Influence of heat treatment on the microstructure and surface groups of Stöber silica

Abstract

Heat treatment is routinely utilized in preparing nanoporous materials (including Stöber silica), and can substantially affect their performance in diverse application fields. However, the effects of heat treatment at different temperatures on the structure and surface properties of Stöber silica have not been systematically investigated before. In this work, Stöber silica (washed with water or ethanol) was calcined at different temperatures (from 250 °C to 1000 °C), and the heat-treated samples were characterized through nitrogen adsorption at 77 K, scanning electron microscopy, simultaneous thermal analysis, elemental analysis, and Fourier-transform infrared spectroscopy. The results show that there is no significant difference in the morphology and particle size of the calcined samples. The internal micropores almost collapse after calcination at 500 °C, and the pores with a smaller diameter are the first to shrink during calcination. The variation in the number of the surface hydroxyl groups and ethoxyl groups with the calcination temperature is discussed in detail. The carbon content analysis and differential scanning calorimetry curves reveal that the surface ethoxyl groups (for the samples washed with ethanol) are completely removed after calcination at 500 °C. After calcination at temperatures above 800 °C, the hydroxyl groups almost completely condense into siloxanes. The specific surface area calculated according to the thermogravimetric mass loss and surface hydroxyl density is found to be significantly different from the measured Brunauer–Emmett–Teller specific surface area. Our results may offer practical guidance for the application of Stöber silica subjected to similar heat-treatment processes.