Abstract
The techniques we describe herein are straightforward yet incredibly efficient in providing a favorable environment for two-dimensional (2D) zinc oxide nanoflakes (ZnONFs) to greatly increase the intensity of surface-enhanced Raman scattering (SERS) and produce significant photoinduced-enhanced Raman scattering (PIERS). Rhodamine 6G (R6G) was used as a Raman probe, and silver nanoparticles (AgNPs) as the SERS mediator. Two types of heterostructure composites, ZnONFs-R6G-AgNPs and ZnONFs-AgNPs-R6G, were fabricated onto four different base substrates (aluminum, copper, , stainless steel, and silicon-wafer) by a simple sequential drop deposition technique. Intriguingly, ZnONFs-R6G-AgNPs could yield a ∼3.2-fold larger SERS enhancement factor (EF) than ZnONFs-AgNPs-R6G, which rendered the ZnONFs-R6G-AgNPs heterostructure a good choice for comprehensive investigations. For inducing PIERS, ZnONFs-R6G-AgNPs stacks were photoirradiated with a 365 nm UV light having energy just above the bandgap of ZnO (band edge ∼400 nm). Remarkably, ZnONFs-R6G-AgNPs could produce gigantic SERS and PIERS intensities of R6G compared to AgNPs alone, yielding 8.2- and 12.0-fold larger magnitudes of the EF, respectively. The observed enormous magnitudes of SERS and PIERS intensities are attributed to the synergistic interplay of ZnONFs evolved with AgNPs. Of the four base substrates, aluminum was found to be the best for observing a remarkable synergy of ZnONFs on SERS, while silicon wafer was the best for PIERS, presumably due to the contributions of additional charge carriers photogenerated by the 785 nm (≈1.6 eV) laser excitation. Thanks to the synergy that ZnONFs conferred, we were able to discriminate the Raman signal of R6G and aldicarb (AlC) pesticide at concentrations as low as 10−14 M on both aluminum and silicon-wafer substrates. Furthermore, our observation demonstrates that the particular heterostructure composite offers the unique opportunity to study the two seemingly opposing functions of ZnONFs (synergy on SERS/PIERS intensity enhancement and triggering the photocatalytic activity) through a single experimental platform.