82
5 Synthesis of Three-Dimensional Nanomaterials
5.2 Sol-Gel Method
In the field of advanced materials, the sol-gel process is an approach to produce
solid materials derived from the small molecules. This method is often utilized for
the synthesis of oxides, especially the titanium oxides or silicon oxides. This method
involves the transformation of monomers into a colloidal solution (sol) that works
as the precursor for an integrated network (or gel) of either network polymers or
discrete particles.
This method can date from mid-nineteenth century. Ebelman found the glassy
appearance of SiO 2 after the hydrolysis of tetraethyl orthosilicate. Meanwhile, the
water in SiO 2 gel replaced by organic solvent was found by Graham, attracting
chemists’ attention. After a long period of exploration, the colloid chemistry was
gradually formed. Sol-gel has been an effective method for preparing ultrafine
particles.
Hydrothermal carbonization (HTC) of carbohydrate is one of the promising candidates for the fabrication of carbon-based materials, since it can provide a facile, inexpensive, and eco-friendly route (Enterría and Figueiredo 2016). Nevertheless, the
preparation of highly porous carbon materials through a direct HTC process is quite
difficult. Herein, the solubilizing technique of micelles was proposed by Wang and
co-workers to direct the HTC of fructose, utilizing an amphiphilic block copolymer,
that is, poly-(4-vinylpyridine)-block-poly-(ethylene glycol) (P4VP-PEG) shown in
Fig. 5.2a, as a structure-directing agent (Wang et al. 2014). Based on this strategy,
porous hierarchical carbon materials with tunable properties were successfully manufactured. The P4VP-PEG micelles could solubilize fructose and meanwhile restrict
the appearance of primary carbon domains upon the sol-gel processes. Moreover, the
size of micelle could be easily controlled through adjusting the synthetic conditions.
Thereby, the particle size of the resultant carbon materials was effectively tuned at
the range of 20–100 nm based on the direction of the initial micelle size.
Besides, uniform and porous yolk-shelled carbon spheres (YS-CSs) with a hierarchical structure have been successfully prepared via a novel gradient sol-gel strategy
with surfactant-directing co-assembly by utilizing cationic surfactant cetyltrimethylammonium bromide (CTAB) as a template, resorcinol-formaldehyde (RF) as a
carbon source along with tetraethoxysilane (TEOS) as an assistant pore-forming
reagent (by Wang et al. 2015). On the basis of the abovementioned observations,
the synthesis of yolk-shell-structured carbon spheres undergoing a gradient sol-gel
process with surfactant-directing co-assembly has been proposed (Fig. 5.2b). Firstly,
in an alkaline solution with ethanol-water mixed solvent, RF precursor forms negatively charged emulsion droplets through the hydrogen bonding of ammonia, water,
alcohol, resorcinol, and formaldehyde, silicate oligomers hydrolyzed from TEOS
are also negatively charged, while CTAB with positive charged (CTA
+ ) can bind to
the surface of the formed RF emulsion droplets and silicate oligomers via electrostatic interactions. Under the catalysis of ammonia molecules, the cross-linkage of
RF droplets and silicate oligomers occurs but with different rates. In our case, the
hydrolysis polymerization of RF is much faster than that of TEOS at the sol-gel
5 Synthesis of Three-Dimensional Nanomaterials
5.2 Sol-Gel Method
In the field of advanced materials, the sol-gel process is an approach to produce
solid materials derived from the small molecules. This method is often utilized for
the synthesis of oxides, especially the titanium oxides or silicon oxides. This method
involves the transformation of monomers into a colloidal solution (sol) that works
as the precursor for an integrated network (or gel) of either network polymers or
discrete particles.
This method can date from mid-nineteenth century. Ebelman found the glassy
appearance of SiO 2 after the hydrolysis of tetraethyl orthosilicate. Meanwhile, the
water in SiO 2 gel replaced by organic solvent was found by Graham, attracting
chemists’ attention. After a long period of exploration, the colloid chemistry was
gradually formed. Sol-gel has been an effective method for preparing ultrafine
particles.
Hydrothermal carbonization (HTC) of carbohydrate is one of the promising candidates for the fabrication of carbon-based materials, since it can provide a facile, inexpensive, and eco-friendly route (Enterría and Figueiredo 2016). Nevertheless, the
preparation of highly porous carbon materials through a direct HTC process is quite
difficult. Herein, the solubilizing technique of micelles was proposed by Wang and
co-workers to direct the HTC of fructose, utilizing an amphiphilic block copolymer,
that is, poly-(4-vinylpyridine)-block-poly-(ethylene glycol) (P4VP-PEG) shown in
Fig. 5.2a, as a structure-directing agent (Wang et al. 2014). Based on this strategy,
porous hierarchical carbon materials with tunable properties were successfully manufactured. The P4VP-PEG micelles could solubilize fructose and meanwhile restrict
the appearance of primary carbon domains upon the sol-gel processes. Moreover, the
size of micelle could be easily controlled through adjusting the synthetic conditions.
Thereby, the particle size of the resultant carbon materials was effectively tuned at
the range of 20–100 nm based on the direction of the initial micelle size.
Besides, uniform and porous yolk-shelled carbon spheres (YS-CSs) with a hierarchical structure have been successfully prepared via a novel gradient sol-gel strategy
with surfactant-directing co-assembly by utilizing cationic surfactant cetyltrimethylammonium bromide (CTAB) as a template, resorcinol-formaldehyde (RF) as a
carbon source along with tetraethoxysilane (TEOS) as an assistant pore-forming
reagent (by Wang et al. 2015). On the basis of the abovementioned observations,
the synthesis of yolk-shell-structured carbon spheres undergoing a gradient sol-gel
process with surfactant-directing co-assembly has been proposed (Fig. 5.2b). Firstly,
in an alkaline solution with ethanol-water mixed solvent, RF precursor forms negatively charged emulsion droplets through the hydrogen bonding of ammonia, water,
alcohol, resorcinol, and formaldehyde, silicate oligomers hydrolyzed from TEOS
are also negatively charged, while CTAB with positive charged (CTA
+ ) can bind to
the surface of the formed RF emulsion droplets and silicate oligomers via electrostatic interactions. Under the catalysis of ammonia molecules, the cross-linkage of
RF droplets and silicate oligomers occurs but with different rates. In our case, the
hydrolysis polymerization of RF is much faster than that of TEOS at the sol-gel