electrical conductivity of microwave heated polyaniline ... - JMPEE

Electrical Conductivity of Microwave 

Heated Polyaniline Nanotubes and Possible 

Mechanism of Microwave Absorption by 

Materials 

Takahiro Murai, Ryo Fukasawa, Tohru Muraoka, Hiroyuki Takauchi, 

Yasuo Gotoh, Tokihiro Takizawa and Takehiro Matsuse * 

Faculty of Textile Science and Technology, Shinshu University, 

Ueda, Nagano 386-8567, Japan 

* 

matsuse@shinshu-u.ac.jp 

In the course of experiments to perform deprotonation and carbonization of doped polyaniline 

(PANI) nanotubes (NTs) by irradiating directly 2.45 GHz microwave (MW) in our microwave 

heating system (MWHS), we have discovered that the PANI-NTs self heat by absorbing the MW, but 

the temperature of the PANI-NTs stops rising around 300 °C in spite of the heightened MW power. 

Furthermore, we have found that the MW irradiated PANI-NTs have transferred from electrical 

conductor to insulator depending on the temperature of the PANI-NTs. 

By measuring electron spin resonance (ESR) spectra of the MW heated PANI-NTs, the 

existence of the unpaired electrons is shown to have a strong correlation between the degree of 

MW absorption and the transition in the electrical conductivities. 

In order to deprotonate and carbonize further the PANI-NTs, we have performed heat treatment 

for the PANI-NTs up to a temperature (T HT 

) of about 1200 °C in the same MWHS using carbon 

fiber which self heats by absorbing MW. 

The chemical transformations in the PANI-NTs induced by the heat treatments are discussed 

by measuring the X-ray photoelectron spectroscopy (XPS) spectra. Finally, the temperature 

dependence of electrical conductivities of the PANI-NTs are measured in order to investigate the 

mechanism of electrical conduction of the heat treated PANI-NTs. 

Submission Date: 9 September 2008 

Acceptance Date: 13 February 2009 

Publication Date: 16 March 2009 

INTRODUCTION 

Carbon nanotubes (CNTs) [Iijima,1991] 

have been expected to be one of the most 

versatile materials in the field of materials 

science. However, the applications are limited 

because metallic elements used as catalysts for 

Keywords: PANI nano-tube, microwave absorption, 

deprotonation and carbonization, electrical conductivity, 

ESR-spectra, unpaired electron, XPS-spectra 

synthesizing CNTs remain inside the CNTs. 

To overcome the limitations, a synthesis of the 

CNTs in the metal free processes could be an 

option. 

It has been pointed out that the doped 

PANI-NTs are expected to be very promising 

process [Langer et al., 2007; Kyotani, et al., 

2008] because the doped PANI has been found 

to form tubular shaped self assembly at room 

temperature [Wan et al., 2000; Zhang et al., 

43-1-34 Journal of Microwave Power & Electromagnetic Energy ONLINE Vol. 43, No. 1, 2009

Figure 1. Morphologies of pristine-PANIs, (a) SEM and (b) TEM images. 

2005]. 

In this report, MW heat treatments are 

applied to deprotonate and carbonize the PANI- 

NTs which are synthesized self assembly by 

the same method shown in Zhang et al. [2005], 

and the properties of the heat treated PANI- 

NTs are investigated by measuring electrical 

conductivities, ESR, and XPS spectra. 

In the low T HT 

region (T HT 

< 300°C), a 

possible mechanism of MW absorption by the 

doped PANI-NTs is discussed by comparing 

the strength of measured ESR-spectra with 

the transitions of electrical conductivities 

which depend strongly on the temperature of 

MW irradiated PANI-NTs. The conduction 

mechanism is discussed from the point of view 

of the variable range hopping (VRH) of unpaired 

electrons with localized wave function [Mott et 

al., 1979; Kobayashi et al., 1993]. 

The measuring of the XPS spectra of the 

MW irradiated and heat treated PANI-NTs 

reveal the kind of chemical transformations 

that have been induced depending on the heat 

treated temperature of the PANI-NTs. 

PREPARATION of PANI-NTs 

The PANI-NTs were synthesized self assembly 

according to a method presented in the 

paper by Zhang et al. [2005] using (±)-10- 

camphorsulfonic acid (CSA) as a dopant. Firstly, 

aniline 0.12 mol and CSA 0.06 mol were mixed 

in 600 mL of distilled water. Before oxidative 

polymerization, the solution was cooled in an 

ice bath (0 – 5 °C). Next, an aqueous solution 

of ammonium per sulfate 0.12 mol pre-cooled 

in 300 mL of distilled water was added to the 

above-cooled mixture solution and reacted in 

the ice bath for 15 hours. The precipitate was 

filtered and washed with distilled water and 

methanol several times, and dried in vacuum 

at 40 °C for 24 hours. Finally, the raw PANI 

doped with CSA was obtained as a powder 

with a dark green color. 

Morphologies of the obtained PANI are 

shown in Figure 1(a) and (b) in the images of 

SEM and TEM observations, respectively. We 

have then confirmed that the PANI synthesized 

self assembly forms NTs with an outer diameter 

of about 100 nm, inner diameter of about a few 

10 nm and a length of a few μm. 

MICROWAVE HEATING SYSTEM 

To deprotonate and carbonize the PANI doped 

with CSA (pristine PANI-NTs), we have tried 

to irradiate 2.45 GHz MW to the pristine PANI 

NTs using our microwave heating system 

(MWHS) shown schematically in Figure 2. 

The continuous power controllable MW is 

generated by the use of an MW oscillator. The 

power of the MW is controlled by monitoring 

the temperature of the MW heated sample, 

which is installed in a quartz test tube. The 

International Microwave Power Institute 43-1-35

Figure 2. Principal layout of MWHS in a side view of E-face of H 01 

-mode waveguide. (1) 

microwave oscillator, (2) circulator, (3) power monitor, (4) short circuit plunger, (5) quartz test 

tube as reaction vessel, (6) junction, (7) condenser and trap, (8) gas outlet, (9) gas inlet, (10) 

thermocouple, and (11) controller. 

temperature is measured using a thermocouple 

of alumel-chromel covered by thin quartz glass 

inserted in the quartz test tube parallel to the 

E-face. To control the ambient atmospheric 

condition of the sample, gas is injected through 

a thin quartz glass tube inserted in the bottom 

of the test tube. 

MW direct irradiation 

Firstly, we have irradiated directly MW to heat 

the pristine PANI-NTs in our MWHS under the 

circulation of the N 2 

gas with a flow rate around 

300mL/min. The applicator part of the MWHS 

is presented schematically in a cross-section 

of the H 01 

-wave guide shown in Figure 3(a). 

The MW irradiation time dependence of the 

measured temperature rise of the pristine PANI- 

NTs are shown in Figure 3(b). It is confirmed 

that the pristine PANI-NTs self-heat in the 

lower temperature region through absorption 

of irradiated MW. However, the temperature 

rises are recognized to stop around 300 °C in 

spite of the higher power of MW irradiation 

of 1kW. Furthermore, the temperature of 

MW heated PANI-NTs revealed a tendency to 

descend to lower temperatures during longer 

MW irradiation with constant power. 

MWHS with an external heat source 

To heat up the pristine PANI-NTs to a 

temperature region higher than approximately 

300 °C, we have changed the heating method 

to carbon fiber (Besfight® produced by TOHO 

TENAX CO., LTD) which self heats rapidly 

by absorbing 2.45 GHz MW. A quartz test 

tube is wound around the carbon fiber (shown 

schematically in Figure 4(a)), then the samples 

installed in the quartz tube are treated thermally 

by the heat of the MW absorbed carbon fiber. 

To demonstrate the performance of MWHS 

as an external heating system, MW irradiation 

time dependence of the measured temperature 

of air inside the quartz test tube are shown in 

Figure 4(b) for several cases of the MW power 

under circulation of air with a flow rate of about 

1000mL/min. These demonstrations show that 

the MWHS is used as an external heating system 

by controlling the power of MW oscillator. 

Next, we have performed the pristine 

PANI-NTs’ heat treatment using our MWHS 

in the temperature region from T HT 

=300 °C 

to about 1200 °C under N gas circulation of 

2 

flow rate about 300mL/min. Of course we 

have confirmed that the properties of the heat 

treated PANI-NTs of T HT 

=300 °C have the same 


Figure 3. (a) Schematic view of applicator parts of MWHS shown Figure 2 in cross section of 

H 01 

mode wave guide, (b) MW power and irradiation time dependences of measured temperature 

rise of MW heated pristine PANI. 

Figure 4. (a) Schematic view of applicator part with reaction vessel winded by carbon fiber of 

MWHS in cross section of H 01 

mode wave guide, (b) MW power and irradiation time dependences 

of measured temperature rise of the atmosphere of the reaction vessels under the circulation of 

air with flow rate about 1000mL / min. 


Figure 5. Morphologies of heat treated PANIs at T HT 

= 900 °C (a) SEM and (b) TEM images. 

properties of the MW directly irradiated PANI- 

NTs for T HT 

= 300 °C. 

Figure 5 shows images of SEM and TEM, 

indicating that the heat treated PANI-NTs are 

confirmed to have almost the same morphologies 

as the pristine PANI-NTs. However, the surface 

of the heat treated PANI-NTs has become 

smoother than that of the pristine PANI-NTs 

shown in Figure 1. 

RESULTS AND DISCUSSION 

The whole PANI-NTs were treated by keeping 

the T HT 

for 30 minutes by controlling the 

power of the microwave oscillator shown in 

Figure 2. The measured electrical resistivities 

at room temperature of the MW irradiated and 

heat treated PANI-NTs are shown in Figure 6, 

dependent upon the heat treatment temperature 

T HT 

up to 1200 °C. To measure the electrical 

resistivities of the powdery PANI-NTs, the 

powder is screwed in a hole opened Teflon rod 

with two screws with electrode. The electrical 

resistivities are then measured by the usual two 

terminal methods. 

What is shown in the present study on the 

resistivity measurements is similar globally 

to those mentioned in a paper by Langer et al. 

[2007]; here the mechanism of transition from 

conductor phase to insulator phase in electrical 

conductivity is investigated from the view point 

of changes of electronic states and chemical 

transformations in the doped PANI-NTs by 

heat treatment. 

From the measured electrical resistivities, 

we have confirmed that the heat treated PANI- 

NTs of T HT 

from about 300 °C to about 600 °C 

transferred to the insulator. These temperature 

regions where the heat treated PANI-NTs 

transferred to the insulator are emphasized 

to coincide completely with the temperature 

region where the pristine PANI-NTs stopped to 

self heat by MW direct irradiation as shown in 

Figure 3. 

The doped PANI-NTs in the low T HT 

region 

of about 200 °C have electrical conductive 

properties with resistivity of about a few 

10Ω⋅cm, but the resistivity changed suddenly 

to much higher values in higher T HT 

than 300 

°C. In the T HT 

region higher than 600 °C, the 

heat treated PANI-NTs transfer gradually to 

conductive materials. 

To investigate the degree of deprotonation 

and carbonization in the doped PANI-NTs, we 

have measured XPS spectra of the heat treated 

PANI-NTs. The XPS measurements of the 

PANI-NTs were made on a system of KRATOS 

ANALYTICAL AXIS ULTRA DLD HAS SV2- 

003A with a MgK α 

X-ray source (1253.6eV 

with energy resolution about 0.7eV). 

From the XPS spectra shown in Figure 7, 

where the intensities are normalized to those of 

C1s core levels, the main origin of deprotonation 

of the doped PANI-NTs is attributed to the 

evaporation of sulfur elements. These results 

on the deprotonation processes are almost the 

same as those mentioned in a paper by Langer 

et al. [2007]. Of course, the rapid evaporation 


Figure 6. T HT 

dependences of measured resistivity at room temperature of the heat treated PANI. 


dependence of measured XPS spectra of the heat treated PANI in T HT 

region


dependence of measured XPS 

spectra of S2p core level of the heat treated 

PANI in low T HT 

region < 500°C. 

of oxygen elements is recognized, but nitrogen 

elements remain up to the higher heat treatments 

of about 1000°C. 

For the XPS spectra of S2p core levels, 

the heat treatment dependences are shown in 

Figure 8 in the low T HT 

region. The intensities 

and profile of S2p of PANI-NTs of T HT 

=100 °C 

did not change those of the pristine PANI-NTs. 

The XPS-spectra of the PANI-NTs of T HT 

= 

300 °C decreased the changing profile, but the 

intensities remained about half of those of the 

pristine PANI-NTs. The intensities of S2p of 

PANI-NTs became negligibly small at T HT 

= 

500 °C. 

From these T HT 

dependences of XPS spectra 

of S2p of PANI-NTs, the evaporation of sulfur 

does not seem enough to explain the change of 

degree of MW absorption (Figure 3) and the 

transition of electrical conductivities (Figure 

6). 

For the XPS spectra of N1s core levels, the 


dependence of measured XPS 

spectra of N1s core level. Dotted lines are the 

results of Gaussian deconvolution. 

heat treatment dependences up to T HT 

= 1200°C 

are shown in Figure 9. To identify the type of 

chemical bonds that compose the spectra, the 

results obtained by the Gaussian deconvolution 

are also shown using dotted lines. 

From the heat treatment dependences of 

the XPS spectra of N1s core levels, we can 

recognize that the chemical transformations 

induced by changes of chemical bonding in 

nitrogen coincide clearly with transitions of 

electrical conductivities of the PANI-NTs 

shown in Figure 6. 

The assigned binding energies by Gaussian 

deconvolution for pristine PANI-NTs and 

the MW irradiated PANI-NTs of T HT 

= 100°C 

and T HT 

=200°C almost coincide with those of 

well established conductive PANIs [Kang et 

al., 1998]. Accompanying the deprotonation 

induced by evaporation of sulfur, the N1s core 

levels of the heat treated PANI-NTs of T HT 

= 

300°C, 400°C and 500°C shifted to the lower 



dependence of measured X-band (9.15 GHz) ESR spectra of the heat treated 

PANI-NTs in low T HT 

region < 300 °C. 

binding energy region by about 2.5eV. These 

energy shifts of approximately 2.5eV are 

large when compared to binding energy shifts 

induced by chemical reduction methods in 

standard PANIs [Kang et al., 1998]. 

For the PANI-NTs of the heat treatment 

temperature higher than 600°C, the binding 

energies shift gradually depending on the heat 

treatment temperature. These tendencies of 

measured XPS spectra of heat treated PANI- 

NTs seem to be globally similar to that of the 

measured IR absorption presented in Langer et 

al. [2007]. 

We can infer the type of chemical bonding 

concerned with nitrogen by using the wellestablished 

binding energies of standard PANIs 

[Monkman et al., 1991; Kang et al., 1998], and 

we can also identify the character of the bonding 

by comparing the measured FTIR spectra with 

the well-established spectra of PANI [Kang et. 

al., 1998]. The detailed investigations on the 

change of chemical bonding will be presented 

elsewhere. 

To find out the fundamental mechanism 

of MW absorption by the pristine PANI-NTs, 

we have measured the ESR spectra of the heat 

treated PANI-NTs. The ESR measurements 

were made using a system of JEOL JES- 

RE200S. The measured ESR spectra are shown 

in Figure 10. 

The measured magnetic field H dependence 

of differential intensity dP/dH in ESR spectra 

is shown for the PANI-NTs heated by MW 

direct irradiation. 

Thus, the obtained ESR-spectra of heat 

treatment dependence for the doped PANI- 

NTs are very instructive to compare the ESRspectra 

for heat treated phenolformaldehyde 

resin [Tanaka, 1987] where the strength of dP/ 

dH has been measured to become weaker with 

increasing the heat treatment temperature. 

The measured ESR-spectra for the pristine 

PANI-NTs are considered to be a profile similar 

to that shown in PANI doped with 2-acrylamido- 

2-methyl-1-propane sulfonic acid [Sitaram 

et al., 2005]. Furthermore, the drastic change 

observed in the spectra from T HT 

= 200 °C to 

300 °C has been confirmed. It is then expected 

that the lower temperature heat treated PANI- 

NTs are composed of more localized wave 

functions occupied with unpaired electrons. 

For the higher temperature heat treated 


Figure 11. Measured temperature T dependence of electrical conductivities of the heat treated 

PANI-NTs plotted in a scale T -1/2 . 

PANI-NTs, the strength of the ESR spectra 

becomes weak, and the changes suggest that 

spin density with unpaired electrons decreases 

suddenly at around T HT 

= 300 °C as the results 

of the sulfur elements evaporation shown in 

XPS spectra (Figures 7 and 8) and chemical 

transformations explored in XPS spectra of 

N1s core levels (Figure 9). 

Therefore, it should be mentioned that 

the temperature rise stopping around 300°C 

in MW direct heating as shown in Figure 

3(b) is explained by the elimination of the 

unpaired electron in heat treated PANI-NTs 

and the electrical transitions from conductor to 

insulator. 

To identify the elimination of the unpaired 

electron depending on the heat treatment, we 

have measured temperature T dependence of 

electrical conductivities of the heat treated 

PANI-NTs. The results are shown in Figure 11 

using a variable T -1/2 . 

From the temperature T dependence of 

electrical conductivities σ of the heat treated 

PANI-NTs, the conductivities in low heat 

temperature treated PANI-NTs are understood 

by the variable range hopping (VRH) mechanism 

with localized unpaired wave function [Mott et 

al., 1979; Kobayashi et al., 1993]. In the higher 

temperature heat treated PANI-NTs of T HT 

>900 

°C, the electrical conductivities change to the 

characteristics of temperature dependences of 

metallic conductor depending on the degrees of 

the carbonization. 

CONCLUSION 

We have discovered a method for transferring 

the electrical conducting PANI-NTs to 

insulating ones by utilizing the characteristic 

response of the PANI-NTs to MW. Namely, the 

conducting PANI-NTs with unpaired electron 

absorb MW and self heat. The self heat 

drives the deprotonation of the PANI-NTs by 

accompanying evaporation of sulfur and the 

chemical transformation. 

Finally, the PANI-NTs that lost the unpaired 

electron transfer to the electrical insulator 

which does not absorb MW. Using our MWHS 

with N 2 

gas circulation, this method has been 

demonstrated by showing the relations between 

the ESR spectra and electrical conductivities. 

We can then summarize that the mechanism 

of the MW absorption by materials depends 

strongly on the existence of macroscopic 


electrical current with unpaired electrons in the 

PANI-NTs. 

To perform further the heat treatment of 

the PANI-NTs transferred to insulator, we have 

utilized the carbon fiber which self heats by 

absorbing MW. We have also carried out heat 

treatment for the PANI-NTs up to about T HT 

= 

1200 °C in the same MWHS. 

By performing the MW assisted heat 

treatments, we have found that the electrical 

insulating PANI-NTs generated by MW direct 

irradiation have been transformed finally to 

conductor depending on the degree of the 

chemical changes and carbonization in the heat 

treated PANI-NTs. 

What we have demonstrated in this report 

will be applied for fabricating the new materials 

by utilizing the characteristic response to MW. 

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749. 

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in Non-Crystalline Materials.” Clarendon Press, 

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Sitaram, V., Sharma, A., Bhat, S. V., Mizoguchi, K., and 

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ACKNOWLEDGMENTS 

This work was supported partly by Grant-in- 

Aid for “Global COE Program” by the Ministry 

of Education, Culture, Sports, Science, and 

Technology, Japan. 

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electrical conductivity of microwave heated polyaniline ... - JMPEE

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