MOF-derived hierarchical ZnO/ZnFe2O4 hollow cubes for enhanced acetone gas-sensing performance

Article abstract of DOI:10.1039/c7ra04437b

ZnO/ZnFe₂O₄ hollow cube composites with heterogeneous structure are synthesized by a facile strategy through simple and direct pyrolysis of Fe^III-modified Zn-based metal-organic frameworks. The as-synthesized ZnO/ZnFe₂O₄ hollow cubes have well-defined cube morphology with an ～2 μm and multiple porous shell constructed from interpenetrated ZnO and ZnFe₂O₄ heterogeneous nanoparticles, providing structurally combined mesoporous channels for facilitating the diffusion and surface reaction of gas molecules. In addition, a comparative sensing performance investigation between ZnO/ZnFe₂O₄ hollow cubes and singular ZnO demonstrates that, in contrast with ZnO, the ZnO/ZnFe₂O₄ hollow cubes show significantly enhanced chemical sensing sensitivity towards low-concentration acetone. Furthermore, the ZnO/ZnFe₂O₄ hollow cubes exhibit good reproducibility and selectivity towards gaseous acetone. The enhanced sensing performance of the MOF-derived ZnO/ZnFe₂O₄ hollow cubes is ascribed to the unique hierarchical structure with high specific surface area, abundant exposed active sites with surface-adsorbed oxygen species and heterojunctions formed at the interfaces between ZnO and ZnFe₂O₄.

Full text of DOI:10.1039/c7ra04437b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
MOF-derived hierarchical ZnO/ZnFe2O4 hollow
cubes for enhanced acetone gas-sensing
performance†
Cite this: RSC Adv., 2017, 7, 34609
a
a
a
Xiang Ma, ab Xinyuan Zhou,ab Yan Gong,ab Ning Han, Haidi Liu and Yunfa Chen
*
ZnO/ZnFe2O4 hollow cube composites with heterogeneous structure are synthesized by a facile strategy
through simple and direct pyrolysis of FeIII-modified Zn-based metal–organic frameworks. The as-
synthesized ZnO/ZnFe2O4 hollow cubes have well-defined cube morphology with an ꢀ2 mm and
multiple porous shell constructed from interpenetrated ZnO and ZnFe2O4 heterogeneous nanoparticles,
providing structurally combined mesoporous channels for facilitating the diffusion and surface reaction
of gas molecules. In addition, a comparative sensing performance investigation between ZnO/ZnFe2O4
hollow cubes and singular ZnO demonstrates that, in contrast with ZnO, the ZnO/ZnFe2O4 hollow cubes
show significantly enhanced chemical sensing sensitivity towards low-concentration acetone.
Furthermore, the ZnO/ZnFe2O4 hollow cubes exhibit good reproducibility and selectivity towards
gaseous acetone. The enhanced sensing performance of the MOF-derived ZnO/ZnFe2O4 hollow cubes
is ascribed to the unique hierarchical structure with high specific surface area, abundant exposed active
sites with surface-adsorbed oxygen species and heterojunctions formed at the interfaces between ZnO
and ZnFe2O4.
Received 20th April 2017
Accepted 28th June 2017
DOI: 10.1039/c7ra04437b
rsc.li/rsc-advances
widespread interest in gas sensors because the unique structure
offer large specic surface area with abundant active sites,
1. Introduction
modulative interfacial electron transfers with enhanced redox
capacities compared with their singular solid building block
units.16–18 The self-sacricial template strategy based on ther-
molysis of solid precursors remains the most attractive one in
the fabrication of hollow micro-/nanostructures and selecting
the right template material is the key factor, as the self-
sacricial precursor should have the ability to be removed
easily under thermal annealing condition as well as preserve the
original stereoscopic shape.
Metal–organic frameworks (MOFs) are emerging as a class of
very important materials offering high levels of porosity with
considerable control over pore size and composition. Such
properties play a crucial role in functional applications in gas
adsorption,19 sensing,20 and catalysis.21 Besides, the
morphology of MOFs can be easily tuned through different
metal ions and organic bridging ligands under different
synthesis conditions.22–26 Utilizing MOFs as templates offers
a valid strategy for designing novel structure of porous carbon
and metal oxides composites. For example, NiCo2O4/NiO hollow
dodecahedra composite were synthesized by thermolysis of Ni
containing ZIF-67 MOFs as template for enhanced lithium-ion
battery.27 a-Fe2O3 porous nanorods were prepared from MIL-
88A MOFs with enhanced gas sensing properties.28 MOF-5 was
employed as template to fabricate hierarchical hollow ZnO
nanocages with enhanced VOCs gas-sensing performance.29
Fe3C nanocrystallite embedded N-doped carbon matrix derived
Gas sensors based on metal oxide semiconductors have received
widespread interest during the past decades owing to their low
power consumption, easy manufacture, low cost, as well as
portable characteristics in the detection of various explosives,
and harmful, and toxic gases.1–3 To date, many metal oxide
semiconductors such as ZnO,4 SnO2,5 In2O3,6 a-Fe2O3,7 NiO,8
and WO3 (ref. 9) have been extensively investigated for gas
sensing. It is well acknowledged that the performance of
sensors including increased sensitivity, enhanced selectivity,
outstanding reproducibility and long-term stability strongly
depend on the gas sensing materials so that great efforts have
been made to enhance the sensing properties such as noble
metal loading,10 designing novel material structures,11 and
construction of heterojunctions.12 These efforts have indeed
improved the sensing properties based on the fact that surface
morphology, microstructure, crystalline size and composition
could affect the sensing behaviors of the metal oxide semi-
conductors.13–15 Hollow micro-/nanostructures have attracted
aState Key Laboratory of Multiphase Complex System, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing, 100190, China. E-mail: chenyf@ipe.ac.cn; Tel:
+86 1082544896
bUniversity of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra04437b
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34609–34617 | 34609

View Article Online
RSC Advances
Paper
ꢁ
from MIL-88B@ZIF-8 dual-MOFs exhibit remarkable electro- reuxed at 100 C for 8 h. The umber precipitant was washed
catalytic activity for the ORR in a basic electrolyte.30 These great with ethanol several times by centrifugation. As-obtained
progresses were achieved due to the remarkable properties with precipitant was dried at 80 ꢁC under vacuum for 24 h. The
selectable metal ion, multifunctional organic ligands, well- collected powders were used as precursors to prepare ZnO/
developed porosity and diverse morphologies of MOFs.
ZnFe2O4 samples by temperature-programmed thermal
Among many metal oxide semiconductors, ZnO is an decomposition process with a slow rate of 1 ꢁC minꢂ1 from
attractive n-type semiconductor with a direct wide bandgap room temperature to 500 ꢁC and kept at 500 ꢁC for 2 h under air
(3.37 eV) as a popular gas sensing material because of its rela- atmosphere. The synthesis of singular ZnO nanoparticle and
tively high sensitivity and ease of fabrication.31 However, the singular ZnFe2O4 are presented in the ESI.†
sensing properties of pure ZnO are still unsatisfactory for
applications in real industrial purposes due to the high oper-
ating temperature and poor selectivity.32 Recently, many re-
ported investigations have demonstrated that the sensing
performances of ZnO could be improved by the heterostructure
formation techniques. For example, Qu et al. have fabricated
gas sensors using ZnO/ZnCo2O4 hollow core–shell nanocages
synthesized by the thermolysis of ZIF-8/Co–Zn hydroxide
precursor, showing enhanced sensitivity to xylene and excellent
reversibility.33 Liu et al. have synthesized ZnFe2O4 nanoparticles
decorated on ZnO microowers by a simple mild solution
method, demonstrating higher response to acetone compared
with single ZnO microower.34 Although some improvements
had been accomplished, there still existed some unnegligible
drawbacks related to the unsatisfactory low detection limit of
the sensing materials.
In this work, we show a generally applicable strategy for the
synthesis of ZnO/ZnFe2O4 hollow cubes which includes the
synthesis of FeIII-modied IRMOF-3 cubes nanostructure
precursor and subsequent transformation to uniform ZnO/
ZnFe2O4 hollow cubes by thermal annealing of the as-prepared
precursor in air atmosphere. Moreover, gas sensors based on
the above products were fabricated, and their gas sensing
2.3 Characterization of materials
Powder X-ray diffraction (XRD) patterns of the catalysts were
measured on a Panalytical X'Pert PRO system using Cu-Ka
radiation. Thermogravimetric analysis (TGA) was performance
on a NETZSCH STA449F3 apparatus with a heating rate of 10 ꢁC
minꢂ1 from 35 ꢁC to 900 ꢁC under air atmosphere. The N2
adsorption–desorption isotherms were measured on automatic
surface analyzer (SSA-7300, China). Before the measurement,
the samples were outgassed at 100 ꢁC for 10 h. The morphology
of materials were characterized by eld-emission scanning
electron microscopy (SEM) via an electron microscope (JEOL
2100F, operating at 15 kV) and transmission electron micros-
copy (TEM) with an accelerating voltage of 200 kV. The chemical
compositions of the samples were measured by energy disper-
sive X-ray spectrometry (EDS) that was attached on TEM. Surface
species of the as-prepared catalysts were determined by X-ray
photoelectron spectroscopy (XPS) using an XLESCALAB 250 Xi
electron spectrometer from VG Scientic with a monochromatic
Al Ka radiation.
2.4 Gas sensing measurement
performance was investigated. It is revealed that the ZnO/ The gas sensing experiments were performed on a homemade
ZnFe2O4 hollow cubes displayed a higher response to acetone at tube-furnace sensor test system. The diameter of tube-furnace is
a lower working temperature than the singular ZnO particles. 5 cm, and length is 1 m. Typically, two Pt wires were xed on an
The formation of the heterojunction between ZnO and ZnFe2O4 Al2O3 substrate by silver paste (Wuhan Double bond Chemical
and porous structure of ZnO/ZnFe2O4 materials are likely to be Co., Ltd,. China). Aer that, a certain amount of the as-
the sources of the enhancement in gas sensing properties.
synthesized ZnO/ZnFe2O4 powders were dispersed into
ethanol and then drop-coated onto the Al2O3 substrate to form
a lm. The obtained gas sensor was placed into the tube-furnace
2. Experimental
ꢁ
and kept at thermostable temperature of 300 C over night to
2.1 Materials
ensure good ohmic contacts. The ohmic contacts were
conrmed by the Keithley 2601 Sourcemeter (Keither Instru-
ment Inc., USA) with a rectangular sweep voltage (continuously
set at 5 V, 2.5 V, 0 V, ꢂ2.5 V, and ꢂ5 V). The detected gases with
different concentrations were obtained through mixing the
synthetic air (20.9 vol% O2, 79.1 vol% N2) and standard gases
(50 ppm in synthetic air for acetone, ethanol, toluene, benzene
and 50 ppm in N2 for formaldehyde and ammonia, Beijing Hua
Yuan Gas Chemical Industry Co., Ltd., China) in a mixer before
entering into the tube furnace. The total ow velocity of gases is
Zn(NO3)2$6H2O was purchased from Aladdin. Fe(acac)3 and 2-
aminoterephthalic acid (H2N-BDC) were purchased from Alfa.
Polyvinyl pyrrolidone (PVP, M. W. 30 000), N,N-dimethylforma-
mide (DMF) and ethanol were purchased from Sinopharm. All
chemicals were used directly without further purication. The
water was made from Millipore Milli-Q water (15 MU cm).
2.2 Preparation of ZnO/ZnFe2O4 hollow cubes
The FeIII-modied IRMOF-3 precursor was synthesized accord- 600 mL minꢂ1. The voltage bias was xed at 5 V and the current
ing to the previously reported method.35 Typically, 3.34 g of was recorded using Keithley 2601 sourcemeter. The sensor
Zn(NO3)2$6H2O, 4.32 g of Fe(acac)3, 0.68 g of H2N-BDC and response is dened as R ¼ Rair/Rgas where Rair and Rgas are the
14.2 g of PVP were dispersed into a 480 mL of mixed solvent sensor resistance in air and in the test gas, respectively. The
(DMF/ethanol ¼ 5 : 3). The resulting solution was stirred for 1 h response time is dened as the time required for sensor resis-
to obtained a uniformly dissolution of the reactants, and then tance reaching to 90% of the nal value aer exposed to the
34610 | RSC Adv., 2017, 7, 34609–34617
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
target gas. The recovery time is dened as the time required for heterogeneous phases get conrmed. The XRD patterns of
reducing to 10% of the saturation value aer exposed to clean singular ZnO and ZnFe2O4 are shown in the Fig. S1 and S2,†
air.
which is indexed to the wurtzite crystal phase of ZnO and spinel
ZnFe2O4 respectively. The thermogravimetric analysis (TGA) of
FeIII-modied IRMOF-3 precursor was investigated and shown
in Fig. 1c. A two-step thermal decomposition process was
observed includes the rst weight loss of 8% from 35 ꢁC to
150 ꢁC which is attributed to the loss of adsorbed H2O and
alcohol. Whileꢁthe second weight loss was about 60.1% between
3. Result and discussion
3.1 Characterizations of materials
The crystallographic structure of the synthesized FeIII-modied
IRMOF-3 precursor and ZnO/ZnFe2O4 materials were deter-
mined by XRD measurements. The detected diffraction peaks of
FeIII-modied IRMOF-3 precursor in Fig. 1a are in good agree-
ment with previously reported simulated IRMOF-3 patterns.36 It
was conrmed that the crystal structure of IRMOF-3 could be
maintained in the precursor even adding the Fe ions, which
may be attributed to a small quantity of Fe doping.37 It should
be pointed out that the intensities of diffraction peaks at 2q ¼
6.9ꢁ and 9.7ꢁ in FeIII-modied IRMOF-3 precursor have been
changed compared with IRMOF-3 which could be induced by Fe
ions modication. Fig. 1b illustrates the XRD patterns of the
nal products aer annealing in air which are indexed to the
combined wurtzite crystal phases of ZnO (JCPDS no. 036-1451)
and spinel ZnFe2O4 (JCPDS no. 077-0011), indicating co-existing
of two heterogeneous phases in the synthesized products
including ZnO and ZnFe2O4. It should be pointed out that the
diffraction peaks of ZnFe2O4 are not obvious, probably because
of the large content of ZnO or the overlap of ZnFe2O4 and ZnO
peaks. But obviously, the color of ZnO/ZnFe2O4 is very different
from that of bare ZnO shown in Fig. 1b, from which the
ꢁ
150 C to 500 C, which most likely resulted from the decom-
position of organic ligands and PVP surfactant resulting in
releasing CO2, NOx and H2O. No obvious weight loss was
observed aer 500 ꢁC and ZnO/ZnFe2O4 was subsequently
formed. The N2 adsorption–desorption isotherms of the as-
prepared ZnO/ZnFe2O4 material is presented in Fig. 1d, and
feature type IV isotherm, indicating mesoporous properties of
ZnO/ZnFe2O4 hollow cubes. The pore diameter of ZnO/ZnFe2O4
is about 8 nm in the inset of Fig. 1d and the BET surface area is
50 m2 gꢂ1. Evidently, the as-prepared ZnO/ZnFe2O4 hollow
cubes possess typical hierarchical structures with micro-/
nanoscale particulate systems, which offer sufficiently inter-
face and active sites for the gaseous adsorption–desorption and
the chemical reaction between analyte molecules and ZnO/
ZnFe2O4. The N2 adsorption–desorption isotherm of the
singular ZnO and ZnFe2O4 nanoparticles are shown in Fig. S3.†
The morphologies and microstructures of FeIII-modied
IRMOF-3 precursors and their derivative ZnO/ZnFe2O4 hollow
cubes are shown in Fig. 2. The FeIII-modied IRMOF-3
Fig. 1 XRD patterns of (a) FeIII-modified IRMOF-3 precursor and (b) ZnO/ZnFe2O4 hollow cubes (the insets are the photographs of ZnO/ZnFe2O4
and singular ZnO); (c) TGA curve of FeIII-modified IRMOF-3 precursor; (d) N2 adsorption–desorption isotherm of the ZnO/ZnFe2O4 hollow cubes
(the inset is the corresponding BJH pore size distribution).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34609–34617 | 34611

View Article Online
RSC Advances
Paper
Fig. 2 SEM images of (a) FeIII-modified IRMOF-3 precursor and (b) ZnO/ZnFe2O4 hollow cubes; (c) TEM image of the edge of ZnO/ZnFe2O4
hollow cubes; (d) HRTEM image taken from the frame marked region in (c).
precursors in Fig. 2a illustrated well-dened cube morphologies mapping spectra of selected individual ZnO/ZnFe2O4 hollow
with average particle sizes of about 2 mm. As shown in Fig. 2b, cube in Fig. 3 show that the elements of Zn and Fe are uniformly
aer calcination process at 500 ꢁC, both the cube shape and size dispersed within the shell, which further conrms the hollow
of the precursors remain intact except that the outside surface structure of ZnO/ZnFe2O4. It should be pointed out that the Fe
become little collapsing inwards indicating the hollow structure content is relative low (3.34 wt%) in ZnO/ZnFe2O4 sample.
of ZnO/ZnFe2O4 cubes. The hollow structure is further Fig. 2d shows the HRTEM image of selected region from Fig. 2c,
conrmed from the TEM image in Fig. 2c, which shows the edge indicates the ZnO–ZnFe2O4 hetero-junctions of in-shell nano-
of ZnO/ZnFe2O4 composites with clearly hollow shell and the particles with two sets of clear lattice fringes and unclear
shell of individual ZnO/ZnFe2O4 hollow cubes was constructed interfacial domains, indicative of crystalline detects and
by ultrane interconnected nanoparticles. Moreover, the EDS distortions due to two-phase transition at heterogeneous
Fig. 3 (a) TEM image of ZnO/ZnFe2O4 hollow cube; (b) EDS analysis of the elemental composition of ZnO/ZnFe2O4 hollow cube; (c and d) EDS
mapping images of ZnO/ZnFe2O4 hollow cube.
34612 | RSC Adv., 2017, 7, 34609–34617
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
interfaces. The lattice spacings of 0.247 nm and 0.254 nm to the surface adsorbed oxygen and the small peak at binding
correspond to the (101) plane of the wurtzite ZnO structure and energy of 533.1 eV is ascribed to the adsorbed OH groups and
the (311) plane of the spinel ZnFe2O4 structure, respectively. molecular water.40–42 It is well known that the surface-absorbed
The good interfacial contact between ZnO and ZnFe2O4 oxygen species is closely with the electron-donor oxygen
contribute to easy electron transfer between the two phases, vacancies, which adsorb the oxygen molecules, and ionize them
which is favorable for the enhanced gas sensing performances. into reactive surface adsorbed oxygen species (O2ꢂ, Oꢂ, O2ꢂ
The TEM and SEM images of singular ZnO and ZnFe2O4 are which is related to the gas sensing performance of sensing
shown in Fig. S4.† material. The contents of adsorbed oxygen species in ZnO/
)
XPS analysis was performed to examine the surface chemical ZnFe2O4 are clearly higher than that of singular ZnO (29.4% for
composition and valence state of the elements of the as- ZnO/ZnFe2O4 and 23.7% for ZnO). Therefore, it is anticipated
synthesized ZnO/ZnFe2O4 sample, and the XPS spectra are that the ZnO/ZnFe2O4 sample performs a better gas-sensing
presented in Fig. 4. Fig. 4a shows the wide spectrum of the ZnO/ behavior than the singular ZnO counterparts.
ZnFe2O4 sample, conrming the existence of Zn, Fe and O
elements. Zn 2p spectrum in Fig. 4b displays two asymmetric
3.2 Gas sensing performance
peaks, corresponding to the Zn 2p1/2 and Zn 2p3/2 states of Zn 2p
To demonstrate the signicantly enhanced gas sensing prop-
orbits, respectively. The tting peaks with binding energy of
erties of the as-prepared ZnO/ZnFe2O4 hierarchical hollow
1045.1 eV and 1022.0 eV are assigned to Zn 2p1/2 and Zn 2p3/2
cubes with interpenetrated nanoparticles as blocking units, the
singular ZnO nanoparticles were chosen as blank reference. It is
photoelectron responses of spinel-structural ZnFe2O4, and
other two tting peaks located at 1044.4 eV and 1021.3 eV are
well-known that the operating temperature has a great impact
on the reaction between the adsorbed gas and sensing mate-
rials. Therefore, the temperature dependent properties of the
as-fabricated gas sensors were measured rst. As shown in
Fig. 5a, the response of the two kinds of sensors increased with
the increasing temperature until the value reached maximum at
an optimum operating temperature, and then decreased with
further increasing of the operating temperature. The optimum
operating temperature ranges from 200 ꢁC to 250 ꢁC for the
synthesized ZnO/ZnFe2O4 sample, which is lower than that of
assigned to divalent Zn in the ZnO,38 which is also in accordance
with singular ZnO sample presented in Fig. S4.† As for Fe 2p
spectrum in Fig. 4c, the binding energy at 724.7 eV and 710.4 eV
are ascribed to Fe 2p1/2 and Fe 2p3/2. The shake-up satellite
peaks at binding energy are at range of 715.7–720.0 eV, which
are characteristic of Fe3+ states in spinel ZnFe2O4 phase.39 The
XPS signal of Fe 2p spectrum is weak because of the relative low
content of Fe element. Besides, the asymmetrical O 1s signal
can be tted with three components: one peak at binding
energy of 530.4 eV is attributed to the lattice oxygen; another
peak at the medium with binding energy of 532.8 eV is assigned
ꢁ
ꢁ
250 C to 300 C for the singular ZnO counterparts. Therefore,
the further measurements were performed at 250 ꢁC. Fig. 5b
Fig. 4 XPS spectra of (a) survey of the spectra; (b) Zn 2p spectra; (c) Fe 2p spectra and (d) O 1s spectra of the as-prepared ZnO/ZnFe2O4 sample.
(OI: lattice oxygen; OII: surface adsorbed oxygen; OIII: adsorbed OH groups and molecular water).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34609–34617 | 34613

View Article Online
RSC Advances
Paper
provides the gas responses of the two kinds of gas sensors as
a function of the concentration of acetone at 250 ꢁC. The results
showed that the gas responses of the two kinds of sensors
increased with the increasing concentration of acetone and
almost linear response versus concentration trend for the ZnO/
ZnFe2O4 hollow cubes indicates a perfect potential in the
quantitative gas analysis. It is obvious that the ZnO/ZnFe2O4
hollow cubes revealed a quite enhanced gas response compared
with that of singular ZnO at each concentration of acetone. It
can be observed that when the acetone concentration was as low
as 0.5 ppm, it still had a response of 2.4, indicating the ultra-low
detection limit of ZnO/ZnFe2O4 hollow cubes. Moreover, the
response of ZnO/ZnFe2O4 hollow cubes towards 5 ppm acetone
is much higher than that of singular ZnFe2O4 nanoparticles
which is only 1.36 at 250 ꢁC. The dynamic response–recovery
curve of ZnO/ZnFe2O4 hollow cubes and singular ZnO towards
different acetone concentrations at 250 ꢁC are shown in Fig. 5c.
The response/recover times are 5.9/7.4 min, 5.7/6.5 min, 5.8/
6.3 min, 5.6/6.0 min for 0.5, 1.0, 2.0, 5.0 ppm acetone, respec-
tively, longer than other ZnO/ZnFe2O4-based sensors towards
relative high-concentration acetone. This may be ascribed to the
slow surface adsorption–desorption kinetics of gaseous acetone
molecules for the lower concentration acetone detection in our
selected experimental conditions.43,44 Importantly, the ZnO/
ZnFe2O4 hollow cubes sensor performs higher response towards
acetone sensing compared with previously reported ZnO/
ZnFe2O4 sensing materials with different microstructure and
summarized in Table 1.
Other important gas sensing parameters including repro-
ducibility long-term stability and selectivity were also investi-
gated using ZnO/ZnFe2O4 hollow cubes as sensing material and
shown in Fig. 6. Obviously, the ZnO/ZnFe2O4 hollow cubes
illustrate good reproducibility towards 5 ppm acetone at 250 ꢁC
operating temperature aer six on–off cycles, and the response
value is 9.4 ꢃ 7.1%. Moreover, ZnO/ZnFe2O4 hollow cubes based
sensor to 5 ppm acetone at a constant working temperature of
250 ꢁC during a long-term stability measurement of 14 days was
conducted. The results showed that the response of the ZnO/
ZnFe2O4 sensor changed a little, which conrmed good stability
of ZnO/ZnFe2O4 hollow cubes. In addition, the selectivity of
ZnO/ZnFe2O4 hollow cubes was also tested at 250 ꢁC towards
5 ppm teat gases including acetone, ethanol, formaldehyde,
toluene, benzene and ammonia which are common indoor
gaseous contaminants. Signicantly, the ZnO/ZnFe2O4 sensor
possesses good selectivity to detect acetone at 250 ꢁC, and shows
low response or almost insensitive to other interference gases at
the same operating temperature and shown in Fig. 6c. The good
selectivity for acetone may be ascribed to the bond dissociation
energy (BDE) of different gases. The BDE of H–CH2COCH3 (393
kJ molꢂ1) in acetone is less than that of H–OCH2CH3 (436 kJ
molꢂ1) in ethanol, H–C6H5 (431 kJ molꢂ1) in benzene, but
similar with CH3–C6H5 (389 kJ molꢂ1) in toluene. Another factor
may be associated with the number of electrons released during
the redox reaction at 250 ꢁC operating temperature presented in
the following equations. Therefore, the ZnO/ZnFe2O4 hollow
cubes show good selectivity towards acetone with relatively
lower bond dissociation energy and more released electrons.
Fig. 5 (a) Gas sensing response of ZnO/ZnFe2O4 hollow cubes and
singular ZnO to 5 ppm acetone at different operating temperature; (b)
comparison of responses between ZnO/ZnFe2O4 hollow cubes and
singular ZnO operating at 250 ꢁC; (c) the dynamic response–recovery
curve of ZnO/ZnFe2O4 hollow cubes and singular ZnO towards
different acetone concentration.
34614 | RSC Adv., 2017, 7, 34609–34617
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Table 1 Comparison of acetone sensing performance of gas sensors based on various ZnO/ZnFe2O4 microstructures
Materials
Temperature (ꢁC)
Concentration (ppm)
Response (Rair/Rgas
)
Reference
ZnFe2O4/ZnO hollow microspheres
ZnFe2O4/ZnO microowers
ZnFe2O4 decorated ZnO rod
ZnFe2O4 nanosheets on ZnO sphere
ZnO/ZnFe2O4 hollow cubes
320
250
260
250
250
100
50
50
10
5
7.5
8.3
3.4
4.8
9.4
45
34
46
47
This work
Fig. 6 (a) The reproducibility of ZnO/ZnFe2O4 hollow cubes towards 5 ppm acetone at 250 ꢁC operating temperature; (b) the long-term stability
test of ZnO/ZnFe2O4 hollow cubes towards 5 ppm acetone at 250 ꢁC; (c) the selectivity test of ZnO/ZnFe2O4 hollow cubes for 5 ppm teat
gaseous acetone, ethanol, formaldehyde, toluene, benzene and ammonia at 250 ꢁC operating temperature.
The above results demonstrate that ZnO/ZnFe2O4 hollow cubes nanostructure and formation of ZnO–ZnFe2O4 heterojunctions
with enhanced sensing performance having the potential for at the ZnO/ZnFe2O4 interfaces. First of all, the hollow cube
the detection of low-concentration acetone.
structure with mesopores can provide a high accessible surface
area, thus more surface active sites are available for the sensing
reactions between teat gas molecules and adsorbed oxygen ion
species, contributing to higher gas sensing performance.
Besides, the sensing mechanism of n-type semiconducting
metal oxides can be explained in terms of depletion layers by
oxygen adsorption, which may result in change in resistance of
the sensor when exposed to different gas.48,49 Fig. 7 illustrates
the sensing reaction mechanism of the ZnO/ZnFe2O4 hollow
cubes sensor in detecting acetone gas. When the ZnO/ZnFe2O4
expose to air, oxygen molecules can adsorb on the surface of
ꢂ
CH3COCH3 + 8O(ads) / 3CO2 + 3H2O + 8eꢂ
(1)
(2)
ꢂ
C6H6 + 15O(ads) / 6CO2 + 3H2O + 15eꢂ
3.3 Gas sensing mechanism
The enhanced gas sensing performance of ZnO/ZnFe2O4 hollow
cubes can mainly be attributed to the mesoporous
Fig. 7 Schematic diagram of the sensing reaction mechanism of the ZnO/ZnFe2O4 composites sensor for detecting acetone.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34609–34617 | 34615

View Article Online
RSC Advances
Paper
sensing materials to form surface adsorbed oxygen (O2ꢂ, Oꢂ,
O2ꢂ) by capturing electrons from the conduction band of
sensing material, leading to formation of electron deletion layer
in the surface regions of both ZnO and ZnFe2O4. On the other
hand, the ZnO–ZnFe2O4 hetero-junctions are created in the
interface regions between ZnO and ZnFe2O4, resulting in the
interfacial charge separation and the increase of free electron
density and plays an important role in enhancing the gas
sensing properties. Since the electron depletion layer can be
generated on the surface region of ZnO, ZnFe2O4 and in the
vicinity of ZnO/ZnFe2O4 interfaces, a thicker electron depletion
layer and a larger amount of adsorbed negatively charged
4 Z. H. Jing and J. H. Zhan, Adv. Mater., 2008, 20, 4547–4551.
5 B. Wang, L. F. Zhu, Y. H. Yang, N. S. Xu and G. W. Yang, J.
Phys. Chem. C, 2008, 112, 6643–6647.
6 J. T. Zai, J. Zhu, R. R. Qi and X. F. Qian, J. Mater. Chem. A.,
2013, 1, 735.
7 C. Z. Wu, P. Yin, X. Zhu, C. Z. Ouyang and Y. Xie, J. Phys.
Chem. B, 2006, 110, 17806–17812.
8 C. J. Dong, X. H. Xiao, G. Chen, H. T. Guan, Y. D. Wang and
L. Djerdj, RSC Adv., 2015, 5, 4880–4885.
9 R. A. Kadir, W. Zhang, Y. C. Wang, J. Z. Qu, W. Wlodarski,
A. P. O'Mullane, G. Bryant, M. Taylor and K. Kalantar-
zadeh, J. Mater. Chem. A., 2015, 3, 7994–8001.
oxygen ion species are formed in ZnO/ZnFe2O4 material than 10 K. P. Kamioth, Chem. Rev., 2008, 108, 387–399.
the singular ZnO upon exposure to air. When the sensing 11 W. T. Koo, S. J. Choi, S. J. Kim, J. S. Jang, H. L. Tuller and
material is exposed to acetone, these acetone gas molecules can
react with the surface adsorbed oxygen ion species to produce 12 H. L. Tian, H. Q. Fan, G. Z. Dong, L. T. Ma and J. W. Ma, RSC
CO2 and H2O, and release the trapped electrons back to the Adv., 2016, 6, 109091–109098.
conduction band, which narrows the electron depletion layer 13 P. Rai, S. M. Majhi, Y. T. Yu and J. H. Lee, RSC Adv., 2015, 5,
and decreases the resistance of ZnO/ZnFe2O4 nanostructure 76229–76248.
sensor. Because more trapped electrons can turn back to the 14 D. P. Volanti, A. A. Felix, M. O. Orlandi, G. Whiteld,
I. D. Kim, J. Am. Chem. Soc., 2016, 138, 13431–13437.
conduction band of the ZnO/ZnFe2O4 nanostructure than the
singular ZnO, the greater resistance change can be found, thus
resulting in an enhanced response.
D. J. Yang, E. Longo, H. L. Tuller and J. A. Varela, Adv.
Funct. Mater., 2013, 23, 1759–1766.
15 M. S. Yao, W. X. Tang, G. E. Wang, B. Nath and G. Xu, Adv.
Mater., 2016, 28, 5229–5234.
16 X. Zhou, W. Feng, C. Wang, X. D. Hu, X. W. Li, P. Sun,
K. Shimance, N. Yamazoe and G. Y. Lu, J. Mater. Chem. A.,
2014, 2, 17683–17690.
4. Conclusions
In summary, uniform ZnO/ZnFe2O4 hollow cubes composites
were synthesized by a facile strategy through simple and direct 17 J. R. Huang, C. C. Shi, G. J. Fu, P. P. Sun, X. Y. Wang and
pyrolysis of FeIII-modied Zn-based metal–organic frameworks.
C. P. Gu, Mater. Chem. Phys., 2014, 144, 343–348.
The material composition and microstructure were examined 18 Z. C. Wu, K. Yu, S. D. Zhang and Y. Xie, J. Phys. Chem. C,
by SEM, TEM, XRD, and XPS analysis. In particular, a compar- 2008, 112, 11307–11313.
ative gas sensing investigation clearly showed that the ZnO/ 19 X. Ma, H. D. Liu, W. M. Li, S. P. Peng and Y. F. Chen, RSC
ZnFe2O4 material exhibited better acetone-sensing performance Adv., 2016, 6, 96997–97003.
compared with singular ZnO nanoparticles, including higher 20 M. Drobek, J. H. Kim, M. Bechelany, C. Valicari, A. Julbe and
response and good reproducibility. Besides the ZnO/ZnFe2O4 S. S. Kim, ACS Appl. Mater. Interfaces, 2016, 8, 8323–8328.
material have good selectivity to acetone, indicating its prom- 21 J. Song, Z. Luo, D. K. Britt, H. Furukawa, O. M. Yaghi,
ising application as gas sensor for detecting polluting gases and
selectivity to acetone. The enhanced sensing properties could be
K. I. Hardcastle and C. I. Hill, J. Am. Chem. Soc., 2011, 113,
16839–16846.
mainly attributed to the unique hierarchical structure with high 22 S. L. Zhao, H. J. Yin, L. Du, L. C. He, K. Zhao, L. Chang,
specic surface area, abundant exposed active sites with
surface-adsorbed oxygen species and heterojunctions formed at
the interfaces between ZnO and ZnFe2O4.
G. P. Ying, H. J. Zhao, S. Q. Liu and Z. Y. Tang, ACS Nano,
2014, 8, 12660–12668.
23 D. Li, H. Wang, X. Zhang, H. Sun, X. P. Dai, Y. Yang, L. Ran,
X. S. Li, X. Y. Ma and D. W. Gao, Cryst. Growth Des., 2014, 14,
5856–5864.
24 S. Y. Zhang, H. Liu, C. C. Sun, P. F. Liu, L. C. Li, Z. H. Yang,
X. Feng, F. W. Huo and X. H. Lu, J. Mater. Chem. A., 2015, 3,
5294–5298.
Acknowledgements
The authors are grateful to the Strategic Project of Science and
Technology of Chinese Academy of Science (No. XDB05050400)
and the National Science and Technology support (No. 25 L. Pan, T. Mummad, L. Ma, Z. F. Huang, S. B. Wang,
2014BAC21B00).
L. Wang, J. J. Zou and X. W. Zhang, Appl. Catal., B, 2016,
189, 181–191.
26 Y. Bai, Y. B. Dou, L. H. Xie, W. Rutledge, J. R. Li and
H. C. Zhou, Chem. Soc. Rev., 2016, 45, 2327–2367.
27 C. C. Sun, J. Yang, X. H. Rui, W. N. Zhang, Q. Y. Yan, P. Chen,
F. W. Huo, W. Huang and X. C. Dong, J. Mater. Chem. A.,
2015, 3, 8483–8848.
References
1 J. H. Lee, Sens. Actuators, B, 2009, 140, 319–336.
2 D. R. Miller, S. A. Akbar and P. A. Morris, Sens. Actuators, B,
2014, 204, 250–272.
3 J. Zhang, X. H. Liu, G. Neri and N. Pinna, Adv. Mater., 2016, 28 P. Y. Gao, R. Liu, H. H. Huang, X. Jia and H. B. Pan, RSC Adv.,
28, 795–831.
2016, 6, 94699–94705.
34616 | RSC Adv., 2017, 7, 34609–34617
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
29 W. H. Li, X. F. Wu, N. Han, J. Y. Chen, X. H. Qian, Y. Z. Deng, 39 Y. Hou, X. Y. Li, Q. D. Zhao and G. H. Chen, Appl. Catal., B,
W. X. Tang and Y. F. Chen, Sens. Actuators, B, 2016, 225, 158–
2013, 142, 80–88.
´
´
´
166.
40 K. Jiratova, J. Mikulova, J. Klempa, T. Grygar, Z. Bastl and
30 Y. Guan, L. Yu and X. W. Lou, Energy Environ. Sci., 2016, 9,
3092–3096.
31 P. Mitra, A. P. Chatterjee and H. S. Maiti, Mater. Lett., 1998,
35, 33–38.
F. Kovanda, Appl. Catal., A, 2009, 361, 106–116.
´
´
41 B. Puertolas, A. Smith, I. Vazquez, A. Dejoz, A. Moragues,
T. Garcia and B. Solsona, Chem. Eng., 2013, 229, 547–558.
42 W. X. Tang, H. D. Liu, X. F. Wu and Y. F. Chen, Ozone: Sci.
Eng., 2014, 36, 502–512.
32 N. S. Ramgir, M. Kaur, P. K. Sharma, N. Datta,
S. Kailasaganapathi, S. Bhattacharya, A. K. Debnath, 43 X. W. Li, X. Zhou, H. Guo, C. Wang, J. Y. Liu, P. Sun, F. M. Liu
D. K. Aswal and S. K. Gupta, Sens. Actuators, B, 2013, 187,
313–318.
and G. Y. Lu, ACS Appl. Mater. Interfaces, 2014, 6, 18661–
18667.
33 F. D. Qu, H. F. Jiang and M. H. Yang, Nanoscale, 2016, 8, 44 Y. Lin, W. Wei, Y. Wang, J. R. Zhou, D. M. Sun, X. D. Zhang
16349–16356. and S. P. Ruan, J. Alloys Compd., 2015, 650, 37–44.
34 C. Liu, B. Q. Wang, T. S. Wang, J. Y. Liu, P. Sun, X. H. Chuai 45 S. R. Wang, X. L. Gao, J. D. Yang, Z. Y. Zhu, H. X. Zhang and
and G. Y. Lu, Sens. Actuators, B, 2017, 248, 902–909. Y. S. Wang, RSC Adv., 2014, 4, 57967–57974.
35 Y. J. Wu, S. L. Zhao, K. Zhao, T. X. Tu, J. H. Zheng, J. Chen, 46 S. R. Wang, J. X. Zhang, J. D. Yang, X. L. Gao, H. X. Zhang,
H. F. Zhou, D. J. Chen and S. X. Li, J. Power Sources, 2016,
311, 137–143.
Y. S. Wang and Z. Y. Zhu, RSC Adv., 2015, 5, 10048–10057.
47 X. W. Li, C. Wang, H. Guo, P. Sun, F. M. Liu, X. H. Liang and
G. Y. Lu, ACS Appl. Mater. Interfaces, 2015, 7, 17811–17818.
48 L. W. Wang, S. R. Wang, M. J. Xu, X. J. Hu, H. X. Zhang,
Y. S. Wang and W. P. Huang, Phys. Chem. Chem. Phys.,
2013, 15, 17179–17186.
36 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter,
M. O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469–472.
37 H. Sun, H. X. Su, X. Y. Ma, P. F. Zhang, X. Zhang, X. P. Dai,
J. S. Gao, C. Chen and S. G. Sun, Electrochim. Acta, 2016, 205,
53–61.
49 C. X. Wang, L. W. Yin, L. Y. Zhang, D. Xiang and R. Gao,
Sensors, 2010, 10, 2088–2106.
38 L. R. Hou, L. Lian, L. H. Zhang, G. Pang, C. Z. Yuan and
X. G. Zhang, Adv. Funct. Mater., 2015, 25, 238–246.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 34609–34617 | 34617