Synthesis and Evaluation of a Photocatalytic TiO₂-Ag Coating on Polymer Composite Materials

Abstract

This study explores the development and optimization of TiO₂-based photoactive coatings enhanced with silver (Ag)—to boost photocatalytic performance—for application on glass-fiber-reinforced polyester (GFRP) and epoxy (GFRE) composites. The influence of Ag content on the structural, physicochemical, and functional properties of the coatings was evaluated. The TiO₂-Ag coating showed the best performance and was tested under UV-A irradiation and visible light (Vis), with high efficiency in VOC degradation, self-cleaning, and microbial activity. The tests were repeated in multiple runs, showing high reproducibility in the results obtained. In GFRP, pollutant and microorganism removal ratios of more than 90% were observed. In contrast, GFRE showed a lower adhesion and stability of the coating. This result is attributed to incompatibility problems with the epoxy matrix, which significantly limited its functional performance. The results highlight the feasibility of using the TiO₂-Ag coating on GFRP substrates, even under visible light. Under real-world conditions for 351 days, the coating on GFRP maintained its stability. This type of material has high potential for application in modular building systems using sandwich panels, as well as in facades and automotive components, where self-cleaning and contaminant-control properties are essential.

1. Introduction

Deteriorating environmental conditions and increased air pollution are producing harmful effects on building materials [1,2,3]. Building materials exposed to the environment are attacked by airborne pollutants such as smoke, dust, or microorganisms [4], leading to a reduction in the service life of the materials [1,5,6]. Therefore, the development of building materials with functional properties is a growing research topic.

In recent years, there has been growing interest in developing strategies to prevent soiling of building surfaces using a photochemical approach. This method uses solar energy, an unlimited resource, as a natural and sustainable cleaning mechanism that reduces the need for manual maintenance [7,8,9]. The key to this technology lies in the creation of sunlight-sensitive materials, an area of study that is constantly evolving.

Building materials with photocatalytic and antimicrobial properties can be kept clean by degrading and removing organic residues through photochemical processes, while inorganic contaminants can be removed by external agents such as rain or wind [10]. Among these materials, titanium dioxide (TiO₂) stands out for its remarkable ability to decompose impurities when activated by sunlight [9,11,12,13,14]. This phenomenon allows the generation of high-energy electrons and holes, facilitating the conversion of pollutants into CO₂ and H₂O, as well as possessing antibacterial and deodorizing properties [15]. It is also worth mentioning the wide field of applications of TiO₂ in other areas, including wastewater purification, removal of atmospheric pollutants, fabrication of sensors, solar cells, energy storage devices, and antibacterial materials [16].

Despite its advantages, TiO₂ presents certain challenges, such as the tendency of its particles to agglomerate, which reduces its photocatalytic effectiveness [17,18]. Also, its limited number of active sites and low pollutant adsorption capacity restrict its performance in self-cleaning and decontamination applications, which has prompted the search for improvements in its formulation and structure by combining TiO₂ with doping with other elements [19,20,21].

Over time, research efforts have focused on improving the efficiency of photocatalysts by optimizing their activity and stability, modifying their structure, doping with different metals [22], and developing hybrid nanomaterials. These improvements aim to solve the biggest challenge present in this methodology, the low adsorption in the visible spectrum (at wavelengths from 400 to 760 nm), for which the introduction of metals such as silver (Ag) is of special relevance. On the other hand, the functionality of TiO₂ to improve antimicrobial properties has been extensively studied on textile substrates [23,24,25], water purification [26], or organic dyes [27].

For this study, we aimed to synthesize TiO₂-based photocatalytic coatings with high performance when exposed to UV-A light and coatings that, in addition to being based on TiO₂, contain Ag nanoparticles [28,29] that enhance the coating by making it effective when exposed to visible light (Vis).

The developed coatings were applied to polymeric composite materials. When coated with the photoactive coating, polymeric composite materials have as a main application the construction and transport sector, where the photocatalytic coating provides self-cleaning and decontamination of outside air in vehicles, facades, and modular constructions, providing virus- and microorganism-free spaces in interior modular constructions of sanitary spaces or places where it is necessary to maintain an atmosphere free of pathogens [30].

Today, composites and polymeric materials have revolutionized design and manufacturing in sectors with high economic impact, such as construction, automotive, aeronautics, water treatment, and medicine [31,32,33,34,35]. Composite and polymeric materials offer a unique combination of lightness, mechanical strength, and durability, making them a highly efficient alternative to traditional materials such as metal or wood [36,37]. These composite materials are made up of a combination of several different components, a continuous phase called the matrix, and a dispersed phase that reinforces the matrix, usually using fibers or particles. The most commonly used composite materials are glass fiber composites, carbon, and plant fibers such as hemp; these fibers give the composite an excellent strength-to-weight ratio [38,39,40,41].

On the other hand, polymeric materials such as polyester, epoxy, or polyurethanes have evolved over time to offer significant properties such as chemical resistance, flexibility, and recyclability. Their great versatility and range of applications make these materials indispensable for the manufacture of high-performance, low-cost products [42,43,44]. In the search for more efficient and sustainable solutions, composite and polymeric materials have proven to be a key alternative in different sectors. With low density, high strength, and low costs, they can replace traditional options, reducing electricity consumption and CO₂ emissions in important sectors such as those reported in [45,46,47].

The photocatalytic coating developed on the different substrates considered in this research is based on heterogeneous photocatalysis processes. This advanced technology is based on the interaction between a semiconductor material and light to promote chemical reactions [48]. This process takes place at a solid–liquid or solid–gas interface, where a photocatalyst (TiO₂ in the case of this study) absorbs radiation of a certain wavelength (UV light), generating electron–hole pairs capable of inducing reduction reactions. Heterogeneous photocatalysis has gained great importance in current environmental and energy applications, such as the degradation of organic pollutants, water purification, air decontamination, and hydrogen production by water splitting [19,49,50]. The main advantage offered by this methodology is the ability to carry out advanced oxidation processes without the need for additional chemical reagents, using light, at different wavelengths, as the only energy source.

For this work, TiO₂ and TiO₂-Ag-based sol syntheses were performed, expecting an improvement in the visible spectrum range of the latter. The synthesis of a TiO₂ sol is the preparation of a stable colloidal dispersion of TiO₂ nanoparticles in a liquid medium. This type of methodology is widely used in the fabrication process of membrane and thin-film photocatalytic coatings [51,52]. The most widely used process in sol synthesis and applied in this study is the sol–gel method [53,54]. This process, based on hydrolysis and condensation of metal precursors, offers controlled particle size, high purity, and uniformity in dispersion. In addition, it has the advantage of something very important in this study, which is the possibility of modifying the properties of the sol with the doping of metallic materials or pH adjustments.

In order to provide greater added value to the recycled sandwich panels with composite and polymeric materials developed in previous research [55], the photoactive coating developed in this research was applied to the outer face of these panels for their application in sustainable construction. The recycled sandwich panels with the photocatalytic coating are an interesting proposal for covering facades in buildings or modular constructions, where the coating would be responsible for decarbonizing the air that comes into contact with the panel, as well as keeping the façade clean thanks to the self-cleaning capability that is activated exclusively by exposure to sunlight. The proposed panels are also a very valid alternative for interior constructions in hospitals, sanitary areas, or places where perishable products are stored, where the photocatalyst present in the panel is activated by UV light, which keeps the interior free of microorganisms. This proposal represents advanced technological development, combining material sustainability, real photocatalytic functionality under visible conditions, and experimental validation on polymeric substrates.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Synthesis of the Coatings

For the fabrication of the coatings, titanium (IV) isopropoxide (Ti(OCH(CH₃)₂)₄ (hereafter, TTIP) (97% v/v, Sigma-Aldrich (St. Louis, Missouri, EEUU) was used as the TiO₂ precursor. For the preparation of the TiO₂-based coating, the precursor was dissolved in ethanol (C₂H₅OH) (96% v/v, PanReac) and glacial acetic acid (C₂H₄O₂) (PanReac) (Castellar del Vallès, Spain) was added dropwise in a 1:10:0.5 ratio (mL). The solution was stirred for 30 min at 200 rpm on a Nahita model 690-1 magnetic stirrer. During stirring, the pH of the solution was monitored and kept between 4 and 5 [56], and an off-white solution was obtained (Figure 1a). In the case of pH deviations, glacial acetic acid (CH₃COOH) was used as an acidifying agent.

To produce the TiO₂ coating doped with silver nanoparticles (TiO₂-Ag), a solution of silver nitrate (2 mL) (AgNO₃) 0.5 % w/v (0.1 M, PanReac) was used as a dopant precursor and dissolved in methanol (CH₃OH) (99%, PanReac) and C₂H₄O₂. The aim of incorporating silver is to promote photocatalytic properties under visible light conditions. The solution was kept stirred for 5 min on a magnetic stirrer, and then the precursor solution prepared earlier was added. The solution was kept stirred for 30 min at 200 rpm, and a dark grey solution was obtained (Figure 1b). Both solutions were kept for 24 h at ambient temperature. This aging time was adopted following protocols widely used in the literature [51,53]. Although no systematic optimization was performed in this study, this parameter may influence the particle size, colloidal stability, and photocatalytic activity of the final coating [57]. Finally, TiO₂ and TiO₂-Ag gels were obtained.

The properties of the TiO₂ and TiO₂-Ag coatings on a glass substrate were evaluated by sol–gel chemical analysis using X-ray diffraction (XRD). The measurements were performed using PANalytical’s X’Pert Pro equipment, operating with a Cu-Kα radiation source (λ = 1.5406 Å). The analyses were performed over a range of 4° to 70° (2θ) for 120 min with a step size of 0.0167° and an optimized integration time to improve the signal-to-noise ratio. The tube was operated at 40 kV and 40 mA.

XRD patterns of TiO₂ nanoparticles and Ag-doped TiO₂ nanoparticles are shown in Figure 2. In the obtained diffractograms, characteristic peaks of TiO₂ were identified, which are associated with specific crystallographic planes [29]. The presence of the anatase phase (A) was confirmed by identifying its most representative peaks at the 2θ positions corresponding to crystallographic planes (101), (004), (200), and (105).

On the other hand, the diffractogram corresponding to Ag-doped TiO₂ (TiO₂-Ag) reveals that the incorporation of Ag does not significantly alter the crystal structure of the anatase phase, suggesting that the doping process does not induce major structural transformations. The possible presence of silver in a metallic state was not evident in the diffraction patterns, which could be attributed to its low concentration or to its dispersion in the TiO₂ matrix, making its detection by XRD difficult. Additionally, the presence of an amorphous fraction in the coatings, especially on the surface, is not ruled out due to the sol–gel process conditions, although there are no complementary analyses (such as Raman spectroscopy) to confirm this at this stage. As for the comparative crystallinity between the TiO₂ and TiO₂-Ag coatings, no significant differences in the intensity or width of the anatase peaks were observed, suggesting that the overall structural order level is maintained after Ag incorporation. Despite this, a remarkable increase in photocatalytic activity was observed under visible light irradiation, which is usually associated with plasmonic effects characteristic of Ag nanoparticles. This phenomenon, characteristic of noble metals such as Ag, allows a more efficient absorption of light in the visible region, facilitating the generation of additional charge carriers and improving the efficiency of the photocatalytic process [58]. This observation indirectly supports the presence of metallic Ag, although it does not constitute conclusive proof of its crystalline state. To confirm the presence of Ag, we resorted to SEM-EDX analysis, which detected this element in the sample. However, this technique does not allow determining its crystalline state, so the use of complementary techniques such as XPS or TEM with EDX mapping is considered essential in future works to obtain a more accurate characterization of the chemical and structural state of silver.

The size of the dopant is a key factor in its ability to incorporate into the TiO₂ crystal lattice. The atomic radius of Ag⁰ (0.126 nm) is considerably larger than that of Ti⁴⁺ (0.061 nm). Due to this difference, the diffusion of Ag⁰ within the crystal lattice of TiO₂ is complicated, suggesting that Ag is predominantly deposited on the crystal surface rather than replacing titanium atoms in the structure. This phenomenon has been reported in several studies [59,60] and could influence the optical and catalytic properties of the material, as the presence of Ag nanoparticles on the surface can facilitate light absorption and enhance photocatalytic activity through plasmonic effects. These effects, generated by Ag nanoparticles on the surface of TiO₂, amplify light absorption, especially in the visible range, and promote electron transfer, which improves photocatalytic efficiency by generating more electrons and holes for catalytic reactions.

Using the Carl Zeiss Merlin scanning electron microscope (SEM), the morphology of pure (TiO₂) and Ag-doped (TiO₂-Ag) nanoparticles was observed (Figure 3). The TiO₂ nanoparticles showed a predominantly spherical morphology with a homogeneous nanometric size. In addition, aggregated microcrystals were observed on the surface of these nanoparticles. Concerning the Ag-doped TiO₂ nanoparticles, SEM images revealed an inhomogeneous distribution of Ag on the surface of the TiO₂ particles. Small, dispersed structures attributed to the presence of Ag nanoparticles deposited on the TiO₂ matrix were observed.

These structures could be associated with Ag segregation on the surface, which would influence the optical and catalytic properties of the material. The morphology of the doped particles showed a slight tendency towards aggregate formation compared to pure TiO₂ particles.

Energy-dispersive X-ray spectra (EDS) confirmed the chemical composition of the analyzed samples. In pure TiO₂ nanoparticles, EDS spectra revealed the presence of titanium (Ti) and oxygen (O) along with traces of vanadium (V) (Figure 3a). On the other hand, EDX spectra of Ag-doped nanoparticles (Figure 3b) showed signals corresponding to Ti, O, and silver (Ag), which evidences the presence of the dopant in the analyzed sample.

2.1.2. Substrate Preparation

Three types of substrates were used to evaluate the functionality of the coatings on different surfaces. The first glass substrate was used to characterize the coatings on an inert material and to select the optimal one for application on the polymeric substrates.

The polymeric substrates were chosen because of their strength and lightweight properties. These two substrates were designed for application in different scenarios, such as building envelopes or modular constructions with a focus on the healthcare sector [29].

The first type of substrate is made of glass-fiber-reinforced polyester (GFRP) (Figure 4a). The fabrication procedure consisted of applying a layer of gel coat (Euromere) with an approximate thickness of 0.5 mm that gelled at ambient temperature (21 ± 5 °C) for 15 min. Subsequently, a layer of spun glass fiber was placed on the surface and impregnated with polyester resin 5-1026, supplied by Sumarcoop, to which 2% Promox P200TX hardener was added to optimize curing time and mechanical resistance. This procedure was repeated to form a second layer composed of resin and glass fiber, obtaining a panel with a thickness of 1.4 mm and dimensions of 300 × 300 cm².

The second substrate corresponds to glass-fiber-reinforced epoxy (GFRE) (Figure 4b), with dimensions of 300 × 300 cm² and a thickness of 1.4 mm. Its fabrication followed a similar process to that of the GFRP, but using SikaBiresin CR75 epoxy resin and SikaBiresin CH75-1 hardener in a 100:40 weight ratio.

For the experimental phase, 3 panels of each type of substrate were used. Figure 5 shows the layer arrangement of the substrates.

Polyester resin has a higher viscosity than epoxy resin (4000 cps (20 °C) and 1800 cps (25 °C), respectively), which is inversely proportional to the density values (1.109 and 1.160 Kg/m³, respectively). The handling times of the resins were determined to ensure efficient application. During the open time period, both resins present a gel state that allows adequate workability and molding. During the curing phase, the panels reach about 60% of the final strength. The curing time represents the total time necessary to obtain 96–98% of the total strength.

Table 1. Technical parameters of the resins.

Resin Type	Curing Temperature (°C)	Time Types	Waiting Time Range (Minutes)	Viscosity (cps)	Density (Kg/m3)
Polyester resin 5-1026	25	Open time	16–20	4000 (20 °C)	1.109
		Hardening time	32–37
		Curing time	1440
SikaBiresin CR75	20–22	Open time	10–15	1800 (25 °C)	1.160
		Hardening time	30–35
		Curing time	1440

shows the technical characteristics of the resins used.

2.2. Coating Methodology and Characterization

The TiO₂ and TiO₂-Ag solutions were first applied on three glass substrates in order to evaluate and characterize both coatings on an inert surface. The coating procedure consisted of roller application of the coatings. Although this technique is not automated, it was selected for its suitability for larger surfaces, and specific measures were applied to improve uniformity and thickness control. These included the use of a fixed volume of solution (20 g/m²) and the application with fine pore foam rollers using unidirectional and controlled passes.

After the characterization of the coatings, the TiO₂-Ag coating was applied on the 2 types of previously fabricated polymeric substrates, which proved to be the one with the best properties, using a roller coating procedure (Figure 5) to ensure a homogeneous distribution of the material on the surface of the substrates. The TiO₂-Ag coating was applied on 3 GFRP substrates and 3 GFRE substrates.

The first layer of the solution was applied to the substrates, ensuring complete and uniform coverage. Subsequently, the coated substrates were subjected to a curing process at room temperature (21 ± 5 °C) for a period of 3 h, allowing the evaporation of the solvents and the initial stabilization of the coating.

Once the first curing process was completed, the second layer was applied, reinforcing the integrity of the coating and improving its functional properties. After this second application, the substrates were cured again at room temperature (21 ± 5 °C) for 48 h, ensuring the consolidation of the coating and obtaining a stable and adherent structure on the substrate surface.

The coating was applied in 2 consecutive coats to optimize adhesion and deposit uniformity. A total amount of 20 g/m² was deposited for the coating on the 3 substrates.

Figure 6 shows the coating methodology of the substrates.

The properties of the coatings and substrates were studied by performing structural, morphological, photocatalytic, antimicrobial, and electronic analyses, which are detailed in

Table 2. Parameters, standards, and equipment used.

Parameter	Standard	Equipment
UV–Vis spectrophotometry	-	Spectrophotometer Shimadzu UV-1800 (Shimadzu, Kyoto, Japan)
UV-A spectrophotometry	-	Spectroradiometer Ramses ACC-UV (Trios GmbH, Rastede, Germany)
Diffuse reflectance UV–Vis	-	Spectrophotometer Agilent 8453 (Agilent Technologies Deutschland GmbH, Waldbronn, Germany)
Methylene blue (MB)	ISO 10678:2010 [61]	Spectrophotometer Shimadzu UV-1800 (Jenck S.A., Buenos Aires, Argentina)
Oleic acid (AO)	ISO 27448:2009 [62]	Krüss Easy Drop (Krüss GmbH, Hamburg, Germany)
TCE degradation	ISO 22197-1:2016 [63]	Spectrophotometer Shimadzu UV-1800 and Spectroradiometer Ramses ACC-UV (Trios GmbH, Rastede, Germany)
Ellipsometry	-	Spectroscopic ellipsometer GES-5E (Sopra Semilab, Budapest, Hungary)
DRX	-	X’Pert Pro PANalytical (Malvern Panalytical, Almelo, The Netherlands)
SEM-EDX	-	Microscope Carl Zeiss Merlin (Carl Zeiss AG, Oberkochen, Germany)
Antimicrobial properties	ISO 21702:2019 [64] ISO 18061:2014 [65]	Spectroradiometer Ramses ACC-UV (Trios GmbH, Rastede, Germany)

under the corresponding standards.

2.2.1. Coating Thickness Analysis

The spectroscopic ellipsometer GES-5E (model Sopra Semilab) was used to determine the thickness of the coatings, operating in a spectral range from 190 to 900 nm. The equipment employed a light beam with a spot size of 3 mm and an angle of incidence ϕ = 65° to optimize measurement precision. The ellipsometry technique allowed obtaining nondestructive and highly accurate measurements of the thicknesses of the deposited layers.

For characterization, two layers of each coating were deposited on a glass substrate (3 repetitions were carried out), which was used as a reference due to its optically inert nature, its excellent transmission in the UV–Vis range, and its chemical stability [66]. The choice of this substrate ensured that thickness measurements were not influenced by light absorption or scattering effects.

2.2.2. Analysis of Photocatalytic Activity

The photocatalytic activity of the coatings and substrates was analyzed by the degradation of VOCs using low-power UV-A and UV–Vis. The photocatalytic activity of the coatings was evaluated by degradation of volatile organic compounds (VOCs in air) under the ISO 22197-1:2016 standard [63]. In this case, trichloroethylene (C₂HCl₃) (TCE), a compound commonly used as a solvent in various industrial applications, was used [67,68].

The TCE degradation experiment was carried out in a photocatalytic reactor designed to evaluate the efficiency of the coatings (Figure 7). The operating procedure was based on introducing a TCE gas stream with a total gas flow rate of 300 mL min⁻¹, with concentrations of 25 ppm. As illumination sources, an 8 W/m² UV-A lamp and visible light LEDs (UV–Vis) were used, allowing for evaluation of the activity under different spectral conditions.

The study of photocatalytic activity was carried out following 5 reaction stages described according to the standard. The first stage consisted of an air inlet in bypass mode for 30 min. During the second stage, the air was mixed with the air with the pollutant in bypass mode for a period of 30 min, and then during the third stage, the air was worked in reactor mode for the same period of time. In the fourth stage, the air mixture and the contaminant were exposed in reactor mode to radiation for 300 min, while in the final stage, air cleaning was performed for 30 min.

2.2.3. Analysis of Photocatalytic Performance

The self-cleaning photocatalytic activity was measured by evaluating the degradation capacity of an organic pollutant, methylene blue (MB). The assay was performed under UV–Vis irradiation using the ISO 10678:2010 standard [61] to analyze the photocatalytic performance of the TiO₂-Ag coating, which was previously selected for its better properties, and the performance of GFRP and GFRE substrates with the coating. This parameter was evaluated by studying the concentration (C/C₀) of MB under UV light. The degradation mechanism is shown below:

$${\mathrm{C}}_{16}{\mathrm{H}}_{18}{\mathrm{N}}_3\mathrm{S}\mathrm{C}\mathrm{l}+{\displaystyle \frac{51}{2}}{\mathrm{O}}_2\stackrel{\mathrm{h}\mathrm{v}}{\to }\mathrm{H}\mathrm{C}\mathrm{l}+{\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4+3{\mathrm{H}\mathrm{N}\mathrm{O}}_3+16{\mathrm{C}\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}$$

Additionally, the ISO 27448:2009 standard [62] was applied, using oleic acid (OA) as a contaminant, in order to determine the self-cleaning efficiency of the coating under specific light exposure conditions. The tested specimens had dimensions of 8.0 × 5.0 × 0.2 cm³ and, after the impregnation process, were dried for 15 min at 70 ± 2 °C. The degradation capacity of AO was evaluated by measuring the variation in the contact angle of a droplet (WCA) on the surface with a UV-A radiation exposure time of 1.0 ± 0.1 mW/cm² at 23 ± 5 °C and relative humidity between 40 and 50%. A pretreatment of the specimens with UV-A light with irradiance of 2.0 ± 0.2 mW/cm² for 20 h was previously established. The AO concentration was 0.5% vol. in n-heptane. WCA measurements were performed with Krüss Easy Drop equipment. The degradation mechanism is shown below:

$${\mathrm{C}}_{18}{\mathrm{H}}_{34}{\mathrm{O}}_2+{\displaystyle \frac{51}{2}}{\mathrm{O}}_2\stackrel{\mathrm{h}\mathrm{v}}{\to }18{\mathrm{C}\mathrm{O}}_2+17{\mathrm{H}}_2\mathrm{O}$$

After subjecting the specimens to pretreatment with UV-A radiation for 20 h, the initial contact angle was measured. Subsequently, a layer of AO was applied by immersing the specimen in the solution for 15 min. After extraction, it was allowed to dry at 70 °C for the same period of time.

Then, the contact angle of the impregnated specimens was measured, considering it as the initial time (t = 0) and recording the corresponding value as WCA. From this point, the specimens were exposed to UV-A radiation, adjusting the exposure time until five consecutive measurements were obtained in which WCA remained close to 10°. Additionally, the properties of the substrate without photocatalytic coating were analyzed both in the absence of light and under UV-A irradiation.

2.2.4. Optical Analysis

The optical response of the TiO₂-Ag coating on the different substrates was characterized by UV–Vis diffuse reflectance spectroscopy using an Agilent 8453 spectrophotometer in a spectral range from 300 to 900 nm.

The optical absorption values of the TiO₂-Ag-coated polymeric materials were calculated using the Kubelka–Munk Equation (1):

$$K-M={\displaystyle \frac{(1-R{)}^2}{2R}}$$

(1)

where R is the diffuse reflectance of the material.

2.2.5. Analysis of Crystalline Properties

A structural analysis by X-ray diffraction (XRD) was performed to identify the crystalline phase of the material, evaluate the presence of possible impurities, and determine the degree of crystallinity of the TiO₂-Ag coating on GFRP and GFRE substrates. This analysis allowed correlating the crystalline structure with the photocatalytic and self-cleaning properties of the material.

Measurements were performed using PANalytical’s X’Pert Pro equipment, operating with a Cu-Kα radiation source (λ = 1.5406 Å). The analyses were performed over a range of 20° to 60° (2θ) for 120 min with a step size of 0.0152° and an optimized integration time to improve the signal-to-noise ratio. The tube was operated at 40 kV and 40 mA.

2.2.6. Microscopic Analysis

The microscopic analysis of the coating was carried out by Carl Zeiss Merlin scanning electron microscopy (SEM), which allowed examining the structure and distribution of the nanoparticles, as well as the homogeneity of the deposit on the selected substrates.

SEM analysis was carried out using an accelerating voltage of 15 keV and a magnification of 279x, with a field of view of 1.50 mm and a working distance of 5.88 mm. A beam current of 1 nA and a spot size of 4.10 nm were used to improve image resolution. The Everhart–Thornley (E-T) detector was used. The E-T detector is optimized for secondary electron detection, which allows highlighting the topography of the sample. In addition, the selected scanning mode was ultra-high resolution (UH-RESOLUTION), ensuring a detailed visualization of the analyzed surface.

2.2.7. Analysis of Antimicrobial Properties

Tests were carried out to evaluate the antimicrobial properties of the TiO₂-Ag coating applied on the GFRP substrate, which presented the best self-cleaning capacity. The objective was to determine its effectiveness in eliminating microorganisms and its possible application on surfaces with high hygienic requirements. For this purpose, the ISO 21702:2019 [64] and ISO 18061:2014 [65] standards were followed. The former standard establishes a protocol for measuring antiviral activity on plastics and other non-porous surfaces, while the latter focuses on the evaluation of photocatalytic surfaces with antiviral properties. Because of this, both methodologies were combined using vaccinia virus (VACV ATCC VR-1508), an enveloped virus of the Poxviridae family, analyzing its infectious capacity in the presence of UV-A radiation (2.0 mW/cm²).

Following regulatory specifications, six photocatalytic-coated polymer samples (3.0 × 3.0 cm²) were prepared, on which viral inocula were deposited. Additionally, nine reference samples without photocatalytic coating were included and used as controls. These tests were performed both in dark conditions and under exposure to UV-A radiation to evaluate the influence of the coating on virus inactivation.

After the contact period, virus recovery was carried out by washing the surfaces. Then, the obtained solutions were seeded in wells, incubated, and subjected to serial dilution to determine the reduction in the viral load.

The antiviral activity was calculated according to Equation (2):

R = (Ut − U₀) − (At − U₀) = Ut − At

(2)

And the percentage of reduction was calculated according to Equation (3):

% Conversion= ((Ut − At)/Ut) × 100

(3)

where R represents the biocidal antiviral activity; U₀ is the average of the infectivity titer of the solutions recovered from the 3 specimens without additives at time 0; Ut indicates the average infectivity titer of the solutions recovered from the 3 specimens without additives exposed to ultraviolet light; and At is the average effective titer of the recovered solutions of the 3 treated test pieces exposed to ultraviolet light.

2.2.8. Analysis of Self-Cleaning Performance

The self-cleaning performance was evaluated by studying the effect of the TiO₂-Ag coating on the GFRE and GFRP substrates in the laboratory and in a real environment in real climates and environments. For this purpose, a visual and tangible analysis of the behavior of the substrates coated with the photocatalytic layer was performed. This study was carried out using substrates of dimensions 400 × 300 cm², which were divided into two parts to apply the TiO₂-Ag coating on one of the parts. The substrates were placed at an inclination of 90°, simulating their location on an exterior wall, and fly ash was poured on them as a contaminant material.

The real environment analysis was evaluated by placing the substrates outside in a 90° position to be subjected to extreme summer and winter climates, with temperatures ranging from 0 °C to 50 °C (directly exposed to the sun). The test time was 351 days.

3. Results

3.1. Characterization of Photoactive Coatings

Figure 8 shows the variation in the thicknesses of the TiO₂ and TiO₂-Ag coatings. It was observed that the TiO₂ coating presented a thickness of (52.5 ± 0.3 nm), which is slightly lower than TiO₂-Ag (59.7 ± 0.2 nm). This slight difference in thickness could be related to the modification in the morphology and structure of the coating induced by the incorporation of Ag. In addition, the presence of the dopant could influence the coating density and growth rate during the deposition process [69].

The thickness variability between replicates was less than 3%, indicating good reproducibility in the deposition conditions used. This thickness control is relevant, since significant differences can affect the photocatalytic efficiency of the coating (limiting or favoring light penetration) and its adhesion to the substrate, which are key factors for its functional performance.

Figure 9 shows the TCE conversion under different illumination conditions (UV-A and UV–Vis) for the TiO₂ and TiO₂-Ag coatings. It was observed that the photocatalytic activity varies as a function of the thickness of the photoactive coating, the type of coating, and the illumination source used [70,71].

It was observed that the TiO₂-Ag coating shows higher TCE conversion compared to pure TiO₂ under both illumination conditions. Under UV-A irradiation, the TCE conversion using TiO₂ was 72.9%, while the TiO₂-Ag coating reached a significantly higher value of 94.1%, indicating an increase in photocatalytic activity due to the incorporation of Ag. Similarly, in the UV–Vis region, TiO₂-Ag achieved a conversion of 62.6%, in contrast to 45.7% obtained with pure TiO₂. These results suggest that the addition of silver improves the absorption in the visible spectrum, promoting the generation of reactive species capable of degrading TCE more efficiently. The improvement in the photocatalytic activity of TiO₂-Ag can be attributed to the ability of silver to act as an electron scavenger, reducing the recombination of electron–hole pairs and increasing the efficiency of the photocatalytic process.

Following the improvements observed in the TCE conversion values of the TiO₂-Ag coating, this sol was selected for the evaluation of its self-cleaning properties. For this purpose, methylene blue (MB) (82%, Sigma Aldrich (St. Louis, MO, EEUU)), a dye widely used in photocatalytic studies due to its high stability and strong absorption in the UV–Vis region [72,73,74,75,76], was used as an indicator.

Figure 10 shows the evolution of the absorption of methylene blue as a function of UV–Vis exposure time between 200 and 400 nm. A progressive decrease in the intensity of the characteristic peaks of MB can be observed, indicating its photodegradation throughout the experiment. After 3 h of irradiation, the absorption is reduced to practically zero, suggesting an almost complete degradation of the dye.

Furthermore, optical observation of the samples reinforces these results, showing an evident visual change in the solution. Initially, the dye presents an intense blue hue, but with increasing exposure time, the color progressively fades until it becomes completely transparent, confirming the photodegradation of MB. These results demonstrate that the TiO₂-Ag sol exhibits excellent self-cleaning properties, which are attributed to the synergy between TiO₂ and Ag nanoparticles.

3.2. Characterization of Photoactive Substrates

After characterization of the TiO₂ and TiO₂-Ag substrates, as well as thorough evaluation of their photocatalytic properties, the TiO₂-Ag coating was selected for application on the two previously prepared polymeric substrates. This choice was based on its higher efficiency in pollutant conversion, which was demonstrated in previous photocatalytic degradation tests.

The main objective of this stage was to analyze the influence of the type of substrate on the performance of the photocatalytic coating and whether this could affect the properties of the coating and, therefore, the photocatalytic activity.

3.2.1. Crystalline and Optical Properties

The crystalline and optical properties were determined by XRD (Figure 11) and UV–Vis diffuse reflectance spectroscopy (Figure 12). The results obtained by both techniques showed the presence of the rutile (R) crystalline phase in the GFRP and GFRE substrates. In both materials, the presence of the crystalline phase TiO₂-anatase (A) was observed. This material is part of the gel coat composition and is generally used as a pigment. Therefore, the presence of this crystalline phase in the substrate makes it difficult to characterize the optical and crystalline properties of the photoactive coating. Regarding the UV–Vis spectra, an intense absorption band was observed in the uncoated polymeric materials, with an absorption around 400 nm. The incorporation of TiO₂-Ag in both polymers produces improvements in light absorption. The displacement of the absorption band is related to the incorporation of Ag nanoparticles that produce modifications in the electronic structure of TiO₂, reduce the recombination of electron–hole pairs and favor the generation of reactive species [77].

3.2.2. Photocatalytic Activity Analysis

An evaluation of the photocatalytic activity of GFRP and GFRE samples in the degradation of volatile organic compounds (VOCs) under irradiation with low-power UV-A and visible light (Figure 13) was carried out at different flow rates between 300 and 900 mL/min.

The results evidenced a high photocatalytic efficiency in the presence of UV-A and UV–Vis irradiation, being especially remarkable at low flow rates, which is attributed to the longer residence time of the pollutant on the photocatalytic surface. Regarding the influence of substrate type, GFRP showed superior conversion compared to GFRE; however, this difference attenuates as the contaminant flow rate increases, suggesting a lower influence of the substrate at high flow rates. Nevertheless, both samples have similar behavior for both types of irradiations.

3.2.3. Photocatalytic Performance Analysis

The photocatalytic self-cleaning activity of the prepared coatings was evaluated by the MB decolorization test under UV irradiation and by oleic acid degradation; these methodologies are described in Section 2.2.

In both cases, the samples showed capacity for MB photooxidation. However, there is a variation in the time required for dye disappearance. Figure 14 shows the behavior of GFRP and GFRE in dye degradation. To distinguish the effect of the photoactive coating from the possible impact of other physical or chemical processes, dark reference samples, corresponding to the control polymers without irradiation, were included. These samples showed minimal variations in dye concentration, confirming that the observed degradation is a direct result of coating activation under UV-A irradiation and that the substrates studied do not possess degradation capacity by themselves. The GFRP substrate achieved a degradation efficiency of MB close to 90% in 60 min of exposure to UV-A irradiation, while GFRE showed a lower degradation (82%). This decrease is attributed to the more hydrophobic nature of GFRE, which affects the interaction of the TiO₂-Ag coating with the dye, and, on the other hand, to the worse dispersion of the coating on this type of substrate. At 300 min, both substrates showed almost complete degradation efficiency.

The lower dispersion of the coating on the GFRE substrate was evidenced by SEM-EDX imaging, where a less uniform distribution of TiO₂-Ag was observed compared to GFRP, which is possibly related to its surface composition. Although the contact angle or adsorption capacity was not quantified, these factors could have a joint influence and will be the subject of more detailed analysis in future studies.

3.2.4. Humectability

The results obtained are corroborated by oleic acid degradation.

Table 3. Contact angle in GFRE and GFRP without photocatalytic coating.

GFRE	GFRP
79.0° ± 3.6	78.2° ± 2.9

shows the WCA of the uncoated substrates. The WCA value decreased for the TiO2-Ag-coated samples (

Table 4. Contact angle variation in GFRP and GFRE samples in the photocatalytic degradation of AO with UV-A radiation.

Sample	WCA0	WCAf
GFRE—TiO2-Ag	60.2° ± 4.2	10.3° ± 3.4
GFRP—TiO2-Ag	58.4° ± 4.0	9.7° ± 2.8

) after 12 h of UV-A irradiation. Figure 15 shows the variation in WCA of the TiO₂-Ag coatings on GFRP and GFRE substrates before and after UV irradiation. Both samples showed a change in hydrophilicity under UV irradiation. Generally, a WCA equal to or less than 10° is related to better hydrophilic performance [1,78,79]. The initial contact angle (WCA₀) of GFRP is slightly lower (58.4°) than that of GFRE (60.2°). The higher hydrophilicity is related to a higher number of oxygen vacancies in the coating. This is attributed to the oxygen vacancies formed by water molecules that tend to be occupied, thus increasing the presence of adsorbed O-H groups and consequently conferring the surface of the doped coatings with higher hydrophilicity [19,80].

3.2.5. Self-Cleaning Performance

The self-cleaning performance of the coatings was evaluated by analyzing the effect of a contaminant material, in this case biomass fly ash (OBFA), on substrates with and without the TiO₂-Ag coating. For this purpose, GFRP and GFRE samples were used, and, in both cases, the right side of each substrate was coated with TiO₂-Ag, while the left side remained uncoated to serve as a reference in the comparison of the behavior towards the pollutant.

The experimental procedure consisted of the random deposition of a controlled amount of OBFA on both areas of the substrates, which were arranged vertically (90°) to simulate real environmental exposure conditions. On the uncoated part of the substrates, strong adhesion of the contaminant particles was observed, indicating a lower detachability of the accumulated material. In contrast, the surface coated with TiO₂-Ag showed a significant reduction in contaminant adhesion, evidencing a higher resistance to fouling and an improvement in self-cleaning capacity (Figure 16). This behavior can be attributed to the hydrophilic properties of the coating, which facilitate the entrainment of contaminant particles with water. The effectiveness of this mechanism is shown in Videos S1 and S2, where the difference in contaminant adhesion between the coated and uncoated areas can be seen.

3.2.6. Analysis in Real Environmental Conditions

In addition to the evaluation under controlled conditions, a durability study of the coatings was conducted by prolonged exposure of the GFRP and GFRE panels to real environmental conditions. Over a period of 351 days, the samples were subjected to various weather conditions, including variations in temperature, relative humidity (RH), solar radiation, and precipitation (Figure 17, Figure 18, Figure 19 and Figure 20).

After this exposure period, visual and colorimetric analysis indicated that the GFRP substrate retained its self-cleaning ability without showing signs of degradation, significant wear, or alterations in material coloration, as evidenced in Figure 21a. These results confirm the long-term stability and effectiveness of the TiO₂-Ag coating on the GFRP substrate, making it a viable option for applications in environments where dirt and contaminant accumulation present a challenge. On the other hand, the GFRE substrate showed a progressive yellowish discoloration attributed to the unfavorable interaction between the epoxy resin and the active photocatalytic layer (Figure 21b). This discoloration is associated with photodegradation of the epoxy matrix induced by the photocatalytic activity of the coating under solar radiation, which in turn compromises the chemical stability of the system. Prolonged exposure generated oxidation intermediates at the interface, leading to evident yellowing. In addition, the lack of a primer or surface pretreatment to improve the chemical compatibility between the coating and the epoxy substrate contributed to the loss of adhesion and functionality of the coating. However, in order to assess the maintenance of the functional properties of the coating, it would be necessary to perform additional tests after aging, such as photocatalytic activity, contact angle, and antimicrobial tests. The absence of these tests represents a limitation of the current study, which is proposed to be addressed in future research to validate the functional durability of the system.

3.2.7. Morphological Analysis

SEM-EDX analysis was carried out in order to analyze the morphology and microstructure of the TiO₂-Ag-coated substrates. By means of SEM analysis, the distribution and homogeneity of the coating on the different substrates were evaluated, allowing the identification of possible variations in the adhesion and dispersion of the photocatalytic material.

The results obtained, shown in Figure 22, show that the TiO₂-Ag coating on the GFRE substrate was not uniformly distributed despite using the same application methodology as on the GFRP substrate. This inhomogeneity can be attributed to the previously reported incompatibility between the photocatalytic layer and the GFRE epoxy resin, which generates an uneven dispersion of the coating in certain areas.

On the other hand, EDX microanalysis confirmed the presence of TiO₂-Ag in specific areas, indicating an accumulation of the material in certain areas of the substrate. In contrast, the analysis of the GFRP substrate, represented in Figure 23, showed a more homogeneous distribution of the coating, suggesting a better adhesion of TiO₂-Ag on this type of material. This difference in dispersion could be related to the chemical composition and surface properties of each substrate, directly influencing the interaction with the photocatalytic layer and, consequently, the efficiency of the coating.

3.2.8. Antimicrobial Properties

A detailed analysis of the antimicrobial properties of TiO₂-Ag-coated GFRP, which was the substrate with the best properties, was carried out under UV-A light irradiation conditions following the protocols established in ISO 21702:2019 [64] and ISO 18061:2014 [65]. For this purpose, the antiviral activity of the material against vaccinia virus was evaluated.

The results obtained reflected a remarkable reduction in the viral concentration in the presence of photocatalytic material. The control sample, without photocatalytic treatment, presented an initial log10 DICT₅₀ viral concentration value of 6.8. In contrast, the GFRP sample with the photocatalytic coating reached values of 1.9 after 4 h of UV-A irradiation, evidencing a high efficiency in virus inactivation.

The values obtained in this study were as follows: U₀ (initial viral concentration): 5.63 DICT₅₀₀/cm²; U_t (viral concentration after treatment): 3.30 DICT₅₀/cm²; and Aₜ (antiviral activity): 0.78 DICT₅₀/cm². Antiviral activity (R) reached a value of 2.51, corresponding to 80% conversion of vaccinia virus under UV-A irradiation. Under dark conditions, the photocatalytic-coated samples presented an antiviral activity value of 1.25, which is equivalent to a 24% reduction in the viral load.

Figure 24 shows the microbiological analysis of the cytopathic effect observed in infected cells. Figure 24a shows no cytopathic effect, while Figure 24b shows the cells damaged as a result of photocatalytic action after 4 h of exposure to UV-A light.

Among the damage, cell rounding indicative of cytopathic damage was observed. Although specific molecular markers were not assessed, the correlation between morphological damage and quantified reduction in viral load reinforces the efficacy of the treatment.

These observations support the antiviral action of the coating. The antiviral action is attributed to the generation of reactive species and the direct action of silver nanoparticles, which can induce oxidative stress, alter membrane integrity, and destabilize viral or cellular structures [81].

4. Conclusions

This study evaluates the development of a photocatalytic coating based on Ag-doped TiO₂ on different polymeric substrates based on glass fiber, epoxy, and polyester resins. By studying the wettability, photocatalytic, optical, crystalline, antimicrobial, and microstructural properties, the following conclusions were drawn:

The incorporation of silver ions in TiO₂-Ag coatings resulted in a substantial improvement of the photocatalytic efficiency, reaching up to 94.1% pollutant degradation under UV-A irradiation, compared to 72.9% for pure TiO₂. This increase highlights the superiority of TiO₂-Ag coatings over other photocatalytic coatings in terms of degradation efficiency.

• The TiO₂-Ag coatings presented a contact angle close to 10°, indicating an outstanding improvement in hydrophilic properties, a behavior that translates into higher interaction with water and higher efficiency in photocatalytic applications.
• Morphological and microstructural analysis showed that the dispersion of the TiO₂-Ag coating was not homogeneous on the GFRE substrate, presenting agglomeration in certain areas, suggesting a worse adhesion to the material.
• Aging tests conducted under outdoor conditions showed that the TiO₂-Ag coating on GFRP exhibits enhanced durability compared to other coatings, such as those based on GFRE, which showed signs of incompatibility between the resin and the photocatalytic layer after 351 days of exposure. This aging resistance gives TiO₂-Ag coatings on GFRP substrates a considerable advantage in outdoor applications where durability is crucial. As a future line, the photocatalytic and antimicrobial properties will be reevaluated.
• The antiviral activity of the TiO₂-Ag coating achieved 80% vaccinia virus conversion under UV-A irradiation, evidencing its ability to efficiently inactivate pathogens. This performance positions TiO₂-Ag as a powerful option for applications in environments requiring microbial inactivation.
• As future lines, it is proposed to extend the application to biopolymers and to assess the environmental impact by means of life cycle analysis (LCA), promoting more sustainable solutions in construction and transport.
• Although the TiO₂-Ag coating showed good performance, toxicity testing, leaching, and life cycle or cost analyses, key aspects for indoor application were not performed. These evaluations are considered future work.

Based on these findings, it can be stated that TiO₂-Ag sol applied on a glass fiber-based substrate with polyester resin is a viable alternative for application in modular sandwich panel constructions, in the car industry, or as a facade coating.