Two-Dimensional Iron Oxyhydroxide for Visible-Light-Driven NO₂ Sensing

Nitrogen dioxide (NO 2 ), even at parts-per-billion (ppb) concentrations, can greatly threaten human health. Two-dimensional (2D) nonlayered metal oxides demonstrate the strong potential of detecting ppb-level NO 2 gas at low temperatures or room temperature with unique selectivity, owing to their physisorption behaviors of NO 2 gas molecules as well as abundant active sites based on dangling bonds and defects. In this work, we reveal the ppb-level NO 2 detection capability of 2D nonlayered FeOOH as a derivative of 2D metal oxides, which are synthesized from a facile water-dissolvable template with abundant surface hydroxide groups acting as active sites of NO 2 adsorption. 2D FeOOH exhibits large lateral dimensions of ∼1–1.5 μm with thicknesses of ∼4–5 nm. An optical energy band gap of 2.2 eV is found, which is suitable for visible-light-driven optoelectronic NO 2 gas sensing. An impressive response factor of 36 for 37.5 ppb NO 2 is achieved at 160 °C under blue light excitation with full reversibility, excellent long-term stability in months, unique selectivity, and a detection limit of 0.38 ppb, which is significantly improved over those of ppb-level NO 2 sensors based on metal oxides. This work reveals 2D nonlayered metal oxyhydroxides as a group of suitable candidates for future high-performance NO 2 gas sensors.