Partial Exchange between Inorganic and Organic Anions in MgAl Layered Double Hydroxide Nanosheets for Humidity Sensing

Layered double hydroxides (LDHs) are a class of layered materials that show intercalant-dependent properties. Precise control over the type and concentration of intercalants in LDHs is important to tailor their properties and functions. Herein, through an anion exchange process, we partially replaced the NO₃^2-ions in MgAl-LDH nanosheets with terephthalic acid (BDC) ions and systematically studied their ionic conduction and humidity sensing properties at different NO₃^2-/BDC ratios. Our results show that moderate BDC intercalation boosted the humid sensing response to 3 orders of magnitude compared to the original MgAl-LDH intercalated with NO₃^2-ions. This is because the BDC molecules could help improve the hydrophilicity of the LDH and thus the ability to absorb water molecules. However, too high a BDC concentration resulted in a considerable reduction in the ionic conduction of the LDH because this might block ionic conductive paths. As a demonstration, the MgAl-LDH-NO₃^2--BDC hybrid was fabricated into a flexible humidity sensor, which showed potential as a wearable breath sensor.