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A facile controlled in-situ synthesis of monodisperse magnetic carbon nanotubes nanocomposites using water-ethylene glycol mixed solvents
Journal of Alloys and Compounds 657 (2016) 138e143
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A facile controlled in-situ synthesis of monodisperse magnetic carbon nanotubes nanocomposites using water-ethylene glycol mixed solvents Renjie Liu a, b, Yu Qiao a, Yan Xu a, Xinbin Ma a, b, Zhenhua Li a, b, * a b
Key Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2015 Received in revised form 9 October 2015 Accepted 12 October 2015 Available online xxx
In this paper, a facile and controllable method for in-situ synthesis of the magnetic carbon nanotubes nanocomposites (Fe/CNTs) in water-ethylene glycol (EG) mixed solvents is reported by the deposition eprecipitation method following annealing. The effect of water/EG ratio on the physico-chemical properties of magnetic Fe/CNTs is investigated by X-ray diffraction, transmission electron microscope, scanning electron microscopy, thermogravimetric analysis and physical property measurement system. The results indicate that the iron particle size distribution and grain size can be well-tuned by adjusting the water/EG ratio. With the variation of EG fraction in the mixed solvent, the nucleation, growth and crystallization of magnetic iron oxides with a controllable morphologies and particle sizes attached on the exterior surface of CNTs can be achieved. The as-prepared Fe/CNTs nanocomposites display superparamagnetic property at room temperature and the water/EG ratio determines the magnetization of the sample. Possible formation mechanism for magnetic Fe/CNTs is proposed based on the characterization results. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic Carbon nanotubes Nanocomposites Ethylene glycol
1. Introduction Carbon nanotubes (CNTs) with the unique electrical, thermal, mechanical and optical properties has been widely used in energy storage materials, sensors, electronics and catalysis [1,2] since Iijima discovered the CNTs in 1991 [3]. Magnetic nanoparticles such as magnetite and maghemite have attracted growing attention since their promising application in magnetic force, water treatment, biosensors and drug delivery [4e7]. To combine the advantages of both, the state-of-the-art of preparation methods have been developed to assemble the magnetic material such as deposition on functionalized CNTs, electrochemical deposition, electroless deposition and physical approaches [8]. Among these methods, nanoparticle deposition on functionalized CNTs is hopefully a simple and efficient chemical method to synthesize the magnetic Fe/CNTs catalyst. The monodispersion and particle size distribution
* Corresponding author. Key Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.jallcom.2015.10.099 0925-8388/© 2015 Elsevier B.V. All rights reserved.
of magnetic material determine their own properties. There are two main categories of the preparation method for nanoparticle dispersion on functionalized CNTs: one is adhering the iron particles onto the CNTs surface through covalent or noncovalent bond; the other is in situ synthesis [9]. In situ synthesis is a time-saving, cost-effective and reproducible process for large-scale production compared to the former method. During the synthesis process, some challenges should be faced to improve the performance and enlarge the applications of magnetic CNTs nanocomposites, such as the suitable dimension of magnetic nanoparticles, a narrow size distribution, high crystallinity and the reproducible procedure without any purification operation. As far as the fine-tuned size distribution of the magnetic nanoparticles, the precipitation method is restrained due to only kinetic factors deciding the growth of the crystal [5]. The above difficulties are real challenges faced by researchers attempting to develop a facile, reproducible and cost-saving method. It is noted that the precipitation process involves two steps, namely, nucleation and growth. To get a monodisperse and narrow size distribution of iron oxide particles, these two steps should be seriously separated, that is, the nucleation should be avoided in the growth process [10]. Ethylene glycol (EG) is the simplest diol with enormous
R. Liu et al. / Journal of Alloys and Compounds 657 (2016) 138e143
potential in various applications such as antifreeze formulations in automobiles, a deicing fluid for windshields and aircraft, a desiccant for natural gas production, and a raw material for manufacture of polyester fibers and resins industry [11]. It should be noted that EG has extensively been used as a reducing agent for the preparation of metal and metal oxide nanoparticles [12]. Though application of EG has recently proliferated in the area of nanoparticles synthesis, efforts in using it as a solvent are lacking. EG is a polar solvent and is thoroughly miscible with water at any ratio. After mixing them, the EG and water molecules are homogeneously mixed [13] and hence the physicochemical properties could be tuned by mixing them. It is reported that the liquid with surface tension higher than 0.1e0.2 N m1 can't get the CNTs wetted [14]. Therefore, the CNTs could be wetted by the water and EG mixture. By filling the tube with solvent, the magnetic nanoparticles attached on exterior surface of CNTs would be obtained subsequently. In early reports, a surfactant would be added to prevent the iron particle from agglomeration during the formation of magnetic particles. Here, the EG was used as not only a solvent but also a stabilizer. By changing the solvent ratio (water/EG ratio), EG would demonstrate its versatility in the formation, growth and crystallization of magnetic oxides with a controllable morphologies and particle sizes as expected. Hence, a nearly monodisperse magnetic particles decorated CNTs nanocomposites can be manufactured via a facile in-situ synthesis method: depositioneprecipitation process at different water/EG ratios followed by annealing. Additionally, the influence of the water/EG ratio on the physicochemical properties of magnetic nanocomposites was investigated by a series of characterization techniques. To further confirm the unique properties of EG, the influence of solvent on the magnetic Fe/CNTs nanocomposites preparation was investigated in this paper. In this study, a facile and controlled in-situ synthesis method in the solvent with different water/EG ratio is reported to fabricate magnetic Fe/CNTs catalyst followed by annealing. The size distribution of magnetic nanoparticles located on the CNTs surface are fine-tuned by adjusting the water/EG ratio in this method. Furthermore, the possible formation mechanisms of the magnetic nanocomposites are discussed. 2. Experimental section 2.1. Synthesis of magnetic CNTs Pristine CNTs (multi-wall CNTs) was obtained from Chengdu Organic Chemical Co. of Chinese Academy of Sciences. Iron (Ⅲ) nitrate nonahydrate, ammonium carbonate and EG were supplied by Tianjin Kermel Co., LTD of China. All the reagents used in this research were analytical grade without any further purification. Before synthesizing the magnetic composite, CNTs were functionalized as follows: 5.0 g raw CNTs were refluxed at 122 C (azeotropic point) for 6 h in 300 mL concentrated nitric acid (65 wt.%). After cooling down to room temperature, the carbon material was filtered and washed with deionized water until a neutral pH was reached, followed by drying at 110 C overnight for further use. This acid treatment can not only remove the residual contaminants but also introduce more functionalized carbon sites on the surface [15]. The magnetic Fe/CNTs nanocomposites were prepared by the depositioneprecipitation method followed by annealing. In a typical experiment, 2.0 g purified CNTs and 3.607 g iron nitrate nonahydrate were dispersed in 60 mL water and heated to 60 C in a round bottomed flask. Ammonium carbonate (1.714 g, dissolved in 120 mL EG) was added drop-wise to the mixed solution under continuous stirring for 2 h and the resulting precipitant was kept in
139
this medium overnight. After filtration and washing three times with ethanol and distilled water, the aged suspension was dried at 110 C for 12 h and calcined in an Ar flow at 400 C for 5 h with a rate of 3 C/min. The resulting sample was named as Fe/CNTsE2W1. Fe/CNTs-W2E1 (120 mL water/60 mL EG) was derived by exchanging the solvent with Fe/CNTs-E2W1. The samples Fe/CNTsW and Fe/CNTs-E were prepared when the solvents used were water and EG, respectively. To investigate the effect of the solvent, the ethanol was used as solvent and the corresponding sample was denoted as Fe/CNTs-e with the same preparation method mentioned above. 2.2. Characterization of materials The morphological features of the samples were measured on a Hitachi S-4800 Scanning electron microscopy (SEM) at an accelerating voltage of 3.0 kV. The structures of samples were characterized with a JEM-2100F transmission electron microscope (TEM) at 200 kV. Sample specimens for TEM studies were prepared by ultrasonic dispersion of the catalysts in ethanol and the suspension were dropped onto a carbon-coated copper grid before TEM images were recorded. The powder X-ray diffraction (XRD) characterization was performed on a RigakuD/max-2500 diffractometer with a CuKa radiation (40 kV, 200 mA). The scan speed was 8 /min, with a scanning angle ranged from 10 to 80 . The magnetic measurements of the samples were conducted on SQUDI-VSM to the field strength of 2 T. Thermogravimetric analysis (TGA) was carried out using a thermal analysis system (STA449F3, NETZSCH Crop.). The sample was heated from room temperature to 800 C with a heating rate of 10 C/min in air (50 mL min1 ). 3. Results and discussion 3.1. Magnetic CNTs morphology and structure Fig. 1 illustrates the morphologies of the as-prepared samples investigated by SEM. As seen in Fig. 1, some aggregation of iron particles outside the CNTs for Fe/CNTs-W (Fig. 1a) and Fe/CNTsW2E1 (Fig. 1b) can be observed, while the other samples display a uniform coated with the iron particles along the CNTs (Fig. 1c and d). The representative TEM images of as-prepared samples further verify the decoration of magnetic particles on CNTs and are shown in Fig. 2. It is evident that the distribution of iron magnetic nanoparticles becomes better with the increase of EG content in the mixed solvent. An obvious iron particles agglomeration on the external surface of CNTs are observed for Fe/CNTs-W (Fig. 2a) and Fe/W2E1 (Fig. 2b) with more serious agglomeration on Fe/CNTs-W, whereas for Fe/CNTs-E2W1 and Fe/CNTs-E, an uniform iron particles without detectable aggregation is found on the exterior surface of CNTs (Fig. 2c and d). Furthermore, both Fe/CNTs-E2W1 and Fe/ CNTs-E show a comparable narrow size distribution of iron particles. The magnetic nanoparticles tend to be aggregated and agglomerated due to the anisotropic dipolar attraction [16]. From the above results, we can deduce that the distribution and dispersion of iron magnetic nanoparticles located on CNTs surface would be well controlled by adjusting the water/EG ratios. Fig. 3 shows the XRD patterns of the Fe/CNTs nanocomposites prepared by different water/EG ratios. As shown in Fig. 3, the diffraction peak at 25.9 and 43.0 are attributed to the (0 0 2) and (1 0 0) planes for graphitic walls of CNTs [17]. It is interesting to note that the crystal phases for Fe/CNTs-W and Fe/CNTs-W2E1 are a mixture of a-Fe2O3 (hematite) and g-Fe2O3 (maghemite), while for the other samples they are single phase g-Fe2O3 according to the JCPDS cards no.33-0664 and no.39-1346 [18e20]. No peaks
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Fig. 1. SEM images of as-prepared samples: (a) Fe/CNTs-W, (b) Fe/CNTs-W2E1, (c) Fe/CNTs-E2W1, (d) Fe/CNTs-E.
assigned to impurities are detected. Obviously, a crystalline phase transformation from g-Fe2O3 to a-Fe2O3 may be on account of the residual dissolved oxygen oxidized effect derived from the different properties during the preparation [21]. With the aid of EG, magnetic Fe/CNTs nanocomposites can be formed in solvent phase, which not only prevents oxidation by excluding air from the area but also prevents g-Fe2O3 nanoparticles aggregation. Additionally, the average crystallite sizes of the iron particles for Fe/CNTs-E and Fe/CNTs-E2W1 samples calculated by Scherrer equation with the most intense diffraction peak at 2q ¼ 35.6 are 8.4 and 5.7 nm, respectively [22]. While for the Fe/CNTs-W and Fe/CNTs-W2E1 samples, the average crystallite sizes could not be obtained due to the mixed phases of g-Fe2O3 and a-Fe2O3. It is noted that the average particle sizes of iron oxides estimated by XRD are in good agreement with the TEM results. TGA of the as-prepared samples and CNTs were carried out to investigate the effect of water/EG ratio on the thermal stability of the samples. As shown in Fig. 4, the samples display a remarkable shift to lower oxidation temperature than CNTs support, which suggests that the iron supported on the CNTs has a catalysis effect on the CNTs oxidation in air [15,23]. The shifts of oxidation temperature for the Fe/CNTs-E and Fe/CNTs-E2W1 are more pronounced than that of the others due to the different iron catalytic effects, which may be related to high dispersion of iron particles (shown in Figs. 1 and 2). From TG profiles, the iron loadings on these samples could be calculated from the residual weight. The mass of remnants (supposing it was Fe2O3) for all the samples after calcination are near 28%, corresponding to 19.6% Fe which is close to the theoretical Fe loading (20% wt.%) on the CNTs. The magnetic properties of as-prepared samples were tested using a physical property measurement system (PPMS) from Quantum Design and the magnetization hysteresis curves of the samples conducted at room temperature were shown in Fig. 5. Obviously, all samples exhibit superparamagnetic behavior at room temperature and no coercivity and remanence is observed [9]. The
Fe/CNTs-E2W1 sample shows the highest saturation magnetization with 14 emu g1 at 2 T among all the samples, while the Fe/CNTs-W possesses the lowest saturation magnetization (9 emu g1 at 2 T). The discrepancy of iron phase distribution and iron particle size may be responsible for the difference in the saturation magnetizations of the samples. This observation is consistence with the SEM, TEM and TGA results. To show the dispersion and magnetic properties, the asprepared magnetic nanocomposites are dispersed in water as shown in Fig. 6. With the effect of external magnetic field, the Fe/ CNTs-E2W1 sample is quickly attracted toward the magnet (Fig. 6, right column). From the above discussion, a novel, facile and in-situ synthesis route for magnetic CNTs nanocomposites with a prevalently application in catalysis, water treatment, sensors, biomaterial technology and renewable resources can be provided. 3.2. Synthetic mechanism According to the above results, the water/EG ratio plays a key role in the dispersion and morphologies of self-assembly of magnetic CNTs nanocomposites. To get a better understanding of the synthetic mechanism of fabricating magnetic particles on CNTs, the effect of solvent with different water/EG ratios on the formation of iron nanocomposites is discussed here. The synthesis procedures are illustrated schematically in Fig. 7. Firstly, the CNTs are pretreated by nitric acid to create the oxygen-containing functional groups such as carboxyl and hydroxyl groups. When it is dispersed in solvent E2W1, the protons are replaced by the high-valent cations (Fe3þ) through electrostatic effect and then the Fe3þ disperses homogeneously along the surface of CNTs. When the precipitating agent is added to the mixture of Fe3þ and CNTs, the contact of the precipitating agent and Fe3þ is suppressed since the precipitation agent is surrounded by EG, resulting in controllable nucleation and growth of iron species. It is noted that carbonate iron is unstable in aqueous solution, which is prone to be instantly hydrolyzed to form
R. Liu et al. / Journal of Alloys and Compounds 657 (2016) 138e143
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ferric hydroxide. The electrostatic interaction might be responsible for the adhesion and anchor of magnetic nanoparticles on the outside surface of CNTs after the drying and heat annealing [19].
Thus, the magnetic CNTs nanocomposites with an uniform and good dispersion without agglomeration of iron particles can be successfully in-situ synthesized by tuning the nucleation and
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Fig. 3. XRD patterns of as-prepared samples.
Fig. 6. Photograph of magnetic CNTs nanocomposites (Fe/CNTs-E2W1) dispersed in water (left) and its response to a magnet (right).
Fig. 4. TG profiles of the CNTs and as-prepared samples.
Fig. 7. Sketch scheme for the anchor of iron oxides on CNTs surface.
growth of the iron particles. When water is replaced by EG, the hydrogen-bond (H-bond) between the OH groups of EG and the oxygen-containing groups in CNTs surface segregates the Fe3þ from the CNTs surface [24]. Therefore, the Fe3þ ions present a disorder distribution on the outside of CNTs surface. When the precipitant is titrated to the 15
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mixture of CNTs and solvent, the precipitation process occurs relatively faster and then subparticles nucleate and grow to a critical size due to the role of EG. Here, EG functions as a stabilizer and a dispersant. So, the iron particle sizes are a little bigger and the dispersion of iron particles is worse than that of Fe/CNTs-E2W1. While for Fe/CNTs-W2E1, the iron particle size is bigger than those of Fe/CNTs-E2W1 and Fe/CNTs-E since some EG is replaced by water. The Fe3þ distributes disorderly outside the CNTs surface just as the sample of Fe/CNTs-E because of the H-bond segregation and thus the growth of iron particles without restraint effect of EG is responsible for the bigger particle size for Fe/CNTs-W2E1. In the case of Fe/CNTs-W, the iron particle sizes are the biggest with the most heavily aggregation on the CNTs surface among all the samples. The subparticles form, grow, crystallize and agglomerate severely due to the absence of the EG. In term of iron nanoparticle dispersion, the Fe/CNTs-E2W1 exhibits the best, followed by Fe/ CNTs-E, Fe/CNTs-W2E1 and Fe/CNTs-W. To investigate the influence of solvent, EG is substituted for ethanol and the sample prepared with the same procedure is named Fe/CNTs-e. Fig. 8 shows the representative TEM image of Fe/ CNTs-e and severe agglomeration of iron particles outside the CNTs surface is similar to that of the Fe/CNTs-W (Fig. 2a). This observation demonstrates that the solvent does affect the formation of the magnetic composites and the nucleation, growth, and aggregation
R. Liu et al. / Journal of Alloys and Compounds 657 (2016) 138e143
Fig. 8. TEM image of Fe/CNTs-e prepared by using ethanol as solvent.
of iron nanoparticles can be controlled by EG content in the mixed solvent as expected. EG functions as a dispersant and stabilizer during the precipitation process. 4. Conclusions A novel, facile and reproducible in-situ synthesis of nearly monodisperse magnetic Fe/CNTs nanocomposites using depositioneprecipitation method following annealing is reported. By adjusting the water/EG ratio, magnetic Fe/CNTs with a good dispersion and narrow particle size distribution of iron oxides attached the external surface of CNTs are synthesized through controlling the nucleation and growth process. Magnetic measurements show that the as-prepared samples are superparamagnetic at room temperature and their magnetizations are determined by the water/EG ratio. The method described in this paper might provide a new way for the design and synthesis of nanoparticles/CNTs nanohybrids. Acknowledgments We greatly appreciate the financial support from the National Natural Science Foundation of China (U1462204) and the Program of Introducing Talents of Discipline to Universities (B06006). References [1] A. Saha, C. Jiang, A.A. Martí, Carbon nanotube networks on different platforms, Carbon 79 (2014) 1e18.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A facile controlled in-situ synthesis of monodisperse magnetic carbon nanotubes nanocomposites using water-ethylene glycol mixed solvents Renjie Liu a, b, Yu Qiao a, Yan Xu a, Xinbin Ma a, b, Zhenhua Li a, b, * a b
Key Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2015 Received in revised form 9 October 2015 Accepted 12 October 2015 Available online xxx
In this paper, a facile and controllable method for in-situ synthesis of the magnetic carbon nanotubes nanocomposites (Fe/CNTs) in water-ethylene glycol (EG) mixed solvents is reported by the deposition eprecipitation method following annealing. The effect of water/EG ratio on the physico-chemical properties of magnetic Fe/CNTs is investigated by X-ray diffraction, transmission electron microscope, scanning electron microscopy, thermogravimetric analysis and physical property measurement system. The results indicate that the iron particle size distribution and grain size can be well-tuned by adjusting the water/EG ratio. With the variation of EG fraction in the mixed solvent, the nucleation, growth and crystallization of magnetic iron oxides with a controllable morphologies and particle sizes attached on the exterior surface of CNTs can be achieved. The as-prepared Fe/CNTs nanocomposites display superparamagnetic property at room temperature and the water/EG ratio determines the magnetization of the sample. Possible formation mechanism for magnetic Fe/CNTs is proposed based on the characterization results. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic Carbon nanotubes Nanocomposites Ethylene glycol
1. Introduction Carbon nanotubes (CNTs) with the unique electrical, thermal, mechanical and optical properties has been widely used in energy storage materials, sensors, electronics and catalysis [1,2] since Iijima discovered the CNTs in 1991 [3]. Magnetic nanoparticles such as magnetite and maghemite have attracted growing attention since their promising application in magnetic force, water treatment, biosensors and drug delivery [4e7]. To combine the advantages of both, the state-of-the-art of preparation methods have been developed to assemble the magnetic material such as deposition on functionalized CNTs, electrochemical deposition, electroless deposition and physical approaches [8]. Among these methods, nanoparticle deposition on functionalized CNTs is hopefully a simple and efficient chemical method to synthesize the magnetic Fe/CNTs catalyst. The monodispersion and particle size distribution
* Corresponding author. Key Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.jallcom.2015.10.099 0925-8388/© 2015 Elsevier B.V. All rights reserved.
of magnetic material determine their own properties. There are two main categories of the preparation method for nanoparticle dispersion on functionalized CNTs: one is adhering the iron particles onto the CNTs surface through covalent or noncovalent bond; the other is in situ synthesis [9]. In situ synthesis is a time-saving, cost-effective and reproducible process for large-scale production compared to the former method. During the synthesis process, some challenges should be faced to improve the performance and enlarge the applications of magnetic CNTs nanocomposites, such as the suitable dimension of magnetic nanoparticles, a narrow size distribution, high crystallinity and the reproducible procedure without any purification operation. As far as the fine-tuned size distribution of the magnetic nanoparticles, the precipitation method is restrained due to only kinetic factors deciding the growth of the crystal [5]. The above difficulties are real challenges faced by researchers attempting to develop a facile, reproducible and cost-saving method. It is noted that the precipitation process involves two steps, namely, nucleation and growth. To get a monodisperse and narrow size distribution of iron oxide particles, these two steps should be seriously separated, that is, the nucleation should be avoided in the growth process [10]. Ethylene glycol (EG) is the simplest diol with enormous
R. Liu et al. / Journal of Alloys and Compounds 657 (2016) 138e143
potential in various applications such as antifreeze formulations in automobiles, a deicing fluid for windshields and aircraft, a desiccant for natural gas production, and a raw material for manufacture of polyester fibers and resins industry [11]. It should be noted that EG has extensively been used as a reducing agent for the preparation of metal and metal oxide nanoparticles [12]. Though application of EG has recently proliferated in the area of nanoparticles synthesis, efforts in using it as a solvent are lacking. EG is a polar solvent and is thoroughly miscible with water at any ratio. After mixing them, the EG and water molecules are homogeneously mixed [13] and hence the physicochemical properties could be tuned by mixing them. It is reported that the liquid with surface tension higher than 0.1e0.2 N m1 can't get the CNTs wetted [14]. Therefore, the CNTs could be wetted by the water and EG mixture. By filling the tube with solvent, the magnetic nanoparticles attached on exterior surface of CNTs would be obtained subsequently. In early reports, a surfactant would be added to prevent the iron particle from agglomeration during the formation of magnetic particles. Here, the EG was used as not only a solvent but also a stabilizer. By changing the solvent ratio (water/EG ratio), EG would demonstrate its versatility in the formation, growth and crystallization of magnetic oxides with a controllable morphologies and particle sizes as expected. Hence, a nearly monodisperse magnetic particles decorated CNTs nanocomposites can be manufactured via a facile in-situ synthesis method: depositioneprecipitation process at different water/EG ratios followed by annealing. Additionally, the influence of the water/EG ratio on the physicochemical properties of magnetic nanocomposites was investigated by a series of characterization techniques. To further confirm the unique properties of EG, the influence of solvent on the magnetic Fe/CNTs nanocomposites preparation was investigated in this paper. In this study, a facile and controlled in-situ synthesis method in the solvent with different water/EG ratio is reported to fabricate magnetic Fe/CNTs catalyst followed by annealing. The size distribution of magnetic nanoparticles located on the CNTs surface are fine-tuned by adjusting the water/EG ratio in this method. Furthermore, the possible formation mechanisms of the magnetic nanocomposites are discussed. 2. Experimental section 2.1. Synthesis of magnetic CNTs Pristine CNTs (multi-wall CNTs) was obtained from Chengdu Organic Chemical Co. of Chinese Academy of Sciences. Iron (Ⅲ) nitrate nonahydrate, ammonium carbonate and EG were supplied by Tianjin Kermel Co., LTD of China. All the reagents used in this research were analytical grade without any further purification. Before synthesizing the magnetic composite, CNTs were functionalized as follows: 5.0 g raw CNTs were refluxed at 122 C (azeotropic point) for 6 h in 300 mL concentrated nitric acid (65 wt.%). After cooling down to room temperature, the carbon material was filtered and washed with deionized water until a neutral pH was reached, followed by drying at 110 C overnight for further use. This acid treatment can not only remove the residual contaminants but also introduce more functionalized carbon sites on the surface [15]. The magnetic Fe/CNTs nanocomposites were prepared by the depositioneprecipitation method followed by annealing. In a typical experiment, 2.0 g purified CNTs and 3.607 g iron nitrate nonahydrate were dispersed in 60 mL water and heated to 60 C in a round bottomed flask. Ammonium carbonate (1.714 g, dissolved in 120 mL EG) was added drop-wise to the mixed solution under continuous stirring for 2 h and the resulting precipitant was kept in
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this medium overnight. After filtration and washing three times with ethanol and distilled water, the aged suspension was dried at 110 C for 12 h and calcined in an Ar flow at 400 C for 5 h with a rate of 3 C/min. The resulting sample was named as Fe/CNTsE2W1. Fe/CNTs-W2E1 (120 mL water/60 mL EG) was derived by exchanging the solvent with Fe/CNTs-E2W1. The samples Fe/CNTsW and Fe/CNTs-E were prepared when the solvents used were water and EG, respectively. To investigate the effect of the solvent, the ethanol was used as solvent and the corresponding sample was denoted as Fe/CNTs-e with the same preparation method mentioned above. 2.2. Characterization of materials The morphological features of the samples were measured on a Hitachi S-4800 Scanning electron microscopy (SEM) at an accelerating voltage of 3.0 kV. The structures of samples were characterized with a JEM-2100F transmission electron microscope (TEM) at 200 kV. Sample specimens for TEM studies were prepared by ultrasonic dispersion of the catalysts in ethanol and the suspension were dropped onto a carbon-coated copper grid before TEM images were recorded. The powder X-ray diffraction (XRD) characterization was performed on a RigakuD/max-2500 diffractometer with a CuKa radiation (40 kV, 200 mA). The scan speed was 8 /min, with a scanning angle ranged from 10 to 80 . The magnetic measurements of the samples were conducted on SQUDI-VSM to the field strength of 2 T. Thermogravimetric analysis (TGA) was carried out using a thermal analysis system (STA449F3, NETZSCH Crop.). The sample was heated from room temperature to 800 C with a heating rate of 10 C/min in air (50 mL min1 ). 3. Results and discussion 3.1. Magnetic CNTs morphology and structure Fig. 1 illustrates the morphologies of the as-prepared samples investigated by SEM. As seen in Fig. 1, some aggregation of iron particles outside the CNTs for Fe/CNTs-W (Fig. 1a) and Fe/CNTsW2E1 (Fig. 1b) can be observed, while the other samples display a uniform coated with the iron particles along the CNTs (Fig. 1c and d). The representative TEM images of as-prepared samples further verify the decoration of magnetic particles on CNTs and are shown in Fig. 2. It is evident that the distribution of iron magnetic nanoparticles becomes better with the increase of EG content in the mixed solvent. An obvious iron particles agglomeration on the external surface of CNTs are observed for Fe/CNTs-W (Fig. 2a) and Fe/W2E1 (Fig. 2b) with more serious agglomeration on Fe/CNTs-W, whereas for Fe/CNTs-E2W1 and Fe/CNTs-E, an uniform iron particles without detectable aggregation is found on the exterior surface of CNTs (Fig. 2c and d). Furthermore, both Fe/CNTs-E2W1 and Fe/ CNTs-E show a comparable narrow size distribution of iron particles. The magnetic nanoparticles tend to be aggregated and agglomerated due to the anisotropic dipolar attraction [16]. From the above results, we can deduce that the distribution and dispersion of iron magnetic nanoparticles located on CNTs surface would be well controlled by adjusting the water/EG ratios. Fig. 3 shows the XRD patterns of the Fe/CNTs nanocomposites prepared by different water/EG ratios. As shown in Fig. 3, the diffraction peak at 25.9 and 43.0 are attributed to the (0 0 2) and (1 0 0) planes for graphitic walls of CNTs [17]. It is interesting to note that the crystal phases for Fe/CNTs-W and Fe/CNTs-W2E1 are a mixture of a-Fe2O3 (hematite) and g-Fe2O3 (maghemite), while for the other samples they are single phase g-Fe2O3 according to the JCPDS cards no.33-0664 and no.39-1346 [18e20]. No peaks
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Fig. 1. SEM images of as-prepared samples: (a) Fe/CNTs-W, (b) Fe/CNTs-W2E1, (c) Fe/CNTs-E2W1, (d) Fe/CNTs-E.
assigned to impurities are detected. Obviously, a crystalline phase transformation from g-Fe2O3 to a-Fe2O3 may be on account of the residual dissolved oxygen oxidized effect derived from the different properties during the preparation [21]. With the aid of EG, magnetic Fe/CNTs nanocomposites can be formed in solvent phase, which not only prevents oxidation by excluding air from the area but also prevents g-Fe2O3 nanoparticles aggregation. Additionally, the average crystallite sizes of the iron particles for Fe/CNTs-E and Fe/CNTs-E2W1 samples calculated by Scherrer equation with the most intense diffraction peak at 2q ¼ 35.6 are 8.4 and 5.7 nm, respectively [22]. While for the Fe/CNTs-W and Fe/CNTs-W2E1 samples, the average crystallite sizes could not be obtained due to the mixed phases of g-Fe2O3 and a-Fe2O3. It is noted that the average particle sizes of iron oxides estimated by XRD are in good agreement with the TEM results. TGA of the as-prepared samples and CNTs were carried out to investigate the effect of water/EG ratio on the thermal stability of the samples. As shown in Fig. 4, the samples display a remarkable shift to lower oxidation temperature than CNTs support, which suggests that the iron supported on the CNTs has a catalysis effect on the CNTs oxidation in air [15,23]. The shifts of oxidation temperature for the Fe/CNTs-E and Fe/CNTs-E2W1 are more pronounced than that of the others due to the different iron catalytic effects, which may be related to high dispersion of iron particles (shown in Figs. 1 and 2). From TG profiles, the iron loadings on these samples could be calculated from the residual weight. The mass of remnants (supposing it was Fe2O3) for all the samples after calcination are near 28%, corresponding to 19.6% Fe which is close to the theoretical Fe loading (20% wt.%) on the CNTs. The magnetic properties of as-prepared samples were tested using a physical property measurement system (PPMS) from Quantum Design and the magnetization hysteresis curves of the samples conducted at room temperature were shown in Fig. 5. Obviously, all samples exhibit superparamagnetic behavior at room temperature and no coercivity and remanence is observed [9]. The
Fe/CNTs-E2W1 sample shows the highest saturation magnetization with 14 emu g1 at 2 T among all the samples, while the Fe/CNTs-W possesses the lowest saturation magnetization (9 emu g1 at 2 T). The discrepancy of iron phase distribution and iron particle size may be responsible for the difference in the saturation magnetizations of the samples. This observation is consistence with the SEM, TEM and TGA results. To show the dispersion and magnetic properties, the asprepared magnetic nanocomposites are dispersed in water as shown in Fig. 6. With the effect of external magnetic field, the Fe/ CNTs-E2W1 sample is quickly attracted toward the magnet (Fig. 6, right column). From the above discussion, a novel, facile and in-situ synthesis route for magnetic CNTs nanocomposites with a prevalently application in catalysis, water treatment, sensors, biomaterial technology and renewable resources can be provided. 3.2. Synthetic mechanism According to the above results, the water/EG ratio plays a key role in the dispersion and morphologies of self-assembly of magnetic CNTs nanocomposites. To get a better understanding of the synthetic mechanism of fabricating magnetic particles on CNTs, the effect of solvent with different water/EG ratios on the formation of iron nanocomposites is discussed here. The synthesis procedures are illustrated schematically in Fig. 7. Firstly, the CNTs are pretreated by nitric acid to create the oxygen-containing functional groups such as carboxyl and hydroxyl groups. When it is dispersed in solvent E2W1, the protons are replaced by the high-valent cations (Fe3þ) through electrostatic effect and then the Fe3þ disperses homogeneously along the surface of CNTs. When the precipitating agent is added to the mixture of Fe3þ and CNTs, the contact of the precipitating agent and Fe3þ is suppressed since the precipitation agent is surrounded by EG, resulting in controllable nucleation and growth of iron species. It is noted that carbonate iron is unstable in aqueous solution, which is prone to be instantly hydrolyzed to form
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ferric hydroxide. The electrostatic interaction might be responsible for the adhesion and anchor of magnetic nanoparticles on the outside surface of CNTs after the drying and heat annealing [19].
Thus, the magnetic CNTs nanocomposites with an uniform and good dispersion without agglomeration of iron particles can be successfully in-situ synthesized by tuning the nucleation and
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Fig. 3. XRD patterns of as-prepared samples.
Fig. 6. Photograph of magnetic CNTs nanocomposites (Fe/CNTs-E2W1) dispersed in water (left) and its response to a magnet (right).
Fig. 4. TG profiles of the CNTs and as-prepared samples.
Fig. 7. Sketch scheme for the anchor of iron oxides on CNTs surface.
growth of the iron particles. When water is replaced by EG, the hydrogen-bond (H-bond) between the OH groups of EG and the oxygen-containing groups in CNTs surface segregates the Fe3þ from the CNTs surface [24]. Therefore, the Fe3þ ions present a disorder distribution on the outside of CNTs surface. When the precipitant is titrated to the 15
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mixture of CNTs and solvent, the precipitation process occurs relatively faster and then subparticles nucleate and grow to a critical size due to the role of EG. Here, EG functions as a stabilizer and a dispersant. So, the iron particle sizes are a little bigger and the dispersion of iron particles is worse than that of Fe/CNTs-E2W1. While for Fe/CNTs-W2E1, the iron particle size is bigger than those of Fe/CNTs-E2W1 and Fe/CNTs-E since some EG is replaced by water. The Fe3þ distributes disorderly outside the CNTs surface just as the sample of Fe/CNTs-E because of the H-bond segregation and thus the growth of iron particles without restraint effect of EG is responsible for the bigger particle size for Fe/CNTs-W2E1. In the case of Fe/CNTs-W, the iron particle sizes are the biggest with the most heavily aggregation on the CNTs surface among all the samples. The subparticles form, grow, crystallize and agglomerate severely due to the absence of the EG. In term of iron nanoparticle dispersion, the Fe/CNTs-E2W1 exhibits the best, followed by Fe/ CNTs-E, Fe/CNTs-W2E1 and Fe/CNTs-W. To investigate the influence of solvent, EG is substituted for ethanol and the sample prepared with the same procedure is named Fe/CNTs-e. Fig. 8 shows the representative TEM image of Fe/ CNTs-e and severe agglomeration of iron particles outside the CNTs surface is similar to that of the Fe/CNTs-W (Fig. 2a). This observation demonstrates that the solvent does affect the formation of the magnetic composites and the nucleation, growth, and aggregation
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Fig. 8. TEM image of Fe/CNTs-e prepared by using ethanol as solvent.
of iron nanoparticles can be controlled by EG content in the mixed solvent as expected. EG functions as a dispersant and stabilizer during the precipitation process. 4. Conclusions A novel, facile and reproducible in-situ synthesis of nearly monodisperse magnetic Fe/CNTs nanocomposites using depositioneprecipitation method following annealing is reported. By adjusting the water/EG ratio, magnetic Fe/CNTs with a good dispersion and narrow particle size distribution of iron oxides attached the external surface of CNTs are synthesized through controlling the nucleation and growth process. Magnetic measurements show that the as-prepared samples are superparamagnetic at room temperature and their magnetizations are determined by the water/EG ratio. The method described in this paper might provide a new way for the design and synthesis of nanoparticles/CNTs nanohybrids. Acknowledgments We greatly appreciate the financial support from the National Natural Science Foundation of China (U1462204) and the Program of Introducing Talents of Discipline to Universities (B06006). References [1] A. Saha, C. Jiang, A.A. Martí, Carbon nanotube networks on different platforms, Carbon 79 (2014) 1e18.
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