Atomic-Scale Surface Segregation in Copper-Gold Nanoparticles.

We combine electron microscopy measurements of the surface compositions in Cu-Au nanoparticles and atomistic simulations to investigate the effect of gold segregation. While this mechanism has been extensively investigated within Cu-Au in the bulk state, it was never studied at the atomic level in nanoparticles. By using energy dispersive x-ray analysis across the (100) and (111) facets of nanoparticles, we provide evidence of gold segregation in Cu_{3}Au and CuAu_{3} nanoparticles in the 10 nm size range grown by epitaxy on a salt surface with high control of the nanoparticles morphology. To get atomic-scale insights into the segregation properties in Cu-Au nanoparticles on the whole composition range, we perform Monte Carlo calculations employing N-body interatomic potentials highlighting a complete segregation of Au in the (100) and (111) facets for gold nominal composition above 70% and 60%, respectively. Furthermore, we show that there is no size effect on the segregation behavior since we evidence the same oscillating concentration profile from the surface to the nanoparticle's core as in the bulk. These results shed new light on the interpretation of the enhanced reactivity, selectivity, and stability of Cu-Au nanoparticles in various catalytic reactions.

Synthesis and structural properties of high-entropy nanoalloys made by physical and chemical routes.

The development of synthesis methods with enhanced control over the composition, size and atomic structure of High Entropy Nano-Alloys (HENA) could give rise to a new repertoire of nanomaterials with unprecedented functionalities, notably for mechanical, catalytic or hydrogen storage applications. Here, we have developed two original synthesis methods, one by a chemical route and the other by a physical one, to fabricate HENA with a size between 3 and 10 nm and a face centered cubic structure containing three (CoNiPt), four (CoNiPtCu and CoNiPtAu) or five (CoNiPtAuCu) metals close to the equiatomic composition. The key point in the proposed chemical synthesis method is to compensate the difference in reactivity of the different metal precursors by increasing the synthesis temperature using high boiling solvents. Physical syntheses were performed by pulsed laser ablation using a precise alternating deposition of the individual metals on a heated amorphous carbon substrate. Finally, we have exploited aberration-corrected transmission electron microscopy to explore the nanophase diagram of these nanostructures and reveal intrinsic thermodynamic properties of those complex nanosystems. In particular, we have shown (i) that the complete mixing of all elements can only occur close to the equiatomic composition and (ii) how the Ostwald ripening during HENA synthesis can induce size-dependent deviations from the equiatomic composition leading to the formation of large core-shell nanoparticles.

Revealing Size Dependent Structural Transitions in Supported Gold Nanoparticles in Hydrogen at Atmospheric Pressure.

The enhancement of the catalytic activity of gold nanoparticles with their decreasing size is often attributed to the increasing proportion of low-coordinated surface sites. This correlation is based on the paradigmatic picture of working gold nanoparticles as perfect crystal forms having complete and static outer surface layers whatever their size. This picture is incomplete as catalysts can dynamically change their structure according to the reaction conditions and as such changes can be eventually size-dependent. In this work, using aberration-corrected environmental electron microscopy, size-dependent crystal structure and morphological evolution in gold nanoparticles exposed to hydrogen at atmospheric pressure, with loss of the face-centered cubic crystal structure of gold for particle size below 4 nm, are revealed for the first time. Theoretical calculations highlight the role of mobile gold atoms in the observed symmetry changes and particle reshaping in the critical size regime. An unprecedented stable surface molecular structure of hydrogenated gold decorating a highly distorted core is identified. By combining atomic scale in situ observations and modeling of nanoparticle structure under relevant reaction conditions, this work provides a fundamental understanding of the size-dependent reactivity of gold nanoparticles with a precise picture of their surface at working conditions.

Quantitative In Situ Visualization of Thermal Effects on the Formation of Gold Nanocrystals in Solution.

Understanding temperature effects in nanochemistry requires real-time in situ measurements because this key parameter of wet-chemical synthesis simultaneously influences the kinetics of chemical reactions and the thermodynamic equilibrium of nanomaterials in solution. Here, temperature-controlled liquid cell transmission electron microscopy is exploited to directly image the radiolysis-driven formation of gold nanoparticles between 25 °C and 85 °C and provide a deeper understanding of the atomic-scale processes determining the size and shape of gold colloids. By quantitatively comparing the nucleation and growth rates of colloidal assemblies with classical models for nanocrystal formation, it is shown that the increase of the molecular diffusion and the solubility of gold governs the drastic changes in the formation dynamics of nanostructures in solution with temperature. In contraction with the common view of coarsening processes in solution, it is also demonstrated that the dissolution of nanoparticles and thus the Ostwald ripening is not only driven by size effects. Furthermore, visualizing thermal effects on faceting processes at the single nanoparticle level reveals how the competition between the growth speed and the surface diffusion dictates the final shape of nanocrystals.

Corrosion Products Formed on MgZr Alloy Embedded in Geopolymer Used as Conditioning Matrix for Nuclear Waste-A Proposition of Interconnected Processes.

Geopolymer has been selected as a hydraulic mineral binder for the immobilization of MgZr fuel cladding coming from the dismantling of French Uranium Natural Graphite Gas reactor dedicated to a geological disposal. In this context, the corrosion processes and the nature of the corrosion products formed on MgZr alloy in a geopolymer matrix with and without the corrosion inhibitor NaF have been determined using a multiscale approach combining in situ Grazing Incidence hard X-ray Diffraction, Raman microspectroscopy, Scanning and Transmission Electron Microscopies coupled to Energy Dispersive X-ray Spectroscopy. The composition, the morphology, and the porous texture of the corrosion products were characterized, and the effect of the corrosion inhibitor NaF was evidenced. The results highlighted the formation of Mg(OH)2-xFx. In addition, in presence of NaF, NaMgF3 forms leading to a decrease of the thickness and the porosity of the corrosion products layer. Moreover, a precipitation of magnesium silicates within the porosity of the geopolymer was evidenced. Finally, we propose a detailed set of interconnected processes occurring during the MgZr corrosion in the geopolymer.

Studying the Effects of Temperature on the Nucleation and Growth of Nanoparticles by Liquid-Cell Transmission Electron Microscopy.

Temperature control is a recent development that provides an additional degree of freedom to study nanochemistry by liquid cell transmission electron microscopy. In this paper, we describe how to prepare an in situ heating experiment for studying the effect of temperature on the formation of gold nanoparticles driven by radiolysis in water. The protocol of the experiment is fairly simple involving a special liquid cell with uniform heating capabilities up to 100 °C, a liquid-cell TEM holder with flow capabilities and an integrated interface for controlling the temperature. We show that the nucleation and growth mechanisms of gold nanoparticles are drastically impacted by the temperature in liquid cell. Using STEM imaging and nanodiffraction, the evolution of the density, size, shape and atomic structure of the growing nanoparticles are revealed in real time. Automated image processing algorithms are exploited to extract useful quantitative data from video sequences, such as the nucleation and growth rates of nanoparticles. This approach provides new inputs for understanding the complex physico-chemical processes at play during the liquid-phase synthesis of nanomaterials.

Introducing cobalt as a potential plasmonic candidate combining optical and magnetic functionalities within the same nanostructure.

The control of magnetic properties at the nanoscale is a current topic of intense research. It was shown that combining both magnetic and plasmonic nanoparticles (NPs) led to the improvement of their magneto-optical signal. In this context, common strategies consist of the design of bimetallic NPs. However, the understanding of the physics leading to the coupling between magnetic and plasmonic NPs is lacking, preventing any significant progress for the development of future photonic devices. In this article, we propose to focus our attention on an efficient and commonly used magnetic metal, cobalt, and evaluate its plasmonic properties at the nanoscale through the use of NP regular arrays, as a potential candidate combining both optical and magnetic functionalities within the same metal. We show that such NPs display plasmonic properties within a large spectral range from the UV to the NIR spectral range, with efficient quality factors, when the inter-particle distance is properly selected. These as-fabricated simple materials could find applications in integrated photonic devices for telecommunications.

Selective shortening of gold nanorods: when surface functionalization dictates the reactivity of nanostructures.

The selective shortening of gold nanorods (NRs) is a directional etching process that has been intensively studied by UV-Vis spectroscopy because of its direct impact on the optical response of these plasmonic nanostructures. Here, liquid-cell transmission electron microscopy is exploited to visualize this peculiar corrosion process at the nanoscale and study the impacts of reaction kinetics on the etching mechanisms. In situ imaging reveals that anisotropic etching requires a chemical environment with a low etching power to make the tips of NRs the only reaction site for the oxidation process. Then, aberration-corrected TEM and atomistic simulations were combined to demonstrate that the disparity between the reactivity of the body and the ends of NRs does not derive from their crystal structure but results from an inhomogeneous surface functionalization. In a general manner, this work highlights the necessity to consider the organic/inorganic natures of nanostructures to understand their chemical reactivity.

On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts.

Fe-N-C catalysts containing atomic FeNx sites are promising candidates as precious-metal-free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of Fe-N-C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe-N-C structure and four-times higher ORR activity loss when performing load cycling AST in O2 - vs. Ar-saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O2 , even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load-cycling AST in O2 leads to loss of a significant fraction of FeNx sites, as shown by energy dispersive X-ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O2 is due to reactive oxygen species produced between H2 O2 and Fe sites via Fenton reactions.

Nanostructured Nickel Aluminate as a Key Intermediate for the Production of Highly Dispersed and Stable Nickel Nanoparticles Supported within Mesoporous Alumina for Dry Reforming of Methane.

Two routes of preparation of mesoporous Ni-alumina materials favoring the intermediate formation of nanostructured nickel-aluminate are presented. The first one involves an aluminum containing MOF precursor used as sacrificial template to deposit nickel while the second is based on a one-pot synthesis combined to an EISA method. As shown by a set of complementary techniques, the nickel-aluminate nanospecies formed after calcination are homogeneously distributed within the developed mesoporous alumina matrices whose porous characteristics vary depending on the preparation method. A special attention is paid to electron-microscopy observations using especially STEM imaging with high chemical sensitivity and EDS elemental mapping modes that help visualizing the extremely high nickel dispersion and highlight the strong metal anchoring to the support that persists after reduction. This leads to active nickel nanoparticles particularly stable in the reaction of dry reforming of methane.

Reshaping Dynamics of Gold Nanoparticles under H2 and O2 at Atmospheric Pressure.

Despite intensive research efforts, the nature of the active sites for O2 and H2 adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue in heterogeneous catalysis. This stems from the absence of a clear picture of the structural evolution of Au NPs at near reaction conditions, i. e., at high pressures and high temperatures. We hereby report real-space observations of the equilibrium shapes of titania-supported Au NPs under O2 and H2 at atmospheric pressure using gas transmission electron microscopy. In situ TEM observations show instantaneous changes in the equilibrium shape of Au NPs during cooling under O2 from 400 °C to room temperature. In comparison, no instant change in equilibrium shape is observed under a H2 environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O2, atomic oxygen, and H2 is predicted using a multiscale structure reconstruction model. Excellent agreement between TEM observations and theoretical modeling of Au NPs under O2 provides strong evidence for the molecular adsorption of oxygen on the Au NPs below 120 °C on specific Au facets, which are identified in this work. In the case of H2, theoretical modeling predicts no interaction with gold atoms that explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O2 and H2 adsorption properties of Au NPs under real working conditions and shows a way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.

Structural analysis of single nanoparticles in liquid by low-dose STEM nanodiffraction.

Liquid-cell TEM has enabled an interdisciplinary community of scientists to carry out atomic- / nano-scale studies of solid/liquid interfaces. Nevertheless, the restricted resolution of TEM in liquid media and the necessity to reduce the electron dose to avoid harmful radiolytic effects induced by the beam have limited the use of high resolution imaging to study the atomic structure of nanomaterials in liquid. Here we show that STEM nanodiffraction can be exploited in liquid-cell TEM experiments to overcome these two limitations. We evidence that this technique allows quick analysis of the structure of single gold nanoparticles whatever their zone axis orientation, which substantially increases the percentage of analysable nanostructures with respect to HRTEM investigations. Moreover, STEM nanodiffraction can also be used in very low dose conditions. The electron dose irradiating the analyzed nanostructures during data acquisition can be reduced by almost four orders of magnitude compared to conventional HRTEM analysis. Finally, dynamical analyses in reciprocal space are used to provide new insights into the shape-dependent rotation of nanocrystals in the liquid-cell.

Thermodynamics of faceted palladium(-gold) nanoparticles supported on rutile titania nanorods studied using transmission electron microscopy.

Many physical properties of nanoparticles (NPs) are driven by their equilibrium shape (ES). Thus, knowing the kinetic and thermodynamic parameters that affect the particle morphology is key for the rational design of NPs with targeted properties. Here, we report on the thermodynamic ES of supported monometallic palladium and bimetallic palladium-gold (Pd-Au) single-crystalline truncated nano-octahedra (TOs) studied using aberration-corrected transmission electron microscopy (TEM). Monometallic palladium and bimetallic Pd62Au38 and Pd43Au57 TOs were grown by pulsed laser deposition on rutile titania (r-TiO2) nanorods exposing mainly (110) facets. Particle structure and dimension were first obtained from aberration-corrected high resolution TEM (HRTEM) images acquired parallel to the metal-oxide interface. By fitting an extended Wulff-Kaishev rule to the HRTEM data of the truncated octahedral thermodynamic ES in the size range of 2 to 5 nm, we secondly determined the interface and excess line energies associated with the particle-oxide-vacuum triple phase junction in Pd and Pd43Au57 TOs in the epitaxial relationship Pd(-Au)(111)101‖r-TiO2(110)[1-1-1] and in Pd62Au38 TOs in the epitaxial relationship Pd62Au38(100)101‖r-TiO2(110)[1-10]. Our results show a decrease in particle adhesion to the oxide support upon alloying Pd with Au. The loss in adhesion is tentatively attributed to an increase of the lattice strain induced at the metal-oxide interface as gold atoms are added to the palladium lattice.

Monitoring the dynamics of cell-derived extracellular vesicles at the nanoscale by liquid-cell transmission electron microscopy.

Cell-derived extracellular vesicles (EVs) circulating in body fluids hold promises as bioactive therapeutic agents and as biomarkers to diagnose a wide range of diseases. However nano-imaging methods are needed to characterize these complex and heterogeneous soft materials in their native wet environment. Herein, we exploit liquid-cell transmission electron microscopy (LCTEM) to characterize the morphology and dynamic behavior of EVs in physiological media with nanometer resolution. The beam-induced controlled growth of Au nanoparticles on bilayer membranes is used as an original in situ staining method to improve the contrast of EVs and artificial liposomes. LCTEM provides information about the size distribution and concentration of EVs that are consistent with Cryo-TEM and nanoparticle tracking analysis measurements. Moreover, LCTEM gives a unique insight into the dynamics of EVs depending on their liquid environment. The size-dependent morphology of EVs is sensitive to osmotic stress which tends to transform their spherical shape to ellipsoidal, stomatocyte or discocyte morphologies. In the liquid-cell, EVs exhibit a sub-diffusive motion due to strong interactions between the Au nanoparticles and the liquid-cell windows. Finally, the high-resolution monitoring of EV aggregation and fusion illustrate that LCTEM opens up a new way to study cell-membrane dynamics.

Direct Measurement of the Surface Energy of Bimetallic Nanoparticles: Evidence of Vegard's Rulelike Dependence.

We use in situ transmission electron microscopy to monitor in real time the evaporation of gold, copper, and bimetallic copper-gold nanoparticles at high temperature. Besides, we extend the Kelvin equation to two-component systems to predict the evaporation rates of spherical liquid mono- and bimetallic nanoparticles. By linking this macroscopic model to experimental TEM data, we determine the surface energies of pure gold, pure copper, Cu_{50}Au_{50}, and Cu_{25}Au_{75} nanoparticles in the liquid state. Our model suggests that the surface energy varies linearly with the composition in the liquid Cu-Au nanoalloy; i.e., it follows a Vegard's rulelike dependence. To get atomic-scale insights into the thermodynamic properties of Cu-Au alloys on the whole composition range, we perform Monte Carlo simulations employing N-body interatomic potentials. These simulations at a microscopic level confirm the Vegard's rulelike behavior of the surface energy obtained from experiments combined with macroscopic modeling.

Driving reversible redox reactions at solid-liquid interfaces with the electron beam of a transmission electron microscope.

Liquid-cell transmission electron microscopy (LCTEM) has opened up a new way to study chemical reactions at the interface between solids and liquids. However, understanding the effects of the electron beam in the liquid cell has been clearly identified as one of the most important challenges to assess correctly and quantitatively LCTEM data. Here we show that the electron beam can be used to drive reversible deposition/dissolution cycles of copper shells over gold nanoparticles in methanol. Besides revealing the influence of irreversible processes on the kinetic of growth/etching cycles, this study of nanostructure behaviour as a function of the dose rate highlights the possibility to switch the oxidising or reducing nature of liquid environment only with the electron beam. The chemical and electronic processes possibly involved in these tunable redox reactions are qualitatively discussed together with their possible impacts on electrochemical LCTEM experiments.

Exploring the Formation of Symmetric Gold Nanostars by Liquid-Cell Transmission Electron Microscopy.

The shape-dependent properties of gold nanostars (NSs) have motivated massive research efforts in the field of colloidal chemistry to gain a better control over the morphology of these promising nanostructures. Nevertheless, this challenge requires a better understanding of the atomic-scale processes leading to the formation of stellated nanoparticles. We hereby report an unprecedented in situ study focused on the seed-mediated synthesis of symmetric gold NSs performed by radiolysis in methanol. We take advantage of the spatial and temporal resolutions of liquid-cell transmission electron microscopy to unravel the key effects of the growth speed, seed-crystal morphology, and dimethylamine functionalization on the formation mechanisms, shape, and stability of NSs enclosed by high-index facets. Surprisingly, the stellation processes transforming icosahedral nanoparticles into NSs with 20 sharp arms entails a continuous restructuring of NS facets driven by surface diffusion, which provide a fresh look at faceting mechanisms.

Atomic-Scale Snapshots of the Formation and Growth of Hollow PtNi/C Nanocatalysts.

Determining the formation and growth mechanism of bimetallic nanoparticles (NPs) with atomic detail is fundamental to synthesize efficient "catalysts by design". However, an understanding of the elementary steps which take place during their synthesis remains elusive. Herein, we have exploited scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy, operando wide angle and small-angle X-ray scattering, and electrochemistry to unveil the formation and growth mechanism of hollow PtNi/C NPs. Such NPs, composed of a PtNi shell surrounding a nanoscale void, catalyze efficiently and sustainably the oxygen reduction reaction (ORR) in an acidic electrolyte. Our step-by-step study reveals that (i) Ni-rich/C NPs form first, before being embedded in a NixByOz shell, (ii) the combined action of galvanic displacement and the nanoscale Kirkendall effect then results in the sequential formation of Ni-rich core@Pt-rich/C shell and ultimately hollow PtNi/C NPs. The electrocatalytic properties for the ORR and the stability of the different synthesis intermediates were tested and structure-activity-stability relationships established both in acidic and alkaline electrolytes. Beyond its interest for the ORR electrocatalysis, this study also presents a methodology that is capable to unravel the formation and growth mechanism of various nanomaterials including preferentially shaped metal NPs, core@shell NPs, onion-like NPs, Janus NPs, or a combination of several of these structures.

Ostwald-Driven Phase Separation in Bimetallic Nanoparticle Assemblies.

The compositional stability of bimetallic nanoparticles (NPs) is crucial for many applications. We have studied the coarsening of amorphous carbon-supported Au-Pd NPs during annealing at 873 K. Using scanning transmission electron microscopy and energy-dispersive spectroscopy measurements, we show that, despite a complete miscibility of the two metals, the particle assembly undergoes a phase separation during annealing, which leads to two distinct populations: Au-rich NPs with a mean radius of 3.5 nm and large Pd-rich NPs with a mean radius of 25 nm. Thermodynamic calculations and kinetic Monte Carlo simulations explain this behavior that is driven by the competition between surface and mixing energy and by the different mobilities of the two atomic species.