Black SiO2 nanoparticles obtained by pyrolysis of rice husk

Rapid pyrolysis at high temperatures is a promising process to obtain black SiO2 nanoparticles, which can be applied, for their characteristic color, in different areas, such as cosmetics, graphite, paints, ceramics and polymer industry (thermoplastics, rubber and tires). In this work, the physico-chemical and toxicological characteristics of rice husk (RH) and rice husk ash (RHA) pyrolysis were investigated. The main characteristic of the method is the production of black nanoparticles in a short time of synthesis and with a high thermal stability. The ash and organic matter contents obtained were 15.7 and 84.3% respectively, showing that the method is efficient to obtain black particles and with a high yield. The chemical elements determination of the black nanoparticles was confirmed with predominance of Si and K. Optical properties were extremely important to demonstrate that the black pigmentation was maintained, as low reflectance indices were detected for all the samples, even for the sample obtained at the temperature of 1200 °C. The pyrolysis of RHA was characterized by thermogravimetric analysis and 38% loss of carbon mass was identified. The X-ray diffraction (XRD) denoted a greater presence of amorphous SiO2, so that the SiO2 nanoparticles are a mixture of amorphous and crystalline particles. The transmission electron microscopy (TEM) results show the formation of nanoparticles having a particle size of about 10–20 nm through scanning electron microscopy (SEM) images, the cellular porous structure, known as the silica skeleton, was noted, and BET surface area of about 114 m2/g. In the biocompatibility test, it was possible to observe that the RHA presented values below 70%, showing that there was no cell growth and, therefore, the product did not showed toxicity; for this reason, the product does not need to undergo purification steps when used in more noble areas such as cosmetics. Thus, it was concluded that the pyrolysis method was efficient and that, even after a second heat treatment at 1200 °C, the SiO2 nanoparticles remained black and not presented the typical white color reported in several studies in the literature.