Surface compositional changes upon heating cobalt oxalate dihydrate in vacuum

Abstract

X-ray photoelectron spectroscopy (XPS) was chosen to explore changes occurring in the surface composition, structural and chemical state of atoms during the course of thermal decomposition of CoC2O4·2H2O under ultra-high vacuum conditions. Accordingly, a large set of high-resolution core-level and Auger spectra was recorded as a function of temperature and time. Some Co-related XPS characteristics, including Auger parameter, were found to be sensitive to the local chemical and structural environment of Co atoms, demonstrating non-monotonic variation in the temperature range of intense dehydration. Isothermal decomposition was shown to proceed via formation of a two-phase system composed of metallic Co and oxygen-deficient oxalate and could be described by Avrami–Erofeev or Prout-Tompkins kinetic models. The long-term isothermal decomposition of the oxalate at 380 °C produced nearly oxygen-free Co covered by a substantial amount of carbon in carbidic form (~19 at%) and as graphitic overlayer (~26 at%). On the contrary, heating of the oxalate to 500 °C produced Co particles covered by surface CoO (~29 mol%) with traces of carbidic and graphitic carbon. XRD, TG and DTA coupled with mass spectrometry were employed as complementary techniques.