MoS2 Nanosheets as Substrates for SERS-Based Sensing

The chemical enhancement of semiconductor-based surface-enhanced Raman scattering (SERS) substrates is an exciting hot topic. Herein, a simple hydrothermal method is developed to prepare molybdenum disulfide (MoS2), which can be easily exfoliated into monolayer nanosheets (MoS2NSs) by sonication, even in the absence of any surfactant. The obtained MoS2NSs contain two types of defects, namely, one caused by the incorporation of Mo atoms of high valence states and one caused by the incorporation of S22–. The density of the two types of defects can be easily tuned by controlling the ratio of Na2S and Na2MoO4 in the raw materials. The unique properties and the clear surface make the obtained MoS2NSs ideal models to investigate the effect of defects on the SERS activity of MoS2. It is found that the SERS activity of the obtained MoS2NSs increases dramatically with the defects caused by Mo atoms of high valence states, while it first increases and then decreases with the increase of defects caused by S22–. On the basis, MoS2NSs with high SERS activity and a low detection limit of 5.0 × 10–9 mol/L toward crystal violet (CV) are obtained. Moreover, the mechanism of defects affecting the SERS activity of MoS2NSs is revealed. The defects on one hand provide a large amount of dangling bonds that can combine CV molecules to form MoS2NS–CV complex and on the other hand provide extensive induced local dipoles and enhance the overall SERS spectrum of CV.