Polyaniline–Zn0.2Mn0.8 Fe2O4 ferrite core–shell composite: Preparation, characterization and properties

Journal of Alloys and Compounds 608 (2014) 283–291

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Polyaniline–Zn0.2Mn0.8 Fe2O4 ferrite core–shell composite: Preparation, characterization and properties M. Khairy ⇑ Chemistry Department, Faculty of Science, Benha University, Benha, Egypt

a r t i c l e

i n f o

Article history: Received 10 January 2014 Received in revised form 11 April 2014 Accepted 19 April 2014 Available online 29 April 2014 Keywords: Composite materials Electrical properties Thermal analysis Dye adsorption Gas sensors

a b s t r a c t Zn0.2Mn0.8Fe2O4 ferrite nanoparticles coated with polyaniline, forming a composite structure, were synthesized by in situ chemical oxidation polymerization at different ferrite weight ratio (PANI/ZnMn ferrite = 30% and 50%). The ZnMn ferrite nanoparticles were prepared by the sol–gel combustion method using metal nitrates as precursors. The structure, morphology, electrical, gas sensing and magnetic property of the ferrite and composite samples were characterized by XRD, FT-IR, TEM, TGA, VSM, LCR instrument and sensing measurements. The results of XRD, FTIR and UV–vis spectra confirmed the formation of ZnMn ferrite/PANI composites. TEM study showed that the composites exhibited the core–shell structure. The results of TGA showed that ZnMn ferrite particles improved the thermal stability of composites indicated the existence of some interactions at the interface of polyaniline macromolecule and ZnMn particle, which influences the physical and chemical properties of the composite. The nanocomposites under applied magnetic field exhibited the hysteresis loops of superparamagnetic nature at room temperature. The ac conductivity and dielectric properties were studied in the frequency range from 102 to 107 Hz. The electronic and magnetic properties of the nanocomposites were tailored by controlling the ferrite content. The adsorption efficiency of the materials to remove toxic dye from waste water was also tested at different pH and it was found that the composite containing 50 wt% ZnMnFe2O4 exhibit excellent adsorption at pH 8. The nanocomposites have been also investigated as sensors for each of NH3, chloroform and acetone vapors in ppm level concentration. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, the conducting polymer nanocomposites have been considered as a new class of materials due to their improved properties when compared with those of pure conducting polymer [1–5]. Of all the conducing polymers, polyaniline (PANI) has been extensively studied because of its ease of synthesis and environmental stability and applications in several fields, such as in electrochemical displays, sensors, catalysis, redox capacitors, electromagnetic shielding, as well as in secondary batteries [2–7]. Polyaniline has a variety of oxidation states and three different states of them are usually referred to in literature [8]: leucoemeraldine base, emeraldine base (EB) and pemigraniline base. Emeraldine base is the most attractive one because it can be doped with protonic acid to become emeraldine salt (ES) and the conductivity of the ES is increased. However, conducting PANI is difficult to be processed through traditional methods as the most conducting polymers. In order to improve the problem of polymer process ability and thus enlarge the applications of the conducting ⇑ Tel.: +20 1270405481; fax: +20 133222578. E-mail address: [email protected] http://dx.doi.org/10.1016/j.jallcom.2014.04.130 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

polymers, the incorporation of inorganic fillers such as ferrites into PANI matrix to form composites has been used. The ferrites of the spinel type are currently key materials for advancements in electronics, magnetic storage, ferro-fluid technology, and many bioinspired applications such as drugs carriers for magnetically guided drug delivery and as contrast agents in magnetic resonance imaging [9,10]. Manganese zinc ferrite spinel considered as one of technologically important materials because of their high permeability and low dielectric loss. Therefore, conjugated polymers combined with magnetic spinel ferrite (Zn0.2Mn0.8Fe2O4) nanoparticles to form ferromagnetic nanocomposites provide an exciting system to investigate the possibility of exhibiting novel functionality. One of this functionality is the adsorption of pollutant, which is considered to be superior to other techniques because of low cost, simplicity of design, availability and ability to treat dyes [11]. Suitable nanoparticles for magnetically assisted separation are super paramagnetic nanoparticles. The application of a magnetic field of low intensity induces the magnetization of the material and thus makes the use of a magnetic force possible, but when the magnetic field is cut of, the magnetization immediately decreases to zero. This last point is important for the release of particles after adsorption of the waste [12].

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The present study involves the synthesis and characterization of PANI/ Zn0.2Mn0.8Fe2O4 nano-composites (30, 50 wt% ferrite) by in situ polymerization of aniline with Zn0.2Mn0.8Fe2O4 nanoparticles and studying their electrical, thermal, magnetic, sensing properties toward NH3, chloroform and acetone vapor as well as their adsorption properties for acid red dye. 2. Experimental 2.1. Materials

The magnetic measurements have been performed at room temperature using magnetometer (VSM) model (VSM-9600-1 LDJ, USA) with a maximum magnetic field of 10 KOe. For electrical measurements the samples are crushed into fine powder in an agate mortar in the presence of acetone. The powder is pressed to form pellets of 7 mm diameter and thickness which varies from 1.5 to 2 mm. The electrical measurements on these samples were made using the silver paint as electrodes on both sides. The dielectric constant and dielectric loss were determined by Agilent Technologies 4285A Precision LCR meter at room temperature in the frequency range from 102 to 107 Hz. The value of dielectric constant (e0 ) was calculated using the formula:

e0 ¼ Ct=e0 A

Aniline (ANI), ammonium peroxydisulfate (NH4)2S2O8 (APS), absolute ethanol, diethanolamine, Mn(NO3)24H2O, Fe(NO3)39H2O, and Zn(NO3)26H2O have all been procured from Merck. Acid red 88 (AR88), C20H13N2O4SNa, a textile dye that absorbs in the visible region (kmax = 505 nm), obtained from Sigma–Aldrich. The monomers aniline has been purified by distillation in vacuum before use while all other chemicals have been used as such. 2.2. Preparation of Zn0.2Mn0.8Fe2O4 nanoparticles The Zn0.2Mn0.8Fe2O4 nanoparticles were prepared by the sol–gel combustion method using Mn(NO3)24H2O, Zn(NO3)26H2O, and Fe(NO3)39H2O as starting materials. Stoichiometric quantities of metal nitrate were dissolved in absolute ethanol to form 0.25 M precursor solution. Diethanolamine were added as a fuel into the alcoholic solution. The molar ratio of metal nitrates to diethanolamine was 1:1.5. The mixed solution was constantly stirred at 200 °C using a magnetic stirrer to transforming it into a dried gel that auto-combusted to form dark brown powder. The powder was gradually heated to 1000 °C at the rate of 20 °C and kept at this temperature for 5 h. The sample will be denoted as ZnMn ferrite

ð3Þ 2

where e0 is the electrical constant equal to 8.854  10 pF/cm; C is the capacitance of the specimen; t = thickness of the specimen in cm and; A is the area of the specimen in sq. cm. The complex dielectric constant e00 of the samples was calculated using the relation:

e00 ¼ e0 tan d

ð4Þ

where tan d is the dielectric loss tangent. The sensing performance of the materials was tested by subjecting the material pellets to the vapors of NH3 as well as two volatile organic compounds: acetone and chloroform vapors at room temperature. Hexane was used as diluents, for preparing various analyst concentrations. The dc electrical resistance of the materials exposed to analyst vapor was measured by hanging the pellet in a closed glass tube containing the organic compound. The sensitivity, S, was calculated using the relation:

S ¼ DR=Ra ¼ ðRa  Rg Þ=Ra

ð5Þ

where Ra is the dc-resistance in air and Rg is the dc-resistance of the material after its exposure to the test gas.

2.3. Synthesis of PANI and PANI/ZnMn ferrite nanocomposites

2.5. Adsorption experiments

The polyaniline–ZnMn nanocomposites containing 30 and 50 wt% ferrite were prepared as follows. 0.93 g of aniline was dissolved in 300 ml distilled water containing 10 ml 1 M HCl solution. The mixed solution was pre-cooled at 0 °C in an ice bath. Appropriate amounts of the Zn0.2Mn0.8Fe2O4 powder were added to the above pre-cooled mixed solution with continuous stirring. Then, 11.5 ml of 1 M aqueous solution of ammonium peroxydisulfate was slowly added into the above reaction mixture. During the synthesis, the mixture was stirred at about 400– 500 rpm for 16 h and the temperature was kept at 0 °C. Finally, PANI-coated ferrite nanoparticles were separated on a filter, and then washed repeatedly with a large amount of de-ionized water and ethanol until the filtrate was colorless. The wet PANI-coated particles were then dried in a vacuum oven at 80 °C for 24 h. PANI/ ZnMn3 and PANI/ZnMn5 will be denoted for the composites containing 30 and 50 wt% ferrite, respectively. For comparison purposes, PANI was also prepared as follows: 1.86 g of aniline was injected to 20.24 ml of 1 M HCl. 22 ml of 1 M aqueous solution of the ammonium peroxydisulfate was dropped into the solution with constant magnetic stirring. The polymerization was processed for 16 h at 0 °C. The product was filtered, washed and dried at the same condition to obtain fine dark green powders.

The adsorption properties of the synthesized materials in removing toxic textile Acid red 88 (AR88) were studied. For each experimental run, 50 ml of dye with known concentration, pH and the known amount of the adsorbent were taken in a 100 ml stoppard conical flask. This mixture was agitated in a temperature controller by shaking water bath at a constant speed of 150 revolutions per minutes (rpm) at 30 ± 1 °C. Samples were withdrawn at appropriate time intervals and supernatant liquid portions centrifuged at 5000 rpm for 20 min and analyzed for the remaining dye concentration spectrophotometrically at a wavelength maxima (kmax) of 505 nm using UV-spectrophotometer (Schimadzu, 2501 pc). Experiments were carried out at initial pH values ranging from 3 to 12; those controlled via addition of dilute HCl or NaOH solutions. The percentage of dye removal and the equilibrium adsorption uptake, qe(mg/g), was calculated using the following relationships:

2.4. Characterization The XRD patterns of the samples were identified by X-ray (Rijaku Rint 700 and Bruker axs D8, Germany using Cu Ka radiation, k = 1.79 Å) diffractometer in the range of 2h = 10–80o, and the average crystallite size was computed by applying Scherrer’s formula [13]. The Fourier transform infrared spectroscopy (FTIR) spectra of the composite samples in KBr pallets were obtained using a Brucker-FTIR (vector 22) in the range of 400–4000 cm1. The particle size and the morphology of Zn0.2Mn0.8Fe2O4 ferrite and polymer composites have been examined using a transmission electron microscopy TEM (JEOL JEM-2010). UV–vis absorption spectra were recorded on a Shimadzu UV-2501PC spectrophotometer in the range of 300–800 nm. N,N-dimethylformamide was used as a solvent to prepare the sample solution. Thermogravimetric analysis (TGA) measurements were carried out using a Shimadzu TGA-50 thermogravimetric analyzer, at a heating rate of 10 °C/min in nitrogen atmosphere with a flow rate of 40 ml/min. The sample porosity of PANI, ferrite and composites powders compacting in a disc shape was calculated using the relation:

q ¼ 1  ðd=dx Þ

ð1Þ

The bulk density d, was evaluated from the sample’s weight and from its dimensions, dx is the theoretical density. The specific surface area (Sspec) was determined using the equation [14]:

Sspec ¼ s=ðv xdÞ where s and

v are the particle surface and volume respectively.

ð2Þ

Percentage removal ¼ 100ðC o  CÞ=C o

ð6Þ

Amount adsorbent qe ¼ ðC o  C e ÞV=Wðmg of adsorbate=g of adsorbedÞ

ð7Þ

where Co is the initial sorbate concentration (mg/L), Ce is the equilibrium sorbate concentration (mg/L); V is the volume of the solution (l) and W is the mass of the adsorbent (g).

Acid red 88 . Blank runs, with only the adsorbent in 50 ml of double-distilled water, were conducted simultaneously at similar conditions to account for any color leached by the adsorbent and adsorbed by glass containers.

3. Results and discussion 3.1. Characterization X-ray diffraction patterns of the investigated samples are presented in Fig. 1. The XRD pattern of ZnMn ferrite (Fig. 1b) shows diffraction peaks at 2h values 30.11°, 35.31°, 43.19°, 53.51°, 57.14°, 62.11° ascribed to the reflections planes of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) respectively of the spinel crystal

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θ Fig. 1. XRD for: (a) PANI, (b) ZnMn ferrite, (c) PANI/ZnMn5 composite and (d) PANI/ ZnMn3 composite. Fig. 2. FT-IR spectra of: (a) PANI, (b) PANI/ZnMn3 composite, (c) PANI/ZnMn5 composite and (d) ZnMn ferrite.

structure. There is no additional peak for other compositions, indicating that pure ZnMn ferrite crystallizes solely in single-phase cubic structure with Fd-3 m space group [15]. The crystallite size obtained is estimated from the most intense (3 1 1) reflection peak using the Scherrer formula [13]: DXRD = 0.9k/bcos h, where DXRD is the average crystalline size, k is the wavelength of Cu Ka, b is the full width at half maximum (FWHM) of the diffraction peaks, and h is the Bragg’s angle. The calculated grain size is found to be 45 nm. The XRD pattern of PANI (Fig. 1a) shows that it achieves partial crystalline structure and indicates two broad peaks at 2h = 20.41° and 25.61° due to the densely packed phenyl rings those exhibit an extensive interchain p–p orbital overlap [16–18]. The estimated mean size of the later calculated using the Scherrer formula was found to be 20 nm. Fig. 1(c and d) also shows the XRD patterns of the PINI/ZnMn ferrite composites upon varying the composition ratio of ferrite from 30% to 50%. It can be seen that the two composite samples contain both the diffraction peaks of ferrite and the broad peaks of PANI with somewhat change in the intensity. The grain sizes of ZnMn ferrite in the composites estimated from the most intense (3 1 1) reflection peak were found to be 50 and 61 nm for 30 and 50 wt% ferrite, respectively (Table 1). On the other hand, a decrease in ferrite crystallinity due to increasing the amount of aniline was observed. Fig. 2 shows the FT-IR spectra of the investigated samples. The main characteristic bands of polyaniline (Fig. 2a) are assigned as follows: the bands located at 1558 and 1461 cm1 are the characteristic of C@N and C@C stretching of the quinoid and benzenoid rings while the bands observed at 1291 and 789 cm1 may be assigned respectively, to the CAN stretching of the secondary aromatic amine and the aromatic CAH out of plane bending vibration [18]. In the region 1010–1170 cm1, aromatic CAH in-plane bending mode was observed. The peak at 1189 cm1 is ascribed as the electronic-like band, which is associated with the vibration mode of N@Q@N (Q refers to the quinonic-type rings) [19,20]. Fig. 2d shows an absorption band at 430 cm1 attributed to an octahedral stretching vibration of MAO and a peak at 580 cm1 due to the tetrahedral MAO stretching vibration of the ZnMn ferrite spinel

Table 1 XRD, TEM data and magnetic parameters of ZnMn ferrite, PANI and PANI/ZnMn ferrite nanocomposites. Samples

Particle size DXRD (nm)

Particle size DTEM (nm)

Ms (emu/g)

Hc (Oe)

PANI ZnMn ferrite PANI/ZnMn3 composite PANI/ZnMn5 composite

20 45 50 61

20 40 55 70

0.04 104 78.0 48.4

0.0 1.0 5.5 7.4

structure [21]. In Fig. 2b and c, all the characteristic peaks of ferrite and PANI are observed in the PANI/ZnMn ferrite composites. Comparing with Fig. 2a, the peaks at 1558, 1461, 1291 and 580 cm1 show a shift to lower wave numbers. Moreover, with an increase in the PANI content, the intensity of the bands corresponding to PANI characteristics increases distinctively, where the intensity of peaks at 470 and 580 cm1 corresponding to ZnMn ferrite was greatly diminished. These results reveal that there exists an interaction between ferrite particles and PANI chains. This is produced from the the r–p interaction between ferrite and PANI, which includes (1) the p molecular orbital of PANI overlaps the empty d-orbital of metal ions to form the r-bond where metallic empty d-orbital is the electron pair acceptor; (2) the p* molecular orbital of PANI overlaps the d-orbital of metal ions to form the p-bond, in which the metal ions is the electron pair donor. In addition, the hydrogen bonding interaction between the polyaniline chains and the oxygen atoms on the ferrite surface occurs in the composites, which make ferrite particles be embedded into polymer chain of PANI [18,22]. TEM micrographs were used to evaluate the morphology of ZnMn ferrite, PANI and PANI/ZnMn ferrite nanocomposites. The images obtained are shown in Fig. 3. Fig. 3b shows that ZnMn ferrite particles are spherical, and agglomerated to some extent because of magnetic dipole interactions between ferrite particles and the high surface energy of the nanoparticles. The average particle size is found to be 40 nm. TEM images of the composite samples (Fig. 3c and d) reveals that the ferrite particles are embedded in PANI matrix forming the core–shell structure. The black core is the magnetic ferrite particles, and the light colored shell is PANI in the composite, due to the different electron penetrability. The size of the ferrite particle in the composites are approximately 55 and 70 nm for composites with 30 wt% and 50 wt% ferrite content, respectively (Table 1). The difference between the crystallite sizes estimated by the Scherrer’s formula and that found by TEM images is mainly attributed to the different approach of the two techniques. In XRD, the estimated crystal size of the crystals and the accuracy of the Scherrer’s equation are affected by many factors such as diffraction line width, defects and surface tension, so the Scherrer’s formula may induce some errors in measuring the absolute value of the crystallite size [23]. Thermal studies of PANI conducting polymers are particularly important when it is considered to use elevated temperatures to process PANI and its blends into technologically useful forms. In order to see the effect of temperature on the thermal behavior of the studied materials, thermogravimetric analysis has been carried out from 25 to 700 °C. The decomposition temperature obtained

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Fig. 4. TGA of: (a) PANI, (b) PANI/ZnMn3 composite, (c) PANI/ZnMn5 composite and (d) ZnMn ferrite.

Fig. 5. Optical absorbance for (a) PANI, (b) PANI/ZnMn3 composite and (c) PANI/ ZnMn5.

3.2. Optical properties

Fig. 3. TEM microphotographs of: (a) PANI, (b) ZnMn ferrite, (c) PANI/ZnMn3 composite and (d) PANI/ZnMn5 composite.

from the thermogram of pure PANI shows three steps weight loss (Fig. 4a). The first step indicates a weight loss at a temperature up to 120 °C which may be attributed to the expulsion of water molecules and the dopant (HCl) from PANI chains. The second step is observed in the temperature range of 120–340 °C which may be due to the volatilization of lower weight PANI. The final step at higher temperatures may be due to the thermal degradation of PANI chains, and only about 4% mass remains for pure PANI at 700 °C. It is seen from Fig. 4b and c that the thermal stability of the composite is higher than that of pure PANI. For instance, the TGA curve of the PANI/ZnMn3 composite reveals a weight loss of 50% at 445 °C. In contrast, the pure PANI shows the same weight loss at 503 °C. This enhancement in the thermal stability can be due to either the formation of an ionic bond between ZnMn ferrite and the amine group of aniline ring or owing to the formation of a coordinate bond via FeAN or MnAN linkages. This can be attained because Fe and Mn atoms have incomplete d-orbitals which can accept their lone pairs of electrons from ANH2 group. This restricts the thermal motion of PANI chains in the composite and then enhanced its thermal stability.

UV–vis spectroscopy was used to investigate the optical properties of synthesized samples. The UV–vis spectra of PANI and PANI- ferrite nanocomposite are shown in Fig. 5. PANI spectrum (Fig. 5a) shows two characterization absorption bands at around 335 and 621 nm attributed to p–p transition of the benzenoid ring and n–p* transition of benzenoid to quinoid, respectively [24,25]. The UV spectra of nanocomposites (Fig. 5b and c) shows similar vibrations to that of PANI except shifting in the absorption band of the p–p* transition peak into a longer wavelength (red shift). The noticed increment to the red shift is related to ferrite (wt%) content. This suggests that an exhibited interaction between oxygen of the ferrite and NH group of the PANI can take place [26,27]. 3.3. Electrical properties PANI is an inherently conducting material in nature due to the presence of a conjugated p electrons system in its structure. Also, PANI is not charge conjugation symmetric, meaning that both the valence and conduction bands are asymmetric to a great extent [28,29]. The partial oxidation of PANI usually leads to bonds reorganization, resulting in an increase in the electronic conductivity. Hence, conductivity of PANI can be influenced and tuned by changing the charging level, the degree of protonation and the nature of dopant [26,29–34]. Accordingly, the range of electrical conductivity of PANI can be changed from 1010 to 103 S/cm based on the acid dopant and fillers [35,36]. The frequency dependence of ac-conductivity (rac) of the studied materials, at room temperature, is illustrated in Fig. 6. The PANI shows a very slow increase in ac conductivity with increases in

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Fig. 6. Frequency dependence of conductivity rac: (a) rac PANI, (b) PANI/ZnMn3 composite and (c) PANI/ZnMn5 composite.

Fig. 8. Frequency dependence of dielectric loss e00 for: (a) PANI, (b) PANI/ZnMn3 composite and (c) PANI/ZnMn5 ferrite.

Fig. 7. Frequency dependence of dielectric constant e0 for: (a) PANI, (b) PANI/ ZnMn3 composite and (c) PANI/ZnMn5 composite.

in the decrease in e0 and e00 and tend to be independent at higher frequency. The reason for this decrease in dielectric permittivity (e0 and e00 ) simply lies in the decrease of electron exchange between Fe2+ and Fe3+ (Fe2+ ? Fe3+ + e) at octahedral sites [41]. The observed dielectric behavior can be also explained in the light of space charge polarization and hopping model [42,43]. The dielectric constant e0 and dielectric loss e00 were also found to be dependent on the amount of ferrites in the composites, Figs. 7 and 8. The high values of the dielectric constant can be attributed to the interfacial polarization produced. Greater interfacial area leads to greater average polarization, which consequently increases the dielectric constant. As the ZnMn ferrite filler loading was increased, the number of dipoles available also increased, and dipolar polarization resulted in an enhancement of the dielectric properties of the composites. 3.4. Gas sensing characteristics

frequency even at high frequency as compared to PANI/ZnMn ferrite composites. Increase in the frequency enhanced electronic exchange occurring among the cations with two and three valences existing in the sublattice of spinel ZnMn ferrite that results more ac conductivity [17,37]. The increase in rac at higher frequencies can be also explained as due to contribution of polarons, which are moving along small distances in the polymer chain. The multiphase variation in ac-conductivity with frequency may be attributed to lattice polarization around a charge in localized state and also due to the variation in the distribution of ferrite crystalline islands in polyaniline matrix as revealed by TEM. The obtained results also show that at all frequencies (especially at high frequencies) the ac-conductivity increases with increasing the nano ferrite content in the sample. This is probably attributed to the enhancement in the electron hopping rate [37,38]. The dielectric constant is represented as e = e0 je00 . The first term is the real part of the dielectric constant and describes the stored energy but the second term is the imaginary part of the dielectric constant, which describes the dissipated energy [37,39]. The presence of insulating ZnMn ferrite in the conducting matrix (polyaniline) results in the formation of more interfaces and a heterogeneous system due to some space charge accumulating at the interface. This produced polarization occurring due to the presence of polaron/bipolaron and other bound charges leads to high value of e0 and e00 . For all the studied samples, the frequency dependence of both e0 and e00 showed dispersion curves, Figs. 7 and 8. At high frequency, both e0 and e00 are almost constant and increases sharply at lower frequencies. The dispersion is coming from that the orientation polarization are playing a good role in decreasing and increasing the dielectric constants [40]. At high frequency of the applied field, the dipoles in the system cannot reorient themselves fast enough to respond to applied electric field, resulting

The sensing measurements were performed for NH3, chloroform and acetone on each of PANI, ZnMn ferrite and composite particles. Performance of the sensor of tested materials was determined by measuring its response to various concentrations of tested vapors formed in the closed chamber. The response is defined by (Rg  R)/R or DR/Rair where Rair is the resistance in air of the material (resistance before exposure to vapor gas) and Rg is the final resistance (resistance after exposure to tested vapor). All tested materials, except ZnMn ferrite, showed gas sensitivity toward the tested vapors. The responses of the studied materials to various concentrations of the tested vapors are shown in Fig. 9 and summarized in Table 2. For all tested gases, an approximately linear relation is obtained in the range of 100–1000 ppm. In case of NH3, the resistivity of PANI and its composites increases because of the undoping or the reduction of charge carriers by adsorption of ammonia on the surface of materials [44]. The sensor was reproducible by exposure to flow of dry air for 5–10 min depending on the pre-exposure concentration of ammonia gas. So far, a lot of researches on NH3 sensors have been reported, included inorganic, inorganic oxide/dioxide and conducting polyaniline polymers [4,45–48], which may be expensive, operated at high temperature, high power consumption or low sensitivity. Tuan et al. [4] have used PNAI nanowires to detect NH3 in air in the concentration range of 25–500 ppm at room temperature. Liu et al. [45] have used a single polyaniline nanowire sensor for detecting NH3 at a very low concentration of 0.5 ppm. The response of sensors do not attain stable resistance for 750 s and recovery after exposure to 59 ppm ammonia is not complete even after 15 min. Bis(2-ethyl hexyl) hydrogen phosphate (DiOHP) DiOHPdoped polyaniline sensor showed sufficient detection sensitivity for NH3 at concentrations of 25 ppm [46]. Ultrathin film of

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B

Response (%)

Resistance (ohm/cm)

Response (%)

A

Gas concentration (ppm) x 10-2 Fig. 9. (A): Response of the studied sensors for 100 ppm of tested gases: (1) PANI sensor for NH3; (2), (4), (6): ZnMn3 composite sensor for chloroform and (3), (5), (7) ZnMn5 composite sensor for acetone. (B) Response versus concentrations of tested gases: (1) PANI sensor for NH3; (2), (4) and (6): ZnMn3 composite sensor for NH3, chloroform and acetone, respectively; (3), (5), (7): ZnMn3 composite sensor for NH3, chloroform and acetone, respectively.

Table 2 Sensing characteristics of Pure PANI, PANI/ZnMn ferrite nanocomposites toward different gases. Sensor material

Porosity (%)

Analyte vapor

concentration of analyte vapor (ppm)

Response time (min)

Recovery time (min)

S (%)

Pure PAN

25.8

NH3

PANI/ZnMn3 composite

36.1

CHCl3

100 1000 100 1000 100 1000 100 1000 100 1000

3 2.5 4.5 4 3.5 3 4 3.5 3.5 3

3 3 4.5 5.5 4 5 5 6 5 6

4.2 23.0 3.5 22.1 4.8 23.5 4.0 22.1 5.1 26.9

Acetone PANI/ZnMn3 composite

42.3

CHCl3 Acetone

polystyrene sulfonic acid PANI-PSSA/TiO2 on interdigital gold sensor at room temperature and under optimal conditions displayed high sensitivity (26.5% and 81.2% toward NH3 of 10 and 100 ppm, respectively) [47]. As discussed in literature [48], nanomorphology of the sensor materials (should be considered among the main reasons determining the strength and rate of the sensor response. Therefore, our composite sensors which are reliable, inexpensive and able to operate at room temperature are significant for human demand. The conductivity of the polyaniline is proportional to the number of conduction sites, which are distributed on the polymer surface. These sites can adsorb species that affect the conductivity. Ammonia gas molecules withdraw protons from, N+-H sites to form energetically more favorable NH+4 due to which PANI changes from the conducting emeraldine salt state to insulating emeraldine base state, leading to the reduced hole density in PANI and thus an increased resistivity [49,50]. When the sensor is purged with dry air, the process is reversed; NH+4 decomposes to form NH3 and resistance is recovered. To confirm the suggested mechanism, the response of the PANI sensors was checked in presence of HCl vapor, where an increase in conductivity was observed. This is probably due to doping of the PANI molecules. In the cases of chloroform and acetone, pure PANI shows a small decrease in resistance value upon initial exposure of organic vapor but fails to return to the baseline value (initial resistance) after being transferred to air, Fig. 9. On the contrary, the PANI/ferrite

nanocomposite sensors showed stable baselines and improved responses to organic vapor at all concentrations and their dc-resistivity values were also found to decrease when exposure to organic vapors and recovery fast when exposure to air, Fig. 9(a) and Table 2. Both chloroform and acetone vapors affect the conductivity of the materials via dipolar interaction with the radical cation and the protonated imine centre and by the subsequent deprotonation process. The polar solvents, instead of deprotonation, take part in disruption of p-electron delocalisation and charge carrier transport through interaction between radical cations and their negative dipole ends. All these interactions, irrespective of their strength and type, are reversible which enables the materials to be efficient and viable sensor systems. The response of sensors toward acetone (dipole moment 2.88 D) is higher than that toward chloroform (dipole moment 1.04 D) indicating that the dipole moment of the tested gases is related with the response level [51]. The good recovery of sensor also confirms that the interaction mechanism is a physical interaction instead of strong chemical bonds. For both chloroform and acetone vapors, it is observed that the response of the composite sensor increases with the increase in the amount of ZnMn ferrite nanoparticles incorporated into PANI. This goes in parallel way with increasing the porosity of the studied materials, Table 2. Since the gas response of a sensor mainly depends on the surface interactions between the gas and the sensing material, therefore a greater surface area of the sensor leads to

M. Khairy / Journal of Alloys and Compounds 608 (2014) 283–291

stronger interactions between the adsorbed gases and the sensor surfaces [52]. The specific surface areas and porosities of the pure PANI and its composites are calculated from density measurements using Eqs. (1) and (2). The specific area values are found to be 2.2, 3.4, 3.6 and 3.2 for PANI, PANI/ZnMn3, PANI/ZnMn5 and ZnMn ferrite, respectively. The porosity results are listed in Table 2. The composite containing 50 wt% ferrite showed the highest porosity and surface area which imply a much more active surface toward test gases. As reported in the literature polyaniline (PANI) emeraldine salt (PANI–SO2 4 ) and a composite with an inorganic metal complex dopant, chromium(III) trioxalate (CTO) were used as sensors for detection of CHCl3 [53]; the sensing mechanism involving adsorption–desorption of chloroform vapors on metal cluster surfaces [54]. The acetone sensing behavior of PANI and PANITPA ((12-tungstophosphoric acid (TPA) are found to be 16.4% and 44.8%, respectively [55]. Polyaniline (PANI) synthesized onto the Au/Al2O3 are employed to monitor the level of acetone in gas phase. The sensitivity of acetone gas sensor based on PANI is greater than 5.20 ppm and recovery times of acetone gas sensor is 3.0–10.0 min [56]. 3.5. Magnetic properties Fig. 10 shows the hysteresis loops of PANI, ZnMn ferrite and PANI–ZnMn ferrite composites at room temperature. The magnetization of the ferrites and the composite samples exhibits a clear hysteretic loops referring to a superparamagentic behavior. The magnetic parameters such as saturation magnetization (MS) and coercivity (HC) of the nanocomposites determined by the hysteresis loops are given in Table 1. It can be seen that the saturation magnetization Ms values of the investigated samples exhibit an appreciable difference, where the saturation magnetization of the composite decreases with the increase in PANI content. It is expected that MS of the composite depends mainly on the volume fraction of the magnetic ferrite particles (u) according to the equation MS = ums, where ms is the saturation moment of a single particle [57]. But, the results obtained showed that the MS-values of the composites are not proportion to volume fraction of the ZnMn ferrite particles, due to the possible charge transfer between the ferrite surface and PANI coatings. The PANI coatings may change the electron density at the ferrite surfaces; via an expected interaction between magnetic domains in the ferrite and free electrons in the conducting polyaniline, and thus affecting magnetic relaxation processes in the system. It is known that ferrite nanocrystals have an irregular structure, geometric and crystallographic nature, such as pores, cracks and

Fig. 10. Magnetic hysteresis of: (a) PANI, (b) PANI/ZnMn3 composite, (c) ZnMn5 composite and (d) ZnMn ferrite.

289

surface roughness [58]. In the polymerization process, PANI is deposited on the ferrite surface, crystallite boundaries, and covers the ferrite surface defects, such as pores and cracks. Moreover, there may be the surface spin pinning of magnetic moments at ferrite nanoparticle/support interface [58], which leads to a decrease in magnetic surface anisotropy of ferrite particles. Consequently, the nanocomposites show lower values of coercivity compared to that of ZnMn ferrite.

3.6. Adsorption of Acid red dye 88 (environmental applications) The application of the studied materials in the adsorption of the toxic dye, such as Acid red 88 (AR88), from colored wastewater was investigated. When AR88 is dissolved in water, it dissociated to anionic form bearing – SO 3 group which is adsorbed via chemical interaction with the positively charged backbone of PANI emeraldine salt, and Na+ ions interact with the chloride ions that are present in PANI. The adsorption process was followed by the change of the absorbance of acid red dye at 505 nm. The absorbance values obtained were used to calculate the unknown concentrations of dye with the help of a calibration curve with a correlation coefficient (Rc2) of 0.98. Fig. 11 shows the adsorption of the dye on the studied materials. To determine the optimum condition of adsorption, different parameters (pH of solution, initial concentration of dye and dose of adsorbent) were studied. The results obtained are summarized in Table 3, from which it can be seen that: (1) For pure PANI and composites, the adsorption capacity increases with increasing pH up to 8, but a further rise in pH value causes a decrease in the adsorption capacity. However, for ZnMn ferrite sample, the adsorption capacity decreases when the pH of the dye solution is increased over the whole studied range. (2) For all samples, the efficiency toward dye removing increased with the decrease in initial dye concentration. (3) The adsorption efficiency increased with increasing the dosage of adsorbent materials. The adsorption efficiency was remarkably higher in case of pure PANI as compared to ferrite and the composites. Kinetics of the adsorption rate of AR88 was also studied. The adsorption rates fit a pseudo first-order model in terms of higher correlation coefficient values R2 > 0.98, in the light of using the integral equation [59] ln (qe  qt) = ln qe  kt, where qe and qt are the adsorption density at equilibrium and at time t, respectively, and k is the rate constant of pseudo-first order sorption. Moreover, the q values calculated (qe,cal) from pseudo first-order model were

Fig. 11. Adsorption of AR88 over studied materials at pH = 8, initial dye concentration 20 ppm and adsorbent dosage of 1 g/L, (a) ZnMn ferrite, (b) PANI/ZnMn5 composite, (c) PANI/ZnMn3 composite, and (d) PANI.

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Table 3 Adsorption data of MR88 over the studied materials.

a b c

Effect of pHa ZnMn ferrite PANI/ZnMn3 PANI/ZnMn5 PANI

qe,mg/g qe,mg/g qe, mg/g qe,mg/g

Effect of dye concentration (mg/L)b ZnMn ferrite PANI/ZnMn3 PANI/ZnMn5 PANI Effect of adsorbent dose (g/L)c ZnMnFerrite PANI/ZnMn3 PANI/ZnMn5 PANI

3

5 3.3 8.5 7.5 11.3

7 2.8 9.4 8.6 10.2

8

3.6 7.6 6.9 8.5

2.6 9.7 8.9 11.6

9 2.2 8.8 8.3 9.8

Efficiency% Efficiency% Efficiency% Efficiency%

10 16.1 51.5 45.7 59.1

20 13.0 48.5 44.5 58.0

30 12.3 43.0 41.2 46.1

40 11.1 39.0 38.1 41.5

50 8.9 33.5 31.4 37.2

Efficiency% Efficiency% Efficiency% Efficiency%

0.05 12.8 43.5 41.5 47.5

1 16.1 51.5 45.7 59.1

2 18.5 53.5 55.3 60.4

3 22.2 56.0 58.8 62.6

11 1.9 7.0 6.8 7.5

(Temperature = 30 °C, contact time = 360 min, initial dye concentration = 20 mg/L, adsorbent dosage = 1 g/L). (Temperature = 30 °C, contact time = 360 min, pH = 8, adsorbent dosage = 1 g/L). (Temperature = 30 °C, contact time = 360 min, pH = 8, initial dye concentration = 10 mg/L).

Table 4 The values of parameters obtained by pseudo first order, pH = 8, initial dye concentration 20 mg/L and adsorbent dosage 1 g/L. Sample

qe (mg/g)

k (h1)

R c2

ZnMn ferrite

2.62 [2.6] 9.73 [9.7] 8.77 [8.9] 11.68 [11.6]

0.184

0.997

0.252

0.992

0.246

0.989

0.249

0.996

PANI/ZnMn3 PANI/ZnMn5 PANI

[ ] experimental value.

found to be more consistent with the experimental q values (qe,exp) than those calculated from other kinetic models. The kinetic data are summarized in Table 4. From which it can be seen that the rate of dye adsorption over ZnMn ferrite is lower than that of PANI and its composites. 4. Conclusions The sol–gel was used to prepare Zn0.2Mn0.8Fe2O4 nanoparticles. The core–shell nanocomposites of PANI containing magnetic ZnMn ferrite were synthesized by in situ polymerization of aniline in the presence of ZnMn ferrite nanoparticles via employing ammonium persulphate as an oxidant in HCl medium. The results of TGA, FTIR and UV–vis spectra indicated that ZnMn ferrite particles improve the thermal stability of composite, and there are interactions between ferrite particles and PANI macromolecule. TEM images showed that nanocomposites present the core–shell structure with a magnetic core of ferrite and an amorphous shell of PANI. The nanocomposites under applied magnetic field exhibit the hysteretic loops of the superparamagnetic nature. The results show that as the ZnMn ferrite content increases the dielectric permittivity (e0 ), dielectric loss (e0 ), ac conductivity (rac) and the magnetization increase. The dielectric behavior is explained in terms of electron exchange between Fe2+ and Fe3+, suggesting that the polarization due to heterogeneity of the samples. The values obtained for the dielectric constant and dielectric loss in these nanomagnetic samples making them as good candidates for applications as inductive and capacitive materials in capacitors used in electronic devices like memory cards and computers. Pure PANI can be used as sensor for NH3 gas and its composites as sensors for chloroform and acetone vapors at concentrations less than <100 ppm. The ZnMn

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