Preparation of Titanium Dioxide (TiO2) thin films by sol gel dip coating method

Mansor Abdul Hamid1 and Ismail Ab. Rahman2

1 AMREC, SIRIM Berhad, Lot 34, Jalan Hi-Tech 2/3, Kulim Hi-Tech Park, 09000 Kulim 2 School of Chemical Sciences,Universiti Sains Malaysia,11800 Minden, Penang.

1 Corresponding author. E-mail: mansor@sirim.my

Abstract : Titanium dioxide thin films have been prepared from tetraisopropyl-orthotitanate solution and ethanol as a solvent by sol gel dip-coating technique. The films and dry powder from the sol gel solution were calcined at 500oC for 1 hour. The morphology, surface structure and composition of the films were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray Diffraction (XRD). It was found that the film consisted of fine titanium dioxide grains homogeneously deposited on the indium tin oxide coated (ITO) glass.

Keywords: titanium dioxide, sol gel, thin films, dip coating.

Abstrak : Tipisan nipis titanium dioksida telah disediakan secara kaedah penyalutan –celup sol-gel daripada larutan tetrapropil-ortotitanat dan etanol sebagai pelarut. Tipisan dan serbuk yang disediakan itu telah dikalsinkan pada suhu 500 oC selama 1 jam. Morfologi, struktur permukaan dan komposisi tipisan dianalisis dengan menggunakan mikroskop imbasan elektron (SEM), mikroskop daya atom (AFM) dan pembelauan sinar-X (XRD). Tipisan nipis yang terhasil terdiri daripada butiran halus titanium dioksida yang tersebar secara seragam di atas permukaan kaca ITO.

Kata kunci : titanium dioksida, sol gel, tipisan nipis, penyalutan-celup.

Received : 14.06.02; accepted : 29.08.03 Introduction

Titanium dioxide (TiO2) thin films have been receiving much attention in the past as their chemical stability, high refractive index, and high dielectric constant allow their use as components in optoelectronic devices, sensors and photocatalysis [1].

Titanium oxide films have been made by a variety of techniques such as e-beam evaporation, magnetron sputtering technique, anodization, chemical vapour deposition (CVD) and sol gel technique. Among the different methods for the preparation of thin TiO2 electrochromic layer, sol gel method has many advantages, particularly the possibility of producing large surfaces[2-5].

The films are generally deposited by dip-coating, but may also be deposited using spin coating[6,7]. The sol gel processes are particularly efficient in producing thin, transparent, multi-component oxide layers of many compositions on various substrates, including glass.

The hydrolysis and the polycondensation of titanium alkoxides proceed according to the following scheme[3] :

Ti(OR)4 + H2O → Ti(OR)3 (OH) + ROH Ti(OR)4 + Ti(OR)3 (OH) → Ti2O(OR)6 + ROH

The reaction stops with the inclusion of two water molecules

Ti(OR)4 + 2H2O → TiO2 + 4ROH

Numerous literature reports on the fabrication of TiO2 thin films by sol-gel dip coating technique using many types of titanium alkoxides as precursors. Chrysicopoulou et al.[1] used titanium tetraethoxide as a precursor, ethanol as a solvent and HNO3 as a catalyst in the presence of a small amount of water. Harizanov[3] and Nilgun et al.[7] used titanium ethoxide dissolved in butanol or ethanol and glacial acetate acid as modifier. Bell et al[4] reported that TiO2 can be produced by hydrolyzing titanium propoxide in ethanol. The use of tetraisopropyl-orthotitanate as precursor was also reported[8-11]. Others[6,12] used titanium butoxide and H2O2 as starting materials for making TiO2 thin films.

The TiO2 films are able to change their optical properties in a reversible and consistent way under the influence of an external applied potential. The electrochromism phenomenon has been extensively studied because of its extreme importance in architectural and automotive markets. The reactions describing the intercalation/deintercalation of ionic species can be represented, schematically, as

TiO2 + xLi+ + xe- ↔ LixTiO2 (colorless) (colored)

Malaysian Journal of Chemistry, 2003, Vol. 5, No. 1, 086 - 091

where TiO2 is transparent and the lithiated material is absorbing. There are indications that the full 0 < x < 1 range can be covered, at least for protons[5].

In this work, we use tetraisopropyl-orthotitanate (TIP, Ti( O-i-C3H7)4 ) as a precursor and ethanol as a solvent to form TiO2 thin films onto indium tin oxide (ITO) coated glass by sol-gel dipping technique. Acetate modification was performed in order to stabilize the sol by introducing of glacial acetic acid. Similar works[1,3,10] used titanium tetra-ethoxide and titanium ethoxide as precursors and butanol as solvent. Experimental

The sol solution was prepared by adding 5 ml tetraisopropyl-orthotitanate, TIP, ( Fluka), to a 50 ml beaker containing a mixture of glacial acetic acid (J.T Baker) and ethanol (Fluka) that had been mixed for five minutes. The mixture was vigorously stirred using a magnetic stirrer during addition and for a further sixty minutes after addition of the precursor at room temperature ( Figure 1).

A TIP-ethanol gel film was formed on the ITO glass substrate by dipping it into the solution and

pulling it up at a constant rate of 1.4 mm s-1 by a dipping machine (KSV Instrument). This process is optimal for producing highly uniform coatings, by simple control of the thickness through control of the speed of withdrawal from the coating solution. The gel films on the ITO substrate contain residual ethanol and very probably water from the condensation reaction. The dip coated ITO substrate was therefore left to dry at ambient temperature followed by heating at 200 oC in an oven under clean room environment for a minimum of 30 min.

The dried films were further heated in a muffle furnace to 500 oC at a heating rate of 2 oC min-1 and maintained at this temperature for 60 minutes . Then, the films were cooled down to room temperature at similar cooling rate.

SEM observation and thickness measurement were obtained by using Leica Cambridge S360. The surface morphology of films was observed with an Atomic Force Microscope (Shimadzu model SPM-9500 J2). The crystal structure of the films was characterized by X-ray diffractometry (Bruker D8 Advance Diffractometer).

multiple dipping.

Figure 1 : Flow chart for preparing titanium dioxide thin films

Mixing

Preparation of coating solution

Preparation of substrate (Size : 1” x 3” )

Cleaning

Dipping (1.4 mm s-1)

Withdraw (1.4 mm s-1)

Drying ( 200oC)

Heating ( 500oC, 1 hour)

Characterisation

087 Mansor Abdul Hamid and Ismail Ab. Rahman Preparation of Titanium Dioxide (TiO2) thin films by sol gel dip coating method.

Results and Discussion 1. Preparation and formation of TiO2 thin films

The chemical composition of the starting alkoxide solution was Ti( O-i-C3H7)4 : C2H5OH : CH3COOH = 1 : 45: 0.3 in molar ratio and it was left at room temperature for a period of at least 2 hour allowing hydrolysis reaction to take place, and ensuring the formation of transparent TiO2 sol. Our preliminary works showed that, the chosen composition was able to produce a good coating based on morphology, homogeneity and transparency of TiO2 thin films.

The process was exothermic and the pH of the solution about 4 to 5. The viscosity of the solution was 64 cP. This experiment was conducted in a controlled atmosphere box due to high reactively of alkoxide towards humidity. The solutions were found to be stable over 7 days after first dipping when glacial acetic acid was used. This carboxylic acid act not only as an acid catalyst, but also as a ligand and changes the alkoxide precursor at a molecular level

therefore modifying the whole hydrolysis and condensation process resulting in a more stable sol[13].

The sol solution applied to substrate using the dip-coating technique, produced an amorphous TiO2 thin film on the glass substrate, which was then converted to a microcrystalline TiO2 grains after heating at 500 oC for 1 hour[11].

2. Scanning Electron Microscope (SEM).

The TiO2 thin films were gold-covered and examined in a scanning electron microscope (SEM) to investigate their structure and surface characteristics. It was observed that the coating was transparent and homogeneous without any visual cracking over a wide area (Figure 2). Multiple coating increased thickness, but did not affect the uniformity of the film [7,9]. A cross-sectional SEM micrograph of a coated glass sample is shown in Figure 3. Based on SEM measurement, the thickness of film is about 157 nm after 5 dips.

Figure 2 : Scanning Electron Microscope of the TiO2 thin films on glass.

Figure 3 : Thickness measurement by SEM of the TiO2 thin films on glass.

088 Mansor Abdul Hamid and Ismail Ab. Rahman Preparation of Titanium Dioxide (TiO2) thin films by sol gel dip coating method.

3. Atomic Force Microscope (AFM) In our work, a Shimadzu SPM 9500-J2 Atomic

Force Microscope was used to examine the morphology of the films. To avoid sample damage, the microscope was operated in non-contact mode using silicon tips (resistivity 0.01-0.02 �cm). From the AFM images (Figure 4) a uniform agglomeration of well defined, small particles was observed and the grains was elongated along the direction of the withdrawal of the substrate from the solutions, during

the films formation. It was deduced that the equivalent diameters of the grain sizes is about 20 nm. Chrysicopoulou at al.[1] and Avellaneda et al.[11] reported that the mean average size from 13 to 100 nm. The roughness profiles (Rms) of the film surfaces have been measured to be in the range of 1.41 to 1.45 nm. The values were smaller than those reported in the literature [1, 10] at 2 – 4 nm. The roughness measured was less than 5 nm, thus indicating that the films were optically smooth [1].

(a)

(b)

Figure 4 : Atomic force microscope of TiO2 thin films on ITO glass,

(a) 2D surface analysis, and (b) 3D topographic image.

089 Mansor Abdul Hamid and Ismail Ab. Rahman Preparation of Titanium Dioxide (TiO2) thin films by sol gel dip coating method.

4. X-ray Diffraction (XRD) The crystal structures of the dry solution

(powder) and thin films were investigated by Bruker D8 Advance X-ray diffraction (XRD). The dry powder derived from the basic sol-gel solution and heated up to 500 oC for 1 hour were measured by X-ray powder diffraction pattern with Cu K� radiation (commonly know as Bragg Brentano technique). In this technique, almost all the information about the crystal structure comes from the lattice planes parallel to the substrate surface[6]. The titanium oxide powder derived from the basic solution and heated up to 200oC is amorphous in structure[3]. Thermal treatment at 500 oC for 1 hour leads to a

well crystalized anatase-type TiO2 without any other phase (Figure 5(a).

For a films coated on ITO glass, the X-ray diffraction pattern was obtained by grazing incidence angle (GIA) technique. The diffracted intensity was measured at 0.5o incidence angle, in the 2θ range between 20o and 90o, with a step size of 0.01o for 2 hours exposure. In GIA technique, the information was obtained from the planes perpendicular as well as parallel to the substrate surface. The fixed grazing incidence geometry has the advantage of substantially limiting the substrate diffraction peak intensity, since the X-rays penetrate less into the layers[6].

(a)

T i0 2 on ITO G lass

Lin

(Cou

nts)

0

10

20

30

40

50

60

70

2-T he ta - S cale

20 30 40 50 60 70 80 90

d=3.4896 ,

d=2 .3521 ,

d=2.3152 ,

d=2 .4377 ,

d =1 .1678 , d=1.6627 ,

d=1.8828,

d=1.7117 ,

d=1.364 4,

d=1.3 344 ,

d =1 .26 52 ,

(b)

Figure 5 : X-ray diffraction patterns of TiO2 firing at 500 oC at 1 hour, (a) dry powder solution, and ( b). thin films on ITO glass

Lin

(Cou

nts)

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10

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40

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100

110

120

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160

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180

190

200

210

220

230

2-Theta - Scale

20 30 40 50 60 70 80 90

d=3.5136

d=2.4219

d=2.3725

d=1.8918

d=1.6994d=1.6663

d=1.4792

d=1.3630

d=1.3346 d=1.2638

090 Mansor Abdul Hamid and Ismail Ab. Rahman Preparation of Titanium Dioxide (TiO2) thin films by sol gel dip coating method.

Figure 5 (b) shows the x-ray diffraction pattern of TiO2 film coated on ITO glass after firing at 500 oC for 1 hour. The XRD pattern confirmed the presence of anatase TiO2 in the films at 500 oC, within the range of 400 o – 600 oC as reported elsewhere[7].

Conclusion

A transparent, optically smooth and homogeneous TiO2 thin films was prepared through hydrolysis and condensation of tetraisopropyl-orthotitanate, ethanol and glacial acetic acid. The thickness of this film is about 157 nm after five dips. The films consisted of anatase TiO2 after heating at 500 oC for 1 hour. Acknowledgements.

We would like to acknowledge SIRIM Berhad for the scholarship to Mansor Abdul Hamid to pursue postgraduate studies in Universiti Sains Malaysia, and AMREC for characterization of the samples using Atomic Force Microscope and X-ray Diffraction. References 1. P. Chrysicopoulou, D. Davazoglou, Chr.

Trapalis, G. Kordas (1998) Thin Solid Films, 323, 188-193

2. C. J Brinker, G.W Scherrer (1990) Sol Gel Science: The Physic and Chemistry of Sol gel Processing, Academic Press, San Diego.

3. O. Harizanov, A. Harizanova (2000) Solar Energy Materials & Solar Cells, 63, 185-195

4. J.M Bell, J. Barczynska, L.A Evans, K.A MacDonald, J. Wang, D.C Green and G.B Smith (1994) SPIE-The International Society for Optical Engineering, 2255, 324-331.

5. C G Granqvist (1995) Handbook of in Organic Electrochromic Materials, Elsevier.

6. A. R Phani, M. Passacantando, S. Santucci (2002) Journal of Physics and Chemistry of Solids, 63, 383-392.

7. Nilgun Ozer, Selmar De Souza and Carl M. Lampert (1995) SPIE-Proceeding, 2531,143-151.

8. Lianyong Su, Zhong Lu (1998) J. Phys. Chem. Solids, 59 (8), 1175-1180

9. Koichi Kajihara, Kazuki Nakanishi, Katsuhisa Tanaka, Kazuyuki Hirao and Naohiro Soga (1998) J. Am. Ceramic Society, 81, 2670-2676.

10. Maria Zaharescu and Maria Crisan (1998) Journal of Sol-gel Science and Technology, 13, 769-773.

11. C.O Avellaneda and A. Pawlicka (1998) Thin Solid Films, 335, 245-248.

12. Zhongchun Wang, Ulf Helmerson and Per-Olov Kall,(2002) Thin Solid Films, 405, 50-54.

13. S. Doeuff, M. Henry, C. Sanchez and J. Livage (1987) Journal of Non-Crystalline Solids, 89, 206-216.

14. Kil Dong Lee (1999) Solar Energy Materials & Solar Cells, 57, 21-30.

091 Mansor Abdul Hamid and Ismail Ab. Rahman Preparation of Titanium Dioxide (TiO2) thin films by sol gel dip coating method.