Self-assembled double-shelled ferrihydrite hollow spheres with a tunable aperture.

Novel double-shelled hierarchical ferrihydrite hollow spheres (Fe(10)O(14)(OH)(2).4H(2)O) were successfully synthesized on a large scale by using a facile medicine-inspired solution-phase approach. Sodium nitroprusside (SNP), an ordinary and inexpensive medicine for expanding blood vessels, served as both a ferric source and an in situ formed gas-bubble template with the presence of sodium dihydrogen phosphate as a pH regulator, coordinator, and stabilizer. A twice-gas-bubble template model has been proposed for the formation of the double-shelled hollow spheres to take advantage of the dissociation and hydrolyzation of the two kinds of ligands in the SNP precursor. The size of the double-shelled ferrihydrite hollow spheres can be tuned by varying the experimental parameters. As-obtained ferrihydrite is highly sensitive to ethanol gas, which indicates potential applications in the field of sensing devices.