Phase Transformation of RGO/SiO₂ Nanocomposites Prepared by the Sol-Gel Technique

Abstract

In a previous paper (Shalaby A, Yaneva V, Staneva A, Aleksandrov L, Iordanova R, Dimitriev Y, Nanoscience & nanotechnology – nanostructured materials application and innovation transfer (14), ISSN 1313-8995, 2014) we studied reduced graphene oxide (RGO)/SiO₂ composite material by adding a small amount of RGO to silica in order to avoid the aggregation process and to solve the problems connected with the exfoliation and distribution of the sheets inside the composites. But from a practical point we needed to study the effect of high amounts of RGO on the composites at different temperatures. The purpose of this investigation is to study the effect of RGO on phase transformations of the composites heated at 200, 400 and 800 °C. The sol-Gel method was used to obtain the RGO/SiO₂ composite by mixing high amounts of RGO with tetraethyl orthosilicate (TEOS). Data are presented for the transformation of the nanocomposites with increasing temperature in air atmosphere. RGO nanosheets were prepared by chemical exfoliation of purified natural graphite using the Hummers and Offeman method (Hummers WS, Offeman RE, J Am Chem Soc 80:1339, 1958) to obtain graphite oxide. Then the material was exfoliated to reduced graphene nanosheets by ultrasonication and reduction process using sodium borohydride (NaBH₄). Characterization of the material was performed by X-ray powder diffraction (XRD), infrared (IR) spectroscopy and scanning electron microscopy (SEM) analysis. In our previous studies we found that all samples with small amounts of RGO are amorphous up to 800 °C. By increasing the amount of RGO (20 %) crystal phases appear at 200, 400 and 800 °C. The carbon phases disappear above 400 °C and cristobalite is found at 800 °C. From the IR spectra it was established that the band related to Si-OH vibration is converted to a small shoulder at 400 °C, and the bands corresponding to vibrations of water molecules around 3,448 and 1,635 cm⁻¹ were drastically reduced by increasing the temperature. The dominant band at 1,099.2 cm⁻¹ (800 °C) is connected with the stretching vibration of Si-O bonds in SiO₄ tetrahedrons. SEM images of RGO/SiO₂ nanocomposites present randomly aggregated stacks of small sheets imbedded in an amorphous matrix.