Low-temperature fluorination of silica by a nonaqueous solution of NH4F

The fluorination of silica by ammonia fluoride in several organic solvents has been studied by diffuse reflectance Fourier transform infrared spectroscopy with in situ thermal treatment, F-19 magic angle spinning nuclear magnetic resonance, H-1 --> Si-29 and F-19 --> Si-29 cross-polarization techniques, and temperature-programmed analysis followed mass spectrometrically. The extent of surface fluorination and the type of fluorinated species depend on the solvent and treatment procedure. The silicas that were studied show evidence of the formation of two types of surface-fluorinated silicon species: suface-bonded tetrahedral species with a general formula of O-n-Si(OH)(4-n-m)F-m and, to a lesser extent, octahedral species [FnSi(OH)(6-n)](2-), which were formed already at low temperature. The thermal decomposition of [FnSi(OH)(6-n)](2-) leads to the removal of corresponding silicon fluorides and hydroxyfluorides, and surface-bonded O3Si-F and O-SiF3 can be additionally formed. Heating to 500-600 degreesC causes the surface migration of fluorine in O3Si-F over siloxane bridges and the formation of silicon fluorides and their subsequent removal from the surface (complete removal at 600 degreesC). The hydrophobic nature of fluorinated silica has been shown by infrared spectroscopy and by the thermal dependence of water desorption from the surface.