Dissolution Kinetics of Oxidative Etching of Cubic and Icosahedral Platinum Nanoparticles Revealed by <i>in Situ</i> Liquid Transmission Electron Microscopy

Dissolution due to atom-level etching is a major factor for the degradation of Pt-based electrocatalysts used in low-temperature polymer electrolyte membrane fuel cells. Selective surface etching is also used to precisely control shapes of nanoparticles. Dissolution kinetics of faceted metal nanoparticles in solution however is poorly understood despite considerable progress in understanding etching of two-dimensional surfaces. We report here the application of <i>in situ</i> liquid transmission electron microscopy for quantitative analysis of oxidative etching of cubic and icosahedral Pt nanoparticles. The experiment was carried out using a liquid flow cell containing aqueous HAuCl₄ solution. The data show that oxidative etching of these faceted nanocrystals depends on the location of atoms on the surface, which evolves with time. A quantitative kinetic model was developed to account for the mass lost in electrolyte solutions over time, showing the dissolutions followed the power law relationship for Pt nanocrystals of different shapes. Dissolution coefficients of different surface sites were obtained based on the models developed in this study

Dissolution Kinetics of Oxidative Etching of Cubic and Icosahedral Platinum Nanoparticles Revealed by <i>in Situ</i> Liquid Transmission Electron Microscopy

Dissolution due to atom-level etching is a major factor for the degradation of Pt-based electrocatalysts used in low-temperature polymer electrolyte membrane fuel cells. Selective surface etching is also used to precisely control shapes of nanoparticles. Dissolution kinetics of faceted metal nanoparticles in solution however is poorly understood despite considerable progress in understanding etching of two-dimensional surfaces. We report here the application of <i>in situ</i> liquid transmission electron microscopy for quantitative analysis of oxidative etching of cubic and icosahedral Pt nanoparticles. The experiment was carried out using a liquid flow cell containing aqueous HAuCl₄ solution. The data show that oxidative etching of these faceted nanocrystals depends on the location of atoms on the surface, which evolves with time. A quantitative kinetic model was developed to account for the mass lost in electrolyte solutions over time, showing the dissolutions followed the power law relationship for Pt nanocrystals of different shapes. Dissolution coefficients of different surface sites were obtained based on the models developed in this study

Ag–Pt Compositional Intermetallics Made from Alloy Nanoparticles

Intermetallics are compounds with long-range structural order that often lies in a state of thermodynamic minimum. They are usually considered as favorable structures for catalysis due to their high activity and robust stability. However, formation of intermetallic compounds is often regarded as element specific. For instance, Ag and Pt do not form alloy in bulk phase through the conventional metallurgy approach in almost the entire range of composition. Herein, we demonstrate a bottom-up approach to create a new Ag–Pt compositional intermetallic phase from nanoparticles. By thermally treating the corresponding alloy nanoparticles in inert atmosphere, we obtained an intermetallic material that has an exceptionally narrow Ag/Pt ratio around 52/48 to 53/47, and a structure of interchangeable closely packed Ag and Pt layers with 85% on tetrahedral and 15% on octahedral sites. This rather unique stacking results in wavy patterns of Ag and Pt planes revealed by scanning transmission electron microscope (STEM). This Ag–Pt compositional intermetallic phase is highly active for electrochemical oxidation of formic acid at low anodic potentials, 5 times higher than its alloy nanoparticles, and 29 times higher than the reference Pt/C at 0.4 V (vs RHE) in current density

Metastability and Structural Polymorphism in Noble Metals: The Role of Composition and Metal Atom Coordination in Mono- and Bimetallic Nanoclusters

This study examines structural variations found in the atomic ordering of different transition metal nanoparticles synthesized <i>via</i> a common, kinetically controlled protocol: reduction of an aqueous solution of metal precursor salt(s) with NaBH₄ at 273 K in the presence of a capping polymer ligand. These noble metal nanoparticles were characterized at the atomic scale using spherical aberration-corrected scanning transmission electron microscopy (C_s-STEM). It was found for monometallic samples that the third row, face-centered-cubic (fcc), transition metal [(3M)Ir, Pt, and Au] particles exhibited more coherently ordered geometries than their second row, fcc, transition metal [(2M)Rh, Pd, and Ag] analogues. The former exhibit growth habits favoring crystalline phases with specific facet structures while the latter samples are dominated by more disordered atomic arrangements that include complex systems of facets and twinning. Atomic pair distribution function (PDF) measurements further confirmed these observations, establishing that the 3M clusters exhibit longer ranged ordering than their 2M counterparts. The assembly of intracolumn bimetallic nanoparticles (Au–Ag, Pt–Pd, and Ir–Rh) using the same experimental conditions showed a strong tendency for the 3M atoms to template long-ranged, crystalline growth of 2M metal atoms extending up to over 8 nm beyond the 3M core

Growth of Au on Pt Icosahedral Nanoparticles Revealed by Low-Dose In Situ TEM

A growth mode was revealed by an in situ TEM study of nucleation and growth of Au on Pt icosahedral nanoparticles. Quantitative analysis of growth kinetics was carried out based on real-time TEM data, which shows the process involves: (1) deposition of Au on corner sites of Pt icosahedral nanoparticles, (2) diffusion of Au from corners to terraces and edges, and (3) subsequent layer-by-layer growth of Au on Au surfaces to form Pt@Au core–shell nanoparticles. The in situ TEM results indicate diffusion of Au from corner islands to terraces and edges is a kinetically controlled growth, as evidenced by a measurement of diffusion coefficients for these growth processes. We demonstrated that in situ electron microscopy is a valuable tool for quantitative study of nucleation and growth kinetics and can provide new insight into the design and precise control of heterogeneous nanostructures

Direct Synthesis of H₂O₂ on AgPt Octahedra: The Importance of Ag–Pt Coordination for High H₂O₂ Selectivity

H₂O₂ production by direct synthesis (H₂ + O₂ → H₂O₂) is a promising alternative to the energy-intensive anthraquinone oxidation process and to the use of chlorine for oxidation chemistry. Steady-state H₂O₂ selectivities are approximately 10-fold greater on AgPt octahedra (50%) than on Pt nanoparticles of similar size (6%). Moreover, the initial H₂O₂ formation rates and selectivities are sensitive to the fractional coverage of Pt atoms and their location on the surfaces of AgPt octahedra, which can be controlled by exposing these catalysts to either CO or inert gases at 373 K to produce Pt-rich (16% initial H₂O₂ selectivity) or Pt-poor (36% initial H₂O₂ selectivity) surfaces. Increasing the coordination of Pt to Ag significantly modifies the electronic structure of Pt active sites, which is reflected by a shift in the ν­(C=O) singleton frequency in ¹³CO from 2016 cm^–1 on Pt to ∼1975 cm^–1 on AgPt. These bimetallic AgPt catalysts present lower activation enthalpies (<i>Δ<i>H</i></i>^⧧) for H₂O₂ formation (29 kJ mol^–1 on Pt to 5 kJ mol^–1 on AgPt) but a lesser decrease for H₂O formation (26 kJ mol^–1 on Pt to 16 kJ mol^–1 on AgPt). Comparisons of H₂O₂ selectivities, <i>Δ<i>H</i></i>^⧧ values, and differences among the ¹³CO singleton frequencies show that a combination of coordinating Ag to Pt and inducing strain modifies the electronic structure of individual Pt atoms, causing them to bind η¹-species (e.g., CO) more strongly than on Pt nanoparticles. Yet the dramatic increase in the number of isolated Pt atoms increases H₂O₂ selectivities by decreasing the number of Pt atom ensembles of sufficient size to cleave O–O bonds and form H₂O

Nanoscale Spin-State Ordering in LaCoO₃ Epitaxial Thin Films

The nature of magnetic ordering in LaCoO₃ epitaxial thin films has been the subject of considerable debate. We present direct observations of the spin-state modulation of Co ions in LaCoO₃ epitaxial thin films on an atomic scale using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and <i>ab initio</i> calculations based on density functional theory (DFT) calculations. The results of an atomic-resolution STEM/EELS study indicate that the superstructure is not associated with oxygen vacancies; rather, it is associated with a higher spin state of Co³⁺ ions and their ordering. DFT calculations successfully reproduced the modulation of lattice spacing with the introduction of spin ordering. This result identifies the origins of intrinsic phenomena in strained LaCoO₃ and provides fundamental clues for understanding ferromagnetism in Co-based oxides

High-Index Facets in Gold Nanocrystals Elucidated by Coherent Electron Diffraction

Characterization of high-index facets in noble metal nanocrystals for plasmonics and catalysis has been a challenge due to their small sizes and complex shapes. Here, we present an approach to determine the high-index facets of nanocrystals using streaked Bragg reflections in coherent electron diffraction patterns, and provide a comparison of high-index facets on unusual nanostructures such as trisoctahedra. We report new high-index facets in trisoctahedra and previous unappreciated diversity in facet sharpness

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

Passivation Dynamics in the Anisotropic Deposition and Stripping of Bulk Magnesium Electrodes During Electrochemical Cycling

Although rechargeable magnesium (Mg) batteries show promise for use as a next generation technology for high-density energy storage, little is known about the Mg anode solid electrolyte interphase and its implications for the performance and durability of a Mg-based battery. We explore in this report passivation effects engendered during the electrochemical cycling of a bulk Mg anode, characterizing their influences during metal deposition and dissolution in a simple, nonaqueous, Grignard electrolyte solution (ethylmagnesium bromide, EtMgBr, in tetrahydrofuran). Scanning electron microscopy images of Mg foil working electrodes after electrochemical polarization to dissolution potentials show the formation of corrosion pits. The pit densities so evidenced are markedly potential-dependent. When the Mg working electrode is cycled both potentiostatically and galvanostatically in EtMgBr these pits, formed due to passive layer breakdown, act as the foci for subsequent electrochemical activity. Detailed microscopy, diffraction, and spectroscopic data show that further passivation and corrosion results in the anisotropic stripping of the Mg {0001} plane, leaving thin oxide-comprising passivated side wall structures that demark the {0001} fiber texture of the etched Mg grains. Upon long-term cycling, oxide side walls formed due to the pronounced crystallographic anisotropy of the anodic stripping processes, leading to complex overlay anisotropic, columnar structures, exceeding 50 μm in height. The passive responses mediating the growth of these structures appear to be an intrinsic feature of the electrochemical growth and dissolution of Mg using this electrolyte