Variability of composition and structural disorder of nanocrystalline CoOOH materials

Abstract

In this study, three distinct preparative routes to nanocrystalline (nc) CoOOH yielding compositionally and structurally specifiable materials, which are all closely related to clean CoOOH are presented. Simple precipitation–oxidation and oxidation–precipitation routes using atmospheric oxygen or Br₂ dissolved in an aqueous solution of NaOH as an oxidising agent lead to the various products. The purified solids were characterised by means of X-ray powder diffraction (XRPD), chemical, thermal and thermogravimetric analyses, magnetic measurements, and, based on synchrotron data, by means of pair distribution function (PDF) analyses. The chosen routes led to nanocrystalline materials differing in the mean size of the coherently scattering domains (7 nm and 24 nm), in the lattice parameter c, in the relative amount of released mass in the course of conversion into cobalt oxide spinel and the residual relative amount of non-oxidised Co(II). The incomplete oxidation of Co(II) is associated with a molar ratio n_H/n_O > 0.5 that is found to be the origin of stacking fault disorder in the materials. Possible models of disorder were derived from structural considerations and tested by DIFFaX simulations. A recursive routing for refining the microstructure by global and local optimisation of the measured powder patterns was applied to each sample. Crystal intergrowth of the (AcB)(BaC)(CbA) stacking pattern of clean CoOOH and that of (AcB)(AcB) brucite type Co(OH)₂ was identified as a main feature of the microstructures of the three diverse CoOOH-type samples. PDF analyses corroborate the derived microstructural models.