Abstract:
A hydrophilic-hydrophobic transformable composite film is disclosed, which has an iron-containing substrate; a silicon-containing layer formed on the iron-containing substrate; and a titanium oxide layer formed on the silicon-containing layer. The hydrophilic-hydrophobic transformable composite film uses UV-light to switch its hydrophilic and hydrophobic property. Also, a method of fabricating the above composite film is disclosed. According to the present invention, an anatase oxide layer can be obtained from the sol-gel techniques, in which the particle size of the titanium dioxide is controlled and a modified surface coating with evident photo-catalyticity is achieved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a hydrophilic-hydrophobic transformable composite film and, more particularly, to a hydrophilic-hydrophobic transformable composite film switched by UV light illumination. 
         [0003]    2. Description of Related Art 
         [0004]    In many situations, the ability to control the surface wettability of solid substrate is very important. In 1972, Fujishima and Honda constructed an electrochemical cell in which a TiO 2  electrode was connected with a platinum electrode through an external circuit. When the surface of the TiO 2  electrode was irradiated, current flowed from the platinum electrode to the TiO 2  electrode through the external circuit. The direction of the current reveals that the oxidation reaction occurs at the TiO 2  electrode and the reduction reaction occurs at the platinum electrode. Thereby, Fujishima and Honda suggested that even without the application of any external voltage, water could be decomposed into hydrogen and oxygen by visible light (A. Fujishima and K. Honda, Nature, 238 (1972) 37-38). Since then, significant amounts of research involved with solar energy transformation, and photochemical catalysis, synthesis, and degradation have been widely performed. 
         [0005]    Anatase titanium oxide has many advantages such as excellent light-catalysis activity, UV light-excited catalysis activity, self-cleaning property, anti-bacterial property, anti-fogging property, and super-hydrophilicity and therefore has been applied into many types of sanitary equipment manufacture. Among these properties, the photocatalytic properties result from the promotion of electrons (on the surfaces of TiO 2  layer) from the valance band to the conduction band under the influence of ultraviolet (UV) or near-UV radiation. Then the hydroxyl free radical or oxygen free radical (O 2− ) generated may further react with organic matters or water and undergo a series of photocatalytic reactions. 
         [0006]    The super-hydrophilicity results from the repulsion between the excited electrons and the oxygen atoms locating on the outer surface of the TiO 2  layer. An abundance of oxygen-holes is created after the leaving of oxygen atoms, and following with the refilling of the oxygen-holes with the water molecules. The hydrogen atoms of those water molecules bind to the oxygen atoms of TiO 2  molecules having strong reducing activity and therefore a large amount of hydroxyl group forms on the TiO 2  layer. Consequently, a TiO 2  layer with super-hydrophilicity on the surface is formed. 
         [0007]    Anatase titanium oxide is usually in a form of nano particles, which need to be immobilized before practical usage. Accordingly, reactivity and durability should be considered for the immobilization process. When an organic substrate is used, the decomposition of organic substrate itself should be concerned first. Meanwhile, if an inorganic substrate is used, the surfaces of the photo-catalyst may be shielded, thus, results in the lowering of the catalytic efficiency. Therefore, the immobilizing method of anatase titanium oxide is still a challenge to those skilled in this art and thus is of particular interest to researchers. 
         [0008]    The TiO 2  coating layer applied to the fabrication of commercial products such as sanitary equipment usually has only single hydrophilic property, either hydrophilic or super-hydrophilic, but does not has hydrophilic-hydrophobic transformation ability, and therefore is limited in many application situations. For example, the cleaning efficiency cannot be largely improved if only hydrophilic property appears in the outer coating layer of the sanitary equipment. 
         [0009]    Hence, it is desirable to provide an improved hydrophilic-hydrophobic transformable composite film that the hydrophilic or hydrophobic property thereof can be changed by a simple step, and can be applied into the fabrication of several kinds of apparatus such as sanitary equipment, vehicle outer shells, or exterior wall tiles, and this improves the cleaning efficiency of those applied usages. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a hydrophilic-hydrophobic transformable composite film, which comprises an iron-containing substrate; a silicon-containing layer formed on the iron-containing substrate; and a titanium oxide layer formed on the silicon-containing layer. 
         [0011]    The hydrophilic and hydrophobic properties of the present composite film are switched by UV light illumination. In detail, the composite film is hydrophobic before being exposed to UV light, but changes to hydrophilic after being exposed to UV light, and changes back to hydrophobic after removing the UV source for a period of time (for example, about 24 hours). According to contact angle testing results, a great difference between contact angles of the present composite films before and after exposing to UV light is shown, which proves the excellent hydrophilic-hydrophobic transforming ability of the composite films of the present invention. 
         [0012]    According to the hydrophilic-hydrophobic transformable composite film of the present invention, the silicon-containing layer may preferably be a silicon layer, a silicon oxide layer, or a combination thereof. 
         [0013]    According to the hydrophilic-hydrophobic transformable composite film of the present invention, the iron-containing substrate is not specially limited, and can be made of one selected from the group consisting of: interstitials free (IF) steel, transformation induced plasticity (TRIP) steel, dual phase (DP) steel, Twinning Induced Plasticity (TWIP) steel, high strength (HS) steel, high strength low alloy (HSLA) steel, ceramics doped with iron, and glass doped with iron. The iron-containing substrate preferably is made of interstitials free (IF) steel due to its low price and being easily commercially reachable. 
         [0014]    According to the hydrophilic-hydrophobic transformable composite film of the present invention, the silicon-containing layer formed on the iron-containing substrate can be made by sputtering or by a sol-gel process, but preferably is made by sputtering. 
         [0015]    According to the hydrophilic-hydrophobic transformable composite film of the present invention, the titanium oxide layer is preferably made by sol-gel process in order to adjust the thickness of the titanium-containing sol-gel film to enable the forming of porous film (anatase titanium oxide) that further becomes an excellent catalyst. The porous film (anatase titanium oxide) is advanced in many advantages comprising (1) good solidifying ability; (2) well-controlled particle size of TiO 2  particles wherein the minimum particle size is 100 nm or less, which may result in excellent quantum effect; and (3) high uniformity of TiO 2  particle distribution that means the aggregation phenomenon is reduced, which results in the increasing of the contact area (exposing area) to UV light and providing an evident photo-catalyticity. Besides, the thickness of the titanium oxide layer preferably is 0.1˜μm, more preferably is 0.2˜0.4 μm. The particle size of the titanium oxide particle preferably is 100 nm or less. 
         [0016]    The present invention also provides a method of fabricating a hydrophilic-hydrophobic transformable composite film, which comprises (A) forming a silicon-containing layer on an iron-containing substrate; (B) coating a titanium-containing sol-gel film on the said silicon-containing layer; and (C) heating the iron-containing substrate with the silicon-containing layer and the titanium-containing sol-gel film thereon. Thereby, a hydrophilic-hydrophobic transformable composite film can be easily fabricated by the method of the present invention. The hydrophilic and hydrophobic properties of the composite film fabricated are switched by UV light illumination. In detail, the composite film is hydrophobic before being exposed to UV light, but changes to hydrophilic after being exposed to UV light, and changes back to hydrophobic after removing the UV source for a period of time, for example, about 24 hours. 
         [0017]    According to the method of the present invention, the silicon-containing layer may preferably be a silicon layer, a silicon oxide layer, or a combination thereof. 
         [0018]    According to the method of the present invention, the iron-containing substrate is not specially limited, and can be made of one selected from the group consisting of: interstitials free (IF) steel, transformation induced plasticity (TRIP) steel, dual phase (DP) steel, Twinning Induced Plasticity (TWIP) steel, high strength (HS) steel, high strength low alloy (HSLA) steel, ceramics doped with iron, and glass doped with iron. The iron-containing substrate preferably is made of interstitials free (IF) steel due to its low price and being easily commercially reachable. 
         [0019]    According to the method of the present invention, the step (B) is preferably repeated once or more, for example, the titanium-containing sol-gel can be coated on the silicon-containing layer for twice, three times, or more in order to adjust the thickness of the titanium-containing sol-gel film to enable the forming of porous film (anatase titanium oxide) that further becomes an excellent catalyst. The titanium-containing sol-gel is preferably made by titanium-containing precursor such as TIP (titanium isopropoxide) or TTIP (titanium tetraisopropoxide). 
         [0020]    According to the method of the present invention, the silicon-containing layer in the step (A) is preferably formed by sputtering or coating, more preferably by sputtering, a silicon-containing sol-gel (e.g. TEOS (tetraethoxysilane)) on the iron-containing substrate following with drying process. 
         [0021]    According to the method of the present invention, the heating temperature of step (C) is preferably 350-450° C., more preferably 400° C. 
         [0022]    Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Example 1  
       [0023]    Preparation of Titanium-Containing Sol-Gel 
         [0024]    0.025 mol of titanium isopropoxide (TIP) is added to 30 ml of isopropyl alcohol (IPA), 0.01 mol of 2-ethylcaproic acid (EA) and 0.6 g of polyethylene glycol (PEG) is then added thereto. Isopropyl alcohol is added to dilute the above mixture volume to 50 ml followed with 2 hrs of ultrasonic mixing, thus the titanium-containing sol-gel is prepared. 
         [0025]    Preparation of Hydrophilic-Hydrophobic Transformable Composite Film 
         [0026]    A silicon layer is formed on a pre-cleaning substrate by using a sputtering process, in which the sputtering conditions are as following. 
         [0027]    Distance between the target and the substrate: 5 cm 
         [0028]    Processing time: 20 min 
         [0029]    Chamber bottom pressure: 3.5×10-5 torr 
         [0030]    Working pressure: 8.5×10-3 torr 
         [0031]    Argon flow rate: 30 sccm 
         [0032]    Operating power: 125W 
         [0033]    The silicon layer made by sputtering may have a preferred heat resistance, but the method of making the silicon layer is not limited thereto. Subsequently, the prepared titanium-containing sol-gel is coated twice at 1500 rpm on the above substrate having the silicon layer formed thereon. Further, the substrate is heated at 400° C. (the pre-heating rate is 5° C. /min) for 1 hour of calcination followed with furnace cooling to the room temperature. Therefore, the hydrophilic-hydrophobic transformable composite film of the present example is fabricated. 
         [0034]    The hydrophilic and hydrophobic properties of the present composite film are switched by UV light illumination. In detail, the composite film is hydrophobic before being exposed to UV light, but changes to hydrophilic after being exposed to UV light, and changes back to hydrophobic after releasing without further exposure to UV light in a period of time, for example, about 24 hours. 
         [0035]    The substrate used in the present example should be an iron-containing substrate, which can be made of stainless steel or interstitials free (IF) steel etc. Because interstitials free (IF) steel is a commercially reachable material with low price, an iron-containing substrate made of interstitials free (IF) steel is used in the present example. 
         [0036]    The coating times of the titanium-containing sol-gel film on the silicon layer, which is formed on the iron-containing substrate, are not limited. The titanium-containing sol-gel film can be coated on the silicon layer for once, twice, three times, or more in order to adjust the thickness of the titanium-containing sol-gel film to enable the forming of porous film (anatase titanium oxide) that further becomes an excellent catalyst. The porous film is advanced in many advantages comprising (1) good solidifying ability; (2) well-controlled particle size of TiO 2  particles wherein the minimum particle size is 100 nm or less, which may result in excellent quantum effect; and (3) high uniformity of TiO 2  particle distribution that means the aggregation phenomenon is reduced, which results in the increasing of the contact area with UV light. 
       Example 2  
       [0037]    The same method as described in Example 1 is used to fabricate the hydrophilic-hydrophobic transformable composite film of the present example, except that the coating of the titanium-containing sol-gel film is performed three times instead of twice and the silicon layer is replaced by a silicon oxide layer. 
         [0038]    According to the three-times-coated titanium-containing sol-gel film, the distribution depth of the Ti element is 1.3 μm and the concentration in the depth of 1.0 μm is about 10 at. % after being calcined. 
       Example 3  
       [0039]    Preparation of Titanium-Containing Sol-Gel 
         [0040]    0.025 mol of titanium isopropoxide (TIP) is added to 30 ml of isopropyl alcohol (IPA), 0.01 mol of 2-ethylcaproic acid (EA) and 0.6 g of polyethylene glycol (PEG) is then added thereto. Isopropyl alcohol is added to dilute the above mixture volume to 50 ml followed with 2 hrs of ultrasonic mixing, thus the titanium-containing sol-gel is obtained. 
         [0041]    Preparation of Silicon Oxide Sol-Gel 
         [0042]    5 ml of 0.1N acetic acid and 0.025 mol of tetraethoxysilane (TEOS) is added to 30 ml of isopropyl alcohol. After mixing, more isopropyl alcohol is added to dilute the above mixture volume to 50 ml to form the silicon sol-gel. 
         [0043]    Preparation of Hydrophilic-Hydrophobic Transformable Composite Film 
         [0044]    The above-prepared silicon sol-gel is coated on a pre-cleaned interstitials free (IF) steel substrate followed with drying. Then, a titanium-containing sol-gel film is formed by coating the titanium-containing sol-gel for three times on the dried silicon sol-gel layer following with heating at 400° C. (the pre-heating rate is 5° C./min) for 1 hour of calcination. Finally, the substrate is subjected to furnace cooling to the room temperature, thus the hydrophilic-hydrophobic transformable composite film of the present example is obtained. 
       Comparative Example 1  
       [0045]    A pre-cleaned interstitials free (IF) steel substrate is heated at 400° C. (the pre-heating rate is 5° C./min) for 1 hour followed with furnace cooling to the room temperature. Accordingly, the sample of the present comparative example is prepared. 
       Comparative Example 2  
       [0046]    Use the same method as described in Example 1 to prepare titanium-containing sol-gel, then coat the prepared titanium-containing sol-gel on a pre-cleaning interstitials free (IF) steel substrate. The coated IF steel substrate is then heated at 400° C. (the pre-heating rate is 5° C./min) for 1 hour of calcination followed with furnace cooling to the room temperature. Accordingly, the sample of the present comparative example is prepared. 
       Experiment Example 1  
       [0047]    Take the hydrophilic-hydrophobic transformable composite films of Examples 1 and 3, and the samples of Comparative Examples 1 and 2 for water-contacting angle test to compare the contact angles of those films and samples before and after exposing to UV light. The results are listed in Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Contact angle before UV 
                 Contact angle after UV 
               
               
                   
                 light exposing 
                 light exposing 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                  93.10° 
                    0° 
               
               
                 Example 3 
                  99.36° 
                  25.85° 
               
               
                 Comparative 
                 117.08° 
                 110.00° 
               
               
                 Example 1 
               
               
                 Comparative 
                 133.99° 
                 107.65° 
               
               
                 Example 2 
               
               
                   
               
             
          
         
       
     
         [0048]    According to the results of Examples 1 and 3 shown in Table 1, a great difference between contact angles of the films before and after exposing to UV light is shown, in which the contact angle of the film made from Example 1 after exposing to UV light is 0°, which means the hydrophilic-hydrophobic transformable composite film of Example 1 is completely changed to hydrophilic after exposing to UV light. Namely, from the testing results, it is shown that the composite film of the present invention has excellent hydrophilic-hydrophobic transforming ability. 
         [0049]    Comparing the testing results of composite films of Examples 1 and 3, it can be seen that the composite film in which the silicon-containing layer is formed by sputtering (Example 1) shows a better hydrophilic-hydrophobic transforming ability compared to the composite film in which the silicon-containing layer is formed by sol-gel process (Example 3). 
         [0050]    However, there is no apparent hydrophilic-hydrophobic transforming shown from testing results of Comparative Examples 1 and 2 before and after UV light illuminating, which means substrates having titanium-containing sol-gel films but no silicon-containing layer formed thereon can not obtain the expected hydrophilic-hydrophobic transforming ability. 
         [0051]    As mentioned above, the hydrophilic-hydrophobic transformable composite film of the present invention has excellent hydrophilic-hydrophobic transforming efficiency wherein the hydrophilic-hydrophobic transforming is switched by UV light exposing. The composite film of the present invention is hydrophobic before being exposed to UV light, but changes to hydrophilic after being exposed to UV light, and changes back to hydrophobic after releasing without further exposure to UV light in a period of time. Moreover, the contact angle of the composite film of the present invention after exposing to UV light is 0°, which means the composite film is completely changed to hydrophilic after exposing to UV light and such phenomenon cannot be reached by the prior arts. 
         [0052]    The hydrophilic-hydrophobic transformable composite film of the present invention can be extensively applied into the fabrication of various apparatus such as sanitary equipment, vehicle outer shells, exterior wall tiles, etc., and therefore can improve the self-cleaning property of these apparatus when being applied. Accordingly, the hydrophilic-hydrophobic transformable composite film and the method fabricating thereof are indeed more practical than the material of merely hydrophilic properties of the prior arts. 
         [0053]    Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed.