3.1 XRD
Normally, solid-state materials are classified into two types: crystalline and amorphous. Most of the time, researchers consider only crystalline materials suitable for XRD analysis. The lattice structure and phase compositions of the samples were characterized by XRD patterns, which are presented in Fig. 1. The production of a polydisperse crystalline nano-material is demonstrated by diffraction peaks 2θ at 27.57, 32.06, 45.98, 54.56, 57.22, 67.19, 76.48 and 85.47, which correspond to the planes (111), (200), (220), (311), (222), (400), (420), and (422), respectively. The peaks demonstrate that the synthesized nanoparticles are extremely polycrystalline in nature. The AgCl structure of the resultant data matches JCPDS database card no. 01-85-1355. Furthermore, signature peaks for the Au (JCPDS card no. 00-001-1174) and Pd (JCPDS card no. 00-001-1174) structures are observed in 2q at 38.26 (111) and 39.92 (111), where overlays and broad peaks form. In XRD, peak 2θ is at 44.6, which corresponds to the plane (200). The Rietveld refinement approach using the Fullprof program was used to conduct the crucial structural study of the polycrystalline AgCl, Pd and Au (details are provided in the supplementary information, which includes supporting Figures S1.1–S1.3 and table S1). Table no. S2 displays the MI range of experimental AgCl@Pd/Au bimetallic NPs, which ranges from 0.5804 to 0.636. It is related to particle size (range: 56.58361–59.24302 nm). With a small deviation, the calculated data show that MI is directly proportional to particle size in nm, and the obtained results are shown in Figure S2, respectively. The variations and interrelationships between them are indicated by the linear fit.
3.2 FTIR
FT-IR spectra indicate absorption bands that indicate the presence of different functional groups on the surface of a nanomaterial. Different functional groups correlate with various wave numbers and frequencies. From Fig. 2, a very wide and strong band of transmittance was found at 3425 cm− 1 which could be attributed to the existence of the -OH group in AgCl@Pd/Au bimetallic NPs. The absorption band of IR in the range 2920 − 2852 cm− 1 corresponds to the C-H bond stretching vibration[36]. The peak at 2310 cm− 1 was attributed to atmospheric residual CO2. The vibrational peaks at 1604 cm− 1 are owing to the asymmetric stretching of the C = O group with amide(I/II)[33]. The 1388 cm− 1 band corresponds to aromatic amine C-N stretching vibrations or aromatic amine C = C stretching. The prominent absorption band at 1094 cm− 1 is due to the stretching of C-O-H (alcohol) or C-O-C (ether)[7, 37]. The band at 604 cm− 1 observed due to C-Cl stretching and having alkyl halides[38].
3.3 Raman
The Raman spectra of synthesized AgCl@Pd/Au bimetallic NPs are given in Figure S3. The small, sharp peak at 230 cm− 1 is obtained because of Ag-Cl bonds stretching vibrations, which implies that proteins are attached to the surface of the particles via amino and/or carboxylate bonds. Additionally, strong peaks were observed at 1370 and 1595 cm− 1, as they are correlated to symmetric and asymmetric C = O stretching vibrations in the carboxylate (-COO−) group and/or phenyl ring stretch. The significant interaction of the carboxylate group with the bimetallic NP surface is indicated by an increase in the intensity of the C = O stretching vibration bands[39]. The ID/IG ratio obtained was 0.88, accordingly. The calculated ID/IG ratio is relatively less than 1 i.e., the AgCl@Pd/Au bimetallic NPs have no defects. The higher the ratio, the sp2 crystal size of the cluster also increases, as they are directly proportional to each other. But in the case of AgCl@Pd/Au bimetallic NPs, the ratio is less than 1 and the cluster size also decreases[40].
3.4 Morphology study
Electron microscopy is a strong method for studying nanoparticles geometry, size, and shape. The morphology of composite images of NPs analyzed by Scanning electron microscopy is shown in Fig. 3a. From SEM images, it is revealed that AgCl@Pd/Au bimetallic NPs exhibit distinct structures with a cubic shape. Also, the average particle size of AgCl@Pd/Au bimetallic NPs is around 197 nm. The homogenous shape of cubic AgCl@Pd/Au bimetallic NPs may help in the increase of catalytic activities, including dye degradation and organic transformation processes, since nanocatalysis is a surface phenomenon activity41. Figure 3 (b-d) shows a FE-SEM photograph of the synthesized material, demonstrating the nanoscale production of cubic, poly-dispersed cluster morphology.
In this investigation, TEM, HR-TEM, and selected area electron diffraction (SAED) patterns were employed to examine the nanostructure and interface composition of the AgCl@Pd/Au bimetallic NPs in order to better understand their structure and shape. The detailed morphological structures of these NPs are rarely examined using the TEM technique because the destabilizing nanocomposites will be destroyed by high-energy transmission electron beam irradiation[41]. The successful completion of TEM and HRTEM testing for AgCl@Pd/Au bimetallic NPs, which contain AgCl, Au, and Pd nanomaterials synthesis by biogenic route. Figure 4 (a–b) shows the group of cubic structures formed by NPs that are interconnected and accreted with one another and have relatively rough decorations on Au and Pd surfaces. The results demonstrated that the AgCl@Pd/Au bimetallic NPs displayed exceptional crystal lattice fringes with a d-spacing of 0.22 nm (Fig. 4c), which relates to the AgCl species' crystal plane (200) of the face-centered cubic (FCC) structure of AgCl[42]. The multiple diffraction rings present in the sample's SAED pattern (see Fig. 4d) show that it is polycrystalline, which is suitable for procuring extra catalytically active sites and facilitating the isolation and transport of the carriers. The lattice planes (200), (220), (311), and (420), respectively, were computed from the SAED patterns and determined to be 2.77, 1.96 and 1.24 nm (Fig. 4d). The dots might be indexed in accordance with the lattice planes of reflection. The unit cells of AgCl@Pd/Au bimetallic NPs were supported by all of the lattice planes[43].
3.5 EDS
The presence of constituent elements such as Ag, Cl, Au and Pd is confirmed by energy dispersive analysis spectroscopy and the recorded spectrum is presented in Figure S4. The spectrum clearly illustrates the strong signals of the four Ag, Cl, Au and Pd atoms at appropriate KeV, which confirm the formation of AgCl@Pd/Au bimetallic NPs. Furthermore, it was found that the weight percent yields of Ag, Cl, Au, and Pd in AgCl@Pd/Au bimetallic NPs are in the range of 81.23, 4.75, 13, and 1.30%, respectively. Information about the elemental mapping of AgCl@Pd/Au bimetallic NPs revealed that the nanoparticles are dispersed uniformly over the length of cubic NPs, as presented in Figure S5 (a–f). Resulted in Ag, Cl, Au and Pd's presence in the nanomaterial and the formation of nanocomposites structure.
3.6 XPS
A surface-sensitive XPS technique was used on these samples to examine the exterior surface chemical composition and oxidation of metal (Ag, Cl, Au, and Pd) constituents in AgCl@Pd/Au bimetallic NPs, as demonstrated in Figs. 5(a–e). The survey spectrum for AgCl@Pd/Au bimetallic NPs shows that they contain Ag, Cl, Pd, Au, C, and N elements (Fig. 5a). In Fig. 5b, the XPS spectrum of Ag shows two peaks at binding energies (BE) of 368.8 eV of 3d5/2 and 374.8 eV of 3d3/2. This shows that Ag is present. The 197.84 and 199.43 eV peaks are Cl 2p3/2 and Cl 2p1/2 of AgCl in the Cl 2p XPS spectrum, respectively (Fig. 5c)[44]. The Cl 2p XPS peaks in Fig. 5b, which have been derived from AgCl NPs, can be deconvoluted into two peaks at 197.84 and 199.43 eV, which are each to be identified as distinct pairs of Cl 2p3/2 and Cl 2p1/2 respectively[45]. The BE of Au 4f7/2 transitions in (Fig. 5d) at 84 eV, suggesting the coexistence of Au(0) in AgCl@Pd/Au bimetallic NPs. The third atom peak of Pd (0) is confirmed at a binding energy of 533.64 of 3p3/2, respectively (see Fig. 5e).
3.7 Zeta potential and DLS
A comparison graph of the cubic AgCl@Pd/Au bimetallic NPs hydrodynamic size distribution by DLS is displayed in Fig. 6a, respectively. According to the results, AgCl@Pd/Au bimetallic NPs are in the average size range of 446.63 nm. However, based on the positioning and shape of independent peaks, it is clear that the measured mean diameter of AgCl@Pd/Au bimetallic NPs is marginally larger compared to the one calculated by the FE-SEM[46]. It was also demonstrated that the AgCl@Pd/Au bimetallic NPs are not agglomerated, as the size distribution was identical to that obtained. On the other hand, Fig. 6b shows the zeta potential values of -18.8 mV along with 0.018 mS*cm− 1 conductivity, demonstrating that the AgCl@Pd/Au bimetallic NPs are of excellent quality and have considerably greater stability[47].
3.8 Reduction of 4-Nitrophenol (4-NP)
The catalytic study of 4-NP is as follows: The readily prepared 4-NP is a green yellow solution that has been transformed into a colorless solution by giving a sharp absorbance peak at 400 nm. This means that nitrophenolate ions are formed by the addition of NaBH4 solution. A new absorption peak is obtained because of the nitro (-NO2) group at 300 nm, confirming that the amino (-NH2) group is present in the reaction, making the solution colorless. After addition of AgCl@Pd/Au bimetallic NPs to the solution, the peak intensity decreases at 400 nm within 420 sec, as shown in Fig. 7a. Thus, the peak at 400 nm indicates the reaction mixture has become colorless, giving zero absorbance value, and confirms the complete conversion of 4-NP to 4-AP, with a percentage yield of 93.28% (Fig. 7f). Therefore, plotting the graph of time (min.) versus ln(A0/At) (Fig. 7e), the slope gives the rate constant (k) of 0.00652 sec− 1 1[3, 46].
3.9 Mechanism of 4-Nitrophenol reduction
The 4-NP shows a pale yellow color with a maximum absorption at 317 nm, Due to the extended conjugation of nitrophenolate anions in alkaline media, the solution becomes bright yellow and exhibits a significant red-shift absorption peak at 400 nm in the presence of the newly prepared NaBH4 solution. The intensity of absorption at 400 nm decreases as the reduction progresses in the presence of a catalyst, while a distinct peak at 300 nm rises, which is that of colorless 4-AP. Two isosbestic points can be detected at 311 and 280 nm, indicating a clean conversion and the generation of a single product. To easily observe the reaction, a UV-vis spectrophotometer can be used[48].
The reduction of 4-NP to 4-AP occurs through hydrogenation, and NaBH4 is the source of hydrogen[49]. Ionization of H+ from water is contributed by NaBH4. The reduction reaction takes two steps: the first is the adsorption of a hydrogen atom, and the second is the removal of a water molecule. A strong repulsive force is present between BH4− and 4-NP, which makes the reduction process difficult[50]. Because of the repulsion between the BH4− and 4-NP, the solution of NaBH4 and 4-NP is stable[51]. Only NaBH4 makes this solution stable. NaBH4 plays an important role in this whole reduction process. After adding NaBH4 to water, BH4− ions are formed, and in the case of AgCl@Pd/Au bimetallic NPs, hydrogen transfers from BH4− to 4-NP[52]. Depending on the surface property of the catalyst, different hydrogenation catalytic reactions occur. Electron transfers from BH4− to 4-NP on the metal NPs surface, which acts as a catalyst[52]. Nanocatalysts (AgCl@Pd/Au bimetallic NPs) overcome the kinetic barrier formed by BH4− and 4-NP, by giving electrons. The formation of an intermediate 4-nitrophenolate ion occurs, which promotes the reduction[50].
The O-atom of Nitro group in the 4-NP adsorbs one free hydrogen atom. Then another H-atom combines the cluster. This creates a dihydroxyl-like structure. Next, 4-NP is formed by forming and removing one water molecule of hydroxyl dehydration. Then, at last, 4-AP is formed[49].
3.10 Reduction of various nitroarene
In this study, apart from the reduction of 4-NP, another three-nitro group containing compounds catalytic efficacy and organic transformation reaction were investigated using prepared AgCl@Pd/Au bimetallic NPs. Under the same conditions, the reduction of various nitroarenes, like 4-NA, 3-NA, and 2-NA, was investigated by utilizing Ag/Pd/Au (AgCl@Pd/Au bimetallic alloy) NPs as a nano-catalyst. For 4-NA, 3-NA and 2-NA, the wavelengths (Fig. 7b-d) at which containing the nitro-group exhibit their strongest absorption (UV-Vis) peaks at 382, 361 and 411 nm, respectively. Meanwhile, the peak intensity of the conversion of 4-NA to 4-aminoaniline (4-AA), 2-NA to 2-aminoaniline (2-AA), and 3-NA to 3-aminoaniline (3-AA) decreases as the reaction progresses, indicating that the conversion will take 330, 150 and 330 sec., respectively. The linear relationship is observed (Fig. 7e) in the graph of ln (A0/At) with time (min). As per the catalytic reduction reaction of nitroarenes, the first-order kinetic model along with the rate constant ‘k’ have values of 0.00729, 0.02104, and 0.00753 sec− 1. Approximately 88.32, 78.74, and 90.75% of the reactant is converted into product during the aforesaid time, as shown in Fig. 7f respectively. Finally, at the surface of the nanocatalyst, the reduction reaction of aromatic chemicals with the nitro-group takes place and the mechanism is that NaBH4, is attached to the active nanocatalyst surface[53]. The comparative study of nitroarenes reduction using different nanocatalysts and nitroarenes, reducing agents, molar ratios and rate constants is given in Table 1. Moreover, a comparison of different parameters such as nitroarenes and reducing agents molar ratios, volume, concentration, amount of nanocatalyst and their rate constants were taken into consideration. As more exposed Pd/Au atoms decorated on the surface and a higher surface-to-volume ratio are features of the smaller size, these exposed atoms might potentially serve as catalytic sites. The overall comparative catalytic efficiency is 2–10 fold higher.
Table 1
Comparative study of various nanocatalyst, concentration of nitroarenes, reducing agent, molar ratio and rate constant.
Sr. No. | Nano-catalyst | Aromatic Nitroarene Chemical | Nitroarene conc. in mM (vol. in ml) | NaBH4 Conc. in mM (vol. in ml) | Catalyst conc. in mg/ml (vol. in µl) | Molar Ratio | Rate constant (sec− 1) | Reference |
1. | Ag/Au/Pd | 4-NA | 0.2 (1.5) | 15 (1.275) | 2.0 mM (75) | 1:63.74 | 12.79 | [63] |
2. | Hollow trimettalic Ag/Au/Pd | 4-NP | 2 (2) | 150 (2) | 5 (1500) | 1:75 | 0. 77 x 10− 3 | [30] |
3. | Pd NP’s | 4-NP | 0.1 (1.5) | 50 (1) | 0.05 (500) | 1:500 | 2.1 x 10− 3 | [64] |
4. | AgCl@ Pd/Au Bimetallic NP’s | 4-NP | 0.15 (1) | 12 (1) | 0.05 (50) | 1:80 | 6.0 x 10− 3 | This work |
2-NA | 0.15 (1) | 12 (1) | 0.05 (50) | 3.2 x 10− 3 |
3-NA | 0.15 (1) | 12 (1) | 0.05 (50) | 2.61 x 10− 3 |
3.11 Dye degradation study for RhB, MO and CR
The reduction process of dye for the substrate NaBH4 i.e., without catalyst (dissociation of NaBH4 to Na + and BH4− ions in an aqueous atmosphere). These BH4− ions subsequently provide the electrons required for dye degradation. The absorption peak for NaBH4 was observed at 554 nm. Even after 44 minutes, the xanthene structure (chromophore) at 554 nm had not diminished, only 93.83% of RhB had been degraded, and total degradation of RhB had not occurred even after a month, indicating that the reaction is kinetically limited[54]. The kinetic catalysis for NaBH4 and rate constant are shown in Fig. 8d. The catalytic degradation of RhB is performed in the presence of the reducing agent NaBH4. The degradation of dye in the presence of NaBH4 occurs slowly. So, an intermediate, i.e., catalyst cubic AgCl@Pd/Au bimetallic NPs, is used for the degradation of dye along with NaBH4. The concentration of the catalyst is 50 µL (0.02 mg/mL) with NaBH4 1.0 mL of 50 mM. The UV-Vis spectrophotometer could be used to analyze the degradation of the aqueous dye solution, where the absorption peak was observed at 554 nm with a time interval. The reduction process for the dye to form pink to the colorless solution took about 3 minutes. The rate constant for the catalytic degradation of RhB is 0.0143 sec− 1. The reaction follows the Langmuir-Hinshelwood model with pseudo-first-order kinetics. The graph for the kinetics with rate constant is shown in Fig. 8d. The graph of UV-vis spectra and degradation of RhB dye without catalyst (Figures S6 and S7) and for the MO and CR dyes in the presence of a catalyst is shown in Fig. 8b-c. Consequently, studies were carried out with AgCl@Pd/Au bimetallic NPs to confirm the role of these particles as nano-catalysts for the degradation of MO and CR in the presence of NaBH4[55]. However, in the case without a catalyst, there was no deterioration seen in Figures S8 and S9. According to this, MO, and CR dyes cannot be degraded by NaBH4 on their own[56]. In order to reduce the MO and CR dyes, AgCl@Pd/Au bimetallic NPs effectively relayed the electrons from NaBH4 (donors) to the MO, and CR dyes (acceptors). The RhB, MO and CR dyes (without nanocatalyst) were not degraded even after 42, 72 and 88 min, respectively. The findings show that the dyes were rapidly reduced after adding cubic AgCl@Pd/Au bimetallic NPs and that the times required for MO and CR to become colorless are 300 and 420 sec, respectively. The reactions correspond to first-order kinetics, as seen (in Fig. 8d) by the strong linear connection between time and ln (Ao/At) at the maximal absorption wavelengths of MO and CR. The rate constants obtained are 0.0676 and 0.05 min− 1 for MO and CR, accordingly[57]. According to the literature, operational factors including dye concentration, reducing agent, catalyst dosage, molar ratio and rate constant appear to have an impact on how quickly dye degrades. Table 2 summarizes some significant research on the degradation of RhB, MO, and CR dyes when compared to other nanocatalysts, concentration of dyes, reducing agent, molar ratio and rate constant.
Table 2
Comparative study of various nanocatalyst, concentration of dyes, reducing agent, molar ratio and rate constant.
Sr. No. | Nano-catalyst | Aromatic Nitroarene Chemical | Dye conc. in mM (vol. in ml) | NaBH4 Conc. in mM (vol. in ml) | Catalyst conc. in mg/ml (vol. in µl) | Molar Ratio | Rate constant (sec− 1) | Reference |
1. | Ag-AgCl | RhB | 1 (5) | 26 (5) | 2 (10) | 1:26 | 0.0017 | [56] |
2. | Ag-AgCl | MO | 1.5 (5) | 26 (5) | 2 (10) | 1:17.3 | 0.0025 | [56] |
3. | LSR/LSA Ag NP’s | CR | 9.76 (5) | 1 (1.5) | 0.1 (1) | 1:32.53 | 0.0019 | [65] |
4. | AgCl@ Pd/Au Bimetallic NP’s | RhB | 20 (1) | 50 (1) | 0.02 (50) | 1:2.5 | 0.0143 | This work |
MO | 30 (1) | 50 (1) | 0.02 (50) | 1:1.66 | 0.00676 |
CR | 14 (1) | 50 (1) | 0.02 (50) | 1:3.57 | 0.005 |
Table 3
Effect of AgCl@ Pd/Au Bimetallic NP’s on growth of bacterial species.
Sr.no. | Concentration of AgCl@ Pd/Au Bimetallic NP’s in mg/mL | Zone of inhibition for E. coli | Zone of inhibition for S. aureus |
1 | 0.5 | 15 | 11 |
2 | 0.25 | 12 | 09 |
3.12 Possible mechanism for dye degradation
The redox potential of the reducing agent NaBH4 i.e., electron donor, and the dyes RhB, MO and CR, i.e., electron acceptor, contains the redox potential of AgCl@Pd/Au bimetallic NPs in between them. The degradation of dye using NaBH4 as a reducing agent with AgCl@Pd/Au bimetallic NPs contains two steps: in the first step of the redox reaction, NaBH4, which is an electron donor, gets split into Na+ and BH4− ions, and the electron donated by the BH4− gets absorbed onto the surface of the AgCl@Pd/Au bimetallic NPs. The charge formation and free radicals from oxygen and water are caused by surface trapping. The formation of various free radicals like hydroperoxyl radicals (˙HO2), anionic superoxide (˙O2) and hydroxyl (˙OH) is revealed. When NaBH4 is added to an aqueous solution, it promptly dissociates, lowering its reductive efficacy. Since BH4− is extremely nucleophilic, it has a huge ability to provide electrons that are necessary for dye degradation processes. In the second step, surface reactivity, accompanied by dye adsorption onto AgCl@Pd/Au bimetallic NP surfaces and free radical degradation of dyes. Because dye molecules are electrophilic, they quickly react with free radicals[46]. As a result, the chromophore structures of RhB dye degrade to generate simple molecules (such as CO2 and H2O). A significant peak of intensity 462 nm was seen after the AgCl@Pd/Au bimetallic NPs were added. This suggests that MO molecules were attracted to the AgCl@Pd/Au bimetallic NPs and that the azo bond, the chromophoric group, may have been broken, potentially discoloring the solution. This suggests that AgCl@Pd/Au bimetallic NPs catalyze the hydrogenation process of –N = N- and = C = N- groups and eliminate the dyes' substantial conjugated structure[56].
The maximum absorbance for MO was obtained at 465 nm. A new peak at 252 nm that manifests as an increase in intensity over the course of the reaction indicates the formation of aromatic products and the -NH2 group. This implies that the reduction of the azo bond to two or more potential chemical components with amines (NH2) resulted in the mixture's successful degradation of the MO. The appearance of a peak shows that, rather than just an adsorption process, the MO’s catalytic degradation was caused by dye degradation from the solution. Dimethyl-4-phenylenediamine and sulfanilic acid are the two products that are being suggested. The mechanism for MO degradation using AgCl@Pd/Au bimetallic NPs can be proposed as follows, based on the findings discussed above: MO was initially adsorbed on the surface of the NPs before the catalyst was introduced. The AgCl NPs and the other two compounds (Au and Pd) then worked as a nanocatalyst to break the N = N (double) bond and produce single phenyl ring compounds with amine groups as potential byproducts, which resulted in the decolorization of the MO dye[58]. The maximum absorbance peak for CR dye at 498 nm is diminished, followed by the formation of two new peaks at 252 and 285 nm, which are aromatic rings and the -NH2 group, respectively[59].
3.13 Antimicrobial Activity of AgCl@Pd/Au bimetallic NPs
The zone of inhibition, measured in mm, is shown in Table 1. A higher inhibition zone is observed for E. coli as compared to S. aureus. Figure S10 shows photographs revealing the antimicrobial activity of the synthesized nanomaterials. This could be due to a difference in cell wall thickness. Literature reveals[60, 61] E. coli has a thin peptidoglycan layer, having a thickness of 5 to 10 nm, while S. aureus has a dense, homogeneous peptidoglycan layer of 20 to 40 nm[62]. Furthermore, the concentration of NPs, the agar concentration and the sonication time affect the inhibition zone. No antifungal activity was observed for the NPs.
3.14 FRAP Activity of AgCl@Pd/Au bimetallic NPs
The magnitude of the reduction in absorbance is displayed in Fig. 9. In the case of ascorbic acid, the absorbance went from 0.333 (5 µg/mL) to 2.101 (50 µg/mL), showing a significant increase. AgCl@Pd/Au bimetallic NPs demonstrated the same patterns of behavior. The absorbance of the bimetallic NPs made of AgCl@Pd/Au changed from 0.151667 (5 µg/mL) to 0.708 (50 µg/mL) as a result of the experiment. The NPs that were produced have activity for ferric reduction similar to that of ascorbic acid.