Ultrathin 2D Zirconium Metal–Organic Framework Nanosheets: Preparation and Application in Photocatalysis

Article abstract of DOI:10.1002/smll.201703929

Synthesizing ultrathin 2D metal–organic framework nanosheets in high yields has received increasing research interest but remains a great challenge. In this work, ultrathin zirconium-porphyrinic metal–organic framework (MOF) nanosheets with thickness down to ≈1.5 nm are synthesized through a pseudoassembly–disassembly strategy. Owing to the their unique properties originating from their ultrathin thickness and highly exposed active sites, the as-prepared ultrathin nanosheets exhibit far superior photocatalysis performance compared to the corresponding bulk MOF. This work highlights new opportunities in designing ultrathin MOF nanosheets and paves the way to expand the potential applications of MOFs.

Full text of DOI:10.1002/smll.201703929

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Ultrathin 2D Zirconium Metal–Organic Framework
Nanosheets: Preparation and Application in Photocatalysis
Ting He, Bing Ni, Simin Zhang, Yue Gong, Haiqing Wang, Lin Gu, Jing Zhuang,
Wenping Hu, and Xun Wang*
coordination polymers constructed by
Synthesizing ultrathin 2D metal–organic framework nanosheets in high yields
has received increasing research interest but remains a great challenge. In
this work, ultrathin zirconium-porphyrinic metal–organic framework (MOF)
nanosheets with thickness down to ≈1.5 nm are synthesized through a
pseudoassembly–disassembly strategy. Owing to the their unique properties
originating from their ultrathin thickness and highly exposed active sites, the
as-prepared ultrathin nanosheets exhibit far superior photocatalysis perfor-
mance compared to the corresponding bulk MOF. This work highlights new
opportunities in designing ultrathin MOF nanosheets and paves the way to
expand the potential applications of MOFs.
coordination bonds between metal clus-
ters and organic linkers with periodic
structural units.[12–14] Compared to the 3D
bulk MOF crystals, 2D MOF nanosheets
possess extremely high percentages of
exposed catalytic active sites and larger
surface area, which are beneficial for the
ensure rapid mass transport and high cat-
alytic activity. These properties endow 2D
MOFs with great potential for applications
in catalytic, energy storage, and electronic
sensors.[15–18]
Conventionally, there are two strate-
gies to synthesize 2D MOF nanosheets.
High-aspect-ratio 2D materials with extended lateral dimen-
sions and atomic or molecular thickness often exhibit unique
physical and chemical properties radically different from those
of their bulk counterparts.[1–4] As a result, 2D nanostructures
are highly interesting components for functional materials
in the applications of separation, optoelectronics, catalysis,
and so on.[5–10] Recently, 2D metal–organic framework (MOF)
nanosheets have emerged as a new member of the 2D family
and drawn tremendous attention.[11] MOFs are crystalline
One is the top–down method, in which MOF nanosheets are
obtained directly from layered bulk MOFs through sonication
exfoliation or mechanical delamination.[19–22] Unfortunately,
shortcomings such as restacking of the obtained sheets, mor-
phological damage, low yields (normally <15%), and species
limitation (weak interactions between layers in 3D bulks) are
often associated to these methods. Recently, defect-mediated
transformation[23] and chemically labile intercalating agent
methods[24] have been developed to promote exfoliating and
improve yields but still require multistep approaches. The
other is the direct bottom-up synthesis[16,25–31] (e.g., three-layer
synthesis strategy, surfactant-assisted synthesis), which is pre-
ferred to prepare well-dispersed MOF nanosheets in high yield.
However, it is still difficult to achieve surfactant-free and atomic
thickness nanosheets. Furthermore, compared to the rich syn-
thetic chemistry of other MOF nanosheets, there have been no
reports of the synthesis of zirconium-based MOF (Zr-MOF)
nanosheets to the best of our knowledge. It is attributed to that
metal–ligand bonds of Zr-MOF are too strong to break.
T. He, B. Ni, S. Zhang, H. Wang, J. Zhuang, Prof. X. Wang
Key Lab of Organic Optoelectronics and Molecular Engineering
Department of Chemistry
Tsinghua University
Beijing 100084, China
E-mail: wangxun@mail.tsinghua.edu.cn
T. He, Prof. W. Hu
Tianjin Key Laboratory of Molecular Optoelectronic Sciences
Department of Chemistry
School of Sciences
Herein, for the first time, we develop a novel strategy for the
high-yield synthesis of atomic thickness ultrathin zirconium-
porphyrinic MOF nanosheets (UNs) in one-pot solvothermal
reaction through precisely controlling the concentration of
monocarboxylic acids (“modulator,” they are commonly used
in Zr-MOF system to obtain larger single crystals[32]) and water.
The growth mechanisms are investigated by examining the
samples obtained at different reaction durations. Unexpect-
edly, a pseudoassembly–disassembly forming process has been
observed under transmission electron microscopy (TEM). This
new growth mechanism is further verified by quantitative dis-
solution/nuclear magnetic resonance (NMR) spectrometry and
small-angle X-ray diffraction (SAXRD) patterns. As a proof-
of-concept application, these as-prepared UNs exhibit singlet
Tianjin University & Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin)
Tianjin 300072, China
T. He
School of Chemistry and Chemical Engineering
Qinghai Normal University
Xining 810000, China
Y. Gong, Prof. L. Gu
Beijing National Laboratory for Condensed Matter Physics
Institute of Physics
Chinese Academy of Sciences
Beijing 100190, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201703929.
DOI: 10.1002/smll.201703929
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Figure 1. a) TEM image (scale bar: 500 nm) (inset, high magnification image, scale bar: 50 nm), b,d,e) aberration-corrected HAADF-STEM image
(scale bar: b,d: 20 nm; e: 5 nm), c) illustration of the grid-like structure, f) AFM image and corresponding height profile, and g) EDX elemental map-
ping of UNs-FA.
oxygen (1O2) generation and superior photocatalytic activity due
to their intrinsic 2D character and high efficiency in light har-
vesting of porphyrin units.
adjacent secondary building units (SBUs) in the crystal model
(17.143 Å) (Figure 1c). The aberration-corrected HAADF-STEM
result with high magnification further confirmed that the Zr6
clusters (bright spots) with reticular architectures were well-
ordered (Figure 1e). The as-prepared UNs-FA were measured
to be 1.48 0.22 nm (Figure 1f) in thickness by atomic force
microscopy (AFM), which was close to the van der Waals diam-
eter of monolayer of Zr6 SBUs. Elemental mapping and line-
scanning spectra using energy-dispersive X-ray spectroscopy
(EDX) showed that the elements C, N, O, Ni, and Zr were
well-dispersed in the nanosheets (Figure 1g, Figure S3, Sup-
porting Information). The porosity of UNs-FA was analyzed
by nitrogen adsorption–desorption isotherms (Figure S4, Sup-
porting Information) and the Brunauer–Emmett–Teller surface
area was calculated to be 398.230 m2 g−1. The thermogravi-
metric curves taken in nitrogen were presented in Figure S5 in
the Supporting Information and the products showed superior
thermal stability and began to collapse at 400 °C. Fortunately,
when other monocarboxylic acids (acetic acid, lauric acid, and
oleic acid) were used as modulator, nanosheets could also be
successfully obtained (Figures S6–S9, Supporting Information).
Slight stacking of nanosheets appeared along with lengthening
of the aliphatic chain of modulator.
The solvothermal reactions of ZrCl4, nickel-tetrakis(4-
carboxyphenyl)-porphyrin (Ni-TCPP), formic acid (FA), and
water in N,N-dimethylformamide (DMF) at 120 °C for 24 h
generated ultrathin Zr-MOF nanosheets (UNs-FA) in 85%
yield. The porous morphology, composition, and structure of
UNs-FA were investigated by various characterization tech-
niques. TEM images (Figure 1a) showed that the UNs-FA
were uniform with ≈200 nm in dimensions. The powder
X-ray diffraction (PXRD) pattern of the nanosheets (Figure S1)
gave three obvious peaks assigned to (0, 0, 1), (2, 0, 1), and
(3, −1, 1), respectively, which were consistent with the reported
single crystals of PCN-222[33] (porous coordination network).
The absence of some peaks was ascribed to the nature of 2D
nanosheet, which was similar to other 2D-MOF materials.[28,30]
To our delight, the lattice fringes were observed clearly in aber-
ration-corrected high-angle annular dark-field scanning TEM
(HAADF-STEM) (Figure 1b,d) and HAADF-STEM (Figure S2,
Supporting Information) images. The Zr6 clusters appeared
as white spots on the image as compared to the nonmetallic
atoms and background. Careful observation demonstrated
that the distances between adjacent clusters in the HAADF-
STEM image (17.23 Å) matched well with that between two
To reveal the probable growth mechanism involved in
the formation of UNs. A series of control experiments
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Figure 2. a–c) TEM images of UNs-AA collected at 4, 12, and 24 h. Scale bar: a) 10 µm (inset, 500 nm); b) 2 µm; c) 500 nm, d) PXRD patterns of
UNs-AA collected at different reaction time (inset, enlarged green line district), e) small-angle X-ray diffraction (SAXRD) patterns of UNs-AA-4h and
3D-AA, f) quantitative dissolution/1H NMR spectra of UNs-AA-4h and 3D-AA.
were systematically carried out. We carried out the time-
dependent experiments and monitored the process by TEM
and XRD. Unexpectedly, an interesting evolutionary pro-
cess in the UNs-AA-system was observed compared to no
obvious information in the UNs-FA-system. When the reac-
tion time was shorted to 4 h, 3D micron-scale bulks were
dominating in the system (Figure 2a). Careful observation
of high magnification image demonstrated that bulks began
to disassemble to ultrathin nanosheets from the outside
surface (Figure 2a, inset). Prolonging the reaction time to
12 h, a large number of nanosheets appeared and only few
bulks remained (Figure 2b). Uniform UNs structures were
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ultimately produced after a 24 h solvothermal treatment
(Figure 2c). Accordingly, the peaks indexed as (2, −1, 1),
(3, −1, 1), and (4, −1, 1) began to disappear or weaken in the
PXRD patterns and the (2, 0, 1) peak became a broad peak due
to the decreasing of dimensions, which were in accordance
with the exfoliation process of nanosheets (Figure 2d). It was
also consistent with corresponding TEM observations above.
As a comparison, 3D-AA was synthesized following the pub-
lished procedure.[34] In order to realize the difference between
3D-AA sample and UNs-AA-4h sample, the quantitative dis-
solution/NMR spectroscopy was carried out to identify the
impact of monocarboxylic acid. The 3D-AA and UNs-AA-4h
were digested in 1 m NaOH (in D2O), then the same amount
of dimethyl sulfoxide (DMSO) was added into the mixture and
structure while large quantities of monocarboxylic acid occu-
pied the coordination sites to weaken the interactions of inter-
layers. This was confirmed by quantitative dissolution/NMR
and SAXRD results. On the disassembly step, the unstable 3D
bulks began to disassemble and ultrathin nanosheets formed
in order to minimize the total surface energy when prolonging
the reaction time. The concentration of monocarboxylic acid
must be precisely controlled during the whole formation pro-
cess of UNs. 3D bulk nanocrystals yielded at last when the
monocarboxylic acid was insufficient,[34] while high concen-
tration led to the formation of needle-shaped microcrystals
(Figure S14, Supporting Information).[32] The reason why the
pseudoassembly–disassembly process was not observed in
UNs-FA-system might be that the size of formic acid molecule
was smaller than monocarboxylic acid with longer aliphatic
chain. The process was too fast to observe due to fast move-
ment of formic acid molecule.
As we know, porphyrin compounds are extensively used for
1O2 generation in photocatalysis due to their high efficiency in
light harvesting.[24,35,36] A series of photochemical experiments
was conducted. The UV–vis diffuse reflectance spectra of zirco-
nium-porphyrinic MOFs demonstrated strong absorption in the
region of ≈300–700 nm, indicating the photon absorption ability
upon visible-light irradiation (Figure 4a). To clarify whether 1O2
species was actually generated in the system, 1, 3-diphenyliso-
benzofuran was used upon visible-light irradiation and the
characteristic absorption (at 410 nm) decreased in the presence
1
recorded by H NMR (Figure 2f). The single peak at 1.8 ppm
could be assigned to acetate fragment. We were confident that
the acetic acid was incorporated into the MOF structure due
to the rigorous washing and activation procedure for samples.
The peaks at 2.2, 2.5, and 4.7 ppm could be assigned to N,N-
dimethyl fragments (originating from DMF hydrolysis during
synthesis procedures), DMSO and D2O, respectively. The loss
of porphyrinic acid peaks was attributed to the insolubility in
D2O. Interestingly, when the integrals of DMSO peak were
set as 100, the integral values of acetate peaks of 3D-AA and
UNs-AA-4h were calculated to 5.25 and 6.19, respectively.
More acetic acid molecules were coordinated with the struc-
ture during the synthesis process of UNs-AA compared to
3D-AA. SAXRD measurements were also performed to verify
the impact of acetic acid. As shown in Figure 2e, the peaks
of UNs-AA-4h shifted toward small angle, indicating that the
interlayer distance increased. This observation indicated that
large quantities of acetic acid molecules were inserted into
the interlayers to weaken the interaction of interlayers. To
our delight, this regularity was also found in UNs-OA-system
(Figures S10–S13, Supporting Information).
In consideration of all above phenomenon, a pseudoas-
sembly–disassembly mechanism was proposed to explain the
evolutionary process (Figure 3). The first step was pseudoas-
sembly process. When the reactants were mixed in the auto-
clave, the monocarboxylic acids were preferred to coordinate
with Zr clusters on account of the higher concentration than
porphyrin acid (e.g., the ratio of FA:Ni-TCPP was 150:1). As
the reaction proceeding, the deprotonated Ni-TCPP competed
with monocarboxylic acid to coordinate with Zr clusters, the
intermediate products formed in according to the crystal
1
of UNs-FA, revealing the formation of O2 (Figure 4b). More-
over, 2,2,6,6-tetramethyl-4-piperidone hydrochloride (4-oxo-
TMP) was able to trap O2 to afford the stable nitroxide radical
1
4-oxo-TEMPO, which could be measured through electron
spin resonance (ESR). To our delight, a triplet with a g value
of 2.006 assigned to the characteristic signal of 4-oxo-TEMPO
for UNs-FA was successfully detected, which further confirmed
the 1O2 generation (Figure 4c).[36] As a proof-of-concept applica-
tion, the UNs-FA was used as catalyst for the photooxidation of
1,5-dihydroxynaphthalene (1,5-DHN) to synthesize juglone. As
shown in Figure 4d, the absorption at 419 nm demonstrated
continuous increasing in the presence of UNs-FA, indicating
the formation of juglone. The UNs-FA showed higher catalytic
activity compared to that of the PCN-222 bulk counterparts[33]
(Figure 4e). The TEM and PXRD results of UNs-FA after
photocatalytic reaction demonstrated the outstanding stability
of nanosheets (Figures S15,16, Supporting Information). The
superior catalytic performance of UNs-FA might be ascribed to
Figure 3. Schematic illustration for the synthesis process of UNs through pseudo-assembly–disassembly strategy.
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Figure 4. a) UV–vis diffuse reflectance spectra of UNs-FA and PCN-222 bulks, b) UV–vis absorption spectra of 1,3-diphenylisobenzofuran upon visible-
light irradiation in the presence of UNs-FA, c) ESR spectra of the samples after mixing 4-oxo-TMP solution in the absence or presence of UNs-FA
under visible-light irradiation or in dark, d) photooxidation of 1,5-dihydroxynaphthalene catalyzed by UNs-FA, e) absorbance of juglone (λ = 419 nm)
as a function of reaction time with UNs-FA and 3D bulks.
that: (1) nanometer thickness to allow rapid mass transport; (2)
extremely high percentages of exposed catalytic active surfaces.
In conclusion, we have successfully developed a novel
strategy to synthesize ultrathin zirconium-porphyrinic MOF
nanosheets with thickness down to 1.5 nm in high yields.
Quantitative dissolution/NMR spectrometry and SAXRD
experiments are performed carefully to explore the growth
and unique properties. We believe that our strategy demon-
strates a great potential in designing and synthesizing other
MOF nanosheets with improved performance and multifunc-
tional applications.
Supporting Information
mechanism and
a pseudo-assembly–disassembly mecha-
nism was proposed. The UNs present excellent photocatalytic
performance owing to their intriguing aesthetic architectures
Supporting Information is available from the Wiley Online Library or
from the author.
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(2016YFA0202801), National Program on Key Basic Research Project
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Keywords
2D, metal–organic frameworks, photocatalysis, ultrathin nanosheets,
zirconium–porphyrin
Received: November 11, 2017
Revised: January 7, 2018
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