Extreme small-sized titanium dioxide (TiO2) particles (nanocrystals) have been extensively used in the field of heterogeneous photo-oxidation catalysis for environmental cleanup. We have measured the optical absorption of mixed TiO2 nanocrystals with rutile and anatase structures (10–40 nm diameter) by photoacoustic (PA) spectroscopy. The values of apparent band-gap energies Eg of the TiO2 nanocrystals with different rutile contents were determined by the theory of interband transition. The value of Eg of a mixed TiO2 nanocrystal with rutile and anatase structures decreases continuously with the increase of rutile content. This indicates that Eg can be controlled by mixing the nanometer-sized TiO2 powders with rutile and anatase structures, hence, we can obtain TiO2 materials with an arbitrary apparent Eg between rutile and anatase structures. The PA spectrum of anatase-type TiO2 nanocrystals with a diameter of 15 nm shows a maximum around a photon energy of 3.9 eV, suggesting a quantum confinement effect with decreasing particle size. Applying the effective mass approximation to the PA spectrum, we can deduce the value of the hole effective mass in anatase-type TiO2 as 0.01 m0 (where m0 is electron rest mass).

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Semiconductor particles with sizes of the order of a few nanometers in a glass matrix show very attractive properties completely different from bulk materials. They show promise both for the basic study of the three-dimensional confinement in semiconductors and for applications in the field of optoelectronics devices for switching, optical signal processing, solar energy conversion, and so on. The study of optical properties, especially optical absorption properties, is most important to understand the behavior of semiconductor nanocrystals. In general, the absorption spectra can be measured by the usual transmitted absorption measurement method, but it needs the samples to be made thin enough and have good quality surfaces by pretreatment. The photoacoustic (PA) method is one of photothermal detection, and it has proved to be useful for investigating optical absorption and thermal properties of various materials by measuring nonradiative processes. In this study, we studied the change in the electronic state due to size confinement for CdTe nanocrystals embedded in a GeO2 glass matrix by PA spectroscopy. The samples were prepared in two steps. First, base glass was produced by heating a mixture of GeO2 (0.91 mol %), NaGeO3 (0.09 mol %), and CdTe (5 wt %) powders up to 600 °C by 40 min and keeping at 600 °C for 12 h, and then heating up to 1180 °C for 5 h, keeping at 1180 °C for 4 h, and finally cooling in water. The CdTe nucleus was formed during this process. Second, the base glasses were annealed at 525 °C from 15 to 120 min to obtain various average sizes of CdTe nanocrystals. The PA intensity gradually increases towards higher energy regions at about 3.25 eV for all samples, probably due to the optical absorption of GeO2 base glasses. New peaks appeared at about 2.41 and 1.85 eV in the PA signal intensity spectra for the samples annealed for 30 and 60 min, respectively. These peaks correspond to the optical absorption of the CdTe nanocrystals in the glasses. The absorption edges and peaks shift to lower energy regions with the increase of annealing time. The average nanocrystal radii were estimated to be 3.5 and 5.5 nm in the two annealed samples from the PA spectra.