Visible light-induced charge transfer to improve sensitive surface-enhanced Raman scattering of ZnO/Ag nanorod arrays

Abstract

Noble-metal/semiconductor heterostructures that show extended multiple functionalities have attracted ever-increasing attention for using as sensitive surface-enhanced Raman scattering (SERS) substrates. Thus the studies of enhanced mechanism for SERS activity are favorable to fabricate other noble-metal/semiconductor heterostructure substrates and exploit their applications. In this study, we take an insight into the enhanced mechanism for SERS activity of Ag nanoparticle-decorated ZnO nanorod arrays (ZnO/Ag NAs) through studying the Raman vibrational signals of rhodamine 6G (R6G) molecules and photoresponse performance under visible light irradiation. Visible light photoresponse performance shows that the “hot electrons” of plasmonic Ag nanoparticles (NPs) are excited to the conduction band (CB) of ZnO nanorod arrays (ZnO NAs) due to the surface plasmon resonance (SPR). Used as a surface-enhanced Raman scattering (SERS) substrate to test Raman signals of rhodamine 6G (R6G) molecules, the prepared ZnO/Ag NAs show superior SERS performance compared to pure Ag NPs substrate. The visible light photoresponse activities and Raman performances experimentally show that the superior SERS performance of ZnO/Ag NAs is attributed to an effective charge transport within the plasmonic Ag, semiconductor ZnO and R6G molecule. In the charge transport process, the “hot electrons” of Ag NPs are first excited by visible light to the CB of semiconductor ZnO, and then to the lowest unoccupied molecular orbital (LUMO) level of R6G molecule. This work demonstrates that the SERS activity of semiconductor-based hybrid Raman substrate can be significantly improved by effectively turning the charge transport within the hybrid substrate.