Controlled synthesis of graphene-based hybrids on hydration-dehydration treated bio-shell-derived catalysts

Graphene-based hybrids such as carbon nanotube/graphene nanosheet (CNT/GNS), carbon nanofiber/graphene nanosheet (CNF/GNS), and CNF-CNT/GNS have attracted extensive attention in energy-related applications due to their excellent properties. The selective synthesis of GNS-based hybrids usually relies on the precise design of metal catalysts and multiple tedious processes. Here, a facile one-step synthesis of GNS-based hybrids was successfully performed by chemical vapor deposition (CVD) on waste bio-shell derived catalysts without the addition of extra metal components, and GNS-based hybrids were selectively synthesized through the hydration and dehydration (H-D) treatment of catalysts. When eggshells were calcined at high temperature, the obtained E-CaO catalyst only synthesized GNSs. After E-CaO was treated by the H-D method, the obtained E-CaO-H-1 catalyst grew CNF/GNS. E-CaO-H-2 catalyst was further prepared by the H-D treatment of E-CaO-H-1, which produced CNF-CNT/GNS. Multiple characterizations of catalysts demonstrated that the high-temperature calcination of eggshells led to the aggregation of CaO and incorporation of Fe into CaO lattices, and thus only GNSs were synthesized on CaO. However, after the H-D treatment of E-CaO, the Fe element was released from CaO lattices and the surface area of CaO enhanced with the decrease of particle size, leading to the dispersion of Fe oxides on the CaO surface, and Fe particles were subsequently formed during the CVD synthesis, which synthesized CNFs on GNS. Further, with the size reduction of Fe particles after the second H-D treatment of the catalyst, smaller Fe particles facilitated the CNT growth, leading to the production of CNF-CNT/GNS. Similarly, the catalysts formed by calcination of other bio-shells (oyster shells, shrimp shells, and crab shells) only synthesized GNSs, whereas these catalysts after H-D treatment produced CNF-CNT/GNS with enhanced carbon yields. This study offers a new method for utilizing the metal species in waste bio-shells for the facile production of structurally tunable GNS-based hybrid materials.