Scalable and Environmentally Friendly Synthesis of Hierarchical

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Scalable and Environmentally–Friendly Synthesis of Hierarchical Magnetic Carbon Nanosheet Assemblies and Their Application in Water Treatment Yi Shen, Ling Li, and Zhihui Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00426 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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The Journal of Physical Chemistry

Scalable and Environmentally–Friendly Synthesis of Hierarchical Magnetic Carbon Nanosheet Assemblies and Their Application in Water Treatment Yi Shen,* Ling Li and Zhihui Zhang School of Food Science and Technology, South China University of Technology, Wushan Road, Tianhe District, Guangzhou, 510640, China. ABSTRACT: Large–scale assembling graphitic carbon nanosheets to a three dimensional hierarchical structure is a great challenge. Herein, we report a facile synthesis of hierarchical magnetic carbon nanosheet assemblies (MCNSAs) via an ambient–pressure chemical vapor deposition method. To explore the formation mechanism, the as–prepared MCNSAs as well as the intermediates of synthesis were extensively characterized. It was revealed that two different carbon deposition processes, i.e., the dissolution–precipitation process and graphitic defects trigged catalytic decomposition of methane were involved in the formation of MCNSAs. The disclosed method is simple and environmentally–friendly, which is favorable for the large–scale production. The resulting MCNSAs possess large surface areas, bimodal pore structures, abundant defective sites, excellent chemical stability and sufficient magnetism. Such features afford significant advantages for application in water cleaning. As a proof of concept, the sorption performance of MCNSAs is demonstrated by using Congo red and Pb2+ as model pollutants. The characteristics of sorption process including kinetics, isotherms, recovering, regeneration and recycling are investigated. The results indicate that the MCNSAs are a promising sorbent for water cleaning.

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1. INTRODUCTION Accompanying with industrial development, increasing pollutants are entering into water sources, resulting in detrimental impacts on human health and ecological systems.1 Certainly, it is critical to develop efficient and cost–effective methods to remove water pollutants. To this end, sorption is considered to be one of the most viable techniques because of its simplicity, high efficiency and minimal secondary pollution. So far, many materials such as carbons, zeolites, and oxides have been explored as sorbents.2 However, their widespread application in water cleaning is limited by unsatisfactory capacity, regeneration and recyclability.3 As a result, continuing efforts are directed in searching for new sorbents which can be practically applied in water treatment. Carbon materials, such as activated carbon (AC),4 ordered mesoporous carbon (OMC),5 carbon nanotubes (CNTs),6 have been extensively studied for pollutant removal. Recently, graphene and its derivatives have stimulated great interest because of the excellent chemical stability and large surface areas.7 In the literature, graphene–based sorbents are always prepared by the reduction of exfoliated graphite oxide, resulting in considerable hazardous metal ions and poisonous gases,8 and moreover, these reported graphene–based sorbents cannot be easily separated and/or regenerated. To address these issues, efforts have been devoted to preparing free–standing graphene–based gels.9–14 However, the mechanical strength of graphene–based gels is still a great concern for practical application. As an alternative strategy, to benefit from magnetic properties, magnetic materials, such as iron and iron oxide nanoparticles (NPs) are assembled to graphene surface to prepare magnetic composite sorbents.15–18 Notwithstanding the tremendous advances on exploring graphene, the large–scale synthesis and rational design of nanostructures still remain a great challenge.19 Herein, we report a facile method to synthesize hierarchical magnetic carbon nanosheets assemblies (for convenience abbreviated as MCNSAs). The as– prepared MCNSAs possess a unique combination of large surface areas, bimodal pore structures, abundant defective sites and sufficient magnetism. The MCNSAs are utilized as a sorbent for water purification. It is demonstrated that the MCNSAs show remarkable performance for pollutant removal in terms of high capacity, fast kinetics, facile separation and regeneration, and excellent recyclability.


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2. EXPERIMENTAL SECTION 2.1 Synthesis. The MCNSAs were synthesized using an ambient–pressure chemical vapor deposition method.20,21 In a typical process, 0.5 g of iron oxide catalyst precursor was loaded into a quartz tube reactor and reduced in-situ before the methane decomposition process. After the complete reduction of iron oxide to iron particles by a mixture of nitrogen and hydrogen (v/v 3:1), a feedstock of methane and nitrogen was introduced and the temperature was increased to a predetermined value at a rate of 5 °C min–1. The reactor was heated by an electric furnace with a temperature programmed controller and the temperature of the reactor was accurately monitored using a K–type thermocouple. The flow rates of the gases were controlled by mass flowmeters. To study the effects of temperature, the chemical vapor deposition process was conducted at four different temperatures, i.e., 1000, 1050, 1100 and 1150 °C. Iron oxides were prepared using a precipitation method as reported in our previous work.20 In brief, iron (III) nitrate nonahydrate and sodium hydroxide were used as metal precursor and precipitant, respectively. After the precipitation reaction, the suspension was aged in 150 °C for 12 hours. CNTs were obtained from the catalytic decomposition of methane as reported in our previous work.21 OMC was synthesized by the nanocasting method using SBA-15 as template.22 An activated carbon, namely, DLC Super 50, was purchased from Norit Company. 2.2 Characterization. A field emission scanning electron microscope (JSM–7600F, JEOL) and a transmission electron microscope (JEM2010, JEOL) were used to observe the morphology of the samples. High-resolution TEM micrographs were obtained from an alternative JEM– 2010F (JEOL) microscope. An energy dispersive X–ray analyzer equipped in the FESEM and an axis-ultra X–ray photoelectron spectrometer (Kratos-Axis Ultra System) with monochromatized Al–Kα radiation were used to analyze the elemental composition of the samples. XRD patterns were obtained by a diffractometer (PW1830, Philips) equipped with Cu–Kα radiation of 1.54 Å. The N2 adsorption–desorption isotherms were obtained using the accelerated surface area porosimetry system (ASAP 2020, Micromeritics). Raman spectra were recorded with a Renishaw Raman microscope using 633-nm excitation at room temperature. 2.3 Adsorption Tests. Stock solution of CR was prepared by dissolving CR powder in double-deionized water and that of Pb2+ was supplied from National Institute of Metrology, China (Beijing). The stock solutions were diluted to the desired concentrations with deionized water. In a typical batch adsorption test, a predetermined amount of sample was added into 100 3 ACS Paragon Plus Environment


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mL of pollutant solution and agitated at 180 rpm using a mechanical shaker at room temperature. The initial pH value of the solution was kept at 5 ± 0.1. The variations in the concentrations of CR and metal ions as a function of time were monitored using a UV-visible spectrophotometer and inductively coupled plasma atomic emission spectrometry (ICP-AES), respectively. The adsorption up-take of adsorbent qt (mg g–1) at time t (min) was calculated using Equation (1)

qt =

(C0 − Ct )V W

(1)

where C0 (mg L–1) is the initial pollutant concentration and Ct (mg L–1) is the concentration at time t (min) in the liquid phase, V (L) is the volume of the solution and W (g) is the weight of the monolith. To determine the equilibrium adsorption capacity qe (mg g–1), the sorbent was immersed in the pollution solution for at least 24 h to achieve the equilibrium state of adsorption. The adsorption isotherms are fitted using two models, i.e., the Langmuir (Equation (2)) and Freundlich models (Equation (3)) Ce 1 1 = + Ce qe qmK L qm

(2)

where Ce (mg L–1) is the equilibrium concentration, qm (mg g-1) is the maximum adsorption capacity, and KL (L mg–1) is a constant.

1 ln q e = ln K F + ln Ce n

(3)

where KF (mg g–1 (L mg)1/n) and n are constants. To study the kinetics of the adsorption of the pollutants, three kinetic models including the pseudo-first-order, the pseudo-second-order and the intraparticle diffusion models were applied to analyze the experimental data. The pseudo-first-order kinetic model can be expressed by Equation (4)

log(qe − qt ) = log qe −

k1 t 2.303

(4)

where k1 (min–1) is the adsorption rate constants of the first order kinetic model. The pseudosecond-order kinetic model is expressed by Equation (5)


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t 1 1 = + t 2 qt k2 qe qe

(5)

where k2 (g (mg·min)–1) is the adsorption rate constants of the second order kinetic model. The intraparticle diffusion model is expressed by Equation (6)

qt = k p t 1/ 2 + C

(6)

where kp (mg (g h1/2)–1) is the intraparticle diffusion rate constant and C (mg g–1) is a constant related to the thickness of the boundary layer.

3. RESULTS AND DISCUSSION 3.1 Synthesis and Characterization of MCNSAs The MCNSAs are synthesized by an ambient–pressure chemical vapor deposition (APCVD) method.20,21 As illustrated in Scheme 1, the overall experimental procedure consists of three continuous steps. In step I, iron oxide nanorods prepared by a simple precipitation method are utilized as catalyst precursor and in–suit reduced by hydrogen at a temperature range of 600 to 800 °C to obtain metallic iron catalyst before APCVD process. In step II, with introducing methane as carbon source, the catalytic decomposition of methane occurs on the surface of iron NPs, producing graphitic carbon nanosheets (CNSs) and gaseous hydrogen. The resulting CNSs closely wrap the iron NPs, thereby keeping methane molecules from further dissociating on the catalyst. Nevertheless, the CNSs contain a large number of defective sites, e.g., unsaturated edges and vacancies, which can serve as alternative sites to catalyze the decomposition of methane at high temperatures. As a result, in step III, carbon atoms continuously accumulate on the surface of graphitic layer encapsulated iron NPs, leading to the formation of hierarchical MCNSAs. The synthesis of MCNSAs is continuous and simple, which does not involve any sophisticated instrument and complicated fabrication process. Moreover, the synthesis process is totally environmentally–friendly and affords hydrogen as a valuable byproduct. With the above– described procedure, at a reaction temperature of 1100 °C, 20 g of sample was obtained in a reaction time of 12 h as shown in Figure 1a. It should be pointed out that, owing to the simplicity of synthesis process, the yield of this method can be easily scaled up by increasing catalyst loading and reaction time.



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Scheme 1. Schematic illustration of synthesis procedures of MCNSAs.

Figure 1. (a) Digital photo of 20 g of sample, (b and c) FESEM, (d–g) TEM, (h–k,) elemental mapping images of as–prepared MCNSAs obtained from 1100 °C (h overall image, i carbon, j oxygen and k Fe element). 6 ACS Paragon Plus Environment

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The morphology of as–prepared MCNSAs was observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). FESEM images shown in Figure 1b,c reveal that interconnected CNSs strongly anchor to nanospheres, leading to a highly porous structure. TEM images shown in Figure d-f reveal that the MCNSAs exhibit a three dimensional hierarchical structure with diameters of 300–500 nm. The high–resolution TEM image shown in Figure 1g indicates that CNSs are mainly composed of 3–12 graphene layers. The composition of MCNSAs was analyzed by energy dispersive X–ray (EDX) spectroscopy. Elemental mapping images (Figure 1h–k) resolve the distribution of carbon, oxygen and iron elements. The structures of the MCNSAs were further studied by Raman and X–ray photoelectron (XPS) spectroscopy. Shown in Figure 2a, four peaks located at 1326, 1567, 1610 and 2658 cm–1 are observed in the Raman spectrum, corresponding to the D, G, D’ and 2D band of CNSs, respectively. Notably, the D band possesses relatively high peak intensity. Three elements including carbon, oxygen and iron are identified in the XPS spectra as shown in Figure 2b–d, which is consistent with the EDX results.

Figure 2 (a) Raman and (b–d) XPS spectra of MCNSAs.



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To explore the formation mechanism of MCNSAs, the intermediates of synthesis process were also systematically characterized. The iron oxide NPs, i.e., catalyst precursor, exhibit a rod–like morphology with lengths of 100–200 nm and widths of 10–30 nm as shown in Figure S1. After reduction, iron oxide nanorods were converted to metallic iron NPs with sizes of 80– 300 nm (Figure S2). As a product of step II, graphitic layer encapsulated iron NPs with sizes of 100–300 nm were obtained (Figure S3). It is revealed that iron NPs are tightly encapsulated by CNSs. The crystallographic properties of the intermediates were investigated by X–ray diffraction (XRD). Shown in Figure S4, for each intermediate, the diffraction peaks in the XRD profile can be well indexed. A close examination on the XRD profiles confirms the conversion of iron oxide to metallic NPs in step I and the formation of graphitic CNSs in step II, validating the overall synthetic procedure as shown in Scheme 1. As inferred from Scheme 1, the synthesis of MCNSAs involves two different carbon deposition mechanisms. The first one is the so–called dissolution–precipitation mechanism.23 Carbon source, i.e., methane first adsorb and dissociate on iron surface to produce carbon atoms. The resulting carbon species dissolve into metal NPs and precipitate to form graphitic carbon. The formation of graphitic CNSs via the catalytic decomposition of methane on iron surface was reported in the literature.24 In general, to maintain a continuous growth of graphitic CNSs, the dissociation rate and precipitation rate have to be well balanced at proper reaction temperatures (850–1000 °C).20 In this work, owing to the increased temperature (e.g. 1100 °C), the dissociation rate is faster than the precipitation rate, resulting in the encapsulation of iron NPs by graphitic CNSs in step II. The second carbon deposition process is attributed to the catalyzing effects of defective sites of graphitic CNSs. It was reported that the defective sites, e.g., unsaturated edges and vacancies in CNSs could effectively catalyze many reactions such as thermal decomposition of water,25 disproportionation of carbon monoxide,26 benzene oxidation27 and dissociation of methane.28 Indeed, the CNSs obtained in step II contain numerous defects. To reveal the structural characteristics, CNSs were detached from iron surface by ultrasonication in ethanol and characterized by TEM. Figure S5a shows a typical high–resolution TEM image of detached CNSs. It is clearly shown that numerous edges are present in the CNSs. To further verify the presence of defects, Raman and XPS tests were conducted. Shown in Figure S6a, the graphitic CNSs exhibit a large D band peak. The D band is a disorder–activated mode and its intensity is 8 ACS Paragon Plus Environment

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closely related to the number of defective sites.29 The intensity ratio of ID/IG was calculated to be 0.82, indicating the presence of numerous defective sites.29 Figure S6b shows the XPS profile of CNSs. It is established that the peak width at half height (FWHH) of C1s spectra is dependent on the number of defective sites.30 The high–resolution XPS spectra of C 1s was carefully decomposed. The resulting binding energies (BEs), FWHM values and peak areas are listed in Figure S6b. The C1s spectra reveal the coexistence of sp3 and sp2 carbon species, as confirmed by the peak at 285.2 and 284.8 eV, respectively. These results verify that the CNSs contain a large number of structural defects, which serve as alternative sites to trigger the decomposition of methane after the encapsulation of iron NPs. More interestingly, our experiments also suggest that the decomposition of methane initiated by graphitic defects can be tuned as a self–sustained process because catalytic sites can be regenerated from deposited CNSs. Such a prominent feature of this process holds great application in hydrogen production by catalytic decomposition of methane.

Figure 3 FESEM images of samples obtained from 1000 (a, b), 1050 (c, d) and 1150 °C (e, f) 9 ACS Paragon Plus Environment


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Figure 4 TEM images of samples obtained from 1000 (a–c), 1050 (d–f) and 1150 °C (g–i) Based on the mechanistic studies, it is inferred that the reaction temperature plays a critical role on the formation of hierarchical MCNSAs. To verify this, the decomposition of methane was conducted at four temperatures including 1000, 1050 1100 and 1150 °C. At 1000 °C, the resulting CNSs in step II are readily detached from iron surface. Meanwhile, the defects in CNSs exhibit negligible activity toward methane dissociation. Therefore, instead of MCNSAs, isolated CNSs are obtained as shown in Figures 3a,b. At 1050 °C, though most CNSs are attached to iron NPs, the defects show insufficient activity to activate methane molecules. This is clearly evidenced from the limited amount of CNSs anchored to iron NPs as shown in Figure 3c,d. At 1150 °C, CNSs closely encapsulate iron NPs and the defects in the resulting CNSs possess sufficient activity to trigger methane decomposition, leading to the formation of three 10 ACS Paragon Plus Environment

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dimensional MCNSAs (see Figure 3e,f). Further inspection reveals that unlike the CNSs formed via the dissolution–precipitation mechanism (Figure 4a–c, d–f), the CNSs stemmed from the catalyzing effects of defects exhibit a silk ribbon–like microstructure with numerous wrinkles and folders (see Figure 4g–i). Raman spectroscopy was employed to characterize the structural properties of the products. As illustrated in Figure S7, the normalized intensity of D peak linearly increases with increasing reaction temperature from 1000 to 1150 °C, indicating that the carbon derived from the catalyzing effects of graphitic defects contains more structural defects and disorders. The Brunauer–Emmett–Teller (BET) surface areas and pore size distributions of products were determined from the nitrogen adsorption–desorption isotherms. The BET surface areas are calculated to be 78, 163, 242 and 266 m2 g–1, for the sample obtained from temperatures of 1000, 1050, 1100 and 1150 °C, respectively. The BET surface area increases with increasing reaction temperature. This is attributed to the enhanced activity of defective sites in the CNSs, which facilitates the carbon deposition. The pore size distribution was also analyzed (Figure S8b). The samples obtained from 1000 and 1050 °C show a single peak centered at 16 and 38 nm, respectively. In contrast, the samples obtained from 1100 and 1150 °C exhibit a bimodal pore distribution. Besides large pores, small pores with an average pore size of 2.8 nm are present in the samples.

3.2 Removal of CR The unique structural features of MCNSAs, including abundant defects, large surface areas, bimodal pore structures and adequate magnetism, make them a promising candidate for pollutant removal in water sources. To demonstrate the sorption performance of MCNSAs, notorious Pb2+ ions were first utilized as a model pollutant.31,32 50 mg of sample was added into 100 mL of a Pb2+ solution with an initial concentration of 200 mg L–1. The concentration of Pb2+ was monitored using an inductively plasma emission spectroscope. Shown in Figure 5a, the concentration of Pb2+ rapidly decreases during the initial period of 60 min, manifesting fast sorption. Within 4 h, the concentration of Pb2+ decreases from 200 to 12 mg L–1, corresponding to a removal rate of 94%. The kinetics of sorption process was studied by the pseudo–first–order, pseudo–second–order and intraparticle diffusion models. It is found that the experimental data are best fitted by using the pseudo–second–order equation with a largest correlation coefficient (R2) of 0.997 (see Figure S9). The adsorption isotherm was recorded as shown in Figure 5b. The Langmuir and Freundlich models were employed to analyze the sorption data. The data can be



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better described by the Langmuir model, yielding a maximum adsorption capacity of 554 mg g–1. To further demonstrate the versatility in water treatment, the MCNSAs were utilized to remove Congo red (CR), a widely used organic dye in textile industries,33–35 from aqueous solutions. The curve of concentration vs contact time displayed in Figure 5c indicates that the MCNSAs are capable of removing 83% of CR within 120 min and achieving a removal efficiency of 92.9% within 300 min. Kinetic analyses reveal that the sorption data also agree best with the pseudo– second–order model (R2 = 0.994, Figure S10). The isotherm (Figure 5d) fitted by the Langmuir model gives a maximum adsorption capacity of 980 mg g−1.

Figure 5. Adsorption curves of (a, b) Pb2+and (c, d) CR.

Figure 6. (a) Adsorption capacities of four carbons, (b) hysteresis loop of MCNSAs and (c) cyclic tests of CR adsorption. 12 ACS Paragon Plus Environment

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For comparison, three other typical carbon materials including CNTs, OMC and AC were obtained. The structures of these three carbons were characterized by FESEM, TEM and N2 adsorption–desorption isotherms (See Figures S11 and 12, and Table S1). The comparative sorption tests were done under the same conditions. Shown in Figure 6a, the MCNSAs have an experimentally determined maximum sorption capacity of 453 mg g–1 for Pb2+ removal, which is 2.2, 3.4, 9.3 times of that obtained from the OMC, AC and CNTs, respectively. In the case of CR removal, the MCNSAs give a maximum sorption capacity of 724 mg g–1, which is also much larger than that of 94.6, 286.8 and 328.3 mg g–1 as obtained from the CNTs, AC and OMC, respectively. Although not the highest values ever reported, the capacities of MCNSAs are higher than most of the reported sorbents as shown in Tables 1 and 2. Such remarkable sorption capacities of MCNSAs are attribute the hierarchical porous structure as well as the large surface areas with abundant defects, which serve as effective sites for absorbing pollutants.

Table 1 Summary of adsorption capacities of various adsorbents for Pb2+ Adsorbent CNT nanosheets Oxidized multiwalled-CNTs Activated carbon from eucalyptus bark Oxidized CNTs Palm shell activated carbon Granular activated carbon Carbon nanotube–graphene hybrid aerogels Flower-like magnesium oxide Nickel oxide nanoflowers Urchin- & flower-like WO3·n H2O Urchin-like α-FeOOH hollow Spheres Titanate/Fe3O4 Chrysanthemum-like α-FeOOH Titanate nanotubes Titanate nanoflowers, nanotubes & nanowires MCNSAs

Maximum capacity (mg g-1) 75.8 50 109.7 28 95.2 29.4 104.9 1980 125 248.9 & 315.0 80 382.3 103.0 216 304, 147 & 105.6 453


Reference 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 This study


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Table 2 Summary of adsorption capacities of various adsorbents for CR Maximum Capacity (mg g-1) 6.72 171 148 2409 80 82.9 &151.7 525 93.5 & 84.5 160 40 240 & 66.7 109.2 275 56.3 & 58.2 53 90 724

Sample Activated carbon from Coir pith Rice hull ash HNO3 treated multiwalled CNTs Porous MgO MnO2 Hollow Ni(OH)2 & NiO nanosheets Nickel oxide nanoflowers Hollow α -Fe2O3 & Fe2O3 nanowires Hollow Nestlike α Fe2O3 Flower-like Fe3O4 3D self-assembled iron hydroxide & oxide Sea urchin-like iron oxide Urchin-like FeOOH hollow spheres Iron hydroxide & oxide mesoporous α-Fe2O3 Spindle-like γ-Al2O3 MCNSAs

Reference 51 52 53 54 55 56 44 57 58 59 60 61 46 62 63 64 This study

The aforementioned experiments demonstrate that MCNSAs can absorb a quantity of Pb2+ and CR from aqueous solutions. More importantly, the MCNSAs can be easily separated using an external magnetic field and repeatedly used after regeneration. Shown in Figure 6b, the hysteresis loop of MCNSAs displays typical superparamagnetic features. The saturated magnetization of the MCNSAs is found to be 36 emu g–1, which is less than that of pure iron NPs as a consequence of the encapsulation of iron NPs by CNSs.65 Nevertheless, such a saturated magnetization is strong enough to achieve a facile magnetic separation. As shown in the inset photograph in Figure 6b, the MCNSAs are readily collected from the aqueous suspension by an external magnet. In addition, the CR–adsorbed MCNSAs can be facilely regenerated at 300 °C via combustion in air, owing to the high resistance to oxidation. The MCNSAs retain the high sorption capacities after being regenerated for six cycles (Figure 6c). FESEM observation on the Pb2+–adsorbed (Figures S13) and regenerated MCNSAs (Figures S14) reveal that the MCNSAs maintain the hierarchical structure after cyclic sorption and regeneration processes. These findings confirm that MCNSAs are a promising sorbent for water purification. 14 ACS Paragon Plus Environment

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4. CONCLUSION In summary, we present a simple approach to synthesize hierarchical MCNSAs via an APCVD method. The synthesis of MCNSAs relies on the process of catalytic decomposition of methane. The decomposition of methane is initiated by iron catalyst, producing graphitic CNSs via the dissolution–precipitation mechanism. After the encapsulation of iron NPs, the defects in the resulting CNSs serve as alternative sites for further carbon deposition. These two carbon deposition process hold the key for the formation of hierarchical MCNSAs. The reaction temperature plays a crucial role and high reaction temperatures are required for the synthesis of MCNSAs. The reported approach is simple, scalable and environmentally–friendly, and affords hydrogen as a valuable byproduct. Such features are favorable for the large–scale production and further practical applications. As an important fundamental study, this work also demonstrates that carbon materials can be tailored to be catalytically active for methane decomposition and that the catalytic decomposition of methane may be tuned as an autocatalytic process. These findings could provide new insights on designing carbon catalysts for hydrogen production by methane decomposition. The resulting MCNSAs possess a unique combination of large surface areas, bimodal pore structures, abundant defect sites, excellent chemical stability and adequate magnetism, rendering them an ideal candidate for water purification. Using CR and Pb2+ ions as model pollutants, the MCNSAs show remarkable sorption performance including high capacity, fast kinetics, facile separation and regeneration, and excellent recyclability. It is expected that, owing to the above–mentioned merits, the MCNSAs will show great application in capacitors, batteries and fuel cells.

ASSOCIATED CONTENT Supporting Information Raman, XPS, TEM, SEM, EDX and XRD data shown in Figures S1–14 and Table S1 are included. The Supporting Information is available free of charge on the ACS Publications website

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected]



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The Project was financially supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and the Natural Science Foundation of Guangdong Province, China (2014A030310315).

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Scalable and Environmentally Friendly Synthesis of Hierarchical

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