Polyvinylpyrrolidone-induced anisotropic growth of gold nanoprisms in plasmon-driven synthesis.

After more than a decade, it is still unknown whether the plasmon-mediated growth of silver nanostructures can be extended to the synthesis of other noble metals, as the molecular mechanisms governing the growth process remain elusive. Herein, we demonstrate the plasmon-driven synthesis of gold nanoprisms and elucidate the details of the photochemical growth mechanism at the single-nanoparticle level. Our investigation reveals that the surfactant polyvinylpyrrolidone preferentially adsorbs along the nanoprism perimeter and serves as a photochemical relay to direct the anisotropic growth of gold nanoprisms. This discovery confers a unique function to polyvinylpyrrolidone that is fundamentally different from its widely accepted role as a crystal-face-blocking ligand. Additionally, we find that nanocrystal twinning exerts a profound influence on the kinetics of this photochemical process by controlling the transport of plasmon-generated hot electrons to polyvinylpyrrolidone. These insights establish a molecular-level description of the underlying mechanisms regulating the plasmon-driven synthesis of gold nanoprisms.

Silica-Encapsulated Pt-Sn Intermetallic Nanoparticles: A Robust Catalytic Platform for Parahydrogen-Induced Polarization of Gases and Liquids.

Recently, a facile method for the synthesis of size-monodisperse Pt, Pt3 Sn, and PtSn intermetallic nanoparticles (iNPs) that are confined within a thermally robust mesoporous silica (mSiO2 ) shell was introduced. These nanomaterials offer improved selectivity, activity, and stability for large-scale catalytic applications. Here we present the first study of parahydrogen-induced polarization NMR on these Pt-Sn catalysts. A 3000-fold increase in the pairwise selectivity, relative to the monometallic Pt, was observed using the PtSn@mSiO2 catalyst. The results are explained by the elimination of the three-fold Pt sites on the Pt(111) surface. Furthermore, Pt-Sn iNPs are shown to be a robust catalytic platform for parahydrogen-induced polarization for in vivo magnetic resonance imaging.

Parahydrogen-induced polarization by pairwise replacement catalysis on Pt and Ir nanoparticles.

Pairwise and random addition processes are ordinarily indistinguishable in hydrogenation reactions. The distinction becomes important only when the fate of spin correlation matters, such as in parahydrogen-induced polarization (PHIP). Supported metal catalysts were not expected to yield PHIP signals given the rapid diffusion of H atoms on the catalyst surface and in view of the sequential stepwise nature of the H atom addition in the Horiuti-Polanyi mechanism. Thus, it seems surprising that supported metal hydrogenation catalysts can yield detectable PHIP NMR signals. Even more remarkably, supported Pt and Ir nanoparticles are shown herein to catalyze pairwise replacement on propene and 3,3,3-trifluoropropene. By simply flowing a mixture of parahydrogen and alkene over the catalyst, the scalar symmetrization order of the former is incorporated into the latter without a change in molecular structure, producing intense PHIP NMR signals on the alkene. An important indicator of the mechanism of the pairwise replacement is its stereoselectivity, which is revealed with the aid of density matrix spectral simulations. PHIP by pairwise replacement has the potential to significantly diversify the substrates that can be hyperpolarized by PHIP for biomedical utilization.

Parahydrogen enhanced NMR reveals correlations in selective hydrogenation of triple bonds over supported Pt catalyst.

Parahydrogen induced polarization using heterogeneous catalysis can produce impurity-free hyperpolarized gases and liquids, but the comparatively low signal enhancements and limited scope of substrates that can be polarized pose significant challenges to this approach. This study explores the surface processes affecting the disposition of the bilinear spin order derived from parahydrogen in the hydrogenation of propyne over TiO2-supported Pt nanoparticles. The hyperpolarized adducts formed at low magnetic field are adiabatically transported to high field for analysis by proton NMR spectroscopy at 400 MHz. For the first time, the stereoselectivity of pairwise addition to propyne is measured as a function of reaction conditions. The correlation between partial reduction selectivity and stereoselectivity of pairwise addition is revealed. The systematic trends are rationalized in terms of a hybrid mechanism incorporating non-traditional concerted addition steps and well-established reversible step-wise addition involving the formation of a surface bound 2-propyl intermediate.

Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para-Hydrogen-Enhanced Polarization.

Intense para-hydrogen-enhanced NMR signals are observed in the hydrogenation of propene and propyne over ceria nanocubes, nano-octahedra, and nanorods. The well-defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet-dependent, the pairwise selectivity is low (2.4% at 375 °C), which is consistent with stepwise H atom transfer, and it is the same for all three nanocrystal shapes. Selective semi-hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7% at 300 °C), indicating product formation predominantly by a non-pairwise addition. Ceria is also shown to be an efficient pairwise replacement catalyst for propene.

Argon Cluster Sputtering Source for ToF-SIMS Depth Profiling of Insulating Materials: High Sputter Rate and Accurate Interfacial Information.

The use of an argon cluster ion sputtering source has been demonstrated to perform superiorly relative to traditional oxygen and cesium ion sputtering sources for ToF-SIMS depth profiling of insulating materials. The superior performance has been attributed to effective alleviation of surface charging. A simulated nuclear waste glass (SON68) and layered hole-perovskite oxide thin films were selected as model systems because of their fundamental and practical significance. Our results show that high sputter rates and accurate interfacial information can be achieved simultaneously for argon cluster sputtering, whereas this is not the case for cesium and oxygen sputtering. Therefore, the implementation of an argon cluster sputtering source can significantly improve the analysis efficiency of insulating materials and, thus, can expand its applications to the study of glass corrosion, perovskite oxide thin film characterization, and many other systems of interest.