In the present review we try to give a comprehensive and most up to date view to the field, with an emphasis on the currently most investigated anodic TiO2 nanotube arrays. We will first give an overview of different synthesis approaches to produce TiO2 nanotubes and TiO2 nanotube arrays, and then deal with physical and chemical properties of TiO2 nanotubes and techniques to modify them. Finally, we will provide an overview of the most explored and prospective applications of nanotubular TiO2.

One-dimensional titanium dioxide nanomaterials: nanotubes.

A dense rutile TiO2 thin film was synthesized by the thermal oxidation of a sputtered titanium metal film in ambient air. The effects on optical properties of TiO2 films of the crystal structure and microstructural evolution at various oxidation temperatures were investigated. The Ti films transformed into single-phase rutile TiO2 at temperatures ⩾ 550 °C without going through an anatase-to-rutile transformation. Instead, an additional crystalline Ti2O phase was detected at 550 °C only. An increase in the oxidation temperatures ranging between 700 and 900 °C led to an increase in both the refractive index and absorption coefficient, but a decrease in the band gap energy (Eg). According to the coherent potential approximation model, the band gap evolution of the oxidized films was primarily attributed to the electronic disorder due to oxygen deficiency at a higher oxidation temperature rather than the presence of an amorphous component in the prepared films.

https://ir.nctu.edu.tw:443/bitstream/11536/30200/1/000089552800026.pdf

Structural evolution and optical properties of TiO2 thin films prepared by thermal oxidation of sputtered Ti films

Anatase and rutile thick films generated from the same source of titania were found to exhibit different types of conductivity upon exposure to CO and CH 4 at 600°C in a background of 5% O 2 /95% N 2 . Anatase behaved as a n-type semiconductor, with a decrease in resistance with reducing gas, whereas rutile exhibited p-type conductivity. The morphology of the particles were different, with anatase consisting of spherical particles of 100–200 nm dimensions, whereas rutile appeared as elongated rods of ∼1 μm lengths. The n-type behavior of anatase can be explained based on the oxygen vacancies. For explanation of the p-type behavior of rutile, impurities in the sample have to be taken into account. The impurity contents in both samples were similar, and doping of the lower valent impurities into the TiO 2 lattice should lead to creation of interstitial titanium defects. During anatase to rutile conversion at temperatures of 1000°C, the titanium interstitials can help incorporate excess oxygen, leading to formation of holes and p-type conductivity in the rutile phase. Resistance changes upon interaction of reducing gas with composites of anatase–rutile was also studied. It was found that samples with 4 followed a similar pattern. However, since the overall response of CH 4 was smaller than that of CO, the 75% rutile sample showed no change upon exposure to CH 4 , while exhibiting an n-type response to CO, indicative of a selective CO sensor at temperatures of 600°C. A polychromatic percolation model was developed to explain the electrical data. Two independent, parallel pathways involving the n-type anatase and p-type rutile were considered to be important in the conductivity. Using experimental data related to the extent of sintering, and appropriate particle sizes, the model predicted that n–n percolation would occur from 0 up to 94.5% rutile and p–p percolation would begin at 75.1% rutile. In between 75.1 and 94.5% rutile, both n- and p-pathways would percolate, resulting in the observed diminished changes in resistance.

Composite n–p semiconducting titanium oxides as gas sensors

Semiconducting oxides are employed as gas-sensitive resistors both for monitoring changes in oxygen partial pressure and for monitoring small concentrations of impurity gases in air. A review of the performance of different materials in this function focuses attention on the mechanisms by which responses are generated and provides an indication of the ultimate selectivity that is likely to be achievable between gases.

Materials selection for semiconductor gas sensors

High temperature gas sensors based on catalytic metal-insulator-silicon carbide (MISiC) devices are developed both as capacitors and Schottky diodes. A maximum operation temperature of 1000 degrees C is obtained for capacitors based on 4H-SiC, and all sensors work routinely for several weeks at 600 degrees C. Reducing gases like hydrocarbons and hydrogen lower the flat band voltage of the capacitor and the barrier height of the diode. The time constants for the gas response are in the order of milliseconds and because of this good performance the sensors are tested for combustion engine control. For temperatures around 600 degrees C total combustion occurs on the sensor surface and the signal is high for fuel in excess and low for air in excess. At temperatures around 400 degrees C the response is more linear. The high temperature operation causes interdiffusion of the metal and insulator layers in these devices; and this interdiffusion has been studied. At sufficiently high temperatures the inversion capacitance shows different levels for hydrogen free and hydrogen containing ambients, which is suggested to be due to a reversible hydrogen annealing effect at the insulator-silicon carbide interface.

W.J. Kaiser

Papers

TiO2 film oxygen sensors made by chemical vapour deposition from organometallics