The charge carrier dynamics of several TiO2 powders are investigated u
sing femtosecond time-resolved diffuse-reflectance spectroscopy. Ultra
fast pump-probe measurements of transparent solutions of 2-nm TiO2 nan
oclusters and the transient diffuse reflection observed in dry powders
of TiO2 are found to be qualitatively similar. The comparison allows
(i) the decay kinetics observed by time-resolved diffuse reflection in
powders to be attributed to electron-hole recombination and (ii) the
unequivocal assignment of the long-lived absorption in nanocluster sol
utions of TiO2 to trapped electrons. The conclusions are further suppo
rted by an intensity-dependent study of the electron-hole recombinatio
n kinetics of TiO2 powder which was consistent with a second-order pro
cess. The sensitivity of the technique is demonstrated by comparison o
f the recombination dynamics of powders of differing size and crystal
structure and with Fe(III) dopants. The results are discussed in terms
of the photoreactivities of the various TiO2 materials.