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 [22–27]. 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 <sup>×</sup> 1 mm with eleven cuts of <sup>3</sup> <sup>4</sup> (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 [22–25]. 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.


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

#### **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 3.** TEM images of the SiO2@TiO2 powder sample (10×).

**Figure 4.** SEM micrographs of the SiO2@TiO2 coating in a power sample.

**Figure 5.** EDS analysis, elemental mapping images of the SiO2@TiO2 power sample: (**A**) EDS area; (**B**) Silicon; (**C**) Titanium and (**D**) Oxygen.

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−<sup>1</sup> and weak peaks at 395 cm−1, 515 cm−<sup>1</sup> and 638 cm<sup>−</sup>1. The Raman spectrum of SiO2 contained a strong peak at 450 cm−<sup>1</sup> and weak peaks at 80 cm<sup>−</sup>1, 90 cm−<sup>1</sup> and 980 cm<sup>−</sup>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<sup>−</sup>1, 816.1 cm<sup>−</sup>1, 830.1 cm−<sup>1</sup> and 882.4 cm<sup>−</sup>1, these peaks are characteristic of the PDMS compound [27].

**Figure 6.** XRD pattern of the SiO2@TiO2 coating.

**Figure 7.** Raman spectra in the range of 80–1200 cm−<sup>1</sup> from TiO2, SiO2 and SiO2@TiO2.

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.

**Figure 8.** UV-Vis transmittance spectra of SiO2@TiO2 coated on glass (red) and glass (black).

#### *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 *R*<sup>4</sup> = 25% and *R*<sup>26</sup> = 55%, the developed SiO2@TiO2 coating satisfies the boundaries as to what can be considered photocatalytic material (*R*<sup>4</sup> > 20% and *R*<sup>26</sup> > 50%) [19]. However, the use of sonochemistry showed an improvement in the efficiencies of degradations of *R*<sup>4</sup> = 30.4% and *R*<sup>26</sup> = 70.5%. Similar values have been also reported by a photocatalytic coating applied on mortar samples [28].

**Figure 9.** Comparison of the degradation of RhB, before and after UV-A irradiation. (**a**) 0 h; (**b**) 4 h; (**c**) 26 h.

**Figure 10.** Rhodamine B removal efficiencies of the TiO2, SiO2 and SiO2@TiO2 coated mortar samples under UV-A irradiation (4 and 26 h).

On the other hand, as expected, TiO2 coated samples displayed the best activity with *R*<sup>4</sup> = 79% and *R*<sup>26</sup> = 92% [29]. In contrast, the SiO2 coated samples exhibited a significantly lower degradation efficiency (*R*<sup>4</sup> = 0.5%, *R*<sup>26</sup> = 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 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.

**Figure 11.** Water contact angles of TiO2, SiO2 and SiO2@TiO2 coated mortar samples before (0 h) and under UV-A irradiation (4 and 26 h).



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 *R*<sup>4</sup> = 44% and *R*<sup>26</sup> = 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 *R*<sup>4</sup> = 25% and *R*<sup>26</sup> = 55% establishing itself as a photocatalytic material, while the SiO2@TiO2 coating coupled with sonochemistry showed *R*<sup>4</sup> = 30.4% and *R*<sup>26</sup> = 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.

**Acknowledgments:** A. Rosales thanks CONACyT for the scholarship granted and also thanks to UniValle and PUJ Cali for the facilities in the international mobility exchange. C. Guzmán and K. Esquivel thanks to Luis A. Ortiz-Frade from CIDETEQ for the SEM and EDS analysis, to QFB. Lourdes Palma from UNAM for the TEM images and to Luis Escobar-Alarcón from ININ for the Raman analysis.

**Author Contributions:** K. Esquivel and A. Maury-Ramírez conceived and designed the experiments; A. Rosales performed the experiments; all the authors analyzed the data; R. Mejía-De Gutiérrez and C. Guzmán contributed reagents/materials/analysis tools; A. Rosales, A. Maury-Ramírez and K. Esquivel wrote the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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