carbon hollow spheres with superior capacities for heavy metal removal

carbon hollow spheres with superior capacities for heavy metal removal

Journal of Colloid and Interface Science 425 (2014) 131–135 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.e...

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Journal of Colloid and Interface Science 425 (2014) 131–135

Contents lists available at ScienceDirect

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

Short Communication

One step solvothermal synthesis of functional hybrid c-Fe2O3/carbon hollow spheres with superior capacities for heavy metal removal Hao-Jie Cui a, Jie-Kui Cai a,b, Huan Zhao a, Baoling Yuan c, Cuiling Ai b, Ming-Lai Fu a,⇑ a

Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China College of Civil Engineering, Fuzhou University, Fuzhou 350108, China c College of Civil Engineering, Huaqiao University, No. 668, Jimei Road, Xiamen, Fujian 361020, China b

a r t i c l e

i n f o

Article history: Received 9 January 2014 Accepted 14 March 2014 Available online 27 March 2014 Keywords: c-Fe2O3/carbon hollow spheres Functional groups Adsorption Heavy metal

a b s t r a c t One-step hydrothermal method was developed to prepare hybrid c-Fe2O3/carbon hollow spheres with a predominant orientation (1 1 1) plane of c-Fe2O3 and rich oxygen-containing functional groups on carbon. The resulting functional hybrid exhibited extremely high adsorption capacities for toxic Pb(II) and Cr(VI) ions in solutions with easy magnetic separation. The ease of synthesis and low cost, coupled with the efficient and rapid removal of toxic heavy metal ions, make hybrid c-Fe2O3/carbon hollow spheres an attractive adsorbent for the purification of waste and contaminated water. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Hollow micro-/nanostructures have attracted increasing attention because of their unique structural features and a wide range of important applications, such as energy storage, catalysis, sensors, biomedicines, and adsorbents [1–4]. Over the past decades, several strategies, including hard/soft template and template-free methods, have been developed to achieve numerous hollow structures with different morphologies to meet the demands of many scientific and technological applications [5–8]. Among them, hollow structures with multicomponent are highly explored, and can be expected to further tune the properties of materials by manipulating the constituent and structure of hollow materials on the micro-/nano-scale [9–10]. Hybrid hollow structures, which contain multiple functional components, are more desirable for their novel properties or improved performance in a variety of applications compared to single-component hollow structure [11–14]. Unfortunately, most routes to synthesize such hollow structures usually require multiple steps and post-treatment [11–14]. Therefore, it is still a big challenge to develop facile and reliable methods for the controlled synthesis of hybrid hollow structures. The application of nanomaterials in environmental remediation has received considerable attention in recent years [15,16]. As very

⇑ Corresponding author. Fax: +86 592 6190762. E-mail address: [email protected] (M.-L. Fu). http://dx.doi.org/10.1016/j.jcis.2014.03.049 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

promising adsorbents, magnetic carbon nanocomposites offer significant improvements over conventional adsorbents for coupling the sorption process with easy separation [17–20]. Moreover, the surface of carbon can probably be easily functionalized to enhance the adsorption capacity. For example, it has been found that increasing the oxygen-containing functional groups content of samples has obviously enhanced adsorption capacity for heavy metals [21]. The combination of magnetic materials and carbons has enlarged the application span of materials because of the existence of some synergistic effects. However, most strategies used for assembling such magnetic functional hybrid nanocomposites involved time-consuming synthetic procedures, where a separate step is usually required to deposit the carbon on the surface of the pre-synthesized iron oxides. Thus, the development of a straightforward synthetic route to assemble such functional composite materials could be a hard target. Recently, unprecedented research efforts have been focused on the controllable preparation of micro- and nano-crystals with various geometries and exposed surfaces to improve their performance in sensor, energy, and environmental applications [22–24]. Herein, c-Fe2O3/C hollow nanostructures with a predominant orientation (1 1 1) plane of c-Fe2O3 and rich oxygen-containing groups on carbon were successfully synthesized by one-step hydrothermal treatment of Fe(NO3)3, glucose, and acrylic acid (AA) mixture solutions. The addition of glucose and acrylic acid led to the formation of hybrid c-Fe2O3/C hollow nanostructures. The as-obtained functional c-Fe2O3/C hybrid hollow nanostructures exhibit extremely high performance in water treatment.

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2. Materials and methods 2.1. Preparation of c-Fe2O3/carbon hollow spheres In a typical procedure, stoichiometric amounts of Fe(NO3)39H2O and glucose were added to 40 mL mixed solvent of deionized water (38 mL) and acrylic acid (AA, 2 mL) with vigorous stirring. The resulting solution was transferred to a Teflon autoclave, which was then heated at 160–200 °C in an electric oven for 9–20 h. The products were collected from the solution with an external magnet and washed with water and ethanol for several times. Finally, the black products were dried in an oven at 60 °C for 24 h. 2.2. Characterizations of samples X-ray powder diffraction (XRD) was carried out using a Bruker D8 ADVANCE X-ray diffractometer equipped with monochromated Cu Ka radiation (k = 0.1541 nm) at a tube voltage of 40 kV and a tube current of 30 mA. Scanning electron microscopy (SEM) images were obtained with a Hitachi S-4800 emission scanning electron microscope. High-resolution electron microscopy (HRTEM) was performed on sample suspensions dried on a carbon coated grid (200 mesh, 3.05 mm in diameter) with a JEOL JEM 2010 FEF electron microscope operated at 200 kV. A Quantachrome Autosorb-1 instrument was used to measure the surface areas and micropore size distributions of the materials. Samples were degassed in a vacuum at 250 °C for about 10 h to remove water and other physically adsorbed species. N2 isothermal adsorption and desorption experiments were performed at relative pressures (P/P0) from 10 6 to 0.9916 and from 0.9916 to 0.047, respectively. X-ray photoelectron spectroscopy (XPS) spectra of the samples were recorded on a PHI Quantum 2000 Scanning ESCA Microprobe spectrometer with an Al Ka incident X-ray beam. The X-ray source was operated at 35 W, and the spectra were recorded at 15 kV. The analysis chamber pressure was 5  10 8 Pa. Hysteresis loops was collected on a Quantum Design superconducting quantum interference device

magnetometer (LakeShore 7307) at 300 K. The thermal degradation of c-Fe2O3/C hollow nanostructures was studied with a thermo-gravimetric analysis (TGA, Netzsch TG 209 F3) from 25 to 800 °C with an air flow rate of 30 mL/min and a heating rate of 10 °C/min. 2.3. Heavy metal ions removal experiments To study the equilibrium adsorption isotherm tests, 30 mg of

c-Fe2O3/C hollow nanostructures was added to 30 mL of aqueous Pb(II) or Cr(VI) solutions with different concentrations. The adsorption experiments were carried out in conical flasks mounted on a shaker at 260 rpm under ambient conditions for 20 h. Then, the suspension was filtered with a 220 nm membrane filter, and ICP–OES was used to measure the concentration of metal ions in the solution. All of the adsorption tests were performed in triplicate. The adsorbents were regenerated with 0.1 mol L 1 HNO3 or NaOH to investigate the recycle stability. 3. Results and discussion Fig. 1a is a typical XRD pattern of the products, and all peaks in the pattern can be well indexed as Fe3O4 (JCPDS no. 75-1609) or c-Fe2O3 (JCPDS no. 39-1346) phases due to their similar XRD patterns [25]. The phase of samples is further confirmed by XPS analysis due to its sensitivity to Fe(II) and Fe(III) cations. Fig. 1 S1 indicates that a well-resolved satellite peak is found at 719.0 eV, which indicate the absence of the Fe(II) ion [26]. Hence, from the XRD and XPS analyses, it is evident that the simple process employed here leads to the formation of the c-Fe2O3 phase in the products. Notably, in contrast to the standard pattern, the XRD pattern clearly exhibits an enhanced peak for the (1 1 1) reflection and a relatively low diffraction intensity for (3 1 1), which indicates that the growth of iron oxide crystals has a preferred orientation along [1 1 1] direction. No obvious diffraction peaks corresponding to carbon are found in the pattern, suggesting the existence of

Fig. 1. XRD pattern (a), SEM (b), and TEM (c and d) images of the as-synthesized c-Fe2O3/C hollow spheres.

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amorphous carbon. The SEM image shows that the as-prepared c-Fe2O3/C compositions are irregular spheres with a diameter of about 400–600 nm, and the broken sphere reveals that the spheres are hollow (Fig. 1b). The EDX spectra of the c-Fe2O3/C hollow spheres confirm the presence of Fe, O, and C elements (Fig. S2), and SEM/EDX was utilized to further verify the elemental composition of the c-Fe2O3/C hollow spheres, the EDX mapping images of Fe, O, and C elements show that the distribution of C is consistent with that of Fe and O, proving that C distributes uniformly on/in the particles (Fig. S3). In agreement with the above SEM observations, the hollow structure can be observed in the TEM image, and the thickness of the shells is estimated to be about 50–100 nm (Fig. 1c). The HRTEM image of the hollow spheres reveals that each shell of the hollow spheres is composed of numerous c-Fe2O3 nanocrystals of 5 nm sizes, and these nanoparticles are well-crystallized with (1 1 1) fringes running and a recognized spacing by 0.48 nm, confirming [1 1 1] as the preferred growth direction for c-Fe2O3 in the hollow spheres (Fig. 1d). The grain boundaries between the c-Fe2O3 particles further corroborate the crystal formation via oriented attachment of these nanoparticles. In the present simple hydrothermal system, the AA and glucose additive were found to play a critical role in the formation of hollow spheres, and no hollow spheres were formed in the absence of either AA or glucose (Figs. S4a and S4c). Moreover, the XRD patterns indicate that only a-FeOOH phase was obtained in the absence of glucose (Fig. S4b), and c-Fe2O3 polyhedrons without a

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preferred orientation along (1 1 1) planes formed in the absence of AA (Fig. S4d). Furthermore, a time-dependent experiment was carried out to investigate the formation of hollow spheres in the present system. Fig. S5 shows the SEM and TEM images of the samples obtained with different reaction durations. As seen in Figs. S5a and S5e, only aggregations of the nanoparticles were collected after a reaction time of 1 h. When the reaction duration was prolonged to 3 h, few hollow spheres were formed (Figs. S5b and S5f). Further prolonging the reaction duration to 6 h, the product contains a large portion of hollow spheres (Figs. S5c and S5g). As the reaction duration went on to 9 h, the products were completely transformed into well-defined hollow spheres (Figs. S5d and S5h). The diffraction peaks of the resultant materials in XRD patterns of Fig. S6 suggest the formation and growth of a-FeOOH nanocrystals at an early reaction stage (1–6 h), and a-FeOOH transformed into c-Fe2O3 with further prolonging the reaction duration (9 h). Based on the above experimental observations, a plausible formation mechanism of the hollow structures is proposed (Fig. 2). In simple terms, primary a-FeOOH nanocrystals first nucleate, resulting from the hydrolysis of Fe3+ ions. Afterward, AA coordinates in some fashion to nucleate FeOOH, resulting in arrested growth and stabilization of the a-FeOOH nanoparticles. In the meantime, carbonaceous materials abundant with carboxylic group were obtained by hydrothermal carbonization of glucose in the presence of AA. Subsequently, the a-FeOOH nanoparticles gradually self-assembled into a hollow structure, driven by the interaction of functional carbon nanoparticles which are adsorbed on the surface of the

Fig. 2. Schematic illustration of the formation process of the as-obtained c-Fe2O3/C hollow spheres.

Fig. 3. XPS spectra of C 1s (a) and TG curve (b) of the as-synthetic c-Fe2O3/C hollow spheres.

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a-FeOOH nanoparticles. During the a-FeOOH phase transform into c-Fe2O3 process, the adjacent primary nanocrystals align in an orderly way by an oriented-attachment mechanism so that the particles share a planar interface in a common crystallographic orientation. As shown in Fig. 3a, the XPS spectrum of C 1s could be deconvoluted into three peaks with binding energies at about 284.7, 286.0, and 288.3 eV, which are attributed to the CAC (58.6%), CAOH (17.0%), and [email protected] (24.4%), respectively [27]. These results demonstrated that a large number of carboxylate and hydroxyl groups are present in the c-Fe2O3/C hollow spheres. The TG curve shows that c-Fe2O3/C hollow spheres have three weight loss steps from room temperature to 800 °C under air atmosphere (Fig. 3b). The drop of 2.5 wt% below 150 °C results from the removal of absorbed water. The combustion of carbon was complete at a relatively low temperature (<400 °C). From the weight change between 150 °C and 400 °C, the organic matter content (carbon and oxygen containing surface groups) was determined to be 30.9 wt%. In order to examine the pore characteristic of the as-obtained c-Fe2O3/C hollow spheres, Brunnauer–Emmett–Teller (BET) N2 adsorption/desorption measurements were performed. The isothermal plots of N2 adsorption/desorption for the c-Fe2O3/C hollow spheres show type IV isotherms with an apparent hysteresis loop in the range 0.5–1.0 P/P0 (Fig. S7), indicating the presence of mesopores. Based on the BET equation, the specific surface area of the c-Fe2O3/C hollow spheres is 62.6 m2 g 1. The pore size distributions of the products, calculated from desorption data using Barett–Joyner–Halenda (BJH) model, show a wide peak centered at 11.0 nm (inset in Fig. S7), and the pore volume is 0.2 cm3 g 1. However, the pore size distributions of the c-Fe2O3/C hollow spheres are not limited to mesopores and clearly trespass in the domain of macroporosity. Compared with ordered mesoporous materiales, their pore size distributions are broader due to the irregular voids formed by the c-Fe2O3/C hollow spheres. Magnetization curve was measured for the as-synthesized c-Fe2O3/C hollow spheres at room temperature. As depicted in Fig. S8, it is hardly to see an obvious hysteresis loop at the full scale for the c-Fe2O3/C hollow spheres, indicating the superparamagnetism of magnetic hollow spheres. It can be indexed from Fig. S8 that the magnetization saturation value is about 34.7 emu g 1 for the c-Fe2O3/C hollow spheres. The magnetization value is lower than those of the c-Fe2O3 prepared by other methods [26]. Such decrease might be due to the presence of amphous carbon in the

shells of the hollow spheres and the smaller particle size of the c-Fe2O3. Despite the reduction in magnetic strength, the materials are still efficency in magnetic manipulation and recovery of the sorbent in the water treatment, as shown in inset in Fig. S8. To further verify the advantage of the porous c-Fe2O3/C hollow spheres in water treatment, we evaluate the adsorption capabilities for toxic heavy metal ions at ambient temperature. The adsorption isotherms of Pb(II) and Cr(VI) were obtained with different initial concentrations ranging from 10 to 1000 mg L 1, as shown in Fig. 4. Experimental data were fitted well with the Langmuir adsorption model isotherm, suggesting that uptake occurs on homogeneous surface by monolayer sorption. The maximal adsorption capacity of the porous c-Fe2O3/C hollow spheres is about 614 mg g 1 for Pb(II) and 449 mg g 1 for Cr(VI), respectively. It is worthwhile to mention that these values are extremely higher than those of previously reported nanomaterials, such as Fe3O4 micron-spheres (43.5 mg g 1 for Cr(VI)), magnetic hollow carbon nanospheres (200 mg g 1 for Cr(VI)), urchin-like a-FeOOH hollow spheres (80 mg g 1 for Pb(II)), magnetic porous ferrospinel MnFe2O4 (67 mg g 1 for Pb(II)), and porous magnetic Mn doped ferrite nanowires (131 mg g 1 for Pb(II) and 67.3–73.9 mg g 1 for Cr(VI)) [4,28–31]. The regeneration and reuse of the porous c-Fe2O3/C hollow spheres show that the adsorption capacities for heavy metal slightly decreased after regeneration (Fig. S9). To further understand the adsorption mechanisms of Pb(II) and Cr(VI), the porous c-Fe2O3/C hollow spheres before and after reaction with Pb(II) and Cr(VI) were analyzed with FT-IR and XPS. The FT-IR spectra of the porous c-Fe2O3/C hollow spheres before and after Pb(II) and Cr(VI) adsorption show that the Pb(II) and Cr(VI) species mainly attached to the oxygen-containing groups and certain chemical bonds were formed, which caused the decrease or shift of the vibration frequency of these surface chemical groups (Fig. S10). The XPS analysis indicates that Cr are adsorbed as Cr(III) (62.4%) and Cr(VI) (37.6%) (Fig. S11), suggesting that most of adsorbed Cr(VI) anions was reduced to Cr(III) by contact with surface carboxyl groups (the electron-donor groups) of the carbon materials. On the other hand, Fe(III) ions in (1 1 1) planes of c-Fe2O3 are coordinated to only three oxygen atoms [32], which promotes the interactions between the surface Fe(III) and Cr(VI) anion through electrostatic interaction. Thus, the high adsorption capacities for heavy metal ion could be attributed to the porous hollow sphere nanostructures, and a large number of oxygen-containing surface functional groups of carbon and surface active Fe(III) in (1 1 1) planes of the c-Fe2O3.

4. Conclusions The multifunctional c-Fe2O3/C hollow nanostructures, containing a preferred orientation (1 1 1) plane in c-Fe2O3 and rich oxygencontaining groups on carbon, were successfully prepared by one step hydrothermal method. The as-prepared multifunctional c-Fe2O3/C hollow nanostructures show excellent abilities to remove Pb(II) and Cr(VI) with easy magnetic separation property. This method may provide a simple and scalable synthesis approach for preparing advanced materials based on various multicomponent hollow structures for multipurpose application.

Acknowledgments

Fig. 4. Adsorption isotherms of Pb(II) and Cr(VI) using the porous c-Fe2O3/C hollow spheres.

We thank the National Natural Science Foundation of China (Nos. 41001139 and 51278481), National High Technology Research and Development Program (‘‘863’’ Program) of China (No. 2012AA062606), and Xiamen Distinguished Young Scholar Award (No. 3502Z20126011) for financial support.

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