Effect of Heat Treatment on Water Vapor Adsorption by Opal Structures and Their Effective Refractive Index

Abstract

We have studied water vapor sorption properties of close-packed opal structures made up of monodisperse spherical silica particles 150 to 350 nm in diameter. The results attest to polymolecular water adsorption followed by capillary condensation in meso- and micropores within the structure-forming particles. The maximum adsorption value, ~0.13 g of H₂O per gram of dry SiO₂, essentially coincides with the specific volume of inner pores in the particles. The annealing temperature has been shown to influence the adsorption properties and effective refractive index of the opal photonic crystals. The refractive index of dry and adsorbed water-filled opal structures has been determined as a function of annealing temperature.