Reactivity of gaseous PtO2 and RhO2 with LaNiO3 thin films: a systematic XPS study

Abstract

The reaction between volatile platinum and rhodium gas species and bulk materials (alloys or oxides) is of high practical importance in the chemical process industry. Herein, an X-ray photoelectron spectroscopy (XPS) study has been conducted to understand how PtO₂(g) and RhO₂(g) react with LaNiO₃ (LNO) thin films grown via atomic layer deposition (ALD) at elevated temperatures (900 °C). In this study, XPS data for reference powders of La₂NiPtO₆, LaNi_0.95Pt_0.05O₃, LaNi_0.88Rh_0.12O₃ and LaRhO₃ provide a library for the interpretation of oxidation states of platinum and rhodium after reaction with LNO thin films. Upon short exposure to PtO₂(g) and RhO₂(g), platinum appear as Pt(III) in the LNO surface, but after prolonged exposure, Pt(IV) appears. Complementary diffraction analysis shows that LaNi_1−xM_xO₃ solid solution forms initially where Ni, Pt and Rh are predominantly +III, however, quickly accompanied by the formation of a La₂Ni_2−2xM_2xO₆ (M = Pt, Rh) double perovskite phase with its characteristic (004) peak. Rhodium appears as Rh(III) and Rh(IV) in both the Rc and the double perovskite like phases. Furthermore, platinum and rhodium depth profiles were generated by the combination of XPS and Ar⁺ sputtering. Platinum depth profiles reveal major differences between as-grown ALD films and post-annealed films when exposed to PtO₂(g). Pt penetrates faster into the LNO material for films without post annealing, probably due to its smaller grain size and larger surface area and grain boundaries. When exposed to a mixture of PtO₂(g) and RhO₂(g), the Rh : Pt ratio in LNO increases upon prolonged reaction time. These results may have implications on the use of LNO as platinum and rhodium capture material in the Ostwald process.