Signature of metallic behavior in the metal-organic frameworks M3(hexaiminobenzene)2 (M = Ni, Cu)

Article abstract of DOI:10.1021/jacs.7b07234

The two-dimensionally connected metal- organic frameworks (MOFs) Ni₃(HIB)₂ and Cu₃(HIB)₂ (HIB = hexaiminobenzene) are bulk electrical conductors and exhibit ultraviolet-photoelectron spectroscopy (UPS) signatures expected of metallic solids. Electronic band structure calculations confirm that in both materials the Fermi energy lies in a partially filled delocalized band. Together with additional structural characterization and microscopy data, these results represent the first report of metallic behavior and permanent porosity coexisting within a metal-organic framework.

Full text of DOI:10.1021/jacs.7b07234

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Communication
Signature of Metallic Behavior in the Metal-Organic
3
2
Frameworks M(hexaiminobenzene) (M = Ni, Cu)
Jinhu Dou, Lei Sun, Yicong Ge, Wenbin Li, Christopher H Hendon,
Ju Li, Sheraz Gul, Junko Yano, Eric A. Stach, and Mircea Dinca
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07234 • Publication Date (Web): 14 Sep 2017
Downloaded from http://pubs.acs.org on September 14, 2017
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Signature of Metallic Behavior in the Metal-Organic Frame-
works M3(hexaiminobenzene)2 (M = Ni, Cu)
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Jin-Hu Dou,† Lei Sun,† Yicong Ge,† Wenbin Li, ‡ Christopher H. Hendon,† Ju Li,§ Sheraz Gul,∇ Junko
Yano,∇ Eric A. Stach,# and Mircea Dincă*,†
†Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts
02139, United States
‡Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massa-
chusetts 02139, United States
§Department of Nuclear Science and Engineering, Department of Materials Science and Engineering, Massachusetts Institute
of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States
∇Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, California
94720, United States
#Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
Supporting Information Placeholder
ands and square-planar mononuclear metal nodes define two-di-
mensional (2D) sheets whose electronic structure enables excellent
electron delocalization through continuous conjugation.
ABSTRACT: The two-dimensionally connected metal-organic
frameworks (MOFs) Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaimino-
benzene) are bulk electrical conductors and exhibit ultraviolet-pho-
toelectron spectroscopy (UPS) signatures expected of metallic sol-
ids. Electronic band structure calculations confirm that in both ma-
terials the Fermi energy lies in a partially filled delocalized band.
Together with additional structural characterization and micros-
copy data, these results represent the first report of metallic behav-
ior and permanent porosity coexisting within a metal-organic
framework.
Relevantly, theoretical studies of these 2D materials predicted
that some should behave as bulk metals,13,15–17 yet experimental ev-
idence for metallic behavior in these remains elusive.13 We sur-
mised that replacing 2,3,6,7,10,11-hexaiminotriphenylene (HITP)
by the smaller hexaiminobenzene (HIB) would reduce the distance
and provide better overlap between the electronic wavefunctions of
neighboring
metals/ligands
in
Ni3(hexaiminobenzene)2
(Ni3(HIB)2), leading possibly to metallicity. However, recently re-
ported field effect transistors (FETs) made from films of M3(HIB)2
(M = Ni, Co, Cu) exhibited responses characteristic of insulators or
materials with very low conductance, at best.18 We contended that
this surprisingly insulating behavior was due to the severe disorder
and poor crystallinity of the films, as evidenced also by selected
area electron diffraction (SAED), and that more crystalline samples
would reveal different information about the intrinsic properties of
these materials. Accordingly, on the premise that HIB-based frame-
works continue to be ideal candidates for metallic MOFs, we set
out to devise new synthetic pathways for accessing high quality and
crystalline samples of these materials and explore their electrical
properties.
Recent progress in the synthesis of electrically conductive metal-
organic frameworks (MOFs) has enabled applications that were
previously thought impractical for this traditionally insulating set
of materials.1 Examples include the use of neat MOFs as active
electrodes in electrocatalysis,2,3 batteries,4,5 chemiresistive sen-
sors,6,7 thermoelectric devices,8 supercapacitors,9 electrochromic
devices,10 and field effect transistors.11 The high porosity and sur-
face area of these materials offer new functionalities and opportu-
nities, especially in sensing and supercapacitors, where a large
amount of host-guest interaction sites are critical.6,7,9 Many of these
applications are served better by semiconducting MOFs, others
would see considerable improvements if the MOFs were metallic.
However, there are thus far no examples of metallic MOFs.
Demonstrating metallic behavior remains an important challenge
of fundamental interest. Along this line, the only materials in this
class that exhibit a bulk conductivity greater than 10 S/m at room
Here, we show that crystalline Ni3(HIB)2 and Cu3(HIB)2, char-
acterized by X-ray diffraction, spectroscopic analysis, and high res-
olution transmission electron microscopy (HRTEM), are indeed
excellent conductors, with pellet conductivities averaging approxi-
mately 1000 S/m at 300 K and under vacuum. Significantly, ultra-
violet-photoelectron spectroscopy (UPS) and electronic band struc-
ture calculations evidence intrinsic metallic behavior in both mate-
rials.
temperature
are
Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2
(Ni3(HITP)2),12 Ni3(benzenehexathiol)2 (Ni3(BHT)2),13 and
Cu3(benzenehexathiol)2 (Cu3(BHT)2).14 All three belong to a sub-
class of hexagonal layered structures wherein trigonal organic lig-
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Scheme 1. Synthesis of Ni3(HIB)2 and Cu3(HIB)2.
Ni3(HIB)2 – simulated
Ni3(HIB)2 – experimental
Cu3(HIB)2 – simulated
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Cu3(HIB)2 – experimental
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2theta (degree)
b
c
Black microcrystalline samples of M3(HIB)2 (M = Ni, Cu) were
isolated from reactions of hexaaminobenzene trihydrochloride
(HAB·3HCl) with ammoniacal solutions of Ni(NO3)2·6H2O or
CuSO4·5H2O in mixtures of water and dimethylsulfoxide heated at
60 °C in air for two hours. The presence of O2 is essential for the
isolation of crystalline materials; the absence of air yields only
amorphous grey powders. Scanning electron microscopy (SEM)
showed that the products consist of irregularly shaped nanoparti-
cles smaller than 100 nm (Figure S3). Inspection of the M(2p) and
N(1s) regions of the high resolution X-ray photoelectron spectra
(XPS, Figures S4 and S5) evidenced a chemical environments con-
sistent with square planar metal ions and anilinic N atoms in both
materials. Additional extra-framework M2+ or NH4+ ions, the only
possible charge-balancing cations for a potentially negatively
charged MOF would appear at different regions in the XPS; their
absence confirms the neutral state of M3(HIB)2.
= Ni, Cu
Ni3(HIB)2 Cu3(HIB)2
Figure 1. (a) Powder X-ray diffraction patterns. (b,c) Calculated
structures of M3(HIB)2.
Synchrotron powder X-ray diffraction (PXRD) confirmed a high
degree of crystallinity: both materials exhibit prominent diffraction
peaks at 2θ = 2.54°, 5.18°, 6.81°, 8.10°, 9.07°, 14.27°, and 18.97°
(Figures 1, S8 and S9). Indexing of these peaks revealed hexagonal
unit cells with lattice parameters a = 13.5(2) Å and c = 3.3(1) Å.
To determine the precise interlayer stacking sequence, we per-
formed Ni and Cu K-edge extended X-ray absorption fine structure
(EXAFS) analysis. Ni···Ni and Cu···Cu scattering paths at R =
3.60±0.09 Å and R = 3.33±0.10 Å were required to fit the EXAFS
spectra of Ni3(HIB)2 and Cu3(HIB)2, respectively (Figure S10).
Both M···M distances determined by XAS analysis exceed the in-
terlayer distance observed by PXRD, suggesting that the 2D layers
are not perfectly eclipsed and that significant (ab) shifting occurs
in both materials between neighboring layers. Taken together, the
PXRD and XAS data give structural models where neighboring
layers are shifted by approximately one quarter cell along one edge
of the hexagonal unit cell, thereby lowering the symmetry to ortho-
rhombic. Indeed, Le Bail fitting of the PXRD patterns of the two
materials gave best fits for space groups Cmcm and C2221 for
Ni3(HIB)2 and Cu3(HIB)2, respectively (Figure S11), with other-
wise identical unit cell parameters a = 13.5(2) Å, b = 23.3(5) Å and
c = 6.6(1) Å.
Thermogravimetric analysis (TGA) revealed that both Ni3(HIB)2
and Cu3(HIB)2 desolvate above 100 °C and likely decompose
above 200 °C, as evidenced by the more pronounced weight losses
above this temperature (Figure S6). Evacuation at 120 °C under dy-
namic vacuum for 24 h followed by N2 sorption analysis reavealed
type II isotherms with uptakes of 82 and 65 cm3/g at 77 K and ap-
parent Brunauer–Emmett–Teller (BET) surface areas of 152 m2/g
and 114 m2/g for Ni3(HIB)2 and Cu3(HIB)2, respectively (Figure
S7). These values are in line with those of other 2D materials ex-
hibiting similar pore sizes (vide infra).
To understand the difference in symmetry between the Ni and
Cu materials, we computed optimized geometries for individual 2D
sheets using density functional theory (DFT) (Figure S12). This re-
vealed hexagonal lattice constants of 13.48 Å, in agreement with
the experimental values. It also revealed that the local symmetry of
the Ni2+ ions is strictly D4h, giving rise to completely planar
Ni3(HIB)2 monolayers, whereas CuN4 units buckle out-of-plane
and give rise to overall buckled 2D layers in Cu3(HIB)2 (Figures 1c
and S12). We therefore attribute the lack of mirror symmetry in
Cu3(HIB)2 to the local coordination environment of the Cu2+ ions
(Figures 1c and S12).
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density of states exhibits considerable contributions from the metal,
C, and N orbitals at the Fermi level, confirming the high degree of
in-plane π-conjugation expected for these materials.
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The electronic band structures of bulk orthorhombic M3(HIB)2
plotted along the high symmetry points in the 1st Brillouin zone are
shown in Figure 3 and Figure S14. Salient features include bands
crossing the Fermi level in the in-plane Γ–Y, T–Z, Z–R, and Γ–S
directions in both materials, which indicates the metallic character
of both solids. Notably, because no bands cross the Fermi level in
the out-of-plane directions (Y–T and R–S), the bulk materials are
expected to be metallic only in the ab directions and semiconduct-
ing in the c direction.
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Figure 2. HRTEM image of (a) Ni3(HIB)2 and (b) Cu3(HIB)2 taken
at 300 kV. Insets are high magnification HRTEM images (left) and
the corresponding FFT analysis (right).
The bulk electrical properties of M3(HIB)2 were determined
from pelletized samples of the two materials, obtained by pressing
powders at approximately 1 GPa. Upon degassing under dynamic
vacuum (~1 × 10-5 torr) at 150 °C, the pellets were imaged with
SEM, which revealed small particles with prominent grain bound-
aries and voids (Figure S15). The electrical conductivity of the pel-
lets, measured by the van der Pauw method (Figure S16) under vac-
uum and in the dark20 was 800 S/m for Ni3(HIB)2 and 1300 S/m in
Cu3(HIB)2 at 300 K. These are among the highest values observed
for MOFs, and compare favorably even with the more relevant sub-
set of electrically conducting 2D MOFs.1
HRTEM provided further evidence of two-dimensional long-
range order in M3(HIB)2. Most notably, fast Fourier transform (FFT)
analysis of the images in Figure 2 reavealed honeycomb lattices
with a lattice parameter a = 13.5 Å, in excellent agreement with the
values obtained from PXRD analysis and DFT computations. Intri-
guingly, our microscopy and structural data contrasts with that pre-
viously reported for these materials.18 In particular, it suggests a
higher degree of crystallinity and potentially a different structure
for our materials, which encouraged us to investigate the electronic
properties of M3(HIB)2 samples as synthesized here.
Although a signature of bulk metallic behavior is thermally de-
activated transport,21 temperature-dependence conductivity meas-
urements revealed that electrical transport in both Ni3(HIB)2 and
Cu3(HIB)2 correlates positively with temperature. Indeed, the elec-
trical conductivity in polycrystalline pellets of both MOFs in-
creases linearly as the temperature increases from 200 K to 420 K
(Figure 4a). PXRD confirmed that no structural transitions occur in
this temperature range (Figure S17). We attribute the observed
thermally activated transport, more characteristic of semiconduct-
ing rather than metallic behavior, to interparticle rather than intra-
particle (i.e. intrinsic) transport. In other words, thermally-activated
hopping over grain boundaries (i.e. interparticle transport) domi-
nates the temperature dependence of conductivity in the bulk poly-
crystalline pellets, giving rise to apparent semiconducting behavior
in otherwise intrinsically metallic solids. Indeed, this behavior is
well documented for granular metals where inter-grain charge hop-
ping mediates charge transport (Figure S18).22 Further complicat-
ing the interpretation of the bulk conductivity of M3(HIB)2 is the
bimodal transport expected from DFT band structure calculations:
metallic in the plane of the MOF sheets, but semiconducting normal
to the 2D sheets (vide supra). Because the orientation of particles
in the pellets is random, both in-plane and out-of-plane charge
transport contribute to bulk conductivity, the latter in line with the
observed temperature dependence.
a
b
1800
1600
1400
1200
1000
800
Cu3(HIB)2
Ni3(HIB)2
Cu3(HIB)2
Figure 3. (a,b) Calculated electronic band structure of bulk
Ni3(HIB)2 and Cu3(HIB)2, respectively. (c) The corresponding first
Brillouin zone and high-symmetry k-points.
Ni3(HIB)2
600
400
Importantly, DFT electronic band structure calculations sug-
gested that both monolayer and bulk M3(HIB)2 should be metallic.
Monolayers of both Ni and Cu MOFs exhibit electronic band struc-
tures featuring two wide bands forming a Dirac cone at the K point,
reminiscent of the classic Kagome bands (Figure S13).19 The Dirac
bands cross the Fermi level in both Γ–K and Γ–M directions and
have wide band dispersions of approximately 0.8 eV. The projected
200
250
300
350
400
1.0
0.0
-1.0
Binding energy (eV)
Temperature (K)
Figure 4. (a) Variable-temperature electrical conductivity of
pressed pellets of M3(HIB)2 measured by the van der Pauw method
under vacuum. (b) UPS of M3(HIB)2 acquired at 300 K.
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Experimental confirmation of the metallic nature of both MOFs
characterization and device fabrication was performed at the Har-
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came from UPS, which informs on the intrinsic properties of even
polycrystalline samples by measuring the density of states near the
Fermi level. Grazing incident wide-angle X-ray scattering
(GIWAXS) of Ni3(HIB)2 and Cu3(HIB)2 films prepared on highly-
doped silicon (Figure S19), as required for UPS measurements,
confirmed the identity of the two MOFs and showed that they pref-
erably orient with a typical face-on packing mode where the 2D
sheets are parallel to the silicon substrate (Figure S20).23 The UPS
measurments at 300 K revealed Fermi edges for both Ni3(HIB)2 and
Cu3(HIB)2 (Figure 4b and S21), which are indicative of electronic
bands crossing the Fermi level, and are strong evidence for metallic
behavior in M3(HIB)2.13,24
vard Center for Nanoscale Systems (CNS), a member of the Na-
tional Nanotechnology Infrastructure Network (NNIN), which is
supported by the National Science Foundation under NSF award
no. ECS-0335765 This work used the Extreme Science and Engi-
neering Discovery Environment (XSEDE), which is supported by
the National Science Foundation grant number ACI-1053575.
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In conclusion, we show that reaction of hexaaminobenzene with
Ni2+ or Cu2+ under carefully controlled conditions gives rise to po-
rous crystalline materials with bulk electrical conductivities ex-
ceeding 800 S/m. UPS measurements and DFT computational stud-
ies evidenced rare metallic behavior in MOFs, a class of notori-
ously insulating materials. Most importantly, we demonstrate for
the first time that metallic behavior and porosity are compatible in
these materials. These results encourage further fundamental phys-
ical studies and advanced electronic applications, which continue
to depend critically on the development of techniques to grow and
study single crystals of 2D MOFs, an area of current efforts in our
group.
ASSOCIATED CONTENT
Supporting Information
Additional experimental details and characterization data. This ma-
terial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*mdinca@mit.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the Army Research Office (grant num-
ber W911NF-17-1-0174). W.L. and J.L. acknowledge support by
the Center for Excitonics, an Energy Frontier Research Center
funded by U.S. Department of Energy, Office of Science, Basic En-
ergy Sciences under Award No. DE-SC0001088. We thank Dr.
Xiaolong Li at beamline BL14B1 (Shanghai Synchrotron Radia-
tion Facility) for assistance with the GIWAXS experiments. Aber-
ration-corrected TEM was carried out at the Center for Functional
Nanomaterials, Brookhaven National Laboratory, which is sup-
ported by the U.S. Department of Energy. Use of the Advanced
Photon Source at Argonne National Laboratory was supported by
the U. S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357. Part
of this work (XAS data collection) was carried out at Stanford Syn-
chrotron Radiation Lightsource, SLAC National Accelerator La-
boratory, supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Contract No. DE-
AC02-76SF00515. XAS studies were performed with support of
the Office of Science, OBES, Division of Chemical Sciences, Ge-
osciences, and Biosciences (CSGB) of the DOE under contract no.
DE-AC02-05CH11231 (J.Y.). We thank Dr. Charles Settens for as-
sistance with in situ X-ray diffraction measurements. Part of the
(24) Stassen, I.; Burtch, N.; Talin, A.; Falcaro, P.; Allendorf, M.; Ameloot,
R. Chem. Soc. Rev. 2017, 46, 3185-3241.
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