Effect of H2O content in electrolyte on synthesis and field emission property of anodized TiO2 nanotubes

The effect of H2O contents, from 2% to 30 vol%, in an ammonium fluoride/ethylene electrolyte on the field emission property of anodized TiO2 nanotubes was investigated. The morphology and composition of the samples were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively, which were relatively independent of the H2O content in the electrolyte. The porous TiO2 films were produced at low H2O content of 2-10 vol% due to slow chemical dissolution, and were mainly composed of TiO2 · nH2O complex. Moreover, the well-aligned TiO2 nanotubes were grown at moderate H2O content of 15 vol% with a little TiO2 · nH2O complex. However, with increasing H2O content to 20-25 vol%, nanotube clusters of TiO2 with enlarged inter-cluster distance and sharp tops of each nanotube in clusters were formed due to electric field induced dissolution. The optimal turn-on field for electron field emission of TiO2 nanotubes was obtained as low as 1.9 V/μm. It was suggested that the reduced local field screening effect could significantly contribute to the improved field emission property of TiO2 nanotubes.