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Nanocrystalline alumina was synthesized by using the sol−gel method with aluminum sec-butoxide as the precursor. FTIR, TG, XRD, and crystalline structure refinement were used to analyze the phase transformations and their crystalline structures. When samples were annealed at 200 °C, they had two nanocrystalline phases γ-boehmite and γ*-boehmite, which had the same crystalline structure but different lattice parameters and defect numbers in the structure. After sample annealing above 400 °C, the γ-boehmite and γ*-boehmite phases were transformed into nanocrystalline γ-Al2O3 and θ-Al2O3. The γ-alumina crystalline structure contained hydroxyls that brought about cationic defects in aluminum sites, mainly in those of octahedral symmetry. In the 27Al NMR spectra of the samples calcined at 400, 600, and 800 °C, a chemical shift around 33 ppm was observed, which occurred between that of aluminum in oxygen octahedra and aluminum in oxygen tetrahedra. Its origin can be explained by assuming the existence the substitution of some lattice oxygen ions in the octahedral symmetry by hydroxyl groups.

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.

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