Rational design of highly efficient visible light driven photocatalyst for enhanced H2 evolution
Ngaw, Chee Keong
Date of Issue2016-10-05
Interdisciplinary Graduate School
This thesis focuses on the development of highly active nanomaterials for H2 evolution based on the rationale design of semiconducting materials. Core shell Au@TiO2 nanocomposite was designed by incorporating the Au (nanoparticles) NPs into the TiO2 hollow cavity, and their H2 activity was further compared with conventional Au-P25. Higher H2 rates generated by the Au@TiO2 can be attributed to its larger surface area and the encapsulation of Au NPs inside the hollow nanocomposites. The encapsulated Au NPs were developed to increase the degree of light absorption through multiple light scattering and induced a stronger localized SPR effect for higher H2 generation. Based on the excellent H2 photocatalytic activity exhibited by the Au@TiO2 hollow spheres, these photocatalysts were fabricated into a photoelectrode, and further employed in a solar assisted microbial hybrid device (Au-TiO2-hybrid device) for H2 generation. This hybrid device, which was driven by solar energy (from PECs) and the metabolism of microorganisms (in MFCs), exhibited higher photocurrent output than conventional standalone PEC devices. The hybrid system employing Au-TiO2 hollow spheres and DSSN+ generated a photocurrent of ~0.35 mA/cm2 at zero bias (0 V vs. Pt), while the conventional stand-alone PEC cell employing only Au-TiO2 hollow spheres was ~0.04 mA/cm2. The enhancement was attributed to the additional electrons originated from the MFC in the hybrid system. Results also show that when these hybrid devices were chemically modified with conjugated oligoelectrolytes (COEs), their photocurrent output and H2 evolution were further enhanced due to better charge collection of the MFCs in the modified hybrid system. The H2 performance of a photocatalyst can also be enhanced by morphological control because the catalytic properties of nanocrystals depend strongly on the enclosed facets derived from a particular morphology. NaInS2 nanostructures with different morphologies, including octahedral nanocrystals (NCs), nanosheets and microspheres were prepared, and the effect of morphologies on the performance for H2 evolution was investigated. The octahedral NaInS2 NCs exhibited the highest H2 evolution and the superior performance can be attributed to the active facets enclosed on these NCs, which provided more active sites to accelerate the water splitting process for H2 generation. To further improve the performance of the octahedral NaInS2 NCs, a quaternary sulphide (Ca-ZnIn2S4) NCs was synthesized via a cation-exchange template strategy. As the morphology and the crystallinity of the NCs were well preserved during the cation-exchange reaction, these obtained quaternary Ca-ZnIn2S4 NCs were able to produce new enhanced properties, while inheriting properties from its parent ternary NaInS2 structure. Higher H2 rates were observed for the Ca-ZnIn2S4 NCs and the enhancement can be attributed to the incorporation of the Ca2+ and Zn2+ cations in the NaInS2 structure, which results in a significant improvement to its light absorption, charge separation and stability of the material. In summary, high performance novel photocatalyst were developed by encapsulating Au NPs in hollow nanocomposites, morphological control and through the formation of quaternary-based nanocomposites.