Heterophase‐Engineered Boron Oxide–Carbon Interfaces Enable Ultrasensitive Metal‐Free SERS via Interfacial Charge Transfer

ABSTRACT Elucidating structure–electronic property relationships in metal‐free surface‐enhanced Raman scattering (SERS) substrates remains challenging. Here, temperature‐mediated heterophase engineering is used to tailor boron‐modified biochar for efficient Raman enhancement. Among samples carbonized at 400°C–900°C, B‐BC400 exhibits the strongest SERS response, enabling statistically validated methylene blue (MB) detection down to 10 −8 M. After spectral deconvolution, the MB component near 1625 cm −1 remains 3.98 fold above the 3σlocal‐background threshold with a signal‐to‐noise ratio of 11.93, corresponding to a concentration‐normalized analytical enhancement factor of 2.51 × 10 7 . Despite its relatively low surface area, B‐BC400 outperforms the higher‐temperature materials. HRTEM and STEM‐EDS reveal intimate nanoscale association between boron oxide rich domains and disordered carbon, while selective heterophase leaching markedly suppresses the SERS response. UPS further reveals modification of the substrate electronic structure and adsorption‐induced electronic redistribution following MB interaction. Together with XPS, photoluminescence spectroscopy, and density functional theory calculations, these observations support heterointerface‐mediated chemical enhancement involving charge‐transfer interactions. These findings demonstrate heterophase engineering of biomass‐derived carbon as an effective strategy for developing sensitive metal free SERS substrates.