Factors influencing surface morphology of anodized TiO2 nanotubes

Abstract

This paper investigates the formation mechanism of ribs on the outer wall of anodized TiO 2 nanotubes (NTs) prepared in a NaF/Glycerol electrolyte containing 2wt% of water. The effect of potential and time on the morphology of the NTs is evaluated along with growth efficiency (% of total charge at the electrode used to form the oxide) and X-ray Photoelectron Spectroscopy (XPS) measurements, providing an insight into the mechanism of formation of ribs. XPS analysis confirms the presence of fluorine, as TiF 6 2-, and carbon as impurities in the anodic film. The growth efficiency of the process decreases from ~70% at 10V to 55-58% at 20-30V and 14% at 40V. Similarly, the anodic growth factor (migration of ions expressed as the maximum radius of the "oxide cell" per applied potential, nm V-1) decreases at higher potentials, due to oxygen bubbles evolving at the anode at 20-40V and disrupting the anodizing process. The formation of gas bubbles also affects the morphology of the NTs; while NTs are smooth at 10V, oxide rings appear over the range 20-40V. Partial dissolution of the oxide rings due to fluorine ions eventually reshapes the NTs forming ribs, whereas excessive dissolution over extended anodizing times tends to smoothen the NTs and eventually leads to collapse of the NTs. On the basis of these observations, we suggest oxygen evolution (requiring a minimum amount of water in the electrolyte and a sufficiently high potential 20-40V) plays a primary role on the formation of ribs on anodized TiO 2 NTs. Ribs are also observed on NTs grown in aqueous electrolytes, although since dissolution is more difficult to control the resulting structure is more irregular than in organic media.

Effects of various applied voltages on physical properties of TiO2 nanotubes by anodization method

“…The porosity of the nanostructures may improve the absorption of the incident photon in comparison with the flat surface. Porosities of the TiO 2 nanotubes can be calculated using following equation [36]:…”

“…At first Masuda and Fukuda suggested anodization process to grow alumina (Al 2 O 3 ) nanostructures in 1995 [32] and then Zwilling et al promoted this methods for synthesizing TiO 2 nano porous on Ti metal in electrolyte containing HF [33] and at last the TiO 2 nanotubes which had acceptable arrays' length were anodized by Schmuki et al [34,35]. However, the nanotubes, which are synthesized by anodization are not top open channel and the suggested solution is two step anodization process to produce highly ordered and open channel arrays of TiO 2 nanotube [36,37]. In this method, the first anodized foils were ultra sonicated to remove the nanotubes and later second anodization steps can be performed.…”

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

“…The formation of oxygen bubbles also affects the morphology of the ATNTs. 29 When the anodizing voltage is low (10 V), oxygen bubbles at the anode cannot be observed and the nanotubes are smooth. While the anodizing voltage is high (20∼40 V), growth efficiency is observed to decrease with increasing anodizing voltage, and then the rings on the outer wall of the ATNTs are formed.…”

“…25,29 As the rings are more exposed to the electrolyte in the gaps among the ATNTs, partial dissolution of the oxide rings due to fluorine ions eventually leads to formation of the ribs around the ATNTs, whereas excessive dissolution over extended anodizing times tends to eliminate the rings and ribs. 29 Eventually ribs on the upper part of the ATNTs are completely dissolved and hybrid morphology of the ATNTs can be observed, with a smooth top and a ribbed bottom. 29 They suggested oxygen evolution plays a primary role on the formation of ribs around the ATNTs.…”

“…The generation of the ribs is suggested to be a consequence of access of electrolyte between nanotubes, due to chemical dissolution of a fluoride-rich boundary layer, enabling transient formation of films around the external tube walls. 25 In 2012, Regonini et al 29 also proposed some original and interesting viewpoints about the formation mechanism of ribs on the outer wall of ATNTs. The formation of oxygen bubbles also affects the morphology of the ATNTs.…”