Article pubs.acs.org/IECR
Alterable Superhydrophobic−Superhydrophilic Wettability of Fabric Substrates Decorated with Ion−TiO2 Coating via Ultraviolet Radiation Yunjie Yin, Ning Guo, Chaoxia Wang,* and Qingqing Rao Key Laboratory of Eco-Textile, Ministry of Education, School of Textiles and Clothing, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China ABSTRACT: To prepare a functional fiber surface with alterable superhydrophobic−superhydrophilic wettability on fabrics, a TiO2 hybrid sol was synthesized with tetrabutyl titanate and a fluoride silane coupling agent, 1H,1H,2H,2Hperfluorooctyltrimethoxysilane, via sol−gel technology. Within irradiation under UV light, the decay of the contact angle from superhydrophobicity to superhydrophilicity on the irradiated front fabric treated with the F−TiO2 hybrid sol was the fastest, and the contact angle on the irradiated reverse cotton fabric hardly changed. During this process, the contact angles of cotton, polyester, and wool fabrics kept within a tiny change (4°) with different doping ratios, and the UV irradiation had no effect on the reverse contact angles. During the storage process in the dark, the loss ratio of the front contact angle was decreased to 20% from 40% as the storage temperature increased to 60 °C from 30 °C.
1. INTRODUCTION On the basis of extreme wettability, like superhydrophobicity,1,2 the understanding and fabrication of stimuli-responsive surfaces with controlled wetting properties have generated significant research interest due to their potential to attain unprecedented levels of control over biomolecule adsorption processes and interactions at engineered interfaces, such as micro-/nanofluidic devices and micro-/nanoarrays.3,4 Various research groups have also tried to develop responsive surfaces that can switch their wetting properties effectively in response to an environmental stimulus by controlling the coating composition on a molecular level and the morphology on the micro- and nanometer scales.5−7 These research efforts were mainly focused on altering their wettability in response to changing temperatures, electric potentials, mechanical deformations, pH values, and others.6,8,9 The sol−gel process has been widely used in the research area of superhydrophobicity due to its unique advantages, such as low temperature processing, easy functionalization of surfaces, and high homogeneity of final products.2,10−13 Photoresponsive materials ranging from inorganic nanomaterials to small organic molecules have been widely studied.3,14−16 For example, some semiconductor oxides, such as TiO2, ZnO, SnO2, and WO3, exhibited photoinduced reversible wettability conversions.17−20 The mechanism of the switchable wettability was involved in photogenerated electrons and holes as well as absorbed water.21 As a kind of eco-friendly inorganic material, TiO 2 represented an exclusive platform on which UV-light-stimulated wettability changes were triggered concertedly with the semiconductor photocatalytic activity.22 When TiO2 was irradiated in UV and/or visible light, the wetting properties before and after UV irradiation differed significantly.23,24 Bayati et al.25 prepared TiO2/YSZ/Si (001) single crystalline heterostructures by pulsed laser deposition, and the surface presented a rapid hydrophobic/hydrophilic switching property. © 2014 American Chemical Society
Such a rapid response set new routes for use of TiO2 in intelligent membranes, sensors, smart catalysts, and multifunctional devices. Watanabe et al.26 coated TiO2 thin films with various surface morphologies by a metalorganic chemical vapor deposition process. The TiO2 thin films became highly hydrophilic by ultraviolet irradiation and returned to their initial relatively hydrophobic state by visible-light irradiation. However, the coated samples have to be stored in the dark for a prolonged period of time to return to the hydrophobic or superhydrophobic state, and the recovery process is supposed to one direction in practical applications for its long process. In the previous study, to prepare a functional surface with UV-switchable wettability on cotton and polyester substrates by hybrid coating, the TiO2 hybrid sol was synthesized with tetrabutyl titanate via sol−gel technology. However, the switchable process was slow, and the switchable cycle of the cotton substrate was 84 h.27 In this paper, the main focus is on the improvement of switchable sensitivity between superhydrophobicity and superhydrophilicity by different doped ions, including Ba2+, Mg2+, Fe3+, F−, and N3+ in different fabric substrates, respectively. Besides, the dynamically modifiable wettability was investigated in detail.
2. EXPERIMENTAL SECTION 2.1. Materials. The poplin, 100% cotton substrate weighting 141.0 g/m2, and 100% polyester substrate weighting 120.1 g/m2 were produced by Jiangsu Hongdou Industrial Co., Ltd. (China). The pure wool gabardine substrate (220.6 g/m2) was offered by Wuxi Xiexin Worsted Spinning Weaving And Dyeing Co., Ltd. (China). The tetrabutyl titanate (TBT), ethanol, and HCl were supplied by Sinopharm Chemical Received: Revised: Accepted: Published: 14322
June 15, 2014 July 23, 2014 August 8, 2014 August 8, 2014 dx.doi.org/10.1021/ie502338y | Ind. Eng. Chem. Res. 2014, 53, 14322−14328
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Figure 1. Colloid performances of F−TiO2 hybrid sol with different doping ratio: (a) particle size; (b) PDI.
Table 1. Front Contact Angles of Treated Cotton Fabric under UV Light for Different Times metal ion treated time, h 0 2 4 6 7 8 9 10 11 13 15 18
undoped sample, deg 155.6 146.1 136.3 129.5 123.8 115.1 113.6 112.2 105.1 103.5 101.1 91.1
± ± ± ± ± ± ± ± ± ± ± ±
3.5 2.6 4.5 6.5 5.7 6.2 2.5 6.2 3.4 7.1 3.2 2.7
3+
Fe , deg 154.3 149.4 147.4 136.1 130.6 130.1 128.4 126.6 125.5 122.8 121.7 121.6
± ± ± ± ± ± ± ± ± ± ± ±
4.8 8.4 6.9 1.1 1.5 2.1 2.2 2.7 4.1 3.6 2.7 4.1
2+
Mg , deg 148.2 145.4 130.5 114.0 107.9 97.8 95.3 86.0 74.9 68.2 54.2 39.7
± ± ± ± ± ± ± ± ± ± ± ±
6.1 6.0 2.6 3.1 2.3 8.2 2.9 2.1 4.3 2.4 3.5 2.3
nonmetallic ion 2+
Ba , deg 151.1 146.8 146.8 127.0 121.0 115.8 116.4 115.8 107.4 104.7 97.9 65.0
± ± ± ± ± ± ± ± ± ± ± ±
1.6 3.3 4.5 2.4 3.1 2.7 3.2 1.1 2.8 4.1 3.0 2.3
3+
N , deg 145.5 143.2 133.4 120.3 118.9 102.9 100.0 92.8 80.3 73.9 64.0 48.9
± ± ± ± ± ± ± ± ± ± ± ±
2.0 3.3 2.8 3.1 2.4 4.1 3.9 4.3 3.7 1.8 7.1 2.0
F−, deg 157.9 136.9 116.0 92.7 70.7 58.7 49.7 38.2 16.2 11.6 0 0
± ± ± ± ± ± ± ± ± ±
1.8 2.9 5.3 2.6 3.0 2.2 1.5 2.5 3.8 1.3
an appropriate tension in a curing oven (Mini Thermo 350, Thermo Co., Ltd. USA). 2.4. Irradiation and Dark Treatments. To induce superhydrophilicity, the fabrics coated with ion−TiO2 hybrid sol were irradiated with the UV light (light intensity 21.0 mW/ cm2, distance 40.0 cm, predominantly wavelength 253.7 nm). The contact angle was measured with the Krüss DSA100 Drop Shape Analysis System during the irradiating process. When the contact angle was decreased to 0°, the fabrics were placed in a darkbox under ambient environment (with relative humidity of 30−40%), and the contact angle was measured. 2.5. Particle Size and ζ-Potential Measurement. The particle size and ζ-potential distributions of the ion−TiO2 hybrid sol were measured by a Nano-ZS90 Zetasizer Nano series supplied by Malvern Instruments Ltd. (UK). The dish containing about 2 mL of ion−TiO2 hybrid sol was placed in the sample room at 25 °C. The scanning data about particle size and ζ-potential distribution curve were analyzed by DTS software. 2.6. Contact Angle Measurement. The contact angle values were recorded after 3 s when the water drop began to still on the fabric substrates using the Krüss DSA100 Drop Shape Analysis System. The experiments were carried out under ambient conditions at temperature 25 °C and the relative humidity 40%. 2.7. Washing Properties. The washing fastness was tested according to the standard of ISO 105-C10:2006 (E) using 3 g/ L soap at 40 °C for 30 min with the 12-A washing fastness tester supplied by Wenzhou Darong Textile Instrument Co., Ltd. (China).
Reagent Co., Ltd. (China). Magnesium chloride (MgCl2), urea, ammonium fluoride (NH4F), ferric trichloride (FeCl3), and barium chloride (BaCl2) were also supplied by Sinopharm Chemical Reagent Co., Ltd. (China). All the chemicals are analytical reagent grade. The 1H,1H,2H,2H-perfluorooctyltrimethoxysilane was supplied by Harbin Xeogia Fluorine-Silicon Chemical Co. (China). 2.2. Preparation of Ion−TiO2 Hybrid Sol. The ion−TiO2 hybrid sol was prepared by adding diluted TBT 28.0 g (TBT 8.0 g, ethanol 20.0 g) into a mixture (70.0 g) containing HCl (1.0 mol/L) and H2O (HCl 15.0 g, H2O 55.0 g) in a flask with a magnetic stirring apparatus (500 rpm) at room temperature. After that, the appropriate amount of ammonia−water (NH3· H2O, 1.0 mol/L) was added and the pH value was kept in the range of 5.0−5.5. Then, the 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (4.0 g) was added into the flask. 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 5.0%, and 10.0% ion doping agents including BaCl2 (Ba2+), MgCl2 (Mg2+), FeCl3 (Fe3+), NH4F (F−), and urea (N3+) were blended into the mixture, respectively. Finally, the mixture was stirred at 30 °C for 4 h with a magnetic stirring apparatus and the solution was aged for 120 h. 2.3. Fabric Modification. The cotton, polyester, and wool fabrics were precleaned in deionized water and then dried at 50 °C for 2 h before other treatments. The fabrics were dipped into the ion−TiO2 hybrid sol for 5 min and padded two times at room temperature with a P-130 padder produced by Taiwan Rapid Co., Ltd. (China). The wet pick-up was 70−90%, which is the ratio of the weighting increment of solution after padding and the original weight of the fabric. Then the fabrics were dried at 60 °C for 20 min and baked at 140 °C for 2 min with 14323
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Figure 2. Photocatalysis mechanism and process of F−TiO2 coating.
Table 2. Front Contact Angles of Treated Cotton Fabric in Dark for Different Times metal ion storage time, h 0 7 12 13 24 28 36 44 50
undoped sample, deg 0 65.2 73.7 86.1 96.4 109.1 113.2 116.0 116.4
± ± ± ± ± ± ± ±
3.2 3.9 5.0 4.8 6.2 2.1 2.5 3.5
Fe3+, deg 0 96.3 102.1 109.2 113.7 116.1 123.7 124.0 124.2
± ± ± ± ± ± ± ±
4.4 3.0 6.2 1.2 1.9 2.6 4.5 5.7
Mg2+, deg 0 36.8 42.2 51.8 76.6 96.0 112.9 115.0 115.4
3. RESULTS AND DISCUSSION 3.1. Colloid Performances of F−TiO2 Hybrid Sol. A series of F−TiO2 hybrid sols containing 0, 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 5.0%, and 10.0% F−, respectively, were prepared and diluted 20 times by absolute ethyl alcohol. The particle sizes were measured according to the diluted F−TiO2 hybrid sols. According to Figure 1a, the particle sizes of the F−TiO2 hybrid sols containing 0−5% F− were kept at a stable value (about 125 nm). With increasing the F− content (more than 5%), the particle sizes of the F−TiO2 hybrid sols increased remarkably. For the particle size of the tiny fluorine atom, F−, nearly had no effect on the particle size of TiO2. Grain boundary migration is the basic condition of grain growth. The higher interfacial energy between crystalline grains is beneficial to promote the more difference of crystalline grains.28,29 The polydispersity index (PDI) is calculated from two parameters fitted to the correlation data called cumulants analysis. It is a significant parameter to evaluate solutions. In Figure 1b, the PDI curve of the F−TiO2 hybrid sol presented a similar change to the particle size curve. Actually, a small PDI value of a colloid solution indicates a stable colloid status. When the doping ratio of the F−TiO2 hybrid sol was in the range of 0−5%, the PDI was lower than 0.25 and the stability, dispersion property, and the transparency of the F−TiO2 hybrid sol were favorable (Figure 1b). Whereas, the PDI increased evidently (from 0.23 to 0.48 at the doping ratio 10%) at a higher doping ratio (higher than 5%), and the transparency of the F−TiO2 hybrid sol turned inferior. 3.2. Contract Angles of Ion−TiO2 Hybrid Sol. In Table 1, the radiated fabric coated with TiO2 or ion−TiO2 hybrid sol showed a decreased front contact angle under the UV light. Compared with the undoped sample, the fabrics were improved by the doped Mg2+, Ba2+, N3+, or F−. The cotton fabric treated
± ± ± ± ± ± ± ±
2.1 2.0 3.1 4.8 5.4 5.9 7.3 5.1
nonmetallic ion Ba2+, deg 0 70.7 78.5 86.9 90.0 108.3 118.9 120.1 120.2
± ± ± ± ± ± ± ±
3.7 4.8 3.0 5.3 4.9 4.3 4.4 5.1
N3+, deg 0 40.2 69.0 74.4 97.4 112.4 113.8 115.9 115.0
± ± ± ± ± ± ± ±
2.7 5.6 4.9 6.0 1.4 2.9 4.2 4.9
F−, deg 0 31.1 40.6 58.2 82.0 94.5 122.1 122.3 122.3
± ± ± ± ± ± ± ±
0.4 2.2 3.6 3.9 4.5 6.7 0.9 4.6
with the F−TiO2 hybrid sol presented the most notable change, and the front contact angle reduced to 0° when the sample was radiated for 15 h. This result indicated that the F− was the best for the conversion from superhydrophobicity to superhydrophilcity. The commonly accepted mechanism of superhydrophilicity in TiO2 is based on the generation of e− pairs at the conduction and valence bands, which are excited by photons with energies higher than the band gap energy.25,30 This phenomenon initiates various redox reactions at the semiconductor surface. The photogenerated electrons and holes were trapped by surface Ti4+ and O2− to produce Ti3+ and oxygen vacancies, respectively (Figure 2).31 Also, the creation of photoinduced hydrophilic surfaces involved formation of partially reduced TiO2. This model stipulated that dissociative adsorption of water molecules occurs at the defect sites.27 The reduction of Ti4+ to the Ti3+ state and formation of oxygen vacancies by photon illumination induced the generation of a superhydrophilic surface.25,32,33 Especially, there are two F formats in the F−TiO2 hybrid coating: hybridization and chemisorption (surface fluorination) .34 In the hybridization format, the F replaced the O in the TiO2 crystal lattice, whereas the surface fluorination can improve the hydroxyl radical (·OH).35,36 The covalent radius of fluorine was 64 pm and was the smallest among these five atoms, which indicated that the fluorine atom (F) can easily enter into the TiO2 crystal lattice. Besides, F presents a great electron withdrawing ability due to its high electronegativity, and the fluoride ion presents positive effects at stabilizing highoxidation states. The OH· is easily dragged by the fluoride ion, and becomes oxygen vacancies. The OH· is changed to hydrophilic OH−, and the cotton fabric recovers the hydrophilic property. 14324
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In Table 2, the coated fabrics containing Fe3+ or F− showed a better recoverability, and the front contact angles increased to the largest after the storage in the dark for 36 h. The observed photocatalytic behavior was attributed to the formation of oxygen vacancies and titanium interstitial defects.25 As the strong oxidability, Fe3+ presents intense electron withdrawing ability. However, Fe3+ can not enter into the TiO2 crystal lattice. In microcosm, the hydroxyl is bonded and dissociated into hydroxyl radicals. The fabric presents a hydrophobic ability from a hydrophilic ability. On the basis of the comprehensive discussion of Tables 1 and 2, the cotton fabric treated with a hybrid TiO2 sol containing F− presented the smallest switching cycle (51 h). This switching cycle was smaller than that of the cotton fabric treated with a TiO2 hybrid sol (84 h), which was verified in our previous work.27 The future discussion about the switchable wettability is focused on the fabrics treated with hybrid TiO2 sol containing F−. Considering the colloid stability and hydrophobic property, a 2% doping ratio was adopted to prepare the F−TiO2 hybrid sol. After UV radiation, the front contact angles of cotton, polyester, and wool fabrics treated with F−TiO2 hybrid sol were decreased gradually (Figure 3). TiO2 in the F−TiO2
cotton fabric is hydrophilic, however, the polyester and wool fabrics are hydrophobic. Thus, the treated cotton fabric presented a fast sensitivity. Compared with polyester, the wool fabric presented characteristic scaling and crimp construction. Whereas, the surface of polyester is smooth; therefore, the damping time of the rough wool fabric was the longest. According to the curves of the front contact angles in UV light, the fitting equations were linear. The R2 are more than 0.9, which indicated the reliable fitting equations (Table 3). The contact angles of cotton, polyester, and wool fabrics treated with the F−TiO2 hybrid sol, which contain different contents of F−, were hardly changed, respectively (smaller than 4°) with the different doping ratios (2% and 10%) (Figure 4),
Figure 4. Effects of doping ratio and fabric on the contact angles (a, cotton; b, polyester; c, wool).
and all fabric were superhydrophobic. After irradiation under UV light for 20 h, the reverse contact angles of the coated cotton, polyester, and wool fabrics were nearly the same as those without irradiation, respectively. For the same treated process and fabric organizational structures, the front and reverse contact angles were nearly the same without UV irradiation. UV light has a weak permeability and only irradiates on the surface of a fabric. The TiO2 crystal lattice of the fabric reverse is not changed; thus, the reverse contact angles were consistent. 3.3. Dark Storage Process. The cotton, polyester, and wool fabrics were treated with F−TiO2 hybrid sol containing different concentrations of F−. The fabrics were irradiated under UV light and the front contact angle decreased to 0° (Figure 3). In the dark environment, the front contact angle recovered gradually. The front contact angle stored in the dark for 5 days was defined to the maximum recovery front contact
Figure 3. Front contact angles under UV light and related fitting curves.
hybrid coating was a photosensitive material. When the fabric was irradiated by UV light, the photogenerated hole in TiO2 reacted with lattice oxygen formed surface oxygen vacancies, which water molecules kinetically coordinated (Figure 2). This change improved the surface hydrophilicity of fabrics greatly and resulted in a front contact angle of 0°. In Figure 3, among cotton, polyester, and wool fabrics, the damping time of the front contact angle for cotton was the fastest (15 h). The damping time of the front contact angle for polyester was 32 h, and the wool had the longest damping time of the front contact angle (36 h). The damping time was related to the fiber component and the surface morphology. Original
Table 3. Fitting Equations of Front Contact Angle Decaying Rates on Different Fabrics intercept
gradient
statistics
fabric
value
standard error
value
standard error
simulation equation
adj. R2
cotton polyester wool
158.6 148.0 164.5
5.864 3.383 5.313
−10.976 −4.625 −4.211
0.6433 0.2190 0.3380
y = 158.6 − 10.976x y = 148.0 − 4.625x y = 164.5 − 4.211x
0.9788 0.9752 0.9441
14325
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ratios of different fabrics before and after washing were higher than 0.9. However, the treated polyester fabric showed a relatively low front contact angle ratio. The mean front contact angle ratio was 0.72 at the doping ratio between 0 and 2%. At a higher level of doped ratio (5−10%), the mean front contact angle ratio was 0.82. There are abundant −OH and −COOH groups on the surface of cotton and wool, and they can bond with the −OH of the F−TiO2 hybrid sol particle by hydrogen and ester bonds. The main component of polyester is polyethylene glycol terephthalate (PET), and there is no distinct adsorbability between polyester and sol particles. The surface of the cotton fiber is rough, and there are abundant groove constructions and natural distortions (Figure 7). The surface of wool consists of
angle, and it would not continue to increase any more as the storage time prolonged. For the −OH group adsorption, the surface of TiO2 transformed into an energetically metastable state. The adsorbed −OH groups could be replaced by atmospheric oxygen gradually when the fabric was placed in the dark. Then, the surface reverted back to its original state, and the surface wettability changed to hydrophobic again. In Figure 5, the recovery front contact angles were slightly increased with the increase of doping ratio. During the storage
Figure 5. Maximum restoring front contact angles with different doping ratios and fabrics.
Figure 7. Morphologies of cotton fibers before (a) and after (b) coating with F−TiO2 hybrid sol.
process in the dark, the loss ratio of the front contact angle was 20% at 60 °C in a darkbox, and it was significantly higher than that of at 30 °C, which was 40%. The recovery front contact angle was improved at a high temperature. It indicated that the high temperature was beneficial to recovery of the front contact angle. At a higher temperature, the reaction activity was improved and the replaced rate of adsorbed −OH groups by atmospheric oxygen was increased followed by the high recovery level. 3.4. Washing Process. Before and after fabric washing, the front contact angle ratio of the fabrics is used to characterize the washing resistance of the hydrophobicity. The front contact angle ratios of cotton, polyester, and wool fabrics treated with F−TiO2 hybrid sols containing different concentrations of F− are listed in Figure 6. In Figure 6, the cotton and wool fabrics treated with F−TiO2 hybrid sols containing different concentrations of F− kept a stable washing resistance. Nearly all the front contact angle
characteristic scaling and crimp construction. Both of these fibers are easy to combine with the F−TiO2 hybrid sol particle. However, the smooth surface of polyester is profitless to incorporate with the sol particle.
4. CONCLUSIONS A novel sol−gel route to prepare a functional surface with dynamically alterable wettability between superhydrophobicity and superhydrophilicity on cotton, polyester, and wool fabrics was developed. By investigating the ion kind and doping ratio, the F− with a 2% doping ratio in hybrid TiO2 sol was adopted. The damping time of the front contact angles for cotton, polyester, and wool fabrics were 15, 32, and 36 h, respectively, which was determined by the fiber component and surface morphology. At higher temperatures, the reaction activity and the replaced rate of adsorbed −OH groups by atmospheric oxygen were improved, and the recovery level was high. The reverse contact angles of cotton, polyester, and wool fabric treated with F−TiO 2 hybrid sols containing different concentrations of F− changed slightly. The cotton and wool fabrics treated with F−TiO2 hybrid sol kept a stable washing resistance ability. Besides, the washing properties of cotton and wool fabrics were better than that of polyester fabric. Therefore, the method with UV irritation and dark storage was effective, which was expected to perform important applications on superhydrophobic-superhydrophilic functional clothing materials.
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AUTHOR INFORMATION
Corresponding Author
*C. Wang. Email:
[email protected]. Tel: 86-051085912105. Fax: 86-0510-85912105. Notes
Figure 6. Front contact angle ratio of different fabrics before and after washing.
The authors declare no competing financial interest. 14326
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ACKNOWLEDGMENTS The authors are grateful for the financial support of the National Natural Science Foundation of China (21174055 and 51403083), the Six Kinds of Outstanding Talent Foundation of Jiangsu Province (2012-XCL-007), the 333 Talent Project Foundation of Jiangsu Province (BRA2011184), IndustryAcademia-Research Joint Innovation Fund of Jiangsu Province (BY2014023-10) and the Fundamental Research Funds for the Central Universities (JUSRP11446).
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