Hematite α-Fe ₂ O ₃ nanorods and laser-induced graphene for sustainable chemiresistive sensing of 1-butanol at room temperature

Volatile organic compounds (VOCs) present in workplace and domestic settings present risks to human health, <i>e.g.</i>, 1-butanol concentrations >100 ppm can cause central nervous system depression and respiratory/skin irritation. Traditional chemiresistive metal-oxide gas sensor platforms frequently rely on noble metal contact electrodes (Au,Pt) and high-temperature operation (200-600 °C), increasing cost and environmental footprint impacts. Consequently, there is an urgent need for sustainable and affordable materials for chemiresistive gas sensors that can operate at room temperature. Our approach combines hematite (α-Fe₂O₃) nanorods, synthesized <i>via</i> a low-impact co-precipitation method, with 3D porous laser-induced graphene (LIG) electrodes for room-temperature chemiresistive sensing of VOCs. Relative humidity (RH) plays a key role in charge transport through these LIG-contacted α-Fe₂O₃ nanorod assemblies, with baseline device resistance <i>R</i> ₀ decreasing quasi-exponentially with increasing humidity. Device resistance increases upon exposure to 1-butanol, with resistance response Δ<i>R</i>/<i>R</i> ₀ ∼ 185 ± 25% (<i>n</i> = 8) to 100 ppm 1-butanol at ∼55% RH, with 50-300 ppm linear range and limit of detection, LOD = 36 ± 11 ppm. Device response, Δ<i>R</i>/<i>R</i> ₀, increases with increasing relative humidity from ∼20-60% RH, highlighting the key role of the hydrated α-Fe₂O₃ surfaces on the sensing mechanism. Measured response values represent a ∼10-fold improvement in sensitivity <i>vs.</i> reported room-temperature performance for devices based on α-Fe₂O₃ nanocubes. Further, the estimated cumulative energy demand (CED) for the α-Fe₂O₃ nanorod active nanomaterial is ∼1000 times lower than reported data for devices with comparable sensitivity, which employed α-Fe₂O₃ nanocubes and reduced graphene oxide hybrids. Estimated CED values for the 3-D porous LIG electrodes also show orders of magnitude reduction <i>vs.</i> values for conventional metal contact electrodes. Finally, we show that the response time constants of these LIG-contacted α-Fe₂O₃ nanorod devices can be used together with chemiresistive Δ<i>R</i>/<i>R</i> ₀ response for effective discrimination of 1-butanol <i>vs.</i> other short-chain alcohols (methanol, ethanol, 2-propanol) and non-polar VOCs (acetone, toluene, hexane).