Design, synthesis and applications of functional carbonaceous nanospheres

Abstract

Carbon nanoparticles have gained considerable attention due to their distinguishing features including hydrophobic, conducting and thermostable properties. Other features include the fine control over the ductile properties, porosity, and surface area of the nanoparticles. These features allow the nanoparticles to be designed for applications in drug and vaccine delivery, nanoreactors for catalytic reactions, electrode material for energy storage and conversion. With the advance in new synthetic techniques, carbon nanoparticles can now be designed with control over the particles morphology, which include microporous, mesoporous, hollow, core-shell and yolk-shell structures. However, due to the inert nature of carbon, there remains a great challenge to prepare carbon nanoparticles not only with a desired morphology and porosity, but with desired functionality. Accomplishing this will provide a powerful synthetic methodology to prepare the next generation of carbon nanoparticles, improving on the many known applications and possibly generate new technologies. This thesis aims to develop highly effective and facile methods to prepare carbon nanoparticles with not only controlled morphology and porosity, but producing composite nanoparticles (e.g. heteroatom-doping and metal nanoparticle inclusion) and greater surface functionality. The additional features have provided the design capability to develop better supercapacitors and nanocatalysts. The first aspect discussed in this thesis is the development of monodisperse Ag@carbon coreshell nanospheres by an extended Stober method. The synthetic method is based on the the similarity between the organic sol-gel of resorcinol/formaldehyde (RF) resins and silicate solgel process. This important finding represents a unique method to make carbon nanoparticles with morphologies similar to that found with silica. In particular, the well-know Stober method is extended to enable the preparation of Ag, AgBr@RF core-shell nanospheres, which after being carbonized under N2 gas produced Ag@carbon core-shell nanospheres. The shape and thickness of the shell can be tuned by simply adjusting the synthesis parameters. The simple one-pot route can be extended to the preparation of core-shell spheres with other metals/metal oxides as cores. The rattle-type Ag, AgBr@meso-SiO2 and yolk-shell structured Ag@carbon@meso-SiO2 have been selectively synthesized by calcinations of double layered Ag, AgBr@RF@meso-SiO2 under air or nitrogen respectively. This synthesis approach is considered to be low cost and more suitable for industrial production. Because of the II tunability and functionality of both cores and shells, these complex core-shell and yolk-shell spheres have the potential application as nanocatalysts, which have been confirmed by our preliminary photocatalysis study. The second aspect discussed in this thesis is the design and synthesis of N-doped mesoporous carbon nanosphere (N-MCN) via a dual surfactants soft template method for fuel cell applications. Here, the technique allows a homogeneous distribution of the doped heteroatom nitrogen into the carbon framework, while at the same time the particle size can be adjusted from 40 to 750 nm. This method also allows the preparation of N-doped hollow mesoporous N-doped carbon nanospheres with a different shell thickness (from 9 to 26 nm) via an outersilica- assisted method. This general synthesis protocol has been further extended to synthesize N-doped hollow microporous carbon and core-shell structured spheres by a onestep aminophenol formaldehyde polymer resins coating and polymerisation process. Interestingly, graphitic as well as dual heteratoms doped mesoporous carbon spheres have been successfully synthesized by a post treatment method. All the resulting materials showed, due to their unique mesoporous and heteroatom nitrogen doping nanostructure, greater energy conversion performance, for example, in the oxygen reduction reaction (ORR). A general and versatile method has also been developed to decorate diverse noble-metal (Pt, Au, Rh, Ru, Ag, Pd, Ir) nanoparticles on N-doped mesoporous carbon nanoparticle (N-MCN) via a simple impregnation and reduction method. Based on this strategy, confined noblemetal nanoparitlces in the cage of N-doped hollow mesoporous carbon nanospheres (NHMCN) have been fabricated. This method can be further extended to fabricate bimetallic nanoparticles (AuPt, AuRh and PtRh) loaded carbon spheres. All the nanostructure morphology, porosity and their alloy degree have been controlled, and their catalytic hydrogenation performance was evaluated. The last part of this thesis reports the synthesis of hierarchical mesoporous yolk-shell structured carbon nanospheres (YSCNs) with ordered mesoporous carbon core and microporous carbon shell via a versatile Stober coating method. The YSCNs can be synthesized with a unique core, void and shell structure with a controlled and different porosity in each structure. For example, the porosity of the shell, core and void contained micorpores (l2 nm), mesopores (2-50 nm) and macropores (g50 nm), respectively. The pore size on the shell can be tailored from a micropore scale (0.6 nm) to a large mesopore scale (~40 nm). The hollow space also can be varied by tunning the silica layer coating thickness. III Yolk-shell structured carbon spheres with graphitic carbon or N-doped carbon shell have also been designed by controlling the synthesis conditions. These yolk-shell structured carbon nanospheres have excellent performances as supercapacitors due to their combined hierarchical porous structures. The findings reported in this thesis provide a series of well-designed carbonaceous nanoparticles in mesoporous, hollow, core-shell and yolk-shell structure. These multifunctional compartments have great potential for the design of nanoreactors and delivery vehicles, thus opening a new platform for a wide range of applications.