Facile synthesis of a ZnO nanoplate/Ag nanoparticle hybrid as a highly sensitive SERS substrate for indigo carmine detection

This work presents the utilization of a hydrothermal treatment and a reduction reaction to synthesize a heterogeneous ZnO nanoplate (NPl)/Ag nanoparticle (NP) nanostructure for application in surface-enhanced Raman scattering (SERS). Under hydrothermal conditions, at 180 °C and 20 h, ZnO NPls with a thickness of 40 nm and edgewise size of 200 nm × 350 nm were prepared from precursors containing zinc acetate (CH₃COO)₂Zn and sodium hydroxide (NaOH). Then, Ag NPs with an average diameter of 17 nm were deposited onto the surface of the ZnO NPls by reducing AgNO₃ using trisodium citrate (TSC). The structural, morphological, and compositional behaviors of the prepared heterostructure were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray spectroscopy (EDS). The optical properties of the as-prepared products were analyzed using Raman, ultraviolet-visible (UV-Vis) absorption and Fourier transform infrared (FTIR) spectroscopies and photoluminescence (PL) technique. Results confirmed the formation of a ZnO NPl/Ag NP heterostructure, with the Ag NPs adhering to the surface of the 2D semiconducting ZnO NPls. The SERS signal from the chemisorbed indigo carmine (IC) molecules on the ZnO/Ag surface was observed at various concentrations between 5 × 10⁻⁹ M and 10⁻⁴ M. The produced SERS substrate demonstrated superior SERS performance in detecting IC, with a low limit of detection (LOD) of 5 × 10⁻⁹ M, a high enhancement factor (EF) of 1.57 × 10⁵, and good uniformity with a relative standard deviation (RSD) of 3.6%. Raman scattering signals from IC adsorbed on this ZnO/Ag heterostructure showed a significant enhancement compared with those of the same molecules adsorbed on a glass substrate. The surface-enhanced Raman scattering of ZnO/Ag was owing to the hotspots at the Ag NPs and effective charge transport among plasmonic Ag NPs, semiconducting ZnO NPls, and the IC molecules. The most captivating aspect of this study is that the molecular structure of IC was compared using computational and experimental methods; in particular, density functional theory (DFT) calculations using the B97 (d,p) basis set were performed to obtain the optimized geometric structure and frontier molecular orbital of IC molecules. This study provides definitive experimental validation underpinning the phenomenon of SERS on metal oxide semiconductor/noble metal hybrids, which can effectively enhance Raman signals owing to the synergistic action of the electromagnetic (EM) and chemical (CM) mechanisms.