Direct Observation of Morphological and Chemical Changes During the Oxidation of Model Inorganic Ligand-Capped Particles

TL;DR

This study combines X-ray techniques to observe how oleic acid ligands on NaYF4 nanoparticles oxidize and change morphology under low oxygen pressure and X-ray exposure, revealing asymmetric reactions and steady-state products.

Contribution

It introduces a combined APXPS and GIXS approach to simultaneously monitor chemical and morphological changes during nanoparticle oxidation, providing new insights into ligand behavior.

Findings

Oxidation occurs at O2 pressures below 1 mbar with X-ray exposure.

Oxidation leads to asymmetric ligand layer formation on particles.

Steady state mainly contains CHx and -COOH groups.

Abstract

Functionalization and volatilization are competing reactions during the oxidation of carbonaceous materials and are important processes in many different areas of science and technology. Here we present a combined ambient pressure X-ray photoelectron spectroscopy (APXPS) and grazing incidence X-ray scattering (GIXS) investigation of the oxidation of oleic acid ligands surrounding NaYF4 nanoparticles (NPs) deposited onto SiOx/Si substrates. While APXPS monitors the evolution of the oxidation products, GIXS provides insight into the morphology of the ligands and particles before and after the oxidation. Our investigation shows that the oxidation of the oleic acid ligands proceeds at O2 partial pressures of below 1 mbar in the presence of X-rays, with the oxidation eventually reaching a steady state in which mainly CHx and -COOH functional groups are observed. The scattering data reveal that the oxidation and volatilization reaction proceeds preferentially on the side of the particle facing the gas phase, leading to the formation of a chemically and morphologically asymmetric ligand layer. This comprehensive picture of the oxidation process could only be obtained by combining the X-ray scattering and APXPS data. The investigation presented here lays the foundation for further studies of the stability of NP layers in the presence of reactive trace gasses and ionizing radiation, and for other nanoscale systems where chemical and morphological changes happen simultaneously and cannot be understood in isolation.