Photophysical Study of Electron and Hole Trapping in TiO₂ and TiO₂/Au Nanoparticles through a Selective Electron Injection

Abstract

The photophysics surrounding the electron and hole trapping in TiO2 do not have a scientific consensus. Herein, we studied the steady-state photoluminescence and time-resolved spectroscopy features from TiO2 and TiO2/Au nanoparticles (NPs). In TiO2/Au NPs, time-resolved photoluminescence indicates that the electrons from bandgap excitation decay slower (∼30 ps) than in TiO2 (<24 ps). We conclude this as a result of the band bending passivation effect on the surface electron traps. Meanwhile, electron trapping is proved as the dominant surface depopulation process because of the easy-fill characteristics of surface hole traps even under low excitation density, which also interprets the slow surface hole trapping (∼2 ns) in TiO2. Through plasmon-assisted electron injection, we distinguished the electron and hole behaviors at varied photon fluences and then obtained the intrinsic bulk trapping of electrons and holes in the ∼50 and ∼400 ps time range, respectively.

Ultrafast Infrared-to-Visible Photon Upconversion on Plasmon/TiO2 Solid Films

“…Power-dependent transient photoluminescence experiments exciting at 800 nm (Figure c and Figure S4 of the Supporting Information) revealed a cubic dependence of the signal, suggesting the involvement of three photons in the upconversion process. This contrasts with the linear dependence of the emission at 550 nm observed when the system was excited at 400 nm, as we have reported in a previous publication . The cubic dependence also favors an upconversion process rather than a nonlinear optical process, such as Raman scattering.…”

“…Room-temperature excitation of TiO 2 at 320 nm (excitation of O 2– → Ti 4+ ) 22 revealed two emissive states ( Figure 3 a), with the most prominent state centered at 525 nm, ascribed to trap electron and trap hole charge recombination, 23 and a weaker state in the infrared region (800–1200 nm). 22 , 24 , 25 A detailed photophysical study of TiO 2 and Au/TiO 2 after TiO 2 bandgap excitation was performed in our previous publication, 26 and its therefore outside this contribution. However, the kinetic trace analysis presented in Figure 3 b is consistent with what has been measured before but, as it will be shown, very different from what happens when exciting below the bandgap energy.…”

Interfacial States in Au/Reduced TiO2 Plasmonic Photocatalysts Quench Hot-Carrier Photoactivity

“…Our results offer a glimpse of the impact of interfacial defects on (i) the hot charge carrier recombination processes and (ii) the plasmonic quenching of the Au NPs from charge carriers generated into TiO 2 . As evidenced by our results, such hybrid plasmonic nanosystems are extremely complex and we expect our findings to refine the understanding of interfacial defects in hybrid nanostructures. ,,,, …”

“…As evidenced by our results, such hybrid plasmonic nanosystems are extremely complex and we expect our findings to refine the understanding of interfacial defects in hybrid nanostructures. 9,11,12,15,30 ■ METHODS Au NPs on TiO 2 Ultrathin Film Fabrication. The Au-TiO 2 nanostructures were fabricated on indium tin oxide (ITO) coated glass slides with a size of 25 × 25 mm 2 (Ossila, U.K.).…”

“…As evidenced by our results, such hybrid plasmonic nanosystems are extremely complex and we expect our findings to refine the understanding of interfacial defects in hybrid nanostructures. 9 , 11 , 12 , 15 , 30 …”