Degradation of organic dyes by Si/SiOx core-shell nanowires: Spontaneous generation of superoxides without light irradiation.

Recently, silicon nanowires (SiNWs) have been proven to be highly active in the photocatalysis of dye degradation. However, the unstable hydrogen-terminated surface and the need for constant light irradiation hinder their extensive use. In this work, a stable silica shell was intentionally formed on the surface of SiNWs to produce Si/SiOx core-shell silicon nanowires (S-SiNWs). Light-illuminated or not, S-SiNWs showed almost identical degradation ability for the degradation of indigo carmine (IC) in both conditions, which meant neither hydrogen termination nor light irradiation was a prerequisite for the degradation activity of S-SiNWs. UV/Vis spectroscopy and liquid chromatography/mass spectrometry showed that IC was converted into isatin sulfonic acid in this process. Quenching studies and electron paramagnetic resonance spectroscopy revealed that this bleaching ability was highly dependent on superoxides. A possible mechanism was accordingly suggested. In addition, the recently discovered reductase-like activity of SiNWs can be explained by the superoxides generation.
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