Fig. 3. IR and NIR transient absorption spectroscopy of injected electrons in the conduction band of TiO₂.

(A) Experimental setup and cartoon illustration of visible pump and NIR or IR probe transient absorption spectroscopy to interrogate the dynamics of injected electrons through direct and indirect charge transfer into the TiO₂ conduction band for AuNR@TiO₂ heterostructures. (B) Steady-state absorption spectrum of a AuNR@TiO₂ film. (C) Ultrafast IR transient absorption spectra of AuNR@TiO₂ heterostructures probed at 4.9 to 5.1 μm following 515-nm excitation in units of mili optical density (mOD). The green box indicates the probe wavelength of 5 μm used for analyzing the dynamics shown in (D). (D) Transient absorption dynamics at 5 μm for TiO₂ with 345-nm (black) and 515-nm excitation (cyan), AuNRs@TiO₂ with 515-nm excitation(green), and AuNRs with 515-nm excitation (pink). (E) Experimentally derived values of ΔA_max are shown as a function of absorbed (abs.) photon density using pump fluence–dependent experiments for the TiO₂ film with 345-nm pump and 5-μm probe (black circles) and for AuNRs@TiO₂ with 515-nm pump and 5-μm probe (black squares). Similar experiments with a 1.15-μm probe are shown for TiO₂ excited with 266-nm pump (red circles) and for AuNRs@TiO₂ with 515-nm pump (red squares) and 650-nm pump (red triangles and inset) as a function of absorbed photon density. Table S1 lists the fitted slopes.