Synthesis of high-purity CuO nanoleaves and analysis of their ethanol gas sensing properties

Abstract

CuO nanocrystals with as-designed morphologies such as uniform quasi-spherical nanoparticles and high-purity nanoleaves were synthesized by adjusting the addition of sodium hydroxide and hydrazine hydrate in aqueous solution at room temperature (25 °C). The increase of sodium hydroxide would accelerate the reaction rate and favor the nucleation of CuO nanocrystals. The decrease of the surface energy will promote the oriented attachment of nanocrystallites along the [−111] direction into nanowires and the final formation of two dimensional (2D) nanoleaves. Increasing the quantity of hydrazine hydrate could decrease the solution system energy and promote the aggregation of CuO nanocrystals from 2D nanoleaves into 3D quasi-spherical nanoparticles. All the CuO nanocrystals with different morphologies were characterized via transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The CuO nanoleaves exhibit excellent gas sensing performance in response to ethanol, showing the strongest response value of 8.22 at 1500 ppm ethanol for 260 °C.

Full text

Synthesis of high-purity CuO nanoleaves and
analysis of their ethanol gas sensing properties†
Yu Cao,‡
a
Shiyu Liu,‡
a
Xian Jian,‡*
a
Gaolong Zhu,
a
Liangjun Yin,*
a
Le Zhang,
a
Biao Wu,
a
Yufeng Wei,
a
Tong Chen,
a
Yuqi Gao,
a
Hui Tang,*
a
Chao Wang,*
b
Weidong He
a
and Wanli Zhang*
c
CuO nanocrystals with as-designed morphologies such as uniform quasi-spherical nanoparticles and high-
purity nanoleaves were synthesized by adjusting the addition of sodium hydroxide and hydrazine hydrate in
aqueous solution at room temperature (25 C). The increase of sodium hydroxide would accelerate the
reaction rate and favor the nucleation of CuO nanocrystals. The decrease of the surface energy will
promote the oriented attachment of nanocrystallites along the [111] direction into nanowires and the
final formation of two dimensional (2D) nanoleaves. Increasing the quantity of hydrazine hydrate could
decrease the solution system energy and promote the aggregation of CuO nanocrystals from 2D
nanoleaves into 3D quasi-spherical nanoparticles. All the CuO nanocrystals with different morphologies
were characterized via transmission electron microscopy (TEM), field emission scanning electron
microscopy (FESEM) and X-ray diffraction (XRD). The CuO nanoleaves exhibit excellent gas sensing
performance in response to ethanol, showing the strongest response value of 8.22 at 1500 ppm ethanol
for 260 C.
Introduction
Cupric oxide (CuO) has been extensively studied because it
exhibits a narrow band gap (1.2 eV) and a number of other
interesting properties in catalysts, sensors and lithium ion
batteries. For instance, Huang et al. have reported CuO nano-
sheets with a superior catalytic performance for CO oxidation
(47.77 mmol
CO
g
1CuO
h
1
at 200 C).
1
Xu et al. have revealed a
high specic capability and good cycle performance of CuO
mesocrystals.
2
Qin et al. have studied the excellent sensing
performance with high sensitivity, quick response, and recovery
of the H
2
S sensor prepared by CuO hollow spheres.
3
Recently,
the effectively controlling and adjusting the size, morphology
and structure of metal oxide nanocrystals have been advanced
remarkably.
4–7
Meanwhile, the morphologies and sizes of CuO
directly determined the properties and applications. Hence,
many methods have been employed towards the synthesis of
various structures, for instance, thermal oxide,
8
wet chemical
method,
9
electrochemical method,
2
pulsed wire explosion,
10
and chemical vapor deposition (CVD).
11
And the structures such
as ribbons, platelets, spheres, owers, hollow structures and so
on, have been synthesized successfully.
12–16
Although investigations have been focused on exploring
synthetic methods, it is still a crucial and fascinating problem
to adjust numerous nanostructures with a simple method.
17–19
In order to modulate numerous nanostructures with a simple
method, great efforts of the structure of metal oxide have been
investigated. To classify these modulating approaches, it could
be mainly divided into choosing various synthetic approaches,
several parameters of the growth process and different kinds of
protected reagents.
20–23
Recently, some new approaches have
been reported. For instance, Ghosh et al. synthesized meso-
porous cube-shaped CuO with a multishell microcarpet-like
patterned interior via a facile aqueous-based process using
copper nitrate, oxalic acid, and phosphoric acid in TBC.
24
Huang et al. provided the method to tune the reaction kinetics
by metal cations such as Zn
2+
,Ag
+
and Al
3+
.
1
The researches and
the creations of approaches for controlling the structures of
metal oxide greatly enrich the eld of nanoscience and
nanotechnology.
In this work, we have reported a facile method for modu-
lating diversity structures of CuO nanocrystal such as high-
purity nanoleaves, uniformed quasi-spherical nanoparticles
a
Clean Energy Materials and Engineering Centre, School of Energy Science and
Engineering, State key Laboratory of Electronic Thin Films and Integrated Devices,
Centre for Information in Biomedicine, University of Electronic Science and
Technology of China, Chengdu, 611731, China. E-mail: jianxian20033835@163.
com; tanghui@uestc.edu.cn
b
Laboratory of Precision Manufacturing Technology, Institute of Machinery
Manufacturing Technology, China Academy of Engineering Physics, Mianyang,
621900, China. E-mail: wangchaohit@126.com
c
State key Laboratory of Electronic Thin Films and Integrated Devices, Insitute of
Microelectronic & solid-state electronics, University of Electronic Science and
Technology of China, Chengdu, 610054, China. E-mail: wlzhang@uestc.edu.cn
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra03497c
‡These authors contributed the equally to this work.
Cite this: RSC Adv.,2015,5, 34788
Received 26th February 2015
Accepted 8th April 2015
DOI: 10.1039/c5ra03497c
www.rsc.org/advances
34788 |RSC Adv.,2015,5,34788–34794 This journal is © The Royal Society of Chemistry 2015
RSC Advances
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and ne nanoparticles in aqueous solution via mixing and
tuning the additions of copper chloride, sodium hydroxide and
hydrazine hydrate without other surfactants at room tempera-
ture (25 C). A series of experiments are carried out to perform
the changes of various morphologies. The mechanism of the
whole synthesis process is revealed by the analysis of reaction
kinetics and an oriented attachment growth step. We found that
a quick reaction rate would favor the nucleation of CuO nano-
crystals, which promote the decrease of the surface energy and
the self-assembly of CuO nanocrystallites into nanowires along
the [111] direction, and the nal formation of CuO nanoleaves
from nanowires perpendicular to the [111] direction. The
hydrazine hydrate could eliminate the nanocrystals energy,
decrease the reaction barrier and obviously shorten the reaction
time from 40 h to 8 h and decrease the reaction temperature
from 80 Cto25C.
25,26
The response to testing gas at different
concentrations of ethanol in various temperatures was taken as
a case to demonstrate the well performance of the CuO
nanoleaves-base sensor. The response to ethanol is close to
those of gas sensor report recently.
27
Further experiments and
studies had been carried out and demonstrated that CuO
nanoleave-based sensor displays the strongest response of
8.22 at 1500 ppm ethanol in 260 C. The well property in sensing
ethanol may be attributed to the 2D structure of CuO nano-
leaves. The puried and uniformed CuO nanostructures
synthesized by the facile aqueous solution method may have an
extensive application in catalysts, lithium ion batteries and
solar cells.
Experimental
Synthesize the quasi-spherical nanoparticles and nanoleaves
All reagents in this experiment were analytical grade and used
without further purication. In a typical experiment, 1 mmol of
CuCl
2
$H
2
Owasrstly dissolved in 300 ml deionized water under
a constant magnetic stirring at room temperature (25 C). Blue
precipitates appeared quickly aer Xmmol NaOH (0.2 mol L
1
)
added into CuCl
2
solutions. Then, a total of hydrazine hydrate
(0.1 mol L
1
) with the additions of Ymmol was added into the
above solutions. The resultant solutions turned into light yellow
in a short time and nally changed into grey in a longer period
of 8 h. The precipitates of each sample were washed more
than three times with deionized water and ethanol. Finally, the
products were ltered and dried at 100 C for 3 h in the drying
oven. The morphologies of as-prepared products were modu-
lated from nanoparticle to nanoleaves when the values of
Xrange from 3.2–4.8 mmol while the value of Yis 0.4 mmol and
the morphologies were modulated from nanocubics twined
with nanoleaves to nanoparticled when the Yrange from 0.4–2.0
mmol while the value of Xis 3.4 mmol.
Gas sensor preparation
The as-prepared CuO (the value of Xand Yis 4.0 and 0.4 mmol,
respectively) was initially grinded with deionized water into grey
slurry and coated uniformly upon an alumina ceramic tube
printed with a pair of Au electrodes at the two ends. The
CuO-coated substrate was dried in the air for 3 h and then
annealed at 500 C for 2h.ANi–Cr alloy wire as a resistance
heater was inserted into the ceramic tube to provide the
working temperature for the gas sensor. The electrical and
ethanol sensing performances of as-prepared CuO were detec-
ted by a measurement system of WS-30A (Zhengzhou Winsen
Electronic Technology Co. Ltd., China).
Characterizations
The size and morphology of Cu nanocrystals were observed with
a JSM-7600F eld emission scanning electron microscope
(FESEM; JEOL, Ltd, Tokyo, Japan) and a JEM 2100F trans-
mission electron microscope (TEM; JEOL, Ltd, Tokyo, Japan)
with an operating voltage of 200 kV. X-ray diffraction (XRD)
patterns were recorded on an X-ray diffractometer (XRD-7000;
Shimadzu, Cu Ka,l¼0.154178 nm).
Results and discussion
Fig. 1 describes the typical synthetic process of various CuO
nanocrystals in aqueous solution with constant magnetic stir-
ring at room temperature (25 C) for 8 h. By adding different
additions of NaOH (Xmmol, 0.2 mol L
1
) and hydrazine hydrate
(Ymmol, 0.1 mol L
1
), CuO ne nanoparticles, high-purity
nanoleaves and quasi-spherical nanoparticles were obtained.
Adjusting CuO morphologies with hydrazine hydrate
Various CuO nanocrystals were prepared by changing the
additions of hydrazine hydrate when the NaOH solution was
3.4 mmol. Fig. 2 shows the scanning electron microscope (SEM)
images of the as-prepared products when the additions of
hydrazine hydrate range from 0.4 to 2.0 mmol. From the SEM
observation, various CuO nanocrystals including nanoleaves
accompanied with nanocubics, irregular nanoparticles and
uniformed quasi-spherical nanoparticles were prepared. In the
sample prepared at 0.4 mmol hydrazine hydrate, CuO nano-
leaves and nanocubic formed in this case. The length of CuO
nanoleaves are 600 nm and the width of CuO nanocubics are
350 nm on average. With the addition of hydrazine hydrate up
Fig. 1 Scheme illusion of the synthetic process of various CuO
nanocrystals by adjusting the additions of NaOH (0.2 mol L
1
) and
hydrazine hydrate (0.1 mol L
1
).
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5,34788–34794 | 34789
Paper RSC Advances

to 0.8 mmol, the as-prepared CuO nanocrystal displayed irreg-
ular shapes, and their sizes ranged from 400 nm to 1400 nm and
770 nm on average (Fig. 2a). When the hydrazine hydrate
addition increased to 1 mmol, uniformed CuO nanoparticles
sized about 500 nm were prepared (Fig. 2b). The CuO nano-
particles got more round, uniform and larger in size (1080
nm) in the case of 2.0 mmol hydrazine hydrate (Fig. 2d). From
the SEM observation, it is obvious that the increasing quantity
of hydrazine hydrate has a signicant inuence on modulating
the morphologies of CuO nanocrystals ranging from two-
dimension (2D) to three-dimension (3D). Large quantities of
hydrazine hydrate favour the agglomeration of CuO nano-
particles into quasi-spherical nanoparticles and results in the
larger size of as-prepared nanoparticle.
Adjusting CuO morphologies with NaOH
Typical spherical nanoparticles have been well studied in
depth,
28,29
while as a novel and basical structure, nanoleaves
and its potential applications have been advanced remark-
ably.
30–32
In order to prepare high-purity CuO nanoleaves, the
other parameters that affect the shape of product is further
considered and designed carefully. The additions of NaOH are
controlled when the addition of hydrazine hydrate is 0.4 mmol.
The morphologies of these samples produced at room temper-
ature are displayed in Fig. 3. We found that in the case of
4.0 mmol NaOH, high-purity CuO nanoleaves were obtained
successfully.
Fig. 3a shows the SEM image of as-obtained CuO ne
nanoparticles obtained when the addition of NaOH is 3.2 mmol.
The smaller crystallite size of CuO nucleus rstly formed, and
then assembled, nally aggregated to form CuO ne nano-
particles. As shown in Fig. 3b, when we increase the addition of
NaOH to 3.6 mmol, plenty of incomplete CuO nanoleaves mixed
with few CuO nanocubics and owerlike CuO nanostructures
are produced. Fig. 3c and d clearly shows the uniformed high-
purity CuO nanoleaves without other shapes as increasing the
addition of NaOH to 4 mmol. These CuO nanoleaves, which
perform well 2D nanostructure, are at and stretched with the
length, width and thickness of 600–900 nm, 250–300 nm and
10–20 nm, respectively. Uniform high-purity CuO nanoleaves
would exist steadily and tend to aggregate with each other as the
additions of NaOH increase to 4.4 and 4.8 mmol (Fig. 3e and f).
The SEM observation conrms that the morphologies of CuO
were modulated from ne nanoparticles to nanoleaves with
increasing the amount of NaOH from 3.2 to 4.8 mmol.
Furthermore, increasing quantity of NaOH favors the nucle-
ation of CuO and the growth of high-purity CuO nanoleaves,
might due to the enhancement of the interaction between OH

and copper ions.
Structure characterization of CuO nanoleaves
From the XRD patterns of CuO nanoleaves and quasi-spherical
nanoparticles (Fig. 4), it is evident that these patterns could
be indexed to the pure monoclinic CuO (space group Cc;
a¼4.689 ˚
A, b¼3.420 ˚
A, c¼5.130 ˚
A, b¼99.57; PDF 01-089-
5899) and no other phases were found. The most strong peaks
with 2qvalues of 35.558 and 38.759 correspond to (111) and
(111) crystal plane of monoclinic CuO, respectively. It is sug-
gested that aer the nucleation of monoclinic CuO, the solution
systems are benet to the growth of (111) and (111) crystal
plane. The crystallite size of CuO for the most intense peak
(111) plane was determined from the X-ray diffraction data
employing the Debye–Scherrer formula:
D¼kl
bcos q
where Dis the crystallite size, k¼0.89 is a correction factor to
account for particle shapes, bis the full width at half maximum
Fig. 2 SEM images of CuO nanocrystals under different molar ratio of
hydrazine hydrate (0.1 mol L
1
): (a) 0.4 mmol, (b) 0.8 mmol, (c)
1.0 mmol, (d) 2.0 mmol.
Fig. 3 SEM images of CuO nanocrystals prepared adjusting the molar
ratio of NaOH into (a) 3.2 mmol, (b) 3.6 mmol, (c and d) 4.0 mmol, (e)
4.4 mmol and (f) 4.8 mmol.
34790 |RSC Adv.,2015,5,34788–34794 This journal is © The Royal Society of Chemistry 2015
RSC Advances Paper

(FWHM) of the most intense diffraction peak (111) plane,
l¼1.5406 ˚
A is the wavelength of Cu target, and qis the Bragg
angle. The average crystallite sizes of the as-product CuO
powders are calculated as 19.69, 18.44 and 14.22 nm while the
molar addition of NaOH is 4.0, 4.4 and 4.8 mmol, respectively.
It means that the CuO nanoleaves contain a numerous of
CuO nanocrystallites, which is in agreement with the observa-
tion of HRTEM images.
TEM images of high-purity CuO nanoleaves as the addition of
4.0 mmol NaOH are shown in Fig. 5a, the owerlike shape is
mainly because the overlapping of different CuO nanoleaves.
Small crystallite particles with a diameter of about 18 nm (Fig. 5b
and c) are observed, the result is in agreement with the calcu-
lation of Debye–Scherrer formula. Needlelike CuO nanowires
composed of many small particles assemble with each other, the
length of these nanowires range from 350 to 950 nm, while the
width of these nanowires range from 10 to 50 nm (Fig. 5a–c).
Fig. 5d shows the HRTEM image of one nanoleaf originating
from the marked area in Fig. 5b. A clear and continuous lattice-
fringe indicates that the CuO nanoleaves share the same crys-
tallographic orientation. The small crystallites composing to
CuO nanoleaves are monocrystalline with the interfringe
distance of 0.25 nm corresponding to the (111) plane of
monoclinic CuO, which is in agreement with the results of XRD
pattern. A Fast Fourier Transformation (FFT) pattern originating
from marked area in Fig. 5d reveals that the high-purity CuO
nanoleaves have the same crystallographic orientation. From the
above analysis, it is evident that the needlelike CuO nanowires
perpendicular to the [111] direction to form high-purity CuO
nanoleaves, the needlelike CuO nanowires were formed by the
assembly of CuO nanoparticles along [111] direction with the
perfect crystallographic orientation of (111), and oriented
attachment goes through these entire course.
The EDX result demonstrates the existence of Cu and O
elements, and the atomic ratio of Cu : O is 48.88 : 51.12,
indicating the high-purity of as-prepared CuO nanoleaves
(Fig. S1 and Table S1 in the ESI†). The UV-vis spectrum of the as-
obtained CuO nanoleaves well dispersed in ethanol shows a
broad absorption peak centered at 278 nm. The band gap of
CuO nanoleaves can be determined via UV-vis spectrum by
employing Tauc/Davis–Mott Model.
33
The direct band gap
energy of the as-obtained CuO nanoleaves is calculated to be
2.17 eV (Fig. S2†).
Growth mechanism of CuO nanocrystal
The chemical reaction of the overall synthetic process is
assumed as follows:
Cu
2+
+ 2OH

5Cu(OH)
2
Y(1)
CuðOHÞ2
!
hydrazine hydrate CuO þH2O(2)
It is suggested that the synthetic process goes through two
steps to form CuO. The rst step is the formation of a meta-
stable phase of light blue Cu(OH)
2
in a short period which is
easily to transforms into more stable CuO.
34
Due to the revers-
ibility of the rst step, the increase of the OH

concentration
may accelerate the reaction rate. What's more, the low solubility
product (K
sp
)is19.32 for Cu(OH)
2
, which indicates that the
acceleration of the forward reaction rate and the low supersat-
urated degree favor the nucleation and the growth of Cu(OH)
2
.
The second step is the dehydration and condensation of
Cu(OH)
2
, and the formation of CuO nanocrystals. Compared
with Xu's experiments in the preparation of CuO nanoleaves at
35 C for 40 h by adjusting the pH ¼12,
26
our experiments are
more facile by employing hydrazine hydrate agent at 25 C for
8 h. The colour of the suspension changed from light yellow into
grey, which indicates the second reaction process may include
the reduction and oxidation of copper ions. Since the solubility
of CuO is much less than that of Cu(OH)
2
, the transition from
Cu(OH)
2
to CuO could decrease the free energy of the reaction
system. In this case, it is suggested that the hydrazine hydrate
Fig. 4 XRD pattern of the CuO nanoleaves prepared of 4.0, 4.4 and
4.8 mmol NaOH (0.2 mol L
1
) within 0.4 mmol hydrazine hydrate (0.1
mol L
1
) and quasi-spherical nanoparticles prepared of 2 mmol
hydrazine hydrate (0.1 mol L
1
) within 3.4 mmol NaOH (0.2 mol L
1
)at
room temperature by constant stirring for 10 h.
Fig. 5 Morphology of the high-purity CuO nanoleaves (X¼4 mmol),
(a) low magnification TEM image, (b) TEM image of CuO nanowires, (c)
TEM image of well-aggregated CuO nanoleaves, (d) HRTEM image of
as-prepared CuO nanoleaves and the corresponding fast Fourier
transform (FFT) patterns.
This journal is © The Royal Society of Chemistry 2015 RSC Adv.,2015,5,34788–34794 | 34791
Paper RSC Advances

decreases the reaction barrier of the formation of CuO and has
the function of driving the second step faster. From the above
analysis, the precursor Cu(OH)
2
is rstly prepared by the mixing
of CuCl
2
and NaOH solutions, and the low supersaturated
degree of Cu(OH)
2
in aqueous solution and the high concen-
trations of OH

favor the nucleation and the growth of Cu(OH)
2
.
The CuO nanocrystals are nally prepared by the dehydration of
Cu(OH)
2
and the hydrazine hydrate promote this process to
accelerate the transition and cut downs the transition time,
which may open a novel eld by adjusting the morphology in
controlling the reaction kinetics.
It is further found that the Gibbs Free Energy (GFE)
decreases with the shape factor and increases with decreasing
of particle size.
35
With the increasing sizes of the spherical
nanocrystal, the GFE decreases and the stability of the CuO
nanoparticles increase. It indicates that the increase of the
addition of hydrazine hydrate may achieve the elimination of
the GFE of reaction system and modulate 2D nanoleaf to 3D
spherical morphology of lager CuO particles sized of 1080 nm.
Summarily, the increase of the additions of NaOH favors the
nucleation of CuO and the growth of CuO nanoleaves. Mean-
while, the tendency of the surface energy decreasing of smaller
CuO nanoparticles drives the assembly along [111] direction
with the perfect crystallographic orientation of (111) into
needlelike CuO nanowires, and needlelike CuO nanowires
perpendicular to the [111] direction to form high-purity CuO
nanoleaves (as shown in Fig. 6). Oriented attachment goes
through entire course.
Ethanol gas sensing performance
The response (the ratio of the resistance of sensor, R
gas
/R
air
) and
recover properties of the CuO nanoleaf-based sensor at different
temperatures, concentration is investigated by using ethanol as
a targeted gas analyte. In order to study the best operating
temperature for sensing ethanol, the response at different
temperatures under the concentration of 1500 ppm of ethanol
is investigated. Fig. 7a shows the response of the CuO nanoleaf-
based sensor at the concentration of 1500 ppm of ethanol, with
the increase of the testing temperature from 100 to 370 C, the
response curve increases at initial stage and has the maximum
value at 260 C, and decreases nally. It indicates that the best
operating temperature for sensing ethanol is about 260 C.
Fig. 7b shows the response curves of the CuO nanoleaf-based
sensor with the concentrations of ethanol at the range of
50–1500 ppm at 260 C. The response curve increases contin-
uously with the concentration of ethanol increasing from 50 to
1000 ppm, and the sensing response of ethanol becomes satu-
rated when the concentration is higher than 1500 ppm. The
minimum response of ethanol is 1.38 at 260 C at concen-
tration of 10 ppm ethanol.
Fig. 8 demonstrates the response and recovery curves of CuO
nanoleaf-based sensor to ethanol of which concentrations
range from 50 to 1500 ppm at 260 C. The resistance values
increase sharply when CuO nanoleaf-based sensor is exposed to
ethanol, while the resistance values decrease quickly when the
ethanol is removed. The responses/recover time (the time
required to reach 90% of the equilibrium value of resistance) is
about 23–55 s and 20–57 s, respectively. The CuO nanoleaf-
based sensor displays a sensitive response to ethanol at a low
testing temperature and ethanol concentration. It is mainly
because of the small size and 2D structure of CuO nanoleaves.
It is well known that the adsorption and desorption of
testing gas on the surface of CuO-based gas sensor attribute to
the changes of electric resistance and directly lead to the
response to the testing gas. As a p-type semiconductor, the
Fig. 6 Scheme of the growth of CuO nanoleaves and CuO nano-
particles under different conditions.
Fig. 7 (a) Relationship of the testing temperature versus response to
ethanol of CuO nanoleaf-based sensor under 1500 ppm ethanol. (b)
Relationship between the concentration and the response to ethanol
of CuO nanoleaf-based sensor at 260 C.
Fig. 8 Response and recovery curves of CuO nanoleave-based sensor
toward different concentrations of ethanol at 260 C.
34792 |RSC Adv.,2015,5,34788–34794 This journal is © The Royal Society of Chemistry 2015
RSC Advances Paper

resistance of CuO-based gas sensor increases when it is exposed
into the testing gas (ethanol) and then decreases when it is
exposed into air. Herein, the chemical reaction of the C
2
H
5
OH
upon the surface of CuO nanoleaves at 100–370 C is assumed
as follows:
27,36,37
C
2
H
5
OH (ads) + 3O
2
(ads) /2CO
2
(g) + 3H
2
O (g) + 3e

(3)
The formation of O
2
(ads) is attributed to the adsorption of
oxygen molecules to the surface of CuO nanoleaves. At less than
240 C, the ethanol molecules without enough energy don't
react with the O
2
(ads) on the surface of CuO nanoleaves
resulting in the lower sensitivity of CuO gas sensor. With the
increase of temperature to 240 C, the adsorbed oxygen mole-
cules change from O
2
(ads) to O

(ads), and the thermal energy
of CuO gas sensor is high enough to maintaining the react
activation energy. On the other hand, the adsorption of ethanol
molecules on the surface of CuO nanoleaves turns to be difficult
and some ethanol molecules may react with the oxygen in the
air, which leads to the dropped response when the operating
temperature is more than 260 C. The sensor behavior is similar
to the case of CO gas in the CuO sensing device reported by
Chang et al.
38
In this case, the electrons from valence band of
CuO are trapped via oxygen which leads to the accumulation of
holes and the increase of carrier's concentration. When the CuO
nanoleaf-based sensor is exposed into the testing gas (ethanol),
the production of electrons would neutralize and decrease the
amount of holes, resulting in the sharp increase of the resis-
tance. Compared with that reported by Wang's,
39
the response
to ethanol of CuO nanoleaf-based sensor is more sensitive and
strong. Repeated testing have been carried out during twenty
days and the response for ethanol of the CuO nanoleaf-based
sensor has no signicant variation, which indicates the well
stability of the CuO nanoleaf-based sensor.
The signicant various in response to ethanol may be
attributed to the 2D structure advantages of CuO nanoleaves, of
which promote the diffusion and adsorption/desorption of
ethanol molecules upon the nanoleaves. The well conductivity
of the quasi single crystal of CuO nanoleaves prepared in
aqueous solution through the oriented attachment can facili-
tate the transportation of holes carriers in the sensing process.
All of these elements directly favor the well performed sensing
response of ethanol. The 2D structure of CuO nanoleaves
provide a new choice for the application in gas sensor and CuO
nanoleaf-based sensor may have potential application in
detecting ethanol in low concentration at low temperature,
further investigations will be carried out.
Conclusions
The morphology of CuO nanocrystal could be modulated simply
by adjusting the ratio of sodium hydroxide and hydrazine
hydrate, the uniformed CuO nanoparticles and high-puried
CuO nanoleaves are obtained in control. Increasing the usage
of sodium hydroxide will modulate the morphology of CuO
from ne nanoparticle to high-purity nanoleaves and favor the
acceleration of the reaction rate and the nucleation of CuO
nanocrystal. The decreases of the surface energy will promote
the oriented attachment of nanocrystallites into nanowires and
the nal formation of 2D nanoleaves. Increasing the quantity of
hydrazine hydrate could decrease the solution system energy
and promote the aggregation of CuO nanocrystals from 2D
nanoleaves into 3D nanoparticles. The 2D CuO nanoleaves
exhibit excellent gas sensing performance in responding
ethanol. CuO nanoleaf-based sensor displays the strongest
response of 8.22 at 1500 ppm ethanol in 260 C. This work
provides a facile and effective method in modulating CuO
morphology including uniformed CuO nanoparticles and high-
purity CuO nanoleaves, and the well sensing prosperity of CuO
nanoleaves to ethanol. The as-prepared CuO nanostructures
may have great potential applications in the eld of catalyst,
lithium ion batteries and solar cells.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant no. 51202228, 51302029, 51402040,
51402045), the Open Foundation of State Key Laboratory of
Electronic Thin Films and Integrated Devices (KFJJ201411) and
the National Hi-Tech Research and Development Program
(863 Program) of China (no. 2015AA034202).
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