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 nano- fibers (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 environ- mental remediation, 4,9,12,16 heterogeneous catalysts for catalytic reactions, 10 and electrodes for fuel cells 10 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 functionaliz- ing 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 (CH 3 CN)/acrylonitrile, 4 ethane-ammo- nia (NH 3 ), 10 ferrocene (Fe(C 5 H 5 ) 2 )-melamine, 10 methane- NH 3 , 10 Fe(C 5 H 5 ) 2 -NH 3 , 10 Fe(C 5 H 5 ) 2 -aniline-toluene, 10 Fe- (C 5 H 5 ) 2 -benzylamine, 9,10 acetylene (C 2 H 2 )-CH 3 CN. 14 A single compound as the combined source of C and N has also been used for doping, for example, triazine, 14 dimethyl- formamide, 15 CH 3 CN, 15 and pyridine. 13,15 In these studies, N- CNSs were grown on a metal or metal oxide substrate such as cobalt, alumina, silica, or magnesia. A drawback of such methods is that the substrate must be removed before using N- CNSs 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 catalysts 20 and biomolecules, 21 and electrodes for sensors 22 and microbial fuel cells. 23 In the present study, we describe the synthesis of N- enriched CNFs in a one-step method. N-CNFs were grown on the ACFs, using CVD with CH 3 CN as the carbon source. CH 3 CN also served as the source of N that was in situ incorporated into the CNFs during CVD. The prepared N- CNFs/ACFs were used as efficient adsorbents to remove cadmium (Cd 2+ ) and lead (Pb 2+ ) 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(NO 3 ) 2 ·6H 2 O), sodium dodecyl sulfate (SDS), CH 3 CN, cadmium nitrate tetrahydrate (Cd(NO 3 ) 2 · 4H 2 O), and lead nitrate (Pb(NO 3 ) 2 ) were purchased from Merck (Germany). Nitric acid (HNO 3 ) was purchased from Fisher Scientific (United States). All reagents were of analytical grade with high purity. The high-purity hydrogen (H 2 ) and nitrogen (N 2 ) and C 2 H 2 (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 Published: April 21, 2015 Article pubs.acs.org/IECR © 2015 American Chemical Society 5172 DOI: 10.1021/ie505016d Ind. Eng. Chem. Res. 2015, 54, 5172-5178