The acid-treated CNT films demonstrate a general increase in electrical conductivity by at least a factor of 2. The 30 min sample has the most significant enhancement with the electrical conductivity of 4990 ± 636 S/cm, much larger than 960 S/cm of SOCl2-treated buckypaper [52] and ~ 2000S/cm of SOCl2 -treated CNT film [53]. This enhancement can be attributed to two factors where the acid treatment first purifies the CNTs [54] and possibly introduces some functional groups into the CNT films [55]. Fig. 5b compares the Raman spectra of the as-prepared and acid-treated specimens. The higher ratio of the intensity of G peak to the intensity of D peak (IG/ID) indicates the less defective structure of the acid-treated film, suggesting the purification effect of the acid treatment. As the presence of impurities (amorphous carbon, highly defective CNTs and catalysts) are known to impede electrical conduction [56], purification of the film through the use of acid treatment is believed to have resulted in the removal of some impurities, thereby improving conductivity. Another factor that contributed to the increase can be the addition of functional groups onto the CNTs which may
generate hydrogen bonding or dipole-dipole interaction [39]. These stronger interactions between nanotubes can result in further densification of the CNT bundles [40], as shown in Fig. 4h. As mentioned previously, densification of CNT films reduces contact resistance between individual CNTs resulting in better electrical conductivity. Furthermore, it has been reported that the functional groups might improve the conductivity by serving as efficient electron pathways [57].However ,a sufficient functionalization of CNTs usually leads to a decreased IG/ID indicating a more defective CNT structure [40,42]. Unlike these studies, the increased IG/ID in this study suggests the degree of the functionalization may be very slight during the 30 min treatment. Hence, the main effects of the acid treatment in this work are suggested to be the purification and densification of the CNT films.