In situ growth and optical gas adsorption performance of Zn(ii) metal–organic framework membranes at room temperature

A series of Zn(ii) metal-organic framework membranes, [Zn2(bdc)2(dpNDI)]n (where bdc = terephthalic acid and dpNDI = N,N'-di(4-pyridyl)-1,4,5,8-naphthalenediimide), were grown on a tin-diffused glass substrate using an in situ growth method at room temperature (20 °C). [Zn2(bdc)2(dpNDI)]n thin films were optically transparent and characterized by absorbance, thickness, refractive index and fluorescence techniques. After 20 h of growth, they displayed a complete honeycomb framework with a larger pore size (380 nm), uniform surface and lower refractive index. Also, it is demonstrated that [Zn2(bdc)2(dpNDI)]n thin films exhibited a greater adsorption response to meta-xylene gas with the coexistence of interferents like toluene, styrene and benzene gases, as measured by the planar optical waveguide (POWG) gas detection system under ultraviolet light (395 nm) irradiation. Moreover, when the [Zn2(bdc)2(dpNDI)]n thin film adsorbs meta-xylene gas, the intramolecular hydrogen bonds in the thin film framework were strengthened because of the accumulation of meta-xylene. At room temperature, the meta-xylene gas adsorption kinetics followed a pseudo-second-order (PSO) model, and the adsorption capacity on the unit surface was 6.46 μg cm-2. The highly selective meta-xylene adsorption performance of the [Zn2(bdc)2(dpNDI)]n thin film makes it an ideal sensing material for the recognition of harmful gases that are poisonous to the human body and the environment.