Layered Heterostructure of Graphene and TiO2 As a Highly Sensitive and Stable Photoassisted NO2 Sensor

As an atomically thin electric conductor with a low density of highly mobile charge carriers, graphene is a suitable transducer for molecular adsorption. In this study, we demonstrate that the adsorption properties can be significantly enhanced with a laser-deposited TiO2 nanolayer on top of single-layer CVD graphene, whereas the effective charge transfer between the TiO2-adsorbed gas molecules and graphene is retained through the interface. The formation of such a heterostructure with optimally a monolayer thick oxide combined with ultraviolet irradiation (wavelength 365 nm, intensity <1 mW/mm2) dramatically enhances the gas-sensing properties. It provides an outstanding sensitivity for detecting NO2 in the range of a few ppb to a few hundred ppb-s in air, with response times below 30 s at room temperature. The effect of visible light (436 and 546 nm) was much weaker, indicating that the excitations due to light absorption in TiO2 play an essential role, while the characteristics of gas responses imply the involvement of both photoinduced adsorption and desorption. The sensing mechanism was confirmed by theoretical simulations on a NO2@Ti8O16C50 complex under periodic boundary conditions. The proposed sensor structure has significant additional merits, such as relative insensitivity to other polluting gases (CO, SO2, NH3) and air humidity, as well as long-term stability (>2 years) in ambient air. The results pave the way for an emerging class of gas sensor structures based on stacked 2D materials incorporating highly charge-sensitive transducer and selective receptor layers.