Materials Chemistry and Physics 78 (2002) 184–188
Preparation and characterization of nano-TiO2 powder Baorang Li∗ , Xiaohui Wang, Minyu Yan, Longtu Li Department of Materials Science a nd Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China
Received 5 December 2001; received in revised form 18 March 2002; accepted 23 May 2002
Abstract
This paper reports the results of an investigation aiming at finding what affects the grain size of nano-TiO 2 powder during synthesis. Nano-sized TiO2 powders have been prepared by a sol–gel method. The crystalline structures and morphologies of the powder have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The result shows that the different preparation conditions conditions such as concentration, concentration, pH value, value, calcination calcination time and calcination calcination temperature temperature have a lot of influences influences upon the properties of nano-TiO2 powders. The smallest grain size of TiO2 powder we have obtained is 6 nm by controlling the process conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Concentration; pH value; Calcination time; Calcination temperature; Nano-TiO 2 powder
1. Introducti Introduction on
2. Experimenta Experimentall
Since Gleiter’s report [1] report [1] on on the nano-materials, more attention has been paid upon the research of nano-materials. Compared with the traditional materials, nano-phase materials processes unusual chemical, mechanical, optical, electrical and magnetic properties [2] properties [2].. Titanium dioxide is mainly applied as pigments, adsorbents, and catalytic supports. In almost all of these cases, the size of the titanium dioxide particles is an important factor affecting the performance of the materials. It is not surprising, therefore, that much research has been focused upon the reduction of particle size. Sol–gel route is regarded as a good method to synthesize ultra-fine metallic oxide [3] oxide [3] a and nd has been widely employed for preparing titanium dioxide particles [4–7] particles [4–7].. It was usually found that different routes often produce different results. Even for the same route, using different amount of the starting materials the powder size obtained is different [8] [8].. So it is regarded as necessary for us to investigate in detail the factors which may have important effect upon the particle size. In this paper, titanium dioxide nano-powders were prepared by the hydrolysis of tetra-n-butyl -butyl titanate. titanate. Using various various techniques techniques,, including including transmission electron microscopy (TEM), X-ray diffraction (XRD), powders obtained were studied in order to find the possible elements of affecting the microstructures and grain size.
TiO2 nano-powders were prepared via a sol–gel method using using tetratetra-n-buty -butyl-t l-tita itanat natee and deioni deionized zed water water as the starti starting ng materi materials als.. Concen Concentra tratio tions ns (the (the volum volumee ratio ratio of tetra-n-butyl -butyl titanate:d titanate:deioni eionized zed water) are chosen chosen as 1:2, 1:6, 1:6, 1:12, 1:12, 1:20, 1:20, 1:50, 1:50, 1:100. 1:100. Tetraetra-n-butyl-tita butyl-titanate nate was dropped into deionized water while magnetic agitating continuously. In order to investigate the effect of the pH value upon the grain size, hydrochloric acid or aqueous ammonia were were droppe dropped d into into the solution solution to get gel with with differ different ent pH values. The obtained gel was then dried at 105 ◦ C for several hours until it was turned into yellow block crystal. After ball milling the dried gel obtained was calcined at different different temperature temperaturess for 2 h. X-ray X-ray powde powderr diffra diffracti ction on (XRD) (XRD) for the powde powders rs calcalcined at various temperatures were recorded on a D/max-RB diffractometer using Cu K radiation. The particle size was calculated using the Scherrer equation and confirmed by TEM which was performed on a H-800 electron microscope.
∗
Corresponding Corresponding author. author.
E-mail address:
[email protected] (B. Li).
3. Results Results and discussion discussion 3.1. Calcination temperatur temperaturee
XRD patterns of TiO2 nano-powders calcined at different temperatures are shown in Fig. 1. 1. It can be obviously seen from the XRD that partial crystallization appears just after drying and the phase structure of the powder calcined
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 2 2 6 - 2
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B. Li et al. / Materials Chemistry and Physics 78 (2002) 184–188
growth rate is given by Eq. (1) [10]: u
=
a0 v0 exp
−
Q
KT
1 − exp
−
F v
KT
(1)
where a0 is the particle diameter, v0 the atomic jump frequency, Q the activation energy for an atom to leave the matrix and attach itself to the growing phase, F v the molar free energy difference between the two phases. For non-crystallization F v KT , so Eq. (1) can be reduced to Eq. (2): u
Fig. 1. The XRD curves of nanocrystalline TiO 2 at different calcination temperatures while the calcination time is 2 h, concentration 1:6 and pH value 7.
=
a0 v0 exp
−
Q
KT
(2)
When the calcination temperature is high, the activation energy is very small, the growth rate is large. So the grain size increases very quickly as the increasing calcination temperature; when the calcination temperature is low, the activation energy is very large, respectively, the growth rate becomes slow. So the grain size increases very slowly as the calcination temperature increases. Fig. 3 shows a set of the typical TEM micrographs of the nano-TiO2 powders calcined at 350, 500 and
at temperatures below 600 ◦ C is mainly of anatase type. The phase transformation from anatase to rutile occurred at about 600 ◦ C and completed at about 800 ◦ C while in Haro-Poniatowski’s report [7], the presence of the rutile was at about 545–550 ◦ C. The grain size of the powder as a function of calcination temperature is plotted in Fig. 2. Obviously, the grain size increases with the increasing calcination temperature. It grows slowly at low calcination temperatures and then becomes very fast at high calcination temperatures. This is similar to the result in Ref. [9] and can be explained as below. The
Fig. 2. The curve of grain size versus calcination temperatures while the calcination time is 2 h, concentration 1:6 and pH value 7.
Fig. 3. TEM micrographs of the nano-TiO 2 powders calcined at different calcination temperatures: (A) 350; (B) 500; (C) 600 ◦ C.
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600 ◦ C. It is easily found that about 6 nm nano-TiO2 powders can be obtained at 350 ◦ C, 13nm at 500 ◦ C, 36nm at 600 ◦ C, which is coincident to the result of XRD as shown in Fig. 2. Because the grain size depends upon the temperature strongly, it is believed that grain size of TiO2 particles in the dried gel should be smaller than 6nm. 3.2. Calcination time
The effect of the calcination time upon the grain size of TiO2 powders is shown in Fig. 4. At low calcination temperatures the prolongation of calcination time has little influence upon the particle size. But when the calcination temperature increases to 1000 ◦ C, the obvious influence upon grain size was found as shown in Fig. 4c. At relatively high temperatures the calcination time seems to have greater effect upon the grain size. Just as shown in Fig. 1, when the calcination temperatures are 350, 500, 1000 ◦ C the total crystallization had been completed and the corresponding phase type is anatase or rutile. The calcination time’s effects upon the grain size was thought to be controlled mainly by diffusion. At this time the following equation is given [11]: u
∝
(α Dt )1/2
(3)
where u is the grain growth rate, D the diffusion coefficient, t the calcination time, α the constant data. Compared with the low calcination temperature the diffusion coefficient at high calcination temperature is large. Huge drive force for diffusion is present. So the growth rate for the particle is fast at relatively high calcination temperatures and the grain size tends to change very greatly as the calcination time prolong.
Fig. 4. Grain size change as a function of the calcination time at different temperatures while the concentration is 1:6 and pH value 7: (a) 350; (b) 500; (c) 1000 ◦ C.
Fig. 5. Grain size change as a function of pH value while calcination temperature is 350 ◦ C, calcination time 2 h and the concentration 1:6.
3.3. pH value
Effect of pH value upon the grain size is shown in Fig. 5. It is found that when the pH value is below 7 the value of grain size is almost constant, which means acid solution could restrain grain growth. When the pH value is beyond 7, however, the line goes up very quickly which indicates that a total alkali environment would enhance grain growth. In this paper nano-TiO2 powders were obtained mainly by controlling hydrolysis and condensation reactions of tetra-n-butyl-titanate. It is well known that acid is usually used to restrain hydrolysis while alkali can accelerate hydrolysis during reaction. When pH is beyond 7 which means environment do benefit to accelerate hydrolysis, the large aggregated particles are formed and grain tend to grow quickly.
Fig. 6. The grain size of the nano-TiO 2 as a function of concentration while calcination time is 2 h, the calcination temperature 350 ◦ C and pH value 7.
B. Li et al. / Materials Chemistry and Physics 78 (2002) 184–188
Fig. 7. XRD of the nano-TiO2 dry-gel powder with different concentrations while calcination time is 2 h and pH value 7.
3.4. Concentration
The grain size of the gel powder via different concentrations is shown in Fig. 6. It is obvious that the grain size do not grow up as the concentration changes. Therefore, the different concentrations may have little effects upon the grain size of the gel powder.
Fig. 8. XRD for nano-TiO 2 with different concentrations while the calcination temperature is 400 ◦ C, the calcination time 2 h and pH value 7.
187
Fig. 9. XRD of the nano-TiO 2 powder with different concentrations while the calcination temperature is 600 ◦ C, the calcination time 2 h, and pH value 7.
The gel powders with different concentrations were calcined at different temperatures. The patterns of XRD are shown in Figs. 7–9. It can be seen from Figs. 7 and 8 that below 400 ◦ C there are no obvious phase transformation difference with the different concentrations; however, when the calcination temperature increases further above 400 ◦ C as shown in Fig. 9, obvious phase transformation difference occurred. This behavior is very interesting. The fraction of rutile during phase transformation from anatase to rutile influenced by the concentration is shown in Fig. 10. The phase
Fig. 10. The fraction of rutile of the nano-TiO 2 powders with different concentrations at different calcination temperatures during phase transformation from anatase to rutile.
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transformations from anatase to rutile for the samples with different concentrations seemed to start at 400 ◦ C but was completed at different temperatures. For the concentrations chosen as 1:20 and 1:50, almost 90% of anatase phase have been transformed into rutile below about 600 ◦ C but for the concentrations chosen as 1:100 and 1:6, the same amount transformation of anatase phase were only completed at about 700 and 800 ◦ C, respectively. It seems that even the concentration has no obvious influence upon the grain size of the gel as described above, but it really affects the phase transformation. However, further studies are needed for explained this phenomenon.
4. Conclusions
Nano-TiO 2 powders have been prepared by sol–gel method successfully. By controlling the conditions properly, nano-TiO2 powders with the grain size less than 6 nm grain size of nano-TiO2 powders could be obtained. Among the elements which may have effect upon the grain size and microstructure of nano-TiO2 powders, the calcination temperature and pH value were found to be more effective compared with the calcination time and concentration. The grain size tends to increase with increasing temperature and the increase in pH value.
Different calcination time was found to produce different effects upon the grain size depending upon calcination temperature. The higher is the calcination temperature, the greater is the effect of calcination time upon the grain size. The most important behavior which is found for the first time is that the phase transformation process of the nano-TiO 2 from anatase to rutile was influenced greatly by the concentration. References [1] R. Birrnlyer, H. Gleiter, H.P. Klein, et al., Phys. Lett. A 102 (8) (1984) 365. [2] H. Gleiter, Prog. Mater. Sci. 33 (1989) 223. [3] S. Sakka, Am. Ceram. Soc. Bull. 64 (1985) 1463. [4] S. Doeuff, M. Henry, C. Sanchen, J. Livage, J. Non-Cryst. Solids 89 (1987) 206–216. [5] H. Kumazuawa, H. Otsuki, E. Sada, J. Mater. Sci. Lett. 15 (1996) 839–840. [6] D.C. Hague, M.J. Mayo, J. Am. Ceram. Soc. 77 (1994) 1957–1959. [7] E. Haro-Poniatowski, R. Rodriguez, O. Cano-Corona, J. Mater. Res. 9 (1994) 2102–2107. [8] C.U.I. Zoulin, J. Mater. Sci. Technol. 15 (1999) 71–74. [9] W.F. Sullivan, S.S. Cole, J. Am. Ceram. Soc. 42 (1959) 127–133. [10] D. Turnbull, Solid State Phys. 3 (1956). [11] H.B. Aaron, D. Fainstein, G.R. Kottter, J. Appl. Phys. 41 (1970) 4404.
