By virtue of their high uniformity and stability, metal oxide-based surface-enhanced Raman spectroscopy (SERS) substrates have attracted enormous attention for molecular trace detection. However, strategies for further enhancing the SERS sensitivity are still desired. Herein, MoOx/WOx nano-heterojunctions are constructed by mixing MoOx and WOx together (MoOx/WOx hybrid) with diverse weight ratios. Using a 532 nm laser as the excitation source and R6G as the Raman reporter, it is shown that the Raman signal intensity (for the peak @ 1360 cm-1) obtained on the optimal MoOx/WOx hybrid (MoOx/WOx = 1:1/3) is twice that observed on a pure MoOx or WOx substrate. Moreover, a limit of detection of 10-8 M and an enhancement factor of 108 are achieved. In the SERS enhancement mechanism investigation, it is revealed that MoOx and WOx form a staggered band structure. During the SERS measurement, electron-hole pairs are generated in the nano-heterojunction using the incident laser. They are then separated by the built-in potential with the electrons moving toward WOx. The accumulated electrons on WOx are further transferred to the R6G molecules through the coupling of orbitals. Consequently, the molecular polarizability is amplified, and SERS performance is enhanced. The abovementioned explanation is supported by the evidence that the contribution of the chemical enhancement mechanism in the optimal MoOx/WOx hybrid substrate is about 2.5 times or 5.9 times that in the pure WOx or MoOx substrate.
Keywords: SERS; charge transfer; molybdenum oxides; nano-heterojunction; tungsten oxides.