Magnetite/Bi-doped carboxylate-rich carbon spheres - A highly efficient magnetic photocatalyst based on dimetallic Fe^II/Fe^III and Bi^III/Bi^IV 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 (λ > 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. A dimetallic photocatalytic system based on magnetite/Bi-doped carboxylate-rich carbon spheres (Bi-MCRCSs) shows prominent visible-light-photocatalytic properties. The synergistic effects between an Fe^II/Fe^III cycle and a Bi^III/Bi ^IV photocatalytic cycle promote the regeneration of Fe^II ions in the Fe^II/Fe^III cycle and, hence, accelerate the degradation of organic pollutants.