Effect of a Ni-Substituted Cu-MOF Precursor toward Efficient NO₂ Gas Detection: A Comprehensive Comparative Gas-Sensing Study of MOF-Derived and Traditionally Synthesized CuO/NiO-Based Nanocomposites

Abstract

With the progression of research on gas sensors, numerous studies are being carried out on the tuning of metal−organic framework (MOF)-derived semiconductor metal oxides (SMOs) for gas-sensing studies in lieu of traditionally synthesized SMOs to attain the desired result at lower temperatures and enhance the sensor response. Still, no comparative gassensing study between traditionally synthesized SMOs and MOF-derived SMOs has been conducted to understand the reasons behind such a higher activity. To gain insights into the enhanced activity and find the reasons behind the improved sensor response, a comparative gas-sensing study was carried out using octahedral Cu-MOF as a template to fabricate an MOF-derived CuO/ NiO (CuO/NiOM) heterostructure and traditionally synthesized CuO/NiO (CuO/NiOD) heterostructure via the combustion route. More importantly, our study revealed that the substitution of Ni 2+ ions into the MOF template did not destroy the porous hierarchical framework of Cu-MOF either during the calcination process or when the Ni 2+ ions were added into the precursor medium. A higher surface area (35 m 2 /g) and porosity obtained during the process would have provided sufficient permeability channels for the sorption of NO 2 molecules, leading to the formation of the maximum concentration of active sites. Our results revealed that MOF-template-derived CuO/NiOM showed a comparatively higher response of S% = 13.9%, with response and recovery times of 18 and 29 s, respectively, at a temperature of 110 °C compared to traditionally synthesized CuO/NiOD. Further, to attain efficient room-temperature activity with a higher sensor response and lower response and recovery times, a p−n heterojunction was formed at their interface by modifying n-type thermally reduced graphene oxide (rGO) with CuO/NiOM. The rGO-CuO/NiOM gas sensor showed a p-type behavior with a comparatively higher response of 23.9% toward 60 ppm of NO 2 at room temperature, with the lowest response and recovery times of 10 and 15 s, respectively. In addition, the sensor also possessed ultralow detection ability up to 5 ppm of NO 2 with fast response and recovery.