**3. Results and discussion**

Thermogravimetric and thermodifferential analyses are shown in Figures 2a for hydrolysis of alkoxides together and 2b for alkoxide hydrolysis separately. The curves obtained are very similar, initially, with a great loss of mass between approximately 40 °C and 200 ºC, which is due, probably, to dehydration, accompanied by an endothermal peak due to the energy consumed to release volatiles. An exothermal mass can be seen in Figure 2a, at 703 ºC. It is ascribed to crystallization of the orthorhombic phase of zirconium titanate. According to Khairulla and Phule (1992), the peaks by differential thermal analysis (DTA) at 350 and 550 °C are related to the removal of organic compounds and an exothermal event without loss of mass at approximately 710 °C is caused by the formation of the crystalline phase of ZrTiO4. Macan *et al*12, describe that a first loss of mass seen in the TGA and DSC curve as accompanied by an endothermal peak at 83 °C due to the evaporation of adsorbed water. Loss of mass diminishes constantly as temperature increases as a function of the slow degradation of residual organic matter, and the exothermal peak at 703°C is due to the crystallization of amorphous ZrTiO4, confirmed in the preliminary research (Consentino *et al*, 2003).

In Figure 2b two exothermal peaks were observed, 261°C and 460 °C, accompanied by loss of mass, due to the burning of the remaining organic matter. Bhattacharya *et al* suggest that this inflexion is due to the removal of structural anionic species. As the temperature increases, decomposition reactions occur and a broad exothermic peak at 460 °C. The weight loss continues and stabilizes at about 500 °C. At 703°C no exothermal peak that could be attributed to the crystallization of zirconium titanate was observed. During the cooling process endothermal events occurred, with hardly any loss of mass at 1260, 948 and 897 °C, which may be related to crystalline phase transitions, as described by Park *et al* (1996) in the solid-state reaction of oxide mixtures between ZrO2 and TiO2 at elevated temperatures.

Synthesis and Characterization of Crystalline Zirconium Titanate Obtained by Sol-Gel 307

According to Navio *et al*, Figure 3 shows different structures for ZrTiO4. The structure I could be postulated as the amorphous and it is expected to change to the structure II, III

Fig. 3. Structures for ZrTiO4. The structure I could be postulated as the amorphous and it is expected to change to the structure II, III and IV as the temperature increases (Navio *et al*,

The ZrO2-TiO2 phase diagram, Figure 4, has been investigated by McHale and Roth (1986). Above 1100 °C there exists a stable solid solution, Zr1+*x*Ti1-*x*O4, with approximate limits −0.17 < *x* < 0.1 (*x* ~ − 0.17 corresponds to Zr5-Ti7O24). At lower temperatures ZrTiO4 is not thermodynamically stable, but a solid solution centered around the composition ZrTi2O6 is stable. However, a wide range of compositions in the solid solution Zr1+*x*Ti1-*x*O4 can be

and IV (crystalline structures) as the temperature increases.

Fig. 4. ZrO2-TiO2 phase diagram (Troitzsch *et al*, 2005).

1992b).

Fig. 2. Thermal analyses of powders after drying obtained by hydrolysis of the alkoxides, NTZ and TPT: (a) together (Santos *et al*, 2010) and (b) separately.

**exo endo**

**TGA**

**TGA**

**(a)**

**exo endo**

**0 200 400 600 800 1000 1200 1400**

**0 200 400 600 800 1000 1200 1400**

**Temperature (°C)**

Fig. 2. Thermal analyses of powders after drying obtained by hydrolysis of the alkoxides,

NTZ and TPT: (a) together (Santos *et al*, 2010) and (b) separately.

**Temperature (°C)**


**-0,6**

**-0,5**

**-0,4**

**-0,3**

**Lost weight (%)**

**-0,2**

**-0,1**

**0,0**




**Lost weight (%)**



**(a)**

0,0

**-50**

**-50**

**0**

**50**

**DTA**

**100**

**150 200**

**DDP (mV)**

**250 300**

**350 400**

**0**

**50**

**DTA**

**100**

**150 200**

 **DDP (mV)** 

**250 300**

**350 400** According to Navio *et al*, Figure 3 shows different structures for ZrTiO4. The structure I could be postulated as the amorphous and it is expected to change to the structure II, III and IV (crystalline structures) as the temperature increases.

Fig. 3. Structures for ZrTiO4. The structure I could be postulated as the amorphous and it is expected to change to the structure II, III and IV as the temperature increases (Navio *et al*, 1992b).

Fig. 4. ZrO2-TiO2 phase diagram (Troitzsch *et al*, 2005).

The ZrO2-TiO2 phase diagram, Figure 4, has been investigated by McHale and Roth (1986). Above 1100 °C there exists a stable solid solution, Zr1+*x*Ti1-*x*O4, with approximate limits −0.17 < *x* < 0.1 (*x* ~ − 0.17 corresponds to Zr5-Ti7O24). At lower temperatures ZrTiO4 is not thermodynamically stable, but a solid solution centered around the composition ZrTi2O6 is stable. However, a wide range of compositions in the solid solution Zr1+*x*Ti1-*x*O4 can be

Synthesis and Characterization of Crystalline Zirconium Titanate Obtained by Sol-Gel 309

**(a)**

**(b)**

∗

**Intensity (a.u.)**

**700°C**

**600°C**

**500°C**

**600°C**

**Intensity (a. u.)**

**500°C**

2010) and (b) separately (A=anatase, B= baddeleyite, \* = ZT).

**700°C**

**0 10 20 30 40 50 60 70 80**

**0 10 20 30 40 50 60 70 80**

2θ **(degree)**

Fig. 5. Diffractograms of synthetized powders by hydrolysis of the alkoxides, after thermal treatment at 500, 600 and 700 °C for 8 h, NTZ and TPT: (a) together (\*= ZT) (Santos et al,

**2**θ **(degree)**

**B**∗

**B**

**A**

obtained in metastable form either by wet chemical methods (35-75 mol % Ti) or by quenching from high temperatures as the decomposition kinetics are very slow. The structure of ZrTiO4 was first investigated in 1967. It was reported to be a close structural relative of columbite, (Fe,Mn)Nb2O6, a compound that has an α-PbO2 substructure (Newnham, 1967).

The results of XRD of powders treated thermally at 500, 600 and 700 °C for 8 h are shown in Figure 5a (joint hydrolysis) (Santos *et al*, 2010) and 5b (separate hydrolysis). Figure 5a shows that below 700°C, the samples are amorphous. No presence of crystalline phases related to titania could be seen, probably because it presents in an amorphous form. The peak of orthorhombic ZrTiO4 (2θ=30.595°) was observed in the diffractogram of the sample at 700 °C (López-López *et al*, 2010; Santos *et al*, 2010). López *et al.* prepared ZrTiO4 films, deposited between 450 and 550 °C. At 450 °C, a weak intensity peak at 2θ = 30.48° indicates the presence of crystalline zirconium titanate, peaks at 2θ = 32-35° also suggest the presence of phases for Ti2O3, TiO2 and ZrO2. Raising the temperature to 550 °C the intensity of reflections (1 1 1) characteristic of ZrTiO4 and the reflections (0 2 0), (2 0 0) and (2 2 2) at 2θ = 32.6°, 35.7° and 63.3 can be identified. Under these conditions only the contributions of ZrTiO4 can be observed, indicating the formation of a single crystalline phase of ZrTiO4. The crystallographic structure for the films deposited, containing tin presents an intense peak at 450°C at 2θ = 33.06° associated with reflection (1 0 4) of Ti2O3. Peaks at 2θ = 32-35° suggest the presence of phases for SnO2, TiO2 and ZrO2, while a weak intensity peak indicates the crystallization of Zr0.8Sn0.2TiO4 (0 0 2) (López-López *et al*, 2008). Figure 5b shows that baddeleyite, JCPDS data for baddeleyite (13-0307), is formed at 2θ = 28.22°, at 600 °C and 700 °C from powder treated thermally at 500 °C. Besides the crystalline phase related to zirconium, the peak of titania, JCPDS data for anatase (04-0477), was observed in the anatase crystalline form and the peak of orthorhombic zirconium titanate, JCPDS data for zirconium titanate (34-415), at angles 25.2° and 30.6°, respectively.

Rodrigues et al (2010) reported the synthesis and characterization of nanostructures of sodium titanate/zirconium oxide obtained from the hydrothermal treatment of mixed oxide. Based on these results, we showed that the morphology and crystal structure of the productsobtained via hydrothermal treatment depend of the x value of the precursor Ti1 xZrxO2-nH2O (Ti/Zr molar ratio). For example, for sample with x equal to 0, only the presence of sodium titanate nanotubes were observed, while for small x values (less than 0.50) the nanoparticles showed morphology of nanoribbons and the presence of the sodium titanate phase and tetragonal ZrO2. For large values of x (greater than 0.50, high amount of zirconium) no morphological changes occurred and the tetragonal ZrO2 phase was observed for samples. Furthermore, only for the product obtained from x equal to 0.15, we observed the presence of three-dimensional flower-like arrangements. Thus, the influence that the Ti/Zr molar ratio of the precursor plays on the phase and morphology of the hydrothermal products obtained is significant. Second Rodrigues, the preferential coordination of Zr4+ ion is between 7 and 8, while for Ti4+ it is always 6. The ionic radius of Zr4+ in ZrO2 is 0.84 , assuming the coordination number equal to 8, and for the Ti4+ in TiO2 it is 0.61 Å , assuming the coordination number equal to 6. This significant difference in the ionic radius results in the differences in behavior of incorporation of the Ti4+ into the lattice of the ZrO2 and of the Zr4+ into the titanate lattice. When Zr4+ was introduced into the titanate structure this ion tends to interrupt the crystal arrangement of the titanate. On the other hand, the Ti4+ ion can

obtained in metastable form either by wet chemical methods (35-75 mol % Ti) or by quenching from high temperatures as the decomposition kinetics are very slow. The structure of ZrTiO4 was first investigated in 1967. It was reported to be a close structural relative of columbite, (Fe,Mn)Nb2O6, a compound that has an α-PbO2 substructure

The results of XRD of powders treated thermally at 500, 600 and 700 °C for 8 h are shown in Figure 5a (joint hydrolysis) (Santos *et al*, 2010) and 5b (separate hydrolysis). Figure 5a shows that below 700°C, the samples are amorphous. No presence of crystalline phases related to titania could be seen, probably because it presents in an amorphous form. The peak of orthorhombic ZrTiO4 (2θ=30.595°) was observed in the diffractogram of the sample at 700 °C (López-López *et al*, 2010; Santos *et al*, 2010). López *et al.* prepared ZrTiO4 films, deposited between 450 and 550 °C. At 450 °C, a weak intensity peak at 2θ = 30.48° indicates the presence of crystalline zirconium titanate, peaks at 2θ = 32-35° also suggest the presence of phases for Ti2O3, TiO2 and ZrO2. Raising the temperature to 550 °C the intensity of reflections (1 1 1) characteristic of ZrTiO4 and the reflections (0 2 0), (2 0 0) and (2 2 2) at 2θ = 32.6°, 35.7° and 63.3 can be identified. Under these conditions only the contributions of ZrTiO4 can be observed, indicating the formation of a single crystalline phase of ZrTiO4. The crystallographic structure for the films deposited, containing tin presents an intense peak at 450°C at 2θ = 33.06° associated with reflection (1 0 4) of Ti2O3. Peaks at 2θ = 32-35° suggest the presence of phases for SnO2, TiO2 and ZrO2, while a weak intensity peak indicates the crystallization of Zr0.8Sn0.2TiO4 (0 0 2) (López-López *et al*, 2008). Figure 5b shows that baddeleyite, JCPDS data for baddeleyite (13-0307), is formed at 2θ = 28.22°, at 600 °C and 700 °C from powder treated thermally at 500 °C. Besides the crystalline phase related to zirconium, the peak of titania, JCPDS data for anatase (04-0477), was observed in the anatase crystalline form and the peak of orthorhombic zirconium titanate, JCPDS data for zirconium titanate

Rodrigues et al (2010) reported the synthesis and characterization of nanostructures of sodium titanate/zirconium oxide obtained from the hydrothermal treatment of mixed oxide. Based on these results, we showed that the morphology and crystal structure of the productsobtained via hydrothermal treatment depend of the x value of the precursor Ti1 xZrxO2-nH2O (Ti/Zr molar ratio). For example, for sample with x equal to 0, only the presence of sodium titanate nanotubes were observed, while for small x values (less than 0.50) the nanoparticles showed morphology of nanoribbons and the presence of the sodium titanate phase and tetragonal ZrO2. For large values of x (greater than 0.50, high amount of zirconium) no morphological changes occurred and the tetragonal ZrO2 phase was observed for samples. Furthermore, only for the product obtained from x equal to 0.15, we observed the presence of three-dimensional flower-like arrangements. Thus, the influence that the Ti/Zr molar ratio of the precursor plays on the phase and morphology of the hydrothermal products obtained is significant. Second Rodrigues, the preferential coordination of Zr4+ ion is between 7 and 8, while for Ti4+ it is always 6. The ionic radius of Zr4+ in ZrO2 is 0.84 , assuming the coordination number equal to 8, and for the Ti4+ in TiO2 it is 0.61 Å , assuming the coordination number equal to 6. This significant difference in the ionic radius results in the differences in behavior of incorporation of the Ti4+ into the lattice of the ZrO2 and of the Zr4+ into the titanate lattice. When Zr4+ was introduced into the titanate structure this ion tends to interrupt the crystal arrangement of the titanate. On the other hand, the Ti4+ ion can

(Newnham, 1967).

(34-415), at angles 25.2° and 30.6°, respectively.

Fig. 5. Diffractograms of synthetized powders by hydrolysis of the alkoxides, after thermal treatment at 500, 600 and 700 °C for 8 h, NTZ and TPT: (a) together (\*= ZT) (Santos et al, 2010) and (b) separately (A=anatase, B= baddeleyite, \* = ZT).

Synthesis and Characterization of Crystalline Zirconium Titanate Obtained by Sol-Gel 311

<sup>ν</sup>**O-H**

**100 °C**

**500 °C**

**Transmittance (u.a.)**

**600 °C**

**700 °C**

100, 500, 600 and 700 °C, without addition HNO3.

Unlike preferred complexing anions such as SO42<sup>−</sup>

functionality (Hoebbel *et al*, 1997).

**O-H vibrations**

4000 3500 3000 2500 2000 1500 1000 500

**Wavenumbers (cm-1)**

Fig. 7. FT-IR of the dried and annealed ZT powders at different temperatures ranging from

With the thermal treatment at the temperatures evaluated in this study, the particle size increased. The mean size of the particle (D50) was 23 µm, determined by LASER diffraction spectrometry for powders after thermal treatment at 600°C. Generally in the sol-gel process, the particles are polydispersed and sometimes they can be multimodal, and/or nonspherical. In the sol-gel particles generally exhibit nanometer size, but in this work, the particle sizes presented in the order of micrometer size due to aggregation between the particles. Research on oxide-based materials have shown that dissolution reactions of titanium are initiated by the surface coordination of the material with H+ and ligands that polarize, weaken and tend to break the metal-oxygen bonds of the surface (Blackwood *et al*, 2002). Therefore, durability of titanium oxide in acidic solutions can be envisaged as occurring by a parallel dissolution mechanism involving H+ and ligands existing in solution.

complexing agent, with very dilute HNO3 known to be non-oxidising (Housecroft, 2005). The pronounced durability of zirconium oxide in nitric acid is well known for nanoparticles and films and can be extended here to zirconium titanate mesoporous materials from the results obtained (Gao *et al*, 1996; Andreeva *et al*, 1961). Moreover, Hoebbel *et al* suggest that a low hydrolytic stability of metal oxide complexes without functionality result in an additional, mostly indefinite number of H+ groups at the metal centres which causes a higher degree of hydrolysis reactions compared to stable complexes with a defined organic

It was possible to prepare ZrTiO4 powders from zirconium *n*-propoxide and titanium isopropoxide, at a Zr:Ti molar ratio of 1:1, in propanol, with and without addition of nitric acid using the sol-gel process at ambient temperature (20 °C) and low thermal treatment.

or Cl<sup>−</sup>

, NO3 −

is a much weaker

be easily introduced into the ZrO2 structure, because of its smaller radius and lower coordination number. However, because of its greater radius and coordination number Zr4+ ions could not be easily introduced into the titanate structure.

Figure 6 and 7 shows spectra obtained by means Fourier transform infrared spectroscopy (FT-IR) analysis of the dried and annealed ZT powders at different temperatures ranging from 100 °C to 800 °C with and without addition of nitric acid, respectively. The bands at 3380 cm-1 and 1565 cm-1 correspond to the vibration of stretching and deformation of the O−H bond due to the absorption of water and coordination water, respectively. As the annealing temperature increased, the formation of these bands gradually decreased, eventually disappearing. The absorption band at 466 cm-1 can not be observed, this band is related to the vibration of the Zr−O bond (Hao *et al*, 2004). According to Devi *et al* the bands between 3.500 and 3.300 cm-1 was assigned to fundamental stretching vibration of hydroxyl groups. Another peak related to hydroxyl group was found at 1.650 cm-1. The set of overlapping peaks in the range of 810–520 cm-1 are related to Zr–O and Zr–O–Ti groups, respectively. From the above results it is indicated the presence of zirconium titanate on 316L SS (AISI stainless steel) substrate was confirmed. Similar results were also observed by Zhu *et al* (2009). The peaks between 500 and 710 cm-1 are related to Ti–O and Zr–O–Ti vibrations.

From the evaluation of the infrared spectra can be observed that the presence of nitric acid influences the absorption bands of OH, Zr−O and Zr−O−Ti. Hoebbel *et al* (1997) suggest that a low hydrolytic stability of metal oxide without functionality result in an additional, mostly indefinite number of H+ groups, of the HNO3 at the metal centres which causes a higher degree of hydrolysis reactions compared to stable complexes with a defined organic functionality.

Fig. 6. FT-IR of the dried and annealed ZT powders at different temperatures ranging from 100, 500, 600 and 700 °C, with addition of nitric acid.

be easily introduced into the ZrO2 structure, because of its smaller radius and lower coordination number. However, because of its greater radius and coordination number Zr4+

Figure 6 and 7 shows spectra obtained by means Fourier transform infrared spectroscopy (FT-IR) analysis of the dried and annealed ZT powders at different temperatures ranging from 100 °C to 800 °C with and without addition of nitric acid, respectively. The bands at 3380 cm-1 and 1565 cm-1 correspond to the vibration of stretching and deformation of the O−H bond due to the absorption of water and coordination water, respectively. As the annealing temperature increased, the formation of these bands gradually decreased, eventually disappearing. The absorption band at 466 cm-1 can not be observed, this band is related to the vibration of the Zr−O bond (Hao *et al*, 2004). According to Devi *et al* the bands between 3.500 and 3.300 cm-1 was assigned to fundamental stretching vibration of hydroxyl groups. Another peak related to hydroxyl group was found at 1.650 cm-1. The set of overlapping peaks in the range of 810–520 cm-1 are related to Zr–O and Zr–O–Ti groups, respectively. From the above results it is indicated the presence of zirconium titanate on 316L SS (AISI stainless steel) substrate was confirmed. Similar results were also observed by Zhu *et al* (2009). The peaks between 500 and 710 cm-1 are related to Ti–O and Zr–O–Ti vibrations.

From the evaluation of the infrared spectra can be observed that the presence of nitric acid influences the absorption bands of OH, Zr−O and Zr−O−Ti. Hoebbel *et al* (1997) suggest that a low hydrolytic stability of metal oxide without functionality result in an additional, mostly indefinite number of H+ groups, of the HNO3 at the metal centres which causes a higher degree of hydrolysis reactions compared to stable complexes with a defined organic

**O-H vibrations**

**Zr-O-Ti**

4000 3500 3000 2500 2000 1500 1000 500

**500 °C Zr-O**

**Wavenumbers** (**cm-1**)

Fig. 6. FT-IR of the dried and annealed ZT powders at different temperatures ranging from

ions could not be easily introduced into the titanate structure.

<sup>ν</sup>**O-H**

**700 °C**

100, 500, 600 and 700 °C, with addition of nitric acid.

**600 °C**

**Transmittance (u.a.)**

**100 °C**

functionality.

Fig. 7. FT-IR of the dried and annealed ZT powders at different temperatures ranging from 100, 500, 600 and 700 °C, without addition HNO3.

With the thermal treatment at the temperatures evaluated in this study, the particle size increased. The mean size of the particle (D50) was 23 µm, determined by LASER diffraction spectrometry for powders after thermal treatment at 600°C. Generally in the sol-gel process, the particles are polydispersed and sometimes they can be multimodal, and/or nonspherical. In the sol-gel particles generally exhibit nanometer size, but in this work, the particle sizes presented in the order of micrometer size due to aggregation between the particles. Research on oxide-based materials have shown that dissolution reactions of titanium are initiated by the surface coordination of the material with H+ and ligands that polarize, weaken and tend to break the metal-oxygen bonds of the surface (Blackwood *et al*, 2002). Therefore, durability of titanium oxide in acidic solutions can be envisaged as occurring by a parallel dissolution mechanism involving H+ and ligands existing in solution. Unlike preferred complexing anions such as SO4 2− or Cl<sup>−</sup> , NO3 − is a much weaker complexing agent, with very dilute HNO3 known to be non-oxidising (Housecroft, 2005). The pronounced durability of zirconium oxide in nitric acid is well known for nanoparticles and films and can be extended here to zirconium titanate mesoporous materials from the results obtained (Gao *et al*, 1996; Andreeva *et al*, 1961). Moreover, Hoebbel *et al* suggest that a low hydrolytic stability of metal oxide complexes without functionality result in an additional, mostly indefinite number of H+ groups at the metal centres which causes a higher degree of hydrolysis reactions compared to stable complexes with a defined organic functionality (Hoebbel *et al*, 1997).

It was possible to prepare ZrTiO4 powders from zirconium *n*-propoxide and titanium isopropoxide, at a Zr:Ti molar ratio of 1:1, in propanol, with and without addition of nitric acid using the sol-gel process at ambient temperature (20 °C) and low thermal treatment.

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The thermodynamics of formation, molar heat capacity, and thermodynamic

Synthesis together with the alkoxides was obtained crystalline orthorhombic phase, confirmed by XRD and an exothermal peak without loss of mass by DTA analysis. When the synthesis was performed with separate alkoxides was not possible to obtain only zirconium titanate, probably due to the fact formed crystalline forms anatase, and baddeleyite observed by XRD, which competes with the formation of ZrTiO4, which can be detected only at 700 °C. The ZrTiO4 powders treated thermally at 700°C present. The thermal analysis showed also a great loss of mass, between 40 and 200 ºC approximately, probably due to dehydration. The peaks in DTA at 350 and 550 °C are related to the removal of organic compounds. The water loss was confirmed in the spectrum FTIR. It was observed that the formation of the bands related to the presence of hydroxyl group showed a decrease with the increase of annealing temperatures, and at 700 °C they disappeared, indicating that the material structure no longer contains zirconium hydroxide. The sol-gel process was efficient in the preparation of ZrTiO4 using the two routes compared, but should be studied the addition of other acids, as nitric acid caused the aggregation of the powers.
