The Role of Surface States in the Ultrafast Photoinduced Electron Transfer from Sensitizing Dye Molecules to Semiconductor Colloids

Abstract

Investigations on the ultrafast electron injection mechanism from the dye alizarin to wide band gap semiconductor colloids in aqueous medium are presented, combined with detailed studies on population, depopulation, and relaxation phenomena in trap states and their influence on the injection process. Because of the very strong electronic coupling between dye and semiconductor in an alizarin/TiO 2 system, a very fast electron injection from the excited dye to the conduction band of TiO 2 is expected. Our measurements show an injection time τ inj < 100 fs, suggesting that the electron transfer follows an adiabatic mechanism. Furthermore, we present experiments over a wide spectral range on the recombination reaction of the electron in the conduction band of the semiconductor colloid and the dye cation to the ground state. We find highly multiphasic recombination dynamics with time constants from 400 fs to the nanosecond time scale. The nonexponential character of the recombination reaction is attributed to fast relaxation processes. The crucial contribution of surface trap states and their influence on the observed dynamics was investigated with alizarin adsorbed on the insulating substrate ZrO 2 . Since the conduction band edge lies far above (≈1 eV) the S 1 state of alizarin, the electron injection into this band is completely suppressed. Despite this fact our spectroscopic investigations show that on ultrafast time scales the formation of an alizarin cation occurs. This observation, is explained by fast electron injection into surface trap states near the docking site on the colloid. For the alizarin/ZrO 2 system the time scale for the injection into these traps is determined to be faster than 100 fs. The relaxation processes in the traps and the repopulation of the S 1 state occur within 450 fs, the subsequent ground-state relaxation takes 160 ps. The ultrafast injection dynamics into the traps, recorded for alizarin/ ZrO 2 , underlines the importance of surface states for the initial charge separation also for systems with a lower band edge such as TiO 2 . We show that in the dye/ZrO 2 system the process of electron injection is not suppressed but "stopped" after the ultrafast transition into trap states. It is therefore a valuable system for probing the electron dynamics in surface states.

Modulation of the Rate of Electron Injection in Dye-Sensitized Nanocrystalline TiO2 Films by Externally Applied Bias

“…We note however that several groups have reported fast, < 100 fs, electron injection kinetics for other 5 metal oxide films, and discussed such kinetics both in terms of adiabatic electron transfer theory, and "hot" electron injection from unrelaxed dye excited states. 5,7,10,12 Given the complexity of our observed electron injection kinetics, it is possible that the fast (<150 fs) phase of injection, which accounts for up to 40% of the total injection yield at moderate potentials, may correspond to such processes. However such considerations are not necessary to account for the overall behavior of the electron injection kinetics of the RuL 2 (NCS) 2 -sensitized TiO 2 films studied in this paper.…”

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

“…Such complex dynamics are not unusual and were reported previously for the system alizarin-ZrO 2 . 49 …”

Electron donor-acceptor distance dependence of the dynamics of light-induced interfacial charge transfer in the dye-sensitization of nanocrystalline oxide semiconductors

“…1,2 Recent ultrafast studies have shown charge injection from excited dye molecules into the conduction band of oxide semiconductors to occur in the femtosecond to picosecond time domain. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The time constant for ET has been found in these studies to vary from 6 fs 7 to hundreds of ps. [16][17][18] Charge transfer times 50 fs indicate that the corresponding electronic coupling strengths are approaching the value of the thermal energy kT (~ 200 cm -1 ) and, thus, that the reactions are likely to have reached the adiabatic limit.…”