Green synthesis of zerovalent copper nanoparticles for efficient reduction of toxic azo dyes congo red and methyl orange

2.1 Materials

All the chemicals and solvents used were of analytical grade. Sodium borohydride (NaBH₄) and copper sulfate pentahydrate (CuSO₄⋅5H₂O) were purchased from BDH Chemicals England. MO and CR dyes were purchased from Merck (Germany). In the synthesis of Cu nanoparticles, deionized (DI) water was used.

2.2 Preparation of aqueous fruit extract

Ripened Duranta erecta fruits were used for the preparation of aqueous extract. D. erecta fruits weighing 30 g were carefully washed with double-distilled water, dried, and then crushed into 100 ml DI water using an electric blender for extract preparation. Then, it was filtered through Whatman No. 42 filter paper (pore size 90 μm) followed by filtration through 25 μm size filter paper. The filtrate was stored in a refrigerator (−4°C).

2.3 Synthesis of CuNPs

For aqueous phase synthesis of CuNPs, D. erecta fruit extract was used as capping as well as reducing agents. An approximately 5 mm solution of CuSO₄⋅5H₂O was prepared in DI water and used for the synthesis of CuNPs. For the preparation of CuNPs, 85 ml of CuSO₄⋅5H₂O solution was taken in 200 ml Erlenmeyer flasks to which 15 ml of aqueous fruit extract was then added. The reaction mixture was allowed to stand with constant stirring at 80°C. The color of the solution changed from light yellow to dark brown after 1 h, indicating the formation of monodispersed CuNPs. The reaction mixture of green synthesized CuNPs was then subjected to centrifugation at 6000 rpm for 10 min. The resulting pellet was washed with ethanol followed by DI water, and then dried at room temperature.

2.4 Catalytic reduction study of dyes

For the catalytic reduction study, a quartz cuvette was used as a reaction vessel. For the reaction, approximately 3 ml of dye solution (0.03 mm) was taken in a cuvette in which 0.5 ml of freshly prepared NaBH₄ (0.2 m) was then added and its UV-visible spectra was recorded. For the reduction of dyes, 5.0 mg of dried CuNPs was introduced into the reaction mixture. The variation in the absorption intensity was monitored continuously after each minute using a UV-visible spectrophotometer. Upon completion of the reduction, the nanoparticles were recovered from the reaction by centrifugation and washed with freshly prepared DI water and dried for the next use.

2.5 Instruments

3.1 Synthesis of CuNPs and UV-visible analysis

In the current work, D. erecta fruit extract was used for the synthesis of stable Cu nanoparticles in aqueous phase. The formations of CuNPs were confirmed using UV-visible spectra due to surface plasmon resonance. The UV-visible absorption spectra of synthesized CuNPs were recorded at fixed wavelengths from 200 to 800 nm. The UV spectrum of D. erecta fruit extract shows absorption bands at 338 and 226 nm, which is characterized due to the absorbance of benzoyl-related systems of the biomolecules [21], [22], [23]. The characteristic absorption peak of Cu colloids was observed at 588 nm, as shown in Figure 1. Surface plasmon is the electron excitation in the conduction band of the nanoparticles surface. The metal nanoparticles providing the characteristic absorption spectra in the UV-visible region are called surface plasmon resonance of nanoparticles. It has been reported in the literature that the surface plasmon resonance band of Cu nanoparticles provides absorption from 570 to 600 nm [24], [25]. The CuNPs surface plasmon band stability at 588 nm confirms the formation of Cu nanoparticles and also shows that particles in the solution are monodispersed with no sign of agglomeration.

Figure 1:

UV-visible spectra of D. erecta fruit extract and synthesized CuNPs (inset shows the change in color after the formation of CuNPs).

3.2 FT-IR discussion of CuNPs

FT-IR spectroscopy is an important tool for the detection of functional groups of biomasses, which are responsible for capping and stabilizing agents of the prepared CuNPs. The FT-IR spectrum of D. erecta fruit extract show absorption bands at 3373, 2936, 1617, 1493, 1269, 1149, and 815 cm⁻¹ (Figure 2). The broad band at 3373 cm⁻¹ may be attributed to the stretching vibration of primary and secondary amine and amide linage of proteins [26]. The band at 2936 may be assigned to the C-H of methylene [27]. The intense peak at 1617 and 1493 cm⁻¹ could be assigned to the carbonyl (C=O) stretching vibration of the amide linkage and stretching vibration of C-N of aromatic amine, respectively [28], [29]. The band at 1269 cm⁻¹ may be assigned to C-O stretching [30], [31]. The intense absorption band at 1052 cm⁻¹ could be assigned to C-O-C stretching vibration [32], whereas the band at 815 cm⁻¹ is assigned to p-disubstituted benzene [26], [27].

Figure 2:

FT-IR of D. erecta extracts and synthesized CuNPs.

The bands observed in the FT-IR spectrum of CuNPs were at 3360, 1620, 1415, 1288, 1052, and 528 cm⁻¹, as shown in Figure 2. The spectrum of CuNPs show all the characteristic peaks of D. erecta extract along with a new peak at 528 cm⁻¹, which corresponds to the Cu-O bond [33]. The slight shift in the FT-IR bands of CuNPs from that of D. erecta extract along with the new peak at 528 cm⁻¹ confirmed the formation of CuNPs. The results obtained from the FT-IR spectra confirmed that nanoparticles are coated with biomolecules, especially with the amino acid residues of the proteins. The amino acid residues of proteins have a strong ability to bind with metal by coating their surface and preventing them from aggregation and are responsible for stabilizing. Thus, from the above discussion, it can be concluded that biomass (protein, phenolic, and alcoholic compounds) perform the function of stabilizing agents for CuNPs.

3.3 FE-SEM analysis of CuNPs

For identification of size and surface morphology, FE-SEM was used. FE-SEM images of the green synthesized CuNPs were found to be spherical in shape without aggregation (Figure 3). From the images, it has been confirmed that the synthesized CuNPs are stabilized by biomolecules present in D. erecta fruit extract. It has been reported in the literature that the optical and electronic properties of metal nanoparticles are shape dependant [34]. For size and surface morphology determination, FE-SEM has previously been used by several investigators for the characterization of metal nanoparticles. The average size of the synthesized CuNPs was found to be 70 nm having monodispersity (Figures 3B and 4A). Higher magnification FE-SEM images show the well-defined surface modification and coating of biomolecules, which stabilize the fine surface of CuNPs.

Figure 3:

Low to high magnification FE-SEM images of prepared CuNPs.

Figure 4:

(A) High-resolution FE-SEM image and (B) EDX investigation of prepared CuNPs.

3.4 EDX investigation of CuNPs

The energy-dispersive X-ray (EDX) profile of the prepared nanoparticles shows strong elemental signals of copper and confirms the presence of copper in nano form, as shown in Figure 4B. This shows that prepared CuNPs contain only metallic copper lacking any other impurities. The EDX pattern clearly shows that the CuNPs are crystalline in nature. The EDX profile obtained from D. erecta extract capped CuNPs confirmed the presence of CuNPs mostly showed strong signal energy peaks for the copper atoms at 1.0 and 8.0 keV. The previous studies carried out for individual spherical-shaped CuNPs also show strong signal peaks at 1.0 and 8.0 keV [35], [36], [37]. Approximately 50.5% weight of copper was present in the prepared nanoparticles whereas the remaining 49.5% were carbon and oxygen present in organic molecules, which act as capping molecules surrounding the nanoparticles. The excess carbon is due to the coating that was used in the analysis. These values show agreement with the TG analysis of the prepared CuNPs, which also show a 49.2% weight loss of the capping agents. Because the nanoparticles are spherical, isolated, and monodispersed without impurities, monodispersities of CuNPs are very useful features for material scientists to improve various applications related to CuNPs. The weight percentages of different elements in the CuNPs were 32.61% C, 16.89% O, and 50.50% Cu, as shown in Figure 4B.

3.5 TG analysis of CuNPs

The presence of biomolecules, which act as capping as well as reducing agents present on the surface of CuNPs, was further confirmed by TGA, as shown in Figure 5. Three distinct exothermic peaks were observed in the DTA thermogram recorded in the temperature range of 27°C to 800°C. The three different weight losses observed were at 39°C to 115°C, 162°C to 192°C, and 220°C to 400°C. The first 13% weight loss observed from 39°C to 115°C is attributed to the physically adsorbed water molecules on the surface of CuNPs. Afterward, there was continuing weight loss occurring until 400°C. The second and third weight losses that appeared in the thermogram were found to be at approximately 36.25%. It is believed that the total weight loss amounting to 49.25% of the sample weight is due to physically adsorbed water molecules and desorption of biomolecules that are responsible for stabilization as well as also for reduction of CuNPs. Thus, the D. erecta fruit extract are likely to be made up of biomolecules such as proteins, enzymes, and polyphenoles that are responsible for the stabilizing of CuNPs, as reported previously in the literature [38], [39]. The total weight loss observed in the thermogram was 49.25%, which shows that 50.75% of the metallic copper is present in the prepared CuNPs. This evidence also supports the EDX analysis of prepared CuNPs.

Figure 5:

TGA/DTA thermogram of D. erecta synthesized CuNPs.

3.6 XRD analysis of CuNPs

The XRD patterns of D. erecta synthesized CuNPs is shown in Figure 6. The XRD pattern of the synthesized CuNPs is similar to previous reports [40]. All the possible peaks observed indicate metallic copper, which also show the polycrystalline nature of CuNPs. In the XRD pattern, two distinct diffraction peaks for CuNPs are observed with a 2θ value of 43.74° and 50.78° representing Bragg’s reflections of [111] and [200] planes of fcc crystal structures of CuNPs [40], [41]. The average crystallites size of copper was calculated from the Scherrer equation. The average crystallite size of the CuNPs was 76 nm, which is ably supported by high-resolution FE-SEM images as well. The small diffraction peak at 35.5°, which corresponds to the Bragg’s reflections of [002] may be due to the formation of copper oxide [42]. Here, the formation of copper oxide in small quantities cannot be ignored. Some unidentified peaks at 29.05°, 38.2°, 41.53°, and 42.53° were assigned (*). These unassigned peaks might be due to the crystallization of biomolecules.

Figure 6:

XRD pattern of D. erecta synthesized CuNPs.

3.7 Catalytic reduction of dyes

3.7.1 Catalytic reduction of MO

With the development of industries, human beings face serious environmental disputes due to the release of a large number of pollutants, especially dyes and other organic molecules, in the aqueous environment. The release of polluted water into the ecosystem is a major threat to aquatic life as well as human health. The intense dark color of azo dyes is due to the presence of azo (–N=N–) groups in their chemical structure [43], [44]. Among the dyes, the azo group constitutes the largest group of organic synthetic dyes and is widely used by the cosmetic, food, paper and pulp, and textile industries. An anionic azo dye MO has been used frequently as an indicator in laboratories as well as in the plastics, cosmetics, paper, textile, pharmaceutical, rubber, and printing industries [45], [46]. Due to the lower biodegradability of MO, the presence of the azo group makes it hazardous for the environment. MO leads to several health risks such as vomiting, diarrhea, breathing, and nausea [47]. Therefore, the elimination and degradation of MO dye from aqueous environments are important. In the past decades, there has been growing interest in catalytic hydrogenation by heterogeneous catalyst [48], [49], [50].

The discharge of azo dye waste effluents from various industries, especially the textile industries, into the aqueous environment is a serious threat. Thus, the development of new methodologies for the degradation/removal of azo dyes is of great interest. The aqueous MO solution gives a strong peak at 468 nm and a small absorption peak at 278 nm. It has been reported in the literature that the reduction of MO dye alone by NaBH₄ is favorable thermodynamically but, kinetically, this reaction is extremely slow [51]. In the absence of a suitable catalyst, the peak intensity at 468 nm remains constant for MO even for several hours [8], [52].

For catalytic reduction of MO, firstly, 3 ml of 0.03 mm solution was taken in a quartz cuvette and then 0.5 ml of aqueous NaBH₄ solution was added. UV-visible spectra of MO gives a strong peak at 468 nm and a weak peak at 278 nm. In the absence of a catalyst, the intensity of the peak remains constant at 467 nm. However, the decrease in peak intensity at both 278 and 468 nm starts after the introduction of 5 mg of CuNPs. The catalytic activity of CuNPs was recorded by measuring their UV-visible spectra with a time interval of 30 s between measurements, as shown in Figure 7A. With the addition of CuNPs, the intensity of both peaks at 278 and 468 nm decreases. However, the peak maxima at 278 nm slightly shifted toward the lower wavelength with an increase in intensity and finally reached 250 nm, as shown in Figure 7A. Thus, the gradual disappearance of the peak at 468 nm was due to the degradation of MO. Generally, NaBH₄ in the presence of active catalyst reduced MO molecule at the azo site by generating low molecular free amino group compounds. Thus, the formation of a new peak at 250 nm was from the generation of free amino groups (hydrazine derivative) compounds during the MO reduction [15].

Figure 7:

UV-visible spectra of (A) MO reduction by NaBH₄ and CuNPs, (B) CR reduction by NaBH₄ and CuNPs, (C) comparative percent reduction of MO and CR, and (D) plot of ln(A_t/A₀) versus time for MO and CR. Experimental conditions (3 ml of aqueous dye solution+0.5 ml of 0.2 m aqueous NaBH₄ solution+5 mg of CuNPs).

Percent reduction of the MO was calculated from the plot of UV-visible spectra by using equation (2)

(2)Percent reduction=100−(At×100/A0)

where A₀ is the initial absorbance at λ_max 467 nm and A_t is the absorbance at time t. Here, a 96% reduction of MO in the presence of NaBH₄ and CuNPs takes place in 4 min, as shown in Figure 7C. The reduction of MO follows the pseudo-first-order reaction as reported in the literature. The apparent rate constant (k_app) of the pseudo-first-order kinetic was calculated from the plot of ln(A_t/A₀) versus time of reduction (as given in Figure 7D) using equation (3)

(3)ln At/A0=−kKt=−kapp

Thus, the k_app calculated from the slope was 8.6×10⁻³ s⁻¹.

3.8 Stability and reusability of CuNPs

Nowadays, the recyclability and reusability of the catalyst is an important issue in catalysis because a large number of catalysts are deactivated after the first or second cycle and then discarded. Reusability and stability of catalysts are important for determining the performance of catalyst as the active and stable catalyst could considerably decrease the cost of the process. Here, we used MO dye to study the reusability of CuNPs catalyst. The reusability of the CuNPs was checked four times for a 95% MO reduction. The reusability of the CuNPs was carried out after centrifugation of catalyst from the reaction mixture and then washing with freshly prepared DI water and then reused again following the same experimental procedure of MO reduction. For the first use, CuNPs takes 4 min for 96% MO degradation, whereas for the second use, it took 6 min for 95% MO degradation. Similarly, it took 10 and 15 min for a 95% reduction on the third and fourth cycles, respectively, as shown in Figure 8. The reusability study showed that CuNPs possess outstanding stability and recyclability. Thus, it was concluded that CuNPs catalyst could be used several times in various chemical reactions as an active catalyst. The decrease in the catalytic activity may be due to the oxidation of copper into copper oxide.

Figure 8:

Reusability of CuNPs in the catalytic reduction of MO.