Integrating active C3N4moieties in hydrogen-bonded organic frameworks for efficient photocatalysis

Article abstract of DOI:10.1039/d1ta00100k

Hydrogen-bonded organic frameworks (HOFs) provide a platform to self-assemble numerous functional species into an ordered structure. Herein, a well-known photoactive C₃N₄moiety was integrated into an HOF structure (PFC-42) with the merits of high porosity and crystallinity. Under visible-light irradiation, the Pt nanoparticle-loaded PFC-42 (PFC-42-Pt) continuously produces hydrogen from water in the presence of scavengers with the evolution rate of 11.32 mmol g^?1, which is outstanding among all the reported Pt/porous composite materials. The significantly high H₂evolution of PFC-42-Pt compared with that of amorphous analogue bulk C₃N₄-Pt and nanosheet C₃N₄-Pt demonstrates that the ordered arrangement of photosensitizers dramatically improves the photocatalytic activity of the material, which is further proved by the recrystallization experiment. This study represents the first example of HOF capable of photocatalysis, not only demonstrating the great application potentials of HOF in heterogeneous photocatalysis but also rendering an excellent opportunity to reveal structure-activity relations.

Full text of DOI:10.1039/d1ta00100k

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Integrating active C3N4 moieties in hydrogen-
bonded organic frameworks for efficient
photocatalysis†
Cite this: J. Mater. Chem. A, 2021, 9,
4687
b
ab
Tao Li,ab Bai-Tong Liu,b Zhi-Bin Fang,
Qi Yin,b Rui Wangb and Tian-Fu Liu
Received 5th January 2021
Accepted 1st February 2021
*
DOI: 10.1039/d1ta00100k
rsc.li/materials-a
Hydrogen-bonded organic frameworks (HOFs) provide a platform to cost.11 However, these materials are usually nonporous, which
self-assemble numerous functional species into an ordered structure. limits the transmission of active species across the catalyst
Herein, a well-known photoactive C3N4 moiety was integrated into an surface. Moreover, it is hard to know the structure–activity
HOF structure (PFC-42) with the merits of high porosity and crystal- relationship due to the lack of precise structural information,
linity. Under visible-light irradiation, the Pt nanoparticle-loaded PFC- and the amorphous form always brings in difficulties on precise
42 (PFC-42-Pt) continuously produces hydrogen from water in the structure control and modication.12 Therefore, it is not hard to
presence of scavengers with the evolution rate of 11.32 mmol gꢀ1
,
imagine that the self-assembly of photoactive ligands into
which is outstanding among all the reported Pt/porous composite a porous and ordered structure would promote the activity of
materials. The significantly high H2 evolution of PFC-42-Pt compared the catalytic species.
with that of amorphous analogue bulk C3N4–Pt and nanosheet C3N4–
HOFs, as a class of porous crystalline materials self-
Pt demonstrates that the ordered arrangement of photosensitizers assembled by organic ligands through hydrogen bonds, have
dramatically improves the photocatalytic activity of the material, drawn signicant interest for a wide range of applications such
which is further proved by the recrystallization experiment. This study as in gas separation13,14 and storage,15,16 sensing,17–19 biomedi-
represents the first example of HOF capable of photocatalysis, not cine,20,21 and photoluminescence.22,23 Despite their versatility,
only demonstrating the great application potentials of HOF in the application of HOFs in the eld of photocatalysis is still very
heterogeneous photocatalysis but also rendering an excellent rare. If one can integrate photoactive species into HOFs, the
opportunity to reveal structure–activity relations.
ordered structure combined with the tunable pore size and
large surface area should greatly benet the photocatalytic
performance. Moreover, the explicit structural information
could help in the establishment of structure–performance
relationships and provide insights into the photocatalytic
mechanism. In particular, exogenous catalytic species can be
stabilized inside the pores of HOFs via one-pot synthesis or
post-synthetic modication. Given these considerations, we are
encouraged to exploit HOF materials as photocatalysts.
In this study, we have successfully designed and synthesized
an organic linker comprising the well-known organic photo-
active carbon nitride unit (heptazine, abbreviated as C3N4). The
self-assembly of this ligand gave rise to a two-dimensional
layered HOF named as PFC-42 (PFC ¼ porous materials from
FJIRSM, CAS). The post functionalization of HOF afforded a Pt
nanoparticle well-dispersed HOF composite denoted as PFC-42-
Pt. The photocatalytic reaction revealed that PFC-42-Pt exhibi-
ted an excellent H2 production rate of 11.32 mmol gꢀ1 under
visible-light illumination (l > 400 nm). The bulk PFC-42-Pt
photocatalytic activity is much higher than that of bulk C3N4–
Pt, which is 3.5 times higher than that of nanosheet C3N4–Pt.
Introduction
Photocatalytic water splitting has been regarded as a feasible
strategy for the effective use of solar energy to solve increasingly
serious environmental and energy problems.1–3 Although most
reported photocatalysts for water splitting are based on inor-
ganic semiconductors,4–6 organic semiconductors such as
polymeric carbon nitride, poly(azomethine)s,7 conjugated
microporous polymers (CMPs),8,9 and linear conjugated poly-
mers10 have also demonstrated their promising potentials for
photocatalysis with the merits of abundant reserves and low
aCollege of Chemistry and Materials Science, Fujian Normal University, Fuzhou
350002, P. R. China. E-mail: tiu@irsm.ac.cn
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China
† Electronic supplementary information (ESI) available. CCDC 2035078. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1ta00100k
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Single-crystal (Fig. S1†) X-ray diffraction reveals that PFC-42 (NHE)), and the VB of the material was calculated to be z1.79 V
crystallizes in the trigonal space group P21/n (Table S1†). PFC- (NHE) based on the bandgap obtained from the Kubelka–Munk
42 exhibits a distinctive two-dimensional (2D) framework. plot. These results manifest that the band alignment of PFC-42
Each H3HTB molecule interacts with two neighboring ones meets the thermodynamic demands for the reduction half-
through two O–H/O hydrogen bonds, extending into a one- reactions of water spitting.
˚
dimensional layer (Fig. 1a). The O–H/O distance is 2.52 A,
Although bare PFC-42 powder is little active for water split-
falling into the hydrogen bond range according to literature ting, the activity is very low, as observed by other pure g-C3N4
˚
˚
(2.49 A to 3.15 A). These chains expand to a 2D square layer (sql) systems. To solve this problem, Pt was introduced into PFC-42
via van der Waals interactions. Adjacent 2D square layers (sql) as a cocatalyst via the post-synthetic metalation as usually
stack in the ABAB mode via intermolecular p–p interactions done in other photocatalysis reactions. The high magnication
˚
˚
(Fig. 1b), leaving one-dimensional channels of 5.52 A ꢁ 8.63 A transmission electron microscopy (TEM) image of PFC-42-Pt
along the c-axis (Fig. S2†). The PLATON analysis indicates that indicates Pt nanoparticles (PtNPs) with a diameter of about 3
3
˚
the simulated solvent-accessible void space is 765.2 A , which to 5 nm were evenly embedded in the porous PFC-42 (Fig. 3a
accounts for 26.5% of the entire unit cell volume. The experi- and b). Pt 4f core-level XPS spectra show that Pt(II) (Pt 4f7/2 peak
mental PXRD pattern of PFC-42 is consistent with the corre- at 72.5 eV) and Pt(0) species (Pt 4f7/2 peak at 70.9 eV) coexisted in
sponding theoretical pattern deduced from single-crystal X-ray the catalyst, which was probably caused by the incomplete
data, indicating the phase purity of the bulk PFC-42 (Fig. S3†). reduction of the Pt precursor or partial oxidation when exposing
Thermogravimetric analysis indicated that PFC-42 did not show the sample in the air (Fig. 3d).24,25 Moreover, the binding energy
obvious weight loss until 395 K (Fig. S4†). PXRD patterns of PFC- of N 1s (from heptazine) shied from 401.6 eV to 401.9 eV in
42 aer being immersed in methanol and water, respectively, XPS spectra aer Pt loading (Fig. S8†).26 These results suggest
match well with that of the as-synthesized one (Fig. S5†). All the that the PtNP in the framework probably attached to the hep-
above-mentioned results manifest that PFC-42 possesses good tazine nitrogen atoms and caused a decrease in the electron
thermal, water, and chemical stability.
density of N atoms. PXRD of PFC-42-Pt did not show any peak
As revealed by the UV/Vis absorption curve (Fig. S6†) and from Pt particles due to the small grain size, low content, and
Kubelka–Munk plot, the absorption edge of PFC-42 is 539 nm even distribution of Pt particles (Fig. S9†).
with a bandgap of 2.30 eV. As an efficient photocatalyst, the
The hydrogen evolution activities of PFC-42-Pt were evalu-
material needs to have not only an appropriate bandgap, but ated under visible-light irradiation in the presence of electron
also matched conduction band (CV) and valence band (VB) scavengers. The H2 production rates varied with the Pt loading
relative to the redox potential of the photocatalytic reaction. The amount in PFC-42 (Fig. 2b). Among which, 1.54% Pt loading
conduction-band position was estimated via the Mott–Schottky gave rise to the highest rate of 11.32 mmol gꢀ1 in 5 h. FT-IR
plot (Fig. 2a). The positive slope indicates a typical n-type (Fig. 2c), PXRD (Fig. S9†), and XPS analyses (Fig. S12†) before
semiconductor characteristic for PFC-42. The CB of PFC-42 and aer the catalytic reaction indicate that PFC-42 is a durable
was estimated to be zꢀ0.51 V (vs. normal hydrogen electrode photocatalyst for H2 evolution.
In order to manifest the merit of the ordered arrangement of
photosensitizers for photocatalytic activity, we also synthesized
the amorphous analogue of PFC-42, bulk C3N4 (Fig. S13†), and
nanosheet C3N4 (Fig. S14†) for comparison, followed by the
photocatalysis investigation. Aer the same Pt loading proce-
dure as conducted on PFC-42, the Pt-modied bulk C3N4
(named as bulk C3N4–Pt) barely showed any activity towards H2
evolution under the same reaction conditions. Nano-sheet C3N4
with PtNP loading (sheet C3N4–Pt) shows activity towards H2
production, which is much lower than PFC-42-Pt (Fig. 3c). These
results indicate that the ordered arrangement of the photo-
sensitizer combined with a large accessible surface greatly
boosts the photocatalytic activity of the material. This can also
be conrmed by the phenomena that detracting the crystallinity
and porosity of PFC-42-Pt via the acid treatment would
dramatically decrease its catalytic activity (Fig. S15†). The crys-
tallinity of the acid-damaged PFC-42-Pt can be repaired by
simply recrystallizing in methanol, and the photocatalytic
activity was recovered to 85.6% of its original rate (Fig. S16†).
Therefore, we demonstrated that the ordered arrangement of
photosensitizers greatly boosts the photocatalytic activity and
indicates the unique advantage of easy recovery and recycla-
bility of HOF catalysts.
Fig. 1 (a) Crystal structure of PFC-42 composed of heptazine-based
building blocks. Color codes: blue N, red O, grey C. (b) ABAB packing
mode of PFC-42 viewed along the b-axis.
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Fig. 2 (a) Mott–Schottky plot of the PFC-42 electrode and the estimated band edge positions of PFC-42 (inset: band structure diagram). (b) The
photocatalytic H2 evolution rate of PFC-42 with different Pt loading amounts. (c) FT-IR spectra of PFC-42 before and after photocatalysis.
distributions with the average lifetimes of about 80.54 ns, 122.5
ns, and 41.05 ns, respectively. The much longer lifetime of PFC-
42 than that of amorphous ligand again indicates that the
ordered arrangement of C3N4 building units signicantly
promotes the electron transfer. Pt loading in PFC-42 causes
a noticeable decrease in the lifetime, suggesting the possible
electron transfer between PFC-42 and Pt. Moreover, photo-
electrochemical (PEC) measurements, including photocurrent
response (Fig. 4c) and electrochemical impedance spectra (EIS)
(Fig. 4d), further implied that PFC-42-Pt enabled effective
charge separation during the photocatalysis reaction.
Based on the above discussion, the possible mechanism of
PFC-42 for water reduction is illustrated and shown in Fig. 5.
First, It should be pointed out that the Pt loaded on the surface
of PFC-42 is not active for water reduction under light irradia-
tion; however, it can act as a cocatalyst to accelerate the charge
separation of the PFC-42 photocatalyst and the surface catalysis
reaction kinetics.29 Under light irradiation, photogenerated
carriers are separated by the strong electron coupling of two-
Fig. 3 (a) TEM micrographs of PFC-42 with the uniform distribution of
Pt nanoparticles. (b) High-resolution TEM image of 3–5 nm Pt nano-
particles on PFC-42, showing the planes of Pt with the d spacing of
0.229 nm. (c) Photocatalytic H2 generation activity of bulk C3N4–Pt,
sheet C3N4–Pt, and PFC-42-Pt under the same reaction conditions.
(d) High-resolution Pt 4f XPS spectrum.
The inuence of the cocatalyst on the photophysical property
of PFC-42 was further evaluated by the steady-state photo-
luminescence (PL) study (Fig. 4a). PFC-42-Pt showed signi-
cantly lower PL intensity compared with the pristine PFC-42 and
the amorphous ligand, implying that the long-term ordered
arrangement of photosensitizers and the incorporation of the Pt
cocatalyst can effectively accelerate the photogenerated charge
separation and transfer. Moreover, the superposition of two-
dimensional PFC-42 layers is expected to induce strong elec-
tron coupling between the overlapped hybrid orbitals and form
a larger conjugate system, thus facilitating photoinduced
carrier transportation,27,28 which should be another factor
contributing to the excellent photocatalytic performance of
PFC-42-Pt.
Fig. 4 (a) Room temperature steady-state photoluminescence (PL)
spectra under 420 nm excitation for the ligand, PFC-42, and PFC-42-
Pt. (b) PL decay spectra monitored at 563 nm under 375 nm excitation
for the ligand, PFC-42, and PFC-42-Pt. (c) Electrochemical impedance
spectra of the ligand, PFC-42, and PFC-42-Pt. (d) Transient photo-
current response for the ligand, PFC-42, and PFC-42-Pt.
To evaluate the efficient interfacial charge transfer, time-
resolved uorescence decay spectroscopy was performed to
provide information about the average lifetime of photo-excited
electrons (Fig. 4b). The uorescence decay curves of the PFC-42-
Pt, PFC-42, and ligand are in accordance with two exponential
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China (21871267, 21802142, 22071246), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB20000000).
Notes and references
1 C. Gao, J. Low, R. Long, T. Kong, J. Zhu and Y. Xiong, Chem.
Rev., 2020, 120, 12175–12216.
2 Y. Qi and F. Zhang, Joule, 2020, 4, 1364–1366.
3 Z. Wang, C. Li and K. Domen, Chem. Soc. Rev., 2019, 48,
2109–2125.
4 Y.-J. Gao, X.-B. Li, X.-Z. Wang, N.-J. Zhao, Y. Zhao, Y. Wang,
Z.-K. Xin, J.-P. Zhang, T. Zhang, C.-H. Tung and L.-Z. Wu,
Matter, 2020, 3(2), 571–585.
5 X.-B. Li, C.-H. Tung and L.-Z. Wu, Nat. Rev. Chem., 2018, 2(8),
160–173.
6 X.-B. Fan, S. Yu, X. Wang, Z.-J. Li, F. Zhan, J.-X. Li, Y.-J. Gao,
A.-D. Xia, Y. Tao, X.-B. Li, L.-P. Zhang, C.-H. Tung and
L.-Z. Wu, Adv. Mater., 2019, 31, 1804872.
Fig.
5 The proposed mechanism for visible-light-driven photo-
catalytic H2 production with PFC-42.
7 M. G. Schwab, M. Hamburger, X. Feng, J. Shu, H. W. Spiess,
X. Wang, M. Antonietti and K. Muellen, Chem. Commun.,
2010, 46, 8932–8934.
8 C. Yang, B. C. Ma, L. Zhang, S. Lin, S. Ghasimi, K. Landfester,
K. A. I. Zhang and X. Wang, Angew. Chem., Int. Ed., 2016, 55,
9202–9206.
9 L. Wang, X. Zheng, L. Chen, Y. Xiong and H. Xu, Angew.
Chem., Int. Ed., 2018, 57, 3454–3458.
10 R. S. Sprick, B. Bonillo, R. Clowes, P. Guiglion,
N. J. Brownbill, B. J. Slater, F. Blanc, M. A. Zwijnenburg,
D. J. Adams and A. I. Cooper, Angew. Chem., Int. Ed., 2016,
128, 1824–1828.
dimensional PFC-42 between the overlapped hybrid orbitals
and transferred to the surface of the photocatalyst through
a crystallized delocalized system.30 Then, photogenerated elec-
trons can be transferred to PtNP due to Pt loading in PFC-42
causing a noticeable decrease in the lifetime, highest photo-
current response, and lowest electrochemical impedance
spectra. The deposited Pt will be used as the active site for
hydrogen production; moreover, the hole is consumed by TEOA,
leading to the restraining of the recombination.
11 X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin,
J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater.,
2009, 8, 76–80.
Conclusions
In summary, for the rst time, a novel heptazine-based HOF
PFC-42 has been successfully constructed and carefully char-
acterized. With loading PtNP as the cocatalyst, the obtained
material PFC-42-Pt shows excellent photocatalytic hydrogen
evolution activity (11.32 mmol gꢀ1). The signicantly high H2
evolution of PFC-42-Pt compared with that of amorphous
analogue bulk C3N4–Pt and nanosheet C3N4–Pt along with
recrystallization experiment demonstrate that the ordered
arrangement of photosensitizers greatly boosts the photo-
catalytic activity of the material. We believe that this study as the
rst HOF material for photocatalysis will open a new door for
the application of HOFs in the photocatalytic eld.
12 V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier,
´
R. Silbey and J.-L. Bredas, Chem. Rev., 2007, 107, 926–952.
13 Y. He, S. Xiang and B. Chen, J. Am. Chem. Soc., 2011, 133,
14570–14573.
14 Y. L. Li, E. V. Alexandrov, Q. Yin, L. Li, Z.-B. Fang, W. Yuan,
D. M. Proserpio and T.-F. Liu, J. Am. Chem. Soc., 2020, 142,
7218–7224.
15 D. W. Kang, M. Kang, H. Kim, J. H. Choe, D. W. Kim,
J. R. Park, W. R. Lee, D. Moon and C. S. Hong, Angew.
Chem., Int. Ed., 2019, 58, 16152–16155.
16 A. Pulido, L. Chen, T. Kaczorowski, D. Holden, M. A. Little,
S. Y. Chong, B. J. Slater, D. P. McMahon, B. Bonillo,
C. J. Stackhouse, A. Stephenson, C. M. Kane, R. Clowes,
T. Hasell, A. I. Cooper and G. M. Day, Nature, 2017, 543,
657–664.
Conflicts of interest
There are no conicts to declare.
17 I. Hisaki, J. Inclusion Phenom. Macrocyclic Chem., 2020, 96,
215–231.
18 I. Hisaki, Y. Suzuki, E. Gomez, Q. Ji, N. Tohnai, T. Nakamura
and A. Douhal, J. Am. Chem. Soc., 2019, 141, 2111–2121.
Acknowledgements
Authors gratefully thank for the support from the National Key 19 B. Wang, R. He, L. H. Xie, Z. J. Lin, X. Zhang, J. Wang,
Research
and
Development
Program
of
China
H. Huang, Z. Zhang, K. S. Schanze, J. Zhang, S. Xiang and
(2018YFA0208600), National Natural Science Foundation of
B. Chen, J. Am. Chem. Soc., 2020, 142, 12478–12485.
4690 | J. Mater. Chem. A, 2021, 9, 4687–4691
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20 W. Liang, F. Carraro, M. B. Solomon, S. G. Bell, 25 S. Bi, C. Yang, W. B. Zhang, J. S. Xu, L. M. Liu, D. Q. Wu,
H. Amenitsch, C. J. Sumby, N. G. White, P. Falcaro and
C. J. Doonan, J. Am. Chem. Soc., 2019, 141, 14298–14305.
X. C. Wang, Y. Han, Q. F. Liang and F. Zhang, Nat.
Commun., 2019, 10, 10.
21 Q. Yin, P. Zhao, R. J. Sa, G. C. Chen, J. Lu, T. F. Liu and 26 Q. Zuo, T. Liu, C. Chen, Y. Ji, X. Gong, Y. Mai and Y. Zhou,
R. Cao, Angew. Chem., Int. Ed., 2018, 57, 7691–7696. Angew. Chem., Int. Ed., 2019, 58, 10198–10203.
22 S. Cai, H. Shi, Z. Zhang, X. Wang, H. Ma, N. Gan, Q. Wu, 27 X. Ding, J. Guo, X. Feng, Y. Honsho, J. Guo, S. Seki,
Z. Cheng, K. Ling, M. Gu, C. Ma, L. Gu, Z. An and
W. Huang, Angew. Chem., Int. Ed., 2018, 57, 4005–4009.
P. Maitarad, A. Saeki, S. Nagase and D. Jiang, Angew.
Chem., Int. Ed., 2011, 50, 1289–1293.
23 B. T. Liu, E. P. Liu, R. J. Sa and T. F. Liu, Adv. Opt. Mater., 28 T. Banerjee, K. Gottschling, G. Savasci, C. Ochsenfeld and
2020, 8, 2000281. B. V. Lotsch, ACS Energy Lett., 2018, 3, 400–409.
24 G. Zhang, Z. A. Lan, L. Lin, S. Lin and X. Wang, Chem. Sci., 29 Y. Xiao, Y. Qi, X. Wang, X. Wang, F. Zhang and C. Li, Adv.
2016, 7, 3062–3066.
Mater., 2018, 30, 1803401.
30 Z. Zhang, Y. Zhu, X. Chen, H. Zhang and J. Wang, Adv.
Mater., 2019, 31, 1806626.
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