Synthesis of surface-modified iron oxide nanocrystals using supercritical carbon dioxide as the reaction field

In the synthesis of surface-modified nanocrystals (NCs), a simple and green chemistry approach to reduce liquid waste, particularly a solventless process, has been desired. In this study, we applied the supercritical CO₂ technology, which is an excellent solventless process, to the synthesis of surface-modified iron oxide NCs. The synthesis was performed at 30.0 ± 0.8 MPa of CO₂, 18 h and 100 °C, where iron(iii) acetylacetonate, pure water and decanoic acid were used as starting materials. As a result, the supercritical CO₂ medium gave the NCs of α-Fe₂O₃ and γ-Fe₂O₃ with unimodal size distribution, where the mean size was 7.8 ± 2.0 nm. In addition, they were self-assembled on the TEM substrate and the mean nearest-neighbor spacing was close to the chain length of decanoic acid. Furthermore, FT-IR and TG analyses indicate that decanoic acid chemically attaches to the surface of iron oxide NCs that are dispersed in cyclohexane. These results suggest that the supercritical CO₂ medium could be the new appealing reaction field to fabricate densely modified NCs without liquid waste.