Fabrication of sponge-like Zn-dpNDI-MOF film optical waveguides for study of improved optical gas adsorbility and kinetics

A sponge-like zinc metal-organic framework film, [Zn2(TpA)2(dpNDI)]n (Zn-dpNDI-MOF, where, TpA = terephthalic acid; dpNDI = N,N′-dipyridylnaphthalenediimide), was prepared on a tin-doped glass optical waveguide (OWG) substrate using a solvothermal method. The effect of the solvothermal growth time on the morphology, optical characteristics, and gas adsorption properties of the Zn-dpNDI-MOF film was investigated. The gas adsorption sensitivity was assessed by comparing the growth of Zn-dpNDI-MOF film OWGs under in situ and UV-light illumination conditions. The results revealed that the Zn-dpNDI-MOF films were optically transparent and exhibited a sponge-like structure after 20 h of continuous growth. Additionally, they demonstrated a higher adsorption response to xylene and ethylenediamine (EDA) gases because of the intrinsic electron deficiencies of the framework and the higher molar refractive indices and dipoles of xylene and EDA compared to other gases. Within the xylene concentration range of 10 ppb–1000 ppm, the OWG film exhibited a fast, improved response. In the static gas adsorption process performed at 283–313 K, the xylene adsorption behaviour of the Zn-dpNDI-MOF film followed a pseudo-second-order (PSO) kinetic model. Within 0–60 min of adsorption time, the amount of xylene gas adsorbed on the unit surface was shows as high as 6.21 μg cm−2. Compared to the in situ grown film, the Zn-dpNDI-MOF film OWGs showed higher sensitivity to xylene gas owing to their higher refractive indices.