XAS study of oxygen plasma-treated micronized iron oxide pigments

The stability of commercially available yellow (Bayferrox 3920) and red (Bayferrox 140) micronized iron oxide pigments against highly aggressive treatment with oxygen plasma is examined. The red pigment microparticles with hematite crystal structure are stable, while the yellow pigment changes colour during the treatment. Structural analysis with powder X-ray diffraction and X-ray absorption methods, EXAFS and XANES, shows that the yellow pigment microparticles recrystallize from goethite to hematite crystal structure. Larger hematite particles are composed of small highly anisotropic nano-domains. No nanoclusters or amorphous iron oxide phases are detected. The valence state of iron atoms in the pigments is not affected by the strongly oxidative plasma environment.