Role of Kinetics and Thermodynamics in Controlling the Crystal Structure of Nickel Nanoparticles Formed on Reduced Graphene Oxide: Implications for Energy Storage and Conversion Applications

Ultrafastheating has emerged recently to speed up the synthesisprocesses of nanoparticles and control their morphology. However,it is not clear how the heating rate affects the formation of metalnanoparticles, particularly those formed on substrates. Here, we exploredthe formation of nickel (Ni) nanoparticles on graphene oxide (GO)substrates under slow (20 degrees C/min) and ultrafast (10(3) degrees C/s) heating rates. The experiments were performed in situon heating microchip devices using an aberration-corrected transmissionelectron microscope. Interestingly, the GO structure was the mosteffective in controlling the stability of nanoparticles when ultrafastheating was employed, leading to a hexagonally close-packed Ni phase(hcp-Ni) because of less lattice mismatch with thegraphitic substrate. On the contrary, fcc-Ni nanoparticlesformed under a slow heating process where no strong correlation withthe GO crystal structure was observed. Additionally, ultrafast heatingresulted in smaller-size nanoparticles which could be ascribed torapid reduction, nucleation rate, and higher diffusion barrier of hcp-Ni crystals on rGO. Nevertheless, the stability of thecrystal structure of the nickel nanoparticles remains unaffected bytheir size. These results indicate the crucial role of the substrateon crystal structure during the nonequilibrium processing of materialsand the competing effects of thermodynamics versus kinetics in creatingnovel phases of materials for energy storage and conversion applications.