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of graphite-like sp2 domains, which can get a response from the
FT-IR spectra due to that N incorporated into the bonding network
[53].
Fig. 10, Ni/MWCNTs exhibit the lowest peak intensity of Ni0+ in
comparison with other samples, while the functional group modi-
fied MWCNTs supported catalysts display higher peak intensity of
Ni0+, suggesting that functional group can effectively act on metal
ions by electrostatic attractions and chemical interactions so as to
provide nucleation sites, and thus enhance the reduction degree of
NiO as well as obtain higher proportion of metallic Ni [29,30]. Sim-
ilarly, as can be seen in N doped MWCNTs in Fig. 10, the stronger
peak intensity of Ni0+ than Ni/MWCNT is presented, indicating that
the introduction of N species can effectively boost the reduction of
NiO so as to increase the percentage of Ni0+. And the fact that the
metallic Ni content can be increased by doping N species has also
identified by many researchers [28,31,57,63]. Also, the surface
compositions of the samples are displayed in Table 3. As shown
in Table 3, the content of Ni0+ in N doped MWCNTs increases firstly
and then decreases when the pyrolysis temperature increases from
873 K to 1273 K, and Ni/N-MWCNTs-800 possesses the highest
Ni0+ content. Moreover, pyridinic N is considered as metal-
binding sites for transition-metal [58,60] and active site for pro-
moting NiO reduction [27,28,57], it also can be seen that Ni/N-
MWCNTs-800 presents the highest pyridinic N ratio. Additionally,
it can be seen in Table 3 that the binding energy of Ni species shifts
toward higher value with the increment pyridinic N ratio, the rea-
son may be that the pyridinic N can effectively influence on the
electronic structure of Ni species due to the electron transfer
between Ni and N species. Ning et al. found that pyridinic N can
serve as electron-acceptor and thus result in higher binging energy
of Pt/NCNTs (H2) than other samples [64]. Nevertheless, the influ-
ence of size effect on the electronic structure of metal cannot be
ignored since smaller metal nanoparticles would present higher
binging energy [64,65]. Hence, Ni/N-MWCNTs-800 may possess
smaller Ni nanoparticles in comparison with other samples.
3.1.7. XPS
Fig. 8 shows the C 1s and O 1s XPS patterns of MWCNTs, COOH-
MWCNTs and OH-MWCNTs. The C1s peak can be deconvoluted
into six Gaussian peaks: sp2-hybridized C@C (284.5 eV), sp3-
hybridized CAC (285.3 eV), CAOH (286.3 eV), C@O (287.5 eV),
COOH (288.9 eV) and
p–
p* transition loss peak (291.1 eV)
[26,54,55], meanwhile, the three Gaussian fitted peaks at
531.4 eV, 532.6 eV and 534.4 eV in O 1s spectrum are assigned to
C@O, CAOH and COOH respectively [26,55]. It is obvious that the
C 1s pattern of COOH-MWCNTs (Fig. 8(b)) presents stronger peak
intensity of C@O, CAOH and COOH than MWCNTs, besides, the
peak intensity of C@O and COOH in O 1s spectrum (Fig. 8(b1)) also
become strong, indicating that abundant of oxygen functional
group such as C@O, CAOH and COOH are introduced, especially
C@O and COOH. Additionally, for C 1s spectrum of OH-MWCNTs
in Fig. 8(c), it shows that CAOH and C@O increase in intensity
while COOH bond has little change, moreover, O 1s spectrum of
OH-MWCNTs presents the highest peak intensity of CAOH in
Fig. 8(c1), suggesting that large number of OH have been decorated
on OH-MWCNTs. Therefore, obvious variations in the peak inten-
sity evidently suggest that more OH exist on OH-MWCNTs, while
more COOH are attached on COOH-MWCNTs, these results are con-
sistent with FT-IR characterization in Fig. 7.
Furthermore, N 1s XPS patterns of NH2-MWCNTs and N func-
tionalized MWCNTs are presented in Fig. 9. As shown in Fig. 9(a),
there is only one peak at 399.6 eV, which is ascribed to N species
in NH2 groups, suggesting that NH2 groups are successfully intro-
duced [56], this is in agreement with the FT-IR characterization
in Fig. 7(a). Besides, the N 1s XPS patterns of N functionalized
MWCNTs in Fig. 9(b)–(f) can be fitted into four peaks at 398.4 eV,
400.0 eV, 401.4 eV and 402.9 eV, which are ascribed to pyridinic
N, pyrrolic N, graphitic N and oxide N respectively [27,28,57,58].
And the content and state of nitrogen in N-doped MWCNTs are
summarized in Table 2. As shown in Fig. 9, with the increment of
pyrolysis temperature, the peak intensity of different nitrogen var-
ies significantly, indicating that temperature is the main factor that
affects the active sites of nitrogen. Moreover, it can be seen from
Table 2 that the N content decreases rapidly from 16.68% (N-
MWCNTs-600) to 3.21% (N-MWCNTs-1000), since C-N is easy to
break at high temperature [59]. Graphitic N increases continuously
with the pyrolysis temperature, while pyridinic N increases first
and then declines, the reason is due to that low thermal stability
of pyridinic N and pyrrolic N is quite easy to convert into graphitic
N at high temperature [59]. Notably, N-MWCNTs-800 possesses
the highest content ratio of pyridinic N. Many researchers found
that pyridinic N can be acted as the main active site for stabiliza-
tion of metal or oxide particles on the support surface and thus
affect the nanoparticles size, electronic valence state and catalytic
performance [27,28,57,58]. And more importantly, pyridinic N is
generally considered as the main metal-binding site for
transition-metal species in N-doped carbon catalyst [58,60]. Espe-
cially, Liu et al. constructed the coordination model of isolated
nickel ion with the adjoining non-coordinated pyridinic N atoms,
in which (NiAN4)ꢁ ꢁ ꢁN structure possesses high activity and ultra-
stability in NiANAC catalyst and serve as main active site for var-
ious unsaturated substrates hydrogenation [58].
3.1.8. TEM
The HAADF-STEM images of functional group modified
MWCNTs and their corresponding elemental mappings are pre-
sented in Fig. 11. As shown in Fig. 11, COOH-MWCNTs (Fig. 11
(b)) and OH-MWCNTs (Fig. 11(c)) possess high strength of oxygen
element, while the distribution of oxygen elements for MWCNTs
(Fig. 11(a)) is hardly detectable, intuitively proves that oxygen
functional groups such as COOH and OH have been introduced.
In addition, the HAADF-STEM images of NH2-MWCNTs in Fig. 11
(d) present the distribution of C, O and N, here N is undeniable
originated from NH2, further suggests that the NH2 group exists
in the surface of NH2-MWCNTs. The results are consistent with
the FT-IR in Fig. 7 and XPS in Fig. 8. And Fig. 12 illustrates the
selected TEM images of representative catalysts and their corre-
sponding histograms of nanoparticles distribution. As shown in
Fig. 12(a), there is exist heterogeneous and heavy aggregation for
Ni nanoparticles owing to sintering during the reduction process,
therefore we can see large span Ni nanoparticles distribution of
6 nm to 22 nm in Fig. 12(a2), and mainly centered around 10 nm.
Conversely, the TEM images of functional group modified MWCNT
supported catalysts (Fig. 12(b)–(d)) show good dispersion and nar-
row size distribution of Ni nanoparticles, especially for Ni/OH-
MWCNTs, the reason may be that these functional group can serve
as anchoring sites for Ni species by electrostatic attractions and
chemical interactions accompanied with providing the nucleation
site, thereby promoting the Ni nanoparticles distribution [26,29–
31,33,44,58]. Moreover, Ni/N-MWCNTs-800 (Fig. 12(e) and (e2))
shows the best dispersion of Ni nanoparticles with the narrowest
size distribution mainly centered around 2–3 nm, the reason may
be that large amount of defects formed by doping N species and
the strong electronic interactions between nitrogen species and
Ni can effectively stabilize the nanoparticles, and thus promote
to form ultra-small particles and enhance the dispersion
Fig. 10 presents Ni 2p3/2 XPS patterns of the samples. Ni 2p3/2
spectra can be fitted into three components, the peak around
852.8 eV, 854.5 eV and 856.4 eV are ascribed to Ni0+, Ni2+ and
Ni3+ respectively [61]. In addition, there is a broad peak at high
binding energy of 861.7 eV, which is assigned to the shake-up
satellite accompanied by high valence nickel [62]. As shown in