Magnetic Nano-photocatalysts: Preparation, Structure, and Application

Abstract

Photocatalytic degradation of organic pollutants using suspended and dispersed nanosemiconductor photocatalysts in wastewater has unique advantages including high activity, low cost, solar utilization, and complete mineralization. However recovery and reuse of photocatalysts are difficult because fine particles are easily discharged in the waterflow. Immobilization of photocatalysts on supports such as glass and zeolite decreases activities due to the low specific area and slow mass transfer. Furthermore, a large amount of photocatalysts will lead to color contamination if they are released at industrial scale. Therefore, it is necessary to develop photocatalysts with separation function for reusable and cyclic application. The concept of magnetic photocatalysts with separation function was raised to benefit from high activity and enable nanosemiconductor photocatalysts to be reused.We review the photocatalytic principle of magnetic semiconductor catalysts, the classification of magnetic materials, the preparation, structure and application of magnetic semiconductor catalysts. The major points are: (1) suspended and dispersed composite nano-photocatalysts are a miniature of the photoelectrochemical cell. Semiconductor photocatalysts can nearly decompose all pollutants including refractory organic pollutants by hydroxyl radicals formed under light irradiation due to the high redox potential of the hydroxyl radicals with 2.8 V. (2) For the fabrication of magnetic photocatalysts the three most widely used magnetic materials are magnetite, maghemite, and MFe2O4, where M stands for Ba2+, Ni2+, Co2+, Zn2+ divalent metallic cations. (3) The three-component photocatalysts are predominant because the structure maintains the high activities of the catalysts and endows the photocatalysts with the separation function. The core structure of the three-component photocatalysts comprises a magnetic core with the separation function, an interlayer (SiO2) with the insulation function preventing the recombination of photogenerated electron-hole pairs, and a shell layer (TiO2) with the photocatalytic function for the degradation of organic pollutants. The core magnetic photocatalysts possess great activities whereas the core magnetic photocatalysts have low activities. Compared with that of pure photocatalysts, the core magnetic photocatalysts may have an enhanced activity with magnetic separation function under optimal conditions. (4) In order to prepare the core photocatalysts with a magnetic separation function, one needs to synthesize the magnetic particle as a magnetic core first. Then a sol-gel method is often used to coat the silica insulator on the magnetic core. Finally, the semiconductor nanoparticles are bound to the interlayer insulator. Thus, a core photocatalyst with both magnetic separation function and photocatalytic function can be obtained. The prepared photocatalysts can be recovered using external magnetic force.KeywordsPhotocatalysisMagnetic photocatalystMagnetic separationCyclic utilization