Metal-organic frameworks (MOFs) have demonstrated great potentials in a variety of important applications. To enhance the inherent properties and endow materials with multifunctionality, the rational design and synthesis of MOFs with nanoscale porosity and hollow feature is highly desired and remains a great challenge. In this work, the formation of a series of well-defined MOF (MOF-5, Fe(II) -MOF-5, Fe(III) -MOF-5) hollow nanocages by a facile solvothermal method, without any additional supporting template is reported. A surface-energy-driven mechanism may be responsible for the formation of hollow nanocages. The addition of pre-synthesized poly(vinylpyrrolidone)- (PVP) capped noble-metal nanoparticles into the synthetic system of MOF hollow nanocages yields the yolk-shell noble metal@MOF nanostructures. The present strategy to fabricate hollow and yolk-shell nanostructures is expected to open up exciting opportunities for developing a novel class of inorganic-organic hybrid functional nanomaterials.

Metal-organic frameworks (MOFs) are potentially useful molecular materials that can exhibit structure flexibilities induced by some external stimuli. Such structure transformations can furnish MOFs with improved properties. The shape-controlled growth of MOFs combined with crystal-structure transformation is rarely achieved. Herein, we demonstrate the synthesis of hierarchical Zn/Ni-MOF-2 nanosheet-assembled hollow nanocubes (NAHNs) by a facile surfactant-free solvothermal approach. The unique nanostructures undergo crystal-structure transformation from Zn/Ni-MOF-5 nanocubes to Zn/Ni-MOF-2 nanosheets, which is analogous to the dissolution and recrystallization of inorganic nanocrystals. The present synthetic strategy to fabricate isostructural MOFs with hierarchical, hollow, and bimetallic nanostructures is expected to expand the diversity and range of potential applications of MOFs.

A recently discovered two-dimensional (2D) layered material phosphorene has attracted considerable interest as a promising p-type semiconducting material. In this work, thermal conductivity (κ) of monolayer phosphorene is calculated using large-scale classical non-equilibrium molecular dynamics (NEMD) simulations. The predicted thermal conductivities for infinite length armchair and zigzag phosphorene sheets are 63.6 and 110.7 W m(-1) K(-1) respectively. The strong anisotropic thermal transport is attributed to the distinct atomic structures at altered chiral directions and direction-dependent group velocities. Thermal conductivities of 2D graphene sheets with the same dimensions are also computed for comparison. The extrapolated κ of the 2D graphene sheet are 1008.5(+37.6)(-37.6) and 1086.9(+59.1)(-59.1) W m(-1) K(-1) in the armchair and zigzag directions, respectively, which are an order of magnitude higher than those of phosphorene. The overall and decomposed phonon density of states (PDOS) are calculated in both structures to elucidate their thermal conductivity differences. In comparison with graphene, the vibrational frequencies that can be excited in phosphorene are severely limited. The temperature effect on the thermal conductivity of phosphorene and graphene sheets is investigated, which reveals a monotonic decreasing trend for both structures.

Limited internal phonon coupling and transfer within graphene in the out-of-plane direction significantly affects graphene-substrate interfacial phonon coupling and scattering, and leads to unique interfacial thermal transport phenomena. Through the simultaneous characterization of graphene and SiC Raman peaks, it is possible, for the first time, to distinguish the temperature of a graphene layer and its adjacent 4H-SiC substrate. The thermal probing resolution reaches the nanometer scale with the graphene (≈1.12 nm) and is on the micrometer scale (≈12 μm) within SiC next to the interface. A very high thermal resistance at the interface of 5.30 (-0.46) (+0.46) x 10(-5) Km2 W(-1) is observed by using a Raman frequency method under surface Joule heating. This value is much higher than those from molecular dynamics predictions of 7.01(-1.05) (+1.05) x 10(-1) and 8.47(-0.75) (+0.75) x 10(-10) Km2 w(-1) for surface heat fluxes of 3 × 10(9) and 1 × 10(9) and 1 x 10(10) W m(-2) , respectively. This analysis shows that the measured anomalous thermal contact resistance stems from the thermal expansion mismatch between graphene and SiC under Joule heating. This mismatch leads to interface delamination/separation and significantly enhances local phonon scattering. An independent laser-heating experiment conducted under the same conditions yielded a higher interfacial thermal resistance of 1.01(-0.59) (+1.23) x 10(-4) Km2 W(-1). Furthermore, the peak width method of Raman thermometry is also employed to evaluate the interfacial thermal resistance. The results are 3.52 × 10(-5) and 8.57 × 10(-5) K m2 W(-1) for Joule-heating and laser-heating experiments, respectively, confirming the anomalous thermal resistance between graphene and SiC. The difference in the results from the frequency and peak-width methods is caused by the thermal stress generated in the heating processes.

As the dimensions of nanocircuits and nanoelectronics shrink, thermal energies are being generated in more confined spaces, making it extremely important and urgent to explore for efficient heat dissipation pathways. In this work, the phonon energy transport across graphene and hexagonal boron-nitride (h-BN) interface is studied using classic molecular dynamics simulations. Effects of temperature, interatomic bond strength, heat flux direction, and functionalization on interfacial thermal transport are investigated. It is found out that by hydrogenating graphene in the hybrid structure, the interfacial thermal resistance (R) between graphene and h-BN can be reduced by 76.3%, indicating an effective approach to manipulate the interfacial thermal transport. Improved in-plane/out-of-plane phonon couplings and broadened phonon channels are observed in the hydrogenated graphene system by analyzing its phonon power spectra. The reported R results monotonically decrease with temperature and interatomic bond strengths. No thermal rectification phenomenon is observed in this interfacial thermal transport. Results reported in this work give the fundamental knowledge on graphene and h-BN thermal transport and provide rational guidelines for next generation thermal interface material designs.

The design of cost-effective electrocatalysts and supercapacitor electrode materials with delicate hierarchical morphologies and enhanced functionalities has great potential in energy conversion and storage processes.

Uniform Pt-Cu alloy nanocrystals in the shape of dendrite, yolk-cage, and box structures are prepared via a facile wet-chemical reduction route in which glycine is demonstrated to alter the reduction kinetics of metal cations, critical to the morphology of the obtained product. These alloy nanocrystals exhibit superior specific activity and stability in the electro-oxidation of methanol.

Metal‐organic frameworks (MOFs) are potentially useful molecular materials that can exhibit structure flexibilities induced by some external stimuli. Such structure transformations can furnish MOFs with improved properties. The shape‐controlled growth of MOFs combined with crystal‐structure transformation is rarely achieved. Herein, we demonstrate the synthesis of hierarchical Zn/Ni‐MOF‐2 nanosheet‐assembled hollow nanocubes (NAHNs) by a facile surfactant‐free solvothermal approach. The unique nanostructures undergo crystal‐structure transformation from Zn/Ni‐MOF‐5 nanocubes to Zn/Ni‐MOF‐2 nanosheets, which is analogous to the dissolution and recrystallization of inorganic nanocrystals. The present synthetic strategy to fabricate isostructural MOFs with hierarchical, hollow, and bimetallic nanostructures is expected to expand the diversity and range of potential applications of MOFs.