Dalton Transactions View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
PAPER
Cite this: Dalton Trans., 2014, 43, 9885
View Journal | View Issue
Synthesis, size and magnetic properties of controllable MnFe2O4 nanoparticles with versatile surface functionalities† Buhe Bateer,a,b Chungui Tian,a Yang Qu,a Shichao Du,a Ying Yang,a Zhiyu Ren,a Kai Pana and Honggang Fu*a Size controllable MnFe2O4 superparamagnetic nanoparticles (NPs) were prepared by a solvothermal method using low-cost metal oleate as the source. The particle size, chemical composition and magnetic properties were investigated. The MnFe2O4 nanomaterials exhibit high colloidal stabilities and superparamagnetic properties, with an average particle size in the range of 2 to 10 nm by modifying the reaction condition, such as surfactant, reaction temperature and reaction time. By changing Mn2+ to different cations, CoFe2O4, NiFe2O4, and ZnFe2O4 with different magnetic properties could be obtained. The hydrophobic MnFe2O4 NPs could also be modified with many surfactants, such as cetyltrimethyl
Received 10th January 2014, Accepted 10th April 2014
ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecylsulfate (SDS), with the aim of achieving high zeta-potential hydrophilic bilayer-coated MnFe2O4 NPs. In addition, the
DOI: 10.1039/c4dt00089g
water soluble MnFe2O4 magnetic NPs can be applied to the removal of Pb(II) from waste water with good
www.rsc.org/dalton
recovery under external magnetic field.
Introduction Transition metal oxides with spinel ferrite MFe2O4 (M = Co, Ni, Mn, Zn, Cu, Mg, etc.) nanoparticles (NPs) have been receiving increasing interest in recent years, due to the fact that the magnetic and electrical properties of the MFe2O4 NPs can be tuned by changing the identity of the divalent M2+ cation or by partial substitution.1 Water-soluble monodisperse MFe2O4 magnetic NPs can be widely used in many important technological fields, such as high magnetic moment applications in magnetic resonance image (MRI),2 biomedicine,3 catalysis.4 To promote their practical application, the spinel ferrites need to be prepared at large scale with high stability, controllable size and composition. Thermal decomposition and coprecipitation are the most common methods used for the synthesis of MFe2O4 NPs. The thermal decomposition method can provide NPs with better tuned shape, size, and monodispersion.5 However, the high reaction temperature and expensive
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, 74 Xuefu Road, Nangang District, Harbin 150080, P.R. China. E-mail: [email protected]; Fax: +86-451-86673647; Tel: +86-451-86608458 b School of Materials and Chemical Engineering, Heilongjiang Institute of Technology, 999 Hongqi Street, Xiangfang District, Harbin 150050, P.R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt00089g
This journal is © The Royal Society of Chemistry 2014
materials of this method restrict its greener technological applications.6,7 The coprecipitation method has many advantages such as low cost, high yield and easy operation, whereas, the products obtained by this method usually have a poor size distribution and are crystalline, which restrains its wide application. Therefore, an effective and inexpensive method for the preparation of aqueous soluble MnFe2O4 NPs with good uniformity and dispersibility is greatly desired. Typically, water soluble monodisperse MnFe2O4 NPs are obtained by two steps. The monodispersed MFe2O4 NPs are firstly synthesized by mixing organic M2+ and Fe3+ compounds, such as metal acetylacetonates,8 metal oleates and metal carbonyls,9 in organic solvent in the presence of surfactants such as oleic acid (OA) and oleyl amine.10 Then, the water soluble NPs can be obtained by ligand-exchange reaction. Ligandexchange is a more strategic approach for modifying the surface composition. The hydrophobic surfactants (oleic acid) on the monodispersed MFe2O4 NPs were washed by a hydrogen bond breaker (dimethyl sulfoxide) and replaced by hydrophilic surfactants. Several ligands have been explored to replace the hydrophobic coatings of iron oxide NPs such as dopamine,11 short-chain water soluble polymer poly(ethylene glycol),12 polyethylenimine,13 1,2-diols.14,15 These reports provide an important conceptual foundation for our systemic study of the exchange process. However, the ligand-exchange process suffers from several disadvantages, including incomplete replacement and low stability of the resulting com-
Dalton Trans., 2014, 43, 9885–9891 | 9885
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Paper
pounds in aqueous solution after the reaction.16 The coating of a bilayer surfactant in the preparation is another way to improve the dispersibility of magnetic NPs in aqueous solution. Shen et al.17,18 used n-alkanoic acids with 9–13 carbons as a bilayer surfactant to stabilize magnetic NPs. Hong et al.19 produced aqueous magnetic NPs in which oleate sodium and PEG-4000 were respectively used as the primary and the secondary layer surfactant. These bilayer coated magnetic NPs are quite stable in aqueous solution. However, the effects of different surfactants on bilayer coated NPs are rarely investigated and reported. In the present work, we report the synthesis of MnFe2O4 magnetic NPs with size controllability through a low temperature solvothermal method. Low-cost metal oleate was selected as the precursor. The particle size could be tuned from 2 nm to 10 nm either by changing the reaction parameters or adopting different M2+ (M = Co, Ni, Mn, Zn) cations. The materials with different M2+ cations showing different magnetic properties could be easily functionalized by a secondary surfactant, such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS). The functionalized NPs show great stability in water, which could be applied as a highly efficient adsorbent for Pb(II) removal from contaminated water.
Dalton Transactions
with a volume ratio of 5 : 1 and centrifuged at 6000 rpm for 10 min to remove any undispersed residue. Under identical conditions, the reaction of the (Zn2+ Fe3+)oleate complex, (Co2+ Fe3+)-oleate complex and (Ni2+ Fe3+)oleate complex at 160 °C led to ZnFe2O4, CoFe2O4 and NiFe2O4 NPs. Synthesis of water soluble MnFe2O4 NPs 0.5 g of 5.5 nm MnFe2O4 NPs ( prepared at a reaction temperature of 160 °C) was added to a suspension of CTAB (about 20 mg in 2 mL), SDBS and SDS in deionized water respectively, stirred for 1 h at 80 °C to obtain a water based MnFe2O4 colloidal solution. Characterization X-Ray powder diffraction (XRD) patterns were obtained using a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The transmission electron microscopy (TEM) experiment was performed on a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. Carbon coated copper grids were used as the sample holders. Magnetization was measured at 300 K in a physical properties measurement system (PPMS, Quantum Design Inc.) up to a field of 20 kOe. The hydrodynamic size and surface charges of the NPs in aqueous solution were evaluated using Zetasizer nano series dynamic light scattering (DLS).
Experimental section Chemicals FeCl3·6H2O (99%), NiCl2·4H2O (99%), MnCl2·4H2O (99%), CoCl2·6H2O (97%), ZnCl2·4H2O (99%), Pb(NO3)2, oleic acid, deionized water, heptane, ethanol, sodium oleate (95%), cetyltrimethylammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Co., Ltd. They were used as received without any further purification. Synthesis of metal oleate complexes The mixed-metal (M2+ Fe3+)-oleate complex was prepared by reaction of sodium oleate and a mixture of Fe3+ and M2+ chlorides. In a typical synthesis,16 40 mmol of FeCl3·6H2O, 20 mmol of MnCl2·4H2O, 160 mmol of sodium oleate, 100 mL of H2O, 100 mL of ethanol and 200 mL of hexane were mixed and refluxed at 60 °C for 4 h. The mixed M2+ Fe3+-oleate complex was obtained after separation from water. Synthesis of MnFe2O4 0.5 g oleic acid was added into a heptane solution of the mixed-metal (Mn2+ Fe3+) oleate complex, the whole solution was stirred thoroughly and then transferred into a 50 mL Teflon-lined stainless-steel autoclave to a filling capacity of 40%. The crystallization was carried out under autogenous pressure at temperatures of 140, 160 and 180 °C for 12 h, respectively. After being cooled to room temperature, the sample was washed 5 times with a mixture of ethanol–hexane
9886 | Dalton Trans., 2014, 43, 9885–9891
Results and discussion Size and magnetic properties of controllable MnFe2O4 nanoparticles The thermal decomposition method was used to prepare monodisperse MnFe2O4 NPs at a high reaction temperature (300 °C) from metal oleate,20 as metal oleate can decompose at high temperatures. The solvothermal synthesis includes various wet chemical technologies of crystallizing a substance in a sealed container at a high vapor pressure (generally in the range from 0.3 to 4 MPa). The solvothermal synthesis is prone to obtain highly crystalline MnFe2O4 NPs due to the high vapor pressure required. The formation of NPs is a crystallization process at the nanometer scale. The solvothermal reaction of metal oleate is the primary step in the synthesis of MFe2O4 NPs, and this process includes the formation of thermal free radicals, the resultant addition and/or substitution of free radicals and the combination of M(oleate)3 containing free radicals. The crystal structure during the initial nucleation stage and the subsequent growth stage is greatly affected by the reaction conditions, such as surfactant, temperature, reacting time, etc. Since the first report on the synthesis of monodisperse NPs with inexpensive and nontoxic iron-oleate complex as a reactant by Hyeon’s group, the method has been utilized in the synthesis of many other binary metal oxides with a high temperature decomposition method.21 We synthesized the monodisperse spinel ferrite MFe2O4 NPs with a uniform
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Dalton Transactions
composition by the solvothermal method with a mixed binary metal oleate precursor. The effect of the reaction parameters on the size and magnetic properties of the monodisperse MnFe2O4 NPs were carefully examined. After examining each parameter thoroughly, we found that the surfactant, reaction temperature and reaction time are the key parameters which play crucial roles in the control of the size and magnetic properties of the NPs. To investigate the effect of the reaction temperature on the crystal size, we analyzed the products that were separately formed at 140 (Fig. 1A), 160 (Fig. 1B) and 180 °C (Fig. 1C) for 12 h. TEM was extensively used to investigate the MnFe2O4 NPs. The mean particle size of the MnFe2O4 NPs, determined by measuring the particles at random, is 4, 5 and 6 nm respectively, and the size distribution is from 2 to 7 nm, 4 to 8 nm and 4 to 9 nm. We can draw from Fig. 1 that the size of the MnFe2O4 NPs increased with the increase of reaction temperature. At the lowest reaction temperature (140 °C), the MnFe2O4 NPs have the smallest particle size and the widest size distribution. At 160 °C, they are well crystalline with a medium size and a narrow size distribution, while the NPs at 180 °C are the largest with a good crystalline structure. Therefore, it is possible to propose that when the reaction temperature is higher than 160 °C, although the degree of crystallinity becomes good, the particle size becomes larger and results in polydispersity. When it is lower than 160 °C, the particle size becomes smaller with a wide size distribution. Therefore, 160 °C is the middle level.
Fig. 1 TEM images of the MnFe2O4 NP samples formed at different temperatures using the solvothermal method: (A) 140 °C, (B) 160 °C, (C) 180 °C.
This journal is © The Royal Society of Chemistry 2014
Paper
MnFe2O4 NPs prepared at 140 °C, 160 °C and 180 °C have super-paramagnetic properties as their remanence and coercivity are zero. In addition, their saturation magnetization is 30.67, 46.01 and 51.12 emu g−1, respectively (Fig. S1†). X-Ray diffraction (XRD) was used to record the crystal information of the MnFe2O4 NPs. As shown in Fig. 2, the materials have the diffraction pattern of spinel ferrites (JCPDS no. 10-0319). The peaks grew in intensity as the crystallization temperature increased, suggesting the transformation of MnFe2O4 NPs takes place from a low degree to a high degree of crystallinity with the increasing of the crystallization temperature from 140 to 180 °C. In addition, the peaks become sharper as the reaction temperature increases, which reflects the typical transformation occurring during crystal growth. This effect has to be ascribed to the increase of NP size under the high synthesis temperature. The size of the MnFe2O4 NPs is about 4.5 (140 °C), 5.6 (160 °C) and 7 nm (180 °C) calculated using the Scherrer equation from the half-maximum width of the (311) X-ray diffraction line,22 which are respectively in accordance with the result of the TEM measurements. Further investigation into the synthesis of shape and size controlled NPs focused on the tuning of the surfactant and reaction time. First of all, we adjusted the dosage of the surfactant OA. The shape and size of the MnFe2O4 NPs formed with different OA dosages are shown in the TEM images (Fig. 3a and b). When we decreased the surfactant dosage in the reaction solution (Fig. 3a) the size of MnFe2O4 NPs became smaller. However, when we increased the surfactant amount (Fig. 3b), the MnFe2O4 size is bigger and size distribution was quite wide (4–9 nm). All these results prove the effect of surfactant amount on the particle size. To further understand the formation process of the MnFe2O4 NPs, experiments were carried out with reaction times of 6 h and 24 h, respectively. The TEM images (Fig. 3c and d) show that the NPs formed after 6 h were small-sized (3–6 nm) and the NPs formed after 24 h are large-sized (10 nm), which illustrates
Fig. 2 XRD patterns of MnFe2O4 NPs formed with different reaction temperatures: (a) 140 °C, (b) 160 °C, (c) 180 °C.
Dalton Trans., 2014, 43, 9885–9891 | 9887
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Paper
Fig. 3 TEM images of the MnFe2O4 NPs synthesized by varying the relative amount of OA: (a) 0.25 g, (b) 0.75 g and syntheses with the reaction time of (c) 6 h, (d) 24 h.
the fact that the reaction time is an important factor that affects the formation of MnFe2O4 NPs. The morphology and size of the MFe2O4 (M = Co, Ni, Zn) NPs were studied in detail from the representative TEM. The TEM images (Fig. 4) reveal that the novel MFe2O4 (M = Co, Ni, Zn) NPs prepared at 160 °C for 12 h are nearly spherical. The size distribution of the MFe2O4 (M = Co, Ni, Zn) NPs can be fitted with a log-normal function, which is determined by measuring the particles at random, with the mean particle sizes being 5.3 nm for CoFe2O4, 6.3 nm for NiFe2O4, and
Dalton Transactions
6.4 nm for ZnFe2O4. Compared with Fig. 1, the particle size of CoFe2O4 and MnFe2O4 is less than that of ZnFe2O4 and NiFe2O4. While the NiFe2O4 and ZnFe2O4 NPs have the widest and narrowest particle size distribution, respectively. XRD experiments were performed to identify the crystallographic structure and estimate the particle size (Fig. S2†). All of the materials have a diffraction pattern of spinel ferrites. The average particle size of the NPs is about 6.6 nm for CoFe2O4, 7.8 nm for NiFe2O4, 6.5 nm for MnFe2O4 and 7.6 nm for ZnFe2O4, calculated using the Scherrer equation from the half-maximum width of the (311) X-ray diffraction line. The room-temperature hysteresis loops of the ferrites were measured using a vibrating sample magnetometer (VSM). The magnetization curves, as shown in Fig. 5, display a relatively high saturation magnetization. The magnetic saturation (Ms) values of CoFe2O4, NiFe2O4, MnFe2O4 and ZnFe2O4 are 54.02, 39.93, 46.01 and 9.58 emu g−1, respectively, which is outside their theoretical values of 71.2, 47.5, 120.8 and 36.9 emu g−1 reported in the literature.20 Among them, the MnFe2O4 and ZnFe2O4 NPs exhibit a typical low saturation magnetization, which is much smaller than the theoretical values. The reduced saturation magnetization of the ferrite NPs is generally believed to be due to the decreased particle size and the presence of a magnetic dead or anti-ferromagnetic layer on the surface, the hysteresis loops of the MnFe2O4 and ZnFe2O4
Fig. 5 Magnetic hysteresis curves of the NPs at 300 K (a) CoFe2O4, (b) NiFe2O4, (c) MnFe2O4, (d) ZnFe2O4.
Table 1 Size and magnetic characterization of MFe2O4 NPs at the preparation temperature of 160 °C for 12 h
Fig. 4 TEM images of (A) CoFe2O4 NPs, (B) NiFe2O4 NPs, (C) ZnFe2O4 NPs prepared at 160 °C for 12 h.
9888 | Dalton Trans., 2014, 43, 9885–9891
Particles
dTEM a
dXRD b
MS
Hc
Mr
CoFe2O4 NiFe2O4 MnFe2O4 ZnFe2O4
5.3 6.5 5.5 6.4
6.6 7.8 6.5 7.6
54.02 39.93 46.01 9.58
14 18 0 0
0.4 0.5 0 0
a Average particle size as estimated by TEM assuming a log-normal particle size distribution. b Average particle size estimated by XRD using the Scherrer equation.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
Paper
display zero coercivities (Hc) and no remanence (Mr), as would be expected for superparamagnetic behavior (Table 1). The CoFe2O4 and NiFe2O4 NPs displayed a very low coercivity of 14 and 18 Oe at room temperature and 20 kOe. The remanence of the CoFe2O4 and NiFe2O4 NPs are 0.4 and 0.5 emu g−1. The result show that the samples demonstrate a superparamagnetic behavior with negligible coercivity and remanence.
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Surface functionalization of water-soluble NPs Organic metals have been demonstrated to be a very promising route for the synthesis of monodispersed MFe2O4 NPs with uniform and controllable size and shape.23 However, the surface of the nanocrystalline structure is coated by a layer of hydrophobic organic ligand (such as OA and octadecylamine),24 which makes these NPs difficult to disperse in water. Ligand exchange is an effective surface functionalizing method that can improve the water dispersibility of NPs. For example, the hydrophobic oleic acid molecular coating on the surface of the NPs can be ligand exchanged by hydrophilic dopamine (DA), etc. However, the disadvantages of the ligand exchange method includes incomplete replacement (a small amount of oleic acid molecules remain on the surface of the NPs) and a reduced stability in aqueous solution after the reaction.25 On the basis of the above consideration, MnFe2O4 NPs were stabilized by mono-functional fatty acid molecules. The mono-functional MnFe2O4 NPs were recoated by a secondary ligand to functionalize their surface and improve their water solubility, which is of great importance for their future applications.26,27 This work presents a systematic study of creatinga stable aqueous dispersion of 5.5 nm MnFe2O4 NPs ( prepared at 160 °C for 12 h) through the recoating of NPs with different water soluble surfactants. The coating process is illustrated in Fig. 7, where the OA coated MnFe2O4 NPs are coated a second time by CTAB, SDBS and SDS. To the OA coated MnFe2O4 NPs, the OA molecules can anchor themselves on the surface of the MnFe2O4 NPs through the formation of hydrogen bonds between –COOH of OA and –OH of MnFe2O4. While the recoating of water soluble surfactants with both nonpolar and polar groups can make the nonpolar groups bind with the OA layer through strong van der Waals interactions (dispersion force), and the polar groups can help the NPs become well dispersed in water (Fig. 6). Due to the strong interactions between the surfactants and the OA coated MnFe2O4 NPs, the bilayer-stabilized magnetic NPs have a good solubility and stability in water. An interdigitated structure is expected to be energetically favorable in an aqueous environment because of the maximization of hydrophobic interactions between the interdigitated hydrocarbon chains. On the basis of this result, we propose that the phase transfer of the OA coated MnFe2O4 NPs and their redispersibility in water arise because of the formation of an interdigitated structure as shown in Fig. 7. The zeta-potential is an important parameter that governs interparticle electrostatic interactions and the stability of the NPs in water, where a colloidal system is generally stable if its zeta-potential is higher than 30 mV or smaller than −30 mV.
This journal is © The Royal Society of Chemistry 2014
Fig. 6 NPs.
Schematic illustration of the surface functionalization of the
Fig. 7 Water-soluble MnFe2O4NPs stabilized with CTAB, SDBS and SDS: (a–c) zeta-potential measurements, (d–f ) photographs and (g–i) DLS plots.
Therefore, the zeta-potential value of the MnFe2O4 NPs dispersion is an indicator of its stability. The high absolute values of all three of the MnFe2O4 NPs water dispersions indicated their high stability (Fig. 7a–c). The zeta-potential value of the CTAB, SDBS and SDS coated NPs are about 75, −73 and −52 mV. The zeta-potential of the bare MnFe2O4 NPs is only −8.7 mV (Fig. S3†), which may be caused by the lack of surfactant modification. The OA can anchor on the surface of the MnFe2O4 NPs through the formation of hydrogen bonds between the –COOH of OA and –OH of MnFe2O4. While nonpolar (long-chain alkyl) groups in CTAB, SDBS and SDS are bound with the OA layer through strong van der Waals interactions, polar groups in CTAB, SDBS and SDS are bound with the water (Fig. 7d–f ). Due to the strong interactions, the bilayer-coated MnFe2O4 magnetic NPs have a high stability in water. All the NP water dispersions are transparent without any noticeable precipitation (Fig. 8d–f ). The concentration of the water dispersion was roughly estimated to be about 1 mg mL−1. The hydrodynamic size of the CTAB, SDBS and SDS
Dalton Trans., 2014, 43, 9885–9891 | 9889
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Paper
Fig. 8 Pb(II) sorption kinetics using various types of MnFe2O4 NPs. Experimental conditions: MnFe2O4 = 1 g L−1 as Fe; surfactant = 10 wt% for stabilized MnFe2O4 particles. Solution pH was kept constant at 6.8 ± 0.4 (a) CTAB, (b) SDBS, (c) SDS and (d) non-stabilized.
coated NPs are respectively 64, 92 and 94 nm (Fig. 7g–i). The increase of the hydrodynamic size is caused by the coating of the bilayer surfactants on the surface of the NPs. The magnetic properties of the functionalized superparamagnetic particles will also present superparamagnetic properties. The magnetic saturation of the functionalized magnetic particles is smaller than that of the bare magnetic particles, where the saturation magnetization is 41.41, 41.87 and 42.22 emu g−1, for MnFe2O4/CTAB, MnFe2O4/SDBS and MnFe2O4/SDS, respectively (Fig. S4†). Surface functionalized MnFe2O4 NPs applied to Pb(II) sorption The increasing heavy metal pollution of surface and ground waters, with largely unknown long-term effects on wildlife and human health, can easily lead to serious environmental problems.28 Lead poisoning particularly affects young children, who can absorb up to 50% of ingested lead. Once ingested through the gastrointestinal track, lead accumulates in vital organs like kidneys, liver, or the brain. Leaded exhaust emissions, produced by the combustion of gasoline with tetraethyl lead as an antiknock agent, are released into the atmosphere and water supply causing pollution.29 Some recent work has shown that magnetic NPs stabilized by surfactants can obviously enhance the removal efficiency of heavy metals from contaminated water,20–32 with both starch and CMC being able act as an effective stabilizer for the preparation of highly stable magnetite NPs possessing a much greater arsenic sorption capacity than conventional magnetite particles. The overall goal of this study is to discuss a preparation method of surfactant (CTAB, SDBS and SDS) stabilized MnFe2O4 NPs for enhanced Pb(II) removal from contaminated water. Fig. 8 shows the Pb(II) removal kinetics data of the bare and bilayer coated MnFe2O4 NPs under otherwise identical conditions. Three control tests on the SDBS, SDS, and CTAB stabilized MnFe2O4 water dispersions were carried out. The adsorption capacities of CTAB, SDBS, SDS stabilized MnFe2O4
9890 | Dalton Trans., 2014, 43, 9885–9891
Dalton Transactions
Fig. 9 Pb(II) adsorption isotherms for different contact times on MnFe2O4 NPs stabilized with (a) CTAB, (b) SDBS, (c) SDS and (d) nonstabilized.
NPs and bare MnFe2O4 NPs for Pb(II) are respectively 89.55%, 75.05%, 66.07% and 35.2%. The adsorption capacities of the CTAB, SDBS, and SDS stabilized MnFe2O4 NPs and bare MnFe2O4 NPs for Pb(II) are 17.9, 15.1, 13.2 and 5.2 mg g−1, respectively (Fig. 9). The largest adsorbed percentage for these Pb(II) ions is up to 90%. Thereinto, the CTAB functionalized MnFe2O4 NPs have the highest adsorption capacity. This is because the functionalization of the NPs improves their water dispersibility and increases their contact with Pb(II) in the water, and thus good adsorption occurs in a short time. They may also have a good adsorption performance for other heavy metal ions in contaminated water. The Pb(II) removal kinetics and adsorption capacities of bare MnFe2O4 NPs have been added to Fig. 8 and 9, respectively. The surface functionalized MnFe2O4 NPs sorption of Pb(II) includes both physical and chemical processes. The stabilized MnFe2O4 NPs with a larger specific surface area and smaller size may possess an open pore structure, which allows exterior surface sorption sites for Pb(II) sorption.33 The high zeta-potential of the surface functionalized MnFe2O4 NPs helps the sorption of the target Pb(II) ions. In addition, the better Pb(II) sorption effect can be attributed to the high affinity between the surfactant groups and Pb(II), as well as to the readily available surfactant groups on the surface of the MnFe2O4 NPs.
Conclusions We report the synthesis of OA coated spinel ferrite MFe2O4 (M = Co, Ni, Mn, Zn) NPs with a uniform composition using a simple route that involves a solvothermal method of a mixed binary metal-oleate precursor formed in a low-temperature reaction of the constituent metal halides with sodium oleate. The mean particle size is 5.5 nm for MnFe2O4, 5.3 nm for CoFe2O4, 6.3 nm for NiFe2O4 and 6.4 nm for ZnFe2O4. The magnetic saturation values of CoFe2O4, NiFe2O4, MnFe2O4 and ZnFe2O4 are 54.02, 39.93, 46.01 and 9.58 mg g−1, respectively.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
The use of a secondary ligand (CTAB, SDBS and SDS) to functionalize the surface of the MnFe2O4 NPs makes the NPs water soluble. The zeta-potentials of the CTAB, SDBS and SDS functionalized NPs are about 75, −73 and −52 mV. In addition, the Pb(II) adsorption capacities of the CTAB, SDBS, and SDS functionalized MnFe2O4 NPs are 17.9, 15.1 and 13.2 mg g−1, respectively.
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (no. 21031001), the National Natural Science Foundation of China (no. 91122018, 51102082, 21101061), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (no. 708029), and the Special Research Fund for the Doctoral Program of Higher Education of China (20112301110002).
Notes and references 1 S. Yanez-Vilar, M. Sanchez-Andujar, C. Gomez-Aguirre, J. Mira, M. A. Senarıs-Rodrıguez and S. Castro-Garcıa, J. Solid State Chem., 2009, 182, 685. 2 D. Yoo, J. H. Lee, T. H. Shin and J. Cheon, Acc. Chem. Res., 2011, 44, 863. 3 C. S. S. R. Kumar and F. Mohammad, Adv. Drug Delivery Rev., 2011, 63, 789. 4 V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara and J. M. Basset, Chem. Rev., 2011, 111, 3036. 5 J. Mohapatra, A. Mitra, D. Bahadur and M. Aslam, CrystEngComm, 2013, 15, 524. 6 A. H. Lu, E. L. Salabas and F. Schüth, Angew. Chem., Int. Ed., 2007, 46, 1222. 7 S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. V. Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064. 8 R. Sai, S. D. Kulkarni, K. J. Vinoy, N. Bhat and S. A. Shivashankar, J. Mater. Chem., 2012, 22, 2149. 9 S. Sun and H. Zeng, J. Am. Chem. Soc., 2002, 124, 8204. 10 C. Pereira, A. Pereira, C. Fernandes, M. Rocha, R. Mendes, M. G. A. Guedes, P. B. Tavares, J. M. Grenèche, J. Araújo and C. Freire, Chem. Mater., 2012, 24, 1496. 11 C. Xu, K. Xu, H. Gu, R. Zheng, H. Liu, X. Zhang, Z. Guo and B. J. Xu, J. Am. Chem. Soc., 2004, 126, 9938.
This journal is © The Royal Society of Chemistry 2014
Paper
12 K. Herve, L. Douziech-Eyrolles, E. Munnier, S. CohenJonathan, M. Souce, H. Marchais, P. Limelette, F. Warmont, M. L. Saboungi, P. Dubois and I. Chourpa, Nanotechnology, 2008, 19, 465608. 13 H. Duan, M. Kuang, X. Wang, Y. Wang, H. Mao and S. Nie, J. Phys. Chem. C, 2008, 112, 8127. 14 W. H. Binder, H. Weinstabl and R. Sachsenhofer, J. Nanomater., 2008, 110, 383020. 15 N. Fauconnier, J. N. Pons, J. Roger and A. Bee, J. Colloid Interface Sci., 1997, 194, 427. 16 J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891. 17 L. Shen, A. Stachowiak, T. A. Hatton and P. E. Laibinis, Langmuir, 2000, 16, 9907. 18 L. Shen, P. E. Laibinis and T. A. Hatton, Langmuir, 1999, 15, 447. 19 R. Y. Hong, S. Z. Zhang, Y. P. Han, H. Z. Li, J. Ding and Y. Zheng, Powder Technol., 2006, 170, 1. 20 N. Bao, L. Shen, Y. Wang, P. Padhan and A. Gupta, J. Am. Chem. Soc., 2007, 129, 12374. 21 N. Bao, L. Shen, W. An, P. Padhan, C. H. Turner and A. Gupta, Chem. Mater., 2009, 21, 3458. 22 X. H. Huang, Z. Y. Zhan, X. Wang, Z. Zhang, G. Z. Xing, D. L. Guo, D. P. Leusink, L. X. Zheng and T. Wu, Appl. Phys. Lett., 2010, 97, 203112. 23 B. Wang, B. Wang, P. Wei, X. Wang and W. Lou, Dalton Trans., 2012, 41, 896. 24 H. Qu, H. Ma, A. Riviere, W. Zhou and C. J. O’Connor, J. Mater. Chem., 2012, 22, 3311. 25 Y. Xu, Y. Qin, S. Palchoudhury and Y. Bao, Langmuir, 2011, 27, 8990. 26 Q. Liang, D. Zhao, T. Qian, K. Freel and Y. Feng, Ind. Eng. Chem. Res., 2012, 51, 2407. 27 A. Swami, A. Kumar and M. Sastry, Langmuir, 2003, 19, 1168. 28 J. Lynch, J. Zhuang, T. Wang, D. L. Montagne, H. Wu and Y. C. Cao, J. Am. Chem. Soc., 2011, 133, 12664. 29 W. J. Liu, K. Tian, H. Jiang, X. S. Zhang, H. S. Ding and H. Q. Yu, Environ. Sci. Technol., 2012, 46, 7849. 30 F. He and D. Zhao, Environ. Sci. Technol., 2007, 41, 6216. 31 Q. Liang, D. Zhao, T. Qian, K. Freel and Y. Feng, Ind. Eng. Chem. Res., 2012, 51, 2407. 32 F. He and D. Zhao, Environ. Sci. Technol., 2005, 39, 3314. 33 F. He, W. Wang, J. W. Moon, J. Howe, E. M. Pierce and L. Liang, ACS Appl. Mater. Interfaces, 2012, 4, 4373.
Dalton Trans., 2014, 43, 9885–9891 | 9891
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
PAPER
Cite this: Dalton Trans., 2014, 43, 9885
View Journal | View Issue
Synthesis, size and magnetic properties of controllable MnFe2O4 nanoparticles with versatile surface functionalities† Buhe Bateer,a,b Chungui Tian,a Yang Qu,a Shichao Du,a Ying Yang,a Zhiyu Ren,a Kai Pana and Honggang Fu*a Size controllable MnFe2O4 superparamagnetic nanoparticles (NPs) were prepared by a solvothermal method using low-cost metal oleate as the source. The particle size, chemical composition and magnetic properties were investigated. The MnFe2O4 nanomaterials exhibit high colloidal stabilities and superparamagnetic properties, with an average particle size in the range of 2 to 10 nm by modifying the reaction condition, such as surfactant, reaction temperature and reaction time. By changing Mn2+ to different cations, CoFe2O4, NiFe2O4, and ZnFe2O4 with different magnetic properties could be obtained. The hydrophobic MnFe2O4 NPs could also be modified with many surfactants, such as cetyltrimethyl
Received 10th January 2014, Accepted 10th April 2014
ammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecylsulfate (SDS), with the aim of achieving high zeta-potential hydrophilic bilayer-coated MnFe2O4 NPs. In addition, the
DOI: 10.1039/c4dt00089g
water soluble MnFe2O4 magnetic NPs can be applied to the removal of Pb(II) from waste water with good
www.rsc.org/dalton
recovery under external magnetic field.
Introduction Transition metal oxides with spinel ferrite MFe2O4 (M = Co, Ni, Mn, Zn, Cu, Mg, etc.) nanoparticles (NPs) have been receiving increasing interest in recent years, due to the fact that the magnetic and electrical properties of the MFe2O4 NPs can be tuned by changing the identity of the divalent M2+ cation or by partial substitution.1 Water-soluble monodisperse MFe2O4 magnetic NPs can be widely used in many important technological fields, such as high magnetic moment applications in magnetic resonance image (MRI),2 biomedicine,3 catalysis.4 To promote their practical application, the spinel ferrites need to be prepared at large scale with high stability, controllable size and composition. Thermal decomposition and coprecipitation are the most common methods used for the synthesis of MFe2O4 NPs. The thermal decomposition method can provide NPs with better tuned shape, size, and monodispersion.5 However, the high reaction temperature and expensive
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, 74 Xuefu Road, Nangang District, Harbin 150080, P.R. China. E-mail: [email protected]; Fax: +86-451-86673647; Tel: +86-451-86608458 b School of Materials and Chemical Engineering, Heilongjiang Institute of Technology, 999 Hongqi Street, Xiangfang District, Harbin 150050, P.R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4dt00089g
This journal is © The Royal Society of Chemistry 2014
materials of this method restrict its greener technological applications.6,7 The coprecipitation method has many advantages such as low cost, high yield and easy operation, whereas, the products obtained by this method usually have a poor size distribution and are crystalline, which restrains its wide application. Therefore, an effective and inexpensive method for the preparation of aqueous soluble MnFe2O4 NPs with good uniformity and dispersibility is greatly desired. Typically, water soluble monodisperse MnFe2O4 NPs are obtained by two steps. The monodispersed MFe2O4 NPs are firstly synthesized by mixing organic M2+ and Fe3+ compounds, such as metal acetylacetonates,8 metal oleates and metal carbonyls,9 in organic solvent in the presence of surfactants such as oleic acid (OA) and oleyl amine.10 Then, the water soluble NPs can be obtained by ligand-exchange reaction. Ligandexchange is a more strategic approach for modifying the surface composition. The hydrophobic surfactants (oleic acid) on the monodispersed MFe2O4 NPs were washed by a hydrogen bond breaker (dimethyl sulfoxide) and replaced by hydrophilic surfactants. Several ligands have been explored to replace the hydrophobic coatings of iron oxide NPs such as dopamine,11 short-chain water soluble polymer poly(ethylene glycol),12 polyethylenimine,13 1,2-diols.14,15 These reports provide an important conceptual foundation for our systemic study of the exchange process. However, the ligand-exchange process suffers from several disadvantages, including incomplete replacement and low stability of the resulting com-
Dalton Trans., 2014, 43, 9885–9891 | 9885
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Paper
pounds in aqueous solution after the reaction.16 The coating of a bilayer surfactant in the preparation is another way to improve the dispersibility of magnetic NPs in aqueous solution. Shen et al.17,18 used n-alkanoic acids with 9–13 carbons as a bilayer surfactant to stabilize magnetic NPs. Hong et al.19 produced aqueous magnetic NPs in which oleate sodium and PEG-4000 were respectively used as the primary and the secondary layer surfactant. These bilayer coated magnetic NPs are quite stable in aqueous solution. However, the effects of different surfactants on bilayer coated NPs are rarely investigated and reported. In the present work, we report the synthesis of MnFe2O4 magnetic NPs with size controllability through a low temperature solvothermal method. Low-cost metal oleate was selected as the precursor. The particle size could be tuned from 2 nm to 10 nm either by changing the reaction parameters or adopting different M2+ (M = Co, Ni, Mn, Zn) cations. The materials with different M2+ cations showing different magnetic properties could be easily functionalized by a secondary surfactant, such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS). The functionalized NPs show great stability in water, which could be applied as a highly efficient adsorbent for Pb(II) removal from contaminated water.
Dalton Transactions
with a volume ratio of 5 : 1 and centrifuged at 6000 rpm for 10 min to remove any undispersed residue. Under identical conditions, the reaction of the (Zn2+ Fe3+)oleate complex, (Co2+ Fe3+)-oleate complex and (Ni2+ Fe3+)oleate complex at 160 °C led to ZnFe2O4, CoFe2O4 and NiFe2O4 NPs. Synthesis of water soluble MnFe2O4 NPs 0.5 g of 5.5 nm MnFe2O4 NPs ( prepared at a reaction temperature of 160 °C) was added to a suspension of CTAB (about 20 mg in 2 mL), SDBS and SDS in deionized water respectively, stirred for 1 h at 80 °C to obtain a water based MnFe2O4 colloidal solution. Characterization X-Ray powder diffraction (XRD) patterns were obtained using a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The transmission electron microscopy (TEM) experiment was performed on a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. Carbon coated copper grids were used as the sample holders. Magnetization was measured at 300 K in a physical properties measurement system (PPMS, Quantum Design Inc.) up to a field of 20 kOe. The hydrodynamic size and surface charges of the NPs in aqueous solution were evaluated using Zetasizer nano series dynamic light scattering (DLS).
Experimental section Chemicals FeCl3·6H2O (99%), NiCl2·4H2O (99%), MnCl2·4H2O (99%), CoCl2·6H2O (97%), ZnCl2·4H2O (99%), Pb(NO3)2, oleic acid, deionized water, heptane, ethanol, sodium oleate (95%), cetyltrimethylammonium bromide (CTAB), sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Co., Ltd. They were used as received without any further purification. Synthesis of metal oleate complexes The mixed-metal (M2+ Fe3+)-oleate complex was prepared by reaction of sodium oleate and a mixture of Fe3+ and M2+ chlorides. In a typical synthesis,16 40 mmol of FeCl3·6H2O, 20 mmol of MnCl2·4H2O, 160 mmol of sodium oleate, 100 mL of H2O, 100 mL of ethanol and 200 mL of hexane were mixed and refluxed at 60 °C for 4 h. The mixed M2+ Fe3+-oleate complex was obtained after separation from water. Synthesis of MnFe2O4 0.5 g oleic acid was added into a heptane solution of the mixed-metal (Mn2+ Fe3+) oleate complex, the whole solution was stirred thoroughly and then transferred into a 50 mL Teflon-lined stainless-steel autoclave to a filling capacity of 40%. The crystallization was carried out under autogenous pressure at temperatures of 140, 160 and 180 °C for 12 h, respectively. After being cooled to room temperature, the sample was washed 5 times with a mixture of ethanol–hexane
9886 | Dalton Trans., 2014, 43, 9885–9891
Results and discussion Size and magnetic properties of controllable MnFe2O4 nanoparticles The thermal decomposition method was used to prepare monodisperse MnFe2O4 NPs at a high reaction temperature (300 °C) from metal oleate,20 as metal oleate can decompose at high temperatures. The solvothermal synthesis includes various wet chemical technologies of crystallizing a substance in a sealed container at a high vapor pressure (generally in the range from 0.3 to 4 MPa). The solvothermal synthesis is prone to obtain highly crystalline MnFe2O4 NPs due to the high vapor pressure required. The formation of NPs is a crystallization process at the nanometer scale. The solvothermal reaction of metal oleate is the primary step in the synthesis of MFe2O4 NPs, and this process includes the formation of thermal free radicals, the resultant addition and/or substitution of free radicals and the combination of M(oleate)3 containing free radicals. The crystal structure during the initial nucleation stage and the subsequent growth stage is greatly affected by the reaction conditions, such as surfactant, temperature, reacting time, etc. Since the first report on the synthesis of monodisperse NPs with inexpensive and nontoxic iron-oleate complex as a reactant by Hyeon’s group, the method has been utilized in the synthesis of many other binary metal oxides with a high temperature decomposition method.21 We synthesized the monodisperse spinel ferrite MFe2O4 NPs with a uniform
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Dalton Transactions
composition by the solvothermal method with a mixed binary metal oleate precursor. The effect of the reaction parameters on the size and magnetic properties of the monodisperse MnFe2O4 NPs were carefully examined. After examining each parameter thoroughly, we found that the surfactant, reaction temperature and reaction time are the key parameters which play crucial roles in the control of the size and magnetic properties of the NPs. To investigate the effect of the reaction temperature on the crystal size, we analyzed the products that were separately formed at 140 (Fig. 1A), 160 (Fig. 1B) and 180 °C (Fig. 1C) for 12 h. TEM was extensively used to investigate the MnFe2O4 NPs. The mean particle size of the MnFe2O4 NPs, determined by measuring the particles at random, is 4, 5 and 6 nm respectively, and the size distribution is from 2 to 7 nm, 4 to 8 nm and 4 to 9 nm. We can draw from Fig. 1 that the size of the MnFe2O4 NPs increased with the increase of reaction temperature. At the lowest reaction temperature (140 °C), the MnFe2O4 NPs have the smallest particle size and the widest size distribution. At 160 °C, they are well crystalline with a medium size and a narrow size distribution, while the NPs at 180 °C are the largest with a good crystalline structure. Therefore, it is possible to propose that when the reaction temperature is higher than 160 °C, although the degree of crystallinity becomes good, the particle size becomes larger and results in polydispersity. When it is lower than 160 °C, the particle size becomes smaller with a wide size distribution. Therefore, 160 °C is the middle level.
Fig. 1 TEM images of the MnFe2O4 NP samples formed at different temperatures using the solvothermal method: (A) 140 °C, (B) 160 °C, (C) 180 °C.
This journal is © The Royal Society of Chemistry 2014
Paper
MnFe2O4 NPs prepared at 140 °C, 160 °C and 180 °C have super-paramagnetic properties as their remanence and coercivity are zero. In addition, their saturation magnetization is 30.67, 46.01 and 51.12 emu g−1, respectively (Fig. S1†). X-Ray diffraction (XRD) was used to record the crystal information of the MnFe2O4 NPs. As shown in Fig. 2, the materials have the diffraction pattern of spinel ferrites (JCPDS no. 10-0319). The peaks grew in intensity as the crystallization temperature increased, suggesting the transformation of MnFe2O4 NPs takes place from a low degree to a high degree of crystallinity with the increasing of the crystallization temperature from 140 to 180 °C. In addition, the peaks become sharper as the reaction temperature increases, which reflects the typical transformation occurring during crystal growth. This effect has to be ascribed to the increase of NP size under the high synthesis temperature. The size of the MnFe2O4 NPs is about 4.5 (140 °C), 5.6 (160 °C) and 7 nm (180 °C) calculated using the Scherrer equation from the half-maximum width of the (311) X-ray diffraction line,22 which are respectively in accordance with the result of the TEM measurements. Further investigation into the synthesis of shape and size controlled NPs focused on the tuning of the surfactant and reaction time. First of all, we adjusted the dosage of the surfactant OA. The shape and size of the MnFe2O4 NPs formed with different OA dosages are shown in the TEM images (Fig. 3a and b). When we decreased the surfactant dosage in the reaction solution (Fig. 3a) the size of MnFe2O4 NPs became smaller. However, when we increased the surfactant amount (Fig. 3b), the MnFe2O4 size is bigger and size distribution was quite wide (4–9 nm). All these results prove the effect of surfactant amount on the particle size. To further understand the formation process of the MnFe2O4 NPs, experiments were carried out with reaction times of 6 h and 24 h, respectively. The TEM images (Fig. 3c and d) show that the NPs formed after 6 h were small-sized (3–6 nm) and the NPs formed after 24 h are large-sized (10 nm), which illustrates
Fig. 2 XRD patterns of MnFe2O4 NPs formed with different reaction temperatures: (a) 140 °C, (b) 160 °C, (c) 180 °C.
Dalton Trans., 2014, 43, 9885–9891 | 9887
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Paper
Fig. 3 TEM images of the MnFe2O4 NPs synthesized by varying the relative amount of OA: (a) 0.25 g, (b) 0.75 g and syntheses with the reaction time of (c) 6 h, (d) 24 h.
the fact that the reaction time is an important factor that affects the formation of MnFe2O4 NPs. The morphology and size of the MFe2O4 (M = Co, Ni, Zn) NPs were studied in detail from the representative TEM. The TEM images (Fig. 4) reveal that the novel MFe2O4 (M = Co, Ni, Zn) NPs prepared at 160 °C for 12 h are nearly spherical. The size distribution of the MFe2O4 (M = Co, Ni, Zn) NPs can be fitted with a log-normal function, which is determined by measuring the particles at random, with the mean particle sizes being 5.3 nm for CoFe2O4, 6.3 nm for NiFe2O4, and
Dalton Transactions
6.4 nm for ZnFe2O4. Compared with Fig. 1, the particle size of CoFe2O4 and MnFe2O4 is less than that of ZnFe2O4 and NiFe2O4. While the NiFe2O4 and ZnFe2O4 NPs have the widest and narrowest particle size distribution, respectively. XRD experiments were performed to identify the crystallographic structure and estimate the particle size (Fig. S2†). All of the materials have a diffraction pattern of spinel ferrites. The average particle size of the NPs is about 6.6 nm for CoFe2O4, 7.8 nm for NiFe2O4, 6.5 nm for MnFe2O4 and 7.6 nm for ZnFe2O4, calculated using the Scherrer equation from the half-maximum width of the (311) X-ray diffraction line. The room-temperature hysteresis loops of the ferrites were measured using a vibrating sample magnetometer (VSM). The magnetization curves, as shown in Fig. 5, display a relatively high saturation magnetization. The magnetic saturation (Ms) values of CoFe2O4, NiFe2O4, MnFe2O4 and ZnFe2O4 are 54.02, 39.93, 46.01 and 9.58 emu g−1, respectively, which is outside their theoretical values of 71.2, 47.5, 120.8 and 36.9 emu g−1 reported in the literature.20 Among them, the MnFe2O4 and ZnFe2O4 NPs exhibit a typical low saturation magnetization, which is much smaller than the theoretical values. The reduced saturation magnetization of the ferrite NPs is generally believed to be due to the decreased particle size and the presence of a magnetic dead or anti-ferromagnetic layer on the surface, the hysteresis loops of the MnFe2O4 and ZnFe2O4
Fig. 5 Magnetic hysteresis curves of the NPs at 300 K (a) CoFe2O4, (b) NiFe2O4, (c) MnFe2O4, (d) ZnFe2O4.
Table 1 Size and magnetic characterization of MFe2O4 NPs at the preparation temperature of 160 °C for 12 h
Fig. 4 TEM images of (A) CoFe2O4 NPs, (B) NiFe2O4 NPs, (C) ZnFe2O4 NPs prepared at 160 °C for 12 h.
9888 | Dalton Trans., 2014, 43, 9885–9891
Particles
dTEM a
dXRD b
MS
Hc
Mr
CoFe2O4 NiFe2O4 MnFe2O4 ZnFe2O4
5.3 6.5 5.5 6.4
6.6 7.8 6.5 7.6
54.02 39.93 46.01 9.58
14 18 0 0
0.4 0.5 0 0
a Average particle size as estimated by TEM assuming a log-normal particle size distribution. b Average particle size estimated by XRD using the Scherrer equation.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
Paper
display zero coercivities (Hc) and no remanence (Mr), as would be expected for superparamagnetic behavior (Table 1). The CoFe2O4 and NiFe2O4 NPs displayed a very low coercivity of 14 and 18 Oe at room temperature and 20 kOe. The remanence of the CoFe2O4 and NiFe2O4 NPs are 0.4 and 0.5 emu g−1. The result show that the samples demonstrate a superparamagnetic behavior with negligible coercivity and remanence.
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Surface functionalization of water-soluble NPs Organic metals have been demonstrated to be a very promising route for the synthesis of monodispersed MFe2O4 NPs with uniform and controllable size and shape.23 However, the surface of the nanocrystalline structure is coated by a layer of hydrophobic organic ligand (such as OA and octadecylamine),24 which makes these NPs difficult to disperse in water. Ligand exchange is an effective surface functionalizing method that can improve the water dispersibility of NPs. For example, the hydrophobic oleic acid molecular coating on the surface of the NPs can be ligand exchanged by hydrophilic dopamine (DA), etc. However, the disadvantages of the ligand exchange method includes incomplete replacement (a small amount of oleic acid molecules remain on the surface of the NPs) and a reduced stability in aqueous solution after the reaction.25 On the basis of the above consideration, MnFe2O4 NPs were stabilized by mono-functional fatty acid molecules. The mono-functional MnFe2O4 NPs were recoated by a secondary ligand to functionalize their surface and improve their water solubility, which is of great importance for their future applications.26,27 This work presents a systematic study of creatinga stable aqueous dispersion of 5.5 nm MnFe2O4 NPs ( prepared at 160 °C for 12 h) through the recoating of NPs with different water soluble surfactants. The coating process is illustrated in Fig. 7, where the OA coated MnFe2O4 NPs are coated a second time by CTAB, SDBS and SDS. To the OA coated MnFe2O4 NPs, the OA molecules can anchor themselves on the surface of the MnFe2O4 NPs through the formation of hydrogen bonds between –COOH of OA and –OH of MnFe2O4. While the recoating of water soluble surfactants with both nonpolar and polar groups can make the nonpolar groups bind with the OA layer through strong van der Waals interactions (dispersion force), and the polar groups can help the NPs become well dispersed in water (Fig. 6). Due to the strong interactions between the surfactants and the OA coated MnFe2O4 NPs, the bilayer-stabilized magnetic NPs have a good solubility and stability in water. An interdigitated structure is expected to be energetically favorable in an aqueous environment because of the maximization of hydrophobic interactions between the interdigitated hydrocarbon chains. On the basis of this result, we propose that the phase transfer of the OA coated MnFe2O4 NPs and their redispersibility in water arise because of the formation of an interdigitated structure as shown in Fig. 7. The zeta-potential is an important parameter that governs interparticle electrostatic interactions and the stability of the NPs in water, where a colloidal system is generally stable if its zeta-potential is higher than 30 mV or smaller than −30 mV.
This journal is © The Royal Society of Chemistry 2014
Fig. 6 NPs.
Schematic illustration of the surface functionalization of the
Fig. 7 Water-soluble MnFe2O4NPs stabilized with CTAB, SDBS and SDS: (a–c) zeta-potential measurements, (d–f ) photographs and (g–i) DLS plots.
Therefore, the zeta-potential value of the MnFe2O4 NPs dispersion is an indicator of its stability. The high absolute values of all three of the MnFe2O4 NPs water dispersions indicated their high stability (Fig. 7a–c). The zeta-potential value of the CTAB, SDBS and SDS coated NPs are about 75, −73 and −52 mV. The zeta-potential of the bare MnFe2O4 NPs is only −8.7 mV (Fig. S3†), which may be caused by the lack of surfactant modification. The OA can anchor on the surface of the MnFe2O4 NPs through the formation of hydrogen bonds between the –COOH of OA and –OH of MnFe2O4. While nonpolar (long-chain alkyl) groups in CTAB, SDBS and SDS are bound with the OA layer through strong van der Waals interactions, polar groups in CTAB, SDBS and SDS are bound with the water (Fig. 7d–f ). Due to the strong interactions, the bilayer-coated MnFe2O4 magnetic NPs have a high stability in water. All the NP water dispersions are transparent without any noticeable precipitation (Fig. 8d–f ). The concentration of the water dispersion was roughly estimated to be about 1 mg mL−1. The hydrodynamic size of the CTAB, SDBS and SDS
Dalton Trans., 2014, 43, 9885–9891 | 9889
View Article Online
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Paper
Fig. 8 Pb(II) sorption kinetics using various types of MnFe2O4 NPs. Experimental conditions: MnFe2O4 = 1 g L−1 as Fe; surfactant = 10 wt% for stabilized MnFe2O4 particles. Solution pH was kept constant at 6.8 ± 0.4 (a) CTAB, (b) SDBS, (c) SDS and (d) non-stabilized.
coated NPs are respectively 64, 92 and 94 nm (Fig. 7g–i). The increase of the hydrodynamic size is caused by the coating of the bilayer surfactants on the surface of the NPs. The magnetic properties of the functionalized superparamagnetic particles will also present superparamagnetic properties. The magnetic saturation of the functionalized magnetic particles is smaller than that of the bare magnetic particles, where the saturation magnetization is 41.41, 41.87 and 42.22 emu g−1, for MnFe2O4/CTAB, MnFe2O4/SDBS and MnFe2O4/SDS, respectively (Fig. S4†). Surface functionalized MnFe2O4 NPs applied to Pb(II) sorption The increasing heavy metal pollution of surface and ground waters, with largely unknown long-term effects on wildlife and human health, can easily lead to serious environmental problems.28 Lead poisoning particularly affects young children, who can absorb up to 50% of ingested lead. Once ingested through the gastrointestinal track, lead accumulates in vital organs like kidneys, liver, or the brain. Leaded exhaust emissions, produced by the combustion of gasoline with tetraethyl lead as an antiknock agent, are released into the atmosphere and water supply causing pollution.29 Some recent work has shown that magnetic NPs stabilized by surfactants can obviously enhance the removal efficiency of heavy metals from contaminated water,20–32 with both starch and CMC being able act as an effective stabilizer for the preparation of highly stable magnetite NPs possessing a much greater arsenic sorption capacity than conventional magnetite particles. The overall goal of this study is to discuss a preparation method of surfactant (CTAB, SDBS and SDS) stabilized MnFe2O4 NPs for enhanced Pb(II) removal from contaminated water. Fig. 8 shows the Pb(II) removal kinetics data of the bare and bilayer coated MnFe2O4 NPs under otherwise identical conditions. Three control tests on the SDBS, SDS, and CTAB stabilized MnFe2O4 water dispersions were carried out. The adsorption capacities of CTAB, SDBS, SDS stabilized MnFe2O4
9890 | Dalton Trans., 2014, 43, 9885–9891
Dalton Transactions
Fig. 9 Pb(II) adsorption isotherms for different contact times on MnFe2O4 NPs stabilized with (a) CTAB, (b) SDBS, (c) SDS and (d) nonstabilized.
NPs and bare MnFe2O4 NPs for Pb(II) are respectively 89.55%, 75.05%, 66.07% and 35.2%. The adsorption capacities of the CTAB, SDBS, and SDS stabilized MnFe2O4 NPs and bare MnFe2O4 NPs for Pb(II) are 17.9, 15.1, 13.2 and 5.2 mg g−1, respectively (Fig. 9). The largest adsorbed percentage for these Pb(II) ions is up to 90%. Thereinto, the CTAB functionalized MnFe2O4 NPs have the highest adsorption capacity. This is because the functionalization of the NPs improves their water dispersibility and increases their contact with Pb(II) in the water, and thus good adsorption occurs in a short time. They may also have a good adsorption performance for other heavy metal ions in contaminated water. The Pb(II) removal kinetics and adsorption capacities of bare MnFe2O4 NPs have been added to Fig. 8 and 9, respectively. The surface functionalized MnFe2O4 NPs sorption of Pb(II) includes both physical and chemical processes. The stabilized MnFe2O4 NPs with a larger specific surface area and smaller size may possess an open pore structure, which allows exterior surface sorption sites for Pb(II) sorption.33 The high zeta-potential of the surface functionalized MnFe2O4 NPs helps the sorption of the target Pb(II) ions. In addition, the better Pb(II) sorption effect can be attributed to the high affinity between the surfactant groups and Pb(II), as well as to the readily available surfactant groups on the surface of the MnFe2O4 NPs.
Conclusions We report the synthesis of OA coated spinel ferrite MFe2O4 (M = Co, Ni, Mn, Zn) NPs with a uniform composition using a simple route that involves a solvothermal method of a mixed binary metal-oleate precursor formed in a low-temperature reaction of the constituent metal halides with sodium oleate. The mean particle size is 5.5 nm for MnFe2O4, 5.3 nm for CoFe2O4, 6.3 nm for NiFe2O4 and 6.4 nm for ZnFe2O4. The magnetic saturation values of CoFe2O4, NiFe2O4, MnFe2O4 and ZnFe2O4 are 54.02, 39.93, 46.01 and 9.58 mg g−1, respectively.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Dalton Transactions
The use of a secondary ligand (CTAB, SDBS and SDS) to functionalize the surface of the MnFe2O4 NPs makes the NPs water soluble. The zeta-potentials of the CTAB, SDBS and SDS functionalized NPs are about 75, −73 and −52 mV. In addition, the Pb(II) adsorption capacities of the CTAB, SDBS, and SDS functionalized MnFe2O4 NPs are 17.9, 15.1 and 13.2 mg g−1, respectively.
Published on 10 April 2014. Downloaded by UTSA Libraries on 14/10/2014 20:21:28.
Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (no. 21031001), the National Natural Science Foundation of China (no. 91122018, 51102082, 21101061), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (no. 708029), and the Special Research Fund for the Doctoral Program of Higher Education of China (20112301110002).
Notes and references 1 S. Yanez-Vilar, M. Sanchez-Andujar, C. Gomez-Aguirre, J. Mira, M. A. Senarıs-Rodrıguez and S. Castro-Garcıa, J. Solid State Chem., 2009, 182, 685. 2 D. Yoo, J. H. Lee, T. H. Shin and J. Cheon, Acc. Chem. Res., 2011, 44, 863. 3 C. S. S. R. Kumar and F. Mohammad, Adv. Drug Delivery Rev., 2011, 63, 789. 4 V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara and J. M. Basset, Chem. Rev., 2011, 111, 3036. 5 J. Mohapatra, A. Mitra, D. Bahadur and M. Aslam, CrystEngComm, 2013, 15, 524. 6 A. H. Lu, E. L. Salabas and F. Schüth, Angew. Chem., Int. Ed., 2007, 46, 1222. 7 S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. V. Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064. 8 R. Sai, S. D. Kulkarni, K. J. Vinoy, N. Bhat and S. A. Shivashankar, J. Mater. Chem., 2012, 22, 2149. 9 S. Sun and H. Zeng, J. Am. Chem. Soc., 2002, 124, 8204. 10 C. Pereira, A. Pereira, C. Fernandes, M. Rocha, R. Mendes, M. G. A. Guedes, P. B. Tavares, J. M. Grenèche, J. Araújo and C. Freire, Chem. Mater., 2012, 24, 1496. 11 C. Xu, K. Xu, H. Gu, R. Zheng, H. Liu, X. Zhang, Z. Guo and B. J. Xu, J. Am. Chem. Soc., 2004, 126, 9938.
This journal is © The Royal Society of Chemistry 2014
Paper
12 K. Herve, L. Douziech-Eyrolles, E. Munnier, S. CohenJonathan, M. Souce, H. Marchais, P. Limelette, F. Warmont, M. L. Saboungi, P. Dubois and I. Chourpa, Nanotechnology, 2008, 19, 465608. 13 H. Duan, M. Kuang, X. Wang, Y. Wang, H. Mao and S. Nie, J. Phys. Chem. C, 2008, 112, 8127. 14 W. H. Binder, H. Weinstabl and R. Sachsenhofer, J. Nanomater., 2008, 110, 383020. 15 N. Fauconnier, J. N. Pons, J. Roger and A. Bee, J. Colloid Interface Sci., 1997, 194, 427. 16 J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891. 17 L. Shen, A. Stachowiak, T. A. Hatton and P. E. Laibinis, Langmuir, 2000, 16, 9907. 18 L. Shen, P. E. Laibinis and T. A. Hatton, Langmuir, 1999, 15, 447. 19 R. Y. Hong, S. Z. Zhang, Y. P. Han, H. Z. Li, J. Ding and Y. Zheng, Powder Technol., 2006, 170, 1. 20 N. Bao, L. Shen, Y. Wang, P. Padhan and A. Gupta, J. Am. Chem. Soc., 2007, 129, 12374. 21 N. Bao, L. Shen, W. An, P. Padhan, C. H. Turner and A. Gupta, Chem. Mater., 2009, 21, 3458. 22 X. H. Huang, Z. Y. Zhan, X. Wang, Z. Zhang, G. Z. Xing, D. L. Guo, D. P. Leusink, L. X. Zheng and T. Wu, Appl. Phys. Lett., 2010, 97, 203112. 23 B. Wang, B. Wang, P. Wei, X. Wang and W. Lou, Dalton Trans., 2012, 41, 896. 24 H. Qu, H. Ma, A. Riviere, W. Zhou and C. J. O’Connor, J. Mater. Chem., 2012, 22, 3311. 25 Y. Xu, Y. Qin, S. Palchoudhury and Y. Bao, Langmuir, 2011, 27, 8990. 26 Q. Liang, D. Zhao, T. Qian, K. Freel and Y. Feng, Ind. Eng. Chem. Res., 2012, 51, 2407. 27 A. Swami, A. Kumar and M. Sastry, Langmuir, 2003, 19, 1168. 28 J. Lynch, J. Zhuang, T. Wang, D. L. Montagne, H. Wu and Y. C. Cao, J. Am. Chem. Soc., 2011, 133, 12664. 29 W. J. Liu, K. Tian, H. Jiang, X. S. Zhang, H. S. Ding and H. Q. Yu, Environ. Sci. Technol., 2012, 46, 7849. 30 F. He and D. Zhao, Environ. Sci. Technol., 2007, 41, 6216. 31 Q. Liang, D. Zhao, T. Qian, K. Freel and Y. Feng, Ind. Eng. Chem. Res., 2012, 51, 2407. 32 F. He and D. Zhao, Environ. Sci. Technol., 2005, 39, 3314. 33 F. He, W. Wang, J. W. Moon, J. Howe, E. M. Pierce and L. Liang, ACS Appl. Mater. Interfaces, 2012, 4, 4373.
Dalton Trans., 2014, 43, 9885–9891 | 9891
