3.1 Synthesis and characterization of SCNTs and SNCNTs
As shown in Figure 1a & b, the pristine CNTs have diameters of 10-20 nm and well-graphitized walls with around 10 layers. SCNTs were prepared by calcination of the pristine CNTs in a SO2 and N2 mixed gas. After the S doping post-treatment, the tubular morphology remains almost unchanged (Figure 1c). High resolution TEM observation (Figure 1d) shows that the surfaces of SCNTs become rough, much different with the smooth surface of the pristine CNTs (Figure 1b). The Raman D band to G band ratios (I D/I G) for SCNT-M and SCNT-S are higher than that for the pristine CNTs (Figure 1e), indicating that more defects have been introduced by S doping, well consistent with the high resolution TEM observation. In the XPS survey patterns of SCNT-S and SCNT-SO2 (Figure 1f), S 2p and S 2s peaks are observed, while no such peaks are found in the pristine CNTs. The S content in SCNT-S determined by XPS is 1.19 wt%, lower than that for SCNT-SO2 (4.6 wt%). It is attributed to the fact that the SO2 concentration generated by CaSO3 decomposition is much lower than that of the SO2-N2 mixed gas (16 wt%). In the S 2p patterns of SCNT-S and SCNT-SO2 (Figure 1g), three peaks centered at 163.9, 165.2 and 168.3 eV are attributed to the S atoms in the chemical configurations of C-S, C=S and –SOx(x=1-4),26-28 respectively. The amount of S atoms in the configurations of C-S and C=S account for 89.2 % of all the S atoms. It indicates that most S atoms have been covalently incorporated in carbon frameworks via C-S or C=S bonds. The S content of SCNT-SO2 (determined by elemental analysis) increases from 1.9 to 4.6 wt% when the S doping temperature increases from 700 to 900 ºC (Table S1), showing higher reactivity of SO2 at higher temperature. Therefore, the S doping reaction temperature of CNTs in the following study is fixed at 900 ºC.
SNCNTs were prepared by applying the gaseous S doping treatment onto NCNTs. As shown in Figure 2a & b, bamboo-joint like structures are observed in NCNTs, showing typical characteristics for CNTs with N doping. XPS analysis shows that the N content of NCNTs is 0.61 wt%. Similar with SCNTs, SNCNTs remain the tubular structure (Figure 2c), but the surfaces of SNCNTs are slightly rougher than NCNTs (Figure 2d) due to the introduction of more defects by S doping. Raman spectra of NCNTs and SNCNTs are shown in Figure 2e. TheI D/I G value for NCNTs is higher than that for the pristine CNTs, indicating that more defects and disorders in graphitic layers have been aroused by N doping. The large number of defects in NCNTs are highly active sites in favor of the following S doping. As the result, theI D/I G value of SNCNTs further increases to 1.095, obviously higher than that of SCNT-S (0.909). The S content in SNCNTs determined by XPS analysis is 0.67 atom % (1.75 wt%), obviously higher than that in SCNT-S (0.45 atom%, corresponding to 1.19 wt%). In the XPS survey pattern of SNCNTs (Figure 2f), S 2p, S 2s and N 1s peaks are observed, showing the successful introduction of S and N heteroatoms. In the S 2p peak of SNCNTs (Figure 2g), two strong peaks at 163.7 and 165.0 eV are observed, corresponding to C-S and C=S configurations, there are about 89.9% S atoms are directly connected with carbon framework. It indicates that most S atoms in SNCNTs have also been covalently connected with carbon frameworks via C-S or C=S bonds. In the X-ray diffraction (XRD) patterns (Figure S1), there is no obvious change after S doping, implying that the S doping reaction have aroused more surface defects but not affected the inner carbon layers of CNTs.