The sensitivity of pristine and Pt doped ZnO nanoclusters to NH3 gas: A transition state theory study

The sensitivity of pristine and Pt doped ZnO nanocrystals to ammonia gas are calculated using density functional theory combined with transition state theory. NH3 is adsorbed on the surface of pristine ZnO without reaction. However, Pt catalyzed ZnO reacts by oxidizing NH3 to nitrogen oxide and water. A small increase in the energy gap of pristine ZnO nanocrystal at the order of 0.05 eV transforms the pristine ZnO to a weak p-type ammonia sensor. On the other hand, the oxygen is removed by NH3 reaction from the surface of Pt/ZnO with a reduction of the energy gap (n-type). The sensitivity of the Pt/ZnO sensor increases with temperature due to an increase in the number of reactions of the NH3 molecules until nearly 330 degrees C. However, when the temperature is increased further, the sensor sensitivity decreases due to the reaction of NH3 with oxygen in the ambient air instead of picking oxygen from Pt/ZnO particles. As we reach the autoignition temperature of ammonia at 650 degrees C, most NH3 gas reacts with ambient air before it reaches the Pt/ ZnO surface of the sensor. The present results are in agreement with available experimental results.