Photocatalytic activity of quantum dot–magnetite nanocomposites to degrade organic dyes in the aqueous phase

Abstract

Quantum dots of Cd(Se,S) and fluorescent magnetic nanocomposites (Cd(Se,S)–magnetite) were used as photocatalytic agents in the photodegradation of methylene blue (MB) under UV irradiation at pH 6.5. Quantum dots and magnetic nanocomposites were characterized by X-ray diffraction (XRD), UV-Vis, photoluminescence and Fourier Transform Infrared (FT-IR) spectroscopy. The photo-induced degradation of MB was monitored using High Performance Liquid Chromatography (HPLC) at 660 nm and titanium dioxide (anatase and aeroxide P25 forms) was used as the photocatalyst standard. A degradation of 99.1% and 90.0% of MB was achieved in the presence of 160 mg L⁻¹-quantum dots and the magnetic nanocomposite, respectively, after 4.5 hours of UV-irradiation. Instead, 45.9% and 100% of MB degradation was achieved using 160 mg L⁻¹ of TiO₂ anatase and aeroxide P25, respectively. The degradation products were studied by mass spectrometry (MS) and the results evidenced the formation of azure B, A, C and phenothiazine. The reuse of the magnetic nanocomposites (i.e., after one photo-degradation cycle) allowed a maximum photo-degradation capacity of 65%. The results suggested that the nanocomposite has 10% less photodegradation capacity than the widely used catalytic agents such as TiO₂ aeroxide P25.