Visible Light-Rectified Chemiresistive Sensing in Two-Dimensional Conductive Metal-Organic Frameworks

Photo-activated chemiresistive gas sensors using two-dimensional conductive metal-organic frameworks (2D c-MOFs) hold great promise for low-power, real-time environmental monitoring; however, how visible light modulates sensing responses toward gas molecules with different characteristics remains poorly understood. Here, we systematically investigate the gas-sensing behavior of four representative 2D c-MOFs toward oxidizing and reducing gases under red and blue light illumination, respectively. A pronounced light-rectified sensing behavior is observed across all four materials, with the response to NO₂ and H₂S modulated by both the electronic features of the gaseous analyte and the wavelength of the incident light. Both blue (400 nm) and red (620 nm) light generally enhance the response to the oxidizing gas of NO₂ but suppress the response to the reducing gas of H₂S, in which blue light shows a stronger effect than red light. With blue light, the NiPc-Cu-O MOF built by interconnecting nickel phthalocyanine (NiPc) with Cu ions realizes sensitive and reversible detection for NO₂ at room temperature, achieving a low limit of detection of 0.016 ppm and allowing continuous monitoring of NO₂ at the OSHA exposure limit of 5 ppm. In situ Fourier transform infrared spectroscopy reveals that light can enhance the surface oxidation reaction but alleviate the extent of reduction reaction, which modulates the charge transfer between the oxidizing/reducing analyte and MOFs to result in distinct light-rectified sensing behavior. Density functional theory calculations further confirm the gas-specific adsorption modes and associated electronic structure changes. This work, for the first time, uncovers photoinduced gas sensing behavior of 2D c-MOFs toward small gas molecules and the underlying mechanism, guiding their further advanced gas sensing applications under external irradiation.