New insights into the charge transfer-enhanced SERS effects in ACo2O4 (A = Ni, Zn) bimetallic oxides: Crystal facet engineering

The development of semiconductor-based surface-enhanced Raman scattering (SERS) substrates is often hindered by their limited sensitivity. Crystal facet engineering has been recognized as an effective strategy to improve the SERS performance of semiconductors. However, the underlying mechanism by which different exposed facets influence SERS activity in bimetallic oxide systems remains unexplored. In this study, we successfully synthesized NiCo 2 O 4 and ZnCo 2 O 4 bimetallic oxides with distinct exposed facets by regulating the alkali source. When evaluated using 4-Nitrothiophenol (4-NBT) as a probe molecule, the {110} facet demonstrated the highest SERS activity in both oxide systems. The enhancement factors (EF) reached 5.56 × 10 4 for NiCo 2 O 4 and 7.41 × 10 4 for ZnCo 2 O 4 . The limit of detection (LOD) was as low as 10 −7 M and 10 −8 M, respectively, as well as the excellent repeatability and stability. Energy level structure reveals that the superior SERS properties on the {110} facet can attributed to their higher VB maxima, which enhances the thermodynamic driving force and kinetic efficiency of the interfacial charge transfer (CT) process. Moreover, first-principles calculations further illustrate that the CT efficiency on the {110} facet can be effectively improved. This work provides a clear new direction for designing high-performance semiconductor SERS substrates through precise facet control.