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., α-quartz and α-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 α-quartz and α-cristobalite surfaces. The adsorption capacity of α-quartz toward MB is larger than that of α-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 α-quartz and α-cristobalite surfaces. The O/Si atom ratio related with adsorption of α-quartz is found to be about 1.8:1, which is higher than that of α-cristobalite (about 1.3:1). This suggests that there are two different silanol species (single and germinal) related to adsorption on the surface of α-quartz and α-cristobalite, and the higher O/Si ratio implies a larger proportion of germinal silanols in α-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 α-quartz leads to a larger average tilt angle, while α-cristobalite does conversely.