Microstructure of nanosized TiO, obtained by sol-gel ... - Science Direct

September

ELSJSVIER

1996

Materials Letters 28 (1996) 225-229

Microstructure

of nanosized TiO, obtained by sol-gel

synthesis

M. GotiC a, M. Ivanda ‘, A. SekuliC a, S. MusiC a’*, S. Popovi6 ‘, A. TurkoviC a, K. FuriC a b Deparfmenf

a l&t&r RnSkoc,i6 hsti&te, qf Physics, Fad& of Sciences,

P.O.

Box 1016,

Unitwsit~

Received 5 February

IO001

of Zagreb.

1996; accepted

Zagreb, P.O.

I1 February

Croatia

Box 162, IO001

Zagreb,

Croatia

1996

Abstract Nanosized TiO, was prepared using a sol-gel procedure. The colloidal suspension was stabilized with hydroxypropyl cellulose (HPC). This polymer prevents sintering of TiO, particles during the heating of the starting material in the form of a solid film. The TiO, crystallite size increased from 5 to 12 nm with increase of temperature up to 5OO”C, as determined by X-ray diffraction. XRD phase analysis showed that the studied samples were a mixture of anatase, as the dominant phase, and brookite. A new approach to size determination of nanophase TiO,, by low-frequency Raman scattering, was used. Keywords:

Titanium

oxide: TiO,;

Synthesis;

Sol-gel;

Films; Microstructure

1. Introduction The initial stage of hydrolysis of Ti(IV)-alkoxide can be described by the following reaction: = Ti-OR

+ HOH +

= Ti-OH

+ R-OH.

(1)

The mechanism of this reaction can be considered in terms of (a) nucleophilic attack of Ti by a water oxygen, (b) transfer of the water proton to the OR group of Ti, and (c) release of the resulting ROH molecule [l]. In the next stage the coordination of titanium is increased through the reaction between partially hydrolyzed oligomers (olation reaction). The formation of oxo-bridges, = Ti-0-Ti = , can be described by the following oxolation reaction: E Ti-OX

+ HO-Ti

z -+ = Ti-0-Ti

(X=HorR).

* Corresponding

(2)

author

00 167.577X/96/$12.00 PII

E +XOH

SO 167-577X(96)0006

Copyright l-4

These reactions are sensitive to the type of Ti(IV)-alkoxide and solvent, pH, concentration of organic the reactants, presence of “neutral” molecules, temperature, mixing, etc. For this reason, it is not surprising that many researches have investigated the influence of these parameters on the chemical, structural and physical properties of the solid products obtained by hydrolysis of Ti(IV)-alkoxides. For example, Ogihara et al. [2] investigated continuous processing of monodispersed TiO, powders by hydrolysis of Ti(OC,H,),. The particle size could be controlled between 0.2 and 0.4 Frn. An aging period longer than 10 min was required for production of monodispersed TiO, powders. Terabe et al. [3] investigated the microstructure and crystallization of the TiO, precursor obtained by hydrolysis of Ti(IV)-isopropoxide in the solution C,H,OH-H,OHCl. The quantity of unhydrolyzed alkyls in the TiO, precursor decreased with increasing amounts of Hz0 and HCI. When the amounts of H20 and

0 1996 Elsevier Science B.V. All rights reserved

HCl became large enough, the molecular structure of the TiO, precursor resembled that of the anatase phase. Lopez et al. [4] investigated the correlation between the crystal structure of TiO, (rutile or anatase) and the experimental parameters of its solgel synthesis. An esterification reaction was used to obtain TiOz gel by hydrolysis of Ti(IV)-isopropoxide in isopropanol [5]. Depending on the concentration ratio of acetic acid and Ti(IV)-isopropoxide, the resulting solid products could be classified as crystals, gels or precipitates. Asaoka [6] investigated polycondensation reactions of tetraisopropylorthotitanate with orthoboric acid in pyridine at 60°C. Submicrometer sized TiO, powders were prepared [7] from Ti(IV)-isopropoxide via three different routes: thermal decomposition in the vapor phase, reaction in supercritical ethanol and sol-gel synthesis. The preparation of TiOZ particles by hydrolysis of Ti(IV)-alkoxide in emulsions was also investigated [8-IO]. Kamiya et al. [ 1 I] described the preparation of TiOz fibers from Ti(lV)-isopropoxide. In the present work the microstructure of TiO,, synthesized by a sol-gel procedure, was investigated. The aim of this research was to obtain nanosized TiO, with improved properties which can be useful for development of photoanodes in dye-sensitized solar cells [ 121.

3. Results and discussion Fig. 1 shows the characteristic part of the XRD patterns of the starting material, TiO, + HPC (solid film), and the solid products after the indicated thermal treatment. In the present experiments the HPC polymer had two roles: (a) to prevent aggregation of colloidal particles in the suspension, and (b) to prevent the sintering of TiO, particles during the heating at elevated temperatures. The starting material and the heated samples were identified as a mixture of anatase, as the dominant phase, and brookite. Both phases exhibited a pronounced diffraction broadening which decreased as the heating temperature increased. The crystallite size of the investigated samples was estimated using the wellknown Scherrer equation: D = 0.9h/P

cos 0.

(3)

Ti02

+ HPC

D=5nm

2. Experimental The chemicals supplied by Aldrich and Merck were used. The precipitation of the TiO, precursor was performed by hydrolysis of Ti(IV)-isopropoxide in a specially designed glass apparatus. The mixing of the precipitation system was varied between 500 and 1000 rpm. The glass apparatus and the experiment by itself were designed to prevent direct contact between air atmosphere and the reacting system. Colloidal TiO, was stabilized by hydroxypropyl celof lulose (HPC), M,\. = 100000. After removal “free” water the solid film was obtained, which was further thermally treated in a laboratory tubular furnace (i2”C) at different temperatures. The samples were studied using X-ray powder diffraction (XRD) and laser Raman spectroscopy.

I

I

15

10

@IQ (Cuxa)

Fig. I. Characteristic parts of XRD powder patterns of TiOl recorded at room temperature (A = anatase, B= samples, brookite). The estimated crystallite sizes, D. xe given lor the .\tarting material TiO, + HPC. and its thermal decomposition product\ obtained at 300. 450 and 500°C. respectively. The e\ttmated error in /I was 25%.

M. GotiC et d/Materials

where X is the X-ray wavelength, 0 the Bragg angle and B the full width of the diffraction line at one half of the maximum intensity. The average crystallite size increased from 5 to 12 nm with increase of the temperature up to 500°C. This result was confirmed by high resolution electron microscopy (HREM) [ 121. It is important to note that the samples analyzed in the present work showed the presence of brookite accompanying anatase; this being different from many similar investigations that considered only the transition scheme “hydrous oxide gel + anatase -+ rutile”. Fig. 2 shows Raman spectra of the starting material, TiO, + HPC, and its thermal decomposition products obtained at 450 and 500°C. Since the starting material, TiO, + HPC, also shows the bands of HPC, the reference Raman spectrum of this polymer is shown in Fig. 3. Raman spectra of TiO, polymorphs have been extensively investigated by many authors. In the present paper we pay attention to some selected publications in this field. For example, Tompsett et al. [ 131 showed that the Raman spectra of natural brookite crystals from Switzerland and Brazil and a synthetic brookite powder are characterized by an intense band at 153 cm-‘. Anatase has a band of

1 P

Wave number

I cm-’

Fig. 2. Laser Raman spectra of sample (a) TiOz +HPC (starting material) and its thermal decomposition products at (b) 450°C and (c) 500°C. The spectra were recorded at room temperature.

221

Letters 28 (1996) 225-229

1500

1000

500

10

Wave number I cm-’

Fig. 3. Laser Raman spectrum of hydroxypropyl cellulose (HPC), M, = 100000, recorded at room temperature. (* ) denotes plasma lines.

similar intensity at 143 cm-’ and rutile lacks a strong band in this region. Bersani et al. [14] observed a shift of the anatase peak from 159 to 149 cm-’ with increase of the calcination temperature of gel-derived glassy TiO,. Felske and Plieth [ 151 recorded Raman spectra of in situ electrochemically formed oxide films on Ti electrodes and, also, the reference spectra of anatase and rutile. Raman spectrum of anatase powder showed characteristic bands at 645, 5 12, 395 and 143 cm-‘, while for rutile powder characteristic bands at 612, 447 and 232 cm-’ were observed. TurkoviC et al. [ 16- 181 investigated the changes in Raman spectra as a function of thermal annealing of TiO, thin films. The Raman bands corresponding to anatase and rutile were assigned. Parker and Siegel [ 191 explained the shift to higher wavelengths of the Raman band at 143 cm-’ in nanophase TiO, by the presence of intergrain defects due to oxygen deficiency, i.e. nanophase TiO, was nonstoichiometric on the average. TiO, thin film, obtained by spraying [ 171, showed a shift of anatase from the 143 cm- ’ to higher wavenumbers. XRD analysis of the sample indicated that the size of the TiO, crystallites was 4.7 nm. On the other hand, the sample obtained by CVD (chemical vapor deposition) had a crystallite size of 50 nm. For this sample the most prominent band was positioned at 140 cm-‘. On the basis of this work, the

shift of the Raman band at 143 cm- ’ to greater wave numbers could be used as a fast probe for nanosized TiOz. Fig. 2 shows the shift of most prominent band from 153 to 148 cm- ’ with increase of temperature up to 500°C. These peak positions indicate a substantial difference in comparison with the band at 143 cm-’ [ 16,191 recorded for a single crystal or crystalline thin film of anatase. As XRD analysis of these samples showed the presence of anatase and brookite. there is an overlap of the most prominent Raman bands of these two phases in this spectral region. The decrease of brookite and a corresponding increase of the anatase fraction with temperature can cause the shift of the band at 153 cm-’ to lower wave numbers. Taking into account this result and those of the above mentioned publications it can be concluded that the factors such as nonstoichiometry, poor crystallinity and presence of both anatase and brookite phases can introduce an error in the size determination of nanophase TiO,. In order to eliminate the intluence of the above mentioned factors which can lead to errors, we suggest a new method for the size determination of nanophase TiO,. This method is based on work of Duval et al. [20], originally applied to crystallized cordierite glass in the region of low-frequency Raman scattering. The authors showed that the maximum of the low-frequency Raman band was proportional to the inverse diameter of the spherical spine1 microcrystallites. Based on the theoretical results of Lamb [21] and Tamura et al. [22], the frequency v (in cm- ’ ) of the lowest-energy spherical mode of a free particle of spinel, corresponding to angular momentum I= 0, is: 1, = 0.7 r,,/dc,

(4)

where c is the vacuum light velocity, l‘, is the longitudinal velocity of sound and d is the particle diameter. Duval et al. [20] pointed out that the technique of Raman scattering could be a simple and good method for characterization of nanosized particles. In addition, this simple method is also complementary to small angle neutron and X-ray scattering. Fig. 4 shows the lower frequency Raman spectra of the starting material, TiOl + HPC (Fig. 4a) and the products of its thermal decomposition (Fig. 4bd). The Raman spectra are corrected for linear back-

-120 -100

-80

-60

-40

-20

Wave Fig. 4. Low-frequency ture correction

of (a)

0

20

40

60

60

100

120

number I cm-’

Raman spectra after baseline and temperamaterial)

and its

thermal decomposition products at (h) 3Oo”C, (c) 450°C

TiO,

+HPC

(starting

and (d)

500°C. In the spectra the anti Stokes (&) and Stokes (+ shown. Sharp peak at - 13 cm-’

) sides are

i!, a plasma line.

ground and temperature reduced for a Bose-Einstein factor. Table I presents the positions of low-frequency Raman peak v and the corresponding particle diameter d calculated from Eq. (4). For the longitudinal velocity of sound the average value for rutile [23]. (3,= 9022 ms- ’ , was used. It is supposed that the differences in the longitudinal velocity of sound in different TiO, polymorphs are not large. A larger deviation of the particle size, determined for the sample prepared at 300°C. in relation to the XRD result, can be explained by the nature of the sample. This sample was black due to the presence of carbon, i.e., the HPC at this temperature was not completely oxidized (burned). Therefore, the absorption of the laser beam by the sample was large and in this case

Table I Positions of low-frequency

Raman peak, V, and particle diameter,

(/. determined from peak position Maximum temperature of heating (“C) (TiOl

V(cm-‘)

dblm)

+ HPC) ,’

32

66

300

20

IO.5

550

24

500

19

” Starting material in the form of solld film

x.x II.1

M. GotiC et al./ Materials

satisfactory statistics of the Raman scattering was not obtained. As a consequence, the actual position of the low-frequency Raman band for this sample was difficult to determine. In spite of this fact, we concluded that the size determination of nanophase TiO,, by low-frequency Raman scattering, as presented in Table 1 is in good agreement with the crystallite size measured by XRD. As a conclusion, the present study showed that the method by Duval et al. [20], originally developed for crystailized cordierite glass, can also be applied in size determination of nanophase TiO,, or generally for nanosized metal oxides which are not inside the host matrix, such as in the oxide glass.

Acknowledgements This work was supported by Ministry of Science and Technology, Republic of Croatia (grants No. l-07- 190 and No. l-03-066). The authors gratefully acknowledge the support of the USA (National Institute of Standards and Technology&Croatia (Ministry of Science and Technology) Joint Fund (grant No. JF106). One of the authors (MI) wishes to thank for the Alexander Von Humboldt fellowship.

References 111 M. Kallala, C. Sanchez and B. Cabane, Phys. Rev. E 48 (1993) 3692. [2] T. Ogihara, M. Ikeda, M. Kato and N. Mizutani, J. Am. Ceram. Sot. 72 (1989) 1598. f3] K. Terabe, K. Kato, H. Miyazaki, S. Yamaguchi and A. Imai, .I. Mater. Sci. 29 (1994) 1617.

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229

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