X-ray photoelectron spectroscopic analysis of halloysites with different composition and particle morphology

AbstractThe surface composition of some halloysites with different particle morphology has been investigated by X-ray photoelectron spectroscopy (XPS) before and after removal of external Fe. The Fe(III) 2p3/2 binding energy of external Fe is appreciably smaller than that of structural Fe. Particle morphology is influenced by structural Fe content. The long-tubular halloysite has very little surface Fe, and its concentration tends to increase with the proportion of non-tubular particles in the samples. The spheroidal sample contains the most structural Fe which, however, does not appear to influence particle shape directly. Study by XPS indicates that Fe substitutes for Al in octahedral positions in approximately 1 : 2 proportion. As a result, an increase in octahedral vacancies and cation exchange capacity would be predicted. Further, halloysite layers within a crystal are generally inhomogeneous in composition. Built up like “onion skins”, the surface layers would either be enriched or depleted in Fe depending on the chemical environment in which crystal growth occurs.