Preparation Titanium Dioxide Combined Hydrophobic Polymer with Photocatalytic Self-Cleaning Properties

Titanium dioxide (TiO2) and hydrophobic of TiO2/PDMS (PDMS = polydimethylsiloxane) have been prepared as photocatalytic self-cleaning materials. Synthesis of TiO2 was carried out using the sol-gel method with titanium(IV) isopropoxide (TTIP) as a precursor and acetic acid as a solvent at a temperature of about 10–15 °C, while the synthesis of hydrophobic of TiO2/PDMS composites was carried out by a sonication method under ethanol solution. The results of XRD analysis of synthesized TiO2 showed that TiO2 was anatase phase. The glass-coated TiO2/PDMS were prepared by dip-coating under an ultrasonication bath. TiO2/PDMS composites at a ratio of TiO2/PDMS (1) on the glass plate showed hydrophobic properties, as evidenced by the contact angle of 104° before irradiation and the contact angle of 99.7° after irradiation. The synthesized titanium dioxide has irregular spherical morphology. The increase in PDMS content was correlated with an increase in the roughness of TiO2. PDMS not only acts as low surface energy but also binds TiO2. The hydrophobic behavior of PDMS creates TiO2/PDMS repel each other, gain irregular agglomeration structures. Beside having optimum contact angle, glasscoated TiO2/PDMS (1) is the best composition for degradation of methylene blue in 69.68% for 20 minutes irradiation. Copyright © 2020 BCREC Group. All rights reserved


Introduction
Nowadays, thin-film outer layer on products as doors and windows need self-cleaning glazing. Knowledge about cleanness, withstand contaminating, and performance over time is of excellent importance. We need a scientific method to quantify the self-cleaning effect by choosing a self-cleaning glazing product in preference to a regular product, then considering the actual profit.
In 1997, self-cleaning based on easy-to-clean coatings has been widely used in the surface field using superhydrophilic and photocatalytic materials [1]. Photoinduced super hydrophilicity of titanium dioxide (TiO2) films are commercially applicable for self-cleaning properties were purposive by using composite systems or only pure of TiO2 [2]. Many kinds of self-cleaning nanocoating materials deposited on the surface of any kind of common glass are based on the photocatalytic property of a thin layer of titanium dioxide (TiO2) nanoparticles.
The superhydrophobic surfaces have low free energy surfaces. The developed selfcleaning utilizes the superhydrophobic surface and the photocatalytic properties of materials [2]. Superhydrophobic materials with a water contact angle (WCA) higher than 150° causing water droplets can thus easily roll across the contaminant from the glass substrate [3]. The hydrophobic surfaces are as usual designed through the controlling of the morphology of solid surfaces and chemical compositions [4].
Superhydrophobic polymers, such as PDMS, is widely used because they have high elasticity, environmentally friendly, chemical inertness, and low cost [5−8]. PDMS has a nonpolar group like -CH3-that makes the ability of contact with pollutants, such as: dye and nonpolar group [9]. PDMS has excellent durability and mechanical properties is a stable cross-linked structure, so easy to use with the composition of materials [3].
On the other hand, the advantages of TiO2 are high chemical stability for base-acid, nontoxic, low cost, and high photo-reactivity [10]. The TiO2 have synthesized by sol-gel [11], solvothermal [12], thermal plasma [13], and others. The highest photocatalytic performance of TiO2 is the anatase phase compared with the brookite and rutile phase [11−13].
The combination between TiO2 as photocatalyst and PDMS as hydrophobic polymer generates stability of self-cleaning properties [14]. Fabricating a hydrophobic-hydrophilic multifunctional surface coating is demonstrated by blending various PDMS and TiO2 nanoparticles. After coating, various blend solutions onto a substrate dried at 110 °C. The surface of -CH3 was obtained through the combined contributions of aggregated TiO2 nanoparticles and hydrophobicity properties are contributed by PDMS as the low surface energy [3].
Fabrication of TiO2/PDMS creates superhydrophobic surfaces. The photocatalytic properties of TiO2 make them possible to work in harsh environmental conditions. When exposing to high-energy ultraviolet light irradiation, electron and holes of TiO2 are generated to form free radicals, that can decompose organic pollutants [14]. The selfcleaning glazing needs a hydrophobic surface when a TiO2-based hydrophobic surface is pol-luted by contaminants. The TiO2 can degrade the contaminant and recover the hydrophobic properties. However, the TiO2 will also degrade those low surface energy materials that constitute the hydrophobic films, resulting in extremely poor durability which represents a hurdle to be controlled. So, the combination of TiO2 and PDMS needs to be provided.
Tavares et al. [15] have reported TiO2/PDMS composite for self-cleaning was processed by spray coating and TiO2 was synthesis by high temperature on microwaveassisted hydrothermal. The composite prepared use hexane solution. In 2020, Julian et al. [16] research PDMS-coating TiO2 using v i n y l a n d h y d r o x y l -t e r m i n a t e d polydimethylsiloxane for functionalization of TiO2. However, these methods need to be improved with the adding of TEOS that can prevent the TiO2/PDMS from cracking during drying.
In this work, we focus on preparing composite photocatalytic-hydrophobic TiO2/PDMS coated on the glass substrate for decolorization of methylene blue. The composite TiO2/PDMS using TEOS as silane and ethanol coated on glass substrate were fabricated using the dipcoating technique under the ultrasonicator bath. Before preparation on glass substrate, we focus to produce the anatase phase of TiO2 by a sol-gel method using a weak acid solution. The ability of self-cleaning TiO2/PDMS was evaluated by measuring the water contact angle (WCA) and decolorization of methylene blue.

Synthesis of TiO2
A 10 mL TTIP was added into 100 mL acetic acid glacial in a distilled water bath (15 °C) and continually stirred with heat up to 90 °C using hot plate Thermo SP131320-33 Q until a white solution was obtained. The prepared precipitates were freeze and followed by heated up to 150 °C for 24 hours. After being washed with ethanol and dried for 3 hours at 100 °C, a white powder was obtained. Finally, the prepared powder was annealed at a temperature of 400 °C for 2 h at a 10 °C/min heating rate.

Preparation of TiO2/PDMS
Preparation of TiO2/PDMS composite by modifying a method reported previously [17]. 10 mg TiO2 was added into PDMS in 200 mL ethanol and then sonicated at 25 °C for 30 minutes (Figure 1). The TiO2/PDMS materials have been prepared by variation of TiO2 and PDMS amount as shown in Table 1.

Preparation of the glass substrate
Glass substrate with 7 cm  3 cm  0.3 cm cleaned to remove organic or non-organic pollutant. Glass substrate put into beaker glass with aquadest and ultrasonicated for 10 min and then oven on 100 °C for 10 min. After that, ultrasonicated in ethanol and acetone solution alternately by the same time and temperature.

Photocatalytic Self-Cleaning properties
Photocatalytic self-cleaning of glass substrate coating was evaluated by methylene blue degradation. 5 mL methylene blue 10 ppm was dropped on glass coated TiO2/PDMS and irradiated with halogen lamp by photon energy source in a closed reactor for 20 min with 5 min increment. After that, methylene blue was poured into a beaker glass and then poured into a cuvet for analysis. Degradation of methylene blue was analyzed by spectrophotometer UV-Vis at the range of 500−700 nm.

Characterization
Characterization of TiO2 by XRD (Shimadzu XRD-600) with Cu-=1.5418 Å), operated at 40 kV and 30 mA (range 2θ = 5−80°) to the identification of crystallinity and crystal phase. Morphology and element composition of TiO2/PDMS coating on glass substrate were identified by SEM (FE Inspect-S50). Function group were characterized by FTIR (Fourier Transform Infrared Spectroscopy Prestige 21 model 8201 PC). Degradation of methylene blue was determined by UV Vis Spectrophotometer (Shimadzu UV-2550) with range wavelength is 500−700 nm. The contact angle was taken using a modification WCA box (10  25 cm) by Bengkel Bubut Utama (local market), then measured by ImageJ software.
The TiO2 as synthesized from the hydrolysis of TTIP in acetic acid glacial (acidic solution) and the calcinated at 400 °C shows that it has low crystallinity. Since the synthesis process, peptization in the sol-gel method influences the crystallinity. The calcination temperature affects the crystallinity, at a temperature of 400 °C the amorphous obtained was according to Mahshid et al. [19]. However, high calcination temperature can transform anatase to rutile phase.
In a previous study, on annealing result of TiO2 prepared at 500 °C, the rutile TiO2 was found with a peak at 2θ = 27.50° (d110 = 3.2394 Å), and we have founded more rutile peak at TiO2 annealed at 600 °C and 700 °C, respectively. Some of the anatase peaks of TiO2 disappear, while the peak represented as a rutile TiO2 was increased [20].

Scanning Electron Microscope (SEM)
The scanning electron microscope was used for identified the morphology of TiO2/PDMS composite. The modification mechanism reaction during the synthesis process as shown in Scheme 1. Polydimethylsiloxane binds to Figure 2. X-ray Diffraction spectra of TiO2. Scheme 1. Reaction mechanism of TiO2/PDMS composite coating on the glass substrate.
TiO2 surface with TEOS as a bridge. On the surface of TiO2, oxygen (electronegative) was bind to silicon which derives from TEOS. The morphology of TiO2/PDMS was identified in Figure 3. It shows that the morphology of TiO2 is irregular spheres. The addition of PDMS produces microscale surface roughness which caused by the aggregation of TiO2 particles. The PDMS matrix is bound together with TiO2 particle agglomeration, as was the case in previous studies [8]. There are many bumps that indicated that an increase of PDMS can be wrapped up TiO2.

Hydrophobic-Hydrophilic Properties
The contact angles of glass coating before and after irradiation shown in Figure 5. Based on the measurement, the contact angle of the glass before and after coating by TiO2 is 59.8° and 81.56° shows that coating influence the contact angle. These hydrophobic-hydrophilic properties can be used as self-cleaning agents. Identification as a hydrophobic material is due to it produces water contact angles θ>90° and to be hydrophilic, because it produces contact angles water 0º<θ<90° [22].
Water droplets on the surface of the glass substrate coating with TiO2/PDMS form a circle with a high contact angle. Along with the decrease in the compositions of hydrophobic polymers composite (TiO2/PDMS) and the increase in the composition of TEOS, it causes the contact angle of droplets are lower ( Figure  6 and Figure S1).
The composition of TiO2/PDMS (1) deposition on the glass shows the highest contact angle of 104°, because the presence of the CH3 group (methyl group) in PDMS increases the hydrophobicity properties which is in accordance with the results of previous research [9]. After light irradiation on the glass-coated TiO2/PDMS (1), the contact angle decreased to 99.7° indicating the occurrence of the photocatalytic process.

Photocatalytic Activity of Glass Coated TiO2/PDMS using Methylene Blue
The application of photocatalytic as a selfcleaning material works based on the principle  of photocatalytic reactions on the substrate coating of TiO2. After the TiO2 absorbs the UV light as a photon source, the electron/hole (e − /h + ) was formed. Electron was excited to conduction band, while hole (h + ) are on valence band. Electrons and holes are recombined, but small amounts of them react with O2 electron acceptors and with electron donors OH − and H2O to form •O2 and •OH. These radicals, as detoxifying agents, degrade methylene blue dyes. Detoxifying agents (•O2 and •OH) will break down the methylene blue into environmentally friendly compounds. The photodegradation mechanism of methylene blue is occurred when the •OH group attacks the C−S + =C. The degradation of methylene blue produces CO2, SO2, NO2, and H2O which are not harmful to the environment [23−24].
The photocatalytic self-cleaning activity test of glass coated TiO2/PDMS with time variations shown in Figure 7 ( Figure S2) and the percentage of degradation shown in Table 2. The best degradation of methylene blue with degradation reached 69.68% occurred in TiO2/PDMS composite synthesized from TiO2/PDMS (1)= 1:90 mg. This is because TiO2 and PDMS are physically bonded. Light irradiation direct to TiO2 leads to optimum hydroxyl radical production, because TiO2 was not a big deterrent to photon exposure. Direct light irradiation to TiO2 causes optimal hydroxyl radical production. However, the resistance of surface bonding and the hydrophobic properties of the PDMS in the composites are also other considerations that need to be considered.
Self-cleaning of methylene blue on a glass substrate coated with TiO2/PDMS occurs by after exposure to visible light releases electrons into the conduction band which results in ecb and holes (h + ) in the valence band. then ecb will react with O2 to produce •O2 − . Reaction of O2 radical production is as follows [25]:  •O2 − decomposes methylene blue by reduction process, while •OH was decomposed by oxidation.
The absorbance of methylene blue decreases with increasing irradiation time. These results indicate that the longer time of irradiation can make more electrons are excited whereas electrons take effect of photodegradation selfcleaning activity.

Conclusion
We have successfully synthesis TiO2/PDMS composites to coat in a glass substrate under a ultrasonication bath. The TiO2/PDMS (1) composite on the glass substrate produces stability of self-cleaning properties materials, that are hydrophobic properties with contact angle 104° and characteristic for self-cleaning photocatalytic for degradation of methylene blue under visible light irradiation in 69.68%. Therefore, the TiO2/PDMS composites is possible as candidates for coating materials to apply in the glass substrate for glass selfcleaning technology.