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polymer core–shell nanocomposites
Journal of Alloys and Compounds 578 (2013) 121–131
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Effective dye removal and water purification using the electric and magnetic Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposites M.A. Ahmed a,⇑, Rasha M. Khafagy b, Samiha T. Bishay b, N.M. Saleh c a b c
Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egypt Physics Department, Girls College for Arts, Science and Education, Ain Shams University, Cairo, Egypt Physics Department, Faculty of Science, Western Mountain University, Libya
a r t i c l e
i n f o
Article history: Received 11 October 2012 Received in revised form 25 April 2013 Accepted 26 April 2013 Available online 6 May 2013 Keywords: Water purification and dye uptake Recycling of industrial waste water Ferrite/polymer core–shell nanocomposites AC conductivity Magnetic properties
a b s t r a c t Flash auto combustion method was successfully used to synthesize nanoparticles of Zn0.5Co0.5Al0.5Fe1.46La0.04O4. High resolution transmission electron microscopy (HRTEM) specified the formation of granular nanospheres beside an intermediated phase of nanowires. Polymer-blended magnetic materials were obtained using poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc) and Polyethylene Glycol (PEG) as capping agents. This coating strategy controls the agglomeration of ferrite nanoparticles, and produces a well-designed core–shell nano-assembly with enhanced physical properties. XRD and HRTEM confirmed the formation of ferrite as a core surrounded by various polymeric shells. The nanocomposite with PVP shell resulted in increased ac conductivity (r) of about four orders of magnitude higher than that recorded for the pure ferrite. Curie temperature (TC) decreased from 703 K as recorded for the pure ferrite to less than 440 K for the core–shell nanocomposites containing PVA and PVAc. All prepared samples succeeded in purifying inked-water with high efficiency. Zn0.5Co0.5Al0.5Fe1.46La0.04O4 up-took 76% of the dye content, while the dye-removal efficiency was increased to 90% when Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite was applied. These novel results indicate that such series of core–shell nanocomposites are promising candidates in industrial applications such as purifying and recycling of industrial waste water. Moreover, this study emphasized that polymers are good additive to ferrites when blended in the form of core–shell nanocomposite structure. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to their wide uses in several technologies and applications, spinel ferrites MFe2O4 (where M is often a transition metal atom) are a kind of most important magnetic materials which are still widely investigated [1–4]. Moreover, polymer-blended magnetic materials are now extensively studied because of their high processability, versatility, low cost [5–8], and because of their fascinating physical properties that appear when the polymer is added by any sort to ferrites [9–13] or perovskites [14]. Among recent research articles concerned with polymer-blended magnetic materials, Khafagy [9] found that MgFe2O4/Polyaniline nanocomposite with core–shell structure shows much better conductivity if compared to pure MgFe2O4 nanoparticles. Also, tuning of the total magneto-electrical behavior of this nanocomposite was possible depending on the volume fraction of the magnetic ferrite nanoparticles, and on the contribution of the non-magnetic ⇑ Corresponding author. E-mail address: [email protected] (M.A. Ahmed). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.182
Polyaniline coating layer. Also, Li et al [15] synthesized Cu0.4Zn0.6Cr0.5Sm0.06Fe1.44O4/Polyaniline core–shell nanocomposite that exhibited a hysteresis loop of the ferromagnetic nature, to be used as a soft magnetic material because of its coercivity, which is lower than that of the pure ferrite. Meanwhile, Xiaowei et al. [16] demonstrated a viable strategy for preparing polymer-coated functional metal oxides nanocrystals, which are potentially useful in biological and nanoelectronic applications. In addition, Varshney et al. [8] reported an approach to synthesize nano-stick shaped ferromagnetic iron oxide/polypyrrole, which influenced the properties of polypyrrole, and controlled the morphology of Fe2O3, that lead to form stick-shaped polymer composites decorated with Fe2O3 nanoparticles. This decorating strategy resulted in the formation of conducting/ferromagnetic/polymer nanocomposite with: electrical conductivity of the order of 10 2 S/cm, saturation magnetization of 35 emu/g and microwave absorption as high as SEA 22.5 dB (>99% attenuation) in the frequency range of 12.4–18 GHz (Ku-band). Such results [8,9,13,15,16] prove the fascinating future of polymer-blended magnetic materials, and show that polymers are good additive to ferrites and perovskites.
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Another reason for using the guest–host polymer technique, is that ferrite nanoparticles themselves highly agglomerate, and in the absence of a trapping media or some other form of encapsulation, they coalesce extremely quickly [17–21]. For this reason, bonding of capping agents to ferrite nanoparticles is necessary to provide chemical passivation, and to improve their surface state, which reduces to certain extent the agglomeration of ferrite nanoparticles, and becomes helpful to their decentralization and stabilization [15]. This reduction of agglomeration and maintenance of nano-scale particles, has a significant influence on the optical and electronic properties of ferrite nanoparticles [19] that makes capping agents good idea. Polymers are good choice for this passivation and stabilization, because they can interact with the metal ions of ferrites by complex or ion-pair formation, and can be designed to vary certain physical properties of ferrite nanoparticles. By this incorporation of inorganic particles within the polymeric matrices, homogeneous organic–inorganic hybrid materials with fascinating properties can be obtained. Polymers with 3-dimensional structure, such as poly(vinyl pyrrolidone) (PVP) [22] and poly(vinyl acetate) (PVAc) [23] have rigid pores, that set an upper limit for the growth of ferrite nanoparticles inside such polymeric matrix. One convincing reason for the formation of this 3-dimensional structure, is the crosslinking reactions formed by coordinate bonds between the polymeric chains and the metal ions present in the magnetic ferrite nanoparticles. Moreover, poly(vinyl alcohol) (PVA) usually draws a considerable attention because of the capability of its hydroxyl groups (OH) to retain metal ions through hydrogen bonds (hydrides) leading to a 3-dimensional structure [20]. Also, the ability of PVA and PVAc to crystallize is one of the most important physical properties of these two polymers, as the degree of crystallinity controls the water solubility, and consequently, controls the electrical and magnetic properties of these two polymers [24]. As a result, both poly(vinyl alcohol) and poly(vinyl acetate) have attracted extensive interest in industrial as well as in medical and pharmaceutical applications due to their important combination of: thermal, structural, mechanical, optical, electrical and magnetic properties [25]. Enhancement of PVA and PVAc properties are expected when these two polymers are motivated by the spinel ferrite nanoparticles of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 as an electronic, electrical and magnetic nanofiller [26]. Although there are several papers on the magnetic character of PVA materials [11,27,28], none of them referred to in situ corporation of magnetic nanoparticles within a PVA matrix in the form of core–shell structure, nor with the other chosenhere polymers. Four different polymers have been chosen here to encapsulate the nanoparticles of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 and produce a core–shell design with the magnetic ferrite as a core being capped with the polymeric surfactant shell. Surfactant polymers are: poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), and Polyethylene Glycol (PEG), the properties of which, were intentionally chosen to vary from completely amorphous polymer (PVP), to semi-crystalline polymer (PVA and PVAc), and highly crystalline polymer (PEG). Poly(vinyl pyrrolidone), poly(vinyl alcohol) and poly(vinyl acetate) are related to the ‘‘Vinyl class’’, which has the general structure
, where ‘‘n’’
is the repetition number, while R =
in case of PVP,
R = AOH in case of PVA, and
in case of PVAc.
structure
. The term ‘‘glycol’’ is reserved
for low to medium range molar mass polymer when the nature of the end-group, which is usually a CH3 group, still matters and can result in predominant effects. Reactive polymers including PVP, PVA, PVAc and PEG are expected to be perfectly utilized for core– shell purposes. It is aimed in this research article to get specifically good conductive properties, and tunable magnetic properties from a series of novel electric and magnetic core–shell nanocomposites using Zn0.5Co0.5Al0.5Fe1.46La0.04O4 as a core being surfactant with four different polymeric shells (PVP, PVA, PVAc and PEG). It is aimed also to apply the synthesized samples to some sort of water treatment; such as purifying and recycling of industrial waste water, which is one of the new era applications. Such study might lead to the fabrication of specially tailored materials suitable for applications demanding high electrical conductivity and low Curie temperature. 2. Experimental details 2.1. Preparation of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles were synthesized using the flash auto combustion method [29,30]. Analytical grade (BDH-99.999%) of Fe(NO3)39H2O, Al(NO3)3H2O, La(NO3)39H2O, Co(NO3)2, Zn(NO3)26H2O, and CO(NH)2 were used as starting materials. The stoichiometric ratios of metal nitrates and urea were calculated based on the total oxidizing and reducing coefficients, so that the equivalence ratio is unity and the released energy is maximum [29,30]. All the above chemicals, with their ratios, were perfectly mixed for 90 min to give a homogeneous paste. The beaker containing the paste was inserted on a hot plate until the reaction was completed. After cooling to room temperature, the obtained powder was grinded properly for half an hour in an agate mortar.
2.2. Synthesis of Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymers core–shell nanocomposites Pure grade of PVP (molecular weight 125,000 g mol 1), PVA (molecular weight 115,000 g mol 1), PVAc (molecular weight 100,000 g mol 1) and PEG (molecular weight 40,000 g mol 1) were imported from LoBa Chemie Pvt. Ltd., India. These four polymers were chosen to encapsulate Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles in the form of core–shell design, with the magnetic ferrite as a core, being capped with the polymeric surfactant shell. To obtain Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core– shell nanocomposites [15], 1 g of the ferrite nanoparticles was added to 60 ml of distilled water. In another set of beakers, 1 g of each separate polymer (PVP/PVA/ PVAc/PEG) was dissolved in 30 ml of distilled water. All solutions were stirred thoroughly for 30 min using an electric stirrer. Afterwards, each polymeric solution was added to the corresponding suspended ferrite solution, and the mixtures were then re-stirred for 3 h. The mixtures were dried at 60 °C for 24 h to perform the core– shell structure by four different surfactant polymers capping the same Zn0.5Co0.5Al0.5Fe1.46La0.04O4 core. 2.3. Sample analysis and characterizations All samples were identified by X-ray Philips analytical (X’pert Graphics & Identify) diffractometer. Morphology and particle’s size of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles and core–shell nanocomposites have been investigated using FEI High resolution transmission electron microscope (HRTEM) model Tecnai-G20, super twin, double tilt and gun type LaB6. HRTEM photos were collected at applied voltage of 200 kV and magnification up to 1,000,000 after samples have been dispersed in a distilled water for 10 min. The dc magnetic susceptibility measurements were performed using Faraday’s method from room temperature up to 850 K at three different magnetic field intensities, where a very small amount of the nanopowdered sample was inserted in a cylindrical Pyrex glass tube at the point of maximum gradient. The ac conductivity (r) have been measured over a wide range of temperatures (300 K 6 T 6 700 K) and frequencies (10 and 100 kHz) using RLC Hioki bridge model 3532.
2.4. Application of prepared samples in water purification and de-inking
On the other side, Polyethylene Glycol is related to ‘‘Polyether’s’’ class which has the general formula
, and linear
A standard solution consisting of 100 mL of distilled water plus 0.75 mL of standard ink was prepared. Five tubes were filled by 7 ml of the standard solution in addition to 0.02 g of the prepared samples. The mixture was then mixed using an electric stirrer for 15 min. Color index analysis was carried out for the solutions after 10 h to insure dye removal.
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present in the structure of PVAc has a role in increasing the carbon backbones disturbance more than the (OH) group of PVA do, thus resulting in slightly more crystalline phases in PVAc as shown in its XRD pattern (Fig. 1c). 3.1.3. X-ray diffraction of Polyethylene Glycol (PEG) PEG is a highly crystalline, ordered and polar polymer, that belongs to the ether’s family. High crystallinity of PEG results from the alignment of oxygen atoms in the gauche configuration, with the intermolecular dipole forces oriented along the axis of the helix [7]. These dipolar groups are assigned to monoclinic crystal system [7], which allowed the determination of PEG crystal structure as identified by ICDD card [49-2097], and as confirmed by the sharpness of the crystalline XRD peaks appearing in Fig. 1d. The diffraction pattern of the monoclinic unit cell of PEG contains four molecular chains, in which a = 0.796 nm, b = 1.311 nm, and c = 1.939 nm [20]. The HRTEM photo shown in Fig. 1e illustrates the highly ordered lattice of the PEG shell with d-spacing value of (4.84 Å) as calculated using the HRTEM attached software. This value shows good accordance with the d-spacing estimated from the PEG ICDD card (49-2097) (d-spacing = 4.65 Å).
Fig. 1. X-ray diffraction of pure polymers.
3. Results and discussion 3.1. X-ray characterization 3.1.1. X-ray diffraction of poly(vinyl pyrrolidone) (PVP) Fig. 1a shows the XRD pattern of PVP, which proves the amorphous nature of this polymer. Two broad diffraction peaks extending between 10–15° and 15–24° were detected, which might be associated with two chain’s length and orientation, thus resulting in the presence of two different amorphous phases of PVP, as consistent with literature [31].
3.1.2. X-ray Diffraction of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) Both PVA and PVAc are semi-crystalline polymers as indicated from their XRD patterns illustrated in Fig. 1b and c. The crystalline nature of these two polymers is emphasized by the diffraction peaks at 2h = 19.8°, 41.05° assigned for PVA and at 2h = 19.54°, 40.54° assigned for PVAc, with a hallow shoulder at 2h = 23° representing the amorphous phase in both of them [24]. This similarity in XRD patterns of these two polymer arises from the fact that both polymers can be easily hydrolyzed to each others, as they are parent materials to one another. On a molecular level, the crystalline nature of PVA results from the strong intermolecular interactions between layered PVA chains through hydrogen bonding. Meanwhile, weaker Van Der Waal’s forces operate between double layers. This folded chain structure leads to small ordered regions (crystallites) scattered in unordered amorphous domains [24] as indicated by the XRD pattern shown in Fig. 1b. Moreover, PVA has slightly higher crystallinity than PVAc due to its structure enriched with hydroxyl (OH) groups, which are small enough to fit into the lattice without disrupting the carbon backbones chains. On the other hand, the function group
3.1.4. X-ray Diffraction of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 Phase formation of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 was proven by XRD, that indicated the absence of secondary phases as identified from ICDD card [73-1963]. Fig. 2 shows the XRD pattern for Zn0.5Co0.5Al0.5Fe1.46La0.04O4, which consists of well-resolved broad peaks, that confirms the mono-phasic nature and the nanometric crystallite size of the prepared nano-ferrite. All main peaks are related to the spinel structure of ferrites (AB2O4), which is known to have a cubic symmetry with a face-centered lattice and space group Fd3m (227). XRD peaks are well-indexed to the spinel crystallographic planes at (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3), but the broadening of some essential XRD peaks superimpose some other spinel peaks [32]. For example, the broad peak at 2h = 37° belonging to the plane (3 1 1), superimposes the neighboring peak at 2h = 38° which corresponds to the plane (2 2 2) as identified from ICDD card [73-1963] [32]. 3.1.5. X-ray diffraction of core–shell nanocomposites Fig. 3a–e enables clear comparison between the structure and complexation of the different polymeric shells with the same ferrite core through vertically separated XRD patterns ((a) the polymer, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4, and (c) Zn0.5Co0.5Al0.5Fe1.46 La0.04O4/polymer core–shell nanocomposite). Peak’s intensities of all polymers are suppressed with different ratios and proportionalities in the XRD spectra of the core–shell nanocomposites, which means decreased polymer’s crystallinity. The presence of ferrite nanoparticles during synthesizing the core–shell nanocomposites reduces the intermolecular interactions between the polymeric chains and prevent them from extending. Consequently, this results in decreasing the intensity of XRD peaks which represent the crystalline portions of the polymeric sample [9,13]. The XRD spectra of ferrite/polymer core–shell nanocomposites show that the positions of all diffraction peaks are just the superposition of the peaks for polymer and Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles. The diffraction patterns of PVP (Fig. 3a), and PVAc (Fig. 3b) are the most inhibited in the core–shell nanocomposite, while PVA (Fig. 3c) and PEG (Fig. 3d) maintained their fundamental diffraction peaks with obvious intensities, but they are still suppressed if compared to the XRD pattern for the parent polymer. XRD characterization proved the formation of ferrite/polymer nanocomposite in all prepared samples through interfacial interactions between the ferrite core and the polymeric shell. This
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Fig. 1e. HRTEM photo representing the ordered lattice of the square PEG shell in the Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PEG core–shell nanocomposite.
could be noticed by broadening of the lines or by vanishing of some lines with low intensities. Fig. 4 shows a schematic representation of an assumed interaction mechanism between the PVP polymer as a capping agent [26] and the metal ion precursors, where the divalent ions (such as zinc and cobalt) and trivalent ions (such as iron) are bounded by strong ionic bonds between the metallic ions and the amide group inside the polymeric chains or between the chains themselves. This uniform immobilization of metallic ions in the cavities of the polymeric chains favors the formation of a uniformly-distributed solid solution of the metallic oxides inside the polymer.
3.2. High resolution transmission electron microscopy (HRTEM)
Fig. 2. XRD pattern for Zn0.5Co0.5Al0.5Fe1.46La0.04O4.
is evident from the appearance of all the diffraction peaks belonging to the core and the shell in the XRD patterns of the core–shell nanocomposites. The strongest chemical interaction appeared between PVA and ferrite nanoparticles (Fig. 3c) due to the capabilities of hydroxyl (OH) groups to retain metal ions through hydrogen bonds (hydrides) leading to a 3-dimensional structure [20]. Weaker interaction between the core and the shell is obtained for PEG (Fig. 3d) due to the presence of oxygen atoms, which might maintain hydrogen bonding with metal ions during preparation of core– shell in the aqua’s polymeric solution. In general, the embedding
3.2.1. HRTEM of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 The HRTEM image shown in Fig. 5a proves that flash auto combustion method was a successful and easy method for obtaining large quantities of nanowire-bundle filaments with average lengths of 150 nm and narrow outer diameters with average value of 22 nm. Such nanowires are characterized by their large surface area, which might result in additional behavior which has never been obtained within traditional bulk materials, or nanospheres. In addition to nanowires, large quantities of nanospheres with average size of 30 nm are present. Fig. 5b illustrates the lattice of a single ferrite nanoparticle, with measured d-spacing of 4.7 Å and 2.9 Å, which is consistent with the values estimated from ICDD card (73-1963) that indicates d-spacing of 4.8 Å for the plane (1 1 1) at 2h = 18.38°, and d-spacing of 2.95 Å for the plane (2 2 0) at 2h = 30.24°.
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Fig. 3. Vertically separated XRD patterns of: (i) polymer, (ii) Zn0.5Co0.5Al0.5Fe1.46La0.04O4, and (iii) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposite; (a) PVP, (b) PVAc, (c) PVA and (d) PEG.
incorporated within a polymeric shell with perfect well-designed core–shell nano-assembly. The apparent contrast between the inner and outer portions of the particulate is based on the average atomic weight of the core and the shell. Spherical particles exhibited an expected agglomeration because of the magnetic dipole interactions between magnetic ferrite nanoparticles. in all core– shells, the ferrite core has an average diameter of 18 nm. The average particulate size of ferrite/PVP core–shell nanocomposite is 24 nm, with a spherical shell of thickness 6.6 nm (Fig. 6a). Both ferrite/PVA and ferrite/PVAc core–shell nanoparticles (Fig. 6b and c), have an average diameter of 21 nm, and a spherical shell of thickness 3 nm only. It is noticed that ferrite/PEG core–shell nanocomposite (Fig. 6d) resulted in a unique square shell, with highest average thickness of about 10 nm, leading to a higher than usual core–shell diameter of 41 nm.
3.3. Electrical properties Fig. 4. A schematic representation of an assumed interaction mechanism between the PVP polymer as a capping agent and the metal ion precursors.
3.2.2. HRTEM of core–shell nanocomposites Fig. 6a–d illustrates HRTEM photos for all core–shell nanocomposites. It is clear that abundance of ferrite nanoparticles are
3.3.1. Dependence of room-temperature ac conductivity on the polymer type Fig. 7 illustrates the dependence of ambient ac conductivity (r) on the polymeric surfactant type as compared to that of Zn0.5Co0.5Al0.5Fe1.46La0.04O4. Among all surfactant polymers, ferrite/PVP
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Fig. 5a. HRTEM image for the sample Zn0.5Co0.5Al0.5Fe1.46La0.04O4.
Fig. 5b. HRTEM image representing the lattice of a single ferrite nanoparticle, with a measured d-spacings of 4.7 Å and 2.9 Å.
core–shell nanocomposite resulted in the highest ac conductivity value (r 40 10 6 X 1 cm 1) which is much higher than that of pure ferrite (r 10 10 6 X 1 cm 1). This means that the presence of PVP shell supported the ac conductivity of the ferrite and became an advantage for the electrical properties of such material. Although PVP is an amorphous polymer and is expected to show
the lowest conductivity; such promising result could be correlated to the fact that PVP is one of the well known conductive polymers with aromatic ring in its structure. Hence it contributed to the final conductivity of the nanocomposite. Aromatic rings are known to play an essential role in controlling the efficiency of other conducting polymers such as Polyaniline and PolyPayroll [9,13].
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Fig. 6. HRTEM micrographs for the samples Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposites; where: (a) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA core–shell, (c) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVAc core–shell, and (d) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PEG core–shell.
3.3.2. Temperature dependence of ac conductivity of the core–shell nanocomposites The temperature-dependent ac conductivity (r) of core–shell nanocomposites are quite interesting, and differs from one surfactant polymer to another. It can be observed from Fig. 8a that ‘‘r’’ of ferrite/PVP core–shell nanocomposite, increases up to 330 K, and afterwards it decreases with increasing temperature. In case of ferrite/PVA core–shell nanocomposite, ‘‘r’’ increases up to 315 K and afterwards it decreases with increasing temperature up to 350 K, followed by continuous increase of ‘‘r’’ with a maximum peak at 410 K (Fig. 8b). Ac conductivity of ferrite/PVAc core–shell showed two maxima peaking at 350 K and 420 K
(Fig. 8c), while ferrite/PEG core–shell (Fig. 8d) showed continuous increase of ‘‘r’’ with temperature up to 350 K, after which ‘‘r’’ decreases with further increase of temperature. Such temperature influence is new for ac conductivity measurements, especially, regarding the existence of two different transition temperatures for the same material. Since the conductivity of polymers is primarily dependent on the ordered arrangement of polymeric chains in the crystalline domains [33], any factor that disrupts such arrangement can influence the polymer’s conductivity. However, the presence of water in the polymeric matrices plays an important role in the crystalline arrangement, and correlates the ac conductivity behavior.
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Fig. 6. (continued)
Fig. 8e presents a closer look to the behavior of ac conductivity at fixed frequency of 100 kHz as shown by the different polymers in the core–shell nanocomposites. One can clearly notice that all polymers show an anomalous behavior of ‘‘r’’ within the temperature range 310–380 K at which PVP, PVAc and PEG showed enhanced conductivity with obvious peaks, while PVA resulted in a deteriorated ac conductivity with an obvious minimum. This
behavior is correlated to the dehydration and elimination of water content upon heating the core–shell pellets during performing ac conductivity measurements. Water present inside polymers are known to be found either in a free associated state (free water), or weakly bounded to the polymer (intermediate state) or strongly bounded to the polar groups of the polymer (bounded water) [24]. Gradual heating of the samples eliminates the free water content,
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Fig. 7. The dependence of room temperature ac conductivity at 10 and 100 kHz on the polymer type used in the core–shell nanocomposites as compared to that of Zn0.5Co0.5Al0.5Fe1.46La0.04O4.
which directly affects the crystalline arrangement of the polymeric chains, and results in the evolution of the polar OH groups. Liberated OH groups act as intrinsic sites for charge transfer, and directly enhance the ac conductivity of: PVP, PVAc and PEG as the number of free OH groups increase, after which ‘‘r’’ decreases again. At 380 K all water molecules are liberated from the
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Fig. 8e. Behavior of ac conductivity shown by the different polymers in the core– shell nanocomposites at fixed frequency of 100 kHz.
polymers, then new conduction mechanism starts to appear depending on the nature of polymer type rather than the humidity content. This conclusion is supported with the fact that most polymers undergo more than one degradation step due to dehydration and elimination of water, then cleavage of the main and side chains (degradation of bonds in the skeleton structure), afterwards, dissociations of organic residuals such as CO2, CH4 and other carbonic residues take place [24].
Fig. 8. The temperature dependence of ac conductivity of the core–shell nanocomposites at 10 and 100 kHz; (a) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA core–shell, (c) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVAc core–shell, and (d) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PEG core–shell.
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In contradiction to other polymers, PVA is the only polymer which showed reduced ac conductivity within this temperature range (310–380 K). This is due to the large content of bounded water in its structure, as it is the only polymer enriched with strongly bounded polar OH groups. Upon heating PVA, it undergoes degradation reaction to get rid of this large content of bounded water [24] which requires high energy to be eliminated through an endothermic reaction to overcome the strong H-bonding formed with water. The tendency of hydrophobic groups to cluster, leads to a threedimensional network. This can be viewed as pseudo-crosslinks resulting in an opposing effect, and results in decreasing ‘‘r’’ during consuming heat in breaking the strong H-bonding between PVA
chains and water molecules, due to the tight nature of OH groups in this polymer [34]. After going beyond 380 K, each polymer started to show another conductivity behavior depending on its nature and structure. In case of PVP surfactant ‘‘r’’ decreased dramatically due to the amorphous nature of this polymer. Both PVA and PVAc surfactants showed continuous increase in ac conductivity due to the immobilization of extraneous ions. As the melting temperature of these two polymers is approached, ‘‘r’’ reached maximum values of 1.7 10 5 X 1 cm 1 and 6.5 10 5 X 1 cm 1 at 410 K and 430 K for PVA and PVAc respectively [18,19,34]. Reduced conductivity of PVA is due to the fact that elimination of bounded water from PVA chains results in main chain cleavage and destruction of the polymer’s crystallinity, which affects its ac conductivity at higher temperatures. After reaching the temperatures corresponding to the maximum ac conductivity of all polymers, ‘‘r’’ decreased again due to the cleavage of the main and side chains of the polymers (degradation of bonds in the skeleton structure) and thereafter, dissociations of organic residuals occurs, and the polymers decompose and collapse [22]. PEG was expected to result in highest conductivity, due to its extremely high crystallinity, and the presence of polar ether groups which are sufficiently polar to attract moisture and to provide charge-transfer sites, but the fast melting of this polymer (360 K) is a great constrain [22,23]. 3.4. Magnetic properties Fig. 9a–d shows the dependence of the magnetic susceptibility (vg) on the absolute temperature T(K) for Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposites, as compared to pure ferrite. The Curie temperature (TC) of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 was found to be around 703 K. A notable decrease of about 263 K was obtained in the Curie temperature of ferrite/PVA and ferrite/PVAc core–shell nanocomposites to become around 440 K. PVA and PVAc are known to show magnetic behavior [22], thus they supported the magnetization showed by the nano-ferrite, resulting in an earlier TC value. 3.5. Application of prepared samples in water purification and deinking Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nano-ferrite and the four core–shell nanocomposite samples have been investigated regarding their ability to remove dyes from waste water. Fig. 10 shows the color
Fig. 9. The dependence of the magnetic susceptibility (vg) on the absolute temperature T(K) for: (a) Zn0.5Co0.5Al0.5Fe1.46La0.04O4, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite, (c) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA core–shell nanocomposite, and (d) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVAc core–shell nanocomposite.
Fig. 10. Color index analysis for inked water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
M.A. Ahmed et al. / Journal of Alloys and Compounds 578 (2013) 121–131
index analysis, which proves that all samples succeeded in absorbing the dye content with different efficiencies in the range 76–90%. The magnetic ferrite sample showed dye absorbance efficiency of 76%, while after being surfactant by the polymeric shells, the efficiency of dye uptake was increased to reach 84%, 86%, 88% and 90% for PVAc, PEG, PVA and PVP surfactants respectively. Better dye uptake due to the presence of different polymers, arises from the various polar sites running along the polymeric chains, which are capable of attracting moisture, and acting as detergent molecules to lift the oily stains and various wastes or dyes. Also, the magnetic and electrical properties of the core–shell nanocomposites are key factors in successful dye uptake. The magnetic effect and the electrostatic potential reproduced by different core–shell nanocomposites cause attraction of the organic dye molecules, which are sometimes polar in nature, and let the ferrite/polymer core–shell nanocomposite acts as a detergent molecule that removes dyes from water. This novel result indicates that such core–shell nanocomposites composed of the chosen polymers and ferrite could be very promising candidates in industrial applications such as purifying and recycling of industrial waste water. 4. Conclusions Polymer-blended magnetic materials in the form of core–shell nanocomposites were obtained through interfacial interactions between PVP, PVA, PVAc and PEG as capping agents and Zn0.5Co0.5Al0.5Fe1.46La0.04O4 as a core. Depending on the polymer type, the polymeric shell supported the magnetism and electrical conductivity of the ferrite. The highest ac conductivity (r 40 10 6 X 1 cm 1) was obtained for Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite, which is much higher than that of pure ferrite (r 10 10 6 X 1 cm 1). The lowest Curie temperature (TC 440 K) was obtained for Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA and PVAc core–shell nanocomposites, which is much lower than that of pure ferrite (TC 703 K). All samples were effective in removing dyes from waste water as proven from the color index analysis. Maximum dye removal efficiency (90%) was shown by Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite, which is also too much better than that of pure ferrite (76%). These novel results indicate that such core–shell nanocomposites could be very promising candidates in industrial applications such as purifying and recycling of industrial waste water, and for those applications demanding better conductivity and lower Curie temperature. All results showed that polymers are good addition to ferrites when blended in the form of core–shell.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effective dye removal and water purification using the electric and magnetic Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposites M.A. Ahmed a,⇑, Rasha M. Khafagy b, Samiha T. Bishay b, N.M. Saleh c a b c
Materials Science Lab (1), Physics Department, Faculty of Science, Cairo University, Giza, Egypt Physics Department, Girls College for Arts, Science and Education, Ain Shams University, Cairo, Egypt Physics Department, Faculty of Science, Western Mountain University, Libya
a r t i c l e
i n f o
Article history: Received 11 October 2012 Received in revised form 25 April 2013 Accepted 26 April 2013 Available online 6 May 2013 Keywords: Water purification and dye uptake Recycling of industrial waste water Ferrite/polymer core–shell nanocomposites AC conductivity Magnetic properties
a b s t r a c t Flash auto combustion method was successfully used to synthesize nanoparticles of Zn0.5Co0.5Al0.5Fe1.46La0.04O4. High resolution transmission electron microscopy (HRTEM) specified the formation of granular nanospheres beside an intermediated phase of nanowires. Polymer-blended magnetic materials were obtained using poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc) and Polyethylene Glycol (PEG) as capping agents. This coating strategy controls the agglomeration of ferrite nanoparticles, and produces a well-designed core–shell nano-assembly with enhanced physical properties. XRD and HRTEM confirmed the formation of ferrite as a core surrounded by various polymeric shells. The nanocomposite with PVP shell resulted in increased ac conductivity (r) of about four orders of magnitude higher than that recorded for the pure ferrite. Curie temperature (TC) decreased from 703 K as recorded for the pure ferrite to less than 440 K for the core–shell nanocomposites containing PVA and PVAc. All prepared samples succeeded in purifying inked-water with high efficiency. Zn0.5Co0.5Al0.5Fe1.46La0.04O4 up-took 76% of the dye content, while the dye-removal efficiency was increased to 90% when Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite was applied. These novel results indicate that such series of core–shell nanocomposites are promising candidates in industrial applications such as purifying and recycling of industrial waste water. Moreover, this study emphasized that polymers are good additive to ferrites when blended in the form of core–shell nanocomposite structure. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to their wide uses in several technologies and applications, spinel ferrites MFe2O4 (where M is often a transition metal atom) are a kind of most important magnetic materials which are still widely investigated [1–4]. Moreover, polymer-blended magnetic materials are now extensively studied because of their high processability, versatility, low cost [5–8], and because of their fascinating physical properties that appear when the polymer is added by any sort to ferrites [9–13] or perovskites [14]. Among recent research articles concerned with polymer-blended magnetic materials, Khafagy [9] found that MgFe2O4/Polyaniline nanocomposite with core–shell structure shows much better conductivity if compared to pure MgFe2O4 nanoparticles. Also, tuning of the total magneto-electrical behavior of this nanocomposite was possible depending on the volume fraction of the magnetic ferrite nanoparticles, and on the contribution of the non-magnetic ⇑ Corresponding author. E-mail address: [email protected] (M.A. Ahmed). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.182
Polyaniline coating layer. Also, Li et al [15] synthesized Cu0.4Zn0.6Cr0.5Sm0.06Fe1.44O4/Polyaniline core–shell nanocomposite that exhibited a hysteresis loop of the ferromagnetic nature, to be used as a soft magnetic material because of its coercivity, which is lower than that of the pure ferrite. Meanwhile, Xiaowei et al. [16] demonstrated a viable strategy for preparing polymer-coated functional metal oxides nanocrystals, which are potentially useful in biological and nanoelectronic applications. In addition, Varshney et al. [8] reported an approach to synthesize nano-stick shaped ferromagnetic iron oxide/polypyrrole, which influenced the properties of polypyrrole, and controlled the morphology of Fe2O3, that lead to form stick-shaped polymer composites decorated with Fe2O3 nanoparticles. This decorating strategy resulted in the formation of conducting/ferromagnetic/polymer nanocomposite with: electrical conductivity of the order of 10 2 S/cm, saturation magnetization of 35 emu/g and microwave absorption as high as SEA 22.5 dB (>99% attenuation) in the frequency range of 12.4–18 GHz (Ku-band). Such results [8,9,13,15,16] prove the fascinating future of polymer-blended magnetic materials, and show that polymers are good additive to ferrites and perovskites.
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Another reason for using the guest–host polymer technique, is that ferrite nanoparticles themselves highly agglomerate, and in the absence of a trapping media or some other form of encapsulation, they coalesce extremely quickly [17–21]. For this reason, bonding of capping agents to ferrite nanoparticles is necessary to provide chemical passivation, and to improve their surface state, which reduces to certain extent the agglomeration of ferrite nanoparticles, and becomes helpful to their decentralization and stabilization [15]. This reduction of agglomeration and maintenance of nano-scale particles, has a significant influence on the optical and electronic properties of ferrite nanoparticles [19] that makes capping agents good idea. Polymers are good choice for this passivation and stabilization, because they can interact with the metal ions of ferrites by complex or ion-pair formation, and can be designed to vary certain physical properties of ferrite nanoparticles. By this incorporation of inorganic particles within the polymeric matrices, homogeneous organic–inorganic hybrid materials with fascinating properties can be obtained. Polymers with 3-dimensional structure, such as poly(vinyl pyrrolidone) (PVP) [22] and poly(vinyl acetate) (PVAc) [23] have rigid pores, that set an upper limit for the growth of ferrite nanoparticles inside such polymeric matrix. One convincing reason for the formation of this 3-dimensional structure, is the crosslinking reactions formed by coordinate bonds between the polymeric chains and the metal ions present in the magnetic ferrite nanoparticles. Moreover, poly(vinyl alcohol) (PVA) usually draws a considerable attention because of the capability of its hydroxyl groups (OH) to retain metal ions through hydrogen bonds (hydrides) leading to a 3-dimensional structure [20]. Also, the ability of PVA and PVAc to crystallize is one of the most important physical properties of these two polymers, as the degree of crystallinity controls the water solubility, and consequently, controls the electrical and magnetic properties of these two polymers [24]. As a result, both poly(vinyl alcohol) and poly(vinyl acetate) have attracted extensive interest in industrial as well as in medical and pharmaceutical applications due to their important combination of: thermal, structural, mechanical, optical, electrical and magnetic properties [25]. Enhancement of PVA and PVAc properties are expected when these two polymers are motivated by the spinel ferrite nanoparticles of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 as an electronic, electrical and magnetic nanofiller [26]. Although there are several papers on the magnetic character of PVA materials [11,27,28], none of them referred to in situ corporation of magnetic nanoparticles within a PVA matrix in the form of core–shell structure, nor with the other chosenhere polymers. Four different polymers have been chosen here to encapsulate the nanoparticles of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 and produce a core–shell design with the magnetic ferrite as a core being capped with the polymeric surfactant shell. Surfactant polymers are: poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), and Polyethylene Glycol (PEG), the properties of which, were intentionally chosen to vary from completely amorphous polymer (PVP), to semi-crystalline polymer (PVA and PVAc), and highly crystalline polymer (PEG). Poly(vinyl pyrrolidone), poly(vinyl alcohol) and poly(vinyl acetate) are related to the ‘‘Vinyl class’’, which has the general structure
, where ‘‘n’’
is the repetition number, while R =
in case of PVP,
R = AOH in case of PVA, and
in case of PVAc.
structure
. The term ‘‘glycol’’ is reserved
for low to medium range molar mass polymer when the nature of the end-group, which is usually a CH3 group, still matters and can result in predominant effects. Reactive polymers including PVP, PVA, PVAc and PEG are expected to be perfectly utilized for core– shell purposes. It is aimed in this research article to get specifically good conductive properties, and tunable magnetic properties from a series of novel electric and magnetic core–shell nanocomposites using Zn0.5Co0.5Al0.5Fe1.46La0.04O4 as a core being surfactant with four different polymeric shells (PVP, PVA, PVAc and PEG). It is aimed also to apply the synthesized samples to some sort of water treatment; such as purifying and recycling of industrial waste water, which is one of the new era applications. Such study might lead to the fabrication of specially tailored materials suitable for applications demanding high electrical conductivity and low Curie temperature. 2. Experimental details 2.1. Preparation of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles were synthesized using the flash auto combustion method [29,30]. Analytical grade (BDH-99.999%) of Fe(NO3)39H2O, Al(NO3)3H2O, La(NO3)39H2O, Co(NO3)2, Zn(NO3)26H2O, and CO(NH)2 were used as starting materials. The stoichiometric ratios of metal nitrates and urea were calculated based on the total oxidizing and reducing coefficients, so that the equivalence ratio is unity and the released energy is maximum [29,30]. All the above chemicals, with their ratios, were perfectly mixed for 90 min to give a homogeneous paste. The beaker containing the paste was inserted on a hot plate until the reaction was completed. After cooling to room temperature, the obtained powder was grinded properly for half an hour in an agate mortar.
2.2. Synthesis of Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymers core–shell nanocomposites Pure grade of PVP (molecular weight 125,000 g mol 1), PVA (molecular weight 115,000 g mol 1), PVAc (molecular weight 100,000 g mol 1) and PEG (molecular weight 40,000 g mol 1) were imported from LoBa Chemie Pvt. Ltd., India. These four polymers were chosen to encapsulate Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles in the form of core–shell design, with the magnetic ferrite as a core, being capped with the polymeric surfactant shell. To obtain Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core– shell nanocomposites [15], 1 g of the ferrite nanoparticles was added to 60 ml of distilled water. In another set of beakers, 1 g of each separate polymer (PVP/PVA/ PVAc/PEG) was dissolved in 30 ml of distilled water. All solutions were stirred thoroughly for 30 min using an electric stirrer. Afterwards, each polymeric solution was added to the corresponding suspended ferrite solution, and the mixtures were then re-stirred for 3 h. The mixtures were dried at 60 °C for 24 h to perform the core– shell structure by four different surfactant polymers capping the same Zn0.5Co0.5Al0.5Fe1.46La0.04O4 core. 2.3. Sample analysis and characterizations All samples were identified by X-ray Philips analytical (X’pert Graphics & Identify) diffractometer. Morphology and particle’s size of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles and core–shell nanocomposites have been investigated using FEI High resolution transmission electron microscope (HRTEM) model Tecnai-G20, super twin, double tilt and gun type LaB6. HRTEM photos were collected at applied voltage of 200 kV and magnification up to 1,000,000 after samples have been dispersed in a distilled water for 10 min. The dc magnetic susceptibility measurements were performed using Faraday’s method from room temperature up to 850 K at three different magnetic field intensities, where a very small amount of the nanopowdered sample was inserted in a cylindrical Pyrex glass tube at the point of maximum gradient. The ac conductivity (r) have been measured over a wide range of temperatures (300 K 6 T 6 700 K) and frequencies (10 and 100 kHz) using RLC Hioki bridge model 3532.
2.4. Application of prepared samples in water purification and de-inking
On the other side, Polyethylene Glycol is related to ‘‘Polyether’s’’ class which has the general formula
, and linear
A standard solution consisting of 100 mL of distilled water plus 0.75 mL of standard ink was prepared. Five tubes were filled by 7 ml of the standard solution in addition to 0.02 g of the prepared samples. The mixture was then mixed using an electric stirrer for 15 min. Color index analysis was carried out for the solutions after 10 h to insure dye removal.
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present in the structure of PVAc has a role in increasing the carbon backbones disturbance more than the (OH) group of PVA do, thus resulting in slightly more crystalline phases in PVAc as shown in its XRD pattern (Fig. 1c). 3.1.3. X-ray diffraction of Polyethylene Glycol (PEG) PEG is a highly crystalline, ordered and polar polymer, that belongs to the ether’s family. High crystallinity of PEG results from the alignment of oxygen atoms in the gauche configuration, with the intermolecular dipole forces oriented along the axis of the helix [7]. These dipolar groups are assigned to monoclinic crystal system [7], which allowed the determination of PEG crystal structure as identified by ICDD card [49-2097], and as confirmed by the sharpness of the crystalline XRD peaks appearing in Fig. 1d. The diffraction pattern of the monoclinic unit cell of PEG contains four molecular chains, in which a = 0.796 nm, b = 1.311 nm, and c = 1.939 nm [20]. The HRTEM photo shown in Fig. 1e illustrates the highly ordered lattice of the PEG shell with d-spacing value of (4.84 Å) as calculated using the HRTEM attached software. This value shows good accordance with the d-spacing estimated from the PEG ICDD card (49-2097) (d-spacing = 4.65 Å).
Fig. 1. X-ray diffraction of pure polymers.
3. Results and discussion 3.1. X-ray characterization 3.1.1. X-ray diffraction of poly(vinyl pyrrolidone) (PVP) Fig. 1a shows the XRD pattern of PVP, which proves the amorphous nature of this polymer. Two broad diffraction peaks extending between 10–15° and 15–24° were detected, which might be associated with two chain’s length and orientation, thus resulting in the presence of two different amorphous phases of PVP, as consistent with literature [31].
3.1.2. X-ray Diffraction of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) Both PVA and PVAc are semi-crystalline polymers as indicated from their XRD patterns illustrated in Fig. 1b and c. The crystalline nature of these two polymers is emphasized by the diffraction peaks at 2h = 19.8°, 41.05° assigned for PVA and at 2h = 19.54°, 40.54° assigned for PVAc, with a hallow shoulder at 2h = 23° representing the amorphous phase in both of them [24]. This similarity in XRD patterns of these two polymer arises from the fact that both polymers can be easily hydrolyzed to each others, as they are parent materials to one another. On a molecular level, the crystalline nature of PVA results from the strong intermolecular interactions between layered PVA chains through hydrogen bonding. Meanwhile, weaker Van Der Waal’s forces operate between double layers. This folded chain structure leads to small ordered regions (crystallites) scattered in unordered amorphous domains [24] as indicated by the XRD pattern shown in Fig. 1b. Moreover, PVA has slightly higher crystallinity than PVAc due to its structure enriched with hydroxyl (OH) groups, which are small enough to fit into the lattice without disrupting the carbon backbones chains. On the other hand, the function group
3.1.4. X-ray Diffraction of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 Phase formation of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 was proven by XRD, that indicated the absence of secondary phases as identified from ICDD card [73-1963]. Fig. 2 shows the XRD pattern for Zn0.5Co0.5Al0.5Fe1.46La0.04O4, which consists of well-resolved broad peaks, that confirms the mono-phasic nature and the nanometric crystallite size of the prepared nano-ferrite. All main peaks are related to the spinel structure of ferrites (AB2O4), which is known to have a cubic symmetry with a face-centered lattice and space group Fd3m (227). XRD peaks are well-indexed to the spinel crystallographic planes at (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3), but the broadening of some essential XRD peaks superimpose some other spinel peaks [32]. For example, the broad peak at 2h = 37° belonging to the plane (3 1 1), superimposes the neighboring peak at 2h = 38° which corresponds to the plane (2 2 2) as identified from ICDD card [73-1963] [32]. 3.1.5. X-ray diffraction of core–shell nanocomposites Fig. 3a–e enables clear comparison between the structure and complexation of the different polymeric shells with the same ferrite core through vertically separated XRD patterns ((a) the polymer, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4, and (c) Zn0.5Co0.5Al0.5Fe1.46 La0.04O4/polymer core–shell nanocomposite). Peak’s intensities of all polymers are suppressed with different ratios and proportionalities in the XRD spectra of the core–shell nanocomposites, which means decreased polymer’s crystallinity. The presence of ferrite nanoparticles during synthesizing the core–shell nanocomposites reduces the intermolecular interactions between the polymeric chains and prevent them from extending. Consequently, this results in decreasing the intensity of XRD peaks which represent the crystalline portions of the polymeric sample [9,13]. The XRD spectra of ferrite/polymer core–shell nanocomposites show that the positions of all diffraction peaks are just the superposition of the peaks for polymer and Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nanoparticles. The diffraction patterns of PVP (Fig. 3a), and PVAc (Fig. 3b) are the most inhibited in the core–shell nanocomposite, while PVA (Fig. 3c) and PEG (Fig. 3d) maintained their fundamental diffraction peaks with obvious intensities, but they are still suppressed if compared to the XRD pattern for the parent polymer. XRD characterization proved the formation of ferrite/polymer nanocomposite in all prepared samples through interfacial interactions between the ferrite core and the polymeric shell. This
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Fig. 1e. HRTEM photo representing the ordered lattice of the square PEG shell in the Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PEG core–shell nanocomposite.
could be noticed by broadening of the lines or by vanishing of some lines with low intensities. Fig. 4 shows a schematic representation of an assumed interaction mechanism between the PVP polymer as a capping agent [26] and the metal ion precursors, where the divalent ions (such as zinc and cobalt) and trivalent ions (such as iron) are bounded by strong ionic bonds between the metallic ions and the amide group inside the polymeric chains or between the chains themselves. This uniform immobilization of metallic ions in the cavities of the polymeric chains favors the formation of a uniformly-distributed solid solution of the metallic oxides inside the polymer.
3.2. High resolution transmission electron microscopy (HRTEM)
Fig. 2. XRD pattern for Zn0.5Co0.5Al0.5Fe1.46La0.04O4.
is evident from the appearance of all the diffraction peaks belonging to the core and the shell in the XRD patterns of the core–shell nanocomposites. The strongest chemical interaction appeared between PVA and ferrite nanoparticles (Fig. 3c) due to the capabilities of hydroxyl (OH) groups to retain metal ions through hydrogen bonds (hydrides) leading to a 3-dimensional structure [20]. Weaker interaction between the core and the shell is obtained for PEG (Fig. 3d) due to the presence of oxygen atoms, which might maintain hydrogen bonding with metal ions during preparation of core– shell in the aqua’s polymeric solution. In general, the embedding
3.2.1. HRTEM of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 The HRTEM image shown in Fig. 5a proves that flash auto combustion method was a successful and easy method for obtaining large quantities of nanowire-bundle filaments with average lengths of 150 nm and narrow outer diameters with average value of 22 nm. Such nanowires are characterized by their large surface area, which might result in additional behavior which has never been obtained within traditional bulk materials, or nanospheres. In addition to nanowires, large quantities of nanospheres with average size of 30 nm are present. Fig. 5b illustrates the lattice of a single ferrite nanoparticle, with measured d-spacing of 4.7 Å and 2.9 Å, which is consistent with the values estimated from ICDD card (73-1963) that indicates d-spacing of 4.8 Å for the plane (1 1 1) at 2h = 18.38°, and d-spacing of 2.95 Å for the plane (2 2 0) at 2h = 30.24°.
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Fig. 3. Vertically separated XRD patterns of: (i) polymer, (ii) Zn0.5Co0.5Al0.5Fe1.46La0.04O4, and (iii) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposite; (a) PVP, (b) PVAc, (c) PVA and (d) PEG.
incorporated within a polymeric shell with perfect well-designed core–shell nano-assembly. The apparent contrast between the inner and outer portions of the particulate is based on the average atomic weight of the core and the shell. Spherical particles exhibited an expected agglomeration because of the magnetic dipole interactions between magnetic ferrite nanoparticles. in all core– shells, the ferrite core has an average diameter of 18 nm. The average particulate size of ferrite/PVP core–shell nanocomposite is 24 nm, with a spherical shell of thickness 6.6 nm (Fig. 6a). Both ferrite/PVA and ferrite/PVAc core–shell nanoparticles (Fig. 6b and c), have an average diameter of 21 nm, and a spherical shell of thickness 3 nm only. It is noticed that ferrite/PEG core–shell nanocomposite (Fig. 6d) resulted in a unique square shell, with highest average thickness of about 10 nm, leading to a higher than usual core–shell diameter of 41 nm.
3.3. Electrical properties Fig. 4. A schematic representation of an assumed interaction mechanism between the PVP polymer as a capping agent and the metal ion precursors.
3.2.2. HRTEM of core–shell nanocomposites Fig. 6a–d illustrates HRTEM photos for all core–shell nanocomposites. It is clear that abundance of ferrite nanoparticles are
3.3.1. Dependence of room-temperature ac conductivity on the polymer type Fig. 7 illustrates the dependence of ambient ac conductivity (r) on the polymeric surfactant type as compared to that of Zn0.5Co0.5Al0.5Fe1.46La0.04O4. Among all surfactant polymers, ferrite/PVP
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Fig. 5a. HRTEM image for the sample Zn0.5Co0.5Al0.5Fe1.46La0.04O4.
Fig. 5b. HRTEM image representing the lattice of a single ferrite nanoparticle, with a measured d-spacings of 4.7 Å and 2.9 Å.
core–shell nanocomposite resulted in the highest ac conductivity value (r 40 10 6 X 1 cm 1) which is much higher than that of pure ferrite (r 10 10 6 X 1 cm 1). This means that the presence of PVP shell supported the ac conductivity of the ferrite and became an advantage for the electrical properties of such material. Although PVP is an amorphous polymer and is expected to show
the lowest conductivity; such promising result could be correlated to the fact that PVP is one of the well known conductive polymers with aromatic ring in its structure. Hence it contributed to the final conductivity of the nanocomposite. Aromatic rings are known to play an essential role in controlling the efficiency of other conducting polymers such as Polyaniline and PolyPayroll [9,13].
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Fig. 6. HRTEM micrographs for the samples Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposites; where: (a) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA core–shell, (c) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVAc core–shell, and (d) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PEG core–shell.
3.3.2. Temperature dependence of ac conductivity of the core–shell nanocomposites The temperature-dependent ac conductivity (r) of core–shell nanocomposites are quite interesting, and differs from one surfactant polymer to another. It can be observed from Fig. 8a that ‘‘r’’ of ferrite/PVP core–shell nanocomposite, increases up to 330 K, and afterwards it decreases with increasing temperature. In case of ferrite/PVA core–shell nanocomposite, ‘‘r’’ increases up to 315 K and afterwards it decreases with increasing temperature up to 350 K, followed by continuous increase of ‘‘r’’ with a maximum peak at 410 K (Fig. 8b). Ac conductivity of ferrite/PVAc core–shell showed two maxima peaking at 350 K and 420 K
(Fig. 8c), while ferrite/PEG core–shell (Fig. 8d) showed continuous increase of ‘‘r’’ with temperature up to 350 K, after which ‘‘r’’ decreases with further increase of temperature. Such temperature influence is new for ac conductivity measurements, especially, regarding the existence of two different transition temperatures for the same material. Since the conductivity of polymers is primarily dependent on the ordered arrangement of polymeric chains in the crystalline domains [33], any factor that disrupts such arrangement can influence the polymer’s conductivity. However, the presence of water in the polymeric matrices plays an important role in the crystalline arrangement, and correlates the ac conductivity behavior.
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Fig. 6. (continued)
Fig. 8e presents a closer look to the behavior of ac conductivity at fixed frequency of 100 kHz as shown by the different polymers in the core–shell nanocomposites. One can clearly notice that all polymers show an anomalous behavior of ‘‘r’’ within the temperature range 310–380 K at which PVP, PVAc and PEG showed enhanced conductivity with obvious peaks, while PVA resulted in a deteriorated ac conductivity with an obvious minimum. This
behavior is correlated to the dehydration and elimination of water content upon heating the core–shell pellets during performing ac conductivity measurements. Water present inside polymers are known to be found either in a free associated state (free water), or weakly bounded to the polymer (intermediate state) or strongly bounded to the polar groups of the polymer (bounded water) [24]. Gradual heating of the samples eliminates the free water content,
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Fig. 7. The dependence of room temperature ac conductivity at 10 and 100 kHz on the polymer type used in the core–shell nanocomposites as compared to that of Zn0.5Co0.5Al0.5Fe1.46La0.04O4.
which directly affects the crystalline arrangement of the polymeric chains, and results in the evolution of the polar OH groups. Liberated OH groups act as intrinsic sites for charge transfer, and directly enhance the ac conductivity of: PVP, PVAc and PEG as the number of free OH groups increase, after which ‘‘r’’ decreases again. At 380 K all water molecules are liberated from the
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Fig. 8e. Behavior of ac conductivity shown by the different polymers in the core– shell nanocomposites at fixed frequency of 100 kHz.
polymers, then new conduction mechanism starts to appear depending on the nature of polymer type rather than the humidity content. This conclusion is supported with the fact that most polymers undergo more than one degradation step due to dehydration and elimination of water, then cleavage of the main and side chains (degradation of bonds in the skeleton structure), afterwards, dissociations of organic residuals such as CO2, CH4 and other carbonic residues take place [24].
Fig. 8. The temperature dependence of ac conductivity of the core–shell nanocomposites at 10 and 100 kHz; (a) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA core–shell, (c) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVAc core–shell, and (d) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PEG core–shell.
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In contradiction to other polymers, PVA is the only polymer which showed reduced ac conductivity within this temperature range (310–380 K). This is due to the large content of bounded water in its structure, as it is the only polymer enriched with strongly bounded polar OH groups. Upon heating PVA, it undergoes degradation reaction to get rid of this large content of bounded water [24] which requires high energy to be eliminated through an endothermic reaction to overcome the strong H-bonding formed with water. The tendency of hydrophobic groups to cluster, leads to a threedimensional network. This can be viewed as pseudo-crosslinks resulting in an opposing effect, and results in decreasing ‘‘r’’ during consuming heat in breaking the strong H-bonding between PVA
chains and water molecules, due to the tight nature of OH groups in this polymer [34]. After going beyond 380 K, each polymer started to show another conductivity behavior depending on its nature and structure. In case of PVP surfactant ‘‘r’’ decreased dramatically due to the amorphous nature of this polymer. Both PVA and PVAc surfactants showed continuous increase in ac conductivity due to the immobilization of extraneous ions. As the melting temperature of these two polymers is approached, ‘‘r’’ reached maximum values of 1.7 10 5 X 1 cm 1 and 6.5 10 5 X 1 cm 1 at 410 K and 430 K for PVA and PVAc respectively [18,19,34]. Reduced conductivity of PVA is due to the fact that elimination of bounded water from PVA chains results in main chain cleavage and destruction of the polymer’s crystallinity, which affects its ac conductivity at higher temperatures. After reaching the temperatures corresponding to the maximum ac conductivity of all polymers, ‘‘r’’ decreased again due to the cleavage of the main and side chains of the polymers (degradation of bonds in the skeleton structure) and thereafter, dissociations of organic residuals occurs, and the polymers decompose and collapse [22]. PEG was expected to result in highest conductivity, due to its extremely high crystallinity, and the presence of polar ether groups which are sufficiently polar to attract moisture and to provide charge-transfer sites, but the fast melting of this polymer (360 K) is a great constrain [22,23]. 3.4. Magnetic properties Fig. 9a–d shows the dependence of the magnetic susceptibility (vg) on the absolute temperature T(K) for Zn0.5Co0.5Al0.5Fe1.46La0.04O4/polymer core–shell nanocomposites, as compared to pure ferrite. The Curie temperature (TC) of Zn0.5Co0.5Al0.5Fe1.46La0.04O4 was found to be around 703 K. A notable decrease of about 263 K was obtained in the Curie temperature of ferrite/PVA and ferrite/PVAc core–shell nanocomposites to become around 440 K. PVA and PVAc are known to show magnetic behavior [22], thus they supported the magnetization showed by the nano-ferrite, resulting in an earlier TC value. 3.5. Application of prepared samples in water purification and deinking Zn0.5Co0.5Al0.5Fe1.46La0.04O4 nano-ferrite and the four core–shell nanocomposite samples have been investigated regarding their ability to remove dyes from waste water. Fig. 10 shows the color
Fig. 9. The dependence of the magnetic susceptibility (vg) on the absolute temperature T(K) for: (a) Zn0.5Co0.5Al0.5Fe1.46La0.04O4, (b) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite, (c) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA core–shell nanocomposite, and (d) Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVAc core–shell nanocomposite.
Fig. 10. Color index analysis for inked water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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index analysis, which proves that all samples succeeded in absorbing the dye content with different efficiencies in the range 76–90%. The magnetic ferrite sample showed dye absorbance efficiency of 76%, while after being surfactant by the polymeric shells, the efficiency of dye uptake was increased to reach 84%, 86%, 88% and 90% for PVAc, PEG, PVA and PVP surfactants respectively. Better dye uptake due to the presence of different polymers, arises from the various polar sites running along the polymeric chains, which are capable of attracting moisture, and acting as detergent molecules to lift the oily stains and various wastes or dyes. Also, the magnetic and electrical properties of the core–shell nanocomposites are key factors in successful dye uptake. The magnetic effect and the electrostatic potential reproduced by different core–shell nanocomposites cause attraction of the organic dye molecules, which are sometimes polar in nature, and let the ferrite/polymer core–shell nanocomposite acts as a detergent molecule that removes dyes from water. This novel result indicates that such core–shell nanocomposites composed of the chosen polymers and ferrite could be very promising candidates in industrial applications such as purifying and recycling of industrial waste water. 4. Conclusions Polymer-blended magnetic materials in the form of core–shell nanocomposites were obtained through interfacial interactions between PVP, PVA, PVAc and PEG as capping agents and Zn0.5Co0.5Al0.5Fe1.46La0.04O4 as a core. Depending on the polymer type, the polymeric shell supported the magnetism and electrical conductivity of the ferrite. The highest ac conductivity (r 40 10 6 X 1 cm 1) was obtained for Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite, which is much higher than that of pure ferrite (r 10 10 6 X 1 cm 1). The lowest Curie temperature (TC 440 K) was obtained for Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVA and PVAc core–shell nanocomposites, which is much lower than that of pure ferrite (TC 703 K). All samples were effective in removing dyes from waste water as proven from the color index analysis. Maximum dye removal efficiency (90%) was shown by Zn0.5Co0.5Al0.5Fe1.46La0.04O4/PVP core–shell nanocomposite, which is also too much better than that of pure ferrite (76%). These novel results indicate that such core–shell nanocomposites could be very promising candidates in industrial applications such as purifying and recycling of industrial waste water, and for those applications demanding better conductivity and lower Curie temperature. All results showed that polymers are good addition to ferrites when blended in the form of core–shell.
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