Interfacial potassium induced enhanced Raman spectroscopy for single-crystal TiO₂ nanowhisker

Structural control and element doping are two popular strategies to produce semiconductors with surface enhanced Raman spectroscopy (SERS) properties. For TiO₂ based SERS substrates, maintaining a good crystallinity is critical to achieve excellent Raman scattering. At elevated temperatures (> 600 °C), the phase transition from anatase to rutile TiO₂ could result in a poor SERS performance. In this work, we report the successful synthesis of TiO₂ nanowhiskers with excellent SERS properties. The enhancement factor, an index of SERS performance, is 4.96 × 10⁶ for methylene blue molecule detecting, with a detection sensitivity around 10⁻⁷ mol·L⁻¹. Characterizations, such as XRD, Raman, TEM, UV–vis and Zeta potential measurement, have been performed to decrypt structural and chemical characteristics of the newly synthesized TiO₂ nanowhiskers. The photo absorption onset of MB adsorbed TiO₂ nanowhiskers was similar to that of bare TiO₂ nanowhiskers. In addition, no new band was observed from the UV–vis of MB modified TiO₂ nanowhiskers. Both results suggest that the high enhancement factor cannot be explained by the charge-transfer mechanism. With the support of ab initio density functional theory calculations, we reveal that interfacial potassium is critical to maintain thermal stability of the anatase phase up to 900 °C. In addition, the deposition of potassium results in a negatively charged TiO₂ nanowhisker surface, which favors specific adsorption of methylene blue molecules and significantly improves SERS performance via the electrostatic adsorption effect.