Rational Design of Crystalline Covalent Organic Frameworks for Efficient CO2 Photoreduction with H2O

Article abstract of DOI:10.1002/anie.201906890

Solar energy-driven conversion of CO₂ into fuels with H₂O as a sacrificial agent is a challenging research field in photosynthesis. Herein, a series of crystalline porphyrin-tetrathiafulvalene covalent organic frameworks (COFs) are synthesized and used as photocatalysts for reducing CO₂ with H₂O, in the absence of additional photosensitizer, sacrificial agents, and noble metal co-catalysts. The effective photogenerated electrons transfer from tetrathiafulvalene to porphyrin by covalent bonding, resulting in the separated electrons and holes, respectively, for CO₂ reduction and H₂O oxidation. By adjusting the band structures of TTCOFs, TTCOF-Zn achieved the highest photocatalytic CO production of 12.33 μmol with circa 100 % selectivity, along with H₂O oxidation to O₂. Furthermore, DFT calculations combined with a crystal structure model confirmed the structure–function relationship. Our work provides a new sight for designing more efficient artificial crystalline photocatalysts.

Full text of DOI:10.1002/anie.201906890

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Accepted Article
Title: Rational Crystalline Covalent Organic Frameworks Design for
Efficient CO2 Photoreduction with H2O
Authors: Ya-Qian Lan, Meng Lu, Jiang Liu, Qiang Li, Mi Zhang, Ming
Liu, Jin-Lan Wang, and Da-Qiang Yuan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906890
Angew. Chem. 10.1002/ange.201906890
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http://dx.doi.org/10.1002/ange.201906890

10.1002/anie.201906890
Angewandte Chemie International Edition
COMMUNICATION
Rational Crystalline Covalent Organic Frameworks Design for
Efficient CO2 Photoreduction with H2O
Meng Lu1+, Jiang Liu1+, Qiang Li2, Mi Zhang1, Ming Liu1, Jin-Lan Wang2, Da-Qiang Yuan3, and
Ya-Qian Lan1,*
Solar energy-driven conversion of CO2 into fuels with H2O as
sacrificial agent is still a challenging research field in photosynthesis.
Here, a series of crystalline porphyrin-tetrathiafulvalene covalent
organic frameworks (COFs) are synthesized and used as
photocatalysts for reducing CO2 with H2O, in the absence of
additional photosensitizer, sacrificial agent and noble metal co-
catalyst. The effective photogenerated electrons transfer from
tetrathiafulvalene to porphyrin by covalent bond, resulting in the
separated electrons and holes respectively for CO2 reduction and
H2O oxidation. By adjusting the band structures of TTCOFs, TTCOF-
Zn achieved the highest photocatalytic CO production of 12.33 μmol
with ~100 % selectivity, along with H2O oxidation to O2. Furthermore,
density function theory calculations combined with the crystal
structure model confirmed the structure-function relationship. Our
work provides a new sight for designing more efficient artificial
crystalline photocatalysts.
building blocks or structural components according to specific
demands.[4] Nevertheless, the majority of crystalline porous
materials assembled with coordination bonds such as metal-
organic frameworks (MOFs), few of them strong enough for
performing the overall reaction (i.e. reducing CO2 with H2O as
electron donor).[5] By contrast, covalent organic frameworks
(COFs) with directional structural designability, highly structural
and chemical stabilities are a class of more promising porous
crystalline materials for artificial photosynthesis[6]. Additionally, it
is revealed that COFs with robust structure are able to afford
permanent platform for catalytic active site[7] as well as large
surface areas to fix CO2[8], and their ordered π-array structures
can provide pre-organized pathways for high-rate charge-carrier
transport through delicate design[9]. Taking these considerations
into account, it is feasible to build a multifunctional COFs
photocatalyst system where the integration of CO2 reduction and
H2O oxidation half-reactions can be implemented. Unfortunately,
to the best of our knowledge, few such system has been
Excessive emission of carbon dioxide (CO2) from fossil fuel
burning into the atmosphere has caused severe energy and
environmental issues that are urgent to be solved[1]. It is well-
known that the significance of plant photosynthesis lies in that its
ability to convert CO2 with H2O into carbohydrate and O2 by
investigated in crystalline COF materials field up to now[10]
Herein, series of crystalline 2D rigid porphyrin-
.
a
tetrathiafulvalene COFs (TTCOF-M, M = 2H, Zn, Ni, Cu) were
designed for artificial photosynthesis including CO2 reduction
and H2O oxidation. Electron-deficient metalloporphyrin (TAPP)
complexes are known to have good visible-light collecting ability
and potential CO2 reduction performance[11], while electron-rich
tetrathiafulvalene (TTF) molecule has proven to be a superior
electron donor with rapid electron transfer[12]. Therefore, the
effective covalent coupling between TAPP and TTF within COFs
enables the visible-light driven electrons to efficiently separate
and transfer from TTF to TAPP moiety, resulting in that the
photoexcited electrons (on porphyrin) and holes (on TTF) can be
used for reduction and oxidation reactions, respectively. As
expected, TTCOF-Zn/Cu owing to suitable photocatalytic redox
potentials were successfully used for reducing CO2 with H2O as
electron donor, and TTCOF-Zn exhibited the highest CO
production of 12.33 μmol and selectivity (~100%) combined with
excellent durability under our experimental conditions. Moreover,
the corresponding density function theory (DFT) calculations
agreed with the experimental results, which also gave detailed
explanations about charge carrier transfer process and
photocatalytic reaction pathways.
sunlight in
a
green manner. Consequently, artificial
photosynthesis is expected to mimic above process efficiently
for reducing CO2 with H2O as electron donor, that is, integrating
CO2 reduction reaction (CO2RR) and H2O oxidation half-
reactions in one catalytic system, in which the development of
photocatalysts is the most crucial factor[2]. However, it is a
daunting work to enable these two half-reactions to couple and
interact effectively. At present, only very limited strategies on the
design of efficient photocatalyst can realize the overall reaction,
such as Z-scheme heterojunction, yet they still face many
problems.[3] Therefore, it is essential and pressing to explore
new photocatalytic system with unambiguous structure for
conducting photosynthetic overall reaction so as to obtain more
insights into the structure-function relationship and then promote
the development of this field.
Crystalline porous materials can provide
a favorable
structure-function research platform (including catalytic active
site, charge transfer and reaction mechanism, etc.) for artificial
photosynthesis because their well-defined structures can be
elaborately designed and constructed by selecting appropriate
TTCOF-M were synthesized by Schiff-base condensation
between
metallized
5,10,15,20-tetrakis
(4-aminophenyl)-
porphinato] (TAPP-M, M = 2H, Zn, Ni, Cu) and 2,3,6,7-Tetra (4-
formylphenyl)-tetrathiafulvalene by solvothermal method (Fig 1a).
The crystalline structures of TTCOF-M were determined by the
powder X-ray diffraction (PXRD) characterizations combined
with theoretically structural simulations by Materials Studio
version 7.0. An orthorhombic P222 space group based on
TTCOF-Zn was built and carried out Le Bail refinements of the
PXRD patterns for full profile fitting against the proposed models,
which provided a unit cell parameter of a = 31.5694 Å, b =
21.9744 Å and c = 5.0014 Å, α = β = γ =90°. The simulated
PXRD pattern using AA stacking mode reproduced the
experimentally observed curve while AB stacking not, as
[1]
M. Lu,[+] Dr. J. Liu, [+] M. Zhang, M. Liu, Prof. Y. -Q. Lan*,
Jiangsu Collaborative Innovation Centre of Biomedical Functional
Materials, School of Chemistry and Materials Science, Nanjing
Normal University. No. 1, Wenyuan Road, Nanjing, 210023 (China)
E-mail: yqlan@njnu.edu.cn
Dr. Q. Li, Prof. J. -L. Wang,
School of Physics, Southeast University, Nanjing 211189 (China)
Prof. D. -Q. Yuan,
[2]
[3]
State Key Laboratory of Structural Chemistry Fujian Institute of
Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou, 350002 (China)
[+] These authors contributed equally to this work.
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10.1002/anie.201906890
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revealed by difference plot, with unweighted-profile R factor (Rp)
= 2.544% and weighted-profile R factor (Rwp) = 3.349%,
suggesting the validity of the computational model (Fig 1b).
TTCOF-Zn has intense PXRD peaks at 4.98, 5.60, 6.9, 9.8 and
11.66°, which can be assigned to the 110, 200, 210, 220 and
320 faces, respectively. More details and raw data are given in
SI.
results. The chemical stability of TTCOF-Zn was examined by
immersing it into different solvents at room temperature. The
FTIR and PXRD patterns indicated its structural robustness (Fig
S15-S16). The thermal stabilities of TTCOF-M were confirmed
by thermogravimetric analysis (Fig S17-S20), which showed no
obvious change up to 300 °C .
The morphology of TTCOF-M was characterized using
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM), which shows that TTCOF-Zn is composed of
hollow spheres with 1~3 μm in diameter and the self-assembled
of small rectangular sheet-shaped crystals with about 0.1~0.2
μm in length (Fig 2a and Fig S21-S22). The fast Fourier
transformation image of high-resolution transmission electron
microscopy (HR-TEM) showed the hexagonal pore structure
arrangement along the 001 crystal axis, which can also manifest
the AA-stacking mode of 2D TTCOF-Zn structure (Fig 2b).
Besides, energy-dispersive X-ray spectroscopy (EDS) analysis
reveals that C, N, S and Zn elements are uniformly distributed
over TTCOF-Zn (Fig S23-S25). The Zn content in TTCOF-Zn
was determined to be ~4.3 wt % by inductively coupled plasma
(ICP) optical emission spectrometry (Table S1).
The visible light-driven (420~800 nm) photocatalytic CO2RR
was conducted under pure CO2 (1.0 atm, 298 K) atmosphere in
CO2 saturated H2O solution, without additional photosensitizer
(PS) and sacrificial agents (SA). It is well known that the CO2
absorption ability is an important factor for CO2RR
photocatalyst[15]. The CO2 adsorption isotherms of TTCOFs were
measured and determined to be 28, 52, 42 and 38 cm3/g for
TTCOF-2H/Zn/Ni/Cu, respectively (Fig 2c). The increased CO2
adsorption capacity of TTCOF-Zn/Ni/Cu compared to the host
TTCOF-2H should be attributed to the strong affinity between
the metal ion and CO2 molecule[16]. At the same time, water
vapor adsorption tests for TTCOFs showed characteristic type II
isotherm (Fig S26-S29), which indicated that the internal pore
structure and external surface of these materials are accessible
to water[17]. This hydrophilicity fact can also be demonstrated by
water contact angles tests showing angle of 22-45° (Fig S.30).
Previous studies have revealed that to achieve photocatalytic
CO2RR with H2O, the photocatalyst is required to have a more
negative conduction band minimum (CBM) potential than CO2
reduction potential (CO/CO2, theoretically -0.53 V vs NHE, pH =
7 and -4.37 eV vs Ev, vacuum level) as well as more positive
valence band maximum (VBM) potential than H2O oxidation
potential (O2/H2O, theoretically 0.82 V vs NHE, pH = 7 and -5.67
eV vs Ev) [2b]. UV–vis diffuse reflectance spectroscopy (DRS)
associated with ultraviolet photoelectron spectroscopy (UPS)
were conducted to determine the electronic properties of
Figure 1. a) Schematic of the synthesis of TTCOF-M via the condensation of
TTF and TAPP-M. b) Experimental (black dot) and simulated (red line) PXRD
patterns of TTCOF-Zn c) Top and d) side views and e)13C ssNMR spectrum of
TTCOF-Zn. f) N2 adsorption curve of TTCOF-Zn at 77 K (inset pore-size
distribution profile).
These refinements finally revealed that there is a dual channel
along c axis with theoretical pore sizes of 1.10 nm and 1.57 nm
and pore volume of 0.94 cm3/g (Fig 1c), and the distance
between adjacent stacking 2D sheets is 3.69 Å (Fig 1d). Besides,
TTCOF-2H/Ni/Cu also showed high crystallinity from their PXRD
patterns (Fig S1-S3).
Fourier transform infrared spectroscopy (FTIR) and 13C solid-
state nuclear magnetic resonance spectroscopy (13C ssNMR)
were used to confirm the chemical structure of TTCOF-M. From
FTIR, the C=N stretching vibration band at 1620 cm-1 appeared
in the resultant TTCOF-M, while the C=O stretching vibration
band at 1700 cm-1 and -NH2 vibration band at 3200~3500 cm-1
belonged to reactant monomers that reduced obviously[13] (Fig
S4-S5). Furthermore, the characteristic peak of ∼158.9 ppm in
13C ssNMR corresponded to the carbon atom of the C=N bond[14]
(Fig 1e and Fig S6-S7). These results confirmed the successful
condensation reaction required for TTCOF-M structure. X-ray
photoelectron spectroscopy (XPS) analysis results showed the
divalent state of the central metal ion in porphyrin pocket of
TTCOF-M (M = Zn, Ni, Cu) (Fig S8-S11).
The surface areas and porosity of TTCOF-M were
determined by N2 adsorption isotherms at 77 K (Fig 1f and Fig
S12-S14). The Brunauer-Emmett-Teller (BET) surface area of
TTCOF-Zn was calculated to be 564.6 m2/g. Moreover, the total
pore volume (0.93 cm3/g) and the pore sizes (1.02 and 1.56 nm)
for TTCOF-Zn are in conformity with theoretical calculation
TTCOF-M. These TTCOFs exhibited
a broad visible-light
absorption range, and the corresponding bandgaps (Eg) were
determined to be 1.15, 1.49, 1.33 and 1.39 eV for TTCOF-
2H/Zn/Ni/Cu by tauc-plots (Fig 2d inset and Fig S31-S33),
respectively, suggesting the characteristics of semiconductor[18]
.
The VBM of TTCOF-Zn was calculated to be -5.76 eV (vs. Ev)
from UPS pattern (Fig 2e) and the CBM was thus calculated to
be -4.27 eV (vs. Ev). Mott–Schottky measurements were also
conducted to determine the band positions of TTCOF-M (Fig
S36), which were consistent with the results extracted from UPS.
It is obvious that the CBM of TTCOF-Zn is more negative than
the standard reduction potential of CO/CO2 and its VBM is more
positive than the oxidation potential of O2/H2O, suggesting its
capability of integrating CO2 reduction with H2O oxidation
reactions. Similarly, the electronic properties of other COFs and
COF-366-Zn was also obtained by the above mentioned
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methods (Fig S34-S35 and S37-S44) and the energy-band
alignment results were presented in Fig 2f. As we can see,
TTCOF-Zn/Cu owing to the matched band structure fulfilled the
artificial photosynthesis with H2O oxidation, while TTCOF-Ni and
COF-366-Zn were considered to be unsuitable.
used as reaction solution, 18O2 (m/z = 36) and 18O16O (m/z = 34)
were detected in the gas phase after the reaction (Fig S51),
confirming that the generated 18O2 stemmed from the oxidation
of H218O. Additionally, the photocatalytic durability of our catalyst
system can maintain at least five cycles (Fig 3d). The crystallinity
and structural integrity of TTCOF-Zn were retained after the
reaction, as confirmed by PXRD, FTIR and XPS
characterizations (Fig S52-S54). All of these evidences
confirmed that TTCOF-Zn materials are efficient and selective
heterogeneous photocatalysts for combining CO2 photoreduction
with H2O photooxidation.
Many characterization methods were performed to investigate
the reasons for the different performances of these
photocatalysts. Incident-photon-to-current conversion efficiency
(IPCE) analysis was used to examine the photoinduced electron
transfer (PET) efficiency. This results showed that the photo-
current responses for TTCOF-Ms (metalloporphyrin) are much
stronger than that of TTCOF-2H (metal-free) (Fig S55),
indicating
electron-hole pairs by ligand-to-metal charge transfer effect.
Besides, photoluminescence (PL) and time-resolved
a more efficient separation of photogenerated
fluorescence decay techniques were further conducted to
investigate the charge separation behaviors. The PL intensity
Figure 2. a) TEM images and b) HRTEM image of TTCOF-Zn, inset the pores
are highlighted. c) CO2 adsorption curves of TTCOF-M measured at 298 K. d)
Solid-state UV-vis spectra of TTCOF-M. e) UPS spectra of TTCOF-Zn. f) Band
structure diagram for TTCOF-M and COF-366-Zn.
The photocatalytic reactions were first carried out with TAPP-
M monomers, no observable product was detected. When
TTCOFs were used as photocatalysts, TTCOF-Zn showed the
highest CO evolution of 12.33 μmol under visible light
illumination after 60 hours, which was higher than that of
TTCOF-Cu (8.65 μmol) and TTCOF-Ni (0.462 μmol) (Fig 3a,
black). Each data is an average of three separate trials
performed under the same conditions, and the standard
deviation of these measurements is indicated by the error bar. At
the same time, the yield of O2 was also detected (Fig 3a and Fig
S45-46). As expected, the CO: O2 ratio is about 2:1. We can see
that only the formed COFs exhibit prominent photocatalytic
activity, and the mechanisms behind are discussed below.
Moreover, the time-dependent CO output increased almost
linearly with irradiation time for TTCOF-Zn as catalyst (Fig 3b).
To further confirm the photocatalytic activity of TTCOFs, a series
of control experiments were performed (Table S2). These results
showed that no other gaseous products (except CO and O2)
were detected by gas chromatography (GC), while
Figure 3. a) CO2RR performances of TTCOF-M and COF366-Zn. b) time-
dependent CO production performance by using TTCOF-Zn as photocatalyst c)
MS of 13CO produced from the photocatalytic reduction of 13CO2. d) Durability
tests of TTCOF-Zn.
of TTCOF-Zn was significantly quenched (Fig S56) compared to
molecular TAPP-Zn and TTF. At the same time, TTCOF-Zn has a
longer fluorescence lifetime (τ1 = 53.45 ± 1.6 ns) than that of
TAPP-Zn (45.57 ± 0.76 ns) (Fig S57-S58), suggesting that the
efficient electron transfer within COF is favorable for the
promotion of charge separation efficiency.
Based on the above experiments and analysis, an intrinsic
mechanism was proposed to explain the CO2RR process with
H2O oxidation: under the drive of visible-light irradiation, the PET
process takes place from the TTF moiety (HOMO center) to the
TAPP moiety (LUMO center) after absorbing photons, the
excited electrons then move to catalytically active sites (Zn/Cu in
TAPP) and used for CO2 reduction. Meanwhile, the
photogenerated holes in TTF are capable of oxidizing H2O to O2,
by which the catalytic system gains electrons from H2O to keep
charge balance (Fig 4a).
1
unquantifiable liquid composition was detected by HNMR (Fig
S48), suggesting its high CO selectivity. The isotope labeling
experiments were performed to ascertain the carbon and oxygen
sources of photochemical products. Firstly we use 13CO2 as
substrates, 13CO (m/z = 29) was finally detected by using mass
spectroscopy, which confirmed that the produced CO was
originated from the reactant CO2 instead of decomposition of
catalyst (Fig 3c and Fig S49-50). Moreover, when H218O was
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tetrathiafulvalene molecules within TTCOFs, the electron-hole
pairs induced by visible-light irradiation can be separated and
transferred efficiently for CO2 reduction and H2O oxidation
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production of 12.33 μmol with ~100 % selectivity and superior
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Acknowledgments
This work was financially supported by NSFC [No. 21622104,
21471080, 2170010097 and 21701085, BK20171032]; the NSF
of Jiangsu Province of China [No. SBK2017040708]; the Natural
Science Research of Jiangsu Higher Education Institutions of
China [No. 17KJB150025]; Priority Academic Program
Development of Jiangsu Higher Education Institutions and the
Foundation of Jiangsu Collaborative Innovation Center of
Biomedical Functional Materials.
Keywords: covalent organic frameworks • CO2 photoreduction •
H2O photooxidation • electron transfer
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Text for Table of Contents
Meng Lu, Jiang Liu, Qiang Li, Mi Zhang,
Ming Liu, Jin-Lan Wang, Da-Qiang
Yuan, and Ya-Qian Lan*
A series crystalline covalent organic
frameworks were designed and
applied for CO2 photoreduction
coupled with H2O photooxidation, with
the absence of any photosensitizer
and sacrificial agent, giving more
Page No. – Page No.
Rational Crystalline Covalent Organic
Frameworks Design for Efficient CO2
Photoreduction with H2O
straightforward
and
clear
understanding over the structure-
function relationship of artificial
photosynthesis.
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