Investigation of Photocatalytic Activity of Nanofluids Containing Silver Nanoparticles Synthesized via Ocimum tenuiflorum Leaf Extract

Introduction: Environmental pollution caused by various organic contaminants, including dyes, pharmaceutical residues, aromatics, and volatile organic compounds, has become a significant global concern. These pollutants contribute to the contamination of water, soil, and air [1], posing serious threats to both human and animal health [1]. Consequently, the removal of such organic pollutants from the environment has become a critical priority. Traditionally, the photocatalytic degradation of dyes has been explored as a method to treat wastewater generated by textile, leather, paper, hair-coloring, and food industries [2,3].

In recent years, the photo-induced degradation of dyes, particularly Rhodamine B and Methylene Blue, has emerged as a standard approach to evaluate the visible-light photocatalytic activity of novel materials [4]. Rhodamine B (N,N,N’,N’-tetraethylrhodamine), a member of the oxygen-containing heterocyclic xanthene dye family, is stable under dark conditions in the presence of a photocatalyst and also under light in the absence of a photocatalyst [5]. Studies have shown that Rhodamine B undergoes degradation through an efficient N-deethylation sensitization mechanism when exposed to intense visible light [6,7].

Commonly used photocatalysts include metal oxide and sulfide semiconductors [8,9]. There is a growing demand for low-temperature, cost-effective, and environmentally friendly methods for nanoparticle synthesis. Advances in size- and shape-controlled synthesis have endowed nanoparticles with unique properties compared to their conventional counterparts [10]. However, achieving nanoparticles with desired morphologies and optimal photocatalytic performance remains a challenge, emphasizing the need for facile synthesis strategies [11,12].

Among metallic nanoparticles, silver nanoparticles (Ag-NPs) have been extensively studied for their catalytic activity [13]. Ag-NPs exhibit strong light absorption and significant visible-light photocatalytic activity under both sunlight and visible-light irradiation [13–15]. Their high absorption in the visible spectrum and ability to suppress electron–hole recombination during the photocatalytic process have made Ag-NPs highly attractive for catalytic applications [13,16]. These properties render Ag-NPs suitable for various industrial applications [13,15]. Numerous nanostructures decorated with chemically stabilized Ag-NPs have been synthesized and applied in diverse fields [13,17].

Silver nanoparticles can be prepared using several techniques, including photochemical methods, chemical reduction, electron irradiation, gamma irradiation, and laser ablation [18]. However, these methods are often costly, require high temperatures and pressures, involve toxic chemicals, and generate hazardous byproducts, which pose biological risks [19]. Moreover, nonconductive stabilizers are commonly used to maintain Ag-NP stability [20], which can reduce the electrochemical performance of the resulting nanomaterials. Consequently, the biosynthesis of Ag-NPs using plant extracts has gained significant attention due to its simplicity, eco-friendliness, and avoidance of expensive instruments or toxic byproducts.

In India, the local population has long cultivated Tulsi (Ocimum tenuiflorum L) as a sacred plant, commonly found in homes and religious sites where its leaves are used in worship. Belonging to the mint family (Lamiaceae), Tulsi leaves are also a key ingredient in traditional remedies for ailments such as fever, insect stings, skin itching, headaches, coughs, diarrhea, constipation, warts, worms, and kidney disorders. This has led to its recognition as a natural reducing and stabilizing agent. Tulsi is reported to contain alkaloids, glycosides, tannins, saponins, and aromatic compounds, and has recently attracted interest for its inhibitory activity against HIV-1 reverse transcriptase and platelet aggregation.

Despite its potential, there are no reports on the photocatalytic degradation of Rhodamine B dye under direct sunlight using Ag-NPs synthesized with Ocimum tenuiflorum leaf extract. In this study, we report the optimization of reaction parameters for the synthesis of Ag-NPs using Tulsi leaf extract and evaluate the photocatalytic degradation of Rhodamine B under direct solar irradiation using the synthesized nanoparticles.

Materials and Methods

Synthesis of Silver Nanoparticle Suspension

Fresh leaves of Ocimum tenuiflorum were weighed (0.205 g), thoroughly washed with distilled water to remove dust and fungal residues, finely chopped, and then mixed with 100 mL of distilled water. The mixture was boiled for 1 hour, and the liquid extract was collected by filtering the cooled solution through filter paper.

To investigate the effect of leaf extract volume on the size and stability of silver nanoparticles, the concentration of AgNO₃ was maintained at 1 mM, while the nano-fluid suspensions were prepared in three different ratios by varying the volume of leaf extract. The resulting molar ratios of leaf extract to AgNO₃ were 1:1, 1:5, and 1:8, with the corresponding nano-fluids labeled as 1S1, 1S5, and 1S8.

An additional set of samples with 1:1 and 1:5 ratios was prepared using a constant leaf extract molarity of 3 mM, labeled as 3S1 and 3S5. These nano-fluids were used to study the influence of leaf extract volume and AgNO₃ molarity on the size and stability of the suspended silver nanoparticles.

Characterization of Nanoparticles

The crystal structure of the synthesized silver nanoparticles was characterized using selective area electron diffraction (SAED). Transmission Electron Microscopy (TEM) was employed to determine the particle size and morphology. Optical absorption studies of the nano-fluids were conducted using a UV-VIS spectrophotometer (UV-1800 Shimadzu) in the 400–800 nm wavelength range to confirm the formation of silver nanoparticles.

The concentration of the synthesized silver nanoparticles was quantified using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) with a Thermo Fisher Scientific iCAP RQ instrument.

Photocatalytic Degradation of Rhodamine Dye

The photocatalytic activity of the synthesized nanoparticles was evaluated through the degradation of Rhodamine B (RhB) dye in aqueous solution under direct sunlight. In each experiment, 2.5 mL of the nanoparticle suspension was mixed with 15 mL of RhB solution (20 µmol/L) and exposed to sunlight. The optical absorption of the mixture was measured at regular intervals to monitor dye degradation.

To determine the rate constants for photocatalytic degradation, RhB solutions of varying concentrations (5, 10, 15, 20, 25, 30, and 40 µmol/L) were prepared and their degradation under sunlight was studied using the nanoparticles that showed the highest degradation efficiency in the previous experiments. All degradation studies were conducted under direct sunlight irradiation in Chengannur, India (9.31830° N, 76.6110° E) during March 2019.

Results and Discussion

Optical Absorption Studies

The formation of silver nanoparticles was initially indicated by a visible color change of the transparent AgNO₃ solution to orange-red upon addition of the leaf extract. This color change is attributed to the Surface Plasmon Resonance (SPR) of metal nanoparticles, arising from the collective oscillation of free electrons in resonance with the light wave. The temporal evolution of optical absorption of the synthesized nanoparticles was monitored using UV-VIS spectroscopy.

Figure 1: Temporal dependence of optical absorbance of silver nano-particles at different time intervals.

The spectra for sample 1S5 are given in figure 1 which showed maximum stability. The sample showed absorbance peak at 422.5nm due to surface Plasmon resonance effect which confirms the formation of Ag nano-particles. The spectra were recorded at different time intervals from 2 days to 54 days. The intensity of absorption band increases with time with slight shift in surface Plasmon resonance from 422.5 nm to 432.5 nm.

A useful mathematical equation which relates diameter of the nano spheres to their surface plasmon resonance wavelength (λ_SPR) for silver particles was derived by Hoonacker et al. as

                                              (1)

where d_av is the average diameter of particles [21]. Using relation 1, we estimated the size of the Ag nano-particles. The size of the nano-particles was calculated to be 47 nm, 37 nm and 45 nm for the samples 1S1, 1S5 and 1S8 respectively. The size of the Ag nano-particles for the samples 3S1 and 3S5 were found to be 38 nm and 46 nm respectively using the same relation.

The plasmon resonance for a small spherical metallic nano-particle can be understood by a simple Drude free electron model, assuming that the positively charged metal atoms are fixed in place and that the valence electrons are dispersed throughout a solid sphere of overall positive charge. In the quasi-static limit, where the wavelength of light is much larger than the size of the particle, the force exerted by the electromagnetic field of the incident light moves all the free electrons collectively. Using the boundary condition that the electric field is continuous across the surface of the sphere, the static polarizability can be expressed as

                       (2)

where R is the sphere radius, ε is the complex dielectric function of the metal and ε_m is the dielectric constant of the embedding medium [22]. The polarizability shows a resonance when the denominator is minimized, which occurs when the magnitude of the real part of the complex dielectric function, ε_real, is –2εm. This resonance condition for the polarizability leads to a strong extinction of light at the plasmon resonance frequency. Within this free-electron description, plasmons can be thought of as collective oscillations of the conduction-band electrons induced by an interacting light wave. The plasmon resonance peak can be assigned to Surface Plasmon Resonance (SPR) of silver nanoparticles. Plasmon resonance of nanoparticles can be explained within the Mie theory. According to the Mie theory, the surface resonance can be described by the following equation [23]

                                (3)

where,

is the bulk plasmon resonance which is about 9 eV for Ag

: Dielectric constant for the matrix = 1 for vacuum.

: Dielectric constant for Ag which is ~5.

Based on relation (3), the surface plasmon resonance (SPR) of Ag is 3.67 eV. Embedding Ag particles in a matrix reduces their work function compared to vacuum, lowering the energetic position of the surface resonance state. The extent of this reduction varies with the matrix, suggesting that in our samples, the observed SPR shift originates from the organic matrix surrounding the Ag particles.

Photo-catalytic Activity of samples 1S1, 1S5 and 3S1

Figure 2.Temperal dependence of the optical absorption of 20µM/L Rhodamine B aqueous solution after direct solar light irradiation for 12 hours in the presence of (a)1S1 (b)1S5 (c)3S1

Among the different samples prepared the samples 1S1, 1S5 and 3S1 which exhibited greater stability were used for the photo degradation studies of Rhodamine B dye solution of molarity 20µmol/L. Figure 2 shows the temporal dependence of optical absorption for the dye solution mixed with the respective nano-fluids containing the suspended Ag nano-particles when exposed to direct sunlight. There is progressive decrease in intensity for the absorption peak with time. It is observed that the maximum absorption peak shifts towards the blue region with respect to time and its intensity becomes very low after 12 hours of continuous solar irradiation for 1S5 and 3S1 samples. But for 1S1 sample the degradation is very small which may be due to its lowest absorption of light as evident from the optical absorption spectra. The 1S5 sample shows the highest degradation among the three samples, which may be due to its smaller particle size as evident from the SPR peak which is blue shifted to the maximum among the prepared samples.

Figure 3. (A) Plot of (C/C0) versus time for the samples 1S1, 1S5 and 3S1 (B) Plot of Ln (C₀ /C) versus decay time for degradation of  20µM/L Rhodamine B aqueous solution in the presence of nano-fluid samples 1S1, 1S5 and 3S1.

The kinetic studies of the RhB dye degradation process play an important role in assessing the efficiency and feasibility of treating dye from contaminated water and also assessing the photo catalytic capability of the studied catalyst. Therefore, the kinetic study of RhB dye degradation under direct sunlight has been discussed here. According to Langmuir–Hinshelwood (L–H) model, the rate expression at low initial concentration is given by

                          (4)

where k_app is the apparent first order rate constant with C=C₀ at t = 0 representing the initial concentration of  the dye in the bulk after dark absorption and “t” is the reaction time. Table 1 compares the first order rate constants obtained for the samples. Figure 3 (A) represents the plot of (C/C0) versus time for the samples 1S1, 1S5 and 3S1. It is evident from the plot that the sample 1S5 exhibits higher degradation efficacy relative to 3S1 and 1S1.  Figure 3(B) shows the linear fit to the natural logarithmic plot of (C/Co) versus time for the samples.

Figure 3. (A) Plot of (C/C0) versus time for the samples 1S1, 1S5 and 3S1 (B) Plot of Ln (C₀ /C) versus decay time for degradation of  20µM/L Rhodamine B aqueous solution in the presence of nano-fluid samples 1S1, 1S5 and 3S1.

The kinetic studies of the RhB dye degradation process play an important role in assessing the efficiency and feasibility of treating dye from contaminated water and also assessing the photo catalytic capability of the studied catalyst. Therefore, the kinetic study of RhB dye degradation under direct sunlight has been discussed here. According to Langmuir–Hinshelwood (L–H) model, the rate expression at low initial concentration is given by

                          (4)

where k_app is the apparent first order rate constant with C=C₀ at t = 0 representing the initial concentration of  the dye in the bulk after dark absorption and “t” is the reaction time. Table 1 compares the first order rate constants obtained for the samples. Figure 3 (A) represents the plot of (C/C0) versus time for the samples 1S1, 1S5 and 3S1. It is evident from the plot that the sample 1S5 exhibits higher degradation efficacy relative to 3S1 and 1S1.  Figure 3(B) shows the linear fit to the natural logarithmic plot of (C/Co) versus time for the samples.

Table 1: Rate constant and R² values obtained from the linear fit of the logarithmic plot of (C₀/C) versus time for different nano-fluid samples used to degrade 20µM/L Rhodamine B aqueous solution.

3.3 Photo-catalytic activity of sample 1S5 for the degradation of RhB of different concentrations

Experiments on the photo catalytic degradation of RhB dye were conducted at different RhB concentrations: 10 µmol/L, 15µmol/L, 20µmol/L, 25µmol/L and 30µmol/L under direct sunlight illumination. The plots of ln (C0/C) versus time for photo degradation at these concentrations under direct sunlight illumination are shown in figure 4.

Figure 4: Temporal dependence of optical absorption spectra of RhB of different concentrations- 10 µmol/L, 15µmol/L, 20µmol/L, 25µmol/L and 30µmol/L subjected to photo-catalytic degradation under direct sunlight by sample 1S5.

Figure 5: (A) Plot of (C/C0) versus time for the samples 1S5 and (B) Plot of Ln (C₀ /C) versus decay time for different dye concentrations of in the presence of nano-fluid sample 1S5.

The linear fit between ln (C₀/C) and irradiation time as shown in figure 5 supports the conclusion that the degradation follows first-order kinetics. The values of regression coefficient (R²) of the experimental trials were more than 0.99 indicating that the degradation of RhB by the nano-particles followed an apparent-first-order kinetics. Our results indicate that the initial concentration of the dye plays a critical role in controlling the degradation rate. Table 2 shows the rate constant and R² values obtained from the linear fit of the logarithmic plot of (C₀/C) versus time for different RhB dye concentrations of in the presence of nano-fluid sample 1S5 under direct sunlight.

Table 2: Rate constant and R² values obtained from the linear fit of the logarithmic plot of (C₀/C) versus time for different RhB dye concentrations.

Under dark conditions the RhB dye in the nano-fluid suspension did not undergo any degradation. When the mixture of the dye and the nano-fluid suspension was exposed to direct sunlight irradiation a photo-catalytic degradation pathway may have been activated.The photo-catalytic pathway can be summarized as follows: The localized surface Plasmon resonance (LSPR) of Ag- NPs can lead to very efficient absorption of light. The de-phasing of the resonant oscillation of the conduction electrons generate hot electrons[24].The energies of these hot electrons are ~ 4eV for Ag nano-particles[25]. These hot electrons may be trapped by oxygen to form oxygen species [26]

     e^– +O₂   →∙O₂^–

The Ag+ ion and the newly formed oxygen species further react with water to generate hydrogen peroxide anions (HO2^·) and increased number of hydroxyl radicals (·OH)[27].  The highly reactive oxidative species oxidize the RhB in aqueous solution to produce CO₂, water and some simple mineral acids [28].

   ∙O₂^– +  RhB → CO₂+H₂O+mineral acids.

A clear decrease in absorbance along with a shift in the position of the main absorbance peak is observed for all of the RhB solution in the presence of the nano-fluids containing the Ag nano-particles. This is suggestive of a photo-degradation pathway which consists of an initial N-de-ethylation step [29]. There is an increase in photo-degradation rate with increasing RhB initial concentration. This may be due to the increased photosensitization process with increase in concentration of dye. This process saturates when maximum number of dye molecules are adsorbed onto the Ag-NP surface.

Figure 6: Comparison of peak absorbance as a function of time for 20µmol/L dye solution mixed with sample 1S5 under direct sunlight illumination and under white light source of intensity 60 mW/cm².

In figure 6, a comparison of the photo-catalytic decay in terms of the decrease in peak absorbance of the 20 mmol/L dye solution mixed with sample 1S5 under direct sunlight illumination and under focused halogen light source is shown. It is apparent that under focused light the rate of dye decay is much faster. The rate constant for the degradation of 20 mmol/L concentration of dye solution was found to be 3.02 x 10^-3 min^-1. 

3.4 TEM analysis of sample 1S5

Figure 7: TEM analysis of sample 1S5: (a) The SEAD pattern shows diffuse rings overlaid with distinct spots indicating the presence of crystalline component to an otherwise amorphous material (b) TEM image showing a single grain with the d-spacing of 0.26 nm (c) TEM image showing spherical shape for the Ag-NPs (d) Histogram representing average particle size distribution in the aqueous solution.

The nano-fluid sample 1S5 which showed the highest stability and efficiency in dye degradation was subjected to TEM analysis to understand the structure and morphology of the formed Ag nano-particles. Figure 7 (a) shows the SEAD pattern for the samples. The presence of diffuse rings overlaid with distinct spots is very evident. This is an indication of the presence of crystalline component to an otherwise amorphous material in the nano-fluid 1S1. Up-to three rings could be clearly indexed to the (111), (202) and (222) planes corresponding to the FCC crystal structure of Ag. The diffraction pattern in figure 7(b) focused on a single grain shows the d-spacing of 0.26 nm.  Figure 7 (c) shows spherical Ag-NPs obtained through the green route, i.e. reduction of AgNO₃ by the contents of the leaf extract in the leaf broth. They nano-particles possess nicely faceted morphology as seen. There appears to be more or less homogenous distribution in size of the nano-particles. Figure 7 (d) shows the histogram depicting the average particle size distribution in the aqueous solution. Maximum number of Ag-NPs is found to be having a size in the range ~ 25–30 nm.

3.5 Inductively coupled mass spectrometry (ICP-MS) analysis

ICP-MS analysis was conducted on the sample 1S5 in order to quantify its concentration. Based on the results of the analysis the concentration of silver in the sample was found to be 33.81mg/L.

4. Conclusions

We have optimized a green synthesis method using Ocimum tenuiflorum leaf extract for the formation of silver nanoparticles (Ag-NPs), achieving an average particle size of 25–30 nm. Among the nanoparticles synthesized, the sample with this size range demonstrated the highest stability, remaining stable for over 50 days. Our results indicate that the ratio between the molarity of AgNO₃ and the volume of leaf extract does not significantly influence the particle size of the Ag-NPs.

The photocatalytic activity of the green-synthesized Ag-NPs was evaluated through the degradation of Rhodamine B (RhB) dye. Interestingly, while the Ag-NP size was unaffected by the AgNO₃-to-leaf-extract ratio, this ratio played a critical role in determining the efficacy of photocatalytic dye degradation. The degradation efficiency of RhB dye was tested across a molarity range of 10 µmol/L to 30 µmol/L using the prepared Ag-NPs. The observed rate constant increased with dye concentration, likely due to enhanced photosensitization at higher concentrations.

Kinetic studies of RhB degradation under direct sunlight irradiation revealed that the process follows first-order kinetics. This work demonstrates the effective photocatalytic degradation of RhB dye under sunlight using a nanofluid containing green-synthesized Ag nanoparticles, highlighting the potential of these eco-friendly nanomaterials for practical applications.

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