Band-gap narrowing and electrochemical properties in N-doped and reduced anodic TiO2 nanotube arrays

Electrochemical activity of TiO2 nanotube arrays (NTAs) is restricted by a wide band gap of TiO2. To overcome this restriction, we considered systematic research on two effective methods of doping of TiO2 NTAs such as the N-doping and electrochemical reductive doping and predicting the proper application of them. Band gap narrowing was occurred from 3.16 eV for undoped TiO2 NTAs to 2.9 and 2.7 eV at N-doped and self-doped TiO2 ones respectively. The electrochemical responses of the TiO2 NTAs before and after doping were examined by cyclic Voltammetry (CV) curve. To understand the electrochemical behavior of the undoped and doped TiO2 NTAs, electrochemical impedance spectroscopy (EIS) was used and three equivalent circuit models were also built. The results showed that the undoped TiO2 NTAs is not strictly capacitive but a small quantity of N in TiO2 remarkably decreases the surface resistance of TiO2 electrode. In contrast, self-doped TiO2 NTAs resistance is reduced to very negligible contents of 0.0001322 Omega cm(-2), that not only self-doped sample becomes to completely capacitive but also, it leads to the semiconductor nature of TiO2 NTAs transforms to semi-metallic one, and the two orders of enhancement in capacitance of blue TiO2 NTAs are very astonishing and it has outstanding potential for applications like supercapacitors as the electrochemical response of the self-doped TiO2 NTA sample was found to be a content of about 7 mF cm(-2) that it is improved about 20 times compared with undoped one. Furthermore, it was found that doping of TiO2 NTAs with nitrogen atoms increases the carrier density about 2.82 x 10(21) and self-doped TiO2 NTAs show the higher carrier density about 1.14 x 10(25) compared with N-doped NTAs. These finding help to understand the mechanism of doping in two different methods and select the best one in relevant applications. (c) 2018 Elsevier Ltd. All rights reserved.