Study of amorphous silica by Electron Energy Loss Spectroscopy and electron diffraction <scp>PDF</scp>

Resumen

Amorphous silica (a-SiO2) has many industrial (glass former) and scientific applications. Structurally silica which has SiO4 tetrahedral units connected with bridging O at the corners. To understand and predict interesting properties of such materials depends on the knowledge of the detailed atomic structure. The loss of structural ordering in amorphous materials, carry sharp Bragg diffraction reflections to disappear and only diffuse diffraction pattern are observed. Study of amorphous materials cannot be performed by routine crystallography and techniques like Pair Distribution Function (PDF) can be used for structure analysis using X-Ray, neutron or electron diffraction (ED) [1]. ED related PDF (e-PDF) in TEM has the big advantage over X- Ray PDF technique that allows studying local structural ordering of amorphous materials in nm scale by collecting (ED) patterns in very short time (msec instead of 15-24 hours in X-Ray case). Here we present how e-PDF analysis can be used to study in detail hydrothermal reactions flow process in amorphous silica.Amorphous silica samples were heated with water in hydrothermal container under different time scales and were studied by X-ray, Electron Energy Loss Spectroscopy and e-PDF. Hydrothermal reaction on amorphous silica was performed with reaction times 6, 16, 312 hours [2]. Figure 1 show the Si L2,3 edges for studied compounds. Si peaks are always observed but its giving rise to a broadening of spectra as the intensity increases. Such broadening in the Si L peaks is probably due to Si tetrahedral distortion, since in SiO2 amorphous phase, distances between silicon and the four surrounding oxygen are slightly different. From the collected ED data, pair distribution function (G(r)) was calculated using dedicated ePDF software [3], developed to analyse ED patterns from amorphous and nanomaterials (Figure 2). Our analysis with ePDF has revealed that no peak (corresponding to interatomic distances) was found beyond 5 Å, which confirms that only short range ordering present in the material, even after several hours of hydrothermal reaction. Besides, small changes were observed in the PDF peak positions (corresponding to interatomic Si-Si, Si-O and O-O distances). All such peaks/interatomic distances match well with the distances existing within the SiO2 crystalline structure. Slight peak width change has also been observed with reaction time, which can possibly arise from local strain. Modelling of the amorphous silica reactive product is under process.