The process of oxidation of the Bi2Te3 surface was investigated by x-ray photoelectron spectroscopy (XPS). The oxidized surface layer was found to have a definite thickness, with configurations where O is bonded with Bi and Te, and Bi and Te are bonded with three and four oxygens, respectively. The oxidation time dependence of the oxidized layer thickness d(t) estimated from the XPS behaved as (t-t0)1/2 when d(t) was smaller than the thickness of a single oxidized quintuple atomic layer in our oxide model and behaved as t-t1 when it was larger than that. Experimental data were compared to our oxidation process model for the layered structure with the van der Waals gap and very good agreement was found.

Charge transport in dye-sensitized nanocrystalline TiO2 electrodes was studied by a theoretical model. For studies of recombination processes through trap states, a Shockley−Read model, which represents electron transfer through an effective trap level, was used as a recombination term in the model. The simulation results have demonstrated that thermal release of the electrons trapped in shallow tail states to the conduction band contributes to effective electron diffusion in the nanocrystalline electrodes. However, the electrons captured in the tail states at the lowest energy may recombine with oxidized ions in electrolytes. Short-range screening of externally applied biases in the nanocrystalline electrodes was taken into account as a boundary condition in the model. Calculations of current−voltage characteristics have indicated that relatively high open-circuit voltages can be attained under the boundary condition.

The light transmittance, T, in nanocrystalline TiO2 films was studied as a function of the light wavelength, lambda, the nanocrystal radius, a, and the film thickness, d. Two types of TiO2 nanoparticles were employed: a commercial powder (P25) and synthesized particles from titanium isopropoxide (SP). The X-ray diffraction measurements revealed that both P25 and SP are mainly anatase and the average crystal sizes, 2a, of P25 and SP are 50.3 and 23.7 nm, respectively. Despite the visual difference between micron-order thin films of P25 and SP, the light hemispherical transmittance corrected with the surface specular reflectance has a clear dependence of ln(T) = -0.5beta lambda(-4)a(3)d, with beta = 1.5 x 10(3) from visible to near-infrared wavelengths. The dependence and beta value were successfully explained by the simplest model on the basis of the Rayleigh scattering theory. This indicates that the nanocrystalline TiO2 thin films are a typical medium where the simplest scattering model is a good approximation. However, the model was inapplicable to light scattering in relatively thick P25 films of 1.5-3.0 microm because of nonnegligible internal multiple scattering. For the moderate thickness films, ln(T) proportional to lambda(gamma), where gamma increases from -4 in proportion to the film thickness is an alternative approximation. With these light scattering models, the light absorption rate of the TiO2 crystal was successfully evaluated from experimental extinction rates.