| Author: | Yao, Xiao |
| Title: | Development of functional materials for photocatalytic water-splitting for hydrogen generation |
| Advisors: | Ho, Cheuk Lam (ABCT) Yung, Joseph (ABCT) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Hydrogen as fuel Water -- Electrolysis Photocatalysis Transition metal oxides Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Biology and Chemical Technology |
| Pages: | 324 pages : color illustrations |
| Language: | English |
| Abstract: | With the rising concerns of greater energy demand and a decrease in the global fossil fuel supply, exploring alternative energy sources has become an urgent and challenging research subject. Solar energy is considered one of the most promising solutions to our energy scarcity as it can be collected and transformed into different forms of power using photocatalytic devices, and that the solar-driven photocatalytic water-splitting can provide a clean pathway to produce hydrogen fuel. The first water-splitting system for hydrogen production using a TiO₂ photoanode ever reported was in 1972 by Fujishima and Honda. Following this pioneering research, water-splitting systems for hydrogen production have been developing rapidly throughout past decades. Ruthenium(II) complexes have shown impressive performance for their potential applications in water-splitting systems due to the unusual metal-to-ligand charge transitions (MLCT) and unique photophysical properties. However, to realise the light-harvesting abilities in these photocatalytic systems remains a challenging task. Most systems can only use ultraviolet (UV) and partially visible light, accounting for only 5-7% of sunlight. To improve the water splitting for the hydrogen-production capability, I proposed a series of Ru(II) complexes with different ligands, such as isoquinoline or triphenylamine, which have been proven to significantly increase the hydrogen-production efficiency during water splitting. All the Ru(II) complexes have been used in water-splitting hydrogen-production experiments. Additionally, the influence of different anchoring groups, such as carboxylic acid anchors or phosphonate functional groups of Ru(II) complexes in hydrogen production, are discussed in the thesis. With the best Ru(II) water-splitting system after 236 hours of irradiation, an H2 turnover number (TON) of up to 14232 was recorded. Compared to other metal dyes that include first- and second-row transition metals, Ir(III) cyclometalated complexes have taken on excellent ligand field stabilisation energies in their 5d valence shell. However, Ir(III) cyclometalated complexes often suffer from weak absorbance of visible light. Therefore, I synthesised a series of Ir(III) complexes with different functional groups, such as triphenylamine or aldehyde, for the Pt-TiO2 system, which showed satisfactory hydrogen production results. Different anchoring groups were designed to test the working efficiency of the water-splitting system. Most of the developed Ir(III) photosensitisers for photocatalytic applications possess carboxylic acid anchors. However, these dyes might have poor stability under photocatalytic operating conditions. Furthermore, the hydrolysis characteristic of the carboxylate linkage site has been reported to inhibit electron transfer efficiency from Ir(III) complex to TiO2. In this regard, the linkage of the phosphonate functional group to the TiO2 surface has been presented with higher stability than that of the carboxylate group. A respectable hydrogen TON of 16483 in a platinised TiO2 hydrogen-production system was demonstrated with the best Ir(III) water-splitting system. In most traditional molecular water-splitting systems, expensive noble metals have been used. Such high cost may hinder industrial application of those water splitting systems. However, cadmium sulfide nanorod (CdS NRs) systems, which have large light-absorption coefficients, offer great research potential. Research has been conducted on water-splitting systems with CdS and different earth-abundant metal complexes, particularly Ni. Different electron-donating or withdrawing ligands have been used on templates of salen-type or salophen-type Schiff bases to determine the physical and chemical properties of the earth-abundant metal complexes and their water-splitting performance for hydrogen production. This study has also examined the effects of the size and aspect ratio of the CdS NRs on hydrogen production. A photocatalytic system containing a metal-salophen complex based on nickel achieved a steady and impressive catalytic activity with a TON of 57238 and a turnover frequency (TOF) of 436.9 h-1 over 131 hours under blue-light irradiation. Metal-organic cages (MOCs), assembled from diverse inorganic structures and organic linkers, have been in rapid development recently. MOCs are a new type of material with an individual nanoscale molecular structure assembled through weak interactions while possessing a great potential for photocatalytic water splitting. The supramolecular chemistry nature of MOCs makes them the ideal materials for binding catalyst centres with different linkers. Additionally, the catalytic properties of MOCs can be tuned effectively by proper structural design. In order to achieve effectively H2 production, MOCs are expected to possess significant absorption characteristics. I have developed functional metal cages with various structures, such as phenyl, furan, and thiophene combined with Ni or Co. Their various water-splitting performances and physical and chemical characteristics are discussed in the thesis. |
| Rights: | All rights reserved |
| Access: | open access |
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