Highly Shape-Selective Synthesis, Silica Coating, Self-Assembly, and Magnetic Hydrogen Sensing of Hematite Nanoparticles

The open forced hydrolysis method and controllable silica growth based on bound water to polyvinylpyrrolidone molecules have been developed for the highly shape (including rhombohedra, semispheres, and rods) selective synthesis, self-assembly, and uniform silica coating (in the unprecedented range of 5−200 nm) of hematite nanoparticles. The open system realizes the direct short-range self-assembly of hematite semispheres in their growth process. The bound water method has been extended to coat gold nanoparticles with tunable silica shell and directly assemble the cores into one-dimensional, dimer, and trimer nanostructures during the coating process. The silica coating increases the particle stability and monodispersity even as hematite is modified into ferromagnetic Fe3O4. The hematite@silica core−shell spheres are assembled into long-range ordered structures with considerable photonic bandgap for the first time due to their high monodispersity. By exploiting the hematite antiferromagnetism caused by the...