Metal oxide nanostructures and their gas sensing properties: a review.
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 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.
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ABSTRACT: We have investigated the phase changes in CdTiO3 nanofibers as the annealing temperature of nanofibers was increased from 600C to 1200C. The nanofibers annealed at 600 C were ilmenite with a very small amount of CdO. Upon annealing at 950C, CdO was completely removed. Annealing at 1000C yielded pure perovskite nanofibers and at temperatures above 1100C rutile TiO2 nanofibers were obtained. Brunauer-Emmett-Teller (BET) analysis showed that with increase in annealing temperature the surface area of nanofibers was decreased. The nanofibers annealed at 600C have the higher surface area ~9.41 m2/gm. Then oxygen sensors using CdTiO3 nanofibers annealed at 600C (ilmenite) and 1000C (perovskite) were fabricated. The sensitivity of the ilmenite nanofibers sensor was two times than that of the perovskite nanofibers sensor. The response and recovery times were 120 seconds and 23 seconds respectively for ilmenite nanofibers sensor whereas response and recovery times were 156 seconds and 50 seconds respectively for the perovskite nanofibers sensor. Better oxygen characteristics of ilmenite nanofibers are attributed to their large surface area and porosity. Therefore we believe that ilmenite CdTiO3 nanofibers are potential candidate to develop practical oxygen sensors.ACS Applied Materials & Interfaces 02/2014; · 5.90 Impact Factor
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ABSTRACT: Electrical measurements as a diagnostic tool for gas sensing application were investigated of the SiO2 nanofibers, Bismuth doped SiO2 nanofibers and Bismuth Silicate (Bi4(SiO4)3) nanofibers, which were synthesized by electrospinning. The gas sensing devices were fabricated using thermally evaporated Ni-Cr metals on the glass substrate and then nanofibers were deposited between the contact electrodes. The performance of the sensors was evaluated by recording DC & AC measurements in an oxygen rich environment. The morphology and structural characterization of nanofibers were characterized by scanning electron microscopy (SEM) and X-rays diffraction (XRD). Bi4(SiO4)3 nanofibers exhibited excellent oxygen sensing properties at all temperatures (25oC-127oC). The rapid response time (~ 49 sec) and recovery time (~ 9 sec) with high linearity indicated that Bi4(SiO4)3 nanofibers could be a good candidate for developing practical oxygen gas sensors.Sensors and Actuators B Chemical 09/2013; · 3.84 Impact Factor
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ABSTRACT: This research paper reports the deposition of nanostructured pure and Ni-doped ZnO thin films deposited at the substrate temperature of 523 K using simple and economical spray pyrolysis technique and subsequently post annealed at 673 K in air atmosphere for 3 h. Ni-doping greatly affected the crystallo-graphic orientation, surface morphology, roughness and room temperature sensing response. Noticeable change in the crystallite size, transmittance and electrical properties was observed. The room tempera-ture sensing characteristics like selectivity, response recovery studies, range of detection, stability and reproducibility of the undoped and Ni-doped ZnO thin films were investigated. Especially, the sensing elements exhibited an excellent selectivity towards ammonia. A lower detection limit of 5 and 25 ppm was observed for undoped and Ni-doped ZnO thin films respectively. The upper detection range was widened to 1000 ppm for the Ni-doped film.Applied Surface Science 06/2014; 311:405-412. · 2.54 Impact Factor
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
Sensors 2012, 12
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
Sensors 2012, 12
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 wireHeating wire
Gold wiresGold wires
Sensing materialsSensing materials
Gold electrodesGold electrodes
Sensors 2012, 12
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.