330
A. P. Chebanenko et al.
In [12], it was theoretically shown that at room temperature, both associative
and dissociative adsorption occur on the SnO 2 (110) surface, where the latter being
thermodynamically preferable. The surface oxygen has a significant influence on the
nature of adsorption. The high density of oxygen on the surface, corresponding to the
oxidized surface, promotes associative adsorption, and the low (reduced surface)—
dissociative adsorption. The authors showed that associative adsorption leads to electron enrichment of the metal oxide surface, while the dissociative adsorption causes
surface depletion by electrons. At the same time, author [17] having carried out
theoretical calculations of the interaction of water with all surfaces of tin dioxide,
obtained the opposite result. He showed that water has a tendency to dissociate on
oxidized surfaces and physically adsorb on the reduced ones.
The calculations using the density functional theory and the formalism of Gaussian and plane waves in [13], theoretically showed that dissociative adsorption of
H 2 O is usually observed on SnO 2 (110) surfaces, accompanied by stable associative
configurations of H 2 O. Both associative and dissociative adsorption of water was
stabilized due to intermolecular hydrogen bonds with an increase of water amount
on tin dioxide surface (110).
In [18, 19], the authors experimentally showed the presence of a noticeable amount
of adsorbed oxygen and its effect on the electrical conductivity of the studied films
at room temperature, as well as the dissociative nature of water adsorption on their
surface as the temperature rises to 150 °C. Taking into account these data and theoretical calculations of authors [12, 13, 17], it is concluded about preferably associative
adsorption of water at room temperature in the films, at least up to the voltage of
300 V. It is predictable that a further increase in voltage leads to a change in the
adsorption character from associative to dissociative, due to strong measuring electric field contributing to the dissociation of water molecules. In [18], this is facilitated
by increased temperature. This is reflected in the fall in sensitivity of the studied
films. It can be assumed that the strong electric field applied to the samples promotes
adsorption of both oxygen and water at room temperature, whereas an increased
temperature contributes to a better sensitivity of tin dioxide to various adsorbates.
4 Conclusion
Studies of current relaxation in tin dioxide layers at room temperature under the
alternate inlet of water vapor and dry air showed a reversible exponential nature of
the change in current, and hence the adsorption–desorption processes.
The nature of the adsorption time constant dependence on the applied voltage
indicates an increase in the rate of adsorption processes with voltage growth and the
subsequent stabilization of the adsorption process. The time constants ratio of adsorption and desorption shows a higher rate of desorption processes, which witnesses in
favor of the associative nature of water adsorption on the films at room temperature.
The results indicate that SnO 2 films can be used as sensitive elements for moisture
sensors operating at room temperature.
A. P. Chebanenko et al.
In [12], it was theoretically shown that at room temperature, both associative
and dissociative adsorption occur on the SnO 2 (110) surface, where the latter being
thermodynamically preferable. The surface oxygen has a significant influence on the
nature of adsorption. The high density of oxygen on the surface, corresponding to the
oxidized surface, promotes associative adsorption, and the low (reduced surface)—
dissociative adsorption. The authors showed that associative adsorption leads to electron enrichment of the metal oxide surface, while the dissociative adsorption causes
surface depletion by electrons. At the same time, author [17] having carried out
theoretical calculations of the interaction of water with all surfaces of tin dioxide,
obtained the opposite result. He showed that water has a tendency to dissociate on
oxidized surfaces and physically adsorb on the reduced ones.
The calculations using the density functional theory and the formalism of Gaussian and plane waves in [13], theoretically showed that dissociative adsorption of
H 2 O is usually observed on SnO 2 (110) surfaces, accompanied by stable associative
configurations of H 2 O. Both associative and dissociative adsorption of water was
stabilized due to intermolecular hydrogen bonds with an increase of water amount
on tin dioxide surface (110).
In [18, 19], the authors experimentally showed the presence of a noticeable amount
of adsorbed oxygen and its effect on the electrical conductivity of the studied films
at room temperature, as well as the dissociative nature of water adsorption on their
surface as the temperature rises to 150 °C. Taking into account these data and theoretical calculations of authors [12, 13, 17], it is concluded about preferably associative
adsorption of water at room temperature in the films, at least up to the voltage of
300 V. It is predictable that a further increase in voltage leads to a change in the
adsorption character from associative to dissociative, due to strong measuring electric field contributing to the dissociation of water molecules. In [18], this is facilitated
by increased temperature. This is reflected in the fall in sensitivity of the studied
films. It can be assumed that the strong electric field applied to the samples promotes
adsorption of both oxygen and water at room temperature, whereas an increased
temperature contributes to a better sensitivity of tin dioxide to various adsorbates.
4 Conclusion
Studies of current relaxation in tin dioxide layers at room temperature under the
alternate inlet of water vapor and dry air showed a reversible exponential nature of
the change in current, and hence the adsorption–desorption processes.
The nature of the adsorption time constant dependence on the applied voltage
indicates an increase in the rate of adsorption processes with voltage growth and the
subsequent stabilization of the adsorption process. The time constants ratio of adsorption and desorption shows a higher rate of desorption processes, which witnesses in
favor of the associative nature of water adsorption on the films at room temperature.
The results indicate that SnO 2 films can be used as sensitive elements for moisture
sensors operating at room temperature.