Photocatalytic Degradations of Malachite Green on Magnetically Separable Ni1-xCoxFe2O4 Nanoparticles Synthesized by Using a Hydrothermal Process

Ni1-xCoxFe2O4 (x=0–1.0) ferrite nanoparticles were synthesized by using a hydrothermal process. The nanoparticles had average particle sizes of 14.8−29.4 nm and a narrow band gap of 1.75−1.91 eV, decreasing with increasing Co content. The nanoparticles showed an excellent sunlight-excited photocatalytic activity to malachite green in water. Moreover, the addition of a small amount of H2O2 resulted in an obviously enhanced photodegradation, indicating a Fenton-like reaction. The photodegradation and Fenton-like photodegradation rates increased with increasing Co content. Complete Co substitution for Ni resulted in an optimal effect. Quasi-kinetic rate constant k values were in the range of 2.23–7.2×10-2 min-1 and increased to 1.3–6.0×10-1 min-1 upon the addition of H2O2. The measurement of oxidation-reduction potential of the nanoaprticles in the photodegradation condition indicated a reductive characteristic enhanced with increasing Co content. Furthermore, the nanoparticles possessed high saturation magnetizations of 50.7−64.2 emu·g−1 and low coercivity of 0.4−1.7 kOe, which ensured that they gave magnetic separable in finally degraded dye aqueous solution.