Fast transient bleaching in Rh-6G functionalized T1O2 nanoparticles: Charge transfer dynamics

Summary form only given. The charge-transfer mechanisms from dye to semiconductor nanoparticles and the time scales involved in these processes are a matter of interest for diverse applications, such as dye-sensitized solar cells (DSC) [1] and photocatalysis [2]. In DSC, the primary mechanism encompasses light-induced electron excitation following electron transfer from excited LUMO states of the dye to the conduction band (CB) of the semiconductor. To take place this electron transfer, the semiconductor CB must have a lower energy than the LUMO states of the sensitizer. Titanium dioxide (TiO₂) often meets this requisition and therefore is the most widely semiconductor employed in DSC. Fast electron transfer is mandatory to avoid exciton recombination before charge-transfer and therefore, improve the efficiency of a DSC. Conversely, slow backward charge transfer (from the semiconductor to the dye) increases the efficiency of a DSC.the dye) increases the efficiency of a DSC.Charge transfer from rhodamine 590 (Rh6G) to amorphous TiO2 nanoparticles in colloidal suspension is investigated here via pump-probe transient absorption (TA). The TA kinetics is analyzed taking into account the relevant energy levels of the hybrid nanocomposite and electron transfer rates are estimated from the TA absorption signals. The pump-probe TA experimental apparatus used, as pump beam, a regeneratively amplified Ti:Sapphire laser (800 nm, 100 fs, 1 kHz repetition rate), which was frequency doubled (λ = 400 nm) by a 1 mm long nonlinear BiBO crystal. The pump beam was modulated by an optical chopper operating at 461 Hz and its optical path was controlled by a mechanical delay line. The probe beam was derived from an optical parametric amplifier (OPA) pumped by the Ti:Sapphire laser whose output beam wavelength was tuned to λ = 530 nm at the maximum of the S0-S1 transition of Rh6G. The photodiode signals of the reference and signal beams were sent to two differen- boxcar averagers. The analog processor output voltage was sent to a lock-in amplifier locked at the chopper frequency. With this system, absorbance changes as low as 10-5 could be accurately measured. To functionalize the TiO2 nanoparticles with rhodamine 6G, a modified methanolic dye solution (Si:Rh6G) was prepared by reacting the dye with a silane molecule (3-isocyanatepropyltriethoxysilane) according to [3]. After the reaction, 10 mL of the Si:Rh6G solution was mixed to 5 mL of the TiO2 colloidal solution. The dye is proposed to anchor to the TiO2 nanoparticles via OH groups at the particles surface formed during the hydrolysis reaction. The colloid was kept under reflux at 80 °C for 24 hours. The resulting colloid was washed several times and redispersed in ethanol. The normalized transient bleaching signal shows curves that could be fitted by exponential functions. In this case, the bleaching recovery time is ≈ (2.9 ± 0.5) ns, which is smaller than the rhodamine 590 lifetime in ethanol (≈ 4.0 ns). However, for the dye binded to TiO2, the signal could be fitted by a sum of two exponential functions plus an offset. The time constants derived from the exponential fits to the TA kinetics for the Rh6Gfunctionalized TiO2 particles were τ1 = (160 ± 70) ps and τ2 = (1.0 ± 0.2) ns. The faster decay is assigned as charge transfer from thermalized excited states of the dye to the TiO2 conduction band, while the slower one corresponds to the back electron transfer from TiO2 to the dye. Direct exciton recombination and triplet states also contribute to the slower decay dynamics. The offset is a signature that the bleaching recovery is not completed during the time scale investigated due to trapping of electrons at in-gap states of the semiconductor or trapping of molecules in triplet states. Using the values of extracted from the TA data, the charge transfer rate is in the range (0.5-6.0)x109 s-1. This value