Thermal Decomposition of Nickel Oxalate Dihydrate: A Detailed XPS Insight

X-ray photoelectron spectroscopy (XPS) was employed to study the thermal decomposition in vacuum of nickel oxalate dihydrate. The process is shown to proceed via mainly two stages. Evaluating the high-resolution core-level spectra, Auger and valence band spectra acquired as a function of temperature up to 500 °C allowed changes in the surface composition, electronic structure, and chemical state of atoms in the oxalate to be monitored continuously. In particular, the removal of crystallization water in the dehydration stage was found to affect the Ni-related XPS characteristics such as the O/Ni atomic ratio, full width at half-maximum of the Ni 2p3/2 and Ni LMM peaks, Ni 2p3/2 binding energy, satellite-to-Ni2+ peak intensity ratio, and the Ni Auger parameter, all of which demonstrated local maxima at ∼200 °C, which is the temperature corresponding to the highest rate of dehydration. XPS analysis of the product formed in the decomposition stage at ∼350 °C revealed, in addition to metallic Ni as a major constituent, an intermediate oxygen-deficient phase with a stoichiometry corresponding to NiC2O3 or/and Ni2C4O7. The final product of Ni oxalate decomposition at temperatures above 400 °C was found to be metallic Ni particles partially covered with surface oxide NiO, chemisorbed oxygen atoms, and graphitic/amorphous carbon. The XPS studies were complemented by traditional X-ray diffraction, thermogravimetry, and differential thermal analysis coupled with mass spectrometry.