Molekulární nanostruktury na površích

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Identical Fe-N₄ Sites with Different Reactivity: Elucidating the Effect of Support Curvature
(AMER CHEMICAL SOC, 2025-01-29) Jakub, Zdeněk; Planer, Jakub; Hrůza, Dominik; Trllová Shahsavar, Azin; Pavelec, Jiří; Čechal, Jan
Detailed atomic-scale understanding is a crucial prerequisite for rational design of next-generation single-atom catalysts (SACs). However, the sub-& aring;ngstrom precision needed for systematic studies is challenging to achieve on common SACs. Here, we present a two-dimensional (2D) metal-organic system featuring Fe-N4 single-atom sites, where the metal-organic structure is modulated by 0.4 & Aring; corrugation of an inert graphene/Ir(111) support. Using scanning tunneling microscopy and density functional theory, we show that the support corrugation significantly affects the reactivity of the system, as the sites above the support "valleys" bind TCNQ (tetracyanoquinodimethane) significantly stronger than the sites above the "hills". The experimental temperature stability of TCNQ varies by more than 60 degrees C, while computations indicate more than 0.3 eV variation of TCNQ adsorption energy across the Fe-N4 sites placed atop different regions of the corrugated graphene unit cell. The origin of this effect is steric hindrance, which plays a role whenever large molecules interact with neighboring single-atom catalyst sites or when multiple reactants coadsorb on such sites. Our work demonstrates that such effects can be quantitatively studied using model SAC systems supported on chemically inert and physically corrugated supports.
Direct Observation of Structural Phase Transformations during Phosphorene Formation on Cu(111)
(American Chemical Society, 2025-01-22) David, Jiří; Jeřábek, František; Procházka, Pavel; Černý, Miroslav; Ciobanu, Cristian V.; Průša, Stanislav; Šikola, Tomáš; Kodambaka, Suneel; Kolíbal, Miroslav
Blue phosphorene, a two-dimensional, hexagonal-structured, semiconducting phosphorus, has gained attention as it is considered easier to synthesize on metal surfaces than its allotrope, black phosphorene. Recent studies report different structures of phosphorene, for example, on Cu(111), but the underlying mechanisms of their formation are not known. Here, using a combination of in situ ultrahigh vacuum low-energy electron microscopy and in vacuo scanning tunneling microscopy, we determine the time evolution of the surface structure and morphology during the deposition of phosphorus on single-crystalline Cu(111). We find that during the early stages of deposition phosphorus intermixes with Cu, resulting in copper phosphide structures. With the increasing surface concentration of phosphorus, the phosphide phase disappears, and a blue phosphorene layer forms, followed by the self-assembly of highly ordered phosphorus clusters that eventually grow into multilayer islands. We attribute the unexpected transformation of stable phosphide to a phosphorene layer to the presence of a large concentration of P2 dimers on the surface. Our results constitute direct evidence for a growth mode leading to a flat phosphorene layer via an intermediary phase, which could underpin the growth of other 2D materials on strongly interacting substrates.
CO-Induced Dimer Decay Responsible for Gem-Dicarbonyl Formation on a Model Single-Atom Catalyst
(WILEY-V C H VERLAG GMBH, 2024-04-15) Wang, Chunlei; Sombut, Panukorn; Puntscher, Lena; Jakub, Zdeněk; Meier, Matthias; Pavelec, Jiří; Bliem, Roland; Schmid, Michael; Diebold, Ulrike; Franchini, Cesare; Parkinson, Gareth S.
The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh1 adatoms and minority Rh2 dimers on Fe3O4(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh1 sites results exclusively in stable Rh1CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh1(CO)2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh2 dimers via an unstable Rh2(CO)3 intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis. Time-lapse scanning tunneling microscopy movies are combined with theoretical computations to study CO adsorption on a model Rh1/Fe3O4(001) catalyst under ultrahigh vacuum conditions. Direct CO adsorption at Rh1 sites results in monocarbonyl species. Rh1-(CO)2 gem dicarbonyl species are observed, but from only via the CO-induced break-up of Rh2 dimer species.+ image
Reversible Intercalation of Organic Solvents in Graphite and Its Hindrance by a Strongly Adsorbing Supramolecular Monolayer
(WILEY, 2024-12-01) Badami-Behjat, Arash; Rinkovec, Tamara; Procházka, Pavel; Bazylevska, Anastasiia; RodríguezGonzález, Miriam C.; Cao, Hai; Čechal, Jan; De Feyter, Steven; Lackinger, Markus
At elevated temperatures, the prototypical organic solvents used to study the self-assembly of supramolecular monolayers at liquid-solid interfaces alter a graphite substrate by intercalation. As a consequence, less strongly bound supramolecular monolayers become thermodynamically unstable, as probed by scanning tunneling microscopy. Complementary characterization by atomic force microscopy, confocal Raman spectroscopy and low energy electron microscopy consistently points to subsurface changes in the top few layers of the graphite substrate due to solvent intercalation. High-temperature annealing at 900 degrees C in the vacuum restores the adsorption properties of the graphite substrates, indicating a high activation energy for deintercalation. However, strongly adsorbing hydrogen-bonded monolayers of trimesic acid inhibit solvent intercalation and thus protect the graphite substrate. Mildly solvent-intercalated graphite may prove useful as an easily prepared graphitic material with further weakened adsorption properties. The solvents commonly used for self-assembly studies at liquid-solid interfaces alter graphite substrates at elevated temperatures by intercalation, rendering weakly bound supramolecular monolayers thermodynamically unstable. This solvent intercalation can be reversed by high-temperature vacuum annealing at 900 degrees C or prevented by strong and persistent adsorption of supramolecular monolayers of trimesic acid. image
Rapid oxygen exchange between hematite and water vapor
(Nature Portfolio, 2021-11-10) Jakub, Zdeněk; Meier, Matthias; Kraushofer, Florian; Balajka, Jan; Pavelec, Jiří; Schmid, Michael; Franchini, Cesare; Diebold, Ulrike; Parkinson, Gareth S.
Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline "r-cut" (1 (1) over bar 02) surface of hematite (alpha-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 degrees C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.

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