Designed Synthesis and Electron Microscopy Investigation of Layered Materials Hybrids for Targeted Applications

Abstract

Post-discovery and advancement in graphene, the field of layered materials, have gained enormous research interest. These materials have unique and diverse range of properties. Layered materials further provide an opportunity to tune the properties through control over morphology and heterostructure/hybrid formation based on them - often exploitable for targeted applications in electronics, optoelectronics, electrocatalysis and sensing. In general, heterostructures exhibit properties superior to their individual counterparts. Thus, control over morphology and heterostructure formation is a major challenge owing to the necessity of controlled nucleation and growth of the material for targeted application. Although various physical and chemical vapor deposition methods are available to synthesize the pristine layered materials and their hybrids, solution methods are also promising route for synthesis owing to the simplicity and morphology control. In this work, we demonstrate simple wet-chemical methods to synthesize layered materials and hybrid/heterostructures based on them and further investigated them through different characterization techniques especially using (S)-TEM. A Solvothermal method has been designed to tune the morphology of SnSe2 (a member of layered materials) from 3D to 2D nanosheets. Density function theory has been performed to rationalize the observed morphology control. As-synthesized SnSe2-nanosheets have been integrated with graphene and investigated for photodetection. The graphene-SnSe2 hybrid device exhibits high photo-detectivity for near IR range photodetection. Followed by a simple wet-chemical method has been designed to synthesizeMoSe2 and WSe2. The reaction kinetics studies reveal feasible synthesis of MoSe2 in comparison to WSe2. The reaction kinetic difference in the synthesis of MoSe2 and WSe2 has been exploited to synthesize hierarchical MoSe2@WSe2 hybrid nanostructure. The nanostructure has been investigated as a robust bifunctional electrocatalyst for total water-splitting reactions involving hydrogen and oxygen evolution reactions. The hybrid nanostructure exhibits better electrochemical activity as compared to MoSe2 and WSe2. The hybrid nanostructure also shows better electrochemical stability as compared to commercial Pt/C and RuO2. Thirdly, electron microscopic investigation has been carried out to study the transformation that occurred during anion exchange in layered material considering SnS2 nanosheets as a template. The mechanistic study has shown that the exchange initiates from the side and moves inward. Using the channeled exchange reaction, lateral hybrid nanostructure SnSe2-SnS2 has been synthesized. Finally, phase transformation of layered metal oxide WO3 has been studied through aberration-corrected STEM. The phase transformation of the hydrated WO3 phase reveals that the transformation o-WO3.0.33H2O to monoclinic phase occurs gradually with the coexistence of 2 phases, namely monoclinic and hexagonal. Both phases exist in the same nanostructure with atomically smooth boundaries between them.