Nanocrystalline cobalt hydroxide oxide: Synthesis and characterization with SQUID, XPS, and NEXAFS

Nanocrystalline cobalt hydroxide oxide (nc-CoOOH) particles were synthesized via a precipitation-oxidation mechanism with different oxidation agents. Since the choice of the oxidative compound is of great practical importance, we compared the properties of the nc-CoOOH particles obtained from the oxidation with O2 and Br2. The particles from both synthesis routes were fully characterized with X-ray powder diffraction (XRPD), infrared spectroscopy (IR), superconducting quantum interface device (SQUID) measurements, X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) measurements. It was found that the choice of the oxidation agent greatly affects the properties of the resulting nanoparticles; the most significant differences concern particle size and magnetic moment, due to residual Co2+. Although both, Br2 and O2, lead to the formation of nc-CoOOH, particles obtained from the synthesis involving O2 show a three times larger particle size than their counterparts obtained from the synthesis using Br2 (24 nm vs. 8 nm). Furthermore, using Br2 during the synthesis leads to an 8-fold increase of the amount of residual Co2+ impurities, which, in fact, induce a magnetic moment. A possible enrichment of those Co2+ impurities on the surface of the nanoparticles is discussed based on XPS and NEXAFS data.