Spontaneous formation of inorganic helical fibers and rings

Helical fibers of porous manganese oxide have been prepared from colloidal solutions. Such fibers have diameters ranging from several mm to the micrometer range. These helices are formed, starting from a well-mixed colloidal precursor and applying no external forcing other than its confinement in a capillary or beaker and heating for a period of time. Structural changes occur during gelation, leading from an amorphous to a layered structure. The layered structure can be ion-exchanged with hard cation template ions and then heated to form tunnel structure materials that retain the helical morphology. The helices are semiconductors and permeable. As the concentration of the colloidal precursor is lowered, helices are no longer formed, but instead, two-dimensional rings are produced. The formation process of these helices is related to contraction of the gel during heating with capillary pressure until solvent is removed followed by expansion of the helix.