RE S EAR CH | R E P O R T S

ELECTROCHEMISTRY

Active sites of nitrogen-doped carbon
materials for oxygen reduction
reaction clarified using model catalysts
Donghui Guo,1 Riku Shibuya,2 Chisato Akiba,2 Shunsuke Saji,2Takahiro Kondo,1* Junji Nakamura1*
Nitrogen (N)–doped carbon materials exhibit high electrocatalytic activity for the oxygen
reduction reaction (ORR), which is essential for several renewable energy systems.
However, the ORR active site (or sites) is unclear, which retards further developments ofhigh-performance catalysts. Here, we characterized the ORR active site by using newly
designed graphite (highly oriented pyrolitic graphite) model catalysts with well-defined p
conjugation and well-controlled doping of N species. The ORR active site is created bypyridinic N. Carbon dioxide adsorption experiments indicated that pyridinic N also creates
Lewis basic sites. The specific activities per pyridinic N in the HOPG model catalysts arecomparable with those of N-doped graphene powder catalysts. Thus, the ORR active sitesin N-doped carbon materials are carbon atoms with Lewis basicity next to pyridinic N.

T

he oxygen reduction reaction (ORR) is a
key reaction for fuel cells and other renewable energy technologies such as metalair batteries and dye-sensitized solar cells.
Nitrogen-doped carbon materials as nonmetal catalysts exhibit high electrocatalytic activity for the ORR (1, 2) and high durability,
even under acidic conditions (3). Nitrogen-dopedcarbon materials are thus among the most promising candidates as alternatives to high-cost Pt
catalysts for fuel cell applications. To be developed as high-performance catalysts, they should
be engineered to contain a high concentration

1

Faculty of Pure and Applied Sciences, University ofTsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan.
Graduate School of Pure and Applied Sciences, University ofTsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan.

2*Corresponding author. E-mail: takahiro@ims.tsukuba.ac.jp (T.K.);
nakamura@ims.tsukuba.ac.jp (J.N.)

of active sites without inactive components.
Hence, it is imperative to identify the active sitesof nitrogen-doped carbon materials for the ORR,
albeit still under debate. Currently, the debate
focuses on whether the active sites are createdby pyridinic N (pyri-N, N bonded to two carbon
atoms) or graphitic N (grap-N, N bonded tothree carbon atoms, also called substituted N orquaternary N) (4–13).
The controversy can be ascribed to two reasons.
One is the mixing of different types of nitrogenspecies in the carbon materials, which is inevitable in the doping processes (e.g., by annealing
under NH3 atmosphere). The other lies in theinhomogeneities associated with the morphology and the graphitization level of the evaluatedsamples, which leads to inhomogeneous sizes ofthe p-conjugated system. Indeed, the size ofnitrogen-doped graphene quantum dots has
been reported to affect the ORR performance(14). Thus, it is difficult to determine which typeof nitrogen creates the active site for the ORR by
comparing samples subjected to treatment or
pyrolysis at different temperatures because thesize of the p-conjugated system is also dependent
on the annealing temperature.
To determine the active site conclusively,
we develop four types of model catalysts withwell-defined p conjugation based on highly oriented pyrolytic graphite (HOPG): (i) pyridinic
N-dominated HOPG (pyri-HOPG); (ii) graphitic
N-dominated HOPG (grap-HOPG), and for comparison, (iii) edges patterned on the surface without N (edge-HOPG); and (iv) clean-HOPG (seesupplementary methods and fig. S1). The active
sites and adsorption properties of the nitrogendoped carbon surfaces are examined by ORR,
post-ORR x-ray photoelectron spectroscopy (XPS),
and CO2 temperature programmed desorption(TPD) measurements.
Of the four types of prepared HOPG model
catalysts,preparationofthepyridinicN–dominated
HOPG model catalyst is the most challenging because pyridinic N atoms are preferentially locatedat the edges of graphite. We thus designed anedge-patterned surface by Ar+ etching through amask. Figure 1 (A to D) shows surface morphological characterization of a typical edge-patterned
model catalyst. The ordered uniform rectangulargroove structures were distributed over the surface in a wide range. The atomic force microscopy(AFM) image presented in Fig. 1B shows dark regions corresponding to the grooves etched throughthe slits of the mask by ion beam and bright regions that correspond to nonetched surfaces. The
surface of the bright region is intact and is basically flat. The profile of the blue line in Fig. 1Bshows that the depth of the grooves is about 1200 ±80 nm for this sample (Fig. 1D), which could bevaried from about 100 nm to more than 2 mm bymanipulating the etching energy and duration.
Figure 1E shows XPS N 1s spectra for cleanHOPG, edge-HOPG, grap-HOPG, and pyri-HOPG(C 1s and survey spectra are shown in fig. S2). The
nondoped samples (clean-HOPG and edge-HOPG)

Table 1. Comparison of specific activity per pyridinic N of HOPG model catalysts (in Fig. 2) and N-GNS powder catalysts (in Fig. 4).

SampleNitrogenPyridinic N

concentration(at. %)*

concentration(at. %)*| j| at 0.5 V−2 †(mA cm )Specific activity perpyridinic N
at 0.5 V (e– s−1 pyri-N−1)‡

Model
HOPG-40.60
0.57
0.000410.12............................................................................................................................................................................................................................................................................................................................................
Model
HOPG-54.9
2.2
0.000900.066............................................................................................................................................................................................................................................................................................................................................
Model
HOPG-65.5
3.1
0.00160.082............................................................................................................................................................................................................................................................................................................................................
Model
HOPG-713
3.9
0.0026
0.11............................................................................................................................................................................................................................................................................................................................................
Model HOPG-8
116.5
0.00550.14............................................................................................................................................................................................................................................................................................................................................
N-GNS-1
1.7
0.72
0.085
0.07
............................................................................................................................................................................................................................................................................................................................................
N-GNS-2
2.4
1.9
0.37
0.11
............................................................................................................................................................................................................................................................................................................................................
N-GNS-3
8.1
6.3
1.4
0.13
............................................................................................................................................................................................................................................................................................................................................
*Nitrogen concentration evaluated by XPS.
†ORR current density obtained from current divided by geometric area of electrode surface.
ORR current density (| j|), i.e., number of electrons converted by oxygen reduction per pyridinic N per second (e– s−1 pyri-N), as follows:
Activity per pyridinic N ¼

SCIENCE sciencemag.org‡Activity derived fromnumber of electrons per sec per cm2 of electrode surface
number of pyridinic N per cm2 of electrode surface

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R ES E A RC H | R E PO R TS

are indeed free of N, whereas N 1s peaks areobserved for grap-HOPG and pyri-HOPG. The
N concentrations of grap-HOPG and pyri-HOPG
are 0.73 atomic % (82% for grap-N and 5% for
pyri-N) and 0.60 atomic % (95% for pyri-N and
5% for grap-N), respectively, as estimated by the
analysis of the peak areas for each element (N, C)
by considering the atomic sensitivity of XPS. The
N 1s spectra were analyzed by least-squares fitting
analysis, which includes the components of pyridinic N (398.5 eV), graphitic N (401.1 eV), pyrrolic
N (400.1 eV), and oxidic N (403.2 eV) (15–18).
The catalytic performance was measured by cyclic voltammetry (CV) in acidic electrolyte (0.1 M
H2SO4) (electrode preparation is shown in fig. S3).

Figure 1F shows the ORR curves obtained underoxygen-saturated conditions with the subtractionof data under nitrogen-saturated conditions as
the background, in which the currents are divided
by the geometric surface areas of electrodes (thesame as the exposed catalyst surface areas) as described in the experimental method of the supplementary materials. Figure 1F shows that thepyri-HOPG model catalyst displays high activity
at high voltages, compared to the very low ORR
activities of the N-free model catalysts. The pyriHOPG sample with lower N concentration (N:0.60 at. %) is much more active than the grapHOPG sample with higher N concentration (N:0.73 at. %). Since the pyri-HOPG sample is nearlyFig. 1. Structural and elemental characterization of four types of N-HOPG model catalysts andtheir ORR performance. (A) Optical image of patterned edge-N+-HOPG. (B) The AFM image obtained
for the region indicated by the yellow rectangle in (A). (C) Three-dimensional representation of (B).
(D) Line profile of the AFM image obtained along the blue line in (B). (E) N 1s XPS spectra of model
catalysts. (F) ORR results for model catalysts corresponding to (E). Nitrogen contents of the model
catalysts are shown as the inset in (F).

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free of graphitic N, the ORR results indicate thatpyridinic N rather than graphitic N reduces the
ORR overpotential and creates the active site.
The activity of the grap-HOPG sample could also
be ascribed to the presence of pyridinic N as aminor component (0.04 at. %).
The dependence of the ORR activity of the catalyst on the concentration of pyridinic N was investigated and is presented in Fig. 2. Patterned
HOPG model catalysts with higher nitrogen concentrations (edge-N+-HOPG) were prepared by N+
ion beam etching through a Ni mask, which contain mixtures of different types of nitrogen (fig. S4
for N 1s and fig. S5 for survey and C 1s spectra).
The ORR activities differed because of the different nitrogen-doping conditions (Fig. 2A). The current densities at a specific potential [potential
versus reversible hydrogen electrode (RHE)] foreach ORR were extracted and plotted versus thecorresponding concentrations of pyridinic N andgraphitic N. Figure 2B illustrates the linear relationship between the current densities and concentration of pyridinic N at each investigatedpotential, independent of the preparation method, indicating that the ORR activity is determinedsolely by the pyridinic N concentration. This linear dependence also suggests that the inhomogeneities in the graphitization and size of thep-conjugation system in the samples were overcome successfully in the HOPG model catalysts
by applying the same annealing temperature of
973 K. In contrast, there is no correlation between
the current density and the concentration of graphitic N (fig. S6). As a result, the onset potential (potential versus RHE at current density of 1 mA cm−2)
increases with increasing concentration of pyridinic N (Fig. 2C).
We further investigated the intermediates of
the ORR by ex situ post-ORR XPS measurementsof the HOPG model, which reflects the steadystate surface of the N-HOPG model catalyst under
ORR and provides mechanistic information about
the active sites. Figure 3A illustrates a significant
change in the N 1s peak (fig. S7 for C 1s) after
the ORR half-cell measurement in acidic conditions, in which the concentration of pyridinic N(398.5 eV) decreased from 54 to 38%, and theconcentration of the component corresponding
to the 400.1 eV peak (either pyrrolic N or pyridonic N) increased from 11 to 29%, whereas thesum of the pyridinic N and pyrrolic/pyridonic N
components remained largely constant (from 65
to 67%). By contrast, an N-HOPG sample after
immersion in 0.1 M H2SO4 solution without the
ORR scanning exhibited a negligible change in N
1s spectrum (fig. S8). The difference in the composition of nitrogen species before and after the
ORR suggests that the carbon atoms next topyridinic N react with OH species with consequenttransformation of the pyridinic N to pyridonic N,
as shown in Fig. 3B, suggesting that the active
sites are the carbon atoms next to the pyridinic N
rather than pyridinic N themselves.
Furthermore, we experimentally evaluated therelationship between the basicity and the activityof the HOPG model catalysts. Recently reported
density functional theoretical (DFT) calculations
sciencemag.org SCIENCERE S EAR CH | R E P O R T S

suggested that carbon nanostructures containing
pyridinic N possess Lewis basicity (19). Based on
local scanning tunneling microscopy/spectroscopy(STM-STS) measurements combined with DFT
calculations, it has also been reported that carbon
atoms adjacent to pyridinic N possess a localized
density of states in the occupied region near the
Fermi level (20). This suggests that the carbon
atoms can behave as Lewis bases owing to thepossibility of electron pair donation. We thus examined the adsorption of CO2 as a probe of the
Lewis base site. Figure 3C shows the profiles for
TPD of CO2 from the HOPG model catalysts on
which CO2 was adsorbed at room temperature(fig. S9 shows N 1s XPS spectra). It is found thatacidic CO2 molecule is adsorbed only on the ORRactive pyri-HOPG catalyst, which proves that the
Lewis basic site is created by pyridinic N on the
HOPG surface. It is generally known that oxygen
molecules can be adsorbed on Lewis base sites(21). Because O2 adsorption is the initial step of
the ORR, the Lewis base site created by pyridinic N is thus suggested to be the active site
for ORR.
To compare the HOPG model catalysts withpowder catalysts, we prepared nitrogen-doped
graphene nanosheets (N-GNS) and measured
their ORR activities by the rotating disc method
in 0.1 M H2SO4. Here, the N-GNS catalysts wereprepared by the reaction of GNS with NH3 at

973 K, which is the same temperature applied in
the preparation of the HOPG model catalysts.
Figure 4A shows the N 1s XPS profiles of the
prepared N-GNS powder catalysts. The powder
catalysts have high percentages of pyridinic N,
and the pyridinic N concentration increases
from N-GNS-1 (0.7 at. %) to N-GNS-2 (1.9 at. %) to
N-GNS-3 (6.3 at. %), whereas the graphitic N concentrations are as low as 0.4 to 0.8 at. %. Figure 4B
shows the ORR performances of the N-GNS powder catalysts, in which the currents are divided by
the geometric electrode surface area (0.283 cm2),
with a loading amount of 0.02 mg. The ORR activity increases with increasing nitrogen concentration; e.g., the onset potential increases from
Fig. 2. Catalytic performance of N-HOPG model
catalysts. (A) ORR results obtained for the model
catalysts with different N concentrations. (B) Correlation between current densities of ORR at 0.2,
0.3, and 0.4 V versus RHE and the pyridinic N concentrations. (C) Correlation between onset potentials at 1 mA cm−2 and the pyridinic N concentrations.
Different markers indicate different sample preparation methods. Nitrogen contents of the HOPG
model catalysts are shown as the inset in (A).

Fig. 3. Post-ORR XPS analysis and CO2-TPD of the
N-HOPG model catalysts. (A) N 1s XPS spectra of the
N-HOPG model catalyst before and after ORR, respectively.
(B) Schematic images of the formation of pyridonic N by
the attachment of OH to the carbon atom next to pyridinic
N. (C) CO2-TPD results for the HOPG model catalysts.

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Fig. 4. N 1s XPS spectra and ORR performance of N-GNS
powder catalysts. (A and B) N 1s XPS spectra of N-GNS
powder catalysts with different nitrogen concentrations andthe corresponding ORR results. (C) Correlation between current densities of ORR at 0.5, 0.6, and 0.7 V versus RHE andpyridinic N concentrations.

Fig. 5. Schematic pathway for oxygen reduction reaction on nitrogen-doped carbon materials.

0.77 V for N-GNS-1 to 0.91 V for N-GNS-3. The
ORR activity was further examined in termsof the current densities at different potentials(0.5, 0.6, and 0.7 V versus RHE). The correlation
between the ORR activities and the concentrationof nitrogen species was examined for pyridinic N(Fig. 4C) and graphitic N (fig. S10). Linear relationships between the ORR activities at threedifferent potentials and the concentration of py364

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ridinic N were obtained (Fig. 4C), consistent withthe linear relationships for the HOPG model catalysts (Fig. 2). It is thus confirmed that pyridinic
N creates the active site for ORR in the N-GNS
powder catalysts.
We further compared the ORR specific activities of the N-HOPG model catalysts (in Fig. 2)
and the N-GNS powder catalysts (in Fig. 4). The
activities were evaluated by using the current

densities at 0.5 V, at which the oxygen diffusion
effect is not so pronounced for N-GNS and thecurrent density is not so small for the N-HOPG
model catalysts (comparison at 0.6 V is shown intable S1). As shown in Table 1, the current densities at 0.5 V (absolute value, |j|) for the N-HOPG
model catalysts are approximately three orders ofmagnitude lower than those for the N-GNS
powder catalysts, which can be attributed to thedifference in the absolute number of active sites
per 1 cm2 of the geometric surface area of theelectrodes. The N-HOPG model catalyst is simply a plate with a very low graphite surface areaof about 0.1 cm2, identical to the geometric electrode surface area. By contrast, the BET (BrunauerEmmett-Teller) surface areas of the N-GNS catalysts
on the electrode (0.283 cm2) are about 80 cm2(see supplementary materials). As the pyridinic N
creates the active site for ORR, we calculated thespecific activities per pyridinic N for the modeland powder catalysts and compared them by taking
into account the BET surface area (400 m2 g−1)
for N-GNS (see supplementary methods for detailed calculation). The specific activities per pyridinic N are similar, at ~0.1 e– s−1 pyri-N−1 forboth types of catalysts (0.07 to 0.14 e– s−1 pyri-N−1).
The agreement in the specific activity per pyridinic N indicates that, in general, the active sites
of ORR for various kinds of nitrogen-doped carbon materials are created by pyridinic N.
Finally, we propose a possible mechanism for
the ORR on nitrogen-doped carbon materials(Fig. 5). As the Lewis base site is created by pyridinic N, the oxygen molecule is first adsorbed
at the carbon atom next to the pyridinic N followed by protonation of the adsorbed O2. Twopathways are then possible: One is the four-electron
mechanism taking place at a single site, and theother is the 2 + 2–electron mechanism, which
does not always take place at a single site. In thefour-electron mechanism, the other two protons attach to the two oxygen atoms, leading to
breakage of the O-OH bond and formation of OH
species (“D” in Fig. 5) as observed in post-ORR
sciencemag.org SCIENCERE S EAR CH | R E P O R T S

XPS (Fig. 3). The additional proton then reacts
with the adsorbed OH to form H2O (“E” in Fig.
5). In the 2 + 2–electron pathway, H2O2 is formedby reaction of the adsorbed OOH species withanother proton (“F” in Fig. 5), followed by readsorption of H2O2 and its reduction by two protons to generate H2O. The OH species detectedin the post-ORR XPS measurement may arise
from the four-electron mechanism, but it is also
possible that the OH species next to the pyridinic
N may arise from the reaction with H2O2 in the
2 + 2–electron mechanism. In either pathway,
the carbon atoms next to pyridinic N with Lewis
basicity play an important role as the active sites
at which oxygen molecules are adsorbed as theinitial step of the ORR.
In summary, we have demonstrated that pyridinic N in nitrogen-doped graphitic carbons
creates the active sites for ORR under acidic conditions, based on studies of HOPG model catalysts and N-GNS powder catalysts. Carbon atoms
next to pyridinic N are suggested to be the active
sites with Lewis basicity at which O2 molecules
are adsorbed as the initial step of the ORR.
RE FE RENCES AND N OT ES

1. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 323,
760–764 (2009).
2. L. Dai, Y. Xue, L. Qu, H.-J. Choi, J.-B. Baek, Chem. Rev. 115,4823–4892 (2015).
3. J. Shui, M. Wang, F. Du, L. Dai, Sci. Adv. 1, e1400129 (2015).
4. H.-W. Liang, X. Zhuang, S. Brüller, X. Feng, K. Müllen, Nat.
Commun. 5, 4973 (2014).
5. L. Qu, Y. Liu, J.-B. Baek, L. Dai, ACS Nano 4, 1321–1326(2010).
6. C. V. Rao, C. R. Cabrera, Y. Ishikawa, J. Phys. Chem. Lett. 1,
2622–2627 (2010).
7. T. Xing et al., ACS Nano 8, 6856–6862 (2014).
8. R. Liu, D. Wu, X. Feng, K. Müllen, Angew. Chem. 49, 2565–2569(2010).
9. H. Niwa et al., J. Power Sources 187, 93–97 (2009).
10. H. Kim, K. Lee, S. I. Woo, Y. Jung, Phys. Chem. Chem. Phys. 13,
17505–17510 (2011).
11. N. P. Subramanian et al., J. Power Sources 188, 38–44(2009).
12. L. Lai et al., Energy Environ. Sci. 5, 7936–7942 (2012).
13. W. Ding et al., Angew. Chem. Int. Ed. 52, 11755–11759 (2013).
14. Q. Li, S. Zhang, L. Dai, L. S. Li, J. Am. Chem. Soc. 134,
18932–18935 (2012).
15. S. Maldonado, S. Morin, K. J. Stevenson, Carbon 44, 1429–1437(2006).
16. E. Raymundo-Piñero et al., Carbon 40, 597–608 (2002).
17. J. R. Pels, F. Kapteijn, J. A. Moulijn, Q. Zhu, K. M. Thomas,
Carbon 33, 1641–1653 (1995).
18. I. Kusunoki et al., Surf. Sci. 492, 315–328 (2001).
19. B. Li, X. Sun, D. Su, Phys. Chem. Chem. Phys. 17, 6691–6694(2015).
20. T. Kondo et al., Phys. Rev. B 86, 035436 (2012).
21. H. Metiu, S. Chrétien, Z. Hu, B. Li, X. Sun, J. Phys. Chem. C 116,
10439–10450 (2012).
ACKN OW LEDG MEN TS

This work was financially supported by the New Energy andIndustrial Technology Development Organization and partially
supported by the Japan Science and Technology Agency–
Precursory Research for Embryonic Science and Technology(JST-PRESTO) program, “New Materials Science and Element
Strategy.” We thank T. Kashiwagi and K. Kadowaki for support withsample etching and AFM measurements. The advice of T. Okajima
and T. Ohsaka concerning the electrode preparation methods for
the HOPG model catalysts is gratefully acknowledged. We thank
X. Hao for assistance with sample preparation by photolithography.
We thank all staff of BL07LSU of Spring-8 for assistance withsurface analysis of the HOPG model catalysts. J.N. supervised the
project. D.G. and C.A. prepared the model catalysts and performed
XPS, AFM, and ORR measurements. R.S. performed CO2-TPD
analysis and S.S. prepared the N-GNS powder catalysts and

SCIENCE sciencemag.org

performed ORR measurements. All authors discussed the resultsand D.G., T.K., and J.N. wrote the paper. The authors declare nocompeting financial interests.

Supplementary Text
Figs. S1 to S10
Table S1SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/351/6271/361/suppl/DC1
Materials and Methods

23 July 2015; accepted 8 December 201510.1126/science.aad0832

MOLECULAR FRAMEWORKS

Weaving of organic threads
into a crystalline covalentorganic framework
Yuzhong Liu,1* Yanhang Ma,2* Yingbo Zhao,1* Xixi Sun,1 Felipe Gándara,3Hiroyasu Furukawa,1 Zheng Liu,4 Hanyu Zhu,5 Chenhui Zhu,6 Kazutomo Suenaga,4Peter Oleynikov,2 Ahmad S. Alshammari,7 Xiang Zhang,5,8
Osamu Terasaki,2,9† Omar M. Yaghi1,7†A three-dimensional covalent organic framework (COF-505) constructed from helicalorganic threads, designed to be mutually weaving at regular intervals, has been synthesized
by imine condensation reactions of aldehyde functionalized copper(I)-bisphenanthroline
tetrafluoroborate, Cu(PDB)2(BF4), and benzidine (BZ). The copper centers are topologicallyindependent of the weaving within the COF structure and serve as templates for bringing
the threads into a woven pattern rather than the more commonly observed parallel
arrangement. The copper(I) ions can be reversibly removed and added without loss of the
COF structure, for which a tenfold increase in elasticity accompanies its demetalation.
The threads in COF-505 have many degrees of freedom for enormous deviations to takeplace between them, throughout the material, without undoing the weaving of theoverall structure.

Weaving, the mutual interlacing of longthreads, is one of the oldest and mostenduring methods of making fabric, but
this important design concept has yet tobe emulated in extended chemical structures. Learning how to link molecular building
units by strong bonds through reticular synthesis(1) into weaving forms would be a boon to making
materials with exceptional mechanical propertiesand dynamics. To successfully design weaving of
chains into two- and three-dimensional (2D and

1

Department of Chemistry, University of California, Berkeley,
Materials Sciences Division, Lawrence Berkeley National
Laboratory, and Kavli Energy NanoSciences Institute, Berkeley, CA
94720, USA. 2Department of Materials and Environmental
Chemistry, Stockholm University, SE-10691 Stockholm, Sweden.
3
Department of New Architectures in Materials Chemistry,
Materials Science Institute of Madrid, Consejo Superior deInvestigaciones Científicas, Madrid 28049, Spain. 4Nanomaterials
Research Institute, National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba 305-8565, Japan. 5NSF
Nanoscale Science and Engineering Center (NSEC), University ofCalifornia at Berkeley, 3112 Etcheverry Hall, Berkeley, CA 94720,
USA. 6Advanced Light Source, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA. 7King Abdulaziz City of
Science and Technology, Post Office Box 6086, Riyadh 11442,
Saudi Arabia. 8Material Sciences Division, Lawrence Berkeley
National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA.
9
School of Physical Science and Technology, ShanghaiTech
University, Shanghai 201210, China.
*These authors contributed equally to this work. †Corresponding
author. E-mail: terasaki@mmk.su.se (O.T.); yaghi@berkeley.edu(O.M.Y.)3D) chemical structures (Fig. 1, A and B), longthreads of covalently linked molecules (i.e., 1D
units) must be able to cross at regular intervals.
It would also be desirable if such crossings serveas points of registry, so that the threads can have
many degrees of freedom to move away fromand back to such points without collapsing theoverall structure. Structures have been made byweaving metal-organic chains (2), but designing
well-defined materials and assembling their structures by weaving is challenging, and weaving in
crystalline inorganic or covalent organic extended
structures is undeveloped.
We report on a general strategy and its implementation for the designed synthesis of a woven
material [covalent organic framework-505 (COF505)]. This COF has helical organic threads interlacing to make a weaving crystal structure withthe basic topology of Fig. 1B, and we show that
this material has an unusual behavior in elasticity.
Although terms such as interweaving (3), polycatenated (2), and interpenetrating (4–6) havebeen used to describe interlocking of 2D and 3D
extended objects (Fig. 1, C and D), most commonly
found in MOFs, we reserve the term “weaving” todescribe exclusively the interlacing of 1D units tomake 2D and 3D structures (Fig. 1, A and B).
Weaving differs from the commonly observed
interpenetrating and polycatenated frameworks
because the latter are topologically interlocking
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