Brief introduction of the results
Two-dimensional conjugated metal-organic frameworks (2D C-MOFs) are unique laminated crystalline coordination polymers that are both porous and conductive. However, the controlled synthesis of 2D c-mofs with laminated nanostructures with high crystallinity and custom morphology is essential for energy and electronic devices, but it is still a great challenge. Based on this,Academician Feng Xinliang of Dresden University of TechnologyProfessor Dong Renhao of Shandong University and Researcher Xu Gang of Fujian Institute of Structure of Matter, Chinese Academy of Sciences (co-corresponding author) and othersA template strategy is reported for the synthesis of layered nanostructured 2D c-mofs with high crystallinity, custom morphology, and high porosity by insulating MOFs to C-MOFs.
Kinetic studies have shown that the insulating 3D MOFs are used as the primary template, and the amorphous interlayer generated on the surface of the 3D MOFs is used as the unique secondary template, which determines the geometry of the generated 2D C-MOFs, slows down the coordination reaction kinetics, and thus improves the crystallinity. Experimental and theoretical studies have shown that the thermodynamic driving force contributes to the formation of a more stable coordination bond, resulting in the spontaneous conversion of MOFs to C-MOFs at room temperature. The authors synthesized 12 different 2D C-moFs with high crystallinity, including different structures (mono-, bi-, and multi-shell), different sizes (1D nanotubes, 2D, using different metals (Cu, Co, Ni, Zn, and bimetal Cu Zn)) and variable ligands [benzene-hexathiol (BHT), hexahydroxybenzene, 3, 6, 7, 10, 11-hexahydroxytriphenyl (HHTP)]. NSS, 3D nanocubes, nanospheres and nanoflowers, and 3D NSS assembly skeleton membranes), as well as different substrates (carbon cloth, silicon wafers, and porous nylon 66 films).
The layered nanostructures of these 2D C-Mofs contribute to their high surface area, improve mass transfer, and enhance access to active sites, making them ideal for energy storage and sensing applications. The authors found that the surface area of hollow Cu-BHT nanocubes was increased by 948 m2 g-1, approximately 41 times higher than the bulk Cu-BHT, and the specific capacity of the integrated supercapacitor device in the organic electrolyte reaches 3645 f g-1, which is 2. higher than the bulk Cu-BHT25 times. In addition, the response strength of the Cu-HBB nanoflower-based chemoresistance sensor to H2S is 25 times, one of the best room-temperature conductive polymer-based H2S sensors. The authors construct a breakthrough template strategy that enables a general transition from insulating 3D MOFs to conductive 2D MOFs, with the ability to precisely adjust the target energy and electronics functions of 2D C-MOFs.
Background:
The high conductivity of porous metal-organic frameworks (MOFs) makes them have a wide range of potential applications in the fields of supercapacitors and thermoelectric devices, which cannot be achieved by insulated MOFs. Two-dimensional conjugate MOFs (2D C-MOFs) have attracted extensive attention from researchers due to their high conductivity and customizable electronic bandgaps. Although a variety of sample formats have been realized, due to their inherent microporous and dense layered stacking structure, it is still scarce to achieve high porosity, high mass permeability, high reachable active site, and intrinsic conductivity in 2D C-MOFs, thus inhibiting mass transport properties and limiting the performance of energy and electronic devices.
It is critical to design layered nanostructured 2D c-mofs with customizable morphology, high porosity, and high crystallinity, so that conductive 2D c-mofs have excellent mass transport characteristics and can be used in the energy and electronics fields. The sacrificial template method is a promising strategy for controlling the synthesis of hierarchical nanostructures that can precisely replicate the original template without the need for an additional etching process. However, although the morphological control of 2D C-MoFs using the sacrificial template method is not well developed, the progress of the synthesis of 3D MoFs by the sacrificial template method shows that its crystallinity is low due to the fast kinetics of the coordination reaction. Therefore, the traditional sacrificial template strategy faces great challenges for the realization of hierarchical nanostructures and high-crystallinity 2D c-moFs.
**Reading guide
Based on the thermodynamic driving force, 3D insulating MOFs can be used as a sacrificial template for the synthesis of corresponding 2D C-MOFs crystals. Using density functional theory (DFT) calculations, the authors investigated the thermodynamic feasibility of converting insulating 3D MOF precursors (HKUST-1 (Cu), ZN-BTC, ZIF-8 (ZN) and ZIF-67 (CO)) into 16 2D C-MOFs. At room temperature, the Gibbs free energy change (δg) values of the calculated MoFs to C-MoFs conversion reactions were all less than zero, indicating that all transformations were thermodynamically spontaneous. For the same 2D C-MOFs, the 3D MOF precursor with a metal-nitrogen coordination bond (M-N) exhibited more negative ΔG than the 3D MOF precursor with a metal-oxygen coordination bond (M-O), indicating that the 3D MOF with M-N was more energetically conducive to conversion.
Figure 1Synthesis and characterization of 2D C-Mofs
Figure 2Topography characterization of different 2D C-Mofs
Figure 3Synthesis and characterization of hollow C-Mofs
In the absence of secondary templates, the authors performed a molecular dynamics (MD) simulation model of the reaction process of HHTP molecules with Zn2+ ions. Simulations show that in the absence of an amorphous layer, HHTP molecules and Zn2+ ions aggregate rapidly. The growing position is constantly changing, mainly depending on the mass diffusion. However, when there is an amorphous layer, the reaction rate of HHTP molecules with Zn2+ ions is significantly reduced. In addition, the growth site is located on the surface of the amorphous template. During CT transformation, reaction kinetics also play a crucial role in the morphology of the final 2D C-MOF sample. The results show that Zn2+ ions diffuse into the solution and react with the deprotonated ligand to form needle-like Zn-Hhtp nanowires in the solution.
Figure 4The interaction of HHTP molecules with Zn2+ ions
Figure 5Characterization of Co-hhtp@zn-HHTP NPS
In this paper, the charge storage behavior of hollow cubht and bulk samples was investigated in a 1 m tetrafluoroborate tetraethylammonium acetonitrile organic electrolyte using a three-electrode system. with blocky cubht (161.)92 f g-1), the specific capacity of the hollow cubht nanocubic is greatly increased by 12511% at 0364 at 1 A g-1 current density5 F g-1, which is better than the reported MOFS-based electrode materials. After 2000 times of charging and discharging, the capacity retention rate of the hollow CUBHT nanocube can still reach 8224%, indicating that the transformed CUBHT nanocube has good electrochemical cycling stability. After exposure to H2S, the resistance of nanofloral and NPS increased significantly, and returned to the initial value after air purging, indicating that the redox active sites and semiconductor properties of Cu-HBB were not damaged by H2S. The response strength of Cu-HBB nanoflowers to 100 ppm H2S was 978%, which was 2. higher than that of bulk Cu-HBB NPS5 times. When R=10%, the theoretical limit of detection (LOD) of Cu-HBB nanoflowers was 83 ppb, which was 8 times better than that of bulk Cu-HBB. The response time of Cu-HBB nanoflors to 100 ppm H2s was 582 s, 2 faster than the response time of lumpy materials1 times. At the same time, the Cu-HBB nanoflower gas sensor exhibits excellent repeatability, selectivity and long-term stability for H2S.
Figure 6Performance testing
Bibliographic information
a general synthesis of nanostructured conductive mofs from insulating mof precursors for supercapacitors and chemiresistive sensors. angew. chem. int. ed.,, doi: 10.1002/anie.202313591.