Precise Control of Water Adsorption Behavior in Solid Solutions of Amorphous Coordination Polymers

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Flexible coordination polymers (CPs) and metal-organic frameworks (MOFs) show potential for water-uptake applications due to their structural responsiveness.
• Gate-opening pressure (P$_{go}$) is a critical parameter governing rapid adsorption and structural transitions in these materials.

Purpose of the Study:

• To precisely modulate water uptake behavior in amorphous coordination polymers (Am-CPs) using a solid solution approach.
• To investigate the amorphous-to-crystalline transformation mechanism and its relation to water adsorption in Co$_{x}$M$_{(1-x)}$[Ni(CN)$_{4}$] systems.

Main Methods:

• Synthesis of amorphous solid solutions Co$_{x}$M$_{(1-x)}$[Ni(CN)$_{4}$] (M = Mn, Fe) via a solid solution approach.
• Characterization of water adsorption properties and gate-opening pressure (P$_{go}$).
• Utilized L$_{2,3}$- and K-edge X-ray absorption fine structure (XAFS) to analyze structural changes and metal coordination environments.

Main Results:

• All synthesized amorphous solid solutions exhibited abrupt water adsorption between P/P$_{0}$ = 0.5-0.9.
• The water adsorption was linked to an amorphous-to-crystalline transformation into Hofmann-type structures (Co$_{x}$M$_{(1-x)}$(H$_{2}$O)$_{2}$[Ni(CN)$_{4}$]·4H$_{2}$O).
• The type and ratio of central metals (Co, M) systematically controlled the P$_{go}$ values, as confirmed by XAFS analysis.

Conclusions:

• The solid solution approach allows for precise control over the water uptake behavior of Am-CPs.
• The amorphous-to-crystalline transformation, involving changes in coordination geometry and bond rearrangement, is key to achieving desirable water adsorption characteristics.
• This study provides insights into designing flexible CPs for targeted water-related applications.