Enhanced Visible-Light Photocatalysis Activity of TiO₂/Ag Nanocomposites Prepared by the Ultrasound-Assisted Sol–Gel Method: Characterization and Degradation–Mineralization of Cationic and Anionic Dyes

Abstract

Coupling TiO₂ with various elements could enhance its photocatalytic activity. In this study, an innovative ultrasound-assisted sol–gel method was used to synthesize TiO₂/Ag(x%) by varying Ag–support mass (x = 9.3, 17.1, and 23.6%), followed by calcination at 450 °C for 30 min. The aim was to demonstrate that Ag compositing improves photoactivity under visible light (>400 nm). The synthesized photocatalysts were assessed for their effectiveness in the degradation and mineralization of Methylene Blue (MB) and Acid Orange 7 (AO7) using visible lamps emitting in the range of 400–800 nm. Characterization of the prepared photocatalysts was performed by using Raman spectroscopy, SEM/EDS, pH_pzc, and UV–visible spectroscopy. Raman spectroscopy confirmed the predominance of the anatase phase in all the photocatalysts. The photodegradation efficiencies of the selected dyes, MB and AO7, reached 99% (pH 6) and 95% (pH 3) after 180 min of irradiation, respectively. The best performance for the degradation of the two dyes was observed with TiO₂/Ag9.3%, showing optimal kinetics at this doping concentration. The improved photoactivity of the TiO₂/Ag composite is due to a decrease in the recombination rate of electron/hole (e⁻/h⁺) and a decrease in the band gap from 3.13 to 2.49 eV. The mineralization rate of both dyes under visible light is about 9.3%, indicating the presence of refractory by-products that resist complete degradation. Under UVA irradiation, complete mineralization is obtained. This study confirms the potential of TiO₂/Ag composite as a high-performance and cost-effective photocatalyst for solar environmental remediation, highlighting the role of silver in extending light absorption into the visible region and improving charge separation.

1. Introduction

The contamination of water resources by industrial effluents, particularly synthetic dyes, has become a critical environmental concern in recent years. Industrial processes such as textiles, leather production, plastics manufacturing, and paper industries contribute significantly to the release of refractory organic dyes into water bodies, leading to long-term environmental and health risks [1,2].

Methylene Blue (MB), a cationic dye, and Acid Orange 7 (AO7), an anionic dye, are among the most persistent and toxic dyes [3,4]; they are detected in industrial wastewater due to their widespread use and resistance to conventional wastewater treatment processes [5,6]. Their diverse ionic nature presents a significant challenge in treatment methods and represents the spectrum of contaminants typically found in wastewater, making their removal essential [7,8].

Conventional wastewater treatment methods such as chemical precipitation, coagulation, and biological degradation are often ineffective for the complete removal of these dyes, especially at low concentrations, where their complex aromatic structures make them highly resistant to biodegradation [9]. The persistence of these dyes in the aquatic environment not only poses direct toxicity to aquatic organisms but also results in severe impacts on human health, including carcinogenicity and mutagenicity [10,11]. To address these challenges, advanced oxidation processes (AOPs) have emerged as an effective alternative for the degradation of recalcitrant organic compounds in aqueous environments [12,13]. Among these, heterogeneous photocatalysis has been highlighted as a green and sustainable method, driven by the use of semiconductor materials to generate reactive oxygen species (ROS), such as hydroxyl radicals (^•OH), that degrade organic pollutants into less harmful by-products [14,15].

Titanium dioxide (TiO₂) is one of the most widely studied photocatalysts due to its excellent chemical stability, strong oxidative potential, and low toxicity [16,17]. However, its practical application for wastewater treatment is constrained by its wide band gap (~3.2 eV), which restricts its photocatalytic activity to the ultraviolet (UV) region, representing only about 5% of solar radiation [18,19]. This limitation has prompted significant research efforts aimed at enhancing the visible-light response of TiO₂-based photocatalysts by various doping and structural modification strategies [20,21,22].

One of the most promising approaches to extending TiO₂ activity into the visible spectrum is the development of core–shell nanocomposites incorporating noble metals such as silver (Ag), gold (Au), and platinum (Pt) [23,24,25]. These metals can exploit localized surface plasmon resonance (LSPR) effects, which generate energetic electrons under visible-light irradiation. These “hot electrons” can be transferred to the TiO₂ shell, extending its light absorption into the visible range and improving photocatalytic activity [26,27,28,29]. Ag/TiO₂ core–shell photocatalysts, in particular, have shown considerable promise for the remotion of various pollutants due to their unique electronic properties and high efficiency [30,31,32,33,34].

Numerous studies have demonstrated the effectiveness of noble metal-TiO₂ core–shell composites for the degradation of both cationic and anionic dyes. For instance, Yang et al. (2013) showed that methyl orange was degraded by over 92% in 50 min using Ag/TiO₂ core–shell structures irradiated by UV light [33].

Similarly, Chen et al. (2014) reported a 99% degradation of Methylene Blue within 120 min using Ag/TiO₂ composites under visible-light irradiation, showcasing its potential for cationic pollutant removal [34]. This system benefits from the plasmonic effect of silver, which enhances the photocatalytic reaction by facilitating charge separation and reducing electron–hole recombination [35].

The application of noble metal-TiO₂ core–shell composites extends beyond dye degradation, with significant progress in addressing various organic pollutants. For instance, Wu et al. (2020) achieved over 95% degradation of methyl orange, a cationic dye, within 60 min using Au/TiO₂ nanocomposites under ultraviolet light irradiation [36]. Similarly, Alshammari et al. (2020) reported an 86% reduction in 4-nitrophenol to 4-aminophenol within 10 min using Pd/TiO₂ core–shell structures [37]. Furthermore, a recent study investigated the effectiveness of a Pt/TiO₂ composite for phenol removal using a continuous flow trickle-bed photoreactor, achieving a high and stable phenol degradation with 97% conversion maintained over 52 h [38]. These findings underscore the versatility of noble metal core–shell photocatalysts in treating a wide range of organic contaminants in aqueous environments.

Advancements in the design and optimization of core–shell photocatalysts have further enhanced their photocatalytic performance. Adjustments in silver nanoparticle size and TiO₂ shell thickness have been shown to significantly influence the efficiency of Ag/TiO₂ composites, with smaller core sizes and thinner shells resulting in faster reaction rates due to more efficient charge transfer [39,40]. Additionally, bimetallic core–shell structures, such as Ag-Au/TiO₂, have been investigated for their superior plasmonic properties, leading to improved degradation of dyes and pharmaceuticals [41,42]. These innovations are paving the way for more efficient and scalable water treatment technologies. Despite significant progress in this field, challenges remain in optimizing the synthesis, stability, and scalability of noble metal-TiO₂ core–shell photocatalysts for large-scale wastewater treatment applications. This study focuses on the photocatalytic degradation of Methylene Blue and Acid Orange 7 using Ag/TiO₂ core–shell nanocomposites under visible-light irradiation. This work aims to provide new insights into the structural and functional parameters that influence photocatalytic efficiency and to explore the potential of these materials for practical wastewater treatment applications.

2. Experimental

2.1. Chemicals

Reagents used for the preparation of the visible-light active TiO₂/Ag composite nanoparticles are titanium (IV) isopropoxide TTIP (>97%, Sigma-Aldrich, St. Louis, MO, USA) and spherical silver nano-powder (30–50 nm, 99.99%, nano-shell). The two pollutants selected for this study were Methylene Blue (MB, cationic dye) and Acid Orange (AO7, anionic dye), both of high purity ≥ 99%, supplied both by Sigma-Aldrich. Deionized (DI) water (obtained with Milli-Q, from Millipore Corporation, Burlington, MA, USA) was used to prepare all electrolytic solutions.

2.2. Preparation of TiO₂/Ag Composite Photocatalysts

TiO₂/Ag composite core–shell nanoparticles were synthesized using an ultrasound-assisted sol–gel method. Various amounts of Ag powder nanoparticles (337.33 mg, 674.66 mg, and 1.012 g) were dispersed in 50 mL of distilled water and subjected to ultrasonication for 20 min at a room temperature of 20 °C to achieve TiO₂/Ag photocatalysts with weight percentages of 9.3%, 17.1%, and 23.6% of Ag relative to TiO₂. Subsequently, 12.5 mL of TTIP solution was added dropwise to the Ag suspension under sonication for 10 min at 20 °C. The resulting mixture was then transferred to a crucible, dried at 105 °C for 4 h, and, finally, calcined at 450 °C for 30 min. The samples were named in

Table 1. Identification of prepared nanophotocatalysts.

Sample Name	Sample Description
TiO2	Pure sol–gel TiO2
TiO2/Ag(9.3%)	TiO2 (3.271 g), Ag (337.33 mg)
TiO2/Ag(17.1%)	TiO2 (3.271 g), Ag (674.66 mg)
TiO2/Ag(23.6%)	TiO2 (3. 271 g), Ag (1.012 g)

.

2.3. Nanoparticles Characterization

The point of zero charge (pH_pzc) of prepared semiconductors was determined following the method described elsewhere [43]. The mass titration method was for the determination of the net charge of nanoparticles in solution. This technique consists of measuring the pH value that becomes constant with the increasing mass of the solid in a volume of water. Under these conditions, it has been observed that the pH of the nanoparticle suspension depends on the amount of powder in DI water up to a final value.

The structure, crystallinity, and defects of the synthesized nanoparticles were examined using a Renishaw InVia Raman microscope (Wotton-under-Edge, UK) with a 633 nm excitation from a He-Ne laser. The chemical composition was evaluated by Scanning Electron Microscopy (SEM, JEOL-JSM 6360, Tokyo, Japan) combined with energy-dispersive X-ray spectroscopy (EDS). The specific surface area, volume, and pore diameter were measured from the N₂ adsorption isotherms at 77 K using the BET methodology and a Costech Sorptometer 1042 instrument (Costech International, Pioltello (MI) Italy) on powder samples. Additionally, the UV–visible diffuse reflectance spectra (UV-Vis DRS) of the samples were measured with a Perkin Elmer Lambda 35 spectrophotometer (Waltham, MA, USA) equipped with a reflectance spectroscopy accessory (RSA-PE-20, Labsphere Inc., North Sutton, NH, USA). The band gap values were determined by plotting [F(R_∞)∙hν]² against hν (eV), where F(R_∞) represents the Kubelka–Munk function and R is the absolute reflectance of the samples.

$$F(R_{\infty }) ={\displaystyle \frac{{\left(1-R_{\infty }\right)}^2}{2R_{\infty }}}$$

2.4. Photocatalysis Tests of BM and AO7 Under Visible Light

The photocatalytic activity of TiO₂/Ag composite was evaluated for its ability to degrade MB and AO7 under visible light. Photocatalytic experiments were conducted in a cylindrical open beaker (200 mL capacity, ∅ = 8 cm). Two visible-light lamps (Osram Lumilux T5 8W—640 Blanc Froid 29 cm, Osram, Milano, Italy), each with a power of 8 W and emitting light in the range (400–800 nm), were positioned around the reactor to ensure a uniform illumination of the solution (30 mW/cm²). Oxygen was supplied to the solution thanks to an air distributor at a flow rate of 150 Ncm³/min. Continuous stirring prevented the catalyst from settling to the bottom of the photoreactor during illumination; a cooling fan kept the temperature below 30 °C.

The initial concentrations of MB and AO7 were 7 and 10 mg/L, respectively, treating a 100 mL solution with a photocatalyst dose of 3 g/L. Prior to initiating the photocatalytic reaction, the suspension was kept in the dark for 60 min to achieve adsorption/desorption equilibrium on the photocatalyst surface. Then, the visible-light lamps were turned on for 180 min. At regular intervals, samples (3 mL) were removed from the reactor and the remaining concentrations of MB and AO7 were analyzed by UV-Vis spectrophotometry (Thermo Fisher Scientific, Waltham, MA, USA). The decolorization of the solutions was evaluated by measuring their maximum absorbance of AO7 (λ_max = 485 nm) and MB (λ_max = 663 nm). The photocatalytic efficiency is calculated using the following relation:

$$\% \mathrm{photocatalytic}\mathrm{efficiency} ={\displaystyle \frac{\left(C_0-C_t\right)}{C_0}}100$$

(1)

where C₀ is the initial dye concentration (mg L⁻¹) and C_t is the concentration at time t (mg L⁻¹).

The photoactivity was assessed by measuring the total organic carbon (TOC) of AO7 and MB, a parameter that quantifies the extent of the dye mineralization and was measured over irradiation time. The physical TOC of the solution was determined from CO₂ produced by catalytic combustion at a temperature of 680 °C The gas-phase analysis of CO and CO₂ emitted from the photoreactor was continuously monitored using specific on-line analyzers (ABB Advance Optima, ABB, Milano, Italy). CO₂ generated in the gas phase was monitored using continuous analyzers that measured the concentrations of CO₂, CO (Uras 14, ABB), and O₂ (Magnos 106, ABB) [44,45]. The extent of dye mineralization was calculated from the following equation:

$$\% \begin{array}{l}\mathrm{Mineralization}\mathrm{efficiency}\hfill \end{array}={\displaystyle \frac{\left(TO{C}_0-TO{C}_t\right)}{TO{C}_0}}100$$

(2)

where TOC₀ is the initial value and TOC_t is the value at time t.

3. Results and Discussion

3.1. Characterization of TiO₂/Ag Composites

3.1.1. pH_PZC

Since photocatalysis occurs at the catalyst surface, the pH is a critical parameter for the photocatalytic degradation of dyes. Identifying pH_pzc (point of zero charge) is essential for accurately predicting the surface charge of nanoparticles during photodegradation. Indeed, the performance of the photocatalyst is significantly influenced by the solution pH, as it affects the catalyst’s ability to adsorb pollutants. The pH_pzc values of TiO₂, TiO₂/Ag(9.3%), TiO₂/Ag(17.1%), and TiO₂/Ag(23.6%) were found to be 5.48, 6.55, 7.29, and 7.14, respectively (Figure 1 and

Table 2. Characteristics of prepared nanoparticles.

Sample Name	pHpzc	Gap (eV)	SBET (m2 g−1)
TiO2	5.48	3.13	96
TiO2/Ag(9.3%)	6.55	2.84	89
TiO2/Ag(17.1%)	7.29	2.49	82
TiO2/Ag(23.6%)	7.14	2.62	71

). The high pH_pzc values of TiO₂/Ag composite compared to pure TiO₂ are likely due to the presence of additional surface OH⁻ groups introduced by the Ag doping. Below pH_pzc, the catalyst surface is positively charged, while it becomes negative above pH_pzc.

The impact of Ag in the TiO₂ heterojunction was investigated by UV–Vis spectrophotometry. The UV–Vis absorbance spectra for bare TiO₂ and TiO₂ doped with 9.3%, 17.1%, and 23.6% Ag are illustrated in Figure 2. Unlike TiO₂, which absorbs in the UV region, TiO₂/Ag shifts the absorbance toward the visible-light region (>380 nm, red shift) [46]. This shift indicates that TiO₂/Ag composites perform better as light harvesters, showing a more pronounced UV transmittance peak and enhanced visibility due to increased absorption in the visible region. Increasing the silver percentage from 9.3 to 23.6% enhances the photoactivity of TiO₂/Ag composite, which absorbs the visible light with a reduction in the band gap energy for all doped TiO₂ (Figure 3). The band gap of TiO₂ is 3.13 eV, which is larger compared to 2.84 eV, 2.62 eV, and 2.49 eV for the 9.3%, 17.1%, and 23.3% TiO₂/Ag composite, respectively, shifting the photo activity in the visible region (Table 2). The presence of Ag introduces a Schottky heterojunction barrier at the interface, which improves charge separation and enhances photocatalytic activity by trapping photogenerated electrons from TiO₂ and preventing recombination. The decrease in the band gap can be due to the effect of Ag NPs that act as an electron trap, altering charge recombination dynamics and indirectly affecting the band structure.

Excessive Ag content leads to a higher agglomeration of metallic Ag nanoparticles, which may increase electron–hole recombination, slightly increasing the effective band gap.

3.1.2. SEM/EDX

The morphologies of undoped TiO₂ and TiO₂/Ag composite with different proportions of Ag nanoparticles (9.3, 17.1, and 23.6%) were visualized by SEM analysis. In Figure 4a–c, the nanostructures of undoped TiO₂ and TiO₂/Ag with 9.3 and 17.1% are shown as small nanoparticles forming aggregates. Both are distributed unevenly across the surface, with varying sizes of agglomerations. Comparative analysis reveals no significant differences among these samples, suggesting that Ag nanoparticles have been effectively incorporated into TiO₂ shell structures [47]. In contrast, Figure 4d shows the TiO₂/Ag23.6% composite, where Ag excess is apparent on the surface of some particles. This finding is corroborated by the EDS spectra, which display distinct energy peaks for Ag, confirming its presence on the surface.

EDS was used to analyze the elemental composition of as-prepared nanoparticles, providing details on the percentage of each element. Figure 4 shows the EDS spectra for both pristine and TiO₂/Ag composites. The spectra confirm the presence of pure TiO₂ nanoparticles, evident from distinct peaks corresponding to Ti and O. For TiO₂ doped with 9.3 and 17.1% Ag, the EDS results do not clearly show Ag on the surface, suggesting that Ag is well incorporated into the TiO₂. Conversely, the EDX spectrum for the TiO₂/Ag23.6% composite displays a prominent Ag peak, showing that excess Ag is present on the sample’s surface. No additional peaks were observed in the EDS spectra of either doped or undoped samples, confirming the absence of impurities. These results align with the Raman spectroscopy findings, which support the phase purity of TiO₂-based photocatalysts.

3.1.3. Raman Spectroscopy

The Raman spectra provide valuable insights into the surface stoichiometry and microstructural characteristics of materials, and they are crucial for identifying structural changes due to doping; Figure 5 illustrates the spectra, which reveal significant structural details. The spectra confirm that all samples correspond to the anatase TiO₂ phase with no inorganic crystalline structures like the rutile. For both pristine TiO₂ and TiO₂/Ag composite, the prominent peak at 144 cm⁻¹ and the less intense peaks at 197 cm⁻¹ and 637 cm⁻¹ are associated with the Eg vibrational mode of TiO₂ [48,49,50]. On the other hand, for bare TiO₂ NPs, additional bands are observed at 250 cm⁻¹ (A1g) and 322 cm⁻¹ (B1g), which can be attributed to a brookite phase of TiO₂ [51]. The peaks at 396 cm⁻¹ and 515 cm⁻¹ correspond to the B1g and (A1g + B1g) vibrational modes, respectively. The E1g mode is related to the (101) crystallographic direction and the symmetric bending vibration of O-Ti-O along the c-axis [49,52]. With Ag compositing, no Raman-active bands from Ag are observed due to the crystal symmetry of Ag. Similar results have been observed in the literature [53]. Additionally, the intensity of Raman bands decreases, due to the thinning of the TiO₂ lattice in the presence of the higher amount of Ag nanoparticles support. Anatase with a small amount of brookite was found in the XRD of TiO₂ NPs and TiO₂/Ag(9.1%) sample, with additional Ag peaks for the latter [40].

3.2. Photocatalytic Activity

3.2.1. Effect of Ag-Doping TiO₂ Nanoparticle on Photocatalytic Degradation of MB and AO7 Dyes Under Visible-Light Irradiation

The photoactivity of TiO₂ coatings on Ag nanoparticle supports was evaluated for the degradation of MB and AO7 under white light illumination. Prior to these experiments, the stability of both dyes MB and AO7 was investigated to assess their degradation in the absence of doping under the same illumination conditions used in the photocatalytic tests. The measurements indicated minimal photolysis of the tested molecules after 1 h of illumination, with degradation levels within the experimental error margin (<1%). Furthermore, adsorption of MB and AO7 on the composites was also carried out in the dark.

Figure 6 illustrates the compositing effect of TiO₂ on Ag photocatalysts on the adsorption and photodegradation processes of two dyes. Specifically, the adsorption of the dyes increases from 4 to 30% as the Ag percentage increases from 0 to 9.3% for MB, and from 7 to 14% for AO7. These results can be explained by the effect of electrostatic interactions, following the ionization of the AO7 molecules at pH 3 (R–SO₃⁻), which suggests that AO7 forms a bidentate inner-sphere complex with two protonated hydroxyl groups via its sulfonate group [54,55]. This interaction is influenced by the positive charge on the TiO₂/Ag composite surfaces at pH < pH_pzc [56]. Additionally, the increased adsorption may also be due to the smaller particle size and therefore a larger surface area, as indicated by BET analysis. For the adsorption of MB on TiO₂/Ag composite, the enhanced interaction is attributed to π-π interactions between the aromatic rings of MB and TiO₂/Ag composite [57].

A preliminary analysis of the recorded curves indicates that the photodegradation of MB and AO7 increases with irradiation time, and the photoactivity is significantly enhanced when TiO₂ is combined with Ag. With Ag9.3%/TiO₂ nanophotocatalysts, 99% degradation of MB was achieved after 3 h under white light exposure, while AO7 saw a 95% degradation in the same time frame. However, beyond this level of Ag doping, specifically at 17.1% and 23.6% Ag, the photoactivity for both dyes is significantly diminished over a period of 180 min. The presence of silver in the TiO₂ nano-powders enhances absorption in the visible range, which is attributed to oxygen defects acting as trapping centers for photogenerated electron/hole (e⁻/h⁺) pairs. This enhancement reduces the recombination of photogenerated electron pairs, leading to higher degradation rates of MB and AO7. The improved efficiency is further attributed to the lower band gap energies and smaller crystallite sizes of the TiO₂/Ag composite samples compared to the undoped specimen [58].

A photocatalytic mechanism of TiO₂/Ag composite nanoparticles may be hypothesized by combining the results of this research work with those existing in the literature. Ag nanoparticles present in the TiO₂ crystal lattice play a crucial role in the dye photocatalytic degradation. The electrons of the conduction band (CB) of TiO₂ generated by light are transferred to the Ag sites, which reduces the recombination of charge carriers (h⁺ and e⁻) and increases the electrochemical rates by decreasing the over-potential (ηAg ~ 0.5 V for the current density of 10 mA cm⁻²). Electrons from the conduction band are more likely to accumulate on the Ag particles or clusters (sub-nanoscale) due to Fermi level equalization between the metal and TiO₂. This effective charge separation enables the trapped holes on the surface to more efficiently react with H₂O and OH, forming ^•OH radicals that subsequently react with dye molecules, thereby enhancing the overall photocatalytic degradation. However, an excessive amount of Ag nanoparticles, either individually or in sub-nanoscale clusters, could have the opposite effect by occupying the active sites on TiO₂, thus hindering the photocatalytic activity [59].

3.2.2. Kinetic Studies

To analyze the influence of Ag on the photodegradation of MB and AO7, the kinetic parameters were calculated assuming a pseudo-first-order mechanism for the degradation (Equation (1)). The apparent rate constants (k_app) and correlation coefficients (R²) were determined by linear fitting over the experimental time interval, and the results are presented in

Table 3. First-order kinetic constants for the degradation of MB and AO7 under visible irradiation.

	MB			AO7
Nanoparticles	kapp (min−1)	t1/2 (min)	R2	kapp (min−1)	t1/2 (min)	R2
TiO2	-		-	9.253 × 10−4	749.07	0.9531
TiO2/Ag(9.3%)	2.451 × 10−2	28.28	0.9986	1.895 × 10−2	36.58	0.9682
TiO2/Ag(17.1%)	1.340 × 10−2	51.73	0.989	1.028 × 10−2	67.43	0.934
TiO2/Ag(23.6%)	1.192 × 10−2	58.15	0.9717	1.057 × 10−2	65.58	0.9659

.

$$-{\displaystyle \frac{dC}{dt}}=k_{app} C$$

(3)

k_app (mn⁻¹) is the apparent rate constant. The integration of Equation (3) gives the following:

$$\ln \left({\displaystyle \frac{C_0}{C}}\right)=k_{app} t$$

(4)

The results for the degradation of both dyes are consistent with a first-order kinetic reaction, as confirmed by the R² values. The evolution of the rate constant (k_app) for different Ag doping in TiO₂ shows that k_app for the photocatalysis of both dyes increases within the presence of Ag support, reaching an optimal value of 2.451 × 10⁻² min⁻¹ for MB with a half-life of 28 min (TiO₂/Ag9.3% composite) and 1.895 × 10⁻² min⁻¹ for AO7 with a half-life of 37 min (TiO₂/Ag9.3% composite), evinced in bold in Table 3. The constant k_app for MB is higher than that for AO7, which can be attributed to the lower molecular weight of MB (373.90 g·mol⁻¹) compared to AO7 (350.32 g·mol⁻¹), resulting in faster degradation [60]. However, beyond these values, a decrease in photoactivity is observed, indicating that the excess of supporting Ag nanoparticles is less effective in the degradation of the dyes.

As shown in Table 3, the current research demonstrates that TiO₂/Ag nanoparticles exhibit a superior catalytic activity compared to those reported in the literature (see

Table 4. Comparison of previously reported decolorization studies of different dyes and the present study.

Doped Nano-Particulate TiO2	Dye	Source Illumination	Discoloration (%)	kapp (min−1)	Reference
Ag–TiO2 (50/50)	Methyl orange	Simulating sunlight lamp (300 W)	88	2.43 × 10−3	[59]
Fe(2.5wt)-TiO2	AO7	Visible LED (10 W)	~90	4.2 × 10−3	[62]
W0.6%–TiO25%–AC	Rhodamine B	Visible light	-	1.86 × 10−2	[63]
Ag5%-TiO2	E102 tartrazine	Hg lamp (400 W)	76	4.15 × 10−3	[53]
Fe–TiO2–MWCNT	MB	Tungsten Lamp (200 W)	58	3 × 10−3	[64]
Ag3%-TiO2	Tartrazine	Solar Simulator with Xenon light source	87	8.38 × 10−3	[65]
Ag/Ti3C2/TiO2	Rhodamine-B	Visible light (300 W Xe lamp)	97	4.73 × 10−3	[66]
Ag3 wt%@TiO2	Rhodamine-B	Sunlight	99	4.77 × 10−2	[67]
Ag3%-TiO2	MB	Visible light (Xe lamp)	94	5.57 × 10−2	[61]
TiO2/Ag(9.3%)	MB	Visible light (8 W × 2)	99	2.451 × 10−2	Current work
TiO2/Ag(9.3%)	AO7	Visible light (8 W × 2)	95	1.895 × 10−2	Current work

). The core–shell structure of TiO₂/Ag nanoparticles enhances charge separation efficiency, with the Ag core extending the lifetime of photogenerated electron–hole (e⁻/h⁺) pairs, thus reducing recombination. Additionally, the plasmonic properties of Ag create ‘hot spots’ at the core–shell interface, which improve the light absorption and facilitate the electron transfer to TiO₂ [61]. The high Fermi energy of Ag also enhances the charge transfer kinetics; indeed, the work function of Ag is 4.26 eV. Furthermore, Ag may increase the performance of reactive oxygen species (ROS) such as hydroxyl radicals (^•OH) and superoxide radicals (O₂^•−), leading to more effective dye degradation. Overall, these factors enhance the photocatalytic performance of TiO₂/Ag nanoparticles, boosting their efficiency for dye degradation under visible light.

3.2.3. Stability Tests

Fifth stability cycles with the TiO₂/Ag9.3% photocatalyst were conducted (Figure 7,

Table 5. k values obtained for stability tests.

Cycle	kapp, min−1 (MB)	kapp, min−1 (AO7)
I cycle	0.0238	0.0219
II cycle	0.0238	0.0168
III cycle	0.0187	0.0157
IV cycle	0.0185	0.0181
V cycle	0.0192	0.0193

), using the same operating conditions as the previous experimental tests. The results showed an initial slight deactivation of the photocatalyst, observed in both the degradation and mineralization of the MB pollutant. By the third cycle, the photocatalytic activity stabilized, although lower, with a mineralization efficiency of approximately 20% and an apparent kinetic constant of MB degradation of 0.0185 min⁻¹ after three hours of visible-light irradiation. Similarly, the photocatalyst demonstrated stable activity, in terms of AO7 mineralization, achieving an efficiency of about 24% after three hours of visible-light exposure. However, the degradation kinetics of AO7 revealed an initial decline, before stabilizing from the fourth cycle onward, with an apparent kinetic constant of dye degradation of 0.0181 min⁻¹.

3.2.4. Effect of Water Matrix Nature on Photocatalytic Activity

Inorganic ions present in the water to be treated are known to influence photocatalytic performance [68,69,70]. To investigate this effect, additional experiments were conducted using tap water (characteristics detailed in

Table 6. List of the physicochemical properties of tap water.

Parameter	Value
Conductivity (µS cm−1)	471
Sodium (ppm)	3.25
Potassium (ppm)	1.12
Calcium (ppm)	85.9
Magnesium (ppm)	16.1
Chlorides, Cl− (ppm)	4.8
Sulfates, SO42− (ppm)	17.1
Bicarbonates, HCO3− (ppm)	319
Nitrates, NO3− (ppm)	4

) contaminated with MB or AO7. These tests aimed to evaluate the impact of the water matrix on photocatalytic efficiency.

The results, presented in Figure 8, demonstrate the system’s efficacy in degrading MB (Figure 8a) and AO7 (Figure 8b) even in tap water. After 180 min of visible-light irradiation, the degradation efficiencies for MB and AO7 in tap water reached 92% and 89%, respectively.

However, the apparent kinetic constant for degradation in the tap water matrix was approximately 40% lower than that observed in distilled water (DW). This reduction is likely due to the presence of ions in tap water (TW), which may act as radical scavengers, diminishing the availability of photogenerated reactive oxygen species (ROS) during the photocatalytic reaction [71].

3.2.5. Proposed Photocatalytic Reaction Mechanism on TiO₂/Ag9.3%: Role of Reactive Oxygen Species in AO7 and MB Degradation

The TiO₂/Ag9.3% photocatalyst was investigated to elucidate the role of reactive oxygen species (ROS), including hydroxyl radicals (^•OH), superoxide anions (O₂^•−), and positive holes (h⁺), in the photocatalytic degradation of Methylene Blue (MB) and Acid Orange 7 (AO7). Specific scavenger molecules were employed to probe the contributions of individual ROS: isopropanol (IPA, 10 mM) for hydroxyl radicals [72], benzoquinone (BQ, 1 μM) for superoxide anions [40], disodium ethylenediaminetetraacetate (EDTA, 10 mM) for positive holes [73], and potassium bromate (KBrO₃, 1 mM) [74] for other ROS.

Figure 9 demonstrates significant reductions in photocatalytic activity when the production of hydroxyl and superoxide radicals is suppressed. For instance, in the presence of IPA, which specifically scavenges hydroxyl radicals, the apparent degradation rate constants for MB and AO7 decreased by approximately 95% compared to control experiments without scavengers. These findings strongly suggest that the degradation of these dyes predominantly involves hydroxyl radicals generated via both h⁺ and O₂^•− pathways.

Additionally, the Ag(0) core within the TiO₂/Ag9.3% nanoparticles facilitates electron trapping, enhancing H₂O₂ formation and subsequently increasing hydroxyl radical generation [41]. This synergistic mechanism significantly amplifies the photocatalytic oxidation of organic pollutants.

The proposed mechanism for the mineralization of MB and AO7 is as follows [41,72,75]:

$${\mathrm{T}\mathrm{i}\mathrm{O}}_2/\mathrm{A}\mathrm{g}9.3\%+\mathrm{h}\mathsf{\nu }\to {\mathrm{h}}^{+}+{\mathrm{e}}^{-}$$

(5)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{O}}_2^{-•}$$

(6)

$$\mathrm{O}{\mathrm{H}}^{-}+{\mathrm{h}}^{+}\to \mathrm{O}{\mathrm{H}}^{•}$$

(7)

$${\mathrm{O}}_2^{-•}+{\mathrm{H}}^{+}\to \mathrm{H}{\mathrm{O}}_2^{•}$$

(8)

$$\mathrm{H}{\mathrm{O}}_2^{•}+\mathrm{H}{\mathrm{O}}_2^{•}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$$

(9)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}^{-}\to \mathrm{O}{\mathrm{H}}^{•}+\mathrm{O}{\mathrm{H}}^{-}$$

(10)

$$\mathrm{A}\mathrm{O}7/\mathrm{M}\mathrm{B}+\mathrm{O}{\mathrm{H}}^{•}\to \mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{s}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(11)

$${\mathrm{e}}^{-}+{\mathrm{h}}^{+}\to \mathrm{E}+\mathrm{N}$$

(12)

where $\mathrm{N}$ is a neutral center, and $\mathrm{E}$ denotes the energy released during the recombination of photoexcited electron–hole pairs. This energy may be emitted as light (hν′ ≤ hν) or dissipated as heat.

3.2.6. Mineralization Studies by TOC Analysis

To monitor the mineralization of each dye during photocatalytic treatment, evolved TOC measurements were conducted. As shown in Figure 10, there was a gradual increase in dye mineralization and a corresponding reduction in TOC over time. After 3 h, mineralization was observed at 21% for MB, while MB exhibited 27% mineralization. The mineralization rates of MB and AO7 are significantly lower than their photodegradation rates, and this clearly indicates that the photocatalytic process generated intermediate by-products [76]. In addition, the wavelength of light (>400 nm) has a significant impact on the photocatalytic performance of Ag/TiO₂. In some works, it is worth mentioning that extending the illumination time increases photomineralization, reaching up to 20 h of irradiation [77].

The same trends were obtained by [78] in neutral red dye photocatalytic degradation by a Gd atom-doped TiO₂, which found a mineralization of 41% for 90 min irradiation under a UV light source. Mottola et al. investigated the mineralization of MB and AO7 under visible and UV irradiation using Fe-N-TiO₂, Fe-N-TiO₂ with β-CD, and Fe-N-TiO₂ combined with β-CD micronized by supercritical antisolvent process. For AO7 mineralization, the best efficiency (∼26%) was achieved using Fe-N-TiO₂ after 180 min of visible-light exposure. However, Fe-N-TiO₂/β-CD120 showed the highest TOC removal (~19%) compared to the other hybrid materials [79].

3.2.7. Photocatalytic Mineralization of MB and AO7 Dyes Under UV-A Irradiation

Figure 11 illustrates the photocatalytic behavior of TiO₂/Ag(9.3%) under UV-A irradiation with respect to both of the two dyes. Discoloration occurs at a high level after 120 min of irradiation, with values higher than 90% for both dyes. Correspondently, TOC removal was enhanced by UV-A radiation, as shown in the right part of Figure 9; however, 180 was needed to achieve more than 90% of mineralization.

A final comparison of MB and AO7 photoefficiencies is reported in Figure 12. Meanwhile, the discoloration of both dyes is efficiently achieved after 180 min, the mineralization appears strongly dependent on the energy of exciting radiation, and, with the more energetic UV-A, is faster and almost completely accomplished.

These results indicate that under solar light, composed of UV-A and visible light, the nanocomposite can well perform in the removal of cationic and anionic dyes.

4. Conclusions

The present study investigated the impact of TiO₂ compositing with Ag nanoparticles at various concentrations, detailing the preparation of TiO₂/Ag x% (x = 0, 9.3, 17.1, and 23.6%) photocatalysts, using a simple and cost-effective ultrasound-assisted sol–gel method. The characteristics of these TiO₂/Ag composite nanoparticles were analyzed using Raman spectroscopy, SEM/EDS, and DRS analysis, which provided insights into their structure and crystallinity, particle morphology, and changes in band gap and light absorption properties, respectively. This study further explores the role of these photocatalysts in the visible-light (>400 nm)-driven degradation of Methylene Blue (MB) and Acid Orange 7 (AO7) dyes as model pollutants. The reduction in band gap (Eg) with increasing Ag load in TiO₂, down to 2.49 eV for TiO₂/Ag17.1% compared to 3.13 eV for undoped TiO₂, indicates a significant enhancement in photocatalytic activity under visible light. The maximum photoactivity results demonstrated that the TiO₂/Ag 9.3% nanocatalyst achieved 99% degradation of BM and 95% degradation of AO7 within 180 min. Despite these high degradation rates, the mineralization of BM and AO7 was only 21 and 27%, respectively. This is attributed to the formation of highly refractory by-products during the photocatalytic degradation process. The degradation of MB and AO7 follows first-order kinetics, with apparent rate constants of k_app = 2.451 × 10⁻²  min⁻¹ and 1.895 × 10⁻²  min⁻¹, respectively. The synthesized TiO₂/Ag composite nanoparticles showed efficient separation of photo-generated (e⁻/h⁺) pairs, high reactivity, and stability under visible light. The photoactivity of nanocomposite photocatalysts can be boosted by UV-A radiation, giving rise to almost total TOC removal after 180 min, showing that the optimal photocatalyst is prone to be applied in solar photocatalysis.