Supercritical Hydrothermal Synthesis of Spinel Type Cobalt Gallate Nanoparticles with Controlled Crystal Defects and Their Magnetic Properties

There are significant ongoing studies on the production of spinel-type CoGa2O4 nanoparticles, regarding its enhanced properties that could be applied in fields such as gas sensing and electrocatalysis. To achieve a better performance, the implementation of defects like oxygen and metal vacancies could be considered. However, trials that have reported on the implementation of crystal defects within CoGa2O4 are extremely rare. In this work, we report the synthesis of spinel-type cobalt gallate nanoparticles (Co-Ga NPs) using a fast supercritical hydrothermal synthesis at around 400 ℃ for 10 min without performing the calcination process and effects of pH value of precursor solutions on the final solid production were investigated. As consequences, spinel-type Co-Ga NPs with controllable morphology, particle size (17~58 nm) and Co/Ga molar ratios were successfully produced using precursor solutions with various pH values. Based on the composition measurement by inductively coupled plasma (ICP), it was found that Co-Ga NPs produced from the precursor solutions with pH value of 7, 9 and 11, were having the Co/Ga molar ratios that significantly exceed the stoichiometric molar ratio of 1:2, which implied a totally different spinel-type crystal structure compared to CoGa2O4 . Interestingly, Co-Ga NPs produced by the present technique exhibited a combined magnetic behavior of paramagnetism and ferromagnetism based on the results of magnetization measurements at room temperature. Moreover, prepared Co-Ga NPs were generated mainly from solid starting materials in the precursor solutions containing cobalt gallium nitrate layered double hydrates (Co-Ga-NO 3 - LDHs).