nanostructured materials, vol. 11, no. 8, pp. 1293_1300, 1999

8/12/2019 NanoStructured Materials, Vol. 11, No. 8, Pp. 1293_1300, 1999

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PREPARATION AND CHARACTERIZATION OF NANOSIZED

TiO2 POWDERS FROM AQUEOUS TiCl4 SOLUTION

Q.-H. Zhang, L. Gao* and J.-K GuoThe State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai, Peoples Republic of China, 200050

(Received September 27, 1999)

(Accepted November 5, 1999)

AbstractA solution-based processing method has been used to synthesize nanocrystalline TiO2

powders bycontrolling the hydrolysis of TiCl

4in aqueous solution. As-prepared powder was characterized by TEM, HREM,

XRD, BET and ED techniques. In the presence of a small amount sulphate ions, the powder was pure anatase and itsprimary particle size is finer than that of alkoxide-derived powder, moreover the transformation of anatase to rutilewas retarded. Both hydrolysis temperature and amount of sulfate ions had effects on the morphology and crystalli-zation of the powder. In an optimum process, TiO

2powder in the anatase phase was fabricated without calcination and

its primary size was 4nm, BET surface area was 290m2/g. 2000 Acta Metallurgica Inc.

Introduction

Titania has been studied extensively because of its wide applications in pigments, catalyst supports,fillers, coatings, photoconductors, dielectric materials and so on. In recent years, nanocrystalline TiO

2

is well known as a semiconductor with photocatalytic activities and has a great potential for applicationssuch as environmental purification, decomposition of carbonic acid gas, and generation of hydrogen gas[1,2]. A key requirement to improving its activity is to increase the specific surface area and decreasethe primary particle size of the catalyst. Generally, titania is obtained either from minerals or from asolution of titanium salts or alkoxides. Synthetic routes of TiO

2production usually result in amorphous

solid TiO2or anatase or rutile depending on the preparation route and the experimental conditions. It

is also known that the transformation behaviour from amorphous to anatase or rutile phase is influencedby the synthesis conditions. However, to our knowledge, most of the literature shows that alkoxide-

based sol-gel or precipitation process yield amorphous titania precursors, or powders with the anatasephase [35]. Phase transformation to the rutile structure takes place when the temperature is raised atleast above 450C [5,6]. As precursors of nanocrystalline oxide powders, however, inorganic com-pounds are more economical than alkoxides. At present, there are a few studies on preparation ofmonodispersed titania powders from TiCl

4or Ti(SO

4)2[7,8], also, gas-phase synthesis of titania by

TiCl4oxidation in the presence of dopants was investigated [9,10]. Additionally, other researchers have

prepared crystalline TiO2powder using TiCl

4as the starting material in the chloride process. Ocana et

al. [11] obtained rutile TiO2powder at 98C, using 3M TiCl

4, for measuring its infrared and Raman

spectra, but did not comment on the fabrication method. Terwilliger et al. [12] prepared the rutile phaseTiO

2powder with an average grain size 20nm by doping with a small amount of Sn, so the powder

was a mixture of SnO2and TiO

2. Matijevic et al. [13] reported that precipitation of crystalline TiO

2

*Corresponding author.

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occurred in dilute TiCl4solution after aging at 98C for 37 days in the presence of SO

4

2 ions. Thisis not an economical method because of the long aging time and low productivity.

In this work, a stable aqueous TiCl4solution made from TiCl

4that vigorously reacts with water in

an ice-water bath is used as the starting material to obtain an ultrafine TiO2powder with uniform size.Because crystalline TiO

2precipitates are formed homogeneously at room temperature to 100C with a

productivity 90% by just heating, stirring and neutralizing the TiCl4solution, which is diluted withan appropriate amount of water, neither high temperature nor oxygen gas is needed for oxidation andcalcination to take place during this process. Thus process simplification leads to lower production costsand makes a continuous process possible. Moreover, in an optimum process, titania powder of anatasephase was prepared without calcination, which primary particle size was 4nm and its BET surface areawas 290m2/g. This TiO

2powder is finer than that of alkoxide-derived powders. The relationship

between hydrolysis conditions of titanium tetrachloride and the thermal transformation of the hydrolysisproducts, particularly the effects of the temperature of hydrolysis and the presence of sulphate ions on

the anatase-rutile transformation and crystalline morphology were investigated as a part of study on theproperties of titanium dioxide formed by the hydrolysis of titanium tetrachloride. We also preparedpowders in both anatase and rutile phase which are potential to be applied as photocatalyst with highphotocatalytic activity since their primary particle size is below 10nm and the rutile phase of them canbe excited by the light 365nm, which is the strongest peak emitted by medium or high pressure Hglamp, and below.

Experimental Methods

(1) Preparation of Titania Powders

Titanium tetrachloride (98% TiCl4) was used as a main starting material without any further purifica-

tion. When TiCl4

dissolved in water, the heat of the exothermic reaction explosively generated theformation of orthotitanic acid [Ti(OH)

4]. Since the formation of that species disturbed homogeneous

precipitation, the appreciated amount TiCl4was dissolved in distilled water in an ice-water bath. The

concentration of titanium was adjusted to 3M. This aqueous solution was then mixed with pure distilledwater or (NH

4)2SO

4solution in a glass bottle and placed in a temperature-controlled bath. Temperatures

were varied in the 20-95C range. The mixture was stirred at high speed while the amount of TiCl4

solution necessary for the desired [H2O]:[Ti] molar ratio was added dropwise. Maintaining at the same

temperature for 1 h, the mixed solution were treated with 2.5M dilute NH4OH until the pH value was7. The hydrolysis and condensation reactions started immediately upon mixing, as indicated by the rapidincrease in turbidity and the formation of large, visible flocs, which precipitated at the bottom of thereaction vessel. The reaction conditions were maintained for a period of 1 h, during which thereactions(hydrolysis and particle formation) were assumed to have attained the equilibrium. Subse-quently, the precipitated titanium hydroxide/hydrous titanium oxide (TiO

2nH

2O) was separated from

the solution by using filtration and repeatedly washed with distilled water and dilute NH4OH solution

to make TiO2

nH2O that was free of chloride ions. The hydrous oxide was dried at room temperature

(30C) under vacuum and ground to fine powder. In some cases, the hydrous TiO 2 powder werecalcined at various temperatures for 2 h at the rate 3C/min. For the preparation of a mixture of anataseand rutile phase TiO

2 powder, some concentrated HCl was added instead of (NH

4)2SO

4 and the

concentration of TiCl4was adjusted to 0.5M with distilled water. A flow chart of the technique is shown

in Figure 1.

NANOSIZED TiO2POWDERS1294 Vol. 11, No. 8


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(2) Characterization

X-ray diffraction(XRD) patterns were obtained on an automated diffractomer RX-10. The weightfractions of the anatase and rutile phases in the samples were calculated from the relative intensities ofthe strongest peaks corresponding to anatase and rutile as described by Spurr and Myers [14]:

1/1 0.8IA/IR) (1)

whereis the weight fraction of rutile in the powder, while IAand IRare the X-ray integrated intensitiesof the (101) reflection of anatase and (110) reflection of rutile, respectively.The morphology of the powder as well as electron diffraction patterns obtained by transmission

electron microscopy (TEM) in a JEOL 200CX ultra high resolution TEM operating at 200kV withpoint-to-point resolution of 2.5. The primary particle size was calculated using the Scherrer formulaand obtained by TEM observation. The Brunauer-Emmett-Teller (BET) surface area was determinedusing a Micromeritics ASAP 2010 nitrogen adsorption apparatus.

Results and Discussion

Table 1 showed a summary of the X-ray diffraction and BET surface area results of nanocrystallinepowders prepared under various hydrolysis conditions. The addition of sulfate is quite effective inpromoting the formation of the anatase phase at a low temperature, the primary particle size of powders

Figure 1. Flow chart of the steps involved in the preparation of nanosized TiO2powder.

NANOSIZED TiO2POWDERS 1295Vol. 11, No. 8


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obtained from hydrolysis of TiCl4solution at 70C and 95C was 3.5nm and 4.0nm respectively, which

is finer than those of alkoxide-derived titania powders [36]. The BET surface area of these powderscalcined at 400C for 2h also indicated these products were ultrafine.

XRD patterns in Fig. 2 show that, during the precipitation process, the presence of SO4

2 ions isbenefit to occurrence of anatase phase and promotes 45nm anatase crystallites. But, when TiCl

4

solution hydrolysed at 70C in the absence of SO4

2 ions, powder consists of both rutile and anatasephases at the ratio 0.63. At room temperature, neither the presence of SO

4

2 ions or nor, thepowder dried at RT under vacuum is dominantly amorphous. It is interesting that when sample 3 wascalcined at 400C and 600C for 2h, was equal to 0.58 and 0.72, respectively. While for the sample

1 the temperature at which rutile occurs is around 700C. These results indicate that some amorphoussolid TiO

2exists in the sample 3 dried at 30C under vacuum, it transformed to anatase at elevated

temperature. Assuming the amount of rutile was constant and all amorphous solid transformed toanatase up to 400C, we calculated the percentage of amorphous titania to nanocrystalline product was8.6%, but, as the temperature elevated increasingly to 600C, anatase transformed to rutile partially, the

TABLE 1Comparison of Powders Prepared under Various Hydrolysis Conditions

[Ti]/SO4

2]Hydrolysis

T/CDried at RT

Particle size*/nm

Calcined at 400C

Weightfraction ofrutile ()

Particle size*/nm

BET surfacearea/m2g1

Sample 1 1:2 95 4.0 6.8 189 0Sample 2 20:1 70 3.5 9.5 138 0

0.63(RT)Sample 3 1:0 70 4.4(R) 10.7(A) 73 0.58(400C)

6.0(A) 14.2(R) 0.72(400C)Sample 4 1:0 RT Amorphous 10.1 131 0Sample 5 20:1 RT Amorphous 9.5 136 0

*Calculated by XRD, A denotes anatase and R denotes rutile.

Figure 2. XRD patterns of three titania powders, taken after dried at room temperature under vacuum: (a) sample 5, (b) sample4, (c) sample 1, (d) sample 2 and (e) sample 3.



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percentage of anatase transformed to rutile was calculated to be 33%. So, from the transformation ofsample 3, we found the transformation temperature of anatase to rutile was below 600C andapproximate to that of alkoxide-derived titania powders [3]. The reason resulted in the difference oftransformation temperature between them may be twofold: rutile seeds induced the transformation ofanatase to rutile and trace sulfate ions retarded the transformation in the sulfate process.

To obtain finer nanocrystalline titania in anatase phase, the effect of sulfate on morphology issignificant. HREM micrographs confirmed the 45 nm size of the crystallites and their crystallinity:anatase electronic diffraction ring patterns were observed, as well as continuous lattice fringes on manyparticles in the sample 1dried at room temperature (RT). Its BET surface area was 290m2/g. Fig. 2 gavethe morphology and electron diffraction (ED) pattern, the latter shows that the brightness and intensityof polymorphic ring is weak, so the powder crystallized partially and was somewhat amorphous.

When the samples were calcined at 400C for 2h, they crystallized completely, the intensity of XRDpatterns as well as ED patterns increased. Fig. 3 are their TEM micrographs, the morphology of thesamples is different from each other: the primary particle size of sample 1 is finer than the rest, someparticle was bottle-fused. But, for sample 2, as same as for sample 1, it was all anatase, particles weredecrete. By the TEM observation of Fig. 4, we noticed that the larger particles were cubic-shaped whilethe finer particles were polyhedral, recalling Table 1, for sample 3, the primary particle size was 10.7nmand 14.2nm calculated from Scherrers formula (D 0.89/Bcos) for anatase and rutile, respectively.We conclude that larger cubic-shaped particles are crystallites of rutile. Since a strong acidic environ-ment generated by hydrolysis of TiCl

4, the formation of TiO

2particles was kinetic controlled. During

the condensation process, the formation of the kinked chains of edge-sharing octahedral correspondingto anatase appears more probable than the formation of the straight chains typical of rutile as the pH

value raised resulting from the neutralisation of NH4OH. It means that pure rutile TiO2powder wassynthesized possibly by varying the hydrolytic parameters. In the strong acidic solution of TiCl

4, by

controlling the concentration and hydrolytic time of the solution, we could also obtain pure rutile phasetitania, the (f) in the Fig. 4 was its TEM micrograph and its BET surface area was 166m2/g. This workis being in progress and we will report the process in detail soon. From the TEM observations, wenoticed that the particle size was unifom and also it was in good agreement with that calculated bySherrers fomula.

When TiCl4

hydrolyses, it generates TiO2

particle as well as H and Cl ions, the process isdescribed as equation 2:

TiCl4 2H2O 3TiO2 4H 4Cl (2)

In the present work, the concentration of TiCl4used for preparation of nanocrystalline TiO

2was much

higher than those employed for the preparation of TiO2colloidal particles [15,16]. The concentration

Figure 3. HREM image and ED patterns of sample 1 dried at RT under vacuum.



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of H increased as the hydrolytic reaction went on, this H ions inhibited the complete hydrolysis ofTiCl

4. The hydrolysis may include three steps as shown below:

TiCl4 H2O7TiOH3 H 4Cl (3)

TiOH37 TiO2 H (4)

TiO2 H2O 7 TiO2(hydrous) 2H (5)

The XRD results in Fig. 2 indicate that the nucleation of both anatase and rutile depending on the

hydrolytic conditions especially the temperature. At room temperature, whether the presence of sulfateor not the rate of nucleation is so slow that the powder was dominantly amorphous, however, for thepowder obtained in the presence of sulfate ions, the intensity of diffraction peak at 225.28 was

Figure 4. Bright field TEM image of the samples calcined at 400C for 2h: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample4, (e) sample 5; and (f) rutile phase TiO

2dried at RT under vacuum.



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stronger than those prepared in the absence of SO4

2. When 1M TiCl4solution hydrolyzed at 70C for

1h, the nucleation of rutile carried out but the growth of titania particles is not fast enough at thetemperature in the strong acidic environment, thus the shape of rutile is cubic-shaped. The remaining

species of Ti transformed to anatase as the pH value increased resulting from the neutrallizing of diluteNH

4OH. Thus, the primary particle of this powder in the anatase phase was larger than those prepared

in the presence of SO4

2 at 70C and 95C. Additionally, when (NH4)2SO

4was added prior to

hydrolysis of TiCl4solution, although the concentration of SO

4

2 ions as low as 1/20 th to that of Ti4,it could be observed that this onset of absorption shifted to longer wavelength [17]. It is well knownthat the onset of the UV-Vis absorbance red-shifted as the primary particle size of semiconductorsincreased. We concluded that the presence of sulfate accelerated the growth of TiO

2cluster in the

anatase phase.In the ceramics industry, it is well known that (a) sulphate process generally produces anatase or by

addition zinc or aluminium salts to produce rutile titania; (b) chloride process produces rutile TiO2by

gas-phase oxidization of TICl4. In this work, the reason that the addition of a small amount of SO

4

2

ions promotes anatase may be that SO4

2 ions induce the growth of TiO2clusters to anatase phase,

which is in agreement with the red-shift ofos

in the absorption spectra and XRD results. In the processemployed preparation for sample 3, the concentration of H generated by the hydrolysis of TiCl

4was

so high that the growth of titania clusters is under kinetic control, when the temperature is raised to 70Cfor 1h, the growth of TiO

2clusters to rutile phase is difficult in view of the result of UV-Vis spectra.

However, the solution neutralised with dilute NH4OH solution, the rate of rutile formation accelerates

but the pH value over a critical value, anatase formed. For this reason, the morphology of rutile iscubic-shaped and different from the crystallites grown from clusters of rutile short-range ordered [TiO

6]

octehedra by an aggregative growth process [18].

Conclusion

Nanocrystalline TiO2powders in anatase or a mixture of anatase and rutile phase has been synthesised

from aqueous solution of TiCl4. The addition of a small amount of (NH

4)2SO

4promotes occurrence of

anatase phase and inhibits the anatase-rutile transformation such that the powder is completely anataseafter calcining at 650C for 2h and rutile occurs at around 700C. Otherwise, in the absence of SO

4

2

ions, the powders had both rutile and anatase phase, when they were calcined at 600C, anatase startsto transform to rutile. The transformation temperature of anatase to rutile is approximate to those ofalkoxide-derived process. The concentration of SO

4

2ions has some effects on the primary particle size:as the ratio of Ti to sulfate is equivalent to 1:2, finer titania crystallite can be prepared, it is less than

7nm and BET surface area of this powder is as high as 189m2/g even calcined at 400C for 2h.

References

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11. M. Ocana, V. Fornes, J. V. G. Ramos, and C. J. Serna, J. Solid State Chem. 75, 364 (1988).12. C. D. Terwilliger and Y.-M. Chiang, Nanostruct. Mater. 2, 37 (1993).13. E. Matijetic, M. Budnik, and L. Meites, J. Colloid Interface Sci. 61, 302 (1977).14. R. A. Spurr and H. Myers, Anal. Chem. 29, 760 (1957).15. E. Joselevich and I. Whillner, J. Phys. Chem. 98, 7628 (1994).16. C. Kormann, D. W. Bahnemann, and M. R. Hoffmann, J. Phys. Chem. 92, 5196 (1988).17. Q.-H. Zhang, L. Gao, and J.-K. Guo, Mater. Sci. Eng. A. in review.18. J. Slunecko, M. Kosec, J. Holc, and G. Drazic, J. Am. Ceram. Soc. 81, 1121 (1998).


nanostructured materials, vol. 11, no. 8, pp. 1293_1300, 1999

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