Instead of using expensive and complex methods to produce 1D structures, a simple direct oxidation method has been developed to produce either pure TiO2 nanowire or heterogeneous nanowires under a limited supply of oxygen. The dimension of the nanofibers ranged from 30 to 60 nm in diameter and their length were between 500 nm to several μm depending on Ti alloy samples. Experimental parameters such as flow rate, heat treatment temperature, and gas constituent have effects on the surface morphology of the samples. Based on studies under various conditions, pure TiO2 nanowires were only grown on pure Ti samples at the heat treatment temperature of 600 °C with an Ar flow rate of 200 ml/min. As the heat treatment temperature increased, well faceted crystals were formed and connected to each other to form a continuous oxide scale. As the flow rate increased, the formation of nanowires were hindered and an oxide layer was formed rather than the formation of nanowires. The phase of nanowires was determined to be rutile based on SAED pattern from TEM analyses.
For Ti alloy samples, the growth of nanowires was accelerated in terms of the density and length. The effects of the heat treatment temperature on the surface morphology were similar to those of pure Ti samples. However, the effect of the flow rate was not prominent as those of pure Ti samples. Consequently, the growth processing window for Ti alloys is larger than that of pure Ti samples with respect to the flow rate. The phase characterization of nanowires by XRD and TEM analyses indicated that the inner rutile core was covered by an Al rich outer layer.
While the growth mechanism for nanowire formation is not well understood, it is established that the growth occurs at the tip and that the rate limiting step is the transport of oxygen through the gas boundary layer. Additionally, 1D growth implies surface reaction anisotropy that disappears at high temperature promoting oxide cale growth. the Based on experimental observations, a four-stage mechanism is proposed as a function of time. In stage I, a cracked oxide scale grows followed by the formation of bumps in stage II that act as nucleation sites for nanowires. In stage III, nanowires continue to grow along with nucleation and growth of new nanowires leading to the increase in density. In stage IV, the density of nanowires is so large that the accessibility of oxygen to the base becomes difficult and nanowires continue to grow at the tip.