Design and understanding of a high-performance gas sensing material based on copper oxide nanowires exfoliated from a copper mesh substrate

Article abstract of DOI:10.1039/c5ta06806a

We report a novel and facile route to synthesize a 1D needlelike CuO gas sensor utilizing simple solution-treatment, heat-treatment and sonication processes based on copper meshes (CMs) for the first time. We have demonstrated that the architectural 1D CuO sensor displays a substantial improvement of response magnitude toward a series of toxic organic molecules. We expect that our work may provide a new inspiration for synthesizing 1D semiconductor nanowires for gas sensing application.

Full text of DOI:10.1039/c5ta06806a

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Design and Understanding of High-Performance Gas Sensing
Material Based on Copper Oxide Nanowires Exfoliated from
Copper Meshes Substrate
Received 00th January 20xx,
Accepted 00th January 20xx
Fuchao Yang ac, Jie Guo ab, Mingming Liu ac, Shen Yu ab, Ningbo Yanad, Jing Li a and Zhiguang
Guo*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
We firstly reported a novel and facile route to synthesize 1D received extensive attentions because of their novel
needlelike CuO gas sensor utilizing simple solution-treatment, electrochemical properties related to larger surface area and
heat-treatment and sonication process based on copper meshes more effective surface defect concentration as well as their
(CM). We have demonstrated that the architectural 1D CuO sensor potential advantages in catalysis, electronics, and gas sensors.
displays substantial improvement of response magnitude toward There are various conventional methods applied to generate 1D
a series of toxic organic molecules. We expect that our work may nanostructural material, such as assisted by Pt in vacuum,9
provide a new inspiration for synthesizing 1D semiconductor thermal deposition,10 microwaveꢀassisted hydrothermal
nanowires for gas sensing application.
method,12 electrospinning,13 etc. However, some limitations still
remain in these approaches, including extremely high vacuum,
superhigh temperature, extraꢀhigh voltage or other harsh
growing conditions. It is quite desirable to develop a facile and
versatile route for the controllable production of 1D functional
nanomaterials under atmosphere without depending on any
other complicated devices.
Herein, we demonstrated an interesting, simple, costꢀ
effective and energyꢀsaving strategy for preparing uniform 1D
CuO nanowires, which possessed enhanced gas sensing
properties. An air dry oven and a ultrasound equipment are all
the devices we needed. In short, three steps are involved in this
process as schematically shown in scheme 1: etching by
alkaline solution, dehydrating by heat treatment and exfoliating
by lowꢀpower sonication. To the best of our knowledge, this is
the first report of CuO nanowires fabricated by this facile route
The elaboration and optimizing of gas sensors are present at the
focus of collective research endeavours owing to the fact that it
plays an everꢀincreasing role in outdoor or indoor
environmental monitoring, assessment of food freshness, lung
cancer detection, industrial surveillance and so on.1ꢀ3 Transition
metal oxides have emerged as a viable alternative in fabricating
highly efficient gas sensors because they can provide ppm level
sensitivity, fast response, low cost, low power, good selectivity
and stability.4,5 In particular, cupric oxide (CuO), as a pꢀtype
semiconductor with a narrow band gap of 1.2 eV, is considered
as one of the most appealing inorganic materials for
applications in chemical sensors because of its efficient
catalytic activity, good electrochemical performance, and
excellent surface adsorption property.6ꢀ8 The CuO gas sensor
material is usually made from copper salt, and we propose and
demonstrate that it can be obtained by exfoliating from the asꢀ
designed copper meshes.
In recent years, the rational design and synthesis of oneꢀ
dimensional (1D) nanostructures, such as nanowires,9
nanoribbon,10 nanotubes,11 nanorods12 and nanofibers,8,13 have
a.State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China
b.Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials
and Ministry of Education Key Laboratory for the Green Preparation and
Application of Functional Materials, Hubei University, Wuhan 430062, People’s
Republic of China
c. University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
China
Scheme 1 Schematic illustration of the facile route
involved three steps for the generation of CuO nanowires
d.School of Mechanical & Electrical Engineering, Wuhan Institute of Technology,
Wuhan 430073, People’s Republic of China
† Electronic Supplementary Informaꢀon (ESI) available: Experimental procedure
and figures S1-7. See DOI: 10.1039/x0xx00000x
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Fig.1 FESEM images of alkaline etching CM with low (a)
and high (b) amplification, after 150 C annealing (c), and
o
the 1D CuO powder form (d). The inserts are TEM images
of 1D CuO powder (e) and its corresponding electron
diffraction pattern (f).
for use in gas sensing applications.
The surfaces of metals can often be finely oxidized and
textured by mixed alkaline solution.
cleaned copper mesh (CM) was firstly put into the aqueous
solution of 1.0 M NaOH and 0.05 M K2S2O8 at room
temperature for 0.5 h. After this process of sacrificial oxidation,
the surface of CM was reconstructed by the needlelike structure
(Fig.1a and b) and its colour turned to blue from shiny red
(Fig.S1aꢀb, ESI). This alkali etching and oxidation process can
be understood by the following chemical equation:
Fig.2 (a) UV-visible absorption spectra and photograph
(inset, the left vial show background) of Cu(OH)2 and CuO
ethanol dispersion (b) XRD patterns of original, after alkali
etching, and after alkali etching and annealing CM (c) TGA
curve indicates the transition process from Cu(OH)2 into
CuO covering CM. The XPS full spectrum (d) and the XPS
O1s (e) and Cu2p fine spectrum (f) of CM covered by blue
(1) and dark (2) substance.
14, 15
In our experiment, the
Cu + 2NaOH + K2S2O8
→ Cu(OH)2 +Na2SO4 +K2SO4 (1)
to be 50ꢀ70 nm (65%, Fig.S3a in ESI) and the selected area
electron diffraction pattern exhibit polycrystalline rings,
indicating that the 1D product is composited of polycrystalline
CuO (Fig.1f and Fig.S3b).
The blue substance covering the CM was Cu(OH)2 and further
evidence will be shown later by Xꢀray diffraction (XRD),
thermoꢀgravimetric analysis (TGA) and Xꢀray photoelectron
spectra (XPS). The 1D nanowires CuO covering CM can be
obtained by thermal decomposition of the needlelike Cu(OH)2
In Fig.2a, we provide the photograph (insert) and UVꢀvis
absorption spectra of Cu(OH)2 and CuO ethanol dispersion.
Compared with the pure ethanol given as blank control (left),
Cu(OH)2 (middle) and CuO (right) ethanol dispersion show
light blue and brownish black. Both these two absorption
spectra show much higher absorption in the UV region (200ꢀ
400 nm) than visible region (400ꢀ800 nm). The Cu(OH)2
ethanol dispersion show blue colour which indicate that it
would absorb the visible light with particular colour of
wavelength and other can pass. In other words, it selectively
absorbs the complementary orangeꢀred light (650ꢀ760 nm).
Several absorption peaks among this region can be found in its
partial enlarged drawing, provided as Fig.S4 in ESI. As for
CuO ethanol dispersion, it absorbs strongly throughout the
visible spectrum and thus shows a brownish black colour.
Spectroscopic analysis gives us a deeper insight into the
observed color.17 The holistic view XRD patterns of CM,
Cu(OH)2 or CuO covering CM exhibit similar (111) dominated
diffraction peaks and the difference was beyond distinction as
shown in Fig.S1e, ESI. Fig. 2b shows the enlarged XRD
o
at 150 C in air, accompanying with the colour changing into
black (Fig.S1c).
It is worth to note that the formed CuO inherit the 1D
nanostructure from the needlelike Cu(OH)2, as shown in Fig.1c.
The transformation of Cu(OH)2 into CuO in the solid state by
thermal dehydration often generate the reserved morphology
while the transformation take place in solution usually
accompanies
a
morphological change.16 The CuO was
successfully detached from the CM substrate utilizing
sonication. The morphology of CuO powder form was obtained
by dropping its dispersion on the Si substrate, which was given
in Fig.1d. Several CuO nanorods lie on the substrate and its 1D
nanostructure was not destroyed during the sonication process.
After 30 min sonication, the FESEM images of the residual CM
were given in Fig.S2. Transmission electron microscopy (TEM)
was also employed to characterize its detailed structures,
exhibiting highly dispersed nanowires morphology (Fig.1e).
The diameters of these CuO nanowires were probably estimated
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Cu(OH)2 is centred at 934.28 eV while that of CuO locates at
933.68 eV. This analysis and variation tendency is accordance
with Lee and coworkers.22 Moreover, the two extra shakeꢀup
satellite peaks raised from CuO were evidently observed at
941.08 and 943.68 eV, which indicates the presence of an
unfilled Cu 3d9 shell and thus further confirming the presence
of CuO.21, 23 The Cu 2p1/2 peak raised from Cu(OH)2 is centred
at 954.08 eV while that of CuO locates at 953.58 eV, which
indicates that both Cu 2p3/2 and Cu 2p1/2 peaks shift toward
lower binding energy after heat treatment.
To illustrate the excellent gas sensing performances of the
asꢀprepared needlelike CuO, we also investigated the gasꢀ
sensing property of ordinary CuO in a comparative view. In
briefly, the synthetic method of ordinary CuO was directly
mixed the aqueous solutions of CuSO4ꢁ5H2O and NaOH, and
endured the annealing process just as the condition of
needlelike CuO. The ordinary CuO exhibits spindleꢀlike shape
with a width of 50 ꢀ 200 nm. Its synthetic details and
characterizations were given in ESI. The gas sensing tests were
performed under the condition of relative humidity at about 22
% and environment temperature at about 18 oC. As clearly
shown in Fig.3a, all these two sensors showed typical pꢀtype
gas sensing behavior, i.e., an increase in resistance on reducing
gases.24 The gas sensing performances of needlelike CuO were
greatly enhanced and independent of the concentrations of
acetone. The corresponding response values can be calculated
based on Rg/Ra and the results were given in Fig.3b. As for 50
ppm, the response value of needlelike CuO sensor is 3.82,
which is about 2.3 times as that of ordinary CuO sensor (1.64).
As for 100 ppm, the response value of needlelike CuO sensor is
4.64, which is about 1.9 times as that of ordinary CuO sensor
(2.41). The curves exhibit that the response values increase
rapidly when the concentration is lower than 800 ppm. But
when the concentration is over 800 ppm, the increase rate
become slow, indicating that the sensor is more or less
statured.25 Fig. 3c shows the responses of needlelike and
ordinary CuO sensors to acetone of 200 ppm as a function of
the operating temperature. The sensors of needlelike and
ordinary CuO exhibit the highest response to acetone gas at 260
oC. In general, it is also observable in Fig. 3c that the sensitivity
values increase remarkably at first and then degrade
dramatically with the increase of working temperature.
Combining the catalytic role of CuO with the unique
characteristics of 1D microstructure, we expect that the asꢀ
synthesized needlelike CuO can be promising candidates for
high performance gas sensors to detect some chemical
contaminants and dangerous gases, i.e., ethanol (1), acetone (2),
benzene (3), toluene (4), benzaldehyde (5) and benzyl alcohol
(6). The concentration of these gases is 100 ppm, being
conducted at 260 oC. As demonstrated in Fig. 3d, the needlelike
CuO sensor is superior to ordinary CuO sensor for all kinds of
these target analytes. The needlelike CuO sensor show
excellent selectivity to benzaldehyde (8.89), which is about 4
times as that of sensitivity to benzene (2.25).
Fig.3 Dynamic resistance change curves (a) and the
corresponding response value (b) of the CuO sensors to
acetone of different concentrations. (c) The response of
CuO sensors to 200 ppm acetone at different testing
temperatures (d) Responses of CuO sensors to different
target gases (100 ppm) at 260 oC.
patterns of CM, Cu(OH)2 (blue line) or CuO (dark line)
covering CM. Strong diffraction peaks from the CM substrate
are also detected. The diffraction peaks of blue pattern
correspond well with orthorhombic Cu(OH)2 (Space group
Cmcm(63), JCPDS 35ꢀ0505), which is consistent with the
published paper.18 The diffraction peaks marked with black
asterisk can be indexed to monoclinic CuO (JCPDS 45ꢀ0937)
and similar patterns have been reported by previous works.19,20
It is reported that Cu(OH)2 precursor precipitates transform
itself into CuO particles after heat treatment at 90 oC for 12 h.18
To well understand the thermal conversion process in our
experiment, TGA was performed and the sample measurement
was taken under air atmosphere from room temperature to 800
o
oC at a speed of 10 C/min. At about 150 oC, the blue Cu(OH)2
covering on CM show a stage weight loss pattern (decreasing of
mass by 1%, the insert of Fig.2c) evidently, resulting from the
conversion of Cu(OH)2 into CuO. It is rationally believed that
o
the sharp increase of weight at about 600 C is attributed to the
conversion of part CM substrate into CuO. In Wang’s work,
they studied the conversion of Cu(COOC6H4NH2)2 films into
CuO/C composite films using the same analytic approach.20
Fig. 2dꢀf shows the XPS full spectrum, O1s and Cu 2p fine
spectra obtained from CM covered by blue or dark colour
substance. The related peaks of Cu3s, C1s, O1s and Cu 2p were
detected and centred at the binding energy of 124.1, 285.1,
530.1, and 930ꢀ960 eV, correspondingly. In Fig. 2e, the dark
line spectra of highꢀresolution core level of the O 1s state
shows a broad asymmetric curve which can be fitted into two
Gaussian peaks: a peak at 529.6 eV indicating O2− bonded with
Cu2+ and the second peak at 531.1 eV indicating surface
adsorbed oxygen on CuO nanowires. The above results are in
full accordance with the previous report.21 In the characteristic
highꢀresolution spectrum of Cu 2p, it was split into Cu 2p3/2, Cu
The metrics of gas sensors require high response magnitude
and fast rate. The fast rate here regard gas sensors on going for
a relatively rapid switching capabilities when it was exposed to
2p3/2, and Cu 2p1/2 peaks. The Cu 2p3/2 peak raised from
sat
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or released the target gas.26 The method for obtaining response
time (Ts) and recovery time (Tc) are given in ESI, which are
further illustrated by Fig. 4aꢀb. As it is shown, the Ts and Tc of
needlelike CuO sensor to 1 ppm acetone is 96.6 s and 19.1 s.
The Ts and Tc of needlelike CuO sensor to 10 ppm acetone is
52.6 and 22.9 s. These response and recovery time are
acceptable for practical application.1,26 While it is hard to obtain
the Ts and Tc of ordinary CuO sensor to acetone at this low ppm
level, its response and recovery behavior are submerged by
noise signal when the sensing curves are magnified. The
response and recovery behaviors of needlelike CuO sensor to
benzene, benzaldehyde and benzyl alcohol are also investigated
and are provided as Fig.S6 in ESI. For a utilityꢀtype gas sensor,
the reproducibility and longꢀterm stability are also important
issues. The reproducibility of needlelike CuO sensor is studied
through repetitive injecting or releasing the 5 ppm acetone over
several times. Fig.4c shows the results that the peaks and
valleys are found nearly constant, suggesting that the reliability
of needlelike CuO sensor. The reproducibility of needlelike
CuO sensor to 100 ppm ethanol and benzene was also studied
and was given in Fig.S7b (ESI). The longꢀterm stability of the
sensors was measured to 100 ppm acetone for a month and the
response value of initial, after 15ꢀdays and after 30ꢀdays is
4.64, 4.31 and 4.24 as shown in Fig. 4d, which confirms the
longꢀterm stability of the sensor by our measurement.
Fig.4 The transient response–recovery curves of needlelike
CuO sensor to 1 ppm (a) and 10 ppm (b) acetone. (c) The
reproducibility of needlelike CuO sensor to 5 ppm acetone.
(d) Stability of needlelike CuO sensor to 100 ppm acetone
over 30 days. (e) Schematic illustration for oxygen
chemisorption and gas sensing mechanism.
Finally, we discuss the mechanisms that can lead to a
resistance change in needlelike CuO sensor upon interaction
with analytes, as shown in Fig.4e. When our sensor exposed to
air, the chemisorbed oxygen ions (O2 , O− and O2−) are formed
−
on the surface of CuO by capturing electrons from its
surface.27,28 The concentrations of adsorbed oxygen species
(O2 , O− and O2−) are largely depended on the operating
−
response value of needlelike CuO sensor to 50 ppm acetone is
about 2.3 times as that of ordinary CuO sensor. The needlelike
CuO sensor show excellent selectivity to benzaldehyde, which
is about 4 times as that of sensitivity to benzene. The Ts and Tc
of needlelike CuO sensor to 1 ppm acetone is 96.6 and 19.1 s.
The reproducibility and longꢀterm stability of the needlelike
CuO sensor were also demonstrated. We anticipate that our
work may provide a new facile strategy for synthesizing
metallic oxide semiconductor nanowires based on metal
substrates for gas sensing application.
temperatures, specific surface area and surface active
centers.2,28 The 1D nanostructure often exhibits larger specific
surface area and sufficient surface active centers. 4,5 When our
sensors are exposed to reducing gases (six kinds of volatile
gases in our experiment), these organic gas molecules are
−
oxidized by oxygen species (O2 , O− and O2−), and
simultaneously the electrons are fed back into CuO sensor
body, leading to a recombination of electron and hole and thus
causing a depletion of holes. As is well known, the conductive
ratio of pꢀtype semiconductor is greatly influenced by the
concentration of the holes and so an increased resistance in
needlelike CuO sensor is observed. According to the previous
literatures, 24, 29 metal oxides with acidic (e.g., SnO2 and WO3),
amphoteric (e.g., Fe2O3 and ZrO2), and basic (e.g., CuO and
Acknowledgements
This work is supported by the National Nature Science
Foundation of China (Nos. 11172301 and 51522510), the
“Funds for Distinguished Young Scientists” of Hubei Province
(No. 2012FFA002), the “Western Light Talent Culture”
Project, the Coꢀjoint Project of Chinese Academy of Sciences,
the “Top Hundred Talents” Program of Chinese Academy of
Sciences and the National 973 Project (2013CB632300) for
financial support.
La2O3) surfaces involve di
fferent interaction procedures and
di erent sensing behaviors. Under basic conditions, the
ff
functional group of aldehydes (ꢀCHO) is much more active than
the alcohols and hydrocarbons. 24,27 Hence, benefiting from the
surface basicity, a higher sensing response to benzaldehyde is
observed.
In summary, we reported a simple solutionꢀbased method for
the preparation of 1D needlelike Cu(OH)2 and it conversion
into the morphology reserved CuO gas sensor after heat
treatment and sonication process. Consequently, the gas sensing
performances of needlelike CuO were greatly enhanced. The
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Table of contents
High-Performance Gas Sensing Based on Copper Oxide Nanowires Exfoliated
from Copper Meshes Substrate (CMS).