Low-temperature ammonia annealed TiO2 nanotube arrays: Synergy of morphology improvement and nitrogen doping for enhanced field emission

The field emission (FE) properties of TiO2 nanotube arrays (TNAs) fabricated by anodization are dramatically enhanced by ammonia annealing at a low temperature of 90 °C with various annealing durations of 5-15 h, which can spontaneously tailor the as-anodized amorphous TNAs to be an anatase hierarchical structure of worm-like TiO2 decorated TNAs, and simultaneously implant nitrogen into the hierarchical structure. Compared to the as-anodized TNAs, the turn-on field of ammonia annealed TNAs are significantly dropped from 23.11 to 2.94 V/μm, and related to the annealing time. Importantly, the optimized sample of ammonia annealed TNAs treated for 10 h exhibits excellent FE properties involving both a low turn-on field of 2.94 V/μm, a high FE current density of about 3.35 mA/cm2 at 7.35 V/μm, and a remarkable FE stability. The substantially enhanced FE properties can be attributed to the combination of the optimized morphology, the improved conductivity, and the presence of localized oxygen vacancy states and N 2p states created by N-doping of ammonia annealed TNAs.