  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.
