intrinsic activity of the Fe carbide phase where additional carbon originating from CO can be placed in the Fe carbide surface. The catalytic significance of the model catalyst surface presented here is that it can be seen as a stable Fe carbide phase with intrinsically vacant sites for additional C insertion at elevated pressure, and we propose that such additional C may act as active species in C-C coupling reactions during FTS. The studies pave the way for a better understanding of FTS processes on Fe based catalysts on the basis of a well-defined model surface.

Adsorption of nitrogenous inhibitor molecules on MoS₂ and CoMoS hydrodesulfurization catalysts particles investigated by scanning tunneling microscopy

Salazar, N., Schmidt, S. B., Lauritsen, J. V. — Fri, 01 Feb 2019 21:33:08 +0100

Heterocyclic nitrogen compounds in crude oil, such as pyridine and quinoline, are known to have a strongly disabling effect on the hydrodesulfurization (HDS) reactions on MoS₂-based catalysts. To shed light on the nature and distribution of the adsorption sites of such nitrogen-containing inhibitors, we use atom-resolved scanning tunneling microscopy (STM) on a gold-supported HDS model system to probe the detailed adsorption configurations and diffusion characteristics of pyridine and quinoline molecules on both unpromoted and Co-promoted MoS₂ (CoMoS) nanoparticles. We find that pyridine and quinoline molecules adsorb strongly on the S-edges of reduced MoS₂ and CoMoS nanoparticles through the formation of protonated pyridinium/quinolinium ions. The quinoline molecules adsorbed in a flat configuration on the S-edge are concluded to block significantly more active sites than the pyridine molecules, due to the larger molecular size and higher mobility. The experimental observations also illustrate how proton transfer to the adsorbed N-containing species may affects bonding of the terminal S atoms of the active edge to the Au substrate. The STM findings overall substantiate the notion that these N-bearing compounds compete with the adsorption of sulfur-containing molecules on the same active edge sites in hydrogenation reactions, but also indicate that corner sites may be less affected.

Water Dissociation and Hydroxyl Ordering on Anatase TiO₂ (001)- (1×4)

Beinik, I., Bruix, A., Li, Z., et al. — Thu, 15 Nov 2018 21:33:09 +0100

We studied the interaction of water with the anatase TiO2(001) surface by means of scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory calculations. Water adsorbs dissociatively on the ridges of a (1×4) reconstructed surface, resulting in a (3×4) periodic structure of hydroxyl pairs. We observed this process at 120 K, and the created hydroxyls desorb from the surface by recombination to water, which occurs below 300 K. Our calculations reveal the water dissociation mechanism and uncover a very pronounced dependence on the coverage. This strong coverage dependence is explained through water-induced reconstruction on anatase TiO2(001)-(1×4). The high intrinsic reactivity of the anatase TiO2(001) surface towards water observed here is fundamentally different from that seen on other surfaces of titania and may explain its high catalytic activity in heterogeneous catalysis and photocatalysis.

Step edge structures on the anatase TiO₂ (001) surface studied by atomic-resolution TEM and STM

Ek, M., Beinik, I., Bruix, A., Wendt, S., Lauritsen, J. V., Helveg, S. — Thu, 11 Jan 2018 21:33:09 +0100

Low-coordinate surface sites, such as those present on high-index step edges, often exhibit chemical reactivity that markedly differs from more close-packed facets. To understand the site-specific reactivity, insight into the three-dimensional atomic arrangement of step edges is needed. Here, we employ atomic-resolution transmission electron microscopy (TEM) of nanoparticles in combination with scanning tunneling microscopy (STM) of a single crystal surface to uncover the structure of prevalent step edges on the anatase TiO₂ (001) surface.

The Structure of the Cobalt Oxide/Au Catalyst Interface in Electrochemical Water Splitting

Fester, J., Makoveev, A., Grumelli, D., et al. — Mon, 10 Sep 2018 21:33:09 +0200

The catalytic synergy between cobalt oxide and gold leads to strong promotion of the oxygen evolution reaction (OER)—one half-reaction of electrochemical water splitting. However, the mechanism behind the enhancement effect is still not understood, in part due to a missing structural model of the active interface. Using a novel interplay of cyclic voltammetry (CV) for electrochemistry integrated with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) on an atomically defined cobalt oxide/Au(111) system, we reveal here that the supporting gold substrate uniquely favors a flexible cobalt-oxyhydroxide/Au interface in the electrochemically active potential window and thus suppresses the formation of less active bulk cobalt oxide morphologies. The findings substantiate why optimum catalytic synergy is obtained for oxide coverages on gold close to or below one monolayer, and provide the first morphological description of the active phase during electrocatalysis.

Topotactic Growth of Edge-Terminated MoS₂ from MoO₂ Nanocrystals

Dahl-Petersen, C., Šarić, M., Brorson, M., et al. — Tue, 26 Jun 2018 21:33:09 +0200

Layered transition metal dichalcogenides have distinct physicochemical properties at their edge-terminations. The production of an abundant density of edge structures is, however, impeded by the excess surface energy of edges compared to basal planes and would benefit from insight into the atomic growth mechanisms. Here, we show that edge-terminated MoS₂ nanostructures can form during sulfidation of MoO₂ nanocrystals by using in situ transmission electron microscopy (TEM). Time-resolved TEM image series reveal that the MoO₂ surface can sulfide by inward progression of MoO₂(202):MoS₂(002) interfaces, resulting in upright-oriented and edge-exposing MoS₂ sheets. This topotactic growth is rationalized in the interplay with density functional theory calculations by successive O-S exchange and Mo sublattice restructuring steps. The analysis shows that formation of edge-terminated MoS₂ is energetically favorable at MoO₂(110) surfaces and provides a necessary requirement for the propensity of a specific MoO₂ surface termination to form edge-terminated MoS₂. Thus, the present findings should benefit the rational development of transition metal dichalcogenide nanomaterials with abundant edge terminations.

Visualizing hydrogen-induced reshaping and edge activation in MoS₂ and Co-promoted MoS₂ catalyst clusters

Gronborg, S. S., Salazar, N., Bruix, A., et al. — Thu, 07 Jun 2018 21:33:09 +0200

Hydrodesulfurization catalysis ensures upgrading and purification of fossil fuels to comply with increasingly strict regulations on S emissions. The future shift toward more diverse and lower-quality crude oil supplies, high in S content, requires attention to improvements of the complex sulfided CoMo catalyst based on a fundamental understanding of its working principles. In this study, we use scanning tunneling microscopy to directly visualize and quantify how reducing conditions transforms both cluster shapes and edge terminations in MoS2 and promoted CoMoS-type hydrodesulfurization catalysts. The reduced catalyst clusters are shown to be terminated with a fractional coverage of sulfur, representative of the catalyst in its active state. By adsorption of a proton-accepting molecular marker, we can furthermore directly evidence the presence of catalytically relevant S-H groups on the Co-promoted edge. The experimentally observed cluster structure is predicted by theory to be identical to the structure present under catalytic working conditions.

Controllable etching of MoS₂ basal planes for enhanced hydrogen evolution through the formation of active edge sites

Wang, Z., Li, Q., Xu, H., et al. — Sun, 01 Jul 2018 21:33:09 +0200

The catalytic activity of molybdenum disulfide (MoS₂) is associated with active sites located along the edges, whereas the MoS₂ basal plane is regarded to be inert. However, it is a great challenge to develop a rational way for producing active edges efficiently. Herein, we report a novel, cost-effective top-down process in which we can create a high density of active edge sites on MoS₂ basal plane by selective steam etching. The results show that the etched structure is strongly sensitive to the temperature, which creates 1D nano-channels, 2D in-plane triangular pits and 3D vertical hexagonal cavities on the MoS₂ basal planes by elevating the temperature. The edge configuration is revealed to exhibit a distinct crystallographic orientation. Furthermore, we evaluate the corresponding enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) by measurements of the single etched MoS₂ samples in an electrochemical microcell, where the Tafel slope decrease by 49%, confirming the increased the density of active sites. In addition, the method is not limited to 2D materials in a flat geometry alone, but is also demonstrated on 0D MoS₂ particles by in-situ transmission electron microscopy. The steam etching reported here offers an alternative avenue to engineer the surface structures of MoS₂ facilitating the electrocatalytic applications of MoS₂ for hydrogen production.

Phase Transitions of Cobalt Oxide Bilayers on Au(111) and Pt(111)

Fester, J., Sun, Z., Rodríguez-Fernández, J., Walton, A., Lauritsen, J. V. — Thu, 18 Jan 2018 21:33:09 +0100

Well-characterized metal oxides supported on single crystal surfaces serve as valuable model systems to study fundamental chemical properties and reaction mechanisms in heterogeneous catalysis or as new thin film metal oxide catalysts in their own right. Here, we present scanning tunneling microscopy and X-ray photoelectron spectroscopy results for cobalt oxide nanoislands that reveal the detailed atomistic mechanisms leading to transitions between Co-O bilayer and O-Co-O trilayer, induced by oxidation in O₂ and reductive vacuum annealing treatments, respectively. By comparing between two different noble metal substrates, Au(111) and Pt(111), we further address the influence of the substrate. Overall, nanoisland edges act to initiate both the oxidation and reduction processes on both substrates. However, important influences of the choice of substrate were found, as the progress of oxidation includes intermediate steps on Au(111) not observed on Pt(111), where the oxidation on the other hand takes place at a significantly higher rate. During reductive treatment of trilayer, the bilayer structure gradually reappears on Pt(111), but not on Au(111) where the reduction rather results in the appearance of a stacked cobalt oxide morphology. These observations point to strong differences in the catalytic behavior between Au and Pt supported cobalt oxides, despite the otherwise strong structural similarities.

Single-layer MoS₂ formation by sulfidation of molybdenum oxides in different oxidation states on Au(111)

Salazar Moreira, N. J., Beinik, I., Lauritsen, J. V. — Thu, 01 Jun 2017 21:33:09 +0200

The sulfidation of a MoO₃ precursor into MoS₂ is an important step in the preparation of catalysts for the hydrodesulfurization process that is widely utilized in oil refineries. Molybdenum oxides are also the most commonly used precursors for MoS₂ growth in, e.g., the synthesis of novel two-dimensional materials. In the present study, we investigate the transformation of MoO_x into MoS₂ on a model Au(111) surface through sulfidation in H₂S gas atmosphere using in situ scanning tunneling microscopy and X-ray photoemission spectroscopy. We find that progressive annealing steps of physical vapor deposited MoO₃ powder allow us to control the stoichiometry and oxidation state of the precursor oxide. Subsequently, we investigate the sulfidation of the compounds ranging from pure low-oxygen Mo to fully oxidized MoO₃ oxide sulfidation using two different methods. We find that the prerequisite for the efficient formation of MoS₂ is that Mo stays in the highest Mo6+ state before sulfidation, whereas the presence of the reduced MoO_x phase impedes the MoS₂ growth. We also find that it is more efficient to form MoS₂ by post-sulfidation of MoO_x rather than its reactive deposition in H₂S gas, which leads to rather stable amorphous oxysulfide phases.

Facile embedding of single vanadium atoms at the anatase TiO2(101) surface

Koust, S., Arnarson, L., Moses, P. G., et al. — Fri, 14 Apr 2017 21:33:09 +0200

To understand the structure-reactivity relationships for mixed-metal oxide catalysts, well-defined systems are required. Mixtures of vanadia and titania (TiO2) are of particular interest for application in heterogeneous catalysis, with TiO2 often acting as the support. By utilizing high-resolution scanning tunneling microscopy, we studied the interaction of vanadium (V) with the anatase TiO2(101) surface in the sub-monolayer regime. At 80 K, metallic V nucleates into homogeneously distributed clusters onto the terraces with no preference for nucleation at the step edges. However, embedding of single V atoms into TiO2 occurs following annealing at room temperature. In conjunction with X-ray photoelectron spectroscopy data and density functional theory calculations, we propose that monomeric V atoms occupy positions of regular surface Ti sites, i.e., Ti atoms are substituted by V atoms.

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