&83?/;<3=B90(966981981&83?/;<3=B90(966981981
#/</+;-2!8638/#/</+;-2!8638/
><=;+63+88<=3=>=/09;889?+=3?/+=/;3+6<
"+:/;<
><=;+63+88<=3=>=/09;889?+=3?/+=/;3+6<
+8>+;B
37/=+663-7/=+69;1+83-0;+7/@9;5<09;-98=;966/.-+=+6B=3-1;+:23=3C+=39837/=+663-7/=+69;1+83-0;+7/@9;5<09;-98=;966/.-+=+6B=3-1;+:23=3C+=398
908+89:9;9><-+;,98<908+89:9;9><-+;,98<
381%+81
(+</.+&83?/;<3=B
#+2>6#$+6>852/
+=398+68<=3=>=/09;+=/;3+6<$-3/8-/+:+8
>+,38*2+81
+=398+68<=3=>=/09;+=/;3+6<$-3/8-/+:+8
'3-=9;+61;+<
+=398+68<=3=>=/09;+=/;3+6<$-3/8-/+:+8
?7>9@7+36/.>+>
%+8<3;2+7+.
381$+>.&83?/;<3=B
$//8/A=:+1/09;+..3=398+6+>=29;<
9669@=23<+8.+..3=398+6@9;5<+=2==:<;9>9@/.>+>+337:+:/;<
#/-977/8./.3=+=398#/-977/8./.3=+=398
%+81381$+6>852/#+2>6#*2+81>+,38+61;+<'3-=9;2+7+.%+8<3;6<2/2;3$++.
9,+B+<23 +9B+%9738+5+$+=9<23./)><>5/37>819+8.)+7+>-23)><>5/37/=+663-
7/=+69;1+83-0;+7/@9;5<09;-98=;966/.-+=+6B=3-1;+:23=3C+=398908+89:9;9><-+;,98<
><=;+63+88<=3=>=/09;889?+=3?/+=/;3+6<"+:/;<
2==:<;9>9@/.>+>+337:+:/;<
#/</+;-2!8638/3<=2/9:/8+--/<<38<=3=>=398+6;/:9<3=9;B09;=2/&83?/;<3=B90(9669819819;0>;=2/;3809;7+=398
-98=+-==2/&!(3,;+;B;/</+;-2:>,<>9@/.>+>
37/=+663-7/=+69;1+83-0;+7/@9;5<09;-98=;966/.-+=+6B=3-1;+:23=3C+=3989037/=+663-7/=+69;1+83-0;+7/@9;5<09;-98=;966/.-+=+6B=3-1;+:23=3C+=39890
8+89:9;9><-+;,98<8+89:9;9><-+;,98<
,<=;+-=,<=;+-=
$3816/7/=+69;1+83-0;+7/@9;5<!<-98<=;>-=/.0;97=2/-99;.38+=398,/=@//898/096.7/=+6398<
+8.9;1+83-6385/;<<29@6373=/.0>8-=398+63=3/<@2/8></.+<:;/->;<9;<09;8+89:9;9><-+;,98
7+=/;3+6</;/38@/:;9:9</=97/;1/=2/+.?+8=+1/<90C38-+8.-9,+6=7/=+6<398<38=998/<3816/
!-;B<=+63/,37/=+663-!<%2/9;1+83-6385/;<=2+=-99;.38+=/@3=2-9,+6=398<=/8.=9B3/6.
1;+:23=3--+;,98<+0=/;-+;,983C+=398>8635/=29</,;3.1381@3=2C38-398<.>/=9=2/-98=;966/.-+=+6B=3-
1;+:23=3C+=398,B=2/-9,+6=8+89:+;=3-6/<8=23<@9;5@/./798<=;+=/+0/+<3,6/7/=29.=9+-23/?/
8+89:9;9><-+;,987+=/;3+6<@3=2=+369;/.:;9:/;=3/<38-6>.381<:/-3D-<>;0+-/+;/+:9;/<3C/
.3<=;3,>=398./1;//901;+:23=3C+=398+8.-98=/8=902/=/;9+=97<%2/,37/=+663-!./;3?/.
8+89:9;9><-+;,98+;/<B<=/7+=3-+66B-2+;+-=/;3C/.23126312=381=2/37:9;=+8-/90:;/-3</6B-98=;966381
=2/:;9:/;=3/<90=2/-+;,987+=/;3+6<%23<-+8,/.98/,BD8/6B=>8381=2/-97:98/8=<38=2/,37/=+663-
!:;/->;<9;<+8.=2><./<3183819:=37+6-+;,987+=/;3+6<09;<:/-3D-+::63-+=398<
">,63-+=398/=+36<">,63-+=398/=+36<
%+81$+6>852/##*2+81+61;+<'2+7+.%6<2/2;3$9,+B+<23 %9738+5+$
./)37)+7+>-23)37/=+663-7/=+69;1+83-0;+7/@9;5<09;-98=;966/.-+=+6B=3-
1;+:23=3C+=398908+89:9;9><-+;,98<$-3/8=3D-#/:9;=<
>=29;<>=29;<
381%+81#+2>6#$+6>852/>+,38*2+81'3-=9;+61;+<%+8<3;2+7+.$++.6<2/2;3 +9B+
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%23<49>;8+6+;=3-6/3<+?+36+,6/+=#/</+;-2!8638/2==:<;9>9@/.>+>+337:+:/;<
1
Scientific RepoRts | 6:30295 | DOI: 10.1038/srep30295
www.nature.com/scientificreports
Bimetallic Metal-Organic
Frameworks for Controlled
Catalytic Graphitization of
Nanoporous Carbons
Jing Tang
1,2
, Rahul R. Salunkhe
1
, Huabin Zhang
1
, Victor Malgras
1
, Tansir Ahamad
3
,
Saad M. Alshehri
3
, Naoya Kobayashi
4
, Satoshi Tominaka
1
, Yusuke Ide
1
, Jung Ho Kim
5
&
Yusuke Yamauchi
1,2,5
Single metal-organic frameworks (MOFs), constructed from the coordination between one-fold metal
ions and organic linkers, show limited functionalities when used as precursors for nanoporous carbon
materials. Herein, we propose to merge the advantages of zinc and cobalt metals ions into one single
MOF crystal (i.e., bimetallic MOFs). The organic linkers that coordinate with cobalt ions tend to yield
graphitic carbons after carbonization, unlike those bridging with zinc ions, due to the controlled
catalytic graphitization by the cobalt nanoparticles. In this work, we demonstrate a feasible method
to achieve nanoporous carbon materials with tailored properties, including specic surface area,
pore size distribution, degree of graphitization, and content of heteroatoms. The bimetallic-MOF-
derived nanoporous carbon are systematically characterized, highlighting the importance of precisely
controlling the properties of the carbon materials. This can be done by nely tuning the components in
the bimetallic MOF precursors, and thus designing optimal carbon materials for specic applications.
Hybrid nanoporous carbon materials with controllable pore sizes, shapes, and surface properties have attracted
considerable attention for the development of next-generation high performance electronic devices. Up to now,
various types of carbon materials with dierent dimensionality (D), such as carbon-onions (0-D)
1
, carbon nano-
tubes (1-D)
2
, graphene (2-D)
3
, activated carbons (3-D)
4
, and templated carbons (3-D)
5
, have been explored exten-
sively. e advantageous properties, such as suitable pore size distribution
6–8
, large specic surface area
9
, high
electrical conductivity
10
, and doped heteroatoms
11
, are favorable for energy conversion and storage applications.
Practical improvements related to a specic property causes, however, the performance associated to other prop-
erties to decrease. us, the rational design and synthesis of hybrid carbon materials with controlled physical and
chemical properties is still a challenge and is of great interest from the viewpoint of synthetic chemistry.
In recent years, metal-organic frameworks (MOFs) have been scrutinized as convenient precursors for pre-
paring diverse porous-carbon-based materials
12,13
or metal oxides
14,15
, due to their regular nano-architecture con-
structed from various metal ions/clusters and organic ligands. Even though great progress has been made in using
MOFs as precursors, the properties of the resulting porous carbons or metal oxides are limited by using only
simple MOFs. As a subfamily of MOFs, zeolitic imidazolate frameworks (ZIFs), constructed from the coordina-
tion between zinc (Zn
2+
) or cobalt ions (Co
2+
) and imidazolate-type linkers
16
, have proved to be great candidates
for fabricating morphology-inherited porous carbon materials. e zinc-based ZIF (ZIF-8) or cobalt-based ZIF
(ZIF-67) derived carbons exhibit many advantageous properties, along with specic limitations. In detail, nan-
oporous carbons derived from the typical single-metal ZIFs composed of zinc ions (e.g., ZIF-8) usually possess
a microporous structure, large specic surface area, and high degree of nitrogen doping, but also a low degree
1
Mesoscale Materials Chemistry Laboratory, International Center for Materials Nanoarchitectonics (MANA), National
Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
2
Faculty of Science and
Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan.
3
Department of Chemistry, College
of Science, King Saud University, Riyadh 11451, Saudi Arabia.
4
TOC Capacitor, 1525 Okaya, Nagano, 394-0001,
Japan.
5
Australian Institute for Innovative Materials (AIIM), University of Wollongong, North Wollongong, NSW 2500,
Australia. Correspondence and requests for materials should be addressed to J.H.K. (email: jhk@uow.edu.au) or Y.Y.
(email: Yamauchi.Yusuke@nims.go.jp)
Received: 28 April 2016
Accepted: 01 July 2016
Published: 29 July 2016
OPEN
www.nature.com/scientificreports/
2
Scientific RepoRts | 6:30295 | DOI: 10.1038/srep30295
of graphitization
17
. On the other hand, nanoporous carbons derived from the single-metal ZIFs composed of
cobalt ions (e.g., ZIF-67) generally possess a mesoporous structure and a high degree of graphitization, but a low
specic surface area and nitrogen content
18
. ese examples suggest that having only a single type of metal ions
in the ZIFs comes with both advantages and disadvantages. In contrast to zinc, cobalt is able to catalytically pro-
mote the graphitization of amorphous carbon at high temperature, but at the expense of decreasing the surface
area
19
and concentration of doped heteroatoms
20,21
. erefore, it is desirable to combine the advantages of zinc
and cobalt ions in one single crystal (bimetallic ZIFs) in order to achieve porous carbon materials with tailored
functionalities.
According to our previous research, ZIF-8 (Zn(MeIm)
2
, MeIm = 2-methylimidazolate) and ZIF-67
(Co(MeIm)
2
) are highly compatible due to their isoreticular structure and similar lattice parameters
16,22
. As a
result, our group successfully synthesized core-shell ZIFs (ZIF-8 @ ZIF-67) by using ZIF-8 as seeds and further
coating with ZIF-67 via epitaxial growth
23
. e fabrication of hetero-bimetallic ZIFs was also recently achieved
via the co-precipitation of zinc and cobalt ions with MeIm
24,25
. Unlike the single-metal ZIFs, which only contain
zinc or cobalt ions, here the zinc and cobalt ions coexist indiscriminately in the bimetallic ZIFs. As mentioned
above, the zinc and cobalt ions exhibit dierent functionalities during carbonization. e MeIm coordinated
with zinc ions can be converted into nitrogen doped carbons, and the micropores formed between the MeIm
and zinc ions can be mostly retained. In contrast, the MeIm coordinated with cobalt ions tends to yield graphitic
carbon, while sacricing the microporosity and doped nitrogen. Considering this background, in the present
work, we study the synthesis of nanoporous carbons using bimetallic ZIFs as precursor. e properties of the
bimetallic-ZIF-derived carbons, including the specic surface area, porosity, degree of graphitization, and nitro-
gen doping, are precisely controlled by nely tuning the composition of the bimetallic ZIF precursors.
Results and Discussion
A series of bimetallic ZIFs were prepared by reacting Co
2+
and Zn
2+
ions with 2-methylimidazolate (MeIm) in
methanolic solution. e proposed crystal structure of bimetallic ZIFs is shown in Fig.1a, which is formed by
the mixed-coordination of MeIm with Zn
2+
and Co
2+
, respectively, based on the nets of ZnN
4
16
or CoN
4
22
tetra-
hedra. e bimetallic ZIF crystals are denoted as Co
x
·Zn
1−x
(MeIm)
2
, where x/1− x represent the corresponding
initial molar ratio of Co
2+
/Zn
2+
used for the synthesis, as listed in Table1. It should be noted that ZIF-8 crystals,
only consisting of zinc ions, are white. When the zinc ions are replaced by cobalt ions, the color of the obtained
bimetallic ZIFs gradually changes from white to pink, lavender, and ultimately to purple (ZIF-67), as illustrated
Figure 1. (a) Schematic illustration of the crystal structure of the bimetallic ZIFs (Co
x
·Zn
1−x
(MeIm)
2
).
(b) Photograph of ZIF-8, ZIF-67, and the bimetallic ZIF (Co
x
·Zn
1−x
(MeIm)
2
) crystals. e initial molar ratios
of Co
2+
/Zn
2+
for the synthesis of each bimetallic ZIF is shown on top of the bottles in the form of an irreducible
fraction. (c) TEM image, (d) elemental mapping, and (e) SEM image of the Co
0.1
·Zn
0.9
(MeIm)
2
.
Sample ICP determined molar ratio of Co
2+
/Zn
2+
Feeding molar ratio of Co
2+
/Zn
2+
Co
0.05
·Zn
0.95
(MeIm)
2
0.027 0.053
Co
0.1
·Zn
0.9
(MeIm)
2
0.065 0.111
Co
0.33
·Zn
0.67
(MeIm)
2
0.356 0.500
Co
0.67
·Zn
0.33
(MeIm)
2
1.886 2.000
Table 1. Summary of the molar ratios of Co
2+
/Zn
2+
in bimetallic ZIFs.
www.nature.com/scientificreports/
3
Scientific RepoRts | 6:30295 | DOI: 10.1038/srep30295
in Fig.1b. e metal content in each bimetallic ZIF sample was precisely determined by inductively coupled
plasma (ICP) analysis. As summarized in Table1, the actual molar ratio of Co
2+
/Zn
2+
is slightly less than the
feeding molar ratio used for the synthesis, implying that the coordination interaction between zinc and MeIm is
stronger than that between cobalt and MeIm. e successful preparation of bimetallic ZIFs by incorporation of
Zn
2+
and Co
2+
into one crystal are directly demonstrated by transmission electron microscopy (TEM) and ele-
ment mapping. As shown in Fig.1c,d, the zinc and cobalt species coexist and are dispersed uniformly throughout
the bimetallic Co
0.1
·Zn
0.9
(MeIm)
2
crystals. e adjustable molar ratio of Co
2+
/Zn
2+
in the other bimetallic ZIFs
samples is also conrmed by elemental mapping (Figure S1). As discussed above, ZIF-8 and ZIF-67 are compati-
ble thus, the resulting bimetallic ZIFs inherit the topology from both parent structures. According to the powder
X-ray diraction (XRD) patterns (Figure S2), the diraction peaks of the bimetallic ZIFs match well with the
single-metal ZIF-8 and ZIF-67. e absence of shied peaks reects the crystal compatibility between the parent
structures as the lattice seems not to suer from any distortions. e SEM images in Fig.1e and Figure S3b–e
show that the series of bimetallic ZIFs have a rhombic dodecahedral shape identical to single-metal ZIF-8 and
ZIF-67 (Figure S3a,f).
In order to investigate the eect of the Co
2+
/Zn
2+
molar ratio on the degree of graphitization, specic surface
area, and pore size distribution of the bimetallic-ZIF-derived carbon, the series of bimetallic ZIFs were carbon-
ized at an elevated temperature. As shown in the thermogravimetric (TG) curves (Figure S4), the thermal stability
of bimetallic ZIF crystals under a N
2
atmosphere gradually becomes lower along with the increased Co
2+
/Zn
2+
ratios, which corresponds to the decreased thermal stability from single-metal ZIF-8 to ZIF-67. e weight of
ZIF-8, ZIF-67, and bimetallic ZIF (Co
x
·Zn
1−x
(MeIm)
2
) crystals decreases rapidly as the temperature increases,
ultimately yielding to ~50 wt% at 900 °C. During heat treatment under inert atmospheres, most the organic link-
ers thermally converted into the carbon matrix, while some parts also decomposed and evaporated as small
molecules. e porous carbon materials derived from the bimetallic ZIFs (Co
x
·Zn
1−x
(MeIm)
2
) are labelled as C-y
(y = x/1− x). C-ZIF-8 and C-ZIF-67 are also prepared for comparison by respectively using single-metal ZIF-8
and ZIF-67 as the precursors. As shown in the SEM images (Fig.2a–f), all of the carbon materials kept the rhom-
bic dodecahedral morphology inherited from the parent ZIFs. It is worth mentioning that the surfaces of C-ZIF-
8, C-1/19, and C-1/9 are smooth (Fig.2a–c). When the molar ratio of Co
2+
/Zn
2+
increases above 1/2, however,
the derived carbon materials C-1/2, C-2/1, and C-ZIF-67 are found to have a rough surface and shrunken facets
(Fig.2d–f). A detailed characterization by TEM and high-resolution TEM reveals that the smooth samples of
C-ZIF-8 and C-1/9 only consist of amorphous carbon (Fig.2g–j) whereas the rough samples of C-2/1 and C-ZIF-
67 are composed of graphitic carbon sheets (Fig.2k–n). ese results suggest that the carbonized bimetallic ZIFs
can be eectively graphitized in the presence of enough cobalt species, which explains the rough surface and
distorted facets.
During carbonization, the MeIm from the bimetallic ZIF is converted into a carbon state and the coexisting
Zn
2+
and Co
2+
ions are thermally reduced to metallic Zn and Co nanoparticles, respectively. Incorporating cat-
alytic active transition metals into the carbon precursor (e.g. Fe, Ni, Co) has been demonstrated as an eective
approach for catalytic graphitization of amorphous carbon via solid-state transformation process
26,27
. As a result,
the MeIm organic linkers that surround the cobalt ions tend to be catalytically converted into graphitic carbon.
However, the organic linkers that surround zinc ions tend to yield amorphous carbon because a part of the zinc
evaporates during the high temperature treatment and the residual zinc nanoparticles have a weak catalytic gra-
phitization eect
25
. In other words, the degree of graphitization of C-y can be easily controlled by adjusting the
molar ratio of Co
2+
/Zn
2+
in the parent bimetallic ZIFs.
A close observation of the representative sample C-2/1 by high magnication SEM and TEM images (Figure S5)
conrms the presence of thin graphitic carbon nanotubes grown on the surfaces of C-2/1. Although C-1/2 and
C-ZIF-67 also consist of graphitic carbon, the presence of carbon nanotubes could not be observed (Fig.2d,f).
is suggests that there is an optimal ratio of Co
2+
/Zn
2+
in the bimetallic ZIFs to favor the growth of carbon
nanotubes under inert atmosphere only. In this case, the zinc species in the bimetallic ZIFs separates from the
cobalt species and prevents the excessive growth of cobalt nanoparticles during carbothermal reduction of cobalt
ions, resulting in the formation of abundant, dispersed, and catalytically active cobalt nanoparticles. At the same
time, these cobalt nanoparticles are surrounded by a suitable amount of carbon atoms that will be eectively cat-
alytically converted to be carbon nanotubes
28
.
e degree of graphitization of carbon materials can be characterized by XRD and Raman spectra. As shown
in Fig.3a, the C-ZIF-8, C-1/19, and C-1/9 samples only exhibit two broad diraction peaks at 23° and 44°, which
are indexed to the (002) and (101) diraction planes of amorphous carbon
23
. e broad diraction peak at around
23° shi slightly toward higher angles as the cobalt content is increased from C-ZIF-8 to C-1/19, and to C-1/9,
indicating the gradual formation of graphitized carbon. In the case of C-1/2, C-2/1, and C-ZIF-67, which have
higher ratios of cobalt, the (002) diraction peak is observed at 26°, indicating highly graphitic carbon states
28
.
ese results conrm the importance of cobalt ions on the degree of graphitization in the bimetallic-ZIF-derived
carbon. In addition, as revealed by the XRD patterns in Figure S6, the elevated calcination temperature from 800
to 900 °C also is quite critical to promote the graphitization of carbon, thus helps to magnify the distinction of gra-
phitization degree in the series of bimetallic-ZIF-derived carbons in this study. e increased degree of graphiti-
zation from C-ZIF-8, to C-1/19, C-1/9, C-1/2, C-2/1, and C-ZIF-67 was further observed by Raman spectroscopy
(Fig.3b). Each carbon sample displays two vibration bands. e D band located at 1360 cm
−1
corresponds to
the vibrations of disordered carbon or defects, while the G band located at 1590 cm
−1
is related to the vibrations
of sp
2
-bonded graphitic carbon sheets
29
. e intensity ratio between the D and G band (I
D
/I
G
) provides a good
insight on the degree of graphitization for comparative studies. As shown in Table2, the value of I
D
/I
G
for C-ZIF-8,
C-y, and C-ZIF-67 decreases with increasing the cobalt content in the bimetallic ZIF precursors, suggesting an
improved graphitization.