Metal Oxide-Hierarchical Porous Silica Nanocomposites Prepared by Nanoemulsion Templating and Integrative Synthesis
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Nanoemulsions are templates that have the potential to fill the gap between micellar systems and latex particles in the preparation of porous materials. A nanoemulsion can also be used as a carrier for uploading the desired materials inside the pore formed after the removal of the template. In this research, oil-in-water (O/W) nanoemulsions were prepared by means of a low-energy method based on a phase inversion composition (PIC) technique, using two nonionic surfactants (Tween 80 and Span 80), which can be mixed in order to adjust the hydrophilic-lipophilic balance (HLB). The influence of a number of parameters on the tunability and stability of such nanoemulsions was also studied. The effect of the simultaneous intercrossing of multifactors on droplet size was explored using a process- mixture design, and the size of the nanoemulsion oil droplets was measured by means of dynamic light scattering (DLS). The nanoemulsions were combined with sol-gel method in order to prepare porous silica with a macroporosity in the 50 nm to 400 nm range. The results demonstrate that a precise synergy between the silica source and the nanoemulsions is essential for effective interactions and homogeneous structures. Depending on the nature of such interactions, a variety of materials were observed, from hollow particles to continuous gels. Changing the size of the oil droplet and the volume of the nanoemulsions produced silica with differing pore sizes and varying total pore volumes. The obtained hierarchical porous silica (HPS) were characterized using mercury porosimetry, small angle X-ray scattering (SAXS), nitrogen isotherms, Fourier transform infrared (FTIR) analysis, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The parallel use of the oil vesicles as containers for the further synthesis of metal oxide is a novel method of internally functionalizing the silica. When hydrophobic metal precursors are dissolved into the oil phase before the preparation of the nanoemulsion, they are confined within the globular cavities of the silica. The thermal treatment applied to the material to burn the organics then leads to the final formation of metal oxide nanoparticles, which are larger than the porosity of the silica matrix but entrapped within the large cavities, producing a "rattle-like" structure. This method was demonstrated through the synthesis of Fe2O3, Fe3O4, and Co3O4 nanoparticles, and the results showed that a rather large amount of metal oxide (up to a 60 wt.% of metal oxide in nanocomposites) be generated while still maintaining the nanometric size observed at lower concentrations. This method allows control of the type of metal oxide, the concentration of the metal oxide, and the pore size, which enables the creation of different types of nanocomposites. Metal oxide hierarchical porous silica (MHPS) nanocomposites were characterized based on nitrogen isotherms, TEM and SEM observations, FTIR analysis, X-ray diffraction (XRD), and Mossbauer spectroscopy. Magnetic measurements were also taken. This new method, using the new templating objects, is a perfect illustration of the concept of "integrative synthesis,” whereby the combination of building units and reactional mechanisms leads to complex structures as a result of true synergy among the elements during the reaction. In this case, the size of the nanoemulsion and the total water volume both contribute to the generation of distinctive architectures. In addition, the reaction of the metal oxide precursors within the cavities limits the extension of the final crystal size, but the surrounding solid matrix plays a role as well by keeping the particles apart. The final factor is that the reactive materials cannot leak from the silica because of the rattle-like structure, but the reagents can reach those particles through the porosity of the silica framework.