Abstract: The reductive amination of benzoic acid and its derivatives would be an effective addition to current synthesis methods for benzylamine. However, with current technology it is very difficult to keep the aromaticity intact when starting from benzoic acid, and salt wastes are often generated in the process. Here, we report a heterogeneous catalytic system for such a reductive amination, requiring solely H-2 and NH3 as the reactants. The Ag/TiO2 or Au/TiO2 catalysts can be used multiple times, and very little noble metal is required, only 0.025 mol % Au. The catalysts are bifunctional: the support catalyzes the dehydration of both the ammonium carboxylate to the amide and of the amide to the nitrile, while the sites at the metal-support interface promote the hydrogenation of the in situ generated nitrile. Yields of up to 92% benzylamine were obtained.

Abstract: Nanoporous Pt based nanoparticles (NP's) are promising fuel cell catalysts due to their high surface area and increased electrocatalytic activity toward the ORR In this work a direct double-pulse electrodeposition procedure at room temperature is applied to obtain dendritic Pt structures (89 nm diameter) with a high level of porosity (ca. 25%) and nanopores of 2 nm protruding until the center of the NP's. The particle morphology is characterized using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and electron tomography (ET) combined with field emission scanning electron microscopy (FESEM) and macroscopic electrochemical measurements to assess their activity and stability toward the ORR. Macroscopic determination of the active surface area through hydrogen UPD measurements in combination with FESEM and ET showed that a considerable amount of the active sites inside the pores of the low overpotential NP's were accessible to oxygen species. As a result of this accessibility, up to a 9-fold enhancement of the Pt mass corrected ORR activity at 0.85 V vs RHE was observed at the highly porous structures. After successive potential cycling upward to 1.5 V vs RHE in a deaerated HClO4 solution a negative shift of 71 mV in half-wave potential occurred. This decrease in ORR activity could be correlated to the partial collapse of the nanopores, visible in both the EASA values and 3D ET reconstructions.

Abstract: A series of heterogeneous catalysts consisting of highly dispersed Pt nanoparticles supported on nanosized ZrO2 (20 to 60 nm) was synthesized and investigated for the one-pot transfer hydrogenation between glycerol and cyclohexene to produce lactic acid and cyclohexane, without any additional H-2. Different preparation methods were screened, by varying the calcination and reduction procedures with the purpose of optimizing the dispersion of Pt species (i.e., as single-atom sites or extra-fine Pt nanoparticles) on the ZrO2 support. The Pt/ZrO2 catalysts were characterized by means of transmission electron microscopy techniques (HAADF-STEM, TEM), elemental analysis (ICP-OES, EDX mapping), N-2-physisorption, H-2 temperature-programmed-reduction (H-2-TPR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Based on this combination of techniques it was possible to correlate the temperature of the calcination and reduction treatments with the nature of the Pt species. The best catalyst consisted of subnanometer Pt clusters (<1 nm) and atomically dispersed Pt (as Pt2+ and Pt4+) on the ZrO2 support, which were converted into extra-fine Pt nanoparticles (average size = 1.4 nm) upon reduction. These nanoparticles acted as catalytic species for the transfer hydrogenation of glycerol with cyclohexene, which gave an unsurpassed 95% yield of lactic acid salt at 96% glycerol conversion (aqueous glycerol solution, NaOH as promoter, 160 degrees C, 4.5 h, at 20 bar N-2). This is the highest yield and selectivity of lactic acid (salt) reported in the literature so far. Reusability experiments showed a partial and gradual loss of activity of the Pt/ZrO2 catalyst, which was attributed to the experimentally observed aggregation of Pt nanoparticles.

Abstract: Chirality in inorganic nanoparticles and nanostructures has gained increasing scientific interest, because of the possibility to tune their ability to interact differently with left- and right-handed circularly polarized light. In some cases, the optical activity is hypothesized to originate from a chiral morphology of the nanomaterial. However, quantifying the degree of chirality in objects with sizes of tens of nanometers is far from straightforward. Electron tomography offers the possibility to faithfully retrieve the three-dimensional morphology of nanomaterials, but only a qualitative interpretation of the morphology of chiral nanoparticles has been possible so far. We introduce herein a methodology that enables us to quantify the helicity of complex chiral nanomaterials, based on the geometrical properties of a helix. We demonstrate that an analysis at the single particle level can provide significant insights into the origin of chiroptical properties.

Abstract: Ultrafast laser irradiation can induce morphological and structural changes in plasmonic nanoparticles. Gold nanorods (Au NRs), in particular, can be welded together upon irradiation with femtosecond laser pulses, leading to dimers and trimers through the formation of necks between individual nanorods. We used electron tomography to determine the 3D (atomic) structure at such necks for representative welding geometries and to characterize the induced defects. The spatial distribution of localized surface plasmon modes for different welding configurations was assessed by electron energy loss spectroscopy. Additionally, we were able to directly compare the plasmon line width of single-crystalline and welded Au NRs with single defects at the same resonance energy, thus making a direct link between the structural and plasmonic properties. In this manner, we show that the occurrence of (single) defects results in significant plasmon broadening.

Abstract: Despite recent advances, the synthesis of colloidal InSb quantum dots (QDs) remains underdeveloped, mostly due to the lack of suitable precursors. In this work, we use Lewis acid–base interactions between Sb(III) and In(III) species formed at room temperature in situ from commercially available compounds (viz., InCl3, Sb[NMe2]3 and a primary alkylamine) to obtain InSb adduct complexes. These complexes are successfully used as precursors for the synthesis of colloidal InSb QDs ranging from 2.8 to 18.2 nm in diameter by fast coreduction at sufficiently high temperatures (≥230 °C). Our findings allow us to propose a formation mechanism for the QDs synthesized in our work, which is based on a nonclassical nucleation event, followed by aggregative growth. This yields ensembles with multimodal size distributions, which can be fractionated in subensembles with relatively narrow polydispersity by postsynthetic size fractionation. InSb QDs with diameters below 7.0 nm have the zinc blende crystal structure, while ensembles of larger QDs (≥10 nm) consist of a mixture of wurtzite and zinc blende QDs. The QDs exhibit photoluminescence with small Stokes shifts and short radiative lifetimes, implying that the emission is due to band-edge recombination and that the direct nature of the bandgap of bulk InSb is preserved in InSb QDs. Finally, we constructed a sizing curve correlating the peak position of the lowest energy absorption transition with the QD diameters, which shows that the band gap of colloidal InSb QDs increases with size reduction following a 1/d dependence.

Abstract: The crystallization of nanomaterials is a primary source of solid-state, photonic structures. Thus, a detailed understanding of this process is of paramount importance for the successful application of photonic nanomaterials in emerging optoelectronic technologies. While colloidal crystallization has been thoroughly studied, for example, with advanced in situ electron microscopy methods, the noncolloidal crystallization (freezing) of nanoparticles (NPs) remains so far unexplored. To fill this gap, in this work, we present proof-of principle experiments decoding a crystallization of reconfigurable assemblies of NPs at a solid state. The chosen material corresponds to an excellent testing bed, as it enables both in situ and ex situ investigation using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic force microscopy (AFM), and optical spectroscopy in visible and ultraviolet range (UV−vis) techniques. In particular, ensemble measurements with small-angle XRD highlighted the dependence of the correlation length in the NPs assemblies on the number of heating/cooling cycles and the rate of cooling. Ex situ TEM imaging further supported these results by revealing a dependence of domain size and structure on the sample preparation route and by showing we can control the domain size over 2 orders of magnitude. The application of HAADF-STEM tomography, combined with in situ thermal control, provided three-dimensional single-particle level information on the positional order evolution within assemblies. This combination of real and reciprocal space provides insightful information on the anisotropic, reversibly reconfigurable assemblies of NPs. TEM measurements also highlighted the importance of interfaces in the polydomain structure of nanoparticle solids, allowing us to understand experimentally observed differences in UV−vis extinction spectra of the differently prepared crystallites. Overall, the obtained results show that the combination of in situ heating HAADF-STEM tomography with XRD and ex situ TEM techniques is a powerful approach to study nanoparticle freezing processes and to reveal the crucial impact of disorder in the solid-state aggregates of NPs on their plasmonic properties.

Abstract: Colloidal Cu–Ag nanocrystals measuring less than 10 nm across are promising candidates for integration in hybrid CO2 reduction reaction (CO2RR) interfaces, especially in the context of tandem catalysis and selective multicarbon (C2–C3) product formation. In this work, we vary the synthetic-ligand/copper molar ratio from 0.1 to 1.0 and the silver/copper atomic ratio from 0 to 0.7 and study the variations in the nanocrystals’ size distribution, morphology and reactivity at rates of ≥100 mA cm–2 in a gas-fed recycle electrolyzer operating under neutral to mildly basic conditions (0.1–1.0 M KHCO3). High-resolution electron microscopy and spectroscopy are used in order to characterize the morphology of sub-10 nm Cu–Ag nanodimers and core–shells and to elucidate trends in Ag coverage and surface composition. It is shown that Cu–Ag nanocrystals can be densely dispersed onto a carbon black support without the need for immediate ligand removal or binder addition, which considerably facilitates their application. Although CO2RR product distribution remains an intricate function of time, (kinetic) overpotential and processing conditions, we nevertheless conclude that the ratio of oxygenates to hydrocarbons (which depends primarily on the initial dispersion of the nanocrystals and their composition) rises 3-fold at moderate Ag atom % relative to Cu NCs-based electrodes. Finally, the merits of this particular Cu–Ag/C system and the recycling reactor employed are utilized to obtain maximum C2–C3 partial current densities of 92–140 mA cm–2 at −1.15 VRHE and liquid product concentrations in excess of 0.05 wt % in 1 M KHCO3 after short electrolysis periods.

Abstract: Good’s buffers can act both as nucleating and shape- directing agents during the synthesis of anisotropic gold nanostars (AuNS). Although different Good’s buffers can produce AuNS shapes with branches that are oriented along specific crystallographic directions, the mechanism is not fully understood. This paper reports how an analysis of the intermediate structures during AuNS synthesis from HEPES, EPPS, and MOPS Good’s buffers can provide insight into the formation of seedless AuNS. Electron tomography of AuNS structures quenched at early times (minutes) was used to characterize the morphology of the incipient seeds, and later times were used to construct the growth maps. Through this approach, we identified how the crystallinity and shape of the first structures synthesized with different Good’s buffers determine the final AuNS morphologies.

Abstract: Circularly polarized luminescent (CPL) films with high dissymmetry factors hold great potential for optoelectronic applications. Herei n , we propose a strategy for achieving strongly dissymetric CPL in nanocomposite films based on chira l i t y induction and energy transfer to semiconductor nanocrystals. First, focusing on a purely organic system, aggregation-induced emission (AIE) and CPL activity of organic liquid crystals (LCs) forming helical nanofilaments was detected, featuring green emission with high dissymmetry factors g(lum) similar to 10(-2). The handedness of helical filaments, and thus the sign of CPL, was controlled via minute amounts of a small chiral organic dopant. Second, nanocomposite films were fabricated by incorporating InP/ZnS semi-conductor quantum dots (QDs) into the LC matri x , which induced the chiral assembly of QDs and endowed them with chiroptical properties. Due to the spectral matching of the components, energy transfer (ET) from LC to QDs was possible enabling a convenient way of tuning CPL wavelengths by varying the LC/QD ratio. As obtained, composite films exhibited absolute glum values up to similar to 10(-2) and thermally on/off switchable luminescence. Overall, we demonstrate the induction of chiroptical properties by the assembly of nonchiral building QDs on the chiral organic template and energy transfer from organic films to QDs, representing a simple and versatile approach to tune the CPL activity of organic materials.

Abstract: Cryogenic electronmicroscopy can provide high-resolution reconstructionsof macromolecules embedded in a thin layer of ice from which atomicmodels can be built de novo. However, the interactionbetween the ionizing electron beam and the sample results in beam-inducedmotion and image distortion, which limit the attainable resolutions.Sample charging is one contributing factor of beam-induced motionsand image distortions, which is normally alleviated by including partof the supporting conducting film within the beam-exposed region.However, routine data collection schemes avoid strategies wherebythe beam is not in contact with the supporting film, whose rationaleis not fully understood. Here we characterize electrostatic chargingof vitreous samples, both in imaging and in diffraction mode. We mitigatesample charging by depositing a single layer of conductive grapheneon top of regular EM grids. We obtained high-resolution single-particleanalysis (SPA) reconstructions at 2 & ANGS; when the electron beamonly irradiates the middle of the hole on graphene-coated grids, usingdata collection schemes that previously failed to produce sub 3 & ANGS;reconstructions without the graphene layer. We also observe that theSPA data obtained with the graphene-coated grids exhibit a higher b factor and reduced particle movement compared to dataobtained without the graphene layer. This mitigation of charging couldhave broad implications for various EM techniques, including SPA andcryotomography, and for the study of radiation damage and the developmentof future sample carriers. Furthermore, it may facilitate the explorationof more dose-efficient, scanning transmission EM based SPA techniques.

Abstract: Chirality in gold nanostructures offers an exciting opportunity to tune their differential optical response to left- and right-handed circularly polarized light, as well as their interactions with biomolecules and living matter. However, tuning and understanding such interactions demands quantification of the structural features that are responsible for the chiral behavior. Electron tomography (ET) enables structural characterization at the single-particle level and has been used to quantify the helicity of complex chiral nanorods. However, the technique is time-consuming and consequently lacks statistical value. To address this issue, we introduce herein a high-throughput methodology that combines images acquired by secondary electron-based electron beam-induced current (SEEBIC) with quantitative image analysis. As a result, the geometric chirality of hundreds of nanoparticles can be quantified in less than 1 h. When combining the drastic gain in data collection efficiency of SEEBIC with a limited number of ET data sets, a better understanding of how the chiral structure of individual chiral nanoparticles translates into the ensemble chiroptical response can be reached.

Abstract: Noble metal nanoparticles are widely used as probes or substrates for surface-enhanced Raman scattering (SERS), due to their characteristic plasmon resonances in the visible and NIR spectral ranges. Aiming at obtaining a versatile system with high SERS performance we developed the synthesis of quasi-monodisperse, non-aggregated gold nanoparticles protected by radial mesoporous silica shells. The radial channels of such shells were used as templates for the growth of gold tips branching from the cores, thereby improving the plasmonic performance of the particles while favoring the localization of analyte molecules at high electric field regions: close to the tips, inside the pores. The method, which allows control over tip length, was successfully applied to various gold nanoparticle shapes, leading to materials with highly efficient SERS performance. The obtained nanoparticles are stable in ethanol and water upon thermal consolidation and can be safely stored as a powder.

Abstract: One of the ultimate goals in the study of metal clusters is the correlation between the atomic-scale organization and their physicochemical properties. However, direct observation of the atomic organization of such minuscule metal clusters is heavily hindered by radiation damage imposed by the different characterization techniques. We present direct evidence of the structural arrangement, at an atomic level, of luminescent silver species stabilized in faujasite (FAU) zeolites using aberration-corrected scanning transmission electron microscopy. Two different silver clusters were identified in Ag-FAU zeolites, a trinuclear silver species associated with green emission and a tetranuclear silver species related to yellow emission. By combining direct imaging with complementary information obtained from X-ray powder diffraction and Rietveld analysis, we were able to elucidate the main differences at an atomic scale between luminescent (heat-treated) and nonluminescent (cation-exchanged) Ag-FAU zeolites. It is expected that such insights will trigger the directed synthesis of functional metal nanocluster-zeolite composites with tailored luminescent properties.

Abstract: Herein we demonstrate that meticulous and in-depth analysis of the reaction mechanisms of nanoparticle formation is rewarded by full control of size, shape and crystal structure of superparamagnetic iron oxide nanocrystals during synthesis. Starting from two iron sources – iron(II)- and iron(III) carbonate -a strict separation of oleate formation from the generation of reactive pyrolysis products and concomitant nucleation of iron oxide nanoparticles was achieved. This protocol enabled us to analyze each step of nanoparticle formation independently in depth. Progress of the entire reaction was monitored via matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and gas chromatography (GC) gaining insight into the formation of various iron oleate species prior to nucleation. Interestingly, due to the intrinsic strongly reductive pyrolysis conditions of the oleate intermediates and redox process in early stages of the synthesis, pristine iron oxide nuclei were composed exclusively from wustite, irrespective of the oxidation state of the iron source. Controlling the reaction conditions provided a very broad range of size- and shape defined monodisperse iron oxide nanoparticles. Curiously, after nucleation star shaped nanocrystals were obtained, which underwent metamorphism towards cubic shaped particles. EELS tomography revealed ex post oxidation of the primary wustite nanocrystal providing a full 3D image of Fe2+ and Fe3+ distribution within. Overall, we developed a highly flexible synthesis, yielding multigram amounts of well-defined iron oxide nanocrystals of different sizes and morphologies.