Solar Hydrogen Generation by a CdS-Au-TiO₂ Sandwich Nanorod Array Enhanced with Au Nanoparticle as Electron Relay and Plasmonic Photosensitizer

TLDR

This paper presents a sandwich‑structured CdS‑Au‑TiO₂ nanorod array as the photoanode in a photoelectrochemical cell for hydrogen generation via water splitting. The gold nanoparticles sandwiched between TiO₂ and CdS quantum dots act as an electron relay and plasmonic photosensitizer, enabling charge transfer for wavelengths 525–725 nm and allowing inversion analysis of transient absorption to reveal trap‑state Auger recombination as a dominant factor. The dual role of Au yields a photocurrent of 4.07 mA cm⁻² at 0 V (vs Ag|AgCl) under full solar spectrum and a 2.8 % solar‑to‑chemical efficiency, while increasing charge‑transfer lifetime, reducing trap‑state Auger rate, suppressing back transfer, and demonstrating that plasmonic hot‑carrier transfer and Schottky barrier height modulate interfacial dynamics, highlighting the importance of defect states in QD‑sensitized solar cells.

Abstract

This paper presents a sandwich-structured CdS-Au-TiO2 nanorod array as the photoanode in a photoelectrochemical cell (PEC) for hydrogen generation via splitting water. The gold nanoparticles sandwiched between the TiO2 nanorod and the CdS quantum dot (QD) layer play a dual role in enhancing the solar-to-chemical energy conversion efficiency. First, the Au nanoparticles serve as an electron relay, which facilitates the charge transfer between CdS and TiO2 when the CdS QDs are photoexcited by wavelengths shorter than 525 nm. Second, the Au nanoparticles act as a plasmonic photosensitizer, which enables the solar-to-hydrogen conversion at wavelengths longer than the band edge of CdS, extending the photoconversion wavelength from 525 to 725 nm. The dual role of Au leads to a photocurrent of 4.07 mA/cm2 at 0 V (vs Ag|AgCl) under full solar spectrum irradiation and a maximum solar-to-chemical energy conversion efficiency of 2.8%. An inversion analysis is applied to the transient absorption spectroscopy data, tracking the transfer of electrons and holes in the heterostructure, relating the relaxation dynamics to the underlying coupled rate equation and revealing that trap-state Auger recombination is a dominant factor in interfacial charge transfer. It is found that addition of Au nanoparticles increases the charge-transfer lifetime, reduces the trap-state Auger rate, suppresses the long-time scale back transfer, and partially compensates the negative effects of the surface trap states. Finally, the plasmonic energy-transfer mechanism is identified as direct transfer of the plasmonic hot carriers, and the interfacial Schottky barrier height is shown to modulate the plasmonic hot electron transfer and back transfer. Transient absorption characterization of the charge transfer shows defect states cannot be ignored when designing QD-sensitized solar cells. This facile sandwich structure combines both the electrical and the optical functions of Au nanoparticles into a single structure, which has implications for the design of efficient solar-energy-harvesting devices.