Carbon nanotubes (CNTs), which were first discovered in 1991 by Iijima (Reference Iijima1991), have been examined for many applications owing to their excellent properties. CNTs have great potential for a wide range of applications owing to their electrical, thermal, and mechanical properties (Avouris et al., Reference Avouris, Chen and Perebeinos2007). In addition, the large specific area and small size of the CNTs make them suitable catalyst supports for catalytically active metal and/or metal oxide particles. Therefore, various metal/CNT and metal oxide/CNT composites have been investigated to take full advantage of the multifunctionality of the metal, metal oxide, and CNTs. In the case of metal oxide/CNT composites, CNT can provide a great reaction site because of the large specific area and high electronic conduction path to the metal oxide, which itself has relatively low conductivity (Avouris et al., Reference Avouris, Chen and Perebeinos2007). Therefore, CNTs are expected to increase the electrochemical utilization of metal oxide. Among the various metal oxides, nickel oxide (NiO) is attractive in view of its low cost, low toxicity, well-defined electrochemical redox activity, and the possibility of enhanced performance through different preparative methods (He et al., Reference He, Wu, Zhang and Wang2006). For these reasons, NiO/CNT composites have been studied widely for many possible applications, such as catalysts, electrochromic films, gas sensors, and electrode materials (Xia et al., Reference Xia, Tu, Zhang, Wang, Zhang and Huang2008; Wang et al., Reference Wang, Li and Cheng2008; Lota et al., Reference Lota, Sierczynska and Lota2011).
For many applications, it is important to understand the oxidation resistance and thermal stability of CNTs in a NiO/CNT composite because they can limit the operation temperature and applicability of the composites. In general, CNTs are oxidized at 600–700°C (Ajayan et al., Reference Ajayan, Stephan, Redlich and Colliex1995). On the other hand, catalyst residues, amorphous carbon, and defects can reduce considerably their oxidative stability toward air. In contrast, the oxidative stability can be enhanced by high-temperature vacuum annealing, which produces more graphitic CNTs with a low defect concentration (Osswald et al., Reference Osswald, Flahaut, Ye and Gogotsi2005; Behler et al., Reference Behler, Osswald, Ye, Dimovski and Gogotsi2006). In addition, in NiO/CNT composites, the oxidation resistance and temperature of CNT can be affected by NiO around the CNT, and the oxidation state of the NiO/CNT composite can affect its characteristics and performance (Li et al., Reference Li, Wang, Liang, Wang, Wu, Hu and Liang2004; Aksel & Eder, Reference Aksel and Eder2010). Therefore, the oxidation mechanism and thermal stability of NiO/CNT composite should be examined in detail.
In this study, NiO/CNT composites with different NiO contents were prepared using a solution-based method. Thermogravimetric analysis (TGA) in air and N2 confirmed the NiO loading on CNT and the decrease in oxidation temperature with increasing NiO content. The oxidation mechanism was predicted from these data. Transmission electron microscopy (TEM), with ex situ and in situ heat treatment, was used to observe the microstructure.
Materials and Methods
NiO/CNT composites were prepared by the thermal decomposition of nickel nitrate on CNTs. Multi-walled carbon nanotubes (MWCNT, M95, Carbon Nano-Material Technology Co. Ltd.) and N2NiO6·6H2O with various contents (10, 30, 50 and 80 wt% NiO) were dissolved in ethanol. The samples were heat-treated in vacuum for 1 h at 300°C for the thermal decomposition of N2NiO6·6H2O to NiO (Brockner et al., Reference Brockner, Ehrhardt and Gjikaj2007). Subsequently, TGA (TA Instruments Q500) was conducted to determine the NiO loading on the CNT and examine the variations of the oxidation temperature as a function of the NiO content and atmosphere (air and N2). The samples were analyzed in a platinum pan from room temperature to 900°C at a heating rate of 5°C/min. Raman spectroscopy (Renishaw RM 1000-In Via) was used to examine the quantity of defects in the CNTs. The ex situ heat treatment was conducted at 600°C for 30 min in vacuum or in air. TEM (JEM-2100F, JEOL Co. Ltd.) was employed to observe microstructure of the NiO (50 wt%)/CNT composite. In addition, selected area diffraction (SAD) was used to confirm the phase transition. The in situ heating experiments were performed in the TEM (JEM-3011 with a LaB6 filament operating at 300 keV, JEOL Co. Ltd.). A model EM-21130 JEOL heating holder was used to resistively heat the samples.
Results and Discussion
Figure 1 shows TGA results of the NiO/CNT composite with various NiO contents in air and N2. Weight losses were observed in each sample and derivative plots were obtained in both atmospheres. The NiO loading after CNT oxidation was confirmed by the weight loss curves in air. The amount of residue remaining increased with increasing NiO loading (see Fig. 1a). On the other hand, in N2 atmosphere, there was less residue remaining (see Fig. 1b). Derivative plots (DTG) in N2 revealed peaks at ~540°C (Fig. 1b). In contrast, in air, the peaks were observed at ~400°C with increasing NiO content from 10 to 80 wt% (Fig. 1a). As a reference, the MWCNTs, which were used for this study, were oxidized at ~600°C (data not shown). In other words, NiO affects the oxidation stability and decreases the oxidation temperature of CNT. The main peaks at ~540°C in both atmospheres are believed to have occurred via the same mechanism. Unlike an air atmosphere, which supplies oxygen continuously, in a N2 atmosphere, there is no oxygen source to oxidize the CNTs except for the oxygen in NiO itself. Therefore, the CNTs were oxidized at the interface between NiO and CNT by consuming oxygen from the NiO and reducing NiO to nickel. This was confirmed by TGA in N2 atmosphere, which showed less residue with increasing NiO loading (see Fig. 1b). The number of reaction sites that cause CNT oxidation increased with increasing NiO content, and more oxygen could be consumed through CNT oxidation. Consequently, less residue remained in the N2 atmosphere. In the air atmosphere, however, oxygen vacancies generated by the oxidation of CNT diffuse from the interface between NiO and CNT to the surface of NiO and are regenerated by gas-phase oxygen (Mars & Krevelen, Reference Mars and Krevelen1954; Aksel & Eder, Reference Aksel and Eder2010). Therefore, in air atmosphere, NiO can keep its phase without a transition to nickel, and the amount of residue shown in the TGA results (Fig. 1a) can be understood in the same context.
To confirm and observe the microstructure and phase transition directly through the oxidation of CNT by reducing NiO to nickel, comparative TEM analysis was conducted selectively for the NiO (50 wt%)/CNT composite samples before and after the ex situ heat treatment at 600°C for 30 min in vacuum and air atmosphere. Figure 2 shows the TEM results before and after heat treatment. After heating in a vacuum, the phase transition of NiO to nickel was confirmed by the SAD pattern and fast Fourier transform (FFT) image (Figs. 2b, 2d). During the oxidation of CNT by reducing NiO to nickel in a vacuum, the oxygen in NiO diffuses to the CNT surface and carbon diffuses into nickel because carbon has some solid solubility in nickel (Singleton & Nash, Reference Singleton and Nash1989). This was confirmed by the formation of graphitic shell covering the nickel particles, which was formed by the precipitation of supersaturated carbon from nickel with cooling down (Fig. 2b). In contrast, NiO in the sample after heating in air maintains its phase without a transition because reduction by CNT and reoxidation by gaseous O2 molecules in NiO occur simultaneously (Fig. 2c).
The oxidation mechanism of CNT with reducing NiO can be explained using the Ellingham Diagram. To oxidize CNT, O2 molecules were adsorbed on the CNT surface first, a C–O bond was formed, and the C–O pair was desorbed with breaking C–C bonds (Zhu et al., Reference Zhu, Lee, Lee and Frauenheim2000; Park et al., Reference Park, Choi, Kim, Chung, Bae, An, Lim, Zhu and Lee2001). In the Ellingham Diagram, the position of the line for a given reaction shows the stability of the oxide as a function of temperature and can determine the relative ease of reducing a given metallic oxide to metal. Reactions closer to the top of the diagram are the most noble metals, and their oxides are unstable and reduced easily. Toward the bottom of the diagram, the metals become progressively more reactive and their oxides become more difficult to reduce (Ellingham, Reference Ellingham1944). To examine the reduction of NiO by CNT, the 2Ni + O2 ⇒ 2NiO line and 2C + O2 ⇒ 2CO line can be used because a CO molecule was desorbed from the adsorbed phases by breaking the C–C bonds after O2 adsorption. Both lines are crossed at ~500°C. At temperatures <500°C, the 2C + O2 ⇒ 2CO line takes place at the bottom of the 2Ni + O2 ⇒ 2NiO line and CNT cannot be oxidized by reducing NiO. On the other hand, at temperatures >500°C, the positions of both lines are reversed and the heat of formation of carbon monoxide is smaller than NiO at ~500°C. Therefore, the main DTG peak at ~540°C in the TGA results in air and N2 atmospheres with the same mechanism, i.e., because of the oxidation of CNT by consuming oxygen from NiO and generating oxygen vacancies.
On the other hand, with increasing NiO loading (50 and 80 wt%) in air, the peak moved from ~540 to 400°C. This is caused by another mechanism, and is due to defects of CNT, which occur with increasing NiO content. To quantify the increase in defects due to the NiO loading, Raman spectroscopy was carried out with excitation at 633 nm. Figure 3 shows the Raman spectra of the CNTs with various NiO contents. The number of defects can be estimated from the D band in the Raman spectrum. Two characteristic peaks near 1,320 and 1,600 cm−1 were observed in the NiO/CNT composites. The former, which is known as the D band, originates from the first-order scattering process of sp3 carbon by the presence of in-plane substitutional hetero-atoms, vacancies, grain boundaries, or other defects, and by finite size effects; all of these reduce the crystalline symmetry of the quasi-infinite lattice (Brown et al., Reference Brown, Jorio, Dresselhaus and Dresselhaus2001). The latter, which is known as the G band, represents the sp2 carbon states related to the graphitic hexagon-pinch mode (Osswald et al., Reference Osswald, Havel and Gogotsi2007). Owing to the origin of the respective bands, the R value (i.e., the D band/G band ratio) indicates the quantity of defects (disordered carbon) in the tube walls (Lee et al., Reference Lee, Park, Bae, Lee, Song, Kim, Lee and Yang2011). More defects are generated on CNTs with increasing NiO content, which was demonstrated by an increase in R value from 1.008 for 10 wt% to 1.318 for 80 wt%. The adsorption energy of an O2 molecule on defects is much larger than that on a nondefective tube wall. Furthermore, the desorption barriers of the C–O pair from the defects are much lower than those at nondefective wall (Zhu et al., Reference Zhu, Lee, Lee and Frauenheim2000; Park et al., Reference Park, Choi, Kim, Chung, Bae, An, Lim, Zhu and Lee2001). Therefore, the oxidation rate of disordered carbon and defective tubes by O2 molecules is much faster than that of the nondefective wall and oxidation occurs at ~400°C (Osswald et al., Reference Osswald, Flahaut, Ye and Gogotsi2005; Behler et al., Reference Behler, Osswald, Ye, Dimovski and Gogotsi2006).
The temperature when a phase transition occurs was examined directly by observing the dynamic changes in the diffraction patterns of NiO as a function of temperature through an in situ heating experiment. Figure 4 shows the in situ results. In the SAD patterns, the (200) plane of nickel began to appear at ~500°C, and the (111) and (220) plane of NiO decreased with continued heating. These ex situ and in situ TEM results are consistent with the TGA results.
This study examined the thermal stability and oxidation mechanism of the NiO/CNT composite. The NiO/CNT composites were prepared by a solution-based method with various NiO contents. The oxidation mechanism was suggested based on the comparative TGA and TEM results in air and N2 atmosphere and Ellingham Diagram. In addition, phase transitions were observed directly by in situ heating TEM. NiO decreases the oxidation temperature of CNTs and acts as a catalyst. CNTs can be oxidized by reducing NiO to nickel by consuming the oxygen of NiO, even in a N2 atmosphere. In air, NiO maintains its phase because reduction by CNT and reoxidation by O2 molecules occur simultaneously. Moreover, with increasing NiO content, the oxidation temperature decreased to much lower temperature because of defects in CNT generated by the NiO loading. The thermal stability of the NiO/CNT composite was examined, and it was affected by NiO and the quantity of defects.
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MEST; No. 2011-0017257, No. 2011-0019984, and No. 2011-0030803). The authors gratefully appreciate technical support from the Cooperative Center for Research Facilities (CCRF) at Sungkyunkwan University.