Materials Science and Engineering A 408 (2005) 243–246

Control of grain growth of nanocomposite TiO2

Dong Hyun Kima,∗, Ha Sung Parka, Jae Han Jhoa, Sun-Jae Kimb, Kyung Sub Leea

a Division of Materials Science & Engineering, Hanyang University, Seoul 133-171, Republic of Koreab Department of Nano Sciences & Technology/SAINT, Sejong University, Seoul 143-747, Republic of Korea

Received in revised form 26 July 2005; accepted 10 August 2005

Abstract

Nanocomposite TiO2 powders were synthesized by a mechanical alloying method with different heating times and temperatures. Formation ofNiTiO3 in nanocomposite TiO2 by ball milling was found to control the grain growth of nanosize TiO2 and enhance the high temperature thermalstability.© 2005 Elsevier B.V. All rights reserved.

Keywords: Titanium dioxide; Mechanical alloying; Titanate; Composite; Grain growth

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1. Introduction

Nanosized titanium dioxide has attracted a great deal of atten-tion due to its novel electronic, optical and catalytic propertiesoriginating from the quantum confinement[1]. An importantmotivation of current research into nanosized TiO2 is the need todevelop an understanding of the relationships between its struc-tures and synthesis conditions. In spite of extensive researchefforts in this research field using various synthesis techniques,structural understanding has still not been achieved[2]. Conven-tionally, preparation of nanosized TiO2 needs heat treatment.The heating process causes the grain growth and thermal stabil-ity, which prohibits the production of nanosized TiO2 powder.

In our previous experiment, we improved the photocatalyticproperty by “synthesis of novel TiO2 by mechanical alloy-ing and heat treatment-derived nanocomposite of TiO2 andNiTiO3”. However, no detailed study has been reported aboutgrain growth and thermal stability in nanocomposite TiO2 pro-duced by mechanical alloying. In this paper, we present theinfluence of forming NiTiO3 in nanocomposite TiO2 on itsstructure.

2. Experimental

doping effect, meta-stable rutile powder, not stable TiO2 phasewas selected as the starting powder for mechanical alloyinobtain the meta-stable powder, a TiO(OH)2 precipitate slurrywas first prepared from TiOCl2 using the HPPLT process[3].Then, the solution was filtered using distilled water. The detaHPPLT process has been reported elsewhere[4,5]. The filteredprecipitates were dried at 60◦C for 12 h to obtain the drieTiO2 powder. The dried powder was mechanically alloyed14 h by planetary ball milling (Fritz mill, P-5) with nickel powders (Kojundo Chem. Co. Ltd., 99.9%). The content of thwas 8 wt.%. The ball milling speed was 150 rpm and theto powder weight ratio was 15:1. After milling, the powdwere heat-treated at the temperature range of 400–1000◦C for4 h in order to induce the grain growth. For the thermal stity, heating temperature was fixed at 1000◦C and then heatintime changed in the range 2–10 h. The structural propertithe alloying process and nanocrystallization process inclugrain size determination were characterized by XRD (Cu�,Rigaku D-MAX 3000) and by transmission electron microsc(TEM, 200 kV, JEM 2000).

3. Results and discussion

Nanocomposite TiO2 powders were prepared by mechani-cal alloying (MA) and heat treatment. In order to get a better

Fig. 1shows the XRD patterns of mechanically alloyed pow-der at different heating temperatures and heat-treated rutile TiO2(h so ningr d an

0d

∗ Corresponding author. Tel.: +82 2 2281 4914; fax: +82 2 2281 4914.E-mail address: dhk@ihanyang.ac.kr (D.H. Kim).

921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2005.08.127

without the Ni doping powder) for 4 h at 1000◦C. When theeating temperature was 400–500◦C, the alloyed powder wanly rutile phase with peak broadening. The peak broadeesults from a refinement of the average crystal size an

244 D.H. Kim et al. / Materials Science and Engineering A 408 (2005) 243–246

Fig. 1. XRD patterns of TiO2 powders for 4 h at different temperatures: (a)400◦C, (b) 500◦C, (c) 600◦C, (d) 800◦C and (e) 1000◦C.

increase in the internal strain by mechanical deformation dur-ing ball milling. And the element Ni was not detected. It meansthat Ni was dissolved in the rutile TiO2 matrix. The NiTiO3phase started to appear while the alloyed powders were heat-treated at 600◦C with well crystallized. At 1000◦C, the powderscompletely coexisted as Ni-doped rutile and NiTiO3. Unlikeconventional ceramics results, the grain size of the heat-treatedNi-doped powder did not grow as shown inFig. 2. Using theDebey–Scherrer equation[6], the grain size of Ni-doped rutileand NiTiO3 were less than 10 nm. This small grain size indi-cates that the grain growth did not occur by the heat treatment.In order to decrease the total surface energy of the heat-treatedpowders, grain movement was restricted by the formation of

F s.

Fig. 3. XRD patterns of TiO2 powders for different heating time at 1000◦C: (a)2 h, (b) 4 h, (c) 6 h, (d) 8 h and (e) 10 h.

NiTiO3 which segregated in the Ni-doped TiO2 matrix. Owingto NiTiO3 segregation in Ni-doped TiO2 matrix, grain boundarymobility was controlled by the lattice diffusion of the NiTiO3[7]. This effect is the so-called “drag effect”[8]. However, inthe case of heat-treated rutile TiO2 without the Ni doping, graingrowth of TiO2 is observed at 1000◦C with a grain size ofabout 67 nm. And this result agreed well with other researchers[9].

Fig. 3shows the XRD patterns of mechanically alloyed TiO2powders after heat treatment for different times at 1000◦C.The powders consisted of rutile and NiTiO3 phase. And,despite increasing heating time, grain growth did not occuras shown inFig. 4. The rutile grain sizes were less than

F t1

ig. 2. Grain sizes of Ni-doped rutile and NiTiO3 at different heat temperature

ig. 4. Grain sizes of Ni-doped rutile and NiTiO3 for different heating time a000◦C.

D.H. Kim et al. / Materials Science and Engineering A 408 (2005) 243–246 245

15 nm, and the NiTiO3 grain sizes were about 14 nm. It indi-cates that the powder exhibited enhanced thermal stabilityat high temperatures without grain growth. And the powderwas composed of the nanocomposite of Ni-doped rutile andNiTiO3.

TEM analyses were carried out to observe morphologies ofthe nanocomposite powders.Fig. 5shows TEM micrographs ofnanocomposite TiO2 powder samples at different heating times

and temperatures. After heat treatment for 4 and 10 h at 400 and1000◦C, the nanocomposite powders consisted of black NiTiO3and ashy Ni-doped TiO2 spherical particles. The morphologiesof the nanocomposite TiO2 powders were in the range 10–20 nmfor all heating temperatures. Grain growth was observed at highheat treatment temperatures due to the high surface energy ofthe nanocrystalline particles. However, it was not observed inthe nanocomposite TiO2. It is thought that NiTiO3 controlled

F(

ig. 5. TEM micrographs of the nanocomposite TiO2 powders. Alloyed powder afd) 10 h at 400◦C.

ter heat-treated for: (a) 4 h at 1000◦C, (b) 10 h at 1000◦C, (c) 4 h at 400◦C and

246 D.H. Kim et al. / Materials Science and Engineering A 408 (2005) 243–246

the grain growth by the drag effect. And these results are agreedwell with XRD data (Figs. 1 and 3) and the Debey–Scherrercalculation.

4. Conclusion

Highly thermally stable nanocomposite TiO2 was synthe-sized by mechanical alloying and simple heat treatment. Heatingtime and temperatures did not affect the grain growth of thenanocomposite TiO2 because grain growth was restricted by theformation of NiTiO3 segregated in Ni-doped rutile TiO2. As aresult, the grain growth was controlled by the formation of theNiTiO3 in nanocomposite TiO2.

References

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