Communications
the n˜ =1000–3000 cmꢀ1 range and consists of a well-defined
D band (n˜ =1353 cmꢀ1, corresponding to defective and disor-
dered graphene), G band (n˜ =1600 cmꢀ1, corresponding to
pristine graphene), 2D band (n˜ =2697 cmꢀ1, corresponding to
the overtone of the D band produced from a two-phonon
scattering process), and D+G bands (n˜ =2930 cmꢀ1), which is
similar to the characteristics of RGO (Figure 1d; see also Fig-
ure S3); this suggests the existence of well-exfoliated graphene
nanosheets in both samples.[17] However, close comparison of
their features reveals broadened and strengthened D and
2D bands for N-RGO/ND, which provides evidence for more
disorder and more defective graphene nanosheets on the
composite resulting from inhibition of reagglomeration of the
graphene nanosheet by incorporation of nanodiamond, as no
disordered graphene exists on the nanodiamond alone as con-
firmed by the absence of D and 2D feature bands for the
nanodiamond.[17c–e] Moreover, the Raman band corresponding
to the G band in the N-RGO/ND composite is visibly downshift-
ed from n˜ =1605 to 1587 cmꢀ1 with respect to that in RGO,
whereas the position of the D band in both N-RGO/ND and
RGO remains unchanged at n˜ =1353 cmꢀ1. This phenomenon
can be ascribed to nitrogen doping in the carbon matrix of the
composite; this is consistent with the results of nitrogen-
doped graphene and nitrogen-doped carbon nano-
tubes.[17a–c,e,18] Furthermore, the ID/IG ratio, which is the ratio of
the intensities of the D and G bands, provides a sensitive mea-
sure of the disorder and crystallite size of the graphene layers
(Figure S4). The ID/IG ratio of N-RGO/ND (1.12) is much higher
than that of RGO (0.92), which suggests that there is more dis-
order in the composite and that the crystallite size of the
nanosheets is smaller[16a,17b,c] We calculated the in-plane crystal-
lite sizes (La) of N-RGO/ND, RGO, N-RGO/ND-m, and RGO/ND
by using Equation (1):[17]
confirms the presence of nitrogen (8.2 atom%) in the N-RGO/
ND sample. The C1s peak region in the XPS spectrum (Fig-
ure 3b) was deconvoluted into five peaks at binding energies
of approximately 284.6, 285.2, 286.0, 287.1, and 289.0 eV corre-
sponding to C=C (41.2%, mainly from RGO in the composite),
CꢀN (24.1%), CꢀC/CꢀO (20.0%, mainly from ND), C=O/C=N
(13.0%), and OꢀC=O (1.7%), respectively (Table S1, Fig-
ure S6),[2b,c,15a,17b,c,19b,20] which is in good agreement with the
FTIR spectroscopy results. The N1s XPS peak (Figure 3c, see
also Figure S7 and Table S2) illustrates that the N atoms from
HTM are incorporated into the graphene lattice mainly in
a form of “pyridinic N’” (37.9%) and “graphitic N’” (also called
quaternary N, 46.0%), besides 9.0% of pyrrolic N and 7.1% of
oxidized N, which appear at binding energies of approximately
398.1, 400.7, 399.2, and 403.5 eV, respectively.[2b,c,15a,17c,20a,21] The
O1s XPS spectrum can be deconvoluted into three peaks with
binding energies of 530.8, 532.5, and 533.8 eV, assigned to
C=O (ketonic carbonyl group, 12.0%), O=CꢀO (sum of carbox-
ylic acid, anhydride, lactone, and ester groups, 68.4%), and
CꢀOꢀC/CꢀOH (19.6%) containing groups (Figure 3d, see also
Figure S8 and Table S3),[2a–c,19a,22] which is crucial for its fascinat-
ing catalytic performance.
We evaluated the catalytic performance of the samples for
the DDH reaction. From Figure 4, the rate of formation of sty-
ID
IG
La ½nmꢁ ¼ ð2:4 ꢂ 10ꢀ10Þl4ð Þꢀ1
ð1Þ
in which l is the Raman excitation wavelength (532 nm).
The crystallite sizes of the N-RGO/ND, RGO, N-RGO/ND-m,
and RGO/ND samples are estimated to be 17.2, 20.9, 20.2, and
18.1 nm, respectively. The smaller crystallite sizes of RGO/ND,
N-RGO/ND-m, and RGO/ND relative to that of RGO illustrates
the inhibiting effect resulting from the wrapped nanodiamond
on the graphene nanosheets, which is in agreement with the
results obtained from FESEM, N2 adsorption/desorption, and
XRD analysis.
Figure 4. Catalytic performance in the DDH reaction; inset: stable styrene
rate, time on stream: 20 h. Reaction conditions: Catalyst (25 mg), 5508C,
2.8% ethylbenzene in Ar, 10 mLminꢀ1
.
rene rapidly drops from 3.5 to 2.6 mmolgꢀ1 hꢀ1 over pristine
RGO and from 6.1 to 2.7 mmolgꢀ1 hꢀ1 over ND. However, the
developed mesoporous N-RGO/ND nanocomposite shows
a steady-state styrene rate that is 1.9 and 1.8 times that of the
parent RGO and ND, respectively, and that is 7.0 times higher
than that of the industrially used K-Fe catalyst; this is ascribed
to the high surface area, increased number of catalytically
active sites exposed to the substrates, and the intensified
mass-transfer of graphene-based catalysts through simultane-
ously efficient inhibition of restacking the exfoliated graphene
nanosheets and re-agglomerization of the dispersed nanodia-
mond confirmed by the above characterization results. More-
By incorporating the structural characteristics obtained from
FESEM, TEM, XRD, and Raman spectroscopy, the interinhibiting
effect on restacking of exfoliated graphene and the reagglom-
eration of deagglomerated nanodiamond has been confirmed.
Furthermore, by employing FTIR spectroscopy and X-ray pho-
toelectron spectroscopy (XPS), the surface chemical state of
the developed N-RGO/ND composite was measured. The FTIR
spectra of N-RGO/ND, RGO, and ND (Figure S5) indicate the ex-
istence of surface groups including C=O, CꢀO, CꢀH, and CN,
which is essential for catalysis application.[2a–c,15a,17e,19] The
survey scan of the XPS analysis (Figure 3a) unambiguously
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