Metal foam-carbon nanotube-reduced graphene oxide hierarchical structures for efficient field emission

Carbon nanotubes (CNTs) were directly grown on copper and nickel foams by chemical vapor deposition (CVD) process, followed by the deposition of reduced graphene oxide (rGO) through dip-coating method. These three-dimensional hybrid structures were used as cold cathodes to perform field emission tests under DC bias application. Field emission response of CNTs grown on metallic foams was compared with rGO coated CNTs samples. Field emission results demonstrate that the emission current density and field enhancement factors were significantly enhanced after rGO deposition on CNTs due to the formation of sharp protrusions in rGO sheets. Local field enhancement at the apex of sharp protrusions allows tunneling and hence emission of electrons from tips at considerable lower voltage. rGO coated CNTs based field emitters on nickel foam, among all studied emitters, demonstrated the highest emission current density of 64 mA/cm2, lowest turn-on field of 0.71 and the highest field enhancement factor of 15,999. Excellent field emission characteristics of metal foam based three-dimensional field emitters suggest the potential of the synthesized cathodes in various field-emission applications.