Oriented contraction: a facile nonequilibrium heat-treatment approach for fabrication of maghemite fiber-in-tube and tube-in-tube nanostructures.

We present a simple and effective nonequilibrium heat-treatment approach that allows for the facile fabrication of maghemite (γ-Fe(2)O(3)) fiber-in-tube and tube-in-tube nanostructures by heat-treating electrospun precursor fibers composed of polyvinylpyrrolidone (PVP) and iron citrate with a carefully devised heating rate (R). In this nonequilibrium heat-treatment procedure, R can be easily utilized to tune the temperature gradient established in the inner portion of the fibers and the difference between the cohesive force and the adhesive force at the interface layer between the inner gel and the dense rigid shell generated in situ by a high R. Therefore, the contraction direction of the precursor nanofibers and the final morphology of the resultant γ-Fe(2)O(3) fibers ranging from a simple tube to a fiber in tube to a tube in tube are realized for control. The nonequilibrium heat-treatment approach reported here can be readily extended to the fabrication of other materials with controllable interior structures by fast heating their corresponding gel precursors, which may be fabricated on the basis of electrospinning techniques and others. The resultant γ-Fe(2)O(3) fiber-in-tube and tube-in-tube nanostructures may have important applications in a number of areas, such as magnetic separable catalysts or catalyst supporting materials, sensors, absorbents, microreactors, and so forth, because of their structural characteristics and good magnetic properties.