Fine fabrication of TiO2/MoOx nano-heterojunctions and investigating on the improved charge transfer for SERS application

Abstract

Metal-oxide surface-enhanced Raman spectroscopy (SERS) substrates have attracted enormous attentions for trace-level molecule detection, and efficient photo-induced charge transfer is crucial to further enhance the SERS sensitivity. Herein, TiO2@MoOx nanorods are finely prepared to construct the TiO2/MoOx nano-heterojunctions, and then they are implemented as an efficient charge transfer substrate for SERS application. The SERS performance of the substrate is evaluated by using a 532-nm laser as the excitation source and R6G molecules as the Raman reporter. Experimental results show that R6G's Raman signal intensity is gradually improved with the increase of the nano-heterojunction numbers on TiO2@MoOx nanorods. A limit of detection as low as 10−8 M (R6G@4.0 × 10−13 M/cm2) and Raman enhancement factor of 1.445 × 108 (R6G@10−8 M) can be achieved on the most optimal TiO2@MoOx nanorods. Moreover, the mechanism of the SERS enhancement is investigated by analyzing the TiO2 and MoOx band diagrams. It evidences that TiO2@MoOx forms staggered gap heterojunction. Under the laser irradiation, the photo-generate electron-hole pairs are separated in the depletion region and then free electrons move toward the MoOx surface. Thereafter, the electrons are further transferred to the molecules enhancing the SERS performance. Finally, to verify the aforementioned explanation, the surface potential of a single TiO2@MoOx nanorod is measured by Kelvin prober force microscope, it exhibits that the surface potential distribution shifts to a higher level under the light illumination, confirming the accumulation of free electrons on the MoOx surface, and then enhancing charge transfer between the molecules and heterojunctions.