Abstract
In the context of thin film nanotechnologies, metal-organic frameworks (MOFs) are currently intensively explored in the context of both, novel applications and as alternatives to existing materials. When it comes to applications under relatively harsh conditions, in several cases it has been noticed that the stability of MOF thin films deviates from the corresponding standard, powdery form of MOFs. Here, we subjected SURMOFs, surface-anchored MOF thin films, fabricated using layer-by layer methods, to a thorough characterization after exposure to different harsh aqueous environments. The stability of three prototypal SURMOFs, HKUST-1, ZIF-8, and UiO-66-NH2 was systematically investigated in acidic, neutral, and basic environments using X-ray diffraction and electron microscopy. While HKUST-1 films were rather unstable in aqueous media, ZIF-8 SURMOFs were preserved in alkaline environments when exposed for short periods of time, but in apparent contrast to results reported in the literature for the corresponding bulk powders- not stable in neutral and acidic environments. UiO-66-NH2 SURMOFs were found to be stable over a large window of pH values.
Highlights
Over the last few years, there has been an increasing demand for sustainable alternatives for current and new products and processes
While HKUST-1 films were rather unstable in aqueous media, ZIF-8 supported metal-organic frameworks (SURMOFs) were preserved in alkaline environments when exposed for short periods of time, but in apparent contrast to results reported in the literature for the corresponding bulk powders- not stable in neutral and acidic environments
Müller et al [38] pointed out the possibility of healing defects in HKUST-1 SURMOF films resulting from exposure to water vapor by exposing the samples to its synthesis solvent, ethanol
Summary
Over the last few years, there has been an increasing demand for sustainable alternatives for current and new products and processes. For a growing number of MOF materials, appropriate layer by layer (LBL) synthesis conditions yielding high-quality SURMOFs have been developed, including HKUST-1 [14], ZIF-8 [15], and UiO-66-NH2 [16] Such MOF thin films have been characterized using a wide range of techniques and have been successfully used to fabricate devices for diverse applications delivering high-performance results [13]. The substrate was placed on a holder and alternatively sprayed using the following 6 step, 2 min approach: (1) Spray for 15 s using a 1 mM ethanolic solution of Cu(OAc) (Merck, Darmstadt, Germany), (2) Wait for 35 s, (3) Rinse for 5 s using pure EtOH, (4) Spray for 25 s using a 0.2 mM ethanolic solution of 1,3,5-benzenetricarboxylic acid (Alfa Aesar, Kandel, Germany), (5) Wait for 35 s, and (6) Rinse again with pure EtOH for 5 s
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