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Zeolite-Based Anti-Fogging Coating via Direct Wet Deposition Wan-Ju Hsu, Pei-Sun Huang, Yi-Chen Huang, Ssu-Wei Hu, Heng-Kwong Tsao, and Dun-Yen Kang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03738 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019
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Zeolite-Based Anti-Fogging Coating via Direct Wet Deposition Wan-Ju Hsu,†,# Pei-Sun Huang,†,# Yi-Chen Huang,† Ssu-Wei Hu,‡ Heng-Kwong Tsao‡ and Dun-Yen Kang*,†
†Department
of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC
‡Department
of Chemical and Materials Engineering, National Central University, Taoyuan 32001,
Taiwan, ROC
*D.-Y.K. E-mail:
[email protected] #Both
authors contributed equally to this work.
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ABSTRACT Zeolites are strongly hydrophilic materials that are widely used as water adsorbents. They are also promising candidates for antifogging coatings; however, researchers have yet to devise a suitable method for coating glass substrates with zeolite-based films. Here, we report on a direct wet deposition technique that is capable of casting zeolite films on glass substrates without exposing the glass to highly basic solutions or the vapors used in zeolite synthesis. We began by preparing cast solutions of pure silica zeolite MFI synthesized in hydrothermal reactions of various durations. The solutions were then applied to glass substrates via spin-on deposition to form zeolite films. The resulting zeolite MFI thin films were characterized in terms of transmittance to visible light, surface topography, thin film morphology, and crystallinity. Wetting and antifogging properties were also probed. We found that hydrophilicity and antifogging capability increased with the degree of thin film crystallinity. We also determined that the presence of the amorphous silica in the thin films is critical to transparency. Fabricating high performance zeolite-based antifogging coatings requires an appropriate composition of zeolite crystals and amorphous silica.
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1. INTRODUCTION Tuning the wetting properties of a substrate through surface engineering1-3 is an effective approach to the creation of anti-fogging (AF),4-15 anti-reflective (AR),6-8, 15-17 or self-cleaning9, 14, 18-19 surfaces. Both superhydrophobic (contact angle > 150°)20 and superhydrophilic (contact angle < 5°)21-22 coatings can imbue a surface with AF properties, but for different reasons.5,
10-11, 14, 23
Superhydrophobic surfaces minimize the adhesion between water droplets and substrate surface, thereby preventing water droplets from remaining on the surface. This AF mechanism is referred to as the lotus effect.24 Conversely, superhydrophilic surfaces cause the coalescence of water droplets to form a pseudofilm, which minimizes the scattering of incident light caused by water droplets.25-27 AF coatings also provide a surface with self-cleaning (i.e., antifouling) characteristics, which can extend the working life of microfluidic devices28 and contact lenses.29 However, most superhydrophobic AF coatings strongly interact with organic substances or biomolecules,30-31 making them less attractive to the superhydrophilic materials.
The most popular materials used in the creation of superhydrophilic coatings are nanoparticles of SiO2, 6-8, 17, 32
TiO2, 12, 33-34 or ZnO35-37, oxides which possess intrinsic hydrophilic surfaces. The morphology
of nanoparticles contributes to the roughness of the coated surface results in high roughness on the coating surface, which further increases hydrophilicity. Unfortunately, the superhydrophilic properties of pure TiO2 or ZnO coatings remain dormant until the nanoparticles have been exposed to strong UV irradiation.37-39 Composites of SiO2-TiO2 have been developed.9,
40-41;
however, fabricating high4
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quality composite films requires the use of multiple-step sol-gel deposition methods.39, 42-43 Zeolites are microporous crystalline aluminosilicates with good mechanical properties. The superhydrophilic and good wetting properties of zeolites44-46
makes them good candidates for AF coatings. Zeolite
membranes have been used in gas separation,47-50 desalination,51-52 and the purification of organic solvents.53-54 Zeolite thin films have been used for sensors55-57 and anti-reflection films in optical devices.58-61 However, zeolite-based coatings for AF applications have thus far been largely overlooked.15
We surmise that challenges pertaining to existing deposition techniques may explain the limited work on zeolite-based AF films. The techniques most widely used in the fabrication of zeolite films/membranes include in situ growth,62-64 seeded growth,
65-67
and dry gel deposition.68-70 These
methods require that the substrate be exposed to the synthesis solution or vapor of zeolites, such that zeolite layer formation and zeolite crystallization occur simultaneously. Most of the solutions used in the synthesis of zeolites are highly basic, which greatly hinders the deposition of zeolite on silicabased substrates, such as glass. In this study, we developed a direct wet deposition technique for zeolites. The proposed scheme eliminates the need to immerse the substrate in a basic solution. Instead, it uses stabilized zeolite colloidal solutions, in which the growth of zeolite crystals has been terminated. These cast suspensions are applied via spin-on deposition71-75 or ultrasonic nozzle spray deposition.76
To demonstrate the effectiveness of direct wet deposition in the casting of zeolite-based AF layers on 5 ACS Paragon Plus Environment
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a glass substrate, we casted pure-silica zeolite MFI films on the glass substrate for the subsequent investigation. According to our previous studies on wet deposition of various types of zeolites,71-74,76 the pure-silica MFI films presented the highest uniformity. Transmission electron microscopy (TEM) was used to image the zeolite MFI particles in the cast suspensions. Grazing-incidence X-ray diffraction (GIXRD), ultraviolet-visible (UV-vis) spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the zeolite thin films on the glass substrates. We also investigated how zeolite synthesis conditions affected thin film properties and wetting/antifogging performance.
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2. EXPERIMENTAL SECTION 2.1 Materials Tetrapropylammonium hydroxide solution (TPAOH, 25 wt.% in water) was purchased from Acros Organics. Tetraethyl orthosilicate (TEOS, 99%) and ethanol (99.9%) were purchased from Merck. TWEEN® 80 was purchased from Sigma Aldrich. All chemicals were used without further purification. Corning® EAGLE XG® alkali-free glass samples (25 × 75 × 0.7 mm) were used as a substrate for the deposition of zeolite films.
2.2 Preparation of zeolite MFI suspensions A mixture of 14.9 g of ethanol and 12.7 g of TPAOH solution was prepared in a perfluoroalkoxy alkane (PFA) container (100 ml capacity) and then stirred for 10 min. We subsequently added 12.1 g of TEOS dropwise to obtain a final molar ratio as follows: TEOS: TPAOH: ethanol: H2O = 1: 5.57: 0.36: 12.11. The resulting solution was aged at 30℃ for 24 h before 21.5 g aliquots of it were transferred to a Teflon® lined autoclave (180 mL capacity) to undergo a subsequent hydrothermal reaction in an oven under conventional heating at 135 °C for 3, 4.5, 5, or 6 h. Following the completion of the hydrothermal synthesis process, the zeolite suspensions (21.5 g) were mixed with 2.96 g of TWEEN® 80 as a surfactant. The mixtures were then stirred at 30°C for 24 h to form cast suspensions. The suspensions subjected to the hydrothermal reaction for a period of 3 hours are denoted as MFI-3 h. The notation for the remainder of the samples can be deduced by analogy.
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2.3 Preparation of zeolite MFI thin films Corning® EAGLE XG® Alkali-free glass (0.7 mm thick) was cut into squares measuring approximately 25 × 25 mm prior to spin-on deposition. Cast solution was then applied dropwise onto the substrate until the entire surface was covered. The substrate was subsequently spun at 2000 rpm for 30 s using a spin coater (Laurell Model-WS-650M2-23NPPB), and the resulting samples were placed in an oven and baked at 100°C for 24 h. The samples were then transferred to a furnace for calcination. The temperature in the furnace was increased from room temperature to 450 °C at a rate of 1 °C/min, where it was held for 5 h. Following calcination, the thin-film samples were ready for material characterization.
2.4 Material Characterization The morphology of zeolite MFI particles in the suspensions were analyzed using a Hitachi H-7100 transmission electron microscope (TEM) operated at an accelerated voltage of 75 kV.
Grazing-incidence X-ray diffraction (GIXRD) analysis of zeolite thin film samples was performed using a PANalytical X'pert diffractometer with a Cu Kα source operated at a voltage of 45 kV and an incident angle of 1°. Scanning was performed from 5° to 30° 2θ, with a step size of 0.02° and a duration of 3s per step.
A Hitachi S-4800 field emission scanning electron microscope (SEM) and a Hitachi SU-5000 SEM 8 ACS Paragon Plus Environment
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were used to characterize the morphology of the zeolite thin-film samples. The samples were coated with platinum via sputtering deposition at an acceleration voltage of 25 V for 40 s. SEM imaging was conducted at an acceleration voltage of 10 kV.
An Asylum Research Cypher S. atomic force microscope (AFM) was used to characterize the surface roughness of the thin film samples. The AFM was equipped with a Nanosensor PPP-RT-NCHR cantilever, and the analysis was performed in tapping-mode.
The adhesion test was conducted by pasting and tearing off the KAPTON® tape (polyimide with adhesion to steel: 25 oz/in).
To analyze transmittance, ultraviolet-visible (UV-vis) spectra were obtained for the various thin-film samples using an Agilent Cary 300 ultraviolet-visible spectrophotometer. Specifically, measurements were obtained using a double beam with a spectral bandwidth of 2.0 nm at a scanning rate of 120 nm/min.
The wettability of the thin film samples was determined according to contact angles recorded using a Dataphysics DSA25E (Krüss, Germany) contact angle measurement system. This involved releasing a water droplet of 5 μl from a micropipette onto the thin film surface. To measure the contact angle, recorded images of the droplet on the surface were analyzed using Image J software.77 9 ACS Paragon Plus Environment
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Antifogging properties were evaluated by covering a glass of boiling water (approximately 40 mL) with the thin film samples. Photographic images were recorded after the samples had been exposed to water vapor for 30 s. The samples were then dried using a high-pressure nitrogen gun and stored under ambient conditions for 24 h before the antifogging test was repeated. The aforementioned test was repeated 14 times.
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3. RESULTS AND DISCUSSION Figure 1a presents photographic images of as-made pure-silica zeolite MFI suspensions. Suspensions subjected to hydrothermal reaction for a period of 3 hours are denoted as MFI-3 h. The notation for the remaining samples can be deduced by analogy. The opaqueness of suspensions increased with the duration of the hydrothermal reaction, due to the formation and growth of zeolite MFI crystals. MFI3 h samples were highly transparent, which suggests that low-crystallinity colloids were present in the solutions.72 Conversely, MFI-6 h were strongly opaque, which suggests the presence of highcrystallinity zeolite MFI. Transmission electron microscopy (TEM) was used to elucidate the particle size and morphology of the zeolite suspensions (Figure 1b). The poorly defined objects in the TEM image of MFI-3 h indicates that these particles were likely to be amorphous. Particle morphology became increasingly defined with the duration of the hydrothermal reaction. The image of MFI-6 h shows crystals with a highly defined morphology, which suggests that MFI-6 h suspensions contained high-crystalline zeolite MFI. The image contrast of suspended nanoparticles in MFI-3 h and MFI-4.5 h was low, whereas the image contrast in MFI-5 h and MFI-6 h was high. This indicates that the hydrothermal reaction required five hours for the complete formation of zeolite MFI phase. Previous research that used TEM imaging to elucidate the crystallinity of silica reported similar findings.78-79 For our samples, average particle sizes were as follows: MFI-4.5 h (90 nm), MFI-5 h (110 nm), and MFI-6 h (130 nm). Note that the particle size in MFI-3 h was unmeasurable due to the ill-defined particle morphology.
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The four suspensions were applied to glass substrates via spin-on deposition to form zeolite films, as shown in Figure 2a. The coatings on all four samples appear homogeneous. The transparency of MFI-3 h, MFI-4.5 h, and MFI-5 h was nearly as high as that of bare glass substrate, whereas MFI-6 h presented a milky appearance, which may be due to the loosely packed zeolite MFI particles causing light to be refracted multiple times.80 Nonetheless, all of the zeolite MFI thin film samples prepared using the proposed direct deposition method presented remarkable uniformity, compared to the zeolite MFI grown on glass substrates in situ. This suggests that the direct deposition method developed by our group71-74, 76 provides distinct advantages over conventional methods in terms of zeolite thin film quality.
Ultraviolet-visible (UV-vis) spectra of the thin film samples were recorded so that transparency could be analyzed quantitatively. The raw UV-vis spectra of each sample are presented in Figure 2b, and the average transmittance values in the spectral range of 380-800 nm are presented in Figure 2c. Average transmittance values for MFI-3 h, MFI-4.5 h, and MFI-5 h were as high as that of a bare glass substrate, whereas the average transmittance value of MFI-6 h was relatively low with a high standard deviation, suggesting a lack of uniformity. Note that the raw UV-vis spectra of MFI-3 h, MFI-4.5 h, and MFI-5 h are indicative of interference, which is usually caused by reflection at the interface of two materials.81-83 The interference became increasingly pronounced as the homogeneity of the coating increased.36, 84 Indeed, the interference that appeared in MFI-3 h, MFI-4.5 h, and MFI-5 h samples was not observed in the UV-vis spectrum of MFI-6 h, indicating a difference in the uniformity of the thin 12 ACS Paragon Plus Environment
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films.
Surface topographic images were obtained using atomic force microscopy (AFM) to evaluate the homogeneity of the zeolite thin films. As shown in Figure 3, the surface morphology of MFI-6 h was far more distinct than that of the other three samples in terms of grain size. The surface roughness of MFI-6 h (230 nm, as defined by the root mean square (RMS) value of surface topography) also differed considerably from that of the other samples (
