RESUMEN
Practical applications involving the magnetic bistability of single-molecule magnets (SMMs) for next-generation computer technologies require nanostructuring, organization, and protection of nanoscale materials in two- or three-dimensional networks, to enable read-and-write processes. Owing to their porous nature and structural long-range order, metal-organic frameworks (MOFs) have been proposed as hosts to facilitate these efforts. Although probing the channels of MOF composites using indirect methods is well established, the use of direct methods to elucidate fundamental structural information is still lacking. Herein we report the direct imaging of SMMs encapsulated in a mesoporous MOF matrix using high-resolution transmission electron microscopy. These images deliver, for the first time, direct and unambiguous evidence to support the adsorption of molecular guests within the porous host. Bulk magnetic measurements further support the successful nanostructuring of SMMs. The preparation of the first magnetic composite thin films of this kind furthers the development of molecular spintronics.
RESUMEN
Metal-organic frameworks (MOFs) are crystalline porous materials with designable topology, porosity and functionality, having promising applications in gas storage and separation, ion conduction and catalysis. It is challenging to observe MOFs with transmission electron microscopy (TEM) due to the extreme instability of MOFs upon electron beam irradiation. Here, we use a direct-detection electron-counting camera to acquire TEM images of the MOF ZIF-8 with an ultralow dose of 4.1 electrons per square ångström to retain the structural integrity. The obtained image involves structural information transferred up to 2.1 Å, allowing the resolution of individual atomic columns of Zn and organic linkers in the framework. Furthermore, TEM reveals important local structural features of ZIF-8 crystals that cannot be identified by diffraction techniques, including armchair-type surface terminations and coherent interfaces between assembled crystals. These observations allow us to understand how ZIF-8 crystals self-assemble and the subsequent influence of interfacial cavities on mass transport of guest molecules.
RESUMEN
High-resolution imaging of electron beam-sensitive materials is one of the most difficult applications of transmission electron microscopy (TEM). The challenges are manifold, including the acquisition of images with extremely low beam doses, the time-constrained search for crystal zone axes, the precise image alignment, and the accurate determination of the defocus value. We develop a suite of methods to fulfill these requirements and acquire atomic-resolution TEM images of several metal organic frameworks that are generally recognized as highly sensitive to electron beams. The high image resolution allows us to identify individual metal atomic columns, various types of surface termination, and benzene rings in the organic linkers. We also apply our methods to other electron beam-sensitive materials, including the organic-inorganic hybrid perovskite CH3NH3PbBr3.
RESUMEN
Zinc oxide nanoparticles (ZnO NPs) are widely used nanomaterials and their environmental impacts have received increasing attention. The fate and toxicity of ZnO NPs in the environment are determined by their stability and dissolution. In this study, the influence of water chemistry on aggregation, sedimentation, and dissolution of ZnO NPs was investigated. The stabilized ZnO NPs aggregated and precipitated when the aqueous pH closed to their zero point of charge (pHzpc). Counter-ions neutralized the surface charge of NPs and promoted their destabilization. However, a high concentration of counter-ion (SO42-, >10meq/L) made the NPs more stable because of the inverted surface potential. The stability of ZnO NPs was maintained by high concentration of Suwannee River humic acid (SRHA, 10mg/L) even the concentration of electrolytes was high. The influence of water chemistry on the stability and dissolution of ZnO NPs was further demonstrated in different wastewaters. In one wastewater sample, ZnO NPs was unexpectedly stable and with a high dissolution, which was due to the effects of pH value, organic matter concentration, as well as the concentration of counter ions. Our findings facilitate the predictions of the fate of stabilized ZnO NPs in the environment.
RESUMEN
Nanomaterials are considered to be emerging contaminants because their release into the environment could cause a threat to our ecosystem and human health. This study aims to evaluate the effects of pH, ions, and humic acid on the destabilization and sedimentation of commercial stabilized TiO2 nanoparticles (NPs) in aquatic environments. The average hydrodynamic size of TiO2 NPs was determined to be 52 ± 19 nm by dynamic light scattering. The zero point charge (ZPC) of the commercial TiO2 NPs was found to occur at pH 6. The stability of commercial TiO2 NPs is independent of its concentration in the range of 50-200 mg/L. In the absence of NaCl, the commercial TiO2 NPs rapidly settled down near pHzpc when the aggregated nanoparticle size surpassed 1 µm. However, when the commercial TiO2 NPs aggregated with the increase of NaCl concentrations, the large aggregates (>1 µm) were found to remain suspended. For example, even at the critical aggregation concentration of NaCl (100 meq/L), TiO2 NP aggregates suspended for 45 min and then slowly deposited. This implies an increase in the exposure risk of NPs. In the presence of Suwannee river humic acid (SRHA), the commercial TiO2 NPs did not settle down until the SRHA concentration increased to 20 mg/L, and were seen to restabilize at SRHA concentrations of 50 mg/L. The uncommon behaviors of the commercial TiO2 NPs we observed may be attributed to the different destabilization mechanisms caused by different species (i.e., NaCl and SRHA) in water.