Room-Temperature Resistive CO₂ Sensing with Ultrafast Recovery via NP-PSi/CuO/rGO Trilayer Heterojunction

<title>Abstract</title> <p>In this work, a trilayer resistive CO₂ gas sensor was developed by combining an NP-type porous silicon (NP-PSi) substrate with a sol-gel-derived copper oxide (CuO) nanofilm and a drop-cast reduced graphene oxide (rGO) overlayer, and its sensing performance was systematically evaluated at room temperature under an applied voltage of 2 V. Scanning electron microscopy confirmed that CuO uniformly infiltrated the pore walls of the porous silicon substrate and that the rGO sheets formed intimate interfacial contact across the composite surface. Compared to the bare NP-PSi reference (recovery time: 47–82 s), incorporation of CuO shortened the recovery time to 5–30 s through charge transfer facilitated by the p-CuO/n-PSi heterojunction; subsequent addition of rGO further reduced recovery time to 3–7 s and the baseline resistance to 420–560 kΩ, attributed to the high-speed electron transport network provided by the rGO overlayer. The NP-PSi/CuO/rGO composite sensor achieved a sensing response of approximately 26.5% at 5.66 vol% CO₂. Compared with room-temperature CO₂ sensors reported in the literature under equivalent measurement definitions, the present work demonstrates orders-of-magnitude improvement in recovery kinetics, confirming the viability of this trilayer architecture for low-power resistive CO₂ gas sensing applications. Keywords: NP-type porous silicon NP-PSi, CO₂ gas sensor, Sol–gel method, Reduced Graphene Oxide (rGO), CuO</p>