Pore and Particle Size Control of Gas Sensing Films Using SnO2 Nanoparticles Synthesized by Seed-Mediated Growth: Design of Highly Sensitive Gas Sensors

Gas sensing is an important application of metal oxides. The gas sensor response of metal oxide films is greatly influenced by particle size, pore size, thickness, and surface states. To study the effects of particle and pore sizes of sensing films on sensitivity, we fabricated SnO2-based films with different particle and pore sizes and studied sensor responses to three different gases: H-2, CO, and H2S with different Knudsen diffusion coefficients. The pore size radii of the gas sensing films were successfully controlled from 2.8 to 5.5 nm using SnO2 nanoparticles of different sizes (4-17 nm diameter) that were synthesized by seed-mediated growth under hydrothermal conditions. Sensor response to H-2 increased with decreasing particle size because of the formation of an electron depletion layer within the nanosized crystals. In contrast, the response to CO and H2S increased with increasing particle size and the resultant pore size. Using the Knudsen diffusion-surface reaction equation, we simulated a gas concentration profile within the films, which revealed that the diffusion of CO and H2S is limited by small pores because of their lower diffusion rates compared with H-2. We show that controlling the pore size of the sensing films produces ultrasensitive films, and a large resistance change by 4 orders of magnitude is achieved in response to a low concentration of H2S (5 ppm).