Abstract: Amino acids have huge potential as platform chemicals in the biobased industry. Pd-catalyzed decarboxylation is a very promising route for the valorization of these natural compounds derived from protein waste or fermentation. We report that the highly abundant and nonessential amino acid L-proline is very reactive in the Pd-catalyzed decarboxylation. Full conversions are obtained with Pd/C and different Pd/MeOx catalysts; this allowed the identification of the different side reactions and the mapping of the reaction network. Due to the high reactivity of pyrrolidine, the selectivity for pyrrolidine was initially low. By carefully modifying Pd/ZrO2 with Pb in a controlled manner-via two incipient wetness impregnation steps-the selectivity increased remarkably. Finally, a thorough investigation of the reaction parameters resulted in an increased activity of this modified catalyst and an even further enhanced selectivity under a low H-2 pressure of 4 bar at 235 degrees C in water. This results in a very selective and sustainable production route for the highly interesting pyrrolidine.

Abstract: Bimetallic nanoparticles with tailored size and specific composition have shown promise as stable and selective catalysts for electrochemical reduction of CO2 (CO2R) in batch systems. Yet, limited effort was devoted to understand the effect of ligand coverage and postsynthesis treatments on CO2 reduction, especially under industrially applicable conditions, such as at high currents (>100 mA/cm2) using gas diffusion electrodes (GDE) and flow reactors. In this work, Cu–Ag core–shell nanoparticles (11 ± 2 nm) were prepared with three different surface modes: (i) capped with oleylamine, (ii) capped with monoisopropylamine, and (iii) surfactant free with a reducing borohydride agent; Cu–Ag (OAm), Cu–Ag (MIPA), and Cu–Ag (NaBH4), respectively. The ligand exchange and removal was evidenced by infrared spectroscopy (ATR-FTIR) analysis, whereas high-resolution scanning transmission electron microscopy (HAADF-STEM) showed their effect on the interparticle distance and nanoparticle rearrangement. Later on, we developed a process-on-substrate method to track these effects on CO2R. Cu–Ag (OAm) gave a lower on-set potential for hydrocarbon production, whereas Cu–Ag (MIPA) and Cu–Ag (NaBH4) promoted syngas production. The electrochemical impedance and surface area analysis on the well-controlled electrodes showed gradual increases in the electrical conductivity and active surface area after each surface treatment. We found that the increasing amount of the triple phase boundaries (the meeting point for the electron–electrolyte–CO2 reactant) affect the required electrode potential and eventually the C+2e̅/C2e̅ product ratio. This study highlights the importance of the electron transfer to those active sites affected by the capping agents—particularly on larger substrates that are crucial for their industrial application.

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: Electron tomography (ET) is a powerful tool to determine the three-dimensional (3D) structure of nanomaterials in a transmission electron microscope. However, the acquisition of a conventional tilt series for ET is a time-consuming process and can therefore not provide 3D structural information in a time-efficient manner. Here, we propose surface-sensitive secondary electron (SE) imaging as an alternative to ET for the investigation of the morphology of nanomaterials. We use the SE electron beam induced current (SEEBIC) technique that maps the electrical current arising from holes generated by the emission of SEs from the sample. SEEBIC imaging can provide valuable information on the sample morphology with high spatial resolution and significantly shorter throughput times compared with ET. In addition, we discuss the contrast formation mechanisms that aid in the interpretation of SEEBIC data.

Abstract: Cobalt nanorods possess ideal magnetic properties for applications requiring magnetically hard nanoparticles. However, their exploitation is undermined by their sensitivity toward oxygen and water, which deteriorates their magnetic properties. The development of a continuous metal shell inert to oxidation could render them stable, opening perspectives not only for already identified applications but also for uses in which contact with air and/or aqueous media is inevitable. However, the direct growth of a conformal noble metal shell on magnetic metals is a challenge. Here, we show that prior treatment of Co nanorods with a tin coordination compound is the crucial step that enables the subsequent growth of a continuous noble metal shell on their surface, rendering them air- and water-resistant, while conserving the monocrystallity, metallicity and the magnetic properties of the Co core. Thus, the as-synthesized coreshell ferromagnetic nanorods combine high magnetization and strong uniaxial magnetic anisotropy, even after exposure to air and water, and hold promise for successful implementation in in vitro biodiagnostics requiring probes of high magnetization and anisotropic shape.

Abstract: The ability to determine the atomic arrangement and termination of various facets of surfactant-coated nanocrystals is of great importance for understanding their growth mechanism and their surface properties and represents a critical piece of information that can be coupled to other experimental techniques and to calculations. This is especially appealing in the study of nanocrystals that can be grown in strongly anisotropic shapes, for which the relative growth rates of various facets can be influenced under varying reaction conditions. Here we show that in two representative cases of rod-shaped nanocrystals in the wurtzite phase (CdSe(core)/CdS(shell) and ZnSe(core)/ZnS(shell) nanorods) the terminations of the polar facets can be resolved unambiguously by combining advanced electron microscopy techniques, such as aberration-corrected HRTEM with exit wave reconstruction or aberration-corrected HAADF-STEM. The [0001] and [000-1] polar directions of these rods, which grow preferentially along their c-axis, are revealed clearly, with one side consisting of the Cd (or Zn)-terminated (0001) facet and the other side with a pronounced faceting due to Cd (or Zn)-terminated {10-1-1} facets. The lateral faceting of the rods is instead dominated by three nonpolar {10-10} facets. The core buried in the nanostructure can be localized in both the exit wave phase and HAADF-STEM images.

Abstract: Hydrophobic interactions constitute one of the most important types of nonspecific interactions in biological systems, which emerge when water molecules rearrange as two hydrophobic species come close to each other. The prediction of hydrophobic interactions at the level of nanoparticles (Brownian objects) remains challenging because of uncontrolled diffusive motion of the particles. We describe here a general methodology for solvent-induced, reversible self-assembly of gold nanoparticles into 3D clusters with well-controlled sizes. A theoretical description of the process confirmed that hydrophobic interactions are the main driving force behind nanoparticle aggregation.

Abstract: The properties and applications of metallic nanoparticles are inseparably connected not only to their detailed morphology and composition but also to their structural configuration and mutual interactions. As a result, the assemblies often have superior properties as compared to individual nanoparticles. Although it has been reported that nanoparticles can form highly symmetric clusters, if the configuration can be predicted as a function of the synthesis parameters, more targeted and accurate synthesis will be possible. We present here a theoretical model that accurately predicts the structure and configuration of self-assembled gold nanoclusters. The validity of the model is verified using quantitative experimental data extracted from electron tomography 3D reconstructions of different assemblies. The present theoretical model is generic and can in principle be used for different types of nanoparticles, providing a very wide window of potential applications.

Abstract: Using electron tomography combined with electron energy loss spectroscopy (EELS), we are able to map the valency of the Ce ions in CeO2-x nanocrystals in three dimensions. Our results show a clear facet-dependent reduction shell at the surface of ceria nanoparticles; {111} surface facets show a low surface reduction, whereas at {001} surface facets, the cerium ions are more likely to be reduced over a larger surface shell. Our generic tomographic technique allows a full 3D data cube to be reconstructed, containing an EELS spectrum in each voxel. This possibility enables a three-dimensional investigation of a plethora of material-specific physical properties such as valency, chemical composition, oxygen coordination, or bond lengths, triggering the synthesis of nanomaterials with improved properties.

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.

Abstract: Multifunctional metal nanoparticles (NPs) such as anisotropic multimetallic NPs are crucial for boosting nanomaterial based applications. Advanced synthetic protocols exist to make a large variety of such nanostructures. However, a major limiting factor for the usability of them in real life applications is their stability. Here, we show that Au/Pd octopods, 8-branched nanocrystals with Oh symmetry, with only a low amount of Pd exhibited a high thermal stability and maintained strong plasmon resonances up to 600 ◦C. Furthermore, we study the influence of the composition, morphology and environment on the thermal stability and define key parameters for the design of thermally stable multifunctional NPs.

Abstract: Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl3-L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu+ for Ga3+ in Cu2-xS nanocrystals. We find that noncomplexed GaCl3 etches the nanocrystals by S2- extraction, whereas GaCl3-TOP is unreactive. Successful exchange of Cu+ for Ga3+ is only possible when GaCl3 is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu2-xS-GaCl3-L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga3+ insertion and Cu+ extraction can occur. This requisite is only met if GaCl3 is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl3-DPP to convert CuInS2 into (Cu,Ga,In)S-2 nanocrystals, which increases the photoluminescence quantum yield 10 -fold, while blue -shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.

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.