Fabrication and Properties of Hydrophobically Modified ZnO–SiO2 Nanocomposite with Polysiloxane

Studies on the wettability properties of solid surfaces are very important in any of the scientific and industrial fields. The most common principle for a superhydrophobic self-cleaning surface is the lotus effect induced by surface roughness. In this study, silicate compounds have been used to produce hydrophobic surfaces. In this way, firstly, SiO2–ZnO nanocomposite was produced, and then vinyl trimethoxy silane was used to increase the water contact angle (WCA). The structure and morphology of nanocomposites were investigated by infrared spectroscopy (FT-IR), X-ray diffraction pattern (XRD), scanning electron microscopy (SEM) and energy-dispersion spectrometer (EDS) analyses. The thermal stability of nanocomposite coatings was examined by thermogravimetric analysis (TGA). In order to investigate the wetting properties, the surface roughness was measured using an atomic force microscope (AFM), where the subsurface roughness average was obtained at 37.79 nm. The WCA of the coated surfaces with ZnO–SiO2 and ZnO–SiO2 @Polysiloxane nanocomposites were measured at 69 and 160°, respectively, indicating the surface superhydrophobic properties of ZnO–SiO2 @Polysiloxane nanocomposites. Finally, superhydrophobic properties of nanocomposites were investigated by the Cassie-Baxter model. The value of the f2 parameter in the model was estimated at 0.9556. This means that air occupies about 95.56% of the contact area between the water droplet and nano-coating, which is responsible for the superhydrophobic property of the surface.


Introduction
The superhydrophobic property is inspired by nature. In the 1990s, with the advent of electron microscopes, more detailed studies on the structure of materials became possible. Earlier in the decade, two German scientists studied 200 species of hydrophobic plants, including the lotus, and discovered the self-cleaning properties of the plant's leaves [1,2]. Their studies reported that the lotus leaf had a very rough surface structure in which protrusions and depressions (including cell wall structures and cuticle folds) with a height of about 3-10 µm coated with hydrophobic nanoparticles. Such a structure (presence of nanoscale bumps on rough microstructures) is called a hierarchical structure. In addition to the hierarchical structure, the lotus leaf hydrophobic is also related to its low surface free energy. In fact, the combination of surface roughness and low surface free energy has caused this property in lotus flowers because the hierarchical structure (protrusions and depressions) reduces the area of contact with water and hydrophobic nanoparticles reduce surface free energy. As a result, water cannot wet this type of surface, and therefore water droplets on the surface become almost spherical and, in their path, they remove impurities from the surface [3]. Therefore, the superhydrophobic surface is inspired by nature due to attractive properties such as waterproofing, self-cleaning and reduction of biological powder [4][5][6], and also the strong potential for application in a wide range of industries, including automotive, construction, food packaging and textiles [7], has attracted much scientific and technological attention in the field of research and development of hydrophobic coatings. Such surfaces usually show a water contact angle of more than 150° and a rolling angle of less than 10° [8,9]. Most preparation of such gripping surfaces focuses on generating a nano/microscale hierarchical roughness on low-energy surfaces or coating low-energy materials on nano/microstructural surfaces [10][11][12]. One of the methods to reduce surface energy is using polydimethylsiloxane (PDMS) and polyhydromethylsiloxane (PHMS) [13,14], superhydrophobic copolymer poly-(styrene)-b-poly (dimethylsiloxane) (PSb-PDMS) [15], organic materials such as alkylene, polycarbonate and polyamide [16][17][18] and inorganic materials such TiO 2 [19]. Another efficient method to make a superhydrophobic surface is applying biopolymers directly to the surface [20]. For example, Polyethylene glycol and 1,1,2,2tetrahydro perfluorodecyl acrylate, among others, have been grafted to cellulose to induce hydrophobicity [21].
Using nano dimension particles to enhance the surface roughness of a hydrophobic surface should be feasible. ZnO nanoparticles have been primarily studied lately due to their many notable physical and chemical properties. However, an appropriate surfactant is essential to obtain a good dispersion and weak interfacial adhesion [22,23]. Surfactants, including reactive functional groups such as silane coupling agents [24] and polymers such as fluorinated polysiloxane, can improve the hydrophobic properties of ZnO [22,25].
Silica nanoparticles have been widely used to produce films with controlled roughness using various methods such as rotating coatings, spray coatings, etc., and the superhydrophobic properties are achieved by surface modification by fluoroalkyl silane compounds [26,27]. Bhagat et al. Showed that silica-based superhydrophobic coatings could be applied to stainless steel using a practical and cost-effective method such as the immersion method [28]. Fluoroalkyltrimethoxysilane (FAS) molecules often perform modification of surfaces with silanol groups [29,30]. This function combines with roughness and creates hydrophobicity. Silica nanoparticles with a diameter of approximately 10 nanometers are obtained by SiCl 4 pyrolysis in the presence of oxygen and hydrogen gas. Silica nanoparticles agglomerate at high temperatures, which increases the grain-to-mass ratio (greater than 100 m 2 g −1 ), which can be adjusted by the amount of agglomeration [31].
Polydimethylsiloxane is a reasonable possibility for the expansion of composite coating films, as its essential chain ((Si-O-Si)n) has excellent bond energy and significant bond angle, which supplies beneficent thermal stability and elasticity, in addition to hydrophobic characteristics [32].
ZnO is structurally similar to silica due to its Zn-O-Zn bond, so it is expected to be easily dispersed into the SiO 2 matrix and established the ZnO-SiO 2 nanocomposite [33][34][35]. The different methods were applied to synthesize superhydrophobic ZnO-SiO 2 nanocomposites such as hydrothermal, sono chemical, co-precipitation, and spray pyrolysis. However, the main challenge of these methods is the phenomenon of agglomeration. The high surface energy of ZnO is one of the essential factors causes of agglomeration [36][37][38][39].
Synthesis and the investigation of superhydrophobic properties of ZnO-SiO 2 @Polysiloxane nanocomposite are the leading purpose of this research as there is no evidence that ternary composite of ZnO-SiO 2 @ poly vinyl trimethoxy silane (polysiloxane) as superhydrophobic coatings has been reported.
Moreover, we have used the economic method for synthesizing SiO 2 , polysiloxane, ZnO-SiO 2 and ZnO-SiO 2 @Polysiloxane nanocomposites. For this purpose, tetraethyl orthosilicate, zinc acetate dehydrated, and vinyl trimethoxy silane were used as precursors receptively. After heat treatment, the zinc acetate complex can be decomposed to ZnO nano crystallites and reinforced by the SiO 2 superhydrophobic. These films can adhere to the substrate through Zn-O-Si linkages. It is also noteworthy that the synthesized ZnO can be homogeneously distributed within the SiO 2 matrix. It is expected that the hydrophobic properties ZnO-SiO 2 @Polysiloxane will increase significantly compare with ZnO-SiO 2 nanocomposite. The nanocomposites were characterized by FTIR, XRD, TGA, DLS, SEM, AFM, and water repellence abilities were investigated by WCA analyses.

Synthesis of SiO 2
To produce SiO 2 powder, 4 ml of TEOS was first dissolved in 50 ml of acetic acid with distilled water (1:1) for 5 min by a magnetic stirrer. Then the mixture of ethanol and ammonia (2:1) was added dropwise to the above solution. In the next step, 50 ml of water and ethanol solution in a ratio of (1:1) was added as hydrolysis solution. The suspension was stirred in an ultrasonic bath for 2 h. In order to evaporate the solvent, the resulting solution was dried at 150 °C for 2 h, and finally, the obtained gel was calcinated at 550 °C for 3 h.

Synthesis of ZnO-SiO 2 Nanocomposite
To synthesize the ZnO-SiO 2 composite, 0.3 g of SiO 2 was dispersed in 50 ml of acetone with a ratio of (1:1) by sonication method. Then, 60 ml of zinc acetate dehydrated (0.5 M) and 60 ml of sodium hydroxide (0.5 M) were added to the suspension and placed in a Teflon autoclave at 150-180 °C for 3 h. The fabricated product was dried at 140 °C for 3 h in the next step. Finally, the ZnO-SiO 2 was prepared after calcination at 650 °C for 2 h.

Synthesis of Polysiloxane
To produce polysiloxane polymer, 8 ml of vinyl trimethoxy silane (VTMS) in 50 ml of pure ethanol was dissolved at room temperature for 1 h by magnetic stirring. The pH of the solution was adjusted to 6, and the stirring was continued for 4 h, then the pH of the solution was adjusted to 9, and the process was continued for 6 h. The obtained gel was filtered, washed with distilled water, and dried in a vacuum oven at 60 °C for 12 h.

Synthesis of ZnO-SiO 2 @Polysiloxane Nanocomposite
In this process, first 0.5 g of synthesized ZnO-SiO 2 composite was homogenized in 70 ml of distilled water. Then 2 g of synthesized Polysiloxane was added to the suspension and dissolved with a magnetic stirrer for 10 min. After obtaining a suspension of the main precursors, the sample was placed in an autoclave at 110 °C for 4 h. In the last step, centrifugation and washing with distilled water and pure ethanol were performed several times, and the final product was dried in an oven at 190 °C for 12 h.

Deposition of ZnO-SiO 2 @Polysiloxane
For deposition of ZnO-SiO 2 @polysiloxane, a mixture of polydimethylsiloxane precursor and hardener is prepared in a ratio of 1:10, and the resulting mixture is dissolved in ethanol on a magnetic stirrer. This mixture is prepared with a concentration of 0.04 g/l. The resulting mixture was ultrasonicated for 16 min. To create a hydrophobic coating on the glass surface, the glass surface was washed with acetone, ethanol and water and dried under nitrogen. After preparing the substrate, 5 mg of the synthesized ZnO-SiO 2 @Polysiloxane was added to the polydimethylsiloxane suspension.
In the next step, to remove large macroscopic particles of the suspension, it was filtered, poured into a sprayer and sprayed on the desired glass. Finally, it was exposed to 50 °C for 5 min. Figure 1 indicates the schematic illustration of deposition of ZnO-SiO 2 @Polysiloxane nanocomposite on glass substrate.

Characterization
The FT-IR of the samples was performed using an FT-IR spectrophotometer (Tensor 27, Broker, Germany) in the range of 4000-400 cm −1 wave number. The XRD pattern was acquired with Cu Kα radiation (λ = 1.5418 Å) using an X-ray diffractometer (D8 Advance, Broker, Germany) at angles of 2θ (from 3° to 80°) with steps of 0.1°, the stoppage time of 0.1 sec per step, and the room temperature. In this investigation, an SEM (the TeScan-Mira III model, Czechia) equipped with an EDS and AFM (ENTEGRA AFM NT-MDT model) were used to assess the levels and quantitatively investigate the chemical surface structure of the material. The WCA test measured hydrophilicity with a precise goniometer (KRÜSS G10, GmbH Co., Hamburg, Germany). TGA curves were obtained using the NETZSCH TG 209 F1 Libra instrument. The applied temperature range was 25-620 °C, with a ramp rate of 10 °C min −1 . The hydrodynamic size of the nanocrystals in ethanol and water was determined using the dynamic light scattering (DLS) technique with a Zetasizer Nano ZS90 (Malvern Instruments, Worcestershire, UK). All the measurements were performed at room temperature, at a concentration of 100 µg/ml, sonicating each sample for 10 min before the acquisition. The WCA of the surface was measured with an optical contact angle meter CAM 200 (KRUSS G10 Instrument Ltd, Germany).

Characteristics of the Nanocomposite Coatings
The FT-IR was used to confirm the existence of functional groups and bonds or linkages formed between silanol groups, hydroxyl groups, zinc, siloxane and other functional groups (Fig. 2). According to the spectrum obtained from ZnO-SiO 2 nanocomposite (Fig. 2), two important peaks in the range of 564 and 895 cm −1 have been observed, which are related to Zn-O and Si-O-Si, respectively [48]. It is clear that the vibration peak located at 470 cm −1 is broadened due to the combination of stretching vibration of Zn-O bands and bending vibration of Si-O-Si bands. During the decomposition of zinc acetate, it reacts with some residual silanol (Si-OH) groups, and some decomposed Zn species bonded with silica (Zn-O-Si). It is shown as the shoulder at 975 cm −1 [49]. After synthesis of the ZnO-SiO 2 @polysiloxan composite, new peaks appear at about 754 and 956 cm −1 that they, respectively, are Si-C stretching and Zn-O-Si band. In addition, there are peaks around 1000-1040 cm −1 , which belong to Si-O-Si stretching band. The peak of around 1120 cm −1 is related to the Si-O stretching band, and the peaks at 1276-1400 cm −1 and 1600 cm −1 are related to Si-CH 3 and Si-OH, respectively [50,51]. The obtained results represent of ZnO-SiO 2 composite in the matrix of polysiloxane. Figure 3 shows the X-ray diffraction pattern of SiO 2 , ZnO-SiO 2 composite, polysiloxane and ZnO-SiO 2 @Polysiloxane nanocomposite. The SiO 2 powder shows a broad peak of around 20-23° due to the amorphous nature of SiO 2 in the Hydrothermal process. On comparing our obtained XRD spectrum from the JCPDS Card No. 361451, the ZnO-SiO 2 nanocomposite indicates three peaks of outstanding at 31.76°, 36.25°, and 38.78° correspond to ZnO (100), ZnO (002), and ZnO (101), respectively. Notably, the crystallinity improves progressively with the addition of ZnO-SiO 2 . However, there are no characteristic peaks of SiO 2 in the ZnO-SiO 2 composite, indicating the total dispersion of SiO 2 particles into the ZnO matrix Also, the SiO 2 particles composed of the ZnO nanoparticles were in an amorphous state [37,52]. Moreover, these results implied that adding silica particles does not change the crystal structure of the ZnO particles [53]. The polysiloxane shows a broad peak around 22.1° due to the diffraction occurring at the polymer's interplanar spacing, indicating polysiloxane's amorphous nature [54]. For the ZnO-SiO 2 @polysiloxan nanocomposite, the ZnO peaks are seen in the XRD pattern but with lower intensities than those of peaks observed for the ZnO-SiO 2 nanocomposite. Scherer's formular was used to determine the crystallite size of the nanoparticles for the most intense as shown below: [55] where D is the crystallite size measured in (nm), β is the full width of the diffraction at half of the maximum intensity measured in radians, θ is the Bragg's angle and λ is X-RAY wavelength of CuKα (0.154 nm). The crystallite size of ZnO was obtained about 37 nm. Figure 4 shows the SEM image of SiO 2 , ZnO-SiO 2 , polysiloxane, and ZnO-SiO 2 @polysiloxan. Figure 4a and b illustrates the SEM morphology of SiO 2 particles. In the present research, the sample of SiO 2 was fabricated by hydrothermal (1) D = 0.94 COS Fig. 2 The FT-IR of ZnO-SiO 2 and ZnO-SiO 2 @polysiloxane nanocomposites Fig. 3 The XRD patterns of SiO 2 , ZnO-SiO 2 , polysiloxane, and ZnO-SiO 2 @polysiloxane nanocomposite methods. As indicated in Fig. 4a, silica particles of dried at 150 °C, agglomerate and form discrete structures. It is observed that the SiO 2 particles calcinated at 550 °C has a spherical morphology and is regular in shape. Also, the SiO 2 particles have a smooth surface with an average particle size of 500 nm. As demonstrated in Fig. 4c and d, the surface of the ZnO@SiO 2 nanocomposite is much rougher than that of SiO 2 , which is due to the surface coating of SiO 2 with ZnO nanoparticles [49]. Moreover, poor aggregation was observed, which indicated the homogenous distribution of the ZnO nanoparticles on the surface of SiO 2 powder. In Fig. 4d, nanometric particles of ZnO are shown with white colors. The particle size distribution result by image analyzer software indicated that the crystallite size average of ZnO nanoparticles is estimated at about 23.18 ± 2.4 nm (Fig. 4e). It seems that the morphology of the ZnO@SiO 2 composite is spherical-like. Figure 4f demonstrates the morphology of the polysiloxane spherical particles with an average particle size of 300 nm. Figure 4g and h show the SEM images ZnO-SiO 2 @polysiloxane nanocomposite. The particles with hierarchical shape structures and micron size were considered to appear from the agglomeration of the polysiloxane particles on the surface. The diameter of the smaller and To study the distribution of Si, Zn, and O in the ZnO-SiO 2 and ZnO-SiO 2 @polysiloxane nanocomposites, the EDX spectrum and elemental mapping analysis were performed (Figs. 5 and 6). The EDX analysis verified the existence of Si, Zn, and O in nanocomposites (Figs. 5a 6a). However, Si atomic percentage in ZnO-SiO 2 @polysiloxane nanocomposite was more than ZnO-SiO 2 . It can be concluded that the polysiloxane polymer is properly combined with the ZnO-SiO 2 nanocomposite. Moreover, the Zn density in the elemental distribution of ZnO-SiO 2 @polysiloxane indicates the proper distribution on the surface of the siloxane polymer (Fig. 6b).

DLS
The DLS proves the colloidal stability of nanoparticles into nanocomposites. Figure 7 indicates the DLS diagrams of the obtained ZnO-SiO 2 and ZnO-SiO 2 @polysiloxane nanocomposites dispersions. According to Fig. 7a, two types of peaks were observed in the ZnO-SiO 2 nanocomposite. One is related to ZnO nanoparticles, and the other is associated with SiO 2 particles in the ZnO-SiO 2 . The calculated average size of ZnO nanoparticles was about 37.5 nm; for SiO 2 particles, the size determined was at about 108.8 nm. In the case of ZnO-SiO 2 @polysiloxan nanocomposite particles, the size distribution is bimodal, indicating the presence of two populations, with the average size at about 37.2 and 1122 nm Fig. 7b. The extensive range of ZnO-SiO 2 @polysiloxane composite nanoparticle size is due to the formation of the aggregated particles. The polysiloxane can be masked by the SiO 2 and ZnO particles. Particles size were indicated by SEM images previously ( Fig. 4f and g).

Thermal Stability Analysis
TGA was used to characterize the thermal properties of polysiloxane, ZnO-SiO 2 and ZnO-SiO 2 @polysiloxan nanocomposites heated from room temperature up to 622 °C, which are shown in Fig. 8. The polysiloxane loses weight of 5 wt% below 400 °C and 6.3 wt% between 400 and 622 °C. The total weight loss is 11.3 wt% by 622 °C, while the mass loss of ZnO-SiO 2 nanocomposite was very slow, and the final mass remained about 95%. However, ZnO-SiO 2 @polysiloxane nanocomposite was decomposed at 475 °C, resulting in a mass loss at about 9 wt% up to 622 °C, which was mainly caused by the decomposition of vinyl end-groups and longchain alkyl on its surface [43,56]. These results indicate that ZnO and SiO 2 properly combined with polysiloxane polymer as illustrated at FT-IR, SEM and elemental mapping analysis

AFM
AFM in three-dimensional mode was used to evaluate the surface roughness. As shown in Fig. 9a, the surface roughness of ZnO-SiO 2 @polysiloxane nanocomposites was found on the glass surface, which was higher in some areas. However, the surface roughness is such that the smooth part is not visible. It can reduce the contact surface between the substrate and water droplets. So, as a result, the surface hydrophobicity will increase on the rough surface. Figure 9b shows the thickness difference diagram of ZnO-SiO 2 @polysiloxane coating. According to this diagram, the average roughness was estimated at about 37.79 nm [57,58].

WCA
To determine the degree of hydrophobicity of the surface after coating of ZnO-SiO 2 and ZnO-SiO 2 @Polysiloxane nanocomposites, the WCA between the water drop and the coated surface has been investigated ( Fig. 10a and b), respectively. According to the Cassie and Baxter model [59], if the surface roughness increases and become closer to each other, the air trapped in the pores prevents water droplets' penetration into the cavities. Nevertheless, if the size of the water droplets is smaller or equal to the pores, the possibility of penetrating the droplets into the cavities increases. Therefore, one of the methods to make hydrophobicity is to create a rough surface with small cavities. On the other hand, polysiloxane, as a surface energy-reducing group, diminished the interactions between the surface and water, resulting in increased hydrophobicity. [60,61] Therefore, due to the roughness on each of the surfaces and the addition of polysiloxane polymer as a surface energy reducing agent to the ZnO-SiO 2 nanocomposite, it can be seen that the laminated sample with ZnO-SiO 2 @ Polysiloxane nanocomposite has a greater CA, compared to the ZnO-SiO 2 composite. As indicated in Fig. 10, after the addition of polysiloxane to the ZnO-SiO 2 , the WCA increased from 69° to 160°. The SEM images indicate many gaps between ZnO-SiO 2 @Polysiloxane particles (Fig. 4f) compared with ZnO-SiO 2 nanocomposite (Fig. 4d). Much air can be trapped among ZnO-SiO 2 @Polysiloxane particles when water drop was dropped on the sample surface. Moreover, the DLS peaks showed the ZnO-SiO 2 @polysiloxane, the large agglomerates (Fig. 7b), which these aggregates cause gaps between nanocomposite components. Generally, the superhydrophobic surface can be explained by the Cassie-Baxter model [59], where the ZnO-SiO 2 @Polysiloxane superhydrophobic surface is regarded as a porous medium composed of air pockets. The apparent WCA is formulated as: in which A is the water contact angle on the glass surface coated with ZnO-SiO 2 nanocomposite ( A =69 • ), f 1 and f 2 are the fraction of the solid/water interface and that of air/ water interface at the rough surface, respectively (f 1 + f 2 = 1), D is the water contact angle on the super hydrophobic surface of ZnO-SiO 2 @Polysiloxane nanocomposite ( D = 160 • ). The value of f 1 and f 2 was calculated to be 0.0443 and 0.9556, respectively. This means that air occupies about 95.56% of the contact area between the water droplet and nanostructures, which is responsible for the superhydrophobic property of the surface. Qing et al. reported a facile method to prepare superhydrophobic fluorinated polysiloxane/ZnO nanocomposite coatings. According to Cassie-Baxter model, f 1 and f 2 are estimated 0.0628 and 0.9372, respectively [22]. A superhydrophobic nano-silica coating was fabricated on a glass surface by Su et al. The value of f 2 was estimated to be about 0.9705 [26]. In other research, the values of f 1 and D for surfaces prepared by colloidal silica nanoparticles coated with fluoroalkyl groups were obtained at 0.15 and 150 • , respectively [27]. Li et al. fabricated SiO 2 shell on ZnO nanoflake arrays with pulsed laser deposition (PLD) process and hydrophobized with hexadecyltrimethoxysilane (HDTM). The WCA on the surface of ZnO nanoflake arrays modified by HDTM was calculated at about 165.6°. Moreover, f 1 estimated about 0.08 for ZnO nanoflake arrays. The Cassie equation describes their superhydrophobic surface formation mechanism [42]. Based on the investigation results stated above, it can be concluded that our nanocomposite coating in this research can be considered super hydrophobic.

Conclusion
In brief, we have developed an easy, facile, and low-cost method for fabricating superhydrophobic coating by combining ZnO-SiO 2 nanocomposite particles with polysiloxane polymer on the glass substrates. The results of spectroscopy, EDS mapping and TGA analyses showed that the ZnO-SiO 2 @Polysiloxane nanocomposite was excellent manufactured. A superhydrophobic surface (WCA = 160) can be obtained when the mass ratio of ZnO-SiO 2 nanocomposite to polysiloxane was 1:4. The ZnO-SiO 2 @ Polysiloxane nanocomposite SEM morphology indicated that the combination of the ZnO nanoparticles, SiO 2 and polysiloxane polymer, with different dimensions and shapes, allow suitable gaps between ZnO-SiO 2 @Polysiloxane particles and consequently the increase of roughness, WCA value and hydrophobicity. Thus, the as-fabricated ZnO-SiO 2 @Polysiloxane nanocomposite coating is expected to have hydrophobicity practical applications on the surface of materials.