Nuclear Instruments and Methods in Physics Research B 97 (1995) 198-201

ELSEVIER

Beam Interactions with Materials 8 Atoms

Local order in sol-gel derived glassy TiO,

G. Antonioli a, D. Bersani a, P.P. Lottici a3 * , I. Manzini a, G. Gnappi b, A. Montenero ’

a Physics Department, University of Parma. Viale delle S&me, 43100 Parma, Italy b CORIVE, Consorzio Ricerca Innovazione Veiro, Viale delle Scienze, 43100 Parma, Italy

’ Chemistry Department, Utkersity of Parma, Viale delle Scienze, 43100 Parma, Italy

Abstract The short range order around Ti atoms in sol-gel derived bulk amorphous TiO, is investigated by X-Ray Absorption

Spectroscopy (EXAFS and XANES). Two Ti-0 distances are found: the longer distance is attributed to the presence of

anatase micro-crystallites, whereas the shorter one is due to a residual of the alkoxide precursor. XANES measurements give

no evidence of tetrahedrally coordinated Ti, both in the organic precursor or in the amorphous phase. The X-Ray absorption data confirm that thermal treatments up to 350°C on bulk amorphous TiOz give crystalline anatase and up to 750°C give

crystalline rutile.

1. Introduction

Since the development of the sol-gel technique, great attention has been paid to glasses with mixed TiO,-SiO,

composition [I], which are interesting for applications in fiber and integrated optics. Less interest has been devoted

to pure TiO, in its amorphous form, most studies being on titania films [2], even if many technological applications

have been proposed also for the bulk phase. The transfor- mations occurring during heat treatments on TiOa by a sol-gel technique were previously investigated by Raman

spectroscopy and X-Ray Diffraction (XRD) [3]. Here an X-Ray absorption study on the organic titanium precursor, on the bulk amorphous TiO,, on the anatase and rutile crystalline phases is reported, aiming at a better knowledge of the changes around the titanium atoms from a disor- dered (liquid or amorphous) state to the crystalline phases.

2. Experiment and data analysis

Titaniumisopropoxide Ti(OPr i)4 is commonly used as

the precursor compound for the sol-gel process to obtain TiOz. To reduce the hydrolysis reaction rate [4], the pre- cursor was modified by changing the isopropyl group by methoxyethyl group R’ = -CH,-CH,-0-CH,). The ob- tained tetramethoxyethoxytitanium Ti(O-R’), (TMET) was added to a solution of water, HCl and methoxyethanol. In about 60 h the gelation process was completed. The gel

* Corresponding author, Tel. +39 521 905238, Fax +39 521

905223, E-mail: lottici@vaxpr.pr.infn.it.

was dried at 110°C for 24 h and then powdered. Differen- tial Thermal Analysis on the TiO, dried gel showed [.5]

three transformations, corresponding to the combustion of organic compounds, to the phase transformation to anatase and to a further combustion of residual organic materials,

respectively. XRD revealed that the dried gel powder was amorphous. Heat treatments were then performed on the powder at a heating rate of lOO”C/h. Wide crystalline

peaks grew up to 350°C where the XRD of the anatase was obtained. At higher temperatures, both crystalline

TiOz forms were present up to complete rutile at 750°C. Room temperature X-ray absorption spectra in trans-

mission mode were obtained at the titanium absorption K edge using synchrotron radiation at LURE, Orsay (France), from DC1 storage ring operating at 250 mA and 1.85 GeV. A copper foil was used to calibrate the photon energy. Monochromatic X-ray beams were obtained by a Si(311) double crystal and the energy resolution was about 1.5 eV. The energy was scanned with a 0.2 eV step around the edge and at 1 eV step up to 900-1000 eV beyond the edge. The EXBACK routine [6] was used for background

removal on the raw absorption data. The Fourier Trans- form (FT) of the experimental modulation x(k). weighted by k3 (k is the photoele$ron wavenumbfr), was taken over the range kmi, = 2.6 A-‘, k,,, = 12 A-‘. The mag- nitude of the FTs is shown in Fig. 1: it represents a radial distribution function (RDF) plot around the titanium atoms when properly phase-shift corrected.

The peaks in the RDF of anatase and rutile correspond to the crystalline distances given in Table 1. Apart from the structure in the first peak, a close resemblance is observed between amorphous and anatase RDF up to the second and third coordination shells. To have true dis-

0168-583X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-583X(94)00390-4

G. Antonio/i et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 198-201 199

-- ri 4 6 8 10

N_

-/-

R (A)

Fig. 1. Fourier transform magnitude of the k’x(k) functions of

sol-gel derived TiO, from the amorphous phase to rutile. Com-

mercial rutile is reported for comparison.

tances, the first two main peaks were isolated and back

Fourier transformed. The filtered x(/c) functions were

analyzed in k-space by using the EXCURV90 program [6],

which calculates ab initio phase shifts and back scattering amplitudes [7]. The EXCURV90 parameters AFAC (in- elastic processes) and VP1 (finite photoelectron lifetime effects) were fixed at the values 0.63 and -4, respec-

tively, as to obtain a good fit with anatase. Free parameters are the coordination numbers N, of the jth shell, the distances R, to the Ti atoms, the Debye-Wailer like

parameter 2 a,’ and the energy shift A&, used to correct for energy offset of the photoelectron at zero wave vector. The fitting parameters obtained are reported in Table 1. The distances determined by EXAFS analysis in anatase

and rutile are shorter than the mean distances determined by XRD by about 0.01-0.02 A. No significant improve-

ment was obtained by using two shells parameters for the first peak in anatase and rutile. In the amorphous phase a two shells fit of the first peak was necessary and two

oxygen distances at 1.79 and 1.94 A were found. When taken as fitting parameters, the coordination numbers and

the Debye-Waller parameters are affected by large uncer- tainties [8] and this is particularly evident for rutile (Table 1). This fact deserves further investigations.

Table 1

EXAFS results for first- and second shell coordination, Ti-0 and Ti-Ti distances and disorder parameters. In parentheses are reported the

crystallographic values [18]. The fit parameter AE,, is also indicated

Sample N, R, ,

2a,- N, R, 2 3

a; 14, 0 [Al [AZ ] T; rs [AZ 1 kvl

TiOZ 7.5 1.92 0.015 3.9 2.96 0.02 13.3 Rutile (6.0) (1.96) (2) (2.956)

TiO, 6.5 1.92 0.015 3.5 2.95 0.02 17.9 Rutile (sol-gel)

TiO, 6.0 1.92 0.011 3.0 3.01 Ana;ase

0.007 11.7 (so-gel) (6.0) (1.95) (4) (3.04)

TiO,

Amorphous

3.5

1 .o 1.93 0.009

1.79 0.003 1.1 3.02 0.009 13.0

Table 2

Peak positions (eV) in the XANES spectra. The peaks are labelled as in Ref. [lo]

Sample

TiOz rutile (sol-gel)

TiOz anatase (sol-gel) TiOz amorphous

TMET

A, ‘4, A3 B c, c2 4967.1 _ 4969.6 4972.8 4979.1 _

4966.2 4968.1 4969.3 4971.5 4977.7 4980.9 4966.6 4967.9 4969.0 4971.2 4976.6 _

4966.6 4967.9 4968.9 4970.9 4979.5 _

IV. GLASSES

200 C. Antonioli et al. /Nucl. Instr. and Meth. in Phys. Res. B 97 (1995) 198-201

a)

TMET

/-

(i

4960 4970 4980 4990

Energy WI

c

TiO, Anatase

sol-gel

4960 4970 4980

Enew WI

4990 4960

TiO, Amorphous /-

4970 4980

Energy W

4990

TiO, Rutile

sol-gel

4970 4980

Energy WI

4990

Fig. 2. XANES spectra of the organic precursor TMET (a), amorphous (b), anatase from sol-gel (cl, rutile from sol-gel (dl determined by background removal and normalization to the edge jump. The peaks are labelled as in Ref. [lo].

The absorption in the edge region (XANES) shows characteristic peaks related to the Ti symmetry, coordina- tion and oxidation states [9]. The XANES spectra of TiO, anatase, rutile, amorphous and liquid precursor TMET are shown in Figs. 2a-2c. The energy of the features (labelled as in Ref. [lo]) may be determined using Gaussian fits and are listed in Table 2. In rutile and anatase one obtains a good agreement with other XANES measurements [9-111.

3. Discussion

EXAFS results reveal an atomic arrangement in the amorphous titania similar, up to second and third coordina- tion shells, to that found in anatase. This suggests a micro-crystallite structure for the amorphous phase, with particle sizes inaccessible by X-ray diffraction. In the amorphous TiO, a first shell double distribution of dis-

G. Antonioli et al. / Nucl. Instr. and Meth. in Phys. Res. B 97 (I 995) 198-201 201

tances is found. Even if no data on the Ti coordination are

tvailable for the TMET precursor, the short distance (1.79 A) is comparable with those observed in different organic

titanium precursors [12,13] as in Ti(OPr’),. This distance cannot be ascribed to square based pyramidal TiO, units, measured by EXAFS at about 1.65 A [14,1.5]. Since in the

tetrahedrally coordinated compound Ba,TiO, the first shell radius ranges from 1.63 to 1.82 A [16], the short Ti-0 distance could be ascribed to a four-coordinated Ti. The XANES data, however, give no evidence of a tetrahedral coordination (see the following). The short Ti-0 coordina- tion distance may therefore be due to residual of the Ti alkoxyde in the amorphous phase. From the coordination

numbers shown in Table 1, even if largely inaccurate, one could roughly estimate a percentage of Ti atoms of about 60% with anatase-like environment.

The XANES spectra of Ti bearing silicates and oxides

where Ti atoms are found mainly in octahedral sites [9,10]

show typical three-fold or four-fold pre-edge features as those shown in Figs. 2c-2d. For tetrahedrally coordinated

titanium, as found in Ti(OPr’), [ 121 and BazTiO, [16], however, the XANES spectra show a characteristic intense single pre-edge peak at N 4967.5 eV. The intensity of the A, peak is rather low compared to that found in Ti(OPr’),:

furthermore each Ti alkoxyde studied in Ref. [12] shows a single relatively intense pre-edge peak between 4967.0 and 4967.4 eV and a bump on the rising shoulder at nearly the same energy in all samples (_ 4973.4 eV) whereas in

TMET only the typical triplet feature characteristic of octahedrally coordinated Ti is observed. A four-fold coor- dination for Ti may therefore be ruled out. TMET has been

in fact described as having a dimeric structure with trigo- nal bipyramidal type of geometry around Ti atoms (five- fold coordination) [17].

The A, peak in the amorphous shows a relatively high

intensity compared to the same peak in anatase and its energy is compatible with a four-fold or a five-fold coordi- nation as in TMET. However, since from EXAFS it is found that roughly 40% of Ti atoms in amorphous material have environment different from that of anatase, the A, peak intensity in amorphous material is still too low to be

assigned to a pure tetrahedrally Ti coordination.

As concerns the crystalline phases obtained by heat treatments, Table 1 shows that the crystalline phases are formed in the correct anatase-rutile order, without coexist- ing rutile microcrystals in the amorphous to anatase phase

131.

4. Conclusions

EXAFS investigations in the sol-gel derived bulk

amorphous titania indicate a micro-crystalline structure locally similar to that found in anatase with a residual short Ti-0 coordination. The XANES spectra suggest that this short distance cannot be ascribed to four-fold coordi- nated Ti but to a residual of the organic precursor TMET.

The crystalline structure following the first phase trans-

formation at about 350°C is completely anatase with no evidence of rutile micro-crystals. At temperatures above 750°C the transformation is to pure rutile.

References

ill

121 [31

[41

[51

161

[71

[81

[91

[lOI

1111

[121

[I31

1141

1151

[161

[171

[I81

B.E. Yoldas, .I. Non-Cryst. Solids 38/39 (1980) 81. A.J. Perry and K. Pulker, Thin Solid Films 124 (198.5) 323. P.P. Lottici, D. Bersani, M. Braghini and A. Montenero, J.

Mater. Sci. 28 (1993) 177. R.C. Mehrotra, Proc. Winter School on Glasses and Ceram-

ics from Gels, SAo Carlos (1989) 17.

M. Braghini, Degree Thesis, Universita degli Studi di Parma.

AA 1990-1991. Chemistry Department

N. Binsted, S.J. Gurman and P.C. Stephenson, SERC Dares-

bury Laboratory Programs: EXBACK, EXCURV90.

S.J. Gurman, N. Binsted and I. Ross, J. Phys. C 17 (1984)

143.

B.K. Teo, EXAFS: Basic Principles and Data Analysis

(Springer, Berlin, 1986).

GA. Waychunas, Am. Mineral. 72 (1987) 89.

R. Brydson, H. Sauer, W. Engel, J.M. Thomas, E. Zeitler, N.

Kosugi and H. Kuroda, J. Phys. Condens. Matter 1 (1989)

797.

S. Quartieri, G. Antonioli, G. Artioli and P.P. Lottici, Eur. J.

Mineral. 5 (1993) 1101.

F. Babonneau, S. Doeuff, A. Leaustic, C. Sanchez, C. Cartier

and M. Verdaguer, Inorg. Chem. 27 (1988) 3166.

S. Barboux-Doeuff, C. Sanchez, Mat. Res. Bull. 29 (1994) 1.

CA. Yarker, P.A.V. Johnson, A.C. Wright, J. Wong, R.B.

Greeegor. F.W. Lytle and R.N. Sinclair, J. Non-Cryst. Solids

79 (1986) 117.

E. Fargin, C. Duchesne, R. Olazcuaga, G. Le Flem, C.

Cartier. L. Canioni, P. Segonds, L. Sarger and A. Ducassc, J.

Non-Cryst. Solids 168 (1994) 132.

R.B. Greegor, F.W. Lytle, D.R. Sandstrom, J. Wong and P.

Schultz, J. Non-Cry% Solids 55 (1983) 27.

S.D. Ramamurthi, D.A. Payne, J. Am. Ceram. Sot. 73

(1990) 2547.

L. Bragg, G.F. Claringbull, Crystal Structures of Minerals

(G. Bell, London, 1965).

IV. GLASSES