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Abstract: High-energy irradiation of materials can lead to void formation due to the aggregation of vacancies, reducing the local stress in the system. Studying void formation and its interplay with vacancy clusters in bulk materials at the atomic level has been challenging due to the thick volume of 3D materials, which generally limits high-resolution transmission electron microscopy. The thin nature of 2D materials is ideal for studying fundamental material defects such as dislocations and crack tips and has potential to reveal void formation by vacancy aggregation in detail. Here, using atomic-resolution in situ transmission electron microscopy of 2D monolayer MoS2, we capture rapid thermal diffusion of S vacancies into ultralong (∼60 nm) 1D S vacancy channels that initiate void formation at high vacancy densities. Strong interactions are observed between the 1D channels and void growth, whereby Mo and S atoms are funneled back and forth between the void edge and the crystal surface to enable void enlargement. Preferential void growth up to 100 nm is shown to occur by rapid digestion of 1D S vacancy channels as they make contact. These results reveal the atomistic mechanisms behind void enlargement in 2D materials under intense high-energy irradiation at high temperatures and the existence of ultralong 1D vacancy channels. This knowledge may also help improve the understanding of void formation in other systems such as nuclear materials, where direct visualization is challenging due to 3D bulk volume.

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Abstract: Metal-organic framework crystal-glass composites (MOF-CGCs) are materials in which a crystalline MOF is dispersed within a MOF glass. In this work, we explore the room temperature stabilization of the open-pore form of MIL-53(Al), usually observed at high-temperature, which occurs upon encapsulation within a ZIF-62(Zn) MOF glass matrix. A series of MOF-CGCs containing different loadings of MIL-53(Al) were synthesized and characterized using X-ray diffraction and nuclear magnetic resonance spectroscopy. An upper limit of MIL-53(Al) that can be stabilized in the composite was determined for the first time. The nanostructure of the composites was probed using pair distribution function analysis and scanning transmission electron microscopy. Notably, the distribution and integrity of the crystalline compo-nent in a sample series was determined, and these findings related to the MOF-CGC gas adsorption capacity in order to identify the optimal loading necessary for maximum CO2 sorption capacity.

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Abstract: Zeolite encapsulated metal nanoparticle catalysts hold great promise for several green and sustainable processes, ranging from environmental remediation to renewable energy and biomass conversion. In particular, the microporous zeolite framework keeps the nanoparticles in a firm grip that can control selectivity and prevent sintering at high temperatures. While progress in the synthesis of mesoporous zeolites continues, the encapsulation of metal nanoparticles remains a challenge that often requires complex procedures and expensive additives. Here, we report a general method to encapsulate both base and noble metal nanoparticles inside the internal voids of a compartmentalized mesoporous zeolite prepared by carbon templating and steam-assisted recrystallization. This results in a remarkable shell-like morphology that facilitates the formation of small metal nanoparticles upon simple impregnation and reduction. When the materials are applied in catalysis, we for instance demonstrate that zeolite encapsulated Ni nanoparticles are highly active, selective and stable catalysts for CO2 methanation (49% conversion with 93% selectivity at 450°C). A reaction where catalysts often suffer from sintering due to the high reaction temperatures. While the introduction of Ni nanoparticles prior to the steam-assisted recrystallization results in the formation of inactive nickel phyllosilicates, noble metals such as Pt do not suffer from this limitation. Therefore, we also demonstrate the synthesis of an active catalyst prepared by the formation of Pt nanoparticles prior to the shell synthesis. We tested the zeolite encapsulated Pt nanoparticles for hydrogenation of linear and cyclic alkenes with increased chain length. The catalysts are active for hydrogenation of oct-1-ene (66% conversion) and cyclooctene (79% conversion) but inactive for the large cyclododecane (<1% conversion), which show that this type of catalyst is highly selective in size selective catalysis. All catalysts are characterized by XRD, TEM, XPS and N2 physisorption.

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Abstract: Developing cost-effective and durable air-cathodes is crucial for improving metal-air batteries. Most reports of cathode formulation involve preparing bi-functional electrocatalysts from wet chemistry or solid-state synthesis, followed by pasting onto a substrate. In this work, the cathodes generated from electrochemical activation of normal carbon paper substrates were directly used in Zn-air batteries. The self-activated carbon paper substrate without any additional electrocatalysts exhibits an impressive cycling stability (more than 165 hours for 1,000 cycles) and a small discharge-charge voltage gap. After the activation, the maximum power density and electrochemical surface area were increased by over 40 and 1,920 times respectively. It is discovered that substrates after activation can be directly used as a cathode. The new method is scalable, inexpensive and produces near best in class performance. The mechanism behind this enhancement is due to the creation of oxygen functional groups within the cathode, which overcame slow kinetics, enhanced wettability and enabled optimum three-phase boundaries.

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Abstract: We show that Pt nanoclusters preferentially nucleate along the grain boundaries (GBs) in polycrystalline MoS2 monolayer films, with dislocations acting as the seed site. Atomic resolution studies by aberration-corrected annular dark-field scanning transmission electron microscopy reveal periodic spacing of Pt nanoclusters with dependence on GB tilt angles and random spacings for the antiphase boundaries (i.e., 60°). Individual Pt atoms are imaged within the dislocation core sections of the GB region, with various positions observed, including both the substitutional sites of Mo and the hollow center of the octahedral ring. The evolution from single atoms or small few atom clusters to nanosized particles of Pt is examined at the atomic level to gain a deep understanding of the pathways of Pt seed nucleation and growth at the GB. Density functional theory calculations confirm the energetic advantage of trapping Pt at dislocations on both the antiphase boundary and the small-angle GB rather than on the pristine lattice. The selective decoration of GBs by Pt nanoparticles also has a beneficial use to easily identify GB areas during microscopic-scale observations and track long-range nanoscale variances of GBs with spatial detail not easy to achieve using other methods. We show that GBs have nanoscale meandering across micron-scale distances with no strong preference for specific lattice directions across macroscopic ranges.