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In this study, the thermal stability of Au-Al2O3 core-shell and Au nanowires was investigated by in situ scanning transmission electron microscopy and other techniques. The nanowires were synthesized by the helium droplets method and deposited on various substrates. The in situ characterization of Au-Al2O3 thermal stability demonstrated a substantially enhanced stability as compared to that of pure Au nanowires, which can be a transformative approach to design more durable Au-based nanocatalysts. Our study also revealed the existence of strong metal-support bonding in the Au/Al2O3 system.
RESUMEN
In a recently reported helium droplet-mediated deposition experiment to produce copper-coated magnesium core-shell nanoclusters, structural inversion was observed, which resulted in copper in the nanocluster interior, surrounded by oxidized magnesium on the copper surface. This study utilizes density functional theory methods to model the migration of copper atoms into the interior of a magnesium nanocluster to probe the energetics of this process and to compare it to the complementary process of magnesium atom migration into the interior of a copper nanocluster. Potential energy surfaces describing the forced migration of copper (magnesium) atoms into the interior of a 30-atom magnesium (copper) cluster were generated using the B3PW91 hybrid generalized gradient approximation functional with the augmented correlation consistent core-valence polarized triple-ζ basis set for magnesium and a pseudopotential plus valence-only basis set for copper. The estimated barrier for atomic copper to penetrate the surface of Mg30 is 0.6 kcal mol-1. In contrast, the migration of atomic magnesium into the interior of Cu30 crosses an estimated barrier of 6 kcal mol-1. These results are qualitatively consistent with the observed structural inversion of copper-coated magnesium nanoclusters and also suggest that inversion of a magnesium-coated copper cluster is less likely to occur.
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Nanoparticles (NPs) are revolutionizing many areas of science and technology, often delivering unprecedented improvements to properties of the conventional materials. However, despite important advances in NPs synthesis and applications, numerous challenges still remain. Development of alternative synthetic method capable of producing very uniform, extremely clean and very stable NPs is urgently needed. If successful, such method can potentially transform several areas of nanoscience, including environmental and energy related catalysis. Here we present the first experimental demonstration of catalytically active NPs synthesis achieved by the helium nanodroplet isolation method. This alternative method of NPs fabrication and deposition produces narrowly distributed, clean, and remarkably stable NPs. The fabrication is achieved inside ultralow temperature, superfluid helium nanodroplets, which can be subsequently deposited onto any substrate. This technique is universal enough to be applied to nearly any element, while achieving high deposition rates for single element as well as composite core-shell NPs.
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The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8-15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and the parallel hydrogenation-dehydrogenation ethylidyne-producing route are considered, uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to structure insensitivity found for larger particles. The onset of catalysed hydrogenation occurs for Ptn (n ≥ 10) clusters at T>150 K, with maximum room temperature reactivity observed for Pt13. Structure insensitivity, inherent for specific cluster sizes, is induced in the more active Pt13 by a temperature increase up to 400 K leading to ethylidyne formation. Control of sub-nanometre particle size may be used for tuning catalysed hydrogenation activity and selectivity.
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Employing rationally designed model systems with precise atom-by-atom particle size control, we demonstrate by means of combining noninvasive in situ indirect nanoplasmonic sensing and ex situ scanning transmission electron microscopy that monomodal size-selected platinum cluster catalysts on different supports exhibit remarkable intrinsic sintering resistance even under reaction conditions. The observed stability is related to suppression of Ostwald ripening by elimination of its main driving force via size-selection. This study thus constitutes a general blueprint for the rational design of sintering resistant catalyst systems and for efficient experimental strategies to determine sintering mechanisms. Moreover, this is the first systematic experimental investigation of sintering processes in nanoparticle systems with an initially perfectly monomodal size distribution under ambient conditions.
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The photocatalytic water reduction reaction on CdS nanorods was studied as function of Pt cluster size. Maximum H2 production is found for Pt46. This effect is attributed to the size dependent electronic properties (e.g., LUMO) of the clusters with respect to the band edges of the semiconductor. This observation may be applicable for the study and interpretation of other systems and reactions, e.g. H2O oxidation or CO2 reduction.
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We introduce size-selected subnanometer cluster catalysts deposited on thin films of colloidal semiconductor nanocrystals as a novel platform to obtain atomic scale insight into photocatalytic generation of solar fuels. Using Pt-cluster-decorated CdS nanorod films for photocatalytic hydrogen generation as an example, we determine the minimum amount of catalyst necessary to obtain maximum quantum efficiency of hydrogen generation. Further, we provide evidence for tuning photocatalytic activities by precisely controlling the cluster catalyst size.