In-situ atomic tracking of intermetallic compound formation during thermal annealing.

Intermetallic compounds (IMCs) with ordered atomic structure have gained great attention as nanocatalysts for its enhanced activity and stability. Although the reliance of IMC preparation on high-temperature annealing is well known, a comprehensive understanding of the formation mechanisms of IMCs in this process is currently lacking. Here, we employ aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) to track the formation process of IMCs on carbon supports during in-situ annealing, by taking PtFe as a case study within an industry-relevant impregnation synthesis framework. We directly discern five different stages at the atomic level: initial atomic precursors; Pt cluster formation; Pt-Fe disordered alloying; structurally ordered Pt3Fe formation, and final Pt3Fe-PtFe IMC conversion. In particular, we find that the crucial role of high-temperature annealing resides in facilitating the diffusion of Fe towards Pt, enabling the creation of alloys with the targeted stoichiometric ratio, which in turn provides the thermodynamic driving force for the disorder-to-order transition.

Interfacial Charge-Transfer Excitonic Insulator in a Two-Dimensional Organic-Inorganic Superlattice.

Excitonic insulators are long-sought-after quantum materials predicted to spontaneously open a gap by the Bose condensation of bound electron-hole pairs, namely, excitons, in their ground state. Since the theoretical conjecture, extensive efforts have been devoted to pursuing excitonic insulator platforms for exploring macroscopic quantum phenomena in real materials. Reliable evidence of excitonic character has been obtained in layered chalcogenides as promising candidates. However, owing to the interference of intrinsic lattice instabilities, it is still debatable whether those features, such as the charge density wave and gap opening, are primarily driven by the excitonic effect or by the lattice transition. Herein, we develop an intercalation chemistry strategy for obtaining a novel charge-transfer excitonic insulator in organic-inorganic superlattice interfaces that serves as an ideal platform to decouple the excitonic effect from the lattice effect. In this system, we observe a narrow excitonic gap, formation of a charge density wave without periodic lattice distortion, and metal-insulator transition, providing visualized evidence of exciton condensation occurring in thermal equilibrium. Our findings identify self-assembly intercalation chemistry as a new strategy for developing novel excitonic insulators.

Highly Crystalline Iridium-Nickel Nanocages with Subnanopores for Acidic Bifunctional Water Splitting Electrolysis.

Developing efficient bifunctional materials is highly desirable for overall proton membrane water splitting. However, the design of iridium materials with high overall acidic water splitting activity and durability, as well as an in-depth understanding of the catalytic mechanism, is challenging. Herein, we successfully developed subnanoporous Ir3Ni ultrathin nanocages with high crystallinity as bifunctional materials for acidic water splitting. The subnanoporous shell enables Ir3Ni NCs optimized exposure of active sites. Importantly, the nickel incorporation contributes to the favorable thermodynamics of the electrocatalysis of the OER after surface reconstruction and optimized hydrogen adsorption free energy in HER electrocatalysis, which induce enhanced intrinsic activity of the acidic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Together, the Ir3Ni nanocages achieve 3.72 A/mgIr(η=350 mV) and 4.47 A/mgIr(η=40 mV) OER and HER mass activity, which are 18.8 times and 3.3 times higher than that of commercial IrO2 and Pt, respectively. In addition, their highly crystalline identity ensures a robust nanostructure, enabling good catalytic durability during the oxygen evolution reaction after surface oxidation. This work provides a new revenue toward the structural design and insightful understanding of metal alloy catalytic mechanisms for the bifunctional acidic water splitting electrocatalysis.

Precise arrangement of metal atoms at the interface by a thermal printing strategy.

The kinetics and pathway of most catalyzed reactions depend on the existence of interface, which makes the precise construction of highly active single-atom sites at the reaction interface a desirable goal. Herein, we propose a thermal printing strategy that not only arranges metal atoms at the silica and carbon layer interface but also stabilizes them by strong coordination. Just like the typesetting of Chinese characters on paper, this method relies on the controlled migration of movable nanoparticles between two contact substrates and the simultaneous emission of atoms from the nanoparticle surface at high temperatures. Observed by in situ transmission electron microscopy, a single Fe3O4 nanoparticle migrates from the core of a SiO2 sphere to the surface like a droplet at high temperatures, moves along the interface of SiO2 and the coated carbon layer, and releases metal atoms until it disappears completely. These detached atoms are then in situ trapped by nitrogen and sulfur defects in the carbon layer to generate Fe single-atom sites, exhibiting excellent activity for oxygen reduction reaction. Also, sites' densities can be regulated by controlling the size of Fe3O4 nanoparticle between the two surfaces. More importantly, this strategy is applicable to synthesize Mn, Co, Pt, Pd, Au single-atom sites, which provide a general route to arrange single-atom sites at the interface of different supports for various applications.