Embedding Ultrasmall Au Clusters into the Pores of a Covalent Organic Framework for Enhanced Photostability and Photocatalytic Performance

Article abstract of DOI:10.1002/anie.201916154

Gold clusters loaded on various supports have been widely used in the fields of energy and biology. However, the poor photostability of Au clusters on support interfaces under prolonged illumination usually results in loss of catalytic performance. Covalent organic frameworks (COFs) with periodic and ultrasmall pore structures are ideal supports for dispersing and stabilizing Au clusters, although it is difficult to encapsulate Au clusters in the ultrasmall pores. In this study, a two-dimensional (2D) COF modified with thiol chains in its pores was prepared. With ?SH groups as nucleation sites, Au nanoclusters (NCs) could grow in situ within the COF. The ultrasmall pores of the COF and the strong S?Au binding energy combine to improve the dispersibility of Au NCs under prolonged light illumination. Interestingly, Au–S–COF bridging as observed in this artificial Z-scheme photocatalytic system is deemed to be an ideal means to increase charge-separation efficiency.

Full text of DOI:10.1002/anie.201916154

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Bridging Ultrasmall Au Clusters into the Pores of a Covalent
Organic Framework for Enhanced Photostability and
Photocatalytic Performance
Authors: Yang Deng, Zhen Zhang, Peiyao Du, Xingming Ning,
Yue Wang, Dongxu Zhang, Jia Liu, Shouting Zhang, and
Xiaoquan Lu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201916154
Angew. Chem. 10.1002/ange.201916154
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10.1002/anie.201916154
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RESEARCH ARTICLE
Bridging Ultrasmall Au Clusters into the Pores of a Covalent
Organic Framework for Enhanced Photostability and
Photocatalytic Performance
Yang Deng,[a] Zhen Zhang,[a] Peiyao Du,*[a] Xingming Ning,[a,b] Yue Wang,[a] Dongxu Zhang,[a] Jia Liu,[a]
Shouting Zhang,[a] and Xiaoquan Lu*[a,b]
Dedication ((optional))
Abstract: Gold (Au) clusters loading on various supports were
recent years, as a promising metal supports, covalent organic
widely used in the fields of energy and biology. However, the poor frameworks (COFs) have attracted widely attention and been
photostability of Au clusters on support interface under longtime continually developed.[7] Compared with some traditional
illumination usually results in loss of catalytic performance. Covalent supports (g-C3N4, metal oxide, etc.), the highly ordered structure
organic frameworks (COFs) with periodic and ultrasmall pore
and ultrasmall pores of COFs not only can limit the growth of
structures are ideal supports for dispersing and stabilizing Au NCs but also realize uniform dispersion of NCs on COF
clusters, while it is difficult to encapsulate Au clusters into the supports.[8] Via a simple post-treatment, Wang and co-workers
ultrasmall pore structures. Herein, a two-dimensional (2D) COF
prepared a Pd(II)-containing COF showing excellent catalytic
modified with thiol chains (-SH) in its pores was prepared. Taking - performance for the Suzuki-Miyaura coupling reaction.[9] Zhang
SH as nucleation sites, Au NCs can in situ grow within the COF. et al. embedded Pd NPs and Pt NPs inside the pores of a COF
Ultrasmall pore structure of COF and the strong binding energy of S- with a size distribution of 1.7±0.2 nm, which exhibited enhanced
Au provide double assurance for improving the dispersibility of Au catalytic performance for Suzuki-Miyaura coupling reaction and
NCs under longtime light illumination conditions. Interestingly, the nitrophenol reduction.[10] Ma et al. provided a new strategy of
artificial Z-scheme photocatalytic system was constructed due to the immobilizing a linear ionic polymer with catalytically active into
formation of Au-S-COF bonding bridge, which is deemed to be an the channels of a Lewis acid-modified COF as a heterogeneous
ideal means to increase the charge separation efficiency. Such new catalyst.[11] All these excellent works demonstrate the unique
strategy provides a facile guideline for rationally designing COF advantages of COFs as decorating supports. It is worth noting
support catalysts with controllable activity and high stability.
that the porous and modification performance of COFs as
supports have been fully developed, however, the interaction
between metal and COF supports has not been deeply explored.
Introduction
Metal nanoparticles (NPs) and nanoclusters (NCs) have always
been
a hot topic for their unique optical, electronic and
mechanical properties and widely employed in catalysts.[1]
Achieving controllable adjustment to their catalytic performance
is significant.[2] “The quantum size effect” proves the properties
of NPs can be greatly influenced by their size.[3] And previous
works have reported that the surface free energy will be
enhanced with size minimizing.[4] When the size of NPs are
smaller than 2 nm, high surface free energy will keep the NPs in
a “boiling state”, which further causes poor stability tending to
aggregate and loss of catalytic performance.[5]
Loading nanomaterials on high-surface-area supports as one
of the most important heterogeneous catalysts is a feasible
strategy to improve stability and catalytic performance.[6] In
[a]
Y. Deng, Dr. Z. Zhang, Dr. P. Du, X. Ning, Y. Wang, D. Zhang, Dr. J.
Liu, Dr. S. Zhang, Prof. X. Lu, Tianjin Key Laboratory of Molecular
Optoelectronic, Department of Chemistry, School of Science, Tianjin
University, Tianjin, 300072, P. R. China
Figure 1. Synthetic scheme of Au@COF.
Au NPs as a prototype with adjustable size among various
NPs have drawn extensive attention for their excellent catalytic
performance.[12] However, Au NCs are easier to aggregate for
higher surface energy compared with other various NCs and
NPs.[13] Transitional strategy of loading Au NCs on metal oxide,
E-mail: luxq@tju.edu.cn; peiyao.du@tju.edu.cn
[b]
Prof. X. Lu, Key Laboratory of Bioelectrochemistry & Environmental
Analysis of Gansu Province, College of Chemistry & Chemical
Engineering, Northwest Normal University, Lanzhou 730070, P. R.
China
E-mail: luxq@nwnu.edu.cn
Y. Deng and Z. Zhang contribute equally to this work.
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Figure 2. a) Structure of COF-V; b) SEM image of COF-V; c) PXRD patterns of COF-V and COF-S-SH; d) Structure of COF-S-SH; e) SEM image of COF-S-SH; f)
N2 adsorption isotherms of COF-V and COF-S-SH; g) Graphic view of COF-V structure; h) Graph of COF-S-SH structure.
SiO2 is difficult to maintain the stability under longtime in situ
photo-irradiation conditions, which is bottleneck for the
development of Au NCs-based catalysts in practical
application.[14] Thus, it is necessary and urgent to develop novel
Au NCs catalysts with enhanced stability.
and Au, thiol group inside the pores served as nucleation sites
where HAuCl4•3H2O can be in situ reduced by L-glutathione (L-
GSH) to obtain the final Au@COF. Au NCs loaded on COF
support are ultrafine (1.8±0.2 nm), highly dispersed and shows
highly photostability under in situ photo-irradiation (λ>420 nm)
for 5 h. Furthermore, the Z-scheme photocatalytic system was
successfully constructed via Au-S-COF bonding bridge between
COF and Au NCs, which was confirmed by the calculation of
binding energy and the results of photoelectrochemical (PEC)
experiments. As far as we know, this is a rare work to deeply
explore the PEC process between metal NCs and COF support.
Such strategy provides a guideline for rationally designing COF
support catalysts with controllable activity and high stability.
a
Herein, a thiol chains post-modified COF-S-SH was selected
as the support of Au NCs.[15] The ultrasmall pores (2.8 nm) of
this COF not only can limit the growth of Au NCs but also play a
role of anchoring Au NCs. Thiol chains were chosen to build a
bridge between COF and Au NCs. On one hand, the strong
binding energy of S-Au is obviously helpful to stabilize Au NCs.
On the other hand, Z-scheme photocatalytic system was
constructed due to the formation of Au-S-COF bonding bridge,
which possesses the advantages of increasing the charge
separation efficiency. Specifically, vinyl-functionalized monomer
1 (2,5-divinylterephthalalde-hyde) and monomer 2 (1,3,5-tris(4-
aminophenyl)-benzene) were utilized to construct COF-V, where
vinyl groups allowed for further chemical modifications with thiol
chains (Figure 1). Due to the strong binding interaction of -SH
Results and Discussion
First, Fourier-transform infrared (FT-IR) spectra were explored to
verify construction of COFs (Figure S1). Compared with
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Figure 3. a) Structure of Au@COF; b) SEM image of Au@COF; c), d), e) TEM images of Au@COF; Inset in c) is distribution statistics of Au NPs size; f) EDS-
mapping of Au@COF; TEM images of Au@COF radiated using a 300 W Xe arc lamp for g) 0 h and h) 5 h.
monomer 1 and 2, a new peak at 1645 cm-1 in the spectrum
appeared, which assigned to the C=N stretching vibration. At the
same time, the characteristic peak corresponding to C=O (1690
cm−1) of monomer 1 and N-H (3352 cm−1) of monomer 2
disappeared in the FT-IR spectrum of COF-V. Furthermore, FT-
IR spectrum of COF-S-SH shows a peak of thiol group at 2522
cm-1, which confirmed the successful integration of thiol groups
(Figure S2). Solid-state 13C NMR spectrum of COF-S-SH also
indicates the characteristic peak of vinyl groups at 114.2 ppm
disappears and the appearance of the alkyl C species of thiol
chains at 38.8 ppm (Figure S3).[15] Scanning electron
microscopy (SEM) images show the COF-V and COF-S-SH
display uniform nanowires with a diameter of approximately
50±5 nm (Figure 2b and 2e). The morphology reveals no
obvious change before and after pore-surface modification with
thiol chains. Powder X-ray diffraction (PXRD) was utilized to
evaluate the periodic structures of as-synthesized COFs. As
Figure 2c represents, a high intensity peak of COF-V appeared
in the low-angle region at 2θ = 2.8 degrees related to the (1 0 0)
facet of hexagonal lattice, which is consistent with reported
literature.[15] While COF-S-SH shows a relatively weak peak at
2.8 degrees and a broad peak from 15 to 30 degrees, which
result from the appearance of flexible thiol chains distorting the
long-range order structure and π-π stacking between layers.
Nitrogen-sorption isotherm indicates the porosity and specific
surface area of COF-S-SH (290 m2 g−1) is much lower than that
of COF-V (736 m2 g−1), which also proves the appearance of
thiol chains in the accessible pores of COF (Figure 2f).
Thermogravimetric analysis (TGA) was utilized to evaluate the
thermal stability of as-synthesized materials. Figure S4 shows
COF-V and COF-S-SH exhibit decomposition temperature (10%
weight loss) respectively at 300 ℃ and 175 ℃, which confirm
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Figure 4. a) The UV-vis spectra of RhB in different time under dark condition by adding Au@COF; Inset is optical photos before (left) and after (right) adding
Au@COF; b) The removal efficiency of RhB over Au@COF and COF-V under dark condition; c) Recycling removal efficiency of RhB over Au@COF under dark
condition; d) The UV-vis spectra of RhB in different time under different irradiation time after Au@COF adsorption saturation; e) The degradation efficiency of RhB
over COF-V and Au@COF; f) Recycling degradation efficiency of RhB over Au@COF under visible light irradiation.
the excellent thermal stability. And weight loss rate of COF-S-SH
is faster than COF-V, which results from the appearance of thiol
chains, proving the successful modification of thiol chains. All
these above results can demonstrate the successful
construction of as-synthesized two COFs. Moreover, the
structure of as-synthesized COFs was simulated by Materials
Studio software. After geometrical energy minimization, graphic
view of COF-V shows periodic pores with a diameter of 28 Å and
an interlayer distance of 3.6 Å (Figure 2g).
on COF supports (Figure 3c). The ultrasmall Au NCs possess an
average diameter of 1.8±0.2 nm, matching well with the size of
pores, which further proves that Au NCs is anchored inside the
pores of COF support (Figure 3d). High-resolution transmission
electron microscope (HR-TEM) image in Figure 3e indicates
crystal faces Au NCs exposing is (1 1 1). And energy dispersive
spectrometer (EDS) mapping of Au@COF proves the existence
of Au and S (Figure 3f). FT-IR spectrum of Au@COF was given
in Figure S5. The disappearance of characteristic peak
corresponding to -SH at 2522 cm-1 indicates the formation of S-
Au bond. X-ray photoelectron spectroscopy (XPS) was also
utilized to further verify the successful grafting of thiol groups
and Au NCs. XPS spectra of COF-S-SH shows the sulfur signal
at 162.1 eV (Figure S6a), which shift to 163.6 eV in Au@COF
(Figure S6c), revealing the successfully post-synthetic
modification of S-Au.
Previous studies have reported that thiol-functionalized COFs
exhibited effective removal ability for mercury ion due to the
porous structure and strong binding interaction between thiol
group and mercury ion.[16] Inspired by this strategy, it is expected
that thiol-functional COFs can be utilized to induce nucleation
and growth of metal NCs inside the accessible pores of COFs.
Taking advantage of strong binding energy of S-Au, the thiol
groups of COF-S-SH can serve as nucleation sites for Au NCs,
and the ultrasmall pores can confine the growth of Au NCs.
Such design not only avoids aggregation of Au NCs providing
more active sites for the ultrasmall size but also achieves
uniform dispersion for order structure of COFs. Specifically, the
Au precursor was adsorbed inside the pores of COF-S-SH by
dropping a certain concentration of HAuCl4•3H2O solution into a
suspension of COF-S-SH and stirring for 4 h. Then L-GSH was
introduced into above mixture and stirred for 48 h to obtain
Au@COF. Transmission electron microscope (TEM) images of
Au@COF show that ultrasmall Au NCs are uniformly distributed
In contrast, composite without thiol modified was also
synthesized (Figure S9a). As can be seen from Figure S9c and
S9d, Au NCs loaded on COF-S-SH are uniform and highly
dispersed. Without thiol chains modified on COF support, Au
NCs loaded on COF-V aggregate to bigger NPs easily (Figure
S9b). In addition, transitional strategy of loading Au NCs on
metal oxide and TiO2 supports fail to achieve the long-term
stability under in situ illumination conditions, which limits further
development of Au NCs-based catalysts in practical
application.[12] Thus, the longtime photostability was explored by
exposing Au@COF under light radiation (λ>420 nm) for 5 h.
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Figure 5. a) Transient photocurrent densities of COF-V and Au@COF under light irradiation (λ > 420 nm); b) EIS Nyquist plots of COF-V and Au@COF; c) Time-
resolved photoluminescence emission decay spectra of COF-V and Au@COF; d) VB-XPS spectra of COF-V; e) Band alignment of Au NCs, COF-V, H2O/•OH,
-
O2/•O2 ; f) The estimated band gap of COF-V and Au@COF; g) Cyclic voltammogram of Au NCs (electrolyte: acetonitrile containing 0.1 M tetrabutyl ammonium
perchlorate); h) ESR spectra of Au@COF in the presence of DMPO as radical scavenger before and after light irradiation; i) Radical trapping experiments of
Au@COF under light irradiation.
TEM images show Au NCs supported on COF-S-SH maintain
ultrafine and high dispersion before and after continuous light
irradiation (Figure 3g and 3h). The result suggests that
ultrasmall pore structure of COF and the strong binding energy
of S-Au provide double assurance for stabilizing Au NCs under
longtime light illumination conditions.
calculated. We found that almost 99.5% of RhB were captured
after 30 min when COF-V as adsorbent, while Au@COF can
only adsorb about 98.3% of RhB within 1 h, which mainly
attribute to the blocked porous structure of COF by Au NCs
(Figure 4b). And the removal efficiency can maintain more than
93.2% after 5 cycles (Figure 4c). Adsorption saturation of
Au@COF was also studied by mixing a solution of RhB dye (20
mL, 10 mg/mL) and Au@COF (1 mg). After 30 min, adsorption
reaches a saturated state (Figure S13b). By the standard curve
of concentration of RhB versus absorbance (Figure S13a),
adsorption capacity is calculated to be 170 mg/g.
In order to assess the effects of as-constructed Au@COF on
photocatalytic performance, RhB dye was chosen as a model to
evaluate controllable photocatalytic performance of Au@COF.
Considering the excellent adsorption capacity of COF materials,
the removal efficiency of Au@COF to RhB was investigated.
Typically, 10 mg of COF-V or Au@COF added to 50 ml RhB
solution (10 mg/L). Then, the absorbance value was monitored
with an ultraviolet-visible (UV-vis) spectrophotometry analyzer.
As shown in Figure 4a and S12a, upon the addition of COF-V or
Au@COF, the characteristic absorbance peak of RhB at 557 nm
decreased. Almost all the RhB in system can be adsorbed after
60 min and 10 min for Au@COF and COF-V, respectively.
According to the adsorption curves, removal efficiency was
Photodegradation efficiency of RhB over COF-V and
Au@COF was evaluated after absorbing to saturation. Figure 4d
and S12b show the absorbance intensity of RhB (100 mL, 10
mg/L) when Au@COF (10 mg) and COF-V (10 mg) appeared,
respectively. As depicted in Figure 4e, the degradation
percentage of 97.3% was obtained after visible light irradiation
for 30 min when Au@COF was added, which shows 2-fold
higher efficiency compared with COF-V. The degradation kinetic
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data were fitted according to -ln(C/C0)=kt where C and k are the
under irradiation. And previous works showed Au NCs cannot
concentrations of RhB and rate constant. Compared with COF-V, exhibit degradability to RhB for fast electron-hole recombination,
RhB degradation rates significantly increase from 5.79×10-2 min-1
to 9.04×10-2 min-1. And the removal efficiency can maintain more
than 95.4% after 5 cycles (Figure 4f).
which also proves that Au NCs cannot drive the reaction of
-
O2/•O2 . However, ESR experiment shows Au@COF can
-
achieve the reduction of O2 to •O2 , which proves COF support
The above results indicate the enhanced photocatalytic
performance. To better understand the constructed model
between COF support and Au NCs, it is significant to deeply
can efficiently inhibit the electron-hole recombination of Au
clusters. The above results indicate the constructed model
between COF support and Au NCs matches well with the Z-
investigate their specific mechanism of enhanced photocatalysis. scheme charge rule where thiol group plays the role of bonding
The transient photocurrent responses of COF-V and Au@COF
with several on-off cycles under intermittent visible light
irradiation were displayed in Figure 5a. Au@COF possesses
higher photocurrent intensity than COF-V, which proves the
dispersed Au NCs on COF support successfully promote the
generation of more photoexcited charges. As shown in Figure 5b,
the electrochemical impedance spectrum (EIS) Nyquist plot of
Au@COF is smaller confirming its better electron transport
capability. Time-resolved photoluminescence emission decay
spectrum were utilized to evaluate the photogenerated carrier
separation efficiency. Figure 5c shows the electronic lifetime of
Au@COF is longer, which indicates the higher photogenerated
carrier separation efficiency. We have further explored the
detailed degradation mechanism of RhB. Numerous researches
reported that RhB can be efficiency degraded by high oxidative
bridge for carriers transportation (Figure 6).[18]
Figure 6. Photogenerated electron transport rules between Au NCs and COF
support (e- in LUMO and h+ in HOMO represent photogenerated electron and
hole of Au NCs, respectively; e- in CB and h+ in VB represent the
photogenerated electron and hole of COF-V, respectively).
-
species (•OH, •O2 and hole h+).[17] Radical trapping experiments
The novel composite Au@COF shows efficient adsorption and
degradation abilities increasing the feasibility of the material in
practical applications, which possesses industrially valuable to
purify flowing wastewater. Thus, we explored the purification
ability of Au@COF for a continuous flowing pollution system. A
round filter paper with Au@COF coating onto its surface was
prepared. Figure 7a illustrates the detailed SEM images of filter
paper. The simplified schematic of purification device as
illustrated in Figure 7b, a sieve plate is put at the bottom of
extraction column, which prevents Au@COF from flowing out.
After optimizing the parameters for the continuous flow process,
the device worked well at room temperature. Utilizing RhB
solution as a model, a clarified solution flowed out from tail tube
of the column. Then the filter paper was taken out from column
and irradiated with light radiation (λ>420 nm). Figure 7b shows
the color of filter paper changed from red to yellow after
irradiating for 30 min, which proved the complete degradation of
adsorbed RhB. The same process was carried out 5 cycles, UV-
vis spectrum and corresponding images were showed in Figure
S16 and Figure 7b, which proves Au@COF still retains high
photocatalytic activity and adsorption capacity. It is obvious that
Au@COF have great potential in practical applications.
and electron spin-resonance spectroscopy (ESR) were carried
out to detect the roles of possible active species in
photocatalysis process. Figure 5i shows the degradation ratio of
RhB decreases to 40.1% and 45.3% after adding EDTA-2Na
(hole scavenger) and BQ (superoxide radical scavenger),
respectively, while a weak inhibition after adding TBA (hydroxyl
radical scavenger). Furthermore, ESR experiments were
conducted to confirm the radicals generated in the photocatalytic
-
process (Figure 5h). No signal can be detected for both •O2 and
•OH under dark condition. When applying light irradiation (λ>420
-
nm), there are four characteristic peaks corresponding to •O2
can be observed, while no characteristic peak of •OH appears.
These results reveal that •O2- and h+ are the main active species.
Subsequently, the transport path of carriers was deeply explored
by comparing energy band structures. The valence band X-ray
photoelectron spectroscopy (VB-XPS) of COF-V is located at
2.07 eV (Figure 5d). The band gap of COF-V and Au@COF can
be estimated to be 2.31 eV and 2.18 eV, respectively (Figure 5f),
which can be calculated by Kubelka-Munk formula utilizing UV-
vis diffuse reflectance spectra (DRS) (Figure S15). To further
assess the redox properties of Au NCs, the cyclic
voltammogram (CV) was employed to obtain highest occupied
molecular orbital (HOMO = 1.70 V) and lowest unoccupied
molecular orbital (LUMO = -0.45 V) of Au NCs (Figure 5g). The
energy band structures of COF-V and Au NCs were given in
Figure 5e. As we can see, the VB of COF-V is not positive
enough to oxide H2O to •OH (H2O/•OH 2.72 V), which is
Bisphenol A (BPA) as one of typical endocrine disruptive
compounds (EDC) is extremely harmful to the normal operation
of the liver and kidney in the human body, and even induces
heart disease.[19] Motivated by the efficient adsorption capacity
and enhanced photocatalytic activity of Au@COF towards dyes,
thus, the ability of Au@COF to degrade BPA in water was
investigated. A process similar to RhB experimental tests was
implemented. After 30 min, almost 98.44% of BPA were
captured in the dark (Figure S18a). After adsorbing to saturation
-
matched with experiment result. The potential level of O2/•O2 (-
0.33 V) is between CB of COF-V and LUMO of Au NCs, which
-
indicates the COF-V cannot drive the reaction process of O2/•O2
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10.1002/anie.201916154
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Project (2017-D-01). We also thank Prof. Wenping Hu (Tianjin
University) and Prof. Baodui Wang (Lanzhou University) for
valuable suggestions and help.
state for BPA, the photocatalytic degradation efficiency was
subsequently evaluated. More than 90% of BPA can be
degraded by Au@COF after 30 min under light irradiation
(Figure S18b). And the kinetic constant k is calculated to be
1.14×10-1 by fitting the kinetic curves (Figure S18c). Moreover,
high degradation efficiency of BPA over Au@COF can retain
after 5 cycles (Figure S18d).
Keywords: Z-scheme photocatalytic system • covalent organic
framework • Au cluster • photostability • pores
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Figure 7. Schematic diagrams of a) filter paper model and b) filter device.
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Conclusion
In summary, a facile strategy to bridge ultrasmall Au NCs into
pores of
a
covalent organic framework for enhanced
photostability and photocatalytic performance has been
developed. Specifically, a novel composite material of loading
Au NCs on a thiol-modified COF was prepared. The ultrasmall
pores of COF support and strong binding energy of S-Au
provided double assurance for the photostability of Au NCs.
Furthermore, PEC experiments show Z-scheme photocatalytic
system is constructed by the formation of COF-S-Au bonding
bridge, which can effectively promote charge separation,
enhancing photocatalytic performance. Such design provides a
new way to develop COF support catalysts with controllable
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Acknowledgements
We are thankful to the Program of Tianjin Science and
Technology Major Project and Engineering (19ZXYXSY00090);
the Program for Chang Jiang Scholars and Innovative Research
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