Introduction

A major problem in the world today involves the pollution of water resources by organic pollutants produced by businesses [1,2]. Untreated industrial wastewater contains a large concentration of organic contaminants, which can be harmful to the environment and human health [3,4]. One important class of pollutants is the presence of dyes in wastewater. Methylene blue (MB) is one of the most widely used dyes, and produces large amounts of organic pollutants Methylene blue (MB) is utilized in various sectors, including paper, printing, textile, and food, and as a result, it is an important organic pollutant. In addition, as MB dye is a carcinogenic, harmful, and mutagenic pollutant [5], it is crucial to remove this pollutant from aquatic systems. To date, many chemical, physical, and biological techniques for removing these pollutants from wastewater have been developed [6-9]. Various adsorbents have been studied to remove dyes from water, and the adsorption approach is thought to be one of the most efficient and straightforward methods among other techniques [10]. For instance, the activated carbon is frequently employed and is thought to be a superior adsorbent. However, because of high cost of regeneration, it was occasionally used as a one-time adsorbent [9]. The resistant nature of synthetic dyes renders ineffectual traditional biological treatment procedures, and using adsorption, coagulation, or sedimentation techniques leads to additional contaminants [11].

As a result, a more promising method for the decolorization and degradation of textile dyes has been thoroughly explored. This technology is based on oxidation processes [11]. In this context, scientists are continuously trying to develop cost-effective and efficient semiconductor photocatalysts that limit the use of H₂O₂ under sunlight [5]. In the realm of nanomaterials, the ones that have garnered the most interest are TiO₂ nanoparticles (NPs) because of their chemical properties like high catalytic activity in organic reactions, light absorbing ability, availability, and low price [2, 12]. TiO₂ photocatalyst has been widely used to remove organic pollutants [13].

Due to the outstanding photocatalytic capabilities of TiO₂ NPs, they have found widespread applications in the sewage treatment [14, 15].

The process of decolorization of a dye by TiO₂ catalyst, as a photocatalyst, is explained through electron transformation from the valence band to the conduction band, which then an electron-hole pair (e^-_cb and h⁺_vb) is generated [16,17].

These species can move to the TiO₂ surface and redox reaction occurs with other species, for example,  h⁺_vb reacts with H₂O on the surface to form active radicals (^•OH), whereas the reaction of  e^­_cb with O₂ generates O₂^•- [16,17].

OH and O₂^•- species reacting with dye to degrade it to small molecules [16,17].

Among the three crystalline polymorphic phases of TiO₂, rutile, anatase, and brookite, the anatase phase is the most effective and widely used TiO₂ photocatalyst [18]. However, its high bandgap (3.2 eV) and high rate of charge carrier recombination under visible light make it not an efficient photocatalyst [5, 18-21]. To reduce the energy gap and enhance visible light absorption and photocatalytic performance, TiO₂ has been modified in several ways, including metals or metal oxides [22-24]. non-metals [25,26], organisms [27,28], ionic liquids [29], and miscellaneous compounds like CDs [30,31]. For example, doping of TiO₂ with Fe₃O₄ (8 wt. %) increased the photocatalytic activity of TiO₂ for MB degradation from 75% to 93% under sunlight irradiation [32]. Also, a comparison of the photocatalytic activity of TiO₂ with its composites TiO₂/SiO₂ and TiO₂/SiO₂-CdS exhibited that the activity was increased from 51% (TiO₂) to 74% (TiO₂/SiO₂), and 85% (TiO₂/SiO₂-Cds) in the degradation of MB dye under sunlight irradiation [19]. Nano and microscale silver-based materials with high photosensitivity have been used as TiO₂ doping agents [22, 33-35], which can enhance the photocatalytic activity of TiO₂ under visible light.

Cyclodextrins (CDs) are cyclic oligosaccharides made of the hydrophobic interior cavity and the hydrophilic exterior surface. The number of α-D-glucose units in cyclodextrins ranges from six to eight, and they are joined to one another by glycosidic α-1,4 linkages. Several adsorbents that have been functionalized with β-CD have been reported recently to remove harmful pollutants from industrial effluents [36]. Reports showed that the β-CD presence in the photocatalysts structure can increase photodegradation properties because β-CD can interact effectively with organic pollutants. For example, the removal of bisphenol A enhanced in aqueous media in the presence of β-CD [31]. Removal of Cr(VI) and phenol in aqueous solution was increased using rGO/β-CD/TiO₂ nanocomposite [37]. Other examples include graphene oxide and TiO₂ stabilized by β-CD led to improved methylene blue photodegradation [38], and β-CD/Fe₃O₄/TiO₂ also exhibited to be an efficient catalyst in photodecomposition of bisphenol A and dibutyl phthalate [39].

Methylene blue has an amphiphilic nature, as it can interact with hydrophobic and hydrophilic sections of β-CD. The positive nitrogen of methylene blue may also form electrostatic interactions with the oxygen atoms on the hydrophilic surface. According to the dimensions of methylene blue molecules and the hydrophobic cavity of β-cyclodextrin, MB may enter a β-cyclodextrin cavity and thus be absorbed, or several β-cyclodextrin cavities simultaneously enclose an MB molecule and through chemical interaction cause the MB absorption (Figure 1) [40,41].

Figure 1: The interaction of methylene blue and beta-cyclodextrin

Herein, we report the synthesis and characterization of TiO₂/β-cyclodextrin (TiO₂/CD) and TiO₂/Ag/β-cyclodextrin (TiO₂/Ag/CD) nanocomposites and the evaluation of their photocatalytic activity in the degradation of MB in aqueous solutions.

The covalently bounded CD to the modified TiO₂/Ag and/or TiO₂ NPs would enhance the stability and performance of photocatalytic activity of the prepared nanocomposites in the degradation of methylene blue in aqueous solution.

Experimental

Materials and Methods

Titanium chloride (TiCl₄), ethanol, deionized water (DW), β-cyclodextrin (CD), bromine, dimethyl sulfoxide (DMSO), N,N’-dimethylformamide (DMF), triphenylphosphine (PPh₃), silver nitrate (AgNO₃), 3-aminopropyltrimethoxysilane (APTMS), diethyl ether, and dichloromethane were obtained from Sigma-Aldrich and Merck Chemical Companies and used unpurified. 

The UV-Visible absorption spectra were recorded using a spectrophotometer from Analytik Jena (SPEKOL 2000). A Bruker Tensor 27 Spectrometer was used to record IR spectra using the KBr disc technique. DRS analysis was performed via a Diffuse Reflectance Spectroscopy (DRS, S_4100 SCINCO). A D8 Advance-Bruker X-ray diffractometer was utilized under Cu-Kα radiation (λ = 1.54178 Å) to study the crystal structure of samples. Thermogravimetric analysis (TGA, STA 1500 Rheometric Scientific) was performed at 25 to 600 °C with a heating rate of 10 °C min^-1 in an air atmosphere. Field emission scanning electron microscopy (FESEM; MIRA3 Tescan, Switzerland) was used to study examine the surface morphology of synthesized materials. Energy-dispersive X-ray mapping spectroscopy (EDX-MAP, MIRA2 Tescan, Switzerland) was utilized to record elemental detection with sufficient sensitivity to determine the corresponding atomic numbers. Inductive Coupled Plasma- Mass Spectrometry (ICP-MS Agilent 7500) ELAN 6100 DRC-e was used to measure determine the Ti and Ag contents of the TiO₂/CD and TiO₂/Ag/CD nanocomposites.

Preparation of Aloe Vera Extract

The healthy leaves of the Aloe Vera plant were harvested and thoroughly washed to remove dust and impurities. Then, the leaves were washed again with distilled water and left to dry at room temperature. Aloe Vera leaves were chopped, the inside gel was collected (100 g), and heated in distilled water (250 mL) for 2 h at 80 ^ºC. The aqueous mixture was filtered and the filtrate was kept at 10 °C [42].

TiO₂ Anatase NPs Biosynthesis

An aqueous Aleo Vera extract (20 mL) was added slowly to an aqueous solution of TiCl₄ (100 mL, 1 M) with vigorous stirring (pH~7). The reaction mixture was stirred for 4 h at room temperature. The obtained TiO₂ NPs were collected by centrifugation (10,000 rpm) and washed several times with ethanol and water. Then dried at 100 °C for 7 h. The solid powder was calcined at 500 °C for 3 h, and a soft white powder was obtained [42,43].

Preparation of C.‎‎‎‎‎­pentagyna Extract

Fresh fruit was thoroughly rinsed with sterile double-distilled water to eliminate any medium components from the biomass. Then, 50 g of C.pentagyna fruit was extracted by maceration using 300 milliliters of methanol as the extraction solvent at room temperature. The methanolic solution was subsequently filtered and concentrated using a rotary evaporator until a rudimentary solid extract was obtained. The extract was filtered and stored at 4 °C. The final extract was utilized as a capping and reducing agent in the synthesis of silver nanoparticles [44].

Biosynthesis of TiO₂/Ag Nanoparticles (TiO₂/AgNPs)

A mixture of TiO₂ nanoparticles (0.5 g, 6.26 mmol) and silver nitrate (0.31 mmol, 0.053 g) was stirred in distilled water (25 mL) for 35 min, then C. pentagyna aqueous fruit extract (12.5 mL, pH=12) was added slowly to the mixture and refluxed for 5 h. The white nanoparticles turned black, indicating the reduction of Ag⁺ to Ag⁰ and the formation of TiO₂/Ag nanoparticles, which were then centrifuged (10,000 rpm) for 10 min, washed with distilled water, and dried in an oven for 24 h [44,45].

TiO₂/Ag Nanoparticles Modification

TiO₂/Ag (0.5 g) was suspended in 50 mL of deionized water (DI) and ultrasonicated for 10 min. The dispersion was then treated with a silane coupling agent (APTMS, 2 mL), and the mixture was refluxed for 24 h. After that, the dispersed particles were centrifuged (10 min at 10,000 rpm) and washed with ethanol and water to remove unreacted silane. Before centrifuging the solid particles were ultrasonicated for 10 min to ensure a well-dispersed suspension was restored. Following the completion of the operation, the collected particles were dried in an oven at 100 °C for 24 h [46].

TiO₂ Nanoparticles Modification

TiO₂ nanoparticles (0.5 g) were suspended in DI water (50 mL) and ultrasonicated for 10 min. APTMS (2 mL) was added and refluxed for 24 h. Then the reaction mixture was centrifuged (10 min at 10,000 rpm) and washed with ethanol and water to remove unreacted silane. The collected solid was dried in an oven at 100 °C for 24 h [46].

Synthesis of Heptakis(6-bromo-6-deoxy)-β-cyclodextrin (CDBr)

A solution of bromine (4 mL, 77.6 mmol) in DMF (15 mL) was slowly added to a cold solution (ice bath) of PPh₃ (20.35 g, 77.6 mmol) in DMF (60 mL) and stirred. After 30 min, 4.2 g (3.7 mmol) of dry CD was added, and the reaction mixture was agitated at 80 ᵒC for 15 h. Under a high vacuum, the mixture was concentrated to half its original volume, and the pH was adjusted to 9-10 using fresh CH₃ONa/CH₃OH (3 M, 60 mL) and immediately refrigerated for 30 min. Then reaction mixture was warmed to ambient temperature, stirred for 30 min, and poured into cold water. The reaction mixture was filtered and the precipitate was washed consecutively with water, ether, and dichloromethane and dried overnight in a vacuum to give a quantitative yield of the desired product (5.7 g).

Synthesis of Titanium-Silver Modified Cyclodextrin Nanocomposite (TiO₂/Ag/CD)

CDBr (0.59 mmol, 1 g) was dissolved in DMSO (30 mL) at 95 °C after stirring for 1 h. APTMS-modified TiO₂/Ag nanoparticles (4.14 mmol ~ 1.3 g) were added to the reaction solution, which was then stirred for 24 h at 80 °C. After reaching room temperature, the resulting mixture was transferred to 200 mL of water with stirring. The precipitate was filtered off, rinsed with water, and then dried in a vacuum oven at 90 °C for 24 h to yield TiO₂/Ag/CD nanocomposite with a mass yield of 97% [47].

Synthesis of Titanium-Modified Cyclodextrin Nanocomposite (TiO₂/CD)

CDBr (0.59 mmol, 1 g) was dissolved in DMSO (30 mL) at 95 °C after stirring for 1 h, and then APTMS-modified TiO₂ (1.3 g) was added to the solution and stirred at 80 °C for 24 hours. Afterwards, the reaction mixture was cooled to room temperature and transferred to 200 mL of water with stirring. The precipitate was filtered off, rinsed with water, and then dried in a vacuum oven at 90 °C for 24 h to yield 93% TiO₂/CD nanocomposite [47].

Hotocatalytic Process

An aqueous MB solution was chosen as the contaminant solution model to evaluate the photocatalytic activity of the prepared TiO₂/Ag/CD under sunlight irradiation. 10 mg of photocatalyst was dispersed in a 10 ppm MB solution (100 mL) using an ultrasonic bath, and then stirred for 60 min in the dark to establish the equilibrium of adsorption and desorption between the photocatalyst and MB dye. Next, the solution was exposed to the sunlight and 5 mL of the solution was sampled every 15 minutes using a pipette. The suspension solution was centrifuged, the nano photocatalyst was separated, and the MB concentration was measured using a UV-Vis spectrophotometer at 665 nm [48].

Results and Discussion

In this work, anatase TiO₂ nanoparticles were synthesized by addition of aqueous Aloe vera leaf extract to a solution of TiCl₄ in water followed by residue calcination at 500 °C (Scheme 1).

TiO₂/Ag NPs were prepared by mixing TiO₂ NPs with AgNO₃ in the presence of C. pentagyna aqueous fruit extract at pH=12 as shown in Scheme 2.

Modification of TiO₂/Ag NPs with APTMS in deionized water afforded compound (1), which then reacted with CDBr (2) (prepared by bromination of CD with Br₂ and pph₃ in DMF) in DMSO to give the nanocomposite TiO₂/Ag/CD in 97% mass yield (Scheme 3).

Scheme 1: Biosynthesis of anatase TiO₂ nanoparticles

Scheme 2: Biosynthesis of TiO₂/Ag nanoparticles

Scheme 3: Synthesis steps of TiO₂/Ag/CD nanocomposite

Similarly, TiO₂ NPs were modified with APTMS followed by reacting with CDBr (2) to obtain TiO₂/CD nanocomposite. FT-IR, UV, TGA, DRS FESEM, ICP, EDX, MAP, and XRD were used to identify the structure and morphology of nanocomposites.

FTIR Analysis

Figure 2 depicts the FTIR spectra of β-CD, β-CD-Br, TiO₂, TiO₂/CD, and TiO₂/Ag/CD.

β-CD: 3417.4 cm^-1 (O-H stretching), 1641.2 cm^-1 (O-H bending), 2927.5 (C-H stretching), 1419.4 and 1369.2 cm^-1 (C-H bending), 1157.1, 1081.9, and 1029.8 cm^-1 (C-O and  C-O-C stretching), and 946.9 cm^-1  (R-1,4 bond skeleton vibration) [49].

CD-Br: Characteristic peaks of β-CD, 1263.2 cm^-1 (C-Br stretching), and 586.2 cm^-1 (C-Br bending) [47].

TiO₂: 3600–3300 cm^-1 (O-H stretching), 1633.4 cm^-1  (O-H bending), and  500-800 cm^-1 (Ti-O-Ti stretching) (50, 47].

APTMS modified TiO₂ NPs: 3448 cm^-1 (NH₂ stretching), 2927.8 cm^-1 (C-H stretching), 1627.8 cm^-1 (O-H bending), 1544.9 and 1459 cm^-1 (Si-O-C characteristic peaks), 1130.2 cm^-1 (Si-O-Si stretching), and 999 cm^-1 (Ti-O-Si stretching) [47]. The FTIR spectra of TiO₂/Ag/CD and TiO₂/CD contain all the of the anticipated absorption peaks related to cyclodextrin and TiO₂ moieties.

TiO₂/Ag/CD and TiO₂/CD: The spectra exhibited all the related peaks of TiO₂ and β-CD.

Figure 2:  The FTIR spectrum of the synthesized products

EDX Mapping and DRS Analysis

The components of the composites were identified using energy-dispersive X-ray analysis (EDX) and determined the approximate amount of carbon, nitrogen, silicon, silver, titanium, and oxygen elements (Figure 3). The results showed that the quantity of carbon and oxygen in the silver-containing composite is greater. This shows that the TiO₂/Ag/CD composite has more cyclodextrin. According to the DRS analysis results, the energy gap of TiO₂ was 3.2ev, which decreased to 3.1ev and 2.5ev in TiO₂/CD and TiO₂/Ag/CD, respectively. The lowering of the energy gap in the silver-containing composite is expected to make it softer and more reactive with cyclodextrin [5, 47, 51].

Figure 3: EDX and MAPPING images 3a related to TiO₂/CD and 3b related to TiO₂/Ag/CD

TGA and ICP Analysis

TGA was used to assess the thermal stability of the nanocomposites. The TiO₂ nanoparticles TGA curve reveals that the nanoparticles are stable up to 600 ᵒC (Figure 4a). Thermal degradation of CD begins at > 250 ^ᵒC, and pyrolysis occurs at > 300 ᵒC (Figure 4b). The weight of synthetic composites decreases with rising temperature in proportion to the amount of organic chemicals present. In the TGA curve of TiO₂/CD two weight losses are observed (Figure 4c). The first weight loss occurs at <150 °C which is related to the removal of surface water. The second weight loss is seen at 200-600 °C, which could be because of pyrolysis of organic moieties (CD and silane agent). The TGA curve of TiO₂/Ag/CD exhibits two weight reductions at <120 °C and 150-600 °C, which is attributed to the loss of surface water and organic moieties (CD and silane agent), respectively (Figure 4d). The lower char yield of TiO₂/Ag/CD compared with TiO₂/CD is due to the higher amount of CD in its structure.

The amounts of Ag and Ti in the structure of TiO₂/Ag/CD nanocomposite were measured using ICP analysis Ag=6% and Ti= 35%. The Ti percentage in the TiO₂/CD was also determined by ICP analysis to be 66% [52].

Figure 4: TGA variation diagram of synthesized samples

XRD Analysis

Figure 5 presents the XRD patterns of TiO₂ NPs (anatase), TiO₂/CD, and TiO₂/Ag/CD. All patterns exhibited strong peaks at 2θ = 25.2°, 37.8°, 48°, 54.04°, and 55.1° related to 101, 004, 200, 105, and 211 lattice planes, respectively, confirmed the TiO₂ anatase phase [53,54] . In addition, the peaks of Ag were not clear, which could be because of low silver content in the composite or the (111) reflection of Ag might be overlapped with the (101) reflection of anatase TiO₂ [53]. However, the intensity of diffraction peaks of anatase TiO₂ nanoparticles in TiO₂/Ag/CD nanocomposite decreases with the addition of silver, which could be due to the presence of silver nanoparticles. XRD analysis of β-CD, anatase TiO₂ nanoparticles, and TiO₂/CD and TiO₂/Ag/CD was enhanced for better distinction in patterns by enlarging Figures 6b-e [54-58]. In Figure 6b, two green peaks are observed for the TiO₂/Ag/CD in the 2θ ranges from 21 to 24 degrees, which is related to the presence of a significant amount of cyclodextrin in this nanocomposite.

Figure 5: XRD patterns of TiO₂, CD, TiO₂/CD, and TiO₂/Ag/CD

In Figure 6d, we can see small green peaks for the TiO₂/Ag/CD in the 2θ ranges from 39 to 47 degrees, which corresponds to the presence of a significant amount of cyclodextrin in this nanocomposite. Although the presence of CD in TiO₂/CD has been confirmed in other analyses, the characteristic peaks of the CD are significantly less or not seen, which is due to the lower amount of cyclodextrin in the TiO₂/CD relative to TiO₂/Ag/CD.

Figure 6: Magnified images of XRD analysis related to β-CD, anatase TiO₂ nanoparticles, and TiO₂/CD and TiO₂/Ag/CD

FESEM Analysis

The electron microscope is one of the most powerful and efficient techniques for examining the surface morphology of nanoparticles and nanocomposites. Therefore, FESEM images with a magnification of 500 nm for TiO₂, CD, TiO₂/CD, and TiO₂/Ag/CD are shown in Figure 7. FESEM images of CD show that CD has a smooth, dense, and amorphous surface morphology. On the other hand, it can be seen from the images related to TiO₂ nanoparticles that TiO₂ has a spherical and relatively rough surface morphology. After modifying the surface of TiO₂ nanoparticles, changes were made in the roughness and compactness of the particles, so that it is clear that a more compact and relatively smoother structure has been created in the TiO₂/CD nanocomposite than the TiO₂ nanoparticles. Through the Ag introduction onto the TiO₂ surface and the subsequent formation of the TiO₂/Ag/CD nanocomposite, it becomes evident that the surface morphology exhibits a higher degree of homogeneity, smoothness, and density compared to that of the TiO₂/CD nanocomposite. In essence, it appears that a greater quantity of the CD exists inside the TiO₂/Ag/CD system, which is consistent with the TGA results. In contrast, the more uniform and stronger binding of CD to the TiO₂ core can increase the efficiency of the nanocomposite in terms of photocatalytic performance.

Based on the findings obtained from the FESEM observations, it can be deduced that the incorporation of silver Ag for surface modification of titanium dioxide TiO₂ exerts a notable impact, specifically a synergistic effect, on the formation and efficacy of robust interactions between the core and CD. Consequently, this leads to the development of a nanocomposite possessing a more homogeneous and enduring structure throughout the synthesis procedure [47].

Figure 7: FESEM images with a magnification of 500 nm for TiO₂, CD, TiO₂/CD, and TiO₂/Ag/CD

Using the Figure 8 and origin programs, the dimensions of the produced nanoparticles and nanocomposites were measured and their histograms were drawn (Figure 8).

Figure 8: Calculation of the size of nanoparticles using a histogram chart

Photocatalytic Studies

According to the obtained data related to the structural analysis of the produced nanocomposites, it was predicted that TiO₂/Ag/CD would show a better photocatalytic effect than TiO₂ and TiO₂/CD because it contains more CD in its structure and also the presence of silver, which reduced the energy gap leading to facilitate the visible light adsorption. Preliminary experiments were conducted to verify this prediction. The results of the experiments exhibited the order of photocatalytic activity to be: TiO₂/Ag/CD > TiO₂/CD > TiO₂. However, the photocatalytic investigation is explained for the TiO₂/Ag/CD nanocomposite to determine the optimal conditions in the following sections. All the experiments were repeated in triplicate and the reported values are the mean values.

Optimization of Dye Concentration

The optimum initial concentration of dye solution on the removal percentage was investigated at a dosage of 0.01g TiO₂/Ag/CD and 100 mL of the dye solution for a photocatalysis period of 105 min. For this purpose, the dye solutions were initially mixed with 0.01 g of photocatalyst for 1 h in the dark until the adsorption and desorption equilibrium of the photocatalyst and MB dye was established. Then the solutions were exposed to sunlight for 105 min. Figure 9 and Table 1 indicates that the degradation percentage increases with decreasing the initial dye concentration. Percentage degradation values of 99.35, 98.18, 91.29, and 82.93% were obtained for 5, 10, 15, and 20 ppm dye solutions, respectively.

As can be seen in Figure 9, the dye concentration of 5 ppm was slightly better than the 10 ppm solution, thus 10 ppm initial concentration was chosen as the optimum value for further experiments.

Figure 9: Effect of initial dye concentration on its degradation, C_o=5, 10, 15, 20 ppm, 0.01g TiO₂/Ag/CD, 100 mL, 105 min

Table 1: Optimization of dye concentration

Optimum Contact Time

The MB degradation at different contact times is demonstrated in Figure 10 and Table 2. The degradation by 0.01 g of TiO₂/Ag/CD in 100 mL of 10 ppm dye solution increases continuously with time. This increase continues up to 120 min (99.38% degradation), and then reaches equilibrium. The rapid degradation of MB in the early stages of the process can be attributed to the numerous empty holes of CD and the presence of TiO₂ and silver, which facilitate visible light harvesting.

Figure 10: Effects of contact time on the MB degradation, at 0.01g TiO₂/Ag/CD, 100 Ml

Table 2: Optimum contact time

Effect of TiO₂/Ag/CD Dosage

The effect of TiO₂/Ag/CD dosage on the MB removal was studied and the results are shown in Figure 11 and Table 3 for initial dye concentrations (0.002, 0.005, 0.01, 0.02, 0.03, and 0.04) g. The degradation percentage was highest at 0.01 mg TiO₂/Ag/CD.  It can be seen in this Figure that by increasing the catalyst dosage from 0.002 to 0.005 and 0.01 g the MB degradation percentage increases from 67.40% to 85.6% and then to 99.38% for 0.01 g catalyst. Further increasing the catalyst did not enhance the rate of degradation and even decreased slightly. Therefore, by increasing the amount of catalyst to 0.02, 0.03, and 0.04 g, the degradation percentage was 99.29, 98.76, and 97.42, respectively.

Figure 11: Effects of catalyst dosage on the MB degradation, at different amounts of TiO₂/Ag/CD, 100 mL, 10 ppm dye concentration, 120 min

Table 3: Dose effect of TiO₂/Ag/CD photocatalyst

This might be due to the presence of an excess catalyst, which causes opacity of the mixture, it leads to preventing the passage and penetration of radiated light [16, 59]. As shown in Figure 11, TiO₂/Ag/CD exhibits the highest degradation efficiency at a dose of 0.01 g. Therefore, this value was selected as the optimal dose.

Investigating the Effect of Photocatalyst

Under optimal conditions, the photocatalytic activity of the synthesized TiO₂ nanoparticles, TiO₂/CD, and TiO₂/Ag/CD nanocomposite was evaluated. To ascertain their absorption, three 100 mL solutions of 10 ppm MB were simultaneously mixed with 0.01 g of photocatalyst for 1 h in the dark. After being exposed to sunlight for 120 min, the solutions were sampled each 15 min. Examining the outcomes revealed that, for all three photocatalysts, the quantity of MB degradation increases as contact time increases. During 120 min, Figure 12 and Table 4 demonstrates that TiO₂/Ag/CD, TiO₂/CD, and TiO₂ degraded 99.38%, 84.1%, and 63.76 % of MB, respectively. The greater rate of TiO₂/Ag/CD in MB degradation can be attributed to the higher concentration of CD and the presence of silver in its structure.

Figure 12: Investigating the effect of photocatalyst, at 0.01 g photocatalyst, 10 ppm MB, 100 mL, 120 min

Table 4: Investigating the effect of photocatalyst

Investigating the Kinetics of MB Photocatalytic Degradation Reaction

To provide useful information about the effectiveness of the photocatalyst, and subsequently develop the optimal conditions for the ongoing removal of MB by TiO₂/Ag/CD at an industrial level the kinetic models were investigated. Two well-known kinetic models of pseudo-first-order and second-order were applied at variable times from 15 to 120 min and the results are presented in Table 5. The linear plot of kinetic models for the photocatalytic degradation of MB on TiO₂/Ag/CD is indicated in Figure 13, and their derived parameters are illustrated in Table 5. According to the outcomes, the pseudo-second-order model demonstrated excellent linearity with correlation coefficients of 1.00 for photocatalytic removal of MB onto TiO₂/Ag/CD.

Figure 13: The linear equation of the first degree and the second degree are shown respectively

Table 5: The kinetics parameters for photocatalytic degradation of methylene blue onto TiO₂/Ag/CD composites

TiO₂/Ag/CD Recycling

Catalyst recycling is one of the primary concerns for practical applications. The stability of TiO₂/Ag/CD as a photocatalyst for MB degradation was thus investigated. To better investigate the recovery of the photocatalyst, the photocatalytic dye degradation reaction was carried out on a larger scale. For this purpose, one liter of 10 ppm solution of MB was prepared, and then 100 mg of TiO₂/Ag/CD was dispersed in it, and after stirring in the dark for one hour, it was exposed to sunlight. Experiments on repeatability were conducted for eight continuous cycles. After each run, the catalyst was recovered by washing with water and ethanol and drying at 80 ^ᵒC for one day before the subsequent cycle. After eight cycles, the results indicate that the catalytic efficacy remains essentially unchanged and the MB degradation is maintained at 93.08%. The stability of photocatalyst up to eight consequitive cycles could be because of covalently bounded CD to the modified TiO₂/Ag NPs. During photocatalysis, a few TiO₂/Ag/CD particles are leached into the solution, which contributes to the modest decrease in degradation (Figure 14).

Figure 14: Reusability of the TiO₂/Ag/CD catalyst for the photodegradation of MB, under optimal conditions (100 mL, 10 ppm, 0.01 g TiO₂/Ag/CD, 120 min)

Conclusion

With the aid of a plant extract, TiO₂ and TiO₂/Ag nanoparticles were synthesized, and their surfaces were modified by forming a chemical bond with a silane agent. The modified products were used to create TiO₂/CD and TiO₂/Ag/CD nanocomposites, and then it was demonstrated that the TiO₂/Ag/CD nanocomposite has superior photocatalytic properties compared to the other two photocatalysts under the same conditions, degrading MB dye up to 99.38% under sunlight irradiation. Under the same conditions, synthesized TiO₂ nanoparticles and TiO₂/CD nanocomposite destroyed 63.76 % and 84.1% of the dye, respectively. This is explained because the TiO₂/Ag/CD nanocomposite structure contains silver. It was demonstrated that using DRS analysis that doping silver to TiO₂ nanoparticles causes the reduction of the energy gap from 3.2 eV to 2.5 eV, facilitating simpler electron transfers. This increases the photocatalytic activity of TiO₂. TGA analysis confirmed that the TiO₂/Ag/CD structures contained more CD than the TiO₂/CD nanocomposite. Higher amounts of CD provide a larger substrate (MB dye) adsorption on the photocatalyst surface, thus more MB is available for decomposition by TiO₂, and the presence of silver reduces the amount of energy required by TiO₂ for dye decomposition. The results of this study revealed that the TiO₂/Ag/CD nanocomposite with a lower energy gap and higher content of CD makes it a potential photocatalyst for the degradation of pollutants in aqueous solutions. The order of photocatalytic activity was evaluated to be: TiO₂/Ag/CD > TiO₂/CD > TiO₂-NPs. The results of this research revealed that the synthesized nanocomposite can be used to remove various other organic pollutants. This is under investigation in our laboratory and the results will be reported shortly. Besides, the antibacterial and anticancer activity of the nanocomposite TiO₂/Ag/CD are under investigation.

Acknowledgments

The authors are thankful to the University of Mazandaran for partial support of this project.

Disclosure Statement

No potential conflict of interest was reported by the authors.

Funding

This study did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' Contributions

All authors contributed toward data analysis, drafting, and revising the article and agreed to be responsible for all aspects of this work.

Conflict of Interest

The authors declare that there is no conflict of interest in this study.

ORCID

Amirhosein Zamani

https://orcid.org/0000-0002-3934-4826

Sakineh Asghari

https://orcid.org/0000-0002-6608-9525

Mahmood Tajbakhsh

https://orcid.org/0000-0002-4856-1937

HOW TO CITE THIS ARTICLE

Amirhosein Zamani, Sakineh Asghari, Mahmood Tajbakhsh. Synthesis of TiO₂/CD and TiO₂/Ag/CD Nanocomposites and Investigation of Their Visible Light Photocatalytic Activities in the Degradation of Methylene Blue. Chem. Methodol., 2024, 8(3) 177-199

DOI: https://doi.org/10.48309/CHEMM.2024.432168.1753

URL: https://www.chemmethod.com/article_190858.html