Semimetallic TiO₂ Nanotubes

Abstract

The highly defined morphology of self-organized anodic TiO 2 nanotube layers found in recent years has applications in fields such as biotechnology, [1,2] photo-catalysis, [3,4] or dyesensitized solar cells. [5] While the semiconducting nature of TiO 2 is crucial for many of these applications, the limited conductivity prevents an even broader and efficient use in applications that require a fast electron transport, such as functional electrodes or as electrocatalyst supports. Herein we demonstrate how to overcome this limitation by using a robust carbo-thermal reduction treatment converting the TiO 2 into an oxy carbide compound that shows stable semimetallic conductivity. These TiO x C y nanotubes can be used, for example, as an inert electrode with substantial overpotential for O 2 evolution, or as a highly efficient support for electrocatalytic reactions, such as in methanol-based fuel cells.To produce TiO 2 nanotube layers we use self-organizing electrochemical anodization, [6][7][8][9] a process leading to oxide nanotubes on a range of refractory metals (e.g. titanium, [10] zirconium, hafnium [11,12] ). TiO 2 in its anatase or rutile form is a wide band-gap semiconductor material (E g % 3.0-3.2 eV) and as such particularly suitable for applications based on UV-or X-ray-induced interactions.[13] To achieve enhanced control over the electronic and optical properties of these semiconducting nanotubes, various doping approaches with suitable elements, for example, C [14,15] and N, [16][17][18] or various transition metals [19] were reported. However, for highthroughput electrodes, or electrocatalyst supports, the conventional doping approaches do not provide sufficient electrical conductivity. Herein we use a high-temperature treatment in acetylene that converts the semiconducting anatase phase into carbon rich (MagnØli-type) phases that show semimetallic conductivity. The process can be carried out without loss of the ordered nanotubular morphology as illustrated in Figure 1. In this case self-organized TiO 2 nanotubes were grown with a diameter of 80 nm to a layer Figure 1. SEM images of the nanotubes after a) anodization formation and b) after a thermal acetylene treatment at 850 8C for 10 min. The lower insets are the corresponding cross-sectional views. c) X-ray diffraction results for the acethylene-treated tubes and those of anatase TiO 2 nanotubes and highly C-doped TiO 2 nanotubes. d) XPS spectra showing the Ti 2p peak of thermal-carbonized nanotubes and of anatase TiO 2 nanotubes and highly C-doped TiO 2 nanotubes.

TiO2 Nanotubes: Synthesis and Applications

“…Such MagnØli-phase titanium oxides are highly conductive (almost as high as graphite) and can be formed under reductive high-temperature treatment in H 2 [210] or C 2 H 2 . [212,213] In fact, many properties of TiO 2 strongly depend on bulk or surface structural defects, and in particular on the formation or presence of bulk or surface TiO 2 nanotubes, after electrochemical formation, are amorphous, and some reports [103,118,214] indicate the presence of nanocrystallites in the tube wall, particularly if anodization is carried out at higher voltages. For example, in acidic electrolyte (H 2 SO 4 ), depending on the anodization conditions (sparking, potentistatic, or galvanostatic), the as formed oxide film may consist of anatase, a mixture of anatase and rutile, or rutile crystallites.…”

“…Highly successful carbon doping and conversion of the tubes can be achieved using acetylene. [212,299] 4.3. Conversion of Tubes (Titanates, Semimetallic Phases) TiO 2 nanotubes can comparably easily be converted into their perovskite oxide.…”

One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes

“…37,476 With increasing temperature, the signal intensity of Ti 3+ increases and reaches a maximum value at 600 ˚C, while the signal intensity of OV remains constant from 400-520 ˚C. 476 but also acetylene treatments at more moderate temperatures were reported to convert TiO 2 nanotubes to such suboxides (or oxycarbides) 441 . Such Magneli-type anodic nanotubes show a semi-metallic behavior.…”

Morphology Control of Single Crystalline Rutile TiO2Nanowires

“…Titanium dioxide (TiO 2 ) nano-structures such as nanotubes, 1 nanowires, 2 nanowhiskers, 3 nanorods, 4 nanobelts, 5 and nanofibers 6 have been recently studied by many researchers because of its wide range of applications in gas sensors, 7 photocatalysis, 8,9 dye-sensitization for solar cells, 10,11 and solar water splitting for hydrogen production.…”