Comparison of Radio-Frequency-Plasma- and Ion-Beam-Induced Surface Modification of Kaolinite

Abstract

Modification of the surface of kaolinite mineral particles was carried out in nitrogen, argon, helium, and air environments using the radio frequency (RF) plasma technique. The modified surfaces of the kaolinite were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS) to determine the surface speciation, the nature of the newly formed surface functional groups, and the interaction mechanism between the RF plasma and the kaolinite surface. Comparison with Ar+ ion beam modification of kaolinite has established that the surfaces of kaolinite undergo low-energy ion bombardment with very limited sputtering in the presence of the Ar plasma but with no observable ion implantation. Surface restructuring with modified bonding is implied by this evidence. The steady-state charge behavior and surface structure of the kaolinite processed in the RF plasma showed significantly different characteristics from those of both the untreated kaolinite and an ion-beam-treated kaolinite. New infrared (IR) absorption bands were observed on the surfaces of RF-plasma-treated kaolinite at 1407, 2805, 3010, and 3100 cm-1, along with some broad, partially resolved absorption bands which occurred near 1440 and 3280 cm-1. The intensities of the newly formed IR bands were a function of the period of plasma treatment and the energy applied but were generally independent of the nature of the gas used for the plasma treatment. They are attributed to plasma-induced surface species arising from hydrocarbon contamination in the gas phase. In contrast to ion beam modification, the skeletal vibrations of the kaolinite lattice are not measurably affected in plasma treatments implying that restructuring, giving rise to the new IR bands, is confined to the surface layers.