Synthesis of Cadmium sulfide Nanoparticles via Green Route for Photocatalytic Degradation of Methylene Blue Dye

Introduction

Recent advancements in nanotechnology have accelerated to meet societal demands, particularly in material control through diverse synthesis methods such as hydrothermal, sol-gel, ultrasonication, and microwave irradiation¹⁻⁵. These methods produce nanomaterials with distinct chemical, physical, and optical characteristics. Among these, nanoparticles have gained interest for use in photocatalytic degradation due to their size, shape, and surface area⁶⁻⁹. NPs, with sizes varying from 1 nm to 100 nm, can be synthesized through a variety of techniques. However, many traditional approaches such as hydrothermal and solvothermal techniques involve toxic chemicals, high costs, generation of toxic waste, and significant energy use, emphasizing the need for eco-friendly synthesis methods using plants extracts and microorganisms.¹⁰⁻¹²

Among green synthesis approaches, the utilization of plant extracts has attracted significant attention. Plants, including their leaves, fruits, roots, and stems, act as natural reducing and stabilizing agents due to their rich content of active compounds. Biosynthesis using the plant Hibiscus rosa-sinensis has gained attention due to its active compounds like tannins, alkaloids, phenols, and flavonoids facilitating the NP synthesis in reducing and stabilizing the metal salts.¹³˒¹⁴

Among photocatalysts, CdS NPs belong to the group of chalcogenides; a II-IV group semiconductor NPs which are promising because of their narrow band gap, excellent photo response under visible light, and ability to breakdown organic pollutants like synthetic dyes.¹⁵⁻¹⁸ With growing industrialization, pollutants like MB – a widely used but toxic synthetic dye pose significant environmental and health risks. MB’s resistance to natural degradation disrupts aquatic ecosystems, necessitating eco-friendly removal strategies.¹⁰˒¹⁹˒²⁰

Photocatalysis, which uses light energy to generate electron-hole pairs and drive redox reactions, is an effective method for organic pollutants degradation. This process produces highly oxidized hydroxyl radicals (•OH) that decompose contaminants. Unlike traditional methods such as adsorption, ozonation, and membrane filtration, photocatalysis is cost-effective, highly efficient, low cost, and environmentally compatible, making it a preferred choice for removing synthetic dyes.⁶˒²¹⁻²⁴

The present work explores the green production of CdS NPs utilising extract from Hibiscus rosa-sinensis leaves as a natural reducing agent. Structural and morphological aspects of the prepared NPs were analyzed, and their photocatalytic efficacy was assessed by degrading MB dye under sunlight. This study emphasizes a sustainable strategy for NP synthesis and wastewater treatment.

Materials and Methods

Synthesis of CdS nanoparticles

Hibiscus rosa-sinensis leaves were washed with de-ionized water. Then they were dried and crushed to make fine powder with the aid of a blender. A mixture of 150 mL of de-ionized water, 10 mL of methanol and 10 g of the powdered leaves were heated for 30 min at 100°C. The solution was left to cool until it reached room temperature and then subjected to filtration with Whatman No. 1 filter paper²⁵. A 10 mL of aqueous Hibiscus rosa-sinensis leaf extract was introduced to 150 mL of cadmium sulfate solution (0.1 M) and agitated at 60°C for 2 hours. Hydrogen sulfide gas was then introduced into the mixture until a bright yellow precipitate of CdS NP was formed. Then the solution was allowed to cool and settle down. Finally, the mixture was filtered and the precipitate was given wash first with acetone followed by deionised water to remove the contaminants if present and dried completely for 24 hours at 50^oC.

Characterization of CdS nanoparticles

The FTIR spectrum of CdS NPs was measured over the span of 4000 to 600 cm⁻¹ with the help of a PerkinElmer spectrophotometer. The XRD pattern was analysed using Rigaku Powder X-ray diffractometer using Cu-Kα radiation (λ: 1.5406 A⁰) scanning over 2θ interval of 20° to 65°. The surface features and elemental mapping of CdS NPs were investigated by using a Carl Zeiss SEM coupled with EDS.

Results and discussion

FTIR analysis

Figure 1: FTIR spectrum of Hibiscus rosa-sinensis mediated CdS NPs Click here to View Figure

The IR spectrum of CdS NPs synthesized using Hibiscus rosa-sinensis extracts is depicted in Figure 1. A broad and distinct peak at 3350 cm⁻¹ is assigned to the O‒H stretching modes of hydroxyl (-OH) units. A weak band occurring at 2982 cm⁻¹ is attributed to the C‒H bond stretching of alkanes and alkyl groups. A signal at 1613 cm⁻¹ is linked to the C=O bond stretching vibrations, while the signals at 940 cm⁻¹ and 1108 cm⁻¹ are associated with the stretching of aromatic C-H and C-O, respectively. These absorption bands are predominantly by the functional moieties contained in the extract of leaf. Additionally, an absorbance band at 650 cm⁻¹ marks the synthesis of CdS NPs.

XRD Pattern Analysis

The diffraction pattern of CdS NPs exhibited sharp diffraction peaks (Figure 2), signifying a well-defined crystalline structure. The diffraction signals appearing at 2θ angles of 26.56°, 43.92°, and 51.88° confirmed the cubic phase of CdS NPs^26,27. Additionally, the absence of any diffraction peaks corresponding to potential impurity phases indicated the successful synthesis of phase-pure cubic CdS NPs. 

Figure 2: XRD pattern of Hibiscus rosa-sinensis mediated CdS NPs Click here to View Figure

Scanning electron microscopy and elemental study

Figure 3: SEM micrographs of the eco-freindly synthesized CdS NPs at magnifications: (a) x 5, (b) x 200, (c) x 300 and (d) x 400. Click here to View Figure

The structural features of the synthesized CdS NPs was carried out using SEM, as depicted in Figure 3. The micrographs reveal that the nanoparticles formed are close to being spherical and exhibit significant agglomeration. Their sizes vary, maintaining a predominantly spherical structure with some degree of clustering.

Further, the elemental composition of green synthesized CdS NPs was determined by Energy Dispersive X-Ray Spectroscopy (EDS). EDS spectrum in Figure 4 clearly showed the characteristic signals for cadmium (Cd) and sulfur (S). This analysis revealed that the sample consists predominantly of Cd and S, with atomic percentages of 54.93 % and 45.07 %, respectively. The EDS spectrum further confirmed the existence of peaks for Cd and S, indicating the generation of absolute CdS without any additional elemental impurities.

Figure 4: EDS pattern of CdS NPs synthesized using Hibiscus rosa-sinensis leaf extract. Click here to View Figure

Photocatalytic Activity of CdS Nanoparticles

The green synthesized CdS NPs were utilized as photocatalysts for MB dye degradation. A comparative study was conducted to assess the efficiency of CdS NPs as photocatalysts in dye degradation. In a typical experiment, first set (Set 1) involved adding 5 mg of CdS NPs to 100 mL of an aqueous MB solution (100 ppm). In the second set (Set 2), the MB solution was prepared with the same concentration and volume as that of the Set 1 but without the addition of CdS NPs. Both the sets were agitated under dark condition for 30 min. to attain chemisorption equilibrium. Subsequently, the solutions were subjected to sunlight for 2 h while stirring to initiate dye photo degradation. At 30 min intervals during the process, 5 mL of the solution was withdrawn from each set into centrifuge tubes. After centrifugation, the solutions were analysed with a Dual Beam Cary 60 UV-Visible spectrophotometer to monitor dye absorbance over time and assess the degradation progress. 

Figure 5: Absorbance spectrum of set 1 (with CdS NPs) for MB degradation Click here to View Figure

Figure 6: Absorbance spectrum of set 2 (without CdS NPs) for MB degradation. Click here to View Figure

For Set 1, the absorbance of MB progressively decreased over time due to its degradation, as shown in Figure 5. The degradation percentage reached 30.55% within 120 min. On the other hand, the Set 2 exhibited minimal variation in absorbance, indicating negligible degradation (Figure 6). Figure 7 illustrates the percentage degradation over time intervals for both the sets as determined through UV-Visible analysis. The degradation percentage was determined using the following equation:

Here, C₀ (mg L⁻¹): initial concentration, and Cₜ (mg L⁻¹): concentration of MB dye at time t after degradation.

Figure 7: Degradation percentage of MB at various intervals of time for Set 1 and 2. Click here to View Figure

Conclusion

The current study demonstrates the successful green synthesis of CdS NPs utilising Hibiscus rosa-sinensis leaf extract. The synthesized CdS NPs exhibited significant photocatalytic activity, achieving up to 30.55% degradation of 100 ppm concentration of MB dye within 120 min in the presence natural sunlight. This highlights their potential as an efficient and bio-based photocatalyst for the elimination of synthetic dyes present in wastewater. The structural, morphological, and compositional analyses confirmed the formation of high-purity, phase-specific CdS NPs, which can serve as a sustainable alternative for environmental remediation. While cadmium is known to be hazardous to the environment, utilizing its NPs in very small amounts for significant environmental pollution remediation is a justifiable approach, balancing efficiency with controlled usage.

Future Perspectives

This work opens up the way for further exploration into optimizing synthesis conditions and investigating key factors such as NP concentration, temperature, reaction time, solution pH, and dye concentration to improve CdS NPs photocatalytic performance. Additionally, studies on the long-term stability and reusability of these NPs are essential to ensure their practical and cost-effective application. Expanding the scope to include the degradation of various hazardous organic and inorganic pollutants in wastewater can further enhance their utility. By addressing these critical aspects, green synthesized CdS NPs can contribute to water pollution management and sustainable environmental solutions.

Acknowledgement

The authors are grateful to St Aloysius (Deemed to be University) Mangaluru, 575003, Karnataka, India for providing the resources and laboratory facilities required for carrying out this research work.

Funding Sources

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Conflict of Interest

The author(s) do not have any conflict of interest.

Data Availability Statement

This statement does not apply to this article.

Ethics Statement

This research did not involve human participants, animal subjects, or any material that requires ethical approval.

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