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.Anodic TiO 2 nanotubes (NTs) and other porous anodic oxides have attracted considerable scientific interests due to their various applications (e.g., solar energy materials, magnetic semiconductors and biosensors) 1-3 and mysterious formation mechanisms. 4,5 Different mechanisms of TiO 2 NTs have been reported in many electrochemical journals in recent years. 4-8 It is well known that field-assisted dissolution (FAD) (TiO 2 + 6F − + 4H + → [TiF 6 ] 2− + 2H 2 O) of the oxide leads to pore formation in anodic titania films, 8-10 similar to that in porous anodic alumina (PAA) films (Al 2 O 3 + 6H + → 2Al 3 + + 3H 2 O), 11-14 despite a lack of direct experimental evidence that confirms this expectation. 14 As the formation mechanism is impossible to be derived by direct in situ experimental methods, much remains to be done along these directions. 15 Garcia-Vergara et al. 16,17 proposed the field-assisted 'plastic flow' model, the constant thickness of the barrier layer is maintained by flow of oxide from the pore bottom toward the pore wall, driven by compressive stresses from electrostriction and possibly through volume expansion. 16 In fact, the plastic flow is contrary to expectations of the FAD. 16 The behavior of incorporated species in PAA is always incompatible with the FAD model. 16 The flow model has been recognized and exploited for explaining the formation of TiO 2 NTs and serrated nanochannels. 18,19 However, Zhou et al. 12 indicated that both the FAD and the flow models cannot explain the formation of gaps among nanotubes. In recent tracer studies on Ti thin films, the expansion factors increase from 1.5 to 3.0, 20,21 these findings cannot be clarified. Furthermore, anodized TiO 2 NTs have been achieved in an aqueous H 2 SO 4 solution as well as other fluoride free solutions, 12,22,23 this fact puts the flu...

The formation mechanism of porous anodic TiO 2 nanotubes (PATNT) still remains unclear. A special approach is proposed in this paper to investigate the forming process of nanopores in the preformed nanotubes. A novel and not easily brittle nanostructure, called triple-layered TiO 2 nanotube array, has been fabricated by changing the electrolytes during the electrochemical anodizing processes. The first porous layer was fabricated in fluoride-containing electrolyte, the middle compact layer was formed in fluoridefree electrolyte and the second porous layer was formed in the same fluoride-containing electrolyte. The results show that middle compact layer becomes thicker with the increase of the third time anodizing voltage. At the same time, it needs more time for the fourth time anodization to reach the equilibrium current, where the nanotubes begin to develop steadily. Furthermore, a possible mechanism for the growth of the triple-layered nanotubes is discussed by comparison with the normal PATNT. The present results may be helpful to understand the mechanism of PATNT and facilitate assembling diverse nanostructures for extensive applications in photocatalysis, dye-sensitized solar cells, and biomedical devices.Self-ordering porous anodic alumina (PAA) 1-3 and porous anodic TiO 2 nanotubes (PATNT) 4,5 have been extensively investigated due to their various applications. The formation mechanisms of PAA 6-8 and PATNT 9,10 have received considerable attention. Several models proposed include the field-assisted dissolution (FAD) model, 6,9 the oxide viscous flow model, 8,10,11 oxygen bubble and electronic current model, 7,12,13 etc. For more than 60 years, it has been assumed that fieldassisted dissolution leads to pore formation in PAA, despite a lack of direct experimental evidence that confirms this expectation, 14 because the formation mechanism is hardly derived by in situ experimental methods. 15 In fact, the viscous flow model and anionic incorporation into PAA are both contrary to expectations of the FAD model. 8,11 Moreover, as Hebert et al. indicated that the relationships between porous morphology and the processing parameters (current-time or voltage-time transients) were not yet well understood. 11 It is well known that there are two different types of anodic oxide films for aluminum and titanium, the compact-type film and poroustype film. 16,17 For a constant voltage anodization, the current-time transients of the compact-type and porous-type films are very different. 16,17 Initially both transients are identical; as the initially formed barrier layer thickens, the electric field strength decreases and the current density decreases rapidly. At a special point D p , the two curves now begin to diverge; the compact-type film current continues to decrease exponentially, while the porous-type film current, after a short period of continuing decrease, begins to increase. 16 To the best of our knowledge, most of the researchers assembling the PATNT take fluoride-containing solutions as the anodizing electroly...

Anodic TiO 2 nanotubes (ATNTs) have been studied extensively for many years. However, their mysterious formation mechanism still remains unclear. The formation of gaps and ribs around the nanotubes has not been elucidated. Here, various surface and cross-section morphologies of ATNTs obtained under different anodizing conditions and their evolution process have been investigated in detail. Based on many experimental facts, new explanations for the gaps and ribs are presented. An entire surface layer covered on the nanotubes plays a primary role on the formation of gaps and ribs. The gaps result from the radial distribution of the electric field at the pore bottom. No newly-formed oxide will exist along the gap direction, because the electric filed along the gap is the minimum. The ribs result from the electrolyte entering into the wider gaps among the ATNTs due to the rupture of the entire surface layer. The rings or ribs on the outer wall of ATNTs are formed at the electrolyte/Ti interface due to the discontinuous existence of a small amount of electrolyte within the gap base. The present viewpoint was demonstrated by an original micro-dam, which can block the electrolyte entering into the gaps and avoid the formation of ribs.