Electronic Structure and Photocatalytic Mechanism of Graphitic Carbon Nitride Modified with Plasmonic Ag@SiO₂ Core-shell Nanoparticles by X-ray Absorption Spectroscopy
Yu-Cheng Huang1*, Jeng-Lung Chen2, Jie Chen3, Shaohua Shen3, Ying-Ru Lu2, Wu-Ching Chou1, Chung-Li Dong4
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3International Research Center for Renewable Energy State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, China
4Department of Physics, Tamkang University, New Taipei City, Taiwan
* Presenter:Yu-Cheng Huang, email:ychuang0129.04g@g2.nctu.edu.tw
Graphitic carbon nitride (g-C₃N₄) modified with plasmonic Ag@SiO₂ core-shell nanoparticles has attracted considerable interest as a means to enhance photocatalytic solar hydrogen evolution under visible light. High-rate charge carrier recombination is a key factor limiting the photocatalytic activity of g-C₃N₄. In this study, the SiO₂ shell generated a nanogap separating the plasmonic silver nanoparticles and g-C₃N₄. The plasmon resonance energy transfer (PRET) and energy-loss Förster resonance energy transfer (FRET) induced by the localized surface plasmon resonance (LSPR) in the silver nanoparticles could be perfectly balanced by engineering the size of the nanogap. The LSPR of the Ag nanoparticles could enhance the visible-light photoactivity of graphitic carbon nitride. Nanosized gaps between the plasmonic Ag nanoparticles and g-C₃N₄ were created and precisely modulated to be 8, 12, 17, and 21 nm by coating SiO₂ shells on the surface of Ag nanoparticles. For this study, the PRET effect and the FRET effect were well balanced with the photocatalytic solar hydrogen evolution performance achieved at a nanogap of 12 nm. In situ X-ray absorption spectroscopy (XAS) was employed to investigate the electronic structure of these photocatalysts. The C and N K-edges were conducted to reveal both the density of unoccupied states in the conduction band and how these states changing at different illumination conditions. In situ XAS directly probe the dynamic charge redistribution indicated that the shift of the conduction band edge as well as the modification of the density of the unoccupied states engendered the improved photocatalytic activity. The SiO₂ shell between the Ag nanoparticles and g-C₃N₄ limit the energy loss of the FRET process by limiting the photocatalytic activity of g-C₃N₄/Ag@SiO₂ to g-C₃N₄/Ag. These results reveal a strong correlation between the dynamics of the semiconductor structure and its electronic properties, which explains the LSPR effect in the photocatalytic mechanism.

Keywords: Graphitic carbon nitride(g-C₃N₄), Localized surface plasmon resonance, X-ray absorption spectroscopy, Photocatalytic, Solar Hydrogen Conversion