Search

Structural and compositional inhomogeneity is common in zeolites and considerably affects their properties. Thickness-limited lateral resolution, lack of depth resolution, and electron dose-constrained focusing limit local structural studies of zeolites in conventional transmission electron microscopy (TEM). We demonstrate that a multislice ptychography method based on four-dimensional scanning TEM (4D-STEM) data can overcome these limitations. Images obtained from a ~40-nanometer-thick MFI zeolite exhibited a lateral resolution of ~0.85 angstrom that enabled the identification of individual framework oxygen (O) atoms and the precise determination of the orientations of adsorbed molecules. Furthermore, a depth resolution of ~6.6 nanometers allowed probing of the three-dimensional distribution of O vacancies, as well as the phase boundaries in intergrown MFI and MEL zeolites. The 4D-STEM ptychography can be generally applied to other materials with similar high electron-beam sensitivity.

Graphdiyne (GDY) is an artificial carbon allotrope that is conceptually similar to graphene but composed of sp- and sp2-hybridized carbon atoms. Monolayer GDY (ML-GDY) is predicted to be an ideal two-dimensional (2D) semiconductor material with a wide range of applications. However, its preparation has not been achieved experimentally due to difficulties with synthesis. Here, we report that in-situ polymerization of hexaethynylbenzene within the sub-nanometer interlayer space of MXene can effectively prevent out-of-plane growth or vertical stacking of the material, resulting in crystalline ML-GDY. The subsequent exfoliation process successfully yields free-standing GDY monolayers with micrometer-scale lateral dimensions. The fabrication of field-effect transistor on free-standing ML-GDY makes the first measurement of its electronic properties possible. The measured electrical conductivity (5.1×103 S m-1) and carrier mobility (231.4 cm2 V−1 s−1) at room temperature are remarkably higher than those of the previously reported multilayer GDY materials. The space-constrained synthesis using layered crystals as templates provides a new strategy for preparing 2D materials with precisely controlled layer numbers and long-range structural order.

Atomically thin two-dimensional (2D) materials have long been considered as ideal platforms for developing separation membranes. However, it is difficult to generate uniform subnanometer pores over large areas on 2D materials. Herein, we report that the well-defined defect structure of monolayer MoS2, namely, eight-membered ring (8-MR) pores typically formed at the boundaries of two antiparallel grains, can serve as molecular sieves for efficient water/ion separation. The 8-MR pores (4.2 × 2.4 Å) in monolayer MoS2 allow rapid single-file water transport while rejecting various hydrated ions. Further, the density of grain boundaries and, consequently, the density of pores can be tuned by regulating the nucleation density and size of MoS2 grains during the chemical vapor deposition process. The optimized MoS2 membrane exhibited an ultrahigh water/NaCl selectivity of ~6.5 × 104 at a water permeance of 232 mol m−2 h−1 bar−1, outperforming the state-of-the-art desalination membranes. When used for direct hydrogen production from seawater by combining the forward osmosis and electrochemical water splitting processes, the membrane achieved ~40 times the energy conversion efficiency of commercial polymeric membranes. It also exhibited a rapid and selective proton transport behavior desirable for fuel cells and electrolysis. The bottom-up approach of creating precise pore structures on atomically thin films via grain boundary engineering presents a promising route for producing large-area membranes suitable for various applications.

Understanding surface reconstruction of nanocrystals is of great importance to their applications, however it is still challenging due to lack of atomic-level structural information under reconstruction conditions. Herein, through in situ spherical aberration corrected scanning transmission electron microscopy (STEM), the reconstruction of nanocrystalline SnO2 (110) surface was studied. By identifying the precise arrangements of surface/subsurface Sn and O columns through both in situ bright-field and high-angle annular dark-field STEM images, an unexpected added Sn2O model was determined for SnO2 (110)-(1 × 2) surface. The protruded Snδ+ of this surface could act as the active sites for activating O2 molecules according to our density functional theory (DFT) calculations. On the basis of in situ observation of atomic-level reconstruction behaviors and DFT calculations, an energy-driven reconstruction process was also revealed. We anticipate this work would help to clarify the long-standing debate regarding the reconstruction of SnO2 (110) surface and its intrinsic property.

Anatase TiO2 nanotubes array (ATONA) has attracted tremendous attention owing to its promising applications in solar cells, water splitting and organic pollutants photocatalytic degradation. However, the activity of ATONAs was greatly suppressed by the grain boundaries existed in the tube walls, which acted as carrier scattering and recombination centers. Herein, we report a novel strategy to prepare array of single crystalline anatase TiO2 nanotubes (SC-ATONAs) with significant enhancement of the photocatalytic activity and UV photo response performance compared to the polycrystalline counterparts. The growth of SC-ATONAs was achieved by establishing a crystallization temperature gradient along the tube wall, which ensured the crystallization started from the nucleation of a single nucleus at the bottom of amorphous TiO2 nanotubes prepared by Ti anodization. These findings pave the way to prepare single-crystalline nanotubes with superior performance for other materials.

Recent advances in high-speed pixelated electron detectors have substantially facilitated the implementation of four-dimensional scanning transmission electron microscopy (4D-STEM). A critical application of 4D-STEM is electron ptychography, which reveals the atomic structure of a specimen by reconstructing its transmission function from redundant convergent-beam electron diffraction patterns. Although 4D-STEM ptychography offers many advantages over conventional imaging modes, this emerging technique has not been fully applied to materials highly sensitive to electron beams. In this Outlook, we introduce the fundamentals of 4D-STEM ptychography, focusing on data collection and processing methods, and present the current applications of 4D-STEM ptychography in various materials. Next, we discuss the potential advantages of imaging electron-beam-sensitive materials using 4D-STEM ptychography and explore its feasibility by performing simulations and experiments on a zeolite material. The preliminary results demonstrate that, at the low electron dose required to preserve the zeolite structure, 4D-STEM ptychography can reliably provide higher resolution and greater tolerance to the specimen thickness and probe defocus as compared to existing imaging techniques. In the final section, we discuss the challenges and possible strategies to further reduce the electron dose for 4D-STEM ptychography. If successful, it will be a game-changer for imaging extremely sensitive materials, such as metal-organic frameworks, hybrid halide perovskites, and supramolecular crystals.

The anatase TiO2 (001) surface has attracted extensive attention in surface science and for practical applications because of its excellent performance. However, the highly reactive (001) surface usually reconstructs to a (1 × 4)-(001) surface, which may potentially alter its properties. As a result, there are still controversial issues concerning the intrinsic performance of this surface. Recently, significant advances have been made in the surface science study of TiO2 (001), which have elucidated its intrinsic physical and chemical properties. In this review, we summarize our present understanding of the structure and properties of the reconstructed TiO2 (001) surface. First, we provide a brief background of the anatase TiO2 surfaces, including its low-index surface structures and the synthesis of anatase TiO2 nanocrystals with the (001) surface exposed. Next, we focus on the structure, formation mechanism, stability, and surface chemistry of the reconstructed TiO2 (001) surface. Finally, we finish this review by highlighting the current challenges of this subject and providing future research perspectives.

Metal catalysts are of great importance in the modern chemical industry. It is well-known that the structures of metal catalysts determine their properties. However, recent studies suggested that t...

Oxygen electrocatalysis involving the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) plays a vital role in cutting-edge energy conversion and storage technologies. In situ studies of the evolution of catalysts during oxygen electrocatalysis can provide important insights into their structure - activity relationships and stabilities under working conditions. Among the various in situ characterization tools available, in situ electron microscopy has the unique ability to perform structural and compositional analyzes with high spatial resolution. In this review, we present the latest developments in in situ and quasi-in situ electron microscopic techniques, including identical location electron microscopy, in situ liquid cell (scanning) transmission electron microscopy and in situ environmental transmission electron microscopy, and elaborate their applications in the ORR and OER. Our discussion centers on the degradation mechanism, structural evolution and structure - performance correlations of electrocatalysts. Finally, we summarize the earlier discussions and share our perspectives on the current challenges and future research directions of using in situ electron microscopy to explore oxygen electrocatalysis and related processes.

The anatase TiO2 nanocrystals with dominant (001) facets attracted tremendous attention in the past decade. However, the reported intrinsic property of the (001) surface is still a cause of controversy. A crucial reason is that the (001) surface usually undergoes a (1 × 4) reconstruction, which may result in a remarkable difference in property. Herein, we performed an in situ environmental transmission electron microscopy (ETEM) study regarding the formation and stability of the (1 × 4)-(001) surface of TiO2 nanocrystals. The systematic ETEM studies confirmed that the (1 × 4)-(001) surface could be generated at elevated temperature (above 300 °C) when the surface contaminants were removed and the formed reconstruction could survive under different conditions, which indicate the surface reconstruction should be taken into account in related property research. In addition, an oriented layer-by-layer beam damage process on the (001) surface is confirmed, and optimal imaging conditions were also investigated,...

CeO2–Nd2O3 microspheres were successfully prepared by internal gelation process using M(OH)m and [MCit∙xH2O] (M = Ce3+, Ce4+, and Nd3+, Cit is (C6O7H5)3−) as precursors. The effects of Nd(NO3)3 content on the stability of precursor solution and on the microstructure of the sintered microspheres were investigated. The gelled microspheres and sintered composite microspheres were characterized by Fourier transform-infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and X-ray fluorescence (XRF) spectroscopy. The distribution of Nd3+ in the microspheres was also investigated by line scanning of SEM. The results indicated that the citrate salt in the gelled microspheres was [MCit∙xH2O] (M = Ce3+ and Nd3+). Compared with composite microspheres prepared with M(OH)m (M = Ce4+ and Nd3+) as a precursor, the mass fraction of Nd2O3 in composite microspheres prepared with [MCit∙xH2O] as a precursor highly coincided with the theoretical value. There was no concentration gradient in the microspheres and the distribution of Nd3+ was homogeneous. Phase composition of the composite microspheres was Ce0.75−xNd0.25+xO1.85 and CeO2.

Cerium dioxide microspheres were successfully prepared by internal gelation process with copolymerization using acrylic acid. The effects of acrylic acid (AA)/hexamethylenetetramine (HMTA) molar ratio and concentration of solution containing urea and HMTA on the stability of precursor solution and on the prepared microspheres were investigated. The results indicated that acrylic acid could modify the urea resin and slow down the decomposition of urea resin. The stability of precursor solutions was improved with the increase of AA/HMTA molar ratio, but the surfaces of gelled microspheres became rough and cracked when the ratio was greater than 1. The solution containing urea and HMTA had a significant effect on the stability of precursor solution and on the surface morphology, specific surface area, pore volume and average pore diameter of the microspheres. The porous CeO2 microspheres were obtained by heat treatment at 600 °C for 2 h.

Cerium dioxide microspheres were successfully prepared by internal gelation with cerium citrate as the precursor. The effects of solution containing hexamethylenetetramine (HMTA) and urea on the stability of precursor solution and on the sintered microspheres were investigated. The results indicate that the gelled microspheres were composed of [CeCit∙xH2O]. Propylene glycol methyl ether was a good solvent for solvothermal treatment. The solution containing HMTA and urea had an important effect on the stability of precursor solution; it also greatly affected the surface morphology, specific surface area, pore volume, and average pore diameter of microspheres. The porous CeO2 microspheres were obtained by sintering at 600 ℃ for 2 h. CeO2 microspheres prepared with cerium citrate as precursor had a larger pore volume than microspheres prepared with cerium hydroxide as the precursor.

Cross-organizational workflow appears to be more and more important in global business. It is a better solution to the problem of cross-organizational collaboration that state machine workflow is able to model business processes based on discrete points. Data will be transferred among different organizations in their collaborations and more human interactivities are needed. Further more, the event driven approach makes it more flexible to state transitions. Therefore, an approach for state machine workflow modeling based on event driven is presented. Using data-dependent methods, it can analyze and optimize the data exceptions to improve process efficiency and data accuracy before compiling workflow processes.