Phosphorus stimulated unidirectional growth of TiO2 nanostructures

Abstract

Previously reported TiO₂ nanowire fabrication from Ni catalysts shows a surprising amount of phosphorous (P) contamination incorporated into the seed particle. We proposed this unintentional P-doping of Ni particles aids the mechanism for nanowire growth and occurs by an alternative pathway from the Vapor–Liquid–Solid (VLS) mechanism. To confirm this new mechanism, mixed phase NiP/Ni₂P (Ni_xP_y) and Ni₂P nanoparticles were fabricated and the central role of phosphorous in TiO₂ nanowire synthesis confirmed. This newly developed P-assisted fabrication method yielded crystalline rutile TiO₂ nanowires. In this mechanism solid, quasi-spherical catalyst particles attached to the ends of nanowires and surrounded by a Ni/P liquid shell are responsible for the nanowire growth. The growing end of the nanowire appears to form a “tangent-plane” to the solid catalyst core with the liquid shell wetting and occupying the interstice between the catalyst and the nanowire. In Ni_xP_y assisted growth, nanowire diameters occurred as small as 12.3 nm, some of the thinnest yet reported TiO₂ nanowires resulting from atmospheric-pressure chemical vapor deposition (APCVD) growth.