Strain-Driven Magnetic Transition of Intercalated Pressure-Stabilized Cobalt and Cobalt Oxide Nanoparticles in Linked Graphene Oxide Nanoscale Reactors

The study of the evolution and metamorphosis of nanoparticles under high pressure and in nanoscale confinement is a rapidly developing field that promises a diverse range of fundamental research and application opportunities. Here, we demonstrate how a linked and strained graphene oxide (GO)-based confinement system, functioning as a <i>nanoscale reactor</i> at high pressures, allows the evolution of the magnetic properties of an <i>in situ</i> generated composite cobalt (Co) nanoparticle system and further enables the retention of such properties when the pressure of the system is returned to ambient. We posit that this phenomenon is due to an <i>‘induced pressure’</i> created by strain, on the flexible planes of the 2D GO system, by the intercalated Co-based nanoparticles. For graphene/GO this strain is characterized by shifts in the Raman "G-band". Specifically, the studied system comprises <i>in situ</i> generated Co-containing nanoparticles confined between linked GO layers upon pressurization between 0 and 25 GPa. After quenching to ambient pressure, each of these samples exhibited innate ferromagnetic behaviors, demonstrating that our confinement system can be used to <i>‘lock in’</i> phase changes created by the application of transient high pressure. Importantly, while the unpressurized sample exhibited antiferromagnetic Co₃O₄ nanoparticles of ~4 nm size, the samples pressurized to 10 GPa and 25 GPa, presented ferromagnetic order on the shell of an antiferromagnetic core of Co₃O₄ and CoO.