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 prepare d by using a sol-gel method in acidic media. The crystalline structures of the various phases calcined at temperatures rang ng from 70 to 800 degreesC were studied by using the Rietveld technique in combination with XRD exper iments. The average crystallite size of the phases, lattice cell parameters , phase concentrations, and titanium cationic defects in the crystalline st ructures of different samples were quantitatively determined. Both iron con tent and calcination temperature strongly affected phase transformation and solid-state reaction mechanism. Below 400 degreesC 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 tita nium-iron solid solution when the samples were calcined at 80, 200, and 400 degreesC. However, when the temperature was 800 degreesC, Fe2TiO5 was prod uced 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 ti tania phases were distorted at higher calcination temperature. For the firs t time, it is possible to show that titanium lattice defects related to the hydroxyl ions in the crystalline structures were created in anatase and ru tile phases. The concentration of titanium defects remained almost constant below 400 degreesC but-decreased as the calcination temperature was higher than 600 degreesC due to the decrease of the hydroxyls in the crystalline structure.