Synergistic Effects of Carbon Dots and Palladium Nanoparticles Enhance the Sonocatalytic Performance for Rhodamine B Degradation in the Absence of Light.

Carbon dot (CD) and palladium nanoparticle (Pd NP) composites are semiconducting materials having tremendous applications in catalysis with suitable band gaps. However, their combination with a suitable polymer matrix in sonophotocatalysis has not been explored. Herein, we have synthesized and characterized a new nanohybrid catalyst from a polyamide cross-linked CD-polymer and subsequent deposition of Pd NPs. A sonocatalytic activity of 99% rhodamine B dye degradation was achieved in mere 5 min in the dark. A model catalyst replacing CDs with benzene and other control studies revealed that the synergistic effects of CDs and Pd NPs enhance the sonocatalytic activity of the nanohybrid catalyst. Interestingly, visible light did not influence the activity significantly. Mechanistic investigations suggest that generation of reactive oxygen species on the surface of the CD-polymer initiated by ultrasound, which is further facilitated by Pd NPs, is the key for remarkable catalytic activity (a rate constant of 0.99 min-1). Recyclable heterogeneous catalysts under ambient conditions are promising for exploring sono-assisted dark catalysis for several avenues.

Introduction

Water pollution by industrial dye effluents has seriously affected the aquatic ecosystem and the human population.[1] Synthetic dyes are widely used in the textile industry and their disposal poses a serious concern for carcinogenic and teratogenic problems because of their chemical stability.[2] Tremendous efforts have been made during the past several decades to reduce their deleterious impacts, including physical adsorption using activated carbon,[3] clays,[4] biological degradation using microorganisms,[5] and chemical degradation using catalyst-mediated advanced oxidation processes.[6] However, the obvious drawbacks of these methods, such as low efficiency, high cost, and/or short service life, continuously drive the exploration of advanced materials for the degradation of synthetic dyes in polluted water.[7] Various catalytic systems such as amorphous alloys,[8] semiconductor metal heterostructures,[9] polymer-based nanocomposites,[10] graphene oxide (GO),[11] carbon quantum dots,[12] amorphous thin ribbons,[13] nanopowders, and nonporous structures[14] have been shown to exhibit relatively satisfactory performance in wastewater remediation for removing synthetic dyes and other organic pollutants.

A semiconductor photocatalyst generates redox centers at the conduction band (CB) and valence band (VB) upon irradiation of light to generate the reactive oxygen species (ROS) such as a hydroxyl radical and superoxide which eventually take part in the degradation process of the organic pollutants.[15] Krishnan et al. have reported the synergistic effect of MoS2-reduced GO doping enhancing the photocatalytic performance of ZnO nanoparticles (NPs) and similarly N-doping onto the ZnO–MoS2 binary heterojunctions enhancing the catalytic activity under visible light.[16] However, light is the limiting factor as every catalyst responds to the visible and/or UV lights according to its band-gap structure. Hence, fabricating a semiconductor material which can develop ROS even in the dark would be desirable because of energy saving, avoiding restriction of light requirement, and indoor applications or places with scarcity of sunlight.[17] Only few reports were shown to exhibit catalytic activity even in the dark, for example, Suib et al. reported that methyl orange can be fragmented into intermediates using a perovskite-type material (LaNiO3−δ) in the dark under ambient conditions in 4 h time.[18] Hussain et al. showed metal nanocomposite (Ag–In–Ni–S)-catalyzed methylene blue (MB) degradation within 12 min under dark conditions.[19] Rajagopalan et al. have reported the ZnO2/polypyrrole nanocomposite and its synergistic effect in dye degradation of MB in the dark and under light irradiation.[20] He et al. showed polymer-coated TiO2 nanocomposites to catalyze the degradation in the dark because of the intrinsic difference of their disordered atomic packing arrangement compared with the well-defined atomic ordering in crystalline materials.[21] However, there is a need to improve catalytic performance further to see the light of practical applications by lowering the cost of the catalysts and conserving the energy. This can be achieved by increasing the reaction rate and reducing the reaction time with good recyclability (Table S2, Supporting Information).

It has been reported that the combination of sonocatalysis and photocatalysis outperformed either of them alone in catalytic degradation of dyes.[22−26] The combination of semiconductors and ultrasound (US) is considered to be a viable option for sonocatalysis because of safe and economical use of nanomaterials.[24,27−29] It has been reported that the sonocatalytic activity of NPs is dependent on the types of NPs as well as materials with the increasing order of montmorillonite clay < anatase–TiO2 < ZnO < rutile–TiO2 < Fe3O4 for the sonocatalytic degradation of 2-hydroxyethyl cellulose.[30] To the best of our knowledge, so far, only one report mentioned about sonocatalytic dye degradation under dark conditions using CdSe–graphene as the catalyst, but this catalyst is toxic and less reactive.[31]

To date, various photocatalysts have been tried, and among them, palladium NPs (Pd NPs) and light-responding carbon materials have shown promising catalytic activity.[32−36] It has been shown that Pd NPs decorated on a carbon support enhance catalytic activity greatly for clean hydrogen generation.[34] Also, Pd NPs supported by GO were found to degrade dye solutions in the presence of NaBH4 in aqueous solution.[35] However, we hypothesize that the catalytic activity can be further improved by exhuming the synergistic characteristics of these materials with the help of US energy. To achieve this, we have designed the nanohybrid catalyst comprising Pd NPs anchored onto the amido-amine-functionalized carbon dot (CD)–polymer matrix. Herein, we describe the sonocatalytic efficiency of the said catalyst on the dye degradation of rhodamine B (RhB) and demonstrate the existence of the synergistic effect of Pd NPs and CDs mediated by US. An initial version of this article was deposited on ChemRxiv on June 28, 2020.[37]

Results and Discussion

Synthesis and Characterization of Polymers (CD-CONH and BTC-CONH) and Nanohybrid Catalysts (Pd@CD-CONH and Pd@BTC-CONH)

The final Pd-doped hybrid nanocatalysts (Pd@CD-CONH and Pd@BTC-CONH) were synthesized by two steps as shown in Scheme . The first step involves the polymerization of the acid chloride-functionalized monomers of CDs and benzene-1,3,5-tricarboxylic acid (BTC), leading to the formation of the amide-linked polymers CD-CONH and BTC-CONH, respectively, upon reaction with benzene-1,4-diamine (BDA) at room temperature. The acid chloride derivatives were in turn freshly prepared by the treatment of their corresponding acids (CD-COOH and BTC-COOH) with thionyl chloride (SOCl2). CD-COCl and BTC-COCl are highly soluble in polar organic solvents such as tetrahydrofuran (THF), CH3CN, and acetone, which can be easily isolated by evaporation of the excess thionyl chloride from the reaction mixture. The acid chloride derivatives are highly adaptable toward surface modification which does not require complex purification procedures because of their high reactivity and the absence of nonvolatile byproducts. The carboxylic acid-capped CD (CD-COOH) was prepared by following the reported procedure by the thermal decomposition of citric acid.[38] The second step was the formation of Pd NPs onto the amido-amine-functionalized polymers of CD-CONH and BTC-CONH by treatment with PdCl2 under refluxing in EtOH as a solvent to provide the corresponding Pd NP-doped hybrid nanocatalysts Pd@CD-CONH and Pd@BTC-CONH, respectively.

Scheme 1

Synthesis of the Catalyst Pd@CD-CONH and the Model Catalyst Pd@BTC-CONH

The surface modifications of the COOH-functionalized CD and BTC into the CONH-functionalized CD and BTC, followed by Pd NP-impregnated polymers of CD-CONH and BTC-CONH, were characterized by Fourier transform infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM) analysis.

As shown in Figure a,b, from the FT-IR spectra of CD-COOH and BTC-COOH, conversion into the amide bond formation with BDA is clearly evident from the partial disappearance of peaks at 1720 cm–1 (CD-CONH) and 1725 cm–1 (for BTC-CONH) corresponding to C=O stretching frequencies of −COOH and the new peak appearance at 1630 cm–1 (CD-CONH) and 1634 cm–1 (BTC-CONH) for the C=O stretching frequencies of the amide bond. Also, the presence of the free amine-terminated polymer is indicated by the new N–H peak appearance at 3335 cm–1 for CD-CONH and at 3412 and 3336 cm–1 for BTC-CONH.[39] Similarly, the Pd NPs anchored onto the amide N–H (CD-CONH) are indicated by the shift in the stretching frequency from 1630 to 1588 cm–1 (Figure a). Also, it is further evidenced by the peak shift of the free NH2 peak from 3335 to 3400 cm–1. Similar to CD-CONH, BTC-CONH also exhibited a peak shift from 1634 to 1612 cm–1 upon binding with Pd NPs.[40] These observations clearly point out the partial amide formation with BDA, leading to the formation of CD-CONH and BTC-CONH polymers. Further, Pd NPs are supported by the CONH and NH2 functional groups in the catalysts Pd@CD-CONH and Pd@BTC-CONH. These results are also corroborated by 1H NMR investigation (Figures S1 and S6, Supporting Information). The presence of aromatic protons corresponding to the BDA moiety in the CD-CONH spectrum indicates the surface modification of CD-COCl with BDA (Figure S1, Supporting Information). The 1H NMR spectra of BTC-CONH and Pd@BTC-CONH clearly indicate the presence of both the linker and BTC protons, which matches well with a similar polymer reported previously.[41] The 1H NMR spectrum of the BTC-CONH polymer showed peaks for amide protons of the terminal BDA moiety in the upfield region at 10.33 and 10.23 ppm because of the presence of the amine in the para position (Figure S6a, Supporting Information). However, the inner chain amide appeared at around 10.67 and 10.75 ppm. The 1H NMR spectrum (Figure S6b, Supporting Information) of Pd@BTC-CONH showed significant upfield shift values of 0.04 and 0.03 ppm for terminal amide protons upon treatment with Pd. However, inner chain amide protons showed a significant shift of 0.06 ppm for the 10.69 ppm peak, indicating partial binding of amide protons. This suggests that not all amide protons are to be available for metal binding, probably because of the stacking nature of the polymer. In contrast, the CD polymer catalyst showed binding of the inner chain polymer, as indicated by the inner phenyl proton shift (Figure S1). This is also facilitated by the porous nature of the CD polymer, as indicated by the TEM images (Figures S3a and S7a, Supporting Information).

Figure 1

(a) FT-IR spectra of precursors and the nanohybrid catalyst Pd@CD-CONH and (b) model catalyst Pd@BTC-CONH.

After the structural characterization of CD-CONH, we studied the formation of Pd NPs onto CD-CONH. The presence of Pd NPs is evidenced by X-ray diffraction (XRD), FT-IR spectroscopy (vide supra), X-ray photoelectron spectroscopy (XPS), TEM, and Raman studies. We believe that upon addition of Pd2+ ions into the EtOH solution of CD-CONH, the Pd2+ ions interact with the N-atoms of the amido-amine group and get reduced to Pd(0).[42] The powder XRD (PXRD) spectrum of Pd@CD-CONH showed the formation of the crystal face-centered cubic (fcc) lattice of Pd(0) indicated as (100), (111), (200), (220), (311), and (222),[43] suggesting that PdCl2 has been reduced completely to form Pd NPs, as shown in Figure . Similarly, the PXRD spectrum of Pd@BTC-CONH showed the formation of the crystal fcc lattice of Pd(0) indicated as (101), (102), (110), and (203),[43] suggesting that PdCl2 has been reduced completely to form Pd NPs (Figure ).

Figure 2

PXRD spectra of Pd@CD-CONH and Pd@BTC-CONH.

Further, the shape and size of the amido-amine-functionalized CD polymer CD-CONH were determined by high-resolution TEM (HR-TEM) (Figure S3c, Supporting Information). The intralayer spacing of 2.4 Å showed lattice fringes corresponding to the (100) plane. The TEM image of CD-CONH showed the presence of spherical NPs interlinked with each other having an average size of 5.5 ± 2.3 nm with a relatively broad size distribution (Figure S3b, Supporting Information). The PXRD spectrum of CD-CONH showed a diffraction peak centered at 29.8°, which corresponds to a lattice spacing of 3 Å, similar to the (200) reflection (d002 = 3.4 Å) (Figure S2a, Supporting Information).

The TEM image of as-prepared Pd@CD-CONH showed the formation of spherical Pd NPs deposited on the amido-amine polymer (CD-CONH) with an average diameter of ∼3.25 nm (Figure ). Figure b shows the HR-TEM image with an intralayer spacing of 2.3 Å, showing lattice fringes corresponding to the (111) plane. The Pd peak is observed in the energy-dispersive X-ray (EDX) mapping of Pd@CD-CONH along with C, N, and O, indicating that the Pd NPs have been clearly homogeneously distributed throughout the entire CD polymer (Figure ). The atomic force microscopy (AFM) images of Pd@CD-CONH have shown that the spherical-shaped Pd NPs have been successfully deposited over the CD-CONH polymer (Figure S4, Supporting Information). The particle size distribution (PSD) of the as-prepared Pd@CD-CONH as well as the model catalyst Pd@BTC-CONH has shown a similar size distribution of ∼3 nm (Figures and S7).

Figure 3

(a) TEM and (b) HR-TEM images of Pd@CD-CONH and (c) histogram.

Figure 4

(a) TEM images of Pd@CD-CONH, (b) EDX mapping of some areas of the catalyst, (c) carbon, (d) oxygen, (e) nitrogen, and (f) Pd.

We carried out XPS analysis to get insight into the surface electronic structure of as-synthesized Pd NPs. As shown in Figure a, the XPS survey spectrum of Pd@CD-CONH indicates the presence of palladium, nitrogen, and carbon with the corresponding characteristic binding energy (BE) peaks at 335.6, 340.9, 398.95, and 284.4 eV, respectively. There are two chemical states indicated in Pd NPs with a lower BE of the Pd 3d profile. The presence of BEs at 335.6 and 340.9 eV is due to Pd(0), whereas the other higher BE at 336.4 and 342.4 eV is due to the presence of Pd(II) (Figure b).[42,44] Furthermore, high magnification of the N 1s profile in CD-CONH showed a BE of 397.93 eV, indicating the presence of N-functionality in the polymer. However, the N 1s profile in Pd@CD-CONH showed a significant shifting of BE from 397.93 to 398.95 eV (Figure c).[42,45] We infer that this shift is due to the Pd NPs and their interaction with N-functionality. However, there is no significant change in the peak fitting of the C 1s (Figure d). These results indicate that Pd NPs have been successfully impregnated onto CD-CONH.

Figure 5

XPS spectra of (a) common elements, (b) Pd 3d of Pd@CD-CONH, (c) N 1s of CD-CONH and Pd@CD-CONH, and (d) C 1s of CD-CONH and Pd@CD-CONH.

UV–Vis Absorption Study of the Nanohybrid Pd@CD-CONH and Model Pd@BTC-CONH Catalysts

The UV–vis absorption spectra of Pd@CD-CONH and model catalysts in EtOH in comparison with the corresponding monomer and polymeric precursors are shown in Figure . The formation of Pd NPs is clearly indicated by the appearance of the surface plasmon resonance band centered at around 410 nm in the catalyst, which is absent in the polymer precursors. Furthermore, the direct optical band gap (Eg) based on the onset value was found to be 2.43 eV (510 nm), which is significantly lower (0.2 eV) than that of the CD polymer with Pd NPs.[46] Though both the catalysts showed a similar PSD (vide supra), the model catalyst with Pd NPs has shown a high energy band gap of 3.35 eV, indicating a superior semiconducting property for the nanohybrid Pd@CD-CONH catalyst owing to the better absorption of carbon quantum dots.

Figure 6

UV–vis spectra of (a) Pd@CD-CONH and (b) Pd@BTC-CONH in EtOH at 25 °C.

Sonocatalytic Activities of the Nanohybrid Pd@CD-CONH and Model Pd@BTC-CONH Catalysts

The sonocatalytic performances of 50 mg of the synthesized hybrid nanocomposites Pd@CD-CONH and Pd@BTC-CONH were tested for the degradation of 100 mL of 10–5 M aqueous solution of the RhB organic dye under sonication in the dark. To our delight, the degradation reaction was completed within 5 min, as indicated by the disappearance (99.9%) of the absorption band λmax (554 nm) of RhB in the dark using the catalyst Pd@CD-CONH (Figure a). Various control studies were performed to unravel the role of each parameter which is responsible for this excellent activity (Figure b). Initially, the degradation experiments were carried out in the absence of the catalyst and with light alone or with US + visible light or with US + dark. None of these conditions led to significant degradation of RhB dye molecules. In contrast, the nanohybrid catalyst Pd@CD-CONH and US under dark conditions showed excellent activity. Surprisingly, the same reaction in the presence of room visible light did not exhibit superior performance, probably because of the poor absorption in the visible region, as indicated by the UV–vis spectrum (Figure a). Further, similar to other control experiments, we tested the sonocatalytic activities of individual components such as the Pd powder and the CD-CONH polymer as well as the semiconductor ZnO (Figure b).

Figure 7

Time-dependent sonocatalytic degradation absorption spectra of RhB in the presence of (a) Pd@CD-CONH and (b) various control catalysts.

Interestingly, the semiconductor ZnO as well as Pd powder catalysts along with US showed negligible sonocatalytic activity in the absence of light, indicating that the reaction was completely sealed from the light source (Figure b). However, the non-metal-containing CD polymer with US has shown a significant activity of 16% in the dark. However, prolonging the reaction time up to 24 h showed no further activity, suggesting that dye physisorption on the polymer coupled with the semiconducting nature of the CD in the presence of US might generate hydroxyl (•OH) and hydrogen (H•) radicals from water upon the formation of cavitation bubble implosion (CBI) as it has been reported that US can generate even without the catalyst.[23,47−49] These results indicate that the observed sonocatalytic activity is not due to any one of the precursors but due to the synergistic effects of Pd NPs, CDs, and US.

This is also further corroborated by the comparison with the model catalyst Pd@BTC-CONH, that is, CDs replaced with benzene on the Pd@CD-CONH catalyst under similar conditions (Figure a). TEM analysis indicated that the PSD of Pd NPs in the control catalyst Pd@BTC-CONH (2 to 6 nm centered around 3 nm, Figure S7) was found to be quite similar to that of Pd@CD-CONH (2 to 5 nm centered around 3.25 nm, Figure ). Other than ensuring that the model catalyst is structurally similar, this is important for a fair comparison as the PSD plays a critical role in the catalytic activity. Figure b shows the rate constant of 0.99 and 0.47 min–1 for Pd@CD-CONH and Pd@BTC-CONH, respectively, indicating that at least 50% reduced sonocatalytic activity was observed compared to the catalyst with CDs under similar conditions. Furthermore, we have also synthesized another Pd NP capped with coffee ingredients by following the reported protocol, that is, Pd@coff,[50] with a PSD of 2 to 7 nm centered around 5 nm (Figure S8, Supporting Information). This catalyst also showed 48% reduced activity over Pd@CD-CONH (Figure b). The inferior sonocatalytic activity for the model Pd@BTC-CONH is more evident when the catalyst amount is reduced from 50 to 10 mg, which led to the partial completion of the reaction for the model catalyst, while Pd@CD-CONH retained the catalytic activity (Figure S9, Supporting Information). Upon decreasing the catalyst amount, the reaction time increased to 80 min for the 93% degradation of RhB for Pd@CD-CONH, while 46% degradation was observed for the model catalyst Pd@BTC-CONH. Continuation of the reaction up to 24 h also did not yield complete degradation for the model catalyst (48% up to 24 h), whereas the nanohybrid catalyst Pd@CD-CONH showed complete degradation for the same time. The outstanding catalytic activity for Pd@CD-CONH over the model as well as Pd@coff catalyst despite having a similar PSD clearly implies that the synergistic effects between the Pd NPs and CDs play a major role in the catalytic activity mediated by US in the dark. Notably, in comparison with other reported catalytic systems in the dark reported in the literature, the sonocatalytic degradation efficiency of the Pd@CD-CONH nanohybrid catalyst was found to be better (Table S2 in the Supporting Information).[22,27,31,47,48]

Figure 8

Time-dependent sonocatalytic degradation absorption spectra of RhB in the presence of the (a) Pd@BTC-CONH model catalyst and (b) rate constant graphs of RhB degradation with Pd@CD-CONH and Pd@BTC-CONH catalysts.

Mechanistic Studies

The presented nanohybrid catalyst Pd@CD-CONH belongs to heterogeneous catalysis driven by adsorption of the dye on the polymeric support mediated by surface reaction. As the reaction occurs under dark ambient conditions, the thermal effect provided by US might conduct the reaction. To test the effect of temperature, we have varied the temperature of the sonicator bath to 273, 298, and 323 K and found that the reaction rate increased considerably higher, thus reducing the reaction time from 20, 5, and 3 min, respectively (Figure S10, Supporting Information). It is well accepted that the thermal catalysis process using heterogeneous catalysts undergoes the major step of adsorption[51,52] (eq ), followed by the US-mediated generation of holes in the VB of the semiconducting catalyst which converts the water molecules into hydroxyl radicals and H+ driven by the CBI due to US waves (eq ). In our control studies (Figure b), we found that only with room light, no reaction was observed, whereas only US and dark conditions yielded slight (4%) degradation, indicating that ROS generation occurs because of water splitting by US as it has been reported that in the presence of US, water can undergo splitting because of CBI.[52] The generated electrons in the CB would react with dissolved oxygen to generate the superoxide radical anion (O2•–) (eq ). Subsequently, the superoxide radical anion can react with H+ to generate the hypoperoxyl radical (HO2•), which can further form hydrogen peroxide (H2O2) upon dimerization. The hydroxyl radical can be generated by the decomposition of H2O2 in the presence of US with electrons on the surface of the catalyst. Thus, three species, O2•–, •OH, and HO2•, are responsible for the degradation of dye molecules under sonocatalytic conditions (eq ). To verify that these species are formed in the reactions, we have conducted a study on the RhB oxidation over the Pd@CD-CONH nanohybrid catalyst using different scavengers such as p-benzoquinone (p-BQ) as the superoxide scavenger, isopropanol (IPA) as the hydroxyl radical (•OH) scavenger, and ethylenediaminetetraacetic acid (EDTA) as the hole scavenger. Because the Pd@CD-CONH nanohybrid catalyst could mediate the degradation even in the dark upon using these scavengers, the rate of the reaction should be altered. Figure shows that the rate of RhB dye degradation greatly reduced up to 84% in the case of the superoxide scavenger (p-BQ), whereas 52 and 18% were observed for the hydroxyl and hole scavengers, respectively, indicating that the superoxide ion plays a major role in the degradation pathway, while the hydroxyl ion significantly contributes to the degradation. The origin of enhanced nanohybrid sonocatalytic activities comes from the efficient separation of electron–hole pairs, which either directly react water with h+ or contribute to forming active species, hydroxyl radicals (•OH), and superoxide radicals (O2•–) through the reduction of the electron.[22,52]

Figure 9

Effect of scavengers on the degradation of RhB (100 mL of 10–5 M in water) using Pd@CD-CONH (50 mg).

Based on these results, the following equations are proposed for the mechanistic steps involved in the thermal catalysis process under dark ambient conditions.

Figure shows the schematic representation of the whole mechanism involved in the sonocatalytic dye degradation process catalyzed by the nanohybrid catalyst.

Figure 10

Proposed mechanism for US-mediated degradation of the RhB dye using the Pd@CD-CONH nanohybrid catalyst. The black star indicates CBI.

In order to ensure that the catalytic degradation of RhB is not mediated by the phosphorescent nature of the catalyst and/or because of UV–vis light assistance, we have studied the lifetime measurement using the time-correlated single-photon counting (TCSPC) method as well as an alternate dye degradation using MB. The TCSPC data revealed that the catalyst showed the nanosecond lifetimes of 2.6 (76%), 2.3 (18%), and 1.9 (10%), clearly indicating that it is not a phosphorescent material (Figure S11, Supporting Information). Further, we have tested the catalytic performance of the Pd@CD-CONH (50 mg) nanohybrid catalyst for the degradation of 100 mL of 10–5 M aqueous solution of MB in the dark (Figure ). It was found that the decrease in the absorbance at 664 nm and the band got disappeared almost completely (99%) within 20 min in the dark.

Figure 11

Time-dependent sonocatalytic degradation absorption spectra of MB in the presence of the Pd@CD-CONH nanohybrid catalyst.

In order to ensure that the sonocatalytic degradation is indeed happening and not due to simple adsorption on the catalyst, we have conducted an FT-IR study of the fresh and recovered catalyst, which clearly show that dye molecules are not adsorbed onto the catalyst (Figure S14, Supporting Information). To corroborate this aspect, the degradation products were studied using liquid chromatography–mass spectrometry (LC–MS) in order to determine the end small molecules (Figure S12, Supporting Information). Without any catalyst, RhB has shown the molecular ion peak at 443.5 Da. Whereas in the presence of the nanohybrid catalyst, the LC−MS spectra obtained during the course (time intervals 0−5 min) of the reaction showed gradual fragmentation of peaks and led to small molecules of 74 and 81 Da at the end of the reaction. The possible degradation products are provided in Figure S12 of the Supporting Information, and the observed fragmentation molecules match well with the previous reports.[53−56]

Recyclability and Stability

The recycling experiments for the sonocatalytic degradation of RhB in the dark were performed in mid- and high-catalytic regimes in order to study the efficient recyclability and stability of the as-prepared Pd@CD-CONH nanohybrid catalyst (Figure S13, Supporting Information). The catalyst was collected upon centrifugation and washed with water, followed by filtration using a Whatman filter paper and the subsequent drying of the catalyst. Interestingly, the sonocatalytic activity of Pd@CD-CONH was tested for up to three cycles in mid- and high-catalytic regimes, and no significant loss in the sonocatalytic activity was observed. The TEM image of the reused nanohybrid catalyst after three runs did not show any significant change in the morphology and size of the Pd NPs (Figure S13c, Supporting Information). Further, the FT-IR spectra analysis of the fresh and recovered catalyst showed clearly that the recovered catalyst is free of the RhB dye as well as the characteristic peaks corresponding to the catalyst are retained (Figure S14, Supporting Information). These results demonstrate that the Pd@CD-CONH nanohybrid catalyst is stable during the degradation process.

Conclusions

In this work, we have successfully polymerized the CDs using the organic linker (BDA) which efficiently anchored the Pd NPs in order to utilize the promising properties of CDs and Pd NPs in catalysis. Further, we demonstrate the sonocatalytic activity of Pd@CD-CONH against RhB dye degradation without shining of any light. Comparison studies with model compounds suggest that the synergistic effects of Pd NPs and CDs greatly enhance the generation of ROS for the fast degradation of RhB (5 min) in the dark. LC–MS and recovered catalyst IR studies clearly indicate that the disappearance of the absorption peak is due to sonocatalytic degradation and not because of adsorption. Detailed mechanistic investigation suggests that the origin of enhanced sonocatalytic activities of the nanohybrid catalyst comes from the efficient separation of electron–hole pairs and the subsequent reaction with water mediated by US, which contributes to form ROS such as •OH, O2•–, and HO2• that degrade the dye molecules. A simple protocol for the synthesis of the Pd@CD-CONH nanohybrid catalyst combined with fast degradation and filtration-based good recyclability along with nonrequirement of other parameters (radiation, temperature, pressure, and electric filed) is promising from the perspective of industrial wastewater management. This study opens up a new avenue for developing sono-assisted dark catalytic systems for other reactions as well, for example, biomass conversion, and our lab is currently working in this direction.

Experimental Section

Materials and Methods

All reagents were of analytical grade and used without further purification. Citric acid and thionyl chloride were supplied by Thermo Fisher Scientific. Palladium chloride, BDA, and BTC (99%) were supplied by TCI. Methanol (MeOH) and 1,4-benzoquinone were supplied by Sigma-Aldrich. Dimethylformamide (DMF) and diethyl ether were supplied by Spectrochem. Triethylamine, ethanol (EtOH), isopropanol, and EDTA were procured from SDFCL, Spectrum, Merck, and SRL, respectively. THF was dried over the sodium metal and benzophenone under an inert atmosphere.

Synthesis of the Nanohybrid Catalyst Pd@CD-CONH

The target catalyst Pd@CD-CONH comprising Pd NPs supported by the CD polymer was synthesized in two steps starting from CD-COOH. Step 1 involves the synthesis of the CD-CONH polymer by treating BDA with CD-COCl.[38] Step 2 involves the decoration of Pd NPs onto the surface of the CD-CONH polymer via the interaction of palladium ions with the nitrogen (N) atoms of the CD-CONH polymer.[42]

Step 1. Synthesis of the Amido-Amine-Functionalized CD Polymer (CD-CONH)

A solution of freshly synthesized CD-COCl (1 g) in dry THF (50 mL) was treated with Et3N (5 mL) at 25 °C for 10 min. After that, BDA (0.23 g, 2.13 mmol) was added, and stirring was continued for 15 h. The solvent was evaporated in vacuo at 40 °C. The reaction mixture was transferred to the sintered funnel and washed with water (80 mL), hexane (20 mL), and diethyl ether (Et2O, 20 mL). Then, the residue was dissolved in MeOH. Evaporation in vacuo at 40 °C afforded CD-CONH (1.12 g) as a deep-brown semisolid.

Step 2. Synthesis of Pd@CD-CONH

A solution of CD-CONH (0.25 g) in EtOH (30 mL) was treated with PdCl2 (0.5 g) in the dark at 25 °C and stirred for 15 h. The solvent was evaporated in vacuo at 40 °C. The reaction mixture was then transferred to the sintered funnel and washed with water (80 mL). The residue was dried in a vacuum oven at 60 °C for 12 h to afford 0.6 g of Pd@CD-CONH as a deep-brown solid.

Synthesis of the Model Catalyst (Pd@BTC-CONH)

Synthesis of the model catalyst (Pd@BTC-CONH) was achieved by a similar protocol followed in Section except for replacing CD-COOH with BTC (BTC-COOH).

The target model catalyst comprising Pd NPs supported by the BTC-polymer Pd@BTC-CONH was synthesized in two steps starting from BTC. Step 1 involves the synthesis of the BTC-CONH polymer by treating BDA with BTC-COCl. Step 2 involves the generation of Pd NPs by treating PdCl2 with the BTC-CONH polymer.

Step 1. Synthesis of the Amido-Amine-Functionalized BTC-Polymer (BTC-CONH)

A solution of BTC (1.5 g) with neat thionyl chloride (SOCl2) (20 mL) and a few drops of DMF under a N2 atmosphere was refluxed at 80 °C. After 2 h, excess SOCl2 was removed by vacuum, affording BTC-COCl.[41] The freshly synthesized BTC-COCl (1 g) was dissolved in dry THF (15 mL) by stirring under N2 in the dark for 10 min at 25 °C. Then, BDA (0.3 g) was added, and stirring was continued for 15 h. The solvent was evaporated in vacuo, and then, the residue was transferred into the sintered funnel and washed with water (80 mL) and Et2O (20 mL). The residue was dissolved in MeOH, followed by evaporation in vacuo at 40 °C to afford 1.4 g of BTC-CONH as a black powder.

Step 2. Synthesis of Pd@BTC-CONH

A solution of BTC-CONH (0.16 g) in EtOH (40 mL) was treated with PdCl2 (0.5 g) and stirred for 15 h in the dark at 25 °C. The solvent was evaporated in vacuo at 40 °C. The reaction mixture was then transferred to the sintered funnel and washed with water (80 mL). The residue was dried in the vacuum oven at 60 °C for 12 h to afford the nanohybrid catalyst Pd@BTC-CONH (0.2 g) as a black solid.

Characterization of the Nanohybrid Catalyst Pd@CD-CONH and the Model Catalyst Pd@BTC-CONH

FT-IR spectra were recorded on an Agilent Cary 660 spectrometer using the KBr pellet technique in the range of 4000–400 cm–1 to monitor the chemical synthesis at each step. Absorption spectra were recorded on a Shimadzu UV–vis spectrophotometer in 3 mL quartz cuvettes having a path length of 1 cm. Thermogravimetric analysis (TGA) was performed to determine the degradation/decomposition behavior of samples using a thermogravimetric analyzer (PerkinElmer STA 8000) at a N2 flow rate of 10 mL min–1 and a heating rate of 10 °C min–1. 1H NMR spectra were measured on a Bruker ADVANCE-II spectrometer at 400 MHz in DMSO-d6 or MeOD. The chemical shift was reported in parts per million (ppm) relative to tetramethylsilane as the internal standard. XRD was performed using a Bruker D-8 advanced diffractometer in the 2θ range of 10–90°. The average crystallite size of NPs with and without surface coating was estimated using the Scherrer equation. The elemental composition and composite homogeneity of the samples were investigated using an SEM–EDX scanning microscope (JEOL-JSM IT 300) attached with a Bruker signal processing unit. TEM images were acquired on a JEOL 2100 HR operating at 200 kV. Samples were prepared by depositing a drop of diluted NP suspension on a 300 mesh TEM grid (gold-coated carbon films on a 300 mesh) and dried under vacuum for 15 h. TEM grids were purchased from Beeta Tech India Pvt. Ltd. AFM images were acquired using a Bruker Multimode 8, and sample analysis was done in the tapping mode. The samples were deposited on silicon wafers, and analysis was performed at different sections at room temperature and an ambient atmosphere. XPS experiments were performed on an Auger electron spectroscopy module, model no. PHI 5000 VersaProbe II FEI Inc., using monochromatic Al Kα radiation (1486.6 eV) operating at an accelerating X-ray power of 50 W 15 kV. Before the measurement, the sample was outgassed at room temperature in an ultrahigh vacuum chamber (<5 × 10–7 Pa). The sample charging effects were compensated by calibrating all BEs with the adventitious C 1s peak at 284.6 eV. This reference gave BE values with an accuracy of ±0.1 eV. An XPS survey of the as-prepared CD-CONH and Pd@CD-CONH indicates the presence of the carbon, nitrogen, and palladium photoelectron peaks (C 1s, N 1s, and Pd 3d signals, respectively, and their Pd LMM Auger), the oxygen peaks (O 1s and its OKLL Auger), and the photoelectron peak of the adventitious carbon (C 1s). Fluorescence spectra were recorded on an Edinburgh FS5 spectrofluorimeter. The fluorescence lifetimes were measured with a Horiba Jobin-Yvon FL-1057 Fluorolog using a 390 nm nano-LED.

Evaluation of the Sonocatalytic Activities of the Nanohybrid Pd@CD-CONH and Model Pd@BTC-CONH Catalysts

For sonication: An ultrasonic cleaner (model SK2210HP, serial no. 14E0388) was used with an operating frequency of 53 kHz, an input of 220 V, and a power consumption of 100 W. The ultrasonication temperature was set to 25 °C all throughout the experiment. The water bath assisted in keeping the temperature under control, that is, around 25–30 °C. Unless otherwise mentioned, all degradation reactions were performed with sonication and in complete darkness. The dark conditions were maintained by not exposing the solution to any light source by covering with aluminum foil and exposure to minimum room light. The concentration change of the RhB dye in the solution during the sonocatalytic experiments was monitored using a UV–vis spectrometer at different time intervals in the presence and absence of the catalysts. For sonocatalytic degradation of RhB, 0.05 g of the Pd@CD-CONH nanohybrid catalyst was added to the beaker containing 100 mL of the deionized RhB (4.79 mg L–1) dye solution. The reaction mixture (3 mL) was taken out with different reaction times and centrifuged and filtered through a 0.2 μL syringe filter in order to remove the catalyst particles prior to the analysis. The filtrate was analyzed for the absorbance measurements at a wavelength of 554 nm using a spectrophotometer.