Ascorbic acid-driven formation of defect-rich Cu₂O Nanocubes toward durable and versatile Raman detection

Preparing semiconductor-based surface-enhanced Raman scattering (SERS) substrates remains challenging, as it requires simultaneous control over morphology, defect chemistry and stability. Here, l-ascorbic acid (AA) served a dual function as both a shape-directing and mild reducing agent in the hydrothermal synthesis of Cu-based nanostructures (denoted as Cu₂O_AAn, where n represents the volume of AA added). The addition of AA initially induces a phase transition from bulk-type CuO to Cu₂O nanocubes. Subsequent increases in AA further facilitate morphological evolution, leading to smaller and well-faceted Cu₂O structures. The optimized Cu₂O_AA30 nanocubes exhibited abundant copper vacancies, as confirmed by EPR analysis, which introduce charge-carrying holes. Zeta potential analysis further revealed that SERS enhancement could not be explained by simple electrostatic adsorption, as even negatively charged probes such as methyl orange displayed strong amplification. Instead, the dominant enhancement pathway was attributed to defect-mediated charge transfer, consistent with the facet-dependent electronic structure of Cu₂O. As a result, Cu₂O_AA30 substrates consistently delivered strong SERS responses for both cationic and anionic analytes and maintained stable performance for more than two months under ambient conditions, highlighting the long-term stability and practical applicability.