Multilayer (Cr/Au) n /PAA Nanoporous Membrane for Ultrafast, Selective CO₂ Detection at Room Temperature

Abstract

Achieving high-performance CO₂ sensing at room temperature remains a significant challenge due to persistent trade-offs between sensitivity, selectivity, and energy efficiency. In this study, we present a robust and scalable gas sensing platform based on nanoporous anodic aluminum oxide (PAA) membranes functionalized with sequentially deposited nanolayers of Cr and Au. Three distinct multilayer sensor structures, (Cr/Au) n /PAA with n = 1, 2, and 3, were developed using a modified fabrication process that combined two-step anodization, pore widening, and RF sputtering. These configurations were thoroughly characterized through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), UV-Vis spectroscopy, and electrical response analysis under CO₂ exposure. Among them, the (Cr/Au) 3 /PAA sensor exhibited the most pronounced performance, achieving a high relative response of 160.69% at 4440 ppm CO₂, with rapid response and recovery times ranging from 60.4-95.95 s and 24.42-99.04 s, respectively. These enhancements are attributed to the optimized multilayer architecture, which increases surface roughness, active site density, and charge transfer efficiency. The sensing mechanism is governed by reversible interactions between CO₂ molecules and pre-adsorbed oxygen species, modulating the carrier concentration within the metallic layers. Long-term stability tests over 30 days confirmed excellent repeatability, while selectivity measurements revealed a strong preference for CO₂ over interfering gases such as C₂H₂ and H₂. This work demonstrates a promising route toward low-power, high-fidelity gas sensors and establishes a versatile framework for tailoring gas selectivity and sensitivity through nanoscale interface engineering.