Facile synthesis of surface-functionalized C₆₀ micro-rods and their humidity sensing potential

Here, we report a simple hydrothermal method for the fabrication of rod-shaped C₆₀ solvated crystals with hydrophilic group-modified surfaces using NaOH as the hydroxylation reagent. The NaOH content in the hydrothermal solvent plays a vital role in the degree of functional group modification and surface morphology by affecting the reaction process and decomposition-recrystallization process of fullerenols on the sample surface under hydrothermal conditions. Strikingly, the modified samples exhibit excellent hydrophilic properties and can serve as a moisture-sensitive layer for capacitive humidity sensors. These sensors show remarkable sensitivity even in low humidity. This is attributed to the formation of a continuous water layer on the fullerenol-modified surface, which facilitates proton conduction and ion diffusion. The unique porous structure of the stacked C₆₀ rods further enhances their interaction with water molecules in high humidity. The highest sensitivity achieved for the modified C₆₀ rod humidity sensor is 39 439 pF/% RH (25 °C in air), which is more than 400 times that of the unmodified sample. Our work provides a simple route to fabricate the hydrophilic C₆₀ micro-rods, which show great potential as effective materials for humidity sensor applications.