Transformation of Oligoaniline Microspheres to Platelike Nitrogen

Article pubs.acs.org/JPCC

Transformation of Oligoaniline Microspheres to Platelike Nitrogen-Containing Carbon Zuzana Morávková, Miroslava Trchová,* Elena Tomšík, Alexander Zhigunov, and Jaroslav Stejskal Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Aniline oligomers are important in relation to one of the most important conducting polymers, polyaniline. In the present study, aniline oligomer microspheres have been prepared during oxidation of aniline with ammonium peroxydisulfate under alkaline conditions in 0.2 or 1.0 M ammonium hydroxide. They were subsequently converted to disordered nitrogen-containing carbons during heating to 650 °C in an inert atmosphere. Raman spectra confirm the transformation of the aniline oligomers into a material displaying two peaks assigned to graphitic and disordered modes. In contrast to polyaniline, which retains its morphology after carbonization, the microspherical morphology of aniline oligomers was completely destroyed and replaced by two-dimensional plates. The evolution of the new morphology was followed by scanning electron and optical microscopies, and the corresponding changes in molecular structure after heating are discussed on the basis of Fourier transform infrared and Raman spectroscopies. The process of carbonization was followed by thermogravimetric analysis. The degree of crystallinity observed by wide-angle X-ray scattering was reduced after the carbonization.

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INTRODUCTION Conducting polymers, such as polyaniline (PANI), are unique among polymers in their ability to produce a variety of nanostructures, such as nanofibers or nanotubes, in the course of their preparation by the oxidation of aniline in acidic aqueous media.1 The conversion of PANI to nitrogen-containing carbons has been observed after heating to temperatures above 600 °C in an inert atmosphere.2−9 The morphology of PANI was retained after carbonization, as demonstrated on nanotubes,2,3 colloidal particles,4 globules,5 or thin films,8 which allows the preparation of various carbonaceous nanostructures. When aniline oxidation was started under alkaline conditions, microspheres composed of aniline oligomers (= oligoanilines) and having the diameter up to several micrometers were produced instead.10−12 Aniline at currently used concentrations is not completely miscible with the aqueous medium. Aniline microdroplets present in the alkaline medium act as templates that become coated with aniline oligomers.1,11,13 The oxidation does not proceed to PANI, unless the acidity of the reaction medium increases. Similarly to PANI, aniline oligomers are also converted to disordered carbonaceous material after heating at elevated temperature, but in contrast to PANI, the morphology of aniline oligomers changes from microspheres to plates.13 The properties of carbon materials are sensitive to the presence of heteroatoms, such as nitrogen.14−20 The pyrolysis of aromatic polymers containing nitrogen atoms is a simple way how to prepare nitrogen-containing carbon analogues.21−24 Such carbonaceous materials find uses in electrocatalysis,6,23,24 as electrode materials,25,26 adsorbents,27 supercapacitors,28,29 or in electrorheology.30 In the present study, the aniline oligomer © 2013 American Chemical Society

microspheres were prepared at various basicity of the medium, in 0.2 and 1.0 M ammonium hydroxide, and the transformation of their molecular structure to carbon-like structure has been followed by Fourier transform infrared and Raman spectroscopies and wide-angle X-ray scattering.

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MATERIALS AND METHODS Synthesis. Aniline (0.2 M; Aldrich) was oxidized with ammonium peroxydisulfate (0.2 M; Lach-Ner, Czech Republic), in 0.2 or 1.0 M ammonium hydroxide, at room temperature. Both reactants, aniline and ammonium peroxydisulfate, were dissolved separately in ammonium hydroxide solutions, and the oxidation of aniline was started by mixing both solutions. The reactions were carried out in 500 mL volumes. The progress of oxidation was followed by acidity changes with a laboratory pH meter. Silicon, glass, and gold-coated glass substrates were immersed into the reaction mixture to be coated with aniline oligomers. The substrates were rinsed with acetone and dried at room temperature in air and then in a desiccator over silica gel. Aniline oligomers produced in the bulk of the reaction mixture were isolated by filtration, rinsed with acetone, and dried as above. Aniline oligomers were placed in an electric oven in a nitrogen atmosphere. The heating was switched on and the temperature increased by 15 °C min−1 up to the target temperature. The heating was then switched off, and the sample Received: July 9, 2012 Revised: January 2, 2013 Published: January 9, 2013 2289

dx.doi.org/10.1021/jp306783m | J. Phys. Chem. C 2013, 117, 2289−2299

The Journal of Physical Chemistry C

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was left to cool to ambient temperature, still in a nitrogen atmosphere. Characterization. UV−vis spectra were measured on the layers deposited during reaction on glass supports with a Lambda 20 spectrometer (PerkinElmer, UK). Scanning and transmission electron microscopy (SEM, TEM) images were taken with a JEOL 6400 and JEOL JEM 2000FX microscope, respectively. Elemental analysis was performed using an elemental analyzer VARIOEL III (Elementar). Thermogravimetric analysis (TGA) coupled with Fourier transform infrared spectroscopy (FTIR) of released gases was performed using a Pyris 1 TGA analyzer (PerkinElmer) coupled with an IR spectrometer Spectrum 100T FT-IR (PerkinElmer) through a transfer line TL 8000 (PerkinElmer). The analyses were performed under flowing nitrogen at 20 mL min−1. The heating rate was 10 °C min−1, from room temperature to 800 °C. The infrared spectroscopic cell and the coupling system to TGA were kept at 260 and 250 °C, respectively, to prevent condensation of evolved gases or vapors. FTIR spectra were continuously collected during the whole analysis, and they were recorded in the range of 650−4000 cm−1, at 2 scans per spectrum at 4 cm−1 resolution. Wide-angle X-ray scattering (WAXS) diffraction patterns were obtained using a powder diffractometer HZG/4A (Seifert GmbH, Germany) in transmission mode. The radiation Cu Kα (wavelength λ = 0.154 nm) monochromatized with a Ni foil (β-filter) was used for diffraction. The measurement was done in the angular range 2Θ = 1.4°−50° with steps of 0.1°. Exposure time at each step was 10 s. The degree of crystallinity was obtained by comparing integral intensity of the whole curve with the sum of integral intensities of crystalline peaks. Before the analysis of the crystalline phase, the background and amorphous halo were subtracted and intensity was normalized. FTIR spectra of samples dispersed in potassium bromide pellets (approximately 0.7 mg of sample in 30 mg of potassium bromide) were recorded in the range 400−4000 cm−1 at 64 scans per spectrum at 2 cm−1 resolution using a fully computerized Thermo Nicolet NEXUS 870 FTIR spectrometer with a DTGS TEC detector. Raman spectra excited with HeNe 633 nm and near-infrared diode 785 nm lasers were collected on a Renishaw inVia Reflex Raman microspectrometer. A research-grade Leica DM LM microscope with an objective magnification ×50 was used to focus the laser beam on the sample placed on an X−Y motorized sample stage. The microscope has also been used for the collection of optical images. The scattered light was analyzed by the spectroscope with holographic gratings 1800 and 1200 lines mm−1 for the corresponding lasers, respectively. A Peltiercooled CCD detector (576 × 384 pixels) registered the dispersed light. In order to avoid damage of the samples by laser beam, the spectra were recorded starting with lowest power obtainable and by gradually increasing the laser power until the spectrum started to change. The spectrum used was then recorded with the highest power that did not alter the sample.

Figure 1. (a) pH profile and (b) UV−vis spectra of the layers deposited on glass supports during oxidation of 0.2 M aniline with 0.2 M ammonium peroxydisulfate in 0.2 and 1.0 M ammonium hydroxide.

1−5 μm in diameter are produced (Figures 2−5).1,11,13 Transmission electron micrographs confirm the spherical shape and the fact that they are compact (Figure 2). Under alkaline conditions, aniline is not completely miscible with the aqueous medium and forms microdroplets that act as templates for the deposition of higher aniline oligomers. The aniline solubility is also reduced due to the salting-out effect of inorganic salts, such as ammonium peroxydisulfate and later ammonium sulfate, in the reaction mixture. The UV−vis spectrum of these microspheres (Figure 1b) displays a band around 340 nm assigned to π−π* transitions.31 The band at 430 nm also indicates formation of phenazine-type oligomers.32−35 When the oxidation of aniline proceeds in 0.2 M ammonium hydroxide, ammonium hydroxide is neutralized by generated sulfuric acid and the medium becomes even acidic during the oxidation (Figure 1a). Under such conditions, in the final stages of aniline oxidation, thin PANI coating is formed on the surface of the microspheres shells that have already been produced at earlier stages of the reaction (Figure 2a). In the UV−vis spectrum of this material we can observe a broad band located around 800 nm typical of emeraldine salt and attributed to π−polaron transition (Figure 1b).8,35−37 Carbonization. The evolution of the morphology of oligoaniline microspheres after heating in an inert nitrogen atmosphere has been studied at first by SEM. The microspheres prepared in 1.0 M ammonium hydroxide are transformed into two-dimensional plates below 200 °C, and this morphology is found also after heating at 600 and 800 °C (Figure 3). It seems

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RESULTS AND DISCUSSION Preparation of Aniline Oligomers. Oxidation of aniline results in the coupling of aniline molecules, and aniline oligomers are produced. Hydrogen atoms abstracted from aniline molecules during this process are released as protons,1 so sulfuric acid is a byproduct, and pH drops in the course of aniline oxidation (Figure 1a). When the oxidation of aniline starts under alkaline conditions, such as in 1.0 M ammonium hydroxide solution, microspheres 2290



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that the sheets arouse from larger plates as they broke into pieces during solidification from the melt during cooling to ambient temperature (see the similar shape of the borders in Figure 3). The same tendency of morphological evolution is observed in the case of microspheres prepared in 0.2 M ammonium hydroxide (Figure 4). The layered structure of the plates is well visible in the last case. The transformation of the morphology of aniline oligomers deposited during the oxidation of aniline in 1.0 M ammonium hydroxide on silicon after exposure to the temperature of 500 °C is shown in Figure 5. It is well seen that the initial microspheres are melted into dispersed “flowing” morphology on the silicon surface. At higher temperatures they melt completely. The transition was observed in more detail by optical microscopy of microspheres deposited on glass (Figure 6). The microspherical morphology is destroyed between 160 and 180 °C, i.e., at the temperatures close to boiling point of aniline, 180 °C. Additional information on these processes can be obtained from TGA in an inert nitrogen atmosphere. This method may be regarded as an analytical way of carbonization.2 Both samples lose a few percent of their mass around 125 °C (Figure 7a), which is associated with the loss of adsorbed humidity. The next loss of mass between 150 and 200 °C is associated with the evaporation of aniline and confirmed by the increase in overall infrared absorption (Figure 7b). The content of residual aniline in the microspheres prepared in 1.0 M ammonium hydroxide is more probable than for the sample prepared in 0.2 M ammonium hydroxide. The decrease in mass is also somewhat faster for the sample prepared in 1.0 M ammonium hydroxide in this temperature region. The interior of the microspheres prepared in 0.2 M ammonium hydroxide thus contains aniline, and at lower pH, this aniline converts to soluble aniline salt,

Figure 2. Transmission electron images of aniline oligomers prepared by the oxidation of aniline with ammonium peroxydisulfate in (a) 0.2 M and (b) 1.0 M ammonium hydroxide.

Figure 3. SEM images of aniline oligomers prepared in 1.0 M ammonium hydroxide after the exposure to various temperatures. (Adapted from ref 1.) 2291



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so that the microspheres become hollow (Figure 2a). Another drop in mass connected with increased infrared absorption is located around 270 °C. We may speculate that evaporation of phenylenediamines (boiling point 252 °C for ortho-isomer, 283 °C for meta-isomer, and 267 °C for para-isomer), potential decomposition products, takes place in this temperature range. The continuous decrease in mass in the temperature range from 250 to 600 °C provides information about the conversion of oligoaniline microspheres into disordered carbonaceous material, which is left as a ≈50 wt % residue at 800 °C. Ammonia and amines (whole temperature range), small aromatic molecules (150−600 °C), sulfur dioxide (300−600 °C), carbon monoxide (350−500 °C), carbon dioxide (elevated absorbance at 600−800 °C), and small organic compounds containing multiple bonds, oxygen, and nitrogen atoms (600−800 °C) were detected in the FTIR spectra of the gases released during TGA (see Supporting Information). Elemental Analysis. Elemental analysis (Table 1) was used to check the nitrogen-containing carbon formation. The content of oxygen was proposed to be higher for aniline oligomers than for PANI.13 The content of hydrogen dropped gradually during the heating as expected. The fraction of carbon decreased below 650 °C during the loss of mass between 350 and 600 °C seen in the TGA (Figure 7a). The oxygen-containing structures were the most stable. It is important to note that the content of nitrogen does not change during these experiments (Table 1). The product of carbonization is mostly a nitrogen-containing carbon with some oxygen and sulfur atoms present in the structure, probably as sulfonate groups.13 Wide-Angle X-ray Scattering. WAXS has been used to characterize the structural changes in aniline oligomers induced by heating. Results are close to each other for both oligomers, so we present below only the results for the samples prepared in 0.2 M ammonium hydroxide. It is well-known, that the initial WAXS profiles of aniline oxidation products significantly depend on the synthetic routes.38 The crystallinity of PANI is well developed only for some specific methods of preparation such as oxidation of aniline at subzero temperatures,39 and the potential presence of PANI fraction in the present samples cannot contribute to the degree of crystallinity. Aniline oligomers are more crystalline.40−43 The obtained microspheres have indeed the degree of crystallinity 46% (Figure 8). All sharp peaks obtained by fitting procedure were proposed as crystalline ones except the peak at 7°. Pronounced peaks at 20.1°, 21.5°, 25.1°, 27.5°, and 28° on an amorphous halo centered at 24° (dashed line in Figure 8) could be observed in the diffraction patterns of the sample before heating at room temperature. After heating to 200 °C and subsequent cooling back the diffraction pattern changed. Intense peaks at 20.1° and 21.5° disappeared and made a contribution into amorphous constituent. This leads to a shift of amorphous halo to lower angles. Many peaks corresponding to a new crystalline phase appeared: 15.7°, 18.3°, 21.9°, 24.5°, 30.5°, 31.2°, 31.9°, 35.9°, 37.8°, and 41.2°. A broader peak at 7° indicates the presence of less organized structure. Despite the fact that some peaks are more pronounced and correspond to larger crystalline sizes, according to Scherrer equation, the crystallinity dropped to 19%. This could be explained by a hypothesis that the fraction of oligomers originally partially crystalline became amorphous and another fraction crystallized after heating to 200 °C and cooling back to room temperature. Practically all peaks disappear after heating at temperatures of 300 and 400 °C; only broad maxima remained around 7° and 21°. On one hand, the chemical bonds are broken, and on

Figure 4. Detailed SEM images of aniline oligomers prepared in 0.2 M ammonium hydroxide (a) before and (b) after heating to 650 °C.

Figure 5. SEM images of aniline oligomers deposited in 1.0 M ammonium hydroxide on silicon (a) before and (b) after the exposure to the temperature 500 °C. 2292



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Figure 6. Optical micrographs of aniline oligomers prepared in 1.0 M ammonium hydroxide on glass substrate and exposed to various temperatures.

the other, random cross-linking takes place. After heating to even higher temperatures, the maximum around 21° moves to higher angles around 26°, which corresponds to decreased mean distances after cooling to the room temperature. Infrared Spectroscopy. In contrast to the UV−vis spectra (Figure 1b), the infrared spectra of both oligoanilines are very close to each other, and for that reason also in this section, we present only the infrared spectra of the sample prepared in 0.2 M ammonium hydroxide (Figure 9). Infrared spectra of the sample prepared in 1.0 M ammonium hydroxide can be found in the Supporting Information. The interaction with electronic structure of the oligomers is more sensitive to the presence of thin shell of PANI produced on their surface than vibrational spectroscopy. The presence of PANI is detected by infrared (and Raman) spectroscopy only when the sample is prepared in lower concentration than 0.1 M ammonium hydroxide.34 The infrared spectrum of aniline oligomers is close to the spectra reported by Surwade et al.44 for the compound produced by the reaction between aniline and p-benzoquinone and also to the spectra reported for similar products in the literature.12,45,46 The interpretation of this spectrum is still under discussion.13,45−48 The spectrum of aniline oligomers changes after heating to 200 °C (Figure 9). Two sharp peaks at 3262 and 3200 cm−1, which correspond to the hydrogen-bonded imine and N−H groups, disappeared. The intensity of the main bands observed at 1587 cm−1 (quinonoid ring stretching vibrations mixed with benzene modes) and 1503 cm−1 (in analogy to PANI, benzenoid ring stretching vibrations mixed with N−H deformations) decreased, and the sharp peaks at 1445 cm−1 (CC stretching vibration of the substituted aromatic ring), 1412 cm−1 (ring stretching of the phenazine constitutional units), and 1366 cm−1

(C−N stretching vibrations) practically disappeared together with the peak of the sulfonic group at 1034 cm−1. A band at 1092 cm−1 and a sharp peak at 613 cm−1 appeared in the spectrum. They can be attributed to the vibrations of sulfate or hydrogen sulfate counterions. In addition to the band at 827 cm−1 of the C−H out-of-plane bending vibrations of two adjacent hydrogen atoms on a 1,4-disubstituted benzene ring, those at 742 and 695 cm−1 are still observed in the spectrum of aniline oligomers. They correspond to the C−H out-of-plane bending and out-of-plane ring deformations of a monosubstituted benzene ring, respectively.13,48 The relative intensity dramatically decreased after heating at 300 °C, and the maxima of the main bands shifted to 1587, 1503, 1294, 1242, 1170, 827, 742, 695, and 613 cm−1. The possible presence of p-benzoquinone and phenazine-like units is supported by the peak observed at 1700 cm−1 and a shoulder at 1620 cm−1, respectively. After heating to 400 °C the intensity of the band at 1503 cm−1 decreased and the bands connected with sulfate anion disappeared, as a result of thermal deprotonation. The material started to decompose. After heating to 500 °C the band observed at 1587 cm−1 shifted to 1620 cm−1. At higher temperatures of 600, 700, and 800 °C, larger organic fragments are released due to the thermal decomposition of the material. All these changes are in agreement with the described results of TGA coupled with FTIR spectroscopy of released gases (see Supporting Information). In the spectra corresponding to these temperatures we observe a maximum at 1620 cm−1, a broad band with the maximum at about 1200 cm−1, and an intermediate band at about 1460 cm−1. The shape of the spectra is close to that of the spectra of a carbon-like material with the Raman-active 2293



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Figure 8. Evolution of WAXS of aniline oligomers prepared in 0.2 M ammonium hydroxide after the exposure to various temperatures.

Figure 7. (a) Thermogravimetric analysis of aniline oligomers prepared in 0.2 and 1.0 M ammonium hydroxide and (b) the integrated infrared absorbance of the gases released during TGA.

Table 1. Results of Elemental Analysis of the Sample Prepared in 1.0 M Ammonium Hydroxide and Heated to Various Temperatures in a Nitrogen Atmosphere T, °C

C, wt %

H, wt %

N, wt %

S, wt %

other (O), wt %

25 200 400 650 800

69.4 70.4 73.3 64.0 65.9

4.4 4.1 2.9 2.0 1.4

14.7 14.3 14.0 16.9 14.1

2.2 1.3

Transformation of Oligoaniline Microspheres to Platelike Nitrogen

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