Aqueous foams stabilized solely by CoOOH nanoparticles and the resulting construction of hierarchically hollow structure

First aqueous non-silica foams, solely stabilized by CoOOH nanoparticles, were prepared by a new colloidal strategy. Highly water-soluble CoOOH nanoparticles with size of 1.5 nm were produced from the reaction between Co(OH)2 nanosheets and ammonia. The foams were formed through the hand-shaking of the high-concentration colloid formed by the redisolving of CoOOH nanoparticles in water. The stability of the foams was attributed to the reduction of surface tension induced by the hydrophilic CoOOH particles and the support of particulate network in the void between bubbles. These two factors also provided resistance for the bubble collapse during the evaporation of solvent, resulting in the keeping of the original foam morphology after drying and the formation of a hierarchically hollow structure of CoOOH. Herein, we proved that the highly hydrophilic nanoparticles also can stabilize liquid foams. This conclusion is different from the classic theory on foams, which may provide new insight for foams and also a pathway for the preparation of inorganic hollow structure.