Kinetic Study on the Crystal Transformation of Fe-Doped TiO2 via In Situ High-Temperature X-ray Diffraction and Transmission Electron Microscopy.

Titanium dioxide (TiO2) is widely used in various major industries owing to its different crystal forms and functions. Therefore, fabricating suitable crystalline TiO2 through reasonable processes is necessary. In this study, Fe-doped TiO2 precursors were prepared via hydrolysis. Further, in situ high-temperature X-ray diffraction and transmission electron microscopy were used to transform the synthesized precursor in its crystal form. The Rietveld full-spectrum fitting method could accurately yield two different crystal forms at instant temperatures. Additionally, the rate relation between the crystal form transformation and reaction conditions was obtained. Results showed that the addition of Fe increased the temperature of phase transition of TiO2 anatase to rutile and accelerated the anatase → rutile transformation process. Further, crystal phase transition kinetic analysis showed that the phase transition kinetic model of Fe-doped TiO2 matched the Johnson-Mehl-Avrami-Kohnogorov (JMAK) model and that its phase transition was affected by crystal defects. Finally, Fe3+ in Fe-doped TiO2 was reduced to Fe2+ to generate oxygen vacancies, thus promoting the rate of transformation from titanium ore to rutile.

Introduction

Titanium dioxide (TiO2) is a very widely used material that cannot easily undergo chemical changes.[1] Currently, its two crystal types, which are used in many industrial applications, are anatase (A) and rutile (R). Anatase can undergo an irreversible phase change to rutile under certain conditions. Rutile exhibits a better crystal structure, denser atomic arrangement, higher dielectric constant, stronger ultraviolet (UV) shielding ability, and better stability than anatase.[2] Because the structure of rutile is denser than that of anatase, it exhibits little distortion; therefore, its catalytic ability is poorer than that of anatase. Different crystal types show different properties, functions, and applications.[3−5] In the production process of TiO2, generally, raw ore is directly used; hence, Fe impurities cannot be avoided, which somewhat affect the crystal transformation of TiO2.[6] The phase transition process of TiO2 continuously changes with time and temperature. Under normal pressure, only anatase → rutile phase transition occurs;[7−9] the phase transition temperature is approximately 600 °C. Higher temperature or longer holding time leads to a more complete transformation from anatase to rutile. Because rutile is the most stable phase, this phase transition process is irreversible.[6,10] The application prospects of TiO2 depend on its crystal type, and a reasonable process is required to prepare TiO2 using a suitable crystal type to achieve its optimal performance in application areas. Therefore, understanding the stability of the TiO2 crystal form and its crystal transformation kinetics and determining techniques to control the transformation process to achieve single-phase or multiphase TiO2 are very important.[11−15]

In this study, Fe-doped TiO2 was synthesized via the hydrolysis and coprecipitation of titanium oxide sulfate (TiOSO4) and ferric chloride (FeCl3·6H2O).[16,17] Additionally, TiO2 with varying Fe contents was calcined using in situ high-temperature X-ray diffraction (XRD)[18−21] technology to simulate the real-time reaction process of the actual titanium mineral-calcining reaction system. Moreover, Rietveld full-spectrum fitting, Williamson–Hall, and other methods were used to study the kinetics of the Fe to TiO2 crystal transformation process under high-temperature conditions. Research results showed that the addition of Fe to TiO2 increases the transformation temperature of TiO2 anatase to rutile and accelerates the anatase → rutile transformation process.[22−24] Further, Fe-doped TiO2 agreed well with the Johnson–Mehl–Avrami–Kohnogorov (JMAK) phase transition kinetic model. The JMAK formula showed that the TiO2 phase transition process was affected by crystal defects. The addition of Fe was conducive to the generation of vacancy defects, and the defects accelerated the transformation speed, thus accelerating the anatase → rutile transformation process.[25]

Experimental Section

TiOSO4 and FeCl3·6H2O were hydrolyzed and precipitated to prepare the Fe-containing TiO2 precursor. The samples were doped with Fe impurities with doping amounts of x = 0, 0.5, 1, 2.5, 5, and 7.5, termed TF0, TF1, TF2, TF4, TF7, and TF10, respectively. First, titanium tetrachloride (TiCl4) was subjected to an ice-water bath and then added to a sulfuric acid solution. Then, the mixture was stirred until it was completely dissolved, thereby forming a TiOSO4 solution. Thereafter, a certain amount of FeCl3·6H2O was added to the TiOSO4 solution by the mass ratio, and the mixture was stirred. The pH of the solution was adjusted to 6 using a saturated NaOH solution to form solid precipitates. The formed precipitate was washed several times with distilled water until SO24– and Cl– could not be detected when saturated BaCl2 and AgNO3 solutions were used. Then, to obtain the Fe-containing TiO2 precursor, the washed precipitate was dried in an electrically heated blast drying oven at 80 °C for 24 h. All in situ high-temperature phase transition analyses of TiO2 were performed using an X-ray diffractometer (PANalytical X’Pert PRO MRD). The used target was Cu, λKα1 = 1.5406 nm, and the voltage and current were 40 kV and 40 mA, respectively. The scanning range and time were 2θ = 10–90° and 10 min, respectively, and the step size was 0.026°. The obtained diffraction pattern was refined using HighScore Plus 4.8 software, and the relative content of the rutile phase was determined using the Rietveld full-spectrum fitting method and the Williamson–Hall mapping method. The crystal structure of the sample was observed using a high-resolution transmission electron microscope (Tecnai G2 F20).

Results and Discussion

In Situ High-Temperature XRD Phase Transition Analysis

Figure a–f shows the in situ high-temperature XRD patterns of the six samples (TF0–TF10). The test started at 300 °C, and it was performed at an interval of every 25 °C until reaching 1000 °C. However, during the cooling process, the test was performed at an interval of 200 °C. The diffraction peak appearing at 2θ = 45° (Figure a–f) was formed after the platinum bar of the heated sample was exposed, and it does not correspond to the TiO2 diffraction peak. In the subsequent analysis, this nonsample peak was eliminated. At 300 °C, TiO2 was still in its anatase phase. With an increase in temperature, the diffraction peak intensity of the anatase phase gradually increased. In the vicinity of 2θ = 37, 55, and 72°, multiple anatase phase peaks were observed with relatively large peak widths. The rutile phase began to appear after 600 °C. When the temperature was further increased, the XRD peak of the anatase phase became weak until it disappeared. Moreover, the diffraction peak of the rutile phase continued to become strong, and the peak width became narrow, indicating that the rutile phase was continuously growing. Compared with the non-Fe-doped samples, after increasing the temperature to a certain value, Fe-doped TiO2 exhibited peaks at 2θ = 25.2, 32.5, 48.9, and 60°, which corresponded to the peak of the Fe2TiO5 (FTO) phase, in addition to the anatase and rutile phases.

Figure 1

In situ high-temperature XRD patterns of (a) TF0, (b) TF1, (c) TF2, (d) TF4, (e) TF7, and (f) TF10.

Figure a,b shows the XRD patterns before and after the formation and disappearance temperatures of the TF0 phase. The XRD patterns of TF0 revealed only the anatase phase at 575 °C. When the temperature was increased to 600 °C, a rutile phase diffraction peak was observed at 2θ = 27.4°, which was the strongest peak of the rutile phase and first appeared during rutile phase transition. Thus, the temperature at which this peak began to appear in the rutile phase is the TiO2 phase transition start temperature. Similarly, the temperature at which all diffraction peaks of the anatase phase disappeared was the termination temperature of the TiO2 phase transition. Generally, the characteristic peak of the anatase phase, which disappeared last, was observed at 2θ = 25.3°, which is the strongest peak of the anatase phase (Figure b). When the temperature reached 825 °C, only the diffraction peak of the rutile phase remained. The change temperature of each phase in TF0–TF10 was obtained, as shown in Table .

Figure 2

TF0 phase transition start and stop temperature: (a) rutile appearance temperature and (b) anatase termination temperature.

Table 1

Phase Transition Temperature of TiO2 with Different Fe Contents

	TF0	TF1	TF2	TF4	TF7	TF10
R appear.	600	625	675	675	675	675
A disappear.	825	800	825	825	825	825
FTO appear.		800–cooling	900	775	725	725

It can be seen from Table that the R phase transition temperature of TF0 was 600 °C and that the Fe addition increases the appearance temperature of the R phase. When the doping amount was ≥1.15% (TF2), the rutile phase transition temperature was maintained at 675 °C. The temperature at which the anatase phase disappeared did not change considerably. Next, all samples were maintained between 800 and 825 °C; however, because of the Fe doping, the temperature range of the anatase → rutile transition became smaller, 225 and 175 °C for TF0 and TF1, respectively, and 150 °C for TF2, TF4, TF7, and TF10.

Figure a–d shows the relative content diagram of each phase during the phase transition process of different samples obtained by performing Rietveld full-spectrum fitting of the XRD spectrum data in Figure . From Figure b, the amount of the rutile phase generated at the beginning of the phase transition process was relatively small, after which it entered the rapid growth phase. Finally, the growth curve became stable until the anatase phase disappeared. For the non-Fe-doped sample (TF0), the initial phase transition temperature was 600 °C. After 650 °C, the rutile phase started to rapidly increase. Thereafter, the phase transition curve became nearly stable at 750 °C. For the Fe-doped samples, as the Fe content increased, the formation temperature of the FTO phase decreased, indicating that the formation of Fe2TiO5 is rapid when the Fe content is increased. Moreover, the rutile phase of TF1 started to rapidly increase after the temperature was increased by 50 °C and that of the rest of the samples started from 675 °C. At the beginning of the anatase → rutile phase transition, at 700 °C, the samples entered a rapid growth stage; then, they steadily increased after 750 °C until the anatase → rutile phase transition was complete. After reaching 850 °C, the relative content of the rutile phase slightly decreased.

Figure 3

The relative content of the phases of TiO2 is trending with temperature: (a) anatase, (b) rutile, and (c) pseudobrookite. (d) TF0-700 °C Rietveld full-spectrum fit quantitative analysis results.

Table shows the relative content of each phase in the phase transition process of Fe-doped TiO2. By analyzing the relative change in the content, it can be inferred that Fe2TiO5 was formed by the reaction of rutile TiO2 and Fe and that the addition of Fe increased the temperature of the anatase → rutile phase transition but reduced the temperature range of the transition process. The end temperature of the sample anatase → rutile transition basically ended at the same temperature of 825 °C.

Table 2

The Relative Content of Each Phase in the Phase Transition of Iron-Doped TiO2

	TF4/%			TF7/%			TF10/%
T/°C	A	R	FTO	A	R	FTO	A	R	FTO
700	89.9	10.1	0.0	74.3	25.7	0.0	67.2	32.8	0.00
725	43.1	56.9	0.0	23.1	76.6	0.3	26.6	67.9	5.50
750	12.1	87.9	0.0	11.3	84.2	4.5	15.7	78.7	5.70
775	2.50	97.0	0.4	5.80	89.8	4.3	9.20	84.2	6.60
800	1.30	96.5	2.2	0.90	93.3	5.8	7.90	83.1	9.00
825	0.00	97.3	2.7	0.00	93.2	6.8	1.40	86.5	12.1
850	0.00	97.2	2.8	0.00	92.6	7.4	0.00	87.6	12.4

For further investigation, to study the holding time effect on the rutile TiO2 content, the samples with the different Fe contents were tested using high-temperature in situ XRD. The samples were retained at their starting phase transition temperatures (Table ), and a test was performed once every 10 min from the beginning to the temperature. Each test time was 10 min, for a total of 5 h. Figure presents the obtained in situ XRD pattern of the holding time.

Figure 4

In-situ XRD diagram of each sample at phase change temperature.

It can be seen from Figure that at the phase transition temperature, the sample with the low iron content demonstrated a little phase variation during the same holding time.TF0 (600 °C) exhibited only 6.3% of the rutile phase at the end of the holding time, while TF1 (625 °C) exhibited 8%. The samples with high Fe contents exhibited more of the rutile phase, and the rutile phase content in TF10 (675 °C) reached 72.6% at the end of heat preservation. Although the temperature at which the Fe-doped samples started undergoing the anatase → rutile phase transition was high, the heat preservation results further indicated that Fe doping increased the reaction rate. When retained at 750 °C, all samples achieved reaction equilibrium after 50 min. The rutile phase content was calculated using the Rietveld full-spectrum fitting and Williamson–Hall mapping methods, and the calculation results are shown in Figure a–f.

Figure 5

Content of the rutile phase of (a) TF0, (b) TF1, (c) TF2, (d) TF4, (e) TF7, and (f) TF10 at the phase transition temperature, 700 °C, and 750 °C.

Kinetic Analysis of the Crystal Transformation of Fe-Doped TiO2

The holding phase content of different TiO2 samples was analyzed with respect to time at different temperatures, and the kinetic parameters of the TiO2 phase transition were solved using the change in the rutile phase. First, the change in TiO2 was analyzed while holding the phase transition temperature to determine the phase transition kinetic model of Fe-doped TiO2. Figure shows the corresponding change in the phase content.

Figure 6

Different Fe content-doped TiO2 kept warm at phase change temperature and the relative content of each phase changing over time: (a) anatase-A and pseudobrookite-FTO and (b) rutile-R.

As shown in Figure a, within 5 h, the TF10 sample formed an FTO phase after 80 min; the other Fe-doped samples did not form FTO phases. It can be seen from Figure a that in the early stage of holding TF10, the FTO phase began to appear from 75 min because the formation of the FTO phase can lead to excessive defects in the crystal. Moreover, the rutile phase growth decreased, indicating the presence of excessive Fe; hence, TF10 could no longer drive the anatase → rutile phase transition at this temperature. In Figure a, the anatase phase of TF10 decreased with time. Furthermore, TF7 tended to decelerate; however, the Fe amount was relatively small, and the FTO phase transition did not occur yet. In Figure b, the different samples were retained at the temperature at which their phase transition began. Moreover, the rutile phase increased with the Fe content and in the slope of the rutile phase growth. The phase content of TF0–TF4 linearly increased within 5 h of the holding time. After TF7 and TF10 rapidly grew at the beginning, their growth began to decelerate, the relative content growth curve of the rutile phase became slower and flatter, and the TF10 performance became more obvious.

The matching of the phase transition kinetics of TiO2 and the solution of the related parameters generally use the standard first-order kinetic modelor the JMAK modelIn the above equation, t is the time, α is the mass fraction of the phase transition, k is the kinetic reaction rate constant, and n is the Avrami index, which is related to the phase transition mechanism.

The above two models were used to fit the insulated samples, and the kinetic model fitting diagram was obtained, as shown in Figure . By comparing the goodness of fitting rutile in Figure a and b, TF0, TF1, TF2, TF4, and TF7 better fit the standard first-order kinetic model, TF10 better matches the JMAK model than the standard first-order kinetic model, and TF4 and TF7 show good matching degrees with the two models. Overall, the matching of the two models was relatively good. Generally, a low Fe content or pure TiO2 conforms to the first-order kinetic model of nucleation growth. When the impurity concentration was high, other growth methods were observed. The Fe content in TF4, TF7, and TF10 was relatively high. Some problems occurred when these samples were fitted with the JMAK model. Next, the Avrami index n of the JMAK formula is related to the phase transition reaction mechanism, and n is the slope of the equation obtained by fitting. When n = 1, the JMAK model corresponds to a first-order kinetic model.[26] However, when n = 2/3, the phase transition is related to crystal defects (dislocations and vacancies).[27,28] Based on the ln[−ln(1 – α)]–ln t curve diagram obtained using the JMAK kinetic equation, the kinetic reaction rate constant k can be calculated. From the Arrhenius equation, the reaction rate and temperature show the following relationshipIn the formula, k0 is a constant, R is the general gas constant, and T is the isothermal absolute temperature. By considering the natural logarithm of both sides of the above formula, we achieveAfter plotting an ln k–1/T curve, the linear regression fitting can yield the activation energy of the TiO2 phase transition.

Figure 7

Kinetic model fitting: (a) standard first-order kinetic model and (b) JMAK model.

It can be seen from Figure that with the increase in temperature, the time required for TiO2 to reach the A → R phase transition equilibrium became shorter. Further, a high Fe content leads to a short transition equilibrium time. The change rule was similar to the JMAK model; hence, ln[−ln(1 – α)]–ln t was used for plotting as shown in Figure a–f, and the obtained ln k value and phase change activation energy are shown in Table .

Figure 8

ln(−ln(1 – α))∼ln diagram of holding time at various temperatures (a) TF0, (b) TF1, (c) TF2, (d) TF4, (e) TF7, and (f) TF10.

Table 3

Phase Transformation Activation Energy of TiO2 for Different Iron-Doped Contents

samples	temperature (K)	ln k	E (kJ/mol)
TF0	873	–5.93375	212.30
	973	–4.97477
	1023	–1.01169
TF1	898	–6.3588	28.32
	973	–9.67976
	1023	–5.2047
TF2	948	–9.81289	741.63
	973	–14.6201
	1023	–4.22434
TF4	948	–10.3711	759.97
	973	–10.8263
	1023	–3.83374
TF7	948	–9.75908	550.51
	973	–3.1779
	1023	–3.77046
TF10	948	–5.92342	451.57
	973	–6.90546
	1023	–2.16794

It can be seen from Table that the activation energy of TF0 is smaller than that of TF2, TF4, TF7, and TF10, with TF4 being the largest and TF7 and TF10 decreasing sequentially. The activation energy of the Fe-doped samples increased, indicating that a higher temperature phase transition was required. Moreover, the activation energy first increased and then decreased with an increase in the Fe content, which can be related to the accelerated phase transition mechanism.

The findings of the kinetic study revealed that the phase transition process of Fe-doped TiO2 was mainly controlled by crystal defects. Because the ionic radius of Fe3+ (0.064 nm) is similar to that of Ti4+ (0.061 nm), Fe3+ can enter TiO2 instead of Ti4+ to form a solid solution. Moreover, because Fe is a variable element, Fe3+ can be reduced to Fe2+ during the phase change; thus, the produced Fe2+ can form more oxygen vacancies. Vacancy defects promote the conversion of anatase to rutile, which was mainly reflected in the conversion rate of the A → R phase transition. In metal-cation-doped TiO2, the cation generally enters the TiO2 lattice to replace Ti4+ to form a replacement solid solution. When the ion radius significantly differs from the Ti4+ radius, the TiO2 lattice will be distorted, and excess energy will be stored. Anatase will first release this part of energy during the phase transition and then transform into rutile. After the Fe doping, the A → R transition temperature increased. After analysis, the samples with low Fe contents well fitted the standard first-order kinetic model, the samples with high Fe contents well matched the JMAK model, and the phase transition process of the Fe-doped samples was controlled by the crystal defects. The activation energy of the Fe-doped samples was higher than that of the non-Fe-doped TiO2 sample.

Figure a–d shows the TEM image of TF0 and TF4, which were calcined at 700 °C. As shown in the figure, the phase observed using a transmission electron microscope at 700 °C mainly corresponded to the anatase phase. From the XRD analysis results, this phase was observed at 700 °C. The rutile phase already appeared underneath; however, the rutile amount was relatively small at 700 °C; therefore, observing the rutile phase lattice fringes under the TEM is difficult. It can be seen from Figure a,b that the average grain size of the Fe-doped sample TF4 is smaller than that of TF0.

Figure 9

TEM images of (a,c) TF0 and (b,d) TF4 after calcination at 700 °C, crystalline size scribed perpendicular to the lattice fringe.

Figure a,b shows the TEM images of TF0 and TF4 when calcined at 800 °C. In the figure, the interplanar spacing d = 0.246 nm was close to the spacing of the rutile phase (101) crystal plane. Moreover, d = 0.32 nm was close to the rutile phase (110) spacing. At 800 °C, the anatase phases of TF0 and TF4 were basically transformed into rutile phases.

Figure 10

TEM images of (a) TF0 and (b) TF4 after calcination at 800 °C.

From the above findings, it can be inferred that Fe-doped TiO2 was transformed from the anatase phase to the rutile phase. Next, the prepared precursor TiO2 mainly comprised anatase crystals with a relatively small number of crystal grains, in addition to non-Fe-doped and Fe-doped crystals. The grain size was less than 10 nm, and high-temperature calcination led to a phase change; however, it was limited by the critical size. The anatase first grew to approximately 11 nm, and the phase change began. However, Fe inhibited the grain growth; thus, a higher temperature was required for growth. After the beginning of the phase transition, Fe3+ in Fe-doped TiO2 generated more oxygen vacancies, formed more defects, accelerated the A → R transition process, and finally transformed into the rutile phase. Therefore, the growth of Fe-doped TiO2 was first controlled by the defects caused by the Fe valence. Second, it was affected by the initial grain size.

The doped Fe was Fe3+, which plays a key role in the phase change of TiO2. In the A → R phase transition, Fe3+ changed to Fe2+ to generate oxygen vacancies and accelerate the A → R phase transition.

Additionally, the TEM results showed that Fe inhibited the TiO2 crystal growth. The phase transition of Fe-doped TiO2 was affected by the change in the Fe valence state as well as the initial grain size. After Fe inhibited the grain growth, the anatase phase requires a higher temperature to achieve the energy needed to reach the critical size of the phase transition. Consequently, the A → R transition temperature increased.

Conclusion

In this study, TiOSO4 and FeCl3·6H2O were hydrolyzed and precipitated to prepare Fe-doped TiO2. Using in situ high-temperature XRD technology, the Williamson–Hall method, and phase transition kinetic analysis, the effect of the Fe mechanism on the phase transition of TiO2 was discussed. XRD results showed that TiO2 prepared via the hydrolysis and precipitation of TiOSO4 was mainly of the anatase type. The initial grain size was below 10 nm. Increasing the Fe doping amount further decreased the size of the crystal particle. The relative content of the anatase and rutile phases during the heating and calcination processes using the Rietveld full-spectrum fitting and Williamson–Hall methods showed that the addition of Fe increased the TiO2 anatase → rutile phase transition temperature and that Fe inhibited the growth of TiO2 crystals. When Fe doping was increased, the inhibition became more obvious. However, Fe accelerated the A → R phase transition process. Before the A → R transformation, Fe was in the form of Fe3+ in TiO2. After the phase transition began, Fe3+ was reduced to Fe2+, forming more vacancy defects and accelerating the A → R phase transition process. After analyzing the phase transition kinetics, we found that Fe-doped TiO2 well matched the JMAK model. The index n ≈ 0.6 in the JMAK model showed that the phase transition process of TiO2 was affected by defects. Finally, the addition of Fe was conducive to the generation of vacancy defects, which accelerated the rate of the phase change.