SiO₂@TiO₂ Coating: Synthesis, Physical Characterization and Photocatalytic Evaluation

Abstract

Use of silicon dioxide (SiO₂) and titanium dioxide (TiO₂) have been widely investigated individually in coatings technology, but their combined properties promote compatibility for different innovative applications. For example, the photocatalytic properties of TiO₂ coatings, when exposed to UV light, have interesting environmental applications, such as air purification, self-cleaning and antibacterial properties. However, as reported in different pilot projects, serious durability problems, associated with the adhesion between the substrate and TiO₂, have been evidenced. Thus, the aim of this work is to synthesize SiO₂ together with TiO₂ to increase the durability of the photocatalytic coating without affecting its photocatalytic potential. Therefore, synthesis using sonochemistry, synthesis without sonochemistry, physical characterization, photocatalytic evaluation, and durability of the SiO₂, SiO₂@TiO₂ and TiO₂ coatings are presented. Results indicate that using SiO₂ improved the durability of the TiO₂ coating without affecting its photocatalytic properties. Thus, this novel SiO₂@TiO₂ coating shows potential for developing long-lasting, self-cleaning and air-purifying construction materials.

1. Introduction

Current environmental problems observed in big cities, such as air pollution and associated infrastructure deterioration, encourage research for the development of new technologies and products that mitigate these modern, urban threats. Among the different environmentally-friendly technologies, heterogeneous photocatalytic oxidation using TiO₂ has become an interesting technology due to its durability and high photocatalytic activity [1]. Recently, the incorporation of TiO₂ (e.g., coatings or additives) into construction materials used in urban infrastructure, such as concrete and mortars, has been an interesting approach to reduce NO_x and VOCs (volatile organic compounds) at outdoor concentrations using sunlight as the only energy source; these are the so-called air purifying properties. TiO₂ under UV-A light irradiation can generate oxidative (·OH) and reductive (·O₂) species, which are able to degrade different organic and inorganic compounds [2,3,4]. Furthermore, exposure to UV-A light enhances the superhydrophilic effect on the TiO₂ surface, which makes it easier to remove the fouling substances on TiO₂ loaded surfaces; this is the so-called self-cleaning ability [5,6,7]. However, recent applications of photocatalytic building materials in urban pilot projects have demonstrated that maintaining the durability of the air-purifying and self-cleaning properties remains challenging, especially for the application of photocatalytic building materials under outdoor conditions [6]. Among other environmental factors, dust and oil accumulation have been reported as major factors affecting the properties of photocatalytic construction materials at an urban scale [7].

On the other hand, hydrophobic surfaces have also received attention for their self-cleaning, anti-flogging, anti-adherent and anti-polluting properties. The natural model for the design of superhydrophobic synthetic films is the lotus plant, which is known for its self-cleaning properties that allow the capture of air under water droplets that contribute to the rolling water droplet, a characteristic of well-designed superhydrophobic surfaces [8]. Due to the nano-manufacturing technologies that have been established for silicon substrates, silicon has been widely used for producing superhydrophobic surfaces; moreover, this kind surfaces, for instance, promotes durability in structures by avoiding the incrustation of corrosive salts (Cl⁻ and ${SO}_4^{-}$) that promote cracking or surface erosion [9]. To make superhydrophobic surfaces of intrinsically hydrophilic materials, a two-step process is usually required, i.e., first, make a rough surface and second, modify it with a coating of chemicals, such as organosilane, which may offer low surface energy after binding to the rough surface [9,10]. This is the case for polydimethylsiloxane (PDMS), which can be easily processed to make a hydrophobic surface with a rough texture and reduced free surface energy [11,12]. The methods to create hydrophobic surfaces have very long reaction times and strict chemical conditions. A method that uses sonochemistry has smaller reaction times, is more likely to undergo a complete chemical reaction and more ordered crystallization. Sonochemistry is a process of cavitation that refers to the rapid growth and collapse of implosion bubbles in a liquid in an unusual reaction environment [13,14]. Therefore, this article reports the development of a SiO₂@TiO₂ coating applicable to cement based materials, such as mortars and glass. The SiO₂ matrix, based on PDMS (polydimethylsiloxane), has the potential to increase the adherence of TiO₂ particles and to improve their photocatalytic efficiency [15].

2. Materials and Methods

As a strategy to develop an efficient SiO₂@TiO₂ coating, pure TiO₂ and pure SiO₂ coatings that used the same precursors, proportions, and two different synthesis methods were evaluated.

2.1. Synthesis of SiO₂@TiO₂ Coating Coupled with Sonochemistry

For TiO₂ sol production, titanium Iso-propoxide (97%, Sigma Aldrich, St. Louis, MO, USA) was added dropwise into an organic solvent (isopropyl alcohol, 99%, Sigma Aldrich), previously stirred under an inert nitrogen atmosphere for 5 min.

For SiO₂ synthesis, sonotrode equipment (Hielscher Ultrasound Technology UP200Ht, Teltow, Germany) was used working at 100% cavitation and 20% amplitude. A solution of distillated water, absolute ethyl alcohol and oxalic acid, in a 5/5/0.1 molar relation was prepared and stirred sonochemically for 15 min. Afterwards, tetraethyl orthosilicate was added dropwise and the mixture was stirred sonochemically for 3 min. Next, polydimethylsiloxane was added dropwise and continuously stirred for 3 min.

Finally, the titanium dioxide sol and the silicon dioxide sol were mixed. Beforehand, sonotrode working conditions were modified from the initial conditions to 100% cavitation and 60% amplitude. Immediately after mixing, 10 mL of distilled water was added and mixed continuously using sonotrode conditions for 20 min. The resultant mixture was applied on glass and mortar surfaces and left to dry at room temperature.

Synthesis of SiO₂@TiO₂ Coating without Sonochemistry

For TiO₂ sol preparation, after sol formation, a hydrolysis process was carried out with the addition of distilled water, added dropwise. The resulting solution was filtered, washed with distilled water and dried at room temperature for 18 h. Finally, a calcination process was carried out at 450 °C for 3 h.

For SiO₂, a solution of distillated water, absolute ethyl alcohol and oxalic acid, in a 5/5/0.1 molar relation was prepared and stirred for 15 min. Afterwards, tetraethyl orthosilicate was added dropwise and stirred for 3 min. Then, polydimethylsiloxane was added dropwise and stirred continuously for 3 min.

Finally, the titanium dioxide particles and the silicon dioxide sol were mixed for 20 min. The resultant mixture was applied on mortar surfaces and dried at room temperature for at least 1 h.

2.2. Preparation of Mortar Samples

The mortar samples were manufactured using a previous mix design (

Table 1. Mortar components and proportions for 1 m³.

Material	Proportions Related to Cement Content	Mass (kg)	Absolute Volume (dm3)
Water	0.59	324.00	324.00
Cement	1.00	549.00	186.70
Aggregate	2.66	1421.40	489.30

), the same local materials, and the ASTM method C192/C192M [16,17,18].

2.3. Physical Characterization of SiO₂@TiO₂ Coating

The microstructures of the materials were examined by transmission electron microscopy (TEM) using a JEOL JEM-1010 (Tokyo, Japan), operating at a voltage of 200 kV. The crystallinity of the SiO₂@TiO₂ coating was determined by X-ray diffraction (XRD) using Bruker D8 equipment (Billerica, MA, USA) with a sealed copper tube to generate Cu–Kα radiation (λ = 1.15406 Å) with angles of 10 < 2θ < 80° in a pitch of 0.01°. To verify the crystallinity, the structures of the obtained samples were characterized using Raman spectroscopy with the LabRAM HR equipment (Horiba Scientific, Kyoto, Japan), which used an NdYGA laser (λ = 532 nm). The samples were analyzed using a microscope with an objective of 10× at a power of 6 mW over a circle 1.5 μm in diameter. The optical transmittance of the glass substrates coated with SiO₂@TiO₂ was measured with a Cary 5000 UltraViolet-Visibe-Near-Infra-Red spectrophotometer (Agilent, Santa Clara, CA, USA) at wavelengths ranging from 350 to 800 nm. Water contact angle was measured using an optical tensiometer (Analyzer-DSA100W Krüss, Hamburg, Germany), which produces water droplets with a volume adjusted to 10 μL using a needle (stainless steel, model NE60).

2.4. Photocatalytic Evaluation

2.4.1. Evaluation of Photocatalytic Properties

The photocatalytic performance of the coating was evaluated using the method (UNI 11259-2016) based on Rhodamine B (RhB) degradation on the sample exposed to UV-A irradiation [19,20]. To monitor dye removal, the mortar samples were divided into three parts, in which TiO₂, SiO₂ and SiO₂@TiO₂ coatings were applied, as shown in Figure 1. Using a pipette, an RhB solution with a concentration of 50 ppm was evenly applied to 3 standardized positions on the samples, and they were left to dry overnight.

Then, the dye-contaminated samples were exposed to UV-A irradiation for 26 h using the UV reactor shown in Figure 2. In this reactor, UV-A irradiation was provided by an Electrolux T8 20 W/BLB. This lamp type emits light with a peak wavelength of 360 nm and an intensity of 10.3 Wႃm⁻² at a distance of 5 mm. Finally, changes in color at 0, 4, and 26 h were measured using a portable X-rite Ci60 spectrophotometer (Photometric Solutions International, Victoria, Australia). Measurements were reported in the L*, a*, b* colorimetric coordinates of the CIE LAB system (32/64 bit software), which corresponds to the white and black color range, red and green color range and yellow and blue color range, respectively, where the a* coordinate is the comparison parameter. Based on these measurements, the following parameters were calculated, as shown in Equations (1) and (2). Where R stands for color removal at time 0, 4, and 26 h of UV light exposition.

$$R_4=\frac{\left(R_4-R_0\right)}{R_0}\times 100$$

(1)

$$R_{26}=\frac{\left(R_{26}-R_0\right)}{R_0}\times 100$$

(2)

2.4.2. Water Contact Angle Measurements

To evaluate the water behavior on the TiO₂, SiO₂ and SiO₂@TiO₂ coated samples, a preliminary test using the “rising drop” method was employed, using a camera and digital measurements, before and after the UV A light exposure (0, 4, and 26 h).

2.5. Durability

To assess the durability of the SiO₂, TiO₂ and SiO₂@TiO₂ coated samples, an adherence test was performed following the ASTM 3359 [21] and the appropriate references [22,23,24,25,26,27]. In this case, the corresponding test for thin films with thicknesses less than or equal to 2 mm was selected. To perform this test, a grid of 1 mm × 1 mm with eleven cuts of ¾ (20 mm) in length was drawn on top of the coated mortar sample. Subsequently, a piece of scotch tape, three inches long, was placed in the center of the grid and soft pressed with an eraser. A change in color of the tape indicated complete contact. Then, the scotch tape was removed from the opposite end of the application, forming a 180° angle. Next, the coated area was compared with the patterns presented in

Table 2. Detachment patterns and classification of different coated surfaces after the adherence test (Modified from ASTM-3359-02 classification chart).

Classification	Area Removed (%)	Cross-Cut Surface Area with Adhesion Range by Percent
5B	None 0%
4B	<5%
3B	5–15%
2B	15–35%
1B	35–65%
0B	>65%

[22,23,24,25]. In addition, the adherence test was carried out before and after UV light exposure (0, 4 and 26 h), to determine the photocatalytic activity of the SiO₂@TiO₂ coating.

3. Results and Discussion

3.1. Physical Characteristics

TEM images (10×) of a power sample from the SiO₂@TiO₂ coating are shown in Figure 3. From these, it can be seen that layered agglomerates are formed by amorphous silicon dioxide while titanium dioxide is not visible. In general, the reported agglomerates vary in shape and size, ranging from 20–600 nm. In addition, it was observed that the morphology of the SiO₂@TiO₂ coating was not affected by the use of sonochemistry.

Figure 4 shows the SEM micrographs of the SiO₂@TiO₂ coating. It can be observed that the surface was rugged and the morphology was not uniform due to the formation of denser particles and their agglomeration. Additional cross-selection elemental mappings, combined with EDS analysis for SiO₂@TiO₂, showed the presence of Si, Ti and O as elements (Figure 5) as expected.

Figure 6 shows an X-ray diffractogram of the SiO₂@TiO₂ coating. It shows the high intensity peak of silicon dioxide at 24°, characteristic of the amorphous SiO₂ phase. In addition, signals observed at 12.8° and 22.6° are characteristic of the PDMS compound. On the other hand, signals of titanium dioxide that presented at peaks of 27.3° (110) and 55.5° (220) corresponded to the rutile crystalline phase [24] and the peaks of 25.3° (101), 38.6° (004) and 48.08° (200) corresponded to the anatase crystalline phase [25]. Additionally, it was observed that the diffractogram of the SiO₂@TiO₂ coating was not affected by the use of sonochemistry on a macro scale. Previous information was obtained using the standard XRD pattern (JCPDS FILES No. 21-1272). Moreover, the reflections corresponding to the silicon covered up the other signals of the TiO₂ phases and PDMS compound. The crystallite size was obtained by Scherrer’s equation [26], which obtained a crystal size of 13 nm, being an amorphous compound due to its matrix of silicon dioxide.

The Raman spectra of TiO₂, SiO₂, SiO₂-PDMS and SiO₂@TiO₂ are presented in Figure 7. The Raman spectrum of TiO₂ contained a strong peak at 143 cm⁻¹ and weak peaks at 395 cm⁻¹, 515 cm⁻¹ and 638 cm⁻¹. The Raman spectrum of SiO₂ contained a strong peak at 450 cm⁻¹ and weak peaks at 80 cm⁻¹, 90 cm⁻¹ and 980 cm⁻¹. These peaks can be attributed to the bending of O–Si–O and Si–O–Si symmetric bond stretching. The Raman spectrum of SiO₂-PDMS exhibited peaks at 680 cm⁻¹, 816.1 cm⁻¹, 830.1 cm⁻¹ and 882.4 cm⁻¹, these peaks are characteristic of the PDMS compound [27].

The Raman spectrum of the SiO₂@TiO₂ nanocomposite exhibited a decrease in the highest intensity peak of titanium dioxide while the other peaks were inhibited. This can be attributed to the highly dispersed titanium dioxide. Furthermore, the signals of silicon dioxide decreased due to the presence of PDMS that modifies the crystallinity and makes noise (fluorescence) on the SiO₂@TiO₂ coating. The Raman spectrum of the SiO₂@TiO₂ coating, without the application of sonochemistry, showed a lower crystallinity for the composite. Furthermore, the TiO₂ signals were decreased and not even located by the Raman Spectroscopy.

Figure 8 shows the UV-visible spectrum of a glass sample coated with SiO₂@TiO₂ with reference to a blank (uncoated glass). The glass substrate had a transmittance of 92–93% (black line). After placing the coating on the glass, the transmittance of sample (blue line) was 85%. This result shows that the coating of SiO₂@TiO₂ has high transparency over a wide wavelength range.

3.2. Photocatalytic Evaluation

By measuring the RhB degradation before (0 h) and after UV-A irradiation (4 and 26 h) as shown in Figure 9, the TiO₂, SiO₂ and SiO₂@TiO₂ coated mortar samples were evaluated (Figure 10). With RhB removal of R₄ = 25% and R₂₆ = 55%, the developed SiO₂@TiO₂ coating satisfies the boundaries as to what can be considered photocatalytic material (R₄ > 20% and R₂₆ > 50%) [19]. However, the use of sonochemistry showed an improvement in the efficiencies of degradations of R₄ = 30.4% and R₂₆ = 70.5%. Similar values have been also reported by a photocatalytic coating applied on mortar samples [28].

On the other hand, as expected, TiO₂ coated samples displayed the best activity with R₄ = 79% and R₂₆ = 92% [29]. In contrast, the SiO₂ coated samples exhibited a significantly lower degradation efficiency (R₄ = 0.5%, R₂₆ = 8%). As there was no photocatalytic material present, the RhB removal was associated with dye photolysis, as previously reported [30]. According to the physico-chemical characterization of the SiO₂@TiO₂ composite previously described, the synthesis coupled with sonochemistry showed a non-significant difference in performance. Nevertheless, in the photocatalytic activity, the use of the sonochemical assisted synthesis helped to improve the Rhodamine B removal. This could be attributed to a better TiO₂ dispersion over the SiO₂-PDMS matrix and a higher anatase phase appearance without any thermal treatment as is used with the conventional sol-gel SiO₂@TiO₂ composite synthesis. However, this effect must be examined in further experiments by an extensive XPS analysis and by modifying the sonochemical synthesis parameters to achieve a macroscopic change in the physico-chemical characterization.

The water contact angle measurements of TiO₂, SiO₂ and SiO₂@TiO₂ coated mortar samples before, during and after UV irradiation (0, 4 and 26 h) are shown in Figure 11. As expected, TiO₂ exhibited hydrophilic behavior with values around 10°. On the contrary, the coated sample with SiO₂@TiO₂ presented water contact angles varying between 100° and 105° after UV-A irradiation. Previous research reports similar contact angles of around 114°–111° for a coating of TiO₂-SiO₂-PDMS [31]. Meanwhile the SiO₂ coated samples remained constant (around 98°) because silicon dioxide has hydrophobic properties.

Table 3. Test of adherence results of the coated mortar samples using TiO₂, SiO₂ and SiO₂@TiO₂.

Coated Sample	% Detachment	Image	Classified under ASTM D3359-02
TiO2	40%		1B
SiO2	5%		4B
SiO2@TiO2	10%		3B

shows the adherence test results of the coated mortar samples using TiO₂, SiO₂ and SiO₂@TiO₂. In the case of the SiO₂@TiO₂, a 10% detachment was quantified using the grid which classifies as 3B according to the ASTM D3359-02 [22,32]. On the other hand, the TiO₂ presented with 40% detachment, which classifies as 1B. Finally, SiO₂ had the highest adherence of the tested coatings and presented with 5% detachment and classifies as 4B.

After the evaluation of durability using the adherence test, Rhodamine B removal and water contact angle measurements were evaluated again on the coated samples. Results indicated that TiO₂ decreased its photocatalytic activity to R₄ = 44% and R₂₆ = 70%. For the SiO₂@TiO₂ coating no difference was noticed, while SiO₂ did not show a change in its photocatalytic activity. The contact angle was also maintained for all the tested materials. The results were 5° for the TiO₂ 90° SiO₂ and 100° SiO₂@TiO₂. Further experiments will be needed to find out the effects of extreme weather conditions on the durability of the coat.

4. Conclusions

In the present study, a hydrophobic and photocatalytic SiO₂@TiO₂ coating for mortar and glass protection was synthesized through a sol-gel with and without sonochemistry assistance. The completed analysis of Scanning Microscopy (SEM), Elemental Analysis (EDS), Transmission Electron Microscopy (TEM), X-ray diffraction (XRD) and Raman Spectroscopy of the SiO₂@TiO₂ coating revealed their composition and microstructure. The TEM images made it possible to observe agglomerates of the composite without a regular shape. But, by mapping the EDS analysis the main elements were found over the entire surface in a homogeneous way. The use of XRD enabled the visualization of the TiO₂ phases formed using sonochemistry. These phases are the rutile and anatase phase. Additionally the SiO₂ remained amorphous. Further, the Raman spectroscopy signals can be attributed to the bending and stretching of the O–Si–O and Si–O–Si symmetric bonds and without the application of sonochemistry a lower crystallinity of the composite and the TiO₂ signals was observed. Finally, according to the physico-chemical characterization of SiO₂@TiO₂, the coating displayed a high transparency over a wide wavelength range.

In addition, the application of sonochemistry in the sol-gel synthesis promoted the photocatalytic phase of the titanium dioxide and improved the removal of the Rhodamine B dye. The transparency of the titanium dioxide coating was around 85% of that compared to glass without a cover.

The photocatalytic activity of the SiO₂@TiO₂ coating showed an RhB removal of R₄ = 25% and R₂₆ = 55% establishing itself as a photocatalytic material, while the SiO₂@TiO₂ coating coupled with sonochemistry showed R₄ = 30.4% and R₂₆ = 70.5% indicating a major photocatalytic activity. The adherence test was used to study the durability, indicating a 3B type adhesion of the SiO₂@TiO₂, in accordance with the ASTM D3359-02 scale. Additionally, the SiO₂@TiO₂ composite after the durability tests showed no photocatalytic activity loss in contrast with the pure TiO₂ coating. These results show the potential of the developed SiO₂@TiO₂ coating for self-cleaning and air-purifying applications.