Comparative studies of solid-state synthesized polyaniline doped with inorganic acids

Materials Chemistry and Physics 90 (2005) 367–372

Comparative studies of solid-state synthesized polyaniline doped with inorganic acids Tursun Abdiryim, Zhang Xiao-Gang∗ , Ruxangul Jamal School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046,PR China Received 19 April 2004; received in revised form 11 October 2004; accepted 19 October 2004

Abstract Polyaniline (PANI) salts doped with inorganic acids (HCl, H2 SO4 and H3 PO4 ) were directly synthesized by using solid-state polymerization method. The FTIR spectra, UV–vis absorption spectra and X-ray diffraction patterns were used to characterize the molecular structures of the PANI salts. Voltammetric study was done to investigate the electrochemical behaviors of all these PANI salts. The PANI salts were affected by varying the protonation media (HCl, H2 SO4 and H3 PO4 ). The FTIR and UV–vis absorption spectra revealed that all PANI salts contained the conducting emeraldine salt phase at different oxidation state. The crystallinity of PANI doped with HCl was better than those doped with H2 SO4 and H3 PO4 . The conductivity of the PANI doped with HCl is the highest among the inorganic acid doped PANI. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyaniline; Solid-state polymerization; Inorganic acids

1. Introduction Polyaniline(PANI) is one of the most interesting conducting polymers due to its environmental stability, ease in preparation, exciting electrochemical, optical and electrical properties and possible applications in rechargeable batteries, microelectronics devices, biosensors, electrochromic displays and chemical sensors[1–8]. PANI occurs in four different oxidation states as shown in Fig. 1. It is insulating with the conductivity of the order of 10−12 S cm−1 . Although insulating in their neutral form, they can be rendered conductive through appropriate oxidation or reduction. Protonation of emeraldine base gives rise to an increase of electronic conductivity by several orders of magnitude [9]. Typically, conducting PANI is synthesized by electrochemical or chemical oxidation of aniline in acidic solutions [10,11]. Alternative methods have been designed to improve the solubility and processibility of the synthesized PANI. Gong et al. have reported solid-state synthesis of PANI doped with ∗

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H4 SiW12 O40 under −20 ◦ C by furbishing in mortar [12]. Kaner et al. have reported a solvent-free mechanochemical route for the synthesis of PANI in which the reaction between aniline salt and the oxidant, ammonium peroxydisulfate was carried out by ball milling the reactants for one hour, in the absence of solvent at ambient temperature [13]. In this paper, PANI doped with inorganic acids synthesized by solid-state polymerization. The results were explained on comparative basis. The effect of inorganic acids on the PANI properties was discussed in detail.

2. Experimental 2.1. Synthesis Aniline monomer was distilled under reduced pressure. Other reagents, such as dopants (HCl, H2 SO4 and H3 PO4 ), oxidant (ammonium peroxydisulfate, APS) were used as received (all the chemicals and aniline used were of AR grade). A typical solid-state polymerization procedure was as following: precooled mortar was put in the ice bath, the

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Fig. 1. Four different redox forms of PANI: (a) leucoemeraldine base (fully reduced form), (b) emeraldine base (half-oxidized form), (c) conducting emeraldine salt (half-oxidized and protonated form) (d) pernigraniline base (fully oxidized form).

temperature was kept at 0 ◦ C. 1 ml 37 wt.% HCl was put into the mortar. Then freshly distilled aniline (1 ml) was added dropwise. After grounding the reactant about 10 min, the mixture became white paste, 2.2 g APS was added by further grounding for 20 min until the color of solid changed to black green. The greenish black precipitate of the polymer was isolated by filtration, washed with ethylether, ethanol and distilled water respectively until the solution is colorless, and then dried under vacuum (0.075 MPa) at 50 ◦ C for 48 h. To get other PANI salts, 1ml 37 wt.% HCl was replaced by 1 ml 96 wt.% H2 SO4 and 1 ml 87 wt.% H3 PO4 , respectively, noted as PANI-H2 SO4 , PANI-H3 PO4 . 2.2. Characterization FT-IR spectra of the polymers were obtained by using a BRUKEREUINOX-55 Fourier transform infrared spectrometer (frequency range 3500–400 cm−1 ). UV–vis spectra of the polymer solution in m-cresol were recorded by using Shimadzu UV-2450 spectrophotometer in the range of 300–900 nm. The X-ray diffraction studies were performed on a D/Max 2400 X-ray diffractometer by using Cu K␣ radiation source (λ = 0.15418 nm). The scan range (2θ) was 10◦ –70◦ . The room temperature conductivity was measured on pressed pellets with a diameter of 1 cm by using two-probe technique. Electrochemical measurements were carried out with a classical three-electrode cell by using CHI660A electrochemical workstation system. The working electrode was a PANI film electrode prepared by casting the DMF solution of respective PANI salts on platinium electrode. The reference electrode was Ag/AgCl, and the counter electrode was a 1 cm2 area Pt flag.

Fig. 2. FT-IR spectra of PANI salts (a) PANI-HCl (b) PANI-H2 SO4 (c) PANI-H3 PO4 .

3. Results and discussion 3.1. IR spectra Fig. 2 shows the IR spectra of PANI salts synthesized by solid-state polymerization method. The characteristic bands at ∼1556–1565 cm−l arises mainly from both C N and C C stretching of the quinoid diimine unit, while the

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Table 1 Assignment of IR spectra of PANI salts Sample 1559 cm−1 1558 cm−1 1558 cm−1

PANI-HCl PANI-H2 SO4 PANI-H3 PO4 a

C Ha

C N 1483 cm−1 1475 cm−1 1475 cm−1

1298 cm−1 1296 cm−1 1296 cm−1

1108 cm−1 1109 cm−1 1124 cm−1

795 cm−1 796 cm−1 796 cm−1

Out-of-plane bending.

band near∼1475–1481 cm−l is attributed to the C C aromatic ring stretching of the benzenoid diamine unit. The ∼1296–1298 cm−1 and ∼795–797 cm−1 bands can be assigned to C N stretching of the secondary aromatic amine and an aromatic C H out-of-plane bending vibration, respectively. These characteristic bands confirm that the PANI salts contain the conducting emeraldine salt phase. The IR assignments of PANI salts were listed in Table 1. The results indicate that the backbone structures of PANIHCl, PANI-H2 SO4 , and PANI-H3 PO4 obtained in this solidstate synthesis method are identical to each other and also to those of PANI salts synthesized previously in conventional chemical and electrochemical methods [10,14,15]. Based on the previous treatments of IR data in which the quinoid and benzenoid units were identified [16–18], the intensity ratio of these two absorption bands at ∼1590 and 1500 cm−l is indicative of the extent of oxidation state of the polymer, which reflects the content of the quinoid diimine and benzene ring structute. The ratio is calculated as following: R(intensity ratio) =

volume (1 ml), the pH value may be different with the [H+ ]. The small amount of water will determine the pH value of them. For 1 ml 37 wt.% HCl, 96 wt.% H2 SO4 and 87 wt.% H3 PO4 , an order of pH (HCl) < pH (H2 SO4 ) < pH (H3 PO4 ) will be discovered. The pH value of the reaction medium appears to be one of the main parameters to affect the oxidation state of the polyaniline, which determines its properties. The higher the pH of the medium and the higher the oxidant concentration, the higher the oxidation state of PANI [19].This is the reason for the ratio of the relative intensity of quinoid to benzenoid ring modes shows the highest ratio in PANIH3 PO4 . To contrast with 37 wt.% HCl and 87 wt.% H3 PO4 , 96 wt.% H2 SO4 is more oxidative acid, which can also cause further oxidation of polyaniline, but that pH (H2 SO4 ) < pH (H3 PO4 ) will benefit for reduction state of polyaniline during the oxidative polymerization [19,20]. So the ratio of the relative intensity of quinoid to benzenoid ring modes shows between PANI-H3 PO4 and PANI-HCl. 3.2. UV–vis spectra

I1590 cm−1 I1500 cm−1

where I is absorption intensity. The values calculated from the FT-IR data are listed in Table 2 are R = 0.870, 0.896 and 0.920 for PANI salts synthesized by solid-state polymerization in the presence of HCl, H2 SO4 , and H3 PO4 , respectively. An order in R (intensity ratio) is PANI-H3 PO4 > PANI-H2 SO4 > PANI-HCl was observed. In each case, the ratio of the two bands is less than 1.0 indicating that there are more benzene units within the polymer. A value of 1.0 defines the emeraldine type structure and the polymer can have higher conductivity [18]. From above results, it can be concluded that the formation of fully reduced phase are predominant in these PANI salts. A comparison of the ratio of the relative intensity of quinoid to benzenoid ring modes shows the highest ratio of 0.92 in PANI-H3 PO4 compared to PANI-H2 SO4 and PANI-HCl. This may be due to the pH value of each acid, because all acids are in the same

Fig. 3 represents the UV–vis absorption spectra of PANI salts in m-cresol solution. These PANI salts show three characteristic absorption peaks at ∼306–324, ∼402–412 and ∼828–835 nm (Table 3). The absorption peak at ∼306–324 nm can be ascribed to π–π* transition of the benzenoid rings, while the peaks at ∼402–412 and ∼828–835 nm can be attributed to polaron–π* transition and π–polaron transition, respectively,[21–27], furthermore the peaks at ∼402–412 and ∼828–835 nm are related to doping level and

Table 2 The absorption intensity and R(intensity ratio) of quinoid and benzenoid units in PANI salts I 1556−1565 cm−1

I 1475−1481 cm−1

R (intensity ratio)

0.234 0.370 0.218

0.269 0.413 0.237

0.870 0.896 0.920

Fig. 3. UV–vis spectra of PANI salt (a) PANI-HCl (b) PANI-H2 SO4 (c) PANI-H3 PO4 .

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Table 3 The assignments of UV–vis absorption peaks of PANI salts Sample

PANI-HCl PANI-H2 SO4 PANI-H3 PO4

Wave length of absorption peak π–π* transition

Polaron–π* transition

π–Polaron transition

A828–835 /A302–3324

324 nm 320 nm 306 nm

403 nm 402 nm 412 nm

835 nm 833 nm 828 nm

0.92 0.77 0.63

formation of polaron [23–25]. Based on the previous research that the extent of doping can roughly be estimated from the absorption spectra of the polyaniline, in which the ratio of absorbances at 828–835 nm (π–polaron) and 306–324 nm (π–π* transition) indicated the doping level of polyaniline [27], it was found that in the case of PANI salts the intensity ratio (A828–835 /A306–312 ) was smallest in PANI-H3 PO4, which meant that the doping level of PANI-H3 PO4 was lower than that of PANI-HCl and PANI-H2 SO4 . 3.3. XRD analysis Crystallinity and orientation of conducting polymer have been of much interest, because more highly ordered systems could display a metallic-like conductive state [28]. Pouget et al. made the detailed and systematical study on the PANI prepared through conventional methods and proposed a pseudoorthorhombic unit cell structure for PANI [29]. In this study, the X-ray diffraction patterns for the PANI powder doped with different inorganic acid are shown in Fig. 4. The Bragg diffraction peaks of ˚ 14.68◦ (d = ∼6.034 A), ˚ 20.33◦ 2θ = ∼9.02◦ (d = ∼9.804 A), ◦ ˚ ˚ (d = ∼4.368 A), 25.30 (d = ∼3.520 A) can be found in the X-ray diffraction patterns of the PANI-HCl. However, most diffraction peaks disappeared in the PANI-H2 SO4 and PANIH3 PO4 . The diffraction peaks of PANI-HCl are relatively sharp and strong. The detailed data are presented in Table 3. ˚ Two peaks centered at 2θ = ∼20◦ (d = ∼4.368–4.461 A) ˚ were observed in X-ray scatand 25◦ (d = ∼3.520–3.560 A)

Fig. 4. The X-ray diffraction patterns for PANI salts (a) PANI-HCl (b) PANIH2 SO4 (c) PANI-H3 PO4 .

tering patterns of all PANI salts. The peak centered at 2θ = ∼20◦ may be ascribed to periodicity parallel to the polymer chain, while the peaks at 2θ = ∼25◦ may be caused by the periodicity perpendicular to the polymer chain [30]. The peak at 2θ = ∼20◦ also represents the characteristic distance between the ring planes of benzene rings in adjacent chains or the close-contact interchain distance [31]. For PANI-HCl, the peak of 2θ = ∼25◦ (110 face) is stronger than that of 2θ = ∼20◦ (100 face), which is similar to that of highly doped emeradine salt [29]. However, for PANI-H2 SO4 and PANIH3 PO4 , the peak of 2θ = ∼25◦ (110 face) is weaker than that of 2θ = ∼20◦ (100 face), which is similar to that of less doped emeradine salt. The d-space corresponding to the peak at 2θ = ∼20◦ is in the order of PANI-H3 PO4 > PANIHCl > PANI-H2 SO4 . The PANI chains produced under more acidic conditions have less structrural defects (e.g., non-para linkages in PANI chains), which will cause the fractions of crystalline phase increasing in PANI [32]. For 1 ml 37 wt.% HCl, 96 wt.% H2 SO4 and 87 wt.% H3 PO4 , an order of pH (HCl) < pH (H2 SO4 ) < pH (H3 PO4 ) will be discovered, which will lead to the different crystallinity. 3.4. Cyclic voltammograms The redox properties of the PANI salts were investigated with cyclic voltammetry. Fig. 5 shows the cyclic voltammograms (CVs) of the polymer films on Pt in 1 mol/L H2 SO4 . The redox peaks of PANI salts are similar to the PANI salts such as PANI-H3 PO4 , PANI-H2 SO4 and PANI-HCl prepared electrochemically that previously reported [33]. The anodic and cathodic potentials are listed in Table 4. Three redox couples are observed in the CVs of PANI-H2 SO4 and PANI-HCl, while the two redox couples are in the CV of PANI-H3 PO4 . In the positive sweep the first redox peak (at ca. 0.26–0.35 V) is well-known as the formation of radical cations (polaronic emeraldine) [34] and the second redox couple at ca. 0.7–0.8 V is the formation of diradical dications (represented by the resonance structures: bipolaronic pernigraniline and protonated quinonediimine) [35] through the oxidation of PANI salts. The intermediate peaks of relatively low intensity absorbed between 0.4–0.6 V, these peaks have been associated to the degradation of PANI salts [36]. The redox peaks are relatively are sharp in CVs of PANI-H2 SO4 , PANI-HCl, however, broad peaks and most positive potential shift in the first redox potential are observed in the CV of PANI-H3 PO4 , these were due to the high pH of H3 PO4 , which will lead to more oxidation state of PANI, on the contrary, the quinoid unit is more elctron-withdrawing unit than

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Table 4 The2θ values, intensity and pseudoorthorhombic indexation(hkl) [28] of PANI salts 2θ (intensity) PANI-HCl 9.02◦

(w) 14.68◦ (m) 20.33◦ (w) 25.30◦ (s)

(hkl) ˚ d (A)

PANI-H2 SO4

˚ d (A)

9.804 6.034 4.368 3.520

10.14◦

(w)

8.723

20.70◦ (s) 24.98◦ (w)

4.291 3.565

˚ d (A)

PANI-H3 PO4

19.90◦ (s) 25.01◦ (s)

001 011 100 110

4.461 3.560

w, weak; m, medium; s, strong.

the benzenoid unit in polymer chain, this cause to the first redox potential shifts to higher potential [37]. 3.5. Conductivity The conductivity of PANI depends on the degree of doping, oxidation state, particle morphology, crystallinity, interor intrachain interactions, molecular weight, etc. [25]. The conductivity of PANI salts is listed in Table 5. From the results of IR spectra, UV–vis spectra and cyclic voltammogram studies, the conductivity differences of PANI salts can be concluded for the characteristics of inorganic acids (e.g., HCl, H2 SO4 and H3 PO4 ). The PANI produced under more acidic conditions has a higher electrical conductivity [31], Polyaniline has proved particularly interesting for its good conductivity upon doping with nonoxidizing acids [10,11]. From Table 6, one can see that the conductivity of polyaniline synthesized in HCl is the largest and that in H3 PO4 is the smallest in the three acids. All these acids were used without further dilution. 96 wt.% H2 SO4 is more oxidative, polyprotic and strong acid while 87 wt.% H3 PO4 is polyprotic, nonoxidizing and not as strong as other acids and 37 wt.% HCl is nonoxidizing strong acid. When in same volume (in this case 1 mL), the amount of water in the acids will determine the pH value and oxidative ability. The amount of water is in the order of HCl > H3 PO4 > H2 SO4 . In the same experimental condition, strong acid is benefit for the yield of PANI and conductivity, while the weak acid has an opposite behavior [19]. Zhang et al. reported an order of PANI-HCl > PANI-H2 SO4 > PANI-H3 PO4 in conductivity Table 5 The redox potentials of PANI salts Sample

PANI-HCl PANI-H2 SO4 PANI-H3 PO4

I (first redox peak)

II (intermediate peaks)

III(second redox peak)

Epa /V

Epc /V

Epa /V

Epc /V

Epa /V

Epc /V

0.26 0.26 0.35

0.08 0.07 0.15

0.56 0.54 0.58

0.49 0.48 0.42

0.79 0.79

0.67 0.69

Table 6 The Conductivity values and yields of PANI salts

Fig. 5. The Cyclic voltammograms of PANI salts in 1 mol/L H2 SO4 solution scan rate: 50 m V/s (a) PANI-HCl (b) PANI-H2 SO4 (c) PANI-H3 PO4 .

Sample

Conductivity (S/cm)

Yield (%)

PANI-HCl PANI-H2 SO4 PANI-H3 PO4

12 3.8 0.8

82 78 75

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absorved, in which the PANI salts are chemically prepared [15]. The conductivities of the solid-state synthesized PANI salts were in the order of PANI-HCl > PANI-H2 SO4 > PANIH3 PO4 , this is well matched with the report above. But the ratio of the relative intensity of quinoid to benzenoid ring modes is related to the conductivity of polyaniline, i.e., the higher the relative intensity of the absorption peaks of quinoid to benzenoid ring modes to that of doped anions, the greater is the conductivity of polyaniline [38]. However, the conductivity of polyaniline is affected by its doping level and crystalline structure. Above all, The lowest conductivity of the PANI-H3 PO4 has may be due to a possible reason that the molecular size of H3 PO4 , which is larger than that of HCl [15], except highest pH value of H3 PO4 , lowest doping level and crystallinity of PANI salt which also leads to lower conductivity [32].

4. Conclusion Polyaniline(PANI) doped with inorganic acids (e.g., HCl, H2 SO4 and H3 PO4 ) were synthesized by solid-state polymerization. Spectroscopic studies showed here is the highest ratio of the relative intensities of the quinoid to benzenoid unit in H3 PO4 doped PANI and leucoemeraldine phase is formed predominantly in PANI salts.These results were further supported by conductivity measurements and cyclic voltammetry. More crystalline PANI salt was obtained by doping with HCl comparing with other inorganic acids in this method. Among these inorganic acids, HCl was found to be more suitable as a protonic acid media in order to attain a PANI with high conductivity in this solid-state polymerization. These differences mainly depended on the characteristics of inorganic acids (e.g., HCl, H2 SO4 and H3 PO4 ): small amount of water in the acids; strong and weak oxidizability of the acids. All these lead to PANI salts in different oxidation state, yield and structure.

Acknowledgements This work was supported by The National Foundation of Science of China (No. 20403014), it was also supported by XiBu ZhiGuang Science Foundation of China.

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