Metal–organic frameworks (MOFs)
are often synthesized using
various additives to modulate the crystallization. Here, we report
the direct imaging of the crystal surface of MOF MIL-101 synthesized
with different additives, using low-dose high-resolution transmission
electron microscopy (HRTEM), and identify three distinct surface structures,
at subunit cell resolution. We find that the mesoporous cages at the
outermost surface of MIL-101 can be opened up by vacuum heating treatment
at different temperatures, depending on the MIL-101 samples. We monitor
the structural evolution of MIL-101 upon vacuum heating, using in
situ X-ray diffraction, and find the results to be in good agreement
with HRTEM observations, which leads us to speculate that additives
have an influence not only on the surface structure but also on the
stability of framework. In addition, we observe solid–solid
phase transformation from MIL-101 to MIL-53 taking place in the sample
synthesized with hydrofluoric acid.
MnCoGe-based compounds undergo a giant negative thermal expansion (NTE) during the martensitic structural transition from Ni2In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution neutron diffraction experiments revealed that the expansion of unit cell volume can be as large as ΔV/V ∼ 3.9%. The optimized compositions with concurrent magnetic and structural transitions have been studied for magnetocaloric effect. However, these materials have not been considered as NTE materials partially due to the limited temperature window of phase transition. The as-prepared MnCoGe-based compounds are quite brittle and naturally collapse into powders. By using a few percents (3-4%) of epoxy to bond the powders, we introduced residual stress in the bonded samples and thus realized the broadening of structural transition by utilizing the specific characteristics of lattice softening enforced by the stress. As a result, giant NTE (not only the linear NTE coefficient α but also the operation-temperature window) has been achieved. For example, the average α̅ as much as -51.5 × 10(-6)/K with an operating temperature window as wide as 210 K from 122 to 332 K has been observed in a bonded MnCo0.98Cr0.02Ge compound. Moreover, in the region between 250 and 305 K near room temperature, the α value (-119 × 10(-6)/K) remains nearly independent of temperature. Such an excellent performance exceeds that of most other materials reported previously, suggesting it can potentially be used as a NTE material, particularly for compensating the materials with large positive thermal expansions.
Binary transition metal phosphides hold immense potential as innovative electrode materials for constructing high-performance energy storage devices. Herein, porous binary nickel-cobalt phosphide (NiCoP) nanosheet arrays anchored on nickel foam (NF) were rationally designed as self-supported binder-free electrodes with high supercapacitance performance. Taking the combined advantages of compositional features and array architectures, the nickel foam supported NiCoP nanosheet array (NiCoP@NF) electrode possesses superior electrochemical performance in comparison with Ni-Co LDH@NF and NiCoO2@NF electrodes. The NiCoP@NF electrode shows an ultrahigh specific capacitance of 2143 F g-1 at 1 A g-1 and retained 1615 F g-1 even at 20 A g-1, showing excellent rate performance. Furthermore, a binder-free all-solid-state asymmetric supercapacitor device is designed, which exhibits a high energy density of 27 W h kg-1 at a power density of 647 W kg-1. The hierarchical binary nickel-cobalt phosphide nanosheet arrays hold great promise as advanced electrode materials for supercapacitors with high electrochemical performance.
The comprehension of the interactions between the building blocks in hybrids can give us an insight into the design and application of highly efficient electromagnetic wave absorption materials. Herein, we report a facile in situ thermal decomposition route for the fabrication of superparamagnetic Fe 3 O 4 nanocrystals anchored on hydrophobic graphene nanosheets as synergistic electromagnetic wave absorbers. The microstructures and interactions of the Fe 3 O 4 -graphene hybrids are systematically investigated, and the results suggest that the Fe 3 O 4 nanocrystals are uniformly decorated and chemically bonded on the surface of graphene nanosheets without obvious conglomeration or large vacancies. The Fe 3 O 4 -graphene hybrids show hydrophobic and superparamagnetic characteristics. Combing the benefits of superparamagnetic Fe 3 O 4 nanocrystals and electrically conducting graphene, the Fe 3 O 4 -graphene hybrids show a maximum reflection loss (RL) of À40 dB at 6.8 GHz with a matching thickness of 4.5 mm, and the effective absorption bandwidth (RL o À10 dB) is 4.6-18 GHz with an absorber thickness of only 2-5 mm. However, due to the lack of dielectric loss, only a weak RL of À5 dB is obtained in bare Fe 3 O 4 nanocrystals. The remarkably enhanced electromagnetic wave absorption properties of the Fe 3 O 4 -graphene hybrids are owing to effective impedance matching and synergistic interaction. Moreover, compared with other reported graphene-based electromagnetic wave absorption materials, the hydrophobic Fe 3 O 4 -graphene hybridsprepared in this work are considered to be more stable and suitable to be applied in some particular environmental conditions, such as rain.
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