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

Abstract

In the present work, we have demonstrated a highly sensitive H2 gas sensor using a lanthanum doped ZnO (La_ZnO) thin film operated at 300 °C. Also, a p-type to n-type carrier reversal activity is revealed in the presence of H2 gas species, which predominantly depends on the operating temperature and doping concentration of lanthanum. Pure and La_ZnO (1–10 at. %) thin films were successfully synthesized using a sol-gel route, where a 5 at. % lanthanum doped ZnO thin film shows an outstanding H2 gas sensitivity (400%) among all other samples with an optimized temperature of 300 °C. Moreover, this sensor actively responds to a wide H2 gas concentration (10–500 ppm) with a sensitivity of 0.9 (∼n). Additionally, H2 gas sensing selectivity and mixed gas sensing performance were investigated in the presence of CO and CO2 gas species at optimized temperature (300 °C). Results show that the pure and 1–3 at. % La_ZnO thin films exhibited n-type H2 gas sensing, while p-type sensing behavior was observed for 5% and 10% La_ZnO thin films at 300 °C. It is further observed that O− species are extremely active to CO gas species operating at a high operating temperature (>250 °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 °C. The improvement of H2 sensing is further interrelated with the defect levels using Raman spectroscopy.