Cu/graphene interdigitated electrodes with various copper thicknesses for UV-illumination-enhanced gas sensors at room temperature

Abstract

Effective detection of NO₂ and NH₃ gases at room temperature (RT) is critical for environmental monitoring and protection. Here, graphene-based gas sensors (Cu/Gr device) of single layer graphene decorated by 6, 8 and 10 nm thick Cu layers with graphene instead of conventional metal as interdigital electrodes are designed and fabricated. The RT performance for both NO₂ and NH₃ detection can be greatly enhanced by UV light illumination which is closely related to the thickness of Cu layers in which the device with 8 nm thickness (8 nm Cu/Gr device) exhibits the best performances. Analysis of XPS reveals that Cu is partly oxidized to Cu⁺ and Cu²⁺ for 6 nm with extra Cu^δ+ (1 < δ < 2) for 8 and 10 nm. The contents and distributions of copper oxides and copper in Cu layers influence the catalytic effects and the heterojunction barrier and thus the performances. The RT responses of −30.9% and −8.1% for 5 and 0.3 ppm NO₂, and of +29.1% and +5.9% for 105 and 10 ppm NH₃ are achieved for the 8 nm Cu/Gr device, respectively. The limits of detection (LODs) for NO₂ and NH₃ are 12 ppb and 17 ppb, respectively. The sensing mechanisms are discussed in terms of density functional theory (DFT) calculations and energy band diagrams. The study demonstrates an effective solution of improving the device performance by modifying the device configuration and incorporating combined oxides naturally oxidized, which provides the novel design alternatives for high performance sensors.