The fabrication and characterization of single-crystalline selenium nanoneedles

A solution-phase approach has been demonstrated for large-scale synthesis of selenium nanoneedles with the stem diameter ranging from 100 to 500 nm and lengths up to tens of micrometers, gradually becoming thinner to form a sharp tip, which were fabricated by reduction of Na2SeO3 with poly( vinyl alcohol) (PVA). An interesting feature of the nanoneedles is their tendency to form branches and junctions. The morphology, microstructure, and chemical compositions of the nanoneedles were characterized using various methods (XRD, XSP, FESEM, TEM, and high-resolution TEM), indicating that the nanoneedles were single crystalline with high purity, structurally uniform, and dislocation-free. On the basis of a series of TEM observations, the nucleation and growth process of selenium nanoneedles could be interpreted by a solid-solution-solid process and PVA-assisted formation mechanism: trigonal (t)-selenium sphere-like nanoparticles initially formed due to the incessant dissolution of the sphere-like amorphous ( a)selenium nanoparticles in the current hydrothermal process; then the wire-like nanostructures were gradually created from the t-selenium particles owing to the intrinsically anisotropic structure of t-selenium. The needle-like end shape was attributed to the higher growth rate on the tip than on the stem of wires since PVA can kinetically control the anisotropic growth via selective absorption on different crystalline faces.