Influence on deposition speed and stirring type in the obtantion of titania films

Materials Chemistry and Physics 85 (2004) 245–250

Materials science communication

Influence on deposition speed and stirring type in the obtantion of titania films Lucas A. Rocha, Katia J. Ciuffi, Hérica C. Sacco, Eduardo J. Nassar∗ Universidade de Franca, Av. Dr. Armando Salles Oliveira 201, P.O. Box 82, 14404-600 Franca, SP, Brazil Received 23 April 2003; received in revised form 3 December 2003; accepted 15 January 2004

Abstract In this work we studied the preparation of titania thin films by sol–gel process and analyzed the influence of parameters such as deposition speed (100, 200 and 300 mm min−1 ) and stirring type (magnetic or ultrasound) during the homogenization of the sol used for deposition. Europium III was incorporated into the titania thin films and used as a structural probe. The prepared films present good transparency. Excitation spectrum presents a large band with maximum in 309 nm. This band can be attributed to charge transfer band (CTB). The emission spectra presented the correspondent bands to the transitions of the excited state (5 D0 ) of the ion for the fundamental (7 FJ=0–4 ). The thickness films depend on deposition speed, we observed a large enhanced when the deposition speed increase of the 100–200 mm min−1 (0.1562–0.4552 nm) and less to 200–300 mm min−1 (0.4552–0.5364 nm). The refractive index for samples obtained in the deposition speed 200 and 300 mm min−1 , are respectively 1.9798 and 1.9803. Both are different from the film deposited at 100 mm min−1 (2.3635). © 2004 Elsevier B.V. All rights reserved. Keywords: Dip-coating; Sol–gel; Europium III

1. Introduction In the last decades the general aspects of the chemistry of metal alkoxides have been studied [1]. Metal alkoxides are the main precursors of the sol–gel process and are used to obtain a vast variety of inorganic oxides of high purity. The preparation of such materials with homogeneity at molecular level is possible because of the ability of any other metal alkoxide to form homogeneous solutions in a great solvent variety and in the presence of other metal alkoxide or derivative [2]. Using these metal alkoxides and the sol–gel process, materials with optical applications can be produced with a high optical quality and appropriate refraction index at very competitive prices compared to other technologies. The several glass types produced using the sol–gel process are natural candidates for such applications, and the synthetic sol–gel process is very attractive technologically [3]. Titania films have attracted attention due to their applications in integrated optical [4], photocatalysis [5], semiconductors [6,7], their mechanic properties and biocompatibilities [8] and, most important, their anti-reflective properties [9–12]. The preparation of titania films through the sol–gel process, starting from titanium alkoxide, presents some dif∗ Corresponding author. E-mail address: [email protected] (E.J. Nassar).

0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.01.037

ficulties mainly for the fast hydrolysis when in contact with the atmospheric humidity. They can be obtained by spin[13] and dip-coating [14]. The microstructures of films depend on the size and aggregation of the species in the sol, on the deposition and on the relative ratios between the condensation and evaporation of the solvent during film deposition. The dip-coating process can be divided into five phases: immersion, start-upon, deposition, drainage and evaporation [8]. When volatile solvents such as ethanol are used, the evaporation usually accompanies the retreat phases (the star-up), deposition, and drainage. The enormous interest in the incorporation of luminescent species in solid matrix produced by sol–gel process is due to the vast application of these materials in lasers, chemical sensors and waveguides. The luminescent species incorporated in the sol–gel samples can also act as a luminescent probe and their optical properties are useful for the characterization of the chemical and physical structural changes, which the sample goes through during all phases of the sol–gel process. These changes involve an initial sol (solution) that produces a gel and after drying turns into xerogel. The rare earths ions, Eu III ions in particular [15–18], are commonly used as probes in the sol–gel process, due to their sensibility to the environmental surroundings where they are inserted, monitoring the synthesis of glasses.

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In this work we studied the preparation of titania thin films by sol–gel process and analyzed the influence of parameters such as deposition speed and stirring type (magnetic or ultrasound) during the homogenization of the sol used for deposition. Europium III was incorporated into the titania thin films and used as a structural probe.

pulse) Xe lamp excitation with a SPEX Fluorolog spectrofluorometer. All spectra were corrected by spectrometer optics, lamp output and detector response. Decay curves were processed with a SPEX 1934 phosphorimeter. The refractive index and thickness of the waveguides were measured by both transverse electric (TE) and transverse magnetic (TM) polarization with an m-line apparatus (Metricon Model 2010) based on the prism coupling technique [14]. The electronic spectra of the films were recorded on a UV-Vis spectrophotometer (Hewlett-Packard 8453, Diode Array).

2. Experimental All solvents and reagents were of commercial grades (Merck and Aldrich) unless otherwise stated. 2.1. Preparation of sol

3. Results and discussion

Sols were prepared by mixing tetraethylorthotitanate (TEOT) (1.0 × 10−3 mol L−1 ) dried in ethanol, followed by the addition of the beta-diketonate-2,4-pentanedione, in the molar ratio 1:1. The ethanolic europium chloride was added in the sol, in different molar ratios. A yellow transparent sol was obtained. The sol was vigorously homogenized by magnetic or ultrasonic stirring for 15 min.

The scope of this work was to study the influence of the deposition speed on the structure film characteristic. The luminescence properties of europium III ion were used as structural probe. Ethanolic europium III was added (0.1 mol%) in the form of an ethanolic chloride solution (0.1 mol L−1 ) into the titanium sol. The sol–gel process was applied for the preparation of the films, using magnetic or ultrasound stirring for 15 min. The prepared films present good transparency. Fig. 1 shows the excitation and emission spectra of the Eu III ion in the titania film obtained in three different speeds. We observed in the excitation spectrum the presence of a large band with maximum in 309 nm, when fix emission Eu III band (612 nm), for the films deposited in the three different speeds. This band can be attributed to charge transfer band (CTB). The CTB denote covalente degree between Eu–ligand, the small energy indicate more covalente [19]. We observed that the films present same degree of cova-

2.2. Films deposition The films were obtained by the dip-coating technique. Borosilicate glass substrates were carefully cleaned and then sunk in the sols. They were withdrawn at a rate of 100, 200 and 300 mm min−1 and the resulting films were dried at room temperature 25 ◦ C. 2.3. Characterization Room temperature excitation and emission spectra were obtained under both continuous (450 W) and pulsed (5 J per

5

Intensity (a.u.)

7

D0 - F2

0,1% - 100 mm/min 0,1% - 200 mm/min 0,1% - 300 mm/min

7

5

D0 - F1

5

7

D0 - F4

5

7

D0 - F0

300

500

5

7

D0 - F3

600

700

Wavelength (nm) Fig. 1. Excitation and emission spectra of the Eu III ion in the films prepared in different speeds, the sol was stirred by magnetic.

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247

Table 1 Relative area of bands correspondent 5 D0 → 7 F0 and 5 D0 → 7 F2 in ratio of 5 D0 → 7 F1 transition, lifetime of the 5 D0 → 7 F2 transitions and yield quantum (φ) 0–0/0–1

0–2/0–1

τ exp (ms)

τ rad (×102 s)

φ (%)

Ultrasonic stirring Speed 100 mm min−1 Speed 200 mm min−1 Speed 300 mm min−1

0.22 0.17 0.14

1.94 2.23 2.62

0.49 0.49 0.47

0.13 0.13 0.11

3.80 3.80 4.27

Magnetic stirring Speed 100 mm min−1 Speed 200 mm min−1 Speed 300 mm min−1

0.15 0.18 018

2.37 2.24 2.28

0.54 0.76 0.74

0.14 0.12 0.14

3.85 6.33 5.29

lence between Eu–ligand. This fact can be due to Eu–oxygen bound in the inorganic chain TiO2 . The emission spectra presented the bands correspondent to the transitions from the excited state (5 D0 ) of the ion to the fundamental 7 FJ=0–4 , as indicated in Fig. 1, when excited at 309 nm. The band corresponding to the transition 5 D0 → 7 F1 is of magnetic dipole nature and its intensity is not affected by the crystalline field to which the ion Eu III is submitted. Therefore, it can be considered a standard to measure the relative intensity of other bands [20]. The electric-dipole character [21], the intensities of the 5 D0 → 7 F0 and 5 D0 → 7 F2 transitions are strongly dependent on the Eu III surrounding. The relative intensities are measured in terms of relative area (RA). Table 1 presents the ratio of RA of the bands 5 D → 7 F and 5 D → 7 F in function of 5 D → 7 F . The 0 0 0 2 0 1 lifetimes of the 5 D0 → 7 F2 transition and the yield quantum are shown in the table too. In Table 1 we can observe similar behavior to the films prepared in the speeds of 200 and 300 mm min−1 , this is an indicative of similar surrounding to Eu III ions. A little difference appears in relation to film deposited at 100 mm min−1 .

The yield quantum (φ) was evaluated as the ratio between the experimental (τ exp ) and the pure radiative lifetimes (τ rad ), φ = τexp /τrad [22]. The transition 5 D0 → 7 F1 , for which the spontaneous emission coefficient is known (A01 = 50 s−1 ), is taken as a Ref. [23]. Therefore, the total radiative contribution for the depopulation of 5 D0 can be evaluated from the transition intensities. Transitions to 7 F5 and 7 F6 were neglected since they could not be observed experimentally. Table 1 also presents the results for φ. The same low values 3, 6 and 5% are obtained for the sample prepared to 100, 200 and 300 mm min−1 , respectively. Since yield quantum is dominated by the non-radiative decay rate in these samples the results suggest that a critical deposition speed in the surrounding of the Eu III ion. This is observed in the lifetime value. Figs. 2 and 3 present the thickness and refractive index in relation to deposition speed for waveguides were studied here and calculated at 632.8 nm using the parameters obtained by the m-line measurements. The thickness films depend on deposition speed, we observed a large enhanced when the deposition speed increase from 100 to 200 mm min−1 (0.1562–0.4552 nm) and less from 200 to 300 mm min−1 (0.4552–0.5364 nm).

0,55 0,50

thickness (nm)

0,45 0,40 0,35 0,30 0,25 0,20 0,15 100

150

200

250

Deposition speed (mm/min) Fig. 2. Thickness films in relation of deposition speed.

300

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Refractive index

2,20 2,15 2,10 2,05 2,00 1,95 100

150

200

250

300

Deposition speed (mm/min) Fig. 3. Refractive index in relation of deposition speed.

The refractive index for samples obtained in the deposition of the speed 200 and 300 mm min−1 , are 1.9798 and 1.9803, respectively. Both are different from the film deposited at 100 mm min−1 (2.3635). These results are in concordance with the results obtained by luminescence data. The films prepared by 200 and 300 mm min−1 deposition speed present similar surrounding to Eu III ion. Fig. 4 presents the absorbance spectra of the films. The number of interference band in UV-Vis spectrum can be related to the thickness. There are more bands for samples prepared at 200 and 300 mm min−1 than for sample at 100 mm min−1 . This confirmed the results obtained by m-line spectroscopy.

Now we present the influence of the stirring type used for the homogenization titanium sol. Magnetic and ultrasonic stirring were used to prepare the titania sol contend 0.1% Eu III as probe. Fig. 5 shows the excitation and emission spectra of the Eu III ion in the films based on titanium deposited in glass substrate in the speeds at 100, 200 and 300 mm min−1 stirring by ultrasonic. We can observe similar behaviors for the two homogenizations types. The maximum excitation appear in 309 nm to the three different speeds used studied. The emission spectrum presented bands correspondent to the transitions of the excited state (5 D0 ) of the ion for the fundamental (7 FJ=0–4 ) of Eu III, when excited in 309 nm.

Absorbance

100 mm/min 200 mm/min 300 mm/min

600

800

1000

Wavelength (nm) Fig. 4. UV-Vis spectra of titanium films deposited at 100, 200 and 300 mm min−1 .

L.A. Rocha et al. / Materials Chemistry and Physics 85 (2004) 245–250

60000

0,1% - 100 mm/min 0,1% - 200 mm/min 0,1% - 300 mm/min

50000

Intensity (a.u.)

249

40000

30000

20000

10000

0 300

500

600

700

Wavelenght (nm) Fig. 5. Excitation and emission spectra of the Eu III ion in the films deposited in different speed, the sol was stirring by ultrasonic.

Absorbance

100 mm/min 200 mm/min 300 mm/min

600

800

1000

Wavelength (nm) Fig. 6. UV-Vis electronic spectrum of titanium films contained 0.1% Eu III prepared by ultrasonic stirring in three different deposition speeds.

The 5 D0 → 7 F1 transition was used as a standard to measure the relative intensity of the other bands, Table 1 presents the ratio of RA of the bands 5 D0 → 7 F0 and 5 D0 → 7 F in function of 5 D → 7 F and the lifetimes of the 2 0 1 5 D → 7 F transition and the yield quantum. As well the 0 2 data of the different type of stirring. For samples homogenized in the ultrasonic we observed an increase of the ratio of 0–2/0–1, indicating a decrease in the symmetry site Eu III. The lifetimes and yield quantum present the same value. The thickness of the films was available by UV-Vis spectroscopy is shown in Fig. 6. The number of interference bands in the UV-Vis electronic spectrum can be related to the thickness.

The thickness of films prepared by 200 and 300 mm min−1 have higher amount of interference bands, which indicated thicker films when compared to films deposited by 100 mm min−1 . The difference in thickness between magnetic and ultrasonic stirring can be ascribed by the particle size formed. The ultrasonic stirring can originate small particles when compared with magnetic this results in thinner films. The lifetimes differ mainly in the films obtained by 200 and 300 mm min−1 , the particles size can be related at lifetime. The decrease in size of the particles provides an increase amount of groups adsorbed in the surface of the particle, mainly –OH groups, which increase in the losses of

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energy by vibrations modes, provides quenching in the Eu III luminescence. 4. Conclusions In this work it was possible to observe the differences between the magnetic and ultrasonic homogenization of the titanium sol, mainly that showing the difference is particles size, this influences directly in the films thickness. We obtained thicker films through magnetic stirring than through ultrasonic stirring. The prepared films in the speeds of 200 and 300 mm min−1 to magnetic stirring present similar behavior; this is an indicative of similar surroundings for Eu III ion. In the ultrasonic stirring the deposition speeds show little influence in the ambient of the Eu III. The absorbance spectrum can evaluate the films thickness. The films obtained present satisfactory transparency, demonstrating that the methodology can be used in the preparation of waveguides, sensor chemical, solid-state lasers and fibers. References [1] S.M. Buckley, M. Greenblatt, J. Chem. Edu. 71 (7) (1994) 599. [2] H.D. Gesser, P.C. Goswani, Chem. Rev. 89 (1989) 765. [3] Y. Haruvy, E. Gilath, M. Maniewictz, N. Eisenberg, Chem. Mater. 9 (1997) 2604. [4] A. Bahtat, B. Jacquier, B. Varrel, M. Bouazaoui, J. Mugnier, Opt. Mater. 7 (1997) 173.

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