Insights into the surface chemistry and electronic properties of sp2 and sp3-hybridized nanocarbon materials for catalysis

Article abstract of DOI:10.1039/c7cc02354e

Ultra-dispersed nanodiamond and its derivatives (UNDDs), including bucky nanodiamond and onion-like carbon, offer superior catalytic behavior relative to other nanocarbons. However, a systematic study of their unique properties has been rarely achieved. Their surface chemistry and electronic properties are therefore studied to reveal the essential differences of UNDDs compared to other nanocarbons for catalysis.

Full text of DOI:10.1039/c7cc02354e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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2
Insights into surface chemistry and electronic properties of sp
and sp -hybridized nanocarbon materials for catalysis
3
Received 00th January 20xx,
Accepted 00th January 20xx
a,b, c
Yangming Lin,
d
e
a,c
c
Zhenbao Feng, Linhui Yu, Qinging Gu, Shuchang Wu,
a, f
DOI: 10.1039/x0xx00000x
and Dang Sheng Su
*
www.rsc.org/
Ultra-dispersed nanodiamond and its derivatives (UNDD), only benefits from the remarkable surface properties of
including bucky nanodiamond and onion-like carbon offer the graphene-based nanomaterials but combines with the intrinsic
superior catalytic behavior relative to other nanocarbons. characters of a diamond core. As increasing the annealing
However, a systematical study on their unique properties has temperature (T≥1500
°C, Fig. S1), BND will further phase
been rarely achieved. Surface chemistry and electronic properties transfer into OLC that is a fascinating non-planar-related
2
are therefore studied to reveal the essential differences of UNDD material with multiply sp curved closed concentric graphite-
to other nanocarbons for catalysis.
like shells.
As new members in the carbonous family, UNDD carry
interesting physicochemical properties, such as superior
thermal and chemical stability, high surface energy and unique
electronic structure. These properties have made UNDD
competitive candidates for catalytic reactions beside
Nanocarbon catalysis has been considered as a potential
candidate to meet the sustainable and green chemistry over
1
the past few years. A large amount of achievement has
demonstrated that carbon materials, such as nitrogen-doped
activated carbon, reduced porous graphene oxide and
nitrogen-doped carbon nanotube arrays, perform excellent
catalytic activity as compared to some industrial transition-
metal based or noble-based catalysts in liquid phase, gas
5
conventional metal-based catalysts. Moreover, some recent
achievements highlighted that UNDD exhibited the superior
2
catalytic behavior in comparison to other sp -hybridized
nanocarbon materials (e.g.,nanographite, nanotubes, activated
carbon, graphene) for some important catalytic reactions, such
2
phase, as well as in electrochemical reactions. Unlike these
2
3
6
-8
sp -hybridized carbon materials, sp -hybridized ultra-dispersed
nanodiamond (ND) and its derivatives (UNDD) materials,
as ethylbenzene dehydrogenation, phenolic oxidation. It
should be pointed out that the essential differences between
3
2
including sp /sp -hybridized bucky nanodiamond (BND) and
2
UNDD system and conventional sp -hybridized nanocarbons in
2
sp -hybridized onion-like carbon (OLC), have emerged as a new
catalytic reactions have been rarely studied, and the origin of
catalytic behavior remains elusive and controversial. As such, it
is therefore highly desirable to clarify the structure-property
relationship in more detail as an effort to elucidate their
underlying catalytic applications. In this work, we compare the
surface chemistry and electronic structures of representative
3
category of the nanocarbon family. ND has the abundant
surface oxygen composition including carboxylic acids,
3
4
ketones, phenols, lactones and so on. BND consists of a sp
2
carbon core covered by few sp graphite-like shell layers that
can be directly synthesized by the graphitization of ND in an
2
3
inert atmosphere or in vacuum (T< 1500
°C, Fig. S1). It not
sp - and sp -hybridized nanocarbon materials by using XPS,
temperature programmed desorption (TPD) and ultraviolet
photoelectron spectroscopy (UPS), and attempt to give
insights into distinctive physicochemical properties of UNDD
and other nanocarbons. Moreover, two probe reactions
(nitrobenzene reduction and selective oxidation of 2, 3, 6-
trimethylphenol) are used to reveal the direct correlation
between electronic structure and catalytic activity.
By using electron energy loss spectroscopy (EELS), the
surface electronic structure and graphitization transformation
of UNDD were firstly studied. As shown in Fig. 1a, the main
peaks located at about 292 eV in the carbon K-edge of UNND
correspond to the characteristic 1s
→σ* transitions and the
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2
C, which are attributed to the phase transformation of sp /sp
3
2
hybridized BND to sp -hybridized OLC. Interestingly, the G-
-
1
-1
bands shift from the 1593 cm of BND to 1574 cm of OLC, in
analogous to GR and MWCNTs. This result manifests that the
surface carbon structures of OLC should consist of ordered
graphite-like layers. On the contrary, the ND and BND are
composed of the highly defective graphite-like shell layer. This
is evidenced by the results of HRTEM (Fig. S2). The surface of
purified ND is covered by amorphous and disordered carbon
(Fig. S2a). The identified interlayer spacing of 0.340 nm in the
outer shell and the lattice spacing of 0.206 nm in the core can
be assigned to the (002) plane of graphite and (111) plane of
5
c
diamond, respectively.
Depending on the calcination
temperature, the number of graphite-like shells of BND
increases from few layers of 900BND to multiple curved
concentric graphite-like shells layers of OLC (Fig. S2d). The
particle sizes of all the ND, BND and OLC samples are about 5-8
nm. The highly defective graphite-like layer of ND and BND
may be in favor of the in-situ formation of active sites during
reaction. Here, due to the non-sensitivity of ND (insulator) to
Fig. 1 a) Electron energy loss spectroscopy (EELS) spectra of
2
UNDD. b) Relationship between the relative fraction of sp
carbon and the specific surface area, conductivity. c) Raman
spectra of representative carbon materials measured with a
5
32 nm laser, we do not find any obvious peaks in Raman
5
32 nm laser. d) XPS O1s spectra of representative carbon
materials. The O1s spectra are divided by fitting the peak
maximum within 0.1 eV and applying a full width at half-
spectra. As displayed in Fig 1d, the XPS O1s spectra of carbon
materials can be deconvoluted into three peaks corresponding
to unsaturated carbonyl group (C=O, ~531.8 eV), ester-like or
anhydride group (C-O, ~532.9 eV) and phenolic group (C-OH,
±
maximum (FWHM) of 1.4~1.6 eV. The value of the mixed
Gaussian-Lorentzian is maintained at 40%.
~
534.0 eV). By comparing the oxygen species of various carbon
materials, it can be found that ND has the highest amount of
oxygen species (9.8 at%), signifying that ND may provide an
potential active platform for catalytic reactions due to its
abundant surface chemistry information. The oxygen contents
of OLC (0.5 at%) and 1200BND (2.4 at%) are close to typical
MWCNTs (0.7 at%) and GR (2.8 at%), but the oxygen species of
the OLC only involve C-O group.
shoulder peaks at 285 eV which are assigned to 1s
→
π*
transition indicate the presence of surface sp carbon. With
increasing the annealing temperature, a decrease in 1s σ*
intensity and a gradual increase in 1s π* intensity are
observed, which show the effective conversion from sp to sp
2
9
→
→
3
2
2
carbon. Moreover, the fraction of sp carbon readily calculated
by EELS (highly oriented pyrolytic graphite (HOPG) as reference
2
material) is shown in Fig. 1b. The content of sp carbon of ND
The surface groups evolution of ND to OLC during the
annealing process is
programmed desorption (TPD) is used to investigate this
dynamic behavior. As increasing the temperature to 700 C,
some representative oxygen functional groups, such as
carboxyl (-COOH, ~450 C), anhydride (O=C-O-C=O, ~550 C),
ethers (C-O-C, ~650 C) and phenol (C-OH, ~700 C) groups
a dynamic process. Temperature
is about 27%. This indicates that the surface of ND may involve
an amorphous core–shell structure that composes of a mixed
2
3
6b,10
°
sp /sp bonding of carbon atoms.
temperature reaches to 1500
When the synthetic
2
C, the corresponding sp
°
°
°
content of OLC is 96 %, suggesting that the phase
3
transformation of sp to sp carbon is almost completed. In
2
°
°
2
addition, the fraction of sp carbon of representative graphene
will be effectively removed, and leading to the occurrence of
both CO and CO2 gases (Fig. 2a-2b). When the annealing
(
GR) and multi-walled carbon nanotubes (MWCNTs) are
2
temperature is above 800
existing oxygen group in theory. The obvious signals of CO and
CO of ND represent the richer surface chemistry properties as
°C, the surface C=O is the only
around 84% and 80%, respectively. The sp graphitization
degree of OLC is much higher than that of GR and MWCNTs.
With respect to specific surface areas, there is a good linear
relationship presented in ND, BND and OLC. The specific
2
compared with other carbon materials. Actually, gradual
detachment of functional oxygen groups causes the formation
2
surface areas of UNDD increases from the 313 m /g of ND to
2
around 460 m /g of OLC. The similar linear tendency can be
found in electrical conductivity of UNDD. With the increase of
2
the sp carbon content, the electrical conductivity of UNDD
-
1
-1
-1
cm for insulator ND to 0.802 Ω cm
-
increases from 0.011
1
for conductor OLC, but the value is still lower than that of GR
Ω
-1
-1
4.3 Ω cm ) and MWCNTs (3.2 Ω cm ).
-1
-1
(
Raman spectroscopy was used to study the surface
structures of samples. As displayed in Fig. 1c, the D-bands
-1
Fig. 2 TPD profiles of various carbon materials in helium at a
(~1325 cm ) of BND and OLC significantly enhance with
-1
2
heating rate of 10 K·min , a) CO, m/z = 28; b) CO , m/z= 44.
increasing the synthesis temperature from 900 °C to 1500
°
2
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of dangling bonds on carbon atoms which can reconstruct and
combine to form π-bonds, indicating that the onset
temperature of graphitization (namely, phase transformation)
approximates to 800–900 °C. The HRTEM result of 900BND
sample further supports this speculation (Fig. S2b). The
2
reconstructive phase transformation generates sp -hybridized
carbon shells on the outside of the ND, followed by continuous
graphitization to the inside of the particle as increasing the
1
1
temperature. Structural defects of the ND surface, deriving
from the inherent or from the detachment of surface
functional groups, increase the reactivity of surface carbon
atoms which greatly facilitates the phase transformation
Fig. 3 a) Valance band spectra of various carbon materials. b)
Magnified region near the Fermi level of carbon materials.
1
2
energies (3.8 eV and 8.5 eV). Moreover, at the region near the
Fermi level (Fig. 3b), BND and OLC show a steeper increase of
DOS as compared to MWCNTs and GR. These results suggest
that the BND with lower π and σ electronic binding energy and
enhanced DOS may be more beneficial to be activated than
that of MWCNTs and GR under the same conditions.
process. As temperatures exceeds 1300
°C, the initially
highly disordered graphite-like shells become increasingly
more graphitic with a lower defect density. A fully transformed
highly ordered OLC can be obtained when the temperature is
above 1500
°C (Fig. S2d). Compared with ND, GR and
MWCNTs do not exhibit the competitive signals of CO and CO2
that can be ascribe to the lower concentration of oxygen
groups and highly ordered graphite structure.
Compared with ND, BND samples show a rise in intensity at
2
3.0 eV, which is suggestive of increase in graphitic sp carbon
~
content (Fig. 1a-1b). It should be noted that the binding energy
of σ of ND (~8.2 eV) is also lower than that of MWCNTs and GR.
Here, the signal of σ band derives from surface disordered
The thermstability and oxidation resistance of UNDD have
been demonstrated to play important roles in gas-phase
catalysis. In other previous studies, Barnard and colleague
applied a calculated model based on the extrapolated atomic
heat of formation, and proposed that the thermstability of
3
carbon of ND rather than bulk sp carbon that is deactivated
under UPS because of its own insulating property. These
results suggest that UNDD may be activated at a lower energy
cost than that of MWCNTs and GR under identical conditions,
that is, UNDD may has a higher surface energy. This fact well
1
3
BND was better than ND. Gogotsi et al. studied the standard
enthalpies of formation at 25 C over various nanostructured
°
carbons, assuming that the thermostability followed the order
graphite > BND> ND > SWCNTs > OLC. The high stability of BND
may be attributed to oxygen-containing functional groups
16
matches the theoretical prediction.
The work function (ɸ) is the needed minimum energy of
inner electrons that can escape from their nucleus, that is, the
lower work function implies that the electrons of samples have
2
14
bonded to the sp structure. In the present work, we found
that OLC exhibited the better thermostability and oxidation
resistance compared with other carbon materials by using the
1
7
a lower excitation energy barrier. As displayed in Fig. 4,
regardless of ND, BND and OLC catalysts exhibit the lower
thermogravimetric analysis (TG) measurement under different work function values than MWCNTs and GR. In general surface
atmospheres (Fig. S3). Although the content of oxygen species groups and electronic structure of samples are two of the main
1
8
among some carbon materials are similar, the weight losses of factors to affect the work function. It is noteworthy that
compared with GR (2.8 at%) and MWCNTs (0.7 at%), 1200 BND
GR (7.8%) and MWCNTs (4.2%) are higher than that of BND
and OLC with similar oxygen contents (2.4 and 0.5 at%,
respectively) have a lower work function. Moreover, the result
(
1.3%~3.4%) and OLC (1.0%), but being lower than ND (13.8%)
under an argon atmosphere. Additionally, the oxidation onset
temperature of OLC is up to 600 C, suggesting its feasibility in
2
of fitting curve of BND indicates that the content of sp carbon
°
is also not the key factor for affecting the work functions
among GR, MWCNTs and BND. All these facts demonstrate
catalysis under a high-oxidation atmosphere. ND, with the
highest concentration of oxygen species, exhibits a similar
oxidation resistance to 900BND and 1100BND.The observed
behavior may correlate with their specific core-shell surface.
Ultraviolet photoelectron spectroscopy (UPS) source can
excite only valence band electrons and thus it can provide
information about the distribution of electrons in the valence
band, work function and density of states (DOS) near the Fermi
level. As shown in Fig. 3a, valance band spectra of carbon
materials exhibit three kinds of characteristic peaks located at
about 3.0, 7.0 and 8.5 eV, which can be assigned to the C2p-π,
1
5
π-σ and σ electrons, respectively. Using HOPG as a reference,
2
there is no obvious difference on the binding energy of π (~3.0 Fig. 4 Relationship between the fraction of sp carbon and the
eV) and σ (~7.5 eV) in the case of BND and OLC, but π and σ work function over carbon materials. Work functions were
bands of multiwalled carbon nanotube (MWCNTs) and determined from the secondary electron cutoff of UPS He I
spectra using nickel metal as a reference.
graphene (GR) were found to shift to higher binding
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hybridized carbon structures and electronic properties on
catalytic reactions and mechanism.
This work was supported by
“Strategic Priority Research
Program”of the Chinese Academy of Sciences (Grant No.
XDA09030103) and the NSFC of China (11504386).
Notes and references
Fig. 5 The dependence of work functions on the catalytic
activity over different carbon materials for reduction of
nitrobenzene a) and selective oxidation of 2, 3, 6-
trimethylphenol b). The reaction conditions: a) 25 mg catalyst,
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and BND could be explained in terms of destabilization of the
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degree. It is therefore reasonable to expect that specific
electronic structure (π and σ) of UNDD as compared to other
carbon materials are energetically favorable to activate
electrons originating from the valence band of the catalyst
under a low energy condition, and to possess a higher surface
energy and hence may lead to the facile formation of activated
complexes for some potential catalytic reactions.
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structure and catalytic activity, the reduction of nitrobenzene
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9
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2
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6-trimethylphenol) further demonstrate the above
8
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