Control over the structures and properties of carbon nanomaterials for oxygen electrocatalysis and photocatalysis

Abstract

As the world population and the usage of high-tech electronic devices increase, the global energy consumption is getting increased along with many environmental issues including global warming, and depletion of fossil fuel. Hence, it is of great importance to develop nanomaterials applicable for sustainable and clean energy technologies. Recently, many different types of carbon-based catalysts for photochemical and electrochemical reactions have been designed through modulation of their physical and chemical structures by doping with heteroatoms and controlling their configurations. However, very harsh conditions such as strong acid treatment and high temperature annealing are still required to control the compositions and nanostructures of the carbon-based catalysts. The limitations of the previous methods are driving development of new synthetic approaches for effective control over the nanostructures and chemical compositions of the catalysts. In this thesis work, a novel and mild synthetic method based on Metal-accelerated Solvothermal Radical Reaction (MSRR) was developed for control over the physical and chemical structures of carbon-based nanocatalysts. In Chapter 1, a simple and mild synthetic method, MSRR was developed for the synthesis of Mo-doped carbon dots as a photocatalyst. The carbon dots were prepared by heating N-methyl-2-pyrrolidone (NMP) in the presence of molecular oxygen and MoS2 nanosheets. Molybdenum and sulfur were doped into the carbon dots via formation of amorphous molybdenum sulfide in the carbon matrix during a process of the MSRR. In addition, the carbon dots had a hollow-sphere structure (Mo-CDs), while non-hollow carbon structure was prepared in the absence of MoS2 nanosheets (N-CDs). It was found that a larger amount of water molecule was produced in the presence of MoS2 during the oxidation process of NMP in the MSRR, leading to nanobubbles to produce a hollow structure. The Mo-CDs with more pyridinic nitrogen exhibited much higher photocatalytic activity for the oxidative coupling reaction of amines under visible light irradiation than N-CDs. The synthetic and reaction mechanisms for the photocatalysis were also revealed. In Chapter 2, the molecular engineering of organic molecules and Co ions was employed in the MSRR to control the structure and composition of carbon-based electrocatalysts. Two different organic precursors, 4-aminopyridine (4-AP) and pyrrole-2-carboxylic acid (PCA), and two cobalt precursors, CoSO4·7H2O and Co(CN)2·2H2O, were used in the MSRR. Controlling the interactions between the organic precursors and the cobalt precursors allowed the formation of four different types of the electrocatalysts, including atomic Co-doped carbon nanosheet (Co-C), Co nanoparticle immobilized carbon nanosheet (AP-CoNP-C), atomic Co-doped carbon hollow sphere (PCA-Co-C), and Co nanoparticle imbedded carbon sponge (PCA-CoNP-C) with unique N/Co configurations and structures. The prepared catalysts were successfully applied for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high activity and stability. The computational study revealed that the pyridinic N-Co played a critical role in enhancement of the catalytic activity for ORR and OER, and especially the carbon near the cobalt cluster on the backside of the catalysts was found to be an active site. The Zn-air battery composed of AP-CoNP-C and PCA-Co-C was prepared, and its performance was compared with that of Pt/C and RuO2. The Zn-air battery composed of AP-CoNP-C and PCA-Co-C showed greater performance and better charge-discharge stability. The developed MSRR and molecular engineering approaches can be extended to design diverse chemical and nanostructures of carbon nanocatalysts with desired photocatalytic and electrocatalytic activities.