acetone sensing characteristics of zno hollow spheres prepared by

Acetone sensing characteristics of ZnO hollow spheres prepared by
one-pot hydrothermal reaction
Peng Song
n
, Qi Wang, Zhongxi Yang
School of Material Science and Engineering, Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China
article info
Article history:
Received 4 June 2012
Accepted 18 July 2012
Available online 27 July 2012
Keywords:
ZnO
Hollow sphere
Microstructure
Sensors
abstract
ZnO hollow spheres were one-pot fabricated by hydrothermal treatment. Zinc nitrate were dissolved
together with urea and glucose in water, and the mixtures were heated at 180 1C for 24 h. During the
hydrothermal treatment, carbon spheres are formed with zinc ions incorporated into the hydrophilic
shell. The removal of carbon via calcinations yields hollow zinc oxide spheres. The as-prepared samples
were characterized by X-ray diffraction and the field emission gun scanning electron microscope. The
results indicated that the products were pure hexagonal ZnO with the structure of hollow sphere, and
the formation mechanism of ZnO hollow spheres was discussed. Consequently, the ZnO hollow spheres
exhibited good sensing performance towards acetone gas with rapid response when operated at 300 1C.
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
Chemical sensors play an important role in the areas of
emissions control, environmental protection, public safety, and
human health [1]. The general mechanism of the oxide semicon-
ductor sensors is based on the changes in electrical properties

before and after exposure to the target gases or vapors [2].
Acetone, a common reagent widely used in industries and labs,
is harmful to health. Inhalation of acetone causes headache,
fatigue and even narcosis and harmfulness to nerve system.
Hence it is necessary to monitor acetone concentration in the
environment for health and the workplace for safety [3–5].
Zinc Oxide, as an n-type semiconductor material, has been
widely investigated as a field-effect transistor [6], optical device
[7], dye-sensitized solar cell [8], and solid-state gas sensor [9,10].
Recently, ZnO-based sensors have been investigated for the
detection of acetone vapor at various concentration levels
[11–14]. For chemical sensor applications, hollow structural
features provide enhanced surface activities, high surface-to-
volume ratio and fast diffusion, which allow easy gas penetration
into the sensing layers. Furthermore, both the outer and inner
shells actively interact with target molecules. So, several promis-
ing approaches have been developed to produce hollow architec-
tures such as spheres, hemispheres and inorganic tubes [15].Up
to now, the most important methods for hollow structures rely on
the use of sacrificial templates, and the desired hollow interiors
are generated upon the removal of templates by calcination or
dissolution [16]. Recently, a novel method for the fabrication of
metal oxide hollow spheres has been developed. Titirici et al. [17]
have reported synthesis of various oxide hollow microspheres
(such as Fe
2
O
3
,Co
2

O
3
, CeO
2
, MgO and CuO) using carbonaceous
polysaccharide microspheres prepared from sacharide solution as
template. However, preparation of well-crystallized ZnO hollow
spheres with controllable surface morphology and high gas
response is still a great challenge. In this contribution, ZnO hollow
spheres are prepared by the one-pot hydrothermal reaction. The
study focuses on the formation mechanism of ZnO hollow spheres
and the effect of hollow morphology on the acetone sensing
characteristics.
2. Experimental
Preparation of ZnO hollow sphere: ZnO hollow spheres were
prepared by a hydrothermal approach using zinc nitrate
(Zn(NO
3
)
2
Á 6H
2
O) as a zinc source. In a typical synthesis, glucose
monohydrate (C
6
H
12
O
6
Á 6H

2
O, 75 mmol), 5 mmol zinc nitrate and
urea (CO(NH
2
)
2
, 50 mmol) were dissolved in 15 mL of distilled
water under stirring, respectively. The above two solutions were
mixed immediately before the experiment and placed in a 50 mL
capacity Teflon-lined stainless steel autoclave, which was heated
in an oven to 180 1C for 24 h. After hydrothermal treatment, the
black precipitates were centrifuged, and then with distilled water
and absolute alcohol washed several times. The washed precipi-
tates were dried in a vacuum oven at 60 1C for 12 h. After
synthesis, the zinc-carbon composites were calcined in air at
500 1C (heating rate of 1 1C/min) for 4 h to remove the carbon
core, leading to ZnO hollow spheres.
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Materials Letters
0167-577X/$ -see front matter & 2012 Elsevier B.V. All rights reserved.
/>n
Corresponding author. Tel.: þ 86 531 82765473; fax: þ 86 531 87974453.
E-mail address: (P. Song).
Materials Letters 86 (2012) 168–170
Characterization: The crystalline phase in the samples were
characterized by an X-ray diffraction (XRD, Bruker D8 Advance)
using CuK
a
radiation (

l
¼0.15406 nm) at 30 kV and 40 mA at a
scanning rate of 21 at 2
y
min
À1
ranging from 201 to 701. The
FESEM micrographs were obtained on a FEI Sirion 200 field
emission gun scanning electron microscope (FESEM, Hitachi
S4800). FESEM measurements were mounted on aluminum studs
using adhesive graphite tape and sputter coated with gold before
analysis.
3. Results and discussion
Phase and morphology of the products:TypicalXRDpatternofthe
as-prepared ZnO is shown in Fig. 1, where one can see that all the
diffraction peaks are in good agreement with t hose of t he hexagonal
structure of ZnO (JCPDS card 36-1451). No other diffraction peaks are
found, indicating that the products are pure ZnO and t he carbon
microsphere t empla tes w ere completely removed. In addition, it can
be found that several diffraction peaks are strong and sharp, w hich
indicates that the prepared ZnO are hi ghly cry stallized yet p olycrys-
talline. The average crystallite size of ZnO samples was estimated
according to the li ne width analysis o f the diffraction peaks based on
the Scherrer formu la, D¼0.89
l
/
b
cos
y
, which was calculated to be

about 1 9.8 nm.
Fig. 2(a) shows FESEM image of the as-prepared zinc-carbon
composite m icrospheres obtained b y the hyd rothermal treatment
before calcinations. The diameter of spheres is about 4–7
m
m. Many
spheres are aggregated and linked to each other and their surf a ces
are smooth. After calcinations, we obtained ZnO hollow spheres
with diameters of about 1–2
m
m, as shown in Fig. 2(b). More details
can be found in Fig. 2(c) and (d), some small o penings in the
spheres can be seen clearly, implying the hollow structure of t he
spheres. And, this porous network is believed to be favorable for gas
sensor, which can facilitate the inward and outward ga s d iffusion.
Furthermore, FESEM images of the spheres before and after
calcinations reveal a considerable shrinkage ( from approximately
4–7
m
m to 1–2
m
m in diameter) of the structures during calcina-
tions tr eatment, showing a transition from loosely adsorbed
zinc ions to a dense zinc oxide network in the hollow spheres.
The formation mechanism o f ZnO hollow spheres is discussed, as
shown in Fig. 3. Firstly, the formation of the carbon spheres involves
the dehydration of the carbohydrate and subsequent carbonization
of the organic compounds. The surface of carbon spheres is
hydrophilic and has a distribution of OH and C¼O g roups, which
are f ormed from non- or just partially dehydrated carbohydrates.

The secondary step is the embedding of zinc precursors (Zn(OH)
4
2À
)
into the hydrophilic shell of as-prepared carbon spheres due to the
fact that the functional groups in the surface layer can bind Zn
cations through coordination or electrostatic interactions. Finally,
the removal of carbon core and densification of incorporati ng Zn
cationic ions in the layer via calcinations results in the formation of
hollow zinc oxide s pheres. As we all kn own, ZnO nuclei form f rom
dehydration of Zn(OH)
4
2À
ions in alk a line environment [18 ,19], and
the reactions are as follows:
Zn
2 þ
þ4OH
À
-ZnðOHÞ
4
2À
ð1Þ
ZnðOHÞ
4
2À
-ZnðOHÞ
2
þ2OH
À

ð2Þ
ZnðOHÞ
2
-ZnOþH
2
O ð3Þ
Acetone sensing properties: Fig. 4 displays the concentration
dependent sensitivity of the sensor based on ZnO hollow spheres
for acetone detection a t a n operating tempera ture o f 3 00 1C. It can
be seen that the sensitivity rapidl y increases with increasing acetone
concentration. Furthermore, a quick response/recovery time was
observed with this sensor. T he t ypical dynamic response curve of
ZnO hollow spheres sensor toward 500 ppm acetone at 300 1Cis
shown in t he inset of Fig. 4. We can find that the response of the
sensor increased abruptly on the injection of acetone, and then
decreased rapidly and recovered to its initial value after the test gas
was released. T he response time and recovery time of the sensor
were less t han 10 s.
ZnO is well-known as an n-type semiconductor, characterized
by its high free carrier concentration. When the ZnO hollow
spheres were exposed to air, oxygen molecules are firstly
adsorbed on the inner and outer surface of ZnO hollow spheres
and capture free electrons from the conduction band to produce
chemisorbed oxygen species (O
À
,O
2À
or O
2
À

). When ZnO hollow
spheres are exposed to acetone gas at higher temperature
(300 1C), acetone gas molecules can react with adsorbed oxygen
species on the inner and outer surface. This liberates free
electrons in the conduction band, leading to an increase in the
resistance of an n-type semiconductor. The final reaction takes
place as
C
3
H
6
Oþ 8OÀðadsÞ-3H
2
Oþ 3CO
2
þ8e
À
ð4Þ
Therefore, the specific surface area of the sensors plays an
important role in the contact and subsequent reaction of oxygen
species with the tested gas. In our case, the high response of the
ZnO hollow spheres sensor can be ascribed to the larger specific
surface area (not only the outer but also the inner surface) of the
sensing materials. Furthermore, the porous structure and open
framework of ZnO hollow spheres may also contribute to the
improved sensor response.
4. Conclusions
In summary, ZnO hollow spheres were prep ared by the glucose-
mediated, one-pot hydrothermal synthesis of Z n-coated carb on
spheres and then calcined at 500 1C, the hollow ZnO microspheres

with diameters of 1–2
m
m were gradually transformed into solid
microspheres. The ZnO hollow spheres sensor shows high response,
10 20 30 40 50 60 70 80
2 Theta / degree
Intensity (a.u.)
(100)
(002)
(101)
(102)
(110)
(103)
(200)
(112)
(201)
Fig. 1. XRD pattern of as-prepared ZnO hollow spheres.
P. Song et al. / Materials Letters 86 (2012) 168–170 169
low d etection a nd fast response and r ecovery towards acetone gas.
The excellent sensing performances are attributed to the hollow and
porous microstructu re.
Acknowledgment
This work was financially supported by National Natural
Science Foundation of China (No. 61102006), Natural Science
Foundation of Shandong Province, China (No. ZR2010EQ009),
Shandong Distinguished Middle-aged and Young Scientist Encou-
rage and Reward Foundation (No. BS2009CL056).
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5 µm
1 µm
100 µm
500 nm

Fig. 2. FESEM images of (a) zinc-carbon composite microspheres via hydrothermal synthesis, (b) low magnification image of ZnO hollow spheres after calcinations, (c) and
(d) high magnification images of ZnO hollow spheres.
Glucose
Urea
Zinc nitrate
Hydrothermal
treatment
Zinc precursor
Carbon sphere
Calcination
ZnO hollow sphere
Fig. 3. Synthetic scheme of ZnO hollow spheres fabricated by hydrothermal treatment.
0 200 400 600 800 1000 1200
2
4
6
8
10
12
14
16
Sensitivity (Ra/Rg)
Acetone concentration (ppm)
T = 300°C
0 102030405060
0
2
4
6
8

10
12
Time (s)
500 ppm
Sensitivity
Fig. 4. Sensitivity of ZnO hollow spheres versus acetone concentration. The inset
shows the response and recovery charact eristic s of the sensor to 500 ppm acetone at
300 1C.
P. Song et al. / Materials Letters 86 (2012) 168–170170