Monodisperse Surface-Charge-Controlled Black Nanoparticles for Near-Infrared Shielding

A simple and quick preparation of black polymer nanoparticles by the microwave-assisted polymerization of 1,5-dihydroxynaphthalene and 1,3,5-trimethyl-1,3,5-triazinane under high temperature and pressure was demonstrated. The average diameter of the nanoparticles ranged from 20 to 700 nm, depending on the preparation conditions such as reaction solvent, monomer concentrations, and monomer ratio in the seed mixture. The surface charge of the nanoparticles was also determined by the monomer ratio used in the seed mixture. The nanoparticles could be dispersed in polar solvents such as water, methanol, and ethanol, probably because the phenolic hydroxyl groups and primary/secondary amine groups remained on the surface of the nanoparticles. The reflectance spectroscopic measurements from 200 to 2200 nm showed that the nanoparticles expressed selective reflectivity of NIR wavelengths. The absorption and reflectance properties could be tuned by microwave-assisted wet calcination of the nanoparticles. The particles were initially dark green, but when the particles were heated at 250 °C for 10 min in ethylene glycol, they absorbed light in the UV–visible region (reflectance was less than 3% from 200 to 750 nm), indicating the color of the particles had become perfectly black. Reflection in the NIR region from 1250 to 2150 nm, however, remained more than 50% after calcination. The calcinated nanoparticles could be dispersed in water, and the surface charge was positive at lower pH and negative at higher pH. The isoelectric point shifted slightly from 5.3 to 4.4 after wet calcination at 250 °C. The amount of nitrogen in the nanoparticles decreased remarkably after calcination; therefore, the phenolic hydroxyl groups must have remained on the surface of the calcinated nanoparticles preferentially to the amine groups. These black nanoparticles with selective reflectance in the NIR region could be applied to black heat-shielding materials such as paints for buildings and automobiles.