Surface morphology analysis processing. Scanning electron microscopy (SEM).
The produced Nano-silica composites were investigated by scanning electron microscopy (SEM) to determine the surface structure. The Nano-silica particles were embedding in the polymer host. Figure 1 (a, b, c) depicts the SEM images with different magnifications (15000x, 100000x, 120000x), (10000x, 80000x, 120000x), and (5000x, 80000x, and 120000x) of C1, C2, and C3, respectively.
Those images explain the dispersion of the Nano-silica particles. It was found that the sizes of Nano-particles at the polymer surface were in the range from 12–50 nm and the Nano-silica particles present in the aggregation degree are relatively homogeneous at an applied magnification.
The energy dispersive X-ray (EDX) analysis processing
In Fig. 2 (a, b, c) the EDX analysis shows the elements contents of the Nano-silica composite, whereas the weight percentage of silicone (Si) and oxygen (O) in NSC was varied in three different Aloe Vera gel concentrations as tabulated in Tables 1,2,3. Whereas the wt.% value of Si in C1 samples is higher than that obtained in both C2 and C3 samples, respectively. In addition, the wt% value of O in C1 is higher than that obtained in both C2 and C3 samples, respectively. While, the wt% value of C in C2 is higher than that obtained in both C3 and C1 samples, respectively. The minerals such as magnesium (Mg), aluminum (Al), and calcium (Ca) originates from the extracted Aloe Vera gel composition were also detected. The Aloe Vera gel plays as a precursor for a gradual reduction of Na2SiO3 as depicted in Tables 1, 2, 3.
Table 1
The elements in wt% and At% exist in the Nano-silica composite C1 by EDX- analysis.
Element | C | O | Na | Mg | Al | Si | Ca |
Wt% | 2.74 | 29.55 | 7.12 | 1.01 | 0.97 | 47.36 | 11.25 |
At% | 5.15 | 41.70 | 6.99 | 0.93 | 0.81 | 38.08 | 6.34 |
Table 2
The elements in Wt % and At % exist in the Nano-silica composite C2 by EDX- analysis.
Element | C | O | Na | Al | Si | Ca |
Wt% | 3.52 | 25.19 | 8.17 | 2.30 | 43.38 | 17.44 |
At% | 6.83 | 36.72 | 8.28 | 1.99 | 36.02 | 10.15 |
Table 3
The elements in wt% and At% exist in the Nano-silica composite C3 by EDX- analysis.
Element | C | O | Na | Mg | Si | Ca |
Wt% | 3.26 | 23.08 | 4.38 | 1.43 | 42.93 | 24.92 |
At% | 6.60 | 35.06 | 4.63 | 1.43 | 37.16 | 15.12 |
Fourier Transform Infrared Spectroscopy (FTIR) analysis processing
Figure 3 depicts the FTIR spectra of Nano-silica composites at three different concentrations of natural polymer Aloe Vera gel (C1 = 20 ml), (C2 = 40 ml), (C3 = 60 ml) with peaks of C1, C2, and C3 at (1635 cm− 1, 1641 cm− 1, 1651 cm− 1), respectively. It was found that the peak was shifted with increasing the concentrations of Aloe Vera gel. This peak was indicated to bending of O–H as can be seen at 1635–1651 cm− 1 41. Under alkaline conditions, the synthesis of silica Nano-particles from sodium silicate was given vibration bending of the water trapped molecules in the silica, which could not be completely removed through heating 42. The peaks located at 1412 cm− 1, 1420 cm− 1, and 1414 cm− 1 of C1, C2, and C3, respectively were indicated to the present of functional group Si—CH = CH2. The peaks located at (1016 cm− 1, 1011 cm− 1, and 1009 cm− 1) of C1, C2, and C3 respectively, were indicated to the predominant band of asymmetric vibration of Si–O–Si of siloxane at 1009 cm− 1 − 1016 cm− 1. It can also be confirmed that the presence of Siloxanes with one or more very strong infrared bands in the region of (1009 cm− 1 − 1016 cm− 1) 43. As the siloxane chains become longer or branched, the Si-O-Si absorption becomes broader and more complex, showing two or more overlapping bands. The peaks located at (785 cm− 1, 788 cm− 1, 788 cm− 1) of C1, C2, and C3 respectively, were indicated to the vibration symmetry of Si-O-Si and Si-C frameworks 44. The peaks located at (432-433-433 cm− 1) of C1, C2, and C3 respectively, were indicated to the bending vibrations of Si–O–Si framework 45. The absorption bands showed the bending vibration of siloxane groups of Si–O–Si at 432 cm− 1 and 788 cm− 1 46.
Thermo-gravimetric analysis (TGA) and data thermal analysis (DTA)
The analysis by Thermo-gravimetric analysis TGA - DTA systems for three different concentrations of Aloe Vera gel were depicted in Fig. 4 and Fig. 5a, b, c. In the TGA analysis of samples, it was found that the weight % of Nano-silica composite C1 was increased from 100.6 wt% to 106.8 wt% till reached 70° C and decreased to reach the real mass at 150°C (100.6 wt%). It can be noted that this composite has high porosity and adsorbed moisture. The thermal decomposition in the range 150°C − 300°C, was decreased in wt% from 100.6 wt% to 91.6 wt% after that high thermal degradation from 300°C − 700°C this indicate that the composite (C1) has high porosity, adsorbed moisture, and high thermal stability. In addition, the thermal decomposition of C2 sample takes place in the range 350°C – 700°C, whereas the thermal decomposition was increased in wt% from 100.3 wt% to 109.8 wt% until reached 80° C after that decreased to reach the real mass at 345°C (100.3 wt%). It can also be noted that this composite has high porosity, adsorbed moisture, and high thermal stability. While, the thermal decomposition of C2 sample takes place in the range 350°C − 700°C as depicted in Fig. 4.
In the case of the Nano-silica composite (C3), the thermal decomposition was decreased in wt. from (109.5 mg to 89.2 mg) at a temperature range from 100°C − 150°C, which indicated to the dehydration of C3 and the Thermal decomposition was take placed at 200°C − 300°C. High thermal degradation was started from 300°C − 700°C. The TGA analysis of the Nano silica composite depicted that height thermal stability of C1, C2, and C3 of the samples was obtained. The thermal stability of C2 sample was higher than C1, C3 samples and the stability of polymer chain was increased. The weight loss at 345°C for Nano-silica composite (C2) may be due to the degradation of small crystalline chains and evolution of CO2 and water. The reasons of losing weight at 500°C – 700°C range, firstly, the degradation may be due to small and medium structure of long chains. Secondly, the reason loss weight may be due to the degradation of long chains which polymerized with grafted silica surface and needed to height temperature of decomposition 47.
The DTA analysis in Fig. 5 showed the thermal stability behavior of the Nano-silica composites samples C1, C2, and C3, respectively. In the diagrams of the DTA analysis, all peaks of C1, C2, and C3, are exothermic reactions, that indicating to the thermal stable of all composites. Peak 1 in C1, C2, and C3 samples was attributed to the dehydration of the composites. While the second transition phase indicated to the exothermic peaks in the C2, and C3 samples in the temperature range (200°C − 270°C) which attributed to the thermo-degradation of the polymer chains and loss of the functional parts in polymers 48. The DTA curves indicated to the broad exothermic peaks for C1, C2, and C3 Nano-silica composites with high temperature which may be due to the oxidation and degradation of long polymer chains of the Nano-composites. The TGA and DTA analyses revealed that the Nano-silica composite C2 more stable than that obtained in the C1 and C3 Nano-silica composites. The Nano-silica composite C2 remains stable at 345ºC.
The X-ray diffraction analysis processing
The state of the Nano silica composites was evaluated via X-ray diffraction (XRD) analysis. Figure 6 depicts the XRD spectral analysis of Nano-silica composite particles (C1, C2, and C3), respectively. The structure of modification Nano-silica composite during XRD analysis depicts the presence of low intensive peaks, and wide range. The disappearance of crystalline phases confirmed to the amorphous state of the samples and contains pure silicone oxide (SiO2) 49.
The Nano-silica composites C1, C2, and C3 have broad peaks and low intensive peaks. It can be seen that the XRD spectra were shifted in 2θ for three different concentrations of Nano-composites (C1, C2, and C3), which attributed to the modified chains of polymers with grafted silica surface 50–52. In addition, the spectra showed stability of the Nano-silica composites
AFM analysis of C2
The C2 is more stable than C1 and C3 so C2 used in fabrication of hydrophobic surface The topographic view of Nano-silica composite (C2) was observed by atomic force microscope (AFM) as depicted in Fig. 7. The AFM images reveal that the accumulated spikes and spherical of silica particles creating hump with hollow portion. The images sizes were at 100 nm and 200 nm. The observation of cross-sectional profiles gave irregular spikes, troughs and aggregation of particles with various sizes and different height distribution to surface features of the samples. In Fig. 7, it can be seen that all AFM images gave clear pictures about the roughness of sample surfaces. The hierarchical morphology enables the surface to repel liquid (hydrophobic surface).
Contact angel measurement processing
The contact angle before and after treatment process of Nano-silica composite (C2) on the surface of the solar mirror was determined using a contact angle instrument as shown in Fig. 8 (a, b). The analysis of contact angel reveals that the surface becomes hydrophobic surface after treatment process of Nano-silica composite (C2) on the surface of the solar mirror. The contact angle was measured as 25.7º for solar mirror without treatment on the surface, while it becomes 92.3º for solar mirror after treatment of Nano-silica composite (C2) on the surface. Hence, it can be concluded that the contact angle values were changed before and after treatment of Nano-silica composite (C2) on the surface and became hydrophobic surface.
The Zeta potential analysis of Nano-silica composite (C 2 ). The zeta potential measurement of the produced Nano-silica composite was illustrated in Fig. (9). The produced data confirm that the negative surface charges of Nano-silica composite with a value of − 14.0 mV was found. Nevertheless this value of zeta potential and its distributions has also revealed that the synthesized nanoparticles inside polymer matrix are stable in nature.
Applications of NSC (C 2 ) on parabolic trough mirrors in El-Kuriemat solar thermal power station in Egypt
The used Nano-silica composites materials on solar mirrors with its type parabolic trough mirrors (case of the study) were located in El-Kuriemat solar thermal power station in Giza governorate Zone-Egypt as depicted in Fig. 10.
Fabrication of hydrophobic surface on solar mirrors for self-cleaning of the mirrors leads to reducing the amount of the deionized water that used in cleaning of mirrors and it must be taken into consideration the reflectance of the mirrors. Therefore, by measuring the values of reflectance before and after applied treatment process, it is found that the reflectance's have the same value of 92.5. This indicates that there are no changes in reflectance of solar mirrors by using the proposed material (Nano silica composite) and has no negative effect on efficiency of solar mirrors after spraying process. The solar mirrors were exposed under weather conditions after spraying NSC on solar mirrors. The reflectance is measured by reflectometer before and after exposure to weather conditions every day at constant amount of demi-water for cleaning solar mirrors with starting reflectance 92.5 as can be seen in Table 4. By using the NSC as hydrophobic layer, it was observed that the NSC layer were reduced the amount of accumulated dust on solar mirrors over time. The NSC layer were also improved the “wash ability” of the solar mirrors that gives more effective cleaning process for solar mirrors.
Table 4
Reflectance's before and after cleaning with position of solar mirrors under weather conditions.
days | Reflectance of solar mirror without using NSC (A) | Reflectance of solar mirror by using NSC (B) |
Before cleaning (A1) | After cleaning (A2) | Before cleaning (B1) | After cleaning (B2) |
| 92.5 (at start) | 92.5 (at start) |
10/10/2023 | 84 | 89.5 | 85.4 | 91.8 |
11/10/2023 | 80.7 | 89.2 | 82 | 91 |
12/10/2023 | 82.4 | 90.7 | 84 | 91.7 |
13/10/2023 | 86.7 | 90.8 | 86.8 | 92 |
14/10/2023 | 88.5 | 90.8 | 89.6 | 91.9 |
15/10/2023 | 86.8 | 91 | 88 | 92 |
16/10/2023 | 87 | 90.8 | 88.3 | 91.7 |
17/10/2023 | 83.1 | 89.5 | 83 | 90.2 |
18/10/2023 | 78.8 | 89.2 | 79.9 | 91.2 |
19/10/2023 | 83.7 | 90.5 | 84.5 | 90.8 |
20/10/2023 | 82.2 | 88.5 | 83.7 | 90.5 |
21/10/2023 | 85.5 | 88.6 | 86.3 | 89.9 |
22/10/2023 | 83.1 | 87.6 | 83.8 | 88.8 |
23/10/2023 | 84.7 | 86.7 | 85.9 | 88.2 |
24/10/2023 | 80.4 | 84.4 | 80.9 | 86.7 |
25/10/2023 | 79.1 | 83.4 | 84.4 | 85.6 |
It was observed that the reflectance's of the treated solar mirrors with Nano-silica composites have highest values than that obtained in comparison with the reflectance's of solar mirrors without using Nano-silica composites before and after cleaning process. The obtained results indicated that the sprayed Nano-Silica composites on solar mirrors give hydrophobic surfaces because the Nano-silica composites. The hydrophobic surfaces make self-cleaning in solar mirrors by adding a little amount of demi-water in cleaning of the solar mirror surfaces. Whereas the self-cleaning mechanism had the capability to expel the dust particles away under the action of spherical water droplets [60]. The self-cleaning process had two ways on solar mirror surfaces, either by rolling driven or by sliding motion of water droplets.
In this work, the motion of water droplets was sliding motion. The coated glass surfaces should be transparent to give the same values of reflectance's before and after spraying the Nano - Silica composites. The spraying process of the Nano-Silica composites on solar mirrors did not effect on the reflectance's of solar mirrors and did not make any scattering of sun radiation. Therefore the produced compounds which have siloxane group can be used in self-cleaning process of solar mirrors to form hydrophobic surfaces using the Nano- silica composites. This will be benefit for a lot of industrial applications 53–60.