The Effect of Inorganic Salt on the Morphology and Nucleation of Polyaniline

Polyaniline nanobers were fabricated through the addition of inorganic salt such as NaCl, MgSO 4 and AlCl 3 into the micelle-like composed of aniline and camphor sulfonic acid (CSA). The inuence of types and concentration of inorganic salts, doped acids and temperature on polyaniline was studied by TEM, Uv-vis and FTIR spectroscopy. In addition, in situ Uv-vis and 1 H NMR were applied to observe the process of aniline polymerization, and it was indicated the polymerization rate of aniline changed after the addition of inorganic salt NaCl into the initial solution.


Introduction
Polyaniline and its composites have attracted lots of attention in various elds such as adsorption [1][2][3] , anticorrosion [4][5][6] , cathode materials [7][8][9] and sensors [10][11][12] , due to abundant morphologies, simple preparation, excellent environmental stability, unique physical and chemical property and high electrochemical property. Nanostructured polyaniline with abundant shapes have been synthesized in micelle-like system [13][14][15][16][17] , but there are many factors affecting the morphology and the property of the product in micelle-like system, such as ionic strength [18][19][20] , the types and the concentration of doped acid [21] , temperature [22] , and the external eld [23][24][25] . At present, inorganic salts as the additives have been applied to prepare nanostructured polyaniline. Zhang and his team [18] successfully prepared chrysanthemum ower-like polyaniline composed of nano bers with high speci c capacitance and high crystallinity by adding inorganic salts via a self-assembly process; Pahovnik and his college [19] synthesized PANI with different nanostructures in the presence of inorganic electrolyte and organic electrolyte; Liu and his coworker [20] synthesized polyaniline nanosheets with good cycling stability and high speci c capacitance in saturated NaCl solution with HCl as doped acid. The effect of inorganic salt on the morphology and electrochemical property has been investigated, but the effect of inorganic salt on the nucleation of polyaniline was not clear. Hence, polyaniline nano bers were synthesized in the micellelike system formed by aniline and camphor sulfonic acid in this paper, and in situ Uv-vis spectroscopy and 1 H NMR were executed to investigate the effect of inorganic salt on the polymerization process of aniline. and AlCl 3 ) were prepared, and kept standing for 24 h at 5°C; and then the equivalent APS solution as the oxidant was quickly added into the above solution to initiate polymerization; after 24 h, the samples were washed with different solvent such as water, ethanol and acetone, dried for 12 h at 60°C, and nally collected.

Synthesis of polyaniline in the presence of D-CSA
A series of aqueous solution with 0.086 mol/L aniline, 0.086 mol/L D-CSA and 0.2 mol/L inorganic salts, and the subsequent steps were the same as above-mentioned; and the samples were collected in nal.

Characterizations
The morphology of the samples was characterized through transmission electron microscopy (JEM-2100); A small amount of polyaniline was ultrasonically dissolved in NMP for 5 min, and the Uv-vis spectra of the solution was recorded by Uv-vis spectrometer (Cary 5000); the structures of polyaniline were measured through FTIR (Bruker Vector 22); a series of in situ Uv-vis spectra and 1 H NMR spectra were used to monitor the evolution information of the oligomers in the polymerization process of aniline, and the CHI660B electrochemical workstation was applied to measure the electrochemical properties of polyaniline, cyclic voltammetry and galvanostatic charge-discharge were performed through three  Figure 1 displayed the morphology of polyaniline synthesized at the different conditions. For the initial systems in the absence of D-CSA, the shape of polyaniline prepared without inorganic salts was nano obers with rough surface in Figure 1a; when inorganic salts (NaCl, MgSO 4 and AlCl 3 ) was added, the morphology of PANI was still nano bers with large diameter and rough surface, as seen in Figure 1b-1d. For the initial system containing D-CSA, the morphology of polyaniline synthesized without salts was nano bers with smooth surface in Figure 1e; when NaCl, MgSO 4 and AlCl 3 was added into polymerization, the morphology of polyaniline was still nano bers, but the diameter of polyaniline nano bers reduced and the surface of polyaniline nano bers became smooth. Compared with the system without D-CSA, D-CSA was helpful to the formation of ner nano bers. Figure 2 and Figure 3 showed the Uv-vis spectra and FTIR spectra of as-synthesized polyaniline in the absence and presence of D-CSA. As Figure 2 shown, the similar peaks of polyaniline synthesized the initial system without D-CSA in the absence of salts and in the presence of NaCl, MgSO 4 and AlCl 3 were observed at 270 nm and 370 nm, which was attributed to π-π * transition of benzenoid rings and polyaniline [23] , and the weak peak at 650 nm was attributed to the doping degree, resulting from the participation of sulfuric acid produced by hydrolysis of APS oxidant into the polymerization. For the initial system including D-CSA, the similar peaks of polyaniline produced with inorganic salts occurred at 330 and 630 nm in Figure 2b.
The characteristic peaks of polyaniline without D-CSA observed were similar in FTIR spectra, as Figure 3a shown. The peaks located at 1616 and 1492 cm −1 were attributed to the stretching vibration of C=C on quinone ring and benzene ring, respectively; the peak at 1384 cm −1 corresponded to the stretching vibration of C-N on aromatic amines from secondary structure; the peak at 1153 cm −1 was attributed to the bending vibration of aromatic C-H; the peak at 833 cm −1 was caused by the out of plane deformation of C-H on 1,4-disubstituted benzene ring [24] . When the organic acid D-CSA was added into the polymerization, the characteristic peaks of polyaniline at 1588 cm −1 , 1488 cm −1 , 1300 cm −1 and 1163 cm −1 were similar to the peaks reported in Figure 3b, but the new peaks were observed, such as the peaks located at 1027 and 673 cm −1 were the absorption characteristic peak of -SO 3 group; the peaks at 701 and 580 cm −1 belonged to the stretching vibration of C-S and S-O group [23] .

The in uence of the concentration of inorganic salt and the reaction temperature on polyaniline
The introduction of inorganic salts such as NaCl, MgSO 4 and AlCl 3 has similar effects on the morphology and structure of the polyaniline on the basis of the above-mentioned analysis, regardless of the presence or absence of D-CSA. Therefore, NaCl was selected and acted as the representative, the effect on the shape and structure of polyaniline was discussed by changing the concentration of NaCl. Meanwhile, to investigate the in uence of the temperature on polyaniline, the temperature (5°C and 25°C) was applied to prepare polyaniline. A series of solution including 0.086 mol/L aniline and 0.086 mol/L D-CSA were prepared, and the amount of NaCl was added into the solution; the mixture solution was kept standing for 24 h, and then the polymerization triggered by APS solution proceeded for 24 h at the different temperature. Figure 4 and Figure 5 displayed TEM images and Uv-vis spectra of as-synthesized polyaniline. As Figure 4 shown, the morphology of polyaniline fabricated at 5 and 25°C was nano bers, but the diameter of polyaniline nano bers synthesized at 5°C was small, the surface was smooth when the concentration of NaCl was less to 0.2M; with the concentration of NaCl increasing, polyaniline nano bers became large and rough; while polyaniline nano bers synthesized at 25°C had no obvious difference in morphology and the size in Figure 4(e-h).
According to the Uv-vis spetra in Figure 5, the peaks of polyaniline synthesized at the different temperature occurred at 340 and 640 nm, and the ratio of the two intensity increased as the temperature increased, which indicated the doping degree of polyaniline increased, when the concentration of NaCl was 0.4M, the doping degree of polyaniline markedly increased, compared with that synthesized at 5°C.

In situ Uv-vis and 1 H NMR of polymerization process
To investigate the in uence of NaCl on the nucleation of polyaniline, in situ Uv-vis and 1 H NMR were applied to observe the evolution of aniline polymerization. Figure 6 displayed a series of UV-vis spectra recorded the evolution information of aniline polymerization with the different concentration of NaCl at 5°C and 25°C. Generally speaking, the new peak at 410 nm occurred in the initial stage of polymerization, which was attributed to o-aminodiphenylamine structure produced in the stage; the broad peak was observed at 600-800 nm, which was related to the doping state and the polaron of polyaniline [26] ; and the intensity of the two peaks increased with the polymerization time increasing, which indicated the amount of polyaniline increased; when the intensity increased to a certain degree and began to decrease, resulting from the precipitation of polyaniline in the solution. The datailed analysis was as follows: At the low temperature, the ladder phenomenon was observed at 600-850 nm, owing to the mist on the surface of the cuvette caused by the low temperature, but the spectra, especially the peak at 410 nm, can still provide us with the information of the polymerization process. For the initial solution with NaCl, the polymerization of aniline at the different temperature was shown in Figure 6a. When the temperature was low, the broad peak was newly observed at 7 min, and the intensity of the peak increased while the width of the peak reduced with the reaction time prolonging, in comparison with the peak at the different time of polymerization (10 min and 22 min); when the polymerization time was 31min, the intensity of the peak at 410nm decreased and then increased while the width of the peak became narrow. When the temperature was room temperature, the intensity of the peak at 410nm gradually increased in the rst 13 min, and then the broad peak at 600-800 nm occurred. These changes suggested that the type and quantity of produced oligomers were constantly changing in the polymerization process. Therefore, the polymerization of aniline without NaCl was slow, the reason was that the movement rate of aniline molecules and oligomers was reduced, the effective movement and collision between molecules, and the reaction speed was also reduced due to the low temperature.
When the concentration of NaCl was equal to 0.2 M, in situ Uv-vis spectra was almost no signi cant change in the rst 20 min, the intensity of the peak at 410 nm gradually increased in the following 20 min, but the shape of the peak kept unchanged; and then the intensity of the peak at 410 nm markedly increased in the following 3min, in the meantime, the broad peak at 600-800 nm occurred, and the intensity of the broad peak increased withe the time increasing. Compared with the polymerization without NaCl at low temperature, the polymerization process was quickly low, which indicated the introduction of NaCl reduced the reaction rate. However, when the reaction with 0.2 M NaCl proceeded at the room temperature, the peak at 410 nm gradually increased in the rst 16 min, and then the broad peak at 600-800 nm occurred and increased step by step; compared with that without NaCl at room temperature, the rate of polymerization at rst was slow and then increased, it was indicated that the effect of NaCl on the polymerization was different from that at low temperature.
The polymerization of aniline in the presence of 0.4 M NaCl was displayed in Figure 6c. Uv-vis spectra recorded at the low temperature was similar during the rst 25 min, the peak at 410nm changed in the following 6min, but the broad peak at 600-800 nm occurred at 45 min. The reaction rate of aniline at the low temperature reduced as the concentration of NaCl increased, while the polymerization rate increased at the room temperature.
In situ 1 H NMR spectra of the evolution in the process of aniline polymerization with the different concentration of NaCl at the low temperature was shown in Figure 7 and Figure 8, the spectra was divided into four sections, according to the signals of the species such as the anilinium, produced oligomers, water and D-CSA. When the polymerization of aniline proceeded in the absence of NaCl in Figure 7, the signals at 7.35, 7.53 and 7.45 ppm was attributed to ortho-H, meta-H and para-H of aniline in the initial solution, and the signals of D-CSA occurred during 4.0-0.6 ppm. The introduce of D-CSA had the signi cant in uence on para-H of aniline, which was in agreement with the micelle-like system composed of aniline and salicylic acid [117,156] , the reason was that the positive charge of anilinium cation was averaged by the aniline molecules around it, and these averaged aniline and D-CSA formed the micellelike. After the addition of APS oxidant, the polymerization of aniline was divided three stage, according to the evolution information of the signals of aniline, oligomers and water, 1) the signals of anilinium occurred at 7.54 and 7.42 ppm, and the signals of the dimers produced occurred at 7.18 and 7.00 ppm in the rst stage in Figure 7a. and meanwhile, the signal of water shifted down eld, and the signals of D-CSA had no obvious change; 2) the signal of water moved toward downshift, and the new signal was observed at 6.87 ppm, which belonged to the p rotons in phenazine structure, but the signals of anilinium cation and D-CSA kept unchanged; 3) the signal of anilinium cation became wide, and the signal of water shift down eld, the signals of phenazine structure oligomers did not change signi cantly and the signals of D-CSA became weak, due to the separation of D-CSA from micellar like structure.
Compared with the initial solution without NaCl, the signals of aniline and D-CSA in the initial solution containing 0.2 M NaCl were no evident changes; and the polymerization process of aniline after the addition of APS was similar in the rst and second stage, but the signal intensity of water and phenazine oligomers gradually increased in the third stage, which suggested that the introduction of NaCl made aniline monomers in the micelle-like diffuse outward and continue to react with the aniline dimer on the micelle surface.

The electrochemical properties of polyaniline
In order to investigate the effect of the introduction of inorganic salts on the electrochemical properties of nanostructured polyaniline, cyclic voltammetry experiments were carried out on the obtained samples, and the results are shown in Figure 9. There were two groups of redox peaks of as-synthesized PANI, due to the doping and dedoping reactions in polyaniline. The peaks of 0.24/0.02 V belonged to the intermediate state of polyaniline from the reduced state of polyaniline losing two electrons, and the peaks at 0.50/0.41 V corresponded to the oxidized state of polyaniline from the intermediate state of polyaniline losing two electrons. The area of cyclic voltammetry curve represents the capacitance, so it can be seen that the capacitance of as-synthesized polyaniline increases with the increase of ionic strength in solution.
The chronopotentiometry experiment was used to further investigate the effect of inorganic salts on the speci c capacitance of polyaniline. Charge and discharge were carried out at different current densities (0.4, 0.6, 1.0, 2.0 and 4.0 A / g). The speci c capacitance calculated according to the mass speci c capacitance formula were shown in Figure 10. Compared with polyaniline synthesized without no salt in Figure 10a, the speci c capacitance of polyaniline synthesized with inorganic salts decreases with the increase of current density. When the current density was 0.4 A / g, the speci c capacitance of polyaniline synthesized with NaCl, MgSO 4 and AlCl 3 was the maximum, which was 180, 234 and 200 F / g, respectively. When the current density increased up to 4.0 A / g, the speci c capacitance decreases to 75, 80 and 125 F / g. When the current density increasede 10 times, the speci c capacitance of PANI synthesized by AlCl 3 decreased by about 37.5%, and the value of polyaniline prepared under other conditions signi cantly decreased. It is shown that inorganic salts have a signi cant effect on the electrochemical properties of polyaniline, and the increase of ionic strength of inorganic salt is helpful to improve the electrochemical properties of polyaniline.

Conclusion
Polyaniline nano bers were synthesized in the presence of inorganic salt, the in uence of types and concentration of salts, doping acid and reaction temperature on polyaniline was studied through TEM, Uvvis and FTIR, the introduction of inorganic salt was helpful to improve the surface of polyaniline nano bers in the absence of D-CSA, while polyaniline nano bers with rough surface was obtained in the presence of D-CSA, and the electrochemical property of polyaniline increased with the increase of the ion strength; the in uence of NaCl concentration on the nucleation of aniline polymerization was investigated through in situ UV-vis and 1 H NMR, the rate of aniline polymerization reduced after the addition of NaCl at the low temperature; in turn, the reaction rate decreased at room temperature.  In situ Uv-vis spectra of PANI prepared at different concentration of NaCl at 0° C (Left) and 25° C (Right), In situ 1H NMR spectra of polyaniline prepared in the micelle-like system composed of aniline and CSA at 5°C