3.1. Characterization of As-Prepared NPs
In a previous work, the XRD patterns of pure and doped TiO
2 nanopowders showed that anatase phase with tetragonal geometry (JCPDS-782486) was obtained [
6]. Based on the XRD data, the average crystallite, lattice parameters and lattice volume were estimated. The values related to all samples are illustrated in
Table 1. After doping pure NPs with silver ions, a slight decrease in crystal size was observed; this fact demonstrates that silver ions slightly inhibit the crystal growth of pure TiO
2 NPs. The obtained results are in contrast with those reported by Pham and Lee [
24] who stated that Ag
+ ions, when they are localized on the surface of TiO
2 NPs, are able to hinder the crystallization of the TiO
2 anatase phase. In our study, it can be revealed that doping TiO
2 by Ag ions slightly affects the lattice parameters and even the lattice volume of pure TiO
2 NPs. However, the detected reduction may be attributed to measurement and/or instrumental errors during the performance of the analysis. The negligible variation in lattice volumes after doping TiO
2 with silver ions suggests that Ag
+ is not inserted inside the TiO
2 lattice, as would be expected if the difference in ionic radius of Ti
4+ (0.68 A°) and Ag
+ (1.26 A°) were to be considered [
25]. Indeed, if this had happened, a volume inflation would have been noticed in Titanium dioxide lattice, which is not the case in this study. Although the replacement of Ti from its lattice by Ag requires an important amount of energy and is not easy because of the high ionic radius of silver ions (Ag
+) compared to Ti
4+, a small fraction of Ti
4+ could be substituted by Ag
+, resulting in the formation of only small amounts of Ti
3+ [
24].
Additional data on the phase crystallinity of the obtained NPs is provided by Raman spectroscopy. Based on our previous work, the six Raman active modes (A1g + 2B1g + 3Eg) of TiO
2 anatase phase have been observed [
6]. A shift in the signals, particularly in Eg mode, is observed as the Ag content increases in the samples. It appears that the Eg mode of TiO
2 was also observed to shift from 142.73 cm
−1 to a higher wavenumber, and we expect that samples doped with 3 mol% Ag shift back to 142.9 cm
−1 (
Figure 1 and
Table 2). It has been reported that the shift in the Raman peak might probably occur because of an alteration in the structure, the particle size and the nature of defects, etc. [
26]. This alteration is due to the introduction of silver ions into TiO
2 network. Besides, a broadening of the peaks, due to doping effect, can be observed in
Figure 1a and was validated by the calculation of the full width at half maximum [FWHM] of the E
1g mode at 142 cm
−1 (
Table 2).
The broadening of the Raman-active bands can be related to the concentration of oxygen vacancies on the photocatalysts [
27]. In fact, the introduction of the dopant promotes the formation of oxygen vacancies on the oxide surface, raising system disorder. As previously mentioned, the substitution of Ti
4+ by Ag
+ on the surface of the photocatalyst would create oxygen vacancies. These oxygen vacancies are predicted to promote the separation of photo-generated charge carriers and consequently improve the photocatalytic activity. In fact, it was stated that oxygen vacancies and surface defects enhance electron-trapping and light-generated (e
−/h
+) separation processes [
28]. The anatase crystal structure was validated by SAED technique; one SAED ring diffraction pattern with marked Miller indices of the anatase titanium dioxide nanoparticles (JCPDS Card No. 21-1272), as is shown in
Figure 1b. The ring pattern proved that the resulting nanopowders are polycrystalline, while the grainy appearance of rings is related to the fact that the size of the constituent crystallites is in the range of 9 nm
Figure 1c.
An SEM micrograph (
Figure 2a) shows that pure TiO
2 nanopowders are constituted by agglomerated spheres with an approximate size of 500 μm. These spheres are formed from an enormous amount of small particles (10–20 nm), as can be revealed from
Figure 2b.
3.2. Photocatalytic Degradation
The photo-degradation efficiency of pure/doped NPs with a different doping % of silver ions was evaluated through the photo-decomposition of the MB stain, under both UV and visible light irradiation. The K
ap values of the catalyst (
Table 3) show that the decomposition of MB dye increases considerably with the incorporation of Ag ions. In fact, bare TiO
2 shows a very low decolorization rate, while the degradation rate constant (K
ap) for Ag-doped TiO
2 NPs is considerably increased. In particular, under UV irradiation, K
ap values are found to be increased by 2.6, 2.5, 2.3 and 2 times for samples doped with 0.1, 1, 3 and 5 Ag mol%, respectively, compared to pure TiO
2. Under the visible light, 0.1% doping induces the highest increase rate (by 4.4 times), while NPs doped with 1, 3 and 5 Ag mol% showed a comparable behavior with an average MB degradation rate constant 3.7 times larger than undoped TiO
2. Therefore, doping with the lowest Ag amount (0.1%) corresponds to the highest increase in the highest photo-degradation rate under visible and ultraviolet light illumination.
In order to understand the photocatalytic process and the role of reactive species (HO
●, h
+, O
2●) involved in the degradation of MB, Ali et al. [
29] performed scavenger analysis. It is based on using scavengers, dissolved separately into the MB dye solution, to trap hydroxyl radicals (HO
●), hole (h
+) and superoxide radicals (O
2●). Results showed that holes and superoxide radicals play little role in the degradation of MB dye, while hydroxyl radicals were the major active species. MB dye can be degraded by the photocatalysis process. When Ag-doped TiO
2 samples are exposed to visible illumination, an electron is transferred from the valance band (VB) of TiO
2 NPs to the localized states created by Ti
3+, and thereafter to the conduction band (CB) according to Equation (1), which might be transferred to the silver NPs deposited on the surface (
Figure 3). The above processes would assure an effective separation of photogenerated positive and negative charge carriers.
The accumulated electrons in the TiO2-Ag NPs react with adsorbed oxygen and produce ROS such superoxide anion O2−●. On the other hand, hydroxyl radicals are produced due to a reaction between holes present in the valance band of TiO2 and water molecules or the OH group. The generated hydroxyl radicals and separated charge carriers then contribute to the decomposition of organic pollutants.
3.3. Antibacterial Activity
The inactivation of Gram-negative and Gram-positive bacterial strains, after contact with pure and doped suspensions, was investigated under visible light and in the dark at two contact times (12 and 24 h). After each contact time, the bacteria was incubated at 28 °C during 48 h to determine the percentage of surviving cells. The results are illustrated in the histogram, as shown in
Figure 4. Outcomes showed a significant difference in the antibacterial behavior of silver-doped TiO
2 NPs compared to pure NPs. In fact, a total inactivation of
S. maltophilia was observed after being in contact with Ag-TiO
2 suspensions during 12 h under both visible irradiations and in the dark, regardless of the amount of silver ions, as can be shown in
Figure 4a. After 12 h of contact,
M. luteus inactivation was observed only after being in contact with both 3 and 5 mol% Ag-doped TiO
2 nanopowders in both working conditions (
Figure 4b). The variation in the antibacterial ability with Gram-positive strains has been exhibited by Ag-doped TiO
2 due to the varying amounts of Ag used for doping. The time required for the complete inactivation of
M. luteus under solar light, after contact with 0.1–1 mol% Ag doped samples, was longer than the time required for
S. maltophilia inactivation (24 h). Doping TiO
2 NPs with silver not only takes part in the improvement of the photo-response activity of nanoparticles, but also implemented their antibacterial performances. In fact, the inclusion of Ag ions added an extra antibacterial capability, in addition to those associated with TiO
2 photodegradation.
After 15 days, some colonies started to grow near the controls and 0.1% Ag-TiO
2 nanoparticles (
Figure 5), whereas no growth was observed near 1%Ag-TiO
2 NPs. These findings reveal the efficiency of 1 mol% Ag-TiO
2 NPs in inhibiting microbial growth.
The antibacterial activity of silver ions has been studied for a long time in detail [
30,
31]. It was reported that silver ions are known to induce denaturation of proteins present in bacterial cell walls and inhibit bacterial growth [
32]. Denaturation of proteins is a process in which proteins lose their native state due to some external stress, such as radiation, heat or notable compound, resulting in the disruption of cell activity and possibly cell death. Lehninger et al. [
33] reported that silver ions interact with proteins by reacting with the sulfhydryl (SH) groups present in bacteria, inducing the inactivation of the proteins. Indeed, the primary molecular target for silver ions resides in cellular -SH groups, which are of ultimate importance for the activity of many enzymes and protein structures [
34]. A further study performed by Liau et al. [
35] stated that the interaction of Ag
+ with thiol groups played an essential role in bacterial inactivation. In fact, Ag ions may participate in photocatalytic oxidation reactions between oxygen molecules in the cell and thiol groups, promoting the formation of disulfide bonds (R-S-S-R) [
29], and consequently causing the blocking of respiration and the cell death of the bacteria [
36]. More recently, Feng et al. [
37] studied the antibacterial activity of Ag
+ ions against a Gram-negative (
E. coli) and Gram-positive (
S. aureus) bacteria. Results revealed the existence of silver and sulfur elements in the cytoplasm. Moreover, a high amount of phosphorus, the primary component of DNA molecules, was detected in the middle of the cells. Therefore, it was believed that DNA loses its replication ability and cellular proteins become inactivated due to Ag
+ treatment. Another report announced that, in addition to the fact that silver ions inhibit several functions in the cell and consequently induce their damage, the generation of reactive oxygen species, possibly produced as a result of the respiratory inhibition of the enzyme(s) by silver ions, contributes to the attack of the cell itself [
38].
3.4. Application of Protective Coatings on Serena Stone Specimens
As a first step, nanoparticles were mixed with PDMS at different powder/PDMS ratios (0.1, 0.2, 0.5 and 1%
w/
v TiO
2) to investigate the effect of nanopowder concentrations on the chromatic and hydrophobic features of tested stones. The different formulations were applied to the SS specimens (1.5 g of each formulation corresponds to 6 g/m
2), and results showed that all coatings displayed water-repellent features (α > 90°) but unacceptable chromatic variations, since ∆E* was above the recommended standard value (ΔE* < 5), as shown in
Table 4 [
17].
In order to deeply understand the influence of nanocomposite thin films on each chromatic coordinate (L*, a*, b*) of SS, results are graphically presented in
Figure 6. The ∆L*, ∆a* and ∆b* values correspond to the difference between the value of each chromatic coordinate of the stone specimen before and after treatment. Results showed that L* is the most affected chromatic coordinate by all the applications of PDMS while b* and a* underwent less significant variations. Outcomes also indicate that ∆L* assumes negative values, which means that the applied coating caused a darkening of the SS surface. After introducing TiO
2 NPs into the binder, ∆L* showed only a slight variation compared to the plain binder, suggesting that PDMS is mainly responsible for the SS chromatic coordinate alterations. Particularly, samples treated with the highest NPs concentration showed the lowest ∆L* alteration. Considering these outcomes, we decided to perform additional experiments with a nanocomposite containing 1%
w/
v NPs, and to dilute it before application with an appropriate solvent in order to moderate the darkness induced by the polymer on the stone surface. Moreover, concerning the doped material, we focused on the material containing 1 mol% Ag-TiO
2 NPs, since they showed optimal performances in terms of (i) microbial inhibitor behavior (see
Figure 4) and (ii) photocatalytic effect under both UV and visible light irradiation (see
Table 3). In addition, the corresponding nanocomposite provided less drastical chromatic variation in the substrate, compared to the other tested materials containing doped NPs with different Ag contents.
As a second step, PDMS was mixed with
t-butanol at 1:10 ratio, and 1.5 g of the resulting product (corresponding to 6 g/m
2) was applied to SS specimens. The same dilution was performed even for all corresponding nanocomposites. The obtained findings of the contact angle and chromatic changes measurements are resumed in
Table 5. All treatments, even with diluted PDMS, provided a contact angle higher than 90°, which reflects the hydrophobic character of the resulting coatings. It is worth noting that PDMS coatings are hydrophobic, and based on the obtained results, the addition of nanoparticles does not exhibit a significant variation in the hydrophobic character of coatings. However, it can be also deduced that dilution of PDMS was not sufficient to obtain acceptable chromatic variation, as suggested by ΔE* values (∆E* = 6–11). Here also, it can be mentioned that the L* is the most affected chromatic coordinate, and it shifted to having positive values (i.e., lightness) when samples were treated with PDMS/TBA + 1 mol% Ag-TiO
2 NPs. The variation in b* is more relevant in this case as it moved from the blue to yellow color, since only negative ∆b* values were obtained, particularly in the presence of NPs.
In order to verify if lower amount can reduce the ∆L* induced by PDMS, we decided to test lower amounts of applied product; thus, the protective coating (PDMS:
t-butanol 1:10) was applied according to 2 and 3 g/m
2, in addition to the amount already considered (6 g/m
2). Interestingly, samples treated with the lowest amount caused acceptable color modification, as can be proved by ΔE* values that are lower than or equal to 3 (
Table 6). Decreasing the polymer amount applied did not significantly affect the water-repellent features of the coatings that showed contact angle values greater than 90° as can be revealed from
Table 6, approving the hydrophobic behavior of the treated stone surface.
Based on the obtained findings, we decided to set the applied amount at 2 g/m
2 and the powder/binder ratio at 1%
w/
v and then investigate the suitability of PDMS, mixed with neat and 1 mol% Ag-TiO
2 NPs at these conditions. Results showed that all treated samples exhibited water-repellent characteristics (i.e.,
α above 90°) and color alteration lower than 5 (
Figure 7), which is considered to be lower than the limit for human eye perception [
17].
From these results, we can conclude that working with diluted PDMS (with TBA), applying only 2 g/m2 of product and fixing NPs/binder ratios at 1% w/v are the optimal conditions to obtain an appropriate nanocomposite coating for the preservation of Serena stone-made historical materials.
In order to investigate the distribution of the nanoparticles (doped NPs with the binder) on the Serena stone surface, SEM analysis was performed, and the obtained results were reported in the
Figure 8. For instance, it clearly showed the changes that appeared on the stone surface after the application of the protective coating (
Figure 8c,d). As mentioned in the introduction, Serena stone is a sandstone which has low porosity (5–10%), and its natural morphology can be easily observed in
Figure 8a,b. Moreover, a higher magnification image clearly showed the grains in the Serena matrix to be made of silicates and/or Quartz (with the chemical formula SiO
2). On the contrary, the doped NPs-PDMS treatment homogeneously dispersed on the stone surface by covering those silicate grains as well as the other compounds present on the stone surface, which provided good protective coating to the Serena stone. In addition, the higher magnification image shows how doped NPs englobed inside the polymer matrix by conjugating each other, which provided the good coating layer to the stone surface as reported in our previous papers (
Figure 8d) [
1,
6].
All the results obtained from the preliminary analyses enabled us to understand the acceptable proportions and the suitable amount of nanocomposite material when it is applied as a protective coating to the Serena stone surface. As reported in our previous papers [
1,
6,
7], further analyses should be performed to confirm the protecting effectiveness of this coating on this type of stone. Further experimental work on this is in progress in the laboratory.
The performance of the self-cleaning effect and anti-microbial effect is expected to be the same, since the PDMS binder did not affect the photocatalytic and the photo-killing performances of 1 mol% Ag-TiO
2 NPs, as reported in our previous work [
6]. Results showed that the hydrophobic character of PDMS makes the microbial suspensions form a droplet shape on the stone surface, and prevent them from spreading overall across the stone surface; therefore, biofilm was formed only where the drops were present. On the other hand, PDMS, due to its water-repellent performance, prevents MB dye from penetrating inside the pores, and a stain remained on the surface making degradation more feasible. The outcomes showed clearly that the properties given by the NPs were not affected by the stone substrates. Nevertheless, the self-cleaning performances of nanocomposite-coated Serena stone are under investigation.
On the other hand, all our previous studies [
1,
6,
7] suggested that doped NPs with PDMS (ZrO
2-ZnO-PDMS, Ag-TiO
2-PDMS, and Gd-TiO
2-PDMS) coating provided good protective properties to the considered stone substrates (Lecce stone, Marble, Bricks) for a long time without changing self-cleaning properties (the photo-protection and anti-microbial properties). It means that, when NPs ARW englobed in the polymer matrix, they create bonds with PDMS due to the presence of hydroxyl groups. So, they have no ability to easily move, and they stay with a binder material (PDMS). PDMS binds with stone matrix as a coating, together with NPs. In fact, until the coating decays, the NPs stay with PDMS, and they are neither able to go inside the stone matrix nor to the environment alone. As another old polymer coating (acrylic polymer: Paraloid B-72), it can probably be removed using emulsion, in the cleaning process which is commonly performed before restoration of the artefacts. However, the long-time investigation needs to confirm this process, and we are progressing our analysis on these points.