Water Harvesting at the Single-Crystal Level.

Area of Science:

• Materials Science
• Chemical Engineering
• Spectroscopy

Background:

• Metal-organic frameworks (MOFs) are promising for atmospheric water harvesting due to their water uptake capabilities.
• Current bulk-level assessments do not reflect the true water harvesting potential of pristine MOFs at the single-crystal level.
• Fluidized bed reactors in devices utilize MOFs' water harvesting at the single-crystal scale.

Purpose of the Study:

• To introduce a novel Raman spectroscopy-based methodology for sub-micrometer resolution water adsorption isotherm and kinetic analysis.
• To investigate water adsorption and desorption in isolated MOF-801 single crystals.
• To differentiate between intra- and inter-particle contributions to water harvesting performance in MOF-801.

Main Methods:

• Utilized spontaneous Raman spectroscopy for in situ analysis of isolated MOF-801 particles.
• Acquired water adsorption-desorption isotherms and kinetic curves at the single-crystal level.
• Employed correlative imaging (four-wave mixing and coherent anti-Stokes Raman scattering) to localize water within MOF-801.

Main Results:

• Discovered a nearly 20-fold faster water uptake in undisturbed MOF-801 single crystals compared to bulk measurements.
• Identified inter-particle condensation as a significant factor contributing to discrepancies between bulk and single-crystal performance.
• Determined an upper water harvesting limit for MOF-801 of approximately 91.9 L/kg_MOF/day.

Conclusions:

• The novel Raman spectroscopy technique provides crucial insights into single-crystal MOF behavior for water harvesting.
• Single-crystal analysis is essential for accurately assessing MOF water harvesting potential, revealing limitations of bulk measurements.
• MOF-801 exhibits significant water harvesting capacity, with its performance influenced by inter-particle condensation effects.