Physico-chemical properties of TiO2 nanotube arrays are very close to the properties of TiO2 nanoparticles, due to chemical and structural similarities. However, some unusual behaviour of nanotube arrays is observed, resulting from the specific nanotubular morphology of materials, and is considered in this section.
Figure 3.18 shows the typical transmittance spectra of a 400 nm thick TiO2 nanotube array film on glass substrate.83 The optical behaviour of the TiO2 nanotube array is quite similar to that of mesostructured titanium dioxide. The TiO2 nanotube absorption coefficient, a', can be calculated from transmittance, T0:
where L is the film thickness (or the length of the nanotubes). The absorption coefficient depends on the energy of the incident light (hPn) as follows:
hPn where EG is the semiconductor bandgap, A' is a proportionality coefficient, n is a number (which is equal to 1/2, 2, 3/2 or 3 for allowed direct, allowed indirect, forbidden direct or forbidden indirect transitions in the semiconductor,
respectively). TiO2 nanotubes have allowed indirect optical transitions and the Tauc plot in (ahPn)05 vs. hn coordinates allow us to estimate the optical band gap by dropping a line from the maximum slope of the curve to the x-axis, as 3.34 eV as seen in Figure 3.18b. The reported band gap value of anatase phase in bulk is 3.2 eV (ref. 44). A slight blue shift in the value might be due to a quantization effect in the nanotubular film where the wall thickness is about 12 nm. A band tail to 2.4 eV is observed. The degree of lattice distortion is likely to be relatively higher for nanotube array films, thus causing an aggregation of vacancies acting as trap states along the seams of nanotube walls leading to a lower band-to-band transition energy.
The uniformity of the TiO2 nanotube array film thickness may also result in appearance of an interference pattern in the absorption spectrum of nanotubes (see the features at 475 and 1100 nm in Figure 3.18a), where the wavelength of the incident light and the thickness of the film have values in the same order magnitude. The spacing between the interference patterns, which is created by the interaction of the transmitted wave and the wave reflected back from the top of the nanotubes, reduces with increasing nanotube length. The position of the interference peaks also affected the thickness of the film.
The electrical conductivity of the TiO2 nanotube array is comparable with that of conventional mesoporous TiO2 films. However, the conductivity can increase by several orders of magnitude in the presence of hydrogen at room temperature, which interacts with the surface of the nanotubes and modifes their properties. In Chapter 6, details of such hydrogen sensing materials are provided.
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