Forming Process of Anodic TiO₂Nanotubes under a Preformed Compact Surface Layer

Anodic TiO 2 nanotubes (NTs) have been studied extensively for many years. However, the growth kinetics still remains unclear, because it is hardly derived by direct in situ methods. Here, an interesting approach is proposed to overcome this challenge. A combinatorial anodization was exploited to monitor the pore initiation and nanotube growth under a preformed compact surface layer (CSL). The preformed CSL and the NTs under the CSL (UCSL-NTs) were formed in fluoride-free and fluoride-containing electrolytes, respectively. The forming process of UCSL-NTs was discussed as compared with that of the general NTs, mainly focusing on the differences of current-time curves and electric charge quantity (Coulomb). The results show that pore embryos of UCSL-NTs have already been achieved under the CSL before the CSL is dissolved. There are five stages in the current-time curve of UCSL-NTs, which is significantly different from three stages of the general NTs. A new growth model, based on a comprehensive review of the existing theories, is proposed to explain the current decrease and increase. And the forming process of TiO 2 NTs is considered to be dominated by the oxide plastic flow around the oxygen bubbles.