Photoinduced hydrophilicity of titanium dioxide thin films prepared by cathodic electrodeposition

Photoinduced hydrophilicity of titanium dioxide thin films prepared by cathodic electrodeposition

ARTICLE IN PRESS Vacuum 80 (2006) 494–498 www.elsevier.com/locate/vacuum Short communication Photoinduced hydrophilicity of titanium dioxide thin fi...

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ARTICLE IN PRESS

Vacuum 80 (2006) 494–498 www.elsevier.com/locate/vacuum

Short communication

Photoinduced hydrophilicity of titanium dioxide thin films prepared by cathodic electrodeposition S. Karuppuchamya,b,, J-M. Jeonga, D.P. Amalnerkarc, H. Minourab a Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan Environmental and Renewable Energy Systems Division, Graduate School of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan c Thick Film Materials & Electronic Packaging Division, Centre for Materials for Electronics Technology, Panchawati, Off Pashan Road, Pune 411008, India

b

Received 6 April 2005; received in revised form 1 June 2005

Abstract Crystal structure and microstructural properties of titanium dioxide thin films prepared by cathodic electrodeposition on indium-tin-oxide coated glass substrates from aqueous peroxo-titanium complex solutions have been investigated as a function of sintering temperature (25–500 1C) for the first time. We have noticed pronounced photoinduced hydrophilicity for such thin films on exposure to ultraviolet (UV) light illumination. It was observed that all the films, irrespective of their crystalline nature (amorphous and crystalline), display transformation from hydrophobic to superhydrophilic behavior upon UV illumination. This observation can be correlated with typical nanoporous morphology of electrodeposited TiO2 films. r 2005 Elsevier Ltd. All rights reserved. Keywords: Electrodeposition; Heat treatment; Titanium dioxide; Hydrophilicity

1. Introduction Nanostructured semiconductor oxide materials have recently attracted a great deal of attention Corresponding author. Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 8208555, Japan. Tel./fax: +81 948 22 7336. E-mail address: [email protected] (S. Karuppuchamy).

owing to their excellent optical [1], chemical [2], photoelectrochemical [3] and electronic properties [4]. Among the metal oxides, nanocrystalline TiO2 is one of the most investigated materials owing to its important applications in environmental cleanup [5], photocatalysis [6], dye-sensitized solar cells [3,7–9] and gas sensors [10]. Furthermore, the TiO2 surface shows super-hydrophilic character with an irradiation of ultraviolet (UV) light. Hence, the TiO2 thin films coated on a window glass, a light bulb, ceramic

0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.06.005

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tiles or a wall can clean the polluted air in the room and can self-clean or defog their surfaces [11]. The mechanism of photoinduced hydrophilicity in TiO2 has been intensively investigated and, consequently, it has been suggested that hydrophilic behavior of TiO2 could be attributed to the water adsorption on the vacancies generated by the dissociation of bridging oxygen bonds during UV light radiation [11]. Additionally, it is believed that the surface morphology of the films other than adsorption properties could also influence the hydrophilic property of TiO2 [12,13]. Hence, it can be presumed that certain factors affecting the surface morphology of the films, such as film preparation technique also could play a vital role in the hydrophilic/hydrophobic conversion cycle. The fabrication methods reported in many previous studies dealing with photoinduced hydrophilicity in TiO2 thin films have been mostly sol–gel [12] or sputtering [13,14]. We explored the possibility of using electrodeposition technique, which offers a convenient and versatile route to deposit TiO2 thin films [9,15,16] from aqueous solutions on the large area substrates, in order to establish surface morphology–hydrophilicity correlation. It has several advantages over other techniques such as (i) rigid control on film thickness, uniformity, and deposition rate; (ii) stringent regulation of reaction parameters such as solution concentration and composition; and (iii) facile adaptation to conformal deposition on substrates of complex shape and geometry. In this communication, we report that a surface of high hydrophilicity can be realized by UV illumination of amorphous and crystalline TiO2 films, which were fabricated by cathodic electrodeposition from aqueous solution. This preliminary attempt highlights that the super-hydrophilic TiO2 surface created by our simple and environmentally friendly process is a promising technique for selfcleaning layers.

2. Experimental 2.1. Preparation of TiO2 thin films All chemicals were purchased from commercial sources and were of the highest purity available.

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They were used without further purification. Doubly distilled and ion exchanged water was used in all the experiments. Optically transparent indium tin oxide coated glass plates (ITO, 10 O/ sq., obtained from Musashino fine glass) were used as the substrates. Prior to film deposition, the ITO substrates were ultrasonically cleaned sequentially in acetone, 2-propanol and water each for 15 min. A standard single compartment, three-electrode electrochemical cell was used for the film preparation. A Pt foil counter-electrode and an Ag/AgCl reference electrode, along with ITO glass substrate as the working electrode, completed the cell set up. Electrodeposition was carried out on a potentiogalvanostat (HOKUTO DENKO, HABF 501). TiO2 thin films were cathodically electrodeposited from an aqueous peroxo-titanium solution that has been described elsewhere [9]. Briefly, the bath consisted of (0.01–1 M) TiOSO4, (0.01–0.5 M) H2O2 and 0.1 M NH4NO3 (pH ¼ 1:5–3 maintained at 10 1C). Cathodic electrodeposition was carried out at 1.2 V (vs. Ag/AgCl), which led to the formation of TiO(OH)2 xH2O gel film on the electrode. Subsequently, this gel was subjected to a heat treatment in air for 1 h to obtain crystalline TiO2 thin film. The electrodeposited samples were stored in air under the dark for 2 weeks before the characterization.

2.2. Characterization The crystallization behavior of the electrodeposited TiO2 films was analyzed by X-ray diffraction (XRD) using a RIGAKU RAD-2R diffractometer with Cu Ka radiation. The surface morphological features of the samples were examined using a scanning electron microscope (SEM, TOPCON ABT-150FS). Film thickness was determined by using a KOSAKA SE-2300 surface profilometer. The photoinduced hydrophilicity was evaluated by the water contact angle measurement, which was performed at ambient conditions (i.e., 25 1C, relative humidity (RH) 65%) using a commercial contact angle meter (Kyowa Kaimen Kagaku). UV illumination was carried out using Hg lamp with an optical fiber coupler, employing a filter to obtain light with

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a wavelength centered at 365 nm. The water droplet size used for the measurements was 10 mL.

(a)

3. Results and discussion X-ray diffraction patterns showing the transition from amorphous to crystalline state in TiO2 thin films are presented in Fig. 1. It can be noted that no diffraction peak due to TiO2 appears in the as deposited and relatively low-temperature heattreated (200 1C) films. However, some diffraction peaks ascribable to anatase TiO2 appear after annealing the samples at 300 and 500 1C for 1 h, in turn, indicating that the films tend to become crystalline. The morphologies corresponding to as-deposited and the heat-treated (at 300, 500 1C) TiO2 films are reproduced in Fig. 2. The surface morphology of the as-deposited gel film reveals an open porous structure with the grain size falling mostly in the submicron range. Although the appearance of the surface indicates open porous structure with clumpy morphological features, X-ray diffraction evidences only amorphous character. The SEM images of the samples before and after annealing show quite similar features. Care-

(b)

(c)

ITO

Intensity (a.u.)

anatase

(d) (c) (b) (a)

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Fig. 2. SEM photographs of electrodeposited TiO2 thin films (a) as-deposited and heated at (b) 300 and (c) 500 1C for 1 h in air.

Fig. 1. X-ray diffractograms of electrodeposited TiO2 thin films (a) as-deposited and heated at (b) 200, (c) 300 and (d) 500 1C for 1 h in air.

ful observation of the SEM pictures advocates that each grain is made up with an aggregate of very small (few tens of nano-meters in size) crystallites.

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40 2/degree (CuKα)

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The UV-light-induced hydrophilicity of the films (ca. 0.5 mm) was investigated by measurements of the water contact angle during the film exposure to light irradiation. Irradiation of the as-deposited and heat-treated samples by UV light at 5 mW/cm2 intensity in atmospheric air-induced changes in the water contact angle. Fig. 3 (left) shows the spherical water droplet with a water contact angles of 1181, 1131, 1071 and 841 for the as-deposited, heated films at 200, 300 and 500 1C respectively, before light illumination. Upon UV-light irradiation, the water droplet spreads out on the film, resulting in a contact angle of about 01 (Fig. 3, right). These results indicate that the wettability of the films characteristically changes from hydrophobicity to super-hydrophilicity. Further, it is quite interesting to note that the electrodeposited amorphous and crystalline films exhibit relatively high contact angle compared to the TiO2 films,

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which are prepared by different techniques [14,17]. The surfaces of these thin films are inherently hydrophobic owing to entrapment of air in the nanoporous network structures associated with such surfaces. Therefore, the unique nanostructures of the electrodeposited TiO2 thin film surface are responsible for the hydrophobic properties. Fig. 4 shows time dependence of the water contact angle upon UV-light illumination for the electrodeposited samples (a) as-deposited and heated at (b) 200, (c) 300 and (d) 500 1C. As depicted in the figure, water contact angle of all the samples decreases with irradiation time. But the conversion time for hydrophobic to super-hydrophilic behavior differs from sample to sample. The film, which was heated at 500 1C, exhibits the largest contact angle reduction from 841 to 01 within 45 min of UV illumination. The other films including crystalline one (heated at 300 1C) show the angle reduction to 01 after UV illumination over 75 min, suggesting relatively inferior quality of the resultant TiO2 films than that of films heated at relatively higher temperature of 500 1C. The largest angle reduction of the above film may be attributed to the well-crystallized anatase TiO2 structure, as revealed by its XRD spectrum. Although, the film heated at 300 1C shows anatase 140 as-deposited 120 Contact angle (degree)

200 °C 100

300 °C

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Fig. 3. Photographs of water droplet shape on the electrodeposited TiO2 thin films (a) as-deposited and heated at (b) 200, (c) 300 and (d) 500 1C, before (left) and after (right) UV-light illumination (5 mW/cm2).

50 UV light irradiation time (min)

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Fig. 4. Variations of water contact angle as they were induced by the UV-light illumination (5 mW/cm2) of the electrodeposited TiO2 films (a) as-deposited and heated at (b) 200, (c) 300 and (d) 500 1C, in air at RH of 65% and temperature of 25 1C.

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phase, it takes little longer time to become superhydrophilic. This may be presupposedly due to the less crystallinity or the mixed amorphous and crystalline character of such films [14]. The present results indicate that the well-crystallized TiO2 film had the better hydrophilicity than that of films displaying amorphous or the mixed (amorphous/ crystalline) behavior. It should be noted that the similar behavior was reported in the previous studies [18,19]. It can be considered that the best UV-induced hydrophilicity of the well-crystallized structure may be due to the transportation of the charge carriers in the bulk crystalline state to the surface state where they interact with the adsorbate molecules [14,18]. Consequent investigations on the UV-light-induced hydrophilicity as a function of microstructure of the cathodically electrodeposited TiO2 thin films are in progress.

4. Conclusions The photoinduced hydrophilicity of the electrodeposited TiO2 thin films under ultraviolet (UV) light illumination was investigated. It was observed that both amorphous and crystalline TiO2 films demonstrate transformation from hydrophobic to super-hydrophilic behavior upon UV illumination of light, irrespective of their crystalline nature. The present study indicates that the hydrophilic TiO2 surface created by our simple cathodic electrodeposition route is a promising technique for self-cleaning layers.

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