Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111)

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH₃–O₂ overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O₂ molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O₂ molecules. Different hole–hole distances are observed with 0.73 nm most common. These holes in the O₂ network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH₃–O₂ complex that serves as a precursor to ammonia oxydehydrogenation

Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111)

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH₃–O₂ overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O₂ molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O₂ molecules. Different hole–hole distances are observed with 0.73 nm most common. These holes in the O₂ network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH₃–O₂ complex that serves as a precursor to ammonia oxydehydrogenation

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching with High Controllability and Reproducibility

Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching with High Controllability and Reproducibility

Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility

Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C–O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state

Ligand Field Effect at Oxide–Metal Interface on the Chemical Reactivity of Ultrathin Oxide Film Surface

Ultrathin oxide film is currently one of the paramount candidates for a heterogeneous catalyst because it provides an additional dimension, i.e., film thickness, to control chemical reactivity. Here, we demonstrate that the chemical reactivity of ultrathin MgO film grown on Ag(100) substrate for the dissociation of individual water molecules can be systematically controlled by interface dopants over the film thickness. Density functional theory calculations revealed that adhesion at the oxide–metal interface can be addressed by the ligand field effect and is linearly correlated with the chemical reactivity of the oxide film. In addition, our results indicate that the concentration of dopant at the interface can be controlled by tuning the <i>drawing effect</i> of oxide film. Our study provides not only profound insight into chemical reactivity control of ultrathin oxide film supported by a metal substrate but also an impetus for investigating ultrathin oxide films for a wider range of applications

Direct Pathway to Molecular Photodissociation on Metal Surfaces Using Visible Light

We demonstrate molecular photodissociation on single-crystalline metal substrates, driven by visible-light irradiation. The visible-light-induced photodissociation on metal substrates has long been thought to never occur, either because visible-light energy is much smaller than the optical energy gap between the frontier electronic states of the molecule or because the molecular excited states have short lifetimes due to the strong hybridization between the adsorbate molecular orbitals (MOs) and metal substrate. The S–S bond in dimethyl disulfide adsorbed on both Cu(111) and Ag(111) surfaces was dissociated through direct electronic excitation from the HOMO-derived MO (the nonbonding lone-pair type orbitals on the S atoms (n_S)) to the LUMO-derived MO (the antibonding orbital localized on the S–S bond (σ*_SS)) by irradiation with visible light. A combination of scanning tunneling microscopy and density functional theory calculations revealed that visible-light-induced photodissociation becomes possible due to the interfacial electronic structures constructed by the hybridization between molecular orbitals and the metal substrate states. The molecule–metal hybridization decreases the gap between the HOMO- and LUMO-derived MOs into the visible-light energy region and forms LUMO-derived MOs that have less overlap with the metal substrate, which results in longer excited-state lifetimes

Evolution of Graphene Growth on Pt(111): From Carbon Clusters to Nanoislands

We study the growth of graphene on a Pt(111) surface in stages by varying the annealing temperature of the precursor hydrocarbon decomposition through an atomic-scale analysis using scanning tunneling microscopy (STM) and studying the geometry-affected electronic properties of graphene nanoislands (GNs) through scanning tunneling spectroscopy. STM reveals that graphene grows on a Pt(111) surface from dome-shaped carbon clusters to flat GNs with the intermediate stages of dome-shaped and basin-shaped hexagonal GN structures. Density functional theory calculations confirm the changes in direction of the concavity upon increase in the size of the GNs. The structural changes are also found to have a significant effect on the electronic properties. Landau levels arise from strain-induced pseudomagnetic fields because of the large curvature, and the nanoscale-size effect promotes electron confinement

Elucidation of Isomerization Pathways of a Single Azobenzene Derivative Using an STM

The predominant pathway for the isomerization between <i>cis</i>- and <i>trans</i>-azobenzeneseither (i) inversion by the bending of an NNC bond or (ii) rotation by the torsion of two phenyl ringscontinues to be a controversial topic. To elucidate each isomerization pathway, a strategically designed and synthesized azobenzene derivative was investigated on a Ag(111) surface. This was achieved by exciting the molecule with tunneling electrons from the tip of a scanning tunneling microscope (STM). Structural analyses of the molecularly resolved STM images reveal that both inversion and rotation pathways are available for isomerization on a metal surface and strongly depend on the initial adsorption structures of the molecule. On the basis of the potential energy diagrams for the isomerization, it is concluded that isomerization pathways on a metal surface are not simply related to the excited states

Atomic-Scale Dynamics of Surface-Catalyzed Hydrogenation/Dehydrogenation: NH on Pt(111)

Low-temperature scanning tunneling microscopy (LT-STM) was used to move hydrogen atoms and dissociate NH molecules on a Pt(111) surface covered with an ordered array of nitrogen atoms in a (2 × 2) structure. The N-covered Pt(111) surface was prepared by ammonia oxydehydrogenation, which was achieved by annealing an ammonia–oxygen overlayer to 400 K. Exposing the N-covered surface to H₂(g) forms H atoms and NH molecules. The NH molecules occupy face-centered cubic hollow sites, while the H atoms occupy atop sites. The STM tip was used to dissociate NH and to induce hopping of H atoms. Action spectra consisting of the reaction yield <i>versus</i> applied bias voltage were recorded for both processes, which revealed that they are vibrationally mediated. The threshold voltages for NH dissociation and H hopping were found to be 430 and 272 meV, corresponding to the excitation energy of the N–H stretching and the Pt–H stretching modes, respectively. Substituting H with D results in an isotopic shift of −110 and −84 meV for the threshold voltages for ND dissociation and D hopping, respectively. This further supports the conclusion that these processes are vibrationally mediated