Effect of ZnO thickness on gas sensing behavior of WS2-ZnO p-n heterojunction nanosheets towards reducing gases

Optimization of materials in heterojunction gas sensors is vital for achieving the highest gas-sensing performance. In this study, we synthesized pristine ZnO, pristine WS2 nanosheets (NSs), and ZnO with different thicknesses (10-50 nm) on WS2 NSs to study their gas-sensing properties. It was found that at 150 degrees C and 300 degrees C, the sensors with ZnO layers of 10 and 30 nm, respectively, exhibited the highest response to reducing gases. At low temperatures, the role of WS2 in gas sensing was dominant, whereas at high temperatures, ZnO played a dominant role. Therefore, the sensor with the thinnest ZnO layer demonstrated the best performance at low temperatures, while at higher temperatures, the sensor with a ZnO thickness of 30 nm exhibited optimal performance. Also, at 300 degrees C, WS2 was oxidized to WO3. Furthermore, to decrease overall power consumption, we conducted sensing measurements under self-heating conditions on a WS2-ZnO (10 nm) p-n heterojunction NS sensor and found that the gas sensor, under a low applied voltage of 4.2 V, exhibited the highest response to reducing gases. The enhanced gas sensing of the optimized gas sensors was attributed to the formation of heterojunctions and the optimal thickness of the ZnO layer on WS2 at appropriate temperatures.