Chemical Physics Letters 494 (2010) 232–236
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Paramagnetic polyaniline nanospheres Kaushik Mallick a,*, Michael Witcomb b, Michael Scurrell c, André Strydom d,** a
Advanced Materials Division, Mintek, Private Bag X3015, Randburg 2125, South Africa Microscopy and Microanalysis Unit, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa c Institute of Molecular Sciences, School of Chemistry, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa d Physics Department, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa b
a r t i c l e
i n f o
Article history: Received 17 March 2010 In final form 28 May 2010 Available online 2 June 2010
a b s t r a c t Polyaniline nanomaterials have gained significant interest because of their unique electronic properties, simple synthesis process and their environmental stability. The morphology of polyaniline has been widely studied due to both its strong influence on the properties and its various applications. We report on the cerium(IV) ammonium nitrate mediated synthesis of polyaniline nanospheres using an interfacial polymerization technique in which polyaniline serves as a guest of the cerium(III) ion, a paramagnetic species produced during the synthesis condition. Cerium(III) ionic species bonded with the chain nitrogen of the polyaniline and the supramolecular system show the paramagnetic behavior throughout the experimental temperature range 400–1.9 K. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured polyaniline has attracted intensive interest as a result of its physical properties and potential applications [1–7]. Considerable efforts have been made with regard to the synthesis of polyaniline nanofibers or nanotubes by chemical or electrochemical oxidative polymerization of the corresponding monomer [8]. The chemical synthesis method of polyaniline is common for its large scale production. There have been reports of a variety of other chemical methods that have been utilized to obtain unique morphologies of the polyaniline. These approaches include the use of templates or surfactants [9,10], electro-spinning [11], interfacial polymerization [12], seeding [13] and oligomer [14] assisted polymerization. Of these methods, interfacial polymerization is one of the easier, inexpensive, environmental friendly and one step method to obtain a fiber-like morphology for a bulk production of polyaniline. Among the known conducting polymers, polyaniline is unique due to its doping adjustable electrical properties and metal-like transport property at both room and low temperatures [15]. The electrical conductivity of the polyaniline can be varied over the full range from insulator to metal by doping. Through doping, the chemical potential (Fermi level) can be moved into the region of energy of the high density of electronic states either by a redox reaction or by an acid–base reaction. Doped polyaniline is a good
* Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (K. Mallick),
[email protected] (A. Strydom). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.05.096
conductor due to the fact that doping introduces charge carriers into the electronic structure and the attraction of an electron in one repeat unit to the nuclei in the neighboring unit leads to carrier delocalization along the polymer chain and to charge carrier mobility, which is extended into three dimensions through interchain electron transfer [16]. Paramagnetic behavior in highly protonic acid doped polyaniline has also reported at low temperatures [17]. EPR study of the camphor–sulphonic acid doped conducting polyaniline showed temperature independent Pauli susceptibility within the temperature range 300–50 K. The Curie contribution to the electronic paramagnetic susceptibility of the heavily doped polyaniline arises from a disordered metallic state close to the metal–insulator transition. This has been observed only below 50 K [17]. Paramagnetism in polyaniline has been reported by introducing paramagnetic metal nanoparticles in the polyaniline matrix [18,19]. In the present communication we report on an in situ synthesis technique for the preparation of a paramagnetic polyaniline–cerium(III) supramolecular composite material by applying an ‘in situ polymerization and composite formation’ (IPCF) type of reaction [20] using cerium(IV) ammonium nitrate (CAN) as an oxidizing agent for polymerizing aniline. CAN is most extensively used in synthetic organic chemistry as an oxidant (reduction potential value of +1.61 V vs. NHE). Other advantages of CAN are low toxicity, ease of handling, experimental simplicity and solubility in a number of solvents [21]. During the polymerization process each step is associated with a release of electron and that electron reduces the Ce4+ ion to form Ce3+ ion. The Ce3+ ion binds with the chain nitrogen of the polyaniline which causes the emergence of paramagnetism in polyaniline.
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2. Experimental section 2.1. Materials Cerium ammonium nitrate and aniline were purchased from Sigma–Aldrich and Fluka, respectively. Aniline was distilled at a reduced pressure over zinc metal. The middle fraction was collected and stored at 10 °C under argon. Ultra-pure water (specific resistivity >17 MX cm) was used to prepare the solution of cerium ammonium nitrate (10 2 mol dm 3). Toluene was purchased from Merck and was used to dilute the aniline. 2.2. Characterization techniques Scanning electron microscopy (SEM) studies were undertaken in a FEI FEG Nova 600 Nanolab at 5 kV. For UV–vis spectra analysis, a small portion of the solid sample was dissolved in methanol and scanned within the range 300–800 nm using a Varian, CARY, 1E, digital spectrophotometer. Raman spectra were acquired using the green (514.5 nm) line of an argon ion laser as the excitation source. Light dispersion was undertaken via the single spectrograph stage of a Jobin–Yvon T64000 Raman spectrometer. Power at the sample was kept very low (0.73 mW), while the laser beam diameter at the sample was 1 lm. A Perkin–Elmer 2000 FT-IR spectrometer, operating within the range 850–1700 cm 1 with a resolution 4 cm 1, was used for the infrared spectra analyses. For this study, the sample was deposited in the form of a thin film on a NaCl disk. X-ray photoelectron spectra (XPS) were collected in a UHV chamber attached to a Physical Electronics 560 ESCA/ SAM instrument. Magnetic measurements were performed under controlled temperatures using a Magnetic Properties Measurement System (MPMS) (Quantum Design, USA).
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covered with asperities. These asperities are perhaps an indication of the formation of secondary nucleation centers [8]. Fig. 1B is the histogram (size of the particles vs. particle frequency) drawn based on 181 particles obtained from the SEM image (Fig. 1A). From the histogram it is clear that most of the polymer spheres are within the size range between 60 and 360 nm and very few spheres are larger than 360 nm. Closer inspection of Fig. 1A indicates the formation of a fiber-like morphology, see arrows. A higher magnification SEM image (Fig. 1D) reveals that the fibers comprise of selfassembled polyaniline nanospheres of comparatively smaller sizes (35 ± 10 nm). A typical EDX analysis (Fig. 2A) obtained from the electron beam being focused onto various locations of the polymer surface indicates the presence of cerium all over the polymer and in similar concentrations. 3.2. XPS analysis Various kinds of information are available from the principal features of typical XPS spectra [22]. From the characteristic binding energies of the photoelectrons, the elements involved can be identified, whereas the peak intensities can be directly related to the atomic concentration of the elements in the sample. The ionic state of cerium was confirmed by XPS analysis. Cerium 3d core level XPS spectra of the cerium doped polyaniline showed the relative concentration of the Ce3+ and the Ce4+ oxidation states (Fig. 2B). Area calculations indicate that 72% of the Ce3+ ions were present in the sample, the remaining 28% being unreacted Ce4+ ions. The peaks between 875 and 895 eV belong to the Ce 3d5/2 level [23], the spectra clearly shows a difference in the population density of the oxidation states of cerium ions present in polyaniline. After the addition of CAN in the aniline solution, the Ce4+ ions present in CAN were converted to Ce3+ ions as a result of the reduction reaction during the polymerization of aniline.
2.3. Synthesis of Ce3+ ion doped polyaniline sphere In a typical experiment 0.5 g of aniline was diluted in 10 mL of toluene in a 50 mL conical flask. The flask was then placed on a magnetic stirrer and a mild stirring condition applied. Cerium ammonium nitrate, CAN, (15 mL) having a concentration of 10 2 mol dm 3 was added slowly drop wise to the aniline solution. A dark green colour developed at the bottom of the conical flask during the addition of CAN. After all the CAN was added, the solution was kept under static conditions for another 10 min. A green precipitation slowly formed at the bottom of the flask. The material was then allowed to settle for an additional 15 min. The whole process was carried out at room temperature (25 °C). Subsequently, 5 lL of the colloidal precipitation was taken from the bottom of the flask and pipetted onto lacey, carbon-coated, copper TEM grids for SEM analysis. The rest of the solution was filtered, washed with distilled water and was kept under vacuum overnight. A small portion of the dried powder was used for UV–vis, IR and Raman analysis. For the magnetic property study, the powder was made into the form of a pellet under high pressure and then cut into an appropriate size and shape and measured under controlled temperatures using a MPMS system. 3. Results and discussion 3.1. Microscopic characterization A SEM image, Fig. 1A, reveals that the polyaniline obtained using CAN is composed exclusively of nanospheres. Higher magnification SEM images such as Fig. 1C indicate the formation of nanospheres having a wide distribution range. In addition, Fig. 1C also shows that the surface of the nanospheres is not smooth, but is
3.3. Optical characterization The optical characterization techniques confirmed the formation of polyaniline. The Fourier transform infrared (FT-IR) spectrum (Fig. 3A) of the polyaniline showed the five major vibrational bands within the region between 1750 and 500 cm 1. The groups N–B–N, where B represents a benzenoid ring, and N@Q@N, where Q represents a quinoid ring, absorb at 1500 and 1595 cm 1, respectively. A broad band with a peak position at 1300 cm 1 corresponds to the C–N stretching vibration with aromatic conjugation. The band at 1170 cm 1 represents an aromatic C–H in-plane bending mode, whereas, the vibrational band at 830 cm 1 is indicative of 1,4substituted benzene rings. The electronic absorption spectrum of the hydrochloride acid doped polyaniline salt has been documented previously [24]. The polyaniline salt shows three absorption peaks at 310–360, 400– 440, and above 700 nm. The absorption peak at 310–360 nm is due to the p–p* transition of the benzenoid rings. The peak at 400–440 nm is due to the polaron–bipolaron transition, whereas the broad absorption band appearing above 700 nm is due to the benzenoid-to-quinoid excitonic transition. Earlier we reported [25] the UV–vis spectra of polyaniline with two different kinds of morphology, which were synthesized with two different solvents, showed three absorption bands with peaks at 315, 435 and 760 nm (methanol as a solvent) and 320, 430 and 720 nm (water as a solvent). The polyaniline formed was either neutral or basic medium because the benzenoid–quinoid excitonic transition showed a broad absorption peak at 620 nm [26]. The shifting of the excitonic peak depends on various factors, such as the counter ions, solvent, pH value, chemical structure, and morphology of the polymer [27,28].
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Fig. 1. (A) SEM image showing the formation of polyaniline nanospheres. (B) Histogram (size of the particles vs. particle frequency) drawn based on the particles obtained from (A). From the histogram it is clear that most of the polymer spheres are within the size range between 60 and 360 nm and very few spheres are larger than 360 nm. (C) A higher magnification SEM image revealing that the surface of the nanospheres is decorated with asperities. These asperities are the indication of the formation of secondary nucleation centers. A fiber-like morphology is also observed in (A), see arrows. (D) A high magnification SEM image of such an area indicated in (A) revealing that the fibers comprise of self-assembled polyaniline nanospheres of a comparatively smaller size, 35 ± 10 nm.
Fig. 2. (A) EDX spectrum obtained from the electron beam being focused onto various locations of the polymer surface indicates the presence of cerium all over the polymer in similar concentrations. (B) Cerium 3d core level XPS spectrum of the Ce3+ doped polyaniline acid showing the relative concentration of the Ce3+ and the Ce4+ oxidation states. Area calculations indicate that 72% of the Ce ions are present in the form of Ce3+ ions, the remaining 28% being unreacted Ce4+ ions.
Fig. 3B is the UV–vis spectrum of the Ce3+ doped polyaniline showing an absorption peak at 310 due to the p–p* transition of the benzenoid rings, and two broad absorption bands approximately within the region of 500–370 and 650–525 nm, these corresponding to the polaron/bipolaron transition and the benzenoid-to-quinoid excitonic transition, respectively. In the Raman spectrum, the bands in the region of 1700 and 1100 cm 1 are sensitive to the polyaniline oxidation state [29].
The Raman spectrum (Fig. 3C) of the polyaniline nanospheres reveals C–C deformation bands of benzenoid ring at 1621 and 1558 cm 1, which are characteristic of semiquinone rings [30,31]. Further, the 1526 cm 1 band corresponds to the N–H bending deformation band of the Ce3+ incorporated amine. The band at 1351 cm 1 with a shoulder at 1376 cm 1 corresponds to the C– N+ stretching modes. The band at 1252 cm 1 can be assigned to the C–N stretching mode of the polaronic units. The position of
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Fig. 3. (A) Fourier transform infrared (FT-IR) spectrum from the nanospheres showing the presence of benzenoid and quinoid rings at 1500 and 1595 cm 1, respectively. (B) UV–vis spectrum of the Ce3+ doped polyaniline showing one sharp absorption peak at 310 due to the p–p* transition of the benzenoid rings. Two other broad bands in the region of 500–370 and 650–525 nm correspond to the polaron/bipolaron transition and the benzenoid-to-quinoid excitonic transition, respectively. (C) Raman spectrum of the material within the range 1100–1700 cm 1. The spectrum indicates the presence of both the benzenoid and the quinoid structure in the polyaniline. (D) Raman spectrum within the range 375–900 cm 1 indicating conformation dependent features of the polyaniline.
the benzene C–H bending deformation band at 1177 cm 1 is characteristic of reduced and semiquinone structures. In the typical Raman spectrum, the band at 1455 cm 1 corresponds to the C@N stretching mode of the quinoid units, while the vibrational band at 1219 cm 1 results from the C–N stretching mode of single bonds. The positions of the C@C and C–H benzene deformation modes are positioned at 1589 and 1177 cm 1, respectively, indicating the presence of quinoid rings. The Raman spectrum within the range 375–900 cm 1 indicates many conformation dependent features of the polyaniline [32,33]. Depending on the torsion angle between two aniline rings two outof-plane C–H bonds shift their position. In the spectrum of polyaniline nanospheres within this range (Fig. 3D) a broad band appears in the region of 850–805 cm 1 with a shoulder like appearance at 780 cm 1 suggesting the existence of a mixture of various torsion angles. The positions of C–N–C out-of-plane deformation modes are also dependent on the torsion angle, these were observed at 415 cm 1 in the Raman spectrum. Intense vibrational bands can be seen in the region 475–675 cm 1 with the peak positions at 527, 546 and 614 cm 1 originating from the deformation mode of the amine groups indicating the strong coordination between the nitrogen and the Ce3+ ions. Other peaks at 704 and 733 cm 1 are due to the C–C ring deformation indicative of a spherical structure of the polyaniline.
tion of the Ce3+ ionic species is confirmed by XPS analysis and has been discussed earlier. The Ce3+ ion forms a covalent type of bond with the nitrogen of the polyaniline chain. The growth and formation mechanism of the spherical structure of the Ce3+ ion doped polyaniline can be proposed as follows. The mechanism of the growth can be understood by reference to the classical nucleation theory. The mechanism is of a two step process, the generation of the nucleation centers followed by successive growth [34]. In the first step, we can assume that initially the oligomeric form of aniline can occur due to the presence of a suitable oxidizing agent which acts as the nucleation centers. As the generation of the Ce3+ ions and the oligomerization process occur simultaneously, a strong bond formation is expected between the Ce3+ ions and the chain nitrogen of the oligomer. Two neighboring Ce3+ ions pull together the two neighboring nitrogen of the aniline oligomer causing deformation of the C–C benzene ring as also indicated by the Raman spectra. This deformation ultimately results in a spherical structure while also acting as a nucleation center. The nucleation centers have an important role for the growth process since they act as the seed and also as the catalytic center where the oxidation process of the remaining monomers present in the solution take place. The seed mediated catalytic growth ends with the formation of the Ce3+ doped polyaniline having a larger sphere-like morphology.
3.4. Formation and growth mechanism 3.5. Magnetic property study The in situ formation of Ce3+ and polyaniline suggests the following formation mechanism. The presence of lone pair of electrons on nitrogen in aniline forms the electrostatic bond with Ce4+ cation. Due to the electrostatic attachment, C6H5–H2N: Ce4+ like species are formed which undergo polymerization in the presence of the oxidizing agent. Each step of the polymerization process is associated with the release of an electron, which is utilized to reduce the Ce4+ ion to form the Ce3+ ion. The forma-
The enhanced stability of the vacant f shell in Ce4+ accounts for the ability of cerium to exist in the +4 oxidation state. The large reduction potential value of 1.61 V (vs. NHE) endowed to Ce4+ makes CAN a superior oxidizing agent. Cerium in its ground state has an electronic configuration of [Xe] 4f26s2, where Xe represents the xenon configuration. The electronic configuration of the Ce4+ ion is [Xe] 4f0 while for the Ce3+ ion it is [Xe] 4f1.
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in this work however, and hence we conclude that our magnetic measurements are gauging a purely incoherent behavior of the magnetic Ce3+ species in the supramolecular complex system. 4. Conclusions
Fig. 4. Temperature dependence of the magnetic susceptibility in the form emu/ (g T) of Ce3+ doped polyaniline. Inset: low-temperature inverse susceptibility to emphasize the strong temperature dependence which develops towards very low temperatures.
Fig. 4 shows the magnetic measurements data obtained from the resultant material. Due to the fact that the precise cerium ion concentration is undetermined, we employed the elementary susceptibility units of measured magnetic moment per unit of applied field which was then normalized to the sample mass. The overall magnetic moment values are very small and monotonously varying with temperature towards elevated temperatures. It is important to note that the susceptibility values are positive throughout the temperature range (1.9–400 K), which means that the presumably diamagnetic host susceptibility in the polyaniline–Ce3+ macromolecular complex, which has a negative and generally temperature independent susceptibility, is dominated by a paramagnetic species (positive susceptibility). Below about 70 K (inset in Fig. 4) the measured moment starts increasing rather rapidly and towards low temperatures a dramatic variation with temperature develops. This response is characteristic of coupling between the applied magnetic field and one or more magnetic species in the sample. The increase in magnetic response towards lower temperatures is ascribed to the lessening effect of thermal energy which tends to randomize a spin magnetic moment, competing with the component of magnetic moment that may be excited (or aligned) by the applied field. The magnetic species is naturally concluded to be the Ce3+ ionic species, since Ce4+ bears no f-electron magnetic moment. The rise in the apparent magnetic moment at low temperatures follows from the weakening thermal energy upon lowering the sample temperature, compared to interaction strengths between the intrinsic Ce3+ magnetic moment stemming from its single 4f electron, and its environment. These interactions may be between the magnetic moment of Ce3+ and nearby free electrons which interact through a spin magnetic moment of such electrons, or with other nearby Ce3+ ions. The former of these interactions require an appreciable density of free electrons, which is not likely in the present in the polyaniline supported Ce3+ ionic complex. The interaction between adjacent Ce3+ ions on the other hand may be of an indirect nature, which require electrons to mediate the magnetic coupling, or it may be of a direct nature. In either of these a relatively high concentration of Ce3+ is needed for the interaction to become evident. This would result in an easily detectable cooperative ordering phenomenon such as longrange magnetic ordering or short-range spin-glass type as the defining achievement of well-developed interaction strength. No such effects have been detected among the samples investigated
In summary, the above results demonstrate that two diamagnetic reactant species form a single supramolecular paramagnetic product due to the in situ formation of the paramagnetic Ce3+ species at the reaction condition. We believe that this material holds promise for magnetic applications by tuning magnetism (nature and strength of exchange interaction between the magnetic species) through suitable concentration levels of the ionic dopant. A systematic investigation into the relation between the concentration of the cerium ion in a polymer matrix and the nature of the ensuing magnetism is highly desirable in order to further our knowledge with regard to this frontier of magnetic research. The resultant paramagnetic organic compound could be useful in the application of flexible magnetic materials in the future. Acknowledgments K.M. acknowledges financial support from NIC, Mintek, South Africa while A.S. acknowledges financial aid from the University of Johannesburg Research Committee and the Faculty of Science, as well as from the SA National Research Foundation under Grant 2072956. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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