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Microwave-assisted hydrothermal synthesis and characterization of tremella-like polyaniline–vanadium oxide nanocomposite nanosheets
Materials Science and Engineering B 168 (2010) 199–203
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Microwave-assisted hydrothermal synthesis and characterization of tremella-like polyaniline–vanadium oxide nanocomposite nanosheets Mohini A. Jagtap, Milind V. Kulkarni ∗ , Sanjay K. Apte, Sonali D. Naik, Bharat B. Kale Nanocomposite/Glass Laboratory, Centre for Materials for Electronics Technology (C-MET), Department of Information Technology, Govt. of India, Panchawati, Off Pashan Road, Pune 411 008, India
a r t i c l e
i n f o
Article history: Received 26 September 2009 Received in revised form 16 December 2009 Accepted 7 January 2010 Keywords: Nanocomposite Polyaniline Vanadium oxide Tremella-like morphology
a b s t r a c t The organic–inorganic hybrid nanocomposite of polyaniline–V2 O5 was synthesized by microwaveassisted hydrothermal method. The nanocomposite was separated by filtration and washed with deionized water. Structural study of nanocomposite was performed using XRD. The microstructural study of nanocomposite performed using field emission scanning electron microscopy (FESEM) shows self-assembled plates with curly petal or tremella-like morphology. The nanocomposite was subjected for spectroscopic (UV–vis and FT-IR) analysis. The UV–vis spectra revealed the presence of conducting emeraldine salt phase of the polymer which was further supported by FT-IR analysis. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The study of hybrid organic (polymer)–inorganic materials is a recent but very fruitful and prolific enterprise. Self-assembled structures with highly specific morphology and novel properties are of great interest. These hybrid materials represent the natural interface between two worlds of chemistry each with very significant contributions to the field of materials science [1]. The activities of organic and inorganic species combine to reinforce or modify each other. These materials with different combinations have promising physical properties and many potential applications in various fields, such as, optics, electrochemistry, electronics, biology, chemical sensors, light emitting diodes (LED), electrochromic displays (ECD), and EMI shielding. Polyaniline (PANI) is unique among conducting polymers, which can be rapidly converted between the base (insulator) and the salt form (conductor) by treatment with acid or base or electrochemical means. PANI also has good environmental stability and high electrical conductivity and also can be easily and cheaply synthesized [2]. It has a great variety of potential applications including anti-corrosion coatings, batteries, and sensors [3,4]. Vanadium oxide (V2 O5 ) has been extensively used as a well-known transition metal oxide. It displays the novel properties including electrochromism. The charge storage behavior of
∗ Corresponding author. Tel.: +91 20 2589 8390 9273, 2589 9273; fax: +91 20 2589 8085. E-mail addresses: [email protected], [email protected] (M.V. Kulkarni). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.016
deposits of hydrated V2 O5 is of special interest in rechargeable lithium batteries. It is often employed in secondary lithium batteries to improve the capacity, voltage, reversibility and stability [5,6]. Li et al. [7] obtained a mesostructured nanocomposite vanadium oxide/PANI, the conductivity of which is higher than that for vanadium oxide xerogel. Malta et al. [8] prepared V2 O5 /PANI nanocomposite with fibrillar morphology by hydrothermal treatment of nanocomposite V2 O5 /PANI and hexadecylamine. Ferreira et al. [9,10] synthesized nanoscale composites of polyaniline (PANI) and vanadium oxide (V2 O5 ) via the electrostatic layer-by-layer (LBL) technique. These composites enhance the charge storage capability of the films. Asim et al. synthesized core-shell PANI/V2 O5 nanocomposite via microemulsion method [11]. The hydrothermal synthesis of the polyaniline–vanadium oxide nanocomposite involves mostly reactions between heterogeneous precursors, the time scale of such a synthesis is very long (generally a few days) [12], while in the sol–gel method aging of V2 O5 takes 1 week to 1 month prior to the synthesis of PANI/V2 O5 nanocomposite [8]. PANI/V2 O5 nanocomposites synthesized by chemical method involves stirring for 16 h in air and further its isolation, drying and its characterization [13]. Considerable attention is paid to the simplification or improvement of existing methods widely used in the synthesis of organic–inorganic hybrid nanocomposite. Compared with the conventional heating, microwave heating is promising due to its unique effects, such as rapid volumetric heating, higher reaction rates and shorter reaction time, higher reaction selectivity and energy saving.
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M.A. Jagtap et al. / Materials Science and Engineering B 168 (2010) 199–203
Microwave-induced reactions have already made an enormous impact in modern science. The formation of uniform nanosized particles demands a uniform growth environment, and microwave heating affords this. In the case of microwave-induced synthesis, the reactions are driven by the intense collision and friction of the molecules. Microwaves are electromagnetic waves, which contain electric and magnetic field components. The force derived from these changes direction rapidly, which causes heat because, the assembly of molecules cannot respond to it instantaneously, creating friction which manifests itself as heat. The goal of this article is to report a facile synthesis method to self-assembled polyaniline–vanadium oxide nanocomposite nanosheets under microwave conditions. 2. Experimental 2.1. Materials Aniline (AR grade), vanadium oxide (V2 O5 , AR grade), hydrochloric acid (HCl, AR grade) were obtained in high purity from S.D. Fine Chem. Ltd. (India). Deionized and double distilled water were used for the preparation of solutions. 2.2. Synthesis PANI/V2 O5 nanocomposite was synthesized by in situ intercalation of V2 O5 powder (0.18 g) and aniline (0.006 g) in 40 ml distilled water. Hydrochloric acid was added to the mixture until the pH of the solution becomes ∼3. The mixture was transferred into a Teflon liner autoclave in a microwave digestion system and reaction was carried out by using ‘Microwave Labstation ms 1200 mega’ (Germany) at 500 W for 15 min in air. After completion of the reaction, autoclave was air-cooled to room temperature. The resultant dark green product was filtered and washed with distilled water and alcohol several times and then dried in vacuum at 60 ◦ C for 24 h. The yield of the product was ∼94%. 2.3. Characterization The UV–vis spectrum of the nanocomposite was recorded by using Hitachi-U3210 spectrophotometer in the range of 300–900 nm. The FT-IR spectrum was taken on a Perkin-ElmerSpectrum 2000 spectrophotometer in the range 400–4000 cm−1 . The sample was prepared in the pellet form using a spectroscopic grade KBr powder. Morphological studies were performed with the help of a FEI QUANTA 200F field emission scanning electron microscope (FESEM). The X-ray diffraction (XRD) measurements were performed using Brucker-AXS Model D8 Advance, X-ray diffractometer. Thermogram of the polymer sample was recorded using Mettler-Toledo 851 thermogravimetric analyzer in presence of N2 atmosphere from RT to 900 ◦ C at a heating rate of 10 ◦ C/min. Electrical conductivity was measured by the two probe technique. Dry powdered sample was made into pellet using a steel die of 1.5 cm diameter in a hydraulic press under a pressure of 7 tons. 3. Results and discussions The optical absorption spectrum of the neat polyaniline and polyaniline/V2 O5 is presented in Fig. 1. In water, polyaniline shows a sharp peak at 320 nm, shoulder at 580 nm and increasing absorption at ∼800 nm. The sharp peak at 320 nm corresponds to the –* transition of the benzenoid rings and the shoulder at 580 nm and increasing absorption at ∼800 nm represents insulating pernigranilne and conducting emeraldine salt phase of the polymer respectively. This clearly shows the formation of mixed phases
Fig. 1. UV–vis spectra of (A) polyaniline; and (B) polyaniline–vanadium oxide nanocomposite nanosheets.
in the polymer which is further supported by the FT-IR studies. The UV–vis spectrum of polyaniline–V2 O5 exhibits a sharp peak at 320 nm and shoulder at 420 nm and 830 nm. The peak at 320 nm corresponds to the –* transition of the benzenoid rings, while the shoulder at 420 nm corresponds to the polaron absorption band. However, the increasing absorption or small peak ∼820 nm is assigned to the conducting emeraldine salt phase of the polymer. Localized polaron bands at around 420 nm and 830 nm are characteristic peaks of emeraldine salt of PANI [2,14]. On comparison of the spectras of neat polyaniline with the nanocomposite there is not much variation is observed, except the formation of polaronic peak at 420 nm in the nanocomposite which is absent in the neat polyaniline spectrum. Fig. 2 represents the FT-IR spectra of the neat polyaniline and polyaniline/V2 O5 nanocomposite synthesized by in situ polymerization method and the peak positions related to the corresponding chemical bonds are listed in Table 1. The peaks in 1000–1600 cm−1 region, attributed to the framework vibration of polyaniline, suggest no evidence for the presence of anilinium ion or free aniline. The presence of the two bands in the vicinity of 1500 cm−1 and 1600 cm−1 is assigned to the benzene stretching modes. The higher frequency vibration at 1600 cm−1 has a major contribution from the quinoid rings while, the lower frequency mode at 1466 cm−1 depicts the presence of benzenoid ring units. The presence of these two bands clearly shows that the polymer is composed of the amine and imine units. The bands below 1100 cm−1 correspond to oxide absorption in composite. The absorption at 1001 cm−1 corresponds to V O stretching modes while at 758 cm−1 and 537 cm−1 is for V–O–V stretching modes. The FT-IR results show intercalation reaction of aniline and V2 O5 [11,15,17] and these bands are absent in the neat polyaniline spectrum. Further, this also supports the UV–vis characterization, discussed earlier, where, the different phases are observed in the spectrum. The presence of other charac-
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Table 1 Characteristic frequencies of polyaniline–vanadium oxide nanocomposite nanosheets. Wavenumber (cm−1 )
Band characteristics
PANI
PANI/V2 O5
814 – – – 598 1126 1300 1494 1572 3200
– 537 758 1001 – – 1306 1466 1629 3421
Paradisubstituted aromatic rings Symmetric V–O–V stretching modes Asymmetric V–O–V stretching modes V O stretching modes C–H out-of-plane bending vibration C–H in plane bending vibration Aromatic C–N stretching indicating secondary aromatic amine group C–N stretching of benzenoid rings C–N stretching of quinoid rings N–H stretching vibration
teristic bands confirms the presence of conducting emeraldine salt phase in the polymer. Fig. 3 shows the FESEM micrographs of nanocomposite recorded at different magnifications. The micrographs of the nanocomposite synthesized by in situ polymerization method offer very interesting morphological features, i.e. tremella-like morphology of nanocomposite nanosheets. In the surface study, it was observed that the thickness of the nanosheets is between 15 nm and 20 nm. At higher magnification (Fig. 3B), the clarity of the morphology is found to be further enhanced and it is observed that the nanosheets are self-assembled into lamellar structures. The dimensions of the nanosheets are in the range of hundreds of nanometers (to several micrometers). Fig. 4 shows the XRD pattern of polyaniline, mesostructured V2 O5 precursor and PANI/V2 O5 nanocomposite under microwave conditions. The peaks in Fig. 4(A) can be attributed to orthorhombic
crystalline V2 O5 phases. The XRD spectrum of the neat polyaniline shows the absence of sharp peak indicating amorphous nature of the polymer. It is seen that, the diffraction peaks of V2 O5 become wider and get reduced after intercalation of aniline (Fig. 4(C)). This indicates the intercalated product is relatively with low crystalinity ascribed to the limited short-range order [5]. It is suggested that aniline is actually intercalated into the V2 O5 layer and in situ polymerization takes place in the layer.
Fig. 2. FT-IR spectrum of (A) polyaniline; and (B) polyaniline–vanadium oxide nanocomposite nanosheets.
Fig. 3. Scanning electron micrographs PANI/V2 O5 nanocomposite nanosheets at the magnifications of 40,000× (A); and 80,000× (B).
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Fig. 5. Thermogram of (A) polyaniline; and (B) polyaniline–vanadium oxide nanocomposite nanosheets.
Fig. 4. XRD patterns of the initial (A) V2 O5 precursor; (B) polyaniline; and (C) polyaniline–vanadium oxide nanocomposite nanosheets.
The thermal stability of neat polyaniline and the polyaniline/V2 O5 nanocomposite was examined by TGA and represented in Fig. 5(A) and (B) respectively. From the careful observation of thermogram of polyaniline it is observed that a sharp weight loss up to 100 ◦ C is observed which is corresponding to the loss of the water/moisture present in the polymer matrix. After 200 ◦ C there is continuous weight loss up to 900 ◦ C, this could be mainly due to the loss of the dopant ion and the degradation and decomposition of the polymer backbone. However, in the PANI/V2 O5 nanocomposite, there is three-step weight loss over the entire temperature range in the nitrogen atmosphere. The thermogram shows ∼4% of weight loss below 170 ◦ C, due to the elimination of weakly bound water, impurities and unreacted monomer if any. Between 170 ◦ C and 460 ◦ C, the weight loss
could be assigned to the loss of strongly bound water or oxygen functional groups (–OH O) attached sometimes to conducting polymer rings by oxidative addition of oxygen competing with the oxidative polymerization. While, the continuous weight loss from 460 ◦ C (∼31%) onwards is attributed to the degradation and decomposition of the skeletal polymer backbone [16]. The steps of the intercalative polymerization in the mesostructured V2 O5 are presented schematically in Scheme 1. The in situ intercalation reaction of aniline with bulk V2 O5 was carried out under microwave conditions. It is a redox reaction in which the aniline monomer is oxidatively polymerized and the V2 O5 layers are reduced. At the beginning of the reaction, to avoid any polymerization in solution, aniline was converted to the anilinium cation in an acidic environment and allowed to diffuse into the inter-layer spaces of V2 O5 and then anilinium cations were subjected to oxidative polymerization inside layered V2 O5 , as it acts as a oxidizing agent [7]. An instant colour change of the solution is observed from yellowish-orange to dark green indicating the reduction of V2 O5 . The bulk layered vanadium oxide was exfoliated to form polyaniline–vanadium oxide nanocomposite by partly reducing to V4+ [5]. It means that layered V2 O5 could be feasible for the intercalation of aniline and itself the oxidative catalyst, which could accelerate the oxidative polymerization of intercalated aniline. Kanatzidis and Kwon et al. have already discussed the intercalation reaction and found that the growth of polymer goes on mainly within the interlamellar space with the consumption of oxygen. Aging in air causes both partial reoxidation of the
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the polymer was confirmed from the UV–vis and FT-IR spectroscopic measurements. The result indicates that the PANI/V2 O5 nanocomposite synthesized, bear tremella-like morphology with the thickness of the nanosheets ranging 15–20 nm. The advantages of reported method include the simplicity, the ease and timesaving synthesis of the tremella-like morphology of the polyaniline–vanadium oxide nanocomposite nanosheet. The PANI/V2 O5 has rather good thermal stability. The conductivity of the polymer was found to be ∼2.27 × 10−5 S/cm. Acknowledgements Authors would like to thank Dr. D.P. Amalnerkar, Executive Director, C-MET, Pune for his constant encouragement throughout this work and DIT, New Delhi for the financial support. Authors also would like to thank the Nanocrystalline Materials Group C-MET, Pune. References Scheme 1.
inorganic host, that is auto-recovery of V5+ from V4+ , and the postpolymerization of the organic intercalate [1,17,18]. The conductivity of the PANI/V2 O5 nanocomposite was measured by using standard two probe conductivity measurement techniques. Pressed pellets of the nanocomposite having 1.5 cm diameter was prepared using stainless steel die by applying 7 tons pressure with the help of hydraulic press. The room temperature conductivity was found to be ∼2.27 × 10−5 S/cm. 4. Conclusions The polyaniline–vanadium oxide nanocomposite nanosheet was successfully synthesized by intercalation and subsequent in situ polymerization of aniline under microwave digestion conditions. The presence of conducting emeraldine salt phase of
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
P. Gomez-Romero, Adv. Mater. 13 (2001) 63. I. Karatchevtseva, Z. Zhang, J. Hanna, V. Luca, Chem. Mater. 18 (2006) 4908. J. Huang, Pure Appl. Chem. 78 (2006) 15. M. Kulkarni, A. Viswanath, R. Aiyer, P. Khanna, J. Polym. Sci., Part B: Polym. Phys. 43 (2005) 2161. L. Li, Z. Yan, J. Nat. Gas Chem. 14 (2005) 35. A. Cao, J. Hu, H. Liang, L. Wan, Angew. Chem. Int. Ed. 44 (2005) 4391. Z.F. Li, E. Ruckenstein, Langmuir 18 (2002) 6956. M. Malta, G. Louarn, N. Errien, Electrochem. Commun. 5 (2003) 1011. M. Ferreira, F. Huguenin, V. Zucolotto, et al., J. Phys. Chem. B 107 (2003) 8351. M. Ferreira, V. Zucolotto, C.J.L. Constantino, et al., Synth. Met. 137 (2003) 969. N. Asim, S. Radiman, M. bin Yarmo, Mater. Lett. 62 (2008) 1044. S. Pang, G. Li, Z. Zhang, Macromol. Rapid Commun. 26 (2005) 1262. B. Rajesh, K.R. Thampi, J.-M. Bonard, N. Xanthapolous, H.J. Mathieu, B. Viswanathana, Electrochem. Solid-State Lett. 5 (2002) E71. M. Kulkarni, A. Viswanath, J. Macromol. Sci. Part A Pure Appl. Chem. 41 (2004) 1173. S. De, A. Dey, S. De, Solid-State Commun. 137 (2006) 662. M. Lira-CantuH, P. GoHmez-Romero, J. Solid State Chem 147 (1999) 601. A. Murugan, C. Kwon, S. Hwang, M. Delville, J. Portier, G. Campet, Act. Passive Electron. Compon 27 (2004) 229. C. Kwon, A. Murugan, G. Campet, Act. Passive Electron. Compon. 26 (2003) 171.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Microwave-assisted hydrothermal synthesis and characterization of tremella-like polyaniline–vanadium oxide nanocomposite nanosheets Mohini A. Jagtap, Milind V. Kulkarni ∗ , Sanjay K. Apte, Sonali D. Naik, Bharat B. Kale Nanocomposite/Glass Laboratory, Centre for Materials for Electronics Technology (C-MET), Department of Information Technology, Govt. of India, Panchawati, Off Pashan Road, Pune 411 008, India
a r t i c l e
i n f o
Article history: Received 26 September 2009 Received in revised form 16 December 2009 Accepted 7 January 2010 Keywords: Nanocomposite Polyaniline Vanadium oxide Tremella-like morphology
a b s t r a c t The organic–inorganic hybrid nanocomposite of polyaniline–V2 O5 was synthesized by microwaveassisted hydrothermal method. The nanocomposite was separated by filtration and washed with deionized water. Structural study of nanocomposite was performed using XRD. The microstructural study of nanocomposite performed using field emission scanning electron microscopy (FESEM) shows self-assembled plates with curly petal or tremella-like morphology. The nanocomposite was subjected for spectroscopic (UV–vis and FT-IR) analysis. The UV–vis spectra revealed the presence of conducting emeraldine salt phase of the polymer which was further supported by FT-IR analysis. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The study of hybrid organic (polymer)–inorganic materials is a recent but very fruitful and prolific enterprise. Self-assembled structures with highly specific morphology and novel properties are of great interest. These hybrid materials represent the natural interface between two worlds of chemistry each with very significant contributions to the field of materials science [1]. The activities of organic and inorganic species combine to reinforce or modify each other. These materials with different combinations have promising physical properties and many potential applications in various fields, such as, optics, electrochemistry, electronics, biology, chemical sensors, light emitting diodes (LED), electrochromic displays (ECD), and EMI shielding. Polyaniline (PANI) is unique among conducting polymers, which can be rapidly converted between the base (insulator) and the salt form (conductor) by treatment with acid or base or electrochemical means. PANI also has good environmental stability and high electrical conductivity and also can be easily and cheaply synthesized [2]. It has a great variety of potential applications including anti-corrosion coatings, batteries, and sensors [3,4]. Vanadium oxide (V2 O5 ) has been extensively used as a well-known transition metal oxide. It displays the novel properties including electrochromism. The charge storage behavior of
∗ Corresponding author. Tel.: +91 20 2589 8390 9273, 2589 9273; fax: +91 20 2589 8085. E-mail addresses: [email protected], [email protected] (M.V. Kulkarni). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.016
deposits of hydrated V2 O5 is of special interest in rechargeable lithium batteries. It is often employed in secondary lithium batteries to improve the capacity, voltage, reversibility and stability [5,6]. Li et al. [7] obtained a mesostructured nanocomposite vanadium oxide/PANI, the conductivity of which is higher than that for vanadium oxide xerogel. Malta et al. [8] prepared V2 O5 /PANI nanocomposite with fibrillar morphology by hydrothermal treatment of nanocomposite V2 O5 /PANI and hexadecylamine. Ferreira et al. [9,10] synthesized nanoscale composites of polyaniline (PANI) and vanadium oxide (V2 O5 ) via the electrostatic layer-by-layer (LBL) technique. These composites enhance the charge storage capability of the films. Asim et al. synthesized core-shell PANI/V2 O5 nanocomposite via microemulsion method [11]. The hydrothermal synthesis of the polyaniline–vanadium oxide nanocomposite involves mostly reactions between heterogeneous precursors, the time scale of such a synthesis is very long (generally a few days) [12], while in the sol–gel method aging of V2 O5 takes 1 week to 1 month prior to the synthesis of PANI/V2 O5 nanocomposite [8]. PANI/V2 O5 nanocomposites synthesized by chemical method involves stirring for 16 h in air and further its isolation, drying and its characterization [13]. Considerable attention is paid to the simplification or improvement of existing methods widely used in the synthesis of organic–inorganic hybrid nanocomposite. Compared with the conventional heating, microwave heating is promising due to its unique effects, such as rapid volumetric heating, higher reaction rates and shorter reaction time, higher reaction selectivity and energy saving.
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Microwave-induced reactions have already made an enormous impact in modern science. The formation of uniform nanosized particles demands a uniform growth environment, and microwave heating affords this. In the case of microwave-induced synthesis, the reactions are driven by the intense collision and friction of the molecules. Microwaves are electromagnetic waves, which contain electric and magnetic field components. The force derived from these changes direction rapidly, which causes heat because, the assembly of molecules cannot respond to it instantaneously, creating friction which manifests itself as heat. The goal of this article is to report a facile synthesis method to self-assembled polyaniline–vanadium oxide nanocomposite nanosheets under microwave conditions. 2. Experimental 2.1. Materials Aniline (AR grade), vanadium oxide (V2 O5 , AR grade), hydrochloric acid (HCl, AR grade) were obtained in high purity from S.D. Fine Chem. Ltd. (India). Deionized and double distilled water were used for the preparation of solutions. 2.2. Synthesis PANI/V2 O5 nanocomposite was synthesized by in situ intercalation of V2 O5 powder (0.18 g) and aniline (0.006 g) in 40 ml distilled water. Hydrochloric acid was added to the mixture until the pH of the solution becomes ∼3. The mixture was transferred into a Teflon liner autoclave in a microwave digestion system and reaction was carried out by using ‘Microwave Labstation ms 1200 mega’ (Germany) at 500 W for 15 min in air. After completion of the reaction, autoclave was air-cooled to room temperature. The resultant dark green product was filtered and washed with distilled water and alcohol several times and then dried in vacuum at 60 ◦ C for 24 h. The yield of the product was ∼94%. 2.3. Characterization The UV–vis spectrum of the nanocomposite was recorded by using Hitachi-U3210 spectrophotometer in the range of 300–900 nm. The FT-IR spectrum was taken on a Perkin-ElmerSpectrum 2000 spectrophotometer in the range 400–4000 cm−1 . The sample was prepared in the pellet form using a spectroscopic grade KBr powder. Morphological studies were performed with the help of a FEI QUANTA 200F field emission scanning electron microscope (FESEM). The X-ray diffraction (XRD) measurements were performed using Brucker-AXS Model D8 Advance, X-ray diffractometer. Thermogram of the polymer sample was recorded using Mettler-Toledo 851 thermogravimetric analyzer in presence of N2 atmosphere from RT to 900 ◦ C at a heating rate of 10 ◦ C/min. Electrical conductivity was measured by the two probe technique. Dry powdered sample was made into pellet using a steel die of 1.5 cm diameter in a hydraulic press under a pressure of 7 tons. 3. Results and discussions The optical absorption spectrum of the neat polyaniline and polyaniline/V2 O5 is presented in Fig. 1. In water, polyaniline shows a sharp peak at 320 nm, shoulder at 580 nm and increasing absorption at ∼800 nm. The sharp peak at 320 nm corresponds to the –* transition of the benzenoid rings and the shoulder at 580 nm and increasing absorption at ∼800 nm represents insulating pernigranilne and conducting emeraldine salt phase of the polymer respectively. This clearly shows the formation of mixed phases
Fig. 1. UV–vis spectra of (A) polyaniline; and (B) polyaniline–vanadium oxide nanocomposite nanosheets.
in the polymer which is further supported by the FT-IR studies. The UV–vis spectrum of polyaniline–V2 O5 exhibits a sharp peak at 320 nm and shoulder at 420 nm and 830 nm. The peak at 320 nm corresponds to the –* transition of the benzenoid rings, while the shoulder at 420 nm corresponds to the polaron absorption band. However, the increasing absorption or small peak ∼820 nm is assigned to the conducting emeraldine salt phase of the polymer. Localized polaron bands at around 420 nm and 830 nm are characteristic peaks of emeraldine salt of PANI [2,14]. On comparison of the spectras of neat polyaniline with the nanocomposite there is not much variation is observed, except the formation of polaronic peak at 420 nm in the nanocomposite which is absent in the neat polyaniline spectrum. Fig. 2 represents the FT-IR spectra of the neat polyaniline and polyaniline/V2 O5 nanocomposite synthesized by in situ polymerization method and the peak positions related to the corresponding chemical bonds are listed in Table 1. The peaks in 1000–1600 cm−1 region, attributed to the framework vibration of polyaniline, suggest no evidence for the presence of anilinium ion or free aniline. The presence of the two bands in the vicinity of 1500 cm−1 and 1600 cm−1 is assigned to the benzene stretching modes. The higher frequency vibration at 1600 cm−1 has a major contribution from the quinoid rings while, the lower frequency mode at 1466 cm−1 depicts the presence of benzenoid ring units. The presence of these two bands clearly shows that the polymer is composed of the amine and imine units. The bands below 1100 cm−1 correspond to oxide absorption in composite. The absorption at 1001 cm−1 corresponds to V O stretching modes while at 758 cm−1 and 537 cm−1 is for V–O–V stretching modes. The FT-IR results show intercalation reaction of aniline and V2 O5 [11,15,17] and these bands are absent in the neat polyaniline spectrum. Further, this also supports the UV–vis characterization, discussed earlier, where, the different phases are observed in the spectrum. The presence of other charac-
M.A. Jagtap et al. / Materials Science and Engineering B 168 (2010) 199–203
201
Table 1 Characteristic frequencies of polyaniline–vanadium oxide nanocomposite nanosheets. Wavenumber (cm−1 )
Band characteristics
PANI
PANI/V2 O5
814 – – – 598 1126 1300 1494 1572 3200
– 537 758 1001 – – 1306 1466 1629 3421
Paradisubstituted aromatic rings Symmetric V–O–V stretching modes Asymmetric V–O–V stretching modes V O stretching modes C–H out-of-plane bending vibration C–H in plane bending vibration Aromatic C–N stretching indicating secondary aromatic amine group C–N stretching of benzenoid rings C–N stretching of quinoid rings N–H stretching vibration
teristic bands confirms the presence of conducting emeraldine salt phase in the polymer. Fig. 3 shows the FESEM micrographs of nanocomposite recorded at different magnifications. The micrographs of the nanocomposite synthesized by in situ polymerization method offer very interesting morphological features, i.e. tremella-like morphology of nanocomposite nanosheets. In the surface study, it was observed that the thickness of the nanosheets is between 15 nm and 20 nm. At higher magnification (Fig. 3B), the clarity of the morphology is found to be further enhanced and it is observed that the nanosheets are self-assembled into lamellar structures. The dimensions of the nanosheets are in the range of hundreds of nanometers (to several micrometers). Fig. 4 shows the XRD pattern of polyaniline, mesostructured V2 O5 precursor and PANI/V2 O5 nanocomposite under microwave conditions. The peaks in Fig. 4(A) can be attributed to orthorhombic
crystalline V2 O5 phases. The XRD spectrum of the neat polyaniline shows the absence of sharp peak indicating amorphous nature of the polymer. It is seen that, the diffraction peaks of V2 O5 become wider and get reduced after intercalation of aniline (Fig. 4(C)). This indicates the intercalated product is relatively with low crystalinity ascribed to the limited short-range order [5]. It is suggested that aniline is actually intercalated into the V2 O5 layer and in situ polymerization takes place in the layer.
Fig. 2. FT-IR spectrum of (A) polyaniline; and (B) polyaniline–vanadium oxide nanocomposite nanosheets.
Fig. 3. Scanning electron micrographs PANI/V2 O5 nanocomposite nanosheets at the magnifications of 40,000× (A); and 80,000× (B).
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Fig. 5. Thermogram of (A) polyaniline; and (B) polyaniline–vanadium oxide nanocomposite nanosheets.
Fig. 4. XRD patterns of the initial (A) V2 O5 precursor; (B) polyaniline; and (C) polyaniline–vanadium oxide nanocomposite nanosheets.
The thermal stability of neat polyaniline and the polyaniline/V2 O5 nanocomposite was examined by TGA and represented in Fig. 5(A) and (B) respectively. From the careful observation of thermogram of polyaniline it is observed that a sharp weight loss up to 100 ◦ C is observed which is corresponding to the loss of the water/moisture present in the polymer matrix. After 200 ◦ C there is continuous weight loss up to 900 ◦ C, this could be mainly due to the loss of the dopant ion and the degradation and decomposition of the polymer backbone. However, in the PANI/V2 O5 nanocomposite, there is three-step weight loss over the entire temperature range in the nitrogen atmosphere. The thermogram shows ∼4% of weight loss below 170 ◦ C, due to the elimination of weakly bound water, impurities and unreacted monomer if any. Between 170 ◦ C and 460 ◦ C, the weight loss
could be assigned to the loss of strongly bound water or oxygen functional groups (–OH O) attached sometimes to conducting polymer rings by oxidative addition of oxygen competing with the oxidative polymerization. While, the continuous weight loss from 460 ◦ C (∼31%) onwards is attributed to the degradation and decomposition of the skeletal polymer backbone [16]. The steps of the intercalative polymerization in the mesostructured V2 O5 are presented schematically in Scheme 1. The in situ intercalation reaction of aniline with bulk V2 O5 was carried out under microwave conditions. It is a redox reaction in which the aniline monomer is oxidatively polymerized and the V2 O5 layers are reduced. At the beginning of the reaction, to avoid any polymerization in solution, aniline was converted to the anilinium cation in an acidic environment and allowed to diffuse into the inter-layer spaces of V2 O5 and then anilinium cations were subjected to oxidative polymerization inside layered V2 O5 , as it acts as a oxidizing agent [7]. An instant colour change of the solution is observed from yellowish-orange to dark green indicating the reduction of V2 O5 . The bulk layered vanadium oxide was exfoliated to form polyaniline–vanadium oxide nanocomposite by partly reducing to V4+ [5]. It means that layered V2 O5 could be feasible for the intercalation of aniline and itself the oxidative catalyst, which could accelerate the oxidative polymerization of intercalated aniline. Kanatzidis and Kwon et al. have already discussed the intercalation reaction and found that the growth of polymer goes on mainly within the interlamellar space with the consumption of oxygen. Aging in air causes both partial reoxidation of the
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the polymer was confirmed from the UV–vis and FT-IR spectroscopic measurements. The result indicates that the PANI/V2 O5 nanocomposite synthesized, bear tremella-like morphology with the thickness of the nanosheets ranging 15–20 nm. The advantages of reported method include the simplicity, the ease and timesaving synthesis of the tremella-like morphology of the polyaniline–vanadium oxide nanocomposite nanosheet. The PANI/V2 O5 has rather good thermal stability. The conductivity of the polymer was found to be ∼2.27 × 10−5 S/cm. Acknowledgements Authors would like to thank Dr. D.P. Amalnerkar, Executive Director, C-MET, Pune for his constant encouragement throughout this work and DIT, New Delhi for the financial support. Authors also would like to thank the Nanocrystalline Materials Group C-MET, Pune. References Scheme 1.
inorganic host, that is auto-recovery of V5+ from V4+ , and the postpolymerization of the organic intercalate [1,17,18]. The conductivity of the PANI/V2 O5 nanocomposite was measured by using standard two probe conductivity measurement techniques. Pressed pellets of the nanocomposite having 1.5 cm diameter was prepared using stainless steel die by applying 7 tons pressure with the help of hydraulic press. The room temperature conductivity was found to be ∼2.27 × 10−5 S/cm. 4. Conclusions The polyaniline–vanadium oxide nanocomposite nanosheet was successfully synthesized by intercalation and subsequent in situ polymerization of aniline under microwave digestion conditions. The presence of conducting emeraldine salt phase of
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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