The intensity ratios of the two peaks (i.e., I D/I G), which

has frequently been used to appraise the crystallinity of CNTs [17], were estimated. The resultant I D/I G values, as listed in Table  1, indicated that the I D/I G values were seldom changed by coating of the find more Al interlayers, but they were significantly reduced by thermal treatment, such as 0.57 to 0.59 for the as-deposited CNTs and 0.40 to 0.43 for the thermally treated CNTs. This may have been because the amorphous carbonaceous by-products, residual binders, and other impurities that were adsorbed on the CNTs’ outer walls were somewhat removed during the thermal treatment. Accordingly, it can be inferred from the FESEM and Raman results that the enhanced electron emission of the thermally treated CNTs may be due to the improvement of their crystal qualities

[18]. Figure 2 The Raman spectra of the CNTs. The estimated I D/I G values are also displayed for all of the CNTs. The X-ray photoelectron spectroscope (XPS; MultiLab 2000, Thermo, Pittsburgh, PA, USA) was used to analyze the chemical bonds of the CNTs. Figure  3a,b shows the XPS spectra of the C 1 s state for all of the CNT samples. The C 1 s spectra were composed of several characteristic peaks, such as two peaks due to the carbon-carbon interactions including C-C sp 2 bonds at the binding energy of 284.4 to 284.7 eV Selleckchem BYL719 and C-C sp 3 bonds at 285.1 to 285.5 eV, and two relatively weak peaks due to the carbon-oxygen interactions including C-O bonds at 286.4 to 286.7 eV and C = O bonds at 287.8 to 288.1 eV [19]. Also, the variations of the peak intensities Inositol oxygenase due to thermal treatment were calculated, which are expressed in Figure  3a,b as the intensity ratios of thermally treated CNTs (i.e., CNT-B or CNT-D) to as-deposited

CNTs (i.e., CNT-A or CNT-C) for each peak (e.g., CNT-B/CNT-A = 1.08 for the C-C sp 2 peak as shown in Figure  3a). The results show that after the thermal treatment, the C-C sp 2 bonds increased, but the C-C sp 3 bonds decreased. This implies the improvement of the CNTs’ crystal qualities, which corresponds to the Raman analysis as shown in Figure  2. After the thermal treatment, furthermore, both of the C-O and C = O peaks were observed to be reduced. These carbon-oxygen peaks indicate that oxygen contaminants such as the carbonyl (C = O), carboxyl (-COOH), and hydroxyl (O-H) groups, which may be generated inevitably by acid treatment during the purification process [20], exist in the CNTs. Accordingly, the decrease of the carbon-oxygen peaks in the XPS spectra indicated that the decomposition of the oxygen contaminants occurred via the thermal treatment [21]. Figure 3 The XPS spectra for C 1  s states of the CNTs. (a) The XPS spectra of the CNT-A and CNT-B samples. (b) The XPS spectra of the CNT-C and CNT-D samples.