synthesis and microstructural control of nanocrystalline titania powders via a stearic acid method

Materials Science and Engineering A328 (2002) 108–112

Synthesis and microstructural control of nanocrystalline titaniapowders via a stearic acid method

Juan Yang, Dan Li, Xin Wang *, Xujie Yang, Lude LuMaterials Chemistry Laboratory, Nanjing Uni�ersity of Science and Technology, Nanjing 210094, People’s Republic of China

Received 29 January 2001; received in revised form 6 June 2001

Abstract

Nanocrystalline titania powders doped with alumina with good dispersity were prepared by a novel method, the so-called stearicacid method (SAM). The preparative process was studied by Fourier transform infrared spectroscopy (FTIR). The influence ofthe alumina dopant concentration on the microstructure of resulting TiO2 powder was investigated by X-ray diffraction (XRD)and transmission electron microscopy (TEM). Compared with the conventional sol-gel technique, SAM is faster and convenient,and can be used to prepare various mixed metal oxides. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Stearic acid method; Nanocrystalline titania; Doping; Microstructure

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1. Introduction

Nanocrystalline titania (TiO2) has received great at-tention in recent years due to its unique dielectric,optical, catalytic properties and potential applicationsin many fields [1–5]. It has been extensively demon-strated that the properties of titania are strongly depen-dent on its crystalline phase, particle size and thesurface structure [6–9]. Therefore, manipulation of themicrostructure of titania, especially of nanocrystallinepowders, is very important in the preparative process.

One of the effective strategies used to control thestructure and properties of nanocrystalline titania isdoping it with other metallic ions [7,10–14]. The dopingprocess was usually performed via the conventionalsol-gel technique, which involved the hydrolysis reac-tion of titanium alkoxide and other metal precursors.However, the hydrolysis rates of different metal precur-sors were usually different. This caused segregation ofsome components during the formation process of thegel. Therefore, the dopant distribution in the resultingmaterials was not so uniform. In addition, the conven-tional method is too time-consuming because the gela-tion process usually needs several days or even months.

We previously developed a non-hydrolytic method,the so-called stearic acid method (SAM) to preparevarious complex oxide nanocrystallines using stearicacid as dispersant [15–17]. Recently, this technique wasdeveloped to prepare metallic ion doped TiO2

nanocrystallines. It was found that this method wasvery convenient and some properties of the obtainedmaterials using this method were different from thoseprepared by the conventional sol-gel technique. In thiswork, we take Al2O3 doped titania as an example todescribe the process of preparing metallic ion dopedtitania by SAM and the influence of the Al2O3 on themicrostructure and photocatalytic activity. The resultswere also compared with samples obtained by the con-ventional sol-gel method.

2. Experimental

2.1. Preparation and characterization of TiO2 powders

Tetrabutyl titanate (Ti(O-Bu)4) and hydrous alu-minium nitrate (Al(NO3)3·9H2O) were used as the pre-cursors of titania and alumina respectively. Stearic acid(C17H35COOH) was used as the solvent and dispersant.The molar ratio of these reactants was: Ti(O-Bu)4:C17H35COOH:Al(NO3)3·9H2O=1:1.5:r (r=0,

* Corresponding author.E-mail address: wxin@publicl.ptt.js.cn (X. Wang).

0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0 9 2 1 -5093 (01 )01674 -4

J. Yang et al. / Materials Science and Engineering A328 (2002) 108–112 109

0.025, 0.05, 0.075). Firstly, an appropriate amount ofstearic acid was heated and melted. Into it, a givenamount of Al(NO3)3·9H2O was added. This mixturewas thoroughly stirred by a magnetic mixer to eliminatethe water. Then the Ti(O-Bu)4was added with vigorousstirring. After 2 h, a homogenous transparent solutionwas formed. The solution was ignited in air and theobtained powders were calcined at different tempera-tures ranging from 450 to 900 °C for 2.5 h in air.

For comparison, the titania powder samples werealso prepared by the conventional sol-gel method ac-cording to the procedure described in Ref. [10].The molar ratio of the reactants was: Ti(O-Bu)4:H2O:EtOH:HCl:Al(NO3)3·9H2O=1:4:15:0.3:r (r=0,0.05). Ti(O-Bu)4 was diluted with half the prescribedamount of ethanol at first, then into it, Al(NO3)3·9H2Owas added, and this mixture was stirred thoroughly bya magnetic mixer. Then H2O and HCl dissolved in theremaining ethanol were added dropwise to the solutionwith continuous stirring to form homogenous sol. Afterbeing kept at room temperature for about a week, thesol was changed to a transparent orange gel. The gelswere dried in a vacuum (10−1 Pa) furnace at 60 °C for6 h and then calcined at different temperatures for 2.5h in air.

Changes in the structures of these powders withtemperature were investigated by XRD experiments ona D8 ADVANCE X-ray diffractometer. The averagegrain sizes of the powders were measured by the XRDline profile analysis using the Scherrer equation [18] andTEM observations on an H-800 transmission electronmicroscope.

2.2. Photocatalytic acti�ity measurement

The photocatalytic activities of the samples wereevaluated by methyl orange decomposition under UV

irradiation. The irradiation was provided by a 300 Whigh pressure Hg lamp with a wavelength centred at365 nm. The initial concentration of methyl orange in aquartz reaction vessel was fixed at approximately 20 mgl−1 with a catalyst (as-prepared titania powders) load-ing of 0.5 g l−1. The reaction cell (450 ml) was bubbledthrough with air at a flow rate of 40 ml min−1. Theextent of methyl orange decomposition was determinedby measuring the absorbance value at 465 nm using aUV-1100 UV–Vis spectrometer.

3. Results and discussion

3.1. Influence of preparation method on themicrostructure of prepared TiO2 powders

Preparative methods have a great effect on the crystalstructure of the obtained nanocrystalline TiO2. Usingthe conventional sol-gel method, calcining at 450 °Cand even 500 °C only led to the formation of anatase,and the anatase to rutile (A�R) transformation oc-curred at �550 °C. However, the rutile structure evenappeared before 450°C using SAM, and the A�Rtransformation took place at much lower temperature(Fig. 1a).

Preparative methods also influenced the grain sizeand dispersity of the obtained powders. Fig. 2 showsthe TEM micrographs of pure TiO2 powders preparedby both SAM and sol-gel methods calcined at 800 °C.The average grain size of TiO2 powders prepared bySAM is about 35 nm, which is a little larger than thatof the sol-gel method (ca. 25 nm). But the powdersobtained by SAM showed better dispersity. The differ-ence in dispersity may result from the difference in theamount of surface hydroxyls. It could be confirmed bythe FTIR spectra of the newly prepared products(shown in Fig. 5) that the characteristic band of hy-droxyl at about 3400 cm−1 could only be observed inproducts prepared by the sol-gel method (Fig. 5e). Thesurface hydroxyls of oxides was considered to be one ofthe main reasons driving the aggregation of grains dueto the hydrogen bonding between these hydroxyls [19].The surface of the powders obtained by sol-gel was richin hydroxyl as a result of hydrolysis [20]. However, forthe powders resulting from SAM, surface hydroxylswere scarce because the preparative process was non-hydrolytic.

3.2. Influence of doping concentration on themicrostructure of prepared TiO2 powders

In the conventional sol-gel method, the addition ofAl2O3 could improve the thermal stability of anataseTiO2 and prevent the A�R transformation [8,14]. Thesame phenomenon was also found in SAM (Fig. 1b). In

Fig. 1. XRD patterns of (a) pure TiO2 powders, (b) Al2O3 doped(r=0.05) TiO2 powders calcined at different temperatures.

J. Yang et al. / Materials Science and Engineering A328 (2002) 108–112110

Fig. 2. TEM micrographs of pure TiO2 powders prepared by (a) SAM, (b) sol-gel method calcined at 800°C.

pure TiO2, rutile TiO2 appeared at �450 °C, but whena small amount of Al2O3 (r=0.05) was added to thesystem, the A�R transformation did not occur untilthe annealing temperature was elevated to 700 °C. Thedoping of only 2.5% Al2O3 prohibited completely theformation of the rutile phase, implying indirectly thatAl2O3 dopant was uniformly distributed in the powders.If Al2O3 had not been well dispersed, the rutile phaseshould appear. The considerable influence of the Al2O3

dopant concentration on the A�R transformation andthe grain size as discussed below also confirmed thisconclusion.

The influence of the amount of dopant on the A�Rtransformation in SAM was investigated. The contentof the rutile phase (XR) in TiO2 crystalline was calcu-lated according to the following equation [21]:

XR=IR/IAK

1+IR/IAK

Where IR is the height or area of the peak of therutile phase in the XRD pattern, while IA is that of theanatase phase, K is a constant equal to 0.79. The resultis shown in Fig. 3. From Fig. 3, it can be seen that theA�R transformation was evidently delayed by theincrease in the amount of dopant.

Fig. 4 shows the anatase crystalline size of TiO2

doped with different amounts of Al2O3. The graingrowth rate of anatase crystallites was reduced with anincrease in doping concentration.

3.3. Characterization of the preparati�e process ofSAM

In order to investigate the uniform distributionmechanism of the dopant in the preparative process,Fourier transform infrared (FTIR) (Bruker Vector 22)

Fig. 3. Content of rutile phase in Al2O3 doped nanocrystalline TiO2

prepared by SAM calcined at different temperatures.

Fig. 4. Change of average grain size of anatase nanocrystalline TiO2

calcined at different temperatures.

J. Yang et al. / Materials Science and Engineering A328 (2002) 108–112 111

Fig. 5. FTIR spectra of (a) stearic acid, (b) gel of Al(NO3)3·9H2O instearic acid, (c) gel of Ti(O-Bu)4 in stearic acid, (d) Al2O3 doped TiO2

powders calcined at 500°C (SAM), (e) Al2O3 doped TiO2 powderscalcined at 500°C (sol-gel).

between metal elements and stearic acid, Ti(IV) andAl3+ were uniformly dispersed in the stearic acid,attaining molecular level distribution.

In SAM, stearic acid was excessive, there was ap-proximately 30% stearic acid with no direct interactionbetween metallic ions, which may function as dispersantin the system. In addition, the mixing process is per-formed in a melted state, i.e. liquid state. Therefore,metallic ions were well dispersed and separated bystearic acid in this mixing system. The highly dispersedprecursor guaranteed that the resulting titania powderwas uniformly doped by alumina.

3.4. Photocatalytic acti�ity measurement

Photocatalytic activity was one of the typical proper-ties of nanocrystalline TiO2 and the property wasstrongly dependent on the surface structure of thematerial [23,24]. In our experiment, photocatalytic de-composition of methyl orange was utilized to probe thesurface structure.

Fig. 6 shows the change in the concentration ofmethyl orange with photoirradiation time. The concen-tration decreased with time in the four systems. But thephotocatalytic activity of undoped TiO2 prepared bythe sol-gel method was better than that from SAM.This may be explained by its anatase phase structureand rich hydroxyls in the powder surfaces. It has beenfound that these two aspects can enhance the photocat-alytic reaction [7,25].

As for doped TiO2, the doping of Al2O3 via thesol-gel route prevented the photodegradation of methylorange. This is similar to Choi et al.’s result [13]. Butthe doping via SAM had little effect on the photoactiv-ity of TiO2 powders. Although we are not sure aboutthe detailed mechanism, these results further confirmthat the microstructure of nanocrystalline TiO2 pre-pared by SAM was different from that resulting fromthe sol-gel method.

4. Conclusion

Al2O3 doped TiO2 nanocrystalline powders withgood dispersity were successfully prepared by SAM.The A�R transformation and grain size of TiO2 pow-ders can be controlled by the dopant concentration andthe calcining temperature. During the preparative pro-cess, the metal precursors were highly dispersed in thestearic acid through the strong interaction betweenthem, which facilitated the formation of uniformlydoped nanocrystalline TiO2.

Compared with the conventional sol-gel method,SAM is more rapid, convenient and general. It is aneffective technique to produce metallic ion doped, evenlarge amount of metallic ion doped, nanocrystalline

Fig. 6. Change of methyl orange concentration against photoirradia-tion time.

was utilized to monitor the structural changes of thecompounds during the preparative process. It wasfound that strong chemical interaction existed betweenmetal precursors and stearic acid. After Al(NO3)3·9H2Owas added into stearic acid, the characteristic absorp-tion bands of –COOH (1704 and 943 cm−1) decreasedwhile two bands at 1560 and 1410 cm−1 were observed,which are assigned to the stretching vibration of –COO− (see Fig. 5), indicating that aluminium stearatewas formed. After Ti(O-Bu)4was added into stearicacid, two new bands at 1590 and 1540 cm−1 appeared.The bands at 1430–1470 and 1550–1590 cm−1 wereattributed to the COO− stretching vibration for biden-tate Ti(IV)–carboxylic acid complex [22], indicatingthat a strong coordination interaction between Ti(IV)and stearic acid existed. Through the strong interaction

J. Yang et al. / Materials Science and Engineering A328 (2002) 108–112112

titania, and it may be extended to the preparation ofvarious nanostructured doped metal oxides or complexoxides.

Acknowledgements

The authors thank the Postdoctoral Science Founda-tion of China and Natural Science Foundation of Ji-angsu province for financial support.

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