Synthesis of Multifunctional Two-Dimensional Structures and Their Applications

Abstract

Two dimensional (2D) materials with uniformly decorated heteroatoms represent a huge challenge and aroused immense interest beyond graphene due to their exceptional electronic and optoelectronic properties. The most widely studies 2D materials graphene and hexagonal boron nitride (h-BN) are the monoatomic layered materials with similar geometry and pole apart electronic structure and properties. Graphene is metallic due to extremely high carrier mobility and h-BN is wide band gap insulator, which limits their potential application in electronics. Despite recent explorations in materials science, an easy and scalable to produce uniformly doped 2D materials are limited. To overcome these problems, I first designed and synthesized a suitable monomer, hexaaminobenezene (HAB) hydrochloride, for the first time in crystalline form by using an easy procedure. The needle like crystal structure of the HAB hydrochlorides was confirmed by single crystal X-ray diffraction study. HAB was further used to fabricate a new layered 2D network structure with uniformly distributed holes and nitrogen atoms and the overall stoichiometry is thought to be C2N. The 2D structure can be efficiently prepared by simple wet-chemical reaction from the logically designed monomers. The structure of the C2N was confirmed by scanning tunneling microscopy (STM) and the calculated and experimental band-gaps are in the semiconductor region about 1.7 and 2.0 eV, suggesting a clear advantage over conducting graphene and insulating h-BN. I synthesized another 2D structure with C3N stoichiometry with uniformly placed nitrogen atoms in the carbon framework by carbonization of hexaaminobenezene hydrochloride single crystal at 500 °C. The topological and electronic structure of the C3N 2D structure was studied by scanning tunneling microscopy. C3N structure could be a new class of 2D materials with novel properties that can be emerged from the unique structure. C2N structure was used to encapsulate iron particles by in situ reducing and subsequent annealing to give Fe@C2N-h2D. Fe@C2N-h2D material shows interesting oxygen reduction reaction ability both in acid and alkaline solution. C2N structure was also used to encapsulate cobalt oxide by the same way for catalytic hydrogen evolution from the sodium borohydride solution. Co@C2N shows outstanding property of hydrogen evolution compared to the pure metal catalysts. Moreover, I also designed and synthesized organic conjugated triazine polymer demonstrating room temperature ferromagnetism derived from purely organic compounds. The polymer was synthesized through self-polymerization of tetracyanoquinodimethane (TCNQ). Electron spin resonance and magnetic characterization revealed the presence of spin ½ moments, which partially lead to ferromagnetic ordering with a critical temperature that is higher than room temperature.