A recently developed imaging mode called “wet-STEM” and new developments in environmental scanning electron microscopy (ESEM) allows the observation of nano-objects suspended in a liquid phase, with ...a few manometers resolution and a good signal to noise ratio. The idea behind this technique is simply to perform STEM-in-SEM, that is SEM in transmission mode, in an environmental SEM.
The purpose of the present contribution is to highlight the main advances that contributed to development of the wet-STEM technique. Although simple in principle, the wet-STEM imaging mode would have been limited before high brightness electron sources became available, and needed some progresses and improvements in ESEM. This new technique extends the scope of SEM as a high-resolution microscope, relatively cheap and widely available imaging tool, for a wider variety of samples.
Great progress has been made in the preparation and application of multi-shelled hollow micro-/nanostructures during the past decade. However, the synthetic methodologies and potential applications ...of these novel and interesting materials have not been reviewed comprehensively in the literature. In the current review we first describe different synthetic methodologies for multi-shelled hollow micro-/nanostructures as well as their compositional and geometric manipulation and then review their applications in energy conversion and storage, sensors, photocatalysis, and drug delivery. The correlation between the geometric properties of multi-shelled hollow micro-/nanostructures and their specific performance in relevant applications are highlighted. These results demonstrate that the geometry has a direct impact on the properties and potential applications of such materials. Finally, the emerging challenges and future development of multi-shelled hollow micro-/nanostructures are further discussed.
Recent advances in multi-shelled hollow micro-/nanostructures were reviewed, and the correlation between their geometric properties and specific performance was highlighted.
We report on the fabrication of photochromic polymersomes exhibiting photoswitchable and reversible bilayer permeability from newly designed poly(ethylene oxide)-b-PSPA (PEO-b-PSPA) diblock ...copolymers, where SPA is spiropyran (SP)-based monomer containing a unique carbamate linkage. Upon self-assembling into polymersomes, SP moieties within vesicle bilayers undergo reversible phototriggered isomerization between hydrophobic spiropyran (SP, λ2 > 450 nm irradiation) and zwitterionic merocyanine (MC, λ1 < 420 nm irradiation) states. For both SP and MC polymersomes, their microstructures are stabilized by multiple cooperative noncovalent interactions including hydrophobic, hydrogen bonding, π–π stacking, and paired electrostatic (zwitterionic) interactions, with the latter two types being exclusive for MC polymersomes. Control experiments using analogous block copolymers of hydrophobic SP monomer with a carbonate linkage (SPO) and conventional spiropyran methacrylate monomer (SPMA) containing a single ester functionality were then conducted, revealing that carbamate-incurred hydrogen bonding interactions in PEO-b-PSPA are crucial for polymersome stabilization in the zwitterionic MC state. Moreover, reversible phototriggered SP-to-MC polymersome transition is accompanied by membrane polarity and permeability switching from being nonimpermeable to selectively permeable toward noncharged, charged, and zwitterionic small molecule species below critical molar masses. Intriguingly, UV-actuated MC polymersomes possess two types of release modules: (1) sustained release upon short UV irradiation duration by taking advantage of the unexpectedly slow spontaneous MC-to-SP transition kinetics (t 1/2 > 20 h) under dark conditions; (2) on-demand and switchable release under alternated UV–vis light irradiation. We further demonstrate photoswitchable spatiotemporal release of 4′,6-diamidino-2-phenylindole (DAPI, cell nuclei-staining dye) within living HeLa cells.
Developing low-cost, efficient, and bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is an appealing yet challenging task. Herein, for ...the first time, a NiS microsphere film was grown in situ on Ni foam (NiS/Ni foam) via a sulfurization reaction as an efficient bifunctional electrocatalyst for overall water splitting with superior activity and good durability. This NiS/Ni foam electrode delivers 20 mA cm(-2) at an overpotential of 158 mV for the HER and 50 mA cm(-2) at an overpotential of 335 mV for the OER in 1.0 M KOH. This bifunctional electrode also enables a high-efficiency alkaline water electrolyzer with 10 mA cm(-2) at a cell voltage of only 1.64 V, which could be promising in water splitting devices for large-scale hydrogen production.
Reduced graphene oxide (rGO)-conjugated Cu2O nanowire mesocrystals were formed by nonclassical crystallization in the presence of GO and o-anisidine under hydrothermal conditions. The resultant ...mesocrystals are comprised of highly anisotropic nanowires as building blocks and possess a distinct octahedral morphology with eight {111} equivalent crystal faces. The mechanisms underlying the sequential formation of the mesocrystals are as follows: first, GO-promoted agglomeration of amorphous spherical Cu2O nanoparticles at the initial stage, leading to the transition of growth mechanism from conventional ion-by-ion growth to particle-mediated crystallization; second, the evolution of the amorphous microspheres into hierarchical structure, and finally to nanowire mesocrystals through mesoscale transformation, where Ostwald ripening is responsible for the growth of the nanowire building blocks; third, large-scale self-organization of the mesocrystals and the reduction of GO (at high GO concentration) occur simultaneously, resulting in an integrated hybrid architecture where porous three-dimensional (3D) framework structures interspersed among two-dimensional (2D) rGO sheets. Interestingly, “super-mesocrystals” formed by 3D oriented attachment of mesocrystals are also formed judging from the voided Sierpinski polyhedrons observed. Furthermore, the interior nanowire architecture of these mesocrystals can be kinetically controlled by careful variation of growth conditions. Owing to high specific surface area and improved conductivity, the rGO-Cu2O mesocrystals achieved a higher sensitivity toward NO2 at room temperature, surpassing the performance of standalone systems of Cu2O nanowires networks and rGO sheets. The unique characteristics of rGO-Cu2O mesocrystal point to its promising applications in ultrasensitive environmental sensors.
Three-component microspheres containing an SiO2-coated Fe3O4 magnetite core and a layered double hydroxide (LDH) nanoplatelet shell have been synthesized via an in situ growth method. The resulting ...Fe3O4@SiO2@NiAl-LDH microspheres display three-dimensional core–shell architecture with flowerlike morphology, large surface area (83 m2/g), and uniform mesochannels (4.3 nm). The Ni2+ cations in the NiAl-LDH shell provide docking sites for histidine and the materials exhibit excellent performance in the separation of a histidine (His)-tagged green fluorescent protein, with a binding capacity as high as 239 μg/mg. The microspheres show highly selective adsorption of the His-tagged protein from Escherichia coli lysate, demonstrating their practical applicability. Moreover, the microspheres possess superparamagnetism and high saturation magnetization (36.8 emu/g), which allows them to be easily separated from solution by means of an external magnetic field and subsequently reused. The high stability and selectivity of the Fe3O4@SiO2@NiAl-LDH microspheres for the His-tagged protein were retained over several separation cycles. Therefore, this work provides a promising approach for the design and synthesis of multifunctional LDH microspheres, which can be used for the practical purification of recombinant proteins, as well as having other potential applications in a variety of biomedical fields including drug delivery and biosensors.
An accelerating voltage of 100–300kV remains a good choice for the majority of TEM or STEM specimens, avoiding the expense of high-voltage microscopy but providing the possibility of atomic ...resolution even in the absence of lens-aberration correction. For specimens thicker than a few tens of nm, the image intensity and scattering contrast are likely to be higher than at lower voltage, as is the visibility of ionization edges below 1000eV (as required for EELS elemental analysis). In thick (>100nm) specimens, higher voltage ensures less beam broadening and better spatial resolution for STEM imaging and EDX spectroscopy.
Low-voltage (e.g. 30kV) TEM or STEM is attractive for a very thin (e.g. 10nm) specimen, as it provides higher scattering contrast and fewer problems for valence-excitation EELS. Specimens that are immune to radiolysis suffer knock-on damage at high current densities, and this form of radiation damage can be reduced or avoided by choosing a low accelerating voltage. Low-voltage STEM with an aberration-corrected objective lens (together with a high-angle dark-field detector and/or EELS) offers atomic resolution and elemental identification from very thin specimens. Conventional TEM can provide atomic resolution in low-voltage phase-contrast images but requires correction of chromatic aberration and preferably an electron-beam monochromator.
Many non-conducting (e.g. organic) specimens damage easily by radiolysis and radiation damage then determines the TEM image resolution. For bright-field scattering contrast, low kV can provide slightly better dose-limited resolution if the specimen is very thin (a few nm) but considerably better resolution is possible from a thicker specimen, for which higher kV is required. Use of a phase plate in a conventional TEM offers the most dose-efficient way of achieving atomic resolution from beam-sensitive specimens.
•100–300kV accelerating voltage is suitable for TEM specimens of typical thickness.•Lower voltage is preferable for thin specimens not damaged by radiolysis.•For very thin specimens, scattering contrast (but not phase contrast) is better at low voltage.•For thick specimens, higher voltage provides better resolution for STEM and EDX analysis.•For EELS elemental analysis, higher accelerating voltage is generally preferable.
Two porous covalent organic frameworks (COFs) with good biocompatibility were employed as drug nanocarriers, where three different drugs were loaded for subsequent drug release in vitro. The present ...work demonstrates that COFs are applicable in drug delivery for therapeutic applications.
Model catalysts with uniform and well-defined surface structures have been extensively employed to explore structure–property relationships of powder catalysts. Traditional oxide model catalysts are ...based on oxide single crystals and single crystal thin films, and the surface chemistry and catalysis are studied under ultrahigh-vacuum conditions. However, the acquired fundamental understandings often suffer from the “materials gap” and “pressure gap” when they are extended to the real world of powder catalysts working at atmospheric or higher pressures. Recent advances in colloidal synthesis have realized controlled synthesis of catalytic oxide nanocrystals with uniform and well-defined morphologies. These oxide nanocrystals consist of a novel type of oxide model catalyst whose surface chemistry and catalysis can be studied under the same conditions as working oxide catalysts. In this Account, the emerging concept of oxide nanocrystal model catalysts is demonstrated using our investigations of surface chemistry and catalysis of uniform and well-defined cuprous oxide nanocrystals and ceria nanocrystals. Cu2O cubes enclosed with the {100} crystal planes, Cu2O octahedra enclosed with the {111} crystal planes, and Cu2O rhombic dodecahedra enclosed with the {110} crystal planes exhibit distinct morphology-dependent surface reactivities and catalytic properties that can be well correlated with the surface compositions and structures of exposed crystal planes. Among these types of Cu2O nanocrystals, the octahedra are most reactive and catalytically active due to the presence of coordination-unsaturated (1-fold-coordinated) Cu on the exposed {111} crystal planes. The crystal-plane-controlled surface restructuring and catalytic activity of Cu2O nanocrystals were observed in CO oxidation with excess oxygen. In the propylene oxidation reaction with O2, 1-fold-coordinated Cu on Cu2O(111), 3-fold-coordinated O on Cu2O(110), and 2-fold-coordinated O on Cu2O(100) were identified as the active sites, respectively, to produce acrolein, propylene oxide, and CO2. Ceria rods enclosed with the {110} and {100} crystal planes, ceria cubes enclosed with the {100} crystal planes, and ceria octahedra enclosed with the {111} crystal planes exhibit distinct morphology-dependent oxygen vacancy concentrations and structures that can be well correlated with the surface compositions and structures of exposed crystal planes. Consequently, the metal–ceria interactions, structures, and catalytic performances of ceria-supported catalysts depend on the CeO2 morphology. Our results comprehensively reveal the morphology-dependent surface chemistry and catalysis of oxide nanocrystals that not only greatly deepen the fundamental understanding of oxide catalysis but also demonstrate a morphology-engineering strategy to optimize the catalytic performance of oxide catalysts. These results adequately exemplify the concept of oxide nanocrystal model catalysts for the fundamental investigations of oxide catalysis without the “materials gap” and “pressure gap”. With the structure–catalytic property relationships learned from oxide nanocrystal model catalyst studies and the advancement of controlled-synthesis methods, it is promising to realize the structural design and controlled synthesis of novel efficient oxide catalysts in the future.
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