Fabrication and characterization of zeolitic imidazolate framework-90 (ZIF-90) absorbers for carbon dioxide sensing at near infrared band

Abstract

In this work, layers of crystalline zeolites formed by tetrahedrally-coordinated Zn ions bridged by imidazolate (ZIF-90) were deposited in order to investigate the desorption and absorption of CO2 and apply these layers as pre-concentrators for CO2 detection. For the deposition of ZIF-90 layers by casting, it was proposed an alternative chemical solution which employs ethanol as solvent instead of methanol, as reported in the literature, to provide a less toxic process to humans and allows one applications of CO2 storage. Several layers were deposited using a solution prepared from a mixture of zinc nitrate (4.3g) and 2-methylimidazole (9.7g) with several dilutions in ethanol to vary the pH in the range of 7.2 to 8.2. As a result, repetitive layers of approximately 7.5μm in thickness were deposited on the silicon wafers by casting. After annealing these layers at a temperature of 150 °C for 48h in ultra-pure nitrogen, nanocrystals with size distributions in the range of 5 to 400nm with a ZIF-90 crystal structure were achieved only for pH next to 7.2. From infrared (IR) measurements of the ZIF-90 layers, it was observed a band located at 2337cm-1 that increases with the increase of the CO2 pressure and with the exposure time to this pressure. In addition to the band at 2337cm-1, it was observed a second band at 2360 cm-1 indicating two different responses: (i) the band at 2337cm-1 is related to a substantial quantity of the CO2 molecules absorbed into the layer along the contours of the nanocrystals or within the crystal structure and (ii) the band at 2360cm-1 is related to the portion of CO2 molecules adsorbed on the surface. Also, if the ZIF-90 layer is exposed to CO2 at atmospheric pressure for at least 2h, a 10ppm sensitivity to CO2 is achieved considering the minimum absorbance as being 0.001 and the loading time at the atmospheric pressure as at least 20min.