Growth, characterization and application of 2D pnictogens - graphene heterostructures

Abstract

Two Dimensional (2D) pnictogens are predicted to have thickness dependent transitionsin electronic properties and hence constitute key interest in energy and electronics applications. The ultra-smooth surface of 2D materials like graphene facilitates the realization of high lattice mismatched layers to grow atop each other via Van-der-Waalsepitaxy. The thus grown polymorphic structures of these pnictogens on graphene have also been found to be superior anode materials for batteries. This study aims to realize such heterostructures and investigate the properties of their rich phases.Aberration corrected atomically resolved scanning transmission electron microscopy was employed to probe the evolution of structure and morphology at the interfaces inthe 2D antimonene (Sb) - graphene heterostructures and bismuthene (Bi) - grapheneheterostructures grown via a physical vapor deposition technique of thermalevaporation. These pnictogens were found to simultaneously exist in two different textures of their thermodynamically preferred layered 2D β-phase, namely: β-(001) and β-[2-21] phases on graphene. Importantly, a strong indication for preferred relative crystallographic orientations with respect to the supporting graphene monolayer lattice was observed for both the phases, indicating the existence of rotational Van-der-Waalsepitaxy in 2D Sb/Bi - graphene heterostructures. The studies here also highlight other interesting properties of the grown pnictogens: (1) exceptional resilience of both phases against environmental oxidation was revealed via electron energy loss spectroscopy (both low-loss and core-loss); (2) electron beam induced in situ crystallizations processes and associated plasmon dynamics were studied with valence electron energyloss spectroscopy etc. Sensitivity of exact Sb / Bi growth results for both phases ongraphene on employed processing and substrate properties incl., notably, the nature of the support type underneath the direct graphene support was found. This work there by provides fundamental insights into the phase wealth and epitaxy landscape in 2D Sb/Biand 2D Sb/Bi- graphene heterostructures.As another promising cost-effective and rational technique to scalable synthesize 2D materials, route of liquid phase exfoliation was explored to exfoliate pnictogens, where exfoliation of Bi under the here adopted processing conditions resulted in the synthesis of laterally large 2D sheets of bismuth-oxy-carbonate. The efficacy of five different common solvents in exfoliating Bi was examined. The synthesized exfoliated sheets/flakes of bismuth-oxy-carbonates were later utilized as catalyst component by integrating them with titanium dioxide as photocatalyst via simple mix and match method. The composite showed improved hydrogen evolution rates upon watersplittingas compared to respective individual components. This work there by furtherdemonstrates the application of bismuth-oxy-carbonate as a catalyst component to drive photocatalytic water splitting reactions to produce hydrogen.