Improved surface-enhanced Raman scattering (SERS) sensitivity to molybdenum oxide nanosheets via the lightning rod effect with application in detecting methylene blue

MoO2 nanomaterials show a superior surface-enhanced Raman scattering (SERS) property due to their high concentration of free electrons and low resistivity. However, the physical process of semiconductor-based SERS is still elusive because there are many factors that affect the local electromagnetic field intensity and the subsequent Raman intensity of the molecules in close proximity to the semiconductor nanomaterials. Herein, we investigate the important contribution of surface morphology to molybdenum oxide SERS. The MoO3/MoO2 nanosheets (NSs) are synthesized by oxidizing MoO2 NS, and the surface roughness of MoO3 can be controlled through adjusting the oxidization time. Compared with the MoO2 NS before oxidization, the MoO3/MoO2 NSs exhibit a much stronger SERS signal, which favors their application as a SERS substrate to detect trace amounts of methylene blue molecules. The minimum detectable concentration is up to 10-9 M and the maximum enhancement factor is about 1.4 × 105. Meanwhile, excellent signal reproducibility is also observed using the MoO3/MoO2 NSs as the SERS substrate. A simulated electric field distribution shows that a stronger electric field enhancement is formed due to the lightning rod effect in the gap of corrugated MoO3 NSs. These results demonstrate that the surface topography of molybdenum oxide may play a more important role than their oxidation state in SERS signal enhancement.