Nonequilibrium Synthesis of Silica-Supported Magnetite Tubes and Mechanical Control of Their Magnetic Properties
2012-09-19T00:00:00Z (GMT) by
Materials synthesis far from thermodynamic equilibrium can yield hierarchical order that spans from molecular to macroscopic length scales. Here we report the nonequilibrium formation of millimeter-scale iron oxide–silica tubes in experiments that tightly control the tube radius and growth speed. The experiments involve the hydrodynamic injection of an iron (II,III) solution into a large volume of solution containing sodium silicate and ammonium hydroxide. The forming tubes are pinned to a motorized glass rod that moves at a predetermined speed. X-ray diffraction and electron microscopy, as well as Raman and Mössbauer spectroscopy, reveal magnetite nanoparticles in the range of 5–15 nm. Optical data suggest that the magnetite particles follow first-order nucleation–growth kinetics. The hollow tubes exhibit superparamagnetic behavior at room temperature, with a transition to a blocked state at TB = 95 K for an applied field of 200 Oe. Heat capacity measurements yield evidence for the Verwey transition at 20 K. Finally, we show a remarkable dependence of the tubes’ magnetic properties on the speed of the pinning rod and the injection rate employed during synthesis.