High-Temperature Anomaly in the Synthesis of Large (d > 100 nm) Silicon Nanoparticles from Hydrogen Silsesquioxane

Hydrogen silsesquioxane (HSQ) is known to disproportionate at elevated temperatures, resulting in nanoscale elemental silicon inclusions within a matrix of SiO2. Previous investigations suggest a continuous and direct relationship in which particle size can be increased by increasing either the processing temperature and/or processing time. Recent attempts at synthesizing large particles (d > 100 nm) using temperatures above 1400 °C and dwell times greater than 1 h uncovered anomalies in both nanoparticle size, and the composition of the resulting composite materials. Silicon nanoparticle (SiNP) growth occurs as predicted at temperatures up to and including 1400 °C, but prolonged heating between 1500–1600 °C results in SiO2 being the sole product of the reaction, and no nanoparticles are recovered. While longer processing times are typically associated with larger SiNPs, increasing the dwell time for samples within the anomalous temperature zone results in the opposite effect and particle formation is suppressed. The standard trends regarding particle formation, size, and relative SiNP:SiO2 composition reemerge beyond this temperature region and are restored at 1700 °C. In addition to quantifying the boundaries of this parameter space, our characterization of the resulting materials suggests that silicon monoxide formation, promoted through the crystallization of cristobalite SiO2, is responsible for the strange behavior of HSQ at high temperatures.