Enhance Raman scattering for probe methylene blue molecules adsorbed on ZnO microstructures due to charge transfer processes

It is generally understood that the chemical enhancement process is essential in surface-enhanced Raman scattering (SERS) of semiconductor-based substrates. Understanding the chemical enhancement mechanism is one of the most significant issues in improving the SERS activity of semiconductor substrates and designed hybrid substrates. Nevertheless, understanding the chemical enhancement process, on the other hand, remains controversial and elusive. In this study, ZnO semiconductor microstructures are prepared to investigate SERS activities in probing methylene blue (MB). The SERS mechanism is well elucidated based on the chemical enhancement mechanism related to both the formation of charge-transfer band and charge transfer from the second ionization of the oxygen vacancy defect (Vo••) to lowest unoccupied molecular orbital (LUMO) of MB. Remarkably, the charge-transfer band was evidenced by examining the ground state absorption spectra with a large band centred at 600 nm. These explanations are consistent with the strong SERS substrate behaviour of ZnO microstructures, with the SERS enhancement factor (EF) of ZnO plate substrates being highest among four investigated samples (ZnO tube, hexagonal, plate and flory-rod) at 2.9 × 104 at a concentration of MB of 10−4 M. To the best of our knowledge, this is the first investigation of detecting MB molecules using a pure semiconductor surface with a strong SERS activity. [Display omitted] •We have successfully fabricated various morphologies of semiconductor ZnO microstructures by hydrothermal route, including tube, hexagonal, plate and flory-like structures.•These products were studied surface-enhanced Raman scattering (SERS) behaviour for probing methylene blue (MB) molecules with the enhancement factor (EF) achieving 2.9 × 104 (10−4 M).•The charge-transfer band corresponding to the interaction between the ZnO surface and the MB molecule was evidenced by examining the ground state absorption spectra with a large band centred at 600 nm.•The SERS mechanism is well elucidated based on the chemical enhancement mechanism related to both the formation of charge-transfer band and charge transfer.•The combination of the existence of charge-transfer band and charge-transfer resonance leads to the large enhancement factor of using pure ZnO as SERS surface.