Insights into the Thermal Decomposition of Co(II) Oleate for the Shape-Controlled Synthesis of Wurtzite-Type CoO Nanocrystals

Abstract

Fatty acid salts of transition metals are known to undergo thermal decomposition in high-boiling organic solvents. Although it is a straightforward, promising approach for generating colloidal metal oxide nanocrystals in high yield, its widespread implementation is hindered by irreproducibility. Subtle structural variations and impurities are often introduced during preparation of the carboxylate precursors, which exhibit strong influence on thermal decomposition, and the resulting nucleation and growth of oxide nanocrystals. Here, we studied the colloidal synthesis of wurtzite-type CoO (wz-CoO) nanostructures via thermal decomposition of Co(II) oleate complex [Co(OL)]2. Using Fourier transform infrared spectroscopy, we discovered that the conventional method for preparing [Co(OL)]2 gives rise to an isolable impurity containing a free hydroxide moiety. Furthermore, [Co(OL)]2 did not thermally decompose within the expected temperature range when the impurity was removed. In contrast, pencil-shaped wz-CoO nanorods were synthesized when no measures were taken to remove the impurity, suggesting that the hydroxide functionality may facilitate the thermal decomposition. These insights enabled us to prepare size-and shape-controlled wz-CoO nanocrystals using purified [Co(OL)]2 solutions, by incorporating additives that mimic the action of the hydroxide impurity. We also demonstrate a simplified pathway to wz-CoO nanocrystals that does not require chemical additives for thermolysis.