Optimum shell thickness and underlying sensing mechanism in p-n CuO-ZnO core-shell nanowires

We report on the sensing properties of p-n CuO-ZnO core-shell nanowires (C-S NWs) for reducing gases. The C-S NWs were synthesized by a two-step process: first, core p-CuO nanowires were prepared by thermal oxidation on patterned interdigital electrodes, forming a network; and second, the n-ZnO shell layers were subsequently deposited by atomic layer deposition (ALD). The ZnO shell thickness was controlled by changing the number of ALD cycles between 5 and 110 nm. The sensing properties of the C-S NWs were investigated for the typical reducing gases CO and C6H6. At 35 nm shell thickness, the C-S NWs showed the highest CO and C6H6 sensing ability, superior to that of pristine p-CuO nanowires. The sensing mechanism of the p-n C-S NWs is based on the radial modulation of an electron-depletion region in the ZnO shell layer, which occurs during the interaction between the reducing gas molecules and the adsorbed oxygen species and causes a pronounced change in resistance. This demonstrates that the radial modulation of the conducting channel is a universal sensing principle operating in p-n type C-S structures.