Dp. Colombo et Rm. Bowman, DOES INTERFACIAL CHARGE-TRANSFER COMPETE WITH CHARGE-CARRIER RECOMBINATION - A FEMTOSECOND DIFFUSE-REFLECTANCE INVESTIGATION OF TIO2 NANOPARTICLES, Journal of physical chemistry, 100(47), 1996, pp. 18445-18449
The charge carrier dynamics of opaque, aqueous suspensions of Degussa
P-25 TiO2 are probed with femtosecond time-resolved diffuse reflectanc
e spectroscopy. Comparison of ultrafast pump-probe diffuse reflectance
measurements of P-25 suspensions with dry P-25 powder and the transie
nt absorption of transparent, aqueous Q-TiO2 solutions allows the obse
rved kinetics to be assigned to charge carrier recombination. The elec
tron-hole recombination kinetics are consistent with a second-order pr
ocess as demonstrated by a laser fluence dependence study. Interfacial
hole transfer dynamics of the P-25 TiO2/SCN- complex are probed as a
function of thiocyanate ion concentration. A dramatic increase in the
population of trapped charge carriers is observed within the first few
picoseconds, demonstrating that interfacial charge transfer of an ele
ctron from the SCN- to a hole on the photoexcited TiO2 effectively com
petes with electron-hole recombination on an ultrafast time scale. The
experimental dependence of the charge carrier dynamics are shown to b
e consistent with a kinetic model of competing second-order processes.
The implications of the results on the use of nanoscale TiO2 for phot
ocatalysis are discussed.