A Microscopy Approach to Investigating the Energetics of Small Supported Metal Clusters

Metal clusters are partway between molecular and bulk systems and thus exhibit special physical and chemical properties. Atoms can rearrange within a cluster to form different structural isomers. Internal degrees of freedom and the interaction with the support, both of which are dependent on cluster size, can promote diffusion across a support. Here, we show how fast scanning tunneling microscopy (FastSTM) can be used to investigate such dynamical behavior of individual clusters on the example of Pdn (1 ≤ n ≤ 19) on a hexagonal boron nitride nanomesh on Rh(111), in particular pertaining to minority species and rare events. By tuning the cluster size and varying the temperature to match the dynamics to the FastSTM frame rate, we followed steady state diffusion of clusters and atoms inside the nanomesh pore and reversible cluster isomerization in situ. While atoms diffuse along the rim of a pore, a small cluster experiences a corrugation in the potential energy landscape and jumps between six sites around the center of the pore. The atom and cluster diffusion between pores is strongly influenced by defects

Tunable Induced Circular Dichroism in Thin Organic Films

In this work, we study the induced optical activity (OA) of spin-coated molecular films made of a pristine optically inactive laser dye (Rhodamine 110) in the visible range. The OA is brought about by a “transparent” chiral modifier (1,1′-Bi-2-naphthol). For the first time, it was experimentally shown how two separate mechanisms yield induced circular dichroism (ICD) in such a film and how they are describable through a model. In one, ICD occurs via a direct interaction between dye and modifier molecules. In the other, ICD takes place in optical transitions of present dye aggregates. It is proposed that this is caused by a chiral bias through an enantiomeric excess in inherently chiral and optically active dye aggregates promoted by the chiral modifier in a sergeant-soldier manner. Further, the strength of ICD strongly depends on the ratio between the modifier and the dye as well as on the preparation conditions. The resulting high tunability of the OA paired with a straightforward film preparation opens up a route for fabrication of homogeneous thin films with customized OA

Catalytic Non-Oxidative Coupling of Methane on Ta₈O₂⁺

Mass-selected Ta8O2+ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H2, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C–C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study

Preparation and Spectroscopic Properties of Monolayer-Protected Silver Nanoclusters

Silver nanoclusters protected by 2-phenylethanethiol (<b>1</b>), 4-fluorothiophenol (<b>2</b>), and l-glutathione (<b>3</b>) ligands were successfully synthesized. The optical properties of the prepared silver nanoclusters were studied. The absorption signal of Ag@SCH₂CH₂Ph in toluene can be found at 469 nm, and Ag@SPhF in THF shows two absorption bands at 395 and 462 nm. Ag@SG in water absorbs at 478 nm. Mie theory in combination with the Drude model clearly indicates the peaks in the spectra originate from plasmonic transitions. In addition, the damping constant as well as the dielectric constant of the surrounding medium was determined. In addition, the CD spectra of silver nanoclusters protected by the three ligands (<b>1</b>–<b>3</b>) were also studied. As expected, only the clusters of type <b>3</b> gave rise to chiroptical activity across the visible and near-ultraviolet regions. The location and strength of the optical activity suggest an electronic structure of the metal that is highly sensitive to the chiral environment imposed by the glutathione ligand. The morphology and size of the prepared nanoclusters were analyzed by using transmission electron microscopy (TEM). TEM analysis showed that the particles of all three types of silver clusters were small than 5 nm, with an average size of around 2 nm. The analysis of the FTIR spectra elucidated the structural properties of the ligands binding to the nanoclusters. By comparing the IR absorption spectra of pure ligands with those of the protected silver nanoclusters, the disappearance of the S–H vibrational band (2535–2564 cm^–1) in the protected silver nanoclusters confirmed the anchoring of ligands to the cluster surface through the sulfur atom. By elemental analysis and thermogravimetric analysis, the Ag/S ratio and, hence, the number of ligands surrounding a Ag atom could be determined

Size and Coverage Effects of Ni and Pt Co-Catalysts in the Photocatalytic Hydrogen Evolution from Methanol on TiO₂(110)

In the past decade, hydrogen evolution from photocatalytic alcohol oxidation on metal-loaded TiO2 has emerged as an active research field. While the presence of a metal cluster co-catalyst is crucial as a H2 recombination center, size and coverage effects on the catalyst performance are not yet comprehensively understood. To some extent, this is due to the fact that common deposition methods do not allow for an independent change in size and coverage, which can be overcome by the use of cluster sources and the deposition of size-selected clusters. This study compares size-selected Ni and Pt clusters as co-catalysts on a TiO2(110) single crystal and the resulting size- and coverage-dependent effects in the photocatalytic hydrogen evolution from alcohols in ultrahigh vacuum (UHV). Larger clusters and higher coverages of Ni enhance the product formation rate, although deactivation over time occurs. In contrast, Pt co-catalysts exhibit a stable and higher activity and size-specific effects have to be taken into account. While H2 evolution is improved by a higher concentration of Pt clusters, an increase in the metal content by the deposition of larger particles can even be detrimental to the performance of the photocatalyst. The acquired overall mechanistic picture is corroborated by H2 formation kinetics from mass spectrometric data. Consequently, for some metals, size effects are relevant for improving the catalytic performance, while for other co-catalyst materials, merely the coverage is decisive. The elucidation of different size and coverage dependencies represents an important step toward a rational catalyst design for photocatalytic hydrogen evolution

Thermal Control of Selectivity in Photocatalytic, Water-Free Alcohol Photoreforming

The selective oxidation of alcohols has attracted a great deal of attention. While most photocatalytic studies focus on the generation of hydrogen from alcohols, there is also a great potential to replace inefficient thermal reaction pathways (as e.g. the formox process) by light-driven reactions. In this work we focus on the photoreforming of methanol, ethanol, cyclohexanol, benzyl alcohol, and tert-butanol on well-defined Ptx/TiO2(110) under UHV. It is found that, with the exception of tert-butanol, alcohol oxidation can produce the respective water-free aldehydes and ketones along with the formation of stoichiometric molecular hydrogen with 100% selectivity. While α-H-containing alcohols usually exhibit only a disproportionation reaction with the release of H2, another reaction pathway is detected for methanol (and to a much lower extent benzyl alcohol) to yield the respective ester, methyl formate (or benzyl benzoate, respectively). The formation of this product occurs via a consecutive photoreaction and is strongly influenced by temperature. In general, higher temperatures lead to a higher selectivity toward formaldehyde, as product desorption is favored over the consecutive photoreaction. For tert-butanol two parallel photoreactions occur. In addition to the splitting of a C–C bond yielding a methyl radical, hydrogen, and acetone, dehydration to isobutene is observed. The branching ratios of both reaction pathways can be strongly controlled by temperature, by changing the reaction regime from adsorption to desorption limited. The high selectivities toward aldehydes are attributed to the absence of O2 and water, which inhibits an unwanted overoxidation to acids or CO/CO2. This study shows that photocatalysis under such conditions provides a prospective approach for a highly selective and water-free aldehyde production under mild conditions

Nonvolatile Memristive Switching in Self-assembled Nanoparticle Dimers

The selective formation of heterogeneous nanoparticle (NP) agglomerates and their memristive switching properties are reported. Following surfactant-mediated self-assembly in suspension, agglomerates of silver nanocubes (AgNCs) and spherical silicon nanoparticles (SiNPs) are prepared in a bottom-up approach. Dimers comprising one AgNC and one SiNP are electrically contacted, and pronounced bipolar memristive switching is demonstrated. The formed nanoscale memristor junctions show in particular narrow distributions of the SET voltage, ON/OFF-resistance ratios in excess of three orders of magnitude, and good retention properties. Ex situ scanning electron microscopy studies directly support a switching model based on Ag-conductive filament formation in the polycrystalline SiNPs. We propose our self-assembled NP memristors as building blocks for the bottom-up construction of future 3D memristive networks

Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au20 was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir−Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O2 molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O2 molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au20 nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects