Photoelectrochemical properties of N doped black TiO2 nanotube arrays

TiO2 nanotube arrays (TNT) were grown by anodization on Ti foils in ethylene glycol electrolyte. After electrochemical reduction and calcination under nitrogen process, Ti3+ and interstitial N in the lattice were found by XPS and Raman test. The co-doped black TNT sample yielded a high photocurrent density of 2.54 mA·cm−2 without bias voltage, while pure TNT has photocurrent density of 0.38 mA·cm−2, it is much higher than ever reported black TiO2. The electron transport resistances decreased from 594.8 ohm·cm−2 to 274.3 ohm·cm−2 and the carrier density increased from 1.35 × 1022 cm−3 to 6.68 × 1022 cm−3 after N, Ti3+ co-doped. Valence-band XPS spectra and Mott-Schottky plot indicated that presence of interstitial N, Ti3+ state in the lattices can reduce the energy gap from 3.2 eV to about 2.0 eV. In addition, TNT cathodic polarization in 0.1 M NaH2PO4 has better photoelectrochemical properties than that in 0.5 M Na2SO4 solution.