- Email: [email protected]
Preparation of TiO2 thin films by modified spin-coating method using an aqueous precursor
Materials Letters 57 (2003) 1775 – 1780 www.elsevier.com/locate/matlet
Preparation of TiO2 thin films by modified spin-coating method using an aqueous precursor K.R. Patil*, S.D. Sathaye, Y.B. Khollam, S.B. Deshpande, N.R. Pawaskar, A.B. Mandale Physical Chemistry Division, National Chemical Laboratory, Pashan, Pune 411 008, India Received 26 June 2002; accepted 8 July 2002
Abstract Modified spin-coating method has been applied to grow thin films of titanium dioxide. With this modification, it is possible to use ammonium titanyl oxalate in the form of its aqueous solutions as starting material. The resulting thin films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV – visible spectroscopy and X-ray photoelectron spectroscopy (XPS). The results show that aqueous precursor form anatase TiO2 thin films, which are qualitatively comparable to the films grown by other wet chemical techniques. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Spin coating; Titanium dioxide; Electron spectroscopy
1. Introduction Thin films of inorganic compound semiconductors are taking vital role in the advanced technologies of the modern society. To improve the performance of thin-film-based devices, it is necessary to control the film properties during its processing. However, the techniques for the film processing should be developed in the view of both aspects of favorable economy and ecology, to make the technique applicable for the increasing demand in the future. Although many sophisticated techniques, namely, vacuum evaporation [1], sputtering [2], molecular beam epitaxy [3], laser-assisted vacuum evaporation [4], etc. are being used, these techniques are energy-intensive and *
Corresponding author. Fax: +91-20-3937044. E-mail address: [email protected] (K.R. Patil).
involve high temperature. Also, these sophisticated methods have merit in having control over film growth and the feasibility to obtain a pure material. However, researchers also studied wet chemical processes from the economic considerations and some other advantages, namely, simplicity and low temperature processing, etc. Spin-coating process [5] is widely applied amongst many wet chemical processes of thin film deposition, namely, chemical bath deposition [6], electrodeposition [7], Langmuir Blodgett technique [8], spray pyrolysis [9], liquid –liquid interface reaction technique [10] and sol – gel process [11] considered for various applications. In the present technique of spin coating, organometallic precursors in the organic solvents are used. This is necessitated by the constrains of precise requirements of spin processing parameters, namely, viscosity of precursor solution and spin speed. The viscosity
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 6 7 - 4
1776
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
needs to be in the range 4 – 10 cp and spin speed depends on the viscosity of the precursor solutions. In this communication, we are reporting the deposition of the thin films of TiO2 by the modified spincoating technique in which inorganic precursor aqueous salt solutions are used. The choice of TiO2 was based on its importance in environmental purification and treatment of water and air being a part of the increasing number of recent environmental problem in the world [12]. Conventional powder catalyst suffers from disadvantages in stirring during the reaction and getting it separated after the reaction [13]. Preparation of the catalyst coated as thin film will be useful to overcome the disadvantages, namely, separation of conventional powder catalyst after reaction is difficult. Also, industrial applications of TiO2 thin films, as a basic unit of antibacterial ceramic tile or self-cleaning glass [12], look promising. Recent studies have proved that TiO2 is an important proposition for many photocatalytic reactions [14]. In some cases, doped TiO2 is more active for photocatalysis [15]. In the conventional spin-coating process using organometallic precursors, the doping is difficult. The present modification would be advantageous in forming doped TiO2 coatings. Small particle size vis-a`-vis high surface area of coating materials become important for catalytic application as well as other modern applications. Low temperature processing and possible control over morphological aspects of the coatings become important. In such applications, modified spin coating would prove an important development for these requirements. We have earlier shown that by modifying the spin processing assembly, the critical requirement of the precursor solution viscosity is relaxed making it possible to use aqueous precursor solutions for the deposition of CoFe2O4 films [16]. In the present communication, we apply the modified spin-coating process to deposit TiO2 films and their characterization.
(Ranbaxy, 99%) were used as supplied. Double distilled deionized water was used in all experiments. 2.2. Modified spin-coating method In the conventional spin-coating process [5], the viscosity of precursor sol/solution in the range of 4– 10 cp is a prerequisite and spinning speed is a dependent parameter. This brings serious limitations to its application, as the viscosity of aqueous sol/ solution is not suitable. Therefore, use of costly solutions of organometallic precursors in organic solvents becomes inevitable. Alternatively, aqueous precursor solutions with certain additives for viscosity adjustment [17] become necessary, which complicates the process. Fig. 1 shows the conventional and modified assembly for spin coating. The modified assembly is made up of a tandem of two substrates separated by spacers between them and maintaining a uniform gap of 1– 3 mm. This assembly can be fixed on a rotatable support chuck having an axis of rotation passing through the center of gravity of the said assembly. The space between the parallel plates consisting of microscopic slide substrate and a cover substrate is used as a reservoir to place aqueous precursor solution leading to the formation of desired precursor solid film on the substrate when subjected to spin processing. All other procedural features are the same in conventional spin coating and its modification.
2. Experimental procedure 2.1. Materials Titanium tetrabutoxide (Albert Victor), oxalic acid, ammonium oxalate (SD chemical) and isopropanol
Fig. 1. Schematic illustration of the spin coating method.
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
1777
using an electric oven. The thickness of the films was adjusted by repeating the cycle from spinning to heat treatment. 2.4. Characterization
Fig. 2. X-ray diffraction pattern of TiO2 films deposited on (a) fused quartz, (b) Ni metal.
The structure of the TiO2 films was determined by X-ray diffraction (XRD) using Philips XRD system with CuKa radiation. Surface morphology of the films was observed using the scanning electron microscope (Phillips, Model XL-30). Spectroscopic analysis of TiO2 films was performed using a UV –visible spectrophotometer (Hewlett-Packard 8452A) with a wavelength range 300– 800 nm, with a resolution of 2 nm. The TiO2 films were analyzed by X-ray photoelectron spectroscopy (XPS) using a V.G. Scientific (UK) ESCA 3000 spectrometer equipped with two ultrahigh-vacuum chambers. The pressure in the chambers during the experiments was about 10 9 Pa. A MgKa (1253.6 eV) X-ray source was used. The analyzer was operated at 50 eV pass energy. The Xray photoelectron spectra were referenced to the C1s peak (285.0 eV).
2.3. Preparation of films Precursor solutions for TiO2 films were prepared by the following method [18]. Titanium tetrabutoxide (0.1 M) in isopropanol was mixed with 0.1 M oxalic acid in water + isopropanol and stirred continuously. A 0.1 M of ammonium oxalate in distilled water was added to the above mixture. A white precipitate obtained was dissolved by adding distilled water when clear solution containing soluble titanium species (ammonium titanyl oxalate) was obtained. Soda lime glass, quartz and nickel metal were used as the substrates for the thin films. The TiO2 films formed on the substrates were prepared from the aqueous precursor ammonium titanyl oxalate. The base substrate is flooded by the precursor solution so that cover substrate (e.g. quartz) is in contact with it (see Fig. 1). This assembly is subjected to spinning operation (3000 RPM, 60 s). A mixture of fine crystallites of the precursor coats covers substrate uniformly. The substrates coated with the TiO2 precursor films were heat-treated at 500 jC for 1 h in air
Fig. 3. Typical UV – visible spectrum of TiO2 nanoparticulate films.
1778
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
3. Results and discussion In the conventional spin-coating process, if aqueous solution of precursors are used, no coating on the substrate could be obtained. During the spinning process, solution along with solute is physically thrown away from the substrate surface by the centrifugal force. Thus, film formation by crystallization of precursors does not take place. This can be attrib-
uted to the low contact time of spinning solution with the substrate surface. Centrifugal force overcomes the forces that are responsible to maintain the contact between a solution and the substrate surface. These forces mainly consist of surface tension and viscosity that are characteristic properties of the materials. In the modified spin-coating method, the centrifugal force that arises when this assembly is spinning would be of the same order as in the conventional
Fig. 4. SEM image of the surface of TiO2 film deposited on fused quartz.
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
spin-coating assembly. However, the fluid dynamics of the modified assembly is indeed complex. Researchers have studied the fluid dynamics for the flow of fluid between rotating parallel plates. Abbot and Walters [19] have obtained an exact solution for the Newtonian fluids and Rajagopal [20] has considered a second order fluid. It is to be noted that the modified assembly is not exactly a parallel plate viscometer, but the study by Abbott and Walters has some relevance for the modified assembly. Without going into the fundamentals, it would be safe to assume that the fluid held between the two substrates gets stabilized due to the secondary flow, which along with the surface tension overcomes the influence of the centrifugal force. Therefore, the loss of the fluid is due to the evaporation alone. This slow evaporation of the fluid initiates the crystallization/gelation/precipitation of the molecules in a thin film form. It is observed that cover substrate is uniformly coated as compared to the base substrate. The process of recrystallization is considerably fast and therefore the deposited material consists of fine crystallites. When the aqueous solution of ammonium titanyl oxalate is subjected to the modified spin-coating process, the cover substrate is coated with the fine crystallites of the ammonium titanyl oxalate. The crystallite size of the deposited material is very small. No peaks in XRD could be observed for the deposited mixture. The coating of the deposited mixture on calcination at 500 jC decompose to its oxide, namely, TiO2. Fig. 2 shows XRD patterns of TiO2 films on quartz and nickel metal substrate heated at 500 jC for 2 h. The TiO2 on quartz shows anatase phase (Fig. 2a). It has also been found that TiO2 prepared under similar condition on nickel substrate and heated at 500 jC shows peaks at 2h = 25.3j, 37.3j and 38.5j, respectively, matching with peaks reported for anatase phase. (ASTM card no. 4-0477). This confirms the formation of anatase phase removing the uncertainty of phase on quartz substrate arising due to the peak of highest intensity, i.e. peak at 2h = 25.3j was getting suppressed in a diffused peak due to quartz substrate. The XRD pattern shows broad peaks of the anatase phase, suggesting the small crystallites existing in the sample. The size of TiO2 crystallites has been estimated from the full-width at half maximum (fwhm) of the (001) diffraction peak using Scherrer’s equation.
1779
The average crystallite size works out to be c 25 nm from three different experiments. This is in agreement to the size of the particles seen in the SEM pictures. Fig. 3 shows the UV – vis spectra in the wavelength range 300 – 800 nm for TiO2 film on quartz substrate. The absorption edge of the TiO2 film is observed at a shorter wavelength. The shift is ascribed to the smaller particle size. The bands due to the interference color of the films appeared in the wavelength 380– 800 nm. These observations are comparable with the observation on the TiO2 thin film prepared by sol – gel technique by Yu and Zhao [14a] The morphology of the TiO2 film obtained by the present method is shown in Fig. 4a and b, which are different regions of the same film. Most of the TiO2 particles have shapes with sizes in nano range; however, some nonspherical shapes are seen. These particles are agglomerated to form aggregates of different shapes and sizes (Fig. 4a and b). Fig. 5 shows the XPS survey spectra taken at the sample surface of TiO2 film on quartz, heat-treated at 500 jC for 2 h. It can be seen from Fig. 5 that TiO2 film deposited on quartz contains elements Ti, O and
Fig. 5. XPS survey spectra for the surface of TiO2 film.
1780
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
C. The photoelectron peak for Ti2p appears at a binding energy at 458.2 eV, O1s at 531.3 eV and C1s at 285 eV. This is consistent with the literature value [14a]. The element C in the film is attributed to the residual carbon. The atomic ratio calculated from the area under curve of Ti2p and O1s peaks gives the ratio 1:1.95 for Ti to O. The ratio shows the stoichiometric formation of TiO2.
4. Conclusions The modified spin-coating method wherein aqueous precursor solution, namely, ammonium titanyl oxalate is used for spinning to prepare TiO2 is a versatile technique. This method has advantages over the conventional methods in respect of simplicity and low temperature formation of thin films of titanium oxides with particle size in nano range. UV – visible absorption studies show a blue shift in the visible range for TiO2 films, which is a typical characteristic of nano semiconductor.
Acknowledgements We are thankful to Dr. T. Seth of Center for Materials for Electronics Technology, Pune for recording SEM photographs.
References [1] N. Romeo, G. Sberveglieri, L. Tarricane, Appl. Phys. Lett. 32 (1978) 807. [2] D.B. Fraser, H. Melchior, J. Appl. Phys. 43 (1972) 3120. [3] S. Fujita, Y. Kawakami, S. Fujita, J. Cryst. Growth 164 (1996) 196. [4] P.C. Dorsey, P. Lubitz, D.B. Chirisey, J.S. Horwitz, J. Appl. Phys. 79 (1996) 6338.
[5] F. Cheng, Z. Peng, C. Liao, Z. Xu, S. Gao, C. Yan, D. Wang, J. Wang, Solid State Commun. 107 (1998) 471. [6] N.R. Pawaskar, C.A. Menezes, A.P.B. Sinha, J. Electrochem. Soc. 124 (1977) 743. [7] H.S. Potdar, R.I. Hegade, S. Badrinarayan, N.R. Pawaskar, Sol. Energy Mater. 6 (1982) 183. [8] D.T. Amm, D.J. Johnson, T. Laursen, S.K. Gupta, Appl. Phys. Lett. 61 (1992) 522. [9] I.S. Mulla, H.S. Soni, V.J. Rao, A.P.B. Sinha, J. Mater. Sci. 21 (1986) 1288. [10] S.D. Sathaye, K.R. Patil, D.V. Paranjape, A. Mitra, S.V. Awate, A.B. Mandale, Langmuir 16 (2000) 3487. [11] (a) S. Sakka, Science in Sol – Gel Method, Agune, Tokyo, 1998, Chapter 6. (b) J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 8 (1988) 259. [12] N. Negishi, T. Iyoda, K. Hashimota, A. Fujishima, Chem. Lett. (1995) 841. [13] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [14] (a) J. Yu, X. Zhao, Mater. Res. Bull. 35 (2000) 1293. (b) Y. Paz, A. Heller, J. Mater. Sci. 12 (1997) 471. (c) Y. Paz, Z. Luo, L. Rabenberg, A. Heller, J. Mater. Sci. 10 (1995) 2842. [15] B. Pal, M. Sharon, G. Nogami, Mater. Chem. Phys. 59 (1999) 254. [16] S.D. Sathaye, K.R. Patil, S.D. Kulkarni, P.P. Bakre, S.D. Pradhan, B.D. Sarwade, S.N. Shintre, J. Mater. Sci. (communicated). [17] (a) B.P. Gorman, V. Sprenkle, H.U. Anderson, Ceram. Trans. 109 (2000) 105. (b) J.J. Lee, Y.W. Kim, W. Cho, I.T. Kim, H. Je, J. Park, Yoop Hakhoechi 36 (1999) 893. (c) J. Hong, H. Him, H. Park, Thin Solid Films 332 (1998) 449. (d) F. Horowitz, A.F. Michels, E.M. Yeatman, J. Sol-Gel Sci. Technol. 13 (1998) 707. (e) M. Azuma, M.C. Scott, C.A. Paz, L.D. Mcmillan, US Patent 5,612,082, 1997. [18] (f) S. Prasad, A. Vijayalakshmi, N.S. Gajbhiye, Therm. Anal. Calorim. 52 (1998) 595. [19] H.S. Potdar, S.B. Deshpande, A.S. Deshpande, Y.B. Khollam, S.D. Pradhan, S.K. Date, Int. J. Inorg. Mater. 3 (2001) 613. [20] T.N.G. Abbott, K. Walters, J. Fluid Mech. 40 (1970) 205. [21] K.R. Rajagopal, J. Non-Newton. Fluid Mech. 9 (1981) 185.
Preparation of TiO2 thin films by modified spin-coating method using an aqueous precursor K.R. Patil*, S.D. Sathaye, Y.B. Khollam, S.B. Deshpande, N.R. Pawaskar, A.B. Mandale Physical Chemistry Division, National Chemical Laboratory, Pashan, Pune 411 008, India Received 26 June 2002; accepted 8 July 2002
Abstract Modified spin-coating method has been applied to grow thin films of titanium dioxide. With this modification, it is possible to use ammonium titanyl oxalate in the form of its aqueous solutions as starting material. The resulting thin films are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV – visible spectroscopy and X-ray photoelectron spectroscopy (XPS). The results show that aqueous precursor form anatase TiO2 thin films, which are qualitatively comparable to the films grown by other wet chemical techniques. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; Spin coating; Titanium dioxide; Electron spectroscopy
1. Introduction Thin films of inorganic compound semiconductors are taking vital role in the advanced technologies of the modern society. To improve the performance of thin-film-based devices, it is necessary to control the film properties during its processing. However, the techniques for the film processing should be developed in the view of both aspects of favorable economy and ecology, to make the technique applicable for the increasing demand in the future. Although many sophisticated techniques, namely, vacuum evaporation [1], sputtering [2], molecular beam epitaxy [3], laser-assisted vacuum evaporation [4], etc. are being used, these techniques are energy-intensive and *
Corresponding author. Fax: +91-20-3937044. E-mail address: [email protected] (K.R. Patil).
involve high temperature. Also, these sophisticated methods have merit in having control over film growth and the feasibility to obtain a pure material. However, researchers also studied wet chemical processes from the economic considerations and some other advantages, namely, simplicity and low temperature processing, etc. Spin-coating process [5] is widely applied amongst many wet chemical processes of thin film deposition, namely, chemical bath deposition [6], electrodeposition [7], Langmuir Blodgett technique [8], spray pyrolysis [9], liquid –liquid interface reaction technique [10] and sol – gel process [11] considered for various applications. In the present technique of spin coating, organometallic precursors in the organic solvents are used. This is necessitated by the constrains of precise requirements of spin processing parameters, namely, viscosity of precursor solution and spin speed. The viscosity
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 6 7 - 4
1776
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
needs to be in the range 4 – 10 cp and spin speed depends on the viscosity of the precursor solutions. In this communication, we are reporting the deposition of the thin films of TiO2 by the modified spincoating technique in which inorganic precursor aqueous salt solutions are used. The choice of TiO2 was based on its importance in environmental purification and treatment of water and air being a part of the increasing number of recent environmental problem in the world [12]. Conventional powder catalyst suffers from disadvantages in stirring during the reaction and getting it separated after the reaction [13]. Preparation of the catalyst coated as thin film will be useful to overcome the disadvantages, namely, separation of conventional powder catalyst after reaction is difficult. Also, industrial applications of TiO2 thin films, as a basic unit of antibacterial ceramic tile or self-cleaning glass [12], look promising. Recent studies have proved that TiO2 is an important proposition for many photocatalytic reactions [14]. In some cases, doped TiO2 is more active for photocatalysis [15]. In the conventional spin-coating process using organometallic precursors, the doping is difficult. The present modification would be advantageous in forming doped TiO2 coatings. Small particle size vis-a`-vis high surface area of coating materials become important for catalytic application as well as other modern applications. Low temperature processing and possible control over morphological aspects of the coatings become important. In such applications, modified spin coating would prove an important development for these requirements. We have earlier shown that by modifying the spin processing assembly, the critical requirement of the precursor solution viscosity is relaxed making it possible to use aqueous precursor solutions for the deposition of CoFe2O4 films [16]. In the present communication, we apply the modified spin-coating process to deposit TiO2 films and their characterization.
(Ranbaxy, 99%) were used as supplied. Double distilled deionized water was used in all experiments. 2.2. Modified spin-coating method In the conventional spin-coating process [5], the viscosity of precursor sol/solution in the range of 4– 10 cp is a prerequisite and spinning speed is a dependent parameter. This brings serious limitations to its application, as the viscosity of aqueous sol/ solution is not suitable. Therefore, use of costly solutions of organometallic precursors in organic solvents becomes inevitable. Alternatively, aqueous precursor solutions with certain additives for viscosity adjustment [17] become necessary, which complicates the process. Fig. 1 shows the conventional and modified assembly for spin coating. The modified assembly is made up of a tandem of two substrates separated by spacers between them and maintaining a uniform gap of 1– 3 mm. This assembly can be fixed on a rotatable support chuck having an axis of rotation passing through the center of gravity of the said assembly. The space between the parallel plates consisting of microscopic slide substrate and a cover substrate is used as a reservoir to place aqueous precursor solution leading to the formation of desired precursor solid film on the substrate when subjected to spin processing. All other procedural features are the same in conventional spin coating and its modification.
2. Experimental procedure 2.1. Materials Titanium tetrabutoxide (Albert Victor), oxalic acid, ammonium oxalate (SD chemical) and isopropanol
Fig. 1. Schematic illustration of the spin coating method.
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
1777
using an electric oven. The thickness of the films was adjusted by repeating the cycle from spinning to heat treatment. 2.4. Characterization
Fig. 2. X-ray diffraction pattern of TiO2 films deposited on (a) fused quartz, (b) Ni metal.
The structure of the TiO2 films was determined by X-ray diffraction (XRD) using Philips XRD system with CuKa radiation. Surface morphology of the films was observed using the scanning electron microscope (Phillips, Model XL-30). Spectroscopic analysis of TiO2 films was performed using a UV –visible spectrophotometer (Hewlett-Packard 8452A) with a wavelength range 300– 800 nm, with a resolution of 2 nm. The TiO2 films were analyzed by X-ray photoelectron spectroscopy (XPS) using a V.G. Scientific (UK) ESCA 3000 spectrometer equipped with two ultrahigh-vacuum chambers. The pressure in the chambers during the experiments was about 10 9 Pa. A MgKa (1253.6 eV) X-ray source was used. The analyzer was operated at 50 eV pass energy. The Xray photoelectron spectra were referenced to the C1s peak (285.0 eV).
2.3. Preparation of films Precursor solutions for TiO2 films were prepared by the following method [18]. Titanium tetrabutoxide (0.1 M) in isopropanol was mixed with 0.1 M oxalic acid in water + isopropanol and stirred continuously. A 0.1 M of ammonium oxalate in distilled water was added to the above mixture. A white precipitate obtained was dissolved by adding distilled water when clear solution containing soluble titanium species (ammonium titanyl oxalate) was obtained. Soda lime glass, quartz and nickel metal were used as the substrates for the thin films. The TiO2 films formed on the substrates were prepared from the aqueous precursor ammonium titanyl oxalate. The base substrate is flooded by the precursor solution so that cover substrate (e.g. quartz) is in contact with it (see Fig. 1). This assembly is subjected to spinning operation (3000 RPM, 60 s). A mixture of fine crystallites of the precursor coats covers substrate uniformly. The substrates coated with the TiO2 precursor films were heat-treated at 500 jC for 1 h in air
Fig. 3. Typical UV – visible spectrum of TiO2 nanoparticulate films.
1778
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
3. Results and discussion In the conventional spin-coating process, if aqueous solution of precursors are used, no coating on the substrate could be obtained. During the spinning process, solution along with solute is physically thrown away from the substrate surface by the centrifugal force. Thus, film formation by crystallization of precursors does not take place. This can be attrib-
uted to the low contact time of spinning solution with the substrate surface. Centrifugal force overcomes the forces that are responsible to maintain the contact between a solution and the substrate surface. These forces mainly consist of surface tension and viscosity that are characteristic properties of the materials. In the modified spin-coating method, the centrifugal force that arises when this assembly is spinning would be of the same order as in the conventional
Fig. 4. SEM image of the surface of TiO2 film deposited on fused quartz.
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
spin-coating assembly. However, the fluid dynamics of the modified assembly is indeed complex. Researchers have studied the fluid dynamics for the flow of fluid between rotating parallel plates. Abbot and Walters [19] have obtained an exact solution for the Newtonian fluids and Rajagopal [20] has considered a second order fluid. It is to be noted that the modified assembly is not exactly a parallel plate viscometer, but the study by Abbott and Walters has some relevance for the modified assembly. Without going into the fundamentals, it would be safe to assume that the fluid held between the two substrates gets stabilized due to the secondary flow, which along with the surface tension overcomes the influence of the centrifugal force. Therefore, the loss of the fluid is due to the evaporation alone. This slow evaporation of the fluid initiates the crystallization/gelation/precipitation of the molecules in a thin film form. It is observed that cover substrate is uniformly coated as compared to the base substrate. The process of recrystallization is considerably fast and therefore the deposited material consists of fine crystallites. When the aqueous solution of ammonium titanyl oxalate is subjected to the modified spin-coating process, the cover substrate is coated with the fine crystallites of the ammonium titanyl oxalate. The crystallite size of the deposited material is very small. No peaks in XRD could be observed for the deposited mixture. The coating of the deposited mixture on calcination at 500 jC decompose to its oxide, namely, TiO2. Fig. 2 shows XRD patterns of TiO2 films on quartz and nickel metal substrate heated at 500 jC for 2 h. The TiO2 on quartz shows anatase phase (Fig. 2a). It has also been found that TiO2 prepared under similar condition on nickel substrate and heated at 500 jC shows peaks at 2h = 25.3j, 37.3j and 38.5j, respectively, matching with peaks reported for anatase phase. (ASTM card no. 4-0477). This confirms the formation of anatase phase removing the uncertainty of phase on quartz substrate arising due to the peak of highest intensity, i.e. peak at 2h = 25.3j was getting suppressed in a diffused peak due to quartz substrate. The XRD pattern shows broad peaks of the anatase phase, suggesting the small crystallites existing in the sample. The size of TiO2 crystallites has been estimated from the full-width at half maximum (fwhm) of the (001) diffraction peak using Scherrer’s equation.
1779
The average crystallite size works out to be c 25 nm from three different experiments. This is in agreement to the size of the particles seen in the SEM pictures. Fig. 3 shows the UV – vis spectra in the wavelength range 300 – 800 nm for TiO2 film on quartz substrate. The absorption edge of the TiO2 film is observed at a shorter wavelength. The shift is ascribed to the smaller particle size. The bands due to the interference color of the films appeared in the wavelength 380– 800 nm. These observations are comparable with the observation on the TiO2 thin film prepared by sol – gel technique by Yu and Zhao [14a] The morphology of the TiO2 film obtained by the present method is shown in Fig. 4a and b, which are different regions of the same film. Most of the TiO2 particles have shapes with sizes in nano range; however, some nonspherical shapes are seen. These particles are agglomerated to form aggregates of different shapes and sizes (Fig. 4a and b). Fig. 5 shows the XPS survey spectra taken at the sample surface of TiO2 film on quartz, heat-treated at 500 jC for 2 h. It can be seen from Fig. 5 that TiO2 film deposited on quartz contains elements Ti, O and
Fig. 5. XPS survey spectra for the surface of TiO2 film.
1780
K.R. Patil et al. / Materials Letters 57 (2003) 1775–1780
C. The photoelectron peak for Ti2p appears at a binding energy at 458.2 eV, O1s at 531.3 eV and C1s at 285 eV. This is consistent with the literature value [14a]. The element C in the film is attributed to the residual carbon. The atomic ratio calculated from the area under curve of Ti2p and O1s peaks gives the ratio 1:1.95 for Ti to O. The ratio shows the stoichiometric formation of TiO2.
4. Conclusions The modified spin-coating method wherein aqueous precursor solution, namely, ammonium titanyl oxalate is used for spinning to prepare TiO2 is a versatile technique. This method has advantages over the conventional methods in respect of simplicity and low temperature formation of thin films of titanium oxides with particle size in nano range. UV – visible absorption studies show a blue shift in the visible range for TiO2 films, which is a typical characteristic of nano semiconductor.
Acknowledgements We are thankful to Dr. T. Seth of Center for Materials for Electronics Technology, Pune for recording SEM photographs.
References [1] N. Romeo, G. Sberveglieri, L. Tarricane, Appl. Phys. Lett. 32 (1978) 807. [2] D.B. Fraser, H. Melchior, J. Appl. Phys. 43 (1972) 3120. [3] S. Fujita, Y. Kawakami, S. Fujita, J. Cryst. Growth 164 (1996) 196. [4] P.C. Dorsey, P. Lubitz, D.B. Chirisey, J.S. Horwitz, J. Appl. Phys. 79 (1996) 6338.
[5] F. Cheng, Z. Peng, C. Liao, Z. Xu, S. Gao, C. Yan, D. Wang, J. Wang, Solid State Commun. 107 (1998) 471. [6] N.R. Pawaskar, C.A. Menezes, A.P.B. Sinha, J. Electrochem. Soc. 124 (1977) 743. [7] H.S. Potdar, R.I. Hegade, S. Badrinarayan, N.R. Pawaskar, Sol. Energy Mater. 6 (1982) 183. [8] D.T. Amm, D.J. Johnson, T. Laursen, S.K. Gupta, Appl. Phys. Lett. 61 (1992) 522. [9] I.S. Mulla, H.S. Soni, V.J. Rao, A.P.B. Sinha, J. Mater. Sci. 21 (1986) 1288. [10] S.D. Sathaye, K.R. Patil, D.V. Paranjape, A. Mitra, S.V. Awate, A.B. Mandale, Langmuir 16 (2000) 3487. [11] (a) S. Sakka, Science in Sol – Gel Method, Agune, Tokyo, 1998, Chapter 6. (b) J. Livage, M. Henry, C. Sanchez, Prog. Solid State Chem. 8 (1988) 259. [12] N. Negishi, T. Iyoda, K. Hashimota, A. Fujishima, Chem. Lett. (1995) 841. [13] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [14] (a) J. Yu, X. Zhao, Mater. Res. Bull. 35 (2000) 1293. (b) Y. Paz, A. Heller, J. Mater. Sci. 12 (1997) 471. (c) Y. Paz, Z. Luo, L. Rabenberg, A. Heller, J. Mater. Sci. 10 (1995) 2842. [15] B. Pal, M. Sharon, G. Nogami, Mater. Chem. Phys. 59 (1999) 254. [16] S.D. Sathaye, K.R. Patil, S.D. Kulkarni, P.P. Bakre, S.D. Pradhan, B.D. Sarwade, S.N. Shintre, J. Mater. Sci. (communicated). [17] (a) B.P. Gorman, V. Sprenkle, H.U. Anderson, Ceram. Trans. 109 (2000) 105. (b) J.J. Lee, Y.W. Kim, W. Cho, I.T. Kim, H. Je, J. Park, Yoop Hakhoechi 36 (1999) 893. (c) J. Hong, H. Him, H. Park, Thin Solid Films 332 (1998) 449. (d) F. Horowitz, A.F. Michels, E.M. Yeatman, J. Sol-Gel Sci. Technol. 13 (1998) 707. (e) M. Azuma, M.C. Scott, C.A. Paz, L.D. Mcmillan, US Patent 5,612,082, 1997. [18] (f) S. Prasad, A. Vijayalakshmi, N.S. Gajbhiye, Therm. Anal. Calorim. 52 (1998) 595. [19] H.S. Potdar, S.B. Deshpande, A.S. Deshpande, Y.B. Khollam, S.D. Pradhan, S.K. Date, Int. J. Inorg. Mater. 3 (2001) 613. [20] T.N.G. Abbott, K. Walters, J. Fluid Mech. 40 (1970) 205. [21] K.R. Rajagopal, J. Non-Newton. Fluid Mech. 9 (1981) 185.