Structural rearrangements of assembly of silica nanoparticles under pressure through in-situ experimental studies

This study aims to explore the formation of a novel amorphous phase resulting from the pressure-induced coalescence of silica nanoparticles, and to compare its structural properties with those of compressed bulk silica. Then, we performed in situ and ex-situ experimental study on silica nanoparticles powder under high pressure and compared it to bulk vitreous silica compression. For that, in-situ Raman spectroscopy under pressure at room temperature permits us to follow structural evolution of assembly of silica nanoparticles with a mean 15 nm diameter. Average and distribution of inter-tetrahedral Si-O-Si angles, distribution of ring size and structural homogeneity from analysis of Raman spectra have been evaluated. Thanks to the granular structure of our powders, the pressure seen locally by the vitreous silica can be locally much greater than the pressure applied to the powder. We observed a reversibility limit, at very low pressure (lower than 2.5 GPa) compared to the elastic limit of 9 GPa for bulk silica, due to high pressure between nanoparticles but also due to high energy surface level which promote local rearrangement of silica between surfaces. Finally, an unexpected decreasing of 3-fold rings at the nanoparticle surface after high pressure cycle is observed. Indeed, during compression cycle, structural surface reorganization occurs, from two 3-fold rings, a creation of 6-fold ring is promoted and explains the reducing of 3-fold rings population. For higher pressure, the evolution of 3-fold rings increases again, due to the large local strain in the glass obtained from nanoparticles, as is the case with bulk vitreous silica.