The charge transfer effect on SERS in a gold-decorated surface defect anatase nanosheet/methylene blue (MB) system

Abstract

Revealing the substrate–molecule vibronic coupling enhancement, especially the photoinduced charge transfer (CT) process, is hugely important for understanding the surface-enhanced Raman spectroscopy (SERS) enhancement mechanism. In this work, oxygen atoms were extracted from anatase phase TiO₂ nanosheets to form oxygen vacancies, and then Au nanoparticles were deposited on defect TiO₂ nanosheets. Through this approach, the Raman scattering intensity of methylene blue (MB) had a remarkable enhancement with an enhancement factor (EF) of 1.7 × 10⁸ at the 1621 cm⁻¹ peak. The defect level located below the conduction band (CB) of anatase calculated by means of density functional theory (DFT) and the Fermi level of gold both generated new photoinduced charge transfer channels from metal through molecule to semiconductor when the incident laser energy was higher than the energy gap barriers. The CT contribution to the SERS enhancement using a parameter ρ_CT was calculated quantitatively, which showed that the CT process played a more important role in our system compared with the electromagnetic mechanism (EM). The charge transfer degrees of TiO₂, defective TiO₂ with oxygen vacancies and defective TiO₂ deposited with gold nanoparticles decrease with increasing Raman enhancement (10⁻⁴ M MB), which confirm that the increase in the transition routes of the CT process was also responsible for the Raman scattering enhancement. The metal oxygen semiconductor intelligently adjusted by artificial control of the surface offer a simple and effective way to gain high EFs, which can expend the application of semiconductor SERS substrates.