Fabrication and Formation Mechanism of Triple-Layered TiO₂Nanotubes

Abstract

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...

Formation Mechanism of Gaps and Ribs Around Anodic TiO2Nanotubes and Method to Avoid Formation of Ribs

“…We come to the conclusion that the field-assisted dissolution current across the barrier layer is impossible to be formed. 39,41 At the same time, it is very difficult to understand why the dissolution reaction takes place from the turning point of Figure 8d, rather than from the beginning. In principle, the phenomena that the current increases from the turning point in Figure 8d and the current efficiency decreases in Figure 8e are both independent of the field-assisted or chemical dissolutions.…”

“…[36][37][38]41 The ionic current and the electronic current are interdependent and independent, and they are a couple of driving forces during the formation of TiO 2 nanotubes. [36][37][38][39][40][41] Distributional diagram of the electric currents through three parts of the electrochemical cell (i.e., the electrolyte, the metal anode and the growing oxide) and the formation mechanism of the nanotubes were detailed in our previous works, 38,41 and here, the entire surface layer covered on the nanotube arrays will be mainly focused on.…”

“…[34][35][36] The model and conclusion of the papers above were reinforced by our subsequent works. [37][38][39][40][41] In 2012, the theoretical expressions for time dependent ionic current and electronic current were successfully derived from the anodizing processes of aluminum and titanium, 37 many questions proposed by Diggle's and Hebert's groups 7,18,30 could be successfully elucidated in our recent works, [36][37][38][39][40][41] such as, the correlation between dissolution rate and anodizing current, dissolution nature, dissolution balance and constant barrier layer, the correlation between the porous morphology and current-time curve (three stages). The complicated growth kinetics could be also quantitatively explained by the ionic current and the electronic current.…”

Morphology Defects Guided Pore Initiation during the Formation of Porous Anodic Alumina

“…29 That is to say, the so-called dissolution current across the barrier oxide layer is impossible to be formed. 29 Here, the total anodic current (J t ), composed by ionic current (J ion ) and electronic current (J e ), is taken into account to interpret the unusual morphologies evolution. We have proposed that the formation of oxide film is determined by J ion , and J e gives rise to the evolution of oxygen gas which is responsible to the pore formation.…”

Effective approach to strengthening TiO2 nanotube arrays by using double or triple reinforcements

“…The compact TiO 2 films are generally formed in fluoride-free electrolytes, while the porous films (TNTAs) are formed in fluoride-containing electrolytes [24][25][26]. More recently, we have proposed a facile method to enhance the adhesion of TNTAs to the underlying Ti substrate, i.e., the introduction of an additional compact layer (ACL) [18].…”