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

Abstract

Abstract 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.