Study Of Iron-doped TiO₂ By XAFS And M(?)ssbauer Spectra

This thesis presents the investigation on Fe-doped TiO₂ nano-composites prepared by sol-gel method, using X-ray absorption spectra and M(?)ssbauer spectra techniques. The structure parameters including valence, position, distribution, coordination environment and oxygen vacancies are obtained. The ultraviolet-visible reflecting spectra are measured. How doping Fe atoms affect the photo-catalytic activity of TiO₂ is analyzed. The best Fe-doping concentration is obtained. Furthermore, the measurements of magnetic property are also carried out. Careful research indicates the nature magnetic property of TiO₂ with Fe atoms substituting Ti atoms is paramagnetic. According to the theoretical explanation presented in previous references, the reason of the ferromagnetism absence in Fe-doped TiO₂ is discussed. This work provides theoretical and experimental support to achieve better photocatalyst, and is also will be helpful to find the diluted magnetic semiconductors.In Chapter 1, the thesis refers previous works about Fe-doped TiO₂. Fe-doped TiO₂ is a new-type material with potential applications in both spintronics and photocatalyst. Doping Fe atoms affects strongly the photo-catalytic and magnetic property. However, previous works focus on concentrate in the information of TiO₂, including phase compositions and how Fe doping affects phase transformation from anatase to rutile. The information about the local structure around Fe atoms and distribution of Fe atoms in TiO₂ have not been reported yet. In addition, contradictory results on the nature magnetic property also have been reported. To solve this problem, careful research on the local structure, including valence, position, distribution, coordination environment and oxygen vacancies, is necessary. XAFS and M(?)ssbauer spectra are powerful tools to obtain these parameters. Therefore we introduce the EXAFS principle based on multiple scattering method, experimental method, experimental equipment and software package for fitting data. The principle of M(?)ssbauer Effect and hyperfine parameters are also illustrated. Especially superparamagnetism relaxation, which appears in our work, is explained.The main body of the thesis is presented as following: 1. Structure characterization of Fe-doped TiO₂ nano-composites.Fe-doped TiO₂ nano-composites with different Fe concentration are prepared by sol-gel method, and are sintered at 923K. Measurements of XRF, TEM and XRD are performed. The XRF results indicate that the metal atoms are mainly composed of Fe and Ti, besides a few Zn impurities. The XRD patterns show that TiO2 in all the samples mainly exists as anatase phase. No peaks of ferric compounds can be observed in the samples with low Fe concentration (1 wtï¼…, 2wtï¼…and 5 wtï¼…), while a weak peak attributed to the (104) ofα-Fe₂O₃ appears in the highly Fe-doped TiO₂(10wtï¼…and 15wtï¼…). The average grain size calculated by Scherer formula is 13nm, which is in well agreement with the results from TEM.2. Local structure around iron in the Fe-doped TiO₂ nano-composites.The XAFS measurements of Fe-doped TiO₂ nano-composites with different Fe concentration are carried out. The results demonstrate that for 1 wtï¼…and 2 wtï¼…Fe-doped TiO₂, Fe atoms enter the lattice of anatase and substitute Ti atoms. When Fe concentration reaches 5 wtï¼…, part of Fe atoms aggregate and formα-Fe₂O₃. For 10 wtï¼…Fe-doped TiO₂, Fe atoms mainly exist asα-Fe₂O₃. The fitting results for 1 wtï¼…Fe-doped TiO₂ show that the average length of Fe-O bond is about 1.98 (?) and is 0.02 (?) larger than that of Ti-O bond, while the coordination distance between the center Fe atom and the first-nearest Ti atoms is 3.01 (?), 0.04 (?) smaller than that between the center Ti atom and the first-nearest Ti atoms. This abnormal variation suggests that Fe ions substituting the Ti ions affect the Fe-O and Fe-Ti shells, and cause local distortion of the anatase lattice. To build a detail model of Fe atoms in the lattice of TiO₂, the FEFFS.20 software package are used to simulate the XANES of 1 wtï¼…Fe-doped TiO₂. We find that forα-Fe₂O₃ the simulation result is in agreement with experimental curve, but for anatase there are a large discrepancy between theoretical curve and experimental result. We doubt whether FEFFS.20 software package is appropriate to simulate the XANES of Fe substituting model, which is based on the crystal structure of anatase. Further research is necessary to solve this problern.3. Distribution of Fe atoms in Fe-doped TiO₂ nano-composites.Room temperature M(?)ssbauer spectra of Fe-doped TiO₂ nano-composites (2 wtï¼…, 5 wtï¼…, 10wtï¼…and 15 wtï¼…) are measured, besides that of 10 wtï¼…Fe-doped TiO₂ at 80K. For 10 wtï¼…Fe-doped TiO₂, the area of sextet in the M(?)ssbauer spectra at room temperature is smaller than that at 80K. This demonstrates that at room temperature superparamagnetic relaxation occurs in this sample because the grain size ofα-Fe₂O₃ is too small. Fe atoms substituting Ti atoms act as doublet, indicating the magnetic property of these Fe atoms is paramagnetic. The M(?)ssbauer spectra are fitted based on three kinds of Fe site: substituting site, site of superparamagneticα-Fe₂O₃ and site of anti-ferromagneticα-Fe₂O₃. The fitting results indicate that the concentration of Fe atoms substituting Ti atoms is limited to about 1.5 wtï¼…and is independent on Fe doping concentration. For the concentration of superparamagneticα-Fe₂O₃, there is a maximum, about 4 wtï¼…in the 10 wtï¼…Fe-doped TiO₂. The concentration of antiferromagneticα-Fe₂O₃ increases sharply with Fe doping concentration.4. The ultraviolet-visible spectra and magnetic property of Fe-doped TiO₂ nano-composites.The measurements ultraviolet-visible spectra of Fe-doped TiO₂ nano-composites are performed. For 1 wtï¼…Fe-doped TiO₂ only one stage located at 430nm (2.88eV) can be seen observed in the ultraviolet-visible spectrum. This stage is attributed to Fe substituting Ti atoms. This demonstrated that Fe doping shift the absorption edge of TiO₂ to low energy indeed and TiO₂ is able to can utilize solar energy directly. While for 5 wtï¼…, 10 wtï¼…and 15 wtï¼…Fe-doped TiO₂, beside this stage, another stage positioned at 600nm (2.07eV) are also observed, arising fromα-Fe₂O₃. Previous works show thatα-Fe₂O₃ decrease the photo-catalytic activity largely. Therefore, to obtain a better TiO₂ photocatalyst, Fe doping concentration should be chosen to be 1.5 wtï¼…. In addition, the magnetic property is also measured using SQUID equipment and the M-T curves are obtained. For highly Fe-doped TiO₂, there is a region of transition to weak ferromagnetism in the range 180-250 K, instead of sharp jump for bulkα-Fe₂O₃ in the range of 245-250K, which was observed by Neel. The magnetic property of TiO₂ with Fe atoms entering the lattice of anatase is paramagnetic. The M-T curves are fitted using the formulaχ=χ₁+χ₂=C₁／(T-θ_?)+χ₂. For 1 wtï¼…Fe-doped TiO₂ the obtained parameter C₁ is 7.7×10^-4, and is agreement with the parameter C6.2×10^-4 calculated by the formula C=(Nμ_B²g_?²J(J+1))／3k. This indicates that no exchange exists between Fe atoms substituting Ti atoms because of large distance caused by its low concentration. The magnetic property of TiO₂ with Fe atoms substituting Ti atoms is mainly attributed to isolated Fe atoms. These results indicate Fe-doped TiO₂ is not an ideal diluted magnetic semiconductor because the solubility of Fe atoms in the lattice of anatase is rather low.