Facile One-Step Synthesis of Nitrogen-Doped Carbon Nanofibers for

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Facile One-Step Synthesis of Nitrogen-Doped Carbon Nanofibers for the Removal of Potentially Toxic Metals from Water Akshay Modi,† Bhaskar Bhaduri,† and Nishith Verma*,†,‡ †

Department of Chemical Engineering, ‡Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, India 208016 ABSTRACT: Nitrogen-doped carbon nanofibers (N-CNFs) were prepared in a one-step method. N-CNFs were grown on an activated carbon fiber (ACF) substrate using catalytic chemical vapor deposition. Acetonitrile served as the single source of carbon and nitrogen. An approximately 10-fold increase of N-content, relative to the ACF substrate, was achieved in the densely and uniformly grown N-CNFs/ACFs. The prepared material, used as adsorbents for cadmium (Cd) and lead (Pb) in water, exhibited a significantly large adsorption capacity of approximately 161 ± 5 and 88 ± 3 mg/g for Cd and Pb, respectively, attributed to the lone pair of electrons in N atoms, which facilitated the formation of coordinate covalent bonds with the metal ions. The one-step method of preparing the N-enriched multiscale web of carbon micronanofibers in this study is simple, and the prepared material may be used as an efficient adsorbent for the removal of toxic metal ions from wastewater.

1. INTRODUCTION Carbon-based nanostructures (CNSs) such as carbon nanofibers (CNFs) and carbon nanotubes (CNTs) have received significant attention in the last two decades. The superiority of such materials over traditional carbon-based materials is due to the remarkable mechanical, physicochemical, electrical, and thermal properties. At present, these materials are extensively used for energy, environmental, and catalytic reaction applications.1−3 Recently, CNFs and CNTs have been doped with nitrogen (N) with a view to increasing the functionality of the materials.4−16 The lone pair of electrons in N atoms activates the π electrons of carbon materials via conjugation, thereby enhancing the surface chemical reactivity.9 Such N-doped carbon materials have been used as adsorbents for environmental remediation,4,9,12,16 heterogeneous catalysts for catalytic reactions,10 and electrodes for fuel cells10 and electrochemical sensors.11 Catalytic chemical vapor deposition (CVD) is a preferred method of doping CNSs with N because the method is simple and may be scaled up for producing a large quantity of the materials.10,14 N-doping is performed either ex situ by first growing carbon structures on a substrate and then functionalizing the structures,10,16 or in situ by incorporating N-content into the material during synthesis.4,10,12,14 Therefore, an additional postgrowth step is required in ex situ methods for the surface functionalization. Moreover, an excess doping may cause pore blockage in the surface of the materials.16 In situ doping has been performed using a mixture of two or more compounds as the source of carbon (C) and N, such as ethylene−acetonitrile (CH3CN)/acrylonitrile,4 ethane−ammonia (NH3),10 ferrocene (Fe(C5H5)2)−melamine,10 methane− NH3,10 Fe(C5H5)2−NH3,10 Fe(C5H5)2−aniline−toluene,10 Fe(C5H5)2−benzylamine,9,10 acetylene (C2H2)−CH3CN.14 A single compound as the combined source of C and N has also been used for doping, for example, triazine,14 dimethylformamide,15 CH3CN,15 and pyridine.13,15 In these studies, NCNSs were grown on a metal or metal oxide substrate such as © XXXX American Chemical Society

cobalt, alumina, silica, or magnesia. A drawback of such methods is that the substrate must be removed before using NCNSs in an end-application. Recently, CNFs were grown on an activated carbon microfiber (ACF) substrate using CVD.17,18 The main advantage of using ACF as a substrate is that the prepared multiscale web of carbon micronanofibers (CNFs/ACFs) may be directly used in an end-application without removing the support in a postsynthesis step. The CNFs/ACFs have been efficiently used as adsorbents,19 support to metal catalysts20 and biomolecules,21 and electrodes for sensors22 and microbial fuel cells.23 In the present study, we describe the synthesis of Nenriched CNFs in a one-step method. N-CNFs were grown on the ACFs, using CVD with CH3CN as the carbon source. CH3CN also served as the source of N that was in situ incorporated into the CNFs during CVD. The prepared NCNFs/ACFs were used as efficient adsorbents to remove cadmium (Cd2+) and lead (Pb2+) toxic metal ions from water.

2. MATERIALS AND METHODS 2.1. Materials. The phenolic resin precursor-based ACFs were procured from Gun Ei Chemical Industry Co. Ltd. (Japan). Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium dodecyl sulfate (SDS), CH3CN, cadmium nitrate tetrahydrate (Cd(NO3)2·4H 2O), and lead nitrate (Pb(NO3) 2) were purchased from Merck (Germany). Nitric acid (HNO3) was purchased from Fisher Scientific (United States). All reagents were of analytical grade with high purity. The high-purity hydrogen (H2) and nitrogen (N2) and C2H2 (AAS grade) gases were purchased from Sigma Gases (India). All aqueous solutions used in this study were prepared in Milli-Q water. Received: December 25, 2014 Revised: March 29, 2015 Accepted: April 21, 2015

A

DOI: 10.1021/ie505016d Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2.2. Synthesis of N-CNFs/ACFs. The pretreated ACF samples were impregnated with an aqueous solution of 0.4 MNi(NO3)2·6H2O salt, using the wet incipient impregnation method.19 Approximately 0.3% (w/w) SDS surfactant was mixed into the solution to achieve a uniform and monodispersion of the salts without agglomeration and to maximize their transfer to the ACF surface. After impregnation, the ACF samples were dried for 6 h at room temperature (30 ± 2 °C) and then for another 12 h at 120 °C in a hot air-oven. Calcination and reduction were performed on the dried ACF samples for 4 h at 400 °C, using N2 at 60 standard cc per min (sccm) and for 2 h at 550 °C, using H2 at 120 sccm, respectively. Ni(NO3)2 dispersed in the ACFs was converted into nickel oxide (NiO) upon calcination, and NiO was converted into the metallic (Ni0) state upon reduction. Next, CVD was performed on the reduced ACF sample in a CVD reactor. The N2 gas was bubbled at 200 sccm through the liquid CH3CN contained in a bubbler at room temperature. CVD was performed for 2 h at 800 °C. After the CNFs were grown, the reactor was cooled to room temperature and the samples were removed from the reactor. The post-CVD samples were then subjected to a mild ultrasonication for 10 min in a 0.05 M HNO3 to dislodge (remove) the Ni nanoparticles (NPs) from the tips of the CNFs.19 The ultrasonicated samples were denoted as N-CNFs/ACFs for the reference purposes in this study. The detailed description of the reactors used for performing calcination, reduction, and CVD is available in a previous study.20 Few samples were also prepared using C2H2 as a non-N-containing hydrocarbon source during the CVD step for comparison purposes. These samples were denoted as CNFs/ACFs. All samples were prepared in triplicate to check the reproducibility. Figure 1 is a schematic illustration of the one-step preparation of N-CNF/ACF and its application as an adsorbent

solution were kept in an orbital shaker at room temperature. The adsorption equilibrium times for Cd and Pb were determined to be approximately 12 and 24 h, respectively. The solutions were periodically collected and analyzed for the metal ion concentrations. An atomic absorption spectrometer (AAS; Varian AA-240, United States) was used for the measurements. The AAS analysis was performed at 228.8 and 217 nm wavelengths for Cd and Pb metals, respectively. Calibration was performed prior to the analysis. All tests were performed in triplicate to check the reproducibility.

3. MATERIAL CHARACTERIZATION The prepared ACF, N-CNF/ACF, and CNF/ACF samples were characterized using different analytical techniques such as AAS, C−H−N elemental analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Brunauer−Emmett−Teller (BET) surface area (SBET) and pore size distribution (PSD) measurements, and Fourier transform infrared (FT-IR) spectroscopy. AAS analysis was performed to determine the Ni metal loading in the ACFs by measuring the Ni concentration of the solution before and after impregnation, using the AAS (Varian, AA-240, United States). The elemental compositions including N were determined using the C−H−N elemental analyzer (Exeter Analytical Inc., CE 440, United States). The surface morphology was examined using field emission SEM (Tescan, MIRA3, Czech Republic), TEM (FEI Technai, 20 U Twin, United States), and EDX (Zeiss, EVO 50, Germany). The SEM and TEM images and EDX spectra were taken at a number of locations on the samples. The SBET and total pore volume (VT) were determined from the N2 adsorption−desorption multipoint isotherms, determined at 77 K using the Quantachrome Autosorb 1C system (United States). Prior to the measurement, the samples were degassed at 150 °C for 12 h in the outgassing station of the instrument to remove adsorbed moisture or entrapped gases in the samples. The SBET was determined using the linearized BET equation fitted to 20 data points corresponding to the amount of N2 adsorbed versus relative pressure (P/Po) ranging from 0.05 to 0.35. Density functional theory and the Barrett−Joyner−Halenda method were used to determine the microporosity and meso-porosity of the prepared materials, respectively. The VT was measured from the amount of vapor adsorbed at the relative pressure close to unity (0.999). The FT-IR spectra of the samples were acquired in the attenuated total reflectance mode using a Ge crystal, for the wavenumbers ranging from 500 to 4000 cm−1, using the FT-IR spectrophotometer (Bruker, Tensor 27, United States).

Figure 1. Schematic illustration showing (A−D) the one-step synthesis of N-CNF/ACF and (E) adsorption of potentially toxic metal ions by N-CNF/ACF.

for the removal of potentially toxic metal ions. The figure depicts the catalytic decomposition of CH3CN into N−C species over the catalytic Ni NP at the ACF substrate; the diffusion of the decomposed species through the metal NP; the growth of an N-CNF, thus forming the multiscale N-CNF/ ACF; the ultrasonicated N-CNF/ACF after the removal of the Ni NPs from the tips of the CNFs; and the adsorption of potentially toxic metal ions. 2.3. Batch Adsorption Experiments. The separate stock solutions of 300 and 150 ppm of Cd(NO3)2·4H2O and Pb(NO3)2, respectively, were prepared. The adsorption tests were performed over the concentration ranges of 0−100 ppm and 0−50 ppm for Cd and Pb ions, respectively. Approximately 0.1 g of the prepared adsorbents was mixed into 150 cc salt solutions of different concentrations. The flasks containing the

4. RESULTS AND DISCUSSION 4.1. Ni Loading. The Ni loading in the impregnated ACF samples with SDS was determined to be ∼450 mg/g, which was ∼35% higher than that determined in the samples impregnated without SDS. The higher metal loading was attributed to a uniform dispersion without agglomeration of the metal salt in the ACFs, which in turn produced a dense and uniform growth of N-CNFs on ACFs (discussed later). 4.2. C−H−N Elemental Analysis. Table 1 summarizes the results of the elemental analysis. The N/C ratio in N-CNFs/ ACFs was determined to be ∼10 times greater than it was in the ACF substrate and ∼6 times larger than it was in CNFs/ ACFs. The elemental analysis confirmed a significant increase B



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ultrasonication, opening up the tips (inset of Figure 2D). The texture of the nanofibers, however, remained intact after ultrasonication. Figure 2E shows the EDX elemental mapping of the N-CNF/ACF sample. The mapping confirmed a uniform dispersion of N on the surface. No foreign impurities were detected. Figure 2F shows the TEM image of an N-CNF. The average internal diameter of the fiber was measured to be ∼25 nm, using ImageJ software. The image shows an irregular and interlinked corrugated morphology (bamboo-shaped) of the nanofiber, attributed to the in situ doping of N in the nanofibers. Such structures with defects in the sidewalls have also been observed in other N-CNSs,10,14,15 and these defects were responsible for a high reactivity of the N-CNFs.9,24 Panels A and B of Figure 3 show the SEM images of the material after adsorption with Pb and Cd, respectively. A distinct change in the surface morphology of the metal-adsorbed samples may be observed (see Figure 2D for the comparison). Panels C and D of Figure 3 show the corresponding EDX elemental mapping. The mapping confirmed the adsorption of the toxic metals by the material. 4.4. SBET, VT, and PSD Analysis. Table 2 presents the data of the SBET, VT, and PSD analysis. The SBET and VT of the ACFs

Table 1. Elemental (C−H−N) Analysis Data (% w/w) material

C

H

N

residue

ACFs N-CNFs/ACFs CNFs/ACFs

80.44 85.21 94.99

1.44 0.92 1.67

0.38 4.00 0.72

17.74 9.87 2.62

of N-content in N-CNFs/ACFs and thereby an improved surface chemical reactivity of the material (discussed later). 4.3. Surface Morphology. Figure 2A shows the SEM image of the ACFs. The ACF surface was rough, and the macro- and meso-pores were visible. Figure 2B shows the SEM image of the N-CNF/ACF sample. A uniform dispersion and loading of the metal in the ACFs facilitated the dense and uniform growth of the N-CNFs across the surface of the substrate. The shiny NPs were observed at the tips of the CNFs. Figure 2C shows the point-EDX spectra of the N-CNF/ ACF sample. The spectra established the shiny NP to be a Ni, confirming that the growth of the N-CNFs followed the tipgrowth mechanism. Figure 2D shows the SEM image of the NCNF/ACF sample that was ultrasonicated. Most of the metal NPs were dislodged from the tips of N-CNFs during

Figure 2. SEM images of (A) ACF and (B) Ni NPs-dispersed N-CNF/ACF; (C) point-EDX analysis of Ni NPs-dispersed N-CNF/ACF; (D) SEM image of N-CNF/ACF; (E) EDX elemental mapping of N-CNF/ACF; and (F) TEM image of N-CNF. C



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Figure 3. SEM images of N-CNF/ACF after (A) Cd-adsorption and (B) Pb-adsorption; EDX elemental mappings of N-CNF/ACF after (C) Cdadsorption and (D) Pb-adsorption.

Table 2. SBET, VT, and PSD of the Prepared Materials

samples are assigned to free O−H groups, attributed to the presence of water molecules. Figure 4B shows the FT-IR spectra of the N-CNF/ACF sample before and after adsorption. The peak observed at ∼1640 cm−1 in the preadsorbed sample, attributed to CN stretch, was shifted to ∼1585 cm−1 in the postadsorbed sample, with a considerable increase in the intensity. The peak at ∼1530 cm−1 in the preadsorbed sample, attributed to CN stretch, was shifted to ∼1490 cm−1 in the postadsorbed sample, with a decrease in the intensity. An increase in the peak intensity was observed at ∼2350 cm−1. The change in the intensity and/or shifting of the peaks indicates the activity of the N-surface functional groups of N-CNFs/ACFs in the adsorption process. A plausible conclusion drawn from the FT-IR analysis is that N atoms of N-CNF/ACF acted as the active adsorption sites for the metal ions, forming the coordination covalent bonds with the ions during adsorption.26,27 4.6. N-CNFs/ACFs as Adsorbents. Figure 5 shows the adsorption isotherms for Cd2+ on N-CNFs/ACFs, CNFs/ ACFs, and ACFs. The solute loading, q (milligrams per gram), was determined using the species balance: q = V × (CO − C)/ w, where CO and C are the concentrations (milligrams per liter) of the solute in the solution before and after equilibrium, respectively; V is the volume (liters) of the solution in contact with the adsorbents, and w is the adsorbent dose (grams). The adsorption capacity of the materials showed the following trend: N-CNFs/ACFs > CNFs/ACFs > ACFs, with the adsorption capacity of N-CNFs/ACFs determined to be approximately 2- and 2.5-times greater than that of CNFs/ ACFs and ACFs, respectively. Figure 6 shows the adsorption isotherms for Pb2+ ions on ACFs, CNFs/ACFs, and N-CNFs/ ACFs, and a similar trend was observed, with the adsorption capacity of N-CNFs/ACFs determined to be approximately 2and 11-times greater than that of CNFs/ACFs and ACFs, respectively. The adsorption capacity of N-CNFs/ACFs was

PSD (%) material

SBET (m2/g)

VT (cm3/g)

micro

meso

macro

ACFs N-CNFs/ACFs CNFs/ACFs

1329 890 656

0.727 0.540 0.461

89.20 76.68 67.68

6.34 16.94 21.41

4.46 6.38 10.91

were determined to be 1329 m2/g and 0.727 cm 3/g, respectively. The ACFs were highly microporous, with the microporosity content determined to be ∼89% of the total pore volume. The SBET and VT decreased in N-CNFs/ACFs and CNFs/ACFs, with more than 30% decrease in microporosity. However, the mesoporosity content increased with CNF growth in both samples. 4.5. FT-IR Analysis. Figure 4A shows the FT-IR spectra of the ACF, CNF/ACF, and N-CNF/ACF samples. The peaks observed at ∼2340, ∼1620, ∼1480, and ∼680 cm−1 in the ACF sample may be attributed to CN (nitrile group), CO stretch (carbonyl and carboxylic acid groups), N−O stretch (nitro group), and CC stretch (alkene group), respectively. The band appearing in the range of ∼1100−1000 cm−1 may be attributed to C−N stretch (amine group). These groups are inherently present in ACFs. The presence of the nitrogencontaining CN, C−N, and N−O stretches are attributed to HNO3 used for the leaching of ACF samples. The peak at ∼1585 cm−1 in the CNF/ACF sample is attributed to the C C stretch. The two high-intensity peaks observed at ∼1640 and ∼1530 cm−1 in the N-CNF/ACF sample are assigned to the CN stretch. These peaks signify that the in situ incorporation of N-functionality into the C framework was successful during CNF growth.8,25 The increase in the intensity of peaks over the range of ∼1100−1000 cm−1 and at ∼2340 cm−1 also indicates the incorporation of N-functionality in the N-CNFs/ACFs.8 The peaks over the range of ∼3860−3500 cm−1 in all three D



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Figure 6. Adsorption isotherms for Pb on N-CNF/ACF, CNF/ACF, and ACF.

respectively. The relatively higher adsorption capacity for Cd is attributed to the vacant “d” orbitals of the penultimate shell in Cd, a transition metal, which facilitates the formation of coordinate covalent bonds with the lone pair of electrons of the N atoms in the adsorbent.28 The higher adsorption capacity for Cd metal ions is also attributed to its smaller ionic radius (0.97 Å), lower hydration energy (−1806 kJ/mol), and higher charge-to-radius ratio (2.06) or ionic potential, compared to those of Pb metal ions. The corresponding values are 1.19 Å, −1480 kJ/mol, and 1.68, respectively.29−32 The adsorption efficiency of N-CNFs/ACFs was compared to that of the other adsorbents discussed in the literature, and the results are presented in Tables 3 and 4 for Cd2+ and Pb2+ Figure 4. FT-IR spectra of (A) the prepared materials and (B) NCNF/ACF before and after adsorption.

Table 3. Comparison of the Adsorption Capacity of NCNFs/ACFs with the Literature Data for Cd2+ reference present study 9 12 12 34 35 36

aqueous phase concentration (ppm)

adsorption capacity (mg/g)

N-CNFs/ACFs

0−100

0−161 (±5)

oxidized N-doped MWCNTs HNO3-oxidized Ndoped CNTs oxidized CNFs ethylenediaminefunctionalized SBA15 oxidized AC mesoporous silica

60

31

56

20

60 187

20 100

79 70

146 30

adsorbent

ions, respectively.9,12,19,28,33−41 The adsorption capacity of NCNFs/ACFs for Cd2+ is significantly greater than that of the functionalized adsorbents including those which have been derived from low-cost materials. The adsorption capacity of NCNFs/ACFs for Pb2+ is significantly greater than that of CNTs, functionalized CNTs, and the other carbon-based adsorbents and is comparable to that of mesoporous silica. The adsorption capacity of N-CNFs/ACFs is, however, smaller than that of the nanosized MnO2 and micron-sized resin particles. However, it is important to mention that the practical use of such nano- or

Figure 5. Adsorption isotherms for Cd on N-CNF/ACF, CNF/ACF, and ACF.

approximately 161 ± 5 and 88 ± 3 mg/g for Cd2+ and Pb2+ ions, respectively. The adsorption capacity of the prepared material for Cd and Pb metal ions may also be expressed as 1.42 and 0.43 mmol/g, E


Industrial & Engineering Chemistry Research Table 4. Comparison of the Adsorption Capacity of NCNFs/ACFs with the Literature Data for Pb2+ reference present study 9 19 28 34 36 37 38 39 40 41

aqueous phase concentration (ppm)

adsorption capacity (mg/g)

N-CNFs/ACFs

0−50

0−88 (±3)

oxidized N-CNTs CNFs/ACFs chitosan/PVA beads ethylenediaminefunctionalized SBA15 mesoporous silica granular palm shell AC commercial carbon hydrous MnO2 gel type weak acid resin CNTs

72 0−50 6.25 217

29 0−40 0.95 360

125 50 50 100 40 5

83 10 36 400 510 2

adsorbent

ACKNOWLEDGMENTS

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REFERENCES

The authors gratefully acknowledge the Gun Ei Chemical Industry Co. Ltd., Japan for supplying ACFs. The authors are also thankful to the Center for Environmental Science and Engineering at IIT Kanpur for carrying out the research.

(1) De Jong, K. P.; Geus, J. W. Carbon nanofibers: Catalytic synthesis and applications. Catal. Rev.: Sci. Eng. 2000, 42, 481−510. (2) De Volder, M. F.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon nanotubes: Present and future commercial applications. Science 2013, 339, 535−539. (3) Tran, P. A.; Zhang, L.; Webster, T. J. Carbon nanofibers and carbon nanotubes in regenerative medicine. Adv. Drug Delivery Rev. 2009, 61, 1097−1114. (4) Lim, S.; Yoon, S.-H.; Mochida, I.; Jung, D.-H. Direct synthesis and structural analysis of nitrogen-doped carbon nanofibers. Langmuir 2009, 25, 8268−8273. (5) Sumpter, B. G.; Meunier, V.; Romo-Herrera, J. M.; Cruz-Silva, E.; Cullen, D. A.; Terrones, H.; Smith, D. J.; Terrones, M. Nitrogenmediated carbon nanotube growth: Diameter reduction, metallicity, bundle dispersability, and bamboo-like structure formation. ACS Nano 2007, 1, 369−375. (6) Terrones, M.; Ajayan, P. M.; Banhart, F.; Blase, X.; Carroll, D. L.; Charlier, J. C.; Czerw, R.; Foley, B.; Grobert, N.; Kamalakaran, R.; Kohler-Redlich, P.; Rühle, M.; Seeger, T.; Terrones, H. N-doping and coalescence of carbon nanotubes: Synthesis and electronic properties. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 355−361. (7) Tang, C.; Bando, Y.; Golberg, D.; Xu, F. Structure and nitrogen incorporation of carbon nanotubes synthesized by catalytic pyrolysis of dimethylformamide. Carbon 2004, 42, 2625−2633. (8) Lai, S. H.; Chen, Y. L.; Chan, L. H.; Pan, Y. M.; Liu, X. W.; Shih, H. C. The crystalline properties of carbon nitride nanotubes synthesized by electron cyclotron resonance plasma. Thin Solid Films 2003, 444, 38−43. (9) Perez-Aguilar, N.; Muñoz-Sandoval, E.; Diaz-Flores, P.; RangelMendez, J. Adsorption of cadmium and lead onto oxidized nitrogendoped multiwall carbon nanotubes in aqueous solution: Equilibrium and kinetics. J. Nanopart. Res. 2010, 12, 467−480. (10) Mabena, L.; Sinha Ray, S.; Mhlanga, S.; Coville, N. Nitrogendoped carbon nanotubes as a metal catalyst support. Appl. Nanosci. 2011, 1, 67−77. (11) Goran, J. M.; Lyon, J. L.; Stevenson, K. J. Amperometric detection of L-lactate using nitrogen-doped carbon nanotubes modified with lactate oxidase. Anal. Chem. 2011, 83, 8123−8129. (12) Andrade-Espinosa, G.; Muñoz-Sandoval, E.; Terrones, M.; Endo, M.; Terrones, H.; Rangel-Mendez, J. R. Acid modified bambootype carbon nanotubes and cup-stacked-type carbon nanofibres as adsorbent materials: Cadmium removal from aqueous solution. J. Chem. Technol. Biotechnol. 2009, 84, 519−524. (13) Sen, R.; Satishkumar, B. C.; Govindaraj, A.; Harikumar, K. R.; Raina, G.; Zhang, J.-P.; Cheetham, A. K.; Rao, C. N. R. B−C−N, C−N and B−N nanotubes produced by the pyrolysis of precursor molecules over Co catalysts. Chem. Phys. Lett. 1998, 287, 671−676. (14) Tetana, Z. N.; Mhlanga, S. D.; Bepete, G.; Krause, R. W. M.; Coville, N. J. The synthesis of nitrogen-doped multiwalled carbon nanotubes using an Fe-Co/CaCO3 catalyst. S. Afr. J. Chem. 2012, 65, 39−49. (15) van Dommele, S.; Romero-Izquirdo, A.; Brydson, R.; de Jong, K. P.; Bitter, J. H. Tuning nitrogen functionalities in catalytically grown nitrogen-containing carbon nanotubes. Carbon 2008, 46, 138−148. (16) Bhaduri, B.; Verma, N. Preparation of asymmetrically distributed bimetal ceria (CeO2) and copper (Cu) nanoparticles in nitrogen-doped activated carbon micro/nanofibers for the removal of nitric oxide (NO) by reduction. J. Colloid Interface Sci. 2014, 436, 218−226.

micron-sized materials as adsorbents requires a suitable support. Figures 5 and 6 also show the equilibrium data fitted with the Langmuir isotherm equation. A reasonably good match is observed, suggesting the monolayer coverage of the surface with the solute molecules. The N-containing surface functional groups of N-CNF/ACF, as was confirmed by the FT-IR spectra, enhanced the chemical reactivity of the adsorbent toward the metal ions. The N atoms have an ability to donate their lone pair of electrons.6−10 Consequently, the N-functional groups present in the adsorbent chemically interacted with the metal ions, forming coordinated covalent bonds.12,26−28 Therefore, the adsorption of the metals by N-CNFs/ACFs was greater than that by CNFs/ACFs, with chemisorption being the predominant mechanism of the removal by the Ndoped materials.

5. CONCLUSIONS A nitrogen-enriched multiscale forming web of carbon micronanofibers was successfully synthesized in a one-step method, using CVD with CH3CN as the single source of C and N. The elemental analysis confirmed an approximately 10-fold increase of N-content in the synthesized material relative to the ACF substrate. The growth of the CNFs across the ACF surface was uniform and dense. TEM image showed the bamboo-shaped compartments in the CNFs, similar in structure to that of the other N-doped carbon nanostructures discussed in the literature, which also confirmed the in situ incorporation of N in the CNFs. The N-containing surface functional groups significantly enhanced the chemical reactivity of the prepared material toward Cd2+ and Pb2 ions in water by forming the coordinate covalent bonds with the ions. A significant adsorption capacity of N-CNF/ACF for Cd and Pb indicates its suitability as an effective adsorbent for the removal of potentially toxic metal ions from wastewater. The production method described in this study is novel, simple, and scalable.

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