Nano-SiO2 is a green and environment-friendly inorganic nonmetallic material, commonly known as ultrafine white carbon black, referred to as white carbon black for short. It has hydroxyl groups on the surface, and the diameter of particles ranges from 1 nm to 100 nm. It is an amorphous white powder. The microstructure is spherical and shows a reticular and flocculent quasi granular structure under transmission electron microscope. Nano-SiO2, with the unique four effects of nanomaterials, has good optical, electrical, thermal, mechanical, magnetic, absorption, radiation, and other special properties, as well as high toughness, high strength, and good stability at a high temperature, so that nano-SiO2 can be widely used in various fields [1, 2]. Optically, the surface effect and small size effect shown by the particle size of several nanometers to more than ten nanometers make it resist UV rays . At the same time, the quantum tunneling effect and volume effect of nano-SiO2 make it produce osmosis, penetrate a polymer’s pi bond, overlap with its electron cloud, form a spatial network structure, thus greatly enhancing the mechanical strength of polymer compounds, and increasing the antiaging properties and wear resistance of other materials. Dispersion of nano-SiO2 particles into other materials can improve the comprehensive properties of products. For example, when added to polymer materials, it can resist UV aging and thermal aging. When used to modify polymers, it can improve the optical, electrical, thermal, mechanical, and processing properties of materials, and improve the strength, toughness, flame retardancy, and heat resistance of polymers [5, 6]. Nano-SiO2 has a wide range of applications. As one of the most widely used nanomaterials at present, it is applied in many fields, including composites, electronic packaging materials, coatings, pigments, plastics, cosmetics, glass carriers, adhesives, and drug carriers .
At present, the raw materials for preparing nano-SiO2 are mainly tetraethyl orthosilicate (TEOS) , 3-aminopropyl triethoxysilane (APTES), methyl orthosilicate (TMOS), and other organosilicon compounds as silicon sources, while there are few reports on the preparation of Nano-SiO2 using silica sol . Although nano-SiO2 with good dispersion can be prepared using a silicone source, it is expensive and requires extreme conditions, which is not suitable for mass production . It is also reported in the literature that silica sol is used as the silicon source , a colloidal protective agent NCMC is directly added under stirring, a back-extractant isopropanol is added dropwise, and then dried in an oven. The silica sol particles are dehydrated to gel and then removed by natural sedimentation or filtration. Most of the aqueous solutions containing isopropanol are removed. The transparent wet gel is dried at 200 ℃ in an oven for 6 h. The pure white nano-SiO2 can be obtained by natural cooling in the rear dryer. Then, the dried nano-SiO2 is calcined in an 800 ℃ muffle furnace for 1 h to remove the residual surface hydroxyl water, colloidal protective agent, and other impurities to obtain high-purity white nano-SiO2. However, 100 g of silica sol requires 3 g of a colloidal protective agent NCMC and 105 g of antiextractant isopropanol to form SiO2 hydrogel, and a hydrogel is added to isopropanol for 6 h after drying in a 200 ℃ oven. After calcination in a 800 ℃ muffle furnace for 1 h, nano-SiO2 powder can be obtained after natural cooling. However, this method consumes a large amount of back-extractant isopropanol, which evaporates into the air after being dried in a 200 ℃ oven for 6 h, resulting in high cost and air pollution. In addition, it requires high-temperature calcination at 800 ℃ to whiten the chromaticity and consumes a high amount of energy . Domestic researchers also used an alkaline silica sol and inorganic acid as the raw materials, water and methanol as the reaction medium, and added an appropriate dispersant (sodium hexametaphosphate and polymer stable dispersant C). The reaction was carried out for a certain time under appropriate reaction temperature and pH conditions, and a stable nano-SiO2 powder was obtained through vacuum dehydration drying and ultrafine screen screening. However, the particle size distribution is uneven, and due to the addition of inorganic acids such as hydrochloric acid and sulfuric acid, the local acidity in the silica sol is too high, making it easy to agglomerate and deteriorate. In contrast, it increases the viscosity, because it loses fluidity and becomes a gel, causing difficulties for mixing. Therefore, it is difficult to obtain nanoparticles with a uniform particle size and monodispersity . At the same time, the particle size of the obtained nano-SiO2 powder was determined using its production process and raw material ratio, and there is basically no repeatability.
Nano-SiO2 also has many unique properties [14–16]. For example, nanoparticles have a small particle size, spherical appearance, hydroxyl group and adsorbed water exist on the surface, a large specific surface area, good dispersion performance, and strong surface adsorption. Nano-SiO2 has superior stability, reinforcement, thixotropy, and excellent optical properties. Therefore, SiO2 nanoparticles are widely used in the preparation of catalysts, ceramics, electronic materials, fillers, and cosmetics. Sol particles are used in chemical mechanical polishing, coating, and precision casting .
Owing to the great application potential of nano-SiO2 powder materials, it is particularly important to study and develop methods for preparing ultrafine SiO2 materials. At present, the methods for preparing nano-SiO2 mainly include precipitation , gas-phase method, sol–gel method, microemulsion method, and micelle method .
There are few reports on the preparation of nano-SiO2 using silica sol. Because the surface of silica sol contains a large number of hydroxyl groups, when concentrated and gelled, the hydroxyl groups shrink through hydrogen bonding, and the colloidal particles merge to form large particles of SiO2 . Shi Li  added a protective agent to combine with the surface hydroxyl groups of silica sol, so that the hydroxyl groups of the colloidal particles cannot be dehydrated and condensed when the silica sol gels, so as to obtain nano-SiO2. However, there are certain defects, such as the use of flammable isopropanol, and the cost is higher due to a large amount of use. During the preparation, with the vacuum concentration of isopropanol, it is easy to agglomerate, and it is difficult to obtain well-dispersed nano-SiO2.
In this study, silica sol was used as the silicon source. For the first time, the citric acid and silyl hydroxyl groups that complex with strong complexation were used as the protectant, and ethyl acetate was used as a latent acid reagent. By slowing down the rate of gelation reaction of SiO2 colloidal particles directly with latent acid reagents in silica sol and controlling the gelation temperature, reaction time, complexing protectant, and precipitant amount in the reaction process, a large surface area was obtained, and uniformly dispersed nano-SiO2 was prepared.