Molybdenum Oxide/Tungsten Oxide Nano-heterojunction with Improved Surface-Enhanced Raman Scattering Performance

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, MoO/WO nano-heterojunctions are constructed by mixing MoO and WO together (MoO/WO 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 MoO/WO hybrid (MoO/WO = 1:1/3) is twice that observed on a pure MoO or WO 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 MoO and WO 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 WO. The accumulated electrons on WO 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 MoO/WO hybrid substrate is about 2.5 times or 5.9 times that in the pure WO or MoO substrate.