Analysis of Unusual and Instantaneous Overshoot of Response Transients in Gas Sensors

Hydrogen is known to diffuse through particular planes of ZnO crystals. However, in spite of being an established gas sensor material, such diffusion related effects have not been reported in gas sensor properties of ZnO in the literature to the best of our knowledge. Here, we report a prominent overshoot observed in the response transients of single crystalline large ZnO microrods sensor upon exposure to hydrogen gas. This overshoot intensity and time has been found to vary systematically with the temperature as well as gas concentration. In spite of larger size of rods (diameter approximate to 1-3 mu m), it shows a very high response of nearly 950% for 1000 ppm hydrogen at 300 degrees C. The observed overshoot is very prominent at temperatures below 300 degrees C. This unusual behavior has been attributed to the bulk diffusion of atomic hydrogen into the subsurface of ZnO microrods. The coefficient of diffusivity, estimated from the time of overshoot, is found to be typically 10(-15) m(2) s(-1), at 250 degrees C, and its thermal activation energy has been found to be 0.22 eV. The lack of grain boundaries within the microrods and surface gradient driven diffused hydrogen species have been proposed as the origin of unusual overshoot profiles.