Gas sensing with oxide nanostructures is increasingly important to detect gaseous compounds for safety monitoring, process controls, and medical diagnostics. For such applications, sensor sensitivity is one major criterion. In this study, to sensitively detect volatile organic compounds (VOCs) at very low concentrations, we fabricated porous films using SnO2 nanocubes (13 nm) and nanorods with different rod lengths (50-500 nm) that were synthesized by a hydrothermal method. The sensor response to H2 increased with decreasing crystal size; the film made of the smallest nanocubes showed the best sensitivity, which suggested that the H2 sensing is controlled by crystal size. In contrast, the responses to ethanol and acetone increased with increasing crystal size and resultant pore size; the highest sensitivity was observed for a porous film using the longest nanorods. Using the Knudsen diffusion-surface reaction equation, the gas sensor responses to ethanol and acetone were simulated and compared with experimental data. The simulation results proved that the detection of ethanol and acetone was controlled by pore size. Finally, we achieved ultrahigh sensitivity to ethanol; the sensor response (S) exceeded S = 100 000, which corresponds to an electrical resistance change of 5 orders of magnitude in response to 100 ppm of ethanol at 250 °C. The present approach based on pore size control provides a basis for designing highly sensitive films to meet the criterion for practical sensors that can detect a wide variety of VOCs at ppb concentrations.
Keywords: Knudsen diffusion; SnO2; gas sensor; nanocubes; nanorods; pore control.