Flexible humidity-tolerant γ-Fe2O3-rGO-based nanohybrids for energy efficient selective NO2 gas sensing

The present manuscript reports an energy efficient gamma-Fe2O3-rGO-based humidity tolerant nanohybrids, fabricated on flexible tempered glass (FTG), for NO2 gas sensing at room temperature. An interaction between n-type gamma-Fe2O3 and p-type rGO semiconductors assisted in forming a p-n heterojunction at their interface, creating oxygen vacancies (O-v) in the gamma-Fe2O3 phase, as revealed by XPS analysis. The O-2 adsorbed on these sites produced O-2(-) species, which facilitated the charge-transfer from O-2(-) to NO2 to eventually reduce the resistivity. The voltage at 0.3 volt restricted the effect of humidity on NO2 gas sensing in a wide RH range (15-97%), making it not only humidity tolerant but also energy efficient. The bending of the FTG up to about 150 degrees resulted in a negligible change in %response (from 56% to 50%) with a minor increase in recovery time, suggesting its potential for usage in flexible electronic devices. Its high selectivity for NO2 gas sensing is manifested by a minor decrease in %response from 56% to 49% in the presence of common air pollutant gases like Cl-2, NO2, CO, NH3, C3H6O, H2S, and C2H5OH at 10-ppm of each. The role of rGO in the nanohybrids towards enhancing the conductivity is corroborated by the significantly lower %response observed (9%) for bare gamma-Fe2O3 film on FTG. The efficient sensing of NO2 gas has been correlated based on the comparison of the electron affinity (eV) values with other probe gases, following the order: NO2 (2.30) > CO (1.32) > H2O (1.3) > NH3 (0.16) > C3H6O (0.00152) > H2S (-1.16). Thus, the as-designed gamma-Fe2O3-rGO-based NO2 gas sensor, operating at 0.3 V at RT, demonstrating selective sensing with rapid response/recovery times (0.08 min/0.25 min) for 0.5 ppm NO2, having high %response, reproducibility, humidity tolerance in a wide RH range and long-term stability (>52 weeks), is suggested to be a novel NO2 gas sensing device.