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Precisely Oriented Porous Crystalline Films and Patterns

Periodic Reporting for period 2 - POPCRYSTAL (Precisely Oriented Porous Crystalline Films and Patterns)

Reporting period: 2019-11-01 to 2021-04-30

Metal-Organic Frameworks (MOFs) are porous crystalline solids composed of organic linkers and inorganic nodes. Both linkers and nodes are chemically mutable and can be judiciously chosen and assembled into porous crystals to afford multi-functional materials, either by themselves or via infiltration of functional guests. These properties make MOFs highly desirable for use in technological applications, such as electronics, optics, sensing and separation. However their positioning on a substrate and their pore alignment in a film remains a barrier for their application to devices.
The POPCRYSTAL project will investigate ceramic-to-MOF conversions for the fabrication of fully oriented nanoporous films and patterns. Crystalline ceramics will be used as substrates for the controlled growth of oriented, porous, crystalline films and patterns. These novel platform materials will be employed for the fabrication of a functional device.
Property-directional dependant (anisotropic) materials are commonly observed in nature and underpin essential structural and biological functions (e.g. wood, bones). Mankind has adopted this concept as the basis of important technologies such as microelectronic, visualization and lightning technologies, etc. Therefore, the discovery of a protocol that can controlled structure-property relationship are of significant technological interest.
During the 5 years of the project, the research team will investigate protocols for the fabrication of different oriented MOFs films and patterns. Their functional properties will be studied and tested.
Activities during the reported period:
1) Building up a team of young researchers.
2) Evaluation, selection, purchase, and setting up equipment for the planned research activities. This included the purchase of an XRD, an AFM, and a Raman microscope. Improvements were made to existing equipment, (e.g. Dip Coater).
3) Since the project start, progress has been made on all WPs of the POPCRYSTAL project. The following points highlight the main studies and results so far.

A new protocol was developed for the automatic deposition of aligned Cu(OH)2 nanobelts (NBs). This new method produces highly oriented samples with increased reproducibility. A related manuscript was finalised and submitted to Adv.Mat.Interf.
Investigation of the key compositional variables for an optimized conversion from Cu(OH)2 NBs to aligned Cu-BDC MOF crystals (BDC=1,4-benzendicarboxylic acid) with respect to the type of organic solvent, the amount of water as co-solvent, the ligand concentration and temperature) is in progress. This activity was planned to be performed at the synchrotron (in situ GI-SAXS) but the COVID-19 and other issues affected the plan. With respect to the original protocol, preliminary improved conditions were found [Chem.Sci. 2020].

Investigation of different MOF ligands:
We have studied the effect of the carboxilic-acid functionalized linkers (L) for the preparation of oriented 2D Cu2(L)2 and 3D Cu2(L)2DABCO MOF films from Cu(OH)2 NBs. The experimental research has successfully extended from the original use of BDC to heteroepitaxially aligned MOFs based on other ligands, such as 1,4-naphthalenedicarboxylic acid; 2,6-naphthalenedicarboxylate; biphenyl-4,4’-dicarboxylic acid (BPDC). [Chem. Sci. 2020]

Investigation of diverse ceramic feedstock materials useful for the fabrication of oriented MOF films.
•Cu2(CO3)H2O was converted into HKUST-1 (MOF) [R. Riccò et al. Chem.Mater. 2018]
•CuO was transformed into Cu-CDC and Cu-BDC MOFs [T. Stassin et al. Chem.Comm. 2019]
•ZnO membranes were converted into ZIF-8 MOFs [R. Bo et al., Adv.Sci. 2020]. ZIF-8 was also produced in water in presence of proteins (ongoing research).
•MOF-on-MOF systems (e.g. Cu-BPDC on Cu-BDC) were prepared from Cu(OH)2 [K. Ikigaki et al. Angew.Chem. 2019]

Since the project has started, we have fabricated 3 MOF-based devices:
1. An optical switch that allows for polarization-dependent optical responses. [Chem.Sci. 2020]
2. A Li-S battery that used a 23 µm thick MOF separator as a perm-selective membrane. [Adv Sci. 2020]
3. A vapor sensor based on a MOF micro-pattern. [M. Tu et al., Nat.Mater. 2020]

Details and additional information are provided in the Scientific Report
WP1: Understanding – First objective: development of an automatic procedure for the deposition of reproducible films of aligned Cu(OH)2 nanobelts has been developed. With respect to the original manual transfer of the Cu(OH)2 precursor from a solution to a substrate, the automated process allows for the deposition of reproducible and highly aligned NBs films irrespective of the operator. This improvement is a key processing step since we have determined that the alignment of the MOF crystals in the films depends critically on the alignment of the precursor Cu(OH)2 NBs. The experimental study was completed, and a related manuscript was prepared and submitted to Adv.Mat.Interf. Sharing this protocol will help with the progress of this research field. - Second objective: understanding of the key compositional variables for the optimization of the conversion from Cu(OH)2 NBs to aligned Cu2(BDC)2 MOF crystals. We have performed preliminary in-situ studies of the conversion from Cu(OH)2 nanobelts to Cu2(BDC)2 with the AFM and SAXS to shed light on the underlying mechanisms that afford oriented MOF films. We believe that this study will help with the understanding of the conditions needed to apply the heteroepitaxial growth of MOFs to other systems.
WP2: Extension – We have been able to extend this ceramic-to-MOF concept to other materials, such as Cu2(CO3)H2O into HKUST-1, CuO into Cu-CDC and Cu-BDC MOFs, and ZnO into ZIF-8 monoliths. Additionally, the heteroepitaxial growth has been demonstrated for a multi-layer MOF-on-MOF approach creating complex composite materials. During this process, as described in the previous report, we found that ZnO can convert into ZIF-8 in presence of biomacromolecules. We think this is a very important discovery; indeed, the conversion occurs at room temperature without compromising the bio-activity of several tested biomacromolecules and this could be a key aspect for the fabrication of bio-hybrid devices. The investigation of different crystalline phases is ongoing to elucidate the properties of the different polymorphs of ZIF-8 when combined with biomacromolecules.
WP3: Control – A study was performed, with results suggesting that it is possible to tune on-demand the orientation of 1-D nanochannels on MOF films by changing the concentration of L and thus the acidity of the solution that induces different growth mechanisms which afford different orientation of the resulting MOF film: dissolution and precipitation growth mechanism (out of plane), or a heteroepitaxial growth mechanism (in-plane orientation).
WP4: Fabrication – The final goal will be to apply the previous results in the development of a proof-of-concept device. The functional properties of the material will be investigated and the device tested. Some progress has been made on different devices that demonstrate the capability of ceramic-based MOF films and membranes for optical, sensor, and separation devices (see previous point 1.2). Additional output is expected by combining the new lithographic method we published in Nature Materials 2020 (i.e. the use of photolithography with halogenated ligands) with MOF films produced via heteroepitaxial mechanisms.
Conversion from Malachite (Cu2CO3(OH)2) to the MOF HKUST-1.
1D nanochannel structures of Cu2(linker)2DABCO.
Vapour phase conversion from CuO to oriented Cu-MOFs
SEM images at each step of the heteroepitaxial growth of multilayer MOF-on-MOF films.
Semi-automated deposition of oriented Cu(OH)2 nanobelts.