Epitaxy Growth of Hofmann MOF on Monolayer MXene for ppb‐Level Room‐Temperature NOx Sensing

Nitrogen oxides (NO x ) monitoring requires sensor platforms that deliver ppb‐level sensitivity, ambient‐temperature operation, and molecular discrimination. However, rapid and selective NO x detection at room temperature remains elusive. Although 2D transition metal carbides/nitrides (MXenes) show promise in chemiresistive sensing, their susceptibility to ambient degradation and poor selectivity limit practical deployment. Here, a room‐temperature NO x sensor is developed by epitaxially growing Hofmann‐type metal–organic frameworks (Ni‐pyz) on oxygen‐terminated monolayer large‐size MXene (Ti 3 C 2 T x ). The Ni‐pyz/Ti 3 C 2 T x heterostructure exhibits enhanced sensitivity (4.8 and 5.5 times compared to Ti 3 C 2 T x ) and low detection limits (8.8 and 6.9 ppb) for NO and NO 2 , along with high selectivity and good stability (>85% response after 8 weeks). The enhanced sensitivity results from electronic coupling between the Ti 3 C 2 T x and Ni(CN) 4 , promoting charge transfer with NO x . The improved selectivity arises from the pore confinement effect of Ni‐pyz and the specific adsorption of NO x through Lewis acid‐base interactions with open Ni sites. Importantly, the dynamic response of Ni‐pyz/Ti 3 C 2 T x closely mirrors that of a trace‐level chemiluminescence NO x analyzer at low NO x concentrations, enabling high‐temporal‐resolution, long‐term atmospheric monitoring. This functional strategy overcomes the insufficiency of MXene‐based gas sensors, paving the way for the development of high‐performance sensors.