Template-annealing-assisted fabrication of Au/TiO2/Ni nanopetal arrays as ultrasensitive and reproducible SERS substrates with super long-term stability

Abstract

Although (Au or Ag)/TiO₂ hybrid structures have been widely used for surface-enhanced Raman scattering (SERS) detection, nanostructured TiO₂ still faces significant drawbacks of low response to visible light and the resulting poor Raman response. Template-annealing-assisted fabrication of three-dimensional (3D) petal-like Au/TiO₂/Ni nanoarrays provides a way to realize ultrasensitive and reproducible SERS responses with super long-term stability for TiO₂ in the visible-light region. Thermal annealing treatment of TiO₂/Ni nanopillar arrays in an air atmosphere at high temperatures (particularly at 650 °C) brought about partial oxidation of Ni to NiO, transformation of amorphous TiO₂ (a-TiO₂) to non-stoichiometric TiO_2−x, and a 3D petal-like morphology with a large specific surface area, which effectively increased visible-light absorption. Dense Au nanoparticles, ordered Ni nanoarrays, and the closely packed nanopetal-like morphology with numerous Raman active sites, as well as each component, including Au, Ni, TiO_2−x, NiO, and a-TiO₂, and their synergies, not only enabled the remarkable enhancement of surface plasmon resonance, but also strongly facilitated the photoinduced charge-transfer effect between the substrate and adsorbed molecules. 3D Au/TiO₂/Ni nanopetal arrays exhibited an ultrasensitive and reproducible SERS response to rhodamine 6G (as low as 10⁻¹² M) and crystal violet molecules (as low as 10⁻¹³ M) with super long-term stability (372 days) at 632.8-nm excitation. A lower concentration detection limit could be realized with precise regulation of the Au deposition thickness. This work presents a cost-effective and efficient alternative for optimizing the SERS performance of wide-bandgap oxides.