Thermal Analysis of Multi-walled Carbon Nanotubes by Kissinger ’ s Corrected Kinetic Equation

Carbon nanotubes (CNTs) have been a popular material in recent years, but their thermal characteristics have not been understood completely. We investigated the unique thermal stability of multi-walled carbon nanotubes (MWCNTs) and used nitric acid (HNO3) to purify MWCNTs to promote its activation energy (Ea). The study used differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), and Fourier transform infrared (FTIR) spectrometer to analyze as-grown MWCNTs and modified MWCNTs. For DSC, the heating rate was chosen to be 0.25 to 2.0 °C/min. From DSC results, Ea and exothermic onset temperature (T0) of the modified MWCNTs increased with increasing HNO3 concentration. The TGA results showed that both as-grown and modified MWCNTs’ decomposition temperatures were higher than 500°C in air. The infrared spectra of as-grown MWCNTs and modified MWCNTs have shown that the gas phase composition is CO2 after TGA linked with FTIR. By Kissinger’s corrected kinetic equation, Ea increased with increasing HNO3 concentration. Through this study, we realized that as-grown MWCNTs and modified MWCNTs are thermally hazardous materials with high potential heat of decomposition, especially under fire exposure. Thus, it is important to know the thermal hazard characteristics of material with a measure to prevent its thermal damage during perturbed situations.


INTRODUCTION
In 1991, Dr. Sumio Iijima discovered multi-walled carbon nanotubes (MWCNTs) via arc-discharge (Iijima, 1991).Since then, carbon nanotubes (CNTs) have attracted tremendous interest globally.They have unique physicochemical as well as electrical properties and have various emerging applications in field emission, hydrogen storage, and chemical sensors (Lu et al., 2005).They not only can adsorb volatile organic compounds (VOC) (Cheng, 2008;Sone et al., 2008) and heavy metal (Li et al., 2003) but also act as a filter (Tsai et al., 2008).
CNTs have van der Waals' attraction so that commonly causes significant agglomeration.This characteristic of CNTs could obstruct its application with other materials.Attaching functional groups can markedly increases the dispersion of nanotube materials (Park, et al., 2003).To modify CNTs can improve the mechanical properties of nanocomposites.There are various modification methods for CNTs that have been reported, such as microwave-assisted heating, thermal oxidation, and chemical oxidation (Chin et al., 2007).Among them, chemical oxidation is frequently used, including nitric acid (HNO 3 ), hydrogen peroxide (H 2 O 2 ), mixture of H 2 O 2 and hydrochloric acid (HCl) or a mixture of sulfuric acid (H 2 SO 4 ) and HNO 3 (Hou et al., 2008).
However, reports of thermal analysis for as-grown MWCNTs and modified MWCNTs are still scanty in the open literature (Pritchard, 2004;Jin et al., 2007).In this study, the thermal decomposition of MWCNTs was characterized for the thermal hazards, investigated, as well as the thermal stability and the gas phase composition identified during TGA test.Finally, we used Kissinger's corrected kinetic equation to analyze and elucidate the thermokinetic parameters derived from the experimental data.

Modification of MWCNTs
The first step used the as-grown MWCNTs with 3 N, 5 N, 10 N and 15 N HNO 3 in a round bottomed flask equipped with a condenser.The dispersion was refluxed under magnetic stirring at 60°C for 12 hr (Ovejero et al., 2006).Moreover, the resulting dispersion was diluted in water and filtered.The resulting solid was washed up to neutral pH.Finally, the sample was dried in an oven.

Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) Spectrometer
The second step used attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectrometer to identify the surface functional groups of the modified MWCNTs.First, the surface of ATR was cleaned by methanol before the sample was applied.The reason was to ensure that the infrared reflecting medium, germanium (Ge), was not polluted.After waiting for the methanol to be evaporated completely, the powder of as-grown MWCNTs and modified MWCNTs were covered with the infrared reflecting medium and fixed on ATR-FTIR.The frequency range of FTIR was set to be from 4,000 to 400 1/cm.Finally, the FTIR spectrum was taken to compare with the standard spectra.We could observe whether the functional group followed our expectations of bond formation on modified MWCNTs or not.

Differential Scanning Calorimetry (DSC)
The third step used the DSC 821 e Mettler TA 8000 system and STAR e analysis software to conduct the experiment (Mettler Toledo, 2005).The thermal curve from DSC can be preliminarily used to evaluate the thermal properties for the material of interest.The first purpose was to understand experimental data on a material's thermal parameters, such as exothermic onset temperature (T 0 ) and ΔH, with respect to time.Then, the experimental data were taken to analyze the thermokinetics, for instance, activation energy (E a ), reaction order, extent of reaction and so on.
As-grown MWCNTs and modified MWCNTs both were packed in an aluminum pan and heating rates were set from 0.1 to 10 °C/min (Chang et al., 2009).In this way, the precision and reproducibility of the experimental results were examined and evaluated with a sound response.For the sake of better precision, the heating rate was chosen to be from 0.25 to 2.0 °C/min.

Thermogravimetric Analyzer (TGA) Linked with Fourier Transform Infrared (FTIR) Spectrometer
The fourth step adopted the PerkinElmer Paris 1 system to perform a weight loss percentage and decomposition experiment (PerkinElmer, 2006).Initially, the temperature was set from 30 to 950°C for TGA.Then, sample was taken in 21 vol.%oxygen environment and the heating rate was 4 °C/min.By this experiment, we can gain the weight loss percentage and decomposition temperature of as-grown MWCNTs and modified MWCNTs.
By means of TGA linked with FTIR, the gas phase sample compositions could be identified simultaneously.The frequency range of FTIR was from 4,000 to 400 1/cm (PerkinElmer, 2006;Yang et al., 2007).

Kissinger's Corrected Kinetic Equation
Kissinger's corrected kinetic equation is the most typical and extensive model that is prevailingly applied to evaluate the kinetics.For the results a line can be drawn through the experimental data from thermal analysis and Kissinger's corrected kinetic equation.Eq. ( 1) expresses the Kissinger's corrected kinetic equation: where β is heating rate (°C/min); A is pre-exponential factor (1/sec); E a is activation energy (kJ/mol); T p is peak temperature (K); R is gas constant (= 8.314 J/mol K).Therefore, using the slope of the line and Eq. ( 1), the E a of the sample can be determined (Kissinger, 1957;Soliman, 2007;Chou et al., 2008;You et al., 2009).
Fig. 1 shows the FTIR spectrum of as-grown MWCNTs, MWCNTs modified with 5 N HNO 3 , and MWCNTs modified with 15 N HNO 3 .It can be seen that the three curves have similar peaks at 3,273 1/cm (O-H) and 1,735 1/cm (C=O).Otherwise, the hydroxyl group and carbonyl group peaks of 15 N HNO 3 curve are more intense than the other two curves.Consequently, we proved our study can succeed to bond on carboxyl group on the surface of MWCNTs (Ovejero et al., 2006).

DSC Tests
The study applied DSC 821 e to conduct a heating experiment in order to understand the sample's stability and exothermic phenomenon of thermal decomposition reaction.By DSC scanning, the results of as-grown MWCNTs are presented in Table 1 and Fig. 2. As temperature was increased, the sample was triggered for exothermic reaction.The conclusion could be generalized from DSC scanning results: when heating rate is increased, the corresponding peak temperature (T p ) and T 0 also increase.This phenomenon is called the "thermal delay effect".
Therefore, the results showed that MWCNTs decomposition occurred under oxidation when the temperature reached 300°C in air.The peak from the thermal curve was distributed from 512 to 601°C and heat of decomposition was from 30,810 to 40,360 J/g.
After that, the same method was used to analyze modified MWCNTs with different HNO 3 solution concentrations, as can be seen in Table 2 and Fig. 3.When increasing HNO 3 concentration, E a and T 0 of the modified MWCNTs were augmented.For 3 N, T 0 was 358°C.However, when concentration was raised to 5, 10 and 15 N,    Through the experimental results, we inferred HNO 3 concentration is a key factor to affect E a .The literature indicates that when MWCNTs are dipped in HNO 3 solution, a bond of MWCNTs is formed on carboxyl groups at the surface of MWCNTs (Kuznetsova et al., 2000).The carboxyl groups at the surface of the MWCNTs became a hydrogen bond, as depicted in Fig. 4. All of the carboxyl groups at the surface of MWCNTs came to form a hydrogen bond net, so that T 0 and E a were increased, leading to the thermal delay effect.

TGA Tests
Fig. 5 shows the as-grown MWCNTs result received from TGA tests.Heating rate of heating furnace was 0.25, 0.5, 1.0 and 2.0 °C/min.As as-grown MWCNTs presented to lose weight, the onset temperature of as-grown MWCNTs was 486°C, 522°C, 537°C, and 551°C.This result reveals that when heating rate is increased, the decomposition temperature is also enhanced.
After that, the modified MWCNTs were loaded in a platinum pan for TGA at 2.0 °C/min.In Fig. 6, the decomposition temperatures are 575°C, 589°C, 594°C, and 597°C corresponding to 3 N, 5 N, 10 N, and 15 N of HNO 3 .Hence, T 0 of the modified MWCNTs was increased with increasing HNO 3 concentration at the same heating rate.We can infer that such results are also related to the hydrogen bond, as disclosed in Fig. 4. The carboxyl group at the surface of MWCNTs grew to form a hexagon hydrogen bond net.This could enhance E a of MWCNTs and increase the thermal stability of the material, so that decomposition temperature rose.The greater the HNO 3 concentration, the more carboxyl group can be bonded on MWCNTs.Furthermore, decomposition temperature also increased obviously.
Results of FTIR Fig. 7 demonstrates the infrared spectrum of product which was detected by FTIR during TGA test.The FTIR spectrum showed two intense peaks at 2,360 and 668 1/cm in Fig. 8.By comparing with standard infrared spectra, the stretch peak of carbon dioxide (CO 2 ) is at 2,360 1/cm.Therefore, the gas phase product of MWCNTs was validated as CO 2 .
The FTIR spectra of modified MWCNTs which were modified by different HNO 3 concentrations were investigated by the same way.All FTIR spectra were similar to as-grown MWCNTs, as denoted in Fig. 7.In every spectrum a peak of CO 2 was present.Therefore, we can conjecture CO 2 is the product of as-grown and modified MWCNTs after oxidation.Here, the TGA test was a complete combustion process.(3) Through transposing Eq. ( 3), a value of E a could be obtained:

Kissinger's Corrected Kinetic Equation
As mentioned above, E a was determined step by step to correspond to concentration, as can be seen in Fig. 8.By Kissinger's corrected kinetic equation, the E a of as-grown MWCNTs and modified MWCNTs through four different HNO 3 concentrations can be acquired.Furthermore, HNO 3 concentration and E a formed a graph that became a curve and had a significant tendency, as can be seen in Fig. 9. Consequently, E a was augmented by increasing HNO 3 concentration.On the other hand, when modifying MWCNTs by HNO 3 in the future, this curve can be applied to determine which concentration corresponds to a specific E a .

CONCLUSIONS
According to the DSC and TGA data, the thermal delay effect on an increasing heating rate increased with the T 0 .From DSC results, E a and T 0 of the modified MWCNTs increased with increasing HNO 3 concentration.The TGA results showed the same phenomenon.Both MWCNTs' and modified MWCNTs' decomposition temperatures were higher than 500°C in air.By linking TGA with FTIR, the gas phase product of MWCNTs was identified as CO 2 .
As a result of Kissinger's corrected kinetic equation from DSC data, E a was augmented by increasing HNO 3 concentration.In principle, the carboxyl group at the surface of MWCNTs is made up of a carbonyl group and hydroxyl group.The carboxyl group at the surface of MWCNTs becomes a hydrogen bond.With the increase of hydrogen bond, it becomes a hydrogen bond net around the outer layer MWCNTs.In this situation, E a of modified MWCNTs will be enhanced.Consequently, E a of modified MWCNTs is higher than as-grown.This situation could also prove that modified MWCNTs' decomposition temperatures change because of the E a by TGA decomposition experiment.Through the above-mentioned, we can investigate "concentration vs E a " to correspond to the degree of thermal hazard.
Through this study, we determined that both as-grown MWCNTs and modified MWCNTs are thermally hazardous materials with high potential heat, especially under fire exposure.Thus, it is important to know the thermal hazard characteristics of a material with a measure to prevent thermal damage during perturbed situations.

AKNOWLEDGMENTS
We are indebted to the donor of the National Science Council (NSC) in Taiwan under the contract No. NSC 97-2622-E-224-002-CC3 for financial support that has made this paper possible.

A
pre-exponential factor, m 3 /mol sec E a activation energy, kJ/mol R gas constant, 8.314 J/K mol T 0 exothermic onset temperature, °C T p peak temperature, °C β heating rate, °C/min

Fig. 4 .Fig. 5 .
Fig. 4. Carboxyl group at surface of MWCNTs become hydrogen bond.The dotted line stands for hydrogen bond.

Table 1 .
Results of experimental data for as-grown MWCNTs by DSC tests at heating rates of 0.25-2.0°C/min.

Table 2 .
Results of experimental data for as-grownMWCNTs and modified MWCNTs by DSC tests at heating rate of 1.0 °C/min.