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Article Cite This: ACS Omega 2017, 2, 8198-8204
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Ultraviolet-Durable Superhydrophobic Nanocomposite Thin Films Based on Cobalt Stearate-Coated TiO2 Nanoparticles Combined with Polymethylhydrosiloxane Jiawei Xiong,†,‡,§ Dilip Kumar Sarkar,*,†,‡ and X. Grant Chen†,‡ †
University Research Center on Aluminum (CURAL) and ‡Aluminum Research Centre-REGAL, University of Québec at Chicoutimi, Chicoutimi, Quebec G7H 2B1, Canada S Supporting Information *
ABSTRACT: Ultraviolet (UV)-durable superhydrophobic nanocomposite thin films have been successfully fabricated on aluminum substrates by embedding cobalt stearate (CoSA)-coated TiO2 nanoparticles in a hydrophobic polymethylhydrosiloxane (PMHS) matrix (PMHS/TiO2@CoSA) using the sol−gel process. When compared to the sharp decrease of water contact angle (WCA) on the superhydrophobic PMHS/TiO2 thin films, the PMHS/TiO2@CoSA superhydrophobic thin films exhibited a nearly constant WCA of 160° under continuous UV irradiation for more than 1 month. The designed scheme of the TiO2@CoSA core−shell structure not only increased the hydrophobic properties of the TiO2 nanoparticle surface but also confined the photocatalytic efficiency of TiO2 nanoparticles. A plausible model has been suggested to explain the UV-durable mechanism of the superhydrophobic nanocomposite thin films based on PMHS/ TiO2@CoSA. Furthermore, the elongated lifetime in the exposure of the solar light imparts this superhydrophobic nanocomposite thin film with potential practical applications where UV-resistant properties are emphasized including corrosion-resistant building walls, anti-icing airplanes, selfcleaning vehicles, and so forth.
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as metals, cottons, and fabrics.6,16−24 Most works used TiO2 nanoparticles as a UV absorber and organic compounds to lower the surface energy to fabricate UV-protective superhydrophobic surfaces.6,16−21 Recently, Gao et al. used fluoroalkylsilane (FAS)-modified TiO2 nanoparticles to fabricate superhydrophobic thin films that converted to hydrophilic [contact angle (CA) = 2°] following an exposure to UV radiation for 24 h.16 This conversion from superhydrophobicity to hydrophilicity was an expected phenomenon as the fabricated thin film contained only a monolayer of FAS on TiO2. However, contradictory reports over FAS-coated superhydrophobic surfaces state that it is possible to maintain the superhydrophobic properties upon prolonged exposure to UV radiation.24 Wang et al. have demonstrated stable superhydrophobic properties on steel surfaces even after 50 h of exposure to UV radiation.24 The UV durability was ascribed to the long chain of FAS-17 on the surface, which could provide a large number of C−F bonds (i.e., −(CF2)7−CF3 chain). This C−F bond, with a bond energy of 485 kJ/mol, cannot be broken by the UV light (314−419 kJ/mol).24 In another work by Gao et al., epoxy resin-covered TiO2 nanoparticles were used to fabricate superhydrophobic filter papers. These authors showed that the WCA on the surface of
INTRODUCTION Superhydrophobicity, as inspired by the “Lotus effect” in nature, features a water contact angle (WCA) above 150°. The water repellency of a surface is governed by a combination of chemical composition and geometrical surface structure. Superhydrophobic thin films have attracted great attention in a wide range of applications including antifouling paints,1 waterproof clothes,2 corrosion inhibition,3 water and oil separation,4 and so forth. Recently, superhydrophobic thin films incorporating wide band gap semiconductor oxide nanoparticles such as TiO2 (band gap: 3.2 eV), ZnO (3.4 eV), and CeO2 (3.1 eV) have attracted increased interest because of their ultraviolet (UV) absorption and self-cleaning properties.5−12 These superhydrophobic thin films were converted to hydrophilic films because of the photocatalytic properties of these nanoparticles. In other words, these thin films changed their wetting properties from the Cassie state13 to the Wenzel state14 via the exposure to UV radiation.5−12 Nishimoto et al.8 reported the hydrophilic transformation of a superhydrophobic surface based on TiO2 porous layers passivated with a self-assembled monolayer of octadodecylphosphonic acid because of UV radiation in just 30 min. Similarly, polydimethylsiloxane (PDMS)-coated TiO2 nanoparticles turned hydrophilic in 6 h because of the UV exposure as reported by Zhang et al.15 In the recent years, several works have been performed on UV-protective superhydrophobic thin films on substrates such © 2017 American Chemical Society
Received: October 17, 2017 Accepted: October 23, 2017 Published: November 20, 2017 8198
DOI: 10.1021/acsomega.7b01579 ACS Omega 2017, 2, 8198−8204
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Figure 1. (a) Low-angle XRD patterns of (I) SA powder and (II) CoSA thin films and (b) ATR−FTIR spectra of (I) SA powder and (II) CoSA thin films. The inset in (a) shows the corresponding UV−vis absorption spectrum of CoSA coated on quartz substrates.
the filter paper slightly decreased to 151° ± 0.5° from 153° ± 1° after more than 300 min of UV exposure.17 However, there was no information on the stability of the superhydrophobic properties after a long-term exposure as presented in our work. In another work by Wang et al., the superhydrophobic surfaces prepared by TiO2/polybenzoxazine nanoassemblies became hydrophilic upon exposure to UV radiation of only 300 s.19 Qing et al. used TiO2 nanoparticles covered by PDMS layers to make superhydrophobic thin films which demonstrated corrosion resistance. These films, however, failed to maintain their superhydrophobic properties after only 20 min of exposure to UV radiation, and their CA reduced drastically from 162.3° to 75.6° converting to hydrophilic surfaces. Surprisingly, a postheat treatment of these surfaces at 120 °C for 30 min restored the superhydrophobicity.18 In another work, the reversibly switchable wettability of the FAS−TiO2/ poly(vinylidene difluoride) composite surface was investigated through the serial alternating of UV exposure and heat treatment.6 They observed that the CA decreased sharply as the UV irradiation time increased, converting a superhydrophobic surface with a CA of 160° to superhydrophilic with a CA of 0° just after 60 min. Interestingly, it was observed that it took only 40 min to revert to the original CA when these surfaces were heated at a temperature of 180 °C.6 Several smart ideas have been utilized to obtain UV-durable superhydrophobic surfaces.5,7,10,25−27 Ding et al.26 reported a UV-durable superhydrophobic fluorinated polysiloxane/TiO2 nanocomposite coating because of the fact that Si−O and C−F bonds in the fluorinated polymer matrix were stable against photocatalytic reactions of TiO2 nanoparticles. It is noteworthy to mention that fluorinated silanes are quite expensive, environmentally unfriendly, and not appropriate for practical applications. Similarly, in a study of Gao et al.,5 a transparent and UV-durable superhydrophobic surface was fabricated by passivating the arrays of the SiO2-coated ZnO nanorods with perfluorodecyltriethoxysilane. The physical barrier of the SiO2 layers was responsible for the UV resistance property. However, both of their fabrication processes are complex, and a further modification of the hydrophilic SiO2 shell with low-surfaceenergy materials is always inevitable. In the present study, a facile, low-cost, and effective sol−gel spin-coating method has been developed to fabricate UVdurable superhydrophobic thin films. With this method, initially, TiO2 nanoparticles are coated with cobalt stearate (CoSA) and this core−shell (TiO2@CoSA) material is further incorporated in the ethanolic polymethylhydrosiloxane (PMHS) solution. The sol−gel spin-coated thin films prepared
with this mixture are found to be superhydrophobic with high UV durability properties. A model has been proposed to explain the UV durability mechanism of this material.
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RESULTS AND DISCUSSION Figure 1a-I,II shows the X-ray diffraction (XRD) patterns of pure stearic acid (SA) powder and CoSA films, respectively, on aluminum substrates in the 2θ scan range of 3°−30°. Figure 1aI shows the characteristic peaks at the 2θ values of 6.76°, 20.35°, 21.61°, and 24.29°, assigned to the SA (#JCPDS stearic acid: 00-009-0618). The XRD pattern of CoSA (Figure 1a-II) shows a series of equidistant diffraction peaks situated at 3.61°, 5.42°, 7.22°, 9.02°, 10.81°, 12.62°, 14.44°, and 16.24°. The origin of these equidistant peaks in the diffraction pattern is attributable to the layered structure of CoSA. The average distance between the two peaks has been calculated to be 1.80° ± 0.01°. Hence, an initial peak would be expected to be situated at 2θ equal to 1.81°. Luo et al.28 have reported the appearance of a peak in a position similar to that in the Langmuir−Blodgett thin film of CoSA. We have not observed this first peak in our study as we have performed the experiment at the initial angle of 3°. Lowering the diffraction angle may cause permanent damage to the detector because of the possibility of direct bombardment of the high intensity of X-ray to the detector. It is noteworthy to mention that we have observed 12 distinct characteristic diffraction peaks of CoSA as compared to only the 4 peaks reported by Luo et al.28 The inset of Figure 1a shows the ultraviolet−visible (UV− vis) absorption spectrum of the drop-coated CoSA films on quartz substrates in the light wavelength range of 200−400 nm. A strong absorption peak is observed at around 210 nm. This result is comparable to the UV−vis spectrum of copper stearate.29 It is noteworthy to mention that the UV−vis spectrum of CoSA is not available in the literature. Figure 1b-I,II reveals the attenuated total reflectance Fourier transform infrared (ATR−FTIR) spectra of pure SA powder and the drop-coated CoSA film, respectively. In the highfrequency region of the two spectra, the adsorption peaks at 2847 and 2914 cm−1 are attributed to the asymmetric and symmetric stretching vibrations of −CH2 groups, respectively. One tiny peak assigned to the asymmetric stretching of −CH3 is observed at 2939 cm−1.3,30 In the mid-frequency region, a distinct sharp peak of the carboxyl (−COOH) group of SA is observed at 1701 cm−1, as shown in Figure 1b-I. It is noteworthy to mention that this peak, associated with the carboxyl group, disappears in the CoSA thin films (Figure 1b8199
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Figure 2. (a) Variation of the WCA on the PMHS/TiO2 (red circle) and PMHS/TiO2@CoSA (black square) superhydrophobic thin films as a function of UV irradiation time. The inset in (a) shows the water drop on the corresponding surfaces; (b) ATR−FTIR spectra of (I) PMHS liquid and (II) PMHS/TiO2@CoSA superhydrophobic thin films. The inset in (b) shows the ATR−FTIR spectra specifically in the range of 3050−2750 cm−1; (c) XRD patterns of (I) aluminum substrates, (II) PMHS/TiO2, and (III) PMHS/TiO2@CoSA; and (d) EDS spectra of (d-I) PMHS/TiO2@ CoSA and (d-II) PMHS/TiO2.
Figure 3. (a) SEM image and (b) schematic model of the PMHS/TiO2@CoSA superhydrophobic thin films on the aluminum substrate. The inset in (a) shows water drops on these thin films.
brief, XRD, ATR−FTIR, and UV−vis results confirm that CoSA has been successfully synthesized. Anatase phase of the TiO2 nanoparticle is well-known for the use of photodegradation11,33,34 of organic compounds because of the wide photonic band gap of 3.2 eV. Therefore, we have used anatase TiO2 nanoparticles of size 100 nm to study photodegradation as well as to counter the photodegradation using our synthesized CoSA in the thin films. It is extremely significant to develop UV-durable superhydrophobic thin films
II). Interestingly, and on the contrary, Figure 1b-II shows two distinct peaks observed at 1410 and 1550 cm−1 corresponding to the carboxylate (−COOCo) symmetric and asymmetric stretching vibrations, respectively.31 Another peak in between these two peaks appearing at 1446 cm−1 is ascribed to −CH2 bending.31,32 In the low-frequency range, a sharp absorption peak at 720 cm−1 is attributed to the in-plane rocking vibrations of −(CH2)n long carbon chains in the CoSA molecules.31 In 8200
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hardly 2% of that of Si−O−Si or Si−CH3 bonds. It is a known fact that the signals of FTIR peaks are proportional to the number of corresponding chemical bonds. In the mid-frequency region, the peak at 2162 cm−1 arising from the Si−H groups of PMHS shown in Figure 2b-I is no longer observed in the spectrum of the superhydrophobic PMHS/TiO2@CoSA surfaces in Figure 2b-II. This indicates that the cross-linked polymer matrix has been formed by PMHS organosilanes through condensation reactions of Si−H groups during the sol−gel process.42 The two peaks at 1270 and 764 cm−1 correspond to Si−CH3 groups. In addition, the double peaks present at 1112 and 1026 cm−1 are ascribed to the Si−O−Si groups of PMHS organosilanes after the formation of oligomers.42 Furthermore, the absorption peak of Ti−O groups is displayed at quite a low frequency, in the region near 500 cm−1, indicating the existence of TiO2 nanoparticles in our UVdurable superhydrophobic thin films. Figure 2c depicts the XRD spectra of the as-received aluminum substrates (Figure 2c-I) and PMHS/TiO2 (Figure 2c-II) and PMHS/TiO2@CoSA superhydrophobic thin films (Figure 2c-III) in the scan range of 22°−58°. The two distinct peaks observed at 38.4° and 44.7° on the XRD pattern of the as-received aluminum substrates are attributed to the characteristic peaks of Al(111) and Al(200). In addition, the XRD patterns of PMHS/TiO2 and PMHS/TiO2@CoSA superhydrophobic thin films coated with aluminum substrates show the characteristic peaks of aluminum as well as the characteristic peaks of TiO2(101), TiO2(200), TiO2(105), and TiO2(211) at 25.3°, 48.1°, 53.9°, and 55.1°, respectively, confirming the incorporation of TiO2 nanoparticles in the thin films. Figure 2d shows the energy-dispersive spectrometry (EDS) spectra of different elements in the superhydrophobic thin films PMHS/TiO2@CoSA (Figure 2d-I) and PMHS/TiO2 (Figure 2d-II). The spectra show the common elements in each of the two thin films, which include C, Ti, O, and Si. The element Co, in particular, can only be detected in the EDS spectra of PMHS/TiO2@CoSA, confirming the existence of CoSA in this superhydrophobic nanocomposite film. The atomic composition of the coating is found to be C/O/Si/Ti/Co = 25.7:54.1:7.9:12.1:0.1, as revealed from the EDS data. Figure 3a shows the morphology of the superhydrophobic PMHS/TiO2@CoSA thin film on the aluminum substrate, as revealed by the scanning electron microscopy (SEM) measurements. PMHS molecules, CoSA sheets (as found from XRD), and TiO2 nanoparticles (100 nm) have agglomerated together producing microsized random particles during the sol−gel and spin-coating processes. The roughness of the thin film is found to be ∼3.3 μm with a WCA as high as 160°. The hydrophilic TiO2 nanoparticles covered with a low-surface-energy CoSA sheet as well as PMHS molecules form binary structures, providing superhydrophobic as well as UV-protective thin films. Figure 3b shows a schematic model of the thin film composed of a core−shell structure of TiO2@CoSA covered with a PMHS network (PMHS/TiO2@CoSA). The CoSA shell on the TiO2 nanoparticles stops the photocatalytic reaction of TiO2 with PMHS by suppressing the movements of the electron−hole pairs generated from TiO2 because of UV absorption. In the literature, SiO2 and Al2O3 shells have been used around TiO2 to stop such reactions to fabricate UVprotective superhydrophobic thin films.7,27,43 However, it should be pointed out that the adoption of the hydrophilic SiO2 shell around the hydrophilic TiO2 nanoparticles would
especially for painting the exterior surfaces such as building walls which are normally exposed to the UV light directly from the sun.35 A comparative UV degradation study has been performed on two superhydrophobic thin films prepared by a sol−gel process using (i) TiO2 nanoparticles and PMHS (coded as PMHS/TiO 2 ) and (ii) CoSA-modified TiO 2 nanoparticles and PMHS (coded as PMHS/TiO2@CoSA) in our UV chamber developed in-house. The red plot of Figure 2a shows the variation of the WCA on the PMHS/TiO2 thin films with respect to the UV irradiation time. The initial superhydrophobic PMHS/TiO2 thin films exhibit a WCA of ∼152° (shown in the inset) and a contact angle hysteresis (CAH) of ∼6°. After 1 h of exposure to UV radiation, the WCA of this film decreased to ∼119°, losing the rolling-off properties. With the UV irradiation time increasing to 2 h, the WCA of the PMHS/TiO2 thin films was found to be only ∼44°. It is important to note that the static and dynamic CAs may be influenced by the size, shape, and movement of the water drop.36,37 The superhydrophobic PMHS/TiO2 thin films were completely converted to superhydrophilic films after UV irradiation of 4 h because of the photocatalytic effect of anatase TiO2 nanoparticles.33 The low-surface-energy PMHS was decomposed by highly reactive superoxide and hydroxyl radicals generated by TiO2 nanoparticles under UV irradiation.33,38 Similar photodegradation and polymerization processes induced by ZnO nanoparticles have been studied recently by Schmitt.39,40 He further reported the formation of silanes and siloxanes by Raman spectroscopy.41 A model has been presented in Figure 3b to explain the degradation mechanism. By contrast, Figure 2a (black square) shows that the WCA remains nearly constant after more than 300 h of UV irradiation for the PMHS/TiO2@CoSA superhydrophobic thin films. Although the data have been presented for 300 h, we have observed that these films retain their superhydrophobic properties even after more than 1000 h of UV irradiation. The initial WCA of 160° (CAH = 2°) reduced only slightly to 156° (CAH = 5°) after 300 h of UV irradiation. The intensity of the UV light in our in-house UV chamber is much stronger than that in the direct sunlight, indicating that the PMHS/TiO2@CoSA superhydrophobic thin films have great potential in outdoor applications. This highly UV-durable superhydrophobic thin film with water roll-off properties can be regarded as a potential candidate for creating an excellent self-cleaning material as well. It is to be noted that the photo-Kolbe reaction leads to radicals which might result in a protective layer around the embedded TiO2 particles similar to that reported by Schmitt on ZnO nanoparticles.39 Figure 2b shows the ATR−FTIR spectra of the chemical groups of the pure PMHS organosilane liquid (Figure 2b-I) and the superhydrophobic PMHS/TiO2@CoSA thin films (Figure 2b-II). In the high-frequency region, the spectrum of PMHS displays a single peak at 2969 cm−1, which is assigned to the asymmetric stretching mode of −CH3 groups in the PMHS molecule. The spectrum of the superhydrophobic PMHS/ TiO2@CoSA thin film (the inset top graph of Figure 2b) shows two peaks at 2914 and 2853 cm−1 attributed to the symmetric and asymmetric stretching modes of −CH2 groups, respectively, arising from CoSA, beside the absorption peak of −CH3. In the mid-frequency region, a tiny peak is observed at 1450 cm−1 because of the −COOCo bonds. It can be seen that the intensity of this peak is very low because the number of −COOCo bonds in the composite films is calculated to be 8201
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thin films has been explained using a reasonable schematic model. These UV-durable superhydrophobic thin films formed by incorporating TiO2@CoSA nanoparticles have potential to be utilized in a wide range of practical applications.
increase the risk of producing hydrophilic surfaces rather than the superhydrophobic ones. In our case, the naturally hydrophobic CoSA shell covering the cores of the TiO2 nanoparticles not only improved the hydrophobic properties of the nanocomposite surfaces but also inhibited the photoinduced degradation by TiO2 nanoparticles, providing a dual protection against UV degradation as well as water adhesion. As a result, the hydrophobic PMHS matrix will not be decomposed by the photoinduced degradation reactions of TiO2. Therefore, the superhydrophobic nanocomposite thin films fabricated with a TiO2@CoSA core−shell structure embedded in the hydrophobic PMHS matrix sustain UV irradiation while exhibiting and maintaining their superhydrophobic properties. In the literature, few works have been reported on the simultaneous UV degradation study of the nanoparticles dispersed in the liquid of methylene blue (MB) or rhodamine B (RhB) as well as UV degradation in the thin films.20,21 For instance, in a work by Xu et al.,20 an organically modified silica (ormosil) aerogel with a high surface area and high porosity was loaded with TiO2 nanocrystals to synthesize TiO2−SiO2 nanocomposites. Both ormosil and TiO2−SiO2 nanocomposites show superhydrophobic properties when coated on fabric surfaces. The authors further studied the photocatalytic performance of the TiO2−SiO2 composite particles and observed decolorization of RhB diluted in water.20 It is interesting to note that these catalyst nanoparticles are more UV active when they are dispersed in the MB or RhB solutions than that in the form of a solid in the thin films. Although the decolorization of RhB was due to photocatalytic reactions of TiO2−SiO2 nanocomposites under UV radiation, the coatings on fabrics remained superhydrophobic after UV radiation.20 Similarly, PDMS-coated SiO2 and N-TiO2 were used by Lee et al.21 to prepare UV-resistive superhydrophobic coatings. They reported that nearly 50:50 mixture of PDMS-coated SiO2 and N-TiO2 provided a film with stable superhydrophobicity; however, it suffered the maximum photocatalytic activity when dispersed in the MB solutions, degrading upon exposure to UV. In the present case, we have shown that our coatings retain their superhydrophobic properties even after more than 300 h of UV exposure, which is much longer than those of other works reported previously.6,16,18−21,24 Further work is in progress to understand the UV degradation mechanism of MB or RhB by our newly developed core−shell CoSA@TiO2 (TiO2 in the core) nanoparticles in the liquid as found in the literature.20,21
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EXPERIMENTAL SECTION Materials and Reagents. An AA6061-T6 aluminum alloy with a chemical composition of Al 97.9 wt %, Mg 1.08 wt %, Si 0.63 wt %, Mn 0.52 wt %, Cu 0.32 wt %, Fe 0.17 wt %, Ti 0.02 wt %, and V 0.01 wt % has been used as substrates for coating. The aluminum (AA6061 alloy) substrates of dimension 1″ × 1″ were ultrasonically degreased in a soap solution and cleaned in ethanol and deionized water for 30 min each. The ultrasonically cleaned aluminum samples were dried for 24 h at 70 °C before further coating processes. The reagents including PMHS, cobalt nitrate, SA, and ammonium hydroxide were purchased form Sigma-Aldrich. TiO2 nanoparticles (average particle size: 100 nm) were purchased from MKnano. All of the reagents were of analytical grade. Deionized water with a resistivity of 18.2 MΩ· cm was used in all of the experiments. Preparation of PMHS/TiO2 Sol. In the first beaker, a mixture was prepared using 2 mL of PMHS and 40 mL of ethanol. Similarly, in the second beaker separately, 3 mL of ammonium hydroxide (28 wt %) was diluted in 10 mL of ethanol. This diluted ammonium hydroxide solution was added drop by drop into the first beaker to make the sol of PMHS. The prepared PMHS sol was aged for 48 h at room temperature while stirring continuously using a magnetic Teflon stirrer. In the third beaker, 1.5 g of anatase TiO2 nanoparticle was dispersed in 50 mL of ethanol, by ultrasonication for an hour. In the same beaker, 25 mL of PMHS aged sol was added drop by drop while constantly stirring to prepare the sol of TiO2/PMHS. This mixed solution was further aged for 5 days at room temperature while being constantly stirred prior to spin-coating on clean aluminum surfaces. Preparation of CoSA. CoSA was prepared using 17 mL of cobalt nitrate (0.01 M) and 34 mL of SA (0.01 M) ethanolic solution as well as 150 μL of ammonia (28 wt %) catalytic solution; the mixture was stirred overnight. Preparation of PMHS/TiO2@CoSA Sol−Gel. Anatase TiO2 nanoparticles (1.5 g) were dispersed in 51 mL of ethanolic CoSA solution prepared as above. Further, 25 mL of PMHS aged sol as prepared above was added drop by drop while constantly stirring to prepare the sol of TiO2@CoSA− PMHS. This mixture was also aged further similar to the PMHS/TiO2 sol for 5 days at room temperature prior to spincoating on clean aluminum surfaces. Spin-Coating Process. The spin-coating process was carried out using a Single Wafer Spin Processor (Laurell Technologies WS-650SZ-6NPP/LITE) to prepare thin films from PMHS/TiO2 and PMHS/TiO2@CoSA sol. UV Degradation Process. A UV degradation chamber developed in-house, composed of two UV lights with wavelengths of 302 and 365 nm, was used to study the UV degradation process of the coated thin films. Characterization. The morphological and elemental analyses of the thin films prepared by the spin-coating process were performed using a scanning electron microscope (JEOL JSM-6480LV) equipped with an energy-dispersive X-ray spectroscopy instrument. The chemical compositional and structural analyses were carried out using XRD (D8 discover with a Cu Kα wavelength of 0.154 nm) as well as by ATR−
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CONCLUSIONS CoSA thin films have been prepared and characterized by XRD, ATR−FTIR, and UV−vis spectroscopy. The synthesized CoSA has been utilized to prepare a UV-durable superhydrophobic nanocomposite thin film by embedding TiO2 nanoparticles in a hydrophobic PMHS matrix on AA6061 aluminum substrates. This PMHS/TiO2@CoSA superhydrophobic thin film exhibited a nearly constant WCA of 160° under continuous UV irradiation for 1 month, whereas the superhydrophobic PMHS/ TiO2 thin films, without CoSA shells on TiO2 nanoparticles, were drastically converted to the superhydrophilic state after UV irradiation of only 4 h. The synthesized CoSA not only increased the hydrophobicity in the surface of TiO2 nanoparticles but also confined the photocatalytic efficiency of TiO2. The UV durability mechanism of the superhydrophobic TiO2 8202
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(4) Guo, P.; Zhai, S.; Xiao, Z.; An, Q. One-step fabrication of highly stable, superhydrophobic composites from controllable and low-cost PMHS/TEOS sols for efficient oil cleanup. J. Colloid Interface Sci. 2015, 446, 155−162. (5) Gao, Y.; Gereige, I.; El Labban, A.; Cha, D.; Isimjan, T. T.; Beaujuge, P. M. Highly transparent and UV-resistant superhydrophobic SiO2-coated ZnO nanorod arrays. ACS Appl. Mater. Interfaces 2014, 6, 2219−2223. (6) Qing, Y.; Yang, C.; Yu, N.; Shang, Y.; Sun, Y.; Wang, L.; Liu, C. Superhydrophobic TiO2/polyvinylidene fluoride composite surface with reversible wettability switching and corrosion resistance. Chem. Eng. J. 2016, 290, 37−44. (7) Wang, L.; Zhang, X.; Fu, Y.; Li, B.; Liu, Y. Bioinspired preparation of ultrathin SiO2 shell on ZnO nanowire array for ultraviolet-durable superhydrophobicity. Langmuir 2009, 25, 13619− 13624. (8) Nishimoto, S.; Becchaku, M.; Kameshima, Y.; Shirosaki, Y.; Hayakawa, S.; Osaka, A.; Miyake, M. TiO2-based superhydrophobic− superhydrophilic pattern with an extremely high wettability contrast. Thin Solid Films 2014, 558, 221−226. (9) Xu, Q. F.; Liu, Y.; Lin, F.-J.; Mondal, B.; Lyons, A. M. Superhydrophobic TiO2−polymer nanocomposite surface with UVinduced reversible wettability and self-cleaning properties. ACS Appl. Mater. Interfaces 2013, 5, 8915−8924. (10) Duan, W.; Xie, A.; Shen, Y.; Wang, X.; Wang, F.; Zhang, Y.; Li, J. Fabrication of Superhydrophobic Cotton Fabrics with UV Protection Based on CeO2 Particles. Ind. Eng. Chem. Res. 2011, 50, 4441−4445. (11) Kamegawa, T.; Shimizu, Y.; Yamashita, H. Superhydrophobic surfaces with photocatalytic self-cleaning properties by nanocomposite coating of TiO2 and polytetrafluoroethylene. Adv. Mater. 2012, 24, 3697−3700. (12) Feng, X.; Zhai, J.; Jiang, L. The fabrication and switchable superhydrophobicity of TiO2 nanorod films. Angew. Chem., Int. Ed. Engl. 2005, 44, 5115−5118. (13) Cassie, A. B. D.; Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (14) Wenzel, R. N. Resistance of solid surfaces to wetting by water. Ind. Eng. Chem. 1936, 28, 988−994. (15) Zhang, X.; Guo, Y.; Zhang, Z.; Zhang, P. Self-cleaning superhydrophobic surface based on titanium dioxide nanowires combined with polydimethylsiloxane. Appl. Surf. Sci. 2013, 284, 319−323. (16) Gao, L.; Lu, Y.; Zhan, X.; Li, J.; Sun, Q. A robust, anti-acid, and high-temperature−humidity-resistant superhydrophobic surface of wood based on a modified TiO2 film by fluoroalkyl silane. Surf. Coat. Technol. 2015, 262, 33−39. (17) Gao, Z.; Zhai, X.; Liu, F.; Zhang, M.; Zang, D.; Wang, C. Fabrication of TiO2/EP super-hydrophobic thin film on filter paper surface. Carbohydr. Polym. 2015, 128, 24−31. (18) Qing, Y.; Yang, C.; Sun, Y.; Zheng, Y.; Wang, X.; Shang, Y.; Wang, L.; Liu, C. Facile fabrication of superhydrophobic surfaces with corrosion resistance by nanocomposite coating of TiO2 and polydimethylsiloxane. Colloids Surf., A 2015, 484, 471−477. (19) Wang, S.-F.; Kao, T.-H.; Cheng, C.-C.; Chang, C.-J.; Chen, J.-K. Reversible manipulation of the adhesive forces of TiO 2/ polybenzoxazine nanoassembled coatings through UV irradiation and thermal treatment. Appl. Surf. Sci. 2015, 357, 1634−1646. (20) Xu, B.; Ding, J.; Feng, L.; Ding, Y.; Ge, F.; Cai, Z. Self-cleaning cotton fabrics via combination of photocatalytic TiO2 and superhydrophobic SiO2. Surf. Coat. Technol. 2015, 262, 70−76. (21) Lee, J. H.; Park, E. J.; Kim, D. H.; Jeong, M.-G.; Kim, Y. D. Superhydrophobic surfaces with photocatalytic activity under UV and visible light irradiation. Catal. Today 2016, 260, 32−38. (22) Bayat, A.; Ebrahimi, M.; Nourmohammadi, A.; Moshfegh, A. Z. Wettability properties of PTFE/ZnO nanorods thin film exhibiting UV-resilient superhydrophobicity. Appl. Surf. Sci. 2015, 341, 92−99. (23) Jiang, T.; Guo, Z. Robust superhydrophobic tungsten oxide coatings with photochromism and UV durability properties. Appl. Surf. Sci. 2016, 387, 412−418.
FTIR (Agilent Technologies Cary 630 FTIR). UV−visible absorption properties were characterized using a UV−vis spectrophotometer (Agilent 8453 UV−visible Spectroscopy System). The surface wettability investigations of the coated samples were conducted by measuring static and dynamic CAs of a 10 μL water drop using a First Ten Angstrom CA goniometer. The dynamic WCA was measured by holding the water drop with a stationary needle in contact with the sample surface and moving the goniometer stage in one direction. The CAH, which is the difference between the advanced and receding CAs, was measured as described in previous reports.3,36,44,45 The roughness measurements were carried out using the MicroXAM-100-HR 3D surface profilometer.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01579. ATR−FTIR spectra of the PMHS/TiO2@CoSA film of as-received and after one-month UV irradiation; a schematic chemical model of CoSA; comparison of morphological features of the PMHS/TiO2@CoSA and PMHS/TiO2 films revealed by SEM images; ATR−FTIR spectra of PMHS liquid, PMHS/TiO2, and PMHS/ TiO2@CoSA superhydrophobic thin films; and studies of UV degradation of the PMHS/TiO2 thin film by ATR− FTIR spectra and WCA (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (D.K.S.). ORCID
Dilip Kumar Sarkar: 0000-0001-7695-6488 Present Address §
Body & Exterior Dept., Pan Asia Technical Automotive Center Co., Ltd., 3999, Longdong Avenue, Shanghai, China 201201 (J.X.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are very grateful for the financial support provided by the Natural Science and Engineering Research Council of Canada (NSERC) and Aluminum Research Center (REGAL). The authors thank Saleema N., NRC-Saguenay, Canada for the critical reading of the manuscript.
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