Elemental surface enrichment in flame synthesis: A mechanism associated with particle melting-solidification

Elemental surface enrichment is important for functionalities of flame-synthesized particle materials, but its mechanism is poorly understood. In this paper, a mechanism associated with particle melting-solidification is proposed based on an experimental study. Y₂O₃:Eu particles were generated by flame assisted spray pyrolysis (FASP), using H₂/O₂ flames or H₂/air flames. The particles were analyzed by transmission electron microscopy (TEM), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). H₂/O₂ flames resulted in particles with Eu surface enrichment, i.e. the surface Eu concentration was several times higher than the overall Eu concentration; there was no elemental surface enrichment in particles from H₂/air flames. The Eu surface enrichment in H₂/O₂ flames was attributed to elemental partitioning during solidification of molten Y₂O₃:Eu particles; in H₂/air flames the particles did not melt and hence there was no elemental surface enrichment. The findings of this study suggest that elemental surface enrichment may be a common phenomenon for binary metal oxide particles that experience melting-solidification. Such particles should be examined for elemental surface enrichment, both for understanding their functionality and for their potential biological effects.