Selective H2 sensing using lanthanum doped zinc oxide thin film: A study of temperature dependence H2 sensing effect on carrier reversal activity

In the present work, we have demonstrated a highly sensitive H-2 gas sensor using a lanthanum doped ZnO (La_ZnO) thin film operated at 300 degrees C. Also, a p-type to n-type carrier reversal activity is revealed in the presence of H-2 gas species, which predominantly depends on the operating temperature and doping concentration of lanthanum. Pure and La_ZnO (1-10at.%) thin films were successfully synthesized using a sol-gel route, where a 5at.% lanthanum doped ZnO thin film shows an outstanding H-2 gas sensitivity (400%) among all other samples with an optimized temperature of 300 degrees C. Moreover, this sensor actively responds to a wide H-2 gas concentration (10-500ppm) with a sensitivity of 0.9 (similar to n). Additionally, H-2 gas sensing selectivity and mixed gas sensing performance were investigated in the presence of CO and CO2 gas species at optimized temperature (300 degrees C). Results show that the pure and 1-3at.% La_ZnO thin films exhibited n-type H-2 gas sensing, while p-type sensing behavior was observed for 5% and 10% La_ZnO thin films at 300 degrees C. It is further observed that O- species are extremely active to CO gas species operating at a high operating temperature (>250 degrees C). Therefore, despite the emerging p-type behavior of the sensor, the ejected electrons are expected to dominantly reduce the sensor resistance in the presence of CO gas species at 350 degrees C. The improvement of H-2 sensing is further interrelated with the defect levels using Raman spectroscopy.