The catalytic performance of nanoparticles is primarily determined by the precise nature of the surface and near-surface atomic configurations, which can be tailored by post-synthesis annealing effectively and straightforwardly. Understanding the complete dynamic response of surface structure and chemistry to thermal treatments at the atomic scale is imperative for the rational design of catalyst nanoparticles. Here, by tracking the same individual Pt3Co nanoparticles during in situ annealing in a scanning transmission electron microscope, we directly discern five distinct stages of surface elemental rearrangements in Pt3Co nanoparticles at the atomic scale: initial random (alloy) elemental distribution; surface platinum-skin-layer formation; nucleation of structurally ordered domains; ordered framework development and, finally, initiation of amorphization. Furthermore, a comprehensive interplay among phase evolution, surface faceting and elemental inter-diffusion is revealed, and supported by atomistic simulations. This work may pave the way towards designing catalysts through post-synthesis annealing for optimized catalytic performance.

Transition-metal (TM) macrocyclic complexes have potential applications as nonprecious electrocatalysts in polymer electrolyte membrane fuel cells. In this study, we employed density functional theory calculation methods to predict the molecular and electronic structures of O2, OH, and H2O2 molecules adsorbed on TM porphyrins, TM tetraphenylporphyrins, TM phthalocyanines, TM fluorinated phthalocyanines, and TM chlorinated phthalocyanines (here TM = Fe or Co). Relevant to their performance on catalyzing oxygen reduction reaction (ORR), we found for the studied TM macrocyclic complexes: (1) The type of the central TM is the most determinant factor in influencing the adsorption energies of O2, OH, and H2O2 (chemical species involved in ORR) molecules on these macrocyclic complexes. Specifically, the calculated adsorption energies of O2, OH, and H2O2 on the Fe macrocyclic complexes are always distinguishably lower than those on the Co macrocyclic complexes. (2) The peripheral ligands are capable of modulating the binding strength among the adsorbed O2, OH, and H2O2, and the TM macrocyclic complexes. (3) A N–TM–N cluster structure (like N–Fe–N) with a proper distance between the two ending N atoms and a strong electronic interaction among the three atoms is required to break the O–O bond and thus promote the efficient four-electron pathway of the ORR on the TM macrocyclic complexes.

Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn5O8 pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge–discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn5O8 suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn2+/Mn4+ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn5O8.

Surface segregation-the enrichment of one element at the surface, relative to the bulk-is ubiquitous to multi-component materials. Using the example of a Cu-Au solid solution, we demonstrate that compositional variations induced by surface segregation are accompanied by misfit strain and the formation of dislocations in the subsurface region via a surface diffusion and trapping process. The resulting chemically ordered surface regions acts as an effective barrier that inhibits subsequent dislocation annihilation at free surfaces. Using dynamic, atomic-scale resolution electron microscopy observations and theory modelling, we show that the dislocations are highly active, and we delineate the specific atomic-scale mechanisms associated with their nucleation, glide, climb, and annihilation at elevated temperatures. These observations provide mechanistic detail of how dislocations nucleate and migrate at heterointerfaces in dissimilar-material systems.

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