Nanocrystalline metal oxides are used for the creation of materials applied in gas sensors, electrochemical devices, solar cells, optoelectronic transducers. These materials are usually electronic and in some cases ionic or mixed electronic-ionic conductors. The relation between the conductivity components varies with the hydratation degree variation and depends on the crystallite size. The latter factor is of great importance for the ceramics with nanometer scaled grain size. The present work is aimed to establish the correlation between the mean crystallite size and the ionic and electronic contributions to the conductivity for nanocrystalline In₂O₃ and SnO₂ with crystallite size in range of 3-40 nm. Samples of nanocrystalline In₂O₃ and SnO₂ were synthesised by precipitation of α-stannic acid and indium hydroxide gel. Powders of hydrated oxides were calcined in air at 300, 500, 700, and 1000^oC during 24 hours to obtain nanocrystalline samples with different crystallite size. Impedance spectroscopy and static current-voltage measurements were applied to investigate charge transport. Grain size increase results in reduction of ionic (proton) conductivity at the room temperature. On the contrary electron conductivity reduces as the crystallite size varies from 40 to 20 nm. The following reduction of the grain size down to 3 nm does not affect the electron conductivity value that may be associated with complete depletion of grains. Activation energy of the electron conductivity is practically independent on grain size and equals to 500-600 meV. That is significantly higher than ionisation energy of the oxygen vacancy of 130 meV. This argument indicates the existence of the barrier conductivity mechanism.
This work was done under a partial financial support of RFBR Grant N 03-03-32586 and Grant of President of Russian Federation N MK-1710.2003.03.

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1. Non-homogeneous nanocrystalline oxide materials for gas sensor applications