5.2 Sol-Gel Method
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Fig. 5.2 a Mechanism for the fabrication of hierarchically porous aggregated carbon materials.
Reprinted from Ref. Wang et al. (2014), copyright 2014, with permission from The Royal Society
of Chemistry. b The schematic illustration of the three-step synthetic processes of the yolk-shell
porous carbon spheres: (1) gradient sol-gel process, (2) carbonization process, (3) silica removal
by HF. Reprinted from Ref. Wang et al. (2015), copyright 2015, with permission from Elsevier.
c Diagram of the GO-RF aerogel preparation process. Reprinted from Ref. Lim et al. (2015a, b),
copyright 2015, with permission from Elsevier
process beginning. Thus only few silicate oligomer stakes part in the co-assembly
of CTAB and RF through the electrostatic interaction, leading to the core formation resulted from the condensation and growth of the assembled hybrid CTAB/RF
aggregates. With the sol-gel process prolonged, the concentration of RF emulsion
was gradually decreased due to the RF consumed for the core growth, resulting in
a slower polymerization rate which well matches the hydrolysis polymerization rate
of TEOS. Thus the gradient hydrolysis and condensation of silicates begin. The silicate oligomers from the hydrolysis of TEOS together with RF emulsion droplets can
interact with CTAB to co-assemble at the surface of the cores, thereby creating the
hybrid silica/CTAB/RF shells and forming core-shell structured products. Obviously,
the component differences between the core and shell strongly rely on the gradient
sol-gel process of RF and TEOS under alkaline conditions.
Similarly, spinel NiCo 2 O 4 was successfully synthesized by an easy sol-gel
approach with the participation of three crucial chelating agents including citric acid