Excitation-Dependent Ultrafast Carrier Dynamics of Colloidal TiO₂ Nanorods in Organic Solvent

Abstract

The relaxation dynamics of charge carriers of organic capped TiO2 nanorods dispersed in chloroform was investigated by femtosecond transient absorption in a weak-excitation regime. Anisotropic TiO2 nanocrystals were excited in the UVvis range, using different pump wavelengths, namely above (300 nm), close to (350 nm), and below (430 nm) the direct band gap of anatase TiO2. We show that the ultrafast dynamics strongly depends on excitation wavelength and determine the time constants of all the processes entering the relaxation. Moreover, we demonstrate that two transient absorption bands at 500 and 700 nm, typically attributed to trapped h(+) and e, respectively, are accessible only when TiO2 is photoexcited well above the band gap, while there is no evidence of such bands when TiO2 is photoexcited close to or below its band gap. In such cases the observed dynamics are attributed to trapped excitons

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO 2 reduction

“…The same broad, featureless absorption in the visible region is observed for all samples; however, I-250 shows a significantly higher absorbance in the near-IR, which does not decay down to the baseline even at long delay times (>3 ns) when measured in air. The broad, featureless absorption in the visible region observed in all spectra is indicative of the formation of coulombically bound excitons (22,27,42). The presence of enhanced near-IR absorption is attributed to charge carriers in shallow trap states created by defects within the material, such as oxygen vacancies and surface hydroxyl groups (21,22,26).…”

“…Using nearband-gap excitation, trap states and shallow donor (acceptor) states near the conduction (valence) band edge can be directly excited (27). The lower potential energy of the photogenerated charge carriers means they are more likely to be trapped at these localized surface states, leading to longer-lived excited states.…”

“…surface oxygen vacancies can act as traps for excited-state electrons (27)(28)(29), whereas surface hydroxyl groups typically function as hole traps (21,27). Transient absorption (TA) spectroscopy has been used extensively to understand the processes that govern the relaxation dynamics of photoexcited electrons and holes and has been successfully applied to the study of midgap states in metal oxides (21,22,26,27,(30)(31)(32)(33)(34).…”

“…Transient absorption (TA) spectroscopy has been used extensively to understand the processes that govern the relaxation dynamics of photoexcited electrons and holes and has been successfully applied to the study of midgap states in metal oxides (21,22,26,27,(30)(31)(32)(33)(34). By studying highly defected materials and modeling the TA decay as a function of defect concentration, time, and excitation wavelength, a more detailed understanding of the dominant charge carrier relaxation processes can be obtained (22)(23)(24).…”

“…Conversely, they can also act as charge recombination centers that facilitate the rapid recombination of photoexcited electrons and holes, reducing excited-state lifetimes. Their chemical nature and location within the band gap largely determine their role in the relaxation pathway (21)(22)(23)(24)27). In metal oxides, both bulk and…”