Magnetite/Bi-Doped Carboxylate-Rich Carbon Spheres-A Highly Efficient Magnetic Photocatalyst Based on Dimetallic FeII/FeIII and BiIII/BiIV Photoredox Cycles

A new magnetically separable visible-light photocatalyst, magnetite/Bi-doped carboxylate-rich carbon spheres (Bi-MCRCSs), was synthesized under ultrasonic irradiation by using magnetite/carboxylate-rich carbon spheres (MCRCSs) as a precursor. The Bi-MCRCSs showed much better photocatalytic activity than MCRCSs in the degradation of methylene blue (MB) under visible-light irradiation (>420 nm). Compared with MCRCSs, the Bi-MCRCSs show more intensive photoabsorption in the whole UV and visible region. In particular, Bi-MCRCSs display a broad absorption band centered at 550 nm, which give Bi-MCRCSs advantages over MCRCSs in the utilization of visible light for the degradation of organic pollutants. Owing to the Bi3+ doping, the Bi-III/Bi-IV redox cycle can be established in the Fe-II/Fe-III redox cycle system, which result in the establishment of dimetallic Fe-II/Fe-III and Bi-III/Bi-IV photoredox cycles for Bi-MCRCSs. This dimetallic Fe-II/Fe-III and Bi-III/Bi-IV photoredox cycle possesses a significantly enhanced photocatalytic degradation rate compared to that of the monometallic photocycles of Fe-II/Fe-III for MCRCS. The synergistic effects between the Fe-II/Fe-III cycle and the Bi-III/Bi-IV cycle promote the regeneration of Fe-II ions in Fe-II/Fe-III cycle and, hence, accelerate the degradation of organic pollutants. In addition, the magnetic saturation (M-s) value is about 20 emu/g. After completion of the reaction, the Bi-MCRCSs could be rapidly separated under an applied magnetic field.