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Abstract: Single-atom catalysts (SACs) have recently attracted broad research interest as they combine the merits of both homogeneous and heterogeneous catalysts. Rational design and synthesis of SACs are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report a general approach to a series of monodispersed atomic transition metals (for example, Fe, Co, Ni) embedded in nitrogen-doped graphene with a common MN4C4 moiety, identified by systematic X-ray absorption fine structure analyses and direct transmission electron microscopy imaging. The unambiguous structure determination allows density functional theoretical prediction of MN4C4 moieties as efficient oxygen evolution catalysts with activities following the trend Ni > Co > Fe, which is confirmed by electrochemical measurements. Determination of atomistic structure and its correlation with catalytic properties represents a critical step towards the rational design and synthesis of precious or nonprecious SACs with exceptional atom utilization efficiency and catalytic activities.

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Abstract: Palladium-catalysed cross-coupling reactions, central tools in fine-chemical synthesis, predominantly employ soluble metal complexes despite recognized challenges with product purification and catalyst reusability. Attempts to tether these homogeneous catalysts on insoluble carriers have been thwarted by suboptimal stability, which leads to a progressively worsening performance due to metal leaching or clustering4. The alternative application of supported Pd nanoparticles has faced limitations because of insufficient activity under the mild conditions required to avoid thermal degradation of the substrates or products. Single-atom heterogeneous catalysts lie at the frontier. Here, we show that the Pd atoms anchored on exfoliated graphitic carbon nitride (Pd-ECN) capture the advantages of both worlds, as they comprise a solid catalyst that matches the high chemoselectivity and broad functional group tolerance of state-of-the-art homogeneous catalysts for Suzuki couplings, and also demonstrate a robust stability in flow. The adaptive coordination environment within the macroheterocycles of ECN facilitates each catalytic step. The findings illustrate the exciting opportunities presented by nanostructuring single atoms in solid hosts for catalytic processes that remain difficult to heterogenize.

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Abstract: Integrating fast readout direct electron detectors into the scanning transmission electron microscope (STEM) is revolutionizing imaging. This paper provides a brief review of work in this area at Glasgow.

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Abstract: When secondary domains nucleate and grow on the surface of monolayer MoS2, they can extend across grain boundaries in the underlying monolayer MoS2 and form overlapping sections. We present an atomic level study of overlapping antiphase grain boundaries (GBs) in MoS2 monolayer-bilayers using aberration-corrected annular dark field scanning transmission electron microscopy. In particular we focus on the antiphase GB within a monolayer and track its propagation through an overlapping bilayer domain. We show that this leads to an atomically sharp interface between 2H and 3R interlayer stacking in the bilayer region. We have studied the micro-nanoscale “meandering” of the antiphase GB in MoS2, which shows a directional dependence on the density of 4 and 8 member ring defects, as well as sharp turning angles 90°–100° that are mediated by a special 8-member ring defect. Density functional theory has been used to explore the overlapping interlayer stacking around the antiphase GBs, confirming our experimental findings. These results show that overlapping secondary bilayer MoS2 domains cause atomic structure modification to underlying anti-phase GB sites to accommodate the van der Waals interactions.

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Abstract: We reveal a self-limiting mechanism during the formation of a specific type of circular nanopore in monolayer WS2 that limits its diameter to sub-nm. A single W atom vacancy (triangular nanopore) is transformed into the self-limiting nanopore (SLNP) through the atomic restructuring of S atoms around the area, reducing the number of dangling bonds at the nanopore edge by shifting them further in-plane with W–W bonding instead. Bond rotations in WS2 help accommodate the electron beam induced atomic loss and ensure the stability of the SLNP. The SLNP shows significant improvement in diameter stability during electron beam irradiation compared to other triangular nanopores in WS2 that typically continue to expand in diameter during atom loss. The atomic structure of these SLNPs is studied using aberration-corrected scanning transmission electron microscopy with an in situ heating holder, revealing that the SLNPs are mostly formed at a temperature of ∼500 °C, which is a balance between thermally activated S vacancy diffusion and sufficient S vacancy density to initiate local atomic reconstruction. At higher temperatures (i.e., 1000 °C), S vacancies quickly migrate away into long line vacancies, resulting in low S vacancy density and rapidly expanding holes generated at the edges of the line vacancies. At room temperature, S vacancy migration is low and vacancy density is very high, which limits atomic reconstruction, and instead many small holes open up. These results provide insights into the factors that lead to uniform sized nanopores in the sub-nm range in transition-metal dichalcogenides.