A novel method to produce arrays of single crystal TiO2 nanofibers by one step heat treatment in a non-combustive H2/N2 atmosphere has been developed. The dimension of the nanofibers was ranged between 15 – 50 nm in diameter and 1 – 5 ìm in length. The nanofibers were formed by selective and anisotropic etching and it is dubbed “nano-carving”. Although nanocarving utilizes H2/N2 gas during treatment, N2 did not affect the nanofiber formation. Major impurities on disk surfaces were Al, Si, Ni, Fe, and C. Nanoparticles composed of metallic Fe and Ni were observed by TEM and it was likely to have formed on end tips of nanofibers. From XRD, XPS and TEM SAD analyses, the phase of nanofibers was rutile TiO2, and not a suboxide although the nanofibers were generated by a heat treatment in H2/N2. From TEM analyses, it was determined that the preferred direction of etching for the nanofiber formation was the <001> direction.
The rate of nanocarving of polycrystalline TiO2 increased as the sintering temperature and time decreased. The optimum H2/N2 heat treatment temperature ranged from 680 – 740 °C. Below 680 °C, the etching process was not active and above 740 °C, anisotropic etching disappeared because etching in other directions became comparable. Nanowhiskers were grown on nanofiber formed surface by reoxidation at 700 °C for 8 hrs in air. Heat treatment of TiO2 in a H2O atmosphere also created tiny nanofibers on the surface via an apparent deposition process. TiO2 nanofibers showed enhanced sensitivity as a sensor of H2(g), relative to sintered material without nanofibers.
TGA analyses indicated that excess Ti cations from gas phase reaction with H2 should also have been removed from the surface during nanocarving. The excess Ti cations from H2/N2 heat treatment were moved to the bulk by diffusion of Ti interstitials. Weight loss during H2/N2 heat treatment was also dependent on the grain size, which indicated that grain boundary diffusion was an important mode of Ti interstitial diffusion.