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© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Phys. Status Solidi C, 1–5 (2016) / DOI 10.1002/pssc.201510301
A comparative study of the gas
sensing properties of hierarchical
ZnO nanostructures
Martine Capochichi-Gnambodoe, Yamina Ghozlane Habba, and Yamin Leprince-Wang*,1
Université Paris-Est, ESYCOM (EA 2552), UPEM, 5 bd. Descartes, 77454 Marne-la-Vallée, France
Received 16 November 2015, revised 11 February 2016, accepted 22 February 2016
Published online 15 March 2016
Keywords ZnO nanowires, hierarchical nanostructure, gas sensing
* Corresponding author: e-mail yamin.leprince@u-pem.fr
Two types of ZnO nanostructure have been fabricated to
make a comparative study on their gas sensing perform-
ance: the conventional ZnO nanowire arrays were syn-
thesized by hydrothermal method and the hierarchical
ZnO nanowires/nanofibers nanostructures were prepared
through a combination of the hydrothermal and electro-
spinning methods. Field emission scanning electron
microscopy study showed a quiet homogeneous morphol-
ogy both for both nanostructures. Three kinds of
commonly used gases, such as ethanol, acetone and am-
monia were chosen for ZnO nanostructure gas sensing
property study. The UV-Visible spectroscopy measure-
ments showed a higher detection sensitivity of ZnO NWs
for ammonia compared to ethanol and acetone, and an
enhanced sensing performance for the hierarchical nano-
structure, which has a higher surface to volume ratio. On
the other hand, the enhancement was more obviously in
the case of ethanol sensing.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Chemical pollution in the air has a
growing threat to our health and our ecosystem in modern
society due to the increasing variety gas emission into the
atmosphere from industrial production and automobile.
The research on environmentally friendly materials having
high gas sensing performance and high air purification ca-
pacity becomes a challenge in material science for sustain-
able development.
ZnO is currently a material of intensive research due to
its excellent and versatile intrinsic properties making it a
very promising material for a wide range applications such
as gas sensors [1, 2], transparent conductors [3, 4], photo
catalysts [5, 6], and optoelectronic devices [7]. Further-
more, since one decade, much effort has been devoted to
the development of reliable technologies to prepare ZnO
with desirable structures in a controllable manner. To date,
a number of ZnO nanostructures have been successfully
synthesized in order to meet the specific requirement for
different applications such as nanowires [8, 9], nanorods
[10, 11], nanoneedles [12], and nanosheets [13]; as well as
the hierarchical flower-like nanostructure [14].
As a one-dimensional (1D) semiconductor metal oxide
(SMO), the nontoxic ZnO nanostructures have attracted in-
creasing attention in the construction of nanodevices for
chemical sensors. The most literature reported SMO gas
sensors have a high working temperature (> 200 °C)
[15-17]. In addition to the temperature effect for increasing
the gas sensing sensitivity, some metallic elements have
been used, such as tin Sn [18], nickel Ni [19], or platinum
Pt [20], for ZnO nanostructure doping.
In this work, we studied the gas sensing property at
low temperature (< 100 °C) using two types of ZnO nano-
structure in order to compare their sensing performance re-
garding their morphology. ZnO nanowires (NWs) arrays
were synthesized via hydrothermal method [9] and ZnO
nanowires /nanofibers (NWs/NFs) homo-hierarchical
nanostructures were prepared by combining the hydro-
thermal and electrospinning methods [21]; both nanostruc-
tures are fabricated via low cost and low temperature proc-
esses. As-grown ZnO nanostructures have a high surface to
volume ratio, but NWs/NFs samples have a higher specific
surface compared to the samples of NWs array. Three
commonly used gasses such as ethanol, acetone, and am-
monic have been employed for ZnO nanostructures gas de-
tection. Widely used in industry and in research laboratory,
ethanol, acetone and ammoniac in gaseous form are more
or less toxic, and can cause irritation and burn of the respi-
ratory system and ocular mucosa [22]. Thus, their detec-
2 M. Capochichi-Gnambodoe et al.: Gas sensing properties of hierarchical ZnO nanostructures
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tion and capture performance is important of the security
both for working people and the environment. The sensing
property measurements were carried out using UV-Visible
spectroscopy.
2 ZnO nanostructure for gas sensing The prin-
ciple of the semiconductor metal oxide gas sensor is based
on the adsorption of a gas molecule on the solid surface.
According to the interaction force between the gas mole-
cule and the solid surface, there are two adsorption types:
physisorption and chemisorption. Physisorption is a weak
interaction between the gas molecule and the solid surface
where the Van der Waals or electrostatic forces are in-
volved with energy E < 0.4 eV and without electronic ex-
change. While chemisorption occurs when the interaction
between the gas molecule and the solid surface involves a
higher energy force (E > 0.4 eV) and with electronic ex-
change between the two species which will alter the elec-
tronic properties of the surface of the host material [23].
For ZnO based gas sensor, the proposed reaction
mechanisms are strongly related to the sensor operating
temperature (Top). For Top > 200 °C, the degradation of gas
adsorbed on the ZnO surface follows the chemisorption
process [24]. In our study, Top < 100 °C, the gas will be ad-
sorbed on the ZnO surface via physisorption, a non-
destructive and reversible process [25]. In this case, the gas
sensing mechanism is based on the interaction between the
ZnO and the oxygenated species adsorbed on the ZnO sur-
face. Indeed, the synthesised ZnO consists of oxygen va-
cancies; they will naturally react with the oxygen present
in the atmosphere. Thus, oxygen is the gas detection pre-
cursor. The gas sensing mechanism will be described in
discussion using ethanol as reducing gas example.
3 Experimental details The ZnO NWs samples
were prepared using two-step hydrothermal method. The
seed layer has been obtained by spin-coating on the Si sub-
strate with a buffer solution containing a mixture of 2g of
poly(vinyl alcohol) (PVA) and 0.25 g of zinc acetate in
25 ml of DI water. Then the as spin-coated seed layer has
been annealed at 500 °C during 3 hours for PVA evapora-
tion and to form ZnO seeds. The second step was the ZnO
NW growth via hydrothermal process at 95 °C in the aque-
ous solution prepared with a mixture of the 25 mM of zinc
nitrate (Zn(NO3)2) and the 12.5 mM of methenamine.
The ZnO hierarchical NWs/NFs samples were obtained
by combining the hydrothermal method and the electro-
spinning process. Firstly, a buffer solution was prepared by
adding 2 g of polyvinylpyrrolidone (PVP) and 1 g of zinc
acetate into a mixed solution of 2 mL of acetic acid, 8 mL
of ethanol, and 5 mL of DI water. The electrospinning
process was carried out by applying 8 kV between the nee-
dle and the collector with a separating distance of 10 cm.
The collected microfibers have been annealed at 700 °C
during 2 hours for PVP evaporation and to form the nano-
scale ZnO seeds which will play the seed layer rule for the
step of the ZnO NW hydrothermally growth.
The morphology of the ZnO nanostructures was char-
acterized using a field emission scanning electron micros-
copy (FEG–SEM, NE-ON 40 ZEISS). The gas sensing
process has been followed in situ by an UV-Vis spectrome-
ter (Maya2000Pro–Ocean Optic), equipped with an optical
fiber (OF) probe. The absorbance spectra measurements
were carried out in spectral range of 300–700 nm. The ex-
perimental setup is shown in Fig. 1. The gas atmosphere
was created in a container of 12 L with 5 mL of solvent
evaporated by a hot plate heating. The heating temperature
was 90, 70, and 60 °C for ethanol, acetone, and ammonia,
respectively, in order to evaporate those gases regarding to
their boiling temperature of 79, 56, and 38 °C, respectively.
The corresponding atmosphere temperature inside of the
container was maintained about 43, 37, and 34 °C, respec-
tively, during the sensing measurements. It would be worth
noting that the obtained spectra stem directly from the
sample surface, thus, it is an accurate reflexion of the gas
absorption state by ZnO nanostructure. The sensing per-
formance is determined by the relative gas elimination rate
which is defined as X = [(A0-A)/A0]·100%, where A0 and
A are the absorbance values corresponding to the original
and final measurements for λ = 375 nm.
Figure 1 Experimental setup for gas sensing process.
Figure 2 SEM images of ZnO nanostructures: (a) cross-section
view of a ZnO NW array with 4 h growth time; (b) and (c) elec-
trospun ZnO fibers before and after PVP evaporation, respec-
tively; (d) ZnO NWs/NFs hierarchical structure morphology.
Hot plate
Solvent vapor
atmosphere
Sam
p
le
OF probe
1 µm 2 µm
2 µm 2 µm
(a) (b)
(c) (d)
Phys. Status Solidi C (2016) 3
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4 Results and discussion Figure 2(a) shows a
typical morphology of ZnO NW arrays grown on the Si
substrate by hydrothermal method. For the 4 h growth time,
the NWs have a high aspect ratio of about 20. Figure 2(b,
c) show the electrospun ZnO fibers before and after an-
nealing. We can note that the fibers are thinner after PVP
evaporation. Figure 2(d) shows the ZnO NWs 4 h hydro-
thermally grown on the electrospun nanofibers. This 3D
homo-hierarchical nanostructure has a very high specific
surface (surface to volume ratio).
Figure 3 shows the comparative results obtained from
three gases sensing UV-Vis spectroscopic measurements,
from which we can note that the ZnO nanostructures ex-
hibit sensitivity more or less notable depending on the gas
nature. With the presence of the ZnO NWs sample, ammo-
nia had a relative elimination rate of X = 59% vs. 35% for
acetone and 21% for ethanol. That means the ammonia has
a better sensitivity for the sensor based on the ZnO NWs
array. With the presence of the hierarchical NWs/NFs
sample, which has a larger specific surface, the effect of
the surface to volume ratio is very significant in the case of
ethanol: X value increases from 21 to 64%. This may be
due to the ability of O-H bond formation between the hy-
drogen atom from ethanol gas molecule and the oxygen on
the ZnO surface and the big number of available sites on
hierarchical structure [16]. On the other hand, the large
specific surface of hierarchical structure seems less benefi-
cial for two other gases due to their higher relative gas
eliminating rate: X values increase from 35 to 48% for ace-
tone, and from 59 to 69% for ammonia, respectively.
Acetone &
Z
nO NWs
/
NFs
Acetone &
Z
nO NWs
Ethanol &
Z
nO NWs / NFs
Ethanol &
ZnO NWs
Ammonia &
ZnO NWs / NFs
Ammonia &
ZnO NWs
X ~21% X ~64%
X ~35% X ~48%
X ~59% X ~69%
(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
Figure 3 (a, d, g) Gas sensing UV-Vis spectra obtained from ZnO NW array sample for ethanol, acetone, and ammonia,
respectively. (b, e, h) Gas sensing UV-Vis spectra obtained from ZnO NWs/NFs sample for ethanol, acetone, and ammonia,
respectively. (c, f, i) Comparative curves plotted from two types of nanostructure at λ = 375 nm showing the relative gas
elimination rate for ethanol, acetone, and ammonia, respectively.
4 M. Capochichi-Gnambodoe et al.: Gas sensing properties of hierarchical ZnO nanostructures
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com
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Moreover, the physisorption process taking place on
the ZnO surface does not affect the ZnO intrinsic proper-
ties. The ZnO gap was not changed, only a decrease in ab-
sorbance was observed while the gas was injected. These
results confirm those we already observed in our previous
work [25]. The adsorption process was reversible when we
opened the container and an almost total desorption can be
obtained by a post-annealing.
Indeed, ZnO is an n-type semiconductor with the in-
trinsic donors, which will introduce the electrons in the
conduction band ( ,
vac
CB
n Fig. 4(a)). In air, a part of these
electrons will be trapped by the oxygen adsorbed on the
ZnO surface, the electrons population in the conduction
band (CB) will be described by ,
air
CB
n(Fig. 4(b)). The ZnO
with a reduced gas interaction will release the previously
trapped electrons; the new electron population will be de-
fined as gas
CB
n (Fig. 4(c)). The electron density will be in-
creased, which is proportional to the adsorbed gas, will re-
sult in a decrease of ZnO surface resistance.
Figure 4 n-ZnO valence band and conduction band representa-
tion in different environment: (a) vacuum, (b) air, (c) ethanol.
By absorbing an energy of a photon with hv = E2–E1,
an electron-hole pair will be created in CB and VB, respec-
tively (Fig. 5(a)). In contact with a reduced gas, the re-
injection of the electrons in the CB which will block par-
tially the electron transition from VB to CB (Fig. 5(b))
leading a decrease of absorbance spectrum intensity, as we
can see in Fig. 3.
Conduction
Band
Valen ce
Band
CB
e−
2,
Oads
−
2,
Oads
−
2,
Oads
−
nair
CB
h
ν
E
1
E
2
Conduction
Band
Valen ce
Band
CB
e−
2,
Oads
2,
Oads
2,
Oads
n
g
as
CB
EtOH
EtOH
EtOH
h
ν
X
(a) (b)
Figure 5 (a) Creation of electron-hole pair by absorbing of pho-
ton energy; (b) interdict electron transition due to the presence of
the reduced gas.
The acetone and ammonia are also reducing gasses, so they
have a similar gas sensing mechanism to the case of the
ethanol [25, 26].
5 Conclusions Two types of ZnO nanostructure have
been used to comparatively study the gas sensing perform-
ance: ZnO NW arrays and ZnO NWs/NFs hierarchical
nanostructures, synthesized by hydrothermal and electro-
spinning methods. Using the UV-Visible spectroscopy, gas
sensing property study, carried on three commonly used
gases (ethanol, acetone, and ammonia), has been investi-
gated. The results showed a higher detection sensitivity of
ZnO NWs for ammonia with the relative gas elimination
rate of X = 59% compared to ethanol (21%) and acetone
(35%); and an enhanced sensing performance for the hier-
archical NWs/NFs nanostructure: X = 64%, 48% and 69%
for ethanol, acetone, and ammonia, respectively, due to its
higher specific surface in contact with the gases molecules.
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