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Insights for phase control in TiO2 nanoparticles from polymeric precursors method
Journal of Alloys and Compounds 466 (2008) 435–438
Insights for phase control in TiO2 nanoparticles from polymeric precursors method Celia M. Ronconi a,1 , Caue Ribeiro b , Luiz O.S. Bulhoes a,2 , Ernesto C. Pereira a,∗ a
LIEC, Universidade Federal de S˜ao Carlos, Departamento de Qu´ımica, Rod. Washington Luiz, km 235, 13565-905 S˜ao Carlos, SP, Brazil b EMBRAPA Instrumenta¸ ca˜ o Agropecu´aria, Rua XV de Novembro, 1452, 13560-970, CP 741 S˜ao Carlos, SP, Brazil Received 14 May 2007; received in revised form 9 November 2007; accepted 16 November 2007 Available online 22 November 2007
Abstract This work reports the synthesis of titanium oxide (TiO2 ) in anatase and rutile phases prepared by the polymeric precursors method. The synthesis was investigated in function of the precursors’ molar ratios and calcination temperature, in long treatment times. The characterization revealed the complete retention of anatase phase in high organic:metal molar ratios. From surface area measurements, this work evidences the existence of a critical size for anatase stabilization, as observed in previous works. © 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Chemical synthesis; Sol–gel synthesis
1. Introduction Titanium oxide (TiO2 ) is a functional material with several technological applications related to its crystalline structure and nanocrystal size and morphology [1–3]. TiO2 occurs in three different crystalline phases, rutile, brookite and anatase. Both anatase and rutile have a tetragonal unit cell, with the anatase phase having a more open structure, and the brookite phase has an orthorhombic structure [4]. Despite rutile is known as the most stable phase at room temperature [5], anatase phase gained attention in the last years by its remarkable activity as photocatalyst for the chemical oxidation of organics [6,7] and its use in dye-sensitized solar cells [1]. These topics motivated several works in order to control the synthesis of TiO2 polymorphs [8–12]. The importance of particle shape [13] and size [14,15] in the stabilization of the phases was only recently explored. Gribb and Banfield [14] experimentally observed a critical size of 14 nm for the stability of
∗
Corresponding author. E-mail address: [email protected] (E.C. Pereira). 1 Present address: Universidade Federal do Rio de Janeiro, Instituto de Qu´ımica, Avenida Athos da Silveira Ramos, Ilha do Fund˜ao, 21941-909 Rio de Janeiro, RJ, Brazil. 2 Also at Centro Universit´ ario Central Paulista, UNICEP, Rua Miguel Petroni, 5111, 13565-905 S˜ao Carlos, SP, Brazil. 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.060
anatase crystals, but phase transformation occurred at larger sizes resulting rutile nanocrystals. In fact, anatase nanocrystals are commonly observed for particle sizes below 15 nm [8,16,11], although higher crystal sizes, from 15 to 50 nm [16,13], have also been reported in the literature. This fact is explained by the dependence of anatase stabilization of several parameters, such as starting material, pH media and synthesis temperature [17]. Another important factor observed in recent studies was the effect of adsorption of counterions in TiO2 surfaces during phase stabilization. Penn and Banfield [18] observed the retention of anatase phase for larger sizes by the contamination of synthesis environment with Cl− . This proposition was confirmed by the theoretical work from Barnard and Zapol [13]. In this sense, an interesting method to investigate these suppositions is the polymeric precursors method [19] or in situ polimerizable complex method [20]. This process is based on the ability of poly-carboxylic acids, particularly citric acid (CA), to form very stable water-soluble complexes. Even cations with a high tendency to become hydrolyzed, such as Ti4+ , can be chelated by CA in a water solution, preventing the hydrolysis and precipitation of hydrous metal oxide. The CA complex thus formed can be immobilized in a solid organic resin through a polyesterification reaction with ethylene glycol (EG). This process leads to the formation of a polymeric precursor with the cations homogeneously distributed in a three-dimensional solid lattice, avoiding precipitation or phase segregation during the
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synthesis of the metal oxide [19]. The method was successfully tested in the synthesis of complex multicomponent oxides [19,20] and deposition of thin oxide films [21]. Those features can lead the method as a good candidate to control the size and surface contamination of TiO2 nanoparticles obtained by calcination routes. Thereof, the main goal of this work is investigate the formation of anatase and rutile phases in TiO2 nanopowders obtained by the polymeric precursors method. Variations in molar ratios of the precursors were done in order to obtain different distances between the primary TiO2 nucleus during calcination, and the treatment temperature was changed in order to verify the interference of surface contamination by the organic subproducts. These results can shed further light on the discussion about phase stabilization of anatase TiO2 , underpinning synthesization strategies and technological applications. 2. Experimental TiO2 powders were prepared by dissolution of the precursor salt (titanium tetraisopropoxide, Hulls-AG) in an aqueous solution of citric acid (Merck), at 80 ◦ C. After complete dissolution and homogenization by magnetic stirring, the polymerization (polyesterification) of the obtained Ti citrate was done by adding ethylene glycol (Merck). The solutions were prepared with molar ratios of 1:8:32, 1:4:16 and 1:2:8 (titanium tetraisopropoxide:citric acid:ethylene glycol). The resins were poured into aluminum oxide crucibles and then subjected to thermogravimetric (TGA) and differential thermal (DTA) analysis using a Netzch model STA 409 C apparatus, at a heating rate of 10 ◦ C min−1 in air atmosphere with a flux of 50 cm3 min−1 . The above resins were thermally treated at 450, 525 and 600 ◦ C (heating rate of 5 ◦ C min−1 and plateau time of 12 h). The titanium oxide powders obtained were characterized by X-ray diffraction using a Siemens X-ray diffractometer ˚ anode. The phase concentramodel D5000, using a Cu (Cu K ␣, λ = 1.5418 A) tion was calculated based on the area of (1 0 1) anatase and (1 1 0) rutile peaks. In this study, the peaks were fitted using a pseudovoigth function in order to calculate the peak area. The measurements of the surface area were carried out in ASAP 2000 equipment using the BET isotherm. The equivalent particle sizes were calculated by the expression size =
6 ρSA
Fig. 1. X-ray diffraction patterns of TiO2 obtained from 1:8:32, 1:4:16 and 1:2:8 solutions thermally treated at 525 ◦ C for 12 h.
[4]—the formation of the most stable polymorph (rutile) is preceded by the unstable (anatase). In general, the Ostwald step rule is observed even at high temperatures. To check this point, in Fig. 3 is presented the effect of the atmosphere on the quantity of anatase phase for the sample prepared at 600 ◦ C. In this figure the 1:4:16 solution was thermally treated at 600 ◦ C under O2 flux and in the presence of a static air atmosphere. The retention of the anatase phase in a O2 -rich atmosphere is expected, as proposed by Shannon and Pask [22], due the formation of oxygen vacancies was avoided. Since the transformation rutile → anatase is unfavorable [3], the retention of anatase phase even at 600 ◦ C is a good indicative that the precursor did not act as a preorganizing agent of the structure in the rutile phase, confirming that the anatase phase formed is preceding the rutile phase. The role of the polymeric precursor in structure organization was discussed by Leite et al. [23], in SrTiO3 synthesis, and by Kakihana et al. for BaTiO3 . Both authors observed by spectroscopic
(1)
where ρ is the oxide density (assumed here as 4.00 g cm−3 indistinctly) and SA is the measured surface area.
3. Results and discussion Fig. 1 shows X-ray diffraction patterns for the TiO2 powders prepared at 525 ◦ C for 12 h. It was observed that the amount of anatase and rutile phases changed considerably as the titanium precursor content change. This fact is more evident in Fig. 2, which illustrates the quantities of rutile phase in the powders prepared at different temperatures in a three-dimensional plot. This figure shows that a significant variation in the quantity of anatase and rutile phases occurred when the amount of the organic part of the precursor solution was modified. However, considering that the ratio between the citric acid and ethylene glycol did not change for the different solutions prepared, important variations in the degree of polymer crosslinking and chains length were not expected. The preferential formation of anatase TiO2 at low temperatures is expected as consequence of the Ostwald step rule
Fig. 2. Amount of rutile phase on TiO2 obtained with different titanium tetraisopropoxide contents as a function of thermal treatment temperature.
C.M. Ronconi et al. / Journal of Alloys and Compounds 466 (2008) 435–438
Fig. 3. X-ray diffraction patterns of TiO2 obtained from the 1:4:16 solution thermally treated at 600 ◦ C for 2 h in the (a) static air atmosphere and (b) presence of oxygen atmosphere with a flux of 1 L min−1 .
data the regular distribution of the different metallic cation along the polymer network formed. However, Kakihana et al. pointed out that the coordination in the short range of the metallic ions is the same expected for the crystalline phase formed. In long range, any ordering was observed, and this result is coherent with the final results of the works: the particles formed appeared as non-crystalline when calcined in temperatures enough only to eliminate the organic phase (around 350 ◦ C). In TiO2 structure both anatase and rutile phases are primary coordinated as TiO6 4− octaedras [5]. The differences between the polymorphs will appear in the organization of such octaedras. Then, it is possible that the Ti4+ cations were six-fold coordinated in the polymeric precursor, but this would not affect the preferential formation of any phase. Those effects from the precursor was also investigated by termogravimetric and differential termoanalysis, presented at Fig. 4. The first derivative of the thermogravimetric curves (DTG) for the resins is observed in the main part of Fig. 4, showing similarity in the weight loss processes. The DTG curves show four main process, with small dislocations for high temperatures following the increment in organic part of the solutions. In the 1:8:32 solution the presence of two small peaks (i and ii) denotes the beginning of different weight loss processes, however, with intensities not significant compared to the main processes (I to IV). The similarities are more evident in Table 1, were the characteristic temperatures of the weight loss processes (peaks I to IV) are compared. In the same way, no significant variations in the DTA curves can be observed for the solutions prepared with
437
Fig. 4. First derivative of the thermogravimetric curves for the (a) 1:2:8, (b) 1:4:16 and (c) 1:8:32 solutions. Heating rate of 10 ◦ C min−1 , air atmosphere flux of 50 cm3 min−1 . (Inset) DTA curves for the same resins.
different amounts of metal salt in the polymeric precursor, as seen in the inset of Fig. 4. For temperatures lower than 200 ◦ C, endothermic processes were observed probably associated with water evaporation, since the polymerization of the resins is a condensation polymerization, i.e., implies in water formation. For temperatures higher than 200 ◦ C a large exothermic process is observed in all the samples, associated to the polymer degradation. In addition, comparing the DTG with the DTA curves, it is possible to observe that the temperature values for the endothermic peaks in the DTA curve in the 100–190 ◦ C range were close to the temperatures in the DTG curves where the beginning of the weight loss process took place. Therefore, it is proposed that the thermal decomposition process is almost the same for the three solutions investigated, and that it plays a minor role in the process of obtaining different quantities of anatase and rutile phases as it was expected, since citric acid and ethylene glycol ratio was kept constant—and in this case, any precursor effect should be negleted.
Table 1 Characteristic temperatures of weight loss processes for the different solutions, from the DTG data following notation in Fig. 4 Solution
I (◦ C)
II (◦ C)
III (◦ C)
IV (◦ C)
1:2:8 1:4:16 1:8:32
136 147 165
185 177 185
243 248 252
323 341 358
Fig. 5. Quantity of the rutile phase as function of the surface area for TiO2 powders prepared.
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Table 2 Surface area measurements, average particle size (by Eq. (1)) and anatase retention for TiO2 powders obtained from different solutions thermally treated at different temperatures for 12 h Temperature (◦ C)
Solution
Area (m2 g−1 )
450
1:8:32 1:4:16 1:2:8
63.27 43.78 35.86
23.7 34.3 41.3
100 100 92.5
525
1:8:32 1:4:16 1:2:8
38.75 26.84 16.85
38.7 55.9 89.0
87.8 73.7 61.5
600
1:8:32 1:4:16 1:2:8
16.48 10.72 4.15
91.0 139.9 361.4
55.9 38.2 17.8
Size (nm)
Anatase (%)
The equivalent sizes were calculated supposing TiO2 density as 4.00 g cm−3 , indistinctly.
A strong indicative of the origin of the phases formation can be observed by surface area measurements. Fig. 5 compares the values of surface area with the anatase retention or rutile formation. Significant variations in the surface area with the phase composition are noticeable. The variation of surface area can also be interpreted in terms of particle sizes. The calculated particle size, as seen in Table 2, shows that the retention of anatase phase is associated with lower sizes, as expected [14]. However, the observed particle sizes of the 1:8:32 and 1:4:16 treated at 450 ◦ C (full anatase retention) are quite bigger than the reported as the critical size for anatase formation (around 13 nm [14]). This interesting fact could be explained as function of eventual surface contamination, stabilizing the phase even in larger sizes. In fact, Yanagisawa and Ovenstone [16] observed anatase nanoparticles around 30 nm using a hydrothermal apparatus. The authors explained the results as an effect of the diffusional growth in the aqueous media, where the surface contamination could act impeding the formation of oxygen vacancies in surface and maintaining large anatase nanoparticles. In the results presented in this work, the particles are formed during the thermal degradation of the polymeric precursor, and this organic phase should be responsible for the reduced mobility of the boundaries by steric impediment, forming nanoparticles. However, after the particles formation, the applied temperatures may induce the fast growth of the nanoparticles by particle–particle contact, and in this case, the growth of the nanoparticles is strictly determined by solid-state interactions, i.e., the growth depends only of diffusion process through the interface between particles. In fact, a fast growth was observed in 1:2:8 solutions, and reduced particle sizes in the solutions with higher organic contents. By these results, the surface contamination probably can break the solid reordering of the pristine surfaces. Finally, it is noticeable the complete retention of anatase phase in surface areas around 44 m2 g−1 , a result that shows experimental conditions to obtain higher anatase particle sizes.
4. Conclusions Different amounts of anatase phase were obtained by polymeric precursor method by modifying the organic content of the precursor solutions, ranging from total retention to 20% anatase phase (80% rutile). The Ostwald step rule was confirmed in the system, i.e., the rutile phases observed were provenient from pristine anatase particles. The results showed that the method is effective for anatase retention in larger ratios organic:metallic salt, probably by the surface contamination resultant from the decomposition of the organic part of the precursor. The phase retention by surface contamination was also indicated by the particle sizes in compositions with full retention of anatase, which are larger than the reported as critical sizes for anatase → rutile transformation. These results can help further strategies for large producion of nanoparticles with adequate size and phase control. Acknowledgements The authors thank FAPESP (process no. 03/09933-8) and CNPq for the financial support. References [1] B. Oregan, M. Gratzel, Nature 353 (1991) 737–740. [2] A. Navrotsky, O.J. Kleppa, J. Am. Ceram. Soc. 50 (1967) 626. [3] M.R. Ranade, A. Navrotsky, H.Z. Zhang, J.F. Banfield, S.H. Elder, A. Zaban, P.H. Borse, S.K. Kulkarni, G.S. Doran, H.J. Whitfield, Proc. Natl. Acad. Sci. U.S.A. 99 (2002) 6476–6481. [4] A. Navrotsky, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 12096–12101. [5] R.D. Shannon, J. Pask, Am. Miner. 49 (1964) 1707–1717. [6] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1) (1993) 341–357. [7] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (3) (1995) 735– 758. [8] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 (1988) 5196–5201. [9] S. Han, S.H. Choi, S.S. Kim, M. Cho, B. Jang, D.Y. Kim, J. Yoon, T. Hyeon, Small 1 (2005) 812–816. [10] J.H. Jean, T.A. Ring, Langmuir 2 (1986) 251–255. [11] G. Oskam, A. Nellore, R.L. Penn, P.C. Searson, J. Phys. Chem. B 107 (2003) 1734–1738. [12] T.J. Trentler, T.E. Denler, J.F. Bertone, A. Agrawal, V.L. Colvin, J. Am. Chem. Soc. 121 (1999) 1613–1614. [13] A.S. Barnard, P. Zapol, Phys. Rev. B 70 (2004), 235403–1–13. [14] A.A. Gribb, J.F. Banfield, Am. Miner. 82 (1997) 717–728. [15] H.Z. Zhang, J.F. Banfield, Chem. Mater. 17 (2005) 3421–3425. [16] K. Yanagisawa, J. Ovenstone, J. Phys. Chem. B 103 (1999) 7781–7787. [17] J. Ragai, W. Lotfi, Colloids Surf. 61 (1991) 97–109. [18] R.L. Penn, J.F. Banfield, Geochim. Cosmochim. Acta 63 (1999) 1549–1557. [19] P.A. Lessing, Am. Ceram. Soc. Bull. 68 (1989) 1002–1007. [20] M. Kakihana, M. Yoshimura, Bull. Chem. Soc. Jpn. 72 (1999) 1427–1443. [21] A.V. Rosario, E.C. Pereira, Thin Solid Films 410 (1–2) (2002) 1–7. [22] R.D. Shannon, J.A. Pask, J. Amer. Ceram. Soc. Phys. 48 (1965) 391–398. [23] E.R. Leite, C.M.G. Sousa, E. Longo, J.A. Varela, Ceram. Int. 21 (1995) 143–152.
Insights for phase control in TiO2 nanoparticles from polymeric precursors method Celia M. Ronconi a,1 , Caue Ribeiro b , Luiz O.S. Bulhoes a,2 , Ernesto C. Pereira a,∗ a
LIEC, Universidade Federal de S˜ao Carlos, Departamento de Qu´ımica, Rod. Washington Luiz, km 235, 13565-905 S˜ao Carlos, SP, Brazil b EMBRAPA Instrumenta¸ ca˜ o Agropecu´aria, Rua XV de Novembro, 1452, 13560-970, CP 741 S˜ao Carlos, SP, Brazil Received 14 May 2007; received in revised form 9 November 2007; accepted 16 November 2007 Available online 22 November 2007
Abstract This work reports the synthesis of titanium oxide (TiO2 ) in anatase and rutile phases prepared by the polymeric precursors method. The synthesis was investigated in function of the precursors’ molar ratios and calcination temperature, in long treatment times. The characterization revealed the complete retention of anatase phase in high organic:metal molar ratios. From surface area measurements, this work evidences the existence of a critical size for anatase stabilization, as observed in previous works. © 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Chemical synthesis; Sol–gel synthesis
1. Introduction Titanium oxide (TiO2 ) is a functional material with several technological applications related to its crystalline structure and nanocrystal size and morphology [1–3]. TiO2 occurs in three different crystalline phases, rutile, brookite and anatase. Both anatase and rutile have a tetragonal unit cell, with the anatase phase having a more open structure, and the brookite phase has an orthorhombic structure [4]. Despite rutile is known as the most stable phase at room temperature [5], anatase phase gained attention in the last years by its remarkable activity as photocatalyst for the chemical oxidation of organics [6,7] and its use in dye-sensitized solar cells [1]. These topics motivated several works in order to control the synthesis of TiO2 polymorphs [8–12]. The importance of particle shape [13] and size [14,15] in the stabilization of the phases was only recently explored. Gribb and Banfield [14] experimentally observed a critical size of 14 nm for the stability of
∗
Corresponding author. E-mail address: [email protected] (E.C. Pereira). 1 Present address: Universidade Federal do Rio de Janeiro, Instituto de Qu´ımica, Avenida Athos da Silveira Ramos, Ilha do Fund˜ao, 21941-909 Rio de Janeiro, RJ, Brazil. 2 Also at Centro Universit´ ario Central Paulista, UNICEP, Rua Miguel Petroni, 5111, 13565-905 S˜ao Carlos, SP, Brazil. 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.060
anatase crystals, but phase transformation occurred at larger sizes resulting rutile nanocrystals. In fact, anatase nanocrystals are commonly observed for particle sizes below 15 nm [8,16,11], although higher crystal sizes, from 15 to 50 nm [16,13], have also been reported in the literature. This fact is explained by the dependence of anatase stabilization of several parameters, such as starting material, pH media and synthesis temperature [17]. Another important factor observed in recent studies was the effect of adsorption of counterions in TiO2 surfaces during phase stabilization. Penn and Banfield [18] observed the retention of anatase phase for larger sizes by the contamination of synthesis environment with Cl− . This proposition was confirmed by the theoretical work from Barnard and Zapol [13]. In this sense, an interesting method to investigate these suppositions is the polymeric precursors method [19] or in situ polimerizable complex method [20]. This process is based on the ability of poly-carboxylic acids, particularly citric acid (CA), to form very stable water-soluble complexes. Even cations with a high tendency to become hydrolyzed, such as Ti4+ , can be chelated by CA in a water solution, preventing the hydrolysis and precipitation of hydrous metal oxide. The CA complex thus formed can be immobilized in a solid organic resin through a polyesterification reaction with ethylene glycol (EG). This process leads to the formation of a polymeric precursor with the cations homogeneously distributed in a three-dimensional solid lattice, avoiding precipitation or phase segregation during the
436
C.M. Ronconi et al. / Journal of Alloys and Compounds 466 (2008) 435–438
synthesis of the metal oxide [19]. The method was successfully tested in the synthesis of complex multicomponent oxides [19,20] and deposition of thin oxide films [21]. Those features can lead the method as a good candidate to control the size and surface contamination of TiO2 nanoparticles obtained by calcination routes. Thereof, the main goal of this work is investigate the formation of anatase and rutile phases in TiO2 nanopowders obtained by the polymeric precursors method. Variations in molar ratios of the precursors were done in order to obtain different distances between the primary TiO2 nucleus during calcination, and the treatment temperature was changed in order to verify the interference of surface contamination by the organic subproducts. These results can shed further light on the discussion about phase stabilization of anatase TiO2 , underpinning synthesization strategies and technological applications. 2. Experimental TiO2 powders were prepared by dissolution of the precursor salt (titanium tetraisopropoxide, Hulls-AG) in an aqueous solution of citric acid (Merck), at 80 ◦ C. After complete dissolution and homogenization by magnetic stirring, the polymerization (polyesterification) of the obtained Ti citrate was done by adding ethylene glycol (Merck). The solutions were prepared with molar ratios of 1:8:32, 1:4:16 and 1:2:8 (titanium tetraisopropoxide:citric acid:ethylene glycol). The resins were poured into aluminum oxide crucibles and then subjected to thermogravimetric (TGA) and differential thermal (DTA) analysis using a Netzch model STA 409 C apparatus, at a heating rate of 10 ◦ C min−1 in air atmosphere with a flux of 50 cm3 min−1 . The above resins were thermally treated at 450, 525 and 600 ◦ C (heating rate of 5 ◦ C min−1 and plateau time of 12 h). The titanium oxide powders obtained were characterized by X-ray diffraction using a Siemens X-ray diffractometer ˚ anode. The phase concentramodel D5000, using a Cu (Cu K ␣, λ = 1.5418 A) tion was calculated based on the area of (1 0 1) anatase and (1 1 0) rutile peaks. In this study, the peaks were fitted using a pseudovoigth function in order to calculate the peak area. The measurements of the surface area were carried out in ASAP 2000 equipment using the BET isotherm. The equivalent particle sizes were calculated by the expression size =
6 ρSA
Fig. 1. X-ray diffraction patterns of TiO2 obtained from 1:8:32, 1:4:16 and 1:2:8 solutions thermally treated at 525 ◦ C for 12 h.
[4]—the formation of the most stable polymorph (rutile) is preceded by the unstable (anatase). In general, the Ostwald step rule is observed even at high temperatures. To check this point, in Fig. 3 is presented the effect of the atmosphere on the quantity of anatase phase for the sample prepared at 600 ◦ C. In this figure the 1:4:16 solution was thermally treated at 600 ◦ C under O2 flux and in the presence of a static air atmosphere. The retention of the anatase phase in a O2 -rich atmosphere is expected, as proposed by Shannon and Pask [22], due the formation of oxygen vacancies was avoided. Since the transformation rutile → anatase is unfavorable [3], the retention of anatase phase even at 600 ◦ C is a good indicative that the precursor did not act as a preorganizing agent of the structure in the rutile phase, confirming that the anatase phase formed is preceding the rutile phase. The role of the polymeric precursor in structure organization was discussed by Leite et al. [23], in SrTiO3 synthesis, and by Kakihana et al. for BaTiO3 . Both authors observed by spectroscopic
(1)
where ρ is the oxide density (assumed here as 4.00 g cm−3 indistinctly) and SA is the measured surface area.
3. Results and discussion Fig. 1 shows X-ray diffraction patterns for the TiO2 powders prepared at 525 ◦ C for 12 h. It was observed that the amount of anatase and rutile phases changed considerably as the titanium precursor content change. This fact is more evident in Fig. 2, which illustrates the quantities of rutile phase in the powders prepared at different temperatures in a three-dimensional plot. This figure shows that a significant variation in the quantity of anatase and rutile phases occurred when the amount of the organic part of the precursor solution was modified. However, considering that the ratio between the citric acid and ethylene glycol did not change for the different solutions prepared, important variations in the degree of polymer crosslinking and chains length were not expected. The preferential formation of anatase TiO2 at low temperatures is expected as consequence of the Ostwald step rule
Fig. 2. Amount of rutile phase on TiO2 obtained with different titanium tetraisopropoxide contents as a function of thermal treatment temperature.
C.M. Ronconi et al. / Journal of Alloys and Compounds 466 (2008) 435–438
Fig. 3. X-ray diffraction patterns of TiO2 obtained from the 1:4:16 solution thermally treated at 600 ◦ C for 2 h in the (a) static air atmosphere and (b) presence of oxygen atmosphere with a flux of 1 L min−1 .
data the regular distribution of the different metallic cation along the polymer network formed. However, Kakihana et al. pointed out that the coordination in the short range of the metallic ions is the same expected for the crystalline phase formed. In long range, any ordering was observed, and this result is coherent with the final results of the works: the particles formed appeared as non-crystalline when calcined in temperatures enough only to eliminate the organic phase (around 350 ◦ C). In TiO2 structure both anatase and rutile phases are primary coordinated as TiO6 4− octaedras [5]. The differences between the polymorphs will appear in the organization of such octaedras. Then, it is possible that the Ti4+ cations were six-fold coordinated in the polymeric precursor, but this would not affect the preferential formation of any phase. Those effects from the precursor was also investigated by termogravimetric and differential termoanalysis, presented at Fig. 4. The first derivative of the thermogravimetric curves (DTG) for the resins is observed in the main part of Fig. 4, showing similarity in the weight loss processes. The DTG curves show four main process, with small dislocations for high temperatures following the increment in organic part of the solutions. In the 1:8:32 solution the presence of two small peaks (i and ii) denotes the beginning of different weight loss processes, however, with intensities not significant compared to the main processes (I to IV). The similarities are more evident in Table 1, were the characteristic temperatures of the weight loss processes (peaks I to IV) are compared. In the same way, no significant variations in the DTA curves can be observed for the solutions prepared with
437
Fig. 4. First derivative of the thermogravimetric curves for the (a) 1:2:8, (b) 1:4:16 and (c) 1:8:32 solutions. Heating rate of 10 ◦ C min−1 , air atmosphere flux of 50 cm3 min−1 . (Inset) DTA curves for the same resins.
different amounts of metal salt in the polymeric precursor, as seen in the inset of Fig. 4. For temperatures lower than 200 ◦ C, endothermic processes were observed probably associated with water evaporation, since the polymerization of the resins is a condensation polymerization, i.e., implies in water formation. For temperatures higher than 200 ◦ C a large exothermic process is observed in all the samples, associated to the polymer degradation. In addition, comparing the DTG with the DTA curves, it is possible to observe that the temperature values for the endothermic peaks in the DTA curve in the 100–190 ◦ C range were close to the temperatures in the DTG curves where the beginning of the weight loss process took place. Therefore, it is proposed that the thermal decomposition process is almost the same for the three solutions investigated, and that it plays a minor role in the process of obtaining different quantities of anatase and rutile phases as it was expected, since citric acid and ethylene glycol ratio was kept constant—and in this case, any precursor effect should be negleted.
Table 1 Characteristic temperatures of weight loss processes for the different solutions, from the DTG data following notation in Fig. 4 Solution
I (◦ C)
II (◦ C)
III (◦ C)
IV (◦ C)
1:2:8 1:4:16 1:8:32
136 147 165
185 177 185
243 248 252
323 341 358
Fig. 5. Quantity of the rutile phase as function of the surface area for TiO2 powders prepared.
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Table 2 Surface area measurements, average particle size (by Eq. (1)) and anatase retention for TiO2 powders obtained from different solutions thermally treated at different temperatures for 12 h Temperature (◦ C)
Solution
Area (m2 g−1 )
450
1:8:32 1:4:16 1:2:8
63.27 43.78 35.86
23.7 34.3 41.3
100 100 92.5
525
1:8:32 1:4:16 1:2:8
38.75 26.84 16.85
38.7 55.9 89.0
87.8 73.7 61.5
600
1:8:32 1:4:16 1:2:8
16.48 10.72 4.15
91.0 139.9 361.4
55.9 38.2 17.8
Size (nm)
Anatase (%)
The equivalent sizes were calculated supposing TiO2 density as 4.00 g cm−3 , indistinctly.
A strong indicative of the origin of the phases formation can be observed by surface area measurements. Fig. 5 compares the values of surface area with the anatase retention or rutile formation. Significant variations in the surface area with the phase composition are noticeable. The variation of surface area can also be interpreted in terms of particle sizes. The calculated particle size, as seen in Table 2, shows that the retention of anatase phase is associated with lower sizes, as expected [14]. However, the observed particle sizes of the 1:8:32 and 1:4:16 treated at 450 ◦ C (full anatase retention) are quite bigger than the reported as the critical size for anatase formation (around 13 nm [14]). This interesting fact could be explained as function of eventual surface contamination, stabilizing the phase even in larger sizes. In fact, Yanagisawa and Ovenstone [16] observed anatase nanoparticles around 30 nm using a hydrothermal apparatus. The authors explained the results as an effect of the diffusional growth in the aqueous media, where the surface contamination could act impeding the formation of oxygen vacancies in surface and maintaining large anatase nanoparticles. In the results presented in this work, the particles are formed during the thermal degradation of the polymeric precursor, and this organic phase should be responsible for the reduced mobility of the boundaries by steric impediment, forming nanoparticles. However, after the particles formation, the applied temperatures may induce the fast growth of the nanoparticles by particle–particle contact, and in this case, the growth of the nanoparticles is strictly determined by solid-state interactions, i.e., the growth depends only of diffusion process through the interface between particles. In fact, a fast growth was observed in 1:2:8 solutions, and reduced particle sizes in the solutions with higher organic contents. By these results, the surface contamination probably can break the solid reordering of the pristine surfaces. Finally, it is noticeable the complete retention of anatase phase in surface areas around 44 m2 g−1 , a result that shows experimental conditions to obtain higher anatase particle sizes.
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