MOF-derived NiWO4@NiO p-p heterostructure for distinguish detection of TEA and xylene by temperature regulation

Article abstract of DOI:10.1016/j.jallcom.2021.160015

Accurate detection of outdoor air quality requires a large number of gas sensors to be used in various fields. Thus, it is greatly significance to develop multifunctional sensors. In this work, the NiWO₄ decorated NiO p-p type heterostructure were successfully synthesized using the metal organic framework (MOF) as a self-sacrificing template with annealing process. The gas sensitive measurements reveal that the 12 wt% NiWO₄ decorated NiO based sensor exhibits the higher response value (65) than that of the pure NiO sensor (2.5) to 50 ppm triethylamine (TEA) at 240 °C. The selectivity of the NiWO₄ decorated NiO based sensors could be changed when the operating temperature over 260 °C, and the 12 wt% NiWO₄ decorated NiO based sensor shows the higher response value (28) than that of the pure NiO sensor (2) to 50 ppm xylene at 270 °C, indicated that as-prepared the NiWO₄ decorated NiO heterostructure can be as a promising material applied to fabricate the dual sensor to detect TEA and xylene with high sensitivity as well as distinguish by adjusting the temperature. The improved gas-sensing mechanisms of the NiWO₄ decorated NiO based on sensor are discussed in detail.

Full text of DOI:10.1016/j.jallcom.2021.160015

Journal of Alloys and Compounds 875 (2021) 160015
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
MOF-derived NiWO4@NiO p-p heterostructure for distinguish detection
of TEA and xylene by temperature regulation
Dongxue Wanga, Kuikun Gua, Jing Zhanga, Dejun Xingb,⁎, Mingzhe Zhanga,⁎
a State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, People’s Republic of China
b Department of Medical Oncology, Jilin Cancer Hospital, Changchun, Jilin 130012, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Accurate detection of outdoor air quality requires a large number of gas sensors to be used in various fields.
Thus, it is greatly significance to develop multifunctional sensors. In this work, the NiWO4 decorated NiO p-
p type heterostructure were successfully synthesized using the metal organic framework (MOF) as a self-
sacrificing template with annealing process. The gas sensitive measurements reveal that the 12 wt% NiWO4
decorated NiO based sensor exhibits the higher response value (65) than that of the pure NiO sensor (2.5) to
50 ppm triethylamine (TEA) at 240 °C. The selectivity of the NiWO4 decorated NiO based sensors could be
changed when the operating temperature over 260 °C, and the 12 wt% NiWO4 decorated NiO based sensor
shows the higher response value (28) than that of the pure NiO sensor (2) to 50 ppm xylene at 270 °C,
indicated that as-prepared the NiWO4 decorated NiO heterostructure can be as a promising material applied
to fabricate the dual sensor to detect TEA and xylene with high sensitivity as well as distinguish by adjusting
the temperature. The improved gas-sensing mechanisms of the NiWO4 decorated NiO based on sensor are
discussed in detail.
Received 19 February 2021
Received in revised form 15 April 2021
Accepted 16 April 2021
Available online 23 April 2021
Keywords:
Metal organic framework
NiWO4@NiO p-p heterostructure
Dual gas sensor
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
respectively. However, discover the source of the leaked toxic,
flammable and explosive gases in time is conducive to solving the
In the recent years, with the rapid development of science and
industry, the types and concentrations of the volatile organic com-
pounds (VOC) gases exposed to the air have been increased, which
poses a serious threat to the atmospheric environment, people also
realize the importance of monitoring air quality in real time. Both
triethylamine (TEA) and xylene are VOC gases, TEA is mainly used as
catalyst, inhibitor, corrosion inhibitor, and synthetic dyes, etc. the
concentration of TEA shall be below 10 ppm in the air, it will strongly
stimulate the eyes, mucous membranes and the high concentrations
of TEA vapor encounter the open flame, high temperature, strong
oxidizer could cause burning and explosion danger [1–3]. Xylene is
widely used in paint, resin, ink, medicine, and other industries as a
solvent, the concentration in the air shall be below 14 ppm [4,5] and
high levels of steam can irritate the respiratory system, nausea and
even cause dizziness, the American Conference of Government In-
dustrial Hygienists (ACGIH) classified as an A4 [6]. It has been re-
ported that a single sensor has been used to detect the two gases,
problem as soon as possible and protected the safety of human and
property, this requires a large number of sensors to be put into each
possible leakage source, so developing a single sensor with dual or
even multifunctional detection is still a challenge.
Among many sensors, the chromatographic analysis and fluor-
escence spectrum can accurately analyze harmful gas molecules, but
the large detection instruments are not easy to carry and expensive
detection costs limit their wide application. In contrast, metal oxide
semiconductor (MOS) sensors, which are easy to manufacture and
low cost, have attracted much attention [7,8]. For n-type metal oxide
sensors, such as SnO2 [9], WO3 [10], ZnO [11], Fe2O3 [12] and In2O3
[13], they usually have higher response to ethanol and formaldehyde
but poor selectivity to some amines and aromatic hydrocarbon
gases, however, p-type metal oxide sensors show good selectivity to
amines and aromatic due to their good catalytic activity [14,15], their
practical application are limited due to the low response value of
single p-type metal oxide. Therefore, it is still necessary to improve
the gas-sensitive performance of p-type sensitive materials. Ac-
cording to the literature reports, the response value and selectivity
can be improved by adjusting the micromorphology [16], controlling
the working temperature [17], precious metal surface modification
[18], ion doping [19] and forming heterogeneous structure RuO2/
⁎
Corresponding authors.
E-mail addresses: 13944113398@163.com (D. Xing),
zhangmz@jlu.edu.cn (M. Zhang).
https://doi.org/10.1016/j.jallcom.2021.160015
0925-8388/© 2021 Elsevier B.V. All rights reserved.

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
LaFeO3 [20], Co3O4@NiMoO4 [21]. Among them, NiO is a typical p-
type metal oxide with excellent electrical properties and chemical
stability make it has a good application prospect in gas sensing
[22,23], and through composite different MOS materials to form the
heterostructure is a simple and effective method to improve the
response value and selectivity of NiO sensors, such as WO3-NiO [24],
NiO/NiCr2O4 [25], CuO/NiO [26].
MOF as sacrificed template is considered as a promising method
to prepare porous materials via thermal calcination, which is ad-
vantageous to be applied in gas sensing materials. In this paper, the
porous small size NiO nanorods were prepared by using MOF as
template, In order to improve the gas sensitive properties of single
NiO, the NiWO4@NiO p-p heterostructure was prepared by the
second step composition. In the gas sensitive test, the working
temperature plays the key role of regulating the selectivity of TEA
and xylene. Compared with a single NiO sensor, the NiWO4@NiO p-p
heterostructures exhibit the improved response value, good se-
lectivity, fast response and recovery time and good stability. It is
proved that the NiWO4@NiO p-p heterostructure is significant for
the development of feasible dual sensors.
2.2. The preparation of the NiWO4@NiO
Putting the 0.05 g above precursor powder dissolved in 15 ml
ethanol stirring 10 min, at the same time, 0.001 g, 0.0025 g and
0.005 g Na2WO4.9 H2O into 5 ml H2O, respectively. Then Na2WO4
aqueous solution was slowly added into the above mix ethanol so-
lution, after stiring 1 h, the precipitates were obtained by cen-
trifuged and washed with deionized water and ethanol solution and
put it into a blowing drying oven at 60 °C for one night. At last the
three products with different compound contents were obtained
under annealed in air at 400 °C for 2 h. named 6 wt% NiWO4@NiO,
12 wt% NiWO4@NiO and 24 wt% NiWO4@NiO, respectively.
2.3. Characterization and fabrication of gas sensor
The phase structure of the as-synthetic product has been ana-
lyzed via X-ray powder diffraction (XRD Rigaku D/max-2500) with
Cu-Kα1radiation (λ = 1.54056 Å). Morphology characteristics of the
specimen were observed through Field-emission scanning electron
microscopy (SEM Magellan 400 FEI microscope), and transmission
electron microscopy (TEM), high resolution TEM (HRTEM) images as
well as selected area electron diffraction (SAED) were carried out on
a JEOL JEM-2200FS microscope. The chemical composition of the as-
prepared product were determined by X-ray photoelectron spec-
troscopy (XPS) (ESCALAB MK II). Nitrogen adsorption-desorption
analysis was examined using a Micromeritics ASAP 2420 apparatus.
The Fig. 1 shows the test equipments, the sensor structure and
the diagram of the test circuit. To fabricate the gas sensor, 0.01 g
sample powders were dispersed in 0.5 ml ethanol and ultrasound
3 min to obtain uniform pastes, and evenly covered on the surfaces
of ceramic tubes with a pair of gold electrodes connected to the four
Pt wires, putting a Ni–Cr wire through the tube to control the
temperature of sensor. In order to make ethanol volatilize as well as
making the material in close contact with the surface of ceramic
tube, the sensors were aged at 200 °C for 24 h. The electrical
2. Experimental
2.1. The synthesis of the pure NiO
In a typical procedure, 0.145 g nickel nitrate, 0.99 g terephthalic
acid and 0.3 g PVP were added in 5 ml ethanol and 25 ml DMF so-
lution stirring 1 h for fully dissolved, then the mix clear green
transparent solution was poured into a Teflon reactor with capacity
of 50 ml and maintained at 180 °C for 6 h. At end of the reaction, the
green precipitation was obtained by centrifuged and alternate wa-
shed with DMF and ethanol solution, then keep it at 60 °C for one
night. Finally, the pure NiO powders were obtained under annealed
in air at 400 °C for 2 h.
Fig. 1. The Schematics of the test equipment, sensor and simplified test circuit.
2

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
performances were tested by a CGS-8 intelligent gas sensing analysis
system, to introduce the target gas, using a micro-syringe to inject
target analyte which has been calculated concentration into 1 L
testing chamber mixed with air. The liquid volume of target analyte
was calculated according to the following formula:
By using a Gaussian fitting method, the chemical state of W element
in 12 wt% NiWO4@ NiO can be seen in Fig. 3b, there are two main
peaks detected in the W 4 f spectrum. One peak located at 35.6 eV
and another peak around at 37.7 eV corresponding to W 4 f7/2 and W
4 f5/2 are separated by 2.1 eV, which are the characteristic of W6+ in
the NiWO4 phase [29]. the chemical states of Ni element in our
synthetic NiO is shown in Fig. 3c, the high resolution separation
curve of Ni 2p3/2 spectrum exhibit two peaks which center at 854.2
and 856.1 eV attributed to Ni2+ and Ni3+, then Ni 2p1/2 is located at
872.8 eV [30], the Ni2+/Ni3+ ratio is determined to be 0.7 in the pure
NiO. The Fig. 3d shows the corresponding peak splitting spectrum of
12 wt% NiWO4@ NiO, however, the Ni2+/ Ni3+ ratio increased to 1.45
when NiWO4@ NiO, presumably because the combination of NiO and
NiWO4 that changed the electronic environment around nickel
atoms. The above results are consistent with XRD pattern and de-
monstrate that the successfully fabricated the NiWO4@NiO hybrid
materials. The Fig. 3e and f display the O 1 s high-resolution spec-
trum of NiO and NiWO4@NiO, the three peaks mainly locate at
529.6 eV is representative crystal lattice (OL), 531.4 eV is resulted
from oxygen-vacancy (OV) and 532.65 eV is corresponded to che-
misorbed oxygen (OC) in NiO [31].
V × × M
22.4 × d ×
273 + TR
273 + TB
9
Q =
× 10
×
Where Q (ml) is the liquid volume of analyte, V (ml) is the volume of
the testing chamber (1 L), φ (ppm) is the required analyte volume
fraction, M (g/mol−1) is the molecular weight of the analyte, d (g/ cm−3
)
is the specific gravity, and ρ is the purity of the liquid analyte, TR and
TB (°C) are the ambient and working temperature, respectively [27]. In
this paper, the response was defined as Rg/Ra for p-type semi-
conductors Here, Rg and Ra is the resistances of the sensors exposed in
the test gas and clean air, respectively. The response and recovery time
were determined as the time taken by the sensors to achieve 90%
of the total resistance change of test in air and in TEA and xylene,
respectively [28].
3. Results and discussion
The morphology characteristics of our synthesized NiO precursor,
pure NiO and 12 wt% NiWO4@NiO were studied through the SEM
showed in Fig. 4, from the low magnification image Fig. 4a, the
synthesized NiO precursor shows homogeneous nanorods, the high
magnification image Fig. 4b shows that the nanorods are about
1.23 µm long and 456 nm wide with smooth surface. Annealed pure
NiO images are shown in Fig. 4c and d, the size of the nanorods
barely change and there are many holes on the surface of pure NiO
3.1. Structure and morphology analysis
The crystal structure of the pure NiO and NiWO4 decorated NiO
were investigated via the XRD, from the Fig. 2, the five main peaks of
all samples that can be observed indexed to standard NiO JCPDF card
71–1179, The sharp and intense peaks confirmed that the well-
crystallization of structure, the diffraction peak of NiWO4 is still very
weak due to the low composite concentration, but the peak of
NiWO4 gradually obvious with the increase of composite con-
centration. It can be observed from the separate spectrum show that
a main peak corresponding to the (−1 1 1) surface of NiWO4 at
2θ = 30.927, which is consistent with the JCPDS card 15–0755, in
addition, the position of the NiO peak isn't offset and no other im-
pure peaks appear indicated that the prepared composite hetero-
structure materials don’t change the crystal structure.
2-
nanorods. After immersion in NaWO4 aqueous solution, the WO4
reacted with the leached Ni2+ions to form NiWO4 after heating
treatment, the morphology and size are still keep uniform exhibit in
Fig. 4e. From the the high magnification image Fig. 4f, we can clearly
observe that NiO is surrounded with a thin layer of the flocculent
NiWO4. To further explore the structures of as-prepared pure
NiO and 12 wt% NiWO4@NiO, the TEM images were showed in
Fig. 5a and b, by comparing the pure NiO and 12 wt% NiWO4@NiO
TEM images, the solid structure with regular surfaces are transformed
into corroded surface structure, further indicated that the NiO na-
norods are wrapped by floccule NiWO4, through the HRTEM and SAED
images of pure NiO nanorods are shown in Fig. 5c and inset 5c, The
lattice fringe of 0.208 is matched with the 200 planes of NiO, and the
For further investigation the surface chemical state and ele-
mental composition of products, the XPS spectrums were tested. The
Fig. 3a shows the full spectrum of pure NiO and 12 wt% NiWO4@ NiO,
there are no impure elements can be find in the spectras and all the
spectrums are fitting with the C 1 s located at 284.5 eV as reference.
Fig. 2. XRD patterns of pure NiO and NiWO4 decorated NiO.
3

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
Fig. 3. (a) The full survey XPS spectra of NiO and 12 wt% NiWO4@NiO. (b) The W 4f spectrum of 12 wt% NiWO4@NiO. (c) and (d) The Ni 2p3/2, Ni 2p1/2 spectra of NiO and 12 wt%
NiWO4@NiO, respectively. (e) and (f) The O 1s spectras of NiO and 12 wt% NiWO4@NiO, respectively.
4

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
Fig. 4. (a) (b) Different magnification SEM images of the NiO precursor and (c) (d) different magnification SEM images of the pure NiO, respectively. (e) and (f) Different
magnification SEM images of the 12 wt% NiWO4@NiO.
clear diffraction ring proved that NiO is polycrystalline behaviour. The
Fig. 5d shows the HRTEM of 12 wt% NiWO4@NiO, There is a clear
crystal boundary between NiWO4 and NiO structure. In addition to
the lattice fringe of NiO, clear NiWO4 fringe 0.288 corresponded to
(−111) crystal surface, which is consistent with XRD test results. It is
further proved that NiWO4 attached to the surface of NiO.
3.2. Gas properties
The gas sensing properties of pure NiO and NiWO4 decorated NiO
were investigated and compared in detail in order to explore the
roles that heterostructure played on improved sensing performance.
The working temperature has great regulating effect on the response
5

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
Fig. 5. (a) and (b) The TEM images of pure NiO and 12 wt% NiWO4@NiO. (c) The HRTEM image of the pure NiO, inset of (c) the SAED image of pure NiO. (d) The HRTEM image of
the 12 wt% NiWO4@NiO.
Fig. 6. (a) The base resistance and response value of the pure NiO sensor toward to 50 ppm TEA and xylene varies with 220–290 °C. (b) Selectivity of pure NiO sensor to other VOC
gases at 230 °C and 270 °C. (c) and (d) The base resistance and response value of the NiWO4@NiO sensors toward to 50 ppm TEA and xylene varies with 220–290 °C. (e) and (f) The
response value of the NiWO4@NiO sensors to various test gases (concentration of 50 ppm) at 240 °C and 270 °C, respectively.
6

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
Fig. 7. (a) and (b) Repeatability curves of sensors to 50 ppm TEA and xylene at 240 °C and 50 ppm xylene and TEA at 270 °C, respectively. (c) and (d) The transient response curve
of the 12 wt% NiWO4@NiO sensors towards 50 ppm TEA and xylene at 240 °C and 50 ppm xylene and TEA at 270 °C, respectively.
value of gas sensor, so the base resistance of sensors in air and the
response value varies with increasing working temperature were
determined firstly. From the Fig. 6a, shows that the base resistance of
NiO decreases as the working temperature increases, which con-
formed to the characteristics of semiconductor materials. Then the
gas-sensitive responses of sensors to triethylamine and xylene at
different working temperatures were investigated, respectively. The
maximum response value of the pure NiO sensor is 2.5–50 ppm TEA
and the response value to 50 ppm xylene is 2 at the optimum
working temperature 230 °C, after 230 °C, the response values of the
two gases are close to each other and gradually decrease and Fig. 6b
shows a poor selectivity compared with other interference gases at
230 °C and 270 °C. In Fig. 6c the resistances of the NiWO4@NiO also
decrease as the working temperature increase, but the resistances of
the NiWO4@NiO obvious higher than pure NiO, it’s due to forming
NiWO4@NiO heterostructure, extra electrons will move from NiWO4
to NiO, the electron hole pair recombination resulted in the hole
concentration decrease on the side of NiO. From the Fig. 6d, the gas
response curves of the three NiWO4@NiO sensors show the parabolic
trend as temperatures range from 220 °C to 290 °C, and we can ob-
vious detect the NiWO4@NiO sensors show different selectivity at
different operating temperatures 240 °C and 270 °C, the 12 wt%
NiWO4@NiO based sensor shows the maximum response value to
50 ppm TEA is 65 at 240 °C, which is 26 time higher than that of NiO
sensor. At the same time, the response value of TEA to xylene is
3 times. As the temperature increases, the response value of trie-
thylamine decreases rapidly and the response value of xylene still
tend to rise until reach 270 °C, the maximum response value to
xylene is 28, the response value of xylene to TEA is 2.8 times at
270 °C. The all response values for 6 wt% NiWO4@NiO and 24 wt%
NiWO4@NiO sensors are lower than 12 wt% NiWO4@NiO sensor, the
6 wt% NiWO4@NiO and 24 wt% NiWO4@NiO sensors to 50 ppm TEA
is 10 and 25.8 at 240 °C, respectively, the 6 wt% NiWO4@NiO and
24 wt% NiWO4@NiO sensors to 50 ppm xylene at 270 °C is 5.5 and 8.3
respectively. To further explore the selectivity, the response values of
other interfering gases were tested at 240 °C and 270 °C as shown in
Fig. 6e and f. At 240 °C, all sensors manifested the highest response
value to TEA contrast with xylene, toluene, acetone, methanol and
aniline. But show a higher response to xylene than TEA, toluene,
acetone, methanol and aniline at 270 °C, It indicated that the de-
tection of TEA and xylene can be realized using a single sensor by
adjust the working temperature.
The dynamic repeatability curves of the four sensors to 50 ppm
TEA and 50 ppm xylene at 240 °C as well as the same tests at 270 °C
were investigated, as shown in Fig. 7a and b, respectively. Through
the eight times of dynamic testing cycle in TEA and xylene, all of
sensors can restore the initial resistance value rapidly, in addition,
the response values of sensors hardly change in 50 ppm TEA and
xylene, respectively, this explain that the sensors have good re-
peatability. The operating temperature not only affects the response
value of the sensor, but also plays an important role in regulating the
recovery time. From the Fig. 7c and d the recovery time of the 12 wt%
NiWO4@NiO to 50 ppm TEA is 16 s, to 50 ppm xylene is 40 s at
240 °C, when the temperature up to 270 °C, the recovery time of the
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D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
Fig. 8. (a) and (b) Dynamic cycling curve of the sensors exposed to different concentrations of TEA at 240 °C and xylene at 270 °C, respectively. (f) The long term stability of the
sensors to 50 ppm TEA at 240 °C and xylene at 270 °C, respectively.
12 wt% NiWO4@NiO to 50 ppm xylene is 21 s, to 50 ppm TEA is 10 s,
further clarification that the optimum operating temperature is
270 °C for xylene.
TEA and xylene, the resistance of sensors can quickly back to the
initial state, respectively, indicate that the NiWO4@NiO sensors have
good stability. Further to track the long-term stability of the pure
NiO and NiWO4@NiO based sensors, the gas response values of every
3 days to 50 ppm TEA and xylene in one month were investigated,
illustrate in Fig. 8c and d, the slight fluctuation of the response value
indicates the sensors exhibit good long-term stability. The Tables 1
and 2 shown the prepared 12 wt% NiWO4@NiO sensor contrast with
the other TEA and xylene sensors which have been reported. As-
prepared 12 wt% NiWO4@ NiO based on sensor shows high response
value as well as fast recovery time.
The circular curves of four sensors to different concentrations of
TEA at 240 °C and xylene at 270 °C are shown in Fig. 8a and b, re-
spectively. when the sensors in target gases, the sensors response
value increases when the TEA and xylene concentration increases,
For the NiWO4@NiO sensors, the detection limit is 1 ppm (the re-
sponse value to 1 ppm TEA is 1.76 at 240 °C, to xylene is 1.45 at
270 °C), however, the pure NiO sensor has little response to 1 ppm
TEA and xylene, after six circulations of the different concentrations
Table 1
The comprehensive comparisons between the 12 wt% NiWO4@NiO based sensor and others reported TEA sensors.
Materials
TEA (ppm)
T (°C)
Res time (s)
Rec time (s)
Res. (Rg/Ra or Ra/Rg)
Ref.
Porous CuO Particles
V2O5 hollow spheres
100 ppm
100 ppm
100 ppm
100 ppm
50 ppm
50 ppm
50 ppm
50 ppm
50 ppm
230
370
300
305
280
250
300
260
240
34
40
4
12
4
31
10
9
103
125
203
28
370
182
48
17
7.9
39
42
76
11
45
30
65
[32]
[33]
[34]
[35]
[36]
[21]
[37]
[38]
Au@SnO2/α-Fe2O3 nanoneedle
ZnFe2O4/Fe2O3 porous microrods
α-Fe2O3/α-MoO3 nanostructure
Co3O4@NiMoO4 composite arrays
ZnO micropyramids
Au/Mg-doped TiO2/SnO2 nanosheets
12 wt% NiWO4@NiO
95
15
48
This work
8

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
Table 2
The comprehensive comparisons between the 12 wt% NiWO4@NiO based sensor and others reported xylene sensors.
Materials
xylene (ppm)
T (°C)
Res time (s)
Rec time (s)
Res. (Rg/Ra or Ra/Rg)
Ref
Sn-doped NiO
α-MoO3/α-Fe2O3
Flower-like Co3O4 structures
NiO/NiCr2O4 nanoparticles
Zr-MoO3
100 ppm
100 ppm
100 ppm
100 ppm
100 ppm
200 ppm
50 ppm
225
206
150
225
206
375
270
298
87
131
1217
32
223
190
250
591
64
20.2
6.8
[15]
[39]
[40]
[25]
[41]
[42]
This work
79.8
66.2
7.99
8.74
28
W-Doped NiO
12 wt% NiWO4@NiO
178
40
152
21
3.3. Enhanced gas sensing mechanism
shown below:
O2 (gas) → O2 (ads)
(1)
(2)
(3)
The NiWO4@NiO heterostructures exhibit improved sensing
performance can be relationship with various factors. Firstly, the
changes of specific surface area by forming NiWO4@NiO was ex-
plored showed in Fig. 8, the BET surface area of pure NiO and 12 wt%
NiWO4@NiO is 98.25 and 130.2 m2 g−1, respectively. It is certified
that the surface area increased by compositing NiWO4, the pore size
distributions of our prepared samples is about 2–30 nm. The large
specific surface area can provide more active sites for oxygen ad-
sorption and target gas, the porosity accelerates the diffusion of gas
molecular across the material surface. Combined with the XPS
analysis, the oxygen vacancy regions (OV) and chemisorbed oxygen
(OC) significantly increase after loading the NiWO4. The increase of
OV component could further promote the oxygen adsorption on the
surface of the NiWO4@NiO materials, and the abundant OC com-
ponent lead to more surface chemisorbed oxygen species could
participate in the oxidation-reduction reaction, which beneficial to
enhance the gas sensitive performance [43].
−
O2 (ads) + e− → O2 (ads)
O2 (ads) + e− → 2O− (ads)
−
On this account, the concentration of the holes relatively in-
creases lead to a hole accumulation layer formed on the NiO material
surface. After forming p-p heterostructure, due to the difference of
original Fermi levels of the NiWO4 and NiO, electrons will be
transferred from the NiWO4 to NiO until their Fermi energy levels
reach the equilibrium, the electron hole pair recombination result in
the hole concentration decreases for NiWO4@NiO, thus the re-
sistance of NiWO4@NiO is higher than pure NiO conformed to pre-
vious Ra test rules. When the sensor contact with the TEA or xylene
the chemical reactions take place on the surface of material are
described as follows, respectively [44–47]:
2 (C2H5)3N + 43O− (ads) → 2 NO2 + 12 CO2 + 15H2O +43 e−
C8H10+ 21O− (ads) →8 CO2 + 5H2O +21 e−
(4)
(5)
The electronic sensitization mechanism descripted that the
charge carrier transfer due to forming the NiWO4@NiO hetero-
structure, can be considered as the main reason. When sensors are in
air, the oxygen molecules combined with electrons which capture
from the conduction band of NiO to form O2− O− (below 300 °C), as
In this process, the resistance of sensors increase because of the
liberated electrons back to the gas-sensitive materials. The gas
sensing mechanism diagrams of the whole process are illustrated in
Fig. 9. The schematic of improved gas sensing mechanism of p-p NiWO4 decorated NiO.
9

D. Wang, K. Gu, J. Zhang et al.
Journal of Alloys and Compounds 875 (2021) 160015
the Fig. 9. In addition, at near the NiWO4@NiO heterostructure in-
terface, the released electrons disrupt the dynamic balance of NiO
and NiWO4 and further shrink the hole accumulation layer on the
surface of NiO. The change in the resistance of the NiWO4@NiO
based sensor is significantly increased and the corresponding re-
sponse value is improved [48–50].
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The enhanced gas response value can be explained by a high
surface area, abundant oxygen defects, and forming p-p hetero-
structure in the present study. Finally notice that pure NiO sensor
does not have an obvious distinction between TEA and xylene, for
this reason, the temperature-dependent gas selectivity may be at-
tributed the different catalytic promotions of the NiWO4@NiO het-
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which also indicates the importance of catalytic promotion in the
gas-sensitive reaction. In addition, the target gases reaction activity
is related to their bond energy, the bond energy of CeN (TEA), CeC
(xylene), OeH (methanol), C]C (benzene), and C]O (acetone), is
307, 345, 458.8, 610.3, and 798.9 kJ/mol, respectively [51]. TEA and
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species (benzaldehyde, benzyl alcohol and benzoic acid), when the
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4. Conclusion
In this study, the pure NiO nanorods and NiWO4@NiO hetero-
structures have been synthesised through hydrothermal synthesis
with annealing process and their gas sensitivity were explored. The
12 wt% NiWO4@NiO based sensor exhibit dual sensing characteristic
and higher response value 65–50 ppm TEA at 240 °C and 28–50 ppm
xylene at 270 °C, which is 26 and 14 times higher than that of the
pure NiO sensor, fast response/recovery speed as well as good sta-
bility. The boosted gas sensitivity is closely attributed to the elec-
tronic sensitization of p-p NiWO4@NiO heterostructure, large
specific surface area and abundant oxygen vacancy. This research has
great meaning for fabrication dual functionality gas sensor to detect
TEA and xylene with outstanding performances.
CRediT authorship contribution statement
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Dongxue Wang: Conceptualization, Data curation, Formal
analysis，Investigation, Methodology，Writing
- original draft.
Kuikun Gu: Investigation，Formal analysis. Jing Zhang:
Investigation，Formal analysis. Dejun Xing: Writing - review &
editing. Mingzhe Zhang: Formal analysis, Funding acquisition,
Writing - review & editing.
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The authors declare that they have no known competing fi-
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Acknowledgments
This work was supported by the Major Research plan of the
National Natural Science Foundation of China, no. 91963101.
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