Concave ZnFe2O4 Hollow Octahedral Nanocages Derived from Fe-Doped MOF-5 for High-Performance Acetone Sensing at Low-Energy Consumption

Article abstract of DOI:10.1021/acs.inorgchem.7b02425

Herein, through a morphology-inherited annealing treatment of a hollow Fe-doped MOF-5 octahedron as the self-sacrificing template, we report the synthesis of concave ZnFe₂O₄ hollow octahedral nanocages as sensing materials, which exhibited high performance, including unprecedented excellent sensitivity, good selectivity, and cyclic stability at ultralow working temperature (120 °C).

Full text of DOI:10.1021/acs.inorgchem.7b02425

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Concave ZnFe2O4 Hollow Octahedral Nanocages Derived from Fe-
Doped MOF‑5 for High-Performance Acetone Sensing at Low-Energy
Consumption
Xue-Zhi Song,† Yu-Lan Meng,† Zhenquan Tan,† Liang Qiao,† Teng Huang,† and Xiao-Feng Wang*,‡
†School of Petroleum and Chemical Engineering and ‡School of Mathematics and Physics Science, Dalian University of Technology,
2 Dagong Road, Liaodongwan New District, Panjin 124221, Liaoning China
S
* Supporting Information
other fields.24−27 Previously, Wang et al. demonstrated that the
ABSTRACT: Herein, through a morphology-inherited
annealing treatment of a hollow Fe-doped MOF-5
octahedron as the self-sacrificing template, we report the
synthesis of concave ZnFe2O4 hollow octahedral nanoc-
ages as sensing materials, which exhibited high perform-
ance, including unprecedented excellent sensitivity, good
selectivity, and cyclic stability at ultralow working
temperature (120 °C).
sensing behavior of hollow microspheres, especially for multi-
shell hollow structures with higher specific area, outperformed
that of the bulk counterparts.28 Besides other well-developed
synthetic technologies,29,30 however, the most powerful and
conventional strategy to generate porous and hollow TMOs
relies on the thermal decomposition of hollow metal−organic
framework (MOF) precursors. Recently, MOFs have been
chosen as versatile precursors or sacrificial templates for the
synthesis of TMO nanomaterials, which have been used in the
electrocatalysis and energy storage fields.31−35 Nevertheless, the
research of porous TMOs from MOFs as gas sensors is still in its
infancy.36−38 By choosing Zn3[Fe(CN)6]2·xH2O as the
precursor, our group has just synthesized ZnO/ZnFe2O4 triple-
shelled hollow microspheres as high-performance sensors toward
acetone.39 Despite numerous fruits about MOF nanomaterials,
only a few of the hollow MOFs were presented, for instance,
zeolitic imidazolate framework (ZIF-8) nanobubbles, multi-
shelled hollow chromium(III) terephthalate MOFs (MIL-101),
and well-defined FeIII-MOF-5 hollow nanocages, by a selective
etching process and a surface-energy-driven mechanism.40−42
As a proof of concept, we herein propose the hollow bimetallic
MOF precursor-directed synthesis of concave ZnFe2O4 porous
hollow octahedral nanocages by thermal decomposition
(Scheme 1). As sensing materials, the integrated chemiresistive
gas sensor can deliver a high sensitivity of 64.4 toward 200 ppm
cetone, a hazardous chemical gas, can cause skin and eye
A
irritation, mood swings, and nausea and anesthetize the
central nervous system.1,2 Therefore, it is of great significance
and urgency to fabricate a highly efficient acetone gas sensor for
environmental monitoring and human health protection.
Chemiresistive gas sensors based on transition-metal oxide
(TMO) semiconductors have been paid more and more
attention because of their low cost, synthetic ease, facile
integration and portability, and excellent sensing ability,
rendering them as superior techniques over other gas detection
methods.3,4 Up to now, single TMOs, TMO composites, and
single-phase multi-TMOs as sensing materials integrated into
prospective sensors have been successfully fabricated.5−12
ZnFe2O4, a typical n-type semiconductor nanomaterial, has
been used in many fields as a photodegradation catalyst, an anode
material in lithium-ion batteries, and electrocatalyst for hydrogen
evolution reaction and even overall water splitting.13−18 Notably,
great endeavors dedicated to optimizing the microstructures of
ZnFe2O4, including yolk−shell microspheres, nanosheet-as-
sembled hollow microspheres, porous nanospheres, mesoporous
materials, and nanotubes, have been made to improve the gas-
sensing performances.19−23 Notwithstanding the achievements,
the sometimes complicated or high-energy consumption
synthetic process of sensing materials, the unsatisfactory gas-
sensing sensitivity, and the high operating temperature of the
sensors restrict their scalable and really practical application. The
enlarged surface area, the promotion of permeability, and the
improvement of the utility factor of the sensing body will achieve
superior TMO-based gas analyzers because gas sensing involves a
surface reaction between the target gas and sensing materials.
Thus, the porous hollow structures are envisaged to be excellent
candidates as advanced sensing materials.
Scheme 1. Schematic Preparation Process of Concave Porous
Hollow ZnFe2O4 Octahedral Nanocages
Hollow nanostructures have earned much more attention for
their intriguing structure-induced properties and applications in
catalysis, drug delivery, energy storage/conversion, and many
Received: September 22, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02425
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
acetone vapor. Remarkably, it also shows excellent selectivity and
good cyclic stability.
distance of O2 flux from the surface/edges (Figure 1b).44 TEM
observations revealed that the shell (a thickness of approximately
26 nm) of the concave hollow nanocage consisted of a large
number of ultrafine nanoparticles of ≈6−9 nm size (Figure 1c).
The nanoparticle building blocks are homogeneously distributed
with interconnected, numerous pores left. This appealing porous
hollow feature would greatly favor for generation of more
reactive sites and acceleration of mass transport. Clear lattice
fringes with a spacing of ≈0.25 nm in the high-resolution TEM
(HRTEM) image can be indexed to the (311) lattice plane of the
spinel ZnFe2O4 (Figure 1d). Moreover, the selected-area
electron diffraction (SAED) pattern (Figure 1d, inset) with a
series of concentric rings reveals the polycrystalline character-
istics and corresponds well to the (440), (400), and (311) planes
of ZnFe2O4. The elemental mapping images (Figure 1f−h)
evidently reveal the homogeneous distribution of the Zn, Fe, and
O elements throughout the whole hollow nanocage. The X-ray
photoelectron microscopy (XPS) survey spectrum (Figure S5)
also suggests the presence of the Zn, Fe and O elements in the
final ZnFe2O4 nanocages. The fitting peaks in the high-resolution
Zn 2p spectrum centered at ≈1044.37 and ≈1021.3 eV
corresponding to Zn 2p1/2 and Zn 2p3/2, respectively, reveal
the ZnII oxidation state to be in line with the octahedral site (B
site) of ZnFe2O4. The binding energies (BEs) for Fe 2p3/2 at
≈712.9 and ≈711.0 eV are assigned to the tetrahedral (A site)
and octahedral (B site) sites, respectively. Moreover, another two
peaks at ≈725.0 and ≈719.2 eV are attributed to Fe 2p1/2 and the
shakeup satellite structure, respectively. All of the above results
verify the FeIII oxidation state in the ZnFe2O4 phase. The high-
resolution XPS spectrum of O 1s can be deconvoluted into two
peaks at BEs of ≈531.47 and ≈529.86 eV, which are
Well-defined FeIII-doped MOF-5 hollow octahedra as the
starting self-sacrificial templates were synthesized through a one-
pot solvothermal method.42 No obvious diffraction peak in the
powder X-ray diffraction (PXRD) pattern reveals that these
octahedra are typically amorphous (Figure S2), which refers to
infinite coordination polymers.43 A panoramic field-emission
scanning electron microscopy (FESEM) image displayed in
Figure 1a clearly demonstrates that well-defined uniform MOF
characteristic of surface-adsorbed oxygen species, such as
−
O2
and O−ads, and a surface lattice oxygen (O2−),
ads
respectively.45 The adsorbed oxygen species are the reactive
species with target gas molecules, playing a significant role in the
improvement of the gas-sensing performances.46 The high
Brunauer−Emmett−Teller (BET) specific surface area of the
final ZnFe2O4 nanocages was calculated to be 150.5 m2·g−1. The
pore size distributed hierarchically and concentrated around 2.7,
4.0, 6.8, and 13.1 nm, indicating a mesoporous characteristic
(Figures S6). Such a mesoporous hollow nanocage with high
specific surface area would provide ample reactive sites and
facilitate target gas diffusion, enabling the construction of a high-
performance sensor device.
Figure 1. (a) SEM images of an FeIII-doped MOF-5 precursor (inset
scale bar 500 nm). (b) SEM images (inset scale bar 250 nm), (c) TEM
images (inset scale bar 100 nm), (d) HRTEM images, with the inset
showing the SAED pattern, (e) HAADF-STEM image, (f−h) Energy-
dispersive spectroscopy elemental mapping images of ZnFe2O4
octahedral nanocages.
The temperature-dependent sensing behavior of a gas sensor
based on as-prepared ZnFe2O4 hollow octahedral nanocages was
first investigated to disclose its optimum operating temperature
because the temperature affects gas adsorption/desorption
processes and surface reactions. As shown in Figure 2a, with an
increase in the operating temperature, the responses toward all of
the tested gases display an “increase−maximum−decrease”
tendency. At a low temperature, the reaction between the tested
gas molecules and surface-adsorbed oxygen species was too inert
to give a high response. As the operating temperature increased,
the test gas molecules were activated enough to overcome the
activation energy barrier to react with more surface-absorbed
oxygen species, leading to a significantly enhanced response.
Because of the low utilization rate of the sensing material caused
by limited gas adsorption and diffusion, the response of the
sensor was reduced at higher temperature. The optimum
operating temperature of 120 °C was applied for further
investigations of the sensing performance because the largest
response to an acetone value higher than those of other
particles with an average size of around 280 nm (an average edge
length of 240 nm) were obtained. From the magnified FESEM
image, the different contrast in the octahedral particle with a
smooth surface reveals that it is hollow (Figure 1a, inset). A
transmission electron microscopy (TEM) image further
discloses the hollow nature of these octahedral nanostructures,
consistent with the high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) image
(Figure S3). The homogeneous coexistence of the Fe and Zn
elements in the precursor provides the synthetic possibility of a
multimetal oxide after an annealing treatment in air. The
broadening PXRD peaks of the annealed product can be indexed
to cubic spinel ZnFe2O4 (JCPDS 65-3111), also suggesting a
nanocrystalline nature with ultrafine crystallite sizes. Interest-
ingly, the annealed product well preserves the original octahedral
morphology of the precursor. It is notable that the surfaces of the
octahedral nanocages are quite rough and more acutely inward
compared to the precursor because of the stress difference caused
by gas release and the diffusion discrepancy in the rate and
B
DOI: 10.1021/acs.inorgchem.7b02425
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
nanocages to form active oxygen species (O2 ads, O−ads, and
−
O2−ads) by capturing electrons from the conduction bands of
ZnFe2O4, resulting in a thick electron depletion layer, a relatively
high potential barrier on the surface region, and a high resistance.
After the introduction of a reducing gas, the acetone molecules
could react with the former active oxygen species to generate
CO2 and H2O and reinject the trapped electrons back to the
conduction band, consequently reducing the electron depletion
layer thickness and lowering the resistance of the sensor (Figure
3).
Figure 2. (a) Response of the sensor to different gases (200 ppm) at
different working temperatures. (b) Dynamic response−recovery
transients of the sensor to acetone at different concentrations. (c)
Responses of the gas sensor to various gases (100 ppm). (d)
Reproducibility of the sensor on successive exposure to 200 ppm
acetone.
Figure 3. Schematic diagram of the acetone sensing mechanism.
counterparts was achieved. With increasing acetone concen-
tration, the response values increased significantly from 3.27,
6.27, 9.71, 20.51, 35.52, and 64.42 to 127.06 at concentrations of
5, 10, 20, 50, 100, 200, and 500 ppm, respectively, observed from
the dynamic response−recovery transients in Figure 2b.
In summary, through annealing treatment of hollow Fe-doped
MOF-5 octahedra, concave ZnFe2O4 hollow octahedral
nanocages have been successfully prepared with high specific
surface area and porous nanostructures assembled by nano-
particles as primary building blocks. As a sensing material, this
chemiresistive sensor possesses unprecedentedly high sensitivity
with its response value of 64.4−200 ppm acetone, as well as good
selectivity and cyclic stability at an ultralow working temperature
(120 °C). This work may pave the way toward a universal
fabricative methodology of an advanced gas sensor based on
TMO nanomaterials from proper MOF nanoprecursors.
Compared with other previously relevant acetone gas sensors
(Table S1), such as ZnFe2O4 nanoparticles (39.5, 200 ppm, and
200 °C),47 ZnFe2O4 yolk−shell microspheres (near 40.5, 100
ppm, and 200 °C),19 ZnFe2O4−ZnO composite hollow
microspheres (20.8, 200 ppm, and 320 °C)48 and ZnFe2O4
nanosheets on ZnO hollow spheres (16.8, 100 ppm, and 250
°C),49 our sensor response value is much higher or comparable
to the above examples at much lower working temperature,
rendering it as a highly sensitive, safer, really applied gas sensor at
low-energy consumption. The appealing highly sensitive sensing
ability may be ascribed to the high BET surface area, porous
feature, and nanoparticle building block size comparable with the
electron depletion layer thickness of hollow ZnFe2O4 nanocages,
favoring more active oxygen species, gas diffusion, and complete
depletion, which are in agreement with the three key factors (the
receptor function, utility factor, and transducer function) in
semiconductor oxide gas sensors proposed by Yamazoe et al.50,51
A bar graph in Figure 2c shows a comparison of the responses
of the sensor toward various gases (acetone, ethanol, form-
aldehyde, toluene, methanol, and hexane) at 100 ppm
concentration to demonstrate the good selectivity because the
response toward acetone is remarkably higher as opposed to
those of the other counterparts (at least about twice that of
ethanol). The response−recovery behavior could be well-
repeated with no clear attenuation in response upon alternate
exposure to air and 200 ppm acetone gas for several cycles, which
indicates the superb robustness and repeatability of the as-
fabricated sensor (Figure 2d).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02425.
■
S
Experimental section, TEM images, XPS spectra, and
PXRD patterns (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: wangxf@dlut.edu.cn.
■
ORCID
Zhenquan Tan: 0000-0002-1730-7290
Xiao-Feng Wang: 0000-0002-5413-4798
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The most accepted sensing mechanism of n-type semi-
conductor oxide ZnFe2O4 belongs to the surface-controlled
model, which is based on a resistance change of the semi-
conductive sensor caused by the adsorption/desorption of
different gaseous molecules. In an air atmosphere, oxygen
molecules would be adsorbed on the surface of ZnFe2O4 hollow
The authors are grateful for financial support from the National
Natural Science Foundation of China (Grants 21601027,
51602035, and 21571028), the Fundamental Research Funds
for the Central Universities [Grants DUT17LK33 and
DUT16RC(4)69], and the Xinghai Scholars Program from
Dalian University of Technology.
C
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DOI: 10.1021/acs.inorgchem.7b02425
Inorg. Chem. XXXX, XXX, XXX−XXX