Highly Transparent and Zirconia-Enhanced Sol-Gel Hybrid Coating on Polycarbonate Substrates for Self-Cleaning Applications

To improve the efficacy of polymer-based substrate hybrid coatings, it is essential to simultaneously optimize mechanical strength and preserve the optical properties. In this study, a mixture of zirconium oxide (ZrO2) sol and methyltriethoxysilane modified silica (SiO2) sol-gel was dip-coated onto polycarbonate (PC) substrates to form zirconia-enhanced SiO2 hybrid coatings. Additionally, a solution containing 1H, 1H, 2H, and 2H-perfluorooctyl trichlorosilane (PFTS) was employed for surface modification. The results show that the ZrO2-SiO2 hybrid coating enhanced the mechanical strength and transmittance. The average transmittance of the coated PC reached up to 93.9% (400–800 nm), while the peak transmittance reached up to 95.1% at 700 nm. SEM images and AFM morphologies demonstrate that the ZrO2 and SiO2 nanoparticles were evenly distributed, and a flat coating was observed on the PC substrate. The PFTS-modified ZrO2-SiO2 hybrid coating also exhibited good hydrophobicity (WCA, 113°). As an antireflective coating on PC, with self-cleaning capability, the proposed coating has application prospects in optical lenses and automotive windows.


Introduction
Transparent engineering plastics have gained considerable interest due to their affordability, robust impact resistance, and lightweight nature [1]. The demand for polymer materials to replace traditional oxide glass has risen considerably in a variety of industries, such as transportation and electronics [2,3]. Polycarbonate has emerged as the material of choice for an extensive range of engineering applications, such as photovoltaic devices [4,5], display panels, optical lenses [6,7], optical discs, security windows, and automotive glass [8,9]. Compared with other inorganic transparent materials, such as glass, polycarbonate (PC) offers particular advantages in terms of reduced manufacturing costs and product weight. Despite its high fracture resistance and impact strength, PC has several drawbacks such as low ordinary transmittance (below 90%), poor resistance to UV exposure, and vulnerability to scratching [10,11], all of which contribute to reduced service performance and service life [12,13]. As such, the functionalization of the polymer surface has become a crucial step [14,15]. At present, an effective means of improving the performance and multifunctionality of PC products to meet the increasingly complex environmental requirements is achieved through plating or coating the surface of PC products with organic, inorganic, or hybrid film layers, utilizing various film-forming processes [16]. As energy automobiles and optical devices have developed, there has been increasing demand for PC with self-cleaning and antireflection, which can be realized through different coating technologies. Such methods include several coating methods, namely vapor phase and vacuum deposition of coating materials by physical vapor deposition (PVD) or chemical vapor deposition (CVD) [17]. Sun et al. reported

Treatment of Substrate Materials
The PC was ultrasonic treated in anhydrous EtOH for 30 min. The PC surface was cleaned by washing with purified water to remove surface dust, and the cleaned PC samples were placed on a stand with the uniform front side facing up and dried in an electric blast dryer at 80 • C for 1 h to remove moisture and solvent. The treated PC sheets were wrapped in non-woven fabric.

Synthesis of Mixed Zirconium Oxide and Silica Sols
First, TEOS, EtOH, AcOH, and H 2 O were mixed together in a beaker with stirring at a molar ratio of 1:35:1:4, in which the volume of EtOH was 20 mL. Then the obtained sol-gel was transferred to a closed beaker and stirred in a water bath at 40 • C for 1 h. To modify the sol-gel, MTES was introduced and added gradually to achieve a TEOS/MTES molar ratio of 1, using a graduated dropper. Subsequently, 0.1 mL of GPTMS silane coupling agent was added, and the mixture was stirred vigorously for 2 h. Under the described conditions, six samples were prepared by adding ZrO 2 sol in the amounts of 0.25 g, 0.50 g, 1.00 g, 1.50 g, 2.00 g, and 2.50 g, before stirring thoroughly for 3 h to obtain stable ZrO 2 and SiO 2 mixed sol-gel samples. Finally, the sol-gel samples were aged at room temperature.

Preparation of Mixed Sol-Gel Coating
The coating was made by means of a simple dip-lift method, in which the pre-treated PC was dipped into the mixed sol using a dip-coating machine, dipped in at 80 mm/min, lifted out, air-dried at room temperature, and then placed in a blast drying oven at 90 • C for 2 h.

Surface Modification of the Hybrid Coating
The mixed coating was modified with PFTS solution to enhance the hydrophobic properties. The PFTS solution was prepared as follows: PFTS (3.5 × 10 −3 mol) was gradually added to EtOH (1.73 mol) with stirring for 20 min at room temperature. The surface of the mixed coating was then immersed in PFTS solution for 10 min and dried at 90 • C for 1 h.

Characterizations
The surface state of the sample was observed by using a scanning electron microscope (SEM, JSM-IT200, Tokyo, Japan). The surface of the sample was coated with a thin layer of gold to improve the conductivity before the test. The chemical composition of the hybrid coating surface was analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA). The optical transmittance of the hybrid coating was analyzed using a UV-VIS-NIR spectrophotometer (UV-2600, SGLC, Shanghai, China). The surface roughness of the coatings was analyzed by means of atomic force microscopy (AFM, Bruker, Billerica, MA, USA). Water contact angle measurements of the hybrid coatings were taken using a water contact angle analyzer (JC2000, Shanghai, China) at room temperature. Static water contact angle values were measured three times and averaged. The adhesion between the substrate and the coating was evaluated using an acoustic emission test mode with a WS-2005 coating adhesion automatic scratching instrument (Lanzhou, China). The maximum load was 25 N, with a loading speed of 25 N/min. Figure 1a shows the transmittance of the coated PC that contained different contents of ZrO 2 . All the coated PC samples exhibited superior transmittance in contrast to the bare PC, and the average transmittance was increased by 3-5%. Such results could likely be attributed to the combined effect of silica with low refractive index and zirconium oxide with high refractive index, which contributes to the refractive index matching between PC and the inorganic particles, thereby minimizing the reflection loss [39,40]. As listed in Table 1, the 0.5 g ZrO 2 -SiO 2 sample has the best optical property with an average transmittance of 93.9% in the visible wavelength range of 400-800 nm, and a peak transmittance of 95.1%. Comparatively, the peak transmittance of the bare PC is 90.0%. Figure 1b shows the transmittance of the 0.5 g ZrO 2 -SiO 2 sample after modification with PFTS solution. The results indicate that the transmittance was unchanged and unaffected by the PFTS modification. As such, considering the transmittance and stability of the coating, the PFTS-modified 0.5 g ZrO 2 -SiO 2 was the optimal sample for the subsequent tests. hybrid coatings were taken using a water contact angle analyzer (JC2000, Shang China) at room temperature. Static water contact angle values were measured three ti and averaged. The adhesion between the substrate and the coating was evaluated u an acoustic emission test mode with a WS-2005 coating adhesion automatic scratch instrument (Lanzhou, China). The maximum load was 25 N, with a loading speed o N/min. Figure 1a shows the transmittance of the coated PC that contained different cont of ZrO2. All the coated PC samples exhibited superior transmittance in contrast to the PC, and the average transmittance was increased by 3-5%. Such results could likel attributed to the combined effect of silica with low refractive index and zirconium o with high refractive index, which contributes to the refractive index matching between and the inorganic particles, thereby minimizing the reflection loss [39,40]. As listed in ble 1, the 0.5 g ZrO2-SiO2 sample has the best optical property with an average trans tance of 93.9% in the visible wavelength range of 400-800 nm, and a peak transmitta of 95.1%. Comparatively, the peak transmittance of the bare PC is 90.0%. Figure 1b sh the transmittance of the 0.5 g ZrO2-SiO2 sample after modification with PFTS solution. results indicate that the transmittance was unchanged and unaffected by the PFTS m fication. As such, considering the transmittance and stability of the coating, the PF modified 0.5 g ZrO2-SiO2 was the optimal sample for the subsequent tests.     Figure 2a shows a photograph of the coated PC, evaluated on a macroscopic scale, which demonstrates the antireflective properties of the coated PC. The visual quality of the bare PC is greatly reduced under the effect of light reflection, while the transparency and visual quality of the coated PC are greatly improved, and the text under the PC can be clearly seen through the coated PC. Figure 2b,c shows the SEM images of the PFTSmodified 0.5 g ZrO 2 -SiO 2 sample. The results indicate that the overall surface of the coating was flat, and there are no obvious abnormal bumps and depressions. Figure 2d,e show the 2D and 3D morphology of the PFTS-modified 0.5 g ZrO 2 -SiO 2 sample analyzed using AFM. The ZrO 2 and SiO 2 nanoparticles were uniformly dispersed throughout most regions of the coating surface, without any instances of agglomeration or phase separation. The number-average roughness (Ra) and mean-square roughness (Rq) of the coating were determined to be 1.07 nm and 1.66 nm, respectively. Such results indicate that the coating surface was substantially flat.

2.50
92.3 95.0 Figure 2a shows a photograph of the coated PC, evaluated on a macroscopic sca which demonstrates the antireflective properties of the coated PC. The visual quality the bare PC is greatly reduced under the effect of light reflection, while the transparen and visual quality of the coated PC are greatly improved, and the text under the PC c be clearly seen through the coated PC. Figure 2b,c shows the SEM images of the PFT modified 0.5 g ZrO2-SiO2 sample. The results indicate that the overall surface of the co ing was flat, and there are no obvious abnormal bumps and depressions. Figure 2d,e sho the 2D and 3D morphology of the PFTS-modified 0.5 g ZrO2-SiO2 sample analyzed usi AFM. The ZrO2 and SiO2 nanoparticles were uniformly dispersed throughout most gions of the coating surface, without any instances of agglomeration or phase separatio The number-average roughness (Ra) and mean-square roughness (Rq) of the coating we determined to be 1.07 nm and 1.66 nm, respectively. Such results indicate that the coati surface was substantially flat.

Chemical Compositions
The FITR spectra analysis of the PFTS-modified 0.5 g ZrO2-SiO2 sample is shown Figure 3. The coating showed a characteristic peak at 3445 cm −1 , which could be ascrib to the stretching vibration of the hydroxyl group on Si-OH, indicating the presence o large number of hydroxyl groups on the surface; the peak at 1385 cm −1 corresponded the in-plane Si-CH3 bending vibration, indicating the incorporation of the MTES. The C stretching vibration produced a characteristic peak located at 1276 cm −1 , which could attributed to PFTS surface modification. The characteristic peak at 774 cm −1 could be cribed to the Si-O-Si antisymmetric stretching vibration, indicating the hydrolytic conde sation during the TEOS as well as the MTES sol-gel process. The peak at 448 cm −1 cor sponded to the bending vibration of Si-O, while the characteristic peak at 573 cm −1 cou be ascribed to the vibration of Zr-O, which also indicates the presence of ZrO2.

Chemical Compositions
The FITR spectra analysis of the PFTS-modified 0.5 g ZrO 2 -SiO 2 sample is shown in Figure 3. The coating showed a characteristic peak at 3445 cm −1 , which could be ascribed to the stretching vibration of the hydroxyl group on Si-OH, indicating the presence of a large number of hydroxyl groups on the surface; the peak at 1385 cm −1 corresponded to the in-plane Si-CH 3 bending vibration, indicating the incorporation of the MTES. The C-F stretching vibration produced a characteristic peak located at 1276 cm −1 , which could be attributed to PFTS surface modification. The characteristic peak at 774 cm −1 could be ascribed to the Si-O-Si antisymmetric stretching vibration, indicating the hydrolytic condensation during the TEOS as well as the MTES sol-gel process. The peak at 448 cm −1 corresponded to the bending vibration of Si-O, while the characteristic peak at 573 cm −1 could be ascribed to the vibration of Zr-O, which also indicates the presence of ZrO 2 .     As is well known, the presence of silanol (Si-OH) groups leads to a large number of hydroxyl groups (-OH) on the surface of the hybrid coating, and the abundant hydroxyl groups (-OH) also provide conditions for their reaction with PFTS. As shown in the high-resolution spectra presented in Figure 4b,c, the C1s peaks observed at 291.80 eV and 294.20 eV and the F1s peaks observed at 688.2 eV [41] could be attributed to the -CF 2 and -CF 3 bonds. Such findings provide clear evidence of the successful modification of PFTS onto the surface of the hybrid coating. The formation of fluorocarbon molecules leads to a low surface area, which corresponds to the low surface energy required to achieve hydrophobicity. Peak 532.74 eV of the O1s spectrum and peak 103.23 eV of the Si2p spectrum correspond to -Si-O. The XPS spectrum of Zr element has two peaks, namely Zr3d5/2 peak at 183.3 eV and Zr3d3/2 peak at 185.7 eV, which correspond to the binding energy of ZrO2 according to the standard spectrum. As shown in Figure 4e,f, the high-resolution spectra of Si2p and Zr3d successfully indicate the hydrolysis of TEOS and the success of the hybrid coating of ZrO 2 .  Figure 4a shows the XPS survey spectra of the PFTS-modified 0.5 g ZrO2-SiO2 sam the peaks of F, O, C, and Si can be clearly observed. Figure 4b-f shows the high-resolu XPS spectra of C1s, F1s, O1s, Si2p, and Zr3d. As is well known, the presence of silano OH) groups leads to a large number of hydroxyl groups (-OH) on the surface of the hy coating, and the abundant hydroxyl groups (-OH) also provide conditions for their r tion with PFTS. As shown in the high-resolution spectra presented in Figure 4b,c, the peaks observed at 291.80 eV and 294.20 eV and the F1s peaks observed at 688.2 eV could be attributed to the -CF2 and -CF3 bonds. Such findings provide clear evidenc the successful modification of PFTS onto the surface of the hybrid coating. The forma of fluorocarbon molecules leads to a low surface area, which corresponds to the low face energy required to achieve hydrophobicity. As shown in Figure 4e,f, the high-res tion spectra of Si2p and Zr3d successfully indicate the hydrolysis of TEOS and the suc of the hybrid coating of ZrO2.   Figure 5a shows the WCA of the coating. Due to the presence of MTES in the coa several hydrophobic methyl groups will exist on the surface of the coating, as show Figure 5b. The WCA of the coating increased from 67° to 83° because of the hydroph methyl groups. There were a large number of hydroxyl groups on the surface of the ing, which provided the reaction center for the surface modification. During the mo cation process of the PFTS solution, the PFTS hydrolyzed and condensed with the droxyl group on the surface of the coating to form a low surface energy film, which proves the hydrophobicity (113°) of the coating, as shown in Figure 5c. As shown in Fi 6a-c, the ferric oxide powder was spread on the coating surface. The water droplets ro on the surface and picked up most of the solid particles to clean the surface, the demonstrating that the PFTS-modified coating possesses a certain self-cleaning abilit  Figure 5a shows the WCA of the coating. Due to the presence of MTES in the coating, several hydrophobic methyl groups will exist on the surface of the coating, as shown in Figure 5b. The WCA of the coating increased from 67 • to 83 • because of the hydrophobic methyl groups. There were a large number of hydroxyl groups on the surface of the coating, which provided the reaction center for the surface modification. During the modification process of the PFTS solution, the PFTS hydrolyzed and condensed with the hydroxyl group on the surface of the coating to form a low surface energy film, which improves the hydrophobicity (113 • ) of the coating, as shown in Figure 5c. As shown in Figure 6a-c, the ferric oxide powder was spread on the coating surface. The water droplets rolled on the surface and picked up most of the solid particles to clean the surface, thereby demonstrating that the PFTS-modified coating possesses a certain self-cleaning ability. cation process of the PFTS solution, the PFTS hydrolyzed and condensed with the hydroxyl group on the surface of the coating to form a low surface energy film, which improves the hydrophobicity (113°) of the coating, as shown in Figure 5c. As shown in Figure  6a-c, the ferric oxide powder was spread on the coating surface. The water droplets rolled on the surface and picked up most of the solid particles to clean the surface, thereby demonstrating that the PFTS-modified coating possesses a certain self-cleaning ability.   Figure 7a,b shows the schematic diagram of the sandpaper abrasion test and the variation in WCA on the coated PC after 20 cycles of sandpaper abrasion. As shown in Figure  7b, the coating still had good hydrophobic performance after 20 cycles of sandpaper abrasion, and the WCA was over 100°. Figure 7c shows the SEM image of the coating after 20 cycles of sandpaper abrasion. An observation can be made that the coating suffered some damage to the surface after 20 cycles of abrasion, but there was no large cracking or peeling of the coating. At the same time, the PC surface was protected to a certain extent, which   Figure 7a,b shows the schematic diagram of the sandpaper abrasion test and the variation in WCA on the coated PC after 20 cycles of sandpaper abrasion. As shown in Figure  7b, the coating still had good hydrophobic performance after 20 cycles of sandpaper abrasion, and the WCA was over 100°. Figure 7c shows the SEM image of the coating after 20 cycles of sandpaper abrasion. An observation can be made that the coating suffered some damage to the surface after 20 cycles of abrasion, but there was no large cracking or peeling of the coating. At the same time, the PC surface was protected to a certain extent, which  Figure 7b, the coating still had good hydrophobic performance after 20 cycles of sandpaper abrasion, and the WCA was over 100 • . Figure 7c shows the SEM image of the coating after

Sand Hit Test
A sand hit test was conducted on the coated PC, and a sand grain with a parti of about 1 mm was used to hit the coated PC from a height of 30 cm in free fall, as in Figure 8a. Figure 8b shows the transmittance of the PFTS-modified 0.5 g ZrO2-SiO ple coated PC after the sand hit test. An observation can be made that although th mittance was reduced after the sand hit test, the transmittance was still higher th of the bare PC. Figure 8c,d shows the WCA of the PFTS-modified sample after th hit test. An observation can be made that the WCA of the sample did not change g indicating that the coating still had good hydrophobicity. As shown in Figure 8e, peeling phenomenon was observed on the coating; however, this did not result in cant cracking or peeling of the coating. Such findings suggest that the coating ex certain degree of stability, robustness, and impact resistance.

Sand Hit Test
A sand hit test was conducted on the coated PC, and a sand grain with a particle size of about 1 mm was used to hit the coated PC from a height of 30 cm in free fall, as shown in Figure 8a. Figure 8b shows the transmittance of the PFTS-modified 0.5 g ZrO 2 -SiO 2 sample coated PC after the sand hit test. An observation can be made that although the transmittance was reduced after the sand hit test, the transmittance was still higher than that of the bare PC. Figure 8c,d shows the WCA of the PFTS-modified sample after the sand hit test. An observation can be made that the WCA of the sample did not change greatly, indicating that the coating still had good hydrophobicity. As shown in Figure 8e, a local peeling phenomenon was observed on the coating; however, this did not result in significant cracking or peeling of the coating. Such findings suggest that the coating exhibits a certain degree of stability, robustness, and impact resistance.

Adhesion Scratch Test
The bonding strength of the coating and the substrate was tested using a coating ad hesion auto-scratcher. Figure 9 shows the coating substrate bonding strength of the coat ing with the acoustic emission signal. As shown in Figure 9, the acoustic emission signa did not change significantly throughout the experiment, indicating that the coating has sufficient bonding strength with the substrate.

Adhesion Scratch Test
The bonding strength of the coating and the substrate was tested using a coating adhesion auto-scratcher. Figure 9 shows the coating substrate bonding strength of the coating with the acoustic emission signal. As shown in Figure 9, the acoustic emission signal did not change significantly throughout the experiment, indicating that the coating has sufficient bonding strength with the substrate.

Water-Drop Impact Test and Stability Test
To further investigate the mechanical stability of the hydrophobic coating surface, water-drop impact simulation experiments were conducted on the PFTS-modified 0.5 g ZrO2-SiO2 sample. The schematic diagram of the water-drop impact test is shown in Figure 10a. The coating maintained hydrophobicity after 54,000 water droplet impacts, indicating that the coating could withstand the test and has good resistance to water droplet impacts. Figure 10b shows the effect of exposure time on the WCA of the surface. The PFTS-modified 0.5 g ZrO2-SiO2 sample coated PC was exposed to air to check its stability under natural aging conditions. After 8 weeks of exposure to air, the WCA of the coating did not change significantly, indicating that the hydrophobic coating has good durability and stability.

Conclusions
In conclusion, a mixture of zirconium dioxide(ZrO2) sol and methyltriethoxysilanemodified silica(SiO2) sol-gel was successfully dip-coated on PC substrates to form ZrO2enhanced SiO2 hybrid coatings. Additionally, a solution containing PFTS was employed for surface modification as demonstrated by both FTIR, where characteristic peaks corresponding to SiO2 and ZrO2 could be observed, and XPS, which indicated the presence and

Water-Drop Impact Test and Stability Test
To further investigate the mechanical stability of the hydrophobic coating surface, water-drop impact simulation experiments were conducted on the PFTS-modified 0.5 g ZrO 2 -SiO 2 sample. The schematic diagram of the water-drop impact test is shown in Figure 10a. The coating maintained hydrophobicity after 54,000 water droplet impacts, indicating that the coating could withstand the test and has good resistance to water droplet impacts. Figure 10b shows the effect of exposure time on the WCA of the surface. The PFTS-modified 0.5 g ZrO 2 -SiO 2 sample coated PC was exposed to air to check its stability under natural aging conditions. After 8 weeks of exposure to air, the WCA of the coating did not change significantly, indicating that the hydrophobic coating has good durability and stability.

Water-Drop Impact Test and Stability Test
To further investigate the mechanical stability of the hydrophobic coating surf water-drop impact simulation experiments were conducted on the PFTS-modified 0 ZrO2-SiO2 sample. The schematic diagram of the water-drop impact test is shown in ure 10a. The coating maintained hydrophobicity after 54,000 water droplet impacts, i cating that the coating could withstand the test and has good resistance to water dro impacts. Figure 10b shows the effect of exposure time on the WCA of the surface. PFTS-modified 0.5 g ZrO2-SiO2 sample coated PC was exposed to air to check its stab under natural aging conditions. After 8 weeks of exposure to air, the WCA of the coa did not change significantly, indicating that the hydrophobic coating has good durab and stability.

Conclusions
In conclusion, a mixture of zirconium dioxide(ZrO2) sol and methyltriethoxysil modified silica(SiO2) sol-gel was successfully dip-coated on PC substrates to form Z enhanced SiO2 hybrid coatings. Additionally, a solution containing PFTS was emplo for surface modification as demonstrated by both FTIR, where characteristic peaks co sponding to SiO2 and ZrO2 could be observed, and XPS, which indicated the presence

Conclusions
In conclusion, a mixture of zirconium dioxide(ZrO 2 ) sol and methyltriethoxysilanemodified silica(SiO 2 ) sol-gel was successfully dip-coated on PC substrates to form ZrO 2enhanced SiO 2 hybrid coatings. Additionally, a solution containing PFTS was employed for surface modification as demonstrated by both FTIR, where characteristic peaks corre-sponding to SiO 2 and ZrO 2 could be observed, and XPS, which indicated the presence and correct valence states of the elements. The PFTS-modified ZrO 2 -SiO 2 hybrid coatings show high transparency (average transmittance up to 93.9% and peak transmittance up to 95.1% at 700 nm, respectively), hydrophobicity (WCA, 113 • ), and good mechanical strength. After a series of tests, the coating could still maintain its properties. Therefore, the belief of the present authors is that the PFTS-modified ZrO 2 -SiO 2 hybrid coatings can contribute to the surface functionalization of other polymer substrates and have promising applications in optical lenses, automotive windows, and self-cleaning antifouling.