RESUMO
Some characteristics of silica-based materials, such as the control/adjustment of their physical and chemical properties, compatibility, and friendly-use synthesis methods, have held the attention of several scientific groups over the years. This condition of prominence becomes even more evident when we seek these characteristics at the micro- and/or nanoscale. Among existing methods to obtain these micro/nanomaterials, the Stöber method is the focus of this review. This method is known to enable the production of silica micro- or nanoparticles from reagents of medium-easy manipulation under mild conditions using equipment that is common in most laboratories. However, this method has many nuances that must be considered to guarantee accurate results, either in size or distribution, and to ensure result reproducibility. Thus, in this review, we discuss the effects of the primary components used in the synthesis of these materials (i.e., TEOS, ammonia, and water), as well as those of other reaction conditions, such as solvent, temperature, and ionic strength. Therefore, we discuss studies involving the synthesis and characterization of micro- and nanoparticles over the years to establish discussions between their experimental observations and proposed models. This review provides experimental observations about the synthesis of these materials, as well as discussions according to complementary and/or contradictory evidence found over the years. This review seeks to help those who intend to work with this method and provide certain key points that, in our experience, can be important to obtain desired results.
RESUMO
In industry, silica nanoparticles (NPs) are obtained by the fuming and the precipitation method. Fumed silica NPs are commonly used in the preparation of nanocomposites because they have an extremely low bulk density (160-190 kg/m3), large surface area (50-600 m2/g), and nonporous surface, which promotes strong physical contact between the NPs and the organic phase. Fumed silica has fewer silanol groups (Si-OH) on its surface than the silica prepared by the Stöber method. However, the number of -OH groups on the fumed silica surface can be increased by pretreating them with sodium hydroxide (NaOH) before further surface modification. In this study, the effectiveness of the NaOH pretreatment was evaluated on commercial fumed silica NPs with a surface area of 200 m2/g. The number of surface -OH groups was estimated by potentiometric titration. The pretreated fumed NPs, and the precipitated NPs (prepared by the Stöber method) were modified with 3-aminopropyltriethoxysilane (APTES) to obtain A200S and nSiO2-APTES, respectively. The NPs were characterized using electron dispersive scanning (EDS), scanning electron microscopy (SEM), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), BET (Brunauer-Emmett-Teller) analysis, and ζ-potential. XRD confirmed the presence of the organo-functional group on the surface of both NPs. After the amino-functionalization, the ζ-potential values of the nSiO2 and A200 changed from -35.5 mV and -14.4 mV to +26.2 mV and +11.76 mV, respectively. Consequently, we have successfully synthesized functionalized NPs with interesting, specific surface area and porosity (pore volume and size), which can be attractive materials for chemical and energy industries.