Nanostructured Co₃O₄ Semiconductors for Plasmon-Free SERS Sensing Platforms

Nanostructured semiconductors have garnered interest as nonplasmonic surface-enhanced Raman spectroscopy (SERS) platforms, as they promise to mitigate the drawbacks of metallic substrates. In this work, we report the synthesis and characterization of Co3O4-based nanoflowers and nanosheets and demonstrate the role of each morphology in intensifying the Raman signal of rhodamine 6G (R6G). We carried out a systematic experimental procedure aimed at optimizing the SERS process, which involved relevant (and commonly largely neglected) aspects such as solution mixing and surface concentration of analytes. We show that 2D-Co3O4 detected R6G down to 10–8 mol L–1 and exhibited superior performance compared to its 3D counterpart, possibly due to the greater availability of SERS-active regions (e.g., vacancies, oxygenated sites, etc.) in this conformation. Around 600 spectra were parametrized and analyzed with the aim of evaluating the incidence and reproducibility of the SERS process across the substrate. Finally, we propose a mechanistic path for the Raman enhancement process that identifies charge transfer as the main phenomenon behind the observed SERS.