Quantitative determination of titanium lattice defects and solid-state reaction mechanism in iron-doped TiO₂ photocatalysts

Iron-doped titania photocatalysts with different iron contents were prepared by using a sol-gel method in acidic media. The crystalline structures of the various phases calcined at temperatures ranging from 70 to 800 °C were studied by using the Rietveld technique in combination with XRD experiments. The average crystallite size of the phases, lattice cell parameters, phase concentrations, and titanium cationic defects in the crystalline structures of different samples were quantitatively determined. Both iron content and calcination temperature strongly affected phase transformation and solid-state reaction mechanism. Below 400 °C of calcination, all the samples had some brookite and a majority of anatase phase. Iron ions were uniformly distributed in the interstices of titania crystals to form a titanium-iron solid solution when the samples were calcined at 80, 200, and 400 °C. However, when the temperature was 800 °C, Fe₂TiO₅ was produced in the sample containing 5 wt% Fe by a reaction between interstitial iron ions and lattice titanium ions, and in the 10 wt% Fe sample through a reaction of hematite with titania phases. The crystalline structures of titania phases were distorted at higher calcination temperature. For the first time, it is possible to show that titanium lattice defects related to the hydroxyl ions in the crystalline structures were created in anatase and rutile phases. The concentration of titanium defects remained almost constant below 400 °C but decreased as the calcination temperature was higher than 600 °C due to the decrease of the hydroxyls in the crystalline structure.