Ultrasonic synthesis of 2D doped metal hydroxides from liquid metals for rare humidity sensing application

Liquid metals (LMs) have attracted significant attention in the preparation of two-dimensional (2D) materials due to their unique self-limiting oxidation reactions. However, a single LM element needs to be heated and melted before being used to prepare 2D materials, whereas the addition of other elements for alloying can significantly reduce the melting point of the LMs, as sonicated LM techniques enable the high-yield production of 2D materials. Enlightened by this, 2D Bi-doped In(OH)3 with an average thickness of 2.95 nm was successfully prepared for the first time from an eutectic bismuth-indium alloy LM using a one-step ultrasonic process, which enabled two-dimensionalization and doping of the material simultaneously. Such prepared 2D Bi-doped In(OH)3 based humidity sensors exhibited a high sensitivity (98.94% towards 90% RH) and a low hysteresis (1.21% at 44% RH) over a wide relative humidity (RH) range of 9-90% RH, realizing the rare application of metal hydroxides in the field of humidity sensing. The enhanced humidity sensing performance can be attributed to the 2D Bi-In(OH)3 structure, which offers an abundance of -OH groups and a high specific surface area. These characteristics synergistically promote the adsorption of water molecules, thereby improving the overall sensitivity of the humidity sensor. This study provides a novel approach for synthesizing 2D metal hydroxides, with the ability to extend to other low-melting metals and alloys, thereby broadening the application range of LM-based nano-functional materials.