Calcination Temperature (°C)

Figure 9. Effect of calcination temperature on the crystallite size for the sol-gel derived nanocrystalline SnO2 [9, 12-14, 21, 22, 25].

the surface adsorbed O—ds or O-^adsj species and subsequently electrons are reintroduced into the electron depletion layer, leading to decrease in the potential barrier. The sensitivity of the nanocrystalline SnO2 thin film is usually determined by the ratio (Ra — Rg )/Rg, where Ra and Rg are the resistance of the sensor in air and reducing gas, respectively.

A typical response of the nanocrystalline SnO2 sensor is schematically shown in Figure 12. The figure shows that the sensor exhibits a resistance "Ra" in air. When the reducing gas comes in contact with the sensor, its resistance decreases rapidly initially, then decreases at lower rate with increasing time and attains a steady-state resistance value (Rg). The total time taken by the sensor to attain 90% of the steady-state value is known as the "response time" of the sensor. If the air is flown back over the sensor, it retains its original resistance in air. The time required to reach 90% of the original resistance is known as the "recovery time" of the sensor. The gas sensitivity, response time, and recovery time are the three most important parameters of the nanocrystalline gas sensor. During the preparation of the nanocrystalline SnO2 thin film, the sol-gel synthesis parameters are optimized to maximize the gas sensitivity and to minimize the response and recovery times.

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