Surface Heterogeneity of SiO2 Polymorphs: An XPS Investigation of α-Quartz and α-Cristobalite

Silica minerals, one of the most abundant mineral species on the earth, play important roles in geochemistry and environment processes. The diversity of the SiO4 tetrahedron polymerization style might result in the heterogeneity of the surface microstructures and properties of SiO2 polymorphs. The surface properties of two common crystalline SiO2 polymorphs, i.e., alpha-quartz and alpha-cristobalite, have been investigated by the surface site density measurement, batch methylene blue (MB) adsorption, and X-ray photoelectron spectroscopy (XPS). The Langmuir adsorption isotherms suggest the formation of monolayer MB on both alpha-quartz and alpha-cristobalite surfaces. The adsorption capacity of alpha-quartz toward MB is larger than that of alpha-cristobalite, which positively correlates with the density of surface site. XPS spectra reveal that the adsorption takes place between the nitrogen atom of the dimethylamino groups in MB and silanols on alpha-quartz and alpha-cristobalite surfaces. The O/Si atom ratio related with adsorption of alpha-quartz is found to be about 1.8:1, which is higher than that of alpha-cristobalite (about 1.3:1). This suggests that there are two different silanol species (single and germinal) related to adsorption on the surface of alpha-quartz and alpha-cristobalite, and the higher O/Si ratio implies a larger proportion of germinal silanols in alpha-quartz. The Nlow/Nhigh ratio (Nlow stands for the N atoms with lower binding energy (399.2 eV), and Nhigh for the N atoms with higher binding energy (399.7 eV)) changes to about 2:1 with the adsorption saturation, implying that the space arrangement of MB adsorbed on the surface was adjusted with the increase of adsorption amount by lifting the average tilt angle between the long axis of the MB molecule and the sample surface. The higher surface site density of alpha-quartz leads to a larger average tilt angle, while alpha-cristobalite does conversely.