利用同步輻射X光吸收光譜對銀電漿子/二氧化矽核-殼結構奈米顆粒修飾石墨氮化碳電子結構及光催化機制之研究

Abstract

近來由於綠色能源的議題受到高度重視，氫燃料電池與光觸媒材料的研究與應用被視為相當重要的課題。過去數十年間研究人員致力於研究高性能之水分解觸媒材料，分別應用於催化產氫反應，其利用光能驅動以化學燃料的形式儲存，滿足乾淨能源的需求。過去研究發現，二氧化矽包覆銀修飾石墨氮化碳，用以作為增強可見光下的光催化太陽能產氫，有良好的光催化效果，並且發現銀金屬奈米結構有特別的侷域表面電漿共振現象，可提供石墨氮化碳在可見光照射下的光催化反應。在本研究中，二氧化矽殼層分離銀電漿子奈米顆粒與石墨氮化碳。透過調整奈米殼層的尺寸大小，銀奈米顆粒中的局域表面電漿共振，誘導的電漿共振能量轉移，和能量損失的螢光共振能量轉移來達平衡，造成電子與原子結構變化與其光催化反應機制。在銀奈米顆粒上包覆二氧化矽殼層，產生銀奈米顆粒和石墨氮化碳之間的奈米間隙，並將其精確調製為8,12,17和21奈米。本研究中電漿共振能量轉移效應與螢光共振能量轉移，在12奈米殼層下呈現的光催化太陽能氫性能平衡良好。本研究利用X光吸收光譜及更進一步的原位X光吸收光譜分析，建構出過去對此類觸媒材料研究中無法得知的電子結構與其在可見光下照射下之光催化反應的微觀機制。可以直接說明電荷重新分佈的情形、導電帶近邊緣的移位以及未佔據狀態的密度的改變產生了改善的光催化活性。我們發現銀奈米顆粒和石墨氮化碳之間的二氧化矽殼層，必需通過限制石墨氮化碳/銀二氧化矽，對石墨氮化碳/銀的光催化活性，來限制螢光共振能量轉移的能量損失。

Graphitic-like 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.