Applied Surface Science 255 (2009) 8328–8333

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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Preparation of uniform TiO2 nanostructure film on 316L stainless steel by sol–gel dip coating N. Barati a, M.A. Faghihi Sani a, H. Ghasemi b,*, Z. Sadeghian c, S.M.M. Mirhoseini a a

Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, P.O. Box 11365-9466, Tehran, Iran Department of Mechanical Engineering, University of Toronto, 5 King’s College Road, Toronto M5S 3G8, Ontario, Canada c Department of Gas, Research Institute of Petroleum Industry (RIPI), P.O. Box: 18745-4163, Tehran, Iran b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 February 2009 Received in revised form 10 May 2009 Accepted 19 May 2009 Available online 27 May 2009

Sol was prepared by the mixing of tetra-h-butyle titanat, ethyl aceto acetate, and ethanol in an optimized condition. Polished 316L specimens were coated with the sol by dip-coating method. The influences of drying condition, withdrawal speed, calcination temperature, addition of dispersant, and pH of sol on TiO2 nanostructure coating were investigated. Choosing of alcohol as drying atmosphere hindered the crack formation. The relation between coating thickness and withdrawal speed was evaluated. The optimum temperature to create a uniform distribution of nanoparticles of anatase was derived as 400 8C. Average roughness of coating was found about 10.61 nm by AFM analysis. Dispersant addition promoted formation of a uniform film as well as prevention of agglomeration. Acidic sol provided smaller particles than neutral sol. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Sol–gel Anatase Nanostructure coating Dip-coating

1. Introduction Recently, titanium oxide (TiO2) thin films have been investigated with regard to their remarkable optical, electrical, and photo electrochemical properties. TiO2 thin films have been a source of interest in environmental cleaning such as a photocatalytic purifier [1], in solar energy converters such as photochemical solar cell [2], and in other applications such as gas sensors [3], ceramic membrane [4], and waveguide [5]. Several methods have been employed to fabricate TiO2 thin films, including e-beam evaporation [6], sputtering [7], chemical vapour deposition [8], and sol–gel process [9]. Due to several advantages such as low processing temperature, low equipment cost and good homogeneity, the sol–gel process is one of the most appropriate technologies to prepare thin films [10]. The preparation of TiO2 thin film on different substrates using several sol–gel deposition techniques have been studied [11,12]. Crystalline TiO2 exists in three phases: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) [13]. Formation of each crystalline phase depends on deposition method, calcination temperature, and sol composition [1]. TiO2 film can transform from amorphous phase into crystalline anatase and then into rutile phase during calcination. The photocatalytic property of TiO2 film depends on the type and

* Corresponding author. Tel.: +1 416 978 5107. E-mail address: [email protected] (H. Ghasemi). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.048

grain size of crystallite phases [14]. In case of photocatalytic efficiency, anatase is superior to the rutile [15]. In this work uniform and nanocrystalline TiO2 thin film was prepared on 316L stainless steel by dip-coating sol–gel method. To hinder formation of cracks and achieve uniform coating, the drying process of the coating was controlled. Effects of withdrawal speed, calcination temperature, dispersant addition, and pH value of sol on structural properties of the TiO2 thin film were investigated. 2. Material and methods The sol was prepared from tetra-h-butyle titanat (TBT) as the starting material according to the following process: 91 molar percent ethanol and 2 molar percent ethyl aceto acetate (EAcAc) were mixed at ambient temperature, and then 4 molar percent TBT was added by the rate of 1 cc/min to the mixture. The solution was continuously stirred for 8 h. Within the first hour, 3 molar percent deionized water was carefully added to the solution for hydrolysis. All molar percents are based on total mole of final solution. The solution was aged for 24 h to complete the reactions. The 316L stainless steel samples (20 mm  15 mm  5 mm) were grounded with No. 80–1500 emery papers, and then polished with 0.3 and 0.05 mm alumina powder. Finally, the surfaces of samples were cleaned by ethanol and acetone. The TiO2 coatings were formed on the stainless steel substrate by dip-coating method. Various withdrawal speeds (3, 10, 15, and 25 cm/min) were used to investigate the effect of withdrawal speed on the coating thickness. Effect of dispersant on the microstructure of films was determined

N. Barati et al. / Applied Surface Science 255 (2009) 8328–8333

by adding different amounts of ammonium polyacrylate to the sol. To investigate the effect of pH on homogeneity and particle size, sols with different pH were prepared by adding HCl and NaOH. Two different pre-drying conditions were performed immediately after dipping: natural drying in an ambient condition and drying in a solvent bath. Then, the pre-dried samples were heated in an oven at 150 8C for 30 min. Finally, the samples were heattreated in a temperature range from 350 to 550 8C for 1 h with increasing temperature rate of 5 8C/min in a furnace. X-ray diffraction (XRD) patterns of TiO2 coatings after preparation with various calcination temperatures were obtained using CuKa radiation (l = 1.5406 A˚) with a scanning rate of 18 min1, ranging 2u from 108 to 708. The effects of withdrawal speed, calcination temperature, dispersant, and pH value on microstructure and morphology of the TiO2 thin films were investigated using a scanning electron microscopy (XL30, Philips). Surface areas of the powders calcined at different temperature, were analyzed by nitrogen gas adsorption. The surface topography of the coatings was characterized by Atomic Force Microscopy (AFM) with 0.1 nm accuracy in Z-axis direction. The multimode nanoscope SPM from digital instruments (veeco metrology group) was used. Finally, the purity of the powders was investigated by TGA from ambient temperature to 900 8C.

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Fig. 2. OM image of coated sample after drying in controlled condition (solvent bath).

3. Results and discussion The optical microscopy (OM) images of TiO2 coating surface after dipping and natural drying in an ambient condition and after drying in an oven are shown in Fig. 1(a) and (b), respectively. Existence of cracks in Fig. 1 indicates that these cracks are formed during natural drying step (before drying in oven), due to very fast evaporation of the solvent. To reduce the solvent removal rate and avoid crack formation, the samples were dried in a solvent bath, immediately after dip-coating. Fig. 2 shows the optical microscopy image of TiO2 thin film after drying under the controlled condition. As shown in Fig. 2, increase in the gas pressure of the solvent by using solvent bath reduces the rate of solvent removal and therefore prevents cracking. Thickness of the prepared films was evaluated using AFM analysis. Fig. 3 shows the cross-section of this film. By a crosssection analysis program of AFM, it can be easily concluded that the average thickness of the film with withdrawal speed of 3 cm/ min is about 140 nm. The relation between the film thickness and withdrawal speed is shown in Fig. 4. It is found that the thickness of the prepared TiO2 thin films increases as the withdrawal speed increases. It agrees with landau–levich equation (1) [16].

h ¼ 0:94

Fig. 1. OM images of coated samples after drying in an air flow (a) before drying at oven and (b) after drying at oven (150 8C).

ðh:vÞ2=3

g 1=6 ðr:gÞ1=2

(1)

Fig. 3. Cross-section image of TiO2 film coated with withdrawal speed about 3 cm/ min.

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N. Barati et al. / Applied Surface Science 255 (2009) 8328–8333

Fig. 4. Effect of Withdrawal Speed on thickness of TiO2 films calcined at 400 8C.

Fig. 5. XRD patterns of the TiO2 thin films calcined at different temperatures.

where h, h, n, g LV, r, and g are film thickness, viscosity, withdrawl speed, liquid–vapor surface tension, density, and gravitational acceleration, respectively. To clarify, the gelation process in the thin film is due to solvent removal. Higher withdrawl speed will cause higher evaporation rate and acceleration of gelation process. Furthermore, the probability of sol dripping is lower at higher withdrawl speed.

Fig. 7. SEM micrographs of TiO2 thin films calcined at various temperatures for 1 h: (a) 350 8C; (b) 400 8C; (c) 450 8C; (d) 500 8C; (e) 550 8C.

Fig. 6. Crystallite size of TiO2 films at different temperatures.

N. Barati et al. / Applied Surface Science 255 (2009) 8328–8333

Fig. 5 shows the XRD patterns of the TiO2 thin films after calcination for 1 h in the air at various temperatures. From 350 8C, a broad and weak peak of anatase phase appears. As the calcination temperature increases, intensity of the anatase peaks increases implying an improvement in crystallinity. The crystallite size of TiO2 thin films can be deduced from Scherer equation (2) [17]. D¼

kl b cos u

(2)

where D is the crystallite size of TiO2 thin films, k is a constant (0.94), l is the wavelength of X-ray (CuKa=1.5406 A˚), b is the full width at half-maximum (FWHM) of the diffraction peak, and  is the half diffraction angle of the centroid of the peak in degree. The results are shown in Fig. 6. As the temperature increases, the TiO2 crystallite size increases. In order to obtain photocatalytic activity, it is necessary to make proper anatase nanostructure thin films. SEM micrographs of the TiO2 thin films calcined at various temperatures are shown in Fig. 7. No individual grains can be distinguished for sample calcined at 350 8C [Fig. 7(a)], implying the fineness and low crystallinity of TiO2 crystal grains. This could be expected from XRD results, as presented in Fig. 5. The TiO2 crystal grains grow as calcination temperature increases from 350 to 550 8C, as shown in Fig. 7, respectively. For instance, the film calcined at 450 8C is composed of particles of about 90–100 nm in size. Fig. 8(a) shows the AFM image of the TiO2 coating surface, calcined at 400 8C for 1 h. It indicates that the coating is smooth

Fig. 8. AFM images of TiO2 film coated with withdrawal speed 3 cm/min (a) surface image and (b) 3-D image.

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and uniform. Fig. 8(b) shows three-dimensional AFM image of this film. Smoothness of surface of the TiO2 thin film from AFM analysis is as high as about 10.61 nm. To avoid agglomeration and obtain homogenous films, it is essential to use the appropriate concentration of dispersant. Ammonium polyacrylate is one of the polyelectrolyte dispersants, most widely used to prepare suspensions of titania. The solutions with different ammonium polyacrylate concentrations were prepared. Fig. 9(a) shows the thin film structure without dispersant addition. The agglomeration of particles is evident in this figure. After dispersant addition, the results showed that 0.12 wt% of the dispersant is the optimum concentration to avoid agglomeration in the films as shown in Fig. 9b. The adsorbed ammonium polyacrylate on the particle surface of titania increases the electrostatic repulsive force between colloidal particles by increasing the absolute value of the zeta potential and therefore prevents agglomeration. Each dispersant has an optimum effective concentration in aqueous media and excess dispersant has a negative effect. The destabilizing effect of excess dispersant is not well understood. The optimum concentration is termed as the saturation adsorption limit of the polyelectrolyte. Addition of polymer over the adsorption saturation limit only serves to provide excess polymer in solution. Excess polymer can destabilize the dispersion by a mechanism called depletion flocculation, resulting from an increase in osmotic pressure, thereby forcing particles together. Since particle sizes are very sensitive to temperature, to ensure the formation of nanoparticles, surface area of the powders, dried and after calcined at different temperatures, were measured by gas

Fig. 9. SEM images of TiO2 films calcined at 400 8C (a) dipped in sol without dispersant and (b) dipped in sol including 0.12 wt% ammonium polyacrylate additives.

N. Barati et al. / Applied Surface Science 255 (2009) 8328–8333

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Fig. 11. TGA curve of the gel from ambient temperature up to 900 8C.

of the titania powder as temperature increases especially in the interval of 400–450 8C. Fig. 10 shows the SEM images of TiO2 thin films coated at different pH values with the same calcination temperature (400 8C). When the pH value of the sol is about 2.3 [Fig. 10(a)], it is difficult to identify the particles in the film. As pH value increases, the crystallite size of the TiO2 thin film increases [Fig. 10(b) and (c)]. The phase transformation from Ti(OH)4 to anatase (TiO2) is assumed to be inhibited by excessive adsorption of OH ions to the TiO2 clusters. It is noteworthy to point out that the excessive adsorption of OH may inhibit the nucleation of anatase particles rather than encourage their growth [18]. Therefore the final particle size of anatase increases with increasing pH. Decrease in the nucleation rate is due to decrease in the concentration of Ti(OH)4 groups with increasing pH values. To investigate the purity of thin films powders, thermal gravimetric analysis (TGA) was done on the powder using Netzsch analyser with a heating rate of 5 8C/min as shown in Fig. 11. There is a sharp weight loss from ambient temperature to near 80 8C in Fig. 11. This initial weight loss is primarily due to ethanol evaporation after which only water evaporation is continued. The next drop in weight is occurring around 195 8C which is attributed to the removal of ethyl aceto acetate (EAcAc). Decomposition of tetra-h-butyle titanat (TBT) can continue up to 310 8C. After 400 8C, the change in the sample weight is very low and it can be concluded that titania is the only present phase in the sample after 400 8C. Also, due to low thickness of titania thin film, the removal rate of organic components is rapid. Fig. 10. SEM images of TiO2 films prepared on 316L stainless steel substrates at 400 8C with different pH values (a) 2.3; (b) 4.5; (c) 7.

adsorption. Nitrogen gas adsorption on powders calcined at different temperatures was performed using Micromeritics ASAP 2010. The data and BET equation were used to determine the BET isotherm parameters and powders surface area. The results are listed in Table 1. The results indicate an increase in the surface area Table 1 Surface areas of the powders calcined at different temperatures; data are taken by N2 adsorption measurements and using BET adsorption isotherm. Calcination temperature (8C)

Crystallite sizes

SBET (m2/g)

350 400 450 500 550

7 8.5 13 17 31

191 187 132 106 65

4. Conclusions In this work a uniform and homogenous TiO2 nanostructure thin film was prepared on the 316L stainless steel by sol–gel method. By drying TiO2 thin films in a solvent bath, crack formation was avoided. Agglomeration of particles in the thin film was prevented by adding ammonium polyacrylate as a dispersant. The XRD patterns of thin films calcined at 350–550 8C intervals, confirmed anatase phase formation. The surface morphology and crystal structure of TiO2 thin films were influenced by calcination temperature and pH values of sol. In acidic region, the crystallite size decreased by decreasing pH values of the sol. References [1] [2] [3] [4]

E.O. Zayim, J. Mater. Sci. 40 (2005) 1345–1352. M. Graetzel, J. Commun. Inorg. Chem. 12 (1991) 93. H. Tang, K. Prasad, R. Sanjine’s, F. Le’vy, J. Sens. Actuators B 26–27 (1995) 71. Y. Yan, S.R. Chaudhuri, A. Sarkar, J. Am. Ceram. Soc. 79 (1996) 1061–1065.

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Zhai Jiwei, Yang Tao, Zhang Liangying, Yao Xi, J. Ceram. Int. 25 (1999) 667. Y.L. Wang, K.Y. Zhang, J. Surf. Coat. Technol. 140 (2001) 155. S. Takeda, S. Suzuki, H. Odaka, H. Hosono, J. Thin Solid Films 392 (2001) 338. H.Y. Ha, S.W. Nam, T.H. Lim, I.H. Oh, S.A. Hong, J. Membr. Sci. 111 (1996) 81. L. Hu, T. Yoko, H. Kozuka, S. Sakka, J. Thin Solid Films 219 (1992) 18. C.J. Brinker, G.W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, Inc., New York, 1990. [11] B.H. Kim, J.H. Ahn, J.H. Jeong, Y.S. Jeon, K.O. Jeon, K.S. Hwang, J. Ceram. Int. 32 (2006) 223–225. [12] Z. Zainal, C.Y. Lee, J. Sol–Gel Sci. Technol. 37 (2006) 19–25.

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[13] Y.J. Yun, J.S. Chung, S. Kim, S.H. Hahn, E.J. Kim, J. Mater. Lett. 58 (2004) 3703– 3706. [14] C.J. Brinker, G.L. Frye, A.J. Hurd, Cs. Ashlet, J. Thin Solid Films 201 (1991) 97. [15] K. Tanaka, T. Hisanaga, A.P. Rivera, Photocatalytic Purification and Treatment of Water and Air, 1993, p. 169. [16] S. Kalliadasis, U.W.E. Thiele, Thin Films of Soft Matter, vol. 490, Springer, 2007,, p. 130. [17] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Pub, Notre Dame, 1978. [18] T. Sugimoto, X. Zhou, J. Colloid Interface Sci. 252 (2002) 347–353.

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