Kinetic insights into Ag-doped TiO2 photocatalysts for dye degradation: advances in materials driving renewable energy technologies

Introduction: Water pollution, stemming from various sources such as pharmaceuticals waste, pesticides, herbicides, textile dyes, resins, and phenolic compounds, is a significant concern in today’s world. Even small amounts of water pollutants can have detrimental effects on human health and ecosystems. Therefore, proper treatment of industrials wastewater through sewage treatment plants is essential for effective wastewater management.

Method: In this study, undoped TiO₂ and silver-doped TiO₂ nanoparticles were synthesized via Sol-Gel method and thoroughly characterized using X-Ray Diffraction (XRD), UV-Visible Spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-rays (EDX), and Fourier Transform Infrared Spectroscopy (FT-IR).

Results: Results revealed a red shift in the UV-Visible spectrum, with increasing silver percentage (2%–4% weight), and silver doping effectively tuned the band gap from undoped TiO₂ from 3.03 eV to 2.47. XRD analysis revealed an average crystallite size range of 8–12 nm. SEM studies demonstrated morphological changes due to doping, while EDX confirmed the elemental composition of the nanoparticles. FT-IR analysis affirmed sample purity and vibrations of the required functional groups.

Discussion: The synthesized nanoparticles exhibited efficient degradation of Malachite Green and Rose Bengal dyes under varied conditions, with 2% weight TiO₂ displaying the highest Photodegradation Efficiency (PDE) of 98% for Malachite Green at optimal conditions of 4 mg catalyst concentration, 85 μM dye concentration, and pH 9 and 2 mg catalyst concentration, 10 μM dye concentration, and pH 3 for Rose Bengal. Kinetic analysis revealed pseudo first order kinetics with observed rate constant of 0.9438 min^-1 for Malachite Green and 0.18118 min^-1 for Rose Bengal, supporting the efficiency of heterogeneous photocatalysis for wastewater treatment. This study underscores the significance of the optimizing degradation parameter for practical applications.

1 Introduction

Water pollution, stemming from various sources such as pharmaceuticals waste, pesticides, herbicides, textile dyes, resins, and phenolic compounds, is a significant concern in today’s world. With underground water resources deleting and inadequate management of water, ensuring access to safe water has become increasingly challenging (Grung et al., 2015; Neyens and Baeyens, 2003). Even small amounts of water pollutants can have detrimental effects on human health and ecosystems. Therefore, proper treatment of industrials wastewater through sewage treatment plants is essential for effective wastewater management (Schneider et al., 2014; Ahmed and Xinxin, 2016; Gupta et al., 2012). Numerous wastewater treatment technologies exist, but cost-effective and time-efficient methods are important for ensuring widespread access to safe water. Advances in oxidation processes (AOPs), particularly photocatalysis, have been emerged as a powerful tool for wastewater treatment. Photocatalysis involves accelerating chemical reactions by using light source, and it has been demonstrated to effectively degrade the harmful chemical pollutants present in wastewater (Fujishi et al., 1972). In the industrial sector, numerous dyes are employed, many of which are non-biodegradable and pose significant health risks. Carmine, Malachite Green and Rose Bangle dyes are mainly used in textile, food and cosmetic industries. However theses dyes are highly toxic and can cause various sorts of diseases such as irritation, gastrointestinal issues and even carcinogenic effect upon ingestion (Ahmed and Xinxin, 2016). Therefore, it is important to focus on the degradation of these types of dyes, which is parent in wastewater to mitigate its adverse impact on the human health and the environment (Schneider et al., 2014; Alderman, 1985; Korb et al., 2008). One pioneering candidate for photocatalysis is the water splitting process on the TiO₂ (Titania) electrode under UV light irradiation, as demonstrated by Fujishima and Honda in 1972. Since then, photocatalysis has garnered significant attention from researchers as a mean to achieve complete waste water degradation (Hoffmann et al., 1995; Pelaez et al., 2012). In recent years, the use of metal oxide semiconductors nanomaterials has gained prominence in addressing environmental challenges. Among these TiO₂ stands out due to its unique properties, including recoverability, eco-friendliness, and nontoxicity. TiO₂ possess favorable characteristics such as suitable position of both valence band maximum and conduction band minimum to favor several redox reactions high chemical and photostability, corrosion resistance, high refractive index and low cost (Ola and Maroto-Valer, 2015; Luttrell et al., 2014; Silva and Faria, 2009).

TiO₂ exists in three crystalline phases: anatase, rutile, and brookite, with the anatase phase being the most active for the photocatalytic application. However, the wide bandgap of anatase TiO₂ (about 3.20 eV) limits its photocatalytic activity in visible light, which constitutes a significant portion of the solar spectrum (Ali et al., 2017; Tsega and Dejene, 2017). Efforts have been made to enhance TiO₂’s activity in visible light and increase dye degradation efficacy by reduction the recombination rate of charge carries. This can be achieved through engineering of pure TiO₂ via the inclusion of the semiconductors or doping with non-metals, transition metals and noble metals (Bensouici et al., 2015; Yu et al., 2019; Sukhadeve et al., 2022). TiO₂ is often doped with various metals such is Ag, Ce, Co, Cu, Fe, Mn, Ni, Y, V, Cr and Zr, which enhance its degradation efficiency under visible light (Liu et al., 2013; Devi et al., 2010; Kim et al., 2008; Inturi et al., 2014). Furthermore, reducing the particle size and modifying the catalysts through doping and synthesis processes have been shown to improve photocatalytic activity. For instance, doping of TiO₂ with metals like Ag, Zr, Co, and Mn has been found to enhance its adsorption properties (active sites on the surface area) and activity under visible light. Additionally, co-doping with Fe and N has demonstrated improved photocatalytic performance under natural visible light by hindering electron–hole pairs recombination (Gao et al., 2010; Choina et al., 2014; Sadanandam et al., 2013; Deng et al., 2011). Among noble metals, Ag is particularly appealing due to its low cost, excellent photocatalytic activity along with its lower work function compared to the noble metals like gold and platinum. Silver can be incorporated into the TiO₂ lattice using various synthesis methods including sol-gel route, which offers advantages such as short reaction time, low temperature synthesis, high purity and good homogeneity (Liu et al., 2012; Li et al., 2011; Rabhi et al., 2019; Aysin et al., 2013; Dong et al., 2019).

The current study focuses on the synthesis of TiO₂ nanoparticles (NPs) and doping it with silver through a sol-gel method. The resulting catalysts are then subjected to uniform temperature calcination, followed by analytical characterization. The dopant materials are observed to enhance the photocatalytic performance of the catalyst under sunlight conditions for the degradation of Malachite Green and Rose Bangle dyes. The study provides insights into the visible light photocatalysis mechanism and corresponding experimental observation, offering potential solutions for the wastewater treatment.

2 Materials and methods

2.1 Reagents

All chemicals and reagents procured from Sigma Aldrich and utilized as purchased without additional purification.

2.2 Synthesis of undoped and silver doped TiO₂ nanoparticles

Through the use of the sol-gel process Figure 1, undoped TiO₂ NPs were synthesized. In this example reaction, 4.73 g of titanium tetra isopropoxide were dissolved in 8.2 g of IPA (iso-propyl alcohol) and stirred continuously for 10 min using a magnetic stirrer. Drop by drop of 1.023 g of water and 5.10 g of IPA were added to the above alkoxide solution. The mixture was stirred once the iso-propanol solution and water had been added for around 24 h at ambient temperature. The white precipitate was then calcined in a furnace at 500 °C after being dried in an air oven at 100 °C. Silver nitrate was added in a weighed amount (2%–8%) as dopant, then same sol-gel method was used for the synthesis of silver doped TiO₂ NPs.

Figure 1

Figure 1. Flow chart for the synthesis of undoped and silver doped TiO₂ NPs.

2.3 Photocatalytic degradation of malachite green and Rose Bengal dyes

The Malachite Green and Rose Bangle are stable dyes and did not degrade without a catalyst. First of all, for the preparation of 1 mM stock solution, calculated amount of Malachite Green and Rose Bengal dyes added to 50 mL of distilled water. The stock solution is then diluted to prepare different concentrations of solution. For at least 30 min, the mixture was kept in the dark while being constantly stirred to maintain the equilibrium between adsorption and desorption. 1 mg of photo catalyst was then added to the mixture. After 30 min, the absorption was examined and given the termed (A₀) in the formula shown below. After placing the mixed solution in the sunlight, the absorbance was checked every 10 min. Various synthesized nanomaterials were used for the photocatalyst degradation of each dyes.

For the most effective photocatalyst, the same approach should be followed to consider the effects of dye concentration, catalyst quantity and pH (Arunachalam et al., 2012). The following mathematical formula was used to compute the photocatalytic degradation efficiency:

$\text{PDE\hspace{0.17em}}\left(\%\right)=\left({\mathrm{A}}_0-{\mathrm{A}}_{\mathrm{t}}\right)/{\mathrm{A}}_{\mathrm{o}}\times 100$

Where, “A₀” is dye absorption before exposure to sunlight and “A_t” stands for the dye’s absorption at various time periods.

2.4 Analysis

Dyes were subjected to photocatalytic degradation while exposed to direct sunlight. Degradation of Malachite Green and Rose Bengal was seen at 617 nm and 562 nm respectively. With the help of a double beam spectrophotometer, absorbance was calculated. With undoped and silver doped TiO₂ NPs, the photodegradation efficiency of dyes was measured. PDE (%) was plotted against several photocatalysts. The search for the most effective photocatalyst continued. By keeping other factors constant, it was possible to study the effects of many variables on dyes, including the influence of dye concentration, the amount of catalyst loading and pH. Through PDE analysis, optimal conditions for the catalyst were investigated.

2.5 Kinetic studies

Investigations were made into the photodegradation kinetics of the Malachite Green and Rose Bengal dyes when undoped and silver doped TiO₂ NPs were used as catalysts. Plotting lnA₀/A_t vs. time, and comparing slope and related constants, the best performing photocatalist was selected.

2.6 Characterizations

X-ray diffraction analysis (conducted using PANalytical XPERT high-score Diffractometer) was employed to assess the crystallinity and phase purity of the synthesized nanomaterials within a 2 theta range of 10°–80°. The Debye-Scherrer equation was utilized to determined crystallite size.

(the equation and related parameters are given below)

$d=\frac{k\lambda }{\mathrm{\beta }\cos \mathrm{ɵ}}$

Scanning electron microscopy (SEM) was utilized for particle morphology analysis, with an energy dispersive X-ray system investigated with SEM to verify the elemental purity. The optical characteristics of the materials were investigated via a double-beam shimadzu-1700 UV-visible spectrometer, enabling the calculation of band gaps from UV-visible absorption spectra. Fourier transform infrared spectroscopy (perform using BRUKER platinum ATR instrument) covered the range of 4,000–500 cm^-1 to identify specific functional groups and detect impurities.

3 Results and discussion

3.1 UV/Vis studies

The UV/Vis absorption spectra of Titania (TiO₂) and (2%, 3% and 4%) silver doped TiO₂ NPs are shown in Figure 2. The optical absorption of TiO₂ reaches wavelength of 303 nm. The optical absorption at this wavelength nm is primarily attributed to electron transitions from the valence band to the conduction band (band-to-band transition, O 2p to Ti 3d), according to the energy band structure of TiO₂ (Ali et al., 2017). In cases of different percentages of silver used as a dopant the maximum absorption of TiO₂ has been shifted (red-shift) toward higher wavelengths: for 2%, 3% and 4% silver doped TiO_2, the maximum absorptions is observed at 324 nm, 337 nm and 351 nm respectively.

Figure 2

Figure 2. UV/Vis spectra of TiO₂ NPs and (2%, 3%, and 4%) silver doped TiO₂ NPs.

Figure 3 shows the energy band gaps of titania and 2, 3 and 4 wt% Ag-doped TiO₂ nanoparticles, calculated using Tauc’s relation. A clear red shift of the absorption edge is observed with increasing Ag content, and the corresponding band-gap values (summarized in Table 1) decrease systematically as the silver concentration increases. This behavior can be attributed to the incorporation of Ag into the TiO₂ lattice, which introduces dopant-related electronic states and leads to partial overlap between the TiO₂ conduction band and Ag 4d levels, thereby narrowing the band gap and extending the absorption into the visible region. Similar dopant-induced red shifts and band-gap narrowing effects have been reported for modified Ag/TiO₂ systems and Ag-containing nanostructures, where Ag-related states and plasmonic contributions enhance visible-light absorption and charge separation (Zhao et al., 2008; Santos et al., 2015; Liu et al., 2020; Chen et al., 2023). In our case, the Ag dopant is expected to create trapping sites within the TiO₂ lattice, which facilitate the separation of photo-excited electron–hole pairs and thus support improved photocatalytic activity under visible-light irradiation.

Figure 3

Figure 3. Energy Gaps of TiO₂ NPs and (2%, 3%, and 4%) silver doped TiO₂ NPs.

Table 1

Table 1. Band gaps values of the TiO₂ NPs and (2%, 3%, and 4%) silver doped TiO₂ NPs.

3.2 XRD studies

XRD patterns of TiO₂ and (2%, 3% and 4%) silver doped TiO₂ NPs (calcined at 500 °C) are plotted to evaluate the crystal structure. It highlights the fact that even at the maximum doping concentration, no diffraction peaks related to impurity phases like silver, or its oxides have been seen, proving that doping does not affect the anatase phase. Additionally, Lack of impurity phases proved that Ag²⁺ ions were effectively incorporated into the TiO₂ lattice without silver oxide building up on the surface of the material. The anatase phase of TiO₂ corresponds to all of the diffraction peaks Figure 4, which are matched with JCPSD card # 21-1272. The presence of lattice strain in the samples may be the reason why prominent diffraction peaks move towards the lower 2 theta value and broaden with increasing doping ion concentration (Ali et al., 2018).

Figure 4

Figure 4. XRD patterns of TiO₂ NPs and (2%, 3%, and4%) silver doped TiO₂ NPs.

Using the Scherrer formula, the average crystallite sizes of the synthesized samples have been calculated (Li et al., 2011; Sukhadeve et al., 2021).

$d=\frac{k\lambda }{\mathrm{\beta }\cos \mathrm{ɵ}}$

Where d is the average crystallite size, k is Scherer’s constant, $\lambda$ is the x-ray wavelength, $\mathrm{ɵ}$ is the diffraction angle, and$\mathrm{\beta }$ is the full width half-maximum of the diffraction. Table 2 shows the differences in the average crystallite sizes of all the nanomaterials with doping concentrations.

Table 2

Table 2. Average crystallite sizes of TiO₂ and (2%, 3% and 4%) silver doped TiO₂ NPs.

3.3 FT-IR studies

FTIR Spectra were taken in the wavenumber range of 4,000–400 cm^-1 to explore the behavior of reaction intermediate and functional groups included in the synthesized samples, as shown in Figure 5. The O-H stretching vibration, which causes a broad band to appear at 3,270 cm^-1, and O-H bending vibration, which causes a band at 1631 cm^-1, come from chemically adsorbed water molecules, because the hydroxyl groups operate as the primary scavengers of photogenerated electron and hole, which leads to the creation of hydroxyl radical (OH^•) necessary for the degradation of dye. The presence of hydroxyl groups plays a critical role in improving photocatalytic activity. The presence of metal oxygen bonding is confirmed by the broad band at 500–850 cm^-1, which is related to the Ti-O bending mode and dopant (silver) mode of vibrations (Ali et al., 2018; Vukoje et al., 2016).

Figure 5

Figure 5. FT-IR spectra of TiO₂ NPs and (2%, 3%, and 4%) silver doped TiO₂ NPs.

3.4 SEM studies

SEM analysis revealed that the synthesized Ag–TiO₂ nanoparticles are predominantly quasi-spherical in shape with slight agglomeration, and the average particle size lies within the nanometer range. After photocatalytic interaction with the dyes, no significant change in particle size or morphology was observed, indicating good structural stability of the photocatalyst during the degradation process (Ullah et al., 2020). Minor surface adsorption of dye molecules was evident, but it did not alter the overall nanoparticle dimensions, confirming the reusability and durability of the synthesized catalyst. The SEM analysis of undoped and Ag-doped TiO₂ NPs, conducted at 100 nm magnification, Figures 6a,b revealed distinctive morphological characteristics. Both types exhibited a spherical shape, with undoped TiO₂ particles forming small blocks of agglomerates. In contrast, Ag-doped TiO₂ NPs displayed a dispersed arrangement, indicating improved particle distribution and small cavites inbetween the particles. The observed differences in aggregation suggest that the introduction of silver modifies the nanoparticle morphology, potentially influencing electronic, optical, or catalytic properties. These findings have significant implications for applications such as photocatalysis, where particle morphology plays a crucial role. The dispersed nature of Ag-doped TiO₂ NPs suggests enhanced potential for specific technological applications.

Figure 6

Figure 6. SEM images of (a) undoped TiO₂ and (b) Ag-doped (4%) TiO₂ NPs.

3.5 EDX studies

Energy Dispersive X-ray Spectroscopy (EDX) was employed to analyze the elemental composition of undoped and Ag-doped TiO₂ NPs are shown in Figures 7a,b. The spectra confirmed the presence of titanium (Ti), silver (Ag), and oxygen (O) in both samples. Undoped TiO₂ exhibited peaks for Ti and O, while Ag-doped TiO₂ additionally showed peaks corresponding to silver. This confirmation is crucial for understanding the success of the synthesis process and predicting the nanoparticles’ behavior in practical applications, especially considering the potential impact of silver doping on electronic and catalytic properties.

Figure 7

Figure 7. EDX spectra of (a) undoped TiO₂ and (b) Ag-doped TiO₂ NPs.

3.6 Photocatalytic degradation of malachite green dye

Malachite green is a synthetic dye that has been historically used as a green dye for fabrics, paper, and leather. To evaluate the photocatalytic activity of the synthesized materials for this particular dye, undoped and silver doped TiO₂ NPs were used for the photocatalytic degradation of Malachite dye. When dye solution having the photo catalyst was exposed to sunlight for 70 min, the intensity of the dye peak decreases gradually with time. Malachite has the highest absorption peak at a wavelength of 614 nm. This was also confirmed by the elimination of dye color, so dye chromophoric structure was degraded. In order to find out the optimum conditions for degradation process, degradation was performed in different steps. The first step is the selection of silver percentage as dopant, and it is termed as dopant optimization.

3.7 Optimization of dopants

Optimizing dopant Ag concentrations in TiO₂ was the first step towards determining the ideal conditions for dye degradation. Gradually, a noticeable decrease in the absorption peak intensity was noted, indicating that the dye was still degrading. The disintegration of the dye’s chromophore structure is the precise mechanism of degradation. Two milligrams of each manufactured catalyst were added to a dye solution with a 30 μM concentration. Additionally, a noticeable disappearing of the dye solution’s color was noted after 70 min. The UV-Visible spectra of Malachite green dye degradation by pristine and Ag (2%–4%) doped TiO₂ is shown in Figure 8.

Figure 8

Figure 8. Photodegradation of Malachite Green dye by (a) undoped TiO₂ and (b–d) (2-4) % Ag-TiO₂ NPs.

The ability of the photocatalysts to adsorb dye molecules onto their surface is what determines how effective they are in photocatalysis. With different percentages (2%–4% weight) of silver doping, the photocatalytic degradation efficiency (%PDE) was evaluated for both undoped TiO₂ and different percentages of silver doped TiO₂ NPs. The Figure 8b shows that 2% Ag-TiO₂ NPs shows the maximum photodegradation efficiency. This remarkable result is ascribed to a decrease in electron-hole recombination, which could potentially be triggered by an increase in surface active sites enabled by the existence of silver ions. The amount of catalyst adsorbed on the dye’s surface determines how much the dye degrades; a higher percentage of PDE indicates more dye molecules are being adsorbed on the catalyst’s surface Supplementary Figure S1.

Exposure to sunlight degrades the dye’s concentration. The graphs demonstrate Figure 9 how the ratio of A/A_o decreases as time increases. The degradation of dyes appears to follow pseudo first order kinetics just based on data in Figure 9.

Figure 9

Figure 9. Determination of observed rate constant for photodegradation of Malachite Green dye by (a) undoped TiO₂ NPs and (b–d) (2-4) % Ag-TiO₂ NPs.

3.8 Effect of catalyst concentration

The impact of a catalyst on the rate of degradation was thoroughly examined in the study’s second phase. By adding different amounts of the catalyst to a dye solution with a 30 μM concentration, the goal was to find the best catalyst dose conditions. Applying 2% Ag-TiO₂ NPs, in particular, showed a notable improvement in photodegradation efficiency. As a result, additional studies using dye solutions were carried out using1-4 mg dosages of this catalyst. Figure 10 shows the intensity of peaks for different catalyst dosages.

Figure 10

Figure 10. Photocatalytic degradation of Malachite Green dye by 1–4 mg (a-d) of 2% Ag-TiO₂ photocatalyst.

The degradation of dye was significantly affected by the addition of more catalyst concentration from 1 mg to 4 mg. For the photocatalytic degradation of Malachite green dye, the use of 4 mg of the 2% catalyst showed the best percentage of photodegradation efficiency (%PDE), as shown in Supplementary Figure S2. The catalyst surface’s active sites, which promote greater light scattering, low e/h recombination, screening and let UV light into the solution, are responsible for the higher efficiency. The degradation rate increased as the catalyst concentration increased up to 4 mg Supplementary Figure S3.

3.9 Effect of dye concentrations

The effect of changing Malachite Green dye concentrations on degradation efficiency was methodically studied in an effort to optimize the photodegradation process. In this investigation, three different concentrations (85 μM, 90 μM and 95 μM) were used. In all three solutions, 4 mg of 2% Ag-TiO₂ NPs were used in the experiment. The results of the degradation process at these various concentrations are shown by the absorption spectra, which are shown in Supplementary Figure S4. For 85 μM, 90 μM of dye concentrations the percentage photodegradation efficiency of 96% was observed, when the concentration was raised from 85 to 90 μM. Interestingly, though, at 95 μM, there was a subsequent decrease in photodegradation efficiency. Important insights can be gained from this complex interaction between dye concentration and photodegradation effectiveness.

The absorption spectra in our investigation showed clear differences in the percentages of degradation efficiency across various concentrations of malachite green dye when silver doped TiO₂ NPs was used for photocatalysis under sun light exposure. As a photocatalyst, silver doped TiO₂ NPs produces free radicals, especially hydroxyl radicals (^•OH), which are essential to the degradation process. It's interesting to note that we found a clear relationship impacting degradation rates between dye concentration and the generation of ^•OH radicals on the photocatalyst’s surface. Reduced generation of ^•OH radicals and decreased degradation efficiency were linked to higher dye concentrations. This phenomenon can be explained by the photocatalyst’s surface having fewer active sites available at higher dye concentrations, which hinders the catalytic activity. Furthermore, catalytic effectiveness may be further reduced if photons are absorbed by the dye before they reach the catalyst. On the other hand, lower dye concentrations demonstrated slower rates of degradation, which could be attributed to inadequate adsorption on the photocatalyst surface. This led to a restricted percentage of photocatalytic degradation efficiency (%PDE). In addition, the observed decrease in catalytic activity at higher dye concentrations could be explained by the deactivation of active molecules. These results help to clarify the complex relationship between sun light exposure, dye concentration, and silver doped TiO₂ catalytic effectiveness and offer important new information for improving photocatalytic degradation processes. The %PDE is shown in Supplementary Figure S5, 6.

3.10 Effect of pH

The pH changes have a major impact on dyes’ ability to favour photocatalytic processes. A thorough analysis of the pH effect was carried out, which included the preparation of several solutions with pH values ranging from 3, 5, 7 and 9. The addition of HCl and NaOH to the solution permitted the adjustment of acidity and basicity of the solutions. An ideal amount of catalyst, 4 mg of 2% Ag-TiO₂ photocatalyst was added to each solution. Figure 11 shows the absorption spectra of the solutions at different pH values.

Figure 11

Figure 11. Absorption spectra of different pH values, 3,5,7,9 (a-d) for degradation of Malachite Green dye by 4 mg of 2% Ag-TiO₂ photocatalyst .

Figure 12 shows a significant difference in percentage degradation efficiency between different pH values. Remarkably, the degradation efficiency approaches 98% in 9 pH basic media. More specifically, at high pH values, a negative charge is produced on the photocatalyst surface. The negatively charged photocatalyst surface and the Malachite cation experience strong electrostatic forces of attraction. Malachite’s adsorption on the photocatalyst surface is facilitated by this interaction, especially around pH 9, when the surface acquires a negative charge. The common oxidation process in an alkaline solution is primarily driven by hydroxyl radicals (^•OH), here resulting from the Malachite green cation and the negatively charged photocatalyst surface. The tendency of the NPs to aggregate makes it more difficult to achieve the ideal surface area needed for efficient dye adsorption and photon absorption from sunlight. The degradation efficiency is considerably reduced at acidic pH values. In these circumstances, the malachite cation and the positively charged photocatalyst surface form a strong attraction that affects the degradation process’s overall efficiency.

Figure 12

Figure 12. PDE for Different pH values of Malachite dye degradation by 4 mg of 2% Ag-TiO₂ photocatalyst.

3.11 Photocatalytic degradation of Rose Bengal dye

The photocatalytic activity of the undoped and Ag-doped TiO₂ NPs were also tested in the photocatalytic degradation of Rose Bengal dye. This experiment was repeated at room temperature and in the presence of sun irradiation, exactly as it was done for a previous dye. The UV-Visible spectra revealed that Rose Bengal exhibits its maximum absorption peak at 562 nm. To enhance the photocatalytic activity of the dye, optimal conditions were systematically investigated, considering key factors such as dopant optimization, catalyst dosage, dye concentration, and pH. The initial focus of the study was on dopant optimization.

3.12 Dopant optimization

Optimizing dopant concentrations was the first step towards determining the ideal circumstances for dye degradation. Gradually, a noticeable decrease in the absorption peak intensity was noted, indicating that the dye was still degrading. The disintegration of the dye’s chromophore structure is the precise mechanism of degradation. Two milligrams of each manufactured catalyst were added to a dye solution with a 10 μM concentration. Additionally, a noticeable disappearance of the dye solution’s color was noted after 70 min. The UV-Visible spectra of Rose Bengal dye degradation by undoped TiO₂ and Ag (2%–4%) doped TiO₂ NPs is shown in Figure 13.

Figure 13

Figure 13. Photodegradation of Rose Bengal dye by undoped TiO₂ (a) and (2%–4%) (b-d) Ag-TiO2 NPs.

The photocatalytic efficiency was assessed through the percentage of photodegradation efficiency (PDE). Notably, undoped TiO₂ NPs demonstrated a maximum PDE value of 97%, highlighting their robust photocatalytic performance. However, the introduction of silver doping into TiO₂ NPs resulted in a reduction in photocatalytic efficiency. The maximum PDE Supplementary Figure S7 for Ag-doped TiO₂ NPs was consistently lower compared to undoped TiO₂. This phenomenon was attributed to the emergence of surface defects caused by the surface functionalization induced by Ag ions. For the comparision of both dyes the variation in photocatalytic degradation efficiency (PDE) between undoped and Ag-doped TiO₂ nanoparticles (NPs) is due to how silver doping affects the catalyst’s electronic properties and surface interactions with different dyes. Ag doping reduces the bandgap of TiO₂, allowing it to absorb more visible light, which can enhance photocatalysis. However, it also introduces defects that act as recombination centers for the electron-hole pairs, reducing the overall photocatalytic efficiency in some cases. In malachite green degradation, Ag-doped TiO₂ likely improves light absorption and provides a surface more favorable for dye adsorption, resulting in a higher PDE than undoped TiO₂. Conversely, for Rose Bengal degradation, the undoped TiO₂ shows higher PDE because it avoids the excessive recombination seen in Ag-doped TiO₂, making the undoped version more efficient for this dye. The interaction between the catalyst surface and the specific dye plays a critical role in these outcomes, with the nature of the dye influencing how well the photocatalyst performs. The reduction in photocatalytic efficiency when silver (Ag) is doped into TiO₂ nanoparticles (NPs) is primarily due to the creation of recombination centers. While Ag doping reduces the bandgap of TiO₂, allowing it to absorb more visible light and potentially improving activity, it can also introduce defects in the crystal structure. These defects act as sites where electron-hole pairs, generated during the photocatalytic process, recombine prematurely before contributing to the reaction. This increases electron-hole recombination, which reduces the overall photocatalytic efficiency. Additionally, excessive doping can block active sites on the TiO₂ surface, further lowering the degradation performance.

A critical observation revealed that the agglomeration of particles on the photocatalyst’s surface occurred with an increase in dopant concentration. This agglomeration phenomenon was identified as a key factor contributing to the decline in degradation efficiency. The findings underscore the intricate interplay between dopant concentration, surface defects, and adsorption capacity in influencing the overall photocatalytic performance of silver doped TiO₂ NPs. These insights contribute valuable information to the understanding of nanomaterial based photocatalysis, particularly in the context of environmental remediation applications Supplementary Figure S8.

3.13 Effect of catalyst concentration

The impact of catalyst concentration on the dye degradation was examined in the following phase. Finding the ideal catalyst dosage is necessary to extract the maximum degradation rate. In every subsequent reaction, undoped TiO₂ was utilized because of its optimized photocatalytic degradation efficiency. While the catalyst dosage ranged from 1 to 4 mg, the dye concentration was kept constant at 10 µM. Figure 14 shows the absorption spectra of undoped TiO₂ NPs at different dosage concentrations.

Figure 14

Figure 14. Absorption spectra for the Photocatalytic degradation of Rose Bengal dye by different dosage (1-4mg) (a-d) of undoped TiO₂ NPs.

When the catalyst dosage was increase from 1 to 2 mg, the photodegradation efficiency reached 97%. The increase of active sites, which significantly enhance Rose Bengal dye molecule interactions. The improved photocatalytic degrading efficiency Supplementary Figure S9 can be explained by the increasing presence of functional groups. However, adding more catalyst than 2 mg had a negative impact since the photocatalytic degradation efficiency dropped to 76% and 91%. This is related to the build-up of catalyst, which stops light from entering the surface. Furthermore, the turbidity of the suspension reduces the amount of light that may pass through it, reducing the efficiency of photodegradation.

3.14 Effect of dye concentration

The photodegradation of Rose Bengal dye was examined at various concentrations between 10 and 14 μM when a given quantity of catalyst is present. The optimized procedure was followed while using the 2 mg of undoped TiO₂ NPs. The absorption spectra of various dye doses are shown in Figure 15.

Figure 15

Figure 15. Absorption spectra of photocatalytic degradation of Rose Bengal dye for different concentrations (10-14 μM) (a-c) by 2 mg of undoped TiO₂ NPs.

The absorption spectra showed different photocatalytic degradation efficiency at different concentrations. High concentration of dye resulted a decrease in ^•OH radical formation on the catalyst’s surface, which in turn resulted in lower degradation rates. At high concentration of dye less active sites are available due to which degradation is slow. Before reaching the catalyst, some photons may be absorbed by the dye, which can also lower the catalytic efficiency. %PDE is high for low concentration of dye because it is insufficient to adsorb on the catalyst’s surface and there are large less active sites availability. Activation of activated molecules may also cause an increase in catalytic performance of dyes. The %PDE for Rose Bengal dye is shown in Supplementary Figure S10, 11.

3.15 Effect of pH

The pH parameter is the most significant in the study of photocatalytic degradation of dyes. For this purpose, HCl and NaOH were used to modify the pH of the solution while keeping the amount of photocatalyst and dye solution concentrations constant. About 2 mg of undoped TiO₂ NPs were treated with the optimum (10 µM) dye concentration. The system was exposed to light irradiation at pH ranges from 3 to 9. The UV-Visible spectra of Rose Bengal dye degraded at various pH levels are shown Figure 16.

Figure 16

Figure 16. Absorption spectra of Rose Bengal dye at different pH (3-9) values (a-d) respectively.

The Rose Bengal dye solution’s color shift at pH3 indicates a change in its chemical structure, which may lead to a modification of the chromophore group. Interestingly, higher photocatalytic rates in acidic pH ranges were found by the photodegradation analysis. Remarkably, pH3 was found to have the maximum percentage of photodegradation efficiency (PDE). This suggests that somewhat acidic environments favor the production of reactive intermediates, especially hydroxyl free radicals, which raises the rate of reaction as a whole. H⁺ ions produced by the catalyst’s surface ionization and Rose Bengal’s deprotonation are essential to the photocatalytic action. It is interesting to note that the spontaneous generation of reactive intermediates is less likely to occur in pH5 and pH7 environments. This data emphasizes how crucial it is to keep the environment high acidic in order to maximize the generation of hydroxyl free radicals and, in turn, improve the photocatalytic process’ efficiency. The oxidant power of holes is greater than that of hydroxyl radicals (^•OH), even though hydroxyl radicals can be easily generated in alkaline environments with an abundance of hydroxyls. Alkaline circumstances consequently result in a slower rate of deterioration. The comparison between different pH levels and the percentage of photodegradation efficiency (PDE) is shown in Figure 17.

Figure 17

Figure 17. PDE at different pH levels of Rose Bengal dye degradation by undoped TiO₂ NPs.

3.16 Mechanism of dyes degredation

Under natural sunlight irradiation, the TiO₂ and Ag–TiO₂ photocatalysts absorb photons with energy equal to or greater than their band gap. This promotes electrons from the valence band (VB) to the conduction band (CB), leaving behind positive holes:

${\text{TiO}}_2\text{\hspace{0.17em}or\hspace{0.17em}Ag}–{\text{TiO}}_2+\mathrm{h}\nu \to {\text{eCB}}^{-}+{\text{hVB}}^{+}$

Because the CB potential of TiO₂ (and Ag–TiO₂) is more negative than the O₂/·O₂⁻ redox couple, the photogenerated electrons can readily reduce dissolved oxygen to superoxide radicals:

${\mathrm{O}}_2+\text{eCB}-{\to }^{·}{\mathrm{O}}_2^{-}$

These superoxide species can further participate in a series of reactions leading to the formation of additional reactive oxygen species:

$·{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}{\to }^{·}{\text{HO}}_2$

$2·{\text{HO}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$

${\mathrm{H}}_2{\mathrm{O}}_2^{+}{\text{eCB}}^{-}\to ·\text{OH}+{\text{OH}}^{-}$

At the same time, the photogenerated holes in the VB are sufficiently oxidizing to react with surface-adsorbed water molecules or hydroxide ions, generating hydroxyl radicals:

${\mathrm{H}}_2\mathrm{O}+\text{hVB}+{\to }^{·}\text{OH}+\mathrm{H}+$

${\text{OH}}^{-}+\text{hVB}+{\to }^{·}\text{OH}$

In the case of Ag–TiO₂, metallic Ag acts as an electron sink, capturing CB electrons and thereby suppressing recombination with holes. This enhances the lifetime and concentration of the reactive radicals (^·O₂⁻ and ^·OH) at the catalyst surface.

Malachite Green (MG, C₂₃H₂₅ClN₂⁺) and Rose Bengal (RB) molecules adsorbed on the catalyst surface are then attacked by these reactive species:

$\text{Dye\hspace{0.17em}}\left(\text{MG\hspace{0.17em}or\hspace{0.17em}RB}\right){+}^{·}\text{OH}{/}^{·}{\mathrm{O}}_2^{-}\to \text{intermediates}\to {\text{CO}}_2+{\mathrm{H}}_2{\mathrm{O}}^{+}\text{inorganic\hspace{0.17em}ions}$

Thus, the overall photocatalytic degradation of MG and RB in our system proceeds through (i) photoexcitation of TiO₂/Ag–TiO₂, (ii) generation of ·O₂⁻ and ·OH via reduction of O₂ and oxidation of H₂O/OH⁻, and (iii) subsequent oxidative mineralization of the dye molecules adsorbed on the photocatalyst surface to CO₂, H₂O and simple inorganic by-products.

Table 3 provides a benchmarking overview of our photocatalysts against a range of TiO₂-based and doped TiO₂ systems reported in the literature. This comparison places the present undoped TiO₂ and Ag–TiO₂ materials within the broader context of existing dye-degradation photocatalysts and highlights that, despite their simple composition and preparation, their performance is comparable to that of more complex modified TiO₂ systems.

Table 3

Table 3. Benchmarking of representative TiO₂-based photocatalysts from the literature and the present undoped TiO₂ and Ag–TiO₂ systems, summarizing catalyst type, TiO₂ configuration, target dye and corresponding degradation performance.

4 Conclusion

In this study successfully synthesized undoped TiO₂ and silver-doped TiO₂ NPs via the Sol-Gel method, characterized using various techniques, and explored their efficacy as photocatalyst. The doping of silver into TiO₂ resulted in a notable red in the absorption spectrum, accompanied by an increase in λ_max from 303 to 351 nm. Moreover, Tauc’s plots indicated a decrease in the band gap of the NPs with increasing silver content, from 3.03 to 2.47 eV. XRD analysis revealed that the crystallite size of the NPs ranged from 8 to 12 nm. Elemental composition analysis via EDX confirmed the presence of all elements in the NPs in different proportions, while SEM analysis depicted spherical morphologies. FT-IR analysis further validated the purity of the materials. Importantly, the synthesized NPs exhibited remarkable photocatalytic activity, with an efficiency of 98% for the degradation of Malachite Green and Rose Bengal dyes. The study investigated various parameters affecting the photocatalytic processes, including catalyst amount, dye concentration, and pH. Optimal conditions were determined with 4 mg of 2% Ag-doped TiO₂ catalyst for Malachite Green degradation at pH, and 2 mg of undoped TiO₂ catalyst for Rose Bengal degradation at pH 3. Furthermore, the catalyst activity was found to follow pseudo first-order kinetics. These finding underscore the potential of silver–doped TiO₂ NPs as efficient photocatalysts for environmental remediation applications.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, on request.

Author contributions

ID: Methodology, Writing – original draft. LS: Conceptualization, Writing – original draft. ZR: Project administration, Supervision, Writing – review and editing. FG: Software, Writing – review and editing. NR: Validation, Writing – review and editing. FM: Formal Analysis, Writing – review and editing. AS: Visualization, Writing – review and editing. PW: Data curation, Resources, Writing – review and editing. SA-S: Funding acquisition, Resources, Writing – review and editing. MZ: Data curation, Funding acquisition, Writing – review and editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This study is supported by Higher Education Commission of Pakistan, This work was also supported by the Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R58), Riyadh, Saudi Arabia.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fctls.2025.1710435/full#supplementary-material

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