Anodic TiO2 nanotubes: double walled vs. single walled

Abstract

Electrochemical formation of self-organized TiO2 nanotube layers has been a highly active research field for more than 10 years. In the present manuscript we investigate the formation of two distinctly different anodic TiO2 nanotube morphologies, 'single walled' and 'double walled' tubes, which are formed mainly depending on the nature of the anodization electrolyte. While the widest used electrolytes are ethylene glycol (EG) based, forming double walled structures, tubes formed in dimethyl sulfoxide (DMSO) based electrolytes show a single tube walled morphology. Here we provide reasons for the formation of double walled tubes, characterize tubes for their composition, structure and certain properties, and give measures to suppress or minimize double wall formation. Except for the fact that in DMSO single walled tubes are formed, we also show that they grow sufficiently slowly to allow partial crystallization of the tubes during growth--this, in turn drastically influences their electronic properties. Finally we discuss the effects and potential consequences of double or single wall growth for TiO2 nanotube applications.

One-step topological preparation of carbon doped and coated TiO2 hollow nanocubes for synergistically enhanced visible photodegradation activity

“…40 These values are greater than the I D /I G ratios (0.78-0.81) previously reported. 41 The C@TiO 2 -HNB sample is therefore indicative of the presence of less graphitic carbon.…”

One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes

“…For TiO 2 nanotubes grown in many organic electrolytes, as shown in Figure 8.e, a multiwall structure is obtained 173,229,230 . For example, Figure 8.e shows a high resolution TEM image of tubes grown in an ethylene glycol (EG) based electrolyte where a carbon rich oxide layer at the inner part of the nanotubes is found.…”

“…In aqueous electrolytes, the inner tube layer is typically more hydroxide-rich than the outer layer, as is the case of porous anodized aluminum, in acidic aqueous electrolytes. In contrast to EG, in other organic solvents such as dimethyl sulfoxide (DMSO) or EG/DMSO mixtures, a single tube wall can be achieved (essentially only the pure outer part of the oxide is present), with significantly less carbon species incorporated, 214,229,230 as shown in Figure 8.f-h.…”

Anodic TiO2 nanotube layers: Why does self-organized growth occur—A mini review

“…Typically the inner shell of TiO 2 nanotubes is of inferior quality. Only under specific conditions [44,45] or using etching treatments this inner shell can be removed, and so-called "single-walled" tubes can be generated.…”

“…Together with voltage control and -alteration procedures, this allowed to establish smooth walled tube layers of several 100 μm thickness, bamboo or stack morphologies, branching, diameter control in the range from 10-800 nm, and the creation of single/double walled morphologies, for an overview, see e.g. refs [1,44,45]. It should be noted though that the as-formed tubes are amorphous and contain significant amount of fluorides and typically carbon, and for many applications need to be thermally crystallized to anatase or mixed anatase/rutile structures [46].…”