Quantitative Determination of Titanium Lattice Defects and Solid-State Reaction Mechanism in Iron-Doped TiO₂ Photocatalysts

Abstract

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.

Characterization and Photocatalytic Evaluation (UV-Visible) of Fe-doped TiO2 Systems Calcined at Different Temperatures

“…In addition, the sample s-0.05-620, has a very low surface area, (less than 5 m 2 /g). This sudden drop in surface area is associated primarily with the presence of rutile in this sample (25), as well as with the synthesis method. When these results are compared with those of the crystal sizes in Figure 2, a correlation between the increase in crystal size and the decrease in surface area for the effect of increasing calcination temperature is observed (31).…”

“…In addition, mixtures of oxides are unlikely because the amount of iron is very small. According to Wang et of 5% or more (25). Other researchers state that during the calcination process, the preadsorbed precursor (iron nitrate, for example) is decomposed, and the iron initially present at the surface diffuses into the bulk, producing a solid solution.…”

“…The shift to the visible spectrum is attributed to charge transitions (or excitation) between the 3d electrons of the Fe 3+ ion and the TiO 2 conduction band (25,27,(38)(39). We estimated the indirect band gap for the samples, as shown in Table 2.…”

Effect of Fe Loading Condition and Reductants on CO2 Reduction Performance with Fe/TiO2 Photocatalyst

“…It was reported that doping transition metal was a useful technique for extending the absorbance of TiO 2 into the visible region [16]. For doping, various metal ions have been used, but among them, Fe 3+ is considered as a strong candidate as it has a similar radius to Ti 4+ (Fe 3+ = 78.5 pm, Ti 4+ = 74.5 pm) [17] and can easily fit into the crystal lattice of TiO 2 [16,18,19] [16,18,19].…”

“…For doping, various metal ions have been used, but among them, Fe 3+ is considered as a strong candidate as it has a similar radius to Ti 4+ (Fe 3+ = 78.5 pm, Ti 4+ = 74.5 pm) [17] and can easily fit into the crystal lattice of TiO 2 [16,18,19] [16,18,19]. Due to easy availability as well as the above described characteristics, Fe is selected as the dopant in the present study.…”