Synthesis of Ag nanoparticles decoration on magnetic carbonized polydopamine nanospheres for effective catalytic reduction of Cr(VI)

Journal of Colloid and Interface Science 526 (2018) 1–8

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Synthesis of Ag nanoparticles decoration on magnetic carbonized polydopamine nanospheres for effective catalytic reduction of Cr(VI) Changlun Chen a,b,c,⇑, Kairuo Zhu a,b, Ke Chen a, Ahmed Alsaedi c, Tasawar Hayat c,d a

CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, PR China c NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia d Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 14 March 2018 Revised 22 April 2018 Accepted 24 April 2018 Available online 25 April 2018 Keywords: Ag NPs Fe3C NPs Cr(VI) Catalysis Reduction

a b s t r a c t More discrete and active Ag nanoparticles (Ag NPs) were fabricated by decorating them on the surface of magnetic nanoparticles encapsulated in carbonized polydopamine nanospheres (M/C-PDA/Ag) via in-situ solid-state decomposition process. The morphology, structure, surface compositions and textural properties of the M/C-PDA and M/C-PDA/Ag catalyst were characterized. The results revealed a dispersion of Ag NPs with average particle size of less than 50 nm on C-PDA nanospheres uniformly embedded with Fe3C NPs of only 3–5 nm in size. With the synergistic effect of Ag NPs, nitrogen doping, and hierarchical mesopores, M/C-PDA/Ag displayed a superior catalytic capability for catalytic reduction of toxic Cr(VI) to lesstoxic Cr(III) by formic acid as a reductant. Moreover, M/C-PDA/Ag maintained good physicochemical structure and stable activity even after several cycles of reactions. According to the results, the simple synthetic strategy, good stability, highly catalytic activity, and easy magnetic separation property of M/C-PDA/Ag hybrid make it serve as a promising environmentally friendly catalyst for the elimination of Cr(VI). Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author at: CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Plasma Physics, Chinese Academy of Sciences, P. O. Box 1126, Hefei 230031, PR China. E-mail address: [email protected] (C. Chen). https://doi.org/10.1016/j.jcis.2018.04.094 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

Cr(VI) is very highly toxic and has a significant threat to living organisms like humans and animals because of its potential toxicity, mutagenicity, and carcinogenicity [1–4]. As such, the world health organization has put a strain on Cr(VI) level to limit a maximal allowable concentration of 0.05 mg
L1 in drinking water [5].

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

Many channels have so far been implemented to capture Cr(VI) from water including adsorption, ion exchange, and membrane separation etc [6–8]. Notably, Cr(III), unlike the toxic hexavalent counterpart, is much less soluble in water and less-toxic to human [9]. Thereby, reducing Cr(VI) to less-toxic and immobile Cr(III) is considered as one of the most efficient approach to eliminate the Cr(VI)-containing contamination, and has been extensively explored recently. Formic acid (FA), a green chemical produced from renewable biomass resource, is a most promising reductant for the transformation of Cr(VI) into Cr(III) due to the strong reducing ability and without the production of any toxic intermediates [10,11]. To date, various catalysts including AuPd@Pd [12], Pd@ZIF-67 [9], 3D Ni@N-C [13], etc. have been employed to reduce Cr(VI) to Cr(III) in the presence of FA. However, these catalysts either suffer from expensive manufacturing cost or may have complicated fabrication processes, which hinder their large-scaled preparation. Hence, developing novel environmentally benign functional materials with relatively simple fabrication process for efficient reduction of Cr(VI) with FA is still underway. More recently, noble metal nanoparticles are frequently used for catalytic Cr(VI) reduction [14–16]. Especially the Ag, owing to its lower price as compared with Pd, Pt, Au, has received more and more attention. Since Ag NPs are prone to aggregate and hard to recover, immobilizing them on magnetic materials as supports is regarded as one of the most facile and efficient approaches [17–19]. One of the most important natures of magnetic supports is their separation properties. Due to the supermagnetic character of the supports, it assures trouble-free separation of the noble metal from the reaction mixture using an external magnet and eliminates the necessity of filtration or centrifugation [20]. Nevertheless, magnetic NPs are unstable in acidic medium and are prone to the formation of large aggregates, so the synthesis of magnetic composite supports with a protective coat has been developed to prevent magnetic NPs from being eroded in acidic and base media or aggregation [21]. Among these techniques, the design and synthesis of the ‘‘glue layer” is an important issue for incorporating Ag NPs onto magnetic supports to realize the multifunctional catalysts. Dopamine (DA), a nontoxic and hydrophilic biomolecule that contains catechol and amine functional groups, can selfpolymerize in alkaline solution at room temperature to form polydopamine (PDA). It is also an outstanding carbon source that can yield thin carbon coatings [22]. Carbonized PDA can provide better chemical stability for use as recyclable catalyst supports, facilitating the overall performance for the noble metal NPs [23]. More importantly, DA can interact with metal ions (i.e. Fe, Ni, Co) via immobilization of a ligand and subsequent metal coordination to form metal/PDA complexes as desirable precursors of magnetic NPs embedded in C-PDA coatings [24,25]. Therefore, C-PDA appears to meet all the relevant criteria to be the ideal ‘‘glue layer” for the multifunctional catalysts. However, to the best of our knowledge, there were no previous reports combined the two strategies of DA to form a novel functional nanocomposite and utilized in the field of catalysis. Inspired by this proposal, we report a novel self-template strategy for the synthesis of the distributed magnetic NPs encapsulated in carbonized polydopamine nanospheres decorated with Ag NPs (M/C-PDA/Ag) through in-situ solid-state decomposition method, using Fe/PDA complex nanospheres as the precursor of C-PDA embedded with magnetic NPs and the support of Ag NPs, wherein the initially formed Fe/PDA coordination polymer acts as the buildin template during the subsequent annealing process [26]. To illustrate this concept, Scheme 1 shows the synthetic procedure for the multifunctional M/C-PDA/Ag composite. We firstly synthesized Fe/ PDA complex nanospheres via one-pot Fe3+-mediated polymerization of DA. Subsequently, the Fe/PDA complexes were employed to

+

Ag decoration

self-polymerization

= Fe3+

= DA

= Magnetic NPs

= Carbonized PDA

= Ag NPs

= Annealed Ag NPs

Annealing

2

Scheme 1. Synthetic procedure of the Ag nanoparticles decoration on magnetic CPDA nanospheres used as catalyst for the Cr(VI) reduction.

capture Ag NPs, where the PDA serves as a reductant to convert Ag+ into metallic Ag and anchor the Ag NPs [27]. Upon heat treatment, the Fe/PDA complex nanospheres can be easily converted to C-PDA nanospheres with evenly distributed magnetic NPs of only 3–5 nm in size [23], and the annealing process facilitated to improve the crystallinity of the Ag NPs without destroying the connection [28]. Based on to the current report, M/C-PDA was often fabricated by three steps like the formation of magnetic NPs, the selfpolymerization of DA and in-situ thermolysis of M/PDA [29–32]. Hence, the synthetic procedure of the M/C-PDA in this investigation is facile and practical. The as-synthesized M/C-PDA/Ag was used to reduce Cr(VI) with FA, which could transform Cr(VI) into Cr(III), and its stability was also assessed. More importantly, the M/C-PDA/Ag could be readily recovered under an external magnetic field. In addition, the possible catalytic mechanism for the excellent performance of the M/C-PDA/Ag catalyst was also elucidated. 2. Experimental section 2.1. Chemicals All common chemicals were purchased from Sigma-Aldrich and used without further purification. 106 mg
L1 of Cr(VI) stock solution was prepared by dissolving an appropriate amount of K2Cr2O7 in deionized water. 2.2. Preparation of the M/C-PDA/Ag Fe/PDA nanospheres were synthesized on the basis of the previous reports [23]. First, 1.0 g of DA was added into 1000 mL of deionized water. Then, 1.32 mmol of FeCl3
6H2O was added. After 30 min, 10 mmol of Tris was added to adjust the pH and the reaction mixture was constantly stirred for 30 h. The obtained Fe(III)/PDA complexes were separated by centrifugation, washed with deionized water, and then freeze-dried for one day. The Ag NPs were deposited onto Fe/PDA nanospheres via a simple dip/ redox process [33]. In a typical experiment, the 0.3 g Fe/PDA was dispersed into 40 mL of deionized water, then added 10 mL of AgNO3 solution into above suspensions and kept still for another 6 h. After freeze-dried under a vacuum condition for 24 h, the obtained sample was annealed at 650 °C for 2 h under the argon atmosphere. Following the above steps, AgNO3 solutions with the different concentrations of 0.1, 0.01 and 0.001 mol/L were employed to deposit various quantities of Ag NPs. The composites were represented as M/C-PDA/0.1Ag, M/C-PDA/0.01Ag and

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

3

M/C-PDA/0.001Ag, corresponding to the 0.1, 0.01 and 0.001 mol/L AgNO3 solution, respectively. 2.3. Catalytic evaluation For the catalytic reduction of Cr(VI), K2Cr2O7 was the source of Cr(VI) and FA was the electron donor under an acidic solution. Typically, a mixture containing 100 mL of K2Cr2O7 solution (106 mg/L), 6 mL of FA (88%) and M/C-PDA/0.1Ag (0.1 g) was placed in a stirred batch-fed beaker at 50 °C, the pH of mixture was measured at 0.35, a certain volume of the solution was sampled at given time intervals, and then filtered (0.22 µm, PTFE) to remove the catalyst solids. The Cr(VI) content in the reaction solution was monitored by measuring UV–vis absorbance of K2Cr2O7 at 353 nm [34]. As a comparison, the control experiments were implemented in a similar way using diverse catalysts. 2.4. Characterization Scanning electron microscopy (JSM-6700F, JEOL) was applied to examine the surface morphology of the materials, along with transmission electron microscopy (JEM-2100F, JEOL). X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Shimadzu) was used to analyze the surface elements of the materials. X-ray diffraction (XRD) was analyzed with a Rigaku/Max-3A X-ray diffractometer (Cu Ka radiation, k = 1.5418 Å) with an operation voltage of 40 kV and current of 200 mA, and N2 adsorption-desorption isotherm data were studied by using the Brunauer-Emmett-Teller method. Raman spectra were collected on a J-Y T64000 Raman spectrometer with 514.5 nm wavelength incident laser light. Thermogravimetric analysis (TGA) was conducted using thermoanalytical equipment (SDTQ600, USA) from 25 to 800 °C in air. Magnetic measurements were performed on powder samples by a MPMS-XL SQUID magnetometer. The chemical state and surface composition of the composites were analyzed by X-ray photoelectron spectroscopy (XPS) using an ESCALAB250 instrument (Thermo-VG Scientific, UK). 3. Results and discussion 3.1. Structure and composition characterization of the M/C-PDA/Ag Fig. 1 confirmed the formation of mC-PDA/Ag after annealing the Fe/PDA/Ag at 650 °C. For Fe-PDA/Ag, three insignificant Ag (1 1 1), (2 0 0) and (2 2 0) diffraction peaks is found, indicating that the Ag+ on the Fe-PDA nanospheres was well reduced to Ag species [14]. And after fast pyrolysis, these peaks was also detected in the M/C-PDA/Ag as well as increased with increasing Ag contents. In addition, the M/C-PDA/Ag samples showed a broad C(0 0 2) diffraction peak, demonstrating the decomposition of the PDA structure during the heat treatment process, while new diffraction peaks are observed in the XRD pattern of the M/C-PDA/Ag, attributing to the Fe3C crystal planes (JCPDS 85-1317) [24]. On the contrary, the characteristic peaks intensity of Fe3C decreased as the Ag contents increased. Both characteristic peaks from Ag and Fe3C were detected in M/C-PDA/Ag composites, indicating the successful synthesis of M/C-PDA/Ag with high purity. Notably, the diffraction peaks in the XRD pattern of the Fe/N-C-650 correspond to the Fe3O4 crystal planes (JCPDS 79-0419), indicating that Ag nanoparticles decoration on Fe/PDA improved the graphitization degree of M/C-PDA. The morphology and structural evolution from Fe/PDA to M/CPDA/Ag were well manifested by SEM. After polymerization for 30 h, Fe/PDA complex obtained from the respective suspension exhibit spherical morphology as disclosed by SEM studies (Fig. 2A),

Fig. 1. XRD patterns of as-prepared Fe/PDA/Ag, M/C-PDA, M/C-PDA/Ag samples and JCPDS card #85-1317, #87-0597, #79-0419 for the corresponding Fe3C, Ag, and Fe3O4, respectively.

and the size of the complex spheres is approximately 200 nm (Fig. S1A). After heat treatment, the Fe/PDA nanospheres are in situ converted into M/C-PDA (Fig. 2B). Of interest is that the spherical profile can be maintained in M/C-PDA, and there is apparent change in the particle size (Fig. S1B). As we can see from the Fig. 2C, Ag NPs, the majority of which are less than 50 nm in size, are hard deposited to the surface of M/C-PDA (Fig. S2A). The loading amount of Ag NPs is restricted by the AgNO3 concentration reducing from 0.1 to 0.001 M, while holding the reduction time. The size distribution of the Ag NPs in different M/C-PDA/Ag affirms the small size (<50 nm) and reveals that the Ag NPs size is in an enhancive tendency with the incremental AgNO3 concentration (Fig. S2). The nanoparticle loading amount increases obviously with the rise of AgNO3 concentration, from a limited number of nanoparticles (M/C-PDA/0.001Ag) to large area coverage (M/CPDA/0.1Ag). The TEM image displays a similar result to the SEM image, except that many of the small sized Ag NPs decorated on the C-PDA layer were identified and observed (Fig. 2D and E), smaller sized Fe3C NPs uniformly embedded in the C-PDA matrix with size of only approximately 3–5 nm are also observed. The inset in Fig. 2E shows the high-resolution TEM image focusing on Ag NPs and Fe3C NPs, Ag NPs are a single crystal with lattice spacing of 0.23 nm for plane (1 1 1) as well as Fe3C NPs are the perfectly crystallized nanoparticles with lattice spacing of 0.24 nm for plane (2 1 0). As the results from the EDS analysis of the M/C-PDA/0.1Ag shown in Fig. 2F exhibits C, Fe, Ag and N were detected. From the above results, we can conclude that M/C-PDA/Ag hybrids were successfully fabricated. Raman spectra of M/C-PDA and M/C-PDA/0.1Ag were shown in Fig. 3A. Typically, the D band at 1350 cm1 corresponds to defect sites or disordered sp2-hybridized carbon atoms of graphite, the G band at 1580 cm1 corresponds to the phonon mode in-plane vibration of sp2-bonded carbon atoms. The intensity ratio of D band to G band (ID/IG) is universally applied to estimate the degree of crystallization or defect density of carbon materials [35]. The ID/IG values of M/C-PDA and M/C-PDA/0.1Ag are 0.86 and 0.87, respectively. The increased ID/IG values of M/C-PDA/0.1Ag are aroused from the formation of pores and increased structural

4

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

Fig. 2. SEM images of Fe/PDA, M/C-PDA and M/C-PDA/0.1Ag; TEM images of M/C-PDA/0.1Ag: (D) low-magnification and (E) high-magnification; the corresponding EDS spectra of M/C-PDA/0.1Ag.

100

A

B Weight (%)

Intensity (a.u.)

80

M/C-PDA/0.1Ag

60

M/C-PDA/0.1Ag 40

M/C-PDA M/C-PDA

20 300

600

900

1200

1500

100

Wavenumber (cm-1)

M/C-PDA/0.1Ag M/C-PDA

0.01

40

Magnetization (emu·g-1)

60

Pore volume (cm3·g-1·nm-1)

Volume adsorbed (cm3·g-1)

80

C

0.03

0.02

M/C-PDA/0.1Ag SBET = 101.24 m2·g-1 M/C-PDA SBET = 74.91 m2·g-1

0.00 1

2

5

10

20

300

400

500

600

700

800

Temperature (°C)

120 100

200

30 40 50

Pore width (nm)

40

D

20

0

-20

σs = 45.53 emu·g-1

20 -40 0 0.0

0.2

0.4

0.6

Relative pressure (P/P0)

0.8

1.0

-30000 -20000 -10000

0

10000

20000

30000

Magnetic filed (Oe)

Fig. 3. The Raman spectra (A), TGA curves (B), Nitrogen adsorption-desorption isotherms and pore-size distributions (C) of M/C-PDA, M/C-PDA/0.1Ag, and room temperature hysteresis loops of M/C-PDA/0.1Ag (D). The inset is a digital photograph of the magnetic separation.

5

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

corresponding to the micropores and mesopores, respectively [40]. Meanwhile, the BJH desorption pore volumes of M/C-PDA and mC-PDA/0.1Ag are 0.05 and 0.16 cm3
g1, respectively. The increased surface area and pore volume of the as-synthesized M/C-PDA/0.1Ag are beneficial to the exposure of active sites and rapid transportation of Cr(VI) species for reduction. The magnetization curve of the as-synthesized M/C-PDA/0.1Ag is presented in Fig. 3D. The saturation magnetization value was measured to be 45.53 emu
g1, the inset digital picture suggested the easy separation of the catalyst from the solution with a magnet. The elemental valences of the as-synthesized M/C-PDA/0.1Ag were studied by XPS (Fig. 4). The survey spectrum clearly detected the existence of Fe, Ag, O, N and C elements (Fig. 4A), while Fig. 4B to F collected the high resolved XPS spectra of Ag 3d, N 1s and C 1s, respectively. The high resolution Ag 3d spectrum displays two doublet high intensity peaks, including Ag 3d3/2 (374.7 eV) and Ag 3d5/2 (368.7 eV), which can be due to the zero valent Ag species [41]. The high-resolution N 1s XPS spectra can be deconvoluted into three different peaks at 398.64, 400.64, and 404.00 eV, attributing to pyridinic N, graphitic N, and oxidized N, respectively [42]. Among those, the pyridinic N and graphitic N dominate the majority. As reported in previous literatures, pyridinic N and graphitic N were conducive to enhanced catalytic performance in oxygen reduction reaction because they could reduce the energy barrier and accelerate the electron transfer [10]. The deconvolution of the C 1s spectrum displays three types of C species: 76.19% C@C, 8.61% C@N & CAO, 6.14% CAOAC & CAN, and 9.06% AOAC@O, indicating the existence of carbon atoms connected to N and O heteroatoms.

disorder in the carbonization process [36]. It’s noteworthy that there are the obvious peak series between 200 and 600 cm1 in M/C-PDA/0.1Ag, which is commonly indexed to the phases of Fe3C [37]. Consequently, both the XRD and Raman results suggest that Fe3C species were formed during the pyrolysis process. The content of Ag in the M/C-PDA/0.1Ag was evaluated by TGA analysis (Fig. 3B). After the combustion in air, the N-doped carbon became CO2 and NO2 gas, the Fe3O4/Fe3C and Ag were oxidized into Fe2O3 and Ag2O, respectively [37–39]. The results demonstrated that the content of Fe2O3 was 19.46 wt% in M/C-PDA as well as the content of Fe2O3 and Ag2O was 41.54 wt% in M/C-PDA/0.1Ag. Thus, the Ag content was calculated to be 24.32 wt% in M/CPDA/0.1Ag. The specific surface area and porosity of M/C-PDA and M/CPDA/0.1Ag were investigated by measuring the N2 adsorptiondesorption isotherm. As shown in Fig. 3C, there is a quick increase at low relative pressure (P/P0 < 0.1) finding in the nitrogen adsorption isotherms, which could be assigned to the fast filling of the micropores of the composite with N2. Notably, the increase of gas adsorption quantity gets smooth with the rise of relative pressure from 0.20 to 0.80, suggesting that all of the pores are filled. While a sudden rise in gas adsorption is observed at relative pressures more than 0.80, which is attributed to the interparticle condensation of N2. Moreover, an extraordinary phenomenon is saw in the isotherms therein the adsorption and desorption branches didn’t close perfectly in the low relative pressure region. The specific surface area of M/C-PDA/0.1Ag is 101.24 m2
g1, which is larger than that of M/C-PDA (74.91 m2
g1). Notably, the pore diameter distribution of the M/C-PDA/0.1Ag is mainly located at 1.75 and 3.31 nm,

C 1s

O 1s Fe 2p

800

Ag 3p

600

B

Ag 3d N 1s

400

200

0

Ag 3d3/2

380

378

376

Binding energy (eV)

374

372

370

N 1s

C=C

Intensity (a.u.)

Intensity (a.u.)

graphitic N

oxidized N

404

402

400

Binding energy (eV)

366

C 1s

D

pyridinic N

406

368

Binding energy (eV)

C

408

Ag 3d

Ag 3d5/2

Intensity (a.u.)

Intensity (a.u.)

A

398

396

C=N&C-O C-O-C&C-N -O-C=O

294

292

290

288

286

284

282

280

Binding energy (eV)

Fig. 4. The XPS spectra of M/C-PDA/0.1Ag: survey spectrum (A), high resolution Ag 3d (B), high resolution N 1s (C), and high resolution C 1s (D).

6

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

activity for Cr(VI) reduction. As a comparison, the Cr(VI) concentrations in two control experiments which used only M/C-PDA/0.1Ag or FA (Fig. 5C) suggest that the Cr(VI) reduction process cannot proceed without a catalyst or FA. When the pristine M/C-PDA without Ag NPs was employed as a catalyst (Fig. 5C), the Cr(VI) concentration only slightly reduced within 16 min, manifesting that the activity of the M/C-PDA/0.1Ag in the Cr(VI) reduction was primarily involved in the superb catalytic properties of the Ag NPs. The FA-induced Cr(VI) reduction was also studied by M/CPDA/0.1Ag, M/C-PDA/0.01Ag and M/C-PDA/0.001Ag under similar experimental conditions and the results were summarized in Fig. 5C. Specifically, the catalytic performance of the M/C-PDA/Ag could be observed to increase first and then decrease thereafter with the reduction of Ag NPs. And when the content of AgNO3 was up to 0.1 M, the composite gave the highest the catalytic activity. Such phenomenon revealed the existence of the synergistic effect between Ag NPs and C-PDA. C-PDA, a special sp2 C-cominant N-doped carbon sub-micrometer spheres with a tunable size, not only effectively prevented the aggregation of Ag NPs, conduced to their uniform distribution, increase the reaction surface area, but also decline the mass transfer resistance, which jointly promoted catalytic reaction of the composite [44]. In addition, C-PDA with high electrical conductivity was also conducive to electron transfer and then improved the catalytic activity of the composite [44]. Last but not least, due to the electrostatic interaction between C-PDA and Cr(VI), Cr(VI) could adsorb on the surface of C-PDA, leading to the collision probability between reactants increasing, which was also beneficial to the catalytic reaction. Therefore, an optimal ratio of Ag NPs and C-PDA existed for catalyzing Cr(VI) reduction.

3.2. Catalytic activity for Cr(VI) reduction The catalytic activity of the M/C-PDA/0.1Ag was investigated by the catalytic reduction of Cr(VI) with FA, which was monitored by UV–vis spectroscopy. The system temperature was fixed at 50 °C, the high temperature reduces the energetics of the Cr(VI) reduction reaction and enhances the activity of catalyst. As displayed in Fig. 5A, the intensity of the Cr(VI) characteristic absorption peak of at 353 nm reduces markedly with the rise of reaction time and vanished within reaction time of 16 min, suggesting that Cr(VI) can be powerfully transformed by FA with the catalysis of M/CPDA/0.1Ag. Meanwhile, the color of the reaction solution rapidly changed from yellow to colorless in this reaction process, visually confirming the thorough reduction of Cr(VI) into Cr(III) (Fig. 5A inset) [43]. After Cr(VI) reduction reaction, typical Cr XPS peaks were clearly observed on the used M/C-PDA/0.1Ag catalyst (Fig. 5B). After XPS-peak-differentiating analysis the Cr 2p spectrum was resolved into four peaks, which corresponded to Cr(VI) and Cr(III) species, implying that Cr(VI) and Cr(III) coexisted on the N-doped carbon surface. The percentages of Cr(VI) and Cr(III) to total Cr species were computed at 41.2% and 58.8%, respectively. Moreover, the pseudo-first-order kinetic Eq. (1) was applied to fit the reduction of Cr(VI) into Cr(III) in the condition of excess FA.

lnðC t =C 0 Þ ¼ kt

ð1Þ

Where C0 is the initial concentration of Cr2O2 7 , Ct is the concentrations of Cr2O2 7 at reaction time t and k is the apparent rate constant. The value of k was evaluated from the slope to be 0.474 min1 for the M/C-PDA/0.1Ag composite (Fig. S4), which demonstrated the composite is endowed with remarkable catalytic

Cr 2p

B

A after 16 min

Cr(I

350

400

450

(40

.3

Intensity (a.u.)

I) (V Cr

Absorbance

0 300

%)

1

59.7

0 min 1 min 2 min 4 min 8 min 12 min 16 min

2

II) (

%)

3

500

590

Wavenumber (nm)

585

580

575

570

Binding energy (eV)

1.0

C

D

100

FA M/C-PDA/

0.8 0.1Ag

80

FA

0.6

C/C0

Conversion (%)

M/C-PDA+ M/C-PDA/ 0.001Ag+FA M/C-PDA/ 0.01Ag+FA M/C-PDA/ 0.1Ag+FA

0.4

0.2

60

40

20

0.0 0

5

10

15

Time(min)

20

25

0 1

2

3

4

Number of cycle

Fig. 5. (A) Catalytic activity evaluation of the M/C-PDA/0.1Ag in the aqueous catalytic reduction of Cr(VI) by FA monitored by UV–vis spectra. (B) Cr 2p XPS spectra of the used M/C-PDA/0.1Ag with the adsorbed Cr(VI) species. (C) the remaining fraction of Cr(VI) (C/C0) versus reaction time under different reaction conditions. (D) the cycle performance of the M/C-PDA/0.1Ag in the catalytic reduction of Cr(VI) with FA.

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

Furthermore, the stability of M/C-PDA/0.1Ag catalyst was studied. Briefly, the M/C-PDA/0.1Ag composite was magnetically separated on the bottom of flasks after each experiment, and washed with water, then dried before the next catalytic cycle. The recycled catalyst displays favorable activity for the another cycles of catalytic experiments (Fig. 5D), with initial Cr(VI) reduction efficiencies being about 95.8%. Nevertheless, it should be noted, when the M/C-PDA/0.1Ag was recycled for the fourth time, the reduction efficiencies decreased to 78.9%, revealing that the catalytic activity of the M/C-PDA/0.1Ag may reduce after several catalytic recycling runs. The slight decrease was primarily due to the catalyst loss in recycling use, which is inevitable and frequently found in many research investigations [45]. Table S1 shows the comparison of M/C-PDA/0.1Ag catalyst for the Cr(VI) removal with the other materials using different treatment methods. Obviously, the Cr (VI) removal performance of the M/C-PDA/0.1Ag catalyst is much higher than those of 1-NO3 (ion exchange) [46], PET/PAN/GO/ Fe3O4 (membrane separation) [47], PVA/chitosan/A-Fe3O4 nanofibers (membrane separation) [48], Fe3O4@NiO (adsorption) [49], Ni/Mg/Al-LDHs (adsorption) [50], and Ag@biochar (catalysis/FA) [14]. Since the preparation of the PET/PAN/GO/Fe3O4 and PVA/ chitosan/A-Fe3O4 nanofibers membrane needed the complex assembly process, meanwhile, the Ni/Mg/Al-LDHs and Fe3O4@NiO adsorbent displayed the poor recycle use, and the obtained M/CPDA/0.1Ag catalyst is a promising candidate with the low-cost and high reuse for the Cr(VI) removal in wastewater cleanup. The reaction mechanism invoked for catalytic reduction of Cr (VI) in the presence of FA over the M/C-PDA/0.1Ag catalyst is demonstrated based on the earlier reports [14]. First, FA was decomposed into H2O and CO with the catalysis of Ag NPs, as the following:

7

4. Conclusions In short, we have successfully synthesized the Ag NPs decorated on magnetic NPs encapsulated in carbonized polydopamine nanospheres hybrid material (M/C-PDA/Ag) by fast thermolysis of the Ag+ loaded Fe/PDA and investigated its catalytic Cr(VI) reduction with FA. The M/C-PDA/0.1Ag proved to have an excellent catalytic activity for this reaction, which can commendably catalytically reduce Cr(VI) to Cr(III) in water within 20 min. The particle size of Ag NPs decoration on the M/C-PDA was demonstrated to be dependent on Ag+ concentration and played a vital part in the Cr (VI) reduction. The catalyst can be very easily separated from the solution by using a common magnet without exterior energy. This investigation showed that the as-prepared highly active M/CPDA/0.1Ag catalyst provided very attractive prospects and could be extended for practical applications in Cr(VI) contaminated wastewater cleanup. Acknowledgements Financial supports from the National Natural Science Foundation of China (21477133), CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences are acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jcis.2018.04.094. References

HCOOH!H2 O + CO

ð2Þ

Then, CO was activated by the Ag NPs to produce an alive Agcarbonyl complex, which further adsorbed the Cr(VI) to generate a Ag-carbonyl-Cr(VI) complex. Thenceforward, the Cr(VI) in the complex was reduced to Cr(III) by CO under acidic conditions, which experienced several procedures of oxygenation-reduction reactions to generate the Cr(III) species.

2CrO4 2 + 10Hþ + 3CO!2Cr3þ + 3CO2 + 5H2 O

ð3Þ

A simply experiment was carried out to testify the above mechanism. First, CO was used as the alternative for FA to reduce Cr(VI) with the addition of H2SO4. As showed in Fig. S5, with the absence of the M/C-PDA/0.1Ag catalyst, the Cr(VI) concentration slightly decreases within 30 min. While with the presence of the catalyst, the Cr(VI) concentration decreases markedly within 60 min. The result implied that CO can serve as a reductant for the catalytic reduction of Cr(VI) with M/C-PDA/Ag. Nevertheless, as compared with the reactions with FA (Fig. 5C), the indispensable time for the Cr(VI) reduction with CO is longer. The phenomenon can manifested that the generated CO in the FA decomposition process is more active than the purchased CO gas and can efficiently react with Cr(VI) in aquatic media. Notably, N-doped carbon plays a vital role in the Cr(VI) reduction process. The proposed mechanism between N-doped carbon and Cr(VI) reduction with the FA catalysis is clarified as follow: (i) N-doped carbon sufficiently attracts the negatively charged Cr2O2 7 due to the electrostatic interaction, leading to the enrichment of Cr2O2 7 on the N-doped carbon surface. (ii) Based on the concept of Lewis base, nitrogen functionality can offer abundant basic sites that can provide a more delocalized electron concentration for enhancing a reductive environment [51].

[1] K.R. Zhu, C.L. Chen, H. Xu, Y. Gao, X.L. Tan, A. Alsaedi, T. Hayat, Cr(VI) reduction and Iimmobilization by core-double-shell structured magnetic polydopamine@zeolitic idazolate frameworks-8 microspheres, ACS Sustain. Chem. Eng. 5 (2017) 6795–6802. [2] K.R. Zhu, Y. Gao, X.L. Tan, C.L. Chen, Polyaniline-modified Mg/Al layered double hydroxide composites and their application in efficient removal of Cr(VI), ACS Sustain. Chem. Eng. 4 (2016) 4361–4369. [3] Y. Gao, C.L. Chen, X.L. Tan, H. Xu, K.R. Zhu, Polyaniline-modified 3D-flower-like molybdenum disulfide composite for efficient adsorption/photocatalytic reduction of Cr(VI), J. Colloid Interface Sci. 476 (2016) 62–70. [4] M.C. Liu, T. Wen, X.L. Wu, C.L. Chen, J. Hu, J. Li, X.K. Wang, Synthesis of porous Fe3O4 hollow microspheres/graphene oxide composite for Cr(VI) removal, Dalton Trans. 42 (2013) 14710–14717. [5] P. Veerakumar, P. Thanasekaran, K.C. Lin, S.B. Liu, Biomass derived sheet-like carbon/palladium nanocomposite: An excellent opportunity for reduction of toxic hexavalent chromium, ACS Sustain. Chem. Eng. 5 (2017) 5302–5312. [6] Z.S. Wei, S. Luo, R.Y. Xiao, R. Khalfin, R. Semiat, Characterization and quantification of chromate adsorption by layered porous iron oxyhydroxide: an experimental and theoretical study, J. Hazard. Mater. 338 (15) 472–481. [7] Z.S. Wei, R. Semiat, Applying a modified Donnan model to describe the surface complexation of chromate to iron oxyhydroxide agglomerates with heteromorphous pores, J. Colloid Interface Sci. 506 (2017) 66–75. [8] S. Rengaraj, K.-H. Yeon, S.-H. Moon, Removal of chromium from water and wastewater by ion exchange resins, J. Hazard. Mater. 87 (2001) 273–287. [9] H.C. Li, W.J. Liu, H.X. Han, H.Q. Yu, Hydrophilic swellable metal-organic framework encapsulated Pd nanoparticles as an efficient catalyst for Cr(vi) reduction, J. Mater. Chem. A 4 (2016) 11680–11687. [10] Y.L. Guo, D. Wang, X.Y. Liu, X.D. Wang, W.S. Liu, W.W. Qin, Synthesis and characterization of the nickel@carbon dots hybrid material and its application in the reduction of Cr(vi), New J. Chem. 38 (2014) 5861–5867. [11] L.Z. Liu, J.J. Xue, X.Y. Shan, G.Y. He, X. Wang, H.Q. Chen, In-situ preparation of three-dimensional Ni@graphene-Cu composites for ultrafast reduction of Cr (VI) at room temperature, Catal. Commun. 75 (2016) 13–17. [12] F.Q. Shao, J.J. Feng, X.X. Lin, L.Y. Jiang, A.J. Wang, Simple fabrication of AuPd@Pd core-shell nanocrystals for effective catalytic reduction of hexavalent chromium, Appl. Catal. B 208 (2017) 128–134. [13] Y.J. Yao, C. Lian, G.D. Wu, Y. Hu, F.Y. Wei, M.J. Yu, S.B. Wang, Synthesis of ‘‘sea urchin”-like carbon nanotubes/porous carbon superstructures derived from waste biomass for treatment of various contaminants, Appl. Catal. B 219 (2017) 563–571. [14] W.J. Liu, L. Ling, Y.Y. Wang, H. He, Y.R. He, H.Q. Yu, H. Jiang, One-pot high yield synthesis of Ag nanoparticle-embedded biochar hybrid materials from waste biomass for catalytic Cr(vi) reduction, Environ. Sci. Nano 3 (2016) 745–753.

8

C. Chen et al. / Journal of Colloid and Interface Science 526 (2018) 1–8

[15] Y. Choi, M.S. Koo, A.D. Bokare, D.H. Kim, D.W. Bahnemann, W. Choi, Sequential process combination of photocatalytic oxidation and dark reduction for the removal of organic pollutants and Cr(VI) using Ag/TiO2, Environ. Sci. Technol. 51 (2017) 3973–3981. [16] L. Wang, C.B. Zhang, F. Gao, G. Mailhot, G. Pan, Algae decorated TiO2/Ag hybrid nanofiber membrane with enhanced photocatalytic activity for Cr(VI) removal under visible light, Chem. Eng. J. 314 (2017) 622–630. [17] P. Wang, H.Z. Liu, J.R. Niu, R. Li, J.T. Ma, Entangled Pd complexes over Fe3O4@SiO2 as supported catalysts for hydrogenation and Suzuki reactions, Catal. Sci. Technol. 4 (2014) 1333. [18] Y. Hu, L.J. Meng, L.Y. Niu, Q.H. Lu, Facile synthesis of superparamagnetic Fe3O4@polyphosphazene@Au shells for magnetic resonance imaging and photothermal therapy, ACS Appl. Mater. Interfaces 5 (2013) 4586–4591. [19] J. Zheng, Y. Dong, W. Wang, Y. Ma, J. Hu, X. Chen, X. Chen, In situ loading of gold nanoparticles on Fe3O4@SiO2 magnetic nanocomposites and their high catalytic activity, Nanoscale 5 (2013) 4894–4901. [20] M.Y. Zhu, C.J. Wang, D.H. Meng, G.W. Diao, In situ synthesis of silver nanostructures on magnetic Fe3O4@C core–shell nanocomposites and their application in catalytic reduction reactions, J. Mater. Chem. A 1 (2013) 2118– 2125. [21] M. Chen, P.P. Liu, C.J. Wang, W.J. Ren, G.W. Diao, Fast catalytic reduction of an azo dye by recoverable and reusable Fe3O4@PANI@Au magnetic composites, New J. Chem. 38 (2014) 4566–4573. [22] C. Li, Y. Huang, J.J. Chen, Dopamine-assisted one-pot synthesis of graphene@Ni@C composites and their enhanced microwave absorption performance, Mater Lett. 154 (2015) 136–139. [23] J.M. Ang, Y. Du, B.Y. Tay, C. Zhao, J. Kong, L.P. Stubbs, X. Lu, One-Pot synthesis of Fe(III)-polydopamine complex nanospheres: Morphological evolution, mechanism, and application of the carbonized hybrid nanospheres in catalysis and Zn-air battery, Langmuir 32 (2016) 9265–9275. [24] K.R. Zhu, C. Chen, M.W.C. Xu, K. Chen, X.L. Tan, M. Wakeel, N.S. Alharbi, In situ carbothermal reduction synthesis of Fe nanocrystals embedded into N-doped carbon nanospheres for highly efficient U(VI) adsorption and reduction, Chem. Eng. J. 331 (2018) 395–405. [25] J.H. Kong, S.I. Seyed Shahabadi, X.H. Lu, Integration of inorganic nanostructures with polydopamine-derived carbon: tunable morphologies and versatile applications, Nanoscale 8 (2016) 1770–1788. [26] C. An, Y.J. Wang, L.F. Jiao, H.T. Yuan, Mesoporous Ni@C hybrids for a high energy aqueous asymmetric supercapacitor device, J. Mater. Chem. A 4 (2016) 9670–9676. [27] F. Wei, J. Liu, Y.N. Zhu, X.S. Wang, C.Y. Cao, W.G. Song, In situ facile loading of noble metal nanoparticles on polydopamine nanospheres via galvanic replacement reaction for multifunctional catalysis, Sci. China Chem. 60 (2017) 1236–1242. [28] Y. Wei, J. Kong, L. Yang, L. Ke, H.R. Tan, H. Liu, Y. Huang, X.W. Sun, X. Lu, H. Du, Polydopamine-assisted decoration of ZnO nanorods with Ag nanoparticles: an improved photoelectrochemical anode, J. Mater. Chem. A 1 (2013) 5045–5052. [29] R. Liu, Y.L. Guo, G. Odusote, F.L. Qu, R.D. Priestley, Core-shell Fe3O4 polydopamine nanoparticles serve multipurpose as drug carrier, catalyst support and carbon adsorbent, ACS Appl. Mater. Interfaces 5 (2013) 9167– 9171. [30] J. Jiang, Y.N. Liu, L.P. Li, J.H. Zhu, M.W. Xu, C.M. Li, Smart magnetic interaction promotes efficient and green production of high-quality Fe3O4@carbon nanoactives for sustainable aqueous batteries, ACS Sustain. Chem. Eng. 6 (2018) 757–765. [31] W.Q. Zhou, M.M. Liu, C. Cai, H.J. Zhou, R. Liu, Highly regenerable carbon-Fe3O4 core–satellite nanospheres as oxygen reduction electrocatalyst and magnetic adsorbent, J. Solid State Chem. 246 (2017) 357–362. [32] G. Cheng, M.D. Zhou, S.Y. Zheng, Facile synthesis of magnetic mesoporous hollow carbon microspheres for rapid capture of low-concentration peptides, ACS Appl. Mater. Interfaces 6 (2014) 12719–12728.

[33] X.Z. Zhang, L.L. Wang, Y. Zou, X. Peng, M. Zhang, S. Gao, J.L. Xu, K.B. Tang, D.L. Zhao, Synthesis of Ag decoration on carbon coated Zn2GeO4 nanorods and its enhanced properties as anode materials for lithium-ion batteries, Mater Lett. 166 (2016) 243–246. [34] B. Vellaichamy, P. Periakaruppan, B. Nagulan, Reduction of Cr6+ from wastewater using a novel in situ-synthesized PANI/MnO2/TiO2 nanocomposite: renewable, selective, stable, and synergistic catalysis, ACS Sustain. Chem. Eng. 5 (2017) 9313–9324. [35] Y.N. Gong, D.L. Li, C.Z. Luo, Q. Fu, C.X. Pan, Highly porous graphitic biomass carbon as advanced electrode materials for supercapacitors, Green Chem. 19 (2017) 4132–4140. [36] R. Qiang, Y.C. Du, H.T. Zhao, Y. Wang, C.H. Tian, Z.G. Li, X.J. Han, P. Xu, Metal organic framework-derived Fe/C nanocubes toward efficient microwave absorption, J. Mater. Chem. A 3 (2015) 13426–13434. [37] S.W. Zhang, M.Y. Zeng, J. Li, J.X. Li, J.Z. Xu, X.K. Wang, Porous magnetic carbon sheets from biomass as an adsorbent for the fast removal of organic pollutants from aqueous solution, J. Mater. Chem. A 2 (2014) 4391–4397. [38] S.W. Zhang, X.X. Wang, J. Li, T. Wen, J.Z. Xu, X.K. Wang, Efficient removal of a typical dye and Cr(vi) reduction using N-doped magnetic porous carbon, RSC Adv. 4 (2014) 63110–63117. [39] L. Gharibshahi, E. Saion, E. Gharibshahi, A.H. Shaari, K.A. Matori, Structural and optical properties of Ag nanoparticles synthesized by thermal treatment method, Materials 10 (2017) 402. [40] G.Q. Zou, H.S. Hou, G.G. Zhao, Z.D. Huang, P. Ge, X.B. Ji, Preparation of S/Ncodoped carbon nanosheets with tunable interlayer distance for high-rate sodium-ion batteries, Green Chem. 19 (2017) 4622–4632. [41] K. Tian, W.J. Liu, S. Zhang, H. Jiang, One-pot synthesis of a carbon supported bimetallic Cu-Ag NPs catalyst for robust catalytic hydroxylation of benzene to phenol by fast pyrolysis of biomass waste, Green Chem. 18 (2016) 5643–5650. [42] Y.H. Su, Y.H. Zhu, H.L. Jiang, J.H. Shen, X.L. Yang, W.J. Zou, J.D. Chen, C.Z. Li, Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions, Nanoscale 6 (2014) 15080–15089. [43] K. Bhowmik, A. Mukherjee, M.K. Mishra, G. De, Stable Ni nanoparticle-reduced graphene oxide composites for the reduction of highly toxic aqueous Cr(VI) at room temperature, Langmuir 30 (2014) 3209–3216. [44] K. Ai, Y.L. Liu, C.P. Ruan, L.H. Lu, G.Q. Lu, Sp2 C-dominant N-doped carbon submicrometer spheres with a tunable size: a versatile platform for highly efficient oxygen-reduction catalysts, Adv. Mater. 25 (2013) 998–1003. [45] K.R. Zhu, S.H. Lu, Y. Gao, R. Zhang, X.L. Tan, C.L. Chen, Fabrication of hierarchical core-shell polydopamine@MgAl-LDHs composites for the efficient enrichment of radionuclides, Appl. Surf. Sci. 396 (2017) 1726–1735. [46] L.L. Li, X.Q. Feng, R.P. Han, S.Q. Zang, G. Yang, Cr(VI) removal via anion exchange on a silver-triazolate MOF, J. Hazard. Mater. 321 (2017) 622–628. [47] S. Koushkbaghi, P. Jafari, J. Rabiei, M. Irani, M. Aliabadi, Fabrication of PET/PAN/ GO/Fe3O4 nanofibrous membrane for the removal of Pb(II) and Cr(VI) ions, Chem. Eng. J. 301 (2016) 42–50. [48] S. Koushkbaghia, A. Zakialamdarib, M. Pishnamazic, H.F. Ramandid, M. Aliabadie, M. Irani, Aminated-Fe3O4 nanoparticles filled chitosan/PVA/PES dual layers nanofibrous membrane for the removal of Cr(VI) and Pb(II) ions from aqueous solutions in adsorption and membrane processes, Chem. Eng. J. 337 (2018) 169–182. [49] E. Mahmoudi, M.A. Behnajady, Synthesis of Fe3O4@NiO core-shell nanocomposite by the precipitation method and investigation of Cr(VI) adsorption efficiency, Colloids and Surf. A 538 (2018) 287–296. [50] C.S. Lei, X.F. Zhu, B.C. Zhu, C.J. Jiang, Y. Le, J.G. Yu, Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr (VI) ions, J. Hazard. Mater. 321 (2017) 801–811. [51] F.Q. Lin, Y.H. Wang, Z. Lin, One-pot synthesis of nitrogen-enriched carbon spheres for hexavalent chromium removal from aqueous solution, RSC Adv. 6 (2016) 33055–33062.