Mechanically Robust Hybrid Coatings for Antifogging, Antireflection, and Self‐Cleaning Applications

In this work, a novel multifunctional film with durable antifogging performance and remarkable optical property is designed. By doping and hybridization to build an organic–inorganic siloxane network, the surface achieves long‐lasting wettability and superhydrophilicity with a water contact angle of 0°, which is maintained within 10° for over 130 days. The low refractive index endows the hybrid film with excellent optical property, which the average transmittance (380–1100 nm) of the film is 97.60% with the peak transmittance of 99.48%. By this design, the film performs well in the felt abrasion test, water impact test, X‐cut tape test, tape‐peeling test, chemical stability test, and anti‐dust test. Due to the abundant hydroxyl radicals on the surface, the film also exhibits excellent performance, achieving 92.73% degradation of methylene blue solution (10 ppm) and 39.53% degradation of rhodamine B (10 ppm) with good reproducibility in cyclic measurements.


Introduction
Fogging has always been an issue that cannot be ignored which must be properly addressed in practical applications where favorable optical properties are required. [1,2] The survey found that the unintended effects of fogging exist everywhere. During the COVID-19 pandemic, fogging on medical equipments such Adv. Mater. Interfaces 2023, 10,2300159 trade-off between antifogging and antireflection properties, and a universal strategy to realize multifunctional antifogging films for complex conditions and a prolonged period of time remains a prevailing challenge. [18] Functionalized antifogging materials are widely reported to be used in various applications including self-cleaning, [19,20] antifrost, [21] antireflection, [22] antibacterial, [23,24] and self-healing. [25] So far, different preparation techniques have been developed to fabricate antifogging coatings, such as spin-coating, layer-bylayer assembly, dip-coating, and spray-coating. [26][27][28] In comparison, antifogging coatings based on inorganic materials have been developed with preferable performance. By depositing inorganic nanoparticles, such as TiO 2 , SiO 2 , or ZnO, on the surface of the target substrate, the coatings tend to have superior mechanical strength and adhesion compared to organic materials, which endows the coatings with excellent wear resistance. [29] As superhydrophilic inorganic materials exhibit great potential in automotive, display panels, and other optoelectronic applications, its coatings possessed of both antifogging and antireflection functions attract significant attention in recent years. According to the surface wetting pattern, it can be divided into two categories: intrinsic and light-induced wetting. The first category consists of materials such as SiO 2 , graphene oxide in various forms, and nanostructures exhibit hydrophilicity via chemical modification and hybridization before or after film formation, obtaining ephemeral antifogging surface, particularly for outdoor exposure and cycle uses. [30] The second category consists of photoactive materials such as TiO 2 or ZnO, which need to be induced by UV light to gain superhydrophilic surface. [31] Generally, TiO 2 and SiO 2 -based antifogging coatings are widely investigated. Almost all superhydrophilic states of TiO 2 -based films show timely antifogging surfaces. However, an unavoidable difficulty is that TiO 2 -based coatings cannot maintain long-term stability without UV light exposure, along with the degradation of organic components on TiO 2 . [32] In contrast, the introduction of SiO 2 -based coatings can circumvent the problems above, and the lower refractive index of SiO 2 coatings provides higher optical transmittance than TiO 2 coatings only. The modification of SiO 2 -based coatings has been intensively explored to support antifogging, their superhy-drophilicity was reported to be related to the concentration of hydroxyl groups (SiOH), rough surface, and film porosity. [33] Practically, the realization of long-term multifunctional antifogging coatings is still challenging, which requires the materials to be wearable and resistance to contaminants to ensure their outdoor applications. [34] Thus, there is an urgent need to design a new material system to ensure the antifogging and antireflective properties, achieving a breakthrough in multifunctionality to be compatible with the existing issues above.
In this work, we design a novel multifunctional film with remarkable high transmittance and durable antifogging performance. As shown in Scheme 1, we propose a new scheme based on the sol-gel method and combine doping and hybridization into the preparation of the coatings, achieving superhydrophilic surfaces with roughness less than 1 nm. Without any form of energy excitation, a water contact angle of 0° can be obtained. The film exhibits long-lasting superhydrophilicity and antifogging performance, along with that the water contact angle is lower than 10° for over 130 days. Compared with the bare glass, the average transmittance (380-1100 nm) of the film is 97.60% with peak transmittance of 99.48%. The film performs well in the felt abrasion test, water impact test, X-cut tape test, tapepeeling test, and dust resistance test. Notably, benefiting from the abundant hydroxyl radicals on the surface, the film presents excellent self-cleaning performance with 92.73% degradation of methylene blue solution (10 ppm) and 39.53% of rhodamine B (RhB, 10 ppm) under xenon lamp irradiation, and also displays high degradation efficiency for outdoor tests.

Chemical Composition and Morphology of the Films
Fourier-transform infrared (FTIR) spectra are carried out to characterize the surface chemical composition derived from hybriding and doping with mesoporous silica coatings (MSC). [35] We use MT to denote the films prepared by silica-titanium hybrid sols without any doping, MTF and MTP refer to the films after doping Fe 3+ or polyethylene glycol (PEG2000) only in the hybrid sols, respectively, and films further doped with Fe 3+ on the basis of MTP is named as MTPF. As shown in Figure 1a, all coatings are observed with three absorption peaks at 803, 962, and 1227 cm −1 , illustrating the existence of the typical bands of silica and confirming the existence of the network in the sol-gel reaction. The doping with a small amount of titanium dioxide shows a certain degree of concavity at 962 cm −1 , which can be attributed to the asymmetric stretching vibration of the added TiOSi bond. It is believed that TiOSi bonding is one of the reasons for long-lasting superhydrophilicity, because of that much more hydroxyl groups can be generated from the bondings. [36] The adsorption band at 1636 cm −1 is attributed to the bending vibration of the HOH bond, which is caused by chemisorption of water. The adsorption bands at about 3394 and 3633 cm −1 are assigned to the stretching mode of the OH bond. [37] The doping of titanium dioxide, PEG2000, and trivalent iron ions all results in a more pronounced down-concave of the peak representing OH, which indicates the improved surface energy and potential hydrophilic performance of the surface. Among them, MTPF is particularly noticeable compared to MTP, where the doped trivalent iron ions enhance the adsorption of hydroxyl groups, suggesting a synergistic effect with the nearby oxygen vacancies.
As we mentioned above, the surface structure has a great impact on its wettability. Figure S2, Supporting Information, shows top-view scanning electron microscopy (SEM) images of MSC, MT, MTP, and MTPF with similar surface morphology at the same magnification. For further comparison, the surface morphology of original MSC and hybrid MTPF films is inspected by SEM images at different magnifications. Seen from Figure 1b, the surface of MTPF shows a compact organization without noticeable micro-nano structure change after hybridization/modification. From the second column of images, we can clearly observe the pristine finger-print like microchannels derived from MSC, which are arranged in a highly ordered network. Similarly, it is not difficult to observe the same surface morphology on MTPF, which may contribute to improve the surface wettability and optical properties. However, from cross-sectional images of films in Figure S3, Supporting Information, the MTPF shows more compact organizations compared with MSC, accompanying with higher refractive index with less porosity, according to the results from spectroscopic ellipsometer in Table S1, Supporting Information.

Superhydrophilicity and Long-Lasting Antifogging Performance
In order to understand the superhydrophilic properties and the long-lasting antifogging performance of the films in detail, we first investigated the role of PEG2000 in the hybrid system. PEG2000 is a commonly used porogenic agent, which usually affects the surface morphology by controlling porosity and void size to regulate the surface energy. After the post-annealing treatment, the hybrid network with new surface nano-structures was formed after the removal of poly(ethylene glycol)block-poly (propylene glycol)-block-poly (ethylene glycol) (F127) micelles and decomposition of PEG2000 aggregates. As illustrated in Figure 2a, the roughness of the films increase as the molar ratio of PEG2000 in the sols expands (the average roughness values are given in Figure S4, Supporting Information), which gives rise to a regular decrease in the average transmittance from 380 to 1100 nm, as shown in Figure 2b. To obtain a film that is highly transparent with superhydrophilic surface, the amount of PEG2000 dopant was strictly controlled. It is seen that the doping of PEG2000 makes little change to the film morphology, the refractive index, and porosity of MT and MTP (as shown in Table S1, Supporting Information) remain almost constant before and after doping, which in principle promotes the transmittance originated from MSC films. SEM images of five different doping ratios are displayed in Figure S5, Supporting Information, from which tiny change is observed from the surface morphology. It is well known that proper roughness enhances the specific surface area, making contribution to achieve hydrophilicity. However, it is presumable that the participation of PEG2000 in this work mainly modified the surface with more in situ hydroxyls, which was verified from the FTIR analysis with a more pronounced downward concavity at 3394 and 3633 cm −1 . Figure 2c schematically displays the surface chemical composition before and after doping with PEG2000. Figure 2d shows the water contact angle of the coated samples with different PEG2000 mole fractions. It is observed that the film reaches 0° for the first time when the mole fraction of PEG2000 is 1.02%. As a result, a high surface energy, optical flat, and superhydrophilic surface can be achieved through fine synthetic tuning, especially the regulation of PEG2000.
To gain insight into the antifogging ability of MTPF in external environment with big temperature difference and high  relative humidity, MSC, MT, and MTP were also fabricated and investigated as comparison to better elucidate the function of TiO 2 , PEG2000, and Fe 3+ . Optical photographs have been used by most researchers to qualitatively show the antifogging performance. As displayed by the images in Figure 3a, MTPF films are covered on the beaker filled with 85 °C of hot water. Immediately, the bare glass side has a visible fog layer, and the letters beneath can barely be read. On the MTPF coated side, the sample exhibits evident antifogging performance without fogging in the early stage of testing. The MTPF surface can absorb and spread the water vapor to form a thin water layer, allowing the surface to prevent the aggregation of isolated droplet and reduce light scattering, which can be attributed to the wettability and water diffusion via the effect from surface chemistry and structure. On the other hand, the cold method of the antifogging test is often applied to evaluate the frost formation on the surface when transferred into a warm environment from a cold atmosphere. As shown in Figure 3b, we put the MTPF film into the refrigerator at −6 °C for 24 h, and took it out at room temperature of 20 °C and humidity of 70%. Apparently, the MTPF film shows low haze with good antifogging performance, as evidenced by the fact that the logos and letters are completely unaffected from observation through the coated section. Also, we study the hydrophilicity and high temperature antifogging performance of four samples at the same time. As displayed in Figure 3c, we notice that MTP, MT, and MSC films all show good antifogging performance in their initial state, and the corresponding water contact angle of MSC is 7.9° and that (3.5°) of MT, while the water contact angle decreases to 0° after modifying with PEG2000 ( Figure 3d). MTPF doped with trivalent iron ions is supposed to have better superhydrophilicity with a water contact angle of 0° and 2 µL water droplets spread completely within 5s (A video of the water contact angle of MTPF is shown in Video S1, Supporting Information). Though all four samples involved in this work exhibit a gradual increase in contact angle, the relative stable hydrophilicity surface will open new possibilities of realizing the long-term antifogging purpose. Therefore, the variations of water contact angle and antifogging properties of the four samples over 130 days have been inspected in Figure 3e and Figure 4, respectively. It is observed that the water contact angle of MSC increases over 10° quickly within 2 days, along with its antifogging effect only presents in the initial stage. MT and MTP surfaces show analogous trend with the contact angle of both increase to 10° in 13 and 32 days respectively, which gives rise to more durable antifogging performance within 10

Optical Performance
In addition to the surface morphology and hydrophilic properties, the optical performance of the films is subsequently scrutinized. As demonstrated by the transmittance and reflectance spectra of the glass substrate coated with and without MTPF film in Figure 5a, the average transmittance (380-1100 nm) of bare glass shows only 91.64%, while the average transmittance (380-1100 nm) of MTPF increases up to 97.60%, indicating that the broadband antireflection property derives from the low refractive index of the porous MTPF film. To visualize the transmission enhancement of MTPF, we put the samples into the real background indoors and outdoors, both of the coated sides intuitively reveal the excellent antireflection effect, which suppresses most of the reflection light, seen from Figure 5b,c. Besides, the original MSCs films doping with titanium dioxide and PEG2000 only are also characterized by the ultraviolet-visible-near infrared (UV-vis-NIR) spectrum. Compared to the bare glass, all of those hybrid thin films enhance the light transmission due to their low refractive indices, and the optical performances are summarized in Table 1 and in Figure S6, Supporting Information. For original MSC, the average transmittance (380-1100 nm) reaches 97.81%, 6.71% higher than that of bare glass. Its maximum transmittance is optimized high up to 99.87%. However, the transmittance of the coatings has tiny change after subsequently doping/ modifying with titanium dioxide, PEG2000, and iron ions. For MT, MTP, and MTPF films, the peak transmittance is reduced from 99.87% to 99.58%, 99.77%, and 99.48%, individually. Among them, the average transmittance (380-1100 nm) of MT decreased the most. After the introduction of PEG2000, the average transmittance (380-1100 nm) of MTP increased by 0.21% compared with MT, which may be attributed to the higher surface porosity originated from PEG2000 aggregates.

Mechanical Stability
As the long-term stability is very important for coatings in outdoor service, it is of great value to study the mechanical robustness and stability of the films. [38][39][40] Felt abrasion test, water impact test, X-cut tape test, and tape-peeling test were performed to evaluate the robustness of the coatings. First, the film was checked by felt abrasion test, and a standard felt was selected as the friction material, and 40 reciprocating motions were performed under a load of 15 N. Figure 6a   comparative change in transmittance before and after felt abrasion test, which shows an average transmittance (400-2000 nm) decay of 0.57%, where the inset shows the device used in this test. To verify the adhesion of the film under wet conditions, we perform the water impact test, which is often used to simulate outdoor rainy conditions. After a stream of water slams against the surface for 2 h, the transmittance of the film does not decrease apparently (Figure 6b), and the average transmittance (400-2000 nm) attenuation is only 0.15%. Additionally, we conduct X-cut tape tests according to the standard of ASTM-D3359-2017 to verify the adhesion strength and the quality of the film. The SEM image in Figure 6c displays the surface mor-phology after X-cut tape test. According to the grading requirements in the standard, the film exhibits the highest level of 5A without any visible damage is observed even at the edges of the scratches. Moreover, tape-peeling tests were carried out to confirm the adhesion strength of the films during cleaning and maintenance in actual use. The tape (Scotch 600#, 3M) was fully adhered to the surface, where a 1 kg weight was rolled over the tape three times to ensure that all air bubbles were removed within 30 s, and then the tape was peeled off. The transmittance of the film decreases slightly as the number of tape peeling cycle increases (Figure 6d   reinforced robustness. No visible crack or detachment was observed in the SEM images (as shown in Figure S7, Supporting Information) after the mechanical tests above, manifesting the strong covalent bonding and intermolecular interactions between MTPF coatings and glass substrates via condensation within silanol groups. [41,42] Furthermore, surface wettability and antifogging properties after tests are equally important. In Figure 6e, the graphs from left to right reveal the water contact angle and antifogging performance of the films after abrasion test, water impact test, and tape-peeling test, respectively. Though all the results presented here show the slight increase of the water contact angle, the film still maintains good antifogging performance as before. The corresponding mechanism was organized and displayed in Figure S8, Supporting Information. Finally, as illustrated in Figure S9, Supporting Information, we verified the chemical stability by exposing the films to ethanol, isopropanol, ammonia, and hydrochloric acid (0.12 m) for 30 min. The results indicate that the films maintained their high optical properties and significant antifogging performance throughout the tests.

Self-Cleaning Performance
We evaluated the self-cleaning performance of MTPF films via the degradation efficiency of organic pollutants. First, we selected methylene blue solution (MB) as the organic pollutant for the relevant tests. Figure 7a displays the variation of the degradation efficiency of methylene blue solution (MB) in the case with/without MTPF under xenon lamp irradiation as a function of irradiation time. The case with MTPF exhibits a higher degradation efficiency for MB (10 ppm) compared to the case without MTPF. When the coated glass was inserted in the solution, 92.73% degradation of MB (10 ppm) was achieved in 4 h. And also, we verified the reusability of MTPF by performing  Figure 6. a) UV-vis-NIR transmission spectra of the MTPF coating after abrasion test (inset on the lower right corner: photograph of the wear test equipment). b) UV-vis-NIR transmission spectra of the MTPF coating after water impact test (inset on the lower right corner: digital image of water impact test). c) SEM image of the MTPF coating after X-cut tape test. d) Degradation in normalized T AVE 400-2000 nm after 0-40 tape-peeling cycles (image below: schematic diagram of the tape-peeling test). e) The graphs from left to right show the water contact angle and the antifogging performance of the films after the abrasion test, the water impact test, and the tape-peeling test, respectively.
five degradation cycles of MB (10 ppm) under xenon light irradiation. Figure 7b reveals that the degradation efficiency of MB (10 ppm) remained almost constant after five cycles. To further investigate the application of MTPF in practice, we carried out the test outdoors on a sunny summer day in Ningbo, Zhejiang province. We recorded the changes in temperature and humidity, as shown in Figure S10, Supporting Information, while the light intensity of the sun at every time point could be observed in Figure 7c. As a result, 95.43% degradation of MB (10 ppm) is also achieved by MTPF under outdoor conditions, which presents slightly higher than that of results from indoors due to the higher temperature and relative unpredictable light intensity outdoors. Second, we also selected rhodamine B (RhB) for the same tests as a control. The degradation efficiency of RhB (10 ppm) with/without MTPF under xenon lamp irradiation is displayed in Figure 7d. Although, the degradation efficiency of RhB (10 ppm) is only 39.53% and 39.09% under indoors and outdoors light irradiation, we notice that MTPF presents similar reusability (as shown in Figure 7e,f). The color change of MB (10 ppm) and RhB (10 ppm) with/without MTPF during xenon lamp irradiation were provided in Figure S11, Supporting Information. Without MTPF, RhB solution displays negative degradation efficiency under sunlight irradiation. This may be due to the higher outdoor temperature, which makes the evaporation rate of deionized water faster than the degradation rate of the dye itself. At last, we conducted anti-dust test to verify the contamination effect of airborne dust on MTPF. Figure 7g shows the optical photographs of coated glass and bare glass before and after the anti-dust test, and the relative change in transmittance was measured as well (the process of anti-dust tests for MTPF and bare glass are provided in Video S3, Supporting Information). It is observed that the surface of the coated glass contains less dust residue compared to the base glass after the test. Meanwhile, the reduction of normalized average transmittance is displayed in the histogram shown in Figure 7g. The normalized T AVE400-2000 nm value for bare glass decreases to 99.20% of the beginning, while that of coated glass remains 99.92% of the initial, indicating that the superhydrophilicity and nanoporous structure also endow MTPF coatings with good self-cleaning property.

Conclusion
We have successfully fabricated a multifunctional hybrid films on glass with high transparency, good mechanical robustness, durable antifogging performance, and self-cleaning properties. Owing to the large number of hydroxyl radicals on the surface, the film shows long-lasting superhydrophilicity along with antifogging performance, and displays excellent self-cleaning properties. The low roughness and nanoporous structure also maintains the excellent optical properties of the film. In addition, we uncover that the large number of SiOSi networks on the surface allows strong covalent bonds and intermolecular interactions between MTPF and bare glass through condensation within the silanol groups, which leads to well performance in various mechanical tests. The antifogging film has a mean square roughness of only 0.47 nm, achieving a water contact angle of 0° without any extra energy excitation, and maintaining its antifogging performance for over 130 days. The average transmittance (380-1100 nm) of MTPF film is 97.11%, 5.96% improvement to bare glass in absolute. After 40 times of felt abrasion tests with 15 N load, the average transmittance (400-2000 nm) attenuation is 0.57%, while after 2 h of water impact test, the average transmittance (400-2000 nm) attenuation is only 0.15%. Meanwhile, the film has good adhesion to the substrate, reaching 5A level. During self-cleaning tests, the film demonstrates strong degradation of the organic pollutants, exhibiting good dust resistance for inorganic particles. This work provides a new strategy to realize a multifunctional antifogging film, opening up a new guide for materials design and fabrication, which presents a great potential for long lasting practical application.
Preparation of Silica-Titanium Hybrid Sols: Before the preparation of hybrid sols, the silica sols were synthesized by following the previous work. [43] First, TEOS was used as the silica source and mixed with EtOH, dilute HCl (0.12 m) and H 2 O in the molar ratio of 1:4.2:6 × 10 −5 :1.2 and refluxed at 50 °C for 2-3 h. The obtained precursor sols were kept in a sealed container and cooled to room temperature for use. Thereafter, F127 was added to a mixture of EtOH, H 2 O, and dilute HCl (0.12 m) and stirred vigorously for about 1 h at room temperature to obtain stable F127 dispersion. The molar ratio was set as EtOH:H 2 O:dilute HCl (0.12 m):F127 = 18:3.5:4.0 × 10 −3 :8.5 × 10 −3 . MSC sols were prepared by adding the F127 dispersion dropwise to the precursor sols and stirring for 2 h. After aging in a sealed container at room temperature for 3 days, the resulting silica sols were ready for use, named as MSC (mesoporous silica coating) sols. Then, titanium sols were prepared by mixing TTBT, EtOH, and concentrated HCl. The molar ratio of TTBT, EtOH, and concentrated HCl was set as 1:49:0.0014. Finally, PEG2000 and FeCl 3 ·6H 2 O were added sequentially under vigorous stirring for 6 h to obtain homogeneous mixture, and stored in a closed container at room temperature for 24 h to obtain the final hybrid sols. The molar ratio was Si:Ti:PEG2000:Fe 3+ = 19:1:0.21:0.31.
Preparation of Films on Glass Substrates: Prior to the dip-coating process, the glass substrates from Taiwan Glass Industry Group were thoroughly washed twice with detergent, followed by ultrasonic cleaning with water, ethanol, and acetone for 10 min each time. The glass substrates were dip-coated with the hybrid sols via evaporation-induced self-assembly process at 20 °C and 35% of relative humidity, and the coated glass samples were calcined at 400 °C (heating rate: 5 °C min −1 ) for 1 h to obtain hybrid films, designated as MTPF films. A routine of the preparation process is illustrated in Scheme 1.
Characterization: The wavelength-dependent ellipsometric parameters were determined with the aid of a spectroscopic ellipsometer (M2000-DI, J.A. Woollam Co.), and the Cauchy model was used to simulate and fit the relevant ellipsometric curves to obtain the refractive index of the films. Optical transmittance of glass substrates and films in the range of 380 to 2000 nm was measured by a UV-vis-NIR spectrophotometer (Lambda 950, Perkin Elmer). Fourier-transform infrared spectrometer (NICOLET 6700, Thermo) was used to measure the infrared reflectionabsorption spectra of films at a grazing reflection angle of 80°. Atomic force microscopy (AFM, Dimension 3100, Vecco) was used to characterize the surface topology of the films. The morphology of the films was observed by SEM (FEI Quanta FEG 250, FEI) in a vacuum environment and the specimens were sputtered with platinum prior to testing. The static water contact angles were examined by a droplet shape analysis system (Data Physics OCA20). A total volume of 2 µL deionized water droplets was used. Abrasion test machine (HT-NM-339L, Huitai Machinery Co.) was used to check the mechanical stability of films. The outside ambient temperature and relative humidity were recorded by a temperature/humidity sensor (SHT 31, Sensirion, Switzerland), while the solar radiance was recorded by a weather station (PC-4, Jinzhou Weather, China). To investigate the self-cleaning properties of the films, anti-dust experiments were conducted in a test chamber with standard ash inside (the simulation equipment was shown in Video S3, Supporting Information). Degradation experiments were carried out in beakers at 25 °C under magnetic stirring. Methylene blue and RhB were selected as target contaminants. The photocatalytic degradation experiments were carried out both indoors and outdoors. In the indoor experiment, a 1000 W xenon lamp (Sol3A 94063A, Newport) was used as the light source, simulating sunlight irradiation, and the measured light intensity was 100 mW cm −2 . The absorbance change of the organic dye solutions were measured by UV-vis-NIR spectrophotometer (723S, Lengguang Tech) at certain time intervals during the experiment. The initial absorbance (A 0 ) of the organic dye was measured at its maximum absorption peak, and then the MTPF films were immersed in the organic dye solutions and stored in the dark for 50 min to reach adsorptiondesorption equilibrium prior to irradiation. After irradiating for a period of time, the absorbance value of the organic dye was marked as A t . The degradation efficiency (η) of the organic dye solutions was calculated by the following formula

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.