| Author: | Zhang, Xiandi |
| Title: | Development of copper- and tungsten-based phosphide nanostructures for photocatalytic reduction reactions |
| Advisors: | Lee, Yoon Suk Lawrence (ABCT) |
| Degree: | Ph.D. |
| Year: | 2021 |
| Subject: | Photocatalysis Transition metal phosphides Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Biology and Chemical Technology |
| Pages: | xiii, 179 pages : color illustrations |
| Language: | English |
| Abstract: | The ever-increasing demand for high quality energy sources and the concerns about environmental degradation have driven the global efforts on seeking sustainable and renewable energy resources and devices. Solar light-driven catalytic reaction is considered as one of the most promising strateties to address these issues. By harvesting solar light, photocatalysts drive the energetically uphill reactions, such as hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR), to convert water or carbon dioxide (CO2) to value-added chemicals, which abates the pollution problems as well as relieves the concerns on the depletion of fossil fuels. The development of effective and sustainable photocatalysts based on transition metal is the key to the successful photocatalytic conversions. Despite the intensive effort devoted in the past decades, the photocatalytic performances of transition metal oxides and sulfides are largely limited by the fast recombination of electron-hole pairs and unstable crystal structure, resulting in a low energy conversion efficiencies and poor reaction stabilities. Recently, transition metal phosphides (TMPs) have gained broad interest as a new member of active catalysts. Owing to the advantage of chemical and mechanical durability, electrical conductivity, and low cost, TMPs have been applied to various energy conversion reactions including HER, oxygen reduction reaction (ORR), and methanol oxidation reaction. However, the current studies are focused mainly on the application of Ni, Co, Fe, and Mo-based single metal TMPs as electrocatalysts. So far, the photocatalytic application of TMPs has rarely been reported due to their metallic properties lacking the light absorptivity. In this Thesis, a new synthetic approach and structure modification strategies are engaged to prepare copper- and tungsten-based phosphides (Cu3P and WP) and carefully tune their properties to apply toward photocatalytic HER and CO2RR. Cu3P has an ideal band gap of 1.3 - 1.5 eV for photocatalytic reduction reactions, but insufficient active sites and weak oxidation ability of Cu3P have restricted its application as a host photocatalyst. By doping foreign elements, unique Cu3P-based nanostructures are synthesized, which show optimal electronic configuration for HER and enhanced stability toward photoreduction reactions. In addition, the use of metallic TMP as an effective co-catalyst in photocatalytic CO2RR is explored. The interfacial bonding environment between TMPs and semiconductors is investigated to reveal the mechanism of the performance promotion. In Chapter I, the mechanisms of photocatalytic HER and CO2RR are summarized with a brief review on the recent progress of transition metal-based photocatalysts. Some strategies widely used for improving the photocatalytic activity are also introduced. In Chapters II and III, heteroatom-doping is employed to modify the structure of Cu3P and study its effect on photocatalytic HER. A sulfur-doped Cu3P (Cu3P|S) nanoplates containing active phosphosulfide (PS) structures is described in Chapter II. The enhanced photocatalytic HER rate of Cu3P|S is credited to the PS structure that is formed by the partial replacement of P with S, which drives the Gibbs free energies (ΔGH*) for the intermediate states to be close to 0 eV and increases the number of active sites for H adsorption. In Chapter III, another heteroatom, boron, is used to prepare boron-doped Cu3P (B-Cu3P) nanocrystals that exhibits an impressive enhancement on the HER rate with a prolonged stability. The B-induced changes in the atomic configuration and bonding environments are shown to endow the B-Cu3P with enhanced photostability and charge carrier generation, as well as the improved conductivity and optimized Gibbs free energy of H adsorption by downshifting the Fermi level. Chapter IV and V describe the developments of TMPs as CO2RR photocatalysts. In Chapter IV, ultra-small copper phosphosulfide (us-Cu3P|S) nanocrystals are coupled with 2D g-C3N4 to form an S-Scheme junction and thus enable the photocatalytic CO generation from CO2. Systematic studies on the interaction between us-Cu3P|S and g-C3N4 reveal the interfacial P-N chemical bond that acts as an electron transfer channel, facilitating the migration and separation of charge carriers. Chapter V discusses another heterostructure constructed by coupling ultra-small WP nanoparticles embedded in N-doped C (WP-NC) with 2D g-C3N4 (WP-NC/CN). The Schottky junction formed at the intimate interface between WP-NC and g-C3N4 via P-N chemical facilitates the flow of photoexcited electrons, preventing the recombination of charge carriers and thus significantly enhancing the photocatalytic CO production rate from CO2RR. The role of WP as a co-catalyst in photocatalytic CO2RR is revealed. Finally, in Chapter VI, the conclusions of all research works in this Thesis are summarized with the perspectives for future research direction. |
| Rights: | All rights reserved |
| Access: | open access |
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