Stability of Monolithic MOF Thin Films in Acidic and Alkaline Aqueous Media
Authors: Evgenia Bogdanova
Alaa Mohamed
Tawheed Hashem
Christof Wöll
Elvia P Valadez Sanchez
Matthias Schwotzer
Anna Ugodchikova
Mohamed H Alkordi
Journal: Membranes (2021)
Discipline: CHEMISTRY
Keywords: MOF; SURMOF; layer-by-layer; liquid phase epitaxy; pH stability.
Affiliations: Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany., School of Nuclear Science & Engineering, National Research Tomsk Polytechnic University, Lenina Avenue 30, 634050 Tomsk, Russia., Center for Materials Science, Zewail City of Science and Technology, October Gardens, 6th of October, 12578 Giza, Egypt.

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. 

Retracted: No

PubPeer comments:
 I am a bit puzzled by: **Figure 3. XRD patterns and SEM images of UiO-66-NH2 SURMOF films immersed in a: (A) pH 2 solution, (B) pH 7 solution, (C) pH 10 solution, and (D) pH 11 solution.** ![file] The images at initial time (t = 0) in pH 2 and 11 appear identical, despite the different pH. It could be that the same image was used since it is the initial conditions, but that opens two questions: a. The scale bars are different (600 u03bcm vs 600 nm), maybe a typo? b. Why are there different images at pH 7 and 10 for t = 0? Could the authors comment on this? , 
 #1 Dear Dr. Coudert, Thank you for your comment. Yes, it is right, the images at the initial time (t=0) are the same. As I explained in the methodology section that at the beginning we used one big sample (for SEM and XRD), then we split it into two pieces and study each piece at different ph values (ph2 and 11). Therefore, you can easily see that both SEM and the XRD spectrum are the same for both pH2 and pH11. Regarding the scale bars, it should be 600 nm for all sem images in this figure. u00b5m is a totally wrong scale here. Thank you again for your comments and please feel free to contact me if you have any further questions. Regards, Tawheed Hashem , 
 Dear Dr. Hashem, Thank you for your response. I was wondering: if the sample at t = 0 is the same, why are images at pH = 7 and pH = 10 different? , 
 #3 Dear Dr. Coudert, generally, in our lab, we are using two different sizes of gold coat substrate, 1 cm X 2 cm and 1 cm X 1 cm. In the case of using the big-size samples such as in Figure 3 a and 3d, and Figure 2a and 2d, we split the sample into 2 pieces. In the case of using small 1 cm X 1 cm samples, we use it as it is without splitting as in Figures 3 b and 3c. , 
 Dear Dr. Hashem, Thank you, that is very clear. ,