Metal-doped bipyridine linked covalent organic framework films as a platform for photoelectrocatalysts

Article abstract of DOI:10.1039/d1ta00396h

The development of efficient photoenergy conversion systems is highly demanded from the viewpoint of solving energy and environmental problems. Covalent organic frameworks (COFs) have attracted much attention as novel photofunctional materials because of their wide-range visible-light absorption, which is related to their large π-conjugation systems. The other important property of COFs is their ability to support a wide variety of metals via coordinate bonds; metal-doped COFs exhibit various electron-transfer catalytic activities depending on the metal species. Here, we newly synthesized free-standing metal-doped COF films as photoelectrocatalysts and evaluated the photoelectrochemical oxygen reduction reaction (ORR) properties of Cu atoms doped in a COF. The photocurrent corresponding to the ORR started to increase under 670 nm irradiation, and the photocurrent for the Cu-doped COF was five times larger than that for the COF without Cu. The action spectra, fluorescence spectra, and in situ X-ray absorption spectra indicate that the electrons photoexcited in the COF were transferred to oxygen via the Cu atoms. Thus, the Cu atoms coordinated to the COF serve as active catalytic sites for the photoinduced ORR. This journal is

Full text of DOI:10.1039/d1ta00396h

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Materials Chemistry A
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PAPER
Metal-doped bipyridine linked covalent organic
framework films as a platform for
photoelectrocatalysts†
Cite this: J. Mater. Chem. A, 2021, 9,
11073
bc
Tomoya Hosokawa,a Masaki Tsuji,a Kosei Tsuchida,a Kazuyuki Iwase,
b
abd
abd
*
*
Takashi Harada,
Shuji Nakanishi
and Kazuhide Kamiya
The development of efficient photoenergy conversion systems is highly demanded from the viewpoint of
solving energy and environmental problems. Covalent organic frameworks (COFs) have attracted much
attention as novel photofunctional materials because of their wide-range visible-light absorption, which
is related to their large p-conjugation systems. The other important property of COFs is their ability to
support a wide variety of metals via coordinate bonds; metal-doped COFs exhibit various electron-
transfer catalytic activities depending on the metal species. Here, we newly synthesized free-standing
metal-doped COF films as photoelectrocatalysts and evaluated the photoelectrochemical oxygen
reduction reaction (ORR) properties of Cu atoms doped in a COF. The photocurrent corresponding to
the ORR started to increase under 670 nm irradiation, and the photocurrent for the Cu-doped COF was
five times larger than that for the COF without Cu. The action spectra, fluorescence spectra, and in situ
X-ray absorption spectra indicate that the electrons photoexcited in the COF were transferred to oxygen
via the Cu atoms. Thus, the Cu atoms coordinated to the COF serve as active catalytic sites for the
photoinduced ORR.
Received 15th January 2021
Accepted 5th April 2021
DOI: 10.1039/d1ta00396h
rsc.li/materials-a
found to promote photocatalytic carbon dioxide (CO2) reduc-
tion to carbon monoxide (CO) under visible-light irradiation.15
Introduction
Another attractive property of COFs is their ability to
immobilize single metal atoms via coordinate bonds with
abundant heteroatoms with a lone electron pair, such as N, S,
and O, and to exhibit unique functions derived from metal
centers, similar to homogeneous organometallics.16–26 That is,
the use of COFs is a novel approach to achieve heterogeneous
single-atom catalysts as the analogues of homogeneous organ-
ometallics. For example, Ding et al. have reported that a Pd-
modied imine-linked COF catalyzes coupling reactions with
high robustness and easy recyclability.27 Gunasekar et al.
showed that Ir-modied triazine-linked COFs catalyze highly
efficient carbon dioxide hydrogenation and exhibit high
stability in air.28 In addition to these thermocatalytic properties
of metal-doped COFs (M-COFs), several groups including us
have recently demonstrated that M-COFs exhibit various unique
electrocatalytic functions depending on the metal species.29–35,78
Cu- and Ni-doped COFs, for example, function as electro-
catalysts for the oxygen reduction reaction (ORR) and carbon
dioxide reduction reaction (CRR), respectively.18,33 In our cata-
lysts, the adsorption energy of the substrates or key interme-
diates adsorbed onto the metal atoms in COFs can be precisely
tuned through ne control of the coordination environment,
leading to unique reaction activity and selectivity.
Covalent organic frameworks (COFs), which are crystalline
cross-linked microporous polymers, have received considerable
research interest as a novel polymeric platform because of their
unique physicochemical properties including high robustness,
nanoporous structure, and high design exibility.1–5 In addition
to these advantages, COFs exhibit wide-range visible-light
absorption because of their large p-conjugation systems.6–8
Thus, COFs are expected to be developed as next-generation
photofunctional materials.7–13 For example, highly crystalline
sulfone-containing COFs modied with Pt nanoparticles exhibit
high activity toward photochemical hydrogen evolution (as high
as 16.3 mmol gꢀ1 hꢀ1).14 Metal-free carbazole-triazine-based
COFs, which have a narrow bandgap of 2.04 eV, have been
aGraduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan. E-mail: kamiya@chem.es.osaka-u.ac.jp;
nakanishi@chem.es.osaka-u.ac.jp
bResearch Center for Solar Energy Chemistry, Osaka University, 1-3 Machikane-yama,
Toyonaka, Osaka 560-8531, Japan
cInstitute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku
University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
dInnovative Catalysis Science Division, Institute for Open and Transdisciplinary
Research Initiatives (ICS-OTRI), Osaka University, Suita, Osaka 565-0871, Japan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ta00396h
Considering electron-transfer catalytic ability that depends
on their metal centers and wide-range visible-light absorption,
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metal-modied COFs are expected to be an attractive platform
for materials for photoelectrochemical applications. However,
the general process to fabricate COF electrodes is to paste the
nanopowders on conductive substrates using organic binders,
which are basically insulative and scatter and/or absorb irradi-
ated light.36–39 To tackle this obstacle, we attempted to utilize
free-standing lms of COFs as binder-free photo-
electrocatalysts.40–48 In the present work, we here newly
synthesized free-standing metal-doped bipyridine-linked COF
(bpy-COF) lms and applied them as binder-free photo-
electrocatalysts. The details of the photoinduced charge trans-
fer process were investigated using photoelectrochemical action
spectra, photoluminescence spectra, and in situ spectroscopy by
employing the ORR mediated by Cu sites doped in the bpy-COF
(Cu-bpy-COF).
Fig. 1 (a) Schematic illustration of the synthesis of the Cu-bpy-COF
film. (b) Photographs of the free-standing bpy-COF (left) and the Cu-
bpy-COF (right) films. (c) Experimentally (red) and simulated (blue)
obtained X-ray diffraction patterns of the Cu-bpy-COF film and the
eclipsed structure of Cu-bpy-COF. C: brown, O: red, N: silver, H: pale,
and Cu: blue.
Results and discussion
The targeted reaction system
We rst explain in detail the photoelectrochemical ORR using
Cu-bpy-COF lms and why they were chosen as the represen-
tative reaction system of the M-COF photoelectrocatalysis in the
present work. The photoinduced ORR is an attractive and well-
studied process as the cathodic reaction of light-driven fuel cells
or rechargeable metal–air batteries and as a method for the
photosynthesis of hydrogen peroxide.49–54 Especially, the ORR in
neutral solutions has attracted keen attention for application in
enzymatic and microbial fuel cells.55 Imine- and bpy-linked
COFs were adopted as the backbone structure of organic
frameworks because single metal centers can be immobilized at
the bipyridine moiety through coordinate bonds. Especially,
free-standing lms of bpy-COFs will provide a unique approach
to developing binder-free photoelectrochemical devices.56–59 In
a previous paper, we used rst-principles calculations to
demonstrate that coordinatively unsaturated Cu sites in bpy-
COFs facilitate the electrochemical ORR.33,60 Thus, the single Cu
sites coordinated with the bipyridine moiety are expected to
possess unsaturated coordination environments and efficiently
catalyze the ORR.
micropores with 1.2–2.1 nm (Fig. S1b†), as reported in the
previous papers.56,61,62 The Brunauer–Emmett–Teller (BET)
specic surface area was 1270 m2 gꢀ1, which basically corre-
sponded to the surface area of bpy-COF powder.56 The
morphology was characterized by scanning electron microscopy
(SEM) and atomic force microscopy (AFM). The AFM height
histogram of the bpy-COF (Fig. S2†) indicates that the average
thickness of the bpy-COF was approximately 400 nm. Needle-
like COF crystallites were observed in the high-magnication
SEM image (Fig. S3†), consistent with the results reported in the
previous paper.56
We physically characterized the Cu-bpy-COF using various X-
ray techniques. The X-ray diffraction (XRD) pattern of the Cu-
bpy-COF shows peaks at 2q ¼ 3.5ꢁ and 26.5ꢁ, which are assigned
to the (100) and (001) planes of the Cu-bpy-COF, respectively
(Fig. 1c). These XRD patterns indicate that the bpy-COF has an
eclipsed structure (Fig. 1c), consistent with the previously re-
ported structure for the bpy-COF.56,62 Notably, the characteristic
peaks at 2q ¼ 7.5ꢁ and 14ꢁ assigned to the staggered stacking are
Physical and morphological characterization of COF lms
The bpy-COF lms were synthesized by oil/water interfacial almost negligible.52
synthesis using an aqueous solution of 2,2-bipyridine-5,50-
Table S1† lists the surface elemental compositions calcu-
diamine (upper layer, 0.70 mmol Lꢀ1) and a dichloromethane lated by X-ray photoelectron spectroscopy (XPS). The atomic
solution of 2,4,6-triformylphloroglucinol (Tp) (lower layer, 0.75 ratio of the Cu atoms in the Cu-bpy-COF was 1.8%, and the Cu/
mmol Lꢀ1).56 The polymerization spontaneously proceeded at N ratio was 0.13. In addition, the Cu amount evaluated by the
ꢁ
the oil/water interface at 30 C for 72 h. We then impregnated ICP was 1.1 at%, which is about two-thirds of the value obtained
the resulting lm with 5 mM CuCl2 aqueous solution to dope by XPS. Thus, the Cu concentration in the near-surface was
Cu2+ ions into the bpy-COF lm (Cu-bpy-COF) (Fig. 1a). Fig. 1b a little higher than that in the bulk. We analyzed the Cu 2p XPS
shows images of the bpy-COF and Cu-bpy-COF, respectively. and X-ray absorption near edge structure (XANES) spectra to
The orange transparent bpy-COF lm (2 cm ꢂ 2 cm) turned determine the oxidation states of the Cu atoms. The Cu 2p3/2
reddish aer the impregnation, which suggests that Cu atoms XPS peaks generated by the Cu-bpy-COF at 932.7 eV correspond
were successfully doped into the bpy-COF (for the details in to Cu(II) ions (Fig. S4†).63 The Cu K-edge XANES spectra of Cu
reddish color, see Fig. 3). N2 isotherm analysis of the bpy-COF metal, Cu2O, and CuO are shown in Fig. 2a. The absorption
was performed at 77 K to analyze its pore structure (Fig. S1a†). A edge of the Cu-bpy-COF was 8980 eV, which is similar to that of
typical type-I isotherm was observed, indicating the existence of
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Fig. 2 (a) Cu–K edge XANES spectra and (b) k3-weighted Fourier
transform of EXAFS spectra for the Cu–K edge of the Cu-bpy-COF
(red), Cu2O (blue), CuO (pink), Cu foil (black), and Cu TPP (orange).
CuO. These results indicate that the Cu(II) oxidation state was
dominant in the Cu-bpy-COF. The Fourier transformations of
the k3-weighted EXAFS oscillations for the Cu-bpy-COF, Cu-
5,10,15,20-tetraphenyl-21H,23H-porphine (Cu-TPP), and Cu
metal are shown in Fig. 2b. The Cu–N peak for the Cu-bpy-COF
Fig. 3 (a) UV-vis diffuse reflection spectra (100 ꢀ reflectance/%) of
bpy-COF and Cu-bpy-COF films. (b) Tauc plots of the Cu-bpy-COF
and bpy-COF. (c) Total density of states (DOS) and partial density of
states (PDOS) of the bpy-COF (left) and Cu-bpy-COF (right). Black, red,
blue, and pink lines represent the DOS of all, C, N, and Cu, respectively.
˚
is clearly observed at 1.6 A, respectively. In contrast, peaks
˚
corresponding to Cu–Cu bonds (2.2 A) and Cu–O–Cu bonds (2.7
33
˚
A) were not detected. The EXAFS results indicate that the Cu
(Photo)electrocatalytic activity of the Cu-bpy-COF
atoms in the Cu-bpy-COF are individually isolated and
anchored onto the bpy-COF through coordination with N atoms We evaluated the electrochemical ORR activity of the Cu-bpy-
COF under dark conditions. A transparent conductive oxide lm
(indium-doped tin oxide, ITO) was coated with the Cu-bpy-COF
as a working electrode without a binder (Fig. S7†). The rough-
Band structure of the Cu-bpy-COF
ness factor (Rf ¼ 1.62) for the Cu-bpy-COF lm calculated from
Here, the energy band structures of the COFs are investigated.
the electrochemically accessible surface area (EASA) was similar
to that for the reported powder Co-bpy-COF electrode16 (for
details, see Fig. S8a–c† and its caption). Changes in the current
density (j) at different potentials (U) were examined for the Cu-
bpy-COF, bpy-COF, and a bare ITO electrode in O2-saturated 0.1
M phosphate buffer solution (pH 7.0) (Fig. S8d†). The ORR
onset potential for the Cu-bpy-COF was ꢀ0.15 V vs. Ag/AgCl,
which is approximately 300–500 mV more positive than the
value associated with the bpy-COF or the bare ITO electrode.
The origin of the catalytic site for the ORR on the Cu-bpy-
COF was examined using DFT calculations. The Gibbs free
energy changes (DG) at 0.62 V vs. computational hydrogen
electrode (CHE), which are associated with the formation of
OOH*, OH*, and O*, are shown in Fig. S9.† The overall reaction
pathway of the Cu-bpy-COF becomes exergonic, whereas the
rst electron transfer process (OOH* formation) is endergonic
for the bare bpy-COF. Thus, the Cu site can reduce the activa-
tion barrier for the rst electron transfer to O2 molecules to
form OOH*. Although Cu-macrocycles such as Cu-porphyrins or
–phthalocyanines are known to exhibit poor ORR activity
because of weak OOH* adsorption, the lower N coordination to
Cu atoms in the Cu-bpy-COF than in Cu-macrocycles would lead
to stronger adsorption of OOH and a lower DG, as demonstrated
in the previous papers.60,73
The diffuse reectance spectra of the Cu-bpy-COF and bpy-COF
are shown in Fig. 3a. The absorption intensity of the bare bpy-
COF begins to increase at 700 nm, corresponding to the p–p*
transition of the bpy-COF (for details, see DFT calculation
results). The bandgap calculated from the Tauc plot is 2.25 eV
(Fig. 3b). We calculated the band structure of the Cu-bpy-COF
using density functional theory (DFT) methods. Fig. 3c shows
the density of states of the Cu-bpy-COF and bpy-COF. The
bandgap of the bpy-COF, as determined from the DFT calcula-
tions, is 1.5 eV. Notably, the bandgap calculated using the
generalized gradient approximation (GGA) is known to be
underestimated.64,65 The valence-band maximum (VBM) and
conduction-band minimum (CBM) are composed of C 2p and C
2p* orbitals, respectively. Cu 3d orbitals are located in the
bandgap of the Cu-bpy-COF, indicating that the photoexcited
electrons in the conduction band can be thermodynamically
transferred to Cu sites. On the basis of the DFT results, the new
absorption at 700–800 nm in the spectrum of the Cu-bpy-COF is
assigned to the Cu(II) d–d transition and/or the direct charge
transfer transition from the VB to Cu(II).66–68 We next carried out
electrochemical measurements in an organic electrolyte to
determine the absolute band position of the bpy-COFs.69–72 The
cyclic voltammograms of the bpy-COF lms in acetonitrile
containing 0.1
M tetrabutylammonium tetrauoroborate
Next, we evaluated the photoelectrochemical ORR properties
of the Cu-bpy-COF. The photocurrents were measured under
monochromated light using the electrochemical cell shown in
Fig. S10.† Fig. S11† shows the representative photocurrent vs.
time curve at ꢀ0.2 V and 0.2 V vs. Ag/AgCl under 650 nm light.
(Bu4NBF4) are shown in Fig. S5.† The CBM of the bpy-COF was
ꢀ0.77 V vs. the normal hydrogen electrode (NHE), calculated
from the threshold of the cyclic voltammograms. The band
structures of the bpy-COF determined on the basis of the UV-vis,
cyclic voltammetry (CV), and DFT results are shown in Fig. S6.†
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The cathodic current clearly appeared under visible-light irra- (Fig. S12a†), suggesting that the cathodic current originated
diation. The apparent photocurrent was observed under even from the ORR, not from the degradation of COFs. In addition,
650 nm irradiation, indicating that our M-COF lm can utilize we also measured the Fourier transform infrared (FT-IR) spectra
almost all visible light regions. The photocurrent action spectra, of Cu-bpy-COF lms before and aer the photoelectrochemical
which represent the incident-photon-to-current-efficiency test for 30 min (Fig. S12b†). The peaks assignable to C–N, C]C
(IPCE) (l), were then examined to obtain conclusive evidence of and C]O were observed, which corresponded to the reported
the origin of the photoresponse (Fig. 4a). Interestingly, Fig. 4a data for the bpy-COF.56,62 Notably, the change in the spectrum
shows that the deposition of Cu into the bpy-COF drastically was not observed aer a 30 min photoelectrochemical test. The
increased (by more than vefold) the cathodic photocurrent. stable photocurrent response and this FT-IR spectrum sug-
The IPCE for the Cu-bpy-COF started to increase under 670 nm gested that our bpy-COF is stable under photoelectrochemical
irradiation, which clearly corresponds to the absorption spec- conditions within this time scale. Fig. 4b shows the potential
trum of the Cu-bpy-COF. These results clearly indicate that the dependence of IPCE for the Cu-bpy-COF. The change in the
photocurrent originated from the photoabsorption of the Cu- polarity of photocurrent occurred at 0 V vs. RHE. In the more
bpy-COF. We observed the stable photocurrent for over 30 min positive region, the anodic photocurrent was also enlarged in
the presence of Cu in a similar manner to the cathodic photo-
current (Fig. S13†).
Photoinduced charge transfer pathway in Cu-bpy-COF lms
We subsequently investigated the electron transfer mechanism
of the Cu-bpy-COF in the presence of O2 using photo-
luminescence spectroscopy and in situ XANES. Fig. 5a shows the
photoluminescence spectra of the bpy-COF and Cu-bpy-COF
excited by 380 nm radiation. Fluorescence quenching was
clearly observed aer the addition of Cu into the bpy-COF,
which indicates that the excited electrons quickly transferred to
Cu(II); thus, charge separation efficiently occurred.74 Photoin-
Fig. 4 (a) Action spectrum for the cathodic IPCE of the Cu-bpy-COF
(red dots) and COF (black dots), and the absorption spectrum of the duced charge transfer was also conrmed by gas-phase in situ
Cu-bpy-COF (solid red line) at ꢀ0.2 V vs. RHE in the presence of O2. (b)
Potential dependence of the value of photocurrent of the bpy-COF
(black dots) and Cu-bpy-COF (red dots). These photocurrent
responses were measured under the constant potentials. The photo-
current in (a) and (b) was obtained under the irradiated light shown in
Fig. S10b and c,† respectively.
XANES. The Cu-bpy-COF was placed in an X-ray-transparent
plastic bag lled with Ar gas (Fig. S14†), and the XANES spectra
were acquired under visible-light irradiation (Fig. 5b). When the
sample was irradiated, the Cu–K absorption edge shied to the
lower-energy region (curve (1) to (2) in Fig. 5b), corresponding to
Fig. 5 (a) Photoluminescence spectra (lexc ¼ 380 nm) of the bpy-COF (black) and Cu-bpy-COF (red). (b) Cu K-edge XANES spectra under Ar/
dark (1, black), Ar/light (2, red), and O2/light (3, blue). (c) The proposed photoinduced electron transfer process at the Cu-bpy-COF under visible-
light irradiation.
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a lower oxidation state of the Cu species. Approximately 3.4% of acid monohydrate salt (p-TsOH) (38.7 mg) dissolved in water
the Cu(II) ions were likely reduced to Cu(I) (see the section of the (100 mL) were added slowly on top of the spacer solution over
band structure of the Cu-bpy-COF).67 Aer the sample was a period of 30 min. The resultant bpy-COF lm was washed with
exposed to air (i.e., O2), the spectrum recovered to the initial distilled water. The Cu-doped bpy-COF (Cu-bpy-COF) lm was
state (curve (2) to (3) in Fig. 5b), indicating that Cu(II) was synthesized by a simple impregnation method by dipping a bpy-
regenerated by the ORR. Photoelectrochemical measurements, COF lm into a Cu-ion solution. Cu was impregnated into the
photoluminescence, and in situ XANES analyses clearly show bpy-COF lm for 12 h at room temperature using 5 mM CuCl2
that the photoexcited electrons in the bpy-COF were transferred aqueous solutions. The resultant Cu-bpy-COF lms were
to O2 molecules via the Cu atoms, as shown in Fig. 5c. The washed with distilled water twice and then dried under air.
remaining holes in the VBs reached the ITO electrode, gener-
ating the cathodic photocurrent. Besides, the direct charge
transfer transition from the VB to Cu(II) might occur to some
extent in the longer wavelength region (see the section of the
band structure of the Cu-bpy-COF). In addition, although the
anodic photocurrent observed at more positive potentials than
0 V is outside the scope of the present paper, its mechanism
might be explained as follows: when the applied potential was
more positive than the Cu(I/II) redox potential, the photo-
generated electrons in Cu sites were transferred to the ITO
electrode more rapidly than to O2 molecules.
Physical characterization
X-ray photoelectron spectra (Axis Ultra, Kratos Analytical) were
acquired with monochromated Al Ka X-rays at hn ¼ 1486.6 eV.
The binding energies were calibrated using the C 1s peak at
284.8 eV. For calculation of the atomic concentration, back-
grounds of core-level spectra were subtracted using the Shirley
method. Inductively coupled plasma optical emission spec-
troscopy (ICP-OES) measurements were performed using an
Optima 8300 (PerkinElmer). The uorescence spectra of the
synthesized bpy-COF lm on a transparent slide glass were
recorded using a spectrouorometer (FP-8500, JASCO). Field-
emission scanning electron microscopy (FE-SEM) experiments
were carried out on a SU-8000 (Hitachi). The N2 adsorption–
desorption isotherms at 77 K were obtained using a 3 Flex
(Micromeritics). The prepared bpy-COF lm was ground into
a ne powder in a mortar before measurement. Prior to the
adsorption measurements, ne powder of the bpy-COF wꢀas4
Conclusions
We newly synthesized metal-modied bpy-COF lms and
applied the Cu-doped COF as a photoelectrocatalyst for the
ORR. The synthesized COF showed absorption over the entire
visible-light region (to 750 nm). The cathodic photocurrent
attributed to the ORR was clearly observed for the COF lms,
and the current was drastically enhanced by the addition of Cu
into the pores of the COFs. The photoluminescence and in situ
X-ray absorption spectra indicated that the excited electrons
were rapidly transferred to Cu sites and consumed by O2
reduction. This work is the rst demonstration of the potential
for COF-based lms to serve as photoelectrocatalysts. Given that
M-COFs can exhibit various unique electron-transfer catalytic
properties depending on their metal center, the M-COF lms
ꢁ
degassed at 120 C for 12 h under dynamic vacuum (1 ꢂ 10
Pa). The powder X-ray diffraction (XRD) patterns of the samples
were obtained on an X'PERT-PRO diffractometer (PANalytical)
˚
using Cu Ka radiation (l ¼ 1.54 A, 45 kV, 40 mA). The data were
collected over the 2q range from 2.5ꢁ to 40ꢁ at a speed of 1ꢁ
minꢀ1. UV-visible diffuse-reectance spectra were obtained in
diffuse reectance mode at room temperature using a UV-vis
spectrometer (U-4100, Hitachi) with an integrating sphere.
Barium sulde was used as a reference and standard sample. X-
ray absorption ne structure (XAFS) spectra were obtained
using the BL01B1 beamline at the Spring-8 facility of the Japan
Synchrotron Radiation Research Institute (JASRI). Double-
crystal Si (111) monochromators were used to obtain XAFS data.
The measurements for as-prepared Cu-bpy-COFs were con-
ducted in the uorescence-yield mode using 19 elements of Ge
solid-state detectors. Reference XAFS spectra were collected in
transmission mode. Extended X-ray absorption ne structure
(EXAFS) spectra were analyzed using the ATHENA soware
package. The FT-IR experiments were conducted in trans-
mission mode using a Cary 670 FTIR (Agilent). The prepared Cu-
bpy-COF lm before and aer the 30 minute photocurrent
measurements was ground into a ne powder with KBr powder
in a mortar before measurements.
are potentially
a novel polymeric platform for photo-
electrocatalysts toward articial photosynthesis. Furthermore,
free-standing lms will provide us with novel articial photo-
synthesis systems. For example, the laminated heterojunction
of anodic and cathodic M-COF lms like the membrane elec-
trode assembly (MEA) could be used as an all-solid-state Z-
scheme laminated photocatalyst.75 Such MEA-type photo-
catalysts would separately evolve CRR products and O2 mole-
cules as products of water oxidation.
Experimental
Synthesis of Cu-doped bpy-COF (Cu-bpy-COF) lms
The bpy-COF lms were synthesized via liquid–liquid interfacial
synthesis using the reported method with a slight modication
using 2,4,6-triformylphloroglucinol (Tp) (TCI) and 5,50-diamino-
2,20-bipyridine (bpy) which were prepared by ourselves (see ESI
Evaluation of the HOMO/LUMO energies of the bpy-COF lm
for details†) as monomers.56 Tp (15.7 mg) dissolved in Cyclic voltammograms in organic electrolytes were obtained
dichloromethane (100 mL) was poured into a glass beaker. A using a standard one-compartment, three-electrode electro-
spacer layer of water (60 mL) was gently added on top of the Tp chemical cell. The Ag/Ag+ electrode and Pt wire were used as
solution. Finally, bpy (20.8 mg) and amine-p-toluene sulfonic reference and counter electrodes, respectively. A bpy-COF lm
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supported on a glassy carbon (GC) electrode was used as the geometrical surface area of the electrode was 0.50 cm2. Ti wire
working electrode. 0.1 tetrabutylammonium hexa- and Ag/AgCl (KCl sat.) were used as the counter and reference
M
uorophosphate (Sigma-Aldrich) solution in acetonitrile was electrodes, respectively. The anodic and cathodic compart-
used as the electrolyte. The potential range was set between ments were separated by a Naon membrane. The working
ꢀ2.5 V and 0.5 V, and the scan rate was 50 mV sꢀ1. The ferro- electrodes were prepared by placing the bpy-COF lms on
cenium/ferrocene (Fc/Fc+) redox potential was measured at the transparent conductive electrodes. The electrodes were washed
end to calibrate the Ag/Ag+ electrode. The lowest unoccupied with boiled acetone for 30 min and then sonicated in water for
molecular orbital (LUMO) of the bpy-COF was calculated using 30 min before use.
eqn (1) and (2):
Oxygen reduction reaction
ELUMO (eV) ¼ (E[onset,red vs. Fc/Fc+] + 5.1)
ELUMO (eV) ¼ (E[onset,red vs. NHE] + 4.75)
(1)
(2)
The electrochemical ORR activity was investigated under 1 atm
O2 conditions in a 0.1 M phosphate buffer solution (pH 7.0).
Before reduction, O2 gas was bubbled for 30 min. The scan rate
The highest occupied molecular orbital (HOMO) was esti- for the current–voltage curves was 100 mV sꢀ1
.
mated by the bandgap energies obtained from the UV-vis
reectance spectra. First, we transformed the absorption (6):
spectra by applying the Kubelka–Munk function (F(RN)) via eqn
(3):
All measured potentials were converted to E(RHE) using eqn
E(V vs. RHE) ¼ E(V vs. Ag/AgCl) + 0.197 + 0.059 ꢂpH (6)
2
K
S
ð1 ꢀ RNÞ
2RN
The incident-photon-to-current-efficiency (IPCE) is dened
as eqn (7):
FðRNÞ ¼
¼
(3)
where RN ¼ Rsample/Rstandard. K and S are the absorption and
scattering coefficients, respectively. The bandgap energy was
obtained from the Tauc plot transformed according to eqn (4):
1
1240
IPCEð%Þ ¼
ꢂ
ꢂ 100
(7)
P
l
where I is the photocurrent density (mA cmꢀ2), P is the intensity
of the light irradiated onto the working electrode (mW cmꢀ2),
and l is the wavelength (nm). I was calculated using eqn (8):
1
2
À
Á
N
ðFðR ÞhnÞ ¼ B hn ꢀ Eg
(4)
where h is Planck's constant, n is the photon frequency, Eg is the
bandgap energy, and B is a constant.
I ¼ IL ꢀ ID
(8)
where IL is the average light current density for the rst 1 min
and ID is the average dark current density just before the light
irradiation.
Details for calculating the portions of Cu(II) reduced to Cu(I)
by visible light irradiation
Normalized absorbance (mN) was calculated by mN ¼ ln(I0/I),
where I0 is the intensity of the incident X-rays, and I is that of
the uorescent X-rays. Here, the mN values at 8980 eV were
0.08578, 0.06318, and 0.5815 in the presence of the Cu(II)-bpy-
COF before reaction, Cu(II)-bpy-COF during reaction under Ar
and visible light irradiation, and Cu2O, respectively. Thus, eqn
(5) is valid:
The IPCE was measured using light (400–780 nm in 20 nm
increments) from a Xe lamp (LA-410UV-3, HAYASHI-REPIC)
passing through a band-pass lter (half-width ¼ 10 nm, Asahi
Spectra). The IPCE was measured under an applied potential at
ꢀ0.2 V and +0.2 V vs. Ag/AgCl in 0.1 M phosphate buffer solu-
tion (pH 7.0). The potential dependence of the value of photo-
current was measured under visible light (l > 650 nm) using
a Xe lamp (LA-410UV-3, HAYASHI-REPIC) with a long-wave pass
lter (LV0650 Asahi Spectra).
ꢀ
ꢁ
ꢀ
ꢁ
I0
I0
0:06318 ¼ ln
; 0:5815 ¼ ln
;
I
I
ðIÞ
ðIIÞ
Computational details
I0
We carried out density functional theory (DFT) calculations for
the ORR activity of the Cu-bpy-COF using the OpenMX code.76,77
The generalized gradient approximation of the Perdew–Burke–
Ernzerhof model (GGA-PBE) was used. The kinetic cut-off
energy was selected at 120 Ryd, and the vacuum layer was set to
0:08578 ¼ ln
(5)
aIðlÞ þ ð1 ꢀ aÞI
ðllÞ
where I(II) and I(I) are the intensities of the uorescent X-rays in
the presence of Cu(II) and Cu(I), respectively, and a is the portion
of Cu(II) converted to Cu(I). In this way, a was calculated to be
0.0342 (3.4%).
˚
be greater than 15 A; (2 ꢂ 2 ꢂ 1) k-points were used. We
calculated all atoms within the relaxations in all directions. The
atomic relaxations were conducted until the maximum force on
(Photo)electrochemical measurements
any atom was less than 10ꢀ4 Hartree Bohrꢀ1
.
A double-chamber electrochemical cell and an electrochemical
For the electrocatalytic ORR process, the model of the
measurement system (HZ-5000, Hokuto Denko) equipped with computational hydrogen electrode was used to calculate the
a three-electrode system were used to evaluate the electro- or energy of a proton–electron pair, where the energy of a proton–
photoelectrocatalytic activity of the M-bpy-COFs. The electron pair was approximated as one-half of the free energy of
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Journal of Materials Chemistry A
H2 molecule under standard conditions.60 The Gibbs free energy 14 X. Wang, L. Chen, S. Y. Chong, M. A. Little, Y. Wu,
(DG) for each electrochemical process was calculated using eqn
(9):
W.-H. Zhu, R. Clowes, Y. Yan, M. A. Zwijnenburg,
R. S. Sprick and A. I. Cooper, Nat. Chem., 2018, 10, 1180.
15 K. Lei, D. Wang, L. Ye, M. Kou, Y. Deng, Z. Ma, L. Wang and
Y. Kong, ChemSusChem, 2020, 13, 1725.
16 H. B. Aiyappa, J. Thote, D. B. Shinde, R. Banerjee and
S. Kurungot, Chem. Mater., 2016, 28, 4375.
17 S. M. J. Rogge, A. Bavykina, J. Hajek, H. Garcia, A. I. Olivos-
Suarez, A. Sepulveda-Escribano, A. Vimont, G. Clet,
P. Bazin, F. Kapteijn, M. Daturi, E. V. Ramos-Fernandez,
F. X. L. i. Xamena, V. Van Speybroeck and J. Gascon, Chem.
Soc. Rev., 2017, 46, 3134.
DG ¼ DE + DEZPE ꢀ TDS
(9)
where the values of DE, DEZPE, and DS denote the changes in the
DFT energy, the zero-point energy, and the entropy at 298 K,
respectively.
Conflicts of interest
18 P. Su, K. Iwase, T. Harada, K. Kamiya and S. Nakanishi,
Chem. Sci., 2018, 9, 3941.
There are no conicts to declare.
19 W. Zhong, R. Sa, L. Li, Y. He, L. Li, J. Bi, Z. Zhuang, Y. Yu and
Z. Zou, J. Am. Chem. Soc., 2019, 141, 7615.
20 K. Iwase, K. Kamiya, M. Miyayama, K. Hashimoto and
S. Nakanishi, ChemElectroChem, 2018, 5, 805.
21 O. Buyukcakir, S. H. Je, S. N. Talapaneni, D. Kim and
A. Coskun, ACS Appl. Mater. Interfaces, 2017, 9, 7209.
22 J.-D. Yi, R. Xu, Q. Wu, T. Zhang, K.-T. Zang, J. Luo,
Y.-L. Liang, Y.-B. Huang and R. Cao, ACS Energy Lett., 2018,
3, 883.
23 J.-D. Yi, R. Xu, G.-L. Chai, T. Zhang, K. Zang, B. Nan, H. Lin,
Y.-L. Liang, J. Lv, J. Luo, R. Si, Y.-B. Huang and R. Cao, J.
Mater. Chem. A, 2019, 7, 1252.
Acknowledgements
This research was also supported by a JSPS KAKENHI Program
(grant no. 20H02568). This work was also supported by CREST
(grant no. JPMJCR18R3) of the Japan Science and Technology
Agency (JST). Synchrotron radiation experiments were per-
formed using the BL01B1 Beam Line of SPring-8 with the
approval of the Japan Synchrotron Radiation Research Institute
(JASRI; Proposal Nos. 2017A1790, 2018A1349, 2019A1394,
2019B1159, 2020A1254 and 2020A1426). We express gratitude to
Prof Y. Aso and Dr S. Tamba for his kind support in the
synthesis of 2,2-bipyridine-5,50-diamine.
24 T. Li, C. Atish, K. Silambarasan, X. Liu and A. P. O'Mullane,
Electrochim. Acta, 2020, 362, 137212.
25 D. A. Popov, J. M. Luna, N. M. Orchanian, R. Haiges,
C. A. Downes and S. C. Marinescu, Dalton Trans., 2018, 47,
17450.
Notes and references
1 S.-Y. Ding and W. Wang, Chem. Soc. Rev., 2013, 42, 548.
2 X. Feng, X. Ding and D. Jiang, Chem. Soc. Rev., 2012, 41, 6010.
26 E. M. Johnson, R. Haiges and S. C. Marinescu, ACS Appl.
Mater. Interfaces, 2018, 10, 37919.
3 J. L. Segura, M. J. Mancheno and F. Zamora, Chem. Soc. Rev., 27 S.-Y. Ding, J. Gao, Q. Wang, Y. Zhang, W.-G. Song, C.-Y. Su
2016, 45, 5635.
and W. Wang, J. Am. Chem. Soc., 2011, 133, 19816.
4 F. Vilela, K. Zhang and M. Antonietti, Energy Environ. Sci., 28 G. H. Gunasekar, K. Park, V. Ganesan, K. Lee, N.-K. Kim,
2012, 5, 7819. K.-D. Jung and S. Yoon, Chem. Mater., 2017, 29, 6740.
5 S. Kandambeth, K. Dey and R. Banerjee, J. Am. Chem. Soc., 29 R. Kamai, K. Kamiya, K. Hashimoto and S. Nakanishi, Angew.
2019, 141, 1807. Chem., Int. Ed., 2016, 55, 13184.
6 E. Jin, J. Li, K. Geng, Q. Jiang, H. Xu, Q. Xu and D. Jiang, Nat. 30 R. Kamai, S. Nakanishi, K. Hashimoto and K. Kamiya, J.
Commun., 2018, 9, 4143. Electroanal. Chem., 2017, 800, 54.
7 J. Lv, Y.-X. Tan, J. Xie, R. Yang, M. Yu, S. Sun, M.-D. Li, 31 K. Kamiya, R. Sugimoto, T. Tatebe, T. Harada and
D. Yuan and Y. Wang, Angew. Chem., Int. Ed., 2018, 57, 12716.
S. Nakanishi, ChemSusChem, 2020, 13, 3462.
8 J. Lee, O. Buyukcakir, T.-w. Kwon and A. Coskun, J. Am. 32 K. Kamiya, T. Tatebe, S. Yamamura, K. Iwase, T. Harada and
Chem. Soc., 2018, 140, 10937. S. Nakanishi, ACS Catal., 2018, 8, 2693.
9 C. Liu, W. Zhang, Q. Zeng and S. Lei, Chem.–Eur. J., 2016, 22, 33 K. Iwase, T. Yoshioka, S. Nakanishi, K. Hashimoto and
6768. K. Kamiya, Angew. Chem., Int. Ed., 2015, 54, 11068.
10 G. Lin, H. Ding, R. Chen, Z. Peng, B. Wang and C. Wang, J. 34 S. Kato, K. Iwase, T. Harada, S. Nakanishi and K. Kamiya,
Am. Chem. Soc., 2017, 139, 8705. ACS Appl. Mater. Interfaces, 2020, 12, 29376.
11 D. Wang, Z. Zhang, L. Lin, F. Liu, Y. Wang, Z. Guo, Y. Li, 35 X. Zhao, P. Pachfule, S. Li, T. Langenhahn, M. Ye,
H. Tian and X. Chen, Biomaterials, 2019, 223, 119459.
12 M. Calik, F. Auras, L. M. Salonen, K. Bader, I. Grill,
C. Schlesiger, S. Praetz, J. Schmidt and A. Thomas, J. Am.
Chem. Soc., 2019, 141, 6623.
M. Handloser, D. D. Medina, M. Dogru, F. Loebermann, 36 R. Joseph Kline, M. D. McGehee and M. F. Toney, Nat.
D. Trauner, A. Hartschuh and T. Bein, J. Am. Chem. Soc.,
2014, 136, 17802.
13 K. Zhang, D. Kopetzki, P. H. Seeberger, M. Antonietti and
F. Vilela, Angew. Chem., Int. Ed., 2013, 52, 1432.
Mater., 2006, 5, 222.
37 G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar and
C. A. Grimes, Sol. Energy Mater. Sol. Cells, 2006, 90, 2011.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A, 2021, 9, 11073–11080 | 11079

View Article Online
Journal of Materials Chemistry A
Paper
38 R. K. Yadav, A. Kumar, N.-J. Park, K.-J. Kong and J.-O. Baeg, J. 57 M. A. Khayum, S. Kandambeth, S. Mitra, S. B. Nair, A. Das,
Mater. Chem. A, 2016, 4, 9413.
39 K. Kamiya, R. Kamai, K. Hashimoto and S. Nakanishi, Nat.
Commun., 2014, 5, 5040.
S. S. Nagane, R. Mukherjee and R. Banerjee, Angew. Chem.,
Int. Ed., 2016, 55, 15604.
58 Q. Hao, C. Zhao, B. Sun, C. Lu, J. Liu, M. Liu, L.-J. Wan and
D. Wang, J. Am. Chem. Soc., 2018, 140, 12152.
40 D. Liu, X. Zhang, Z. Sun and T. You, Nanoscale, 2013, 5, 9528.
41 X. Huang, S. Tang, X. Mu, Y. Dai, G. Chen, Z. Zhou, F. Ruan, 59 S. Kandambeth, B. P. Biswal, H. D. Chaudhari, K. C. Rout,
Z. Yang and N. Zheng, Nat. Nanotechnol., 2011, 6, 28.
42 X. Kong, K. Xu, C. Zhang, J. Dai, S. Norooz Oliaee, L. Li,
X. Zeng, C. Wu and Z. Peng, ACS Catal., 2016, 6, 1487.
43 J. W. Hong, Y. Kim, D. H. Wi, S. Lee, S.-U. Lee, Y. W. Lee,
S. Kunjattu H, S. Mitra, S. Karak, A. Das, R. Mukherjee,
U. K. Kharul and R. Banerjee, Adv. Mater., 2017, 29, 1603945.
60 K. Iwase, S. Nakanishi, M. Miyayama and K. Kamiya, ACS
Appl. Energy Mater., 2020, 3, 1644.
S.-I. Choi and S. W. Han, Angew. Chem., Int. Ed., 2016, 55, 61 A. R. Millward and O. M. Yaghi, J. Am. Chem. Soc., 2005, 127,
2753.
17998.
44 X. Zhang and Y. Xie, Chem. Soc. Rev., 2013, 42, 8187.
45 C. S. Diercks, S. Lin, N. Kornienko, E. A. Kapustin,
E. M. Nichols, C. Zhu, Y. Zhao, C. J. Chang and
O. M. Yaghi, J. Am. Chem. Soc., 2018, 140, 1116.
46 T. Sick, A. G. Hufnagel, J. Kampmann, I. Kondofersky,
62 D. B. Shinde, H. B. Aiyappa, M. Bhadra, B. P. Biswal,
P. Wadge, S. Kandambeth, B. Garai, T. Kundu, S. Kurungot
and R. Banerjee, J. Mater. Chem. A, 2016, 4, 2682.
63 V. Hayez, A. Franquet, A. Hubin and H. Terryn, Surf. Interface
Anal., 2004, 36, 876.
M. Calik, J. M. Rotter, A. Evans, M. Doeblinger, S. Herbert, 64 L. L. Jensen, J. T. Muckerman and M. D. Newton, J. Phys.
K. Peters, D. Boehm, P. Knochel, D. D. Medina, Chem. C, 2008, 112, 3439.
D. Fattakhova-Rohlng and T. Bein, J. Am. Chem. Soc., 65 F. Tran and P. Blaha, J. Phys. Chem. A, 2017, 121, 3318.
2018, 140, 2085.
66 M. Shukla, N. Srivastava, S. Saha, T. R. Rao and S. Sunkari,
Polyhedron, 2011, 30, 754.
67 H. Irie, K. Kamiya, T. Shibanuma, S. Miura, D. A. Tryk,
T. Yokoyama and K. Hashimoto, J. Phys. Chem. C, 2009,
113, 10761.
68 T. Tatebe, T. Harada, K. Kamiya and S. Nakanishi,
Photochem. Photobiol. Sci., 2018, 17, 1153.
69 C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale and
G. C. Bazan, Adv. Mater., 2011, 23, 2367.
70 P. Makula, M. Pacia and W. Macyk, J. Phys. Chem. Lett., 2018,
9, 6814.
47 K. Takada, R. Sakamoto, S.-T. Yi, S. Katagiri, T. Kambe and
H. Nishihara, J. Am. Chem. Soc., 2015, 137, 4681.
48 R. Sakamoto, K. Hoshiko, Q. Liu, T. Yagi, T. Nagayama,
S. Kusaka, M. Tsuchiya, Y. Kitagawa, W.-Y. Wong and
H. Nishihara, Nat. Commun., 2015, 6, 6713.
49 D. Zhu, Q. Zhao, G. Fan, S. Zhao, L. Wang, F. Li and J. Chen,
Angew. Chem., Int. Ed., 2019, 58, 12460.
50 B. Zhang, S. Wang, W. Fan, W. Ma, Z. Liang, J. Shi, S. Liao
and C. Li, Angew. Chem., Int. Ed., 2016, 55, 14748.
51 N. T. Hong Trang and K. Ishii, J. Phys. Chem. C, 2018, 122,
3539.
71 Y. Zhao, H. Liu, C. Wu, Z. Zhang, Q. Pan, F. Hu, R. Wang,
P. Li, X. Huang and Z. Li, Angew. Chem., Int. Ed., 2019, 58,
5376.
52 Y. Sun, L. Han and P. Strasser, Chem. Soc. Rev., 2020, 49,
6605.
53 L. Guo, K. Liang, K. Marcus, Z. Li, L. Zhou, P. D. Mani, 72 J. Chen, X. Tao, C. Li, Y. Ma, L. Tao, D. Zheng, J. Zhu, H. Li,
H. Chen, C. Shen, Y. Dong, L. Zhai, K. R. Coffey, R. Li and Q. Yang, Appl. Catal., B, 2020, 262, 118271.
N. Orlovskaya, Y.-H. Sohn and Y. Yang, ACS Appl. Mater. 73 D. Wang, C. Ao, X. Liu, S. Fang, Y. Lin, W. Liu, W. Zhang,
Interfaces, 2016, 8, 34970.
X. Zheng, L. Zhang and T. Yao, ACS Appl. Energy Mater.,
2019, 2, 6497.
ˇ
´
54 M. Jakesova, D. H. Apaydin, M. Sytnyk, K. Oppelt, W. Heiss,
N. S. Saricici and E. D. Głowacki, Adv. Funct. Mater., 2016, 74 S. Barazzouk, P. V. Kamat and S. Hotchandani, J. Phys. Chem.
26, 5248. B, 2005, 109, 716.
55 S. Calabrese Barton, J. Gallaway and P. Atanassov, Chem. 75 H. Li, W. Tu, Y. Zhou and Z. Zou, Adv. Sci., 2016, 3, 1500389.
Rev., 2004, 104, 4867. 76 T. Ozaki, Phys. Rev. B, 2003, 67, 155108.
56 K. Dey, M. Pal, K. C. Rout, S. Kunjattu H, A. Das, 77 T. Ozaki and H. Kino, Phys. Rev. B, 2004, 69, 195113.
R. Mukherjee, U. K. Kharul and R. Banerjee, J. Am. Chem. 78 K. Kamiya, Chem. Sci., 2020, 11, 8339–8349.
Soc., 2017, 139, 13083.
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