Sensors 2012, 12, 2610-2631; doi:10.3390/s120302610
Metal Oxide Nanostructures and Their Gas Sensing Properties:
Yu-Feng Sun 1,2,3, Shao-Bo Liu 3, Fan-Li Meng 2,*, Jin-Yun Liu 2, Zhen Jin 2, Ling-Tao Kong 2
and Jin-Huai Liu 2
1 Department of Mechanical and Automotive Engineering, Anhui Polytechnic University, Wuhu
241000, China; E-Mail: firstname.lastname@example.org
2 Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent
Machines, Chinese Academy of Sciences, Hefei 230031, China; E-Mails: email@example.com (J.-Y.L.);
firstname.lastname@example.org (Z.J.); email@example.com (L.-T.K.); firstname.lastname@example.org (J.-H.L.)
3 Wuhu Returned Overseas Students’ Enterprise Park, Wuhu 241000, China;
* Author to whom correspondence should be addressed; E-Mail: email@example.com;
Tel.: +86-551-559-5607; Fax: +86-551-559-2420.
Received: 19 December 2011; in revised form: 19 January 2012 / Accepted: 2 February 2012 /
Published: 27 February 2012
Abstract: Metal oxide gas sensors are predominant solid-state gas detecting devices for
domestic, commercial and industrial applications, which have many advantages such as
low cost, easy production, and compact size. However, the performance of such sensors is
significantly influenced by the morphology and structure of sensing materials, resulting in
a great obstacle for gas sensors based on bulk materials or dense films to achieve
highly-sensitive properties. Lots of metal oxide nanostructures have been developed to
improve the gas sensing properties such as sensitivity, selectivity, response speed, and so
on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing
properties from the aspects of particle size, morphology and doping. When the particle size
of metal oxide is close to or less than double thickness of the space-charge layer, the
sensitivity of the sensor will increase remarkably, which would be called “small size
effect”, yet small size of metal oxide nanoparticles will be compactly sintered together
during the film coating process which is disadvantage for gas diffusion in them.
In view of those reasons, nanostructures with many kinds of shapes such as porous
nanotubes, porous nanospheres and so on have been investigated, that not only possessed
large surface area and relatively mass reactive sites, but also formed relatively loose film
Sensors 2012, 12
structures which is an advantage for gas diffusion. Besides, doping is also an effective
method to decrease particle size and improve gas sensing properties. Therefore, the gas
sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed
in this article. The effect of doping is also summarized and finally the perspectives of metal
oxide gas sensor are given.
Keywords: metal oxide; gas sensing; nanostructure; size effect; doping
The issue of air quality is still a major concern in many countries. A clean air supply is essential to
our health and the environment. The human nose serves as a highly advanced sensing system which
may differentiate between hundreds of smells but fails if absolute gas concentrations or odorless gases
need to be detected. The demand for detecting toxic and deleterious gases is accordingly urgent to
support or replace human nose. Although a large number of gas detecting systems have currently been
used in process control and laboratory analytics [1–4], high performance gas sensors with high
sensitivity, high selectivity and rapid response speed are also needed to improve the levels of gas
Metal oxide gas sensors have been widely used in portable gas detection systems because of their
advantages such as low cost, easy production, compact size and simple measuring electronics [5,6].
However, the performance of such sensors is significantly influenced by the morphology and structure
of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films
to achieve highly-sensitive properties. Gas sensors based on nanomaterials are a greatly developing
direction to improve gas sensing properties in sensitivity, selectivity and response speed. Although
there are already some reviews on metal oxide gas sensor [7–9], it is still necessary to systematically
summarize the features of metal oxides from the perspective of nanoscience and nanotechnology. In
this review, we provide a brief summary on metal oxide nanostructures and their gas sensing properties
from the aspects of particle size, morphology and doping. Most of the examples are given based on
n-type metal oxides which are more extensively investigated and applied among the metal oxide
2. Gas Sensing Mechanism
It is necessary to reveal the sensing mechanism of metal oxide gas sensors which is helpful for
designing and fabricating novel gas sensing materials with excellent performance. Although the exact
fundamental mechanisms that cause a gas response are still controversial, it is essentially responsible
for a change in conductivity that trapping of electrons at adsorbed molecules and band bending
induced by these charged molecules. Herein, a brief introduction to the sensing mechanism of n-type
metal oxides in air is given based on the example of SnO2. Typically, oxygen gases are adsorbed on
the surface of the SnO2 sensing material in air. The adsorbed oxygen species can capture electrons
from the inner of the SnO2 film. The negative charge trapped in these oxygen species causes a
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depletion layer and thus a reduced conductivity. When the sensor is exposed to reducing gases, the
electrons trapped by the oxygen adsorbate will return to the SnO2 film, leading to a decrease in the
potential barrier height and thus an increase in conductivity. There are different oxygen species
including molecular (O2−) and atomic (O−, O2−) ions on the surface depending on working temperature.
Generally, below 150 °C the molecular form dominates while above this temperature the atomic
species are found [9,10].
The overall surface stoichiometry has a decisive influence on the surface conductivity for the metal
oxides. Oxygen vacancies act as donors, increasing the surface conductivity, whereas adsorbed oxygen
ions act as surface acceptors, binding elections and diminishing the surface conductivity. Figure 1
shows the energy diagram of various oxygen species in the gas phase, adsorbed at the surface and
bound within the lattice of SnO2 [11,12]. On SnO2 films the reaction O2−ads + e− = 2O−ads takes place as
the temperature increases. The desorption temperatures from the SnO2 surface are around 550 °C for
O−ads ions and around 150 °C for O2−ads ions. At constant oxygen coverage, the transition causes an
increase in surface charge density with corresponding variations of band bending and surface
conductivity. From conductance measurements, it is concluded that the transition takes place slowly.
Therefore, a rapid temperature change on the part of the sensors is usually followed by a gradual and
continuous change in the conductance. The oxygen coverage adjusts to a new equilibrium and the
adsorbed oxygen is converted into another species which may be used in measurement method of
dynamic modulated temperature as reported previously [13–19].
Figure 1. Energy diagram for various oxygen species in the gas phase adsorbed at the
surface and bound within the lattice of SnO2. Reprinted with permission from .
Copyright (2007) Nova Science Publishers.
3. Device Structure
Gas sensors based on metal oxide nanostructures generally consist of three parts, i.e., sensing film,
electrodes and heater. Metal oxide nanostructures react in the form of a film which will change in
resistance upon exposure to target gases. A pair of electrodes is used to measure the resistance of the
sensing film. Usually the gas sensors are furnished with a heater so that they are heated externally to
reach an optimum working temperature. Currently, metal oxide nanostructures sensors have been
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characterized in three ways: conductometric, field effect transistor (FET) and impedometric ones .
FET type is usually exploited to fabricate sensors based on single or arrays of one-dimensional (1D)
semiconducting nanomaterials, which have a complex fabrication process. Impedometric type sensors
are based on impedance changes and are operated under alternating voltage upon exposure to target
species, which has not yet attracted much attention. The conductometric type is the most common gas
sensor which is suitable for most nanomaterials. There are two types of device structures in
conductometric sensors: directly heated and indirectly heated. A directly heated type structure means
the heater is contacted with the sensing material, which may lack stability and anti-interference ability,
so most of the current nanostructure-based gas sensors are indirectly heated type structures which can
be divided into two types, i.e., cylindrical and planar layouts, as shown in Figures 2 and 3. Alumina
ceramics (wafers or tubes) are generally used as substrates to support sensing films. In the ceramic
tube-based device, a piece of heating wire is placed in the interior of the ceramic tube, while, in the
ceramic wafer-based device, heating paste is placed on the backside of the ceramic wafer. Some silica
wafers can also be used as the substrate, which is advantageous in manufacturing small sized gas
sensor because of its compatibility with integrated circuits.
Figure 2. Device structure based on ceramic wafer substrate.
Figure 3. Device structure based on ceramic tube substrate.
4. Nano Effect of Small Size of Metal Oxide Nanoparticles
The “small size effect” of metal oxides has been reported by many publications [21–27]. As shown
in Figure 4, a sensor is considered to be composed of partially sintered crystallites that are connected
Ceramic tube Ceramic tube
Heating wire Heating wire
Gold wiresGold wires
Sensing materials Sensing materials
Gold electrodesGold electrodes
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to their neighbors by necks. Those interconnected grains form larger aggregates that are connected to
their neighbors by grain boundaries . On the surface of the grains, adsorbed oxygen molecules
extract electrons from the conduction band and trap the electrons at the surface in the form of ions,
which produces a band bending and an electron depleted region called the space-charge layer. When
the particle size of the sensing film is close to or less than double the thickness of the space-charge
layer, the sensitivity of the sensor will increase remarkably. Xu et al. explained the phenomena by a
semiquantitative model . Three different cases can be distinguished according to the relationship
between the particle size (D) and the width of the space-charge layer (L) that is produced around the
surface of the crystallites due to chemisorbed ions and the size of L is about 3 nm for pure SnO2
material in literatures [30–34]. When D >> 2L, the conductivity of the whole structure depends on the
inner mobile charge carriers and the electrical conductivity depends exponentially on the barrier height.
It is not so sensitive to the charges acquired from surface reactions. When D ≥ 2L, the space-charge
layer region around each neck forms a constricted conduction channel within each aggregate.
Consequently, the conductivity not only depends on the particle boundaries barriers, but also on the
cross section area of those channels and so it is sensitive to reaction charges. Therefore, the particles
are sensitive to the ambient gas composition. When D < 2L, the space-charge layer region dominates
the whole particle and the crystallites are almost fully depleted of mobile charge carriers. The energy
bands are nearly flat throughout the whole structure of the interconnected grains and there are no
significant barriers for intercrystallite charge transport and then the conductivity is essentially
controlled by the intercrystallite conductivity. Few charges acquired from surface reactions will cause
large changes of conductivity of the whole structure, so the crystalline SnO2 becomes highly sensitive
to ambient gas molecules when its particle size is small enough.
Based on Xu’s model, many new sensing materials are developed to achieve high gas sensing
properties [35–37]. Typically, the nanocomposite of SnO2 and multiwall carbon nanotube (MWCNT)
was exploited to detect persistent organic pollutants (POPs) which possess stable chemical properties
and are ordinarily difficult to detected with metal oxides . The preparation of materials with size
and porosity in the nanometer range is of technological importance for a wide range of sensing
applications. The ultrasensitive detection of aldrin and dichlorodiphenyltrichloroethane (DDT), has
been carried out using the nanocomposite of small SnO2 particles and MWCNTs. The nanocomposite
shows a very attractive improved sensitivity compared with a conventional SnO2 sensor. A sharp
response of low limiting concentration about 1 ng was observed in both aldrin and DDT, suggesting
potential applications as a new analytical approach. One major advantage of this sensing material is its
stable attachment between sub-10 nm SnO2 nanoparticles and carbon nanotubes shown is Figure 5.
Besides, the SnO2/MWCNT nanocomposite synthesized by a wet chemical method may control the
size of SnO2 particles under 10 nm and form highly porous three dimensional (3D) structures. Among
the highly porous 3D structures, MWCNTs can be regarded as the framework and the SnO2 particles
uniformly packed on them, which may enhance the ability of gas diffusion into and out of the sensing
film. The high sensitivity can also be attributed to an effect of p-n junction formed between p-type
carbon nanotubes and n-type SnO2 nanoparticles. The investigation results make SnO2/MWCNT
nanocomposites attractive for the purpose of POPs detection.
Sensors 2012, 12
Figure 4. Schematic model of the effect of the crystallite size on the sensitivity of
metal-oxide gas sensors: (a) D >> 2L; (b) D ≥ 2L; (c) D < 2L.
Figure 5. (a) Low- and (b) high-magnified TEM images of the SnO2/MWCNT
nanocomposites. Reprinted with permission from . Copyright (2010) RSC Publishing.
5. Porous Film of Metal Oxides
Commonly, metal oxide sensing films are divided into dense and porous . In dense films, the
gas interaction takes place only at the surface of the film since the analyte cannot penetrate into the
sensing film. In porous films, the gas can penetrate into the film and interact with the inner grains. In
fact, metal oxide films are usually produced with a certain overall porosity through several processes,
which is yet insufficient for gas sensing.
Apart from large surface-to-volume ratios, well-defined and uniform pore structures are particularly
desired for metal oxides to improve sensing performance. Porous materials are classified into several
kinds according to their size. According to the definition of the International Union of Pure and
Applied Chemistry (IUPAC) , microporous materials have pore diameters of less than 2 nm and
mmethod of M
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6. Porous Nanostructures of Metal Oxide and Their Gas Sensing Properties
In the past few years, many efforts have been devoted to improve the sensitivity of gas sensors.
Sakai et al. found that the porous structure of the sensing film played a critical role in the performance
of the sensor because it decided the rate of gas diffusion . Xu et al. found that the particle size
heavily affected the sensitivity of sensor . Although many methods have been reported to
synthesize monodisperse nanoparticles of metal oxides [60–64], small size of nanoparticles are not
stable which may easily congregate and grow up under heating conditions . Besides, small sized
nanoparticles will be compactly sintered together during the film coating process which is a
disadvantage for gas diffusion in them. If porous nanostructures are used as gas sensing materials, the
gas sensing properties will be much improved. On the basis of those reasons, nanoparticles-assembled
nanostructures with many kinds of shapes such as porous nanowires, porous nanotubes, porous
nanospheres and so on are reviewed in this chapter, which exhibited excellent gas sensing properties
because they not only possessed large surface area and relatively mass reactive sites, but also formed
relatively loose film structures.
6.1. Porous Nanowires
One-dimensional or quasi-1D metal oxide nanostructures possess very large surface-to-volume
ratios which is advantageous in gas sensing. Besides, other factors also make these nanostructures
particularly suitable for conductimetric gas sensing as follows: (i) the comparability of the Debye
screening length of nanostructured metal oxides with their lateral dimensions and (ii) the ability to
fabricate them routinely with significant lengths providing a long semiconducting channel. All these
make 1D or quasi-1D nanostructures such as nanowires, nanotubes and nanorods highly sensitive and
efficient transducers of surface chemical processes into electrical signals .
Nanowires as a kind of important one-dimensional nanostructures have been used in many
field [66,67]. Many kinds of semiconductor nanowires, such as SnO2 [68–70], In2O3 [71,72],
ZnO [73–75], TiO2 [76,77] and so on, have been widely applied in gas sensors. However, smooth
nanowires only adsorb gases at their surfaces which results in a great obstacle to achieve
highly-sensitive properties. Porous nanowires have attracted great interests due to their high
surface-to-volume ratio and porous structure which allows adsorbing gases not only on the surface but
also throughout the bulk. Wang et al. [78,79] have prepared porous SnO2 nanowires based on glycolate
precursors under mild conditions which showed good sensitivity to some gases such as C2H5OH, CO
and H2. Guo et al. have prepared highly porous CdO nanowires as shown in Figure 7 by calcining the
hydroxy- and carbonate-containing cadmium compound precursor nanowires . The precursor
converted into porous CdO nanowires, which were polycrystalline structure, through heat treatment in
air without changing the wire-like topography. Due to the highly porous structure, the highly porous
CdO nanowires showed rapid response, low detection limit, high signal-to-noise ratio and selectivity to
nitrogen oxide which is one of the most dangerous air pollutants.
S Sensors 201
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The In2O3 nanospheres obtained have a uniform diameter of around 200 nm and hollow structures
with thin shells of about 30 nm. It is just the hollow and porous structure that In2O3 nanospheres have
much larger surface area, so the In2O3 nanospheres exhibit a good response and reversibility to volatile
organic compounds such as methanol, alcohol, acetone and ethyl ether. Wang et al. also prepared
hollow SnO2 nanospheres by carbonaceous spheres as templates which also showed high sensitivity to
triethylamine and ethanol .
7. Doping of the Metal Oxide Nanostructures in Nanoscale Levels
Doping of metal oxide sensing film is a traditional technology for gas sensors. The traditional
concept of doping is to enhance catalytic activity and adjust electrical resistance of the intrinsic metal
oxide [103–105]. The dopant is usually high active, which make it react preferentially with adsorbed
molecules. As shown in Figure 11, the dopant is generally dispersed on the metal oxide matrix so that
they are available near all the intergranular contacts. In air, the oxygen molecules react preferentially
with the dopant forming oxygen anions and then spill over to the metal oxide matrix. When the target
gases are adsorbed on to the surface of the dopant and then migrate to the oxide surface to react with
surface oxygen species thereby increasing the surface conductivity .
Figure 11. Oxygen spillover process in the surface of doped metal oxides.
However, as the development of nanotechnology, doping is given many novel meanings. A typical
doping phenomenon concerns the fact that the particle size of the doped metal oxide becomes smaller
than the pure one [27,28] which can be explained by Nae-Lih Wu’s theory , i.e., because of the
interaction on the boundaries between host and dopant crystallites, the motion of crystallites is
restricted [107–109]. In other words, the advancing of grain boundaries which is required for crystal
growth is stunted. As a result, the size of crystallites is decreased by the doping of impurities.
Gong et al. have investigated the role of the Cu doping in enhancing the capability to adsorb CO
molecules . According to their results, the Cu site in ZnO film plays an important role to adsorb
CO molecules at both low and high temperatures. When CO molecules are adsorbed on the film, they
are preferably adsorbed on the Cu sites to form bonds between Cu and CO. The interacting bonding
between Cu and CO consists of the donation of CO 5σ electrons to the metal and the back donation of
π electrons from d-orbitals of Cu to CO. That adsorption results in the enhancement of the reactivity to
CO. The CO adsorption mainly takes place at the Cu sites but not at the Zn sites, and then CO
molecules migrate from the Cu to the Zn sites , by which the Cu sites enhance the CO adsorption
and thus the reaction of CO with oxygen species.
S Sensors 201
im mproved re
If the dop
Xue et al. h
amount of o
such as ther
Liu et al.
se the dep
, resulting in
ping is integ
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d SnO2 holl
ther gases i
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images of C
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nd (b) FES
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due to the
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H2S of Cu
as shown i
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in Figure 1
35 °C [116
be seen th
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Figure 13. Selectivity for H2S gas from gas mixtures. Reprinted with permission
from . Copyright (2009) Springer.
Figure 14. Schematic illustration of the plasma-assisted strategy for preparing highly dense
In-doped SnO2 coral-like nanostructures for gas-sensing applications. Reprinted with
permission from . Copyright (2011) IOP Publishing Ltd.
Firstly, coral-like SnO2/carbonaceous nanocomposites were synthesized via a hydrothermal route.
Then, the nanocomposites were functionalized by plasma treatment. The densities of some functional
groups, such as hydroxyl and carboxyl, can be greatly increased on the surface of nanocomposites,
which is significant for further adsorbing In3+ ions to achieve dense doping. The plasma-treated
SnO2/carbonaceous nanocomposites were ultrasonically dispersed in In3+ ion solution and left static for
a long time and subsequently washed and centrifugated. Finally, the In-doped SnO2 coral-like
nanostructures combined with porous and hollow structures were prepared by following an annealing
process to remove the sacrificed carbonaceous templates. In gas-sensing measurements, the In-doped
SnO2 coral-like nanostructures with plasma treatment exhibited highly sensitive to chlorobenzene with
a high response and short response and recovery times.
Sensors 2012, 12
8. Conclusions and Perspectives
Although metal oxide gas sensors are predominantly solid-state gas detecting devices with many
advantages such low cost, easy production, and compact size, and thus have been widely-used in many
fields such as public safety, pollutant monitoring and so on, there is still room to improve the gas
sensing performance of such sensors by controlling the morphology and structure of sensing materials.
Here, gas sensing mechanisms have been reviewed first for better understanding their working
principles. Then, the influences of size effect, porous nanostructure and doping on nanoscale levels
have been described. By considering those influencing factors on nanoscale, novel metal oxide
nanostructures will be developed and then gas sensing properties of metal oxides will be much
On the basis of current progress in the field of metal oxide gas sensors, it is anticipated that the
following aspects would be promising directions for developing in the future: (1) novel nanostuctures
or nanocomposites which may achieve super-sensitive detection; (2) combining porous nanostructures
which possess fast responses and recovery characteristics to a chromatographic technique; (3)
exploiting first principles to further investigate the gas sensing mechanisms. The research on gas
sensors is related to many fields such as physics, chemistry, electronics and mathematics. Addressing
those problems will be one of the great challenges and it is important to enhance interdisciplinary
This work is supported by the National Basic Research Program of China (No. 2011CB933700 and
No. 2007CB936603), the National Natural Science Foundation of China (No. 61071054, 61174012
and 21177131) and the Youth Scientific Funds, National Natural Science foundation of China (No.
51002157 and 61104205).
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