Heterogeneous stacking of reduced graphene oxide on ZnO nanowires for NO2 gas sensors with dramatically improved response and high sensitivity

Graphene or two-dimensional materials have been intensively studied as a new generation of gas sensing ma-terials due to their large specific surface area and high mobility. However, fabrication processes for oxide and 2D materials lead to non-uniform structures of flakes of graphene or its derivatives and oxide nanowire, are randomly suspended to devices, resulting in poor and unrepeatable sensing performances. Here, we report on the heterogeneous stacked interface of reduced graphene oxide (rGO) on the surface of ZnO nanowires and their demonstration as a NO2 gas sensor. Compared to the conventional surface decoration using noble metals such as Au, Ag, and Pd, the present sensor shows excellent sensing performances including 22 times faster response behavior. Moreover, this interface-based rGO-ZnO gas sensor showed outperforming sensitivity and recovery time to reported 2D and 2D/oxide based gas sensor. The active sites of rGO are more favorable for chemisorption of oxygen molecules due to functional groups on rGO surfaces. Moreover, the gas-sensing mechanism is firstly elucidated by the finite-difference time-domain (FDTD) simulation, confirming that mono-to-few layers of rGO on ZnO act a role of bridge, facilitating the migration of electrons from ZnO to NO2, leading to higher increment of depletion region and corresponding sensor response. Our approaches may offer the new opportunities and strategies for highly sensitive and fast recoverable 2D materials/oxide hybrid sensors.