2.1. FT-IR
Figure 2 depicts the FT-IR spectra in the range of 1600–400 cm
−1 of the hybrid xerogels synthesized at different molar percentages of organic precursor. The spectra in the range of 4000–2750 cm
−1 are displayed in
Figure S1 of the
Supplementary Material.
In
Figure 2, the characteristic bands of the amorphous silica matrix can be observed: (i) Rocking of O–Si–O at 455 cm
−1 (ρ O–Si–O), (ii) symmetric Si–O–Si stretching vibration at 800 cm
−1 (ν
s Si–O–Si), (iii) Si-O bond stretching vibration of the silanols on the surface at 955 cm
−1 (ν
s Si–OH), (iv) asymmetric Si–O–Si stretching vibration at 1090 cm
−1 (ν
as Si–O–Si), and (v) a wide and intense shoulder from 1350 to 1120 cm
−1 associated with various vibrational modes of the Si–O–Si [
45]. Additionally, a slight shoulder at 550 cm
−1 can be observed, which is associated with the presence of 4-member siloxane rings, (SiO)
4 [
46]. In the spectra, as the molar percentage of organic precursor gradually increases, a new incipient shoulder becomes more evident at 1140 and 1155 cm
−1, and the band at 1080 cm
−1 narrows and stands out, similar to an emerging peak. These changes could indicate that the organic precursor is favoring the formation, within the silica matrix, of new structures with vibration modes (ν
as Si–O–Si) at different specific frequencies [
40]. In the spectral region of 400–2750 cm
−1 (
Figure S1 of the
Supplementary Material), it is possible to observe the stretching bands of superficial silanols (ν Si–OH a 3450 cm
−1) and those resulting from the interaction of these groups through hydrogen bonds (ν Si–OH–H at 3660 cm
−1) [
45].
The presence of the chlorophenyl moiety in the hybrid materials can be confirmed by a set of bands observed in the spectra: (i) The stretching bands of the hydrogens of the benzene ring in the range of 3090–3010 cm
−1 (ν (C-H),
Figure S1), (ii) three C=C bond stretching bands in the spectral range of 1450–1000 cm
−1 (1380, 1085 and 1015 cm
−1), and finally, (iii) three bands corresponding to the deformation vibrations of the C-H bonds in the aromatic rings (at 815, 760, and 500 cm
−1), and a band due to the stretching vibration of the C–Cl bond (at 710 cm
−1) [
43,
47,
48]. It should be noted that the spectrum of the xerogel 20ClPh is not shown in
Figure 2 because a heterogeneous monolith with two well-differentiated phases was obtained: A non-colored transparent phase above (monolith) and an opaque phase below (precipitate), whose spectra turned out to be very different (
Figure S2c in the
Supplementary Material). This difference might be because the organic precursor favors the formation of structures with lower solubility in the reaction media, and thus precipitates. To test this hypothesis and obtain information on those structures, a material using only ClPhTEOS was synthesized, resulting in a white, soft, and extremely hydrophobic solid (100ClPh) (
Figure S2d in the
Supplementary Material).
Figure 3 depicts the FT-IR spectra of the reference, 15ClPh, 20ClPh (monolith and precipitate), and 100ClPh, while
Table 1 displays the list of bands observed in the spectrum of 100ClPh, the vibrations and structures assigned to those bands, and the literature consulted for the assignment.
The bands due to the vibrations of ν
as (Si–O–Si) in the spectral range of 1400–1000 cm
−1 (
Table 1) correspond to simple structures (linear siloxane chains and (SiO)
4 or (SiO)
6 rings) and more compact and complex structures (i.e., polyhedral oligomers of silsesquioxanes, POSS). Oligomers known as open or closed cages (T
7 and T
8, respectively) and short ladders (SLd) are among the best known POSS, which are formed by the fusion of two or more four-membered rings ((SiO)
4) (
Figure 4) [
52,
53,
54,
55,
56].
The formation of the rings that constitute these structures ((SiO)
4) is thermodynamically favored in the oligomerization of siloxanes in acidic media [
45,
57], in contrast to the six-membered rings ((SiO)
6), kinetically favored and typical of amorphous materials [
39,
58]. To study how the precursor affects the formation of ordered structures, it is necessary to know the proportion of (SiO)
4 in the silica matrix. For this purpose, the deconvolution of the FT-IR spectra in the range of 1300–980 cm
−1 was performed using the non-linear least-squares method, obtaining the Gaussian–Lorentzian components. The different distances and degrees of torsion of Si–O–Si bonds in (SiO)
4 and (SiO)
6 rings allow us to distinguish bands belonging to the optical modes of vibration; two in transverse mode between 1100 and 1000 cm
−1 (TO
4 and TO
6) and two in longitudinal mode between 1250 and 1100 cm
−1 (LO
4 and LO
6) [
40,
44,
59,
60]. In this work, a modification of this method has been carried out, consisting of the adjustment of additional bands (
Table 1) corresponding to (i) C=C and C-H vibrations of the chlorophenyl (1450–700 cm
−1); (ii) vibrations of the siloxane groups and Si-O bonds, characteristic of amorphous silica (950 and 800 cm
−1, respectively); (iii) vibrations of the (SiO)
4 rings that make up the POSS: ν
ring-s and ν
ring-as for the “open” species (T
7 and short ladders), and only ν
ring-as for the “closed” species (T
8), due to the ν
ring-s vibration mode being forbidden [
49]; and (iv) ν
as (Si–O–Si) vibration of linear siloxanes. As an example, the calculated spectra and the bands generated for the reference xerogel and 100ClPh are displayed in
Figure 5. The calculated bands and the fit of 5ClPh, 10ClPh, 15ClPh, and the two phases of 20ClPh are depicted in
Figure S3 of the
Supplementary Material.
The percentage area of each component and the residual value (difference between the real spectrum and the fit) are exhibited in
Table S1 of the
Supplementary Material. The percentage of four-membered and six-membered rings was determined by applying the following equations to the Gaussian–Lorentzian bell areas of the TO and LO components:
where A(LO)
6 is the area of the band at 1190 cm
−1, (TO)
6 is the area of the band at 1030 cm
−1, A(LO)
4 is the sum of the areas of the three LO
4 bands, 1160, 1135, and 1120 cm
−1, and A(TO)
4 is the area of the band at 1050 cm
−1.
Table 2 displays the proportion of rings calculated by applying Equations (1) and (2). It can be observed that increasing the molar percentage of ClPhTEOS in the xerogels favors the formation of (SiO)
4 rings (from 46.54% in the reference to 97.35% in 100ClPh).
Another noteworthy observation is the great difference in the percentage of these rings between both phases of 20ClPh: The monolithic phase has a percentage of rings similar to that of the reference 0ClPh, and the phase that precipitates is similar to 100ClPh. This might be explained by taking into account that an increase in ClPhTEOS favors the formation of POSS, which are mainly formed by four-membered rings, and, as their abundance increases, there is a critical point at which these species exceed the molar solubility in the reaction media and segregate as a precipitate [
61].
2.2. 29Si Nuclear Magnetic Resonance (NMR)
29Si NMR spectra of the hybrid materials were obtained to determine the relationship between the molar percentage of the precursor and the relative abundance of silicon species in the xerogels.
Figure 6a depicts the spectra of the hybrid materials normalized with respect to the signal of the dominant species, Q
3, the most intense in all cases.
The signals associated with the less condensed species (Q
1 and T
1) in
Figure 6a are not observed. The dominant species corresponding to the hybrid precursor is the semi-condensed T
2, whose intensity is greater than that of the more condensed T
3 in all the materials. In addition,
Figure 6b displays both the evolution of the relative abundance of Q (Q
2 + Q
3 + Q
4) and T (T
2 + T
3) species with respect to the molar percentage of ClPhTEOS, as well as that of each species. For example, the Q
2 relative abundance increases slightly to stabilize at 11% and Q
4 increases up to 5% ClPhTEOS and then decreases at higher molar percentages of organic precursor.
Table 3 exhibits the chemical shifts of each
29Si species in the spectra and the integrals of the T species.
There is no significant displacement of the chemical shifts with the increase in ClPhTEOS, indicating that the environment of the silicon atoms does not change substantially. The chemical shifts of the T signals (organic precursor) are less negative than those of the Q signals (TEOS) because the chlorophenyl moiety removes less electronic charge from the silicon atom than oxygen, favoring the Shielding Effect [
62,
63]. Additionally, a higher positive charge density in the silicon atom favors nucleophilic attacks and therefore condensation [
64,
65]; however, the more abundant species is the least condensed and not T
3, indicating that the inductive effect exerted by the chlorine atom of the chlorophenyl moiety is weaker than its steric effect, preventing total condensation in the materials. The increase in T
3 species is related to the presence of POSS in the material, since the silicon atoms that form (SiO)
4 rings are mainly condensed species T
3, Q
3, or Q
4 (less condensed structures T
7 and SLd also contain T
2 and Q
2) [
49]. In fact, the shifts of T
3 species (
Table 3) are closer to those observed in T
8 structures (−77 ppm) than to those of the aliphatic R-Si-O
1.
5 species (−66 to −67 ppm) [
66,
67]. Finally, it is worth mentioning that 100ClPh material only contains T units and, as has been verified in the analysis of its FT-IR spectra, it is composed almost exclusively of (SiO)
4 rings.
2.3. X-ray Diffraction (XRD)
Figure 7 depicts the X-ray diffraction patterns of the hybrid materials synthesized at different molar percentages of organic precursor.
All the diffractograms showed a broad diffraction maximum at 2θ~24°, characteristic of the amorphous silica and associated with the distance between the silicon atoms linked by siloxane bridges [
68]. This maximum slightly decreases with the increase in the molar percentage of the organic precursor. Interestingly, another maximum can be observed at 2θ < 10° when the molar percentage of the precursor is increased (10ClPh and 15ClPh). This new diffraction maximum is associated, in the literature, with the presence of ordered domains composed of POSS in the form of cages (T
7 or T
8) or short ladders within the amorphous matrix of the material [
60]. The emergence of this maximum as the one at 24° decreases is consistent with the greater local structuration of the materials as we increase the molar percentage of the organic precursor. This behavior was already seen in previously studied ClRTEOS xerogels (R = M, methyl; E, ethyl, or P, propyl), where it was found that the maximum at 2θ < 10° appeared at lower molar percentages than their analogous RTEOS. In these chlorinated series, the minimum molar percentages containing the new maximum were 30, 1, and 10% for ClMTEOS, ClETEOS, and ClPTEOS, respectively [
37]. The ClPhTEOS series not only share this minimum percentage with the ClPTEOS but also have more T
2 than T
3 species, which could indicate that, although the chlorine atom in the precursors favors the condensation and the subsequent formation of POSS, the steric effect of the bulkier organic moieties (both propyl and phenyl) could generate less condensed POSS, such as T
7 or SLd. In all the patterns, another low-intensity maximum is observed around 45°, which is associated, according to Bragg’s law (
n = 2 in
nλ = 2
d sin
θ), with a replica of the maximum at 2θ~24°, indicating long-range order in the materials.
Table 4 displays the angles, intensities, and distances calculated from Bragg’s law for each maximum in the diffractograms of
Figure 5.
In 1ClPh and 5ClPh materials, the Si–O–Si bond elongates proportionally to the molar percentage of the precursor (maximum shifts to smaller angles), consistent with what was observed in the
29Si NMR spectra, that is, the increase in T species (from the organic precursor) causes a decrease in the average positive charge density of the silicon atoms, and therefore, the siloxane bonds are less polarized. However, as the molar percentage of the organic precursor increases in 10ClPh and 15ClPh, the bond becomes shorter (the maximum shifts to higher angles). This effect, inverse to the one discussed above, is related to the appearance of a diffraction maximum at 2θ < 10° in 10ClPh and 15ClPh, since this maximum is associated with ordered POSS-type structures, where the siloxane bridges (Si–O–Si) that make up the (SiO)
4 rings are more compact than those of the amorphous silica (formed mainly by (SiO)
6) [
40], thus explaining the decrease in Si–O–Si distances associated with the maximum at 2θ~24°. The calculated distances for the additional maximum at 2θ < 10° (displayed in
Table 4) are similar to those associated with the organic moiety in the cage-like structures (1–3 nm) [
39,
54], and to that of the interplane between short ladders [
56,
69]. Additionally, the X-ray diffractogram of 100ClPh is depicted in
Figure S4 (
Supplementary Material). In the diffractogram, a sharp and intense diffraction maximum at 2θ = 6.84° (1.3 nm) can be observed, which confirms that the 20ClPh precipitate contains a large amount of POSS. In a recent study, Nowacka et al. reported an interplane distance of 1.24 nm for ladder-like phenylsilsesquioxane oligomers, suggesting that these are the species formed in 100ClPh [
60].
2.5. N2 and CO2 Adsorption Isotherms
N
2 and CO
2 molecules have a similar size; however, the temperature at which the adsorption takes place is very different, being −196 °C for N
2 isotherms and 0 °C for CO
2 isotherms. If the pores are very thin, N
2 molecules cannot access them due to kinetic restrictions; however, CO
2 molecules can. On the other hand, the high saturation pressure of CO
2 vapor (3.5 MPa) allows the finer microporosity to be explored in detail, which is covered at very low relative pressures. This fact makes the data provided by both isotherms complementary and makes it possible to differentiate the micropores of less than 0.7 nm and even the finest mesopores. If there are no kinetic constraints, N
2 adsorption provides the volume of pores of less than 50 nm, CO
2 adsorption of those sized less than 0.7 nm, and the difference of these values would provide the microporosity between 2 and 0.7 nm. This divergence can be explained by the different adsorption mechanisms taking place in both microporous intervals; in the so-called “primary micropore filling”, the ultramicropores are accessed at a very low relative pressure (
p/
p0 = 0.03) and the adsorbent–adsorbate interactions predominate over those of the adsorbate–adsorbate, whereas, in wider micropores, the adsorbate–adsorbate interactions predominate, favoring a cooperative process of adsorption [
70,
71].
The N
2 adsorption isotherms (at −196 °C) and the CO
2 isotherms (at 0 °C) of the hybrid xerogels are shown in
Figure 9. The isotherm of the reference material has an open knee, a sign of a wide micropore size distribution, characteristic of type I(b) isotherms. Moreover, its slope is pronounced in the adsorption, and it presents a hysteresis loop in the desorption (H2(a)), which is characteristic of a type IV isotherm typical of mesoporous materials. Therefore, the reference material can be considered micro-mesoporous with a type I(b)-IV(a) mixed isotherm [
71]. Except for 1ClPh and 15ClPh materials, the higher the percentage of an organic precursor the smaller the pore volume, obtaining a type I(a) isotherm in all cases, typical of microporous materials with a narrow pore distribution. The isotherm of 1ClPh is a type IV(a) with an H1 hysteresis loop, indicating that, in this case, the organic precursor increases the total volume of pores and mesopores with respect to the reference, an effect that was also observed in analogous hybrid materials previously reported, which is associated with a change in the morphology of the pores, from cone-shaped to inkwell-shaped pores [
27,
37]. The 15ClPh isotherm is type I(a) but, surprisingly, it adsorbs more N
2 than 10ClPh, 7.5ClPh, and ClPh5. This fact is consistent with the value obtained for its skeletal density (
Figure 8), which suggests that in this material, the limit of the organic precursor that the xerogel accepts is practically reached and is therefore heterogeneously distributed in the silicon matrix.
Table 5 exhibits the textural parameters obtained from the adsorption isotherms.
A decrease in the specific surface area (a
BET) with the increase in the molar percentage of ClPhTEOS is observed, except for 15ClPh, which has a larger area than expected. Additionally, the table displays the volume of micropores obtained from the N
2 adsorption data (V
micro(N
2)), and of the narrowest micropores (V
micro(CO
2), where φ < 0.7 nm), determined by applying the Dubinin–Raduskevich equation to the CO
2 adsorption data. Both volumes decrease with an increase in the molar percentage of ClPhTEOS. The average pore size determined by the Barrett–Joyner–Halenda method (BJH APS) indicates that their mesoporosity narrows with the molar percentage of ClPhTEOS until microporous materials are obtained. In comparison with the ClRTEOS materials with the same percentage of the organic precursor, 10ClPh has a higher surface area and V
micro(N
2) than those of ClPTEOS and ClETEOS, but lower than that of ClMTEOS [
37]. This implies, once again, that these materials are less condensed due to the predominance of T
2 over T
3 species and the bulky nature of the chlorophenyl group.
Figure 10 depicts the pore size distribution by applying DFT calculations to the N
2 and CO
2 isotherm data.
All the materials are microporous, with an internal width close to 1 nm (
Figure 10). The distribution shows that the materials do not present a significant volume of mesopores, except for the reference and 1ClPh.