Carbon dioxide sensing at near infrared using zeolitic imidazolate framework-8 (ZIF-8) absorbers

In this work, it is presented the carbon dioxide sensing at near infrared using zeolitic imidazolate framework-8 (ZIF-8) absorbers. ZIF-8 was synthesized by dissolving 4.3 g of zinc nitrate hexahydrate and 9.7 g of 2-methylimidazole in 800 mL of ethanol. The solution was stirred for 60 min at room temperature and 10mL of the fresh solution was dripped and spread over the rough side of a Si wafer to be dryed at the room temperature during 10 min and annealed at 150°C during 5 h in ultrapure N₂. As a result, 11.7-μm thick films were obtained on the Si-p wafers. From FTIR spectroscopy, it was worthy of note that the signal of the asymmetric stretching mode at 2337 cm^-1 increases with the increase of the CO₂ pressure and with the time interval in which the pressure is kept. Besides the component at 2337 cm^-1, a second component was also observed at 2360 cm^-1, indicating two different mechanisms of CO₂ adsorption. The component at 2337 cm^-1 was interpreted as a substantial portion of CO₂ molecules inserted into the ZIF-8 film at the contours of the nanocrystals or inside of the framework and the component at 2360 cm^-1 was understood as a portion of CO₂ molecules adsorbed directly on the surface of the ZIF-8 film. In addition, it was observed a sensitivity of 100 ppm for CO₂ if the ZIF-8 loading time at the atmospheric pressure is at least twenty minutes in order to achieve a measurable absorbance inferior limit of 0.01.