Grain size control in nanocrystalline In2O3 semiconductor gas sensors

In2O3 thin films prepared by sol–gel method make it possible to detect low levels (several hundreds ppb) of nitrogen dioxide in air. The possibility of grain size control in indium oxide-sensing layers has been established by using of two preparation methods—electron beam evaporation (EB) and sol–gel technique (SG).

SG-prepared samples show smaller particles sizes (down to 5 nm), higher state of agglomeration, higher semiconductor gas sensor resistance in air and higher response to NO2 in comparison to EB samples. Sol–gel technique leads to the preparation of polycrystalline indium oxide with particle sizes of about 5–6 nm after calcination at 400°C and 20 nm after calcination at 700°C.

The initial state of particle agglomeration in initial indium hydroxide sol (IHS), stabilized with nitric acid, influences the structure and surface morphology of the resulting indium oxide. While the In2O3 layer prepared by using low agglomerated IHS is smooth and porous, In2O3 layers prepared from highly agglomerated IHS consist of two regions—thin layer and crystallite agglomerates in cubic and rectangular parallelepiped form. The last shows the best results in terms of NO2 sensitivity. Sensor resistance and NO2 sensitivity increase with the decrease of the grain sizes in In2O3.

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