Effect of noble metal element on microstructure and NO2 sensing properties of WO3 nanoplates prepared from a low-grade scheelite concentrate

Article abstract of DOI:10.1016/j.jallcom.2019.152927

To break the limitation of raw materials for preparing functional WO₃ nanomaterials, a low-grade scheelite concentrate was selected as the tungsten source, and WO₃ nanoplates doped with Ag, Pd, Au and Pt were synthesized through three combined processes including NaOH leaching, chemical precipitation and acidification. The microstructure and NO₂ sensing properties of pure and noble metal-doped WO₃ nanoplates were investigated. The microstructure characterization demonstrated that all WO₃ products were composed of interlaced and irregular nanoplates with the thickness of 10–30 nm, and the length and width of these nanoplates were in the range of several hundred nanometers. NO₂ sensing properties indicated that WO₃ nanoplates doped with noble metal nanoparticles exhibited obviously higher responses and shorter response times than pure WO₃ nanoplates. Especially, noble metal-doped WO₃ nanoplates exhibited distinct behaviors in terms of the enhancement of sensing properties. Pd-doped WO₃ nanoplates exhibited highest response to NO₂, and Ag-doped WO₃ nanoplates exhibited fastest response speed. Additionally, Ag-, Pd- and Pt-doped WO₃ nanoplates exhibited a relatively lower optimal operating temperature. The enhanced NO₂ sensing performance can be ascribed to the large specific surface area of WO₃ nanoplates, the catalytic activities of noble metal nanoparticles, and the varied work function energies together with the lower activation energies.

Full text of DOI:10.1016/j.jallcom.2019.152927

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Effect of noble metal element on microstructure and NO sensing properties of WO
2
3
nanoplates prepared from a low-grade scheelite concentrate
Tingting Li, Yanbai Shen, Xiangxi Zhong, Sikai Zhao, Guodong Li, Baoyu Cui, Dezhou
Wei, Kefeng Wei
PII:
S0925-8388(19)34173-8
DOI:
https://doi.org/10.1016/j.jallcom.2019.152927
JALCOM 152927
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 23 July 2019
Revised Date: 4 November 2019
Accepted Date: 5 November 2019
Please cite this article as: T. Li, Y. Shen, X. Zhong, S. Zhao, G. Li, B. Cui, D. Wei, K. Wei, Effect of noble
metal element on microstructure and NO sensing properties of WO nanoplates prepared from a low-
2
3
grade scheelite concentrate, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/
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Effect of noble metal element on microstructure and NO sensing
2
properties of WO nanoplates prepared from a low-grade
3
scheelite concentrate
a
a,
a
a
a
Tingting Li , Yanbai Shen *, Xiangxi Zhong , Sikai Zhao , Guodong Li ,
a,
a
b
Baoyu Cui *, Dezhou Wei , Kefeng Wei
a
b
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China
Shen Kan Engineering and Technology Corporation, MCC., Shenyang, 110169, China
E-mail: shenyanbai@mail.neu.edu.cn; Tel/Fax: +86-24-83687381.cuibaoyu@mail.neu.edu.cn.
The noble metal-doped WO nanoplates were synthesized from a low-grade scheelite
3
concentrate for NO detection with enhanced sensing performances.
2

Effect of noble metal element on microstructure and NO sensing
2
properties of WO nanoplates prepared from a low-grade scheelite
3
concentrate
a
a,*
a
a
a
Tingting Li Yanbai Shen , Xiangxi Zhong , Sikai Zhao , Guodong Li ,
,
a,*
a
b
Baoyu Cui , Dezhou Wei , Kefeng Wei
a
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China
Shen Kan Engineering and Technology Corporation, MCC., Shenyang, 110169, China
b
*Corresponding author. Tel. & Fax: +86-24-8368-7381.
E-mail: shenyanbai@mail.neu.edu.cn (Y. Shen). cuibaoyu@mail.neu.edu.cn (B. Cui)
1
/ 37

ABSTRACT
To break the limitation of raw materials for preparing functional WO3
nanomaterials, a low-grade scheelite concentrate was selected as the tungsten source,
and WO nanoplates doped with Ag, Pd, Au and Pt were synthesized through three
3
combined processes including NaOH leaching, chemical precipitation and
acidification. The microstructure and NO sensing properties of pure and noble
2
metal-doped WO nanoplates were investigated. The microstructure characterization
3
demonstrated that all WO products were composed of interlaced and irregular
3
nanoplates with the thickness of 10–30 nm, and the length and width of these
nanoplates were in the range of several hundred nanometers. NO sensing properties
2
indicated that WO nanoplates doped with noble metal nanoparticles exhibited
3
obviously higher responses and shorter response times than pure WO nanoplates.
3
Especially, noble metal-doped WO nanoplates exhibited distinct behaviors in terms
3
of the enhancement of sensing properties. Pd-doped WO nanoplates exhibited
3
highest response to NO , and Ag-doped WO nanoplates exhibited fastest response
2
3
speed. Additionally, Ag-, Pd- and Pt-doped WO nanoplates exhibited a relatively
3
lower optimal operating temperature. The enhanced NO sensing performance can be
2
ascribed to the large specific surface area of WO nanoplates, the catalytic activities
3
of noble metal nanoparticles, and the varied work function energies together with the
lower activation energies.
Keywords: Scheelite concentrate; Noble metal doping; WO nanoplates; NO ; Gas
3
2
sensing mechanism
2
/ 37

1
. Introduction
Over the past few decades, air pollution has attracted more and more attention
due to its serious harm to environment and human health. Therefore, there is an
increasing requirement for the detection of toxic and explosive gases like NO , H S,
x
2
H , SO , NH and so on [1, 2]. Among these detrimental gases, NO is a sort of
2
2
3
2
corrosive and poisonous gas, which is mainly derived from the exhaust of industrial
plants and automobile engines as well as high-temperature combustion of fossil fuels
[
3-6]. It is a main contributor to acid rain and also can bring about a variety of
adversely environmental effects such as visibility reduction, ozone layer thinning and
acidification [7-9]. More importantly, NO is extremely harmful to human health,
2
which can damage human’s respiratory system at a relatively low concentration [10].
Therefore, numerous efforts have been made to develop NO sensor for detecting
2
low-concentration even ppb-level NO2.
Recently, semiconductor metal oxides (SMO), such as WO , ZnO, In O , NiO
3
2
3
and SnO have received considerable attention in the field of gas sensors such as
2
atmosphere detection, medical diagnosis, automobile emission inspection, fossil fuel
combustion control, food processing, and homeland security due to their obvious
advantages such as low cost, high response, simple implementation, outstanding
selectivity, and compatibility with micro-assembly technology [11-18]. Especially
among the numerous gas sensing materials, tungsten oxide (WO ) with a wide band
x
gap from 2.4 to 2.8 eV, an important n-type functional semiconductor material, has
attracted more and more attention due to its outstanding sensing performance such as
3
/ 37

high sensitivity, good stability, fast response/recovery speed, and extraordinary
selectivity towards various gases, specially towards NO [3, 8, 19-22]. Over the past
2
few years, for the sake of meeting the needs of practical application, considerable
efforts have been devoted to improve gas sensing properties. As a consequence,
various nanoscale WO nanomaterials, including nanoplates [23, 24], nanoparticles
3
[
25], nanotubes [26], nanorods [27], nanofibers [28], and nanospheres [29] have been
successfully prepared and showed enhanced sensing properties for different detected
gases. However, as far as we know, the raw materials for preparing functional WO3
nanomaterials are mainly analytical pure reagents such as sodium tungstate,
ammonium metatungstate and tungsten hexachloride, or tungsten metal. These
chemicals are relatively expensive and complex in preparation process, which
extremely restricts their large-scale application and inevitably brings about some
environmental pollution in their preparation processes. Therefore, using a cheaper
tungsten-containing raw material to synthesize WO nanomaterials will provide a new
3
idea for the development of functional tungsten nanomaterials. To the best of our
knowledge, there are few reports on the synthesis of functional WO nanomaterials by
3
using some tungsten-containing raw materials. Hence, it is believed that this will be
an interesting insight in material science, which will be an efficient and cost-effective
method to prepare functional nanomaterials.
Additionally, a variety of noble metal elements may exist in tungsten ores, which
are beneficial to the improvement of the gas sensing performance of functional WO3
nanomaterials. As is well known, some noble metal elements (Pd, Ag, Pt and Au) can
4
/ 37

be used to enhance the sensing performance of WO nanomaterials due to their
3
sensitization effects together with thermally and chemically stable for long-term
operation [30, 31]. Some studies demonstrated that the enhanced sensing properties
could be ascribed to the reduced activation energy of the chemical reactions between
sensing material and the detected gas due to noble metal’s catalytic effect, so that it
could more easily react with detected gases [32, 33]. Several works have been carried
out to identify the sensitization role of noble metal dopants. For instance, Akbar et al.
[
34] reported that the addition of noble metal had a wide influence on catalytic
activity, gas adsorption, and band bending of semiconducting oxide nanomaterials.
Kamble et al. [35] prepared Ag-decorated WO gas sensor, which exhibited an
3
obviously enhanced NO sensing performance compared with pure one and could be
2
used for the detection of sub-ppm level NO concentration. Wu et al. [36] prepared the
2
Pd-doped mesoporous WO sensor, which had a higher response to H in comparison
3
2
with pure one. Zhang et al. [7] synthesized bilayer Au nanoparticle-decorated WO3
porous thin films, revealing that Au doping could not only improve NO sensing
2
performance but also reduce the optimal operating temperature as well as the
response/recovery speed. Gu et al. [31] studied the atomically dispersed Pt(II) on
WO for catalytic oxidation and highly selective sensing of trimethylamine. They
3
indicated that the atomically dispersed Pt(II) on WO not only decreased the
3
activation energy, but also increased the number of active sites, which resulted in an
enhancement in the gas sensing properties. These studies mainly focused on the effect
of a single noble metal on WO sensing materials. However, there are few reports
3
5
/ 37

related to the modification of nanostructured WO sensing materials with different
3
noble metal elements.
Herein, a low-grade scheelite concentrate was selected as the tungsten source,
where the sodium tungstate-containing leaching solution was firstly obtained by a
high-pressure leaching method with sodium hydroxide as the leaching agent. Then
calcium tungstate was prepared by a chemical precipitation method. Subsequently,
using the as-obtained calcium tungstate as the precursor, pure and noble metal-doped
WO nanoplates were synthesized at room temperature by a simple acidification
3
method. It is expected that such combined process will provide a new insight into
preparing WO functional nanomaterials by using a cheaper scheelite concentrate as
3
the tungsten source as well as improving sensing properties by utilizing the noble
metal element existed in this mineral materials. The in-situ doping of noble metal
elements onto WO sensing materials was performed to simulate their existence in
3
tungsten raw material, and their effects on microstructure and NO sensing
2
performance of the products were also investigated systemically.
2
. Experimental
2
.1. Materials
In this work, a low-grade scheelite concentrate was used as the tungsten source,
which was purchased from Gansu Xinzhou Mining Co., Ltd. And all chemical
reagents (Kemiou Chemical Reagent Co., Ltd) used for the present work were of
analytical grade without any subsequent purification treatment. Deionized water was
6
/ 37

directly used in all experimental operations.
The X-ray Fluorescence spectrometry (XRF) and X-ray diffraction (XRD) were
performed to determine the chemical compositions of the low-grade scheelite
concentrate before the experiments, and the corresponding results are illustrated in
Table 1 and Fig. 1, respectively. It can be observed from Table 1 that the main
valuable composition of the scheelite concentrate sample is WO , and the main
3
impurity components are SiO , CaO, P O , Fe O , MgO and SO . It can be seen from
2
2
5
2
3
3
Fig. 1 that tungsten in the scheelite concentrate exists in the form of CaWO4.
2
.2. Synthesis of WO nanoplates
3
The synthesis of pure, Ag-, Pd-, Au-, and Pt-doped WO nanoplates mainly
3
includes the following three steps.
Firstly, the scheelite concentrate was converted into water-soluble Na WO
2
4
compound by leaching experiments. All the leaching experiments were performed in a
microform high-pressure autoclave with a volume of 250 mL. The apparatus equipped
with a stirring speed controller and a temperature controller was purchased from
Shanghai Yanzhen Co., Ltd. In the preparation process, 5 g of scheelite concentrate,
3
.51 g of sodium hydroxide, and 5 mL of deionized water were added into the above
autoclave. Then, the leaching experiments were performed for 120 min at the reaction
o
temperature of 180 C with a stirring speed of 400 rpm. After naturally cooling down
o
to 50 C, the digested slurry was removed and then treated by washing and filtering to
obtain the sodium tungstate-containing leaching solution. The main chemical
compositions of the as-obtained leaching solution were analyzed by inductively
7
/ 37

coupled plasma atomic emission spectrometry (ICP-AES), and the corresponding
results are shown in Table 2. The obtained leaching solution mainly contains W and
trace of impurities such as Si and Mo. The content of other impurities in the leaching
solution are close to those in tungsten-containing chemical purity or analytical purity,
which have no effect on the later preparation of WO nanomaterials.
3
Secondly, the sodium tungstate in leaching solution was converted to calcium
tungstate by a simple chemical precipitation. 5 mL of the above leaching solution was
added into a 250 mL beaker and then 70 mL of deionized water was added to dilute
the leaching solution. The pH value of the diluted leaching solution was above 13.
Subsequently, to obtain Ag-, Pd-, Au-, and Pt-doped WO nanoplates, a certain
3
amount of silver nitrate solution (1.699 g/L), palladium chloride solution (1.773 g/L),
chloroauric acid solution (5.850 g/L) or chloroplatinic acid solution (6.706 g/L) was
added into the above diluted leaching solution to ensure the molar ratio of Ag/W,
Pd/W, Au/W or Pt/W to be 0.7 mol%, and the as-obtained homogeneous solution was
determined as solution A. 2 g of CaCl was added into 20 mL of deionized water and
2
continuously stirred to form solution B. Subsequently, the solution B was slowly
added into the solution A and vigorously stirred for 10 min. In the case, a large
amount of white precipitate, namely calcium tungstate, was formed. The obtained
white precipitate was separated by washing and filtering by using deionized water,
and subsequently dried in a drying oven.
Thirdly, WO nanoplates doped with Ag, Pd, Au and Pt were prepared at room
3
temperature by a simple acidification method. The dried white product was added into
8
/ 37

2
0 mL of 4 M HNO and continuously stirred for 24 h. Then, the as-obtained products
3
were washed and filtered by using deionized water, and subsequently dried in a drying
oven for 12 h. To obtain more stable WO nanoplates, the dried products were
3
o
annealed for 4 h at 400 C in a tube furnace.
2
.3. Structural characterizations
The elemental components of the scheelite concentrate were determined by
X-ray Fluorescence spectrometry (XRF, Shimadzu XRF-1800). The crystalline
structure and phase purity of the scheelite concentrate and the as-synthesized WO3
nanoplates were analyzed by X-ray diffraction (XRD, PANalytical X’Pert Pro) with
o
o
Kα radiation and copper target in diffraction angle ranges of 10–90 and 15–70 ,
respectively. The main elemental compositions of the leaching solution were
examined by inductively coupled plasma atomic emission spectrometry (ICP-AES,
Thermo Fisher Scientific). The surface morphology and microstructure of the WO3
samples were characterized by using a field-emission scanning electron microscope
(FESEM, ZEISS Ultra Plus) and a high-resolution transmission electron microscope
2
(
TEM, FEIG -20) equipped with an energy-dispersive spectroscope (EDS). The
elemental components and elemental valence of the WO products were examined by
3
X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi) using Al Kα radiation.
The main chemical groups of the as-synthesized WO nanoplates were analyzed by
3
Fourier transform infrared spectroscopy (FTIR, NICOLET 380). The specific surface
areas of the as-obtained WO samples were analyzed by a Brunauer-Emmet-Teller
3
(BET, Micro-meritics ASAP 2020 instruments, USA) method.
9
/ 37

2
.4. Fabrication of gas sensor device
The WO nanoplates-based gas sensor devices were prepared as below: firstly, a
3
moderate amount of the as-synthesized WO nanomaterial and absolute ethanol were
3
put into 1.5 mL centrifugal tube and then ultrasonically dispersed for 15 min to form a
viscous paste. Secondly, the paste was uniformly coated on the outer surface of an
Al O ceramic tube equipped with four Pt lead wires and two Au electrodes to form
2
3
the sensing film. As a resistance heater, a Ni-Cr alloy heating coil was inserted
through the ceramic tube to control the desired operating temperature. Subsequently,
the WO nanoplates-coated Al O ceramic tube was directly welded to a pedestal with
3
2
3
six probes to assemble the final sensor device. Finally, to enhance the long-term
stability and repeatability of the WO nanoplates, all sensor devices were aged in
3
o
ambient air at 300 C for 24 h before use.
2
.5. Gas sensing measurements
Gas sensing measurements of pure, Ag-, Pd-, Au-, and Pt-doped WO nanoplates
3
were performed in a fume hood with a relative humidity (RH) of below 30% by using
a static gas-sensing system (WS-30A, Weisheng Electronics Co., Ltd). The operating
o
temperature (T ) of the WO sensor was adjusted from 75 to 225 C by changing the
op
3
heating voltage. In order to obtain the fixed NO concentration in the range of 50
2
ppb–5 ppm, a certain volume of NO with the original concentration of 0.02% was
2
injected into the test chamber with a capacity of 18 L by a syringe. The resistance
value of the as-prepared WO nanoplates-based sensor was determined by monitoring
3
the voltage signal, where 5 V dc bias was used between two Au electrodes. The
1
0 / 37

responses (S) of the sensor exposed to reducing and oxidizing gases were calculated
based on the equations of R /R and R /R , respectively, where R and R referred to
a
g
g
a
g
a
the sensor resistance in the presence and absence of detected gas, respectively [37].
The response time (T ) was defined as the time for the sensor to reach 90% of the
res
whole resistance change in the case of the chemisorption of NO molecules, and the
2
recovery time (T ) was the required time for the sensor to fall to 10% of its base
rec
resistance in the case of the desorption of NO molecules [38].
2
3
. Results and discussion
3
.1. Structural and morphological characteristics
The EDS analysis of pure WO nanoplates was firstly carried out to determine
3
whether there were other impurities in the WO sample, and the result is illustrated in
3
Fig. 2(a). It demonstrates that the WO products consist of two elements of W and O,
3
demonstrating that the products show high purity. Fig. 2(b) further investigates the
phase purity and crystal structure of pure and noble metal-doped WO samples by
3
XRD. As observed in Fig. 2(b), the peaks of all the as-synthesized WO samples can
3
be matched well with monoclinic-structured WO , which are in good agreement with
3
the standard data file (JCPDS No. 43-1035). The peaks of (200), (020), and (002)
planes are all narrow and strong, indicating that all the as-prepared WO products
3
have good crystallinity. Obviously, the main peaks of pure and noble metal-doped
WO samples have the same position in the XRD patterns. Additionally, the
3
11 / 37

diffraction peaks corresponding to other components are not observed, further
revealing that no other impurities are observed in the WO products prepared from a
3
low-grade scheelite concentrate. More importantly, no diffraction peaks related to Ag,
Pd and Pt elements are found, which may be ascribed to the low content of noble
metal in the WO products. A small reflection peak correlated to (111) from Au
3
(JCPDS No. 04-0784) can be found, which indicates the existence of Au in WO3.
Fig. 3 illustrates the low- and high-magnification SEM images of pure and noble
metal-doped WO nanoplates. It can be clearly seen from the low-magnification SEM
3
images (Fig. 3a, 3d, 3j and 3m) that compared with other WO samples, the Au-doped
3
WO nanoplates sample (Fig. 3j) has the worst dispersibility. As seen from the
3
high-magnification SEM images, all the WO products are composed of interlaced
3
and irregular nanoplates with the thickness of 10–30 nm, and the length and width of
the nanoplates are in the range of several hundred nanometers. These WO nanoplates
3
in different directions loosely stack together and correspondingly create a large
number of porous passageways, which is helpful to the diffusion of gaseous
molecules and the subsequent interaction between gaseous molecules and sensing
material.
TEM and HRTEM were also employed to further determine the microstructure
and crystallinity of the as-prepared WO nanoplates, and the corresponding results are
3
presented in Fig. 4. As clearly seen from the TEM images (Fig. 4a, 4c, 4e and 4g),
except for many nanoplates, there are some black rod-shaped shadows in the field of
view. These shadows are the nanoplates perpendicular to the plane, which further
1
2 / 37

confirms the hierarchical nanoplate structure of the as-prepared WO samples. Such
3
results are in good accordance with the SEM observation, which is helpful to improve
gas sensing properties due to its porosity structure. The clear lattice fringes can be
seen from HRTEM images, revealing the high crystallinity of these WO3
nanomaterials. The inter-planar spacings of 0.364 nm (Fig. 4b, 4d and 4f) and 0.384
nm (Fig. 4h) can match well with the (200) and (002) planes of monoclinic-structured
WO , respectively, which are in accordance with the XRD results. The interplanar
3
spacings of 0.243 and 0.181 nm in Fig. 4b can be in good accordance with the (111)
plane of Ag and the (102) plane of Ag O, respectively, demonstrating the existence of
2
0
Ag in the form of silver metal (Ag ) and silver oxide (Ag O). The inter-planar
2
spacings of 0.215 nm in Fig. 4(d), 0.204 nm in Fig. 4(f), and 0.226 nm in Fig. 4(h)
can match well with the (111) plane of PdO , the (200) plane of Au, and the (111)
2
plane of PtO , respectively, indicating that these noble metal elements exist in the
2
form of metallic or metallic oxide nanoparticles. The SAED pattern inserted in each
TEM image further reveals that the noble metal-doped WO nanoplates belong to
3
polycrystalline structure.
The elemental mapping images were investigated to further verify the existence
and distribution state of noble metal elements in the WO samples and the
3
corresponding results are illustrated in Fig. 5. As observed in this figure, each WO3
sample consists of W, O, and corresponding noble metal element. More importantly,
for each WO sample, the noble metal elements are uniformly distributed in the
3
visible area.
1
3 / 37

The surface chemical components and electronic structures of noble metal-doped
WO nanoplates measured by XPS are illustrated in Fig. 6. The binding energies of all
3
the WO samples were calibrated with reference to the C 1s peak (284.8 eV). As seen
3
in Fig. 6(a), the W 4f spectra of noble metal-doped WO nanoplates have two strong
3
peaks centered at 35.68–35.88 eV and 37.88–37.98 eV, which can be identified to the
binding energies of W 4f7/2 and W 4f , respectively, demonstrating that the tungsten
5/2
6
+
in the WO crystal exists as W [25, 39]. As observed in Fig. 6(b), the peaks of noble
3
metal-doped WO nanoplates centered at 530.38–530.58 eV are corresponding to O 1s,
3
2
-
which can be contributed to lattice oxygen (O ) in WO lattice [8, 40]. Fig. 6(c)
3
displays that the doublets located at 368.38 and 374.28 eV are attributed to Ag 3d5/2
0
and Ag 3d , respectively, of which one is related to a single silver metal (Ag ) and
3
/2
the other is in accordance with a silver oxide (Ag O) due to the partial surface
2
oxidization of Ag [15, 41]. Fig. 6(d) shows that the peak situated at 338.11 eV is
contributed to Pd 3d of a palladium oxide (PdO ) [42, 43]. Fig. 6(e) presents that two
2
strong peaks situated at 83.99 and 87.68 eV, respectively, which correspond to Au
4
f7/2 and Au 4f , implying the existence of Au nanoparticles in the Au-doped WO
5/2 3
crystals [44]. Fig. 6(f) shows that the peak located at 79.26 eV is ascribed to Pt 4f,
indicating that Pt exists in the WO sample in the form of PtO [45].
3
2
Fig. 7 illustrates the FTIR spectra of pure and noble metal-doped WO3
-1
nanoplates in the wavenumber range of 500–4000 cm . It can be clearly observed that
there are some differences in these FTIR spectra. For all the WO samples, the peak
3
positions are basically same, but the intensities of the peaks related to some special
1
4 / 37

functional groups change slightly. Meanwhile, for each WO sample, no peaks
3
correlated to corresponding noble metal element are found because of the low content
of noble metal in the WO nanoplates. Furthermore, there are no other peaks related to
3
some impurity components, demonstrating that all the WO samples are of high purity.
3
-1
The peaks situated at ~3425 cm are corresponding to –OH stretching vibration [25].
-1
The peaks positioned at ~1631 cm are contributed to O-H bending vibration band.
-1
-1
The peaks at ~968 cm are assigned to W=O bond stretching. The peaks at ~732 cm
are attributed to the W-O-W stretching vibration of the bridging oxygen [25].
Fig. 8 illustrates the N adsorption-desorption isotherms of pure and noble
2
metal-doped WO samples. All the WO samples exhibit type-IV isotherms with H3
3
3
hysteresis loops, indicating that the WO samples have typical mesoporous structure.
3
The average specific surface areas of the pure, Ag-, Pd-, Au-, and Pt-doped WO3
2
samples are determined to be 27.57, 34.93, 36.38, 42.84 and 42.31 m /g, respectively.
It can be concluded that the specific surface area of pure WO nanoplates is greatly
3
improved by noble metal doping, which is conducive to enhancing the sensing
properties of the samples.
3
.2. Gas sensing properties
The responses of pure and noble metal-doped WO nanoplates to 5 ppm NO at
3
2
o
7
5–225 C are illustrated in Fig. 9(a). As is well-known, the sensor response mainly
depends on the operating temperature due to its influence on the thermodynamics and
kinetics of the chemisorption of gaseous molecules on the surface active sites of the
1
5 / 37

sensing materials. For all sensors, there are similar change trends, i.e., the responses
of all WO nanoplates to 5 ppm NO increase gradually and then reach a maximal
3
2
value along with increasing the operating temperature. As a result, an optimal
o
o
operating temperature of 175 C for pure and Au-doped WO nanoplates, and 150 C
3
for Ag-, Pd- and Pt-doped WO nanoplates are respectively achieved to balance the
3
activation and desorption of gas molecules, thereby obtaining a peak response for
each sensor [37]. Obviously, the responses of noble metal-doped WO nanoplates are
3
significantly higher than those of pure WO nanoplates. Especially, the Pd-doped
3
WO nanoplates exhibit highest response value at a relatively low operating
3
o
temperature of 150 C. Compared with pure and Au-doped WO nanoplates, Ag-, Pd-,
3
and Pt-doped WO nanoplates obviously reduce the optimal operating temperature at
3
which the peak response can be obtained. The above results can be contributed to the
different catalytic activities of the noble metal nanoparticles as well as the varied
work function energies of WO and noble metal elements.
3
Fig. 9(b) presents the base resistance values of pure and noble metal-doped WO3
o
nanoplates at 75–225 C in air. Obviously, all the base resistance values of noble
metal-doped WO nanoplates are higher than those of pure WO nanoplates due to the
3
3
different work function energies of WO and noble metal elements, resulting in the
3
formation of Schottky junctions in the contact interfaces of WO and noble metal. It
3
should be mentioned that the base resistance value of Au-doped WO nanoplates at 75
3
o
C is not presented here because its base resistance is too large to be detected at a
o
relatively low operating temperature. When the operating temperature is below 100 C,
1
6 / 37

the base resistance values of all the as-synthesized WO sensing materials are higher
3
than 100 MΩ. However, the values rapidly decrease as the operating temperature
further increases. Especially, the values obtained at the optimal operating temperature
o
o
of 150 C for Ag-, Pd- and Pt-doped WO nanoplates or 175 C for pure and
3
Au-doped WO nanoplates are less than 100 MΩ, revealing that all the as-prepared
3
WO sensing materials can meet the requirement of the practical application. As
3
observed from the inserted image, the base resistance value has a good linear
o
relationship with the operating temperature in the range of 125–175 C. Therefore, the
o
base resistances at 125–175 C are chosen to calculate the activation energies of the
reactions between the as-synthesized WO sensing materials and NO molecules. The
3
2
activation energies are calculated according to the Arrhenius’ equation of
ln(1/R)=A-E/KT, where R is resistance, A is Arrhenius constant, T is Kelvin
temperature, K is Boltzmann constant, and E is energy barrier [32]. The calculated
results are illustrated in Fig. 9(c). According to the calculated results, the activation
energies of the reactions between pure, Ag-, Pd-, Au-, and Pt-doped WO nanoplates
3
and NO are 0.84, 0.22, 0.20, 0.58 and 0.68 eV, respectively. Correspondingly, the
2
o
response values to 5 ppm NO at 150 C are obviously improved in the following
2
order: pure, Pt-, Au-, Ag-, and Pd-doped WO nanoplates. This further proves that the
3
sensor response has a direct relationship with the activation energy of the reaction
between sensing material and NO . Noble metal doping can obviously reduce the
2
activation energy of the reaction between sensing material and NO , which is helpful
2
to the chemical adsorption of oxygen species and the acceleration of the surface
1
7 / 37

chemical reactions of the sensing materials, and the subsequently enhancement of
NO sensing performance. In addition, the larger specific surface area can also be
2
beneficial to the enhancement of the sensor response.
Fig. 9(d) shows the dynamic response curves of pure and noble metal-doped
WO nanoplates to 5 ppm NO at their optimal operating temperatures. For each
3
2
sensor, the response quickly increases along with the injection of NO into test
2
chamber, and then decreases along with the release of NO , implying that the WO
2
3
nanoplates are classified to n-type metal oxide semiconductor material. Meanwhile, it
also shows that the sensor responses at their optimal operating temperatures are
obviously improved in the order as follows: pure, Pt-, Ag-, Au-, and Pd-doped WO3
nanoplates, demonstrating that the catalytic effect of Pd is obviously stronger in
comparison with those of other noble metals and the catalytic effect of Pt is relatively
weaker in this case.
Fig. 9(e-f) illustrates the response times and recovery times of pure and noble
o
metal-doped WO nanoplates to 5 ppm NO at 75–225 C. It can be seen in Fig. 9(e)
3
2
that the response speed of noble metal-doped WO nanoplates is significantly faster
3
than that of pure WO nanoplates at a fixed operating temperature, indicating that
3
noble metal doping can effectively accelerate the chemisorption of detected gaseous
molecules on the surface active sites and subsequently promote their chemical
reactions with sensing materials. This is mainly attributed to the catalytic activity of
noble metal nanoparticles and the low activation energies of the reactions between
sensing materials and NO . However, the changes in the recovery time of noble
2
1
8 / 37

metal-doped WO nanoplates are relatively complex. It can be clearly observed in Fig.
3
9
(f) that the recovery times of noble metal-doped WO nanoplates are usually longer
3
o
than those of pure one below 125 C, and the corresponding values are shorter than
o
those of pure one above 175 C. It should be mentioned that the recovery time of pure
o
WO nanoplates at 75 C is not illustrated in Fig. 9(f) because it takes a long time to
3
recover to its initial value at such low operating temperature.
Fig. 10(a-b) illustrates the dynamic response curves of the WO3
nanoplates-based sensors towards 50 ppb, 100 ppb, 300 ppb, 500 ppb, 1 ppm, 3 ppm
and 5 ppm NO at their optimal operating temperatures. For each sensor, the
2
amplitude change in the sensor response increases as NO concentration increases.
2
For a fixed NO concentration, the increase in the response follows the injection of
2
NO , and subsequently decreases after the release of NO from the test chamber,
2
2
indicating a good reversibility. Additionally, the lowest detection concentration is 50
ppb, indicating that the as-obtained WO nanoplates can be used for the detection of
3
ppb-level NO2.
The corresponding responses of pure and noble metal-doped WO nanoplates to
3
various concentrations of NO at their optimal operating temperatures are illustrated
2
in Fig. 10(c-d). For each sensor, the response increases as NO concentration
2
increases, demonstrating that the sensor does not reach the saturation state for the
adsorption of NO molecules. This may be attributed to the large effective specific
2
surface area of the WO nanoplates, which can supply more surface sites for the
3
chemisorption reactions of gas molecules.
1
9 / 37

The correspondingly enlarged response images of pure and noble metal-doped
WO nanoplates-based sensors in the NO concentration range of 1–5 ppm at their
3
2
optimal operating temperatures are shown in Fig. 10(e). The linear correlation
2
coefficients (R ) of the sensor response curves as a function of NO concentration are
2
above 0.90, indicating a good linearly relationship with NO concentration for each
2
sensor. The slopes calculated based on the above curves of pure, Ag-, Pd-, Au- and
Pt-doped WO nanoplates are 8.11, 30.61, 51.77, 25.83 and 9.32, respectively.
3
Therefore, Pd-doped WO nanoplates can supply more surface active sites for the
3
chemisorption of NO molecules along with further increasing NO concentration and
2
2
exhibit highest response in comparison with other sensors, which further proves that
the lower activation energy induced by the catalytic effect of noble metal is helpful to
the enhancement of the sensor response.
The relationship between sensor response and NO concentration can be
2
m
estimated by the theory of power laws: S=k[CNO2] +1, where CNO2 refers to NO2
concentration, S is sensor response, m is an exponent, and k is a constant. The
calculated results are illustrated in Fig. 10(f). As seen in this figure, the sensor
response linearly increases as NO concentration increases from 50 ppb to 5 ppm for
2
each sensor, which can be used to assess NO concentration based on its sensor
2
response value.
A summary presented in Fig. 11 is made to further determine the effect of noble
metal element on NO sensing properties of the WO nanoplates in terms of response,
2
3
response/recovery time as well as operating temperature. Some important results are
2
0 / 37

summarized as below. (1) Compared with Au doping, the doping of Ag, Pd and Pt can
effectively reduce optimal operating temperature. (2) Noble metal doping can
obviously improve the sensor response, especially Pd doping. (3) Noble metal doping
can significantly reduce response time, especially Ag-doped WO nanoplates-based
3
sensor shows the shortest response time. (4) Compared with response time, the
recovery time changes relatively complex after noble metal doping, and Pd doping
obviously reduces the recovery time, while Ag doping increases the recovery time.
Although Au doping cannot reduce the optimal operating temperature, the peak
response is still higher, and the response/recovery time is relatively shorter. Pt doping
can reduce the optimal operating temperature, but the enhancement of NO sensing
2
properties is not obvious. The above results are mainly ascribed to the synergistic
effect of the large specific surface area, the sensitization effects of noble metal
nanoparticles and the varied work function energies of WO and noble metal elements
3
together with the different activation energies of the reactions between sensing
materials and NO . The detail sensing mechanism will be discussed later.
2
To further estimate NO sensing performance of the as-synthesized WO
2
3
nanomaterials, some comparisons between the as-prepared WO nanoplates and other
3
nanostructured WO materials were conducted in terms of operating temperature,
3
response, response/recovery time as well as detection limit. The results are illustrated
in Table 3. Obviously, these WO sensors in previous reports still have more or less
3
drawbacks, such as low response, slow response/recovery speed and high operating
temperature, which restrict their practical applications. Although the Ag-, Pd-, Au-,
2
1 / 37

and Pt-doped WO nanoplates are synthesized from a low-grade scheelite concentrate,
3
the WO nanoplates-based sensors show excellent NO sensing performance
3
2
compared with other WO sensors reported in the literatures. Additionally, such
3
sensors can be used to detect low-concentration NO , and the detection limit is even
2
as low as 50 ppb.
Fig. 12(a-b) presents the reproductive cycles of pure and noble metal-doped WO3
nanoplates-based sensors exposed to 5 ppm NO at their optimal operating
2
temperatures. The results show that the amplitude change in the sensor response is
basically same at a fixed concentration of NO . It suggests good repeatability and
2
excellent stability of the present WO nanomaterials, which is beneficial to their
3
practical application. Fig. 12(c) illustrates the long-term stability of the WO3
nanoplates-based sensors to 5 ppm NO at their optimal operating temperatures. As
2
seen in this figure, all the sensors exhibit a good stability during the test cycle of 30
days. Fig. 12(d) presents the selectivity of pure and noble metal-doped WO3
nanoplates-based sensors towards various kinds of toxic gases, including NO2,
methanol, NH , acetone, methylbenzene, methanal, and SO at their optimal operating
3
2
temperatures. Obviously, all the sensors exhibit extraordinary selectivity towards NO2
in comparison with other detected gases. The prominent NO selectivity can be
2
attributed to the chemical reactions on the surfaces of WO sensing materials between
3
NO molecules and chemisorbed oxygen species.
2
3
.3. Gas sensing mechanism
As a typical n-type metal oxide semiconductor, WO3 belongs to
2
2 / 37

surface-controlled type of the sensing material. Hence, the change in the sensor
resistance is relevant to the chemisorption of detected gaseous molecules as well as
the chemical reactions between these gaseous molecules and chemical components on
the WO surface [48]. The commonly accepted sensing mechanism of the WO
3
3
nanomaterial is the surface-depletion model [49]. The schematic illustration of NO2
sensing mechanism of noble metal-doped WO nanoplates is presented in Fig. 13.
3
The enhanced NO sensing properties of noble metal-doped WO nanoplates can
2
3
be ascribed to the large specific surface area of WO nanoplates, the sensitization
3
effects of noble metal nanoparticles, and the varied work function energies of metal
noble and WO together with the different activation energies of the reactions between
3
WO sensing materials and NO . The work function energies of WO and noble
3
2
3
metals [3, 5, 50-54], and the activation energies of the reactions between the WO3
sensing materials and NO are listed in Table 4. As seen from Table 4, noble metal
2
doping can significantly reduce the activation energy of the reaction between WO3
sensing material and NO induced by the catalytic effect of noble metal, which is
2
helpful to the improvement of gas sensing properties. It can also be seen from the
table that the work function energy of each noble metal is different from that of WO3,
resulting in the formation of Schottky junctions in the contact interfaces of noble
metal and WO . Subsequently, the electrons transfer is accelerated, leading to a
3
broadening in the width of interfacial electron depletion layer. Correspondingly, the
WO sensing materials are more sensitive to the detected gas molecules. According to
3
the results illustrated in Fig. 10(b), the base resistance values in air of noble
2
3 / 37

metal-doped WO nanoplates are higher than those of pure WO nanoplates at 75–225
3
3
o
C, demonstrating that the width of electron depletion layer increases after doping
noble metals [55]. As actively catalytic components, noble metal nanoparticles in
WO nanoplates can accelerate the diffusion of oxygen molecules onto the surface
3
sites and extract free electrons from the WO conduction band by the conversion of
3
gas molecules and ions, which improves the quality and quantity of the chemical
2
–
–
–
adsorbed oxygen ions (O , O and O ) and simultaneously generates more surface
2
effective adsorption sites [34, 56]. These effective adsorption sites are beneficial to
–
the fast diffusion of NO molecules and the subsequent ionization of NO to NO
2
2
2
–
–
(
NO +e →NO ) on the WO surface. Because the optimal operating temperature for
2 2 3
o
pure and noble metal-doped WO nanoplates is between 100 to 300 C, the oxygen
3
–
species on the surface of WO mainly exist as O . Thus, the chemical reaction relative
3
to the above can be presented as follows [17, 25].
O (gas) → O (ads)
(1)
(2)
(3)
(4)
(5)
(6)
ꢀ
ꢀ
ꢁ
ꢁ
O (ads) + e → O (ads)
ꢀ
ꢀ
ꢁ
ꢁ
ꢁ
O (ads) + e → 2O (ads)
ꢀ
NO (gas) → NO (ads)
ꢀ
ꢀ
ꢁ
ꢁ
NO (ads) + e → NO (ads)
ꢀ
ꢀ
ꢁ
ꢁ
ꢁ
ꢀꢁ
NO (ads) + O (ads) + 2e → NO (ads) + O (ads)
ꢀ
ꢀ
Meanwhile, the noble metal nanoparticles on the WO surface will create more
3
adsorption sites to adsorb and catalytically dissociate oxygen molecules, which are
conducive to improving the sensitivity [57]. Compared with other noble metal-doped
2
4 / 37

WO nanoplates, the Pd-doped WO nanoplates exhibit a highest response to NO and
3
3
2
o
a shortest recovery time at 150 C due to the smallest activation energy, indicating that
the catalytic effect of Pd is relatively stronger in comparison with those of other noble
metals. The Pt-doped WO nanoplates show a relatively lower response to NO , and a
3
2
longer response/recovery time because of the relatively higher activation energy,
indicating that the catalytic effect of Pt are relatively lower.
4
. Conclusions
The interlaced and irregular WO nanoplates with a single monoclinic structure
3
were synthesized from a low-grade scheelite concentrate by three combined processes
including NaOH leaching, chemical precipitation, and acidification. Meanwhile, the
in-situ doping of noble metal elements into WO nanoplates was carried out to
3
simulate their existence in tungsten raw material and their effects on microstructure
and NO2 sensing properties were also investigated. The microstructure
characterization demonstrated that the thickness of all WO nanoplates was 10–30 nm,
3
and the length and width of these nanoplates were several hundred nanometers. The
noble metal nanoparticles were uniformly distributed in the corresponding noble
metal-doped WO sample. NO sensing properties indicated that noble metal doping
3
2
could effectively enhance the sensor response to NO as well as reduce response time.
2
Especially, the doping of Ag, Pd and Pt could obviously reduce optimal operating
temperature. Different behaviors in the improvement of NO sensing properties could
2
2
5 / 37

be explained by the specific surface area of WO nanoplates, the catalytic activities of
3
noble metal elements, the work function energies of WO and noble metal as well as
3
the activation energies of the reactions between sensing materials and NO . In
2
addition, this work provides a new idea to prepare functional WO sensing
3
nanomaterials by using a low-grade scheelite concentrate as the tungsten source as
well as improving gas sensing properties by simulating the existence of noble metal
elements in tungsten raw materials. The noble metal-doped WO nanoplates
3
synthesized from a low-grade scheelite concentrate exhibited obviously enhanced
NO sensing properties, which have a potential application in environmental
2
detection.
Acknowledgement
The project was supported by the National Natural Science Foundation of China
(
51674067, 51422402), Fundamental Research Funds for the Central Universities
N180102032, N180106002, N180408018, N170106005), LiaoNing Revitalization
(
Talents Program (XLYC1807160), Liaoning BaiQianWan Talents Program
(
201892127), and Open Foundation of State Key Laboratory of Mineral Processing
BGRIMM-KJSKL-2019-12).
(
2
6 / 37

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