Structure and transmittance behavior of sol–gel silica nanoparticles synthesized using pH-stable alkanolamines

Although sol–gel silica nanoparticles are widely used in academic and industrial applications, only a few studies have focused on amine catalysts other than ammonia to control the particle size. In this study, we systematically investigated how the particle size is affected by various amine catalysts such as ammonia (NH₃), ethanolamine (EOA), diethanolamine (DEOA), and triethanolamine (TEOA) in a wide range of catalyst amount. Different amine catalysts had a different effect on the particle size with respect to their content, and it was shown that the pKa and molecular size of the amine catalyst are the main factors controlling the particle size during the sol–gel reactions. It was suggested that the amines behave not only as catalysts, but also as surfactants or dispersants. While the pH of the ammonia solution exhibited a gradual decrease due to the high volatility of NH₃, the other types of amines exhibited high pH stability, which is an important aspect for reproducibility and industrial application. After coating the silica nanoparticles on glass substrates, the transmittance was significantly affected by the coating thickness and the type of amine catalyst. TEOA lead to higher transmittance than NH₃, which is attributed to the more porous structure of TEOA-catalyzed silica nanoparticles.