Dual-plasmonic Ag/MoO_3−x nanoflowers for ultrasensitive SERS sensing of aldehyde VOCs gas

The electromagnetic field coupling between two kinds of noble metal nanoparticles endows high surface-enhanced Raman scattering (SERS) activity but is accompanied by uneven hot spots. Using two-dimensional semiconductors with localized surface plasmon resonance (LSPR) effects instead of one of the noble metal components can effectively improve uniformity. Hence, the Ag nanoparticles (Ag NPs) loaded MoO_3−x nanoflowers (Ag/MoO_3−x) were engineered to exploit dual-plasmonic coupling and realize the trace detection of aldehyde volatile organic compounds (VOCs) gas. The finite-difference time-domain (FDTD) simulation results proved that there is an obvious electromagnetic field coupling effect between Ag NPs and MoO_3−x semiconductors, which can amplify the molecular dipole moment significantly. The chemical enhancement mechanism in the Ag/MoO_3−x substrate was clarified by band structure analysis, in which the free electrons accumulated at the bottom of the conduction band of the MoO_3−x semiconductor can promote the charge transfer process between Ag/MoO_3−x and the 4-aminothiophenol (4-ATP) molecule. Moreover, the electron delocalization of 4-ATP molecule was enhanced after being absorbed on Ag/MoO_3−x nanoflowers, facilitating the charge transfer between 4-ATP and Ag/MoO_3−x substrate effectively. Importantly, using the 4-ATP molecule as a probe, the trace detection of a variety of aldehyde VOCs gas was realized by Ag/MoO_3−x substrate with a low limit of detection (LOD) of 10 ppb. This work provided a new idea for the design of noble metal-plasmonic semiconductor heterostructure substrates.