In Situ Spectroscopy of Calcium Fluoride Anchored Metal–Organic Framework Thin Films during Gas Sorption

Abstract

Surface-mounted metal-organic frameworks (SUR-MOFs) show promising behavior for am anifold of applications.AsMOF thin films are often unsuitable for conventional characterization techniques,understanding their advantageous properties over their bulk counterparts presents ag reat analytical challenge.Inthis work, we demonstrate that MOFs can be grown on calcium fluoride (CaF 2)w indows after proper functionalization. As CaF 2 is optically (in the IR and UV/Vis range of the spectrum) transparent, this makes it possible to study SURMOFs using conventional spectroscopic tools typically used during catalysis or gas sorption. Hence,wehave measured HKUST-1 during the adsorption of CO and NO.We show that no copper oxide impurities are observed and also confirm that SURMOFs grown by al ayer-by-layer (LbL) approach possess Cu + species in paddlewheel confirmation, but 1.9 times less than in bulk HKUST-1. The developed methodology paves the way for studying the interaction of any adsorbed gases with thin films,n ot limited to MOFs,l ow temperatures,o rt hese specific probe molecules,p ushing the boundaries of our current understanding of functional porous materials.

Power of Infrared and Raman Spectroscopies to Characterize Metal-Organic Frameworks and Investigate Their Interaction with Guest Molecules

“…When adsorption of NO was performed at low temperature, in addition to the band at 1888 cm –1 , another band was registered at 1876 cm –1 and attributed to NO adsorbed on another kind of Cu 2+ sites . Also bands at 1737 cm –1 (mononitrosyls of Cu + ), 1860 cm –1 ( cis -dinitrosyls of Cu + ) and 1770–1760 cm –1 ( trans -dinitrosyls of Cu + ) were reported.…”

“…When adsorption of NO was performed at low temperature, in addition to the band at 1888 cm −1 , another band was registered at 1876 cm −1 and attributed to NO adsorbed on another kind of Cu 2+ sites. 581 Also bands at 1737 cm −1 (mononitrosyls of Cu + ), 1860 cm −1 (cis-dinitrosyls of Cu + ) and 1770−1760 cm −1 (trans-dinitrosyls of Cu + ) were reported. In another work 14 the same team proposed a slightly different assignment: bands, at 1857 and 1763 cm −1 were attributed to the ν s and ν as modes, respectively, of Cu + (NO) 2 dinitrosyls.…”

“…When the CO adsorption experiments are performed at low temperature, an intense band around 2178 cm –1 is reported. ,,,,,,,− ,− , Consensually, the band is assigned to Cu 2+ –CO species formed with the structural copper ions. ,,,,− ,, Minor contribution of π-back donation has also been considered . At high coverage, the band is red shifted to 2179–2170 cm –1 . , This phenomenon is interpreted as formation of dicarbonylic species. ,, However, it was latter suggested that the shift is not due to adsorption of a second CO molecule to the same Cu 2+ site; instead the shift arises as a result of the occupation of the second site in the Cu 2+ pairs by CO (pseudo dicarbonyls) .…”

“…When the CO adsorption experiments are performed at low temperature, an intense band around 2178 cm −1 is reported. 70,284,409,465,522,541,570,[572][573][574][576][577][578]581 Consensually, the band is assigned to Cu 2+ −CO species formed with the structural copper ions. 70,284,465,541,[572][573][574]576,580 Minor contribution of π-back donation has also been considered.…”