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Enhanced Hydrophilic Property of TiO<sub>2</sub> Thin Film Deposited on Glass Etched with O<sub>2</sub> Plasma
Enhanced Hydrophilic Property of TiO2 Thin Film Deposited on Glass Etched with O2 Plasma
Transactions on Electrical and Electronic Materials. 2013. Jun, 14(3): 152-155
Copyright ©2013, The Korean Institute of Electrical and Electronic Material Engineers
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use,distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : March 03, 2013
  • Accepted : April 04, 2013
  • Published : June 25, 2013
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About the Authors
Hwa-Min Kim
Department of Advanced Energy Material Science & Engineering, Catholic University of Daegu, Kyeongsan 712-702, Korea
Sung Bo Seo
Department of Electronics Engineering, Catholic University of Daegu, Kyeongsan 712-702, Korea
Dong Young Kim
Department of Electronics Engineering, Catholic University of Daegu, Kyeongsan 712-702, Korea
Kang Bae
Department of Electronics Engineering, Catholic University of Daegu, Kyeongsan 712-702, Korea
Sun Young Sohn
Department of Electronics Engineering, Catholic University of Daegu, Kyeongsan 712-702, Korea
sysohn@cu.ac.kr
Abstract
TiO 2 films were deposited on glass substrates with and without O 2 plasma etching by using the RF-magnetron sputtering method. We focused on the effect of surface structure on the photoinduced hydrophilic properties of TiO 2 films, fabricated on different surface conditions according to the presence or absence of the O 2 plasma treatment on glass substrates. The wettability and photoinduced hydrophilic properties of the TiO 2 films were investigated according to the changes in water contact angles under UV light irradiations with a very low intensity of 0.1 mW/cm 2 . The photoinduced hydrophilic properties on the TiO 2 formed above the plasma treated glass were also superior to those on the TiO 2 formed above the bare glass. This enhanced TiO 2 film has been used practically for self cleaning and anti-fogging glasses.
Keywords
1. INTRODUCTION
Titanium dioxide (TiO 2 ) has been well known as an efficient phtocatalyst material [1 - 4] . When ultra-violet (UV) light is irradiated on, electron and hole pairs are generated in the TiO 2 and they, respectively, reduce and oxidize adsorbates on the surface, generating radical species such as O 2 and OH. These radicals can decompose most organic compounds [5 , 6] , and extensive research has been performed on TiO 2 in terms of applications for water and air purification [7] .
In particular, the generation by UV illumination of a superhydrophilic TiO 2 surface with a water contact angle (WCA) of 0° has attracted significant attention [8 - 13] . This material has been successfully applied as a transparent super-hydrophilic coating with anti-fogging [14] and self-cleaning properties [15] . In general, the wettability of a solid surface is well known to be governed by both the chemical composition and the geometrical microstructure of the surface [16] .
In this work, the glass substrates are etched with O 2 plasma prior to the coating of TiO 2 in order to form the geometrical microstructure on the TiO 2 surface, and the microstructure dependence of TiO 2 thin films on the hydrophilic property is investigated. In particular, we focused on the effect of surface structure on the photoinduced hydrophilic properties of TiO 2 films, fabricated on different surface conditions of glass substrates according to the presence or absence of O 2 plasma etching. The sputtered TiO 2 film has been used practically for glass with self-cleaning and anti-fogging properties.
2. EXPERIMENTS
A radio frequency (RF) magnetron sputtering apparatus was used to fabricate TiO 2 film on soda-lime glass substrates with a size of 50×50 mm 2 . After thoroughly stirring TiO 2 powder (99.99%) for 2 hours using a Ball-Mill, the powder was calcined at 500℃ in the air for 2 hours. The calcined powder was molded as a cylindrical type pellet with a diameter of 2 inch and a height of 50 mm with a pressure of 11 tons. This pellet was used as a sputter target for TiO 2 films. Prior to deposition of TiO 2 , the glass substrates were etched with plasma for 30 min in an oxygen atmosphere of 2×10 -2 Torr. The applied RF-power was fixed at 50 W for all films. The thicknesses of the TiO 2 films deposited on the glass substrates were measured from 50 nm to 200 nm, which were controlled by deposition time. The surface morphologies of TiO 2 films were analyzed using a field emission scanning electron microscopy (FESEM: Jeol Co.). The hydrophilicity and crystalline phase of TiO 2 films were investigated by measuring the water contact angle and X-ray diffraction (XRD? Co.) pattern, respectively. The contact angle measurements were carried out at room temperature using a Kruss DSA100 goniometer following a very standard and commonly used experimental procedure as reported in the literature [17] .
The photoinduced hydrophilic conversion was evaluated according to the changes in the water contact angle under UV light irradiation using black light bulbs (BLB, Toshiba lighting & Technology). The UV light irradiation was stopped when measuring the water contact angle. The photo-catalytic activity of the film was assessed for the degradation of commercial gear oil (TOYOTA gear oil super GL-5). Prior to UV irradiation, the as-deposited films were contaminated by dropping oil onto the surfaces and wiping to ensure even coverage. The samples were washed with distilled water, to remove excess oil. During the photo-catalytic degradation, the humidity was maintained at 40% RH at room temperature.
3. RESULTS AND DISCUSSION
Figure 1 shows the thickness change as a function of deposition time for TiO 2 films deposited on glass substrates with RFpower of 50 W. It is shown that the thicknesses of TiO 2 films linearly increase as deposition time increases. Therefore, the deposition rate calculated from the slope of Fig. 1 is found to be 1.67 nm/min. In addition, the XRD pattern of the 200 nm-thick TiO 2 film is represented in the inset of Fig. 1 . Since no crystalline peaks are observed in the X-ray diffraction pattern, the asdeposited TiO 2 films have an amorphous structure.
Figure 2 shows the change of WCA as a function of thickness for TiO 2 films deposited on the glass etched with oxygen plasma. For comparison, the change of WCA as a function of thickness for TiO 2 films deposited on the bare glass substrate is also shown. In the case of TiO 2 /glass, the WCAs are almost constant with a thickness of between 17° and 18°. However, TiO 2 films deposited on the etched glass show lower WCAs of between 4° and 7° compared with those of TiO 2 /glass. This enhanced hydrophilic property of TiO 2 film can be explained by Wenzel’s model. Consider a rough solid surface with a typical size of roughness detail smaller than the size of the droplet, as shown in Fig. 3 (a). For a droplet in contact with a rough surface without air pockets, referred to as a homogeneous interface (or Wenzel’s model), according to Wenzel, the contact angle is given as follows.
Lager Image
  • Rf: roughness factor
  • θ: contact angle for the rough surface
  • θ0: contact angle for a smooth surface
Figure 3 (b) shows that the dependence of the contact angle on the roughness factor is predicted for various values of θ0 , based on Wenzel’s model. The model predicts that a hydrophobic surface
Lager Image
Changes of thickness as a function of deposition time for TiO2films deposited on the glass substrate under RF-power of 50 watt.
Lager Image
Change of WCA as a function of thickness for TiO2 films accordingto presence (rectangular) or absence (circle) of oxygen plasmaetching on the surface of glass substrates.
Lager Image
(a) Schematic of a liquid droplet in contact with (i) a smoothsolid surface (contact angle, θ0) and (ii) a rough solid surface (contactangle, θ). And (b) the dependence of the contact angle on the roughnessfactor is predicted for various values of θ0, based on Wenzel’smodel, where, θ0=30° (for a flat glass), θ0=72° (for a flat p-Si), θ0=103°(for a flat p-Si coated with PTFE), and θ0=116° (for a flat Al surface).
( θ >90°) becomes more hydrophobic with an increase in Rf , and that a hydrophilic surface ( θ <90°) becomes more hydrophilic with an increase in Rf .
Thus, the enhanced hydrophilic property of TiO 2 films deposited on the etched glass shown in Fig. 2 is attributed to the surface roughness formed after the plasma etching on the glass substrate. The roughness is observed in Fig. 4 , which shows SEM images for (a) unetched glass and (b) etched glass and TiO 2 films with various thicknesses deposited on them. In the case of glass substrate, nano protrusions are formed on the surface of the glass after plasma etching, as shown in Fig. 4 (b), which is compared to the smooth surface of unetched glass.
A calculated roughness factor of the etched glass, based on Wenzel’s model, is 1.088, where WCAs of unetched and etched glass surfaces are considered to be 28° (= θ0 ) and 16° (= θ ), respectively. These roughness factors will decrease with increasing thickness of TiO 2 layer because the thicker films may further alter the geometrical microstructure formed on the etched glass surface. The roughness factors corresponding to each thickness of TiO 2 films deposited on the etched glass, based on Wenzel’s model, are described in Fig. 2 , where WCA ( θ0 ) on the flat TiO 2 surface was considered to be 17°, which is an averge value of WCAs for TiO 2 films deposited on the unetched glass, showing that the TiO 2 film has a hydrophilic surface.
The photoinduced hydrophilic properties are shown in Fig. 5 , where Fig. 5 (a) indicates the changes in water contact angles on both films under UV light irradiation with intensity of 0.1 mW/ cm 2 . The reciprocal of water contact angles is also plotted against irradiation time, a linear relationship is obtained, as shown in figure (b). This straight line is defined as the rate constant for the hydrophilic conversion process. A distinguished difference was observed in the critical water contact angles and the rate constants for the hydrophilic conversion between TiO 2 /etched glass films and the TiO 2 /glass films. It is clear that the TiO 2 /etched glass film became highly hydrophilic compared to the TiO 2 /glass film. This indicates that the photoinduced hydrophilic reaction of the TiO 2 surface was also enhanced by oxygen plasma etching on the surface of the glass.
When TiO 2 is illuminated with UV light, electron and hole pairs are generated, which reduce and oxidize adsorbates on the surface, respectively. These reactions are known as photocalysis [1 - 6] .
Thus, the enhanced photoinduced hydrophilic reaction TiO 2 film deposited on the etched glass in comparison with that of TiO 2 coated on the bare glass may be attributed to an increase of the light receiving area due to the nano protrusions formed on the glass after the plasma etching, which results in an increase of photogenerated electron and hall pairs.
Because of the high hardness, adhesion, and highly photoinduced hydrophilic reaction of the sputtered TiO 2 film, the sputtered TiO 2 films are suitable for application in many outdoor uses, especially in the exterior rear view mirrors and windshields of automobiles (see Fig. 6 ). The TiO 2 films have been used practically for large-area glass with anti-fogging [14] and self-cleaning properties [15] . These products promise aclear view on rainy days.
4. CONCLUSIONS
In this work, TiO 2 films were deposited on glass substrates with and without O 2 plasma etching by using the RF-magnetron sputtering method, and the wettability and sensitization of photocatalytic reactions were investigated. We focused on the effect of surface structure on the photoinduced hydrophilic properties of TiO 2 films, fabricated on different surface conditions of glass according to the presence or absence of the O 2 plasma etching. The surface structures of TiO 2 films were observed from SEM images and their wettability and photoinduced hydrophilic properties were investigated according to the changes in water contact angles under UV light irradiations with intensity of 10 mW/cm 2 . As a result, the enhanced hydrophilic property of TiO 2 film deposited on the etched glass can be explained by the increase of roughness factor calculated, based on Wenzel’s model.
On the other hand, the photoinduced hydrophilic properties on the TiO 2 formed above the etched glass were also superior to those on the TiO 2 formed above bare glass. The enhancement of
Lager Image
SEM images for surfaces of (a) bare and TiO2 films with variousthicknesses coated on the bare glass and (b) plasma treated glassand TiO2 films with various thicknesses coated on the plasma treatedglass.
Lager Image
(a) Change in water contact angles of the 200 nm-thick TiO2films deposited on the etched glass and unetched glass under 0.1mW/cm2 UV light irradiation. (b) The TiO2 layers were coated at athickness of 200 nm reciprocal of the water contact angles of bothfilms.
Lager Image
Comparison between the visibilities of (a) the plasma treatedglass with TiO2 layer and (b) that of bare glass.
the hydrophilic conversion of the TiO 2 film/etched glass can be explained by an increase of the light receiving area due to the surface structure formed on the surface of the TiO 2 film, resulting in further accumulation of photogenerated electrons and hole pairs at the surface of TiO 2 . The sputtered TiO 2 films are applicable for outdoor uses because of the high hardness, adhesion to the glass substrates, and highly photoinduced hydrophilic reaction, the TiO 2 films have been used practically for glass with selfcleaning and anti-fogging properties. Therefore, we suggest that these products can be applied to the exterior rear view mirrors and windshields of automobiles to ensure clear vision on rainy days.
Acknowledgements
This work was supported by the Gyeonbuk Science & TechnologyPromotion Center (GBSP) grant funded by the Korea government(MEST) (GBSP-001-111228-002).
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