A particularly stable thiolate-protected gold nanocluster, Au25(SR)18, was structurally characterized from X-ray crystallography in 2008, and concomitantly its electronic and optical properties were ...analyzed via density functional theory. The robust geometry and a well-understood electronic structure of this cluster motivate explorations of properties of extended systems made out of Au25(SR)18 building blocks. As a first step in this direction, we analyze here structural, vibrational, electronic, and optical properties of the Au25 cluster anion as it was observed in the crystalline environment and predict properties of cluster dimers, where the Au25 units are linked together via an aromatic dithiolate linker. We show that properties of each Au25 unit of the dimer can be quite independently modified from the other by doping with a nonmagnetic (Pd) or magnetic (Mn) metal atom. We anticipate that material systems with interesting properties could be made from these building blocks, provided that a suitable chemistry for their controlled linking can be found.
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42.
Size Effects in MgO Cube Dissolution Baumann, Stefan O; Schneider, Johannes; Sternig, Andreas ...
Langmuir,
03/2015, Volume:
31, Issue:
9
Journal Article
Peer reviewed
Stability parameters and dissolution behavior of engineered nanomaterials in aqueous systems are critical to assess their functionality and fate under environmental conditions. Using scanning ...electron microscopy, transmission electron microscopy, and X-ray diffraction, we investigated the stability of cubic MgO particles in water. MgO dissolution proceeding via water dissociation at the oxide surface, disintegration of Mg2+–O2– surface elements, and their subsequent solvation ultimately leads to precipitation of Mg(OH)2 nanosheets. At a pH ≥ 10, MgO nanocubes with a size distribution below 10 nm quantitatively dissolve within few minutes and convert into Mg(OH)2 nanosheets. This effect is different from MgO cubes originating from magnesium combustion in air. With a size distribution in the range 10 nm ≤ d ≤ 1000 nm they dissolve with a significantly smaller dissolution rate in water. On these particles water induced etching generates (110) faces which, above a certain face area, dissolve at a rate equal to that of (100) planes. The delayed solubility of microcrystalline MgO is attributed to surface hydroxide induced self-inhibition effects occurring at the (100) and (110) microplanes. The present work underlines the importance of morphology evolution and surface faceting of engineered nanomaterials particles during their dissolution.
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The density functional theory is used to explore structural, energetic, vibrational, and optical properties of thiolated gold clusters (MeSAu) x with x = 2−12. Clusters up to (MeSAu)4 adopt Au−S ring ...conformations, and crownlike structures are formed for larger sizes. The clusters are essentially polymeric and show convergence in structural and energetic properties at (MeSAu)4. The nature of the Au−S bond is polar covalent with a degree of cyclic electron delocalization. Vibrational analysis reveals characteristic Au−S stretch vibrations at ∼300 cm-1. Effects of ligand substitution are studied in the case of the tetramer by comparing the results for methylthiolate with hexylthiolate, benzenethiolate, and glutathionate (GS). The choice of ligand has clear effects on electronic properties. For example, the optical gap is ∼1.5 eV lower for (GSAu)4 than for (MeSAu)4.
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Amorphous titania (am.-TiO2) has gained wide interest in the field of photocatalysis, thanks to exceptional disorder-mediated optical and electrical properties compared to crystalline TiO2. Here, we ...study the effects of intrinsic Ti3+ and nitrogen defects in am.-TiO2 thin films via the atomic layer deposition (ALD) chemistry of tetrakis(dimethylamido)titanium(IV) (TDMAT) and H2O precursors at growth temperatures of 100–200 °C. X-ray photoelectron spectroscopy (XPS) and computational analysis allow us to identify structural disorder-induced penta- and heptacoordinated Ti4+ ions (Ti5/7c 4+), which are related to the formation of Ti3+ defects in am.-TiO2. The Ti3+-rich ALD-grown am.-TiO2 has stoichiometric composition, which is explained by the formation of interstitial peroxo species with oxygen vacancies. The occupation of Ti3+ 3d in-gap states increases with the ALD growth temperature, inducing both visible-light absorption and electrical conductivity via the polaron hopping mechanism. At 200 °C, the in-gap states become fully occupied extending the lifetime of photoexcited charge carriers from the picosecond to the nanosecond time domain. Nitrogen traces from the TDMAT precursor had no effect on optical properties and only little on charge transfer properties. These results provide insights into the charge transfer properties of ALD-grown am.-TiO2 that are essential to the performance of protective photoelectrode coatings in photoelectrochemical solar fuel reactors.
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The influence of different approximations on adsorbate entropies is investigated for density functional theory based mean-field kinetic modeling. Using CO oxidation over Pt(111) as a prototypical ...reaction, we compare four approximations: the harmonic approximation, the hindered translator, the free translator, and complete potential energy sampling (CPES). The CPES method results in particularly good agreement with previously measured experimental data. Given its general applicability and moderate computational cost, the CPES method stands out as a preferable option to describe adsorbate entropies.
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The intrinsic activity of Pd(100) and Pd(111) for methane oxidation is investigated by Density Functional Theory (DFT)-based mean-field microkinetic modeling. The model includes 32 reaction steps, ...and the calculated turnover frequencies together with reaction orders compare favorably with experimental data. On both surfaces, the reaction proceeds via complete dehydrogenation of methane to elemental carbon followed by different mechanisms for carbon oxidization. Pd(100) is found to be more active than Pd(111) at temperatures from 400 to 1000 K. For both surfaces, the reaction order in methane approaches unity with increasing temperature. The reaction order in water is positive at low temperatures owing to water-promoted carbon oxidation. Methane dissociation is the main rate-controlling step for Pd(111), whereas formation of COH and CO is also controlling the rate over Pd(100). The present work uncovers the detailed reaction mechanisms for complete methane oxidation over palladium, which can be used in catalyst design to target the rate-controlling steps.
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Cu(NH 3 ) 2 + -pairs in chabazite (CHA) have been suggested to activate oxygen during low-temperature selective catalytic reduction of nitrogen oxides with ammonia (NH 3 -SCR). As charge neutrality ...requires that each Cu-complex is associated with a framework Al, the Al-distribution may affect Cu(NH 3 ) 2 + -pair formation and subsequent oxygen activation. Here, density functional theory calculations in combination with ab initio molecular dynamics simulations are used to explore Cu(NH 3 ) 2 + -pair formation and oxygen activation in Cu–CHA. The Al-distribution is found to markedly affect the probability for Cu(NH 3 ) 2 + -pair formation. Moreover, the molecular dynamics simulations reveal a low-energy reaction path for O 2 activation and dissociation. The facile O 2 dissociation suggests that Cu-pair formation rather than O 2 activation governs the low-temperature NH 3 -SCR activity. The results indicate that precise synthesis of Cu-exchanged chabazite with respect to Al-distribution may enhance the catalytic activity.
In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps ...of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.
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Platinum is the most widely used and best performing sole element for catalyzing the oxygen reduction reaction (ORR) in low-temperature fuel cells. Although recyclable, there is a need to reduce the ...amount used in current fuel cells for their extensive uptake in society. Alloying platinum with rare-earth elements such as yttrium can provide an increase in activity of more than seven times, reducing the amount of platinum and the total amount of catalyst material required for the ORR. As yttrium is easily oxidized, exposure of the Pt–Y catalyst layer to air causes the formation of an oxide layer that can be removed during acid treatment, leaving behind a highly active pure platinum overlayer. This paper presents an investigation of the overlayer composition and quality of Pt3Y films sputtered from an alloy target. The Pt3Y catalyst surface is investigated using synchrotron radiation X-ray photoelectron spectroscopy before and after acid treatment. A new substoichiometric oxide component is identified. The oxide layer extends into the alloy surface, and although it is not completely removed with acid treatment, the catalyst still achieves the expected high ORR activity. Other surface-sensitive techniques show that the sputtered films are smooth and bulk X-ray diffraction reveals many defects and high microstrain. Nevertheless, sputtered Pt3Y exhibits a very high activity regardless of the film’s oxide content and imperfections, highlighting Pt3Y as a promising catalyst. The obtained results will help to support its integration into fuel cell systems.
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