A simple strategy for engineering heterostructures of Au nanoparticle-loaded metal-organic framework nanosheets to achieve plasmon-enhanced photocatalytic CO2 conversion under visible light

Article abstract of DOI:10.1039/c9ta01840a

Carbon dioxide selectively concentrated in micropores of metal-organic frameworks (MOFs) and light absorption units as bridged ligands or guest molecules incorporated into MOF matrices are positive effects on upgrading reactivity of metal catalytic centers towards CO₂ conversion through high-efficient quasi-intramolecular electron transfer among them. However, in contrast to the surfaces of MOFs, sluggish mass transfer of CO₂, as well as of products, in micropores could negatively affect photocatalytic activity for CO₂ reduction. Moreover, interactions between light and matter are heavily blocked if light absorption units are buried deep in bulky MOF matrices. In this research, hybrids of thin porphyrin paddle-wheel framework-3 (PPF-3) nanosheets (PPF-3-1) anchored with AuNPs are prepared by an electrostatic interaction to address these questions. This mainly involves improvement of PPF-3 morphology and the assembly mode between Au nanoparticles (AuNPs) and PPF-3 nanosheets. On the one hand, thin nanosheets can induce faster charge transfer rate and higher mass transport capability than thick nanosheets in the photocatalytic process; on the other hand, plasmonic AuNPs loaded onto the surfaces of nanosheets lead to more effective light absorption than AuNPs encapsulated in the matrices of nanosheets. Finally, the hybrids exhibit superior photocatalytic activity for CO₂ conversion into HCOOH in an acetonitrile/ethanol system by plasmon resonance energy transfer process compared with pure PPF-3-1 or hybrids of thick PPF-3 nanosheets (PPF-3-2) supporting AuNPs. The research offers a novel and universal strategy to build high-efficiency MOF-based photocatalysts with enhanced performance for selective photocatalytic CO₂ reduction.

Full text of DOI:10.1039/c9ta01840a

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
View Journal
Journal of
Materials Chemistry A
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Chen, Y. Wang,
F. Yu, X. Shen and C. Duan, J. Mater. Chem. A, 2019, DOI: 10.1039/C9TA01840A.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–330
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry A
Accepted Manuscripts are published online shortly after
Materials for energy and sustainability
www.rsc.org/MaterialsA
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021PAlsepaosseePdtyoLtnd.ot adjust margins
Page 1 of 8
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C9TA01840A
Journal Name
ARTICLE
A simple strategy for engineering heterostructures of AuNPs
loaded metal-organic framework nanosheets to achieve plasmon-
enhanced photocatalytic CO2 conversion under visible light
Received 00th January 20xx,
Accepted 00th January 20xx
Liyong Chen,*a Yanxin Wang a, Fengyang Yu a, Xiaoshuang Shen b and Chunying Duan*a
DOI: 10.1039/x0xx00000x
CO2 selectively concentrated in micropores ofmetal-organic frameworks (MOFs) and light absorption units as bridged ligands
or guest molecules incorporated into MOF matrices are positive effects on upgrading reactivity of metal catalytic centers
towards CO2 conversion through high-efficient quasi-intramolecular electron transfer among them. However, in contrast to
surfaces of MOFs, sluggish mass transfer of CO2, as well as products, in micropores could negatively affect photocatalytic
activity for CO2 reduction. Moreover, interactions between light and matter are greatly blocked if light absorption units are
buried deeply in bulky MOFs matrices. In the research, hybrids of thin porphyrin paddle-wheel frameworks-3 (PPF-3)
nanosheets (PPF-3_1) anchored AuNPs are prepared by an electrostatic interaction to address these questions. This mainly
involves in improvement of PPF-3 morphology and assembly mode between AuNPs and PPF-3 nanosheets. On the one hand,
thin nanosheets can induce faster charge transfer rate and higher mass transport capability than thick nanosheets in the
photocatalytic process; on the other hand, plasmonic AuNPs loaded on the surfaces of nanosheets lead to more effective
light absorption than AuNPs encapsulated into the matrices of nanosheets. Finally, the hybrids exhibit the superior
photocatalytic activity towards CO2 conversion into HCOOH in acetonitrile/ethanol system by a plasmon resonance energy
transfer process compared to pure PPF-3_1 or hybrids of thick PPF-3 nanosheets (PPF-3_2) supported AuNPs. The research
offers a novel and universal strategy to build high-efficient MOF-based photocatalysts with the enhanced performance for
selective photocatalytic CO2 reduction.
www.rsc.org/
intramolecular electron transfer between organic chromophore
and metal ion.18-23 However, owing to small photo-absorption
cross section, ensemble optical absorption units shows
Introduction
The conversion of CO2 into commodity carbon-based chemicals
and fuels is a viable way to fully use waste CO2 and decrease its
content in our planet’s atmosphere as well.1-3 Solar-energy-
driven photocatalytic CO2 reduction is generally considered as
the most promising manner for energy conversion and CO2
relatively low optical absorption capability.24 Decrease of
incident light intensity with increase of incident depth
overwhelmingly weakens interaction between matter and
photons,25 resulting in low optical absorption efficiency of
photoactive sites embedded into MOF matrices. In addition,
adsorption of substrates and desorption of products in
micropores show lower mass transport rate than those on
surfaces. Therefore, to address the fundamental challenges, on
the one hand, other light harvesting antennas such as metal
nanoparticles with large photo-absorption cross section are
used to improve light absorption;24, 26, 27 on the other hand,
MOFs are needed to trim into small particles in nanoscale,
which not only fully utilizes light absorption units to harvest
solar energy but also facilitates mass transport, as well as
electron transfer in the catalysis process.28-31
5
utilization.4, Till now, various inorganic materials, such as
semiconductor-based
composites
and
metal-free
photocatalysts, have been vastly utilized for photocatalytic CO2
reduction.6-13 In contrast, metal-organic frameworks (MOFs) as
emerging materials have gradually attracted significant
attention due to precisely tunable components and pore
environments, favourable for selective adsorption,14,
15
activation of substrates to produce desirable products,16 and
preparation of inorganic catalysts as well.17, In particular,
18
metal-organic frameworks (MOFs) with variable-valence metal
ions and photoactive chromophore molecules can be used as
Ultra-thin nanosheets, such as carbon-based materials, metals,
and metal dichalcogenides, are generally served as
heterogeneous catalysts with intriguing performance, which is
beneficial for easily accessible catalytic sites and acceleration of
mass transfer/electron transfer compared to other
morphologies.32-34 Two dimensional (2D) layered structural
MOFs with variable valence metal ions are needed to prepare
sheet-like photocatalysts for CO2 reduction. In the case, hence,
high-performance
photocatalysts
based
on
quasi-
a. State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
116024, China. lychen@dlut.edu.cn; cyduan@dlut.edu.cn.
b. School of Physical Science & Technology, Yangzhou University, Yangzhou, 225002,
China.
Electronic Supplementary Information (ESI) available: Experimental procedures,
XRD, TEM, SEM, AFM, XPS, IR, Zeta potential, CV, and optical image. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021PAlsepaosseePdtyoLtnd.ot adjust margins
Journal of Materials Chemistry A
Page 2 of 8
ARTICLE
Journal Name
porphyrin paddle-wheel frameworks (PPFs) nanosheets by Noted that the as-made PPF-3_2 was thermally treated using
View Article Online
coordination of Co2+ with tetrakis(4-carboxyphenyl) porphyrin the same procedure.
DOI: 10.1039/C9TA01840A
(TCPP) pillared by 4,4’-bipyridine were selected as MOF AuNPs (~1 mg) were collected by centrifugation at 17000 g from
photocatalysts, designated as PPF-3.35 To further improve AuNPs colloidal solution (21.2 mL) and dispersed into ethanol
photoactive of PPF-3 by surface plasmon resonance (SPR) (10 mL). The treated PPF-3_1 (10 mg) were dispersed into a 250-
excitation, hybrid of plasmonic AuNPs loaded on the surface of mL round-bottom flask containing 100 mL ethanol, and
PPF-3 nanosheets (designated as Au/PPF-3) was successfully subsequently, AuNPs were added. The resultant mixture was
prepared by electrostatic interaction. It not only simplifies gently stirred for 24 h at room temperature. The final sample
synthetic procedure, but sufficiently enhances light absorption was collected by centrifugation at 6000 g, and dried in vacuum
efficiency of AuNPs. Finally, an enhanced photocatalytic at 80 oC for 12 h.
performance of Au/PPF-3 hybrid for CO2 conversion in Noted that the loading amount of AuNPs on PPF-3 nanosheets
acetonitrile/ethanol medium upon visible light illumination was can be adjusted by change of their original mass ratio, including
achieved by plasmon resonance energy transfer from Au to PPF- 1:1, 1:10 and 1:50.
3. As expected, the photocatalytic activity of the hybrid was
significantly affected by thickness of PPF-3 nanosheets.
Photocatalytic CO2 reduction measurements
The photocatalyst powder (1 mg) was dispersed into a 20-mL
quartz glass reactor containing 4 mL of acetonitrile and 1 mL of
ethanol. Before photocatalytic reaction, the reactor was sealed
Experimental section
with a rubber septum after being bubbled carefully with CO2 for
30 min. The photocatalytic system was illuminated using a
Photocatalyst preparation
Preparation of thin porphyrin paddle-wheel frameworks-3 (PPF-
3) nanosheets (PPF-3_1). Co(NO3)2·6H2O (2.2 mg, 0.0075 mmol),
4,4'-bipyridine (0.78 mg, 0.005 mmol), and polyvinylpyrrolidone
(PVP) (10 mg, Mw: 40000) were dissolved into a mixture solvent
of N,N-dimethylformamide (DMF) and ethanol (6 mL, V:V = 3:1)
in a vial. A solution of TCPP (2.0 mg, 0.0025 mmol) in DMF and
ethanol (2 mL, V:V = 3:1) was prepared and then added
dropwise to the aforementioned vial. After sonication for 25
min, the resultant mixture was thermal treatment for 24 h at 80
oC. The final brick-red product was washed with ethanol,
collected by centrifugation at 8000 g, and dried in vacuum at 80
oC for 12 h.
Preparation of thick porphyrin paddle-wheel frameworks-3
(PPF-3) nanosheets (PPF-3_2). Co(NO3)2·6H2O (4.4 mg, 0.015
mmol), and 4,4'-bipyridine (1.56 mg, 0.01 mmol) were dissolved
into a mixture solvent of DMF and ethanol (6 mL, V:V = 3:1) in a
vial. A solution of TCPP (4.0 mg, 0.005 mmol) in DMF and
ethanol (2 mL, V:V = 3:1) was prepared and then added
xenon lamp (300 W, Beijing Perfectlight Technology Co. Ltd)
with a UV-cutoff filter (>400 nm) or a monochromatic light filter
(475nm and 540 nm) for 4 h under magnetic stirring gently at
room temperature with a condensation water system. The final
mixture was treated by centrifugation to remove the
photocatalysts and by rotary evaporation to remove solvent.
Subsequently, deionized water (10 mL) was added, and the
liquid product was analyzed by ion chromatograph (Dionex ICS-
5000). The gas product taken from the reactor by a syring was
analyzed using a gas chromatograph (GC7900 Techcomp)
equipped with a flame ionized detector and methanizer.
Noted that each set of measurements was repeatedly
performed three times.
Results and discussion
Synthesis of photocatalysts
dropwise to the aforementioned vial. After sonication for 25 Thin PPF-3 nanosheets (PPF-3_1) were produced by a PVP-
min, the resultant mixture was thermal treatment for 24 h at 80 assisted solvothermal synthetic route. In the synthetic process,
oC. The final brick-red product was washed with ethanol, PVP molecules are employed as structural direction agents to
collected by centrifugation at 6000 g, and dried in vacuum at 80 confine growth along c-axis direction, leading to formation of
oC for 12 h.
sheet-like structures.36 Finally, the as-made PPF-3_1 was
Preparation of citrate-modified AuNPs (15 nm). Briefly, 100 mL confirmed by X-ray diffraction (XRD) pattern (Fig. S1†), in which
de-ionized water was poured into a 250-mL round-bottom flask, all diffraction peaks were consistent with those of simulated
and was heated to boiling at 110 oC. Afterwards, HAuCl4 PPF-3. The sample showed the morphology of quasi-square
aqueous solution (1 mL, 1%w.t.) was added and heated for nanosheets with ~1 μm in width, characterized by transmission
another 15 min under vigorously stirring. Subsequently, sodium electron microscopy (TEM) and scanning electron microscopy
citrate aqueous solution (3 mL, 1%w.t.) was induced to the (SEM) images (Fig. 1a and 1d). After careful observation, a few
solution one shot. After heating the solution for 30 min, a deep- irregular nanosheets with small size were also present in both
red citrate-stabilized AuNPs was obtained.
Preparation of Au/PPF-3 hybrids.
images. The magnified TEM image (Fig. 1b) revealed a regular
nanosheet of PPF-3_1 with slight fragmentary on its edges but
The as-made PPF-3_1 (10 mg) was dispersed into a 100-mL plausibly smooth surface along crystalline [001] direction of
round-bottom flask containing 50 mL DMF, and the mixture was PPF-3, corresponding to TCPP linkers and Co2(COO)4 secondary
thermally treated at 80 oC for 12 h. The treated PPF-3 was building units forming layered structures. AFM image (Fig. 1c)
collected and washed thrice with ethanol.
showed somewhat uneven surface of PPF-3_1 with a thickness
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 3 of 8
Journal of Materials Chemistry A
Please do not adjust margins
Journal Name
ARTICLE
of less than 20 nm, which is favourable for electron migration pattern, corresponding to the {111} crystalline planes of Au. The
View Article Online
from interior to surfaces of the MOFs.
morphological change of quasi-squareDPOPI:F1-03.1_0139/nCa9nToAs0h18e4e0tAs
cannot be found after loading of AuNPs. The loading amount of
AuNPs on PPF-3_1 can be controlled by tuning their original
mass ratio. For example, if increasing the mass ratio of AuNPs
to PPF-3_1 from 1:10 to 1:1 in the self-assembly system, the
high dense AuNPs dispersed on the surfaces of PPF-3_1
nanosheets were found (designated as Au/PPF-3_1B); in
contrast, if decreasing their mass ratio to 1:50, the sparse
AuNPs were on the PPF-3_1 nanosheet surfaces ((designated as
Au/PPF-3_1C, Fig. S9). By careful observation, AuNPs were not
completely adsorbed on the surfaces of PPF-3_1 nanosheets,
and a few AuNPs were scattered in blank areas while the mass
ratio of AuNPs to PPF-3_1 was 1:1.
Fig. 1 (a. b) TEM images with different magnifications, and (c) SEM and (d) AFM images
of PPF-3_1; (e) SEM and (f) TEM images of Au/PPF-3_1A. Inset is the profile of a PPF-3_1
nanosheet in the AFM image.
Electrostatic interaction originated from their different surface
charges is one of effective driving forces to achieve self-
assembly of materials.37 In the research, citrate modified AuNPs
with a SPR absorption peak of ~525 nm were prepared by
sodium citrate reduction of HAuCl4 (Fig. S2†).38 Negatively
charged surfaces were further analyzed by zeta potential of ~-
19.4 mV (Fig. S3†). Thereby, positively charged surfaces of PPF-
3_1 were needed to form Au/PPF-3_1 hybrid through an
electrostatic interaction. However, zeta potential of PPF-3_1
was -5.9 mV, revealing negatively charged surfaces. Citrate
modified AuNPs were difficultly loaded on the surfaces of PPF-
3_1 nanosheets (Fig. S4†). PPF-3_1 was thermally treated firstly
Fig. 2 High-resolution Co 2p XPS spectra of (a) treated PPF-3_1 and (b) Au@PPF-3_1A.
X-ray photoelectron spectroscopy (XPS) measurement was used
to analyze surface chemical state of Co element, whose redox
potential can greatly determine photoreduction ability of PPF-
3_1. From the high-resolution Co 2p XPS spectrum of treated
PPF-3_1 (Fig. 2), Co 2p3/2 and Co 2p1/2 core level photoemission
peaks at 781.0 eV and 796.3 eV, respectively, were found,
accompanied with two satellite peaks at 785.8 and 802.0 eV
corresponding to Co2+ shakeup excitation.39, 40 The presence of
the two satellite peaks confirms PPF-3_1 consisting of Co2+
species. Obvious shifting of Co binding energy was not found
after loading of AuNPs, and Co 2p core level region showed two
peaks at 780.95 eV and 796.2 eV together with two satellite
peaks at 785.9 and 802.0 eV in Au/PPF-3_1A. Hence,
photocatalytic reduction of CO2 could be related to redox
o
in DMF at 80 C for 12 h to expect alteration of its surface
properties. In the event, its zeta potential was changed into +4.9
mV, signifying successful transformation of PPF-3_1 nanosheet
surface properties from negative charge to positive charge. We
speculate that this experimental observation is possibly
attributed to (i) desorption of PVP from PPF-3_1 nanosheet
surfaces although PVP-related signal cannot be found in Fourier
transform infrared spectroscopy (FTIR) spectra of PPF-3_1
possibly due to small amount of PVP (Fig. S5†), and (ii) Co2+ ions
diffusion to surfaces. If the treated PPF-3_1 was re-dispersed
into PVP solution in DMF, its zeta potential was negatively
shifted (Fig. S3†). The morphology and the structure of PPF-3_1
nanosheets were not obviously changed after thermal
treatment in DMF (Fig. S6†). Both materials of AuNPs and PPF-
3_1 nanosheets (mass ratio: 1:10) were separately dispersed
into ethanol, and then were mixed together with gentle stirring
for 24 h. The purple precipitate was separated from the final
mixture that was kept for 8 h without disturbance, leaving a
colorless supernatant (Fig. S7†). In UV-Vis absorption spectrum
of the supernatant, SPR characteristic peak of AuNPs cannot be
found. These results manifest that AuNPs are completely
adsorbed on PPF-3_1 nanosheet surfaces in the suspension.
AuNPs were uniformly dispersed on PPF-3_1 nanosheet
surfaces when the mass ratio of Au and treated PPF-3_1 in the
self-assembly media was 1:10 (designated as Au/PPF-3_1A; Fig.
1e, 1f, and S8). A diffraction peak at 38.1o was appeared in XRD
2+
potential of ECo
/
Co
+ that was evaluated to be -0.64 V (vs NHE)
by cyclic voltammetry measurement of PPF-3_1 (Fig. S10†).
Noted that the position of Co 2p binding energy of PPF-3_1
cannot be alerted by thermal treatment, but the intensity is
weakened (Fig. S11†). The result reveals the change of surface
properties of PPF-3_1 is partly related to Co2+ ions migration.
The optical absorption ability is an important factor for
photocatalytic activity of Au/PPF-3_1 hybrid. UV-Vis absorption
spectra of treated PPF-3_1 and Au/PPF-3_1A showed strong
optical absorption in the range of UV to visible light (Fig. 3).
These absorption peaks were assigned to Soret band at low
wavelength and Q band between 500 and 700 nm related to
TCPP.41 Moreover, absorbance of Au/PPF-3_1A in the
wavelength region larger than 520 nm was significantly
increased compared to treated PPF-3_1, possibly due to SPR
absorption of AuNPs. In addition, photoluminescence (PL)
emission of TCPP was quenched in the treated PPF-3_1 and
Au/PPF-3_1A (Fig. 3b). The possible scenario is attributed to
photo-induced electron transfer (PET) from the excited TCPP to
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins