Electrical Properties of Micro-Nano Hierarchically Controlled Tin Dioxide Gas Sensors
Authors: Mashiba Felipe Hiroshi , Suematsu Koichi , Watanabe Ken , Shimanoe Kengo 
Published: December 2024

Metal oxide semiconductor gas sensors, particularly those utilizing tin dioxide, are indispensable across various sectors, including medical diagnostics. The medical industry demands high sensitivity and selectivity to detect low gas concentrations. In our previous proposal, we integrated pulse-driven techniques with a micro gas sensor to achieve this goal. Additionally, nanostructure control, involving nanoparticles, nanoclusters, and nano/mesopores, is crucial for attaining these properties. Typically, this type of gas sensor exhibits high resistance at elevated temperatures. However, maintaining low electrical resistance at temperatures between 200°C to 400°C poses a challenge due to the associated low cost of electronic circuits. At lower temperatures, sensor resistance in the air often surpasses measurement limits. In the microsensors' sensing film, it is believed that as the sizes of secondary and tertiary particles draw closer to the electrode gap due to particle agglomeration, the contact area of these larger particles with the electrodes decreases, resulting in higher electrical resistance. Although metal oxide semiconductor gas sensors have primarily focused on the nano effect of particles, the combined nano and micro effects arising from micro-size agglomeration have not been extensively explored. Therefore, our research aims to investigate the micro-size effect of tin dioxide on micro gas sensors and evaluate its impact on electrical properties such as electrical resistance and sensor response.

To synthesize tin dioxide, we utilized the reverse homogeneous precipitation method using SnCl4 and NH4HCO3, followed by the elimination of Cl ions through centrifugation. The resulting sol was dried, ground, and calcined in wet-O2 at 600°C for 3 hours. The obtained powder was mixed with water and separated into fine and coarse agglomeration using sedimentation and centrifugation method. Material characterization included X-ray diffraction (XRD) for analyzing crystallite structure, scanning electron microscopy (SEM) for observing microstructure, Brunauer-Emmett-Teller and Barrett-Joyner-Halenda (BET-BJH) analysis to assess pore structure from nitrogen adsorption-desorption isotherm, and particle size distribution (PSD) analysis. Micro gas sensor devices were fabricated for sensor measurements, with a focus on assessing the electrical resistance of SnO2 micro gas sensors in air and their response to various gases.

XRD analysis revealed a rutile structure pattern in all tin dioxide samples, with no distinct average crystallite sizes (around 18 nm). SEM results showed fine SnO2 agglomerates predominantly less than 1 μm, while coarse SnO2 agglomerates were less than 5 μm, consistent with PSD analysis showing narrower distributions for fine and coarse SnO2, with mean particle sizes of 0.2 μm and 2 μm, respectively. BET results indicated that fine SnO2 had a specific surface area of 22 m2g-1, while coarse SnO2 had 19 m2g-1. BJH results showed that fine SnO2 had a specific pore volume of 0.17 cm3g-1, whereas coarse SnO2 had 0.09 cm3g-1. These pore properties suggest that fine SnO2 has a larger specific surface area and pore volume than coarse SnO2. Measurements of electrical resistance in air were conducted across temperatures ranging from 350°C to 200°C. Sensors containing fine SnO2 particles exhibited lower resistance compared to conventional tin dioxide sensors, indicating a potential influence of finer agglomerates on electrical properties. The sensor's response to various gases will be shown during the presentation.