Physicochemical and Morphological Properties of Hybrid Films Containing Silver-Based Silica Materials Deposited on Glass Substrates

Abstract

The main goal of this study was to present a facile and inexpensive approach for the preparation of hybrid coatings by the deposition under ambient air conditions of silver-based silica materials on glass substrates, which can be used to improve solar cells’ performance. The silica materials containing silver nanoparticles (AgNPs) were synthesized by the hydrolytic condensation of tetraethylorthosilicate (TEOS), triethoxymethylsilane (MTES), and trimethoxyhexadecylsilane (HDTMES), under acidic conditions, at room temperature (25 ± 2 °C). The silver nitrate solution (AgNO₃, 0.1 wt. %) was used as a source of Ag⁺ ions. The final samples were investigated through Fourier Transform Infrared Spectroscopy–Attenuated Total Reflectance (FTIR–ATR), Transmission Electron Microscopy equipped with energy dispersive X–ray (TEM–EDX), UV–Vis spectroscopy, Atomic Force Microscopy (AFM), and Raman Spectroscopy (RS). The TEM images confirmed the formation of AgNPs and were found to be around 3 nm. It was observed that AgNPs were embedded in the silica matrix. EDX also confirmed the presence of the resulting AgNPs within the silica material. AFM images demonstrated that the morphology of the hybrid films’ surfaces can be changed as a function of sol–gel composition. RS analysis indicated that silanol groups were significantly present on the silver-based silica film surface. The UV–Vis spectra revealed that the hybrid coatings presented a reflectance of ~8%, at 550 nm. This study will enhance the value of nanocoating technology in optoelectronics, particularly in the development of nanostructures that improve the performance in thin-film solar cells.

1. Introduction

Silver-based nanomaterials have received considerable interest because of their potential for applications, such as catalysis, sensing, food packaging, water purification, and antimicrobial coatings [1,2,3]. In general, metal nanoparticle-based materials have become the focus of many research groups due to their great performance, reduced costs compared to fine silver (99.9% purity), and significant chemical resistance [4]. Some benefits of silver-based silica materials have been recognized by many industries because of the good antimicrobial properties of silver (Ag) against various pathogens from microbial classes (Gram-positive/negative bacteria, funguses, viruses, and parasites) and the low frequency of resistance development [5,6].

Several methods have been used to prepare silver-based silica, such as multi-target sputtering [7], the ion exchange process [8], electrospinning [9], and the sol–gel process [10]. Hybrid silica composite layers on different substrates, in order to achieve coatings with different thicknesses or densities, have been realized through techniques such as deep-coating, spin-coating, plasma spraying, the vapor phase technique, vacuum deposition, and pulsed laser deposition [11,12]. These techniques have some drawbacks, such as high evaporation temperature, high process cost, potential poor bonding of the coating, and the requirement of external energy.

The sol–gel method has been regarded as the frequent approach for the realization of silver-based silica nanocomposites, because it has many advantages, such as versatility, feasibility, lower maintenance and operational costs, low-temperature processing, and the ability to control the shape of particles and the size distribution [13,14].

Many researchers have used templates (e.g., aluminum oxide (Al₂O₃), titanium dioxide (TiO₂), and silica (SiO₂)) to maintain the quality of silver nanoparticles (AgNPs) [15,16]. Several reports have shown that silica (SiO₂) is a good matrix for the stabilization of AgNPs, because it is chemically inert, does not affect reactions at the core surface, and can be easily prepared [17,18,19]. Silica/titanosilicate films based on AgNPs, deposited over the silica substrates, were prepared by Arun et al. [20]. The results showed that these films presented a self-cleaning property. Bois et al. [21] obtained silver nanoparticles in mesoporous silica films using a triblock copolymer. They demonstrated that the size distribution of the particles may be affected by the matrix. Organic–inorganic hybrid films based on AgNPs, fabricated by the deposition of a solution containing tetraethoxysilane and poly(acrylic acid sodium salt) through the dip-coating technique, were reported by Rivero et al. [22]. The results confirmed that the final films presented good mechanical and optical properties, and the coatings may be applied in various areas of activity such as in buildings and pharmaceutical tools. Wei et al. [23] assembled nonconductive silica nanoparticles on silver nanowires to form Ag–nanosilica hybrids. They demonstrated that the obtained hybrids presented good mechanical adhesion, flexibility, and thermal stability. Huang et al. [24] prepared surface mesoporous silica microspheres doped with AgNPs by using a combination between the sol–gel and ultrasonic wave reaction methods. The obtained results indicated that the resulting silica microspheres could be applied as low-cost reusable materials in the field of water purification systems. Gangadoo et al. [25] reported the production of coatings based on AgNPs assembled in clusters, deposited on copper surfaces. They used the ion exchange and reduction–oxidation reactions, and afterward, a surface silanization step, in order to obtain these coatings. Das et al. [26] prepared nanosilica with silver nanoparticles that can be used as antifouling nanomaterial for sustainable water purification. Procaccini et al. [27] related that the coatings can be prepared through the hydrolytic condensation of tetraethoxysilane and methyltriethoxysilane, using silver nitrate. They demonstrated a relationship between the silver nanoparticles’ size and their behavior against microorganisms. AgNP thin films based on system metal nanoparticle/ionic silsesquioxane, deposited on glass substrates, were obtained by Schneid et al. [28]. It was observed that the thin films were thermally stable. Mejía et al. [29] produced silver-based thin coatings through an environmentally friendly method that includes the silanization of clay nanoparticles, using (3-glycidoxypropyl) trimethoxysilane. Li et al. [30] fabricated coatings using mesoporous silica particles loaded with silver chloride. The obtained results indicated that the hybrid coatings possessed good adhesion to the poly(methyl methacrylate) substrate, and high hardness. Pilipavicius et al. [31] produced colloidal silica/silver nanoprism composite coatings on the substrate of glass slides using the dip-coating technique. The results showed that the functionalization of the surface with different silanes (having various functional groups) was necessary in order to achieve a layer of self-assembled silver nanoprisms.

In this paper, we report a simple, rapid, and low-temperature synthesis method to prepare the silica materials containing silver nanoparticles, under acidic conditions, using various silane precursors (tetraethylorthosilicate (TEOS), triethoxymethylsilane (MTES), and trimethoxyhexadecylsilane (HDTMES)) and a silver nitrate solution (AgNO₃). The main aim of this study was to achieve the hybrid coatings by the deposition of obtained silver-based silica materials on glass substrates, which can enhance the efficiency in thin-film solar cells. Physicochemical and morphological properties of the obtained materials were investigated by FTIR–ATR, TEM–EDX, UV–Vis spectroscopy, AFM, and Raman Spectroscopy (RS).

2. Materials and Methods

2.1. Materials

Tetraethylorthosilicate (TEOS, 98% purity), triethoxymethylsilane (MTES, 99% purity), and trimethoxyhexadecylsilane (HTDMES, 85% purity) were purchased from Aldrich (Saint Louis, MO, USA). Tetraisopropyl orthotitanate (TIP, 97% purity) was purchased from Aldrich (Saint Louis, MO, USA) and was added as a reaction catalyst. Maleic anhydride (MA, 99.7% purity) was purchased from Fluka (Philadelphia, PA, USA) and was used as a complexing agent. 2-propanol (99.9% purity) was purchased from Chimreactiv S.R.L. (Bucharest, Romania) and was introduced as a solvent. Hydrochloric acid solution (HCl, 0.1 N) was purchased from Sigma-Aldrich (Saint Louis, MO, USA) and was added as an acid catalyst. Silver nitrate solution (AgNO₃ (0.1 wt. %)) was purchased from Aldrich (Saint Louis, MO, USA) and was added to obtain the silver-based silica material. D(−)glucose solution (1 wt. %) was introduced as a reduction agent of the silver ions (Ag⁺) in solution. This solution was prepared by dissolving the D(−) glucose powder (purchased from Aldrich (Saint Louis, MO, USA)) in distilled water. The reagents were used as received without any further purification.

2.2. Synthesis of Silica Material and of Silver-Based Silica Material

The silica material and silver-based silica material were synthetized via the sol–gel method, under acidic conditions and in an ambient atmosphere, using different silane precursors: tetraethylorthosilicate (TEOS) as a silica source, and triethoxymethylsilane (MTES) and trimethoxyhexadecylsilane (HDTMES) as modifier silane agents. A silver nitrate solution (AgNO₃, 0.1 wt. %) was used for the synthesis of silver-based silica material.

For the preparation of silica materials (without silver nanoparticles (sample A) and with silver nanoparticles (sample B)), the synthesis procedure was as follows: 15 mL of 2-propanol, 0.6 mL of hydrochloric acid solution (HCl, 0.1 N) or 0.9 mL of silver nitrate solution (AgNO₃, 0.1 wt. %), 3.15 mL of TEOS, 2.82 mL of MTES, and 1.05 mL of HDTMES were mixed and stirred for 2 h at room temperature (25 ± 2 °C). Afterward, 0.06 g of MA, 0.45 mL of TIP, and 0.6 mL of hydrochloric acid solution (HCl, 0.1 N) or D(−)glucose solution (1 wt. %) were added to the mixture. The stirring was continuous for 2 h at room temperature (25 ± 2 °C).

In the obtaining process, the silanol groups were generated due to the hydrolysis of TEOS in acidic medium. Then, condensation took place between the silanol groups and alkyl groups from MTES and HDTMES, forming the hydrolyzed species. Meanwhile, the Ag ions on the silica matrix were reduced to Ag during this process due to the addition of the reduction agent (D(−)glucose solution). In this way, the Ag ions occupied the interstitial sites of the sol.

Finally, two materials were made through the sol–gel method. The quantities of silane precursors, added to achieve both materials, are shown in Table 1. These materials were analyzed as white powders (placed into plastic vials, dried, and milled) and as hybrid films (put over glass substrates using the drawdown sample coating with a manual applicator). Before their use, the glass substrates were cleaned by soap and water for 30 min in an ultrasonic bath. Then, the substrates were cleaned with acetone, ethanol, and deionized water. This cleaning procedure was repeated three times. Afterward, the glass substrates were dried in a desiccator under vacuum for 24 h to remove the possible residues and chemical impurities.

The schematic representation of the sol–gel method to obtain the silica materials (without AgNPs (sample A) and with AgNPs (sample B)) and to prepare the hybrid films is shown in Figure 1. The resulting samples were dried and kept at room temperature (25 ± 2 °C) overnight for the solvent evaporation and then characterized by FTIR–ATR, TEM–EDX, UV–Vis, AFM, and RS spectroscopy, in order to analyze the structural, morphological, and optical properties.

2.3. Analysis of the Sol–Gel Silica Materials (without and with Silver Nanoparticles) and of Hybrid Films

2.3.1. FTIR–ATR

The infrared spectra of the materials (obtained as powders) were registered using FTIR equipment (Jasco FT–IR 6300 instrument from JASCO Int. Co., Ltd. (Tokyo, Japan)), assisted with a diamond-crystal ATR accessory. Data were acquired in the spectral range of 400–4000 cm⁻¹ (resolution of 4 cm⁻¹ and a total of 30 scans per spectrum), recorded at ambient temperature.

2.3.2. TEM–EDX

The resulting materials (as powders) were contacted with the Lacey Formvar/Carbon film, copper grids (Ted Pella Inc., Redding, CA, USA) and thoroughly shaken afterward to remove large or loose particles from the grid. Micrographs in bright-field mode (BF–TEM) were obtained using a TECNAI F20 G² TWIN Cryo-TEM (FEI Company, Phillips, The Netherlands) at an accelerating voltage of 200 kV. The associated EDX spectra were obtained using the X-MaxN 80T detector (Oxford Instruments, Oxford, UK), installed on the device.

2.3.3. AFM Analysis

The hybrid films morphology (achieved by deposition of silica material and of silver-based silica material on glass substrates) was tested via Atomic Force Microscopy (AFM). The AFM measurements were registered with an XE100 AFM from Park (Park Systems, Suwon, Korea), assisted with flexure-guided, crosstalk-eliminated scanners set in noncontact mode, thus minimizing the tip–sample interaction. PPP–NCHR sharp tips from Nanosensors^TM were used in all AFM experiments, having the following characteristics: less than 8 nm tip radius, ~125 μm length, ~30 μm mean width, ~4 μm thickness, ~42 N/m force constant, and ~330 kHz resonance frequency. The XEI program (version 1.8.0, Park Systems, Suwon, Korea) was used for displaying the AFM images and for evaluating the roughness. The Scanning Probe Image Processing (SPIP™) software package (version 4.6.0.0) was used for the particles’ histogram distribution, calculated based on the watershed method (SPIP™ User’s and Reference Guide, 2007).

2.3.4. Raman Spectroscopy (RS)

The chemical structure of hybrid films was studied by micro-Raman spectra (μ-RS) recorded in a Horiba Jobin Yvon LabRam HR micro-spectrometer (Horiba Jobin Yvon, Tokyo, Japan) using a 325 nm excitation laser and a ×40 UV optical lens in an Olympus microscope. Analyzed zones, ~1 μm in diameter, were selected with the aid of an optical microscope, with ×10, ×50, and ×100 objective lenses.

2.3.5. UV–Vis Spectroscopy

Transmittance and diffuse reflectance spectra of hybrid films were analyzed by the UV–VIS–NIR spectrophotometer (Jasco V–570 from JASCO Int. Co., Ltd., Tokyo, Japan), in the range of 380–780 nm, provided with an integrating sphere (Jasco ILN–472, against Spectralon). Each hybrid film was measured three times (standard deviation ± 0.2%, at 550 nm).

3. Results and Discussion

3.1. FTIR–ATR

The FTIR–ATR analysis was monitored to investigate the structure of silica materials (without AgNPs (sample A) and with AgNPs (sample B)), as shown in Figure 2. The samples were studied as powders.

Analyzing Figure 2, it can be detected that all samples presented the absorption band localized at 1020 cm⁻¹, which was assigned to the Si–O–Si linkage (stretching vibrations). The peak at 910 cm⁻¹ was attributed to the Si–O group (bending vibrations) [32]. The peak that appeared at ~560 cm⁻¹ corresponded to the Si–O–Si bonds of silica (symmetric stretching vibrations) [33]. These bands indicated that, during the hydrolysis and condensation reaction, linear silica networks were formed [34].

In addition, in both samples, other peaks were identified in the range of 2970–2853 cm⁻¹ and were assigned to the C–H bonds from –CH₃ and –CH₂– groups (stretching vibration), in the aliphatic chains of the organosilane [35]. The peak detected at 770 cm⁻¹ was attributed to C–H bonds (out-of-plane vibration) [36]. The band situated at ~3300 cm⁻¹ corresponded to O–H stretching in H-bonded water [37]. The band at 1631 cm⁻¹ was due to –OH bending vibration and could be assigned to adsorbed water [38,39]. The peak localized at 1271 cm⁻¹ corresponded to the Si–CH₃ bonds (stretching vibrations) [40].

The presence of these peaks, detected in the FTIR–ATR spectra of both samples, revealed that the hydrolysis and condensation reactions occurred effectively between the silane species.

After FTIR–ATR spectra evaluation of the obtained materials, it can be observed that no significant modifications were detected between the samples.

3.2. TEM–EDX

The morphological analysis and elemental compositions of the samples (as powders) were evaluated through the TEM–EDX. The TEM images and EDX analysis of silica material (sample A) and of silver-based silica material (sample B) are depicted in Figure 3. EDX analysis was performed to evaluate the silica and silver as indicators of the silver nanoparticles (AgNPs) attachment on the silica matrix. These samples were investigated as powders.

The TEM image of silica material (sample A) showed that this sample had an amorphous structure [41,42]. It can be seen that the organic and inorganic materials were hydrolyzed and covalently bonded in the sol–gel process and any phase separation was not observed. Analyzing the TEM image of silver-based silica material (sample B), it can be observed that the silver nanoparticles (AgNPs) were embedded in the silica matrix. The main population of AgNPs could be detected (scale of 20 nm) and the average size of AgNPs was approximately 3 nm. The TEM image of sample B showed the formation of the AgNPs with spherical shape, uniformly distributed in the amorphous silica matrix [29]. The alkyl groups of silane precursors could interact attractively with AgNPs in the silica matrix [43]. It can also be observed that, during the sol–gel process, some of the silver nanoparticles tended to agglomerate in some places. This fact could be due to the unattached silane precursors to the AgNPs surface.

The particle size distribution for silver-based silica material (sample B) represented a wide nanoscale range (2.94 ± 0.08 nm). The histogram raw data were fitted by a Gaussian distribution. The obtained result indicated that the proposed sol–gel synthesis route is a good method for silver-based silica nanomaterial synthesis.

From EDX analysis, the presence of C, O, and Si signals could be observed, in both samples, originating from silica materials that were obtained by hydrolytic condensation of silane precursors. The Ti signal was detected and originated from tetraisopropyl orthotitanate (TIP) that was added as a catalyst in the sol–gel reactions. Considering the Ca and Cu lines scan, it can be concluded that the carbon foil on the TEM copper grid contributed to the signals.

The EDX spectrum in analysis mode for silver-based silica material (sample B) indicated the presence of the Ag signal. This result confirmed that the silver nanoparticles were attached to the silica template.

Silver nanoparticles (AgNPs) were used in this process in order to obtain the antimicrobial effect, and the results are shown in Figure S1 and Table S1 of the Supporting Information, Section 3.

3.3. AFM Analysis

The AFM analysis was achieved in two areas in order to observe the layer uniformity, defects, or cracks (scale of 8 μm × 8 μm), and to observe the porosity of hybrid films and the presence of Ag on the surface (scale of 0.75 μm × 0.75 μm).

Figure 4 displays the 2D AFM images of the hybrid films (achieved by the deposition of silica material (sample A) and of silver-based silica material (sample B) onto glass substrates, registered at the scale of 8 μm × 8 μm. Below each AFM image, the characteristic line-scans are presented and collected at the position specified in each image by a horizontal red line, in the horizontal (fast) scanning direction.

By examining the AFM images, it can be detected that the silica–hybrid film (see Figure 4a) exhibited random protruding clusters of silica material, in the form of rounded hills, not exceeding 20 nm in height. These hills-like silica clusters had a mean diameter of 183.1 ± 9.6 nm, as estimated from the Gaussian fit of their diameter histogram, presented in Figure 4b. The root-mean-square (RMS) roughness of the silica–hybrid film was 5.3 nm, and it had a peak-to-valley parameter of 49.0 nm. In contrast, the silver-based silica material (Figure 4c) exhibited a structure of superficial pores, which were no deeper than a few tens of nanometers (as estimated from the AFM images, and within the limit of the tip dimensions), as noticeable in the line scan depicted below the AFM image from Figure 4c. The Gaussian fit of their diameter histogram was 98.0 ± 18.4 nm. The RMS roughness of the silver-based silica film was 4.1 nm, and the corresponding peak-to-valley parameter was 54.2 nm.

The specific morphology of the hybrid films was revealed by AFM scanning of small areas, 0.75 μm × 0.75 μm, as exhibited in Figure 5, outside of defects such as protrusion and pores.

From Figure 5a, it was estimated that, locally, at this very small scale, the RMS roughness of the silica-hybrid film was 0.83 nm, and it had a peak-to-valley parameter of 49.0 nm. The mean diameters of the very small superficial silica particles were assessed to be 14.2 ± 0.3 nm, as can be obtained from the corresponding particles’ diameter histogram presented in Figure 5b. Meanwhile, the silver-based silica material presented larger superficial grains, possibly because of the presence of the silver at the surface (as suggested by TEM analysis), which may enhance the nucleation of the superficial particles. Their diameter was estimated at 54.2 ± 2.0 nm, from the particles’ diameter histogram presented in Figure 5d. These superficial larger particles were determined to have a larger RMS roughness of 3.4 nm and a peak-to-valley parameter of 41.4 nm.

3.4. Raman Spectroscopy (RS)

The homogeneity of the hybrid films achieved by the deposition of silica material (sample A) and of silver-based silica material (sample B) was further studied by μ-RS. The micrographs in Figure 6 were taken from the Raman optical microscope, in order to identify the areas analyzed by the μ-RS. Hill-like regions in sample A, described in the AFM results, displayed the same Raman features than those measured in intermediate regions between them. The μ-RS demonstrated very good reproducibility of the Raman features in each analyzed zone, including hill-like areas in sample A.

Figure 6 shows the region between 400 and 1300 cm⁻¹ of the μ-RS, associated with the silicate network vibration modes. The region below 700 cm⁻¹ corresponded to vibrations of inter-tetrahedral Si–O–Si links. The broadband at 450–500 cm⁻¹ corresponded to the Si–O–Si links (bending vibrations) between adjacent, coupled SiO₄ tetrahedral groups [44], thus named ‘tOt’ band in Figure 6. The Raman shift of the band maximum depends on the average angle between the tetrahedral; therefore, the band width is associated with the distribution of these angles. The presence of this broadband in the sol–gel-derived coatings indicated a well-formed network of inter-tetrahedral bridges within the amorphous silica structure. The peaks at ~500 and ~609 cm⁻¹ corresponded to the D₁ and D₂ defect modes, related to vibrations of oxygen atoms in four- and three-membered rings, respectively [45]. The number of both four-membered rings (D₁) and three-membered rings (D₂) seemed to increase in the silver-based silica film. The band situated at ~800 cm⁻¹ corresponded to the Si–O bonds (stretching vibrations) [46,47]. The low-frequency region of the μ-RS revealed a similar tetrahedral network structure in silica and silver-based silica films.

The high-frequency bands above 900 cm⁻¹ corresponded to the silicate tetrahedral units (symmetric silicon–oxygen stretching vibrations), with four, three, two, and one nonbridging oxygen atoms [48]. These units are commonly named as the Q⁰, Q¹, Q², and Q³ species [49]. The band at ~950 cm⁻¹ corresponded to Q² species and the strong wide band at ~1050–1150 cm⁻¹ corresponded to Q³ species [48]. The number of Q² and Q³ species showed no significant change between the two types of silica films, while the stronger contribution at the 950–1000 cm⁻¹ region in sample B, seen as a shoulder of the Q² band in sample B, corresponding to the characteristic band of (Si–OH) silanol groups [46,47] around ~980 cm⁻¹, indicated that the silanols groups were significantly present on the silver-based silica film surface.

3.5. Optical Properties

The UV–Vis spectra of hybrid films (achieved by the deposition of silica material (sample A) and of silver-based silica material (sample B) onto glass substrates) were measured in the wavelength range between 380 and 780 nm, as revealed in Figure 7a,b. The glass substrates were covered by depositing materials on one side only.

From Figure 7a, it can be detected that the glass substrate covered with silica material (sample A) showed a slight increase in the transmittance of about 92% in the wavelength range of 400–650 nm as compared to the uncoated glass and to the glass substrate coated with silver–silica material (sample B) (~88%). This result indicated that the increase in transmission was achieved by the smooth transition of the incident light due to the antireflective coating (index matching) [12,50]. In the case of a glass substrate coated with silver-based silica material (sample B), the transmittance was lower than uncoated glass. For this hybrid film, the mirror effect did not appear, due to the formation of the silver maleate on the film surface, which led to the decrease in transmittance. The plasmon resonance energy was affected by the metal nanoparticles’ closeness and due to the modification of AgNPs from the aqueous dispersion to film medium. Moreover, it should be considered that the resonance effects may cause destructive film interference between scattered and unscattered light beams, which may produce the reflective surfaces.

The diffuse reflectance spectra are revealed in Figure 7b and show the antireflective character of hybrid films. The spectra of these hybrid films were evaluated at 550 nm because the human eye is most susceptible in the yellow–green area of the visible light spectrum [51].

Analyzing Figure 7b, it can be seen that the reflectance of the coated substrates decreased by the deposition of hybrid films. For glass substrates coated with silica material (sample A) and coated with silver-based silica material (sample B), a low decrease in reflection in the visible range was detected, compared with the uncoated glass substrate. The diffuse reflectance of these hybrid films was ~8%, at 550 nm. It was demonstrated that the antireflective properties of films with a single layer were influenced by the destructive interference of the reflected light [52]. Furthermore, the reflection may be caused by the film optical interference, which resulted from the multiple reflections between the surface and the film–substrate interface [53].

Yang et al. [54] reported that a glass covered with surfaces that present the antireflective property and have a transmittance of about 96% may increase the effectiveness of a solar collector and the thermal efficiency of solar energy systems.

From data shown in Figure 7a,b, it can be concluded that the obtained hybrid films could suppress the light reflection from the substrate and also improve the transmittance of light.

4. Conclusions

In this paper, we described the preparation of hybrid coatings based on silver-based silica materials through the sol–gel method, under acidic conditions. Silica materials, without and with silver nanoparticles, were synthesized using TEOS, MTES, and HDTMES. Thin films were achieved by the deposition of obtained materials on glass substrates. The structure, composition, and morphology of obtained materials were determined by FTIR–ATR, TEM–EDX, AFM, UV–Vis, and RS spectroscopy. FTIR–ATR spectra indicated that the hydrolysis–condensation process between silane precursors occurred successfully. The TEM image showed that the silver nanoparticles, with an average size of 3 nm, were attached on the silica matrix. EDX also confirmed the presence of the resulting silver nanoparticles within the silica material. In summary, it was demonstrated that the silver nanoparticles may be embedded in the amorphous silica matrix. AFM images showed that the morphology of hybrid films may be changed as a function of the sol–gel systems. Raman results indicated the glassy structure of both silica and silver-based silica films. UV–Vis spectra showed that the hybrid films had a reflectance of about 8%, at 550 nm. Therefore, this study provides a feasible solution for the realization of silica nanostructures that can enhance the efficiency in thin-film solar cells.

Author Contributions

Conceptualization, V.P. and V.R.; methodology, V.P. and V.R.; formal analysis, F.M.R., M.A., M.P., M.C., C.F. and G.C.I.; investigation, A.R., F.M.R., M.C., C.F. and R.Ş.; data curation, M.P., R.Ş. and S.C.; validation, A.R., M.A. and G.C.I.; writing—original draft preparation, V.P., V.R., M.A. and S.C.; writing—review and editing, V.P., V.R., M.A. and S.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the INCDCP ICECHIM Bucharest 2019–2022 Core Program PN. 19.23-Chem-Ergent, Project No.19.23.03.04.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data described in this article are not publicly available, as it contains information that could compromise the confidentiality of research participants.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 1. Schematic representation of the sol–gel method to obtain the silica materials (without AgNPs (sample A) and with AgNPs (sample B)) and to prepare the hybrid films.

Figure 2. FTIR–ATR spectra of silica material (sample A) and of silver-based silica material (sample B), obtained as powders.

Figure 3. TEM images and EDX analysis of silica material (sample A) and of silver-based silica material (sample B), obtained as powders, and diameter histogram of the silver-based silica material (sample B).

Figure 4. (a) 2D AFM images of the hybrid films formed by deposition of silica material. (b) Diameter histograms of the superficial protruding clusters formed on the surface of silica film and of silver-based silica material. (c) 2D AFM images of the hybrid films on glass substrates, registered at the scale of 8 μm× 8 μm, accompanied by representative line-scans. (d) Diameter histograms of the superficial protruding clusters and on the surface of silver-based silica film.

Figure 5. (a) 3D AFM images of the hybrid films formed by deposition of silica material. (b) Diameter histograms of the superficial particles formed on the surface of silica film (c) 3D AFM images of silver-based silica material on glass substrates, registered at the scale of 0.75 μm × 0.75 μm. (d) Diameter histograms of the superficial particles formed on the surface of silver-based silica film.

Figure 6. Optical micrographs and μ-RS from hybrid films (glass substrates coated with silica material (sample A) and with silver-based silica material (sample B)).

Figure 7. (a) Transmittance and (b) diffuse reflectance spectra of hybrid films (glass substrates coated with silica material (sample A) and with silver-based silica material (sample B)).

Table 1

Reagents used for synthesis of silica material (sample A) and silver-based silica material (sample B).

Sample	TEOS(mL)	MTES(mL)	HDTMES(mL)	Concentration of AgNO3 Solution (wt. %)
A	3.15	2.82	1.05	-
B				0.1

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings12020242/s1, Figure S1: Antimicrobial activity of silica material (sample A) and of silver–silica material (sample B), against S. aureus and C. albicans; Table S1: Diameter of inhibition zone obtained by the diffusion method through spot inoculation for the silica material (sample A) and the silver–silica material (sample B), against S. aureus and C. albicans.

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