Postsynthetic Zn(II) Species Modification Regulates the Surface States of TiO2 Nanoparticles for Resonance-Assisted SERS

Abstract

Engineering interfacial electronic states at the nanoscale is a powerful yet underdeveloped strategy for improving the selectivity of semiconductor-based molecular sensing. Here, we report a simple postsynthetic surface-associated Zn(II) species modification approach to engineer the surface states of TiO2 nanoparticles for resonance-assisted surface-enhanced Raman scattering (SERS). Structural and spectroscopic analyses indicate that Zn(II)-containing species are mainly associated with the TiO2 surface rather than incorporated into the lattice, allowing the nanoparticle framework and morphology to be preserved while modulating the near-surface electronic structure. This nanoscale surface-state engineering enriches defect-related states and narrows the effective band gap of TiO2, leading to improved energy-level matching with methylene blue (MB) under 532 nm excitation. As a result, the Zn-TiO2 substrate exhibits an enhanced Raman response relative to bare TiO2, with a maximum enhancement factor of 1.22 × 104 and acceptable reproducibility. More importantly, the selective amplification of specific MB vibrational bands indicates that the dominant contribution arises from a surface-state-mediated resonance-assisted charge-transfer process. Rather than relying on bulk doping, oxygen-vacancy engineering, or complex heterostructures, this work establishes surface metal-species association as a mild and structurally preserving route for tailoring semiconductor interfacial states. These findings provide a nanoscale design principle for selective molecular sensing and broaden the use of TiO2-based nanomaterials in interfacial spectroscopy and related optoelectronic applications.