An infrared and Raman spectroscopic study of polyanilines co-doped with metal ions and H+

Spectrochimica Acta Part A 66 (2007) 1364–1368

Short communication

An infrared and Raman spectroscopic study of polyanilines co-doped with metal ions and H+ Sun Tao a , Bi Hong a,b,∗ , Zhu Kerong c a

b

Department of Chemistry, Anhui University, Hefei 230039, China The Key Laboratory of Environment-Friendly Polymer Materials of Anhui Province, Anhui University, Hefei 230039, China c Center of Modern Experimental Technology, Anhui University, Hefei 230039, China Received 23 March 2006; received in revised form 4 August 2006; accepted 7 August 2006

Abstract Polyanilines doped with (HCl + KCl) and (HCl + CoCl2 ) were prepared by co-doping method, respectively. For comparison, polyaniline emeraldine salt (ES) by doping with HCl and its emeraldine base (EB) form were also synthesized. The co-doped polyanilines, ES and EB samples were all characterized by Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy aiming to understand the transformations in the different doping status. The results show that the doping degree of K+ ions is considerably higher than that of Co2+ ions under the same co-doping conditions possibly due to different pseudoprotonation constants of EB with K+ ions and Co2+ ions. Moreover, morphology difference of polyaniline co-doped with alkaline metal ions or transition meal ions may arise from different coordination geometry of metal ions. Nevertheless, there are similar chemical transformations of quinoid units to benzenoid ones on polyaniline backbones for the ES and both co-doped samples. And the polyaniline backbones co-doped with H+ and metal cations are found to attain weaker charge delocalization than the ES which is doped solely with H+ . © 2006 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Raman spectroscopy; Polyaniline; Co-doping method

1. Introduction Polyaniline (PANI) has received a great deal of attention over the past two decades [1] due to its high electrical conductivity [2], unique redox properties [3] and favorable environmental stability. The excellent processability and the presence of a number of intrinsic redox states have enhanced polyaniline in potential practical applications [4] such as in chemical sensor devices [5,6], light-emitting diodes [7], and electromagnetic interference (EMI) shielding materials [8], etc. It is well known that PANI has a variety of oxidation states that are both pH and potential dependent. Three different base forms of PANI are usually distinguished in the literature: leucoemeraldine (LEB, fully reduced), emeraldine (EB, half-oxidized) and pernigraniline (PNB, fully oxidized) [9]. Especially, emeraldine base can be doped with protonic acid to become emeraldine salt (ES) due to the coordination of the protons with the imine

∗

Corresponding author. Tel.: +86 551 3935397; fax: +86 551 5106094. E-mail address: [email protected] (B. Hong).

1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2006.08.011

nitrogen of emeraldine base (EB) [10,11]. Protonic acid doping creates, to various degrees, the charge delocalization on the polymer backbone resulting in increased dc conductivity [12,13] and increased spin density [14]. On the contrary, in LiCl-doped polyaniline the charge carriers are strongly localized so that the electrical conductivity and magnetic susceptibility of LiCltreated samples were determined to be much smaller than those for protonated EB [15]. Similarly, it was found that conductivity and spin concentration of the ZnCl2 -treated polyaniline emeraldine salts are less than those of the ES [16]. Since polyaniline doped with transition metal ion, i.e. Pd(II) was reported to be a selective catalyst in dehydrogenative oxidation [17] and hydrogenation reactions [18], in recent years, increasing research interests have been focused on studies of the interactions between polyaniline chain and metal ions. It is considered that the polyaniline is doped via pseudoprotonation of the imine nitrogen by lithium or zinc cations [19]. Hasik and co-workers have investigated the interactions between polyanilines and palladium ions both in aqueous and nonaqueous media [20,21]. They found that in more acidic aqueous solution, highly protonated poly-emeraldine is obtained and Pd2+ enters into the

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polymer matrix without changing of its oxidation level. In contrast, in solutions of low acidity, some Pd2+ is reduced to Pd0 with simultaneous oxidation of the polymer backbone towards polypernigraniline [20]. Moreover, the complexation of PANI with Cu2+ in CuCl2 N-methylpyrrolidinone (NMP) solutions has also been reported [22]. Dimitriev and Kislyuk reported a novel conductive complex formation of polyaniline with europium chloride [23], and the effect of metal cations on the polyaniline film morphology when doped by transition metal salts [24]. Recently, Yang et al. reported that polyanilines doping with transition metal ions (Fe3+ , Co2+ , Ni2+ and Cu2+ ) were synthesized by oxidizing the complex of aniline with metal ions in solution, respectively. Compared with the ES, transition metal ions doped PANI exhibit two to three orders of magnitude lower electrical conductivity which was attributed to the complexation of polyaniline with the transition metal ions [25]. All these findings prompted us to investigate the interaction of metal ions with the ␲-conjugate polyaniline chains. This work reports that the polyaniline backbones co-doped with HCl and KCl or CoCl2 are found to attain weaker charge delocalization than the ES which is doped solely with HCl. Our assertion is based on the measurement of the optical absorptions of the codoped polyanilines, ES and its EB form. The study is helpful to fully understand the effect of different kinds of metal cations on the ␲-conjugated polymer backbones. 2. Experimental Polyaniline EB and its ES (PANI–HCl) form were prepared by a standard procedure of aniline oxidation according to the literature [26]. In order to exclude the influence of different counter-ions, chlorides KCl and CoCl2 were selected as dopants for co-doping with HCl. Salts of KCl and CoCl2 ·6H2 O were dissolved in H2 O to prepare aqueous solutions with desirable concentrations. Polyaniline with co-doping of (HCl + KCl) or (HCl + CoCl2 ) have been synthesized as follows: 5 g aniline was mixed with 100 mL aqueous acidic (pH = 1) salt solution with the molar ratio 2:1 of aniline monomer to metal ions. Then 5 g (NH4 )2 S2 O8 was dissolved in 40 mL H2 O and added drop-wise into the above solution in 30 min. The reaction was carried out under the temperature of 5 ◦ C with constant stirring for 24 h, and then the precipitate was filtered and washed with 95% ethanol and acetone for several times, finally the obtained product was dried under vacuum for 48 h. The resulting powder samples were labeled as PANI–(HCl + KCl) and PANI–(HCl + CoCl2 ), respectively. The doping contents of Co2+ and K+ ions in the sample are determined by inconductively coupled plasma atomic emission spectrometer (ICP, Model Atomscan Advantage, Thermo Jarrell Ash Corporation, USA). The morphologies of samples PANI–HCl, PANI–(HCl + KCl) and PANI–(HCl + CoCl2 ) were observed by using a JEOL 100CX transmission electron microscopy (TEM). The samples used for TEM were prepared by suspending the powder sample in ethanol with sonication for 15 min, then placing a drop of the suspension on a carbon-coated Cu grid and drying naturally. Fourier transform infrared (FTIR) spectra of all samples were recorded on a Nicolet 200SXV FTIR

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spectrometer in KBr pellets. The UV/vis absorption spectrum of EB dissolved in NMP was recorded on a Shimadzu UV-2501 PC spectrophotometer. Raman spectra have been measured by using a France JY HR800 Raman Spectrometer with the 488 nm line of air-cooled Ar+ laser used for excitation. The resolutions of the IR spectrometer and the Raman spectrophotometer were 2.0 and 0.8 cm−1 , respectively. 3. Results and discussion 3.1. Doping contents of metal ions and co-doping mechanism It is known that EB can be doped with protonic acid to become ES due to the coordination of the protons with the imine nitrogen of EB which is called protonation process. Fig. 1 shows a schematic formula of EB and its transformation to ES by protonation selected from reference [20]. Once EB is doped with alkaline metal or transition metal cations, a pseudoprotonation process of the imine nitrogen by the cations takes place. In our experiment, since the EB is co-doped with H+ and K+ or Co2+ cations, it is reasonable to assume that the protonation and pseudoprotonation processes undertake simultaneously. Although both of the co-doped samples were synthesized in aqueous acidic salt solution with the molar ratio 2:1 of aniline monomer to metal ions, the actual doping contents of metal cations in the samples should be measured. According to the ICP data, the exact mass doping content of Co2+ ions in the codoped sample PANI–(HCl + CoCl2 ) is 0.17 %, and that of K+ ions in the sample PANI–(HCl + KCl) is 0.25%. Considering the molar mass of Co is 58.933 g/mol and that of K is 39.098 g/mol, the ratio of molar doping content of K+ ions to that of Co2+ ions is 6.39 × 10−5 :2.86 × 10−5 . It means doping degree of K+ ions is 2.23 times higher than that of Co2+ ions under the same co-doping conditions. We’ve repeated the experiment for several times, and almost the same results were obtained. This remarkable difference of doping degree may arise from different pseudoprotonation constants of polyaniline EB with K+ ions and Co2+ ions, to our knowledge, no literature data has been found on pseudoprotonation constants of polyaniline EB with alkaline and transition metal cations. A further study on pseudoprotona-

Fig. 1. Schematic formula of EB and its transformation to ES by protonation.

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Fig. 2. TEM images of (a) PANI–HCl, (b) PANI–(HCl + CoCl2 ), and (c) PANI–(HCl + KCl).

tion constants of EB with various metal cations was undergoing by potentiometric measurements, it will be helpful to prepare EB-membrane ion-selective electrode for various metal cations in the future although polyaniline-modified ion-selective electrodes for anion recognition has already been presented [27]. 3.2. Morphology It is recognized that transition metal ions as dopants are able to interact with polyaniline chains directly and to induce morphology changes of polyaniline film [24,25], which is often observed by a SEM. Before observation, the film is prepared by dissolving dry polyaniline powder in NMP and spin-casting or drop-casting onto a quartz plate. Normally, this process can be considered as a result of competition of the metal cation-to-solvent coordination and the metal cation-to-polymer coordination, with shift of the equilibrium to the latter process upon removal of a competitive ligand [24]. That is to say, this process cannot exclude the influence of the organic solvent on the interactions between metal ions and polymer chains. Therefore, dry powder samples suspending in ethanol were used directly to investigate the morphologies and thus the corresponding interactions between metal ions and polymer chains. Morphologies of samples PANI–HCl, PANI–(HCl + CoCl2 ), and PANI–(HCl + KCl) are shown by TEM images of the same magnification of 50,000 in Fig. 2. It can been seen clearly that TEM image of sample PANI–HCl (Fig. 2a) as well as that of sample PANI–(HCl + KCl) (Fig. 2c) is in irregular platelike morphology, whereas sample PANI–(HCl + CoCl2 ) exhibits morphology of linked quasi-spherical particles with average size of 80–130 nm as shown in Fig. 2b, which differ apparently from both of Fig. 2a and c. This morphology difference is likely to derive from a change in coordination geometry—transitionmetal ions such as Co2+ own coordination number (n) less than or equal to 6 and generally with octahedral coordination geometry, it means one Co2+ ion may bind to more than one nitrogen sites in a polyaniline chain or form inter-chain linkage among several adjacent polyaniline chains by coordination, both intra-chain and inter-chain connections may lead to a more coil-like conformational change or a more twisted aggregation of polyaniline chains and thus a quasi-spherical morphology. On the contrary, each alkaline-metal ion such as K+ can only bind to one nitrogen site via “pseudoprotonation”, although the doping content of K+ ions is remarkably higher than that of Co2+ ions in the EB, the

more homogenous doping results in a homogenous morphology with no tendency of phase separation. 3.3. FTIR spectra As shown in Fig. 3, the FTIR spectra of samples EB, PANI–HCl, PANI–(HCl + CoCl2 ), and PANI–(HCl + KCl) are essentially the same, indicating that the chain protonation reaction is dominant. The positions of the main IR absorption bands of the four samples and the corresponding assignments are collected in Table 1. The spectral changes caused by the doping can be briefly described as follows: (1) The peak at 1162 cm−1 in EB (Fig. 3a) has red-shifted to 1104 cm−1 in PANI–HCl (Fig. 3b). It can be assigned to the doped PANI chain with H+ and was commonly observed in the spectra of EB protonated with strong acids [20]. (2) In fact, it can be seen from Table 1 that almost all peaks in EB have red-shifted to lower frequency after doping or co-doping in our experiment. Typically, the bands

Fig. 3. FTIR spectra of (a) EB, (b) PANI–HCl, (c) PANI–(HCl + CoCl2 ), and (d) PANI–(HCl + KCl).

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Table 1 Main IR vibrational frequencies (cm−1 ) and the corresponding assigned modes of samples EB, PANI–HCl, PANI–(HCl + CoCl2 ), and PANI–(HCl + KCl) PANI EB

PANI–HCl

PANI–(HCl + CoCl2 )

PANI–(HCl + KCl)

Stretching mode

1587 1495 1303 1259 1162 829

1557 1473 1297 1241 1104 795

1559 1480 1297 1242 1126 796

1560 1481 1297 1242 1126 798

N Q N stretching N–B–N stretching N–H bending C–N stretching B–NH+ –B or Qa NH+ –B Out-of-plane C–H bending

a

Q denotes quinoid units of PANI; B denotes benzenoid units of PANI.

at 1587 cm−1 (N Q N stretching; Q: quinoid unit) and 1495 cm−1 (N–B–N stretching; B: benzenoid unit) in EB have an obvious red-shift to 1557 and 1473 cm−1 , respectively, which is exactly in consistence with that of previous literatures [20,28]. Similarly, both bands also have a red shift in PANI–(HCl + CoCl2 ) (Fig. 3c) and PANI–(HCl + KCl) (Fig. 3d), respectively. However, it is noteworthy that the two bands appear at a little higher frequency in Fig. 3c and d compared to that in Fig. 3b. The red shift of the two IR absorption peaks is referred to be a signature of the conversion of the quinoid rings to the benzenoid rings by proton-induced spin-unpairing mechanism [28], which was considered to be an indication of increasing degree of charge delocalization on the polyaniline backbone due to protonation [29]. However, the bands at 1104, 1557, and 1473 cm−1 in Fig. 3b all shifted to a little higher frequencies in Fig. 3c and d, it implies that co-doping with (HCl + CoCl2 ) or (HCl + KCl) instead of doping solely with the protonic acid HCl may lead to a less degree of charge delocalization on the polyaniline backbone. (3) In EB, there exist two types of sites available for metal ions bonding via either protonation or complexation (i.e., formation of a coordination Pd–N bond): amine (–NH–) and imine (–N ) nitrogen atoms [20]. It has been demonstrated that in the case of protonation with simple inorganic acids, the imine sites are protonated preferentially [30]. It is confirmed by our result (as shown in Table 1) that band at 1303 cm−1 in EB assigned to N–H bending mode only shifted a little to 1297 cm−1 in PANI–HCl and keeps unchanged in both PANI–(HCl + KCl) and PANI–(HCl + CoCl2 ). On the contrary, band at 1162 cm−1 in EB assigned to –N vibration has shifted sharply to 1104 and 1126 cm−1 after doping and co-doping, respectively. (4) It is noted that the IR spectra of PANI–(HCl + KCl) and PANI–(HCl + CoCl2 ) are the same (Table 1) even though the ion contents in the two samples are considerably different. This characteristic cannot be explained clearly now, but the final chemical state of the polyaniline backbone depends on a comprehensive effect of protonation and pseudoprotonation processes which are assumed to undertake simultaneously during co-doping with strong protonic acid and metal cations. The same IR of the two co-doped samples imply that total effect of K+ ions by pseudoprotonation is almost equal to that of Co2+ ions although actual doping content of K+ ions is 2.23 times higher than that of Co2+ ions.

3.4. Raman spectra Fig. 4 shows all of the 488 nm Ar+ laser-excited Raman spectra of EB, PANI–HCl and the co-doped PANI samples. The sample of EB dissolved in NMP exhibited two absorption bands locating at 332 and 638 nm in UV–vis spectrum as previously reported [20]. It means that the 488 nm Ar+ -laser excitation line is not the resonance frequency for the as-synthesized EB sample, but it will increase both quinoid unit and semi-quinoid unit vibrational bands [31]. Furthermore, laser power was always kept below 5.0 mW at the samples, and the integral time were controlled in 60 s for all the samples, aiming to avoid structural change caused by long, strong photo excitation [31,32]. Regarding the bands at 1609 cm−1 (C–C stretching of benzoid units) and 1589 cm−1 (C C stretching of quinoid units) in EB (Fig. 4a), the former band blue shifted to a little higher frequencies (1619, 1621, 1624 cm−1 ) in Fig. 4b–d, and its intensity increased sharply after doping or co-doping whereas the latter band disappeared. In the case of the doped samples, the band observed at 1337 cm−1 is the most characteristic Raman band of the radical cation (C–N+• stretching) [30,32,33] which is non-observable in that of EB. This typical band is expected

Fig. 4. Raman spectra of (a) EB, (b) PANI–HCl, (c) PANI–(HCl + CoCl2 ), and (d) PANI–(HCl + KCl).

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when quinoid rings are converted to benzoid rings. Another obvious change is the 1497 cm−1 band in Fig. 4a assigned to N C N stretching of the quinoid di-imine units diminishing in Fig. 4b–d, which indicates the decreasing of the amount of quinoid units in the polymer chain due to the doping processes. Moreover, the 1413 cm−1 band in Fig. 4a diminished in Fig. 4b and c and almost disappeared in Fig. 4d, this phenomenon has also been found in Pd-doped PANI [20]. One possible reason of this change with different degree for the two co-doped samples may be due to higher doping content of K+ ions than that of Co2+ ions. Additionally, a blue shift of the 1164 cm−1 band in Fig. 4a to 1191 cm−1 in Fig. 4b–d must be noted, the 1164 cm−1 band is assigned to the in plane C–H bending of quinoid units while the 1191 cm−1 band is attributed to the same mode in benzoid segments [34]. The bands in range of 800–900 cm−1 includes many information about deformation of benzoid ring, which also become more salient in all of the doped and codoped samples. All these changes confirmed the transformation of quinoid units into benzoid ones in the PANI backbones after doping and co-doping and that the nitrogen atom is the “reductive point”. 4. Conclusions There is a chemical transformation of quinoid units to benzenoid ones in EB backbone for both ES and co-doped samples. However, co-doping with H+ and metal cations instead of doping solely with the protonic acid may lead to a less degree of charge delocalization on the polyaniline backbone. It is found that doping degree of K+ ions is considerably higher than that of Co2+ ions under the same co-doping conditions possibly due to different pseudoprotonation constants of EB with K+ ions and Co2+ ions. Moreover, morphology difference of polyaniline co-doped with H+ and metal ions may arise from different coordination geometry of metal ions. However, the IR spectra of PANI containing Co2+ and K+ ions are the same even though the ion doping contents in the samples are different. This result is based on the assumption that when the PANI is co-doped with H+ and metal cations, the protonation and pseudoprotonation processes undertake simultaneously, the final chemical state of the polyaniline backbone depends on a comprehensive effect of protonation and pseudoprotonation processes. Further study on the origin of weakening delocalization on the PANI backbone by pseudoprotonation of metal cations is still in progress. Acknowledgement All of the authors sincerely thank for the financial support of National Key laboratory of Solid State Microstructure, Nanjing University, under the grant no. M031408.

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