| Semiconductor photocatalysts, mainly TiO2, have attracted much attention in last decade because of their potential application in the removal of all kinds of organic and inorganic pollutants in air or water. In water and wastewater treatment field, a slurry type reactor is the most commonly applied method because of its high specific surface area and dispersion. However, the use of TiO2 slurry reactor is still limited mainly due to difficult separation of TiO2 particles from treated water. To overcome the problem, titania beads, TiO2 based thin film, fiberglass loaded with titania, and encapsulated titania within a zeolite framework have been prepared and used in fixed bed. However, the activity of TiO2 photocatalyst is reduced to a considerable extent in the application of these immobilizations because the effective surface area of photocatalysts decreases considerably. In addition, though titania shows excellent photocatalytic activity for removal of environmental pollutants under ultraviolet irradiation, it is a wide bandgap semiconductor (3.2 eV for anatase) and can only absorb about 5% of sunlight in the ultraviolet region, which greatly limits its practical applications. So, it is an urgent and important task to develop an easily separable titania photocatalyst with visible light activity.The nanosized magnetic particles can solve the difficulty of photocatalyst separation from the treated water by applying an external magnetic field due to its magnetism property. In this study, magnetically separable photocatalysts with UV and visible light activity have been prepared through the combination of the nanosized magnetic particles and nanosized photocatalyst particles. The morphology and structure of the samples have been characterized using analytical techniques of VSM, XRD, TEM, EDS, XPS, FT-IR, BET and DRS etc. The relationship between the microstructure and photocatalytic properties was investigated. Some conclusions have been made as follows:1. NiFe2O4 nanoparticles were prepared by liquid catalytic phase transformation method at low temperature. The prepared samples show the characteristics of excellent dispersion, high magnetic property, stable crystalline phase and the superparamagnetic nature. The magnetic property of the prepared samples is very strong, when the action conditions are 0.6～1.5 mol·L-1 of total concentration, 8.5～10.3 of pH value, 0.02 of mole ratio for Fe2+/ Fe3+ and 2～4 h of boiling and refluxing time. The minute Fe2+ ions play the role of remarkable catalysis for the preparation of NiFe2O4 nanoparticles. The results of TEM and XRD testing indicate that the crystalline phase of prepared NiFe2O4 nanoparticles is spinel phase, and their size is less than 5 nm. The dissolution/reprecipitation and solid-state transformation mechanisms explain the reason why the minute Fe2+ ions play the role of remarkable catalysis.2. Silica-coated NiFe2O4 nanoparticles based on the prepared NiFe2O4 nanoparticles were prepared by a chemical precipitation method. A magnetically separable photocatalyst TiO2/SiO2/NiFe2O4 (TSN) with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO2/NiFe2O4 (SN) dispersion and P-25 titania were mixed, sonificated, refluxed, separated, dried, and calcined, showing high photocatalytic activity for the degradation of methyl orange in water under UV irradiation. Transmission electron microscope (TEM) and X-ray diffractometer (XRD) were used to characterize the structure of photocatalyst TSN, indicating that the magnetic SN particle was compactly enveloped by P-25 titania, and TiO2 shell was formed. The effect of a thin SiO2 layer between NiFe2O4 and TiO2 shell on the magnetic property and photocatalytic activity of photocatalyst TSN is least when the weight ratio of SiO2/NiFe2O4 is 2:1. In photocatalyst TSN, a thin SiO2 layer between NiFe2O4 and TiO2 shell prevented effectively the leakage of charges from TiO2 particles to NiFe2O4, which gave rise to the increase in photocatalytic activity.3. Silica-coated NiFe2O4 nanoparticles based on the prepared NiFe2O4 nanoparticles were prepared by a chemical precipitation method. A magnetically separable nitrogen-doped photocatalyst TiO2-xNx/SiO2/NiFe2O4 (N-TSN) with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO2/NiFe2O4 (SN) dispersion and the visible-light-active photocatalyst TiO2-xNx were mixed, sonificated, dried, and calcined at 400°C, showing photocatalytic activity for the degradation of methyl orange in water under visible light irradiation (λ> 400 nm). Transmission electron microscope (TEM) and X-ray diffractometer (XRD) were used to characterize the structure of photocatalyst N-TSN. The results indicated that the magnetic SN nanoparticles adhered to the surface of TiO2-xNx congeries. The prepared photocatalyst N-TSN show the characteristics of high specific surface area, photocatalytic activity under visible light irradiation, stable photocatalytic activity after several cycles and the superparamagnetic nature.4. Silica-coated NiFe2O4 nanoparticles based on the prepared NiFe2O4 nanoparticles were prepared by a chemical precipitation method. A magnetically separable photocatalyst Bi12TiO20/SiO2/NiFe2O4 (BSN) with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic SiO2/NiFe2O4 (SN) dispersion and the visible-light-active photocatalyst Bi12TiO20 prepared by a simple coprecipitation processing were mixed, sonificated, dried, and calcined at 550°C. The spectrum of UV-Vis absorption indicated that its absorption edge was at 450 nm. The prepared photocatalyst BSN showed photocatalytic activity for the degradation of phenol in water under visible light irradiation (λ> 400 nm). Transmission electron microscope (TEM) and X-ray diffractometer (XRD) were used to characterize the structure of photocatalyst BSN. The results indicate that the magnetic SN nanoparticles adhere to the surface of Bi12TiO20 congeries. The prepared photocatalyst BSN show the characteristics of photocatalytic activity under visible light irradiation and the superparamagnetic nature.5. Magnetic SiO2/NiFe2O4 (m-SN) nanospheres based on the prepared NiFe2O4 nanoparticles were prepared by reverse micelle technique. A magnetically separable TiO2/SiO2/NiFe2O4 (e-TSN) photocatalyst nanosphere with egg-like structure was prepared by chemical precipitating TiO2 onto the surface of m-SN nanospheres, showing high photocatalytic activity for the degradation of methyl orange in water under UV irradiation. Transmission electron microscope (TEM) and X-ray diffractometer (XRD) were used to characterize the structure of e-TSN photocatalyst nanospheres, indicating that nickel ferrite core nanoparticles were completely enveloped into monodisperse silica nanospheres as carrier and titania nanoparticles aggregates coated onto the surface of m-SN nanospheres to form a imperfect TiO2 shell for photocatalysis. Magnetic m-SN nanospheres can be prepared by reverse micelle technique when the weight ratio of NiFe2O4/SiO2 is 15 wt%, the effect of NiFe2O4 nanoparticles on the magnetic property and photocatalytic activity of e-TSN photocatalyst nanospheres being least. The photocatalytic activity of the recycled e-TSN photocatalyst nanospheres has no noticeable change after several cycles under UV irradiation.
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