Surface plasmon resonance (SPR)-enhanced photocatalytic degradation by periodically ordered δ-MnO2 arrays with alkali metals doping

Periodically ordered δ-MnO₂ arrays of different periods have been fabricated by using polystyrene (PS) nanosphere lithography (NSL) combined with following hydrothermal growth. By choosing different precursors, δ-MnO₂ arrays with different alkali metals doping were obtained. It was observed that the K-doped δ-MnO₂ array of 500 nm period exhibited superior photocatalytic degradation performance of methylene blue (MB) under visible light irradiation relative to those of Na- or Mg-doped δ-MnO₂ arrays of 500 nm period, as well as K-doped δ-MnO₂ array of 800 nm period. The best performance of K-doped δ-MnO₂ array of 500 nm period can be firstly attributed to enhanced surface plasmon resonance (SPR) effect caused by alkali metal doping, where K-doped MnO₂ could provide more free electrons than those of Na- or Mg- doped MnO₂. Furthermore, for the nanomaterials with SPR effect the surface plasmon polaritons (SPP) can change the propagation path of incident light to vertical direction after scattering on them. Since the MnO₂ array of 500 nm period possessed the distance between neighbor unites smaller than the wavelength of irradiation light, and thus the scattered light interfered with each other after the incident light was scattered by the MnO₂ unites. Then the electric field (E-field) intensities generated by SPR effect near each MnO₂ unit were elevated due to the interference effect. Therefore, our finding may provide a peculiar way to improve SPR-mediated photocatalytic performance of non-metallic materials by combination of doping and arrays fabrication of particular periods.