In the past decade, nanostructured materials have increasingly played an important role in the area of gas sensing. Tin oxide SnO₂ is one of the most widely investigated sensor materials and is commonly used for environmental monitoring.
SnO₂ nanoparticles with small particle size distribution (σ <1.1), and diameters of 10nm, 15nm and 20nm have been deposited on platinum interdigital transducers with 2μm finger width and 2μm spacing. Impedance spectroscopy measurements in synthetic air, taken in the temperature range between 373 K and 573 K show a clear dependence of the measured impedance on the particle size. Three different contributions to the overall impedance have been resolved for 10nm particles and assigned to the contribution of bulk, intergranular contact (IC) and electrode contact (EC). Particles of larger diameters do not allow such a clear distinction. The experimental observations have been analyzed by fitting the measured data with equivalent circuit functions, where typical serial connections of R and parallel RCs are used for fitting. As expected for a semiconducting material, the resistance decreases with increasing temperature. Activation energies of the different charge carrier transport processes have been determined from Arrhenius plots. The calculated values are -499meV for IC, -126meV for EC and -27meV for bulk contribution. The measured data indicate that the gas sensing mechanism is dominated by the EC and that the IC is dependent on the particle size.
For detailed information on these correlations, short period sensor arrays have been produced. Interdigital structures with a total width of ~ 55μm and length of ~ 100μm were written using electron beam lithography. The width of the contacts and their distances are varied between 100 nm and 1000 nm. Nanosized SnO₂ particles from a microwave reactor were mass separated within a particle mass spectrometer and consequently deposited on different interdigital structures.

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