The crystallographic properties of silver nanoparticles with different morphologies (two different kinds of spheres, cubes, platelets, and rods) were derived from X-ray powder diffraction and electron microscopy. The size of the metallic particle core was determined by scanning electron microscopy, and the colloidal stability and the hydrodynamic particle diameter were analyzed by dynamic light scattering. The preferred crystallographic orientation (texture) as obtained by X-ray powder diffraction, including pole figure analysis, and high resolution transmission electron microscopy showed the crystallographic nature of the spheres (almost no texture), the cubes (terminated by {100} faces), the platelets (terminated by {111} faces), and the rods (grown from pentagonal twins along [110] and terminated by {100} faces). The crystallite size was determined by Rietveld refinement of X-ray powder diffraction data and agreed well with the transmission electron microscopic data.
Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations.
Metadislocations are a novel type of structural defect in complex metallic alloys. We present an experimental characterization of metadislocation loops in ξ′-Al–Pd–Mn by means of transmission electron microscopy. Furthermore we discuss the mode of metadislocation motion (glide vs. climb) and the geometrical structure of metadislocations in other orthorhombic complex metallic alloys.
Shape-controlled octahedral Pt–Ni alloy nanoparticles exhibit remarkably high activities for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR), which makes them fuel-cell cathode catalysts with exceptional potential. To unfold their full and optimized catalytic activity and stability, however, the nano-octahedra require post-synthesis thermal treatments, which alter the surface atomic structure and composition of the crystal facets. Here, we address and strive to elucidate the underlying surface chemical processes using a combination of ex situ analytical techniques with in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), and in situ electrochemical Fourier transformed infrared (FTIR) experiments. We present a robust fundamental correlation between annealing temperature and catalytic activity, where a ∼25 times higher ORR activity than for commercial Pt/C (2.7 A mgPt–1 at 0.9 VRHE) was reproducibly observed upon annealing at 300 °C. The electrochemical stability, however, peaked out at the most severe heat treatments at 500 °C. Aberration-corrected scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDX) in combination with in situ electrochemical CO stripping/FTIR data revealed subtle, but important, differences in the formation and chemical nature of Pt-rich and Ni-rich surface domains in the octahedral (111) facets. Estimating trends in surface chemisorption energies from in situ electrochemical CO/FTIR investigations suggested that balanced annealing generates an optimal degree of Pt surface enrichment, while the others exhibited mostly Ni-rich facets. The insights from our study are quite generally valid and aid in developing suitable post-synthesis thermal treatments for other alloy nanocatalysts as well.
Octahedral-shaped PtNi-alloy nanoparticles are highly active oxygen reduction reaction catalysts for the cathode in proton exchange membrane fuel cells. However, one major drawback in their application is their limited long-term morphological and compositional stability. Here, we present a detailed in situ electron microscopy characterization of thermal annealing on octahedral-shaped PtNi catalysts as well as on doped octahedral PtNi(Mo) and PtNi(MoRh) catalysts. The evolution of their morphology and composition was quantified during both ex situ and in situ experiments using energy dispersive X-ray spectroscopy in a scanning transmission electron microscope under a hydrogen atmosphere and in vacuum. Morphological changes upon heating, i.e., a gradual loss of the octahedral shape and a continuous rounding of the particles, were observed, as well as evidence for increased alloying. Furthermore, the evolution of the shape of the PtNi(Mo) nanoparticles was quantified using in situ experiments under hydrogen atmosphere in a transmission electron microscope. The shape change of the particles was quantified using segmentation maps created by a neural network. It has been demonstrated that morphological changes crucially depend on the composition and surface doping: doping with Mo or Mo/Rh significantly stabilizes the structure, allowing for persistence of a truncated octahedral shape during heat treatments.
A noble metal alloy at the nanoscale: The internal structure and the surface composition of bimetallic silver–gold nanoparticles of 30–40 nm diameter were probed by transmission electron microscopy, X-ray diffraction, and electrochemistry (cyclovoltammetry, underpotential deposition). The elements are not evenly distributed inside each nanoparticle; there is more gold in the core and more silver in the shell. More information can be found in the Full Paper by M. Epple, K. Tschulik, et al. on page 9051.
Octahedral PtNi alloy nanoparticles show a very high catalytic activity for the oxygen reduction reaction. However, their integration into membrane electrode assemblies (MEAs) is challenging, resulting in low fuel cell performance. We report the application of three strategies that are promising to improve the MEA-based fuel cell performance of octahedral PtNi alloy nanoparticles: (1) Rh surface doping to stabilize the morphology, (2) high Pt weight percentage loading on carbon to decrease the catalyst layer thickness (at parity of geometric-area-normalized Pt loading), and (3) N-functionalized carbon supports to more homogeneously distribute the ionomer. The surface chemistry of the Rh dopants is analyzed by in situ X-ray absorption spectroscopy (XAS) under applied potentials in a liquid half-cell. The Rh dopants are present at the catalyst surface with a local coordination to oxygen atoms as in Rh oxide and show potential dependent changes in the oxidation states. A rotating disk electrode (RDE) screening showed advantages in using Ketjen Black EC300J instead of carbon Vulcan XC72R to accommodate high Pt weight percentage loading (∼30 Pt wt %). Finally, a Rh-doped PtNi nanoparticle catalyst was grown on 3% nitrogen-doped Ketjen Black and tested in a MEA-based single cell after being annealed and acid washed. The results showed modest mass activity (MA), 0.35 A mgPt–1 at 0.9 V, but significantly high performance at high current density for octahedral PtNi nanoparticles, 1500 mA cm–2 at 0.6 V, to our knowledge the highest to date for this class of catalysts. Despite this achievement, the full potential of N doping could not be utilized, with samples showing negligible differences with respect to undoped carbon in both high- and low-humidity MEA testing. Even though no enhancement of mass transport at high current density by better distribution of the ionomer on the N-doped carbon was seen in MEA, this could be due to the diffusion of Ni cations, affecting ionomer interaction and overwhelming the effect of nitrogen species on the support.
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