Study of aluminophosphate oxynitride (AlPON) materials by X-ray photoelectron (XPS) and diffuse reflectance Fourier transform IR spectroscopy (DRIFTS)

Abstract

A series of amorphous aluminophosphate oxynitrides (AlPON) have been obtained by nitridation of AlPO₄ in a tubular furnace at 750 °C under a dry NH₃ stream. Bulk nitrogen content is controlled by the reaction time and quantified by alkaline digestion with molten KOH. Samples have been analysed by X-ray photoelectron spectroscopy (XPS) and diffuse reflectance IR spectroscopy (DRIFTS). XPS results indicate that nitrogen selectively replaces oxygen atoms bonded to phosphorus in the [PO₄ ] tetrahedra of AlPO₄ leading to a mixture of [PO_xN_4–x] units in AlPON. Simultaneously, an Al₂O₃-type phase develops. Compositions obtained by XPS are in good agreement with those expected from bulk formulation. Nitridation reaction starts at the surface of AlPO₄ by breaking P–O bonds and generating terminal P–NH₂ . Once surface P–NH₂ saturation is achieved (above 3.6 mass% N), bulk nitridation occurs. In nitrogen saturated AlPON samples (ca. 20 mass% N) the presence of a surface P₃N₅ phase is proposed to be responsible for the slight surface nitrogen enrichment observed. The presence of structural (–NH–) groups, isoelectronic to (–O–), and their growing concentration with nitridation time have been detected by DRIFTS. Diffuse reflectance IR spectroscopy also suggests the presence of cyclic (P–N–P) structures.