2.1. Hybrid Pyrolysis Carbonization
Pyrolysis is an endothermic process, and pyrolytic carbonization refers to the conversion of biomass or other organic matter into carbon materials by thermochemical decomposition under atmospheric pressure in an inert atmosphere (such as N
2 or Ar) [
55,
56,
57]. This preparation method involves the pyrolysis of organic precursors followed by carbonization, with the release of various hydrocarbon gaseous species, to obtain residues with high carbon content (carbonaceous residues). The pyrolysis of organic precursors and the carbonization of carbon-containing residues occur continuously; thus, pyrolysis and carbonization usually overlap with each other [
58]. During the preparation process, the carbon precursor homogeneously mixed with the silica material is first carbonized by the pyrolysis treatment, while the silica material retains a relatively rigid structure during the carbonization process due to its high melting point. As the pyrolytic carbonization is completed, the two components of carbon and silica will be closely combined to form a carbon-silica composite material. The preparation conditions and resultant material properties of the carbon-silica composites prepared by the hybrid pyrolysis carbonization method are listed in
Table 3.
Taking this synthetic strategy, Aranda et al. [
62] reported a method for preparing carbon-silica foam using microwave irradiation to greatly shorten the preparation time without using a foaming agent and strong acid; this carbon-silica composite could be used to remove soluble toxic compounds in water pollution. Sucrose was used as the carbon precursor, and microporous silica gel was used as the inorganic carrier for the chemical conversion of sucrose. When the carbon and silica precursors were homogeneously mixed, the sucrose was initially converted into caramel by short-term microwave irradiation. The microwave irradiation time during the caramelization step was strictly controlled to avoid carbonization. The carbon/silica material was subsequently obtained by heat treatment (
Figure 2). Notably, water plays a significant dual role in this process: first, it homogenizes the sucrose/silica mixture and makes it easier to convert sucrose to caramel, and second, a part of the water is adsorbed in the micropores of the silica, where the slow release of steam during heating contributes to the formation of macropores. In addition to acting as an adsorbent for water, the porous silica gel also serves as an adsorbent for volatile organic compounds generated during the thermal decomposition of carbon precursors. Once these volatile compounds become air bubbles, they can be used as a pore-forming agent for carbon-silica foam. In a similar way, Tomaszewski et al. [
63] used silica gel as the carrier and potato starch as the carbon precursor to prepare a carbon-silica composite adsorbent and investigate the influence of structural characteristics on its solid-phase extraction performances for explosive nitramines. The textural results showed that the deposition of carbon nanoparticles on the silica gel led to a decrease in the pore volume but an enhanced proportion of micropores. In this way, the recovery rate of the explosive nitramines was also increased due to the more accessible surface area of the carbon phase. In addition to biomass precursors, other organics can also be adopted as the carbon precursors (e.g., dichloromethane or surfactant). For example, Gao et al. [
64] prepared a mesoporous carbon-silica composite by carbonizing the cationic surfactant CTAB in mesoporous silica under 800 °C with the assistance of concentrated sulfuric acid. When using surfactant template as carbon source, the problems caused by using exogenous carbon sources, such as pore blocking and coating unevenness, can be avoided.
The volume expansion of silica-based materials during the charging and discharging process can lead to excessive capacity decay and poor conductivity, which is adverse to rapid charging and discharging. For this reason, Xing et al. [
65] successfully prepared a new type of silica-based material using a simple pyrolysis strategy to solve the volume expansion problem. By virtue of this strategy, nitrogen doping and carbon nanosheet coating on a silica precursor could be simultaneously realized in one step. Firstly, silica nanoparticles of different sizes were prepared by the hydrolysis and condensation reaction of TEOS; then, a certain amount of anhydrous glucose, dicyandiamide, and silica nanosphere powder were ground. The obtained white slurry was then dried and calcined in a protection atmosphere, and a nitrogen-doped carbon nanosheet–silica composite material was finally obtained. Through this method, the particle size of the synthesized silica nanospheres could be well controlled while the size-controllable nanosilica spheres were embedded into the carbon nanosheets, which greatly enhanced the electronic conductivity of the composite. In addition, the existence of carbon nanosheets could also serve as a buffer layer to effectively alleviate the volume expansion of electrode materials during delithiation (wherein Li ions depart from the electrode and dissolve into the electrolyte) and intercalation (wherein Li ions move from the electrolyte to the lattice of the electrode). In addition to the high capacity of silica, a moderate doping of nitrogen in the graphitic matrix further improved the Li-ion storage performance and electron transfer rate of the composites [
65].
Thermal stability is a long-standing bottleneck in the application of traditional activated carbon. In order to solve the safety problem during the use of activated carbon, Kong et al. [
66] prepared a carbon-silica composite material by extrusion molding and heat treatment over a commercial activated carbon, sodium silicate, and sodium carboxymethyl cellulose mixture. First, the powdered activated carbon and sodium carboxymethyl cellulose were fully mixed; second, the dissolved sodium silicate was added to the mixture during the mixing process; and the sample was finally extruded by manual hydraulic pressure and calcined in a tube furnace under a N
2 atmosphere. After heat treatment and vacuum drying, a column-shaped carbon-silica composite material was obtained (
Figure 3a). More importantly, the adoption of sodium silicate during the synthesis further promoted the thermal stability of the composite by both the cooling and gas dilution effect (derived from the hydrated water in sodium silicate) and the intumescent-like flame effect (derived from its decomposition;
Figure 3b). The ignition point of the column-shaped carbon-silica composite material increased by at least 40 °C when compared with the traditional activated carbon.
The studies discussed above mostly focused on carbon-silica composites using biomass or hydrocarbon organics as the carbon precursor and TEOS as the Si precursor. In recent years, metal–organic frameworks have proven to be ideal precursors for the synthesis of carbon or metal oxide/carbon nanomaterials due to their excellent electrochemical properties and high porosity. However, the combination of silicon precursors and MOFs remains challenging due to their distinct properties. Wang et al. [
67] reported a synthetic strategy for preparing carbon–silicon composites using SiAl alloy microspheres as self-sacrificing templates. The SiAl alloy/Al-MOF core-shell precursor was formed by the self-etching reaction of silicon–aluminum alloy microspheres in organic acid solution. Carbon–silicon composites were then prepared by the annealing and etching of SiAl/Al-MOF precursors. As a consequence, the composite had a superior core-shell structure consisting of an amorphous carbon shell and a porous silicon microsphere core. The MOF-derived carbon shell and porous silicon microsphere core promoted the electronic conductivity, enhanced the electrochemical kinetics and, most importantly, alleviated the volume expansion effect (
Figure 4).
2.2. Hydrothermal Carbonization
Hydrothermal carbonization is a thermochemical conversion technology where carbon precursors, such as biomass, are converted into energy and chemicals without pre-drying [
68]. The hydrothermal carbonization process takes place in aqueous solution in a closed vessel at relatively low temperature (180–250 °C) and saturation pressure (2–10 MPa). The carbon precursors undergo hydrolysis, dehydration, decarbonylation, decarboxylation, and polymerization, and are finally converted into charcoal. The dehydration and decarboxylation processes can reduce both the oxygen and hydrogen contents of the carbon precursors, in the form of either water or CO
2 [
68,
69,
70,
71,
72,
73]. The hydrothermal product distribution mainly depends on the feedstock and treating temperature, as well as the reaction time and the carbon–water ratio [
69]. Different hydrothermal parameters result in a varied coalification degree of the precursors [
68]. For the preparation of carbon-silica composite materials, biomass is usually used as the carbon source. The carbon and silica precursors are fully mixed in an aqueous solution, transferred into a high-pressure reactor for hydrothermal reaction, and finally a carbon-silica composite is obtained. The preparation conditions and material properties of the carbon-silica composites prepared by the hydrothermal carbonization method are listed in
Table 4.
Separation is one of the most important unit operations in chemical engineering. In order to prepare a stationary phase with hydrophilic interaction for liquid chromatography, Zhao et al. [
77] used silica microspheres as a carrier and cyclodextrin as a carbon source to prepare a carbon-coated composite by hydrothermal carbonization. Cyclodextrin and polyvinylpyrrolidone were first added to a Teflon liner containing deionized water along with silica microspheres. After the hydrothermal reaction, the slurry-packed capillary columns containing the carbon-silica stationary phase exhibited excellent chromatographic repeatability, separation selectivity, and pH stability for polar compounds, such as phenols and endocrine disrupting chemicals (EDCs;
Figure 5). Since the trade-off between the polarity and selectivity, especially for polar organics, is a common issue in adsorption, Yang et al. [
78] used waste lithium–silicon powder and commercial activated carbon as resources for preparing a zeolite-activated carbon composite material, so as to combine the advantages of activated carbon and molecular sieve. The lithium–silicon powder was first treated with hydrochloric acid, and the treated powder was added into a reactor together with activated carbon and sodium hydroxide for hydrothermal reaction. The specific surface area of the obtained zeolite–activated carbon composite was much higher than that of zeolite and lithium silica fume, reaching 660 m
2/g. To further promote the adsorption performance and hydrophobicity of silica materials, Lu et al. [
76] introduced a carbon source to silica gel and prepared hydrophobic carbon-silica composites with hierarchical pore structures via hydrothermal treatment. In this method, the silica gel was first immersed in sucrose solution. After filtration, the mixture was transferred to a stainless steel autoclave for hydrothermal reaction. The carbon/silica composite material was then collected after calcination at 800 °C for 4 h. Accordingly, the synthesized carbon-silica composite had significantly improved adsorption capacity and desorption efficiency for toluene.
In addition, Guo and Liang et al. [
79] took advantage of hydrothermal carbonization to decorate ordered and disordered mesoporous silica surfaces with carbon layers and obtained mesoporous carbon-silica composites with uniform carbon coating and controllable thickness. First, SBA-15 or spherical silica gel was dried, added to toluene solution containing 3-aminopropyl trimethoxysilane (APTMS) and stirred at 110 °C in a N
2 atmosphere. Then, the obtained product was washed with toluene, dichlorotoluene, methanol, water, and methanol in turn to introduce amino groups. The amino-modified SBA-15 or spherical silica gel was later mixed with a glucose precursor and reacted hydrothermally in an autoclave at 160–220 °C, followed by carbonization in an Ar atmosphere at 600–800 °C for 3 h to obtain SBA-15–carbon or spherical silica–carbon composites. The application results showed that the composites demonstrated a high adsorption capacity for dyes and good separation performances for oligosaccharide isomers (
Figure 6).
As a superior separation method, solid-phase microextraction is widely applied in trace chemical detection; however, the conventional preparation procedure of solid-phase microextraction coatings is rather complicated and time-consuming, and the obtained coatings have poor thermal and mechanical stability. To solve this problem, Saraji et al. [
80] proposed a simple one-step hydrothermal method. TEOS was used as the silica source, while carbon nanotubes were used as the carbon source and glucose as the connecting agent between the silica and carbon nanotubes. Carbon nanotubes were first added to an ethanol solution containing glucose. The resulting black mixture was hydrothermally treated in an autoclave at 180 °C for 5 h after sonication. The dried hydrothermal product was added into ethanol again for ultrasonic stirring. During the stirring process, TEOS was added dropwise to the solution, which was stirred for a period of time, and a carbon nanotube–silica composite material was obtained after centrifugation and drying, without any further treatments. The carbon nanotube–silica composite, when used as a microextraction coating, showed excellent thermal and mechanical stability.
Graphene is now a prevalent material for many applications. Qian et al. [
81] found that the disturbance of the water–oil interface (e.g., by rapid stirring) would cause the partial hydrolysis of TEOS and result in its interaction with surfactants to form vesicle structures. During this process, both the organic solvent and TEOS are encapsulated in the vesicles. With hydrothermal treatment, the TEOS in the vesicles is gradually hydrolyzed and then adsorbed on the polar end of the surfactant via electrostatic interaction, while the organic solvent is adsorbed on the hydrophobic end through hydrophobic interactions. Therefore, TEOS can be rapidly hydrolyzed and combined with surfactants to form a silica layer during hydrothermal reaction, which can prevent the leakage of organic solvent from the vesicles. Meanwhile, the organic solvents between the silica layers can interact with each other to form agglomerated pairs of benzadiene or its oligomers, which further form graphene after calcination. In this way, the deteriorated performance caused by graphene agglomeration can be avoided, while the properties of graphene and porous silica are integrated (
Figure 7).
The processes discussed above are frequently used in carbon–SiO
2 composites. In addition, the carbon precursor can also be coated on porous Si. Zhang et al. [
82] prepared a nanostructured carbon–silicon spherical porous composite. Silicon nanoparticles were first dispersed in a glucose solution containing Pluronic F127 using ultrasound, and the suspension was sealed in a stainless steel autoclave and hydrothermally treated at 180 °C in an Ar atmosphere. As a result, each silicon particle was coated with a thin, porous carbon shell. The carbon shell could effectively prevent the aggregation of Si nanoparticles and buffer the volume expansion of the Si nanoparticles. Furthermore, Yang et al. [
83] proposed a 3D porous silicon preparation method involving grinding and heating for silicon and magnesium powder. Using glucose as the carbon source, the 3D porous silicon powder and the glucose solution were stirred under negative pressure for 3 h to boost the introduction of the glucose solution into the pores of the 3D porous silicon. Following a hydrothermal reaction and thermal treatment, a porous silicon/carbon composite was obtained without the use of toxic and corrosive hydrofluoric acid (
Figure 8).
2.3. Sol-Gel Method
The sol-gel method has been widely used in material science and ceramic engineering in recent years. This method is primarily used to fabricate materials with integrated networks starting from chemical solutions serving as precursors. The sol-gel method has the merits of convenient operation, low price, low reaction temperature, and uniform particle size [
84,
85]. Besides, it provides a suitable route to combine the inorganic and organic compounds into a homogeneous hybrid, in a chemically linking or physically mixing state [
86]. In general, the sol-gel process can be primarily described in five key steps: hydrolysis, polycondensation, aging, drying, and thermal decomposition [
85]. Materials prepared by sol-gel methods have been widely used in gas separation, coating films, fibers, catalysis, optics, electronics, etc. [
87]. The preparation conditions and material properties of the carbon-silica composites prepared by the sol-gel method are listed in
Table 5.
Aerogel is an ultralight porous material obtained from a gel in which the liquid part of the gel is replaced by gas [
91]. Notably, aerogels exhibit great differences in bulky properties compared to other matter states. Although aerogels maintain a fixed volume and shape like solids, the density of aerogels can vary from over 1000 kg/m
3 (solid density) down to about 1 kg/m
3 (lower than air density), which results in a great variation in properties. Since aerogels not only have a high porosity like other foams, but also have dual structural properties, i.e., microscopic (nanoscale framework) and macroscopic (condensed matter) characteristics, they have now shown several unique properties, such as an ultra-low thermal conductivity, refractive index, sonic velocity, modulus, and dielectric constant, and a high surface area and ultra-wide tunable range of density [
92]. As a typical example, silica aerogels are amorphous materials with remarkable features, including very high porosity and surface area, as well as low bulky density and thermal conductivity. However, the potential applications of these superior materials are restricted by their poor mechanical strength. Despite this, silica aerogels can easily incorporate different components into their structure, which enables the preparation of materials with different properties from natural silica aerogels [
93].
As carbon foam has good mechanical properties but poor thermal insulation, Liu et al. [
94] prepared a foamed carbon-silica aerogel composite to take advantage of the good thermal insulation properties of silica aerogel. First, carbon foams were prepared by heating commercial coal tar pitch at 420 °C under N
2 atmosphere. The product was then fixed in a tube furnace and heated again under N
2 atmosphere to obtain the carbon foam. The carbon-silica composite was thereafter obtained by mixing TEOS, deionized water, and ethanol, followed by the addition of carbon foam and hydrochloric acid, adjusting the pH value, aging, and drying. Obviously, the traditional carbon foam preparation process is complicated and costly, which limits the wider application of carbon foam. For this reason, Liu et al. [
95] proposed a new carbon foam preparation method using melamine foam as a raw material under a N
2 atmosphere. TEOS, water, and ethanol were mixed in turn, along with the addition of hydrochloric acid and diluted ammonia. The foamed carbon was then soaked in the mixed solution, and the composite of carbon foam and silica aerogel was obtained after drying. According to the microscopic morphology of the sample, it was found that the carbon foam had a 3D network structure comprised of intertwined dendritic fibers, and triangular fiber cross-section. In the tests for material compressive strength, the maximum compressive stress of the foamed carbon/silica aerogel reached about 1.0 MPa.
One major drawback of aerogels is their high brittleness, which makes them easily damaged by external stress. However, the mechanical strength of aerogels can be improved by mixing them with multi-walled carbon nanotubes, which have a high mechanical strength. Yet another problem is that carbon nanotubes are difficult to disperse in solution; thus, the carbon nanotube surface needs to be functionalized by acid treatment first to have better dispersibility. Kyu et al. [
96] successfully combined functionalized carbon nanotubes with a methyltrimethylsilane silica aerogel. First, multi-walled carbon nanotubes were treated in a mixture of sulfuric acid and nitric acid to introduce carboxyl groups on their surface, which made it easier for the carbon nanotubes to disperse in solution. Then, the functionalized multi-walled carbon nanotubes were added to methyltrimethylsilane silica sol together with ammonium hydroxide (for condensation) and methanol (for aging); finally, the resultant wet gel was converted into gas by supercritical drying to obtain a functionalized composite composed of multi-walled carbon nanotubes and methyltrimethylsilane silica aerogels. The composite aerogel had good hydrophilicity and exhibited high flexibility and rigidity. Another method to improve the structural rigidity of aerogels is the use of fibers; however, the surface modification of fibers and chemical bonding between the fibers and the silica gel are two remaining problems in this field. Agnieszka et al. [
97] used a sol-gel method to prepare silica aerogel–carbon fiber nanocomposites with stronger structural durability. In their methods, two solutions were mixed together—solution A (a mixture of tetramethyl orthosilicate (TMOS) and methanol) and solution B (a mixture of NH
4OH and methanol). The mixing of these two solutions gave rise to a hydrolysis reaction, thereby producing a sol, which was further converted into a gel during condensation (gelation). In a subsequent step, the gel was aged, first in a mixture of water and methanol, and later in methanol solvent. Meanwhile, the carbon fibers were chemically treated with hot nitric acid before they were introduced into the TMOS solution. Thereafter, the carbon fibers were washed and dried in air. Finally, the silica aerogel and its composite with carbon fibers were obtained by removing the solvent from the nanostructure of the aerogel. By this means, a much better mechanical strength was witnessed in the composite with the assistance of carbon fibers.
As mentioned above, even though silica aerogels have excellent properties, their mechanical strength relies on the combination of a second rigid component in their highly porous matrix, such as carbon nanotubes. However, the high loading of carbon nanotubes is hindered because they can easily form aggregates due to string van der Waals interactions, which negatively affects the performance of composites composed of carbon nanotubes and silica aerogel. Morales-Florez et al. [
90] successfully dispersed carbon nanotubes uniformly into highly porous silica through a two-step sol-gel process and used acid-base catalysis to achieve rapid control of the gelation process and prevent carbon nanotubes from reaggregation (
Figure 9). The content of carbon nanotubes in the prepared composite material was as high as 2.5%, and the low density of the silica aerogel was maintained (the density was lower than 80 mg/cm
3), while the specific surface area of the composite material was 600 m
2/g and the pore volume greater than 4 cm
3/g. Furthermore, FTIR results demonstrated the existence of Si-O-C bridges, which inferred the good dispersion of carbon nanotubes and the improvement in the composite’s mechanical strength. Another approach to enhance the dispersity of carbon nanotubes is the addition of polymers or surfactants to form non-covalent polymer-encapsulated or surfactant-encapsulated carbon nanotubes. Lu et al. [
98] used the sol-gel method to coat silica on the surface of carbon nanotubes functionalized with sodium polystyrene sulfonate to synthesize multi-walled carbon nanotubes/silica composites. First, commercial carbon nanotubes were dispersed in a solution of sodium polystyrene sulfonate consisting of deionized water and ethanol; after stirring, ammonium hydroxide and TEOS were added dropwise. The resulting precipitate was washed to obtain carbon nanotube–silica composites. Similarly, Tsygankov et al. [
99] used TEOS as a silica source, which was mixed with isopropanol and aqueous citric acid solution in the sol-forming stage. Thereafter, surfactant Triton x-100 and carbon nanotubes were added in turn to form an emulsion. After the gelation and aging stages, a carbon nanotube–silica hybrid aerogel was finally obtained by supercritical drying. (
Figure 10).
Finally, a carbon dot–silica composite has shown great potential for the future development of electronic devices. A sol-gel synthetic route was recently proposed to produce this kind of composite, which has the unique advantages of high purity, a uniform distribution of the dispersed phase in the matrix, and controllable texture and porosity. Given that nitrogen doping has been proven to be an effective way to enhance the quantum efficiency of carbon dots, Casula et al. [
100] used the sol-gel method to prepare nitrogen-doped carbon dot–silica porous composites. Through different gel drying strategies, three kinds of composites with different pores were prepared. The effect of porosity on the fluorescence emission of composites was also investigated; the microporous structure of the composite material made the emission centers close to each other, resulting in a greatly reduced fluorescence, while the open texture of the macroporous composite material enabled the carbon dots to be effectively dispersed. According to the emission tests, aerogel composites were the most efficient emissive composites due to their unique ductility and large mesopores. To encapsulate carbon dots in more regular spherical MCM-41 nanoparticles, Vassilakopoulou et al. [
101] synthesized spherical-like MCM-41 by the standard sol-gel method using TEOS as a silica precursor and CTAB as a structure-directing agent. During gel preparation, an aqueous solution of carbon dots and TEOS were added into a solution containing ammonia, ethanol, and CTAB surfactant. After stirring, the white powder was filtered, washed, and dried to obtain MCM–carbon dot composites. The entrapped carbon dots retained their photoluminescence properties even with thermal treatment at 550 °C, which indicated a strong protective effect of the porous matrix.