Novel photocatalyst: Titania-coated magnetite. Activity and photodissolution

Magnetic photocatalysts were synthesized by coating titanium dioxide particles onto colloidal magnetite and nano-magnetite particles. The photoactivity of the prepared coated particles was lower than that of single-phase TiO2 and was found to decrease with an increase in the heat treatment. These observations were explained in terms of an unfavorable heterojunction between the titanium dioxide and the iron oxide core, leading to an increase in electron-hole recombination. Interactions between the iron oxide core and the titanium dioxide matrix upon heat treatment were also seen as a possible cause of the observed low activities of these samples. Other issues considered include the physical and chemical characteristics of the samples, such as surface area and the presence of surface hydroxyl groups. Depending on the calcination conditions, these photocatalysts were found to suffer from varying degrees of photodissolution. Photodissolution tests revealed the greater the extent of the heat treatment, the lower the incidence of photodissolution. This was explained in terms of the stability of the iron oxide phases present, as well as the photoactivity of the titanium dioxide matrix. Our studies revealed that the observed photodissolution was in fact to be induced photodissolution. That is, the photogenerated electrons elevated to the conduction band of the titanium dioxide nanocrystals were being injected into the lower lying conduction band of the iron oxide core, leading to its reduction.