Synthesis of porous organic-inorganic nanocomposites based on urea-bonded molecular networks and study on their electrocatalytic and photocatalytic properties

Abstract

Functionalization of surfaces is an important technique. Adjusting, enhancing, improving, or changing the properties of a surface is a critical issue in introducing the desired functionality to an object. Coating the surface of a porous material with a thin layer is an attractive way to provide maximum functionality. The reason is that molecules grafted onto the surface can provide the desired function to the substrate. This paper shows that porous carbon can be made using urea-based network nanogels and introduced on various substrate surfaces. Furthermore, studies on these materials' electrochemical and photochemical catalytic activity are presented. The first section proposes a template-free approach for mesopore-rich hierarchical porous carbon. Mesopore-rich, hierarchically porous carbon monolith was prepared by carbonizing a polyisocyanurate network derived by thermal rearrangement of a polyurea network. The initial polyurea network was synthesized by cross-linking polymerization of tetrakis(4-aminophenyl)methane (TAPM) and hexamethylene diisocyanate (HDI) in the sol-forming condition, followed by precipitation into nanoparticulate solids in a nonsolvent. The powder was molded into a shape and then heated at 200-400 °C to obtain the porous carbon precursor comprised of the rearranged network. Thermolysis of urea bonds to amine and isocyanate groups and the subsequent cyclization of isocyanates and vaporization of volatiles caused sintering of the nanoparticles into a monolithic network with micro-, meso-, and macropores. The rearranged network was carbonized to obtain a carbon monolith. It was found that the rearranged network with a high isocyanurate ratio led to the porous carbon with a high mesopore ratio. The electrical conductivity of the resulting carbon monoliths exhibited a rapid response to carbon dioxide adsorption, indicating the efficient gas transport through the hierarchical pore structure. The second section reports the facile synthesis of the graphene sandwiched in microporous carbon layers, using a urea-bonded network nanogel as a carbon precursor while exploiting its unique ability to form a uniform nanofilm on the graphene surface and rearrange into a microporous carbon layer upon moderate thermal treatment. The initial urea-bonded network was synthesized by cross-linking polymerization of TAPM and HDI in the sol-forming condition. The facile processability of urea-bonded network sol allows optimizing the carbon-graphene composition, providing a conductive framework of the nitrogen-doped graphene-in-carbon sandwiched structure with hierarchical porosity, consisting of the macropores between the carbon-sandwiched graphene flakes and micro/mesopores within the carbonaceous coating layer. The graphene-in-carbon composite exhibit excellent molecular and electron transport characteristics. The tandem arrangement of graphene and carbon layers facilitated oxygen reduction in water with negligible formation of peroxides. The study shows that wrapping nanomaterials in the urea-bonded covalent network nanogel is a promising method for synthesizing functional carbon-based nanocomposites and preparing efficient electrocatalytic materials. In the third section, we demonstrate that the solution process of the porphyrin-based network nanogel enables synthesizing a porphyrin-based network nanogel coated TiO2 nanoparticle with superior photocatalytic activity. The initial porphyrin-based urea-bonded network was synthesized by cross-linking polymerization of 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) and HDI in the sol-forming condition. The grafting of porphyrin-based urea-bonded networks nanogel on the TiO2 surface homogeneously forms porphyrin aggregate within the covalent network layer. The porphyrin-based urea-bonded network shows high charge separation efficiency for H2O2 synthesis. The composite shows high photocatalytic activity toward H2O2 synthesis. In the fourth section, we prepare a metalloporphyrin-based urea-bonded network coated graphene. Metalloporphyrin thin layer on graphene is simply prepared by mixing a porphyrin-based urea-bonded network with graphene oxide and immersing in the metal precursor solution. This study shows that wrapping nanomaterials with a thin porous layer made from the urea-bonded networks is a promising method for synthesizing functional layer coated nanocomposites and preparing efficient electro- and photocatalysts.