Deep Surface Trap Filling by Photoinduced Carriers and Interparticle Electron Transport Observed in TiO₂ Nanocrystalline Film with Time-Resolved Visible and Mid-IR Transient Spectroscopies

TiO 2 nanocrystal and TiO 2 − x SiO 2 core/shell films were investigated by band gap excitation followed by detection of the photoinduced carriers with time-resolved transient absorption spectroscopy in both the visible and mid-IR range. The passivation of the TiO 2 nanocrystal with the SiO 2 shell has two goals, that is, reducing the surface-trapped states and segregating the TiO 2 nanoparticles in the sintered film to prevent interparticle carrier migration. The results show that within the accessible mid-IR range (5−6.5 μm) the observed transient absorptions of the TiO 2 crystal and TiO 2 − x SiO 2 core/shell films exhibit an IR spectrum typical of intraband transition of free electrons where the observed absorption intensity is proportional to λ p (λ = wavelength/μm), and p was determined as 2.6 for the TiO 2 nanoparticle film and 4.9 for the TiO 2 − x SiO 2 core/shell composite film, respectively. Three typical kinetic phases for the transient IR absorption decay profile of the TiO 2 film have been observed, that is, a rapid decay beyond the instrumental response time (∼80 ns), a bleaching recovery with a rising time constant around 1.8 μs, and a very slow absorption decay component. The rapid decay phase is assigned as the free carrier recombination within the bulk crystal, whereas the bleaching recovery corresponds to the filling of the deep trapping sites by the shallow trapped electrons, and finally the very slow absorption component is assigned to the interparticle electron transport and deep trapped electron−hole recombination. For the TiO 2 − x SiO 2 core/shell film, only the rapid decay phase can be observed. During the visible light detection, a broad-band transient absorption with a peak located at about 650 nm has been observed for the TiO 2 nanoparticle film, which is expected for the absorption from the shallow trapped electrons, while the corresponding carrier relaxation kinetics can be fitted by a monoexponential decay with a time constant of about 1.8 μs, correlating well to the deep trap filling process revealed by IR absorption. It is thus assigned as the filling of the deep trapped sites by the shallow trapped electrons. In contrast, no observable transient absorption can be detected at 650 nm for the TiO 2 − x SiO 2 core/shell film. As a result, the facts from both the IR and visible light detections indicate that the passivation of SiO 2 reduces the density of the trapped state on TiO 2 nanoparticle surface substantially and blocks the interparticle carrier migration efficiently.