A novel Co-O cluster based coordination polymer for efficient hydrogen production photocatalysis

Article abstract of DOI:10.1016/j.jphotochem.2019.112137

The development of efficient and economical catalysts for photocatalytic water splitting and hydrogen production is one of the promising methods to alleviate the current energy crisis. In this work, we synthesized a novel Co-O cluster based coordination p

Full text of DOI:10.1016/j.jphotochem.2019.112137

JournalofPhotochemistry&PhotobiologyA:Chemistry387(2020)112137
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A novel Co-O cluster based coordination polymer for efficient hydrogen
production photocatalysis
Mei-Juan Weia, Jian-Hua Zhanga, Wei-Ming Liaob, Zhang-Wen Weia, Mei Pana,⁎, Cheng-Yong Sua
a MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275,
China
b School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The development of efficient and economical catalysts for photocatalytic water splitting and hydrogen pro-
duction is one of the promising methods to alleviate the current energy crisis. In this work, we synthesized a
novel Co-O cluster based coordination polymer, [Co2(HL)2·(DMF)2·H2O]·solvents (Co-CP) by a facile sol-
vothermal method from amino-functionalized ligand and non-precious cobalt salt. The Co-CP possesses a
bandgap of 2.65 eV, whose conduction and valence band are -0.8 and 1.85 eV, respectively. Efficiently, the
visible-light driven photocatalytic hydrogen evolution rate of Co-CP reaches 1778 μmol g−1 h−1, due to the
rapid charge transfer and efficient suppression of electron-hole recombination as determined by photocurrent
response and photoluminescence measurement. Possible photocatalytic mechanisms are proposed, for which the
Co-O clusters are predicted to be the active sites.
Coordination polymer
Non-precious photocatalyst
Hydrogen production
1. Introduction
precious metal catalysts for water splitting, while its practical industrial
application suffers from such problems as instability in the acid-base
With the fast growing of global energy crisis, hydrogen stands out as
one of the green renewable energy sources most likely to replace tra-
ditional fossil energy [1]. Since Fujishima firstly achieved photo-
catalytic hydrogen evolution from water by using TiO2 in 1972 [2],
photocatalytic water splitting has been proved to be an efficient and
feasible way to transform the endless solar energy into clean chemical
energy. However, semiconductor catalysts such as TiO2, CdS, BiVO4, g-
C3N4, and so on [3–5], often suffer from insufficient activity due to their
large bandgaps, which can only absorb ultraviolet light, accounting for
only 4% of the whole solar spectrum.
Coordination polymers (CPs), a kind of crystalline material self-as-
sembled from metal ions and organic linkers, have been widely studied
in gas adsorption and separation, sensing, drug delivery, etc [6–10].
Benefiting from the adjustable porous structure, functionally modified
ligands, and uniformly dispersed active sites, CPs can serve as potential
photocatalysts for hydrogen production [11]. In attempt to obtain
photocatalytically active CPs, there are multiple strategies as proposed
in the literatures. One of the strategies is to design CPs containing
precious metal catalytic centers, such as Pt-, Ru-, Rh-, and so on, by in-
situ or post-synthetic method [12,13]. Another strategy is to use non-
precious metal-based CPs, which is definitely more economical. Among
which, cobalt-based complexes are proved to be the most versatile non-
environment and non-recyclability, which should be solved [19,20]. On
the other hand, there is another strategy to improve the photocatalytic
activity of CPs by applying organic ligand with visible light absorbing
abilities, such as dye-sensitized ligands [14], porphyrins [15], and
amino-functionalized ligands, etc [16–18].
Inspired by the researches mentioned above, we designed and
synthesized a Co(II)-CP that can absorb visible light by using earth-rich
metal cobalt and amino-modified ligand (2′-amino-[1,1′:4′,1″-ter-
phenyl]-3,3″,5,5″- tetracarboxylic acid (H4L)). The photocatalytic hy-
drogen production of Co-CP under visible light irradiation and involved
mechanisms were explored in detail.
2. Experimental sections
2.1. Chemicals and materials
2,5-dibromoaniline was purchased from Sigma-Aldrich. 3, 5-bis
(methoxycarbonyl) benzeneboronic acid pinacolester was purchased
from UCHEM Inc. Cesium fluoride, tetrakis (triphenylphosphine) pal-
ladium and Co(NO3)2·6H2O were purchased from Alfa aesar. Ru
(bpy)3Cl2 was purchased from HWRK Chem. Triethanolamine (TEOA)
was purchased from Sigma-Aldrich. All chemicals and solvents
⁎ Corresponding author.
E-mail address: panm@mail.sysu.edu.cn (M. Pan).
https://doi.org/10.1016/j.jphotochem.2019.112137
Received 13 August 2019; Received in revised form 25 September 2019; Accepted 2 October 2019
Availableonline04October2019
1010-6030/©2019ElsevierB.V.Allrightsreserved.

M.-J. Wei, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry387(2020)112137
purchased from the commercial sources were of AR grade and used
without further purification.
2.2. Characterization and methods
Electron Paramagnetic Resonance (EPR) experiments were carried
out by using Bruker Electron ParaMagnetic Resonance (A300). The
powder sample was sealed into the capillary and tested at X-band mi-
crowave frequency in Continuous Wave (CW) mode at 100 K. Elemental
analysis (EA) measurement was performed on a Vario EL CHNS ele-
mental analyzer. The UV–vis diffuse reflectance spectra were recorded
on a SHIMADZU UV-3600. Photoluminescence spectra were tested by
an EDINBURGH FLS980 fluorescence spectrophotometer. FT-IR spectra
were performed on a Nicolet/Nexus-670 spectrometer in the range of
4000-500 cm−1. The thermogravimetric analysis (TGA) curve was re-
corded on a STA 449 F3 Jupiter instrument under nitrogen flow at a
heating rate of 5 °C per minute. Powder X-ray diffraction (PXRD) were
measured on a Rigaku SmartLab X-ray diffractometer with Cu–Kα ra-
diation (λ =1.54056 Å) at room temperature. Single-crystal X-ray
crystallography (SCXRD) data were collected on a Rigaku Oxford
SuperNova X-RAY diffractometer system equipped with a Cu sealed
tube (λ =1.54184 Å) at 150 K. X-ray photoelectron spectroscopy (XPS)
was performed on a Thermo Fisher Scientific Nexsa with a monochro-
matic Al Kα X-ray source.
2.3. Synthesis of [Co2(HL)2·(DMF)2·H2O]·solvents (Co-CP)
The
ligand
2′-amino-[1,1′:4′,1″-terphenyl]-3,3″,5,5″-
tetra-
carboxylic acid (H4L) was synthesized according to the previous lit-
erature [21], and characterized by 1H NMR (400 MHz, DMSO-d6) δ
13.38 (s, 4 H), 8.47 – 8.42 (m, 2 H), 8.40 (d, J =1.5 Hz, 2 H), 8.23 (d, J
=1.5 Hz, 2 H), 7.23 (d, J =1.6 Hz, 1 H), 7.20 (d, J =7.9 Hz, 1 H), 7.05
(dd, J = 7.9, 1.6 Hz, 1 H), 5.20 (s, 2 H). The Co-CP was synthesized by a
mild solvothermal method. Briefly, H4L (0.0105 g, 0.025 mmol) and Co
(NO3)2·6H2O (0.0291 g,0.1 mmol) were dissolved in a DMF-methanol-
H2O (v/v/v 5/2/1) mixture solution (8 mL) and ultrasound for 30 min,
which was then sealed in a 20-mL Teflon reactor and heated in an oven
at 85 °C for 24 h. After being centrifuged and washed with DMF and
water for several times, the obtained purple crystals were finally dried
at 80 °C for 2 4 h in a vacuum oven. Among these crystals, the appro-
priate one was selected for SC-XRD measurement (Yield: 62.3%, based
on
H4L).
EA
calcd.
for
C34H51Co2N5O19
([Co2(HL)2·(DMF)2·H2O]·DMF·6H2O): C 42.90%, H 5.36%, N 7.36%;
found: C 42.78%, H 5.39%, N 7.07%. (CCDC number: 1945367).
Fig. 1. The crystal structure of Co-CP: a) coordination mode of Co-O clusters; b)
linking mode of the ligand; c) 3D packing mode along a axis. Color code: red, O;
gray C; blue N; purple Co; H atoms are omitted for clarity.
2.4. Photocatalytic hydrogen evolution
Typically, the photocatalytic experiments were carried out in a
55 mL Pyrex reactor, in which 4 mg of Co-CP as photocatalyst, 4 mg of
Ru(bpy)3Cl2 as photosensitizer (PS) and 1 mL of TEOA as sacrificial
agent were dispersed in 5 mL of 4:1 (v/v) mixed solution of DMF-H2O.
Before irradiation, the suspension was purged by nitrogen for at least
20 min. Then the resultant mixtures were irradiated by a white LED
lamp (λ = 420–780 nm, 100 mW cm−2) and stirred, cooling with the
flowing air. After the reaction, 200 μL headspace samples were ana-
lyzed for H2 concentration by a gas chromatograph (GC9790, Fuli
Analytical Instrument Co., Ltd) equipped with TCD detector.
2 mg of the Co-CP were dispersed into a mixed solution with 20 μL
Nafion and 2 mL of ethanol to form a suspension, which was then ap-
plied by drop coating onto a 0.5 cm*0.5 cm indium-tin oxide (ITO) glass
electrode. As for photocurrent tests, a 300 W Xenon lamp with a UV-cut
off filter (λ > 420 nm) was used as light source. Mott-Schottky tests
were performed with the potentials ranging from -0.8 to 0.8 V at dif-
ferent frequencies (500, 1000, and 1500 Hz, respectively), measuring
by Impedance-Potential technique. The amplitude was fixed at 10 mV.
Cyclic voltammetry (CV) measurements were also carried out with a
three-electrode system, in which 0.1 M tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) was used as electrolyte in DMF solvent.
2.5. Electrochemical measurement
The photoelectrochemical measurement was performed on a CHI
660e electrochemical work station (Chenhua Instrument, Shanghai,
China) in a standard three-electrode system with a platinum plate as the
counter electrode, a standard Ag/AgCl (saturated KCl) as a reference
electrode and a photocatalyst-coated ITO as the working electrode. A
0.2 M Na2SO4 aqueous solution was used as electrolyte. To be specific,
3. Results and discussion
3.1. Crystal structure and characterization
Co-CP was obtained by the mild reaction of Co(NO3)2·6H2O and H4L
2

M.-J. Wei, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry387(2020)112137
(2′-amino-[1,1′:4′,1′'-terphenyl]-3,3′',5,5′'-tetracarboxylic acid) ligand
in DMF-Methanol-H2O mixed solvent at 85 °C for 24 h. Single-crystal X-
ray crystallography shows a 3D framework crystallized in a chiral
monoclinic space group P21 (Table S1). As shown in Fig. 1a, there are
two different coordination environments for Co2+ ions. Co1 is hexa-
coordinated by six oxygen atoms, four of which are from chelating
coordination carboxylate groups, and the other two are from mono-
dentate coordinated carboxylate groups. Co2 is also hexa-coordinated
by six oxygen atoms, while one of them comes from a coordinated
water molecule, two from DMF, and the remaining three oxygen atoms
are from monodentate coordinated carboxylate groups. The thus
formed {Co2O6} clusters are linked by the tetrapodal organic ligands
(Fig. 1b). In general, a chiral network is formed in the crystal structure
(Fig. 1c). However, since crystals with opposite chirality are co-existed,
the bulk samples are racemic as a whole. Meanwhile, as calculated by
PLATON software [22], after removing solvent, the total potential
solvent accessible void volume is 1029.1 Å, which accounts for about
40.2% of the total crystal volume. To investigate the phase purity of Co-
CP, powder X-ray diffraction (PXRD) experiment was performed, whose
diffraction peaks matched well with the simulated one (Fig. S1).
Fourier transform infrared (FT-IR) spectrum was measured to prove
the coordination effect between cobalt ions and organic ligands.
Compared with the free ligand, the hydroxyl vibration peak of −COOH
ranging from 2500 to 3200 cm−1 disappears in the Co-CP, and the
stretching vibration peak of the carbonyl group shifts to a low wave
number at around 1663∼ 1623 cm−1 (Fig. S2). According to the
thermogravimetric analysis curve, the first weight loss peak appeared at
around 100 °C due to the loss of the coordinated and free water mole-
cules in the framework. The second weight loss peak appeared at
around 150 °C should be attributed to the departure of DMF molecules.
And the framework collapse of Co-CP occurred up to 450 °C, revealing
the excellent thermal stability (Fig. S3). Co-CP also exhibited good
solvent stability. There was no significant change in the powder dif-
fraction peaks after soaking for 12 h in DMF, acetonitrile, acetone,
ethanol, trimethylamine, and DMF/H2O mixed solvent (v/v 4/1) (Fig.
S4).
3.2. Optical and electrochemical properties
As shown in UV–vis diffuse reflectance spectra (Fig. 2a), H4L owns
an absorption band edge at 450 nm, which can be attributed to the K
and B bands of aromatic group. Compared with the ligand, Co-CP
possess a broad absorption in the visible region ranging from 420 to
650 nm due to the typical d-d transition absorption of Co2+ ions. Also,
the introduction of the amino groups in the ligand also results in the
strong visible-light absorption [23]. Calculated by the Kubelka-Munk
method, the bandgap is about 2.65 eV for Co-CP, as shown in the Tauc
plot curve (Fig. S5). For photocatalytic hydrogen production, in addi-
tion to having a suitable bandgap to separate photo-generated electrons
and holes, the matching positions of the conduction band (CB) and
valence band (VB) are critical. Mott-Schottky measurements were used
to confirm the energy levels, which indicated the n-type property of the
Co-CP and the flat band position was determined to be -0.90 V vs. Ag/
AgCl (-0.70 V vs. NHE, Fig. 2b). In general, the bottom of the conduc-
tion band of a n-type semiconductor is 0.10 V more negative than the
flat band position [24], therefore the bottom of the conduction band of
Co-CP is estimated to be -0.80 V, which is more negative than the redox
potential of H+/H2. Combining the bandgap of 2.65 eV obtained by
UV–vis diffuse reflectance spectra and the CB, the valence band position
should be 1.85 eV, which was well corroborated from the XPS spectra
(Fig. S6). Considering the remarkable light absorption and suitable
band position like semiconductors, Co-CP is a kind of potential catalyst
for photocatalytic hydrogen evolution.
Fig. 2. a) UV–vis diffuse reflectance spectra of Co-CP and H4L ligand; b) Mott-
Schottky plots of the Co-CP. The illustration shows the predicted band edge
positions. c) Photocatalytic hydrogen production efficiencies of Co-CP. Reaction
conditions: Solvent, 4 mL DMF and 1 mL H2O; 1 mL TEOA as sacrificial reagent;
4 mg Co-CP as photocatalyst, 4 mg Ru(bpy)3Cl2 as photosensitizer, irradiated by
white-light LED lamp with a power of 100 mW·cm−2
.
3.3. Visible-light photocatalytic hydrogen production and proposed
mechanism
The Co-CP powder was evaluated for visible-light driven photo-
catalytic water splitting to produce hydrogen at room temperature in
the presence of TEOA serving as sacrificial agent and Ru(bpy)3Cl2 as
photosensitizer under a white-light LED lamp (100 mw·cm−2). As
shown in Fig. 3c, comparative tests manifest that without light, pho-
tosensitizer, or sacrificial agent, there was no hydrogen product de-
tected. Without water or catalyst, only traces of hydrogen was ob-
served, which was ascribed to proton reduction from the sacrificial
agent. It can be seen that the Co-CP, light irradiation, photosensitizer,
sacrificial agent and water all play decisive role in the hydrogen pro-
duction process. Furthermore, the solvent ratio of DMF to H2O was
adjusted to investigated the photocatalytic hydrogen production effi-
ciencies, in which Co-CP showed the best catalytic performance in the
3

M.-J. Wei, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry387(2020)112137
there was no signal of the low spin (S = 1/2) CoII species occurred to
both two samples, disclosing the weak 6-coordination in Co-CP. While
exposing Co-CP to visible light for photocatalysis, an intense new signal
with a g value of 3.49 (at around 120 m T) was observed, which is
characteristic of an octahedral high-spin (S = 3/2) CoII species. The
photo-induced EPR signal indicated that there was charge transfer in
the Co-CP during photocatalysis [29].
The cyclic voltammetry (CV) measurement was performed on a
DMF solution of 0.1 M Bu4NPF6 and the Co-CP coated electrode, which
revealed the photo-induced electron transition inside the catalyst. As
shown in Fig. 3d, there was two irreversible reduction peaks at -2.21 V
and -1.47 V, respectively, which could be ascribed to the signal electron
transfer from [CoIICoII] to [CoICoII] and then [CoICoII] to [CoICoI] [16].
It was generally believed that in the photocatalytic process, the pho-
tosensitizer [Ru(bpy)3]2+ underwent an excited state [Ru(bpy)3]2+* to
quenching process, possibly by oxidative quenching or reduction
quenching pathway [18]. In particular, the excited state [Ru(bpy)3]2+
*
Fig. 3. a) PL emission spectra of Co-CP and H4L ligand. b) XPS spectra of Co-CP.
c) CW EPR spectra of pristine Co-CP and the Co-CP after photocatalytic hy-
drogen production. d) The Cyclic voltammogram of Co-CP. Measured condi-
tions: Ag/AgCl was used as the reference electrode, indium-tin oxide (ITO) glass
electrode was used as the working electrode, and a platinum plate was used as
counter electrode; electrolyte, a DMF solution of Bu4NPF6 (0.1 M); scan rate,
0.1 V/s.
might be oxidized by the Co-CP catalyst, forming [Ru(bpy)3]3+ species,
or might be reduced by the sacrificial agent TEOA to form [Ru(bpy)3]1+
species, both of which accompanied with an electron-transfer (ET)
process. In attempt to verify the quenching pathway of [Ru(bpy)3]2+*,
the photoluminescence measurements of original Ru(bpy)3Cl2 solution
and the solution with added 1 mL of TEOA were carried out (Fig. S11).
There was no fluorescence quenching but a fluorescence enhancement
occurred while adding 1 mL of TEOA into original mixed solution, in-
dicating an oxidatively quenching pathway for the excited state [Ru
(bpy)3]2+*. Additionally, comparing the UV–vis diffuse reflectance
spectra of Co-CP and the emission spectrum of [Ru(bpy)3]2+, partial
spectra overlap illustrated a feasible energy-transfer (EnT) from [Ru
(bpy)3]2+ to the catalyst Co-CP (Fig. S12) [18].
DMF/H2O (4/1, v/v) mixed solvent (Fig. S7). Through a series of
control experiments, it was found that triethanolamine (TEOA) worked
best as a sacrificial agent (Fig. S8). Screening the above conditions,
4 mg Co-CP, 4 mg Ru(bpy)3Cl2, and 1 mL TEOA were added into 5 mL of
DMF/H2O (4/1, v/v) pre-mixed solvent and exposed to visible light for
photocatalytic hydrogen evolution, which could reach an activity of
1778 μmol g-1 h-1 at 3 h. The Co-CP-catalyzed HER reaction was further
investigated by recycling experiments, in which Co-CP was centrifuged
from the catalytic reaction solution, washed with DMF for 3 times, and
redistributed in the fresh solvent after every 2 h illumination. As shown
in Fig. S9, Co-CP maintained catalytic activity within three cycles, in-
dicating that the catalyst is recyclable and stable for photocatalytic
hydrogen.
In attempt to study the separation and recombination of photo-in-
duced electron-hole pairs in the catalyst, photoluminescence (PL)
measurements were performed on Co-CP and H4L ligand. Both excited
by 460 nm, H4L ligand presented a maximum emission peak at 575 nm,
while the PL of the Co-CP was almost quenched, showing the rapid
transfer of photo-generated electrons and lower recombination rate of
the photo-induced electron-hole (Fig. 3a). Furthermore, the photo-
current response of Co-CP was recorded versus Ag/AgCl under visible
light. As soon as the light was turn on, a fast and apparent current signal
was observed, while the signal decayed rapidly with the light turned
off. The fast, uniform and invertible photocurrent response shown in
Fig. S10 suggested the efficient separation of photo-generated charge
carriers in Co-CP.
Taking into account the above factors, we propose the possible
mechanism of Co-CP photocatalytic hydrogen production as following.
In the presence of the visible light, the photosensitizer [Ru(bpy)3]2+
was excited to from photoactive [Ru(bpy)3]2+*, which then underwent
an oxidatively quenching pathway accompanied by the generation of
[Ru(bpy)3]3+ species. The excited photoelectron formed from the
LUMO state of Ru(bpy)3Cl2 transfer to the unsaturated CoII sites. The
Co2+ in the catalyst Co-CP was reduced from the [Ru(bpy)3]2+* to
form an intermediate state [CoIICoI] or [CoIICoI], and the [Ru(bpy)3]3+
immediately obtained electron from sacrificial agent TEOA, achieving a
loop. Finally, the protons gathered on the catalyst surface by hydrogen
bonding react with the photoelectrons on the unsaturated CoII cites to
continuously generate hydrogen. In general, Co-CP provides a good
platform to promote the transfer of photoelectrons and inhibit the
combination of photo-generated electron-holes of the Ru(bpy)3Cl2. The
photocatalytic process is shown in Scheme 1.
High-resolution X-ray photoelectron spectroscopy (XPS) measure-
ment was used to probe the surface chemical state of cobalt in Co-CP. As
shown in Fig. 3b, the peaks at 781.3 and 797.5 eV were attributed to Co
2p3/2 and Co 2p1/2 core level photo emission. And the two adjacent
satellite peaks at 785.8 and 801.9 eV were originated from Co2+ shake-
up excitation [25,26], which indicating the existence of Co2+ species in
Co-CP. To elucidate the mechanism of the photocatalytic process,
continuous wave (CW) EPR spectroscopy was used to determine the
active intermediates formed during visible-light irradiation over the Co-
CP at 100 K. Recorded at X-band microwave frequency (9.85 GHz), the
CW EPR spectra of pristine Co-CP in the dark and after visible-light
driven photocatalytic hydrogen production were presented in Fig. 3c.
According to literature, the downfield signal at g = 2.04 both occurred
to the pristine Co-CP and the catalyzed sample can be attributed to the
spatially confined amino-groups in the Co-CP [27,28]. Furthermore,
Scheme 1. Plausible mechanism for visible-light driven photocatalytic hy-
drogen production by Co-CP.
4

M.-J. Wei, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry387(2020)112137
4. Conclusion
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To sum up, we synthesized a new amino-functionalized 3D cobalt
coordination polymer (Co-CP) with efficient photocatalytic hydrogen
production ability under visible-light irradiation. Benefiting from the
appropriate bandgap of the framework and the rapid charge transfer as
well as the efficient suppression of electron-hole recombination, Co-CP
exhibits a photocatalytic activity of 1778 μmol g−1 h−1. This study
might promote further exploration for the design and application of
noble-metal-free coordination polymers in artificial photosynthesis.
Declaration of Competing Interest
No conflict of interests are reported.
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This work was supported by NSFC (21771197, 21890380,
21821003,21573291), Local Innovative and Research Teams Project of
Guangdong Pearl River Talents Program (2017BT01C161), and FRF for
the Central Universities.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.
112137.
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