Figure 2 TEM images of the NCNTs (a), (b) and the SNCNTs (c), (d); Raman spectra (e), XPS survey (f) of NCNTs and SNCNTs; S 2p patterns of SNCNTs (g).
The gaseous S doping has significantly improved the hydrophilicity of CNTs. CNTs-water suspensions with a CNT content of 0.2 wt% were prepared by violently stirring and ultrasonification for 20 min. As shown in Figure 3a, all the CNT samples are evenly dispersed in water in the initial state. After standing for 1 h (Figure 3b), obvious precipitation appears in the aqueous suspension containing the pristine CNTs, almost without CNTs dispersed in the supernatant. In contrast, highly stable dispersion of CNTs is observed for the SCNT-S and SNCNT suspensions. It indicates that S doping has markedly enhanced the hydrophilicity of CNTs. Obvious precipitation in the SCNT-M suspension is observed. It implies that the hydrophilicity of SCNT-S is better than SCNT-M. The contact angles between different CNT tablets and water droplets were further measured. As shown in Figure 3c-f, the contact angles for SCNT-S, SCNT-M and SNCNTs are obviously smaller than that for the pristine CNTs, and the samples with gaseous S doping (SCNT-S and SNCNTs) exhibit smaller contact angles than the S doped one using MgSO4 as S dopant (SCNT-M). The contact angle test results are well consistent with the stability observation (Figure 3a & b). Considering that the gas-solid contact between SO2and CNTs would be better than the solid-solid contact between MgSO4 and CNTs, it is reasonable to consider that the gaseous S doping process would lead to more even distribution of S heteroatoms on the surface of CNTs, as compared to the solid phase S doping with MgSO4 as S dopant. As the result, the evenly-distributed hydrophilic S-containing groups in SCNT-S have led to the better hydrophilicity as compared to SCNT-M.
The conductivity of SCNT-S, determined by a powder conductivity test, is 1230 S/m, significantly higher than SCNT-M (780 S/m), as listed in Table 1. It can be concluded that the gaseous S doping is more effective to promote the hydrophilicity and conductivity of CNTs, as compared to the S doping introduced by MgSO4. The conductivity of NCNTs is lower than CNTs, probably because the defects on surface and the breakages in graphite layers aroused by N doping have increased the contact resistance and lowered the long-distance conductivity.