Hierarchical nanostructures incorporated with different enhancement strategies for improved photoelectrochemical performance

• Main Author: Wang, Zhiwei
• Other Authors: Alfred Tok Iing Yoong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Nanostructured materials
• Online Access: https://hdl.handle.net/10356/138681
• Institution: Nanyang Technological University

Photoelectrochemical (PEC) water splitting makes it possible to harvest solar energy and then convert it to storable hydrogen fuel in a one-step reaction. As the product of hydrogen combustion is water, PEC water splitting offers enormous promise when it comes to reducing carbon emission. Since the investigation into PEC water splitting with TiO2 thin films as the photoelectrode, various semiconductors with different nanostructures have been developed as photoelectrodes for enhanced PEC performance. An ideal PEC photoelectrode is characterized by an efficient visible absorption, quick charge collection ability, fast charge transfer and high photostability. As a single semiconductor cannot fulfill all of the requirements, several targeted strategies were thus proposed to satisfy more requirements. However, the incorporation of several targeted strategies into one nanostructure to achieve synergistic effects still remains a challenge due to uncertainties in the interaction between these strategies. Herein, a series of hierarchical nanostructures were reported which incorporate different targeted strategies, such as elemental doping, enlarged surface area, heterojunction formation, surface protection, and increased light scattering ability. Through investigating the interactions between these targeted strategies, synergistic effects were achieved which contributed to an improved PEC performance. Firstly, the heterojunction formation and depressed interface charge recombination strategies were combined in a SnO2 nanosheets (NSs)/TiO2 thin film/CdS quantum dots (QDs). The introduced TiO2 thin film fully covered the porous SnO2 NSs and the bare FTO glass substrate, resulting in a depression in interface charge recombination. Besides, the introduced TiO2 thin film has a proper band alignment with SnO2 and CdS, which is beneficial to the formation of a heterojunction. Thus, porous SnO2 NSs/TiO2 thin film/CdS QDs showed a higher photocurrent in the low voltage area and a lower onset potential than SnO2 NSs/CdS QDs. Secondly, the interaction between the surface area and light penetration was investigated in the WO3 NSs/CdS nanorods (NRs)/amorphous TiO2 film photoanode. The large space between adjacent WO3 NSs provides sufficient areas for the secondary growth of 1D CdS NRs, which can increase the contact area with the electrolyte, favoring the oxidation process. However, excessive loading of CdS NRs occupies all the space, which ends up blocking light penetration to the bottom area. Through morphology tuning, a co-existence of CdS NRs and short CdS NRs on the surface of WO3 NSs was achieved, reporting a higher photocurrent than pristine short CdS NRs and long CdS NRs. This enhancement results from achieving a balance between the amount of light penetration and the increased surface area. The photostability of CdS NRs is further improved by coating with a thin layer of amorphous TiO2 film. Thirdly, the interaction between surface area and electron collection was also investigated in the hierarchical FTO inverse opals (IOs)/SnO2 nanocrystals (NCs)/TiO2 film photoanode. Although longer SnO2 NCs can offer larger surface areas, short SnO2 NCs have lower charge transfer resistance, resulting in better PEC performance. Finally, FTO IOs/CdS NRs/CdSe clusters were fabricated based on previous results. A co-existence of long CdS NRs and short CdS NRs were grown on FTO IOs, which were then coated with narrow bandgap CdSe clusters. Conductive 3D FTO IOs played the role of a quick electron collector, while CdS and CdSe acted as visible light absorbers for charge generation. The CdS/CdSe heterojunction further enhanced the charge separation. Furthermore, the unique 3D/1D hierarchical nanostructure enhanced light scattering ability and thus enabled higher light absorption. Due to the synergistic effects of these different components, FTO IOs/CdS NRs/CdSe clusters showed a higher photocurrent than pristine CdS NRs/CdSe clusters. This work investigated the interaction between different targeted strategies and their influence on the PEC performance, providing useful guidance on the future fabrication of photoanodes for efficient PEC water splitting.