Surface Functionalization of a γ-Graphyne-like Carbon Material via Click Chemistry

Article abstract of DOI:10.1002/asia.202100125

Surface functionalization of carbon materials is of interest in many research fields, such as electrocatalysis, interfacial engineering, and supercapacitors. As an emerging carbon material, γ-graphyne has attracted broad attention. Herein, we report that the surface functionalization of a γ-graphyne-like carbon material (γ-G1) is achieved by immobilizing functional groups via the click chemistry. Texture analysis of aberration-corrected microscopy, X-ray photoelectron spectroscopy, and electrochemistry confirm the successful surface modification of γ-G1 through a strong covalent linkage 1,2,3-triazole. The direct linkage of functional groups on γ-G1 via the click chemistry represents a general method for preparing other functional materials by using γ-graphyne-like materials as a skeleton.

Full text of DOI:10.1002/asia.202100125

DOI: 10.1002/asia.202100125
Communication
Surface Functionalization of a γ-Graphyne-like Carbon Material via
Click Chemistry
Huatian Xiong+,[a] Haiyuan Zou+,[b] Hong Liu,[a] Mei Wang,[a] and Lele Duan*[a, b]
or hydrophobic forces, which can dramatically change the
Abstract: Surface functionalization of carbon materials is of
physicochemical characteristics of the host materials, making
interest in many research fields, such as electrocatalysis,
them promising candidates in various fields.[15–17] In comparison,
interfacial engineering, and supercapacitors. As an emerg-
ing carbon material, γ-graphyne has attracted broad
attention. Herein, we report that the surface functionaliza-
tion of a γ-graphyne-like carbon material (γ-G1) is achieved
the covalent bond can certainly provide a stronger interlinkage
between modifiers and host materials, reinforcing the stability
of the bounded molecules and further broadening their
applications, particularly under harsh operating conditions.[18]
by immobilizing functional groups via the click chemistry.
Nevertheless, surface functionalization of the aforementioned
Texture analysis of aberration-corrected microscopy, X-ray
carbonaceous materials via the covalent interaction is syntheti-
photoelectron spectroscopy, and electrochemistry confirm
cally challenging due to the inert nature of benzene rings and
the successful surface modification of γ-G1 through a
strong covalent linkage 1,2,3-triazole. The direct linkage of
functional groups on γ-G1 via the click chemistry repre-
lack of reactive ligands on the surface; introducing additional
specific functional groups, such as À NH2, À CN and À COOH, is
then required, as shown in Scheme 1a and 1b.[19–21] Therefore,
sents a general method for preparing other functional
seeking a new carbon material with intrinsic reactive groups to
materials by using γ-graphyne-like materials as a skeleton.
establish a strong covalent interaction with surface modifiers is
desired but remains largely elusive.
Graphyne, as a newly emerging two-dimensional carbon
Surface functionalization is one of the powerful methods to
prepare host materials with desired merits and is currently
adapted in various processes.[1–5] Especially, the immobilization
of homogenous molecules/complexes on heterogeneous sub-
strates is expected to combine the advantages of both
homogenous and heterogeneous systems, providing advanced
hybrid materials.[6–8] The surface-immobilized molecules/com-
plexes usually feature well-defined structures at the atomic
level, and their structures and functions are readily tunable,
imposing competent characteristics on host materials.
From the viewpoint of molecular interaction forces, surface
immobilization strategies can be briefly summarized as follows:
(i) covalent attachment, (ii) electrostatic interactions, (iii) hydro-
gen bonding, (iv) van der Waals forces involved in the
physisorption of molecules on solid surfaces, and (v) physical
entrapment inside pores.[9–14] Prevalently, most decorated
molecules stabilized on the substrate surface undergo weak
interactions like electrostatic adsorption or van der Waals
attraction. For example, on the typical carbonaceous material of
graphite, graphene, and carbon nanotubes, the immobilization
of molecular modifiers is primarily achieved by π-π interaction
allotrope and constituted of sp2-hybridized benzene rings and
sp-hybridized alkyne linkages, has gained increasing attention
due to their fascinating structures and properties.[22–31] Accord-
ing to the difference in construction, the graphyne family can
be classified into a series of subtypes, including α-, β-, γ-
graphyne, graphdiyne, and 6,6,12-graphyne.[32–35] Within these
frameworks, the acetylenic linkages enable the possible chem-
ical reaction with introduced molecules through mature
reactions, such as the Diels-Alder reactions and the click
reactions.[36,37] Up to now, the proof-of-concept study on
covalent attachment of homogeneous molecules/complexes on
graphyne has yet to be tapped, and we report herein the
synthesis of defective γ-graphyne-like carbonaceous (γ-G1) with
[a] H. Xiong,+ H. Liu, Prof. Dr. M. Wang, Prof. Dr. L. Duan
State Key Laboratory of Fine Chemicals
Dalian University of Technology
Dalian, 116024, P. R. China
[b] H. Zou,+ Prof. Dr. L. Duan
Department of Chemistry and Shenzhen Grubbs Institute
Southern University of Science and Technology
Shenzhen, 518055, P. R. China
E-mail: duanll@sustech.edu.cn
[+] These authors contributed equally.
Scheme 1. Some examples of surface modification by covalent bonding. (a)
Reprinted from Ref. 20; (b) reprinted from Ref. 21; (c) this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202100125
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Communication
terminal alkyne groups and the first successful covalent surface
functionalization via click reactions.
azide, were immobilized on γ-G1 through the 1,2,3-triazole
linkage formed via the click reaction, yielding two functional-
ized materials γ-G1-Fc and γ-G1-TMS (Scheme 1c), respectively.
The morphology of γ-G1 was investigated by transmission
The α-carbon in fluorobenzene can react with nucleophilic
reagents, due to the strong electronegativity of F.[38] Therefore,
the γ-graphyne-like carbonaceous γ-G1 was prepared through
the solvothermal treatment of the deprotonated 1,3,5-triethyn-
yl-2,4,6-trifulorobenzene (Scheme S1, Figures S1–S6) via the
aromatic nucleophilic substitution of ArÀ F by the ArÀ C�CÀ
anion (Scheme 2). The XPS analysis of the resulting γ-G1
(Table S1) showed a very low F:C ratio of 1:83, in line with the
leaving of F from the aromatic ring; in addition, the FÀ anion
generated from the reaction was detected by the ion
chromatography (Figure S7), confirming the nucleophilic sub-
stitution pathway. The as-prepared γ-G1 material was charac-
terized by Raman and XPS (see details below) and these results
are in agreement with the proposed γ-graphyne-like
structure.[32] The presence of terminal, unreacted alkyne groups
in the as-prepared γ-G1 allowed us to carry out the surface
modification reaction with azide compounds via the well-
known click chemistry. As a proof-of-concept study, two azide
compounds, azidoferrocene (Scheme S2) and trimethylsilyl
electron microscopy (TEM; Figure 1a), revealing
a porous
structure. The energy dispersive spectrometry (EDS) images of
γ-G1 show certain amounts of element F (Figure S8), suggesting
that the aromatic nucleophilic substitution reaction is not
complete and γ-G1 should contain abundant terminal alkyne
groups. In addition, the nitrogen adsorption-desorption studies
were carried out to evaluate the porosity of γ-G1 (Figure S9).
The material of γ-G1 exhibits the type IV isotherm with the
pore size distribution in the range of 0.6 nm, 1.3 nm, and
34.0 nm, indicating the microporous and mesoporous structure
of γ-G1. The Brunauer-Emmett-Teller (BET) surface area is
788.05 m2 gÀ 1. Besides, high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) of γ-G1-Fc
clearly shows that the element Fe was distributed uniformly in
γ-G1-Fc (Figure 1b), which is consistent with the EDS images
(Figure 1c–f).
Raman spectra of γ-G1 and γ-G1-Fc smoothed by FFT filter
are presented in Figure 1g (unsmoothed spectra shown in
Figure S10). The D band at around 1381 cmÀ 1 corresponds to
the structure defects, including the amorphous carbon atoms
and edges.[36] The strong
G
band at around 1584 cmÀ 1
represents the stretching vibration of sp2 hybridized carbon
atoms in benzene rings, suggesting that the samples possess
abundant benzene rings. Two weak bands at around 2036 cmÀ 1
and 2157 cmÀ 1 are ascribed to the stretching vibration of sp
hybridized carbon atoms in the acetylene group. The Raman
spectrum of the as-prepared γ-G1 is consistent with that of γ-
graphyne reported by the Cui group, revealing the γ-graphyne-
like nature of γ-G1.[32,35,39] No obvious difference was noticed
between Raman spectra of γ-G1 and γ-G1-Fc, suggesting that
their main structures of these two materials are the same.
Fourier transform infrared spectroscopy (FT-IR) demonstrated
strong evidence for the successful immobilization of Fc groups.
As shown in Figure 1h, the bands at 825 cmÀ 1 and 1031 cmÀ 1
are due to the vibration of CÀ H bonds of the Cp rings of
ferrocene. These two vibrational bands were observed for γ-G1-
Fc but not for γ-G1.
Scheme 2. The synthetic scheme of γ-G1.
Besides, X-ray photoelectron spectroscopy (XPS) technology
was performed to provide more information about the
elementary composition and bonding structure of γ-G1 and γ-
G1-Fc. The atomic ratio of elements C and F in the XPS
spectrum of γ-G1 (83:1) is much larger than that of 1,3,5-
triethynyl-2,4,6-trifulorobenzene in theory (4:1). The XPS sur-
veys in Figure 2a showed that the γ-G1 and γ-G1-Fc are mainly
composed of carbon and oxygen, while a small amount of
nitrogen and iron elements (2.67% and 0.94% atoms, respec-
tively) were detected in γ-G1-Fc. The XRD data also confirm
that γ-G1 is a carbon material (Figure S11). The C 1s XPS
spectrum of γ-G1 in Figure 2b could be fitted into four
subpeaks located at 284.5, 285.2, 286.6, and 288.5 eV, which can
be assigned to the C 1s orbitals of CÀ C(sp2), CÀ C(sp), CÀ F and
C=O, respectively. The presence of element O in C=O drives
from the adsorbed oxygen on the surface or the oxidized group
Figure 1. (a) TEM image of γ-G1; (b) HAADF-STEM and (c-f) EDS images of γ-
G1-Fc; (g) Raman spectrum of γ-G1 and γ-G1-Fc; (h) FT-IR spectrum of γ-G1,
Fc-pt, and γ-G1-Fc.
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G1-Fc was studied. Figure 3 shows the cyclic voltammetry (CV)
of γ-G1-Fc and γ-G1 in 1.0 M HClO4. The CV of γ-G1-Fc shows
well-behaved oxidation and reduction waves at 0.596 V and
0.460 V vs. AgjAgClj3 M KCl. These redox peaks are closed to
the redox peaks of Fc-pt (0.555 V and 0.508 V vs. AgjAgClj3 M
KCl) under the same conditions (Figure S12), confirming the
formation of triazole and the electrochemical activity of γ-G1-
Fc.
Another azide, trimethylsilyl azide that is commercially
available was used to test the generality of the click reaction on
γ-G1, and the XPS was performed to study the corresponding
product (γ-G1-TMS). Due to the low load of trimethylsilyl azide
(about 0.37% of Si atoms), the Si 2p signal in the XPS survey of
γ-G1-TMS is weak but a more obvious N 1s signal can be
detected (Figure S13a). The
C 1s XPS spectrum can be
deconvoluted into six subpeaks including CÀ N (286.4 eV), CÀ Si
(285 eV), and the subpeaks from γ-G1 (Figure S13b). Besides,
the weak peak in the Si 2p XPS spectrum was attributed to SiÀ C
(102.3 eV) (Figure S13c).[46] The formation of triazole was con-
firmed by the N 1s XPS spectrum that was fitted into two
subpeaks (399.71 eV and 401.01 eV), and the area ratio of those
two subpeaks is around 2 (Figure S13d). Apparently, the click
reaction has a broad substrate scope on γ-G1. Immobilization
of transition metal complexes-based molecular catalysts (such
as carbon dioxide reduction catalysts and water splitting
catalysts) on γ-G1 (Figure 4) is ongoing.
In conclusion, a γ-graphyne-like carbon material, γ-G1, was
synthesized by the aromatic nucleophilic substitution reaction
and surface functionalization of γ-G1 via click reaction was
successfully demonstrated. Various techniques, such as HAADF-
STEM, XPS, IR, Raman, and CV, were employed to confirm the
attachment of functional groups and the formation of the
triazole-based covalent linkage. This study has paved the way
to functionalize graphyne-based carbon materials using azide
compounds via the click reaction, and the resulting hybrid
materials shall have potential applications in broad fields.
Figure 2. (a) XPS surveys spectra and (b) C 1s XPS spectra of γ-G1, Fc-pt,
and γ-G1-Fc; (c) Fe 2p and (d) N 1s XPS spectra of Fc-pt, and γ-G1-Fc.
of some exposed terminal alkyne groups, as previously reported
in carbon materials.[40,41] Notably, the area ratio of the sp2 and sp
hybridized carbon atoms in γ-G1 is 0.99, suggesting that γ-G1
has almost the same carbon composition with γ-graphyne. For
γ-G1-Fc, the subpeaks deconvoluted from γ-G1 together with a
subpeak at 286.4 eV due to the CÀ N were considered in the
fitting. To our delight, the area ratio of the sp2 and sp hybridized
carbon atoms in γ-G1-Fc is slightly higher than that in γ-G1
(1.01 versus 0.99). This is due to that (i) the sp2 hybridized
carbon atoms of the Cp rings in the ferrocene are introduced
and (ii) some sp hybridized carbon atoms in terminal alkyne
groups were transformed into the sp2 hybridized carbon atoms
in triazole.
The oxidation state of Fe in γ-G1-Fc was determined by
comparing the Fe 2p XPS spectra between Fc-pt and γ-G1-Fc
(Figure 2c). The two spin-orbit components Fe 2p3/2 and Fe 2p1/2
of Fc-pt and γ-G1-Fc are almost the same, all at around
707.8 eV and 720.5 eV, respectively. Thereby, the oxidation state
of Fe in γ-G1-Fc is +2.[42,43]
Most importantly, the N 1s XPS spectra of γ-G1-Fc and Fc-pt
further prove the hypothesis that the attachment of the
azidoferrocene on γ-G1 is via the click reaction (Figure 2d). Two
subpeaks (399.96 eV and 401.26 eV) of γ-G1-Fc can be
deconvoluted from the N 1s XPS spectrum. Those two subpeaks
are associated with the photoelectron emission from nitrogen
atoms of the immobilized triazole moiety,[44,45] and can be
ascribed to the sp2 and sp3 N atoms, which are almost identical
to the subpeaks of the N 1s spectrum of Fc-pt (399.82 eV and
401.12 eV). The ratio of the subpeaks area is close to 2, which is
consistent with the structure of triazole in theory. If the
azidoferrocene was physically adsorbed on the γ-G1, the N 1s
signal should be found at ~403 eV, which could not be
detected or fitted in the N 1s peak of γ-G1-Fc.[44]
To further confirm that the attachment of the azidoferro-
cene via click reaction is successful, the electrochemistry of γ-
Figure 3. CVs of γ-G1 and γ-G1-Fc in 1.0 M HClO4 at a scan rate of 0.10 V/s.
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Keywords: Surface chemistry · Graphyne · Carbon material ·
Click chemistry · Covalent bond
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Manuscript received: February 5, 2021
Revised manuscript received: March 11, 2021
Accepted manuscript online: March 17, 2021
Version of record online: March 22, 2021
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