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SENSORS BASED ON NANOSTRUCTURED MIXED METALLOXIDES FOR

DETECTION OF TOXIC, EXPLOSIVE AND FLAMMABLE SPECIES

E.L.Shangina, V.A.Seleznev, B.M.Voronov, A.P.Melnikov, G.N.Goltsman

Moscow State Pedagogical University

L.I.Trakhtenberg, V.F.Gromov, G.N.Gerasimov, T.V.Belysheva, E.Yu.Spiridonova

Karpov Institute of Physical Chemistry, Moscow

Currently, sensors of different types are used for detection of toxic, explosive and

flammable air content. Conductometric sensors are the most simple, reliable and low-

price ones among them. The operation principle of the aforementioned sensors is based

on the change of the conductivityof the sensor film as a result of chemosorption of the

analyzed chemical compounds on the surface of the film. The efficiency of

semiconductor nanostructered films as conductometric sensors is proven due to thefact that the conductivity of these films is changed as a result of volatile species

chemosorption on the surface of nanoparticles and subsequent reactions with

chemosorbed molecules.

The most wide-spread conductometric sensors are based on semiconductor-type

oxides of metals,primarily SnO2 , that detect the content of such gases as H2, CO,

CH4 [1], O3 [2] in the atmosphere. On the other hand, there are practically no sensors of

this kind for detection of complex organic toxic or explosive compounds.

This work is part of the project oriented on the research and development of fast

nanostructured metalloxide conductometric sensors for selective detection of toxicorganic compounds, mainly organic amins that work at a temperature of 200-250 C.

In this project, the electron metallic oxide semiconductors, such as SnO2 and In2O3,have

been chosen as the base of the sensors. Earlier research showed that one can detect

small concentrations of toxic non-symmetric dimethyl hydrazine in the air using these

sensors [3].

The sensitive layer is deposited on a sapphire substrate. First, platinum electrodes

were formed on the top side of this substrate to measure the conductivity of sensitive

layer. Then, a platinum layer was deposited on the back side of the substrate to heat the

sensitive layer. The SEM pictures of the sensor are shown in Fig 1.

Currently, paste-like compositions are usually used to fabricate the sensitive layer.

These compositions contain nanosize powders of appropriate semiconductor oxides.

Such a paste is deposited on the substrate as a thin layer, for example, by the silk-

screening method and then treated at a certain temperature.

It should be noted that this method is rather time-consuming. Furthermore, in this

case it is very hard to obtain films that have uniform thickness and identical

properties. Additional problems are related to the relatively low adhesion of the layer to

the substrate surface. Thus, we propose and apply a new method of sensitive layer

deposition. The method is based on the aerosol spraying of the solution that contains the

correspondingprecursors. The solution is sprayed on the surface of the substrate


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a b

Fig.1 . The SEM pictures of the sensor:

a) top view with platinum electrodes;

b) back side of the substrate with meander-like platinum heater.

heated to a certain temperature and then the obtained layer is annealed. It is importantto note that the method of the aerosol spraying of the sensitive layer allowed us to get

more homogeneous films with particles of smaller sizes compared to the silk-screen

printing: the mean size of the particles in the first case is about 30 nm. Such a

difference in the size should lead to a higher efficiency of the sensors produced by the

aerosol spraying method .

The conductivity of the nanostructured SnO2 films fabricated by both the aerosol

spraying method and the silk-screen printing one was investigated. The thickness of the

films made by the aerosol spraying was about 100 nm while the silk-screen printed

films were as thick as about 1-2 m. The measurements were performed in the

temperature range from 250 to 500 C.

We observed the Arrhenius temperature dependence of the conductivity of the

films containing up to 12mole percent of In2O3 (see Table 1 ). In the case when the film

contains 19 mole percent and more of In2O3 it reveals non-monotonic temperature

dependence of the conductivity. Thus we may conclude that the basic conductivity

mechanism is changed when the fraction of In2O3 in SnO2 increases.

Table 1. The activation energy E of the

conductivity of nanostructured films of

different content, fabricated by the aerosolspraying method.

The film content , kcal/mol

SnO2 10,8

SnO2 + 6% In2O3 12,3

SnO2 + 12% In2O3 8,1

SnO2 + 2%PdO 5,0

SnO2 + (1:1) 5,6


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In spite of the fact that the temperature dependence of the conductivity does not

change when one adds oxides of palladium and cobalt to SnO2, the introduction of

these oxides significantly affects the value of the activation energy of these films. It

can be easily seen from the data presented in Table 1 that the activation energy of the

conductivity for mixed films containing CoO or PdO in addition to SnO2 is

substantially lower than for the pure SnO2 film. Addition of 6 mole percent of indiumoxide to SnO2 film does not practically change the activation energy of the conductivity

of nanostructured film. On the contrary, the addition of 12 mole percent of indium

oxide leads to a significant decrease of the activation energy of the conductivity.

It should also be noted that the activation energy of the conductivity of SnO2 film

appreciably depends on the method of the film fabrication: for the films deposited by

the aerosol spraying the activation energy E=10,8 kcal/mol, for silk-screen printed

ones E=11,8 kcal/mol.

Based on the conductivity data for the obtained nanostructured metaloxide films

we consider the possibility to fabricate the sensitive layers with the optimal compositionfor detection of some gases-deoxidants, for example, hidrogen, carbon monoxide, and

organic compounds.

We gratefully acknowledge the financial support of the Russian Foundation for Basic Research (Grants #08-03-00029 and #09-03-00194) and the Federal Agency forEducation (Grant #2.1.1/4240).

1. Gromov V.F., Gerasimov G.N., Belysheva T.V., Trakhtenberg L.I. Mechanisms of

sensor effect in SnO2 conductometric detectors for gases-deoxidants // Rossiiskii

Khimicheskii Zhurnal. 2008. V.52. N 5. P.80 87.

2. Belysheva T.V., Gerasimov G.N., Gromov V.F., Trakhtenberg L.I. Sensor

properties of Fe2O3 * In2O3 films: detection of low concentrations of ozone in

air // Zhurnal Fizicheskoi Khimii. 2008. V.82. N 10. P.1921 1926.

3. Belysheva T.V., Kazachkov E.A., Bogovtseva L.P., Kubyshkin V.N., Vokhontsev

V.M. Electrophysical properties of gas sensitive In23 and WO3 semiconductor

films as detectors of non-symmetric dimethyl hydrazine in air // Zhurnal

Analiticheskoi Khimii. 2006. V.61. N 7. P.731 739.

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