Engineering 2D Graphitic Carbon Nitride/Graphene Nanosheets for Enhancing Photocatalytic Hydrogen Evolution under Visible Light Irradiation

Abstract

Solar energy is the most sustainable energy replacement for fossil fuels as it is a plentiful, widely distributed, and continuously renewable source of energy. Water is the basis for all life on Earth, which contains hydrogen and oxygen. Water is considered the most favorable hydrogen energy carrier because it has the potential to exist for as long as civilization. The polymeric semiconductor, g-C3N4 as a novel noble metal-free photocatalyst with a bandgap of 2.7 eV, a delocalized π system, and diverse synthetic modularity has emerged as a promising visible light-responsive photocatalyst due to its facile synthesis, appealing optical bandgap and semiconducting properties, high chemical and thermal stability as well as a variety of textures and structures. Graphene, a sheet of sp2-bonded carbon atoms packed in a honeycomb lattice one atom thick, is used as a supporting material for photocatalysts due to its superior electron mobility at room temperature and its superior electrical conductivity, which allows it efficiently to accept and mediate photoinduced electrons from excited electron donors. More effective separation and migration of electron-hole pairs prevent the recombination of charge carriers. Hence, photocatalytic hydrogen evolution reaction by half-reaction of water-splitting has been intensively studied for green, low-cost, and large-scale production of H2 fuels. This thesis focuses on the nanostructural design of 2D graphitic carbon nitride, graphene, and the engineering oxidation state of platinum cocatalyst for photocatalytic green hydrogen production. In order to develop an advanced nanostructure of graphitic carbon nitride to maximize catalytic accessible active sites, thus reducing material cost and enhancing the efficiency of the photocatalytic process for commercial technologies, this work provides a strategy to exfoliate bulk graphitic carbon nitride into a single-layer, generating holey structure of graphitic carbon nitride for enhancing photoactivity and discovering the formation of self-assembled graphene phenomenon. Furthermore, this work studies fundamental processes that occur in solar-driven hydrogen production by photocatalyst systems. The online gas chromatography assessment method and an optimization method for hydrogen evolution performance were introduced for a more accurate and transparent photocatalytic hydrogen production measurement benchmark. Multiple characterizations have been performed to understand crystallographic nature, chemical composition, morphology and microstructure, optical properties, photoinduced charge carrier separation and transfer, and newly phenomenon of graphene formation via the solvothermal treatment of graphitic carbon nitride. Moreover, density functional theory calculations were applied to study the adsorption energy of used organic solvents on g-C3N4 sheets and the charge density differences for all adsorbates on the s-heptazine sheet. The distinguished behaviors of interaction between the O atom of the adsorbate and the C atom of the sheet further verified the mechanism of solvolysis of graphitic carbon nitride to form a holey-defect structure. Methyl alcohol, isopropyl alcohol, tetrahydrofuran, and dimethylformamide were chosen for exfoliating and modifying graphitic carbon nitride sheets based on their compatibility with graphitic carbon nitride in Hansen parameters. Uniformly holey-defects on 2D graphitic carbon nitride nanosheets were successfully engineered using tetrahydrofuran solvent in a facile solvothermal process. The introduction of N vacancies in heptazine units and the formation of the holey structure of tetrahydrofuran-modified graphitic carbon nitride (C3N4-THF) sample led to excellent photocatalytic performance due to enhanced mass transfer, shortening of the charge diffusion lengths, increased charge separation during the photocatalysis process, and maximizing accessible reducing and oxidizing active sites. As a result, the C3N4-THF sample achieved superior photocatalytic activity with a hydrogen evolution rate at the stationary point as high as 31,256.9 µmol h-1 g-1 under 1 Sun illumination of a Solar Simulator, which was 13.9 times higher than that of the bulk sample. Furthermore, this work studies the solid-state structure transformation through the solvothermal treatment of bulk graphitic carbon nitride in an environmentally sustainable ethyl alcohol solvent for photocatalytic mass production of green hydrogen. The new structure of grid-like monolayered graphitic carbon nitride nanosheets was achieved using a facile solvothermal process. The formation of self-assembled graphene on graphitic carbon nitride during the process facilitated charge transfer and separation on the photocatalyst, as confirmed by density functional theory calculations. A higher proportion and a better distribution of oxidized platinum cocatalyst enable the ability to suppress hydrogen oxidation reactions, which prefer the direction of hydrogen generation. In order to get a step closer to a homogeneous photocatalyst, this work generates a highly dispersion of holey graphene/graphitic carbon nitride composite in aqueous by using holey graphene as the carrier in 2D/2D photocatalyst hybrid. A high-surface-area-inspired water-soluble holey graphene/graphitic carbon nitride colloidal nanoparticles for hydrogen evolution was successfully synthesized using a mere mixing process of holey graphene oxide with oxidized graphitic carbon nitride, followed by hydrogen reduction in aqueous to restore the initial photocatalytic active sites but maintain highly soluble in reaction media. The recent extreme weather gets a boost from climate change and the global energy security conundrum has posed extensive studies on boosting the conversion of solar energy to chemical energy through photocatalysis. The demonstrations of photocatalytic solar green hydrogen production based on a facile method of synthesizing an advanced nanostructure of 2D graphitic carbon nitride/graphene photocatalyst and the optimization of the performance evaluation system and assessment method for this photocatalyst indicates the large-scale applicability viability as well as technical challenges toward practical mass green hydrogen production.