Superhydrophobic SiO₂/Trimethylchlorosilane Coating for Self-Cleaning Application of Construction Materials

Abstract

This study has demonstrated, for the first time, the potential application of coatings to protect bricks or architectures against detrimental atmospheric effects via a self-cleaning approach. In this research, a facile fabrication method was developed to produce amorphous SiO₂ particles and their hierarchical structures via applying trimethylchlorosilane (TMCS). They were fully characterized by various surface analytic tools, including a goniometer, SEM, AFM, zeta sizer, and a spectroscopic technique (FTIR), and then applied as super hydrophobic coatings on glass and sand. The characterization results revealed that the SiO₂ particles are amorphous, quasi-spherical particles with an average diameter of 250–300 nm, and the hierarchical structures in the film were assembled from building blocks of SiO₂ and TMCS. The wettability of the films can be controlled by changing the pH of the SiO₂/TCMS dispersion. A super hydrophobic surface with a water contact angle of 165° ± 1° was achieved at the isoelectric point of the films. The obtained translucent super hydrophobic SiO₂/TMCS coatings show good self-cleaning performances for glass and sand as construction materials. This study indicated that the superhydrophobic coatings may have potential applications in the protection of buildings and construction architectures in the future.

1. Introduction

In recent times, pollutants from climate and environmental change are causing serious damage to the outdoor surfaces of buildings. These contaminants not only bring damage to the surface, but also remain on the surface of the building [1,2,3,4,5]. If these surfaces are not cleaned over time, they will depreciate the wear resistance of the building and affect the quality of the surface. Cleaning the exterior of a building is not only expensive, but also a very perilous practice. Developing multifunctional super hydrophobic materials is attractive to many researchers, since they hold surface self-cleaning and protection properties, and reduce costs. It is well known that superhydrophobic materials have excellent self-cleaning properties [6,7,8,9,10,11,12], and their practical applications include their use as waterproof, anti-fouling, anti-corrosion [13] and anti-rust materials [14,15]. Among them, corrosion resistance is one of the unique properties of superhydrophobic materials [16,17]. Furthermore, water contact angle (WCA) and roll-off angle (SA) are the two main important parameters used to characterize their superhydrophobic property [18]. A superhydrophobic surface possesses low surface energy and comprises a hierarchical rough structure [19]. As both WCA and SA are greater than 150° and less than 10°, respectively, they become ideal candidates for the self-cleaning application. Therefore, both a high static WCA and a low SA are major requirements for self-cleaning materials [18,20,21,22]. On the self-cleaning superhydrophobic surface, water droplets can easily roll and simultaneously carry away dust/dirt attached to the surface. Moreover, superhydrophobic materials have been developed from various materials, such as carbon nano-structures [22,23,24], silica-based nano-composites [25,26,27,28], fluorinated silanes [29,30,31], siloxane polymers [32,33,34] and two-dimensional (2D) transition metal dichalcogenides [35]. Significant efforts have been made in developing self-cleaning superhydrophobic surfaces [19].

However, protection of concrete is still a challenging task in the structural engineering field, since the desired coating is not only substrate-dependent, but also the cost has to be taken into consideration. The synthesis process of some organic particles is complex, time-consuming, harsh and costly, and can only be prepared in the laboratory [36]. In addition, to produce the self-cleaning hydrophobic coating, some superhydrophobic coatings need further baking and heating treatment after coating on the substrate surface [37,38].

Thus, the development of silica-based coatings is interesting to study since they are cost effective, easy processable materials and they have unique properties, such as excellent chemical inertness, high thermal resistance, strong mechanical strength, and tunable optical properties.

In this study, for the first time, we reported a facile fabrication method of a silica-based superhydrophobic coating system on glass and sand substrates using trimethylcholorsilane-modified SiO₂ nanoparticles and tested self-cleaning construction materials. The experimental results showed that the as-prepared superhydrophobic SiO₂/trimethylchlorosilane (TMCS) coating had a contact angle up to 165°, with a roll-off angle of less than 10° and demonstrated self-cleaning properties.

2. Materials and Methods

2.1. Materials

Tetraethoxysilane (TEOS, 98%), ethanol (96% purity), trimethylchlorosilane (TMCS, 99%), ammonium hydroxide solution (28% NH₃ in H₂O, 99.99%), and DI water were used in all the experiments. All the chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA) and were used without further purification.

2.2. Synthesis of SiO₂ Nanoparticles

As in a typical synthesis process, 30 mL of ethanol was mixed with 10 mL of DI water. Later on, 2 mL of TEOS was added drop-wise. The obtained solution was heated at 30 °C and vigorously stirred (700 rpm) for 5 min. Then, a different amount of 0.5 M NH₄OH (depending on the pH level) was added and left to react for 1 h to form white turbid silica solution. A schematic illustration of the process is shown in Figure 1.

2.3. Preparation of Superhydrophobic Surfaces

An amount of 25 mL of as-prepared silica solution and 5 mL of TMCS in a volume ratio of 5:1 were stirred at 700 rpm for 5 h to form the colloidal dispersion. The initial pH of the dispersion was 2.8 and different pH levels were achieved using 0.5 M NH₄OH. The dispersion with a pH range from 2 to 12 was used for the coating and the contact angles were measured for each pH level after spraying. The hydrophobic colloidal dispersion was sprayed onto substrates with a household sprayer. After coating, the substrates were dried under ambient conditions for 1 h, and then heated at 150 °C for 1h with an increase of 1 °C/min in order to remove the residual solvent.

2.4. Characterization

A zeta sizer (Malvern Zetasizer Nano ZS, Worcestershire, UK) was applied to measure the zeta (ζ) potential of the colloidal particles. The formation of silica particles and their modification were investigated in powder form using an FT-IR spectrometer (ANicolet iS10, Thermo Scientific, Waltham, MA, USA). Contact angles (OCA) of water droplets on the glass and sand were measured by a goniometer (Dataphysics OCA 15Pro, Filderstadt, Germany). Scanning electron microscopy (SEM) (Zeiss Auriga Crossbeam 540, Carl Zeiss, Oberkochen, Germany) and X-ray diffraction (XRD, SmartLab, Rigaku, Tokyo, Japan) were utilized to investigate the morphology and crystal structure of nano-SiO₂ and functionalized SiO₂/TMCS.

3. Results and Discussion

In this study, to develop a superhydrophobic self-cleaning coating for the exterior of buildings or construction, silica-based particle systems were proposed based on the following advantages: (1) abundance on earth; (2) cost-effectiveness; (3) easily processed; and (4) similar nature to buildings. For this purpose, hydrophobic-silica particle coatings were developed to test their self-cleaning performance on glass and sand.

3.1. Fabrication of Superhydrophobic SiO₂/TMCS Coating

Pure SiO₂ NPs were synthesized by hydrolysis of TEOS in the presence of ammonia aqua solution using the Stober method [34], and then TMCS was introduced onto the surface of silica particles. Superhydrophobic properties were achieved by substitution of hydroxyl groups on the surface of the SiO₂ NPs with methyl groups from TMCS (Figure 2).

The zeta potential, which depends on the surface charge, is important for the stability of nanoparticles in suspension. When the pH decreases, the particles tend to form agglomerates due to the absence of electrostatic stabilization of the nanoparticles that keeps them apart. As a result, silica nanoparticles functionalized with TMCS dispersion showed some precipitates. When the pH is at about 9, it reaches the isoelectrical point, where the negative and positive charges are equal on the surface of the silica particles. As the pH value increases from 9, the Z potential becomes more and more positive, indicating the increase in positive charge on the particle surface. As a result, the repulsion forces overcome the attractive forces between the nanoparticles and result in electrostatic stabilization [39]. In addition, the isoelectric point (IEP) of the as-synthesized bare silica NPs was found at pH = 2–3 (Figure 3a). This might be a feature of the condensation reaction of the silicic acids (Si(OH)₄). As the pH increases from 3, the negative charges increase, the repulsion forces between nanoparticles also increase and suspension tends to stabilize [40].

As shown in Figure 3b, broad absorption bands at 3400 cm⁻¹ and 1620 cm⁻¹ corresponded to the –OH groups adsorbed at the SiO₂ surface in the FTIR spectrum. The broad absorption bands at around 3400 cm⁻¹ and 1600cm⁻¹ corresponded to the –OH groups adsorbed at the SiO₂ surface, which disappeared after applying TMCS on the surface of SiO₂ and after annealing (Figure 3b, red line). The absorption peaks at around 1099 cm⁻¹ were assigned to the Si–O–Si asymmetric stretching and bending vibrations. Likewise, the characteristic peaks at 2972 cm⁻¹ and at 1392 cm⁻¹ were assigned to C–H of the CH₃ antisymmetric stretching vibration to the symmetric vibration and in-plane vibration of the –CH₃ group, respectively. It indicated the presence of CH₃ (hydrophobic functional group) on the surface of SiO₂ particles. Moreover, the peak at 765 cm⁻¹ that corresponded to the symmetric stretching vibration of the Si–CH₃ group showed the hydrophobic modification of the silica particles. The reaction between SiO₂ particles and TMCS was completed through the formation of the Si–O–Si bonds.

3.2. Investigation of Surface Morphology of SiO₂/TMCS Coating

The morphology of the bare SiO₂ NPs and the SiO₂ modified with TMCS coating onto glass was examined by SEM and AFM techniques. According to the SEM images (Figure 4a), bare SiO₂ NPs had a spherical monodisperse uniform shape and were 15–20 nm in size, and also had an amorphous structure with a wide peak at 2θ = 22.5° (Figure 4c). However, after the SiO₂ particles were modified by TMCS (Figure 4b,d), the surface of the materials became rough. Moreover, SiO₂ modified with TMCS particles formed a uniformly distributed film on the glass substrate, with sizes ranging from 100 to 300 nm and exhibited a rough surface covered by the low surface free energy of methyl groups (Si–CH₃) at 765cm⁻¹, as confirmed by FTIR (Figure 3b).

Particles of different sizes and shapes in the system resulted in the rough surface of the coatings, which is beneficial for the formation of the superhydrophobic properties. The rough surface structure is a crucial parameter to create superhydrophobic properties [41,42,43,44].

3.3. Contact Angle (OCA) Study

The wettability of glass and sand coated with SiO₂/TMCS was studied by using a goniometer (an OSA-15EC instrument). Figure 5 shows the contact angles of water droplets on the SiO₂ modified with TMCS superhydrophobic materials after being applied to the surface of glass and sand. Glass surfaces (Figure 5a,b) treated with SiO₂/TMCS slurry produced a superhydrophobic effect with a contact angle above 150°. The reason for the superhydrophobic coatings is the formation of the rough surface at IEP due to the van der Waals attraction.

Figure 6 shows the effect of the pH of the SiO₂/TMCS dispersion on the contact angle. The wettability of coated glass was increased with the increasing pH of SiO₂, reaching a superhydrophobic state with a contact angle of 152° at pH = 7, 159° at pH = 8, and 165° at pH = 9. This increase in contact angle can be explained by an increase in surface roughness, due to the formation of a network of silica particles via Vander Waals attraction at the IEP point. However, after pH = 9, the contact angle sharply decreased, reaching 129°. The diagram shows that, despite the influence of the pH of SiO₂/TMCS dispersion, all samples showed a hydrophobic state.

The high contact angle of hydrophobized sand (151°) and glass (165°) confirmed the formation of superhydrophobic coatings (Figure 7). To evaluate the anti-icing properties of the SiO₂/TMCS coating, surface-coated brick and concrete were placed in a freezer at −30°C, then water was dropped over them. The water contact angle was 135°. An image of the experiment is given in the Supporting Information (Figures S1 and S2). It can be observed from the image that the SiO₂/TMCS coating can be applied in extreme cold temperatures for the purpose of coating building materials. Fluidity testing of the SiO₂/TMCS coating on the brick can also be observed in the video in the supporting information. It can be observed that hydrophobic surfaces do not easily become wet in contact with water. The phenomenon is due to unbalanced molecular forces at the water/solids interface, which causes surface tension. Low surface energy resists wetting [45,46].

3.4. Self-Cleaning Study of Construction Materials

Self-cleaning is a crucial property of superhydrophobic materials for practical applications. A series of experiments were conducted to demonstrate the self-cleaning properties of the coated glass. The experiments were executed following two methods, as shown in Figure 8. In the first experiment, blue color water droplets (contaminated water) were rolled from glass substrates, and the result showed no remains of tailings (Figure 8a,b). In the second experiment, dust from the sands was applied as dirt onto a superhydrophobic glass substrate for visualization of the self-cleaning process. Then, the process was observed to determine the capability of water droplets to remove the dust particles (Figure 8c,d).

These results indicate that the coated glass substrates have good self-cleaning capabilities because the dyed water and dust did not remain on the coated glass slide after the removal of dirt by rolling water droplets. SiO₂ modified with TMCS coatings possesses superhydrophobic properties due to the surface roughness by creating the hierarchical rough structure and low surface free energy of methyl groups. These properties could maintain the water droplets in the ‘solid–vapor–liquid phase’ through forming a high-water contact angle (90° < θ < 150°) and a low-water roll-off angle (10°). Consequently, the two mentioned characteristics of the as-prepared superhydrophobic glass caused the water droplets to easily roll off over the modified surface without moistening the surface, meaning that contaminants will be easier to remove.

4. Conclusions

Bare SiO₂ nanoparticles and SiO₂ modified with TMCS nanocomposites were successfully synthesized, characterized and used as self-cleaning coatings. The results showed that nano-sized SiO₂ particles had spherical, monodisperse and uniform shapes. XRD analysis of SiO₂ indicates a wide peak located approximately at 2θ = 22.5°, which suggests an amorphous structure. The SEM results indicate that, after SiO₂ particles were modified with TMCS, the surface of the coatings became rough. It was found that the coatings demonstrated excellent superhydrophobic and self-cleaning properties. The highest contact angle was recorded as 165° at pH = 9. The proposed coatings may have potential applications in the protection of buildings and constructions.