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Facile synthesis of poly (benzylamine) brushes stabilized silver nanoparticle catalyst for the abatement of environmental pollutant methylene blue
Materials Chemistry and Physics 229 (2019) 421–430
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Facile synthesis of poly (benzylamine) brushes stabilized silver nanoparticle catalyst for the abatement of environmental pollutant methylene blue
T
Arumugam Ramesha, Mathialagan Neelavenia, Perumal Tamizhduraia, Ramadoss Ramyaa, Natarajan Sasirekhab, Kannan Shanthia,∗ a b
Department of Chemistry, Anna University, Chennai, 600 025, Tamil Nadu, India Department of Crystallography and Biophysics, University of Madras, Chennai, 600 025, Tamil Nadu, India
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of poly (benzylamine) • Synthesis brushers at room temperature and act as a stabilizing agent.
recoverable Ag nanocompo• Magnetic site catalysts. prepared catalyst was ex• Newly amined through the reduction of methylene blue.
preparation method along with • Simple its recyclable capacity and high activity.
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly (benzylamine) brushes Silver nanoparticles Magnetic nanoparticles Methylene blue Catalyst
A novel type of polymer brushes as a stabilizing agent was synthesized by benzylamine at room temperature. The synthesized polymer brushes can act as an excellent stabilizing agent and the method of synthesis is also a simple one. Three different types of magnetically recoverable nano-catalysts were prepared through individual immobilization of silver nanoparticles (Ag NPs) onto the surface of Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4-PBA NPs viz., Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs, Fe3O4-PBA-Ag NPs catalysts, respectively. The synthesized catalysts were characterized by XRD, 1H NMR, GPC, Raman, FT-IR, UV-vis, VSM, SEM and HR-TEM. The catalytic potential of the newly prepared catalyst was examined through the reduction/degradation of methylene blue by NaBH4 under pseudo-first order reaction condition. The apparent rate constant (Kapp) and TOF of Fe3O4-Ag NPs, Fe3O4OA-Ag NPs, Fe3O4-PBA-Ag NPs are 2.0 × 10−2 s−1, 3.3 × 10−2 s−1, 15.6 × 10−2 s−1 and 5.0 × 10−3 s−1, 83.3 × 10−3 s−1, 285.7 × 10−3 s−1 respectively. Among the catalysts, based on Kapp value, catalytic activity of Fe3O4-PBA-Ag NPs (15.6 × 10−2 s−1) is eight times higher than Fe3O4-Ag NPs (2.0 × 10−2 s−1). The Fe3O4PBA-Ag NPs catalyst was tested to estimate the material stability up to seven runs without losing catalytic activity by recovery catalysts. This new method of prepared catalysts is expanded for the application of magnetic recoverable nano-catalysts in various industrially important reactions because of its simple preparation method along with its recyclable capacity and high activity.
∗
Corresponding author. E-mail address: [email protected] (K. Shanthi).
https://doi.org/10.1016/j.matchemphys.2019.03.034 Received 28 March 2018; Received in revised form 8 March 2019; Accepted 9 March 2019 Available online 13 March 2019 0254-0584/ © 2019 Published by Elsevier B.V.
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1. Introduction
above-mentioned technologies have been not considered because of economically not suitable and also require further treatment. Therefore, the existing catalysts cannot pack into continuous operation mode as these catalysts do not meet the industrial requirements; hence there arises a need for the development of new catalysts. The present work focuses on the development of environmentally viable catalyst, which has high regeneration capacity to degrade environmental pollutants. One of the region which need more attention in developing a nano-catalyst is to maintain the stability of nanoparticles. In view of this, an alternative simple method has been developed to fabricate magnetic nanoparticles with improved catalytic stability using benzylamine, which is a cheap polymer to address cost-effectiveness. The magnetic nanoparticles are supported over poly (benzylamine) brushes stabilized/immobilized silver nanocatalyst. Owing to its high efficiency, stability, low cost, simple preparation method, easy handling and reusability, the prepared nanocatalysts are expected to be a potential candidate for degradation of dyes in industrial effluents, especially in a continuous operation mode.
Recently, numerous researchers are focusing on the synthesis of high-quality Fe3O4 nanoparticles with well-controlled size and shape. Metal nanoparticles with one dimension and less than 100 nm in size have garnered much attention because of their attractive physical and chemical properties for various applications viz., therapeutic effect in human body, microelectronics [1], catalysis [2], photo catalyst [3], magnetic devices [4], chemisorption [5], energy storage devices [6], bio-sensors [7] etc. Song et al. studied various magnetic nano-materials for sensing purposes of iron oxides [8]. Particularly, Fe3O4 have been most widely used due to their simple preparation process, excellent magnetism and biocompatibility [9]. Thus far, the reported work on magnetic nanoparticles as catalyst supports/carriers mainly focused on magnetic iron oxide NPs coated with silica, carbon, gold, silver etc. [10–17]. These types of magnetic core supported nanoparticles have some drawbacks such as metal leaching, oxidation and aggregation [18]. To overcome these limitations, magnetic NPs are often coated with some inert shells such as polymer (linear/branching). Sun and Li et al. recently developed another interesting coating material, amorphous carbon, because of its hydrophilic nature and the presence of abundant amounts of –OH and –COO- groups on the surface, its disparity in water is enhanced and facilitated further by coupling interaction with some guest molecules [19]. Surface modification of magnetic core with different types of polymer shell to form a core-shell metal nanoparticle has become an important route to stabilize/immobilize metal nanoparticles [20–22]. Polyelectrolytes act as an excellent stabilizing agent for growth of polymer shell which may be used for immobilization of metal nanoparticles [23]. Schrinner et al. also supported the stabilization of gold nanoparticles by spherical polyelectrolyte brushes [24]. Shchukin et al. published the stabilization of silver nanoparticles by polyelectrolyte and poly (ethylene glycol) by layer to layer assembly to prevent aggregation and increase in mono dispersity [25]. These types of homogeneous form of polyelectrolyte stabilized metal nanoparticles catalyst have some recoverability problem in aqueous/organic phase during reaction condition. Hence, Ag nanoparticles stabilized with magnetic nature of Fe3O4 NPs solve the recovery of the catalyst both in organic and aqueous phase. Methylene blue is a common dye mostly used in textile, rubber, plastics, leather, cosmetics, pharmaceutical and food industries [26,27]. All the available methods have got significant disadvantages such as removal of incomplete ions or other waste products in treating the effluents. Hence, further disposal methods are needed such as biodegradation [28], chemical oxidation [29–31], foam flotation [32], electrolysis [33], adsorption [34], electro-coagulation [35] and photocatalysis [36,37] to completely remove the dye [38]. However, these
2. Experimental method 2.1. Materials FeSO4 7H2O (SRL), FeCl3 6H2O (Loba Chemie), AgNO3 ((Sigma Aldrich), benzylamine (Sigma Aldrich), epichlorohydrin (Sisco Research Laboratories (SRL)), oleic acid (Merck), HCl (Merck), methylene blue (Merck), NaBH4 (SRL) and NH4OH (SRL) were purchased. Other solvents such as methanol, CH2Cl2, CHCl3, CDCl3, C2H5OH, THF, acetone, diethyl ether and deionized water were purchased from SRL and used further without any purification.
2.2. Preparation of magnetic Fe3O4 NPs Magnetic Fe3O4 nanoparticles were prepared by co-precipitation method [39]. Initially, 0.5 mol L−1 FeCl3 6H2O and 0.25 mol L−1 FeSO4 7H2O were dissolved in 100 mL of deionized water with mechanical stirrer. Then, NH4OH solution was added slowly by adjusting the pH 11.0–12.0, with continuous stirrer for 2 h at 65 °C. It was then followed by addition of 30 mL of oleic acid (OA) in a slow paced manner to the above reaction mixture and the pH 6.0–7.0 was adjusted by dilute HCl, with continuous stirrer for 6 h at 80 °C (Scheme 1). The resultant product, the black precipitate was collected, washed and later dried at 60 °C for overnight. The obtained product, Fe3O4-OA NPs was characterized by FT-IR, SEM, VSM, and XRD.
Scheme 1. Synthesis of Poly (benzylamine) and Fe3O4-PBA-AgNPs. 422
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catalysts and concentration of MB. The kapp can be obtained from the slope of linear plots of ln (Ct/C0) versus time. The % of degradation efficiency determined is calculated by the following formula
2.3. Synthesis of Poly (benzylamine) brushes Poly (benzylamine) brushes [PBA] were prepared by solvent free reaction under room temperature. Initially, 10 g of benzylamine (BA) and 9.29 g of epichlorohydrin (ECH) were poured into 100 mL RB flask equipped with a magnetic stirrer at room temperature (RT) for 6 h (Scheme 1). The complete polymerization was allowed at RT for overnight and thus the white precipitate was obtained. This was later dissolved in CHCl3 and again precipitated with diethyl ether. The obtained PBA was filtered and dried at 40 °C for overnight and the product was characterized by 1H NMR (Fig. S1) and GPC (Fig. S2).
X = (C0 − C / C0) × 100%
Where, C0 and C are the solution concentration or absorbance before and after the degradation, respectively. The concentrations of MB were estimated from the absorbance value of UV-vis spectra. 2.7. Instrumentation
2.4. Synthesis of Poly (benzylamine) brushes grafting on to Fe3O4 NPs
FT-IR analysis was carried out on BRUKER, TENSOR 27 model instrument. The UV-vis spectra were measured on shimadzu instrument (UV-2450 series). The ultra-sonication of all magnetic materials was done by PCi analytics sonication bath at 40 KeV. The surface morphology study was performed using VEGA3 TESCAN model scanning electron microscope (SEM). The X-ray diffractometer using a Cu kλ monochromatic radiation source (λ = 1.54045). The magnetic susceptibility values were recorded for prepared magnetic core-shell based nanocomposites using Lakeshore Model 7404 with high and low-temperature attachments, at the maximum magnetic field −2.17 T (0.6 air gap). FEI TECNAI G2 Model T-30 was used for measuring morphology by HR-TEM, Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer (SII) under nitrogen atmosphere in a temperature range of 30–750 °C at a heating rate of 8 °C min−1. Synthesized poly (benzylamine) was analyzed by 1H NMR spectra using CDCl3 as a solvent in a JEOL spectrometer at 500 MHz, Proton chemical shifts (δ) are expressed in parts per million and are related to TMS, which is an internal standard. Gel permeation chromatograms were used for determining molecular weight by Shimadzu instrument (CTO-20 A) with THF. EZRaman 785 series, Diode Laser power 60 mW. Powder was spread on the glass slide with an integration of 10 and box car 2.
Initially, 0.5 g of Fe3O4-OA NPs was dissolved in ethanol (40 mL) by ultra-sonication for 30 min, and then added to 0.25 g of PBA/THF (40 mL) mixture. The above reaction mixture was stirred mechanically at 80 °C for 6 h (Scheme 1). The resulting reaction mixture was filtered, washed and then dried at 60 °C for overnight. The obtained magnetic core coated PBA was labeled as Fe3O4-PBA NPs and thus characterized by FT-IR and Raman analyses. 2.5. Synthesis of three different types of magnetic core-shell supported nanoparticles catalyst The stabilization/immobilization of magnetic core-shell supported Ag NPs catalyst was investigated using three different magnetic core supports viz., Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4 PBA NPs and AgNO3 as metal precursor. Initially, 0.5 g of Fe3O4NPs, Fe3O4-OA NPs and Fe3O4 PBA NPs were taken individually in three different 100 mL RB flask and dispersed in ethanol (50 mL) under ultra-sonication for 30 min. After that, 1.4 × 10−3mM of AgNO3 (10 mL) was added individually to three different RB flasks containing Fe3O4 NPs, Fe3O4-OA NPs, and Fe3O4-g-PBA NPs supports, and then the reaction mixture was refluxed with mechanical stirring at 60 °C. Then, 1 mL of NaBH4 aqueous solution (10 mM) was added individually at room temperature for 4 h. The observation color changed from brown to black, which thereby indicating the formation of Ag0 from Ag+. The resulting product was filtered, washed and dried at 60 °C for overnight (Scheme 1). Then, Ag NPs immobilized/stabilized on Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4-gPBA NPs magnetic core-shell nano-composite catalysts were labeled as Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs, respectively, and thus characterized by FT-IR, VSM and Raman analyses.
3. Results and discussion 3.1. XRD analysis In order to confirm the presence of crystalline magnetite within the polymeric poly (benzylamine) brushes, the structure of each core-shell nanoparticles were analyzed with XRD. The XRD patterns of Fe3O4 NPs, Fe3O4-OA NPs, and Fe3O4-PBA-Ag NPs are shown in Fig. 1A respectively. The XRD pattern of Fe3O4 shows the characteristic diffraction peaks for 2θ values at 30.49 (220), 35.78 (311), 43.37 (400), 57.34 (511) and 62.92° (440). It was found to be in good agreement with the standard pattern of Fe3O4 that proved the existence of cubic spinal phase as shown in Fig. 1(A)a. The average diameter of the Fe3O4 was estimated using Debye Scherrer equation and the average crystalline size was calculated as 15.31 nm. The magnetic Fe3O4 nanoparticles were stabilized using OA as capping agent. Fig. 1(A)b XRD pattern resembles to that of magnetic Fe3O4 nanoparticles with decreased intensity thereby confirming that the structure of Fe3O4 nanoparticles is retained even after coating of OA as a stabilizer. The immobilization/ stabilization of Ag NPs onto the core-shell nanocomposites (Fig. 1(A)c) have shown 2θ peaks corresponding to Ag NPs at 38.11 (111), 44.27 (200), 58.2 (220) & 64.42° (311) and this confirms the stabilization of Ag NPs onto the surface of Fe3O4-PBA NPs. The average diameter of the Ag NPs was estimated by Debye Scherrer equation and the crystallite size was calculated as 23.12 nm. The size of the nanoparticles was found to increase in the order of Fe3O4 (15.31 nm), Fe3O4-OA (17.31 nm) and Fe3O4-PBA-AgNPs (23.12 nm).
2.6. Catalytic efficiency of Fe3O4-AgNPs, Fe3O4-OA-AgNPs and Fe3O4-gPBA-AgNPs catalysts for reduction of methylene blue The three different types of nano-composite catalysts viz., Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4 PBA-Ag NPs were tested for MB reduction/degradation under the pseudo-first order reaction condition. The kinetic experiments were performed following parameters like 50 mL of MB (1 × 10−3 mM), 5 mL of NaBH4 (1 × 10−1 mM) and 6 mg of respective catalyst viz., Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4PBA-Ag NPs were used. The reaction was monitoring the UV-Vis spectrum range from 200 to 700 nm at regular intervals of time (3 min). The rate of the reaction was used to determine the decreasing trend of absorbance at 664 nm NaBH4 was used in the reduction of methylene blue; a pseudo-first-order kinetics was involved to carry out the rate of the reaction [37,40]. Therefore, the apparent rate constant (kapp) can be defined by the following equation, -dcMB/dt = kappCMB
(2)
(1)
3.2. 1H NMR and GPC analysis
Where, Kapp is the apparent rate constant, which is related to the concentration of silver content in the polymer brushes stabilized nano-
The formation of Poly (benzylamine) by using BA and ECH at room 423
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Fig. 1. (A) XRD analysis of a) Fe3O4 NPs, b) Fe3O4-OA NPs and c) Fe3O4–PBA-Ag NPs (B) FT-IR spectra of a) Fe3O4 NPs, b) Fe3O4-OA NPs, c) Fe3O4–PBA NPs and d) Fe3O4 –PBA-AgNPs, (C) VSM spectra of a) Fe3O4 OA NPs and b) Fe3O4-PBA-AgNPs (D) TG analysis of a) Fe3O4-OA-AgNPs and b) Fe3O4-PBA-AgNPs catalysts.
temperature reaction was confirmed by 1H NMR spectrum as shown in Fig. S1. It can be observed that –N–CH2– and –CH–CH2–N– protons are appear in the region of 2.3–2.8 ppm, the same way, the addition of peaks was observed at 3.47 ppm which can be assigned as –CH–OH protons. Further, 1H NMR spectrum shows signals at 7.5–7.0 ppm absorbing the phenyl proton and 3.41 ppm for C6H5–CH2– protons. The equine-molar ratio of benzylamine and Epic chlorohydrin was used to synthesize polymer brushes as a stabilizing agent by condensation reaction in the room temperature. The molecular weight of PBA was confirmed by GPC [35]. Based on the GPC analyzed result as shown in Fig. S2, the average molecular weight was found to be 539 (g/mol) and poly disparity index, Mw/Mn as 1.37.
was analyzed through FT-IR, as shown in Fig. 1 (B)d. Compared with Fig. 1(B)c-d showed a shift in the peak at 1095 cm−1 in Fe3O4-PBA NPs to 1114 cm−1, which confirmed the immobilization of Ag NPs on Fe3O4-PBA NPs catalyst. 3.4. VSM analysis The prepared material was tested for magnetic properties of catalysts such as Fe3O4 OA NPs & Fe3O4-PBA-Ag NPs by VSM technique. Fig. 1(C)a and Fig. 1(C)b show a hysteresis loop of typical Fe3O4 & Fe3O4-PBA-AgNPs nano-composite catalysts measured by sweeping the external field between −2 and 2 T at room temperature. The saturated mass magnetization is to be 63.4 & 17.08 emu/g, respectively. However, the prepared magnetic nano-composite catalyst via, Fe3O4-PBAAgNPs can be easily recovered by applying the external magnetic fields, which may be a separate nano-catalyst used for the further reaction. This is because, on surface coating of PBA onto the Fe3O4-OA NPs, the Ms value was found to decrease as the thickness of the PBA layer becomes dense due to the formation of PBA grafted over Fe3O4-OA NPs.
3.3. FTIR analysis The FTIR spectrum of Fe3O4 NPs, Fe3O4-OA NPs, Fe3O4-PBA NPs, and Fe3O4-PBA-AgNPs are shown in Fig. 1(B)a-d respectively. From Fig. 1(B)a, the characteristic peak was noticed at 578 cm−1 due to Fe-O band, this peak is an evidence for the formation of Fe3O4 NPs. From the FTIR spectrum of Fe3O4-OA NPs shows the characteristic peaks at 3429 cm−1, 578 cm−1 and 451 cm−1 thereby corresponding to the –OH (hydroxyl group), Fe2+-O2−and Fe3+-O2- respectively. Fig. 1(B)b shows the peaks at 2929 cm−1and 2853 cm−1 which can be attributed to the saturated-C–H groups in oleic acid. Further, the characteristic peak appeared at 1708 cm−1 and it can be assigned as stretching vibration of carbonyl group (ʋC]O) of OA. All these peaks evidenced that oleic acid was coated onto the Fe3O4 NPs. Further, PBA was coated onto the OA-Fe3O4 NPs (Fe3O4-PBA), which might be the strong interaction between PBA and OA-Fe3O4 NPs. The FTIR spectrum of Fe3O4-PBA NPs is shown in Fig. 1(B)c. When compared with Fig. 1(B)b, Fig. 1(B)c showed the presence of new characteristic peaks at 1628 & 1263 cm−1 for an aromatic ring, 1095 cm−1 for C-N and 3392 cm−1 for OH. These peaks evidenced that the PBA was coated onto the Fe3O4-OA NPs. The immobilization/stabilization of Ag NPs onto the Fe3O4-PBA NPs and thus obtained product
3.5. TG analysis Thermogravimetric analysis was employed to study the thermal behavior of the magnetic polymer brushes stabilized nanoparticle catalysts. Fig. 1D shows the TGA studies of Fe3O4-OA-Ag NPs and Fe3O4PBA-Ag NPs, which confirms the weight change during the PBA stabilized Ag NPs. The weak thermal stability of magnetic Fe3O4 nanoparticle is already reported [37,41]. The stability of magnetic Fe3O4 NPs can be enhanced by the addition of OA as a capping agent. Fig. 5a shows a major weight loss of about 12.9% in the temperature range between 175 and 371 °C due to the decomposition of OA, after that no weight loss occurs because of the stability of magnetic Fe3O4-OA NPs. Fig. 5b shows the TGA results of Fe3O4-PBA-Ag NPs, where the grafting of PBA over magnetic supports as Fe3O4-OA NPs. The magnetic PBA stabilized Ag NPs shows only one major weight loss of 20% in range 424
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due to the presence of more amounts of Ag NPs stabilized by the newly developed PBA, coated Fe3O4-OA. 3.7. Raman analysis Raman spectra of Fe3O4NPs, Fe3O4-OA NPs, Fe3O4-PBA NPs and Fe3O4-PBA-AgNPs are shown in Figs. S3a–d respectively. Fig. S3a shows a characteristic peak at 680 cm−1 due to the formation of Fe-O-Fe bond. After coating with oleic acid onto the surface of Fe3O4, the peak intensity of Fe-O-Fe (680 cm−1) decreased as shown in Fig. S3b. Further coating with PBA onto Fe3O4-OA NPs, the peak intensity of Fe-O-Fe (680 cm−1) decreased again as in Figs. S3c and a new peak was appeared at 602 cm−1, due to C-N stretching band, which implied that the PBA was coated onto the Fe3-O4-PBA. After immobilization/stabilization of metal nanoparticles onto Fe3O4-PBA NPs (Fig. S3d), the characteristic peak for C-N stretching band at 602 cm−1 was shifted to 610 cm−1, these peaks confirmed the immobilization/stabilization of metal nanoparticles onto the PBA coated magnetic nanoparticles. Fig. 2. UV-vis spectra of a) Fe3O4-OA-AgNPs and b) Fe3O4-PBA-Ag NPs catalysts.
3.8. HR-SEM and TEM studies HR-SEM was used to study the morphology of most of the stabilizing catalysts, including Fe3O4-PBA-Ag NPs. Fig. 3a-b shows spherical morphology of Fe3O4-PBA-Ag NPs. The presence of spherical particles even after PBA grafting interpret that the morphology of F Fe3O4 NPs and Fe3O4-OA NPs may also be spherical in nature. Also, Ag NPs was used to stabilize/immobilize Fe3O4-PBA NPs. The intense spherical black dot observed in SEM images indicates the presence of Ag NPs. HR-TEM images of the Fe3O4-PBA-Ag NPs catalyst are shown in Fig. 3c-d which confirms its spherical morphology. The TEM images also illustrate the presence of core-shell nanocomposites of Fe3O4-PBA NPs surrounded by fine dispersion of Ag NPs. From the intense black dots and patches, the average particle size was calculated to be 7 nm. The presence of grey and black particles in Fig. 3c confirmed the coverage of PBA polymer stabilizing agent over the surface of Fe3O4 NPs. The black particles may be due to the presence of Fe3O4 NPs and grey region due to PBA polymer stabilizing agent. Fig. 3d indicates springer, which is due to Ag NPs onto the surface of Fe3O4 NPs. The morphology observed from TEM results are in accordance with SEM images. The sharp intense XRD peaks also confirm the absence of bulk formation of
from 180 to 350 °C, which may be due to presence of polymer brushes coated onto the surface of Fe3O4-OA NPs. The TGA result also confirmed the growth of polymers over the surface of Fe3O4-OA NPs and revealed that the polymer brushes stabilized Ag NPs catalyst better on the material as compared to the other catalysts. 3.6. UV-vis analysis The conformation of Ag NPs and its immobilization onto the Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4-PBA NPs were established through UVVis analyses. For understanding and clarity, the UV-Vis spectrum obtained from Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs are shown in Fig. 2a-c. From, each UV-Vis spectrum, it can be observed that the typical Surface Plasmon Resonance (SPR) peak for Ag NPs appeared at 410 nm. Therefore, the appearance of SPR peak at 410 nm has undoubtedly the formation of Ag NPs over the Fe3O4-PBA NPs matrix. The peak intensity of Fe3O4-PBA-AgNPs was greater than the peak intensity of Fe3O4-Ag NPs and Fe3O4-OA-Ag NPs, which may be
Fig. 3. Electron microscopy images of Fe3O4-PBA-AgNPs catalyst: (a) and (b) SEM images, (c) and (d) HR-TEM images. 425
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Table 1 Comparative catalytic activity of apparent rate constant, TOF and the efficiency towards reduction of MB using MNPs catalyst. S.No
Catalysts
Kapp × 10−2 (s−1)
TOF × 10−3 (s−1)
DE (%)
R2
Ref
1 2 3 4 5 6 7 8 9 10
Fe3O4-Ag NPs Fe3O4-OA-Ag NPs Fe3O4-PBA-Ag NPs Fe3O4-PAC-AuNPs Fe3O4-HEA-AuNPs Fe3O4–PAMAM-G (0)-AuNPs Fe3O4–PAMAM-G (1)-AuNPs Fe3O4–PAMAM-G (2)-AuNPs NiFe2O3 Fe3O4/PS
2.0 3.3 15.6 2.90 2.44 0.65 1.30 3.55 – –
5.01 83.3 285.7 1.18 1.0 – – – – –
52.5 67.3 99.1 44.23 29.15 43 72 85 33.7 80
0.9564 0.9800 0.9923 0.9921 0.9925 – – – – –
This work This work This work [42] [42] [43] [43] [43] [44] [45]
Kapp = Apparent rate constant, TOF = Turn over frequency, DE = degradation efficiency, R2 = Correlation coefficient.
nanocomposites.
3.9. Effect of stabilization on the catalytic activity of Fe3O4-PBA-Ag NPs The catalytic activity of the Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs catalysts was studied individually for the reduction of MB with NaBH4 under identical pseudo first order reaction condition to calculate the Kapp and TOF based on the above mentioned formula (1) and (2). The reaction mesearued by the peak intensity disappered which may be due to the reduction of MB. The effective reduction was calculated and given in Table 1. These values were used to estimate the catalytic activity of individual nano-composite catalysts. The reduction/degradtion of MB was monitored at 664 nm which decreased against time as shown in Fig. 4. A control observation used to was test without catalyst whereby they cannot alter the reaction as there is no occurrence of reduction, even after two days, henceforth confirmed that the reaction kinetics was restricted. The kapp, and TOF were calculated with the same amount of catalyst for Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs catalysts through the reduction/degradation of MB. The apparent rate constant and TOF of Fe3O4-Ag NPs, Fe3O4-OA-Ag
Fig. 4. UV-Vis spectra of MB reduction using Fe3O4-PBA-AgNPs catalyst.
Fig. 5. Normalized absorbance at λmax of MB vs. Ag NPs catalysts, Ct is the conc. of MB at real time ‘t’ and C0 is the conc. of MB. (B) Ln (Ct/C0) at 664 nm vs. reaction time for the reduction of MB. (C) Figure of apparent rate constant (Kapp) as a function of different catalysts (D) Figure of TOF as a function of different catalysts. 426
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Fig. 6. GC-MS images of a) Initial concentration of MB and B) After reaction take place in presence of Fe3O4-PBA-Ag NPs catalysts.
NPs and Fe3O4-PBA-Ag NPs catalysts are 2.0 × 10−2 s−1, 3.3 × 10−2 s−1 & 15.6 × 10−2 s−1 and 5.0 × 10−3 s−1, 83.3 × 10−3 s−1 and 285.7 × 10−3 s−1, respectively. Among the different types of Ag NPs catalysts studied, the rate constant and TOF is considered to be better for Fe3O4-PBA-Ag NPs. Similarly, the degradation efficiency of the catalysts was investigated and found to be that the catalytic activity of synthesized poly (benzylamine) brushes onto magnetic Fe3O4-OA NPs used to stabilize/immobilize Ag NPs was better than other catalysts. This may be due to the stability of the Ag NPs stabilized/immobilized by PBA as compared to the rest of the catalysts and values as given in Table 1 and the corresponding plot is also given in Fig. 5. Further, the superior catalyst viz., Fe3O4-PBA-Ag NPs was examined through the reaction condition by altering the concentration. Brushes onto magnetic Fe3O4-OA NPs and stabilization/immobilization of Ag NPs is considered to be the superior catalytic activity because of the size of the MNPs stabilization/immobilization as compared to rest of the catalyst values as given in Table 1 and the corresponding plot is also given in Fig. 5. In order to establish the right reaction condition for effective reduction of MB, the reaction has to be optimized. Hence, this superior catalyst viz., Fe3O4-PBA-AgNPs was employed to determine the through kinetics of the same reduction/degradation of MB by varying the experimental parameters such as [catalyst] and [MB].
PBA-Ag NPs under the optimum reaction condition (50 mL of MB (1 × 10−3 mM), 5 mL of 1 × 10−1 mM NaBH4, 6 mg, time 21 min) and the efficiency of the catalysts was measured and analyzed by GC-MS (Fig. 6a). There is clear evidence from the mass spectra of the experiment done before and after degradation. The peaks appeared at low retention time (Fig. 6b) indicating the fragmentation of the MB dye to small molecules. The degradation mechanism of MB can be understood as follows. The surface active sites of Fe3O4-PBA-Ag NPs can react the BH4− (a strong nucleophile). This supplies electron to MB thereby degradation of MB (a good electrophile) occurs faster. The faster degradation of MB with BH4− an electron rich moiety may be due to columbic interaction. Similarly, the diffusive nature and high electron injection capability of BH4− help to transfer electron to the substrate MB via Fe3O4-PBA-Ag NPs and thus overcome the kinetic barrier for the degradation MB [37]. Further, the stability of the superior catalyst viz., Fe3O4-PBA-AgNPs was examined through the same reaction under the identical condition and by employing its recovered catalyst up to 7th cycle. 3.9.2. Effect of catalyst weight The effect of catalyst weight on the catalytic efficiency of the superior catalyst viz., Fe3O4-PBA-AgNPs was examined by varying the catalyst weight from 2 to 10 mg and keeping another parameter as constant. The Kapp values (Fig. 7) are in the range of 2.85 × 10−2 to 30 × 10−2 s−1 for catalyst weight from 2 to 10 mg. Also, the TOF value
3.9.1. GC-MS analysis for reduction of MB The photocatalytic degradation of MB was carried out on Fe3O4427
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Fig. 7. (A) Normalized absorbance at λmax of MB vs Fe3O4-PBA-AgNPs catalysts, Ct is the conc. of MB at real time t and C0 is the conc. of MB, (B) ln (Ct/C0) at 664 nm vs. reaction time for the reduction of MB, (C) Figure of apparent rate constant (kapp) as function of different catalysts (D) Figure of TOF as function of different catalyst.
Table 2 Effect of catalysts for apparent rate constant, TOF and the efficiency towards reduction of MB. S.No
Fe3O4-PBA-Ag NPs
Kapp × 10−2s−1
TOF × 10−3 s−1
Degradation Efficiency (%)
Correlation coefficient
1 2 3 4 5
2.0 mg 4.0 mg 6.0 mg 8.0 mg 10.0 mg
2.87 6.54 9.0 16.54 30.74
230 250 285 313 333
93 96 99 99.9 99.9
0.9931 0.9904 0.9931 0.9905 0.9903
Fig. 7 a, b, c and d, respectively [42]. 3.9.3. Effect of MB concentration Effect of MB concentration of the reaction was also studied by using different concentrations of the methylene blue such as 6.0 × 10−3 mM, 8.0 × 10−3 mM, 10 × 10−3 mM, 12 × 10−3 mM & 14 × 10−3 mM and other parameter as constant. The plot of C/C0 and ln (C/C0) values are given in Fig. S4 that shows a decrease in Ct/Co and ln (Ct/Co) value with an increase in [MB]. This may be due to the availability of more molecules for excitation and consecutive degradation. The degradation efficiency of MB was found to be decreasing in the Ct/C0 and ln (Ct/C0) value on increasing MB concentration. The increase in the dye concentration always leads to lesser amount of active sites over the surface of the catalyst as all the active sites will be occupied by the reactant molecules and eventually the activity will decrease. In this case, with an increase in time and concentration of methylene blue, the degradation efficiency of the catalyst decreases.
Fig. 8. Recycle efficiency of Fe3O4-PBA-Ag NPs catalyst.
ranges from 230 × 10−3 s−1 to 329 × 10−3 s−1 as illustrated in Fig. 7. The corresponding Kapp, TOF, degradation efficiency and correlation co-efficient for the effect of catalyst weight on the catalytic efficiency of Fe3O4-PBA-Ag NPs are given in Table 2. Both the apparent rate of the reaction and TOF increases with an increase in the concentration of catalyst. The active sites of the surface may enhance the electron transfer to the reaction mixture and also as noticed there was faster degradation with increased Ct/C0, ln (Ct/C0), Kapp, and TOF as shown in
3.9.4. Recycling efficiency Catalyst stability of Fe3O4-PBA-Ag NPs during the reaction was also tested. In the industrial reaction conditions, used catalyst was recycled with an applied external magnetic field to remove the catalyst followed by washing of the solvents. Then, the recycled catalyst was tested under same reaction condition and the Kapp value obtained initially was 15.6 × 10−3 s−1. Fe3O4-PBA-Ag NPs which was later examined up to 7th cycles and the obtained kapp value was 14.0 × 10-3 s−1, which was later found to be almost similar irrespective of the cycle as shown in 428
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Fig. 8. The small dropping of kapp value after several times was due to the washing of Ag NPs catalyst.
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4. Conclusion In conclusion, three types of magnetic recoverable catalysts were developed by a simple, reproducible, non-toxic, highly active and facile method. The electrostatic interaction between the PBA and Ag NPs may stabilize the metal nanoparticles over the surfaces, plentiful nanoparticles with 5–7 nm in size indicates the surface of Fe3O4-PBA matrix to form Fe3O4-PBA-Ag NPs catalyst. These AgNPs immobilized nanoparticles exhibit excellent catalytic potential for the reduction of MB with NaBH4 compared with Fe3O4-AgNPs and Fe3O4-OA-AgNPs. The kapp, and TOF for Fe3O4-AgNPs, Fe3O4-OA-AgNPs and Fe3O4-PBAAgNPs catalysts with same amount of catalyst were examined for the reduction/degradation of MB and found to be 2.0 × 10−2 s−1, 3.3 × 10−2 s−1 & 15.6 × 10−2 s−1 and 5.0 × 10−3 s−1, 83.3 × 10−3 s−1 and 285.7 × 10−3 s−1, respectively. Among the different types of Ag NPs catalysts studied, Fe3O4-PBA-Ag NPs were found to have apparent better rate constant and TOF than other catalysts. Moreover, the TGA analysis gives appropriate information about the polymer brushes stabilized magnetic core-shell; these nanocomposites have 20% of weight loss. On the other hand, the saturation magnetization of grafted material value is 17.08 emu/g. Besides, the recycling efficiency of magnetic core-shell catalyst viz, Fe3O4-PBA-Ag NPs was examined up to seventh cycle & no loss of catalytic activity. It was observed that the grafted magnetic core-shell Fe3O4-PBA-Ag NPs nanocomposites catalyst exhibited higher catalytic activity and reproducibility towards the degradation of MB. Hence, the magnetic core-shell nanocomposite is an excellent and environmentally friendly method for removal of environmental pollutant MB. Conflicts of interest The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgments The authors are grateful for the financial support provided by UGCSAP, New Delhi, India the grant sanction no. 540/3/DRS-III/2016 (SAP-I) dt 21.05.2018. Authors have gratefully acknowledged the University Grants Commission, Department of Science & Technology, Defence Research and Development Organization (DRDO) New Delhi and Department of Chemistry, Anna University, Chennai to support and carry out this research work. Finally, the authors acknowledge National Centre for Catalysis Research (NCCR). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchemphys.2019.03.034. References [1] Y. Xu, C. Liu, D. Khim, Y.Y. Noh, Development of high-performance printed organic field-effect transistors and integrated circuits, Phys. Chem. Chem. Phys. 17 (2015) 26553–26574. [2] M.M. Mohamed, M.S. Al-Sharif, One pot synthesis of silver nanoparticles supported on TiO2 using hybrid polymers as template and its efficient catalysis for the reduction of 4-nitrophenol, Mater. Chem. Phys. 136 (2012) 528–537. [3] M.J.S. Mohamed, S.U. Shenoy, D. K Bhat, Novel NRGO-CoWO4-Fe2O3 nanocomposite as an efficient catalyst for dye degradation and reduction of 4-nitrophenol, Mater. Chem. Phys. 208 (2018) 112–122. [4] J.S. Cheng, Y. Wu, Z.H. Xn, B. Hu, J.M. Bai, J.P. Wang, J. Shen, Hydration dynamics of oriented DNA films investigated by time-domain terahertz spectroscopy, Appl. Phys. Lett. 90 (2007) 233902. [5] P.V. Kamat, Meeting the clean energy Demand: Nanostructure architectures for solar energy conversion, J. Phys. Chem. C 111 (2007) 2834–2860.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Facile synthesis of poly (benzylamine) brushes stabilized silver nanoparticle catalyst for the abatement of environmental pollutant methylene blue
T
Arumugam Ramesha, Mathialagan Neelavenia, Perumal Tamizhduraia, Ramadoss Ramyaa, Natarajan Sasirekhab, Kannan Shanthia,∗ a b
Department of Chemistry, Anna University, Chennai, 600 025, Tamil Nadu, India Department of Crystallography and Biophysics, University of Madras, Chennai, 600 025, Tamil Nadu, India
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
of poly (benzylamine) • Synthesis brushers at room temperature and act as a stabilizing agent.
recoverable Ag nanocompo• Magnetic site catalysts. prepared catalyst was ex• Newly amined through the reduction of methylene blue.
preparation method along with • Simple its recyclable capacity and high activity.
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly (benzylamine) brushes Silver nanoparticles Magnetic nanoparticles Methylene blue Catalyst
A novel type of polymer brushes as a stabilizing agent was synthesized by benzylamine at room temperature. The synthesized polymer brushes can act as an excellent stabilizing agent and the method of synthesis is also a simple one. Three different types of magnetically recoverable nano-catalysts were prepared through individual immobilization of silver nanoparticles (Ag NPs) onto the surface of Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4-PBA NPs viz., Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs, Fe3O4-PBA-Ag NPs catalysts, respectively. The synthesized catalysts were characterized by XRD, 1H NMR, GPC, Raman, FT-IR, UV-vis, VSM, SEM and HR-TEM. The catalytic potential of the newly prepared catalyst was examined through the reduction/degradation of methylene blue by NaBH4 under pseudo-first order reaction condition. The apparent rate constant (Kapp) and TOF of Fe3O4-Ag NPs, Fe3O4OA-Ag NPs, Fe3O4-PBA-Ag NPs are 2.0 × 10−2 s−1, 3.3 × 10−2 s−1, 15.6 × 10−2 s−1 and 5.0 × 10−3 s−1, 83.3 × 10−3 s−1, 285.7 × 10−3 s−1 respectively. Among the catalysts, based on Kapp value, catalytic activity of Fe3O4-PBA-Ag NPs (15.6 × 10−2 s−1) is eight times higher than Fe3O4-Ag NPs (2.0 × 10−2 s−1). The Fe3O4PBA-Ag NPs catalyst was tested to estimate the material stability up to seven runs without losing catalytic activity by recovery catalysts. This new method of prepared catalysts is expanded for the application of magnetic recoverable nano-catalysts in various industrially important reactions because of its simple preparation method along with its recyclable capacity and high activity.
∗
Corresponding author. E-mail address: [email protected] (K. Shanthi).
https://doi.org/10.1016/j.matchemphys.2019.03.034 Received 28 March 2018; Received in revised form 8 March 2019; Accepted 9 March 2019 Available online 13 March 2019 0254-0584/ © 2019 Published by Elsevier B.V.
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1. Introduction
above-mentioned technologies have been not considered because of economically not suitable and also require further treatment. Therefore, the existing catalysts cannot pack into continuous operation mode as these catalysts do not meet the industrial requirements; hence there arises a need for the development of new catalysts. The present work focuses on the development of environmentally viable catalyst, which has high regeneration capacity to degrade environmental pollutants. One of the region which need more attention in developing a nano-catalyst is to maintain the stability of nanoparticles. In view of this, an alternative simple method has been developed to fabricate magnetic nanoparticles with improved catalytic stability using benzylamine, which is a cheap polymer to address cost-effectiveness. The magnetic nanoparticles are supported over poly (benzylamine) brushes stabilized/immobilized silver nanocatalyst. Owing to its high efficiency, stability, low cost, simple preparation method, easy handling and reusability, the prepared nanocatalysts are expected to be a potential candidate for degradation of dyes in industrial effluents, especially in a continuous operation mode.
Recently, numerous researchers are focusing on the synthesis of high-quality Fe3O4 nanoparticles with well-controlled size and shape. Metal nanoparticles with one dimension and less than 100 nm in size have garnered much attention because of their attractive physical and chemical properties for various applications viz., therapeutic effect in human body, microelectronics [1], catalysis [2], photo catalyst [3], magnetic devices [4], chemisorption [5], energy storage devices [6], bio-sensors [7] etc. Song et al. studied various magnetic nano-materials for sensing purposes of iron oxides [8]. Particularly, Fe3O4 have been most widely used due to their simple preparation process, excellent magnetism and biocompatibility [9]. Thus far, the reported work on magnetic nanoparticles as catalyst supports/carriers mainly focused on magnetic iron oxide NPs coated with silica, carbon, gold, silver etc. [10–17]. These types of magnetic core supported nanoparticles have some drawbacks such as metal leaching, oxidation and aggregation [18]. To overcome these limitations, magnetic NPs are often coated with some inert shells such as polymer (linear/branching). Sun and Li et al. recently developed another interesting coating material, amorphous carbon, because of its hydrophilic nature and the presence of abundant amounts of –OH and –COO- groups on the surface, its disparity in water is enhanced and facilitated further by coupling interaction with some guest molecules [19]. Surface modification of magnetic core with different types of polymer shell to form a core-shell metal nanoparticle has become an important route to stabilize/immobilize metal nanoparticles [20–22]. Polyelectrolytes act as an excellent stabilizing agent for growth of polymer shell which may be used for immobilization of metal nanoparticles [23]. Schrinner et al. also supported the stabilization of gold nanoparticles by spherical polyelectrolyte brushes [24]. Shchukin et al. published the stabilization of silver nanoparticles by polyelectrolyte and poly (ethylene glycol) by layer to layer assembly to prevent aggregation and increase in mono dispersity [25]. These types of homogeneous form of polyelectrolyte stabilized metal nanoparticles catalyst have some recoverability problem in aqueous/organic phase during reaction condition. Hence, Ag nanoparticles stabilized with magnetic nature of Fe3O4 NPs solve the recovery of the catalyst both in organic and aqueous phase. Methylene blue is a common dye mostly used in textile, rubber, plastics, leather, cosmetics, pharmaceutical and food industries [26,27]. All the available methods have got significant disadvantages such as removal of incomplete ions or other waste products in treating the effluents. Hence, further disposal methods are needed such as biodegradation [28], chemical oxidation [29–31], foam flotation [32], electrolysis [33], adsorption [34], electro-coagulation [35] and photocatalysis [36,37] to completely remove the dye [38]. However, these
2. Experimental method 2.1. Materials FeSO4 7H2O (SRL), FeCl3 6H2O (Loba Chemie), AgNO3 ((Sigma Aldrich), benzylamine (Sigma Aldrich), epichlorohydrin (Sisco Research Laboratories (SRL)), oleic acid (Merck), HCl (Merck), methylene blue (Merck), NaBH4 (SRL) and NH4OH (SRL) were purchased. Other solvents such as methanol, CH2Cl2, CHCl3, CDCl3, C2H5OH, THF, acetone, diethyl ether and deionized water were purchased from SRL and used further without any purification.
2.2. Preparation of magnetic Fe3O4 NPs Magnetic Fe3O4 nanoparticles were prepared by co-precipitation method [39]. Initially, 0.5 mol L−1 FeCl3 6H2O and 0.25 mol L−1 FeSO4 7H2O were dissolved in 100 mL of deionized water with mechanical stirrer. Then, NH4OH solution was added slowly by adjusting the pH 11.0–12.0, with continuous stirrer for 2 h at 65 °C. It was then followed by addition of 30 mL of oleic acid (OA) in a slow paced manner to the above reaction mixture and the pH 6.0–7.0 was adjusted by dilute HCl, with continuous stirrer for 6 h at 80 °C (Scheme 1). The resultant product, the black precipitate was collected, washed and later dried at 60 °C for overnight. The obtained product, Fe3O4-OA NPs was characterized by FT-IR, SEM, VSM, and XRD.
Scheme 1. Synthesis of Poly (benzylamine) and Fe3O4-PBA-AgNPs. 422
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catalysts and concentration of MB. The kapp can be obtained from the slope of linear plots of ln (Ct/C0) versus time. The % of degradation efficiency determined is calculated by the following formula
2.3. Synthesis of Poly (benzylamine) brushes Poly (benzylamine) brushes [PBA] were prepared by solvent free reaction under room temperature. Initially, 10 g of benzylamine (BA) and 9.29 g of epichlorohydrin (ECH) were poured into 100 mL RB flask equipped with a magnetic stirrer at room temperature (RT) for 6 h (Scheme 1). The complete polymerization was allowed at RT for overnight and thus the white precipitate was obtained. This was later dissolved in CHCl3 and again precipitated with diethyl ether. The obtained PBA was filtered and dried at 40 °C for overnight and the product was characterized by 1H NMR (Fig. S1) and GPC (Fig. S2).
X = (C0 − C / C0) × 100%
Where, C0 and C are the solution concentration or absorbance before and after the degradation, respectively. The concentrations of MB were estimated from the absorbance value of UV-vis spectra. 2.7. Instrumentation
2.4. Synthesis of Poly (benzylamine) brushes grafting on to Fe3O4 NPs
FT-IR analysis was carried out on BRUKER, TENSOR 27 model instrument. The UV-vis spectra were measured on shimadzu instrument (UV-2450 series). The ultra-sonication of all magnetic materials was done by PCi analytics sonication bath at 40 KeV. The surface morphology study was performed using VEGA3 TESCAN model scanning electron microscope (SEM). The X-ray diffractometer using a Cu kλ monochromatic radiation source (λ = 1.54045). The magnetic susceptibility values were recorded for prepared magnetic core-shell based nanocomposites using Lakeshore Model 7404 with high and low-temperature attachments, at the maximum magnetic field −2.17 T (0.6 air gap). FEI TECNAI G2 Model T-30 was used for measuring morphology by HR-TEM, Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer (SII) under nitrogen atmosphere in a temperature range of 30–750 °C at a heating rate of 8 °C min−1. Synthesized poly (benzylamine) was analyzed by 1H NMR spectra using CDCl3 as a solvent in a JEOL spectrometer at 500 MHz, Proton chemical shifts (δ) are expressed in parts per million and are related to TMS, which is an internal standard. Gel permeation chromatograms were used for determining molecular weight by Shimadzu instrument (CTO-20 A) with THF. EZRaman 785 series, Diode Laser power 60 mW. Powder was spread on the glass slide with an integration of 10 and box car 2.
Initially, 0.5 g of Fe3O4-OA NPs was dissolved in ethanol (40 mL) by ultra-sonication for 30 min, and then added to 0.25 g of PBA/THF (40 mL) mixture. The above reaction mixture was stirred mechanically at 80 °C for 6 h (Scheme 1). The resulting reaction mixture was filtered, washed and then dried at 60 °C for overnight. The obtained magnetic core coated PBA was labeled as Fe3O4-PBA NPs and thus characterized by FT-IR and Raman analyses. 2.5. Synthesis of three different types of magnetic core-shell supported nanoparticles catalyst The stabilization/immobilization of magnetic core-shell supported Ag NPs catalyst was investigated using three different magnetic core supports viz., Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4 PBA NPs and AgNO3 as metal precursor. Initially, 0.5 g of Fe3O4NPs, Fe3O4-OA NPs and Fe3O4 PBA NPs were taken individually in three different 100 mL RB flask and dispersed in ethanol (50 mL) under ultra-sonication for 30 min. After that, 1.4 × 10−3mM of AgNO3 (10 mL) was added individually to three different RB flasks containing Fe3O4 NPs, Fe3O4-OA NPs, and Fe3O4-g-PBA NPs supports, and then the reaction mixture was refluxed with mechanical stirring at 60 °C. Then, 1 mL of NaBH4 aqueous solution (10 mM) was added individually at room temperature for 4 h. The observation color changed from brown to black, which thereby indicating the formation of Ag0 from Ag+. The resulting product was filtered, washed and dried at 60 °C for overnight (Scheme 1). Then, Ag NPs immobilized/stabilized on Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4-gPBA NPs magnetic core-shell nano-composite catalysts were labeled as Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs, respectively, and thus characterized by FT-IR, VSM and Raman analyses.
3. Results and discussion 3.1. XRD analysis In order to confirm the presence of crystalline magnetite within the polymeric poly (benzylamine) brushes, the structure of each core-shell nanoparticles were analyzed with XRD. The XRD patterns of Fe3O4 NPs, Fe3O4-OA NPs, and Fe3O4-PBA-Ag NPs are shown in Fig. 1A respectively. The XRD pattern of Fe3O4 shows the characteristic diffraction peaks for 2θ values at 30.49 (220), 35.78 (311), 43.37 (400), 57.34 (511) and 62.92° (440). It was found to be in good agreement with the standard pattern of Fe3O4 that proved the existence of cubic spinal phase as shown in Fig. 1(A)a. The average diameter of the Fe3O4 was estimated using Debye Scherrer equation and the average crystalline size was calculated as 15.31 nm. The magnetic Fe3O4 nanoparticles were stabilized using OA as capping agent. Fig. 1(A)b XRD pattern resembles to that of magnetic Fe3O4 nanoparticles with decreased intensity thereby confirming that the structure of Fe3O4 nanoparticles is retained even after coating of OA as a stabilizer. The immobilization/ stabilization of Ag NPs onto the core-shell nanocomposites (Fig. 1(A)c) have shown 2θ peaks corresponding to Ag NPs at 38.11 (111), 44.27 (200), 58.2 (220) & 64.42° (311) and this confirms the stabilization of Ag NPs onto the surface of Fe3O4-PBA NPs. The average diameter of the Ag NPs was estimated by Debye Scherrer equation and the crystallite size was calculated as 23.12 nm. The size of the nanoparticles was found to increase in the order of Fe3O4 (15.31 nm), Fe3O4-OA (17.31 nm) and Fe3O4-PBA-AgNPs (23.12 nm).
2.6. Catalytic efficiency of Fe3O4-AgNPs, Fe3O4-OA-AgNPs and Fe3O4-gPBA-AgNPs catalysts for reduction of methylene blue The three different types of nano-composite catalysts viz., Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4 PBA-Ag NPs were tested for MB reduction/degradation under the pseudo-first order reaction condition. The kinetic experiments were performed following parameters like 50 mL of MB (1 × 10−3 mM), 5 mL of NaBH4 (1 × 10−1 mM) and 6 mg of respective catalyst viz., Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4PBA-Ag NPs were used. The reaction was monitoring the UV-Vis spectrum range from 200 to 700 nm at regular intervals of time (3 min). The rate of the reaction was used to determine the decreasing trend of absorbance at 664 nm NaBH4 was used in the reduction of methylene blue; a pseudo-first-order kinetics was involved to carry out the rate of the reaction [37,40]. Therefore, the apparent rate constant (kapp) can be defined by the following equation, -dcMB/dt = kappCMB
(2)
(1)
3.2. 1H NMR and GPC analysis
Where, Kapp is the apparent rate constant, which is related to the concentration of silver content in the polymer brushes stabilized nano-
The formation of Poly (benzylamine) by using BA and ECH at room 423
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Fig. 1. (A) XRD analysis of a) Fe3O4 NPs, b) Fe3O4-OA NPs and c) Fe3O4–PBA-Ag NPs (B) FT-IR spectra of a) Fe3O4 NPs, b) Fe3O4-OA NPs, c) Fe3O4–PBA NPs and d) Fe3O4 –PBA-AgNPs, (C) VSM spectra of a) Fe3O4 OA NPs and b) Fe3O4-PBA-AgNPs (D) TG analysis of a) Fe3O4-OA-AgNPs and b) Fe3O4-PBA-AgNPs catalysts.
temperature reaction was confirmed by 1H NMR spectrum as shown in Fig. S1. It can be observed that –N–CH2– and –CH–CH2–N– protons are appear in the region of 2.3–2.8 ppm, the same way, the addition of peaks was observed at 3.47 ppm which can be assigned as –CH–OH protons. Further, 1H NMR spectrum shows signals at 7.5–7.0 ppm absorbing the phenyl proton and 3.41 ppm for C6H5–CH2– protons. The equine-molar ratio of benzylamine and Epic chlorohydrin was used to synthesize polymer brushes as a stabilizing agent by condensation reaction in the room temperature. The molecular weight of PBA was confirmed by GPC [35]. Based on the GPC analyzed result as shown in Fig. S2, the average molecular weight was found to be 539 (g/mol) and poly disparity index, Mw/Mn as 1.37.
was analyzed through FT-IR, as shown in Fig. 1 (B)d. Compared with Fig. 1(B)c-d showed a shift in the peak at 1095 cm−1 in Fe3O4-PBA NPs to 1114 cm−1, which confirmed the immobilization of Ag NPs on Fe3O4-PBA NPs catalyst. 3.4. VSM analysis The prepared material was tested for magnetic properties of catalysts such as Fe3O4 OA NPs & Fe3O4-PBA-Ag NPs by VSM technique. Fig. 1(C)a and Fig. 1(C)b show a hysteresis loop of typical Fe3O4 & Fe3O4-PBA-AgNPs nano-composite catalysts measured by sweeping the external field between −2 and 2 T at room temperature. The saturated mass magnetization is to be 63.4 & 17.08 emu/g, respectively. However, the prepared magnetic nano-composite catalyst via, Fe3O4-PBAAgNPs can be easily recovered by applying the external magnetic fields, which may be a separate nano-catalyst used for the further reaction. This is because, on surface coating of PBA onto the Fe3O4-OA NPs, the Ms value was found to decrease as the thickness of the PBA layer becomes dense due to the formation of PBA grafted over Fe3O4-OA NPs.
3.3. FTIR analysis The FTIR spectrum of Fe3O4 NPs, Fe3O4-OA NPs, Fe3O4-PBA NPs, and Fe3O4-PBA-AgNPs are shown in Fig. 1(B)a-d respectively. From Fig. 1(B)a, the characteristic peak was noticed at 578 cm−1 due to Fe-O band, this peak is an evidence for the formation of Fe3O4 NPs. From the FTIR spectrum of Fe3O4-OA NPs shows the characteristic peaks at 3429 cm−1, 578 cm−1 and 451 cm−1 thereby corresponding to the –OH (hydroxyl group), Fe2+-O2−and Fe3+-O2- respectively. Fig. 1(B)b shows the peaks at 2929 cm−1and 2853 cm−1 which can be attributed to the saturated-C–H groups in oleic acid. Further, the characteristic peak appeared at 1708 cm−1 and it can be assigned as stretching vibration of carbonyl group (ʋC]O) of OA. All these peaks evidenced that oleic acid was coated onto the Fe3O4 NPs. Further, PBA was coated onto the OA-Fe3O4 NPs (Fe3O4-PBA), which might be the strong interaction between PBA and OA-Fe3O4 NPs. The FTIR spectrum of Fe3O4-PBA NPs is shown in Fig. 1(B)c. When compared with Fig. 1(B)b, Fig. 1(B)c showed the presence of new characteristic peaks at 1628 & 1263 cm−1 for an aromatic ring, 1095 cm−1 for C-N and 3392 cm−1 for OH. These peaks evidenced that the PBA was coated onto the Fe3O4-OA NPs. The immobilization/stabilization of Ag NPs onto the Fe3O4-PBA NPs and thus obtained product
3.5. TG analysis Thermogravimetric analysis was employed to study the thermal behavior of the magnetic polymer brushes stabilized nanoparticle catalysts. Fig. 1D shows the TGA studies of Fe3O4-OA-Ag NPs and Fe3O4PBA-Ag NPs, which confirms the weight change during the PBA stabilized Ag NPs. The weak thermal stability of magnetic Fe3O4 nanoparticle is already reported [37,41]. The stability of magnetic Fe3O4 NPs can be enhanced by the addition of OA as a capping agent. Fig. 5a shows a major weight loss of about 12.9% in the temperature range between 175 and 371 °C due to the decomposition of OA, after that no weight loss occurs because of the stability of magnetic Fe3O4-OA NPs. Fig. 5b shows the TGA results of Fe3O4-PBA-Ag NPs, where the grafting of PBA over magnetic supports as Fe3O4-OA NPs. The magnetic PBA stabilized Ag NPs shows only one major weight loss of 20% in range 424
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due to the presence of more amounts of Ag NPs stabilized by the newly developed PBA, coated Fe3O4-OA. 3.7. Raman analysis Raman spectra of Fe3O4NPs, Fe3O4-OA NPs, Fe3O4-PBA NPs and Fe3O4-PBA-AgNPs are shown in Figs. S3a–d respectively. Fig. S3a shows a characteristic peak at 680 cm−1 due to the formation of Fe-O-Fe bond. After coating with oleic acid onto the surface of Fe3O4, the peak intensity of Fe-O-Fe (680 cm−1) decreased as shown in Fig. S3b. Further coating with PBA onto Fe3O4-OA NPs, the peak intensity of Fe-O-Fe (680 cm−1) decreased again as in Figs. S3c and a new peak was appeared at 602 cm−1, due to C-N stretching band, which implied that the PBA was coated onto the Fe3-O4-PBA. After immobilization/stabilization of metal nanoparticles onto Fe3O4-PBA NPs (Fig. S3d), the characteristic peak for C-N stretching band at 602 cm−1 was shifted to 610 cm−1, these peaks confirmed the immobilization/stabilization of metal nanoparticles onto the PBA coated magnetic nanoparticles. Fig. 2. UV-vis spectra of a) Fe3O4-OA-AgNPs and b) Fe3O4-PBA-Ag NPs catalysts.
3.8. HR-SEM and TEM studies HR-SEM was used to study the morphology of most of the stabilizing catalysts, including Fe3O4-PBA-Ag NPs. Fig. 3a-b shows spherical morphology of Fe3O4-PBA-Ag NPs. The presence of spherical particles even after PBA grafting interpret that the morphology of F Fe3O4 NPs and Fe3O4-OA NPs may also be spherical in nature. Also, Ag NPs was used to stabilize/immobilize Fe3O4-PBA NPs. The intense spherical black dot observed in SEM images indicates the presence of Ag NPs. HR-TEM images of the Fe3O4-PBA-Ag NPs catalyst are shown in Fig. 3c-d which confirms its spherical morphology. The TEM images also illustrate the presence of core-shell nanocomposites of Fe3O4-PBA NPs surrounded by fine dispersion of Ag NPs. From the intense black dots and patches, the average particle size was calculated to be 7 nm. The presence of grey and black particles in Fig. 3c confirmed the coverage of PBA polymer stabilizing agent over the surface of Fe3O4 NPs. The black particles may be due to the presence of Fe3O4 NPs and grey region due to PBA polymer stabilizing agent. Fig. 3d indicates springer, which is due to Ag NPs onto the surface of Fe3O4 NPs. The morphology observed from TEM results are in accordance with SEM images. The sharp intense XRD peaks also confirm the absence of bulk formation of
from 180 to 350 °C, which may be due to presence of polymer brushes coated onto the surface of Fe3O4-OA NPs. The TGA result also confirmed the growth of polymers over the surface of Fe3O4-OA NPs and revealed that the polymer brushes stabilized Ag NPs catalyst better on the material as compared to the other catalysts. 3.6. UV-vis analysis The conformation of Ag NPs and its immobilization onto the Fe3O4 NPs, Fe3O4-OA NPs and Fe3O4-PBA NPs were established through UVVis analyses. For understanding and clarity, the UV-Vis spectrum obtained from Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs are shown in Fig. 2a-c. From, each UV-Vis spectrum, it can be observed that the typical Surface Plasmon Resonance (SPR) peak for Ag NPs appeared at 410 nm. Therefore, the appearance of SPR peak at 410 nm has undoubtedly the formation of Ag NPs over the Fe3O4-PBA NPs matrix. The peak intensity of Fe3O4-PBA-AgNPs was greater than the peak intensity of Fe3O4-Ag NPs and Fe3O4-OA-Ag NPs, which may be
Fig. 3. Electron microscopy images of Fe3O4-PBA-AgNPs catalyst: (a) and (b) SEM images, (c) and (d) HR-TEM images. 425
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Table 1 Comparative catalytic activity of apparent rate constant, TOF and the efficiency towards reduction of MB using MNPs catalyst. S.No
Catalysts
Kapp × 10−2 (s−1)
TOF × 10−3 (s−1)
DE (%)
R2
Ref
1 2 3 4 5 6 7 8 9 10
Fe3O4-Ag NPs Fe3O4-OA-Ag NPs Fe3O4-PBA-Ag NPs Fe3O4-PAC-AuNPs Fe3O4-HEA-AuNPs Fe3O4–PAMAM-G (0)-AuNPs Fe3O4–PAMAM-G (1)-AuNPs Fe3O4–PAMAM-G (2)-AuNPs NiFe2O3 Fe3O4/PS
2.0 3.3 15.6 2.90 2.44 0.65 1.30 3.55 – –
5.01 83.3 285.7 1.18 1.0 – – – – –
52.5 67.3 99.1 44.23 29.15 43 72 85 33.7 80
0.9564 0.9800 0.9923 0.9921 0.9925 – – – – –
This work This work This work [42] [42] [43] [43] [43] [44] [45]
Kapp = Apparent rate constant, TOF = Turn over frequency, DE = degradation efficiency, R2 = Correlation coefficient.
nanocomposites.
3.9. Effect of stabilization on the catalytic activity of Fe3O4-PBA-Ag NPs The catalytic activity of the Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs catalysts was studied individually for the reduction of MB with NaBH4 under identical pseudo first order reaction condition to calculate the Kapp and TOF based on the above mentioned formula (1) and (2). The reaction mesearued by the peak intensity disappered which may be due to the reduction of MB. The effective reduction was calculated and given in Table 1. These values were used to estimate the catalytic activity of individual nano-composite catalysts. The reduction/degradtion of MB was monitored at 664 nm which decreased against time as shown in Fig. 4. A control observation used to was test without catalyst whereby they cannot alter the reaction as there is no occurrence of reduction, even after two days, henceforth confirmed that the reaction kinetics was restricted. The kapp, and TOF were calculated with the same amount of catalyst for Fe3O4-Ag NPs, Fe3O4-OA-Ag NPs and Fe3O4-PBA-Ag NPs catalysts through the reduction/degradation of MB. The apparent rate constant and TOF of Fe3O4-Ag NPs, Fe3O4-OA-Ag
Fig. 4. UV-Vis spectra of MB reduction using Fe3O4-PBA-AgNPs catalyst.
Fig. 5. Normalized absorbance at λmax of MB vs. Ag NPs catalysts, Ct is the conc. of MB at real time ‘t’ and C0 is the conc. of MB. (B) Ln (Ct/C0) at 664 nm vs. reaction time for the reduction of MB. (C) Figure of apparent rate constant (Kapp) as a function of different catalysts (D) Figure of TOF as a function of different catalysts. 426
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Fig. 6. GC-MS images of a) Initial concentration of MB and B) After reaction take place in presence of Fe3O4-PBA-Ag NPs catalysts.
NPs and Fe3O4-PBA-Ag NPs catalysts are 2.0 × 10−2 s−1, 3.3 × 10−2 s−1 & 15.6 × 10−2 s−1 and 5.0 × 10−3 s−1, 83.3 × 10−3 s−1 and 285.7 × 10−3 s−1, respectively. Among the different types of Ag NPs catalysts studied, the rate constant and TOF is considered to be better for Fe3O4-PBA-Ag NPs. Similarly, the degradation efficiency of the catalysts was investigated and found to be that the catalytic activity of synthesized poly (benzylamine) brushes onto magnetic Fe3O4-OA NPs used to stabilize/immobilize Ag NPs was better than other catalysts. This may be due to the stability of the Ag NPs stabilized/immobilized by PBA as compared to the rest of the catalysts and values as given in Table 1 and the corresponding plot is also given in Fig. 5. Further, the superior catalyst viz., Fe3O4-PBA-Ag NPs was examined through the reaction condition by altering the concentration. Brushes onto magnetic Fe3O4-OA NPs and stabilization/immobilization of Ag NPs is considered to be the superior catalytic activity because of the size of the MNPs stabilization/immobilization as compared to rest of the catalyst values as given in Table 1 and the corresponding plot is also given in Fig. 5. In order to establish the right reaction condition for effective reduction of MB, the reaction has to be optimized. Hence, this superior catalyst viz., Fe3O4-PBA-AgNPs was employed to determine the through kinetics of the same reduction/degradation of MB by varying the experimental parameters such as [catalyst] and [MB].
PBA-Ag NPs under the optimum reaction condition (50 mL of MB (1 × 10−3 mM), 5 mL of 1 × 10−1 mM NaBH4, 6 mg, time 21 min) and the efficiency of the catalysts was measured and analyzed by GC-MS (Fig. 6a). There is clear evidence from the mass spectra of the experiment done before and after degradation. The peaks appeared at low retention time (Fig. 6b) indicating the fragmentation of the MB dye to small molecules. The degradation mechanism of MB can be understood as follows. The surface active sites of Fe3O4-PBA-Ag NPs can react the BH4− (a strong nucleophile). This supplies electron to MB thereby degradation of MB (a good electrophile) occurs faster. The faster degradation of MB with BH4− an electron rich moiety may be due to columbic interaction. Similarly, the diffusive nature and high electron injection capability of BH4− help to transfer electron to the substrate MB via Fe3O4-PBA-Ag NPs and thus overcome the kinetic barrier for the degradation MB [37]. Further, the stability of the superior catalyst viz., Fe3O4-PBA-AgNPs was examined through the same reaction under the identical condition and by employing its recovered catalyst up to 7th cycle. 3.9.2. Effect of catalyst weight The effect of catalyst weight on the catalytic efficiency of the superior catalyst viz., Fe3O4-PBA-AgNPs was examined by varying the catalyst weight from 2 to 10 mg and keeping another parameter as constant. The Kapp values (Fig. 7) are in the range of 2.85 × 10−2 to 30 × 10−2 s−1 for catalyst weight from 2 to 10 mg. Also, the TOF value
3.9.1. GC-MS analysis for reduction of MB The photocatalytic degradation of MB was carried out on Fe3O4427
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Fig. 7. (A) Normalized absorbance at λmax of MB vs Fe3O4-PBA-AgNPs catalysts, Ct is the conc. of MB at real time t and C0 is the conc. of MB, (B) ln (Ct/C0) at 664 nm vs. reaction time for the reduction of MB, (C) Figure of apparent rate constant (kapp) as function of different catalysts (D) Figure of TOF as function of different catalyst.
Table 2 Effect of catalysts for apparent rate constant, TOF and the efficiency towards reduction of MB. S.No
Fe3O4-PBA-Ag NPs
Kapp × 10−2s−1
TOF × 10−3 s−1
Degradation Efficiency (%)
Correlation coefficient
1 2 3 4 5
2.0 mg 4.0 mg 6.0 mg 8.0 mg 10.0 mg
2.87 6.54 9.0 16.54 30.74
230 250 285 313 333
93 96 99 99.9 99.9
0.9931 0.9904 0.9931 0.9905 0.9903
Fig. 7 a, b, c and d, respectively [42]. 3.9.3. Effect of MB concentration Effect of MB concentration of the reaction was also studied by using different concentrations of the methylene blue such as 6.0 × 10−3 mM, 8.0 × 10−3 mM, 10 × 10−3 mM, 12 × 10−3 mM & 14 × 10−3 mM and other parameter as constant. The plot of C/C0 and ln (C/C0) values are given in Fig. S4 that shows a decrease in Ct/Co and ln (Ct/Co) value with an increase in [MB]. This may be due to the availability of more molecules for excitation and consecutive degradation. The degradation efficiency of MB was found to be decreasing in the Ct/C0 and ln (Ct/C0) value on increasing MB concentration. The increase in the dye concentration always leads to lesser amount of active sites over the surface of the catalyst as all the active sites will be occupied by the reactant molecules and eventually the activity will decrease. In this case, with an increase in time and concentration of methylene blue, the degradation efficiency of the catalyst decreases.
Fig. 8. Recycle efficiency of Fe3O4-PBA-Ag NPs catalyst.
ranges from 230 × 10−3 s−1 to 329 × 10−3 s−1 as illustrated in Fig. 7. The corresponding Kapp, TOF, degradation efficiency and correlation co-efficient for the effect of catalyst weight on the catalytic efficiency of Fe3O4-PBA-Ag NPs are given in Table 2. Both the apparent rate of the reaction and TOF increases with an increase in the concentration of catalyst. The active sites of the surface may enhance the electron transfer to the reaction mixture and also as noticed there was faster degradation with increased Ct/C0, ln (Ct/C0), Kapp, and TOF as shown in
3.9.4. Recycling efficiency Catalyst stability of Fe3O4-PBA-Ag NPs during the reaction was also tested. In the industrial reaction conditions, used catalyst was recycled with an applied external magnetic field to remove the catalyst followed by washing of the solvents. Then, the recycled catalyst was tested under same reaction condition and the Kapp value obtained initially was 15.6 × 10−3 s−1. Fe3O4-PBA-Ag NPs which was later examined up to 7th cycles and the obtained kapp value was 14.0 × 10-3 s−1, which was later found to be almost similar irrespective of the cycle as shown in 428
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Fig. 8. The small dropping of kapp value after several times was due to the washing of Ag NPs catalyst.
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4. Conclusion In conclusion, three types of magnetic recoverable catalysts were developed by a simple, reproducible, non-toxic, highly active and facile method. The electrostatic interaction between the PBA and Ag NPs may stabilize the metal nanoparticles over the surfaces, plentiful nanoparticles with 5–7 nm in size indicates the surface of Fe3O4-PBA matrix to form Fe3O4-PBA-Ag NPs catalyst. These AgNPs immobilized nanoparticles exhibit excellent catalytic potential for the reduction of MB with NaBH4 compared with Fe3O4-AgNPs and Fe3O4-OA-AgNPs. The kapp, and TOF for Fe3O4-AgNPs, Fe3O4-OA-AgNPs and Fe3O4-PBAAgNPs catalysts with same amount of catalyst were examined for the reduction/degradation of MB and found to be 2.0 × 10−2 s−1, 3.3 × 10−2 s−1 & 15.6 × 10−2 s−1 and 5.0 × 10−3 s−1, 83.3 × 10−3 s−1 and 285.7 × 10−3 s−1, respectively. Among the different types of Ag NPs catalysts studied, Fe3O4-PBA-Ag NPs were found to have apparent better rate constant and TOF than other catalysts. Moreover, the TGA analysis gives appropriate information about the polymer brushes stabilized magnetic core-shell; these nanocomposites have 20% of weight loss. On the other hand, the saturation magnetization of grafted material value is 17.08 emu/g. Besides, the recycling efficiency of magnetic core-shell catalyst viz, Fe3O4-PBA-Ag NPs was examined up to seventh cycle & no loss of catalytic activity. It was observed that the grafted magnetic core-shell Fe3O4-PBA-Ag NPs nanocomposites catalyst exhibited higher catalytic activity and reproducibility towards the degradation of MB. Hence, the magnetic core-shell nanocomposite is an excellent and environmentally friendly method for removal of environmental pollutant MB. Conflicts of interest The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgments The authors are grateful for the financial support provided by UGCSAP, New Delhi, India the grant sanction no. 540/3/DRS-III/2016 (SAP-I) dt 21.05.2018. Authors have gratefully acknowledged the University Grants Commission, Department of Science & Technology, Defence Research and Development Organization (DRDO) New Delhi and Department of Chemistry, Anna University, Chennai to support and carry out this research work. Finally, the authors acknowledge National Centre for Catalysis Research (NCCR). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchemphys.2019.03.034. References [1] Y. Xu, C. Liu, D. Khim, Y.Y. Noh, Development of high-performance printed organic field-effect transistors and integrated circuits, Phys. Chem. Chem. Phys. 17 (2015) 26553–26574. [2] M.M. Mohamed, M.S. Al-Sharif, One pot synthesis of silver nanoparticles supported on TiO2 using hybrid polymers as template and its efficient catalysis for the reduction of 4-nitrophenol, Mater. Chem. Phys. 136 (2012) 528–537. [3] M.J.S. Mohamed, S.U. Shenoy, D. K Bhat, Novel NRGO-CoWO4-Fe2O3 nanocomposite as an efficient catalyst for dye degradation and reduction of 4-nitrophenol, Mater. Chem. Phys. 208 (2018) 112–122. [4] J.S. Cheng, Y. Wu, Z.H. Xn, B. Hu, J.M. Bai, J.P. Wang, J. Shen, Hydration dynamics of oriented DNA films investigated by time-domain terahertz spectroscopy, Appl. Phys. Lett. 90 (2007) 233902. [5] P.V. Kamat, Meeting the clean energy Demand: Nanostructure architectures for solar energy conversion, J. Phys. Chem. C 111 (2007) 2834–2860.
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