Hydrothermal Microemulsion Synthesis of Oxidatively Stable Cobalt Nanocrystals Encapsulated in Surfactant/Polymer Complex Shells

Air-stable magnetic cobalt nanocrystals have been conveniently prepared via a reverse micellar synthesis, followed by a hydrothermal treatment. The synthesis was carried out by first mixing an aqueous solution containing cobalt chloride and poly(sodium 4-styrenesulfonate) (PSS) with an organic mixture containing cetyltrimethylammonium bromide (CTAB) to form reverse micelles, followed by reducing cobalt ions with sodium borohydride. The resultant nanoparticles were then undergone a hydrothermal treatment at 165 °C for 8 h to generate well-dispersed CTAB/PSS-encapsulated cobalt nanocrystals with an average diameter of 3.5 ± 0.5 nm. The nanoparticles were highly crystalline with a hexagonal close-packed crystal phase. The presence of CTAB/PSS complex coatings was identified by FT-IR and UV−vis spectroscopies as well as thermogravimetry analyses. The nanocrystals exhibited superparamagnetic property at room temperature with a saturation magnetization (M s) of 95 emu/g. The magnetization could be largely preserved after storage at room temperature for 4 months as the M s value only slightly decreased to 88 emu/g (measured at 300 K). Thus, the polymer encapsulation could not only improve thermal stability of the micelles for the growth and nucleation of Co atoms but also protect the resulting cobalt nanocrystals from oxidation through forming an oxygen impermeable sheath.