Materials Chemistry and Physics 78 (2002) 184–188

Preparation and characterization of nano-TiO2 powder

Baorang Li∗, Xiaohui Wang, Minyu Yan, Longtu LiDepartment of Materials Science and 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-TiO2 powder during synthesis.Nano-sized TiO2 powders have been prepared by a sol–gel method. The crystalline structures and morphologies of the powder have beencharacterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The result shows that the different preparationconditions such as concentration, pH value, calcination time and calcination temperature have a lot of influences upon the properties ofnano-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-TiO2 powder

1. Introduction

Since Gleiter’s report[1] on the nano-materials, more at-tention has been paid upon the research of nano-materials.Compared with the traditional materials, nano-phase mate-rials processes unusual chemical, mechanical, optical, elec-trical and magnetic properties[2].

Titanium dioxide is mainly applied as pigments, adsor-bents, and catalytic supports. In almost all of these cases, thesize of the titanium dioxide particles is an important factoraffecting the performance of the materials. It is not surpris-ing, therefore, that much research has been focused uponthe reduction of particle size. Sol–gel route is regarded as agood method to synthesize ultra-fine metallic oxide[3] andhas been widely employed for preparing titanium dioxideparticles[4–7]. It was usually found that different routes of-ten produce different results. Even for the same route, usingdifferent amount of the starting materials the powder sizeobtained is different[8]. So it is regarded as necessary forus to investigate in detail the factors which may have im-portant effect upon the particle size. In this paper, titaniumdioxide nano-powders were prepared by the hydrolysis oftetra-n-butyl titanate. Using various techniques, includingtransmission electron microscopy (TEM), X-ray diffraction(XRD), powders obtained were studied in order to find thepossible elements of affecting the microstructures and grainsize.

∗ Corresponding author.E-mail address: libaorang99@mails.tsinghua.edu.cn (B. Li).

2. Experimental

TiO2 nano-powders were prepared via a sol–gel methodusing tetra-n-butyl-titanate and deionized water as thestarting materials. Concentrations (the volume ratio oftetra-n-butyl titanate:deionized water) are chosen as 1:2,1:6, 1:12, 1:20, 1:50, 1:100. Tetra-n-butyl-titanate wasdropped into deionized water while magnetic agitating con-tinuously. In order to investigate the effect of the pH valueupon the grain size, hydrochloric acid or aqueous ammoniawere dropped into the solution to get gel with differentpH values. The obtained gel was then dried at 105◦C forseveral hours until it was turned into yellow block crystal.After ball milling the dried gel obtained was calcined atdifferent temperatures for 2 h.

X-ray powder diffraction (XRD) for the powders cal-cined at various temperatures were recorded on a D/max-RBdiffractometer using Cu K� radiation. The particle size wascalculated using the Scherrer equation and confirmed byTEM which was performed on a H-800 electron microscope.

3. Results and discussion

3.1. Calcination temperature

XRD patterns of TiO2 nano-powders calcined at differ-ent temperatures are shown inFig. 1. It can be obviouslyseen from the XRD that partial crystallization appears justafter drying and the phase structure of the powder calcined

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B. Li et al. / Materials Chemistry and Physics 78 (2002) 184–188 185

Fig. 1. The XRD curves of nanocrystalline TiO2 at different calcinationtemperatures while the calcination time is 2 h, concentration 1:6 and pHvalue 7.

at temperatures below 600◦C is mainly of anatase type.The phase transformation from anatase to rutile occurredat about 600◦C and completed at about 800◦C while inHaro-Poniatowski’s report[7], the presence of the rutile wasat about 545–550◦C.

The grain size of the powder as a function of calcinationtemperature is plotted inFig. 2. Obviously, the grain size in-creases with the increasing calcination temperature. It growsslowly at low calcination temperatures and then becomesvery fast at high calcination temperatures. This is similar tothe result in Ref.[9] and can be explained as below. The

Fig. 2. The curve of grain size versus calcination temperatures while thecalcination time is 2 h, concentration 1:6 and pH value 7.

growth rate is given byEq. (1) [10]:

u = a0v0

[exp

(− Q

KT

)] [1 − exp

(−�Fv

KT

)](1)

wherea0 is the particle diameter,v0 the atomic jump fre-quency,Q the activation energy for an atom to leave thematrix and attach itself to the growing phase,�Fv themolar free energy difference between the two phases. Fornon-crystallization�Fv � KT, so Eq. (1) can be reducedto Eq. (2):

u = a0v0

[exp

(− Q

KT

)](2)

When the calcination temperature is high, the activation en-ergy is very small, the growth rate is large. So the grain sizeincreases very quickly as the increasing calcination temper-ature; when the calcination temperature is low, the activationenergy is very large, respectively, the growth rate becomesslow. So the grain size increases very slowly as the calcina-tion temperature increases.

Fig. 3 shows a set of the typical TEM micrographsof the nano-TiO2 powders calcined at 350, 500 and

Fig. 3. TEM micrographs of the nano-TiO2 powders calcined at differentcalcination temperatures: (A) 350; (B) 500; (C) 600◦C.

186 B. Li et al. / Materials Chemistry and Physics 78 (2002) 184–188

600◦C. It is easily found that about 6 nm nano-TiO2 pow-ders can be obtained at 350◦C, 13 nm at 500◦C, 36 nmat 600◦C, which is coincident to the result of XRD asshown in Fig. 2. Because the grain size depends uponthe temperature strongly, it is believed that grain size ofTiO2 particles in the dried gel should be smaller than6 nm.

3.2. Calcination time

The effect of the calcination time upon the grain sizeof TiO2 powders is shown inFig. 4. At low calcinationtemperatures the prolongation of calcination time has littleinfluence upon the particle size. But when the calcinationtemperature increases to 1000◦C, the obvious influenceupon grain size was found as shown inFig. 4c. At rela-tively high temperatures the calcination time seems to havegreater effect upon the grain size.

Just as shown inFig. 1, when the calcination tempera-tures are 350, 500, 1000◦C the total crystallization had beencompleted and the corresponding phase type is anatase orrutile. The calcination time’s effects upon the grain size wasthought to be controlled mainly by diffusion. At this timethe following equation is given[11]:

u ∝ (αDt)1/2 (3)

whereu is the grain growth rate,D the diffusion coefficient,t the calcination time,α the constant data. Compared withthe low calcination temperature the diffusion coefficient athigh calcination temperature is large. Huge drive force fordiffusion is present. So the growth rate for the particle isfast at relatively high calcination temperatures and the grainsize tends to change very greatly as the calcination timeprolong.

Fig. 4. Grain size change as a function of the calcination time at differenttemperatures 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 calcinationtemperature 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 inFig. 5.It is found that when the pH value is below 7 the valueof grain size is almost constant, which means acid solutioncould restrain grain growth. When the pH value is beyond7, however, the line goes up very quickly which indicatesthat a total alkali environment would enhance grain growth.

In this paper nano-TiO2 powders were obtained mainlyby controlling hydrolysis and condensation reactions oftetra-n-butyl-titanate. It is well known that acid is usu-ally used to restrain hydrolysis while alkali can acceleratehydrolysis during reaction. When pH is beyond 7 whichmeans environment do benefit to accelerate hydrolysis, thelarge aggregated particles are formed and grain tend to growquickly.

Fig. 6. The grain size of the nano-TiO2 as a function of concentrationwhile calcination time is 2 h, the calcination temperature 350◦C and pHvalue 7.

B. Li et al. / Materials Chemistry and Physics 78 (2002) 184–188 187

Fig. 7. XRD of the nano-TiO2 dry-gel powder with different concentrationswhile calcination time is 2 h and pH value 7.

3.4. Concentration

The grain size of the gel powder via different concentra-tions is shown inFig. 6. It is obvious that the grain size donot grow up as the concentration changes. Therefore, the dif-ferent concentrations may have little effects upon the grainsize of the gel powder.

Fig. 8. XRD for nano-TiO2 with different concentrations while the calci-nation temperature is 400◦C, the calcination time 2 h and pH value 7.

Fig. 9. XRD of the nano-TiO2 powder with different concentrations whilethe calcination temperature is 600◦C, the calcination time 2 h, and pHvalue 7.

The gel powders with different concentrations were cal-cined at different temperatures. The patterns of XRD areshown inFigs. 7–9. It can be seen fromFigs. 7 and 8thatbelow 400◦C there are no obvious phase transformation dif-ference with the different concentrations; however, when thecalcination temperature increases further above 400◦C asshown in Fig. 9, obvious phase transformation differenceoccurred. This behavior is very interesting. The fraction ofrutile during phase transformation from anatase to rutile in-fluenced by the concentration is shown inFig. 10. The phase

Fig. 10. The fraction of rutile of the nano-TiO2 powders with differentconcentrations at different calcination temperatures during phase transfor-mation from anatase to rutile.

188 B. Li et al. / Materials Chemistry and Physics 78 (2002) 184–188

transformations from anatase to rutile for the samples withdifferent concentrations seemed to start at 400◦C but wascompleted at different temperatures. For the concentrationschosen as 1:20 and 1:50, almost 90% of anatase phase havebeen transformed into rutile below about 600◦C but for theconcentrations chosen as 1:100 and 1:6, the same amounttransformation of anatase phase were only completed atabout 700 and 800◦C, respectively. It seems that even theconcentration has no obvious influence upon the grain sizeof the gel as described above, but it really affects the phasetransformation. However, further studies are needed for ex-plained this phenomenon.

4. Conclusions

Nano-TiO2 powders have been prepared by sol–gelmethod successfully. By controlling the conditions prop-erly, nano-TiO2 powders with the grain size less than 6 nmgrain size of nano-TiO2 powders could be obtained.

Among the elements which may have effect upon the grainsize and microstructure of nano-TiO2 powders, the calcina-tion temperature and pH value were found to be more effec-tive compared with the calcination time and concentration.The grain size tends to increase with increasing temperatureand the increase in pH value.

Different calcination time was found to produce differ-ent effects upon the grain size depending upon calcina-tion temperature. The higher is the calcination temperature,the greater is the effect of calcination time upon the grainsize.

The most important behavior which is found for thefirst time is that the phase transformation process of thenano-TiO2 from anatase to rutile was influenced greatly bythe concentration.

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