3.1 FTIR analysis
FTIR method analyzed the possible reactions between nano-SiO2 and GPTMS during modification process, which shown in Scheme 1.
Fig 1 shows FTIR spectra of the GPTMS, pure nano-silica and surface modified samples with 30 and 50 wt.% GPTMS, respectively. The main IR absorption peaks of GPTMS were observed at 817 cm-1 (aromatic C-H bending), 910 cm-1 (silane group), 1070 cm-1 (Si-O-Si asymmetric stretching), 1340 cm-1 (aromatic C-H stretching), 1469 cm-1 (C-CH2), 1729 cm-1 (carbonyl group C=O), 1897 cm-1 (CO2+ (NO)2), 2840 cm-1 (C-H symmetric stretching), 2940 cm-1 (C-H asymmetric stretching), 3500 ̴ 3600 cm-1 (O-H stretching) [19, 24–29]. Further, major bonds of the pure sample can be observed at 1110 cm-1 (symmetric Si-O-Si) and 3419 cm-1 (hydrogen bonding between hydroxyl groups) [30, 31]. Moreover, in IR spectra of functionalized SiO2 NPs; symmetric Si-O-Si, C-CH2, C-CH3 and N-H stretching (amide bonds) occurred at 1110 cm-1, 2877 cm-1, 2935 cm-1 and 3426 cm-1, respectively [30, 32–34].
Consequently, proceed in modification process was confirmed by FTIR due to disappearing of silane group absorption peak and also shifting the hydroxyl group bonds in to amide bonds in either modified samples. Moreover, disappearing of the silane group peak in IR spectra of functionalized samples illustrated that all the silane groups were consumed in either sample during functionalization process. Furthermore, intensifying the N-H, C-CH3 and C-CH2 peaks by increasing of silane concentration (from 30 wt.% up to 50 wt.%) at 3425 cm-1, 2935 cm-1 and 2877 cm-1, respectively, showed that 30 wt.% silane was not sufficient to complete the surface modification process.
So, the study was followed by performing other tests on the samples which modified with 50 wt.% and more silane concentrations.
3.2 FESEM
Fig 2 shows the morphology of the pure and functionalized nano-silica samples with FESEM micrographs. The histogram of the samples also illustrate in Fig 3. The measured average diameters of the NPs in pure sample (Fig 2-a) was 25.72 nm. Mean NPs diameters in functionalized samples with 50, 80 and 110 wt.% of silane, shown in Fig 2-b, 2-c and 2-d, were 25.29, 25.63 and 26.26 nm, respectively. The decrease in NPs average diameter of the functionalized samples compared to the pure sample may be due to the use of hydrochloric acid in functionalization process [35].
According to Fig 2 and 3, as the silane concentration increases in the functionalization process, the NPs average diameter increases. It might be due to increase in bonds between the NPs and coupling agent or silane deposition on the NPs [23].
According to Fig 2-a, nano-SiO2 particles agglomerated in some areas due to very high surface energy. As shown in Fig 2-b, NPs modification using 50 wt.% GPTMS, appeared to be insufficient because some aggregations were visible. However, NPs functionalization with 80 wt.% GPTMS (Fig 2-c) should be more effective because of the least visible NPs aggregations. Next, Fig 2-d shows that functionalization of the NPs with 110 wt.% GPTMS led to some deposition on the NPs instead of the surface modification.
3.3 Thermal analysis
Thermo Gravimetric analysis was performed on the samples and the resulted curves illustrates in Fig 4. The curves were described in three separated stages in terms of temperature ranges.
In the first stage, in range 25 and 95°C, there were 0.63, 0.8, 0.7 and 1.16% weight losses in pure, 50, 80 and 110 wt.% GPTMS- SiO2 nanoparticles, due to evaporation of absorbed water during the modification process. The obtained results showed that the amount of water absorption in the last sample was distinctly different from other samples and this might be due to differences in how silane reacted with nano-silica.
The second stage related to thermal stability of the samples against decomposition of the NPs surfaces’ silane groups, in range 130 and 380°C. In this stage, it can be seen 0.3, 11.37, 11.2 and 14.65 weight losses percentage in pure, 50, 80 and 110 wt.% GPTMS-nanosilica, respectively. Lack of functional groups on pure sample surfaces caused negligible weight loss during this stage. Thermal stability in this analysis is directly proportional to bonding strength between coupling agent and NPs. Therefore, in TGA, the higher weight losses percentage, the less bonding strength between silane and samples’ surfaces. The third stage corresponded to decomposition of nano-silica structures in range 483.5 and 560 °C.
Consequently, TGA confirmed nano-silica functionalization, because there were at least 11.2% weight loss in the specimens, which related to decomposition of silane compounds from nano-silica surfaces in range 130, and 380 °C. As a result, 80 wt.% GPTMS-SiO2 NPs was known as better modified sample due to the least weight loss and more stability of grafted coupling agent on the NPs at related temperature range.
3.4 XRD analysis
Fig 5, shows the XRD patterns of the pure and modified nano-silica samples with different coupling agent concentrations. Tetrahedral arrangement of O atoms around the Si atoms caused two dimensional structure with a local short range ordering in amorphous nano-silica [36]. Therefore, there was not any sharp peak in XRD patterns’ and as shown in Fig 5, it can be seen only a broad peak related to quartz in range 20 and 22 degree [37–39].
In case of silanization of nano-silica, the characteristic peak in the XRD patterns tends to crystalline mode due to some changes in the ordering of amorphous structure.
The XRD analysis results summarize in Table 2. According to the analysis results, nano-silica functionalization with different amounts of silane groups caused a little change in the broad peaks position in XRD pattern. Therefore, it would be possible to survey the effectiveness of silane concentration on functionalization process with comparing the XRD patterns.
Table 2. XRD analysis data for pure and modified nano-silica with different silane concentrations
Specimen
|
Pos. [°2θ.]
|
d-spacing[Å̊]
|
FWHM Left. [°2θ.]
|
D-crystallite size (nm)
|
Pure nano-silica
|
20.792
|
4.2687
|
0.062
|
4.0124
|
50 wt.% GPTMS/ nano-silica
|
20.856
|
4.2558
|
0.054
|
4.6073
|
80 wt.% GPTMS / nano-silica
|
21.310
|
4.1661
|
0.049
|
5.0812
|
110 wt.% GPTMS / nano-silica
|
22.287
|
3.9856
|
0.077
|
3.2388
|
As shown in Fig 5-a, the broad peak was appeared at 20.792° which was related to pure amorphous nano-silica.
According to Fig 5-b and Table 2, nano-silica modification with 50 wt.% silane didn’t make much differences in the broad peak shape and position; however, increasing the crystallite size in this sample (at about 15%) compared with pure nano-silica, showed progress in modification process.
Fig 5-c shows that increase in silane concentration (up to 80 wt.%) in NPs modification process caused to be more crystalline properties of the peak in such a way that intensifies the height of the peak as well as reduces its width. Further, increase in crystallite size (at about 10.3%) in comparison with sample B, confirmed more modification progress in sample C.
Finally, with respect to Fig 5-d, increase in silane concentration up to 110 wt.% in functionalization process of sample D, increased the width of the peak again; and also, according to Table 2, the least crystallite size was reported for this sample (decreasing the crystallite size at about 19.3 % in comparison with the pure sample). It might be due to precipitate the silane on nano-silica instead of modification and increase in disordering of atomic arrangement in microstructure of sample D.