CdS-based Photocatalysts : The Study of Tip Decoration, Charge Dynamics, and Heterointerfaces

Abstract

Photocatalytic water splitting to produce H2 fuel has drawn considerable attention due to ever-increasing environmental concerns and the rising global energy demand for renewable and clean energy formation. Since last century, the semiconductor photocatalysts have been studying to explore their photocatalytic properties for solar-driven H2 evolution from water. Among the various H2-evolving photocatalysts, CdS with excellent visible-light absorption and suitable band potentials is one of the most studied photocatalysts. Although cocatalyst-decorated CdS nanorods (NRs) offer a promising H2 evolution performance, further benefits invoked by spatial cocatalyst decoration are worth exploring. Given that noble metal cocatalysts were most deposited on the tips of CdS NRs, the tip deposition of non-noble metal cocatalysts on CdS NRs was explored in the first stage of PhD research. For the first time, it was found that amorphous MoOxSy preferentially photodeposited on CdS NR tips, which enhanced photocatalytic H2 evolution. Detailed characterization and studies determined the composition and possible formation pathways of the amorphous MoOxSy on the CdS tips. The enhancement was ascribed to the effective trapping of photoinduced electrons by the MoOxSy resulting from its lower surface work function compared to CdS. However, amorphous MoOxSy is unstable in most hole scavengers except methanol. Consequently, the MoOxSy was vulcanized into MoS2, with MoS2 presenting either on the only tips, on the tips and random walls, or having an overall coating. MoS2-tipped CdS NRs exhibited a better H2 evolution activity compared to other MoS2-loaded CdS NRs. Kelvin probe force microscopy and time-resolved photoluminescence spectroscopy were used to study spatial charge separation, transfer, and lifetime. The superior photoactivity of MoS2-tipped CdS NRs was assigned to spatially intensified charge separation along the long axis of NRs, thereby lowering the charge recombination rate. To further understand the effects of MoS2 distribution on CdS NRs, MoS2 was parallelly or perpendicularly placed on polar CdS (001) surfaces (representing CdS NR tips) and non-polar CdS (110) surface (representing CdS NR wall) to construct different CdS/MoS2 models for DFT calculations. It was verified that CdS NR is capable of the longitudinal charge transfer due to charge polarization on polar (001) surfaces. The distinct electronic properties (i.e. shift in core-level energies of Cd and S orbitals, interfacial charge accumulation and depletion) of different CdS/MoS2 heterostructures, induced by the different interfacial bonding arrangements and MoS2 orientation, were observed and analyzed. In addition, two types of MoS2-loaded CdS NRs (MoS2 being only on the tips or only on the walls of CdS NRs) were fabricated to obtain their BEs of Cd 3d and S 2p by XPS for comparison, which is consistent with the expected DFT results. The band edges of CdS component in the as-prepared samples were also determined by combining Tauc plots and VB XPS measurements; the band shifts in experiments approximately correspond to DFT results. As a result, the MoS2 distribution and orientation affect the electronic properties of CdS/MoS2 heterostructures and thereby giving a different photoactivity. Overall, this thesis reveals that the origins of photocatalytic H2 evolution advantages induced by spatially located cocatalyst and gives insights into the activity enhancement mechanism for tip-decorated CdS NRs, which guides the construction of more efficient CdS-based photocatalysts.