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Study of crystallization behavior of Ce4+-modified titania gels
Scripta Materialia 50 (2004) 885–889 www.actamat-journals.com
Study of crystallization behavior of Ce4þ-modified titania gels Z.M. Shi *, W.G. Yu, Xin Bayar College of Science, Inner Mongolia Polytechnic University, Aimin road 221, Hohhot 010062, China Received 27 October 2003; received in revised form 24 November 2003; accepted 5 December 2003
Abstract The addition of Ce4þ has a significant effect on the temperatures for conversion of gels into anatase and anatase into rutile, and depresses the growth of anatase crystal. It is suggested that Ce4þ addition helps to prepare ultrafine anatase crystals and enhance the thermal stability of anatase. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Sol–gel; Differential thermal analysis (DTA); X-ray diffraction (XRD); Rare earth; Functional ceramics
1. Introduction Titania ceramics and films have aroused great interest in recent years because of their extensive application in preparation of photocatalysts [1–5], catalyst carriers [6], gas sensors [7–9] and photoelectric components [10–13]. The physicochemical activities of titania are associated with the microstructure of titania, which, in turn, depends to a great extent on phase transformation and the size of the crystals. Many of the studies have been, naturally, focused on the crystallization behavior. However, the preparation technique and element doping are also two major factors affecting the crystallization processes. Karthikeyan and Almeida [14] used sol–gel/spin coating and subsequent heat-treatment to prepare titania films on single crystal silicon wafers. Liu et al. [15] prepared TiO2 nanoparticles using a sol–gel method. TiO2 particles were obtained by redissolving dry-gel treated at 100 °C, rather than by calcining. Wu et al. [16] prepared nanosized TiO2 particles from Ti(SO4 )2 solution by forced hydrolysis under boiling reflux condition. They found that the reactant concentration and acidity have an obvious effect on the size of TiO2 particles. Liao et al. [17] used high-pressure/low-temperature sintering to produce bulk nanocrystalline rutile with a high density and a grain size less than 20 nm. Gouma et al. [18] studied the stability of thin films of nanostructured *
Corresponding author. Tel.: +86-471-657-6143; fax: +86-471-6503298. E-mail address: [email protected] (Z.M. Shi).
titania at elevated temperatures. The initial films consisted of nanocrystalline anatase and were exposured to temperatures higher than 400 °C, resulting in the nucleation and rapid growth of rutile grains. Nair et al. [19] found that Cu2þ and Ni2þ enhanced the transformation of anatase to rutile; La3þ retarded the transformation and densification. Wang et al. [20] synthesized Fe3þ -doped titania powders in a flame aerosol reactor. It was confirmed that Fe3þ was incorporated into the titania lattice and promoted the conversion of anatase to rutile, and that the crystal size of the titania particles decreased with the increase of iron dopant. Bonini et al. [21] proved that the addition of dopants of Nb, Ga and Ta inhibits the grain growth by suppressing the conversion of anatase to rutile for nanostructured thick films of the doped TiO2 fabricated by screen-printing technology. Rare earths as modifiers have been widely used to improve the properties of ceramics owing to their extremely active physical and chemical properties [22,23]. However, few works have dealt with their application in the preparation of titania films and ceramics. In previous work [24–27], it has been confirmed that the addition of Ce4þ /CeO2 decreases the temperature of phase transformation of a sol–gel derived amorphous phase to a cordierite product, depresses the growth of cordierite crystals, improves the density of cordierite ceramics and makes available the preparation of cordierite ceramics with unique properties. Owing to the simplicity of the operation, the sol–gel method has been widely used for the preparation of titania films and fine ceramics [28–30]. Since the effects
1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.12.006
886
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889
of Ce4þ /CeO2 on the phase transformation, microstructure and properties of cordierite ceramics are quite desirable, it is intended to use the sol–gel method to prepare Ce4þ -modified TiO2 gels and to study their crystallization behavior in terms of the crystallization temperature and the growth character of anatase crystals. Further, it is expected that the addition of Ce4þ can enhance the sintering process of the gels and promote the preparation of ultrafine anatase crystals.
2. Experimental procedures 2.1. Preparation of samples All of the gelation procedures were performed at room temperature (25 °C) in air and through intense stirring. Titanium tetraisopropoxide and cerium nitrate (AR) were used as the precursory materials. Titanium tetraisopropoxide (120 ml) was first hydrolyzed in 120 ml of absolute alcohol. Then, the hydrolyzed titanium tetraisopropoxide sol was evenly divided into three parts and injected respectively into three beakers. Different amounts of cerium nitrate were added into these beakers (0, 1.2, 2.6 g respectively) to obtain colloidal sols. After ageing for 3 h, dilute acetic acid (acid: water ¼ 10 vol%) was titrated into the sols until gels are formed. Afterwards, the gels were aged for another 24 h and calcinated at 100 °C, to yield transparent xerogels with different colors, which were then sintered at different temperatures for different holding times in air. The sintered products were finely ground by a ball-mill and screened into powders with a granularity range of 74–63 lm (ASTM E11-58T). Thus, the samples for XRD analysis are made ready. The compositions are listed in Table 1.
Based on the data examined by XRD, the size of anatase crystals is calculated by using the Scherrer equation [31]: D¼
0:89k B cos h
ð1Þ
where, D is the size of crystals; k––the wavelength of CuKa1 radiation; h––the diffraction angle corresponding to a specified diffraction peak; Bð¼ Bs Br Þ––the width (rad) of the diffraction angle at half of the peak height, of which Bs is that measured for the samples; and Br ––for the reference sample of quartz. This method applies only to the case in which the size is smaller than 100 nm, and the smaller the crystal size, the more accurate value D will be. Thus, by measuring h and Bs of the (0 1 0) peak of anatase, the size of anatase crystals can be calculated by using the above equation.
3. Results 3.1. Phase transformations Fig. 1 presents the DTA curves of the three samples. The exothermic peaks at about 280–300 °C are the result of the oxidation of organic compounds in the samples, while the exothermic peaks at above 300 °C represent the reactions relative to crystallization of the gels to anatase. It can be found that, along with the increase of Ce4þ content, the crystallization temperature appears to rise first and then drop, indicating that a small quantity of Ce4þ causes an increase of the crystallization temperature while a larger quantity of Ce4þ makes it decrease. Moreover, the fact that the exothermic peaks are reduced in temperature and broaden gradually with the increase of Ce4þ content demonstrates that the addition
2.2. Examination procedures The crystallization process and weight loss of samples were examined by a differential thermal analyzer (DTA/ TG, Universal V2.5H), using a-Al2 O3 powder as the reference sample, in atmospheric condition and at a heating rate of 10 °C/min. The phases formed in the samples were identified by an X-ray powder diffractometer (XRD, Rigaku D/max-2400X), with copper K radiation, 40 kV, 120 mA and at a scanning speed of 4°/ min.
Temperature Difference,oC
332.1 285 378.0 10CT
294 357.5
5CT 282
0CT
Table 1 Reduced composition (wt.%) of samples Sample
TiO2
CeO2
OCT 5CT 10CT
100 95 90
0 5 10
150
300
450
600
Temperature,oC Fig. 1. DTA curves of samples.
750
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889 + (010) Rutile TiO2
Intensity
.
o
550 C
..
+
+
+
+
++
+
+
350 C ++
++
+ o
450 C
+
o
20
30
40
50
60
2θ / o
(a) + (010)
+ Anatase TiO2
o
550 C
+
+
+
+
++
+
+
++
++
+ o
Intensity
450 C
+ o
350 C
20
30
40
(b)
50
60
2θ / o (010)
+ Anatase TiO2
o
550 C
o
450 C
+
o
350 C
+ 20
30
++
+ 40
50
60
2θ / o
(c)
Fig. 3. XRD patterns of samples sintered at different temperature for one hour (a) sample OCT, (b) sample 5CT and (c) sample 10CT.
+
100
(010)
.
o
600 C+3h +
+ Anatase TiO2 Rutile TiO2 +
++
+
++
10CT
+
90 +
80
126
70
.
377
+
+
60 0
200
5CT
+
.
Intensity
Weight percentage, %
. .. . .
+ Anatase TiO2
Intensity
of Ce4þ reduces the speed of the crystallization of anatase. This ratiocination can be confirmed by the XRD analysis in latter sections. However, the exothermic peaks representing the conversion of anatase to rutile have not been identified in the present test. This phenomenon was also found for the 3 mol% Yttria-doped titania gels [32], for which the peak of rutile was not identified in the DTA curve, but rutile appears at about 700 °C in the XRD curve So, it can be considered that, the heat of the transformation of anatase to rutile, especially for the case of Ce4þ -modifying, is so small that rutile is difficult to be identified by the DTA method. The thermo-gravimetric (TG) curve of sample 5CT is shown in Fig. 2. The gel loses weight rapidly before 100 °C and slowly afterwards, which corresponds respectively to the volatilization of ethanol and the oxidation of hydroxyls and carbon. This indicates that the organic compositions can be completely removed from the samples when they are calcinated at above 400 °C. Therefore, this is the lowest sintering temperature for preparing anatase crystals with high purity. Fig. 3 shows the XRD patterns for the crystallization of the gels at different sintering temperatures and with different Ce4þ additions. With an increase of temperature, the height of the main peak (0 1 0) of anatase in all the samples increases monotonically. The difference consists in that, for sample OCT, the main peak height increases quite rapidly, so that a small amount of rutile at 550 °C appears (Fig. 3(a)); while for samples 5CT and 10CT, the increase of heights is not significant, nor is any rutile formed (Fig. 3(b) and (c)) even when sintering is done at 600 °C (Fig. 4). In contrast, only little anatase remains in sample OCT at this temperature. This indicates that Ce4þ addition inhibits both conversions of the gels into anatase and of anatase into rutile. The XRD results of samples sintered for different holding times are shown in Fig. 5. It can be observed that increasing holding time strongly affects the crystallization for sample OCT, but it has little effect on samples 5CT and 10CT. Since the sharpening of diffraction peaks is attributed to the coarsening of crystals, it is obvious that the addition of Ce4þ can effectively
887
400
600 o
Temperature, C Fig. 2. TG curve of sample 5CT.
800
20
+ 30
. . . 40
2θ /
.
OCT
. + 50
60
ο
Fig. 4. XRD patterns of samples sintered at 600 °C for 3 h.
888
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889 1000
+ : Anatase
Intensity
o
5CT
Granularity, A
800
0CT
o
600
600 C+3h
400 200
+
+
0
10CT
+ ++
+ 20
30
40
50
20
60
30
+
+ ++
40
50
0
5
10
CeO2, wt%
60
Fig. 8. Relation of granularity of anatase to Ce4þ addition.
2θ / O
2θ / (a) 30 min O
(b) 180 min
Fig. 5. XRD patterns of sample sintered at 450 °C for different holding times.
inhibit the growth of anatase crystals and the formation of rutile. 3.2. Growth of anatase crystals Figs. 6 and 7 show the relation of granularity to the sintering temperature and to the holding time respectively. Increasing the sintering temperature and holding time result in a remarkable growth of anatase in sample OCT but an insignificant growth in samples 5CT and
10CT. This proves that Ce4þ addition does have a considerable effect on the inhibition of the growth of anatase. Therefore, unlike Ce4þ -free anatase, Ce4þ modified anatase is stable even at temperatures as high as 600 °C, since neither the conversion into rutile nor crystal growth occur. Fig. 8 shows the relationship of the granularity of anatase crystals and the Ce4þ additions. It can be noticed that 5 wt.% of CeO2 is sufficient to inhibit the growth of anatase.
4. Discussion 4.1. Variation of crystallization temperature of anatase
O
Granularity, A
400 OCT 5CT 10CT
300
200
100
0 350
400
450
500
550
Sintering temperature,oC Fig. 6. Relation of granularity of anatase to sintering temperature (holding for 1 h).
o
Granularity, A
200 0CT 5CT 10CT
160
120
80 30
60
90
120
150
180
Holding time, min Fig. 7. Relation of granularity of anatase to holding time (sintered at 450 °C).
Amorphous TiO2 in xerogels possesses a network structure, in which tetrahedrons of [TiO4 ] should be connected by bridging oxygen. However, in such a network there, certainly, exist some kinds of discontinuation, such as gaps, non-bridging oxygen, remaining hydroxyls and other organic components, which are likely to result from the imperfect sintering of the preparation. Moreover, since the radius of Ce4þ (six-coordination, is much larger than that of Ti4þ (four-coordi0.87 A) [33], Ce4þ can only be located in the gaps. nation, 0.42 A) In this case, Ce4þ connects the hydroxyls and nonbridging oxygen to the tetrahedrons. Thus, the continuity of the amorphous network is enhanced due to the addition of Ce4þ . The compacted network will certainly cause an increase in the crystallization temperature of anatase. This phenomenon probably occurs when only a small quantity of Ce4þ is added, for example, to sample 5CT. On the other hand, when excessive Ce4þ is added to the amorphous TiO2 , the network may get loosened by the extra Ce4þ , consequently, the continuity of the network will be reduced. Besides, because the ultrafine CeO2 particles can precipitate prior to the crystallization of anatase when excessive Ce4þ is added [12], it may play a role as the nucleation site of anatase, resulting in the easier formation of anatase than in sample 5CT. Thus, the crystallization temperature of anatase decreases again (in the case of sample 10CT).
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889
4.2. Growth of anatase crystals and their transformation into rutile 4þ
As regard to the question why Ce can inhibit the growth of anatase crystals, it can be considered the solid solution and solid-state diffusion of Ce4þ in anatase. Anatase is a typical n-type semiconductor, in which there are oxygen vacancies. Although Ce4þ can dissolve in anatase [34], its solubility in anatase is much smaller than that in the amorphous TiO2 . As the anatase crystals grow, extra Ce4þ ions have to be discharged out of the crystals through solid diffusion. However, the diffusion speed of Ce4þ is rather slow because of its large radius. Thus, Ce4þ ions are enriched around the anatase crystals, suppressing Ti4þ diffusion towards the crystals. As a result, the growth is slowed down or completely terminated, even for sample 10CT with a relatively low crystallization temperature. In addition, the diffusion of Ce4þ also participates in the conversion of anatase into rutile. Rutile has a more compact structure than anatase [35,36], therefore, only when Ce4þ is discharged from anatase, can the conversion to rutile proceed. For the same reason as mentioned above, Ce4þ addition also inevitably retards the conversion of anatase into rutile.
5. Conclusions With the increase of Ce4þ addition, the crystallization temperature of TiO2 gels converting into anatase first increases and then decreases. Even a small quantity of Ce4þ addition can significantly inhibit the growth of anatase crystals and the conversions both of TiO2 gels into anatase and of anatase into rutile. Among the three samples, the best crystallinity has been observed in the sample with 5 wt.% of CeO2 when sintered at 450 °C for 3 h. Anatase crystals with sizes of the order of nanometer can be obtained for the Ce4þ modified samples, and their high-temperature stability is much higher than that of Ce4þ -free titania. It is suggested that the influence of Ce4þ to inhibiting the crystallizations of TiO2 gels to anatase and of anatase to rutile is correlated to the difficulty of diffusion of Ti4þ towards them, which is caused by the difference in the solubility of Ce4þ in amorphous, anatase and rutile crystals.
Acknowledgements The project is supported by National Natural Science Foundation of China (NSFC), No. 50262002, China.
889
References [1] Shi LY, Zhang Y, Fang DY, Li CZ, C H. J Mater Synth Proc 1999;7:357–63. [2] Ding Z, Hu X, Lu GQ, Yue PL. Langmuir 2000;16:6216–22. [3] Grzechulska J, Hamerski M, Morawski A, Antoni W. Water Res 2000;34:1638–44. [4] Sano T, Negishi N, Kutsuma S, Takeuchi K. J Mol Cat, A: Chemical 2001;168:233–40. [5] Reddy BM, Ganesh I. J Mol Cat, A: Chemical 2001;169:207–23. [6] Rynkowski J, Farbotko J, Touroude R, Hilaire L. Appl Cat, A: General 2000;203:335–48. [7] Sharma RK, Bhatnagar MC. Sensor Actuat B: Chem 1999;56:215–9. [8] Akbar SA, Younkman LB. J Elec Soc 1997;144:1750–3. [9] Gouma PI, Dutta PK, Mills MJ. Nanostruct Mater 1999;11:1231– 7. [10] de Tacconi NR, Mrkic M, Rajeshwar K. Langmuir 2000;16:8426– 31. [11] Fick J, Martucci A, Guglielmi M, Schell J. Fiber Integrat Opt 2000;19:43–56. [12] Avellaneda CO, Pawlicka A. Thin Solid Films 1998;335:245–8. [13] Ghodsi FE, Tepehan FZ, Tepehan GG. Electrochim Acta 1999;44:3127–36. [14] Karthikeyan A, Almeida RM. J Non-Cryst Solids 2000;274:169– 74. [15] Liu XH, Yang J, Wang L, Yang XJ, Lu LD, Wang X. Mater Sci Eng A: Struct Mater 2000;289:241–5. [16] Wu RT, Wei Y, Zhang YF. Mater Res Bull 1999;34:2131–5. [17] Liao SC, Colaizzi J, Chen Y, Kear BH, Mayo WE. J Am Ceram Soc 2000;83:2163–9. [18] Gouma PI, Dutta PK, Mills MJ. Nanostruct Mater 1999;11:1231– 7. [19] Nair J, Nair P, Mizukami F, Oosawa Y, Okubo T. Mater Res Bull 1999;34:1275–90. [20] Wang ZM, Yang G, Biswas P, Bresser W. Powder Tech 2001;114:197–204. [21] Bonini N, Carotta MC, Chiorino A, Guidi V, Malagu C, Martinelli G, et al. Sensor Actuat B: Chem 2000;68:274–80. [22] Subbarrao EC, Wallance WE. Science and Technology of Rare Earth Materials. New York: Academic Press; 1980. [23] Gschneidner KA, Jr, Egring LR. Handbook on the physics and chemistry of rare earths, vol. 3, Non metallic compounds. New York: North-Holland Publishing Company; 1979. [24] Shi ZM, Liang KM, Zhang Q, Gu SR. J Mater Sci 2001;36:1–4. [25] Shi ZM, Liu Y, Yang WY, Liang KM, Gu SR. J Eur Ceram Soc 2002;22:1251–6. [26] Shi ZM, Liang KM, Gu SR. Mater Lett 2001;51:68–72. [27] Shi ZM, Pan F, Liu DY, Liang KM, Gu SR. Mater Lett 2002;57(2):409–13. [28] Bersani D, Lottici PP, Montenero A. J Mater Sci 2000;35:4301–5. [29] Viitala RI, Langlet M, Simola J, Linden M, Rosenholm JB. Thin Solid Films 2000;368:35–40. [30] Liu XH, Yang J, Wang L, Yang XJ, Lu LD, Wang X. Mater Sci Eng A: Struct Mater 2000;289:241–5. [31] Plug HP, Alexander LE. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. second ed. New York: Willey; 1974. [32] Wong YT, Wong NB, Shi L. J Mater Sci 2003;38:1973–7. [33] Shannon RD. Acta Crystall A 1976;32:751–67. [34] Lin J, Yu JC. J Photochem Photobio A: Chem 1998;116:63–7. [35] JSPD Card 211272. [36] JSPD Card 211276.
Study of crystallization behavior of Ce4þ-modified titania gels Z.M. Shi *, W.G. Yu, Xin Bayar College of Science, Inner Mongolia Polytechnic University, Aimin road 221, Hohhot 010062, China Received 27 October 2003; received in revised form 24 November 2003; accepted 5 December 2003
Abstract The addition of Ce4þ has a significant effect on the temperatures for conversion of gels into anatase and anatase into rutile, and depresses the growth of anatase crystal. It is suggested that Ce4þ addition helps to prepare ultrafine anatase crystals and enhance the thermal stability of anatase. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Sol–gel; Differential thermal analysis (DTA); X-ray diffraction (XRD); Rare earth; Functional ceramics
1. Introduction Titania ceramics and films have aroused great interest in recent years because of their extensive application in preparation of photocatalysts [1–5], catalyst carriers [6], gas sensors [7–9] and photoelectric components [10–13]. The physicochemical activities of titania are associated with the microstructure of titania, which, in turn, depends to a great extent on phase transformation and the size of the crystals. Many of the studies have been, naturally, focused on the crystallization behavior. However, the preparation technique and element doping are also two major factors affecting the crystallization processes. Karthikeyan and Almeida [14] used sol–gel/spin coating and subsequent heat-treatment to prepare titania films on single crystal silicon wafers. Liu et al. [15] prepared TiO2 nanoparticles using a sol–gel method. TiO2 particles were obtained by redissolving dry-gel treated at 100 °C, rather than by calcining. Wu et al. [16] prepared nanosized TiO2 particles from Ti(SO4 )2 solution by forced hydrolysis under boiling reflux condition. They found that the reactant concentration and acidity have an obvious effect on the size of TiO2 particles. Liao et al. [17] used high-pressure/low-temperature sintering to produce bulk nanocrystalline rutile with a high density and a grain size less than 20 nm. Gouma et al. [18] studied the stability of thin films of nanostructured *
Corresponding author. Tel.: +86-471-657-6143; fax: +86-471-6503298. E-mail address: [email protected] (Z.M. Shi).
titania at elevated temperatures. The initial films consisted of nanocrystalline anatase and were exposured to temperatures higher than 400 °C, resulting in the nucleation and rapid growth of rutile grains. Nair et al. [19] found that Cu2þ and Ni2þ enhanced the transformation of anatase to rutile; La3þ retarded the transformation and densification. Wang et al. [20] synthesized Fe3þ -doped titania powders in a flame aerosol reactor. It was confirmed that Fe3þ was incorporated into the titania lattice and promoted the conversion of anatase to rutile, and that the crystal size of the titania particles decreased with the increase of iron dopant. Bonini et al. [21] proved that the addition of dopants of Nb, Ga and Ta inhibits the grain growth by suppressing the conversion of anatase to rutile for nanostructured thick films of the doped TiO2 fabricated by screen-printing technology. Rare earths as modifiers have been widely used to improve the properties of ceramics owing to their extremely active physical and chemical properties [22,23]. However, few works have dealt with their application in the preparation of titania films and ceramics. In previous work [24–27], it has been confirmed that the addition of Ce4þ /CeO2 decreases the temperature of phase transformation of a sol–gel derived amorphous phase to a cordierite product, depresses the growth of cordierite crystals, improves the density of cordierite ceramics and makes available the preparation of cordierite ceramics with unique properties. Owing to the simplicity of the operation, the sol–gel method has been widely used for the preparation of titania films and fine ceramics [28–30]. Since the effects
1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.12.006
886
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889
of Ce4þ /CeO2 on the phase transformation, microstructure and properties of cordierite ceramics are quite desirable, it is intended to use the sol–gel method to prepare Ce4þ -modified TiO2 gels and to study their crystallization behavior in terms of the crystallization temperature and the growth character of anatase crystals. Further, it is expected that the addition of Ce4þ can enhance the sintering process of the gels and promote the preparation of ultrafine anatase crystals.
2. Experimental procedures 2.1. Preparation of samples All of the gelation procedures were performed at room temperature (25 °C) in air and through intense stirring. Titanium tetraisopropoxide and cerium nitrate (AR) were used as the precursory materials. Titanium tetraisopropoxide (120 ml) was first hydrolyzed in 120 ml of absolute alcohol. Then, the hydrolyzed titanium tetraisopropoxide sol was evenly divided into three parts and injected respectively into three beakers. Different amounts of cerium nitrate were added into these beakers (0, 1.2, 2.6 g respectively) to obtain colloidal sols. After ageing for 3 h, dilute acetic acid (acid: water ¼ 10 vol%) was titrated into the sols until gels are formed. Afterwards, the gels were aged for another 24 h and calcinated at 100 °C, to yield transparent xerogels with different colors, which were then sintered at different temperatures for different holding times in air. The sintered products were finely ground by a ball-mill and screened into powders with a granularity range of 74–63 lm (ASTM E11-58T). Thus, the samples for XRD analysis are made ready. The compositions are listed in Table 1.
Based on the data examined by XRD, the size of anatase crystals is calculated by using the Scherrer equation [31]: D¼
0:89k B cos h
ð1Þ
where, D is the size of crystals; k––the wavelength of CuKa1 radiation; h––the diffraction angle corresponding to a specified diffraction peak; Bð¼ Bs Br Þ––the width (rad) of the diffraction angle at half of the peak height, of which Bs is that measured for the samples; and Br ––for the reference sample of quartz. This method applies only to the case in which the size is smaller than 100 nm, and the smaller the crystal size, the more accurate value D will be. Thus, by measuring h and Bs of the (0 1 0) peak of anatase, the size of anatase crystals can be calculated by using the above equation.
3. Results 3.1. Phase transformations Fig. 1 presents the DTA curves of the three samples. The exothermic peaks at about 280–300 °C are the result of the oxidation of organic compounds in the samples, while the exothermic peaks at above 300 °C represent the reactions relative to crystallization of the gels to anatase. It can be found that, along with the increase of Ce4þ content, the crystallization temperature appears to rise first and then drop, indicating that a small quantity of Ce4þ causes an increase of the crystallization temperature while a larger quantity of Ce4þ makes it decrease. Moreover, the fact that the exothermic peaks are reduced in temperature and broaden gradually with the increase of Ce4þ content demonstrates that the addition
2.2. Examination procedures The crystallization process and weight loss of samples were examined by a differential thermal analyzer (DTA/ TG, Universal V2.5H), using a-Al2 O3 powder as the reference sample, in atmospheric condition and at a heating rate of 10 °C/min. The phases formed in the samples were identified by an X-ray powder diffractometer (XRD, Rigaku D/max-2400X), with copper K radiation, 40 kV, 120 mA and at a scanning speed of 4°/ min.
Temperature Difference,oC
332.1 285 378.0 10CT
294 357.5
5CT 282
0CT
Table 1 Reduced composition (wt.%) of samples Sample
TiO2
CeO2
OCT 5CT 10CT
100 95 90
0 5 10
150
300
450
600
Temperature,oC Fig. 1. DTA curves of samples.
750
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889 + (010) Rutile TiO2
Intensity
.
o
550 C
..
+
+
+
+
++
+
+
350 C ++
++
+ o
450 C
+
o
20
30
40
50
60
2θ / o
(a) + (010)
+ Anatase TiO2
o
550 C
+
+
+
+
++
+
+
++
++
+ o
Intensity
450 C
+ o
350 C
20
30
40
(b)
50
60
2θ / o (010)
+ Anatase TiO2
o
550 C
o
450 C
+
o
350 C
+ 20
30
++
+ 40
50
60
2θ / o
(c)
Fig. 3. XRD patterns of samples sintered at different temperature for one hour (a) sample OCT, (b) sample 5CT and (c) sample 10CT.
+
100
(010)
.
o
600 C+3h +
+ Anatase TiO2 Rutile TiO2 +
++
+
++
10CT
+
90 +
80
126
70
.
377
+
+
60 0
200
5CT
+
.
Intensity
Weight percentage, %
. .. . .
+ Anatase TiO2
Intensity
of Ce4þ reduces the speed of the crystallization of anatase. This ratiocination can be confirmed by the XRD analysis in latter sections. However, the exothermic peaks representing the conversion of anatase to rutile have not been identified in the present test. This phenomenon was also found for the 3 mol% Yttria-doped titania gels [32], for which the peak of rutile was not identified in the DTA curve, but rutile appears at about 700 °C in the XRD curve So, it can be considered that, the heat of the transformation of anatase to rutile, especially for the case of Ce4þ -modifying, is so small that rutile is difficult to be identified by the DTA method. The thermo-gravimetric (TG) curve of sample 5CT is shown in Fig. 2. The gel loses weight rapidly before 100 °C and slowly afterwards, which corresponds respectively to the volatilization of ethanol and the oxidation of hydroxyls and carbon. This indicates that the organic compositions can be completely removed from the samples when they are calcinated at above 400 °C. Therefore, this is the lowest sintering temperature for preparing anatase crystals with high purity. Fig. 3 shows the XRD patterns for the crystallization of the gels at different sintering temperatures and with different Ce4þ additions. With an increase of temperature, the height of the main peak (0 1 0) of anatase in all the samples increases monotonically. The difference consists in that, for sample OCT, the main peak height increases quite rapidly, so that a small amount of rutile at 550 °C appears (Fig. 3(a)); while for samples 5CT and 10CT, the increase of heights is not significant, nor is any rutile formed (Fig. 3(b) and (c)) even when sintering is done at 600 °C (Fig. 4). In contrast, only little anatase remains in sample OCT at this temperature. This indicates that Ce4þ addition inhibits both conversions of the gels into anatase and of anatase into rutile. The XRD results of samples sintered for different holding times are shown in Fig. 5. It can be observed that increasing holding time strongly affects the crystallization for sample OCT, but it has little effect on samples 5CT and 10CT. Since the sharpening of diffraction peaks is attributed to the coarsening of crystals, it is obvious that the addition of Ce4þ can effectively
887
400
600 o
Temperature, C Fig. 2. TG curve of sample 5CT.
800
20
+ 30
. . . 40
2θ /
.
OCT
. + 50
60
ο
Fig. 4. XRD patterns of samples sintered at 600 °C for 3 h.
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Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889 1000
+ : Anatase
Intensity
o
5CT
Granularity, A
800
0CT
o
600
600 C+3h
400 200
+
+
0
10CT
+ ++
+ 20
30
40
50
20
60
30
+
+ ++
40
50
0
5
10
CeO2, wt%
60
Fig. 8. Relation of granularity of anatase to Ce4þ addition.
2θ / O
2θ / (a) 30 min O
(b) 180 min
Fig. 5. XRD patterns of sample sintered at 450 °C for different holding times.
inhibit the growth of anatase crystals and the formation of rutile. 3.2. Growth of anatase crystals Figs. 6 and 7 show the relation of granularity to the sintering temperature and to the holding time respectively. Increasing the sintering temperature and holding time result in a remarkable growth of anatase in sample OCT but an insignificant growth in samples 5CT and
10CT. This proves that Ce4þ addition does have a considerable effect on the inhibition of the growth of anatase. Therefore, unlike Ce4þ -free anatase, Ce4þ modified anatase is stable even at temperatures as high as 600 °C, since neither the conversion into rutile nor crystal growth occur. Fig. 8 shows the relationship of the granularity of anatase crystals and the Ce4þ additions. It can be noticed that 5 wt.% of CeO2 is sufficient to inhibit the growth of anatase.
4. Discussion 4.1. Variation of crystallization temperature of anatase
O
Granularity, A
400 OCT 5CT 10CT
300
200
100
0 350
400
450
500
550
Sintering temperature,oC Fig. 6. Relation of granularity of anatase to sintering temperature (holding for 1 h).
o
Granularity, A
200 0CT 5CT 10CT
160
120
80 30
60
90
120
150
180
Holding time, min Fig. 7. Relation of granularity of anatase to holding time (sintered at 450 °C).
Amorphous TiO2 in xerogels possesses a network structure, in which tetrahedrons of [TiO4 ] should be connected by bridging oxygen. However, in such a network there, certainly, exist some kinds of discontinuation, such as gaps, non-bridging oxygen, remaining hydroxyls and other organic components, which are likely to result from the imperfect sintering of the preparation. Moreover, since the radius of Ce4þ (six-coordination, is much larger than that of Ti4þ (four-coordi0.87 A) [33], Ce4þ can only be located in the gaps. nation, 0.42 A) In this case, Ce4þ connects the hydroxyls and nonbridging oxygen to the tetrahedrons. Thus, the continuity of the amorphous network is enhanced due to the addition of Ce4þ . The compacted network will certainly cause an increase in the crystallization temperature of anatase. This phenomenon probably occurs when only a small quantity of Ce4þ is added, for example, to sample 5CT. On the other hand, when excessive Ce4þ is added to the amorphous TiO2 , the network may get loosened by the extra Ce4þ , consequently, the continuity of the network will be reduced. Besides, because the ultrafine CeO2 particles can precipitate prior to the crystallization of anatase when excessive Ce4þ is added [12], it may play a role as the nucleation site of anatase, resulting in the easier formation of anatase than in sample 5CT. Thus, the crystallization temperature of anatase decreases again (in the case of sample 10CT).
Z.M. Shi et al. / Scripta Materialia 50 (2004) 885–889
4.2. Growth of anatase crystals and their transformation into rutile 4þ
As regard to the question why Ce can inhibit the growth of anatase crystals, it can be considered the solid solution and solid-state diffusion of Ce4þ in anatase. Anatase is a typical n-type semiconductor, in which there are oxygen vacancies. Although Ce4þ can dissolve in anatase [34], its solubility in anatase is much smaller than that in the amorphous TiO2 . As the anatase crystals grow, extra Ce4þ ions have to be discharged out of the crystals through solid diffusion. However, the diffusion speed of Ce4þ is rather slow because of its large radius. Thus, Ce4þ ions are enriched around the anatase crystals, suppressing Ti4þ diffusion towards the crystals. As a result, the growth is slowed down or completely terminated, even for sample 10CT with a relatively low crystallization temperature. In addition, the diffusion of Ce4þ also participates in the conversion of anatase into rutile. Rutile has a more compact structure than anatase [35,36], therefore, only when Ce4þ is discharged from anatase, can the conversion to rutile proceed. For the same reason as mentioned above, Ce4þ addition also inevitably retards the conversion of anatase into rutile.
5. Conclusions With the increase of Ce4þ addition, the crystallization temperature of TiO2 gels converting into anatase first increases and then decreases. Even a small quantity of Ce4þ addition can significantly inhibit the growth of anatase crystals and the conversions both of TiO2 gels into anatase and of anatase into rutile. Among the three samples, the best crystallinity has been observed in the sample with 5 wt.% of CeO2 when sintered at 450 °C for 3 h. Anatase crystals with sizes of the order of nanometer can be obtained for the Ce4þ modified samples, and their high-temperature stability is much higher than that of Ce4þ -free titania. It is suggested that the influence of Ce4þ to inhibiting the crystallizations of TiO2 gels to anatase and of anatase to rutile is correlated to the difficulty of diffusion of Ti4þ towards them, which is caused by the difference in the solubility of Ce4þ in amorphous, anatase and rutile crystals.
Acknowledgements The project is supported by National Natural Science Foundation of China (NSFC), No. 50262002, China.
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