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Abstract: The electron beam for scanning transmission electron microscopy (STEM) provides rich information about the atomic structure and chemical composition of materials from micron to atomic scale. However, the electron probe can also damage the materials of interest, as the high-energy electrons are often focused on very small sample regions. These effects limit the quality of information which can be extracted from experiments on beam-sensitive materials, such as Li-ion battery materials and metal halide perovskites used in solar cell devices. However, with the increasing interest in these materials to address environmental and societal concerns, a detailed understanding of their microstructure and chemical composition at high spatial resolution is needed to improve their performance and stability. For these materials, the correlation between processing and nanoscale structure-property relationships has been difficult to firmly establish. As shown in Fig. 1a-1c, phase change or amorphisation in beam-sensitive materials can be easily caused by a focused electron probe. Fortunately, this problem can be solved through combined scanning electron nano-diffraction (SEND) and energy dispersive X-ray spectroscopy (EDX) with low electron dose conditions, providing nanoscale crystallographic and chemical information from the specimen. However, the signal-to-noise (SNR) of the EDX data is very poor - with just a few counts in any individual scan prohibiting comprehensive materials characterisation (Fig. 1d). To address this, we perform automated SEND-EDX data acquisition under low dose conditions utilising our automated data analysis workflow. By communicating with two different modalities, i.e., Aztec®; Oxford Instruments and MerlinEM; Quantum Detectors, and using our Python-based software, many SEND-EDX data pairs were simultaneously acquired from a metal halide perovskite. The radially flattened diffraction datasets were then be segmented into distinct phases by using an unsupervised learning approach, non-negative matrix factorisation, and the EDX spectra from identical phases classified earlier were summed across all datasets to enable chemical identification with a much higher SNR than one EDX spectrum image (Fig. 1d) as shown in Fig. 2. In this way we can determine the chemical and crystallographic structure of small phase domains in a highly beam-sensitive multi-phase metal halide perovskite. This research will both demonstrate a novel multi-modal, data-fusion based approach to imaging beam-sensitive materials and shed light on the processing and structure-property relationships of these materials on the nanometre length scale to improve their long-term operational stability.

Open Access

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Abstract: Selective catalytic oxidation (SCO) of NH3 to N2 is one of the most effective methods used to eliminate NH3 emissions. However, achieving high conversion over a wide operating temperature range while avoiding over-oxidation to NOx remains a significant challenge. Here, we report a bi-metallic surficial catalyst (PtSCuO/Al2O3) with improved Pt atom efficiency that overcomes the limitations of current catalysts. It achieves full NH3 conversion at 250 °C with a weight hourly space velocity of 600 ml NH3·h−1·g−1, which is 50 °C lower than commercial Pt/Al2O3, and maintains high N2 selectivity through a wide temperature window. Operando XAFS studies reveal that the surface Pt atoms in PtSCuO/Al2O3 enhance the redox properties of the Cu species, thus accelerating the Cu2+ reduction rate and improving the rate of the NH3-SCO reaction. Moreover, a synergistic effect between Pt and Cu sites in PtSCuO/Al2O3 contributes to the high selectivity by facilitating internal selective catalytic reduction.

Open Access

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Abstract: Growth plate cartilage (GP) serves as a dynamic site of active mineralization and offers a unique opportunity to investigate the cell-regulated matrix mineralization process. Transmission electron microscopy (TEM) provides a means for the direct observation of these mechanisms, offering the necessary resolution and chemical analysis capabilities. However, as mineral crystallinity is prone to artifacts using aqueous fixation protocols, sample preparation techniques are critical to preserve the mineralized tissue in its native form. We optimized cryofixation by high-pressure freezing followed by freeze substitution in anhydrous acetone containing 0.5% uranyl acetate to prepare murine GP for TEM analysis. This sample preparation workflow maintains cellular and extracellular protein structural integrity with sufficient contrast for observation and without compromising mineral crystallinity. By employing appropriate sample preparation techniques, we were able to observe two parallel mineralization processes driven by chondrocytes: 1) intracellular- and 2) extracellular-originating mineralized vesicles. Both mechanisms are based on sequestering calcium phosphate (CaP) within a membrane-limited structure, albeit originating from different compartments of the chondrocytes. In the intracellular originating pathway, CaP accumulates within mitochondria as globular CaP granules, which are incorporated into intracellular vesicles (500-1000 nm) and transported as granules to the extracellular matrix (ECM). In contrast, membrane budding vesicles with a size of approximately 100-200nm, filled with needle-shaped minerals were observed only in the ECM. Both processes transport CaP to the collagenous matrix via vesicles, they can be differentiated based on the vesicle size and mineral morphologies. Their individual importance to the cartilage mineralization process is yet to be determined.

Open Access

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Abstract: The induction of structural distortion in a controlled manner through tilt engineering emerges as a potent method to finely tune the physical characteristics of Prussian blue analogues. Notably, this distortion can be chemically induced by filling their pores with cations that can interact with the cyanide ligands. With this objective in mind, we optimized the synthetic protocol to produce the stimuli-responsive Prussian blue analogue AxMn[Fe(CN)6] with A = K+, Rb+, and Cs+, to tune its stimuli-responsive behavior by exchanging the cation inside pores. Our crystallographic analyses reveal that the smaller the cation, the more pronounced the structural distortion, with a notable 20-degree Fe-CN bending when filling the cavities with K+, 10 degrees with Rb+, and 2 degrees with Cs+. Moreover, this controlled distortion offers a means to switch on/off its stimuli-responsive behavior, while modifying its magnetic response. Thereby empowering the manipulation of the PBA's physical properties through cationic exchange.

Open Access

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Abstract: This work reports the thermal and electron beam stabilities of a series of isostructural metal-organic frameworks (MOFs) of type MFM-300(M), where M = Al, Ga, In, or Cr. MFM-300(Cr) was most electron beam stable, having an unusually high critical electron fluence of 1111 e-·Å-2 while the Group 13 element MOFs were found to be less stable. Within Group 13, MFM-300(Al) had the highest critical electron fluence of 330 e-·Å-2, compared to 189 e-·Å-2 and 147 e-·Å-2 for the Ga and In MOFs respectively. For all four MOFs, electron beam-induced structural degradation was independent of crystal size and was highly anisotropic, with the one-dimensional pore channels being the most stable, although the length and width of the channels decreased during electron beam irradiation. Notably, MFM-300(Cr) was found to retain crystallinity while shrinking up to 10%. Thermal stability was studied using in situ synchrotron X-ray diffraction at elevated temperature which revealed critical temperatures for crystal degradation to be 605, 570, 490 and 480°C for Al, Cr, Ga, and In, respectively. The pore channel diameters contracted by ~0.5% on desorption of solvent species but thermal degradation at higher temperatures was isotropic. The observed electron stabilities were found to scale with the relative inertness of the cations and correlate well to the measured lifetime of the materials when used as photocatalysts.