Green-Routed Carbon Dot-Adorned Silver Nanoparticles for the Catalytic Degradation of Organic Dyes

: Herein, a simple, cost-effective, and in-situ environmentally friendly approach was adopted to synthesize carbon dot-adorned silver nanoparticles (CDs@AgNPs) from yellow myrobalan ( Terminalia chebula ) fruit using a hydrothermal treatment without any additional reducing and or stabilizing agents. The as-synthesized CDs@AgNP composite was systematically characterized using multiple analytical techniques: FESEM, TEM, XRD, Raman, ATR-FTIR, XPS, and UV-vis spectroscopy. All the results of the characterization techniques strongly support the idea that the CDs were successfully made to adorn the AgNPs. This effectively synthesized CDs@AgNP composite was applied as a catalyst for the degradation of organic dyes, including methylene blue (MB) and methyl orange (MO). The degradation results revealed that CDs@AgNPs exhibit a superior catalytic activity in the degradation of MB and MO in the presence of NaBH 4 (SB) under ambient temperatures. In total, 99.5 and 99.0% rates of degradation of MB and MO were observed using CDs@AgNP composite with SB, respectively. A plausible mechanism for the reductive degradation of MB and MO is discussed in detail. Moreover, the CDs@AgNP composite has great potential for wastewater treatment applications.


Introduction
Today, nanomaterials with different architecture and supports play key roles in industrial catalysis. They provide exciting opportunities for dealing with various chemical transformations and environmental challenges [1][2][3][4][5]. The catalytic processes are mainly classified into three categories, including homogeneous, heterogeneous, and enzyme/biocatalysis, which are based on the phase nature of the catalyst [6,7]. Among these, the heterogeneous catalytic process is industrially and environmentally important due to its application in biologically important chemical transformations and environmental remedies such as dye degradation, pesticide removal, and photocatalytic dye degradation [8][9][10][11]. Generally, heterogeneous catalysts are solid materials that increase the rate of a chemical reaction and remain unchanged after the completion of the reaction. The unchanged catalysts can be quickly recovered and recycled. Thus, dye degradation with the help of heterogeneous catalysts is an interesting topic among environmental scientists. Based on reports, noble metal nanoparticles, such as gold, silver, and palladium nanoparticles, show excellent catalytic activities in the reduction/degradation of toxic dyes by aqueous sodium borohydride The structural and chemical compositions of the prepared CDs@AgNP composite were characterized by various techniques, such as XRD, Raman, FESEM, TEM, ATR-FTIR, and XPS. The optical properties of the CDs@AgNPs were studied using the UV-vis spectrum. The absorption spectra of the fruit extract and CDs@AgNPs are shown in Figure S1. After the hydrothermal process, the color of the resultant solution changed from pale yellow to brownish yellow, which indicated the formation of the CDs@AgNP composite [27]. The photographic images of the TCE and CDs@AgNP aqueous solutions are displayed in the inset of Figure S1. The TCE exhibited a base peak at 280 nm respective to the π-π* electronic transition of the carbonyl phytoconstituents (tannin) of Terminalia chebula. The CDs@AgNP composite showed a sharp and clear surface plasmon resonance absorbance peak at 425 nm and a shoulder peak at 380 nm. A peak at 425 nm confirmed the formation of the AgNPs [28,29]. Further, the TCE peak intensity decreased and slightly shifted towards a higher wavelength, rising to 320 nm from 280 nm in the CDs@AgNPs. This might be due to the carbonization and complete reduction of the tannins present in the extract. These results revealed the presence of C=C and C=O bonds in the prepared CDs@AgNPs.
The crystal structure of the CDs@AgNPs was characterized by XRD. As shown in Figure 1a, the XRD pattern exhibited four sharp peaks, with the 2θ values of 38.1, 44.2, 64.4, and 76.4 • , which are related to the (111), (200), (220), and (311) planes of the Ag-NPs [30]. This XRD results confirmed the presence of AgNPs in the prepared CDs@AgNPs. However, patterns responsible for the CDs were missing. This could be because of the high intensity of the patterns that correspond to the AgNPs, but the observation of CD patterns was not possible. Further, the CDs@AgNPs were characterized using Raman spectroscopy ( Figure 1b). In contrast to the XRD results, Raman studies revealed the presence of CDs and AgNPs in the CDs@AgNPs. The Raman spectrum of the CDs@AgNPs showed four predominant peaks. The peaks at 1350, 1575, and 2800 cm −1 are contributed by the D, G, and 2D bands of CDs, being typical peaks of a graphitic carbon structure [31,32]. The peaks between 350 and 750 cm −1 are responsible for the AgNPs.
high intensity of the patterns that correspond to the AgNPs, but the observation of CD patterns was not possible. Further, the CDs@AgNPs were characterized using Raman spectroscopy (Figure 1b). In contrast to the XRD results, Raman studies revealed the presence of CDs and AgNPs in the CDs@AgNPs. The Raman spectrum of the CDs@AgNPs showed four predominant peaks. The peaks at 1350, 1575, and 2800 cm −1 are contributed by the D, G, and 2D bands of CDs, being typical peaks of a graphitic carbon structure [31,32]. The peaks between 350 and 750 cm −1 are responsible for the AgNPs.  Figure 2 shows the FESEM images of the prepared CDs@AgNPs with different magnifications. The irregular morphologies of the CDs@AgNPs in the hierarchical process can be observed (Figure 2a-d). This is because of the very small size of the CDs and AgNPs. However, the elements present in CDs@AgNPs were demonstrated using elemental mapping images (Figure 2e-i). This directly indicated that the carbon (C), oxygen (O), nitrogen (N), and silver (Ag) were homogeneously distributed in the CDs@AgNPs. Furthermore, the uniform distribution of C, O, N, and Ag elements is revealed by Figure 2i. The C, O, and N elements in the CDs@AgNPs derive from the plant source, and the element Ag is derived from the AgNO3. Moreover, the EDAX spectrum in Figure S2 revealed the presence of the elements C, N, O, and Ag in CDs@AgNPs, along with the platinum and silicon signals that were used for the sputter coating and crystalline silicon wafer substrate, respectively.   Figure 2i. The C, O, and N elements in the CDs@AgNPs derive from the plant source, and the element Ag is derived from the AgNO 3 . Moreover, the EDAX spectrum in Figure S2 revealed the presence of the elements C, N, O, and Ag in CDs@AgNPs, along with the platinum and silicon signals that were used for the sputter coating and crystalline silicon wafer substrate, respectively.
Consistent with the FESEM results, the TEM/HRTEM observation of the CDs@AgNPs, as indicated by Figure 3, led to the discovery of the irregular morphology of the CDs. The presence of CDs on the AgNP surface is evident from Figure 3a-e. The HRTEM showed that the carbon lattice was enclosed on the AgNPs. The size distribution histogram of the AgNPs (inset Figure 3c) was obtained by measuring the sizes of 100 randomly selected nanoparticles from the TEM image ( Figure 3b). The average AgNP size in the CDs@AgNP composite was about 20 nm. Furthermore, the HETRM images revealed the spherically shaped AgNPs, and the particle size was calculated at about 20 nm and was relatively uniform. The AgNPs had a fine fringe pattern, with a spacing of 0.24 nm, corresponding to the Ag (111) lattice fringes (Figure 3e). The HRTEM image also showed visible crystalline lattice fringes, with bright spots in the selected area electron diffraction (SAED) pattern. This SAED in Figure 3f indicates the crystalline nature of the CDs@AgNPs.
Furthermore, an ATR-FTIR study was performed to identify the functional groups that were present in the CDs@AgNPs ( Figure S3). The spectra of the TCE showed a broad peak centered at 3250 cm −1 , revealing the presence of O-H/N-H bonds. The peaks at 2850 and 2920 cm −1 occurred due to the C-H antisymmetric and symmetric bonds, respectively. The intense peaks at 1620 and 1050 cm −1 are responsible for the stretching vibrations of the C=O/C=C and C-O-C bonds, respectively. For the CDs@AgNPs, the broad peaks between 3500 and 3100 cm −1 are responsible for the O-H/N-H bonds [33]. The C-H stretching vibration was observed at 2870 cm −1 from the graphitic structure of the CDs@AgNPs [34]. A strong peak at 1650 cm −1 was attributed to the C=O/C-C vibrations. The C-N and O-H/N-H/C-OH bonds can be observed at 1430 and 1314 cm −1 , respectively [35]. The peaks at 1050 and 826 cm −1 are responsible for the C-O-C and -CH 2 groups in the CDs@AgNPs, respectively [36]. The presence of the AgNPs confirms the peak at 480 cm −1 . These results reveal that the CDs@AgNPs were rich in oxygen and nitrogen functional groups, such as hydroxyl, carbonyl, carboxylic acid, amide, and amino groups. These nitrogen functional groups could serve as stabilizing agents for the AgNPs.  XPS measurements were carried out to investigate the chemical compositions and elemental states of the CDs@AgNPs. As shown in Figure S4, the survey spectrum disclosed four peaks: C 1 s at 285 eV, Ag 3d at 368 eV, N 1s at 400 eV, and O 1s at 533 eV. Thus, the prepared CDs@AgNPs were composed of the elements C, O, Ag, and N, with values of 60, 27, 7, and 6% (inset of Figure S4). The high-resolution spectrum of C 1s (Figure 4a) resulted in four peaks at 284.8, 286.0, 287.2, and 288.9 eV, which can be assigned to C=C/C-C, C-N/C-O, C=O/C=N, and HO-C=O, respectively [37]. The C=C/C-C peak corresponds to the presence of sp 2 graphitic/sp 3 hybrid structures in the CDs@AgNPs [38]. The reduction process of the AgNO 3 , resulting in the formation of AgNPs, was confirmed by the existence of a peak at the Ag 3d level [39]. With respect to the deconvolution of the Ag 3d level results, two peaks at 368.1 and 374.2 eV can be attributed to the 3d 5/2 and 3d 3/2 , respectively ( Figure 4b). This indicated the presence of metallic Ag in the CDs@AgNPs [27,39]. Figure 4c shows the deconvolution spectrum of the N 1s level that resulted in three peaks at 399.0, 400.0, and 401.6 eV, which can be assigned to C-N-C, C-N-H, and (C) 3 [40]. The XPS showed results consistent with those of the ATR-FTIR, and, thus, the overall results of the XPS and ATR-FTIR validate and confirm that the CDs@AgNPs consisted of many functional groups, such as carboxyl, hydroxyl, amide, amino, and carbonyl groups.  Furthermore, an ATR-FTIR study was performed to identify the functional groups that were present in the CDs@AgNPs ( Figure S3). The spectra of the TCE showed a broad peak centered at 3250 cm −1 , revealing the presence of O-H/N-H bonds. The peaks at 2850 and 2920 cm −1 occurred due to the C-H antisymmetric and symmetric bonds, respectively. The intense peaks at 1620 and 1050 cm −1 are responsible for the stretching vibrations of the C=O/C=C and C-O-C bonds, respectively. For the CDs@AgNPs, the broad peaks between 3500 and 3100 cm −1 are responsible for the O-H/N-H bonds [33]. The C-H stretching vibration was observed at 2870 cm −1 from the graphitic structure of the CDs@AgNPs [34]. A strong peak at 1650 cm −1 was attributed to the C=O/C-C vibrations. The C-N and O-H/N-H/C-OH bonds can be observed at 1430 and 1314 cm −1 , respectively [35]. The peaks at 1050 and 826 cm −1 are responsible for the C-O-C and -CH2 groups in the CDs@AgNPs, respectively [36]. The presence of the AgNPs confirms the peak at 480 cm −1 . These results reveal that the CDs@AgNPs were rich in oxygen and nitrogen functional groups, such as hydroxyl, carbonyl, carboxylic acid, amide, and amino groups. These nitrogen functional groups could serve as stabilizing agents for the AgNPs.
XPS measurements were carried out to investigate the chemical compositions and elemental states of the CDs@AgNPs. As shown in Figure S4, the survey spectrum disclosed four peaks: C 1s at 285 eV, Ag 3d at 368 eV, N 1s at 400 eV, and O 1s at 533 eV. Thus, the prepared CDs@AgNPs were composed of the elements C, O, Ag, and N, with values of 60, 27, 7, and 6% (inset of Figure S4). The high-resolution spectrum of C 1s (Figure 4a) resulted in four peaks at 284.8, 286.0, 287.2, and 288.9 eV, which can be assigned to C=C/C-C, C-N/C-O, C=O/C=N, and HO-C=O, respectively [37]. The C=C/C-C peak corresponds to the presence of sp 2 graphitic/sp 3 hybrid structures in the CDs@AgNPs [38]. The reduction process of the AgNO3, resulting in the formation of AgNPs, was confirmed by the existence of a peak at the Ag 3d level [39]. With respect to

Catalytic Activity of the Synthesized CDs@AgNPs Composite towards the Degradation of Organic Dyes
The catalytic performance of the CDs@AgNP composite was studied in terms of the reductive degradation of MB and MO using aqueous SB solution. The catalytic activity of the CDs@AgNP composite was monitored through visual observation and also by using UV-vis spectroscopy. In order to confirm the catalytic ability of the CDs@AgNP

Catalytic Activity of the Synthesized CDs@AgNPs Composite towards the Degradation of Organic Dyes
The catalytic performance of the CDs@AgNP composite was studied in terms of the reductive degradation of MB and MO using aqueous SB solution. The catalytic activity of the CDs@AgNP composite was monitored through visual observation and also by using UV-vis spectroscopy. In order to confirm the catalytic ability of the CDs@AgNP composite, as a control experiment, degradations of MB and MO without the CDs@AgNP composite were conducted. Figures S5-S8 show the UV-vis absorbance spectra of the MB/MO in the presence of SB (and in the absence of CDs@AgNPs) and in the presence of CDs@AgNPs (and in the absence of SB), respectively. In these control experiments, insignificant changes in the characteristic absorbance peaks for MB and MO, centered at 665 and 465 nm, were observed, respectively. This suggests that the degradation of MO and MB does not only occur when using a catalyst (the CDs@AgNP composite) or the reductive agent SB.
Conversely, after mixing SB and CDs@AgNPs and their application to MB/MO, the reductive degradation reaction commenced quickly with the evolution of the hydrogen gas, which was identified from the color degradation and decrement in the absorbance intensity, as exhibited by the respective time-dependent UV-vis spectra in Figure 5a,b. This result confirmed the catalytic activity of the CDs@AgNPs in the degradation of MB and MO [41][42][43]. Furthermore, the level of degradation (%) and rate of reaction as per the pseudo-first-order kinetic model for both MB and MO were calculated. The achieved degradation levels for the MB and MO dyes were about 99.5 and 99.0%, respectively (Figure 5c). Figure 5d shows the degradation time vs. ln (A t /A 0 ) kinetic graphs of MB and MO. It is noticeable that the kinetic graphs of MB and MO yielded linear straight lines with rate constants (slope values) of 0.204 and 0.128 min −1 , and the respective linearity coefficient (R 2 ) values were about 0.985 and 0.986, respectively. This result suggests that the degradation of MB/MO by the CDs@AgNP composite followed pseudo-first-order kinetics [44,45]. These catalytic reduction reactions can be explained using the Langmuir-Hinshelwood model. In this model, SB acts as an electron donor and hydrogen provider. After the addition of SB to the reaction, the pH of the solution changed to basic, and hence the surface charge of CDs@AgNP composite was changed to positive. Thus, the MB/MO and BH 4 − ions were concurrently adsorbed on the surface of the CDs@AgNP composite. Furthermore, the reduction reaction was initiated by relaying electrons from the donor BH 4 − ions acting as electron transfer mediators to the adsorbed MB/MO molecules through the CDs@AgNP composite. This was evident from the strong effervescence of the hydrogen gas that evolved, and, subsequently, this action led to the reductive degradation of MB and MO [46,47]. Furthermore, the visual observations of the MB/MO degradation in the presence of the CDs@AgNP composites with SB are shown in Figure 6. One can note the decoloring of the MB/MO aqueous solution by increasing the reaction time. The decoloring starts within 1 min, and complete decoloring (degradation product-DP) of the MB and MO was observed within 15 and 20 min, respectively. These photographic images confirm the effectiveness of the CDs@AgNP composite as a catalyst for the degradation of MB/MO. In order to ensure the greater activity of the prepared CDs@AgNP composite, the results of the present work were compared with the recent literature studies on MB/MO degradation by AgNPs-related composites, and the corresponding results are illustrated in Table 1. The CDs@AgNP composite achieved the degradation efficiency rates of 99.5 and 99.0% at the reaction times of 15 and 20 min, with the rate constants of 0.204 and 0.128 min −1 for the MB and MO, respectively, results which are significantly comparable with those of the performances of previously reported Ag-based catalysts [17,44,[48][49][50][51][52][53]. This suggests that the CDs@AgNP composite exerted a high catalytic activity on the MB and MO in the aqueous medium under atmospheric conditions. Furthermore, the FESEM images were captured and EDAX elemental mapping analyses of CDs@AgNP composite were performed after the degradation of the organic dyes, and the corresponding results are shown in Figure S9. The FESEM image of the CDs@AgNP composite revealed the spherical shape of the AgNPs, and one can see that the particles were relatively uniform in shape, with minute variation in size ( Figure S9a). The elemental composition of the CDs@AgNP composite was examined by elemental mapping. The elemental mapping results of the CDs@AgNP composite confirmed that the concentrations of the different elements, including C, O, N, and Ag, were uniform in the composite ( Figure S9b-e). Furthermore, the uniform distribution of the C, O, N, and Ag elements was revealed by overlapping elemental mapping, as shown in Figure S9f. Hence, we could observe an insignificant amount of AgNPs leaching from the CDs@AgNP composite after the degradation of the organic dyes.   Furthermore, the FESEM images were captured and EDAX elemental mapping analyses of CDs@AgNP composite were performed after the degradation of the organic dyes, and the corresponding results are shown in Figure S9. The FESEM image of the CDs@AgNP composite revealed the spherical shape of the AgNPs, and one can see that the particles were relatively uniform in shape, with minute variation in size ( Figure S9a). The elemental composition of the CDs@AgNP composite was examined by elemental mapping. The elemental mapping results of the CDs@AgNP composite confirmed that the concentrations of the different elements, including C, O, N, and Ag, were uniform in the composite ( Figure S9b-e). Furthermore, the uniform distribution of the C, O, N, and Ag elements was revealed by overlapping elemental mapping, as shown in Figure S9f. Hence, we could observe an insignificant amount of AgNPs leaching from the CDs@AgNP composite after the degradation of the organic dyes.

Synthesis of the CDs@AgNP Composite
Terminalia chebula fruits were dried in sunlight followed crushing into a powder using a blender. To every 1 g of powder, 100 mL of distilled water was added and heated at 70 °C overnight. The mixture was cooled down and filtered using filter paper. In an

Synthesis of the CDs@AgNP Composite
Terminalia chebula fruits were dried in sunlight followed crushing into a powder using a blender. To every 1 g of powder, 100 mL of distilled water was added and heated at 70 • C overnight. The mixture was cooled down and filtered using filter paper. In an autoclave reactor, 5 mL of the filtered mixture and 50 mL of 0.01 M AgNO 3 were taken and reacted at 200 • C for 24 h. After the reaction time, the mixture was cooled down and dried using a freeze-dryer. The dried CDs@AgNP composite was utilized for the characterization using X-ray diffractometry (XRD), Raman and field emission scanning electron microscopy (FESEM) together with energy-dispersive X-ray (EDX) spectroscopy, high-resolution transmission electron microscopy (HRTEM), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). For the degradation analyses, the mixture was used as it is before drying. Scheme 1 illustrates the formation of the CDs@AgNP composite from the Terminalia chebula fruits extract (TCE) and AgNO 3 aqueous solution by a simple one-pot hydrothermal method. Scheme 1. Formation of the CDs@AgNP composite from the Terminalia chebula fruit and silver nitrate via the one-step hydrothermal method.

Conclusions
In summary, a simple and effective CDs@AgNP catalyst was hydrothermally developed via a simple one-pot route as a catalyst for the reductive degradation of organic dyes, including MB and MO. From the ATR-FTIR spectra, we discovered that the plant extract could reduce and stabilize to form AgNPs, and the strong UV-vis absorbance of the final product confirmed the formation of the AgNPs. Excellent crystallinity and spherical-shaped morphology, with a lattice distance of 0.24 nm, were observed from the XRD, FESEM, and TEM/HRTEM analyses. The EDAX and XPS results demonstrated the presence of C, N, O, and Ag constituents in the synthesized CDs@AgNP composite. The XPS results revealed the characteristic peak of the Ag and the surface functionalities of oxygen-and nitrogen-presented moieties from the TCE. Furthermore, the ATR-FTIR and XPS revealed functional groups, such as hydroxyl, carbonyl, carboxyl, amide, and amino groups, in the CDs@AgNPs. The reductive catalytic activities of the CDs@AgNP composite in the presence of SB against degradation of MB and MO under ambient conditions were measured as 99.5 and 99.0% within 15 and 20 min, respectively. This degradation followed pseudo-first-order kinetics. The kinetic graphs of MB and MO dye yielded linear straight lines, showing rate constants of 0.204 and 0.128 min −1 , with R 2 values of 0.985 and 0.986, respectively. Thus, the present study suggested that the TCE-assisted CDs@AgNP composite has potential as a reductive catalysts for the degradation of various organic dyes and could act as a nano-catalyst in other applications.
Supplementary Materials: The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal12090937/s1, Materials, instrumentation methods, and catalytic degradation measurement of the synthesized CDs@AgNPs, Figure S1. UV-vis spectra of the Terminalia chebula fruit extract and synthesized CDs@AgNPs, Figure S2. EDAX spectrum of the synthesized CDs@AgNPs, Figure S3. ATR-FTIR spectra of the Terminalia chebula fruit extract and synthesized CDs@AgNPs, Figure S4. XPS survey spectrum of the synthesized CDs@AgNPs, and the inset is a pie chart of the elemental composition of the synthesized CDs@AgNPs, Figure S5. UVvis absorbance spectra of MB in the presence of SB at different reaction times, Figure S6. UV-vis absorbance spectra of MB in the presence of CDs@AgNPs at different reaction times, Figure S7. UV-vis absorbance spectra of MO in the presence of SB at different reaction times, Figure S8. UV-vis absorbance spectra of MO in the presence of CDs@AgNPs at different reaction times, Figure S9.