Crystalline nanoporous materials are one of the most important families of complex functional material. Many questions pertaining to the molecular assembly mechanism of the framework of these ...materials remain unanswered. Only recently has it become possible to answer definitively some of these questions by observation of growing nanoscopic surface features on metal organic frameworks (MOFs) through use of in situ atomic force microscopy (AFM). Here we reveal that a growth process of a MOF, zeolitic imidazolate framework ZIF-8, occurs through the nucleation and spreading of successive metastable unenclosed substeps to eventually form stable surface steps of the enclosed framework structure and that this process is reliant on the presence of nonframework species to bridge the developing pores during growth. The experiments also enable identification of some of the fundamental units in the growth process and the stable crystal surface plane. The former findings will be applicable to numerous nanoporous materials and support efforts to synthesize and design new frameworks and to control the crystal properties of these materials.
A gravimetric study of the solvothermal dimethylformamide-mediated crystallisation of the metal-organic framework ZIF-8 has enabled the rate constants and activation energies of the nucleation and ...crystal growth processes to be determined. The kinetics analyses reveal the crystallisation is nucleation controlled with activation energies for nucleation and crystal growth of 115.1 and 87.7 kJ mol super(-1), respectively. The results are compared to the crystallisation kinetics for other syntheses of ZIF-8 and other MOFs to reveal the importance that different solvents, different metal ion/organic linker ratios and the presence of deprotonating modulators have on these processes. Temporal ex situmonitoring of the crystal morphology and surface topography during crystallisation reveals the crystals evolve to the equilibrium rhombic dodecahedral morphology viaedge-truncated cubic and corner-truncated rhombic dodecahedral habits while relatively smooth surfaces containing large truncated rhombic surface terraces and growth spirals develop through rough surfaces consisting of numerous ill-defined small growth islands and steps. The combination of techniques applied over different size scales provides important understanding for potential control over the crystal properties of MOFs for new application or enhanced performance in current applications.
A gravimetric study of the solvothermal dimethylformamide-mediated crystallisation of the metal-organic framework ZIF-8 has enabled the rate constants and activation energies of the nucleation and ...crystal growth processes to be determined. The kinetics analyses reveal the crystallisation is nucleation controlled with activation energies for nucleation and crystal growth of 115.1 and 87.7 kJ mol
−1
, respectively. The results are compared to the crystallisation kinetics for other syntheses of ZIF-8 and other MOFs to reveal the importance that different solvents, different metal ion/organic linker ratios and the presence of deprotonating modulators have on these processes. Temporal
ex situ
monitoring of the crystal morphology and surface topography during crystallisation reveals the crystals evolve to the equilibrium rhombic dodecahedral morphology
via
edge-truncated cubic and corner-truncated rhombic dodecahedral habits while relatively smooth surfaces containing large truncated rhombic surface terraces and growth spirals develop through rough surfaces consisting of numerous ill-defined small growth islands and steps. The combination of techniques applied over different size scales provides important understanding for potential control over the crystal properties of MOFs for new application or enhanced performance in current applications.
Parallel monitoring at the bulk, single crystal and surface level provide unique insight into the solvothermal crystallisation, and potential control over the crystal properties, of ZIF-8.
Reinforced electrospun PAN nanofibers with carbon nanotubes and carbon nanofibers were produced via electrospinning techniques. The spinning was carried out in a ratio of 6 and 10 wt % of CNT and CNF ...to PAN slurry respectively. The applied voltage was 10.0 kV and the spuns had undergone three stages of thermal treatments, namely, stabilization, carbonization, and graphitization. The studies indicated that the nanofiber diameter, thermal stability and electric conductivity properties of the PAN electrospun were greatly affected by the ratio of the nanoparticles. The morphological study revealed that the fiber diameters of the PAN/CNT electrospun were between 0.62 to 0.70 mm, and the PAN/CNF fiber diameters were between 1.03 to 1.20 mm respectively. For the thermal stability study, it was observed that the thermal degradation of the PAN polymer had increased as the CNT and CNF ratios increased. The electro-conductivity study also revealed that addition of CNT had increased the conductivity of the PAN polymer until 5.4 x 10-4 Scm-1 besides PAN/CNF increased the electrical resistivity of PAN from 2.49 x 10-5 to 1690.2 Ω cm-1 respectively.
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