Highly Efficient Visible-Light-Driven Photocatalysis of Rose Bengal Dye and Hydrogen Production Using Ag@Cu/TiO 2 Ternary Nanocomposites

: In this work, an Ag@Cu/TiO 2 ternary nanocomposite was synthesized by a simple chemical methodology and subsequently studied for the photocatalytic degradation of rose bengal (RB) dye under visible light as well as its hydrogen production. The shape, size and topographical analysis by scanning and transmission electron microscopy revealed that all the constituents are well intercalated and are in the nano range. The energy dispersive X-ray analysis of the Ag@Cu/TiO 2 showed the presence of Ti, O, Cu and Ag and the absence of any other impurities, while the mapping analysis showed their uniform distribution. The X-ray photon spectroscopy also showed successful interaction between the components. Furthermore, the changes in the chemical state of Ti2p were examined. The band gap of Ag@Cu/TiO 2 using the Tauc plot relations was found to be the lowest at 2.86 eV in comparison to pure TiO 2 (3.28 eV), binary Ag/TiO 2 (3.13 eV) and Cu/TiO 2 (3.00 eV). The Ag@Cu/TiO 2 displayed the lowest photoluminescence intensity, suggesting the highest degradation efficiency and lowest recombination rate. The application of Ag@Cu/TiO 2 toward the photocatalytic degradation of RB dye exhibited a degradation rate of ~81.07%, which exceeds the efficiency of pure TiO 2 by 3.31 times. Apart from this, the hydrogen production of Ag@Cu/TiO 2 was found to be 17.1 µ mol h − 1 g − 1 , suggesting that copper and silver synergistically contributed, thereby resulting in the increased hydrogen production of pure TiO 2 .


Introduction
Water is an essential element for the survival of life on the Earth and is present on the Earth in the form of rivers, seas, lakes and ground water.The rapid growth of the population, development, Industrial Revolution, and climate change have contaminated every aspect of water resources [1].The main contaminants in water are tannery, leather, rubber, dyes, cosmetics, food pharmaceuticals, printing paint, etc. [2,3].Among them, textiles, pharmaceuticals, printing paints and fabric industries discharge colored dye up to 17-20% into water bodies, which has created water pollution [4].Synthetic colored dyes are non-biodegradable and highly photostable [5].The dye inhibits sunlight, decreases the solubility of molecular oxygen in water, which affects the aquatic life of organisms as well as the ecosystem of water, and has long-term toxic and carcinogenic effects for all ecosystems.
Among the most widely used organic dye is rose bengal (RB) dye, which is used in the textiles, printing, and cosmetic photochemical industries.It must be removed from water since it causes itching, redness, inflammation, irritation, and blistering when it comes into direct contact with skin.Different methods, including membrane filtration, coagulation, flocculation, and adsorption, have recently been used to degrade dye pollution [6][7][8].However, all these methods have limitations, including effluent, sludge development, and difficulty reactivating the adsorbent.Photocatalysis is one of a most common and most cost-effective methods to decompose RB dyes from water using efficient nanocatalysts [9].Visible light photocatalysis is a process that utilizes the energy from visible light to initiate a chemical reaction.It involves the use of a photocatalyst, which can absorb light in the visible spectrum and promote the conversion of reactants into the desired product [10].Recently, this technique has drawn a lot of interest owing to its environmental friendliness, versatility, selectivity, efficiency, cost-effectiveness and sustainability, which make it highly applicable for a large number of uses.
Nanoparticles show different properties to the bulk materials or molecular species or atoms from which they are derived due to the high surface volume ratio [11].The various types of nanoparticles of transition metals [12], transition metal oxides and chalcogenides are used for varied applications, such as energy storage, magnetic data storage, sensors, and ferrofluids, and photocatalytic degradation of organic dye.Nanoparticles, particularly metal oxide nanoparticles, are widely used in the field of visible light photocatalysis.These nanoparticles have unique properties that make them effective in photocatalysis for various reactions.For instance, metal oxide nanoparticles such as titanium oxide (TiO 2 ) [13], zinc oxide (ZnO) [14], and tungsten oxide (WO 3 ) [15] have been extensively studied for their photocatalytic capabilities.However, there are certain drawbacks to using nanoparticles for photocatalysis, such as their high band gap and the requirement that the photocatalyst's band gap must match the energy of the visible light photons in order to achieve effective visible light photocatalysis.Unfortunately, many metal oxide nanoparticles have a band gap that corresponds to ultraviolet (UV-Vis) light, thereby limiting their visible light photocatalytic activity.During the photocatalysis process, nanoparticles may agglomerate, which results in a decrease in the surface area as well as limiting the reaction-active sites that are available.This aggregation can occur due to various factors, such as a high concentration, pH changes, or the presence of impurities [16].To overcome this limitation, researchers have developed various strategies, such as doping, surface modification, and band gap alteration, to activate metal oxide nanoparticles under visible light illumination.These modifications enable the nanoparticles to absorb a broader range of light, including visible light, thereby improving their photocatalytic efficiency [17].Numerous transition metal oxide, such as ZnO, TiO 2 , NiO, and CuO, and metal chalcogenides, such as ZnS and MoS 2 , have been used for the removal of colored dye, industrial pollutants and pharmaceutical water waste [18][19][20].The mostly studied transition metal oxide for the photocatalytic degradation of contamination as well as dye is TiO 2 , which has a strong oxidizing power, is non-toxic, and is extremely stable toward both photo and chemical corrosion.Photocatalysis is a promising method because it is inexpensive and environmentally friendly.TiO 2 has a 3.2 ev band gap and absorbs wavelengths between 360 and 380 nm in the UV-Vis region.Several studies show the decreased band gap of TiO 2 by the doping of novel transition metals such as Cu, Ag, Au, Pt, Ni, and Co, as well as polyaniline (PANI), polypyrrole (PPY) and reduced graphene oxide, which extends the spectral shift in the visible region and increases the photocatalytic activity of TiO 2 [21].It has been shown that methyl blue (MB) and methyl orange (MO) exhibit 96.3% and 97% degradation under UV-Vis light irradiation using TiO 2 synthesized by the modified solvothermal method.Lin et al. [22] demonstrated the photocatalytic degradation (UV-VIS) of toxic 2-chlorophenol by using a TiO 2 catalyst.Similarly, Zhang et al. [23] investigated the photoelectric effect and photocatalytic properties of TiO 2 by doping it with Ag nanoparticles to enhance the photoelectric and photocatalytic degradation.Mingxuan Sun et al. [24] showed much improved visible region photocatalytic activity, photochemical stability, and low charge recombination from TiO 2 doped with CdS and CdS/MoS 2 .
On the basis of the above discussion, we have synthesized ternary composites of Ag@Cu/TiO 2 by the in situ solvothermal method for the photocatalytic degradation of RB dye in the visible region of light.The synthesized ternary composite Ag@Cu/TiO 2 was characterized for the structural and morphological characteristics.Finally, the photocatalytic degradation of RB dye and the hydrogen evolution phenomenon in the visible region were deeply investigated.

Synthesis of Ag@Cu/TiO 2
The synthesis of Ag@Cu/TiO 2 was performed by the solvothermal methodology.In this method, 6 mL of isopropanol was dispersed in 86 mL of water, and to it, 6 mL of HCl was added dropwise.To the isopropanol and acid mixture, 5 mL of TIP was slowly added, and the entire mixture was put under stirring conditions at 80 • C for 1 h, thereby resulting in a white colloidal solution (termed solution A).This white colloidal solution was subjected to centrifugation and subsequently the collected TiO 2 was washed with an excess of solvents (water and ethanol mixture) and finally dried in an air oven at 80 • C for 6 h to obtain pure TiO 2 .The Cu/TiO 2 was prepared by dissolving 0.4 g of CuSO 4 .5H 2 O in 100 mL (for ~5% loading of Cu on TiO 2 ) of water and using 0.05 M NaOH to keep the pH at 12. To the above solution of CuSO 4 .5H 2 O, 30 mL of 0.05 M ascorbic acid was mixed dropwise and the resultant was heated at 60 • C with continuous stirring for 1 h (Figure 1).The yellow color of the mixture slowly changed to brown, indicative of the formed Cu (the Cu dispersion was termed solution B).Ultimately, both the solutions (A and B) were mixed under continuous stirring and the resultant Cu/TiO 2 was subjected to centrifugation, washed with an excess of solvents (water and ethanol) and finally dried at 80 • C for 6 h to obtain Cu/TiO 2 .The Ag@Cu/TiO 2 was prepared by first preparing the colloidal solution of Ag nanoparticles by dissolving 0.12 g of AgNO 3 in 100 mL H 2 O and then 30 mL of 0.06 M NaBH 4 was added to it dropwise.The Cu/TiO 2 binary composite was added to the Ag colloidal solution and the entire mixture was continuously stirred for 2 h.Thus, the prepared Ag@Cu/TiO 2 composite was washed with an excess of solvents (water and ethanol) and finally dried at 80 • C for 6 h to obtain pure Ag@Cu/TiO 2 .

Photocatalytic Degradation of RB and Hydrogen Production
The photocatalytic activity of the synthesized catalysts, namely pure TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2, was investigated by assessing their effectiveness in degrading RB dye.For this, 200 mg of prepared catalyst was mixed with a 250 mL solution containing 30 mg/L of RB dye and the pH was adjusted to 8. To attain adsorption-desorption equilibrium, the mixture was agitated for 30 m in the dark (refer to Supplementary Figure S4).Afterwards, visible light irradiation was performed using a 500 W halogen lamp.The solutions were collected at various intervals (0, 20, 40, and 60 min), followed by centrifuga-

Photocatalytic Degradation of RB and Hydrogen Production
The photocatalytic activity of the synthesized catalysts, namely pure TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 , was investigated by assessing their effectiveness in degrading RB dye.For this, 200 mg of prepared catalyst was mixed with a 250 mL solution containing 30 mg/L of RB dye and the pH was adjusted to 8. To attain adsorption-desorption equilibrium, the mixture was agitated for 30 m in the dark (refer to Supplementary Figure S4).Afterwards, visible light irradiation was performed using a 500 W halogen lamp.The solutions were collected at various intervals (0, 20, 40, and 60 min), followed by centrifugation and evaluation of the catalytic RB dye degradation using double-beam UV-Vis spectroscopy.
In the provided equation, C o represents the initial concentration of RB dye, while C t signifies the concentration of RB after a specific time interval (0, 20, 40, and 60 min).Afterwards, the apparent rate constant (k) was determined using the following equation.
To evaluate the hydrogen production, methanol was used as a scavenger.In hydrogen production processes, scavengers play a critical role in boosting efficiency.They tackle key challenges by capturing electron-hole pairs and preventing them from wasting generated energy [1].Additionally, scavengers selectively target and eliminate molecules involved in undesired side reactions, protecting the crucial intermediates needed for hydrogen evolution.Furthermore, they can facilitate the movement of photogenerated charges within the system, ensuring their efficient utilization for hydrogen production.Essentially, scavengers act as guardians, optimizing various aspects of the process to maximize the hydrogen output [2].

Surface Morphological Analysis
The morphological analysis of the TiO 2 , Ag/TiO 2 , Cu/TiO 2 and Ag@Cu/TiO 2 was performed by SEM (Figure 2) and TEM (Figure 3).The SEM of the pure TiO 2 shows clusters of small-sized round particles packed into small and big aggregates.The Ag/TiO 2 , Cu/TiO 2 and Ag@Cu/TiO 2 show similar morphology, with all the constituents in the nano range.It can be predicted from the micrographs that all the constituents in the Ag@Cu/TiO 2 are well intercalated with each other.The TEM images of the Ag@Cu/TiO 2 show that all the TiO 2 , Cu and Ag are well intercalated with each other, with the size of some TiO 2 being slightly over 100 nm.
The elemental analysis of the TiO 2 , Ag/TiO 2 , Cu/TiO 2 and Ag@Cu/TiO 2 by EDS shows the presence of Ti and O in the TiO 2 , Ti, O and Ag in the Ag/TiO 2 , Cu, Ti and O in the Cu/TiO 2 and Cu, Ti, O and Ag in the Ag@Cu/TiO 2 , thereby depicting the successful formation of the composite as well as the absence of any impurities.The uniform distribution of the respective elements, as depicted by the mapping analysis (Figure 4), suggests the efficacy of the synthesis methodology.

Surface Compositional and Chemical Analysis
The elemental composition and functional groups of the prepared catalysts (pure TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 ) were studied by X-ray photoelectron spectroscopy (XPS).The survey spectra (Figure 5a) provided insights into the surface elements, while the detected composition is illustrated in Figure 5b.The pure TiO 2 demonstrated a composition comprising 58.1% oxygen and 41.9% titanium.In the case of the Ag/TiO 2 , the elements were present in the percentages of 51.7% oxygen, 45.3% titanium, and 3.0% silver.Similarly, the incorporation of copper into the Cu/TiO 2 was confirmed, with a composition of 50.7% oxygen, 43.0% titanium, and 6.3% copper.The ternary nanocomposite, referred to as Ag@Cu/TiO 2 , exhibited an elemental composition of 35.4% oxygen, 39.2% titanium, 16.7% silver, and 8.7% copper.The chemical state analysis of the Ti2p was further explored to elucidate the chemical interactions in the TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 (Figure 6a-d).In the chemical state analysis of the pure TiO 2 (Figure 6a), distinct peaks at 457.99 eV and 463.67 eV were observed, corresponding to Ti2p3/2 and Ti2p1/2, with contributions of 73.32% and 26.68%, respectively.The introduction of Ag and Cu into the TiO 2 resulted in alterations in the contributions of Ti2p3/2 and Ti2p1/2.Specifically, the Ag/TiO 2 sample exhibited total contributions of 72.05% (Ti2p3/2) and 27.95% (Ti2p1/2).Similarly, the Cu/TiO 2 showed total contributions of 78.38% (Ti2p3/2) and 21.62% (Ti2p1/2).The final ternary composite, Ag@Cu/TiO 2 , displayed total contributions of 72.31% (Ti2p3/2) and 27.69% (Ti2p1/2).In summary, the observed changes in the atomic percentage contributions of Ti2p3/2 and Ti2p1/2 in the pure TiO 2 following the addition of Ag and Cu signify the successful interaction between the components.The elemental analysis of the TiO2, Ag/TiO2, Cu/TiO2 and Ag@Cu/TiO2 by EDS shows the presence of Ti and O in the TiO2, Ti, O and Ag in the Ag/TiO2, Cu, Ti and O in the Cu/TiO2 and Cu, Ti, O and Ag in the Ag@Cu/TiO2, thereby depicting the successful formation of the composite as well as the absence of any impurities.The uniform distribution of the respective elements, as depicted by the mapping analysis (Figure 4), suggests the efficacy of the synthesis methodology.The elemental analysis of the TiO2, Ag/TiO2, Cu/TiO2 and Ag@Cu/TiO2 by EDS shows the presence of Ti and O in the TiO2, Ti, O and Ag in the Ag/TiO2, Cu, Ti and O in the Cu/TiO2 and Cu, Ti, O and Ag in the Ag@Cu/TiO2, thereby depicting the successful formation of the composite as well as the absence of any impurities.The uniform distribution of the respective elements, as depicted by the mapping analysis (Figure 4), suggests the efficacy of the synthesis methodology.

Surface Compositional and Chemical Analysis
The elemental composition and functional groups of the prepared catalysts (pure TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2) were studied by X-ray photoelectron spectroscopy (XPS).The survey spectra (Figure 5a) provided insights into the surface elements, while the detected composition is illustrated in Figure 5b.The pure TiO2 demonstrated a composition comprising 58.1% oxygen and 41.9% titanium.In the case of the Ag/TiO2, the elements were present in the percentages of 51.7% oxygen, 45.3% titanium, and 3.0% silver.Similarly, the incorporation of copper into the Cu/TiO2 was confirmed, with a composition of 50.7% oxygen, 43.0% titanium, and 6.3% copper.The ternary nanocomposite, referred to as Ag@Cu/TiO2, exhibited an elemental composition of 35.4% oxygen, 39.2% titanium, 16.7% silver, and 8.7% copper.The chemical state analysis of the Ti2p was further explored to elucidate the chemical interactions in the TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2 (Figure 6a-d).In the chemical state analysis of the pure TiO2 (Figure 6a), distinct peaks at 457.99 eV and 463.67 eV were observed, corresponding to Ti2p3/2 and Ti2p1/2, with contributions of 73.32% and 26.68%, respectively.The introduction of Ag and Cu into the TiO2 resulted in alterations in the contributions of Ti2p3/2 and Ti2p1/2.Specifically, the Ag/TiO2 sample exhibited total contributions of 72.05% (Ti2p3/2) and 27.95% (Ti2p1/2).Similarly, the Cu/TiO2 showed total contributions of 78.38% (Ti2p3/2) and 21.62% (Ti2p1/2).The final ternary composite, Ag@Cu/TiO2, displayed total contributions of 72.31% (Ti2p3/2) and 27.69% (Ti2p1/2).In summary, the observed changes in the atomic percentage contributions of Ti2p3/2 and Ti2p1/2 in the pure TiO2 following the addition of Ag and Cu signify the successful interaction between the components.The O1s spectra of the pure TiO2 (Figure 7a) exhibit two main peaks at around 530 eV and 532.5 eV, attributed to the interaction between oxygen and Ti at 530 eV and oxygen vacancy at 532.5 eV, respectively [1].Notably, the presence of Ag in the TiO2 (Ag/TiO2), as depicted in Figure 7b, results in an increase in the oxygen vacancy from 12.54% to 13.12%.The O1s spectra of the pure TiO 2 (Figure 7a) exhibit two main peaks at around 530 eV and 532.5 eV, attributed to the interaction between oxygen and Ti at 530 eV and oxygen vacancy at 532.5 eV, respectively [1].Notably, the presence of Ag in the TiO 2 (Ag/TiO 2 ), as depicted in Figure 7b, results in an increase in the oxygen vacancy from 12.54% to 13.12%.This trend continues with the addition of Cu to the TiO 2 (Cu/TiO 2 ) and the ternary Ag@Cu/TiO 2 .The maximum oxygen vacancy, reaching 56.45%, is observed for the Ag@Cu/TiO 2 .These vacancies serve as active capture sites for dye molecules and pollutants, thereby enhancing the photocatalytic activity of the material [2].These findings align with our photocatalytic results, as described later in Figure 9. Additionally, an extra peak is evident at around 534.6 eV for the Ag@Cu/TiO 2 , attributed to the surface oxygen (Os), further contributing to the enhanced photocatalytic activity of the ternary Ag@Cu/TiO 2 compared to the pure TiO 2 , Ag/TiO 2 , and Cu/TiO 2 .
Chemistry 2024, 6, FOR PEER REVIEW 10 The chemical state of the Ag3d in the Ag/TiO2 (Figure 8a) reveals peaks of Ag3d5/2 and Ag3d3/2 at 368 eV and 374 eV, respectively, with contributions of 59.90% and 40.10% [1].However, upon the addition of Cu to the Ag/TiO2 (Ag@Cu/TiO2), as depicted in Figure 8b, these peaks undergo slight changes, with the percentages shifting to 59.03% and 40.97%, respectively.The Cu2p3 analysis of the Cu/TiO2 and Ag@Cu/TiO2 is presented in Figure 9c,d.The peaks observed at 937 eV are attributed to Cu's interaction with oxygen (CuOx), while those at 945.5 eV correspond to the shake-up satellite peak, with contributions of 27.46% and 30.71%, respectively [1].This alteration in the contribution of the shake-up satellite peak further supports the enhancement of the photocatalytic activity of the ternary Ag@Cu/TiO2.The chemical state of the Ag3d in the Ag/TiO 2 (Figure 8a) reveals peaks of Ag3d5/2 and Ag3d3/2 at 368 eV and 374 eV, respectively, with contributions of 59.90% and 40.10% [1].However, upon the addition of Cu to the Ag/TiO 2 (Ag@Cu/TiO 2 ), as depicted in Figure 8b, these peaks undergo slight changes, with the percentages shifting to 59.03% and 40.97%, respectively.The Cu2p3 analysis of the Cu/TiO 2 and Ag@Cu/TiO 2 is presented in Figure 9c,d.The peaks observed at 937 eV are attributed to Cu's interaction with oxygen (CuOx), while those at 945.5 eV correspond to the shake-up satellite peak, with contributions of 27.46% and 30.71%, respectively [1].This alteration in the contribution of the shake-up satellite peak further supports the enhancement of the photocatalytic activity of the ternary Ag@Cu/TiO 2 .

Charge Carrier Movement
The band gap alteration significantly influences the charge carriers' migration, directly effecting the photocatalytic efficiency of the catalyst.Therefore, the band gap calculations for the TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2 (Figure 9a-d) were determined using the Tauc plot relations [26].

α hυ C hυ 𝐸
Here, C is a constant, hν represents the photon energy, and α is the absorption coefficient.The band gap of the pure TiO2 measured at around 3.28 eV aligns well with the previously reported literature [27].For the Ag/TiO2 and Cu/TiO2, the calculated band gap of 3.13 and 3.00 eV, respectively, indicates that the addition of Ag to both Cu and TiO2 has a significant impact on the conduction of the charge carriers.This change in the TiO2 band gap is attributed to the surface plasmon resonance effects [28].The band gap of the Cu/TiO2 is 3.00 eV, with a further reduction observed in the case of the Ag@Cu/TiO2, measuring around 2.86 eV.The band gap reduction in the Ag@Cu/TiO2 indicates the combined effects of Ag and Cu, contributing to the enhanced electron capture through the Schottky effect and surface plasmon resonance [29].This synergistic effect results in a narrowed

Charge Recombination Ratio
The charge recombination ratio is inversely proportional to the photocatalytic performance.The charge recombination ratios of the TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2 were determined through the photoluminescence spectra, as depicted in Figure 10.Analysis of the photoluminescence spectra revealed that the TiO2 exhibited a peak of high intensity in comparison to the Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2, indicating a high recombination ratio for the TiO2, consequently leading to lower photocatalytic effi-

Charge Carrier Movement
The band gap alteration significantly influences the charge carriers' migration, directly effecting the photocatalytic efficiency of the catalyst.Therefore, the band gap calculations for the TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 (Figure 9a-d) were determined using the Tauc plot relations [26].
Here, C is a constant, hν represents the photon energy, and α is the absorption coefficient.The band gap of the pure TiO 2 measured at around 3.28 eV aligns well with the previously reported literature [27].For the Ag/TiO 2 and Cu/TiO 2 , the calculated band gap of 3.13 and 3.00 eV, respectively, indicates that the addition of Ag to both Cu and TiO 2 has a significant impact on the conduction of the charge carriers.This change in the TiO 2 band gap is attributed to the surface plasmon resonance effects [28].The band gap of the Cu/TiO 2 is 3.00 eV, with a further reduction observed in the case of the Ag@Cu/TiO 2 , measuring around 2.86 eV.The band gap reduction in the Ag@Cu/TiO 2 indicates the combined effects of Ag and Cu, contributing to the enhanced electron capture through the Schottky effect and surface plasmon resonance [29].This synergistic effect results in a narrowed band gap, accelerated electrons and hole movements in the Ag@Cu/TiO 2 , consequently resulting in the enhanced photocatalytic degradation of RB.

Charge Recombination Ratio
The charge recombination ratio is inversely proportional to the photocatalytic performance.The charge recombination ratios of the TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 were determined through the photoluminescence spectra, as depicted in Figure 10.Analysis of the photoluminescence spectra revealed that the TiO 2 exhibited a peak of high intensity in comparison to the Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 , indicating a high recombination ratio for the TiO 2 , consequently leading to lower photocatalytic efficiency.Conversely, the Ag@Cu/TiO 2 had a low intensity, suggesting a higher degradation efficiency.These photoluminescence results align with our photocatalytic findings, which indicated the degradation efficiency to be as follows: Ag@Cu/TiO 2 > Cu/TiO 2 > Ag/TiO 2 > TiO 2 .

Photocatalytic Degradation of RB
The time-dependent UV-Vis spectrum of RB under UV-Vis irradiation is depicted in Figure 9a, where the characteristic absorption peak is at λmax of 545 nm, which is the primary feature of RB.Observing Figure 9a, it becomes evident that with the increase in the irradiation time from 0 to 60 m, the absorption peak gradually diminishes, indicating the degradation of RB by the TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2 under visible light.

Photocatalytic Degradation of RB
The time-dependent UV-Vis spectrum of RB under UV-Vis irradiation is depicted in Figure 9a, where the characteristic absorption peak is at λmax of 545 nm, which is the primary feature of RB.Observing Figure 9a, it becomes evident that with the increase in the irradiation time from 0 to 60 m, the absorption peak gradually diminishes, indicating the degradation of RB by the TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 under visible light.
Figure 9b,c illustrate the degradation statistics, demonstrating that the degradation efficiency of the pure TiO 2 is approximately 24.41% within 60 m of irradiation.This relatively low efficiency is due to its large band gap and high charge recombination rate.With the integration of Ag, the degradation efficiency of the pure TiO 2 increased to about 34.73% within the same duration.This enhancement is due to the surface photoluminescence resonance induced by silver, which suppresses the charge carriers' recombination, consequently intensifying the degradation of RB by the Ag/TiO 2 [25].
The degradation efficiency of the Cu/TiO 2 was approximately 68.38%, which is 2.82 times that of the pure TiO 2 .This improvement is due to the reduced charge recombination ratio and the introduction of active sites such as oxygen vacancies in the pure TiO 2 , resulting in the more effective capture of RB molecules and thus enhancing the degradation efficiency of the Cu/TiO 2 [26].Similarly, the combined effect of silver and copper is evident in the photocatalytic efficiency of the ternary Ag@Cu/TiO 2 , which exhibited a degradation rate of approximately 81.07%, which is 3.31 times that of the TiO 2 .The notable improvement is due to the effective reduction in the recombination of the electron-hole pairs and the introduction of more active sites for RB molecule capture.Additionally, the incorporation of silver and copper into the TiO 2 led to heightened light absorption, further contributing to the overall enhancement.Moreover, as the degradation efficiency increases, the reaction rate constant of the TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 also increases.The reaction rate constant (Figure 11d) for the Ag@Cu/TiO 2 was found to be approximately 2.76 × 10 −2 , 5.94 times than that of the TiO 2 .The reaction rate constant for the Ag/TiO 2 and Cu/TiO 2 was around 7.11 × 10 −3 and 1.91 × 10 −2 , respectively.The dye concentration is a crucial factor as it can either enhance or diminish the photocatalytic efficiency of the catalyst.To examine this, a series of reactions were conducted at various concentrations of RB, namely 5, 20, 30, 40, and 50 ppm, while keeping the dosage of the Ag@Cu/TiO2 fixed at 200 mg/250 mL and the light intensity at a 500 W halogen lamp.As depicted in Figure 12a, the degradation of RB was found to be at its peak, around 81.07%, at an RB concentration of 30 ppm.The results revealed a progressive increase in

Effect of RB Concentration
The dye concentration is a crucial factor as it can either enhance or diminish the photocatalytic efficiency of the catalyst.To examine this, a series of reactions were conducted at various concentrations of RB, namely 5, 20, 30, 40, and 50 ppm, while keeping the dosage of the Ag@Cu/TiO 2 fixed at 200 mg/250 mL and the light intensity at a 500 W halogen lamp.As depicted in Figure 12a, the degradation of RB was found to be at its peak, around 81.07%, at an RB concentration of 30 ppm.The results revealed a progressive increase in the degradation rate from 5 ppm to 30 ppm, with degradation values of approximately 49.5%, 57.1%, 70.9%, and 81.07%for 5 ppm, 10 ppm, 20 ppm, and 30 ppm, respectively.Notably, the degradation of RB began to decline beyond 30 ppm, with degradation values of 75.9% and 65.2% for 40 ppm and 50 ppm, respectively.The highly efficient photocatalytic performance of the Ag@Cu/TiO 2 is due to the growing availability of RB dye molecules for excitation and energy transfer with an increasing RB concentration [30].However, upon surpassing a critical concentration level, in our case 30 ppm, the dye molecules begin to absorb the radiation instead of being absorbed by the Ag@Cu/TiO 2 , which results in a lowering of the photocatalytic efficiency of the Ag@Cu/TiO 2 [31].

Effect of Ag@Cu/TiO2 Dosage
The photocatalytic degradation depends on the quantity of the catalyst in the solution.For this, varying the amount of Ag@Cu/TiO2 in the RB solution was studied for the purpose of photocatalysis.The dosages of the Ag@Cu/TiO2 selected were 50 mg/250 mL, 100 mg/250 mL, 200 mg/250 mL, 300 mg/250 mL, and 400 mg/250 mL, while the irradiation time (0-60 m), irradiation light intensity (500 W), and RB concentration (300 ppm) were kept constant.
The results (Figure 12b) revealed that the efficiency of the Ag@Cu/TiO2 toward RB degradation increased with the increase in its dosage.Specifically, the efficiency was approximately 70.41% at 50 mg/250 mL, 75.1% at 100 mg/250 mL, and 81.07%at 200 mg/250 mL.This enhanced degradation efficiency might be due to the increased availability of active regions with the rise in the Ag@Cu/TiO2 dosage, facilitating the capture of RB molecules and intensifying the RB degradation However, it was observed that as the dosage of the Ag@Cu/TiO2 was increased from 200 mg/250 mL to 300 mg/250 mL, there was a slight reduction in the efficiency, which was approximately 80.53%.This efficiency further decreased to 75.54% at 400 mg/250 mL.This decline in efficiency can be attributed to the desorption and scattering of light beyond a certain threshold dosage of Ag@Cu/TiO2 [29].
The cyclic reusability of the Ag@Cu/TiO2 was evaluated over seven consecutive cycles (see Figure S2).Remarkably, the reusability remained stable at 79.96% even after the seventh cycle.These findings underscore the promising potential of Ag@Cu/TiO2 for RB dye

Effect of Ag@Cu/TiO 2 Dosage
The photocatalytic degradation depends on the quantity of the catalyst in the solution.For this, varying the amount of Ag@Cu/TiO 2 in the RB solution was studied for the purpose of photocatalysis.The dosages of the Ag@Cu/TiO 2 selected were 50 mg/250 mL, 100 mg/250 mL, 200 mg/250 mL, 300 mg/250 mL, and 400 mg/250 mL, while the irradiation time (0-60 m), irradiation light intensity (500 W), and RB concentration (300 ppm) were kept constant.
The results (Figure 12b) revealed that the efficiency of the Ag@Cu/TiO 2 toward RB degradation increased with the increase in its dosage.Specifically, the efficiency was approximately 70.41% at 50 mg/250 mL, 75.1% at 100 mg/250 mL, and 81.07%at 200 mg/250 mL.This enhanced degradation efficiency might be due to the increased availability of active regions with the rise in the Ag@Cu/TiO 2 dosage, facilitating the capture of RB molecules and intensifying the RB degradation However, it was observed that as the dosage of the Ag@Cu/TiO 2 was increased from 200 mg/250 mL to 300 mg/250 mL, there was a slight reduction in the efficiency, which was approximately 80.53%.This efficiency further decreased to 75.54% at 400 mg/250 mL.This decline in efficiency can be attributed to the desorption and scattering of light beyond a certain threshold dosage of Ag@Cu/TiO 2 [29].
The cyclic reusability of the Ag@Cu/TiO 2 was evaluated over seven consecutive cycles (see Figure S2).Remarkably, the reusability remained stable at 79.96% even after the seventh cycle.These findings underscore the promising potential of Ag@Cu/TiO 2 for RB dye degradation under visible light.

Proposed Photocatalytic Mechanism
Figure 13 depicts the suggested Ag@Cu/TiO 2 degradation pathway of RB.There are three possible ways for the formation of binary and ternary heterojunctions with TiO 2 .The energy level of Cu's highest occupied molecular orbital is located between TiO 2 's valence band (VB) and conduction band (CB), which facilitates the formation of binary heterojunction.Under visible light irradiation, Cu/TiO 2 facilitates the hole transfer to Cu, leading to electron transport to the CB of the TiO 2 .The large number of holes in Cu react with RB, causing oxidation and subsequent RB degradation.Furthermore, the generated electrons degrade RB by reacting with the dissolved oxygen.Electron transfer from the TiO 2 to Ag's CB occurs in an Ag@TiO binary heterojunction due to the differences in the TiO 2 's and Ag's work functions [32].This shows that Ag has a significant role as an electron reservoir, reducing the charge recombination by creating a Schottky barrier.This, in turn, enhances the photocatalytic degradation under visible light [33].Additionally, Ag's surface plasmon resonance (SPR) increases the photocatalytic activity by encouraging electron migration through photon harvesting.The hole migration to Cu facilitates the production of ternary heterojunctions in the Ag@Cu/TiO 2 , where electrons are transferred to the CB of the TiO 2 and ultimately to the CB of the Ag.Here, they quicken the processes of oxidation and reduction, producing free radicals.The degradation process begins when RB absorbs these radicals.
Chemistry 2024, 6, FOR PEER REVIEW 16 CB occurs in an Ag@TiO2 binary heterojunction due to the differences in the TiO2's and Ag's work functions [32].This shows that Ag has a significant role as an electron reservoir, reducing the charge recombination by creating a Schottky barrier.This, in turn, enhances the photocatalytic degradation under visible light [33].Additionally, Ag's surface plasmon resonance (SPR) increases the photocatalytic activity by encouraging electron migration through photon harvesting.The hole migration to Cu facilitates the production of ternary heterojunctions in the Ag@Cu/TiO2, where electrons are transferred to the CB of the TiO2 and ultimately to the CB of the Ag.Here, they quicken the processes of oxidation and reduction, producing free radicals.The degradation process begins when RB absorbs these radicals.

Hydrogen Production
Using a 20% concentration of methanol as a scavenger, we assessed the photocatalytic hydrogen generation of the TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2.The outcomes, as

Figure 9 .
Figure 9. Band gap analysis of the (a) TiO 2 , (b) Ag/TiO 2 , (c) Cu/TiO 2 , and (d) Ag@Cu/TiO 2 and an inset showing the change in the band gap.

Chemistry 2024, 6 , 14 Figure 11 .
Figure 11.(a) Time-dependent UV-Vis spectrum of RB for the Ag@Cu/TiO2, and (b) degradation statistics and (c) reaction rate constant of the TiO2, Ag/TiO2, Cu/TiO2, and Ag@Cu/TiO2 under the optimized value of 200 mg of each catalyst in a 250 mL solution containing 30 mg/L of RB dye.3.5.1.Effect of RB Concentration

Figure 11 .
Figure 11.(a) Time-dependent UV-Vis spectrum of RB for the Ag@Cu/TiO 2, and (b) degradation statistics and (c) reaction rate constant of the TiO 2 , Ag/TiO 2 , Cu/TiO 2 , and Ag@Cu/TiO 2 under the optimized value of 200 mg of each catalyst in a 250 mL solution containing 30 mg/L of RB dye.