Solid State Microwave Fabrication of Fe3O4-Reduced Graphene Oxide Nanocomposites
Nanoparticle functionalized 2D material networks are promising for a wide range of applications from energy storage to catalysis to separations. in-situ formation of nanoparticles on 2D material networks is commonly limited by rapid particle growth that leads to low particle density and low surface area. Here we demonstrate that synthesis using microwaves yields high density of small and dispersed iron oxide nanoparticles on reduced graphene oxide (rGO) networks. The localized rapid heating of rGO by the microwaves promotes the thermal decomposition of low-cost iron nitrate and generation of iron oxide nanoparticles, but the short heating time and rapid cooling on cessation of microwaves minimize crystal growth for a high density of small Fe3O4 nanoparticles on rGO. In comparison to analogous nanocomposites fabricated by thermal decomposition in a furnace, the microwave processed Fe3O4-rGO composites exhibited higher surface area, smaller average nanoparticle size, and a narrower particle size distribution. When employed in aqueous supercapacitors, much higher initial capacitance (up to 1530 F/g vs. 451 F/g @0.5A/g) and an order of magnitude lower charge transfer resistance is obtained for the microwave synthesized materials. However, these small Fe3O4 nanoparticles supported on rGO tend to be unstable during electrochemical cycling in aqueous supercapacitors with reduction in the electrochemical performance directly correlated with the growth of the Fe3O4 nanoparticles. Through tuning the microwave conditions, a balance between capacitance (692 F/g) and cycling retention (89.7% @500th cycle) can be achieved with an intermediate microwave reaction time (40 s). These results illustrate the ability to fabricate small nanoparticles on 2D materials through rapid localized heating using microwaves in the solid state with tunable size through the microwave process conditions.