Stability and Reversible Oxidation of Sub-Nanometric Cu5 Metal Clusters: Integrated Experimental Study and Theoretical Modeling.

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude "breathing" motions. A chemical phase diagram for Cu oxidation states of the Cu5 -oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu.

Quantitative, Spectro-kinetic Analysis of Oxygen in Electron-Beam Sensitive, Multimetallic Oxide Nanostructures.

The oxygen stoichiometry of hollandite, KxMnO2-δ, nanorods has been accurately determined from a quantitative analysis of scanning-transmission electron microscopy (STEM) X-Ray Energy Dispersive Spectroscopy (XEDS) experiments carried out in chrono-spectroscopy mode. A methodology combining 3D reconstructions of high-angle annular dark field electron tomography experiments, using compressed-sensing algorithms, and quantification through the so-called ζ-factors method of XEDS spectra recorded on a high-sensitivity detector has been devised to determine the time evolution of the oxygen content of nanostructures of electron-beam sensitive oxides. Kinetic modeling of O-stoichiometry data provided K0.13MnO1.98 as overall composition for nanorods of the hollandite. The quantitative agreement, within a 1% mol error, observed with results obtained by macroscopic techniques (temperature-programmed reduction and neutron diffraction) validate the proposed methodology for the quantitative analysis, at the nanoscale, of light elements, as it is the case of oxygen, in the presence of heavy ones (K, Mn) in the highly compromised case of nanostructured materials which are prone to electron-beam reduction. Moreover, quantitative comparison of oxygen evolution data measured at macroscopic and nanoscopic levels allowed us to rationalize beam damage effects in structural terms and clarify the exact nature of the different steps involved in the reduction of these oxides with hydrogen.

Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids.

Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal-organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.

Nanoscale Anatomy of Iron-Silica Self-Organized Membranes: Implications for Prebiotic Chemistry.

Iron-silica self-organized membranes, so-called chemical gardens, behave as fuel cells and catalyze the formation of amino/carboxylic acids and RNA nucleobases from organics that were available on early Earth. Despite their relevance for prebiotic chemistry, little is known about their structure and mineralogy at the nanoscale. Studied here are focused ion beam milled sections of iron-silica membranes, grown from synthetic and natural, alkaline, serpentinization-derived fluids thought to be widespread on early Earth. Electron microscopy shows they comprise amorphous silica and iron nanoparticles of large surface areas and inter/intraparticle porosities. Their construction resembles that of a heterogeneous catalyst, but they can also exhibit a bilayer structure. Surface-area measurements suggest that membranes grown from natural waters have even higher catalytic potential. Considering their geochemically plausible precipitation in the early hydrothermal systems where abiotic organics were produced, iron-silica membranes might have assisted the generation and organization of the first biologically relevant organics.

Regioselective generation and reactivity control of subnanometric platinum clusters in zeolites for high-temperature catalysis.

Subnanometric metal species (single atoms and clusters) have been demonstrated to be unique compared with their nanoparticulate counterparts. However, the poor stabilization of subnanometric metal species towards sintering at high temperature (>500 °C) under oxidative or reductive reaction conditions limits their catalytic application. Zeolites can serve as an ideal support to stabilize subnanometric metal catalysts, but it is challenging to localize subnanometric metal species on specific sites and modulate their reactivity. We have achieved a very high preference for localization of highly stable subnanometric Pt and PtSn clusters in the sinusoidal channels of purely siliceous MFI zeolite, as revealed by atomically resolved electron microscopy combining high-angle annular dark-field and integrated differential phase contrast imaging techniques. These catalysts show very high stability, selectivity and activity for the industrially important dehydrogenation of propane to form propylene. This stabilization strategy could be extended to other crystalline porous materials.

Regioselective Generation of Single-Site Iridium Atoms and Their Evolution into Stabilized Subnanometric Iridium Clusters in MWW Zeolite.

Preparation of supported metal catalysts with uniform particle size and coordination environment is a challenging and important topic in materials chemistry and catalysis. In this work, we report the regioselective generation of single-site Ir atoms and their evolution into stabilized subnanometric Ir clusters in MWW zeolite, which are located at the 10MR window connecting the two neighboring 12MR supercages. The size of the subnanometric Ir clusters can be controlled by the post-synthesis treatments and maintain below 1 nm even after being reduced at 650 °C, which cannot be readily achieved with samples prepared by conventional impregnation methods. The high structure sensitivity, size-dependence, of catalytic performance in the alkane hydrogenolysis reaction of Ir clusters in the subnanometric regime is evidenced.

Apoferritin Protein Amyloid Fibrils with Tunable Chirality and Polymorphism.

Ferritin, a soluble and highly robust protein with subunits packed into well-defined helices, is a key component of the iron regulatory system in the brain and thus is widely recognized as a crucial protein for iron metabolism, but may also bear possible implications in some neurodegenerative disorders. Here, we present evidence of how human recombinant apoferritin can convert into an unusual structure from its folded native state; that is, amyloid fibrils analogue to those found in pathological disorders such as Alzheimer's and Parkinson's diseases. An extensive combination of advanced microscopy, spectroscopy and scattering techniques concur to reveal that apoferritin fibrils possess a common double stranded twisted ribbon structure which can result in a mesoscopic right-handed chirality. We highlight a direct connection between the chirality and morphology of the resulting amyloid fibrils, and the initial protein subunits composition, advancing our understanding on the possible role of misfolding in some ferritin-related pathologies and posing new bases for the design of chiral 1D functional nanostructures.

Confined Pt1 1+ Water Clusters in a MOF Catalyze the Low-Temperature Water-Gas Shift Reaction with both CO2 Oxygen Atoms Coming from Water.

The synthesis and reactivity of single metal atoms in a low-valence state bound to just water, rather than to organic ligands or surfaces, is a major experimental challenge. Herein, we show a gram-scale wet synthesis of Pt1 1+ stabilized in a confined space by a crystallographically well-defined first water sphere, and with a second coordination sphere linked to a metal-organic framework (MOF) through electrostatic and H-bonding interactions. The role of the water cluster is not only isolating and stabilizing the Pt atoms, but also regulating the charge of the metal and the adsorption of reactants. This is shown for the low-temperature water-gas shift reaction (WGSR: CO + H2 O → CO2 + H2 ), where both metal coordinated and H-bonded water molecules trigger a double water attack mechanism to CO and give CO2 with both oxygen atoms coming from water. The stabilized Pt1+ single sites allow performing the WGSR at temperatures as low as 50 °C.

Synthesis of Densely Packaged, Ultrasmall Pt02 Clusters within a Thioether-Functionalized MOF: Catalytic Activity in Industrial Reactions at Low Temperature.

The gram-scale synthesis, stabilization, and characterization of well-defined ultrasmall subnanometric catalytic clusters on solids is a challenge. The chemical synthesis and X-ray snapshots of Pt02 clusters, homogenously distributed and densely packaged within the channels of a metal-organic framework, is presented. This hybrid material catalyzes efficiently, and even more importantly from an economic and environmental viewpoint, at low temperature (25 to 140 °C), energetically costly industrial reactions in the gas phase such as HCN production, CO2 methanation, and alkene hydrogenations. These results open the way for the design of precisely defined catalytically active ultrasmall metal clusters in solids for technically easier, cheaper, and dramatically less-dangerous industrial reactions.

Direct Synthesis of Highly Conductive tert-Butylthiol-Capped CuInS2 Nanocrystals.

tert-butylthiol (tBuSH) is used as the sulfur source, surface ligand and co-solvent in the synthesis of CuInS2 nanocrystals (NCs). The presented method gives direct access to short-ligand-capped NCs without post-synthetic ligand exchange. The obtained 5 nm CuInS2 NCs crystallize in the cubic sphalerite phase with space group F-43m and a lattice parameter a=5.65 Å. Their comparably large optical and electrochemical band gap of 2.6-2.7 eV is attributed to iodine incorporation into the crystal structure as reflected by the composition Cu1.04 In0.96 S1.84 I0.62 determined by EDX. Conductivity measurements on thin films of the tBuSH-capped NCs result in a value of 2.5(.) 10(-2)  S m(-1) , which represents an increase by a factor of 400 compared to established dodecanethiol-capped CuInS2 NCs.

Strain Field in Ultrasmall Gold Nanoparticles Supported on Cerium-Based Mixed Oxides. Key Influence of the Support Redox State.

Using a method that combines experimental and simulated Aberration-Corrected High Resolution Electron Microscopy images with digital image processing and structure modeling, strain distribution maps within gold nanoparticles relevant to real powder type catalysts, i.e., smaller than 3 nm, and supported on a ceria-based mixed oxide have been determined. The influence of the reduction state of the support and particle size has been examined. In this respect, it has been proven that reduction even at low temperatures induces a much larger compressive strain on the first {111} planes at the interface. This increase in compression fully explains, in accordance with previous DFT calculations, the loss of CO adsorption capacity of the interface area previously reported for Au supported on ceria-based oxides.

Synthesis, Internal Structure, and Formation Mechanism of Monodisperse Tin Sulfide Nanoplatelets.

Tin sulfide nanoparticles have a great potential for use in a broad range of applications related to solar energy conversion (photovoltaics, photocatalysis), electrochemical energy storage, and thermoelectrics. The development of chemical synthesis methods allowing for the precise control of size, shape, composition, and crystalline phase is essential. We present a novel approach giving access to monodisperse square SnS nanoplatelets, whose dimensions can be adjusted in the range of 4-15 nm (thickness) and 15-100 nm (edge length). Their growth occurs via controlled assembly of initially formed polyhedral seed nanoparticles, which themselves originate from an intermediate tetrachlorotin-oleate complex. The SnS nanoplatelets crystallize in the α-SnS orthorhombic herzenbergite structure (space group Pnma) with no evidence of secondary phases. Electron tomography, high angle annular dark field scanning transmission electron microscopy and electron diffraction combined with image simulations evidence the presence of ordered Sn vacancy rich (100) planes within the SnS nanoplatelets, in accordance with their slightly S-rich composition observed. When using elemental sulfur instead of thioacetamide as the sulfur source, the same reaction yields small (2-3 nm) spherical SnS2 nanoparticles, which crystallize in the berndtite 4H crystallographic phase (space group P3m1). They exhibit quantum confinement (E(g) = 2.8 eV vs 2.2 eV in the bulk) and room temperature photoluminescence. By means of electrochemical measurements we determined their electron affinity EA = -4.8 eV, indicating the possibility to use them as a substitute for CdS (EA = -4.6 eV) in the buffer layer of thin film solar cells.

Gold nanoparticle-coated apoferritin conductive nanowires.

Gold-metallic nanofibrils were prepared from three different iso-apoferritin (APO) proteins with different Light/Heavy (L/H) subunit ratios (from 0% up to 100% L-subunits). We show that APO protein fibrils have the ability to in situ nucleate and grow gold nanoparticles (AuNPs) simultaneously assembled on opposite strands of the fibrils, forming hybrid inorganic-organic metallic nanowires. The AuNPs are arranged following the pitch of the helical APO protein fiber. The mean size of the AuNPs was similar in the three different APO protein fibrils studied in this work. The AuNPs retained their optical properties in these hybrid systems. Conductivity measurements showed ohmic behavior like that of a continuous metallic structure.

Localization and Directionality of Surface Transport in Bi2Te3 Ordered 3D Nanonetworks.

The resistance of an ordered 3D-Bi2Te3 nanowire nanonetwork was studied at low temperatures. Below 50 K the increase in resistance was found to be compatible with the Anderson model for localization, considering that conduction takes place in individual parallel channels across the whole sample. Angle-dependent magnetoresistance measurements showed a distinctive weak antilocalization characteristic with a double feature that we could associate with transport along two perpendicular directions, dictated by the spatial arrangement of the nanowires. The coherence length obtained from the Hikami-Larkin-Nagaoka model was about 700 nm across transversal nanowires, which corresponded to approximately 10 nanowire junctions. Along the individual nanowires, the coherence length was greatly reduced to about 100 nm. The observed localization effects could be the reason for the enhancement of the Seebeck coefficient observed in the 3D-Bi2Te3 nanowire nanonetwork compared to individual nanowires.

Quantitative 3D Characterization of Functionally Relevant Parameters in Heavy-Oxide-Supported 4d Metal Nanocatalysts.

Accurate 3D nanometrology of catalysts with small nanometer-sized particles of light 3d or 4d metals supported on high-atomic-number oxides is crucial for understanding their functionality. However, performing quantitative 3D electron tomography analysis on systems involving metals like Pd, Ru, or Rh supported on heavy oxides (e.g., CeO2) poses significant challenges. The low atomic number (Z) of the metal complicates discrimination, especially for very small nanoparticles (1-3 nm). Conventional reconstruction methods successful for catalysts with 5d metals (e.g., Au, Pt, or Ir) fail to detect 4d metal particles in electron tomography reconstructions, as their contrasts cannot be effectively separated from those of the underlying support crystallites. To address this complex 3D characterization challenge, we have developed a full deep learning (DL) pipeline that combines multiple neural networks, each one optimized for a specific image-processing task. In particular, single-image super-resolution (SR) techniques are used to intelligently denoise and enhance the quality of the tomographic tilt series. U-net generative adversarial network algorithms are employed for image restoration and correcting alignment-related artifacts in the tilt series. Finally, semantic segmentation, utilizing a U-net-based convolutional neural network, splits the 3D volumes into their components (metal and support). This approach enables the visualization of subnanometer-sized 4d metal particles and allows for the quantitative extraction of catalytically relevant structural information, such as particle size, sphericity, and truncation, from compressed sensing electron tomography volume reconstructions. We demonstrate the potential of this approach by characterizing nanoparticles of a metal widely used in catalysis, Pd (Z = 46), supported on CeO2, a very high density (7.22 g/cm3) oxide involving a quite high-atomic-number element, Ce (Z = 58).

Generation of Subnanometer Metal Clusters in Silicoaluminate Zeolites as Bifunctional Catalysts.

Zeolite-encapsulated subnanometer metal catalysts are an emerging class of solid catalysts with superior performances in comparison to metal catalysts supported on open-structure solid carriers. Currently, there is no general synthesis methodology for the encapsulation of subnanometer metal catalysts in different zeolite structures. In this work, we will show a general synthesis method for the encapsulation of subnanometer metal clusters (Pt, Pd, and Rh) within various silicoaluminate zeolites with different topologies (MFI, CHA, TON, MOR). The successful generation of subnanometer metal species in silicoaluminate zeolites relies on the introduction of Sn, which can suppress the migration of subnanometer metal species during high-temperature oxidation-reduction treatments according to advanced electron microscopy and spectroscopy characterizations. The advantage of encapsulated subnanometer Pt catalysts in silicoaluminate zeolites is reflected in the direct coupling of ethane and benzene for production of ethylbenzene, in which the Pt and the acid sites work in a synergistic way.

Direct assessment of confinement effect in zeolite-encapsulated subnanometric metal species.

Subnanometric metal species confined inside the microporous channels/cavities of zeolites have been demonstrated as stable and efficient catalysts. The confinement interaction between the metal species and zeolite framework has been proposed to play the key role for stabilization, though the confinement interaction is elusive to be identified and measured. By combining theoretical calculations, imaging simulation and experimental measurements based on the scanning transmission electron microscopy-integrated differential phase contrast imaging technique, we have studied the location and coordination environment of isolated iridium atoms and clusters confined in zeolite. The image analysis results indicate that the local strain is intimately related to the strength of metal-zeolite interaction and a good correlation is found between the zeolite deformation energy, the charge state of the iridium species and the local absolute strain. The direct observation of confinement with subnanometric metal species encapsulated in zeolites provides insights to understand their structural features and catalytic consequences.

Spontaneous Hetero-attachment of Single-Component Colloidal Precursors for the Synthesis of Asymmetric Au-Ag2X (X = S, Se) Heterodimers.

Finding simple, easily controlled, and flexible synthetic routes for the preparation of ternary and hybrid nanostructured semiconductors is always highly desirable, especially to fulfill the requirements for mass production to enable application to many fields such as optoelectronics, thermoelectricity, and catalysis. Moreover, understanding the underlying reaction mechanisms is equally important, offering a starting point for its extrapolation from one system to another. In this work, we developed a new and more straightforward colloidal synthetic way to form hybrid Au-Ag2X (X = S, Se) nanoparticles under mild conditions through the reaction of Au and Ag2X nanostructured precursors in solution. At the solid-solid interface between metallic domains and the binary chalcogenide domains, a small fraction of a ternary AuAg3X2 phase was observed to have grown as a consequence of a solid-state electrochemical reaction, as confirmed by computational studies. Thus, the formation of stable ternary phases drives the selective hetero-attachment of Au and Ag2X nanoparticles in solution, consolidates the interface between their domains, and stabilizes the whole hybrid Au-Ag2X systems.

Cytotoxic sub-nanometer aqueous platinum clusters as potential antitumoral agents.

Ligand-free sub-nanometer metal clusters (MCs) of Pt, Ir, Rh, Au and Cu, are prepared here in neat water and used as extremely active (nM) antitumoral agents for HeLa and A2870 cells. The preparation just consists of adding the biocompatible polymer ethylene-vinyl alcohol (EVOH) to an aqueous solution of the corresponding metal salt, to give liters of a MC solution after filtration of the polymer. Since the MC solution is composed of just neat metal atoms and water, the intrinsic antitumoral activity of the different sub-nanometer metal clusters can now fairly be evaluated. Pt clusters show an IC50 of 0.48 µM for HeLa and A2870 cancer cells, 23 times higher than that of cisplatin and 1000 times higher than that of Pt NPs, and this extremely high cytotoxicity also occurs for cisplatin-resistant (A2870 cis) cells, with a resistance factor of 1.4 (IC50 = 0.68 µM). Rh and Ir clusters showed an IC50 ∼ 1 µM. Combined experimental and computational studies support an enhanced internalization and cytotoxic activation.

Tutorial: structural characterization of isolated metal atoms and subnanometric metal clusters in zeolites.

The encapsulation of subnanometric metal entities (isolated metal atoms and metal clusters with a few atoms) in porous materials such as zeolites can be an effective strategy for the stabilization of those metal species and therefore can be further used for a variety of catalytic reactions. However, owing to the complexity of zeolite structures and their low stability under the electron beam, it is challenging to obtain atomic-level structural information of the subnanometric metal species encapsulated in zeolite crystallites. In this protocol, we show the application of a scanning transmission electron microscopy (STEM) technique that records simultaneously the high-angle annular dark-field (HAADF) images and integrated differential phase-contrast (iDPC) images for structural characterization of subnanometric Pt and Sn species within MFI zeolite. The approach relies on the use of a computational model to simulate results obtained under different conditions where the metals are present in different positions within the zeolite. This imaging technique allows to obtain simultaneously the spatial information of heavy elements (Pt and Sn in this work) and the zeolite framework structure, enabling direct determination of the location of the subnanometric metal species. Moreover, we also present the combination of other spectroscopy techniques as complementary tools for the STEM-iDPC imaging technique to obtain global understanding and insights on the spatial distributions of subnanometric metal species in zeolite structure. These structural insights can provide guidelines for the rational design of uniform metal-zeolite materials for catalytic applications.