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.